Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Ahmadijokani, Farhad
Doctor of Philosophy - PhD
University of British Columbia
Metal-organic frameworks (MOFs), specifically Zr-based UiO-66, have gained attention for removing toxic pollutants due to their unique attributes. These include a large surface area, high porosity, active adsorption sites, tunable chemistry, pore control, and robust host-guest interactions. However, the practical use of pure MOF powders in water treatment faces limitations such as insolubility, processability, brittleness, safety concerns, and separation challenges. One of the significant concerns when using MOFs for water treatment is how well they maintain stability when exposed to aqueous environments. Therefore, this thesis demonstrates the stability of UiO-66 MOF over one year in water and chloroform, with minor changes observed in structure when exposed to dimethylformamide (DMF). The results from the stability studies contribute to a better understanding of UiO-66's stability in aqueous media. UiO-66 exhibits effective removal of organic pollutants, including methyl red (MR), methyl orange (MO), malachite green (MG), and methylene blue (MB), which possess different charges. The stable UiO-66 polycrystalline MOFs still grapple with the challenge of effective separation from the treated water. This thesis pursued a magnetic approach for the magnetization of UiO-66, coupled with subsequent post-modification. This strategy aimed to facilitate the efficient separation of UiO-66 from treated water using an external magnet. A two-step post-modification procedure, utilizing 2,4,6-trichloro-1,3,5-triazine (TCT) and 5-phenyl-1H-tetrazole (PTZ) agents, was introduced to enhance the adsorption performance of the magnetic nanocomposite. Despite a reduction in porosity and specific surface area compared to the pristine UiO-66-NH2, the resulting magnetic post-modified UiO-66 exhibited significantly improved adsorption capacity. Notably, magnetic UiO-66-TCT demonstrated a high adsorption capacity of approximately 298 mg/g for methyl orange (MO), offering facile MOF separation via an external magnet. The research further underscores the significance of post-modifying UiO-66 with novel functional groups to create effective adsorbents for the removal of organic contaminants. Combining MOFs with electrospun nanofibers offers efficient water treatment, addressing challenges linked to polycrystalline MOFs and their separation from treated water. This thesis employed a hybrid approach by integrating UiO-66 with thermally oxidized nanodiamond (TOND) to enhance its affinity for removing targeted organic contaminants. These hybrid UiO-66 particles were then incorporated into chitosan/ polyvinyl alcohol (PVA) nanofibers. The resulting nanofiber composite, containing 1.5 wt% hybrid UiO-66, exhibited a twofold increase in the maximum adsorption capacity for anionic Congo red dye (1429 mg/g) compared to the unfilled nanofiber (769 mg/g). Further advancements in UiO-66 nanofiber composite development were achieved by functionalizing UiO-66-NH2 with TCT and PTZ, followed by their incorporation into chitosan/PVA nanofibers. The optimal composite, consisting of 7 wt% functionalized MOF, exhibited a remarkable maximum adsorption capacity for MO (619 mg/g), surpassing most previously reported adsorbents such as activated carbon.
UiO-66 Metal-Organic Frameworks for Water Treatment by Farhad Ahmadijokani M.Sc., Sharif University of Technology, 2017 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE COLLEGE OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) December 2023 © Farhad Ahmadijokani, 2023ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a dissertation entitled: UiO-66 Metal-Organic Frameworks for Water Treatment submitted by Farhad Ahmadijokani in partial fulfillment of the requirements of the degree of Doctor of Philosophy Dr. Mohammad Arjmand, School of Engineering Supervisor Dr. Lukas Bichler, School of Engineering Supervisory Committee Member Dr. Jian Liu, School of Engineering Supervisory Committee Member Dr. Liwei Wang, School of Engineering University Examiner Dr. Sohrab Zendehboudi, Memorial University External Examiner iii Abstract Metal-organic frameworks (MOFs), specifically Zr-based UiO-66, have gained attention for removing toxic pollutants due to their unique attributes. These include a large surface area, high porosity, active adsorption sites, tunable chemistry, pore control, and robust host-guest interactions. However, the practical use of pure MOF powders in water treatment faces limitations such as insolubility, processability, brittleness, safety concerns, and separation challenges. One of the significant concerns when using MOFs for water treatment is how well they maintain stability when exposed to aqueous environments. Therefore, this thesis demonstrates the stability of UiO-66 MOF over one year in water and chloroform, with minor changes observed in structure when exposed to dimethylformamide (DMF). The results from the stability studies contribute to a better understanding of UiO-66's stability in aqueous media. UiO-66 exhibits effective removal of organic pollutants, including methyl red (MR), methyl orange (MO), malachite green (MG), and methylene blue (MB), which possess different charges. The stable UiO-66 polycrystalline MOFs still grapple with the challenge of effective separation from the treated water. This thesis pursued a magnetic approach for the magnetization of UiO-66, coupled with subsequent post-modification. This strategy aimed to facilitate the efficient separation of UiO-66 from treated water using an external magnet. A two-step post-modification procedure, utilizing 2,4,6-trichloro-1,3,5-triazine (TCT) and 5-phenyl-1H-tetrazole (PTZ) agents, was introduced to enhance the adsorption performance of the magnetic nanocomposite. Despite a reduction in porosity and specific surface area compared to the pristine UiO-66-NH2, the resulting magnetic post-modified UiO-66 exhibited significantly improved adsorption capacity. Notably, magnetic UiO-66-TCT demonstrated a high adsorption capacity of approximately 298 mg/g for methyl orange (MO), offering facile MOF separation via an external iv magnet. The research further underscores the significance of post-modifying UiO-66 with novel functional groups to create effective adsorbents for the removal of organic contaminants. Combining MOFs with electrospun nanofibers offers efficient water treatment, addressing challenges linked to polycrystalline MOFs and their separation from treated water. This thesis employed a hybrid approach by integrating UiO-66 with thermally oxidized nanodiamond (TOND) to enhance its affinity for removing targeted organic contaminants. These hybrid UiO-66 particles were then incorporated into chitosan/ polyvinyl alcohol (PVA) nanofibers. The resulting nanofiber composite, containing 1.5 wt% hybrid UiO-66, exhibited a twofold increase in the maximum adsorption capacity for anionic Congo red dye (1429 mg/g) compared to the unfilled nanofiber (769 mg/g). Further advancements in UiO-66 nanofiber composite development were achieved by functionalizing UiO-66-NH2 with TCT and PTZ, followed by their incorporation into chitosan/PVA nanofibers. The optimal composite, consisting of 7 wt% functionalized MOF, exhibited a remarkable maximum adsorption capacity for MO (619 mg/g), surpassing most previously reported adsorbents such as activated carbon. v Lay Summary The UiO-66 family of materials, known as Metal-Organic Frameworks (MOFs), holds great promise for cleaning up water, but there are challenges in using them effectively. MOFs, in their pure powder form, can't be easily used for water treatment because they don't dissolve in water, are difficult to work with, and can be brittle and potentially unsafe. They also have limitations in removing certain contaminants, and it's hard to separate them from water once they've done their job. This thesis aims to show that UiO-66 MOFs can be used effectively in wastewater treatment by making them more stable and improving their ability to capture contaminants. We do this by adding active functional groups with high affinity to organic contaminants to the UiO-66 substrate. We also explored mixing UiO-66 with thermally treated nanodiamonds to make them better at capturing contaminants. By utilizing magnetization and electrospinning techniques, this research addresses concerns about using powdery MOFs in aqueous media. The findings on functionalization, adsorption mechanisms, and processing techniques provide valuable insights that can facilitate the swift application of UiO-66 MOFs in real-world water and wastewater treatment scenarios. vi Preface This dissertation showcases the original research undertaken by Farhad Ahmadijokani under the supervision of Dr. Mohammad Arjmand as the supervisor. Farhad Ahmadijokani was responsible for the materials preparation and characterization, as well as conducting data collection, analysis, and creating figures during the course of his thesis research. Numerous research contributions emerged through partnerships with experts from various Canadian and international research institutions. These collaborations extended to entities such as the Advanced Fibrous Material Lab at UBC Vancouver, the Bioproducts Institute at UBC Vancouver, the Basque Center for Materials (with a focus on Applications and Nanostructures in Spain), and the Institute for Advanced Studies in Basic Sciences in Iran. Below is a list of publications resulting from the research presented in the thesis, contributions of the main collaborators to each work, along with the respective chapters where the work can be found. Journal publications 1. F. Ahmadijokani, H. Molavi, M. Rezakazemi, S. Tajahmadi, A. Bahi, F. Ko, T.M. Aminabhavi, J.-R. Li, M. Arjmand, UiO-66 Metal-Organic Frameworks in Water Treatment: A Critical Review, Progress in Materials Science 125 (2021) 100904. Farhad Ahmadijokani was responsible for writing the first draft of the manuscript and preparing the figures. Hossein Molavi, Mashallah Rezakazemi, Addie Bahi, Frank Ko, Tejraj Aminabhavi, and Mohammad Arjmand performed the revisions. The project was supervised by Farhad Ahmadijokani and Mohammad Arjmand, who also finalized the manuscript in its current online version. vii 2. F. Ahmadijokani, H. Molavi, A. Bahi, R. Fernández, P. Alaee, S. Wu, S. Wuttke, F. Ko, M. Arjmand, Metal‐Organic Frameworks and Electrospinning: A Happy Marriage for Wastewater Treatment, Advanced Functional Materials 32(51) (2022) 2207723. Farhad Ahmadijokani was responsible for preparing the table of contents, writing the first draft of the manuscript, and preparing figures. Farhad Ahmadijokani, Addie Bahi, Frank Ko, Siying Wu, Stefan Wuttke, and Mohammad Arjmand performed the revisions. Mohammad Arjmand supervised the project. 3. F. Ahmadijokani, R. Mohammadkhani, S. Ahmadipouya, A. Shokrgozar, M. Rezakazemi, H. Molavi, T.M. Aminabhavi, M. Arjmand, Superior Chemical Stability of UiO-66 Metal-organic Frameworks (MOFs) for Selective Dye Adsorption, Chemical Engineering Journal 399 (2020) 125346. Farhad Ahmadijokani and Hossein Molavi were responsible for synthesizing and managing aging data collection. Rahman Mohammadkhani, Salman Ahmadipouya, and Atefeh Shokrgozar assisted with conducting XRD, SEM, TGA, and adsorption/desorption data collection. Farhad Ahmadijokani and Hossein Molavi were in charge of gathering and reporting the adsorption data. Farhad Ahmadijokani authored the manuscript, while Mashallah Rezakazemi, Tejraj Aminabhavi, and Mohammad Arjmand oversaw the revisions. Mohammad Arjmand funded and led the project. 4. F. Ahmadijokani, S. Ahmadipouya, M.H. Haris, M. Rezakazemi, A. Bokhari, H. Molavi, M. Ahmadipour, S.-Y. Pung, J.i.J.r. Klemeš, T.M. Aminabhavi, Magnetic Nitrogen-Rich UiO-66 Metal–organic Framework: An Efficient Adsorbent for Water Treatment, ACS Applied Materials & Interfaces 15(25)(2023)30106–30116. viii Farhad Ahmadijokani was responsible for synthesizing the samples. Salman Ahmadipouya, Hossein Molavi, and Mahdi Heidarian Haris assisted with conducting FTIR, BET, FESEM, TEM, CHN, and ICP-OES analyses. Farhad Ahmadijokani and Hossein Molavi carried out adsorption experiments and data reporting. Farhad Ahmadijokani authored the manuscript. Tejraj Aminabhavi, Hossein Molavi, and Mohammad Arjmand reviewed and revised the manuscript, providing guidance in data analysis. Mohammad Arjmand supervised the project. 5. F. Ahmadijokani, H. Molavi, A. Bahi, S. Wuttke, M. Kamkar, O.J. Rojas, F. Ko, M. Arjmand, Electrospun Nanofibers of Chitosan/Polyvinyl Alcohol/UiO-66/Nanodiamond: Versatile Adsorbents for Wastewater Remediation and Organic Dye Removal, Chemical Engineering Journal 457 (2023) 141176. Farhad Ahmadijokani and Hossein Molavi were responsible for synthesizing the particles. Farhad Ahmadijokani conducted the electrospinning process and fabricated the nanofiber composites. Hossein Molavi managed the submission of samples for FTIR, TGA, XRD, and BET characterizations. The experiment design was undertaken by Farhad Ahmadijokani, who also collected the adsorption data along with Hossein Molavi. Farhad Ahmadijokani authored the manuscript. Frank Ko, Orlando Rojas, and Mohammad Arjmand reviewed and revised the manuscript and funded the project, with Mohammad Arjmand acting as the project supervisor. 6. F. Ahmadijokani, H. Molavi, M. Amini, A. Bahi, S. Wuttke, T.M. Aminabhavi, M. Kamkar, O.J. Rojas, F. Ko, M. Arjmand, Waste Organic Dye Removal using MOF-based Electrospun Nanofibers of High Amine Density, Chemical Engineering Journal 466 (2023) 143119. ix Farhad Ahmadijokani and Hossein Molavi synthesized particles, with Farhad Ahmadijokani leading electrospinning. They both conducted adsorption experiments and data interpretation. Farhad Ahmadijokani handled XRD, TGA, BET, and FTIR and drafted the manuscript. Funding and manuscript reviews were conducted by Frank Ko, Orlando Rojas, and Mohammad Arjmand, with Mohammad Arjmand acting as the project supervisor. Conference publications and presentations 1. F. Ahmadijokani, M. Amini, A. Bahi, F. Ko, M. Arjmand, Electrospun UiO-66-NH2 Polyaniline Nanofiber: A Promising Adsorbent for Dye Waste Water Treatment, The 49th World Polymer Congress (Macro 2022). July 17-21, 2022, Winnipeg, Canada. 2. F. Ahmadijokani, H. Molavi, M. Amini, M. Arjmand, A Systematic Study on the Selective Adsorption of Quercetin onto Magnetic Fe3O4@PAA@UiO-66-NH2 Nanoadsorbent, American Chemical Society (ACS) Spring 2021. April 5-16, 2021. x Table of Contents Abstract .......................................................................................................................................... iii Lay Summary .................................................................................................................................. v Preface ............................................................................................................................................ vi Table of Contents ............................................................................................................................ x List of Tables ................................................................................................................................ xv List of Figures ............................................................................................................................. xvii List of Abbreviations ................................................................................................................. xxiii List of Chemicals ....................................................................................................................... xxiv List of Symbols ........................................................................................................................... xxv Acknowledgments...................................................................................................................... xxvi Dedication ................................................................................................................................. xxvii Chapter 1 Introduction ................................................................................................................. 1 1.1 Metal-organic Frameworks (MOFs) ............................................................................... 1 1.1.1 Water-stable MOFs ..................................................................................................... 3 1.2 MOFs and nanofibers .................................................................................................... 10 1.3 Electrospun nanofibers .................................................................................................. 13 1.3.1 Modification of nanofibers ....................................................................................... 16 1.3.2 MOF nanofiber fabrication strategies ....................................................................... 18 1.3.3 Water treatment applications of MOF-based fiber composites ................................ 27 1.4 Research scope .............................................................................................................. 29 xi 1.5 Thesis structure ............................................................................................................. 30 1.6 Gap, novelty, and objectives ......................................................................................... 32 Chapter 2 Research methodology .............................................................................................. 35 2.1 Material synthesis ......................................................................................................... 35 2.1.1 Synthesis of UiO-66 .................................................................................................. 35 2.1.2 Synthesis of UiO-66-NH2 ......................................................................................... 36 2.1.3 Synthesis of magnetic nitrogen-rich UiO-66 ............................................................ 36 2.1.4 Preparation of thermally oxidized nanodiamond (TOND) ....................................... 37 2.1.5 Synthesis of TOND@UiO-66 nanocomposite .......................................................... 38 2.1.6 Preparation of electrospun TOND@UiO-66 adsorbents .......................................... 38 2.1.7 Preparation of high-loading amine-functionalized MOF (Zr-MOF) ........................ 39 2.1.8 Preparation of electrospun MOF-based adsorbent nanocomposites ......................... 40 2.2 Materials characterizations ........................................................................................... 41 2.2.1 Dry-state characterization ......................................................................................... 42 2.2.2 Wet-state characterizations ....................................................................................... 45 2.3 Electrospinning machine ............................................................................................... 52 Chapter 3 Exploring the Stability and Structure-to-Performance Relationship of UiO-66 ....... 53 3.1 Introduction ................................................................................................................... 53 3.2 Long-term aging experiments ....................................................................................... 55 xii 3.3 Batch adsorption performance, kinetics, and equilibrium studies ................................ 56 3.4 Selective separation ...................................................................................................... 56 3.5 Results and discussion .................................................................................................. 57 3.5.1 Adsorbents characterization ...................................................................................... 57 3.5.2 Adsorption kinetics ................................................................................................... 63 3.5.3 Adsorption isotherms ................................................................................................ 66 3.5.4 Effect of initial pH and ionic strength on adsorption capacity ................................. 70 3.5.5 Selective separation .................................................................................................. 72 3.5.6 Regeneration and reusability ..................................................................................... 75 3.6 Chapter summary .......................................................................................................... 76 Chapter 4 Magnetization and Subsequent Modification for the Production of Highly Active Adsorbents 77 4.1 Introduction ................................................................................................................... 77 4.2 Isotherm adsorption experiments .................................................................................. 79 4.3 Desorption and reusability ............................................................................................ 79 4.4 Results and discussion .................................................................................................. 79 4.4.1 Characterization of adsorbents .................................................................................. 79 4.4.2 Adsorption isotherms ................................................................................................ 84 4.4.3 Kinetic study ............................................................................................................. 87 4.4.4 Effect of ionic strength and pH ................................................................................. 90 xiii 4.4.5 Desorption and recyclability ..................................................................................... 92 4.4.6 Adsorption mechanism ............................................................................................. 93 4.4.7 Comparison with other reported adsorbents ............................................................. 95 4.5 Conclusions ................................................................................................................... 96 Chapter 5 Designing Active Mat Adsorbents through UiO-66 Hybridization and Electrospinning 97 5.1 Introduction ................................................................................................................... 97 5.2 Dye removal experiments ............................................................................................. 99 5.3 Results and discussions ............................................................................................... 100 5.3.1 Characterization of adsorbent nanoparticles ........................................................... 100 5.3.2 Characterization of electrospun nanofibers ............................................................ 105 5.3.3 Dye adsorption experiments ................................................................................... 109 5.3.4 Reusability of ECPN/T@U for dye removal .......................................................... 125 5.4 Chapter summary ........................................................................................................ 126 Chapter 6 Waste organic dye removal using MOF-based electrospun nanofibers of high amine density 128 6.1 Introduction ................................................................................................................. 128 6.2 Adsorption experiments .............................................................................................. 131 6.3 Results and discussion ................................................................................................ 131 6.3.1 Characterization of Zr-MOFs ................................................................................. 131 xiv 6.3.2 Characterization of electrospun MOF-based adsorbent nanocomposites ............... 135 6.3.3 Dye removal experiments ....................................................................................... 139 6.3.4 Regeneration and reusability ................................................................................... 148 6.4 Chapter summary ........................................................................................................ 149 Chapter 7 Conclusions, contributions, and future directions ................................................... 151 7.1 Conclusions ................................................................................................................. 151 7.2 Contributions............................................................................................................... 154 7.3 Future direction ........................................................................................................... 156 Bibliography ............................................................................................................................... 158 xv List of Tables Table 1-1. Comparison of different MOF-based nanofiber fabrication techniques ..................... 21 Table 2-1. Electrospun chitosan-polyvinyl alcohol nanofibers with 1.5 wt% of different nanoparticle additives ................................................................................................................... 39 Table 2-2. Electrospun MOF-based adsorbents composed of the parent composition of 0.375 g PVA and 0.125 g chitosan with different contents of the modified MOFs .................................. 41 Table 3-1. BET surface areas and zeta potential of pristine UiO-66 and its solvent-aged samples....................................................................................................................................................... 62 Table 3-2. The kinetics parameters of dye adsorption models for pristine and solvent-aged UiO-66 nanoparticles ............................................................................................................................ 65 Table 3-3. The isotherms constants pertaining to adsorption models for various dyes ............... 68 Table 3-4. Dye adsorption capacity reported for various materials .............................................. 69 Table 4-1. Physical properties of pure and modified adsorbents .................................................. 81 Table 4-2. Relevant calculated parameters from different models .............................................. 86 Table 4-3. Thermodynamic parameters for the adsorption of MO onto m-UiO-66-TCT ............ 88 Table 4-4. Kinetic parameters for the adsorption of MO by the modified adsorbent at various temperatures .................................................................................................................................. 89 Table 4-5. Comparison of adsorption capacity of the developed adsorbents toward MO dye with the reported adsorbents ................................................................................................................. 96 Table 5-1. Textural properties of pristine materials and their composite .................................. 103 Table 5-2. Kinetic parameters for removing MB and CR dyes by the synthesized adsorbent materials ...................................................................................................................................... 116 Table 5-3. Adsorption capacities of different adsorbent materials toward anionic CR dye ...... 120 xvi Table 5-4. Isotherm parameters for the adsorption of MB and CR onto the fabricated nanofiber composites................................................................................................................................... 121 Table 5-5. Thermodynamic parameters for the adsorption of CR and MG dyes onto the ECPN/T@U at various temperatures .......................................................................................... 122 Table 6-1. Physical properties and chemical composition of Zr-MOF ...................................... 134 Table 6-2.Kinetics data for adsorption of MO onto adsorbent nanocomposites. ....................... 141 Table 6-3. Relevant parameters relating to the employed models .............................................. 143 Table 6-4. Adsorption capacities reported for MO dye removal by different adsorbent materials..................................................................................................................................................... 144 Table 6-5. Parameters for adsorption of MO dye onto ECNF-7 ................................................. 145 xvii List of Figures Figure 1-1. (a) Illustration of the crystal structure of UiO-66(Zr); cavities in highly porous UiO-66, (b) tetrahedral cavities, and (c) octahedral cavities. Blue and yellow spheres represent the void regions inside the tetrahedral and octahedral cages, respectively. Used with permission from Elsevier [70] .................................................................................................................................... 6 Figure 1-2. Various applications of UiO-66. Used with permission from Elsevier [70] ............... 8 Figure 1-3. Schematic of plausible mechanisms for adsorption of hazardous materials onto MOFs. Used with permission from Elsevier [70] ......................................................................... 10 Figure 1-4. General overview of the advantages of processing and applying electrospun MOF-based membranes for water remediation. Adapted with permission[114]. Copyright 2022, John Wiley and Sons. ............................................................................................................................ 13 Figure 1-5. Schematic of a) electrospinning setup and b) various electrospinning approaches to produce different nanofiber morphologies Adapted with permission[114]. Copyright 2022. ..... 15 Figure 1-6. Synthesis strategies of MOF-based nanofiber composites. Adapted with permission[114]. Copyright 2022, John Wiley and Sons ............................................................. 19 Figure 2-1. Fe3O4@UiO-66-NH2 functionalization with TCT and PTZ ..................................... 37 Figure 2-2. Schematic representation for the post-synthesis modification of Zr-MOFs ............. 40 Figure 2-3. Digital photo of electrospinning machine ................................................................. 52 Figure 3-1. Chemical structures of various dyes employed in this study .................................... 55 Figure 3-2. XRD patterns of UiO-66 before and after aging in (a) water for 6 (W-UiO-66-6) and 12 (W-UiO-66-12) months, (b) chloroform for 6 (C-UiO-66-6) and 12 (C-UiO-66-12) months, and (c) DMF for 6 (D-UiO-66-6) and 12 (D-UiO-66-12) months ................................................ 58 Figure 3-3. FE-SEM images of pristine UiO-66 and its aged samples for 12 months ................ 59 xviii Figure 3-4. The N2 adsorption/desorption isotherms (desorption in red color) and pore size distribution of pristine UiO-66 and aged samples in (a) water, (b) chloroform, and (c) DMF at different times ............................................................................................................................... 60 Figure 3-5. TGA (a) and DTG (b) profiles of UiO-66 before and after aging in different solvents for 12 months ................................................................................................................................ 63 Figure 3-6. Adsorption capacity of (a) pristine UiO-66, (b) W-UiO-66-6, (c) W-UiO-66-12, (d) C-UiO-66-6, (e) C-UiO-66-12, (f) D-UiO-66-6, and (g) D-UiO-66-12 for different dyes at different contact times ................................................................................................................... 64 Figure 3-7. Adsorption isotherms of (a) MR, (b) MO, (c) MG, and (d) MB onto pristine UiO-66....................................................................................................................................................... 66 Figure 3-8. Model fits of Langmuir (a and b) and Freundlich (c and d) for the adsorption of these dyes onto pristine UiO-66 ............................................................................................................. 67 Figure 3-9. The impact of pH (a) and salt concentration (b) on the adsorption capacity of dyes onto pristine UiO-66 and the pH impact on the zeta potential of pristine UiO-66 (c) ................. 71 Figure 3-10. The UV-Vis spectra and adsorption images of different mixtures, (a) MO/MB, (b) MO/MG, (c) MR/MB, and (d) MR/MG before and after adsorption onto the pristine UiO-66 ... 73 Figure 3-11. The ideal adsorption selectivity of MR and MO dyes against MG and MB dyes onto all adsorbents ........................................................................................................................ 74 Figure 3-12. Regeneration and recyclability of pristine UiO-66 ................................................. 75 Figure 4-1. (a) FTIR spectra, (b) XRD patterns, (c) N2 adsorption-desorption isotherms, and (d) pore size distribution of adsorbents .............................................................................................. 80 Figure 4-2. (a) TGA and (b) DTG curves of pure and modified adsorbents ............................... 83 xix Figure 4-3. (a and b) SEM and (c-e) TEM images of pure and modified adsorbents and (f) photo images of m-UiO-66-TCT suspension in water before and after the magnetic separation .......... 84 Figure 4-4. Adsorption isotherms and fitting experimental MO adsorption data to (b) Freundlich, (c) Langmuir, (d) Tempkin, and (e) Dubinin-Radushkevich isotherms. (Adsorption conditions: dye concentration= 10-250 mg/L, contact time= 180 min, adsorbent weight= 5 mg, dye solution volume =20 mL, temperature= 25°C ) .......................................................................................... 86 Figure 4-5. (a) Adsorption capacity of modified adsorbent towards MO as a function of time at various temperatures and (b) van't Hoff plot. (Adsorption conditions: dye concentration= 20 mg/L, contact time=0-180 min, adsorbent weight= 20 mg, dye solution volume= 50 mL, temperature= 25-45 °C) ................................................................................................................ 89 Figure 4-6. (a) The pseudo-first-order, (b) pseudo-second-order, and (c) intra-particle diffusion plots for MO adsorption onto the modified adsorbent .................................................................. 89 Figure 4-7. Effect of (a) NaCl concentration and (b) pH on the adsorption capacity of MO onto the modified adsorbent. (Adsorption conditions: dye concentration= 20 mg/L, contact time=180 min, adsorbent weight= 5 mg, dye solution volume= 20 mL, temperature= 25°C , NaCl concentration= 0-0.5 mol/L, pH=2-12). (c) zeta potential of the modified adsorbent as a function of pH. (d) XRD patterns of m-UiO-66-TCT nanoparticles before and after stirring in basic (pH ≈ 11) and acidic (pH ≈ 3) solutions for 120 min .............................................................................. 92 Figure 4-8. Reusability of the modified adsorbent towards MO adsorption. (Adsorption conditions: dye concentration= 20 mg/L, contact time=180 min, adsorbent weight= 5 mg, dye solution volume= 20 mL, temperature= 25°C ) ............................................................................ 93 Figure 4-9. FTIR spectra of pure MO dye and after being adsorbed onto (a) Fe3O4@UiO-66-NH2 and (b) m-UiO-66-TCT ........................................................................................................ 94 xx Figure 5-1. Schematic illustration of the fabrication of the electrospun TOND@UiO-66 nanofibers. Used with permission from Elsevier [277] ................................................................ 99 Figure 5-2. a) FTIR spectra, b) XRD patterns, c) N2 adsorption-desorption isotherms, and d) TGA thermograms of nanoparticles ............................................................................................ 101 Figure 5-3. SEM images of the synthesized nanoparticles ........................................................ 105 Figure 5-4. a) FTIR spectra and b) XRD patterns of electrospun nanofibers holding 1.5 wt.% of various nanoparticles .................................................................................................................. 106 Figure 5-5. SEM images of the fabricated electrospun composite nanofibers containing 1.5 wt.% of different adsorbent nanoparticles ............................................................................................ 108 Figure 5-6. Energy Dispersive X-Ray mapping of the ECPN/T@U nanofibers ....................... 108 Figure 5-7. Removal efficiency of the nanoparticles and their electrospun composite nanofibers toward a, c) MB and b, d) CR dyes ............................................................................................. 112 Figure 5-8. FTIR spectra of (a) ND, (b) TOND, (c) UiO-66, and (d) TOND@UiO-66 nanoparticles before and after adsorption of MB and CR dyes .................................................. 113 Figure 5-9. (a) The pseudo-first-order, (b) pseudo-second-order, and (c) intraparticle diffusion plots for the adsorption of MB and CR dyes onto the synthesized adsorbent nanoparticles ...... 114 Figure 5-10. (a)The pseudo-first-order, (b) pseudo-second-order, and (c) intraparticle diffusion plots for the adsorption of MB and CR dyes onto the fabricated composite nanofibers ............ 114 Figure 5-11. Adsorption isotherms of electrospun nanofiber composites for removing a) MB and b) CR dyes, c) Effects of NaCl concentration, and d) pH on the removal efficiency of ECPN/T@U toward MB and CR dyes ....................................................................................... 118 xxi Figure 5-12. Model fit of (a) Langmuir, (b) Freundlich, (c) Dubinin-Radushkevich, and (d) Tempkin isotherms for the adsorption of MB and CR onto the fabricated composite nanofibers..................................................................................................................................................... 119 Figure 5-13. Removal efficiency of the fabricated ECPN/T@U composite nanofibers toward MB and CR mixture and the related photo images as a function of contact time ...................... 120 Figure 5-14. Plausible adsorption mechanisms as a function of pH. Used with permission from Elsevier[277] ............................................................................................................................... 125 Figure 5-15. Cyclic usage of ECPN/T@U for removing CR and MB dyes .............................. 126 Figure 6-1. (a) XRD tracings, (b) FTIR spectrum, (c) N2 adsorption-desorption isotherm and pore size distribution (hollow symbols present desorption), and (d) TGA and DTG profiles of Zr-MOF ............................................................................................................................................ 133 Figure 6-2.SEM images of (a, b) pristine and (c, d) functionalized MOFs at different magnifications ............................................................................................................................. 135 Figure 6-3. (a) XRD tracings and (b) FTIR spectra of the specimens ....................................... 137 Figure 6-4. SEM images of the nanofiber specimens with different contents of Zr-MOF ........ 138 Figure 6-5. Energy Dispersive X-ray mapping of ECNF-7 ....................................................... 138 Figure 6-6. (a) Kinetic of MO adsorption and (b) Model fitting for pseudo-second-order model..................................................................................................................................................... 141 Figure 6-7. (a) Adsorption isotherms of MO, and (b) Langmuir adsorption isotherm fitting to adsorption data ............................................................................................................................ 142 Figure 6-8. Influence of (a) pH and (b) NaCl concentration on adsorptive removal of MO over the ECNF-7 ................................................................................................................................. 147 xxii Figure 6-9. Selective adsorption of ECNF-7 toward MO dye in the mixture of (a) MO/MB and (b) MO/MG dyes as a function of contact time .......................................................................... 148 Figure 6-10. Reusability of the ECNF-7 for the removal of MO dye ........................................ 149 xxiii List of Abbreviations ALD Atomic layer deposition MOFs Metal–organic frameworks COFs Covalent organic frameworks BAS Bottle around ship BET Brunauer–Emmett–Teller CNTs Carbon nanotubes CWA Chemical warfare agent ECNF Electrospun composite nanofibers EDS Energy dispersive spectroscopy ENF PVA/chitosan nanofiber FCC Face-centered cubic FTIR Fourier transform infrared spectroscopy GO Graphene oxide HPLC High-performance liquid chromatography HSAB Hard-soft-acid-base-theory LbL Layer-by-layer MOFs Metal-organic frameworks ND Nanodiamond PC Partition coefficient PCP Porous coordination polymer PSM Post-synthetic modification PSP Post-synthetic polymerization S Adsorption selectivity SBUs Secondary building units SEM Scanning electron microscopy TOND Thermally oxidized nanodiamond WHO World Health Organization XRD X-ray diffraction Min Minute ZIFs Zeolite imidazolate frameworks xxiv List of Chemicals BDC 1,4-benzenedicarboxylic acid CR Congo red CS Chitosan DMF Dimethylformamide DNT Dinitrotoluene EtOAc Ethyl acetate EtOH Ethanol GA Glutaraldehyde HAc Acetic acid HMDA Hexamethylenediamine MB Methylene blue MG Malachite green MO Methyl orange MR Methyl red MR Methyl red PA-66 Nylon PAA Poly(acrylic acid) PAN Polyacrylonitrile PS Polystyrene PTZ 5-phenyl-1H-tetrazole PVA Polyvinyl alcohol PVP Polyvinyl pyrrolidone TCT 2,4,6-trichloro-1,3,5-triazine THF Tetrahydrofuran TNT 2,4,6-trinitrotoluene xxv List of Symbols ∆G0 (KJ/mol) Standard free energy ∆H0 (KJ/mol) Standard enthalpy ∆S0 (J/mol K) Standard entropy PC (mg/g µM) Partition coefficient qe (mg/g) Equilibrium adsorption capacity qt (mg/g) Adsorption capacity S Adsorption selectivity C0 (mg/L) Initial adsorbate concentration Ct (mg/L) Adsorbate concentration at time t Ce (mg/L) Adsorbate concentration at equilibrium qmax (mg/g) Theoretical maximum adsorption capacity (mg/g) KF Freundlich adsorption constant nF Freundlich exponent KL Langmuir constant K1 (min-1), Rate constant for pseudo-first-order model K2 (g/mg.min) Rate constant for the pseudo-second-order model Kid (mg/g min1/2) Intra-particle diffusion rate constan C Constant (Intra-particle diffusion model) RL Dimensionless separation factor (derived from Langmuir model) B (mol2/kJ2) Dubinin-Radushkevich constant (indicator of mean free energy of adsorption) ε (J/mol) Polanyi potential R (J/mol K) Universal gas constant E Mean energy of adsorption BT (kJ/mol) Tempkin constant AT (L/g) Tempkin equilibrium binding constant xxvi Acknowledgments First and foremost, I extend my deepest appreciation to my dedicated supervisor, Dr. Mohammad Arjmand. His unwavering support, expert guidance, and constant encouragement have been instrumental in completing my PhD research. Without his mentorship, this work would not have been possible. Furthermore, I wish to express my appreciation to the members of my supervisory committee: Dr. Lukas Bichler, Dr. Jian Liu, Dr. Liwei Wang, and Dr. Sohrab Zendehboudi, for generously dedicating their time and providing valuable insights and comments. I am also indebted to Dr. Frank Ko and Dr. Orland Rojas. Thank you for hosting me at the UBC Vancouver campus. Their valuable time and constructive feedback significantly enriched the quality of my research. I am truly grateful to my cherished colleagues and friends, including Dr. Addie Bahi, Dr. Milad Kamkar, Dr. Mashallah Rezakazemi, Dr. Stefan Wuttke, Dr. Nikoo Ghahremani Sourani, Dr. Amin Shotorbani, Majed Amini, Dr. Hossein Golriz, Aysan Ghazanfari, Adel Tabbakh, Samira Komachali, Adel Jalali, Dr. Parya Keyvani, Sheri Kianfar, Dr. Hamed Habibi and Tina Raeisi. Their unwavering friendship and unwavering support have been a source of motivation and inspiration throughout my journey. Special thanks to my roommates, Mehdi and Mahyar, for their constant support, encouragement, and efforts in creating a harmonious living environment. My most profound thanks go to my family. To my wonderful wife, Maryam Dehbozorgi, and my kindest siblings, Farzad and Farid, whose love, encouragement, and care have been my pillars of strength. Lastly, my heartfelt appreciation goes to my parents, Nader and Farahnaz, whose enduring love and support have been my guiding light each and every day. xxvii Dedication To my lovely wife and parents 1 Chapter 1 Introduction Water security and providing clean water have been the most challenging issues for modern society because of the steady growth of the global population, modernization, industrialization, and climate change [1]. Recently, various contaminants, including heavy metal ions, organic dyes, pharmaceuticals, personal care products, fluoride, pesticides/herbicides, phosphate, etc., have been frequently detected in different natural water sources [2-9]. Usually, these emerging pollutants are toxic and even carcinogenic; hence, their presence in water sources may cause harmful effects on the environment, ecosystem, and human health [9-14]. Therefore, developing an efficient and cost-effective method for removing these emerging pollutants from water sources is essential. To date, different physicochemical and biological techniques, including chemical precipitation, biological degradation, adsorption, catalytic/photocatalytic degradation, flocculation, flotation, coagulation, ion exchange, membrane filtration, electrochemical treatment, etc., have been applied for the treatment of wastewater containing the emerging pollutants [8, 15-20]. However, these removal methods need expensive chemicals, multiple steps, high energy, and complicated operation conditions. 1.1 Metal-organic Frameworks (MOFs) MOFs have emerged as a novel class of crystalline porous materials that exhibit great potential for water remediations [21]. MOFs are built from the coordination reaction between metal ions or clusters and organic ligands [12, 22, 23]. Due to the large chemical and geometrical variety of organic ligands and the coordination modes of the metal ions/clusters, a broad scope of frameworks with predefined porosity and topology metrics can be designed following the principles of reticular chemistry [23]. In addition, the chemical adaptability of MOFs to be functionalized, both pre and post-synthesis, opens the avenue to tailor their framework and pore 2 space to capture specific inorganic or organic pollutants from aqueous media. Today, the MOFs' library extends to more than 100,000 frameworks [23] without accounting for the isostructural materials decorated with different functions or multivariate or multicomponent MOFs, where the space of chemical variance accessible augments exponentially [24-26]. Overall, MOFs' structural features endow them with low density, high permanent porosity, large specific surface area (theoretically up to 14600 m2/g), chemical versatility, tunable pore size/structure/distribution, and simple functionalization [10, 12, 25, 27, 28]. These exceptional properties opened the perspective to explore MOFs for a wide range of applications since the beginning of the reticular materials. As a natural step when comparing MOFs with porous materials such as zeolites or aluminosilicates, the initial investigations focused on gas storage and separation and heterogeneous catalysts [5, 10, 12, 23, 29-31]. Later, as the landscape and chemical robustness of the MOF materials increased, they were used for more stringent applications such as drug delivery, energy storage, and conversion [23, 26, 32-35], and water remediation. The porous nature and the chemical versatility of MOFs have revealed them as one of the most promising classes of sorbents for water remediation since they can be precisely tuned to capture target inorganic and organic pollutants. For instance, MOFs hold outstanding adsorption capacities, kinetics, and selectivities over organic pollutants such as dyes, drugs, persistent chemicals, or heavy metals such as mercury, lead, chromium, or arsenic. The merits of MOFs to capture organic and inorganic pollutants rival and even overcome the ones of traditional sorbents such as activated carbons, zeolites, or aluminosilicate materials [36]. 3 1.1.1 Water-stable MOFs Chemical stability, exclusively MOF water stability, is a crucial factor in the practical application of porous materials because water molecules' presence in many applications is unavoidable [37]. Water molecules attack the metal-organic ligand coordination bonds, break the ligand-metal bonds (ligand displacement), change the crystal phases, and finally collapse MOFs' structure [38-40]. The parameters, including crystallinity, moisture/water content, metal-ligand coordination geometry, central metal ions, porosity, organic ligands, pore surface, and hydrophobicity, contribute to the water stability of MOFs [39, 41]. Based on hard-soft-acid-base-theory (HSAB), stable MOFs with robust chemical bonds can be generated by the reaction between hard base-hard acid and soft base-soft acid [40]. For example, oxygen-donor ligands are considered hard bases; hence, stable MOFs can be created via the coordination of carboxylate-based ligands and hard acid metal ions, including Fe(III), Al(III), Zr(IV), and Cr(III) [41, 42]. Based on this theory, several water-stable MOFs such as UiO-66, PCN-230, PCN-229, PCN-228, PCN-777, PCN-521, NU-1105, NU-1000, MIL-160, MIL-808, MIL-163, PCN-253, and MIL-121(Al) have been synthesized [39, 43-50]. Soft-base nitrogen donor ligands form the other kind of stable MOFs (e.g., pyrazolates, imidazolates, tetrazolates, and triazolates) with the soft-acid metal ions (e.g., Cd(II), Cu(II), Pd(II), Ni(II), Co(II), and Zn(II)) [40-42]. The most stable MOF representative of this class is the zeolite imidazolate frameworks (ZIFs), which are comprised of Co(II) and Zn(II) as metal ions and imidazolate ligands as linkers to manufacture stable porous structures [41, 51, 52]. Therefore, based on the principles mentioned above, water-stable MOFs are classified into three main categories: (i) MOFs consisting of oxygen-donor ligands and high-valence metal ions, (ii) MOFs consisting of nitrogen-donor ligands 4 and soft-acid metal ions, and (iii) MOFs consisting of modified organic ligands with hydrophobic groups or MOFs functionalized by blocked metal ions [39]. The water stability of MOFs can be considered in terms of kinetic stability (short-term) and thermodynamic stability (long-term) [53]. Generally, when the metal-ligand binding energy is higher than the metal-water binding energy, MOFs' structure can resist hydrolysis and are stable against water [53-55]. Therefore, a high metal-ligand binding energy (high strength bond between organic ligand and metal centers), a high pKₐ of organic ligands, a high oxidation state of the metal ion, and a high coordination number favor producing water-stable MOFs [40, 54, 56-58]. Accordingly, introducing hydrophobic or water-repellent groups onto the organic ligands could efficiently protect the metal centers and prevent water molecules from approaching the metal centers and attacking the framework structures [59-61]. While the organic ligands with short hydrophobic functional groups do not necessarily decrease the MOFs' porosity and adsorption capacity remarkably [62]. Several studies have been published to improve the water stability of porous materials. For instance, CALF-25 was modified with nonpolar alkyl groups to enhance water stability [59]. In other works, the functionalization of MOFs with fluorine groups and superhydrophobic functionalities considerably improved water stability [60, 61]. There has been a significant surge in research focused on developing and synthesizing water-stable MOFs in recent years. Numerous stable MOFs have emerged, exhibiting enhanced structural robustness against water, organic solvents, high-temperature steam, and extreme pH conditions [39, 55, 63, 64]. Among the stable MOFs, Zr-based MOFs have shown exceptional stability against water because of the strong Zr-O bond and a specific geometry that hinders the inclusion of water molecules to mitigate the hydrolysis reactions [45, 54]. The Zr-based MOFs, especially UiO-66, have shown great potential for water-based applications [39, 41, 45, 54, 55] due to their unique 5 properties such as high thermal stability (> 700 K), excellent chemical stability against water, methanol, ethanol, benzene, acetone, chloroform, dimethylformamide (DMF), and aqueous acidic and basic solutions [12, 27]. Moreover, its crystalline structure can be preserved even after exposure to high external pressures [65]. Additionally, this nanoparticle could be post-modified by metal ion or ligand exchange mechanisms; as a result, various types of UiO-66 have been synthesized. 1.1.1.1 UiO-66 UiO-66 (Zr), the prototype of the UiO (stands for the University of Oslo) family, was first manufactured from the zirconium clusters (Zr6O4(OH)4) as the secondary building units (SBUs) and 1,4-benzenedicarboxylic acid (BDC) as the linkers [45]. As shown schematically in Figure 1-1, the UiO-66 has a face-centered cubic (FCC) crystal structure [66]. Accordingly, the Zr ion is 8-coordinated by oxygen atoms in the as-synthesized structure, and six of these clusters form the Zr6O4(OH)4 metal center. Each Zr6O4(OH)4 cluster or metal center is connected to the 12 nearest neighbor metal centers via 12 BDC linkers [65]. As shown in Figure 1-1, the aperture of UiO-66 is composed of four octahedral cages (1.1 nm) and eight tetrahedral cages (0.8 nm), and the accessibility to these cavities is ensured through the triangular windows (0.6 nm) [62, 67]. Furthermore, the strong bonds between metal centers and organic ligands, originating from the high framework connectivity, bond polarization, and high charge density, render UiO-66 high chemical stability under harsh conditions (such as different pHs) [68, 69]. Such features make the UiO-66 a promising candidate in various applications. 6 Figure 1-1. (a) Illustration of the crystal structure of UiO-66(Zr); cavities in highly porous UiO-66, (b) tetrahedral cavities, and (c) octahedral cavities. Blue and yellow spheres represent the void regions inside the tetrahedral and octahedral cages, respectively. Used with permission from Elsevier [70] 1.1.1.2 Large-scale synthesis of UiO-66 The crystalline nature of UiO-66 has held back its large production. Therefore, numerous investigations have been devoted to developing scale-up, green, and repeatable synthesis methods [71-73]. For instance, an environmentally friendly, relatively large-scale, and cheap synthesis method using water-based reflux synthesis of UiO-66(Zr)-(COOH)2 at 100°C was introduced [74]. A similar study on mild (1 atm, 100°C ), green (water as solvent), and large-scale (up to kilograms) modulated hydrothermal synthesis methods can be found in the literature [71], where zirconium nitrate and water/acetic acid mixture were used as the metal precursor and solvent, respectively, to synthesize a series of UiO-66(Zr)-type MOFs. More significantly, it was reported that this method could be extended to UiO-66-derivatives, including UiO-66-(COOH)4, UiO-66-(F)4, and 7 UiO-66-(OCH2CH3)2, which cannot be synthesized via the traditional solvothermal method. A relatively large-scale (pilot-scale, 100 L) solvothermal synthesis method was designed for the UiO-66 preparation using industrial-grade materials and reduced HCl [75]. The obtained UiO-66 exhibited high porosity (BET surface area: 1387 ± 12 m2/g) and product yield (> 93%). Moreover, it was reported that changing the ingredient scale could change the synthesized materials' crystallinity and porosity. On top of this, varying the activation solvents and methods led to synthesized MOFs with different porosities and crystallinities [39]. However, using a large amount of metal precursors, organic solvents (often dimethylformamide (DMF)), organic ligands, and high temperature entails increasing the cost. 1.1.1.3 Chemical stability of UiO-66-based MOFs The water stability of MOFs can be evaluated by different analytical methods such as BET surface area, X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), high-performance liquid chromatography (HPLC), etc., [41]. The related data to the structural stability of UiO-66 against various solvents, viz., water, benzene, acetone, and DMF [45], showed that XRD patterns remained unchanged after exposure to these solvents. Furthermore, the minor changes in the XRD patterns and surface area at pressures up to 104 kg/cm2 also indicated its excellent crystalline mechanical stability. It was thus proposed that the inorganic part (metal center) of the UiO-66 framework, composed of an inert Zr6O4(OH)4 cluster, played the dominant role in controlling the structural stability, where each Zr atom is 8-coordinated via the 1,4-benzenedicarboxylate (BDC) ligands in the structure, the highest possible connectivity among all the MOFs [45, 56]. 8 1.1.1.4 Applications of UiO-66-based MOFs Outstanding properties of UiO-66 offer wide applications in different areas, including catalysis, membrane, photocatalysis, adsorption, and separation in liquid and gaseous phases, drug adsorption and delivery, sensing, luminescence, supercapacitor, and pollutant elimination Figure 1-2 [5, 12, 30, 41, 48, 54, 64, 67, 76-79]. Figure 1-2. Various applications of UiO-66. Used with permission from Elsevier [70] 1.1.1.5 Organic dye elimination Synthetic organic dyes are wide-applicable compounds in multiple industries, such as leather, textiles, paper, cosmetics, wool, and painting [80]. However, due to their high toxicity, 9 carcinogenic, and non-biodegradable nature, releasing these into water bodies has seriously menaced the environment and human beings [81]. Therefore, their removal from wastewater is crucial. Various methods have addressed this issue, such as adsorption, membrane filtration, flocculation, biological treatment, and catalytic degradation [38]. Of all these, adsorption has received much attention due to its high efficiency, simple operation, low cost, environment-friendly nature, and easy regeneration of adsorbents [12, 13, 82]. Organic dye separation using MOFs has been widely explored due to its straightforward experimental nature [27]. UiO-66 eliminates organic dyes from aqueous solutions via the adsorption/separation, degradation, and combining of these processes. Understanding the adsorbate and adsorbent interactions is essential in the adsorption process. Designing the proper MOFs for adsorption processes without a clear idea about adsorption mechanisms is inconceivable. Adsorption mechanisms are mainly dominated by electrostatic interaction, hydrophobic interaction, acid-base interaction, hydrogen bonding, and π-π stacking interaction or a combination of two or more interactions (Figure 1-3) [38, 41, 83-85]. 10 Figure 1-3. Schematic of plausible mechanisms for adsorption of hazardous materials onto MOFs. Used with permission from Elsevier [70] 1.2 MOFs and nanofibers Despite the appealing characteristics of MOFs as sorbents, most are obtained in powder form, a critical feature that restricts their usage in large-scale applications such as water remediation [23, 86-90]. To mention some of their limitations, MOF powders exhibit poor heat/mass transfer, hard processability, unsatisfactory mechanical stability, high-pressure drops in the packed beds, difficulty handling and recovery, or risk of dust formation when processed [41, 87, 90-92]. Thus, it is urgent to shape MOF-powdered materials into easy-handling MOF-based devices or composites without significantly sacrificing their intrinsic properties [11, 23, 41, 90, 93-98]. It is important to note that by shaping the MOF powder, the mechanical properties, 11 permeability, easy recoverability, and retention of the MOF functionality need to be balanced to obtain the best of the compromises in the final material/device for water remediation. For example, MOFs can be processed as 100%-MOF porous gels or foams without losing material functionality, but improving these systems' mechanical properties and water permeability remains challenging. Similarly, polymeric binders have been used to obtain MOF-based composite systems. However, a binder reduces specific surface area and partially blocks the pores [20, 92, 99-102]. Combining the MOFs with nanostructured materials (i.e., silica, alumina, carbon nanotubes, graphene oxide, graphite oxide, metal oxides, nanodiamonds, etc.) has emerged as an effective strategy to fabricate high-performance composites leveraging the features of both nanomaterials [23, 26, 103]. In contrast, shaping and recovering these composites after use remains a challenge similar to those for powdered MOFs. Among the multiple alternatives explored up to date to shape MOFs, incorporating MOF particles into a polymeric matrix is one of the most appealing ones in terms of processability, malleability, thermal, chemical, and mechanical properties that the polymeric component endows to the hybridized MOF-polymer systems [34, 104-107]. In general terms, the role of the polymeric substrate is highly dominant in MOF-composite's final structural and mechanical properties, while the MOF has a clear impact on the functionality of the systems (i.e., higher permeability, selectivity, or adsorption capacity). Hybridization of polymer and MOF worlds is far from being straightforward, as many parameters shape the final properties and performance of the system, such as (i) binding mode between the polymeric matrix and the MOF material [20, 104], (ii) nature of polymeric and MOF components [47, 108, 109], and especially, (iii) the shaping process of the polymer-MOF composites in their final functional forms (i.e., membranes, films, filters, textile fibers, etc.). 12 Among the multiple alternatives to shape and process the polymer-MOF composites, flexible textile fibers are deemed abundant and inexpensive polymeric substrates to host MOF nanoparticles without altering their functionality [5]. Shaping polymeric matrix as a high surface-to-volume ratio fiber structure increases the active surface in contact with the surrounding environment, increasing the chances of surface trapping of organic or inorganic pollutants found in water bodies [26, 76, 110-112]. In addition, fine control over the MOF-polymer fiber structure through a meticulous processing strategy enhances the MOF nanoparticle's stabilization while avoiding the loss of functionality when immobilized into the polymeric host fibers. This is where the electrospinning processing of MOF-polymeric fiber-like materials comes into play. Among various methods for designing nanofibers (i.e., self-assembly, drawing, template synthesis, and fiber spinning), electrospinning has received particular attention in recent years due to the multiple parameters that can be controlled to shape unique features of the MOF-polymeric fibers nanostructure (Figure 1-4), and their final performance [113]. For instance, MOF-polymeric textile fiber-like composites have been successfully applied to remove emergent organic contaminants (i.e., organic dyes, heavy metal ions, pesticides, drugs, oily compounds, etc.) and inorganic highly toxic heavy metals. Interestingly, in some specific cases, the MOF-polymeric composites have outperformed their constituent's performance [5]. 13 Figure 1-4. General overview of the advantages of processing and applying electrospun MOF-based membranes for water remediation. Adapted with permission[114]. Copyright 2022, John Wiley and Sons. 1.3 Electrospun nanofibers Electrospun nanofiber technology has received significant attention in the past two decades due to its versatility and ability to produce nanofibers with controlled morphologies [115]. Nanofibres belong to a new fibrous material class having high aspect ratios and a diameter equal to or less than 100 nm [116-118]. Various methods have been employed for nanofiber fabrication, including wet-spinning, melt or dry-spinning, solution blow spinning, template synthesis, and force spinning [36, 41, 119-122]. However, obtaining nanofibers using electrostatic force via electrospinning is the most commonly employed method [123, 124] due to its simplicity, diversity, and capability to produce nanofibers with a wide range of diameters [116, 125, 126]. This technique applies a high-voltage electric field to polymer solution/melt to produce nanofibers. 14 As shown in Figure 1-5, the electrospinning setup consists of four main parts: a high-voltage power supply, a syringe pump, a spinneret, and a collector. Accordingly, the polymer is melted at a high temperature or dissolved in an appropriate solvent to produce a polymer melt/solution with a proper viscosity. Next, the polymer melt/solution is spun by applying a high-voltage electric field. As the electric field crosses a critical value, the electrical forces will overcome the melt/solution surface tension, and the melt/solution droplet shape in the tip of the needle will transform into a conical shape known as the Taylor cone. Finally, the solvent of the ejected-out charged jet will be evaporated, and nanofibers will form and be collected onto the collector surface [126-129]. The effect of electrospinning conditions on the nanofibers' shape, diameter, and structural morphologies has been comprehensively discussed in the literature [113, 129-133]. The nanofibers' shape, diameter, and surface morphology largely depend on the (i) polymer solution/melt properties (i.e., molecular weight, viscosity, concentration, electrical conductivity, molecular weight distribution, surface tension, polymer conformations, pH value, and the solvent evaporation rate), (ii) processing conditions (i.e., the geometry and size of the collector mandrel, the distance between the spinneret and the collector (working distance), (iii) strength of the electric field, (iv) the flow rate, and, (v) the ambient conditions (i.e., airspeed, temperature, and humidity) [113, 129, 133-137]. 15 Figure 1-5. Schematic of a) electrospinning setup and b) various electrospinning approaches to produce different nanofiber morphologies Adapted with permission[114]. 16 Copyright 2022. On top of that, the nanofiber morphology can be adjusted by varying electrospinning setups, including needleless electrospinning, multi-jet electrospinning, co-axial electrospinning, emulsion electrospinning, and bubble electrospinning [99, 138-145]. Accordingly, a single-layered nanofiber mat is attainable by a simple electrospinning technique, while multi-layered nanofibers are achievable using a triaxial electrospinning setup consisting of three needles linked to the spinneret device core, intermediate, and sheath solutions [139]. In the triaxial electrospinning technique, the core and intermediate solutions should be immiscible, while the intermediate and outer sheath solutions could be miscible to each other [146]. Core-shell nanofibers are fabricated using typical co-axial or simple emulsion electrospinning methods [147]. Multi-layered and core-shell nanofibers are usually applied in biomedical applications because of the high loading capacity and simplicity of poorly soluble drugs loading into the polymeric shell [56, 148-150]. In the bubble electrospinning method, which is designed for potential mass production, electrospinning starts from multiple points on the bubble's surface, and nanofibers are prepared simultaneously [151]. 1.3.1 Modification of nanofibers Several features, such as high porosity, suitable pore size, and high chemical, thermal, and mechanical stabilities, are necessary for electrospun nanofibers to be employed for water treatment because of their exposure to different pHs, high temperatures and stresses, and high vibration in a liquid flow [125, 126]. Despite several advantages of electrospun nanofibers (i.e., high porosity and high surface area-to-volume ratios), some pristine polymeric nanofibers still suffer from some limitations in wastewater treatment [25]. For instance, some polymeric nanofibers are unstable in water (i.e., poly(acrylic acid) (PAA), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), etc.) 17 or have low adsorption capacity for efficient removal of water pollutants (i.e., nylon, polyacrylonitrile (PAN), etc.), and some possess weak mechanical properties like chitosan [125]. Considerable literature has grown up around the theme of improving polymeric nanofibers' performance via surface modification. These studies have successfully enhanced the stability (mechanical, thermal, and chemical), hydrophilicity, wettability properties, and adsorption capacity due to increased active surface sites [152]. Two main strategies have been followed for surface modification of the fibers: (i) one-step modifications (blending and nanocomposite fabrication), which can be performed during electrospinning, and (ii) post-modifications (grafting, plasma, coating, and wet-chemistry), which can be performed after electrospinning [141, 153-157]. Modifying hydrophobic nanofibers with amphiphilic copolymers or hydrophilic homopolymers is common for water treatment [65, 69, 158]. For instance, modifying polyamide-6 with chitosan increased the final nanofiber hydrophilicity, resulting in improved dye removal efficiency [159]. An exciting approach to expanding the polymeric fibers' functionalities for water remediation is the inclusion of nanomaterials within the polymeric mats. Nanofiber composite or mixed matrix nanofibers are polymeric matrices containing organic or inorganic nanoparticles as dispersed fillers. Electrospinning of solutions/melts holding various nanoparticles is popular for modifying polymeric nanofibers' physicochemical properties and water treatment performance. Up to now, different materials such as nanostructured carbon-based (i.e., graphene and graphene oxide, carbon nanotubes, nanodiamond, etc.), metal-oxide nanoparticles, nano clays, zeolites, MOFs, and so on, have been incorporated into the polymeric nanofibers to modify their performance [5, 160, 161]. Applying an appropriate electrospinning strategy to integrate nanomaterials into the nanofibers would lead to polymeric nanocomposites, which enjoy the merits 18 of both polymeric matrix and nanoparticle fillers. Even in some cases, the nanocomposites overperform the average functionalities of their components [82, 162-164]. 1.3.2 MOF nanofiber fabrication strategies As mentioned above, electrospinning is promising for shaping MOF powders into composites with a hierarchical porosity and improved structural properties. As summarized in Figure 1-6, A) the direct electrospinning of MOF polymer melt/solution, B) the direct growth of MOF particles on electrospun polymeric fiber, and C) the surface decoration of the electrospun fibers with presynthesized MOFs are the three main strategies to obtain MOF-polymer composite fibers. It is important to note that the strategies mentioned above can be combined to control fiber nanocomposites' morphology and internal structure. In addition, post-treatment methods can be further applied to modify the MOF-based nanofiber composites. The direct electrospinning of MOF/polymer starts with a slurry mixture of a polymer solution and MOF particles. Accordingly, the mixture is directly electrospun into a MOF-based nanofiber composite (Figure 1-6 A.1). However, the pores of embedded MOF in nanofiber are usually covered with polymer chains, translating into the reduced surface area and declined water treatment performance of MOFs. Disruption of polymer processing and formation of deformed nanofibers are probable in this method. While having a rapid production rate and enjoying high-quality presynthesized MOFs with perfect crystalline structure in the final nanofiber composites make this strategy appealing [165]. A less explored alternative is to follow up polymerization of MOF and monomer during the electrospinning process, as schematically depicted in Figure 1-6 A2. Specific reaction and electrospinning conditions are central to attaining a controllable binding of MOFs and polymer during spinning. 19 Figure 1-6. Synthesis strategies of MOF-based nanofiber composites. Adapted with 20 permission[114]. Copyright 2022, John Wiley and Sons Direct MOF growth on the surface of the polymeric fibers is the most explored strategy to obtain MOF-fiber composites, as depicted in Figure 1-6 B. The decoration via MOF synthesis strategy refers to the in situ growth of MOF particles in or onto the surface of nanofibers. Thus, the surface area, pores, and physical and chemical properties of MOF could be preserved in this method. However, MOF nanoparticles can be formed either on the nanofiber surface or ambient, leading to the loss of some of the precursors and decreasing the nanofiber formation rate. Moreover, this method contains a two-step preparation process, including electrospinning a polymeric nanofiber and subsequent MOF growth. The critical point is the survivability of polymeric fiber substrate in the MOF synthesis ambient because using harsh solvents under high temperatures and pressures is very common for MOF synthesis. In addition, pretreatments such as atomic layer deposition (ALD) might be needed depending on the polymeric matrix to facilitate MOF nucleation growth. As a result, the production rate of MOF-based nanofiber composites in this strategy is relatively lower than that of the former [112, 165]. Several approaches have been applied to overcome these challenges, such as the seeds incorporation into the electrospun fibers, deposition of metal and metal-oxide layers before the MOF growth, the layer-by-layer MOF incorporation to the fibers coordinatively assisted MOF growth that induces the steady replacement of a metal-oxide or metal layer growth on the fiber by MOF crystals, the direct crystallization of MOF layers into electrospun membranes by inkjet printing and hot-pressing (Figure 1-6 B). Last but not least, the assembly of MOF nanoparticles to the polymeric fibers is a less explored but thoughtful approach that requires engineer-specific and complementary surface functionalization of the MOF nanoparticles and polymeric fibers. Finally, a comparison of different MOF-based nanofiber fabrication techniques is in Table 1-1. 21 Table 1-1. Comparison of different MOF-based nanofiber fabrication techniques Techniques Advantages Disadvantages Strategies for improving Direct electrospinning - Control over the nucleation and growth of MOFs - Chance to have MOF crystals with high quality in the nanofiber -Rapid MOF-fiber production rate -Poor MOF dispersion -Pore blockage of MOFs -Only suitable for low MOF loading -Disruption of polymer processing and formation of deformed nanofibers -MOF functionalization -Priming technique -Using blend polymer matrix -Using volatile solvents -In situ polymerization Surface decoration via MOF synthesis -Uniform coatings of MOF crystals -Deposition of MOF layers with controlled thickness and small crystallite size -Good MOF-polymer interfacial interactions -Direct exposure of MOFs` active sites with ambient -Low MOF-fiber production rate -MOF nanoparticles formed in solution bulk instead of surface nanofibers, decreasing MOF loading -Low stability of many common polymer nanofibers under MOF synthesis condition -In situ MOF growth -Using organic-inorganic composite nanofibers -Using compatible solvents for MOF production -Cross-linking the polymeric nanofibers -Seed-assisted in situ MOF growth -ALD-assisted in situ MOF growth -Using LbL MOF growth strategy -Using a coordination replication strategy Surface decoration with presynthesized MOFs -Control over the nucleation and growth of MOF particles and nanofiber fabrication -Direct exposure of MOFs` active sites with ambient -Low MOF-fiber production rate -Poor MOF-polymer interfacial interactions -Requires MOF and nanofibers modifications -Risk of leaching the MOF nanoparticles during handling and under operating conditions in the case of improper interfacial interaction -PSM of MOF particles and polymeric nanofibers -Chemical attachment of MOF particles to polymeric nanofibers via covalent bonding 22 In direct electrospinning of MOF/polymer solution or MOF-first strategy, the presynthesized MOF particles are mixed with a polymer and processed into nanofibers. Accordingly, the MOF composites are fabricated by a simple three-step procedure, including synthesis of MOF, MOF-polymer slurry preparation, and direct electrospinning of slurry [166]. Thus, MOF particles can form hybrids with various polymer solutions (polymer/solvent systems) and undergo electrospinning under proper conditions (i.e., the loading of MOFs, operating voltage, the viscosity of the MOF-polymer slurry, working distance, etc.). Consequently, the three-dimensional MOFs exhibit a morphology resembling composites of one-dimensional nanofiber. The direct electrospinning strategy allows loading a wide range of MOF contents into the polymeric matrix. As a result, the nanofiber structural integrity can be retained at low MOF loadings [34]. However, several examples of high-MOF-content nanofibers with appropriate mechanical stability and improved chemical activity have emerged in the literature [93, 167-171]. The critical point is that the MOF content in the high MOF loading slurry is sometimes greater than the MOF content in the composite, mainly because of solubility differences between ingredients, MOF agglomeration, and settling or sedimentation of MOF particles [166, 172]. Furthermore, the sedimentation of MOFs can exacerbate the risk of needle clogging during electrospinning. Therefore, incorporating accurate contents of MOF in the polymeric matrix is essential because excess MOF loadings can deteriorate the nanoparticle dispersion and nanofiber integrity. On top of that, high MOF content would increase the slurry viscosity, decrease the stability dispersion, and cause several operating problems, such as needle clogging and being energy-intensive [79, 123, 173]. The solvents can affect the structure of nanofiber composites. For instance, Peterson et al. [79] investigated the effect of different parameters, including polymer and MOF weight fractions 23 and solvent type, on the UiO-66-NH2/polystyrene (PS) fiber structure for chemical warfare agent (CWA) detoxification. Accordingly, UiO-66-NH2 nanoparticles were deposited on the surface of PS fibers using pure DMF solvent while changing the solvent to the mixture of 50:50 DMF: tetrahydrofuran (THF) resulted in the deposition of UiO-66-NH2 within the PS fibers. As a versatile method, simple blending has been widely used for electrospinning different fiber nanocomposites. For instance, Wang and colleagues [170] fabricated various nanofibers by mixing different polymeric matrices (i.e., PS, PAN, and polyvinylpyrrolidone (PVP)) with various MOF materials (i.e., UiO-66-NH2, ZIF-8, Mg-MOF-74, and MOF-199). As a result, the flexible MOF-based nanofiber composites with a high MOF content (up to 60 wt%) and proper MOF dispersion were fabricated and employed for air purification. The post-synthetic polymerization (PSP) technique has shown an incredible capacity to attach and disperse the MOF to the polymer substrate covalently. In this regard, Cohen and colleagues [174] functionalized UiO-66-NH2 with adipoyl chloride via a post-synthetic modification to introduce reactive acyl chloride groups on MOF organic ligands. Then the modified UiO-66-NH2 (UiO-66-int, where int refers to intermediate) and adipoyl chloride in ethyl acetate (EtOAc) were added to a water solution containing hexamethylenediamine (HMDA). Subsequently, interfacial polymerization occurred at the phase interface, and the modified MOFs were covalently bonded to the nylon (PA-66) polymer chains. Further, stretching the prepared PA-66-UiO-66-NH2 turned the hybrid into flexible fibers. The nylon-MOF composite showed a substantial improvement in the catalytic hydrolysis of a nerve agent simulant compared to the MOF embedded in nylon via physical adhesion [64]. The direct growth of MOFs as particles or films onto the surface of preformed nanofibers, or the fiber-first method, is the second strategy to integrate the MOF into nanofibers. Several 24 problems according to the previous method, such as poor dispersion of MOFs in the polymeric matrix, pore blockage of MOFs, and weak mechanical properties, are addressed in this process. The deposition of MOF layers with controlled thickness and small crystallite size on the surface of nanofibers are advantages that are not achievable by the direct electrospinning method. Direct exposure of MOFs' active sites with ambient in these composites gives them wide applications in adsorption and catalysis [5, 112]. In direct in situ growth of MOFs, precursors (typically an organic ligand and a metal salt) are dissolved in an appropriate solvent, and the nanoparticle synthesis is followed in the presence of nanofibers. Accordingly, MOF crystals' nucleation, growth, and inter-growth on the surface of preformed nanofibers usually occur simultaneously. The chemical features and thermal stability of polymer nanofiber substrates determine the quality and quantity of growth MOFs. Conventional polymers with large active functional groups (i.e., hydroxyl, amine, and carboxyl) are suitable for incorporating MOF nanoparticles [112, 165, 175, 176]. In the seed-assisted strategy, at the first stage, different materials, including presynthesized MOFs, organic ligands, metal precursors, and reactive inorganic materials, could be added to the polymer solution before electrospinning; subsequently, after the spinning of the solution, the seeded nanofibers would be treated to grow MOF nanoparticles. In the MOF seed-assisted growth, the presynthesized MOF seeds are incorporated into the polymer solution to initiate the secondary growth of the MOFs further. For instance, Li and coworkers [166] reported electrospinning of copolymer (P(St-co-TMSPMA) solution denoted as PST (St and TMSPMA referred to styrene and 3-(trimethoxysilyl)propylmethacrylate, respectively), containing presynthesized Zn-MOF nanoparticles followed by secondary solvothermal growth of MOF. Accordingly, Zn-MOF/PST thin films were formed. Zn-MOF films from the secondary growth had a good fluorescence 25 quenching sensitivity to the nitroaromatic explosives, including 2,4,6-trinitrotoluene (TNT) and dinitrotoluene (DNT). Incorporating reactive inorganic materials in or onto the nanofiber by ALD to promote MOF growth is another strategy for integrating MOFs into polymers. Precoated nanofibers with various metal oxides, such as TiO2, ZnO, Al2O3, etc., by ALD have shown great promise as substrates for synthesizing a large number of MOFs [177]. ALD is an effective technique for depositing conformal thin films, controllable to the atomic scale. This method is usually composed of two half-reactions that occur only at the surface of the substrate to be coated. The first step is to dose a metal-containing precursor, which reacts with the located hydroxyls on the surface of substrates, leaving a metallated surface. The excess reactants would be purged from the system. The second reactant or precursor is an oxygen-containing reactant (i.e., water), which reacts with the metal sites to form a metal oxide. Then, excess water molecules and any reaction products are purged. These two steps can be repeated until the favorable film thickness is obtained. As a result, ALD is particularly useful because the small-scale vapor phase precursors allow for conformal films on the surface of different substrates with complex topologies like fiber mats. The layer-by-layer (LbL) growth strategy is an efficient method to develop MOFs onto fibers at ambient conditions (unlike the hydro/solvothermal synthesis methods that are performed under high reaction temperatures and pressures) [12]. For instance, Morsali and colleagues studied an ultrasound treatment-assisted LbL strategy to grow MOF-5 [178] and Cu-BTC MOF [109] onto a silk fiber surface. The Cu-BTC MOFs were grown by sequential dipping of silk nanofiber into a metal salt solution (aqueous Cu(OAc)2.2H2O solution) and organic ligand solution (H3BTC in a mixed solution of ethanol (EtOH) and DMF) and intermediate washing steps to remove unreacted MOF precursors. 26 Coordination replication from solid precursors is another emerging method that benefits from the coordination replication directly from insoluble and cheap MOF precursors to prepare MOF-based fiber composites. Kitagawa and colleagues [179] proposed this strategy to design a well-structured Al-MOF architecture for the first time. Inspired by geological processes, this strategy relies on the morphological replacement of a shaped sacrificial metal oxide, which acts as a metal source for MOF formation and as an architecture-directing agent by an analogous porous coordination polymer (PCP) architecture. Therefore, this pseudomorphic replication strategy leads to mesoscopic architecture made of PCPs in one, two, and three dimensions [179, 180]. The MOF deposition onto the papers, plastics, and polymeric fibers could effectively widen their applications. For instance, MOF-deposited materials can be used as a platform for photonic or biomedical testing because of their substrates' flexibility, low cost, and biocompatibility [181, 182]. However, only a few studies in this field have been reported. Therefore, developing a general and effective method for the controlled deposition of surface-attached MOFs on such substrates remains challenging. Although conventional patterning techniques (i.e., soft lithography, photolithography, pen-type lithography, electron-beam lithography, and deep X-ray lithography) allow the fabrication of surface-attached MOFs with a high lateral resolution in the micro/nanometer regime, most of these methods consist of complicated multi-step procedures, which usually need complex/costly machinery, relatively producing large amounts of chemical waste, take a long time, and are typically restricted to relatively small overall areas. Moreover, these methods mainly focus on depositing surface-attached MOFs onto non-transparent and non-flexible substrates like metal or metal oxides [136, 183-186]. The hot press synthesis of the solvent- and binder-free MOF synthesis enables rapid MOF coating development on different substrates such as cloths, metal foils, and fibers. The MOF 27 precursor powders (metal ions or organic ligands) can react with metal sites or functional groups on the substrate surface under compression at an applied pressure and temperature to form the first MOF layer [41]. Subsequent MOF growth can then be repeated. As a result, the hot-pressing method can provide an appropriate binding between MOFs and flexible substrates, finally enhancing the flexibility of such MOF-coated devices. Moreover, this strategy allows simultaneous synthesis and adhesion of various MOFs (i.e., MOF-5, ZIF-8, Co-MOF, ZIF-67, Cd-MOF, ZIF-9, and bimetallic Ni-ZIF-8) onto different flexible substrates, including carbon cloth, glass cloth, aluminum oxide film, glass fiber, copper foil, nickel foam, silk clothing and nonwoven fibers [145, 151]. It should be considered that the substrate must be stable at 473 K during the hot-pressing method [165]. The presynthesized MOFs can be integrated into the preformed polymeric nanofibers through covalent and non-covalent interactions. For instance, different polymers as binders can facilitate hydrogen bonding or van der Waals interactions between phases. Moreover, coupling agents can covalently integrate MOFs into the polymeric nanofiber. For instance, spraying the MOF suspension onto electrospun fibers is a simple strategy to fabricate MOF nanofibers [76, 187, 188]. Accordingly, UiO-66 nanoparticles suspended in methanol were sprayed onto silk nanofiber and employed for protection against CWAs [187]. As a result, the nanocomposite with about 60 wt% UiO-66 was obtained; however, the composites severely suffered from poor interaction of UiO-66 and silk nanofibers. Therefore, leaching the MOFs during handling and operating conditions restricted this strategy's potential for industrial applications. 1.3.3 Water treatment applications of MOF-based fiber composites Water security has been one of the most concerning challenges of the 21st century, originating from population growth, industrialization, modernization, and climate change [1, 10, 28 41]. According to World Health Organization (WHO) reports, about one billion people worldwide lack proper access to safe drinking water, and the water shortage might adversely influence more than four billion people by 2050 [189]. In addition, more than 80 countries worldwide encounter severe water shortages, and about 25% of the population worldwide does not have sufficient access to freshwater [190, 191]. On top of that, the direct disposal of industrial and domestic wastes into the water bodies exacerbates the water crisis [27]. As a result, emerging pollutants such as organic dyes, heavy metal ions, drugs, antibiotics, and pesticides threaten water sources, ecosystems, and human health due to their toxicity and high resistance to degradation [9-13, 192]. Therefore, developing cost-effective and more efficient water and wastewater treatment techniques is essential. MOF-based materials are among the most attractive adsorbents for efficiently removing pollutants from contaminated water due to their large surface area, abundant accessible unsaturated coordination sites, and tunable pore shape/size [10, 193]. Combining MOF-based adsorbents with suitable materials can further improve their stability and adsorption performance [123, 194-199]. Water pollution remediation, prevention, and even sensing using MOF-fibre electrospun adsorbents are steadily increasing. MOF-based electrospun fiber adsorbents have shown significant advantages in water treatment applications due to their noticeable properties, including high porosity, large surface-to-volume ratio, good film-forming ability, facilitated recyclability, and excellent flexibility. For instance, these composites have better performance and a more straightforward recovery than pure MOF powders for water treatment applications. Due to organic dyes' carcinogenic and mutagenic characteristics and their toxic effects on humans, fauna, and flora, widespread water contamination by organic dyes is a severe environmental issue [12, 16, 200]. Furthermore, organic dyes in water can reduce light penetration 29 and bioaccumulation and influence the photosynthetic activities of terrestrial and aquatic ecosystems [34, 201]. Additionally, because of their complex structure, high molar mass, and low biodegradation, their removal from water is complicated via conventional methods, including oxidation, biodegradation, and coagulation [202, 203]. On the contrary, the adsorption process is fast, simple, inexpensive, reversible, and can be applied on a large scale; thus, this method has become a proper candidate for water treatment [11, 13, 27, 38]. Recently, a growing number of publications have focused on MOF-based fibers to remove different organic dyes from aqueous solutions. For instance, MOF-5 (a well-known MOF) silk fiber was prepared using the LbL method and employed to remove CR from water [178]. ZIF-67/PAN fiber was prepared in another study by electrospinning the ZIF-67/PAN solution [157]. Accordingly, the adsorption of MG onto the pure ZIF-67 and the ZIF-67/PAN fiber composites followed the Langmuir isotherm model and fitted the pseudo-second-order kinetic model. As a result, the maximum adsorption capacities (Qe) of pure ZIF-67 and the ZIF-67/PAN fiber composite (with a ZIF-67 loading ratio of 54%) were 2545 and 1305 mg/g, respectively, higher than those of most reported adsorbent materials to date. The π-π interactions between the benzene rings of MG molecules and the imidazole ring of ZIF-67 nanoparticles were introduced as the main adsorption mechanisms [48, 189, 204, 205]. More interestingly, the ZIF-67/PAN fiber showed more than 92% of the initial adsorption capacity of fresh ZIF-67/PAN fiber, even after four regeneration cycles via washing with ethanol solution. 1.4 Research scope This thesis aims to showcase the potential of the UiO-66 family of MOFs as the next generation of versatile adsorbents for water treatment. Through their precisely engineered structure, adaptability, chemical stability, porosity, and tunable pore size, these MOFs exhibit 30 unparalleled adsorption capabilities. Compared to conventional adsorbents like activated carbon, the UiO-66 MOFs offer superior features such as enhanced adsorption capacity, kinetics, and regenerability. However, despite their exceptional attributes, much of their modification potential remains unexplored. Moreover, the hindrance posed by their polycrystalline powdery form has limited their widespread application in water treatment. This research presents an investigation of the UiO-66 family of MOFs, encompassing their stability, post-synthetic modification, and post-processing techniques employing electrospinning. Moreover, this dissertation enhances comprehension regarding the underlying chemistry of adsorption under diverse environmental circumstances, including pH, temperature, and the existence of interfering ions. This expansion aims to broaden the applicability of the UiO-66 series of MOFs across various scenarios. This contribution will foster the continued advancement of the versatile UiO-66 family of adsorbents. 1.5 Thesis structure Chapter 1 presents a thorough and evaluative examination of existing research on utilizing the UiO-66 family of MOFs for water treatment through adsorption. Subsequently, the chapter delves into integrating MOFs with electrospinning, elucidating diverse techniques for incorporating MOFs into nanofibers and exploring different morphologies. Finally, the literature on the adsorptive removal of water contaminants using MOF-based nanofibers is discussed. Chapter 2 encompasses the research methodology employed in this thesis, encompassing several vital aspects. These include the synthesis of diverse MOFs, the post-processing of MOFs through electrospinning, material characterizations in both dry and wet states, conducting adsorption experiments, and the subsequent calculation of relevant parameters. 31 Chapter 3 explores the research on UiO-66's stability in diverse media, including water, DMF, and THF, which possess varying polarities. The study elucidates the progressive alterations occurring within the UiO-66 structure and their impact on its distinct adsorption characteristics. Chapter 4 of the thesis delves into the issues associated with separating the powdery form of UiO-66 MOF in water treatment applications, focusing on a magnetization approach. Consequently, the magnetized MOFs are extracted from the treated water using an external magnet, eliminating the need for complex and time-consuming separation techniques such as centrifugation. The proper methodology for the successful growth of MOFs on a magnetic substrate is expounded upon. Subsequently, in a progressive step, the post-modification of magnetic UiO-66 with active nitrogen-containing functional groups, including 2,4,6-trichloro-1,3,5-triazine (TCT) and 5-phenyl-1H-tetrazole (PTZ), is pursued to enhance the adsorption performance of the designed composite. This chapter offers insights into the efficacy and potential applications of the modified magnetic UiO-66 MOF adsorbents for water treatment through thorough experimental investigations and analysis. Chapter 5 showcased the effectiveness of hybridizing UiO-66 with nanodiamonds to enhance the affinity for removing organic contaminants from water. On top of that, the obstacle of utilizing powdery UiO-66/nanodiamond hybrids to remove organic pollutants is addressed through electrospinning. The resultant MOF nanofiber composites were subsequently employed to eliminate organic dye contaminants from wastewater. This chapter delves into an exploration of the adsorption capacity, mechanism, and regeneration capability of the prepared nanofiber composites. Chapter 6 explores a two-step modification approach for enhancing the affinity of UiO-66-NH2 MOF toward removing organic contaminants. The first step involves incorporating nitrogen 32 active sites and phenyl groups into UiO-66-NH2 through the reaction with TCT and PTZ, respectively. This modification aims to increase further the favorability of the framework for organic contaminant removal. Following the modification, the chapter investigates the direct electrospinning technique to prepare electrospun adsorbents based on the modified UiO-66 MOF. This approach offers the advantage of facilitated separation from treated water, overcoming the challenges of using MOF polycrystalline powders. The electrospun adsorbents are examined for their selectivity towards organic contaminants, regeneration ability, and the influence of ambient conditions on their performance. This chapter provides insights into the effectiveness and potential applications of the modified UiO-66 MOF and electrospun adsorbents for water treatment through comprehensive experimental investigations and analysis. In Chapter 7, the culmination of Chapters 3-6 is presented, where significant conclusions are drawn. This chapter provides an overview of the specific contributions made by this dissertation and proposes recommendations for future advancements and assessments in the field of UiO-66 MOF adsorbents. The findings and insights gathered from the preceding chapters are summarized, shedding light on the implications and significance of the research conducted. Furthermore, this chapter outlines avenues for further exploration, emphasizing the importance of continued development and evaluation in the realm of the UiO-66 family of MOF adsorbents. 1.6 Gap, novelty, and objectives In my research area, a significant gap exists despite the valuable attempts and achievements in large-scale and green synthesis of UiO-66 MOFs. These efforts have aimed to transition these materials from laboratory bench experiments to industrial applications. However, the challenge remains in understanding how to effectively implement these materials for practical uses. This gap can be divided into two main aspects: Firstly, there is a knowledge gap regarding the relationship 33 between the structure of UiO-66 MOFs and their performance in target applications. Secondly, there are challenges associated with using polycrystalline powdery materials, as discussed in the introduction. What makes UiO-66 particularly intriguing is its stability and the potential for various post-processing modifications to harness more benefits from its structure. It offers the opportunity to introduce numerous active functional groups to enhance its performance further. For example, the Amine (-NH2), Carboxyl (-COOH), Sulfonic Acid (-SO3H), and Hydroxyl (-OH) groups are known for their effectiveness in removing organic and inorganic contaminants from water bodies. Therefore, there is still a substantial unexplored territory in this area that needs further investigation. This thesis aims to investigate methods for bringing UiO-66 MOFs closer to practical water treatment applications. This exploration encompasses understanding the relationship between structure and application, addressing challenges associated with polycrystalline powdery states through techniques such as magnetization and electrospinning, and enhancing their capability for contaminant removal through hybridization and post-synthetic modifications. Here are the points highlighting the novelty and originality of the research presented in this dissertation: Chapter 1 explores the distinctive characteristics of UiO-66 MOFs and their relevance in water treatment, in addition to elucidating critical strategies for effectively integrating MOFs into nanofibers through the electrospinning technique. The resultant pair of review papers emerging from this chapter serves as a valuable asset, benefiting both newcomers entering the realm of MOFs and experienced researchers looking to advance the frontiers of MOFs in water treatment applications. 34 Chapter 3 highlights the significance of an extensive and one of the most prolonged investigations into the stability of UiO-66 in diverse media featuring varying polarities. This study delves into the effects of aging on UiO-66's adsorption properties and stands out as one of the most prolonged examinations in this regard. Chapter 4 of this work introduces a beneficial approach to creating a magnetic UiO-66 material enriched with a nitrogen-containing functional group tailored for effective dye removal. This chapter underscores the importance of magnetization in streamlining the adsorbent separation process and emphasizes the crucial role of post-modification in achieving an advanced absorbent with enhanced capabilities. Chapter 5 demonstrates the capacity of high-surface-to-volume ratio electrospun nanofibers to serve as hosts for MOFs. To this end, UiO-66 underwent modification through a hybridization approach involving nanodiamonds. A comprehensive investigation was conducted to assess the effectiveness of these MOF nanofiber adsorbents, encompassing evaluations of kinetics, thermodynamics, recyclability, and their performance in the presence of interfering ions. These studies emphasized the engineered MOF nanofibers' capability to remove organic dye contaminants. Building upon the content of Chapters 4 and 5, Chapter 6 presents an exploration of UiO-66-NH2 MOF nanofibers functionalized with TCT and PTZ, specifically designed to remove organic dyes. The comprehensive investigation unveils the optimal MOF nanofiber configuration, analyzing its performance across various environmental conditions, including differences in pH, temperature, and interfering ions. 35 Chapter 2 Research methodology Conducting systematic experiments and characterizations to achieve the above goals was essential. Accordingly, in each experiment, the material synthesis and characterization are undertaken to ensure the quality of MOFs. To do this, well-accepted practices in the MOF community, such as XRD, Thermogravimetric Analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR), Gas Adsorption Analyzers, and SEM, are performed in different chapters. Electrospinning has been followed to fabricate MOF nanofiber composite adsorbents for wastewater treatment. XRD, TGA, XRD, and SEM are appropriate tools to investigate the quality of fabrication procedures and final nanofiber composites. The batch adsorption experiments are followed in all chapters of the thesis. Accordingly, UV-visible spectrophotometry quantifies the amount of organic contaminants in aqueous media. A range of analytical tools and experimental approaches have been utilized to gain insights into the underlying mechanisms of adsorption (physical or chemical interactions). These include Fourier-transform infrared spectroscopy (FTIR), conducting adsorption experiments under diverse conditions, encompassing varying initial dye concentrations, pH levels, and adsorption temperatures. Additionally, investigations have been undertaken to assess adsorption behavior in the presence of interfering ions. 2.1 Material synthesis 2.1.1 Synthesis of UiO-66 For UiO-66 synthesis, 0.39 g of terephthalic acid and 0.54 g of zirconium (IV) chloride (ZrCl4) were dissolved in 31 mL DMF and agitated at 25 °C to obtain a homogeneous solution. The mixture was heated at 120 °C in an autoclave and cooled to ambient temperature, which was then centrifuged at 7000 rpm for 15 min to separate the nanoparticles. Subsequently, white 36 particles were washed twice with DMF and then with chloroform for five consecutive days. In the end, particles were dried at 150 °C under a vacuum overnight. Pristine UiO-66 has a cubic shape with a crystal size of about 100 nm. 2.1.2 Synthesis of UiO-66-NH2 UiO-66-NH2 was synthesized by dissolving 2.27 mmol (0.53 g) of ZrCl4 and 2.27 mmol (0.41 g) of 2-aminoterephthalic acid in 30 mL of DMF at room temperature for 1 h [206, 207]. The homogenous mixture was then transferred to a 50 mL Teflon-lined autoclave and placed in a preheated oven for 24 h at 120 °C. The final product was cooled to room temperature. It was separated by centrifugation at 5000 rpm for 10 min and washed 3 times with DMF and then 3 times with chloroform under sonication to remove unreacted reactants. Finally, the yellow MOF was filtrated and heated at 100 °C under a dynamic vacuum for 24 h. 2.1.3 Synthesis of magnetic nitrogen-rich UiO-66 The magnetic UiO-66-NH2 was prepared through a facile solvothermal method. Accordingly, 200 mg of modified Fe3O4 was dispersed into 30 mL of DMF and sonicated for 30 min. Then, 410 mg of 2-ABDC and 530 mg of ZrCl4 were added to the suspension and stirred with an overhead mixer for another 60 min. After that, the mixture was kept in an autoclave and heated at 120 °C for one day. The solid product was cooled down slowly, separated by an external magnet, washed with DMF and chloroform, and then activated by immersion in chloroform for more than 4 days. Afterward, the sample was dried in a vacuum oven at 100 °C. The sequential tandem functionalization process involved reacting the TCT reagent with amine-tagged UiO-66-NH2. To initiate this reaction, 500 mg of magnetic UiO-66-NH2 was dispersed in 20 mL of dry THF using a bath sonicator. Subsequently, 5 mmol of TCT was introduced and stirred at 273K for 8 hours. 37 Afterward, 10 mmol of PTZ and 950 mg of K2CO3 were added to the mixture, and the stirring continued at 298 K for 4 hours. The mixture was refluxed at 323 K for 24 hours to ensure a complete reaction. Finally, the particles were separated, washed with water and methanol, and dried under vacuum at 333K for 12 hours. The synthesis procedure is depicted in Figure 2-1. The adsorbent designed under the name m-UiO-66-TCT Figure 2-1. Fe3O4@UiO-66-NH2 functionalization with TCT and PTZ 2.1.4 Preparation of thermally oxidized nanodiamond (TOND) Nanodiamonds with a diameter ranging from 4 to 6 nm, possessing a specific surface area (SBET) of 282 m2/g, and a purity level of 98 to 99% were supplied by NaBond Technologies Co. in China. Then, Nanodiamond (ND) particles were oxidized to add carboxylic acid groups on their surface to serve as nuclei sites for UiO-66 growth. While undergoing thermal treatment at elevated temperatures, NDs experience a conversion of most of their functional groups into carboxylic acid groups. This conversion process eliminates other impurities, such as amorphous and graphite carbon residues, generated during the detonation step [208]. Accordingly, ND was heated at 425 °C for 270 min under an air atmosphere [13, 21]. The oxidized nanoparticles were stored in a desiccator to prevent moisture adsorption from the air. 38 2.1.5 Synthesis of TOND@UiO-66 nanocomposite A 'bottle around ship' (BAS) concept was followed to synthesize TOND@UiO-66 nanocomposite [27, 194, 209]. Briefly, 60 mg of TOND nanoparticles were dispersed in 20 mL DMF by sonication for 1 h. Thereafter, 530 mg of ZrCl4 was added to the TOND suspension, sonicated for 1 h, and stirred at room temperature for 12 h. Meantime, 380 mg of BDC was dissolved in 20 mL DMF and stirred to prepare a transparent solution. Then, the BDC solution was added to the TOND suspension, sonicated for 1 h, and the mixture was transferred into a Teflon autoclave and heated at 120 °C for 24 h. The prepared TOND@UiO-66 nanocomposite was separated by filtration, washed with DMF, and further with ethanol, and then solvent-exchanged by Soxhlet extraction in ethanol for 5 days. Finally, the synthesized nanocomposite was dried at 120 °C under the vacuum for 12 h. The same procedure was used to synthesize pure UiO-66 without adding TOND to the DMF solution [17] 2.1.6 Preparation of electrospun TOND@UiO-66 adsorbents Due to the nature of UiO-66 MOFs, a MOF-first strategy [165] was utilized to prepare nanofiber composites. Accordingly, 7.5 mg of TOND@UiO-66 was mixed with 7.5 mL of distilled water using a bath sonicator for 30 min, followed by stirring at room temperature for 12 h. In the next step, 375 mg of PVA powder was added to the suspension, sonicated for 30 min, and finally stirred at room temperature for 12 h. In the meantime, 125 mg of CS (75–85 % deacetylated, medium molecular weight, Sigma-Aldrich, CAS Number: 9012-76-4) powder was dissolved in 2.5 mL of 2% v/v acetic acid (HAc) solution, and stirring was continued at room temperature for 12 h. The chitosan (CS) solution was added to the nanoparticle suspensions plus 30 minutes of sonication and 12 h of stirring at room temperature to prepare homogeneous suspension for electrospinning. 39 High voltage electrospinning (22-25 kV- syringe tip to collector distance: 15 cm), followed by drying nanofibers (under vacuum for a day) and cross-linking with glutaraldehyde (GA), was performed to fabricate the final nanofiber nanocomposite for dye removal. To evaluate the performance of the TOND@UiO-66 nanofiber compared to bare UiO-66, ND, and TOND, the same procedure was employed to fabricate their nanofiber composite. The following terminology (Table 2-1) will be used throughout Chapter 5 for samples with parent composition of 125 mg CS, 375 mg PVA (molecular weight of 145,000, Sigma-Aldrich, CAS Number: 9002-89-5), 9.8 mL water, and 0.2 mL HAc. Table 2-1. Electrospun chitosan-polyvinyl alcohol nanofibers with 1.5 wt% of different nanoparticle additives Specimens Nanoparticles ECPN - ECPN/N ND ECPN/T TOND ECPN/U UiO-66 These nanofibers were used as adsorbents in Chapter 5 of the thesis. 2.1.7 Preparation of high-loading amine-functionalized MOF (Zr-MOF) The stepwise tandem functionalization was done by the reaction of the TCT reagent with amine-tagged UiO-66-NH2. Accordingly, 500 mg of UiO-66-NH2 was dispersed in dry THF (20 mL) using a bath sonicator. Afterward, 5 mmol of TCT was added to MOF suspension and stirred at 273K for 8 h. Then, 10 mmol of PTZ and 950 mg of K2CO3 were added to the mixture and stirred at 298 K for 4 h. The mixture was refluxed at 323 K for 24 h to ensure the complete reaction. 40 Finally, the particles were separated, washed with water and methanol, and kept under vacuum at 333K for 12 h. Zr-MOF is synthesized according to Figure 2-2. Figure 2-2. Schematic representation for the post-synthesis modification of Zr-MOFs 2.1.8 Preparation of electrospun MOF-based adsorbent nanocomposites Electrospun composite nanofibers (ECNF) were fabricated by one-step electrospinning of the highly amine-functionalized MOFs in the polymeric solution (PVA/chitosan mixture). Accordingly, various contents of post-synthetically modified MOFs (Table 2-2) were dispersed homogeneously in water, using a batch sonicator, and stirred vigorously overnight at room temperature (solution A). Meanwhile, 0.375 g of PVA was dissolved in water and stirred overnight to prepare a homogeneous PVA solution (solution B). Later, 0.125 g of chitosan was dissolved in an acidic aqueous solution (2% v/v acetic acid/water) and stirred overnight to prepare a homogeneous chitosan solution (solution C). Finally, solutions B and C were added to solution A, followed by mixing via stirring and sonicating to attain a homogeneous solution. The solution was loaded in a 10 ml syringe. The electrospinning was performed at a high voltage of 15-18 kV, in which the injection rate was 0.5 mL/h, and the distance between the needle tip and the rotating collector (150 rotations/min) was fixed at 20 cm. The electrospun MOF-based nanofibers were 41 dried at 298 K for one dye and finally cross-linked by glutaraldehyde to offer superior nanofiber stability in aqueous media. The nanofibers were crosslinked by placing them in a desiccator containing glutaraldehyde solution and keeping them at room temperature for 48 h. Therein, glutaraldehyde vapor penetrates the nanofibers and reacts with the hydroxyl and amine groups of polymers. After the crosslinking reaction, the nanofibers were washed several times with water to remove the unreacted glutaraldehyde molecules. Finally, the crosslinked nanofibers were dried overnight at 60 °C under a vacuum for further characterization. Table 2-2. Electrospun MOF-based adsorbents composed of the parent composition of 0.375 g PVA and 0.125 g chitosan with different contents of the modified MOFs Adsorbents Zr-MOF Loading (%) ENF 0 ECNF-3 3 ECNF-5 5 ECNF-7 7 ECNF-10 10 These nanofibers were used as adsorbents in Chapter 6. 2.2 Materials characterizations Understanding the structure-function relationships in nanomaterials and nanomaterial composites is of great importance for pushing this family of materials from bench to real water treatment applications. Characterizing MOF is a critical step in understanding the behavior of these particles, which is necessary for translating their performance benefits from laboratory settings to real-world applications. Surveying the literature and community revealed the following characterizations for evaluating nanoparticles and nanofiber composites for water treatment. 42 2.2.1 Dry-state characterization 2.2.1.1 TEM Undoubtedly, transmission electron microscopy (TEM) is among the most crucial techniques for characterizing nanoparticles. This method utilizes a concentrated electron beam to capture high-resolution micrographs of nanomaterials on a thin sample (usually less than 200 nm) with excellent lateral spatial resolution. Modern electron microscopes are capable of achieving resolutions as low as 0.05-0.1 nm, thanks to aberration correctors that minimize image distortion. This enables the acquisition of high-resolution images with atomic precision. TEM also allows for investigating the crystalline structure of specific microscopic areas in crystalline materials by focusing and spatially constraining the incoming beam and then detecting the resulting electron diffraction pattern. To prepare UiO-66 samples for TEM imaging, a tiny amount of MOF samples were dispersed in ethanol using a bath sonicator, followed by stirring. Subsequently, a droplet of the MOF suspension was carefully placed onto the TEM grid. Subsequently, ample time was allowed for the grid to dry thoroughly, and ultimately, the TEM grid was introduced into the TEM machine (Philips CM120). 2.2.1.2 SEM The scanning electron microscope functions by detecting secondary electrons that are emitted from the sample when it interacts with the incoming electron beam. This allows for imaging of the sample's surface. In SEM, samples are imaged at lower energies than in TEM. It is, therefore, sensitive only to the specimen's surface and has limited penetration depth. In SEM analysis, electron energies are moderate, which limits resolution to 2–3 nm but lowers the chance of damage to samples caused by beams. Energy dispersive spectroscopy (EDS) is often used in 43 conjunction with scanning electron microscopy (SEM) to provide elemental information about the surface of a sample. Due to the non-conductive nature of the UiO-66 family of MOFs and their electrospun nanofibers made of chitosan/polyvinyl alcohol, it is necessary to coat the samples with a thin layer of conductive material (gold sputtering). This prevents the buildup of charge during SEM imaging, leading to the acquisition of high-quality images. 2.2.1.3 XRD In crystalline samples, X-ray diffraction can examine a wide range of structural characteristics. It includes information from the microscale to the macroscale, ranging from the size and shape of crystals to the structure of the crystal constituents. It is based on the principle that X-rays interact with atoms in crystals and scatter in a variety of directions as they are directed at them. There is information contained in the diffraction pattern that indicates the arrangement of atoms in the crystal lattice. XRD can identify the MOF structure, thermal stability, and structural changes caused by environmental changes, such as exposure to different gases or solvents. 2.2.1.4 Gas sorption A widely used technique for characterizing porous materials is to analyze the adsorption and desorption of gas molecules, such as nitrogen, on solid surfaces under isothermal conditions. The surface area, pore size distribution, and accessible pore volume of a material can be determined by measuring its adsorption and desorption isotherms. Various properties can be determined by analyzing sorption isotherms using different data analysis techniques, such as the Brunauer–Emmett–Teller (BET) method. According to the experimental data, the investigated materials fall into three categories: microporous (pore size below 2 nm), mesoporous (pore size between 2 and 50 nm), or macroporous (pore size greater than 50 nm). 44 2.2.1.5 FTIR With the help of FTIR, chemical compounds can be identified and quantified in a wide range of samples. A sample is passed through infrared radiation, and the amount of radiation absorbed or transmitted is determined. FTIR can provide valuable information about the chemical bonds present in MOFs, their nanofiber composites, and their surface properties and interactions with guest molecules. Specifically, FTIR spectra of MOFs can be used to identify functional groups present in the organic ligands and to determine the coordination of metal ions with these ligands. FTIR can also be used to study the adsorption of adsorbates in MOFs and their nanofiber composites, which is essential for their applications in wastewater treatment and separation. Monitoring the FTIR spectra of MOFs (MOF nanofiber composites) before and after exposure to a particular adsorbate makes it possible to determine the nature of the interactions between the MOF and the guest molecule. 2.2.1.6 TGA In this thesis, thermogravimetric was used to evaluate the thermal stability of the samples. This involved subjecting the material to a controlled heating program within a nitrogen-inert atmosphere, which was commonly employed throughout the study. By monitoring the material's mass at various temperatures, we were able to extract valuable insights into the thermal stability of the samples. In this thesis, a predetermined amount of samples (ranging from 5 to 10 mg) was vacuum-dried overnight and subsequently placed into the TGA machine. All experiments were conducted under an inert N2 atmosphere with a heating rate of 10 °C /min. 45 2.2.2 Wet-state characterizations 2.2.2.1 Zeta potential A dynamic environment exists at the boundary between the solid and fluid phases. A number of phenomena contribute to the appearance of a net charge on the surface of nanoparticles, such as the presence of dangling bonds or the adsorption or grafting of charged molecules. The zeta potential is a standard indicator of surface charge and colloidal stability. The zeta potential is the potential difference between the surface of the MOF and the surrounding fluid and is a measure of the electrostatic repulsion or attraction between the MOF particles in solution. The zeta potential of MOFs can affect their stability in solution and their interactions with other molecules or surfaces. MOFs with a high zeta potential (either positive or negative) are generally more stable in solution and less prone to aggregation or precipitation. MOFs with a low zeta potential are more likely to form clusters or aggregates due to attractive van der Waals forces between particles. Measuring the zeta potential of MOFs in different solution conditions (e.g., pH, ionic strength) can provide information about the stability of these materials under different environmental conditions and help optimize their applications. 2.2.2.2 Adsorption experiments 2.2.2.2.1 UV-visible spectroscopy UV-visible spectroscopy is a common technique used in adsorption experiments to measure the amount of adsorbate molecules that are adsorbed onto a solid surface, such as MOF. In these experiments, a solution of the adsorbate molecules is prepared and added to the MOF. The MOF-adsorbate mixture is then allowed to equilibrate for a period of time to enable adsorption to occur. After equilibration, the mixture is centrifuged or filtered to separate the MOF particles from the 46 solution. The absorbance spectrum of the solution before and after the adsorption process is measured using a UV-visible spectrophotometer. To calculate the adsorption of a sample using UV-Vis spectroscopy, we followed the steps below: 1. Standard curve preparation: First, a series of known concentrations of a standard adsorbate solution was prepared. The standard solution should be prepared in the same solvent and under the same conditions as the sample. Next, the absorbance of each solution at a specific wavelength using a UV-Vis spectrophotometer was measured. The plot of the absorbance vs. concentration was drawn to obtain a standard curve. 2. Measurement of the absorbance of the sample: Next, the absorbance of the sample at the specific wavelength was measured. 3. Determination of the concentration of the sample: Use the standard curve to determine the concentration of the sample from its absorbance value. The adsorbate uptake into the nanoparticles at the time, t (qt (mg/g)) and equilibrium (qe (mg/g)) are measured using: 𝑞𝑡 =(𝐶0 − 𝐶𝑡)𝑉𝑚 Eq. 1 𝑞𝑒 =(𝐶0 − 𝐶𝑒)𝑉𝑚 Eq. 2 𝑅𝑒𝑚 𝑜𝑣𝑎𝑙(%) =(𝐶0 − 𝐶𝑒)𝐶0× 100 Eq. 3 Where C0, Ct, and Ce (mg/L) are adsorbate concentrations at the start, any time t, and at equilibrium, respectively, adsorbent mass is m (g), and V (L) represents the dye solution volume. 47 Understanding the kinetics of the adsorption process is crucial to evaluating an adsorbent's performance and gaining insight into its mechanisms. Various mathematical models have been developed to describe adsorption reactions and diffusion processes. Sorption kinetic models are mathematical equations that describe the rate at which a solute is adsorbed onto a solid surface. These models can help to understand and optimize the design and operation of adsorption processes. Some of the most commonly used adsorption kinetic models for MOFs and their composites include the linear shapes of pseudo-first-order, pseudo-second-order, and intra-particle diffusion kinetic models, respectively, given as [210]: 𝑙𝑛( 𝑞𝑒 − 𝑞𝑡) = 𝑙𝑛( 𝑞𝑒) − 𝐾1𝑡 Eq. 4 𝑡𝑞𝑡=1𝐾2𝑞𝑒2+𝑡𝑞𝑒 Eq. 5 𝑞𝑡 = 𝐾𝑖𝑑𝑡1/2 + 𝐶 Eq. 6 Here, qt and qe (mg/g) are the amounts of adsorbed dye at any time, t, and equilibrium, respectively; K1 (min-1), K2 (g/mg.min), and Kid (mg/g min1/2) are the rate constants of pseudo-first-order, pseudo-second-order, and intra-particle diffusion models, respectively; C is related to the boundary layer thickness (constant). Selecting the appropriate adsorption equilibrium correlation is critical to investigating new adsorbents and achieving optimal adsorption systems. It is crucial for predicting adsorption parameters and quantitatively comparing adsorbent performance across various systems. Adsorption isotherms, which depict how pollutants interact with adsorbents, are essential for optimizing the adsorption process, characterizing adsorbent surface properties and capacities, and designing effective adsorption strategies. Langmuir and Freundlich isotherm models are two frequently used models to determine the adsorption mechanism as well as the theoretical maximum 48 adsorption capacity. According to the Langmuir model, the adsorbate homogeneously covers the adsorbent surface and makes a uniform monolayer. Freundlich's adsorption model assumes the heterogeneous-multilayer presence of the adsorbate onto the surface of the adsorbent, while adsorption capacity is dependent on equilibrium. The linear forms of the models are given below [211]: 𝐶𝑒𝑞𝑒=1𝐾𝐿𝑞𝑚𝑎𝑥𝐶𝑒𝑞𝑚𝑎𝑥 Eq. 7 𝑙𝑛 𝑞𝑒 = 𝑙𝑛𝐾𝐹 +1𝑛𝐹𝑙𝑛 𝐶𝑒 Eq. 8 Here, qmax is the theoretical maximum adsorption capacity (mg/g) when the dye molecules create a uniform monolayer on the adsorbent surface, while KL(L/mg) is Langmuir constant according to the binding site affinity, which defines the adsorption energy. The KF and nF are Freundlich constants connected to the capacity and strength of adsorption, respectively. The value of 1/n determines the tendency of the adsorption mechanism and the extent of adsorbent surface heterogeneity [212]. A dimensionless constant can be defined to examine the adsorption favorability of adsorbent, as shown in Eq. 9 [213, 214]: 𝑅𝐿 =11 + 𝐶0𝐾𝐿 Eq. 9 Here, C0 is the dye concentration, and KL is the Langmuir constant. The favorable adsorption isotherm usually happens for the RL values in the range of 0 to 1. The linearized Dubinin-Radushkevich and linearized Tempkin are also widely used to describe the adsorption of molecules on solid surfaces. The Dubinin-Radushkevich isotherm model was formulated to incorporate the influence of the porous structure of the adsorbents. This model was derived from adsorption 49 potential theory and posited that the adsorption mechanism was associated with the filling of micropore volumes rather than occurring through layer-by-layer adsorption on pore walls [215]. The linearized Dubinin-Radushkevich isotherm model is expressed as follows : ln 𝑞𝑒 = ln 𝑞𝑚𝑎𝑥 − 𝐵𝜀2 Eq. 10 Where qmax (mg/g) is the theoretical saturation capacity, constant B (mol2/kJ2) is an indicator of mean free energy of adsorption, and ε is the Polanyi potential, which could be obtained based on the following equation : 𝜀 = 𝑅𝑇 𝑙𝑛( 1 +1𝐶) Eq. 11 Where R (J/mol K) and T (K) are the universal gas constant and the absolute temperature, respectively. The mean energy of adsorption (E) can be calculated based on B (the Dubinin–Radushkevich isotherm constant) values using the following equation: 1E2B= Eq. 12 Generally, E is the free energy change according to one-mole ions transformation from infinity to the adsorbent surface. The Tempkin isotherm model is based on the assumption that the adsorption energy decreases linearly with the coverage of the adsorbate on the surface. The linearized Tempkin isotherm model is also described as the following equation : e T eTRTq ln(A C )B= Eq. 13 Where BT (kJ/mol) and AT (L/g) are the Tempkin constants correlated with adsorption heat and the equilibrium binding constant, respectively. R is the gas constant, and T (K) is the absolute temperature. 50 2.2.2.2.1.1 Adsorption thermodynamics Adsorption thermodynamics is the study of the physical and chemical processes that occur when a substance is adsorbed onto a surface. Adsorption thermodynamics involves analyzing the energy changes that occur when a substance is adsorbed onto a surface. This includes the heat of adsorption, which is the amount of energy released or absorbed when a substance is adsorbed, and the entropy of adsorption, which is a measure of the disorder of the system during the adsorption process. Overall, the thermodynamics of adsorption plays an important role in understanding and optimizing adsorption processes in various applications. The adsorption thermodynamic parameters, including standard free energy (∆G0, kJ/mol), standard enthalpy (∆H0, kJ/mol), and standard entropy (∆S0, J/mol K), could be calculated by the following equations [216]: 𝑙𝑛𝑞𝑒𝐶𝑒=𝛥𝑆0𝑅−𝛥𝐻0𝑅𝑇 Eq. 14 𝛥𝐺0 = 𝛥𝐻0 − 𝑇𝛥𝑆0 Eq. 15 T (K) is the absolute temperature, and R (J/mol K) is the universal gas constant. The ∆H0 and ∆S0 values could be determined by plotting lnqe/Ce against 1/T based on Eq. 14, in which these values could be obtained from the y-intercept and the slope of this plot, respectively. Accordingly, the ∆G0 values at different experiment temperatures could be calculated based on Eq. 15. In the case of adsorption, a negative ΔG indicates that the adsorption process is spontaneous and energetically favorable. This means that molecules or ions in the gas or liquid phase tend to adsorb onto the solid or liquid surface because it lowers the overall Gibbs free energy of the system. When the Gibbs free energy change (ΔG) for a process is positive, it indicates that the process is not 51 thermodynamically favorable under the given conditions. Finally, When the Gibbs free energy change (ΔG) for a process is exactly zero, the process is at equilibrium under the given conditions. 2.2.2.2.1.2 Adsorption selectivity Adsorption selectivity (S) measures the adsorbent material's ability to preferentially adsorb one component from a mixture of liquids or gases. S could be determined from the single adsorption isotherms using Eq. 16 [210]: 𝑆 =𝑞𝑖𝐶𝑖𝑞𝑗𝐶𝑗 Eq. 16 2.2.2.2.1.3 Partition coefficient (PC) To better understand the adsorption performances of various adsorbent materials in removing different organic dyes from aqueous solutions, it is critical to determine their real adsorption performance metrics. PC describes the distribution of a solute between a solid adsorbent and a liquid phase[217]. It represents the equilibrium distribution of the solute between these two phases. The PC (mg/g µM) can be calculated using the following equation [218, 219]: 𝑃𝐶 =𝑞𝑒𝐶𝑒 Eq. 17 Where qe (mg/g) and Ce (µM) are the adsorption capacity and concentration of each organic dye at equilibrium, respectively. PC indicates the accurate adsorption performance of each adsorbent because it does not depend on operating conditions. In contrast, the removal efficiency and adsorption capacity mainly depend on operating conditions and can be changed by altering the operating conditions [217, 220]. 52 2.3 Electrospinning machine The electrospinning setup comprises four primary components: a high-voltage power source, a syringe or gear pump, a nozzle or spinneret, and a fiber collector. In this process, the polymer is either dissolved in a suitable solvent or heated to a high temperature until it reaches the desired viscosity, resulting in a polymer solution or melt. Subsequently, the high-voltage electric field is applied to spin the polymer solution or melt. When the electric field surpasses a critical threshold, the electrical forces overcome the surface tension of the solution or melt, causing the droplet at the tip of the nozzle or spinneret to transition into a characteristic conical shape called the Taylor cone. In conclusion, the nanofibers will be generated and gathered on the surface of the collector. Figure 2-3. Digital photo of electrospinning machine 53 Chapter 3 Exploring the Stability and Structure-to-Performance Relationship of UiO-66 The stability of adsorbents is a must that needs to be met for water treatment applications. So, to make sure about the capability of this family of MOFs for water treatment, Chapter 3 of this thesis focuses on the systematic and comprehensive study of chemical stability and the effect of stability on the removal performance of UiO-66 toward four various organic dyes, i.e., MR, MO, MG, and MB. Accordingly, the adsorbent structural stability in water, chloroform, and DMF was studied for one year to understand their chemical stability. The results from this study, as one of the most prolonged studies on the stability of the UiO-66 family of MOFs in the literature, play an essential role in pushing these materials for broader applications in environmental sectors, as low stability of MOFs in aqueous media has always been a concern for water treatment application. 3.1 Introduction Several reports on the dye-removal application of various adsorbents such as activated carbon, clay materials, waste foundry sand, magnetic nanoparticles, graphene and graphene oxide (GO), carbon nanotubes (CNTs), hydrogels, polymers, zeolite, nanodiamond, agricultural waste, and MOFs have been investigated [213, 216, 221]. The MOFs, as a unique family of porous materials, are comprised of organic ligands and the desired metal clusters. Given their three-dimensional spatial network, tunable shapes and pore size, large surface area, and high affinity for various organic dyes, MOFs have received much attention as potential organic dye removal adsorbents [11, 222]. Despite their superiority, MOFs have several drawbacks, such as low thermal stability and low stability under natural air, water, and other chemical solvents, which prohibit their practical applications as attractive adsorbents [223, 224]. 54 Studies on dye adsorption of various MOFs have tried to address shortcomings such as low adsorption capacity, weak adsorption selectivity, and low stability inside the water [225, 226]. Therefore, a few specific MOFs have shown promising performance for dye adsorption. Zhao et al. [227] introduced a new approach to synthesizing various MOFs such as HKUST-1, MOF-5, MOF-505, NbO-type, and NOTT-101. They observed that their water stability and morphological features could be modified by adjusting the water volume ratio to DMF. The HKUST-1 showed the highest structural stability against water [227]. The applications of zirconium-based MOFs (UiO-66-X, where X can be H, COOH, NH2, SO3H, etc.) have created considerable attention because of their high adsorption capacities and exceptional structural stability against water [13, 43, 228]. UiO-66-NH2 possesses higher adsorption capacity compared to UiO-66 for the cationic dyes due to electrostatic attraction arising from the high negative zeta potential [228]. In another study, the water stability of isoreticular Zr-MOFs constructed from different carboxylates was investigated [45]. Still, no significant changes in the XRD pattern of UiO-66 were observed after aging in various environments due to inorganic unit strength, where each Zr6O4(OH)4 unit is 12-coordinated to terephthalic acid ligands [45, 63, 229]. In our previous study, it was observed that the crystalline structure of UiO-66 did not change even after one year of aging in liquid water, but its adsorption affinity for anionic and cationic dyes showed a slight drop after stirring in the water at 30°C for one year [13]. Therefore, in this study, UiO-66 was selected as an adsorbent because of its good water stability, high adsorption capacity toward organic dyes, and good chemical resistance against polar and organic solvents like alcohols, aqueous HCl, acetone, aqueous NaOH, dimethylformamide, benzene, and chloroform [13, 45, 229, 230]. 55 In this study, UiO-66 was synthesized, and its adsorption behaviors were investigated using adsorption kinetics, isotherm behaviors, recyclability, and adsorption selectivity against different organic dyes, including MO, MG, MR, and MB. Then, the structural stability in various solvents like water, chloroform, and DMF after one year of aging was studied. The chemical structures of MR, MO, MG, and MB dyes are depicted in Figure 3-1. Figure 3-1. Chemical structures of various dyes employed in this study 3.2 Long-term aging experiments The long-term structural stability of the MOFs as a challenging bottleneck in industrial applications was assessed in distilled water, DMF, and chloroform media. The suspension of UiO-66 within different solvents was kept at ambient temperature for two periods of 6 and 12 months. Centrifugation was performed to separate the aged materials. Finally, the aged UiO-66 was washed 56 with fresh chloroform under sonication for 15 min and heated in a vacuum oven at 150°C to eliminate any potential residual solvents. 3.3 Batch adsorption performance, kinetics, and equilibrium studies Dye separation and adsorption efficiency of UiO-66 were evaluated in 1000 mg/l stock solutions of MB, MG, MR, and MO. Then, the dye concentration was measured by UV-Vis spectroscopy at the maximum absorbance values of each dye (λmax-MB= 631, λmax–MO= 463, λmax-MR= 435, and λmax-MG= 601) [231]. All adsorption tests were carried out at least thrice, and averages were considered. To investigate the UiO-66 adsorption mechanism, pseudo-first-order (Eq. 4), pseudo-second-order (Eq. 5), and intra-particle diffusion (Eq. 6) models were used. The adsorbate uptake into the nanoparticles at time, t (qt (mg/g)) and equilibrium (qe (mg/g)) were measured using Eq. 1 and Eq. 2 [211, 232]. Langmuir (Eq. 7) and Freundlich (Eq. 8) isotherm models were employed to determine the adsorption mechanism as well as the theoretical maximum adsorption capacity [211]. 3.4 Selective separation A solution of 5 mg UiO-66 and 25 mL dye mixture with a concentration of 20 mg/L (10 mg/L for each dye) was mixed to assess the selectivity and separation behavior of the nanoparticle. After three-hour adsorption at 25 °C , the adsorbent was separated, and UV-Vis spectroscopy was performed to determine the dye concentration. The ideal adsorption selectivity (S) was determined using Eq. 16. 57 3.5 Results and discussion 3.5.1 Adsorbents characterization XRD patterns of pristine and solvent-aged UiO-66 are displayed in Figure 3-2. XRD tracings before and after aging provide insight into the crystalline structure stability of the nanoparticles. As shown in Figure 3-2a, b, profiles of the pristine UiO-66 and the aged samples exposed to water and chloroform are indistinguishable, displaying a consistent pattern that aligns with findings in the existing literature [45]. The appearance of three distinctive peaks at 2θ= 7.38°, 8.56°, and 25.67° relating to (111), (002), and (006) Miller planes [233]indicates that UiO-66 was successfully synthesized, and there is no impurity in its crystalline structure. Moreover, XRD patterns show no crystalline structure failure upon aging in water and chloroform for 12 months, but structural changes in the XRD pattern of UiO-66 aged in DMF (D-UiO-66) are visible. Figure 3-2c demonstrates that observed peak intensities at 2θ= 7.38° and 8.56° decreased, and a new peak appeared at 2θ= 40.02°. Besides, the XRD profile of UiO-66 aged in DMF presents a broad peak at 2θ= 6.5°-10°, indicating that the crystalline structure of UiO-66 has collapsed in the presence of DMF and deformed to an amorphous structure [234]. The emergence of a new peak at 2θ= 40.02° in the XRD profiles of DMF-aged samples might be due to the recrystallization of terephthalate linker within the pores of UiO-66 as a result of solvation reaction [229, 235, 236]. The new peak intensity was enhanced by increasing the aging time. Overall, XRD revealed that the MOF framework did not entirely collapse even after long-term aging in DMF. 58 Figure 3-2. XRD patterns of UiO-66 before and after aging in (a) water for 6 (W-UiO-66-6) and 12 (W-UiO-66-12) months, (b) chloroform for 6 (C-UiO-66-6) and 12 (C-UiO-66-12) months, and (c) DMF for 6 (D-UiO-66-6) and 12 (D-UiO-66-12) months 59 FE-SEM micrographs of UiO-66 before and after aging in various environments are depicted in Figure 3-3. The pristine UiO-66 has a cubic shape with a crystal size of about 100 nm. Based on visual inspection, the morphological observation reveals that the surface of pristine UiO-66 nanoparticles appears rougher compared to the solvent-aged samples, especially in the case of water exposure. This suggests a deformation of UiO-66 crystals on the exterior surface. The nanoparticle`s dimension might have grown in size after aging in water and chloroform to form large agglomerates. However, DMF-aged samples displayed little evidence of agglomeration over the aging samples for 12 months and were also slightly shrunk due to the collapse of the exterior framework, as endorsed by XRD analysis. Figure 3-3. FE-SEM images of pristine UiO-66 and its aged samples for 12 months 60 Structural stability assessment was done by nitrogen adsorption analysis, where N2 adsorption/desorption isotherms and pore features of pristine and aged UiO-66 are presented in Figure 3-4. The corresponding data calculated from the BET analysis, including BET surface area, pore volume, and average pore size, are also given in Table 3-1. As presented in Figure 3-4, it is evident that pristine UiO-66 nanoparticles showed type-I isotherm, illustrating the microporous structure [45], and also displayed a uniform and narrow pore size distribution at 1.7 nm. Figure 3-4. The N2 adsorption/desorption isotherms (desorption in red color) and pore size distribution of pristine UiO-66 and aged samples in (a) water, (b) chloroform, and (c) DMF at different times 61 The nanoparticle aging in various solvents significantly decreased the BET surface area in the first six months, followed by a slight reduction during the second six months (Table 3-1). The surface area was reduced due to aggravation by enhancing the polarity of the solvent. This observation corroborates XRD and FE-SEM data, which showed the failure in crystalline structures and deformations in the outer surface morphology because of exposure to the harsh atmosphere. According to Table 3-1, the UiO-66 sample aged in DMF for 12 months (D-UiO-66-12) experienced the highest reduction in BET surface area (more than 30%), probably due to the framework collapse and partial pore blockage originating from the recrystallization of free terephthalate linker and DMF molecules within the pores. Meanwhile, the sample aged in chloroform displayed a negligible BET surface area loss (14%) after 12 months, reflecting the retention of the porous structure, which is in line with FE-SEM and XRD results. The adsorbent's pore volume and average pore diameter enhanced with aging in different solvents (Table 3-1), which supports the pore size distribution results from Figure 3-4. All the aged samples possess pore diameters> 2 nm (Table 3-1 and Figure 3-4), indicating the mesoporous structures of the aged samples. The long-time exposure of the nanoparticle to different solvents changed the microporous structure of the nanoparticles to mesoporous. 62 Table 3-1. BET surface areas and zeta potential of pristine UiO-66 and its solvent-aged samples Samples SBET (m2/g) % loss Total pore volume (cm3/g) Average pore diameter (nm) zeta potential (mV) Pristine UiO-66 1276 - 0.54 1.7 -0.114 W-UiO-66-6 1014 20 0.62 2.6 13.51 W-UiO-66-12 996 22 0.73 2.9 25.31 C-UiO-66-6 1205 5 0.58 2.1 6.15 C-UiO-66-12 1095 14 0.61 2.4 9.34 D-UiO-66-6 951 25 0.76 3.2 10.58 D-UiO-66-12 886 30 1.05 4.7 15.72 TGA thermograph of pristine UiO-66 in inert atmosphere shows three distinct weight reduction steps in the temperature range of 45-220, 280-370, and 470-600 °C, which are centered at 95, 320, and 550 °C, respectively (Figure 3-5a). The weight reduction at 95 °C might be due to the evaporation of water and solvents captured in the pores or adsorbed onto the surface of the nanoparticles. The weight reduction for W-UiO-66 is higher than the other solvent-aged samples, unveiling greater adsorption of water molecules [229]. The weight reduction centered at 320 °C represents the loss of unreacted organic ligands and trapped solvent in the pores of UiO-66, which happened at a higher temperature than their boiling point due to their strong interactions with the pore walls. The final weight reduction at 550 °C is associated with the collapse and degradation of the framework into CO2, CO, and zirconium oxide (ZrO2) [13, 237, 238]. As can be seen from Figure 3-5b, the samples aged in DMF and chloroform displayed small shifts in the inflection points. Accordingly, in the case of D-UiO-66-12, the first and second weight 63 reductions decreased to 70 and 270 °C, respectively, while the final weight reduction slightly increased to 555 °C, and a new small weight reduction occurred at 440 °C, which might be due to the presence of terephthalate crystals within the pores of UiO-66. The TGA thermogram patterns of D-UiO-66-12 and UiO-66 are nearly alike, implying that the aged samples retained their structural integrity fairly and were not substantially damaged. Figure 3-5. TGA (a) and DTG (b) profiles of UiO-66 before and after aging in different solvents for 12 months 3.5.2 Adsorption kinetics The influence of adsorbent/adsorbate exposure time on adsorption capacity, as a key factor in practical applications, was assessed, and the results are depicted in Figure 3-6. The adsorption capacity of pristine UiO-66 increased rapidly in a short period (15 min), which then experienced a very gentle increment to reach a relatively unchanged value with additional contact time. The adsorption was then stopped after approximately two hours (equilibrium condition) [212]. The MR 64 and MO adsorption capacities of UiO-66 are markedly higher than those of cationic MB and MG dyes (Figure 3-6a). It is apparent from Figure 3-6b and Figure 3-6d that the adsorption capacity of pristine UiO-66 decreased by aging in different solvents, and the adsorption capacity declined with increasing aging time and solvent polarity. Consequently, samples aged in water and chloroform showed the lowest and highest adsorption capacity, respectively. Figure 3-6. Adsorption capacity of (a) pristine UiO-66, (b) W-UiO-66-6, (c) W-UiO-66-12, (d) C-UiO-66-6, (e) C-UiO-66-12, (f) D-UiO-66-6, and (g) D-UiO-66-12 for different dyes at different contact times The adsorption kinetic models were employed to ascertain the rate-limiting step during the kinetics of dye adsorption. The results obtained are compiled in Table 3-2. The correlation coefficients (R2) for the pseudo-second-order kinetic model are slightly greater than the other models but are very close to one. Furthermore, fitting the adsorption capacity values (qe, cal) from the model with the experimental adsorption capacity confirms the validity of the pseudo-second-65 order kinetic model. This observation suggested chemisorption is the rate-limiting step in dye adsorption [239]. Table 3-2. The kinetics parameters of dye adsorption models for pristine and solvent-aged UiO-66 nanoparticles Adsorbents Dye R2 qe (mg/g) K2 (g/mg min) Pseudo-first-order Pseudo-second-order Intra-particle diffusion Pristine UiO-66 MR 0.943 0.998 0.966 80.00 1.710-3 MO 0.969 0.999 0.961 86.21 6.210-3 MG 0.953 0.994 0.792 17.04 5.810-3 MB 0.979 0.999 0.941 72.46 6.210-3 W-UiO-66-6 MR 0.961 0.999 0.971 64.51 2.810-3 MO 0.978 0.999 0.991 68.49 6.410-3 MG 0.986 0.998 0.839 11.72 7.110-3 MB 0.979 0.999 0.941 29.67 5.310-3 W-UiO-66-12 MR 0.868 0.998 0.973 54.94 2.910-3 MO 0.935 0.999 0.986 58.82 7.510-3 MG 0.966 0.994 0.965 8.69 4.310-3 MB 0.974 0.997 0.979 21.83 6.410-3 C-UiO-66-6 MR 0.975 0.999 0.936 77.52 7.910-3 MO 0.974 0.999 0.968 86.96 6.310-3 MG 0.935 0.999 0.995 13.77 3.310-2 MB 0.896 0.998 0.942 60.24 4.410-3 C-UiO-66-12 MR 0.95 0.999 0.936 68.96 6.310-3 MO 0.839 0.999 0.965 81.96 6.710-3 MG 0.887 0.999 0.991 13.74 2.110-2 MB 0.915 0.998 0.955 60.61 3.510-3 D-UiO-66-6 MR 0.966 0.998 0.958 73.52 2.810-3 MO 0.909 0.999 0.981 86.20 3.710-3 MG 0.752 0.998 0.841 14.43 1.810-2 MB 0.959 0.999 0.992 62.51 3.710-3 D-UiO-66-12 MR 0.962 0.999 0.921 68.49 4.510-3 MO 0.949 0.999 0.977 86.20 3.810-3 MG 0.444 0.999 0.903 12.77 3.210-2 MB 0.958 0.999 0.969 57.47 2.710-3 66 3.5.3 Adsorption isotherms Adsorption isotherms at various initial dye concentrations were performed to assess equilibrium data and adsorbate/adsorbent interactions to find the adsorption mechanism. The UiO-66 adsorption capacity at different initial dye concentrations is presented in Figure 3-7. As can be observed, the qe parameter of dyes enhanced with an initial dye concentration increment and reached maximum values of 329, 401, 82, and 326 mg/g for MR, MO, MG, and MB dyes, respectively, for the initial dye concentration equal of 200 mg/L. The adsorption results were matched well with the Langmuir and Freundlich models to evaluate the adsorption mechanism. Figure 3-7. Adsorption isotherms of (a) MR, (b) MO, (c) MG, and (d) MB onto pristine UiO-66 67 Results from Table 3-3 and Figure 3-8 demonstrate that correlation coefficients (R2) for Langmuir isotherm in MR, MO, and MB dyes adsorption are higher than that of Freundlich isotherm, while in the case of MG adsorption, R2 values for Freundlich isotherm are higher than that of Langmuir isotherm. These observations show that the Langmuir model is more appropriate for explaining MO, MB, and MR dyes adsorption onto the pristine UiO-66, while the adsorption of MG followed the Freundlich model. All the RL values are between 0 and 1, and the Freundlich isotherm constant (n) for all the dyes are higher than 1, indicating that the adsorption of these dyes onto the pristine UiO-66 is favorable. Figure 3-8. Model fits of Langmuir (a and b) and Freundlich (c and d) for the adsorption of these dyes onto pristine UiO-66 68 Table 3-3. The isotherms constants pertaining to adsorption models for various dyes Dyes Langmuir constants Freundlich constants qmax (mg/g) KL (L/mg) R2 KF (mg/g) n R2 MR 384.62 7.0110-2 0.993 32.65 1.77 0.946 MO 454.54 9.4910-2 0.991 51.92 1.89 0.973 MG 133.33 9.6710-3 0.847 3.04 1.53 0.975 MB 370.37 7.9110-2 0.978 31.26 1.68 0.921 A comparison of our results with the literature findings (summarized in Table 3-4) suggests that pristine UiO-66 displays the best adsorption performance for anionic MR and MO dyes adsorption from aqueous solutions, as qmax for MO, MB, and MR dyes are considerably greater than those listed. Meanwhile, the adsorption capacity for cationic MG dye is slightly lower than that of other adsorbents. Thus, the qmax of pristine UiO-66 for different organic dyes mainly depends on the size and charge of the dyes. 69 Table 3-4. Dye adsorption capacity reported for various materials Adsorbents Dye Optimum adsorption condition (pH, Temperature (°C )) Residual dye concentration (µM) Adsorption capacity (mg/g) Partition coefficient (mg/g µM) Ref. Activated carbon MR 4, 25 92.24 40 0.434 [240] Pristine UiO-66 MR 5.5, 25 337.99 384 1.137 This study Fe3O4/Al2O3/chitosan MO 6, 25 1985.76 416 0.209 [241] Muli-walled CNT MO 7, 60 49.49 53 1.071 [242] ED-MIL-101 MO -, 25 518.31 160 0.309 [243] UiO-66 MO 5.5, 25 38.19 39 1.021 [43] NH2-UiO-66 MO 5.5, 25 43.99 28 0.636 [43] Pristine UiO-66 MO 5.5, 25 205.66 454 2.208 This study MIL-100(Fe) MG 5, 30 246.64 146 0.592 [226] ZIF-8 MG 3.5, 20 274.04 1611 5.879 Cd-ZIF MG 7, 25 7.54 3324 440.849 [244] Pristine UiO-66 MG 5.5, 25 474.77 133 0.070 This study Activated carbon MB 6, 25 7.82 263 33.632 [245] Graphene oxide MB 6, 25 40.64 243 5.979 [245] Muli-walled CNT MB 6, 25 131.31 196 1.493 [245] NH2-MIL-101 (Al) MB -, 30 515.87 762 1.477 [246] MIL-101-SO3H MB 7, 35 781.62 374 0.478 [247] UiO-66 MB 5.5, 25 5.72 90 15.734 [43] NH2-UiO-66 MB 5.5, 25 1.22 96 78.69 [43] Pristine UiO-66 MB 5.5, 25 274.97 370 1.345 This study 70 The PC in an adsorption system containing organic dye and adsorbent nanoparticles determines the ratio of dye concentration in the solid phase (surface of nanoparticles) to its concentration in the liquid phase (solvent) at equilibrium [217]. As presented in Table 3-4, the pristine UiO-66 showed enhanced adsorption performance for the adsorption removal of anionic MR and MO dyes from aqueous solutions compared with commercial activated carbons with PCs of 1.137 and 2.208 mg/g µM for MR and MB dyes, respectively. Among the various adsorbents listed in Table 3-4, the Cd-ZIF nanoparticles showed the best performance for adsorption removal of cationic MG dye from aqueous solutions with the highest PC of 440.85 mg/g µM. 3.5.4 Effect of initial pH and ionic strength on adsorption capacity The interactions of adsorbents with dye molecules depend on the initial pH of dye solutions due to the influence of pH on the adsorbent surface charge as well as dye molecules. Thus, the effect of initial solution pH on the adsorption capacities (qe) of these dyes at 25 °C and different pHs (from 4 to 12) was assessed, and the results are shown in Figure 3-9a. The qe of all the dyes displays different reactions toward the initial pH value. The qe of anionic MR and MO dyes decreased gradually by increasing the pH and showed the lowest values at pH≈12. A plausible explanation for this observation might be the electrostatic repulsion increment among the anionic dyes and the highly negative charge of the pristine UiO-66. The qe values of UiO-66 for MR and MO dyes at pH≈12 are high, as shown in Figure 3-9a. This suggests the role of other adsorption mechanisms such as π-π stacking interactions between the aromatic structure of dye molecules and benzene ring of UiO-66, hydrogen bonding interactions between the polar functional groups of dye molecules and the surface of UiO-66 nanoparticles in addition to physical adsorption and penetration of dye molecules into the porous structure of pristine UiO-66 [13]. 71 The qe of both cationic MG and MB dyes onto this adsorbent increased rapidly with an increasing pH, which reached the maximum, then decreased by increasing the solution pH. Such an unusual trend could be justified by the varying interactions between the cationic dyes and the surface of the adsorbent nanoparticles caused by the zeta potential variation with pH. Figure 3-9c shows that the surface charge of pristine UiO-66 reduces gradually with the increase of pH, suggesting the presence of a highly negative surface charge. However, in acidic pH, electrostatic repulsion forces between the cationic dyes and the positively charged surface of UiO-66 is predominantly due to the adsorption mechanism, confirming the lowest adsorption capacity. Increasing the pH of dye solutions decreases repulsive forces, and attractive force becomes dominant between the cationic dye molecules and the negatively charged surface of UiO-66, contributing to an increase in qe of the cationic dyes with an enhanced pH. Figure 3-9. The impact of pH (a) and salt concentration (b) on the adsorption capacity of dyes onto pristine UiO-66 and the pH impact on the zeta potential of pristine UiO-66 (c) The impact of salt concentration (ionic strength) on the dye adsorption capacity of pristine UiO-66 was also examined in different ionic strength solutions adjusted by NaCl. It can be seen from Figure 3-9b that the trends of variation of qe of anionic and cationic dyes in pristine UiO-66 are the same, and the adsorption capacities of dyes decreased by increasing the ionic strength of 72 dye solutions. This might be due to the screening influence of chlorine ions (Cl-), which would reduce electrostatic attraction between the dye molecules and the surface of adsorbents [216]. 3.5.5 Selective separation The selective adsorption ability of UiO-66 (Eq. 16)against different dyes in the binary solution containing various dyes with different charges was also investigated, and these data are displayed in Figure 3-10. The UV-Vis absorption spectra show that adsorption peak intensities of anionic MR and MO dyes decrease noticeably after 2 h, while those of the cationic MG and MB dyes decreased slightly. This suggests that pristine UiO-66 could selectively separate the anionic dyes from the anionic/cationic dyes mixture, leading to a color switch from dark to blue. 73 Figure 3-10. The UV-Vis spectra and adsorption images of different mixtures, (a) MO/MB, (b) MO/MG, (c) MR/MB, and (d) MR/MG before and after adsorption onto the pristine UiO-66 The ideal adsorption selectivity of MR and MO against MG and MB onto pristine and solvent-aged adsorbents was calculated based on their single adsorption experiments. Figure 3-11 describes that the ideal adsorption selectivities of MR/MG and MO/MG are significantly higher than those of MR/MB and MO/MB. This might be because of the smaller size of MB than MG, which may penetrate through the small pores of the adsorbents. The highest adsorption selectivity 74 was obtained by W-UiO-66-12, which is attributed to the increase of zeta potential caused by the hydrolysis reaction, which could change the interaction between the dye molecules and adsorbents. As a consequence of hydrolysis, the bond between the inorganic unit and organic ligands is broken, so several carboxylic acid functionalities are created in the outer surface of the aged UiO-66, leading to an increase in the zeta potential of W-UiO-66-12. Additionally, this reaction can create many defect sites on the surface of water-aged UiO-66, which might have increased the adsorption capacity for anionic dyes [56, 236, 248]. In the case of W-UiO-66-12, the electrostatic forces between anionic dyes and the highly positive charge of the adsorbent increased, while electrostatic repulsion between cationic dyes and the adsorbent increased, thereby giving an increase in qe of the anionic dyes with a reduction in the qe values of cationic dyes. Figure 3-11. The ideal adsorption selectivity of MR and MO dyes against MG and MB dyes onto all adsorbents 75 3.5.6 Regeneration and reusability The regeneration and recyclability of adsorbents are critical in large-scale practical applications [189, 249]. Therefore, the adsorption performances of regenerated UiO-66 for these organic dyes were assessed using diluted NaOH solution (0.01 M) and water. The dyes-loaded UiO-66 nanoparticles were dispersed in the NaOH solution under ultrasonic treatment for 20 min, washed several times with water, dried at 100 °C overnight, and then used for further dye adsorption. Figure 3-12 shows the adsorption capacities of these organic dyes onto the pristine UiO-66 after three cycles of regeneration. It can be observed that the adsorption capacities of all organic dyes changed only slightly, indicating that the UiO-66 nanoparticles are capable of being used for multiple adsorption-desorption cycles. Figure 3-12. Regeneration and recyclability of pristine UiO-66 76 3.6 Chapter summary UiO-66, a stable MOF, was successfully synthesized and employed to remove anionic and cationic dyes from the aqueous solutions. The pristine UiO-66 and its solvent-aged samples were characterized by XRD, FE-SEM, BET, TGA, and zeta potential analysis. Pristine UiO-66 displayed good structural stability against water, chloroform, and DMF based on XRD and TGA data, with a negligible reduction of BET surface area due to the excessive structural strength of the inorganic units. While the BET surface area of UiO-66 aged in DMF for 12 months showed a 30% reduction compared to pristine UiO-66, it still has a reasonable value as its adsorption capacity for organic dyes was quite desirable. The effect of contact time, initial pH, ionic strength, aging time, and initial dye concentration were examined, and adsorption kinetics was adequately described by the pseudo-second-order kinetic model, while equilibrium adsorption results were designated by the Langmuir isotherm model. The qmax of pristine UiO-66 for MR, MO, MG, and MB dyes are 384, 454, 133, and 370 mg/g, respectively, while the partition coefficients of pristine UiO-66 for these dyes are about 1.137, 2.208, 0.070, and 1.345 mg/g µM, respectively. Adsorption mechanism studies suggest that adsorption of anionic and cationic dyes onto UiO-66 can be explained by electrostatic interaction, hydrogen bonding, π-π stacking interaction, physical adsorption, and dye molecules may be captured in the porous structure of UiO-66. The UiO-66 nanoparticles aged in water showed a greater tendency to adsorb anionic MO and MR dyes than the cationic MG, mainly due to powerful electrostatic attractions between the unlike charges of anionic dyes and W-UiO-66-12. In conclusion, from the present findings, UiO-66 nanoparticles are potential candidates for selective dye pollutant removal from wastewater sources. 77 Chapter 4 Magnetization and Subsequent Modification for the Production of Highly Active Adsorbents The appealing characteristics of metal-organic frameworks render them well-suited for water treatment. Nevertheless, their powdery form presents a significant hurdle in practical water treatment applications due to the difficulties associated with separating them from treated water. In this chapter, the magnetization of post-modified UiO-66-NH2 using a magnetic core has been followed to address separation challenges by using an external magnet. 4.1 Introduction To date, different MOFs, such as UiO-66-family, MIL-family, ZIF-family, etc., have been widely applied as adsorbents to remove organic dyes from wastewater because of their water stability, good adsorption performance, ease of synthesis, and simple modification [21, 219]. Among them, UiO-66, one of the most important MOF subclasses, has been extensively explored in dye removal from wastewater due to its large surface area (> 1,000 m2/g), high crystallinity, excellent porous and crystalline stability under high mechanical pressure and temperature as well as harsh chemical (basic/acidic) conditions [27]. The UiO-66 and its derivatives are mainly used as pure powders, needing high-speed centrifugation or time-consuming and tedious filtration separation after organic dye adsorption, limiting their dye removal applications [250]. Incorporating UiO-66 within or onto an appropriate substrate efficiently addresses the separation challenges [251]. A large body of literature shows that the stability of MOFs could be remarkably improved against aqueous solutions by combining them with other substrates [114]. The magnetic Fe3O4 nanoparticles have emerged as appropriate substrates due to their magnetic properties, small size, low toxicity, and facile separation from the aqueous solutions by an external magnet [171, 252]. 78 UiO-66 modification via direct or post-synthetic modification (PSM) methods has shown great promise in improving MOF's adsorption capacity towards organic dyes [253]. For instance, Kavak et al. [254] reported that the adsorption of UiO-66 and NH2-UiO-66 towards MB was improved by incorporating ionic liquids. Hydrogen bonding between polar components of organic dyes and unsaturated functional groups of MOFs such as –NH2, -SO3H, and –COOH could actively support dye removal in UiO-66 [25]. It was observed that the stability of UiO-66 could be adversely affected by the modification [70]. Thus, improving the adsorption performance of UiO-66 towards organic dyes without disturbing chemical structural stability seems to be the main issue [39]. This study first addresses the challenges of using polycrystalline powdery MOFs in wastewater treatment via magnetization. Magnetic core preparation and proper functionalities are critical to obtaining robust magnetic MOF composites. Following the post-modification of MOFs, the affinity towards removing the target organic contamination is adjusted to take advantage of the MOF platform's unique advantages. Accordingly, magnetic Fe3O4@UiO-66-NH2 adsorbent with highly active functional groups such as TCT and PTZ is designed using solvothermal and post-synthetic modification approaches (Figure 2-1). Various techniques were employed to investigate different characteristics of the designed MOFs, such as thermal stability, textural properties, and crystalline structure. An advanced MOF composite with easy-handling separation and high affinity to remove MO anionic dye was obtained using magnetization and post-synthetic modification with nitrogen-rich compounds. The critical parameters in industrial applications of the designed MOFs (Fe3O4@UiO-66-TCT), such as recyclability and the mechanisms involved in MO adsorption, were investigated. 79 4.2 Isotherm adsorption experiments The absorbance vs. dye concentration calibration curve at various concentrations (2-50 mg/L) of dye was defined in the first step. After that, the adsorption isotherm experiments were performed at room temperature. About 10 mg of dried adsorbent nanoparticles were added to 20 mL of MO dye solution with concentrations ranging from 10 mg/L to 100 mg/L. After stirring for 3 h, the nanoparticles were separated from the dye solution by an external magnet, and the absorbance of each supernatant solution was measured using UV-Vis spectroscopy. 4.3 Desorption and reusability The regeneration and reusability of the adsorbent as a crucial factor in the practical application of adsorbents were investigated to evaluate the stability of the adsorbent nanoparticles in consecutive adsorption-desorption cycles. The used adsorbent nanoparticles were dispersed in deionized water and sonicated for 30 min. The samples were washed several times with water and 0.01 M NaOH solution to ensure complete desorption of adsorbed MO molecules. Then, the washed nanoparticles were dried at 60 °C for 12 h under a vacuum. Finally, the regenerated adsorbent nanoparticles were reused in the adsorption cycle. 4.4 Results and discussion 4.4.1 Characterization of adsorbents The post-synthetic modification of Fe3O4@UiO-66-NH2 with TCT and further with PTZ was investigated by FTIR. As depicted in Figure 4-1a, two peaks have appeared at 3375 and 3456 cm-1 in the spectrum of pristine adsorbent, attributed to the stretching vibration of the primary –NH2 group of NH2-BDC ligands [12]. The modified adsorbent showed an overlap of these two peaks and the appearance of a broad peak, owing to the conversion of primary amines to secondary 80 amines due to the reaction between –NH2 groups of NH2-BDC ligands and TCT molecules [255]. The appearance of two distinctive peaks in the case of modified adsorbents at 1451 and 1528 cm-1 is related to the stretching vibration of C=N and N=N bonds of the tetrazole rings, indicating Fe3O4@UiO-66-NH2 was functionalized with TCT and PTZ [256]. The increased nitrogen content of the modified adsorbents confirmed the attachment of nitrogen-rich-containing compounds to Fe3O4@UiO-66-NH2 (Table 4-1). Figure 4-1. (a) FTIR spectra, (b) XRD patterns, (c) N2 adsorption-desorption isotherms, and (d) pore size distribution of adsorbents 81 Table 4-1. Physical properties of pure and modified adsorbents Adsorbents SBET (m2/g) Pore volume (cm3/g) Mean pore diameter (nm) Zeta potential (mV) Elemental analysis of adsorbents (%) C H N Zra Fe3O4@UiO-66-NH2 663 0.416 2.51 2.78 37.7 3.15 4.62 32.48 m-UiO-66-TCT 69 0.089 5.12 5.13 35.4 2.86 7.85 18.67 aZr contents were determined by ICP-OES and CHN content with TruSpecRCHN, respectively. XRD patterns of pristine and modified adsorbents were used to assess the stability and changes in the crystal structure of Fe3O4@UiO-66-NH2 after functionalization (Figure 4-1b). The relative intensities and consistent peak positions related to characteristic peaks of pristine and modified XRD patterns in the range of 5° to 10° confirmed that Fe3O4@UiO-66-NH2 has an unaltered crystalline structure [10]. The nitrogen adsorption-desorption isotherms of pristine and modified adsorbents were performed at 77 K, and the parameters are compiled in Table 4-1. It is observed from Figure 4-1d that pristine and modified adsorbents have mesoporous structures. It was reported that the size of the substituent in the structure of organic ligands and the degree of functionalization could significantly affect the available pore volume within the porous structure of the adsorbents as well as their BET surface area [255]. As a result of the modification of Fe3O4@UiO-66-NH2 with the bulky groups, the total pore volume and the BET surface area of the modified adsorbent significantly decreased from 663 m2/g in Fe3O4@UiO-66-NH2 to 69 m2/g in m-UiO-66-TCT. Thermogravimetry was employed to determine the thermal stability of the adsorbents; the TGA and DTG profiles of Fe3O4@UiO-66-NH2 and m-UiO-66-TCT nanoparticles are depicted in Figure 4-2. Thermograms of pristine and modified adsorbents exhibit a four-step weight loss. The initial weight loss occurred at 100 °C due to the evaporation of the adsorbed H2O molecules onto 82 the surface of the adsorbent or trapped in the pores [257]. A second weight loss occurred when the residual solvents (DMF) or the unreacted reactants were removed from the pores of the adsorbents at temperatures above 200 °C. The third weight loss occurred at 300 °C, possibly due to removing different functional groups and organic compounds attached to the surface of the adsorbents [12]. From Figure 4-2, it can be concluded that m-UiO-66-TCT resulted in a significantly more significant weight loss than Fe3O4@UiO-66-NH2, which is attributed to the removal of organic TCTs and PTZs. At temperatures above 400 °C, the fourth and most considerable weight losses were caused by the collapse/decomposition of the frameworks into CO, CO2, iron oxide, and zirconium oxide [45]. TGA curve of Fe3O4@UiO-66-NH2 and m-UiO-66-TCT demonstrated that the decomposition temperature of the modified MOFs was slightly lower than that of pristine MOF, and its fourth weight loss was slightly shifted to lower temperatures. The final residue ash content of the modified MOF as an indicator of thermal stability [258], suggesting higher thermal stability of the pristine MOF compared to the modified adsorbent, which can be ascribed to the presence of different combustible organic compounds attached to the surface of MOF. The differences in thermal stability of pristine and modified adsorbent materials endorsed the grafting of TCT and PTZ onto the surface of Fe3O4@UiO-66-NH2. 83 Figure 4-2. (a) TGA and (b) DTG curves of pure and modified adsorbents Using SEM analysis, Fe3O4@UiO-66-NH2, and m-UiO-66-TCT MOFs were examined for their general morphology. It was found that the SEM images of both MOFs are relatively the same and have round-shaped nanoparticles with particle sizes smaller than 100 nm (Figure 4-3a, b). Moreover, no changes in MOFs morphology or destruction of their structure are evident during functionalization, supporting the structural stability of Fe3O4@UiO-66-NH2 MOFs under the designed post-modification procedure. The growth of UiO-66-NH2 on Fe3O4 and the resulting core-shell structure was verified by TEM images (Figure 4-3c-e). The magnetic separation of the modified adsorbent is shown in Figure 4-3f. The dispersed magnetic nanoparticles were rapidly attracted to the external magnet within 20 s, confirming the appropriate magnetic property of the modified adsorbent. 84 Figure 4-3. (a and b) SEM and (c-e) TEM images of pure and modified adsorbents and (f) photo images of m-UiO-66-TCT suspension in water before and after the magnetic separation 4.4.2 Adsorption isotherms The equilibrium adsorption isotherm was employed to define the maximum saturated adsorption capacity, adsorption mechanism, and the driving force for adsorption. The adsorption isotherms of MO dye onto the adsorbents were obtained at varying initial MO concentrations (from 10 to 250 mg/L) for 180 min at room temperature. The adsorption isotherms of MO dye onto Fe3O4@UiO-66-NH2 and m-UiO-66-TCT at room temperature, depicted in Figure 4-4a, exhibit 85 an increment in the adsorption capacity of MO dye at enhanced dye concentration. Figure 4-4a also demonstrates that the modified adsorbent (m-UiO-66-TCT) has a higher adsorption capacity (≈298 mg/g) compared to the pristine adsorbent (≈271 mg/g) for the MO dye. As a result of electrostatic interaction, hydrogen bonding, and π-stacking interactions, modified nanoparticles and MO molecules interact strongly. Although the modified adsorbent had a smaller BET surface area than the pristine adsorbent, its adsorption capacity for MO dye was slightly higher than that of the pristine adsorbent. The data suggest that the high affinity of the modified adsorbent for the anionic MO was not just a result of its porosity but also adsorption mechanisms, which can be attributed to strong electrostatic interactions between the anionic MO molecules and the adsorbent because of the adsorbent's high positive zeta potential (Table 4-1) [43]. To better evaluate the mechanism of MO molecules adsorption over the synthesized MOFs, well-known adsorption isotherm models, such as Freundlich, Langmuir, Tempkin, and Dubinin-Radushkevich isotherms, were applied and fitted to the obtained adsorption data. The fitted curves to these models are displayed in Figure 4-4 b-e, whereas the calculated constant parameters are listed in Table 4-2. For both pristine and modified adsorbents, Freundlich's isotherm model showed higher correlation coefficients (R2). In this sense, experimental adsorption data match the Freundlich isotherm model more closely, suggesting that MO adsorption occurs at heterogeneous sites with varying energy levels. MO's experimental maximum adsorption capacities over Fe3O4@UiO-66-NH2 and m-UiO-66-TCT were 271 and 298 mg/g, respectively. However, since the isotherm adsorption data did not match the Langmuir and Dubinin-Radushkevich isotherm models, the experimental adsorption capacities were not equal to the values calculated by these isotherm models. This observation confirms that the Freundlich isotherm model is most suitable for describing adsorption data. 86 Figure 4-4. Adsorption isotherms and fitting experimental MO adsorption data to (b) Freundlich, (c) Langmuir, (d) Tempkin, and (e) Dubinin-Radushkevich isotherms. (Adsorption conditions: dye concentration= 10-250 mg/L, contact time= 180 min, adsorbent weight= 5 mg, dye solution volume =20 mL, temperature= 25°C ) Table 4-2. Relevant calculated parameters from different models Adsorbents Freundlich Langmuir Tempkin Dubinin-Radushkevich KF (mg/g) n R2 qmax (mg/g) KL (L/mg) R2 BT (KJ/mol) AT (L/g) R2 qmax (mg/g) B (mol2/kJ2) E (J/mol) R2 Fe3O4@UiO-66-NH2 4.635 1.215 0.9688 555.56 0.0059 0.8174 30.895 0.139 0.9405 160.93 2.0E-5 1.58E+2 0.8313 m-UiO-66-TCT 20.373 1.878 0.9816 370.37 0.0206 0.9233 38.114 0.371 0.8960 178.86 4.0E-6 3.54E+2 0.7614 87 4.4.3 Kinetic study Adsorbent and adsorbate contact times are essential in industrial applications [136]. This has prompted the investigation into the adsorption capacity of m-UiO-66-TCT towards MO dye at different times, temperatures, and initial MO concentrations (20 mg/L was investigated). Accordingly, the initial MO dye adsorption onto m-UiO-66-TCT was very fast (≈ 15 min), which was enhanced slowly with enhancing contact time to reach the equilibrium after 180 min. As depicted in Figure 4-5a, m-UiO-66-TCT exhibited 72.32, 78.54, and 81.04 mg/g of adsorption capacities for MO dye after 180 min at 25, 35, and 45 °C, indicating that adsorption is endothermic, which increased with temperature. Increasing the temperature decreased MO solution viscosity, allowing MO molecules to diffuse across the external boundary layer more quickly. This enabled more MO molecules to transfer into m-UiO-66-TCT particles [91]. Moreover, this observation indicates that the interaction of MO molecules with m-UiO-66-TCT nanoparticles increases the operation temperature, leading to higher adsorption capacity at higher temperatures. Thermodynamic parameters, including the standard enthalpy (∆H0, kJ/mol) and standard entropy (∆S0, J/mol K), were determined from the slope and intercept of the Van't Hoff equation (Figure 4-5b) [232]. Thermodynamic analysis of MO adsorption onto m-UiO-66-TCT at various temperatures shown in Table 4-3 suggests that free energy change (∆G0, kJ/mol) is -5.81 kJ/mol at 25 °C, indicating the adsorption of MO onto m-UiO-66-TCT is spontaneous and thermodynamically favorable. The positive value of ∆H0 implies the endothermic nature of adsorption and the favorability of higher temperatures to remove MO. The calculated value of positive entropy change during MO adsorption indicates the spontaneous adsorption process due to increased randomness [12]. 88 Table 4-3. Thermodynamic parameters for the adsorption of MO onto m-UiO-66-TCT T (°C) ∆G0 (kJ/mol) ∆H0 (kJ/mol) ∆S0 (J/mol K) 25 -5.81 18.48 81.49 35 -6.62 - - 45 -7.44 - - Experimental data have been fitted with the pseudo-first-order, pseudo-second-order, and intra-particle diffusion kinetic models to explore the adsorption kinetic characteristics (Figure 4-6 (a, b, and c) and Table 4-4). Notice that the pseudo-second-order kinetic model's correlation coefficients (R2) are higher than the other models at different temperatures. The equilibrium adsorption capacities of m-UiO-66-TCT calculated from the pseudo-second-order kinetic model are close to the experimental equilibrium adsorption capacities. The pseudo-second-order kinetic model seems more suitable for understanding the adsorption mechanism, demonstrating that chemical adsorption controls the adsorption [216]. As shown in Figure 4-6c, the fitted data for the intra-particle diffusion kinetic model are linear, and the curves did not pass through the origin, indicating that intra-particle diffusion has a remarkable influence on the adsorption of MO onto m-UiO-66-TCT; however, this may not be the only rate-controlling step [259]. 89 Figure 4-5. (a) Adsorption capacity of modified adsorbent towards MO as a function of time at various temperatures and (b) van't Hoff plot. (Adsorption conditions: dye concentration= 20 mg/L, contact time=0-180 min, adsorbent weight= 20 mg, dye solution volume= 50 mL, temperature= 25-45 °C) Figure 4-6. (a) The pseudo-first-order, (b) pseudo-second-order, and (c) intra-particle diffusion plots for MO adsorption onto the modified adsorbent Table 4-4. Kinetic parameters for the adsorption of MO by the modified adsorbent at various temperatures Temperature (°C ) R2 qe (mg/g) K2 (g/mg min) Pseudo-first-order Pseudo-second-order Intra-particle diffusion Experimental Calculated 25 0.9646 0.9977 0.9965 72.3286 74.6269 0.0015 35 0.9800 0.9989 0.9732 78.5421 80.6452 0.0018 45 0.9827 0.9988 0.9919 81.0451 83.3334 0.0015 90 4.4.4 Effect of ionic strength and pH Wastewater from various industries generally contains different types of dissolved toxic compounds such as alkalis, acids, metal ions, and salts apart from the dyes, which can significantly affect the adsorption performance of the adsorbents [260]. For instance, NaCl is often used as an accelerator or stimulator in the dyeing process, which generally increases or decreases the adsorption capacity of the adsorbents via increasing the degree of dissociation of the dye molecules or screening the electrostatic attraction between the opposite charges [261]. In theory, when electrostatic interactions between the adsorbate and adsorbent are repulsive, an increase in salt concentration or ionic strength increases the adsorption capacity of the adsorbents; if electrostatic interaction is attractive, an increment in salt concentration reduces the adsorption capacity of the adsorbents [262]. Therefore, the effect of ionic strength on the adsorption capacity of m-UiO-66-TCT towards MO adsorption was investigated at various ionic strengths adjusted by NaCl. As shown in Figure 4-7a, increasing the NaCl concentration has decreased the adsorption capacity of m-UiO-66-TCT. Under this situation, many Cl- ions in the dye solution could screen the positive sites of m-UiO-66-TCT nanoparticles, reducing electrostatic attraction between m-UiO-66-TCT nanoparticles and MO molecules, thereby following a decrease in adsorption capacity, which follows the reported trends [216]. The pH of the dye solution is another parameter that affects removal efficiency. Solution pH can significantly affect the dissociation of various functional groups located at the adsorption active sites of the adsorbents and the structure of dye molecules. This changes the interactions between the dye molecules and the adsorbents [263]. The impact of the pH of the MO solution on the adsorption capacity of m-UiO-66-TCT was examined by regulating the pH from 2 to 12 by adding diluted NaOH and HCl solutions. According to Figure 4-7b, the m-UiO-66-TCT shows the 91 highest adsorption capacity of ≈72 mg/g at pH 6, which can be attributed to strong electrostatic interactions between the anionic MO molecules and the highly positive-charged m-UiO-66-TCT (Figure 4-7c). The adsorption capacity of m-UiO-66-TCT towards MO decreased to 59 mg/g upon reducing the MO solution pH from 6 to 2.7, suggesting that an acidic environment might reduce the adsorption capacity of m-UiO-66-TCT. The MO molecules are neutral at this condition, while m-UiO-66-TCT has a positive charge, and hence, no electrostatic interactions exist between these materials. It may be expected that at pH lower than pKa of MO molecules, the possible interactions between m-UiO-66-TCT and MO are those of the π-stacking interactions, which can be confirmed by FTIR (Figure 4-9) [253]. As displayed in Figure 4-7c, increasing the solution pH gradually decreased the zeta potential of m-UiO-66-TCT, resulting in a stronger electrostatic repulsion between the anionic MO molecules and the negatively charged m-UiO-66-TCT nanoparticles. These results indicate that the modified adsorbent showed desirable adsorption capacity towards MO dye at acidic and neutral solutions. Another plausible reason for the decreased adsorption capacity of MO by increasing the solution pH is the partial collapse of the m-UiO-66-TCT structure. By comparing the characteristic peaks of UiO-66 MOFs at approximately 2θ = 7.38°, 8.56°, and 25.67° in various environments, valuable insights into the integrity of the crystalline structure can be gained. As shown in Figure 4-7d, the XRD pattern of m-UiO-66-TCT was slightly changed after being stirred in a basic solution with a pH of 11 for 12 min, but its XRD patterns in acidic media remained unchanged. 92 Figure 4-7. Effect of (a) NaCl concentration and (b) pH on the adsorption capacity of MO onto the modified adsorbent. (Adsorption conditions: dye concentration= 20 mg/L, contact time=180 min, adsorbent weight= 5 mg, dye solution volume= 20 mL, temperature= 25°C , NaCl concentration= 0-0.5 mol/L, pH=2-12). (c) zeta potential of the modified adsorbent as a function of pH. (d) XRD patterns of m-UiO-66-TCT nanoparticles before and after stirring in basic (pH ≈ 11) and acidic (pH ≈ 3) solutions for 120 min 4.4.5 Desorption and recyclability The reusability of the modified adsorbent for MO adsorption up to three cyclic adsorption-desorption processes was investigated. After three consecutive adsorption-desorption cycles, the adsorption capacity of m-UiO-66-TCT reached 92% of the initial adsorption capacity value (Figure 4-8). Such a reduction in the adsorption capacity of m-UiO-66-TCT for MO might be due to the occupation of the adsorbent’s active sites with MO molecules even after the desorption and 93 mass loss of the adsorbent during the desorption cycle [264]. The results indicated that m-UiO-66-TCT could be rapidly regenerated for the adsorption of MO, which is crucial in practical industrial applications. Figure 4-8. Reusability of the modified adsorbent towards MO adsorption. (Adsorption conditions: dye concentration= 20 mg/L, contact time=180 min, adsorbent weight= 5 mg, dye solution volume= 20 mL, temperature= 25°C ) 4.4.6 Adsorption mechanism FTIR was used to evaluate the adsorption mechanisms of MO. As seen in Figure 4-9, several distinct differences exist between the FTIR spectra of the synthesized adsorbents before and after the adsorption of MO molecules. As per the FTIR spectra of the adsorbents after the adsorption of MO molecules, the peak intensity at 1550 cm-1 associated with the vibration of the aromatic ring of MO molecules decreased due to π-stacking interaction between the benzene rings of MO molecules and the aromatic ligand of the adsorbents. Additionally, it can be observed that such a reduction in peak intensity for m-UiO-66-TCT is slightly higher than that of Fe3O4@UiO-66-NH2, 94 suggesting more π-stacking interactions between the modified adsorbent and the MO molecules due to the presence of more number of benzene rings. Similar results were observed for methyl red removal with OND-UiO [194] and malachite green adsorption over Fe3O4@PDA-COOH [265]. The two new peaks at 2,850-2,950 cm-1 are due to the stretching vibration of CH groups, and their intensities increased after the adsorption of MO molecules onto the adsorbents. The peaks observed at 1,041 and 1,115 cm-1, which are assigned to the stretching vibration of the –SO3 group of MO, were shifted to lower frequencies of 1,037 and 1,104 cm-1 after being adsorbed onto the adsorbents, mainly due to the formation of H-bonds between the sulfonate group of MO molecules and nitrogen-containing functional groups of the adsorbents. A broad and strong absorption band confirms the hydrogen bonds formed between these adsorbents and the MO molecules at 3,200-3,600 cm-1. In addition, from a study of the effect of MO solution pH and ionic strength on the adsorption capacity of the modified adsorbent, the electrostatic interaction between anionic MO molecules and the positively charged adsorbent plays a vital role in the adsorption mechanism [266]. Therefore, the results indicate that MO molecules were adsorbed onto the adsorbents via hydrogen bonding, electrostatic interactions, and π-stacking interactions. Figure 4-9. FTIR spectra of pure MO dye and after being adsorbed onto (a) Fe3O4@UiO-66-NH2 and (b) m-UiO-66-TCT 95 4.4.7 Comparison with other reported adsorbents The adsorptive performance of the prepared adsorbents towards MO dye removal was compared with other common MOFs in the literature (Table 4-5). It is found that the adsorption capacities of the present adsorbents are comparable to or slightly higher than the other MOFs summarized in Table 4-5. The adsorption capacities of the adsorbents for the MO dye are significantly higher than pure UiO-66 and NH2-UiO-66. This observation might be due to the many nitrogen groups on the surface of these adsorbents (Table 4-1), which can create strong interactions with MO molecules. The differences in operating conditions, such as pH, temperature, adsorbent dosage, and initial dye concentration, can significantly affect the adsorption capacity of various adsorbents. Hence, judging from the adsorbent performance without considering these parameters may not be appropriate [218]. Therefore, the partition coefficient (PC, mg/g µM), which is an independent parameter of the adsorption conditions, was employed to compare the adsorption performance of the synthesized adsorbents with the other adsorbents [27]. Among the adsorbents listed in Table 4-5, CNT showed the best performance for MO adsorption regarding partition coefficient. The pure UiO-66 and NH2-UiO-66 exhibited the best performance for the MO removal from water with high PCs of 1.021 and 0.636 mg/g µM. These results confirmed that UiO-66 and its functionalized derivatives are excellent candidates for removing anionic MO dye from water. 96 Table 4-5. Comparison of adsorption capacity of the developed adsorbents toward MO dye with the reported adsorbents Adsorbents Equilibrium MO concentration (µM) Adsorption capacity (mg/g) Partition coefficient (mg/g µM) Adsorption conditions Ref. pH Time (min) Temperature (C) Initial dye concentration (mg/L) Adsorbent dosage (g/L) Fe3O4/Al2O3/chitosan 1985.76 416 0.209 NA 400 25 800 0.4 [241] MIL-101 528.52 140 0.265 5.6 720 45 200 0.2 [243] ED-MIL-101 518.31 160 0.309 5.6 720 45 200 0.2 [243] NH2-MIL-101 (Al) 574.34 188 0.327 - 720 30 200 0.05 [246] PED-MIL-101 467.42 241 0.516 5.6 720 45 200 0.2 [243] NH2-UiO-66 43.99 29 0.636 NA 150 25 20 0.2 [43] UiO-66 38.19 39 1.021 NA 150 25 20 0.2 [43] Muli-walled CNT 49.49 53 1.071 NA 120 45 20 0.3 [242] Fe3O4@UiO-66-NH2 569.23 270.95 0.476 NA 180 25 250 0.25 This study m-UiO-66-TCT 508.13 297.65 0.586 NA 180 25 250 0.25 This study NA = Not adjusted 4.5 Conclusions A novel magnetic Fe3O4@UiO-66-NH2 composite was synthesized, modified with nitrogen-rich compounds, and characterized by XRD, FTIR, SEM, TEM, BET, TGA, and elemental analysis. The adsorption of m-UiO-66-TCT towards MO dye was investigated regarding kinetics, isotherm, thermodynamics, solution pH, ionic strength, and reusability of the adsorbent. It was demonstrated that the adsorption isotherm and kinetics of MO adsorption onto m-UiO-66-TCT followed the pseudo-second-order kinetic models and the Freundlich isotherm. The electrostatic interactions between the anionic MO molecules and the positively-charged adsorbent, the hydrogen bonds formation between the –SO3H group of MO molecules and amino groups of the adsorbents, and π-stacking interactions between the benzene ring of MO molecules and the aromatic structures of the modified adsorbent dictate the mechanism of adsorption. Furthermore, the modified adsorbent showed excellent regeneration and recyclability, making it an attractive adsorbent for dye-polluted water decontamination. 97 Chapter 5 Designing Active Mat Adsorbents through UiO-66 Hybridization and Electrospinning In the preceding chapters, the utilization and modification of UiO-66 MOFs for water treatment have been demonstrated. However, the practical implementation of polycrystalline powdery MOFs has presented significant challenges. Building upon the findings presented in those chapters, this chapter investigates the promising approach of incorporating UiO-66 MOFs into polymeric nanofiber substrates to overcome the limitations associated with powdery MOFs. Moreover, the effective enhancement of MOFs' adsorption affinity for removing contaminants from aqueous media has been achieved through their hybridization with other nanomaterials. Therefore, the initial step in this study involves modifying the adsorption affinity by hybridizing MOFs with nanodiamond, followed by integrating the hybrid MOF material into the nanofiber substrate. 5.1 Introduction Crystalline MOFs with large surface area, abundant functionalities, tunable pore size, and high affinity to specific molecules have shown great potential for removing water contaminants, such as heavy metal ions, organic drugs, etc. [10, 27, 32, 114, 123, 267, 268]. While considerable efforts have been devoted to scale-up, chemistry, and application of MOFs at the lab scale level [23, 88], exploratory initiatives must be taken to transcend MOFs into practical wastewater treatment. Processability issues and control of MOF superstructure have become critical rate-limiting steps, hindering real-world applications of MOFs [90], especially their prospective use for wastewater treatment. Shaping MOFs into macrostructure by accommodating them into electrospun nanofibers is deemed a game-changer approach to bring powdery MOFs closer to the industrial applications level [114]. 98 Nanofabrication via electrospinning has had a tremendous impact over the past three decades due to its versatility, simplicity, and affordability [165]. Notably, numerous enterprises have invested in electrospinning technology and thereby have pushed the nanofibers' yearly production capacity to the ton-scale [269]. Placing the atomic-level functional MOF materials into nanofibers with a large surface-to-volume ratio not only can address the processing obstacles but also can retain the MOFs' active sites for interacting with pollutants in aqueous media [114]. For instance, mats comprising ZIF-8/PVA [270] and ZIF-67/PAN [157] showed maximum adsorption capacities of 160 and 849 mg/g toward CR, respectively. Several approaches can be considered to further improve the design of MOF-based nanocomposites for water remediation, such as the post-modification of MOFs by adding active functional groups, doping, organic ligand modification, and hybridizing with other nanoparticles [10, 23, 26, 36, 209, 271]. Hybridizing MOFs with inorganic nanoparticles such as GO [202], CNTs [272], nanodiamonds (ND) [194], magnetic nanoparticles [195], covalent organic frameworks (COFs) [273], activated carbon [274], and layered double hydroxide [275] can improve the morphological, stability, kinetic, and physicochemical features for water treatment. For example, the hybrid of ZIF-8 with GO and CNT has shown improved adsorption performance, stability, and reusability for eliminating malachite green (MG) [202]. In another study, Ahmadipouya et al. [195] followed an in situ growth of UiO-66 onto a magnetic Fe3O4 core to address separation challenges. They reported high adsorption capacities of 205 and 244 mg/g toward cationic MB and anionic MO dyes, respectively. In the present work, TOND was selected as the substrate to produce MOF-based nanocomposite mainly because of its relatively high surface area, good chemical stability, high affinity toward specific organic dyes, non-toxicity, and numerous surface bridging sites due to the 99 existence of large oxygen-containing functional groups on its surface [12, 114, 276]. To unravel the benefits of the hybridization of MOFs with other nanomaterials and electrospinning technology to fabricate nanoparticle-filled nanofiber adsorbents, ND, TOND, UiO-66, and their nanocomposite (TOND@UiO-66) were prepared and incorporated into nanofibers formed with a mixture of chitosan and polyvinyl alcohol (CS/PVA) (Figure 5-1). The prepared nanofiber nanocomposites were then cross-linked using GA and tested for cationic MB and anionic CR removal from wastewater. In addition, the systematic adsorbent performance, in terms of adsorption kinetics, isotherms, selectivity, ionic strength, the effect of pH, and reusability, was investigated to verify the applicability of MOFs nanofibers in real wastewater treatment applications. Figure 5-1. Schematic illustration of the fabrication of the electrospun TOND@UiO-66 nanofibers. Used with permission from Elsevier [277] 5.2 Dye removal experiments To conduct the adsorption study, measured amounts of each adsorbent were added to 50 mL of dye solutions (20-500 mg/L) and shaken in a dark place for 180 min at room temperature. Afterward, the concentration of dyes was measured by UV-Vis spectroscopy. Finally, the removal 100 percentage, adsorption capacity (qt) at specific intervals (time t), as well as adsorption capacity (qe) at equilibrium were calculated using Eq. 1, Eq. 2, and Eq. 3.[13]. To fully exploit the adsorbents, they were dried at 80 °C for 12 h under a vacuum before the experiments. 5.3 Results and discussions 5.3.1 Characterization of adsorbent nanoparticles 5.3.1.1 FTIR analysis The surface chemistry of the adsorbents and the interaction of the designed MOF components were studied by FTIR. Figure 5-2a displays the FTIR spectra of ND, TOND, UiO-66, and TOND@UiO-66 hybrid. The FTIR spectrum of the as-received ND shows the characteristic absorption peaks at 1000-1200 cm-1 (ν(C-O-C)), 1630-1750 cm-1 (ν(C=O)), 2850-2950 cm-1 (ν(C-H)), and 3250-3650 cm-1 (ν(O-H)), in agreement with previous findings [12, 222, 278]. The increased absorption peaks at 1722 cm-1 (ν(C=O) and 1100-1200 cm-1 (ν(C-O-C)) were observed in TOND compared to ND, an indicator of carboxylic acid formation from different functional groups [262, 278]. The absorption band in the spectrum of UiO-66 at 3200-3700 cm-1 belongs to the stretching vibration of O-H bonds of BDC molecules. The adsorbed or inter-crystalline water molecules also appeared in the 3200-3700 cm-1 spectrum range [17, 69]. The sharp peaks at around 1700 and 1400 cm-1 are assigned to the symmetrical stretching vibration of the C=O bonds of BDC molecules. Two sharp peaks observed at 1502 and 1578 cm-1 are attributed to the stretching vibration of C=C bonds of BDC molecules [17]. The observation of these characteristic peaks suggests that the signal associated with UiO-66 nanoparticles originated from the presence of BDC ligands in their structures [69, 76]. The peaks at 661 and 765 cm-1 come from the asymmetric and symmetric 101 stretching vibration of O-Zr-O bonds, confirming the formation of coordination bonds between Zr4+ ions as the coordination center and the BDC molecules as bridging ligands [110, 279]. The emergence of characteristic peaks of ND and UiO-66 in the spectrum of TOND@UiO-66 confirms the successful synthesis of the nanocomposite. It should be noted that the absorption peak position belonging to the stretching vibration of C=O bonds slightly shifted to lower wavenumbers in TOND@UiO-66, mainly due to the coordination of the Zr center of UiO-66 with the carboxylic acid functional groups of TOND [194]. A similar observation was reported by Han et al. [71] in the case of the MIL-68 MOF and CNT (CNT@MIL-68) composites. Figure 5-2. a) FTIR spectra, b) XRD patterns, c) N2 adsorption-desorption isotherms, and d) TGA thermograms of nanoparticles 102 5.3.1.2 XRD analysis XRD patterns of the synthesized adsorbent nanoparticles were obtained and shown in Figure 5-2b to study the crystallinity of the parent materials and their nanocomposite. The XRD patterns of ND unfold the presence of all the characteristic diffraction peaks of ND (2θ =16.2°, 21.4°, and 43.5°), consistent with previous reports [12, 280]. TOND demonstrated similar XRD patterns to ND with minor peak position and intensity changes. Accordingly, the peak at 2θ = 21.4°, related to a graphite-type lattice, slightly shifted to lower degrees, and its intensity decreased after thermal oxidation, confirming the reduction of graphite layer number on the surface of ND particles. Three sharp diffraction peaks at 2θ = 7.3°, 8.5°, and 25.8° in the XRD patterns of UiO-66 and TOND@UiO-66 belong to the (1 1 1), (2 0 0), and (6 0 0) crystal planes, respectively [76]. The good agreement of the XRD patterns with simulated UiO-66 and the literature confirms the successful synthesis of UiO-66 specimens in our study [17, 69, 76]. The XRD observation revealed that the presence of TOND in the TOND@UiO-66 didn't interrupt the formation of the UiO-66 crystalline structure. 5.3.1.3 N2 adsorption-desorption analysis N2 adsorption-desorption isotherms assessed the porosity and texture of nanoparticles at 77 K (Figure 5-2c). The isotherms of the TOND are similar to Type IV, demonstrating mesoporous (pore diameter > 2 nm) structures [114]. Pure UiO-66 showed Type I isotherms with a predominantly microporous structure [219]. Figure 5-2c demonstrates that the TOND@UiO-66 isotherms are relatively a combination of Type I and Type IV isotherms, showing a sharp increase in N2 adsorption at sufficiently low relative pressures, continuous growth to high pressures, and hysteresis at high pressures. Moreover, the N2 gas uptake of TOND@UiO-66 increased continuously with the relative pressure from 0.1 to 0.3, confirming the presence of larger 103 micropores, mainly due to the existence of the TOND in the structure. Table 5-1 summarizes the pore textural properties of nanoparticles. TOND and UiO-66 nanoparticles have BET surface areas of 331 and 1050 m2/g, respectively, which aligns with previous reports for these materials [17, 280]. The BET surface area of TOND@UiO-66 experienced a 20% decrease compared to pure UiO-66, mainly due to its higher density of TOND and lower porosity. Table 5-1 shows that the TOND@UiO-66 nanocomposite has a slightly larger pore diameter and pore volume than pure UiO-66, which may be caused by the formation of new pores between UiO-66 crystals and TOND particles. The larger pores in the structure of TOND@UiO-66 nanocomposite facilitate mass diffusion from bulk to intra-particles, which are beneficial to the adsorption of dye molecules [219]. Table 5-1. Textural properties of pristine materials and their composite Samples SBET (m2/g) Pore volume (cm3/g) Average pore diameter (nm) Zeta potential NDa 291 - - -2.61 TOND 331 - - -12.48 UiO-66 1050 0.521 2.1 -0.28 TOND@UiO-66 845 0.541 2.5 -3.81 a data reported from Ref. [114]. 5.3.1.4 Thermogravimetric analysis The thermal stability of nanoparticles and their nanocomposite was investigated with thermogravimetric analysis under N2 atmosphere and displayed in Figure 5-2d. Only a slight reduction in the TOND weight was observed at temperatures higher than 500 °C, which might be attributed to the degradation of various oxygen-containing functional groups on the surface of TOND particles [231]. The somewhat higher thermal stability of TOND in comparison with the 104 as-received ND particles may be due to a reduction in the number of graphite layers on the surface of TOND particles, further confirming the conversion of different functional groups on the surface of the ND particles to carboxylic acid groups during thermal oxidation [280, 281]. UiO-66 and TOND@UiO-66 showed three weight loss steps by increasing the temperature up to 800 °C. Removing guest or physically adsorbed water could account for weight loss at 60-120 °C [79, 209]. The second weight loss in the 120-250 °C range could be attributed to removing solvent molecules such as DMF. The following weight reduction step for UiO-66 and TOND@UiO-66 was observed at temperatures higher than 490 °C, possibly due to the collapse of their framework [272]. According to our results, pristine UiO-66 and TOND@UiO-66 exhibited similar thermal stabilities, demonstrating that the thermal stability of UiO-66 was not significantly affected by TOND. However, the final ash content of the nanocomposite is more than that of pristine UiO-66, primarily due to the high thermal stability of TOND in the nanocomposite structure. 5.3.1.5 SEM analysis The particle size and morphology of nanoparticles were investigated by SEM images and displayed in Figure 5-3. Accordingly, pristine and thermally oxidized ND are nearly spherical, with particle sizes smaller than 40 nm. UiO-66 possesses a cubic shape with a particle size smaller than 200 nm, in line with literature reports [27, 280, 282]. The TOND@UiO-66 morphology is reasonably similar to the UiO-66, with minor changes in particle size irregularity. Accordingly, the particle sizes decreased in TOND@UiO-66, and the crystal became more irregular. Coordinating the Zr4+ center of the UiO-66 framework with the oxygen-containing functional groups (especially carboxylic acid groups) of TOND inhibits MOF nanocrystal aggregation, thereby enhancing their dispersion [272, 282]. 105 Figure 5-3. SEM images of the synthesized nanoparticles 5.3.2 Characterization of electrospun nanofibers 5.3.2.1 FTIR analysis FTIR analysis assessed the nanofiber nanocomposites' interfacial interactions between nanoparticles and matrix. Accordingly, as shown in Figure 5-4a, cross-linked CS/PVA (ECPN) has the characteristic peaks of CS and PVA. The broad absorption bands in the range of 3150-3600 cm-1 could originate from the stretching vibration of –OH and –NH groups and the formation of intramolecular or intermolecular hydrogen bonds of –OH groups of PVA [278, 283]. CS and PVA showed two absorption bands between 2870 and 2980 cm-1, probably due to their aliphatic -C-H stretching vibrations [284]. CS amide groups may be responsible for the strong absorption 106 bands observed in the range of 1500-1650 cm-1 [13, 278]. The strong absorption band at 1380 cm-1 could correspond to the symmetrical deformation mode of –C-H bonds of both CS and PVA. The sharp absorption bands observed in 1000-1100 cm-1 could be assigned to the stretching vibration of –C-O bonds in both polymers [285]. Nanocomposite nanofibers exhibited all characteristic peaks of their constituents, including polymers and nanoparticles. However, the broad absorption band in the range of 3150-3600 cm-1, related to the stretching vibration of hydrogen-bonded –N-H and –O-H groups, shifts to the lower wavenumbers. This red shift confirms the formation of hydrogen bonds between the –N-H and –O-H groups of the polymer matrix and the polar functional groups of nanoparticles [286]. Delavar and Shojaei observed comparable results in their study on thermally oxidized and untreated ND particles [278]. Figure 5-4. a) FTIR spectra and b) XRD patterns of electrospun nanofibers holding 1.5 wt.% of various nanoparticles 5.3.2.2 XRD analysis The possible effects of the incorporated nanoparticles on the crystallinity of nanofibers composed of semicrystalline CS and PVA were investigated using XRD. Figure 5-4b displays the XRD patterns of pure CS/PVA nanofibers and nanofiber composites. The cross-linked CS/PVA 107 matrix exhibits a broad diffraction peak in the range of 15-30° due to its amorphous structure, in agreement with the literature [286, 287]. Nanocomposite nanofibers containing different nanoparticles show XRD patterns nearly similar to pure nanofibers, indicating that the nanoparticles did not significantly affect the crystalline structure of the CS/PVA matrix. It is worth mentioning that the diffraction patterns of each nanoparticle appeared in the same position as their pure crystals but with decreased peak intensities compared to those of pure nanoparticles. This observation also endorses that the crystalline structure of nanoparticles remains intact under the nanofiber fabrication procedure, i.e., electrospinning. 5.3.2.3 SEM analysis The surface morphology, location of nanoparticles, and their effect on the nanofiber diameter and configuration were investigated by SEM analysis and are depicted in Figure 5-5. It can be inferred that ECPNs have smooth and uniform surface morphology, with average diameter sizes smaller than 100 nm and without forming bead defects. Including nanoparticles in the nanofiber nanocomposites did not disturb the nanofiber integrity and surface morphology. The SEM images showed that the nanoparticles were fully embedded inside the nanofibers, likely due to their interactions with the CS/PVA matrix [165]. Furthermore, energy dispersive X-ray spectroscopy (EDS) images (Figure 5-6) of ECPN/T@U nanocomposite nanofibers show that these nanocomposite nanofibers are composed of carbon (70%), oxygen (29%), and zirconium (1%) elements. 108 Figure 5-5. SEM images of the fabricated electrospun composite nanofibers containing 1.5 wt.% of different adsorbent nanoparticles Figure 5-6. Energy Dispersive X-Ray mapping of the ECPN/T@U nanofibers 109 5.3.3 Dye adsorption experiments 5.3.3.1 Adsorption kinetics Figure 5-7 displays the removal efficiency of specimens toward cationic MB and anionic CR dyes. Nanoparticles exhibited fast adsorption kinetics (Figure 5-7a,c) in the first ≈ 20 min, followed by a gradual improvement in removal performance and ultimately reaching equilibrium after an hour. Compared to nanoparticles, nanofiber nanocomposites feature a slower removal rate (Figure 5-7c,d). This can be attributed to the facilitated mass transfer of contaminants from the water bulk to nanoparticles' active adsorption sites compared to their transfer to nanofiber composites adsorption sites. The nanofiber nanocomposites demonstrated a fast kinetic stage at the beginning (60 min) followed by a slow phase, reaching equilibrium within 180 min. It can be inferred from Figure 5-7a that ND showed the lowest adsorption removal toward cationic MB dye, while the TOND showed the highest removal efficiency toward this dye. Perhaps this observation can be elucidated through the higher concentration of carboxylic acid functional groups, thus leading to a greater abundance of negative surface charges on TOND particles. In essence, this could enhance the electrostatic attraction between cationic MB molecules and the negatively charged TOND, leading to augmented adsorption of MB [11, 12]. As a result of the hybridization of UiO-66 with TOND, MB’s removal efficiency increased, primarily due to enhancing the electrostatic attraction between adsorbate-adsorbents as a result of increasing the negative surface charge of UiO-66 (Table 5-1). Accordingly, the removal efficiencies of the nanoparticles toward MB dye are in the following order: ND (18.0%) < UiO-66 (83.8%) < TOND@UiO-66 (90.9%) < TOND (99.6%). However, a completely different trend in the removal performance of nanoparticles toward CR was observed, as shown in Figure 5-7b. Accordingly, ND showed the highest performance by removing 99.1% of anionic CR dye, 110 followed by UiO-66 (68.3%), TOND@UiO-66 (59.9%), and TOND (22.9%). This observation can be justified by the different charges of the dye and their chemical structure. The BET surface area for ND is about 291 m2/g (Table 5-1), which was not too large to account for the high removal performance toward anionic CR, so looking for other contributing factors is imperative. In addition, the negative surface charge of ND could trigger the charge repulsion between ND and CR molecules. Hence, it is plausible that additional adsorption mechanisms, such as hydrogen bonding or hydrophobic-hydrophobic interactions, might also play a role in this process. A comparison between the FTIR spectrum of adsorbents before and after adsorption could provide valuable insight into the adsorption mechanism. As depicted in Figure 5-8, the intensities of absorption bands at 1541 and 1683 cm-1 increased after the adsorption of CR onto all adsorbent nanoparticles due to the presence of azo (-N=N-) groups in the structure of CR molecules. Two –SO3- groups in the chemical structure of the CR molecule and their oxygen atoms could act as a hydrogen-bonding acceptor and create intramolecular hydrogen bonds with the polar groups of the adsorbent nanoparticles. As a result of this interaction, the positions of characteristic bands of this group (at 1401 and 11120 cm-1) slightly shifted to the lower wavenumbers [222]. The formation of hydrogen bonds between CR molecules and the synthesized adsorbent nanoparticles was further confirmed by increasing the intensity of the broad absorption bands at 3150-3600 cm-1 due to the formation of hydrogen bonds between the –NH2 and –SO3 groups of CR molecules and the polar groups of the adsorbent nanoparticles. Other researchers also observed similar results when removing MO and acid orange 7 from water by ND [222] and modified CNTs [136]. For the pure MB, characteristic absorption bands at about 1084, 1316, and 1586 can be ascribed to –CH3, -C-N, and –C=S bonds, respectively. The FTIR spectrum of adsorbents showed a slight increase in the intensity of bands in the range of 2850-2950 cm-1 after the adsorption of 111 MB, perhaps caused by stretching vibrations in –C-H bonds [27]. The aromatic rings in the structure of both dyes promise their π-π interaction with the π electron-rich regions of the nanoparticles, which can be further seen in the reduction of –C=C stretching vibration intensity of nanoparticles [136]. Hence, the nanoparticles are capable of adsorbing anionic CR molecules via hydrogen bonds and electrostatic interactions while adsorbing cationic MB molecules via π-π and electrostatic interactions. Figure 5-7c shows that all nanocomposite nanofibers had relatively low removal efficiencies for cationic MB dye, owing to the protonation of the CS/PVA matrix [287]. Accordingly, the removal efficiency of nanofibers toward cationic MB dye is in the order of ECPN/T@U (10.5%) < ECPN (11.6%) < ECPN/U (11.7%) < ECPN/N (14.1%) < ECPN/T (20.8%), significantly lower than those observed for their parent nanoparticles (Figure 5-7a). There is a possibility that the relatively higher removal efficiency of ECPN/T toward cationic MB dye might be due to the high negative charge of the TOND particles, which reduces the positive charge of the CS/PVA matrix. As shown in Figure 5-7d, adding different nanoparticles significantly improved the removal efficiency of pure nanofibers toward anionic CR dye. Therefore, the removal efficiencies of nanocomposite nanofibers towards anionic CR dye are as follows: ECPN (39.4%) < ECPN/T (85.2%) < ECPN/N (86.2%) < ECPN/U (90.3%) < ECPN/T@U (95.4%). The enhancement in effective surface area and porosity of nanocomposite could be responsible for the latter observation. The electrostatic attraction allows CR to interact strongly with positively charged nanofibers. Therefore, electrostatic attraction and hydrogen bonding may be involved in the adsorption of anionic CR molecules by the nanofibers, as schematically illustrated in Figure 5-14. 112 Figure 5-7. Removal efficiency of the nanoparticles and their electrospun composite nanofibers toward a, c) MB and b, d) CR dyes 113 Figure 5-8. FTIR spectra of (a) ND, (b) TOND, (c) UiO-66, and (d) TOND@UiO-66 nanoparticles before and after adsorption of MB and CR dyes Adsorption of organic dyes on adsorbent materials is a physicochemical process that involves the transfer of the dissolved molecules from the aqueous phase to the surface of adsorbent materials. Consequently, three well-known kinetic models, including pseudo-first-order (Eq. 4), pseudo-second-order (Eq. 5), and intra-particle diffusion (Eq. 6) models were employed to clarify the adsorption behavior of the synthesized adsorbent materials toward dyes [188, 288]. Table 5-2 summarizes the determination coefficients (R2) and kinetic parameters determined by fitting the experimental data to the above-stated kinetic models (Figure 5-9 and Figure 5-10). 114 Figure 5-9. (a) The pseudo-first-order, (b) pseudo-second-order, and (c) intraparticle diffusion plots for the adsorption of MB and CR dyes onto the synthesized adsorbent nanoparticles Figure 5-10. (a)The pseudo-first-order, (b) pseudo-second-order, and (c) intraparticle diffusion plots for the adsorption of MB and CR dyes onto the fabricated composite nanofibers 115 The R2 values from the pseudo-second-order kinetic model are greater than 0.99 for the adsorption of both organic dyes on the synthesized adsorbent materials. Moreover, all the calculated adsorption capacities (qe, ca) are nearly close to those measured experimentally (qe, exp). This suggests that MB and CR dye adsorption behavior over the synthesized adsorbent materials is better predicted with a pseudo-second-order kinetic model, implying that the rate-controlling step in this adsorption process might be chemisorption [12, 188]. 116 Table 5-2. Kinetic parameters for removing MB and CR dyes by the synthesized adsorbent materials Adsorbents Dyes R2 qe (mg/g) K2 (g/ mg*min) Pseudo-first order Pseudo-second order Intraparticle diffusion Calculated Experimental ND MB 0.973 0.999 0.971 18.18 18.01 2.410-2 CR 0.950 0.999 0.953 99.01 99.12 1.810-2 TOND MB 0.948 0.999 0.981 100.00 99.56 2.310-2 CR 0.928 0.998 0.993 23.86 22.92 4.310-3 UiO-66 MB 0.964 0.999 0.996 84.03 83.82 1.910-2 CR 0.949 0.999 0.974 68.49 68.27 6.310-3 TOND@UiO-66 MB 0.995 0.999 0.997 90.91 90.93 2.710-2 CR 0.926 0.999 0.978 60.24 59.89 6.110-3 Composite nanofibers ECPN MB 0.972 0.999 0.929 12.50 11.62 5.810-3 CR 0.979 0.998 0.928 41.16 39.43 6.210-4 ECPN/N MB 0.948 0.998 0.901 15.48 14.11 3.510-3 CR 0.983 0.997 0.902 89.09 86.19 3.010-4 ECPN/T MB 0.963 0.998 0.901 22.62 20.76 2.410-3 CR 0.970 0.996 0.941 87.26 85.19 2.210-4 ECPN/U MB 0.982 0.999 0.931 12.53 11.66 5.810-3 CR 0.983 0.998 0.925 92.69 90.31 2.610-4 ECPN/T@U MB 0.987 0.999 0.878 11.04 10.52 1.010-2 CR 0.993 0.998 0.874 99.69 95.42 3.910-4 117 5.3.3.2 Adsorption isotherms The data relating to the adsorption performance of nanofibers for removing MB and CR in various initial dye concentrations (20-500 mg/L) are depicted in Figure 5-11a,b, respectively. All nanofibers showed a sharp increase in adsorption capacity for removing MB before reaching equilibrium (Figure 5-11a). Accordingly, the maximum equilibrium adsorption capacity of cationic MB dye over the nanofibers was as follows: ECPN (62 mg/g) < ECPN/U (212 mg/g) < ECPN/N (229 mg/g) < ECPN/T@U (244 mg/g) < ECPN/T (307 mg/g), indicating an approximately four times increase in the adsorption capacity of ECPN after adding only 1.5 wt.% of TOND. Comparing Figure 5-11a, b reveals that the maximum equilibrium adsorption capacities of anionic CR dye over nanofibers are much higher than those of cationic MB dye, possibly due to the prominent role of the dye removal mechanism by electrostatic interactions. Accordingly, the maximum equilibrium adsorption capacity of anionic CR dye over the fabricated nanocomposite nanofibers increased in the order of ECPN (644 mg/g) < ECPN/T (719 mg/g) < ECPN/N (722 mg/g) < ECPN/U (733 mg/g) < ECPN/T@U (1085 mg/g), indicating approximately 68% improvement in the adsorption performance of ECPN/T@U in comparison with ECPN. 118 Figure 5-11. Adsorption isotherms of electrospun nanofiber composites for removing a) MB and b) CR dyes, c) Effects of NaCl concentration, and d) pH on the removal efficiency of ECPN/T@U toward MB and CR dyes Four well-known isotherms, including Langmuir, Freundlich, Dubinin-Radushkevich, and Tempkin isotherms, were employed to get further insight into the interaction of dye molecules with nanofiber adsorption active sites. The associated data compiled in Figure 5-12 and Table 5-4 demonstrates that R2 values belonging to the Langmuir isotherm model for removing CR are among the highest values (ranging from 0.990 to 0.998), suggesting monolayer adsorption of CR molecules onto the homogeneous surface of the nanocomposite nanofibers. 119 Figure 5-12. Model fit of (a) Langmuir, (b) Freundlich, (c) Dubinin-Radushkevich, and (d) Tempkin isotherms for the adsorption of MB and CR onto the fabricated composite nanofibers The higher R2 values of the Tempkin isotherm model for eliminating MB and the corresponding nF values higher than one suggest the favorable nature of the adsorption processes [286]. Moreover, the mean energy values for both organic dyes calculated by the Dubinin-Radushkevich isotherm model are below 5000 J/mol, indicating that physical adsorption dominates the adsorption mechanism of both organic dyes onto the fabricated nanocomposite nanofibers [136]. The maximum adsorption capacity of ECPN/T@U towards CR, obtained from the Langmuir model, was compared with those in the literature (Table 5-3). It is inferred that the maximum adsorption capacity of ECPN/T@U (1428.68 mg/g) surpasses most reported adsorbents in the literature. In addition, as illustrated in Figure 5-13, ECPN/T@U showed appreciable selectivity toward anionic CR dye in the CR/MB mixture. Consequently, the designed nanofiber composite appears to be an excellent candidate for effectively and selectively removing the anionic organic dyes. 120 Table 5-3. Adsorption capacities of different adsorbent materials toward anionic CR dye Adsorbents Adsorption capacity (mg/g) Ref. Ni-MOF/GO 2489 [289] Mn-UiO-66@GO-NH 1266 [290] PPy–PANI nanofibers 222 [291] DBPC@ZIF-67 1351 [292] ZIF-67/PAN nanofibers 849 [293] CS/PVA/PANI/SiO2 nanofibers 110 [287] PVA/PAA/GO-COOH@PDA nanofibers 10 [294] Co-MOF 1019 [295] ZIF-67 1045 [295] CA/CS/SWCNT/Fe3O4/TiO2 nanofibers 74 [296] CoCu-MOF 150 [297] CoFe-MOF 1936 [298] Fe-MOF 775 [298] Co-MOF 629 [298] Acid-activated sepiolite fibers 522 [299] ECPN/T@U composite nanofibers 1428 This study Figure 5-13. Removal efficiency of the fabricated ECPN/T@U composite nanofibers toward MB and CR mixture and the related photo images as a function of contact time 121 Table 5-4. Isotherm parameters for the adsorption of MB and CR onto the fabricated nanofiber composites Composite nanofibers ECPN ECPN/N ECPN/T ECPN/U ECPN/T@U Dyes MB CR MB CR MB CR MB CR MB CR Langmuir qmax (mg/g) 75.75 769.23 384.61 833.34 500.0 791.54 416.66 873.66 714.28 1428.68 KL (L/mg) 0.0103 0.0172 0.0042 0.0233 0.0028 0.0226 0.0028 0.0244 0.0013 0.0127 R2 0.998 0.996 0.692 0.988 0.910 0.993 0.614 0.990 0.371 0.991 Freundlich KF (mg/g) 3.23 34.53 2.46 56.38 1.65 57.47 1.62 67.92 1.18 78.61 n 1.97 1.85 1.23 2.11 1.08 2.20 1.16 2.30 1.09 2.15 R2 0.950 0.912 0.853 0.952 0.911 0.972 0.893 0.970 0.935 0.944 Dubinin-Radushkevich qmax (mg/g) 48.84 475.23 181.90 481.16 211.98 462.89 154.34 478.95 153.45 564.25 B 8.010-5 2.010-5 1.010-4 3.010-6 1.010-4 3.010-6 1.010-4 2.010-6 2.010-4 5.010-7 E (J/mol) 790 1500 70 400 70 400 70 500 50 1000 R2 0.807 0.852 0.894 0.624 0.861 0.673 0.874 0.624 0.862 0.521 Tempkin constants BT (KJ/mol) 152.20 16.13 34.59 16.33 24.91 17.65 37.26 17.75 33.04 13.44 AT (L/g) 0.108 0.217 0.072 0.388 0.054 0.396 0.060 0.525 0.04 0.565 R2 0.990 0.991 0.957 0.924 0.983 0.970 0.984 0.928 0.962 0.786 5.3.3.3 Thermodynamics of adsorption To better understand the adsorption process of CR and MB dyes, the effect of temperature on the adsorption performance of ECPN/T@U toward both CR and MB dyes was investigated by changing the adsorption temperature from 25 to 45 °C. The adsorption thermodynamic parameters, including standard free energy (∆G0, kJ/mol), standard enthalpy (∆H0, kJ/mol), and standard entropy (∆S0, J/mol K) were calculated using Eq. 14 and Eq. 15. It can be observed from Table 122 5-5 that by increasing the temperature, the adsorption capacities of both anionic and cationic dyes onto the ECPN/T@U decreased, indicating the exothermic nature of this adsorption process. Moreover, the negative values of ∆H0 further confirmed that the adsorption process is exothermic. The negative values of ∆G0 indicated that the adsorption of these dyes onto the ECPN/T@U is a spontaneous process [300]. However, the negative values of ∆S0 are unfavorable for spontaneous adsorption, demonstrating that the system's randomness decreases during the adsorption of these dyes. Table 5-5. Thermodynamic parameters for the adsorption of CR and MG dyes onto the ECPN/T@U at various temperatures T (°C ) qe (mg/g) ∆G0 (kJ/mol) ∆H0 (kJ/mol) ∆S0 (J/mol K) CR 25 95.43 -11.90 -58.69 -157.24 35 93.44 -10.33 45 82.35 -8.76 MB 25 10.52 -4.78 -23.45 -62.66 35 8.74 -4.16 45 7.03 -3.53 5.3.3.4 Effect of ionic strength and pH Several interfering agents in industrial wastewater, such as surfactants, salts, heavy metal ions, etc., can affect adsorption [10]. For instance, in its simplest form, by altering the ionic strength of the medium [189]. According to two opposite mechanisms, ionic strength can either accelerate or decelerate dye removal, including i) facilitating dye dissociation, which results in an improvement in dye removal efficiency and ii) reducing the electrostatic attraction between dye molecules and adsorbent particles [11, 13]. Therefore, the effect of ionic strength on the removal efficiency of ECPN/T@U toward MB and CR dyes was investigated by adding different amounts of NaCl into the dye solutions. Figure 5-11c displays the removal efficiency at three different 123 concentrations of NaCl. Accordingly, the MB removal efficiency increased by increasing the NaCl concentration to the maximum removal efficiency of 26.3% at the concentration of 0.5 mol/L. This observation might be due to MB molecule dissociation, which may enhance the dye removal efficiency of ECPN/T@U. However, it was found that adding NaCl into the CR solution reduced CR removal efficiency, mainly because of the screening of electrostatic interactions between anionic CR molecules and positively-charged surfaces of ECPN/T@U. The effect of pH was also investigated on the removal efficiency of ECPN/T@U. The pH of the dye solution can affect the adsorption performance by protonation and deprotonation of the CS/PVA matrix [13]. It was reported that the acidic solution is more favorable for removing anionic dyes by pure CS adsorbent compared to basic solutions. However, the cross-linked CS/PVA showed good stability against highly acidic and basic solutions. Accordingly, this study explored the adsorption performance in a wide pH range, from 3 to 13. Figure 5-11d displays the removal efficiency of ECPN/T@U toward MB and CR dyes as a function of solution pH. As shown in Figure 5-11d, the removal efficiency of cationic MB decreased by increasing pH from 3.2 to 6.2 and then increased continuously with a further increment of pH from 6.2 to 12.4. At low pH values (lower than the pKa value of MB ≈ 3.8 [12]), MB molecules are in neutral form and unable to interact by electrostatic interactions with the positively-charged surface of adsorbent materials. As an ideal planar molecule, MB can interact with an adsorbent material's electron-rich regions [136]. Therefore, π-π stacking interactions between MB molecules and ECPN/T@U at low pH could be responsible for the removal of the MB, schematically illustrated in Figure 5-14. In light of CS's pKa of approximately 6.5, it exists in a protonated state in acidic media and deprotonated in the basic environment. Thus, in the pH range of 3.8-6.5, both MB molecules and ECPN/T@U carry positive charges. Hence, increasing 124 the pH enhances the repulsion between adsorbate and adsorbent and will reduce the ECPN/T@U efficiency for cationic MB dye removal. Further enhancement of pH will increase the negative charges on both TOND@UiO-66 and CS/PVA, which is desirable for removing MB via strong electrostatic attractions. ECPN/T@U exhibited a maximum removal efficiency of 99.5 % for anionic CR dye at pH 6.2, similar to the results obtained with CS and its derivatives for adsorptive dye removal [13, 287]. However, as mentioned above, increasing pH will induce a more negative charge on the surface of ECPN/T@U and impede the adsorption of anionic CR molecules due to the creation of electrostatic repulsion between adsorbate-adsorbent. As a result, the removal efficiency of ECPN/T@U toward anionic CR dye constantly decreased with increasing the solution pH from 6.2 to 12.4. 5.3.3.5 Adsorption mechanism The plausible removal mechanisms for ECPN/T@U, according to our tests on the effect of ionic strength, pH, and FTIR spectra of adsorbents before and after adsorption, are illustrated in Figure 5-14. Accordingly, ECPN/T@U is proposed to interact with cationic MB molecules via π-π stacking interactions (pH < 3.8), electrostatic repulsion (3.8 < pH < 6.5), and electrostatic attraction (6.5 < pH). Concerning the interactions with anionic CR molecules, one expects π-π stacking interactions (pH < 3.5), electrostatic attraction and hydrogen bonding (3.5 < pH < 6.5), and electrostatic repulsion (6.5 < pH). 125 Figure 5-14. Plausible adsorption mechanisms as a function of pH. Used with permission from Elsevier[277] 5.3.4 Reusability of ECPN/T@U for dye removal The reusability of ECPN/T@U was evaluated by investigating the removal efficiency of the regenerated nanofibers over repeated adsorption-desorption cycles. Desorption and regeneration of CR-loaded ECPN/T@U were achieved by washing the spent adsorbent with an alkaline ethanol solution (containing 0.5% NaOH), whereas the MB-loaded ECPN/T@U adsorbent was washed with an acidic ethanol solution (containing 0.5% HCl) [287]. By rinsing the samples with an alkaline solution and deprotonating the nanofiber substrate, the electrostatic attraction between the negatively charged CR and the adsorbent decreases, facilitating the regeneration of the adsorbent. This identical approach can also be employed when rinsing the nanofiber with acidic solutions, as substrate protonation reduces the electrostatic interaction between the cationic MB dye and the CS/PVA fiber substrate, ultimately resulting in the regeneration of the sample. Accordingly, after each adsorption cycle, the dye-loaded ECPN/T@U was washed with water and alkaline or acidic ethanol solution to desorb the dye molecules entirely. Then, the 126 regenerated ECPN/T@U was dried at 60 °C under vacuum for 12 h. It can be seen from Figure 5-15 that the removal efficiency of ECPN/T@U toward both anionic and cationic dyes slightly decreased with increasing the number of adsorption-desorption cycles. The acceptable reusability of the spent ECPN/T@U and its relatively high removal efficiency of 90.1% toward CR dye after 6 adsorption-desorption cycles demonstrate the potential of these adsorbent materials for removing anionic CR dye from wastewater. Figure 5-15. Cyclic usage of ECPN/T@U for removing CR and MB dyes 5.4 Chapter summary Facile electrospinning was employed to prepare rationally designed (UiO-66 MOFs)-loaded chitosan/PVA nanofibers for dye-contaminated water treatment. According to the prominent role of polymeric substrate in the adsorption performance of MOF nanofibers, chitosan, a functional biobased polymer with abundant amino and hydroxyl functional groups, was added to polyvinyl 127 alcohol to incorporate active functional groups for better dye removals. The unique features of MOF-based polymeric nanofibers, such as ease of handling, straightforward separation from treated water, and regeneration ability, make them appealing compared to pure MOF powder for real-world applications. Moreover, a hybridization approach by keeping an eye on the final morphology and textural properties was followed by binding thermally oxidized nanodiamond (TOND) with UiO-66, named TOND@UiO-66. The post-treatment, i.e., chemical crosslinking, of the nanofiber nanocomposites with glutaraldehyde made the substrate structurally stable for water treatment. Results show that incorporating 1.5 wt% of TOND@UiO-66 into the pure polymeric nanofiber doubled the maximum Langmuir adsorption capacity of anionic Congo red from 770 to 1430 mg/g). Different mechanisms responsible for removing cationic and anionic dyes in aqueous solutions containing dyes at various pH and in the presence of counter-ions were also clarified. Multiple interactions between dyes and the designed adsorbents, including π-π stacking interactions, electrostatic attraction and repulsion, and hydrogen bonding, could be developed depending on the media’s pH. As a critical factor in applications, the regeneration of adsorbents showed that the designed MOF-based nanofibers could survive repeated adsorption-desorption cycles (after 6 cycles with just a 10 % drop in performance). Given the unique feature of the MOFs and the electrospinning approach, we believe this combination will definitely spur the growth of the nascent area of MOF polymer nanocomposites for water remediation. 128 Chapter 6 Waste organic dye removal using MOF-based electrospun nanofibers of high amine density Good structural stability of UiO-66 MOF and ligands' functional groups render this system a versatile candidate for multiple applications, including wastewater treatment. The possibility of a broad spectrum of post-synthetic modification of UiO-66 further expands the prospective uses of this MOF nanoparticle. However, a commercial application of UiO-66 has been hindered by the polycrystalline nature of the powder. In this research, the post-modification of UiO-66-NH2 and its post-processing into the nanofiber of chitosan and PVA is followed to address the challenges related to the processability of powder UiO-66 and to leverage the post-modification capability of UiO-66-NH2 and advantages of the facile removal of nanofiber substrate. 6.1 Introduction The immense opportunities offered by MOFs for step functionalization and their uses in removing water contaminants render them superior to many inorganic and organic adsorbents [76, 301, 302]. The possibility of functionalizing the inner and outer surfaces through reaction with organic ligands and SBU can be taken as an advantage, which is rarely observed for inorganic materials such as clays, porous silica-based materials, or microporous silicon [25, 302]. In reality, chemical treatment of MOFs can be easier than inorganic materials, mainly because of the organic part of MOFs, which could welcome the insertion of various moieties. MOFs, consisting of inorganic centers linked coordinatively by the bridging organic molecules, exhibit a stiff molecular structure as these feature an extended periodic structure that depends on the topology of their organic linkers. The resulting crystallinity leads to a high degree of control over their morphology, differentiating them from the regular coordination polymers and 129 other inorganic and organic materials. The crystalline structure of MOFs can be visualized as an assembly of cages with defined pore openings and interior space. These porous materials' pore size and surface area can be tuned by designing their linkers. MOFs can also feature excellent functionality, i.e., organic and inorganic constituents of their lattice, which can be functionalized selectively using various methods. UiO-66, a family member of the MOFs with long-term stability in aqueous media, has received tremendous attention in removing organic and inorganic contaminations from wastewater sources [10, 114, 303]. The post-modification of MOFs has opened up an impactful avenue to call for the applications of UiO-66 MOFs to eliminate the target pollutants [104]. The solvothermal synthesis of the highly functionalized UiO-66 is primarily restricted due to (i) missing labile functionality from the ligands at synthesis temperature (≈120ᵒC), (ii) possible coordination of worn-off functionalities with metal ions, and (iii) issues related to the solubility of damaged linkers in the solvent [76]. However, the unique oxophilicity and nuclearity of Zr6O4(OH)4 and SBU of UiO-66 ensure good chemical stability of these nanoparticles to allow for any substitutions on their linkers [304]. Covalent grafting of amines is a sustainable and effective method to increase the adsorption capacity of UiO-66 MOFs towards the target analyte(s) [305]. The attached positively charged active amine sites open up many opportunities to remove emerging water contaminants, such as different organic dyes with negative and positive charges, pharmaceuticals, and heavy metals [304, 306]. The functionalization of MOFs with amine groups imparts new chemical and physical properties to MOFs and expands their applications in pollutant removal [304]. Despite the amino-functionalized MOFs exhibiting good performance for wastewater treatment, most modified MOFs are in powder form, limiting their usage in large-scale wastewater treatment applications. 130 The main limitations of powder MOFs in wastewater treatment applications are complicated processability, poor mass transfer, difficulty separation from treated water, complex regeneration and reuse, and poor mechanical and chemical stability [114]. Consequently, MOF powders need to be shaped into macrostructures that are easy to handle yet possess intrinsic properties. Combining the brittle and polycrystalline UiO-66 MOFs with malleable and processible polymers [104] through electrospinning is a promising strategy for integrating MOFs into wastewater treatment. Nanofibers, with a high surface-to-volume ratio, decent flexibility, structural adjustments, and production scalability, are the better substrates for embedding MOF nanoparticles [114]. Recently, environmentally friendly adsorbents based on bio-polymers have received much attention. For instance, chitosan, a biocompatible, biodegradable, low-cost bio-polymer, has been widely used as an adsorbent to remove various pollutants from water [13]. Chitosan shows low stability in harsh acidic media, while PVA as a modifier can increase the chemical stability of chitosan by forming hydrogen bonds with the amine groups of chitosan. Moreover, the PVA/chitosan blend can be cross-linked with glutaraldehyde to improve the chemical and physical stability of the nanofibers [286]. Effluents containing dyes can cause severe environmental issues, which must be removed before discharge into the water streams [12, 307]. In this research, an active adsorbent to remove MO dye was designed by covalently attaching 5-phenyl-tetrazole (PTZ) functional group (having a high density of active amine sites) to amine-tagged UiO-66-NH2. A two-step post-modification was used, including the primary functionalization of UiO-66-NH2 with TCT, followed by PTZ. The functionalized nanoparticles (Zr-MOF) were then incorporated into the chitosan/PVA solution before the electrospinning to produce nanofibers. The composition of MOF nanofiber adsorbent was then systematically optimized to achieve an optimum removal efficiency for MO. 131 Additionally, the effects of adsorption parameters such as agitation time, dye concentration, temperature, ionic strength, and pH of the solution on the performance of the nanofibers were investigated. 6.2 Adsorption experiments The adsorption performance of the adsorbents, including adsorption kinetics, isotherms, thermodynamics, the effect of interfering ions, and pH on MO dye, was investigated. A specific amount of each adsorbent was agitated with 50 mL of dye solution (20-500 mg/L) for 3 h, after which adsorbents were separated from the solution, and the concentration of MO was measured using a UV-Vis spectrophotometer. The adsorption removal performance of MO dye was calculated using Eq. 1, Eq. 2, and Eq. 3 [300]. To investigate the influence of the pH of dye solutions on the removal efficiency of adsorbents, initially, the pH of MO solution was maintained between 2 and 11 by dropwise addition of HCl (0.1 M) and NaOH (0.1 M) solutions. The effect of ionic strength on the removal efficiency of MO was investigated by adjusting the concentration of NaCl from 0.01 to 0.5 M. 6.3 Results and discussion 6.3.1 Characterization of Zr-MOFs The prerequisite for post-synthetic modification of MOFs is their survivability under modification reaction conditions, where UiO-66-NH2 must be stable under reaction conditions of temperature, solvents, and reagents. XRD analysis is used to ensure the proper synthesis of MOFs as well as to check their stability in different media. The crystalline structure of the modified MOF (Zr-MOF) was investigated by XRD (see results in Figure 6-1a). All the diffraction peaks of Zr-132 MOF at around 2θ = 7.38°, 8.56°, and 25.67° are similar to the simulated XRD patterns of pure UiO-66, confirming the successful synthetic procedure. The agreement of XRD tracings of Zr-MOF with pure UiO-66 confirms the modification of MOF. FTIR spectrum of Zr-MOF ranging from 400 to 4000 cm-1 verify its successful post-modification. Figure 6-1b shows prominent characteristic peaks of Zr-MOF, which include peaks at 1389 and 1426 cm-1 for symmetric modes of –COO, while 1572 cm-1 for the asymmetric mode of –COO, broad absorption peak centered at 3360 and 3464 cm-1 for symmetric and asymmetric modes of –NH2 groups, respectively, and 1256 cm-1 for the vibrational mode of –C-N bonds [10, 12]. The characteristic peaks of modifier agents (TCT and PTZ) have appeared at 1446 and 1524 cm1 that are associated with the stretching vibration modes of –C=N and –N=N groups, respectively. Strong evidence for the successful synthesis of Zr-MOF is its high nitrogen content (10.21%) compared to its pristine counterpart (5.62%) [12] (Table 6-1). Due to the large size of the modifying molecules, only the –NH2 groups located on the surface of MOFs can react with these molecules. Reaction yield is reduced since inner groups cannot be post-functionalized. 133 Figure 6-1. (a) XRD tracings, (b) FTIR spectrum, (c) N2 adsorption-desorption isotherm and pore size distribution (hollow symbols present desorption), and (d) TGA and DTG profiles of Zr-MOF Comparing the porosity of MOFs before and after the treatment can help verify the functionalization. The N2 adsorption-desorption isotherms of Zr-MOF were performed at 77 K; the results are shown in Figure 6-1c. The adsorption isotherms have a sharp increase in N2 adsorption at very low relative pressures with a gradual rise up to high relative pressures, while a small hysteresis appeared at relatively high pressures. From this, it can be concluded that Zr-MOF has a relatively Type IV adsorption-desorption isotherm with micropores and mesopores located throughout its porous structure. Furthermore, the calculated BET surface area of Zr-MOF is 583 m2/g, much smaller than the pristine adsorbent (≈ 1009 m2/g) [12], further confirming the 134 successful synthesis of Zr-MOF. Functionalized MOFs often suffer from reduced BET surface areas due to partial blockage of the pores with new functionalities [302]. The thermal stability of MOFs provides valuable insights into their crystalline structure and functionalities. Accordingly, TGA and DTGA thermograms of Zr-MOF (Figure 6-1d) showed a relatively four-stage weight reduction due to the removal of water molecules (at about 373 K), trapped solvent, and other organic molecules (at about 473 K), decomposition of the attached organic molecules on the surface of Zr-MOF (at about 573 K), and deterioration of the framework (at about 723 K), which are in line with the results reported for the modified MOFs [195]. The SEM images of pristine and functionalized MOFs showed round-shaped particles with particle sizes smaller than 100 nm (Figure 6-2), exhibiting the same morphology that confirms the structural stability of MOFs during the modification procedure. Table 6-1. Physical properties and chemical composition of Zr-MOF Adsorbents SBET (m2/g) Mean pore diameter (nm) Pore volume (cm3/g) Zeta potential (mV) Elemental analysis of adsorbents (%) C H N Zra UiO-66-NH2 1009 3.32 0.56 - 31.80 4.11 5.62 55.68 Zr-MOF 583 2.21 0.34 4.81 30.32 3.87 10.21 34.65 aZr contents were determined by ICP-OES and CHN contents from CHN TruSpecRCHN, respectively 135 Figure 6-2.SEM images of (a, b) pristine and (c, d) functionalized MOFs at different magnifications 6.3.2 Characterization of electrospun MOF-based adsorbent nanocomposites 6.3.2.1 XRD and FTIR analysis XRD tracings of pure PVA/chitosan nanofiber (ENF), Zr-MOF, and their electrospun 136 composite nanofibers (presented in Figure 6-3a) confirm that ENF is amorphous with a broad and weak peak observed at 2θ = 20.28°. According to the literature, PVA has a characteristic XRD peak centered at 2θ = 19.45° [308], while pure chitosan has two distinct diffraction peaks at 2θ = 10.05° and 20.36° [191]. The XRD of ENF showed a decrease in peak intensities and crystallinity, primarily due to the interaction between PVA and chitosan [309]. The XRD of all the MOF nanofibers matches well with those of Zr-MOFs and the polymers. The characteristic peaks of both components (MOF and nanofiber) in the XRD of MOF nanofiber suggest that Zr-MOF and the polymer matrix retained their crystalline structures during electrospinning. As a result of solid interfacial interaction between the two phases via hydrogen bonds, MOF nanofiber peak intensities were lower than those of pure components [309]. FTIR was used to study the possible interactions between Zr-MOF and the polymeric matrix. Figure 6-3b shows the FTIR spectra of Zr-MOF, ENF, and ECNF-7. The spectra of ECNF showed a broad peak between 3000 and 3600 cm-1 due to the stretching vibrations of –NH2 and –OH groups of chitosan and PVA and hydrogen bonds [287]. A weak peak between 2850 and 2950 cm-1 is attributed to -CH stretching in chitosan and PVA. Chitosan amine groups are responsible for absorption peaks at 1535 and 1631 cm-1, while the C-O-C bonds are due to a strong absorption peak at 1092 cm-1 [310]. The sharp peak at 1336 cm-1 can be related to the –CO bond [191, 286]. After the Zr-MOF incorporation, the intensity of the broad absorption band between 3000 and 3600 cm-1 increased, and its position was shifted slightly to lower frequencies. The hydrogen bonds formed between –NH2 and –OH groups of the polymer matrix and functional groups of Zr-MOF are responsible for this shift [286]. 137 Figure 6-3. (a) XRD tracings and (b) FTIR spectra of the specimens 6.3.2.2 Morphological analysis Zr-MOF dispersion and surface morphology of ECNF were evaluated using SEM, shown in Figure 6-4, which indicates that the electrospun nanofibers (50-150 nm diameter) had smooth surfaces without any bead defects. The nanofiber morphology was not significantly altered by adding the Zr-MOF to the polymeric matrix. The hydrogen bond formation between Zr-MOF and the polymeric matrix results in a good interaction between MOF and polymer, thereby preventing aggregation of MOF and their consequent embedding in nanofibers [165]. Due to these effects, Zr-MOF nanoparticles were effectively dispersed in the polymeric matrices for which EDX mapping of ECNF-7, along with the mapping of Zr, provides further evidence (Figure 6-5). 138 Figure 6-4. SEM images of the nanofiber specimens with different contents of Zr-MOF Figure 6-5. Energy Dispersive X-ray mapping of ECNF-7 139 6.3.3 Dye removal experiments 6.3.3.1 Influence of agitation time and adsorption kinetics The polymeric matrix plays an essential role in the performance of MOF nanofibers since properties such as surface charge, chemical structure, active sites, mechanical properties, and thermal properties would help to determine the appropriate application. Water treatment via adsorption, for example, could benefit from the surface charge features and electrostatic interactions resulting from the adsorbent's surface charge. Chitosan/PVA, which is cationic, can remove both anionic and cationic dyes [287]. Organic dyes with a negative surface charge, such as MO, are commonly used in industries, which could cause environmental hazards when discharged into water resources [311]. Adsorption was investigated as a function of agitation time to study the adsorption kinetics of MO dye onto the designed adsorbents. As shown in Figure 6-6a, the removal rates of the adsorbents are rapid during the initial stages of adsorption because of the abundance of unoccupied adsorption sites. With the increasing agitation time, the removal efficiency increased steadily until equilibrium was reached. The decrease in the removal rate at the second step might be due to the gradual blockage of the adsorption sites with the MO molecules [312]. Zr-MOF has a relatively higher removal rate for MO dye than the other MOF nanofibers (i.e., ECNF-m samples), probably because the MOFs have a higher facilitated mass transfer and more accessible active sites than the nanofibers. Thus, the removal rate for MOF is quick during the first 15 min, which reached equilibrium in 40 min. The removal efficiency (≈ 31%) is slightly lower than the nanofiber adsorbents but still higher than UiO-66-NH2 MOF (≈ 29%) [43]. A higher removal efficiency was observed for ENF compared to the pure Zr-MOF (43%), which took longer for ENF (150 min) to 140 reach equilibrium. Therefore, adding Zr-MOF to the electrospun nanofibers dramatically increased the MO removal efficiency, which continued to increase as the MOF content increased. Compared to pure ENF, ECNF-7 as an adsorbent reduced the MO adsorption time from 150 min to 60 min due to the higher surface area and great active sites caused by the presence of Zr-MOF in ECNF-7. The kinetic models, namely pseudo-first-order, pseudo-second-order, and intra-particle diffusion, were employed to investigate the rate-controlling step and adsorption mechanism in removing the MO dye. Table 6-2 provides calculated kinetic parameters for these models, where the regression coefficient (R2) measures how best experimental data match well with the kinetic models. The R2 values calculated for the pseudo-second-order model are higher than other models, suggesting that chemisorption is the rate-controlling step, which can be linked to the adsorption capacity of the solid phase [276]. According to this kinetic model, equilibrium adsorption capacity values (qe, exp) are much closer to the experimental values. The plot of t/qt vs. agitation time (t) is shown in Figure 6-6b, whose slopes and intercepts were used to calculate the equilibrium adsorption capacity (qe, cal) and rate constant (K2), respectively. According to the data presented in Table 6-2, ECNF-7 showed that MO adsorption capacity increased continuously with increasing Zr-MOF loading, offering a maximum adsorption value of 92.27 mg/g. 141 Figure 6-6. (a) Kinetic of MO adsorption and (b) Model fitting for pseudo-second-order model Table 6-2.Kinetics data for adsorption of MO onto adsorbent nanocomposites. Adsorbents R2 qe (mg/g) K2 (mg/mg min) Pseudo-first-order Pseudo-second-order Intra-particle diffusion Calculated Experimental Zr-MOF 0.9608 0.9993 0.9882 31.24 30.97 8.44 ENF 0.9829 0.9998 08751 46.52 42.84 1.41 ECNF-3 0.9439 0.9995 0.7003 70.00 67.21 2.06 ECNF-5 0.9666 0.9998 0.7514 77.15 74.07 1.85 ECNF-7 0.9807 0.9999 0.9758 92.27 91.65 5.57 ECNF-10 0.9325 0.9997 0.7837 84.21 81.35 1.61 6.3.3.2 Influence of dye concentration and adsorption isotherms Understanding the effect of initial dye concentration on the adsorption performance of the adsorbents is critical to optimize the adsorption process. Hence, the removal performance of Zr-MOF, ENF, and ECNF-7 at various initial dye concentrations was investigated (results depicted in Figure 6-7). Consequently, adsorption capacity increased dramatically with the MO 142 concentration reaching a plateau, suggesting increased mass transfer driving force due to the MO concentration gradient in bulk and the solid phase [313]. Therefore, adsorption capacity reduces when MO molecules occupy all the available active sites [314, 315]. For instance, the adsorption capacity of MO over pure Zr-MOF increased from 15.97 to 119.92 mg/g with increasing MO concentration from 20 to 500 mg/L. As a result, the equilibrium adsorption capacity of MO dye over the specimens followed the sequence for Zr-MOF (119.92 mg/g) < ENF (268.04 mg/g) < ECNF-7 (582.81 mg/g). The impressive adsorption capacity of ECNF-7 towards MO dye can be attributed to the abundant active sites both on the surface and within the nanofibers. These active sites are likely a result of the presence of the porous Zr-MOF material. Figure 6-7. (a) Adsorption isotherms of MO, and (b) Langmuir adsorption isotherm fitting to adsorption data An isotherm study was performed to predict the adsorbent-adsorbate interactions and to determine the maximum adsorption capacity. Langmuir isotherm model (Eq. 7), along with Freundlich (Eq. 8) and Tempkin (Error! Reference source not found.) isotherm models (Figure 6-7b), were employed, and relevant data are summarized in Table 6-3. Based on the R2 values (Table 6-3), Langmuir isotherm described the adsorption isotherms of MO dye, which suggests 143 the following aspects: 1) monolayer coverage of the adsorbent surfaces, 2) all adsorption sites have similar adsorption energy and 3) there are no interactions between the adsorbed MO molecules. Hence, the maximum monolayer adsorption of MO dye by these adsorbents was calculated for Zr-MOF (167.51 mg/g) < ENF (330.14 mg/g) < ECNF-7 (618.86 mg/g). The ECNF-7 was a good candidate for removing MO from the wastewater based on its maximum adsorption capacity. Table 6-3. Relevant parameters relating to the employed models Adsorbents Langmuir Freundlich Tempkin qmax (mg/g) KL (L/mg) R2 KF (mg/g) n R2 BT (KJ/mol) AT (L/g) R2 Zr-MOF 167.51 0.0060 0.986 2.99 1.58 0.965 71.68 0.072 0.972 ENF 330.14 0.0091 0.998 13.00 1.95 0.991 38.00 0.119 0.967 ECNF-7 618.86 0.0273 0.981 84.98 3.01 0.988 27.29 1.071 0.952 144 Table 6-4. Adsorption capacities reported for MO dye removal by different adsorbent materials Adsorbents Adsorption capacity (mg/g) Ref. Chitosan/PVA/Zeolite nanofibers 153 [316] Deacetylated Chitosan/PVA nanofibers 183 [191] Glutaraldehyde Crosslinked PVA 44 [317] Activated carbon fiber 357 [315] PVA/Chitin NFs/Fe(III) 810 [308] Mesoporous carbon nanofibers 101 [312] Polypyrrole nanofibers 169 UiO-66 39 [43] UiO-66-NH2 29 [43] Polyacrylonitrile fiber 194 [314] Chitosan/nanodiamond composite 454 [13] Chitosan/PVA Nanofibers Modified with PANI/SiO2 Nanoparticles 90 [287] pTSA-PANI/PLLA nanofiber mat 377 [313] ECNF-7 619 This study 6.3.3.3 Influence of temperature and thermodynamic study The adsorptive removal performance of ECNF-7 towards MO dye was investigated at different temperatures (298-318 K). This data (Table 6-5) suggests that increasing the operating temperature decreased adsorption capacities. This observation may explain the lower interaction between MO molecules and the ECNF-7 at higher temperatures. For a better understanding of the adsorption process, thermodynamic parameters were also calculated using Eq. 14 and Eq. 15 [21]. The calculated enthalpy change values of -85.37 kJ/mol (categorized as high value) indicate a 145 strong interaction between MO molecules and ECNF-7, suggesting adsorption is exothermic, favoring spontaneous adsorption. The ∆S0 (-253.52 J/mol K) indicates that the system's randomness decreased after the adsorption, which is unfavorable for spontaneous adsorption. The negative standard free energy (∆G0) values calculated from Eq. 15 further prove that MO molecules were spontaneously adsorbed onto ECNF-7 in the temperature range examined. Thus, it can be deduced that MO adsorption onto ECNF-7 is controlled by enthalpy changes rather than the entropy effects. Table 6-5. Parameters for adsorption of MO dye onto ECNF-7 T (°C ) qe (mg/g) ∆G0 (kJ/mol) ∆H0 (kJ/mol) ∆S0 (J/mol K) 25 91.65 -9.82 -85.37 -253.52 35 75.96 -7.29 45 55.76 -4.75 6.3.3.4 Influence of pH and salt concentration pH is a critical parameter during adsorption since it affects the dye's structure and surface charge of adsorbent materials. The removal efficiency of ECNF-7 towards MO dye was studied over the pH range of 2 to 11 (Figure 6-8a). Amines and hydroxyl groups on the ECNF-7 surface could protonate at acidic pH, resulting in a positive charge onto the nanocomposite surface. The increased adsorption is observed at acidic pH due to electrostatic interactions between the positively charged ECNF-7 and the negatively charged MO. It may be expected that the adsorbent surface becomes negatively charged in alkaline pH due to the deprotonation of these functional groups. Hence, the adsorption of anionic MO under this condition has decreased mainly due to the electrostatic repulsion between MO molecules and the adsorbent. 146 As shown in Figure 6-8a, the removal efficiency of ECNF-7 against MO increased significantly when the pH of the solution was increased from 2 to 6 but decreased further as the pH increased. Higher removal efficiency was observed (91.65%) at pH 6. Under this condition, the surface charge of ECNF-7 is positive (because the pKa value of chitosan is 6.5), while MO molecules are in anionic form. Therefore, high removal efficiency for MO may be due to the strong electrostatic attraction between the adsorbent and adsorbate. When the solution pH was increased to a value higher than the pKa of chitosan, the surface charge of the adsorbent became negative, and repulsive electrostatic interactions would occur between the anionic MO molecules and the negatively charged ECNF-7, thereby reducing the removal efficiency at increased pH. The optimum pH range for MO removal is 3.47 (pka of MO) to ≈7, at which point the surface charge of ECNF-7 is positive, and MO molecules are in anionic form, leading to the electrostatic attraction between the adsorbent and the adsorbate. Overall, physical processes, such as electrostatic interactions, seem to be the dominant adsorption mechanism during the adsorption process. The influence of ionic strength was also studied to confirm the electrostatic interactions between MO molecules and ECNF-7. Notice that ionic strength originates by adding various salts into the dye solution, which could significantly interfere with the electrostatic interactions by screening effect [13, 40]. Theoretically, when electrostatic interaction is attractive, adding salt to the dye solution reduces adsorption capacity [40]. In this study, different amounts of NaCl were added to the MO solution to determine the effect of salt concentration on the adsorption performance of ECNF-7. Figure 6-8b suggests that the removal efficiency of ECNF-7 for MO decreased after the addition of NaCl, and such reduction was further increased with increasing concentration of NaCl. Therefore, adding the salt could produce an electrostatic screening effect, releasing MO molecules from the adsorbent surface to the dye solution. Here, the electrostatic 147 interactions play a dominant role in the adsorption of MO onto ECNF-7. Figure 6-8. Influence of (a) pH and (b) NaCl concentration on adsorptive removal of MO over the ECNF-7 6.3.3.5 Selective dye removal performance of ECNF-7 From the observation of pH and ionic strength effects, it was found that the surface charge of ECNF-7 is positive at natural pH (without adjusting the pH by HCl or NaOH), making it ideal for removing anionic organic dyes such as MO. Therefore, ECNF-7 was studied to remove the anionic MO dye from the binary MO/MB and MO/MG solutions selectively. As shown in Figure 6-9, the removal efficiency of MO is much higher than that of MB and MG dyes, which can offer high selective MO removal performance for ECNF-7 against MB (≈ 7.9) and MG (≈ 2.6) dyes. So, the color of these mixed solutions changed from dark green to blue after the removal process, demonstrating that MO was selectively removed from the mixed dye solutions, whereas MB and MG remained in the solution. In other words, ECNF-7 has excellent potential to selectively remove anionic MO dye from cationic dyes. 148 Figure 6-9. Selective adsorption of ECNF-7 toward MO dye in the mixture of (a) MO/MB and (b) MO/MG dyes as a function of contact time 6.3.4 Regeneration and reusability An adsorbent's reusability and ease of regeneration are as crucial as its high removal efficiency. Therefore, eight cycles of adsorption and subsequent desorption were performed on ECNF-7 to determine its reusability. ECNF-7 was eluted with ethanol/NaOH 0.1 M (50/50) solution to remove all the MO molecules adsorbed onto its surface. As shown in Figure 6-10, the ECNF-7 was able to remove the MO dye almost with the same capacity (with a slight decrease from 92.23 to 85.32% ) after eight consecutive adsorption-desorption cycles, which suggests its reversible and stable adsorption sites [308, 318]. 149 Figure 6-10. Reusability of the ECNF-7 for the removal of MO dye 6.4 Chapter summary Nanofibers having a large surface-to-volume ratio hold great promise as carriers of MOFs. For this purpose, polycrystalline powdery UiO-66 MOFs are primarily synthesized in DMF, limiting the feasibility of in situ growing onto many polymeric substrates. An alternative protocol was proposed here to overcome this challenge by considering the electrospinning of the blends from the pre-synthesized UiO-66 MOFs and a polymer carrier. With its exceptional stability in various media, UiO-66 can be post-treated and functionalized. To increase the affinity of UiO-66-NH2 to remove dye contaminants from the contaminated water, amine-containing functional groups, namely TCT and PTZ, were attached to UiO-66-NH2. The simplicity and versatility of electrospinning and the strong adsorption performance of UiO-66 MOFs were combined to enable the good removal performance of the model dye (methylene orange, MO). The nanofiber system 150 comprising PVA/chitosan with 7 wt% of Zr-MOF showed an adsorption capacity of ca. 619 mg/g for MO while retaining its performance even after eight adsorption/desorption cycles. 151 Chapter 7 Conclusions, contributions, and future directions 7.1 Conclusions As the prevalence of water contaminants continues to increase exponentially, it has become evident that traditional adsorbents like activated carbon are inadequate in effectively removing these emerging pollutants. Consequently, the introduction of more efficient adsorbents has become imperative. In this study, significant emphasis was placed on investigating a category of porous materials known as UiO-66 MOFs. A comprehensive range of experiments, encompassing the synthesis and processing of the UiO-66 family of MOFs, was undertaken to establish a seamless integration of UiO-66 into water treatment applications. The study adhered to established methodologies within the field to systematically elucidate the relationship between structure and performance, thereby enhancing clarity for readers. When conducting adsorption studies, it is crucial to investigate the impact of key factors, including pH, interference of ions, time, and regeneration. Such investigations aim to gain valuable insights into the performance of adsorbents across various sources of water contamination. Examining these determining factors can give a more comprehensive understanding of adsorbent effectiveness. In the context of this thesis, the experimental study demonstrates achievement in investigating the stability of UiO-66, a known challenge in MOFs, under various media conditions with different polarities for an extensive duration of one year. Consequently, the pristine UiO-66 exhibited good structural integrity when exposed to water, chloroform, and DMF, with a negligible reduction in BET surface area. These findings establish the potential of UiO-66 as a promising alternative to conventional adsorbents. Furthermore, considering the structural changes observed in aging UiO-66, a comprehensive investigation of the structure-property relationship was conducted on the samples. The maximum adsorption capacities (qmax) of pristine UiO-66 for MR, 152 MO, MG, and MB dyes were found to be 384, 454, 133, and 370 mg/g, respectively. Notably, UiO-66 nanoparticles subjected to aging in water exhibited a higher affinity for anionic MO and MR dyes compared to the cationic MG dye, thus underscoring their effectiveness in targeted dye adsorption. In continuing, a dual approach comprising magnetization and post-synthetic modification was employed to enhance the effectiveness of the adsorbent. The utilization of organic and inorganic building blocks within the single crystal structure is a distinctive feature rarely found in conventional adsorbents such as activated carbon. To enhance the effectiveness of UiO-66 MOFs in removing organic contaminants, namely methyl orange, a solvothermal post-modification strategy involving TCT and PTZ was employed on UiO-66-NH2. Notably, the post-modification reaction in MOFs differs from typical chemical reactions due to the intricate nature of the UiO-66 crystalline structure and the accompanying steric hindrances. Consequently, meticulous attention was devoted to optimizing the reaction conditions. As a result, it was observed that the designed adsorbent (m-UiO-66-TCT) has an adsorption capacity of ≈298 mg/g for methyl MO with facile MOF separation using an external magnet. The m-UiO-66-TCT composite exhibited easy separation, high adsorption capacity, and good recyclability, rendering it an attractive candidate for the adsorptive removal of MO dye from aqueous environments. Despite the effectiveness of the UiO-66 family of adsorbents in water contaminant removal, their powdery polycrystalline state presents numerous challenges, including poor processability, brittleness, safety hazards from dust formation, and difficulties in separating them from aqueous solutions. The incorporation of MOFs into polymeric nanofibers has been pursued to address these issues, as they offer favorable attributes such as a high surface-to-volume ratio and improved substrate characteristics. In this thesis, a particular focus was placed on combining UiO-66 with 153 thermally oxide nanodiamond to enhance their affinity for both anionic and cationic dyes. Subsequently, these hybrid materials were integrated into chitosan/polyvinyl alcohol nanofibers. The experimental results demonstrated that incorporating just 1.5 wt% of TOND@UiO-66 into the pure polymeric nanofibers doubled the maximum Langmuir adsorption capacity for anionic Congo red from 770 to 1430 mg/g. Notably, the regeneration of the adsorbents, a critical aspect for practical applications, revealed the robustness of the designed MOF-based nanofibers, as they withstood repeated adsorption-desorption cycles with only a 10% drop in performance after six cycles. These findings highlight the substantial advancements achieved in overcoming the limitations associated with powdery UiO-66 adsorbents through the innovative integration of MOFs into polymeric nanofibers. In the final chapter of this thesis, an approach was implemented to enhance the adsorption kinetics and capacity of UiO-66-NH2 for MO anionic dyes by incorporating a new functionality with abundant nitrogen-containing active functional groups. This was achieved through a stepwise tandem functionalization process, wherein the TCT reagent was reacted with amine-tagged UiO-66-NH2, followed by the attachment of PTZ compound, which consists of a five-membered tetrazole ring with a phenyl group attached to one of the carbon atoms. The resulting modified UiO-66 exhibited a high amine density and was subsequently integrated into PVA/chitosan nanofibers for water treatment applications. Remarkably, the nanofiber system comprising 7 wt% of the modified Zr-MOF exhibited an improved adsorption capacity of approximately 619 mg/g for MO, which was approximately three times higher than the bare nanofiber. This enhancement in adsorption performance demonstrates the successful incorporation of the modified UiO-66 with increased amine density into the nanofiber composite. Moreover, the nanofiber composite demonstrated good durability, as it retained its adsorption capacity even after undergoing eight 154 cycles of adsorption and desorption. This stability further supports the potential of the modified UiO-66-based nanofiber system for sustainable water treatment applications. 7.2 Contributions The following aspects summarize the key contributions emanating from this dissertation: 1. The introduction of this thesis encompasses two comprehensive review papers that explore the application of UiO-66 in water treatment and the utilization of electrospinning for processing MOFs. These papers serve as invaluable resources for newcomers to the field, offering a thorough analysis of the current literature and addressing the existing gaps and challenges. By carefully studying these review papers, researchers can acquire a wealth of expertise, not only in synthesizing and designing a new subfamily of UiO-66 MOFs but also in mastering the post-processing techniques using the versatile method of electrospinning. It is important to emphasize that the methodologies outlined for incorporating MOF nanofibers through electrospinning can be readily adapted for other processing techniques, such as 3D printing. This contribution provides a significant advantage for researchers, equipping them with the necessary knowledge and expertise to excel in the synthesis, design, and post-processing of MOFs, particularly UiO-66, as well as the application of electrospinning in MOF fabrication. Additionally, the inclusion pathways defined for MOF nanofibers in the electrospinning section hold the potential for broader application in other processing methods. 2. In this study, we have unveiled UiO-66's remarkable capability for water treatment applications regarding structural stability. For the first time, we have demonstrated that UiO-66 exhibits stability not only in water but also in chloroform and DMF, which are widely utilized solvents with varying polarities. Our research has elucidated the intricate relationship between structural integrity, long-term porosity retention, and their ultimate contribution to removing water 155 contaminants. By establishing the stability of UiO-66 in chloroform and DMF, we have alleviated serious concerns and expanded the potential scope of its utilization in various fields. Their stability ensures they can withstand the rigors of industrial conditions and maintain their performance over extended periods, making them viable for commercial applications. 3. The approach presented in Chapter 4, which combines magnetization and post-synthetic modification, has proven highly advantageous for tackling challenges associated with powdery UiO-66 MOFs and enhancing their performance. The magnetization process is easily adaptable to other MOF classes due to the abundant carboxylic acid functional groups present in the synthesized core, making them suitable substrates to bind with MOFs. Additionally, the TCT and PTZ functional groups, known for their effectiveness in enhancing adsorption, offer efficient solutions for removing various organic contaminants and can even be applied to gas-phase adsorption. 4. The investigation on the hybridization of UiO-66 with thermally oxidized nanodiamond (TOND) has successfully outlined the necessary procedures to render an organic substrate like TOND suitable for hosting UiO-66 on its surface. Moreover, the findings have revealed that incorporating a minute quantity (1.5 wt%) of the specifically designed MOF can effectively double the adsorption capacity of the electrospun nanofiber adsorbent. This good performance surpasses that of numerous other adsorbents examined in the study. Overall, this research highlights the significant potential and impact of the UiO-66 and TOND hybridization approach, showcasing its ability to enhance the adsorption capacity of nanofiber adsorbents. These outcomes contribute to advancing efficient and high-performance adsorbents for various applications. 5. The last chapter of the study presents an investigation into a unique functionality characterized by high nitrogen loading. This functionality enables the creation of π–π interaction with contaminants through the delocalized pi electrons present above and below the plane of the 156 benzene ring in the phenyl group of TCT. To enhance the adsorption capacity of the nanofiber composite (consisting of PVA/CS), we introduced modified MOFs. This addition resulted in a remarkable increase in the adsorption capacity (almost threefold) for removing organic MO pollutants. Moreover, our study determined the optimal MOF content, which is significant considering the wide range of MOF contents reported in the literature for nanofiber composites. Additionally, we investigated the effect of pH, salt concentration, and the regeneration and reusability of the adsorbent. Our findings demonstrate the potential of the devised MOF nanofiber composite in effectively removing MO contaminants from water resources. 7.3 Future direction 1. A significant shift in the MOF synthesis, such as UiO-66, is observed. Previously, expensive and risky solvents like DMF were employed, but now, efforts are being made to increase water utilization. However, organic solvents are still frequently required for specific modifications due to the inability of water to dissolve certain substances. Consequently, research needs to be conducted to explore methods for greater water utilization and the discovery of safer and more cost-effective approaches. 2. The primary focus of the studies conducted in this thesis revolved around UiO-66 and its adsorption capabilities, employing various mechanisms, including electrostatic interactions, hydrogen bonding, acid-base reactions, and pi-pi stacking. However, the expansive pores present within the MOF structure were underutilized due to the relatively larger size of the contaminants. Therefore, it is worth exploring the adjustment of pore size, particularly the pore aperture, to enable efficient physical trapping of contaminants and closely monitor their effectiveness. 3. In this thesis, a comprehensive investigation was conducted on the stability of bare UiO-66. 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Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Arjmand, Mohammad
UiO-66 metal-organic frameworks for water treatment
Text