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Impact of water solutes on the formation of nitrite under Vacuum UV(VUV) advanced oxidation of nitrate-rich… Han, Mengqi 2020

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Impact of Water Solutes on the Formation of Nitrite under Vacuum UV(VUV) Advanced Oxidation of Nitrate-Rich Water  by Mengqi Han  B.A., East China University of Science and Technology, 2014 M.Sc., Western University, 2015  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemical and Biological Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  February 2020  © Mengqi Han, 2020 ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled:  IMPACT OF WATER SOLUTES ON THE FORMATION OF NITRITE UNDER VACUUM UV(VUV) ADVANCED OXIDATION OF NITRATE-RICH WATER  submitted by Mengqi Han in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemical and Biological Engineering  Examining Committee: Madjid Mohseni, Chemical and Biological Engineering Supervisor  Susan Baldwin, Chemical and Biological Engineering Supervisory Committee Member  Jongho Lee, Civil Engineering Supervisory Committee Member Mark Maclachlan, Department of Chemistry University Examiner Fariborz Taghipour, Chemical and Biological Engineering University Examiner Zhimin Qiang, Chinese Academy of Sciences External Examiner iii  Abstract Advanced oxidation processes (AOPs) are promising treatment options for the degradation of micropollutants in water. Vacuum-UV (VUV), as one such AOPs with UV photons under 200 nm, can directly photolyze water to produce •OH without extra oxidant or catalyst. However, a potential challenge in treating micropollutants by VUV, is the formation of nitrite in nitrate-containing water. Nitrate absorbs 185 nm photons, leading to the potential formation of nitrite. Given the greater toxicity and more stringent regulatory limits of nitrite on its concentration in drinking water, it is essential to examine the mechanisms of nitrite formation during VUV AOP.  This research focused on understanding the effect of common solutes present in water, including dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), chloride, and sulfate on the formation of nitrite in nitrate-containing water during VUV photolysis. Experimental work involved kinetic studies using a custom-made benchtop UV/VUV collimated beam setup. Water samples containing solutes of interests at different concentrations were irradiated and collected for analysis at set intervals, corresponding to UV254 fluences of up to 1200 mJ/cm2.  The results indicated that the formation of nitrite follows a very complex mechanism and is not simply dependent upon the concentration of nitrate. Among the solutes evaluated, DOC and chloride had the greatest impact on nitrite formation. The presence of DOC, through its scavenging of •OH, resulted in increased formation of nitrite. Chloride, on the other hand, led to a significant reduction in nitrite formation due to its competition with nitrate for absorbing VUV photons. Unlike chloride and DOC, sulfate and DIC, at concentrations commonly present in water, had little impact on nitrite formation. Their impact was only evident at extremely high concentrations to slightly reduce nitrite formation. When all the solutes, i.e., DOC, DIC, sulfate and chloride, were present simultaneously, the effect of DOC was more dominant and eventually increased nitrite iv  formation. Finally, dissolved oxygen was determined to decrease nitrite formation through the scavenging of ∙ 𝐻 and 𝑒𝑎𝑞− . The details of the experimental and mechanistic studies can provide scientific guidance towards more effective and optimized application of VUV technology for drinking water treatment.    v  Lay Summary  Emerging micropollutants and their impact on drinking water quality, have received much attention in the past decade. Vacuum-UV (VUV), a UV-based advanced oxidation process, has emerged as a viable solution due to its efficacy at producing hydroxyl radicals without the addition of oxidants or chemicals. However, a potential issue related to the application of VUV is the formation of nitrite when treating water that contains nitrate. This process is highly impacted by the types and amounts of naturally occurring water matrix constituents, including dissolved organics and inorganics, present in water. The goal of this research was to understand the impact of various water solutes on the formation of nitrite during VUV photolysis. The data and information through this work could be used to optimize VUV treatment processes, guide its applications, and predict potential nitrite formation in given applications. vi  Preface  I, Mengqi Han, was the principal author of this thesis. All the literature review, project definition, experiment design, and experiments conduction were done solely by me under the supervision of Dr. Mohseni as the principal investigators for this project. The following is the list of publications from this project in academic journals.   A version of chapter 4 was published in “Water Research” and presented in the following Conferences:  ▪ Mengqi Han and Madjid Mohseni, Impact of organic and inorganic carbon on the formation of nitrite during the VUV photolysis of nitrate-containing water, Water Research, 168 (2020) 115169 ▪ Mengqi Han, Madjid Mohseni. “Study of Vacuum UV Photolysis of Water Contaminated with Nitrate and Formation of Nitrite” CSChE 2017 (Oral Presentation), 2017 October 22-25, Edmonton, Canada. ▪ Mengqi Han, Madjid Mohseni. “Vacuum UV (VUV) Photolysis of Nitrate-containing Water and Formation of Nitrite” Canadian Water Summit (Poster Presentation), 2018 May 20-22. ▪ Mengqi Han, Madjid Mohseni. “Vacuum UV (VUV) Photolysis of Nitrate and Formation of Nitrite” RES’EAU AGM 2017 (Poster Presentation), 2017 May 26-27.  A version of chapter 5 was prepared for publication and presented in the following conferences:  vii  ▪ Mengqi Han, Madjid Mohseni. “The Role of Chloride and Chlorine Radical during Vacuum UV photolysis: Impact on Nitrite Formation” Ready for submission. ▪ Mengqi Han, Madjid Mohseni. “Effect of Water Solutes on Formation of Nitrite during VUV Photolysis of Nitrate Contaminated Water” WQTC 2019 (Oral Presentation), 2019 Nov 3-6, Dallas, USA. ▪ Mengqi Han, Madjid Mohseni. “Effect of Water Matrix on Formation of Nitrite in VUV Photolysis of Nitrate Contaminated Water” WQTC 2018 (Oral Presentation), 2018 Nov 11-15, Toronto, Canada. ▪ Mengqi Han, Madjid Mohseni. “Vacuum UV Photolysis of Nitrate-Containing Water and the Effect of Water Matrix on Formation of Nitrite” IUVA America 2018 (Oral Presentation), 2018 Feb 26-28, Los Angeles, USA.  A version of chapter 6 is prepared for publication: ▪ Mengqi Han, Madjid Mohseni. “Impact of Sulfate on Formation of Nitrite Under VUV Photolysis of Nitrate Rich Water” Ready for submission.  viii  Table of Contents  Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ....................................................................................................................... viii List of Tables ............................................................................................................................... xii List of Figures ............................................................................................................................. xiii List of Symbols .......................................................................................................................... xvii List of Abbreviations ............................................................................................................... xviii Acknowledgements .................................................................................................................... xix Dedication .....................................................................................................................................xx Chapter 1: Introduction ................................................................................................................1 1.1 Micropollutants in Water ................................................................................................ 1 1.2 Treatment of Micropollutants ......................................................................................... 2 1.2.1 Activated Carbon Adsorption ..................................................................................... 2 1.2.2 Membrane Filtration ................................................................................................... 2 1.2.3 UV-based Advanced Oxidation .................................................................................. 3 1.3 Vacuum UV for Treating Micropollutants ..................................................................... 4 1.4 Thesis Layout .................................................................................................................. 5 Chapter 2: Literature Review .......................................................................................................7 2.1 UV-based AOPs for Micropollutants Removal .............................................................. 7 2.1.1 UV/H2O2 ..................................................................................................................... 7 ix  2.1.2 UV/O3 ......................................................................................................................... 9 2.1.3 UV/TiO2 .................................................................................................................... 10 2.2 Vacuum UV and Degradation of Micropollutants ........................................................ 12 2.3 Challenge: Formation of Nitrite .................................................................................... 16 2.3.1 Absorption and Wavelength Dependence ................................................................. 16 2.3.2 Nitrite Formation by MP vs LP UV Photolysis and UV/H2O2 Treatment ............... 18 2.3.3 Nitrite Formation by Vacuum UV Radiation............................................................ 19 2.3.4 The Effect of Background Water Matrix .................................................................. 23 2.3.5 Knowledge Gap ........................................................................................................ 24 2.4 Thesis Scope and Objectives ........................................................................................ 25 2.4.1 Research Questions and Objectives .......................................................................... 25 2.4.2 Significance of This Study ........................................................................................ 27 Chapter 3: Methodology..............................................................................................................28 3.1 Experimental Procedure Overview ............................................................................... 28 3.2 Chemical and Reagents ................................................................................................. 28 3.3 Experimental Setup ....................................................................................................... 29 3.3.1 UV254 Collimated Beam ............................................................................................ 29 3.3.2 UV/Vacuum-UV Collimated Beam .......................................................................... 30 3.3.3 Iodate/iodide actinometry ......................................................................................... 32 3.3.4 Analytical Methods ................................................................................................... 33 3.3.5 185 nm Absorption Coefficient Measurement Method ............................................ 35 Chapter 4: Impact of Organic and Inorganic Carbon on the Formation of Nitrite during the VUV Photolysis of Nitrate Containing Water .....................................................................37 x  4.1 Introduction ................................................................................................................... 37 4.2 Results and Discussion ................................................................................................. 38 4.2.1 Effect of Initial Nitrate Concentration ...................................................................... 38 4.2.2 Effect of Dissolved Oxygen (DO) ............................................................................ 42 4.2.3 Effects of DOC ......................................................................................................... 44 4.2.4 Effects of DIC ........................................................................................................... 48 4.3 Summary ....................................................................................................................... 52 Chapter 5: Effect of Chloride on the Formation of Nitrite during the Vacuum UV Photolysis of Nitrate-contaminating Water ...............................................................................53 5.1 Introduction ................................................................................................................... 53 5.2 Results and Discussion ................................................................................................. 57 5.2.1 The Effect of Chloride .............................................................................................. 57 5.2.2 Acetate and Acetone as Chloride and Hydroxyl Radical Scavengers ...................... 60 5.2.3 CBZ as Probe Compound ......................................................................................... 64 5.3 Summary ....................................................................................................................... 66 Chapter 6: Impact of Sulfate on the Formation of Nitrite Under VUV Photolysis of Nitrate Rich Water ....................................................................................................................................67 6.1 Introduction ................................................................................................................... 67 6.2 Results and Discussion ................................................................................................. 70 6.2.1 Impact of Sulfate Concentration ............................................................................... 70 6.2.2 Impact of Sulfate Radical.......................................................................................... 71 6.2.3 CBZ as Probe Compound ......................................................................................... 75 6.3 Summary ....................................................................................................................... 77 xi  Chapter 7: Interactions of Major Organic and Inorganic Water Solutes and their Combined Effect on Nitrite Formation ......................................................................................78 7.1 Introduction ................................................................................................................... 78 7.2 Results and Discussion ................................................................................................. 79 7.2.1 Comparison of water solutes on the formation rate of nitrite ................................... 79 7.2.2 Combined Effects of DOC and DIC ......................................................................... 81 7.2.3 The Effects of Chloride and DOC ............................................................................ 83 7.2.4 Impact of Sulfate and DOC....................................................................................... 86 7.2.5 Impact of Sulfate and Chloride ................................................................................. 89 7.2.6 The Effect of DOC and Chloride in the presence of DIC and Sulfate ...................... 92 7.3 Summary ....................................................................................................................... 94 Chapter 8: Conclusion and Future Work ..................................................................................95 8.1 Overall Conclusion ....................................................................................................... 95 8.2 Recommendations for Future Work.............................................................................. 99 Bibliography ...............................................................................................................................101 Appendices ..................................................................................................................................115 Appendix A Preliminary test of UV 254 ................................................................................ 115 Appendix B Photon Fraction Calculation ............................................................................... 116 B.1 Sample Calculation for NOM Containing Solution ................................................ 116 B.2 Sample Calculation for Chloride Containing Solution ........................................... 117 Appendix C UV Fluence Sample Calculation ........................................................................ 118 Appendix D Sample of Nitrogen Balance .............................................................................. 120 Appendix E Calibration Curve for Anion Concentration ....................................................... 122 xii  List of Tables  Table 2.1 Reactions involved in nitrate and nitrite chemistry under vacuum UV ........................ 21 Table 4.1 Photon Fractions of nitrate solution in various concentration ...................................... 40 Table 4.2 185 nm molar absorption coefficient at room temperature........................................... 41 Table 4.3 Overview of reaction rate constants between hydroxyl radical and natural organic matter ............................................................................................................................................ 45 Table 4.4 Pseudo-first order rate constant of nitrite generation of DOC contained solutions ...... 48 Table 4.5 Pseudo-first order rate constant of nitrite generation of DIC contained solutions ....... 52 Table 5.1 Comparison of HO● and Cl● reactivities with acetate and acetone ............................. 56 Table 5.2 Some of the main reactions involved in the VUV photolysis of Cl- and nitrate........... 56 Table 5.3 Absorption coefficient at 185nm of different solutes at 25 ℃ ..................................... 59 Table 5.4 Fractions of 185 nm photons absorbed by different species as the concentration of chloride increases in water ............................................................................................................ 59 Table 5.5 Comparison of reaction rates of chlorine radical with nitrate and nitrite ..................... 62 Table 6.1 The Major Reactions Involving 𝑆𝑂4•− During VUV Photolysis of Nitrate .................. 69 Table 7.1 NF value of major water solutes ................................................................................... 80 Table 7.2 Pseudo-first order rate constants of nitrite generation in both DOC and DIC contained solutions ........................................................................................................................................ 83 Table 7.3 Pseudo-first order rate constants comparison ............................................................... 85 Table 7.4 List of Rate Constants of NOM with OH Radical and Sulfate Radical ........................ 86  xiii  List of Figures    Figure 2.1 Schematic representation of the mechanism of photocatalytic activity ...................... 11 Figure 2.2 Transmittance of different quartz glass types and relative spectral emittance from low-pressure mercury lamp (adopted from Schalk et al., 2005 with permission) ............................... 14 Figure 2.3 Absorption spectra of the intermediate species potentially involved in the primary photolysis of aqueous nitrate (reprinted from Madsen et al., 2003 with permission) .................. 17 Figure 2.4 Possible pathways of nitrite formation under VUV photolysis ................................... 20 Figure 3.1 Benchtop UV254 nm collimated beam setup ................................................................ 30 Figure 3.2 The schematic diagram of the VUV collimated beam setup ....................................... 31 Figure 3.3 The Spectrosil quarte cuvette and its copper made cuvette holder ............................. 31 Figure 3.4 Setup for measuring VUV molar absorption coefficient of different solutes in water 36 Figure 4.1 (a) The effect of initial nitrate concentration on nitrite formation (nitrate as N from 1 to 20 mg/L), (b) Initial nitrite formation rate at various nitrate concentration ............................. 39 Figure 4.2 Effect of dissolved oxygen on nitrite formation .......................................................... 42 Figure 4.3 Possible interaction of nitrate and O2(aq) in VUV photolysis ....................................... 43 Figure 4.4 DOC effect on nitrite formation as a function of UV fluence (nitrate as N at 10 mg/L, DOC at 2, 4, 6 mg/L) .................................................................................................................... 46 Figure 4.5 DOC effect on nitrite formation in presence of tert-butanol as a function of UV fluence (nitrate as N at 10 mg/L, DOC various from 2-6 mg/L, tert-butanol at 2mM) ................ 47 Figure 4.6 DIC effect on nitrite formation as a function of UV fluence (nitrate as N at 10 mg/L, NaHCO3 as C at 4, 12 and 48 mg/L) ............................................................................................ 50 xiv  Figure 4.7 DIC effect on nitrite formation in presence of Tert-Butanol as a function of UV fluence (nitrate as N at 10 mg/L, NaHCO3 as C from 4 to 48 mg/L, tert-butanol at 2mM) ......... 51 Figure 5.1 Possible mechanisms and reaction pathways involving chloride ions and formation of nitrite during the photolysis of water containing nitrate (Dash lines mean that conversion happen in multiple reactions) .................................................................................................................... 55 Figure 5.2 Effect of chloride on the formation of nitrite (nitrate-N at 10 mg/L as control, chloride is increased from 0 to 80 mg/L) .................................................................................................... 58 Figure 5.3 Nitrite formation vs. acetone/acetate concentration (nitrate-N 10 ppm, NaCl-Cl 30 ppm with VUV 1 h radiation) ....................................................................................................... 61 Figure 5.4 The influence of chloride in the presence of acetone (Acetone used at 50 mg/L, N10 respect to nitrate-N at 10 mg/L, chloride concentration is various from 0-30 mg/L) ................... 61 Figure 5.5 The influence of chloride in presence of acetate (acetate used at 50 mg/L, nitrate-N are constantly at 10 mg/L, chloride is 0 to 30 mg/L) .................................................................... 63 Figure 5.6 Formation of nitrite and degradation of CBZ in nitrate and chloride contained solution (the initial solution contains nitrate-N at 10 mg/L, acetone at 50 mg/L, CBZ at 3 mg/L and chloride at 0 and 80 mg/L) ............................................................................................................ 65 Figure 6.1 Diagram of sulfate and sulfate radical pathways leading to nitrite formation under VUV photolysis. ........................................................................................................................... 68 Figure 6.2 Impact of different concentrations of sulfate on nitrite formation during VUV irradiation (N10: nitrate-N at 10 mg/L; S5: sulfate-S at 5 mg/L). ................................................ 71 Figure 6.3 Nitrite oxidation and nitrate formation under UV/persulfate system (Na2S2O8 at 1mM, nitrite at 10 mg/L and nitrate at 2mg/L as initial concentration). ................................................. 72 xv  Figure 6.4 Impact of sulfate on nitrite formation in the presence of TBA under VUV radiation: (a) N10: NaNO3-N at 10 mg/L, different sulfate concentrations from S5 to S30 ( Na2SO4-S at 5 to 30 mg/L); TBA at 2mM; (b): N10 (NaNO3 at 45 mg/L); NaNO2 at 10 mg/L; sulfate concentration at 0 and S30) ........................................................................................................... 74 Figure 6.5 Degradation of CBZ and nitrite formation in the presence of sulfate and TBA (Initial nitrate concentration (N10) is at NaNO3-N at 10 mg/L, tert-butanol (TBA) is at 2mM. Sulfate concentration (SO42--S) is at 0 and 30 mg/L (S30). (a). Degradation of CBZ at 2 mg/L. (b) Formation of nitrite) ...................................................................................................................... 76 Figure 7.1 The rate of nitrite formation as a function of the concentration of individual solutes present in water (initial nitrate as N at 10 mg/L). ......................................................................... 79 Figure 7.2 The effects of combination of DOC and DIC on nitrite formation: (a) Influence of DIC in a solution containing nitrate-N at 10 mg/L (N10) and DOC at 6 mg/L (DOC6); (b) Influence of DOC in a solution containing N10 and DIC at 12 mg/L (DIC12). .......................... 82 Figure 7.3 The effect of chloride in presence of DOC containing solution (Nitrate-N at 10 mg/L (N10), chloride ion increasing from 0 to 80 mg/L (Cl 0-80); (a) DOC at 6 mg/L; (b) DOC at 2 mg/L)............................................................................................................................................. 84 Figure 7.4 Impact of sulfate on nitrite formation in the presence of NOM during VUV irradiation of water containing nitrate (N10: NaNO3-N at 10 mg/L, S30: Na2SO4-S at 30 mg/L; (a) DOC at 2 mg/L, (b) DOC at 6 mg/L). ........................................................................................................... 87 Figure 7.5 Impact of NOM on nitrite formation the in presence of sulfate (N10: NaNO3-N at 10 mg/L, S30: Na2SO4-S at 30 mg/L, DOC at 2 or 6 mg/L). ............................................................ 88 Figure 7.6 The combined impacts of sulfate and chloride on nitrite formation. (N10: nitrate-N at 10 mg/L; initial nitrite is zero; Cl20: chloride at 20 mg/L; S5-30: sulfate-S at 5-30 mg/L) ........ 90 xvi  Figure 7.7 The combined impact of chloride and sulfate on nitrite formation in nitrate and nitrite rich water. (The initial nitrate and nitrite concentrations for all the experiments were 40 and 10 mg/L, respectively. Control: no sulfate and chloride; SO4-90: sulfate at 90 mg/L; Cl30: chloride at 30 mg/L; Cl30 SO4-90: chloride at 30 mg/l and sulfate at 90 mg/L) ....................................... 91 Figure 7.8 Effect of combined water solutes (DOC, DIC, Sulfate, and Chloride) on nitrite formation (Nitrate-N at 10 mg/L; DOC at 2 and 6 mg/L; DIC at 12 mg/L; Sulfate at 90 mg/L; Chloride in range of 20-80 mg/L) ................................................................................................. 93 xvii  List of Symbols  [•OH] Hydroxyl radical concentration mol L-1 [Cl•] Chlorine radical concentration mol L-1 A Absorbance   E Irradiance mW·cm-2 Eavg Fluence rate mW·cm-2 I Light intensity  I0 Light intensity of DI water  l Optical path length of the cuvette cm L Distance from lamp to the cell surface cm NF Nitrite formation factor (J/cm2)-1 R Reflected fraction  t Exposure time  sec v Volume  cm3 α Absorption coefficient cm-1 ε Molar absorption coefficient mol-1·cm-1 λ Wavelength  nm Φ Quantum yield Einstein mol-1 xviii  List of Abbreviations  AOPs Advanced oxidation processes CBZ Carbamazepine DBP Disinfection byproduct DIC Dissolved inorganic carbon DO Dissolved oxygen DOC Dissolved organic carbon EDCs Endocrine disrupting compounds HAA Halogenic acetic acids HPLC High performance liquid chromatography IC Ion chromatography LPUV Low pressure ultraviolet MPUV Medium pressure ultraviolet PPCPs Pharmaceutical and personal care products PVC Polyvinyl Chloride SRNOM Suwannee river Natural Organic Matter TBA Tert-butanol THM Trihalomethanes TOC Total organic carbon UV Ultraviolet VUV Vacuum-UV  xix  Acknowledgements My first debt of gratitude owned to Dr. Madjid Mohseni, for providing me this great opportunity, and his continuous self-giving support, generous encouragement and trust through the way of my PhD journey. Without his expert guidance, this research would not have been accomplished to such a degree of scientific rigour. He kindly devoted much time and effort to this project, and he patiently taught me so that I could learn from the times when I was struggling most. Dr. Mohseni also gave me many opportunities to attend many international conferences and communicate with many researchers the World over. What I learned from him not only helped me accomplish this project, but also will continuously benefit me in the future endeavors. I also would like to thank my supervisory committee members Dr. Jongho Lee and Dr. Susan Baldwin, for their kind input and advice throughout this project. I am so grateful for all the instructions, feedback and valuable comments.  I am truly thankful to all my fellow researchers, particularly to Fuhar Dixit, Morteza Jafarikojour, Maryam Dezfoolian, Siddharth Bhartia, Adrian Mora Serrano, Ataollah Kheyrandish, Adel Hajimalayeri, Kai Song, Reza Rezaei, and Karl Zimmermann who offered countless hours of assistance and companionship during my time at UBC.  Special thanks are owed to my parents, whose have supported me throughout my years of education. Their unconditional love always motivates me to work hard towards my goals in life. Finally, I would like to acknowledge the financial support received from the Natural Sciences and Engineering Research Council of Canada (NSERC) and RES’EAU-WaterNET NSERC Strategic Network. xx  Dedication   To my parents  who always love me, support me and encourage me throughout the long challenging journey of my PhD study.  1  Chapter 1: Introduction 1.1 Micropollutants in Water Micropollutants, also termed as emerging contaminants, consist of a vast and expanding array of anthropogenic as well as natural substances (Luo et al., 2014). These include pharmaceuticals and personal care products (PPCPs), endocrine disrupting compounds (EDCs), pesticides, disinfection by-products (DBP), hormones, and industrial chemicals, etc. (Geissen et al., 2015). Some micropollutants formed during domestic usage reach the wastewater treatment plants, while the various other sources are surface run-off from agricultural areas, industrial discharge and stormwater run-off from urban areas. Micropollutants in water are commonly found at trace concentrations, ranging from a few ng/L to several μg/L. The low concentration and diversity of micropollutants not only complicate the associated detection and analysis procedures, but also create challenges for water and wastewater treatment processes.  Current wastewater treatment plants (WWTPs) are not specifically designed to eliminate micropollutants. Thus, many of these micropollutants can pass through wastewater treatment processes and enter surface and natural water supplies. Consequently, many of these compounds may end up in the aquatic environment, introducing threats to wildlife, ecosystems and public health, and as a result, they pose a challenge for drinking water industry.  Given that conventional water treatment processes cannot remove micropollutants effectively, an additional water treatment barrier should be considered and potentially introduced to the treatment trains. Advanced treatment methods such as membranes, adsorption processes, chemical oxidation and advanced oxidation have proven their ability for treating micropollutants, depending on the physical and chemical properties of the compound present in the water.  2  1.2 Treatment of Micropollutants 1.2.1 Activated Carbon Adsorption Adsorption on activated carbon (AC) is one of the well-established techniques for the removal of volatile organic compounds, synthetic organic compounds (SOCs), and chemicals responsible for undesirable tastes and odors (Zimmer et al., 1988). Some pesticides and pharmaceutical compounds like atrazine, 2,4-D, carbamazepine, etc. are also easily removed via activated carbon adsorption. However, some challenges exist in AC adsorption processes. Firstly, adsorbents suffer from progressively deteriorating in capacity with every treatment cycle; once the activated carbon has reached its capacity, it will no longer remove any contaminants. Therefore, the adsorbent must be replaced or regenerated through an expensive process. In addition, the spent adsorbent may be considered as hazardous waste since the contaminants are concentrated on the adsorbent rather than eliminated. Moreover, the presence of natural organic matter (NOM), such as humic substances, proteins, and polysaccharides found in surface water can considerably reduce the adsorptive capacity of AC due to competitive inhibition of NOM with micropollutants on the surface of AC (Kilduff et al., 1996; Kilduff and Karanfil, 2002).   1.2.2 Membrane Filtration Nanofiltration (NF) and reverse osmosis (RO) are two membrane-based processes capable of removing many micropollutants from water (Ojajuni et al., 2015). Indeed, NF and RO are considered effective with removal rates reported as being greater than 80% for many micropollutants (Ojajuni et al., 2015). However, there are several challenges associated with these physical separation technologies: i) The pressure-driven membrane filtration requires high energy and cost; ii) The removal of micropollutants by NF/RO is effective but generates highly 3  concentrated toxic retentate solution that needs further management; iii) Micropollutants with very small molecular size, e.g., 1,4-Dioxane and N-Nitrosodimethylaniline (NDMA), still cannot be removed by UF/RO. The rejection of NDMA, the smallest compound amongst all N-nitrosamines, was consistently found to be lowest by all types of membrane reported in the literature (Fujioka et al., 2012).   1.2.3 UV-based Advanced Oxidation Advanced oxidation processes (AOPs) refer to the processes that generate sufficient hydroxyl radical (•OH) for oxidizing water contaminants at ambient temperature and pressure (Glaze et al., 1987). Hydroxyl radical is a powerful and nonselective oxidant, which reacts very rapidly with most organic and inorganic compounds (Linden and Mohseni, 2014). The most common AOP techniques include ozonation, Fenton’s reaction, and UV-based AOPs, etc. Among all various AOPs, UV-based AOPs have proven to be more efficient in terms of •OH generation (Audenaert et al., 2011).  UV-based AOPs refer to the processes of forming •OH with assistance of UV photons/radiation. UV (ultraviolet) has been widely used for microbial disinfection in both drinking water and wastewater. In addition to its disinfection effectiveness for inactivating microorganisms, UV can also degrade organic compounds by direct photolysis as a consequence of UV absorption of micropollutants or by indirect processes involving the addition of oxidants. The most investigated AOP systems are UV/H2O2, UV/O3, UV/TiO2, and Vacuum UV; among them, H2O2/UV is the one which is currently commercialized and widely applied (Audenaert et al., 2011; Yang et al., 2014a).  4  Although AOPs have shown their effectiveness for contaminant degradation, implementation of AOPs could be cost-prohibitive, particularly in small to medium size water treatment facilities due to the high operational and maintenance costs (Sudhakaran et al., 2013). This necessitates the growing need for identifying robust water treatment processes at a lower cost and with less energy demand yet minimized use of chemicals to promote environmental sustainability (Yasar et al., 2006; Chuang et al., 2017; Krishnan et al., 2017; Mishra et al., 2017).  1.3 Vacuum UV for Treating Micropollutants Vacuum UV is known to be a promising technology for degrading micropollutants, as it can directly degrade most micropollutants without extra oxidants/chemicals. The VUV photons at low wavelength (of less than 200 nm) have sufficient energy to photolyze water and produce hydroxyl radicals. Many studies have demonstrated that VUV presented faster degradation of phenol, natural organic matter (NOM), microcystin-LR, carbamazepine, atrazine, etc., compared to other UV-based AOPs (Buchanan et al., 2006; Alapi and Dombi, 2007; Duca et al., 2017; Chintalapati and Mohseni, 2019; Zhu et al., 2019). The low-pressure mercury lamp is one of the most common sources of vacuum UV at 185 nm. It has around 10-20% of its emission at 185 nm, in addition to the major output at 254 nm. Despite only 10-20% of the output at 185 nm, this source is proven to be quite effective at treating micropollutants.    With growing interest in VUV process for water treatment, many researchers have highlighted a potential challenge associated with the application of VUV: the potential formation of nitrite in nitrate-containing aqueous solutions (Gonzalez et al., 2004; Thomson and Vmlm, 2004; Zoschke et al., 2014). Nitrate, a naturally occurring substance in surface or groundwater, 5  absorbs deep UV and VUV radiations and may produce nitrite under certain conditions. Nitrite has been identified as a potentially harmful compound towards human health. The formation of nitrite can oxidize iron in hemoglobin which can result in detrimental health impacts, especially affecting infants and pregnant women, commonly known as the blue baby syndrome (i.e., methemoglobinemia). Nitrate and nitrite ions are also precursors to a carcinogenic group of amines (i.e., nitrosamines) (NCBI Bookshelf, 1995). Because of their potential health hazards, the United States Environmental Protection Agency (USEPA) and Health Canada have set a maximum water contaminant level of 10 mg/L and 1 mg/L as nitrogen for nitrate and nitrite, respectively.  Previous studies showed that dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) have effects on nitrite formation. Also, other water matrix constituents, such as sulfate and chloride, may to some extent influence the formation of nitrite during VUV photolysis of nitrate-containing water. However, the role of these solutes and their chemical mechanisms are not clear or not studied in detail. Thus, this research has focused on the effect of water matrix and the mechanism of nitrite formation when nitrate-containing water is irradiated by VUV. The results from this research can help better understand the mechanism involved in the transformation of nitrate to nitrite and hence it helps with better strategies towards controlling nitrite.   1.4 Thesis Layout This dissertation compiles all the major results of four years of research at the University of British Columbia (UBC). The methods, experimental work and findings are included in 8 chapters. The structure of this dissertation is as follows:  6  Chapter 1 provides background information on the problem of micropollutants in water supplies and the technologies for the removal of micropollutants. It particularly highlights the benefits of vacuum UV. Chapter 2 presents a literature review of the advanced oxidation processes, especially VUV based AOP, and the potential challenge of nitrite formation. The knowledge gaps, research questions and objectives are highlighted. Chapter 3 demonstrates a detailed description of the experimental setups and research methodologies for achieving different objectives, as well as analytical techniques employed in this research. Chapter 4 mainly discusses the impact of initial nitrate concentration, dissolved oxygen, and organic and inorganic carbon during VUV photolysis and their effect on nitrite formation. Chapter 5 describes the role of chloride under VUV irradiation and evaluates the impact of formation of Cl•, and their influence on nitrite formation. Chapter 6 discusses the impact of sulfate and the consequent formation of sulfate radical (SO4•-) during VUV AOP, and evaluates the formation of nitrite in the presence of sulfate. Chapter 7 integrates the combined effects of various water solutes, such as DOC and DIC, DOC and sulfate, DOC and chloride, and sulfate and chloride, etc. on the formation of nitrite, to assess their overall effects under a nearly natural water condition. Chapter 8 summarizes and highlights the most significant outcomes of this work and exhibits recommendations for future researches. 7  Chapter 2: Literature Review 2.1 UV-based AOPs for Micropollutants Removal 2.1.1 UV/H2O2 UV/H2O2 advanced oxidation processes (AOPs) have been commercially applied as an effective way of treating water with organic micropollutants. For this technology, both medium-pressure (MP) and low-pressure (LP) mercury lamps are applied. Medium-pressure mercury lamps are common in commercial UV water treatment systems. They are characterized by compact construction (small footprint), high output power, and a broad UV emission spectrum from 200−400 nm. However, low photoelectric efficiency in the critical UV region (200−300 nm) and a relatively short lifetime (about 5000 h) may constrain their applications (Ijpelaar et al., 2010). The low-pressure UV (LPUV) lamp is characterized by nearly monochromatic emission at 254 nm, with the merits of high photoelectric efficiency and low manufacturing cost. At present, the highest nominal power of an LP lamp (LP amalgam lamp) has reached about 1 kW (Li et al., 2019).  The dominant pathway of •OH formation in UV/H2O2 is the photolysis of H2O2. The formation of •OH from the energy absorbed by H2O2 has a relatively high quantum yield. This process, shown in reaction (2-1), is considered efficient as it yields two moles of •OH per mole of H2O2. However, two main factors limit the efficiency of the process. First, the molar absorption coefficient of H2O2 is very low compared to other constituents in natural water, whether it is LPUV or MPUV (Ijpelaar et al., 2010). Therefore, a relatively high concentration of H2O2 is required to generate sufficient •OH concentration (Sarathy and Mohseni, 2006). At the same time, H2O2 can scavenge •OH (reaction (2-2)) and reduce the efficacy of the process (Linden and Mohseni, 2014). 8  Hence, the range of peroxide concentration needs to be controlled carefully, and it is usually used in the range of 5–10 mg L-1 for commercial UV-peroxide AOPs. 𝐻2𝑂2 + ℎ𝑣 → 2 ∙ 𝑂𝐻                                                                (2-1) 𝐻2𝑂2 +∙ 𝑂𝐻 → 𝐻𝑂2 ∙ +𝐻2𝑂                                                          (2-2) UV/H2O2 can degrade micropollutants via two distinct mechanisms: 1) direct photolysis and 2) •OH-initiated degradation. Generally, •OH-initiated degradation is the major degradation pathway for contaminants such as carbamazepine, caffeine, 1,4-dioxane, methyl-tert-butyl ether (MTBE), trichloroethanes, metaldehyde, etc., since they lack chromophores and do not absorb the UV radiation, so that they unable undergo degradation through direct photolysis (Shu et al., 2013).  Beside the •OH-initiated degradation, another mechanism involved in the UV/H2O2 process for the degradation of target contaminants, is photon-initiated degradation, or direct photolysis. Two fundamental photochemical parameters, the molar absorption coefficient and quantum yield, both determine the efficiency of direct photolysis process on micropollutants destruction (Stefan, 2018). The combination of large molar absorption coefficients and quantum yields over the radiation wavelengths emitted by the light source could make direct photolysis an effective process. Examples of contaminants degraded by direct photolysis under UV radiation (253.7 nm or polychromatic 200–400 nm) include NDMA (N-Nitrosodimethylamine) and other aliphatic nitrosamines, dimethylnitrosamine, trietazine, triclosan, sulfamethoxazole, sulfisoxazole, diclofenac, ketoprofen and RDX (hexahydro-1,3,5-trinitro-1,3,5 triazine) (Ijpelaar et al., 2010). UV/H2O2 is a well-established technology and shows the effective degradation of many micropollutants. But the treatment efficiency is highly impacted by raw water turbidity, which can reduce UV transmittance. In addition, UV/H2O2 can result in the formation of disinfection 9  byproducts (THMs and HAAs) if post chlorination is followed (Dotson et al., 2010). Another disadvantage is that the use of peroxide requires ultimate quenching to remove residual peroxide.  2.1.2 UV/O3 Low-pressure UV in conjunction with ozone, which is a strong oxidant, can produce •OH. Ozone itself has a high oxidation potential, and is widely used in water treatment as disinfectant and oxidant (Katsoyiannis et al., 2011). However, ozone is selective and reacts with organic compounds with various second-order rate constants (some being relatively low) and may not completely oxidize some species. Compared with O3 alone, combined UV with ozone can improve the efficiency of micropollutant degradation (Peyton and Glaze, 1988). Ozone strongly absorbs UV at 254 nm with a high molar absorption coefficient of 3300 M-1 cm-1. The primary mechanism of UV/O3 is shown in reactions (2-3) to (2-5) (Sarathy and Mohseni, 2006). 𝑂3 + 𝐻2𝑂 + ℎ𝑣 → 𝐻2𝑂2 + 𝑂2                                                      (2-3) 2𝑂3 + 𝐻2𝑂2 → 2 ∙ 𝑂𝐻 + 3𝑂2                                                      (2-4) 𝐻2𝑂2 + ℎ𝑣 → 2 ∙ 𝑂𝐻                                                            (2-5) However, the generation of •OH in the UV/O3 process depends on the formation of H2O2, which can be better achieved by UV/H2O2. The advantage of UV/O3 is that it offers multiple barriers for contaminants in the process as both •OH and O3 are available oxidants, but the major drawback of UV/O3 AOP is the requirement of O3 production which involves high capital and operational costs (Liu et al., 2018), and the extra O3 may require off-gas treatment. In addition, as bromate is formed in the reaction between bromide and ozone (Haag and Holgné, 1983; von Gunten and Hoigné, 1994), bromate formation must be controlled if the raw water contains 10  bromide. Bromate is a regulated carcinogenic contaminant and is required to be lower than the drinking water standard of 10 μg/L.  2.1.3 UV/TiO2 UV/TiO2 is a photocatalysis process and effective for the removal of various organic and inorganic contaminants. Titanium dioxide (TiO2) is the most common photocatalyst, because it is highly photoreactive, chemically and biologically inert and stable, nontoxic and cheap (Sarathy and Mohseni, 2006). The mechanism of hydroxyl radical formation during photocatalytic processes is slightly different from that of other UV-based AOPs. TiO2 as the semiconductor is activated by UV radiation with energy equal or greater than the band gap energy (Eg) of the catalyst. For TiO2, the band gap energy is around 3.0-3.2 eV, and therefore it requires an excitation light wavelength range shorter than 400 nm (Eg = hc/λ ≅ 1240/λ) (Park et al., 2013). Unlike conductors with continuum electronic states, semiconductors possess a void energy region where electrons are promoted from filled energy level (conduction band) to empty energy level states (valence band) by photoactivation (Linsebigler et al., 1995), as shown in Figure 2.1. The energy of photons excites electrons from the valence band (VB) to the conduction band (CB) generating h+ (hole) and e- (electron) pairs. 11   Figure 2.1 Schematic representation of the mechanism of photocatalytic activity  The positive hole oxidizes either pollutants directly or water to produce •OH, whereas the electron in the conduction band reduces oxygen adsorbed to photocatalyst. The mechanism of UV/TiO2 photocatalytic process is represented by reactions (2-6)-(2-11) as shown below (De Lasa et al., 2005): 𝑝ℎ𝑜𝑡𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 (𝑇𝑖𝑂2)ℎ𝑣→ 𝑒− + ℎ+                                                     (2-6) 𝑒− + 𝑂2 → 𝑂2.−                                                                      (2-7) ℎ+ + 𝑂𝑟𝑔𝑎𝑛𝑖𝑐 → 𝑂𝑟𝑔𝑎𝑛𝑖𝑐+                                                     (2-8)  ℎ+ + 𝐻2𝑂 → 𝐻𝑂 ∙ +𝐻+                                                              (2-9) ℎ+ + 𝑂𝐻− → 𝐻𝑂 ∙                                                              (2-10) 𝐻𝑂 ∙ +𝑂𝑟𝑔𝑎𝑛𝑖𝑐+ → 𝐼𝑛𝑡𝑒𝑟𝑚𝑒𝑑𝑖𝑎𝑡𝑒                                                     (2-11) The efficiency of a photocatalyst depends on the competition between charge transfer reactions and the recombination of e--h+ pairs. While many micropollutants have been reported to be effectively degraded via photocatalysis at the laboratory scale, photocatalysis has found limited 12  success in full-scale applications. The efficiency of most photocatalytic processes is low, typically <10%, due to reflection and scattering losses and significant electron-hole recombination at light intensities typically found in photoreactors (Hoffmann et al., 1995; Fujishima et al., 2000).   2.2 Vacuum UV and Degradation of Micropollutants Vacuum UV (VUV) refers to UV radiation below 200 nm. It was named Vacuum UV by German physicist Victor Schumann, because it is strongly absorbed by air (O’Shea and Dionysiou, 2018). In aqueous solutions, the VUV radiation is absorbed exclusively by water since its concentration (55.5 mol L-1) substantially exceeds those of the dissolved compounds. VUV applications for the treatment of micropollutants rely on the formation of reactive species such as ·OH, ·H, eaq-, HO2·, O2·- through the photolysis of water. The two primary reactions are showed below (Fetoff and Schenck, G, 1968): 𝐻2𝑂 + ℎ𝑣<200𝑛𝑚 →∙ 𝑂𝐻 + 𝐻 ∙                    𝜑 = 0.33                               (2-12) 𝐻2𝑂 + ℎ𝑣<200𝑛𝑚 →∙ 𝑂𝐻 + 𝐻+ + 𝑒𝑎𝑞−               𝜑 = 0.045                             (2-13) The formation of the hydroxyl radical depends on the emission power of the lamp, the reactor design, and the optical properties of the irradiated medium (Imoberdorf and Mohseni, 2011).   VUV photons can be generated by different sources, the most common are excimer lamps and low-pressure (LP) mercury vapor (Hg) lamps (VUV–Hg lamps). Excimer and exciplex light sources are based on the formation of noble gas and halogen excimers, or rare gas/halogen exciplex representing a relatively novel lamp generation. One of the most studied excimer lamps is the xenon (Xe2*) excimer lamp. High energy electrons are required to generate excited state noble gas atoms. The photons are emitted from the excited dimer (excimer with the formation of two noble 13  gas atoms in the ground state) (O’Shea and Dionysiou, 2012). The Xe2 excimer lamp can emit at 172 nm. This light source does not require cooling in normal operation, has no startup time, the switching cycle is unlimited, and the estimated lifetime is 2500 h. Since water absorptivity at 172 nm is very high (550 cm-1), photons are mostly absorbed only in a 10 μm layer close to the lamp (Sosnin et al., 2006), which lowers the efficacy of treatment. In practice, nominal VUV efficiency at 172 nm is about 40%. The spectral radiation from LP mercury lamp is dominated by the two Hg resonance lines at 254.7 nm and 184.9 nm. Approximately 10–20% of the radiation is at 185 nm and 80–90% of the radiation is at 254 nm. Compared to UV 172 nm, the absorptivity of water at 185 nm is 1.80 cm-1 (Weeks and Gordon, 1963): in this case, photons are absorbed in approximately 5.5 mm layer close to the lamp (Zoschke et al., 2014). The LP mercury lamp is also used in disinfection and UV/H2O2, but in those applications, the lamps emit only at 254 nm because the photons of 185 nm are blocked when the lamp envelope material is soft glass (Schalk et al., 2006). Figure 2.2 shows the transmittance of various wavelengths of UV light through different qualities of quartz glass used for the sleeve surrounding the UV lamp. The importance of high-purity quartz can be seen by the sharp decline in transmittance for various quartz qualities as the UV wavelength decreases. Hence, even if the UV light source is emitting at 254 and 185 nm, the 185 nm wavelength will only pass through the sleeve if it is made from high-quality quartz.  14   Figure 2.2 Transmittance of different quartz glass types and relative spectral emittance from low-pressure mercury lamp (adopted from Schalk et al., 2005 with permission) Although only 10-20% of all radiation from LP lamps is in the VUV range, the efficiency of VUV on the degradation of micropollutants is quite high. It is reported that the degradation rate of atrazine under VUV is more than 10 times faster than that under UV/H2O2 (10 mg/L), while the fluence rate at 254 nm of VUV lamp is 0.03mW/cm2 and that of UV lamp is 0.29mW/cm2 (Duca et al., 2017). Because the degradation of atrazine is driven primarily by •OH oxidation, it was suggested that the formation of •OH under VUV is much higher than that under UV/H2O2. Also, the degradation of carbamazepine (CBZ) via VUV, UV and UV/H2O2 at pH 7.0 was reported by Zhu et al. (2019b), which showed no significant removal of CBZ within 10 min of irradiation (i.e., direct photolysis) at wavelength of 254 nm alone; when 10 μM H2O2 was added into the UV system, approximately 67% of CBZ was degraded after 10 min. However, nearly 98% degradation of CBZ was achieved at the same time for the VUV system. Another study (Chintalapati and Mohseni, 2019) found that the use of UV/VUV lamps could provide increased degradation of microcystin (MC)-LR beyond that of UV254 photolysis, bringing the concentration of MCLR to 15  well below the Health Canada guideline of 1.5 ppb. Furthermore, with VUV (185 nm) / UV (254 nm) treatment, perfluorinated compounds such as PFOA and PFOS, can be degraded by the 185 nm radiation but cannot be removed by the 254 nm radiation alone. It was suggested that the reaction with eaq− also can work as the mechanistic pathways for the degradation of certain contaminants.  Many factors impact the efficiency of VUV treatment. The degradation efficiency of benzoic acid in VUV system increased significantly with elevating temperature (Xie et al., 2018). The effect of pH on the VUV-irradiated aqueous solutions is complex. The removal of citric acid at pH 3.4 was faster than at pH 11 (Gonzalez et al., 2004); while the degradation rates of 1,4-dioxane, geosmin, and MIB in buffered dechlorinated tap water treated with VUV/UV were reported to decrease with the increase of pH in the 5.5–8.0 range (Matsushita et al., 2015). The inorganic ions, such as NO2- and CO32-, are highly reactive towards •OH, and therefore impact the VUV process performance (Buxton et al., 1988). Nitrate ion has high absorption of VUV photons and may reduce the efficiency of micropollutant degradation (Serrano Mora and Mohseni, 2018). The presence of natural organic matter (NOM), which is a broad term for the complex mixture of thousands of naturally formed organic compounds found in raw water, was an important inhibiting factor on VUV system efficiency (Xie et al., 2018), as NOM can significantly scavenge •OH.  The most important mechanism of VUV AOP is the formation of •OH through photolysis of water. However, the reaction mechanism and pathways involved in the VUV process are quite complex. More than 30 reactions are known to occur during the VUV photolysis of pure water alone (Gonzalez et al., 2004; Imoberdorf and Mohseni, 2012), involving several radical species (•OH, HO2‾, O2•‾, H•, etc.) and stable species (H2O2, HO2‾, O2, etc.). In addition, these photogenerated radicals can react with other species present in the system such as organic and 16  inorganic molecules. Besides, depending on the complexity of the organics degraded, many by-products and intermediate radicals can be formed (Linden and Mohseni, 2014). Therefore, the degradation of micropollutants during VUV photolysis requires further investigation on radical reaction pathways and mechanisms, and need be paid much attention to byproduct formation.  2.3 Challenge: Formation of Nitrite Nitrate is a common constituent of natural surface and groundwater, and its fate during the UV irradiation of water has been the focus of numerous studies (Gonzalez and Braun, 1996; Mack and Bolton, 1999; Sharpless et al., 2003). Nitrite is more toxic than nitrate, as it oxidizes hemoglobin to methemoglobin, which then cannot transfer oxygen to tissues (Van Faassen et al., 2009). The human health concerns surrounding nitrate, and particularly nitrite, have led to them being regulated by the US Environmental Protection Agency (EPA) at 10 mg/L (as N) for NO3- and 1 mg/L (as N) for NO2-. Given that nitrite could be formed during the irradiation of water with lower wavelength UV photons, it is important to determine the extent to which this happens and how various operating and environmental conditions affect the transformation of nitrate to nitrite.  2.3.1 Absorption and Wavelength Dependence The photolysis of nitrate and the formation of nitrite depend greatly on their UV absorptions. The absorption spectrum of NO3- is dominant by a weak n→π* band around 302 nm (ε= 7.2 M-1 cm-1) and a much stronger π→π* band at 200 nm (ε=9900 M-1 cm-1) (Goldstein and Rabani, 2007), which partly overlap with the spectral emission of MP-Hg lamps. It indicates that the nitrite can be easily formed under MPUV. However, NO3- has a small molar absorption coefficient of 4 M-1 cm-1 at 254 nm (Mark et al., 1996), meaning that nitrite formation from nitrate 17  in LPUV photolysis is negligible. Many intermediate radicals and ions are formed during this process, such as •NO, •NO2, ONOO-, etc. The absorption coefficients of these intermediate species in the photolysis of nitrate sharply increase at shorter wavelengths as shown in Figure 2.3. •NO2 (aq) and •O- (aq) both have weak absorption bands (ε<250 M-1cm-1) with maxima at 400 and 240 nm, respectively. •NO (aq) absorbs weakly in the UV-vis region, while its anion, NO- (aq), absorbs below 300 nm and has an extinction coefficient of 1200 M-1cm-1 at 260 nm. Moreover, peroxynitrite (ONOO-) potentially exists as planar cis- and trans-isomers. The absorption band of cis-peroxynitrite is centered at 302 nm with a maximum extinction coefficient of 1670 M-1cm-1. Trans-ONOO- has not been observed in aqueous solution (Madsen et al., 2003).   Figure 2.3 Absorption spectra of the intermediate species potentially involved in the primary photolysis of aqueous nitrate (reprinted from Madsen et al., 2003 with permission)  Many studies have focused on nitrite formation during photolysis of nitrate under MP and LP UV, and confirmed the strong impact of nitrate due to its high UV absorption (IJpelaar et al., 2005; Goldstein and Rabani, 2007; Shah et al., 2011). Some species such as •NO, •NO2, etc. are mainly involved in the reduction of nitrate, while others like ONOO-, NO2-, etc. are directly 18  produced by the photolysis of nitrate. At different wavelengths, a variety of radicals and ions could be formed with different impacts on the generation of nitrite. Therefore, the formation of nitrite is significantly dependent on wavelength and UV light intensity. In other words, the dominant pathways of nitrite formation could switch from either oxidation or photolysis to one another, as the maximum UV absorption of these formed radicals and ions occur at different wavelengths.   2.3.2 Nitrite Formation by MP vs LP UV Photolysis and UV/H2O2 Treatment The photolysis of nitrate during the application of medium-pressure (MP) UV lamps has been studied extensively. Nitrite is reported to be formed when MP lamps are applied for either disinfection or oxidation processes (Sharpless and Linden, 2001; Sharpless et al., 2003). It has been demonstrated that, when photolysis was carried out using the full spectrum MP-UV lamp and germicidal relevant UV doses, NO2- concentrations remained well below the maximum contaminant level of 1 ppm N, even with nitrate initially present at 10 ppm N. It was indicated that NO2- formation should not pose a significant problem for water utilities during UV disinfection of drinking water with MP Hg lamps (Sharpless and Linden, 2001). However, nitrite formation exceeded this limit when MP-UV/H2O2 was applied (Martijn et al., 2014). It was reported that significantly higher nitrite yields were observed during MP-UV/H2O2 treatment of nitrate-rich waters than during the LP-UV/H2O2 treatment (Lekkerkerker-Teunissen et al., 2013).  Nitrate photolysis utilizing low-pressure (LP) UV lamps showed that no or low amounts of nitrite were produced (IJpelaar et al., 2005). Derks (2010) found that nitrite formation under LP-UV photolysis was not influenced by the addition of H2O2. Semitsoglou-Tsiapou et al. (2016) also stated that the increase of H2O2 had an insignificant effect on nitrite formation, while the higher initial nitrate concentration strongly increased nitrite generation. It was indicated that for 19  UV fluences above 1500 mJ/cm2, the resulting nitrite concentration produced by the initial nitrate concentration of 50 mg/L can exceed the 1 mg/L regulatory limit for nitrite, suggesting that nitrite formation by LP-UV advanced oxidation of nitrate-rich waters is important (Semitsoglou-Tsiapou et al., 2016). In addition, the presence of 4 mg/L Suwannee River natural organic matter (SRNOM) led to increased nitrite yields compared to NOM-free controls (Semitsoglou-Tsiapou et al., 2018). Many authors demonstrated that NOM has strong OH radical scavenging effect (Mark et al., 1996; Sharpless and Linden, 2001; Semitsoglou-Tsiapou et al., 2016). However, the effect of NOM on nitrite formation is somewhat complicated because of the complex composition of NOM and its behavior under photolysis or advanced oxidation.   2.3.3 Nitrite Formation by Vacuum UV Radiation As mentioned earlier, NO2− is not a common water constituent but it could be generated in-situ via NO3− photolysis at low UV wavelengths (e.g., with medium-pressure lamps and 185 nm radiation). However, studies investigating nitrite formation under VUV (185nm) radiation are very limited. Current literature mentions that the initial nitrate concentration significantly impacts nitrite formation, and the dissolved oxygen and dissolved organic carbon can influence nitrite concentration under VUV irradiation (Gonzalez and Braun, 1995, 1996). However, the impact of these abovementioned factors has not been discussed or delineated. Another major issue is that previous studies have only focused on the hydroxy radical oxidation processes and did not consider the photolysis of nitrate at shorter wavelengths (i.e. 185 nm).  Nitrate may be converted to nitrite via oxidation or photolysis process (Gonzalez and Braun, 1995; Gonzalez et al., 2004; Zoschke et al., 2014). Out of several possible mechanisms, three pathways and mechanisms are most likely and are presented in Figure 2.4. 20    Figure 2.4 Possible pathways of nitrite formation under VUV photolysis  The first pathway is the oxidation process. Nitrate mainly reacts with hydrate electron and hydrogen radical, producing some intermediates which eventually form nitrite (Gonzalez and Braun, 1995; Mack and Bolton, 1999). The second and third pathways show the direct photolysis of nitrate to form either isomerized form of nitrate, peroxynitrite (ONOO-), which is precursor of nitrite (Sharpless and Linden, 2001; Madsen et al., 2003), or the nitrite ion itself (Mack and Bolton, 1999). During these processes, radicals such as eaq-, •H, •OH, •NO2 and ONOO- anions are the dominant compounds because their reaction rate constants with nitrate or nitrite are quite high. Nitrite formation from the three pathways could happen simultaneously. In addition, many other background water solutes may also be involved in this complex chemistry and impact nitrite formation. The involved reactions and their rate constants are listed in Table 2.1.   21  Table 2.1 Reactions involved in nitrate and nitrite chemistry under vacuum UV No. Reactions Rate constants Reference 1 NO3− + H• → NO3H•− 2.4 × 107M−1s−1 (Gonzalez and Braun, 1995) 2 NO3− + eaq− → (NO3• )2− 1.0 × 1010M−1s−1 (Gonzalez and Braun, 1995) 3 NO3− + HO• → NO3• +HO− < 1 × 105M−1s−1 (Gonzalez and Braun, 1995) 4 NO3H•− → (NO3• )2− + H+ 16s−1 (Gonzalez and Braun, 1995) 5 (NO3• )2− + H+ → NO3H•− 5.× 108M−1s−1 (Gonzalez and Braun, 1995) 6 (NO3• )2− + O2 → NO3− + O2•− 1.6 × 108M−1s−1 (Gonzalez and Braun, 1995) 7 (NO3• )2− + H2O → NO2• + 2HO• 5.5 × 104s−1 (Gonzalez and Braun, 1995) 8 NO3H•− + H2O → NO2• +HO• 2.3 × 105s−1 (Gonzalez and Braun, 1995) 9 2NO2• → N2O4 4.5 × 108M−1s−1 (Gonzalez and Braun, 1995) 10 N2O4 → 2NO2•  6.9 × 103s−1 (Gonzalez and Braun, 1995) 11 N2O4 + H2O → NO3− + NO2−+ 2H+ 1.× 103s−1 (Gonzalez and Braun, 1995) 12 NO3− → ONOO− 48% isomerization λ<280nm (Madsen et al., 2003) 13 ONOO− +∙ OH → ∙ ONOO + OH− 5.× 109M−1s−1 (Mack and Bolton, 1999) 14 [NO3−]∗ → ONOO− ↔ HOONO pKa=6.5 (Mack and Bolton, 1999) 15 ∙ OH + NO2 ∙→ ONOOH 1.3 × 109M−1s−1 (Mack and Bolton, 1999) 16 ONOO− ↔ NO ∙ +O2.− k+=0.023 s-1 k-=5×109 M-1 s-1 (Goldstein and Rabani, 2007) 17 ONOO− + CO2(aq) → ONOOCO2− 3 × 104M−1s−1 (Sharpless and Linden, 2001) 18 NO3− + hv → NO2 ∙ +O.− φ254nm = 0.09 (Mark et al., 1996) 19 NO3− + hv → ONOO− φ254nm = 0.1 (Mark et al., 1996) 20 NO3− + hv → NO2− + O φ254nm = 0.001 (Mark et al., 1996) 22  21 NO2− + HO• → NO2• + OH− 1.0 × 1010M−1s−1 (Mack and Bolton, 1999) 22 NO2− +∙ OH → NO3H•− 2.5 × 109M−1s−1 (Gonzalez and Braun, 1995) 23 NO2− + eaq− → (NO2• )2− 6 × 109M−1s−1 (Gonzalez and Braun, 1995) 24 (NO2• )2− + NO3−→ (NO3• )2− + NO2− 1 × 1010M−1s−1 (Gonzalez and Braun, 1995) 25 (NO2• )2− + H2O → NO ∙ +2OH− 9 × 104M−1s−1 (Gonzalez and Braun, 1995) 26 NO2• + NO ∙→ N2O3 1.1 × 109M−1s−1 (Mack and Bolton, 1999) 27 NO ∙ +HO• → NO2− + H+ 1.0 × 1010M−1s−1 (Mack and Bolton, 1999) 28 NO ∙ +eaq− → NO− 3.1 × 1010 M-1 s-1 (Gonzalez and Braun, 1995) 29 NO ∙ +O2.− → NO3− 3.7 × 107M−1s−1 (Gonzalez and Braun, 1995) 30 N2O3 → NO ∙ +NO2 ∙ 8.4 × 104 s−1 (Gonzalez and Braun, 1995) 31 N2O3 + H2O → 2H+ + 2NO2− 5.3 × 102s−1 (Mack and Bolton, 1999) 32 NO2• + HO ∙→ ONOOH 1.3 × 109M−1s−1 (Mack and Bolton, 1999) 33 NO2• +H ∙→ NOOH 6 × 108M−1s−1 (Mack and Bolton, 1999) 34 NO2 ∙ +eaq− → NO2− 4.6 × 109M−1s−1 (Gonzalez and Braun, 1995) 35 NO2 ∙ +O2.− → NO2− + O2 2 × 108M−1s−1 (Gonzalez and Braun, 1995) 36 O + NO3− → NO2− + O2 3 × 108M−1s−1 (Mack and Bolton, 1999) 37 O + NO2− → NO3− 3 × 109M−1s−1 (Mack and Bolton, 1999) 38 ONOO ∙→ NO ∙ +O2 n.a. (Mack and Bolton, 1999) 39 NO2. + ONOO ∙ +H2O→ 2NO3− + 2H+ n.a. (Mack and Bolton, 1999)  23  2.3.4 The Effect of Background Water Matrix Many of the raw water constituents can either highly absorb VUV photons or create radicals after being exposed to VUV radiation, including dissolved organic carbon, inorganic carbon, sulfate, chloride, etc. The formed inorganic radicals, such as carbonate radical, chlorine radical and sulfate radical can be reactive with many contaminants and impact the efficiency of VUV treatment. The presence of these water solutes can also affect nitrate/nitrite reaction pathways and mechanisms, and therefore, is deemed to influence the formation of nitrite. Current literature includes a few studies that examined the effect of DOC and DIC on nitrite yield during MP-UV and LP-UV photolysis (Gonzalez and Braun, 1996; Sharpless and Linden, 2001; Buchanan et al., 2006; Imoberdorf and Mohseni, 2011). Most studies agree that the presence of DOC can lead to increased nitrite formation since DOC strongly scavenges •OH (Semitsoglou-Tsiapou et al., 2016). However, this proposed mechanism has not yet been rigorously verified and there has been no research to determine the impact of NOM during VUV irradiation.  While there are some general agreements on the effect of DOC, the impact of DIC on nitrite formation is not clear. Some studies found that carbonate ion reacts significantly with OH radical and hence, brings similar impact as DOC (P.Neta and Huie, 1988). Conversely, other studies indicated that the formation of CO2(aq) can reduce the concentration of peroxynitrite (ONOO-) and further reduce nitrite concentration (Sharpless and Linden, 2001). These conflicting reports suggest the need for further research on the impacts of DIC on nitrite formation, especially during the VUV irradiation of water solutions. In addition to DOC and DIC, there are a number of other inorganic solutes that strongly impact the efficacy of VUV treatment (Duca et al., 2017; Furatian and Mohseni, 2018a). Two of those inorganic solutes, chloride, and sulfate, that are widely present in surface water and can 24  participate in the complex photochemical reactions during VUV photolysis of water or treatment of micropollutants. For example, chloride has been shown to increase the degradation rate of carbamazepine (CBZ) dramatically (Furatian and Mohseni, 2018a). Chlorine radical formed by the process, is reported to be reactive with many organic and inorganic compounds (Buxton et al., 2000; Fang et al., 2014). The presence of sulfate can also affect the process since sulfate in the solution produces sulfate radical under VUV irradiation. During the VUV photolysis, the degradation of atrazine increased slightly with increasing the concentration of sulfate (Duca et al., 2017). In addition, the rate of carbamazepine degradation under VUV irradiation increased noticeably with the increase of sulfate concentration (Furatian and Mohseni, 2018b). The sulfate radical formed during the VUV process has also been reported to react with nitrite (Ji et al., 2017), and therefore may lead to formation of nitrate.  The aforementioned reactions indicate that the complexity of the water matrices influences on the VUV treatment and on the array of interfering and simultaneous reactions that occur following UV irradiation. All of these simultaneous and competing reactions from the constituent water matrix, ultimately influence the generation of nitrite during the VUV photolysis of nitrate-rich water. An area that has until now not received much attention in the literature has been focused on this research.  2.3.5 Knowledge Gap VUV photolysis can be used in drinking water treatment, especially for the removal of micropollutants which are harmful but difficult to remove via conventional processes. Despite its many advantages and promising results, the VUV advanced oxidation process is prone to challenges including the potential to form nitrite from nitrate. The relevant research in this area is 25  limited, and particularly the effect of water matrix on nitrite formation under irradiation from VUV range has never been investigated by the research community. The effect of water matrix on nitrite formation, particularly the impact of organics, inorganics, and dissolved oxygen will be valuable for real and commercial applications of VUV process. Thorough understanding of the kinetics of nitrite formation under different water matrices will aid in determining the extent to which VUV may be applied as a viable technology for water treatment.  The kinetics and mechanisms of reactions including the role of OH radical, as well as the contributions of HCO3.−/CO3∙−, Cl ∙, SO4∙−, and their competitions with OH radical, in the formation of nitrite are essential for further development and implementation of this technology.   2.4 Thesis Scope and Objectives 2.4.1 Research Questions and Objectives The main objectives of this research are to study the impact of water matrix composition on nitrite formation during VUV treatment, and to develop a detailed understanding of the phenomena governing the formation of nitrite under various water matrices. These objectives have been achieved through a series of investigations to answer the following research questions: 1. How does the initial nitrate concentration influence the rate of nitrite formation?  2. To what extent does dissolved oxygen (DO) influence the rate of nitrite formation? Specifically, what is the role of DO in nitrate/nitrite photochemical reaction pathways during VUV photolysis? 3. What is the effect of DOC and DIC on nitrite formation? 26  • How does NOM impact on nitrite formation?  • To what extent does the presence of bicarbonate impact the nitrite formation? • What is the influence of different compositions of NOM and bicarbonate on the formation of nitrite? • What are the mechanisms of DOC and DIC, such as the scavenging effects of NOM and bicarbonate for photo-generated OH radical under VUV process? Are there any other DOC and DIC involved pathways influencing nitrite formation? 4. What is the effect of chloride on nitrite formation? • How does chloride at different concentrations influence the formation of nitrite? • What is the role of photo-generated chlorine radical, and will it impact the formation of OH radical?  • What is the influence of NOM and chloride at different compositions on the formation of nitrite? 5. What is the effect of sulfate on nitrite formation? • To what extent does sulfate at different concentrations influence the formation of nitrite? • What is the role of photo-generated sulfate radical, and will its presence impact the formation of nitrite? • What is the effect of combined water solutes, such as sulfate and chloride, sulfate and NOM, on the formation of nitrite? • How does the relative reactivity of •OH, Cl•, and SO4∙−, and their effects on nitrite formation during VUV photolysis?  27  2.4.2 Significance of This Study Vacuum UV, utilizing low-pressure mercury lamps, is one of the most promising processes for simultaneous removal of micropollutants and inactivation of pathogens in water. The degradation of micropollutants during the VUV based AOP takes place without the addition of chemicals and oxidants, which are known to bring significant cost and operational challenges to water treatment process. Despite its great promises, there are concerns around the formation of nitrite if the water contains nitrate.  This research addresses many fundamental and practical questions related to this challenge and provides data and detailed analyses around the kinetics of nitrite formation in the presence of different water solutes including NOM (or DOC), DIC, sulfate, chloride, and their combinations. The methodology and obtained knowledge help guide the water industry towards the optimized application of the UV/VUV technology, and controlling the formation of nitrite during this process. In addition, this study adds to the fundamental understanding around the kinetics of nitrite formation and the roles of various radical species that are formed during the photolysis of water containing different solutes.   28  Chapter 3: Methodology 3.1  Experimental Procedure Overview The experiments for a given water matrix were performed in at least duplicate by the following procedure. The synthetic water samples were prepared using deionized water (DI) and chemicals for the desired water matrix composition including DOC, DIC, chloride, and sulfate, at a certain concentration. Then, water quality parameters, including pH, DOC, alkalinity, and UV absorbance at 254 nm, were measured before the solution was irradiated in a VUV reactor setup (Chapter 3.3.2). pH was always adjusted to 7.5±0.5 using 0.1N NaOH or HCl, to reach neutral pH and eliminate the effect of pH in different water matrices. DOC and alkalinity were measured to confirm the initial concentration of samples which contained SRNOM as a source of DOC and NaHCO3 as a source of DIC. Absorbance at 254 nm was determined for UV actinometry water factor correction (Bolton and Linden, 2003). Iodate/iodide actinometry method was used for measuring the UV fluence rate (Chapter 3.3.3). The irradiation time was set at 20, 40, 60, 80 and 100 mins. Upon collection of samples, the concentrations of nitrate and nitrite were analyzed by ion chromatography. The ion concentrations were identical within the standard deviation of 5% maximum.   3.2  Chemical and Reagents All chemicals used such as sodium nitrate (NaNO3), sodium nitrite (NaNO2), sodium bicarbonate (NaHCO3), sodium chloride (NaCl), sodium sulfate (Na2SO4), sodium persulfate (Na2S2O8), and carbamazepine (CBZ), sodium acetate (CH3COONa), tert-butanol, acetone were analytical grade and purchased from Sigma-Aldrich (St. Louis, MO USA) or Fisher Scientific (Canada). Suwannee River Natural Organic Matter (SRNOM, 2R101N) was obtained from the 29  International Humic Substance Society (St. Paul, MN, USA) as reverse osmosis isolates in freeze-dried powder form. NOM stock solution in 100 mg/L was prepared by dissolving SRNOM powder overnight and passing through 0.45μm membrane filter (Millex-HV Syringe Filter Unit) and storing at 4℃ in the dark. All solutions were made using deionized water which was treated by a Milli-Q water (resistivity 18.2 M.Ω.cm) purification system (Elga Labwater, UK). Ultrahigh purity (99.998%) nitrogen gas purchased from Praxair Canada Inc. (Compressed Nitrogen 4.8, Mississauga ON Canada) was used for purging air from the optical path of the 185 nm enabled collimated beam.  3.3 Experimental Setup 3.3.1 UV254 Collimated Beam Experiments on the impact of 254 nm photons via direct UV photolysis were conducted using a UV collimated beam apparatus (Figure 3.1). The UV254 collimated beam used a low-pressure mercury lamp (LightSources Inc, Orange CT USA), which prevents the emission of 185 nm radiation and is often described by lamp manufacturers as a germicidal UV lamp. The lamp was cooled using ambient air via fans to push air through the lamp housing. A collimation tube, painted black, provided a distance from lamp to liquid sample surface of at least 30 cm with an inner diameter of 6 cm. Samples were held in a cuvette (Figure 3.3) (Starna Cells Inc, Atascadero CA USA), and placed upon a stir plate and mixed using Teflon-coated stir-bars to allow for uniform irradiation. The stir plate was placed upon a laboratory jack stand for accurate vertical position control.   30   Figure 3.1 Benchtop UV254 nm collimated beam setup  3.3.2 UV/Vacuum-UV Collimated Beam The VUV AOP experiments were mainly conducted with a collimated beam bench-scale reactor (Figure 3.2) equipped with a Vacuum-UV Hg lamp, emitting light at 80-90% 254 nm, 10-20% 185 nm and a low percentage of visible. This lamp was mounted in a sealed aluminum tube that purged with nitrogen to provide the no oxygen atmosphere, which guaranteed 185 nm wavelength irradiation. A collimation tube was fitted under a hole from lamp housing, which measured 18 cm in length and 3.2 cm in inner diameter. Samples were enclosed in a transparent synthetic quartz cuvette, which has an interior diameter of 47 mm and the nominal volume of 17.0 mL, and placed at the bottom of a UV/VUV collimated beam apparatus. A shutter of thick card was used via a slot in the shutter housing and operated manually in coordination with a timer to measure exposure times. The distance from the bottom of shutter housing to the surface of the cuvette was only 2 cm and surrounded by a transparent plastic curtain outside to ensure an uninterrupted flow of nitrogen gas over the sample. A magnetic stirrer inside cuvette was used to 31  mix the solution during the irradiation. For temperature control, a copper-made cuvette holder (Figure 3.3) was used to quickly remove heat from the light source and keep temperature difference within 2℃ which can be ignored. During the daily use, nitrogen kept flushing through the setup and the VUV lamp was turned on after nitrogen gas flowed in. The system took at least 10 mins to stabilize prior to samples irradiation.   Figure 3.2 The schematic diagram of the VUV collimated beam setup   Figure 3.3 The Spectrosil quarte cuvette and its copper made cuvette holder 1. Low-pressure mercury lamp 2. Copper ballast 3. Aluminum lamp housing 4. Nitrogen gas ports 5. Teflon collimation tube 6. Shutter 7. Spectrosil quartz cuvette plated in a copper-made cuvette holder 8. Magnetic stir plate 9. Laboratory jack stand 32  3.3.3 Iodate/iodide actinometry The standard measurement of UV254 by iodate/iodide actinometry has been well developed (Bolton and Linden, 2003; Rahn et al., 2006; Bolton et al., 2015). This method relies on the different absorption of triiodide ions and iodate/iodide under 352 nm and is simply measured by spectrophotometer. The actinometer solution consists of 0.6M KI and 0.1 M KIO3 in a 0.01 M Na2B4O7 buffer solution.  8𝐼− + 𝐼𝑂3− + 3𝐻2𝑂 + ℎ𝑣 → 3𝐼3− + 6𝑂𝐻− The photoproduct is triiodide ion (I3-) which exhibits strong absorption at UV 352 nm, where the actinometer’s components do not interfere. Thus, the total photon irradiance is applied to the formation of I3-.  • Firstly, the actinometer solution was prepared by dissolving 0.38g Na2B4O7, 2.14g KIO3 and 9.96g KI in a 100 ml volumetric flask.  • The solution is effective in 4 h, mixing it well to guarantee all chemicals dissolved. • Test the actinometer solution by measuring the absorbance at 300 nm (Abs300), the value should be close to 0.58±0.02, otherwise, the actinometer solution is invalidated.   • Always zero absorbance using DI water when the wavelength changed. • Measure the absorbance at 352 nm (Abs352) of actinometer solution, the value should be lower than 0.02. • The actinometer solution is irradiated under UV 254 nm, and the above photoreaction is happened to form I3-. Triplicated the test and record irradiation time and their absorbance.  • Calculate the irradiance based on the final equation. 33  • 𝐸 = 23.373[𝐴352  (𝑠𝑎𝑚𝑝𝑙𝑒)− 𝐴352 (𝑏𝑙𝑎𝑛𝑘)] [𝐴𝑟𝑒𝑎 (𝑐𝑚2) ×𝐸𝑥𝑝𝑜𝑠𝑢𝑟𝑒 𝑡𝑖𝑚𝑒 (𝑠)]∙ 𝑉𝑜𝑙𝑢𝑚𝑒 (𝑚𝐿) (𝑚𝑊 ∙  𝑐𝑚−2) • Eavg=E × Reflection Factor × Water Factor × Divergence Factor (Bolton and Linden, 2003). • Reflection factor = 1-R, which used to correct for light passes from one medium to another, the reflected fraction R is given by the Fresnel Law (Meyer-Arendt, 1984). In our case, R=0.025. • water factor =1−10−alal Ln(10), accounting for UV absorption of experimental solutions. • divergence factor =L(L+l), correcting for decreased irradiance over the path length l of the cell suspension, L is the distance from lamp to the cell surface. The final Eavg of our UV collimated beam is from 0.2 to 0.28, depending on different water matrix. Usually, the water containing higher DOC absorbs more incident UV lights and reduces the final Eavg.   3.3.4 Analytical Methods Before VUV irradiation, pH was adjusted to neutral, and absorbance at 254 nm was recorded for UV fluence correction. DOC and alkalinity tests were performed when there were additions of NOM or HCO3-, respectively. pH was adjusted for all water samples to 7.5 ± 0.5 by using an Oakton pH meter (OAKTON Instruments, USA), calibrated at pH 4, 7, and 10 with a disposable type gel-filled probe. The absorbance of the solution was measured by UV-Vis Cary 100 spectrophotometer (Agilent Technologies, USA). The concentration of DOC was measured using a TOC analyzer (GE Sievers M5310 C, USA) based on oxidizing regent (ammonium persulfate). Alkalinity was measured by Multiparameter Benchtop Photometer (HANNA HI 34  83300). For experiments on the study of dissolved oxygen (DO) effect, DO was measured using YSI ProODO handheld DO meter (YSI Inc./Xylem Inc, USA). UV energy was measured by iodide/iodate actinometry (Rahn et al., 2006) for determining UV 254nm fluence rate. Note that the lamp power output ratio of the UV 185 nm to UV 254 nm remains constant (around 16%) (Furatian, 2017) provided the operating conditions to stay unchanged. Hence, we monitored and presented UV energy at 254 nm instead of 185 nm, because it allows for convenient comparison with other UV-based AOP studies in the literature. The concentrations of nitrate, nitrite, sulfate and chloride in water were measured using an ion chromatograph (IC) (Dionex ICS-1100, Thermo Fisher Scientific, USA). To be more specific, the autosampler withdrew a 10 μL sample volume from IC vials per injection, multi-injections were applied to reduce variability; Dionex IonPac AS22 column (Thermo Fisher Scientific, USA) with super-fast separation time was equipped and maintained at 35 ℃ in isocratic mode; A mobile phase composed of 0.45 M carbonate and 0.14 M bicarbonate (Thermo Fisher Scientific, USA) were diluted 100 times and applied as eluent solution at a flow rate of 1.2 mL min-1. The concentration of CBZ, which was used as a probe compound in our study, was quantified by HPLC (Agilent Technologies, USA) using a Dionex UltiMate 3000 System. The autosampler of the HPLC system withdraws a volume of 20 μL for each injection. The mobile phase, composed of 30% acetonitrile and 70% water acidified to pH 2.5 with 10 mM phosphoric acid, was used to carry the samples through a Nova-Pak C18 column (Waters Corp., USA) maintained at a temperature of 35 ℃. UV detection of CBZ was performed at the wavelength of 211 nm.  35  3.3.5 185 nm Absorption Coefficient Measurement Method Tert-butanol (TBA) was used as radical scavenger when the role of hydroxyl radical is investigated. Their optimum concentration is chosen mainly based on the fact that they have no influence on the main reaction taking place and be enough to partly scavenge radicals. The molar absorption coefficient of TBA under 185nm was measured in this study. The measurement method and setup refer to Serrano Mora and Mohseni (2018). The unit consisted of a UV/VUV low-pressure amalgam Hg lamp (Light Sources, Inc.), a temperature-controlled cuvette holder (T2, Quantum Northwest) coupled with a temperature controller (TC1, Quantum Northwest), a 185 nm gold cathode sensor (SED185, International Light Technologies) connected with a radiometer (IL1700 Research Radiometer, International Light Technologies). Radiometer can provide us the light intensity after passing solutions in the cuvette. A digital thermometer (HH801B, Omega) coupled with a type T thermocouple (0.81 mm diameter, Omega) was used to measure the temperature inside the quartz cuvette (10mm, Starna). Nitrogen at a flowrate of 5 L min-1 was used to purge the lamp casing (i.e., to prevent ozone formation) and to avoid condensation on the quartz cuvette at lower temperatures. A thermostatic circulator unit (Isotemp 250 LCU, Fisher Scientific) was used to maintain the recirculating water used by the temperature controller at 5 °C. 36   Figure 3.4 Setup for measuring VUV molar absorption coefficient of different solutes in water  For the measurement of tert-butanol (TBA) molar absorption coefficient, the concentration of 2, 4, 6, 8 and 10 mM were selected for 185 nm light intensity (I), by using Lambert-Beer Law, the molar absorption coefficient ε can be calculated by testing I0 and I using the equation below. 𝐴 = − log (𝐼𝐼0) = 𝜀 ∙ 𝑐 ∙ 𝑙 ⇒ 𝜀 = log (𝐼0𝐼)/(𝑐 ∙ 𝑙) Here the I0 is the light intensity of DI water, c is the molar concentration of TBA (mol/L), and l is the path length of the cuvette (cm).   37  Chapter 4: Impact of Organic and Inorganic Carbon on the Formation of Nitrite during the VUV Photolysis of Nitrate Containing Water 4.1 Introduction One concern around the application of VUV to drinking water treatment is the formation of nitrite during the VUV photolysis of nitrate-containing water. This is largely because the absorption coefficient of nitrate increases sharply at shorter wavelengths, leading to the potential formation of nitrite. Nitrate and nitrite have been identified as harmful compounds towards human health. The reduction of nitrate to nitrite has a major role in forming methemoglobin which results in a disease affecting infants and pregnant women, commonly known as the blue baby syndrome or methemoglobinemia. Nitrate and nitrite ions are also precursors to a carcinogenic group of amines and nitrosamines (NCBI Bookshelf, 1995). Because of their potential health hazards, the USEPA has set a maximum allowable contaminant level (MCL) of 10 mg/L for nitrate (10 mg nitrate-N L−1 or 44.3 mg nitrate L−1) and 1 mg/L for nitrite (1 mg nitrite-N L−1 or 3.3 mg nitrite L−1).  In the past years, studies focusing on nitrite formation in UV-based AOPs have highlighted the effect of wavelength, UV fluence, pH, temperature (Thomson and Vmlm, 2004; Lu et al., 2009). The focus of those studies was primarily on the effect of wavelength and pH under low-pressure or medium-pressure UV irradiation of nitrate. Also, only a few studies have shown the effect of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) (Sharpless and Linden, 2001; Moussavi and Mahdavianpour, 2016), which could act as OH radical scavenger (Imoberdorf and Mohseni, 2011) and influence nitrite formation. However, studies on the formation of nitrite during VUV AOP is absent. More specifically, the influence of major organic 38  and inorganic solutes, present in natural waters, on nitrite formation during the UV/VUV photolysis of nitrate-containing water has not been investigated. This research aimed to fill this gap and investigate the VUV photolysis of nitrate and the formation of nitrite in water matrices with different DOC, DIC and dissolved oxygen (DO). In addition, the combined effects of DOC and DIC within a water matrix and their reaction mechanisms were also included in the investigation.   4.2 Results and Discussion 4.2.1  Effect of Initial Nitrate Concentration The five different initial nitrate concentrations were chosen from 1 mg/L to 20 mg/L (𝑁𝑂3− −𝑁). Firstly, the formation of nitrite was not simply linearly correlated with UV fluence (up to 1600 mJ/cm2 in 100 mins irradiation) which represents the amount of UV energy delivered to the solution. As shown in Figure 4.1 (a), with the exception of the highest nitrate concentrations, N10 and N20 (𝑁𝑂3− − 𝑁 at 10 and 20 mg/L, respectively), there was a gradual decline in the rate of nitrite formation as the fluence increased. The effect of initial nitrate concentration can be analyzed at two stages: initial rate and residual rate of nitrite formation (Mack and Bolton, 1999). The initial rate where the nitrite concentration increases linearly with fluence, is dependent on nitrate concentration, whereas for the residual rate the formation of nitrite is somewhat self-inhibited by nitrite reactions (Daniels, 1968):  𝑂 + 𝑁𝑂3− → 𝑂𝑂𝑁𝑂𝑂− → 𝑁𝑂2− + 𝑂2                            𝑘 = 3 × 108𝑀−1𝑠−1                           (4-1)  𝑂 + 𝑁𝑂2− → 𝑁𝑂3−                                                           𝑘 = 3 × 109𝑀−1𝑠−1                           (4-2) 39   Figure 4.1 (a) The effect of initial nitrate concentration on nitrite formation (nitrate as N from 1 to 20 mg/L), (b) Initial nitrite formation rate at various nitrate concentration  The plot of the initial formation rate of nitrite vs. nitrate concentrations, Figure 4.1 (b), shows that the nitrite formation rate increases initially, but reaches a nearly constant value with further increase of nitrate concentration. The concentration of nitrate in water directly influences 00.20.40.60.811.20 300 600 900 1200 1500 1800nitrite concentration (mg/L)UV fluence(mJ/cm2)N1 N2 N5 N10 N20(a)234567890 5 10 15 20 25Initial nitrite formation rate ×10-2(mg L-1mJ-1 cm2 )nitrate-N concentration (mg/L)(b)40  photon absorption fraction (Table 4.1), which can be calculated using Lambert-Beer’s Law with α(H2O)=1.8 cm-1 (Weeks and Gordon, 1963) and 𝜀(𝑁𝑂3−,185𝑛𝑚)=4779 M-1s-1 (Serrano Mora and Mohseni, 2018) (Table 4.2). The photon fraction calculation details can be found in Appendix (Table S2). The increase of nitrate leads to an increase of 185 nm photons being absorbed by nitrate and results in a decrease of photolysis of water. Note, the species ∙H, ∙OH and eaq- which are produced by the photolysis of water have an important role in nitrite formation via pathway (1) in Figure 2.4. The decreased photons fraction absorbed by H2O eventually hinders pathway (1) and reduces the initial formation rate of nitrite at high nitrate concentration. As reactions proceed, nitrite is formed and participates in the complex set of reaction chains, among those being the self-inhibition of nitrite which plays an important role in the residual formation rate. Table 4.1 Photon Fractions of nitrate solution in various concentration Compounds N1 N2 N5 N10 Nitrate 0.16 0.28 0.49 0.65 H2O 0.84 0.72 0.51 0.35  The residual formation rate of nitrite is dependent on pathways shown in reactions (4-1), (4-2) and (4-3), and the ratio of nitrate, nitrite, and other radical species formed during the photolysis of water. The formed nitrite is consumed by ∙OH, forming NO2∙ which can further react with other radicals to form N2O3/N2O4 intermediates and finally convert back to nitrate via (4-3) 41  reactions (4-1) to (4-3). The re-formed nitrate could continue to participate in the VUV photolysis and produce nitrite again via the major three possible pathways as mentioned earlier. Table 4.2 185 nm molar absorption coefficient at room temperature substance Molar absorption coefficient at 185nm (M-1cm-1) Reference Nitrate 4779 (Serrano Mora and Mohseni, 2018) NOM 1402 (Serrano Mora and Mohseni, 2018) Bicarbonate 290 (Furatian and Mohseni, 2018b) tert-Butanol 73 This study  For the higher initial nitrate concentration such as N10, the formation of nitrite increases linearly with fluence. The hypothesis is that nitrate at such high concentration can absorb all or majority of the 185nm photons; thus, nitrite formation is mainly dependent on the UV energy, and the self-inhibition effects caused by nitrite is insignificant due to the relatively low concentration of nitrite formed. Further increase of the nitrate in the solution (i.e., NaNO3-N at 20 ppm) did not lead to an increase in nitrite formation, confirming the aforementioned hypothesis that nitrite formation is limited by 185 nm photon absorption by nitrate. This result is valuable since it demonstrates that the formation of nitrite during the VUV advanced oxidation is independent of the concentration of nitrate, when it is high. Hence, one could determine and control the extent of nitrite formation by simply controlling the irradiation time or UV fluence delivered to the solution.  42  4.2.2 Effect of Dissolved Oxygen (DO) To investigate the effect of dissolved oxygen, experiments were conducted by sparging compressed air/nitrogen gas in water. Figure 4.2 shows that nitrite formation decreased with the increase of DO, which is consistent with the finding of Gonzalez and Braun (1995, 1996). It is mentioned that dissolved oxygen strongly competes with nitrate for ·H and hydrated electrons as shown in reactions (4-4) to (4-7). Note that the reactive species ·H and hydrated electron are produced by 185 nm photolysis of water. 𝑂2 +∙ 𝐻 → 𝐻𝑂2∙                                                 𝑘 = 1 × 1010𝑀−1𝑠−1                                       (4-4) 𝑂2 + 𝑒𝑎𝑞− → 𝑂2∙−                                               𝑘 = 2 × 1010𝑀−1𝑠−1                                        (4-5) 𝑁𝑂3− +∙ 𝐻 → 𝑁𝑂3𝐻∙−                                     𝑘 = 2.4 × 107𝑀−1𝑠−1                                       (4-6) 𝑁𝑂3− + 𝑒𝑎𝑞− → (𝑁𝑂3∙ )2−                                  𝑘 = 1 × 1010𝑀−1𝑠−1                                         (4-7) (𝑁𝑂3∙ )2− +𝑂2 → 𝑁𝑂3− + 𝑂2∙−                         𝑘 = 1.6 × 108𝑀−1𝑠−1                                       (4-8)  Figure 4.2 Effect of dissolved oxygen on nitrite formation  00.20.40.60.811.21.41.61.80 300 600 900 1200 1500 1800nitrite concentration (mg/L)UV fluence(mJ/cm2)DO=0.4 ppm DO=9.0 ppm43  Reaction (4-8) also occurs, resulting in the decreased formation of nitrite. However, based on the reported rate constants, reactions (4-4) and (4-5) should be more dominant than reaction (4-8) in the process of nitrite formation. In addition, the strong absorption of VUV radiation by O2(aq) could modify the light field inside the irradiated system; hence, higher DO concentration leads to lower absorption of photons by nitrate, which could also contribute to the decreased nitrite formation. The overall interactions of DO and the pathways involved in the process are shown in Figure 4.3. Of important note is that the impact of DO on the formation of OH radicals, which are the most effective species for the degradation of micropollutants, is unclear. On the one hand, the formation of ∙OH can be decreased due to the reduced VUV absorption by water. On the other hand, given that DO mainly reacts with ·H, the recombination between ∙H and ∙OH is inhibited, resulting in an increased formation of ∙OH. It is speculated that the decreasing formation of ∙OH is insignificant, since nitrite is strongly affected by ∙OH and the reduced formation of ∙OH should lead to an increase of nitrite, which is conflicting with our results. Overall, injecting air/oxygen and increasing DO could present itself as an effective strategy to decrease nitrite formation during the application of VUV photolysis for micropollutant removal.   Figure 4.3 Possible interaction of nitrate and O2(aq) in VUV photolysis  44  4.2.3 Effects of DOC NOM as one of the major sources of DOC is already found to impact nitrite formation under UV254 or other UV-based AOPs due to its high UV absorbance (Sharpless and Linden, 2001; Thomson and Vmlm, 2004). Figure 4.4 shows the effect of DOC on nitrite formation under VUV photolysis. The results show that the presence of DOC leads to increased concentration of nitrite, and the higher concentration of DOC, the greater formation of nitrite. As seen in Table 4.4, the pseudo-first order rate constant of the sample N10 DOC6 (NO3--N at 10mg/L and DOC-C at 6 mg/L) is 2.4 times greater than that of the sample N10, which means DOC has a large effect on nitrite formation. There are two likely explanations for this phenomenon. First, NOM is known as a strong OH radical scavenger which competes with nitrite for OH radicals. Hence, reaction (4-9) is blocked resulting in an increase in nitrite concentration (Mack and Bolton, 1999).  𝑁𝑂2− +∙ 𝑂𝐻 → 𝑁𝑂2∙ + 𝑂𝐻−                            𝑘 = 1.0 × 1010 𝑀−1𝑠−1                                    (4-9) 𝑁𝑂𝑀 +∙ 𝑂𝐻 → [𝑁𝑂𝑀]∗ + 𝑂𝐻−                    𝑘 = 2.3 × 104 (𝑚𝑔 𝐶/𝐿)−1𝑠−1                        (4-10) Another hypothesis for the increased formation of nitrite is through the VUV photolysis of NOM, since NOM has large molecular structure and contains a small portion of N-based group functions (Westerhoff and Mash, 2002). However, this second hypothesis is unlikely, based on our further investigation. Using a control test involving irradiation of NOM solution (DOC at 6 mg/L), it was determined that nitrite formation was below the detection limit of 10 µg/L.  Considering the effect of VUV photon fraction, the presence of DOC is assumed to reduce the direct photolysis of nitrate since DOC highly absorbs 185 nm photons with the molar absorption coefficient of 1402 M-1cm-1 (Serrano Mora and Mohseni, 2018). Thus, DOC can compete with nitrate for VUV photons. Nonetheless, the relative absorption of VUV photons by 45  DOC compared to that by nitrate is not significant due to the very large molecular weight of NOM (refer to Appendix B Table S3 for detailed calculation).  Table 4.3 Overview of reaction rate constants between hydroxyl radical and natural organic matter NOM Source 𝑘𝑇𝑂𝐶,∙𝑂𝐻 (L mg-1s-1) Reference Average of sixteen NOM isolates 3.00 × 104 (Westerhoff et al., 1999) Average of five surface water DOM 2.30 × 104 (Brezonik and Fulkerson-brekken, 1998) Suwannee River Aquatic NOM 1.14 × 104 (Sarathy, 2009) Suwannee River Fulvic Acid 2.70 × 104 (Goldstone et al., 2002) Suwannee River Humic Acid 1.90 × 104 (Goldstone et al., 2002)  According to the above, the major impact of NOM is related to its OH radical scavenging effect. Table 4.3 shows the reaction rate constants of hydroxyl radical and NOM determined by several studies. All the suggested rate constants of 𝑘𝐷𝑂𝑀,∙𝑂𝐻 are in the order of 104 (mg of C/L)-1s-1. Assuming an average molecular weight of 3000 Da for NOM (Wang et al., 2017), the value of 𝑘𝐷𝑂𝑀,∙𝑂𝐻 for reaction (4-10) will be in the order of 1010 M-1s-1, which is competitive with the reaction of nitrite and OH radical. 46   Figure 4.4 DOC effect on nitrite formation as a function of UV fluence (nitrate as N at 10 mg/L, DOC at 2, 4, 6 mg/L)  It is important to note that NOM can scavenge not only free ∙OH, but also the ∙OH inside the solvent cage of [∙OH + NO2∙] (Daniels et al., 1968), which is formed by photolysis of nitrate (pathway 3 in Figure 2.4) (Mark et al., 1996). These two species are initially surrounded by a cage of water molecules. The scavenging of ∙OH in the cage can inhibit their recombination and enhance the generation of NO2∙ very efficiently (Mack and Bolton, 1999).  To confirm the OH radical scavenging effect of NOM and its impact on nitrite formation, tert-butanol (TBA) was selected as another known OH radical scavenger in the experiments (Figure 4.5). The rate constant of TBA reacting with OH radical is 6×108 M-1s-1 (Buxton et al., 2000). The concentration of TBA was chosen at 2mM, which is known to be a sufficient radical scavenging concentration. The pseudo-first order rate constants of DOC contained solution for nitrite formation are shown in Table 4.4. The presence of TBA prominently increases the rate 00.511.522.530 300 600 900 1200 1500 1800nitrite concentration (mg/L)UV fluence(mJ/cm2)N10 N10 DOC2 N10 DOC4 N10 DOC647  constant of nitrite formation compared with the solution of no TBA. Since TBA as an OH radical scavenger also competes with nitrite, the concentration of OH radical is significantly reduced by the contribution of TBA. Thus, reaction (4-9) is further hindered and nitrite concentration is increased. It is important to note that the absorption coefficient of tert-butanol at 185 nm was measured in this study (refer to Chapter 3.3.5), as 72.77 M-1cm-1 at 20 ℃, which does not significantly affect the photon absorption fraction and radical distributions. Based on these results, it is concluded that NOM is strong enough to compete with nitrite for OH radicals and increase the formation of nitrite, and the presence of NOM in nitrate contaminated water impacts the quality of treated water with respect to nitrite formation.  Figure 4.5 DOC effect on nitrite formation in presence of tert-butanol as a function of UV fluence (nitrate as N at 10 mg/L, DOC various from 2-6 mg/L, tert-butanol at 2mM)   00.511.522.530 200 400 600 800 1000 1200 1400 1600nitrite concentration (mg/L)UV fluence(mJ/cm2)N10 TBA2mMN10 DOC2 TBA2mMN10 DOC4 TBA2mMN10 DOC6 TBA2mM48  Table 4.4 Pseudo-first order rate constant of nitrite generation of DOC contained solutions Water matrix Pseudo-first order rate constant (mg/L/min) DOC DOC with 2mM TBA N10 1.0 × 10−2 2.4 × 10−2 N10 DOC2 1.8 × 10−2 2.7 × 10−2 N10 DOC4 2.1 × 10−2 2.7 × 10−2 N10 DOC6 2.4 × 10−2 2.7 × 10−2  4.2.4 Effects of DIC Bicarbonate is a major dissolved inorganic carbon species at neutral pH, which often influences the efficacy of the treatment processes. The global average concentration of bicarbonate in various surface waters ranges widely, from <5 mg L-1 to 300 mg L-1 (Stumm, W., 1970; Wetzel, 2001). As shown in Figure 4.6, the presence of DIC (in the form of bicarbonate) at low concentrations had little or no impact on nitrite formation, and only at very high DIC concentrations, it was observed to decrease the formation of nitrite. DIC is likely involved in the mechanism of nitrite formation in two different ways that bring opposing influences. On the one hand, bicarbonate can react with and scavenges ∙OH (Buxton et al., 1988): 𝐻𝐶𝑂3− +∙ 𝑂𝐻 → 𝐶𝑂3∙− + 𝐻2𝑂                           𝑘 = 8.5 × 106 𝑀−1𝑠−1                                  (4-11) 𝐶𝑂32− +∙ 𝑂𝐻 → 𝐶𝑂3∙− + 𝑂𝐻−                           𝑘 = 3.9 × 108 𝑀−1𝑠−1                                  (4-12) Accordingly, one might expect that DIC would lead to increased formation of nitrite since the presence of bicarbonate can compete with OH radicals, in reacting with nitrite. This phenomenon, however, was not evident from the results obtained (Figure 4.6). Since the reaction 49  of ∙OH with HCO3- is much slower than that with CO32-, the OH radical scavenging effect might be overestimated.  On the other hand, CO2(aq) can rapidly react with peroxynitrite ions (Lymar and Hurst, 1995), according to reaction (4-13):   𝑂𝑁𝑂𝑂− + 𝐶𝑂2(𝑎𝑞) → 𝑂𝑁𝑂𝑂𝐶𝑂2−                       𝑘 = 3 × 104 𝑀−1𝑠−1                                  (4-13) A high concentration of bicarbonate can promote the formation of CO2(aq) and contribute to reaction (4-13), thereby reducing the concentration of ONOO- which could play a role in the formation of nitrite (refer to pathway 2 Figure 2.4). Stumm and Morgan (1970) presented the equilibrium distribution of solutes in aqueous carbonate solution. For the pure NaHCO3 solution, the concentration of 𝐶𝑂2(𝑎𝑞)  was increased roughly from 10−6~10−7𝑀  to 2.4 × 10−4𝑀 when DIC (NaHCO3-C) increased from 4 to 48 mg/L, respectively, based on the solubility coefficients of CO2 in NaHCO3 solution (Wong et al., 2005). Thus, it is confirmed that CO2(aq) increases with the increase of DIC at neutral pH, thereby facilitating reaction (4-13) and obstructing pathway (2) (Figure 2.4), and finally decreasing the formation of nitrite.  50   Figure 4.6 DIC effect on nitrite formation as a function of UV fluence (nitrate as N at 10 mg/L, NaHCO3 as C at 4, 12 and 48 mg/L)  Another possible mechanism, which may influence nitrite concentration in the solution with higher concentration of bicarbonate, is that involving ∙CO3- formed either in reaction (4-11) or (4-12).  The formed ONOOCO2- can yield NO2∙ and ∙CO3- (Goldstein and Czapski, 1998; Lymar and Hurst, 1998; Sharpless and Linden, 2001), which can further produce CO2(aq) and NO3- (Buxton et al., 1988): 𝑂𝑁𝑂𝑂𝐶𝑂2−30%→  𝑁𝑂2 ∙ +𝐶𝑂3∙−𝑘=1.0×109 𝑀−1𝑠−1→              𝐶𝑂2 + 𝑁𝑂3−                                                     (4-14) The produced CO2 from reaction (4-14) can also contribute to reaction (4-13) and decrease nitrite formation. It is known that carbonate radical is a common product in photoreactions; however, unlike OH radical, the species ∙CO3- is a highly selective reactant with few exceptions 00.20.40.60.811.20 300 600 900 1200 1500 1800nitrite concentration (mg/L)UV fluence(mJ/cm2)N10 N10 DIC4 N10 DIC12 N10 DIC4851  (Chen and Hoffman, 1973; Larson and Zepp, 1988). So far, except reaction (4-14), there are no reactions of carbonate radical and nitrate/nitrite ions found in the open literature.  To further confirm the hypotheses on the negligible impact of OH radical scavenging and significant effect of CO2(aq), nitrite formation was monitored in a solution containing TBA. As shown in Figure 4.7, at very high concentration of DIC, nitrite formation was somewhat suppressed (similar to the results in Figure 4.6), which confirmed the non-negligible effect of CO2(aq) on reducing nitrite formation. Nonetheless, comparing the results presented in Figures 4.6 and 4.7, as well as the pseudo-first order rate constants of all the DIC solutions (details presented in Table 4.5), indicated that the presence of TBA significantly increased the formation of nitrite. This further underscores the significance of OH radical scavenging effects of TBA (and DOC) and their subsequent impact on increased nitrite formation. On the other hand, the results show that bicarbonate does not impact nitrite formation.    Figure 4.7 DIC effect on nitrite formation in presence of Tert-Butanol as a function of UV fluence (nitrate as N at 10 mg/L, NaHCO3 as C from 4 to 48 mg/L, tert-butanol at 2mM) 00.511.522.50 200 400 600 800 1000 1200 1400 1600nitrite concentration (mg/L)UV fluence(mJ/cm2)N10 TBA2mMN10 DIC4 TBA2mMN10 DIC12 TBA2mMN10 DIC48 TBA2mM52   Table 4.5 Pseudo-first order rate constant of nitrite generation of DIC contained solutions Water matrix Pseudo-first order rate constant (mg/L/min) DIC DIC with 2mM TBA N10 1.0 × 10−2 2.4 × 10−2 N10 DIC4 1.0 × 10−2 2.3 × 10−2 N10 DIC12 1.1 × 10−2 2.3 × 10−2 N10 DIC48 0.8 × 10−2 1.8 × 10−2  4.3 Summary In this work, we have demonstrated that the formation of nitrite under VUV photolysis of nitrate-rich water is related to UV fluence, initial nitrate concentration, and the presence of DOC, DIC and DO. The rate of nitrite formation is initially high, but it plateaus with increased irradiation time (i.e., fluence). This brings a great possibility to guide VUV applications for better controlling nitrite formation, based on the initial nitrate concentration, light intensity. DOC has the greatest impact on the formation of nitrite, and its presence leads to an increased nitrite formation by scavenging OH radicals. DIC, on the other hand, has little impact and leads to a slight reduction in nitrite formation at extremely high concentration. DO has a positive role in reducing nitrite formation and brings the opportunity to broaden VUV applications for treating micropollutants in nitrate-rich water.     53  Chapter 5: Effect of Chloride on the Formation of Nitrite during the Vacuum UV Photolysis of Nitrate-contaminating Water 5.1  Introduction VUV has been demonstrated to be a simple and effective process for the degradation of micropollutants (Zoschke et al., 2014). Many micropollutants, such as microcystin-LR (Chintalapati and Mohseni, 2018), atrazine (Duca et al., 2017) p-CBA (Han et al., 2004) carbamazepine (Zhu et al., 2019), etc. have been demonstrated to be degraded efficiently under VUV. However, one of the major concerns in VUV applications is the potential formation of nitrite if the water contains nitrate. Nitrate is an inorganic ion present in some water sources, especially in farming communities due to the use of fertilizer and manure. Both nitrate and nitrite are toxic in the human body; they result in the formation of methemoglobin, which causes an inability to transport oxygen to the tissues (Irina et al., 2014). The United States Environmental Protection Agency (USEPA) enforces the drinking water standard of nitrate and nitrite, expressed as maximum contaminant levels (MCL), at 10 mg nitrate-N L-1 and 1 mg nitrite-N L-1, respectively. Hence, understanding the potential formation of nitrite during the VUV irradiation of nitrate-containing water is essential, in order to ensure the quality of VUV treated drinking water and to provide essential information for the applications of this technology. Nitrite, as an undesirable by-product, is formed through a complex set of reactions when water containing nitrate undergoes VUV photolysis. The primary mechanisms of nitrite formation are the direct photolysis of nitrate (Mack and Bolton, 1999) and the oxidation of nitrate by the reactive species produced by VUV photolysis of water, i.e., H●, 𝑒𝑎𝑞−  and HO● (Gonzalez and Braun, 1995) (refer to Chapter 2.3.3). Once nitrite is formed, it can decrease the efficiency of the 54  VUV process by scavenging HO● with a high reaction rate constant, thereby limiting the reaction between HO● and target contaminants (Buxton et al., 1988). 𝑁𝑂2− + 𝐻𝑂• → 𝑁𝑂2• + 𝑂𝐻−                    𝑘 = 1.0 × 1010 𝑀−1𝑠−1                                            (5-1) Similar to HO●, chloride ion (Cl⁻) and chlorine radical (Cl●) have been speculated to impact nitrite formation due to their closely associated reactions with nitrate, nitrite, and other formed relevant species that may be involved under UV-AOPs. So far, there has been no study in the open literature on the impact of chloride on the formation of nitrite. This research aimed to fill this knowledge gap, by investigating the effect of chloride on nitrite formation under VUV photolysis and VUV-based AOP.  Chloride ions (Cl⁻) are one of the major solutes in water with impacts on the degradation efficiency of contaminants during VUV-AOPs. On the one hand, Cl⁻ strongly absorbs VUV photons due to its high molar absorption coefficient of 3500 M-1cm-1 (Weeks and Gordon, 1963). Thus, the presence of Cl⁻ will decrease the VUV photon fraction absorbed by water, resulting in a decreased formation of HO●. On the other hand, the VUV photolysis of Cl⁻ in water produces a suite of reactive oxidants including Cl●, Cl2●⁻ , and HO●, which could increase the efficiency of the process at degrading the target micropollutant (Jortner et al., 1964; Dainton and Fowles, 1965; Hasegawa and Neta, 1978; Buxton et al., 1998, 2000). 𝐶𝑙−185 𝑛𝑚→     𝐶𝑙• + 𝑒𝑎𝑞−                                           𝜑 = 0.4                                                           (5-2) 𝐶𝑙 ∙ +𝐶𝑙− ⇌ 𝐶𝑙2•−                     𝑘 = 1.4 × 105 𝑀−1𝑠−1                                   (5-3) 𝐶𝑙•/𝐶𝑙2•− + 𝐻2𝑂 ⇌ H+ + 𝐻𝑂𝐶𝑙•− ⇌ 𝐶𝑙− + 𝐻𝑂• + 𝐻+                           (5-4)  Many studies suggested that Cl● could act as an effective oxidant to treat organics or micropollutants. The redox potential of Cl● (E=2.4 V) (Armstrong et al., 2013) is close to that of 55  HO● (E=1.8-2.7 V) (Guan et al., 2011). Cl● is considered as a more selective oxidant, compared with HO●, which also has higher reactivity towards alcohols and organic acids (Hasegawa and Neta, 1978), such as benzoic acid and phenol (Fang et al., 2014; Guo et al., 2017). It is also speculated that Cl● has a higher reaction rate with Suwannee River NOM (SRNOM) than the rate of HO● with SRNOM (Furatian and Mohseni, 2018a). Thus, the presence of chloride and organics like NOM, which can affect the distribution of radical species and further impact the formation of nitrite, were considered in this study.   Figure 5.1 Possible mechanisms and reaction pathways involving chloride ions and formation of nitrite during the photolysis of water containing nitrate (Dash lines mean that conversion happen in multiple reactions) Figure 5.1 summarizes the proposed pathways involving chloride, nitrate, and nitrite during the VUV irradiation. A significant challenge around the study of the effect of chloride is the interconnectedness of the contribution of HO● and Cl● on the formation of nitrite. A possible strategy to overcome this involves the use of acetate and acetone. Buxton et al. (2000) noted acetate and acetone for their preference towards HO● and Cl●, respectively, with significant differences in 56  the rate constants as shown in Table 5.1. Hence, they can be used as HO● or Cl● scavengers to further analyze the reaction pathways and mechanisms.  Table 5.1 Comparison of HO● and Cl● reactivities with acetate and acetone Scavenger (S) 𝑘𝐶𝑙,𝑆 (𝑀−1𝑠−1) 𝑘𝑂𝐻,𝑆 (𝑀−1𝑠−1) 𝑘𝐶𝑙,𝑆 /𝑘𝑂𝐻,𝑆 Ref. Acetate (CH3COO-) 37× 108 0.75× 108 50 (Buxton et al., 2000) Acetone (CH3COCH3) <0.05× 108 1.1× 108 <0.045 (Buxton et al., 2000)  Table 5.2 Some of the main reactions involved in the VUV photolysis of Cl- and nitrate No. Reaction k value (M-1s-1) Ref. 5 𝐶𝑙− + 𝑁𝑂3• → 𝑁𝑂3− + 𝐶𝑙• k=1.7×107 (Herrmann et al., 1999) 6 𝐶𝑙• + 𝐶𝑙− → 𝐶𝑙2•− k=8×109 (Nagarajan and Fessenden, 1985) 7 𝐶𝑙2•− + 𝑁𝑂2− → 2𝐶𝑙− + 𝑁𝑂2• k=2.5×108 (Hasegawa and Neta, 1978) 8 𝐶𝑙− + 𝐻𝑂• → 𝐶𝑙𝑂𝐻•− k=4.3×109 (Fang et al., 2014) 9 𝐶𝑙• + 𝑂𝐻− → 𝐶𝑙𝑂𝐻•− k=1.8×1010 (Fang et al., 2014) 10 𝐶𝑙𝑂𝐻•− → 𝐻𝑂• + 𝐶𝑙− k=6.1×109 (Fang et al., 2014) 11 𝐶𝑙• + 𝑇𝐵𝐴 k=6.2×108 (Buxton et al., 2000) 12 𝐻𝑂• + 𝑇𝐵𝐴 k=6×108 (Buxton et al., 1998) 13 𝐶𝑙2•− + 𝑇𝐵𝐴 k=7×102 (Fang et al., 2014) 14 𝐶𝑙• + 𝑁𝑂𝑀 k=1.3×104 (mg/L)-1s-1 (Fang et al., 2014) 57  15 𝐻𝑂• + 𝑁𝑂𝑀 k=2.3×104 (mg/L)-1s-1 (Brezonik and Fulkerson-brekken, 1998) 16 𝐶𝑙• + 𝑁𝑂3− k=1×108 (Buxton et al., 2000) 17 𝐶𝑙• + 𝑁𝑂2− k=5×109 (Buxton et al., 2000) 18 𝐶𝑙• + 𝐶𝐵𝑍 k=5.6×1010 (Wang et al., 2016) 19 𝐻𝑂• + 𝐶𝐵𝑍 k=2.98×109 (Huber et al., 2003)  Table 5.2 presents some of the additional reactions along with their rate constants, including the reactions of chloride and its radical forms with nitrate, nitrite, NOM, tert-butanol (TBA) and carbamazepine (CBZ). By investigating the effect of chloride and the mechanistic insights on the formation of nitrite during VUV photolysis, this study has further established the importance of direct photolysis of VUV, the formation of Cl● and its competition with HO●, the impact of chloride and NOM on nitrite formation, and the role of Cl● on degradation of target contaminants.   5.2 Results and Discussion 5.2.1 The Effect of Chloride The first stage involved carrying out some control experiments with UV 254 photolysis of nitrate and chloride-containing water to establish nitrite formation under UV 254 nm irradiation (refer to Chapter 3.3.1). The results indicated that the formation of nitrite after 1200 mJ/cm2 irradiation was insignificant and lower than IC detection limit of 0.01 mg/L.  58   Figure 5.2 Effect of chloride on the formation of nitrite (nitrate-N at 10 mg/L as control, chloride is increased from 0 to 80 mg/L) Chloride concentrations in natural water are normally between 1 and 100 mg/L (Hunt et al., 2012). In our experiments, chloride was in the range between 0 and 80 mg/L, and nitrate as N was kept constant at 10 mg/L. Figure 5.2 shows that the formation of nitrite decreased with the increase of chloride concentration. This phenomenon can in part be explained by the strong absorption of 185 nm by chloride. As shown in Table 5.3, the 185 nm absorption coefficient of chloride is very high, and comparable to that of nitrate. Hence, any change in the concentration of chloride can significantly affect the fraction of photons absorbed by different species ( Table 5.4, also refer to Appendix B Table S.4 for detailed calculation) as well as the concentrations of HO● and Cl● formed in the system. With the increase of chloride, 185 nm photon absorption shifts significantly from nitrate to chloride, thereby reducing the formation of nitrite through direct photolysis of nitrate. Another possible mechanism is that a greater fraction of photons absorbed by chloride leads to the increased formation of Cl●. The mechanism shown in 00.20.40.60.811.21.40 500 1000 1500Nitrite concentration (mg/L)UV Fluence (mJ/cm2)N10 N10 Cl10 N10 Cl20 N10 Cl30 N10 Cl8059  Figure 5.1 suggests that the formed Cl● mainly converts to HO● and Cl2●⁻ which can further react with nitrite to generate NO2●. Buxton et al. (1998) found that Cl2●⁻ is heavily favored although it has a self-consuming cycle and reforms to Cl● and Cl⁻. Thus, the increased HO● and Cl2●⁻ result in decreased formation of nitrite under VUV photolysis (Han and Mohseni, 2020).  Table 5.3 Absorption coefficient at 185nm of different solutes at 25 ℃ Substance Absorption Coefficient Ref. H2O 1.8 cm-1 (Weeks and Gordon, 1963) Nitrate 4779 M cm-1 (Serrano Mora and Mohseni, 2018) Chloride 3063 M cm-1 (Serrano Mora and Mohseni, 2018) DOC 1402 M cm-1 (Serrano Mora and Mohseni, 2018)  Table 5.4 Fractions of 185 nm photons absorbed by different species as the concentration of chloride increases in water   N10 N10 Cl10 N10 Cl20 N10 Cl30 N10 Cl80 Nitrate 0.65 0.54 0.46 0.41 0.25 Chloride 0 0.17 0.29 0.38 0.62 H2O 0.35 0.29 0.25 0.21 0.13    Note: Nitrate concentration (N10) is constant at 10 mg N/L The impact of Cl● on the formation of nitrite is less clear. On the one hand, Cl● can react with both nitrate and nitrite with comparable rate constants. One the other hand, Cl● can convert to HO● and Cl2●⁻ which are also reactive with nitrite. Thus, it needs further work to separate the impact of radical species on nitrite formation. 60  5.2.2 Acetate and Acetone as Chloride and Hydroxyl Radical Scavengers To further investigate the mechanisms of Cl● and HO● reactions, and their contribution to nitrite formation, acetate and acetone were selected as Cl● and HO● scavengers, respectively. The rate constants in Table 5.1 indicate the higher reactivity of acetate with Cl● vs. the greater reactivity of acetone with HO●. Therefore, sufficient concentrations of either acetate or acetone in the solution allow for scavenging of Cl● or HO●, respectively, and maintaining their concentrations at steady state. The optimal concentration of these two radical scavengers was determined at 50 mg/L by testing the nitrite formation under various acetate and acetone concentrations, as described in Figure 5.3.  The nitrite concentration was initially increasing with the increase of acetate and acetone concentration. Acetate and acetone both react with HO• and compete with nitrite for HO•, therefore, the presence of acetate and acetone can hinder the reaction of nitrite and HO• which result in the increase of nitrite. However, nitrite tends to decrease with the increase of acetate/acetone concentration over 50 ppm, which may due to the over-concentrated solution of dissolved organics; consequently, the presence of high acetate/acetone concentration reduce the photolysis of nitrate and lead to the decrease of nitrate degradation. Hence, acetate and acetone at 50 ppm were selected as optimal concentration to maximally scavenge Cl• and HO•.  61   Figure 5.3 Nitrite formation vs. acetone/acetate concentration (nitrate-N 10 ppm, NaCl-Cl 30 ppm with VUV 1 h radiation)  Figure 5.4 The influence of chloride in the presence of acetone (Acetone used at 50 mg/L, N10 respect to nitrate-N at 10 mg/L, chloride concentration is various from 0-30 mg/L) Figure 5.4 shows that in the presence of acetone, the increase of chloride has no significant influence on the formation of nitrite. This is in line with the role of acetone in scavenging HO● and 00.20.40.60.811.20 20 40 60 80 100 120Nitrite concentration (ppm)Concentration (ppm)acetone acetate00.20.40.60.811.21.40 500 1000 1500Nitrite concentration (mg/L)UV Fluence (mJ/cm2)N10 N10 Cl10 N10 Cl20 N10 Cl3062  maintaining its concentration at steady state. Note, the formation of HO● is primarily due to the VUV photolysis of water and transformation from Cl●, since Cl● converts to HO● by reaction (5-4) or reactions (5-9) and (5-10) with high rate constants. With the increase of chloride, VUV photons absorbed by water will decrease, thus the formation of HO● from the VUV photolysis of water will decrease. At the same time, since HO● is consumed by acetone, reactions (5-4), (5-9), and (5-10) will transform more of the Cl● to HO● in order to maintain steady state concentration of HO●. Thus, the increased concentration of Cl●, resulting from the increase of chloride, is indirectly consumed by acetone, and the formation of Cl2●⁻ is also limited as all the reactions are towards the formation of HO●. Therefore, in the presence of acetone, nitrite formation is primarily governed by HO● and since its concentration is maintained constant, no change in nitrite formation is observed. In addition, Cl● reacts with nitrate and nitrite at similar rates. Table 5.5 lists the k’ values of reactions (5-16) and (5-17), which indicate C𝑁𝑂3− ∗ 𝑘𝐶𝑙•,𝑁𝑂3− and C𝑁𝑂2− ∗ 𝑘𝐶𝑙•,𝑁𝑂2− are of the same magnitude. In other words, the formation of nitrite from reaction (5-16) is countered by reaction (5-17), which leads to the removal of nitrite. Hence, the impact of Cl● on nitrite formation is insignificant.  Table 5.5 Comparison of reaction rates of chlorine radical with nitrate and nitrite Solutes Conc. (mg/L) Reaction # k (M-1s-1) k’=k*C (s-1) Nitrate 45 5-16 1×108 0.71×105 Nitrite 0-1 5-17 5×109 (0-1.09)×105  63   Figure 5.5 The influence of chloride in presence of acetate (acetate used at 50 mg/L, nitrate-N are constantly at 10 mg/L, chloride is 0 to 30 mg/L) In the presence of acetate, however, nitrite formation increased with the increase of chloride concentration (Figure 5.5). With acetate scavenging Cl●, the concentration of this radical stays constant in the solution, and this consequently affects the 4 pathways shown in Figure 5.1, and described through reactions (5-3), (5-4), (5-16) and (5-17). That is, the impacts of Cl● as well as Cl2●⁻, generated from Cl●, on nitrite formation are minimized. At the same time, the increase of chloride which strongly absorbs 185 nm results in decreased generation of HO● from the photolysis of water, since photon absorption shifts from water to chloride. This decreases the formation of HO●, reaction (5-1), which in turn increases the formation of nitrite.  These results confirmed that the increase of chloride decreased nitrite formation, mainly due to the increased VUV absorption of chloride and decreased photolysis of nitrate. The generated Cl● itself has an insignificant impact, even though it indrectly leads to decreases in nitrite formation, by generating HO● (as shown in Figure 5.1). 00.20.40.60.811.21.41.61.820 500 1000 1500Nitrite concentration (mg/L)UV Fluence (mJ/cm2)N10 N10 Cl10 N10 Cl20 N10 Cl3064  5.2.3 CBZ as Probe Compound Carbamazepine (CBZ), a micropollutant found in different water sources or wastewater effluents (Zhang et al., 2008), was used as a probe compound to further assess the impact of chloride and the potential formation of nitrite. Previous research (Furatian and Mohseni, 2018a) suggested that the increase of chloride positively affected the degradation of CBZ during the VUV AOP. As discussed earlier, the presence of chloride leads to decreased nitrite formation during the VUV irradiation. To verify the combined effect of chloride on the formation of nitrite and degradation of CBZ, water containing both CBZ and nitrate was irradiated by VUV at different fluences and under various chloride concentrations. In addition, acetone as HO● scavenger was used to eliminate the impact of HO● under different chloride concentrations. Figure 5.6 (a) and (b) show that the increase of chloride has no impact on nitrite formation but promotes the degradation of CBZ. During VUV photolysis, the chlorine radical forms quickly in water since chloride has a high quantum yield of 0.43 (Dainton and Fowles, 1965) and a molar absorption coefficient of 3300 M-1cm-1 (Weeks and Gordon, 1963). The formed chlorine radical prefers to react with CBZ instead of nitrate or nitrite. The rate constants of chlorine radical with nitrate and nitrite (reactions (5-16) and (5-17), respectively) are much lower than that with CBZ (reaction (5-18)), as listed in  Table 5.2. In terms of k’ (k*C), the value for CBZ is 7.1×105 s-1 which is around one order of magnitude higher than that for nitrate and nitrite shown in Table 5.5.  65    Figure 5.6 Formation of nitrite and degradation of CBZ in nitrate and chloride contained solution (the initial solution contains nitrate-N at 10 mg/L, acetone at 50 mg/L, CBZ at 3 mg/L and chloride at 0 and 80 mg/L)  These results agree with and confirm earlier results (Furatian and Mohseni, 2018a) in that chloride, and in turn chlorine radical, positively impacts the degradation of CBZ, even in the presence of nitrate. Further, chlorine radicals do not lead to increased formation of nitrite, which 00.20.40.60.811.21.41.60 10 20 30 40 50 60nitrite concentration (mg/L)radiation time (min)N10 acetone50 CBZ3 Cl0 N10 acetone50 CBZ3 Cl80-0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.100 10 20 30 40 50 60Ln (C/C0)radiation time/minN10 acetone50 CBZ3 Cl0 N10 acetone50 CBZ3 Cl80(b)(a) 66  brings a potential opportunity for VUV, in the presence of promoters that form chlorine radicals (e.g., VUV- or UV- chlorine), for the treatment of micropollutants without an increased formation of harmful byproducts such as nitrite.  5.3 Summary This chapter has investigated the effect of chloride during VUV photolysis, and the presence of chloride is found to be sensitive to the formation of nitrite in nitrate-containing water. An increase in chloride concentration significantly reduced nitrite formation. This is due to the relatively high VUV absorption of chloride, which competes for VUV photons with nitrate, and the decreased nitrate VUV photon fraction results in a lower formation of nitrite. However, Cl•, which formed under VUV irradiation, has shown a minor impact on nitrite formation, while •OH can significantly reduce nitrite formation due to its high reaction rate constant with nitrite. Carbamazepine (CBZ) as a typical micropollutant, was used to analyze the effect of Cl• on both the degradation of CBZ and the formation of nitrite. Cl• showed to significantly increase the degradation of CBZ, but it had little impact on the formation of nitrite.    67  Chapter 6: Impact of Sulfate on the Formation of Nitrite Under VUV Photolysis of Nitrate Rich Water 6.1 Introduction VUV has shown its high efficiency in treating micropollutants in water, compared to other UV based AOPs such as UV/H2O2, UV/O3, etc. (Chintalapati, 2017; Duca et al., 2017; Zhu et al., 2019). This is due to the high absorption of water and high quantum yield of •OH formation at such short wavelength. In addition, as VUV is a chemical-free process, several issues such as bromide formation, waste/residual treatment, and quenching are no longer emerged. However, during VUV photolysis, nitrite formation in nitrate-rich water is a potential challenge which may hinder the VUV technology and applications (Thomson and Vmlm, 2004; Buchanan et al., 2006; Kutschera et al., 2009). Nitrite has been found to be a toxic substance to infants and results in methemoglobinemia. Therefore, water with high nitrate concentration under VUV exposure may pose a significant health risk due to the formation of nitrite. The main pathways of nitrite formation from nitrate involve oxidation by H•, e(aq)- and •OH, and direct photolysis of nitrate, which are reviewed and summarized in Chapter 2.3.3 Figure 2.4 & Table 2.1.  Many solutes have been indicated to impact nitrite formation such as dissolved oxygen, natural organic matter (NOM), chloride and sulfate (Furatian and Mohseni, 2018b). This chapter is mainly focused on the effect of sulfate while the impact of other water solutes has been elaborated in previous chapters, including the impact of sulfate and the consequent formation of sulfate radical on nitrite formation during VUV photolysis. The mechanistic pathways of sulfate radical have been established using radical scavenger and degradation of target contaminants.  68  Sulfate, as one of the major inorganics in natural water, has been found to produce 𝑆𝑂4•− under VUV irradiation (Dainton and Fowles, 1965; Dogliotti and Hayon, 1967), shown in reaction (6-1).  𝑆𝑂42−(𝑎𝑞)ℎ𝑣→ 𝑆𝑂4.− + 𝑒𝑎𝑞−                                              ∅ = 0.64                          (6-1) Here, 𝑆𝑂4•− is a strong oxidant with a redox potential of 2.5-3.1 V, which is even greater than that of the hydroxyl radical (HO•) with a redox potential of 1.8-2.7 V (Guan et al., 2011). Recently, many studies have presented that 𝑆𝑂4•− can react quickly with many organics such as NOM, carbamazepine, atrazine, etc., and inorganics such as chloride, nitrite, and bicarbonate (Lutze et al., 2015; Xie et al., 2015; Lian et al., 2017). In addition, 𝑆𝑂4•− can convert to •OH in alkaline condition (Neta et al., 1988). Due to the high reactivity of sulfate radicals and the additional formation of •OH, it was proposed that a small concentration of 𝑆𝑂4•− is likely sufficient to induce a significant impact on VUV water treatment (Furatian and Mohseni, 2018b). It is also speculated that the formation of nitrite under VUV might be influenced because 𝑆𝑂4•− is very active and can react with nitrite at a relatively high rate constant while it poorly reacts with nitrate (Buxton et al., 1988; Neta et al., 1988). However, the relevant research has not been performed and much uncertainty exists on the real impact of sulfate on nitrite formation.   Figure 6.1 Diagram of sulfate and sulfate radical pathways leading to nitrite formation under VUV photolysis. 69  Figure 6.1 shows the possible pathways of sulfate and sulfate radical on nitrite formation. Sulfate radical is speculated to be involved in three possible pathways: firstly, 𝑆𝑂4•− reacts with nitrite to reduce the formation of nitrite; secondly, 𝑆𝑂4•− can convert to •OH, which itself also can strongly react with nitrite; lastly, 𝑆𝑂4•− is self-consumed and forms back to sulfate with the formation of persulfate anion. The related reactions and their rate constants are listed in Table 6.1. The proposed mechanism and its impact on nitrite formation need to be further investigated.   Table 6.1 The Major Reactions Involving 𝑆𝑂4•− During VUV Photolysis of Nitrate # Reactions Rate constant Reference (2) 𝑆𝑂4.− + 𝑁𝑂3− → 𝑆𝑂42− + 𝑁𝑂3 ∙ 𝑘 = 2.1 × 100𝑀−1𝑠−1 (P.Neta and Huie, 1988) (3) 𝑆𝑂4.− + 𝑁𝑂2− → 𝑆𝑂42− + 𝑁𝑂2 ∙ 𝑘 = 8.8 × 108𝑀−1𝑠−1 (Buxton et al., 1988) (4) ∙ 𝑂𝐻 + 𝑁𝑂2− → 𝑂𝐻− + 𝑁𝑂2 ∙ 𝑘 = 1.0 × 1010𝑀−1𝑠−1 (Mack and Bolton, 1999) (5) 𝑆𝑂4.− + 𝑂𝐻− → 𝑆𝑂42− +∙ 𝑂𝐻 𝑘 = 6.5 × 107𝑀−1𝑠−1 (P.Neta and Huie, 1988) (6) 𝑆𝑂4.− + 𝐻2𝑂 →∙ 𝑂𝐻 + 𝐻𝑆𝑂4− 𝑘 = 660𝑠−1 (Hayon et al., 1972; Yang et al., 2014b) (7) 2𝑆𝑂4∙− → 𝑆2𝑂82− 𝑘 = 4.4 × 108𝑀−1𝑠−1 (Huie and Clifton, 1993) (8) 𝑆𝑂4∙− + 𝑆2𝑂82− → 𝑆𝑂42− + 𝑆2𝑂8∙−  𝑘 = 6.1 × 105𝑀−1𝑠−1 (Yu et al., 2004) (9) 𝑆2𝑂82− 𝑈𝑉→ 2𝑆𝑂4.− ∅ = 2 (Dogliotti and Hayon, 1967; Tang et al., 1988) (10) ∙ 𝑂𝐻 + 𝑇𝐵𝐴 𝑘 = 6 × 108𝑀−1𝑠−1 (Buxton et al., 2000) (11) 𝑆𝑂4∙− + 𝑇𝐵𝐴 𝑘 = 8.4 × 105𝑀−1𝑠−1 (P.Neta and Huie, 1988) (12) 𝑆𝑂4∙− + 𝐶𝐵𝑍 𝑘 = 1.92 × 109𝑀−1𝑠−1 (Matta et al., 2011) (13) ∙ 𝑂𝐻 + 𝐶𝐵𝑍 𝑘 = 2.98 × 109𝑀−1𝑠−1 (Huber et al., 2003)  70  6.2 Results and Discussion 6.2.1 Impact of Sulfate Concentration Figure 6.2 illustrates the impact of sulfate concentration on nitrite formation under VUV AOP. The experimental sulfate concentration (Na2SO4-S) is in the range typical of that in surface water (Fawell et al., 2004), i.e., 5-30 mg/L, with an additional extremely high concentration of 300 mg/L. The results indicate that sulfate has a limited impact on nitrite formation under normal sulfate concentration but leads to slightly reduced nitrite formation when it is extremely high. The major mechanisms during VUV irradiation include direct photolysis and oxidation. Since the low 185 nm absorption of sulfate (𝜀𝑆𝑂42−,185 = 160 𝑀−1𝑐𝑚−1) (Furatian and Mohseni, 2018) is not comparable with the high absorption of water (𝜀𝐻2𝑂,185 = 1.8 𝑐𝑚−1) (Weeks and Gordon, 1963), the presence of sulfate is not considered to impact VUV photon fraction of nitrate and water. Nevertheless, both •OH and 𝑆𝑂4•− as strong oxidants are produced under VUV in the presence of sulfate in water. As discussed in chapter 4, the impact of •OH is to reduce nitrite formation mainly via reaction (6-4); note, the •OH reacts with nitrate at a very low rate constant, while it reacts with nitrite very fast (Gonzalez and Braun, 1995). The impact of 𝑆𝑂4•− is also expected to reduce nitrite formation because sulfate radical shows similar reactivity to •OH: the low reaction rate with nitrate (reaction (6-2)) and high reaction rate with nitrite (reaction (6-3)). Thus, the presence of sulfate was speculated to decrease nitrite.  However, the results show that the reducing impact of sulfate on nitrite formation is negligible. The hypothesis is that the self-consuming cycle of sulfate and sulfate radicals have a major impact on decaying the formation of sulfate radicals. Therefore, although sulfate concentration increased, the self-consuming cycle of 𝑆𝑂4•− and the consequent formation of S2O82- and SO42- can decrease the formation of 𝑆𝑂4•−. Evidence from the literature suggests that most of 71  𝑆𝑂4•− is scavenged by S2O82- via reaction (6-7) and (6-8) (Fang et al., 2012). Thus, the impact of sulfate radicals on nitrite formation might be overestimated. Also, reaction (6-5) suggests that 𝑆𝑂4•− can be converted into •OH. However, it has been reported that the formation of •OH from 𝑆𝑂4•− mainly occurs under alkaline conditions (Hayon et al., 1972) while these experiments were performed under neutral pH. Therefore, the formation of •OH from 𝑆𝑂4•− may not be significant with the increase of sulfate.   Figure 6.2 Impact of different concentrations of sulfate on nitrite formation during VUV irradiation (N10: nitrate-N at 10 mg/L; S5: sulfate-S at 5 mg/L).  6.2.2 Impact of Sulfate Radical To investigate the impact of sulfate radicals, separate (control) experiments were carried out with UV254/persulfate (Na2S2O8). Here, 𝑆𝑂4•− can be produced by the photolysis of persulfate via reaction (6-9) with high quantum yield. At the same time, there is very limited formation of •OH under UV 254 irradiation (Mark et al., 1990). The 1 mM persulfate solution was used as 00.20.40.60.811.20 300 600 900 1200 1500 1800Nitrite formation (mg/L)UV fluence (mJ/cm2)N10 S5 N10 S10 N10 S15 N10 S30 N10 S30072  constant in our experiments. Firstly, water synthetic with nitrate at 40 mg/L was photolyzed under UV/persulfate. The result showed that nitrite formation was lower than the ion chromatograph detection limit (<0.1 mg/L) under a maximum of 1600 mJ/cm2 UV radiation, indicating that the sulfate radicals have little impact on nitrate and do not lead to nitrite formation. This was expected because of the very low rate constant of the reaction between nitrate and 𝑆𝑂4•− (reaction (6-2)). Then, water containing 10 mg/L nitrite and 2mg/L nitrate was photolyzed under the same UV fluences. As shown in Figure 6.3, with the increase of UV fluence, nitrite concentration decreased and nitrate concentration increased significantly. In other words, nitrite transformed to nitrate under UV/persulfate. As only 𝑆𝑂4•− was generated in the UV254/persulfate process, it can be concluded that 𝑆𝑂4•− reacted with nitrite and form nitrite radical (reaction (6-3)). Note, nitrite radical is a precursor of nitrate (Mack and Bolton, 1999). Thus, one could confirm that the formation of 𝑆𝑂4•− promote the formation of nitrate from nitrite.   Figure 6.3 Nitrite oxidation and nitrate formation under UV/persulfate system (Na2S2O8 at 1mM, nitrite at 10 mg/L and nitrate at 2mg/L as initial concentration). 0246810120 300 600 900 1200 1500 1800Concentration (mg/L)UV fluence (mJ/cm2)nitrite nitrate73  However, as observed in Figure 6.1, the impact of sulfate radicals is insignificant under VUV photolysis. To further examine the effect of sulfate radicals on nitrite formation, tert-butanol (TBA), a commonly used •OH scavenger, was applied to eliminate the impact of •OH during VUV photolysis. The rate constant of TBA with •OH is 3 orders of magnitude higher than that with 𝑆𝑂4•− (reaction (6-10) and (6-11)). Therefore, the increase of sulfate leads to an increased formation of sulfate radicals, while •OH concentration remains at steady state in the presence of TBA.  The results in Figure 6.4 (a) show the impact of sulfate radical during VUV irradiation and the consequent formation of nitrite from nitrate. As discussed earlier, in the presence of TBA, sulfate radicals are the dominant species and their increased formation, due to higher sulfate concentration, has little influence on nitrite formation. Figure 6.3 has indicated that sulfate radical leads to reduced formation of nitrite. Therefore, an experiment was conducted with a higher initial concentration of nitrite (at 10 mg/L) and the result is shown in Figure 6.4 (b). As expected, the formation of nitrite decreased with the increase of sulfate concentration. It is confirmed that sulfate radical reduces nitrite formation during VUV irradiation, however, this effect is only pronounced at high initial nitrite concentration.  74     Figure 6.4 Impact of sulfate on nitrite formation in the presence of TBA under VUV radiation: (a) N10: NaNO3-N at 10 mg/L, different sulfate concentrations from S5 to S30 ( Na2SO4-S at 5 to 30 mg/L); TBA at 2mM; (b): N10 (NaNO3 at 45 mg/L); NaNO2 at 10 mg/L; sulfate concentration at 0 and S30)  00.511.522.530 300 600 900 1200 1500 1800Nitrite formation (mg/L)UV fluence (mJ/cm2)N10 N10 S5 N10 S10 N10 S15 N10 S3000.20.40.60.811.20 300 600 900 1200 1500 1800Nitrite formation (mg/L)UV fluence (mJ/cm2)no sulfate S30(a) (b) 75  6.2.3 CBZ as Probe Compound Carbamazepine (CBZ), a model micropollutant detected in water, is a suitable probe compound for further assessing the role of sulfate in water that contains nitrate and the consequent formation of nitrite during the degradation of contaminants. Although the UV/VUV lamp emits at both 254 and 185 nm wavelengths, the degradation of CBZ under 254 nm is negligible since the quantum yield of CBZ at 254 nm is very low (𝜑𝐶𝐵𝑍,254𝑛𝑚 = 0.00067) (Furatian and Mohseni, 2018). So, the primary degradation mechanism for CBZ will be through radical species. CBZ possesses similar rate constants with 𝑆𝑂4•− and •OH (reactions (6-12) and (6-13)) (Huber et al., 2003; Matta et al., 2011). Therefore, it is valuable to investigate whether the presence of sulfate impacts the degradation of CBZ, and to verify whether the formation of nitrite is affected by the treatment of micropollutants. Here again, TBA was used to scavenge •OH and eliminate the influence of •OH under different sulfate concentrations. Figure 6.5 shows that the increase of sulfate enhanced the degradation of CBZ, but had no additional impact on nitrite formation. Since the increase of sulfate under VUV photolysis causes greater formation of 𝑆𝑂4•−, the results confirm that sulfate radical is more reactive with CBZ than it is with nitrite. Fortunately, the presence of sulfate has not increased nitrite formation which is consistent with earlier results shown in Figure 6.2, that nitrite formation is stable with different sulfate concnetration from 0 to 30 mg/L. However, tert-butanol, as OH radical scanvenger, has shown to increase nitrite formation in Figure 6.2 and 6.5 (b). The effect of OH radical scavenger on increasing nitrite formation has been deeply discussed in Chapter 4. The results show a potential opportunity for the VUV technology to provide similar or greater removal of micropollutants in the presence of sulfate, without concerns over the formation of nitrite and negative impacts of water quality.  76    Figure 6.5 Degradation of CBZ and nitrite formation in the presence of sulfate and TBA (Initial nitrate concentration (N10) is at NaNO3-N at 10 mg/L, tert-butanol (TBA) is at 2mM. Sulfate concentration (SO42--S) is at 0 and 30 mg/L (S30). (a). Degradation of CBZ at 2 mg/L. (b) Formation of nitrite)  -0.6-0.5-0.4-0.3-0.2-0.100 10 20 30 40 50 60Ln(C/C0)Radiation time (min)no sulfate S3000.20.40.60.811.21.40 100 200 300 400 500 600 700 800Nitrite formation (ppm)UV fluence (mJ/cm2)no sulfate S30(a) (b) 77  6.3 Summary In this chapter, the impact of sulfate and sulfate radicals on nitrite formation under VUV photolysis of nitrate-rich water has been elaborated. The presence of sulfate during VUV photolysis was originally hypothesized to reduce nitrite formation, because the photolysis of sulfate-containing water produces hydroxyl and sulfate radicals, which both react with nitrite but rarely react with nitrate. However, the experimental evidence confirmed that the presence of sulfate and the formation of sulfate radicals had an insignificant influence on nitrite formation. It was only pronounced at very high sulfate concentration that reduced nitrite concentration. It is speculated that the formation of sulfate radical under VUV photolysis is decayed, which may be due to its self-consuming cycle that leads to the formation of S2O82- and SO42-, rather than react to nitrite. Sulfate radical, which can be generated in VUV/sulfate or UV/persulfate process, has been identified to increase the efficiency of carbamazepine (CBZ), a typical micropollutant, degradation in nitrate-rich water without increasing the formation of nitrite. The experimental evidence provided useful pathways for developing a kinetic model, which would be useful for predicting the radical concentrations and exploring the greater mechanistic insights.    78  Chapter 7: Interactions of Major Organic and Inorganic Water Solutes and their Combined Effect on Nitrite Formation 7.1  Introduction Early chapters showed that dissolved organic matter (DOC) can significantly increase nitrite formation under VUV irradiation, as DOC strongly scavenges •OH and hinders the transformation of nitrite to NO2•, a nitrate precursor. The presence of chloride can reduce nitrite formation as chloride strongly absorbs VUV photons and reduces the photolysis of nitrate. Dissolved inorganic carbon (DIC) and sulfate have a minor effect on nitrate and nitrite concentrations, but their effect is more pronounced at high concentration to reduce nitrite formation, and the formed carbonate radical and sulfate radical showed their positive role on reducing nitrite formation. However, the combined impacts of these water solutes when found together and their associated effects are not fully known. Thus, this chapter is focused on the effect of the water matrix on nitrite formation when at least two solutes are present in the water matrix. The results from this chapter further support and strengthen the data from earlier chapters, and help us better understand the transformation of nitrite from nitrate under conditions that mimic real world applications and consequently help with better strategies towards controlling nitrite formation.   79  7.2 Results and Discussion 7.2.1 Comparison of water solutes on the formation rate of nitrite   Figure 7.1 The rate of nitrite formation as a function of the concentration of individual solutes present in water (initial nitrate as N at 10 mg/L).  The effects of DOC, DIC, chloride, and sulfate have been individually investigated and presented in Chapters 4-6. These results showed that the presence of DOC promotes the formation of nitrite under VUV photolysis, thereby potentially affecting the quality of produced water. Other solutes investigated, i.e. DIC, chloride and sulfate, had either no impact on nitrite formation or reduced the formation of nitrite upon treatment with VUV. To further predict the impact of water solutes at various concentration, a new parameter called nitrite formation factor (NF) is defined to describe the formation rate of nitrite as a result of a single water solute:  y = 0.2024x + 0.6405R² = 0.985900.511.520 2 4 6 8Nitrite formation rate (mg/L/(J/cm2 ))DOC concentration (mg/L)DOCy = -0.0067x + 0.6439R² = 0.976600.20.40.60.80 50 100nitrite formation rate (mg/L/(J/cm2))chloride concentrationchloridey = -0.0022x + 0.6545R² = 0.955400.20.40.60.80 20 40 60nitrite formation rate (mg/L/(J/cm2 ))DIC concentration (mg/L)DICy = -0.0002x + 0.677R² = 0.989700.20.40.60.810 200 400 600 800 1000Nitrite formation rate (mg/L/(J/cm2 ))Sulfate concentration (mg/L)sulfate80  𝑁𝐹 =𝑛𝑖𝑡𝑟𝑖𝑡𝑒 𝑓𝑜𝑟𝑚𝑎𝑡𝑖𝑜𝑛 (𝑚𝑔𝐿)𝑈𝑉 𝑓𝑙𝑢𝑒𝑛𝑐𝑒 (𝐽𝑐𝑚2)×𝑤𝑎𝑡𝑒𝑟 𝑠𝑜𝑙𝑢𝑡𝑒 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛(𝑚𝑔𝐿)     (J/cm2)-1 As shown in Figure 7.1, a positive slope means that the rate of nitrite formation increases with the increase of water solute; conversely, a negative slope indicates that the effect of this water solute is towards reducing nitrite formation. The absolute value of NF means the degree of correlation of nitrite formation and water solutes concentration. The higher NF, the closer correlation of nitrite formation and water solutes concentration. The slope of the linear line is referred to NF of each water solute, as listed in Table 7.1. Based on NF value, the impact of water solutes is quantified and showed their order of chloride (-) > DIC (-) > sulfate (-), which have positive correlations on reducing nitrite formation. DOC also presents a strong relationship with nitrite formation, and its NF absolute value (0.2024) is much higher than other solutes. However, the NF for DOC shows an undesirable correlation with an increase in nitrite formation. Table 7.1 NF value of major water solutes  DOC DIC Chloride Sulfate NF ((J/cm2)-1) +0.2024 -0.0022 -0.0067 -0.0002  So far, the impact of these individual water solutes has been understood, but there is always more than one constituent in real waters. Therefore, it is necessary to know their combined effects and the predominant solutes in water on the formation of nitrite during VUV irradiation.  81  7.2.2 Combined Effects of DOC and DIC Figure 7.2 shows the effect of DOC and DIC, when both present in water, on the formation of nitrite. As shown in Figure 7.2 (a), for a given concentration of DOC, DIC did not show any impact on the concentration of formed nitrite, indeed there was no statistically significant difference between the formation of nitrite in the presence or absence of DIC in water. On the other hand, at a given DIC concentration (Figure 7.2 (b)), increases in DOC led to more nitrite formation, much the same as what was observed in Figure 4.4. All these confirm the earlier conclusion that DIC has negligible effects. At the same time, comparing the results of Figure 4.6 and 7.2 (a) reveals that pseudo-first order rate constants of nitrite formation for the water containing both DOC and DIC, were more than 2 times greater than those for the water with DIC only (Table 7.2). Similarly, comparing the results of Figure 4.4 and 7.2 (b), the formation rates of nitrite are very close as long as the DOC concentration is the same (Table 7.2).   82   Figure 7.2 The effects of combination of DOC and DIC on nitrite formation: (a) Influence of DIC in a solution containing nitrate-N at 10 mg/L (N10) and DOC at 6 mg/L (DOC6); (b) Influence of DOC in a solution containing N10 and DIC at 12 mg/L (DIC12).   00.511.522.530 200 400 600 800 1000 1200 1400nitrite concentration (mg/L)UV fluence(mJ/cm2)N10 DOC6N10 DOC6 DIC4N10 DOC6 DIC8N10 DOC6 DIC12N10 DOC6 DIC48(a)00.511.522.530 200 400 600 800 1000 1200 1400 1600 1800nitrite concentration (mg/L)UV fluence(mJ/cm2)N10 DIC12N10 DIC12 DOC2N10 DIC12 DOC4N10 DIC12 DOC6(b)83  Table 7.2 Pseudo-first order rate constants of nitrite generation in both DOC and DIC contained solutions Water matrix Pseudo-first order rate constant (mg/L/min) DIC DIC with 6 mg/L DOC N10 1.0 × 10−2 2.4 × 10−2 N10 DIC4 1.0 × 10−2 2.6 × 10−2 N10 DIC12 1.1 × 10−2 2.6 × 10−2 N10 DIC48 0.8 × 10−2 2.5 × 10−2 Water matrix Pseudo-first order rate constant (mg/L/min) DOC DOC with 12 mg/L DIC N10 1.0 × 10−2 1.1 × 10−2 N10 DOC2 1.8 × 10−2 1.9 × 10−2 N10 DOC4 2.1 × 10−2 2.3 × 10−2 N10 DOC6 2.4 × 10−2 2.6 × 10−2  7.2.3 The Effects of Chloride and DOC  As discussed, DOC in water, resulting from the presence of NOM, reacts with HO● with a second-order rate constant of 2.5×104 (mg/L)-1s-1. This will lead to increases in nitrite formation. At the time, it was demonstrated earlier (Figure 5.2) that higher chloride on its own results in the decrease of nitrite formation. With these in mind and given that Cl● is also reactive with DOC at a similar rate constant of 1.3×104 (mg/L)-1s-1 (Fang et al., 2014), it is necessary to investigate the combined effects of chloride and DOC.  84   Figure 7.3 The effect of chloride in presence of DOC containing solution (Nitrate-N at 10 mg/L (N10), chloride ion increasing from 0 to 80 mg/L (Cl 0-80); (a) DOC at 6 mg/L; (b) DOC at 2 mg/L)  Figure 7.3 (a) and (b) show the formation of nitrite at different chloride concentrations for two DOC concentrations of 6 mg/L and 2 mg/L, respectively. Nitrite formation was unaffected by an increase in chloride at high DOC concentrations, but it decreased slightly with increasing 00.511.522.530 200 400 600 800 1000 1200 1400nitrite concentration (mg/L)UV fluence (mJ/cm2)N10 DOC6N10 DOC6 Cl20N10 DOC6 Cl50N10 DOC6 Cl80(a)00.20.40.60.811.21.41.61.820 200 400 600 800 1000 1200 1400nitrite concentration (mg/L)UV fluence (mJ/cm2)N10 DOC2N10 DOC2 Cl20N10 DOC2 Cl50N10 DOC2 Cl80(b)85  chloride in a solution with low DOC concentration. When water contains chloride, both HO● and Cl● are produced during VUV photolysis. NOM can react strongly with both HO● and Cl● at rate constants of the same order of magnitude. Thus, the presence of NOM can scavenge these two radicals and impact their distribution. With the increase of chloride, Cl• increases and •OH decreases, as more of the 185 nm photons are absorbed by the chloride ion. However, Cl• also can be transferred to •OH (reaction (5-4)). When DOC is at a relatively high concentration, both [HO●] and [Cl●] may stay constant irrespective of chloride concentration, as DOC can consume most radicals and maintain the steady state concentration of free radicals. Therefore, the influences of HO● and Cl● on the formation of nitrite are reduced with the increase of chloride concentration. On the other hand, when DOC concentration is low, the increase of chloride results in a greater formation of chlorine radicals, which in turn results in the increased concentrations of HO● and Cl2●⁻, which consequently react with nitrite and decrease its concentration in the solution.  The pseudo-first order rate constants associated with nitrite formation in the presence of DOC and chloride are listed in Table 7.3. As can be seen, DOC increases nitrite formation, whereas chloride decreases nitrite formation, and the overall effect of their combination on nitrite formation is dependent on the ratio of concentrations of DOC and chloride in the water matrix.  Table 7.3 Pseudo-first order rate constants comparison Water samples Pseudo-first order rate constant of nitrite formation (mg/L/min) DOC0 DOC2a DOC6 N10 1.15×10-2 1.78×10-2 2.41×10-2 N10 Cl20 7.37×10-3 1.66×10-2 2.26×10-2 86  N10 Cl50 n.a.b 1.56×10-2 2.19×10-2 N10 Cl80 2.04×10-3 1.50×10-2 2.26×10-2 a. Results of DOC2 are statistically different at p =0.05 using a t-test for N10 and N10 Cl80. b. Data is not obtained. 7.2.4 Impact of Sulfate and DOC As discussed, natural organic matter (NOM) is one of the major solutes in water that can scavenge OH radicals and increase the formation of nitrite. Also, sulfate at a high concentration showed to decrease nitrite formation. Given that •OH and 𝑆𝑂4•− react with NOM at considerable rate constants (Table 7.4), their presence together could impact the distribution of radical species as well as nitrite formation, and requires further investigation.   Table 7.4 List of Rate Constants of NOM with OH Radical and Sulfate Radical Species Reaction Rate with NOM Reference OH Radical 2.3x104 (mg of C/L)-1s-1 (Brezonik and Fulkerson-brekken, 1998) Sulfate Radical 2x103 (mg of C/L)-1s-1 (Hou et al., 2018) 6.8x103 (mg of C/L)-1s-1 (Wacławek et al., 2017)  87   Figure 7.4 Impact of sulfate on nitrite formation in the presence of NOM during VUV irradiation of water containing nitrate (N10: NaNO3-N at 10 mg/L, S30: Na2SO4-S at 30 mg/L; (a) DOC at 2 mg/L, (b) DOC at 6 mg/L).  The experiments were conducted in the presence of DOC at 2 mg/L and 6 mg/L with various sulfate concentrations. As shown in Figure 7.4, the increase of sulfate had little impact on 00.20.40.60.811.21.41.61.80 300 600 900 1200 1500 1800Nitrite formation (mg/L)UV fluence (mJ/cm2)N10 DOC2N10 DOC2 S5N10 DOC2 S15N10 DOC2 S30(a)00.511.522.530 300 600 900 1200 1500Nitrite formation (mg/L)UV fluence (mJ/cm2)N10 DOC6N10 DOC6 S5N10 DOC6 S15N10 DOC6 S30(b)88  nitrite formation irrespective of DOC concentration. However, increasing the concentration of DOC, when sulfate was constant, significantly increased nitrite formation (Figure 7.5). It is known that 𝑆𝑂4•− and •OH both react with nitrite to form NO2• but rarely react to nitrate, so the presence of these radicals can reduce nitrite formation to some extent; however, the presence of DOC may scavenge 𝑆𝑂4•− and •OH, thereby hindering their impact on decreasing nitrite formation. Thus, when DOC concentration is constant, the increase of sulfate does not significantly influence nitrite formation. Conversely, the increase of NOM can strongly increase nitrite formation, because NOM competes with nitrite for reactions with •OH and 𝑆𝑂4•−, and significantly hinders the conversion of NO2- to NO2•. As a result, nitrite concentration is increased. In summary, NOM concentration contributes most significantly to increasing nitrite formation, whereas the effect of sulfate in NOM-containing water is negligible.  Figure 7.5 Impact of NOM on nitrite formation the in presence of sulfate (N10: NaNO3-N at 10 mg/L, S30: Na2SO4-S at 30 mg/L, DOC at 2 or 6 mg/L).  00.511.522.530 300 600 900 1200 1500 1800Nitrite concentration (mg/L)UV fluence (mJ/cm2)N10 S30 N10 DOC2 S30 N10 DOC6 S3089  7.2.5 Impact of Sulfate and Chloride Previous results suggested that chloride (with its high absorption coefficient of 3063 M cm-1) has a strong effect on decreasing nitrite formation (refer to Chapter 6). However, some researchers reported that chloride ions also react with and scavenge sulfate radicals (reaction 7-1) (McElroy, 1990; Fang et al., 2012). The presence of chloride and sulfate may exhibit dual effects on reducing nitrite formation and change the distribution of radical species, based on the below key reactions:  𝑆𝑂4.− + 𝐶𝑙− → 𝑆𝑂42− + 𝐶𝑙 ∙                                𝑘 = (2 − 6.1) × 108𝑀−1𝑠−1                          (7-1) 𝑆𝑂42− + 𝐶𝑙 ∙→ 𝑆𝑂4.− + 𝐶𝑙−                                 𝑘 = 2.1 × 108𝑀−1𝑠−1                                    (7-2) 𝑆𝑂4.− + 𝑁𝑂2− → 𝑆𝑂42− + 𝑁𝑂2 ∙                          𝑘 = 8.8 × 108𝑀−1𝑠−1                                    (7-3) 𝑆𝑂4.− + 𝑁𝑂3− → 𝑆𝑂42− + 𝑁𝑂3 ∙                          𝑘 = 2.1 × 100𝑀−1𝑠−1                                     (7-4) 𝐶𝑙• + 𝑁𝑂3− → 𝐶𝑙− + 𝑁𝑂3 ∙                               𝑘 = 1 × 108𝑀−1𝑠−1                                       (7-5) 𝐶𝑙• + 𝑁𝑂2− → 𝐶𝑙− + 𝑁𝑂2 ∙                               𝑘 = 5 × 109𝑀−1𝑠−1                                        (7-6) VUV photolysis of chloride and sulfate can produce chlorine and sulfate radicals, respectively (Jortner et al., 1964; Barrett et al., 1965; Dainton and Fowles, 1965; Mcgarvey, 2000). The chlorine radical is found to react with both nitrate and nitrite (reactions (5-6)) (Buxton et al., 1999), while sulfate radical mainly reacts with nitrite (reactions (3-4)) (Buxton et al., 1988). From the earlier part of this research, it has been determined that the chlorine radical has a minor impact on nitrite formation (Chapter 5), whereas the sulfate radical at very high concentrations decreased nitrite (Chapter 6). In both chloride and sulfate-containing solution, the chloride ion can react with sulfate radicals with a very high rate constant (McElroy, 1990); similarly, chlorine radical also reacts with sulfate ions (Buxton et al., 1998). Thus, the presence of chloride could change the distribution of chlorine and sulfate radicals, and might affect nitrite formation.  90   Figure 7.6 The combined impacts of sulfate and chloride on nitrite formation. (N10: nitrate-N at 10 mg/L; initial nitrite is zero; Cl20: chloride at 20 mg/L; S5-30: sulfate-S at 5-30 mg/L)  The experiments were firstly conducted in the presence of chloride at 20 mg/L, with an increase of sulfate-S concentration from 0-30 mg/L. Figure 7.6 shows that the impact of sulfate on nitrite formation is not pronounced in the presence of chloride. With the increase of sulfate concentration, the formation of the sulfate radical increased during VUV photolysis. As sulfate radicals and chlorine radicals can be converted to each other through reactions (7-1) and (7-2), it is unknown which radical is dominant in this equilibrium. If the presence of chloride and the consequent formation of chlorine radicals can help the generation of sulfate radicals, the effect of sulfate radicals on reducing nitrite formation might be pronounced, which is inconsistent with our results. It is speculated that the chlorine radical is more favored than sulfate radical under VUV irradiation, due to the higher absorption coefficient of chloride (𝜀𝐶𝑙−,185 = 3063 𝑀−1𝑐𝑚−1) compared to the sulfate radical, which has a much lower absorption at 185 nm (𝜀𝑆𝑂42−,185 =00.20.40.60.811.20 300 600 900 1200 1500 1800Nitrite concentration (mg/L)UV fluence (mJ/cm2)N10 Cl20 N10 Cl20 S5 N10 Cl20 S15 N10 Cl20 S3091  160 𝑀−1𝑐𝑚−1) (Furatian and Mohseni, 2018). Another possible mechanism is that the formed chlorine radical during VUV photolysis promotes the formation of 𝑆𝑂4•− (reaction 7-2) which itself is also consumed by chloride ions (reaction 7-1) and reforms the sulfate ions. Thus, the effect of chlorine and sulfate radicals may offset each other and minimize their impact and performance on nitrite formation.    Figure 7.7 The combined impact of chloride and sulfate on nitrite formation in nitrate and nitrite rich water. (The initial nitrate and nitrite concentrations for all the experiments were 40 and 10 mg/L, respectively. Control: no sulfate and chloride; SO4-90: sulfate at 90 mg/L; Cl30: chloride at 30 mg/L; Cl30 SO4-90: chloride at 30 mg/l and sulfate at 90 mg/L)  As previously discussed, the impact of sulfate is pronounced when nitrite is present in the solution at a relatively high concentration (Figure 6.4 (b)). To further evaluate the combined impacts of sulfate and chloride in the presence of high nitrite concentration, experiments with different concentrations of chloride and sulfate were conducted in the presence of nitrate at 40 00.10.20.30.40.50.60.70.80.90 300 600 900 1200 1500 1800Nitrite formation (mg/L)UV fluence (mJ/cm2)control SO4-90 Cl30 Cl30 SO4-9092  mg/L and nitrite at 10 mg/L under VUV. Figure 7.7 shows that both sulfate and chloride can reduce the nitrite formation individually. However, the decreasing impact of chloride is stronger than that of sulfate, and the presence of sulfate cannot further decrease the formation of nitrite in chloride-containing solution. It is speculated that the effect of sulfate on reducing nitrite formation is due to the formation of 𝑆𝑂4•− via reaction (7-3), while the effect of chloride on reducing nitrite formation is due to its high absorbance of VUV, which decreases the photolysis of nitrate (as discussed in Chapter 6). Note that sulfate radicals do not react strongly with nitrate due to a very low rate constant for reaction (7-4) (Neta et al., 1988). Therefore, the impact of sulfate radicals on nitrate is negligible. When both sulfate and chloride are present in water, chloride might minimize the effect of 𝑆𝑂4•− by scavenging 𝑆𝑂4•− via reaction (7-2). However, the mechanism is very complex as many reactions are involved, such as the decay of Cl•, the decay of 𝑆𝑂4•−, etc., which require further investigation.  7.2.6 The Effect of DOC and Chloride in the presence of DIC and Sulfate Based on the above discussion, it is concluded that DOC is the dominant waterborne solute influencing nitrite formation. The effect of chloride on reducing nitrite formation is also pronounced at low DOC concentration, while the impact of sulfate is insignificant. These conclusions are quite useful for predicting the formation of nitrite during VUV irradiation. However, the results have been obtained in water matrices with only two solutes, while it is different from the real water matrix which is more complex and contains multiple solutes. Therefore, it is necessary to verify the effects of DOC and chloride, which have a major impact on nitrite formation in more complex water matrices. 93   Figure 7.8 Effect of combined water solutes (DOC, DIC, Sulfate, and Chloride) on nitrite formation (Nitrate-N at 10 mg/L; DOC at 2 and 6 mg/L; DIC at 12 mg/L; Sulfate at 90 mg/L; Chloride in range of 20-80 mg/L)  As shown in Figure 7.8, the initial concentrations of DIC and sulfate are kept constant because their individual effect on nitrite formation has been shown to be insignificant. With the increase of DOC, nitrite formation increased significantly, whereas the impact of chloride was only pronounced at low DOC concentrations. The results of DOC 2 mg/L are statistically different at p=0.05 using a t-test for Cl20 and Cl80. However, at high DOC concentration, chloride has a minor effect on nitrite formation. The results are consistent with previous results for individual and combined water matrices.  However, the inorganics in water can react with each other, which in turn may impact the concentration of radicals and further influence nitrite formation. It is found that Cl•, Cl2•-, SO4•- effectively react with bicarbonate and form CO3•-, as shown in reactions (7-7) and (7-9) (Dogliotti and Hayon, 1967; Buxton et al., 2000; Lutze et al., 2015). The effects of chlorine, carbonate, and 00.511.522.50 200 400 600 800 1000 1200 1400Nitrite formation (mg/L)UV fluence (mJ/cm2)DOC2 Cl20 DOC2 Cl50 DOC2 Cl80DOC6 Cl20 DOC6 Cl50 DOC6 Cl8094  sulfate radicals on reducing nitrite formation have been confirmed to various extents (refer to early chapters). The transformation of these radicals may enhance or weaken their overall effect on nitrite formation. Therefore, it requires future studies to verify the overall effects of these water solutes on the formation of nitrite at normal surface water concentration. 𝐶𝑙 ∙ +𝐻𝐶𝑂3− → 𝐻2𝑂 + 𝐶𝑂3∙−                               𝑘 = 2.4 × 109 𝑀−1𝑠−1                                  (7-7) 𝐶𝑙2∙− + 𝐻𝐶𝑂3− → 2𝐶𝑙− + 𝐻+ + 𝐶𝑂3∙−                 𝑘 = (1.5 × 106~1.6 × 108) 𝑀−1𝑠−1            (7-8) 𝑆𝑂4∙− + 𝐻𝐶𝑂3− → 𝐶𝑂3∙− + 𝑆𝑂42− +𝐻+                𝑘 = 2.4 × 109 𝑀−1𝑠−1                                 (7-9)  7.3 Summary This chapter has demonstrated the combined effects of DOC and DIC, DOC and chloride, DOC and sulfate, as well as the impact of DOC and chloride in the presence of DIC and sulfate. A new parameter called nitrite formation factor (NF) was defined to describe the formation rate of nitrite as a result of one water solute, in order to quantify the impact of water solutes at various concentrations. Based on the earlier experimental results and NF value, the impact of water solutes showed their order of chloride (-) > DIC (-) > sulfate (-) on reducing nitrite formation, while DOC is another dominant solute but leads to increased nitrite concentration. The effect of DIC and sulfate are negligible when DOC (or NOM) is present in the water. Chloride, on the other hand, showed to slightly decrease nitrite, but the effect is only pronounced at low DOC concentrations.   95  Chapter 8: Conclusion and Future Work  8.1 Overall Conclusion This research has investigated the effect of common organic and inorganic water solutes, such as NOM (DOC), DIC, chloride, and sulfate, on nitrite formation during VUV photolysis. Utilizing different radical scavengers, the role of photo-generated inorganic radicals and their relationships, as well as their effect on nitrite formation has been delineated and discussed, including 𝐶𝑂3∙−, 𝐶𝑙 ∙, and 𝑆𝑂4∙−. Through systematic and detailed experimental work and analyses of reaction pathways and mechanisms, this research has provided insights and data on the applications of VUV based treatment of nitrate-containing water. It must be noted that DOC in water needs be given close attention as it can increase the nitrite formation significantly; while the other inorganic ions and the formed inorganic radicals are either insignificant or reduce the formation of nitrite. For a given water solutes concentration in either surface water or groundwater, this study can estimate the formation of nitrite under VUV irradiation, using a nitrite formation factor (NF) which is correlated with water solutes concentration and UV fluence. The overall conclusions obtained from this research are highlighted below:  1. Effect of initial nitrate concentration • This research has examined the relationship between initial nitrate concentration and the amount of nitrite formed during VUV irradiation of water containing nitrate. With the increase of UV fluence, the nitrite formation increased.  96  • At lower initial nitrate concentrations, the formation of nitrite starts to increase and then stays relatively constant with the increase of UV fluence. The higher formation rate at the beginning of UV irradiation is dependent on the nitrate concentration.  • With the increase of nitrite formation, nitrite can react with •OH and reform nitrate; therefore, the formation rate of nitrite is decreased and both nitrate and nitrite concentration can impact the overall nitrite generation.  • At higher initial nitrate concentration, the formation of nitrite increased linearly with respect to UV fluence. Since nitrate concentration is much higher than nitrite, the formation rate of nitrite is mainly dependent on initial nitrate concentration.  2. Effect of Dissolved Oxygen (DO) • A higher DO concentration results in a lower formation of nitrite under the same UV fluence. • DO competes with nitrate for H• and hydrated electron; therefore, the degradation of nitrate by H• and hydrate electron is hindered, and the formation of nitrite is reduced.   3. Effect of Dissolved Organic Carbon (DOC) • The presence of DOC leads to the greater formation of nitrite; the higher the DOC concentration, the higher the formation of nitrite.  • DOC mainly acts as OH radical scavenger and competes with nitrite for •OH. Thus, the reaction between nitrite and •OH is hindered in the presence of DOC, leading to increased formation of nitrite.  97  4. Effect of Dissolved Inorganic Carbon (DIC) • The effect of DIC on nitrite formation is insignificant. At low DIC concentration, there is no significant impact on nitrite formation. However, a very high concentration of DIC (e.g., greater than 40 mg inorganic C/L) leads to a slight reduction in nitrite formation. • Unlike DOC, bicarbonate as the source of DIC does not strongly react with OH radical. However, the increase of bicarbonate can increase the dissolved CO2, which hinders the pathway of peroxynitrite to nitrite and slightly decreases the formation of nitrite. • Carbonate radical (CO3•-) is formed under VUV irradiation, and reacts with nitrite radical (NO2•) to generate dissolved CO2 and NO3-. CO3•- converts nitrite radical to nitrate; therefore, it reduces the formation of nitrite through nitrite radical.  5. Effect of Chloride • The presence of chloride decreases the formation of nitrite. That is, higher chloride concentration leads to the lower formation of nitrite. • The absorption coefficient of chloride at 185 nm is very high, so in the presence of increased chloride concentration, more of the 185 nm photons are absorbed by chloride. Thus, the formation of nitrite is decreased due to the reduced VUV photolysis of nitrate. • Both Cl• and •OH are generated under VUV irradiation, and Cl• can be converted to •OH. The effect of Cl• was examined by using acetone as •OH radical scavenger. In the presence of acetone, the increase of chloride increased the formation of Cl•. The results indicated that the effect of Cl• on nitrite formation is insignificant. • Similarly, acetate was applied as Cl• scavenger. The decrease of •OH increases the formation of nitrite, which confirmed that the role of •OH is to decrease nitrite formation. 98  • Chlorine radical can help the degradation of carbamazepine (CBZ), but it does not increase nitrite formation, which brings an opportunity for the application of VUV for treating nitrate-rich water.  6. Effect of Sulfate • Sulfate shows a minor impact on nitrite formation. Only at the extremely high concentration (e.g., 300 mg S/L), sulfate decreases nitrite formation under VUV irradiation. • Both sulfate radical (SO4.-) and •OH are generated under VUV irradiation. To investigate the effect of SO4.-, UV/persulfate was applied since it only produces SO4.-. It was found that the presence of SO4.- decreased nitrite concentration.  • With further investigation, it is confirmed that the effect of sulfate radical only pronounces at high sulfate or high nitrite concentration.  • Sulfate radical has a similar effect of chlorine radical to help the degradation of CBZ, but it does not increase nitrite formation.   7. Effect of Combined Water Solutes • When both DOC and DIC are present in water, DOC has the dominant impact on nitrite formation. The formation of nitrite is increased as the concentration of DOC increases, and the rate of nitrite formation is similar to that observed when only DOC is present. The nitrite formation rate is independent of DIC concentration. 99  • The effects of combined DOC and chloride are mainly dependent on their relative concentrations. At high DOC concentration, the effect of chloride is not pronounced. At low DOC concentration, the increase of chloride slightly decreases the formation of nitrite.  • In the presence of DOC and sulfate, DOC plays a dominant role in nitrite formation, whereas sulfate only has a minor impact. DOC consumes •OH and reduces its concentration, and as a result, it affects the reaction between nitrite and OH radical, which in turn affects the concentration of nitrite.        • In the presence of chloride and sulfate, the effect of sulfate is insignificant, but the increase of chloride decreases nitrite formation. This is largely because chloride ion has a much higher 185 nm absorption coefficient compared to sulfate. 8.2 Recommendations for Future Work In this research, the UV fluence rate, also called UV irradiance, was measured and expressed at UV 254 nm. The advantage of this method is to bring the UV energy presented under the same scale for micropollutants degradation and UV disinfection, so one can easily compare the results of this research with those by other researchers. However, it is necessary to develop a convenient chemical actinometer and a standard protocol for measuring UV energy at UV 185 nm. This is because the degradation of micropollutants is mainly dependent on UV 185 nm. The available UV 185 nm measurement is methanol actinometry; however, the measurement process is somewhat difficult. Also, methanol is a toxic compound for the environment and requires proper disposal. So, other environmental-friendly chemicals and practical actinometry method is required to be developed.  This research has confirmed that NOM can strongly impact the formation of nitrite under VUV irradiation. Further studies can focus on understanding what fraction in humics is most 100  sensitive to nitrite formation, and which functional groups in NOM structure are dominant in influencing nitrite formation during VUV photolysis. The presence of chloride and sulfate in water showed to increase the degradation of CBZ under VUV irradiation. At the same time, there was no further increase in the formation of nitrite (indeed in some cases, nitrite formation decreased). More micropollutants could be tested for their degradation efficiency in the presence of chloride and sulfate. In addition, it is necessary to know whether new by-products form during the treatment.  A detailed kinetic model and reaction mechanism could be developed based on the results. 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It is to confirm the impact of UV 254 on nitrite formation and provide a baseline of several background water matrices. The synthetic water samples included DOC, DIC, sulfate, and chloride, as listed in Table 2.1. The selected concentrations of each water solute are typically in the range of natural water. The results showed that nitrite formation under UV 254 was lower than IC (ion chromatograph) detection limit (0.1 mg/L). Thus, the impact of UV 254 on nitrite formation is negligible, which is consistent with the literature (Shu et al., 2013). Also, it is confirmed that the nitrite formation under VUV is due to the effect of VUV.  Table S 1 Water Matrices under UV 254nm Sample No. Nitrate-N 10 mg/L DOC 6 mg/L DIC 12 mg/L Chloride 30 mg/L Sulfate-S 15 mg/L 1 ✓     2 ✓ ✓    3 ✓  ✓   4 ✓   ✓  5 ✓    ✓ 6 ✓ ✓ ✓ ✓ ✓  116  Appendix B  Photon Fraction Calculation  By the radiation of VUV collimated beam, all the UV photons should be absorbed by water and the solutes inside. With the known concentration of prepared water samples and the literature reported 185 nm molar absorption coefficient of each solute, the photon fractions can be calculated using the equation below. Note that the total absorption means the overall absorption of each compound, thus the overall photon fraction is equal to 1.  𝑝ℎ𝑜𝑡𝑜𝑛 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 𝐴 =𝜀𝐴 ∗ 𝑀𝐴𝑡𝑜𝑡𝑎𝑙 𝑎𝑏𝑠𝑜𝑝𝑡𝑖𝑜𝑛  The molar absorption coefficient was obtained from literature and listed in Chapter 4 Table 4.2 and Chapter 5 Table 5.3. The absorption of H2O at 185nm is confirmed at 1.8cm-1 (J. L. Weeks, 1963).  The photon fraction of N10 (NaNO3-N at 10mg/L) was calculated using the above equation and shown as an example.  Table S 2. Photon Fractions Calculation of N10 (NO3-N at 10mg/L) Solution Compounds Cons(mg/L) Cons(mol/L) α(M-1cm-1) A(cm-1) A(total) photons fraction Nitrate 44.29 7.14×10-4 4779 3.41 5.21 0.655 water --- --- --- 1.8 0.345  B.1 Sample Calculation for NOM Containing Solution For NOM contained water, NOM can highly absorb 185 nm photons, however, due to the large molecular weight, the impact of NOM on photon fractions was not as much as expected. Assuming an average molecular weight of 3000 Da (g/L) for NOM (Wang et al., 2017), the photon 117  fraction of solution with DOC 6 mg/L was listed below. Note that around 50% (w/w%) of NOM is organic carbon. Table S 3. Photon Fractions Calculation of N10 DOC6 Solution Compounds Cons(mg/L) Cons(mol/L) α(M-1cm-1) A(cm-1) A(total) photons fraction Nitrate 44.29 7.14×10-4 4779 3.41 5.22 0.654 NOM 12 4.00×10-6 1402 0.0056 0.001 water --- --- --- 1.8 0.345  B.2 Sample Calculation for Chloride Containing Solution For chloride contained water, chloride can highly absorb 185 nm photons with the 185 nm absorption coefficient of 3063 M cm-1 (Serrano Mora and Mohseni, 2018). Thus, the presence of chloride can decrease the photon fraction of nitrate and water, as presented below.  Table S 4. Photon Fractions Calculation of N10 Cl80 Solution Compounds Cons(mg/L) Cons(mol/L) α(M-1cm-1) A(cm-1) A(total) photons fraction Nitrate 44.29 7.14×10-4 4779 3.41 13.78 0.248 chloride 80 2.25×10-3 3063 8.56 0.621 water --- --- --- 1.8 0.131  118  Appendix C  UV Fluence Sample Calculation As mentioned in Chapter 3.3.3, the UV fluence is calculated based on the following equation: 𝐸 = 23.373[𝐴352 (𝑠𝑎𝑚𝑝𝑙𝑒) −  𝐴352 (𝑏𝑙𝑎𝑛𝑘)] [𝐴𝑟𝑒𝑎 (𝑐𝑚2) × 𝐸𝑥𝑝𝑜𝑠𝑢𝑟𝑒 𝑡𝑖𝑚𝑒 (𝑠)]∙ 𝑉𝑜𝑙𝑢𝑚𝑒 (𝑚𝐿) (𝑚𝑊 ∙  𝑐𝑚−2) The average fluence rate then calculated with several correction factors (Bolton and Linden, 2003): Eavg=E × Reflection Factor × Water Factor × Divergence Factor  • Reflection factor=1-R, correcting for light passes from one medium to another, the fraction reflected R is given by the Fresnel Law (Meyer-Arendt, 1984). In our case, R=0.025. • Water factor =1−10−alal Ln(10), accounting for UV absorption of experimental solutions. • Divergence factor =L(L+l), correcting for decreased irradiance over the path length l of the cell suspension, L is the distance from lamp to the cell surface. The examples of raw data are listed in the below table.  Table S 5 Raw Data of UV Fluence Rate Calculation Solution Abs254 E Reflection Factor Water Factor Divergence Factor Eavg N10 0.0186 0.32 0.975 0.98 0.90 0.28 N10 DOC6 0.2136 0.32 0.975 0.79 0.90 0.225 N10 DIC12 0.0116 0.32 0.975 0.99 0.90 0.28 N10 Cl80 0.0086 0.30 0.975 0.99 0.90 0.26 119  N10 S30 0.0022 0.28 0.975 0.99 0.90 0.24  The actinometry experiments were conducted every time before using VUV lamp. One of the E value calculation is exhibited as an example.              Diameter of vessel: 4.7 (cm)                   Surface area of vessel: 17 (cm3)             Volume of vessel: 17 (cm2)                     Height of vessel: 1(cm) A (blank)=0.0115, which was used to measure the absorbance at 352 nm (Abs352) of actinometer solution, the value should be lower than 0.02. 𝐸 = 23.373𝐴352 (𝑠𝑎𝑚𝑝𝑙𝑒) −  0.0115 𝐸𝑥𝑝𝑜𝑠𝑢𝑟𝑒 𝑡𝑖𝑚𝑒 (𝑠)(𝑚𝑊 ∙  𝑐𝑚−2)  Table S 6 Raw Data of Actinometry Measurement Run # Irradiation time(s) Absorbance at 352 A(average) Fluence rate (mW/cm2) 1 30.41 0.3941 0.3986 0.407 0.3999 0.298522631 2 30.59 0.3994 0.4012 0.401 0.400533333 0.297249954 3 30.34 0.3838 0.3889 0.3921 0.388266667 0.290249417  120  Appendix D  Sample of Nitrogen Balance The complex mechanism of nitrite formation from nitrate exhibited that nitrate can be degraded to many N-based species. One of the approaches to check the formation of other N-based species, such as NO-, ONOO-, N2O3, N2O4 etc., is to check the nitrogen balance. However, the nitrogen balance calculated from nitrate and nitrite concentration showed that no N difference obtained between various reaction times. One of the examples is listed in the below table.   Table S 7 Raw Data of Nitrite Concentration of Sample N10 Cl30 Irradiation time (min) UV fluence (mJ/cm2) Nitrite (ppm) CNO2- (avg) Standard Deviation 0 0 0 0 0 0 0 20 300 0.1164 0.1148 0.1086 0.1133 0.004120 40 600 0.2102 0.2043 0.2090 0.2078 0.003118 60 900 0.3382 0.3254 0.3252 0.3296 0.007450 80 1200 0.5166 0.5248 0.5188 0.5201 0.004244 100 1500 0.6639 0.6575 0.6689 0.6634 0.005710  Table S 8 Raw Data of Nitrate Concentration of Sample N10 Cl30 Irradiation time (min) UV fluence (mJ/cm2) Nitrate (ppm) CNO3- (avg) Standard Deviation 0 0 44.7849 44.6401 44.7557 44.7269 0.07658 20 300 44.6915 43.9777 44.3375 44.3356 0.35693 40 600 44.172 44.2452 44.3664 44.2612 0.09818 121  60 900 44.2295 44.3643 44.0300 44.2079 0.16823 80 1200 44.0197 43.8230 43.5580 43.8002 0.23169 100 1500 43.5640 43.4829 43.4017 43.4829 0.08115  Table S 9 Nitrogen Balance Calculated from above Raw Data Irradiation time (min) Nitrite-N Standard Deviation Nitrate-N Standard Deviation N total 0 0 0 10.0996 0.01729 10.0996 20 0.0345 0.001254 10.0113 0.08059 10.0457 40 0.0633 0.000949 9.9945 0.02217 10.0577 60 0.1003 0.002267 9.9824 0.03798 10.0827 80 0.1583 0.001292 9.8904 0.05232 10.0487 100 0.2019 0.001739 9.8187 0.01832 10.0206 ΔN 0.2019 N.A. 0.2809 N.A. 0.079  As shown in Table S 9, the difference of nitrogen balance is 0.079, while the standard deviation of nitrate-N is maximum up to 0.08. Therefore, the difference of N balance cannot provide evidence of N loss or formation of other compounds. Further research or techniques require to be applied for deeply understanding the formation of the related N-based species.   122  Appendix E  Calibration Curve for Anion Concentration Nitrite concentration was determined by ion chromatography (IC), the detailed method was described in Chapter 3.3.4. For this method, building a proper calibration curve is critical for ion concentration measurement.  The Thermal Fisher Scientific has provided Dionex combined seven anion standard solution (catalog #057590), which contains fluoride, chloride, nitrite, bromide, nitrate, phosphate and sulfate, however, their concentration is in the same range, with the ratio of 1:1 for most of the ions.  It is not matched to my experimental requirements, because my work need a very low nitrite concentration with relative high chloride, nitrate and sulfate. Therefore, the standard solutions in 7 levels were prepared manually for preparing IC calibration curve. The chemicals used were sodium chloride, sodium nitrite, sodium nitrate and sodium sulfate. In this standard solution, the concentration ratio of chloride, nitrite, nitrate and sulfate is in order of 15:2:10:10, as shown in Table S 10.  Table S 10 Anion Standard Solution for 7 Level IC calibration curve Standard Level Cl- (ppm) NaCl (ppm) NO2- (ppm) NaNO2 (ppm) NO3- (ppm) NaNO3 (ppm) SO42- (ppm) Na2SO4 (ppm) M (mg/mol) 35.5 58.5 46 69 62 85 96 142 1 0 0 0 0 0 0 0 0 2 15 24.7183 2 3 10 13.7097 10 14.7917 3 30 49.4366 4 6 20 27.4194 20 29.5833 4 45 74.1549 6 9 30 41.1290 30 44.375 123  5 60 98.8732 8 12 40 54.8387 40 59.1667 6 75 123.5915 10 15 50 68.5484 50 73.9583 7 90 148.3098 12 18 60 82.2581 60 88.75  The calibration curve was obtained as shown below, the R2=0.99.   Figure. S. 1 IC Calibration Curve for chloride (a), nitrite (b), nitrate (c) and sulfate (d)  y = 0.2184xR² = 0.998905101520250 20 40 60 80 100area (μs*min)anion concentrationchloridey = 0.0811xR² = 0.999900.20.40.60.811.20 5 10 15area (μs*min)anion concentrationnitritey = 0.1048xR² = 0.9986012345670 20 40 60 80area (μs*min)anion concentrationnitratey = 0.1343xR² = 0.998902468100 20 40 60 80area (μs*min)anion concentrationsulfate(a) (b) (c) (d) 

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