Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Injamala, Sharon Kumar
Master of Applied Science - MASc
University of British Columbia
Desiccation cracks in landfill liners pose several problems that can significantly impact the performance and environmental safety of landfills. This study presents two different sustainable stabilizing materials to minimize the cracking properties and improve the geotechnical properties of bentonite for its application in landfill liner systems. One of those sustainable stabilizing materials is biochar which has been widely used to improve soil quality and store atmospheric carbon dioxide. The other material is a biopolymer which helps in the polymerization of clays and has gained significant interest due to its sustainability characteristics. Three different percentages of biochar (5%, 15%, 25%) of sizes less than 0.075 mm and three different percentages of biopolymer (0%, 2%, 4%) are mixed with bentonite clay followed by investigating their volumetric shrinkage behavior, compaction and compressive strength characteristics. The desiccation cracking is assessed with the help of ImageJ analysis in terms of Crack Intensity Factor (CIF) and Crack Density Factor (CDF). The microstructure analysis using a Scanning electron microscope (SEM) is carried out to visualize how the composite materials interact when mixed. Finally, the adsorption studies are conducted to determine the effect of biochar and biopolymer on the adsorption behavior of bentonite using Lead (Pb²⁺) as a contaminant. The compaction and compressive strength values are highest at biochar (5%) and biopolymer (4%). Furthermore, the biochar-amended bentonite displayed improved adsorption behavior, volumetric shrinkage and crack resistance in terms of CDF and CIF. The results indicate that biochar and biopolymer are sustainable amendments to the high-swelling bentonite clay to improve its geotechnical properties for its application as a landfill liner material.
CRACK MITIGATION AND CONTAMINANT IMMOBILIZATION IN BENTONITE CLAY WITH SUSTAINABLE AMENDMENTS by Sharon Kumar Injamala B.Tech, IIT Patna, 2019 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Civil Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) May 2024 © Sharon Kumar Injamala, 2024 ii The following individuals certify that they have read and recommend to the College of Graduate Studies for acceptance, the thesis entitled: Crack Mitigation and Contaminant Immobilization in the Bentonite Clay using Sustainable Amendments: submitted by Sharon Kumar Injamala in partial fulfillment of the requirements of the degree of Master of Applied Science. Examining Committee: Dr. Sumi Siddiqua, School of Engineering Supervisor Dr. Chinchu Cherian, University of Northern British Columbia Supervisory Committee Member Dr. Amir Ardestani-Jaafari, Faculty of Management Supervisory Committee Member Dr. Wenbo Zheng, University of Northern British Columbia Examiner iii Abstract Desiccation cracks in landfill liners pose several problems that can significantly impact the performance and environmental safety of landfills. This study presents two different sustainable stabilizing materials to minimize the cracking properties and improve the geotechnical properties of bentonite for its application in landfill liner systems. One of those sustainable stabilizing materials is biochar which has been widely used to improve soil quality and store atmospheric carbon dioxide. The other material is a biopolymer which helps in the polymerization of clays and has gained significant interest due to its sustainability characteristics. Three different percentages of biochar (5%, 15%, 25%) of sizes less than 0.075 mm and three different percentages of biopolymer (0%, 2%, 4%) are mixed with bentonite clay followed by investigating their volumetric shrinkage behavior, compaction and compressive strength characteristics. The desiccation cracking is assessed with the help of ImageJ analysis in terms of Crack Intensity Factor (CIF) and Crack Density Factor (CDF). The microstructure analysis using a Scanning electron microscope (SEM) is carried out to visualize how the composite materials interact when mixed. Finally, the adsorption studies are conducted to determine the effect of biochar and biopolymer on the adsorption behavior of bentonite using Lead (Pb2+) as a contaminant. The compaction and compressive strength values are highest at biochar (5%) and biopolymer (4%). Furthermore, the biochar-amended bentonite displayed improved adsorption behavior, volumetric shrinkage and crack resistance in terms of CDF and CIF. The results indicate that biochar and biopolymer are sustainable amendments to the high-swelling bentonite clay to improve its geotechnical properties for its application as a landfill liner material. iv Lay Summary Sodium bentonite is the most used material in the construction of dams, landfills and various forms of subsurface barrier systems for preventing migration of contaminants such as volatile organic compounds (VOC), pesticides, heavy metals etc. Due to its high volume changing nature, sodium bentonite undergoes swelling, shrinkage, and cracking. Cement and lime, which are used for the stabilization of sodium bentonite, cause a great deal of environmental pollution and energy consumption. This study proposes using biochar, an industrial byproduct, as a potential alternative for cement and lime, in order to reduce carbon footprint and carbon sequestration. Similarly, biopolymer (sodium alginate) which is derived from wastewater, is used as a sustainable stabilizing material in this study to enhance the performance of sodium bentonite in landfill applications. In brief, the current study explored the feasibility of using various sustainable stabilizing materials to address the challenges associated with sodium bentonite while using it as a landfill liner and contaminant barrier system. v Preface The main goal of this thesis is to examine the mechanical and adsorption behavior of the bentonite clay amended with sustainable amendments like biochar and biopolymer. Sharon Kumar Injamala, carried out the key responsibilities of this research. These responsibilities included conducting a detailed literature review, preparing samples of various composites, undertaking an experimental study from preliminary analysis to advanced testing, analyzing and interpreting the data acquired as well as documenting the thesis under the supervision of Dr. Sumi Siddiqua. The preparation of samples and their mechanical property evaluations took place in the Geomaterials Research Laboratory at the School of Engineering, University of British Columbia's Okanagan campus. Furthermore, analyses using Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDS) and Inductively Coupled Plasma- Optical Emission Spectrometry (ICP-OES) were conducted at the FiLTER – Fipke Laboratory for Trace Element Research at the University of British Columbia, Okanagan campus. vi Table of Contents Abstract ................................................................................................................................... iii Lay Summary ......................................................................................................................... iv Preface .......................................................................................................................................v Table of Contents ................................................................................................................... vi List of Figures ...........................................................................................................................x List of Tables ......................................................................................................................... xii List of Abbreviations ........................................................................................................... xiii Acknowledgement ..................................................................................................................xv Dedication ............................................................................................................................. xvi Chapter 1: Introduction ..........................................................................................................1 1.1 Overview ..........................................................................................................................1 1.2 Motivation ........................................................................................................................2 1.3 Objective ..........................................................................................................................5 1.4 Thesis Organization .........................................................................................................6 Chapter 2: Literature Review .................................................................................................8 2.1 Bentonite Clay as a Landfill Liner ...................................................................................8 2.2 Soil Stabilization ..............................................................................................................9 2.2.1 Mechanical Stabilization .........................................................................................10 2.2.2 Chemical Stabilization ............................................................................................10 2.2.2.1 Traditional Stabilizers ..................................................................................................10 vii 2.2.2.2 Non-traditional Stabilizers ...........................................................................................11 2.3 Interaction of Biochar and Biopolymer with the Bentonite Clay ..................................13 2.4 Performance Criteria of Stabilized Bentonite Clay .......................................................15 2.4.1 Index Properties ......................................................................................................15 2.4.2 Unconfined Compressive Strength .........................................................................15 2.4.3 Optimum moisture content (OMC) & Maximum dry density (MDD) ...................16 2.4.4 Volumetric Shrinkage of Bentonite Composites ....................................................16 2.4.5 ImageJ Analysis ......................................................................................................16 2.5 Microstructural Analysis and Desiccation Cracking .....................................................18 2.6 Adsorption Studies and Environmental Implications ....................................................18 2.7 Summary ........................................................................................................................19 Chapter 3: Materials and Methods ......................................................................................20 3.1 Overview ........................................................................................................................20 3.2 Materials ........................................................................................................................20 3.2.1 Bentonite .................................................................................................................20 3.2.2 Biochar ....................................................................................................................21 3.2.3 Biopolymer (Sodium Alginate)...............................................................................22 3.3 Sample Preparation and Curing .....................................................................................23 3.3.1 Unconfined Compressive Strength .........................................................................24 3.3.2 Volumetric shrinkage ..............................................................................................24 3.3.3 Adsorption Studies ..................................................................................................25 3.4 Experimental Methods ...................................................................................................26 viii 3.4.1 Atterberg Limit Test ...............................................................................................26 3.4.1.1 Liquid Limit Test .........................................................................................................27 3.4.1.2 Plastic Limit .................................................................................................................28 3.4.1.3 Plasticity Index.............................................................................................................29 3.4.2 Compaction Test .....................................................................................................29 3.4.3 Unconfined Compressive Test ................................................................................30 3.4.4 Volumetric Shrinkage .............................................................................................31 3.4.5 Image Analysis........................................................................................................32 3.4.6 Batch Adsorption Study ..........................................................................................33 3.4.7 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) .............35 3.4.8 Microscopic and Fourier Transform Infrared Spectroscopy ..................................36 Chapter 4: Results and Discussions......................................................................................37 4.1 Overview ........................................................................................................................37 4.2 Atterberg Limit ..............................................................................................................37 4.3 Compaction Characteristics of Bentonite Composites ..................................................38 4.4 Compressive Strength of Bentonite Composites ..........................................................43 4.5 Volumetric Shrinkage ....................................................................................................47 4.6 Crack Density Factor (CDF) & Crack Intensity Factor (CIF) .......................................51 4.7 SEM and EDS Analysis .................................................................................................58 4.8Adsorption Properties of Bentonite Composites for Lead (Pb2+) Contaminant ............61 4.9 FTIR Analysis on Batch Sorption Specimens ...............................................................64 Chapter 5: Conclusion ...........................................................................................................67 ix 5.1 Summary ........................................................................................................................67 5.2 Key Findings and Conclusions ......................................................................................68 5.3 Significant Contributions: ..............................................................................................69 5.4 Limitations and Future Work .........................................................................................70 References ...............................................................................................................................72 x List of Figures Figure 3.1 Molecular structure of sodium alginate ................................................................. 22 Figure 3.2 Image of a) Bentonite b) Biopolymer c) Biochar .................................................. 23 Figure 3.3 Volumetric shrinkage sample ................................................................................ 25 Figure 3.4 Batch Adsorption testing and sample preparation ................................................. 26 Figure 3.5 Cone penetrometer setup ....................................................................................... 28 Figure 3.6 Miniature compaction apparatus ........................................................................... 30 Figure 3.7 Unconfined compressive strength test in progress ................................................ 31 Figure 3.8 Volumetric shrinkage mould, prepared and desiccated samples ........................... 32 Figure 3.9 Experimental setup of ICP-OES ............................................................................ 35 Figure 3.10 Experimental setup of SEM and EDS ................................................................. 36 Figure 4.1 Compaction characteristics of bentonite-biochar composites without biopolymer content ...................................................................................................................................... 40 Figure 4.2 Compaction characteristics of bentonite-biochar composites with 2% biopolymer content ...................................................................................................................................... 41 Figure 4.3 Compaction characteristics of bentonite-biochar composites with 4% biopolymer content ...................................................................................................................................... 42 Figure 4.4 Unconfined compressive strength of bentonite composites with varying biochar content ...................................................................................................................................... 44 Figure 4.5 Unconfined compressive strength of bentonite-biochar composites with 2% biopolymer ............................................................................................................................... 45 Figure 4.6 Unconfined compressive strength of bentonite with varying biochar content and 4% biopolymer ......................................................................................................................... 46 Figure 4.7 Volumetric shrinkage of bentonite-biochar composites. ....................................... 48 Figure 4.8 Volumetric shrinkage of bentonite-biochar composites with 2% biopolymer ...... 50 xi Figure 4.9 Volumetric shrinkage of bentonite-biochar composite with 4% biopolymer ........ 51 Figure 4.10 Cracking factors of bentonite-biochar composites without biopolymer ............. 52 Figure 4.11 Cracking factors of bentonite-biochar composites with 2% biopolymer ............ 53 Figure 4.12 Cracking factors of bentonite-biochar composites with 4% biopolymer ............ 54 Figure 4.13 Cracking images of the different bentonite composites ...................................... 56 Figure 4.14 Cracking images of the different bentonite composites using ImageJ analysis .. 57 Figure 4.15 SEM Image of bentonite composite with 5% biochar ......................................... 58 Figure 4.16 SEM image of bentonite-biochar composite and biopolymer ............................. 59 Figure 4.17 SEM and EDS analysis of bentonite-biochar and with biopolymer content of 2%.................................................................................................................................................. 60 Figure 4.18 SEM along with EDS analysis of biopolymer amended bentonite ..................... 61 Figure 4.19 Initial concentration, ci (mg/L) against % of Pb2+ removal ................................. 62 Figure 4.20 Fitted data using Freundlich isotherm ................................................................. 63 Figure 4.21 Fitted data using Langmuir isotherm ................................................................... 64 Figure 4.22 FTIR spectra of bentonite composites of biochar, biopolymer and combination of both before adsorption ......................................................................................................... 66 Figure 4.23 FTIR spectra of bentonite composites of biochar, biopolymer and the combination of both after adsorption with Pb2+ ions ............................................................... 66 xii List of Tables Table 3.1 Properties of bentonite clay ..................................................................................... 21 Table 3.2 Properties of biochar ............................................................................................... 21 Table 3.3 Physical and chemical properties of sodium alginate ............................................. 22 Table 4.1 Atterberg limits of bentonite composites ................................................................ 37 Table 4.2 Compaction characteristics of bentonite composites .............................................. 39 xiii List of Abbreviations ATR Attenuated total reflection. ASTM American Standard for Testing and Materials ALG Alginate BC Biochar BP Biopolymer B Bentonite CDF Crack density factor CIF Crack Intensity factor CO2 Carbon dioxide EDS Energy dispersive X-ray spectroscopy FTIR Fourier transform infrared spectroscopy. GHG Greenhouse gas ICP-OES Inductively Coupled Plasma-Optical Emission Spectrometry L/S Liquid-to-solid-ratio L.L Liquid limit MDD Maximum dry density Na-bentonite Sodium bentonite NOx Nitrogen oxides OMC Optimum moisture content Pb2+ Lead Pb (NO)2 Lead Nitrate PPM Parts per million P Polymer P.L Plastic limit xiv P.I Plasticity Index SEM Scanning electron Microscope SO2 Sulphur dioxide UBC University of British Columbia UCS Unconfined compressive strength VOCs Volatile organic compounds xv Acknowledgement I am very grateful to everyone who motivated and encouraged me during my research. I would like to express my deepest gratitude to my supervisor, Dr. Sumi Siddiqua for her invaluable guidance, patience and expert advice throughout this research. Her mentorship helped me in shaping both direction and success of my research. I am also grateful to members of our research group for their constant support and cooperation in knowledge and motivation from their experiences. I appreciate the hands-on training I received from the PhD student Alok Chandra at the lab. I would also extend my gratitude to the lab managers in the Fipke laboratory for their invaluable support, especially Sudip Shrestha and Mark Button. Finally, I want to express my gratitude to my family members and friends for their unwavering love, encouragement and belief in me. xvi Dedicated to My parents and family members for their endless support and encouragement 1 Chapter 1: Introduction 1.1 Overview Early landfills were simply open dumps for untreated waste. These sites frequently contaminated the surrounding land and waterways. Regulations requiring more responsible waste management emerged in the mid-twentieth century as public awareness of environmental issues grew. This included the need for improved containment to prevent gas and leachate migration. The first landfill liners were simple, consisting primarily of compacted clay. Bentonite clays are recognized for their small particle size which contributes to their effective sealing capabilities when they are densely packed and moistened. The self-sealing nature of bentonite clay is helpful as it allows the soil to mend minor fissures. Hence, bentonite clay soil is beneficial for constructing barriers similar to those used in landfills to prevent environmental contamination. Yet, the very quality that enables the bentonite clay to seal effectively also makes them susceptible to fluctuations in volume in response to moisture variations. This can lead to swelling or shrinking, which in turn may cause the soil to crack. The soil's tendency to change volume can pose challenges as it can compromise structural stability due to the formation of larger cracks (Wei et al., 2016). The heat from decomposing waste in landfills can intensify these issues by drying out the soil and increasing the likelihood of cracking. Moreover, the clay soil is susceptible to pore water evaporation, cracking and swelling under the conditions of elevated temperatures and precipitation fluctuations, which can occur due to periodic or seasonal weather patterns (Freilich et al., 2008), the heat-producing breakdown of solid wastes in landfills or activities related to heat production in mining operations (Hornsey et al., 2010; Thiel & Smith, 2004). There are a few methods to mitigate the above challenges associated with bentonite clays. 2 Soil stabilization is the most common method for improving the geotechnical properties of problematic bentonite clay soils. Stabilizing bentonite clay for use as a landfill liner is a crucial step to ensure the proper containment of waste materials and prevent their leakage into the surrounding environment. Mechanical and chemical stabilization techniques are the two broad categories widely used in geotechnical applications (Estabragh et al., 2014). Mechanical stabilization enhances soil performance without altering its chemical composition. Chemical stabilization induces reactions within the soil matrix leading to enhanced bonding and improved stability (Abduljauwad & AI-Sulaimani, 1993). Problematic soils can be chemically stabilized by adding cement, lime and fly ash. However, they are found to be detrimental to the environment due to high CO2 emissions and other air pollutants released to the atmosphere during their production. Therefore, considerable efforts are being employed in exploring environmentally friendly soil stabilizers. Bentonite clay is a widely used material in landfill liners to avoid contaminant migration. Therefore increasing its resistance to desiccation cracking is of particular interest to researchers and practitioners. However, the use of the environmental stabilizers like enzymes, polymers, surfactants and plant extracts etc in stabilizing bentonite clay to withstand the effect of temperature and moisture changes by assessing their volumetric shrinkage and desiccation cracking has not been reported in literature. 1.2 Motivation Chemical stabilization methods are extensively employed to condition unstable fine-grained soils for application in landfill liners (Barman & Dash, 2022). Conventional materials like cement and lime are the most widely used chemical stabilizers. Stabilization with lime and cement enhances the bonding and strength of problematic clays through the cation exchange capacity and the cementing effects caused by pozzolanic reactions (Kiviranta & Kumpulainen, 2011). However, they pose both beneficial and detrimental effects on the environment. The 3 cement production process is resource-intensive and requires many resources including limestone, clay and energy. Cement production is a major source of carbon dioxide (CO2) emissions, largely because of the chemical processes involved in its production, particularly the calcination of limestone. The manufacturing of every metric ton of Portland cement results in the release of approximately one metric ton of CO2, positioning it as the second-largest source of greenhouse gas emissions; the production of one metric ton of lime, on the other hand, results in the release of approximately 0.86 metric tons of CO2 (Thangaraj & Thenmozhi, 2013). During the cement production process, pollutants such as particulate matter, sulfur dioxide (SO2), nitrogen oxides (NOx) and volatile organic compounds (VOCs) are emitted into the atmosphere. These emissions can degrade air quality and pose health risks to humans. Another environmental problem that can occur with the use of traditional stabilizers is the release of cement dust particles into the air, contributing to particle air emissions in the field (Dash & Hussain, 2012; Meo SA, 2004). Therefore, alternative binders, such as industrial byproducts or natural pozzolans, which may have lower environmental impacts than traditional cement, should be explored. Biopolymers, derived from natural sources such as plants, microorganisms or animals are increasingly being studied as environmentally friendly alternatives for soil stabilization. These biodegradable materials offer a sustainable approach to improving soil properties, reducing erosion and enhancing the stability of soil in various applications. Among the biopolymers, very few researchers have studied the efficacy of sodium alginate in being used as a soil stabilizer. Sodium alginate is derived from alginic acid and exists as a sodium salt. It is classified as an anionic polymer and belongs to the hydrogel group, which enables it to expand or swell when exposed to water. The presence of a hydrophilic functional group (OH-) results in the ability to absorb water. (Elkenawy et al., 2023; Khan et al., 2022) reported that adding sodium alginate to clayey soils improves swelling, hydraulic performance 4 and undrained shear strength. Therefore, biopolymer (sodium alginate), which is extracted from wastewater, was adopted for the polymerization of bentonite. In addition, Biochar, an industrial byproduct, has seen extensive application as an additive in clay liners for landfills in recent years. It has gained attention as a sustainable soil amendment with potential applications in soil stabilization. This is because of its low cost, low carbon footprint and chemical stability (Weber & Quicker, 2018). Biochar is characterized by its high porosity, low density and thermal conductivity as well as its large specific surface area and chemical durability (Cuthbertson et al., 2019; Reddy et al., 2015; Lehmann & Joseph, 2015). The incorporation of biochar into soil has the potential to modify its physical, chemical and hydraulic characteristics by altering pore size distribution, enhancing water retention and promoting aggregate stability (Cyrus et al., 2023 ; Patwa et al., 2021; Jyoti Bora et al., 2021; Hussain et al., 2021; Kong & Zhou, 2020; Reddy et al., 2015). Furthermore, (Gupta et al., 2020; Zand & Abyaneh, 2020; Jayawardhana et al., 2016a) reported that biochar can remove harmful heavy metals from landfill leachates through carbon adsorption which can have drastic effects on human health. Although multiple studies exist on the performance of biochar and biopolymer on the clayey soils individually, their combined influence on the properties of bentonite clay to withstand the effect of temperature and moisture changes by assessing their volumetric shrinkage and desiccation cracking has not been reported in the literature. Also, the use of industrial by-products and naturally occurring materials as soil additives has been of high interest in recent years. Therefore, this study evaluates the influence of adding biochar and biopolymer to the high-swelling bentonite clay in determining compaction, compressive strength, volumetric shrinkage and desiccation cracking. Further, an effort was made to study the efficacy of bentonite clay amended with biochar and biopolymer in removing harmful heavy metal Lead (Pb2+). Lead (Pb2+) is chosen as a model contaminant because of its toxic nature and adverse effects on the human health. 5 1.3 Objective The primary objective of this study is to evaluate the influence of the addition of finer biochar and biopolymer to the high-swelling bentonite clay in determining the mechanical and adsorption properties of bentonite clay amended with sustainable stabilizers. The following objectives were set to achieve the goals of this research. Objective-1 Develop different composites of bentonite clay using biochar and biopolymer (sodium alginate). Understand the performance of the composites in terms of strength and volumetric shrinkage. Tasks 1. Designing bentonite clay composites using various dosages of sodium bentonite, biochar and biopolymer based on the factorial design method. 2. Performing basic geotechnical element tests of composites such as liquid and plastic limit, maximum dry density and optimum moisture content. 3. Conducting unconfined compressive strength tests on the specimens to determine the uniaxial compressive strength after 1 and 7 days of curing. 4. Preparing specimens for the volumetric shrinkage measurements by statically compacting them to maximum dry density (MDD) in an oedometer ring and kept in an oven to determine the volumetric deformation after the end of the drying cycle. 5. Analyzing the cracking patterns using the ImageJ analysis and determining the cracking factors, for instance, crack density factor (CDF) and crack intensity factor (CIF). 6 Objective-2 Conducting batch adsorption studies using Pb2+ as a contaminant along with micro-structural analysis such as SEM-EDS and ATR-FTIR to determine the adsorption properties of the composites and the interactions of the composites at the microstructural level. Tasks 1) Performing 24-hour batch adsorption on the bentonite composites using Lead (Pb2+) as a contaminant to determine the adsorption behavior of the composites. 2) Analyzing the composites at the micro-structural level by conducting Scanning electron microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) and Attenuated Total Reflectance-Fourier Transformed Infrared Spectroscopy (ATR-FTIR). 1.4 Thesis Organization The thesis consists of five chapters as follows: Chapter 1: Introduces the application of bentonite clays as a landfill bottom liner and the possible issues associated with them. Furthermore, it highlights the benefits of sustainable amendments in stabilizing the problematic bentonite clays. Finally, the objectives of the study were specified with the outlined tasks to obtain those goals. Chapter 2: Provides a literature review on sodium bentonite as a landfill liner and different stabilization methods. In addition, the use of biochar and biopolymer as sustainable stabilization materials in enhancing the geotechnical and adsorption properties of the bentonite clay are included. Chapter 3: Presents the details of the materials and methods employed for attaining the objectives of this study. In addition, it provides details of the procedures for sample preparation and experimental studies. 7 Chapter 4: Provides the results and discussions of all experimental studies carried out in this study. The effect of sustainable amendments on the geotechnical and adsorption properties of the bentonite clay are also discussed along with the microstructural analysis. Chapter 5: Presents the overall conclusions of the experimental results and research findings. This chapter further outlines the encountered limitations and potential recommendations for future studies. 8 Chapter 2: Literature Review This chapter presents a comprehensive literature review on various stabilizing materials, including sustainable options, that can be used to stabilize bentonite clay for landfill liner applications. Also, this chapter presents the review of the basic mechanical and adsorption properties of stabilized bentonite clay, including index, compaction, compressibility, volumetric shrinkage and adsorption behavior. 2.1 Bentonite Clay as a Landfill Liner Bentonite is a natural silicate clay mineral layer primarily composed of the montmorillonite clay mineral. It is characterized by a variety of properties, such as an extensive specific surface area, the capacity for cation exchange (CEC), significant absorption and adsorption. Additionally, it exhibits hydration and swelling abilities, thixotropic behavior, the potential for bonding, impermeability, flexibility and a tendency to interact with organic compounds (Rautureau et al., 2017). Owing to these properties, bentonite clay acts as a bottom liner layer for multilayer engineered landfills. It is common to use bentonite alone or in combination with natural soils or other amendments to construct liners for these landfills. Engineered landfills feature a barrier system with low permeability aimed at impeding the flow of hazardous contaminants and leachate into the soil. The United States Environmental Protection Agency (USEPA, 1993) stipulates that the hydraulic conductivity of the liners employed in these landfills must be lower than 10-7 cm/s. Additionally, these liners must have a minimum thickness of no less than 600 mm. According to (Cazaux & Didier, 2000; Brandl, 1992), the liners should last for long life spans and show minimal volume change behavior. However, a bentonite liner has low compressive strength and can be costly to import when unavailable locally. It is also prone to shrinkage cracks after drying (Sivapullaiah & Baig, 9 2011). Desiccation cracking of bentonite affects its compressibility, mechanical strength and permeability (Peron et al., 2013). Bentonite is a key component in landfill liners and waste containment facilities. Furthermore, it is crucial in preventing leachates from reaching groundwater as well as mitigating shrinkage and desiccation cracks. This is important as these cracks can provide extra flow paths for harmful chemical compositions in leachates to reach groundwater. Consequently, enhancing the clay's resistance to desiccation cracking is of particular interest to researchers and practitioners. Bentonite has been stabilized with various additives, including polymers and MICP as well as traditional additives such as lime, fly ash, cement and bitumen to improve its stability and strength (Liu et al., 2020; Gupt et al., 2020a; Castro-Fresno et al., 2011; Katsioti et al., 2008; Mcrory & Ashmawy, 2005). The next section provides various stabilization techniques that can be applied for bentonite stabilization. 2.2 Soil Stabilization Soil stabilization involves improving soil characteristics through the incorporation and mixing of different materials with the soil. This process aims to increase the soil's shear strength parameters and thereby, its bearing capacity. Clayey soils, known for their stiffness when dry and tend to lose this stiffness upon saturation. Soft clays are characterized by their low compressive strength and high settlement potential, making them challenging for construction. The introduction of moisture can significantly reduce soil strength leading to severe damage to buildings and foundations. In order to counteract these issues, stabilizing agents such as cement or lime are added to the soil; the stabilization techniques not only enhance the engineering qualities of the soil but also convert it into a material that is more appropriate for construction purposes. Benefits of soil stabilization include increased soil strength and durability along with a reduction in soil plasticity and the potential for swelling and shrinkage (Dash & Hussain, 10 2012; Naeini et al., 2012; Petry et al., 2002). Stabilization techniques are broadly classified into two categories: mechanical and chemical stabilization methods (Estabragh et al., 2014). 2.2.1 Mechanical Stabilization Mechanical stabilization refers to the process of altering soil gradation to enhance properties such as shear strength and density. This is achieved by compacting and densifying the soil through the application of mechanical energy. Techniques commonly employed include the use of rollers, rammers and vibration methods; occasionally, blasting is also used. The effectiveness of this method largely depends on the inherent properties of the soil material being treated. 2.2.2 Chemical Stabilization Chemical stabilization is achieved through the addition of materials such as cement, lime, fly ash (FA) and various salts (Smaida et al., 2021; Muhammad & Siddiqua, 2021; Lemaire et al., 2013). This method induces reactions within the soil matrix leading to enhanced bonding and improved stability (Abduljauwad & AI-Sulaimani, 1993). Petry et al. (2002) classified these stabilizers as traditional or non-traditional additives. Traditional additives, such as hydrated lime, Portland cement and fly ash, are well-established in soil stabilization practices. Additionally, by-product additives, including lime kiln dust and cement kiln dust are utilized for their stabilizing properties. Conversely, non-traditional additives, such as sulfonated oils, polymers, salt and enzymes, represent a newer approach to soil stabilization and are gaining attention for their potential benefits in this field. 2.2.2.1 Traditional Stabilizers Traditional stabilizers including cement, lime and fly ash are the most utilized materials to enhance the characteristics of weak clay soils (Smaida et al., 2021; Lemaire et al., 2013; N. K. Sharma et al., 2012). In addition to the most common traditional stabilizers, materials such as 11 gypsum, slag, alum, bituminous substances, kiln dust and stone dust have also been used to enhance soil stability (Andavan & Maneesh Kumar, 2020; R. K. Sharma, 2017). Adding cement and lime has been shown to effectively stabilize low-plasticity clay, enhancing its compressive strength, swelling characteristics, shear strength and cohesion through the filling of its micropores, as reported by (Smaida et al., 2021; Lemaire et al., 2013). However, concerns have been raised regarding the potential negative environmental impacts and occupational health and safety issues associated with these practices as reported by Indraratna, Athukorala, and Vinod (2013). The cement production process is resource-intensive and requires a lot of resources including limestone, clay and energy. The chemical processes involved in the cement production process, particularly the calcination of limestone, are a major cause of carbon dioxide (CO2) emissions in the cement industry. Overall, Portland cement emits around one metric ton of CO2 per ton of cement production, while lime emits approximately 0.86 metric tons of CO2 per ton of its production (Thangaraj & Thenmozhi, 2013). Pollutants released into the atmosphere during the cement production process include particulate matter, sulfur dioxide (SO2), nitrogen oxides (NOx) and volatile organic compounds (VOCs). These pollutants can lower air quality and pose risks to human health. Another environmental problem that can occur with the use of traditional stabilizers is the release of cement dust particles into the air contributing to particle air emissions in the field (Dash & Hussain, 2012; Meo SA, 2004). Therefore, alternative binders, such as industrial byproducts or natural pozzolans, which may have lower environmental impacts than traditional cement and lime, should be explored. 2.2.2.2 Non-traditional Stabilizers Non-traditional stabilizers or alternative binders, particularly those derived from industrial byproducts or natural pozzolans, have received a lot of attention in civil engineering. They are being studied and used because of their potential environmental benefits with reduction in CO2 emissions, low cost and unique engineering properties. Some of the alternative binders are fly 12 ash, slag cement, natural pozzolans, rice husk ash, corn cub ash, metakaolin and silica fume. Alternate binders also include liquid or powdered lignin derivatives, polymers, resins, acids, silicates and ions ( Latifi, Meehan, et al., 2016; Latifi, Marto, et al., 2016; Blanck et al., 2014). Many of these materials are found in nature or can be produced within nature.In addition, water-insoluble gel-forming microbial biopolymers, such as xanthan, chitosan, polyglutamic acid, sodium alginate and polyhydroxy butyrate have applications as soil additives. They can be utilized to enclose a bioremediation zone or mitigate soil liquefaction (Dejong et al., 2013; Ivanov & Chu, 2008). Biopolymers, derived from natural sources such as plants, microorganisms or animals are increasingly being explored as environmentally friendly alternatives for soil stabilization. These biodegradable materials offer a sustainable approach to improving soil properties, reducing erosion and enhancing soil stability in various applications. Biopolymers such as xanthan gum, guar gum, agar gum, gellan gum, sodium alginate and beta-glucan have been effective in enhancing the mechanical properties and hydraulic performance as well as strengthening weak soil (Chang et al., 2016; Chang et al., 2015b; Khatami & O’Kelly, 2013; Bouazza et al., 2009;). Among the biopolymers, very few researchers have studied the efficacy of sodium alginate in being used as a soil stabilizer. Therefore, Sodium alginate, which is extracted from wastewater was adopted for the polymerization of bentonite in the current study. Sodium Alginate Sodium alginate, classified as an anionic polymer and part of the hydrogel category, can expand in water. This ability to absorb water is attributed to the presence of hydrophilic functional groups (OH-). Sodium alginate gradually dissolves in water forming solutions that pour smoothly and create gels through reactions with sodium and calcium salts (Bouazza et al., 2009). Sodium alginate has gained attention as a low-carbon cementing agent for soil 13 improvement due to its more environmentally friendly nature compared to traditional cementing materials. (Elkenawy et al., 2023; Khan et al., 2022.) reported that adding sodium alginate to clayey soils improves swelling, hydraulic performance and undrained shear strength. Torfi et al. (2021) identified that cementation bonds were formed at the optimum concentration of sodium alginate, which aided in enhancing inter-particle forces and reducing the free swelling of clayey soils. Biochar Biochar, an industrial byproduct, has seen extensive application as an additive in clay liners for landfills in recent years. It has gained attention as a sustainable soil amendment with potential applications in soil stabilization. This is because of its low cost, low carbon footprint and chemical stability (Weber & Quicker, 2018). Biochar is a porous carbonaceous solid that results from the thermochemical conversion of biomass in the absence of oxygen (Lehmann & Joseph, 2015). It is characterized by its high porosity, low density and thermal conductivity as well as its large specific surface area and chemical durability (Cuthbertson et al., 2019; Reddy et al., 2015; Lehmann & Joseph, 2015). The incorporation of biochar into soil has the potential to modify its physical, chemical and hydraulic characteristics by altering pore size distribution, enhancing water retention and promoting aggregate stability (Cyrus et al., 2023; Patwa et al., 2021; Jyoti Bora et al., 2021; Hussain et al., 2021; Kong & Zhou, 2020; Reddy et al., 2015). Several studies (Gupta et al., 2020; Zand & Abyaneh, 2020; Jayawardhana et al., 2016a) reported that biochar can remove harmful heavy metals from landfill leachates through carbon adsorption which can have drastic effects on human health. 2.3 Interaction of Biochar and Biopolymer with the Bentonite Clay In landfill liner applications, incorporating biochar and sodium alginate with bentonite soil offers several enhancements. Adding biochar to bentonite soil is particularly beneficial for 14 improving the barrier properties of landfill liners. This is due to biochar's porous structure which further decreases the soil's hydraulic conductivity, an essential factor in preventing leachate migration. Additionally, biochar improves the compressive strength of bentonite, a critical attribute in landfill liners that must withstand various stresses without compromising their integrity (Zong et al., 2014). Sodium alginate interacts with bentonite to form a gel-like structure, effectively reducing the permeability of the liner (Bouazza et al., 2009). This gelation is crucial for sealing pores and cracks in the soil, ensuring more efficient containment of waste materials. As a natural polymer, sodium alginate also enhances the overall stability of the bentonite soil providing additional structural support to the landfill liner. This stabilization is particularly beneficial in maintaining the liner's structural integrity over time, an essential aspect of effective landfill management. Using biochar and sodium alginate in landfill liners also aligns with environmental sustainability goals. These natural eco-friendly materials reduce reliance on synthetic polymers and other non-sustainable substances, promoting a more environmentally responsible approach to landfill construction and maintenance. In addition, combining these materials in bentonite soil allows the liner to better adapt to soil movement and settlement, which are everyday occurrences in the dynamic landfill environment. Furthermore, the moisture-retaining properties of both biochar and sodium alginate help prevent desiccation cracking, a common issue in landfill liners due to drying conditions (Jayawardhana et al., 2016b). This moisture retention maintains the liner's integrity even under varying moisture levels. Moreover, biochar's ability to adsorb various contaminants enhances waste containment and neutralization in landfill leachate, adding another layer of effectiveness to the landfill liner system (Han et al., 2019). However, the combined influence of biochar and biopolymer amendment on the temperature and moisture changes, which result in volumetric 15 shrinkage and desiccation cracking of clayey soils, has not been reported in the literature. Also, the use of industrial by-products and naturally occurring materials as soil additives has been highly interesting in recent years. 2.4 Performance Criteria of Stabilized Bentonite Clay In geotechnical engineering, stabilized bentonite soil performance testing is critical in determining the soil's suitability for various applications such as landfill liners, hydraulic barriers and building materials. Furthermore, the properties and performance of stabilized bentonite soil determine the suitability and the efficacy of an additive for a successful landfill liner. Besides its properties, the appropriate mix proportions and mixing methods are also essential to achieve the desired properties of landfill liners. The following are the general testing methods followed in assessing the mechanical and adsorption properties as well as the cracking patterns in bentonite soil with various sustainable amendments. 2.4.1 Index Properties Index properties, for instance, the liquid limit, plastic limit, specific gravity and swell index, are crucial for characterizing bentonites. These parameters are instrumental in describing the moisture behavior and sensitivity of different types of soils (Kiviranta & Kumpulainen, 2011). The liquid limit (LL) refers to the moisture content at which soil changes from a plastic to a liquid state. Conversely, the plastic limit (PL) is the moisture content at which soil shifts from semi-solid to plastic. 2.4.2 Unconfined Compressive Strength The Unconfined Compressive Strength (UCS) test is an important parameter in designing and evaluating landfill liners, especially for those made of compacted clay or amended soils. The UCS test is critical for landfill liner design because it measures the material's ability to 16 withstand axial stresses, indicating its stability and suitability for waste containment without failure. To be suitable for liner construction, the mix should have a minimum strength of 200 kPa and no more than 4% volumetric shrinkage after drying (Ojuri & Oluwatuyi, 2017). 2.4.3 OMC & MDD The Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) hold significant importance in designing landfill liners as they ensure that the soil is compacted to its densest form, achieving minimal permeability. This optimization maximizes the strength and durability of the liner while also reducing settlement and migration of leachate, ensuring effective containment over time. (Y. Lu et al., 2023; Kumar et al., 2020; Weber & Quicker, 2018) reported that the OMC of the soil increases whereas the MDD decreases with the inclusion of biochar. 2.4.4 Volumetric Shrinkage Volumetric shrinkage of soil is an important consideration in landfill liner design because it reflects the potential for the soil to shrink in volume as it dries, which can cause fractures and cracks to form. In the context of landfill liners, designed to be low-permeability barriers that prevent contaminants from migrating into the environment, any cracking may undermine the liner system's integrity. If the soil shrinks and cracks, it loses its barrier function allowing leachate to pass through the liner and contaminate the underlying groundwater or soil. Understanding and mitigating the volumetric shrinkage of soil used in liners is critical to a landfill's long-term containment capabilities. 2.4.5 ImageJ Analysis In analyzing desiccated bentonite composites, image processing techniques are applied to the raw RGB (red, green, blue) images to quantify the patterns of cracks and shrinkage. (Singh et 17 al., 2018; Auvray et al., 2014; Lakshmikantha et al., 2009) applied image analysis techniques to quantify the surface crack patterns in the cohesive clays because of their wetting and drying cycles. Initially, the images are processed to eliminate background noise enhancing the quality of the image for analysis. This step is essential for preparing the image for further processing. Once the image is prepared, it is converted into a greyscale image using a thresholding technique in ImageJ software. This process adjusts the image so that the cracks and shrinkage regions, which appear darker, are differentiated from the intact, lighter soil. The greyscale image is then subjected to segmentation which partitions the image into separate regions according to specific criteria, such as greyscale values. In this approach, a threshold value is established to differentiate the dark pixels representing cracks and shrinkage from the light pixels of the undamaged soil. During the segmentation process, each pixel in the grayscale image is evaluated against this threshold. Pixels that are darker than the threshold are converted to black (with a value of 0) while those that are lighter are converted to white (with a value of 255). This process creates a binary image that distinctly separates the damaged regions from the intact ones. After segmentation, the binary image is then analyzed to determine the patterns of cracking and shrinkage. This involves measuring various parameters, such as the total area of the cracks, the Crack Intensity Factor (CIF) and the Crack Density Factor (CDF) (Lakshmikantha et al., 2009). The CIF indicates the extent of cracking within the remaining intact area of the specimen. At the same time, the CDF represents the combined area of cracks and shrinkage as a percentage of the initial area of the specimen. These metrics are critical for understanding the extent and pattern of damage within clay soil samples. The interactions between the bentonite clay and sustainable amendments at microscopic level are explained in the next section. 18 2.5 Microstructural Analysis and Desiccation Cracking Soil stabilization is one of the important areas of soil mechanics where different composite materials like fibers, fly ash, saw-dust and other industrial by-products are added together to enhance the characteristics of the soil. Different materials interact with soil in distinct ways, influencing properties such as strength, stiffness, permeability and compressibility. Therefore, it is essential to understand the interaction of the different materials with the soil at the micro-level. Microscopic techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) provide information on topography, morphology, composition, chemistry, grain orientation and crystallographic information of the clay specimen. SEM analysis visualizes the interactions between soil particles and agents, highlighting morphological changes (Chegenizadeh et al., 2020). Scanning Electron Microscopy (SEM) is an effective microscopic technique for understanding the interactions between stabilizing materials and soil in geotechnical engineering (Ural, 2021). 2.6 Adsorption Studies and Environmental Implications Adsorption studies are essential in geotechnical engineering, especially when designing and evaluating landfill liners. Landfill liners are critical for preventing soil and groundwater contamination from leachate, a liquid that percolates through waste material in a landfill. Adsorption studies assist in understanding how various contaminants in the leachate are adsorbed by the liner material (Alexander et al., 2019). This is essential for preventing harmful chemicals from leaching into the surrounding environment. Engineers can choose the best material for a landfill by analyzing the adsorption properties of various liner materials like clay, bentonite or geosynthetic liners. This choice is based on the material's ability to absorb and immobilize various types of contaminants. Adsorption studies are critical for evaluating a landfill's environmental impact. They aid in predicting how well the liner will safeguard the 19 soil and groundwater from contamination. Adsorption studies also drive the development of new liner materials with improved adsorption properties, such as modified clays or composite liners, which provide better leachate resistance. Sodium bentonite , a type of expansive clay known for its high cation exchange capacity (CEC), serves as an efficient adsorbent. This effectiveness stems from its negatively charged specific surface area, which exhibits a strong affinity for positively charged heavy metal ions (Uddin, 2017; Gupta & Bhattacharyya, 2012). (Gupt et al., 2020b; Han et al., 2019; Komkiene & Baltrenaite, 2016) conducted an adsorption study, taking Lead (Pb2+) as a contaminant, which is common in municipal solid waste (MSW) and has adverse health effects because of its high concentrations (Kushwaha et al., 2018). Therefore, studying the adsorption properties of landfill liners is an integral part of its design. 2.7 Summary This literature review delves into the application of bentonite clay as an effective landfill liner due to its unique properties, such as high specific surface area, cation exchange capacity and swelling abilities. It highlights the challenges associated with its use, including low compressive strength and susceptibility to desiccation cracking and discusses various stabilization techniques to enhance its properties. Soil stabilization methods, both mechanical and chemical, are explored with an emphasis on improving soil's geotechnical properties and durability. The review also examines the environmental implications of traditional and non-traditional stabilizers, advocating for eco-friendly alternatives like biopolymers and biochar. These efforts aim to not only improve the performance and sustainability of landfill liners but also to address the environmental impacts associated with traditional soil stabilization materials. 20 Chapter 3: Materials and Methods 3.1 Overview The primary aim of this study is to analyze the performance of biochar and biopolymer bentonite composites in terms of strength, volumetric shrinkage and adsorption behavior. To assess the performance of bentonite composites, different laboratory experiments were carried out with varied concentrations of biochar and biopolymer. This chapter describes the materials and methods used in this study and briefly describes the Mechanical and adsorption studies performed on the developed bentonite composites. 3.2 Materials 3.2.1 Bentonite The soil used in the study is sodium bentonite clay. It was obtained from the Brock White Construction Materials, Canada. Natural sodium bentonite exhibits a moderate to high swelling index, attributed to a well-balanced distribution of exchangeable ions predominantly sodium and calcium, resulting in a pH level typically recorded around 9. It has a high cation exchange capacity, which allows it to exchange ions with the environment and it has an exceptional swelling capacity absorbing water several times its dry weight and expanding significantly. Bentonite clay's adsorptive and absorptive properties make it effective at removing impurities. 21 Table 3.1 Properties of bentonite clay Classification UCSC Liquid limit (%) Plastic limit (%) Plasticity index pH CH 419 45 374 9 3.2.2 Biochar British Columbia Biocarbon Ltd produced the BC Biocarbon Black Bear Biochar used for this research. It was produced at high temperatures ranging from 600℃ to 800℃ which ensures low bio-oil residues. It makes the biochar highly stable and helps in long-term carbon removal. The biochar came in as pellet form and consists of different sizes ranging from less than 4.75 to 0.075mm. For the experimental purposes, the biochar is grounded with the help of a grinder to make it fine up to less than 0.075mm. The size of biochar used in this study is the one that passes through a 0.075mm sieve. Table 3.2 Properties of biochar Texture pH value at 20℃ Odour Solubility in water Auto ignition temperature ℃ Density at 20℃ (kg/m3) Black solid 7-9.5 Odourless Insoluble 450 150 22 3.2.3 Biopolymer (Sodium Alginate) The commercial sodium alginate (ALG), derived from alginic acid, was sourced in its pure form from SIGMA-ALDRICH CANADA LTD. Sodium alginate, classified as an anionic polymer and part of the hydrogel category, which can expand in water. This ability to absorb water is attributed to the presence of hydrophilic functional groups (OH-). The molecular structure of sodium alginate is shown in Figure 3.1. Figure 3.1 Molecular structure of sodium alginate Table 3.3 Physical and chemical properties of sodium alginate Appearance (colour) Appearance (form) Spectrum Loss on Drying (%) Viscosity (cps) pH White to beige Solid Conforms to structure <- 15.5 5.0 to 40 5-8 23 (a) (b) (c) Figure 3.2 Image of a) Bentonite b) Biopolymer c) Biochar 3.3 Sample Preparation and Curing All specimens were prepared using deionized water to achieve the Optimum Moisture Content (OMC). Bentonite samples were modified by adding 5%, 15% and 25% biochar (by weight) with the biochar particles being fine enough to pass through a 0.075mm sieve. Predetermined quantities of bentonite and biochar were measured and thoroughly mixed in a large pan with a spatula to prepare the biochar-enriched bentonite samples. Deionized water, equivalent to the OMC of the total dry weight of the bentonite and biochar mixture, was then added. This mixture was stirred until a uniform moisture distribution was achieved. Similarly, bentonite samples were also modified by incorporating 5%, 15% and 25% biochar (by weight) and 2% and 4% biopolymer (by weight) composites, following the same preparation procedure. The samples of bentonite composites with varying biochar (5%,15% and 25%) and biopolymer (2% and 4%) contents were prepared to study compaction properties, unconfined compressive strength, volumetric shrinkage and adsorption behavior. Details of the preparation methods for each bentonite composite are explained below. 24 3.3.1 Unconfined Compressive Strength To prepare the samples for the Unconfined compressive strength (UCS) test, bentonite clay, biochar and biopolymer were thoroughly mixed in a large pan using a spatula. Deionized water, corresponding to the Optimum Moisture Content (OMC) of the total dry weight of the composite mixture, was then added. The mixture was mixed until a uniform moisture distribution was achieved. For compaction, the soil specimens consisting of bentonite, biochar and biopolymer were compressed at the OMC and the Maximum Dry Density (MDD) in three equal layers within a mold of 38mm diameter and 76mm height. After compaction the samples were carefully removed from the mold using a hydraulic jack and then wrapped in several layers of cling film. They were left to cure for periods of 1 day and 7 days in a room with high relative humidity and a controlled temperature of 22 ±2 °C. All samples were tested in triplicates to ensure the reliability and repeatability of the experimental results. 3.3.2 Volumetric shrinkage For the volumetric shrinkage testing, the specimens of bentonite, biochar and biopolymer were prepared at OMC by mixing thoroughly until the homogeneous moisture distribution was observed. The samples were enclosed in airtight bags for 2 days to achieve moisture equilibrium and statically compacted to maximum dry density (MDD) in an oedometer ring with a diameter of 63.3 mm and a height of 22.3 mm, resulting in an initial thickness of 20 mm. The compacted specimens were placed between two filter papers and kept in an oven for 24 hours at 45 ±2° C. 25 Figure 3.3 Volumetric shrinkage sample 3.3.3 Adsorption Studies For the adsorption study, a series of batch tests were conducted on the bentonite composites of biochar (5%) and biopolymer (2%) and combination of both. To create lead (Pb2+) concentration solutions with a range of 50 to 1,000 mg/L, a predetermined amount of lead nitrate (Pb (NO3)2) and a mixture of bentonite, biochar and biopolymer were dissolved in 50 mL of deionized water in a centrifuge tube. A batch sorption study was carried out at a pH of 5 to compare the adsorption potential of various materials (Gupt et al., 2020b). This procedure was carried out to prevent the precipitation of Pb2+ in the solution, which is prone to occur within the pH range of 8-11.5. Following centrifugation of the sample for 30 minutes at 1500 rpm, the supernatant was filtered through a syringe filter with a 0.45µm grade. The adsorbed lead concentration was subsequently determined using ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry). All samples were tested in triplicates to ensure the reliability and repeatability of the experimental results. 26 Figure 3.4 Batch Adsorption testing and sample preparation 3.4 Experimental Methods This section outlines geotechnical characterization techniques including index properties, compaction, unconfined compressive strength, volumetric shrinkage, adsorption and microstructural analysis. 3.4.1 Atterberg Limit Test The liquid limit, plastic limit and plasticity index of bentonite clay, which was enhanced with biochar and biopolymer, were determined by Atterberg limit tests. The Atterberg limit tests followed the procedures outlined in ASTM D4318 - the standard test method for these soil characteristics (ASTM, 2017). 27 3.4.1.1 Liquid Limit Test The liquid limit of the sample refers to the moisture content at which it transitions from a plastic state to a liquid state. The test was conducted using the cone penetration method. The following steps were followed to determine the liquid limit of the bentonite clay composites. • Samples were prepared by mixing the distilled water with almost 200g of bentonite composites until a smooth, uniform paste was observed. • The prepared bentonite composite paste was placed in a container of a specific size, ensuring a smooth and level surface. • The supporting assembly was lowered until the tip of the penetration cone touched the soil's surface, set the position to zero and then the green button was pressed. • The cone was released for 5 seconds using an automatic release device. After the penetration, the depth of the penetration was measured; the entire procedure was repeated by varying the moisture content. • Following each penetration, a small sample of soil near the penetration area was collected to determine its moisture content. • A graph was plotted by plotting penetration depths against corresponding moisture contents. The moisture content that corresponded to 20 mm of penetration was found to be the sample's liquid limit. 28 Figure 3.5 Cone penetrometer setup 3.4.1.2 Plastic Limit The plastic limit of the soil is identified as the moisture content at which it transitions from a plastic to a semi-solid state. Additionally, it represents the lowest water content at which the soil can be reshaped into threads of 3.2 mm in diameter without breaking apart. The specimen was prepared by adding distilled water and making it into a smooth, uniform paste to determine the plastic limit. The following steps were followed to determine the plastic limit of the bentonite clay composites: • A small soil mass was taken to form an ellipsoidal mass that was rolled into thin threads about 3.2mm in diameter. • This process was repeated by re-rolling until the sample was gradually dried and crumbled around this standard diameter. 29 • The procedure was carried out three times to obtain three different moisture content values; the plastic limit was calculated as the mean of these moisture contents. 3.4.1.3 Plasticity Index The Plasticity Index (PI) of soil quantifies its plasticity and gives information regarding the soil's consistency and cohesiveness across varying water contents. It is determined by subtracting the soil's plastic limit (PL) from its liquid limit (LL) as given in equation 3.1 (ASTM D4318-17). 𝑃𝐼 = 𝐿. 𝐿 − 𝑃. 𝐿 (3.1) 3.4.2 Compaction Test In this study, the miniature compaction test was performed on all specimens of bentonite composites prepared with biochar and biopolymer. Initially, biochar and biopolymer were mixed dry with bentonite clay. Following this, distilled water was added and thoroughly combined with the mixture. The wet sample of bentonite composites was compacted in three different layers with a standard compaction effort. The compacted samples of different moisture content were weighed after removing from the mold. A small sample was also taken from each layer to determine the moisture content. At least five data points were recorded for each sample to construct the compaction curve. The maximum dry density (MDD) and optimum moisture content (OMC) were identified through the necessary calculations. 30 Figure 3.6 Miniature compaction apparatus 3.4.3 Unconfined Compressive Strength Test The Unconfined compressive strength (UCS) of the bentonite composites enhanced with biochar and biopolymer was evaluated following ASTM D2166 (ASTM 2006). This UCS test was instrumental in assessing the strength properties of various bentonite clay composites. Furthermore, the UCS test helps to determine the influence of different composites on the strength of bentonite clay. Bentonite clay specimens composed of biochar and biopolymer were compacted with the standard compaction effort to achieve specimens at their optimum moisture content and maximum dry density, in three equal layers, within a mold measuring 38mm in diameter and 76mm in height. After compaction, the samples were extracted from the mold using a hydraulic jack, then enveloped in multiple layers of cling film and stored to cure in a humidity-controlled room at a steady temperature (22 ± 2 °C). All specimens were tested for triplicates and were cured for 1 and 7 days. The application of the vertical load was consistently maintained at a rate of 0.8 mm/min until the samples failed. 31 Figure 3.7 Unconfined compressive strength test in progress 3.4.4 Volumetric Shrinkage The method used to assess the volumetric deformation was obtained from Dash & Hussain (2015). For the volumetric shrinkage testing, specimens of bentonite, biochar and biopolymer were prepared at OMC. The samples were enclosed in airtight bags for 2 days to achieve moisture equilibrium, and statically compacted to maximum dry density (MDD) in an oedometer ring with a diameter of 63.3 mm and a height of 22.3 mm, resulting in an initial thickness of 20 mm. The compacted specimens were then sandwiched between two filter papers and placed in an oven at 45 ±2°C for 24 hours. Three replicates were prepared for the dimension’s measurement purposes. The average height and diameter were measured using a vernier caliper. They were utilized to calculate the volumetric deformations upon the completion of the drying process. During the drying process, shrinkage occurs both vertically (resulting in a reduction in specimen height) and laterally (manifested as an annular gap between the oedometer ring and the shrunken specimen as well as cracks within the specimen) (Julina & Thyagaraj, 2021; Sanchez et al., 2013). 32 Figure 3.8 Volumetric shrinkage mould, prepared and desiccated samples 3.4.5 Image Analysis In analyzing desiccated bentonite composite samples, image processing techniques were applied to the raw RGB images to quantify the patterns of cracks and shrinkage. Initially, the images were processed to eliminate background noise, enhancing the quality of the image for analysis. Once the image was prepared, it was converted into a greyscale image using a thresholding technique in ImageJ software. This process adjusts the image so that the cracks and shrinkage regions, which appear darker, are differentiated from the intact, lighter soil. The greyscale image is then subjected to segmentation, which partitions the image into separate regions according to specific criteria, such as greyscale values. In this scenario, a threshold value was established to differentiate between the dark pixels representing cracks and shrinkage and the light pixels of the intact soil. Each pixel in the grayscale image is evaluated against this threshold during the segmentation process. Pixels darker than the threshold are 33 turned black (value of 0) whereas all others are turned white (value of 255), resulting in a binary image that clearly delineates the damaged areas from the intact areas. After segmentation, the binary image is analyzed to determine the patterns of cracking and shrinkage. This involves measuring various parameters, such as the total area of the cracks, the Crack Intensity Factor (CIF) and the Crack Density Factor (CDF). The CIF indicates the extent of cracking within the remaining intact area of the specimen while the CDF represents the combined area of cracks and shrinkage as a percentage of the initial area of the specimen. CDF = (𝑇𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐𝑟𝑎𝑐𝑘𝑠)/(𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛) (3.2) CIF = (𝑇𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐𝑟𝑎𝑐𝑘𝑠)/(𝑅𝑒𝑑𝑢𝑐𝑒𝑑 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛 𝑎𝑟𝑒𝑎) (3.3) 3.4.6 Batch Adsorption Study A 24-hour batch sorption study is carried out on the bentonite composites amended with biochar and biopolymer according to ASTM D4646 (ASTM (2008)) for soils and sediments. The amount of Pb2+ sorbed on the adsorbent was obtained by Eq. (3.4), and the amount of Pb2+ removed (%) was calculated using Eq. (3.5). Pb2+ adsorption capacity, 𝑞𝑒 = ሾ𝐶𝑖 − 𝐶𝑒ሿ(𝑉𝑀𝑠) (3.4) Percentage Pb2+ removal = ሾ𝐶𝑖 − 𝐶𝑒ሿ/𝐶𝑖 (3.5) Where 𝑞𝑒 is the amount of ions sorbed per unit mass of sorbent (mg/g), 𝑉 is the volume of solution (mL) used in the batch adsorption study, 𝑀𝑠 is the amount of adsorbent (g) used in batch adsorption study, and 𝐶𝑖 and 𝐶𝑒 are the initial and equilibrium concentrations of the solution (mg/L), respectively. Two widely used adsorption isotherms, namely Langmuir and Freundlich, were employed to model the collected data in this study, as described below. In its non-linear form, the Langmuir isotherm was used as per Eq (3.6): 𝑞𝑒 = ൣ𝑞𝑚𝑎𝑥 × 𝑏 × 𝐶𝑒൧/ሾ(1 + (𝑏 × 𝐶𝑒)] (3.6) 34 Where 𝑞𝑒 is the amount of ions sorbed per unit mass of sorbent (mg/g), 𝑞𝑚𝑎𝑥 is the constant representing maximum adsorption capacity (mg/g), 𝑏 is the Langmuir constant representing adsorption energy (L/g), and 𝐶𝑒 is the equilibrium concentration of ions in the solution (mg/L). In its non-linear form, the Freundlich isotherm can be used as per Eq (3.7): 𝑞𝑒 = 𝐾𝑓 × 𝐶1 𝑛Τ (3.7) Where 𝑞𝑒 is the amount of ions sorbed per unit mass of sorbent (mg/g), 𝐾𝑓 is Freundlich’s coefficient (mg1-n Ln kg-1) and 𝑛 is Freundlich’s exponent which expresses the degree of isotherm linearity, and 𝐶𝑒 is the equilibrium concentration of ions in the solution (mg/L). For this purpose, Lead (Pb2+) ions were selected as the metal ions because of their high concentration in landfill leachates and their drastic effects on human health (Wadanambi et al., 2008). A liquid-to-solid ratio 50 was chosen to conduct the batch adsorption study of the biochar and biopolymer-amended bentonite soil. This is due to the bentonite soil forming a thick gel-like consistency at the L/S of 20 (Gupt et al., 2019). Bentonite clay comprises montmorillonite minerals capable of adsorbing lead ions (Pb2+) on both internal and external surfaces. Internal surface adsorption occurs through the cation exchange mechanism while electrostatic adsorption on the external surface is facilitated by forming Si-O and Al-O complexes (Siddiqua et al., 2014). Biochar is also a good adsorbent of heavy metals because of its properties such as microporosity and specific surface area (Komkiene & Baltrenaite, 2016). Simultaneously, the combined composite of bentonite clay, biochar and biopolymer will provide additional adsorption sites to remove Pb2+ ions and enhance the adsorption capacity of the composites. 35 3.4.7 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) is a sophisticated analytical method based on the principles of atomic emission spectroscopy and inductively coupled plasma from analytical chemistry. The filtered adsorption specimens were diluted with 1% concentrated nitric acid (HNO3) to prepare the samples,. These prepared solutions were then introduced into a high-temperature plasma torch as an aerosol where they were ionized. As the excited atoms revert to their ground state, they release energy. The detector measures the emitted wavelengths and intensities of this energy. Following the calibration with standards, the software calculates and reports the concentration of each element in the samples. Figure 3.9 Experimental setup of ICP-OES 36 3.4.8 Microscopic and Fourier Transform Infrared Spectroscopy Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) analysis were conducted to examine the morphological features of the bentonite clay amended with biochar and biopolymer. The bentonite, biochar, and biopolymer samples were saturated and oven-dried for 24 hours at 105℃. Subsequently, the samples were affixed to aluminum pin stubs using double-sided conductive carbon tape and then sputter-coated with a layer of platinum (10nm) to mitigate surface charging and facilitate secondary electron emission. The samples were imaged using a Tescan Mira 3 XMU SEM after coating. The weight percentage of elemental compositions was determined using an Oxford Instruments XMax Energy Dispersive Spectrometer (EDS) detector. Additionally, standard and lead-contaminated specimens of biochar and biopolymer-amended bentonite soil were analyzed using the ATR FTIR sampling technique (De Klein & Ulbert, 1969) to obtain the infrared absorption spectra using Fourier Transform Infrared Spectroscopy. Figure 3.10 Experimental setup of SEM and EDS 37 Chapter 4: Results and Discussions 4.1 Overview This chapter summarizes the results of the experimental program undertaken to investigate the geotechnical properties and adsorption behavior of various bentonite composites as described in Chapter 3. The goal of this chapter is to carefully assess experimental data for the effectiveness of using sustainable amendments (biochar and biopolymer) for bentonite composites. Finally, the adsorption behavior of the bentonite composites is presented in detail using batch adsorption tests and microstructural analysis. 4.2 Atterberg Limit The Atterberg limit tests for bentonite composites prepared with biochar and biopolymer were performed according to ASTM D4318 - the Standard Test Method for Liquid Limit, Plastic Limit and Plasticity Index of Soils (ASTM, 2017). The results are presented in Table 4.1. The results show that the plasticity index of the bentonite composite with 5% biochar decreases by 19.65% and 40.36% respectively, with an increase in biochar to 15% and 25%. A similar trend was observed with the inclusion of 2% and 4% biopolymer in the bentonite-biochar composites. Table 4.1 Atterberg limits of bentonite composites ID Design Mix Weight percentage (%) Liquid Limit (L.L) (%) Plastic Limit (P.L) (%) Plasticity Index (P.I) B BC BP 1 BBC5BP0 95 5 0 354.74 120.66 234.08 2 BBC15BP0 85 15 0 337.54 149.46 188.08 3 BBC25BP0 75 25 0 316.74 177.15 139.59 38 ID Design Mix Weight percentage (%) Liquid Limit (L.L) (%) Plastic Limit (P.L) (%) Plasticity Index (P.I) B BC BP 4 BBC5BP2 93 5 2 329.64 143.07 186.57 5 BBC15BP2 83 15 2 339.37 182.95 156.42 6 BBC25BP2 73 25 2 306.86 180.17 126.69 7 BBC5BP4 91 5 4 373.37 170.7 202.67 8 BBC15BP4 81 15 4 361.14 182.71 178.43 9 BBC25BP4 71 25 4 309.48 156.61 152.87 4.3 Compaction Characteristics of Bentonite Composites Compaction tests were conducted using miniature compaction equipment to estimate the optimal moisture content and maximum dry unit weight of the bentonite composites. The results for the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) are summarized in Table 4.2. Figures 4.1 to 4.3 illustrate the correlation between water content (expressed in percentage) and dry density (g/cm3) of the bentonite clay composites. 39 Table 4.2 Compaction characteristics of bentonite composites ID Design Mix OMC (%) MDD (g/cm3) 1 BBC5BP0 35.89 1.234 2 BBC15BP0 36.5 1.23 3 BBC25BP0 40 1.138 4 BBC5BP2 37 1.18 5 BBC15BP2 42.32 1.134 6 BBC25BP2 42.5 1.066 7 BBC5BP4 33 1.26 8 BBC15BP4 41 1.134 9 BBC25BP4 41.5 1.073 40 Figure 4.1 Compaction characteristics of bentonite-biochar composites without biopolymer content 41 Figure 4.2 Compaction characteristics of bentonite-biochar composites with 2% biopolymer content 42 Figure 4.3 Compaction characteristics of bentonite-biochar composites with 4% biopolymer content As shown in Figure 4.1, the MDD of 1.234 g/cm3 was obtained for the composite prepared with bentonite and biochar of 5% (w/w). The value decreased by 0.32% and 7.8% with the addition of biochar of 15% and 25%, respectively. The decrease in dry density in bentonite composite resulting from biochar content can be attributed to lower density and higher porosity than bentonite clay (Brown et al., 2006; Lehmann et al., 2011; Soinne et al., 2014). The bentonite-biochar composites also display reduced compressibility under a given compaction energy. When the 2% biopolymer was introduced to the bentonite composites, the MDD of the composites with 5% biochar was recorded 1.18 g/cm3 as shown in Figure 4.2. The value further decreased by 4.2% and 10%, with an increase in biochar content of 15% and 25%, respectively. 43 The decrease in the MDD value with biopolymer addition from 0 to 2% is attributed to the addition of materials with low specific gravities in the range of 1.0 and 1.10. Similarly, when 4% biopolymer was added, the MDD of the bentonite composite with 5% biochar was recorded as 1.26 g/cm3 presented in Figure 4.3. This value decreased by 10% and 15% with the addition of 15% and 25% biochar content. Similar results related to the decrease in the MDD value were reported by Vydehi & Moghal (2022) upon amending the soil with biopolymers such as xanthan gum and guar gum. As the biopolymer content increases in the composite, the biopolymer tends to inhibit the direct interaction of bentonite clay particles, increasing pore volume and reducing MDD (Dehghan et al., 2019). The OMC of bentonite composite with 5% biochar was 36%, as in Figure 4.1. The value increased by 6% and 15%, with the addition of biochar by 15% and 25%. The increased surface area and microporosity of the biochar particles, which lead to an increased water absorption capacity of the bentonite-biochar, may potentially be the cause of the composite's rise in OMC (Downie, 2012; Guo et al., 2014; Blanco-Canqui, 2017). When the 2% biopolymer was introduced to the bentonite composites, the OMC of the composite with 5% biochar was recorded as 37% in Figure 4.2. The OMC value further increased by 14% and 17%, with an increase in biochar content by 15% and 25%. Similarly, when 4% biopolymer was added, the OMC of the bentonite composite with 5% biochar was recorded to be 33% in Figure 4.3. The value increased by 24% and 26% with the addition of 15% and 25% biochar content. These changes in the OMC values in composites can be attributed to the microporosity of biochar and the moisture-absorbing ability of the biopolymer (Vydehi & Moghal, 2022). 4.4 Compressive Strength of Bentonite Composites Figures 4.4-4.6 present the unconfined compressive strength results of bentonite composites which underwent curing for 1-day and 7-days. Figure 4.4 presents the variation of unconfined compressive strength (UCS) of bentonite composite with varying biochar content. 44 Figure 4.4 Unconfined compressive strength of bentonite composites with varying biochar content As illustrated in Figure 4.4, the compressive strength of the bentonite composites with the addition of biochar is from 5% to 15%. The UCS of the composites increased by 5.5% and 0.5% after 1 day and 7 days of curing. Previous researchers have also reported similar findings (Hussain et al., 2021; Rojimul Hussain, 2020). The enhanced interlocking between biochar and bentonite clay particles and increased friction within the bentonite-biochar composite account for the increased UCS of the composites (Sudhakar et al., 2017). The decrease in UCS of bentonite composites with more than 15% biochar can be linked to the reduced cohesive bond and frictional interlocking. The reduced cohesion value observed in bentonite-biochar composites is likely due to changes in pore size distribution and overall porosity of the 45 composite mix, resulting in an expanded pore volume, especially in macropores within the soil structure (S. G. Lu et al., 2014; Sun & Lu, 2014). Furthermore, hydrophilic and hydrophobic functional groups on biochar surfaces support interactions with water, creating a complex network among biochar particles and water molecules. This network facilitates an increased separation distance between soil particles (Pardo et al., 2019). Consequently, there is a reduction in the electrostatic force of attraction between soil particles, resulting in decreased cohesive forces (Al-Shayea, 2001). Moreover, the biochar surface contains water-repellent functional groups, such as aliphatic and aromatic nonpolar groups. These features further contribute to the reduction in soil cohesion by affecting the soil's ability to retain water and maintain structural integrity (Briggs et al., 2012; Kinney et al., 2012). Figure 4.5 Unconfined compressive strength of bentonite-biochar composites with 2% biopolymer 46 Figure 4.5 illustrates the variation in UCS of bentonite composites with varying biochar content and 2% biopolymer. As Figure 4.5 indicates, for bentonite composite with 2% biopolymer and biochar from 5% to 15%, UCS increased by 1.56 and 0.57 times after 1 day and 7 days of curing. The UCS of bentonite composite with 25% biochar decreased by 39.6% and 38.1% after curing for 1 and 7 days, respectively. Incorporating 2% biopolymer into the bentonite-biochar composite has increased the UCS at the optimum biochar percentage of 15%. This is in accordance with the results of Chang et al. (2015a) who reported improvement in the UCS values of clayey soils after incorporating biopolymers. As biopolymer used in this study is an anionic polymer, the polymer-clay interactions may be dominated by electrostatic attraction of the positively charged edge of clay surfaces (Ghasemzadeh et al., 2021). Figure 4.6 Unconfined compressive strength of bentonite with varying biochar content and 4% biopolymer 47 Figure 4.6 illustrates the variation in UCS with varying biochar content and 4% biopolymer. In contrast, in the bentonite-biochar composite amended with 4% biopolymer, the compressive strength decreased by 54% and 60% with biochar content from 5% to 15% after 1 and 7 days of curing. Meanwhile, for the biochar addition from 15% to 25%, the UCS of bentonite composite with 4% biopolymer decreased by 17.4% and 19% after 1 and 7 days of curing. The decrease in UCS is attributed to the elevated positive surcharge and subsequent repulsion of soil particles within the soil-polymer mixture. Additionally, the concentration of alkaline additives may have exceeded the threshold necessary for a chemical reaction to occur, further contributing to this reduction. Naeini et al. (2012) also observed a reduction in the UCS value as the concentration of water-borne polymer increased beyond 5%. 4.5 Volumetric Shrinkage of Bentonite Composites The volume shrinkage properties of the bentonite composites are reported in Figures 4.7-4.9 following the methods obtained by Dash & Hussain (2015). Figure 4.7 illustrates how the volumetric shrinkage of bentonite composite with varying percentages of biochar decreased with increasing biochar content under constant desiccation. 48 Figure 4.7 Volumetric shrinkage of bentonite-biochar composites. With the increase in the biochar content from 5% to 15%, the volumetric shrinkage decreased by 32%, whereas from 15% to 25%, it decreased by 95%. This change in volumetric shrinkage is due to finer biochar particles of less than 75µm. The intra-pores of biochar particles contributed to an increase in the number of micropores within the bentonite-biochar composites. Bentonite clay absorbs water and undergoes significant volume changes due to drying and wetting. On the other hand, biochar, a porous carbonaceous material, has a high porosity and surface area. Hence, the volumetric shrinkage of bentonite composite was reduced with the increase in biochar amendment. The porous structure of biochar helps in water retention, thereby preventing excessive drying of bentonite clay (Yargicoglu & Reddy, 2017; Reddy et al., 2015; S. G. Lu et al., 2014). Also, the biochar is rich in organic matter and contains functional groups on its surface, which 49 interact with the clay minerals. The biochar interaction with the bentonite clay particles helps in modifying bentonite clay properties to reduce its shrinkage upon drying. According to Zong et al. (2014), the decrease in shrinkage can be attributed to alterations in the shrinkage properties of clay minerals induced by the presence of carbon particles introduced into the bentonite composite by biochar. The interaction between the carbon particles of biochar and the clay colloids of bentonite would lead to forming clay-carbon complexes, influencing particle behavior within the bentonite-biochar composite system at a colloidal level. Consequently, significant changes at the microstructural level would occur, affecting the shrinkage properties of the bentonite-biochar composite. The volumetric shrinkage of the bentonite-biochar decreased upon the addition of biopolymer. As illustrated in Figure 4.8, with 2% biopolymer, the volumetric shrinkage of the bentonite-biochar composite decreased by 82.5% for biochar content of 5% to 15%. But beyond 15%, the volumetric shrinkage of the mix increased slightly. 50 Figure 4.8 Volumetric shrinkage of bentonite-biochar composites with 2% biopolymer . The same pattern was observed for the bentonite-biochar composites when 4% of a biopolymer was added to them. This is illustrated in Figure 4.9 accordingly; it can be concluded that the bentonite-biochar composite mixed with biopolymer is more suitable for minimizing the volumetric shrinkage of expansive bentonite clay. Sodium alginate, used as the biopolymer, is a hydrophilic substance with a high affinity for water. It has a film-forming property that creates a cohesive network with bentonite and biochar particles. In this manner, the biopolymer helps improve overall soil structure and stability and supports the bentonite clay resist volumetric shrinkage. 51 Figure 4.9 Volumetric shrinkage of bentonite-biochar composite with 4% biopolymer 4.6 Crack Density Factor (CDF) & Crack Intensity Factor (CIF) The cracking properties and patterns of the bentonite composites determined from the ImageJ analysis are reported in Figures 4.10-4.12. The cracking factors obtained from ImageJ analysis were presented in terms of crack density factor (CDF) and crack intensity factor (CIF). As illustrated in Figure 4.10, in the bentonite composite with 5% biochar content, the CDF value decreased by 16.5% and 19.6% with an increase in biochar content by 15% and 25%. 52 Figure 4.10 Cracking factors of bentonite-biochar composites without biopolymer Furthermore, as depicted in the above Figure, the CIF value decreased by 16% and 19% with an increase in biochar content to 15% and 25%, respectively. The CDF and CIF values of the low plastic soils are less because of their low swelling and shrinkage properties. Singh et al. (2018) mixed high-plastic sodium bentonite clay with non-plastic fly ash and observed a reduction in the CDF and CIF values. Brian Albrecht & Benson (2001) reported that severe cracking is typically caused by larger volumetric shrinkage strains, which are typically present in soils with higher clay content and higher plasticity index. The modification of bentonite properties following the addition of biochar is closely linked to the properties of the biochar, which are influenced by the biomass feedstock and preparation process. Biochar, a non-plastic material that does not undergo volume expansion or shrinkage during water absorption or desorption, substitutes a portion of the clay particles in the soil. As a result, decreased values 53 of CIF and CDF were observed in bentonite clay amended with biochar. Plant-based biochar has higher water retention, which helps resist crack formation (Kumar et al., 2020). Y. Lu et al. (2022) found that the finer biochar is better than the coarser biochar in inhibiting cracking because of its higher water retention due to greater specific surface area. Figures 4.11 and 4.12 depict the CDF and CIF for bentonite-biochar composites with 2% and 4% biopolymer, respectively. Figure 4.11 Cracking factors of bentonite-biochar composites with 2% biopolymer 54 Figure 4.12 Cracking factors of bentonite-biochar composites with 4% biopolymer The biopolymer addition to the bentonite composite with 5% biochar content significantly improved the cracking resistance. With the 2% biopolymer added to the bentonite composite with 5% biochar, the CDF and CIF values decreased by 37.2% and 39.3%, respectively with an increase in biochar content from 5% to 15%. With the further increase in the biochar content to 25%, the CDF and CIF values of the bentonite composites of 2% biopolymer increased by 16.4% and 15.5%, respectively. The addition of biopolymer helps to improve tensile strength and resistance to volume change by forming hydrogel bonds between polymers and clay particles (Muguda et al., 2017). Taheri & El-Zein (2023) observed a reduction in the shrinkage potential which, in turn, helps reduce crack development with the polymer addition. But for 55 the bentonite clay composites with 4% biopolymer addition, the CDF and CIF values of bentonite clay composites with 5% biochar content increased by 98% and 86% with the increase in biochar content to 15%. With the further increase in biochar content to 25%, the CDF and CIF values of the bentonite clay composites decreased by 54% and 51%, respectively. The cracking factors of the bentonite clay composites with varying biochar content showed good crack retention properties. As moisture loss is responsible for crack propagation in the soil (V et al., 2023), the combined interaction of biochar and biopolymer with bentonite clay helps modify swelling characteristics (Taheri & El-Zein, 2023; V et al., 2023). Also, the CDF and CIF values are visually presented with the help of the raw images of different bentonite composites in Figure 4.13 as well as the threshold cracking images of different samples in Figure 4.14. From Figures 4.13-4.14, it can be observed that the biochar addition reduced the intensity of cracks. 56 Figure 4.13 Cracking images of the different bentonite composites 57 Figure 4.14 Cracking images of the different bentonite composites using ImageJ analysis 58 4.7 SEM and EDS Analysis The SEM analysis showed how the biochar is distributed within the bentonite clay matrix, as seen in Figure 4.15. The fine biochar particles are evenly distributed in the bentonite matrix and the biochar encapsulates the bentonite particles. Figure 4.15 SEM Image of bentonite composite with 5% biochar 59 Fig 4.16 shows the biopolymer coating of the flocculated clusters of bentonite particles with the fine biochar evenly distributed in them. Figure 4.16 SEM image of bentonite-biochar composite and biopolymer With the addition of the biopolymer to the bentonite-biochar composite, the biochar particles seem to be more closely bonded with bentonite clusters, as seen in Figure 4.17. This supports the CDF & CIF results, where the cracking factors decrease with the increase in the biopolymer content from 2% to 4%. Figure 4.17 illustrates the changes in pore size and connectivity within bentonite soil resulting from the amendment with biochar. 60 Figure 4.17 SEM and EDS analysis of bentonite-biochar and with biopolymer content of 2% Figure 4.18 shows the biopolymer anchoring the bentonite particles by completely coating the surface of bentonite particles. Ramachandran et al. (2021) reported similar findings by mixing the biopolymer with clay to biopolymer stabilized clay soils. The EDS results of the bentonite-biochar composites with 2% biopolymer show the carbon content in the composites. The higher percentage of O and Si can be seen from the EDS results of biopolymer amended bentonite composites. 61 Figure 4.18 SEM along with EDS analysis of biopolymer amended bentonite 4.8 Adsorption Properties of Bentonite Composites for Lead (Pb2+) contaminant Figure 4.19 shows the relationship between % of Pb2+ removal and initial Pb2+ concentration at pH 5. It can be seen from Figure 4.19 that the bentonite-biochar composites have the highest removal percentage for all initial Pb2+ concentrations due to additional adsorption zones provided by the functional groups of biochar. It can also be observed from Figure 4.19 that with the increase in Pb2+ concentration, the percentage removal for all the bentonite-biochar composites with biopolymer content decreased due to the saturation of adsorption zones with high Pb2+concentration. The bentonite-biochar composites recorded the highest removal of 98.8% for the initial Pb2+ concentration of 50 mg/L. For the same initial Pb2+ concentration, the bentonite composite amended with biopolymer and the combination of both biopolymer and biochar recorded the removal percentage of 95.8% and 92% respectively. However, for the maximum initial 62 concentration of Pb2+(1000 mg/L), the bentonite-biochar composite recorded the highest removal percentage of 52% whereas the bentonite composite amended with biopolymer and the combination of both biopolymer and biochar had a removal percentage of 43% and 36%, respectively. The results of the batch study were analyzed using two distinct isotherm models to establish the correlation between the adsorption capacity of Pb2+ and the equilibrium Pb2+ concentration. Figure 4.19 Initial concentration, Ci (mg/L) against % of Pb2+ removal Figures 4.20 and 4.21 present the Freundlich and Langmuir isotherm fit for the bentonite composites amended with biochar, biopolymer and the combination of both. For the Langmuir isotherm, the maximum adsorption capacities (qmax) and the Langmuir constants (b) for bentonite composites with biochar, biopolymer and their combination were found to be 36.6, 63 33.85, 29.74 mg/g and 0.0046, 0.0027, and 0.0022, respectively. Similarly, for the Freundlich isotherm, the Freundlich coefficients (Kf) and Freundlich exponent (n) for bentonite composites containing biochar, biopolymer and a combination of both were recorded as 1.833, 0.676, 0.5464 and 2.36, 1.85, 1.867, respectively. The regression coefficient (R2) obtained for the Langmuir model was (0.92-0.96) and for the Freundlich model was (0.96-0.98). Among both models, the Freundlich isotherm fitted the best, suggesting adsorption occurs on a heterogeneous surface with variable adsorption capacities and possibly multilayer adsorption. Figure 4.20 Fitted data using Freundlich isotherm 64 Figure 4.21 Fitted data using Langmuir isotherm 4.9 Molecular Characteristics of Bentonite Composites Subjected to Batch Adsorption tests using FTIR Analysis Figures 4.22 and 4.23 present the FTIR spectra of bentonite-biochar composites with and without biopolymer, before and after adsorption of Pb2+ at the initial concentration of 50 mg/L. The bands at 1984, 2160, and 2213 cm-1 for the bentonite-biochar composites before adsorption show some minor peaks for bentonite-biochar composites. These peaks in bentonite-biochar composites are associated to stretching vibrations of O=C=O bonds (Yang et al., 2023). All three composites of bentonite clay with biochar and biopolymer showed similar absorption peaks between the range 1100 cm-1 and below. In this range, the depth of the transmittance 65 peaks varies for the different composites of bentonite clay, which is directly proportional to the concentration of chemical bonds or functional groups within that sample. This range is associated with the silicate layers (Si-O-Si) in the clay mineral structure in bentonite clay (Bergaya & Lagaly, 2006). For biochar, this region is associated with the C-O stretching in cellulose and lignin Si-O stretching, similar to what is in bentonite clay (Yang et al., 2023). For sodium alginate biopolymer, the region between 1100 cm-1 and below is associated with the C-O stretching vibrations of the carbohydrate structures (Derkach et al., 2019). The peak represents the glycosidic linkage vibrations in these polysaccharide chains in sodium alginate, which comprises mannuronic and guluronic acid blocks (Aprilliza, 2017). After adsorption with 50 mg/L lead concentration, the bentonite clay composites showed peaks almost like those of bentonite clay composites before adsorption. However, the depth of the peaks is higher for the composites after adsorption. Figure 4.23 shows that the bentonite clay composite of biochar has peaked with more depth than other bentonite clay composites. Further, to ascertain the chemical interaction during Pb2+ adsorption, the elemental composition of the composites before and after adsorption was determined using the acid digestion method. The results of acid digestion and adsorption show a decrease in the concentration of divalent cations such as Ca2+ and Mg2+ of three composites of bentonite clay by 39.1%, 38.2%, 23.4% and 71.6%, 78.1%, 81.5%. The higher percentage decrease in Mg2+ concentrations compared to Ca2+ across the composites suggests that magnesium ions were more readily exchanged with the Lead (Pb2+) ions. This is due to the higher specific surface area of biochar and cation exchange capacity of bentonite clay. 66 Figure 4.22 FTIR spectra of bentonite composites of biochar, biopolymer and combination of both before adsorption Figure 4.23 FTIR spectra of bentonite composites of biochar, biopolymer and the combination of both after adsorption with Pb2+ ions 67 Chapter 5: Conclusion This chapter summarizes the overall findings of the current research and their significant contributions to the geotechnical field. It also provides the limitations of current research and the possibilities of future research directions. 5.1 Summary This study aimed to obtain a sustainable stabilization material to minimize the cracking and improve contaminant immobilization in bentonite clay when used as a landfill bottom liner material. For this purpose, sustainable soil stabilization materials such as biopolymer and biochar were employed to find their potential in enhancing the geotechnical and adsorption properties of bentonite clay. This study emphasizes the importance of developing sustainable stabilization materials in addressing the inherent challenges of the bentonite clay such as desiccation cracking and volumetric changes due to moisture fluctuations. The application of sodium alginate, a biopolymer derived from wastewater, in conjunction with biochar which is an industrial by-product, has shown promising results in improving the compaction characteristics, compressibility behavior and cracking properties of bentonite clay. Moreover, the added benefit of biochar in absorbing harmful heavy metals, specifically lead ions (Pb2+), from landfill leachates emphasizes its role in mitigating environmental contamination. The findings from the experimental research indicate that the use of both biochar and biopolymer amendments not only improves the physical and mechanical characteristics of bentonite clay but also aids in making soil stabilization practices more sustainable by efficiently repurposing waste materials. The effectiveness of the suggested sustainable amendments is further supported by the microstructural analysis using SEM and EDS in conjunction with FTIR spectroscopy, which offers insightful information about the mechanisms of interactions between the biochar, biopolymer and bentonite. 68 5.2 Key Findings and Conclusions The fundamental geotechnical properties essential for soils used in landfill liners, such as compaction characteristics, unconfined compressive strength, volumetric shrinkage, cracking properties (including crack density factor (CDF) and crack intensity factor (CIF)) were examined. Also, the adsorption characteristics of the bentonite composites were investigated through 24-hour batch adsorption studies. Based on the laboratory experiments conducted in this study, the following conclusions were drawn: • With the help of a miniature apparatus, the compaction properties of the bentonite composites were determined. The biopolymer acts as a binding agent when added to the bentonite and improves its cohesion. The bentonite soil amended with biochar and biopolymer shows better compaction properties as the biochar addition improves the aeration and increases porosity, which enhances the water retention capacity. • The unconfined compressive strength of the bentonite soil amended with 4% biopolymer and 5% biochar remains the highest among all the mixes. The biopolymer addition improves the bonding that results in higher densities. Higher densities of clay soil often led to improved strength characteristics. Moreover, the bentonite soil amended with 15% Biochar and 2% biopolymer showed a good improvement in compressive strength. Biochar and biopolymer are found to chemically interact with clay particles by forming bonds that enhance the overall strength of the soil. • Due to its porous nature, which promotes a more stable soil structure, the addition of biochar reduced the volumetric shrinkage of bentonite soil. Since the addition of biochar raises the water-holding capacity, proper moisture levels are maintained and the amount of shrinkage during drying cycles is decreased. 69 • While biopolymers effectively absorb and retain water, they also reduce moisture fluctuations in clay soils, which is why increasing their content decreased the CIF and CDF of mixes with varying amounts of biochar. As a result, bentonite soil is less prone to drying out and developing shrinkage cracks. • The results of the batch adsorption studies indicate that the bentonite composite of biochar, with its additional adsorption sites from the functional groups of both biochar and bentonite, has the highest removal percentage for all initial Pb2+ concentrations. Therefore, the amendment of sustainable additives like biochar and biopolymer helps in carbon sequestration and reduces carbon footprint. Biochar helps in improving water retention and longevity of the liner system while polymer amendments are used to increase the cohesion with the biochar particles and strength of the bentonite. The combination of these two additives is often used to create a barrier system that is more effective at preventing contaminants from leaching into the environment. 5.3 Significant Contributions: One of the practical challenges associated with landfill liners is their ability to crack and shrink under elevated temperatures and precipitation fluctuations. This research’s primary contribution is to mitigate the cracks and volumetric shrinkage in bentonite clay using sustainable amendments. This research introduces a sustainable approach to using biochar and biopolymer as amendments in bentonite clay, showcasing their effectiveness in enhancing the bentonite clay's geotechnical properties and contaminant immobilization capabilities. This approach improves the engineering performance of landfill liners and contributes to the sustainability of soil stabilization practices. 70 This research reported that incorporating biochar and biopolymer into bentonite clay significantly improved the compaction characteristics, compressibility and resistance to desiccation cracking. These findings provide a practical framework for improving landfill liner designs to prevent environmental contamination. Another key finding of this research is the improved adsorption capacity of bentonite composite with biochar amendment for heavy metals, particularly Lead (Pb2+) ions, which are the major pollutant of landfill leachates. This contribution is crucial for mitigating the risk of groundwater contamination, aligning with global efforts towards environmental protection. This research also provided the details of microstructural analysis, which contributed to understanding the interaction mechanisms of biochar, biopolymer and bentonite clay at the microstructure level. In conclusion, this research outlines several directions for future research, particularly in exploring the suitability of various sustainable materials in stabilizing problematic soils. This direction extends the scope of the current study and opens up new possibilities for innovation in geotechnical engineering. 5.4 Limitations and Future Work The research on using sustainable amendments in the field of geotechnical engineering for soil stabilization is the present area of interest for many researchers. In this study, the suitability of using biochar and biopolymer as a sustainable amendment to bentonite clay was studied. In future, a detailed study of using various combinations of different sustainable amendments would be beneficial to mitigate adverse effects on the environment if remained unutilized. By focusing on the use of sustainable materials, such as biochar and biopolymers, the environmental impact of soil stabilization processes can be mitigated while ensuring the structural integrity and environmental safety of landfill liners. Some of the limitations associated with current study and the recommendations for future work are as follows. 71 • Future research should aim to expand the scope of this study by exploring the long-term performance of these amendments under varying environmental conditions and their applicability to other types of problematic soils. • In the current study, we focused on 1-day and 7-day curing periods for UCS tests as the present study focused on the immediate effects of sustainable amendments on the strength of bentonite composites. • However, future studies can expand the curing periods to 7, 14, 28 and 60 days, to investigate the long term impact of biochar and biopolymer interactions on the UCS, enhancing our understanding of their durability and performance over time. • In the current research, we conducted a 24-hour batch adsorption study on bentonite composites, focusing solely on varying Lead (Pb2+) concentrations while keeping temperature, time, and pH constant due to time limitations. • However inorder to understand the adsorption process comprehensively, future studies should expand to include a full adsorption analysis. 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Thesis/Dissertation
eng
Civil Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Siddiqua, Sumi
Crack mitigation and contaminant immobilization in the bentonite clay using sustainable amendments
Text