DEVELOPMENT OF AN ELECTROCHEMICAL SYSTEM FOR THE MANGANESE ELECTROCATALYTIC OXIDATION AND HARDNESS PRECIPITATION IN GROUNDWATER by Mostafa Dorosti B.Sc., Shahid Beheshti University, 2015 M.Sc., University of Tehran, 2019 THESIS SUMBITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN NATURAL RESOURCES AND ENVIRONMENTAL STUDIES UNIVERSITY OF NORTHERN BRITISH COLUMBIA December 2023 © Mostafa Dorosti, 2023 Abstract Manganese is one of the most occurring heavy metals in the source and drinking water of rural and remote communities in Canada. The removal of manganese from groundwater was explored by desiging a biochar-assisted electrochemical technology as a small-scale and chemicalfree method. Electrocatalytic oxidation of manganese was acquired while utilizing wood residue biochar as a coating layer on the surface of activated carbon felt anode. The biochar-coated anode showed enhanced conductivity and surface area, consequently facilitating the electron exchange and manganese removal efficiency. Manganese initial concentration, current intensity, pH, and time were considered the main variables and their effect on the removal efficiency of manganese from groundwater was investigated. Current had the most influence on removal efficiency, as evidenced by the fact that no manganese removal occurred at zero current and the removal efficiency was increased by enhancing current from 25 to 75 mA. With the Mn initial concentration of 2 mg/L, current of 75 mA, pH 9, 97.5 % manganese removal efficiency was acquired, which helped to reduce the contaminant concentration under the maximum acceptable concentration. The results demonstrated a well-fitted pseudo-first-order model with a rate constant of 0.0411 min-1 for electrocatalytic oxidation. Real groundwater of Lheidli T'enneh community in Northern BC was employed to evaluate the impacts of co-existing ions on the manganese removal performance. The results confirmed that not only did the use of real groundwater have no detrimental impact on the system’s efficiency, but also the system was able to decrease the high hardness of 268.2 mg CaCO3/L in the groundwater to 72.2 mg CaCO3/L suitable for drinking purposes. Investigation of the manganese removal mechanisms indicated that various species, such as hydroxide ions, sulfate, and hydroxide radicals can play key roles in transferring or removing manganese from groundwater by oxidation and precipitation pathways. The formation of a black powder ii precipitated on the anode surface and in the cell after treatment proved that oxidation of manganese takes place in the system. Furthermore, the evolution of pH after 90 minutes of reaction further confirmed the presence of hydroxide ions in the system. Overall, the designed system achieved a significant performance in removing manganese and hardness from groundwater while utilizing biochar as a waste and cost-effective material. iii Table of Contents Abstract .............................................................................................................................. ii Table of Contents ............................................................................................................. iv List of Tables ................................................................................................................... vii List of Figures ................................................................................................................. viii Glossary ..............................................................................................................................x Acknowledgements ......................................................................................................... xii Chapter 1: Introduction ....................................................................................................1 1.1 Background ......................................................................................................... 1 1.2 Research Objectives ............................................................................................ 3 1.3 Thesis Organization ............................................................................................ 4 Chapter 2: Literature Review ...........................................................................................5 2.1 Introduction ......................................................................................................... 5 2.2 Water Security .................................................................................................... 6 2.2.1 Water security in rural and remote communities in Canada ........................... 6 2.2.2 Drinking water access challenges ................................................................... 7 2.2.3 Drinking water advisories ............................................................................... 9 2.2.4 Boil water advisory major reasons ................................................................ 12 2.2.5 Water quality issues ...................................................................................... 14 2.2.5.1 Source water quality ............................................................................. 14 2.2.5.2 Drinking water quality .......................................................................... 15 2.2.5.3 Emerging contaminants in rural communities ...................................... 16 iv 2.3 Small water systems .......................................................................................... 16 2.4 Decentralized water treatment systems ............................................................. 18 2.5 Manganese and its removal from water ............................................................ 19 2.5.1 Source control methods................................................................................. 20 2.5.2 Removal methods.......................................................................................... 21 2.5.2.1 Physicochemical methods ..................................................................... 22 2.5.2.2 Biological methods ............................................................................... 23 2.5.3 Manganese recovery after treatment ............................................................. 24 2.6 Water treatment electrification ......................................................................... 25 2.7 Electrochemical technology .............................................................................. 27 2.7.1 Electrodes ...................................................................................................... 29 2.7.2 Water flow modes ......................................................................................... 32 2.7.3 Removal mechanisms ................................................................................... 32 2.7.3.1 Electrochemical Oxidation.................................................................... 32 2.7.3.2 Electrochemical Reduction ................................................................... 33 2.7.3.3 Electrocoagulation and Electroflocculation .......................................... 34 2.7.4 Modification of electrodes ............................................................................ 34 2.7.5 Biochar and its electrochemical attributes .................................................... 35 2.7.6 Integration of adsorption with electrochemical technology.......................... 38 2.7.7 Three-Dimensional electrochemical cell ...................................................... 40 2.8 Summary ........................................................................................................... 41 Chapter 3: Materials and Methods ................................................................................43 3.1 Materials ........................................................................................................... 43 v 3.2 Preparation of Biochar Modified ACF ............................................................. 43 3.3 Experimental Setup ........................................................................................... 44 3.4 Experimental design.......................................................................................... 47 3.5 Groundwater characteristics .............................................................................. 51 3.6 Analytical method ............................................................................................. 54 Chapter 4: Results and Discussion .................................................................................55 4.1 Statistical Analysis ............................................................................................ 55 4.2 Effect of variables and their interactions on removal efficiency ...................... 60 4.3 Effect of anode modification on the manganese removal efficiency ................ 68 4.4 Manganese removal mechanisms analysis........................................................ 69 4.4.1 Manganese hydroxide (Mn(OH2)) ................................................................ 71 4.4.2 Manganese oxides ......................................................................................... 72 4.4.3 Persulfate radicals ......................................................................................... 72 4.5 The influence of real groundwater on manganese removal efficiency ............. 74 4.6 Capability of the designed electrochemical cell for hardness removal from the real groundwater ........................................................................................................... 78 4.7 Evolution of pH during electrochemical reactions ........................................... 80 4.8 Comparison of the designed system efficiency with other methods for manganese removal in the literature ............................................................................. 81 Chapter 5: Conclusion .....................................................................................................84 5.1 Limitations and future research ........................................................................ 85 Bibliography .....................................................................................................................88 vi List of Tables Table 3. 1 Independent variables and their experimental range ....................................... 47 Table 3. 2 Design of experiments. .................................................................................... 50 Table 3. 3 Physical characteristics of the groundwater..................................................... 52 Table 3. 4 Metals concentration in groundwater. ............................................................. 53 Table 4. 1 Analysis of variance (ANOVA). .................................................................... 55 Table 4. 2 Coefficient of determination (R2) of the model. .............................................. 56 Table 4. 3 RSM design and responses value. .................................................................... 59 Table 4. 4 Kinetics parameters of electrochemical oxidation ........................................... 66 Table 4. 5 Comparison of metals concentration in groundwater before and after treatment. ........................................................................................................................................... 77 Table 4. 6 Physical characteristics of the groundwater..................................................... 80 Table 4. 7 Comparison of manganese removal efficiency by proposed methods in recent studies. .............................................................................................................................. 83 vii List of Figures Figure 2. 1 Water distribution methods in rural and remote communities. (Balasooriya et al., 2023) ............................................................................................................................. 9 Figure 2. 2 Status of long-term drinking water advisories in Canada. (Government of Canada, 2022a) ................................................................................................................. 12 Figure 3. 1 Synthesis of BC@ACF. .................................................................................. 44 Figure 3. 2 Experimental setup of the electrochemical cell. ............................................. 46 Figure 4. 1 (a) Predicted response values vs. actual response values, (b) Normal plot of residuals for the model. ..................................................................................................... 57 Figure 4. 2 Perturbation plot for Mn Removal Efficiency ................................................ 60 Figure 4. 3 The 3D response surface plots (a) interactive effect of time and pH, (b) interactive effect of time and current, (c) interactive effect of current and Mn initial concentration. .................................................................................................................... 63 Figure 4. 4 Effect of current on the removal efficiency. ................................................... 65 Figure 4. 5 (a) First-order kinetic of manganese removal at different currents, (b) Pseudofirst-order rate constant of manganese electro-oxidation at various current intensities. .. 67 Figure 4. 6 Effect of biochar modification on the catalytic electro oxidation of manganese. ........................................................................................................................ 68 Figure 4. 7 Various mechanisms of manganese removal from groundwater in the designed electrochemical cell ........................................................................................... 70 viii Figure 4. 8 Removal efficiency of manganese from real groundwater and synthesis water with current of 75mA, 0.05 M Na2SO4 as a supporting electrolyte, and without pH adjustment. ........................................................................................................................ 75 Figure 4. 9 First-order kinetic model of manganese removal from real groundwater and synthesis water at the current of 75 mA............................................................................ 76 Figure 4. 10 Evolution of pH in various experiments in constant current of 75 mA, Mn initial concentration of 2 mg/L, and 0.05 M Na2SO4 electrolyte. ..................................... 81 ix Glossary AC Activated Carbon ACF Activated Carbon Felt AOPs Advanced Oxidation Processes AO Aesthetic Objective ANOVA Analysis of Variance BWA Boil Water Advisory BDD Boron-doped Diamond BBD Box-Behnken Design BET Brunauer–Emmett–Teller CDWQ Canadian Drinking Water Quality CNTs Carbon Nanotubes DBP Disinfection by-Products DNC Do Not Consume DNU Do Not Use DWAs Drinking Water Advisories EAOPs Electrochemical Advanced Oxidation Processes ERPs Electrochemical Reduction Processes EDX Energy-dispersive X-ray Spectroscopy E. Coli Escherichia coli x FE-SEM Field Emission Scanning Electron Microscopy FNHA First Nation Health Authority FTIR Fourier Transform Infrared GF Graphite Felt ICP-OES Inductively Coupled Plasma Optical Emission spectroscopy MAC Maximum Acceptable Concentration MP-AES Microwave Plasma Atomic Emission Spectroscopy MMO Mixed Metal Oxide NMP N-Methyl-2-pyrrolidone PDS Peroxodisulfate PMS Peroxymonosulfate POE Point of Entry POU Point of Use PVDF Polyvinylidene fluoride ROS Reactive Oxygen Species RSM Response Surface Methodology TDS Total Dissolved Solid UNHRC United Nations Human Rights Council WHO World Health Organization xi Acknowledgements First and foremost, I would like to express my gratitude to my dedicated supervisor, Professor Jianbing Li, for his support and invaluable guidance throughout my master's studies. His mentorship has been instrumental in shaping my academic journey and fostering my growth as a researcher. In addition, I extend my sincere appreciation to my committee members: Jun Yin, Natalie Linklater, and Oliver Iorhemen. Their insightful advice, constructive comments, and valuable feedback significantly enriched the quality of my thesis. It is important to acknowledge that my thesis is an integral component of the larger project titled "Water Security in Rural and Remote Communities in Canada," funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). This support has been pivotal in enabling me to contribute to addressing issues related to water security. My profound appreciation extends to Guangji Hu, whose generous sharing of knowledge and extensive experience played a pivotal role in advancing the project. Dr. Todd Whitcombe's invaluable insights and Jordan Wilbey's assistance in the chemistry lab are highly valued contributions to my research journey. I also wish to express my gratitude to the entire team within our research group, as well as my friends, whose support and camaraderie have been a source of motivation during my studies. Beyond the academic realm, I owe an immense debt of gratitude to my wife, Sorour. She has been not only my companion in moments of joy and hardship but also a dedicated co-researcher. Her presence, warmth, and kindness in these years have been a source of strength during challenging times. A special and heartfelt thanks are due to my family, particularly my mother, whose unconditional support and love from far away have been inspiring throughout my educational journey, and I would love to dedicate this work to her. xii Chapter 1: Introduction 1.1 Background Water quality in rural and remote areas has been a widespread problem. Industries and poor sanitation cause contaminants such as heavy metals and pathogens to enter the surface and groundwater. Contaminated water intake can cause severe diseases and death. Centralized water treatment systems are primarily used in developed countries and big cities and the treated water is then distributed to households via a complex piping network. However, in rural communities often due to the lower densities, the cost of using centralized systems is high. For these areas, adopting an on-site water treatment and storage system is a feasible alternative. Decentralized water treatment systems provide an opportunity of removing pollutants from drinking water before consumption (Pooi & Ng, 2018). Manganese is one of the most occurring heavy metals in the rural areas and First Nation communities in Canada (Schwartz et al., 2021a). Although it is an essential microelement for humans, the excessive amount of manganese causes diseases related to central nervous system (Chu et al., 2023). Elevated concentrations of manganese in source waters, such as groundwater, can originate from the discharge of industrial wastewater into the environment. This discharge can lead to the introduction of manganese into groundwater. Additionally, insoluble manganese compounds present in minerals and rocks have the potential to dissolve in groundwater, further contributing to elevated manganese levels (Tobiason et al., 2016). Various technologies, including membranes, filtrations, ion exchange, and adsorption are employed, which are capable of removing and transferring manganese. However, their 1 use might be impeded in many rural areas because of their high cost. Moreover, there is a considerable need for a fit-for-purpose, modular and scalable water treatment method, which is able to eliminate contaminants from various sources to a degree dependent on its intended purpose. Electrochemical technology has gained much attention due to its capability to help in achieving this vision. There are several advantages over other methods, including the ability of removing wide range of contaminants simultaneously in a compact unit, in-situ production of chemicals, removal of contaminants in small concentrations, and limited operation requirement. The electrodes material influences the removal performance since the generation of highly reactive oxygen species (ROS) on the electrode surface is the main contribution toward contaminants removal (Xie et al., 2022a). The application of electrochemical cell is hampered by challenges associated with expensive and low surface area electrodes, small mass transport caused by solid and plate electrodes used in flow-by configuration, and low conductivity (Chaplin, 2019a). In the proposed study, the main goal is to develop an electrochemical cell, which despite the majority of the researches, uses biochar coated on the electrode for the electrocatalytic removal of most occurring heavy metals in rural community’s groundwater, particularly manganese. The gaps and limitations of conventional electrochemical cells, such as high cost of electrodes, low surface area and inadequate capacity of heavy metals removal will be overcome by implementing some modifications. Biochar derived from wood debris, which is a local resource is used as a sustainable and environmental friendly yet cheap method for anode modification. It can provide an opportunity to exploit the solid residue char generated from a natural material for water treatment purposes. Biochar is a potentially promising material owing to its considerable 2 specific surface area, porosity, low-cost and easy production. In the meantime, the graphitic structure of biochar enhances its electron transferability and highly active radicals production (Liu et al., 2021). These unique characteristics of biochar employed in an electrochemical cell improve its capability for the catalytic oxidation of manganese from groundwater. In contrast to numerous research efforts that rely on chlorine production from chloride ions within an electrochemical cell or the addition of conventional chemical oxidants such as chlorine for manganese oxidation in water, this study directs its attention towards alternative methods of manganese oxidation (Shu et al., 2020; Yu et al., 2015). Specifically, it explores the generation of other reactive species like hydroxyl and sulfate radicals. This approach mitigates the potential risks associated with the formation of disinfection by-products (DBP) in water. It is worth noting that certain halogenated organic DBPs are recognized as carcinogenic and genotoxic, underscoring the significance of this alternative approach (Tobiason et al., 2016). 1.2 Research Objectives As a result of the comprehensive literature review and understanding of the challenges and gaps (chapter 2), the main objective of this study is to investigate the removal of manganese from groundwater utilizing an electrochemical cell composed of biocharmodified electrode. Consequently, the subtasks are defined as follows: · Analyze methods for the modification of electrode with biochar and the effect of modification on the electrode characterization and performance. · Design of an electrochemical cell in order to remove manganese from groundwater and the process optimization. 3 · Investigate the effect of main variables such as pH, current intensity, time, and initial manganese concentration on the removal efficiency of manganese from groundwater and determine their interactions simultaneously. · Evaluate the performance of the designed electrochemical cell employing real groundwater as a source water and in the presence of co-existing species such as different cations and anions and their concentrations before and after treatment. · Analyzing data and studying the statistical modelling of the experiments to assess manganese removal optimization. · Investigate the kinetic of electrooxidation within the system and calculate the rate constant in different conditions, as well as the various possible mechanisms and pathways for manganese catalytic electrooxidation. 1.3 Thesis Organization Consequently, in accordance with the stated objectives and subtasks, Chapter 2 encompassed a comprehensive literature review. Chapter 3 meticulously detailed all materials employed in the experiments, including the methodology for electrode preparation, experimental setup, design of experiments, and the analytical techniques applied. The outcomes of the experiments and the ensuing discussions were extensively expounded upon in Chapter 4. Finally, the research was completed with Chapter 5, which presented the conclusions drawn, outlined the limitations encountered, and offered recommendations for future research endeavours. 4 Chapter 2: Literature Review 2.1 Introduction Water is one of the necessities of sustainable development and an essential factor of human health and ecosystem function. Access to safe and reliable drinking water and sanitation has been universally recognized as a human right due to the significance of water in health and well-being. According to the United Nations Human Rights Council (UNHRC), water right "entitles everyone to sufficient, safe, acceptable, physically accessible and affordable water for personal and domestic uses" (Galway, 2016). As reported by World Health Organization (WHO) and UNICEF in 2021, there is a lack of access to safe drinking water services for approximately 25% of the global population. Moreover, 80% of this amount live in rural and remote areas (Nath et al., 2022). Although the provision of potable water in low-income and developing countries is of high importance and the subject of many research papers, there is an increasing concern about the drinking water supply for small, rural, and remote areas in developed and high-income countries (Nath et al., 2022). With 7.6% of its area comprised of water resources, Canada is one of the countries with the highest water availability (Kaur et al., 2019). In Canada, 99% of the population takes advantage of healthy and safe drinking water. However, the majority of rural and Indigenous populations lack access to adequate sanitation and tap water systems, which is an ongoing issue in many Indigenous communities. This is 90 times more likely for an Indigenous household to lack access to tap water, demonstrating the health inequalities encountered by Indigenous people (Bermedo-Carrasco et al., 2018). 5 2.2 Water Security Water security is defined as the capability of having access to drinking water with safe quality and sufficient quantity. Water security problems are growing worldwide owing to several factors, such as limited water resources, demographic alterations, uneven distribution of water sources, and high standards of living (Kaur et al., 2019). On the other hand, water insecurity outlines the inability to access adequate, reliable, and safe water in order to maintain a healthy life and well-being, which reportedly poses a threat to over 80% of the global population. In particular, 844 million people worldwide lack access to essential drinking water services, and 2.1 billion people live without safely managed water (Achore et al., 2020) 2.2.1 Water security in rural and remote communities in Canada 6.2% of the world's population are Indigenous people, and more than 73.4% of them live in rural areas (Balasooriya et al., 2023). One of Canada's most marginalized groups is the First Nations population. Various reports have revealed poverty, poor living conditions, and insufficient access to health and social services among First Nation people (Galway, 2016). About one-third of First Nations living on reserves in Canada face health risks from elevated-risk water systems, making the First Nation drinking water crisis one of the most urgent policy issues (Wilson et al., 2019). For instance, the rate of water-borne diseases in First Nations communities is 26 times greater than the national average in Canada (Galway, 2016). These communities experience several challenges regarding drinking water, including unpredictable source water quality, unsuitable water treatment technologies, inefficient governance systems, and inadequate access to training (Nath et al., 2022). The underlying political, social, and economic disadvantages even exacerbate the issue of 6 limited access to safe potable water (Galway, 2016). Water governance issues and inadequate policy commitment unequally affect Indigenous communities throughout Canada, which necessitates alternative and new approaches concerning water governance (Nath et al., 2022). 2.2.2 Drinking water access challenges Various parameters impact the water security of Indigenous communities, which can be investigated through different lenses of water quantity, quality, accessibility, and affordability (Wilson et al., 2019). One of the main restrictions to safe drinking water is due to the remote location of traditional lands and reserves, which causes many people to use raw water from local sources such as lakes, wells, and springs. As a result, inequalities in providing sustainable sources of potable water increase the vulnerabilities of Indigenous communities to water-borne diseases and possible chemical pollutant exposures and related health consequences (McLeod et al., 2020). Moreover, many Indigenous people still prefer to rely on the traditional untreated water sources, which have been used for decades throughout their territories (Wilson et al., 2019). Many people in these communities use the water delivered by trucks from nearby water treatment plants. Then, the water is stored in household cisterns or tanks for consumption. This method increases the possibility of contamination of drinking water since the truck tanks are not cleaned frequently due to the lack of time and busy delivery schedule. Various methods of water distribution are shown in Figure 2.1. Furthermore, in the process of filling truck tanks from a water treatment plant, airborne water pollution might occur (Baijius & Patrick, 2019). There is also a high cost of operation and maintenance in this water delivery method, as well as the driver salaries and 7 difficulties of water transportation to remote areas in winter. Additionally, water stored in household cisterns for some days elevates the potential growth of bacteria and microbiological contamination (Wilson et al., 2019). The type of source water and its quality is varied in different communities. As a result, the risks to source and drinking water originate from different sources. For instance, the presence of chemicals in soils and subsequent leaching into the source water and fecal contamination of animals entering the waterways can contaminate the water (McLeod et al., 2020). Animal fecal pollution in natural sources such as ponds, streams, and brooks and the disinfection by-products have been reported as the main quality problems in one of the Indigenous communities (Sarkar et al., 2015). In addition, the impacts of climate change, including high rainfall occurrences, excessive temperatures, long-lasting droughts, and overland flooding can contribute to the increase of contamination, particularly in small and remote communities (McLeod et al., 2020). Another significant obstacle to safe drinking water was the disparities in cultural attitudes. As mentioned in one study, Indigenous people believe that drinking water gives spirit to people along with providing hydration to the body. Thus, using bottled water is against their belief since water wrapped in plastic is dead water (Bradford et al., 2016). Because of the challenges to accessing safe and affordable drinking water, there is high consumption of sweetened beverages as an alternative to water among many people in Indigenous communities, particularly for children (Bradford et al., 2016). Given the well-established relationship between soft drink consumption and the risk of diseases such as diabetes and obesity, such practices have substantial adverse health effects (Achore et al., 2020; Sarkar et al., 2015). In another study, it was reported in one community that due 8 to the carrying of water buckets every day, there are adverse health consequences such as mental stress and severe back and shoulder injuries (Sarkar et al., 2015). Iodine deficiency and skin diseases among Indigenous men in Quebec is another problem reported in one research. The former is because of the drinking of tap or spring water, which increases the risk of diseases compared to people who drink bottled water. The latter is due to the overchlorination of water (Bradford et al., 2016). Figure 2. 1 Water distribution methods in rural and remote communities. (Balasooriya et al., 2023) 2.2.3 Drinking water advisories Drinking Water Advisories (DWAs) are preventative measures that can be issued for a variety of reasons, such as issues with the drinking water system, operational problems, and microbiological contamination, in order to safeguard the public from potential health risks of supplying drinking water (Lane et al., 2020). Since First Nation 9 communities may have various water systems, a drinking water advisory does not always indicate a problem with the drinking water supply for the entire community and may only affect one building. There are three types of drinking water advisories utilized by Health Canada, including boil water, do not consume, and do not use advisories. Boil water advisory (BWA) demonstrates the presence of microbiological contaminants such as bacteria and viruses in drinking water. So, boiling will remove these contaminants from water (First Nations Health Authorities, n.d.). Moreover, in some cases, BWA serves as precautionary measures for communities. This refers to situations where the safety of drinking water has been questioned, such as during emergency repairs of water distribution systems or lack of operators for the community's water treatment system (Galway, 2016). Do Not Consume (DNC) advisories issued when the community's drinking water includes a contaminant that poses health risks and is resistant to be removed by boiling, such as heavy metals. While under DNC advisory, people should not use the water for purposes such as drinking, cooking, brushing their teeth, etc. Do Not Use (DNU) advisories issuance is due to the presence of contaminants in water that are not easily eliminated by boiling and further treatment. Exposure to this water may cause skin, eye, and nose irritations (First Nations Health Authorities, n.d.). An analysis of DWAs reveals that from 2004 to 2013, the number of issued advisories increased for First Nation communities in certain parts of Canada, despite the fact that the federal government has acknowledged and tried to mitigate issues regarding drinking water for Indigenous communities (Spicer, 2020). As of august 21, 2023, there are still 28 long-term drinking water advisories in effect on public systems of 26 communities all around Canada (Figure 2.2) (Government of Canada, 2022a). In addition, 10 First Nation communities south of 60, excluding those in the British Columbia region, are subject to 46 short-term drinking water advisories (Government of Canada, 2022b). Longterm advisories are those that last more than 365 days, whereas advisories less than 365 days are short-term advisories. 89% of BWAs in 2021 were issued for drinking water systems serving communities with 500 or fewer residents (Government of Canada, 2022a). Due to small communities' particular difficulties, such as their limited operational capacity and the lack of regular monitoring, BWAs are more common. For instance, a larger city's operation team can usually fix a damaged water main without needing a BWA. However, in a smaller community, the same issue can take longer to fix, and a BWA might be issued during repairs (Canada.ca, 2022). From 2004 to 2014, at least one DWA occurred in 65% of the First Nations communities. According to the monthly summary of First Nation Health Authority (FNHA), As of August 31, 2023, there were 7 Water Quality Advisories, 8 BWAs and 11 DNC advisories for a total of 26 Drinking Water Advisories in effect in 26 Water Systems across 21 First Nation communities in British Columbia (First Nations Health Authority, 2023). 11 Figure 2. 2 Status of long-term drinking water advisories in Canada. (Government of Canada, 2022a) 2.2.4 Boil water advisory major reasons The frequency of BWAs is 2.5 times higher in First Nations communities compared to municipal communities, which can be due to the high turbidity, low chlorine residuals, poor infrastructure, and positive coliform (Lane et al., 2020; Thompson et al., 2017). Based on the government of Canada reports, in 2021, 90% of BWAs were due to problems with water treatment equipment and processes. 2% were due to the presence of Escherichia coli 12 (E. coli), and the remaining 8% were because of other microbiological contamination of water. Between 2010 and 2021, fewer precautionary BWAS were issued because of E. coli and other microbiological problems, whereas more were issued because of equipment and water treatment process issues (Canada.ca, 2022). In one of the studies, the number of DWAs in three provinces was analyzed, and it was found that in most of the months recorded in Nova Scotia and New Brunswick, the number of advisories lifted was roughly equal to the number of DWAs. This demonstrates the necessity of fundamental improvements in the systems infrastructure (Lane et al., 2020). According to another research, it was reported that during 11 years of study, out of 776 First Nation drinking water systems, 515 of them experienced at least one DWA. It was also indicated that there was a total of 1526 DWAs, and approximately 96% of that number were BWAs (Thompson et al., 2017). Operational problems accounted for 40% of DWAs issued, followed by health-related issues (33%). 25% of DWAs are problems with the quality of the water, and 2% have unidentified reasons. More extended advisories occurred in the systems without fully certified operators, and they were more likely to get DWAs related to health issues. In communities with surface water as their source of water, the frequency of advisories was higher. Moreover, systems with surface water sources or groundwater sources under the direct effects of surface water experienced longer-lasting advisories (Thompson et al., 2017). Some practices could help to reduce the frequency of DWAs, including source water protection by improving governance systems, employing multiple disinfection mechanisms on water treatment systems, preventing the storage of treated water before usage due to the possibility of pollution occurring, conducting the traditional and spiritual practices of respecting water, and increasing the collaboration with 13 Indigenous communities and agencies by establishing Indigenous technical service providers to support communities' treatment systems (McLeod et al., 2020). 2.2.5 Water quality issues The quality of water sources and drinking water in rural and remote communities is of high importance. Deficiencies in water quality can be due to the problems in each step of drinking water provision, such as source water quality, treatment system, and distribution services. As a result, a thorough investigation of each part can assist us in addressing the water quality issues in First Nations communities. 2.2.5.1 Source water quality Groundwater is one of the primary sources of drinking water, particularly for people in rural areas. In Canada, one-third of the population relies on groundwater for their drinking. In rural communities across Canada, this number increases even more, and over 80% of the population uses groundwater for drinking water supply (Environment and Climate Change Canada, 2022). Several factors, including rapid climate change, urbanization, deforestation, developing agriculture, and the increased use of pesticides, have huge impacts on groundwater quality and quantity. Another potential hazard is wastewater leakage from the rural septic systems into groundwater. Natural aquatic environments in rural areas with lower population densities are more prone to anthropogenic contaminants like pharmaceuticals. Consequently, the source water quality of rural communities and their water security is at risk (Husk et al., 2019). 14 2.2.5.2 Drinking water quality In a study, the level of metals with operational guidance and aesthetic goals in the drinking water of 1516 households of First Nations communities south of the 60th parallel were assessed as part of a community-based participatory research project. The results demonstrated that the Maximum Acceptable Concentration (MAC) for five metals was found to be exceeded as follows. Lead (8.4% of homes, first draw), manganese (4.0%), uranium (1.6%), aluminum (1.3%), and copper (0.2%). (flushed). The aesthetic goals (flushed) were exceeded regarding manganese 12.8%, sodium 5.1%, iron 3.5%, and copper 0.4% of households. Only arsenic, lead, selenium, and uranium, out of the ten healthrelated metals examined in this study, exceeded the recommended levels by the Guidelines for Canadian Drinking Water Quality (CDWQ) (Schwartz et al., 2021b). Benjamin Ochoo et al. indicated that colour, manganese, Total Dissolved Solid (TDS), and iron were aesthetic factors, whereas trihalomethanes, haloacetic acids, and turbidity were the primary contaminants found in the public water supply. Taste, odour, colour, turbidity, and BWA experience in the past were expressed as the major dissatisfaction reasons with water quality in this research (Ochoo et al., 2017). On First Nations reserves in Manitoba, Canada, a water treatment facility provides piped water to around half of the residences. At the same time, 31 percent of all households have cisterns filled by a water truck. The result of one study showed that the tap water in households with cisterns was relatively more contaminated with coliform bacteria as compared to homes with pipes. Moreover, E. coli frequency and severity were higher in drinking water samples from cisterns, and the amount of total coliform contamination was highest in late spring (Amarawansha et al., 2021). Farenhorst et al. research demonstrated a higher level of total coliform and E. coli 15 in the drinking water supplies of households without running water. The contamination mainly occurred due to the dust contamination with the water truck and buckets, biofilm formation in cisterns, inflow of contaminated water into cracks of pipes and cisterns, and the low level of residual chlorine in the cisterns, pipes, and water trucks (Farenhorst et al., 2017). Harmful pollutants such as phenols, dioxins, mercury, lead, and arsenic were also reported in the First Nation communities' source water (Bradford et al., 2016). 2.2.5.3 Emerging contaminants in rural communities Larger urban areas in Canada that obtain their drinking water supplies mostly from surface water have been the focus of the majority of studies on the presence of pharmaceuticals and pesticides in municipal drinking water. Few studies have focused on this issue in smaller, rural areas, which account for approximately one-third of Canada's population and where groundwater is the primary source of drinking water (Husk et al., 2019). For instance, in a study, the presence of emerging contaminants in the drinking water of 17 rural communities in Quebec was investigated. Among 70 chemicals examined in samples, 15 compounds, including six pesticides and nine pharmaceuticals, were detected. Furthermore, caffeine, atrazine, and naproxen were the three most common emerging contaminants in the drinking water samples (Husk et al., 2019). 2.3 Small water systems In most big cities and municipalities, drinking water is acquired by conventional water treatment methods. However, due to the lower population of rural communities and less water consumption, it is not feasible to utilize the same method as larger cities. Hence, small drinking water systems are employed for water treatment in small, remote, and rural 16 areas, and the majority of Indigenous communities consume the water treated with these systems. In high-income countries, small drinking water systems, particularly in rural and remote areas, encounter considerable challenges (McFarlane & Harris, 2018). For instance, thousands of people in developed countries such as Canada, United States and United Kingdom have been affected by drinking water-related outbreaks in the past few decades (Nath et al., 2022). As a result, despite the progress, there are still high-risk small water systems, which cause frequent drinking water advisories in Indigenous communities (Nath et al., 2022). According to Health Canada, small systems account for more than 80% of drinking water systems in Canada (Health Canada, 2013). Small systems are more prone to contamination and outbreaks than major municipal systems, based on the investigations into water-borne diseases and drinking water advisories in Canada. Small drinking water systems confront a variety of unique challenges, including the use of untreated surface water, shortage of funding, staffing and capacity concerns, and the old distribution systems (Galway, 2016). As reported by a national evaluation of 807 small water systems serving First Nation communities across Canada, 39% of them are classified as high risk, and approximately half of this amount is due to the presence of bacteria in the water delivery system. 34% of systems were at medium risk, and 27% were categorized as low risk (Spicer, 2020). The most significant issues small water treatment systems in Indigenous communities face are as follows: defective water treatment systems that require replacement, inadequate number of operators on reserves, the lack of training in the operation section, poor raw water quality, inappropriate infrastructure, water distribution problems, frequent microbiological contamination due to the inappropriate treatment 17 system, lack of regular testing of raw and treated water quality, and insufficient system inspection (Black & McBean, 2017). Furthermore, the complex governance and ownership structure of small systems and their exemption from regulations cause significant issues compared to larger systems with public owners. As a result, the vulnerability of these systems to contaminants might increase. In particular, it was reported that the lack of monitoring in small systems brings about more advisories compared to the water quality problems (Nath et al., 2022). Distribution services are a significant part of Indigenous drinking water systems. According to a survey's results, 72 % of households in 587 Canada's First Nation communities have pressurized pipe delivery, 13.5 % have cisterns and truck delivery, 13 % have private wells, and 1.5 % have no services. The distribution of water delivery systems is not equal across Canada, and Atlantic, Quebec and British Columbia have the highest percentage of pipe services (Spicer, 2020). In rural and remote communities, the security of drinking water cannot be guaranteed by the availability of water treatment and distribution facilities. A sustainable infrastructure must be efficient, help to increase energy efficiency and reduce environmental impact, be compatible with local resources (financial, human, technological, and supply chain), and be appropriate for the social, economic, and regulatory settings (Nath et al., 2022). 2.4 Decentralized water treatment systems Globally, there is an increasing reliance on decentralized technologies for water treatment. These technologies allow for the local reuse of wastewater or the purification of centrally treated drinking water at the point of use. Moreover, they should be small, minimize chemical storage, produce minimal waste, allow remote operation, and treat 18 water with varying compositions (Garcia-Segura et al., 2020). Decentralized treatment systems are classified based on the level at which the water is treated, including Point of Use (POU) and Point of Entry (POE). They can be used where the application of centralized water systems is not feasible and advantageous, and they decrease the risk of contamination during transport and storage, such as the release of lead from plumbing materials and the regrowth of pathogens in storage tanks. POE systems are employed to treat the entire water entering a house, while POU treatment is designed for specific treatment purposes such as drinking (Deng, 2021). The technologies that are primarily used in POU treatment are membranes, filtration, reverse osmosis, and UV disinfection (Wu et al., 2021). Although they are able to remove a diverse array of contaminants, their utilization is often hindered in rural and remote communities due to their high costs. 2.5 Manganese and its removal from water The presence of excessive heavy metals in water sources has detrimental effects on human beings, aquatic organisms, and the overall ecosystem as a result of their poisonous nature. Manganese (Mn) is one of the common heavy metals found in water, soil, and air (Michael et al., 2022). It primarily originates from various sources and activities in the environment with both geogenic and anthropogenic causes. It is particularly abundant in soil compared to other sources. Human activities, such as mining and landfill leaching, significantly contribute to the influx of manganese into the environment (Nkele et al., 2022a). Manganese can be found in a variety of states, the most prevalent of which are the solid and dissolved phases. Depending on the pH of the water, manganese can be present in different forms (Health Canada, 2016). Manganese frequently exists in three oxidation states. Manganese (II) is the stable form in the absence of oxygen and low pH. However, 19 manganese (III) and manganese (IV) forms, such as MnO2 and MnOOH, are mostly dominant in higher pH and the presence of O2 (Nkele et al., 2022a). Manganese is regarded as a crucial micronutrient for the growth of a wide range of living creatures, including plants and animals. Although manganese is considered a non-carcinogenic substance, exposure to its high concentrations has adverse impacts on human health. It can affect the nervous system functions and cause neurological diseases such as Alzheimer’s and Manganism, specifically in people aged 55 and more. Infants are also more prone to the neurological effects of manganese high concentrations. It can affect their neurodevelopment, leading to neurological abnormalities such as brain disorders, shortterm memory, impaired attention as well as visual challenges (Alvarez-Bastida et al., 2018). The presence of high concentrations of manganese causes the coloration and taste deterioration of water. The aesthetic quality of drinking water was formerly the only method of controlling manganese concentration in drinking water (Hu et al., 2020). By establishing MAC, according to the guidelines for CDWQ, the total manganese amount in drinking water has to be under 120 ppb and the Aesthetic Objective (AO) of under 20 ppb (Health Canada, 2019). 2.5.1 Source control methods Manganese contamination has gained increasing attention due to its adverse health effects and environmental concerns. Addressing this issue necessitates not only the removal of manganese from affected water sources but also proactive source control measures. This section provides a summary of source control strategies for manganese, encompassing various methods and techniques aimed at preventing or mitigating manganese contamination at its origin. 20 Evaluating the sources of water pollution constitutes an initial and crucial step in preventing contaminants from entering water bodies. The utilization of fertilizers, pesticides, and the discharge of untreated industrial wastewater are common contributors to source water pollution. Therefore, it becomes imperative to assess these pollution sources by requiring industries and companies to obtain permission for any activities that could impact the environment (Nkele et al., 2022b). The establishment of monitoring programs and the implementation of proactive measures at early stages are imperative to prevent contaminated water from entering the water treatment plant. Such proactive actions prove highly effective in mitigating the risks associated with polluted raw water (WHO, 2001). Maintenance of water distribution networks can serve as an additional method for controlling manganese levels at the source. The gradual accumulation of even low concentrations of manganese in pipes and other network components over the long term can lead to elevated manganese levels in consumers' tap water. Hence, it holds significant importance to manage manganese buildup through regular cleaning and deposit control measures within the distribution network (Nkele et al., 2022b). 2.5.2 Removal methods There are different methods for manganese removal from water. In order to identify the most effective treatment strategy, it is significant to evaluate the chemistry and microbiology of manganese in drinking water. Various chemical reactions, including oxidation, precipitation, and adsorption, can control the types of manganese species present in drinking water treatment. In the following section, each removal mechanism is briefly introduced. 21 2.5.2.1 Physicochemical methods These methods can be used directly to remove the dissolved form of the manganese, i.e. Manganese (II) from water. The particulate form of manganese, such as manganese (III) and manganese (IV), can be the result of manganese (II) oxidation. Precipitation is mainly employed as a method to remove particulate manganese from water (Nkele et al., 2022a). In the precipitation process, the formation of solid, insoluble metal precipitates, specifically metal hydroxides, occurs through the reaction between metals dissolved in an aqueous solution and precipitating agents. During this process, ultrafine particles are generated, and coagulation and flocculation are employed to facilitate the enlargement of particle size, thereby facilitating their removal as sludge and by additional filtration processes (Nkele et al., 2022a). The precipitation process involves the utilization of oxidation to transform dissolved manganese into its precipitated state. The hydroxide treatment method is extensively employed in precipitation techniques due to its simplicity, convenient automatic pH control, and the cost-effectiveness of precipitants such as lime (Nkele et al., 2022a). The process of adsorption can be classified into two categories: chemical adsorption and physical adsorption. Chemical adsorption occurs when there is an interaction between the surface of the adsorbent and the adsorbate involving electron transfer or chemical bonding. This type of adsorption is irreversible and requires a significant amount of activation energy. However, physical adsorption between the adsorbate and the adsorbent is driven by weak forces such as van der Waals forces and hydrogen bonding (Nkele et al., 2022a). Idrees et al. utilized biochar as an adsorbent, and the manganese removal efficiency 22 was greater than 80% at pH 6. This is because manganese ion adsorption on the negatively charged biochar surfaces is less competitive due to the reduced concentration of hydrogen ions (Idrees et al., 2018). One study investigated the manganese removal with modified alumina, and the result demonstrated that by increasing adsorbent dosage and temperature, the removal efficiency increased due to the higher adsorption sites (Khobragade & Pal, 2014). The filtration method exhibits the ability to eliminate various substances such as organics, inorganics, microorganisms, and suspended solids. The membrane filtration methods encompass reverse osmosis, ultrafiltration, nanofiltration, and microfiltration. The selection of specific membrane filtration techniques for the purification of aqueous solutions depends on the size of the particles involved (Nkele et al., 2022a). The ion exchange technology is able to extract soluble ions from liquids. The process offers several advantages, including its affordability, cost-effectiveness, high treatment capacity, and efficient removal rates. Furthermore, it has demonstrated high efficacy in eliminating heavy metals from aqueous solutions, particularly those containing low concentrations of heavy metals (Nkele et al., 2022a). Kononova et al. demonstrated the capability of ion exchange for the removal of manganese (II) and chromium (VI) from an aqueous solution. The removal efficiency reported was more than 99% (Kononova et al., 2019). 2.5.2.2 Biological methods The biological treatment is another method for removing manganese from aqueous solutions. Bio-filtration is the primary method employed in this regard. There are various mechanisms for dissolved manganese biological treatment, including manganese(II) 23 oxidation catalysis by biopolymers, direct manganese intracellular oxidation, and extracellular adsorption where manganese(II) can bind to negatively charged extracellular polymer molecules (Nkele et al., 2022a). 2.5.3 Manganese recovery after treatment Manganese, a valuable and versatile metal, is utilized in various industrial applications. Its unique properties have sparked a growing demand across industries, underpinning economic growth in regions rich in this resource. Consequently, manganese has emerged as a focal point of economic development in many countries, with its abundant mineral resources driving economic progress (Nkele et al., 2022b). Water treatment processes generate a significant amount of residue known as water treatment sludge. Proper management of this sludge includes options such as recovery, recycling, and reuse. Within these waste materials, there is a potential to recover water and extract value from the sludge, ultimately reducing the costs and environmental footprint of the treatment process. In one study, the optimization of manganese recovery from sludge generated during groundwater treatment was achieved through reductive acid leaching. This optimization aimed to create manganese dioxide adsorbents capable of effectively removing Cu(II) and Pb(II) from simulated electroplating wastewater (Ong et al., 2021). Notably, Anyakora et al. (2012) explored sludge beneficiation for environmental sustainability, employing methods like gravity thickening and air drying with mechanical agitation to recover 3.11% of manganese oxide(Anyakora et al., 2012). Yu et al. (2019) adopted an innovative approach for manganese recovery from manganese-bearing wastewater, utilizing carbon dioxide (CO2) produced from flue gases. Their findings revealed 99.99% manganese (II) recovery (Yu et al., 2019). 24 Additionally, it was reported that oxidation and manganese recovery in a leaching solution derived from manganese ore using chlorine salt (NaClO3) as an oxidant, achieving a 90% manganese recovery (Nkele et al., 2022b). Further research by Ong et al. (2018) delved into the recovery of iron and manganese from groundwater treatment sludge using acid leaching and hydroxide precipitation, achieving a remarkable 100% leached manganese (Ong et al., 2018). Bioleaching studied in another work proved highly efficient, achieving a 99.5% manganese recovery (Kamizela & Worwag, 2020). Lastly, another study evaluated the coextraction of vanadium and manganese from vanadium wastewater, with the precipitation process yielding 91.05% manganese recovery (Liu et al., 2020). 2.6 Water treatment electrification Conventional methods of purifying drinking water entail substantial consumption of chemical coagulants, lime, soda ash, and activated carbon to eliminate suspended solids, divalent cations, and organic compounds. It is noteworthy that these chemicals, used in the removal of these specific constituents, account for 95% of the environmental impact associated with chemical reagent utilization in the drinking water treatment process. The production of these chemicals entails various energy-intensive processes, such as mining for raw materials, polymer precursor synthesis, calcination, thermal activation of materials, and other practices primarily reliant on fossil fuel combustion (Gingerich et al., 2022a). The estimated annual cost of air emissions resulting from the existing treatment processes at drinking water facilities in the United States stands at approximately $500 million USD. These damages are associated with heightened electricity or chemical consumption in water treatment processes (Gingerich & Mauter, 2017). 25 Alternative approaches to electrify drinking water treatment encompass the substitution of processes reliant on externally generated chemicals with two key methodologies. The first involves employing physical processes that solely necessitate electricity as an input, such as membrane separations. The second entails using electricity to generate chemicals in situ. Practical instances of these alternatives include the substitution of chemical coagulation with electrocoagulation, the replacement of chemical softening with nanofiltration membranes, and the substitution of granular activated carbon with packed tower aeration or advanced oxidation processes (Gingerich et al., 2022a). Electrified water treatment processes are categorized as any processes reliant on electrodes powered by electric potential or current, preferably from renewable energy sources. These processes leverage electricity directly to eliminate a wide spectrum of organic, inorganic, and microbial contaminants (Zuo et al., 2023). For instance, electro-coagulation processes create active coagulants on-site, leading to several advantages such as equipment simplicity, reduced costs, limited use of chemicals, and lower and non-toxic sludge production (Sandoval et al., 2021). Electrochemical disinfection can neutralize bacteria such as E. coli by an electrochemical cell composed of graphene sponge as an electrode (Norra et al., 2022). Electrochemical Advanced Oxidation Processes (EAOPs) and Electrochemical Reduction Processes (ERPs) possess the capability to convert hazardous contaminants into benign substances, such as CO2, N2, and H2O, with faster reaction kinetics and diminished chemical consumption compared to traditional chemical or biological methods (Zuo et al., 2023). Electrified membranes are another new method utilized in water and wastewater treatment, which is able to improve the sustainability and efficiency of membrane technologies by introducing electrification. Electrified membranes 26 extend their functionality beyond pure separation by harnessing various electro-based phenomena, encompassing electrochemical oxidation and reduction, electrostatic adsorption and rejection, electrophoresis, and electroporation. Consequently, ElectroMembranes demonstrate a high efficiency in the degradation and transformation of contaminants (Sun et al., 2021a). Incorporating electrified water treatment processes offers multiple advantages beyond carbon emissions reduction. Firstly, it opens up opportunities for small-scale and distributed water treatment by diminishing the need for chemical usage. This reduction not only simplifies logistics, reducing the space required for chemical storage and streamlining supply chain operations but also contributes to more resilient systems by minimizing supply chain complexities. Second, the emissions generated during the manufacturing of chemicals used in drinking water treatment, including nitrous oxides, sulfur dioxide, and particulate matter, result in human health and environmental damages that are more than twice greater than climate damages. Transitioning towards electrified processes assists in substantially mitigating health, environmental, and climate-related damages associated with drinking water treatment (Sun et al., 2021a). 2.7 Electrochemical technology The widening disparity between water demand and supply emphasizes the necessity of tapping into non-conventional water sources and creating effective and efficient treatment methods for ensuring water security at the household and community levels. Electrochemical technologies are able to be promising alternatives as cost-effective and fitfor-purpose systems in rural areas (McBeath et al., 2020a). They can make a significant contribution to decentralized systems by introducing a flexible method capable of treating 27 water from different sources (Shu et al., 2023a). In the past few years, there has been a significant increase in the attention given to the advancement and utilization of electrochemical treatments as substitutes for conventional treatment systems in peerreviewed literature and commercial section. Within the last two decades, approximately 31,000 research articles and books have been published in the area of environmental electrochemistry (Garcia-Segura et al., 2020; Shu et al., 2023b). They can be either used as a separate technology or as part of a treatment series in various applications where it is logistically challenging to rely on chemical-intensive technologies to treat water or they are not capable of eliminating emerging contaminants (Garcia-Segura et al., 2020). Electrochemical cells are able to simultaneously remove various organic, inorganic and microbiological contaminants from water through several mechanisms, including oxidation, reduction and disinfection (Shu et al., 2023a). In-situ production of chemicals on the surface of electrodes with low energy demands enables the electrochemical method to serve as a compact and easy-to-operate water treatment method known as a green technology (Särkkä et al., 2015a). Additionally, the energy footprint of these technologies can be further reduced by providing energy recovery through fuel generation at the cathode (Chaplin, 2019b). Water treatment electrification can be a promising alternative to off-site chemical production. It can occur by providing in-situ or on-site chemical generation using electricity. Hence, the carbon emission from drinking water treatment can be significantly reduced (Gingerich et al., 2022b) (McBeath et al., 2020a). Electrochemical technology can be employed as point-of-entry or point-of-use water treatment systems owing to diverse advantages, such as modular and compact design, easy operation, and 28 transportation/storage-free operation (Shu et al., 2023a). Despite advances in effective electrochemical strategies for environmental remediation, challenges remain in implementing and scaling these strategies, particularly in developing efficient and robust electrodes and catalysts. Furthermore, designing an efficient reactor configuration with elevated mass transport and low capital costs needs further research (Shu et al., 2023a). 2.7.1 Electrodes The efficiency of treatment could be significantly increased by the development of innovative electrode materials. Highly active agents, such as hydroxyl radicals, oxygen radicals, are produced at the cathode and anode surface via electrons, which are known as clean reagents. They play a significant role in the removal of contaminants and the elevation of system efficiency (Shu et al., 2023a). The generation of agents depends on the materials employed as electrodes in an electrochemical cell. Numerous types of electrodes, including metals, metal oxide, polymers, and carbonaceous materials (e.g. aluminum, iron, stainless steel, boron-doped diamond (BDD), PbO2, Ti4O7, SnO2, and carbon fibre cloth) have been reported in different studies so far (Pavithra et al., 2021; Sun et al., 2021b). Electrodes make the most contribution to an electrochemical cell's performance. Therefore, high porosity, electrical conductivity, and stability should be taken into account while choosing electrodes to increase the performance of electro-elimination of contaminants accordingly. BDD is one of the most promising electrodes due to its high anodic stability, hydroxyl radical yield and wide potential range. However, its development on larger scales might be hindered because of the slow and high cost of production(Chaplin, 2019b). In addition, the use of plate and frame electrodes exacerbates the limitations of mass transfer in electrochemical cells. Besides, the production of oxygen reactive agents is only limited 29 to the area near the surface of the electrodes (Radjenovic & Sedlak, 2015). As a result, the efficiency of contaminants removal is diminished. Finding affordable alternatives is crucial to facilitate the use of electrochemical cells and their significant capabilities for treatment purposes. Depending on the type of pollutants present in the water and the mechanism of the removal, which can be either electrochemical reduction, oxidation or both, the material for anode and cathode would be a critical factor for increasing the removal efficiency. Scientists have discovered that for the design and optimization of electrochemical oxidation processes, the anode material has to be significantly taken into consideration. However, the cathode can be materials such as stainless steel, platinum mesh or carbon felt (Alkhadra et al., 2022a). Anodes can be divided into two categories: active and nonactive materials. Carbon, platinum, and iridium oxides are examples of active anodes. Nonactive anodes include lead dioxide, boron-doped diamond, etc. (Alkhadra et al., 2022b; Shu et al., 2023a). In contrast to nonactive anodes, which have high oxygen evolution overpotential and are poor electrocatalysts for oxygen evolution, active anodes have low oxygen evolution overpotential (Abdalrhman et al., 2019a; Shu et al., 2023a). Implementation of porous materials helps overcome the problem of electrodes' diminished performance by providing a higher surface area; thereby, more active agents are produced, and less energy is consumed. As a result, the removal of contaminants is not only limited to the electrooxidation on the anode, but electrodes are used as adsorption units and improve decontamination by other mechanisms. Among various materials, carbon nanotubes (CNTs) have gained significant attention due to their unique physicochemical, mechanical and electronic characteristics, which allow them to serve as 30 both electrode and filtration media. Therefore, by utilizing an electroactive CNT filter, two processes occur. The flow of the contaminated water through the electrode considerably enhances the electrochemical reaction kinetics, and the electrochemical reactions decrease the membrane fouling (Liu et al., 2020). Activated Carbon Felt (ACF) was introduced as a novel three-dimensional electrode due to its high specific surface area, significant adsorption capacity, high electric and catalytic capability, and excellent conductivity. It was reported that the oxygen evolution potential on an ACF electrode is 1.7 V, which is roughly equivalent to the potential on a platinum electrode (Yi et al., 2008). Based on a research study conducted by Ma et al., potassium hydroxide-modified activated carbon fibre felt was utilized to investigate the removal of cobalt ions (II) from simulated radioactive effluent by electrosorption mechanism. It was illustrated that the performance of the system remained more than 90% after six cycles (Ma et al., 2023). Li et al. studied the electrosorption of norfloxacin in an aqueous solution by modified activated carbon fibre felt. The modification of the ACF was employed by 20% nitric acid. It was demonstrated that the adsorption capacity of the electrode was 128.55 mg/g, and the regeneration rate of the electrode was maintained at approximately 96% (Li et al., 2020). In another work, the efficiency of fluoride electrosorption using micropore-dominant activated carbon as an electrode was studied. The results showed that the high capacity of fluoride electrosorption was attributed to the high specific surface area of the synthesized electrode, which was 2130 m2/g, mainly from micropores contribution (Li et al., 2017). 31 2.7.2 Water flow modes The design of an electrochemical cell is a significant determining factor of its efficiency for contaminants removal. Because it affects the mass transfer patterns and energy consumption, which influences the cost-effectiveness of the system (Xie et al., 2022b). In an electrochemical reactor, the contaminated water can be transported in either flow-by or flow-through modes. Traditional electrochemical cells employ the flow-by mode in which the water transfers between two electrodes. Mass transportation in this method is restricted because the reaction rate is controlled by the diffusion of contaminated water on the electrode surface. Therefore, this structure has less potential for larger-scale applications (Xie et al., 2022b). In contrast, in the flow-through mode, porous and high specific surface area electrodes are utilized, and the contaminated water passes directly through them; thereby, mass transfer and reaction rates significantly increase due to the higher electroactive contact area. As a result, operating in flow-through mode, an electrochemical cell is highly capable of contaminant removal even on larger scales (Chaplin, 2019b; Sun et al., 2021a). 2.7.3 2.7.3.1 Removal mechanisms Electrochemical Oxidation Electrochemical oxidation is a chemical process wherein an atom or molecule at the anode undergoes the loss of one or more electrons upon the passage of an electric current through the system. In the realm of water treatment, electrochemical oxidation produces reactive oxidizing agents known as free radicals, which engage with the contaminants and degrade them, which might lead to the mineralization of target 32 compounds. Reactive agents such as hydroperoxyl (HO2•), hydroxyl (HO•), and sulfate (SO4•–) radicals exemplify the oxidizing species that can initiate a chain of radical oxidation, thereby facilitating the degradation of various contaminants (Alkhadra et al., 2022a; Giannakis et al., 2021). Of the various radicals mentioned, the hydroxyl radical (HO•) holds particular significance as the primary oxidizing agent. It is noteworthy that HO• is produced through water oxidation without the need for additional chemical substances(Särkkä et al., 2015b). Additionally, HO• is known as one of the most potent oxidants generated within electrochemical systems (Xie et al., 2022b). Electrochemical technology is applicable for a wide range of purposes, including water disinfection, groundwater and wastewater treatment, and soil remediation (Alkhadra et al., 2022a). Electro-oxidation can take place via two mechanisms. The first mechanism involves the direct oxidation facilitated by hydroxyl radicals generated on the surface of the anode. The second mechanism is an indirect process where oxidizing agents, such as chlorine, hypochlorite, hydrogen peroxide or ozone, are formed at the electrodes(Särkkä et al., 2015a; Zhang et al., 2013a). 2.7.3.2 Electrochemical Reduction Electrochemical reduction involves the acquisition of one or more electrons by an atom or molecule at the cathode when an electric current is applied to the system. Electrochemical reduction can take place either directly at the cathode's surface or indirectly in the solution via reducing agents produced at the electrodes. This method is employed to eliminate heavy metals, inorganic compounds, etc., from a solution. Cathode potential, catalyst loading, and water quality are significant factors that influence the effectiveness of electrochemical reduction (Alkhadra et al., 2022a). 33 2.7.3.3 Electrocoagulation and Electroflocculation By using metal anodes to create flocs during the two-step electrocoagulation and electroflocculation processes, pollutants can be eliminated via settling, sedimentation, precipitation, or flotation. This technique uses sacrificial anodes made of iron and aluminum to treat water and sewage. In situ oxidation of metal surfaces, which produces metal ions that aid in the removal of particulates, organic species, and inorganic chemicals, is a vital step. Electrocoagulation is frequently utilized in industrial settings in order for the treatment of generated water and wastewater and the removal of heavy metals. It has benefits such as in situ cationic coagulant production, less sludge formation, and no requirement for counterion separation. Electrochemical reactions produce OH− ions during the process, eliminating the need for external pH-regulating agents and generating gaseous H2 as a potential fuel source. However, operational challenges such as sludge deposition, anode dissolution inconsistency, and variable coagulant production undermine continuous operation. Periodic anode replacement and post-treatment are required due to anode consumption and residual metal ions. Despite these challenges, electrocoagulation and electroflocculation remain active areas of research due to their simplicity, costeffectiveness, and versatility (Alkhadra et al., 2022a). 2.7.4 Modification of electrodes In some research, the modification of electrodes is reported. So, the oxidation potential of anode is increased by improvements in the production of ROS such as OH radicals. Also, the limitations of low current density and reduced efficiency with time at high anode potentials due to the effects of water oxidation and anode corrosion diminished (Liu et al., 2020). For instance, Zizhen Li et al. used FeOCL catalysts as a novel two34 dimensional material with specific physicochemical properties for the functionalization of CNT filters, and the removal of tetracycline was achieved (Li et al., 2020). In another study, Liu et al. synthesized an electrochemical CNT filter coated with a TiO2 film to enhance the porosity and surface area of CNT and sorption capacity and kinetics improvements accordingly. The TiO2–CNT filter was used to eliminate low concentrations of arsenic from groundwater with both adsorption and oxidation (Liu et al., 2014). In a recent study, a novel 3-dimensional BDD anode was prepared on the Cu/W framework. Compared to conventional 2-dimensional BDD, the electrochemically active surface area was significantly increased and is 7.9 times higher than the 2-D structure. As a result of improving mass transpo rt, the oxidation performance of total organic carbon from water was enhanced by 5.3 times and 7.95 times less energy consumed in the removal process (Miao et al., 2022). 2.7.5 Biochar and its electrochemical attributes Biochar has gained extensive research attention owing to its sustainable carbonaceous structure. Tremendous surface area, porous structure, functional groups, and high conductivity facilitate the electron exchange process and allow biochar derived from pyrolysis to act as catalysts, support, and cathode material (Nidheesh et al., 2021a; Tian et al., 2021a). In addition, containing persistent free radicals enables the generation of ROS by biochar, which are the key contributors in electrochemical water treatment. Due to the less secondary pollution, it has been proven that biochar has great potential applications and economic value. In a recent study, biochar derived from oily sludge was successfully used to remove cadmium and lead from an aqueous solution. Mineral precipitation and complexation were the dominant mechanisms for the adsorption process(Tian et al., 2020). 35 Liu et al. (2021) demonstrated the high capacity of nitrobenzene removal by Zn/Femodified biochar. The removal efficiency of 94% was reported, which was attributed to both adsorption and electrochemical degradation of nitrobenzene molecules. Bao et al. (2020) investigated the redox capacity of biochar derived from oily sludge and demonstrated its high electron transfer capability as an electrode material. In a study, orange peel was used as a raw material for biochar production, and the prepared electrode was used for the electroreduction of nitrate. It showed that the addition of Fe can increase the electron transport efficiency and the nitrate reduction accordingly (Zhang et al., 2022). Electrochemical behaviours of biochar, such as electron donating and accepting capacity, have a significant role in chemical reactions occurring for contaminants removal from water. In the electron exchange process, characteristics of biochar, such as persistent free radicals, oxygen-containing functional groups, graphitic structure, and metal elements, are of high importance(Jiang et al., 2023). For instance, the carboxylic groups of dissolved biochar are able to be involved in both Cr(VI) reduction and As(III) oxidation. It has been indicated that the raw materials properties and the preparation process considerably affect the physiochemical structures of the biochar and, subsequently, its electrochemical behaviour in reactions (Tian et al., 2021a). As an example, by applying different heating treatment temperatures, the functional groups presented are varied, which has an influence on the biochar's tendency to act as an electron acceptor or electron donator. Biochar modification by metallic and non-metallic elements can increase the generation of ROSs, which are the key contributors to pollutant oxidation and reduction. Moreover, it enhances the efficiency of electron transfer and adsorption of contaminants(Nidheesh et al., 2021b). 36 Biochar is produced from many kinds of feedstock, including grass, algae, wood, manure, sewage sludge, etc. However, it was demonstrated that wood-derived biochar has more carbon content and less ash compared to biochar derived from non-wooden material. Moreover, biochar generated from animal waste consists of high nitrogen content and nitrogen bonds and functional groups, accordingly (Chacón et al., 2017). Biochar is considered a substantial and economic catalytic material for peroxymonosulfate (PMS) and peroxodisulfate (PDS) activation. Activated PMS and PDS can help the degradation of pollutants from water and wastewater through various pathways of free and non-free radicals. The catalytic active sites of biochar assist the generations of SO4- and OH radicals, which are of importance in the oxidation of pollutants (Jiang et al., 2023). Functional groups such as -COOH or -OH act as electron donors, and persulfate is activated by accepting electrons; as a result, -COOH or -OH is converted to -COO• and -O•, respectively (Equations 2.1 and 2.2) (Jiang et al., 2023). BC – Surface – OH + S2O82- → BC – Surface – O• + (SO4-)• + HSO4- (2.1) BC – Surface – OOH + S2O82- → BC – Surface – OO• + (SO4-)• + HSO4- (2.2) Biochar-assisted advanced oxidation processes (AOPs) have been utilized for the elimination of various types of contaminants from water. However, their application is mainly investigated for the removal of organic pollutants such as organic dyes, methyl orange, Rhodamine B, etc. There are several works have taken advantage of biochar catalytic methods for removing heavy metals from water. For instance, Yan et al. prepared 37 NiS/NiSe/3D porous biochar for the photocatalytic removal of As3+ from an aqueous solution (Yan et al., 2021). Utilizing biochar as an electrode material for the electrocatalytic oxidation of contaminants has drawn increasing attention in recent years. For example, in a study, an efficient sludge biochar-derived Pd-SAC@Ni electrode was fabricated for the degradation of 4-chlorophenol in wastewater. The result showed that direct and indirect reduction pathways contributed 19.5% and 80.5% for 4-CP degradation, respectively(Zhao et al., 2022). In another work, Yao et al. designed a Pd/N-doped loofah sponge-derived biochar material with a three-dimensional (3D) network structure as an electrode for the electrocatalytic and adsorption of bromate. The highest bromate removal efficiency of 96.7 % was acquired (Yao et al., 2020). 2.7.6 Integration of adsorption with electrochemical technology Integration of electrochemical technology with adsorption allows us to take advantage of both mechanisms simultaneously. In fact, porous and conductive materials as electrodes can also play the role of adsorbent, which assists in removing contaminants through the adsorption process by providing a higher contact area. In addition, it shows significantly enhanced electrochemical efficiency due to their higher mass transfer. This novel hybrid method can be beneficial for removing a wide range of pollutants. The small footprint, low energy consumption, and continuous operation make the electrochemical filtration process one of the most promising options for water treatment in small and rural communities (Pan et al., 2020). Electrosorption technology relies on voltage-induced double-layer formation at the electrode-solution interface, enabling the migration of charged ions from the solution to 38 the oppositely charged electrode for water purification. The choice of electrode material is crucial, necessitating characteristics such as mechanical stability, cost-effectiveness, and strong adsorption capabilities. Common electrode materials include graphite, carbon nanotubes, and activated carbon (AC). AC, known for its ample surface area and stable chemical properties, is a preferred adsorbent. However, AC has drawbacks like low porosity and suboptimal regeneration efficiency. In contrast, activated carbon fibre felt, derived from activated carbon fibre, excels with its superior adsorption capacity, surface micropores, structured design, and rapid adsorption/desorption rates. Activated carbon fibre felt has proven to be an excellent choice for water purification (Li et al., 2020). Liu et al. (2014) employed a graphene-based electrochemical filter with CNTs for the electrooxidation of organic pollutants such as tetracycline, phenol, and oxalate. The high surface area and high conductivity of graphene allowed the simultaneous physical adsorption and electrochemical oxidation of chemical contaminants. McBeath et al. (2020) investigated the removal of Mn2+ in source waters for drinking water supply by electrochemical oxidation. BDD anode was used in a batch configuration with separated anolyte and catholyte solutions, which decreases its application in real situations. Another study aimed to fabricate a MnO2/TiO2 nanotube array-coated titanium anode for efficient electrocatalytic oxidation of As(III) ions from groundwater(Xiong et al., 2021). Mixed metal oxide (MMO) electrodes could be a good alternative for high-cost BBD. However, the experiments conducted in flow by mode can decrease the efficacy of removal by diminishing mass transport. 39 2.7.7 Three-Dimensional electrochemical cell Significant advancements have been achieved in electrochemical technologies for treatment, particularly in the removal of bio-refractory substances. These technologies are highly efficient, environmentally friendly, and versatile. However, despite these advantages, there are still certain limitations that hinder their widespread industrial application. These include the short lifespan of electrode materials and low current efficiency, as well as some inherent challenges such as mass transfer limitations and inadequate area-to-volume ratio, especially when treating low-conductivity wastewater (Zhang et al., 2013b). The utilization of three-dimensional (3D) electrochemical processes is considered a viable approach and alternative to conventional electrochemical systems. Compared to two-dimensional (2D) electrochemical processes, the incorporation of particle electrodes in the 3D configuration offers several advantages. These include a larger specific surface area, enhanced conductivity, and shorter mass transfer distances, which enhance the efficiency and potential of this method for environmental applications (Xie et al., 2021a; Yan et al., 2011a). The particle electrode system entails introducing conducting particles into the region between the anode and cathode, wherein the degradation of contaminants occurs not only on the electrodes’ surface but also on the surface of the conducting particle electrodes (Ghanbarlou et al., 2020a). When a potential is applied, a bipolar system is formed at the surface of these particles, allowing them to function as an independent microelectrochemical system, with one side of the particle acting as an anode and the other side serving as a cathode (Xie et al., 2021b; Zhang et al., 2013b). Various carbon materials, including granular activated carbon, graphene, and activated carbon fibres, are commonly employed in the preparation of particle electrodes. Additionally, 40 some metal oxides, such as modified activated carbon fibres with MnOx, have also been explored as particle electrodes, which facilitated the generation of hydroxyl radicals on their surface (Ghanbarlou et al., 2020b; Liu et al., 2019). 2.8 Summary To summarize this chapter, it was discussed that rural communities often deal with contaminated water sources, which stem from a multitude of sources, including industrial discharges and poor sanitation practices. Such contamination poses a serious threat to public health, as it can lead to the ingress of heavy metals and pathogens into both surface and groundwater, potentially resulting in severe diseases and even fatalities. Manganese is a prevalent heavy metal in the rural regions and First Nation communities of Canada. While manganese is essential in trace amounts for human health, elevated concentrations have been linked to central nervous system disorders. Conventional technologies like filtration, ion exchange, and adsorption have demonstrated their efficacy in removing manganese, yet their high costs present significant barriers in many rural areas. Furthermore, while centralized water treatment systems are effective in urban environments, they are often impractical in rural areas due to cost and infrastructure limitations. Decentralized water treatment systems offer a potential solution, allowing for on-site purification to ensure safe drinking water. As a result, the need for a versatile, modular, and cost-effective water treatment method capable of addressing contaminants from various sources remains pressing. The use of electrochemical technology has advantages such as the capacity to remove a wide range of contaminants in a compact unit, no chemical usage, treatment of low-concentration pollutants, and minimal operational demands. Nevertheless, this technology faces hurdles, such as costly and low-surface-area electrodes, limited mass 41 transport, and low conductivity that need to be addressed. Electrode modifications, flowthrough designs for electrochemical cells, three-dimensional cell configurations, and various other methods have been employed to enhance the efficiency of electrochemical technology and address the challenges discussed. However, additional research is necessary to further advance these techniques and optimize their performance. 42 Chapter 3: Materials and Methods 3.1 Materials Graphite felt (GF) and ACF were purchased from CeraMaterials (Pennsylvania, USA) and used as electrodes for all the experiments. Manganese chloride (MnCl2) was used to prepare the Mn(II) solution. Sodium Sulfate (Na2SO4) was utilized as a supporting electrolyte. Polyvinylidene fluoride (PVDF) and N-Methyl-2-pyrrolidone (NMP) were used for the ACF modification process. Sulfuric acid (H2SO4) and Sodium Hydroxide (NaOH) were employed for pH adjustment. Concentrated nitric acid (65 % w/w) was added for the stabilization of the Mn(II) in the solution. All the aforementioned materials were purchased from Sigma Aldrich (Massachusetts, USA). Wood biochar was purchased from a local company (BC, Canada). Deionized water (DI) was used for all the main experiments. Raw groundwater of the Lheidli T'enneh community was collected to investigate the effect of co-existing ions on Mn removal efficiency. 3.2 Preparation of Biochar Modified ACF The biochar-coated ACF (BC@ACF) was prepared by coating a mixture of biochar powder and PVDF on ACF at a mass ratio of 2:1. Firstly, ACF was washed with deionized water several times. The biochar was ground with an agate mortar for 20 min, and the produced powder was sieved to size smaller than 75 µm. The dried PVDF powder was dissolved in 20 ml NMP under ultrasonic dispersion to obtain a homogeneous solution as a binder. The biochar was added to the prepared binder and dispersed ultrasonically at room temperature. The resulted slurry was uniformly coated on both sides of the ACF and dried in an oven at 80 degrees for 2 hours. This process was repeated three times to achieve a 43 consistent BC@ACF. The final product was washed with deionized water several times and dried at atmosphere temperature (Figure 3.1). Figure 3. 1 Synthesis of BC@ACF. 3.3 Experimental Setup All experiments were conducted within a 400 ml beaker, with the anode and cathode positioned on opposite sides and maintaining a 5 cm separation between them. The cathode, consisting of GF, has dimensions of 8 cm in height, 3 cm in width, and 5 mm in thickness. On the other hand, dimensions of the anode, composed of BC@ACF are 8 cm in height, 3 cm in width, and 2 mm in thickness. To facilitate the transfer of electrons within the system, the anode and cathode were meticulously connected to a power supply, utilizing a graphite rod for the anode and a copper rod for the cathode (Figure 3.2). For each experimental run, a 200 ml solution featuring varying concentrations of Mn(II) was employed. To ensure thorough mixing and homogeneity, a magnetic stirrer was utilized, agitating the solutions at a constant speed of 400 rpm. To enhance the overall 44 conductivity of the solutions and thereby promote efficient ion transport crucial for the electrochemical reactions, 0.05 mol/L Na2SO4 was added as a supporting electrolyte to all solution compositions. This addition played a pivotal role in facilitating the electrochemical processes and the subsequent assessment of Mn removal efficiency. 45 Figure 3. 2 Experimental setup of the electrochemical cell. 46 3.4 Experimental design More than 30 preliminary experiments were carried out to figure out the effect of various parameters and determine the major variables and their range for the main experiments. In every series of experiments, varying quantities were explored for a single variable to determine the optimal range while keeping the remaining parameters constant. All the experiments were carried out in triplicate and the average is reported. According to Table 3.1, Mn initial concentration, current, pH, and time were considered the main independent factors affecting the Mn removal efficiency. Table 3. 1 Independent variables and their experimental range Variables Factor Unit Levels −1 0 +1 Mn initial concentration A mg/L 0.5 2 3.5 Current B mA 25 50 75 pH C ---- 5 7 9 Time D min 30 60 90 In traditional experimentation, parameters are often assessed in isolation while others are held constant, resulting in a limited exploration of parameter influence and significance, as well as the interactions between variables. To address this limitation, Response Surface Methodology (RSM) emerges as a valuable statistical modelling and mathematical technique. RSM facilitates the evaluation of the relationships between various factors and the corresponding measured responses, involving both independent and 47 dependent variables. Moreover, it optimizes conditions to achieve maximum efficiency, thus reducing unwanted costs(Ferreira et al., 2023). Within the realm of optimization for removal processes, various strategies have been applied, including the Box-Behnken Design (BBD), Central Composite Design, Full or Fractional Factorial Design, and Doehlert Matrix Design. In terms of response efficiency, BBD has demonstrated superior outcomes compared to the Central Composite Design and even outperformed the Full or Fractional Factorial Design. BBD is able to deliver essential information about optimization by investigating the operating parameters and their interactions while requiring a smaller number of experiments(Sahu et al., 2018). By combining these potent mathematical tools, a distinct advantage emerges the ability to simultaneously investigate the effects of multiple parameters and their interactions on the desired response, as opposed to the conventional "one-variable-at-a-time" approach. This integrated approach enhances our ability to comprehensively explore and optimize complex systems, offering a more efficient and insightful path for experimental design (Chibani et al., 2022; Ferreira et al., 2023). Design Expert 12 was utilized for manganese removal process optimization. Data analysis was performed through BBD and RSM. This resulted in a total of 27 experimental runs, as shown in Table 3.2, and the assessment of experimental errors was carried out using five center points. In order to assess model accuracy, as well as determine the correlation between variables and the significance of each individual variable, an analysis of variance (ANOVA) was employed. The impacts of the four main factors, including Mn initial concentration, current, pH, and time, on the manganese removal efficiency as the 48 primary response, along with their interdependencies, were illustrated through 3D response surface plots. 49 Table 3. 2 Design of experiments. Variables Run A: Mn Initial B: Current Concentration mg/L C: pH mA D: Time min 1 2 2 2 75 75 7 7 90 30 3 2 75 9 60 4 2 5 2 50 50 9 7 90 60 6 2 50 7 60 7 0.5 75 7 60 8 2 9 2 10 3.5 50 50 50 5 9 7 90 30 90 11 2 12 0.5 75 50 5 9 60 60 13 2 50 7 60 14 2 25 7 90 15 2 16 0.5 17 0.5 50 50 50 7 7 7 60 30 90 18 2 19 3.5 50 75 5 7 30 60 20 2 25 5 60 21 3.5 22 3.5 50 50 9 5 60 60 23 0.5 24 0.5 25 50 7 5 60 60 25 2 26 2 50 25 7 9 60 60 27 3.5 25 7 60 50 The removal efficiency of manganese was calculated based on the following formula: (%) = × 100 (3.1) Additionally, a pseudo-first-order model was used to fit the kinetics of manganese electro-oxidation as follows: ( )= (3.2) where C0 (mg/L) denotes the initial Mn concentration, Ct (mg/L) indicates the final Mn concentration after treatment, k (min-1) denotes the apparent rate constant, and t (min) denotes the reaction time. 3.5 Groundwater characteristics The quality of groundwater samples collected from the community was assessed in order to investigate the potential problems and excess amount of any metals in the samples. Furthermore, a set of experiments were conducted to evaluate the performance of the designed system in removing the targeted contaminant (manganese) in the presence of various co-existing ions in a real groundwater as well as the effect of the system on the concentration of these elements after treatment process. Understanding how the designed system interacts with a complex mixture of ions found in actual groundwater is paramount. It not only sheds light on its ability to selectively remove manganese but also provides insights into potential side effects on other elements present. Table 3.3 and 3.4 demonstrate the summary of the physical properties and various metals concentration in the groundwater, respectively. The results from this comprehensive analysis will aid in 51 understanding the state of the groundwater and serve as a foundation for future actions aimed at improving water quality and removing potential contaminations. Table 3. 3 Physical characteristics of the groundwater. Physical Tests Units pH ----- 7.88 EC μs/cm 502 TDS mg/L 244 Hardness mg CaCO3/L 268.2 52 Table 3. 4 Metals concentration in groundwater. Total Metals Unit Concentration Reportable Limit Aluminum mg/L <0.02 <0.02 Arsenic mg/L <0.02 <0.02 Boron mg/L 0.007 <0.001 Barium mg/L 0.103 <0.0004 Calcium mg/L 75.9 <0.005 Cadmium mg/L <0.0004 <0.0004 Cobalt mg/L <0.002 <0.002 Chromium mg/L <0.0004 <0.0004 Copper mg/L 0.006 <0.002 Iron mg/L 0.173 <0.003 Potassium mg/L 3.04 <0.003 Magnesium mg/L 19.1 <0.001 Manganese mg/L 1.04 <0.001 Molybdenum mg/L <0.003 <0.003 Sodium mg/L 3.06 <0.002 Nickel mg/L <0.001 <0.001 Phosphorus mg/L 0.041 <0.01 Lead mg/L <0.004 <0.004 Sulfur mg/L 1.04 <0.08 Antimony mg/L <0.01 <0.01 Selenium mg/L <0.02 <0.02 Tin mg/L <0.01 <0.01 Uranium mg/L <0.01 <0.01 Vanadium mg/L 0.001 <0.001 Zinc mg/L 0.002 <0.001 53 3.6 Analytical method 0.45 μm PVDF syringe filters were used to filter all samples immediately after sampling to effectively remove particulate matter and impurities, ensuring the purity and quality of the samples. Subsequent to the filtration process, a preparatory step was initiated wherein the samples were subjected to acidification using HNO3 to decrease experimental errors. Moreover, to stabilize the samples, preventing any potential oxidation or alteration of manganese concentrations before analysis such as Mn2+ and OH− interactions (Chu et al., 2023). This precautionary measure played a pivotal role in maintaining the integrity and reliability of our Mn concentration data. Microwave Plasma Atomic Emission Spectroscopy (MP-AES) was utilized in order to measure Mn concentrations. The Electrical conductivity (EC), pH, and TDS of samples were measured by the associated meters. Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES) was utilized to measure metals concentration in groundwater before and after treatment. 54 Chapter 4: Results and Discussion 4.1 Statistical Analysis In order to evaluate the importance of the suggested model and investigate the effect of each variable, an ANOVA was conducted. This analytical approach utilizes a low pvalue (<0.05) to denote the significance of a given parameter. According to Table 4.1, parameters A, B, C, D, AC, BC, CD, A², B², and D² all exhibited p-values below 0.05. This establishes the significance of these parameters within the model. The overall model's pvalue was less than 0.0001, highlighting the significance of the proposed model and its alignment with the data. The lack-of-fit displayed a non-significant p-value, affirming that the data fit the model well. Table 4. 1 Analysis of variance (ANOVA). Source Model A-Mn Initial Concentration B-Current C-pH D-Time AC BC CD A² B² D² Residual Lack of Fit Pure Error Cor Total Sum of Squares 7093.79 994.79 df 1957.46 285.19 2170.28 121.00 76.56 72.25 407.58 568.90 209.92 152.11 84.41 67.70 7245.90 1 1 1 1 1 1 1 1 1 12 8 4 26 14 1 Mean Square 506.70 994.79 F-value p-value 39.97 78.48 < 0.0001 < 0.0001 1957.46 285.19 2170.28 121.00 76.56 72.25 407.58 568.90 209.92 12.68 10.55 16.93 154.43 22.50 171.22 9.55 6.04 5.70 32.15 44.88 16.56 < 0.0001 0.0005 < 0.0001 0.0094 0.0302 0.0343 0.0001 < 0.0001 0.0016 0.6234 0.7364 significant not significant 55 As depicted in Table 4.2, the R² value is close to 1, indicating a strong correlation between the predicted data by the model and the experimental results. The near values of R2 and adjusted R2 confirm the experiments satisfactory results. Table 4. 2 Coefficient of determination (R2) of the model. Std. Dev. 3.56 R² 0.9790 Mean 72.37 Adjusted R² 0.9545 C.V. % 4.92 Predicted R² 0.8977 Adeq Precision 20.8501 In Figure 4.1 a, the graph comparing predicted and actual data, where the distance between experimental data points and the prediction line at a 45° angle signifies the alignment of experimental results with the model and their predictability. Figure 4b demonstrates the consistent distribution of residuals. 56 Figure 4. 1 (a) Predicted response values vs. actual response values, (b) Normal plot of residuals for the model. Equation 4.1 demonstrates a quadratic model that describes the effectiveness of manganese (II) removal utilizing anode made of BC@ACF. This equation was derived using Design Expert 12 software. It established a relationship between removal efficiency and the main variables: the Mn initial concentration (A), Current (B), pH value(C), and time (D). Removal Efficiency = 80.1 + 10.05A + 14.10B + 4.87C + 16.85D + 5.5AC + 4.37BC + 4.25CD + -8.36A2 - 9.88B2 - 6.50833D2 (4.1) In Table 4.3, the outcomes of the conducted experiments are provided for analysis. This table serves as a visual representation of the data collected from these experiments, 57 offering valuable insights into the effects and outcomes of the processes being studied. Within this table, there are two distinct response variables that have been measured and recorded. The first response variable is the Mn final concentration in the solution. This variable indicates the amount of manganese that remains in the solution after the electrochemical treatment or process has taken place. The second response variable provided in the table is the removal efficiency. This parameter quantifies the effectiveness of the electrochemical process in removing manganese ions from the solution. It represents the proportion of initial manganese ions that have been successfully eliminated or converted during electrochemical treatment. A higher removal efficiency indicates a more successful process in terms of reducing the concentration of manganese ions in the solution. By presenting these two response variables, the table offers a comprehensive overview of how changes in experimental conditions, such as initial concentration of manganese ions or current, impact both the final concentration of manganese in the solution and the efficiency of its removal. This information is crucial for evaluating the performance of the electrochemical system and for making informed decisions regarding process optimization or comparison with other methods. 58 Table 4. 3 RSM design and responses value. Variables Run A: Mn Initial B: Current Concentration mg/L Responses C: pH mA D: Time Mn Final Concentration Removal Efficiency min mg/L % 1 2 2 2 75 75 7 7 90 30 0.13 0.78 93.5 61 3 2 75 9 60 0.04 98 4 2 5 2 50 50 9 7 90 60 0.14 0.43 93 78.5 6 2 50 7 60 0.26 87 7 0.5 75 7 60 0.18 64 8 2 9 2 10 3.5 50 50 50 5 9 7 90 30 90 0.13 0.7 0.16 93.5 65 95.4 11 2 12 0.5 75 50 5 9 60 60 0.5 0.21 75 58 13 2 50 7 60 0.39 80.5 14 2 25 7 90 0.73 63.5 15 2 16 0.5 17 0.5 50 50 50 7 7 7 60 30 90 0.47 0.28 0.15 76.5 44 70 18 2 19 3.5 50 75 5 7 30 60 1.03 0.44 48.5 87.4 20 2 25 5 60 0.89 55.5 21 3.5 22 3.5 50 50 9 5 60 60 0.23 0.86 93.4 75.4 23 0.5 24 0.5 25 50 7 5 60 60 0.29 0.19 42 62 25 2 26 2 50 25 7 9 60 60 0.44 0.78 78 61 27 3.5 25 7 60 1.6 54.3 59 4.2 Effect of variables and their interactions on removal efficiency The perturbation plot was utilized in order to investigate and compare the effect of each parameter at a particular point in the design space. The higher the slope of a curve in this plot, the bigger the influence of the associated parameter on the removal efficiency. According to Figure 4.2, time (D) and current (B) have the greatest influence on manganese removal efficiency. However, a very high current intensity has an adverse effect and can decrease removal efficiency. On the other hand, the pH value (C) has the least effect on the removal efficiency. Figure 4. 2 Perturbation plot for Mn Removal Efficiency 60 Figure 4.3a-c. shows the 3D response surface plots, which represent the correlation between two individual variables simultaneously with regard to the removal efficiency. Based on Figure 4.3a, the interaction between time (D) and pH (C) at the constant Mn initial concentration of 3.5 mg/L and current intensity of 75 mA, and their effects on the Mn removal efficiency is depicted. It is indicated that, for instance, in pH 5, by increasing the time from 30 to 90 minutes, the removal efficiency is enhanced. However, this increase is less when in pH 9 the time of experiment is raised from 30 to 90 minutes. It corroborates the fact that the manganese removal efficiency is relatively elevated at higher pH. It can be due to the improved oxidation rate of manganese in high pH. There are more Hydroxide ions (OH-) at higher pH. Thus, this availability of hydroxide ions enhances the reaction between manganese ions and oxygen, promoting the conversion of manganese to its oxidized form. Additionally, the higher pH levels can lead to the formation of manganese hydroxides, which further accelerates the oxidation reaction (J. Shu et al., 2020). Moreover, activated persulfate, which is a strong oxidant produced due to the presence of sulfate ions in water, has a higher efficiency under alkaline conditions, which helps to convert manganese to its oxidized forms (Du et al., 2019). Figure 6b. illustrates the relation between time (D) and current intensity (B) at the constant Mn initial concentration of 3.5 mg/L and pH 9 and their simultaneous influence on the removal efficiency. It is obvious that at lower times, such as 30 minutes, by raising the current from 25 to 75 mA, the removal efficiency is boosted significantly. Furthermore, in higher reaction times, like 90 minutes, the removal efficiency is increased from around 60% to 98%, which confirms the substantial effect of current on the manganese removal efficiency. Due to the higher current, an increased amount of oxygen gas and hydroxide ions are produced on the anode 61 and cathode surfaces, respectively, which leads to an increment in the rate of manganese oxidation. Furthermore, the indirect electro-oxidation of manganese would be facilitated by active intermediates like S2O82-, which will be increased in amount by higher current intensities (Yi et al., 2008). In Figure 6C, the interaction between current (B) and Mn initial concentration (A) and their impact on the removal efficiency is depicted. It is indicated that by enhancing the Mn initial concentration from 0.5 to 3.5 mg/L, the removal efficiency is improved both in low and high currents. This can be due to the effects of mass transfer. In fact, the higher concentration gradient between the solution and the electrode surfaces can facilitate increased mass transfer of manganese ions towards the electrode surfaces, improving their chances of being electrochemically oxidized or removed. 62 (a) (b) (c) Figure 4. 3 The 3D response surface plots (a) interactive effect of time and pH, (b) interactive effect of time and current, (c) interactive effect of current and Mn initial concentration. 63 A series of experiments were conducted to better examine the significant role of current in the electrooxidation of manganese with the Mn initial concentration of 2 mg/L and pH 9, and Na2SO4 concentration of 0.05 M as a supporting electrolyte. Current intensities were set up at 0, 25, 50, 75, and 100 mA in different experiments. Several samples were taken in 10, 20, 30, 60 and 90 minutes in each experiment. As can be seen in Figure 4.4, The results of the conducted electrochemical experiments shed light on the relationship between current and manganese removal efficiency. Specifically, when operating at 0 current, the removal efficiency remained negligible after a 90-minute duration. This outcome validates the essential role of electric current in driving the oxidation reactions necessary for effective manganese removal. The absence of current impedes electron transfer, consequently preventing the electrooxidation of manganese. Furthermore, through the augmentation of current intensity from 25 to 75 mA, a consistent enhancement in the removal efficiency was observed. Notably, when employing current intensity of 75 mA, removal efficiency of 85.5% was achieved after 30 minutes and 97.5% removal efficiency was obtained after 90 minutes. However, at lower current levels, such as 25 and 50 mA, the removal efficiency after 30 minutes is comparatively less pronounced than the efficiency observed at 60 and 90 minutes. This could be due to the low electrooxidation rate and less reactive species at lower currents (McBeath et al., 2020b; Shu et al., 2020). Conversely, an elevated current level, such as 100 mA, exhibited a detrimental impact on the removal efficiency, leading to a reduction to 55% removal after 90 minutes. The observed decline in performance may be attributed to the increased production of hydroxide ions (OH-), resulting in the higher precipitation of manganese as Mn(OH)2. This 64 phenomenon, in turn, impedes the electrooxidation process of manganese. The corroborating evidence for this hypothesis can be found in the yellowish coloration of the water observed during this experimental procedure. The other possible reason can be the excessive oxygen evolution reaction due to the current oversupply leading to low current efficiency. (Wang et al., 2019). Removal Efficiency (%) 100 80 Current 0 mA 60 Current 25 mA Current 50 mA 40 Current 75 mA Current 100 mA 20 0 0 20 40 60 80 100 Time (min) Figure 4. 4 Effect of current on the removal efficiency. As delineated in Table 4.4, these experiments also explored the kinetics of the electrooxidation process under varying current densities. The results manifest a consistent reaction rate and R2 value for each experiment, demonstrating a well-fitted pseudo-firstorder model. Furthermore, a direct and positive correlation is discernible between current intensities and the efficiency of manganese electrooxidation, as well as the corresponding reaction rates. In accordance with the pseudo-first-order model, the reaction rate constants (K) for current densities of 25, 50, and 75 mA were determined to be 0.0129, 0.0310, and 0.0406 min-1, respectively (Figure 4.5). The increase in current density is concomitant with 65 a corresponding rise in the reaction rate, underscoring the enhanced electrooxidation potential achieved at higher current densities. This improvement is attributed to the heightened generation of electrons and, consequently, the increased production of reactive species. The increase of kinetic rate constant by enhancing current intensities was confirmed by the research work conducted by Wang et al. as well (Wang et al., 2019). Table 4. 4 Kinetics parameters of electrochemical oxidation 25 Mn initial concentration (mg/L) 2 50 2 9 0.0310 0.97 75 2 9 0.0406 0.96 Current (mA) pH k (min−1) R2 9 0.0129 0.95 66 (a) 4.5 y = 0.0406x + 0.2964 4 3.5 Ln (C0/Ct) 3 2.5 25mA y = 0.031x + 0.0966 2 50mA 1.5 75mA 1 0.5 y = 0.0129x - 0.0984 0 0 20 40 60 80 100 Time (min) (b) 0.045 0.04 0.035 K (S-1) 0.03 0.025 0.02 R² = 0.9883 0.015 0.01 0.005 0 0 10 20 30 40 50 60 70 80 Current (mA) Figure 4. 5 (a) First-order kinetic of manganese removal at different currents, (b) Pseudo-first-order rate constant of manganese electro-oxidation at various current intensities. 67 4.3 Effect of anode modification on the manganese removal efficiency In an experimental setup, ACF was utilized without the incorporation of biochar to assess the system's efficiency in removing manganese without any modifications to the anode. The results, as depicted in Figure 4.6, clearly indicate a decrease in the efficiency of manganese electrooxidation when compared to the experiment involving BC@ACF. The experiment was conducted with Mn initial concentration of 2 mg/L and current intensity of 75 mA. The enhanced performance observed with the presence of biochar on the anode surface can be attributed to the catalytic active sites inherent to biochar. These active sites play a pivotal role in facilitating the activation of ROSs and enhancing electron transfer by improving conductivity (Tian et al., 2021b; Zhang et al., 2022). Consequently, the electrocatalytic oxidation of manganese occurs more efficiently when employing the Removal Efficiency (%) BC@ACF. 100 90 80 70 60 50 40 30 20 10 0 ACF BC@ACF 30 60 Time (min) 90 Figure 4. 6 Effect of biochar modification on the catalytic electro oxidation of manganese. 68 4.4 Manganese removal mechanisms analysis In an electrochemical cell, various reactions happen as a result of intricate interplays between electrode materials, electrolyte solutions, and the electric potential applied across the cell. The elementary constituents of water are principal actors in these electrochemical systems. The surface of the anode, typically associated with oxidation processes, often witnesses the generation of cations, electrons, and, under certain conditions, solid precipitates (Equation 4.2). At the same time, at the cathode, where reduction reactions typically occur, hydrogen gas (H2), metal deposition, and hydroxide ion (OH⁻) generation frequently take place (Equation 4.3). 2H2O (l) → O2 (g) + 4H⁺(aq) + 4e⁻ (4.2) 2H2O (l) + 2e⁻ → H2 (g) + 2OH⁻(aq) (4.3) 69 Figure 4. 7 Various mechanisms of manganese removal from groundwater in the designed electrochemical cell According to Figure 4.7, it becomes evident that manganese ions possess the capacity to engage with a diverse range of pathways within the electrochemical cell. These pathways serve as avenues for manganese to react with the various species generated during the electrochemical processes. This multifaceted interaction with these pathways is central to understanding the fate and transformation of manganese within the system. Manganese ions may follow one or, in some cases, multiple of these distinct pathways. These pathways lead to the conversion of manganese into different forms, including but not limited to precipitation and various oxide states. It's important to note 70 that these mechanisms can manifest in two primary modes. Firstly, they can occur directly on the BC@ACF, which serves as an electrode within the electrochemical cell. This direct interaction between manganese ions and the electrode plays a significant role in the overall removal and transformation processes. Secondly, these mechanisms may manifest indirectly within the solution phase through the involvement of reactive species such as hydroxyl and sulfate radicals. This indirect route highlights the complexity of manganese's behaviour within the electrochemical cell, as it can interact with various reactive species in the solution, leading to diverse transformation pathways. The possible mechanisms are explained in more detail in the following sections. 4.4.1 Manganese hydroxide (Mn(OH2)) One possible pathway that manganese takes is to produce precipitation of manganese hydroxide by reacting with hydroxide ions produced on the cathode (Equation 4.4). This precipitation mechanism holds paramount significance in the removal of manganese from groundwater, as manganese hydroxide possesses very low solubility in water. The generated solid particles can be readily separated from the treated water through sedimentation, filtration, or other separation techniques. The effectiveness of this pathway for manganese removal depends on several factors, including the pH of the solution, the concentration of manganese ions, and the prevailing electrochemical conditions. An alkaline pH is generally favourable for promoting the formation of manganese hydroxide precipitates, as it facilitates the availability of hydroxide ions for the reaction (Shu et al., 2020; Wilamas et al., 2023). Mn2+ +2OH- → Mn(OH)2 (4.4) 71 4.4.2 Manganese oxides Manganese ions can undergo oxidative processes within electrochemical systems. These oxidation processes can occur either directly at the anode surface or indirectly within the solution phase. The former is described in Reaction 4.5, taking place directly on the anode surface, while the latter, Reaction 4.6, is a consequence of Reaction 4.4 within the electrochemical cell (Shu et al., 2020). Following the experimental procedures aimed at manganese removal, evident observations were made, revealing the presence of a distinct black precipitate that appeared both on the surface of the anode and within the solution. This compelling visual evidence serves to confirm the hypothesis that the electrooxidation of manganese induced the formation of various manganese oxide species in differing forms. This phenomenon underscores the complexity of the electrochemical oxidation process and underscores the potential for the generation of multiple manganese oxide products, which may include various oxidation states and chemical compositions. The characterization and analysis of these manganese oxide species represent critical paths for further investigation, shedding light on the manganese removal mechanisms in electrochemical systems. Mn2+ + 2H2O → MnO2 + 4H+ + 2e- (4.5) Mn(OH)2 + O2 → Mn3O4 + H2O (4.6) 4.4.3 Persulfate radicals The electrochemical generation of persulfate in an electrochemical cell is a possible method for the removal of manganese from groundwater. The process typically involves the oxidation of sulfate ions (SO42-) at the anode to produce persulfate ions (S2O82-) 72 (Equation 4.7)(Jakóbczyk et al., 2022; Shu et al., 2023b). These persulfate ions are strong oxidants and can react with manganese(II) ions in the water, oxidizing them to less soluble and removable forms, such as manganese(III) and manganese(IV) oxides/hydroxides. 2SO42− → S2O82− + 2e− (4.7) SO42− → (SO4−)•+ e− (4.8) Adding supportive electrolyte such as Sodium Sulfate to the solution is reported frequently in research works due to its capability to increase the removal efficiency by forming radicals and stable oxidants as well as reducing the energy consumption by increasing the conductivity of the solution(Jalife-Jacobo et al., 2016; Periyasamy & Muthuchamy, 2018). Hydroxyl radicals produced on the anode surface may also help the manganese removal by direct electro-oxidation (Shu et al., 2020). Hydroxyl radicals are the primary reactive species that resulted from the oxidation of hydroxide ions and the breakdown of water molecules(Wang & Wang, 2020) (Equations 4.9 and 4.10). H2O → HO• + H+ + e− (4.9) OH− → HO• + e− (4.10) Peroxydisulphate and sulphate radicals are generated as a result of Sulfate ion and hydroxyl radical reactions, which is represented in the following reactions (Periyasamy & Muthuchamy, 2018): SO42− + HO• → (SO4−)•+ OH − (4.11) 2HSO4− + 2HO• → S2O82− + 2H2O (4.12) 73 4.5 The influence of real groundwater on manganese removal efficiency Different groundwater sources exhibit varying water quality characteristics, and these differences can significantly impact the efficacy of manganese removal from water. To comprehensively assess the influence of actual groundwater on manganese removal efficiency, an identical batch setup was employed for experimentation. The experimental conditions included a constant current intensity of 75 mA, an initial manganese concentration of 3.33 mg/L, 0.05 M Na2SO4 as a supporting electrolyte, and the original pH of groundwater, which was 7.88 without any adjustments. However, the pH for the experiment with the synthesis water adjusted to 8 to maintain the same pH. As illustrated in Figure 4.8, our system demonstrated remarkable efficiency in removing manganese from real groundwater, achieving an impressive removal rate of 95.20% within a 90-minute timeframe. Notably, this level of performance closely matches the results obtained in experiments conducted with synthetic water, which resulted in 98.5% removal efficiency within 90 minutes. This equivalency in performance highlights the system's robustness and reliability in real-world scenarios, making it a promising candidate for practical applications without the need for chemical additives. Furthermore, our results underscore the system's ability to effectively remove manganese from actual groundwater without necessitating pH adjustments, simplifying its implementation in the field. This feature enhances the method's practicality and ease of use, especially for applications in rural and remote communities. To elucidate the kinetics of manganese electrooxidation, the pseudo-first-order kinetic model was applied, which revealed a rate constant (K) of 0.034 min-1 as depicted in Figure 4.9 for real groundwater experiment. Whereas the rate constant of 0.045 min-1 74 was acquired for the experiment with synthesis water, which confirms the small increase in the rate of electro oxidation process in the absence of co-existing ions. 100 90 Removal Efficiency (%) 80 70 60 50 40 30 20 10 0 30 60 Time (min) Groundwater 90 Synthesis Water Figure 4. 8 Removal efficiency of manganese from real groundwater and synthesis water with current of 75mA, 0.05 M Na2SO4 as a supporting electrolyte, and without pH adjustment. 75 5 4.5 4 Ln (C0/Ct) 3.5 3 2.5 2 1.5 1 0.5 0 0 10 20 30 40 Groundwater 50 60 Time (min) 70 80 90 Synthesis Water Figure 4. 9 First-order kinetic model of manganese removal from real groundwater and synthesis water at the current of 75 mA. In the following table, the water quality parameters of the groundwater after treatment are illustrated (Table 4.5). The comparison of the raw and post-treated groundwater quality demonstrates that the manganese in raw groundwater was 1 mg/L and its concentration was decreased to 0.008 mg/L after treatment, which is less that MAC and AO according to the guidelines for CDWQ. Furthermore, the findings of the table indicate that the various metals concentration were almost the same before and after treatment. The small change in some of the metals can be due to experimental errors. 76 Table 4. 5 Comparison of metals concentration in groundwater before and after treatment. Total Metals Units Raw Groundwater Aluminum mg/L <0.02 Post-treated Groundwater (Filtered) 0.041 Arsenic mg/L <0.02 <0.02 <0.02 Boron mg/L 0.007 0.013 <0.001 Barium mg/L 0.103 0.022 <0.0004 Calcium mg/L 75.9 14.7 <0.005 Cadmium mg/L <0.0004 <0.0004 <0.0004 Cobalt mg/L <0.002 <0.002 <0.002 Chromium mg/L <0.0004 <0.0004 <0.0004 Copper mg/L 0.006 0.024 <0.002 Iron mg/L 0.173 0.013 <0.003 Potassium mg/L 3.04 8.75 <0.003 Magnesium mg/L 19.1 8.62 <0.001 Manganese mg/L 1.04 0.008 <0.001 Molybdenum <0.003 <0.003 <0.003 Nickel mg/L mg/L <0.001 <0.001 <0.001 Phosphorus mg/L 0.041 0.019 <0.01 Lead mg/L <0.004 <0.004 <0.004 Antimony mg/L <0.01 <0.01 <0.01 Selenium mg/L <0.02 <0.02 <0.02 Tin mg/L <0.01 <0.01 <0.01 Uranium mg/L <0.01 <0.01 <0.01 Vanadium mg/L 0.001 <0.001 <0.001 Zinc mg/L 0.002 0.001 <0.001 Reportable Limit <0.02 77 4.6 Capability of the designed electrochemical cell for hardness removal from the real groundwater The groundwater quality analysis revealed a significant presence of magnesium and calcium, resulting in a notably high hardness level of 268.2 mg CaCO3/L in the water. This level exceeds the MAC as stipulated by the Guidelines for CDWQ, indicating a potential concern for water quality. Saito et al. (2022a) investigated the impact of Ca2+, Mg2+ on the catalytic oxidative removal of Mn2+ by powdered activated carbon. The result indicated that the presence of these ions reduced the oxidation and removal efficiency and due to the competitive adsorption. However, our designed electrochemical system exhibited the remarkable ability to not only eliminate manganese from the groundwater without reduced efficiency in the presence of Ca2+, Mg2+ ions, but also effectively remove these ions and reduce water hardness by precipitation on the cathode surface. This transformative treatment process yielded a final concentration of 72.2 mg CaCO3/L, which falls below the MAC threshold (Table 4.6). As a result, this system possesses a dual functionality - it not only succeeds in reducing manganese levels in groundwater to levels below the MAC, addressing a significant concern, but it is also able to potentially tackle the persistent issue of water hardness in the raw groundwater, a long-standing challenge faced by the Lheidli T'enneh community. This multifaceted approach holds the potential to greatly enhance the overall quality of the water. In an electrochemical cell, the precipitation of magnesium and calcium ions can occur due to the high pH and alkaline environment around the cathode surface by the following reactions (Zhi & Zhang, 2016): 78 Mg2+ + 2OH− → Mg(OH)2 ↓ (4.13) Ca2+ + HCO3− + OH− → CaCO3 ↓ + H2O (4.14) As mentioned before, and based on the result of experiments, the pH of the solution during experiments was increasing and the final pH was almost 9 depending on the initial pH of the solution. The removal of hardness within our designed system can be attributed to the elevated pH levels sustained during the experimental process. This is a direct consequence of the substantial production of hydroxide ions at the cathode surface. OH − foster an alkaline environment, thereby promoting the precipitation of hardness both on the cathode surface and within the solution itself. The influence of various concentrations of hardness on both system performance and manganese removal was not explored in this study. However, it is anticipated that an increase in hardness concentration could potentially have a detrimental impact on manganese removal and the overall efficiency of the electrochemical cell. This negative effect may arise due to the substantial precipitation of magnesium hydroxide and calcium hydroxide on the cathode surface, leading to a subsequent reduction in the transfer of current between the anode and cathode. Additionally, the elevated concentrations of other ions, aside from manganese, might deplete the hydroxide ions within the system, hindering the reaction between manganese and hydroxide. To gain a comprehensive understanding of the impact of hardness on this system, further experiments are necessary for thorough investigation. 79 Table 4. 6 Physical characteristics of the groundwater Raw Groundwater Post-treated Groundwater Calcium (mg/L) 75.9 14.7 Magnesium (mg/L) 19.1 8.62 Hardness (mg CaCO3/L) 268.2 72.2 4.7 Evolution of pH during electrochemical reactions In an electrochemical cell, numerous reactions take place that can exert an influence on the pH of the solution. As illustrated in Figure 4.10, in this study, the pH exhibited an ascent to 9 after 90 minutes of electrochemical oxidation, starting from an initial pH of 5. This increase in pH over time may be attributed to the occurrence of the water oxidation reaction at the cathodic surface and the subsequent release of hydroxide ions (OH-) into the aqueous medium during the electrooxidation process. When the initial pH of the solution prior to the commencement of electrochemical oxidation was set at 7, the final pH surged to 10 after 90 minutes. This phenomenon aligns with the earlier hypothesis regarding the pH increment during the experiment, indicating the impact of hydroxide ions production on pH elevation. Furthermore, in experiments initiated with an initial pH of 9, the final pH after 90 minutes recorded a substantial increase to 11. All of the previously mentioned results were obtained at a constant current intensity of 75 mA, 0.05 M Na2SO4, and 2 mg/L of manganese concentration. It is noteworthy to emphasize that the level of current intensity directly impacts the pH of the solution. Higher current intensities lead to a greater flow of electrons within the electrochemical cell, subsequently increasing the production of hydroxide ions (OH-) on the cathode surface as a consequence of water electrolysis. 80 14 12 10 pH 8 Initial pH 6 Final pH 4 2 0 Experiment I Experiment II Experiment III Experiments Figure 4. 10 Evolution of pH in various experiments in constant current of 75 mA, Mn initial concentration of 2 mg/L, and 0.05 M Na2SO4 electrolyte. 4.8 Comparison of the designed system efficiency with other methods for manganese removal in the literature The proposed research presented addresses the critical issue of manganese removal from groundwater. To understand the implications of our work, it is crucial to examine it in the context of recent studies in this field. Various techniques have been explored to tackle the challenge of manganese removal, but our focus is on comparing these methods, particularly in terms of their performance when dealing with groundwater. As presented in Table 4.7, under optimized conditions, our method demonstrated an impressive manganese removal rate of 98.75%. This figure stands as a testament to the effectiveness of our approach, surpassing the results obtained in previous studies. This superior performance highlights the potential of our method to provide a more efficient solution to the issue of 81 manganese contamination. In the evaluation of manganese removal methods, it is crucial to account for treatment time as a critical parameter. This parameter holds particular significance in real-world scenarios where timely water treatment is essential. For instance, in emergencies or situations requiring rapid response, the efficiency of a treatment method can be essential. Under optimized conditions, the designed system is able to achieve a manganese removal rate exceeding 90% in just one hour. This remarkable efficiency underscores the practicality of our method for addressing manganese contamination in reallife situations. Whether it's in disaster relief efforts, urgent public health interventions, or routine water treatment processes, our method demonstrates its capability to deliver results swiftly and effectively. 82 Table 4. 7 Comparison of manganese removal efficiency by proposed methods in recent studies. Method Electrocoagulation Mn Removal Concentration Efficiency 10 mg/L 10 % Reference (McBeath et al., 2021) 1.0 mg/L 75 % (Du et al., 2019) Direct Biofiltration 0.8 mg/L 91% (Granger et al., 2014) Catalytic oxidation 0.033 mg/L 57 % (Saito et al., 2022b) Powdered activated carbon (PAC) and 0.2 mg/L 90 % (Li et al., 2019) 4.562 mg/L 51.07 % (Wilamas et al., Electro-oxidation/coagulation (EO/EC) pretreatment combined with a ceramic ultrafiltration (UF) membrane Chlorine Adsorption onto Permanganate-Modified Bamboo Biochars from Groundwater Electrochemical deposition with Modified 2023) 15.77 mg/L graphite paper treated by anionic 77.01 % (Chu et al., 2023) (after 2 hours) intercalation Electrocatalytic oxidation with biochar 3.5 mg/L 98.75 % This Study modified electrode 83 Chapter 5: Conclusion This study explored a novel approach employing electrochemical technology as a small-scale and green method for manganese removal. The investigation of key variables such as Mn initial concentration, current intensity, pH, and reaction time revealed that current intensity exerted the most pronounced influence on manganese removal. The efficiency was raised by increasing the current from 25 to 75 mA. 97.5% manganese removal efficiency achieved under specific conditions (Mn initial concentration of 2 mg/L, current intensity of 75 mA, pH 9), effectively reducing contaminant levels below the MAC. Notably, the system's efficiency remained uncompromised when dealing with actual groundwater, and it successfully reduced water hardness from 268.2 mg CaCO3/L to 72.2 mg CaCO3/L, making it suitable for drinking purposes. The concentration of other metals in the real groundwater remained constant after the treatment process. Examination of manganese removal mechanisms indicated the pivotal roles of various species, including hydroxide ions, sulfate, and hydroxide radicals, in oxidizing and precipitating manganese from groundwater. The formation of a black powder precipitate on the anode surface and changes in pH during the reaction substantiated the presence of oxide forms of manganese in the system. The performance of the anode decreased by 25% after more than 50 hours of operation in the system. This decline was primarily attributed to the precipitation of manganese oxides on the anode's surface during the experiments. Consequently, this led to a reduction in the electron transportation capability, as the active sites on the anode became less available. In summary, our innovative system, which leverages biochar as a sustainable and cost-effective material, demonstrated remarkable performance in removing manganese in 84 just 30 minutes and reducing water hardness from groundwater. This research not only addresses a critical environmental and health concern in rural and remote Canadian communities but also offers an eco-friendly solution with the potential for broader applications in water treatment and remediation. 5.1 Limitations and future research While our research has investigated the performance of manganese removal from groundwater through the innovative use of an electrochemical cell, it is essential to acknowledge the inherent limitations throughout the experiments and research work. These limitations, which are an integral part of the scientific process, serve as valuable signposts, guiding us toward a more comprehensive comprehension of the complexities involved in real-world applications. In this section, the constraints and boundaries of our laboratorybased investigations are discussed, recognizing that they provide a basis for future research and refinement for effective and sustainable manganese removal solutions. These limitations are listed briefly below: · The experiments were conducted on a batch scale, which does not fully represent the complexities and challenges encountered in real-world applications such as water quality variations, water temperature and flow rate alterations. · Although the performance of the system in real groundwater and the presence of various co-existing ions was evaluated, complex interactions in the presence of other contaminants, such as heavy metals and their impact on manganese removal efficiency were not taken into account. 85 · The durability and maintenance of the electrode materials, especially in extended operational periods, can be of concern. The long-term stability of the BC@ACF electrode under continuous use should be considered. · Electrochemical methods, depending on the materials used and the scale of implementation, can be associated with significant operational and maintenance costs. Exploring the economic feasibility and sustainability of the designed system in practical applications would be important. · Possible mechanisms involved in the removal of manganese are provided based on the observations from the experiments and the results of other similar research works. However, it's important to note that a comprehensive exploration of these mechanisms was constrained by limitations in time and available technology. · Electrochemical processes may generate byproducts or waste materials that need to be managed properly. The disposal or treatment of these byproducts should be taken into account when considering the environmental impact of the method. Based on the previously mentioned limitations, here are some prospects for future research works: · Column Study by BC@ACF Electrodes: Exploring the performance of synthesized BC@ACF electrodes in column studies. This investigation aims to assess their efficiency in removing manganese from groundwater in a flowthrough and continuous electrochemical cell, with a focus on practical applications. · Diverse Electrolyte Investigations: Examining the impact of different types of electrolytes, including NaCl, to evaluate the influence of chloride ions on 86 manganese removal efficiency. This research seeks to compare the effectiveness of manganese removal in the presence of various supporting electrolytes. · Concentration of Supporting Electrolyte: Investigating the effect of varying concentrations of Na2SO4 as a supporting electrolyte on the final concentration of manganese in treated water. This study aims to optimize the concentration of the supporting electrolyte for improved removal efficiency. · Quantifying Reactive Species: Measuring the concentration of reactive species, such as hydrogen and sulfate radicals, generated during experiments. 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