Overcoming the Limitations of Conventional Iron Electrocoagulation: Towards Compact, Fast, and Efficient Process for Treating Arsenic Contaminated Groundwater for Drinking

Hundreds of millions of people around the world are exposed to toxic levels of arsenic dissolved in the groundwater they use for drinking. Low income communities are disproportionately affected by this arsenic poisoning because they lack the necessary technical, managerial, and financial capacity to address the issue. Iron electrocoagulation (FeEC) holds promise to address arsenic contamination in such resource-constrained communities because it relies on locally available materials, a minimal supply chain and low operating costs. This dissertation builds upon an ongoing demonstration of a community scale FeEC plant in West Bengal, India, that has been supplying arsenic safe water to the local community at an affordable price since 2016. This dissertation underpins three main objectives in detail. First, it analyzes the causation behind surface layer growth or passivation of large (1000 mm × 900 mm × 3 mm) Fe(0) electrodes in active use for more than two years. Second, with an aim to develop a compact, rapid and an effective arsenic treatment solution, it advances the standard FeEC to a more effective version, Air Cathode Assisted Iron Electrocoagulation (ACAIE). This study compares arsenic removal efficacies and mechanisms of FeEC and ACAIE over a range of operating conditions varying over an order of magnitude. Third, to assess the practical feasibility of ACAIE, its long-term performance is investigated. A particular emphasis is laid on fouling of the air cathode and its impact on H2O2 generation, and thus on the ability to achieve contaminant removal./nIn FeEC, a commonly noted limitation is the passivation of electrode surfaces via rust accumulation over long-term use. In Chapter 2, we characterized the surface layers formed on the electrode plates and examined the effect of surface layer removal, on the performance of an FeEC plant in India. The electrode surfaces developed three distinct horizontal sections of layers that consisted of different minerals: calcite, Fe(III) precipitates and magnetite near the top, magnetite in the middle, and Fe(III) precipitates and magnetite near the bottom. The interior of all surface layers adjacent to the Fe(0) metal was dominated by magnetite. After removing the surface layers by mechanical cleaning, substantial improvements both in the Fe concentration in the bulk solution, and removal rates of As, P and Si, were observed. Results presented in this chapter show that removing surface layers that accumulate on electrodes over extended periods of operation can restore FeEC system efficiency (mass of solute removed/kWh delivered). Therefore, these findings assert that routine electrode maintenance can ensure robust and reliable FeEC performance over year-long timescales. However, routine (e.g. daily) electrode maintenance may not be feasible in regions (e.g. rural California) where labor costs are high. Also, FeEC requires long operating times (~hours) and a relatively large footprint to remove dissolved arsenic due to inherent kinetics limitations./nIn response to these limitations, ACAIE was developed, with an aim to provide a compact, rapid and an effective arsenic treatment solution. In Chapter 3, the arsenic removal efficiency of FeEC and ACAIE are compared for a range of operating parameters (electrolysis times: 0.5 to 200 minutes, charge dosage rates: 1.5 to 1200 C/L/min, current densities: 0.8 to 156 mA/cm2) for removing As(III) from an initial concentration of ~1500 μg/L in synthetic groundwater. ACAIE consistently outperformed FeEC in bringing arsenic levels to less than World Health Organization (WHO) maximum contaminant level (MCL) of 10 μg/L. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) analysis conclusively showed that poor arsenic removal in FeEC arises from incomplete As(III) oxidation, incomplete Fe(II) oxidation and the formation of Fe(II–III) (hydr)oxides at short electrolysis times (<20 min). Finally, ACAIE robust performance (retention time 19 s) was verified with arsenic contaminated groundwater from rural California./nSustained cathodic production of H2O2 is critical in ACAIE for rapid, effective, and efficient arsenic removal. The air cathodes in ACAIE could undergo fouling (i.e., decrease in their Faradaic Efficiency (FE) of H2O2 production) because of accumulation of precipitates on their surfaces. In Chapter 4, long-term performance of the air cathodes is investigated by performing a series of continuous flow experiments (100 to 200 hours of operation) with simple and complex electrolytes. The H2O2 FE of the fouled air cathode was almost fully recovered, back to its original value, with exposure to 1% ascorbic acid for 16 hours. Scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), Contact angle and Linear sweep voltammetry (LSV) analysis of the fresh, fouled, and regenerated air cathodes conclusively demonstrated that that cathodic reduction of H2O2 at the surface of iron (hydr)oxides deposited on the air cathodes contributed to decrease in H2O2 FE of the fouled cathode. Lastly, in the final chapter, I summarized important findings, discussed future research questions and presented a novel energy efficient design of FeEC with an external oxidizer (e.g. H2O2) addition to address the limitations of FeEC and ACAIE identified in this dissertation.

Advisor: Gadgil, Ashok J