Selenium(VI) Removal from Challenge Waters by Continuous-Flow-Through Iron Electrocoagulation

Iron electrocoagulation (EC) is promising for selenium(VI) removal from water. This study investigated the performance of flow-through iron EC under environmentally relevant conditions. The influence of water composition on Se removal and the mechanisms by which water components affect Se removal were studied. The individual effects of major anions (bicarbonate, sulfate, and nitrate) and humic acid on selenate removal were examined under pH 8 anoxic conditions, at which selenate removal was previously demonstrated to be effective in a simple water composition. Bicarbonate inhibited Se removal by promoting the formation of less reactive iron solids (carbonate-containing green rust). Sulfate inhibited the oxidation and transformation of iron solids, thereby limiting their reactivity with selenate. Nitrate and humic acid had a smaller impact on Se removal compared to bicarbonate and sulfate. Building on the work with individual constituents, flow-through EC was applied to treat challenge waters that represent mining discharge, agricultural runoff, and flue-gas desulfurization wastewater. Sulfate and ionic strength were major inhibitors for Se removal in those waters, and an iron dose of 240 mg/L was required for effective Se removal. Pretreatment to remove sulfate improved the EC performance for Se removal from agriculture and mining wastewater, but it had little effect for flue-gas desulfurization wastewater. The impact of major anions and humic acid on solids formation, redox, and adsorption in flow-through EC processes provided valuable insights into removal mechanisms and practical guidance for predicting EC performance in real-world water treatment.