Selenium(VI) Removal by Continuous Flow-Through Iron Electrocoagulation: Effects of Operating Conditions and Stability of Selenium in Residual Solids

Selenium (Se) contamination is widespread, and Se(VI) removal from water is particularly challenging. This study evaluated Se(VI) removal using iron electrocoagulation (EC) in a flow-through reactor under various water chemistry and operating conditions. Effective Se(VI) removal (>98% from 1000 μg/L Se) was achieved under anoxic conditions with an iron dose as low as 30 mg/L and an EC reactor residence time as short as 11 s that was followed by a 1-h settling period. The removal remained stable over an extended operating time (24 h) and involved the generation of reactive Fe(II)/Fe(III) solids (green rust and magnetite). Oxic conditions were less effective for Se removal because of limited Se adsorption at the elevated pH of the effluent. The immobilized Se in the solids was in a reduced form (-II or 0), but about 70% of Se was oxidized after air exposure. Despite the reduced forms of Se being oxidized, very little Se was released from the solids and the toxicity characteristic leaching procedure indicated that EC-generated solids can be classified as nonhazardous. This study highlights the potential of flow-through iron EC to produce iron-containing adsorbents and reductants that can be tailored for Se(VI) and other oxyanion removal. It also offers practical insights into designing effective treatment systems and ensuring the safe disposal of EC-generated residual solids in real-world applications.