Advancing Selenium(VI) Removal by Iron Electrocoagulation: Roles of Water Chemistry and Operating Conditions

Selenium(VI) (Se(VI)) is a challenging contaminant to remove from water due to its high solubility and limited adsorption affinity. Elevated Se concentrations pose adverse effects to both aquatic ecosystems and human health. Iron electrocoagulation (EC) is a promising method for Se removal by generating iron-containing solids that act as adsorbents and reductants. This study systematically investigated the effects of water chemistry and operating conditions on the rates and extents of Se(VI) removal in batch reactors. A reaction-based model for Se(VI) removal under oxic conditions (DO: 7.5–9 mg/L) was developed to enhance mechanistic understanding. Under oxic conditions, Se(VI) removal occurred through adsorption onto Fe(III) (oxy)hydroxides, with enhanced removal at acidic pH (pH 4–6). Under anoxic conditions (DO ≤ 0.1 mg/L), Se(VI) removal proceeded through chemical reduction on mixed-valence iron-containing solids, achieving better removal at neutral-alkaline pH. Three optimal conditions were identified (1 – pH 4, oxic; 2 – pH 7, anoxic; 3 – pH 8, anoxic). Se(VI) removal rates and extents increased with increasing charge loading rates but were independent of current density. EC demonstrated superior or similar Se(VI) removal extents compared to chemical coagulation at the three optimal conditions. These findings provide valuable insights for the optimal deployment of iron EC to achieve Se(VI) removal from waters.