Coupled Effect of Sulfidation and Ferrous Dosing on Selenate Removal by Zerovalent Iron Under Aerobic Conditions

Both the reactivity and the removal capacity of zerovalent iron (ZVI) for the target contaminant are important for applying ZVI in wastewater treatment. In this study, the feasibility of combining sulfidation treatment and Fe²⁺ dosing (S-ZVI/Fe²⁺) to enhance the performance of ZVI for Se(VI) removal was comprehensively investigated under aerobic conditions. Se(VI) was first adsorbed on the surface of ZVI particles and then reduced to Se(IV) and Se(0) with Se(0) being the final product in S-ZVI/Fe²⁺ system. This system bore the advantages of both sulfidation treatment (S-ZVI) and Fe²⁺ dosing (ZVI/Fe²⁺) for Se(VI) removal. The amounts and rate constants of Se(VI) removal in S-ZVI/Fe²⁺ system were increased by 1.8-32.8 times and 11.7-194.0 times, respectively, compared to those in pristine ZVI system. Sulfidation significantly accelerated the corrosion of Fe⁰ thus improved the removal rate of Se(VI). The promoting effect of Fe²⁺ on Se(VI) sequestration by S-ZVI should be mainly associated with the following facts: Fe²⁺ could maintain a relatively low pH level during Se(VI) removal by S-ZVI; Compared to S-ZVI alone, the consumption of Fe⁰ in S-ZVI/Fe²⁺ by O₂/H⁺ was slower, and thus the electron efficiency of S-ZVI was elevated; Fe²⁺ dosing facilitated electron transfer by forming semiconductive Fe₃O₄.