Unveiling the Role of Mackinawite and Fe0 Components in Arsenic(III) Removal by Sulfidated Zero-Valent Iron under Aerobic Conditions

In this study, we synthesized microscale sulfidated zerovalent iron (S-mZVI) with controllable mackinawite (FeS) content up to nearly 100 wt % and investigated the roles of FeS and Fe0 for arsenite (As(III)) sequestration under aerobic conditions. Batch experiments show that FeS and Fe0 contents determine the kinetics and longevity of As removal by S-mZVI, respectively. The Fe0/FeS galvanic cell accelerates the consumption of Fe0 by dissolved oxygen (DO) while preserving FeS to preferentially remove As as sulfide, which is protected from oxidation by Fe0. In column studies, mZVI and S0 were mixed in sand to form S-mZVI in situ. Results of sequential extraction of reacted S-mZVI particles from different column zones and run stages further indicate that As formed as sulfide by S-mZVI, which was then oxidized by DO after Fe0 depletion to form As-iron (hydr)oxides. X-ray absorption near-edge structure characterization confirmed that As sulfide is mainly in the form of realgar (As4S4). S-mZVI (2 wt % of column sand) reduces total As from 2 mg/L to 10 μg/L, up to 300 bed volumes, with a residence time of 70 min. In situ synthesis and cost advantages make S-mZVI a promising method to address As contamination issues worldwide.