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.