Redox Dynamic\nInteractions of Arsenic(III) with Green\nRust Sulfate in the Presence of Citrate

Arsenic is a global pollutant. Recent studies found that\nFe(II)\ncan oxidize As(III), but the extent of oxidation with mixed-valent\niron minerals and the mechanisms involved are unknown. In this study,\nwe investigated whether As(III) can be oxidized under reducing conditions\nusing green rust sulfate (GR-SO<sub>4</sub>), an Fe mineral containing\nboth Fe(II) and Fe(III). Batch sorption experiments showed that GR-SO<sub>4</sub> (1 g L<sup>–1</sup>) effectively sorbs environmentally\nrelevant concentrations of As(III) (50–500 μg L<sup>–1</sup>) under anoxic, neutral pH conditions with and without citrate (50\nμM). X-ray absorption near-edge structure spectroscopy analysis\nat the As K-edge demonstrated that approximately 76% of As(III) was\noxidized to As(V) by GR-SO<sub>4</sub>. Complete oxidation of As(III)\nwas observed in the presence of citrate. As(III) oxidation can be\nlinked to the phase transformation of GR-SO<sub>4</sub> to goethite,\nresulting in new reactive Fe(III) species that plausibly drive oxidation.\nCitrate enhanced this process by stabilizing Fe on the mixed GR-SO<sub>4</sub>/goethite surface, preventing its reduction back to Fe(II)\nand facilitating further As(III) oxidation without significant Fe\nloss to the solution. This study highlights the cryptic As(III) oxidation\nthat occurs under reducing conditions, providing new insights into\nthe cycling of arsenic in mixed phases of iron-rich, anoxic environments.