Novel insights into the interaction of thioarsenates with ferrous-bearing minerals under anoxic conditions: Evidence from XAS analysis

Thioarsenates (thioAs^V) are key regulators of As geochemistry in anoxic environments based on spectroscopic and chromatographic evidence. The interaction of thioAs^V with ferrous-bearing minerals (magnetite/mackinawite) was studied to investigate the adsorption characteristics of thioAs^V, As speciation change, and adsorption products at various pH values under anoxic conditions. Results indicate that thioAs^V are the most mobile inorganic As species in the presence of the two minerals and their adsorption on magnetite is pH-dependent. Partial reduction of thioAs^V occurred during adsorption, particularly at acidic pH. As K-edge XANES confirmed that thioAs^V was reduced to As^III standard and formed arsenic-sulfur or FeAsS-like phases after contacting magnetite. A similar reduction process was observed during the reaction of thioAs^V with mackinawite, although no FeAsS-like phases formed. In contrast, S^II− was oxidized to elemental S, FeS₂, and S₂O₅²⁻ with no relation to the adsorbates and adsorbents used. In addition to reduction, thioAs^V can also be converted to As^V via desulfidization after adsorption on both minerals, with this process being favored at pH 5 and 7. The findings of this work will further clarify the thioAs^V mobility under anoxic conditions. Moreover, the described transformation processes here may also elucidate the fate of other metal(loid)s in similar environments.