Thioarsenate sorbs to natural organic matter through ferric iron-bridged ternary complexation to a lower extent than arsenite

Understanding processes regulating thioarsenate (H_xAsS_nO_4-n^3-x; n = 1 - 3; x = 1 - 3) mobility is essential to predicting the fate of arsenic (As) in aquatic environments under anoxic conditions. Under such conditions, natural organic matter (NOM) is known to effectively sorb arsenite and arsenate due to metal cation-bridged ternary complexation with the NOM. However, the extent and mechanism of thioarsenate sorption onto NOM via similar complexation has not been investigated. By equilibrating monothioarsenate (representative of thioarsenate) with a peat (model NOM) with different Fe(III) loadings, this study shows that NOM can sorb monothioarsenate considerably via Fe(III)-bridging. Iron and As K-edge XAS analysis of the monothioarsenate-treated Fe-loaded peats revealed that monothioarsenate forms bidentate mononuclear edge-shared (¹E) (R_As···Fe: 2.89 ± 0.02 Å) and bidentate binuclear corner-shared (²C) (R_As···Fe: 3.32 Å) complexes with organically bound Fe(O,OH)₆ octahedra, in addition to direct covalent bonds with oxygen-containing functional groups (e.g., -COOH and -OH) (R_As···C: 2.74 ± 0.02 Å), upon equilibration with the Fe(III)-loaded peat. However, the extent of monothioarsenate sorption was considerably less than that of its precursor As species, arsenite, due to higher electrostatic repulsion between the negatively charged monothioarsenate and peat. This study implies that thioarsenate formation under anoxic conditions would increase As mobility by decreasing its sorption onto the NOM.