LAUSR

Abstract The biogeochemical cycling of arsenic (As) and antimony (Sb) is coupled with sulfur (S). Biogenic sulfide from microbial sulfate reduction under anaerobic conditions may enhance the release of coexisting As and Sb through the formation of labile thioarsenate (AsV-S) and thioantimonate (SbV-S). To understand the competition between As and Sb for thiolation and its effect on their mobility, we investigated the speciation and fate of As and Sb in the presence of indigenous microbiota in sediments during multiple oxic-anoxic cycles. Sediments with Sb contents at low (L-Sb, 570 mg/kg Sb, 50 mg/kg As), medium (M-Sb, 2,560 mg/kg Sb, 90 mg/kg As) and high (H-Sb, 10,090 mg/kg Sb, 190 mg/kg As) levels were collected in the Xikuangshan mining area, the largest Sb mine in the world. Our aqueous speciation analysis indicated that As was released under anoxic conditions whereas Sb was mobilized during oxic cycles in all samples. The As release was not driven by the transformation of Fe-bearing phases, but by biogenic sulfide through the formation of soluble AsV-S. AsV-S was the primary soluble species under anoxic conditions as evidenced by its high percentage, up to 80% in L- and M-Sb sediments, and 60% in H-Sb sediment. AsV-S was formed prior to SbV-S in L- and M-Sb sediments and even suppressed SbV-S formation in H-Sb sediment, indicating that limited biogenic sulfide preferentially bound with As rather than Sb. The Sb mobility was not associated with sulfide, but mainly regulated by its redox transformation between mobile antimonate and immobile antimonite. X-ray absorption near edge structure (XANES) spectra of Sb, As and S showed that no As or Sb sulfide precipitation was formed during the incubation. The Fe K-edge XANES analysis suggested the minor transformation of Fe-bearing phases between goethite (32 ± 8%), hematite (24 ± 7%) and magnetite (25 ± 4%), resulting in a limited effect of Fe cycling on As mobility. The results of our study signify the competition of As and Sb in thiolation and improve our understanding of the contrasting behaviors of As and Sb during oxic-anoxic cycles.