Sulfate-Reducing Bacteria Mobilize Adsorbed Antimonate by Thioantimonate Formation

The biogeochemical cycling of antimony (Sb) is often coupled with sulfur and sulfate-reducing bacteria (SRB). The biogenic sulfide is usually assumed to facilitate Sb immobilization via Sb2S3 precipitation. Here, on the contrary, we discovered that SRB mobilize adsorbed Sb(V). When Sb-V(OH)(6)(-)-bearing goethite was incubated anaerobically with Desulfovibrio vulgaris DP4, an elevated level of antimony was released due to the formation of thioantimonate, which is the dominant Sb species in solution. Our Fourier transform ion cyclotron resonance mass spectrometry analysis revealed multiple six- or five-coordinate thioantimonate intermediates, suggesting stepwise ligand exchange of hydroxyl groups on Sb-V(OH)(6)(-) by biogenic sulfide. Direct H2S elimination reactions resulted in four-coordinate thioantimonate species as the stable end product, which was confirmed by our density functional theory calculations. The thiolation of antimonate is pH-dependent and occurs in neutral environments. The thiolation changed Sb(V) from a six-coordinate octahedral coordination to a four-coordinate tetrahedral coordination, weakening its affinity for iron oxides and thus facilitating its release into the aquatic environment. The results of this study highlight the importance of biogenic sulfide produced by SRB for the fate and transport of Sb.