The biogeochemistry of antimony (Sb) and sulfur (S) are always coupled together because of the strong affinity between Sb and S. Besides the widespread stibnite (Sb₂S₃) mineral, aqueous Sb-S complexes, so-called thioantimony, have been measured or modeled in several previous studies. However, because the instability and detecting difficulty of thioantimony, the role of thioantimony in Sb biotransformation process remains unknown.

Thioantimony are formed during the reaction between sulfide and antimony. Sulfide in natural water is mainly generated by sulfate-reducing bacteria (SRB), and SRB have simply been believed to inhibit Sb mobilization via Sb₂S₃ formation. However, our study found that Sb release was greatly enhanced by SRB because of the formation of dissolved thioantimonate. pe-pH diagrams suggested that in most neutral Sb contaminated water (Sb concentration at μg/L level), it is thioantimonate formed rather than Sb₂S₃ in the presence of biogenic sulfide. The formation of thioantimony promoted the transformation from goethite to ferrihydrite. Even with higher surface reactivity than goethite, the newly formed ferrihydrite failed to reuptake thioantimonate, which finally causing Sb mobility. Fourier transform ion cyclotron resonance mass spectrometric (FT-ICR MS) and the density functional theory (DFT) calculation were conducted to explore the thioantimonate formation mechanism. A new pathway was proposed based on the direct reaction between antimonate and sulfide without redox transformation. Multiple intermediates with six/five- coordinate thioantimonate were detected by FT-ICR MS, and four-coordinate thioantimonate was considered as the final stable product through thermodynamic calculation. The insights gained from this study shed new light on the effect of SRB in Sb mobilization and the role of thioantimony in Sb biogeochemistry cycling.