New Mobilization Pathway of Antimonite: Thiolation and Oxidation by Dissimilatory Metal-Reducing Bacteria via Elemental Sulfur Respiration

Antimony (Sb) mobilization is widely explored with dissimilatory metal-reducing bacteria (DMRB) via microbial iron(III)-reduction. Here, our study found a previously unknown pathway whereby DMRB release adsorbed antimonite (Sb^III-O) from goethite via elemental sulfur (S⁰) respiratory reduction under mild alkaline conditions. We incubated Sb^III-O-loaded goethite with Shewanella oneidensis MR-1 in the presence of S⁰ at pH 8.5. The incubation results showed that MR-1 reduced S⁰ instead of goethite, and biogenic sulfide induced the formation of thioantimonite (Sb^III-S). Sb^III-S was then oxidized by S⁰ to mobile thioantimonate (Sb^V-S), resulting in over fourfold greater Sb release to water compared with the abiotic control. Sb^IV-S was identified as the intermediate during the oxidation process by Fourier transform ion cyclotron resonance mass spectrometry and electron spin resonance analysis. The existence of Sb^IV-S reveals that the oxidation of Sb^III-S to Sb^V-S follows a two-step consecutive one-electron transfer from Sb to S atoms. Sb^V-S then links with Sb^III-S by sharing S atoms and inhibits Sb^III-S polymerization and Sb^III₂S₃ precipitation like a "capping agent". This study clarifies the thiolation and oxidation pathway of Sb^III-O to Sb^V-S by S⁰ respiration and expands the role of DMRB in the fate of Sb.