Antimony (Sb) has been an underappreciated element of environmental concern, until the last decade. The ever-increasing industrial activities such as mineral exploration and extraction liberate extremely high concentrations of Sb. Improved awareness of Sb exposure pathways and toxicity has motivated recent studies on the biogeochemistry of Sb. Nevertheless, compared with arsenic, an element in the same group in the periodic table, Sb biotransformation and mobility in the environment are much less understood.

Sb exists mainly in two oxidation states in the environment. Sb(III) is prevalent in anoxic environments, whereas Sb(V) dominates under oxic conditions. The redox transformation of Sb generally involves microorganisms. Anaerobic microbiological Sb(V) respiration is a newly discovered process regulating the Sb redox transformation in soils. However, little is known about the role microbiological Sb(V) respiration plays in the fate of Sb in the subsurface, especially in the presence of sulfate and electron shuttles. Herein, we enriched a Sb(V) reducing microbiota (SbRM) from the subsurface near an active Sb mine. SbRM was dominated by genus Alkaliphilus (18-36%), Clostridiaceae (17-18%), Tissierella (24-27%), and Lysinibacillus (16-37%). The incubation results showed that SbRM reduced 88% of dissolved Sb(V) to Sb(III), but the total Sb mobility remained the same as in the abiotic control, indicating that SbRM alone did not increase the total Sb release, but regulated the Sb speciation in the subsurface. Micro X-ray fluorescence (μ-XRF) analysis suggested the association of Sb and Fe, and electron shuttles such as anthraquinone-2,6-disulphonic disodium salt (AQDS) markedly enhanced the Sb release due to its ability to facilitate Fe mineral dissolution. Sb L-edge and S K-edge X-ray absorption near edge structure (XANES) results demonstrated that indigenous SbRM immobilized Sb via Sb₂S₃ formation, especially in a sulfur-rich environment.

Our study shows that Sb(V) respiring microbiota play an important role in Sb redox transformation in the subsurface. This conclusion provides a plausible explanation for the occurrence of high Sb(III) in paddy soils and subsurface groundwater. The new insights into Sb biotransformation warrant an extensive investigation of Sb(V) respiring reduction under different environmental conditions.