The antimonite-thioantimonates-antimonate pathway: Insights from sulfidic hot springs and microbial culture experiments for a novel mechanism of abiotic antimonite oxidation

Abstract

Antimony speciation in natural waters is of great environmental significance, and mechanisms involved in antimonite oxidation have been a research hotspot during the past years. In the present study, a new antimonite oxidation mechanism, which could be responsible for occurrence of appreciable antimonate in sulfidic hot springs, was proposed. Within the investigated hydrothermal areas in the Yunnan-Sichuan-Tibet Geothermal Province, much more antimonate formed in the neutral to alkaline sulfidic hot springs with low EH values than in the acidic hot springs low in sulfide and with high EH values, and antimonite-oxidizing microorganisms were rare in both neutral to alkaline and acidic hot springs. Antimonite oxidation experiments designed according to the key hydrochemical and microbial parameters of these hot springs indicated that the antimonite oxidation rates were the highest under abiotic, anoxic and dark, and sulfidic conditions and at pH 10, and there was a significant positive relationship between the proportions of antimonate and thioantimonates in the experimental solutions. Moreover, antimonite was oxidized only to a quite low degree even under microbial culture conditions with microorganisms-bearing hot spring sediments being used. These results implied that the antimonite oxidation in the sulfidic hot springs could occur via thiolation of antimonite followed by further transformation of thioantimonates into antimonate. This antimonite-thioantimonates-antimonate pathway may be prevalent in some other sulfidic environments, like paddy soils, mining areas, or euxinic basins.