Understanding the goethite role on stibnite oxidative dissolution and transformation: Spectroscopic and DFT study

Abstract

The geochemical cycling of antimony (Sb) in aquatic system is primarily influenced by the dissolution and transformation of stibnite (Sb2S3) in the presence of iron minerals. Here, Sb2S3 oxidative dissolution and sequestration on goethite were investigated to mimic the environmental fate of Sb2S3. The results demonstrated that goethite accelerated the rate of Sb2S3 oxidative dissolution by a factor of 9.4 times under sunlight. The significant Sb2S3 oxidation on goethite was attributed to a heterogeneous electron transfer from Sb2S3 to goethite, as proved by XANES analysis. This electron transfer facilitated the generation of hydroxyl radicals (OH) on Sb2S3, and superoxide radicals (O2−) on goethite. Radical trapping experiments confirmed that O2− was the dominant oxidant for Sb(III) oxidation with 91 % contribution. Thus, goethite plays a dominant role in O2− generation and Sb2S3 oxidative dissolution. Meanwhile, the total dissolved Sb was decreased by 69 % in Sb2S3 and goethite coexisting system compared to a single Sb2S3 system, indicating the retention of dissolved Sb on goethite. Density functional theory (DFT) calculations deciphered that Sb(III) oxidation on mineral-water interfaces with O2− radicals was thermodynamically preferential to OH radicals. Additionally, the Sb was anchored on goethite as a bidentate binuclear structure with a favorable adsorption energy. Our findings shed the light to understand the geochemical cycles of Sb2S3 in natural environment.