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

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.