A study of speciation of Sb in bisulfide solutions by X-ray absorption spectroscopy.

Direct evidence of the structure of thioantimonide species in alkaline aqueous solutions is provided by X-ray absorption spectroscopy. Twenty solutions containing thioantimonide species were prepared by dissolving stibnite (Sb₂S₃) in deoxygenated aqueous NaHS solutions; the solution pH range was 8–14, the [Sb_tot] 1–100 mM and the [HS⁻] 0.009–2.5 M. The structural environment of the dissolved Sb was determined by EXAFS analysis of the Sb K-edge over the temperature range 80–473 K.Many of the solutions contain a species with Sb bonded to four S atoms at 2.34 Å, consistent with the presence of a [Sb(V)S₄³⁻] species, demonstrating that oxidation of Sb(III) to Sb(V) has occurred on dissolution. There is evidence that the complementary reduced phase is H₂. In three solutions, the Sb has three nearest neighbor S atoms and two of these solutions have an additional S shell of two atoms at 2.9Å, with one showing evidence of an Sb shell at 4.15 Å. This provides evidence of the presence of multimeric Sb(V) thioantimonide species. Analysis of several solutions reveals the presence of a species with three Sb–S interactions of 2.41–2.42 Å, supporting the presence of a Sb(III) species such as Sb₂S₂(SH)₂. Six solutions have S coordination numbers from 2.7–4 Å and Sb–S distances of 2.37–2.39 Å, and are likely to contain mixtures of at least two species in concentrations such that each make a significant contribution to the EXAFS. There was no clear relationship between either [Sb_tot] or [HS⁻] and the type of species present, but Sb(III) species were only present in the solutions with high pH. The effect of temperature was most significant in one solution, where at 423 K partial hydrolysis occurred and the presence of a species such as Sb₂S₂(OH)₂, with an Sb–O distance of 1.91 Å, is indicated.The study provides new information on the coordination environment of thioantimonide species, complementary to previous studies and provides a basis for a better understanding of Sb speciation in aqueous solutions found in hydrothermal systems, anoxic basins and man-made, high pH environments. In particular it demonstrates the need for Sb(V) to be considered in theoretical and experimental studies of such systems. However, more definitive interpretation of some of the data is inhibited by the presence of mixtures of species and the lack of information on the outer coordination shells that would confirm the presence of multimeric species.