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

Direct evidence of the structure of thioantimonide species in alkaline aque ous solutions is provided by X-ray absorption spectroscopy. Twenty solution s containing thioantimonide species were prepared by dissolving stibnite (S b2S3) in deoxygenated aqueous NaHS solutions; the solution pH range was 8-1 4, the [Sb-tot] 1-100 mM and the [HS-] 0.009-2.5 M. The structural environm ent of the dissolved Sb was determined by EXAFS analysis of the Sb K-edge o ver the temperature range 80-473 K. Many of the solutions contain a species with Sb bonded to four S atoms at 2 .34 Angstrom, consistent with the presence of a [Sb(V)S-4(3-)] species, dem onstrating that oxidation of Sb(III) to Sb(V) has occurred on dissolution. There is evidence that the complementary reduced phase is H-2. In three sol utions, the Sb has three nearest neighbor S atoms and two of these solution s have an additional S shell of two atoms at 2.9 Angstrom, with one showing evidence of an Sb shell at 4.15 A. This provides evidence of the presence of multimeric Sb(V) thioantimonide species. Analysis of several solutions r eveals the presence of a species with three Sb-S interactions of 2.41-2.42 Angstrom, supporting the presence of a Sb(III) species such as Sb2S2(SH)(2) Six solutions have S coordination numbers from 2.7-4 Angstrom and Sb-S dis tances of 2.37-2.39 Angstrom, and are likely to contain mixtures of at leas t two species in concentrations such that each make a significant contribut ion to the EXAFS. There was no clear relationship between either [Sb-tot] o r [HS-] and the type of species present, but Sb(III) species were only pres ent in the solutions with high pH. The effect of temperature was most signi ficant 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 A ngstrom, is indicated. The study provides new information on the coordination environment of thioa ntimonide species, complementary to previous studies and provides a basis f or a better understanding of Sb speciation in aqueous solutions found in hy drothermal systems, anoxic basins and man-made, high pH environments. In pa rticular it demonstrates the need for Sb(V) to be considered in theoretical and experimental studies of such systems. However, more definitive interpr etation of some of the data is inhibited by the presence of mixtures of spe cies and the lack of information on the outer coordination shells that woul d confirm the presence of multimeric species. (C) 2000 Elsevier Science Ltd . All rights reserved.