Antimony(III) sulfide complexes in aqueous solutions at 30 °C: A solubility and XAS study

Abstract

Stibnite solubility experiments and ambient temperature X-ray absorption spectroscopy (XAS) measurements of antimony in solution were undertaken to determine the aqueous speciation of Sb(III) in sulfide solutions having reduced sulfur concentrations from 0.001 to 0.1molkg−1S2−total over the pH range of 3.5 to 12. At 30°C, stibnite solubility curve was defined by a scheme of five species: Sb2S42−, HSb2S4−, H2Sb2S52−, H3SbS2O, and Sb(OH)3(aq). Equilibrium constants were determined for the following heterogeneous solubility reactions:Sb2S3s+HS−⇋Sb2S42−+H+Sb2S3s+HS−⇋HSb2S4−Sb2S3s+2HS−⇋H2Sb2S52−0.5Sb2S3s+0.5HS−+1.5H++H2Ol⇋H3SbS2O0.5Sb2S3s+3H2Ol⇋SbOH3aq+1.5HS−+1.5H+The Sb K-edge XAS measurements of antimony in alkaline (pH=10.9 to 12) sulfide solutions gave average first shell coordination environments for strongly alkaline solutions that were consistent the speciation model derived from solubility experiments (i.e., Sb2S42− and Sb(OH)3(aq)). XAS data enabled the elimination of a speciation model involving only Sb-S monomers at strongly alkaline pH. Antimony speciation in near neutral to strongly alkaline pH's is dominated by dimeric antimony sulfide complexes at sulfide concentrations>0.001molkg−1S2−total. Calculations of stibnite solubility and aqueous antimony speciation for conditions in anoxic marine basins indicate that sulfide species comprise a significant part of the total dissolved Sb(III) when molkg−1S2−total>~0.00005molkg−1 and that stibnite is close to saturation in some anoxic basins.