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 0.1 mol kg − 1  S 2 − total over the pH range of 3.5 to 12. At 30 °C, stibnite solubility curve was defined by a scheme of five species: Sb 2 S 4 2 − , HSb 2 S 4 − , H 2 Sb 2 S 5 2 − , H 3 SbS 2 O, and Sb(OH) 3 (aq). Equilibrium constants were determined for the following heterogeneous solubility reactions: Sb 2 S 3 s + HS − ⇋ Sb 2 S 4 2 − + H + S b 2 S 3 s + HS − ⇋ H S b 2 S 4 − S b 2 S 3 s + 2 HS − ⇋ H 2 S b 2 S 5 2 − 0.5 S b 2 S 3 s + 0.5 HS − + 1.5 H + + H 2 O l ⇋ H 3 S b S 2 O 0.5 S b 2 S 3 s + 3 H 2 O l ⇋ S b OH 3 aq + 1.5 HS − + 1.5 H + 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., Sb 2 S 4 2 − 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 > 0.001 mol kg − 1  S 2 − 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 mol kg − 1  S 2 − total  > ~ 0.00005 mol kg − 1 and that stibnite is close to saturation in some anoxic basins.