Antimony(III) Speciation in Hydrosulfide Solutions from 70 to 400 °C and up to 300 bar

Stibnite solubility in hydrosulfide solutions (S2–total = 0.01–0.06 mol kg–1) between pH 3 and 8 was measured up to 300 bar from 70 to 400 °C using a flow-through autoclave reactor. The stoichiometries of the aqueous antimony(III) complexes and their heterogeneous equilibrium constants were determined from nonlinear least-squares fitting of the solubility data from 70 to 350 °C. Stibnite solubility was explained by a speciation model comprising three aqueous species: HSb2S4–, Sb2S42–, and Sb(OH)3(aq). At circumneutral pH and moderate sulfide concentrations (∼0.01 mol kg–1), antimony sulfide complexes predominated at temperatures <150 °C and antimonous acid (Sb(OH)3(aq)) at temperatures >150 °C. Stibnite solubility in supercritical solutions at ∼400 °C was strongly dependent on pressure up to 300 bar and was consistent with Sb(OH)3(aq) being the dominant species. Between 390 and 406 °C, stibnite solubility decreased from ∼0.015 (∼1800 ppm) at ∼300 bar to ∼0.0002 (∼30 ppm) at 150 bar, indicating that supercritical fluids with vapor-like densities can transport high concentrations of antimony. Circumneutral solutions with sulfide concentrations between 0.0001 and 0.001 mol kg–1 and with antimony concentrations typical of active geothermal systems (tens to hundreds of parts per billion) are undersaturated with respect to stibnite by several orders of magnitude at geothermal reservoir temperatures (200–300 °C) but become saturated with stibnite at temperatures ≤150 °C, depending on the solution pH and sulfide and antimony concentrations.