THE SOLUBILITY OF AS2S3 AND SPECIATION OF AS IN DILUTE AND SULFIDE-BEARING FLUIDS AT 25-DEGREES-C AND 90-DEGREES-C

The solubility of synthetic, crystalline As2S3 in O2-free, dilute and sulphide-bearing solutions has been determined at temperatures of 25 and 90°C. The predominant dissolved As species are H3AsO0 and H2As3S-. Orpiment solubility is limited by the reactions: 0.5As2S3 + 3H2O = H3AsO0 + 1.5HS- + 1.5H+ pk25 = 23.11 ± 0.11 pK90 = 18.83 ± 0.05 1.5As2S3 + 1.5HS- + 0.5H+ = H2As3S- pK25 = -3.61 ± .09 pK90 = -3.42 ± 0.06 A formational free energy for orpiment has been calculated as ΔG0 = -96.72 kJ/mol. Measured concentrations of As in hot springs of the Taupo Volcanic Zone, New Zealand, are consistent with those predicted using the solubility reactions and equilibrium constants of this study. In sampled springs (pH 2-6) 97-100% of the dissolved As is present as H3AsO0. When H3AsO0 is the dominant As species, orpiment deposition is likely to occur in response to fluid cooling in the sub-aerial environment. © 1990.