The solubility of As 2 S 3 and speciation of As in dilute and sulphide-bearing fluids at 25 and 90° C

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Scientific paper

The solubility of synthetic, crystalline As 2 S 3 in O 2 -free, dilute and sulphide-bearing solutions has been determined at temperatures of 25 and 90°C. The predominant dissolved As species are H 3 AsO 0 3 and H 2 As 3 S - 6 . Orpiment solubility is limited by the reactions: 0.5 As 2 S 3 + 3 H 2 O = H 3 AsO 0 3 + 1.5 HS - + 1.5 H + pk 25 = 23.11 ± 0.11 pK 90 = 18.83 ± 0.05 1.5 As 2 S 3 + 1.5 HS - + 0.5 H + = H 2 As 3 S - 6 pK 25 = -3.61 ± .09 pK 90 = -3.42 ± 0.06 A formational free energy for orpiment has been calculated as G 0 f = -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 H 3 AsO 0 3 . When H 3 AsO 0 3 is the dominant As species, orpiment deposition is likely to occur in response to fluid cooling in the sub-aerial environment.

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