Thermodynamic properties and stoichiometry of As(III) hydroxide complexes at hydrothermal conditions

The stoichiometry and thermodynamic properties of As(III) hydroxide complexes were determined from both solubility and Raman spectroscopic measurements. Arsenolite, claudetite, and orpiment solubilities were measured at temperatures to 250 and 300 degrees C, respectively, in acid solutions (pH < 6) at the saturated vapor pressure of the system. Raman spectroscopic measurements were performed on As2O3-H2O solutions (0.02 less than or equal to As less than or equal to 6 m; 0 less than or equal to pH less than or equal to 9) at temperatures from 20 to 275 degrees C. Results indicate that H3AsO30(aq) is the dominant As-bearing species at concentrations up to similar to 1 m over a wide range of pH (0-8) and temperature (20-300 degrees C). At higher As concentrations (greater than or equal to 1-2 m), a polymerization-dehydration of H3AsO30(aq) occurs via the formation of As-O-As bonds, leading to the formation of poly-As aqueous complexes. These experimental results were combined with corresponding properties for arsenolite, claudetite, and orpiment obtained in this study to generate H3AsO30(aq) thermodynamic properties within the framework of the revised HKF equation of state (Helgeson et al., 1981; Tanger and Helgeson, 1988). Calculations carried out using these properties indicate that orpiment, realgar, and native As can control As concentration in epithermal fluids at T less than or equal to 150-200 degrees C. At higher temperatures (greater than or equal to 250 degrees C), it is shown that arsenopyrite in association with pyrite and pyrrhotite or cassiterite can control As deposition in hydrothermal environments.