Densities and heat capacities of aqueous arsenious and arsenic acid solutions to 350 °C and 300 bar, and revised thermodynamic properties of As(OH)3∘(aq), AsO(OH)3∘(aq) and iron sulfarsenide minerals

Densities and heat capacities of aqueous arsenious and arsenic acid solutions of 0.1–0.6 mol/kg were measured using the flow vibrating tube densitometry and differential calorimetry at temperatures to 350 °C and pressures to ∼310 bar. The standard partial molal volumes V∘ and heat capacities Cp∘ of the neutral aqueous AsIII and AsV (oxy)hydroxide species, As(OH)3 and AsO(OH)3, were obtained from these data, via corrections for partial dissociation and extrapolation to infinite dilution. The generated V∘ and Cp∘ values, together with the existing data on AsIII oxide and sulfide minerals solubilities and low-temperature AsIII–AsV aqueous solution equilibria, were used to refine the thermodynamic properties of As hydroxide complexes over a wide temperature–pressure range, in the framework of the revised HKF equation of state and using correlation algorithms recently proposed for aqueous neutral species. These revised properties were combined with solubility data for arsenopyrite (FeAsS) and direct calorimetric heat capacity and enthalpy measurements reported in the literature for arsenopyrite, loellingite (FeAs2), and westerveldite (FeAs), to generate a consistent set of thermodynamic parameters for these iron sulfarsenides. The new Gibbs free energy values of arsenopyrite and loellingite resulting from these properties imply lower solubilities of iron sulfarsenides in aquatic environments than have been assumed. The thermodynamic properties of arsenic aqueous species and solid phases obtained in this study provide quantitative constraints on As-bearing mineral stabilities and arsenic transport by geological fluids.