Solubility and Raman spectroscopic study of As(III) speciation in organic compound-water solutions. A hydration approach for aqueous arsenic in complex solutions

Solubilities of arsenolite (As2O3, cub.) were measured from 22 to 90 degree s C in water-acetone, water-acetic acid, and water-formic acid solutions of compositions ranging from the pure organic compound to pure water. Raman s pectra were obtained at ambient temperature on As-containing water-acetic a cid and water-acetone solutions. Results show that arsenic solvation by the se organic compounds is negligible and hydroxide species dominate As specia tion over a wide range of water activity (a(H2O) > 0.01). The solubility da ta were analyzed using an approach based on stoichiometric hydration reacti ons. Results show that As2O3 solubility can be described as a function of w ater activity, independently of the nature of the organic compound, by invo lving two neutral As hydroxide complexes: As(OH)(3) and As(OH)(3) . 4H(2)O. Stability constants derived for these species indicate that hydration weak ens with increasing temperature. Calculations using these constants show th at at low temperatures the tetrahydrate As(OH)(3) . 4H(2)O is dominant in w ater-rich solutions; by contrast, in high-temperature crustal fluids, As(OH ), becomes the major As species. The proposed hydration model can be used t o analyze solubility of arsenic-bearing minerals and arsenic transport in c omplex H2O-CO2-electrolyte solutions encountered in natural and industrial environments.