CRITICAL EVALUATION OF THERMODYNAMIC DATA FOR SELECTED ARSENIC MINERALS AND AQUEOUS SPECIES WITH APPLICATIONS TO NATURAL WATERS

A critical evaluation of selected arsenic minerals and aqueous species and a separate review of aqueous arsenic speciation were completed recently as part of a new volume on arsenic geochemistry, mineralogy, toxicity, and microbiology (Reviews in Mineralogy and Geochemistry #79). We developed a thermodynamic network based on the most reliable and fundamental data in the arsenic system, using the most direct pathways to build the network, and checking for internal consistency at each step. Arsenolite, As₂O₃-cubic, is the cornerstone or anchor of the network from which thermodynamic properties of the aqueous species of arsenate and arsenite and their hydrolysis products are derived. Thermodynamic data on aqueous arsenate ions are essential for the evaluation of metal arsenate minerals and their solubilities. Very few measurements of metal arsenate and metal arsenite complexes are reported from the literature but by using a consistent chemical model and the same equation of state, several metal arsenate and metal arsenite complexes have been estimated by Marini and Accornero (2007, Environ. Geol. 52, 1343). The solubilities and solubility-product constants of several metal arsenates have been reported but many are of variable quality and in need of better measurements and further evaluation. Scorodite solubility is well characterized, consistent with recent calorimetric data at ambient temperatures. The solubility of some calcium arsenates is well characterized and there is some agreement between investigations but inconsistencies still exist and some phases are poorly characterized or not measured at all. Application of estimated metal-arsenate and metal-arsenite stability constants to waters with a wide range of composition and temperature shows that arsenite speciation is affected very little (98-99% as free forms of hydrolyzed arsenite) but that arsenate speciation can be affected substantially (24-71% of the arsenate species in the free forms). Calcium and magnesium arsenate complexes contribute 0-24%, and sodium arsenate complexes contribute 1.5-75%. These complexes should be included in speciation calculations for any waters involving arsenate species, especially water compositions with moderate or higher concentrations of sodium.