ARSENIC MOBILITY CONTROL BY SOLID CALCIUM ARSENATES: AN ALTERNATIVE MECHANISM OF RETENTION OF ARSENIC IN THE ENVIRONMENT

Lime amendments are widely used to stabilize arsenic-rich residues from mining, metallurgy, and industrial sewage throughout the world. This treatment technology forms solid calcium arsenates whose solubility is highly dependent on the geochemical conditions where these residues are disposed of. We have investigated the geochemical mechanism in a closed perched aquifer in North Central Mexico where tons of arsenic have been mobilized from an inactive mining complex, leading to aqueous concentrations of up to 158 mg/L in some segments of the aquifer. Using wet chemistry data to make chemical speciation calculations and microscopy, we show that calcium ion availability dictates arsenic attenuation from metallurgical wastes through the digenetic precipitation of calcium arsenates in the aerobic aquifer investigated. Arsenic pollution is decreased up to 90% by the precipitation of Ca₅H₂(AsO₄)₄.cH₂O in a 6 km downgradient distance preventing the release of 58.5 tons of dissolved arsenic in a year. This equilibrium is highly fragile and easily disturbed by calcium-binding anions, such as phosphate and sulfate, which readily shift the solubility of the calcium arsenate towards releasing high concentrations of As(V) to the aqueous phase. Our results represent a model for understanding a highly underreported/alternative mechanism of retention of arsenate [As(V)] species likely to dominate in calcium-rich environments, such as those in calcareous sediments and soils, where the commonly reported mechanism of adsorption to iron(III) oxides is not the dominant process.