Arsenic Immobilization by Calcium Arsenate Formation

Abstract

Lime additions to arsenic-containing wastes have been proven to be beneficial in reducing the mobility of dissolved arsenic, presumably through the formation of low-solubility calcium arsenates. However, the role of calcium arsenate formation in reducing the concentrations of dissolved arsenic has not been well established. Therefore, slurries with varying Ca/As ratios were equilibrated, and the compounds that formed at elevated pH values were established. In contrast to the literature, Ca3(AsO4)2 was not observed, rather Ca4(OH)2(AsO4)2·4H2O, Ca5(AsO4)3OH (arsenate apatite), and Ca3(AsO4)2·32/3H2O had formed. The equilibrium concentrations of arsenic were found to be the lowest at high pH. Minimum arsenic concentrations in equilibrium with Ca4(OH)2(AsO4)2·4H2O and Ca5(AsO4)3OH were 0.01 and 0.5 mg/L, respectively. Because arsenate apatite is stable to near-neutral pH values, the extent of its solid solubility with Ca5(PO4)3OH was determined. This was done to assess the effects of phosphate ion on the possible release of arsenate ion. Although equilibrium arsenate ion concentrations increased with decreasing pH, solid solution formation did not occur under ambient conditions. Rather, the arsenate apatite formed at the expense of Ca5(PO4)3OH.