Stability of arsenate minerals in soil under biotically generated reducingconditions

Mining activities and arsenical pesticide applications can introduce arsena te compounds into soils and sediments. Under water-saturated (flooded) soil conditions, arsenate solids are subjected to biotically generated reducing conditions and may undergo reductive dissolution. While thermodynamic calc ulations have been used to predict the conditions under which mineral-assoc iated As undergoes reduction, there is relatively little data from systems in which well-characterized arsenate solids have been subjected to reducing conditions, and a limited amount of information about the reduction of min eral-bound arsenate. In this study, the behavior of five arsenates was obse rved under reducing conditions generated by flooded soils, The apparent sol ubility of the arsenates decreased in the order CaHAsO4 = Na2AsO4. 7H(2)O > AlAsO4. 2H(2)O > MnHAsO4 > FeAsO4. 2H(2)O under oxic conditions: under ano xic conditions (redox potential <0 mV) the apparent solubility was FeAsO4. 2H(2)O greater than or equal to CaHAsO4 = Na2HAsO4. 7H(2)O > AlAsO4. 2H(2)O > MnnAsO(4). Calcium and sodium arsenates completely dissolved under the i nitial oxidizing conditions. X-ray absorption near-edge structure (XANES) s pectroscopy indicated that As in AlAsO4. 2H(2)O rapidly transformed to soli d-phase As(III). Manganese arsenate yielded the least solution and solid-ph ase As(III) of all of the minerals. Iron arsenate underwent reductive disso lution, releasing As(III) to solution and solid phases, and thus may yield solution or solid-phase As(III) if prolonged anoxic conditions prevail.