Stability of arsenate minerals in soil under biotically generated reducing conditions

Mining activities and arsenical pesticide applications can introduce arsenate 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 calculations have been used to predict the conditions under which mineral-associated 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 mineral-bound arsenate. In this study the behavior of five arsenates was observed under reducing conditions generated by flooded soils. The apparent solubility of the arsenates decreased in the order CaHAsO4 = Na2HAsO4(.)7H2O AlAsO4(.)2H2O MnHAsO4 FeAsO4(.)2H2O under oxic conditions; under anoxic conditions (redox potential 0 mV) the apparent solubility was FeAsO4(.)2H2O greater than or equal to CaHAsO4 AlAsO4(.)2H2O MnHAsO4. Calcium and sodium arsenates completely dissolved under the initial oxidizing conditions. X-ray absorption near-edge structure (XANES) spectroscopy indicated that As in AlAsO4(.)2H2O rapidly transformed to solid-phase As(III). Manganese arsenate yielded the least solution and solid-phase As(III) of all of the minerals. Iron arsenate underwent reductive dissolution, releasing As(III) to solution and solid phases and thus may yield solution or solid-phase As(III) if prolonged anoxic conditions prevail