SORPTION AND REDOX TRANSFORMATION OF ARSENITE AND ARSENATE IN 2 FLOODED SOILS

The fate of As in soils is regulated mostly by its participation in sorption reactions and redox transformations. Few studies have examined the rate of arsenite and arsenate reduction or the extent to which these redox transformations may be affected by sorption reactions. The objective of this study was to examine changes in solution concentrations of H3AsO30 and H2AsO4- in two soils subjected to prolonged flooding. The soils, which differed in H3AsO30 and H2AsO4- sorption capacities, were flooded by suspending 1 g of soil in 25 mL of a solution containing 0.01 M CaCl2 and 1 g D-glucose kg-1. The suspensions were amended with NaAsO2 or Na2HAsO4 . 7H2O and were incubated for 0.5 h to 20 d. Changes in solution chemistry (electrode potential [Eh], pH, and dissolved Fe, Mn, H3AsO30, and H2AsO4-) were observed with time. Sorption processes controlled the dissolved concentrations of H3AsO30 and H2AsO4- during initial stages of H2AsO4-. When anaerobic conditions were achieved, dissolution of Fe and Mn oxyhydroxides occurred, causing desorption of H3AsO30 and H2AsO4-. In NaAsO2-amended suspensions, desorbed H3AsO30 disappeared from solution within 10 d. In Na2HaSO4--amended suspensions, desorbed H2AsO4- also disappeared within 10 d. Concurrent with the disappearance of H2AsO4- was the appearance of H3AsO30, indicating that H2AsO4- was rapidly reduced to H3AsO30. First-order plots of H3AsO30 and H2AsO4- disappearance had a linear relationship. Rates of desorption and disappearance of H3AsO30 and H2AsO4- were slower in the soil with higher adsorption capacity, suggesting that sorption processes may influence redox transformations of As oxyanions.