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 so rption reactions and redox transformations. Few studies have examined the rate of arsenite and arsenate reduction or the extent to which the se redox transformations may be affected by sorption reactions. The ob jective of this study was to examine changes in solution concentration s 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 contain ing 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. C hanges in solution chemistry (electrode potential [Eh], pH, and dissol ved Fe, Mn, H3AsO30, and H2AsO4-) were observed with time. Sorption pr ocesses controlled the dissolved concentrations of H3AsO30 and H2AsO4- during initial stages of H2AsO4-. When anaerobic conditions were achi eved, dissolution of Fe and Mn oxyhydroxides occurred, causing desorpt ion of H3AsO30 and H2AsO4-. In NaAsO2-amended suspensions, desorbed H3 AsO30 disappeared from solution within 10 d. In Na2HaSO4--amended susp ensions, desorbed H2AsO4- also disappeared within 10 d. Concurrent wit h the disappearance of H2AsO4- was the appearance of H3AsO30, indicati ng that H2AsO4- was rapidly reduced to H3AsO30. First-order plots of H 3AsO30 and H2AsO4- disappearance had a linear relationship. Rates of d esorption and disappearance of H3AsO30 and H2AsO4- were slower in the soil with higher adsorption capacity, suggesting that sorption process es may influence redox transformations of As oxyanions.