Sorption and Redox Transformation of Arsenite and Arsenate in Two Flooded Soils

Abstract

Abstract The fate of As in soils is regulated mostly by its participation is 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 H 3 AsO 0 3 and H 2 AsO − 4 in two soils subjected to prolonged flooding. The soils, which differed in H 3 AsO 0 3 and H 2 AsO − 4 sorption capacities, were flooded by suspending 1 g of soil in 25 mL of a solution containing 0.01 M CaCl 2 and 1 g d ‐glucose kg −1 . The suspensions were amended with NaAsO 2 or Na 2 HAsO 4 ·7H 2 O and were incubated for 0.5 h to 20 d. Changes in solution chemistry (electrode potential [Eh], pH, and dissolved Fe, Mn, H 3 AsO 0 3 , and H 2 AsO − 4 ) were observed with time. Sorption processes controlled the dissolved concentrations of H 3 AsO 0 3 and H 2 AsO − 4 during initial stages of flooding. When anaerobic conditions were achieved, dissolution of Fe and Mn oxyhydroxides occurred, causing desorption of H 3 AsO 0 3 and H 2 AsO − 4 . In NaAsO 2 ‐amended suspensions, desorbed H 3 AsO 0 3 disappeared from solution within 10 d. In Na 2 HaSO − 4 ‐amended suspensions, desorbed H 2 AsO − 4 also disappeared within 10 d. Concurrent with the disappearance of H 2 AsO − 4 was the appearance of H 3 AsO 0 3 , indicating that H 2 AsO − 4 was rapidly reduced to H 3 AsO 0 3 . First‐order plots of H 3 AsO 0 3 and H 2 AsO − 4 disappearance had a linear relationship. Rates of desorption and disappearance of H 3 AsO 0 3 and H 2 AsO − 4 were slower in the soil with higher adsorption capacity, suggesting that sorption processes may influence redox transformations of As oxyanions.