Sorption and Redox Transformation of Arsenite and Arsenate in Two Flooded Soils
McGeehan, S. L. | Naylor, D. V.
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 H₃AsO⁰₃ and H₂AsO⁻₄ in two soils subjected to prolonged flooding. The soils, which differed in H₃AsO⁰₃ and H₂AsO⁻₄ 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, H₃AsO⁰₃, and H₂AsO⁻₄) were observed with time. Sorption processes controlled the dissolved concentrations of H₃AsO⁰₃ and H₂AsO⁻₄ during initial stages of flooding. When anaerobic conditions were achieved, dissolution of Fe and Mn oxyhydroxides occurred, causing desorption of H₃AsO⁰₃ and H₂AsO⁻₄. In NaAsO₂-amended suspensions, desorbed H₃AsO⁰₃ disappeared from solution within 10 d. In Na₂HaSO⁻₄-amended suspensions, desorbed H₂AsO⁻₄ also disappeared within 10 d. Concurrent with the disappearance of H₂AsO⁻₄ was the appearance of H₃AsO⁰₃, indicating that H₂AsO⁻₄ was rapidly reduced to H₃AsO⁰₃. First-order plots of H₃AsO⁰₃ and H₂AsO⁻₄ disappearance had a linear relationship. Rates of desorption and disappearance of H₃AsO⁰₃ and H₂AsO⁻₄ were slower in the soil with higher adsorption capacity, suggesting that sorption processes may influence redox transformations of As oxyanions. Idaho Agric. Exp. Stn. Manuscript no. 92723.
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