Alteration of Arsenic Sorption in Flooded-Dried Soils

Arsenic is known to be immobilized in soils through oxyanion sorption reactions involving metal (hydr)oxides. While the mechanisms involved in oxyanion sorption are relatively well characterized in aerobic systems, the response of (hydr)oxides to fluctuating soil redox conditions, and its impact on As sorption, is poorly understood. This study investigated the influence of soil flooding and drying on As sorption and Fe fractionation. Four soils exhibiting a range in As sorption capacity were incubated under flooded (anaerobic) conditions. After the flooding period, the pH and redox potentials of each soil suspension were measured and saturation indices were calculated for standard soil minerals. The suspensions were air dried at room temperature and the short-range order Fe (oxalate-extractable) and crystalline Fe (dithionite-extractable) fractions determined. Separate flooded-dried samples were resuspended in 0.1 M NaCI for AsO 4 sorption measurements. Prolonged soil flooding resulted in a decrease in soil redox potential and an increase in dissolved Al, Fe, Mn, and Si. Drying the previously flooded soils resulted in an increase in As sorption. Selective extraction indicated that the flooding-drying treatments increased the short-range order Fe fraction while depleting the free oxide fraction. These changes in Fe mineralogy most likely increased the surface area and number of potential As sorption sites.