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 syst ems, the response of (hydr)oxides to fluctuating soil redox conditions , and its impact on As sorption, is poorly understood. This study inve stigated the influence of soil flooding and drying on As sorption and Fe fractionation. Four soils exhibiting a range in As sorption capacit y were incubated under flooded (anaerobic) conditions. After the flood ing period, the pH and redox potentials of each soil suspension were m easured and saturation indices were calculated for standard soil miner als. The suspensions were air dried at room temperature and the short- range order Fe (oxalate-extractable) and crystalline Fe (dithionite-ex tractable) fractions determined. Separate flooded-dried samples were r esuspended in 0.1 M NaCl for AsO4 sorption measurements. Prolonged soi l flooding resulted in a decrease in soil redox potential and an incre ase in dissolved Al, Fe, Mn, and Si. Drying the previously flooded soi ls resulted in an increase in As sorption. Selective extraction indica ted that the flooding-drying treatments increased the short-range orde r Fe fraction while depleting the free oxide fraction. These changes i n Fe mineralogy most likely increased the surface area and number of p otential As sorption sites.