Arsenic availability in soil with time under saturated and subsaturated conditions

Loss of As from solution in reduced soils has long been attributed to volatilization of As as arsines. This study was conducted to determine whether loss of As from soil solution could be accounted for by increases in surface-bound forms of As. Pickney sand (sandy, siliceous, thermic Cumulic Humaquept) was treated with sodium arsenate and sodium arsenite and incubated under saturated and subsaturated conditions for up to 68 d. Sequential extraction of As by NH4Cl, NH4F, NaOH, and H2SO4 was performed to monitor the redistribution of As between different fractions with time. By determining the As concentration present in sequentially extractable forms, it was demonstrated that As became more recalcitrant in soil with time. Arsenic added in water-soluble form became more tightly bound to the soil as evidenced by the necessity of stronger extractants to remove added As. The conversion (curing) of As to more insoluble forms occurred in saturated soils where the pe/pH status was in the arsenite stability held and in subsaturated soils where the pe/pH status was in the arsenate stability field. The rate at which As was converted to more insoluble (and possibly less bioavailable) forms was as rapid in saturated soils as in unsaturated soils. Arsenic was conserved during the experiment with no apparent loss of As from the system through 68 d.