Determination of arsenic species in soil solution under flooded conditions

Greenhouse experiments were conducted to evaluate the relationships between the species and concentrations of As in the soil solution of flooded soils with other parameters including soil pe, pH, Fe, Mn, and type and amount of As added. Two soils were treated with 0, 5, 15, 25, 35, and 45 mg As kg(-1) soil added as either Na-arsenate or Na-arsenite and planted with rice (Oryza sativa L.). Soil solution samples were collected during a period of 60 d and analyzed for As. Selective hydride generation was employed to evaluate both type and quantity of As present in the samples. Inorganic As in the form of arsenate and arsenite was found in the soil solution of both soils. The conversion of added arsenite to arsenate occurred within the first 10 d of the experiment when the pe/pH of the soil was not conducive to arsenite stability. Added arsenate was converted to arsenite during the course of the experiment as the pe/pH of the soil declined due to flooding. Arsenate reached a maximum in soil solution at 10 to 20 d after flooding while maximum arsenite concentrations occurred at 20 to 30 d after flooding. The total concentration of As in soil solution generally reached a maximum at 20 to 30 d after flooding, after which time precipitous losses of As from soil solution occurred in all but the highest As treatments. Soil solution As concentrations were not statistically different between planted and unplanted controls.