Sorption behavior of selenium and antimony in soils as a function of phosphate ion concentration

Both selenium (Se) and antimony (Sb) are major soil and water pollutants. Their sorption behavior in a soil-plant system was studied. Soil-soil solution distribution coefficients (Ksub(d)s) for Se and Sb were measured, using a radiotracer, as an indicator of their sorption levels. Both Se and Sb behave as oxoanions (SeO4sup(2-), SeO3sup(2-) and SbO3sup(-)) in soil; thus, the effects of concentrations of two major oxoanions (H2PO4sup(-) and SO4sup(2-)) on Se and Sb sorption were also examined. The Ksub(d) values for Se for Japanese soils significantly correlated with the Ksub(d) values for Sb (n=141). The Ksub(d)s of both Se and Sb similarly decreased with increasing H2PO4sup(-) concentration. These results indicated that the sorption of Se and Sb was similarly controlled by a ligand-exchange mechanism such as phosphate sorption in soil. However, an increase in the concentration of SO4sup(2-) did not decrease the Ksub(d)s of Se and Sb. Furthermore, the ligand-exchangeable fractions of stable Se and Sb in major Japanese soils were determined by extraction with 0.1 mol/L Na2HPO4 solution. For both Se and Sb, the phosphate-extractable fractions were 10-fold higher for Se and fivefold higher for Sb than their water-soluble fractions. Although the total Se and Sb amounts in soils were the same, their ligand-exchangeable fractions were different. Approximately 0.9-12% of total Se and 0.2-1.3% of total Sb were extracted by the phosphate solution. These findings suggested that Se was more likely to be mobilized by the addition of phosphate than Sb. The effect of plant-available phosphate in the soil and the phosphate sorption capacity of soil on Se and Sb availabilities for plants were also examined using a pot experiment with soybean plants. The experimental results suggested that a high content of available phosphate and/or low phosphate sorption capacity of soil increased both Se and Sb availabilities to the plant. However, the results also suggested that the soil Se availability to the plant was higher than that of Sb even though the soil total Se and Sb amounts were the same.