Selenite Adsorption and Desorption in Selected South Dakota Soils as a Function of pH and Other Oxyanions

Changes in soil pH and chemical composition of solutions in vadose zone strongly affect selenite (SeO32-) adsorption and desorption properties. In this study, batch experiments were carried out to evaluate the properties of SeO32- adsorption and desorption in four South Dakota soils as a function of pH and the presence of competitive oxyanions. Selenite adsorption capacity of the soils was strongly dependent on soil pH and decreased with increasing pH between 5 and 9. Selenite adsorption capacity increased with increasing specific surface area of the soils, and Langmuir isotherm was described as an SeO32- adsorption behavior. The presence of phosphate (HPO42-) in solution significantly decreased the partition coefficient values and adsorption maximum from Langmuir isotherm on all tested soils. Although HPO42- addition affected SeO32- adsorption in all soils, the effect had depended on the pool size of SeO32- adsorption site. The competitive effect between SeO32- and HPO42- was less apparent in soils with a high adsorption capacity, and the competitive effect was more apparent in low-adsorbing capacity soils. The amount of SeO32- adsorbed per unit area was lower in the presence of HPO42- in solution, but the depression by HPO42- addition was greater in low-adsorbing capacity soils compared with high-adsorbing capacity soils because of the much fewer adsorption sites. Contrary to HPO42-, sulfate in solution had little effect on SeO32- adsorption on all tested soils, which indicates that specific adsorption plays a major role in the adsorption of SeO32-. The desorption of adsorbed SeO32- was found to be dependent on the amount of SeO32- initially adsorbed on soils and HPO42- in solution. Significantly more SeO32- desorbed when HPO42- was in solution compared with sulfate.