Adsorption and Desorption of Selenite and Selenate Selenium on Different Soils

In the laboratory, we studied the adsorption of selenite and selenate selenium and desorption by sulfate and phosphate in normal, calcareous, high organic carbon saline, and alkali soils. Initial selenium solutions contained 5 to 150 μmole Se/g soil, and desorbing solutions contained 500 μmole P or S/g soil. All soils sorbed variable amounts of selenite and selenate in the order: high organic carbon soil > calcareous soil > normal soil > saline soil > alkali soil. Adsorption of both selenite and selenate forms of selenium was influenced positively by organic carbon, clay content, CaCO3, and CEC and negatively influenced by high salt content, alkalinity, and pH. Conventional and rearranged Langmuir equations were tested. Langmuir curve analysis of Eadie-Hofstee was valid in normal and calcareous soils for selenite and in calcareous and saline soils for selenate. The analysis of Syers et al. did not fit well in any case. The adsorption of selenite and selenate showed differential behavior and indicated that a part of both selenite and selenate is adsorbed on differential qualitative sites. Selenate was always sorbed in higher amounts than selenite. The desorption of selenite selenium showed no preference for sulfate or phosphate, but selenate was more effectively desorbed by phosphate than sulfate in all soils.