Sorption and desorption of selenium in different soils of the Mediterranean area

The adsorption of selenium (Se) in the selenate form and its desorption by phosphate in four soils with different physiochemical properties were studi ed in the laboratory. To determine adsorption isotherms for selenate 25 mL of solutions containing 1 to 100 ppm of Se were added to 2.5 g of soil. Des orption isotherms were determined by resuspending the samples in phosphate solution. The selenate sorption process was adequately described by the Fre undlich equation. In pine forest and woodland soils, characterized by the h ighest organic matter content and cation exchange capacity (CEC) values, th e isotherms were classified as L type, since the amount of Se sorbed appear ed to move towards saturation. The organic matter content played the most i mportant part in the adsorption of Se, while pH appeared to have a small ef fect on the ability of the soil to adsorb Se. The high CaCO3 content of the pine forest soil may have contributed in increasing the Se adsorption notw ithstanding the high pH value. The cultivated and arable soils showed a red uced sorption capacity. The sorption could be described by an S type curve. At low concentrations of Se the affinity of the solid phase was less than that of the liquid phase. By increasing the concentration of Se in solution , the affinity of the solid phase increased and the sorption was favored. S elenate desorption by water was negligible, whereas the amount of Se desorb ed by phosphate varied among the different soils. The desorption experiment s indicated that a significant portion of the sorbed Se was irreversibly re tained. This suggests the existence of linkages which allow the release of Se in the soil solution only after physico-chemical variation such as excha nge with phosphate ions.