CHEMICAL FACTORS AFFECTING SELENITE SORPTION BY ALLOPHANIC SOILS

The sorption of selenite by two allophanic soils containing high amoun ts of variable charge materials was studied. Selenite sorption exhibit ed a maximum near pH 4 and decreased, although not proportionally, wit h increasing pH. Only negligible amounts of selenite were sorbed above pH 7. In the two soils, the addition of selenite caused a release of sulphate (SO42-), silicate (Si) and hydroxyl ion (OH-) and an increase in cation (Na+) adsorption. No measurable amount of phosphate (P) was released. Increase in negative charge as measured by Na+ adsorption a ccounted for 48 and 18% of selenite sorbed (soils 1 and 2, respectivel y), the rest being accounted for by release of anions. The results pre sented here are consistent with the widely held view that selenite and phosphate are sorbed onto variable charge surfaces by a similar mecha nism (ligand exchange).