SELENIUM SORPTION BY KAOLINITE AND MONTMORILLONITE

Abstract

Toxic selenium (Se) concentrations were found recently in shallow groundwater on the west side of the San Joaquin Valley in California [USA]. The origin of the Se and the mechanism by which it accumulated in the groundwater are not known. The objective of this work was to study Se sorption by kaolinite and montmorillonite at different pH levels, as a model of Se sorption reactions in soils, and evaluate the role of adsorption in distributing Se between solution and solid phases of soils. Within an Se concentration range of 0 to about 12 .mu.mol/L and pHs between 4 and 8, which cover the pH and Se concentrations in most affected drainage water, Se solubility in the studied suspensions was governed by adsorption. The sorption kinetics were best described by the Elovich equation. The adsorption decreased with elevating pH values and became negligible above pH 8. A modified competitive Langmuir model simulated satisfactorily Se retention by the two adsorbents under the studied experimental conditions. The greater adsorption of Se4 than Se6 by kaolinite and the different relative adsorption of Se4 by kaolinite and montmorillonite above and below pH 7 were expressed in terms of model parameters. Hydroxyl ions, according to the model, were more effective in modifying the anions'' adsorption capacity than competing with Se on common adsorption sites.