Thermodynamics of selenium sorption on alumina and montmorillonite

Abstract

The effect of pH and temperature on the thermodynamics of selenium sorption on alumina and montmorillonite was investigated. The equilibrium constants were obtained from batch experiments carried out at 25 and 40°C, under acidic and near to neutral conditions. Microcalorimetry was used to measure directly the enthalpy change upon sorption. These data were analysed by taking into account the different reactions that occur during the sorption process (the acid–base equilibria in the bulk solution on the one hand and the complexation equilibria between surface sites of the solid and solution species on the other hand). This was done using a simple surface model which assumes that the thermodynamic properties of the aluminol surface sites of both alumina and montmorillonite are identical. Two of the considered reactions were found to predominate, one under acidic conditions and one at near neutral pH. The microcalorimetric data allowed to check that the temperature effect on selenium sorption on alumina and montmorillonite can be correctly predicted simply using the above assumption concerning the sorption sites and by applying van’t Hoff relation both to the homogeneous and heterogeneous reactions.