Adsorption Studies on Clay Minerals: IX. Ion-Exchange Properties of Natural and Thermally Altered Montmorillonite

The ion-exchange behavior of a thermally collapsed montmorillonite clay is compared with that of the natural mineral. Equilibrium measurements in the cesium-barium system at different temperatures demonstrate that the markedly increased selectivity for cesium, at low cesium content, shown by the altered clay is due to changes in the enthalpy of the exchange process partially compensated by a specific entropy effect favoring the sorption of barium. The standard free energy changes at 30C on the natural and altered mineral (for the sorption of 2 moles of cesium) are, respectively, −3.38 and −3.19 kcal. These do not reflect the considerable increase in selectivity for cesium of the heated clay at low cesium content. Variation in the exchange capacity for barium of the heated clay when small amounts of cesium are present implies that this strongly sorbed ion renders many sites inaccessible to barium.