Sorption-analytic, Microcalorimetric and ab initio Study of Ion-exchange Equilibria involving Mg~(2+) and Ca~(2+) Cations on Clinoptilolite

摘要

The exchange equilibrium between Mg2+ and Ca2+ ions and the corresponding microcalorimetric ion-exchange heats were studied on two clinoptilolite (Cli) samples. Using the experimental values of the ion-exchange constants (free energies) and heats (enthalpies), reliable estimates of the entropy were obtained for the direct (Mg~(2+)-Ca~(2+)-Cli) and inverse ion-exchange reactions. It was shown that the sign (negative or positive) of the ion-exchange free energy in the case under study was determined essentially by the entropy constituent of the ion-exchange process. The selectivity of clinoptilolite towards Ca~(2+) cations rather than to Mg~(2+) cations was demonstrated.