Sorption-Analytic,Microcalorimetric and ab Initio Study of Ion-Exchange Equilibria Involving Mg 2+ and Ca 2+ Cations on Clinoptilolite

Abstract

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 (Mg²⁺-Ca²⁺-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 Ca²⁺ cations rather than to Mg²⁺ cations was demonstrated.

The ab initio calculations undertaken confirmed the experimental results and provided structural information concerning the localisation of ions in the crystalline environment. In particular, it was shown that the 10-membered structural clinoptilolite rings provide favourable positions for hydrated Ca²⁺ cations. The energy gain of the system arising from the Ca²⁺-Mg-Cli exchange was -E = 8.0–3.4 kJ/mol depending on the localisation sites for the Ca²⁺ cations; these values are in acceptable quantitative agreement with experimental data for the exchange enthalpy variation –ΔH = (7.3-8.4) kJ/(g equiv). Data were obtained for the energetics of interaction of Ca²⁺ and Mg²⁺ ions with water molecules located in the first and second hydration spheres.