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.