Ion exchange kinetics and thermodynamics of hydrosodalite, a narrow pore zeolite

The ion-exchange properties of a synthetic hydrosodalite (Na-hS) have been investigated by kinetic and thermodynamic analysis of exchange reactions of the original sodium form for lithium, potassium and calcium forms. Kinetic curves, modelled by a Langmuir-type equation, revealed that exchange rate for lithium and for potassium are of the same order, whereas they are two order faster than for calcium. Thermodynamic analysis of the cation exchange isotherms pointed out that sodalite is selective for sodium over the other three cationic forms examined, which is consistent with the preference exhibited by the sodalite type for sodium environments, either in natural or in laboratory crystallization. Na/Li and Na/Ca exchanges are incomplete, whereas unexpectedly Na/K exchange turns out to be complete, even though K+ dimension exceeds the width of the access window to sodalite cages. The obtained results have been discussed in terms of Eisenman-Sherry theory, pointing out agreements and discrepancies