Ion exchange in zeolites. Part 1.—Prediction of Ca2+–Na+ equilibria in zeolites X and Y

Abstract

Exchange of Ca²⁺ in zeolites NaX and NaY was investigated with special emphasis on prediction of Ca²⁺–Na⁺ exchange equilibria over a wide range of ion concentrations in solution. Predictions were good for moderate and higher concentrations, but poor for dilute solutions. This was caused by the strong decrease in selectivities of the zeolites, at a given composition, with increasing dilution of th solution. Re-examination of ion-exchange data of earlier workers implied that this phenomenon, which is inconsistent with current theories of ion exchange, is more common in zeolites than hitherto suspected. The effects of salt imbibition, concurrent hydronium ion exchange and dissolution of the zeolites on the selectivity decrease of the zeolites were considered. None of these could be shown to be responsible for the phenomenon observed.