Selectivity of Low- and High-Silica Clinoptilolites with Respect to Alkali and Alkaline–Earth Metal Cations

The ion-exchange equilibria involving K⁺ and Ca²⁺ cations were studied using the Na-forms of nine different clinoptilolite samples. The high-silica (in the native state, sodium–potassium form) clinoptilolites were shown to have the increased selectivity with respect to K⁺ ions, whereas the low-silica (in the native state, calcium form) clinoptilolites to Ca²⁺ ions. The phenomenon found is interpreted as a structural memory of clinoptilolites with respect to alkali and alkaline-earth metal ions which had been present during zeolite crystallization under natural conditions.