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.