Abstract: Clinoptilolite from the Fort LaClede deposit in Sweetwater County, Wyoming, shows a moderate selectivity for NH4- over Na+ in aqueous solution. At 30°C, the standard free energy of this replacement reaction is −0.7 kcal/mole at an ionic strength of 0.05 M and −0.8 kcal/mole at 0.5 M. The Na+-NH4+ exchange is complete within 3 days in agitated solution and proceeds to the same extent from the clinoptilolite saturated with either cation.
The Ca2+-Na+ exchange also is complete within 3 days in agitated aqueous solution and proceeds to the same extent from either the calcium or the sodium form of the zeolite. Using test methods which take into account the slower equilibration of Ca2+-loaded clinoptilolite, the cation-exchange capacity is substantially the same over the full range of loading by Ca2+ and Na+. Ca2+ replaces Na+ with decreasing selectivity as Ca2+ loading increases to about 80% at 30°C (95% at 63°C), above which the selectivity reverses. The standard free energy of replacement of two Na+ ions by one Ca2+ ion in 0.05 M solution is - 1.2 ± 0.2 kcal/mole at 63°C and −0.3 to −0.8 kcal/mole at 30°C.