Multicomponent Cation Exchange In Natural Zeolites—Experiments and Thermodynamic Model

Natural zeolites exhibit favorable cation-exchange selectivity for certain cations such as Cs⁺, Sr²⁺, and NH₄⁺, and have been studied for potential use in the treatment of industrial, municipal, and nuclear wastewaters and acid mine drainage waters. These minerals also have been studied for potential use in the remediation of sites contaminated with fission products such as ⁹⁰Sr, ¹³⁵Cs, and ¹³⁷Cs and in the remediation of soils contaminated with heavy metals. Numerous ion-exchange studies involving natural zeolites have been published, but the application of thermodynamic models to describe and predict multicomponent ion-exchange equilibria has been hampered by the lack of well-constrained ion-exchange data in multicomponent systems.

In this study, ternary and quaternary ion-exchange experiments involving the cations Na⁺, K⁺, Cs⁺, Sr²⁺, and Ca²⁺ and the zeolite mineral clinoptilolite were conducted. In the ternary ion-exchange experiments, homoionic forms of Na⁺-, K⁺-, and Ca²⁺-clinoptilolite prepared from Death Valley Junction, California, clinoptilolite-rich tuff were reacted with solution mixtures of Na⁺+K⁺+Cs⁺, Na⁺+K⁺+Sr²⁺, and Na⁺+K⁺+Ca²⁺ at constant solution normality. The quaternary ion-exchange experiment studied a Na⁺+K⁺+Ca²⁺+Sr²⁺system.

The ion-exchange equilibria were modeled using a thermodynamic approach based on the Wilson equation. Published ion-exchange data were used to derive the Wilson equation parameters and the equilibrium constants for binary ion exchange. A correlation method that has been applied to predictions of formation constants of aqueous hydroxo-metal complexes was used to help constrain the equilibrium constants. The Wilson model, with parameters derived only from binary ion-exchange data, was used to predict ternary and quaternary ion-exchange equilibria. A comparison of experimental data for ternary and quaternary systems and thermodynamic model predictions indicates that the Wilson model adequately reproduces multicomponent ion-exchange equilibria.

This study was funded by the Southwest Research Institute^® Internal Research and Development Project 20–9211.