Ion exchange in zeolites. The exchange of cadmium and calcium in sodium X using different anionic backgrounds

Abstract

Ion-exchange characteristics have been determined for the binary exchanges of Cd²⁺ and Ca²⁺ in sodium X zeolite using a variety of anionic backgrounds. A thermodynamic model, previously presented for the evaluation of the activity coefficients of salts in mixed electrolyte solutions, is applied to calculate the thermodynamic data for these exchanges. The application of this model shows that changes in cadmium selectivity as a function of changing anionic background can be rationalised entirely in terms of changes in the magnitude of solution-phase activity coefficients.

The reversibility of the systems has been tested carefully in order to obtain information on the degree to which the zeolite is affected by hydronium exchange and/or acid damage of the structure. It is shown that the ability of divalent ions to undergo reversible and stoichiometric exchange in zeolite X is consistent with trends observed in their acid association constants. In addition, and contrary to some exchanges in clay systems, it is shown that the highly negatively charged environment of zeolite X efficiently excludes associated ion pairs.