The double selectivity model for the description of ion exchange equilibria in zeolites

A thermodynamic model has been proposed for the description of cation-exchange equilibria on zeolites. The model is based on the hypothesis that zeolites can be modeled as composed by two different groups of cation sites available for ion-exchange reactions, each characterized by its own selectivity toward any given exchange reaction, and each having ideal behavior concerning the zeolite−cation interaction. The model has been tested by comparing its previsions with equilibrium experimental data, relative to uni−divalent and uni−univalent exchange reactions on clinoptilolite. In general, the comparison between model and experimental results was satisfactory, and in the majority of the cases, the model was capable of reproducing the experimental data obtained at different total normalities in solution. A reasonable agreement was also found between the equilibrium constants predicted by the model and those evaluated on the basis of the experimental data.