Experimental and Modeling Study of Multicomponent Ion-Exchange Equilibria on Zeolite Minerals, 20-9211

Principal Investigators
F. Paul Bertetti
Roberto T. Pabalan

Inclusive Dates: 10/01/00 - 06/02/03

Background - Zeolites are crystalline, hydrated aluminosilicate minerals characterized by an ability to exchange some of their constituent cations with cations in aqueous solutions, without a major change in crystalline structure. Because of their favorable ion exchange selectivity for certain cations, such as Cs⁺ and Sr²⁺, naturally occurring zeolites have been studied for their potential use in the treatment of wastewaters and the remediation of sites contaminated with radionuclides such as ⁹⁰Sr and ¹³⁷Cs. Although numerous theoretical and experimental studies of ion-exchange equilibria have been published, most have focused on ion-exchange reactions involving two cations only (binary exchange). Little attention has been paid to multicomponent ion-exchange equilibria, despite the fact that many natural and industrial processes involve multicomponent systems.

Approach - This study had the objectives of (i) generating well constrained zeolite ion-exchange experimental data at 25 °C, and (ii) developing a general thermodynamic model that can be used to predict zeolite ion-exchange equilibria in multicomponent systems of interest in geochemistry and chemical engineering. Data were generated through experiments in which zeolite minerals were reacted with aqueous mixtures of cations of interest. The experimental data were then used to derive parameters for thermodynamic models of ion exchange and to check the predictive capability of those models. Three tasks comprised the project. In general, the tasks progressed through a series of binary, ternary, and quaternary ion-exchange experiments with appropriate modeling evaluations and updates during each phase.

Accomplishments - Clinoptilolite, a widely occurring natural zeolite mineral commonly used in industry, was selected for detailed investigation. Because a primary goal of the project was to develop a more uniform approach to modeling ion-exchange reactions in zeolites, a solid-solution activity coefficient model was developed using the Wilson equation. The Wilson approach is formulated so that parameters developed from binary ion-exchange systems can be used to represent multicomponent system behavior. Application of the Wilson model in this study demonstrated that it is not as sensitive as alternative models to uncertainties associated with data found at the extreme ends of isotherms. An extensive literature review was used to provide some parameters for the modeling effort. Where experimental data were lacking or unavailable, a correlation method for predicting equilibrium constants was developed based on a technique used to predict formation constants of hydroxo-metal complexes. The correlation method was shown to aid in constraining the range of fitted model parameters, thus helping to reduce parameter variance between experimental data sets. Predictions of experimental behavior compared well to the binary exchange experimental results and to data from the literature. Several binary ion-exchange systems were studied including Na⁺/Cs⁺, K⁺/Cs⁺, K⁺/Na⁺, and Ca²⁺/Sr²⁺. Model predictions for multiple ternary systems and one quaternary system were developed, and experiments designed to test these predictions were completed. The ternary systems studied included Na⁺/K⁺/Cs⁺, Na⁺/K⁺/Sr²⁺, and Na⁺/K⁺/Sr²⁺, while the quaternary system consisted of Na⁺/K⁺/Ca²⁺/Sr²⁺. The results indicate the Wilson model parameters derived from binary systems could be used successfully to predict ternary system behavior. Results of this research suggest that the thermodynamic modeling approach used can be applied to natural zeolite interactions in waters of complex chemistry.

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