Experimental and Modeling Study of Multicomponent Ion Exchange Equilibria on Zeolite Minerals, 20-9211Printer Friendly Version
Inclusive Dates: 10/01/2000 - Current
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 has the objectives of 1) generating well-constrained zeolite ion exchange experimental data at 25 °C, and 2) 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 will be generated through experiments that involve reacting a zeolite mineral with aqueous mixtures of cations of interest. The experimental data will then be used to derive parameters for thermodynamic models of ion exchange and to check the predictive capability of those models. The project is comprised of three tasks. In general, the tasks progress 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 is 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 has also demonstrated that it is not as sensitive as alternative models to uncertainties associated with data found near the extreme ends of isotherms. An extensive literature review was used to develop parameters for the modeling effort. Where experimental data were lacking or unavailable, a correlation method for predicting equilibrium constants was developed based on a method used to predict formation constants of hydroxo-metal complexes. The correlation method was shown to aid in the constraint of the range of fitted model parameters, thus helping to reduce parameter variance between experimental data sets. Predictions of experimental behavior compare well to the Task 1 binary exchange experimental results and to data from the literature. Binary ion exchange systems studied to date include Na+/Cs+, K+/Cs+, and K+/Na+. Model predictions for a ternary system consisting of univalent cations have been developed, and experiments designed to test these predictions have been designed.