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Thermodynamics of Ion-Exchange Between Na+/Sr2+ Solutions and the Zeolite Mineral Cinoptilolite

Published online by Cambridge University Press:  25 February 2011

Roberto T. Pabalan  and
F. Paul Bertetti
Roberto T. Pabalan
Affiliation:
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute 6220 Culebra Road, San Antonio, TX 78238-5166
F. Paul Bertetti
Affiliation:
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute 6220 Culebra Road, San Antonio, TX 78238-5166
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Abstract

Ion-exchange experiments were conducted at 25 °C between the zeolite mineral clinoptilolite and aqueous solutions of varying equivalent ratios of Na+ and Sr2+ and total concentrations of 0.005, 0.05, and 0.5 N. The experiments were designed to investigate the effects of changes in total solution concentration and in the relative concentrations of exchangeable cations on the following ion-exchange equilibrium:

Sr2+ + 2NaZ ⇄ SrZ2 + 2Na+

Using the isotherm data at 0.05 N solution concentration, a thermodynamic model for the ion-exchange reaction was derived using a Margules formulation for the activity coefficients of zeolite components and the Pitzer ion-interaction approach for activity coefficients of aqueous ions. The results of the forward experiments showed that the ion-exchange isotherm strongly depends on the total solution concentration. Additional experiments demonstrated that the above ion-exchange reaction is reversible. The derived equilibrium constant, K, and Gibbs energy of ion-exchange, ΔG°, are equal to 0.321±0.021 and 2,820±170 J/mol, respectively.

Using thermodynamic parameters derived from the 0.05 N isotherm experiment, the model was used to predict isotherm values at 0.005 and 0.5 N, which showed excellent agreement with measured data. Because the thermodynamic model used in this study can be easily extended to ternary and more complicated mixtures, it may be useful for modeling ion-exchange equilibria in multicomponent geochemical systems.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 333: Symposium V – Scientific Basis for Nuclear Waste Management XVII , 1993 , 731
Copyright
Copyright © Materials Research Society 1994

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