Thermodynamics of the ion exchange of Ca2+ and Sr2+ cations on Na-form of natural clinoptilolite

Abstract

The adsorption and direct calorimetric studies of the binary exchange of Ca²⁺ and Sr²⁺ cations on the Na-form of low-silica type natural clinoptilolite (Sokyrnytsya, Ukraine) were performed. The ion exchange of Ca²⁺ and Sr²⁺ cations on the Na-form of natural clinoptilolite was studied under static conditions at a solid to liquid phase ratio of 1 : 100 and a constant ionic strength of the solution of 0.1. The integral heats of ion exchange for these systems were measured using a highly sensitive Tian-Calve microcalorimeter in a special lab-made cell. Also, the ion exchange isotherms and Kielland curves were plotted, and the corrected selectivity coefficients and ion exchange constants were calculated. Changes in the integrated Gibbs free energies, enthalpies, and entropies of the ion exchange of Ca²⁺ and Sr²⁺ cations on the Na-form of clinoptilolite were calculated in entire range of the substitutions of the exchange complex. The thermodynamic parameters of incomplete ion exchange were analyzed in terms of the crystal structure of the zeolite. In addition, the state of exchangeable cations in the channels of clinoptilolite was described in detail. It is shown that structural heterogeneities of cation-substituted forms of clinoptilolite are clearly reflected in the dependence of thermodynamic functions on the degree of exchange, and significant differences are observed between the exchange of Ca²⁺ and Sr²⁺ cations that could not be described by standard thermodynamic values. Although the isotherms of ion exchange have a convex shape, the Na-form of clinoptilolite does not show any thermodynamic affinity to Ca²⁺ cations. This ion exchange reaction is accompanied by positive enthalpy changes in all range of substitutions of the exchange complex. At the same time, thermodynamic selectivity toward Sr²⁺ cations was observed for 40 % of the zeolite exchange centers, and the exchange is accompanied by minor exothermic effects in the range of small substitutions, and the ion exchange isotherm has σ-like form. In general, the thermodynamic affinity of the Na-form of clinoptilolite to the studied metal ions under conditions of incomplete exchange is ordered as following hydration energy trends in the case of the lyotropic properties among alkaline earth metals. Thus, the experimental thermodynamic characteristics of ion exchange equilibria could be a reliable support for the practical using of natural clinoptilolite.