Studies on the thermodynamics of zinc exchange in montmorillonite

The ion exchange equilibria and mechanism of Zn exchange with Na montmorillonite has been studied with the help of adsorption isotherms and thermodynamic parameters. The exchange isotherms, K and △G° values indicated a spontaneity of reaction and a higher preference of Zn for the montmorillonite surface. A stronger binding of Zn and changes in the hydration rates of Zn and Na with increase in order were suggested by enthalpy an entropy effects respectively. The surface phase activity coefficients and the excess thermodynamic functions were indicative of a nonideal heterogeneous exchange in which the mixture of ions on the montmorillonite surface was more stable and more tightly bound with significant differences in the hydration rates of the ions in the mixture with respect to the homo-ionic forms.