Thermodynamics of exchange of zinc with Mg-bentonite and Mg-illite

The ion-exchange equilibria of Zn with Mg-bentonite and Mg-illite have been studied with the use of exchange isotherms and thermodynamic parameters. The exchange isotherms and the equilibrium constants point to a lower preference for Zn than Mg ions by both clays. Preference differed with temperature. Free energy changes were in agreement with this conclusion. Positive enthalpy and entropy changes occurred during Zn exchange with bentonite, whereas the reverse was the case with illite. These changes indicate diversity in the strength of binding and order of distribution of Zn ions on surfaces of the two clays.