Thermodynamic parameters were derived for the exchange reaction between Cu(II) and calcium on bentonite clay in systems essentially devoid of precipitation, hydroxylation, and complex formation. Although the exchange isotherms showed preference of clay for calcium this proved to be only apparent since a large component of the standard free energy change for copper adsorption was due to entropy loss. The stronger binding of copper ions to clay surfaces was revealed, however, by the standard enthalpy changes and their arrangement in a more orderly structure than calcium ions on the surface was inferred from the magnitude of standard entropy changes. The activity coefficients of the adsorbed ions were obtained and were used to test the assumptions inherent in the treatment and the significance of their variations with clay composition was discussed.
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Elsayed et al. (1970) studied this question.