Thermodynamic study of the influence of complexation on exchange equilibria in Wyoming Bentonite clay

Abstract

In Wyoming Bentonite the ion-exchange reactions for the homovalent pairs Ag(en)⁺₂–Cs⁺ and Cu(en)²⁺₂–Ca²⁺ and for the heterovalent pair Ag(en)⁺₂–Cu(en)²⁺₂ are characterized by ΔG^⊖ values of exchange which are very much larger than those for the aqueous metal ions. These effects, which emphasize the importance of ion–ligand versus ion–solvent (water) interactions, are discussed in terms of the relative contributions of the interactions occurring in the surface and bulk solution phase.

A significant decrease is found in bivalent ion selectivity with increasing loading in the Ag(en)⁺₂–Cu(en)²⁺₂ case (two orders of magnitude difference in K_c at both ends of the composition scale). This is opposite to what is commonly found in mono–divalent ion-exchange equilibria in montmorillonite. This effect is discussed in terms of heterogeneity in charge distribution in the exchanger and variations in electrostatic interaction.