Charge density effects in ion exchange. Part 1.—Heterovalent exchange equilibria

Abstract

A thermodynamic study is made on the effect of surface charge density on the sodium–cesium equilibrium in three clay minerals, Otay montmorillonite, hectorite and a series of Camp Berteau montmorillonite samples with reduced layer charge.It is shown that the standard free energy, enthalpy and entropy changes for the reversible displacement of sodium by cesium ions increase linearly with surface charge density. The opposite effect of charge density on cesium selectivity in zeolites and clay minerals is discussed in terms of electrostatic and hydration effects and the swelling properties of clays.