Hofmeister Effects on Cation Exchange Equilibrium: Quantification of Ion Exchange Selectivity

The Hofmeister or specific ion effects are not involved in the existing ion exchange equilibrium theories. In this study, a new cation exchange model considering the specific ion effects was established. The relative adsorption ability was quantified through relative adsorption energy or selectivity coefficients of ions. The quantificational sequence of relative adsorption ratio ((relative adsorption energe for i ion)/(relative adsorption energy for j ion)) calculated from the new model was obtained: Ca > Mg ((Ca2+)/(Mg2+) = 1.407) > K ((Mg2+)/K2+ = 1.467) > Na (K+/(Na2+) = 1.646) > Li ((Na2+)/(Li2+) = 1.110). It provided a new theory for quantificational description of ionic exchange adsorption selectivity for any ion pairs. The experimental data on various materials (montmorillonite, Illite, and Altamont soil) with very different charge densities agree well with each other. Thus we presume that the new model may apply to all charged surfaces.