Ion exchange equilibrium between cation exchange membranes and aqueous solutions of K⁺/Na⁺, K⁺/Ca²⁺, and Na⁺/Ca²⁺

Abstract

Equilibrium between synthetic ion exchangers and solutions of cations has been the subject of this investigation. Competitive ion exchange reactions were studied for two cation exchange membranes (CMX and CRP) involving K⁺, Na⁺, and Ca²⁺ ions. The ionic strength of the equilibrating solutions was maintained constant, but the molar fraction varied; all experiments were conduced with nitrate as nonexchanging anions at 25 °C. Adsorption isotherm for the three binaries systems: K⁺/Na⁺, K⁺/Ca²⁺, and Na⁺/Ca²⁺ were studied. The obtained results show that potassium was the most strongly sorbed and the selectivity order for CMX and CRP membranes is K⁺>Ca²⁺>Na⁺ at 0.1 M, under the experimental conditions. Selectivity coefficients ${\text{K}}_{{\text{K}}^{\text{ + }} }^{{\text{Na}}^{\text{ + }} }$, ${\text{K}}_{{\text{2}}\,{\text{Na}}^{\text{ + }} }^{{\text{Ca}}^{{\text{2 + }}} }$, and ${\text{K}}_{{\text{2}}\,{\text{K}}^{\text{ + }} }^{{\text{Ca}}^{{\text{2 + }}} }$ for the three binaries and for the two membranes were determined at an ionic strength of 0.1 M and at a constant temperature of 25 °C. We remark that all the selectivity coefficient values are quite different from the unit. Ternary equilibrium was taken for the Ca²⁺/K⁺/Na⁺ system. It was found that binary selectivity data could be successfully used to predict the ternary ion exchange equilibrium.