Monovalent/divalent selectivity and the charge separation concept

This article establishes the importance of the active site spacing in the resin matrix on the relative selectivity for monovalent and divalent ions. The selectivity trends of dicarboxylate/chloride exchange on trialkylamine resins provide experimental verification of the hypothesis that resins with closely spaced functionalities have an inherent preference for divalent ions over monovalent ions. For a typical trimethylamine resin, the isotherm data show a monotonic decrease in selectivity with chain length whereas for the triethyl and tributylamine resins, an increase followed by a decrease in selectivity is observed. A theoretical analysis of ion exchange equilibria is developed to quantify the hydrophobic and electrostatic components of the overall dianion selectivity. A phenomenological model is used to estimate the site spacing distance for each resin and the results suggest a nonhomogeneous distribution of ionogenic groups in the gel phase. The charge separation concept, which is applicable to both anion and cation exchangers, is validated with supportive evidence from other research studies.