Affinity of potassium-form cation exchange resin for alkaline earth and transition metals

This research conducted bench-scale batch equilibrium tests to generate new selectivity data for the alkaline earth metals Ca²⁺, Sr²⁺, and Ba²⁺ and the transition metals Cd²⁺ and Co²⁺ for binary cation exchange involving sulfonic acid, polystyrene, gel and macroporous resins using the potassium (K⁺) mobile counterion. Calculations for separation factor, and Langmuir and Freundlich isotherm models were used to investigate the sorption behavior of metals onto the cation exchangers. The separation factor and Langmuir isotherm were used to determine the selectivity sequence of cations for the resins. It was determined that the K-form cation exchangers were more selective for alkaline earth metals than the corresponding sodium (Na⁺) form resin, and the gel resin was more selective than the macroporous resin for all metals. Using the separation factor to determine the selectivity sequence for K-form cation exchange resin showed similar order as published selectivity sequences for Li- and Na-form resins. The hydrated radius and ionic radius of the metals had a strong influence on the selectivity sequence for both types of cation exchange resins. Smaller hydrated radius resulted in an increase in the affinity of the resin for contaminants. Overall, K-form cation exchange resin has the potential to replace Na-form resin, as indicated by higher selectivity and comparable selectivity sequence for common metal contaminants.