Alkaline earth cation exchange with novel Na-3-mica: Kinetics and thermodynamic selectivities

The cation-exchange kinetics and thermodynamic equilibria for alkaline-earth metals by Na-3-mica [chemical composition of Na_2.94(Mg_5.55Ti_0.07Fe_0.04)_oct(Al_2.07Si_5.83)_tetO₂₀F₄·1.72H₂O] with a high layer charge were investigated at room temperature. The Ba, Sr, Ca and Mg uptake kinetics were determined and first-order, modified Freundlich, parabolic diffusion and Elovich kinetic models were examined for the experimental data. The first-order model gave a relatively good fit throughout for these cations and indicated a kinetic selectivity order of Ca²⁺ < Mg²⁺ < Sr²⁺ < Ba²⁺. The cation exchange isotherms for Ba, Sr, Ca and Mg uptakes with the Na-3-mica were also determined. The Kielland plots evaluated from the cation exchange isotherms showed a thermodynamic selectivity order of Ba²⁺> Sr²⁺> Ca²⁺ > Mg²⁺. The above basic studies on the selective cation exchange of the highly charged ion exchanger are of relevance in the separation, immobilization and disposal of ⁹⁰Sr from aqueous nuclear wastes.