Thermodynamics of ion-exchange of alkali metal ions on crystalline niobium(V) phosphate

Exchange isotherms for hydrogen ion/alkali metal ions (H+/Li+, H+/Na+, H+/K+, H+/Rb+, and H+/Cs+) have been studied at 20, 40, and 60 degrees C at a constant ionic strength of 0.1 on the crystalline phase of niobium(V) phosphate. The isotherms show an irregular (S-shaped) behavior without a phase transition. A selectivity reversal has been observed over the entire system of solid compositions. Plots of the selectivity coefficient (log(e) scale) versus equivalent ionic fraction of metal ion in the exchanger phase are linear for all of the systems and at all temperatures studied. The ion-exchange capacities and the maximum uptake at equilibrium is on the order of Li+ < Cs+ < Rb+ < Na+ < K+. The higher selectivity of potassium ion compared to other alkali ions has been explained in terms of steric hindrance and the stiff three-dimensional structure of the exchanger having a fixed pore size. The thermodynamic parameters for the overall and for zero loading have been worked out using the Gaines-Thomas equation.