Ion exchange studies on natural and modified zeolites and the concept of exchange site accessibility

In the present study natural and Na-rich form of clinoptilolite are examined, in respect to ion exchange of Pb²⁺, Cu²⁺, Cr³⁺, and Fe³⁺. Equilibrium and kinetic studies performed, under the same normality (0.01 N). Equilibrium studies demonstrate that Na ⁺ enrichment of clinoptilolite is beneficial in respect to metal uptake for all metals, except Cr³⁺, which is shown to have the same equilibrium behavior in both materials. Kinetic study shows that diffusion coefficients are in the range of 0.16 to 9×10^-9 cm ²/s, and are not always improved in Na-rich form of clinoptilolite. The effect of temperature on diffusion coefficients is also examined, and Arrhenius activation energy is determined to be in the range of 3.02 to 13.9 kcal/mol, for all metals and materials, except Cu²⁺, which have extremely low activation energy in the natural sample, equal to 0.04 kcal/mol.