SODIUM ISOTOPIC EXCHANGE RATE BETWEEN DISODIUM ZIRCONIUM(IV) BIS(ORTHOPHOSPHATE) TRIHYDRATE AND AQUEOUS SOLUTIONS

The sodium isotopic exchange rate between the large crystals of disodium zirconium(IV) bis(orthophosphate) trihydrate and aqueous solutions was determined radiochemically. The exchange proceeded quickly up to 5% of the total, followed by a slow process. The first step was attributed to the instantaneous attainment of the equilibrium between the imperfect surface layers of the crystals and the solution, and the second step was found to be controlled by the diffusion of sodium ions along the direction perpendicular to the layers consisting of zirconium atoms. The diffusion constant, D, was calculated with the equation obtained by Fick’s diffusion law under the conditions of slab geometry and infinite solution volume. From the dependence of D on temperature, the activation energy for the diffusion was evaluated to be 101 kJ mol−1.