Ion exchange in beryllophosphate-G. Part 2.—Ion-exchange kinetics

Abstract

Ion-exchange kinetics plots and diffusion coefficients are presented for beryllophosphate-G, the synthetic beryllophosphate analogue of the zeolite gismondine. Exchanges of K⁺ or NH⁺₄ for Na⁺ were relatively slow, with correspondingly small diffusion coefficients. Reverse exchange of Na⁺ for K⁺ was 200 times more rapid than the forward reaction. This can be explained in part by the unit-cell expansion previously observed on exchange of K⁺ for Na⁺. Exchange of Ca²⁺ for Na⁺ was found to be slower than that of Mg²⁺ for Na⁺, in contrast to the behaviour usually observed for aluminosilicate zeolites. This result is thought to arise from the relatively high electrostatic field of the beryllophosphate framework which enhances the interaction between the high charge density of Mg²⁺ and the framework relative to that of the larger Ca²⁺ ion. In zeolites, lower electrostatic field strengths render these interactions less important and diffusion of the hydrated cations through the framework dominates the exchange processes.