Effect of Film Diffusion on the Ion-Exchange Kinetics of a Tracer Ion in Nafion-117 Membrane from a Mixture of Salt Solution

The ion-exchange kinetics of a tracer ion (Cs⁺ and Ba²⁺) in presence of a bulk ion (Na⁺/H⁺) has been measured in Nafion-117 membrane for a range of concentrations of NaCl/HNO₃ using the nonstationary radiotracer method. A systematic increase in the ion-exchange rate and decrease in the partition coefficients of the tracer ions between membrane and solution have been observed with the increase in bulk ion concentration. The sigmoidal nature of experimental profiles indicates film-diffusion-controlled kinetics even for well-stirred solutions. In the absence of an existing analytical or numerical solution, a simple empirical approach has been proposed to find the variable membrane surface concentration and has been used to solve the membrane diffusion equation by the finite difference method. The fitting of the experimental curves with a single diffusion coefficient for Cs⁺/Ba²⁺ has been achieved. The exchange rate has been found to be independent of the stirring speed beyond a limiting speed.