Mass Transfer and Kinetics of Ion Exchange

While ion-exchange processes were originally used for the treatment of very dilute solutions, many applications for the treatment of concentrated solu- tions have been developed in recent years. In these situations, the mass- transfer bottlenecks are located in the~, rather than the liquid phase. Therefore, the development of quantitative models for ion-exchange kinetics requires knowledge about the conductance characteristics of ions and solvent in the solid phase. A useful approach towards this aim is the study of trans- port characteristics of these species, and of their interactions in solid ion- exchange membranes. Many different transport processes and related phenomena can be observed in membrane-solution systems, e.g., ion migration, electroosmosis, diffusion arid self-diffusion, osmosis, hydraulic flow, hyperfiltration (reverse osmosis) or ultrafiltration, streaming potential and streaming current, and membrane potentials (also called "e;membrane concentration potentials"e;). It is important to correlate all these phenomena so as to avoid a very large number of unnec- essary measurements. Such correlation is often possible [Meares, 1976] since all these phenomena are determined by the ease of migration of the different species across the membrane. Important correlations have been made and summar- ized even before high-capacity ion-exchange membranes became commercially available [Sollner, 1950, 197iJ.