Exchange Kinetics in Heterogeneous Systems

GSF-Forscbungszentrum fur Utnwelt und Gesundbeit, InsHtut fur Strablenscbutz, Neuberberg, Germany 

A detailed kno edge of the kinetics of ion exchange is important not only for economic employment of ion exchange materials in the laboratory and in industry but also for a better understanding of these processes in natural systems, as e.g., in the soil or in biological membranes. For this reason numerous theoretical and experimental investigations on this subject are available. For excellent review articles see e.g., [1-10]. 

In this presentation we, therefore, investigate the kinetics of ion exchange in such mixtures for the case vdiere diffusion of the ions across a hydrostatic boundary layer (Nemst film) surrounding the particles is the rate controlling step (film diffusion). In well-stirred systems, liquid-phase mass transfer will usually be fevored by a low concentration of the external solution, a high ion-exchange capacity, and a small particle size [I]- 

Several methods are available to ascertain whether film diffusion is rate determining for a given system (e.g., interruption test, determination of the Helfferich number) but will not be discussed here in more detail. 

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