ON THE IMPORTANCE OF DIFFUSIVE LAYERS IN ION-EXCHANGE KINETICS

Based on NMR imaging results of the spatial and temporal development o f ion exchange processes in alginate gels, the question of appropriate modelling of these processes is discussed. For rare earth and actinoi d ions, the behaviour is found to correspond qualitatively to the expe ctations of Stefan's model (shrinking core model, SCM). However, quant itative correspondence of the experimental data with the model can onl y be achieved when an additional diffusive layer at the surface of the ion exchange material is assumed. An extension of the SCM with respec t to this problem is derived.