AN ANOMALOUS EFFECT IN KINETICS OF POLYDISPERSE ION-EXCHANGERS

The kinetics of ion exchange involving simultaneously ion-exchanger particles of different diameters was investigated in well-stirred, dilute solutions, where film diffusion is the rate-determining step. A special situation arises, if sufficient ion exchanger is added to a solution to remove a given counterion species almost completely. In this case the concentration of the ions in solution still attains its equilibrium in a time as about expected from the weighted mean of the rate of coefficients of the individual size fractions of the ion exchanger. In the same system, however, the ion-exchanger particles approach their equilibrium more slowly by several orders of magnitude. The kinetic behavior of such mixtures can, therefore, not be obtained by measuring only the concentration changes in the solution phase. The above effect is demonstrated by investigating the isotopic exchange Cs+/Cs+ and the ion exchange Cs+/H+ in 10(-4)-10(-3) M solutions (50 mL each) in the presence of about 100 mg of cation exchanger, sieved to contain only particles with 0.4 and 0.8-mm diameter. Rate equations that describe quantitatively the experimental observations are given. If the particles exhibit different ion-exchange properties (ion-exchange capacity, separation factor), the above effect should be observable also for monodisperse mixtures. The presence of anomalous kinetics has to considered also when the sorption of trace ions (metal ions, radionuclides) by soils or soil components is investigated.