Study On Ion-Exchange Kinetics

The counter ions dissociate only partially from the fixed sites of ion exchangeresins. The dissociation degree of counter ions has a great impact on ion diffusionprocess in resin phase. In addition, ion exchange resins can be regarded as fractalmedia. Diffusion in fractal media does not follow classical laws, which leads to manyanomalous diffusion phenomena. In this paper, the research on ion-exchange kineticswill be carried out from two aspects — the dissociation degree of counter ions and thefractal structure of resin particles. The main content includes: Based on electrical double layer model and Donnan equilibrium theory, thephysical model of ion diffusion in gel resin was set up. With this model, therelationship between selectivity factor and dissociation degree of counter ions wasdeduced in condition of ion exchange equilibrium. The research shows: if counter ionscompletely dissociate, the ion exchange resin would not have selectivity to theexchanged ions. The dissociation degree of counter ions can be determined by conductivity.Through lower frequency measurement of conductivity of resin bed in differentconcentration electrolyte solutions, the conductivity of resin phase can be calculatedwith Hanai's complex dielectric constant equation. The dissociation degree of counterions in resin phase is obtained by the ratio of equivalent ionic conductance in resinphase to that in aqueous solution. Due to tangential migration of counter ions alongresin particles surface, Hanai's equation needs to be modified. For weak acid andweak-alkali ion exchange resins, the conductivity of resin phase can be directlymeasured through isoconductivity point. The valence of ions has great influence onthe dissociation degree of counter ions. The higher the valence of counter ions is, thelower the dissociation degree of counter ions. The dissociation degree of H+ is muchlower than those of other univalent ions. The relationship was deduced between diffusion coefficient and equivalent ionicconductance in dilute solution. Meanwhile, the calculation method of diffusioncoefficient in resin phase was analysed in detail. From three aspects it was explained that the differential equation is deficient todescribe diffusion behaviour of ions in fractal media. Three influential anomalousdiffusion equations were analyzed and compared. The result shows: the Metzler'sfraction diffusion equation not only can reduce to the ordinary diffusion equation inEuclidean space, but also its mean-square displacement and probability density satisfythe scaling relation in fractal media. The mathematical model of ions diffusion in resin phase was set up. Meanwhile,with initial and boundary conditions, the fraction diffusion equation was solved. Themodel fits strong-acid and strong- alkali ion exchange resins well in comparison withexperimental data. For weak-acid and weak-alkali ion exchange resins, thedissociation degree of counter ions is so low that the thickness of electrical doublelayer is less than micropore radius, then, the ion exchange process becomes moresensitive to the solution concentration, so the model doesn't fit well.Taking example for removing Na+ with strong-acid ion exchange resin in watertreatment process, the work behaviour of ion exchange column was simulated. Thesimulated results fit experimental data well.