Transport properties of monovalent-ion-permselective membranes

This paper analyses the separation properties of monovalent-ion-permselective membranes, manufactured and commercialised for several electrodialysis applications (e.g. edible salt production from sea water, recovery of valuable chemicals from industrial waste waters). Experimental data from four separation systems (SO42−Cl−, C2O42−Cl−, Ca2+Na+ and Mg2+Na+), obtained by use of either monovalent-anion- or monovalent-cation-perm-selective membranes (Neosepta ACS or CMS by Tokuyama Soda Co.), are interpreted on the basis of a theoretical set-up based on both thermodynamic and kinetic concepts. Particularly, three models are proposed: the classical solution-diffusion model, a kinetic model (accounting for the dependency of the separation factor on the applied current density) and a combined model (accounting for both equilibrium- and kinetics-based transport mechanisms). The first model turned out to interpret statisfactorily the separation properties of CMS membranes (Ca2+Na+ and Mg2+Na+ separations), whereas for the C2O42−Cl− and SO42−Cl− separations much better results were obtained with the kinetics and the combined model, respectively. The generally good adequacy of the proposed models to data interpretation (standard deviations always lower than 0.15) allows to regard them as good tools for process design purposes.