472314 Influence of Fixed Charge Group Concentration on Ion Sorption in Ion Exchange Polymers
In this study, positively and negatively charged membranes having different fixed charge group concentrations but similar water content were synthesized. Ion concentrations in membranes equilibrated with NaCl solutions of varying concentration were measured using a desorption technique. Counter-ion (i.e., ion with opposite charge to that of the fixed charges) sorption increased with increasing fixed charge group concentration, since a larger number of counter-ions are required to electrically balance the fixed charge groups. Co-ion (i.e., ion with same charge as that of the fixed charges) sorption decreased with increasing fixed charge group concentration, presumably due to enhanced Donnan exclusion. However, despite similar changes in membrane fixed charge group concentrations, co-ion sorption in the negatively charged membranes was suppressed to a greater extent than in the positively charged membranes, presumably due to structural differences between the materials. The ion sorption data were interpreted using a model based on Donnan theory and Manning’s counter-ion condensation theory. The model predicted the experimental ion sorption data in the positively charged membranes with no adjustable parameters. However, agreement between model and experimental data for the negatively charged membranes was poor, presumably due to membrane inhomogeneities introduced during polymerization. Use of the Donnan/Manning model for predicting ion sorption in inhomogeneous materials is discussed.