The effect of conducting spacers on transport properties of ion-exchange membranes in electrodriven separation

A conventional non-conducting spacer (interpolymer of polyethylene styrene-divinylbenzene copolymer) was transformed into an ion-conducting spacer by chemical modifications. Electrochemical characterization of cation-and anion-exchange membranes when it was alone or kept in between the non-, cation- or anion-conducting spacers was carried out by recording membrane potential and current-voltage curves in contact with NaCl solution of 220 ppm concentration at different now rate of the solution. Transport parameters such as the counter-ion transport number, ionic permeability, membrane resistance and permselectivity were estimated for different experimental systems. Introduction of cation-conducting spacer near cation-exchange membrane and anion-conducting spacer near anion-exchange membrane led to a dramatic increase in counter-ion transport number and limiting current density along with the reduction in membrane resistance and thickness of diffusion boundary layer. On the basis of these studies, the position of conducting spacers (CS) in electrodialysis (ED) stack were optimized and packed. The performance of this ED stack has been tested with non-conducting and cation-conducting spacers in electrolytic solution of NaCl, NiCl2 and CuCl2 having concentration 500-1000ppm. The presence of conducting spacers not only suppress the concentration polarization but out put of the unit and its current efficiency also increases by about two times.