NOVEL ION-EXCHANGE SPACER FOR IMPROVING ELECTRODIALYSIS - II - COATEDSPACER

Ion-conducting spacers were prepared by applying an ion-exchange coati ng to commercially available polypropylene netting. Homogeneous and he terogeneous types of coating were used. Homogeneous anion-exchange coa ting consisted of bromomethylated and aminated polysulfone, homogeneou s cation-exchange coating of sulfonated polysulfone. All heterogeneous coatings consisted of ground ion-exchange resin, embedded in crosslin ked poly(vinyl alcohol). All the coated spacers increased the rate of desalting of sodium chloride solutions, at concentrations of 20 mM or less. The effect increased with the ion-exchange capacity of the space r per unit area. The spacers suppress polarization, leading to increas ed current efficiency and decreased cell resistance. As expected, larg est decrease of cell resistance is obtained in dilute solutions, <3 mM . The dearest effect on efficiency was observed in ED with heterogeneo us ion-exchange membranes, which are by themselves highly polarizing. Most experiments were carried out with anion-exchange spacers, minimiz ing the water splitting which takes place at the surface of the ion-ex change membrane. Introduction of an anion-exchange spacer near the het erogeneous anion-exchange membrane and a cation-exchange spacer near t he heterogeneous cation-exchange membrane led to a dramatic increase i n current efficiency. (C) 1998 Published by Elsevier Science B.V.