Novel ion-exchange spacer for improving electrodialysis. II. Coated spacer

Ion-conducting spacers were prepared by applying an ion-exchange coating to commercially available polypropylene netting. Homogeneous and heterogeneous types of coating were used. Homogeneous anion-exchange coating consisted of bromomethylated and aminated polysulfone, homogeneous cation-exchange coating of sulfonated polysulfone. All heterogeneous coatings consisted of ground ion-exchange resin, embedded in crosslinked 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 spacer per unit area. The spacers suppress polarization, leading to increased current efficiency and decreased cell resistance. As expected, largest decrease of cell resistance is obtained in dilute solutions, < 3 mM. The clearest effect on efficiency was observed in ED with heterogeneous ion-exchange membranes, which are by themselves highly polarizing. Most experiments were carried out with anion-exchange spacers, minimizing the water splitting which takes place at the surface of the ion-exchange membrane. Introduction of an anion-exchange spacer near the heterogeneous anion-exchange membrane and a cation-exchange spacer near the heterogeneous cation-exchange membrane led to a dramatic increase in current efficiency.