Effect of Arrangement of Ion Exchange Material on Desalting Performance in Electrodeionization System

The relationship between the arrangements of an ion exchange fabric packed in diluting compartment and the behavior of ion migration and water dissociation in an electrodeionization system (EDI) was investigated. The arrangement of an anion-exchange membrane (AM) / an anion-exchange fabric (AF) / a cation-exchange fabric (CF) / a cation-exchange membrane (CM) significantly enhanced the ion removal efficiency compared to the arrangement including a non-woven fabric made with both cation- and anion-exchange fibers (AM/AFCF/CM). The enhancement of ion removal efficiency was indicated also by current-voltage curves. The difference in the rate of ion removal between various salts was small in the arrangement AM/AF/CF/CM compared with that in the arrangement AM/AFCF/CM, which demonstrates an advantage for the production of ultrapure water by means of the EDI system. The dissociation of water in the arrangement AM/AF/CF/CM was found to be promoted at the contact interface between the ion exchange fabrics with different polarity each other, whereas the water dissociation in other arrangements takes place at the interface of the ion exchange membrane and the ion exchange fabric. The formation of H+ and OH- ions due to the water dissociation in the interior of the diluting compartment should regenerate efficiently the ion exchanger packed in the diluting compartment, a fact facilitating the deionization in the arrangement AM/AF/CF/CM.