Thin Cell EDI
The first commercial EDI devices were thin cell with mixed bed ion exchange resin in the diluting cells. Although they have been modified over the years to improve performance, the basic principles have remained constant and the technology has proven to be effective and reliable.
Thin cell, mixed bed electrodeionization devices require a much greater area of ion exchange membrane per unit volume of water processed, and are therefore not as cost effective as thick cell devices. The following discussion will attempt to explain the subtle difference in removal mechanism between the two.
In thin cell, mixed bed EDI, two distinct zones are created inside the diluting compartments. Strongly ionized substances are removed first and then weakly ionized substances are removed as the water continues down through the flow path. We refer to these zones as enhanced transport and electro regeneration respectively.
In the production of ultrapure water, the feed to a EDI device is pretreated with reverse osmosis. This water contains low amounts of dissolved, ionized solids and some weakly ionized substances such as carbon dioxide and silica. Because of the low load, the device is able to remove most of the strongly ionized substances in the enhanced transport zone. Here, the ion exchange resin simply acts as a conductor to speed the passage of ions from the dilute compartment through the respective membrane, and into the concentrating compartment. This is because the ion exchange resin is several orders of magnitude more conductive than the water. This is shown in the top portion of the resin bed in Figure 3.
After most of the strongly ionized substances have been removed at the top of the cell, conductance of the diluting cell is maintained by the ion exchange resins. At locations where the minimum thermodynamic overvoltage for water splitting is applied, the concentrations of hydrogen (H+) and hydroxyl (OH-) ions are increased. This is shown in the figure in the electro-regeneration zone. The water decomposition reaction is catalyzed by conditions at the resin/resin or membrane/resin interfaces of dissimilar polarities. Here, liberated H+ and OH- ions convert the resins into the regenerated state where weakly ionized substances can react, become ionized and be moved into the concentrating stream.
Thick Cell EDI
Thick cell EDI devices arrived commercially in 1996, and several different types are now available. Besides dilute cell thickness, another thing that differentiates these devices from thin cell EDI is the fact that the dilute cells can use separate resins or a combination of separate resins and mixed bed resins.
Thick diluting channels can be a detriment to performance using mixed bed resin filler due to a lower chance of obtaining a continuous path between membranes. Water splitting can still occur at dissimilar resin/resin and resin/membrane interfaces but much of the split H+ and OH- ions will recombine when they encounter the counter ions traveling in the opposite direction. The basis for ion removal is different in devices that use separate resins in the dilute cells. Because a single type of resin is present at any given point between membranes, the transfer of co-ions is not possible. Therefore, water must decompose to provide H+ and OH- ions for transfer to take place while maintaining electrical neutrality. Therefore, current passage and water splitting are critical for both weak and strong ion removal.