EDI Process

• Anion exchange membrane;

• Cation exchange membrane;

• Anion exchange resin;

• Cation exchange resin;

• Concentrate chamber;

• Desalinate chamber

Furthermore, when the desalinate chamber is filled with ion exchange resin, the liquid flow velocity in the desalinate chamber is much higher than in a conventional electrodialysis unit. The exchange resin also acts as a stirrer, promoting ion diffusion and improving the hydraulic state, thus increasing the conductivity of the desalinate chamber system and correspondingly raising the limiting current density. When the operating current of a packed bed electrodialysis unit exceeds the limiting current, the interface layer near the membrane and resin becomes polarized, causing water to dissociate and produce OH- and H+ ions. Most of these ions, except for some that migrate to the concentrate chamber, will regenerate the anion and cation exchange resins in the desalinate chamber, maintaining their exchange capacity. Simultaneously, the hydrolysis of the exchange resin will partially electrochemically regenerate it.

The packed bed electrodialysis desalination process involves three main processes simultaneously: 1) Electrodialysis, where electrolyte ions in water selectively migrate through ion exchange membranes under the influence of an external electric field; 2) Ion exchange between OH- and H+ ions on the mixed anion and cation exchangers and electrolyte ions in the water (thus accelerating the removal of ions from the desalination chamber); 3) Electrochemical regeneration of the exchanger through the hydrolysis of the exchanger itself and the H+ and OH- ions generated during electrodialysis polarization. The first two processes improve the effluent quality, while the final regeneration process deteriorates the water quality due to the regeneration reaction. However, this regeneration process is essential for the long-term, uninterrupted operation of the packed bed electrodialysis unit. Therefore, by selecting appropriate operating conditions, high-quality pure water can be obtained while simultaneously achieving the self-regeneration of the exchanger.

The operational practice of preparing ultrapure water using packed-bed electrodialysis [6] also shows that the process has two states: when the salt concentration of the water to be demineralized is high, the resin in the desalination chamber is basic; while when the salt concentration is low, the resin will electrochemically convert to hydrogen and hydroxide forms. In this way, electrodialysis and ion exchange are organically and intricately combined, and the reactions and processes that occur together constitute the entire electro-deionization process. Ion exchange enables deep desalination, overcoming the incomplete desalination caused by polarization in electrodialysis; while electrodialysis polarization generates H+ and OH- ions through water ionization, achieving resin self-regeneration and overcoming the drawback of requiring chemical regeneration after resin failure. Thus, the electro-deionization process reaches a near-perfect state. This method is suitable for desalination of water with low salinity, as it can essentially remove all ions from water. Therefore, it has broad prospects for the preparation of ultrapure water, pure water, softened water, and the treatment of radioactive wastewater. Why does EDI have such wide applicability? Below, a practical analytical method of reaction superposition is proposed to illustrate this problem and explain some application examples.