Electro-Deionization Water Purification Technology—a Practical Analytical Method for Reaction Superposition

Based on the extensive experiments and mechanistic analyses of electro-deionization conducted by Huang Yipu et al., this paper adopts an analytical method that first decomposes the electro-deionization process into individual component reactions and then superimposes them for synthesis. According to the order and location of each component reaction, the primary and secondary roles of these reactions in a specific application are determined, and their application analysis focuses on ion exchange. The key points of this practical analytical method are described as follows:

1) The electro-deionization process is decomposed into electrodialysis and ion exchange processes, which are independent of each other and each governed by its inherent laws. Although both remove ions from water, electrodialysis plays the actual role in removing ions, while ion exchange merely serves as an intermediate step in the removal process.

2) Ion exchange resin retains ions, inhibiting electrodialysis and allowing ion exchange to proceed; resin desorbs ions, inhibiting ion exchange and allowing electrodialysis to proceed. These two points can be illustrated as follows:

Electro-deionization resin Ion-retaining resin Desorbed ions Electrodialysis ↓ + Ion exchange ↑ Electrodialysis ↑ + Ion exchange ↓ 3) The ion migration rate during electrodialysis is determined by the ion's migration rate in the aqueous solution and the membrane. The magnitude of the migration rates of various ions determines the ion concentration distribution spectrum as they migrate from the desalination chamber to the concentrate chamber. Under the influence of a DC electric field, the direction of ion migration during electrodialysis is perpendicular to the direction of ion movement carried by the water flow. Therefore, in the desalination chamber, the concentration distribution spectra of anions and cations are respectively biased to the sides.

4) Concentration polarization during electrodialysis leads to water ionization, which promotes resin desorption. The location of concentration polarization occurs randomly at the interface between the aqueous solution and the resin particles or membrane. H+ and OH- ions generated by water ionization at the resin particle surface interface layer can promptly regenerate nearby exhausted resin. One type of ion (H+ or OH-) generated by water ionization at the membrane surface interface layer simply passes through the membrane into the concentrate chamber, acting as a charge carrier and not participating in regeneration; the other type of ion (OH- or H+) migrates laterally and participates in regeneration. The original ion electrodialysis concentration profile is disrupted by this randomly generated water ionization-induced resin desorption, and the phenomenon of ions being repeatedly desorbed and re-adsorbed by the resin can occur.

5) Ion exchange reactions are extremely fast, much faster than the migration rate in electrodialysis. Therefore, the ion exchange process is controlled by diffusion factors. Simultaneously, ions are carried by the water flow, which continuously washes away the resin particles. Most ions in the water are adsorbed and retained by the resin before migrating out of the desalination chamber via electrodialysis, and then gradually desorbed and migrate out of the desalination chamber via electrodialysis. Thus, in the electro-deionization process, the resin is an intermediate in ion transport.

6) Ion exchange in the electro-deionization process should follow the usual distribution rules of the ion exchange chromatography within the column: During ion exchange, for a certain adsorbed ion, the ion exchange layer can be divided into an inactive layer, a working layer, and a protective layer; the selectivity order of each ion's chromatography layer and the order of replacement are determined by their affinity for the resin. The selectivity order for strongly acidic cation exchange resins is:

Fe3+>Ca2+>Mg2+>K+>Na+>H+

The selectivity order for strongly basic anion exchange resins is: SO₄⁻ > NO₃⁻ > Cl⁻ > OH⁻ > HCO₃⁻ > HSiO₃⁻ ion exchange chromatography is the basis for determining the degree of electro-deionization of treated water. The greater the flow rate of the freshwater chamber, the lower the diffusion rate of ions, and the deeper the chromatography depth. The flow rate of the freshwater chamber depends on the inlet and outlet pressure difference and flow resistance. 7) When describing the electro-deionization process, electrodialysis and ion exchange should be organically combined for analysis. Based on the order and location of each component reaction, the primary and secondary roles of each reaction should be determined. Sometimes some reactions of electrodialysis are dominant, and sometimes some reactions of ion exchange are dominant. Finally, they should be superimposed for comprehensive analysis.