Primary and Secondary Processes in Electrodialysis

I. Basic Performance

(1) Operating Pressure: Approximately 0.5–3.0 kg/cm²

(2) Operating Voltage and Current: 100–250V, 1–3A

(3) Power Consumption: Approximately 0.2–2.0 kWh per ton of fresh water

II. During electrodialysis, the following secondary processes also occur:

① Migration of like ions: The selective permeability of ion exchange membranes is often not 100%, therefore a small amount of opposite ions will always permeate the membrane;

② Concentration diffusion of ions: Due to the concentration difference between the solutions in the concentration chamber and the desalination chamber, a small amount of ions will always diffuse from the concentration chamber to the desalination chamber, thus reducing dialysis efficiency;

③ Water permeation: Although the exchange membrane does not allow solvent molecules to permeate, the concentration difference between the desalination chamber and the concentration chamber allows some solvent molecules (water) to permeate into the concentration chamber;

④ In electrodialysis, due to ion hydration and the formation of an electrical double layer, water molecules can migrate from the desalination chamber to the concentration chamber under the influence of a DC electric field.

⑤ In polarization ionization of water, sometimes unfavorable operating conditions force water to ionize into hydrogen ions and hydroxide ions, which can permeate through the exchange membrane into the concentration chamber.

⑥ In pressure osmosis of water, the pressure difference between the concentration and desalination chambers forces water molecules to permeate from the side with higher pressure to the side with lower pressure. Clearly, these secondary processes are detrimental to electrodialysis, but they can be avoided or controlled by changing the operating conditions.