Electrodialysis

I. Principle
The semi-permeable membrane used in electrodialysis is actually an ion-exchange membrane. This type of ion-exchange membrane can be divided into cation-exchange membranes (cation membranes) and anion-exchange membranes (anion membranes) based on the charge properties of ions. In aqueous electrolyte solutions, cation membranes allow cations to pass through while rejecting and blocking anions, and anion membranes allow anions to pass through while rejecting and blocking cations. This is the selective permeability of ion-exchange membranes. During the process of electrodialysis, the ion-exchange membrane does not exchange with specific ions in the aqueous solution like ion-exchange resins; instead, it only selectively permits the passage of ions of different charges, meaning that the ion-exchange membrane does not require regeneration. The compartments composed of electrodes and membranes in the electrodialysis process are called electrode chambers, where the electrochemical reactions occurring are the same as those in ordinary electrode reactions. Oxidation reactions take place in the anode chamber, making the anode water acidic and the anode itself susceptible to corrosion. Reduction reactions occur in the cathode chamber, making the cathode water alkaline and prone to scaling on the cathode.

II. Application Scope

Currently, electrodialyzers have a wide range of applications. They are used in water desalination and purification, seawater concentration for salt production, refined dairy products manufacturing, fruit juice deacidification and purification, as well as in the production of chemical products. They can also be utilized in the food and light industries for the production of pure water, and in the electronics and pharmaceutical industries for pretreatment in the production of high-purity water. Primary softening and desalination of boiler feedwater can transform brackish water into drinking water.

Electrodialyzers are suitable for water treatment in industries such as electronics, pharmaceuticals, chemicals, thermal power generation, food processing, beer production, beverages, printing and dyeing, as well as coating. They can also be employed in physicochemical processes such as material concentration, purification, and separation. Electrodialysis can further be used for the treatment of wastewater and waste liquids, as well as for the recovery of precious metals, such as nickel from electroplating waste liquids.

III. Basic performance

(1) Operating pressure：about 0.5-3.0kg/cm²

(2) Operating voltage and current：100-250V，1-3A

(3) Power consumption of the body：about 0.2-2.0 kWh per ton of fresh water

Ⅳ.Characteristics of the Method

1.Electrodialysis can simultaneously desalinate, concentrate, separate, and purify electrolyte aqueous solutions.

2.It can be used for the purification of non-electrolytes such as sucrose to remove electrolytes from them.

3.In principle, an electrodialyzer is an electrolytic cell with membranes, which can utilize the high efficiency of redox reactions on the electrodes.

Ⅴ.Secondary Processes in Electrodialysis

During the electrodialysis process, the following secondary processes also occur：

1.Migration of Homologous Ions：The selective permeability of ion-exchange membranes is never 100%, so there will always be a small amount of counter-ions passing through the exchange membrane.

2.Concentration Diffusion of Ions：Due to the concentration difference between the concentrated and diluted solutions, a small amount of ions will diffuse from the concentrated chamber to the diluted chamber, thereby reducing the dialysis efficiency.

3.Water Osmosis：Although the exchange membrane does not allow solvent molecules to pass through, the concentration difference between the diluted and concentrated chambers causes some solvent molecules (water) to permeate into the concentrated chamber.

4.Water Electrodialysis：Due to ion hydration and the formation of an electric double layer, water molecules can also migrate from the diluted chamber to the concentrated chamber under the influence of a direct current electric field.

5. Water Polarization Ionization：Sometimes, due to poor working conditions, water is forced to ionize into hydrogen ions and hydroxide ions, which can pass through the exchange membrane into the concentrated chamber.

6.Water Pressure Osmosis：Due to the difference in fluid pressure between the concentrated and diluted chambers, water molecules are forced 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 altering the operating conditions.

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