Environmental Water-Treatment Knowledge: Meaning and Function of Electrodialysis
The electrodialysis process is a combination of an electrochemical process and a dialysis diffusion process; driven by an applied direct-current electric field and utilizing the selective permeability of ion-exchange membranes (i.e., cations can pass through cation-exchange membranes and anions can pass through anion-exchange membranes), cations and anions migrate toward the anode and cathode, respectively. During ion migration, if the fixed charge of the membrane is opposite to the charge of the ion, the ion can pass; if their charges are the same, the ion is repelled, thus achieving the purposes of solution desalination, concentration, refining, or purification [1].
Compared with reverse osmosis, another membrane-separation technology introduced in recent years, electrodialysis is cheaper but has a lower desalination rate. When
the quality of domestically produced ion-exchange membranes is also very stable, and operation and management are very convenient.
The semi-permeable membrane used in electrodialysis is actually an ion-exchange membrane. According to the charge nature of the ions, this ion-exchange membrane can be divided into two types: cation-exchange membranes (cationic membranes) and anion-exchange membranes (anionic membranes). In an electrolyte aqueous solution, the cationic membrane allows cations to pass through while repelling and blocking anions, and the anionic membrane allows anions to pass through while repelling and blocking cations; this is the selective permeability of the ion-exchange membrane. During the electrodialysis process, the ion-exchange membrane does not exchange with a certain ion in the aqueous solution like an ion-exchange resin does, but only plays a selective-permeation role for ions of different electrical natures, i.e., the ion-exchange membrane does not need regeneration. The compartment composed of the electrodes and membranes in the electrodialysis process is called the electrode compartment, where the electrochemical reaction that occurs is the same as an ordinary electrode reaction. In the anode compartment an oxidation reaction occurs, the anode water becomes acidic, and the anode itself is easily corroded. In the cathode compartment a reduction reaction occurs, the cathode water becomes alkaline, and scale easily forms on the cathode [2].
Electrodialysis is a relatively mature technique among membrane-separation processes and has been widely used in brackish-water desalination, being a major method of producing fresh water in some regions of the world. Because newly developed charged membranes have higher selectivity, lower membrane resistance, better thermal stability and chemical stability, and higher mechanical strength, the electrodialysis process is not limited to desalination, but also has many other industrial applications in the food, pharmaceutical, and chemical industries, such as industrial wastewater treatment, mainly including the recovery of acids and metals from waste liquids formed by acid cleaning of metal surfaces; the recovery of heavy-metal ions from electroplating wastewater; the recovery of sulfates from synthetic-fiber wastewater; and the recovery of sulfites from pulp waste liquids. In the food industry it is used for milk desalination to produce infant milk powder; in the chemical industry it is used to separate ionic from non-ionic substances; and in clinical treatment electrodialysis can be used as an artificial kidney, etc. [3].
Automatically controlled electrodialysis with frequent polarity reversal (EDR) makes operation and management more convenient. The raw-water utilization rate can reach 80%, and the general raw-water recovery rate is between 45-70%. Electrodialysis is mainly used for primary desalination of water, with a desalination rate between 45-90%. It is widely used for desalination of seawater and brackish water; primary desalination when producing pure water; and desalination and softening of boiler and power-equipment feed water, etc.
Essentially, electrodialysis can be said to be a desalting technique, because all kinds of water (including natural water, tap water, and industrial wastewater) contain a certain amount of salts, and the cations and anions that make up these salts migrate toward electrodes of opposite directions under the action of a DC electric field. If one anion-exchange membrane and one cation-exchange membrane are inserted into an electrodialyzer, because the ion-exchange membranes are selectively permeable—that is, the cation-exchange membrane allows only cations to pass freely and the anion-exchange membrane allows only anions to pass—the salt concentration in the compartment between the two membranes will decrease due to the directional migration of ions, while the two compartments near the electrodes become concentration compartments for anions and cations, respectively, and finally the purpose of desalination is achieved in the middle desalting compartment.
In practical applications, an electrodialyzer is not composed of just one pair of anion- and cation-exchange membranes (because this is very inefficient), but uses one hundred pairs or even several hundred pairs of exchange membranes, thereby greatly improving efficiency.
At present, electrodialyzers have a wide range of applications: in water desalination and desalting, seawater concentration for salt production, dairy-product refining, fruit-juice deacidification, refinement and purification, and the production of chemical products; they can also be used in the food, light-industry, and other industries for the production of pure water, and as pre-treatment for the production of high-purity water in the electronics, pharmaceutical, and other industries. They are used for primary softening and desalting of boiler feed water, and for desalinating brackish water into drinking water.
Electrodialyzers are suitable for water-supply treatment in the electronics, pharmaceutical, chemical, thermal-power, food, beer, beverage, printing and dyeing, and coating industries. They can also be used for the concentration, purification, and separation of materials in physical and chemical processes.
Electrodialysis can also be used for the treatment of wastewater and waste liquids and the recovery of precious metals, such as the recovery of nickel from electroplating waste liquids.
(1) Operating pressure about 0.5-3.0 kg/cm2
(2) Operating voltage and current 100-250 V, 1-3 A
(3) Body power consumption about 0.2-2.0 kWh per ton of fresh water
1) It can simultaneously perform desalting, concentration, separation, and purification of electrolyte aqueous solutions;
1) Migration of ions of the same name: the selective permeability of ion-exchange membranes can never be 100%, so a small amount of counter-ions always pass through the exchange membrane;
3) In principle, an electrodialyzer is an electrolytic cell with a diaphragm, and the high efficiency of the redox reaction at the electrodes can be exploited.
IV. During the electrodialysis process, the following secondary processes also take place
2) It can be used for the purification of non-electrolytes such as sucrose, to remove the electrolytes therein;
2) Concentration-diffusion of ions: because there is a concentration difference between the solutions in the concentration compartment and the desalting compartment, a small amount of ions always diffuse and migrate from the concentration compartment to the desalting compartment, thereby reducing the dialysis efficiency;
3) Water permeation: although the exchange membrane does not allow solvent molecules to pass through, because there is a concentration difference between the desalting compartment and the concentration compartment, part of the solvent molecules (water) will permeate toward the concentration compartment;
4) Electro-osmosis of water: due to the hydration of ions and the formation of an electric double layer, under the action of a DC electric field, water molecules can also migrate from the desalting compartment to the concentration compartment;
5) Polarization ionization of water: 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 concentration compartment;
6) Pressure permeation of water: because there is a fluid-pressure difference between the concentration compartment and the desalting compartment, water molecules are forced to permeate from the side with higher pressure to the side with lower pressure. Obviously, these secondary processes are unfavorable factors for electrodialysis, but they can all be avoided or controlled by changing the operating conditions.
