What Is Electrodialysis? A Professional Glossary of Wastewater Treatment Terms
The electrodialysis process is a combination of an electrochemical process and a dialysis-diffusion process. Driven by an applied DC electric field and using the selective permeability of ion-exchange membranes (i.e., cations pass through cation-exchange membranes and anions pass through anion-exchange membranes), cations and anions move toward the anode and cathode respectively. During ion migration, if the fixed charge of the membrane is opposite to the ion's charge, 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
domestic ion-exchange membranes are of stable quality, operation and management are also very convenient.
The semi-permeable membrane used in electrodialysis is in fact an ion-exchange membrane. According to the charge nature of the ions, this ion-exchange membrane is divided into two types: cation-exchange membrane (cationic membrane) and anion-exchange membrane (anionic membrane). In an electrolyte aqueous solution, the cationic membrane allows cations to pass and blocks anions, while the anionic membrane allows anions to pass and blocks cations - this is the selective permeability of the ion-exchange membrane. During electrodialysis, the ion-exchange membrane does not, like an ion-exchange resin, exchange with a certain ion in the aqueous solution; it only selectively allows ions of different charges to pass through, i.e., the ion-exchange membrane needs no regeneration. The compartment composed of the electrodes and membranes in the electrodialysis process is called the electrode compartment, where the electrochemical reactions are the same as ordinary electrode reactions. Oxidation occurs in the anode compartment, the anode water becomes acidic, and the anode itself is easily corroded. Reduction occurs in the cathode compartment, the cathode water becomes alkaline, and scale easily forms on the cathode [2].
Electrodialysis is a relatively mature membrane separation technology, widely used for brackish-water desalination and a main method of producing fresh water in some regions of the world. Because newly developed charged membranes have higher selectivity, lower membrane resistance, better thermal 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 recovering acid and metals from waste liquors formed by acid cleaning of metal surfaces; recovering heavy-metal ions from electroplating wastewater; recovering sulfate from synthetic-fiber wastewater; and recovering sulfite from pulp waste liquor. In the food industry it is used, for example, for milk desalination to make infant formula; in the chemical industry for separating ionic from non-ionic substances; and in clinical treatment electrodialysis can be used as an artificial kidney [3].
Electrodialysis with automatic frequent electrode reversal (EDR) makes operation and management more convenient. Raw-water utilization can reach 80%, and the general raw-water recovery rate is between 45% and 70%. Electrodialysis is mainly used for primary desalination of water, with a desalination rate of 45-90%. It is widely used for seawater and brackish-water desalination, primary desalination in pure-water preparation, and desalination and softening of boiler and power-equipment feed water.
Essentially, electrodialysis can be said to be a desalting technology, because various kinds of water (including natural water, tap water, and industrial wastewater) contain a certain amount of salt, and the anions and cations that make up these salts move toward oppositely charged electrodes under the DC electric field. If one anion-exchange membrane and one cation-exchange membrane are inserted into an electrodialyzer, because of the membranes' selective permeability - 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 middle compartment between the two membranes is reduced by the directional migration of ions, while the two compartments near the electrodes become concentration chambers for anions and cations respectively, finally achieving desalination in the middle diluting compartment.
5. Water polarization ionization: sometimes, due to poor operating conditions, water is forced to ionize into hydrogen ions and hydroxide ions, which can pass through the exchange membrane into the concentration chamber;
Currently, electrodialyzers have a wide range of applications: in water desalination and desalting, seawater concentration for salt making, refining dairy products, juice deacidification and purification, and the production of chemical products; they can also be used in the food and light industries to produce pure water, and as pretreatment for high-purity water in the electronics and pharmaceutical industries. They are used for primary softening and desalting of boiler feed water and for desalting brackish water into drinking water.
Electrodialyzers are suitable for water treatment in the electronics, pharmaceutical, chemical, thermal power, food, beer, beverage, dyeing and printing, and coating industries. They can also be used in physical and chemical processes such as concentration, purification, and separation of materials.
Electrodialysis can also be used for the treatment of wastewater and waste liquors and the recovery of precious metals, such as recovering nickel from electroplating waste liquors.
(1) Operating pressure about 0.5-3.0 kg/cm2
(2) Operating voltage and current 100-250 V, 1-3 A
(3) Energy consumption of the unit about 0.2-2.0 kWh per ton of fresh water
1. Can simultaneously desalinate, concentrate, separate, and purify an electrolyte aqueous solution;
2. Can be used for the purification of non-electrolytes such as sucrose, to remove the electrolytes therein;
3. In principle, an electrodialyzer is an electrolytic cell with membranes, which can make efficient use of the redox reactions at the electrodes.
4. During the electrodialysis process, the following secondary processes also take place
1. Co-ion migration: the selective permeability of the ion-exchange membrane is often not 100%, so a small amount of counter-ions always pass through the membrane;
2. Ionic concentration diffusion: because there is a concentration difference between the solutions in the concentration and dilution chambers, a small amount of ions always diffuses from the concentration chamber to the dilution chamber, reducing the dialysis efficiency;
3. Water osmosis: although the exchange membrane does not allow solvent molecules to pass, the concentration difference between the dilution and concentration chambers causes some solvent molecules (water) to permeate toward the concentration chamber;
4. Water electro-osmosis: due to ion hydration and the formation of an electric double layer, water molecules can also migrate from the dilution chamber to the concentration chamber under the DC electric field;
In practical applications, an electrodialyzer is not composed of a single pair of anion- and cation-exchange membranes (because that would be very inefficient) but uses one hundred pairs or even several hundred pairs of membranes, thus greatly improving efficiency.
6. Water pressure permeation: because there is a fluid-pressure difference between the concentration and dilution chambers, water molecules are forced to permeate from the high-pressure side to the low-pressure side. Obviously, these secondary processes are unfavorable to electrodialysis, but they can all be avoided or controlled by changing the operating conditions.
