Water Treatment Basics: The 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, it uses the selective permeability of ion-exchange membranes (i.e., cations can pass through cation-exchange membranes and anions can pass through anion-exchange membranes), so that anions and cations move 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 through; if their charges are the same, the ion is repelled, thereby achieving purposes such as desalination, concentration, refining, or purification of the solution. [1]
Compared with reverse osmosis, another membrane separation technology introduced in recent years, electrodialysis is cheaper but has a lower desalination rate. At
present, 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, such ion-exchange membranes can be divided into two types: cation-exchange membranes (cation membranes) and anion-exchange membranes (anion membranes). In an electrolyte aqueous solution, cation membranes allow cations to pass while repelling and blocking anions, and anion membranes allow anions to pass while repelling and blocking cations -- this is the selective permeability of ion-exchange membranes. In the electrodialysis process, unlike ion-exchange resins, the ion-exchange membrane does not exchange with a certain ion in the aqueous solution but only selectively allows ions of different polarity to pass through, meaning 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 reactions that occur are the same as ordinary electrode reactions. An oxidation reaction occurs in the anode compartment, the anode water is acidic, and the anode itself is easily corroded. A reduction reaction occurs in the cathode compartment, the cathode water is alkaline, and scale easily forms on the cathode. [2]
Electrodialysis is a relatively mature technology among membrane separation processes and has been widely used in brackish-water desalination; it is the main method for 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 no longer limited to desalination. In the food, pharmaceutical, and chemical industries, the electrodialysis process also has many other industrial applications, such as the treatment of industrial wastewater -- mainly including recovering acid and metals from the spent liquor formed when acid cleans 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 purposes such as desalting milk to make infant formula; in the chemical industry, it is used to separate ionic substances from non-ionic substances; and in clinical treatment, electrodialysis can be used as an artificial kidney. [3]
Automatically controlled electrodialysis reversal (EDR) with frequent polarity reversal makes operation and management more convenient. The raw water utilization rate can reach 80%, with the general raw water recovery rate between 45% and 70%. Electrodialysis is mainly used for the primary desalination of water, with a desalination rate between 45% and 90%. It is widely used for the desalination of seawater and brackish water, primary desalination in the preparation of pure water, and the desalination and softening of feed water for boilers and power equipment.
In essence, electrodialysis can be regarded as a desalination technology, because various types 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 in opposite directions toward the electrodes under the action of a direct-current electric field. If one anion-exchange membrane and one cation-exchange membrane are inserted into an electrodialyzer, because ion-exchange membranes have selective permeability -- i.e., cation-exchange membranes only allow cations to pass freely and anion-exchange membranes only allow anions to pass -- the salt concentration in the middle compartment between the two membranes decreases due to the directional migration of ions, while the two compartments near the electrodes become the concentration compartments for anions and cations, respectively, finally achieving desalination in the middle diluting compartment.
In practical applications, an electrodialyzer is not composed of a single pair of anion- and cation-exchange membranes (because this would be very inefficient), but instead 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. They are used in water desalination, seawater concentration for salt production, refining dairy products, deacidification and purification of fruit juice, 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 the production of high-purity water in the electronics and pharmaceutical industries. They are used for the primary softening and desalination of boiler feed water and for desalinating brackish water into drinking water.
Electrodialyzers are suitable for water treatment in industries such as electronics, pharmaceuticals, chemicals, thermal power generation, food, beer, beverages, printing and dyeing, and coating. They can also be used for physicochemical processes such as the concentration, purification, and separation of materials.
Electrodialysis can also be used for the treatment of wastewater and waste liquid and the recovery of precious metals, such as recovering nickel from electroplating waste liquid.
(1) Operating pressure: about 0.5-3.0 kg/cm2
(2) Operating voltage and current: 100-250 V, 1-3 A
(3) Power consumption of the unit itself: about 0.2-2.0 kWh per ton of fresh water
1) It can simultaneously perform desalination, concentration, separation, and purification of an electrolyte aqueous solution;
2) It can be used to purify non-electrolytes such as sucrose in order to remove the electrolytes therein;
3) In principle, an electrodialyzer is an electrolytic cell with a diaphragm, which can utilize the high efficiency of oxidation-reduction at the electrodes.
2) Concentration-difference diffusion of ions: because a concentration difference exists between the solutions in the concentration compartment and the diluting compartment, a small number of ions always diffuse and migrate from the concentration compartment to the diluting compartment, thereby reducing the dialysis efficiency;
1) Migration of co-ions: the selective permeability of ion-exchange membranes can never be 100%, so a small number of counter-ions always pass through the exchange membrane;
IV. During the electrodialysis process, the following secondary processes also occur
3) Osmosis of water: although the exchange membrane does not allow solvent molecules to pass, the concentration difference between the diluting compartment and the concentration compartment causes some solvent molecules (water) to permeate into the concentration compartment;
4) Electro-osmosis of water: due to the hydration of ions and the formation of the electric double layer, water molecules can also migrate from the diluting compartment to the concentration compartment under the action of the direct-current electric field;
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 osmosis of water: due to the difference in fluid pressure between the concentration compartment and the diluting compartment, water molecules are forced to permeate from the higher-pressure side to the lower-pressure side. Obviously, these secondary processes are unfavorable factors for electrodialysis, but they can all be avoided or controlled by changing the operating conditions.
