Cation exchange membranes have negative charges: they allow cations to pass through and repel anions. Meanwhile, anion exchange membranes have a positive charge: they allow anions to pass through and repel cations.
The electrodialysis chambers or cells are organized in packages with several anionic and cationic exchange membranes, which alternate with each other. The anionic and cationic exchange membranes delimit the spaces where the dilution and concentration processes of the liquids are carried out.
The driving force of the ionic fluxes that pass through the membranes of electrodialysis cells is a difference in electrical potential between a cathode and an anode.
How does it work in diluting the water?
Description of a pack of five electrodialysis cells
In the diagram shown (upper image) the location of six ion exchange membranes is indicated: three for cation exchange and three for anion exchange, which is interspersed with each other. These membranes delimit five compartments where electrodialysis fluids circulate (see red and blue arrows).
Anion exchange membranes are positively charged, appearing in the blue color scheme shown. These let the anions pass and repel the cations by electrostatic repulsion.
The cation exchange membranes, meanwhile, have a negative charge, indicated in the green color scheme. They are traversed by cations, however, repelled anions.
Both types of membranes are impervious to water. The diagram shows two spaces where there is a dilution of the saline content of the water. In the scheme are marked in blue aqueous flows into and out of these areas identified as current D.
Likewise, the anode (+) and cathode (-) compartments are indicated in the diagram, through which the current E circulates. (Electrodialysis Advantages and Disadvantages)
How electrodialysis works
The cations are attracted by the charge of the cathode (-), while the anode (+) repels them. Anions are attracted by the anode charge (+) and repelled by the cathode charge (-).
The observation, for example, of cells three, four, and five, indicates the following: in cells three and five concentrated liquid circulates, while in cell four dilution liquid circulates.
The Cl – ion from compartment four is attracted to the charge on the anode and passes through the anion exchange membrane that separates it from compartment five.
Meanwhile, the Na + ion in compartment five is repelled by the positive charge from the anode but is unable to enter compartment four. This is because the membrane that separates compartments four and five is anion exchange, so it repels cations.
On the other hand, the Na + from compartment four is repelled by the anode (+) and crosses the cation exchange membrane (-) that separates it from compartment three.
In summary: the concentration of Na + and Cl – in compartment four tends to decrease. Then, the salt concentration decreases in the dilution compartments and increases in the concentration compartments.
Electrodialysis Advantages and Disadvantages
- Electrodialysis is a simple and continuous operation.
- No chemical treatment is required to regenerate the exchange membranes.
- There is a low cost of operation and maintenance, requiring little space for its operation.
- Particles with a diameter greater than 1 µm can obstruct ion exchange membranes.
- The use of water with a hardness greater than 1 is not recommended, since the calcium carbonate creates a crust in the concentrate cells.
- Requires carbon dioxide removal treatment (CO 2 ), since this dissociation can modify the conductivity of the water.
Difference between dialysis and electrodialysis
Dialysis is the flow of water through a semi-permeable membrane from the lower osmolar compartment to the higher osmolar compartment. The compartment with the lowest osmolarity has a higher concentration of water molecules.
For this reason, water flows, in favor of its concentration gradient, from the compartment with lower osmolarity to the compartment with higher osmolarity.
Electrodialysis, on the other hand, is a flow of ions through an ion exchange membrane that allows the ions to pass or not, depending on their electrical charge. The driving force for ion movement is a difference in electrical potential.
Unlike dialysis, in electrodialysis, there is no flow of water through the membranes and there is only the flow of ions.
Electrodialysis Applications: Electrodialysis Advantages and Disadvantages
There are electrodialysis plants that process water with a high salt content to produce drinking water. Electrodialysis is also used to process wastewater from domestic and industrial use, in order to make it suitable for later consumption.
Electrodialysis is specifically used in the recovery of valuable metals present in industrial effluent waters. It is also used in the removal of salts and acids from aqueous solutions, and in the separation of ionic compounds from neutral molecules.
Although electrodialysis is used in pharmaceuticals, cosmetics, and other industries, its most widespread use is in the food industry. Among some of its applications we can mention the following:
- Demineralization of serum and molasses
- Stabilization of tartaric acid in wine
- Decrease in the acidity of fruit juices
- Production of plant protein isolates
- Fractionation of whey proteins
- Demineralization of soybean and fish oils
- Removal of alkali ions from beet and cane molasses
- Shell waste treatment
- Demineralization of grape must and potato juice
- Concentration and recovery of organic acids
- Production of soy proteins
- Enrichment of antioxidants