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electrodialysis
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell. The cell consists of a feed (dilute) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes. In almost all practical electrodialysis processes, multiple electrodialysis cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation exchange membranes forming the multiple electrodialysis cells. Electrodialysis processes are different than distillation techniques and other membrane based processes (such as reverse osmosis (RO)) in that dissolved species are moved away from the feed stream rather than the reverse. Because the quantity of dissolved species in the feed stream is far less than that of the fluid, electrodialysis offers the practical advantage of much higher feed recovery in many applications.〔Davis, T.A., "Electrodialysis", in Handbook of Industrial Membrane Technology, M.C. Porter, ed., Noyes Publications, New Jersey (1990)〕〔Strathmann, H., "Electrodialysis", in Membrane Handbook, W.S.W. Ho and K.K. Sirkar, eds., Van Nostrand Reinhold, New York (1992)〕〔Mulder, M., Basic Principles of Membrane Technology, Kluwer, Dordrecht (1996)〕〔Sata, T., Ion Exchange Membranes: Preparation, Characterization, Modification and Application, Royal Society of Chemistry, London (2004)〕〔Strathmann, H., Ion-Exchange Membrane Separation Processes, Elsevier, New York (2004)〕〔(ED - Turning Seawater into Drinking Water )〕 Electrodialysis reversal (EDR) is an advanced electrodialysis process which utilizes a flow and polarity reversal to de-scale membrane surfaces and enable high concentration operation. EDR is used in higher salinity commercial applications where brine volume and scaling is a concern.
==Method==
In an electrodialysis stack, the dilute (D) feed stream, brine or concentrate (C) stream, and electrode (E) stream are allowed to flow through the appropriate cell compartments formed by the ion exchange membranes. Under the influence of an electrical potential difference, the negatively charged ions (e.g., chloride) in the dilute stream migrate toward the positively charged anode. These ions pass through the positively charged anion exchange membrane, but are prevented from further migration toward the anode by the negatively charged cation exchange membrane and therefore stay in the C stream, which becomes concentrated with the anions. The positively charged species (e.g., sodium) in the D stream migrate toward the negatively charged cathode and pass through the negatively charged cation exchange membrane. These cations also stay in the C stream, prevented from further migration toward the cathode by the positively charged anion exchange membrane.〔AWWA, Electrodialysis and Electrodialysis Reversal, American Water Works Association, Denver (1995)〕 As a result of the anion and cation migration, electric current flows between the cathode and anode. Only an equal number of anion and cation charge equivalents are transferred from the D stream into the C stream and so the charge balance is maintained in each stream. The overall result of the electrodialysis process is an ion concentration increase in the concentrate stream with a depletion of ions in the dilute solution feed stream (see video ).
The E stream is the electrode stream that flows past each electrode in the stack. This stream may consist of the same composition as the feed stream (e.g., sodium chloride) or may be a separate solution containing a different species (e.g., sodium sulfate).〔 Depending on the stack configuration, anions and cations from the electrode stream may be transported into the C stream, or anions and cations from the D stream may be transported into the E stream. In each case, this transport is necessary to carry current across the stack and maintain electrically neutral stack solutions.
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