Electrodialysis in the Separation of Chemicals
Modern Electrodialysis (ED) is a simple, convenient, flexible process for rapidly moving low-molecular weight organic and inorganic ions from one solution to another by means of highly conductive, ion-selective membranes and low-voltage direct electric current. Two types of membranes are used simultaneously — one type selective to cations such as sodium, hydrogen, potassium, ammonium, guanidinium, tetra-alkyl ammonium, calcium and magnesium, the other type selective to anions such as chloride, hydroxide, nitrate, perchlorate, acetate, phosphate, citrate, sulfate and bicarbonate. The pressure used is low, only enough to pass the solutions which are to be deionized/respectively ionized over the surfaces of the membranes. Very little water goes through the membranes, essentially only the water of hydration of the ions transferred, although in the case of deionization of concentrated solutions of electrolytes, the transport of such water of hydration can be useful in concentrating non-ionized materials present in the solution deionized. The transport of ions is not generally accompanied by the transport of non-ionized materials of molecular weight greater than about 200 although special membranes are available which permit non-ionized materials of molecular weight up to about 500 to be transported with the water of hydration of the ions.
In contrast to ultrafiltration, diafiltration, dialysis and ion-exchange, ED permits the ions which are removed to be recovered in solution more concentrated than that from which they were removed and free of foreign ions. (Concentrations can be obtained up to about 4N or as may be limited by the solubility of the least soluble electrolyte). Often such concentrations will permit the re-use of the recovered electrolytes (e.g. guanidine hydrochloride). Chemical regenerants are not required as in the case of deionization with granular ion-exchangers. Ultrapure water requirements are minimal in contrast to diafiltration and dialysis. Deionization of 99 percent or more is easily achieved.
Batch-mode laboratory and commercial scale ED can be operated at constant low pressure (about 1.5 atmospheres) and constant low voltage (about 0.5 volts per membrane) and is particularly user-friendly. ED is simply continued until the desired electrolyte level is obtained, as may be measured by electrical conductivity. ED membranes and apparatus can be operated continuously at temperatures up to 60°C and can be sanitized-in-place with strong acids and/or alkalies or dilute solutions of active chlorine. ED apparatus, including the membrane modules, can be easily and rapidly dissassembled for inspection.
Owing to its simple hydrodynamics, ED is relatively tolerant of colloids and particulates. In the event that high molecular weight charged materials are concentrated by electrophoresis against one of the membranes, they can be moved back into solution by a brief reversal of the direction of the electric current.
ED is now being used commercially to reduce the high salt concentration (2N NaC1) present in an alpha-interferon eluate and to recover guanidine hydrochloride in high yield. In the laboratory, ED has been used to precipitate euglobulins from plasma, to salt-out proteins (and subsequently to deionize the supernatant) and to deionize protein and poly-peptide solutions (MW as low as 1000).
KeywordsDirect Current Voltage Whey Powder Soluble Electrolyte Enrichment Stream Instal Membrane
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