Electrodialysis anion-exchange

Among electrochemical methods of water purification, one can also list the various electromembrane technologies, electrodialysis in particular. The simplest elec-trodialyzer consists of three compartments separated by semipermeable membranes (usually, cation- and anion-exchange membranes). The water to be purified is supplied to the central (desalination) compartment. In the outer (concentration) compartments, electrodes are set up between which a certain potential difference is applied. Under the effect of the electric field, ions pass througfi the membranes so that the concentration of ionic contaminants in the central compartment decreases. 

FIGURE 26.2 Electrodialysis purifying a feed solution. An alfemation of anion-exchange membranes and cafion-exchange membranes is placed between electrodes in a receiving solution outside the last membranes. Only a central section is shown. 

Figure 14.3 Schematic drawing of an electrodialysis cell. A, Anion-exchange membranes C, cation-exchange membranes.

Another way to obtain acids and bases is to make use of the transport of hydrogen ions through cation-exchange membranes, respectively of hydroxyl ions through anion-exchange membranes, which occurs during electrodialysis if no new counterions are offered by the depleted solution (1, 113, 114, 115). 

Electrodialysis (ED) is a unit operation for the separation or concentration of ions in solutions based on their selective electromigration through semi-permeable membranes under the influence of a potential gradient (Lacey and Loeb, 1972 Strathmann, 1992). Owing to their selectivity, ion-exchange membranes (IEM) allow transport of only cations (cation-exchange membranes) or anions (anion-exchange membranes) and thus can be used to concentrate, remove, or separate electrolytes. 

Elmidaoui, A., Lutin, F., Chay, L., Taky, M., Tahaikt, M., and Alaoui Hafidi, My R. 2002. Removal of melassigenic ions for beet sugar syrup by electrodialysis using a new anion-exchange membrane. Desalination 148, 143-148. 

Grebenyuk, V.D., Chebotareva, R.D., Peters, S., andLinkov, V. 1998. Surface modification of anion-exchange electrodialysis membranes to enhance anti-fouling characteristics. Desalination 115,... 

Lee, H.-J., Choi, J.-H., Cho, J., and Moon, S.-H. 2002a. Characterization of anion exchange membranes fouled with humate during electrodialysis. J. Membr. Sci. 203, 115-126. 

Figure 10.17 Flow schematic of electrodialysis systems used to exchange target ions in the feed solution, (a) An all-cation exchange membrane stack to exchange sodium ions for calcium ions in water softening, (b) An all-anion exchange membrane stack to exchange hydroxyl ions for citrate ions in deacidification of fruit juice...

Many of today s available membranes meet most of these requirements. In particular, the Nafion-type cation-exchange membrane has quite satisfactory properties for applications in the chlorine-alkaline electrolyses as well as in electrodialysis [6], Anion-exchange membranes often show lower stability in strong alkaline solutions than cation-exchange membranes. 

Figure 5.2 Schematic diagram illustrating the principle of desalination by electrodialysis in a stack with cation- and anion-exchange membranes in alternating series between two electrodes.

Stack design in bipolar membrane electrodialysis The key component is the stack which in general has a sheet-flow spacer arrangement. The main difference between an electrodialysis desalination stack and a stack with bipolar membranes used for the production of acids and bases is the manifold for the distribution of the different flow streams. As indicated in the schematic diagram in Figure 5.10 a repeating cell unit in a stack with bipolar membranes is composed of a bipolar membrane and a cation- and an anion-exchange membrane and three flow streams in between, that is, a salt... 

FIGURE 40 (a) Electrodialysis and (b) electrodialysis reversal (EDR). Cation exchange membrane indicated by C and anion exchange membrane by A (Ionics Inc.)... 

Another approach to achieve purification of rinses and recovery in one step, electrodialysis has been suggested for chromic acid recovery and removal of metallic impurities [108]. As the authors point out there are two main process limitations first, the poor stability of most anion-exchange membranes against the oxidative chromic acid solution and secondly the increase in membrane resistance due to the formation of polychromates in the membrane. 

Frenzel, I., Holdik, H., Stamatialis, D.F., Pourcelly, G., and Wessling, A. (2005) Chromic acid recovery by electro-electrodialysis -1. Evaluation of anion-exchange membrane. J. Membr. Sci., 261 (1-2), 49-57. 

Electrodialysis — In electrodialysis electrically charged - membranes and an electrical potential difference are used to separate ionic species from an aqueous solution and uncharged components. It refers to an industrial-scale process of electrolyte concentration/depletion due to separation on anion- and cation-exchange membranes under the influence of an electric field. The electrodialysis cell is constructed like a bipolar filter-press electrolyzer, with anion-exchange membranes sandwiched alternately with cation-exchange membranes, see following Figure. 

On the basis of this study, Sudoh et al. [78] proceeded to generate hydrogen peroxide in an acidic solution (1 M H2SO4) for use in Fenton s reagent oxidation of wastewater streams (Fig. 20). The peroxide, produced at the cathode in an alkaline KOH electrolyte, was transferred by electrodialysis to a central chamber, separated by anion exchange membrane (ACLE-5P, Tokuyama Soda, Japan) on the cathode side and a cation exchange membrane (CM-2, Tokuyama Soda, Japan) on the anode side. At a current of 4 A, 2.2 kmol/m of H2O2 was found to accumulate in the cen-... 

Gering and Scamehorn [86] studied the removal of CdCl2 and CdS04 from water by electrodialysis using a platinum-coated columbium anode and a Hastelloy cathode. Ionics (Watertown, MA) cation and anion exchange membranes were used in the electrodialysis stack. The effective cell pair area, defined as half the area of... 

Chiapello and Gal [93] studied the recovery of cyanide electroplating rinse waters by electrodialysis. The transfer rates of the ionic species present in a copper or zinc cyanide rinse bath was limited by the high resistance of the anion exchange membrane used. The presence of highly charged cyano complexes of copper increased the resistance of the anion exchange membrane. The transfer of zinc was easier at higher... 

The future for electrodialysis-based wastewater treatment processes appears bright. The dilute concentrations of metals in the waste streams do not degrade or foul the cation or anion exchange membranes. The concentrate streams are recirculated to build up their metal content to a level that is useful for further recovery or direct return to the process stream. Ongoing research in the development of cheaper cation exchange membranes, and stable anion exchange and bipolar membranes will allow electrodialysis-based applications to become more competitive with other treatments. 

Unlike electrodialysis, which tends to concentrate and remove or recover species, electromembrane processes transform species present in waste streams by electrolysis and the use of cation and anion exchange membranes. These processes offer chemical cost reduction (by recovery), water consumption reduction, and discharge or reuse of contaminant-free waters. Supplementary gains are also obtained by identifying a market for products obtained from the recovery and transformation processes. 

Finally, electrodialysis may be mentioned, a process widely used to de-salt aqueous solutions. An electric field is applied across a stack of alternating cation-exchange and anion-exchange membranes. Ions in the electrolyte solutions between these membranes are transported till they meet a membrane of the same sign, so that electrolyte-rich and electrol)rte-freed solutions are created. The process involves conduction and electro-osmosis. Obviously, irreversible thermod mamics appears very suitable to describe the various flows... 

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