Ion-exchange membranes in electrodialysis

Permselectivity of Specific Ions through Ion Exchange Membranes in Electrodialysis... 

V.A. Shaposhnik, VI. Vasileva and E.V Reshetnikova, Concentration polarization of ion-exchange membranes in electrodialysis An interferometric study, Russian J. Electrochem., 2000, 36, 773-777. 

Wingerd and Block also accomplished demineralization of whey [77], Among other examples may be mentioned the use of ion-exchange membranes in electrodialysis of acidified aqueous solutions of cinchona bark extract [78] and demineralization of polar solvents (acetone, acetonitrile, nitrobenzene) in high-voltage... 

Josefsson [5] determined soluble carbohydrates in seawater by partition chromatography after desalting by ion exchange membranes. The electrodialysis cell used had a sample volume of 430mL and an effective membrane surface area of 52cm2. Perinaplex A-20 and C-20 ion exchange membranes were used. The water-cooled carbon electrodes were... 

Electrodialysis (ED) is a membrane separation process, which exploits an electrical field as the driving force instead of pressure. Charged compounds are separated by ion-exchange membranes. In the pulp and paper industry, ED is being studied for the... 

Because there is a concentration difference across the ion exchange membrane in practical electrodialysis, the transport number (current efficiency) in the presence of the concentration difference is required. When the current efficiency (dynamic state transport number) is measured in the presence of a concentration difference, the current efficiency in the absence of the concentration difference may be basically obtained by subtracting the amount of electrolyte diffusing through membrane from the measured current efficiency. In all cases, when the transport number is measured, the solutions in both compartments should be vigorously agitated to eliminate the effect of diffusion boundary layers on the transport number. 

Many attempts to prepare ion exchange membranes having permselectivity for specific ions and to find electrodialysis methods to achieve such a purpose have been made. In this chapter, studies on the modification of ion exchange membranes and electrodialysis methods to permeate specific ions through the membrane, mainly in electrodialysis, are explained. 

I. Doi, T. Tanaka and T. Yamabe, Permselectivity of halogen ions through anion exchange membranes in electrodialysis, Nippon Kagaku Zasshi, 1962, 83, 1161 ... 

H.-W. Rosier, F. Maletzki and E. Staube, Ion transport across electrodialysis membranes in the overlimiting current range Chronopotentiometric studies, J. Membr. Sci., 1992, 72, 171-178 R Sistat and G. Pourcelly, Chronopotentiometric response of an ion-exchange membrane in the underlimiting current-range. Transport phenomena within the diffusion layers, J. Membr. Sci., 1997, 123, 121. 

A polyion in the form of a thin membrane is used as ion-exchange membrane in another application of the ion-exchange phenomenon. When exposed to an electrolyte, an ion-exchange membrane will allow counterions to pass through it, but will act as a barrier to the complementary ion, and is therefore said to be permselective. Thus a polyanionic membrane will allow passage of cations and a polycationic membrane that of anions, so that under the influence of an electric current, continuous fluxes of cations and anions, respectively, can be set up across these membranes. This principle is exploited in electrodialysis and in chlor-alkali cells as described later. 

Process Description Electrodialysis (ED) is a membrane separation process in which ionic species are separated from water, macrosolutes, and all uncharged solutes. Ions are induced to move by an electrical potential, and separation is facilitated by ion-exchange membranes. Membranes are highly selective, passing either anions or cations and very little else. The principle of ED is shown in Fig. 20-79. 

Electrodialysis is a process for the separation of an electrolyte from the solvent and is used, for example, in desalination. This process occurs in a system with at least three compartments (in practice, a large number is often used). The terminal compartments contain the electrodes and the middle compartment is separated from the terminal compartments by ion-exchanger membranes, of which one membrane (1) is preferentially permeable for the cations and the other one (2) for the anions. Such a situation occurs when the concentration of the electrolyte in the compartments is less than the concentration of bonded ionic groups in the membrane. During current flow in the direction from membrane 1 to membrane 2, cations pass through membrane 1 in the same direction and anions pass through membrane 2 in the opposite direction. In order for the electrolyte to be accumulated in the central compartment, i.e. between membranes 1 and 2 (it is assumed for simplicity that a uni-univalent electrolyte is involved), the relative flux of the cations with respect to the flux of the solvent, /D +, and the relative flux of the anions with respect to... 

Electrodialysis units recover plating chemicals differently from the recovery units discussed thus far. In electrodialysis, electromotive forces selectively drive metal ions through an ion-selective membrane (in RO, pressure is the driving force in ion exchange, the driving force is chemical attraction). The membranes are thin sheets of plastic material with either anionic or cationic characteristics.33... 

For the sake of simplicity, we shall assume again an ideal permselectivity of the ion-exchange membrane. As mentioned before, this assumption is practically justified, since the co-ion transport numbers in modern monopolar ion-exchange membranes, employed, e.g., in electrodialysis, are limited to a few percent. 

Electrodialysis with ion-exchange membranes can also be used to concentrate electrolytes. In Japan this method meets with much interest for the recovery of sodiumchloride from seawater (104, 105, 156, 172, 188). 

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