Polymeric membranes electrodialysis

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is hnger-ing controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltratiou. Nanofiltration membranes are probably charged porous structures. 

A more detailed study of transport processes in solvent polymeric membranes was initiated recently.72 One aim was to get information on the distribution within the membrane of the carrier and the cation transported after a steady state has built up during an electrodialysis experiment. A further objective was the demonstration of a relaxation of the concentration gradients of both carrier and cation. To this end the transport properties of solvent polymeric membranes containing the carrier l4C-valinomycin (66 wt.% dioctyladipate, 33 wt.% polyvinyl chloride, 1 wt.%, JC-valinomycin) in contact with aqueous solutions of -,H-a-phenylethylammonium chloride were studied. 

The first widespread use of polymeric membranes for separation applications dates back to the 1960-70S when cellulose acetate was cast for desalination of sea and brackish waters. Since then many new polymeric membranes came to the market for applications extended to ultrafiltration, miciofiltration, dialysis, electrodialysis and gas separations. So far ultrafiltration has been used in more diverse applications than any other membrane processes. The choice of membrane materials is dictated by the application environments, the separation mechanisms by which they operate and economic considerations. Table 1.4 lists some of the common organic polymeric materials for various membrane processes. They include, in addition to cellulose acetate, polyamides. 

M. Yoshikawa, J. Izumi, T. Kitao, Enantioselective electrodialysis of amino acids with charged polar side chains through molecularly imprinted polymeric membranes containing DIDE derivatives, Polym. J., 1997, 29, 205. 

Membranes which may be used in the removal of alkali metal ions by electrodialysis are those which are impermeable to anions, but which allow the flow therethrough of cations. Such cation-selective membranes should, of course, possess chemical durability, high resistance to oxidation and low electrical resistance in addition to their ion-exchange properties. Homogeneous-type polymeric membranes are preferred, for example, network polymers such as phenol, phenosulfonic acid, formaldehyde condensation polymers and linear polymers such as sulfonated fluoropolymers and copolymers of styrene, vinyl pyridine and divinylbenzene. Such membranes are well known in the art and their selection for use in the method of the invention is well within the skill of the art. 

Yoshikawa, M. Izumi, J. Kitao, T. Enantioselective electrodialysis of Y-a-acetyltrypto-phans through molecularly imprinted polymeric membranes. Chem. Lett. 1996, 26, 611-612. 

Ion exchange desalination - DESAL , SIROTHERM Membrane desalination - Reverse Osmosis, Electrodialysis Continuing fixed bed counterflow development Condensate polishing systems Ion Chromatography analysis, and Pellicular resins Polymeric adsorbents... 

S. Tsuneda, K. Saito, H. Mitsuhara, T. Sogo, Novel ion-exchange membranes for electrodialysis prepared by radiation-induced graft polymerization, J. Electrochem. Soc., 1995, 142, 3659-3663. 

Figure 5.1 Change in transport number of calcium ions relative to sodium ions, mobility ratio and ion exchange equilibrium constant between calcium ions and sodium ions in a cation exchange membrane having salicylic acid groups as cation exchange groups (condensation type) with the pH of a mixed salt solution. (O) PNaC i (A) KNaCa ( ) UCu/UNa. Electrodialysis of the mixed solution of 0.375 N sodium chloride and 0.125 N calcium chloride (chloride ions 0.500 N), the pH of which was adjusted, was carried out using a cation exchange membrane prepared from condensation polymerization of salicylic acid, phenol and formaldehyde.

Figure 5.14 Change in ratio of calcium ions to sodium ions in the membrane phase during electrodialysis (KNaCa) with polymerization time of aniline. After a 1 1 mixed salt solution of 0.250N calcium chloride and 0.250N sodium chloride (concentration of chloride ions 0.500N) had been electrodialyzed for 1.0 h at 10 mA cm 2 using a membrane with polyaniline layers, the membrane was immediately removed from the cell during electrodialysis and the ratio of calcium ions to sodium ions in the membrane phase was determined (starting membrane NEOSEPTA CM-1).

Although fairly distinct in their capabilities, these processes have several features in common. The membrane materials used are generally polymeric in origin and are extensions, both in mode of preparation and in composition, of the cellulose acetate membranes pioneered by Loeb and Sourirajan. Another common feature shared by most of the processes bsted is the use of pressure as the driving force the exceptions are the different forms of dialysis. Thus, in hemodialysis, concentration replaces pressure as the driving force, while electrodialysis is driven by an electrical potential. 

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