Electrodialysis anion exchange membranes with

Figure 5.42 Change in transport number of sulfate ions relative to chloride ions of an anion exchange membrane with the concentration of diethylene glycol in the desalting side solution. After the anion exchange membrane (cross-linked 10%) had been immersed in diethylene glycol for 48 h at 45 °C, electrodialysis was carried out using a mixed solution composed of 0.20 N sodium sulfate, 0.20 N sodium chloride and diethylene glycol of various concentrations at 80 mA cm 2 for 60 min at 25.0 °C.

Hara [50, 51] employed membranes of styrene—butadiene copolymer for electrodialysis. He showed that the best separation effect could be obtained by using cation- and anion-exchange membranes with specific resistance of 100 and 120 2-cm respectively and with an initial current density of 0.8 A/dm. Under these conditions it was possible within 2 hours to remove completely acidic and basic amino acids from the initial solution, in the central compartment. However, 10-25% of the neutral acids also left the central compartment due to migration [51]. 

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 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... 

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. 

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. 

FIGURE 21.23 Electrodialysis (ED) cell with BPM. (EDBM2C configuration AEM, anion-exchange membrane BPM, bipolar membrane.) (Adapted from Vera Calle, E., Domier, M., Sandeaux, J., and Pourcelly, G., Desalination, 149, 357, 2002.)... 

A composite membrane prepared, e.g., by lamination of a cation exchange membrane with an anion exchange membrane, called a bipolar ion exchange membrane, shows interesting properties, for example water splitting to generate hydrogen ions and hydroxide ions at the interface of both membranes in electrodialysis, or a rectifier effect.110 When the current is passed from one direction. 

Figure 3.18 Change in electrical resistance of an anion exchange membrane (strongly basic anion exchange) with and without anionic polyelectrolyte layers in the presence of sodium tetradecyl sulfate (STS). 1. without the layers and with STS 2. with the layers (immersion time 4 h) and with STS 3. with the layers (immersion time 24 h) and with STS 4. with the layers and without STS 1 left vertical axis 2,3 and 4 right vertical axis. After an anion exchange membrane had been immersed in 100ppm anionic polyelectrolyte (polycondensation product of sodium naphthalene sulfonate and formaldehyde MW ca. 1000) solution for the respective time at room temperature, electrodialysis was carried out at a current density of 2.5 mAcmr2 using 0.10 N sodium chloride solution containing 2.16 X 10 3 mol dm3 of STS.

After anion exchange membranes had been immersed in an aqueous 0.745 mol dm 3 pyrrole solution for 16 h, the membranes were immersed in aqueous 0.111 N ferric chloride solution for 4 h. b Electrodialysis was carried out with 6.0 N HCl/membrane/0.50 N HC1 at 10 mAcrrr2 for 90 min at room temperature. 

T. Sata, K. Teshima and T. Yamaguchi, Permselectivity between two anions in anion exchange membranes cross-linked with various diamines in electrodialysis, J. Polymer Sci., Polym. Chem. Ed., 1996, 34, 1475-1482. 

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