Anion permselectivity

Metal nanotube membranes with electrochemically suitable ion-transport selectivity, which can be reversibly switched between cation-permeable and anion-permselective states, have been reported. These membranes can be viewed as universal ion-exchange membranes. Gold nanotube molecular filtration membranes have been made for the separation of small molecules (< 400 Da) on the basis of molecular size, eg. separation of pyridine from quinine (Jirage and Martin, 1999). 

The dashed lines at the top and bottom of Fig. 12 are the values that would be achieved if the nanotubule membrane showed ideal cation and ideal anion permselectivity, respectively [Eq. (1)]. At negative applied potentials, the nanotubule membrane shows ideal cation permselectivity, whereas at positive applied potentials the membrane shows ideal anion permselectivity. This selectivity occurs because at negative applied potentials, excess electrons are present on the walls of the tubes and excess posi-... 

For any combination of metal and electrolyte, there is a potential called the potential of zero charge (pzc) where there is no excess charge on the metal. At this potential the nanotubule membranes should show neither cation nor anion permselectivity, and should approach 0 mV. for the tubule-containing membrane does, indeed, go from the ideal cation-... 

We have demonstrated that these Au nanotubule membranes can be cation permselective, anion permselective, or nonselective, depending on the potential applied to the membrane [99]. These membranes can be as permse-... 

Another system under investigation is the iron/ chromium redox flow battery (Fe/Cr RFB) developed by NASA. The performance requirements of the membrane for Fe/Cr RFB are severe. The membrane must readily permit the passage of chloride ions, but should not allow any mixing of the chromium and iron ions. An anionic permselective membrane CDIL-AA5-LC-397, developed by Ionics, Inc., performed well in this system. ° It was prepared by a free radical polymerization of vinylbenzyl chloride and dimethylaminoethyl methacrylate in a 1 1 molar ratio. One major issue with the anionic membranes was its increase in resistance during the time it was exposed to a ferric chloride solution. The resistance increase termed fouling is related to the ability of the ferric ion to form ferric chloride complexes, which are not electrically repelled by the anionic membrane. An experiment by Arnold and Assink indicated that... 

A polypyrrole membrane doped with anions of high molecular weight during electrochemical polymerization changes in anion permselectivity or cation perm-... 

About 1.4 million tons of sodium chloride (edible salt)/year has been produced in Japan since 1972 by the electrodialytic concentration of seawater. This technology has been exported to Korea, Taiwan and Kuwait. After seawater is concentrated up to about seven times or more its initial concentration by electrodialysis, an optimal concentration from an economic viewpoint, sodium chloride is produced by evaporating the concentrated solution. Table 6.4 shows a typical composition of the concentrated solution.51 In this process, monovalent cation and anion permselective ion exchange membranes have been used. This technology was developed in Japan to produce sodium chloride by the electro-... 

For the production of table salt by concentration of sea water monovalent cation selective membranes were prepared by forming a thin positively charged layer on the surface of a cation-exchange membrane. Monovalent anion permselective membranes have a thin highly cross-linked layer on the membrane surface have also been developed [25]. By such means the selectivity of sulfate compared to the one of chloride can be reduced from about 0.5 to about 0.01 and of magnesium compared to sodium from about 1.2 to about 0.1. 

In summary, electromembrane processes, especially when used as part of hybrid treatment schemes, can provide an efficient removal of toxic metal ions from water. When a target metal exists in water as a mono-valent species, the use of mono-valent (cation- and/or anion-permselective membranes) is especially attractive. Situations, in which ED appears to be less applicable are for waters of very low salinity (conductivity of less than 0.5 mS cm ), for which EDI or DD can be better choices, and, in cases when besides ions, removal of low molecular mass non-charged compounds from the water is desired. In the latter case, pressure-driven membrane processes such as RO or NF may be preferable. 

Brackish water, containing 3000mg of TDS and 3mgL of F , was tested by Amor et al. (2001). The use of a mono-anion permselective membrane (Neosepta ACS) allowed for maintaining the water sulfate concentration close to its original value (only 5% was removed). The membrane transported the anions in the following order Cl > F > HCO > SO ". 

Fujiwara et al. (2011) showed the possibility of preparing reversible air electrodes that can be used in metal-air storage batteries or unitized regenerative fuel cells. To reduce the impact of atmospheric carbon dioxide the reversible air electrodes were integrated with a polymer anion-exchange membrane which was placed between the cathode s catalytic layer (with Pt and Pt-Ir catalysts) and the alkaline solution. The membrane with anion permselectivity presumably inhibited the permeation of COj cations to the air electrode and thus suppressed precipitation of carbonates in pores of the air electrode. 

Anionic permselectivity . Measurements carried out with BaCli solutions. 

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