Anion permeability

Fig. 35. Schematic diagram of a plate-and-frame electro dialysis stack. Alternating cation- and anion-permeable membranes are arranged in a stack of up to...

Electrodialysis. In reverse osmosis pressure achieves the mass transfer. In electro dialysis (qv), dc is appHed to a series of alternating cationic and anionic membranes. Anions pass through the anion-permeable membranes but are prevented from migrating by the cationic permeable membranes. Only ionic species are separated by this method, whereas reverse osmosis can deal with nonionic species. The advantages and disadvantages of reverse osmosis are shared by electro dialysis. 

Fig. 10 pH-Dependent permeability of the anions Fe(CN)6 and the cation Ru(NH3)g in nanocomposite dendrimer-poly(maleic anhydride) grafts that contain - CO2H/ - C02" and NH2/-NH3 groups in the film with a anion permeability at low pH where the... 

Cell DC-2. Earlier demineralization studies by Lyon (9) employed cell DC-2. This was a sandwich-type cell with Lucite side plates bolted together with two epoxy resin-gasketed graphite electrodes separated by an anion-permeable membrane. The membrane was necessary because a suitable anion-responsive electrode was not then known. The principle of operation is that in the cathode compartment, after several current reversal conditioning cycles, sodium ions are removed by the cathode while chloride ions migrate from the cathode through the membrane to the anion chamber. In the anode chamber, sodium ions, from the previous half cycles, are rejected from the anode. The net result was salt depletion in the cathode chamber and a similar concentration increase in the anode chamber. 

The ion movement can be controlled by ion exchange or ion transfer membranes, thin sheets of cross-linked organic polymers with ion exchange properties—for example, sulfonated polystyrene-divinylbenzene polymers. Both cation-permeable and anion-permeable membranes are available and have been described (3, 9). To achieve demineralization, these membranes are spaced alternately between a cathode and an anode which introduce direct current. The compartment between each pair of membranes is filled with a saline water. The resulting ion motion is controlled by the membranes, so that one set of compartments—for example, the even-numbered compartments—lose ions and the odd-numbered compartments gain ions. The product from the ion-losing cells is collected and comprises electrically demineralized water. 

Electrodialysis — Figure. The electro dialysis cell (a) [i] (a = anion-permeable membrane c = cation-permeable membrane o = positive ion = negative ion ion migration under action of electrical current (b), causes salt depletion in alternate compartments and salt enrichment in adjacent ones (c))... 

A Anion-permeable membrane f Cation-permeable membrane... 

When a constant voltage is applied to the electrodes, all cations migrate towards the cathode, and all anions migrate towards the anode. The cations can pass through the cation-permeable membrane, but they cannot permeate the anion-permeable membrane. The counter argument applies to anions. Alternate compartments contain ionic concentrations that are greater or less than the feed solution. These compartments are then combined to create the brine (waste) stream and the purified water stream. 

Cabantchik, Z. I., and Rothstein, A. (1972). The nature of the membrane sites controlling anion permeability of human red blood cells as determined by studies with disulfonic stilbene derivatives.. Membr. Biol. 10,311-330. 

A ANION-PERMEABLE MEMBRANE C = CATION-PERMEABLE MEMBRANE... 

Many large-scale applications of clectrodialysis use ion-selective membranes and a potential gradient to speed migration of ions through the membranes. Brackish water can be made potable by passage through an array with alternate cation- and anion-permeable membranes, as shown in Fig. 26.13. In half of the spaces, cations move out to one side and anions move out to the other side, leaving... 

The permeability of liposomes to other cations also seems to depend on the size of the vesicle In small vesicles, sodium permeability ranges around 10 14 cm/s, whereas in large vesicles, sodium permeability is about 100-fold greater. The permeability of lipid bilayers to monovalent anions like chloride is consistently higher typical values are in the range of 10 10 cm/s. The question of greater anion permeability has been addressed by Flewelling and Hubbell (22), who concluded that permeability is in part a function of a dipole potential at the bilayer surface that favors permeation of anions. 

The influence of resistance on the cost of demineralization has been estimated. From the operating data of Katz, the average resistance per unit area of one cell pair or repeating unit—i.e., one cation- and one anion-permeable membrane plus... 

The principle of operation of a multicompartmented electrodialysis unit (11) is shown in Fig. 1.11. The cation and anion permeable membranes carry a fixed charge thus they prevent the migration of species of like charge. In a commercial version of Fig. 1.11, there would be several hundred rather than three compartments, multicompartmentalization being required to achieve electric power economics, since electrochemical reactions take place at the electrodes. 

It has been observed experimentally that varions combinations of cation-permeable and anion-permeable membrane pairs have varying capacities to eliminate different ions. It is possible to enhance potassinm elimination by choosing an appropriate pair of membranes, thus achieving tartrate stabilization without greatly modifying the acetic acid content, as this would be unacceptable. Pairs of membranes that eliminate potassium tend to reduce pH, even if the tartaric acid content is reduced. Membranes selected by Moutounet et al. (1994) reduced pH by under 0.2 units and volatile acidify by just a few percent. 

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