Electrodialysis electrodes

Electrodialysis. In electro dialysis (ED), the saline solution is placed between two membranes, one permeable to cations only and the other to anions only. A direct electrical current is passed across this system by means of two electrodes, causiag the cations ia the saline solution to move toward the cathode, and the anions to the anode. As shown ia Figure 15, the anions can only leave one compartment ia their travel to the anode, because a membrane separating them from the anode is permeable to them. Cations are both excluded from one compartment and concentrated ia the compartment toward the cathode. This reduces the salt concentration ia some compartments, and iacreases it ia others. Tens to hundreds of such compartments are stacked together ia practical ED plants, lea ding to the creation of alternating compartments of fresh and salt-concentrated water. ED is a continuous-flow process, where saline feed is continuously fed iato all compartments and the product water and concentrated brine flow out of alternate compartments. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

FIG. 22-56 Schematic diagram of electrodialysis. Solution containing electrolyte is alternately depleted or concentrated in response to the electrical field. Feed rates to the concentrate and dduate cells need not be equal. In practice, there would he many cells between electrodes. 

Electrode isolation is practiced to minimize chlorine production and to reduce fouhng. A flush solution free of chlorides or with reduced pH is used to bathe the electrodes in some plants. Further information on electrodes may be found in a work by David [ Electrodialysis, pp. 496 99, in Porter (ed.), op. cit.]. 

Consider a typical electrodialysis process. What will happen if the electrodes on either side are interchanged Explain the performance of the modified system. 

Eadie-Hofstee plot, 111 Electrode, 14, 15, 72, 73, 75-80 Electrodialysis, 351, 353-356, 393 Elemental composition, 228-230 Embden-Myerhof-Pamas pathway (EMP), 3, 207, 244-251... 

Among electrochemical methods of water purification, one can also list the various electromembrane technologies, electrodialysis in particular. The simplest elec-trodialyzer consists of three compartments separated by semipermeable membranes (usually, cation- and anion-exchange membranes). The water to be purified is supplied to the central (desalination) compartment. In the outer (concentration) compartments, electrodes are set up between which a certain potential difference is applied. Under the effect of the electric field, ions pass througfi the membranes so that the concentration of ionic contaminants in the central compartment decreases. 

FIGURE 26.2 Electrodialysis purifying a feed solution. An alfemation of anion-exchange membranes and cafion-exchange membranes is placed between electrodes in a receiving solution outside the last membranes. Only a central section is shown. 

Much more simply, the same result can be attained with bipolar membranes, membranes consisting of an anion- and cation-permeable (an anion- and cation-exchange) membrane laminated together. At such a membrane, when mounted between electrodes so that the cation-exchange layer faces the anode, water is split into and OH ions so that the acidic and alkaline solutions required for regeneration as above are produced at the respective surfaces of the bipolar membrane. When such membranes are suitably integrated into the sequence of membranes in the electrodialysis unit above, gas evolution at the electrodes is not needed the acid-base pair is produced with about half the power. 

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 are constructed using a plate-and-frame technique similar to filter presses. Alternating sheets of anionic and cationic membranes are placed between two electrodes. The plating or rinse solution to be recovered (electrolyte) circulates past the system s electrodes. Hydrogen and oxygen evolve. Positive ions travel to the negative terminal and negative ions travel to the... 

Atom economy is high. As a reagent, no compounds are needed and consequently none are produced as the electron is immaterial. This results in a greater advantage of electrochemical reactions compared to chemical conversions, namely, an effective contribution to pollution control. The direct ET from the electrode to the substrate avoids the problem of separation and waste treatment of the, frequently, toxic end products of chemical reductions or oxidations. Furthermore, by electrodialysis, organic acids or bases can be regenerated from their salts without the use of, for example, sulfuric acid or... 

An electrodialysis set-up is made out of a sequence of parallel cells (a stack) placed between two power electrodes to which a potential drop (voltage) is applied. This latter divides between the cells according to their resistance. As a result, salt concentration is reduced at the exit from every odd cell (desalination or depletion compartments) and is increased at the exit from every even (brine) compartment. 

Only one basic design of electrodialysis equipment for demineralization appears to be in use. This is an assembly of alternate cation and anion ion exchange sheets separated by spacers in groups of several hundred clamped together between electrodes. The assembly physically resembles a plate-and-frame filter press. Figures 15.21(a) and (b) show such assemblies, and some dimensional data were stated in Section 15.5, Electrodialysis. 

The proposed electrochemical method of saline water demineralization is similar to electrodialysis, in that ions are removed from a solution by electrical transference. If the electrodes are porous, oppositely charged ions will be attracted to them and thus... 

In the study described in Christensen et al. (2004, 230), the arsenic content of the wood before electrodialysis was 837 114 mg kg-1 (95 % confidence level) based on 95 samples. The electrodialytic process was more effective if the wood was first broken down into <2 cm chips and soaked in phosphoric acid followed by oxalic acid (Christensen et al., 2004, 236). The soaking probably leaches a significant amount of the arsenic and metals from the wood, which allows the electrodialytic process to remove most of the remaining arsenic, copper, and chromium (Christensen et al., 2004, 235-236). The most efficient results for all three contaminants, which included >95 % removal of arsenic, used 100 kg of wood chips with a 60-cm spacing between the electrodes. The electrodialysis lasted for 21 days (Christensen et al., 2004, 231). 

Electrodialysis involves the application of an electric field to a colloidal dispersion that has been placed in a chamber arranged so that one or both electrodes are separated from the dispersion by semi-permeable membrane(s). Typically, dissolved ions can flow through the membrane(s) in response to the electric field gradient, while the dispersed particles (or other species) are restrained within the chamber. 

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.

The electrod ialysis stack A key element in electrodialysis is the so-called stack, which is a device to hold an array of membranes between the electrodes that the streams being processed are kept separated. A typical electrodialysis stack used in water desalination contains 100-300 cell pairs stacked between the electrodes. The electrode containing cells at both ends of a stack are often rinsed with a separate solution which does not contain Cl- ions to avoid chlorine formation. 

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