Electrodialysis cation-exchange

Figure 6.7 Model concentration profile at the membrane-solution interface during electrodialysis (cation exchange membrane). F is the Faraday constant, Clf C2, C3 and C4, concentrations of desalting side solution, of the membrane—solution interface desalting side, at the membrane-solution interface (the concentrated side) and of the concentrated solution, respectively <5m, is the thickness of ion exchange membrane, <5i and d2 the thickness of the diffusion boundary layer at the desalting side and concentrated side, respectively t+ and t+ are the transport numbers of the cation in the solution and in the membrane, I is the current density, D and Dm are diffusion coefficients of electrolyte in the solution and in the membrane.

The fourth fully developed membrane process is electrodialysis, in which charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process utilizes an electrodialysis stack, built on the plate-and-frame principle, containing several hundred individual cells formed by a pair of anion- and cation-exchange membranes. The principal current appHcation of electrodialysis is the desalting of brackish groundwater. However, industrial use of the process in the food industry, for example to deionize cheese whey, is growing, as is its use in poUution-control appHcations. 

Dissolved Solids None Dissolved solids is measure of total amount of dissolved matter, determined by evaporation high concentrations of dissolved solids are objectionable because of process interference and as a cause of foaming in boilers Various softening processes, such as lime softening and cation exchange by hydrogen zeolite, will reduce dissolved, solids demineralization distillation reverse osmosis electrodialysis... 

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. 

Figure 14.3 Schematic drawing of an electrodialysis cell. A, Anion-exchange membranes C, cation-exchange membranes.

Another way to obtain acids and bases is to make use of the transport of hydrogen ions through cation-exchange membranes, respectively of hydroxyl ions through anion-exchange membranes, which occurs during electrodialysis if no new counterions are offered by the depleted solution (1, 113, 114, 115). 

Electrodialysis (ED) is a unit operation for the separation or concentration of ions in solutions based on their selective electromigration through semi-permeable membranes under the influence of a potential gradient (Lacey and Loeb, 1972 Strathmann, 1992). Owing to their selectivity, ion-exchange membranes (IEM) allow transport of only cations (cation-exchange membranes) or anions (anion-exchange membranes) and thus can be used to concentrate, remove, or separate electrolytes. 

Figure 10.17 Flow schematic of electrodialysis systems used to exchange target ions in the feed solution, (a) An all-cation exchange membrane stack to exchange sodium ions for calcium ions in water softening, (b) An all-anion exchange membrane stack to exchange hydroxyl ions for citrate ions in deacidification of fruit juice...

Many of today s available membranes meet most of these requirements. In particular, the Nafion-type cation-exchange membrane has quite satisfactory properties for applications in the chlorine-alkaline electrolyses as well as in electrodialysis [6], Anion-exchange membranes often show lower stability in strong alkaline solutions than cation-exchange membranes. 

The key element in electrodialysis with bipolar membranes is the bipolar membrane. Its function is illustrated in Figure 5.11(a), which shows a bipolar membrane consisting of an anion- and a cation-exchange layer arranged in parallel between two electrodes. 

FIGURE 40 (a) Electrodialysis and (b) electrodialysis reversal (EDR). Cation exchange membrane indicated by C and anion exchange membrane by A (Ionics Inc.)... 

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. 

Nation membranes have also been used to carry out other concentration schemes. Hwang et al. employed an electro-electrodialysis approach to raise the concentration of HI within the HI, solution at 110°C. Nation acts as a cation exchange membrane in an electrolysis cell in which HI is formed at the cathode. This raises the HI concentration within the HI catholyte (figure 4.23). In addition to this, the increase in HI content helps to break up the azeotropic between HI, I2, and H2O and facilitates the distillation of HI from HI. Researchers at JAERI have taken this concept... 

Wisniewski, J. and G. Wisniewska (1997). Application of electrodialysis and cation exchange technique to water and acid recovery. Environ. Protection Eng., 23, 3-4, 35-45. 

On the basis of this study, Sudoh et al. [78] proceeded to generate hydrogen peroxide in an acidic solution (1 M H2SO4) for use in Fenton s reagent oxidation of wastewater streams (Fig. 20). The peroxide, produced at the cathode in an alkaline KOH electrolyte, was transferred by electrodialysis to a central chamber, separated by anion exchange membrane (ACLE-5P, Tokuyama Soda, Japan) on the cathode side and a cation exchange membrane (CM-2, Tokuyama Soda, Japan) on the anode side. At a current of 4 A, 2.2 kmol/m of H2O2 was found to accumulate in the cen-... 

Enoch et al. [90] used electrodialysis reversal (EDR) to prepare boiler makeup water for Dutch power stations from several types of surface waters. EDR uses automatic reversal of electrode polarity at regular time intervals to minimize membrane scaling. The EDR unit contained 200 anion and cation exchange membrane pairs, each with a surface area of 0.47 m. Polarity reversal occurred every 15, 20, or 25 min. Samples of surface water were desalted by 96% at an energy consumption of 1 kWh/m of product water and at a current density (8.3 A/cm ) that was 80% of the limiting current density (current density when the surface water cation concentration at the membrane surface drops to zero). 

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. 

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