Electrodialysis

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. 

Electrodialysis is a process in which ions contained in the solution are separated by membranes in the presence of an external electric field. The membranes employed in this process are called ion-exchange membranes. There are two types of membranes cation-exchange membranes, which are penetrable only for cations, and anion-exchange membranes, penetrable for anions only. 

Ionic motion leads to the appearance of an ion concentration gradient in the solution at the membrane surfaces. The resultant polarization of concentration is similar to polarization in the electrolytic cell. This polarization is responsible for the low concentration of salt and great strength of the electric field near the membrane in the dialyzate channel. 

By analogy with the problem of convective diffusion in the channel with a soluble wall (see Section 6.3) and in the channel with membrane walls (see Section 6.4), we can introduce the concepts of the region of concentration development and the region of developed concentration (see Fig. 7.5). 

Assume for simplicity that the ohmic resistance of membranes is small and can be ignored. Also, assume that the electrolyte consists of one positive component and one negative component with equal numbers of ions  

Note that these assumptions are not essential, they are needed only to simplify calculations. 

Electrodialysis (ED) is used to remove ionized substance from hquids through selective ion-permeable membranes. ED is the most widely commercialized electromembrane technology. Desalination of brackish water is the area of electrodialysis application with the largest number of installations. This chemical-free technology competes with reverse osmosis. Electrodialysis shows better resistance to fouling and scaling. It also has an economical advantage in desalination of low-salinity solutions [13]. Also, it should be kept in mind that because of small material consumption ED is the most environmental friendly process for solution desalination [14]. 

Electrodialysis has the ability to concentrate salts to high levels with much less energy consumption than evaporation would require. This capability has been utilized in Japan to make edible salt by recovering NaCl from seawater and concentrating it to 20% before evaporation. The plants there are huge some have greater than 100 000 square meters of membrane. Salt recovered by electrodialysis in Kuwait is the raw material for a chlor-alkali plant there. Electrodialysis has also been used to concentrate salts in reverse osmosis brines [32]. 

Electrodialysis is used in a wide variety of food applications. Throughout the world it is used to remove salt from cheese whey so that the other components of whey can be used as food for humans and animals. In Japan, the mineral composition of cows milk intended for infant formula is altered by electrodialysis to more closely resemble the composition of mother s milk. In Erance, potassium tartrate is removed from wine to prevent its precipitation. In Japan, salt is removed from soy sauce to allow its use by people with hypertension. Salts of organic acids. 

In the food applications mentioned above it is impractical to remove components that could foul the membranes, because these are necessary constituents of the product. In such cases the process is operated under conditions that minimize fouling, and then the fouling that does occur is handled by cleaning in place (CIP). The CIP procedures can include soaking in brine, current reversal and washing with acid, base and nonionic surfactants. 

The mandatory condition for an electrodialysis process to be executed is an alternating order of cation and anion membranes and electric field applied across the entire assembly (Fig. 6.3). Between the alternating membranes are two types of compartments - desalination and concentrating. Ions wiU migrate from the compartments where electric current is passing from an anion membrane to a cation one (the even compartments in Fig. 6.3). They will be transferred to the successive compartments (the odd compartments in Fig. 6.3). These compartments will accumulate the ions because the ion-exchange membrane between them would prevent ions from moving further. Therefore the solution in the even compartments will be demineralized and solution in the odd compartments will be concentrated. 

Electrodialysis consists of applying a direct current across a lx)dy of water separated into vertical layers by membranes alternately permeable to cations and anions. Cations migrate toward the cathode and anions toward the anode. Cations and anions both enter one layer of water, and both leave the adjacent layer. Thus, layers of water enriched in salts alternate with those from which salts have been removed. The water in the brine-enriched layers is recirculated to a certain extent to prevent excessive accumulation of brine. 

In electrodialysis, electrolytes are transferred through solutions and membranes by an electrical driving force. Electrodialysis is used to change the concentration or composition of solutions, or both. The process usually involves multiple, thin compartments of solutions separated by membranes that allow passage of either positive ions (cations) or negative ions (anions) and block the passage of the oppositely charged ions. But the process may be operated with only one membrane that separates two electrode-rinse solutions to purify one of the solutions (e.g., production of salt-free sodium hydroxide). 

As in any membrane process, selective membranes are critically important in electrodialysis. Therefore, the nature of the ion-exchange membranes used in electrodialysis will be discussed first. 

Cation-exchange membranes are permeable to cations but not anions. Anion-exchange membranes are permeable to anions but not cations. The source of this ability to discriminate between cations and anions is discussed with the aid of Fig. 21.2-1. 

FIXED NEGATIVELY CHARGED EXCHANGE SITE I.E.,S03 MOBILE POSITIVELY CHARGED EXCHANGEABLE CATION I.E.,Na POLYSTYRENE CHAIN DIVINYLBENZENE CROSSLINK FIGURE 21.2-1 Representation of an ion-exchange membrane. 

Elias Klein, Richard A. Ward and Robert E. Lacey 

Clearly electrodialysis can be used for concentrating ionic solutions, deionizing salt solutions and separating ionic and non-ionic species (Fig. 11.10). In the context of this chapter, electrodialysis has appHcations for the following  

Electrodialysis has a number of other large-scale applications and these would include the manufacture of pure sodium chloride for table salt (in Japan, electrodialysis is the principal method, production exceeding 10 ton yr ), the demineralization of cows milk (for baby food), cheese whey and sugar solutions, the removal of excess acid from fruit juice and the isolation of organic acids from reaction streams. 

Data for three electro dialysis plants for the desahnation of well water. 

Major ionic species CaS04 NaCl NaCl 

Operation Continuous, four stacks in series Batch, one stack Continuous, one stack 

The desalination of brackwish water (Table 7.3) in order to produce drinking water. Typically sea water or well-water is treated to reduce the total salt concentration from 1000-3000 mg dm to less than 500 mg dm . In most cases, the major ionic component is sodium chloride but this is not always so, e.g. in some well water the chief salt is calcium sulphate. 

The recycling of transition-metal ions, e.g. the rinse waters from nickel plating containing perhaps Igdm nickel sulphate can be concentrated to 60 g dm and recycled directly to the plating bath. The removal of Cr(vi) is discussed in section 7.4. 

Salt removal from effluent waters prior to reuse in industrial processes. Reduction in the concentration of ionic species is often necessary if the water is to be recycled through a chemical process, e.g. to reduce corrosion. 

Capacity/m day raw water purified water Salinity of feed/mgdm Salinity of product/mg dm Major ionic species Operation 

Most importantly, electrodialysis is not a suitable separation/purification method if magnesium or calcium hydroxide is utilized as a neutralizing agent because binary ionic compounds cannot be separated [100]. 

As discussed by Pletcher 24, electrodialysis is an electrically driven membrane separation process. The main use of electrodialysis is in the production of drinking water by the desalination of sea-water or brackish water. Another large-scale application is in the production of sodium chloride for table salt, the principal method in Japan, with production exceeding 106 tonne per annum. 

The membranes in electrodialysis stacks are kept apart by spacers which define the flow channels for the process feed. There are two basic types(3), (a) tortuous path, causing the solution to flow in long narrow channels making several 180° bends between entrance and exit, and typically operating with a channel length-to-width ratio of 100 1 with a cross-flow velocity of 0.3-1.0 m/s (b) sheet flow, with a straight path from entrance to exit ports and a cross-flow velocity of 0.05-0.15 m/s. In both cases the spacer screens are 

The value of /jim is determined by the discontinuity in the dependence of cell current on applied cell voltage which occurs when the interfacial concentration approaches zero. The polarisation parameter is convenient in the design and scale-up of electrodialysis equipment. It can be easily measured in small-scale stacks at a given value of bulk concentration and then used to predict limiting current densities in larger stacks at other concentrations. Most stacks use operating values of the polarisation parameter that are 50-70 per cent of the limiting values. 

Like the other methods, the main purpose of dialysis and electrodialysis is the separation of small and large molecules it is often used for desalting purposes. These are based on the phenomenon that certain compounds can diffuse through a semipermeable membrane, while others cannot. This differentiation is mainly based on molecular size. The principle of dialysis is, in fact, quite similar to ultrafiltration the driving force is not only gravity (assisted by centrifugation) but also osmotic pressure. 

In a typical dialysis experiment, a membrane separates two liquid phases, one of which is the sample (see Fig. 1) and the other is a clean washing liquid. The membrane is permeable for small molecules but retains large ones. Small molecules can therefore diffuse through the membrane into the other liquid phase. This diffusion process goes on until equilibration is reached. In practice, a large amount of washing liquid and a small amount of sample solution are used, so 

A typical example of dialysis is desalting of proteins. About 500 p,l protein solution is put into a dialysis tube, which is immersed in 500 ml buffer solution (see Fig. 1). The salts diffuse from the sample into the buffer solution, while the buffer (since diffusion can occur in the opposite direction too) diffuses into the sample and maintains the pH. This process not only desalts the protein but also can be used to exchange the buffer. 

Various dialysis membranes are used those of 10-15 kDa molecular weight cutoff are most common. Dialysis may also be used to clean small molecules from unwanted macromolecules. It is easy to miniaturize sample volumes as small as a microliter can be used (e.g., one drop of sample placed onto a small filter floating on pure water). Various parameters may influence the efficiency of dialysis, such as the type of the membrane, temperature, the volume of the sample, extractant volume, etc. Efficiency of the dialysis may significantly be decreased if the analytes bind to the membrane either by electrostatic or by hydrophobic interaction. The use of a low-concentration surfactant may decrease this effect. 

In electrodialysis, diffusion of charged compounds through the membrane is aided by an electric potential difference. Naturally, this potential difference acts only on charged species, so in electrodialysis the charge on the analyte has key importance. 

Colloidal silica has been made by various procedures involving electrodialysis whereby sodium ions are removed from a solution of sodium silicate to produce sol. These have been reviewed by Her (8), but in no case were stable products made. Sanchez (87) and Her (88) patented processes of electrolyzing alkali metal silicate solution to continuously remove alkali metal ions until a sol is obtained. 

There is essentially no consumption of acid except the small amount needed at the start of each batch to neutralize a dilute solution of sodium silicate (0.5% SiOj) to pH 9 at 60-90 C to form silica nuclei to start the process. A narrow uniform spac- 

A type of electrolytic process patented by Tripp (91) is used to dissolve an anode of silicon metal in alcohol containing a metal salt catalyst such as copper sulfate to produce a silica organosol. 

In a study of transport of silica through membranes during electrodialysis, Boari et al. (92) found that no transport or silica deposition occurred unless the pH was such that HSiOj ions were present. This is consistent with the observation that it is necessary to carry out electrodialysis at less than pH 9.5 (88) in order not to deposit silica in the membrane. 

Many variations in the ion-exchange procedure have been proposed. Dirnberger 

In this chapter only electromembrane separation processes such as electrodialysis, electrodialysis with bipolar membranes, and continuous electrodeionization will be discussed. 

7 IS Electrodiatysis cdlSn For the concentration of soditim chloride in sea water, (Courtesy Asaht Chemical Industry Co. Ltd.) 

The anion-selective (AX) membranes (Fig. 2b) also consist of cross-linked polystyrene but have positively charged quaternary ammonium groups chemically bonded to most of the phenyl groups in the polystyrene instead of the negatively charged sulfonates. In this case the counterions are negatively charged, eg, Cl , HCO 3, NO 3, or S02 4. 

Commercially available membranes are usually reinforced with woven, synthetic fabrics to improve the mechanical properties. Several hundred thousand square meters of IX membranes are now produced annually, and the mechanical and electrochemical properties are varied by the manufacturers to suit the proposed applications. The electrochemical properties of most importance for ED are (/) the electrical resistance per unit area of membrane (2) the ion transport number, related to current efficiency (3) the electrical water transport, related to process efficiency and (4) the back-diffusion, also related to process efficiency. 

Commercial IX membranes have thicknesses of ca 0.15—0.5 mm and electrical resistances of ca 3-20 fhcm2 at 25°C when in equilibrium with 0.5 N sodium chloride. The electrical resistances are somewhat higher in more dilute solutions because co-ions are more effectively excluded from the membrane by the IX resin. The electrical resistance decreases with increasing temperature at a rate of ca —1.9%/°C. The electrical resistance of an ED apparatus 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Back-diffusion is the transport of co-ions, and an equivalent number of counterions, under the influence of the concentration gradients developed between enriched and depleted compartments during ED. Such back-diffusion counteracts the electrical transport of ions and hence causes a decrease in process efficiency. Back-diffusion depends on the concentration difference across the membrane and the selectivity of the membrane the greater the concentration difference and the lower the selectivity, the greater the back-diffusion. Designers of ED apparatus, therefore, try to minimize concentration differences across membranes and utilize highly selective membranes. Back-diffusion between sodium chloride solutions of zero and one normal is generally ca 2 x 10-6 meq/(s-cm2). 

Schematic representation of (a) cation-exchange resin, and (b), anion-exchange resin. 

In this process, dissolved electrolytes are removed by application of electromotive force across a battery of semipermeable membranes constructed from cation and anion exchange resins. The cation membrane passes only cations and the anion membrane only anions. The two kinds of membranes are stacked alternately and separated about 1mm by sheets of plastic mesh that are still provided with flow passages. When the membranes and spacers are compressed together, holes in the comers form appropriate conduits for inflow and outflow. Membranes are 0.15-0.6 mm thick. A commercial stack may contain several hundred compartments or pairs of membranes in parallel. A schematic of a stack assembly is 

Membranes may be manufactured by mixing powdered ion exchange resin with a solution of binder polymer and pouring the heated mixture under pressure onto a plastic mesh or cloth. The concentration of the ion exchanger is normally 50-70%. They are chiefly copolymers of styrene and divinylbenzene, sulfonated with sulfuric acid for introduction of the cation exchange group. 

Standard cell sizes are up to 30 by 45 in. In an individual stack the compartments are in parallel, but several stacks in series are employed to achieve a high degree of ion exchange. The ion exchange membrane is not depleted and does not need regeneration. The mechanism is that an entering cation under the influence of an emf replaces an H+ ion from the resin and H+ from solution on the opposite face of membrane replaces the migrating cation. 

Like many other specialities, electrodialysis plants are purchased as complete packages from a few available suppliers. Membrane replacement is about 10% per year. Even with prefiltering the feed, cleaning of membranes may be required at intervals of a few months. The comparative economics of electrodialysis for desalting brackish waters is discussed by Belfort (1984) for lower salinities, elecfrodialysis and reverse osmosis are competitive, but for higher ones elecfrodialysis is inferior. Elecfrodialysis has a number of important unique applications, for removal of high contents of minerals from foods and pharmaceuticals, for recovery of radioactive and other substances from dilute solutions, in electro-oxidation reduction processes and others. 

C) SALT DEPLETION OCCURS BETWEEN A PAIR OF ANION- AND CATION-EXCHANGE MEMBRANES 

There are two general types of commercially available ion-exchange membranes heterogeneous and homogeneous. Both types usually contain a reinforcing fabric to increase tensile strength and improve dimensional stability. Heterogeneous membranes have two distinct polymer phases. They are rather 

0 FIXED NEGATIVELY CHARGED EXCHANGE SITE, J. E. S05 MOBILE POSITIVELY CHARGED EXCHANGEABLE CATION I. E., Na+ = POLYSTYRENE CHAIN S2S DIVINYLBENZENE CROSSLINK 

Commercial IX membranes have thicknesses of ca 0.15—0.5 mm and electrical resistances of ca 3-20 at 25°C when in equihbtium with 0.5 N 

How does the system work It has been proposed that the highly reactive hydroxyl (OH) radical is responsible for the strong oxidizing power. The OH radical could be formed by Fenton s reaction  

Since O3 may be formed, it is likely that hydrogen peroxide may also be produced. Fe is readily formed from the iron anode. 

Another possible process which may be occurring depends on the presence of chloride (d ) ions in the treated water. Chloride could be oxidized to chlorine (CI2), which can oxidize the organic matter and kill the pathogenic bacteria. In either case, the electrolysis relies on reactants which are formed during the electrolysis. The simplicity of this process and its reported effectiveness make it an attractive method of recycling water. 

An electrode potential can direct the motion of ions that depends on the polarity of the voltage and the sign of the charge of the ions. This is illustrated in Fig. 15.7 where the flow of cations is directed 

Some water-softening (conditioning) units which are sold for domestic use are advertized as no saltwater conditioners. These units are reported to remove CaCOs from hard water by using a catalyst, which by epitaxial nucleation, and the reduction of pressure by virtue of a change in water velocity, converts the Ca(HC03)2 into CaCOs and CO2. These units are very attractive and are advertised to work with detergents but are not intended for use with soap. This can be interpreted to mean that the calcium ions are not removed from the water system and that a precipitate will form from the calcium salt of the fatty acid from the soap (Eq. 15.14). 

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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. 

F. H. WeUer, ed.. Electrodialysis (ED) and Electrodialysis Reversal (EDR) Technology, Ionics, Inc., Watertown, Mass. 

Oxygen-Evolving Anode. Research efforts to iacorporate the coated metal anode for oxygen-evolving appHcations such as specialty electrochemical synthesis, electro winning, impressed current, electrodialysis, and metal recovery found only limited appHcations for many years. 

Electrodialysis. Electro dialytic membrane process technology is used extensively in Japan to produce granulated—evaporated salt. Filtered seawater is concentrated by membrane electro dialysis and evaporated in multiple-effect evaporators. Seawater can be concentrated to a product brine concentration of 200 g/L at a power consumption of 150 kWh/1 of NaCl (8). Improvements in membrane technology have reduced the power consumption and energy costs so that a high value-added product such as table salt can be produced economically by electro dialysis. However, industrial-grade salt produced in this manner caimot compete economically with the large quantities of low cost solar salt imported into Japan from Austraha and Mexico. 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

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. 

Electrodialysis Reversal. Electro dialysis reversal processes operate on the same principles as ED however, EDR operation reverses system polarity (typically three to four times per hour). This reversal stops the buildup of concentrated solutions on the membrane and thereby reduces the accumulation of inorganic and organic deposition on the membrane surface. EDR systems are similar to ED systems, designed with adequate chamber area to collect both product water and brine. EDR produces water of the same purity as ED. 

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. 

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