Ion Exchange Membranes for Electrodialysis: A Comprehensive Review of Recent Advances

: Electrodialysis related processes are effectively applied in desalination of sea and brackish water, waste water treatment, chemical process industry, and food and pharmaceutical industry. In this process, fundamental component is the ion exchange membrane (IEM), which allows the selective transport of ions. The evolvement of an IEM not only makes the process cleaner and energy-efficient but also recovers useful effluents that are now going to wastes. However ion-exchange membranes with better selectivity, less electrical resistance, good chemical, mechanical and thermal stability are appropriate for these processes. For the development of new IEMs, a lot of tactics have been applied in the last two decades. The intention of this paper is to briefly review synthetic aspects in the development of new ion- exchange membranes and their applications for electrodialysis related processes.


INTRODUCTION
Electrodialysis (ED) is an electro-driven separation process that is used for the highly efficient desalination of brackish water, the production of ultra-pure water, water softening and the removal of other charged impurities from water streams including waste water treatment to recover some valuable elements in chemical industry and production of many important chemicals. The efficiency of this process depends on the use of ion exchange membranes (IEMs) to remove unwanted charged particles from the feed stream. For ED process, the IEMs are expected to possess high permselectivity, less electrical resistance, and good chemical, thermal and mechanical stabilities. Although many developments in IEMs have come from researches for the chloro-alkali industry and fuel cells [1,2], the development of new materials and further fundamental research for the ED process are not numerous.
In fact, ion exchange membranes based electrodialysis rapidly became an industrial process for demineralizing and concentrating electrolyte solutions with the development of stable, highly selective IEM of low electric resistance in 1950 by Juda  properties for different applications, including ED. For example, in 1960s, first salt production from sea water was realized by Asahi Co. with monovalent ion permselective membranes [5]; in 1969, the invention of electrodialysisis reversal (EDR) realized long-term run without salt precipitation or deposition on both membranes and electrodes [6]; in 1970s, a chemically stable cation exchange membrane based on sulfonated polytetra-fluorethylene was firstly developed by Dupont as Nafion® [7]; simultaneously, a composition of cation exchange layer and an anion exchange layer into a bipolar membrane in 1976 by Chlanda et al. [8] brings many novelties in electrodialysis applications today [9]. Therefore, to awake researcher's interest in this field and also to understand the present states of ED research, this review provides a comprehensive overview of IEMs covering the fundamentals as well as the recent development of IEMs in this field, and development of novel ED processes. However, this paper is not intended as a review of the literature in these areas. Instead, it is focused on recent progress in synthesis and some new important applications of major homogeneous ion exchange membranes, hybrid ion exchange membranes, as well as bipolar membranes to illustrate the improvement in ED.     Table  1 [10,11].

Homogeneous Ion Exchange Membrane
In order to supply suitable membranes for ED application and to expand the opportunity to new potential applications, the search for new materials for ion-exchange membranes has kept on the rise. To prepare homogeneous IEMs, various tactics are available to introduce ionic groups. These tactics can be divided into three classes based on the starting materials.

1)
Polymerization or polycondensation of monomers; at least one of them must comprise a moiety that either is or can be made cationic or anionic groups, respectively. Then the charged polymers undergo film processing to form membranes

2)
Charge moieties insert ion to polymer chains followed by the formation of membranes

3)
Introduction of functional charge groups on the already film-formed membranes either directly by grafting of a functional monomer or indirectly by grafting nonfunctional monomer followed by functionalization reactions.

Direct Polymerization from Monomer Units
The direct synthesis of polymer from monomer units gives excellent opportunity to control the functional groups quantity and distribution along the polymer backbones. Moreover, this process helps to regulate both microstructure and properties of the ion-exchange membranes. IEMs via direct polymerization of monomers, in which, at least one of them must contain a moiety that is or can be made anionic or cationic, respectively, have been successfully reported [58][59][60].
If such amembrane is prepared from monomer, styrene and divinyl benzene are most frequently used starting material for a conventional hydrocarbon type ion exchange membrane for industrial uses, from which cation exchange membranes (CEMs) are readily obtained through sulfonation of the aromatic ring with chlorosulfonic acid whereas anion exchange membranes (AEMs) are prepared through halomethylation, especially chloromethylation, followed by quaternization. Usually chloromethyl ether (CME), a carcinogenic and hazardous chemical, was used for chloro-methylation reaction and its use has been restricted since 1970s.
As shown in Figure 1, alternative methods have been proposed to minimize the hazards involved in the preparation of AEMs such as polymerization of halomethyl substituted aromatic monomers (i.e. chloromethylstyrene), followed by quaternization with alkylamine [58], and polymerization of p-methylstyrene and subjected to benzylic bromination followed by amination to obtain quaternary ammonium sites [59].
As electronic properties of conducting polymers (CP) are useful for ED application, they are materials  with an enormous projection. Among them, polythiophene derivatives are very important, since their serious problems of solubility and processability have been solved recently. Poly(2-thiophen-3-ylmalonic acid), a new CP bearing two carboxylate units per thiophene ring, has been derived from the alkaline hydrolysis of the poly(2-thiophene-3-yl-malonic acid dimethyl ester), which was obtained by chemical oxidative coupling polymerization [60].

Direct Modification of Polymer Backbone
For soluble polymers, polyarylene polymers containing aromatic pendant groups on polymer backbones such as poly aryl sulfone, poly aryl ketone, and polyphenylene oxides, the corresponding membranes can be obtained either by introducing anionic or cationic moieties, followed by the dissolving of polymer and casting it into a film [61][62][63][64][65][66][67][68][69][70][71][72][73][74]. For the preparation of IEMs, these polymers are attractive as the polymer matrix due to several reasons: (1) their high mechanical and thermal stability, (2) good processibility, (3) relatively high glass transition temperature (Tg), (4) low cost, and (5) the ability to chemically modify the polymer backbone via the electrophilic substitution at their aromatic skeletons [11]. However, for membrane preparation with these soluble polymers, its chemical stability is not so good and often needs post treatment, such as crosslinking [65,66].

Poly(aryl sulfone)
Poly(arylsulfone)s such as polyether sulfones (PES), have strong mechanical, thermal and chemical stability and have been successfully used as ion exchange membranes in electro-chemical applications [61][62][63][64][65][66]. For use as a cation exchange membrane, PES must be functionalized to incorporate fixed negatively charged groups. For the functionalization of PES, sulfonation is commonly used over other forms such as carboxylation or phosphonation, as the process is comparatively simple and yields membranes show good ion transport properties. There are different sulfonation methods to add sulfonic acid groups to PES chain in heterogeneous or homogeneous media with sulfuric or chlorosulfonic acid. However, if chlorosulfonic acid is used as sulfonating agent, sometimes PES chains cleave or undergo branching and crosslinking reactions by the conversion of the intermediate sulfonic acid group into a partially branched or cross-linked sulfone unit [75]. Additionally, the amount of these side reactions for the sulfonation of PES mostly depends on the reaction conditions and polarity of the solvents used [61].
Also, it is difficult to control the degree of sulfonation and chemical structure of a sulfonated polymer if the polymer is directly sulfonated with chlorosulfonic acid or concentrated sulfuric acid. In the course of the sulfonation process, it is often the main chain that becomes sulfonated, however side chain sulfonation increase the chemical and mechanical properties of the polymer [76]. Hence, for potential application in ED with suitable properties, random and multiblock side-chain sulfonated polyether sulfones (sPES) was prepared by condensation polymerization using 2,5-diphenylhydroquinone, 4,4 -Difluorodiphenyl sulfone, and 4,4dihydroxydiphenyl sulfone as monomer [62].
Noted that electrochemical properties of sPES membranes not only depend on the sulfonation processes and structure of the PES but also on the way of preparation [63,64]. Membrane prepared by the solvent evaporation method shows better electrochemical properties than the membrane prepared by the phase inversion method. Also, by tuning both the wet film thickness and film drying time before immersing in water bath to form membrane sheet, the morphology of the membranes can be easily controlled [64].

Poly(aryl ether ketone)
Poly (aryl ether ketone) has also been used as IEM matrix [67][68][69][70]. The modification of the polymer backbone can be carried out in the similar way used for PES polymer material. However, uncrosslinked sulfonated membranes show high swelling ratio which reduces mechanical properties and ion permselectivity, and thus prohibits them from applying in electromembrane processes, especially at higher temperature. For these reasons, these polymers are often blended with non-functional polymers or crosslinked by different means to enhance and modify the transport properties [69,70].

Polyphenylene Oxides (PPO)
From engineering polymer poly(2,6-dimethyl-1,4phenylene oxide) (PPO), anion exchange membranes have been prepared either by chloroacetylationquaternary amination or by bromination-amination process as shown in Figure 2   harmful to human health, normally used in the conventional preparation procedure of AEMs, and the process can be easily controlled through the Friedel-Crafts reaction, which is a kind of simple and well investigated reactions; (2) the used materials are conventional agents with low toxicity [71]. Via chloroacetylation, the membranes with high IEC can be obtained. Nevertheless, the physical stability of the resultant membranes still requires more improvement. This problem can be solved by the introduction of bromination substitutions (Figure 2b) which can occur on both aryl and benzyl positions. Now it can be easily aminated and can also be created cross-linking to some extend among the functional groups of the membranes [72,73]. It is noted that this crosslinking can also be conducted after the formation of base membrane [10].
The cation exchange membrane can also be obtained with the same way: by bromination and sulfonation (Figure 2c) or directly by sulfonation [10,74].

Charge Induced on the Film Membranes
The IEM can also be prepared by forming the noniongenic polymer films firstly, subsequently by the introduction of charged functional groups onto the formed polymer films either directly by grafting of a functional monomer or indirectly by grafting nonfunctional monomer followed by functionalization reaction.
Both porous and non-porous membranes can be used as the film substrates. Typical examples of grafting substrates include hydrocarbon polymer based films of polyethylene (PE), polypropylene (PP), polyalkene (polyalkene non-woven fabrics (PNF)), and fluorocarbon polymer based films of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).
For the grafting agents, there are two major categories; (1) functional monomers such as acrylic acid, methacrylic acid that can be directly attached to the substrate as charged functional groups and (2) non-functional monomers such as styrene, glycidyl methacrylate (GMA, the ester of methacrylic acid and 2,3-epoxy-propanol that bears a reactive epoxy group) and vinylbenzyl chloride that can be further chemically modified into the ion-exchangeable.
Radiation-induced graft copolymerization is well known for its merits and potential to transform the chemical and the physical properties of pre-existing polymeric materials without altering their intrinsic properties. Using this methodcation exchange membrane for electrodialysis application was early prepared by grafting of methacrylic acid onto preformed polymer film PE [77]. Another type of membranes prepared by grafting of fluorinated monomers such as  methyl trifluoro-proproenoate onto PTFE films was found to be promising for electrodialysis processes as the use of fluorinated monomer imparted more chemical stability to membranes [78].
Both anion and cation exchange membranes can be obtained from the same precursor prepared by the radiation-induced grafting of glycidyl methacrylate (GMA), and subsequent chemical modification of poly(GMA) graft chains by different ways i.e. amination gives anion exchange, sulfonatin or phophonation gives cation exchange membranes [79,80]. The anion exchange membranes can also be prepared by grafting of vinylbenzyl chloride onto fluorinated PVDF films followed by amination reaction to convert the functional groups to amine derivatives [81].
Into Nafion precursor, made by DuPont, usually charge induces through the hydrolysis of this resin with sulfuric acid and hydrogen peroxide to form a perfluorosulfonic polymer. It is a logical support for a thin carboxylic acid-form membrane onto Nafion to decrease water permeability because the sulfonic acid form of this membrane is highly permeable to water and ions. The carboxylate layer could be created by two ways; (1) cast or laminated onto a sulfonate layer (e.g., Nafion 90209) and (2) by modifying the surfaces of a sulfonate-form membrane. To create very thin, defect-free carboxylate layers on already thin starting films, surface modification is the better way [82]. The general surface modification reaction of Nafion has included four primary steps: reduction of sulfonyl fluoride to sulfinic acid, hydrolysis of residual sulfonyl fluoride to sulfonate, oxidation of sulfinic acid to carboxylic acid, and cleaning of the resultant ionomer.

Mixed Matrix Ion-Exchange Membrane
Actually it is difficult to acquire all the targeted properties in one IEM to satisfy the requirements of particular application. In fact, different applications usually require specific membrane properties. For electrodialysis, the ion-exchange membranes are expected to possess high permselectivity, excellent conductivity, and good chemical, thermal and mechanical stabilities. As most polymer based IEMs have some common drawbacks including insufficient mechanical, chemical and thermal stabilities, and poor fouling resistance, they are still insufficient for the ED process. The concept of combining two distinct materials forming a new composite that keeps desirable properties of both components is an alternative method for the development of new IEMs with excellent electrochemical properties and good mechanical stability. Table 2 provides examples of composite (or mixed matrix) ion-exchange membranes prepared from different routes and some of their properties for electrodialysis applications.
Inorganic-organic composite materials have gained increasing attention due to its specific properties  [83,84] sPES-sulfonated silica Blending The composite showed good ionic conductivity, transport properties while maintain acceptable mechanical and thermal stability.

PES-Rasine Blending
Membrane prepared by solvent evaporation method showed higher transport number, permselectivity, and relative transport number compared to gelation method. [102] HIPS-PAni Blending The ion percent extraction for mixed matrix membrane is similar to that observed for the commercial membrane. [103][104][105]  Mixed matrix ion-exchange membrane can be made by several routes including sol-gel process, intercalation, blending, in situ polymerization, molecular self-assembling, but probably physical blending and sol-gel process are the most prominent technique as depicted in Figure 3 [11]. For the first approach, the resultant membranes normally show phase separation from aggregated fillers, causing mechanical instability of the composite membranes. On the other hand, solgel method offers better interconnection between two domains.
It is known that a big problem in development of composite materials is the distribution of inorganic nanoparticles in the organic matrix due to the aggregate tendency of the nanoparticles. However, by enhancing the interaction among them via covalent bond, hydrogen bond, and electrostatic interaction, this problem can be reduced. Frequently used strategies for enlightening the interaction between inorganic and organic matrix are; (1) functionalization of inorganic fillers or/and polymer matrix (2) introduction of the inorganic filler on the polymer chains. The functionalization of mesoporous SiO 2 using sulfonate groups is a good policy to enhance the distribution of the nanoparticles in the polymer matrix [85][86][87][88]. Moreover, the addition of functionalized mesoporous nanoparticles not only increases charged functional groups in the composites, but also improves the ionic transport properties due to their high surface area and well-accessible mesopores.
Mixed matrix ion-exchange membrane can also be made by mechanical incorporation of powered ionexchange resin into sheets of rubber, PVC, acrylonitrile copolymers or some other extrudable or mouldable matrix. Such membranes can be prepared by different methods; (1) dry moulding of inert film forming polymers and ion-exchange particles and then milling the mould stock, (2) calendaring ion-exchange particles into an inert plastic film, and (3) resin particles can be dispersed in a solution containing a film forming binder and then the solvent is evaporated to give ionexchange membrane. To increase mechanical stability, such composite membranes may also be reinforced with a chemically resistance fabric.
Recently, a two-step phase inversion membrane formation technique has been developed for the preparation of composite membrane which allows a good control of the membrane structure, porosity and electrochemical properties [86]. In this procedure polymer with ion exchange group firstly dissolved in a solvent. The polymer solution is then cast on glass  substrates with different solution thickness and then partially dried in a vacuum oven at 60 C with different aging times before precipitating in 60-70 C DI water. By tuning the aging time before precipitating the partly dried film in water bath, membrane porosities can be control via this new technique. Among the IEMs prepared by this method, membranes with interconnecting pore structure show good ionic conductivity but their transport number and selectivity are very poor. On the other hand, membranes with smaller pores and denser surface are more selective, but less conductive.

Bipolarmembrane
A bipolar membrane (BPM) is a composite membrane consists of an anion-exchange layer and a cation-exchange layer and usually has a catalytic intermediate layer (InL) between the two ion exchange layers. Recently bipolar membrane has gained increasing attention as an efficient tool for the production of acids and bases from their corresponding salts by electrically enforced accelerated water dissociation. Actually, water dissociation takes place at the InL between ion exchange layers (CEL and AEL) of BPM and particularly depends on their properties.
However, a noble BPM should have high selectivity, water dissociation efficiency, acid and base stability over wide pH range, low salt diffusion, along with good thermal and mechanical stability. These desired properties can be obtained by suitable membrane forming materials, selection of InL and membrane casting methodology.
For preparing bipolar membranes with suitable properties, various methods has been initiated, such as laminating polymeric films with fixed charges of opposite polarity with heat and pressure or with an adhesive paste [106], preparing by casting a cation exchange polyelectrolyte solution onto an anion exchange membrane or vice-versa [107], or preparing from the same base membrane by simultaneous functionalizing at the two membrane sides [108] or selectively functionalizing on one side to give cation selectivity and on the other side to give anion selectivity [109], etc. Among these, the casting method is the most efficient one for the preparation of such membrane because it is simple, less costly and also allows a bipolar membrane with desired properties [110]. Using this method, novel bipolar membranes have been prepared by casting the sulfonated PPO solution on a series anion exchange membranes [111].
Actually, the function of the two ion exchange layers in a bipolar membrane is to selectively transport the water dissociation products, protons and hydroxyl ions, at the InL and block co-ions. However, water dissociation mainly depends on the charged groups and structure of InL region and thus usually is modified to enhance the bipolar membrane's performance. As catalysts in the bipolar junction, immobile weak acids or bases with an equilibrium constant of the acid/base pair close to that of the water dissociation reaction (pKa = 7) could be used, such as amino groups, sulfonic acid groups, amide groups, hydroxyl groups, pyridine groups [112][113][114][115][116] as well as metal ion/metal ion complexes, such as Aluminum, Magnesium, Iron, Cupper, etc. [116][117][118][119][120]. Also, metal ions are immobilized by placing their insoluble salts or low soluble (multivalent metal ion) hydroxides in the intermediate layer or using a soluble salt with a subsequent treatment [121].
Our research group has developed a series of bipolar membranes to demonstrate the catalytic activity of different macromolecules whose anion exchange layers were modified with a variety of (bio)macromolecules beforehand. The list includes hydrophilic materials such as hyperbranched aliphatic polyesters of the Boltorn series, polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyamphoteric bovine serum albumin (BSA), and a dendrimer molecule polyamidoamine (PAMAM) which possesses higher amino groups [113,[122][123][124][125]. The catalysts can be immobilized before or after the film formation. There have been several techniques applied: spray or dipcoating, incorporating in the polymer material by even dispersion, by electrochemical methods after formation of the layers or by in situ polymerization, electrospray deposition, layer-by-layer (LbL) assembly of polyelectrolyte multilayers [126][127][128][129][130].
The LbL deposition of polyelectrolyte multilayers is an effective method to introduce molecularly thin catalyst groups at this interface of bipolar membranes [130]. By this method, the bipolar membranes are prepared by first modifying an AEM via successive dipping LbL assembly, then casting a thin highly charged intermediate layer followed by casting a CEM.

ELECTRODIALYSIS APPLICATIONS OF ION EXCHANGE MEMBRANES
Electrodialysis (ED) as a novel process has grown rapidly during last decades. At first the ED was mainly used for the separation and production processes based on ion exchange membranes, which mainly includes conventional electrodialysis (CED), electroelectrodialysis (EED), bipolar membrane electrodialysis (BMED or EDBM) and electrodialysis deionization (EDI), etc. There are several comprehensive reviews which summarized ED utilization in organic acid productions [131], in environmental protection [132], for sustainable development [133], in separation process [134], in bioproducts transforming [135]. During last decades of development, ED has also been used for the energy reclamation from concentration gradient that called reverse electrodialysis (RED) [136][137][138]. However, the following will focus on the reclamation of useful resources such as nitrogen, phosphorus, precious metals and organic compounds from factory byproducts, wastewaters and brines, etc.

Recovery of N&P
Nitrogen and phosphorus are very important nutrients for the grown of crops which phosphate account for 2-4% of the dry weight of most cells. During the past decades, the demand for nitrogen and phosphorus increases rapidly for growing dairy-and meat-based diets. However, phosphorus has no substitute in food production and can only be extracted from phosphorous mining; in the same case, the production of ammonia requires nearly 1 m 3 of natural gas per kg, moreover, commercial nitrogen fertilizer cost increase rapidly with increase of raw materials price. The excessively dissolved phosphorus and nitrogen nutrient would induce water deterioration and eutrophication as well as green-house gas emissions. It is important to consider any possible methods to recycle nitrogen and phosphorus from wastewater. Electrodialysis as an efficient separation process has been used for the extraction of nitrogen and phosphate. Akyeva et al. [139], used electrodialysis to treat wastewater effluent from phosphoric acid extraction process. In this process, the phosphate extraction efficiency greatly depends on the applied current density. Particularly, low value of current density is suitable to block transition of triple-charged ions (PO 4 3-) as a consequence; conventional electrodialysis is a potential process for phosphorus reusing. Zhang et al. [48], used electrodialysis configured with selective ion exchange membrane (SED) to recover phosphate from struvite reactor (see Figure 4). The selective membrane only allows the passive of mono-valent ions and blocks the multi-valent ions. To maintain the recovery rate of phosphate, the experiments were carried out at pH 9 which was adjusted by adding NaOH. Here phosphate was removed from feed compartment and recovered at product compartment simultaneously. For the aerobic effluent as the phosphate source, the current efficiency initially reached 72%, with a satisfying phosphate concentration (9 mmol L -1 ). In the experiments with the anaerobic effluent, the phosphate flux was 16 mmolm . A cost evaluation shows that 1 kWh electricity can produce 60 g of phosphate by using a full scale stack, with a desalination rate of 95% on the feed wastewater. Finally, a struvite precipitation experiment shows that 93% of phosphate can be recovered. In our group [17], both conventional electrodialysis and bipolar membrane electrodialysis as well as their integration were used to recover phosphate from excess sludge. Firstly, simulated wastewater was treated with CED to recover phosphate. The concentration of phosphate was enriched to 1600 mg/Land then was pumped into EDBM process and transformed to valuable alkali solution and phosphate acid. Using continuous operation model, 95.8% recovery ratio could be achieved. However, at 50 mA/cm 2 operating current density, the net production of phosphorus acid was about 0.075 mol/L with a current efficiency about 75% and energy consumption rate about 29.3 kWh/(kg H 3 PO 4 ). There also some other phosphorus recovery ED process, such as phosphoric acid from liquid crystal display (LCD) manufacturing process [33], the recovery of organic phosphorus composition from seawater [140], etc.

Recovery of Base
NaOH as its inherent strong absorption ability toward acid gas has been used to remove H 2 S, CO 2 , and CS 2 from the gaseous petroleum fractions and corrosive streams which are usually called sweetening. Simultaneously, in prewash section of a Merox tower, acidic gases (H 2 S, CO 2 ) are eliminated by using caustic solution. This process can be represented by the following reactions: The excess amount of NaOH is often used to achieve complete elimination of acidic gases and hence a large amount of NaOH is periodically discarded via spent caustic stream which would induce great environmental pollution. Keramati et al. [141], used electrodialysis to recover NaOH from waste stream of Merox tower (see Figure 5). Moreover, to improve NaOH recovery ratio and the applied current The results indicate that a maximum NaOH recovery ratio of 75% was obtained for electrodialysis, and it is improved to 85% value by the introducing of cation exchange resin.
Besides, organic base, such as tetramethyl ammonium hydroxide (TMAH), which is often produced from photoresist developer wastewater, is another kind of hazardous material. It was recently recovered and recycled using electrodialysis in our group [15]. By this method, TMAH can be re-concentrated into the range of 7.45-8.33% mass concentration with a process cost about 36.4 $/t besides the recovered TMAH profits.

Recovery of Acid
Inorganic acids such as HCl, H 2 SO 4 , HNO 3 , HF, HI are often produced in industries of electroplating, metallurgy, biological fermentation and rare earth industry, etc. If the concentration is high (>1 mol/L), the acid can be recovered by diffusion dialysis (DD). Due to the concentration limit of DD process, it is not proper for acid with low concentration and this waste with low acid concentration can be concentrated through    [142] used a threecompartment electrodialyser to treat the complex of hydriodic acid and iodine solution. In this process, I moves from feed compartment through anion exchange membrane toward cathode compartment and oxidized to I 2 , at the same time, H + moves toward anode compartment simultaneously where I 2 is reduced to I . Hydriodic acid was also recovered by electroelectrodialysis (EED), a combination of electrolysis and electrodialysis with ion exchange membranes process [143]. Normally, EED mainly composes of two compartments: cathode compartment where reduction occur and anode compartment where oxidation occur, and one ion exchange membrane which was placed between two compartments allowing the passive of counter ions under the potential gradient. In this HI acid recovery process, the current efficiency is influenced by the temperature. At 293K, the current efficient was about 85.1%; on the other hand, at 333K it was about 82.5%. Hydrochloric acid is also a widely used inorganic acid in the field of hydrometallurgical, pharmaceutical, food industry. However, the effluent that contains hydrochloric acid must be treated as their potential threat to environment. There are some works focusing on hydrochloric acid recovery from wasters [144,145]. Our group [20] used integrated diffusion dialysis (DD) and conventional electrodialysis (CED) to recover hydrochloric acid from simulated chemosynthesis aluminum foils wastewater. In this method, initially the simulated wastewater was imputed into a spiral wound diffusion dialyser for recovering hydrochloric acid. Then the recovered low concentrations hydrochloric acid was reconcentrated by a conventional electrodialysis stack with a common configuration. To understand the compatibility and operational uniformity between DD and ED dialysate, flow rate and CED current were adjusted. The results indicate that high hydrochloric acid recovery rate of 74.9% was obtained with the energy consumption of 0.41 kW h and low aluminum leakage of 12.2%. There are some other ED based useful acid reclamation processes from pickling wastewater [146][147][148], recovery of nitric acid [36,149], especially, the recovery of metallic ion from pickling or plating waster as an independent process will be summarized in a separate section hereafter.
Besides inorganic acids, organic acid recovery using electrodialysis has also been studied for a long time. Citric acid, an organic acid, has been widely used for food industry, chemical and textile industry, environmental protection, cosmetics industry and pharmaceutical industry, etc. It is often synthesized using fermentation method with a subsequent addition of sulphuric acid and lime to clarify fermentation broths [150]. Due to this case, ED based separation process has been used as an alternatively to the conventional, such as bipolar membrane electrodialysis [151][152][153][154][155][156]. A comprehensive review has summarized the application of electrodialysis to the production of organic acids in detail [131].

Recovery of Ionic Liquids (ILs)
Ionic liquids (ILs), novel solvents, exhibit excellent properties, such as non-volatility, non-flammability, high electric conductivity, excellent catalytic activities and good phase separation performance. By virtue of these particular properties, ILs have been used for the extraction of biomass. However, the regeneration of biomass from ILs often produces large amounts of dilute waster with ILs. The disposal of ILs dilute wastewater may cause environmental issues due to their slow degradation and toxicity. ILs always consist of salts containing various heterocyclic cations and anions or other complex anions which could dissociate freely in aqueous solution. ED is often used as an optional process for the recovery of ILs. For examples, Trinh et al. [26] used it to recover an ionic liquid1-butyl-3-methylimidazolium chloride [BMIM]Cl from a hydrolysis of lingocellulosic biomass. The results indicate that 64% ILs can be recovered with 63.5 % current efficiency. The similar recovery was also conducted by Lu et al. [157]. Wang et al. [54] used this process for the recovering of 1-hexyl-3methylimidazolium chloride ([Hmim]Cl) and 1-butyl-3methylimidazolium tetrafluoroborate ([Bmim]BF4). The results showed that the highest ILs reached to 85.2% and highest overall current efficiency reached to 80.9% with high specific energy consumption of 1350 g/kWh. Using this noble process, Haerens et al. [52] produced ILs of choline thiocyanate, choline acetate and choline dicyanamide from starting product choline chloride with a high current efficiency between 65% and 78%.

Recovery of Metals from Wastewater
In chemical engineering industries such as hydrometallurgical, machinery manufacturing, electroplating, electronic industry and instrument manufacturing, there often produces large quantity of waste containing metal ions of chromium (Cr), copper (Cu), nickel (Ni), cadmium (Cd), gold (Au), and zinc (Zn). Chromium often exists as Cr (III) or Cr (VI) form in the wastewater produced from electroplating, leather tanning, cement and dyeing/fertilizer/photography industries. Chromium ion, especially hexavalent chromium Cr (VI) has been reported to be toxic to animals and humans and it is known as carcinogenic [158]. The direct discharge of the effluents would induce severe environmental problems. Gayathri et al. [159] used hybrid technology of ED and ion exchange to recover Cr (VI) from chrome plating wastewater. The efficiency of this method to remove and recover the chromium from the effluent was about 100% for the different modes of operation like 1) batch recirculation process; 2) batch recirculation process with continuous dipping; 3) continuous process. Chen et al. [45] used a two-stage ED (TSED) process for the recycling of Cr (VI) from electroplating wastewater (see Figure 6). In this process, the raw wastewater was concentrated by first stage ED at low pH condition. The concentrated stream was then adjusted to pH 8.5 followed by treating with second stage ED which configuring monovalent anion permselective membrane and nonselective cation exchange membranes. The chromate in concentrated stream was concentrated up to 191% in the first stage and in the second stage; chloride was separated about 45% from CrO 4 2 . Nataraj et al. and Peng et al. [160,161] also used ED to recover Cr (VI) from plating wastewaters. Cr (III) as another chromium in the environment is more stable and less toxic compare to Cr (VI). Lambert et al. [22,35] used monovalent cation selective membrane for the selective recovery of Cr (III) from tanning process in the leather industry. The final results demonstrated that separation of trivalent chromium and sodium ion is possible.
To improve the corrosion resistance and provide decorative characteristics, electroplating and metal finishing processes often use several hazardous chemical and toxic compounds such as nickel. However, the effluent always contains nickel salts and organic additives that should be treated. ED as an excellent separation process has been used for the recovery of useful nickel from rinse water. For example, Benvenuti et al. [162] used ED to concentrate and extract nickel (Ni) and its salts from the bright nickel electroplating process. In this extraction process, the recovery rate of nickel reached to 90mA A -1 h -1 cm -2 with an energy consumption of 0.7 kW h for 1 kg NiSO 4 ·H 2 O. Li et al. [163] also used ED to separate nickel ions from pent electroless nickel plating bath. ED combined with ion exchange has also been used to recover nickel from dilute solution [51,53,164,165]. A highest recovery rate of 99% (Ni) was obtained by the using Dowex 50WX-2 resin [51]. After the treatment, the concentration of nickel in the process solution decreases from approximately 5 ppm to less than the detection limit of the analysis (<20 ppb) [164]. Modified ED using special membranes such as liquid membrane and mono-valent selective membrane were also used for the selective recovery of nickel from electroplating industry [166,167].
Besides chromium and nickel, lithium is a growing metal that is often used in production of glass and  ceramic, air conditioning refrigeration systems, primary and second batteries, and nuclear energy production. Generally, lithium was extracted from lake brine or sweater instead of ore. Conventional extraction procedure of lithium requires high energy consumption and would induce sever environmental problem. ED is used as an optional process for the separation of lithium from lake brine, seawater and water. Recently, our group [19] used the integration of ED and bipolar membrane electrodialysis (BMED) for the production of LiOH from lake brine. In this process, the lake brine is treated with Na 2 CO 3 to precipitate Ca and Mg ion followed by concentrating the effluent with low concentration lithium ion. The lithium ions are then extracted from concentrated brine by adding of Na 2 CO 3 as Li 2 CO 3 followed by treating the obtained Li 2 CO 3 with BMED to produce LiOH (see Figure 7). The final results indicate that the concentration of lithium increased from 879 mg/L to 3157 and 3485 mg/L respectively, for two operating voltage of 10V and 15V, as well as a high purity of ca. 98% Li 2 CO 3 powder was obtained by this process. Lithium, an alkali metal abundantly present in sea water, was also recovered by ED using liquid membranes [41][42][43]168]. This modified ED involves the using of ionic-liquid-impregnated organic membrane (IL-i-OM) through which only the Li ions in seawater can permeate through.

SUMMARY AND PERSPECTIVE
This contribution summarizes up-to-date synthetic tactics for the preparation of ion-exchange materials and their applications for electrodialysis related processes from the literature. We feel that most of the upcoming progresses in the area will come from those developers or users that will look at these technologies as tools to cope with their specific treatment requirements. But it should be given emphasis that preparation of ion exchange membranes or materials is the most crucial. IEMs with high performance are important polymeric materials and used in ED for different applications. The main driving force for the development of new IEMs with improved properties is to make those applications more effective.
Actually, the performance of membranes is determined by its properties and separation abilities. IEMs can be designed and prepared by a number of tactics varying from basic polymer reactions to innovative nanotechnology via molecular design and architectural tailoring of composite materials.  Besides the preparation of IEMs, the technical and commercial relevance of the IEM-based ED processes should also be taken into account. As evaluated by Strathmann [181], some of the ED applications can be considered as state-of-the-art technology, such as the applications using conventional electrodialysis, production of pure water using continuous electrodeionization and some specific applications using bipolar membranes, such as production of organic acid from the fermentation broth or recovery of HF and HNO 3 from a waste stream generated by neutralization of a steel pickling bath. In some applications, ED processes provide higher quality products or are more environmentally friendly and will therefore be used in spite of some drawback such as operation cost. Also, increasing costs of raw materials and environmental awareness have expanded the application of ED.
However, the targeted properties of IEMs have to be set up to fulfill the membrane requirements for specific applications, and appropriate synthesis routes for IEMs towards the goals should be specified accordingly. Besides the material development, the ED system design and their operation optimization should also be further developed. In the ED based application of the IEMs, new systems to bring down the energy consumption and production cost require more progress. [18] Ren HY, Sun