Electro-Membrane Processes

ELECTRO-MEMBRANE PROCESSES Ajay K. Singh and Vinod K. Shahi Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR), Bhavnagar, Gujarat, India

1 INTRODUCTION Currently, electromembrane separation processes such as electrolysis, electrodialysis (ED), reverse electrodialysis (RED), diffusion dialysis (DD), etc. are used on large industrial scales in the production of potable and industrial process water as well as in the treatment of industrial effluents (1). Presently, desalination of brackish water (2), blue energy production (3), fuel cells (4), diary applications (5), HI concentration from HIx mixed (HI—H2 O—I2 ) solution (6), chromic acid recovery (7), organic acid separation and recovery (8, 9), ionic liquids synthesis (10), tetrabutylammonium hydroxide (TBAOH) production (11), low water-soluble organic acids from their salts (12), DD for acids and base recovery from waste acids (13), salts separation from mixed salts and organic solutions, separation of cation and anions from wastewater, demineralization of sugar and amino acids (AAs) (14), etc. are the application of electromembrane processes (EMPs). Ion-exchange membranes (IEM) are key components in electrochemical processes for separation, in situ ion substitution, and water splitting, and resemble ion-exchange resins (IERs) in the sheet form. There are two types of IEMs: (i) cation-exchange membranes (CEMs), in which negatively charged groups are fixed into polymer membrane matrix, and (ii) anion-exchange membranes (AEMs) in which positively charged groups are fixed into polymer membrane matrix. Fixed negative charges present in the CEM come to equilibrium with movable cations, as shown in Figure 1. CEMs mainly allow cation transfer through the membrane matrix due to the exclusion of co-ions, while AEMs allow anion transfer through the polymer membrane matrix (Fig. 1). Electrochemical and physicochemical properties of IEMs are dependent on their fixed charge concentration and their hydrophobic/hydrophilic nature. The extent of co-ions excluded from IEMs does not depend only on membrane properties but also on concentration of the feed solution. High permselectivity, low membrane resistance, high mechanical, chemical, and thermal stabilities are desirable properties of membranes that are used in membrane-based electrochemical processes for separation, ion substitution, and water splitting. Encyclopedia of Membrane Science and Technology. Edited by Eric M.V. Hoek and Volodymyr V. Tarabara. Copyright © 2013 John Wiley & Sons, Inc.

1

2

ELECTRO-MEMBRANE PROCESSES

CEM

–

+

+ + – + – + – – + + – – – + + – + + – – + + –

AEM – – +

–

– + + – – +

+

(a)

– + – + + – + – + + – + – + – – + + – – + – – + + – + – – + – + (b)

+ – Fixed ionic groups + – Free ions

FIGURE 1 Schematic presentation of (a) CEM and (b) AEM.

Desired membranes properties can be controlled by functionalization of base material and fixed charge concentration. Fixed charge concentration determines permselectivity and also influences the mechanical and swelling properties. The following moieties are present in the membrane matrix as fixed charges in CEMs: –SO3− , –COO− , PO3 2− , –PHO2 − , –AsO3 2− , and –SeO3− , while in the AEM matrix, they are –N+ H2 R, –N+ HR2 , –N+ R3 , –+ PR3 , and –S+ R2 . Among the all cation-exchange groups, –SO3 H is completely dissociated over the entire pH range, while the −COOH group is undissociated at pH of less than 3.0. The quaternary ammonium group is completely dissociated over entire pH, range while secondary the ammonium group is weakly dissociated. Thus, IEMs should be stable in acidic and alkaline media. The most desired properties of IEM are high permselectivity, low electrical resistance, good mechanical and form stability, high chemical and thermal stabilities, and low production cost. The commercial membranes available today meet most of the required properties (Tables 1 and 2). But there are large differences in the properties of IEM. The fluorocarbon polymer-based Nafion-type® CEM, for example, has satisfactory chemical and thermal stabilities, but its production cost is presently extremely high. Most heterogeneous membrane structures have relatively low production costs, but their permselectivity is generally low and their electrical resistance is high. 2

RECENT DEVELOPMENTS IN ION-EXCHANGE MEMBRANES (IEMs)

Because of the high production cost of the fluorocarbon-based polymer Nafion, a new solid polymer electrolyte for the transport of ions with low cost and the desired properties has been developed recently. Many researchers have developed different types of IEM using different methods. 2.1

Heterogeneous Ion-Exchange Membranes

Heterogeneous IEMs have embedded fine IER particles in an inert binder such as polyethylene (PE), phenolic resins, and polyvinylchloride (PVC). Heterogeneous IEMs

3

AEM AEM AEM CEM CEM AEM CEM CEM AEM CEM CEM CEM CEM AEM AEM

Neosepta AM Neosepta ACS

Neosepta AFN 61CZL386

CR67-HMR AR103QDP Morgane CDS Morgane CRA

Morgane ADP Nafion 117

Nafion 901 IPC HGC IPA HGA

CSMCRI, Bhavnagar, India

DuPont Co., USA

Solvay S.A., Belgium

Ionics Inc., USA

Tokuyama Co., Japan

CEM CEM AEM CEM

FKS FKB FAS Neosepta CMS

FuMA-Tech GmbH, Germany

Type

Membrane

Company

1.1 1.4 0.67–0.77 0.80–0.90 0.40–0.50

1.3–1.7 0.90

2.1–2.45 1.95–2.20 1.7–2.2 1.4–1.8

2.0–3.5 2.60

1.8–2.2 1.4–2.0

0.9 0.8 1.1 2.0–2.5

IEC, mEq/g

0.40 0.14–0.16 0.22 –0.25 0.16–0.18 0.22–0.25

0.13–0.17 0.20

0.53–0.65 0.56–0.69 0.13–0.17 0.13–0.17

0.15–0.18 0.63

0.12–0.16 0.12–0.20

0.09–0.110 0.10–0.115 0.10–0.120 0.14–0.17

Thickness, mm

TABLE 1 Properties of Commercial Ion-Exchange Membranes (CEMs and AEMs)

3.8 1.5–2.0 2.0–4.0 4.0–6.0 5.0–7.0

1.8–2.9 1.5

7.0–11 14.5 0.70–2.1 1.8–3.0

0.20–1.0 9.0

1.3–2.0 3.0–6.0

2.0–4.0 5.0–10 2.0–4.0 1.5–3.5

Area Resistance,  cm2

Standard CEM For EDBPM Standard AEM Monovalent selective Low Rm Monovalent selective Diffusion dialysis Monovalent selective ED ED Standard CEM Acid preconcentration Demineralization EMP (ED and EED) EMP ED and EDI ED ED ED

Remarks

4 1.20–2.2c

1.5–2.1

0.20–0.30

—

—

b

a

Voltage drop at kA/m2 in 0.50 N Na2 SO4 at 30 ◦ C. Efficiency at 1 kA/m2 in 1.0 N NaOH and HCl. c c Water-splitting efficiency of BP-1 for 1.0 N NaCl at 10 A/dm2 current density.

TWBPM

—

MB

Tianwei Membrane Technology Co. Ltd., China

—

Aqualytics BP6

Graver water Co., USA Kuban State University, Russia

—

WSI

WSI Technologies Inc., USA

—

92b

0.450

CaCl2 > MgCl2 > CuCl2 . Membrane selectivity increased towards monovalent ion because the PPY layer on the membrane surface hindered the migration of bulkier ions through membranes (77). Shahi et al. separated monovalent and bivalent ions with the help of ED, and the separation trend was in the order Cu2+ > Mg2+ > Ca2+ (78). Monovalent cation-selective membranes (MCSMs) were prepared by the sol–gel method in an acidic medium, and –SO3 H groups were anchored at inorganic segment by oxidation of –SH groups with H2 O2 as oxidizing agent. The membranes were physicochemically and electrochemically characterized. The electrotransport efficiency (η) of MCSMs for different cations was obtained from the change in concentration after passage of the desired coulombs of electric charge (40). η of MCSMs, especially MCSM (Si-65%), for cations was found to be in the order Na+ > Ca2+ > Mg2+ > Fe3+ . The separation of Na+ and Mn+ was carried out in a threecompartment ED cell by taking the mixed electrolyte solution of equal concentration. On the basis of η of MCSM for individual cations (Mn+ , Na+ , Ca2+ , Mg2+ , Fe3+ ), a separation factor (SF) was estimated from the flux ratio of ηNa+ and ηMn+ (40). Relevant data for MCSM (Si-65%) and Nafion 117 membranes are presented in Figure 23. It was revealed that SF for Na+ /Mn+ was high, especially in case of the Na+ /Fe3+ mixture (40). Amado et al. modified chitosan containing –PO3 H2 or –N+ (CH3 )3 Cl− group and further blended in PVA for the separation of metal ions. The relative permeability of composite membranes for various cations was found to be in the order Na+ > Ca2+ > Mg2+ (79). 4.1.1.2 Separation and Recovery of Anions from Wastewater. The NO− 3 concentration in ground water is growing day by day due to the excessive use of fertilizers as well as to the discharge of human wastes and industrial wastewater. According to the US Environmental Protection Agency (EPA), the maximum permissible limit of NO− 3 in drinking water is 45 mg/l, while European community has recommended a value of 25 mg/l. NO− 3 -contaminated water may cause methemoglobinemia in humans, and nitrosamines

30

ELECTRO-MEMBRANE PROCESSES

Separation factor

10 8

Si-65% Nafion

6 4 2 0 Na+/Ca2+

Na+/Mg2+

Na+/Fe3+

Electrolyte mixed system

FIGURE 23 Separation factor of Si-65% and Nafion 117 membranes for different mixed electrolyte solutions (0.01 M).

(by-products) derived from NO− 3 are carcinogenic (80). Thus, it is important to urgently develop a process for the separation and removal of NO− 3 from drinking and wastewater to protect public health and the environment. Several reports are available in the literature for the separation and recovery of NO− 3 from drinking and wastewater by ED (81, 82). Fluoride is both beneficial and harmful to public health depending on its concentration in drinking water. More than 1.0 mg/l fluoride concentration leads to bone diseases, teeth mottling, lesions of endocrine glands, thyroids, liver, as well as other organs of human body. The World Health Organization has recommended 1.0 mg/l as the maximum permissible limit of fluoride in drinking water. There has been increasing interest to use ED for the treatment of fluoride-rich water, because it is simple and avoids the use of chemical processes (83, 84). Arar et al. (85) separated and recovered F− from wastewater by ED in the presence of interfering ions (SO4 2− /Cl− ). The perchlorate ion (ClO− 4 ) is an emerging contaminant in water and has received much attention among public and private sectors. Thus, there is an urgency to develop innovative processes for separation and removal of ClO− 4 from contaminated wastewater. Wang et al. have developed an ED-assisted catalytic reduction (EDACR) process for − ClO− 4 removal from dilute waste solution. They could remove 88% ClO4 from synthetic wastewater and the developed process could be used for the separation and recovery of ClO− 4 from synthetic solutions in the low concentration range (10–100 ppm) as well (86). 4.1.1.3 Electro-Electrodialysis (EED) for Chromic Acid Recovery from Wastewater. An EMP based on electro-electrodialysis (EED) principles offers many advantages in achieving in situ ion substitution and separation in different electrochemical transformations. This is a combination of membrane electrolysis (ME), ion transport across charged IEMs (CEMs and AEMs), and electrochemical reactions occur at electrodes. During water electrolysis, the following reactions occur at anode and cathode (4): At the anode,

1 2

H2 O → 14 O2 + H+ + e−

At the cathode, H2 O + e− → 12 H2 + OH−

ELECTRO-MEMBRANE PROCESSES

31

Thus, electrode reactions provide H+ and OH− for in situ ion substitution, and the whole reaction is known as water splitting. This process has been successfully used for chromic acid recovery, electrochemical transformation of valuable salts into their corresponding acids and bases, and production of pharmaceutical-grade organic acids from their salts. Shahi et al. developed EED based on an electromembrane reactor (EMR) for in situ substitution, separation, and recovery of chromic acid and heavy metal ions from synthetic individual or mixed solutions of chromate and metal electrolyte solutions. A single-step electrotransport and in situ ion substitution of Cr(VI) was carried out in an EMR. Two steps occurred in this process: (i) electrotransport of CrO4 2− from the central compartment (CC) to anolyte took place across the AEM, while metal ion was transferred from CC to catholyte across the CEM under the influence of applied electrical gradient, and (ii) in situ ion substitution, in which chromic acid was formed by the combination of H+ and CrO4 2− while metal hydroxide was formed in catholyte by the combination of OH− and the metal ion. In this process, the combined effect of electrode polarization and electromigration of CrO4 2− occurred, while the metal ions migrated towards the anode and cathode, respectively. Results have revealed that the developed EMR will be an efficient tool for the separation and recovery of chromic acid from wastewater from the electroplating industry (4). 4.1.1.4 Electrodeionization (EDI) for Ultrapure Water. Electrodeionization (EDI) is a membrane-based separation process that removes ions from water and produce ultrapure water (8.0–18.2 M conductivity). EDI is an advantageous process because of stable product quality and also because there is no need to use an acid or base for resin regeneration. It is a continuous chemical-free deionization process and is based on the EED principle and mixed bed of IER. The EDI cell consist DC, CC, and electrode compartments (Fig. 24).

1/4 O2 + H+ + e–

Feed

Cl–

Na+

Cl–

Cathode

H+

H+

Na+

OH–

H+

H+ OH–

H2O

1/2H2O

1/2 H2 + OH

Na+ OH– Cl–

H2O + e–

Electro regeneration Process

Enhanced transfer process

Na+ Cl–

H2O

H+ Concentrating Diluting Concentrating compartment compartment compartment Concentrate

Product

Anode

Concentrate

FIGURE 24 Schematic presentation of electrodeionization (EDI) for ultrapure water production. (Please refer to the online version for the color representation of the figure.)

32

ELECTRO-MEMBRANE PROCESSES

Under the influence of the applied electric gradient, the ions present in the DC exchange with mixed bed of IER and, simultaneously, get transferred to the CC through IEMs (AEM and CEM). Thus, the DC gets depleted in ions, which accumulate in the CC. The IER plays an important role in EDI process because of (i) faster ion transfer and (ii) reduction in system resistance and back diffusion of ions from the CC to the DC. The rate-determining step in this process is the diffusion of ions from the bulk solution to the IER surface and the simultaneous production of H+ and OH− by water splitting which regenerates the mixed bed of IER (87). Recently, EDI was employed to separate and recover heavy metal ions (Ni2+ , Pb2+ , Zn2+ , Cr(VI), Co2+ , and Cu2+ ) from wastewater solutions (88, 89). EDI has been considered as tool for ED with the ion-exchange process. A mixed bed of IERs extract salt ions from water and act as channels responsible for ion migration. Furthermore, water splitting is not enough to produce sufficient H+ and OH− for resin regeneration because of the low transport efficiency − of H+ (tH+ = 0.00004–0.11) for CEMs and of OH− (tOH = 0.03–0.60) for AEMs. EDI is coupled with EDBPM to enhance the water-splitting efficiency and generation of H+ and OH− for resin regeneration. In this process, more number of H+ and OH− are − provided for resin regeneration due to high water splitting (tH+ and tOH = 0.90–0.96). Continuous electrodeionization (CEDI) with a BPM, that is, CEDI-BPM, is used to obtained ultrapure water (Fig. 25). It consists of CER and anion-exchange resin (AER) beds in the DC while CEM and AEM to separate DC and CC, respectively. Under the influence of the applied electrical gradient, water splitting occurs at the BPM interface, resulting in simultaneous migration of H+ and OH− to the DCs. Reverse osmosis (RO) treated water was fed into DC where mineral cations (Me+ ) were exchanged with H+ , and the exchanged Me+ migrated into the CC through the CEM (90).

OH–

Me+ H+

A–

A– OH–

H+ H2O

1/4 O2 + H+ + e–

Me+

H+

–

OH

H2O

1/2H2O

H2O + e–

1/2 H2 + OH–

Feed

Anode

Cathode

Repeating unit CER AER

AEM CEM

BPM

FIGURE 25 Schematic presentation of CEDI with bipolar membranes (CEDI-BPM) for pure water production. (Please refer to the online version for the color representation of the figure.)

ELECTRO-MEMBRANE PROCESSES

33

4.1.1.5 Bipolar Membrane Electrodialysis (EDBPM) for Production of Acid and Base from Salt. In EDBPM, CEMs and AEMs are placed together with a BPM in an alternating arrangement between electrodes to separate the electrode wash, feed, acid, and base compartments, as depicted in Figure 26. Under the influence of the applied electric gradient, charged species (A+ and X− ) get removed from the feed compartment (AX, salt solution) through AEMs and CEMs. H+ and OH− produced at the BPM interface are responsible for the migration of charged species from one compartment to the other. Thus, the migrating charged species react with H+ and OH− which results in the formation of acid (HX) and base (AOH) while the feed compartment become desalted. EDBPM has been effectively used in industries to recover valuable chemicals from salts without using any hazardous chemicals and additives. EDBPM was carried out in two- and three-compartment cell configurations for the conversion of salt into acid and base (Fig. 27a and b). The two-compartment cell configuration is simple and easy to use, in which base is transported from the cathodic to the anodic compartment. Transport of the base is serious drawback and reduces the process efficiency and product recovery; this can be avoided by using the three-compartment cell configuration (91). EDBPM process has been used to convert NaCl into HCl and NaOH by water splitting. The results showed that recovery and current efficiency (CE) were low, but these parameters could be improved by modifying the EDBPM configuration: that is, by changing the EDBPM cell into the three-compartment configuration. Thus modified cell was also used to convert sodium salts (NH4 NO3 , NaNO3 , and Na3 PO4 ) into acids and base (92). Recently, EDBPM with the three-compartment cell configuration was used to convert NH4 NO3 into HNO3 and NH3 by water splitting, where ammonia was used as the stripping solution (Fig. 27b) (93). 4.1.2

Electromembrane Processes for Biomolecular Separation and Recovery.

4.1.2.1 Separation and Recovery of Amino Acids from their Mixtures. AAs are being extensively used as permitted chemicals, as additives in pharmaceutical products, in Acid(HX) Diluted salt (AX)

EW EW

H+ X–

X–

OH–

A+

A+

X– H+

A+ OH

A+ Anode

Cathode

X–

EW H2O Salt (AX) AEM

CEM

BPM

FIGURE 26 Schematic presentation of electrodialysis with bipolar membrane (EDBPM) for acid and base production from their salts. (Please refer to the online version for the color representation of the figure.)

34

ELECTRO-MEMBRANE PROCESSES

Base: Depleted Acid: salt BOH HX

Acid: HX Base: BOH

(B)

(A) B+ Water B+ X– – X–

B+ H+

X– H+

OH

X–

A+

+ –

OH–

OH– B+

B+X–

H +

X–

A+ X– EW

EW Water

Water

Water AEM

Salt: BX CEM

BPM

FIGURE 27 (a) Two-compartment and (b) three-compartment EDBPM arrangement for converting salts into acid (HX) and base (BOH). (Please refer to the online version for the color representation of the figure.)

solid-phase peptide synthesis, as well as in agrochemical and biomedical sensor. Demand of AAs is growing day by day worldwide, and they are being produced from molasses and raw sugar by fermentation technology. AAs are amphoteric in nature and exist as cations, anions, or zwitterions, depending on the solution pH (94). Separation of AAs from fermentation broths depends on the following factors: (i) solubility, (ii) molecular size, (iii) charge, and (iv) affinity. Generally, ion-exchange process has been used for separation and purification of AAs from fermentation broths but, in this process, large quantities of waste are created during IER regeneration. With growing international concern over environmental issues, waste increases the treatment cost and deteriorates the effectiveness of processes for the separation and recovery of AAs from their mixture. Therefore, there is an urgent need to develop alternate IEM-based separation processes to separate and recover AAs from fermentation broths. The ED/EMP has been used as an alternative process for AA separation and recovery from fermentation broths. Various reports are available in the literature for desalting AAs and separating them from their mixture with salts by ED/EMP (94, 95). Shaposhnik et al. studied AA transport through IEMs by ED, and their investigation showed that maximum ampholyte ions flux was obtained at limiting current density, above which a decline in flux occurred. This decline was due to (i) the barrier effect of the boundary layers, (ii) the decrease in pH at the boundary layer of AEM, and (iii) the increase in pH at the boundary layer of the CEM. Changes in pH recharge the ampholyte ions and further block their migration from one compartment to the other through IEMs under the influence of an applied electrical gradient. The barrier effect may be alternate mechanism to effect AA separation and purification problems occurring in fermentation broths and biotechnology (95). Furthermore, phenylalanine was also separated and recovered from fermentation broths containing at least AA and inorganic salts (Na2 SO4 , (NH4 )2 SO4 ) (96). Electrotransport of tyrosine was carried out using ED, and only diffusive transfer was observed. On the

ELECTRO-MEMBRANE PROCESSES

35

basis of tyrosine transport, separation of tyrosine from binary mixtures (tyrosine/histidine and tyrosine/arginine) was carried at various current densities, and results revealed that the SF increased with the applied current density. Maximum separation was achieved in case of tyrosine/histidine and tyrosine/arginine mixtures. Thus, ED can be efficiently used for the separation of AAs from their binary mixtures present in a fermentation broth. Aghajanyan et al. (97) developed an ED process to separate and recover proline from proline model solution and fermentation broths containing chalk-free l-proline, l-valine, and l-alanine. Desalting of the AA mixture solution was performed in five- and sixcompartment cells. The separation performance was better in the six-compartment cell than in the five-compartment cell. Also, neutral AAs were also separated from biologically active substances and salt solution mixtures (98). Shahi et al. developed an EMP for AA separation from their mixtures by the isoelectric focusing (IEF) technology. Glu and Lys separation from their mixture was carried out in an EMP cell in which the feed chamber (FC) and permeate chambers (PC) were separated by an AEM, while the electrode chambers (catholyte and anolyte) were separated by a CEM. Separation of Glu and Lys (0.01 M) from their mixture was carried out in an EMP cell at a constant applied potential (5.0 V) and pH 8.0. Results revealed that initially the (SF) was low, but reached approximately 7.5 after passage of an appropriate charge because AEM allowed Glu− transport, while Lys+ was retained in FC. The high SF revealed the feasibility of EMP for separation of a Glu and Lys mixture at pH 8.0. Furthermore, Glu and Lys transmission was strongly dependent on charge on transmitting species, solution pH, the nature of charge on the membranes, and the applied electrical gradient. Separation of Glu and Lys was achieved at a constant applied potential as a result of the difference in their pI values, in spite of their very close molecular weights and sizes. Thus, EMP may be an important tool for separation and recovery of AAs with close molecular weights using IEF technology (94). 4.1.2.2 Electrodialysis Fermentation (EDF). Organic acids are produced from bacterial fermentation of carbohydrates on the industrial scale and several processes have been developed for the isolation and recovery of concentrated organic acids from fermentation broths. Recently, EDF was used for this purpose because in this process the pH of the fermentation broths was maintained during the separation of organic acids. EDF was used to separate and recover lactic acid (Lah) from fermentation broth, in which the bacterium Lactobacillus rhamnosus acted as the Lah producer (99). 4.1.2.3 Electrophoretic Membrane Contactor (EPMC) for Biomolecules Separation. Separation and purification of biomolecules (proteins, polysaccharides, vitamins, antibodies, and AAs) is an integral part of bioprocessing industries. Thus, there is an urgent need to develop membrane technology to fractionate biomolecules because their demand has been increasing in the food and pharmaceutical industries (99). Conventional separation techniques such as affinity separation, chromatography, and electrophoresis have been used in bioindustries for this purpose. Electrophoresis is an electrodriven separation technique that has been utilized for the separation and purification of biomolecules in downstream processes. But these processes are not suitable for large-scale production because of the low product recovery and high operational cost. Thus, highly selective and cost-effective membrane-based separation processes for isolation and recovery of biomolecules are urgently required for academic and industrial research (99). Ultrafiltration has been used to fractionate protein mixtures

36

ELECTRO-MEMBRANE PROCESSES

Concentrate (A.B)

B+ –

Dilute (A.B)

A–

Concentrate Dilute A (A.B)

+ –

A–

+

Electrode Electrode Electrode Feed Elution Feed buffer buffer buffer – + (A– and B+) chamber (A and B ) Seprating chamber Seprating chamber Ultrafiltration membrane CEM AEM (b)

Electrode buffer

(a)

FIGURE 28 Schematic presentation of electrophoretic membrane contractor. (a) Separation mode. (b) Elution mode. (Please refer to the online version for the color representation of the figure.)

from fermentation broths (100). But major problem in this process is the decline in protein flux after long runtime because of fouling and poor selectivity of membranes towards specific proteins in the protein mixtures. EMP offers the option to enhance purity and recovery of proteins from their mixture present in fermentation broths without affecting separation efficiency. In the ED process, migration of bulky molecules (more than ∼ 500 Da MW) is impossible due to the dense nature of the IEMs. Thus, use of porous charged membranes in EMP (EPMC) may be a substitute for IEMs for the separation and recovery of biomolecules (polyamino acids, peptides, proteins, and CS oligomers) from their mixture (101). EPMC is a hybrid EMP in which an ultrafiltration membrane (UFM) is used as the molecular barrier and the applied electrical gradient is used as driving force for ion migration from one compartment to another. The separation chamber is composed of two compartments (PC and FC) which are separated by porous IEMs, and the electrodes are separated by dense IEMs (CEM and AEM) to avoid denaturation of biomolecules. Under the influence of the applied electrical gradient, charged molecules (A− and B+ ) migrate towards PC through porous UFMs, and the solute mass flow depends on electrophoretic mobility of biomolecules. EPMC is operated in two different modes, as depicted in Figure 28a and b. This process was used to fractionate biomolecules from a complex feedstock solution (102, 103). EPMC was used to separate α-lactalbumin (α-lg) from its mixture with bovine hemoglobin. It was reported that the separation and production rate were enhanced up to five times with feed concentration (104). β-Lactoglobulin (β-lg) is a major whey protein that is released from the enzymatic hydrolysis of various bioactive peptides. However, it is necessary to fractionate the protein hydrolysates to obtain more purified proteins. Poulin et al. (105) used EPMC to separate and recover β-lg hydrolysate from a mixture. EPMC was also used to fractionate peptides from a β-lg tryptic hydrolysate mixture and separation of chitosan oligomers (106).

ELECTRO-MEMBRANE PROCESSES

4.2

37

General Description and Application of Membrane Electrolysis

4.2.1 Production and Recovery of Organic Acids. There is growing interest in the separation and recovery of organic acids because of their increasing demand in the food and pharmaceutical industries. Organic acids are obtained as a mixture of their salts and used as acidifying agents, preservatives in food industry, detergents, and raw material for production of biodegradable polymers or biosolvents. Various purification steps are necessary to obtain organic acids from fermentation broths, and approximately 50–80% products are obtained after 8–10 unit operation, which reduces the success of the process for their separation and recovery. The level of impurities (sugars, polyols, minerals, and coloring reagents) in organic acid mixtures depends on the manufacturing process and chemical consumption in various separation processes such as precipitation, crystallization, adsorption, or ion exchange. EED-based water-splitting technology was employed to convert salts into acids from fermentation broths. EED was also used for the separation and recovery of formic, acetic, butyric, valeric, adipic, caproic, and oxalic acids from waste fermentation broth solutions, as well as for the production of malic acid, partial electroneutralization of D-α-p-hydroxyphenylglycine (pHPG), conversion of phenoxides into phenols, and Lah production from its sodium salts (LANa) (107–110). Shahi et al. developed an EMR for in situ ion substitution and single-step separation and recovery of Lah from its lactate salts. Experiments were carried at various applied electrical gradients and various LANa concentrations in the CC and DW were fed into anodic compartment. With progress of the experiment, lactate ions (LA− ) migrated from the CC to the AC and, simultaneously, H+ was generated at the anode by water splitting. Two steps were involved in this process: (i) migration of LA− from CC to AC across AEM and (ii) LA− reacting with H+ leading to the formation of Lah in AC. A schematic presentation for LA− migration and electrode reactions is depicted in Figure 29. Single-step in situ ion substitution and separation of 1.0 M ammonium lactate (LANH4 ) solution at 8.0 V/cm was also carried out in a proposed EMR as a representative case. Data revealed that the lactate flux was strongly dependent on the applied electrical gradient and initial feed concentration of LA− in the CC. In this process, LAH

OH–

+H

Anode

Cathode

1/2 H2 + OH

AEM

1/4 O2 + H+ + e–

NaOH

+

Na

H2O

H2O

1/2H2O

H2O + e–

LA–

LANa

FIGURE 29 Schematic presentation of the lactate ion (LA− ) transfer and Lah recovery. (Please refer to the online version for the color representation of the figure.)

ELECTRO-MEMBRANE PROCESSES

NaOH

RH

H2O

+H

Anode

+H

1/4 O2 + H+ + e–

CEM

R–Na+

OH– Na+

H2O + e–

Cathode

1/2 H2 + OH

CEM

1/2H2O

38

H2O

RNa

FIGURE 30 Schematic representation of EMR-3/EMP for the separation and recovery of carboxylic acids (RH) from their sodium salts (RNa). (Please refer to the online version for the color representation of the figure.)

LA− was transported into the AC and uncharged molecules such as polysaccharides, carbohydrates, and other colouring agents were retained in the CC. Moreover, ion substitution and separation of Lah in a single step could be achieved without use of any additives or hazardous chemicals (110). EMP for in situ ion substitution and separation of salicylic acid (SAH) from its sodium salt (SANa), was also developed by same group. In situ ion substitution and separation of SAH from SANa was carried out in an EMP cell. The electrochemical principle of EMP is illustrated in Figure 30. Three steps were involved in this process: (i) formation of OH− and H+ at cathode and anode by reductive and oxidative water splitting; (ii) electrotransport of H+ from the AC to the CC across the CEM; (ii) in situ substitution of Na+ by H+ which led to the formation of SAH; and (iii) electrotransport of the liberated Na+ from CC to the CC, thus forming NaOH. In this process, ion substitution was achieved by the combined effect of electrode polarization reactions and the simultaneous migration of H+ and Na+ . The overall electrochemical reaction for in situ ion substitution and SAH separation from SANa can be written as follows: ONa

OH COOH

COOH

+ H+ + OH–

SANa

+

NaOH

SAH

The developed EMP was a more efficient process for in situ ion substitution and separation for SAH from SANa than ED (7). An EED principle based EMR was developed for in situ ion substitution and separation of ascorbic acid (ASH) from its sodium salt (ASNa). Three step were involved in this process: (i) H+ and OH− generation by oxidative and reductive water splitting; (ii)

39

ELECTRO-MEMBRANE PROCESSES

electromigration of H+ from the AC to the CC across the CEM and in situ substitution of Na+ by H+ , which lead to the formation of ASH; and (iii) electromigration of Na+ from CC to CC across CEM and subsequent formation of NaOH (Fig. 30). The overall electrochemical reaction for in situ ion substitution and ASH separation from ASNa can be written as follows:

O

HO

O

+Na–O

OH ASNa

H2O

O

HO

Acidification

NaOH

O

OH

HO ASH

The developed EMP was more efficient than ED for in situ ion substitution and separation for ASH from ASNa. Production of NaOH as a by-product in CC was a spinoff of the developed EMR (111). Low water-soluble organic acids and their salts formed solid particles or suspended droplets, leading to (i) membrane fouling, (ii) pump erosion, (iii) low product recovery, and (iv) unstable operation. These problems could be solved by using specific organic solvents that induce organic acid solubility. Selection of the organic solvent plays an important role in two-phase electro-electrodialysis (TPEED) and the modified two-phases electro-electrodialysis (MTPEED) processes. TPEED is a combination of two-phase electrophoresis (TPE) and EED. In this process, the two phases are separated by an AEM, and pure organic solvent or a water–organic (W/O) emulsion is used as the extracting medium. Electroosmosis, osmosis of water, and backdiffusion are well controlled in TPEED. Thus, low water-soluble organic acids can be easily concentrated and recovered from very dilute waste fermentation solutions (112). This process has been used to concentrate and recover organic acids from model and fermentation broth solutions (12, 113–115). EDBPM processes have been utilized for the production of organic acids such as Lah, pHPG, propanoic acid, tartaric acid (H2 Tar), pyruvic acid, gluconic acid, formic acid, acetic acid, 2,2-dimethyl-3-hydroxypropionic acid, sebacic acid, and galacturonic acid (GAH) (115, 116). Xu et al. studied the effect of cell configuration on EDBPM process performance for the electro-acidification of H3 A. Three basic cell configurations, namely, AEM–CEM–BPM–AEM–CEM, CEM–BPM–CEM, and BPM–AEM–CEM–BPM, were used for H3 A production from its sodium salt (Na3 A). The energy consumed in the CEM–BPM–CEM configuration was low and followed the trend, CEM–BPM–CEM < AEM–CEM–BPM–AEM–CEM < BPM–AEM–CEM–BPM. It was reported that the CEM–BPM–CEM cell configuration was more appropriate for H3 A production (117). 4.2.2 Production of Tetrabutyl Ammonium Hydroxide (TBAOH). Shahi et al. developed EMRs with two compartments (EMR-2) and three compartments (EMR-3) for the efficient synthesis of tetrabutyl ammonium hydroxide (TBAOH) from tetrabutyl ammonium bromide (TBABr) by in situ ion substitution and separation. Experiments were carried out in EMR-2 at different applied current densities and TBABr concentrations. TBABr of various concentrations was fed into the cathodic compartment, while DW was fed into anodic compartment. In this process, the applied current density was mainly responsible for OH− and H+ formation and migration of Br− , which resulted in the

40

ELECTRO-MEMBRANE PROCESSES

variation of pH in both compartments. The complete electrochemical process for TBABr conversion into TBAOH can be written as follows: (C4 H9 )4 N+ Br− + OH− → (C4 H9 )4 N+ OH− + Br− To evaluate the technical and economic feasibility of TBAOH synthesis in EMR-2, W , CE (%), and recovery (%) of TBAOH were evaluated. The data revealed that W increased, while CE and TBAOH recovery decreased with increase in applied current density, for 0.10 M TBABr feed solution. More OH− was formed at high applied current density. TBAOH formation may be high but this was not the case for more TBAOH production. Reduction in CE may be due to (i) partial TBAOH degradation (Hofmann elimination) and (ii) OH− leakage along with Br− from the cathodic to the anodic compartment. To improve product recovery and CE, the EMR was modified in such a way that TBABr was not in direct contact with the electrodes. Single-step, in situ ion substitution and TBAOH synthesis from TBABr were achieved in EMR-3, as shown in Figure 31. Three steps were involved in this process: (i) OH− generation by reductive water splitting and their migration from the cathodic compartment to the CC across the AEM; (ii) substitution of Br− by OH− ; and (iii) migration of Br− from the CC to the anodic compartment across AEM, forming HBr/Br2 . TBAOH synthesis rates in EMR-2 and EMR-3 were compared under similar experimental conditions as a function of applied current density and initial TBABr concentration in CC. The results revealed that TBAOH flux was two times higher in EMR-3 than in EMR-2. TBAOH synthesis rate in EMR-3 increased with applied current density, while it reduced in EMR-2. Reduction in TBAOH synthesis rate might be due to the high extent of degradation of TBAOH in EMR-2 as compared to EMR-3. The developed EMR-3 may be used for efficient TBAOH synthesis without the use of hazardous chemicals (9). TBAOH

OH–

AEM

+

OH– Br–

H

Anode

Cathode

1/2 H2 + OH

AEM

HBr 1/4 O2 + H+ + e–

H2O

Br–

1/2H2O

H2O + e–

TBA+ H2O

TBABr

FIGURE 31 Schematic presentation of an electromembrane reactor for the synthesis of tetrabutyl ammonium hydroxide (TBAOH) from the corresponding bromide by in situ ion substitution. (Please refer to the online version for the color representation of the figure.)

ELECTRO-MEMBRANE PROCESSES

41

4.2.3 Chlor-Alkali Electrolysis. Electrolysis is used for synthesis of chemicals, in which oxidation and reduction reactions occur at the respective electrodes. In this process, anolyte and catholyte are separated by an IEM to prevent the mixing of the electrolyte solutions. Chemicals can be produced by electrolysis because there is no need to use catalysts or other additives (118). This process is based on NaCl electrolysis and on the migration of ions from one compartment to another across the IEM. The following reaction occurs in NaCl electrolysis by the chlor-alkali process. 2NaCl + 2H2 O → 2NaOH + Cl2 + H2 In this process, two major challenges exist: (i) Cl2 reacts explosively with H2 and (ii) Cl2 dissolves in NaOH and forms a hypochlorite solution. Therefore, it is imperative to separate and recover the reaction products. In an electrolysis cell, the anodic and cathodic compartments are separated by a CEM (Fig. 32), while a saturated solution of NaCl is fed into acidic compartment and DW to the cathodic compartment. Under the applied electrical gradient, Cl2 gas is produced in the anodic compartment due to Cl− oxidation, and, simultaneously, migration of Na+ takes place from the cathodic compartment across the CEM and NaOH is formed. The concentration and pH of saturated brine solution should be in the range 300–305 g/l and 2.0–4.0, respectively, (119) because athigh pH, Cl2 reacts with OH− and forms hypochloric acid (HClO) or chlorate ions, as per reactions below (120): Cl2 + OH− → HClO + Cl− HClO + H2 O + 2e− → 3H+ + Cl− + O2 + − HClO + ClO− → ClO− 3 + 2H + Cl

The membrane used in chlor-alkali electrolysis should (i) have good physical and chemical stability in alkaline and Cl2 media, (ii) have low membrane resistance, (iii) only allow Na+ transport from anolyte to catholyte and Cl− transport from catholyte

Hydrogen H2

NaOH 23–32%

Depleted NaCl brine Chlorine Cl2

OH–

Cl2

H2O Na+

Cl– Na+

Anode

Cathode

CEM

NaCl brine

FIGURE 32 Schematic presentation of the chlor-alkali process for NaOH and Cl2 synthesis. (Please refer to the online version for the color representation of the figure.)

42

ELECTRO-MEMBRANE PROCESSES

to anolyte, (iv) operate at high current densities, and (vi) be impervious to interfering heavy metal ion impurities present in brine solution (120). CEMs (Du Pont, and Asahi Glass Company) have been used for NaCl electrolysis to synthesize NaOH and Cl2 . Removal of metal ions is necessary because they influence the product purity and electrolysis efficiency. Salt splitting have been carried out using the chloro-alkali process by many researchers to convert salts into acids and bases (121). Yazicigil et al. used the chlor-alkali process to convert metallic salts into acids and bases. Potassium salts were fed into the cathodic compartment and acid solutions were fed into anodic compartment, both separated by an AEM. Furthermore, in another set of experiments, CEM was used as the separator and base solutions were fed into cathodic compartment. Results revealed that different salts of potassium were converted into acids and bases by water splitting and migration of ions (anions or cations) across the IEM (AEM or CEM, respectively) (121). 4.2.4 Bipolar Membrane Electroacidification (BPMEA). BPMEA is a process based on water splitting and further acidification for the fractionation of soy proteins, casein production, and chitosan oligomer production, as well as the fractionation of whey proteins. The effect of CEMs (CSV and CMX) permselectivity on cation skim milk protein migration and precipitation was studied by BPMEA (122–126). It was reported that highly pure bovine milk casein (97–98%) could be isolated and used for commercial applications (127). BPMEA was also used to separate the α-lg fraction (98%) from the whey protein isolate (WPI) solution (5%) (128). Electroacidification of WPI solution (20%) at p, 4.65 was studied to investigate the effect of solution conductivity on protein precipitation (129). Cheddar cheese whey protein was acidified by BPMEA, in which whey lipids (30%) precipitated at pH 3.7 (130). 4.3

General Description and Application of Diffusion Dialysis

DD is an IEM separation process driven by concentration gradient and has been applied for the separation and recovery of acid/alkali waste solutions in a cost-effective and environmentally friendly manner. DD has been successfully applied for the recovery of acids and alkalis from the discharges from steel production, metal refining, electroplating, CER regeneration, nonferrous metal smelting, aluminum etching, and tungsten ore smelting. The separation of HCl and NaOH from their feed solution is illustrated in Figure 33 to describe the principle of DD. As shown in Figure 33a, HCl and its metal salts in the feed solution tend to transport to the water side due to the concentration difference across the 3− membrane. Because of the presence of the AEM, the Cl− ions (SO4 2− , NO− 3 , PO4 , etc.) are permitted passage, while the metals in the waste solution are much less likely to pass. The H+ ions, although positively charged, show higher diffusion than metal ions because of their smaller size, lower valence state, and higher mobility. Hence they can 3− diffuse along with the Cl− ions (or SO4 2− , NO− 3 , PO4 , etc.) to meet the requirement of + electrical neutrality. The H transport is a key to the DD process (131–133). Suitable properties of the AEM are also necessary, including stability in acidic solution, high H+ permeability, strong rejection of other metal ions, relatively high water uptake (WR), and poor water permeability. The separation process of NaOH from its feed solution (Na2 WO4 as an example) is illustrated in Figure 33b. NaOH and Na2 WO4 tend to transport to the water side due to the concentration difference across the membrane. Because of the presence of the CEM, the Na+ ions in the feed are permitted passage, while the WO4 2− ions are much less likely to pass through the membrane. Similar to

ELECTRO-MEMBRANE PROCESSES

(a)

(b)

AEM

Water Side Cl–

Feed Side Cl–

43

CEM Water Side

Na−

Feed Side

Na−

M+

Cl–

Cl–

H−

H−

Na–

Na−

OH−

OH−

FIGURE 33 Schematic diagram illustrating the principle of diffusion dialysis. (a) HCl separation process from its feed solution. (b) NaOH separation process from Na2 WO3 solution. (Please refer to the online version for the color representation of the figure.)

H+ through an AEM, the hydroxyl ions (OH− ) diffuse better than the WO4 2− ions and, along with Na+ ions, meet the requirement of electrical neutrality. The OH− transport is also a key to the process (134), and CEMs with high stability in alkali solutions, high OH− permeability, strong rejection for other anions, and relatively high WR, but poor water permeability, are also required. Two methods have been generally used for the DD process: the first one is a batch process, and the second one is a continuous or pilot process. The batch process is schematically presented in Figure 34a, in which two equal volumes are separated by an IEM, and one side is filled with the feed solution and the other side with fresh water. For decreasing concentration polarization, both solutions are stirred by mechanical stirrers continuously. This process is generally used in the laboratory for the calculation of diffusivity andSF. Continuous dialysis is based on the batch process but uses a number of IEMs in a dialyser for the recovery of acid or base (Fig. 34b). For decreasing concentration polarization, both the feed solution and water are moved by a pump. The data obtained in a continuous dialysis can be more valuable for practical reference because the experimental conditions are more similar to those in a practical dialysis unit. 4.3.1 Inorganic Acid Recovery. A number of inorganic acids are used in the industrial processes such as steel production and metal refining, nonferrous metal smelting, etc. For the recycling of acid from the metal salts, different methods are used, such as cooling and crystallization, thermal decomposition, evaporation and crystallization, ion exchange, solvent extraction, distillation, and electromembrane separation, as well as direct disposal and neutralization with alkalis. But all these methods consume considerable amounts of energy, and so new, sustainable methods such as DD, are required. Many workers have recovered H2 SO4 , HNO3 , HCl, and HF from different solutions using IEMs (Table 3). 4.3.2 Organic Acid Recovery. Organic acids have been widely used in chemical, leather, food, fermentation, and pharmaceutical industries. Among them, industries using bacterial fermentation always generate carboxylic acids and carboxylates of various compositions and concentrations (159). Acids are toxic to bacteria during the

44

ELECTRO-MEMBRANE PROCESSES

(a)

Stirrer

Stirrer

M

AE

Cl– Feed Solution

Water Solution

Cl–

M+ Cl–

Cl–

H−

M+

H−

(b)

Recovered acid Depleted Solution

Cl–

AEM

AEM

Cl– Cl–

AEM

Cl– Cl–

Cl– M+

M– Cl–

Feed water

Cl–

Cl–

H−

H−

H−

Cl– H−

Cl–

Cl–

H−

H−

Feed solution

FIGURE 34 Experimental set up for diffusion dialysis. (a) Batch process. (b) Continuousbreak dialysis. (Please refer to the online version for the color representation of the figure.)

fermentation process, and the pH of the broth should be in the range of 5–6 (160, 165). Hence different methods (163, 165), including facilitated membrane extraction, neutralization dialysis, IERs, as well as integrated systems of ultrafiltration and ED have been used to remove acids from the broth. Meanwhile, DD, as a simpler and more economical method, has been studied for the recovery of weak acids from those industries. Some work in the recovery of some carboxylic acids is systematized in Table 3. 4.3.3 Base Recovery. Alkali waste is mainly generated from paper, leather, printing and dying, tungsten ore smelting, and manmade fiber industries (166–169). DD is the lowest cost-effective process for the recovery of alkali from such waste. Around 20 years ago, Astom Corporation of Japan developed an alkali-resistant CEM, which was responsible for the remarkable progress in DD (166). Furthermore, Astom Corporation also successfully developed a DD process to recover NaOH from an aluminum etching

45

Separation of H2 SO4 + CuSO4 mixture H2 SO4 recovery from waste acid solution

H2 SO4

H2 SO4 and Ni recovery from electrolysis spent solution H2 SO4 recovery from waste aluminum surface processing solution Separation of H2 SO4 + ZnSO4 mixture H2 SO4 recovery from rare earth sulfate solution H2 SO4 recovery from titanium white acid by DD H2 SO4 recovery from hydrometallurgy leaching solution

Research and Application Field

Recovered Acid

Neosepta-AFN (Astom Corporation, Japan) DF 120-I (Shandong, China)

Laboratory scale (batch dialysis) Pilot scale: TSD-2 dialysis cell (Tokuyama Ltd. Japan) Pilot scale

Pilot scale

Selemion DSV (Asahi Glass, Tokyo, Japan)

Pilot scale: Asahidialyzer (Model T-O)

DF 120-I (Shandong, China)

DF 120-I (Shandong, China)

Brominated PPO-based AEM (Shandong, China)

Selemion DSV (Asahi Glass, Tokyo, Japan)

Pilot scale: Asahi Type T-0b Dialyzer

Laboratory scale and Pilot runs (industrial scale)

Neosepta-AFN (Astom Corporation, Japan)

Membrane

Laboratory scale (batch dialysis)

Scale 135

Membrane area 62.2 cm2 ; temperature 20 ± 0.5 ◦ C; R CuSO4 > 0.965 in case of C CuSO4 > 0.75 kmol/m3 Membrane area 0.327 m2 ; membranes number 19; R H2 SO4 80–57%; R NiSO4 and R FeSO4 96 and 99%; S H2 SO4 /FeSO4 15–18, S H2 SO4 /NiSO4 140–270 Membrane effective area 2.32 m2 ; R NiSO4 > 96%; R H2 SO4 66–72%

143

142

(continued overleaf)

Membranes number 40; RH2 SO4 80%; RV 96%

139

Membrane area 62.2 cm2 ; transport of negatively charged complexes of Zn2+ being controlling step Membrane area 0.02 m2 ; membranes number 11; one-pass and cycling operation; R H2 SO4 70–80% Membrane area 1.9 m2 ; S H2 SO4 /FeSO4 23.6; mR H2 SO4 > 85%; R FeSO4 > 93%

140, 141

138

Membrane area 0.326 m2 ; membranes number 19; R H2 SO4 82–90%; R Al (SO ) 35.3–38.5% 2 4 3

137

136

References

Process Characteristic

TABLE 3 Application Diffusion Dialysis in Inorganic and Organic Acid Separation

46

HCl

Neosepta-AFN (Tokuyama Co., Japan)

DF120-III (Shandong, China) DF120-I (Shandong in China)

Pilot scale (HKY-001 dialyzer) Laboratory scale; Pilot scale;

Laboratory scale

—

Laboratory scale (cycling operation)

HCl recovery from solution containing NiCl2

DF120-I (Shandong, China)

Pilot scale

Neosepta-AFN (Tokuyama Co., Japan)

DF 120-I (Shandong, China)

Industrial scale

H2 SO4 recovery from titanium white (pigment) waste solution Waste acid recovery from Hua Cheng Foil Factory Acid recovery by DD in chemical fiber factory H2 SO4 recovery from solution containing uranium H2 SO4 recovery from acid leach solution H2 SO4 recovery from waste anodic aluminium oxidation solution HCl recovery Laboratory scale and pilot scale (TSD-2 dialyzer)

Brominated PPO-based AEM (Shandong, China)

Laboratory scale

Research and Application Field

Recovered Acid

Membrane

Scale

(Continued)

TABLE 3

148

149

Membrane area 54 cm2 ; R H2 SO4 30%; R U 70% Membrane area 100 cm2 ; temperature 50 ◦ C; acid flux is proportional to acid concentration, membrane permeability is independent of acid concentration Membrane area 50 cm2 ; fluxes: 1 mol/m2 h for acid, 0.07 mol/m2 h for salt separation factor: 20 for Neosepta AFN, 30 for DSV membrane Separation factor: 20–37 for AFN, about 29 for Selemion DSV; Fluxes 1.5–2.0 for acid, 0.07–0.08 for salt

(continued overleaf)

160

159

156

155

154

153

Membrane area 512 m2 ; R H+ 85%

The permeability of HCl apparently increases with acid concentration

The partial flux of FeCl2 < 5.6%; the concentration gradient of FeCl2 has significant effect on the salt flux R HCl >88%; R Fe 89–77%; R Zn >56%

48 Pilot scale

Continuous dialysis of carboxylic acids

Solubility and diffusivity of carboxylic acids in membrane Laboratory scale (batch dialysis)

Pilot scale

Transport of some carboxylic acids

Transport of formic acid

Laboratory scale (batch dialysis)

Research and Application Field

Recovered Acid

HCOOH

Scale

(Continued)

TABLE 3

SB-6407 (Gelman Sciences); Neosepta AMH, AFN, ACM (Japan)

Neosepta AMH (Astom Corporation, Japan)

Neosepta AMH (Astom Corporation, Japan)

Neosepta AMH (Astom Corporation, Japan)

Membrane

161

Membrane area 62.2 cm2 ; the mass-transfer data of oxalic acid is the largest (1.0 × 10−6 to 1.0 × 10−7 m/s, nearly one order of magnitude higher than others) Membrane area 3.31 × 10−2 m2 ; P values of AMH 7.50 × 10−9 to 3.57 × 10−7 m/s, and its order: PCA < PLA < PTA < PAA < POA The partition coefficients (acid concentration in membrane to that in external solution) decrease with increasing acid in feed; The orders of D values: DTA < DLA < DAA < DOA Membrane area 7.07 cm2 ; AFN membraneshows the highest mass-transfer coefficient

164

163

162

References

Process Characteristic

ELECTRO-MEMBRANE PROCESSES

49

solution. The production technique was industrialized in a California Caspian plant, USA, around 1991 incorporating a diffusion dialyzer (TSD10-300 and TSD25-250) (166). Inspired by initial successes of DD, recovery of alkali from an alkali etching solution from the white tungsten ore smelting industry has been tried with dialyzer TSD-2-20 in Japan (170). The feed alkali solution contains WO3 91.25 g/l and NaOH 1.36 mol/l. When the flow rate of the feed and the recovered solution are controlled at 40 and 10 mol/min, respectively, in a cyclic manner, the concentration of the recovered solution was up by 51% of feed concentration, and the recovery rate was 50% (136). 4.4

General Description and Application of Donnan Dialysis

Donnan dialysis is used to exchange ions between two solutions. The basic principle of Donnan dialysis is schematically presented in Figure 35, which shows a two equal volumes separated by a CEM, with the feed CuSO4 solution on one side and 1 N H2 SO4 on the other. The solution pH on the CuSO4 side is significantly higher than that of the H2 SO4 side. The pH difference drives the H+ ions across the membrane, which increases the H+ concentration in the CuSO4 side. Cation selection membranes only pass the cations and repel the anions (SO4 2− ), which causes a potential difference, and for maintaining equilibrium Cu2+ ions move to solution 2. A similar process is followed by the AEM. 4.4.1 Separation of Salt from the Acid. With the help Donnan dialysis, an acid is easily separated by its salt solution. Figure 36 shows how an intelligent use of the Donnan membrane principle in an anion exchange column allows separation and recovery of relatively pure citric acid from a fermentation broth in the presence of citrate salts (171). CEM Solution 1 CUSO4, PH = 7

H−

SO4–2 Cu2−

Solution 2 H4SO4, PH = 1

H−

SO4–2 Cu2−

FIGURE 35 Schematic diagram illustrating the principle of Donnan dialysis using a CEM and a pH gradient to exchange copper ions against hydrogen ions. (Please refer to the online version for the color representation of the figure.)

50

ELECTRO-MEMBRANE PROCESSES

CEM Solution 1 RCOONa

H−

Solution 2 HCl

H−

RCOO Cl– Na−

Na−

FIGURE 36 Recovery of organic acid from their salt solution. (Please refer to the online version for the color representation of the figure.)

Owing to the Donnan membrane effect, the salt co-ions (e.g., Na+ , Mg2+ , Ca2+ ) are well excluded from the inside of the AERs containing nondiffusible quaternary ammonium (R4 N+ ) groups. 4.4.2 Coagulant Recovery. According to Second Law of thermodynamics, a solute always moves from a higher concentration to a lower concentration, but in Donnan membrane process it is possible, although counterintuitive, to drive ions from a lower to a higher concentration region and concentrate (such ions) even in the absence of electricity (external work). Worldwide, there are approximately 10,000 drinking water treatment plants where commercial alum is used for coagulation of suspended particles. In the coagulation process, aluminum sulfate is converted to undissolved Al(OH)3 and disposed of in land. But in Donnan membrane process, alum is recovered and further utilized. In Figure 37, the Donnan membrane process for the recovery of alum from waste Al(OH)3 is shown. 4.5

General Description and Application of Reverse Electrodialysis (RED)

The production of energy by mixing sea and river water through IEMs is referred to as RED (Fig. 38). The RED process provides clean and sustainable energy, and is based on the concentration gradient. The design of a stack to be used in RED is very similar to that used in ED. The main difference is that the cells are arranged in parallel between the electrodes and are rinsed alternately by sea and by river water. The ions in the sea water (Na+ and Cl− ) permeate into the river water through the corresponding IEM and produce brackish water due to their electrochemical potential gradient. This leads to an electrical current between the cathode and the anode.

51

ELECTRO-MEMBRANE PROCESSES

Water treatment residuals (WTR)

Dilute sulfuric acid

Impermeable Natural organic matter, anions, neutral molecules

Permeable AI3+ Permeable 3H+

AI3+

3H+

Impermeable AI(OH)3(s)

Impermeable SO42–

CI–

FIGURE 37 Single-step Donnan membrane process illustrating selective coagulant recovery from water treatment residuals (WTRs). Source: Reprinted with permission from Reference 171. Copyright (2010) American Chemical Society. (Please refer to the online version for the color representation of the figure.)

River water Condensation

Evaporation

Water Flow

Anode

Cathode Sea water

Na− Cl−

Anion-exchange membrane Cation-exchange membrane

FIGURE 38 Schematic representation of the natural water cycle and energy production by using reverse electrodialysis. (Please refer to the online version for the color representation of the figure.)

The maximum energy recovered in the RED process is equal to Gibbs free energy of mixing (mix G) of fresh water to sea water, which is given by: mix G = Gb − (Gc − Gd )

(21)

mix G = −(nc + nd )T mix Sb − (nc T mix sc − nd T mix sd )

(22)

52

ELECTRO-MEMBRANE PROCESSES

where the subscript c stands for the concentrated salt solution (e.g., sea water), the subscript d is for the dilute salt solution (e.g., river water), and the subscript b is for the resulting brackish salt solution. n is the amount (moles), and T is the temperature; mix s represents the contribution of the molar entropy of mixing (J/mol · K) to the total molar entropy of the corresponding electrolyte solution, according to  xi ln xi (23) mix s = −R i

where R is the universal gas constant (8.314 J/mol · K), and x is the mole fraction of component i (i = Na, Cl, H2 O). The theoretically available amount of energy from mixing 1 m3 of fresh water with 1 m3 of sea water having a total salt content of ca. 35 kg/m3 is ca. 0.4 kWh. Unfortunately, the actual recovered energy is lower than the theoretical value and it can calculated by the open voltage multiplied by current carried by ion diffusion of sea water in to river water and stack resistance. For a completely semipermeable IEM, the open voltage is calculated by the following equation: a RT ln c (24) Usto = F ad where Usto is the open voltage of a stack, R is the gas constant, T is the absolute temperature, and ac and ad are the equivalent activities of the concentrated and diluted cell, respectively. The maximum power that can be generated is given as Wmax = IUsto = I 2 Rst =

(U o )2 Rst

(25)

where Wmax is the maximum power output, I is the current of the stack, and R is the resistance of stack. Stack resistance is the most important parameter for increasing the maximum power density; it can be reduced by making the layers as thin as possible to decrease the concentration polarization area. From the literature, it is found that the maximum power density that is achieved to date in practical application is 0.5–1.0 W/m2 (107). Therefore, it seems that the membrane costs must be drastically decreased to make RED a competitive energy production process. But better and thinner membranes and cells, as well as low cost of membranes, which most likely will be developed, will increase the competitiveness of RED in the future.

5

CONCLUSIONS AND FUTURE PERSPECTIVE

ED, which is based on IEMs, has been well proven for water desalination and purification. However, new applications of ED and other related processes are rapidly developing for separation, downstream process, and water-splitting technology. New challenges in water and wastewater treatment, chemical processing, and food and biotechnology industry will also accelerate the requirement of ED and related processes. For these processes, highly selective, conductive, stable, and durable IEMs are urgently required. Development of specific membrane-forming materials has been receiving interdisciplinary efforts since last two decades. Several types of functionalized polymers and organic–inorganic

53

FIGURE 39

Research status and application of electromembrane processes. (Please refer to the online version for the color representation of the figure.)

54

ELECTRO-MEMBRANE PROCESSES

nanocomposite materials are most appropriate to produce highly stable and permselective IEMs. Intensified researches on IEM-based EMPs have been accomplished during recent years to produce potable water, separation and recovery of toxic components, and downstream processing for biomolecular separation and recovery. The choice of the proper membrane, design, and operational optimization of the process are highly essential for successful commercialization of any process (Fig. 39). Thus, the development of cost-effective membranes by the eco-friendly route is highly urgent for researchers working in the membrane-based electrochemical separation process filed. Current IEM research starts with requirement of membrane properties for a specific process, elucidation of membrane structure, and design of the membranes from molecular-level tailoring. The relationship between separation results and membrane function should be studied properly for successful membrane developments. In addition, the relationship between the membrane microstructure and operation parameters of EMP has also a valuable impact. Some of IEM applications should be considered as state of the art for technology development, such as water desalination/purification and EDI, and BPMs for the production of organic acids from their salts present in fermentation. Often, EMP competes with other mass separation techniques from economic considerations, but in some applications EMPs provide higher quality products and are more environmentally friendly. These processes can be used as alternatives for the separation and recovery in spite of their cost disadvantage. An interdisciplinary approach should help develop EMP for water treatment, biomolecular separation, waste treatment, and energy production, as well as integrate recent advances and knowledge on material, inorganic, polymer science and technology, mathematics, and engineering to solve some multifold problems and design of apparatus for EMPs.

ACKNOWLEDGMENT The authors wish to acknowledge the services of the Analytical Sciences Division, CSMCRI, Bhavnagar, India, for instrumental support. One of the authors (AKS) is grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing a Senior Research Fellowship (Ext).

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