Preparation and characterization of organic–inorganic hybrid anion-exchange membranes for electrodialysis

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JIEC-1991; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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Preparation and characterization of organic–inorganic hybrid anion-exchange membranes for electrodialysis Moonis Ali Khan a,*, Mahendra Kumar b, Zeid Abdullah Alothman a a b

Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia School of Biotechnology, Dublin City University, Dublin 9, Ireland

A R T I C L E I N F O

Article history: Received 14 January 2014 Accepted 4 April 2014 Available online xxx Keywords: Organic–inorganic hybrid anion-exchange membrane Sol–gel Membrane conductivity Electrodialysis NaCl removal

A B S T R A C T

Organic–inorganic hybrid anion-exchange membranes were prepared from quaternized chitosan, anionexchange silica precursor and poly(vinyl alcohol) by the sol–gel method in acidic condition (pH 2). Fourier transform infrared spectroscopy with attenuated total reflection mode, scanning electron microscopy and wide angle X-ray diffraction technique were used to confirm the functional groups in the membranes and their surface morphology. Physicochemical and electrochemical properties of the prepared membranes were determined in detail. The electro-osmotic study was conducted to determine the equivalent pore radius of the membranes. The membranes were used in electrodialytic removal of (0.1, 0.2 and 0.4 M) NaCl solution at 3 V applied potential. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Electrodialysis is an electro-membrane process in which an electrical potential gradient is applied as driving force for the selective separation and recovery of ions from solutions [1–3]. In this process, ions are migrated from the diluate compartment to the concentrated compartment through ion-exchange membranes (IEMs) at an applied electrical gradient across the electrodes. Ion-exchange membranes have gained more attention in academic and industrial research due to their high selectivity toward specific ions and applications in electrodriven membrane based separation processes [1–6]. The ion exchange membranes contain either fixed positive charged groups (i.e. anion-exchange membrane; AEM) or fixed negative charged groups (i.e. cationexchange membrane; CEM). CEMs and AEMs are selective for negatively or positively charged ions [1,2]. AEMs are widely used in electrodialysis, electro-membrane reactors, diffusion dialysis, and Chlor-alkali process because AEMs allow the transport of anions via an electrostatic interaction and oppose the transport of cations [1–3]. However, the permselectivity of the membranes is an important parameter to evaluate the efficiency of AEMs in electro-membrane separation processes [7,8]. Thus, highly

* Corresponding author. Tel.: +966 564505403. E-mail addresses: [email protected], [email protected] (M.A. Khan).

conductive, selective, chemical, thermal and oxidative stable AEMs are urgently required for practical applications in the aforementioned membrane based separation processes. Several AEMs have been prepared from chloromethylated polysulfone, poly(etherimide) and Cardo poly(ether sulfone) as well as styrene–divinylbenzene copolymer derived polymers. The anion-exchange groups (i.e. quaternary ammonium groups; – N+(CH3)3) are anchored by conducting amination reaction using tertiary amine [1,2]. Chloromethylated polymers are synthesized by performing a chloromethylation reaction on polymer main chains using chloro-methyl ether (CME). However, CME is harmful to human health and banned chemical [8–10]. The copolymer of chloromethylstyrene and divinylbenzene with different amine groups (trimethylamine and 4-vinylpyridine) have also been used to fabricate AEMs. The chloromethylstyrene is an expensive chemical and therefore, the manufacturing cost of AEMs is enhanced [10,11]. In addition, the styrene-co-divinylbenzene copolymer derived AEMs are thermally and oxidatively unstable [2,10]. Due to the aforementioned difficulties and reasons, highly selective and stable anion-exchange membranes are required at low manufacturing cost and simple preparation method. AEMs have been prepared by the doping or blending methods using the inorganic fillers with polymer electrolyte solutions. However, the doping or blending methods are not suitable to fabricate AEMs because the inorganic fillers are not homogenously distributed in

http://dx.doi.org/10.1016/j.jiec.2014.04.002 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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List of abbreviations

w Wdry Wwet Lwet Ldry a b m tCl  F Co C Vo V R T r Rmem

L

m

L A V0F Vtp C0F Ctp CE W m M n Q V i t DV ilim

water uptake (%) weight of the dried membrane (g) weight of the wet membrane (g) length of the wet membrane (cm) length of the dried membrane (cm) concentration of AgNO3 solution (mmol dm3) volume of the consumed AgNO3 solution (dm3) chloride ion transport number in the membrane phase Faraday constant (96,500 C) initial concentration of Cl in cathodic compartment final concentration of Cl in cathodic compartment initial volume of the cathodic compartment (mL) final volume of the cathodic compartment (mL) gas constant (8.314 J mol1 K1) absolute temperature (K) equivalent pore radius (A8) membrane resistance (V) conductivity of membrane (mS cm1) thickness of the membrane (cm) membrane area (cm2) initial volume of diluate compartment (L) final volume of diluate compartment (L) initial concentration of NaCl (mol L1) final concentration of NaCl (mol L1) current efficiency (%) energy consumption (kWh kg1) weight of electro-transported NaCl (kg) molecular weight of NaCl (g mol1) stoichiometric number (n = 1 in this case) electricity passed (C) applied potential (V) current (A) the allowed time (s) plateau length (V) limiting current density (mA cm2)

membranes matrix. Further, the fillers are easily leached out from the membrane matrix after the long time application [12,13]. Researchers have been developed various AEMs using the prefunctionalized polymers derived from poly(2,6-dimethyl-1,4phenylene oxide), PVDF-polyvinyl benzyl chloride, and polymeric imidazolium salts [1,10,14,15]. It has been noticed that the pre-functionalized polymers derived membranes are less selective and oxidatively unstable. The organic–inorganic hybrid materials have been developed by the sol–gel method to fabricate selective and oxidative stable AEMs due to the dual properties of inorganic backbone (i.e. oxidative and thermal stable) and organo-functional groups [1,2,12,13,16]. The interpenetrating network between organic and inorganic components is formed in the membrane matrix when the sol–gel method is conducted in acidic/basic hydrolysis of metal alkoxides or silica precursors in polymer electrolyte solutions [2,3,16–18]. Singh et al. [19] had synthesized anion-exchange silica precursor to fabricate organic–inorganic hybrid AEMs by the sol–gel method. The membranes had excellent electrochemical properties, thermal stability and high selectivity with chloride ion [19]. Recently, Wu et al. had prepared AEMs from

quaternized poly(2,6-dimethyl-1,4-phenylene oxide), poly(vinyl alcohol) using phenyl triethoxysilane and tetraethoxysilane as a double cross-linking agent. The membranes were thermally stable and selective for the diffusion of ions [20]. Chitosan has been used in the fabrication of IEMs due to its availability in large quantity, non-hazardous and biodegradable behavior [21–25]. The amino- and hydroxyl groups on the main chain of chitosan provide an opportunity to anchor cation- or anion-exchange groups by chemical modifications. In addition to this, the solubility of modified chitosan is enhanced. Chitosan derivatives containing quaternary ammonium groups have been used in the fabrication of AEMs for electro-chemical separation process, fuel cell and pervaporation [24–28]. The object of this study was to develop thermally and oxidatively stable organic–inorganic hybrid AEMs at low manufacturing cost and simple preparation method for electromembrane separation processes. Quaternized chitosan and anionexchange silica precursor were used to obtain selective and oxidatively stable AEMs. In this study, the fabricated membranes were used in electrodialytic removal of NaCl from its aqueous solution at constant applied potential. 2. Materials and methods 2.1. Materials Poly(ether sulfone), glycidoxypropyltrimethylammonium chloride (GDTMAC), 3-(2-aminoethylamino) propyl-trimethoxy silane (AAPTMS), tetraethoxysilane (TEOS), poly(vinyl alcohol) and chitosan (Mw  200,000; N-deacetylation degree 75–85%) were purchased from Sigma–Aldrich Chemicals. Iodomethane (CH3I), acetone, methanol, formaldehyde, Na2SO4, NaCl and H2SO4 were obtained from Merck Chemicals and used without any further purification. All other chemicals and reagents were of commercial grade. The electrodes used in this study were purchased from Titanium Tantalum Products (TITAN, India). Double distilled water (DDW) was used in this study. 2.2. Preparation of quaternized chitosan and anion-exchange silica precursor Quaternized chitosan was synthesized according to a reported method in literature [26,27]. The method for the preparation of quaternized chitosan (QC) was as follows: 2.5 g of chitosan was dispersed into a round bottom flask containing 42 mL methanol and 58 mL deionized water and subsequently the resulting mixture was stirred for 1 h at 30 8C. Thereafter, 20 mL of CH3I solution in three portions (2:2:1) was added at 3 h interval and the reaction mixture refluxed at 70 8C for 12 h. After 12 h, the yellow color solution was obtained and the temperature of the resulting mixture solution was reduced to 30 8C. Then, 5 wt% solution of NaCl was added in excess and continuously stirred for 24 h to convert the quaternized chitosan in iodide form to chloride. The reaction mixture was poured into excess acetone and the resulting precipitate was collected by vacuum filtration on the filter paper and dried in a vacuum oven at 50 8C for 12 h. Finally, the light yellow color powder was obtained. Anion-exchange silica precursor (AESP) was synthesized from GDTMAC and AAPTMS by an epoxide ring opening reaction at 80 8C [19]. The AAPTMS and GDTMAC in 1:2 molar ratios were added alternatively into a round bottom flask containing the predefined amount of DDW. The reaction continued at 30 8C for 2 h with a constant stirring and then, the temperature was raised to 80 8C for further 1 h reaction. The transparent viscous solution was obtained for anion-exchange silica precursor.

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2.3. Membrane preparation and characterizations The QC, AESP and PVA derived organic–inorganic hybrid AEMs were prepared in two steps: (i) sol–gel method and (ii) chemical cross-linking with formaldehyde in acidic medium. The membrane was prepared as follows: 10 g PVA was dissolved in 100 mL of DDW at 70 8C and 2 g of QC was separately dissolved in 100 mL DDW with constant stirring at 30 8C. Polymer solutions were then mixed and the stirring continued for 12 h to get a homogeneous solution. AESP with varied fraction (wt%) to the total wt% of PVA and QC was added into a homogeneous solution and subsequently, the predefined amount of TEOS was added drop wise. The pH of the resulting solution was adjusted to 2 by adding few drops of 4 M HCl solution and the stirring continued at 30 8C for 12 h. The transparent viscous gel was then obtained which was cast onto a clean and dry glass plate using a glass rod with a gap height of 300 mm [8,16]. Transparent membrane with glass plate was left at 30 8C for 24 h and then the dried membrane kept in an oven at 60 8C for 24 h to remove the traces of moisture. Afterwards, the membrane was placed into the solution containing formaldehyde (54.1 g), sodium sulfate (150 g), sulfuric acid (125 g), and water (470 g) for 2 h at 60 8C for cross-linking [8,16]. The same procedure was adopted to fabricate membranes with varied fraction of AESP. The membranes were designated as A-X (where X is the amount of AESP (%) to the total wt% of QC and PVA): membrane A-50, A-55, A60 and A-70 respectively. Sulfonated polyether sulfone (SPES) based CEM was used in this study. SPES was synthesized by conducting sulfonation reaction for polyether sulfone at 50 8C for 24 h using excess H2SO4 [29]. The 20 wt% of SPES was dissolved in DMF and polymer solution was then cast onto a clean glass plate using a glass rod with a gap height of 200 mm. The transparent membrane with glass plate was left at 30 8C for 3 h and then placed into a vacuum oven at 60 8C to remove the traces of DMF. The CEM and AEM were equilibrated in 1 M HCl and NaOH solutions consecutively for 24 h to convert them in H+ and OH form. The acid and base treated membranes were thoroughly washed with DDW to remove the traces of acid and base. Finally, the cleaned membranes were kept in 1 M NaCl solution before further characterizations and use. FTIR spectrum of AESP was recorded by KBr pellet method in transmission mode. Fourier transform infrared spectrometer in attenuated total reflectance mode (FTIR-ATR; Spectrum GX series 49387 spectrometer) was used to record FTIR spectra of QC and membranes. ATR-FTIR spectra were recorded over a wide range from 800 to 4000 cm1 at an average of 32 scans with a resolution of 4.0 cm1. The wide angle X-ray diffraction (WXRD) pattern of the membranes was recorded using Philips Xpert wide-angle X-ray diffractometer with nickel-filtered CuKa radiation (1.54056). Thermal stability of the membranes was determined by thermo gravimetric analysis (TGA) using thermo gravimetric analyzer (Mettler Toledo TGA/SDTA851 with Starc software). The water uptake of the membranes was determined from the change in weight of the membrane before and after hydration using an electronic balance (Mettler Toledo, Thermo Scientific Co.). The water uptake (w) was calculated using Eq. (1) [3,8]:

’ ð%Þ ¼

W wet  W dry  100 W dry

m Section S2. The transport number of the membranes (tCl  ) was determined by Hittorf’s method [8,19]. The detailed procedure for the determination of membrane transport number is given in supporting information; Section S3. The electro-osmotic permeability measurement was performed to determine the equivalent pore radius of the membranes [8,19]. The equivalent pore radius of the membranes was calculated using Katchalsky and Curran equation (Eq. (2)) [8,30]:

r¼

!1=2 8hF b 0 f1w

(2)

DCl value was calculated from the ionic conductivity of 0.01 M NaCl solution [31,32]. The conductivity of membranes in equilibration with varied concentration of NaCl solution (0.01–0.1 M) was determined using a potentiostat–galvanostat instrument (Auto Lab, Model PGSTAT 30, Eco Chemie, B.V. Utrecht, Netherlands). The conductivity of membrane was calculated using Eq. (3) [8,19]:

Lm ¼

L Rmem  A

(3)

The current–voltage (i–v) polarization curves for the membranes in equilibration with 0.1 M NaCl solution were recorded according to the reported procedure elsewhere [16,19]. 2.4. Electrodialysis performance Electrodialysis experiments were performed to evaluate the performance of membranes in a removal of NaCl from its aqueous solution. ED cell was fabricated using PVC sheets and titanium oxide electrodes coated with a triple precious metal oxide (titanium–ruthenium–platinum). ED cell was divided into four compartments (diluate compartment – DC; concentrated compartment – CC and electrode wash (EW) compartments (anode and cathode)) by placing the two pieces of AEM and CEM between the electrodes. The effective area of membrane and the electrodes was 55 cm2. The schematic presentation of ED cell is depicted in Fig. S1 (supporting information). The parallel-cum-series flow arrangement was used to monitor the flow of each stream in the respective compartments and 0.1 M Na2SO4 solution was used as an electrode wash. Initially, 300 mL of DDW and NaCl solution of varied concentration (0.1, 0.2 and 0.4 M) were fed into CC and DC using the peristaltic pumps at a flow rate of 120 mL min1. Thereafter, 3 V potential was applied across the electrodes using the power supply in direct current mode. ED experiments for the removal of NaCl from its aqueous solution at 3 V applied potential were conducted for 2 h. The variation in current (A) during the experiments was recorded using a digital multimeter in series. The changes in solution pH and conductivity of DC and CC were also regularly monitored by placing the digital pH and conductivity meters into the respective compartments. Moreover, the NaCl removal (%) efficiency of membranes was determined from the change in concentration of NaCl solution in DC after 2 h experiment at 3 V applied potential. The NaCl removal (%) efficiency was calculated using Eq. (4) [8,27]:

(1)

The dimensional swelling ratio (DSR) was calculated from the changes in volume of the membranes in wet and dry states [30]. The detailed procedure is given in supporting information; Section S1. The ion-exchange capacity (IEC) of the membranes was calculated by Mohr method [11,16]. The detailed procedure for the determination of IEC is given in supporting information;

3

NaCl removal efficiency ¼

Ct pV t p  100 ð%Þ C 0F V 0F

(4)

The current efficiency and energy consumption values were calculated using Eqs. (5) and (6) [8,32,33]: CE ð%Þ ¼

mnF  100 MQ

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(5)

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JIEC-1991; No. of Pages 8 M.A. Khan et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

4

and 1

W ðkWh kg

Þ¼

Z 0

t

VIdt m

(6)

3. Results and discussion 3.1. Synthesis and characterization of quaternized chitosan and anion-exchange silica precursor Quaternized chitosan was synthesized by performing the quaternization of chitosan using CH3I at 70 8C. The absorption bands at 1651, 1466 and 1320 cm1 confirmed the success of quaternization of chitosan (Fig. S2, supporting information) [27,34]. AESP was synthesized via an epoxide ring opening of GDTMAC with AAPTMS at 80 8C. In this reaction, the proton transferred from –NH2 groups of AAPTMS into the epoxide ring of GDTMAC and thus the secondary alcohol was formed [19]. The absorption bands at 1641 and 1466 cm1 confirmed the success of reaction and the presence of –N+(CH3)3 groups in AESP (Fig. S3, supporting information). 3.2. Membrane preparation and characterization The membrane forming material was prepared by the sol–gel method and condensation polymerization of AESP in acidic solution containing PVA and QC. The cross-linking of the membranes occurred in two steps. (i) Formaldehyde reacted with –OH groups of PVA so that hemiacetal was formed and (ii) the hemiacetal was reacted with the other –OH group of PVA. Thus acetal was formed in the membrane matrix. The membranes were fabricated at molecular level and PVA, AESP and QC were

connected by the covalent/hydrogen bonding interactions [19,20,29]. The schematic reaction route for the fabrication of membranes is shown in Scheme 1. The ATR-FTIR spectrum for membrane A-70 is depicted in Fig. 1 as a representative example. The broad absorption band at 3330 cm1 attributed to the stretching vibration of –OH groups in the membrane. The absorption band at 2922 cm1 is occurred due to the –CH2 stretching vibrations [19,23]. The bands at 1167 and 1082 cm1 are ascribed to the C–O–C and Si–O–Si linkages in the membranes [16,19,20]. The absorption bands at 1656 and 1390 cm1 are due to the stretching vibration of I and III amine groups in the membranes [34,35]. The peak at 1410 cm1 is attributed to the asymmetric bending vibration of –CH3 groups in –N+(CH3)3 groups [8,19,24]. Results confirmed that organic–inorganic hybrid AEMs were fabricated successful by the sol–gel method and cross-linking. The WXRD patterns for membrane A-50, A-60 and A-70 are depicted in Fig. 2. The hybrid membranes have typical peaks at around 2u = 208 and the intensity of these peaks is systematically decreasing with increasing fraction of AESP. This indicated that the crystallinity of the hybrid membranes was decreased by the AESP because the fraction of quaternary ammonium groups was simultaneously enhanced. It is expected that the intramolecular interaction between PVA, AESP and QC in the membranes suppressed while the intermolecular interaction between PVA, AESP and QC in the membranes established when the fraction of AESP was increased [36,37]. As a result, the amorphous nature of the membrane was dominated, what could favor the more diffusion of the ions [28,36]. The thermal stability of the membranes was investigated by performing TGA study. The obtained data for membrane A-50, A-60 and A-70 are presented in Fig. 3 and three steps weight loss was observed for these membranes. The first weight loss 100 8C is due to the evaporation of water from the membranes [19,37]. The second weight loss in

Scheme 1. Schematic reaction route for the preparation of organic–inorganic hybrid anion-exchange membranes.

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A-50 A-60 A-70

100

Weight loss (%)

Transmitance (%)

80

1656

1320 1410

3330 2922

1167

1082

3500

3000

2500

2000

1500

40

20

847

4000

60

0

1000 100

-1

Wave number (cm )

200

300

400

500

600

o

Temperature ( C)

Fig. 1. ATR-FTIR spectrum of membrane A-70. Fig. 3. TGA curves for organic–inorganic hybrid anion-exchange membranes.

the range from 250 to 430 8C is attributed to the decomposition of quaternary ammonium groups in the membranes [8,19,35]. The third weight loss after 430 8C is ascribed to the decomposition in main polymer chains [19,20]. These results confirmed that the prepared membranes were thermal stable. 3.3. Membrane stability Oxidative stability of the membranes was checked by estimating their loss in weight (%) and IEC after treatment with Fenton reagent (3% H2O2 containing 2 mg/L FeSO4) for 1 hat 70 8C. The obtained data are tabulated in Table S1; supporting information section. The slight loss in weight (%) and IEC values were obtained after Fenton reagent treatment. This indicated that the highly cross-linked silica network had restricted the attack of OH/OOH radicals onto the hydrophilic domains of the membranes [14,16,18]. The lowest loss in weight (%) and IEC were obtained for membrane A-70 because the tight cross-linked silica network in membrane matrix restricted the attack of OH/OOH radicals than that of other membranes. Over all, the results confirmed that the prepared membranes had high oxidative stability. The prepared

Intensity (a.u.)

A-50 A-60 A-70

membranes could be suitable for use in electro-membrane separation processes under oxidative environment. 3.4. Membranes characterization The membranes were characterized physico- and electrochemically by determining their water uptake, DSR, IEC, transport number and equivalent pore radii. The ww values for the membranes are tabulated in Table 1 and these values were systematically increased with fraction of AESP because of high attracting ability of the membranes due to the presence of quaternary ammonium groups [8,19]. The highest value (40.1%) of water uptake was obtained for membrane A-70 because the high fraction of quaternary ammonium groups was contributed by AESP. The DSR values for membranes are tabulated in Table 1. Results demonstrated that the prepared membranes were dimensional stable and their dimensional stability further enhanced with increasing fraction of AESP in matrix (cf. Table 1). The IEC values for the membranes are reported in Table 1 and the obtained values are in range of 0.82– 1.29 mequiv. g1. This could be ascribed to the increase in fraction of anion-exchange groups (i.e. quaternary ammonium groups) in the membranes. These results provided an opportunity for tuning the IEC of the membranes by simply varying the fraction of AESP in casting solutions. The prepared membranes were highly charged, which are required for electro-membrane separation processes. The IEC of the prepared membranes are comparable to other m reported membranes in the literature [8,10,17,38,39]. The tCl  values of membranes are tabulated in Table 1. These values were increased with fraction of AESP because the diffusion of co-ions across the membrane was suppressed with increasing fraction of quaternary ammonium groups (cf. Table 1) [19,39,40]. Among all the prepared membranes, the highest transport number (0.94) was

Table 1 Physicochemical and electrochemical properties of organic–inorganic hybrid AEMs.

0

10

20

30

40

50

Membrane

Fraction of AESP (%)

ww (%)

DSR (%)

IEC (meqiv. g1)

m tCl 

r (A8)

A-50 A-55 A-60 A-70

50 55 60 70

21.7 25.5 35.2 40.1

17.2 16.6 11.4 7.61

0.82 1.02 1.15 1.29

0.86 0.90 0.92 0.94

3.23 3.78 4.16 4.34

60

2 θ (degree) Fig. 2. WXRD patterns for organic–inorganic hybrid AEMs with varied fraction of AESP.

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Fig. 4. Variation in km for: (A) organic–inorganic hybrid AEMs in equilibration with varied concentration of NaCl solution and (B) at varied solution pH in equilibration with 0.1 M NaCl solution.

obtained for membrane A-70. This could be related to the highest suppression of co-ion diffusion by maximum fraction of quaternary ammonium groups in the membrane A-70. Over all, these results are suggested that the prepared membranes were more efficient toward the chloride ions. The obtained values of equivalent pore radii for the membranes are tabulated in Table 1. Results demonstrated that the equivalent pore radii of the membranes were systematically increased with fraction of AESP. This can be explained on the basis of transport of water molecules along with migration of chloride ions under the applied electrical gradient. The water transport rate across the positively charged membranes could be enhanced when the fraction of quaternary ammonium groups is systematically increased. The highest value of equivalent pore radius (4.34 A8) was obtained for membrane A-70 because this membrane had an excellent ability to transport the water molecules along with chloride ion due to high water attracting capacity of quaternary ammonium groups [19,39]. The obtained values of equivalent pore radii confirmed the dense nature of the prepared membranes. The conductivity of membranes is presented in Fig. 4 (A and B). The Lm values depended on the fraction of AESP in the membrane matrix and these values were systematically increased with fraction of AESP. The conductivity of membranes is dependent on the water uptake, IEC and concentration of electrolyte solution. The higher conductivities are obtained for membranes which have high

Current density (mA cm-2 )

14 12

2

10

1 0 0.0

8

A

B

3

0.5

1.0

1.5

2.0

3.5. Membranes performance

2.5

6 4

A-50 A-55 A-60 A-70

2 0

0

1

2

3

water uptake and IEC values [8,19,32,41]. This could be one probable reason for enhancing the conductivity of the membranes in equilibration with 0.1 M NaCl solution. Furthermore, Lm values were also increased with concentration of NaCl solution. It has been reported that electro-neutral solutions of electrolyte are filled in intergel of membranes and the gel phase combines with the microporous charged matrix [32,41]. Therefore, the electrical conductivity of NaCl solution in the intergel of the membrane enhanced at high concentration of NaCl solution. Due to this reason, the conductivity of membranes was increased with concentration of NaCl solution. The value of Lm for membrane A-70 was higher than that of other membranes. In addition, Lm for membrane A-70 was higher, for instance, than those of other reported membranes in the literature [8,19,38]. Effect of solution pH on Lm of membrane A70 was studied by varying pH of NaCl solution in range from 2 to 12 to evaluate its stability in acidic and alkaline medium. The relevant data of Lm for membrane A-70 is presented in Fig. 4(B). The values of Lm for membrane A-70 remained almost same in the studied pH range from 2 to 12. These results suggested that the prepared membranes could be suitable in electro-membrane separation process under acidic or alkaline medium. The i–v polarization curves for the membranes are presented in Fig. 5(A) and the obtained values (DV, Di and ilim) are included in Table 2. The DV and ilim values depended on the fraction of AESP in the membranes and these values were systematically increased with fraction of AESP. This is attributed to the change in surface heterogeneity and conducting area of membranes with fraction of AESP [8,42]. The highest value (2.72 V) of plateau length was obtained for membrane A-70 because this membrane had high conductivity and surface heterogeneity [40,42]. In addition, the variation in current density at low applied potential is presented in inset Fig. 5(B), showing that current density values were improved after the incorporation of AESP in the membranes.

4

5

Applied potential (V) Fig. 5. Current–voltage (i–v) polarization curves for organic–inorganic hybrid AEMs in equilibration with 0.1 M NaCl solution (A) at high and (B) low current density.

After the detailed characterization of membranes, the membranes were used in electrodialytic removal of NaCl from its aqueous solution (cf. Fig. S1; supporting information). Electrodialytic experiments were conducted by feeding 300 mL of 0.1, 0.2 M and Table 2 Characteristics (Di, Dilim and DV) values for organic–inorganic hybrid AEMs in equilibration with 0.1 M NaCl solution. Membrane

Di (mA cm2)

Dilim (mA cm2)

DV (V)

A-50 A-55 A-60 A-70

1.25 1.57 1.8 1.86

1.45 1.8 2.1 2.18

1.54 1.70 1.84 2.72

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Fig. 7. Variation in % removal of NaCl at 3 V applied potential using organic– inorganic hybrid AEMs. Fig. 6. Variation in current density during the ED removal of 0.2 M NaCl solution at 3 V applied potential using organic–inorganic hybrid AEMs.

0.4 M NaCl solution in DC and DDW in CC for 2 h at 3 V applied potential. The variation in current density (mA cm2) during the electrodialytic removal of 0.2 M NaCl solution using the membranes A-50, A-55, A-60 and A-70 is shown in Fig. 6 as a representative example. It can be seen that the current density values were systematically increased with fraction of AESP in the membranes. This could be attributed to the fast electromigration of Cl from diluate compartment to concentrated compartment by the highly charged membranes (cf. Table 1). The highest variation in current density was achieved for membrane A-70 because this membrane facilitated more Cl than that of other membranes. Moreover, the current density values at the initial stage of experiments were high because more Na+ and Cl ions were available for electro-migration across the membranes from DC to CC at 3 V applied potential. The membrane performance was evaluated by calculating NaCl removal efficiency (%), CE (%) W (kWh kg1). The relevant data for NaCl removal (%) efficiency of membranes are presented in Fig. 7. The NaCl removal (%) efficiency of the membranes was systematically increased with fraction of AESP. This could be attributed to the increase in counter ions ability of the membranes with fraction of AESP because the quaternary ammonium groups were proportionally enhanced. The highest removal (94.2%) of NaCl from 0.2 M NaCl solution was obtained for membrane A-70 because Cl ions transported fast from the DC to CC due to high

m IEC and tCl  values (cf. Table 1). Moreover, NaCl removal (%) efficiency was dependent on the feed concentration of NaCl solution. Results are demonstrated that NaCl removal efficiency (%) of the membranes decreased with increase in concentration of NaCl from 0.2 to 0.4 M under same experimental conditions (cf. Fig. 7). The feed concentration of electrolyte solution influenced the % removal of NaCl because limiting current density and water splitting efficiency are dependent on the concentration of feed electrolyte solution [8,19,32]. Due to this reason, the current density became so high (larger than the limiting current density of AEMs) when concentration of NaCl solution was increased to 0.4 M from 0.2 M. Thus, water splitting was started at the membrane interface and more OH produced which had hindered the migration of Cl at high concentration of feed electrolyte solution. Therefore, the exchange rate of Cl across the membrane became slow, which was responsible for further decline in % NaCl removal efficiency of the membranes [32,43]. Moreover, the rate of Cl migration across the membranes from DC to CC became independent above the certain feed concentration of NaCl solution. These results clearly indicated that the developed membranes were efficient in electrodialytic removal of 0.2 M NaCl solution at 3 V applied potential. The obtained values of CE for membranes are presented in Fig. 8(A); CE values depended on the fraction of AESP in the membrane matrix. The CE values were increased with fraction of AESP because % NaCl removal efficiency of the membranes proportionally enhanced with fraction of AESP (cf. Fig. 7). The highest CE (90.4%) was obtained for membrane A-70 because this

Fig. 8. Variation in: (A) CE (%), and (B) W (kWh kg1) during the electrodialytic removal of 0.2 M NaCl solution at 3 V applied potential using organic–inorganic hybrid AEMs.

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membrane had highest % NaCl removal efficiency compared to other membranes. However, CE of membranes was systematically reduced with further increase in concentration of NaCl solution from 0.2 M to 0.4 M. This could be correlated to decline in % NaCl removal efficiency with feed concentration of NaCl solution because CE depended on the rate of NaCl removal [8,19,43]. The obtained values of W for membranes are presented in Fig. 8 (B); W values were decreased with increase in fraction of AESP because electrodialytic process became fast due to high counter ion facilitating ability of the membranes under same experimental conditions. Low energy (4.1 kWh kg1) was consumed when membrane A-70 used in electrodialytic removal of 0.2 M NaCl solution at 3 V applied potential. It was also observed that high energy was consumed during the electrodialytic removal of 0.4 M NaCl solution. This happened due to decline in % NaCl removal efficiency of the membranes with increase in concentration of NaCl solution from 0.2 to 0.4 M (cf. Fig. 7). The obtained results are indicated that the prepared membranes were efficient in electrodialytic removal of 0.2 M NaCl solution at constant applied potential 3 V. 4. Conclusions Organic–inorganic hybrids AEMs were prepared successful using QC, AESP and PVA by the sol–gel method and cross-linking. This approach is simple and avoids the use of carcinogenic and banned chloromethyl methyl ether. The membranes were thermal and oxidative stable at elevated temperature. The membranes were dense as confirmed from the obtained values of equivalent pore radius in the range from 3.23 to 4.34 A8. The membranes had high water uptake, IEC, conductivity and counter ion transport number. The obtained values of w (40.1%), IEC (1.29 meqiv. g1), Lm (11 mS m cm1) and tCl  (0.94) for membrane A-70 suggested that physicochemical and electrochemical properties of the membranes could be tuned by varying the fraction of AESP in the casting solutions. The prepared membranes were highly conducting and selective toward chloride ions. The developed membranes had better NaCl removal efficiency under the optimized conditions. The low energy (4.1 kWh kg1) was consumed for 94% removal of NaCl from 0.2 M NaCl solution at 3 V applied potential when membrane A-70 was used in electrodialytic process. Over all, the developed membranes could be efficient for water desalination and other electro-membrane separation processes. Acknowledgement Authors would like to thank funding from Deanship of Scientific Research (RGP-VPP-052) at King Saud University Riyadh, KSA is gratefully acknowledged.

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