Development of BPPO-based anion exchange membranes for electrodialysis desalination applications

DES-12755; No of Pages 8 Desalination xxx (2015) xxx–xxx

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Development of BPPO-based anion exchange membranes for electrodialysis desalination applications Muhammad Imran Khan, Abhishek N. Mondal, Bin Tong, Chenxiao Jiang, Kamana Emmanuel, Zhengjin Yang, Liang Wu, Tongwen Xu ⁎ CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Anion exchange membranes with Nmethylmorpholine (NMM) groups were developed. • The membrane performance can be controlled by NMM contents. • The membranes possess excellent stability as well as high ED performance.

a r t i c l e

i n f o

Article history: Received 16 October 2015 Received in revised form 24 November 2015 Accepted 24 November 2015 Available online xxxx Keywords: BPPO N-methylmorpholine Ion exchange capacity Anion exchange membrane Electrodialysis

a b s t r a c t In this article, the fabrication of anion exchange membranes from brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) and N-methylmorpholine (NMM) has been reported. The prepared membranes were characterized in terms of ion exchange capacity (IEC), water uptake (WR), linear expansion ratio (LER), thermal stability, tensile strength (TS), alkaline stability, transport number and membrane area resistance. The ion exchange capacity, water uptake and linear expansion ratio of the prepared membranes are found to be increased with the increasing amount of NMM content in the membrane matrix. The membranes NMM-15 and NMM-18 are much more stable than QPPPO membrane in 2 M NaOH at room temperature. Fourier transform infrared spectroscopy was employed to confirm the functional groups in the membrane. The surface morphology of fabricated membranes was studied by scanning electron microscopy. The membranes were used in electrodialytic removal of NaCl from aqueous solution at constant applied voltage. The membrane NMM-18 with higher hydrophilicity (IEC = 1.74 mmol/g & WR = 27.40%) and lower membrane area resistance (1.5 Ω·cm2) showed good ED performance than commercial membrane Neosepta AMX under the same experimental conditions. © 2015 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. E-mail address: [email protected] (T. Xu).

From the perspective of waste water purification, membrane based separation technique has drawn a major attention as it can significantly produce clean water with various advantageous like low cost,

http://dx.doi.org/10.1016/j.desal.2015.11.024 0011-9164/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: M.I. Khan, et al., Development of BPPO-based anion exchange membranes for electrodialysis desalination applications, Desalination (2015), http://dx.doi.org/10.1016/j.desal.2015.11.024

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M.I. Khan et al. / Desalination xxx (2015) xxx–xxx

environment friendly nature as well as easy operation. Among several other available separation techniques present till date electrodialysis appeared to be the prime choice for the modern researchers due to its exclusive properties. Electrodialysis is nothing but an electromembrane driven process where an electrical potential gradient is used to apply as an applied force for selective separation and recovery of ions from the waste solution. Typically, in this process ions are migrated from the dilute compartment to the concentrated compartment through ion exchange membrane at a certain applied electric gradient across the electrodes [1–3]. This process has been widely used for the production of drinking as well as process water from brackish and seawater, the treatment of industrial effluents, the recovery of useful materials from effluents and salt production [4–7]. Electrodialysis showed the possibility of separating different charged organic and inorganic components [1,8]. Ion exchange membranes have expanded its horizon more toward the academic and industrial research due to its higher selectivity toward specific ions and applications in electro-driven membrane based separation process [1–3,9–11]. Traditionally, the ion exchange membranes contain either fixed positive charged groups (i.e. anion-exchange membrane; AEM) or fixed negative charged groups (i.e. cation-exchange membrane; CEM). CEMs and AEMs are selective for cations and anions [1,2]. To be more specific, anion exchange membranes are most widely used in electrodialysis, electro-membrane reactor, diffusion dialysis as well as chlor-alkali process as the anion exchange membranes allows the transportation of anions via an electrostatic interaction and oppose the transportation of cations [1–3]. However, the permselectivity of the membranes is an important endowment to evaluate the efficiency of anion exchange membrane in electro-membrane based separation process [12,13]. Thus, highly conductive, selective, chemical, thermal and oxidative stable anion exchange membranes are urgently required for the practical applications in the aforementioned membrane based separation processes. Many different kinds of polymers have been used in AEM fabrication, for instance, polystyrene [14–16], polysulfone [17], polyether imide [18], poly(arylene ether) [19], poly(phthalazinone ether ketone) (PPEK) [20], and poly(phthalazinone ether sulfone ketone) (PPESK) [21]. These polymers have been subjected to chloromethylation and thereafter converted for distinct membrane utilization such as in fuel cell [22], biomedical devices [23,24], evaporation [25], and solid state polypeptide synthesis [26]. Chloromethyl ether (CME) was employed commonly for chloromethylation process. However, CME is pretty much harmful for human health and is a banned chemical [13,27,28]. To eradicate the above mentioned difficulties, much attention has been paid to the fabrication of anion exchange membrane without using the harmful chloromethylation reagents. Bromination of benzylmethyl and chloroacetylation of phenyl are known to be promising method for the avoidance of CME [29–31]. Polymers such as Benzylmethyl-containing polysulfone [32,33], poly(phenylene) [34], and poly(arylene ether ketone) [35] have been used to attain AEMs via bromination and subsequent quaternization. This approach has, however, not been abused in the fabrication of intermolecular AEMs which have good durability and useful for distinct applications [36,37]. Our previous work reported the preparation of poly(vinylidene fluoride) (PVDF) based anion exchange membrane for water desalination [38]. The hydrophobic nature of PVDF was the serious problem. To overcome this problem, it is desirable to prepare anion exchange membranes from different polymers. Herein, a novel green synthetic route has been proposed for the fabrication of BPPO-based anion exchange membrane because hydrophobic surface of BPPO membranes can be easily modified by grafting hydrophilic materials to get hydrophilic surface. Compared to other polymers such as poly(ether sulfone) (PES), polysulfone (PS) and poly(vinylidine) fluoride (PVDF), BPPO contains abundant –CH2Br functional group which can react easily with amine without penetrating base membrane and use of cross-linkers [39]. BPPO is well known to be a stable polymer and very much susceptible

for the nucleophilic attack. In view of this observation that BPPO is a nucleophilic attack prone polymer, we used N-methylmorpholine as a nucleophile which can very efficiently introduce functionality in the BPPO main chain. The prepared membranes were characterized in term of FTIR, thermal stability, SEM, ion exchange capacity, water uptake, linear expansion ratio, mechanical stability, alkaline stability, membrane area resistance and transport number. Moreover, their applications in electrodialytic removal of NaCl from aqueous solution have been investigated and compared with commercial membrane Neosepta AMX. 2. Experimental 2.1. Material Brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) with aryl substitution degree of 0.29 and benzyl substitution degree of 0.71 was supplied by Tianwei Membrane Co. Ltd., Shandong of China. Commercial anion exchange membrane Neosepta AMX and cation exchange membrane Neosepta CMX were purchased from ASTOM, Japan. Nmethylmorpholine (NMM) was obtained from Sinopharm Chemical reagent Co. Ltd., China. All other reagents used during the expertiments were of analytical grade and commercially available from domestic chemical reagent companies. These reagents were used without further purification. Deionized (DI) water was used throughout the experiments. 2.2. Membrane fabrication The anion exchange membranes NMM-4 to NMM-18 were fabricated by solution-casting method. The casting solution consist of 15% of brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) into NMethyl-2-pyrrolidone (NMP) solvent. Anion exchange membranes were prepared by adding different amounts of NMM into the casting solution to get different properties. The obtained membranes were denoted as NMM-4, NMM-8, NMM-12, NMM-15 and NMM-18 respectively according to the different amount of amine (NMM). The casting solution was stirred vigorously for 15 h to accelerate the reaction among the BPPO and NMM at 40 °C. Finally, the solution was casted onto a glass plate and heated at 60 °C (solvent evaporation) for 24 h. The anion exchange membrane were peeled off from glass plates and washed with deionized water before characterization and study. Chemical structure for the prepared BPPO based membrane is also represented in Scheme 1.

Scheme 1. The preparation of BPPO-based anion exchanged membranes.

Please cite this article as: M.I. Khan, et al., Development of BPPO-based anion exchange membranes for electrodialysis desalination applications, Desalination (2015), http://dx.doi.org/10.1016/j.desal.2015.11.024

M.I. Khan et al. / Desalination xxx (2015) xxx–xxx

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membranes, WR values were calculated [41] as the relative weight gain per gram of the dry sample using following Eq. (1),

2.3. Characterizations 2.3.1. FTIR spectra & thermal stability FTIR spectra of dried membranes were recorded by using the technique attenuated total reflectance (ATR) with FTIR spectrometer (Vector 22, Bruker) having resolution of 2 cm− 1 and a total spectral range of 4000–400 cm− 1. TGA for the prepared membranes was carried out using a Shimadzu TGA-50H analyzer within the temperature range 25 °C to 700 °C under nitrogen flow, with a heating rate of 10 °C/min. 2.3.2. Ion exchange capacity (IEC) In theory, IEC represents the number of exchangeable ionic groups (equivalents) present per dry membrane weight. IEC for the prepared membranes were measured by the classical Mohr method [40] by the following way: firstly, the membrane samples were equilibrated in 1.0 M NaCl solution for 2 days such that all charge sites were converted into the Cl− form. Then, the membranes were washed very carefully with deionized water in order to remove excess amount of NaCl. The washed membranes were then equilibrated with 0.5 M Na2SO4 solutions for 2 days. The amount of Cl− ions liberated was estimated by titration with 0.05 M AgNO3 using K2CrO4 as an indicator. The ionexchange capacity of the membrane was calculated by the equation IEC = VC/m where m, V and C represents the dry weight of the membrane, titre value during titration and the concentration of AgNO3 solution respectively. 2.3.3. Microscopic characterizations for AEMs Membrane morphological characterization was successfully done through a field emission scanning electron microscope (FE-SEM, Sirion200, FEI Company, USA). Surface and cross-sectional views of membranes were taken from dry membranes. The SEM images of different dense BPPO-based membranes were shown as representative cases. 2.3.4. Water uptake, linear expansion ratio and fixed group concentration Membrane hydrophilic nature was investigated by water uptake (WR) measurement. Membrane samples were oven dried and accurately weighed to confirm their dry weight. Then membranes were immersed in water for 72 h at 25 °C and wet weight of those membranes were recorded after removal of surface water with tissue paper. From the difference in mass before and after the complete drying of the

WR ¼

WWET −WDRY  100% WDRY

ð1Þ

where WWET and WDRY are the weights of wet and dry membranes, respectively. Linear expansion ratio (LER) was evaluated by linear expansion ratio (LER) in 25 °C water. All of the membranes were cut into a (2 ∗ 2) cm2 pieces for the experiment. LER is based on the following equation [41]: LER ¼

ðLWET −LDRY Þ  100% LDRY

ð2Þ

where LWET and LDRY are the lengths of wet and dry membranes, respectively. 2.3.5. Mechanical property Tensile strength of dry and hydrates membranes were measured using a Q800 dynamic mechanical analyzer (DMA, TA Instruments) at a stretch rate of 0.5 N/min. 2.3.6. Alkaline stability of membranes The alkaline stability of the membranes was monitored by measuring the changes in IEC values of the membranes before and after being kept in 2 M NaOH at room temperature. 2.3.7. Membrane area resistance Membrane area resistance was measured by commercial cellassembly (MEIEMP-I, Hefei Chemjoy Polymer Material Co., Ltd.) under a constant current mode [42]. It is composed of five compartments: two intermediate compartments equipped with two reference electrodes; one quadrate clip for membrane. Particularly, two pieces of Nafion-117 membrane were assembled between electrode and intermediate chambers, separately to eliminate the influence of electrode reaction. Two intermediate chambers are separated by the membrane clip and tips of the reference electrodes should be closely to the center of membrane. During the measurement, Na2SO4 solution (0.3 mol/L) was fed to the electrode chambers and NaCl (0.5 mol/L) is fed to the intermediate chambers. A constant current is supplied by a direct current power supply (SHEKONIC, Yangzhou Shuanghong Co., Ltd.) and the potential between electrode is read by a digital multimeter (model: GDM

Fig. 1. The schematic setup of ED stack.

Please cite this article as: M.I. Khan, et al., Development of BPPO-based anion exchange membranes for electrodialysis desalination applications, Desalination (2015), http://dx.doi.org/10.1016/j.desal.2015.11.024

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8145), Good will instrument Co. Ltd., Taiwan). Membrane area resistance can be calculated according to the values of current, potential and membrane area (7.07 cm2). 2.3.8. Membrane transport number Membrane transport number was determined by measuring membrane potential. A cell made of Perspex sheet was used for membrane potential measurement and it had two compartments separated by the membrane, circular in shape and having an area of 7.0 cm2. The membrane potential was measured by keeping the ratio of salt concentrations of the higher (0.05 M NaCl) to lower (0.01 M NaCl) side constant at 5.0 The potential difference (Em) across the cell was measured using a multimeter (VC890C+, Shenzhen Victor Hi-Tech. Ltd., China) connected to Ag/AgCl reference electrodes which were responsible up to 0.10 mV. The transport number “t′i” was then calculated using the following modified Nernst equation [43],

Em ¼

RT nF

 
a1 2t0i −1 ln 2 a

ð3Þ

where R is universal gas constant (8.314 J/K·mol), F is the Faraday constant (96,487 C/mol), T is the absolute temperature (K), a1 and a2 are the mean activities of electrolyte solutions and n is the electrovalence of counter-ion (ni = 1 in this case). 2.3.9. Electrodialysis stack The electrodialysis stack used in this work is shown in Fig. 1. It consists of an anode and a cathode of stainless steel sheets coated with platinum and six electrode cell separated by two CEMs and three AEMs as shown in Fig. 1. The effective area of membrane is 7.0 cm2. Anion exchange membranes NMM-15 and NMM-18 and commercial membrane AMX were used in the experiments for comparison. A feed solution of 0.1 M NaCl was pumped in the dilute chamber with a constant flow rate of 60 mL/min. Meanwhile, two electrode cells were fed by 0.3 M Na2SO4 solutions and connected together to prevent pH change. Before the experiment, each cell was circulated for 30 min to eliminate the visible bubbles. Then, it was operated under constant current of 28 mA/cm2. The change in conductivity of NaCl solution in dilute cell and potential over the stack were recorded after every 5 min.

Fig. 3. TGA thermograms for different anion exchange membranes.

3. Results and discussion 3.1. FTIR analysis FTIR analysis was employed to confirm the successful synthesis of Nmethylmorpholine aminated BPPO polymeric membrane. Fig. 2 represents the FTIR spectra of pristine and aminated BPPO membrane. After the reaction with NMM, the characteristic band at 1115 cm−1 appeared in the aminated BPPO membrane which is due to the C–N stretching vibration which is absent in pristine BPPO membrane representing the successful reaction between BPPO and NMM. The band at 1608 cm−1 is attributed to C = C stretching vibration in phenyl groups; the peak at 1190 cm−1 is the characteristic of C–O–C stretching [44]. The bands at 1446 cm−1 are due to stretching of –CH groups (V and δ) [45]. The adsorption peaks of symmetrical and asymmetrical stretching vibration of C–O are at 1200 cm− 1 and 1306 cm− 1 and those of phenyl group at 1470 cm−1 and 1600 cm−1 respectively. 3.2. Thermal stability Thermal stability of the prepared AEMs was investigated by TGA as shown in Fig. 3. The weight loss property of membranes occurred via

Fig. 2. IR spectrum of different anion exchange membranes.

Fig. 4. Ion exchange capacity and water uptake of anion exchange membranes.

Please cite this article as: M.I. Khan, et al., Development of BPPO-based anion exchange membranes for electrodialysis desalination applications, Desalination (2015), http://dx.doi.org/10.1016/j.desal.2015.11.024

M.I. Khan et al. / Desalination xxx (2015) xxx–xxx Table 1 Composition, linear expansion ratio and thickness of membranes NMM-4 to NMM-18. Membranes

NMM-4

NMM-8

NMM-12

NMM-15

NMM-18

BPPO (g) NMM (g) LER (%) Thickness (μm)

1.5 0.07 1.47 89

1.5 0.12 3.84 87

1.5 0.17 6.15 139

1.5 0.22 8.86 133

1.5 0.27 15.0 156

three main stages corresponding to the thermal desorption of water, thermal deamination and thermal oxidation of membrane polymer. During the first stage all the membranes represent a weight loss around 100–150 °C which is attributed to the evaporation of water from the membrane matrix. The second weight loss observed at around 250 °C is associated to the degradation of quaternary ammonium group [46]. The last and final weight loss occurred around 430–450 °C is attributed to the degradation of main polymer chain.

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3.3. Ion exchange capacity, water uptake and linear expansion ratio Ion exchange capacity is a crucial parameter of membranes employed in electrodialysis process. It depends on the density of functional group in the membrane matrix and plays a significant role in selectivity [47,48]. Ion exchange capacities of prepared AEMs were calculated by titration method and are given in Fig. 4. Obtained result represented that the IEC value of prepared membranes is found to be increased with increase in the quantity of NMM in the membrane matrix. Water uptake is a significant parameter of ion exchange membrane and has accessed an important influence on the separation phenomena, dimensional as well as mechanical properties [49–51]. Dissociation of the charged functional groups could be promoted by the presence of water molecules inside the membrane matrix and are very crucial for the transportation of ions [51]. Water uptake (WR) of prepared membranes is found to be increased from 7.28% to 27.40% with the increase in the amount of NMM in the membrane matrix as shown in Fig. 4. It

Fig. 5. Surface (left side) and cross-section (right side) of membranes NMM-4 toNMM-18.

Please cite this article as: M.I. Khan, et al., Development of BPPO-based anion exchange membranes for electrodialysis desalination applications, Desalination (2015), http://dx.doi.org/10.1016/j.desal.2015.11.024

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is attributed to the increased hydrophilic region in the membrane matrix. Water uptake is controlled by the concentration of NMM. Both IEC and WR are significant parameters of anion exchange membrane to describe its hydrophilicity. Linear expension ratio (LER) of anion exchange membranes was investigated in water and obtained data is given in Table 1. The LER values of membranes are found to be increased from 1.47–15% with increase in the amount of ion exchange content in the membrane matrix. It indicates that prepared AEMs have excellent swelling resistance which is responsible for long time running of electodialysis. 3.4. Membrane morphologies Morphologies of prepared membranes were examined through the scanning electron microscopy (SEM) and represented in Fig. 5. It is quite evident from the membrane surface images that with the increase in the amine content better miscibility can be observed. No pores, holes or cracks can be observed in membrane surface as well as in crosssection which suggested homogeneous nature of prepared membranes. For electrodialytic purpose to be operative it is essential to have dense structure of membrane which can be observed here. We can conclude from the above observations that increasing amine content is beneficial for the membrane homogeneity though with highest amine loading little bit of aggregation can still be observed on the surface of membrane NMM-18. Overall, all the prepared membranes possess dense structure. 3.5. Mechanical properties Mechanical properties of the prepared membranes including tensile strength (TS) and elongation at break (Eb) are significant endowments in relation to their practical applications in electodialysis process. Tensile strength (TS) showed the resistance of membranes to mechanical force whereas elongation at break (Eb) indicated the flexibility of membranes. The membrane that shows higher tensile strength always possesses smaller elongation at break values. Tensile strength (TS) and elangation at break (Eb) values of prepared anion exchange membranes are given in Table 2. It can be seen that the prepared membrane NMM18 possesses lowest tensile strength and largest elongation at break representing the higher flexibility of membrane. But the membrane NMM-4 represented highest tensile strength and lowest elongation at break values. The highest Eb value is 12.0% and highest TS value is 28.0 MPa. As expected, the TS values follow a decreasing trend and Eb values follow an increasing trend from membrane NMM-4 to NMM18. Tensile strength (TS) values are found to be in range of 22.20 MPa to 28 MPa which are higher than those of previously studied ion exchange membranes [52] revealing that the prepared membranes possess excellent tensile strength. The elongation at break (Eb) values were in range of 10.12% to12.0% which seems to be lower than PVA/ SiO2 anion exchange hybride membane [53]. As shown in Table 2, the tensile strength of anion exchange membranes gradually decreased from membrane NMM-4 to NMM-18 with increase in the ion exchange capacity (resulting from the incorporation of ion exchang groups into the initially tight polymer chain network) [54]. 3.6. Alkaline stability The alkaline stability is a crucial property of anion exchange membranes. To investigate the alkaline stability of the QPPO, NMM-15 and NMM-18 membranes, the membranes were immersed in 2 M NaOH

Fig. 6. The change in IEC values of QPPO, NMM-15 and NMM-18 membrane after immersion in 2 M NaOH at room temperature.

solution at room temperature for specific time. The changes in IECs of selected membranes were used to estimate their alkaline stability. The results are represented in Fig. 6. It can be found that the IEC values of the NMM-15 and NMM-18 membranes remain at ~ 84% and ~ 85% respectively of their initial values after 240 h while the IEC value of QPPO membrane decreases rapidly to ~ 55% of its initial value under same experimental conditions. Indeed, QPPO membrane lost its mechanical strength, flexibility and appearance within only 50 h. From Fig. 6, it can be noted that the IEC values of studied membranes initially decreased rapidly and then stabilized over longer immersion time. This is due to the degradation of quaternary ammonium group in the membrane matrix at the start of immersion time. It shows that the NMM-15 and NMM-18 are much more stable than QPPO membrane under same experimental conditions. 3.7. Membrane area resistance and transport number Area resistance of an ion exchange membrane is a significant parameter to know the energy using up in the electrodialysis process. From this reason, membrane with lower area resistance is preferred for ED applications. Area resistance of fabricated anion exchange membranes was calculated in 0.5 M NaCl and is given in Table 3. Out of five prepared membranes, only one membrane (NMM-18) has lower area resistance than that of commercial membrane Neosepta AMX as shown in the Table 3. The area resistance is found to be decreased with increase in the amount of NMM in the membrane matrix. Hence, the area resistance of ion exchange membrane depends on their hydrophilicity. In electrodialysis phenomenon, the membrane transport number is interpreted as the current by the counter-ions. A higher transport number of membrane indicates higher per-selectivity to counter-ions (anions here). The obtained transport numbers of the prepared anion exchange membrane are shown in Table 3. These values are found to be increased with increasing the NMM content from membrane NMM-4 to NMM-18 because the diffusion of co-ions across the membrane was suppressed with increasing quaternary ammonium group Table 3 Transport number and area resistance of anion exchange membranes.

Table 2 Tensile strength (TS) and elongation at break of membranes. Membranes

NMM-4

NMM-8

NMM-12

NMM-15

NMM-18

TS (MPa) Eb (%)

28.0 10.12

28.0 11.18

26.54 11.69

25.27 11.80

22.20 12.0

Membranes

AMX

NMM-4

Area resistance (Ω·cm2) Transport number

2.35

810

0.98

0.91

NMM-8

NMM-12

NMM-15

NMM-18

131.6

7.17

4.2

1.5

0.94

0.94

0.96

0.93

Please cite this article as: M.I. Khan, et al., Development of BPPO-based anion exchange membranes for electrodialysis desalination applications, Desalination (2015), http://dx.doi.org/10.1016/j.desal.2015.11.024

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(7.28–27.40%), linear expansion ratios (1.47–15%), tensile strength (22.20–28.0 MPa) and transport number (0.91–0.96). These results suggested that the membranes (NMM-15 & NMM-18) could be used in ED for desalination. The prepared membranes (NMM-15 & NMM18) have good desalination performance under the given experimental conditions. The membrane NMM-18 showed higher desalination performance than commercial membrane Neosepta AMX. Overall, the fabricated membranes are useful for water desalination and other separation process.

Fig. 7. The change in conductivity of NaCl with time.

[55–58]. The membrane NMM-18 represents highest transport number (0.96) among all the fabricated membranes. It could be associated to the maximum suppression of co-ions diffusion by largest amount of quaternary ammonium groups in the membrane NMM-18. Therefore, these conclusions proposed that the prepared membranes are capable toward the chloride ions. 3.8. ED performance After the detailed characterization of all the fabricated membranes, the two membranes (NMM-15 & NMM-18) were employed in electrodialytic removal of NaCl from its aqueous solution to evaluate their desalination performance. The ED experiment was performed by pumping the feed solution (0.1 M NaCl) in the dilute cell with a constant flow rate of 60 mL/min using direct current of 28 mA/cm2. The variation in conductivity of NaCl in dilute cell for both selected membranes (NMM-15 & NMM-18) is represented in Fig. 7. The conductivity of NaCl in dilute cell is found to be decreased with the increase in desalination time for both the membranes as shown in Fig. 7. According to the conductivity model employed by Daniel Aguado et al. [59], the conductivity of the dilute solution cell can be explained by the following reasons: (i) the anion and cation permeated the anion exchange membrane and cation exchange membrane under the applied electric field respectively which resulted to the ion concentration decreasing significantly and (ii) the rate of ionic diffusion increased followed by an increase of molar conductivity because of reduction in concentration of ions in the dilute solution. It shows that a decrease in concentration of ions in the dilute solution is much higher than the molar conductivity increases resulting in a net decrease of conductivity in the dilute solution. This is attributed to the increased ion exchange content in the membrane matrix. Membrane NMM-18 showed higher desalination performance than the prepared membrane NMM-15 and commercial membrane Neosepta AMX because Cl− ions migrated rapidly from dilute cell to concentrated cell due to higher IEC and transport number. Therefore, it can be potentially applied for salt removal from water. 4. Conclusions Anion exchange membranes have been synthesized from BPPO and N-methylmorpholine in NMP solvent. The characterization of membranes by FTIR confirms the successful reaction between BPPO and NMP. The dense morphology of fabricated membranes was confirmed by SEM. The developed anion exchange membranes have excellent thermal stability and good alkaline stability. The membranes have higher ion exchange capacity (0.39–1.74 mmol/g), water uptake

Nomenclature Code full name AEM anion exchange membrane AMX commercial anion exchange membrane BPPO brominated poly(2,6-dimethyl-1,4-phenylene oxide) CMX commercial cation exchange membrane Eb elongation at break ED electrodialysis IEC ion exchange capacity LER linear expension ration NMM N-methylmorpholine NMP N-methyl-2-pyrrolidone R gas constant SEM scanning electron microscopy TS tensile strength WR water uptake Acknowledgments The authors are highly thankful to the National Science Foundation of China (No. 21490581), National High Technology Research and Development Program 863 (No. 2015AA021001) and CAS-TWAS President's fellowship for financial support. References [1] T.W. Xu, Ion exchange membranes: state of their development and perspective, J. Membr. Sci. 263 (2005) 1–29. [2] M. Kumar, M.A. Khan, Z.A. Al-Othman, T.S.Y. Choong, Recent developments in ionexchange membranes and their applications in electrochemical processes for in situ ion substitutions, separation and water splitting, Sep. Purif. Rev. 42 (2012) 187–261. [3] C. Klaysom, R. Marschall, S.-H. Moon, B.P. Ladewig, G.Q.M. Lu, L. Wang, Preparation of porous composite ion-exchange membranes for desalination application, J. Mater. Chem. 21 (2011) 7401–7409. [4] M. Sadrzadeh, T. Mohammadi, Treatment of sea water using electrodialysis: current efficiency evaluation, Desalination 249 (2009) 279–285. [5] Y. Zhang, S. Paepen, L. Pinoy, B. Meesschaert, B. Van der Bruggen, Selectrodialysis: fractionation of divalent ions from monovalent ions in a novel electrodialysis stack, Sep. Purif. Technol. 88 (2012) 191–201. [6] A. Abou-Shady, C. Peng, J. Almeria O, H. Xu, Effect of pH on separation of Pb (II) and NO− 3 from aqueous solutions using electrodialysis, Desalination 285 (2012) 46–53. [7] P.K. Sow, A. Shukla, Electro-electrodialysis for concentration of hydroiodic acid, Int. J. Hydrog. Energy 37 (2012) 3931–3937. [8] Y. Zhang, B. Van der Bruggen, L. Pinoy, B. Meesschaert, Separation of nutrient ions and organic compounds from salts in RO concentrates by standard and monovalent selective ion-exchange membranes used in electrodialysis, J. Membr. Sci. 332 (2009) 104–112. [9] A.R. Khodabakhshi, S.S. Madaeni, T.W. Xu, L. Wu, C. Wu, C. Li, W. Na, S.A. Zolanvari, A. Babayi, J. Ghasemi, S.M. Hosseini, A. Khaledi, Preparation, optimization and characterization of novel ion exchange membranes by blending of chemically modified PVDF and SPPO, Sep. Purif. Technol. 90 (2012) 10–21. [10] L.J. Banasiak, B. Van der Bruggen, A.I. Schäfer, Sorption of pesticide endosulfan by electrodialysis membranes, Chem. Eng. J. 166 (2011) 233–239. [11] F. Karimi, S.N. Ashrafizadeh, F. Mohammadi, Process parameter impacts on adiponitrile current efficiency and cell voltage of an electromembrane reactor using emulsion-type catholyte, Chem. Eng. J. 183 (2012) 402–407. [12] T. Sata, Studies on anion exchange membranes having permselectivity for specific anions in electrodialysis — effect of hydrophilicity of anion exchange membranes on permselectivity of anions, J. Membr. Sci. 167 (2000) 1–31. [13] M. Kumar, S. Singh, V.K. Shahi, Cross-linked poly(vinyl alcohol)–poly(acrylonitrileco-2-dimethylamino ethylmethacrylate) based anion-exchange membranes in aqueous media, J. Phys. Chem. B 114 (2010) 198–206. [14] A.S. Góźdź, W. Trochimczuk, Continuous modification of polyethylene with styrene and divinylbenzene in melt, J. Appl. Polym. Sci. 25 (1980) 947–950.

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Please cite this article as: M.I. Khan, et al., Development of BPPO-based anion exchange membranes for electrodialysis desalination applications, Desalination (2015), http://dx.doi.org/10.1016/j.desal.2015.11.024