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Preparation of anion exchange membranes from BPPO and dimethylethanolamine for electrodialysis
Desalination 402 (2017) 10–18
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Preparation of anion exchange membranes from BPPO and dimethylethanolamine for electrodialysis Muhammad Imran Khan a, Chunlei Zheng b, Abhishek N. Mondal a, Md. Masem Hossain a, Bin Wu a, Kamana Emmanuel a, Liang Wu a, Tongwen Xu a,⁎ a 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 b Hefei ChemJoy Polymer Materials, Co., LTD, Hefei, Anhui 230601, PR China
H I G H L I G H T S • Anion exchange membranes with dimethyethanolammonium were prepared. • Developed membranes have high desalination performance and suitable for ED.
a r t i c l e
i n f o
Article history: Received 23 February 2016 Received in revised form 19 September 2016 Accepted 22 September 2016 Available online xxxx Keywords: Anion exchange membrane BPPO Dimethyethanolamine Area resistance Electrodialysis
a b s t r a c t In this research work, the synthesis of anion exchanges membranes (AEMs) from brominated poly (2, 6-dimethyl-1, 4-phenylene oxide) (BPPO) and dimethyethanolamine (DMEA) has been investigated for electrodialysis (ED) application. Fourier transform infrared spectroscopy was used to confirm the functional groups in the membranes. The morphology of the prepared membranes was investigated by scanning electron microscopy. Physiochemical and electrochemical properties of the prepared membranes were studied in detail. The membranes possess ion exchange capacity of 0.66 mmol/g to 1.38 mmol/g, water uptake of 11.60% to 48.50%, volume expansion ratio of 8.58% to 20.21%, tensile strength of 32.52 MPa to 49.22 MPa and transport number of 0.94 to 0.98. These membranes have higher chemical stability in alkaline medium than QPPO membrane (quaternized with trimethylamine) at room temperature. The membranes DMEA-10 and DMEA-15 were selected for NaCl desalination by ED with a lab-scale electrodialysis cell at constant applied voltage. The prepared membranes DMEA-10 and DMEA-15 showed better performance than the commercial anion exchange membranes Neosepta AMX under the identical conditions. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The electrodialysis (ED) process is based on the transport of ions through selective membranes under the influence of an electrical field. Ion transfer is caused by diffusion and electro migration, while solution migration is caused by the electro-osmosis and concentration-osmosis [1,2]. The ED process is selective for the removal of ionic species from non-ionic species in an aqueous medium and has proven to be a robust, efficient and versatile method for such applications. The advantages of ED over other separation processes includes low energy cost, versatility in term of wide variety of feed streams that can be utilized with minimum requirement of pre-treatment, easier and low cost of maintenance and higher membrane life. Recent development has resulted in the significant usefulness of the ED technique for producing safe drinking ⁎ Corresponding author. E-mail address: [email protected] (T. Xu).
http://dx.doi.org/10.1016/j.desal.2016.09.019 0011-9164/© 2016 Elsevier B.V. All rights reserved.
water and for the separation of hazardous chemicals from water [3–6]. The ED process is more convenient than Reverse Osmosis (RO) because higher brine concentration can be obtained in the former process as there are no osmotic pressure limitations. Thus the main applications of ED process are the purification of salty water, the separation and concentration of high value chemicals and the removal of effluents. Recently ion exchange membranes (IEMs) have attracted great attention from both academic and industrial fields due to their variable applications from desalination of salt-rich water to food production process including waste water treatment to recover some valuable elements in chemical stability industry [7–11]. Traditionally, the IEMs 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 [12,13]. Different process usually requires specific membrane properties. For desalination specially electron driven process such as electrodialysis, which is the scope of interest in the present work, the IEMs were
M.I. Khan et al. / Desalination 402 (2017) 10–18
expected to have high permselectivity, excellent conductivity, good chemical, thermal and mechanical stabilities. To date, many polymers have been used in the preparation of AEMs, for example, 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 applications such as in fuel cell [22], biomedical devices [23,24], pervaporation [25], solid state polypeptide synthesis [26], etc. Chloromethyl ether (CME) was used commonly for chloromethylation process. However, CME is pretty much harmful for human health and is a banned chemical [27–29]. To eliminate the above mentioned problems, much attention has been paid to the preparation of AEMs without using the dangerous chloromethylation reagents. Bromination of benzylmethyl and chloroacetylationl of phenyl are known to be promising method for the avoidance of CME [30–32]. Polymers such as benzylmethyl-containing polysulfone [33,34], poly(phenylene) [35], and poly(arylene ether ketone) [36] have been employed to obtained AEMs via bromination and subsequent quaternization. This approach has, however, not been abused in the preparation of intermolecular AEMs which have excellent durability and excellent for distinct applications [37,38]. Alternatively, it is quite well-known that use of non-toxic inorganic constituents also improves functional properties of the membrane significantly [39]. Recently, Dzyazko et al. showed nanoparticle embedded heterogeneous membrane, which can be a potential solution for upgrading functional properties of the membrane [40]. From our research group, Pan et al. showed silicon dioxide incorporated QPPO membrane which is useful for separation purpose [41]. Our previous work reported the synthesis of N-methylmorpholine (NMM) based AEMs for water desalination by ED [42]. The major problem associated with these membranes are the relatively low alkalinity of NMM (pkb = 6.62) which results in low anion permeability. To overcome these problems, dimethyethanolamine (DMEA) (pkb = 4.77) based AEMs for ED process have been synthesized. These membranes show higher alkaline stability due to non-aromatic character of DMEA which is beneficial for electrochemical applications. Moreover, these membranes possess higher alkaline stability. In our previous work, AEMs were also prepared with BPPO by subsequent quaternary amination with a dimethyethanolamine (DMEA) aqueous solution. It was found that the membrane's intrinsic properties are dependent on DMEA concentration and amination temperature and the conditions were optimized. However, at that case, BPPO based membrane with polyester as a substrate was used and therefore the amination reaction between BPPO and DMEA is heterogeneous (solid-liquid) so the membrane area resistance (30Ω.cm2) is too high to be used in conventional electrodialysis even the membrane had proper IEC of 1.5 mmol/g dry membrane and water content of 30%, [43]. In the present research, cast solutions were firstly prepared from the homogenous reaction between BPPO and DMEA under the optimized conditions and then a series of AEMs were prepared by varying the amount of DMEA in the reaction medium. The prepared membranes were characterized in term of FTIR, thermal stability, SEM, ion exchange capacity (IEC), water uptake (WR), volume expansion ration (VER), mechanical/chemical stability, membrane area resistance and transport number. Moreover, their applications in electrodialytic removal of NaCl from aqueous solution have also been investigated and compared with commercial membrane Neosepta AMX.
Commercial anion exchange membrane Neosepta AMX and cation exchange membrane Neosepta CMX were purchased from ASTOM, Japan. Dimethyethanolamine (DMEA) 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 reagents companies. These reagents were used without further purification. Deionized (DI) water was used throughout the experiments.
2.2. Membrane preparation These membranes were prepared by solution casting method as reported in our previous work [42,44]. In this method, firstly the casting solution of 8% of brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) was prepared into N-methyl-2-pyrrolidone (NMP) solvent. AEMs with different properties were prepared by adding 0.04 g, 0.06 g, 0.08 g and 0.12 g of DMEA into the casting solution. The casting solution was stirred at 40 °C for 12 h to accelerate the reaction among the BPPO and DMEA. Finally, the solution was casted onto a glass plate and heated at 60 °C (solvent evaporation) for 24 h. The membrane were peeled off from glass paltes and washed with deionized water before characterization and study. The obtained membranes were respresented as DMEA-5, DMEA-8, DMEA-10 and DMEA-15 respectively according to the different amount of amine. Chemical structure for the prepared BPPO based membrane is also represented in Scheme 1.
2.3. Preparation of quaternized poly (2, 6-dimethyl-1, 4-phenylene oxide) (QPPO) membrane The preparation of quaternized poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO) membrane was carried out according to our previous reported method [45]. In a typical method, 0.8 g of brominated poly (2, 6dimethyl-1, 4-phenylene oxide) (BPPO) was dissolved into N-methyl2-pyrrolidone (NMP) solvent to get homogeneous solution. After that measured amount of trimethylamine (TMA) was added into the already prepared BPPO solution. The resultant mixture was stirred at 60 °C for 12 h and then caste on hot plate at 60 °C for 24 h. The attained membranes was washed and stored in DI water.
2. Experimental 2.1. Material Brominated poly (2, 6-dimethyl-1, 4-phenyleneoxide) (BPPO) with aryl substitution degree of 0.42 and benzyl substitution degree of 0.58 was provided by Tianwei Membrane Co. Ltd., Shandong of China.
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Scheme 1. The preparation of BPPO-based anion exchanged membranes.
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2.4. Characterizations 2.4.1. FTIR spectra & thermal stability FTIR analysis of prepared AEMs were measured by employing FTIR spectrometer (Vector 22, Bruker) having resolution of 2 cm− 1 and a total spectral range of 4000–400 cm− 1. The thermal stability of prepared membranes was investigated by employing a Shimadzu TGA50H analyzer within the temperature range 25 °C to 700 °C under nitrogen flow, with a heating rate of 10 °C/min. 2.4.2. Ion exchange capacity (IEC) It shows the number of exchangeable ionic groups (equivalents) present per dry membrane weight. It was measured by the classical Mohr method [42,44,46]. In a typical procedure, 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 DI 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 measured by titration with 0.05 (M) AgNO3 using K2CrO4 as an indicator. The ion-exchange capacity (IEC; mmol/g) of the membrane was measured 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. 2.4.3. Water uptake and volume expansion ratio Water uptake (WR) is used to investigate the hydrophilicity of IEM. Membrane samples were dried in vacuum oven at 60 °C for 24 h and accurately weighed to confirm their exact dry weight. Then the membranes were immersed in DI water for 72 h at 25 °C and wet weight of those membranes are recorded after removal of surface water with tissue paper. From the difference in mass before and after the complete drying of the membranes, WR values were measured [42,44,46] as the relative weight gain per gram of the dry sample using following Eq. (1). WR ¼
W WET −W DRY 100% W DRY
ð1Þ
where WWET and WDRY are the weights of wet and dry membranes, respectively. The volume expansion ratio (VER) of prepared AEMs was investigated at room temperature. It was measured by using following equation. VER ¼
LWET W WET T WET −LDRY W DRY T DRY 100% LDRY T DRY W DRY
ð2Þ
where LWET, WWET and TWET is the length, width and thickness of membranes in wet state respectively whereas LDRY, WDRY and TDRY is the length, width and thickness of membranes in dry state respectively. 2.4.4. Chemical stability The chemical stability of the prepared membranes was investigated by calculating the changes in IEC values before and after being kept in 2 M NaOH at ambient temperature. 2.4.5. Mechanical property Tensile strength of dry and hydrates membranes were calculated employing Q800 dynamic mechanical analyzer (DMA, TA Instruments) at a stretch rate of 0.5 N/min. The tested samples were equilibrated in deionized water (DI) for 24 h and then cut into a rectangular shape with dimensions of 4 cm × 1 cm. At least four specimens from each sample were tested. 2.4.6. Microscopic characterizations for AEMs The morphology of prepared membranes was investigated through a field emission scanning electron microscope (FE-SEM, Sirion200, FEI
Company, USA). Surface and cross-sectional images of membranes were taken from dry membranes. The SEM images of prepared membranes were represented as representative cases. 2.4.7. Membrane area resistance It was calculated by commercial cell-assembly (MEIEMP-I, Hefei Chemjoy Polymer Material Co., Ltd.) under a constant current mode [42]. It is made up 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 remove the effect of electrode reaction. Two intermediate chambers are separated by the membrane clip and tips of the reference electrodes should be close to the center of membrane. During the experiment, Na2SO4 solution (0.3 mol/L) was fed to the electrode chambers and NaCl (0.5 mol/L) was fed to the intermediate chambers. A constant current is supplied by direct current power supply (SHEKONIC, Yangzhou Shuanghong Co., Ltd.) and the potential between electrode is read by digital multimeter (model: GDM 8145, Good will instrument Co. Ltd., Taiwan). It can be measured using the values of current, potential and membrane area (7.07 cm2). 2.4.8. Membrane transport number It was calculated by measuring the membrane potential as reported in our previous work [42]. A cell made of Perspex sheet was employed 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 investigated 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 calculated using 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 measured employing the following modified Nernst equation [42]. Em ¼
RT 0 2t i −1 nF
ln
a1 aÞ
ð3Þ
where R is universal gas constant (8.314 J/K.mol), F the Faraday constant (96,487C/mol), T the absolute temperature (K), a1 and a2 the mean activities of electrolyte solutions and n is the charge number of counter-ion (ni = 1 in this case). 2.4.9. Electrodialysis stack The ED performance of prepared AEMs was studied according to the previous reported method [42]. The ED stack employed in this research is depicted in Fig. 1. It is made up 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. Prepared anion exchange membranes (DMEA-10 & DMEA-15) and commercial membrane Neosepta AMX were employed 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 was noted after specific intervals of time. The performance of developed membranes DMEA-10 and DMEA-15 was compared with the commercial membrane Neosepta AMX in term of energy consumption calculated by the following equation [47]. Z U P¼
Idt m
ð4Þ
M.I. Khan et al. / Desalination 402 (2017) 10–18
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Fig. 1. The schematic setup of ED stack.
3. Results and discussion
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. Moreover, the signal for C\\Br stretching in bromobenzyl groups expected at 740 cm−1 to 750 cm−1 was not observed in FTIR spectra of aminated BPPO membranes [48,51].
3.1. FT-IR spectra
3.2. Thermal stability
The successful quaternization of BPPO with DMEA was confirmed by the FTIR spectra analysis. Fig. 2 showed the FTIR spectra of pristine and aminated BPPO membranes. The characteristic band at 750 cm−1 corresponding to the C\\Br stretching in the pristine BPPO membrane [48]. After the reaction with DMEA, the characteristic band at 1090 cm−1 appeared in the aminated BPPO membrane is due to the C\\N stretching vibration which is absent in pristine BPPO membrane representing the successful reaction between BPPO and DMEA. Moreover, the broad peak at 3350 cm− 1 corresponds to the stretching vibration of OH group in the prepared anion exchange membrane. 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 [49]. The bands at 1446 cm−1 are due to stretching of –CH groups (V and δ) [50]. The adsorption peaks of symmetrical and asymmetrical
TGA curves of the prepared membranes are shown in Fig. 3. The weight loss of membranes took place into 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 showed a weight loss below 100 °C which is associated to the evaporation of water from the membrane matrix. The second weight loss occurred around 250 °C is attributed to the degradation of quaternary ammonium group [52]. The final weight loss observed occurred above 400 °C is due to the degradation of main polymer matrix. Two crucial endowments, i.e. thermal degradation temperature (Td) and initial decomposition temperature (IDT) were calculated and are given in Table 1. The IDT values of prepared membranes are in the range of 201 °C to 236 °C whereas the Td values were in the range of 227 °C to 254 °C. Correspondingly, IDT and Td values of BPPO blank
Fig. 2. IR spectrum of pristine and aminated BPPO membrane.
Fig. 3. TGA Thermograms for different anion exchange membranes.
where P = energy consumption of NaCl (KW h/kg), I = current (A), U = applied potential (V), m = mass of removed salt (kg), and t = time (S).
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Table 1 Thermal decomposition temperature (Td) and the initial decomposition temperature (IDT) of the prepared membranes.
Table 2 Composition, theoretical IEC, volume expansion ratio and thickness of membranes DMEA5 to DMEA-15 and commercial membrane Neosepta AMX.
Membranes
BPPO
DMEA-5
DMEA-8
DMEA-10
DMEA-15
Membranes
BPPO (g)
DMEA (g)
IECT
VER (%)
Thickness (nm)
IDT (°C) Td (°C)
185 225
236 254
227 241
208 240
201 227
DMEA-5 DMEA-8 DMEA-10 DMEA-15 AMX
0.8 0.8 0.8 0.8 –
0.04 0.06 0.08 0.12 –
0.54 0.78 1.02 1.47 –
8.58 12.71 16.53 20.41 14.64
57 47 82 64 152
membrane are 185 °C and 225 °C respectively. The IDT and Td values of these AEMs are higher than that of PVA-SiO2 hybrid anion exchange membranes [53] and higher IDT and Td values indicate better thermal stability. This showed that the prepared AEMs possess good thermal stability. 3.3. Ion exchange capacity, water uptake and volume expansion ratio Ion exchange capacity is a significant endowment of IEMs used in ED process. It gives information on the charge density in the membranes which is a crucial parameter related to conductivity and transport properties of membranes. It depends on the density of functional group in the membrane matrix and plays a crucial role in selectivity [54,55]. Ion exchange capacities of prepared membranes were measured by Mohr method. IEC of prepared AEMs and commercial anion exchange membrane Neosepta AMX is shown in Fig. 4. Obtained results showed that IEC of prepared AEMs is found to be enhanced with the increasing the amount of DMEA in the membrane matrix. Further, the commercial membranes Neosepta AMX possess IEC of 1.25 mmol/g [56]. Water uptake is a crucial parameter of IEMs and has accessed a significant effect on the separation phenomena, dimensional as well as mechanical properties [57–59]. Dissociation of the charged functional groups could be promoted by the presence of water molecules inside the membrane matrix and very important for the transportation of ions [59]. Water uptake of prepared AEMs and commercial membrane Neosepta AMX is shown in Fig. 4. For prepared AEMs, it is found to be enhanced from 11.60% to 48.50% with increasing the ion exchange content in the membrane matrix. Moreover, water uptake of commercial anion exchange membrane Neosepta AMX is found to be 16% [56]. It is associated to the increased hydrophilic region in the membrane matrix. Water uptake is controlled by the concentration of DMEA in the membrane matrix. Both IEC and WR are the important parameters to describe the hydrophilicity of ion exchange membrane. The volume expansion ratio (VER) of the prepared membranes was probed in water and attained results are shown in Table 2. The VER values of prepared membranes are found to be increased from 8.58– 20.41% with increasing the concentration of ion exchange content in the membrane matrix. It shows that prepared AEMs have good swelling resistance which is liable for long time running of electodialysis.
Fig. 4. Ion exchange capacity and water uptake of anion exchange membranes.
3.4. Chemical stability To study the alkaline stability of the quaternized poly (2, 6-dimethyl-1, 4-phenylene oxide) (QPPO), DMEA-10 and DMEA-15 membranes, they were immersed into 2 M NaOH solution at ambient temperature for specific time. The variation in IECs of studied membranes was used to evaluate their alkaline stability. The results are shown in Fig. 5. It can be found that the IEC values of the DMEA-10 and DMEA-15 membranes remain at ~ 74% and ~ 69% respectively of their initial values after 250 h whereas the IEC value of QPPO membrane decreases rapidly to ~55% [42] of its initial value under identical experimental conditions. Indeed, QPPO membrane lost its flexibility and mechanical strength within only 50 h. From Fig. 5, it can be observed that the IEC values of studied membranes initially decreased rapidly and then stabilized over longer immersion time. It is due to the degradation of quaternary ammonium group in the membrane matrix at the beginning of immersion time. It indicates that the studied membranes are more stable than QPPO membrane under identical experimental conditions.
3.5. Mechanical properties Mechanical properties of prepared AEMs including tensile strength (TS) and elongation at break (Eb) were investigated under wet condition and results are shown in Table 3. The TS of prepared membranes is found to be decreased with increasing the amount of DMEA in the membranes matrix. Tensile strength values are in range of 32.52 MPa to 49.22 MPa which are higher than those of previously studied ion exchange membranes [60] revealing that the prepared membranes possesses excellent tensile strength. The commercial membrane Neospta AMX possess tensile strength of 40.59 MPa and elongation at break of 27.64%. The relatively high values are attributed to the substrate contained therein. It can be seen that the prepared membrane DMEA15 have the lowest TS and the largest Eb indicating the higher flexibility
Fig. 5. The change in IEC values of QPPO, DMEA-10 and DMEA-15 membrane after immersion in 2 M NaOH at room temperature.
M.I. Khan et al. / Desalination 402 (2017) 10–18 Table 3 Tensile strength (TS) and elongation at break of prepared membranes and commercial membrane Neosepta AMX. Membranes
DMEA-5
DMEA-8
DMEA-10
DMEA-15
BPPO
AMX
TS (MPa) Eb (%)
49.22 23.29
38.85 33.94
37.86 43.29
32.52 55.85
2.02 2.86
40.59 27.64
of membrane. As shown in Table 3, the tensile strength of anion exchange membranes gradually decreased from membrane DMEA-5 to DMEA-15 with increase in the ion exchange capacity (resulting from the incorporation of ion exchang groups into the initially tight polymer chain network) [61]. On the other hand the Eb values of prepared membranes are found to increase with the amount of DMEA in the membrane matrix. The elongation at break (Eb) values of membranes DMEA-5 to DMEA-15 were in range of 23.29% to 55.85% which seems to be lower than PVA based hybride anion exchange membane [62]. However, all the values demonstrate that the prepared anion exchange membranes possess good mechanical stability and are efficient for electodialysis applications. 3.6. Membrane morphologies Morphologies of prepared anion exchange membranes were studied through the scanning electron microscopy (SEM). The surface morphology of all the prepared membranes is seemed to be similar. Therefore, the SEM micrographs of membranes DMEA-5 and DMEA-15 are selected as representative and are shown in Fig. 6. It is quite indisputable from the membrane surface images that with the enhancement of amine content in the membrane matrix better miscibility can be scrutinized.
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No pores, holes or cracks can be observed in membrane surface suggested homogeneous nature of prepared membranes whereas crosssection of the membranes showed the rough nature which is found to be decreased from membrane DMEA-5 to DMEA-15 with increasing the amount of amine in the membrane matrix. For ED process to be operative, it is necessary to have dense morphology of membrane which can be noticed here. We can deduce from the above investigations that increase in the amount of amine is favorable for the membrane homogeneity. Overall, all the membranes have dense structure. 3.7. Membrane area resistance and transport number Area resistance of IEMs is a crucial endowment to know the energy consumption in the ED process. Membranes with lower area resistance are selected for ED process. Area resistance of prepared AEMs was calculated in 0.5 M NaCl and is shown in Table 4. Out of four prepared membranes, DMEA-15 membrane possesses lower area resistance than that of commercial membrane Neosepta AMX as shown in the Table 4. The area resistance of prepared membranes is found to be decreased with enhancing the amount DMEA in the membrane matrix. Prepared membrane DMEA-15 possesses lower area resistance whereas the prepared membrane DMEA-5 shows higher area resistance among all the studied membranes. This is ascribed to the higher hydrophilicity of prepared membrane DMEA-15. Moreover, the higher area resistance of prepared membrane DMEA-5 is associated to its lower hydrophilicity. Therefore, the area resistance of IEMs depends on their hydrophilicity. Transport number is the fraction of total current carried by counter ions passing through the membrane [63]. A higher transport number of membrane shows higher perm-selectivity to counter-ions (anions here). Its value is dependent on membrane properties namely its IEC,
Fig. 6. Surface (left side) and cross-section (right side) of membrane DMEA-5 and DMEA-15.
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M.I. Khan et al. / Desalination 402 (2017) 10–18
Table 4 Transport number, area resistance and energy consumption of prepared membranes and commercial membrane Neosepta AMX. Membranes
AMX
Area resistance (Ω·cm2) Transport number Energy consumption (kw·h/kg)
2.35 62.5 0.98 0.94 88.30 –
DMEA-5 DMEA-8 DMEA-10 DMEA-15 5.62 0.96 –
3.59 0.96 61.66
1.43 0.98 29.19
WU, morphology, binder characteristics, thickness, etc. The transport numbers of prepared membrane are given in Table 4. It is found to be increased from membrane DMEA-5 to DMEA-15 with increasing the amount of DMEA in the membranes matrix because the diffusion of co-ions across the membrane was suppressed with rising quaternary ammonium group [64–67]. The membrane DMEA-15 possesses highest transport number (0.98) among all the prepared membranes. It could be attributed to the maximum suppression of co-ions diffusion by highest content of quaternary ammonium groups in the membrane DMEA-15. On the other hand prepared membrane DMEA-5 represents lower transport number due to lower concentration of DMEA in the polymer matrix. Because of lower amount of DMEA in membrane matrix, it possesses lower hydrophilicity among the prepared membranes which is responsible for its lower transport number. Hence these results suggested that the membranes synthesized here have higher capability toward the chloride ion (Cl−). 3.8. Desalination performance of developed membranes After complete characterization of prepared membranes, ED experiments were performed in a laboratory-scale unit to reveal the desalination performance of two selected membranes (DMEA-10 and DMEA15) at room temperature and results are compared with the commercial membrane Neosepta AMX. This experiment was carried out with the feed solution (0.1 M NaCl) flowing into the dilute cell (DC) with a constant flow rate of 60 mL/min by employing direct current of 28 mA/cm2. The change in the conductivity of NaCl in dilute cell (DC) as well as in concentrated cell (CC) for both membranes is shown in Fig. 7. The conductivity of NaCl in dilute cell (DC) is found to be reduced whereas it is found to be increased in concentrated cell (CC) with increasing the desalination time for both membranes as represented in Fig. 7. According to the conductivity model employed by Daniel Aguado et al. [68], the conductivity of the dilute solution cell can be interpreted by following deductions: (i) the anion and cation permeated the AEM and CEM under the applied electric field respectively which resulted in the ion concentration decreasing significantly or (ii) the rate of ionic diffusion
increased followed by an increase of molar conductivity because of reduction of concentration ions in the dilute solution. It indicates that 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 associated to the increased ion exchange content in the membrane matrix. The membrane DMEA-15 used in the electrodialytic removal of NaCl from aqueous solution showed better desalination performance than the commercial membrane Neosepta AMX because Cl− ions migrated rapidly from dilute cell to concentrated one due to higher IEC and lower area resistance. Therefore, the membrane can be successfully applied for salt removal from water. Table 4 represents the energy consumption for prepared membrane (DMEA-10 & DMEA-15) and commercial membrane Neosepta AMX. As can be seen from this table, under the same conditions the values of energy consumption for both the prepared membranes showed better desalination performance over the commercial membrane Neosepta AMX.
4. Conclusions Anion exchange membranes have been prepared from BPPO and DMEA in NMP solvent. The FTIR analysis confirms the successful reaction between BPPO and DMEA. The dense morphology of the membranes was confirmed by SEM images. The prepared AEMs possess excellent thermal and alkaline stability. The prepared AEMs possess ion exchange capacity of 0.66–1.38 mmol/g, water uptake of 11.60– 48.50%, volume expansion ratio of 8.58–20.21%, tensile strength of 32.52–49.22 MPa and transport number of 0.94–0.98. Especially, the membranes DMEA-10 and DMEA-15 possesses higher ion exchange capacity, water uptake and lower area resistance among all the developed membranes. This showed that the membranes DMEA-10 and DMEA-15 could be successfully employed for desalination by ED. These membranes showed higher desalination performance than commercial membrane Neosepta AMX. Overall, the developed AEMs are suitable for water desalination and other separation process. 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 CC concentrated cell DMEA dimethylethanolamine DC dilute cell elongation at break Eb ED electrodialysis IEC ion exchange capacity theorectical ion exchange capacity IECT LER linear expension ration NMM N-methylmorpholine NMP N-methyl-2-pyrrolidone R gas constant SEM scanning electron microscopy TS tensile strength VER volume expansion ratio water uptake WR
Acknowledgments
Fig. 7. The change in conductivity of NaCl with time.
The authors are highly thankful to the National Science Foundation of China (Nos. 91534203, 21490581), One Hundred Person Project of the Chinese Academy of Sciences (2015-43-D) and CAS-TWAS President's Fellowship for financial support.
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References [1] Y. Tanaka, A computer simulation of continuous ion exchange membrane electrodialysis for desalination of saline water, Desalination 249 (2009) 809–821. [2] F. Helfferich, Ion-exchange, McGraw-Hill Book, New York, 1962 397. [3] S. Mulyati, R. Takagi, A. Fujii, Y. Ohmukai, T. Maruyama, H. Matsuyama, Improvement of the antifouling potential of an anion exchange membrane by surface modification with a polyelectrolyte for an electrodialysis process, J. Membr. Sci. 417–418 (2012) 137–143. [4] 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. [5] N. Tanaka, M. Nagase, M. Higa, Preparation of aliphatic-hydrocarbon-based anionexchange membranes and their anti-organic-fouling properties, J. Membr. Sci. 384 (2011) 27–36. [6] C. Klaysom, R. Marschall, L. Wang, B.P. Ladewig, G.Q.M. Lu, Synthesis of composite ion-exchange membranes and their electrochemical properties for desalination applications, J. Mater. Chem. 20 (2010) 4669–4674. [7] G.S. Gohil, R.K. Nagarale, V.V. Binsu, V.K. Shahi, Preparation and characterization of monovalent cation selective sulfonated poly(ether ether ketone) and poly(ether sulfone) composite membranes, J. Colloid Interface Sci. 298 (2006) 845–853. [8] J. Balster, O. Krupenko, I. Pünt, D.F. Stamatialis, M. Wessling, Preparation and characterisation of monovalent ion selective cation exchange membranes based on sulphonated poly(ether ether ketone), J. Membr. Sci. 263 (2005) 137–145. [9] R. Scherer, A.M. Bernardes, M.M.C. Forte, J.Z. Ferreira, C.A. Ferreira, Preparation and physical characterization of a sulfonated poly(styrene-co-divinylbenzene) and polypyrrole composite membrane, Mater. Chem. Phys. 71 (2001) 131–136. [10] M.-S. Kang, Y.-J. Choi, I.-J. Choi, T.-H. Yoon, S.-H. Moon, Electrochemical characterization of sulfonated poly(arylene ether sulfone) (S-PES) cation-exchange membranes, J. Membr. Sci. 216 (2003) 39–53. [11] A. Elattar, A. Elmidaoui, N. Pismenskaia, C. Gavach, G. Pourcelly, Comparison of transport properties of monovalent anions through anion-exchange membranes, J. Membr. Sci. 143 (1998) 249–261. [12] T.W. Xu, Ion exchange membranes: state of their development and perspective, J. Membr. Sci. 263 (2005) 1–29. [13] 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. [14] A.S. Góźdź, W. Trochimczuk, Continuous modification of polyethylene with styrene and divinylbenzene in melt, J. Appl. Polym. Sci. 25 (1980) 947–950. [15] G. Pozniak, W. Trochimczuk, Chloromethylation of the polyethylene/poly(styreneco-divinylbenzene) system, J. Appl. Polym. Sci. 27 (1982) 1833–1838. [16] G. Poźniak, W. Trochimczuk, Interpolymer anion exchange membranes, I. Properties of weak base membranes based on polyethylene modified by styrene and divinylbenzene in the presence of diluents, Die Angew. Makromol. Chem. 92 (1980) 155–168. [17] G. Wang, Y. Weng, D. Chu, R. Chen, D. Xie, Developing a polysulfone-based alkaline anion exchange membrane for improved ionic conductivity, J. Membr. Sci. 332 (2009) 63–68. [18] G. Wang, Y. Weng, J. Zhao, R. Chen, D. Xie, Preparation of a functional poly(ether imide) membrane for potential alkaline fuel cell applications: chloromethylation, J. Appl. Polym. Sci. 112 (2009) 721–727. [19] X. Li, Q. Liu, Y. Yu, Y. Meng, Quaternized poly(arylene ether) ionomers containing triphenyl methane groups for alkaline anion exchange membranes, J. Mater. Chem. A 1 (2013) 4324–4335. [20] H. Zhang, Z. Zhou, Alkaline polymer electrolyte membranes from quaternized poly (phthalazinone ether ketone) for direct methanol fuel cell, J. Appl. Polym. Sci. 110 (2008) 1756–1762. [21] C. Yan, S. Zhang, D. Yang, X. Jian, Preparation and characterization of chloromethylated/quaternized poly (phthalazinone ether sulfone ketone) for positively charged nanofiltration membranes, J. Appl. Polym. Sci. 107 (2008) 1809–1816. [22] Q.H. Zeng, Q.L. Liu, I. Broadwell, A.M. Zhu, Y. Xiong, X.P. Tu, Anion exchange membranes based on quaternized polystyrene-block-poly(ethylene-ran-butylene)block-polystyrene for direct methanol alkaline fuel cells, J. Membr. Sci. 349 (2010) 237–243. [23] D.F. Stamatialis, B.J. Papenburg, M. Gironés, S. Saiful, S.N. Bettahalli, S. Schmitmeier, M. Wessling, Medical applications of membranes: drug delivery, artificial organs and tissue engineering, J. Membr. Sci. 308 (2008) 1–34. [24] N. Wang, C. Wu, Y. Cheng, T.W. Xu, Organic–inorganic hybrid anion exchange hollow fiber membranes: a novel device for drug delivery, Int. J. Pharm. 408 (2011) 39–49. [25] W. Kujawski, G. Poźniak, Transport properties of ion-exchange membranes during pervaporation of water-alcohol mixtures, Sep. Sci. Technol. 40 (2005) 2277–2295. [26] R.B. Merrifield, Solid phase synthesis (Nobel lecture), Angew. Chem. Int. Ed. Eng. 24 (1985) 799–810. [27] M. Kumar, S. Singh, V.K. Shahi, Cross-linked poly(vinyl alcohol) − poly(acrylonitrile-co-2-dimethylamino ethylmethacrylate) based anion-exchange membranes in aqueous media, J. Phys. Chem. B 114 (2010) 198–206. [28] C. Larchet, L. Dammak, B. Auclair, S. Parchikov, V. Nikonenko, A simplified procedure for ion-exchange membrane characterisation, New J. Chem. 28 (2004) 1260–1267. [29] J. Sun Koo, N.-S. Kwak, T.S. Hwang, Synthesis and properties of an anion-exchange membrane based on vinylbenzyl chloride–styrene–ethyl methacrylate copolymers, J. Membr. Sci. 423–424 (2012) 293–301.
17
[30] T.W. Xu, W. Yang, Fundamental studies of a new series of anion exchange membranes: membrane preparation and characterization, J. Membr. Sci. 190 (2001) 159–166. [31] L. Wu, T.W. Xu, W. Yang, Fundamental studies of a new series of anion exchange membranes: membranes prepared through chloroacetylation of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) followed by quaternary amination, J. Membr. Sci. 286 (2006) 185–192. [32] J. Yan, M.A. Hickner, Anion exchange membranes by bromination of benzylmethylcontaining poly(sulfone)s, Macromolecules 43 (2010) 2349–2356. [33] C.H. Zhao, Y. Gong, Q.L. Liu, Q.G. Zhang, A.M. Zhu, Self-crosslinked anion exchange membranes by bromination of benzylmethyl-containing poly(sulfone)s for direct methanol fuel cells, Int. J. Hydrog. Energy 37 (2012) 11383–11393. [34] J. Yan, M.A. Hickner, Anion exchange membranes by bromination of benzylmethylcontaining poly (sulfone) s, Macromolecules 43 (2010) 2349–2356. [35] M.R. Hibbs, C.H. Fujimoto, C.J. Cornelius, Synthesis and characterization of poly(phenylene)-based anion exchange membranes for alkaline fuel cells, Macromolecules 42 (2009) 8316–8321. [36] Z. Liu, X. Li, K. Shen, P. Feng, Y. Zhang, X. Xu, W. Hu, Z. Jiang, B. Liu, M.D. Guiver, Naphthalene-based poly (arylene ether ketone) anion exchange membranes, J. Mater. Chem. A 1 (2013) 6481–6488. [37] G. Merle, M. Wessling, K. Nijmeijer, Anion exchange membranes for alkaline fuel cells: a review, J. Membr. Sci. 377 (2011) 1–35. [38] G.P.W. Kujawski, Q.T. Nguyen, J. Néel, Properties of interpolymer PESS ion-exchange membranes in contact with solvents of different polarities, Sep. Purif. Technol. 32 (1997) 1657–1667. [39] Z.A.S.M. Hosseini, M. Nemati, F. Parvizian, S.S. Madaeni, Electrodialysis heterogeneous ion exchange membranes modified by SiO2 nanoparticles: fabrication and electrochemical characterization, Water Sci. Technol. 73 (2016) 2074–2084. [40] F. Iacomi, V. Craciun, S. Popescu, K. Mueller, M. Lattuada, Y. Dzyazko, L. Rozhdestveskaya, Y. Zmievskii, Y. Volfkovich, V. Sosenkin, N. Nikolskaya, S. Vasilyuk, V. Myronchuk, V. Belyakov, The selected papers of 10th international conference on physics of advanced materials, ICPAM-10Heterogeneous membranes modified with nanoparticles of inorganic ion-exchangers for whey demineralization, Mater. Today: Proc. 2 (2015) 3864–3873. [41] J. Pan, Y. He, L. Wu, C. Jiang, B. Wu, A.N. Mondal, C. Cheng, T.W. Xu, Anion exchange membranes from hot-pressed electrospun QPPO–SiO2 hybrid nanofibers for acid recovery, J. Membr. Sci. 480 (2015) 115–121. [42] M.I. Khan, A.N. Mondal, B. Tong, C. Jiang, K. Emmanuel, Z. Yang, L. Wu, T.W. Xu, Development of BPPO-based anion exchange membranes for electrodialysis desalination applications, Desalination 391 (2016) 61–68. [43] Z.L.T.W. Xu, Y. Li, W. Yang, Preparation and characterization of type II anion exchange membranes frompoly(2,6-dimethyl-1,4-phenylene oxide) (PPO), J. Membr. Sci. 320 (2008) 332–339. [44] M.I. Khan, A.N. Mondal, C. Cheng, J. Pan, K. Emmanuel, L. Wu, T.W. Xu, Porous BPPObased membranes modified by aromatic amine for acid recovery, Sep. Purif. Technol. 157 (2016) 27–34. [45] Y. He, L. Wu, J. Pan, Y. Zhu, X. Ge, Z. Yang, J. Ran, T.W. Xu, A mechanically robust anion exchange membrane with high hydroxide conductivity, J. Membr. Sci. 504 (2016) 47–54. [46] M.I. Khan, L. Wu, M.M. Hossain, J. Pan, J. Ran, A.N. Mondal, T.W. Xu, Preparation of diffusion dialysis membrane for acid recovery via a phase-inversion method, Membr. Water Treat. 6 (2015) 365–378. [47] M.M. Hossain, L. Wu, Y. Li, L. Ge, T.W. Xu, Preparation of porous poly(vinylidene fluoride) membranes with acrylate particles for electrodialysis application, Sep. Purif. Technol. 150 (2015) 102–111. [48] C.G. Arges, L. Wang, M.-s. Jung, V. Ramani, Mechanically stable poly (arylene ether) anion exchange membranes prepared from commercially available polymers for alkaline electrochemical devices, J. Electrochem. Soc. 162 (2015) F686–F693. [49] Y. Wu, C. Wu, M. Gong, T.W. Xu, New anion exchanger organic–inorganic hybrids and membranes from a copolymer of glycidylmethacrylate and -methacryloxypropyl trimethoxy silane, J. Appl. Polym. Sci. 102 (2006) 3580–3589. [50] C. Wu, Y. Wu, J. Luo, T.W. Xu, Y. Fu, Anion exchange hybrid membranes from PVA and multi-alkoxy silicon copolymer tailored for diffusion dialysis process, J. Membr. Sci. 356 (2010) 96–104. [51] Y. Li, T.W. Xu, M. Gong, Fundamental studies of a new series of anion exchange membranes: membranes prepared from bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) and pyridine, J. Membr. Sci. 279 (2006) 200–208. [52] J. Luo, C. Wu, Y. Wu, T.W. Xu, Diffusion dialysis of hydrochloride acid at different temperatures using PPO–SiO 2 hybrid anion exchange membranes, J. Membr. Sci. 347 (2010) 240–249. [53] Y. Wu, C. Wu, Y. Li, T.W. Xu, Y. Fu, PVA–silica anion-exchange hybrid membranes prepared through a copolymer crosslinking agent, J. Membr. Sci. 350 (2010) 322–332. [54] Z. Zhang, L. Wu, J. Varcoe, C. Li, A.L. Ong, S. Poynton, T.W. Xu, Aromatic polyelectrolytes via polyacylation of pre-quaternized monomers for alkaline fuel cells, J. Mater. Chem. A 1 (2013) 2595–2601. [55] J. Ran, L. Wu, X. Lin, L. Jiang, T.W. Xu, Synthesis of soluble copolymers bearing ionic graft for alkaline anion exchange membrane, RSC Adv. 2 (2012) 4250–4257. [56] C. Jiang, Q. Wang, Y. Li, Y. Wang, T.W. Xu, Water electro-transport with hydrated cations in electrodialysis, Desalination 365 (2015) 204–212. [57] D.B. Spry, A. Goun, K. Glusac, D.E. Moilanen, M.D. Fayer, Proton transport and the water environment in Nafion fuel cell membranes and AOT reverse micelles, J. Am. Chem. Soc. 129 (2007) 8122–8130.
18
M.I. Khan et al. / Desalination 402 (2017) 10–18
[58] C. Wang, C. Wu, Y. Wu, J. Gu, T.W. Xu, Polyelectrolyte complex/PVA membranes for diffusion dialysis, J. Hazard. Mater. 261 (2013) 114–122. [59] T. Chakrabarty, A.K. Singh, V.K. Shahi, Zwitterionic silica copolymer based crosslinked organic-inorganic hybrid polymer electrolyte membranes for fuel cell applications, RSC Adv. 2 (2012) 1949–1961. [60] C. Cheng, Z. Yang, Y. He, A.N. Mondal, E. Bakangura, T.W. Xu, Diffusion dialysis membranes with semi-interpenetrating network for acid recovery, J. Membr. Sci. 493 (2015) 645–653. [61] J. Ran, L. Wu, J.R. Varcoe, A.L. Ong, S.D. Poynton, T.W. Xu, Development of imidazolium-type alkaline anion exchange membranes for fuel cell application, J. Membr. Sci. 415–416 (2012) 242–249. [62] A.N. Mondal, C. Cheng, Z. Yao, J. Pan, M.M. Hossain, M.I. Khan, Z. Yang, L. Wu, T.W. Xu, Novel quaternized aromatic amine based hybrid PVA membranes for acid recovery, J. Membr. Sci. 490 (2015) 29–37. [63] A. Noshay, L. Robeson, Sulfonated polysulfone, J. Appl. Polym. Sci. 20 (1976) 1885–1903.
[64] S. Singh, A. Jasti, M. Kumar, V.K. Shahi, A green method for the preparation of highly stable organic-inorganic hybrid anion-exchange membranes in aqueous media for electrochemical processes, Polym. Chem. 1 (2010) 1302–1312. [65] J.-H. Cheng, Y.-C. Xiao, C. Wu, T.-S. Chung, Chemical modification of P84 polyimide as anion-exchange membranes in a free-flow isoelectric focusing system for protein separation, Chem. Eng. J. 160 (2010) 340–350. [66] M.A. Khan, M. Kumar, Z.A. Alothman, Preparation and characterization of organic– inorganic hybrid anion-exchange membranes for electrodialysis, J. Ind. Eng. Chem. 21 (2015) 723–730. [67] C. Klaysom, S.-H. Moon, B.P. Ladewig, G.M. Lu, L. Wang, The influence of inorganic filler particle size on composite ion-exchange membranes for desalination, J. Phys. Chem. C 115 (2011) 15124–15132. [68] D. Aguado, T. Montoya, J. Ferrer, A. Seco, Relating ions concentration variations to conductivity variations in a sequencing batch reactor operated for enhanced biological phosphorus removal, Environ. Model. Softw. 21 (2006) 845–851.
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Desalination journal homepage: www.elsevier.com/locate/desal
Preparation of anion exchange membranes from BPPO and dimethylethanolamine for electrodialysis Muhammad Imran Khan a, Chunlei Zheng b, Abhishek N. Mondal a, Md. Masem Hossain a, Bin Wu a, Kamana Emmanuel a, Liang Wu a, Tongwen Xu a,⁎ a 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 b Hefei ChemJoy Polymer Materials, Co., LTD, Hefei, Anhui 230601, PR China
H I G H L I G H T S • Anion exchange membranes with dimethyethanolammonium were prepared. • Developed membranes have high desalination performance and suitable for ED.
a r t i c l e
i n f o
Article history: Received 23 February 2016 Received in revised form 19 September 2016 Accepted 22 September 2016 Available online xxxx Keywords: Anion exchange membrane BPPO Dimethyethanolamine Area resistance Electrodialysis
a b s t r a c t In this research work, the synthesis of anion exchanges membranes (AEMs) from brominated poly (2, 6-dimethyl-1, 4-phenylene oxide) (BPPO) and dimethyethanolamine (DMEA) has been investigated for electrodialysis (ED) application. Fourier transform infrared spectroscopy was used to confirm the functional groups in the membranes. The morphology of the prepared membranes was investigated by scanning electron microscopy. Physiochemical and electrochemical properties of the prepared membranes were studied in detail. The membranes possess ion exchange capacity of 0.66 mmol/g to 1.38 mmol/g, water uptake of 11.60% to 48.50%, volume expansion ratio of 8.58% to 20.21%, tensile strength of 32.52 MPa to 49.22 MPa and transport number of 0.94 to 0.98. These membranes have higher chemical stability in alkaline medium than QPPO membrane (quaternized with trimethylamine) at room temperature. The membranes DMEA-10 and DMEA-15 were selected for NaCl desalination by ED with a lab-scale electrodialysis cell at constant applied voltage. The prepared membranes DMEA-10 and DMEA-15 showed better performance than the commercial anion exchange membranes Neosepta AMX under the identical conditions. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The electrodialysis (ED) process is based on the transport of ions through selective membranes under the influence of an electrical field. Ion transfer is caused by diffusion and electro migration, while solution migration is caused by the electro-osmosis and concentration-osmosis [1,2]. The ED process is selective for the removal of ionic species from non-ionic species in an aqueous medium and has proven to be a robust, efficient and versatile method for such applications. The advantages of ED over other separation processes includes low energy cost, versatility in term of wide variety of feed streams that can be utilized with minimum requirement of pre-treatment, easier and low cost of maintenance and higher membrane life. Recent development has resulted in the significant usefulness of the ED technique for producing safe drinking ⁎ Corresponding author. E-mail address: [email protected] (T. Xu).
http://dx.doi.org/10.1016/j.desal.2016.09.019 0011-9164/© 2016 Elsevier B.V. All rights reserved.
water and for the separation of hazardous chemicals from water [3–6]. The ED process is more convenient than Reverse Osmosis (RO) because higher brine concentration can be obtained in the former process as there are no osmotic pressure limitations. Thus the main applications of ED process are the purification of salty water, the separation and concentration of high value chemicals and the removal of effluents. Recently ion exchange membranes (IEMs) have attracted great attention from both academic and industrial fields due to their variable applications from desalination of salt-rich water to food production process including waste water treatment to recover some valuable elements in chemical stability industry [7–11]. Traditionally, the IEMs 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 [12,13]. Different process usually requires specific membrane properties. For desalination specially electron driven process such as electrodialysis, which is the scope of interest in the present work, the IEMs were
M.I. Khan et al. / Desalination 402 (2017) 10–18
expected to have high permselectivity, excellent conductivity, good chemical, thermal and mechanical stabilities. To date, many polymers have been used in the preparation of AEMs, for example, 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 applications such as in fuel cell [22], biomedical devices [23,24], pervaporation [25], solid state polypeptide synthesis [26], etc. Chloromethyl ether (CME) was used commonly for chloromethylation process. However, CME is pretty much harmful for human health and is a banned chemical [27–29]. To eliminate the above mentioned problems, much attention has been paid to the preparation of AEMs without using the dangerous chloromethylation reagents. Bromination of benzylmethyl and chloroacetylationl of phenyl are known to be promising method for the avoidance of CME [30–32]. Polymers such as benzylmethyl-containing polysulfone [33,34], poly(phenylene) [35], and poly(arylene ether ketone) [36] have been employed to obtained AEMs via bromination and subsequent quaternization. This approach has, however, not been abused in the preparation of intermolecular AEMs which have excellent durability and excellent for distinct applications [37,38]. Alternatively, it is quite well-known that use of non-toxic inorganic constituents also improves functional properties of the membrane significantly [39]. Recently, Dzyazko et al. showed nanoparticle embedded heterogeneous membrane, which can be a potential solution for upgrading functional properties of the membrane [40]. From our research group, Pan et al. showed silicon dioxide incorporated QPPO membrane which is useful for separation purpose [41]. Our previous work reported the synthesis of N-methylmorpholine (NMM) based AEMs for water desalination by ED [42]. The major problem associated with these membranes are the relatively low alkalinity of NMM (pkb = 6.62) which results in low anion permeability. To overcome these problems, dimethyethanolamine (DMEA) (pkb = 4.77) based AEMs for ED process have been synthesized. These membranes show higher alkaline stability due to non-aromatic character of DMEA which is beneficial for electrochemical applications. Moreover, these membranes possess higher alkaline stability. In our previous work, AEMs were also prepared with BPPO by subsequent quaternary amination with a dimethyethanolamine (DMEA) aqueous solution. It was found that the membrane's intrinsic properties are dependent on DMEA concentration and amination temperature and the conditions were optimized. However, at that case, BPPO based membrane with polyester as a substrate was used and therefore the amination reaction between BPPO and DMEA is heterogeneous (solid-liquid) so the membrane area resistance (30Ω.cm2) is too high to be used in conventional electrodialysis even the membrane had proper IEC of 1.5 mmol/g dry membrane and water content of 30%, [43]. In the present research, cast solutions were firstly prepared from the homogenous reaction between BPPO and DMEA under the optimized conditions and then a series of AEMs were prepared by varying the amount of DMEA in the reaction medium. The prepared membranes were characterized in term of FTIR, thermal stability, SEM, ion exchange capacity (IEC), water uptake (WR), volume expansion ration (VER), mechanical/chemical stability, membrane area resistance and transport number. Moreover, their applications in electrodialytic removal of NaCl from aqueous solution have also been investigated and compared with commercial membrane Neosepta AMX.
Commercial anion exchange membrane Neosepta AMX and cation exchange membrane Neosepta CMX were purchased from ASTOM, Japan. Dimethyethanolamine (DMEA) 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 reagents companies. These reagents were used without further purification. Deionized (DI) water was used throughout the experiments.
2.2. Membrane preparation These membranes were prepared by solution casting method as reported in our previous work [42,44]. In this method, firstly the casting solution of 8% of brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) was prepared into N-methyl-2-pyrrolidone (NMP) solvent. AEMs with different properties were prepared by adding 0.04 g, 0.06 g, 0.08 g and 0.12 g of DMEA into the casting solution. The casting solution was stirred at 40 °C for 12 h to accelerate the reaction among the BPPO and DMEA. Finally, the solution was casted onto a glass plate and heated at 60 °C (solvent evaporation) for 24 h. The membrane were peeled off from glass paltes and washed with deionized water before characterization and study. The obtained membranes were respresented as DMEA-5, DMEA-8, DMEA-10 and DMEA-15 respectively according to the different amount of amine. Chemical structure for the prepared BPPO based membrane is also represented in Scheme 1.
2.3. Preparation of quaternized poly (2, 6-dimethyl-1, 4-phenylene oxide) (QPPO) membrane The preparation of quaternized poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO) membrane was carried out according to our previous reported method [45]. In a typical method, 0.8 g of brominated poly (2, 6dimethyl-1, 4-phenylene oxide) (BPPO) was dissolved into N-methyl2-pyrrolidone (NMP) solvent to get homogeneous solution. After that measured amount of trimethylamine (TMA) was added into the already prepared BPPO solution. The resultant mixture was stirred at 60 °C for 12 h and then caste on hot plate at 60 °C for 24 h. The attained membranes was washed and stored in DI water.
2. Experimental 2.1. Material Brominated poly (2, 6-dimethyl-1, 4-phenyleneoxide) (BPPO) with aryl substitution degree of 0.42 and benzyl substitution degree of 0.58 was provided by Tianwei Membrane Co. Ltd., Shandong of China.
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Scheme 1. The preparation of BPPO-based anion exchanged membranes.
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2.4. Characterizations 2.4.1. FTIR spectra & thermal stability FTIR analysis of prepared AEMs were measured by employing FTIR spectrometer (Vector 22, Bruker) having resolution of 2 cm− 1 and a total spectral range of 4000–400 cm− 1. The thermal stability of prepared membranes was investigated by employing a Shimadzu TGA50H analyzer within the temperature range 25 °C to 700 °C under nitrogen flow, with a heating rate of 10 °C/min. 2.4.2. Ion exchange capacity (IEC) It shows the number of exchangeable ionic groups (equivalents) present per dry membrane weight. It was measured by the classical Mohr method [42,44,46]. In a typical procedure, 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 DI 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 measured by titration with 0.05 (M) AgNO3 using K2CrO4 as an indicator. The ion-exchange capacity (IEC; mmol/g) of the membrane was measured 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. 2.4.3. Water uptake and volume expansion ratio Water uptake (WR) is used to investigate the hydrophilicity of IEM. Membrane samples were dried in vacuum oven at 60 °C for 24 h and accurately weighed to confirm their exact dry weight. Then the membranes were immersed in DI water for 72 h at 25 °C and wet weight of those membranes are recorded after removal of surface water with tissue paper. From the difference in mass before and after the complete drying of the membranes, WR values were measured [42,44,46] as the relative weight gain per gram of the dry sample using following Eq. (1). WR ¼
W WET −W DRY 100% W DRY
ð1Þ
where WWET and WDRY are the weights of wet and dry membranes, respectively. The volume expansion ratio (VER) of prepared AEMs was investigated at room temperature. It was measured by using following equation. VER ¼
LWET W WET T WET −LDRY W DRY T DRY 100% LDRY T DRY W DRY
ð2Þ
where LWET, WWET and TWET is the length, width and thickness of membranes in wet state respectively whereas LDRY, WDRY and TDRY is the length, width and thickness of membranes in dry state respectively. 2.4.4. Chemical stability The chemical stability of the prepared membranes was investigated by calculating the changes in IEC values before and after being kept in 2 M NaOH at ambient temperature. 2.4.5. Mechanical property Tensile strength of dry and hydrates membranes were calculated employing Q800 dynamic mechanical analyzer (DMA, TA Instruments) at a stretch rate of 0.5 N/min. The tested samples were equilibrated in deionized water (DI) for 24 h and then cut into a rectangular shape with dimensions of 4 cm × 1 cm. At least four specimens from each sample were tested. 2.4.6. Microscopic characterizations for AEMs The morphology of prepared membranes was investigated through a field emission scanning electron microscope (FE-SEM, Sirion200, FEI
Company, USA). Surface and cross-sectional images of membranes were taken from dry membranes. The SEM images of prepared membranes were represented as representative cases. 2.4.7. Membrane area resistance It was calculated by commercial cell-assembly (MEIEMP-I, Hefei Chemjoy Polymer Material Co., Ltd.) under a constant current mode [42]. It is made up 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 remove the effect of electrode reaction. Two intermediate chambers are separated by the membrane clip and tips of the reference electrodes should be close to the center of membrane. During the experiment, Na2SO4 solution (0.3 mol/L) was fed to the electrode chambers and NaCl (0.5 mol/L) was fed to the intermediate chambers. A constant current is supplied by direct current power supply (SHEKONIC, Yangzhou Shuanghong Co., Ltd.) and the potential between electrode is read by digital multimeter (model: GDM 8145, Good will instrument Co. Ltd., Taiwan). It can be measured using the values of current, potential and membrane area (7.07 cm2). 2.4.8. Membrane transport number It was calculated by measuring the membrane potential as reported in our previous work [42]. A cell made of Perspex sheet was employed 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 investigated 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 calculated using 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 measured employing the following modified Nernst equation [42]. Em ¼
RT 0 2t i −1 nF
ln
a1 aÞ
ð3Þ
where R is universal gas constant (8.314 J/K.mol), F the Faraday constant (96,487C/mol), T the absolute temperature (K), a1 and a2 the mean activities of electrolyte solutions and n is the charge number of counter-ion (ni = 1 in this case). 2.4.9. Electrodialysis stack The ED performance of prepared AEMs was studied according to the previous reported method [42]. The ED stack employed in this research is depicted in Fig. 1. It is made up 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. Prepared anion exchange membranes (DMEA-10 & DMEA-15) and commercial membrane Neosepta AMX were employed 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 was noted after specific intervals of time. The performance of developed membranes DMEA-10 and DMEA-15 was compared with the commercial membrane Neosepta AMX in term of energy consumption calculated by the following equation [47]. Z U P¼
Idt m
ð4Þ
M.I. Khan et al. / Desalination 402 (2017) 10–18
13
Fig. 1. The schematic setup of ED stack.
3. Results and discussion
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. Moreover, the signal for C\\Br stretching in bromobenzyl groups expected at 740 cm−1 to 750 cm−1 was not observed in FTIR spectra of aminated BPPO membranes [48,51].
3.1. FT-IR spectra
3.2. Thermal stability
The successful quaternization of BPPO with DMEA was confirmed by the FTIR spectra analysis. Fig. 2 showed the FTIR spectra of pristine and aminated BPPO membranes. The characteristic band at 750 cm−1 corresponding to the C\\Br stretching in the pristine BPPO membrane [48]. After the reaction with DMEA, the characteristic band at 1090 cm−1 appeared in the aminated BPPO membrane is due to the C\\N stretching vibration which is absent in pristine BPPO membrane representing the successful reaction between BPPO and DMEA. Moreover, the broad peak at 3350 cm− 1 corresponds to the stretching vibration of OH group in the prepared anion exchange membrane. 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 [49]. The bands at 1446 cm−1 are due to stretching of –CH groups (V and δ) [50]. The adsorption peaks of symmetrical and asymmetrical
TGA curves of the prepared membranes are shown in Fig. 3. The weight loss of membranes took place into 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 showed a weight loss below 100 °C which is associated to the evaporation of water from the membrane matrix. The second weight loss occurred around 250 °C is attributed to the degradation of quaternary ammonium group [52]. The final weight loss observed occurred above 400 °C is due to the degradation of main polymer matrix. Two crucial endowments, i.e. thermal degradation temperature (Td) and initial decomposition temperature (IDT) were calculated and are given in Table 1. The IDT values of prepared membranes are in the range of 201 °C to 236 °C whereas the Td values were in the range of 227 °C to 254 °C. Correspondingly, IDT and Td values of BPPO blank
Fig. 2. IR spectrum of pristine and aminated BPPO membrane.
Fig. 3. TGA Thermograms for different anion exchange membranes.
where P = energy consumption of NaCl (KW h/kg), I = current (A), U = applied potential (V), m = mass of removed salt (kg), and t = time (S).
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M.I. Khan et al. / Desalination 402 (2017) 10–18
Table 1 Thermal decomposition temperature (Td) and the initial decomposition temperature (IDT) of the prepared membranes.
Table 2 Composition, theoretical IEC, volume expansion ratio and thickness of membranes DMEA5 to DMEA-15 and commercial membrane Neosepta AMX.
Membranes
BPPO
DMEA-5
DMEA-8
DMEA-10
DMEA-15
Membranes
BPPO (g)
DMEA (g)
IECT
VER (%)
Thickness (nm)
IDT (°C) Td (°C)
185 225
236 254
227 241
208 240
201 227
DMEA-5 DMEA-8 DMEA-10 DMEA-15 AMX
0.8 0.8 0.8 0.8 –
0.04 0.06 0.08 0.12 –
0.54 0.78 1.02 1.47 –
8.58 12.71 16.53 20.41 14.64
57 47 82 64 152
membrane are 185 °C and 225 °C respectively. The IDT and Td values of these AEMs are higher than that of PVA-SiO2 hybrid anion exchange membranes [53] and higher IDT and Td values indicate better thermal stability. This showed that the prepared AEMs possess good thermal stability. 3.3. Ion exchange capacity, water uptake and volume expansion ratio Ion exchange capacity is a significant endowment of IEMs used in ED process. It gives information on the charge density in the membranes which is a crucial parameter related to conductivity and transport properties of membranes. It depends on the density of functional group in the membrane matrix and plays a crucial role in selectivity [54,55]. Ion exchange capacities of prepared membranes were measured by Mohr method. IEC of prepared AEMs and commercial anion exchange membrane Neosepta AMX is shown in Fig. 4. Obtained results showed that IEC of prepared AEMs is found to be enhanced with the increasing the amount of DMEA in the membrane matrix. Further, the commercial membranes Neosepta AMX possess IEC of 1.25 mmol/g [56]. Water uptake is a crucial parameter of IEMs and has accessed a significant effect on the separation phenomena, dimensional as well as mechanical properties [57–59]. Dissociation of the charged functional groups could be promoted by the presence of water molecules inside the membrane matrix and very important for the transportation of ions [59]. Water uptake of prepared AEMs and commercial membrane Neosepta AMX is shown in Fig. 4. For prepared AEMs, it is found to be enhanced from 11.60% to 48.50% with increasing the ion exchange content in the membrane matrix. Moreover, water uptake of commercial anion exchange membrane Neosepta AMX is found to be 16% [56]. It is associated to the increased hydrophilic region in the membrane matrix. Water uptake is controlled by the concentration of DMEA in the membrane matrix. Both IEC and WR are the important parameters to describe the hydrophilicity of ion exchange membrane. The volume expansion ratio (VER) of the prepared membranes was probed in water and attained results are shown in Table 2. The VER values of prepared membranes are found to be increased from 8.58– 20.41% with increasing the concentration of ion exchange content in the membrane matrix. It shows that prepared AEMs have good swelling resistance which is liable for long time running of electodialysis.
Fig. 4. Ion exchange capacity and water uptake of anion exchange membranes.
3.4. Chemical stability To study the alkaline stability of the quaternized poly (2, 6-dimethyl-1, 4-phenylene oxide) (QPPO), DMEA-10 and DMEA-15 membranes, they were immersed into 2 M NaOH solution at ambient temperature for specific time. The variation in IECs of studied membranes was used to evaluate their alkaline stability. The results are shown in Fig. 5. It can be found that the IEC values of the DMEA-10 and DMEA-15 membranes remain at ~ 74% and ~ 69% respectively of their initial values after 250 h whereas the IEC value of QPPO membrane decreases rapidly to ~55% [42] of its initial value under identical experimental conditions. Indeed, QPPO membrane lost its flexibility and mechanical strength within only 50 h. From Fig. 5, it can be observed that the IEC values of studied membranes initially decreased rapidly and then stabilized over longer immersion time. It is due to the degradation of quaternary ammonium group in the membrane matrix at the beginning of immersion time. It indicates that the studied membranes are more stable than QPPO membrane under identical experimental conditions.
3.5. Mechanical properties Mechanical properties of prepared AEMs including tensile strength (TS) and elongation at break (Eb) were investigated under wet condition and results are shown in Table 3. The TS of prepared membranes is found to be decreased with increasing the amount of DMEA in the membranes matrix. Tensile strength values are in range of 32.52 MPa to 49.22 MPa which are higher than those of previously studied ion exchange membranes [60] revealing that the prepared membranes possesses excellent tensile strength. The commercial membrane Neospta AMX possess tensile strength of 40.59 MPa and elongation at break of 27.64%. The relatively high values are attributed to the substrate contained therein. It can be seen that the prepared membrane DMEA15 have the lowest TS and the largest Eb indicating the higher flexibility
Fig. 5. The change in IEC values of QPPO, DMEA-10 and DMEA-15 membrane after immersion in 2 M NaOH at room temperature.
M.I. Khan et al. / Desalination 402 (2017) 10–18 Table 3 Tensile strength (TS) and elongation at break of prepared membranes and commercial membrane Neosepta AMX. Membranes
DMEA-5
DMEA-8
DMEA-10
DMEA-15
BPPO
AMX
TS (MPa) Eb (%)
49.22 23.29
38.85 33.94
37.86 43.29
32.52 55.85
2.02 2.86
40.59 27.64
of membrane. As shown in Table 3, the tensile strength of anion exchange membranes gradually decreased from membrane DMEA-5 to DMEA-15 with increase in the ion exchange capacity (resulting from the incorporation of ion exchang groups into the initially tight polymer chain network) [61]. On the other hand the Eb values of prepared membranes are found to increase with the amount of DMEA in the membrane matrix. The elongation at break (Eb) values of membranes DMEA-5 to DMEA-15 were in range of 23.29% to 55.85% which seems to be lower than PVA based hybride anion exchange membane [62]. However, all the values demonstrate that the prepared anion exchange membranes possess good mechanical stability and are efficient for electodialysis applications. 3.6. Membrane morphologies Morphologies of prepared anion exchange membranes were studied through the scanning electron microscopy (SEM). The surface morphology of all the prepared membranes is seemed to be similar. Therefore, the SEM micrographs of membranes DMEA-5 and DMEA-15 are selected as representative and are shown in Fig. 6. It is quite indisputable from the membrane surface images that with the enhancement of amine content in the membrane matrix better miscibility can be scrutinized.
15
No pores, holes or cracks can be observed in membrane surface suggested homogeneous nature of prepared membranes whereas crosssection of the membranes showed the rough nature which is found to be decreased from membrane DMEA-5 to DMEA-15 with increasing the amount of amine in the membrane matrix. For ED process to be operative, it is necessary to have dense morphology of membrane which can be noticed here. We can deduce from the above investigations that increase in the amount of amine is favorable for the membrane homogeneity. Overall, all the membranes have dense structure. 3.7. Membrane area resistance and transport number Area resistance of IEMs is a crucial endowment to know the energy consumption in the ED process. Membranes with lower area resistance are selected for ED process. Area resistance of prepared AEMs was calculated in 0.5 M NaCl and is shown in Table 4. Out of four prepared membranes, DMEA-15 membrane possesses lower area resistance than that of commercial membrane Neosepta AMX as shown in the Table 4. The area resistance of prepared membranes is found to be decreased with enhancing the amount DMEA in the membrane matrix. Prepared membrane DMEA-15 possesses lower area resistance whereas the prepared membrane DMEA-5 shows higher area resistance among all the studied membranes. This is ascribed to the higher hydrophilicity of prepared membrane DMEA-15. Moreover, the higher area resistance of prepared membrane DMEA-5 is associated to its lower hydrophilicity. Therefore, the area resistance of IEMs depends on their hydrophilicity. Transport number is the fraction of total current carried by counter ions passing through the membrane [63]. A higher transport number of membrane shows higher perm-selectivity to counter-ions (anions here). Its value is dependent on membrane properties namely its IEC,
Fig. 6. Surface (left side) and cross-section (right side) of membrane DMEA-5 and DMEA-15.
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M.I. Khan et al. / Desalination 402 (2017) 10–18
Table 4 Transport number, area resistance and energy consumption of prepared membranes and commercial membrane Neosepta AMX. Membranes
AMX
Area resistance (Ω·cm2) Transport number Energy consumption (kw·h/kg)
2.35 62.5 0.98 0.94 88.30 –
DMEA-5 DMEA-8 DMEA-10 DMEA-15 5.62 0.96 –
3.59 0.96 61.66
1.43 0.98 29.19
WU, morphology, binder characteristics, thickness, etc. The transport numbers of prepared membrane are given in Table 4. It is found to be increased from membrane DMEA-5 to DMEA-15 with increasing the amount of DMEA in the membranes matrix because the diffusion of co-ions across the membrane was suppressed with rising quaternary ammonium group [64–67]. The membrane DMEA-15 possesses highest transport number (0.98) among all the prepared membranes. It could be attributed to the maximum suppression of co-ions diffusion by highest content of quaternary ammonium groups in the membrane DMEA-15. On the other hand prepared membrane DMEA-5 represents lower transport number due to lower concentration of DMEA in the polymer matrix. Because of lower amount of DMEA in membrane matrix, it possesses lower hydrophilicity among the prepared membranes which is responsible for its lower transport number. Hence these results suggested that the membranes synthesized here have higher capability toward the chloride ion (Cl−). 3.8. Desalination performance of developed membranes After complete characterization of prepared membranes, ED experiments were performed in a laboratory-scale unit to reveal the desalination performance of two selected membranes (DMEA-10 and DMEA15) at room temperature and results are compared with the commercial membrane Neosepta AMX. This experiment was carried out with the feed solution (0.1 M NaCl) flowing into the dilute cell (DC) with a constant flow rate of 60 mL/min by employing direct current of 28 mA/cm2. The change in the conductivity of NaCl in dilute cell (DC) as well as in concentrated cell (CC) for both membranes is shown in Fig. 7. The conductivity of NaCl in dilute cell (DC) is found to be reduced whereas it is found to be increased in concentrated cell (CC) with increasing the desalination time for both membranes as represented in Fig. 7. According to the conductivity model employed by Daniel Aguado et al. [68], the conductivity of the dilute solution cell can be interpreted by following deductions: (i) the anion and cation permeated the AEM and CEM under the applied electric field respectively which resulted in the ion concentration decreasing significantly or (ii) the rate of ionic diffusion
increased followed by an increase of molar conductivity because of reduction of concentration ions in the dilute solution. It indicates that 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 associated to the increased ion exchange content in the membrane matrix. The membrane DMEA-15 used in the electrodialytic removal of NaCl from aqueous solution showed better desalination performance than the commercial membrane Neosepta AMX because Cl− ions migrated rapidly from dilute cell to concentrated one due to higher IEC and lower area resistance. Therefore, the membrane can be successfully applied for salt removal from water. Table 4 represents the energy consumption for prepared membrane (DMEA-10 & DMEA-15) and commercial membrane Neosepta AMX. As can be seen from this table, under the same conditions the values of energy consumption for both the prepared membranes showed better desalination performance over the commercial membrane Neosepta AMX.
4. Conclusions Anion exchange membranes have been prepared from BPPO and DMEA in NMP solvent. The FTIR analysis confirms the successful reaction between BPPO and DMEA. The dense morphology of the membranes was confirmed by SEM images. The prepared AEMs possess excellent thermal and alkaline stability. The prepared AEMs possess ion exchange capacity of 0.66–1.38 mmol/g, water uptake of 11.60– 48.50%, volume expansion ratio of 8.58–20.21%, tensile strength of 32.52–49.22 MPa and transport number of 0.94–0.98. Especially, the membranes DMEA-10 and DMEA-15 possesses higher ion exchange capacity, water uptake and lower area resistance among all the developed membranes. This showed that the membranes DMEA-10 and DMEA-15 could be successfully employed for desalination by ED. These membranes showed higher desalination performance than commercial membrane Neosepta AMX. Overall, the developed AEMs are suitable for water desalination and other separation process. 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 CC concentrated cell DMEA dimethylethanolamine DC dilute cell elongation at break Eb ED electrodialysis IEC ion exchange capacity theorectical ion exchange capacity IECT LER linear expension ration NMM N-methylmorpholine NMP N-methyl-2-pyrrolidone R gas constant SEM scanning electron microscopy TS tensile strength VER volume expansion ratio water uptake WR
Acknowledgments
Fig. 7. The change in conductivity of NaCl with time.
The authors are highly thankful to the National Science Foundation of China (Nos. 91534203, 21490581), One Hundred Person Project of the Chinese Academy of Sciences (2015-43-D) and CAS-TWAS President's Fellowship for financial support.
M.I. Khan et al. / Desalination 402 (2017) 10–18
References [1] Y. Tanaka, A computer simulation of continuous ion exchange membrane electrodialysis for desalination of saline water, Desalination 249 (2009) 809–821. [2] F. Helfferich, Ion-exchange, McGraw-Hill Book, New York, 1962 397. [3] S. Mulyati, R. Takagi, A. Fujii, Y. Ohmukai, T. Maruyama, H. Matsuyama, Improvement of the antifouling potential of an anion exchange membrane by surface modification with a polyelectrolyte for an electrodialysis process, J. Membr. Sci. 417–418 (2012) 137–143. [4] 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. [5] N. Tanaka, M. Nagase, M. Higa, Preparation of aliphatic-hydrocarbon-based anionexchange membranes and their anti-organic-fouling properties, J. Membr. Sci. 384 (2011) 27–36. [6] C. Klaysom, R. Marschall, L. Wang, B.P. Ladewig, G.Q.M. Lu, Synthesis of composite ion-exchange membranes and their electrochemical properties for desalination applications, J. Mater. Chem. 20 (2010) 4669–4674. [7] G.S. Gohil, R.K. Nagarale, V.V. Binsu, V.K. Shahi, Preparation and characterization of monovalent cation selective sulfonated poly(ether ether ketone) and poly(ether sulfone) composite membranes, J. Colloid Interface Sci. 298 (2006) 845–853. [8] J. Balster, O. Krupenko, I. Pünt, D.F. Stamatialis, M. Wessling, Preparation and characterisation of monovalent ion selective cation exchange membranes based on sulphonated poly(ether ether ketone), J. Membr. Sci. 263 (2005) 137–145. [9] R. Scherer, A.M. Bernardes, M.M.C. Forte, J.Z. Ferreira, C.A. Ferreira, Preparation and physical characterization of a sulfonated poly(styrene-co-divinylbenzene) and polypyrrole composite membrane, Mater. Chem. Phys. 71 (2001) 131–136. [10] M.-S. Kang, Y.-J. Choi, I.-J. Choi, T.-H. Yoon, S.-H. Moon, Electrochemical characterization of sulfonated poly(arylene ether sulfone) (S-PES) cation-exchange membranes, J. Membr. Sci. 216 (2003) 39–53. [11] A. Elattar, A. Elmidaoui, N. Pismenskaia, C. Gavach, G. Pourcelly, Comparison of transport properties of monovalent anions through anion-exchange membranes, J. Membr. Sci. 143 (1998) 249–261. [12] T.W. Xu, Ion exchange membranes: state of their development and perspective, J. Membr. Sci. 263 (2005) 1–29. [13] 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. [14] A.S. Góźdź, W. Trochimczuk, Continuous modification of polyethylene with styrene and divinylbenzene in melt, J. Appl. Polym. Sci. 25 (1980) 947–950. [15] G. Pozniak, W. Trochimczuk, Chloromethylation of the polyethylene/poly(styreneco-divinylbenzene) system, J. Appl. Polym. Sci. 27 (1982) 1833–1838. [16] G. Poźniak, W. Trochimczuk, Interpolymer anion exchange membranes, I. Properties of weak base membranes based on polyethylene modified by styrene and divinylbenzene in the presence of diluents, Die Angew. Makromol. Chem. 92 (1980) 155–168. [17] G. Wang, Y. Weng, D. Chu, R. Chen, D. Xie, Developing a polysulfone-based alkaline anion exchange membrane for improved ionic conductivity, J. Membr. Sci. 332 (2009) 63–68. [18] G. Wang, Y. Weng, J. Zhao, R. Chen, D. Xie, Preparation of a functional poly(ether imide) membrane for potential alkaline fuel cell applications: chloromethylation, J. Appl. Polym. Sci. 112 (2009) 721–727. [19] X. Li, Q. Liu, Y. Yu, Y. Meng, Quaternized poly(arylene ether) ionomers containing triphenyl methane groups for alkaline anion exchange membranes, J. Mater. Chem. A 1 (2013) 4324–4335. [20] H. Zhang, Z. Zhou, Alkaline polymer electrolyte membranes from quaternized poly (phthalazinone ether ketone) for direct methanol fuel cell, J. Appl. Polym. Sci. 110 (2008) 1756–1762. [21] C. Yan, S. Zhang, D. Yang, X. Jian, Preparation and characterization of chloromethylated/quaternized poly (phthalazinone ether sulfone ketone) for positively charged nanofiltration membranes, J. Appl. Polym. Sci. 107 (2008) 1809–1816. [22] Q.H. Zeng, Q.L. Liu, I. Broadwell, A.M. Zhu, Y. Xiong, X.P. Tu, Anion exchange membranes based on quaternized polystyrene-block-poly(ethylene-ran-butylene)block-polystyrene for direct methanol alkaline fuel cells, J. Membr. Sci. 349 (2010) 237–243. [23] D.F. Stamatialis, B.J. Papenburg, M. Gironés, S. Saiful, S.N. Bettahalli, S. Schmitmeier, M. Wessling, Medical applications of membranes: drug delivery, artificial organs and tissue engineering, J. Membr. Sci. 308 (2008) 1–34. [24] N. Wang, C. Wu, Y. Cheng, T.W. Xu, Organic–inorganic hybrid anion exchange hollow fiber membranes: a novel device for drug delivery, Int. J. Pharm. 408 (2011) 39–49. [25] W. Kujawski, G. Poźniak, Transport properties of ion-exchange membranes during pervaporation of water-alcohol mixtures, Sep. Sci. Technol. 40 (2005) 2277–2295. [26] R.B. Merrifield, Solid phase synthesis (Nobel lecture), Angew. Chem. Int. Ed. Eng. 24 (1985) 799–810. [27] M. Kumar, S. Singh, V.K. Shahi, Cross-linked poly(vinyl alcohol) − poly(acrylonitrile-co-2-dimethylamino ethylmethacrylate) based anion-exchange membranes in aqueous media, J. Phys. Chem. B 114 (2010) 198–206. [28] C. Larchet, L. Dammak, B. Auclair, S. Parchikov, V. Nikonenko, A simplified procedure for ion-exchange membrane characterisation, New J. Chem. 28 (2004) 1260–1267. [29] J. Sun Koo, N.-S. Kwak, T.S. Hwang, Synthesis and properties of an anion-exchange membrane based on vinylbenzyl chloride–styrene–ethyl methacrylate copolymers, J. Membr. Sci. 423–424 (2012) 293–301.
17
[30] T.W. Xu, W. Yang, Fundamental studies of a new series of anion exchange membranes: membrane preparation and characterization, J. Membr. Sci. 190 (2001) 159–166. [31] L. Wu, T.W. Xu, W. Yang, Fundamental studies of a new series of anion exchange membranes: membranes prepared through chloroacetylation of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) followed by quaternary amination, J. Membr. Sci. 286 (2006) 185–192. [32] J. Yan, M.A. Hickner, Anion exchange membranes by bromination of benzylmethylcontaining poly(sulfone)s, Macromolecules 43 (2010) 2349–2356. [33] C.H. Zhao, Y. Gong, Q.L. Liu, Q.G. Zhang, A.M. Zhu, Self-crosslinked anion exchange membranes by bromination of benzylmethyl-containing poly(sulfone)s for direct methanol fuel cells, Int. J. Hydrog. Energy 37 (2012) 11383–11393. [34] J. Yan, M.A. Hickner, Anion exchange membranes by bromination of benzylmethylcontaining poly (sulfone) s, Macromolecules 43 (2010) 2349–2356. [35] M.R. Hibbs, C.H. Fujimoto, C.J. Cornelius, Synthesis and characterization of poly(phenylene)-based anion exchange membranes for alkaline fuel cells, Macromolecules 42 (2009) 8316–8321. [36] Z. Liu, X. Li, K. Shen, P. Feng, Y. Zhang, X. Xu, W. Hu, Z. Jiang, B. Liu, M.D. Guiver, Naphthalene-based poly (arylene ether ketone) anion exchange membranes, J. Mater. Chem. A 1 (2013) 6481–6488. [37] G. Merle, M. Wessling, K. Nijmeijer, Anion exchange membranes for alkaline fuel cells: a review, J. Membr. Sci. 377 (2011) 1–35. [38] G.P.W. Kujawski, Q.T. Nguyen, J. Néel, Properties of interpolymer PESS ion-exchange membranes in contact with solvents of different polarities, Sep. Purif. Technol. 32 (1997) 1657–1667. [39] Z.A.S.M. Hosseini, M. Nemati, F. Parvizian, S.S. Madaeni, Electrodialysis heterogeneous ion exchange membranes modified by SiO2 nanoparticles: fabrication and electrochemical characterization, Water Sci. Technol. 73 (2016) 2074–2084. [40] F. Iacomi, V. Craciun, S. Popescu, K. Mueller, M. Lattuada, Y. Dzyazko, L. Rozhdestveskaya, Y. Zmievskii, Y. Volfkovich, V. Sosenkin, N. Nikolskaya, S. Vasilyuk, V. Myronchuk, V. Belyakov, The selected papers of 10th international conference on physics of advanced materials, ICPAM-10Heterogeneous membranes modified with nanoparticles of inorganic ion-exchangers for whey demineralization, Mater. Today: Proc. 2 (2015) 3864–3873. [41] J. Pan, Y. He, L. Wu, C. Jiang, B. Wu, A.N. Mondal, C. Cheng, T.W. Xu, Anion exchange membranes from hot-pressed electrospun QPPO–SiO2 hybrid nanofibers for acid recovery, J. Membr. Sci. 480 (2015) 115–121. [42] M.I. Khan, A.N. Mondal, B. Tong, C. Jiang, K. Emmanuel, Z. Yang, L. Wu, T.W. Xu, Development of BPPO-based anion exchange membranes for electrodialysis desalination applications, Desalination 391 (2016) 61–68. [43] Z.L.T.W. Xu, Y. Li, W. Yang, Preparation and characterization of type II anion exchange membranes frompoly(2,6-dimethyl-1,4-phenylene oxide) (PPO), J. Membr. Sci. 320 (2008) 332–339. [44] M.I. Khan, A.N. Mondal, C. Cheng, J. Pan, K. Emmanuel, L. Wu, T.W. Xu, Porous BPPObased membranes modified by aromatic amine for acid recovery, Sep. Purif. Technol. 157 (2016) 27–34. [45] Y. He, L. Wu, J. Pan, Y. Zhu, X. Ge, Z. Yang, J. Ran, T.W. Xu, A mechanically robust anion exchange membrane with high hydroxide conductivity, J. Membr. Sci. 504 (2016) 47–54. [46] M.I. Khan, L. Wu, M.M. Hossain, J. Pan, J. Ran, A.N. Mondal, T.W. Xu, Preparation of diffusion dialysis membrane for acid recovery via a phase-inversion method, Membr. Water Treat. 6 (2015) 365–378. [47] M.M. Hossain, L. Wu, Y. Li, L. Ge, T.W. Xu, Preparation of porous poly(vinylidene fluoride) membranes with acrylate particles for electrodialysis application, Sep. Purif. Technol. 150 (2015) 102–111. [48] C.G. Arges, L. Wang, M.-s. Jung, V. Ramani, Mechanically stable poly (arylene ether) anion exchange membranes prepared from commercially available polymers for alkaline electrochemical devices, J. Electrochem. Soc. 162 (2015) F686–F693. [49] Y. Wu, C. Wu, M. Gong, T.W. Xu, New anion exchanger organic–inorganic hybrids and membranes from a copolymer of glycidylmethacrylate and -methacryloxypropyl trimethoxy silane, J. Appl. Polym. Sci. 102 (2006) 3580–3589. [50] C. Wu, Y. Wu, J. Luo, T.W. Xu, Y. Fu, Anion exchange hybrid membranes from PVA and multi-alkoxy silicon copolymer tailored for diffusion dialysis process, J. Membr. Sci. 356 (2010) 96–104. [51] Y. Li, T.W. Xu, M. Gong, Fundamental studies of a new series of anion exchange membranes: membranes prepared from bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) and pyridine, J. Membr. Sci. 279 (2006) 200–208. [52] J. Luo, C. Wu, Y. Wu, T.W. Xu, Diffusion dialysis of hydrochloride acid at different temperatures using PPO–SiO 2 hybrid anion exchange membranes, J. Membr. Sci. 347 (2010) 240–249. [53] Y. Wu, C. Wu, Y. Li, T.W. Xu, Y. Fu, PVA–silica anion-exchange hybrid membranes prepared through a copolymer crosslinking agent, J. Membr. Sci. 350 (2010) 322–332. [54] Z. Zhang, L. Wu, J. Varcoe, C. Li, A.L. Ong, S. Poynton, T.W. Xu, Aromatic polyelectrolytes via polyacylation of pre-quaternized monomers for alkaline fuel cells, J. Mater. Chem. A 1 (2013) 2595–2601. [55] J. Ran, L. Wu, X. Lin, L. Jiang, T.W. Xu, Synthesis of soluble copolymers bearing ionic graft for alkaline anion exchange membrane, RSC Adv. 2 (2012) 4250–4257. [56] C. Jiang, Q. Wang, Y. Li, Y. Wang, T.W. Xu, Water electro-transport with hydrated cations in electrodialysis, Desalination 365 (2015) 204–212. [57] D.B. Spry, A. Goun, K. Glusac, D.E. Moilanen, M.D. Fayer, Proton transport and the water environment in Nafion fuel cell membranes and AOT reverse micelles, J. Am. Chem. Soc. 129 (2007) 8122–8130.
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M.I. Khan et al. / Desalination 402 (2017) 10–18
[58] C. Wang, C. Wu, Y. Wu, J. Gu, T.W. Xu, Polyelectrolyte complex/PVA membranes for diffusion dialysis, J. Hazard. Mater. 261 (2013) 114–122. [59] T. Chakrabarty, A.K. Singh, V.K. Shahi, Zwitterionic silica copolymer based crosslinked organic-inorganic hybrid polymer electrolyte membranes for fuel cell applications, RSC Adv. 2 (2012) 1949–1961. [60] C. Cheng, Z. Yang, Y. He, A.N. Mondal, E. Bakangura, T.W. Xu, Diffusion dialysis membranes with semi-interpenetrating network for acid recovery, J. Membr. Sci. 493 (2015) 645–653. [61] J. Ran, L. Wu, J.R. Varcoe, A.L. Ong, S.D. Poynton, T.W. Xu, Development of imidazolium-type alkaline anion exchange membranes for fuel cell application, J. Membr. Sci. 415–416 (2012) 242–249. [62] A.N. Mondal, C. Cheng, Z. Yao, J. Pan, M.M. Hossain, M.I. Khan, Z. Yang, L. Wu, T.W. Xu, Novel quaternized aromatic amine based hybrid PVA membranes for acid recovery, J. Membr. Sci. 490 (2015) 29–37. [63] A. Noshay, L. Robeson, Sulfonated polysulfone, J. Appl. Polym. Sci. 20 (1976) 1885–1903.
[64] S. Singh, A. Jasti, M. Kumar, V.K. Shahi, A green method for the preparation of highly stable organic-inorganic hybrid anion-exchange membranes in aqueous media for electrochemical processes, Polym. Chem. 1 (2010) 1302–1312. [65] J.-H. Cheng, Y.-C. Xiao, C. Wu, T.-S. Chung, Chemical modification of P84 polyimide as anion-exchange membranes in a free-flow isoelectric focusing system for protein separation, Chem. Eng. J. 160 (2010) 340–350. [66] M.A. Khan, M. Kumar, Z.A. Alothman, Preparation and characterization of organic– inorganic hybrid anion-exchange membranes for electrodialysis, J. Ind. Eng. Chem. 21 (2015) 723–730. [67] C. Klaysom, S.-H. Moon, B.P. Ladewig, G.M. Lu, L. Wang, The influence of inorganic filler particle size on composite ion-exchange membranes for desalination, J. Phys. Chem. C 115 (2011) 15124–15132. [68] D. Aguado, T. Montoya, J. Ferrer, A. Seco, Relating ions concentration variations to conductivity variations in a sequencing batch reactor operated for enhanced biological phosphorus removal, Environ. Model. Softw. 21 (2006) 845–851.