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Surface modification of composite ion exchange membranes by polyaniline
Accepted Manuscript Surface modification of composite ion exchange membranes by polyaniline H. Farrokhzad, M.R. Moghbeli, T. Van Gerven, B. Van der Bruggen PII: DOI: Reference:
S1381-5148(14)00163-1 http://dx.doi.org/10.1016/j.reactfunctpolym.2014.08.003 REACT 3420
To appear in:
Reactive & Functional Polymers
Received Date: Revised Date: Accepted Date:
9 May 2014 10 August 2014 18 August 2014
Please cite this article as: H. Farrokhzad, M.R. Moghbeli, T. Van Gerven, B. Van der Bruggen, Surface modification of composite ion exchange membranes by polyaniline, Reactive & Functional Polymers (2014), doi: http:// dx.doi.org/10.1016/j.reactfunctpolym.2014.08.003
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Surface modification of composite ion exchange membranes by polyaniline
H. Farrokhzad1, M.R. Moghbeli2, T.Van Gerven1, B. Van der Bruggen1* 1. Department of Chemical Engineering, Laboratory of Process Engineering for Sustainable Systems (ProcESS), KU Leuven, Leuven, Belgium 2. Chemical Engineering, Iran University of Science and Technology (IUST) * Email: [email protected]; Phone: +32 16 322340
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Abstract The aim of this work is to develop new selective cation exchange membranes (CEMs) from bivalent to monovanet selectivity bysurface functional groups. So, a novel hybrid cation exchange membrane was prepared by polymerization of polyaniline on a composite membrane, made of polyvinylidene fluoride (PVDF) and sulfonated PVDF (S-PVDF). Polyaniline was doped with different doping agents and their effect on total salt extraction and selectivity of bivalent to monovalent cations was evaluated. The chemical and morphological properties of hybrid membranes were characterized by Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD) and scanning electron microscopy (SEM). Ion exchange capacity (IEC), transport number, ion conductivity and water uptake decreased after surface polymerization. Composite membrane has a good selectivity (~2) for bivalent and excellent removal of both cations. The hybrid membrane, doped with pTSA has a very high selectivity for monovalent ions (~7.1) and a high removal of monovalent ions.
Keywords
Polyaniline, surface polymerization, sulfonated PVDF, cation exchange membrane, cation selectivity
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1. Introduction Electrodialysis (ED) is a separation technology in which ions are removed by an alternating arrangement of ion-exchange membranes (IEMs) in a direct electrical field. Among the different basic elements in an ED set-up, IEMs play a key role by allowing selective passage of counter-ions and inhibiting passage of co-ions [1]. In the synthesis of cation-exchange membranes (CEMs), sulfonation of polymers is a common method to provide the required ion exchange capacity (IEC) and cation conductivity in membranes [2-4]. In general, most sulfonated polymers do not possess all the required properties for an excellent CEM, because of the trade-off between IEC improvement and enhanced swelling of membranes after sulfonation [5]. A novel membrane based on sulfonated polyvinylidene fluoride (S-PVDF) was proposed as a promising route for CEM synthesis [6]. One of the important potential applications of ED is selective ion permeation [7]. The selective extraction of ions has many applications as Sodium Chloride (NaCl) concentration for the chlor-alkali industry [8], removal of specific cations from industrial waste water treatment [9] and acid effluents [10], hydrometallurgy [11], water softening [12], etc. Polyaniline (PANi) is an interesting conductive polymer because of its ease of synthesis and chemical stability [13]. PANi polymerization is used for surface modification of IEMs but usually for CEMs [14, 15]. Easy doping/dedoping of PANi with acid/base provides a controlled level of doping by using dopants of different sizes and shapes [16]. The influence of polymerization time and surface formed PANi density on electrochemical properties of membranes have been well-investigated by previous works [14, 15], however, the effect of different doping agents on the membrane performance and especially on cation selectivity has not yet been evaluated. Furthermore, although many studies have been carried out to modify IEMs and evaluate the selective ion transport, the effect of membrane chemistry and morphology on ion permeation through membranes remains unclear. In this work, the influence of PANi surface polymerization with different doping agents on the extraction and selective permeation of cations was investigated, using Na+ and Mg2+ as the monovalent and bivalent cations. The aim is to define a procedure to change in membrane selectivity by keeping the removal of selected cation as high as possible. 3
2. Experimental 2.1.
Materials
PVDF (grade Solef® 6020) was purchased from Solvay. Other materials that were used include 1, 2-dichloroethane from Merck, N-Methyl-2-pyrrolidone (NMP) and aniline (99.8% purity) from Acros, ammonium peroxodisulfate (APS) from ChemLab and chlorosulfonic acid (≥95%) from Fluka. Camphorsulfonic acid (CSA) (98%), para-toluene sulfonic acid (pTSA) (ACS reagent, ≥98.5%), dodecylbenzenesulfonic acid (DBSA) (mixture of isomers, ≥95%) and sulfuric acid 95-97% were purchased from Sigma-Aldrich. Hydrochoric acid (HCl) 1 N, sodium hydroxide (NaOH) (beads with 97% purity) and ammonia 32% (NH4OH) were purchased from VWR chemicals. 2.2.
Synthesis of sulfonated PVDF (S-PVDF)
The sulfonation of PVDF was carried out according to our previous work [6]. 10 g of PVDF powder was mixed in 50 ml chlorosulfonic acid at 80 °C for 45 min. After reaction, the obtained product was precipitated first in 1,2- dichloroethane and then in deionized water. The precipitate was filtered and washed with 1,2dichloroethane and deionized water. The resulting S-PVDF was then dried in a dynamic vacuum oven at 50 °C for 1 h. 2.3.
Synthesis of S-PVDF/PVDF composite membrane
As described elsewhere [6], composite membranes were prepared by using the codissolution method in NMP. A 10% solution of S-PVDF in NMP and a 10% solution of PVDF in NMP were prepared separately. The S-PVDF solution was then added to the PVDF solution under stirring in an appropriate amount to form S-PVDF/PVDF blend with a share of 70% S-PVDF. The blended solution was casted on a glass plate with a casting knife and then dried in a dynamic vacuum oven at 60 °C overnight. Then, the synthesized membrane was peeled off by immersion of the glass plate in water and was put in a 1 M NaCl solution for at least 24 h to reach equilibrium condition (maximum exchange of Na+ and membrane proton).
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2.4.
Surface polymerization of PANi
The membrane with 70% S-PVDF was selected to be modified by surface polymerization. This selection is because of the best cation removal that achieved by this composition in our past work [6]. The polymerization of aniline on the composite membrane was performed by adapting the procedure in the literature [15, 35]. By immersion of a CEM that equilibrated with aqueous anilinum sulfate solution in aqueous ammonium peroxodisulfate solution, aniline polymerizes as a thin layer only on the membrane surface because peroxodisulfate ions cannot enter into the membrane matrix due to Donnan exclusion [7]. The hybrid membranes were synthesized according to the steps shown in Figure 1. Fig. 1.
The membranes then were immersed in a 1 M salt solution (0.5 M MgCl2 + 0.5 M NaCl) to reach the equilibrium condition for 48 h before testing in the electrodialysis system. The membranes are denoted as S-70, S-CSA, S-pTSA and S-DBSA, referring to S-PVDF/PVDF composite membrane with 70% S-PVDF, composite membrane coated by PANi-CSA, PANi-pTSA and PANi-DBSA, respectively. 2.5.
Characterization techniques
To determine the polymerization of PANi with different doping agents on the surface of S-70 membrane, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used. The prepared membranes were characterized by different methods to evaluate the performance of hybrid membranes for selective desalination. For this purpose, the ion exchange capacity (IEC), transport number, permselectivity, ion selectivity, ion conductivity, water uptake and contact angle were measured. 2.5.1.1.
Functional groups
The functional groups of synthesized membranes were analyzed by using a Perkin– Elmer FTIR spectrometer (Spectrum 100) in the range from 4000 to 650 cm–1. 2.5.1.2.
Morphology 5
Scanning electron microscope (SEM) ((Philips XL30 FEG, the Netherlands) was used to visualize morphology of the membranes. SEM images were taken in high vacuum condition at 20 kV. The compactness and crystallinity of membranes were evaluated by X-Ray diffraction (XRD) method. The XRD studies were performed on a Philips X-ray diffractometer (PW1830 model) using a CuKα radiation source (1.542 Å). The scan range (2θ) was 5-50° at a scan rate of 2°min-1. 2.5.1.3.
Water uptake
All membranes were dried in vacuum at 60 °C for 4 h before testing. The sample membranes were soaked in deionized water at room temperature for 24 h. The water remaining on the surface of the wetted membrane was removed using tissue paper before weighing. The water uptake was calculated by: Eq.1 Uptake = [(wwet-wdry)/ wdry] × 100% where wwet and wdry were the masses of dried and wet samples, respectively. 2.5.1.4.
Contact angle
The contact angles of water on membranes were determined at room temperature with a Kruss device (DSA 10-MK2, Germany) according to the plate method. At least three angles were measured and averaged for every sample. 2.5.1.5.
Transport number and permselectivity
The counter-ion transport number in the membrane phase (t+m) was estimated from membrane potential data using Eq. 2: Eq.2 ∆E = 2t − 1 where α1 and α2 are electrovalence of the J K−1 mol−1), F is the temperature in Kelvin
RT α ln nF α
the mean activities of electrolytic solutions, n is the counter-ion, R is the universal gas constant (~8.314 Faraday constant (~96 485.3 C mol−1) and T is the [19]. The membrane potential was measured by a two 6
compartment cell, in which a membrane with 22 cm2 effective area separated two NaCl solutions of different concentrations (0.01M and 0.1 M). The diffusion potential difference across the membrane was measured using a voltmeter connected to a calomel reference electrode. The two solutions in the chambers were mixed vigorously to avoid fouling on the membrane surface. During the experiment, fresh solutions were continuously fed into the chambers while excess solutions were discharged. The membranes were immersed in a 1M NaCl solution for at least 12 h to reach equilibrium, before measurement of the potential. The permselectivity (PS) is a measure for the preferable transport of counter-ions in the membrane, thus, it is related to the ion fluxes in the membrane. The permselectivity is defined as: Eq.3 =
− 1 −
where t+sol is the solution transport number of the counter ion in the membrane (with no concentration profiles in the solution) [20, 21]. Sodium chloride has a transport number of approximately 0.39 for its cation in water at 25 ºC [22]. 2.5.1.6.
Ion conductivity
For the measurement of conductivity, following the procedures described earlier [21], a clip cell was used. This cell is composed of two graphite electrodes fixed in a plexiglass container with an effective area of 22 cm2. The membrane was first equilibrated in 1 M NaCl before being placed in the cell. The resistance of the membranes was measured at room temperature by impedance spectroscopy using a digital LCR meter (HP 4262A) at a frequency of 1500 kHz in 0.5 M NaCl solution. Then, the membrane resistance (Rmem) was calculated by subtraction of the electrolyte resistance (here Rsol is the resistance of electrolyte in the cell measured without any membrane) from the membrane resistance equilibrated in electrolyte solution (Rcell), according to the equation Rmem = Rcell − Rsol. Based on the electrical resistance measurement, the conductivity (σ, Scm−1) of the membranes was calculated according to Eq. 4: Eq. 4
7
=
where L is the thickness of the membrane (cm), and A is the effective area of the membrane (cm2). 2.5.1.7.
Ion exchange capacity (IEC)
The ion exchange capacity of the membranes was estimated by a conventional titration method [23]. A piece of membrane was exchanged with H+ by immersing the sample in 50 ml of 1 M HCl solution for 24 h. The membrane was washed with distilled water until neutral pH to remove any excess of H+ ions. Then, the sample was dried at 55°C overnight in vacuum and weighted (Wdry in g). Afterwards, the membrane was immersed in 25 ml of 2M NaCl solution during 24 h to exchange the H+ with Na+. The resultant solution was then titrated by an aqueous solution of 0.01M (CNaOH) NaOH (equivalent volume = VNaOH in ml). The ion exchange capacity was expressed in milli-equivalents of sulfonic groups per gram of dry polymer (meq/g) and was obtained by the following equation: Eq. 5 IEC = (CNaOH × VNaOH) / (Wdry) 2.5.1.8.
Electrodialysis (ED)
For the ED experiments, a Berghof BEL-500 system was used, which included two anion exchange membranes (AEM) and two cation exchange membranes (CEM). The CEM was in contact with the anode in order to prevent Cl2 production (2Cl‾→Cl2+2e‾) at the anode [24]. The effective surface area of ED stack was 58 cm2 for each membrane. The equipment consisted of three separated circuits for the diluate, the concentrate and electrode rinsing solutions, each with a volume of 3 L and recirculated by a separate centrifugal pump. The initial concentration in the diluate and concentrate compartments was 0.01 M NaCl + 0.01 M MgCl2; the concentration of Na2SO4 in the rinsing circuit was 0.1 M. The membrane stack was connected to a DC electrical potential through TiO2-coated titanium electrodes. During a 2 h experiment, the voltage was kept constant at 5 V. The synthesized membranes were cut in 10 cm×10 cm pieces and perforated to fit in the ED system.
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The ED performance was evaluated by conductivity measurement (using a CDM 83 conductivity meter) of the diluate each 30 min. The ion selectivity was calculated as [14]: Eq. 6
%$P"# =
t $%⁄t "# C$%⁄C"#
where tMg and tNa are the transport numbers of the divalent cation (Mg) and the monovalent cation (Na) in the membrane, and CMg and CNa are the average concentrations of magnesium and sodium during electrodialysis. The concentrations in the initial and final sample were measured by Atomic Absorption Spectroscopy (AAS) (Perkin Elmer AAnalyst 100). 3. Results and Discussion 3.1.
Functional groups
The spectra in Fig. 2 show the characteristics peaks of the conducting form of PANi and of the dopants on the S-70 composite membrane. Fig. 2.
The band ~ 3301 cm-1 is the interchain hydrogen-bonded N–H band between amine and imine groups, while the weak band ~ 3409 cm-1 is due to free N–H vibration. The peaks observed at around 2800–3000 cm-1 can be assigned to the hydrocarbon stretching of the dopants [25]. The peak ~ 1593 cm-1 is attributed to C=N stretching vibration (quinoid unit) and the band at ~1501 cm-1 is due to C=C stretching vibration (benzenoid unit) [26]. These peaks prove the emeraldine structure of PANi. The ratio of quinoid/ benzenoid differs for different dopants according to doping amount. The peak ~ 1040 cm-1 is for SO3- of doping agents [27] and overlaps with the sulfonic group of S-PVDF. The peak ~ 1650 cm-1 is attributed to C=O group that was seen in base S-PVDF as a side reaction [6], but its intensity is more significant for PANi-CSA as a characteristic peak [37]. The peaks observed around 2800–2900 cm-1 can be assigned to the hydrocarbon stretching of the DBSA dopant [25]. 3.2.
Morphology 9
SEM pictures of the surface and cross section of composite and hybrid membranes are presented in Fig. 3. As shown in cross section pictures, a completely dense layer is formed for each membrane, but it is not possible to see a distinct layer for PANi in hybrid membranes. This is due to very thin layer thickness of formed PANi. However, the thin layer of PANi is formed on the rough surface of S-70 and filled the small pores which changed the surface morphology to a smooth surface. This can be seen in surface pictures of hybrid membranes. Fig.3.
The functional groups of PANi obtained from FTIR and change in surface morphology of hybrid membranes knew by SEM, demonstrate the surface polymerization of aniline on top of composite membranes For a deeper investigation of the membrane structure, X-ray diffraction was used to evaluate chain ordering and crystallinity in composite and hybrid membranes. The XRD patterns for composite and PANi coated membranes with different dopants are given in Fig. 4. Fig.4.
Apart from the small crystalline peak for PANi-pTSA (at 2θ ~ 14.2), no distinctive crystal structure was observed. Warren and coworkers [36] found that the ratio of half-width to height (HW/H) of the X-ray diffraction peak reflects ordering in the polymer backbone. The smaller the value of HW/H is, the higher the ordering [28]. Therefore, the values of HW/H were determined for the different membranes (Table 1). As known, different doping agents can change the XRD pattern of PANi [29, 30]. This effect can be seen in the coated membranes. PANi-CSA increased the ordering a little bit, but PANi-DBSA and PANi-pTSA changed the structure to a microcrystalline and more compact form and even a small crystalline peak was observed for PANi-pTSA. Table.1.
3.3. Water uptake & contact angle Water uptake is an important property that influenced on dimensional stability and cation selectivity of membrane. Table 2 summarizes the water uptake and contact angle of different membranes. The water uptake is influenced by the degree of 10
sulfonation, pretreatment of the membrane, hydration state, ambient relative humidity and water temperature [31]. Table.2.
By coating the composite membrane, water uptake decreased for all hybrid membranes. Since the sulfonation degree of all composite membranes was equal, the surface PANi layer can play the main role. This layer can act as a barrier for diffusion of water inside the membrane, while it can be affected by the surface morphology and hydrophilicity. Because PANi-CSA has a less ordered structure and low contact angle, its water uptake does not substantially decrease. PANiDBSA has a higher contact angle because of the hydrophobicity of the long chain alkyl group DBSA, leading to a lower water uptake than in case of PANi-CSA. PANi-pTSA is more hydrophilic than PANi-DBSA, because of its more crystalline structure, and therefore exhibits the lowest water diffusion and water uptake. In Fig. 5, the chemical structure of doping agents is illustrated. Fig.5.
3.4.
IEC & ion conductivity
The IEC and ion conductivity are given in Table 3. IEC values are not significantly decreased for hybrid membranes compared to the composite membrane. This indicates that the concentration of fixed charges on the membrane matrix does not change significantly after PANi coating for different doping agents. However, the conductivity declined for all hybrid membranes. This is due to the formation of a dense layer on the membrane surface with lower charge density than the matrix, thus preventing the facile migration of cations through the membrane. The decrease in water uptake after polymerization can also influence total cation transport through the hybrid membranes. This states that the surface polymerization of PANi even after doping can decrease ion conductivity drastically. Table.3.
3.5.
Transport number and permselectivity
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The transport number and permselectivity are presented in Fig. 5. The membrane potential equation proposed by Meyer-Severs-Teorell [32] is a sum of the Donnan potential at both sides of the membrane and the diffusion potential through the membrane. As mentioned for IEC, coating of a PANi layer has a negligible influence on cation exchange groups and consequently on the Donnan potential, but a significant effect on morphology and on the diffusion potential. A small decrease for PANi-CSA and a huge decrease for PANi-DBSA and PANi-CSA confirms this hypothesis. Also, the surface hydrophilicity is increased for PANiCSA, which facilitates the cation passage through the membrane. Fig.6.
3.6.
Electrodialysis (ED)
In the final step, the performance of the synthesized membranes was evaluated during electrodialysis (ED) by measuring the change in solution conductivity and cation removal in the diluate after 2 h, from which the cation selectivity was calculated. Fig.7.
As presented in Fig. 6, S-70 and PANi-CSA membranes have the same overall salt removal, and perform better than PANi-pTSA and PANi-DBSA. The flux of Na+ and Mg2+ through the membranes is shown in Fig. 7. The concentration change with time (dCcation/dt) in the diluate compartment was calculated as: Eq. 7 J)#*+,- =
V dC)#*+,-⁄dt A
where V is the volume of the circulated solution and A is the membrane area. Fig.8.
Furthermore, the removal of each cation by membranes is provided in Fig. 8. Fig.9. 12
As shown in Fig. 8, the selectivity of Mg2+ to Na+ was evaluated for different
%$membranes and quantified in Table. 4. S-70 has a good selectivity, but P"# is lower for hybrid membranes. The selectivity achieved for PANi-pTSA was found to be the opposite of the other membranes, i.e., it has a high selectivity for the "# monovalent cation (i.e. P$% ≈1/0.14≈7.1). Table.4.
To explain the selectivity of these membranes, the diffusion theory of cations through a CEM can be applied. Firdaous et al. [33] argue that cations diffuse in a membrane by their hydration shell, but the passing of a solution/membrane interface requires overcoming the energy barrier caused by the necessity of partial dehydration of ions. The effective parameter of this barrier breakage is the hydration energy of cations. This can be expressed by the ratio hydrated-ion-hop/ not-hydrated-ion-hop, which is equal to 0.10 and 5.65 for Na+ and Mg2+, respectively. Thus, Na+ cations intend to jump inside the membrane in their dehydrated state, because of the friction of their hydration shell against the polymer matrix of the membrane; while diffusion of Mg2+ in their hydrated shell is more possible. This effect was confirmed in the case of monovalent-ion-selective membranes by hydrophobization of the membrane surface. Van der Bruggen et al. [34] indicated that the ion size may have an additional effect; the larger the ion, the more its transport through the membrane is sterically hindered. As mentioned in Table 4, Na+ is smaller in hydrated state. The membrane structure can have an effect on this parameter. Table.5.
The S-70 membrane, with higher water uptake and a more hydrophilic surface with a less ordered structure provides easier conditions for Mg2+ hop and diffusion and so shows a good selectivity for Mg2+. However, the flux of both cations is too high in this membrane and consequently it has a high cation removal. PANi-CSA has nearly the same structure as S-70 and a more hydrophilic surface, but its water uptake is less, so the diffusion of Na+ is more facile because of more hydrophilic surface and less friction of the hydrated shell compared to Mg2+.
%$Therefore, P"# decreases but the total flux is close to that of S-70. 13
PANi-DBSA has a lower flux for both cations. This may be due to its more hydrophobic surface and ordered structure and the presence of long chain alkyls that prevent the diffusion of cations. However, the diffusion of Na+ is comparable to that of Mg2+, because of its smaller radius and better hop from the hydrophobic surface barrier. Selectivity decreases compared to S-70. This membrane gives the lowest removal for both cations. An attractive result was obtained for PANi-pTSA, particularly in view of the high selectivity for the monovalent cation. pTSA is not as large as DBSA and does not hinder cations to be close to the membrane; the surface is more hydrophilic. This membrane has a crystalline structure on the surface and less water inside the membrane to prevent the diffusion of Mg2+ and Na+.
4. Conclusion Novel hybrid cation exchange membranes were synthesized by surface polymerization of polyaniline (PANi) on S-PVDF/PVDF composite membranes with different doping agents. FTIR and SEM proved the formation of a PANi layer on the membrane surface, leading to a change in surface morphology and functional groups. XRD showed that after surface polymerization, the hybrid membrane structure becomes more ordered than that of the composite membrane, especially for pTSA and DBSA doping agents. The combination of structure information by other membrane properties as water uptake, IEC and permselectivity helped to determine the relation between membrane properties and ED performance. The results show that S-70 is a bivalent selective membrane with high salt extraction. After surface polymerization this selectivity decreases, but the salt extraction remains high for PANi-CSA. PANi-pTSA has a very high selectivity for monovalent cations with relatively high extraction of Na+. All the results are attributed to the membrane structure and water affinity, more than to any other property.
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[27] W. Pan, L. Qu, Y. Chen, “Conductive blends of dodecylbenzene sulfonic acid-doped polyaniline with poly (vinyl pyrrolidone)”, Optoelectronics and Advanced Materials- Rapid Communications, 2010, 4, 2123 – 2128 [28] H. Farrokhzad, T. Van Gerven, B. Van der Bruggen, “Preparation and characterization of a conductive polyaniline/ polysulfone film and evaluation of the effect of co-solvent”, European Polymer Journal, 2013, 49, 3234–3243 [29] S. J. Varma, F. Xavier, S. Varghese, S. Jayalekshmi, “Synthesis and studies of exceptionallycrystalline polyaniline thin films”, Polymer International, 2012, 61, 743–748 [30] H. K. Chaudhari, D. S. Kelkar, “X-ray Diffraction Study of Doped Polyaniline”, Journal of Applied Polymer Science, 1996, 62, 15-18 [31] H. L. Wu, Ch. Ch. M. Ma, F. Y. Liu, Ch. Y. Chen, Sh. J. Lee, Ch. L. Chiang, “Preparation and characterization of poly(ether sulfone)/sulfonated poly(ether ether ketone) blend membranes “,European Polymer Journal, 2006, 42, 1688–1695 [32] T. Sata, “Ion Exchange Membranes: Preparation, Characterization, Modification and Application”, The Royal Society of Chemistry, UK, 2003 [33] L. Firdaous, J. P. Malériat, J. P. Schlumpf & F. Quéméneur,” Transfer of Monovalent and Divalent Cations in Salt Solutions by Electrodialysis”, Separation Science and Technology, 2007, 42, 931–948 [34] B. Van der Bruggen, A. Koninckx, C. Vandecasteele, “Separation of monovalent and divalent ions from aqueous solution by electrodialysis and nanofiltration”, Water Research, 2004, 38, 1347–1353 [35] G. Ćirić-Marjanovića, E. N. Konyushenkob, M. Trchováb, J. Stejskal, “Chemical oxidative polymerization of anilinium sulfate versus aniline: Theory and experiment”, Synthetic Metals, 2008, 158, 200-211. [36] L.F. Warren, J.A. Walker, D.P. Anderson, C.G. Rhodes, L.J. BuckleyJ. “A Study of Conducting Polymer Morphology The Effect of Dopant Anions Upon Order”, Journal of Electrochemical Society, 1989, 136, 2286-2295
18
[37] G. Wu, H. Zhang, “Synthesis and Characterization of Camphor Sulfonic Acid Fully Doped Polyaniline”, Research and application of Material, 2013, 1, 5-8
19
Figures:
Put the membranes in 0.1 M H2SO4 + 5% V/V aniline/water for 1 h
Wash membranes with deionized water
Put in aqueous solution of 0.5 M APS for 4 h (by shaking)
Wash membranes with deionized water
Put in 0.1 M ammonia solution to dedope for 12 h Wash membranes with deionized water
Put in 0.1 M acid solution (CSA, pTSA, DBSA) to redope for 12 h
Fig. 1. Procedure for surface polymerization of PANi on 70% S-PVDF composite membrane (hybrid membrane synthesis diagram)
20
Fig. 2. FTIR spectra of composite membrane (S-70) and hybrid membranes coated with PANi doped with different doping agents (PANi-CSA, PANi-DBSA, PANi-pTSA)
21
Fig. 3. SEM images of membranes surface (a1-d1) and cross section (a2-d2); a1, a2: S-70 composite membrane, b1, b2: PANi-CSA, c1, c2: PANi-DBSA, d1, d2: PANi-pTSA
Fig.4. X-ray diffraction patterns of composite membrane (S-70) and hybrid membranes with different dopants (PANi-CSA, PANi-DBSA, PANi-pTSA)
22
Fig. 5. Chemical structure of different doping agents
Fig. 6. Transport number and permselectivity for composite and hybrid membranes
23
Dluate Conductivity (µS/cm)
2500 2000 S-70
1500
PANi-DBSA PANi-pTSA
1000
PANi-CSA
500 0 0
20
40
60
80
100
120
140
Time (min) Fig. 7. Conductivity measurement of diluate for different membranes by electrodialysis
Fig. 8.Partial flux of Na+ and Mg2+ cations and total cation flux through different membranes (106 mol cm-2 s-1)
24
Fig. 9. Cation removal percentage for different membranes
Tables: Table. 1. X-ray diffraction pattern analysis of composite and hybrid membranes amorphous peak (°) S-70 20.7 PANi-CSA 20.4 PANi-pTSA 20.5 PANi-DBSA 20.6
d /amorphous (Å) 4.28 4.35 4.33 4.31
W (°) 10.3 12.2 18.0 16.2
H (CPS) 64 88 237 238
HW/H 0.080 0.069 0.038 0.034
Table 2. Water uptake and contact angle of S-PVDF/PVDF membranes S-70
PANi-CSA
PANi- DBSA
PANi- pTSA
Water uptake (%)
13.7
9.5
7.7
4.3
Contact angle (º)
66.9
45.4
70.6
60
Table 3. IEC and ion conductivity of S-PVDF/PVDF membranes 25
S-70
PANi-CSA
PANi- DBSA
PANi- pTSA
IEC (meq/g)
0.64
0.51
0.54
0.55
Ion conductivity (mS/cm)
7.2
2.7
0.6
1.1
Table. 4. Cation selectivity for composite and hybrid membranes Membrane
%$P"#
S-70
PANi-CSA
PANi-pTSA
PANi-DBSA
2.01
1.28
0.14
1.26
Table. 5. Physio-chemical properties of studied cations [32] Hydrated ionic radius (nm)
Hydration Energy (kJ/mol)
Na+
0.365
407
Mg2+
0.429
1921
26
S1381-5148(14)00163-1 http://dx.doi.org/10.1016/j.reactfunctpolym.2014.08.003 REACT 3420
To appear in:
Reactive & Functional Polymers
Received Date: Revised Date: Accepted Date:
9 May 2014 10 August 2014 18 August 2014
Please cite this article as: H. Farrokhzad, M.R. Moghbeli, T. Van Gerven, B. Van der Bruggen, Surface modification of composite ion exchange membranes by polyaniline, Reactive & Functional Polymers (2014), doi: http:// dx.doi.org/10.1016/j.reactfunctpolym.2014.08.003
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Surface modification of composite ion exchange membranes by polyaniline
H. Farrokhzad1, M.R. Moghbeli2, T.Van Gerven1, B. Van der Bruggen1* 1. Department of Chemical Engineering, Laboratory of Process Engineering for Sustainable Systems (ProcESS), KU Leuven, Leuven, Belgium 2. Chemical Engineering, Iran University of Science and Technology (IUST) * Email: [email protected]; Phone: +32 16 322340
1
Abstract The aim of this work is to develop new selective cation exchange membranes (CEMs) from bivalent to monovanet selectivity bysurface functional groups. So, a novel hybrid cation exchange membrane was prepared by polymerization of polyaniline on a composite membrane, made of polyvinylidene fluoride (PVDF) and sulfonated PVDF (S-PVDF). Polyaniline was doped with different doping agents and their effect on total salt extraction and selectivity of bivalent to monovalent cations was evaluated. The chemical and morphological properties of hybrid membranes were characterized by Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD) and scanning electron microscopy (SEM). Ion exchange capacity (IEC), transport number, ion conductivity and water uptake decreased after surface polymerization. Composite membrane has a good selectivity (~2) for bivalent and excellent removal of both cations. The hybrid membrane, doped with pTSA has a very high selectivity for monovalent ions (~7.1) and a high removal of monovalent ions.
Keywords
Polyaniline, surface polymerization, sulfonated PVDF, cation exchange membrane, cation selectivity
2
1. Introduction Electrodialysis (ED) is a separation technology in which ions are removed by an alternating arrangement of ion-exchange membranes (IEMs) in a direct electrical field. Among the different basic elements in an ED set-up, IEMs play a key role by allowing selective passage of counter-ions and inhibiting passage of co-ions [1]. In the synthesis of cation-exchange membranes (CEMs), sulfonation of polymers is a common method to provide the required ion exchange capacity (IEC) and cation conductivity in membranes [2-4]. In general, most sulfonated polymers do not possess all the required properties for an excellent CEM, because of the trade-off between IEC improvement and enhanced swelling of membranes after sulfonation [5]. A novel membrane based on sulfonated polyvinylidene fluoride (S-PVDF) was proposed as a promising route for CEM synthesis [6]. One of the important potential applications of ED is selective ion permeation [7]. The selective extraction of ions has many applications as Sodium Chloride (NaCl) concentration for the chlor-alkali industry [8], removal of specific cations from industrial waste water treatment [9] and acid effluents [10], hydrometallurgy [11], water softening [12], etc. Polyaniline (PANi) is an interesting conductive polymer because of its ease of synthesis and chemical stability [13]. PANi polymerization is used for surface modification of IEMs but usually for CEMs [14, 15]. Easy doping/dedoping of PANi with acid/base provides a controlled level of doping by using dopants of different sizes and shapes [16]. The influence of polymerization time and surface formed PANi density on electrochemical properties of membranes have been well-investigated by previous works [14, 15], however, the effect of different doping agents on the membrane performance and especially on cation selectivity has not yet been evaluated. Furthermore, although many studies have been carried out to modify IEMs and evaluate the selective ion transport, the effect of membrane chemistry and morphology on ion permeation through membranes remains unclear. In this work, the influence of PANi surface polymerization with different doping agents on the extraction and selective permeation of cations was investigated, using Na+ and Mg2+ as the monovalent and bivalent cations. The aim is to define a procedure to change in membrane selectivity by keeping the removal of selected cation as high as possible. 3
2. Experimental 2.1.
Materials
PVDF (grade Solef® 6020) was purchased from Solvay. Other materials that were used include 1, 2-dichloroethane from Merck, N-Methyl-2-pyrrolidone (NMP) and aniline (99.8% purity) from Acros, ammonium peroxodisulfate (APS) from ChemLab and chlorosulfonic acid (≥95%) from Fluka. Camphorsulfonic acid (CSA) (98%), para-toluene sulfonic acid (pTSA) (ACS reagent, ≥98.5%), dodecylbenzenesulfonic acid (DBSA) (mixture of isomers, ≥95%) and sulfuric acid 95-97% were purchased from Sigma-Aldrich. Hydrochoric acid (HCl) 1 N, sodium hydroxide (NaOH) (beads with 97% purity) and ammonia 32% (NH4OH) were purchased from VWR chemicals. 2.2.
Synthesis of sulfonated PVDF (S-PVDF)
The sulfonation of PVDF was carried out according to our previous work [6]. 10 g of PVDF powder was mixed in 50 ml chlorosulfonic acid at 80 °C for 45 min. After reaction, the obtained product was precipitated first in 1,2- dichloroethane and then in deionized water. The precipitate was filtered and washed with 1,2dichloroethane and deionized water. The resulting S-PVDF was then dried in a dynamic vacuum oven at 50 °C for 1 h. 2.3.
Synthesis of S-PVDF/PVDF composite membrane
As described elsewhere [6], composite membranes were prepared by using the codissolution method in NMP. A 10% solution of S-PVDF in NMP and a 10% solution of PVDF in NMP were prepared separately. The S-PVDF solution was then added to the PVDF solution under stirring in an appropriate amount to form S-PVDF/PVDF blend with a share of 70% S-PVDF. The blended solution was casted on a glass plate with a casting knife and then dried in a dynamic vacuum oven at 60 °C overnight. Then, the synthesized membrane was peeled off by immersion of the glass plate in water and was put in a 1 M NaCl solution for at least 24 h to reach equilibrium condition (maximum exchange of Na+ and membrane proton).
4
2.4.
Surface polymerization of PANi
The membrane with 70% S-PVDF was selected to be modified by surface polymerization. This selection is because of the best cation removal that achieved by this composition in our past work [6]. The polymerization of aniline on the composite membrane was performed by adapting the procedure in the literature [15, 35]. By immersion of a CEM that equilibrated with aqueous anilinum sulfate solution in aqueous ammonium peroxodisulfate solution, aniline polymerizes as a thin layer only on the membrane surface because peroxodisulfate ions cannot enter into the membrane matrix due to Donnan exclusion [7]. The hybrid membranes were synthesized according to the steps shown in Figure 1. Fig. 1.
The membranes then were immersed in a 1 M salt solution (0.5 M MgCl2 + 0.5 M NaCl) to reach the equilibrium condition for 48 h before testing in the electrodialysis system. The membranes are denoted as S-70, S-CSA, S-pTSA and S-DBSA, referring to S-PVDF/PVDF composite membrane with 70% S-PVDF, composite membrane coated by PANi-CSA, PANi-pTSA and PANi-DBSA, respectively. 2.5.
Characterization techniques
To determine the polymerization of PANi with different doping agents on the surface of S-70 membrane, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used. The prepared membranes were characterized by different methods to evaluate the performance of hybrid membranes for selective desalination. For this purpose, the ion exchange capacity (IEC), transport number, permselectivity, ion selectivity, ion conductivity, water uptake and contact angle were measured. 2.5.1.1.
Functional groups
The functional groups of synthesized membranes were analyzed by using a Perkin– Elmer FTIR spectrometer (Spectrum 100) in the range from 4000 to 650 cm–1. 2.5.1.2.
Morphology 5
Scanning electron microscope (SEM) ((Philips XL30 FEG, the Netherlands) was used to visualize morphology of the membranes. SEM images were taken in high vacuum condition at 20 kV. The compactness and crystallinity of membranes were evaluated by X-Ray diffraction (XRD) method. The XRD studies were performed on a Philips X-ray diffractometer (PW1830 model) using a CuKα radiation source (1.542 Å). The scan range (2θ) was 5-50° at a scan rate of 2°min-1. 2.5.1.3.
Water uptake
All membranes were dried in vacuum at 60 °C for 4 h before testing. The sample membranes were soaked in deionized water at room temperature for 24 h. The water remaining on the surface of the wetted membrane was removed using tissue paper before weighing. The water uptake was calculated by: Eq.1 Uptake = [(wwet-wdry)/ wdry] × 100% where wwet and wdry were the masses of dried and wet samples, respectively. 2.5.1.4.
Contact angle
The contact angles of water on membranes were determined at room temperature with a Kruss device (DSA 10-MK2, Germany) according to the plate method. At least three angles were measured and averaged for every sample. 2.5.1.5.
Transport number and permselectivity
The counter-ion transport number in the membrane phase (t+m) was estimated from membrane potential data using Eq. 2: Eq.2 ∆E = 2t − 1 where α1 and α2 are electrovalence of the J K−1 mol−1), F is the temperature in Kelvin
RT α ln nF α
the mean activities of electrolytic solutions, n is the counter-ion, R is the universal gas constant (~8.314 Faraday constant (~96 485.3 C mol−1) and T is the [19]. The membrane potential was measured by a two 6
compartment cell, in which a membrane with 22 cm2 effective area separated two NaCl solutions of different concentrations (0.01M and 0.1 M). The diffusion potential difference across the membrane was measured using a voltmeter connected to a calomel reference electrode. The two solutions in the chambers were mixed vigorously to avoid fouling on the membrane surface. During the experiment, fresh solutions were continuously fed into the chambers while excess solutions were discharged. The membranes were immersed in a 1M NaCl solution for at least 12 h to reach equilibrium, before measurement of the potential. The permselectivity (PS) is a measure for the preferable transport of counter-ions in the membrane, thus, it is related to the ion fluxes in the membrane. The permselectivity is defined as: Eq.3 =
− 1 −
where t+sol is the solution transport number of the counter ion in the membrane (with no concentration profiles in the solution) [20, 21]. Sodium chloride has a transport number of approximately 0.39 for its cation in water at 25 ºC [22]. 2.5.1.6.
Ion conductivity
For the measurement of conductivity, following the procedures described earlier [21], a clip cell was used. This cell is composed of two graphite electrodes fixed in a plexiglass container with an effective area of 22 cm2. The membrane was first equilibrated in 1 M NaCl before being placed in the cell. The resistance of the membranes was measured at room temperature by impedance spectroscopy using a digital LCR meter (HP 4262A) at a frequency of 1500 kHz in 0.5 M NaCl solution. Then, the membrane resistance (Rmem) was calculated by subtraction of the electrolyte resistance (here Rsol is the resistance of electrolyte in the cell measured without any membrane) from the membrane resistance equilibrated in electrolyte solution (Rcell), according to the equation Rmem = Rcell − Rsol. Based on the electrical resistance measurement, the conductivity (σ, Scm−1) of the membranes was calculated according to Eq. 4: Eq. 4
7
=
where L is the thickness of the membrane (cm), and A is the effective area of the membrane (cm2). 2.5.1.7.
Ion exchange capacity (IEC)
The ion exchange capacity of the membranes was estimated by a conventional titration method [23]. A piece of membrane was exchanged with H+ by immersing the sample in 50 ml of 1 M HCl solution for 24 h. The membrane was washed with distilled water until neutral pH to remove any excess of H+ ions. Then, the sample was dried at 55°C overnight in vacuum and weighted (Wdry in g). Afterwards, the membrane was immersed in 25 ml of 2M NaCl solution during 24 h to exchange the H+ with Na+. The resultant solution was then titrated by an aqueous solution of 0.01M (CNaOH) NaOH (equivalent volume = VNaOH in ml). The ion exchange capacity was expressed in milli-equivalents of sulfonic groups per gram of dry polymer (meq/g) and was obtained by the following equation: Eq. 5 IEC = (CNaOH × VNaOH) / (Wdry) 2.5.1.8.
Electrodialysis (ED)
For the ED experiments, a Berghof BEL-500 system was used, which included two anion exchange membranes (AEM) and two cation exchange membranes (CEM). The CEM was in contact with the anode in order to prevent Cl2 production (2Cl‾→Cl2+2e‾) at the anode [24]. The effective surface area of ED stack was 58 cm2 for each membrane. The equipment consisted of three separated circuits for the diluate, the concentrate and electrode rinsing solutions, each with a volume of 3 L and recirculated by a separate centrifugal pump. The initial concentration in the diluate and concentrate compartments was 0.01 M NaCl + 0.01 M MgCl2; the concentration of Na2SO4 in the rinsing circuit was 0.1 M. The membrane stack was connected to a DC electrical potential through TiO2-coated titanium electrodes. During a 2 h experiment, the voltage was kept constant at 5 V. The synthesized membranes were cut in 10 cm×10 cm pieces and perforated to fit in the ED system.
8
The ED performance was evaluated by conductivity measurement (using a CDM 83 conductivity meter) of the diluate each 30 min. The ion selectivity was calculated as [14]: Eq. 6
%$P"# =
t $%⁄t "# C$%⁄C"#
where tMg and tNa are the transport numbers of the divalent cation (Mg) and the monovalent cation (Na) in the membrane, and CMg and CNa are the average concentrations of magnesium and sodium during electrodialysis. The concentrations in the initial and final sample were measured by Atomic Absorption Spectroscopy (AAS) (Perkin Elmer AAnalyst 100). 3. Results and Discussion 3.1.
Functional groups
The spectra in Fig. 2 show the characteristics peaks of the conducting form of PANi and of the dopants on the S-70 composite membrane. Fig. 2.
The band ~ 3301 cm-1 is the interchain hydrogen-bonded N–H band between amine and imine groups, while the weak band ~ 3409 cm-1 is due to free N–H vibration. The peaks observed at around 2800–3000 cm-1 can be assigned to the hydrocarbon stretching of the dopants [25]. The peak ~ 1593 cm-1 is attributed to C=N stretching vibration (quinoid unit) and the band at ~1501 cm-1 is due to C=C stretching vibration (benzenoid unit) [26]. These peaks prove the emeraldine structure of PANi. The ratio of quinoid/ benzenoid differs for different dopants according to doping amount. The peak ~ 1040 cm-1 is for SO3- of doping agents [27] and overlaps with the sulfonic group of S-PVDF. The peak ~ 1650 cm-1 is attributed to C=O group that was seen in base S-PVDF as a side reaction [6], but its intensity is more significant for PANi-CSA as a characteristic peak [37]. The peaks observed around 2800–2900 cm-1 can be assigned to the hydrocarbon stretching of the DBSA dopant [25]. 3.2.
Morphology 9
SEM pictures of the surface and cross section of composite and hybrid membranes are presented in Fig. 3. As shown in cross section pictures, a completely dense layer is formed for each membrane, but it is not possible to see a distinct layer for PANi in hybrid membranes. This is due to very thin layer thickness of formed PANi. However, the thin layer of PANi is formed on the rough surface of S-70 and filled the small pores which changed the surface morphology to a smooth surface. This can be seen in surface pictures of hybrid membranes. Fig.3.
The functional groups of PANi obtained from FTIR and change in surface morphology of hybrid membranes knew by SEM, demonstrate the surface polymerization of aniline on top of composite membranes For a deeper investigation of the membrane structure, X-ray diffraction was used to evaluate chain ordering and crystallinity in composite and hybrid membranes. The XRD patterns for composite and PANi coated membranes with different dopants are given in Fig. 4. Fig.4.
Apart from the small crystalline peak for PANi-pTSA (at 2θ ~ 14.2), no distinctive crystal structure was observed. Warren and coworkers [36] found that the ratio of half-width to height (HW/H) of the X-ray diffraction peak reflects ordering in the polymer backbone. The smaller the value of HW/H is, the higher the ordering [28]. Therefore, the values of HW/H were determined for the different membranes (Table 1). As known, different doping agents can change the XRD pattern of PANi [29, 30]. This effect can be seen in the coated membranes. PANi-CSA increased the ordering a little bit, but PANi-DBSA and PANi-pTSA changed the structure to a microcrystalline and more compact form and even a small crystalline peak was observed for PANi-pTSA. Table.1.
3.3. Water uptake & contact angle Water uptake is an important property that influenced on dimensional stability and cation selectivity of membrane. Table 2 summarizes the water uptake and contact angle of different membranes. The water uptake is influenced by the degree of 10
sulfonation, pretreatment of the membrane, hydration state, ambient relative humidity and water temperature [31]. Table.2.
By coating the composite membrane, water uptake decreased for all hybrid membranes. Since the sulfonation degree of all composite membranes was equal, the surface PANi layer can play the main role. This layer can act as a barrier for diffusion of water inside the membrane, while it can be affected by the surface morphology and hydrophilicity. Because PANi-CSA has a less ordered structure and low contact angle, its water uptake does not substantially decrease. PANiDBSA has a higher contact angle because of the hydrophobicity of the long chain alkyl group DBSA, leading to a lower water uptake than in case of PANi-CSA. PANi-pTSA is more hydrophilic than PANi-DBSA, because of its more crystalline structure, and therefore exhibits the lowest water diffusion and water uptake. In Fig. 5, the chemical structure of doping agents is illustrated. Fig.5.
3.4.
IEC & ion conductivity
The IEC and ion conductivity are given in Table 3. IEC values are not significantly decreased for hybrid membranes compared to the composite membrane. This indicates that the concentration of fixed charges on the membrane matrix does not change significantly after PANi coating for different doping agents. However, the conductivity declined for all hybrid membranes. This is due to the formation of a dense layer on the membrane surface with lower charge density than the matrix, thus preventing the facile migration of cations through the membrane. The decrease in water uptake after polymerization can also influence total cation transport through the hybrid membranes. This states that the surface polymerization of PANi even after doping can decrease ion conductivity drastically. Table.3.
3.5.
Transport number and permselectivity
11
The transport number and permselectivity are presented in Fig. 5. The membrane potential equation proposed by Meyer-Severs-Teorell [32] is a sum of the Donnan potential at both sides of the membrane and the diffusion potential through the membrane. As mentioned for IEC, coating of a PANi layer has a negligible influence on cation exchange groups and consequently on the Donnan potential, but a significant effect on morphology and on the diffusion potential. A small decrease for PANi-CSA and a huge decrease for PANi-DBSA and PANi-CSA confirms this hypothesis. Also, the surface hydrophilicity is increased for PANiCSA, which facilitates the cation passage through the membrane. Fig.6.
3.6.
Electrodialysis (ED)
In the final step, the performance of the synthesized membranes was evaluated during electrodialysis (ED) by measuring the change in solution conductivity and cation removal in the diluate after 2 h, from which the cation selectivity was calculated. Fig.7.
As presented in Fig. 6, S-70 and PANi-CSA membranes have the same overall salt removal, and perform better than PANi-pTSA and PANi-DBSA. The flux of Na+ and Mg2+ through the membranes is shown in Fig. 7. The concentration change with time (dCcation/dt) in the diluate compartment was calculated as: Eq. 7 J)#*+,- =
V dC)#*+,-⁄dt A
where V is the volume of the circulated solution and A is the membrane area. Fig.8.
Furthermore, the removal of each cation by membranes is provided in Fig. 8. Fig.9. 12
As shown in Fig. 8, the selectivity of Mg2+ to Na+ was evaluated for different
%$membranes and quantified in Table. 4. S-70 has a good selectivity, but P"# is lower for hybrid membranes. The selectivity achieved for PANi-pTSA was found to be the opposite of the other membranes, i.e., it has a high selectivity for the "# monovalent cation (i.e. P$% ≈1/0.14≈7.1). Table.4.
To explain the selectivity of these membranes, the diffusion theory of cations through a CEM can be applied. Firdaous et al. [33] argue that cations diffuse in a membrane by their hydration shell, but the passing of a solution/membrane interface requires overcoming the energy barrier caused by the necessity of partial dehydration of ions. The effective parameter of this barrier breakage is the hydration energy of cations. This can be expressed by the ratio hydrated-ion-hop/ not-hydrated-ion-hop, which is equal to 0.10 and 5.65 for Na+ and Mg2+, respectively. Thus, Na+ cations intend to jump inside the membrane in their dehydrated state, because of the friction of their hydration shell against the polymer matrix of the membrane; while diffusion of Mg2+ in their hydrated shell is more possible. This effect was confirmed in the case of monovalent-ion-selective membranes by hydrophobization of the membrane surface. Van der Bruggen et al. [34] indicated that the ion size may have an additional effect; the larger the ion, the more its transport through the membrane is sterically hindered. As mentioned in Table 4, Na+ is smaller in hydrated state. The membrane structure can have an effect on this parameter. Table.5.
The S-70 membrane, with higher water uptake and a more hydrophilic surface with a less ordered structure provides easier conditions for Mg2+ hop and diffusion and so shows a good selectivity for Mg2+. However, the flux of both cations is too high in this membrane and consequently it has a high cation removal. PANi-CSA has nearly the same structure as S-70 and a more hydrophilic surface, but its water uptake is less, so the diffusion of Na+ is more facile because of more hydrophilic surface and less friction of the hydrated shell compared to Mg2+.
%$Therefore, P"# decreases but the total flux is close to that of S-70. 13
PANi-DBSA has a lower flux for both cations. This may be due to its more hydrophobic surface and ordered structure and the presence of long chain alkyls that prevent the diffusion of cations. However, the diffusion of Na+ is comparable to that of Mg2+, because of its smaller radius and better hop from the hydrophobic surface barrier. Selectivity decreases compared to S-70. This membrane gives the lowest removal for both cations. An attractive result was obtained for PANi-pTSA, particularly in view of the high selectivity for the monovalent cation. pTSA is not as large as DBSA and does not hinder cations to be close to the membrane; the surface is more hydrophilic. This membrane has a crystalline structure on the surface and less water inside the membrane to prevent the diffusion of Mg2+ and Na+.
4. Conclusion Novel hybrid cation exchange membranes were synthesized by surface polymerization of polyaniline (PANi) on S-PVDF/PVDF composite membranes with different doping agents. FTIR and SEM proved the formation of a PANi layer on the membrane surface, leading to a change in surface morphology and functional groups. XRD showed that after surface polymerization, the hybrid membrane structure becomes more ordered than that of the composite membrane, especially for pTSA and DBSA doping agents. The combination of structure information by other membrane properties as water uptake, IEC and permselectivity helped to determine the relation between membrane properties and ED performance. The results show that S-70 is a bivalent selective membrane with high salt extraction. After surface polymerization this selectivity decreases, but the salt extraction remains high for PANi-CSA. PANi-pTSA has a very high selectivity for monovalent cations with relatively high extraction of Na+. All the results are attributed to the membrane structure and water affinity, more than to any other property.
14
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Figures:
Put the membranes in 0.1 M H2SO4 + 5% V/V aniline/water for 1 h
Wash membranes with deionized water
Put in aqueous solution of 0.5 M APS for 4 h (by shaking)
Wash membranes with deionized water
Put in 0.1 M ammonia solution to dedope for 12 h Wash membranes with deionized water
Put in 0.1 M acid solution (CSA, pTSA, DBSA) to redope for 12 h
Fig. 1. Procedure for surface polymerization of PANi on 70% S-PVDF composite membrane (hybrid membrane synthesis diagram)
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Fig. 2. FTIR spectra of composite membrane (S-70) and hybrid membranes coated with PANi doped with different doping agents (PANi-CSA, PANi-DBSA, PANi-pTSA)
21
Fig. 3. SEM images of membranes surface (a1-d1) and cross section (a2-d2); a1, a2: S-70 composite membrane, b1, b2: PANi-CSA, c1, c2: PANi-DBSA, d1, d2: PANi-pTSA
Fig.4. X-ray diffraction patterns of composite membrane (S-70) and hybrid membranes with different dopants (PANi-CSA, PANi-DBSA, PANi-pTSA)
22
Fig. 5. Chemical structure of different doping agents
Fig. 6. Transport number and permselectivity for composite and hybrid membranes
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Dluate Conductivity (µS/cm)
2500 2000 S-70
1500
PANi-DBSA PANi-pTSA
1000
PANi-CSA
500 0 0
20
40
60
80
100
120
140
Time (min) Fig. 7. Conductivity measurement of diluate for different membranes by electrodialysis
Fig. 8.Partial flux of Na+ and Mg2+ cations and total cation flux through different membranes (106 mol cm-2 s-1)
24
Fig. 9. Cation removal percentage for different membranes
Tables: Table. 1. X-ray diffraction pattern analysis of composite and hybrid membranes amorphous peak (°) S-70 20.7 PANi-CSA 20.4 PANi-pTSA 20.5 PANi-DBSA 20.6
d /amorphous (Å) 4.28 4.35 4.33 4.31
W (°) 10.3 12.2 18.0 16.2
H (CPS) 64 88 237 238
HW/H 0.080 0.069 0.038 0.034
Table 2. Water uptake and contact angle of S-PVDF/PVDF membranes S-70
PANi-CSA
PANi- DBSA
PANi- pTSA
Water uptake (%)
13.7
9.5
7.7
4.3
Contact angle (º)
66.9
45.4
70.6
60
Table 3. IEC and ion conductivity of S-PVDF/PVDF membranes 25
S-70
PANi-CSA
PANi- DBSA
PANi- pTSA
IEC (meq/g)
0.64
0.51
0.54
0.55
Ion conductivity (mS/cm)
7.2
2.7
0.6
1.1
Table. 4. Cation selectivity for composite and hybrid membranes Membrane
%$P"#
S-70
PANi-CSA
PANi-pTSA
PANi-DBSA
2.01
1.28
0.14
1.26
Table. 5. Physio-chemical properties of studied cations [32] Hydrated ionic radius (nm)
Hydration Energy (kJ/mol)
Na+
0.365
407
Mg2+
0.429
1921
26