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Sulfonated reduced graphene oxide modification layers to improve monovalent anions selectivity and controllable resistance of anion exchange membrane
Journal of Membrane Science 536 (2017) 167–175
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Sulfonated reduced graphene oxide modification layers to improve monovalent anions selectivity and controllable resistance of anion exchange membrane
MARK
Yan Zhaoa, Kaini Tanga, Huimin Ruana, Lixin Xueb, B. Van der Bruggenc, Congjie Gaoa, ⁎ Jiangnan Shena, a b c
Center for Membrane Separation and Water Science & Technology, Ocean College, Zhejiang University of Technology, Hangzhou 310014, PR China Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
A R T I C L E I N F O
A B S T R A C T
Keywords: Sulfonated graphene oxide Reduced graphene oxide Anion exchange membrane Monovalent selectivity Membrane modification
Graphene oxide with lamellar structure has attracted research interest in various fields. In this study, sulfonated reduced graphene oxide (S-rGO) nanosheets with negatively charged sulfonic acid groups were synthesized via a facile distillation-precipitation polymerization, followed by hydrazine reduction. The sulfanilic acid was grafted on the graphene oxide sheets to separate GO nanosheets each other and provide anion channels for anions selectivity. These nanosheets were used to modify anion exchange membranes (AEMs), and to enhance the membrane monovalent anions selectivity and the modification layer conductivity, in order to meet industrial requirements. The permselectivity and separation efficiency were used to evaluate selectivities of the modified membranes. The results show that the unmodified AEM has no monovalent selectivity, while the permselectivity and separation efficiency of S-rGO modified AEMs (reduced by hydrazine hydrate steam in 10 min) increases from 0.65 to 1.80 and from −0.13 to 0.31 (in 40 min), respectively; and from 0.72 to 2.30 and from −0.07 to 0.28 (in 80 min), respectively.
1. Introduction Anion exchange membranes (AEMs) are one of the core elements of the electrodialysis process, which is usually applied to produce drinking water from seawater and brackish water and to produce salt from seawater [1–4]. Multifunctional, high performance AEMs are of interest to treat wastewater and to be applied in fuel cells [5–9]. To meet the purity requirements for specific anions in water treatment, AEMs allowing for separating monovalent from multivalent anions contained in solution is a primary challenge [10]. Generally, the monovalent selectivity of an AEM is determined by the affinity in equilibrium conditions between the fixed charges of the membrane and the counterions present in the solution [11,12]. The structure and pore size of AEMs are important factors to impede the transport of anions with larger size [11,13–15]. The hydrated anion radius in the solution also determines the transport rate [16]. In addition, under the application of current density conditions, a diffusion boundary layer which close to the surface of AEM, can favor the specific anions transport from the bulk solution to the surface of AEM [14,17]. The above key factors are to be considered to improve the
⁎
Corresponding author.
http://dx.doi.org/10.1016/j.memsci.2017.05.002 Received 9 January 2017; Received in revised form 11 April 2017; Accepted 1 May 2017 Available online 04 May 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
membrane permselectivity for monovalent anions in salt mixtures. Several approaches to obtain monovalent ion selectivity have been developed: bio-inspired artificial single ion pumps and ion channels [13,18], the formation of a thin layer on the membrane surface [19] and layer-by-layer modification of AEMs [20]. These methods create 'intelligent' ion transport channels or form a negatively charged polymer on the membrane surface. A novel AEM is developed through poly (sodium 4-styrene sulfonate) and hydroxypropyltrimethyl ammonium chloride chitosan alternate electro-deposition to enhance the monovalent selectivity was demonstrated in our previous work [21]. However, a higher membrane area resistance is unavoidable, which may limit these modified membranes application in industry. For any material used to modify AEMs, a high permselectivity and conductivity is required. Graphene, a typical two-dimensional (2D) carbon nanostructure, has been widely studied for its excellent electrical conductivity and high mechanical strength properties [22,23]. Graphene oxide (GO) sheets have a high specific surface area and can be separated from each other [24–26]. Due to the presence of oxygencontaining functional groups (such as hydroxyl, epoxy, carbonyl, and carboxyl groups) in GO sheets, it can be well dispersed in water [27].
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by using ultrasone in 40 kHz. Then, the mixture was kept in the oil bath at 60 °C under stirring at 750 rpm. After that, 48 mg of sulfanilic acid was added to the mixture. 12 h later, the samples were washed by pure water three times and then, dried in a blast oven at 60 °C. 8 h later, the dry S-GO was obtained.
Besides, the oxygen-containing functional groups allow GO to be manipulated in water and to be assembled in thin films such as nanochannels with a narrow size [27,28]. Furthermore, GO nanosheets can also be grafted by –NH2 groups and obtain a high proton conduction ability of AEM [29–32]. When GO incorporates -SO3groups, the GO nanosheets will separate from one another to some distance due to the interaction of electrostatic repulsion [33,34]. Polymers with a large number of -SO3- groups are often applied as a coating on a membrane surface to form a thin layer that enhances the selectivity of anions [10,35–37]. Recently, reduced graphene oxide (rGO), reduced by hydrazine, was demonstrated to have an excellent electrochemical performance and is used to improve the conductivity of GO membranes [27,38–40]. In addition, due to the presence of -SO3- groups, a surface with a negatively charged layer is obtained, which will enhance the monovalent anions selectivity [19,41]. In the current approach, a functionalized sulfonated graphene oxide (S-GO) was successfully synthesized and anchored on the surface of a commercial AEM to enhance the anion selectivity. Then the S-GO was further reduced (S-rGO) by hydrazine hydrate steam to improve the conductivity of the modification layer. This requires four main consecutive steps: (1) GO dispersion in water; (2) sulfanilic acid was chose and grafted on the GO sheets to synthesis of S-GO; (3) based on the electrostatic adsorption, attachment of S-GO on an AEM surface; (4) reduction of the S-GO modified AEM by the hydrazine hydrate steam. After the above steps, an S-rGO modified AEM is obtained. The monovalent selectivity of an S-rGO modified AEM was investigated and compared to the unmodified AEM by galvanostatic selectivity experiments of Cl- and SO42-. The results show that the S-rGO modified AEM has an excellent selectivity for monovalent anions, and a low resistance induced by the additional layer.
2.3. Membrane modification of S-rGO The commercial original AEM was first rinsed with pure water to remove impurities from the membrane surface. As shown in Scheme 1b, 200 mg of S-GO was well dispersed in 100 mL 0.05 M NaCl solution by ultrasonic dispersion. Subsequently, the solution was added to a labmade experimental setup while stirring at 100 rpm for 24 h. After that, a yellow layer is observed on the AEM surface, the lab-made experimental setup as shown in Fig. 1. Then particulate impurities on the surface of S-GO were rinsed off by pure water for 2 h, followed by hydrazine reduction for 10 min (referred to as S-rGO-1 AEM) and 60 min (referred to as S-rGO-2 AEM), respectively (the hydrazine reduction is shown in supporting information, Fig. S1). 2.4. Membrane characterization The change of the functional groups on the membrane surface was monitored by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Nicolet 6700, The United States) at room temperature to provide information about the chemical structure of the commercial and modified membranes. To quantify the elemental composition of the surface of the unmodified AEM, S-GO AEM, SrGO-1 AEM and S-rGO-2 AEM, all the membranes were analyzed by Xray photo-electron spectroscopy (XPS, Kratos AXIS Ultra DLD, Japan). The crystallinity of the composite was obtained by X-ray diffraction (XRD) at room temperature using an X′Pert PRO (PANalytical, the Netherlands). The surface morphology of the membrane was observed by scanning electron microscopy (SEM) (S-4700 Hitachi, Tokyo, Japan) and atomic force microscope (AFM) (Nanoscope V, Bruker Dimension, The United States).
2. Experimental 2.1. Materials and chemicals Potassium permanganate and hydrogen peroxide (30%) were purchased from Lingfeng Chemical Reagent Co. Ltd. (Guangdong, China). Sulfanilic acid (analytical reagent) was purchased from Macklin Biochemical Co. Ltd. (Shanghai, China). Graphite powder (99.95%), Potassium persulfate (K2S2O8), H2SO4, NaCl, Na2SO4 and all other chemicals were analytical reagent and obtained from Aladdin Industrial Co. Ltd. (Shanghai, China). The standard AEM was the commercial JAM-II-07 Homogeneous AEM (Yanrun, China) which was used as the reference membrane. A commercial homogeneous CEM (FUJIFILM, Japan) was used to prevent the leakage of anions. The properties of the commercial AEM and CEM are shown in Table 1.
2.5. ζ-potential of the membranes Zeta potential values of the membranes were auto-measured on the basis of streaming potential measurements (Surpass 3, Anton Paar, Austria) with 60 mg/L KCl as the electrolyte solution at pH from 5.5 to 9. 2.6. Membrane surface area resistance A custom-designed cell (shown in Fig. S2. in Supporting Information) was used to measure the membrane surface resistance. The resistance of membranes was calculated by a membrane resistance measurement setup and calculated according to the following equation
2.2. Synthesis of GO and S-GO
Rn = In a typical procedure, GO was prepared from graphite powder by a modified Hummers' method (the synthesis of GO is shown in Supporting Information) [39,42,43]. A procedure for the preparation of S-GO was shown in Scheme 1a. 200 mg of GO was dispersed in 100 mL pure water
Thickness (μm)
AEM CEM
160–230 135
Ion exchange capacity (mmol g−1)
Area resistance (Ω cm2)
Transport number (%)
1.6–2.0 1.04
3.0–6.5 2.7
90–95 97
(1)
where Rn is the n multilayers resistance of the membrane expressed in Ω cm2, U is the voltage of the membrane and U0 is the voltage of blank expressed in V, I is the constant current through the membrane and insure the current fixed at 0.004 A, S is the membrane effective area in this setup (7.065 cm2).
Table 1 Characteristics of the anion exchange membrane and cation exchange membrane (commercial data). Membrane type Homogeneous
U − U0 ×S I
2.7. Monovalent anions selectivity measurement The selectivity of monovalent anions was measured in a four compartment module (shown in Fig. S3. in Supporting Information). The current density was 5 mA/cm2, which is in the suitable range for these membranes. At the beginning, 0.05 M Na2SO4 and NaCl mixed solution (SO42- and Cl-) was applied in both compartments in contact 168
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Scheme 1. (a) Synthesis of GO and S-GO; (b) membrane modified by S-rGO.
expressed in A cm−2. The flux of ions was obtained from the change in concentration of the ions on the dilute side according to
Stirring rods
dc
Ji =
V · dti A
(4)
where V is the volume of the electrolyte solution in dilute compartment which is 1 L, and A is the active area of the membranes, which was 19.625 cm2. To represent the separation efficiency, Van der Bruggen et al. [44] the separation efficiency S between component A and B was evaluated as:
AEM
AEM
S(t) =
Unmodified AEMs
((cA(t))/(cA(0))) − ((cB(t))/(cB(0))) × 100% (1 −(cA(t))/(cA(0))) + (1 −((cB(t))/(cB(0)))
(5)
where S(t ) is the separation efficiency; cA (0 ) and cB (0 ) are the initial concentration of SO42- and Cl-, respectively; and cA(t ) and cB (t ) are the concentration of SO42- and Cl- at time t.
Fig. 1. The reaction route of S-GO modified AEM, S-rGO-1 AEM (with hydrazine reduction 10 min) and S-rGO-2 AEM (with hydrazine reduction 60 min).
3. Results and discussion with the membrane. The electrode solution was a 0.2 M Na2SO4 solution. In the dilute compartment, the concentration of mixtures of SO42- and Cl- was measured by Anion Chromatography, after 40 min and 80 min. The permselectivity of the membranes between chloride − and sulfate ions (Cl- and SO42-) P Cl 2− was calculated by Eq. (2):
3.1. Characterization of GO, rGO, S-GO and S-rGO Fig. 2a illustrates the ATR-FTIR spectrum of GO, rGO, S-GO and SrGO respectively. A broad band between 3600 cm−1 and 3200 cm−1 was distinguished in the spectra of GO and S-GO, which represents the O-H groups of GO bending vibration. Besides, the spectrum of S-GO shows a higher intensity and lower broadness in the O-H groups. Due to the generation of C-N (amide group) junction between GO and sulfanilic acid molecules and water molecules are formed. The spectrum of GO and S-GO also shows all the characteristic bands corresponding to oxygen functional groups, including the stretching vibrations from C=O (1725 cm−1), C-O (1630 cm−1) and C-O-C (1110 cm−1). When the GO or S-GO was reduced by the hydrazine hydrate steam, the spectra of rGO and S-rGO, the absorption peak at 3430 cm−1 were significantly weakened. At the same time, the absorption peak of C=C, C-O and C-O-C of rGO and S-rGO was significantly smaller. When
SO4
−
P Cl 2− = SO4
tCl− / t SO42− cCl
− /c
SO42−
=
JCl−. c SO42− JSO42−∙cCl−
(2)
where ti is the transport number of the ions through the membrane, Ji is the flux of the ions through the membrane expressed in mol/m2 s and c is the concentration of the ions in the diluate compartment, expressed in mol/L. The ti was calculated using Eq. (3):
ti =
Jz i iF I
(3)
where zi is the ion charge, F is Faraday's constant and I is the DC current 169
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Fig. 2. ATR-FTIR spectra a), XPS spectra b) and XRD patterns spectra c) of the GO, rGO, S-GO and S-rGO, respectively; image of S-rGO d).
degree and will stack again forming bulky materials.[46] This confirms the existence of conversion that GO to rGO and S-GO to S-rGO in some degree. The peaks marked by squares indicate –SO3- on the GO or rGO. Compared to GO or rGO, these peaks appear more in S-GO and S-rGO, which can be related to the grafting of sulfanilic acid. Fig. 2d shows a high resolution XPS spectrum of C 1s (S-rGO) and it could be deconvoluted into three peaks of 284.4 (the sp2 hybridized carbon), 285.8 (C-N) and 288.2 (O=C-O), respectively.
Table 2 Average chemical compositions of the different GO types (limit to C, O, N, S). GO types
GO rGO S-GO S-rGO
Elements (at%) C 1s
O 1s
N 1s
S 2p
68.31 84.37 58.75 75.13
30 13.14 32.19 15.9
0.80 1.75 4.72 4.82
0.89 0.74 4.34 4.15
3.2. Membrane modification of S-rGO and characterization sulfanilic acid was grafted onto the GO surface, the spectra of S-GO and S-rGO showed two new bands at about 1080 cm−1 and 1030 cm−1, which were assigned to the two symmetric stretching vibrations of S=O [45]. Fig. 2b illustrates the average chemical compositions (limited to C, O, N, S) spectrum of GO, rGO, S-GO and S-rGO. The peaks of four elements at 529.7 eV, 397.2 eV, 282.2 eV, and 165.8 eV can be assigned to O 1s, N 1s, C 1s and S 2p, respectively. The corresponding element ratios are shown in Table 2. It can be seen that the O 1s changed from GO to rGO and S-GO to S-rGO, the fraction of O 1s were decreased and the fraction of C 1s were increased. At the same time, compared to GO and S-GO, rGO and S-rGO, the corresponding elemental ratio of N 1S and S 2p were obviously increased, because of the grafting with sulfanilic acid. XRD was utilized to investigate the crystalline structure of the as-prepared GO and S-GO, rGO and S-rGO; the patterns are shown in Fig. 2c. A diffraction peak at 9.71 was observed, and assigned to the (001) peak of GO and marked by triangles. Compared to GO or SGO, a new broad (002) peak appears at 24.21 in the pattern of GO or SGO. This peak at 24.21 means that when the GO was reduced to some
The existence of an S-rGO modification layer on the surface of AEMs was confirmed by ATR-FTIR and XPS. As shown in Fig. 3a, the unmodified AEM, S-GO, S-rGO-1 and S-rGO-2 AEM were investigated using Fourier Transform Infrared Spectroscopy. Compared to the unmodified AEM and modified AEM, the existence of sulfonic acid groups on the modified AEM can be confirmed by the peak at about 1180 cm−1 and 1030 cm−1 (symmetric stretching vibrations of the two S=O of -SO3H).[45] In addition, the spectra of the modified AEMs shows absorbance peaks at 1630 cm−1 and 1100 cm−1 which indicate the vibration of the groups of C-O and C-O-C, respectively. Moreover, the peak at 1630 cm−1 and 1100 cm−1 are different in the S-GO, SrGO-1 and S-rGO-2 AEM. The peaks for S-rGO-1 and S-rGO-2 AEMs are lower than those of the S-GO AEM, which is due to the presence of reduced sulfonated graphene oxide. The chemical compositions (limited to C, O, N and S) of the surface of the unmodified and modified AEMs were further analyzed by XPS, as shown in Fig. 3b; the chemical composition of each element is shown in Table 3. In comparison with the spectrum of the unmodified AEM, the peak intensity of C 1s 170
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Fig. 3. ATR-FTIR spectra a) and XPS spectra b) of the unmodified and modified AEMs.
surface modified with the S-GO layer is higher than that of an unmodified AEM. When the S-GO AEM surface was reduced by hydrazine hydrate steam in 10 min (the S-rGO-1 AEM), as shown in Fig. 5c, the height of the roughness on the surface significantly decreased. The reason is probably because the oxygen-containing functional groups such as hydroxyl, epoxy, carbonyl, and carboxyl groups were reduced by the hydrazine hydrate steam and some of them coated with sulfanilic acid will desquamate. However, as shown in Fig. 5d, when the S-GO modification layer reduction time was 60 min (the S-rGO-2 AEM), the hilly type surface structure was suppressed, but the roughness of the membrane surface significantly increased. The primary reason for this phenomenon was attributed to the stability decrease of the modified layer and the nanosheets partially desquamated through the reduction of hydrazine hydrate steam after a sufficiently long time.
Table 3 Average chemical compositions of the unmodified AEM and S-GO, S-rGO-1, S-rGO-2 modified AEMs (limit to C, O, N, and S). AEM types
Unmodified S-GO S-rGO-1 S-rGO-2
Elements (at%) C 1s
O 1s
N 1s
S 2p
63.09 67.72 76.19 76.18
32.34 26.66 18. 90 18.66
4.34 4.67 4.06 4.50
0.23 0.95 0.86 0.66
increased. The S 2p increased between the unmodified AEM and modified AEMs, due to the modified layer containing –SO3- groups. In addition, the elemental composition was different for the modified AEMs. In comparison with the spectra of the S-GO and S-rGO-1 (or SrGO-2) AEM the peak intensity of O 1s decreased while the peak intensity of C 1s increased, as shown in Fig. 3b. This is due to the S-GO layer reduced by the hydrazine hydrate steam, which formed the S-rGO layer. However, the elemental fraction of C 1s, O 1S, and N 1S were not changed between the S-rGO-1 and S-rGO-2 AEM and the element of the S 2p even decreased. This illustrates that the reduction by the hydrazine hydrate steam also affected the S-rGO layer. The morphology of membranes’ surfaces is shown in Fig. 4 (The cross-section SEM images of the unmodified AEM and modified AEMs are shown in supporting Fig. S5). From the SEM image of unmodified AEM (Fig. 4a), the rugged shape surface can be clearly seen on the commercial homogeneous membrane. Fig. 4b shows the S-GO AEM, the membrane surface of the rugged shape disappears while covers of S-GO layers. Fig. 4c and d display the surface change after the S-GO AEM is reduced by hydrazine hydrate steam. Fig. 4c is the SEM of S-rGO-1 AEM, many bulge region can be distinguished on the surface. Because of the GO nanosheets are grafted with large number of sulfonate groups and reduced by hydrazine hydrate steam, the nanosheets separated from each other while linked by tertiary ammonium groups. However, when the modified layer of the surface is reduced 60 min, many part of the modified layer fall off from the surface (as seen in Fig. 4d, the surface of S-rGO-2 AEM). This phenomenon particularly because of the tertiary ammonium groups and sulfonate groups may decomposed by hydrazine hydrate steam. In Fig. 5, surface roughness of unmodified AEM and modified AEMs are presented in AFM 3D images. The unmodified AEM has smooth and flat membrane surface as shown in Fig. 5a. Compared to the unmodified AEM, the surface of the S-GO modified AEM (shown in Fig. 5b) has a more hilly type surface structure. Thus, the roughness of the AEM
3.3. ζ-potential of unmodified and modified membranes The surface ζ-potential of the membrane describes the interaction of the electrical surface charges with their surroundings. Because of the mutative surroundings, an inversion of membrane surface charge is produced by sequential adsorption. Fig. 6 shows the ζ-potential of the unmodified and modified AEM. The unmodified AEM has a large number of –N+R3 groups, so the ζ-potential remains positive. As can be seen in Fig. 6, the ζ-potential of the unmodified AEM was increased to a maximum value of 0.2 mV (pH=8.5), and then decreases again. For modified AEMs, the ζ-potential of the S-GO modified AEM stays negative; with an increase of the pH, the value increases but always stays negative, as can be seen in Fig. 6. In the pH range considered here, the ζ-potential was always between −0.4 mV and −0.7 mV. When the S-GO modification layer was reduced by the hydrazine hydrate steam for 10 min, as shown in Fig. 6 for the S-rGO-1 AEM, the ζ-potential was even lower than the S-GO AEM. This may be the hydrazine hydrate steam reduced some oxygen energy groups of GO sheet, which make the modification multilayer change and then leads to the ζ-potential value changed. Thus, the ζ-potential remains below −0.9 mV and always lowers than that of the S-GO AEM. However, as can be seen in Fig. 6 for the S-rGO-2 AEM, the ζ-potential of the modified AEM is even higher than the S-GO AEM. When the hydrazine hydrate steam time was 60 min, the -SO3- poly-negatively charged groups may also be strongly affected. It can be concluded that the modified layer with negative charge and the reduction with hydrazine hydrate steam also affects the ζ-potential of the modified layer.
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a)
b)
20μm
20μm
c)
d)
20μm
20μm
Fig. 4. SEM images of the surface of the unmodified AEM a), S-GO modified AEM b), S-rGO-1 modified AEM c) and S-rGO-2 modified AEM d).
3.4. Membrane surface area resistance
3.5. Selectivity to monovalent anions
For electro-driven membranes, the membrane area resistance is an important parameter, which may limit the application of a membrane for ion separation. The membrane area resistance of the unmodified membrane is 3.06 Ω cm2, as shown in Fig. 7. When the membrane coat with S-GO, the resistance of the modified membrane is up to 4.26 Ω cm2. Compared with the unmodified AEM, the increase value of the membrane resistance was 1.20 Ω cm2. However, the resistance of the S-rGO-1 AEM is 3.72 Ω cm2 which means that the increase of the membrane resistance was 0.66 Ω cm2. Compared to S-GO layer, the membrane area resistance of S-rGO-1 AEM was lower than 0.54 Ω cm2. This can be explained in terms of the larger number of oxygencontaining functional groups (such as hydroxyl, epoxy, carbonyl, and carboxyl groups) that were reduced by the hydrazine hydrate steam. Besides, the GO incorporated -SO32- groups can provide additional sites inside the modified layer with the interaction of electrostatic repulsion. Thus, with the S-GO layer reduced by the hydrazine hydrate steam, the resistance of membrane modified layer decreases. When the membrane continued reduction by the hydrazine hydrate steam, as shown in Fig. 7 the S-rGO-2 AEM, the area resistance was 3.37 Ω cm2 and the membrane resistance increase value is only 0.31 Ω cm2. This may be because some of the modified layer would be affected and even desorption from the membrane surface.
To investigate the monovalent selectivity of AEMs, the permselectivities and separation efficiency between Cl- and SO42- ions were tested. In Fig. 8a and b, the concentration of anions (Cl- and SO42- ions) in the diluted compartment is shown at 40 min and 80 min, respectively. At 40 min, for the unmodified AEM, the concentration of SO42ions was lower than Cl- ions while the modified AEMs showed an opposite phenomenon. The same phenomenona were observed at 80 min, as can be seen in Fig. 8b. Compared to Fig. 8a and b, the anions (Cl- or SO42- ions) concentration value of unmodified AEM is decreased more than modified AEMs, which means that the modified AEMs limit the anions transptation. The permselectivity and separation efficiency values were calculated by Eq. (2) and Eq. (5) respectively. In Fig. 8c and d, zone I means the permselectivity was higher than 1 and the separation efficiency was over zero, which reflected the membrane shows the excellent monovalent selectivity. At the same time, zone II shows the permselectivity was lower than 1 and the separation efficiency was in negative value, which reflected the membrane shows the no monovalent selectivity. Fig. 8c and d shows the permselectivity and separation efficiency of Cl- and SO42- ions of unmodified and modified AEMs at 40 min and 80 min. For the unmodified AEM, the permselectivity was lower than 1 and the separation efficiency was negative value (as shown in Fig. 8c and d zone II). This suggests that the unmodified membrane has no monovalent anion selectivity. When the membrane surface was modified with a S-GO layer, the permselectivity 172
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Fig. 5. 3D AFM images of the surface of the unmodified AEM a), S-GO modified AEM b), S-rGO-1 modified AEM c) and S-rGO-2 modified AEM d).
Fig. 6. Zeta potential values of the unmodified and modified AEM in different pH scope.
Fig. 7. Membrane surface area resistance of unmodified AEM and S-GO, S-rGO-1, S-rGO-2 AEM (Zone I means the membrane resistance increase value; Zone II means the membrane surface area resistance of original AEM).
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Fig. 8. Temporal evolution of anions transport of unmodified AEM, S-GO, S-rGO-1, S-rGO-2 modified AEMs in 40 min (a) and in 80 min (b), respectively; Permselectivity and separation efficiency of Cl-/SO42- of unmodified AEM, S-GO, S-rGO-1, S-rGO-2 modified AEMs in 40 min (c) and in 80 min (d), respectively.
and from −0.07 to 0.28 (in 80 min), respectively.
increased from 0.65 to 1.75 and the separation efficiency increased from −0.13 to 0.28 after 40 min, as shown in Fig. 8c; the permselectivity increased from 0.72 to 2.07 and the separation increased from −0.07 to 0.26 in 80 min, as shown in Fig. 8d. Compared to the S-GO AEM, the permselectivity and separation of S-rGO-1 AEM in 40 min was 1.80 and 0.31, respectively; the permselectivity and separation efficiency was at 80 min was 2.30 and 0.28, respectively. This suggests that the S-GO modification layer reduced by the hydrazine hydrate steam could improve the current efficiency and enhance the monovalent selectivity. However, the permselectivity and separation of S-rGO-2 decreased significantly, as shown in Fig. 8c and d. This phenomenon was due to the longer hydrazine treatment will make the chemical stability of membrane surface compromised and some of them even degradation. Thus, the results show that modified AEMs have the monovalent anion selectivity. In addition, the selectivity of the modified membranes is different for different hydrazine hydrate steam times.
Acknowledgement The research was supported by the National High Technology Research and Development Program 863 (No. 2015AA030502), National Natural Science Foundation of China (No. 21676249), Natural Science Foundation of Zhejiang Province (No. LY16B060013) and Foundation of Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (Grant No. 2016K05). Appendix A. Supplementary material The commercial AEM was placed into the setup and the experimental electro-deposition setup, membrane surface resistance measurement custom-designed cell and anions in the diluted compartment passing though the original commercial or modified AEM to the concentrated compartment were plotted as a function of time. This material is available free of charge via the Internet at http://www.journals. elsevier.com/journal-of-membrane-science. Supplementary data associated with this article can be found in the online version at http://dx. doi.org/10.1016/j.memsci.2017.05.002.
4. Conclusions In this study, sulfonated reduced graphene oxide was coated on the surface of an anion exchange membrane to enhance the monovalent selectivity of anions. In addition, the conductivity of modified membranes does not significantly decrease. Cl- and SO42- were chosen as a monovalent and multivalent anion, respectively. The permselectivity and selectivities were calculated to evaluate the monovalent selectivity of membranes. The results show that the permselectivity of unmodified AEM was lower than 1 and the separation efficiency was lower than 0, which confirms that it has no monovalent selectivity. In contrast, the permselectivity and separation efficiency of S-rGO AEMs (reduced by hydrazine hydrate steam in 10 min) increases from 0.65 to 1.80 and from −0.13 to 0.31 (in 40 min), respectively; and from 0.72 to 2.30
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Sulfonated reduced graphene oxide modification layers to improve monovalent anions selectivity and controllable resistance of anion exchange membrane
MARK
Yan Zhaoa, Kaini Tanga, Huimin Ruana, Lixin Xueb, B. Van der Bruggenc, Congjie Gaoa, ⁎ Jiangnan Shena, a b c
Center for Membrane Separation and Water Science & Technology, Ocean College, Zhejiang University of Technology, Hangzhou 310014, PR China Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
A R T I C L E I N F O
A B S T R A C T
Keywords: Sulfonated graphene oxide Reduced graphene oxide Anion exchange membrane Monovalent selectivity Membrane modification
Graphene oxide with lamellar structure has attracted research interest in various fields. In this study, sulfonated reduced graphene oxide (S-rGO) nanosheets with negatively charged sulfonic acid groups were synthesized via a facile distillation-precipitation polymerization, followed by hydrazine reduction. The sulfanilic acid was grafted on the graphene oxide sheets to separate GO nanosheets each other and provide anion channels for anions selectivity. These nanosheets were used to modify anion exchange membranes (AEMs), and to enhance the membrane monovalent anions selectivity and the modification layer conductivity, in order to meet industrial requirements. The permselectivity and separation efficiency were used to evaluate selectivities of the modified membranes. The results show that the unmodified AEM has no monovalent selectivity, while the permselectivity and separation efficiency of S-rGO modified AEMs (reduced by hydrazine hydrate steam in 10 min) increases from 0.65 to 1.80 and from −0.13 to 0.31 (in 40 min), respectively; and from 0.72 to 2.30 and from −0.07 to 0.28 (in 80 min), respectively.
1. Introduction Anion exchange membranes (AEMs) are one of the core elements of the electrodialysis process, which is usually applied to produce drinking water from seawater and brackish water and to produce salt from seawater [1–4]. Multifunctional, high performance AEMs are of interest to treat wastewater and to be applied in fuel cells [5–9]. To meet the purity requirements for specific anions in water treatment, AEMs allowing for separating monovalent from multivalent anions contained in solution is a primary challenge [10]. Generally, the monovalent selectivity of an AEM is determined by the affinity in equilibrium conditions between the fixed charges of the membrane and the counterions present in the solution [11,12]. The structure and pore size of AEMs are important factors to impede the transport of anions with larger size [11,13–15]. The hydrated anion radius in the solution also determines the transport rate [16]. In addition, under the application of current density conditions, a diffusion boundary layer which close to the surface of AEM, can favor the specific anions transport from the bulk solution to the surface of AEM [14,17]. The above key factors are to be considered to improve the
⁎
Corresponding author.
http://dx.doi.org/10.1016/j.memsci.2017.05.002 Received 9 January 2017; Received in revised form 11 April 2017; Accepted 1 May 2017 Available online 04 May 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
membrane permselectivity for monovalent anions in salt mixtures. Several approaches to obtain monovalent ion selectivity have been developed: bio-inspired artificial single ion pumps and ion channels [13,18], the formation of a thin layer on the membrane surface [19] and layer-by-layer modification of AEMs [20]. These methods create 'intelligent' ion transport channels or form a negatively charged polymer on the membrane surface. A novel AEM is developed through poly (sodium 4-styrene sulfonate) and hydroxypropyltrimethyl ammonium chloride chitosan alternate electro-deposition to enhance the monovalent selectivity was demonstrated in our previous work [21]. However, a higher membrane area resistance is unavoidable, which may limit these modified membranes application in industry. For any material used to modify AEMs, a high permselectivity and conductivity is required. Graphene, a typical two-dimensional (2D) carbon nanostructure, has been widely studied for its excellent electrical conductivity and high mechanical strength properties [22,23]. Graphene oxide (GO) sheets have a high specific surface area and can be separated from each other [24–26]. Due to the presence of oxygencontaining functional groups (such as hydroxyl, epoxy, carbonyl, and carboxyl groups) in GO sheets, it can be well dispersed in water [27].
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by using ultrasone in 40 kHz. Then, the mixture was kept in the oil bath at 60 °C under stirring at 750 rpm. After that, 48 mg of sulfanilic acid was added to the mixture. 12 h later, the samples were washed by pure water three times and then, dried in a blast oven at 60 °C. 8 h later, the dry S-GO was obtained.
Besides, the oxygen-containing functional groups allow GO to be manipulated in water and to be assembled in thin films such as nanochannels with a narrow size [27,28]. Furthermore, GO nanosheets can also be grafted by –NH2 groups and obtain a high proton conduction ability of AEM [29–32]. When GO incorporates -SO3groups, the GO nanosheets will separate from one another to some distance due to the interaction of electrostatic repulsion [33,34]. Polymers with a large number of -SO3- groups are often applied as a coating on a membrane surface to form a thin layer that enhances the selectivity of anions [10,35–37]. Recently, reduced graphene oxide (rGO), reduced by hydrazine, was demonstrated to have an excellent electrochemical performance and is used to improve the conductivity of GO membranes [27,38–40]. In addition, due to the presence of -SO3- groups, a surface with a negatively charged layer is obtained, which will enhance the monovalent anions selectivity [19,41]. In the current approach, a functionalized sulfonated graphene oxide (S-GO) was successfully synthesized and anchored on the surface of a commercial AEM to enhance the anion selectivity. Then the S-GO was further reduced (S-rGO) by hydrazine hydrate steam to improve the conductivity of the modification layer. This requires four main consecutive steps: (1) GO dispersion in water; (2) sulfanilic acid was chose and grafted on the GO sheets to synthesis of S-GO; (3) based on the electrostatic adsorption, attachment of S-GO on an AEM surface; (4) reduction of the S-GO modified AEM by the hydrazine hydrate steam. After the above steps, an S-rGO modified AEM is obtained. The monovalent selectivity of an S-rGO modified AEM was investigated and compared to the unmodified AEM by galvanostatic selectivity experiments of Cl- and SO42-. The results show that the S-rGO modified AEM has an excellent selectivity for monovalent anions, and a low resistance induced by the additional layer.
2.3. Membrane modification of S-rGO The commercial original AEM was first rinsed with pure water to remove impurities from the membrane surface. As shown in Scheme 1b, 200 mg of S-GO was well dispersed in 100 mL 0.05 M NaCl solution by ultrasonic dispersion. Subsequently, the solution was added to a labmade experimental setup while stirring at 100 rpm for 24 h. After that, a yellow layer is observed on the AEM surface, the lab-made experimental setup as shown in Fig. 1. Then particulate impurities on the surface of S-GO were rinsed off by pure water for 2 h, followed by hydrazine reduction for 10 min (referred to as S-rGO-1 AEM) and 60 min (referred to as S-rGO-2 AEM), respectively (the hydrazine reduction is shown in supporting information, Fig. S1). 2.4. Membrane characterization The change of the functional groups on the membrane surface was monitored by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Nicolet 6700, The United States) at room temperature to provide information about the chemical structure of the commercial and modified membranes. To quantify the elemental composition of the surface of the unmodified AEM, S-GO AEM, SrGO-1 AEM and S-rGO-2 AEM, all the membranes were analyzed by Xray photo-electron spectroscopy (XPS, Kratos AXIS Ultra DLD, Japan). The crystallinity of the composite was obtained by X-ray diffraction (XRD) at room temperature using an X′Pert PRO (PANalytical, the Netherlands). The surface morphology of the membrane was observed by scanning electron microscopy (SEM) (S-4700 Hitachi, Tokyo, Japan) and atomic force microscope (AFM) (Nanoscope V, Bruker Dimension, The United States).
2. Experimental 2.1. Materials and chemicals Potassium permanganate and hydrogen peroxide (30%) were purchased from Lingfeng Chemical Reagent Co. Ltd. (Guangdong, China). Sulfanilic acid (analytical reagent) was purchased from Macklin Biochemical Co. Ltd. (Shanghai, China). Graphite powder (99.95%), Potassium persulfate (K2S2O8), H2SO4, NaCl, Na2SO4 and all other chemicals were analytical reagent and obtained from Aladdin Industrial Co. Ltd. (Shanghai, China). The standard AEM was the commercial JAM-II-07 Homogeneous AEM (Yanrun, China) which was used as the reference membrane. A commercial homogeneous CEM (FUJIFILM, Japan) was used to prevent the leakage of anions. The properties of the commercial AEM and CEM are shown in Table 1.
2.5. ζ-potential of the membranes Zeta potential values of the membranes were auto-measured on the basis of streaming potential measurements (Surpass 3, Anton Paar, Austria) with 60 mg/L KCl as the electrolyte solution at pH from 5.5 to 9. 2.6. Membrane surface area resistance A custom-designed cell (shown in Fig. S2. in Supporting Information) was used to measure the membrane surface resistance. The resistance of membranes was calculated by a membrane resistance measurement setup and calculated according to the following equation
2.2. Synthesis of GO and S-GO
Rn = In a typical procedure, GO was prepared from graphite powder by a modified Hummers' method (the synthesis of GO is shown in Supporting Information) [39,42,43]. A procedure for the preparation of S-GO was shown in Scheme 1a. 200 mg of GO was dispersed in 100 mL pure water
Thickness (μm)
AEM CEM
160–230 135
Ion exchange capacity (mmol g−1)
Area resistance (Ω cm2)
Transport number (%)
1.6–2.0 1.04
3.0–6.5 2.7
90–95 97
(1)
where Rn is the n multilayers resistance of the membrane expressed in Ω cm2, U is the voltage of the membrane and U0 is the voltage of blank expressed in V, I is the constant current through the membrane and insure the current fixed at 0.004 A, S is the membrane effective area in this setup (7.065 cm2).
Table 1 Characteristics of the anion exchange membrane and cation exchange membrane (commercial data). Membrane type Homogeneous
U − U0 ×S I
2.7. Monovalent anions selectivity measurement The selectivity of monovalent anions was measured in a four compartment module (shown in Fig. S3. in Supporting Information). The current density was 5 mA/cm2, which is in the suitable range for these membranes. At the beginning, 0.05 M Na2SO4 and NaCl mixed solution (SO42- and Cl-) was applied in both compartments in contact 168
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Scheme 1. (a) Synthesis of GO and S-GO; (b) membrane modified by S-rGO.
expressed in A cm−2. The flux of ions was obtained from the change in concentration of the ions on the dilute side according to
Stirring rods
dc
Ji =
V · dti A
(4)
where V is the volume of the electrolyte solution in dilute compartment which is 1 L, and A is the active area of the membranes, which was 19.625 cm2. To represent the separation efficiency, Van der Bruggen et al. [44] the separation efficiency S between component A and B was evaluated as:
AEM
AEM
S(t) =
Unmodified AEMs
((cA(t))/(cA(0))) − ((cB(t))/(cB(0))) × 100% (1 −(cA(t))/(cA(0))) + (1 −((cB(t))/(cB(0)))
(5)
where S(t ) is the separation efficiency; cA (0 ) and cB (0 ) are the initial concentration of SO42- and Cl-, respectively; and cA(t ) and cB (t ) are the concentration of SO42- and Cl- at time t.
Fig. 1. The reaction route of S-GO modified AEM, S-rGO-1 AEM (with hydrazine reduction 10 min) and S-rGO-2 AEM (with hydrazine reduction 60 min).
3. Results and discussion with the membrane. The electrode solution was a 0.2 M Na2SO4 solution. In the dilute compartment, the concentration of mixtures of SO42- and Cl- was measured by Anion Chromatography, after 40 min and 80 min. The permselectivity of the membranes between chloride − and sulfate ions (Cl- and SO42-) P Cl 2− was calculated by Eq. (2):
3.1. Characterization of GO, rGO, S-GO and S-rGO Fig. 2a illustrates the ATR-FTIR spectrum of GO, rGO, S-GO and SrGO respectively. A broad band between 3600 cm−1 and 3200 cm−1 was distinguished in the spectra of GO and S-GO, which represents the O-H groups of GO bending vibration. Besides, the spectrum of S-GO shows a higher intensity and lower broadness in the O-H groups. Due to the generation of C-N (amide group) junction between GO and sulfanilic acid molecules and water molecules are formed. The spectrum of GO and S-GO also shows all the characteristic bands corresponding to oxygen functional groups, including the stretching vibrations from C=O (1725 cm−1), C-O (1630 cm−1) and C-O-C (1110 cm−1). When the GO or S-GO was reduced by the hydrazine hydrate steam, the spectra of rGO and S-rGO, the absorption peak at 3430 cm−1 were significantly weakened. At the same time, the absorption peak of C=C, C-O and C-O-C of rGO and S-rGO was significantly smaller. When
SO4
−
P Cl 2− = SO4
tCl− / t SO42− cCl
− /c
SO42−
=
JCl−. c SO42− JSO42−∙cCl−
(2)
where ti is the transport number of the ions through the membrane, Ji is the flux of the ions through the membrane expressed in mol/m2 s and c is the concentration of the ions in the diluate compartment, expressed in mol/L. The ti was calculated using Eq. (3):
ti =
Jz i iF I
(3)
where zi is the ion charge, F is Faraday's constant and I is the DC current 169
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Fig. 2. ATR-FTIR spectra a), XPS spectra b) and XRD patterns spectra c) of the GO, rGO, S-GO and S-rGO, respectively; image of S-rGO d).
degree and will stack again forming bulky materials.[46] This confirms the existence of conversion that GO to rGO and S-GO to S-rGO in some degree. The peaks marked by squares indicate –SO3- on the GO or rGO. Compared to GO or rGO, these peaks appear more in S-GO and S-rGO, which can be related to the grafting of sulfanilic acid. Fig. 2d shows a high resolution XPS spectrum of C 1s (S-rGO) and it could be deconvoluted into three peaks of 284.4 (the sp2 hybridized carbon), 285.8 (C-N) and 288.2 (O=C-O), respectively.
Table 2 Average chemical compositions of the different GO types (limit to C, O, N, S). GO types
GO rGO S-GO S-rGO
Elements (at%) C 1s
O 1s
N 1s
S 2p
68.31 84.37 58.75 75.13
30 13.14 32.19 15.9
0.80 1.75 4.72 4.82
0.89 0.74 4.34 4.15
3.2. Membrane modification of S-rGO and characterization sulfanilic acid was grafted onto the GO surface, the spectra of S-GO and S-rGO showed two new bands at about 1080 cm−1 and 1030 cm−1, which were assigned to the two symmetric stretching vibrations of S=O [45]. Fig. 2b illustrates the average chemical compositions (limited to C, O, N, S) spectrum of GO, rGO, S-GO and S-rGO. The peaks of four elements at 529.7 eV, 397.2 eV, 282.2 eV, and 165.8 eV can be assigned to O 1s, N 1s, C 1s and S 2p, respectively. The corresponding element ratios are shown in Table 2. It can be seen that the O 1s changed from GO to rGO and S-GO to S-rGO, the fraction of O 1s were decreased and the fraction of C 1s were increased. At the same time, compared to GO and S-GO, rGO and S-rGO, the corresponding elemental ratio of N 1S and S 2p were obviously increased, because of the grafting with sulfanilic acid. XRD was utilized to investigate the crystalline structure of the as-prepared GO and S-GO, rGO and S-rGO; the patterns are shown in Fig. 2c. A diffraction peak at 9.71 was observed, and assigned to the (001) peak of GO and marked by triangles. Compared to GO or SGO, a new broad (002) peak appears at 24.21 in the pattern of GO or SGO. This peak at 24.21 means that when the GO was reduced to some
The existence of an S-rGO modification layer on the surface of AEMs was confirmed by ATR-FTIR and XPS. As shown in Fig. 3a, the unmodified AEM, S-GO, S-rGO-1 and S-rGO-2 AEM were investigated using Fourier Transform Infrared Spectroscopy. Compared to the unmodified AEM and modified AEM, the existence of sulfonic acid groups on the modified AEM can be confirmed by the peak at about 1180 cm−1 and 1030 cm−1 (symmetric stretching vibrations of the two S=O of -SO3H).[45] In addition, the spectra of the modified AEMs shows absorbance peaks at 1630 cm−1 and 1100 cm−1 which indicate the vibration of the groups of C-O and C-O-C, respectively. Moreover, the peak at 1630 cm−1 and 1100 cm−1 are different in the S-GO, SrGO-1 and S-rGO-2 AEM. The peaks for S-rGO-1 and S-rGO-2 AEMs are lower than those of the S-GO AEM, which is due to the presence of reduced sulfonated graphene oxide. The chemical compositions (limited to C, O, N and S) of the surface of the unmodified and modified AEMs were further analyzed by XPS, as shown in Fig. 3b; the chemical composition of each element is shown in Table 3. In comparison with the spectrum of the unmodified AEM, the peak intensity of C 1s 170
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Fig. 3. ATR-FTIR spectra a) and XPS spectra b) of the unmodified and modified AEMs.
surface modified with the S-GO layer is higher than that of an unmodified AEM. When the S-GO AEM surface was reduced by hydrazine hydrate steam in 10 min (the S-rGO-1 AEM), as shown in Fig. 5c, the height of the roughness on the surface significantly decreased. The reason is probably because the oxygen-containing functional groups such as hydroxyl, epoxy, carbonyl, and carboxyl groups were reduced by the hydrazine hydrate steam and some of them coated with sulfanilic acid will desquamate. However, as shown in Fig. 5d, when the S-GO modification layer reduction time was 60 min (the S-rGO-2 AEM), the hilly type surface structure was suppressed, but the roughness of the membrane surface significantly increased. The primary reason for this phenomenon was attributed to the stability decrease of the modified layer and the nanosheets partially desquamated through the reduction of hydrazine hydrate steam after a sufficiently long time.
Table 3 Average chemical compositions of the unmodified AEM and S-GO, S-rGO-1, S-rGO-2 modified AEMs (limit to C, O, N, and S). AEM types
Unmodified S-GO S-rGO-1 S-rGO-2
Elements (at%) C 1s
O 1s
N 1s
S 2p
63.09 67.72 76.19 76.18
32.34 26.66 18. 90 18.66
4.34 4.67 4.06 4.50
0.23 0.95 0.86 0.66
increased. The S 2p increased between the unmodified AEM and modified AEMs, due to the modified layer containing –SO3- groups. In addition, the elemental composition was different for the modified AEMs. In comparison with the spectra of the S-GO and S-rGO-1 (or SrGO-2) AEM the peak intensity of O 1s decreased while the peak intensity of C 1s increased, as shown in Fig. 3b. This is due to the S-GO layer reduced by the hydrazine hydrate steam, which formed the S-rGO layer. However, the elemental fraction of C 1s, O 1S, and N 1S were not changed between the S-rGO-1 and S-rGO-2 AEM and the element of the S 2p even decreased. This illustrates that the reduction by the hydrazine hydrate steam also affected the S-rGO layer. The morphology of membranes’ surfaces is shown in Fig. 4 (The cross-section SEM images of the unmodified AEM and modified AEMs are shown in supporting Fig. S5). From the SEM image of unmodified AEM (Fig. 4a), the rugged shape surface can be clearly seen on the commercial homogeneous membrane. Fig. 4b shows the S-GO AEM, the membrane surface of the rugged shape disappears while covers of S-GO layers. Fig. 4c and d display the surface change after the S-GO AEM is reduced by hydrazine hydrate steam. Fig. 4c is the SEM of S-rGO-1 AEM, many bulge region can be distinguished on the surface. Because of the GO nanosheets are grafted with large number of sulfonate groups and reduced by hydrazine hydrate steam, the nanosheets separated from each other while linked by tertiary ammonium groups. However, when the modified layer of the surface is reduced 60 min, many part of the modified layer fall off from the surface (as seen in Fig. 4d, the surface of S-rGO-2 AEM). This phenomenon particularly because of the tertiary ammonium groups and sulfonate groups may decomposed by hydrazine hydrate steam. In Fig. 5, surface roughness of unmodified AEM and modified AEMs are presented in AFM 3D images. The unmodified AEM has smooth and flat membrane surface as shown in Fig. 5a. Compared to the unmodified AEM, the surface of the S-GO modified AEM (shown in Fig. 5b) has a more hilly type surface structure. Thus, the roughness of the AEM
3.3. ζ-potential of unmodified and modified membranes The surface ζ-potential of the membrane describes the interaction of the electrical surface charges with their surroundings. Because of the mutative surroundings, an inversion of membrane surface charge is produced by sequential adsorption. Fig. 6 shows the ζ-potential of the unmodified and modified AEM. The unmodified AEM has a large number of –N+R3 groups, so the ζ-potential remains positive. As can be seen in Fig. 6, the ζ-potential of the unmodified AEM was increased to a maximum value of 0.2 mV (pH=8.5), and then decreases again. For modified AEMs, the ζ-potential of the S-GO modified AEM stays negative; with an increase of the pH, the value increases but always stays negative, as can be seen in Fig. 6. In the pH range considered here, the ζ-potential was always between −0.4 mV and −0.7 mV. When the S-GO modification layer was reduced by the hydrazine hydrate steam for 10 min, as shown in Fig. 6 for the S-rGO-1 AEM, the ζ-potential was even lower than the S-GO AEM. This may be the hydrazine hydrate steam reduced some oxygen energy groups of GO sheet, which make the modification multilayer change and then leads to the ζ-potential value changed. Thus, the ζ-potential remains below −0.9 mV and always lowers than that of the S-GO AEM. However, as can be seen in Fig. 6 for the S-rGO-2 AEM, the ζ-potential of the modified AEM is even higher than the S-GO AEM. When the hydrazine hydrate steam time was 60 min, the -SO3- poly-negatively charged groups may also be strongly affected. It can be concluded that the modified layer with negative charge and the reduction with hydrazine hydrate steam also affects the ζ-potential of the modified layer.
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a)
b)
20μm
20μm
c)
d)
20μm
20μm
Fig. 4. SEM images of the surface of the unmodified AEM a), S-GO modified AEM b), S-rGO-1 modified AEM c) and S-rGO-2 modified AEM d).
3.4. Membrane surface area resistance
3.5. Selectivity to monovalent anions
For electro-driven membranes, the membrane area resistance is an important parameter, which may limit the application of a membrane for ion separation. The membrane area resistance of the unmodified membrane is 3.06 Ω cm2, as shown in Fig. 7. When the membrane coat with S-GO, the resistance of the modified membrane is up to 4.26 Ω cm2. Compared with the unmodified AEM, the increase value of the membrane resistance was 1.20 Ω cm2. However, the resistance of the S-rGO-1 AEM is 3.72 Ω cm2 which means that the increase of the membrane resistance was 0.66 Ω cm2. Compared to S-GO layer, the membrane area resistance of S-rGO-1 AEM was lower than 0.54 Ω cm2. This can be explained in terms of the larger number of oxygencontaining functional groups (such as hydroxyl, epoxy, carbonyl, and carboxyl groups) that were reduced by the hydrazine hydrate steam. Besides, the GO incorporated -SO32- groups can provide additional sites inside the modified layer with the interaction of electrostatic repulsion. Thus, with the S-GO layer reduced by the hydrazine hydrate steam, the resistance of membrane modified layer decreases. When the membrane continued reduction by the hydrazine hydrate steam, as shown in Fig. 7 the S-rGO-2 AEM, the area resistance was 3.37 Ω cm2 and the membrane resistance increase value is only 0.31 Ω cm2. This may be because some of the modified layer would be affected and even desorption from the membrane surface.
To investigate the monovalent selectivity of AEMs, the permselectivities and separation efficiency between Cl- and SO42- ions were tested. In Fig. 8a and b, the concentration of anions (Cl- and SO42- ions) in the diluted compartment is shown at 40 min and 80 min, respectively. At 40 min, for the unmodified AEM, the concentration of SO42ions was lower than Cl- ions while the modified AEMs showed an opposite phenomenon. The same phenomenona were observed at 80 min, as can be seen in Fig. 8b. Compared to Fig. 8a and b, the anions (Cl- or SO42- ions) concentration value of unmodified AEM is decreased more than modified AEMs, which means that the modified AEMs limit the anions transptation. The permselectivity and separation efficiency values were calculated by Eq. (2) and Eq. (5) respectively. In Fig. 8c and d, zone I means the permselectivity was higher than 1 and the separation efficiency was over zero, which reflected the membrane shows the excellent monovalent selectivity. At the same time, zone II shows the permselectivity was lower than 1 and the separation efficiency was in negative value, which reflected the membrane shows the no monovalent selectivity. Fig. 8c and d shows the permselectivity and separation efficiency of Cl- and SO42- ions of unmodified and modified AEMs at 40 min and 80 min. For the unmodified AEM, the permselectivity was lower than 1 and the separation efficiency was negative value (as shown in Fig. 8c and d zone II). This suggests that the unmodified membrane has no monovalent anion selectivity. When the membrane surface was modified with a S-GO layer, the permselectivity 172
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Fig. 5. 3D AFM images of the surface of the unmodified AEM a), S-GO modified AEM b), S-rGO-1 modified AEM c) and S-rGO-2 modified AEM d).
Fig. 6. Zeta potential values of the unmodified and modified AEM in different pH scope.
Fig. 7. Membrane surface area resistance of unmodified AEM and S-GO, S-rGO-1, S-rGO-2 AEM (Zone I means the membrane resistance increase value; Zone II means the membrane surface area resistance of original AEM).
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Fig. 8. Temporal evolution of anions transport of unmodified AEM, S-GO, S-rGO-1, S-rGO-2 modified AEMs in 40 min (a) and in 80 min (b), respectively; Permselectivity and separation efficiency of Cl-/SO42- of unmodified AEM, S-GO, S-rGO-1, S-rGO-2 modified AEMs in 40 min (c) and in 80 min (d), respectively.
and from −0.07 to 0.28 (in 80 min), respectively.
increased from 0.65 to 1.75 and the separation efficiency increased from −0.13 to 0.28 after 40 min, as shown in Fig. 8c; the permselectivity increased from 0.72 to 2.07 and the separation increased from −0.07 to 0.26 in 80 min, as shown in Fig. 8d. Compared to the S-GO AEM, the permselectivity and separation of S-rGO-1 AEM in 40 min was 1.80 and 0.31, respectively; the permselectivity and separation efficiency was at 80 min was 2.30 and 0.28, respectively. This suggests that the S-GO modification layer reduced by the hydrazine hydrate steam could improve the current efficiency and enhance the monovalent selectivity. However, the permselectivity and separation of S-rGO-2 decreased significantly, as shown in Fig. 8c and d. This phenomenon was due to the longer hydrazine treatment will make the chemical stability of membrane surface compromised and some of them even degradation. Thus, the results show that modified AEMs have the monovalent anion selectivity. In addition, the selectivity of the modified membranes is different for different hydrazine hydrate steam times.
Acknowledgement The research was supported by the National High Technology Research and Development Program 863 (No. 2015AA030502), National Natural Science Foundation of China (No. 21676249), Natural Science Foundation of Zhejiang Province (No. LY16B060013) and Foundation of Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (Grant No. 2016K05). Appendix A. Supplementary material The commercial AEM was placed into the setup and the experimental electro-deposition setup, membrane surface resistance measurement custom-designed cell and anions in the diluted compartment passing though the original commercial or modified AEM to the concentrated compartment were plotted as a function of time. This material is available free of charge via the Internet at http://www.journals. elsevier.com/journal-of-membrane-science. Supplementary data associated with this article can be found in the online version at http://dx. doi.org/10.1016/j.memsci.2017.05.002.
4. Conclusions In this study, sulfonated reduced graphene oxide was coated on the surface of an anion exchange membrane to enhance the monovalent selectivity of anions. In addition, the conductivity of modified membranes does not significantly decrease. Cl- and SO42- were chosen as a monovalent and multivalent anion, respectively. The permselectivity and selectivities were calculated to evaluate the monovalent selectivity of membranes. The results show that the permselectivity of unmodified AEM was lower than 1 and the separation efficiency was lower than 0, which confirms that it has no monovalent selectivity. In contrast, the permselectivity and separation efficiency of S-rGO AEMs (reduced by hydrazine hydrate steam in 10 min) increases from 0.65 to 1.80 and from −0.13 to 0.31 (in 40 min), respectively; and from 0.72 to 2.30
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