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Structurally stable graphene oxide-based nanofiltration membranes with bioadhesive polydopamine coating
Accepted Manuscript Title: Structurally stable graphene oxide-based nanofiltration membranes with bioadhesive polydopamine coating Authors: Chongbin Wang, Zhiyuan Li, Jianxin Chen, Yongheng Yin, Hong Wu PII: DOI: Reference:
S0169-4332(17)32482-0 http://dx.doi.org/10.1016/j.apsusc.2017.08.124 APSUSC 36961
To appear in:
APSUSC
Received date: Revised date: Accepted date:
10-4-2017 26-6-2017 18-8-2017
Please cite this article as: Chongbin Wang, Zhiyuan Li, Jianxin Chen, Yongheng Yin, Hong Wu, Structurally stable graphene oxide-based nanofiltration membranes with bioadhesive polydopamine coating, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.08.124 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structurally
stable
graphene
oxide-based
nanofiltration
membranes with bioadhesive polydopamine coating
Chongbin Wanga, Zhiyuan Lic, Jianxin Chenb,c*, Yongheng Yina, Hong Wua
a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,
China b
Engineering Research Center of Seawater Utilization Technology, Ministry of
Education, Hebei University of Technology, Tianjin 300130, China c
School of Chemical Engineering, Hebei University of Technology, Tianjin 300130,
China *Corresponding
author: School of Marine Science & Engineering, Hebei University of Technology, Tianjin 300130, P R China. E-mail: [email protected] (J.X. Chen)
Graphical abstract
Highlights
Composite membranes were prepared by depositing graphene oxide on adhesive platform.
The formation of polydopamine layer is benefit for interface compatibility.
The composite membranes exhibited high water flux and dyes rejection.
The graphene oxide-based membrane showed good structural stability.
Abstract Graphene oxide (GO)-based membranes possess promising potential in liquid separation for its high flux. The state-of-art GO-based membranes need to be supported by a substrate to ensure that the ultra-thin GO layer can withstand transmembrane pressure in practical applications. The interfacial compatibility of this kind of composite membrane remains a great challenge due to the intrinsic difference in chemical/physical properties between the GO sheets and the substrate. In this paper, a structurally stable GO-based composite nanofiltration membrane was fabricated by coupling the mussel-inspired adhesive platform and filtration-assisted assembly of GO laminates. The water flux for the prepared GO-based nanofiltration membrane reached up to 85 L m-2 h-1 bar-1 with a high retention above 95% and 100% for Orange G and Congo Red, respectively. The membrane exhibited highly stable structure owing to the covalent and noncovalent interactions between GO separation layer and dopamine adhesive platform.
Keywords: Graphene oxide; Nanofiltration; Dopamine; Bioadhesion; Stability
1. Introduction Graphene oxide (GO), prepared by sonication of graphite oxide, has attracted great interest owing to its potential applications in various fields including nanoelectronics, photonics and composite materials, etc [1-4]. Recently, GO has been widely studied in membrane-based separation such as gas permeation, reverse osmosis and pervaporation due to its unique features of naturally smooth surface, single-atom thick sheet and nanoscale interlayer channels [4-8]. GO-based membranes with strong hydrophilicity and fast water permeation properties also possess great prospect in liquid separation [9-11]. A growing body of literature has investigated the GO-based liquid separation membrane. Gao et al. prepared ultra-thin GO-based membranes which exhibited high water flux and high dye retention [12]. The membrane also exhibited moderate retention for inorganic salts. Jin et al. prepared a GO-based membrane with a high water flux by facile filtration-assisted assembly [13]. The fabricated
GO-based
membranes
showed
high
water
flux
for
dimethyl
carbonate/water mixtures. These studies show that GO-based membrane exhibits great potential applications in water separation. However, GO-based membranes are easily detached from the substrate in separation processes because of the poor stability between thin GO film and adjacent substrate. Additionally, the GO sheets with abundant hydrophilic groups trend to re-disperse under hydrated conditions. Efforts have been made to circumvent these issues. Liu et al. improved stability of the prepared membranes in which hydroiodic acid was used to reduce the hydrophilic groups of graphene oxide [14]. Yeh et al. fabricated stable GO-based membranes
using metal cation as cross-linking agent [15]. Mi et al. prepared a novel membrane by dipping the polydopamine and trimesoyl chloride modified membrane in the GO solution [16]. However, these methods failed to address the distinct decrease in water flux. It is desirable to construct stable GO-based membranes without sacrificing water permeability, increasing it even better.
Dopamine, an excellent mussel-inspired coating material, has recently drawn great interest as a potential candidate material to fabricate functional adhesive surfaces [17-21]. Polydopamine (PDA) is normally formed by the self-polymerization of dopamine in the presence of oxygen [22]. The polydopamine coating may strongly adhere to almost any type of solid surfaces owing to the various interactions between PDA layer and the substrates including hydrogen bonding, covalent bonding, coordination bonding and electrostatic interaction, etc [23]. Many attempts have been made to enhance interfacial compatibility of composite membranes in which PDA coating acts as an intermediate to connect polymer substrates and inorganic selective layer. For example, Li et al. fabricated a nanocomposite imprinted membrane through depositing PDA coating on the surface of the polyvinylidene fluoride substrate. The results indicated that nanoparticles were tightly bound onto the PDA layer [24]. Li et al. prepared a nanofiltration (NF) membrane with enhanced structural stability via dipping polyether sulfone membrane in aqueous solution of dopamine. The membrane showed improved interactions between polyamide active layer and polyether sulfone substrate in alcohol. Besides producing a highly adhesive platform, the reaction between the groups from PDA and the groups from GO can form strong covalent
bonds to strengthen the structure stability of membranes [16, 25]. Therefore, there is reason to believe that an ideal structurally stable GO-based nanofiltration membranes would be fabricated by a bio-inspired adhesive platform.
Herein, a novel PDA coating modified GO-based membrane (GO-PDA/PES) was prepared to achieve improved separation performance, and strengthened stability. The nanofiltration membranes were fabricated via depositing thin GO sheets on PDA-coated polyether sulfone substrates using filtration-assisted assembly strategy. PDA could not only act as “bridges” connecting ultrathin GO sheets and polyether sulfone membrane, but also act as “anchors” binding GO sheets on the surface of membrane firmly which prohibited GO sheets re-dispersion under hydrated conditions. The separating property of as-fabricated membranes were studied via filtration experiments using different dyes as model systems. The structure stability of the GO-PDA/PES membrane was evaluated with alcohol. 2. Experiment 2.1 Materials and chemicals Polyether sulfone (PES, E6020 P, Mw=59000) supplied by BASF Co. (Germany) was utilized to prepare the substrate layer. Graphene oxide (GO) was obtained by using natural graphite powders supplied from Aladdin. Poly (ethylene glycol) (PEG), as a pore-foaming agent with molecular weight of 2000g/mol, was supplied by Kermel Chemical Reagent Co. (Tianjin, China). Sulfuric acid (H2SO4) and n-heptane were bought from Benchmark Chemical Reagent Co. (Tianjin, China). Tris (hydroxymethyl) aminomethane (Tris) and Dopamine were bought from Sigma
Aldrich. Ethanol and N,N-dimethyl formamide (DMF) were obtained from Guangfu Fine Chemical Research Institute (Tianjin, China).
2.2 Fabrication of PDA coating membranes The PES microporous membranes, as the substrate of GO-PDA/PES membranes, were prepared in laboratory. A phase separation method was applied to fabricate PES substrate with molecular weight retention above 60000 Da which has been described in previous literature [26]. For construction of PDA coated PES membrane, 0.1 g PDA was added into 50 mL HCl-Tris buffer solution (50 mM), then the PES membrane was dipped into the solution. After 2 h of soaking, the coating membrane was take out and the residual unbound PDA on the surface of PES membranes were removed using deionized water. Finally, the coated substrates, named PDA/PES, were stored in ultrapure water before experiment.
2.3 Preparation of GO-based membrane Graphene oxide was prepared utilizing a modified Hummers method [27]. The solution, containing 5mg L-1 graphene oxide, was prepared by dispersing the above obtained GO powder into ultrapure water. The GO was exfoliated to form GO nanosheets using ultrasonic processing. The resulting GO sheets were deposited onto the surface of PDA/PES membranes via filtration-assisted assembly strategy. An ultrafilter with inner diameter of 62 mm (Millipore Co., model 8200) was used as the filtration cell connecting with an inert gases cylinder and a liquid receiver. The thickness of separation layer was regulated by altering the amount of graphene oxide
sheets deposited on PDA/PES membranes surface. The unbound GO sheets on the surface of PDA coating layer were removed with ultrapure water. The prepared membranes, named GO-PDA/PES, were stored in deionized water before use. The plain GO/PES membrane without PDA was also prepared for comparison. 2.4 Characterizations The surface and cross-sectional morphology of the as-fabricated membranes were studied by scanning electron microscope from FEI Co., Ltd. (FE-SEM, Nanosem 430). The morphology of the GO sheets were studied by transmission electron microscopy (TEM, Tecnai G2 F20). The crystalline structure of the GO powder and membranes was analyzed using powder X-ray diffraction (XRD, D8, Bruker). The chemical composition of GO and prepared membranes were studied utilizing a FT-IR analyzer (Bruker Vertex 80 V) in the range of 4000-600 cm-1. X-ray photoelectron spectroscopy (XPS, PHI-1600) was utilized to study the changes of fabricated membranes surface element composition. The surface hydrophilic properties of GO-based membranes were evaluated by a contact angle goniometer. Weight loss investigation of the GO-based membranes was performed to testify the thermal stability of membranes with a thermal gravimetric analyzer (TGA, NETZSCH, TG209 F3). The membranes were treated under a nitrogen flow from 20 °C to 800 °C，The heating rate was 10 °C min-1. 2.5 Separation performance of the prepared membranes Separation properties of the fabricated GO-based NF membranes were studied using a filtration system which was same to the device of filtrating process as
described in section 2.3. Before experiments, all the nanofiltration membranes were pre-compacted under 2 bar for 30 min. Subsequently, the separation test was carried out at 1 bar operation pressure. Several organic dyes including Methyl Orange, Orange G, and Congo Red at a concentration of 100mg/L were used to evaluate the separation properties of the GO-based NF membranes. The water flux (Fw, L/m2hbar) and organic dyes rejection (R%) were evaluated by the equations (1) and (2), respectively, FW
V S t P
(1)
where V (L) stood for volume of the permeate, S (m2) represented the effective area of the GO-based membranes, t (h) was the operation time and P (bar) was the operation pressure. C R 1 a 100% Cb
(2)
where Ca and Cb were organic dyes concentration of permeate and feed solutions, respectively. The concentration of the organic dyes including Methyl Orange, Orange G, and Congo Red was detected by UV-vis spectrophotometer. Each GO-based membrane was measured three times to obtain an exact value.
2.6 Structural stability of the PDA coated GO-based membranes The structure stability of as-prepared composite membranes was calculated by ethanol treatment. The GO based membranes were soaked in pure ethanol for 3 days under room temperature. Then the residual ethanol on the surface of GO based membranes was removed using deionized water. The pure water permeability and
retention to organic dye were evaluated by the aforementioned methods and compared with those of fresh membranes.
3. Results and Discussion 3.1 Fabrication for the GO-based composite nanofiltration membranes
Fig. 1 presented schematic diagram of synthesis process for the GO-PDA/PES membrane. PES microporous membrane as the substrate was prepared by phase separation method. Then, the substrate was immersed into PDA solution to form a coating layer. After removing the residual PDA on the surface of the substrate, an extremely dilute GO solution was deposited upon the PDA/PES via filtration-assisted assembly strategy. The covalent and noncovalent interactions between PDA and GO provided an obvious enhance for the GO-based membranes structure stability. Moreover, the amino group of PDA could react with carboxylic acid group and epoxy group of GO which was also beneficial to reinforce the structure stability of the GO based membrane.
The chemical composition of the prepared GO sheets were analyzed using FT-IR and the results was presented in Fig. 2a. The strong and wide absorption band at 3400 cm-1 was ascribed to the stretching vibration of O-H in GO, the characteristic band located at 1720 cm-1 was assigned to C=O stretching vibration of carboxylic acid group in GO, and that at 1612 cm-1 was owed to the C=C stretching vibration from the exfoliated GO sheets [28, 29]. To confirm the reaction between PDA and GO, the GO-PDA compound was prepared by adding GO into PDA solution, followed by a
thorough washing with water. The FTIR spectrum of obtained GO-PDA compound was presented in the Fig. 2a. Compared with the FTIR spectrum of GO, two new peaks located at 2920 cm-1 and 2850 cm-1 appeared which was assigned to methylene absorption peak from PDA. The C=O stretching vibration peak at 1720 cm-1 weakened obviously, indicating the the reaction between PDA and GO occurred indeed. XRD was explored to analyze the exfoliated GO sheets crystal structure (Fig. 2b). Two characteristic peaks of the exfoliated GO sheets were exhibited at 2θ of 12° and 23°, indicating the successful synthesis of GO sheets [30]. The weak and wide peak at 23° demonstrated the GO sheets were reduced partly [31]. The morphology of GO sheets was studied using TEM. As presented in Fig. 3, the GO was a single nanosheet with smooth surface.
3.3 Characterization of the prepared composite NF membranes
The surface and cross-sectional morphologies of the GO-based NF membranes were investigated using SEM. As presented in the Fig. 4., the substrate membrane possessed a relatively smooth surface. After coating of PDA, some nanometer PDA particles were observed on the top of PES substrate. Both GO/PES membrane and GO-PDA/PES membrane showed a flat surface with some nanoscale ripples. However, compared with GO/PES membrane, the wave-like ripples in the GO-PDA/PES membrane surface increased dramatically, indicating that the interlayer spacing between GO sheets and PES substrate has been distinctly disrupted by the deposited PDA. As shown in Fig. 4e 4f and 4g, the cross-sectional morphologies of
GO-PDA/PES membrane and membrane fabricated without GO sheets possessed similar asymmetric morphology including a fingerlike porous supporting layer and a dense skin layer. Though the graphene oxide layer upon PES substrate was only dozens of nanometers in thickness, it could be randomly bent without crack. The thickness of GO layer with 75.2 mg/m2 GO loading was in the range of 60-70 nm (Fig. 4f). Simultaneously, the results showed that the GO sheets integrated well with the PES substrate, indicating the good integrity of membrane structure after PDA coating.
Fig. 5 presented a comparison of FT-IR spectra of the as-fabricated membranes. For PDA/PES membrane, besides the characteristic peaks of substrate, a new peak appeared at 1522 cm-1 attributed to the deformation vibration of N-H in polydopamine indicated that the polydopamine layer was successfully formed on the surface of substrate membrane. Compared to the PES and PDA/PES membranes, two new characteristic bands at 1635 cm-1 and 1750 cm-1 were observed for GO-PDA/PES membrane, corresponding to the C=C and C=O stretching vibration from GO, confirming the existence of GO sheets on the polydopamine coated membrane surface.
The chemical composition of the as-prepared membranes were further confirmed using Raman spectroscopy. As showed in Fig. 6, in comparison with the margin substrate membrane, PDA/PES membrane exhibited two wide absorption bands at 1250 cm-1 and 1400 cm-1 , attributing to the catechol groups in PDA. The results demonstrated that the PDA coated on the substrate membrane surface. For
GO-PDA/PES membrane, the absorption bands of the substrate membrane and PDA coated membrane disappeared. Simultaneously, two new absorption bands at 1350cm-1 and 1580 cm-1 belong to GO sheets appeared which proved the successful formation of the GO layer on the surface of PDA/PES membrane.
To further explore composition changes for the prepared membranes, we tested the surface element of the as-prepared membranes using XPS measurement. As presented in Fig. 7a, for polydopamine coated membrane, a new N1s peak corresponding to the nitrogen from PDA appeared, indicating the coating of PDA layer successfully. In addition, two weak peaks (S2s, S2p), which existed in the PDA layers, can be observed. These S peaks in the PDA layer derived from the PES substrate. In comparison with the membrane fabricated without GO sheets, the GO-based membrane presented a decreased nitrogen element content from 3.2% to 1.7%, owing to the existence of GO layer. Fig. 7b exhibited the XPS N1s spectra of prepared composite membrane. The position of N1s peak shifted, demonstrating the chemical reaction between GO and PDA occurred. The XPS spectra of C1s for the GO-PDA/PES membrane (Fig. 7c) can be fitted to three main peaks at ~ 284.6 eV, 286.6 eV and 287.9 eV, which can be assigned to C=C, C-O and C=O groups, respectively. Deconvolution of the N1s region (Fig. 7d) results in three peaks at 398.6, 399.9 and 401.9 eV, which are attributed to =N-R, R-NH-R and R-NH2 amine functionalities respectively. Similarly, the O1s region is fit with three peaks (Fig. 7e) at 530.5, 531.7 and 533.0 eV, which are assigned to C=O, C-OH and C-O-C groups, respectively.
The crystalline peaks of the prepared membranes were shown in the Fig. 8. The broad peak at 18° was displayed in these three membranes, which were assigned to the characteristic amorphous substrate diffraction peak [32]. There were no remarkably distinct diffraction patterns between PES, GO/PES and GO-PDA/PES membranes. The diffraction pattern belonging to GO was no appeared, because of the thickness of GO layer was very thin, resulting in the diffraction signal too weak to detect [12].
To explore the hydrophilicity of the fabricated membranes, the surface static water contact angles of the prepared membranes were measured. As presented in Fig. 9, the contact angle decreased from 69.6±1.4° for pristine membrane to 63.4±1.6° for PDA/PES membrane, implying a more hydrophilic surface due to the abundant hydrophilic amine groups of PDA. The value of contact angle further decreased after introducing GO sheets, indicating higher hydrophilicity. This phenomenon could be possibly owe to the existing of hydrophilic groups from GO. Notably, the existence of PDA on the PES surface affected the deposition of GO sheets, due to the the reaction between the functional groups from GO and PDA. As a result, the water contact angle increased from 52.4±1.4° of GO/PES membrane to 57.5±1.5° of the GO-PDA/PES membrane.
TGA was utilized to study the thermal stability of the fabricated membranes and presented in Fig. 10. Two major stages were shown in the decomposition process. The first weight loss stage between 400 °C and 600 °C was related to the degradation of
functional group from PES, PDA and GO. The second weight loss stage in the range from 600°C to 800°C was mainly corresponded to the decomposition of the PES backbone. In addition, the residue for GO-PDA/PES membrane was lower than that for GO/PES membrane in the range of from 400 °C to 600 °C, owing to PDA layer in the membrane. Notably, the fabricated membranes were sufficiently stable below 350°C, which were suitable for nanofiltration process.
3.4 Performances of the prepared GO-based membrane
The water flux for the prepared membranes were evaluated under 1 bar and the separation performance of these membranes was determined utilizing three different organic dye solutions (100 mg/L) including Methyl Orange (MO, MW=327.33), Orange G (OG, MW=452.37), and Congo Red (CR, MW=696.68). The membranes prepared with different loading of GO sheets, in the range from 71.8 mg m-2 to 78.7 mg m-2 were investigated. As shown in Table 1, the water flux for the membranes obviously decreased by further increasing the GO loading. At a constant GO loading, the water flux of the membrane fabricated with PDA layer apparently increased compared to GO/PES membrane. Meanwhile, the separation performance maintained at a higher level. The change in water flux with different amount of GO could be simply explained by the thickness of GO layer. However, the increase in both flux and separation performances for GO-PDA/PES membrane compared to the GO/PES could be attributed to the following factors. First of all, for the GO/PES membrane, the GO sheets might coat the pores of the substrate membrane, leading to decrease in water
flux. For GO-PDA/PES membrane, PDA acted as intermediary could adjust the interlayer spacing between GO sheets and PES substrate which could avoid GO sheets covering the pores of the substrate directly, resulting in the improvement of water flux. Secondly, the existence of PDA on the PES surface enlarged the surface area of GO layer which could provide more chance for water pass through [33]. In addition, due to the poor interaction, the GO sheets tended to detach from adjacent PES substrate which could bring about the decline in the retention for dyes. PDA, as a “ bio-glue ”, immobilized the GO sheets on PES tightly, leading a higher retention. Also, the reaction between the amino group from PDA and carboxyl group and epoxide group from GO further enhanced the structural stability of GO layer. As a consequence, the GO-PDA/PES membrane exhibited higher retention than that membrane fabricated without PDA. The GO-PDA/PES membrane with 75.2mg m-2 GO loading exhibited high water flux and retention for organic dyes, indicating the as-prepared membranes possessed great potential in water treatment.
The major drawback of the polyether sulfone based thin composite membranes is that the membranes are not tolerant to organic solvent. The PES substrate of membrane is susceptible to swelling by treatment with ethanol [34]. The exposure of PES substrate to ethanol has been suggested to cause damage of the membrane integrity, rapidly sacrificing the separation performance [35]. For the purpose of investigating structural stability of fabricated thin composite membranes, the as-prepared composite membranes in ethanol for different period were studied with pure water and methyl orange aqueous solution (1g/L). As illustrated in Fig. 11,
unsurprisingly, GO/PES membrane showed obvious decrease in the performances. The water flux decreased dramatically from 60.6 to 42.0 L m-2 h-1 bar-1. Meanwhile, the rejection to Methyl Orange declined from 68.2% to 40.5% after ethanol treatment for 8 days. However, the GO-PDA/PES membrane evidently exhibited better performances as compared to membrane fabricated without PDA coating. The water flux decreased slightly from 85 to 74.3 L m-2 h-1 bar-1 and the rejection to Methyl Orange declined from 69% to 56.8% under same conditions. The observation indicated that the GO-PDA/PES membrane had a durable construction as exposure to ethanol. It could be concluded that the highly structural stability for GO-based composite nanofiltration membrane was attributed to the presence of adhesion layer between GO and PES substrate. The formed PDA layer firmly immobilized GO sheets upon PES substrate via strong noncovalent interactions. The immobilized GO sheets were also covalently linked to the PDA via the reaction between the functional groups from GO and PDA, further enhanced the structural stability of GO-based composite nanofiltration membrane. The above mentioned forces between supporting layer and separation layer could improve the interfacial compatibility and reinforce the membrane structure significantly [36].
4. Conclusions A versatile adhesive platform was achieved firstly by coating polydopamine onto the surface of polyether sulfone support layer, and then followed by depositing GO laminates to form the separation layer. The separation performance and structural stability were remarkably enhanced which was attributed to the interactions between
GO separation layer and dopamine adhesive platform. The water flux of GO-based composite nanofiltration membrane that fabricated with 75.2 mg m-2 GO loading was up to 85 L m-2 h-1 bar-1 while retention was maintained above 69% to Methyl Orange, 95% to Orange G and 100% to Congo Red, respectively. The prepared membranes, owing to the high performance and structural stability, are expected to be a promising candidate for future liquid separation.
Acknowledge The research was financially supported by the National Natural Science Fundation of China (21476059, 21576189 and 21276063), Hebei Science and Technology Support Program (16273101D) and the Key Project of Natural Science Foundation of Tianjin (16JCZDJC36500).
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H.K. Jeong, Y.P. Lee, M.H. Jin, E.S. Kim, J.J. Bae, Y.H. Lee, Thermal stability of graphite oxide, Chem. Phys. Lett. 470 (2009) 255-258.
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N.A. Patankar, Supernucleating surfaces for nucleate boiling and dropwise condensation heat transfer, Soft Matter 6 (2010) 1613-1620.
[34]
R. Shukla, M. Cheryan, Performance of ultrafiltration membranes in ethanol-water solutions: effect of membrane conditioning, J. Membr. Sci. 198 (2002) 75-85.
[35]
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Fig. 1. The fabrication process of the GO-PDA/PES membrane 3.2 Characterization of the GO
a
b
GO
Intensity
Transmittance
12° -1 1720 cm GO-PDA
-1 -1 2920 cm 3400 cm
23°
-1 2850 cm -1 1612 cm
4000
3500
3000
2500
2000
Wavenumber (cm-1)
1500
1000
10
20
30
40
50
2θ(degrees）
Fig. 2. a) the FTIR spectra of GO and GO-PDA, b) the XRD spectra of GO
Fig.3. TEM images of GO
（a）PES
4 μm
（c）GO-PDA/PES 4 μm
（e）PDA/PES
（a’）PES
2 μm
（c’）GO-PDA/PES 2 μm
4 μm
（b）PDA/PES
4 μm
（b’）PDA/PES
2 μm
（d）GO/PES
4 μm
（d’）GO/PES
2 μm
（f）GO-PDA/PES
2 μm
（g）GO-PDA/PES
1 μm
Fig.4. Surface SEM images of the PES membrane (a, a’), PDA/PES membrane (b, b’), GO-PDA/PES membrane (c, c’), and GO/PES membrane (d, d’); cross-sectional SEM images of the as-prepared membranes (e, f, g) .
PES
Transmittance
1522cm
-1
PDA/PES 1635cm
-1
GO-PDA/PES 1750cm
2000
1800
-1
1600
1400
1200
1000
Wavenumber (cm-1)
Fig.5. FT-IR spectra of the prepared membranes
20000
Intensity（ a.u.)
15000
PES membrane PDA/PES membrane GO-PDA/PES membrane
PES 10000
PDA/PES 5000
GO-PDA/PES 0 1000
1200
1400
1600
1800
2000
Raman shift（ cm-1）
Fig.6. Raman spectroscopy of the prepared membranes
a
b
C1s
O1s
PES PDA/PES GO-PDA/PES
PES
S2s
PDA/PES
PDA/PES
S2p
N1s GO-PDA/PES
GO-PDA/PES
800
600
400
200
0
420
415
Bingding Energy(eV)
410
400
405
395
380
385
390
Bingding Energy(eV)
c C 1s
e O 1s
d N 1s R2NH
C=O
C-OH
C-O-C
C-O
RNH 2
=NR C=O
C=C
300
295
290
285
280
275
270
410
Bingding Energy(eV)
405
400
395
390 542
540
538
Bingding Energy(eV)
536
534
532
530
Bingding Energy(eV)
Fig.7. a) XPS spectra of the prepared membranes. b) XPS N1s core level spectra of
Intensity
the membranes. c, d, e) XPS C 1s, N1s and O 1s core level spectra resolving results
PES GO/PES GO-PDA/PES 10
20
30
40
50
60
70
80
2θ(degrees）
Fig.8. XRD patterns of the PES, GO/PES and GO-PDA/PES membranes
528
526
90
Water contact angle （ °）
80 70 60 50 40 30 20 10 0
GO-PDA/PES GO/PES
PDA/PES
PES
Fig.9. Contact angles for the prepared membranes
PES PDA/PES GO-PDA/PES GO/PES
100 100
90
GO/PES
80
60
Weight(%)
Weight(%)
80
PES
70
PDA/PES
60
GO-PDA/PES
50
40 40
30
400
450
20
500
550
600
Temperature (°C)
200
400
600
800
Temperature (°C)
Fig.10. TGA curves of the prepared membranes
100
80
Flux (L/(m2hbar))
60 60 50 40 40 Flux(GO-PDA/PES membrane) Flux(GO/PES membrane) Rejection(GO-PDA/PES membrane) Rejection(GO/PES membrane)
20
0
Rejection (%)
70
80
30
20 0
2
4
6
8
Immersion time (day)
Fig. 11. Influence of ethanol treatment on the properties for GO/PES and GO-PDA/PES membranes.
Table 1. Rejection to organic dyes for PDA coated GO based membrane with different GO loading GO loading/ -2
mg m
Water flux/L m-2 h-1 bar-1
MO Retention/%
GO-PDA GO/PES
OG Retention/%
GO-PDA/ GO/PES
/PES
CR Retention/%
GO-PDA/P GO/PES
PES
GO-PD GO/PES
ES
A/PES
0
600±20
/
0
/
0
/
25±0.8
/
71.8
80±2.5
104±2.4
55.9±1.7
58.7±1.5
84±2.3
86.4±2.2
100
100
75.2
60.6±2
85±2
68.2±2
69±1.8
92±2.7
95±2.5
100
100
78.7
40±0.7
65±0.9
70.4±2.1
71±2
96.5±2.5
97.8±2.4
100
100
S0169-4332(17)32482-0 http://dx.doi.org/10.1016/j.apsusc.2017.08.124 APSUSC 36961
To appear in:
APSUSC
Received date: Revised date: Accepted date:
10-4-2017 26-6-2017 18-8-2017
Please cite this article as: Chongbin Wang, Zhiyuan Li, Jianxin Chen, Yongheng Yin, Hong Wu, Structurally stable graphene oxide-based nanofiltration membranes with bioadhesive polydopamine coating, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.08.124 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structurally
stable
graphene
oxide-based
nanofiltration
membranes with bioadhesive polydopamine coating
Chongbin Wanga, Zhiyuan Lic, Jianxin Chenb,c*, Yongheng Yina, Hong Wua
a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,
China b
Engineering Research Center of Seawater Utilization Technology, Ministry of
Education, Hebei University of Technology, Tianjin 300130, China c
School of Chemical Engineering, Hebei University of Technology, Tianjin 300130,
China *Corresponding
author: School of Marine Science & Engineering, Hebei University of Technology, Tianjin 300130, P R China. E-mail: [email protected] (J.X. Chen)
Graphical abstract
Highlights
Composite membranes were prepared by depositing graphene oxide on adhesive platform.
The formation of polydopamine layer is benefit for interface compatibility.
The composite membranes exhibited high water flux and dyes rejection.
The graphene oxide-based membrane showed good structural stability.
Abstract Graphene oxide (GO)-based membranes possess promising potential in liquid separation for its high flux. The state-of-art GO-based membranes need to be supported by a substrate to ensure that the ultra-thin GO layer can withstand transmembrane pressure in practical applications. The interfacial compatibility of this kind of composite membrane remains a great challenge due to the intrinsic difference in chemical/physical properties between the GO sheets and the substrate. In this paper, a structurally stable GO-based composite nanofiltration membrane was fabricated by coupling the mussel-inspired adhesive platform and filtration-assisted assembly of GO laminates. The water flux for the prepared GO-based nanofiltration membrane reached up to 85 L m-2 h-1 bar-1 with a high retention above 95% and 100% for Orange G and Congo Red, respectively. The membrane exhibited highly stable structure owing to the covalent and noncovalent interactions between GO separation layer and dopamine adhesive platform.
Keywords: Graphene oxide; Nanofiltration; Dopamine; Bioadhesion; Stability
1. Introduction Graphene oxide (GO), prepared by sonication of graphite oxide, has attracted great interest owing to its potential applications in various fields including nanoelectronics, photonics and composite materials, etc [1-4]. Recently, GO has been widely studied in membrane-based separation such as gas permeation, reverse osmosis and pervaporation due to its unique features of naturally smooth surface, single-atom thick sheet and nanoscale interlayer channels [4-8]. GO-based membranes with strong hydrophilicity and fast water permeation properties also possess great prospect in liquid separation [9-11]. A growing body of literature has investigated the GO-based liquid separation membrane. Gao et al. prepared ultra-thin GO-based membranes which exhibited high water flux and high dye retention [12]. The membrane also exhibited moderate retention for inorganic salts. Jin et al. prepared a GO-based membrane with a high water flux by facile filtration-assisted assembly [13]. The fabricated
GO-based
membranes
showed
high
water
flux
for
dimethyl
carbonate/water mixtures. These studies show that GO-based membrane exhibits great potential applications in water separation. However, GO-based membranes are easily detached from the substrate in separation processes because of the poor stability between thin GO film and adjacent substrate. Additionally, the GO sheets with abundant hydrophilic groups trend to re-disperse under hydrated conditions. Efforts have been made to circumvent these issues. Liu et al. improved stability of the prepared membranes in which hydroiodic acid was used to reduce the hydrophilic groups of graphene oxide [14]. Yeh et al. fabricated stable GO-based membranes
using metal cation as cross-linking agent [15]. Mi et al. prepared a novel membrane by dipping the polydopamine and trimesoyl chloride modified membrane in the GO solution [16]. However, these methods failed to address the distinct decrease in water flux. It is desirable to construct stable GO-based membranes without sacrificing water permeability, increasing it even better.
Dopamine, an excellent mussel-inspired coating material, has recently drawn great interest as a potential candidate material to fabricate functional adhesive surfaces [17-21]. Polydopamine (PDA) is normally formed by the self-polymerization of dopamine in the presence of oxygen [22]. The polydopamine coating may strongly adhere to almost any type of solid surfaces owing to the various interactions between PDA layer and the substrates including hydrogen bonding, covalent bonding, coordination bonding and electrostatic interaction, etc [23]. Many attempts have been made to enhance interfacial compatibility of composite membranes in which PDA coating acts as an intermediate to connect polymer substrates and inorganic selective layer. For example, Li et al. fabricated a nanocomposite imprinted membrane through depositing PDA coating on the surface of the polyvinylidene fluoride substrate. The results indicated that nanoparticles were tightly bound onto the PDA layer [24]. Li et al. prepared a nanofiltration (NF) membrane with enhanced structural stability via dipping polyether sulfone membrane in aqueous solution of dopamine. The membrane showed improved interactions between polyamide active layer and polyether sulfone substrate in alcohol. Besides producing a highly adhesive platform, the reaction between the groups from PDA and the groups from GO can form strong covalent
bonds to strengthen the structure stability of membranes [16, 25]. Therefore, there is reason to believe that an ideal structurally stable GO-based nanofiltration membranes would be fabricated by a bio-inspired adhesive platform.
Herein, a novel PDA coating modified GO-based membrane (GO-PDA/PES) was prepared to achieve improved separation performance, and strengthened stability. The nanofiltration membranes were fabricated via depositing thin GO sheets on PDA-coated polyether sulfone substrates using filtration-assisted assembly strategy. PDA could not only act as “bridges” connecting ultrathin GO sheets and polyether sulfone membrane, but also act as “anchors” binding GO sheets on the surface of membrane firmly which prohibited GO sheets re-dispersion under hydrated conditions. The separating property of as-fabricated membranes were studied via filtration experiments using different dyes as model systems. The structure stability of the GO-PDA/PES membrane was evaluated with alcohol. 2. Experiment 2.1 Materials and chemicals Polyether sulfone (PES, E6020 P, Mw=59000) supplied by BASF Co. (Germany) was utilized to prepare the substrate layer. Graphene oxide (GO) was obtained by using natural graphite powders supplied from Aladdin. Poly (ethylene glycol) (PEG), as a pore-foaming agent with molecular weight of 2000g/mol, was supplied by Kermel Chemical Reagent Co. (Tianjin, China). Sulfuric acid (H2SO4) and n-heptane were bought from Benchmark Chemical Reagent Co. (Tianjin, China). Tris (hydroxymethyl) aminomethane (Tris) and Dopamine were bought from Sigma
Aldrich. Ethanol and N,N-dimethyl formamide (DMF) were obtained from Guangfu Fine Chemical Research Institute (Tianjin, China).
2.2 Fabrication of PDA coating membranes The PES microporous membranes, as the substrate of GO-PDA/PES membranes, were prepared in laboratory. A phase separation method was applied to fabricate PES substrate with molecular weight retention above 60000 Da which has been described in previous literature [26]. For construction of PDA coated PES membrane, 0.1 g PDA was added into 50 mL HCl-Tris buffer solution (50 mM), then the PES membrane was dipped into the solution. After 2 h of soaking, the coating membrane was take out and the residual unbound PDA on the surface of PES membranes were removed using deionized water. Finally, the coated substrates, named PDA/PES, were stored in ultrapure water before experiment.
2.3 Preparation of GO-based membrane Graphene oxide was prepared utilizing a modified Hummers method [27]. The solution, containing 5mg L-1 graphene oxide, was prepared by dispersing the above obtained GO powder into ultrapure water. The GO was exfoliated to form GO nanosheets using ultrasonic processing. The resulting GO sheets were deposited onto the surface of PDA/PES membranes via filtration-assisted assembly strategy. An ultrafilter with inner diameter of 62 mm (Millipore Co., model 8200) was used as the filtration cell connecting with an inert gases cylinder and a liquid receiver. The thickness of separation layer was regulated by altering the amount of graphene oxide
sheets deposited on PDA/PES membranes surface. The unbound GO sheets on the surface of PDA coating layer were removed with ultrapure water. The prepared membranes, named GO-PDA/PES, were stored in deionized water before use. The plain GO/PES membrane without PDA was also prepared for comparison. 2.4 Characterizations The surface and cross-sectional morphology of the as-fabricated membranes were studied by scanning electron microscope from FEI Co., Ltd. (FE-SEM, Nanosem 430). The morphology of the GO sheets were studied by transmission electron microscopy (TEM, Tecnai G2 F20). The crystalline structure of the GO powder and membranes was analyzed using powder X-ray diffraction (XRD, D8, Bruker). The chemical composition of GO and prepared membranes were studied utilizing a FT-IR analyzer (Bruker Vertex 80 V) in the range of 4000-600 cm-1. X-ray photoelectron spectroscopy (XPS, PHI-1600) was utilized to study the changes of fabricated membranes surface element composition. The surface hydrophilic properties of GO-based membranes were evaluated by a contact angle goniometer. Weight loss investigation of the GO-based membranes was performed to testify the thermal stability of membranes with a thermal gravimetric analyzer (TGA, NETZSCH, TG209 F3). The membranes were treated under a nitrogen flow from 20 °C to 800 °C，The heating rate was 10 °C min-1. 2.5 Separation performance of the prepared membranes Separation properties of the fabricated GO-based NF membranes were studied using a filtration system which was same to the device of filtrating process as
described in section 2.3. Before experiments, all the nanofiltration membranes were pre-compacted under 2 bar for 30 min. Subsequently, the separation test was carried out at 1 bar operation pressure. Several organic dyes including Methyl Orange, Orange G, and Congo Red at a concentration of 100mg/L were used to evaluate the separation properties of the GO-based NF membranes. The water flux (Fw, L/m2hbar) and organic dyes rejection (R%) were evaluated by the equations (1) and (2), respectively, FW
V S t P
(1)
where V (L) stood for volume of the permeate, S (m2) represented the effective area of the GO-based membranes, t (h) was the operation time and P (bar) was the operation pressure. C R 1 a 100% Cb
(2)
where Ca and Cb were organic dyes concentration of permeate and feed solutions, respectively. The concentration of the organic dyes including Methyl Orange, Orange G, and Congo Red was detected by UV-vis spectrophotometer. Each GO-based membrane was measured three times to obtain an exact value.
2.6 Structural stability of the PDA coated GO-based membranes The structure stability of as-prepared composite membranes was calculated by ethanol treatment. The GO based membranes were soaked in pure ethanol for 3 days under room temperature. Then the residual ethanol on the surface of GO based membranes was removed using deionized water. The pure water permeability and
retention to organic dye were evaluated by the aforementioned methods and compared with those of fresh membranes.
3. Results and Discussion 3.1 Fabrication for the GO-based composite nanofiltration membranes
Fig. 1 presented schematic diagram of synthesis process for the GO-PDA/PES membrane. PES microporous membrane as the substrate was prepared by phase separation method. Then, the substrate was immersed into PDA solution to form a coating layer. After removing the residual PDA on the surface of the substrate, an extremely dilute GO solution was deposited upon the PDA/PES via filtration-assisted assembly strategy. The covalent and noncovalent interactions between PDA and GO provided an obvious enhance for the GO-based membranes structure stability. Moreover, the amino group of PDA could react with carboxylic acid group and epoxy group of GO which was also beneficial to reinforce the structure stability of the GO based membrane.
The chemical composition of the prepared GO sheets were analyzed using FT-IR and the results was presented in Fig. 2a. The strong and wide absorption band at 3400 cm-1 was ascribed to the stretching vibration of O-H in GO, the characteristic band located at 1720 cm-1 was assigned to C=O stretching vibration of carboxylic acid group in GO, and that at 1612 cm-1 was owed to the C=C stretching vibration from the exfoliated GO sheets [28, 29]. To confirm the reaction between PDA and GO, the GO-PDA compound was prepared by adding GO into PDA solution, followed by a
thorough washing with water. The FTIR spectrum of obtained GO-PDA compound was presented in the Fig. 2a. Compared with the FTIR spectrum of GO, two new peaks located at 2920 cm-1 and 2850 cm-1 appeared which was assigned to methylene absorption peak from PDA. The C=O stretching vibration peak at 1720 cm-1 weakened obviously, indicating the the reaction between PDA and GO occurred indeed. XRD was explored to analyze the exfoliated GO sheets crystal structure (Fig. 2b). Two characteristic peaks of the exfoliated GO sheets were exhibited at 2θ of 12° and 23°, indicating the successful synthesis of GO sheets [30]. The weak and wide peak at 23° demonstrated the GO sheets were reduced partly [31]. The morphology of GO sheets was studied using TEM. As presented in Fig. 3, the GO was a single nanosheet with smooth surface.
3.3 Characterization of the prepared composite NF membranes
The surface and cross-sectional morphologies of the GO-based NF membranes were investigated using SEM. As presented in the Fig. 4., the substrate membrane possessed a relatively smooth surface. After coating of PDA, some nanometer PDA particles were observed on the top of PES substrate. Both GO/PES membrane and GO-PDA/PES membrane showed a flat surface with some nanoscale ripples. However, compared with GO/PES membrane, the wave-like ripples in the GO-PDA/PES membrane surface increased dramatically, indicating that the interlayer spacing between GO sheets and PES substrate has been distinctly disrupted by the deposited PDA. As shown in Fig. 4e 4f and 4g, the cross-sectional morphologies of
GO-PDA/PES membrane and membrane fabricated without GO sheets possessed similar asymmetric morphology including a fingerlike porous supporting layer and a dense skin layer. Though the graphene oxide layer upon PES substrate was only dozens of nanometers in thickness, it could be randomly bent without crack. The thickness of GO layer with 75.2 mg/m2 GO loading was in the range of 60-70 nm (Fig. 4f). Simultaneously, the results showed that the GO sheets integrated well with the PES substrate, indicating the good integrity of membrane structure after PDA coating.
Fig. 5 presented a comparison of FT-IR spectra of the as-fabricated membranes. For PDA/PES membrane, besides the characteristic peaks of substrate, a new peak appeared at 1522 cm-1 attributed to the deformation vibration of N-H in polydopamine indicated that the polydopamine layer was successfully formed on the surface of substrate membrane. Compared to the PES and PDA/PES membranes, two new characteristic bands at 1635 cm-1 and 1750 cm-1 were observed for GO-PDA/PES membrane, corresponding to the C=C and C=O stretching vibration from GO, confirming the existence of GO sheets on the polydopamine coated membrane surface.
The chemical composition of the as-prepared membranes were further confirmed using Raman spectroscopy. As showed in Fig. 6, in comparison with the margin substrate membrane, PDA/PES membrane exhibited two wide absorption bands at 1250 cm-1 and 1400 cm-1 , attributing to the catechol groups in PDA. The results demonstrated that the PDA coated on the substrate membrane surface. For
GO-PDA/PES membrane, the absorption bands of the substrate membrane and PDA coated membrane disappeared. Simultaneously, two new absorption bands at 1350cm-1 and 1580 cm-1 belong to GO sheets appeared which proved the successful formation of the GO layer on the surface of PDA/PES membrane.
To further explore composition changes for the prepared membranes, we tested the surface element of the as-prepared membranes using XPS measurement. As presented in Fig. 7a, for polydopamine coated membrane, a new N1s peak corresponding to the nitrogen from PDA appeared, indicating the coating of PDA layer successfully. In addition, two weak peaks (S2s, S2p), which existed in the PDA layers, can be observed. These S peaks in the PDA layer derived from the PES substrate. In comparison with the membrane fabricated without GO sheets, the GO-based membrane presented a decreased nitrogen element content from 3.2% to 1.7%, owing to the existence of GO layer. Fig. 7b exhibited the XPS N1s spectra of prepared composite membrane. The position of N1s peak shifted, demonstrating the chemical reaction between GO and PDA occurred. The XPS spectra of C1s for the GO-PDA/PES membrane (Fig. 7c) can be fitted to three main peaks at ~ 284.6 eV, 286.6 eV and 287.9 eV, which can be assigned to C=C, C-O and C=O groups, respectively. Deconvolution of the N1s region (Fig. 7d) results in three peaks at 398.6, 399.9 and 401.9 eV, which are attributed to =N-R, R-NH-R and R-NH2 amine functionalities respectively. Similarly, the O1s region is fit with three peaks (Fig. 7e) at 530.5, 531.7 and 533.0 eV, which are assigned to C=O, C-OH and C-O-C groups, respectively.
The crystalline peaks of the prepared membranes were shown in the Fig. 8. The broad peak at 18° was displayed in these three membranes, which were assigned to the characteristic amorphous substrate diffraction peak [32]. There were no remarkably distinct diffraction patterns between PES, GO/PES and GO-PDA/PES membranes. The diffraction pattern belonging to GO was no appeared, because of the thickness of GO layer was very thin, resulting in the diffraction signal too weak to detect [12].
To explore the hydrophilicity of the fabricated membranes, the surface static water contact angles of the prepared membranes were measured. As presented in Fig. 9, the contact angle decreased from 69.6±1.4° for pristine membrane to 63.4±1.6° for PDA/PES membrane, implying a more hydrophilic surface due to the abundant hydrophilic amine groups of PDA. The value of contact angle further decreased after introducing GO sheets, indicating higher hydrophilicity. This phenomenon could be possibly owe to the existing of hydrophilic groups from GO. Notably, the existence of PDA on the PES surface affected the deposition of GO sheets, due to the the reaction between the functional groups from GO and PDA. As a result, the water contact angle increased from 52.4±1.4° of GO/PES membrane to 57.5±1.5° of the GO-PDA/PES membrane.
TGA was utilized to study the thermal stability of the fabricated membranes and presented in Fig. 10. Two major stages were shown in the decomposition process. The first weight loss stage between 400 °C and 600 °C was related to the degradation of
functional group from PES, PDA and GO. The second weight loss stage in the range from 600°C to 800°C was mainly corresponded to the decomposition of the PES backbone. In addition, the residue for GO-PDA/PES membrane was lower than that for GO/PES membrane in the range of from 400 °C to 600 °C, owing to PDA layer in the membrane. Notably, the fabricated membranes were sufficiently stable below 350°C, which were suitable for nanofiltration process.
3.4 Performances of the prepared GO-based membrane
The water flux for the prepared membranes were evaluated under 1 bar and the separation performance of these membranes was determined utilizing three different organic dye solutions (100 mg/L) including Methyl Orange (MO, MW=327.33), Orange G (OG, MW=452.37), and Congo Red (CR, MW=696.68). The membranes prepared with different loading of GO sheets, in the range from 71.8 mg m-2 to 78.7 mg m-2 were investigated. As shown in Table 1, the water flux for the membranes obviously decreased by further increasing the GO loading. At a constant GO loading, the water flux of the membrane fabricated with PDA layer apparently increased compared to GO/PES membrane. Meanwhile, the separation performance maintained at a higher level. The change in water flux with different amount of GO could be simply explained by the thickness of GO layer. However, the increase in both flux and separation performances for GO-PDA/PES membrane compared to the GO/PES could be attributed to the following factors. First of all, for the GO/PES membrane, the GO sheets might coat the pores of the substrate membrane, leading to decrease in water
flux. For GO-PDA/PES membrane, PDA acted as intermediary could adjust the interlayer spacing between GO sheets and PES substrate which could avoid GO sheets covering the pores of the substrate directly, resulting in the improvement of water flux. Secondly, the existence of PDA on the PES surface enlarged the surface area of GO layer which could provide more chance for water pass through [33]. In addition, due to the poor interaction, the GO sheets tended to detach from adjacent PES substrate which could bring about the decline in the retention for dyes. PDA, as a “ bio-glue ”, immobilized the GO sheets on PES tightly, leading a higher retention. Also, the reaction between the amino group from PDA and carboxyl group and epoxide group from GO further enhanced the structural stability of GO layer. As a consequence, the GO-PDA/PES membrane exhibited higher retention than that membrane fabricated without PDA. The GO-PDA/PES membrane with 75.2mg m-2 GO loading exhibited high water flux and retention for organic dyes, indicating the as-prepared membranes possessed great potential in water treatment.
The major drawback of the polyether sulfone based thin composite membranes is that the membranes are not tolerant to organic solvent. The PES substrate of membrane is susceptible to swelling by treatment with ethanol [34]. The exposure of PES substrate to ethanol has been suggested to cause damage of the membrane integrity, rapidly sacrificing the separation performance [35]. For the purpose of investigating structural stability of fabricated thin composite membranes, the as-prepared composite membranes in ethanol for different period were studied with pure water and methyl orange aqueous solution (1g/L). As illustrated in Fig. 11,
unsurprisingly, GO/PES membrane showed obvious decrease in the performances. The water flux decreased dramatically from 60.6 to 42.0 L m-2 h-1 bar-1. Meanwhile, the rejection to Methyl Orange declined from 68.2% to 40.5% after ethanol treatment for 8 days. However, the GO-PDA/PES membrane evidently exhibited better performances as compared to membrane fabricated without PDA coating. The water flux decreased slightly from 85 to 74.3 L m-2 h-1 bar-1 and the rejection to Methyl Orange declined from 69% to 56.8% under same conditions. The observation indicated that the GO-PDA/PES membrane had a durable construction as exposure to ethanol. It could be concluded that the highly structural stability for GO-based composite nanofiltration membrane was attributed to the presence of adhesion layer between GO and PES substrate. The formed PDA layer firmly immobilized GO sheets upon PES substrate via strong noncovalent interactions. The immobilized GO sheets were also covalently linked to the PDA via the reaction between the functional groups from GO and PDA, further enhanced the structural stability of GO-based composite nanofiltration membrane. The above mentioned forces between supporting layer and separation layer could improve the interfacial compatibility and reinforce the membrane structure significantly [36].
4. Conclusions A versatile adhesive platform was achieved firstly by coating polydopamine onto the surface of polyether sulfone support layer, and then followed by depositing GO laminates to form the separation layer. The separation performance and structural stability were remarkably enhanced which was attributed to the interactions between
GO separation layer and dopamine adhesive platform. The water flux of GO-based composite nanofiltration membrane that fabricated with 75.2 mg m-2 GO loading was up to 85 L m-2 h-1 bar-1 while retention was maintained above 69% to Methyl Orange, 95% to Orange G and 100% to Congo Red, respectively. The prepared membranes, owing to the high performance and structural stability, are expected to be a promising candidate for future liquid separation.
Acknowledge The research was financially supported by the National Natural Science Fundation of China (21476059, 21576189 and 21276063), Hebei Science and Technology Support Program (16273101D) and the Key Project of Natural Science Foundation of Tianjin (16JCZDJC36500).
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Fig. 1. The fabrication process of the GO-PDA/PES membrane 3.2 Characterization of the GO
a
b
GO
Intensity
Transmittance
12° -1 1720 cm GO-PDA
-1 -1 2920 cm 3400 cm
23°
-1 2850 cm -1 1612 cm
4000
3500
3000
2500
2000
Wavenumber (cm-1)
1500
1000
10
20
30
40
50
2θ(degrees）
Fig. 2. a) the FTIR spectra of GO and GO-PDA, b) the XRD spectra of GO
Fig.3. TEM images of GO
（a）PES
4 μm
（c）GO-PDA/PES 4 μm
（e）PDA/PES
（a’）PES
2 μm
（c’）GO-PDA/PES 2 μm
4 μm
（b）PDA/PES
4 μm
（b’）PDA/PES
2 μm
（d）GO/PES
4 μm
（d’）GO/PES
2 μm
（f）GO-PDA/PES
2 μm
（g）GO-PDA/PES
1 μm
Fig.4. Surface SEM images of the PES membrane (a, a’), PDA/PES membrane (b, b’), GO-PDA/PES membrane (c, c’), and GO/PES membrane (d, d’); cross-sectional SEM images of the as-prepared membranes (e, f, g) .
PES
Transmittance
1522cm
-1
PDA/PES 1635cm
-1
GO-PDA/PES 1750cm
2000
1800
-1
1600
1400
1200
1000
Wavenumber (cm-1)
Fig.5. FT-IR spectra of the prepared membranes
20000
Intensity（ a.u.)
15000
PES membrane PDA/PES membrane GO-PDA/PES membrane
PES 10000
PDA/PES 5000
GO-PDA/PES 0 1000
1200
1400
1600
1800
2000
Raman shift（ cm-1）
Fig.6. Raman spectroscopy of the prepared membranes
a
b
C1s
O1s
PES PDA/PES GO-PDA/PES
PES
S2s
PDA/PES
PDA/PES
S2p
N1s GO-PDA/PES
GO-PDA/PES
800
600
400
200
0
420
415
Bingding Energy(eV)
410
400
405
395
380
385
390
Bingding Energy(eV)
c C 1s
e O 1s
d N 1s R2NH
C=O
C-OH
C-O-C
C-O
RNH 2
=NR C=O
C=C
300
295
290
285
280
275
270
410
Bingding Energy(eV)
405
400
395
390 542
540
538
Bingding Energy(eV)
536
534
532
530
Bingding Energy(eV)
Fig.7. a) XPS spectra of the prepared membranes. b) XPS N1s core level spectra of
Intensity
the membranes. c, d, e) XPS C 1s, N1s and O 1s core level spectra resolving results
PES GO/PES GO-PDA/PES 10
20
30
40
50
60
70
80
2θ(degrees）
Fig.8. XRD patterns of the PES, GO/PES and GO-PDA/PES membranes
528
526
90
Water contact angle （ °）
80 70 60 50 40 30 20 10 0
GO-PDA/PES GO/PES
PDA/PES
PES
Fig.9. Contact angles for the prepared membranes
PES PDA/PES GO-PDA/PES GO/PES
100 100
90
GO/PES
80
60
Weight(%)
Weight(%)
80
PES
70
PDA/PES
60
GO-PDA/PES
50
40 40
30
400
450
20
500
550
600
Temperature (°C)
200
400
600
800
Temperature (°C)
Fig.10. TGA curves of the prepared membranes
100
80
Flux (L/(m2hbar))
60 60 50 40 40 Flux(GO-PDA/PES membrane) Flux(GO/PES membrane) Rejection(GO-PDA/PES membrane) Rejection(GO/PES membrane)
20
0
Rejection (%)
70
80
30
20 0
2
4
6
8
Immersion time (day)
Fig. 11. Influence of ethanol treatment on the properties for GO/PES and GO-PDA/PES membranes.
Table 1. Rejection to organic dyes for PDA coated GO based membrane with different GO loading GO loading/ -2
mg m
Water flux/L m-2 h-1 bar-1
MO Retention/%
GO-PDA GO/PES
OG Retention/%
GO-PDA/ GO/PES
/PES
CR Retention/%
GO-PDA/P GO/PES
PES
GO-PD GO/PES
ES
A/PES
0
600±20
/
0
/
0
/
25±0.8
/
71.8
80±2.5
104±2.4
55.9±1.7
58.7±1.5
84±2.3
86.4±2.2
100
100
75.2
60.6±2
85±2
68.2±2
69±1.8
92±2.7
95±2.5
100
100
78.7
40±0.7
65±0.9
70.4±2.1
71±2
96.5±2.5
97.8±2.4
100
100