PVDF nanocomposite membranes with improved anti-fouling properties

Journal of Water Process Engineering 32 (2019) 100960

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Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe

Zwitterionic-functionalized GO/PVDF nanocomposite membranes with improved anti-fouling properties

T

Akram Rahimi, Hossein Mahdavi⁎ School of Chemistry, College of Science, University of Tehran, Tehran, Iran

ARTICLE INFO

ABSTRACT

Keywords: Nanocomposite PVDF membrane GO-g-PMSA Hydrophilic modification Anti-fouling performance

Herein, novel anti-fouling nanocomposite membranes were developed by the incorporating of zwitterionicmodified graphene oxide (GO-g-PMSA) sheets bearing –SO3− and –NH3+ groups into the polyvinylidene fluoride (PVDF) matrix. The GO-g-PMSA nanosheets were synthesized by growing of PMSA zwitterionic brushes on GO surface by the redox graft polymerization via the “graft-from” strategy, causing the twofold advantages of increasing the Go dispersibility in the membrane matrix and giving more antifouling property to GO. The incorporation of the zwitterionic additive in the PVDF membranes significantly improved water affinity and surface hydrophilicity of the nanocomposite membranes. Comparing to pure PVDF membranes and GO/PVDF membranes, as-prepared GO-g-PMSA/PVDF membranes showed evident improvement in anti-fouling properties due to the smoother surface and the more hydrophilic surface in presence of PMSA brushes. An optimum antifouling performance was obtained for 1 wt% GO-g-PMSA/PVDF membrane, e.g. FRR = 95.3%, Rr = 32.3% and Rir = 4.7% (testing with 1 g.L-1 Buffer phosphate BSA solution at 0.7 MPa). The salt rejection experiments also showed high salt ions rejection. Taken together, the results confirmed that zwitterionic polymer modified GO as an antifouling additive can be potentially applied to fabricate antifouling nanocomposite membranes.

1. Introduction Due to the world population growth and depletion of fresh water, there is a strong demand for fresh clean water in the modern society. Membrane separation technique has been extensively used in the applications of wastewater treatment, seawater desalination and drinking water treatment [1]. Among the developed polymeric membranes, PVDF is a classical and favorable membrane material due to its extraordinary properties such as good thermal stability, outstanding chemical resistance, high mechanical strength and easily controlled morphology [2]. In addition to these special advantages, the strong inherent hydrophobic characteristic and low surface energy of PVDF, triggering severe membrane protein-fouling, sharp flux decline, low rejection of contaminates and a change in cut-off size, have limited its applications in the membrane separation process of aqueous solutions. Hence, it is indispensable to design and prepare low fouling or non-fouling PVDF membranes by advanced techniques for practical uses [3,4]. It is generally accepted that the hydrophilic modification is an effective applied strategy to reduce the fouling trend of polymeric membranes, and therefore, enhance their performance during separation process [5]. Various approaches have been applied to improve the hydrophilicity and antifouling ability of the PVDF membranes, ⁎

including the membrane surface hydrophilic modification, blending with hydrophilic polymers or fillers and direct modification of the membrane materials [2,6]. Introducing of the hydrophilic polymers to the membrane matrix or surface, which can be carried out whether directly or indirectly, has been proved to be a convenient approach in preparing membranes with better property and performance. Among these polymers, zwitterionic polymers such as polysulfobetaine, polyphosphobetaine and polycarboxybetaine with super antifouling properties have received enormous attention from researchers. Sulfobetaine surfaces can bind a remarkable amount of water molecules to form a “free water” hydration layer via electrostatically induced hydration. Thus the strong hydration of sulfobetaine surfaces based on the positive and negative groups with strong hydrophilicity can prevent nonspecific protein adsorption in the film application process [7,8]. Up to now, various methods such as blending of zwitterionic copolymers, membrane surface coating with zwitterionic polymers and membrane surface grafting with zwitterionic brushes have been applied to enhance the protein adsorption-resistant properties of the hydrophobic membranes [9–12]. Nevertheless, the use of zwitterionic materials in the large-scale membrane applications is restricted owing to harsh conditions and multi-step reactions of these methods as well as poor dissolutions of zwitterionic materials in the organic solvents [13].

Corresponding author. E-mail address: [email protected] (H. Mahdavi).

https://doi.org/10.1016/j.jwpe.2019.100960 Received 23 May 2019; Received in revised form 15 August 2019; Accepted 14 September 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 32 (2019) 100960

A. Rahimi and H. Mahdavi

Incorporation of inorganic nanomaterials such as TiO2 [14], ZnO [15], carbon nanotube (CNT) [16] and graphene oxide (GO) [17] into the polymer membrane matrix to fabricate nanocomposite membranes has been a point of considerable interest over the last decades, for it is simple and can be produced at an industrial scale with ease [18]. GO, an atomic thin carbon material, can be used as a novel nanomaterial in fabrication of high-performance membranes for water separation because of its flexibility in chemical functionalization, high surface area and superior chemical stability. The main challenge in the membrane application of GO nanosheets is its strong tendency to aggregation in the polymer matrix by л-л stacking. Appropriate chemical modification of GO is very effective in providing better processing and interaction with other compounds. On the other hand, functionalization of GO provides them with functional groups for enhancing several properties such as charges and specific interactions with water contaminants in order to produce additives with potential applications in the membrane separation process [19,20]. One of the most popular strategies to improve the GO miscibility with the polymer matrix is grafting of watersoluble polymer to/from their surface via polymerization techniques [21]. Zwittterionic modification of nanofillers carried out by grafting of zwitteronic polymers to their surface can overcome the problems such as the agglomeration of the nanofillers in the membrane matrix, immiscibility between organic phase and inorganic phase and also insolubility of the zwitterionic materials in the organic solvents. Until now, most research groups have used zwitterionic functionalized nanofillers to improve the biocompatibility of materials [22,23]. However, only a few papers have reported the synthesis of the zwitterionic functionalized nanofillers for improving the membrane fouling control by the blending method. Recently, He et al. [24] prepared zwitterionic SiO2 nanoparticles by grafting lysine onto the SiO2 nanoparticles surface and blending them with PVDF to prepare an asymmetric ultrafiltration membrane via phase inversion method. The results indicated that the zwitterionic nanoparticles could enhance antifouling performance of hydrophobic membranes. Liu et al. [25] also observed a remarkable improvement in the filtration performance and antifouling property of the PVDF composite membrane prepared via blending with TiO2-PMMA-PSBMA (polymethyl methacrylate-polysulfobetaine methacrylate) nanoparticles as zwitterionic fillers. Wang et al. [26] prepared a PES ultrafiltration membrane blended with HNTs-MPC (halloysite nanotubes-2-methacryloyloxyethyl phosphorylcholine) to improve the hydrophilicity and antifouling performance of the hybrid membrane. To the best of our knowledge, the use of polyzwitterionicgrafted GO has never been reported to modify the asymmetric membranes for water purification applications. In this paper, poly[2-(methacryloyloxy)ethyl dimethyl-(3-sulfopropyl)ammonium hydroxide]–grafted GO (GO-g-PMSA) containing ammonium and sulfonic acid groups was prepared and used as a new blending additive to prepare GO-g-PMSA/PVDF nanocomposite membranes by the phase inversion process. The cerium (Ce (IV))-induced graft polymerization of MSA was carried out in the presence of functionalized GO as flat macromolecular backbone. The effect of the addition of zwitterionic nanosheets on the separation performance and fouling resistant properties of prepared nanocomposite membranes was systematically investigated. The nanocomposite membranes structure and properties were characterized using AFM, FESEM, EDX, porosity and water contact angle measurements. The main objective of this work was to develop a novel and convenient strategy for fabrication of antifouling PVDF membranes.

purchased from Merck. Sulphuric acid (H2SO4) (Merck, 98%), phosphoric acid (H3PO4) (Merck, 99%), hydrochloric acid (HCl) (Merck, 36 wt%), hydrogen peroxide (H2O2) (Merck, 30 wt%), potassium permanganate (KMnO4) (Aldrich, 99%), nitric acid (HNO3) (Aldrich, 65 wt %), (3-aminopropyl) triethoxysilane (APTES) (Aldrich, 97%), ceric ammonium nitrate (CAN) (Aldrich, 99.99%), 2-(Methacryloyloxy) ethyl dimethyl-(3-sulfopropyl) ammonium hydroxide (MSA) (Aldrich, 97%), N,N-dimethylformamide (DMF) (Merck, 99.9%), bovine serum albumin (BSA) (Aldrich, Mn = 68,000), NaCl (Merck, 99%) and Na2SO4 (Merck, 99%) were used as received. 2.2. Synthesis of GO GO was prepared by the improved Hummer method [27]. Briefly, graphite powder (3.0 g) was added into a 9:1 mixture of H2SO4/H3PO4 (360:40 ml) and the reaction mixture was stirred continuously for 2 h. After the addition of KMnO4 (18.0 g, as an oxidizing agent), the reaction mixture was heated to 50 0C and stirred for 24 h. On completion of the reaction, the mixture was poured onto ice (400 ml) with 30% H2O2 (6 ml). At this stage, a change in the color of the suspension from brown to yellowish brown was observed. The solid was centrifuged and purified by rinsing with 30% HCl (200 ml), ethanol (200 ml) and distilled water (200 ml). Then, the wet sediments were washed with large amounts of distilled water several times until the pH of the filtrate became neutral. After drying the residue in air, GO was used for further modification. 2.3. Functionalization of GO with APTES (GO- NH2) GO (0.5 g) was suspended in a 1:1 mixture of ethanol and DMF (50:50 ml) by sonication. Ammonia solution (30 ml) was then added into the above dispersion. After stirring at 40 0C for 2 h, APTES (2.2 ml) in 50 ml ethanol was added dropwise into the GO dispersion. The reaction mixture was continuously stirred for 48 h under N2 atmosphere at 40 0C. GO sheets modified by APTES (GO-NH2) were obtained after washing with acetone and distilled water, and then dried at room temperature. 2.4. Surface initiated-redox polymerization of MSA (GO-g-PMSA) For redox grafting polymerization, carried out in a 100 ml round bottom flask, the as-prepared dried GO-NH2 (0.05 g) was suspended into an MSA monomer solution (3 g dissolved in a deionized water (5 ml) /methanol (20 ml) mixture). The suspension was degassed by argon gas before the addition of the solution containing CAN (40 mg in 1 mol. L −1 nitric acid). Then, the mixture was stirred at 80 0C for 24 h. The reaction was finally stopped by exposing the mixture to air. The product (GO-g-PMSA) was obtained by centrifugation, washing and drying at room temperature. 2.5. Preparation of the membrane Pure PVDF, GO/PVDF and GO-g-PMSA/PVDF membranes were prepared via the phase inversion process. The measured amount of nanosheets was well dispersed in DMF solvent using sonication for 30 min, and then, 18 wt% PVDF as the bulk material was dissolved in the above solution at 50 0C, followed by further stirring the mixture for 24 h to obtain a homogeneous casting solution. After that bubbles were completely removed from the solution, it was casted on a glass plate with a constant casting rate using a 200 μm-thick casting blade. Then, the glass plate was dipped into a coagulation bath (distilled water at 30 0 C) without any evaporation time. After the membrane formation, the membranes were washed with fresh distilled water to remove the residual DMF, and kept in distilled water for 24 h prior to testing. The composition of the casting solutions for all membranes are given in Table 1. The percentages of the nanosheets denote the corresponding

2. Materials and methods 2.1. Materials PVDF (average Mw = 145,000) was used as polymer matrix. Natural graphite powder (< 20 mm, with purity > 99.85 wt%) was 2

Journal of Water Process Engineering 32 (2019) 100960

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Table 1 Composition of membrane casting mixtures for nanocomposite membranes with different loadings of GO-g-PMSA and GO. Membrane designation

PVDF (%)

DMF (%)

GO (%)

GO-g-PMSA (%)

M0 M (І) M (IІ) M (IІІ) M (ІV) M (V)

18 18 18 18 18 18

82 82 82 82 82 82

– – – – 0.75 1

– 0.5 0.75 1 – –

ε= (w1-w2)/ (A × l×dw)

(1)

Where w1 and w2 are the weight of the wet and dry membrane (kg), respectively, A is the membrane effective surface area (m2), l is the membrane thickness (m) and dw is the water density (0.998 kg. m−3). Furthermore, the membrane mean pore radius (rm) was determined using the Guerout-Elford-Ferry equation, as follows [8]: rm = √(((2∙9-1∙75ε)×8μlQ)/(ε× A × TMP))

(2)

−4

Where μ is the water viscosity (8.9 × 10 Pa s), Q is the volume of the permeated pure water flux per unit time (m3. s-1) and TMP is the operation pressure (0.7 MPa).

weight percentages of nanosheets regarding overall concentration of polymer.

2.8. Filtration and antifouling performance experiments

2.6. Characterization

A homemade dead-end stirred cell filtration system (see Fig. 1) was employed to investigate the filtration and antifouling performance of the prepared membranes. All membranes were pressurized with DI water at 0.7 MPa for 30 min so that a steady flux could be provided. Moreover, prior to the filtration test, the sample membrane was immersed in deionized water for 24 h. The feed pressure of 0.7 MPa and ambient temperature were applied to carry out the cyclic filtration experiments, consisting of three steps. At the first step, the pure water filtration through the membrane samples was carried out at the abovementioned conditions. The second step included the filtration of BSA solution (1.0 g.L−1 in PBS buffer solution, pH = 7.2–7.4) through the membrane. Then, the membrane was taken out and rinsed in deionized water for 3 h to remove the reversibly adsorbed proteins. Finally, the washed membrane was employed at the third step to study the pure water permeation of the membrane for 1 h. The stable flux in each step was defined as Jw1, JBSA and Jw2, respectively, determined by the following Eq. (3):

To investigate the chemical changes in the structure of GO during the modification process, FT-IR analysis (Bruker Equinox 55) was employed. The thermal properties of the modified GO were measured by thermogravimetric analysis (TGA) (STA 6000 Perkin- Elmer), through which the samples (˜ 5 mg) were heated to 800 0C at a heating rate of 20 0 C per minute under nitrogen atmosphere. The cross section morphology of the prepared membranes was studied by FESEM (HITACHI S-41600), in which cross cut samples were prepared by fracturing the membranes in liquid nitrogen, and a thin layer of gold was coated on the surface of all samples before microscopic analysis. FESEM (Zeiss SUPRA 35 V P) equipped with an EDX system was also utilized to study the composition and chemical-element distribution on the surface of the PVDF nanocomposite membrane. The surface roughness of the prepared membranes was studied by an AFM instrument (Ara NanoscopeFull Plus, Iran) using the tapping mode, in which the sample squares of the membranes (approximately 1 cm2) were fixed on a specimen holder before being scanned. The hydrophilicity of the prepared membranes was characterized using static contact angle measurements (Dataphysics instrument, OCA 15 Plus) at room temperature.

J = V/(A × T)

(3)

Where J (L. m−2 h-1) represents the flux, V (L) is the volume of the permeated liquid, T (h) is the operation time of permeation and A is the effective filtration area. Based on the fluxes, flux recovery ratio (FRR) of the membranes was calculated using Eq. (4), as follows:

2.7. Porosity and mean pore size The overall porosity () was calculated using the following Eq. (1) [8]:

Fig. 1. Schematic representation of the dead end filtration cell. Legend: (1) wire gauze, (2) membrane, (3) plastic washer with inner diameter of 4.91 cm2. 3

Journal of Water Process Engineering 32 (2019) 100960

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FRR = J_w2/J_w1 "×" 100

(4)

Not only the successful synthesis of GO-NH2 but also the surface initiated redox polymerization of MSA from GO surface was revealed by FTIR and TGA analyses. The FTIR spectra of GO, GO-NH2 and GO-gPMSA are displayed in Fig. 3. The GO spectrum exhibits an intense and broad stretching OeH bond absorption at 3400 cm−1, reveling the presence of numerous hydroxyl groups. In addition, the GO spectrum also shows the characteristic peaks at 1730 cm−1 (C]O st), 1623 cm−1 (C]C st), 1228 cm−1 (CeO st), 1053 cm−1 (CeOeC) and 868 cm−1 (CeOeC). After the modification of GO with APTES, the NeH stretching vibration at 3410 cm−1, the NeH bending peak at 1616 cm−1, the CeH bending peak at 1480 cm−1 and the Si-O stretching peak at 1100 cm−1 can be observed. Moreover, the presence of CH2 and the CeH stretching vibrations at 2854 and 2925 cm−1 verifies successful covalent interactions between APTES and the surface of GO. Compared with the FTIR spectrum of GO-NH2, the strong peak at 1385 cm−1 and the weak peak at 1220 cm−1 are characteristic peaks of S]O, and the peak appearing at 943 cm−1 can be attributed to the stretching vibrations of O]S]O in the FTIR spectrum of GO-g-PMSA. In addition, the broad absorption band between 3680 and 3000 cm−1 assigned to the NeH/OeH stretching vibrations can be also observed. These results demonstrated the successful coordination of PMSA onto GO through covalent bands [28–30]. TGA analysis was used to study the thermal properties of the functionalized GO. The TGA thermograms of the samples are shown in Fig. 4, by which it is demonstrated that the modified GO possesses a different thermal decomposition behavior in comparison with pristine GO, indicating the successful attachment of the modifier molecules on the surface of GO. Pristine GO is thermally unstable and show significant weight losses at about 100 0C and 200 0C, corresponding to the loss of absorbed water and labile oxygen-containing functional groups, respectively. The small weight loss between 280 0C and 600 0C indicates that the oxygen-containing functional groups have all but degraded below 200 0C. It is noteworthy that GO-NH2 exhibited lower weight loss at about 200 0C as compared to pristine GO, demonstrating the degradation or reduction of some oxygen-containing functional groups on GO during the organo-modification with APTES [31]. Comparing the TGA curves provided by GO-NH2 and GO-g-PMSA, the final char value of the polymer modified GO at temperatures higher than 600

Furthermore, the rejection of Na2SO4 and NaCl was performed using 1000 ppm feed solution under same conditions via a conductivity meter (Oakton CON 110) for the feed and permeate. Both the BSA rejection ratio (RBSA) and salt rejection (RNaCl and RNa2SO4) were calculated by the following equation: R (%) = (1-C_permeate/C_feed) "×" 100

(5)

Where Cpermeate (mg. L−1) and Cfeed (mg. L−1) represent the protein concentration or ion concentration in permeate and feed, respectively. The protein concentration was analyzed with a UV–vis spectroscopy (Shimadzu, UV-1601) at a wavelength of 278 nm and calculated according to a calibration curve. Additionally, the parameters including the reversible membrane fouling (Rr), irreversible membrane fouling (Rir) and total fouling (Rt) were calculated using the following equations: Rr (%) = (J_w2-J_BSA)/J_w1 × 100

(6)

Rir (%) = (J_w1-J_w2)/J_w1 × 100

(7)

Rt (%) = (J_w1-J_BSA)/J_w1 × 100

(8)

3. Results and discussion 3.1. Structures and characteristics of the synthesized GO-g-PMSA The process of zwitterionic modification of GO is illustrated in Fig. 2. Firstly, the amino terminal silanes were introduced onto both the edge sites and basal planes of GO through a silane coupling reaction, carried out between the hydroxyl groups provided by GO as anchoring points and APTES, making it easy to attach polymer chain to the GO surface and also improve the chemical stability of GO. Then, PMSA was introduced to the surface of GO-NH2 by surface initiated redox polymerization. During the grafting reaction, free radical sites were created on the surface of GO-NH2 by the redox reaction of methylene groups, bonded with an amine group on GO-NH2, with Ce (IV)/HNO3. Finally, the radical active sites initiated the graft polymerization of MSA vinyl monomers.

Fig. 2. Surface initiated redox polymerization of MSA from GO surface. 4

Journal of Water Process Engineering 32 (2019) 100960

A. Rahimi and H. Mahdavi

Fig. 3. FTIR spectra of GO-NH2 and GO-g-PMSA.

Fig. 4. TGA thermograms of GO, GO-NH2 and GO-g-PMSA. 5

Journal of Water Process Engineering 32 (2019) 100960

A. Rahimi and H. Mahdavi

phase inversion process, contributing to extended porosity, bigger cavities, finger hole formation as well as changes in the macrovoids structure [32]. Same behavior was reported by Zhang et al. [33] and Xu et al. [20] for a GO/MWCNT/PVDF membrane and a functionalized GO/PVDF one, respectively. On the other hand, the finger like pores in the structure of the GO-gPMSA embedded membranes were slightly wider than those of the GO embedded membranes. The hydrophilic nature of zwitterionic chains on the GO-g-PMSA surface was able to enhance the penetration rate of water molecules into the cast solution during the phase inversion process and, therefore, led to the formation of larger pore channels [20]. Fig. 6 shows EDX spectrum and SEM-EDX elemental mapping micrographs of the M (II) sample. The High element concentration of Nitrogen, sulfur and oxygen as antifouling hopping sites were clearly observed, demonstrating the successful introduction of GO-g-PMSA onto the surface of the nanocomposite membrane. Additionally, the homogeneous distribution of nitrogen, oxygen and sulfur in the micrographs indicated that there were no GO-g-PMSA agglomerations in the membranes matrix. Therefore, zwitterionic modification of GO can be considered as a superb rout for attaining ideal nanocomposite membranes for water desalination application, ensuring an efficient antifouling performance by the membranes. The surface roughness of the pristine and nanocomposite membranes was studied by analyzing the membrane surface topology using AFM measurements. Two and three-dimensional AFM images of the membranes with different amounts of GO-g-PMSA and GO are presented in Fig. 7. Generally, the brightest areas in AFM images illustrate the highest point of the membrane surface, and the dark regions indicate membrane valleys or pores. Moreover, the roughness parameters of the membranes surface, obtained in an AFM scanning size of 5 μm × 5 μm, are presented in Table 2. The surface roughness parameters are reflected in terms of the average root-mean square (RMS) roughness, the mean surface roughness (Sa), the mean difference between the highest peaks and the lowest valleys (Sz) and the root mean square of Z data (Sq). It is well recognized that roughness parameters

C̊ was more than that of amino-silane modified GO, mainly owing to the presence of the grafted polymer chains onto the surface of GO. The difference between the weight loss for GO-NH2 and GO-g-PMSA was attributed to the amount of the redox graft polymerization. The grafting percentage was calculated as 41% for GO-g-PMSA using the following equation: G (%) = (m'800-m800)/m800 ×100

(9)

where m′800 and m800 exhibit the residual weights of GO-NH2 and GO-gPMSA, respectively, at 800 °C. The FTIR and TGA results completely demonstrated that GO-gPMSA was successfully synthesized via surface initiated redox polymerization. These obtained nanosheets were applied as a membranemaking additive for the modification of PVDF membranes. 3.2. Morphologies and surface composition of the nanocomposite membranes In order to investigate the effects of the doped GO-g-PMSA on the PVDF membrane morphology, cross-section SEM micrographs of pristine PVDF membrane (a), GO-g-PMSA/PVDF membranes (b–d) and GO/PVDF membranes (e and f) were provided with two different resolutions, as depicted in Fig. 5. According to the images, all the membranes possessed a typical asymmetric porous structure, comprising of three layers: a selective and dense skin layer on the top surface, a finger-like middle layer and a sponger-like layer on the bottom surface. Compared with the pristine PVDF membrane, beside the sponger structures with slightly increased pore size, some finger-like structures with relatively longer and wider voids could also be observed for PVDF nanocomposite membranes. A possible reason for this observation can be given on the basis of the surface properties of GO and GO-g-PMSA such as the concentration of the active sites and surface functional groups. The hydrophilic nanosheet components in the casting solution were capable of accelerating the interpenetration rate between nonsolvent (water) and solvent (DMF) in the casting solution during the

Fig. 5. SEM images of cross-section of (a) M0, (b) M (I), (c) M (II), (d) M (III), (e) M (IV) and (f) M (V) at low (number 1: 30 μm) and high (number 2: 10 μm) magnifications. 6

Journal of Water Process Engineering 32 (2019) 100960

A. Rahimi and H. Mahdavi

Fig. 6. (a) EDX spectrum and (b) X-ray mapping of the surface of the M (II).

influence membranes fouling [34]. Low surface roughness can increase the fouling resistance ability on the modified membrane surface, leading to the enhancement of flux recovery ratio (FRR) [35]. As can be

seen, the surface roughness of the PVDF membrane exhibited an obvious change, arising from the addition of nanosheets into the casting solution. According to the roughness results, the surface roughness of

Fig. 7. AFM images of the mixed matrix membranes with different loading of nanosheets: (a) M0, (b) M (I), (c) M (II), (d) M (III), (e) M (IV) and (f) M (V). 7

Journal of Water Process Engineering 32 (2019) 100960

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water contact angle and pure water flux (PWF) of the prepared nanocomposite membranes were measured. In general, with decrease in the contact angle of water, the membrane hydrophilicity enhances. It has been well proven that an increase in the hydrophilicity of membranes surface results in the reduction of not only the penetration pressure of water molecules but also the adsorption of pollutants and proteins on the membranes surface. As can be seen in Fig. 8, not only did the unfilled PVDF membrane exhibit the highest water contact angle of 89°, but the contact angle also dramatically decreased to 65° by increase in the GO-g-PMSA content in the polymer matrix from 0 to 1%. Decreasing trend of the static contact angle after the incorporation of hydrophilic GO-g-PMSA was mainly attributed to the introduction of the sulfonic and tertiary amine hydrophilic groups onto the top layer of the nanocomposite membranes, able to adsorb large amounts of free water molecules and, so, form a hydration layer. Generally, to reduce the interface energy during the phase inversion process, hydrophilic modified GO sheets spontaneously migrates towards the top surface of membranes and lead to a more hydrophilic membrane surface [39,32]. This behavior is also clear from the SEM mapping data of the M (II) sample, confirming the presence of functional groups on the nanocomposite membrane surface, as depicted in Fig. 6. Additionally, the digital photographs of the prepared membranes displayed that the top surface (non-solvent exposed side in the phase inversion process) was darker in comparison to the bottom surface (glass side), as shown in Fig. 9 [40]. No significant decrease in the water contact angle was also observed by the addition of 1 wt% unmodified GO. This phenomenon could occur due to the agglomeration of unmodified GO on the membrane surface in high blending ratios, triggering the reduction of functional groups on the membrane surface. These results were in a good agreement with those obtained from AFM analysis. As can be seen in Fig. 8, the water contact angle of the GO/PVDF nanocomposite membranes was higher than that of the GO-g-PMSA/PVDF nanocomposite membranes, indicating that the modification of GO had been able to improve the wetting ability of GO-g-PMSA/PVDF membranes through not only the formation of a hydration layer on the membrane surface but also an excellent dispersion of the modified nanosheets in the membrane matrix. The PWF results obtained for the prepared membranes are also represented in Fig. 8. All the nanocomposite membranes showed higher PWF amount in comparison with the M0 sample. PWF increased from

Table 2 Surface roughness parameters of PVDF membrane and nanocomposite membranes. Membrane designation

M0 M (I) M (II) M (III) M (IV) M (V)

Roughness parameters Ra(nm)

Rz (nm)

Rq (nm)

4/8 ± ) 0.2) 4/4 ( ± 0.4) 3/9 ( ± 0.6) 4/1 ( ± 0.8) 4/3 ( ± 0.4) 4/8 ( ± 0.9)

47/5 39/8 34/5 36/8 37/4 52/5

38/1 35/4 25/7 28/7 33/3 41/2

( ± 1.4) ( ± 1.6) ( ± 2.1) ( ± 1.2) ( ± 1.6) ( ± 2.7)

( ± 2.5) ( ± 2.2) ( ± 1.6) ( ± 1.7) ( ± 1.9) ( ± 3.2)

the nanocomposite membranes was lower than that of the pristine PVDF membrane. Evidently, when GO-g-PMSA content increased from 0% to 1%, the roughness parameters values decreased and the membrane surface became smoother. The results indicated that the proper dispersion of the modified GO and so low electrostatic interactions could lead to a lower amount of agglomeration on the membrane. However, the AFM images showed that by increase in the unmodified GO content from 0.75 wt% to 1 wt%, the surface roughness also increased. The increase in the roughness could be related to the accumulation of unmodified GO sheets on the membrane surface in high amount of GO [36]. It is noteworthy that in the most previous studies, by the addition of GO to the casting solution, the surface of nanocomposite membranes became rougher [32,37]. For instance, Zinadini et al. [17] reported that the surface roughness increased when the GO content increased from 0.5 wt%, arising from the accumulation of GO on the membrane surface. Nevertheless, because of good modification and, therefore, good dispersion of GO in the membrane matrix, nanocomposite membranes possessing smoother surface were achieved even in high concentration of GO-g-PMSA. Similar observations were achieved by Zhao et al. [38] and Xu et al. [20], for isocyanate-modified GO/PSf membranes and organosilane-modified GO/PVDF ones, respectively. 3.3. Contact angle, pure water flux and porosity of the membranes To evaluate the hydrophilicity of the membranes surface, static

Fig. 8. Pure water flux and static water contact angle of the prepared nanocomposite membranes. 8

Journal of Water Process Engineering 32 (2019) 100960

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water permeability. Indeed, the hydrophilic properties of GO-g-PMSA improved the membrane surface hydrophilicity, enhanced PWF by absorbing water molecules inside the PVDF membrane matrix, and thus, made it easier for water molecules to pass through the nanocomposite membrane [41]. According to Fig. 8, despite the enhancement in the hydrophilicity of M (V), it exhibited lower PWF amount in comparison with M (IV), explained by not only the blockage of some pores but also the reduced pore radius provided by M (V), induced from the agglomeration in the membrane matrix containing 1 wt% unmodified GO (see Table 3), triggering less permeability for the membrane. Similar behavior has been reported for GO/PES membranes [39] and HPEI-GO/ PES ones [42]. As presented in Table 3, the incorporation of nanosheets into the membrane structure increased the mean pore radius and porosity of the prepared blended membranes. It could be attributed to the increase in the solvent and non-solvent exchange during the phase inversion process, arising from the presence of the hydrophilic GO in the polymeric matrix which led to a higher porosity in the membrane. This trend of increasing is well matched with the PWF amounts acquired for the membranes (Fig. 8). However, a further increase in the doped unmodified GO to 1 wt% resulted in a less porosity and smaller pore size,

Table 3 Porosity and mean pore size for different membranes. Membrane designation

Porosity (%)

Mean pore size (nm)

M M M M M M

45.8 47.3 50.7 52.2 48.6 46.7

3.12 3.52 4.02 4.18 3.63 3.41

0 (I) (II) (III) (IV) (V)

( ± 1.4) ( ± 1.8) ( ± 2.2) ( ± 2.1) ( ± 1.5) ( ± 1.6)

17.1 L. m−2 h-1 to 42.9 L. m−2 h-1 with the increase in the GO-g-PMSA content from 0 to 1 wt%, described by the compromise between two factors: (1) the addition of GO-g-PMSA to the membrane matrix provided a lower efficient filtration area due to the decrease in the surface roughness with increasing nanosheets content (Table 2). Hence, it was expected that the nanocomposite membranes possessed a lower water flux than the nascent PVDF membrane [16]. (2) The addition of GO-gPMSA led to an increase in the membrane hydrophilicity (Fig. 8), resulting in a promotion in water permeability. Herein, the second factor has played a prominent role in the determination of the membrane

Fig. 9. Digital photographs of top surface ((a) M0 and (b) M (II)) and bottom surface ((c) M (II)).

Fig. 10. Time-dependent fluxes for the nanocomposite membranes at 0.7 MPa during three steps: pure water permeation for 1 h, BSA solution filtration (pH = 7.2–7.4) for 1 h and recovered pure water permeation after 2 h washing with distilled water for 1 h. 9

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could decrease the membrane fouling for nanocomposite membranes. The scheme for the BSA rejection by the nanocomposite membrane containing GO-g-PMSA is illustrated in Fig. 11. It has been reported that zwitterionic sulfobetaine chains could keep the BSA molecules from contacting the hydrophobic PVDF, causing high retention between protein molecules and the membrane surface [12]. In the final step, the fouled membranes were recovered by being rinsed with distilled water, and pure water fluxes were measured again. The FRR values of various membranes are presented in Fig. 12. The FRR values of the membranes increased from 40.2% for M0 to 95.3% for M (III). It is well known that the higher FRR values, the better anti-fouling ability in the membrane filtration. The results indicated that the regenerating capability of the GO-g-PMSA/PVDF membranes was significantly better than that of the pristine membrane and the GO/PVDF ones. Hence, the surface roughness of the PVDF membrane decreased as a result of blending membrane matrix with zwitterionic nanosheets, preventing not only the adsorption but also the deposition of protein foulants onto the membrane surface [44]. The obtained FRR trend can also be explained through the results provided by contact angle analysis (see Fig. 12), based on which a hydrophobic surface increases the fouling possibility of the membrane due to the increasing BSA-surface interactions, while a hydrophilic surface not only takes up some water molecules but also exhibits a strong repulsive force with BSA molecules. Reversible fouling, which can be removed by water washing, takes place through the weak adsorption of foulants, while, in irreversible fouling, the foulants are strongly bound to the membrane. It was found that the reversible and irreversible fouling resistance of the GO-g-PMSA blended membranes were considerably lower than those of the pristine membrane and GO blended membranes, as depicted in Fig. 13. M0 exhibited the highest irreversible fouling amount due to the more hydrophobic adsorption existing between its surface and BSA. Irreversible fouling was reduced from 59.8% to 4.7% with the addition of the GO-gPMSA amount from 0 to 1%. In fact, zwitterionic groups on the modified membranes surface provided antifouling sites, which could effectively prevent the pollutants from aggregating on the membrane surface. The improved fouling resistance of PVDF nanocomposite membranes with GO-g-PMSA as filler was better than that of other reported nanocomposite membranes using various types of nanofillers (see Table 4). It was found that the zwitterionic functionalization of GO provided a convenient strategy to produce graphene-based nanocomposite membranes with high water flux and excellent anti-fouling performance.

Fig. 11. Schematic illustration of antifouling mechanism of nanocomposite membranes containing zwitterionic GO-g-PMSA.

triggering the flux decline of M (V). It could be attributed to the delayed phase separation and increased viscosity, induced from the agglomeration of the doped GO at high concentrations [43]. 3.4. Anti-fouling performance of membranes It is known that organic fouling is one of the main problems taking place when a membrane is employed in separation and purification processes. Hence, in this study, BSA protein was chosen as a foulant agent to investigate the anti-fouling performance of the prepared membranes. In order to evaluate the anti-protein-fouling performance of PVDF nanocomposite membranes in details, irreversible fouling ratio, reversible fouling ratio, total protein fouling and water flux recovery ratio were measured using cyclic filtration tests by pure water and BSA solution. According to the results of time-dependent fluxes (Fig. 10), all the nanocomposite membranes possessed a higher Jw and JBSA compared to M0, arising from the decrease in membrane resistance to water and BSA solution permeation after the incorporation of hydrophilic nanosheets (discussed in the former section). In addition, it can be seen in Fig. 10. that the fluxes decreased for all the membranes when pure water was replaced by BSA solution in the cell, arising from the membrane fouling phenomenon caused by adsorbed protein on the membrane surface. Nevertheless, the decreasing trend of JBSA reduced in the presence of GO-g-PMSA. In fact, the pure membrane was prone to be fouled because of its hydrophobic nature, while hydrophilic GO

Fig. 12. Flux recovery ratio (FRR) of the prepared membranes. 10

Journal of Water Process Engineering 32 (2019) 100960

A. Rahimi and H. Mahdavi

Fig. 13. Fouling resistance ratio of the prepared membranes.

Table 4 Comparison of hydrophilicity and antifouling property of membrane in present study with other nanocomposite membranes reported in the literatures. Membrane composite abbreviation

GO/PVDF/DMAc MWCNT/PES/DMAC GO/PES/DMAc TiO2-MWCNT/PES/DMAc GO /PVDF/DMAc GO-PMSA/PVDF/DMF

GO-based water desalination membrane performance Applied pressure

Water fluxL.m−2 h-1

Optimal contact angle ()̊

FRR %

Ref

100 400 400 500 100 700

26.49 ˜7 20.4 4.35 505 42.9

60.5 ˜ 63 53.2 61.5 68 65

88.6 87.7 90.5 83 74 95.3

[41] [45] [39] [40] [46] This work

kPa kPa kPa kPa kPa kPa

3.5. Salt rejection of the membranes

pressure of 0.7 MPa and in a 1000 ppm salt solution, as shown in Fig. 14. The GO-g-PMSA/PVDF membranes displayed higher rejection than other prepared membranes despite their relatively larger pore size. Such a behavior suggested that the rejection was largely influenced by

To evaluate the salt rejection performance of the prepared membranes, NaCl and Na2SO4 rejection was measured under the operating

Fig. 14. Salt retention performance of the prepared nanocomposite membranes (0.7 MPa, 1000 ppm salt, after 60 min filtration). 11

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A. Rahimi and H. Mahdavi

the interactions of salt ions and PMSA chains. Because the zwitterionic brushes possessed a strong binding affinity to salt ions, the modified membranes could efficiently trap the ions by electrostatic interactions between their sulfonate and ammonium groups and salt ions. Additionally, after the addition of electrolyte solution, negative and positive ions could penetrate to the collapsed PMSA chains and disrupt the inter- and intra- chain association of zwitterion chains, leading to an extended conformation of SMA polymer chains, reducing the effective pore size of the membrane [47].

[10] [11] [12] [13]

4. Conclusion

[14]

Experimental results demonstrated that the anti-fouling modification of PVDF membranes through the blending process of the anchored zwitterionic additive into the hydrophobic membrane was a convenient strategy to achieve extraordinary antifouling performance and hemocompatibility. The anti-fouling zwitterionic polymers were successfully grafted to the surface of GO through the redox initiated surface graft polymerization method. Then, novel PVDF nanocomposite membranes were developed via the phase inversion process by dispersing GO-gPMSA into the casting polymeric solution. The addition of GO-g-PMSA led to wider finger-like pores, a larger mean pore size along with a higher porosity. EDX mapping analysis confirmed the presence of GO-gPMSA on the membrane surface. Data provided by water contact angle measurements indicated the increased surface hydrophilicity of the nanocomposite membranes. Furthermore, with 0.75 wt% GO-g-PMSA content, the pure water flux (Jw) reached the maximum value of 42.9 L.m−2 h-1. The salt retention of the blended membranes was also improved by incorporating GO-g-PMSA. Evaluation of the anti-fouling performance of the membranes was also performed by ternary cyclic filtration of pure water and BSA solution. The results showed that the GO-g-PMSA/PVDF nanocomposite membranes had superior protein anti-fouling properties compared to the nascent PVDF and GO/PVDF membranes, indicating that the foulants could be less likely adsorbed onto the GO-g-PMSA embedded membranes due to the hydrophilic and natural characteristics of the zwitterionic chains on the membranes surface. Together with the high anti-fouling properties, the acquired results proved that the nanocomposite membranes were promising for high-performance membranes in innumerable separation applications.

[15]

[16]

[17]

[18] [19] [20]

[21] [22] [23]

[24] [25]

Acknowledgement

[26]

Authors greatly acknowledge the University of Tehran for the financial support of this work.

[27] [28]

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