Preparation of a Novel Polyvinylidene Fluoride (PVDF) Ultrafiltration ...

Article pubs.acs.org/IECR

Preparation of a Novel Polyvinylidene Fluoride (PVDF) Ultrafiltration Membrane Modified with Reduced Graphene Oxide/Titanium Dioxide (TiO2) Nanocomposite with Enhanced Hydrophilicity and Antifouling Properties Mahdie Safarpour,† Alireza Khataee,*,† and Vahid Vatanpour‡,§ †

Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-14766 Tabriz, Iran ‡ Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran § Novel Technology Research Group, Petrochemical Research and Technology Company, 14977-13115 Tehran, Iran ABSTRACT: A novel polyvinylidene fluoride (PVDF) mixed matrix ultrafiltration membrane containing reduced graphene oxide/titanium dioxide (rGO/TiO2) nanocomposite was prepared by phase inversion method. The synthesized rGO/TiO2 was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy (SEM) techniques. The prepared rGO/TiO2 blended PVDF membranes were characterized by atomic force microscopy, SEM, water contact angle, porosity, permeation measurements, and rejection tests. Due to the high hydrophilicity of the rGO/TiO2 nanocomposite, the rGO/TiO2/PVDF membranes were more hydrophilic and had higher pure water flux and flux recovery ratio than the bare PVDF. The blended membranes showed remarkably good properties and performance when the rGO/TiO2 content of 0.05 wt % was added to the casting solution. The pure water flux of the 0.05 wt % rGO/TiO2 blended membrane was increased by 54.9% compared with the bare PVDF membrane. The antifouling study of the membranes revealed that a 0.05 wt % rGO/TiO2 membrane had the best fouling resistance.

1. INTRODUCTION Membrane technology has attracted much attention over the past 30 years with extensive use in various industrial fields such as water desalination, ultrapure water production, product recycling, and wastewater treatment.1 Among various membrane technologies, ultrafiltration (UF) has been known as an impressive technique in the refinery wastewater systems owing to its proper pore sizes (usually 2−50 nm) and its ability to remove emulsified oil droplets and other organic contaminants.2,3 The UF membranes are mostly classified into two categories: polymeric and inorganic membranes. Despite the advantages of inorganic membranes such as temperature and wear resistance, definition, stable pore structure, and chemical inertness, they display some inherent disadvantages such as relatively high cost, a complicated fabrication process, and low membrane surface per apparatus specific volume. Therefore, the cheap and easy fabrication of polymeric membranes can overcome the membrane market.4−6 Among the polymeric materials, polyvinylidene fluoride (PVDF), as a polymer with great thermal stability and chemical resistance to aggressive reagents such as organic solvents, acids, and bases, is widely used in the preparation process of nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), and pervaporation (PV) membranes. The major drawback of PVDF membranes is their hydrophobic nature, causing severe membrane fouling and permeability decline and also, influencing their application in water and wastewater treatments.7,8 Improvement in the membrane hydrophilicity seems to be an efficient technique to overcome the membrane fouling problem. Several methods have been investigated to increase the © 2014 American Chemical Society

hydrophilicity of PVDF membranes. The most reported techniques are addition of hydrophilic polymers9 or blending of nanoparticles10−12 with casting solution, chemical modification such as sulfonation,13 and immobilization of polymers with hydrophilic segments on the surface of membranes by coating14 or grafting with photo or plasma polymerization.15−17 Among various approaches used to control membrane hydrophilicity and fouling, the blending of inorganic nanostructured materials has attracted great attention.18 Titanium dioxide (TiO2) is the most widely used catalyst in environmental applications due to its high catalytic activity, great stability, low toxicity, and low material cost.19−21 The introduction of TiO2 into polymeric membranes can enhance the hydrophilicity, self-cleaning, antifouling, and antibacterial properties of these membranes.22,23 Besides the numerous advantages of incorporating TiO2 in the membrane matrix, some disadvantages have also been reported; they include both aggregation and agglomeration of TiO2 nanostructures in the prepared membrane. Also, some results have shown that the excessive addition of inorganic additives may negatively affect the morphology and elasticity of PVDF membranes.24,25 To overcome these disadvantages, the search to identify the materials that not only enhance the hydrophilicity of the membrane but also improve its strength can be very important.25 In this regard, the application of carbon nanomaterials for the modification of Received: Revised: Accepted: Published: 13370

June 15, 2014 August 3, 2014 August 8, 2014 August 8, 2014 dx.doi.org/10.1021/ie502407g | Ind. Eng. Chem. Res. 2014, 53, 13370−13382

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic of rGO/TiO2 nanocomposite synthesis.

molecular weight of 29000 g/mol, tetraethyl orthotitanate (titanium(IV) ethoxide, Ti (OC2H5)4), and bovine serum albumin (BSA, MW = 67000) were purchased from Sigma-Aldrich Co., Munich, Germany. 2.2. Synthesis of Graphene Oxide (GO). GO was synthesized by the chemical oxidation of industrial graphite powder using the modified Hummers and Offeman method.31 The specific steps in the synthesis were as follows: first, the desired amount of graphite powder was added to the cooled (0 °C) mixture of concentrated H2SO4 and HNO3 (H2SO4/HNO3 4:1 v/v) under vigorous stirring. Second, 15 g of KMnO4 was slowly added to the above mixture while stirring in an ice−water bath. Then, the mixture was stirred at 35 °C for 2 h, and 250 mL of distilled water was slowly added to the mixture, which increased the temperature to 98 °C, and that temperature was maintained for 30 min. Finally, 750 mL of water was poured quickly into the mixture to terminate the reaction, and 20 mL of 10% H2O2 was added to reduce the residual KMnO4 and MnO2. The resulting suspension was separated and washed several times with distilled water, and the product was dried at 60 °C. The resulting powder was sonicated for 90 min in a bath type sonicator (Sonica 2200 EP S3, Soltec, Milan, Italy) to obtain the relatively pure GO. 2.3. Synthesis of rGO/TiO2 Nanocomposite. First, the desired amount of the prepared GO powder was dispersed in a mixture of ethanol/water (4:1 v/v) by ultrasound and then transferred into a 100 mL Teflon-lined stainless autoclave. Then, 2 mL of tetraethyl orthotitanate (TEOT) was added to the autoclave, and the mixture was stirred for 1 h. After that, 1 mL of concentrated HNO3 was added slowly to the above mixture. Finally, the autoclave underwent a hydrothermal conditioning at 180 °C for 24 h, and then it was allowed to cool to room temperature naturally. As-synthesized rGO/TiO2 nanocomposite was collected and washed with distilled water and absolute ethanol several times to remove residual impurities and then dried at 60 °C for 12 h. Pure TiO2 nanoparticles were also obtained using the same method in the absence of GO. Figure 1 shows the schematic of rGO/TiO2 nanocomposite synthesis.

polymeric membranes has received great attention. Recently, several studies have been published on carbon nanotube (CNT)/organic hybrid membranes, reporting high water fluxes, protein rejections, and enhanced hydrophilic characters for these membranes.26,27 Other potential options to efficiently modify the polymeric membranes are graphene and graphene oxide (GO).25,28,29 Interestingly, graphene has a high aspect ratio as well as low density and high strength and stiffness. Nevertheless, the chemically inert nature of graphene prevents its dissolving in the usual organic solvents. In contrast to that, the affinity provided by hydroxyl, carboxyl, carbonyl, and epoxy groups of GO is more appropriate for fabricating organic− inorganic-blended ultrafiltration membranes.25,30 GO also has high surface area and excellent mechanical properties, and its incorporation into a polymer matrix can considerably improve physical properties such as the mechanical strength of the host polymers at a very low additive amount.25 In this study, to improve the hydrophilicity and antifouling properties of PVDF, rGO/TiO2/PVDF UF membranes were fabricated by the phase inversion technique using hydrophilic rGO/TiO2 nanocomposite additive. It was expected that the use of GO together with TiO2 nanoparticles would not only increase the hydrophilicity, antifouling, and mechanical properties of the prepared PVDF membranes but also better distribute TiO2 nanoparticles and prevent their aggregation and agglomeration in the polymer matrix. To the best of our knowledge, there is no other report on the use of rGO/TiO2 nanocomposite to prepare blended polymeric UF membranes.

2. MATERIALS AND METHODS 2.1. Materials. Industrial graphite was manufactured by the Qingdao Ruisheng Graphite Co., Ltd., China. Analytical grade H2SO4 (purity = 98%), KMnO4 (purity = 99%), H2O2 (30% aqueous solution), polyethylene glycol (PEG) polymers with different molecular weights, ethanol, and N-methyl-2pyrrolidone (NMP, EMPLURA) were purchased from Merck Co., Darmstadt, Germany. PVDF was purchased from Alfa Aesar, Karlsruhe, Germany. Polyvinylpyrrolidone (PVP) with a 13371

dx.doi.org/10.1021/ie502407g | Ind. Eng. Chem. Res. 2014, 53, 13370−13382


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It should be noted that it was not the purpose of this study to reduce GO by hydrothermal treatment. A hydrothermal method was used just for the synthesis of TiO2 nanoparticles. However, in the case of GO/TiO2 composite, the hydrothermal condition led to partial reduction of GO unlike our propensity, so, in the case of GO alone, there was no hydrothermal treatment and reduction of oxygenated groups. 2.4. Preparation of rGO/TiO2/PVDF Membranes. rGO/ TiO2 blended PVDF ultrafiltration membranes were fabricated by the phase inversion technique using PVDF as bulk material, NMP as solvent, rGO/TiO2 nanocomposite as additive, and distilled water as nonsolvent coagulation bath. Briefly, the synthesized rGO/TiO2 nanocomposites (0, 0.01, 0.02, 0.5, 0.075, and 0.1 wt % based on the weight of PVDF) were added to the NMP and dispersed using sonication for 2 h. After that, PVP (1 wt %) and PVDF (21 wt %) were dissolved in the above solution at 70 °C and stirred for 48 h. Afterward, the homogeneous solution was maintained in a drying oven for 24 h to release air bubbles. The casting solution of pure PVDF membranes was prepared by dissolving 21 wt % PVDF and 1 wt % PVP in NMP. After some degassing, the solutions were cast with the thickness of 150 μm on clean glass plates using a casting knife with controlled casting rate. The cast films were then immersed in a coagulation bath (distilled water at 25 °C). The prepared membranes were rinsed with distilled water to remove the residual solvent and preserved in distilled water until they were used. 2.5. Characterization. X-ray diffraction (XRD) patterns of GO, TiO2, and rGO/TiO2 samples were measured by a Siemens X-ray diffraction D5000 diffractometer (Munich, Germany), with Cu Kα radiation (1.54065 Å). An accelerating voltage of 40 kV and emission current of 30 mA were used. For Fourier transform infrared spectroscopy (FT-IR) analysis, the KBr pellets were prepared from the synthesized samples. FT-IR analysis was performed using a spectrophotometer (Tensor 27, Bruker, Leipzig, Germany). Scanning electron microscopy (SEM) analysis of the synthesized samples and the prepared mixed matrix membranes was carried out on a Hitachi SEM model S-4200 (Tokyo, Japan) device after gold plating of the samples. The cross-section samples were obtained by fracturing the membranes after freezing in liquid nitrogen. The hydrophilicity of the membrane surfaces was evaluated using water contact angle analysis (G10, Kruss, Hamburg, Germany) according to the sessile-drop method. The contact angles were measured for at least five random locations of membrane and then averaged to minimize the experimental errors. Atomic force microscopy (AFM) images were recorded using a Nanosurf Mobile S scanning probe-optical microscope (Liestal, Switzerland) equipped with Nanosurf Mobile S software (version 1.8). The AFM analysis was used to investigate the roughness and surface morphology of the fabricated membranes. The surface roughness parameters were introduced regarding the average roughness (Sa), the root-mean-square of the Z data (Sq), and the height difference between the highest peak and the lowest valley (Sy). The overall porosity of membranes (ε) was determined using gravimetric method, as presented in eq 1:32 ε=

ω1 − ω2 A × l × dw

Figure 2. XRD patterns of GO, TiO2, and rGO/TiO2 nanocomposite.

Figure 3. FT-IR spectra of GO, TiO2, and rGO/TiO2 nanocomposite.

The Guerout−Elford−Ferry equation (eq 2) was applied to calculate the mean pore radius (rm) of membranes using the pure water flux and porosity results.32,33 rm =

(2.9 − 1.75ε) × 8ηlQ ε × A × ΔP

(2) −4

In this equation, η is the water viscosity (8.9 × 10 Pa s), Q is the volume of the permeated pure water per unit time (m3/s), and ΔP is the operation pressure (0.3 MPa). 2.6. Membrane Permeation Performances. The permeation flux and rejection measurements of the rGO/TiO2/ PVDF UF membranes were performed by dead-end experimental equipment. The rejection studies were done with an aqueous solution of bovine serum albumin (BSA) (molecular weight = 67000) (0.5 g/L). All experiments were directed at 25 °C with a feed pressure of 0.3 MPa. The measuring procedure used can be briefly described here: for the first 30 min, the prepared flat-sheet membrane was compacted at 0.5 MPa to reach a steady flux of pure water; then the flux was recorded at 0.3 MPa every 3 min for 90 min, and at least four replicates were collected to calculate an average value. After this, the pure water was replaced by 0.5 g/L BSA solution. The concentration of BSA in the feed and permeation solution was measured by a UV spectrophotometer (Shimadzu UV-2450, Tokyo, Japan). The pure water flux, Jw,1 (kg/m2 h), was calculated through eq 3

(1)

where ω1 and ω2 are the weights of the wet and dry membrane, respectively; A is the membrane area (m2), l is the membrane thickness (m), and dw is the water density (0.998 g/cm3). 13372



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Figure 4. SEM images of (a) GO, (b) TiO2, and (d, e) rGO/TiO2 nanocomposite; (c) diameter size distribution of TiO2 nanoparticles.

Jw,1 =

M At

washed with distilled water for 20 min, and their water flux was measured again, Jw,2 (kg/m2 h). To investigate the antifouling property of the prepared membranes, the flux recovery ratio (FRR) was utilized; high FRR indicated superior fouling resistance of the membranes. The equation used for FRR calculation was

(3)

where M is the weight of the collected permeates (kg), A is the membrane effective area (m2), and t is the permeation time (h). 2.7. Antifouling Tests. BSA solutions were immediately replaced in the filtration cell after the pure water tests. The flux of BSA solution Jp (kg/m2 h) was assessed on the basis of the collected water weight at 0.3 MPa for 90 min. The worked membranes were

⎛J ⎞ w,2 ⎟ × 100 FRR (%) = ⎜⎜ ⎟ J ⎝ w,1 ⎠ 13373

(4)



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Figure 5. Surface SEM images of (a) bare PVDF, (b) 0.05 wt % GO/PVDF, (c) 0.05 wt % TiO2/PVDF, and (d) 0.05 wt % rGO/TiO2/PVDF membranes.

3. RESULTS AND DISCUSSION 3.1. Characterization of rGO/TiO2 Nanocomposite. A series of characterization methods was used to confirm the successful synthesis of the prepared samples. Figure 2 shows the XRD patterns of GO, TiO2, and rGO/TiO2 nanocomposite. In the XRD pattern of GO, the characteristic reflection of GO was the peak centered at around 12° and appointed to (002) interplanar spacing and two weak peaks at about 26 and 43°.36 In the XRD pattern of TiO2, there were five distinguished peaks at 25.3, 37.9, 48.0, 54.6, and 62.8°, corresponding to anatase phase (JCPDS 21-1272).19 This confirms that using a hydrothermal method and TEOT as a precursor generates exclusively anatase phase TiO2. The average crystal size of TiO2, estimated from Scherrer’s formula, was 12.05 nm.37 As can be seen, the XRD diffraction pattern of rGO/TiO2 nanocomposite was similar to

To study the fouling process in detail, eq 5 was utilized to explain the fouling resistance of the prepared membranes.26,34 The fouling of the membrane can be represented as the resistance formed during the filtration process. The total fouling ratio (Rt) can be estimated using eq 5: ⎛J − J ⎞ w,2 p ⎟ × 100 R t = ⎜⎜ ⎟ ⎝ Jw,1 ⎠

(5)

2.8. Molecular Weight Cutoff (MWCO). To investigate the surface pore size of the membranes, their MWCO was calculated by measuring the rejection of polyethylene glycols (PEGs) (20, 35, and 100 kDa). The modified Dragendorff reagent method35 was used to measure the amount of PEG in the permeate to calculate the rejection values. 13374



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that of the pure TiO2, indicating that the crystal structure of TiO2 was not changed during the graphene oxide reduction process. In this study, all peaks of rGO/TiO2 nanocomposite were readily indexed to the anatase phase of TiO2 and no diffraction of GO was observed in the XRD pattern, possibly due to the low content (3 wt %) of GO loading or the relatively low diffraction intensity of GO as well as the reduction of GO to graphene sheet during the hydrothermal reaction.38 The functional groups of GO, TiO2, and rGO/TiO2 have been represented using FT-IR spectra (Figure 3). For the synthesized TiO2, a broad absorption peak at 3000−3600 cm−1 and a strong peak at 1640 cm−1 could be attributed to the vibration of OH groups of adsorbed water and the Ti−OH group. The absorption peak at around 500−600 cm−1 was assigned to the signal of Ti−O−Ti bond.39 As it is clear, GO had various functional groups such as aromatic CH bonds, epoxy, organic carbonate, alcoholic OH, aromatic CC bonds, CO, and COH. The peaks at 1730, 1630, and 1075 cm−1 were assigned as the carbonyl groups (CO), hydroxyl groups (OH), and epoxy groups (C−O). After the formation of rGO/TiO2 nanocomposite, the intensity of the mentioned peaks was decreased, indicating the partial reduction of GO.40 Moreover, it was found that the signal of Ti−O−Ti was shifted to a higher wavenumber around 600−700 cm−1 because of the combination of Ti−O−Ti and Ti−O−C vibration signals.41 Figure 4 shows the SEM images of GO, TiO2, and rGO/ TiO2 samples. The SEM image of GO (Figure 4a) obviously showed the wrinkled surface of graphene sheet with a wormlike structure randomly aggregated. This confirmed that the GO sheet had been successfully exfoliated from graphite containing ordered stacking graphene layers. Figure 4b shows the SEM image of hydrothermally synthesized TiO2 nanoparticles. The obtained SEM images of TiO2 nanoparticles were analyzed using manual microstructure distance measurement software (Nahamin Pardazan Asia Co., Mashhad, Iran) to evaluate the diameter size distribution of the synthesized samples. Figure 4c shows the diameter size distribution of TiO2 nanoparticles. As can be seen, the diameter size in >95% of the nanoparticles was 0.05 wt %, the viscosity of the casting solution was increased, thereby reducing the overall porosity and the mean pore radius of the membranes and subsequently, leading to a decrease in the permeability.49 Zinadini et al.48 and Wang et al.25 have reported a similar behavior for the GO blended polymeric membranes. 3.2.3. Determining the MWCO of the Prepared Membranes. The selection of an ultrafiltration membrane for any application is commonly based on the MWCO given by the manufacturer. In general, the assessment of membrane MWCO enables the appropriate engineering of membrane systems to process design and predict their performance.50 A membrane’s MWCO refers to the approximate molecular weight of a dilute globular solute (i.e., a typical protein), which is 90% rejected by the membrane when a range of different MW solutes are filtered in the desired solvent. This indicates that a membrane retention performance should be investigated to calculate a MWCO value. In this study, polyethylene glycols (PEGs) with different molecular weights (20, 35, and 100 kDa) were subjected to pressure filtration tests, and the obtained rejection results were used to determine the membrane’s MWCO value. Figure 9 shows the retention curve of the prepared membranes used to determine apparent MWCO. The apparent MWCOs were calculated as the corresponding PEG molecular weight of 90% was rejected (as shown for two membranes in Figure 9). As can be seen in Table 3, all of the membranes gave the MWCO of around 50−80 kDa. The difference in the MWCO value for bare PVDF and blended PVDF membranes may be related to the interaction between PEG chains and nanoparticles. The low MWCO of the blended membranes, despite their big pore size, can be related to the interactions between PEG macromolecules and functional groups on the blended membranes surface. This led to high retention of PEG solution and low MWCO values in the blended membranes. 3.2.4. Antifouling Property of the Prepared Membranes. Commonly, the flux reduction demonstrates that the fouling phenomena occur on the membrane in which the electrostatic force, hydrogen bonding, hydrophobic, and van der Waals forces are responsible for the fouling of the membrane

Table 1. Surface Roughness Parameters of rGO/TiO2 Blended PVDF Membranes Resulted from Analyzing Three Randomly Chosen AFM Images roughness parameters membrane

Sa (nm)

Sq (nm)

Sy (nm)

bare PVDF 0.01 wt % rGO/TiO2/PVDF 0.02 wt % rGO/TiO2/PVDF 0.05 wt % rGO/TiO2/PVDF 0.075 wt % rGO/TiO2/PVDF 0.1 wt % rGO/TiO2/PVDF 0.05 wt % GO/PVDF 0.05 wt % TiO2/PVDF

7.30 5.12 4.53 1.42 5.53 4.96 2.13 2.67

8.75 7.21 6.20 1.51 7.49 7.06 2.77 3.59

55.35 49.52 47.07 22.05 62.42 47.46 25.09 29.20

nanocomposite to PVDF membrane could improve its hydrophilicity. Table 2 shows the decrease in the contact angle with the addition of rGO/TiO2 in all concentrations of the nanocomposite compared to the bare PVDF. This could be attributed to the presence of oxygenated hydrophilic groups on the rGO/TiO2 surface, which might have a favorable effect on increasing the pure water flux of the blended membranes. The slight increase in the contact angle for rGO/TiO2 concentration >0.02 wt % could be related to the agglomeration of the nanocomposite,42 which reduced the contact area of hydrophilic groups carried by the rGO/TiO2. The overall porosity and mean pore size of the rGO/TiO2/ PVDF membranes are also presented in Table 2. As can be seen, the porosity and mean pore radius of the blended membranes were slightly higher than those of the pure PVDF membrane. According to the mean pore size values reported in Table 2, the mean pore radius of the membranes was increased with rGO/TiO2 amount up to 0.05 wt % and then decreased. This trend was similar to the water permeability of the membranes, which will be discussed below (Figure 8). The results of BSA rejection by rGO/TiO2/PVDF membranes are also presented in Table 2. The BSA rejection was increased for all of the blended membranes with different ratios of the nanocomposite compared to the bare PVDF. As can be seen, all prepared blended membranes rejected the BSA >95%. It was confirmed that improving the hydrophilicity of membranes could increase the pure water flux. Figure 8 shows the pure water flux of the fabricated rGO/TiO2/PVDF membranes. As can be seen, the trend of increase in pure water flux was similar to the contact angle and hydrophilicity trend. It has been well established that the enhanced hydrophilicity of the membranes can increase the water permeability by attracting water molecules inside the membrane matrix and facilitating their permeation through the membrane. The results demonstrated that the pure water flux had the maximum value when

Table 2. Contact Angle, Porosity, and BSA Rejection Values of Membranes membrane bare PVDF 0.01 wt % rGO/TiO2/PVDF 0.02 wt % rGO/TiO2/PVDF 0.05 wt % rGO/TiO2/PVDF 0.075 wt % rGO/TiO2/PVDF 0.1 wt % rGO/TiO2/PVDF 0.05 wt % GO/PVDF 0.05 wt % TiO2/PVDF

contact angle (deg) 78.0 71.3 69.9 71.5 72.5 74.2 74.3 67.7

± ± ± ± ± ± ± ±

1.7 4.1 2.1 3.2 1.2 3.7 2.3 1.7

porosity (%) 72.95 73.45 73.20 83.10 82.10 69.70 81.95 75.90 13377

± ± ± ± ± ± ± ±

2.33 4.31 4.59 2.12 1.12 0.35 1.20 0.85

BSA rejection (%) 94.7 97.5 98.0 98.5 96.2 96.2 96.5 98.2

± ± ± ± ± ± ± ±

2.8 2.3 1.4 1.1 1.4 1.0 2.1 1.1

mean pore size (nm) 55.1 72.4 73.6 75.5 72.3 69.5 70.2 60.8

± ± ± ± ± ± ± ±

1.3 3.1 2.3 2.1 1.1 0.9 1.5 1.1



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Figure 8. Pure water flux of the prepared membranes (after 90 min at 0.3 MPa).

Figure 9. Retention curve of the prepared membranes used to determine apparent MWCO.

respectively. This confirmed the better performance of rGO/TiO2 nanocomposite in improving the permeability and antifouling properties of PVDF membranes compared to the individual using GO or TiO2. Also, the effect of different amounts of the rGO/ TiO2 nanocomposite on the BSA flux and the pure water flux (before and after BSA filtration) is shown in Figure 10b. As can be seen, the 0.05 wt % rGO/TiO2 blended membrane had the highest water permeability and the BSA flux imparted superior antifouling properties of the resulting rGO/TiO2 embedded PVDF membranes in this concentration. On the other hand, as is obvious from panels a and b of Figure 10, the permeate flux declined sharply when pure water was replaced with the BSA solution and then became stable. This could be related to the accumulation of proteins near the membrane surface, which led to the concentration polarization effect and influenced the flux of the membranes severely. The FRR (%) of the membranes after BSA fouling was calculated using eq 4 as presented in Figure 11. The FRR (%) of all blended membranes was clearly higher than that of the bare PVDF membrane. The bare membrane FRR was only 25.5% after 90 min. This low recovery ratio displayed the poor antifouling property of the bare membrane. In contrast, the

Table 3. Apparent Molecular Weight Cutoff (MWCO) Values of the Prepared Membranes membrane

MWCO (kDa)

bare PVDF 0.01 wt % rGO/TiO2/PVDF 0.02 wt % rGO/TiO2/PVDF 0.05 wt % rGO/TiO2/PVDF 0.075 wt % rGO/TiO2/PVDF 0.1 wt % rGO/TiO2/PVDF 0.05 wt % GO/PVDF 0.05 wt % TiO2/PVDF

87 80 64 53 66 85 46 54

surface.51 In this study, the flux changes were monitored for 270 min in pure water or alternating 500 mg/L BSA solution every 90 min, as shown in Figure 10. The permeability performance of bare PVDF, 0.05 wt % GO/PVDF, 0.05 wt % TiO2/ PVDF, and 0.05 wt % rGO/TiO2/PVDF was investigated and is presented in Figure 10a. As can be seen, the BSA flux of the bare PVDF was 50.30 kg/m2 h after 90 min of filtration, whereas this value was 102.64, 88.00, and 111.02 for 0.05 wt % GO/ PVDF, 0.05 wt % TiO2/PVDF, and 0.05 wt % rGO/TiO2/PVDF, 13378



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Figure 10. Time-dependent fluxes of PVDF mixed matrix membranes with (a) different additives and (b) different concentrations of rGO/TiO2 nanocomposite during BSA filtration at 0.3 MPa. The filtration process involves four steps: pure water flux, BSA filtration, water washing, and pure water flux measurement of the cleaned membranes.

rGO/TiO2 arose from various oxygenated groups in the surface of membranes owing to the placement of the rGO/TiO2 nanocomposite in the surface of the prepared membranes. This can decrease the membrane surface contact angle (see Table 2) and increase membrane hydrophilicity, as well as the hydrophobic adsorption between BSA protein and modified membrane surface. These all lead to a reduction of membrane fouling.42 The observed trend in the FRR (%) of the membranes can be also explained by AFM analysis results. As mentioned already, the surface roughness of the bare PVDF membranes was clearly higher than that of the rGO/TiO2 blended membranes. On the other hand, the membrane fouling tendency was increased with

FRR of the 0.05 wt % rGO/TiO2/PVDF membrane was 88.1%, and the flux recovery ratio of the rGO/TiO2 blended membranes with 0.075 and 0.1 wt % nanocomposite was decreased slightly compared with the former. In fact, the causes of membrane fouling are quite complex. The better antifouling property of the nanocomposite membranes can be related to the increasing hydrophilicity, a more negative zeta potential, and the improved smoothness of the membrane surface. Hydrophilicity affects the surface adsorption characteristics of the membrane. Improving the hydrophilicity of a membrane can reduce membrane fouling to some extent. The increased hydrophilicity of membranes modified with 13379



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Figure 11. Flux recovery ratios of the rGO/TiO2/PVDF membranes after protein fouling. (The average of four replications is reported.)

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increasing roughness due to protein accumulation in the “valleys” of the rough membrane surface. As can be seen in Table 1, the membrane containing 0.05 wt % rGO/TiO2 had the smoothest surface (the lowest roughness parameters) and gave the best flux recovery ratio to this membrane (see Figure 11). The roughness parameters of the membranes modified with concentrations >0.05 wt % were increased, leading to a reduction in flux recovery ratio. The roughness increase can be related to the accumulation of the hydrophilic rGO/TiO2 nanocomposite on the membrane surface in the high contents of the nanocomposite in the membrane structure.

4. CONCLUSION In this study, a novel rGO/TiO2 blended PVDF membrane was prepared using the phase inversion method. The incorporation of rGO/TiO2 nanocomposite to the polymer casting solution obviously improved the properties and performance of the membrane. The hydrophilicity and permeability of the blended membranes were enhanced due to the addition of the rGO/ TiO2 nanocomposite containing various oxygenated hydrophilic groups. Also, the rGO/TiO2 blended PVDF membranes had higher flux recovery ratios compared with the bare PVDF membrane. The SEM images confirmed that the fabricated mixed matrix UF membranes had a finger-like structure. On the basis of the achieved hydrophilicity, pure water flux, and antifouling results, the best content of rGO/TiO2 nanocomposite was 0.05 wt % in the casting solution. The results of this study confirmed that the rGO/TiO2 nanocomposite is an excellent antifouling additive, so it can be a promising material for new applications in the membrane field.

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SYMBOLS AND ABBREVIATIONS ε = overall porosity η = water viscosity (8.9 × 10−4 Pa s) ω1 = weight of the wet membrane (g) ω2 = weight of the dry membrane (g) ΔP = operation pressure (MPa) Δt = permeation time (h) A = membrane effective area (m2) AFM = atomic force microscopy dw = water density (0.998 g/cm−3) FRR = flux recovery ratio FT-IR = Fourier transform infrared GO = graphene oxide Jw,1 = water flux (L/m2 h) Jw,2 = flux of cleaned membrane (L/m2 h) l = membrane thickness (m) NMP = N-methyl-2-pyrrolidone PVDF = polyvinylidene flouride PVP = polyvinylpyrrolidone Q = volume of the permeate pure water (l) rGO = reduced graphene oxide rm = mean pore radius (nm) Rt = total fouling ratio Sa = average roughness (nm) Sq = root mean square of the Z data (nm) Sy = mean difference between highest peaks and lowest valleys (nm) SEM = scanning electron microscopy TiO2 = titanium dioxide REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*(A.K.) E-mail: [email protected] or ar_khataee@yahoo. com. Tel.: +98 411 3393165. Fax: +98 411 3340191. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Tabriz (Iran) for all of the support provided. We also sincerely thank the Petrochemical Research & Technology Co. (Tehran, Iran) for support. 13380



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