Preparation of a Novel Polyvinylidene Fluoride (PVDF) Ultrafiltration

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Preparation of a novel PVDF ultrafiltration membrane modified with reduced graphene oxide/TiO2 nanocomposite with enhanced hydrophilicity and antifouling properties Mahdie Safarpour, Alireza Khataee, and Vahid Vatanpour Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie502407g • Publication Date (Web): 08 Aug 2014 Downloaded from http://pubs.acs.org on August 11, 2014

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Preparation of a novel PVDF ultrafiltration membrane modified with reduced graphene oxide/TiO2 nanocomposite with enhanced hydrophilicity and antifouling properties

Mahdie Safarpour 1, Alireza Khataee 1,*, Vahid Vatanpour 2,3

1

Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department

of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-14766 Tabriz, Iran 2

Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran

3

Novel Technology Research Group, Petrochemical Research and Technology Company,

14977-13115 Tehran, Iran

* Corresponding author E–mail address: [email protected] ([email protected]) Tel.: +98 411 3393165; Fax: +98 411 3340191

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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 techniques. The prepared rGO/TiO2 blended PVDF membranes were characterized by atomic force microscopy, SEM, water contact angle, porosity and permeation measurements and rejection tests. Due to the high hydrophilicity of 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 0.05 wt.% rGO/TiO2 membrane had the best fouling resistance.

Keywords: Nanostructured membranes; Ultrafiltration; Graphene oxide; TiO2 nanoparticles; Antifouling.



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1. Introduction Membrane technology has attracted much attention over the last 30 years with extensive use in various industrial fields such as water desalination, ultra-pure 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 it 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. In spite of 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, the complicated fabrication process and the low membrane surface per apparatus specific volume. So, the cheap and easy-fabricating 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 like 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 membrane fouling problem. Several methods have been investigated to increase the hydrophilicity of PVDF membranes. The most reported techniques are addition of hydrophilic polymers

9

or blending of nanoparticles

chemical modification such as sulfonation

13

10-12

with casting solution,

and immobilization of polymers with

hydrophilic segments on the surface of membranes by coating

14

or grafting with photo or

plasma polymerization.15-17 Among various approaches used to control membrane


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hydrophilicity and fouling, the blending of inorganic nanostructured materials has attracted great attention.18 Titanium dioxide (TiO2) is known as the most widely used catalyst in the 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 showed that the excessive addition of the inorganic additives may negatively affect the morphology and elasticity of the PVDF membranes.24-25 To overcome these disadvantages, the search for identification of 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 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 Another potential option to efficiently modify the polymeric membranes is graphene and graphene oxide (GO).25, 28-29 Interestingly, graphene has a high aspect ratio as well as a low density and high strength and stiffness. Nevertheless, the chemically inert nature of graphene prevents its dissolving in usual organic solvents. In contrast to that, the affinity resulted 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 polymer matrix can considerably improve physical properties such as mechanical strength of the host polymers at a very low additive amount.25



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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 it would help better distribution of 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. The analytical grade H2SO4 (purity 98%), KMnO4 (purity 99%), H2O2 (30% aqueous solution), PEG polymers with different molecular weights, ethanol and N-methyl-2pyrrolidone (NMP, EMPLURA®) were purchased from Merck Co., Germany. PVDF was purchased from Alfa Aesar (Germany). Polyvinyl pyrrolidone (PVP) with the molecular weight of 29,000 g/mol, tetraethyl orthotitanate (Titanium (IV) ethoxide, Ti (OC2H5)4) and bovine serum albumin (BSA, MW=67,000) were purchased from Sigma-Aldrich Co., 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: firstly, the desired amount of graphite powder was added to the cooled (0 ºC) mixture of concentrated H2SO4 and HNO3 (H2SO4/HNO3: 4/1 v/v) under vigorous stirring.


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Second, 15 g KMnO4 was slowly added to the above mixture while stirring in an ice-water bath. Then, the mixture was stirred at 35 ºC for 2 h and 250 mL of distilled water was slowly added to the mixture which increased the temperature to 98 ºC and maintained that temperature for 30 min. Finally, 750 mL water was poured quickly into the mixture to terminate the reaction, and 20 mL 10% H2O2 was added to reduce the residual KMnO4 and MnO2. The resulted suspension was separated and washed several times with distilled water, and the product was dried at 60 ºC. The resulted powder was sonicated for 90 min in a bath type sonicator (Sonica, 2200 EP S3, 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 condition at 180 ºC 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 in order to remove residual impurities and then dried at 60 ºC for 12 h. Pure TiO2 nanoparticles were also obtained using the same method in the absence of GO. Fig. 1 shows the schematic of rGO/TiO2 nanocomposite synthesis. It should be noted that it was not the purpose of this study to reduce GO by hydrothermal treatment. Hydrothermal method was used just for synthesis of TiO2 nanoparticles. But, 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 not any hydrothermal treatment and reduction of oxygenated groups.



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2.4. Preparation of rGO/TiO2/PVDF membranes rGO/TiO2 blended PVDF ultrafiltration membranes were fabricated by the phase inversion technique using PVDF as a bulk material, NMP as a solvent, rGO/TiO2 nanocomposite as an additive and distilled water as a non-solvent 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 ºC and stirred for 48 h. Afterward, the homogenous 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 150µm 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 ºC).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 (Germany), with Cu Kα radiation (1.54065 Å). The 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, 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 (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


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membrane surfaces was evaluated using water contact angle analysis (G10, Kruss, 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 (Switzerland) equipped with Nanosurf® MobileS 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 the Eq. (1) 32:

ε=

ω1 − ω2

(1)

A × l ×dw

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). 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 − 1.75 ε ) × 8η l Q ε × A × ∆P

(2)

In this equation, η is the water viscosity (8.9×10-4 Pa s), Q is the volume of the permeated pure water per unit time (m3/s) and ∆P 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= 67,000) (0.5 g/L).



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All experiments were directed at 25 ºC 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 4 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 UVspectrophotometer (Shimadzu UV-2450, Japan). The pure water flux, Jw,1 (kg/m2 h), was calculated through Eq. (3): J w,1 =

M At

(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 based on the collected water weight at 0.3 MPa for 90 min. The worked membranes were 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-resistantability of the membranes. The equation used for FRR calculation was as:

J  FRR (%) =  w,2  ×100 J   w,1 

(4)

In order to study the fouling process in detail, equation (5) was utilized to explain the fouling resistance of the prepared membranes.26, 34 The fouling of the membrane can be represented


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as the resistance formed during the filtration process. The total fouling ratio, (Rt), can be estimated using Eq. (5):

 J − Jp  Rr =  w,2  ×100  J w,1  

(5)

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

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. Fig. 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) inter-planar 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 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 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



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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 (Fig. 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 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, etc.. 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 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 Fig. 4 shows the SEM images of GO, TiO2 and rGO/TiO2 samples. The SEM image of GO (Fig. 4a) obviously showed the wrinkled surface of graphene sheet with a worm-like structure randomly aggregated. This confirmed that GO sheet had been successfully exfoliated from graphite containing ordered stacking graphene layers. Fig. 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., Iran) to evaluate the diameter size distribution of the synthesized samples. Fig. 4c shows the diameter size distribution of TiO2 nanoparticles. As can be seen, the diameter size in more than 95% of the nanoparticles was

Preparation of a Novel Polyvinylidene Fluoride (PVDF) Ultrafiltration

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