Preparation of a novel antifouling mixed matrix PES membrane by embedding graphene oxide nanoplates

Journal of Membrane Science 453 (2014) 292–301

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Preparation of a novel antifouling mixed matrix PES membrane by embedding graphene oxide nanoplates Sirus Zinadini a, Ali Akbar Zinatizadeh a, Masoud Rahimi b,n, Vahid Vatanpour c, Hadis Zangeneh a a

Water and Wastewater Research Center (WWRC), Department of Applied Chemistry, Razi University, Kermanshah, Iran CFD Research Centre, Department of Chemical Engineering, Razi University, Taghe Bostan, Kermanshah, Iran c Faculty of Chemistry, Kharazmi (Tarbiat Moallem) University, Tehran, Iran b

art ic l e i nf o

a b s t r a c t

Article history: Received 11 June 2013 Received in revised form 29 October 2013 Accepted 30 October 2013 Available online 20 November 2013

A novel polyethersulfone (PES) mixed matrix nanofiltration membrane containing graphene oxide (GO) nanoplates was prepared via the phase inversion method. The effect of the embedded nanosheet on the morphology and performance of the fabricated new membranes was investigated in terms of pure water flux, dye removal and fouling parameters. Scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle and porosity measurements were employed to characterize the prepared membranes. FT-IR spectra of the graphene oxide nanoplates revealed that the hydroxyl and carboxylic acid groups are formed on the surface of the graphene oxide. The water flux from the nanocomposite membranes improved significantly after addition of graphene oxide to the casting solution, due to the higher hydrophilicity of the prepared membranes. The water contact angle measurement confirmed the increased hydrophilicity of the modified membranes. The morphology studies by SEM showed the wider finger-like pores of the GO incorporated membranes in comparison with those of the unfilled PES membrane. Evaluation of the nanofiltration performance was performed by investigating the retention of Direct Red 16. It was observed that the GO membranes have higher dye removal capacity than the unfilled PES. Fouling resistance of the membranes assessed by powder milk solution filtration revealed that 0.5 wt% GO membrane had the best antibiofouling property. In addition, the results showed that the 0.5 wt% GO membrane had the highest mean pore radius, porosity, and water flux. The prepared GO nanocomposite membrane showed noteworthy reusability during filtration. & 2013 Elsevier B.V. All rights reserved.

Keywords: Membrane Nanofiltration Antifouling Dye removal Direct Red 16

1. Introduction After successful isolation of graphene into the free standing form in 2004, i.e. a single 2D carbon sheet with one-atom-layerthick and the same structure as the individual layers in graphite [1], research in this field has become more attracting for scientific community. This is due to its unique two-dimensional structure, large specific surface area, good mechanical properties, high transparency, etc [2,3]. Application of graphene and its derivatives such as graphene oxide in the preparation of membranes can be designed from two aspects. The first one is direct use of graphene as a separating layer [4–7] and second one is incorporating graphene in polymer matrix for improving the membrane performance [8–10]. The selective properties of nanoporous graphene membranes have been considered in literature. The investigations show that the nanoporous graphene films can be employed for selective separation of various n

Corresponding author. Tel.: þ 98 8314274530; fax: þ 98 8314274542. E-mail addresses: [email protected], [email protected] (M. Rahimi).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.10.070

gases [4] and ions [5] through design of various nanopores with different shapes, sizes, and chemical functionalities. Han et al. [6] presented a procedure for fabrication of ultrathin (E22–53 nm thick) graphene nanofiltration membranes (uGNMs) on microporous substrates (polymeric microfiltration membranes). They used chemically converted graphene (CCG) for efficient water purification. In another study, Sun et al. [7] fabricated free standing graphene oxide (GO) membranes by a simple drop-casting method. The GO colloidal suspension was drop-cast onto a piece of smooth paper, allowed to dry in air at room temperature and consequently peeled from the underlying paper. The resulted NF membranes showed that sodium salts could be effectively separated from copper salts and organic contaminants. When graphene is appropriately incorporated into a polymer [11,12] or ceramic [13] matrices, the special properties of obtained nanocomposites improves remarkably. Polymeric nanocomposites of graphene derivatives have been used in the preparation of different membranes for fuel cell exchange membrane [8,9], ultrafiltration [10,14,15], nanofiltration [16,17], pervaporation [16] and gas separation [18] applications.

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293

Fig. 1. The schematic of graphene oxide preparation.

Zhang et al. [14] prepared polyvinylidene fluoride (PVDF)– graphene oxide composite ultrafiltration membranes using the phase inversion method. The prepared composite membranes exhibited a bigger mean pore size and higher roughness parameters compared with pristine membranes. Moreover, they demonstrated an impressive prospect for the anti-irreversible fouling performance in multi-cycle operations from bovine serum albumin (BSA) treatment. Wang et al. [15] described fabrication of PVDF/GO organic–inorganic-blended ultrafiltration membranes. Due to the hydrophilic nature of GO, the resulted membranes appeared to be more hydrophilic and have higher pure water fluxes recovery ratio. GO incorporation caused a decrease in the contact angle and improve in antifouling ability. Ganesh et al. [17] reported preparation of GO dispersed polysulfone (PSf) mixed matrix membranes and their performance was tested in terms of pure water flux and salt rejection. The prepared membranes enhanced the salt rejection, pure water flux and hydrophilic properties after GO doping. However, influence of graphene oxide nanoplates on fouling reduction of PSf membrane was not investigated. Polyethersulfone (PES) is one of the most used polymers in the preparation of commercial and laboratory ultrafiltration and nanofiltration membranes [19–21]. However, fouling is the major problem in this type of polymers as well as in other polymeric membranes. Membrane fouling negatively affects membrane performance by decreasing water permeability. In order to mitigate PES membrane fouling, several approaches such as blending with hydrophilic polymers [22,23], grafting with hydrophilic monomers [24,25], grafting with short-chain molecules [26], embedding hydrophilic nanoparticles [20,27], etc. have been suggested. The blending of inorganic nanoparticles into the membrane matrix has also been utilized to reduce fouling in membranes, attributed to an increase in hydrophilicity or change in membrane morphology [28]. Therefore, hydrophilic properties of the membranes can be induced by GO incorporation due to its functional hydrophilic groups. It can also change the roughness and mechanical strength of the host polymer and has an influence in membranes fouling. In the present work, the influences of blending GO on performance and antifouling properties of the polyethersulfone nanofiltration membrane have been investigated. GO was incorporated in PES matrix using the phase inversion method. The membrane structure and properties were characterized using AFM, SEM, FTIR and water contact angle measurements. Nanofiltration performance was tested by rejection of Direct Red 16 dye. Fouling resistance of the prepared mixed matrix membranes was also studied.

2. Experimental 2.1. Materials All chemicals used in the experiments were of reagent grade. Polyethersulfone (Ultrason E 6020P, Mw ¼58,000 g/mol and glass transition temperature Tg ¼225 1C) and dimethylacetamide

Table 1 The compositions of casting solutions. Membrane type

PES (wt%)

PVP (wt%)

GO nanoplate (wt%)

Unfilled PES M1 M2 M3

20.0 20.0 20.0 20.0

1.0 1.0 1.0 1.0

– 0.1 0.5 1.0

(DMAc) as solvent were supplied from BASF Co., Germany. Polyvinylpyrrolidone (PVP) with a molecular weight of 25,000 g/mol, potassium permanganate (KMnO4) and sulfuric acid (H2SO4) (98 wt%) were supplied from Merck. The azo dye, Direct red 16, C26H17N5Na2O8S2 (MW ¼637.26) with purity of 99% was purchased from Alvan Sabet Co., Iran. Extra pure fine graphite with a particle size less than 50 μm was obtained from Merck Co. Deionized water was used throughout this study. 2.2. Preparation of graphene oxide (GO) nanoplate The graphene oxide (GO) was prepared from natural graphite by the Hummers method as reported in the literature [29,30]. In the first stage, 5 g of graphite powder was added to the concentrated H2SO4 in an ice-bath. Consequently, 7 g potassium permanganate was slowly added while preserving the temperature below 20 1C. The mixture was stirred at 35 1C for 30 min and slowly added into the deionized water (250 ml), followed by stirring the mixture at 98 1C for 15 min in order to increase the oxidation degree of the GO product. The treatment of graphene was terminated by adding 750 ml of 2 wt% H2O2 with stirring at 10 1C. The GO slurry was washed with de-ionized water and centrifuged several times to clean out the remained salt until a neutral pH was reached. The final GO slurry was sonicated for 1 h followed by filtering and drying in a vacuum oven at 40 1C for 24 h. A schematic view from the GO preparation process is presented in Fig. 1. 2.3. Fabrication of asymmetric GO nanoplate/PES nanofiltration membranes The asymmetric flat sheet PES dense membranes containing graphene oxide nanoplates were fabricated by phase inversion induced by the immersion precipitation technique. The component of casting solutions was PES (20 wt%), PVP (1 wt%) and measured amounts of GO nanoplates in DMAc as solvent. The compositions of casting solutions for all membranes are listed in Table 1. Precise amounts of GO nanoplates were dispersed into DMAc and sonicated for 30 min to prepare homogenous solutions using DT 102H Bandelin ultrasonic (Germany). After sonication, PES and PVP were dissolved in the dope solution by continuous stirring for 24 h. A sonication-assisted method was again used for 10 min to remove air bubbles. Afterwards, the solution was cast

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Fig. 2. Structure of Direct Red 16 dye.

onto a clean glass plate with 200 mm thickness using self-made casting knife. Subsequently, the glass plate was immediately immersed into the non-solvent bath (distilled water at a temperature of 15 1C) without any evaporation. After primary phase separation and membrane formation, the membranes were stored in fresh distilled water for 24 h to ensure complete phase separation. Finally, for the drying process, the membranes were sandwiched between two sheets of filter papers for 24 h at room temperature. 2.4. Characterization of the GO nanoplate and prepared membranes

thickness (m). All tests were replicated three times and the mean values were taken into account. In addition, in order to determine the membrane mean pore radius (rm), Guerout–Elford–Ferry equation (Eq. (2)) on the basis of the pure water flux and porosity data was used [32,33]. rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2:9  1:75εÞ8ηlQ ð2Þ rm ¼ ε  A  ΔP where η is the water viscosity (8:9  10  4 Pa s), Q is the volume of permeated pure water per unit time (m3/s), and ΔP is the operational pressure (0.4 MPa). 2.6. Permeation and rejection tests

The measurements of the FT-IR spectra of the samples were performed using Bruker spectrometer (TENSOR 27). The morphology and structure of the prepared membranes were analyzed using Philips-X130 and Cambridge scanning electron microscopes (SEM). The membranes were cut into the small pieces and were cleaned with filter paper for removal of probably attached contaminants. The pieces were submerged in nitrogen liquid for 60– 90 s and were frozen. The frozen membranes were broken and kept in air for drying. The dried samples were gold sputtered to provide electrical conductivity. The photomicrographs were taken in very high vacuum conditions at 25 kV. Atomic force microscopy (AFM) images were prepared using a Nanosurfs Mobile scanning probe-optical microscope (Switzerland) equipped with Nanosurfs Mobile software (version 1.8), using the tapping mode in air. Approximately, 1 cm2 membrane samples were fixed on a specimen holder and 8.8 μm  8.8 μm of area were scanned. The surface roughness parameters were reflected in terms of the average roughness (Sa), the root mean square of the Z data (Sq) and the mean difference between the highest peaks and the lowest valleys (Sz). The concentration of the dye in each sample was analyzed using a UV–vis spectrophotometer (DR 5000, Hach, Jenway, USA) by measuring the absorbance at λmax ¼526 nm using an appropriate calibration curve [31]. The contact angle of the prepared membranes was analyzed using a contact angle goniometer (G10, KRUSS, Germany) at 25 1C and a relative humidity of 50%. Images of 2 μL de-ionized water droplets on the membrane surface were captured after reaching to a constant value. This was done in 30– 120 s after the drop placed upon the membrane surface. The average value of at least five random locations was reported to minimize the experimental error.

where CP is the concentration of a particular component of permeate and Cf is the feed concentration.

2.5. Porosity and pore size

2.7. Fouling tests of the membranes

The overall porosity (ε) was determined using gravimetric method, as defined in the following equation [32]:

After water flux tests, a milk powder solution (at concentration of 8000 mg/L as a good fouling agent) was quickly replaced in the stirred cell. The flux for milk powder, Jp (kg/m2 h), was measured based on the water quantity permeated through the membranes at 4 bar for 90 min. After filtration of milk powder, the fouled membranes were washed with distilled water for 15 min. Consequently, the water flux of cleaned membranes, Jw,2 (kg/m2 h), was measured again. The flux recovery ratio (FRR) can be defined as

ε¼

ω1  ω2 A  l  dw

ð1Þ

where ω1 is the weight of the wet membrane; ω2 is the weight of the dry membrane; A is the membrane effective area (m2), dw is the water density (998 kg/m3) and l is the membrane

The performance of the prepared novel nanofiltration membranes was characterized by measuring pure water flux, dye removal, and powder milk fouling tests. A dead-end stirred cell filtration system (200 ml volume) with a membrane effective surface area of 12.56 cm2 connected to a nitrogen gas line was used. Pressurized nitrogen gas was utilized to force the liquid through the membrane. The feed solution was stirred at a rate of 400 rpm. Prior to the permeation test, in order to obtain a steady flux, each membrane was compacted at 0.6 MPa with distilled water for 30 min. Then, the pressure was reduced to the operating pressure of 0.4 MPa. The water flux jw,1 (kg/m2 h) was calculated using the following equation: jw;1 ¼

M AΔt

ð3Þ

where M is the weight of the permeate pure water (kg), A is the membrane effective area (m2) and Δt is the permeation time (h). The experiments were carried out at 2071 1C and the average of three replicates was depicted in figures and tables. For evaluation of nanofiltration performance, the retention of Direct Red 16 (Fig. 2) was studied. An aqueous feed dye solution was prepared with concentration of 30 mg/L, which is within the range of typical concentration in textile wastewaters [34]. Rejection (R) at any point in the process is defined as follow:   Cp Rð%Þ ¼ 1   100 ð4Þ Cf

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follow [32]:   j FRR ¼ w;2  100 jw;1

295

ð5Þ

Generally, higher FRR indicates better antifouling property of the nanofiltration membranes. Also, in order to analyze the fouling process in details, total fouling ratio (Rt), reversible fouling ratio (Rr) and irreversible fouling ratio (Rir) were calculated using the following equations [32,35]:   jp Rt ð%Þ ¼ 1   100 ð6Þ jw;1  Rr ð%Þ ¼

jw;2  jp jw;1

 Rir ð%Þ ¼



jw;1  jw;2 jw;1

 100

ð7Þ

  100 ¼ Rt  Rr

ð8Þ

3. Results and discussion 3.1. Characterization of graphene oxide nanoplates For characterization of the graphene oxide, the Fourier transform infrared (FT-IR) test was carried out. The FT-IR spectrum of GO is shown in Fig. 3. The curve of GO shows a stretching vibration of CQO at 1720 cm  1, and two bands at 3420 cm  1 and 1390 cm  1 corresponding to the stretching vibration and deformation vibration of O–H [17]. The spectrum also shows two bands at 1078 cm  1 and 1234 cm  1 originated from the CQO stretching vibrations of alkoxy. This means that the carboxylic acid groups were formed on the surface of the graphene. The band at 1627 cm  1 is assigned to the vibrations of the adsorbed water molecules and the contributions from the vibration of aromatic CQC [36,37]. Whereas, IR spectrum of graphite is featureless [17], it can be concluded that graphene oxide is effectively functionalized. Other researchers [15,38] also presented similar results. 3.2. Characterization of graphene oxide membranes 3.2.1. Hydrophilicity and pure water flux The hydrophilicity of the membrane surface can be investigated by water contact angle measurement. The initial contact angle measured right away after the distilled water was dropped onto the membrane surface, which can reflect the natural wettability of

Fig. 3. The FT-IR spectrum of synthesized graphene oxide.

Fig. 4. Pure water flux (after 60 min) and static contact angle of the prepared membranes.

the material. Lower contact angle indicates that the membrane surface is more hydrophilic in nature. As shown in Fig. 4, the static contact angle declined considerably with the addition of the graphene oxide nanoplate into the polymer matrix. Unfilled PES membrane showed the highest water contact angle of 65.21. Addition of 0.1 and 0.5 wt% GO reduced the water contact angle to 58.61 and 53.21, respectively. However, the high contact angle of GO (1 wt%) did not have significant effect on decreasing the hydrophilicity. This is probably due to agglomeration and decreased effective surfaces of the nanoplates in the high blending ratio, which lead to reduction of the membrane surface functional groups. In the phase inversion preparation of the nanocomposite membranes, the mixed nanoparticles in the casting solution could migrate spontaneously to the surface of the prepared mixed matrix membranes to reduce the interface energy [35,39]. In the present study, this behavior was seen for the GO/PES blended membranes. The top and the bottom surface photographs of prepared membranes are represented in Fig. 5. The results illustrate that the top surface of the membrane was darker than the bottom-side. During membrane formation, the hydrophilic GO migrates towards the top surface of the membrane as the top layer was more exposed to water (non-solvent). This migration decorates the functional groups of GO on the membrane top surface and improve the membrane hydrophilicity. Similar results were also reported for the GO polysulfone matrix [17] and carbon nanotubes in PES [32,35]. It is well established that improving of hydrophilicity has an influence on the pure water flux. Fig. 4 reveals the pure water flux of the prepared GO/PES membranes. As shown in this figure, the trend of increasing in pure water flux is well matched with hydrophilicity improvement. The permeability of the membranes increased with increasing graphene oxide contents. The result shows that the pure water flux had the highest value when the amount of GO was 0.50 wt%. However, 1 wt% GO content caused a decrease in the flux. This flux reduction of 1 wt% GO embedded membrane can be attributed to pore blocking with high concentration of the nanoplates [27] which is observed as a reduction in porosity (Fig. 6) and mean pore radius (Fig. 7) for the prepared membrane. The hydrophilicity effect of GO can increase the solvent and non-solvent exchange during the phase-inversion process [35]. This can lead to a higher porosity in the membrane surface (Fig. 6) and improve the water permeability. However, when the GO content was greater than 0.50 wt%, the viscosity of the casting solution increases. This reduces the porosity and the mean pore radius of the membrane [40] and causes a decrease in the

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the same, a little higher pure water flux of 1 wt% membrane can be related to its higher contact angle and porosity. It should be mentioned that the rejection of protein, analyzed by the Bradford method [32], was more than 98% for all the prepared nanocomposite membranes, indicating that an increase in flux is not due to defects and cracks in membranes.

Fig. 5. Digital photograph of top and bottom surface of 1 wt% GO embedded PES membranes.

Fig. 6. The overall porosity of the graphene oxide/PES mixed matrix membranes (average and standard deviation of three replicates is reported).

5

4.5 4.5

3.8

Mean pore radius (nm)

4 3.5

3.8

3.2

3 2.5 2 1.5 1 0.5 0

Unfilled PES

M1

M2

M3

Fig. 7. Mean pore radius of the prepared membranes.

water flux. Wang et al. [15] reported a similar behavior for GO blended PVDF membranes. The overall porosity of the prepared GO/PES nanofiltration membranes is depicted in Fig. 6. All of prepared membranes have a suitable porosity in the range between 73% and 83%, which can be ascribed to low polymer concentration and addition of PVP [41]. Since the PVP additive is soluble in water, it can leach out from membrane matrix during the solvent exchange in the coagulation bath. Based on the mean pore radius presented in Fig. 7, the mean pore radius of the membranes enlarged with GO content up to 0.5 wt% and then decreased. This behavior is similar to the way that water passed through the membranes. Comparing the mean pore radius and pure water flux of 1 wt% and 0.1 wt% membranes reveal that although the mean pore radius of both membranes is

3.2.2. Morphology of the GO embedded PES membranes Fig. 8 shows the surface SEM images of 0.5 wt% graphene oxide blended polyethersulfone membrane in two resolutions. As shown in this figure, the surface is relatively smooth and the GO nanoplate mass or lump is not seen in the surface. In many noncarbonic nanoparticles such as TiO2 [27], boehmite [20], silica [28] and alumina [42] prepared by the phase inversion method, the nanoparticles are obviously observed in the membrane surface. Nevertheless, in case of carbon based nanofiller such as carbon nanotube [35,43], accumulation of CNTs was not observed. Therefore, due to carbon based structure of graphene oxide nanoplate and PES polymer, the GO was well dispersed in the polymer matrix and agglomeration on the surface was not observed. In addition, no cracks were distinguished on the surface, which indicates that the membranes did not become brittle by the addition of GO, and that they have good stability. The cross-sectional SEM images of the prepared membranes with different graphene oxide contents are exhibited in Fig. 9. All of the membranes show typical characteristic of asymmetric porous structure with a dense skin top-layer followed by a finger-like porous sub-layer. The finger-like pores for all of the GO embedded membranes are slightly wider than that of the unfilled PES membrane. The hydrophilic nature of graphene oxide increases the mass transfer rate between the solvent and the nonsolvent during phase inversion and lead to the formation of larger pore channels [35]. In addition, some lateral pore structures appeared when the GO content increased specially in 0.5 wt% GO embedded membranes (Fig. 9f). These lateral pores improved the water fluxes of the membranes. In the preparation of GO blended PVDF membranes, the same lateral pores were also observed by Wang et al. [15]. In the present study, the surface roughness of the prepared mixed matrix membranes was investigated using atomic force microscopy (AFM) measurements. Fig. 10 shows the two and three-dimensional AFM images of the membrane surfaces. In these images, the brightest area demonstrates the highest point of the membrane surface and the dark regions illustrate the valleys or membrane pores. The roughness parameters of the surfaces of the GO/PES nanocomposite membranes are presented in Table 2, which are calculated in an AFM scanning area of 8 μm  8 μm. The surface roughness of the unfilled PES membranes was obviously higher than that of the GO mixed membranes. After graphene oxide was blended, the large peaks and valleys were substituted by many small ones, which led to a smooth membrane surface. The same behavior was reported by Zhao et al. [44] for GO/PVDF and Zhao et al. [45] for isocyanate-treated graphene oxide/PSf membranes. For low concentrations of modified carbon based nanofiller, because of low electrostatic interactions, they are regularly collocated in membrane. Therefore, the membrane surface became smooth [40]. The AFM images show that when the GO content increases, the surface roughness also increases. This may be due to the fast exchange of solvent and non-solvent occurring during the phase inversion process because of the hydrophilic nature of GO [46]. However, even in the case of 1 wt% GO, the average roughness of GO blended membrane is lower than unfilled PES membrane. A similar observation was previously reported for mixing of high concentration of MWCNTs in PES membrane [35].

S. Zinadini et al. / Journal of Membrane Science 453 (2014) 292–301

Fig. 8. Surface SEM images of 0.5 wt% GO embedded PES membrane.

Fig. 9. Cross-section SEM images of the prepared membranes. (a and b) Unfilled PES, (c and d) GO 0.1 wt%, (e and f) GO 0.5 wt% and (g and h) GO 1 wt%.

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Fig. 10. AFM images of the GO mixed matrix PES membranes with different concentrations: (a) Unfilled PES, (b) 0.1 wt%, (c) 0.5 wt% and (d) 1 wt%.

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25

Table 2 Surface roughness parameters of graphene oxide blended PES membranes resulted from analyzing three randomly chosen AFM images.

Unfilled PES M1 M2 M3

Unfilled PES M1 M2

20

M3

Roughness parameters Sa (nm)

Sq (nm)

Sz (nm)

20.4 71.7 8.0 71.1 10.2 71.6 12.8 71.9

28.17 2.5 10.0 7 1.2 13.0 7 1.8 19.4 7 2.4

241.7 7 45.6 69.5 7 19.3 169.2 7 22.8 331.9 7 52.7

Flux(kg/m2h)

Membrane

299

15

10

5

0 0

50

100

150

200

250

Time (min) Fig. 12. Flux versus time for the GO blended PES membranes at 0.4 MPa during three steps: water flux for 60 min, 8000 ppm milk powder solution flux (pH¼ 770.1) for 90 min, and water flux for 60 min after 20 min washing with distilled water.

Fig. 11. Dye retention performance of the prepared GO/PES nanofiltration membrane (0.4 MPa, pH ¼ 6.0 7 0.1, 30 mg/L Direct Red 16, after 60 min filtration).

3.2.3. Nanofiltration performance Evaluation of nanofiltration performance of the prepared membranes was performed by an investigation on Direct Red 16 retention. Using a dead-end permeation cell, the nanofiltration membrane dye rejection was tested under operating pressure of 0.4 MPa, pH ¼6 and dye concentration of 30 mg/L. The retention results after 60 min filtration of dye solution are shown in Fig. 11. All the prepared membranes showed a high dye rejection. This is a typical behavior of nanofiltration membranes. However, the rejection capability of the prepared GO blended membranes was higher than that of unfilled PES membrane. It has been reported that the GO can induce surface negative charge throughout the entire pH range [47]. Due to acidic functional groups of graphene oxide [48], it can induce negative charge on the surface of the prepared membranes, causing high retention between negative dye and negative surface. Reduction in retention of dye in 0.5 wt% GO/PES membrane can be attributed to an increase of the membrane pore radius (Fig. 7). The increasing of membranes fluxes resulted in the reduction of dye rejection. 3.3. Antifouling performance The performance of membrane filtration, in terms of fluid separations and usage life time, depends greatly on the membrane fouling. The membrane flux is reduced with many complications such as the blockage or plugging of pores within the membrane as well as the concentration polarization and cake layer formation on the membrane surface [49]. A good quality membrane should possess the characteristics of high flux, low fouling tendency and high rejection rate over a prolonged period of time. The causes of membrane fouling are truly complex. Poor antifouling property is mainly caused by the hydrophobic behavior of membranes surfaces [50]. In order to improve membrane permeability and antifouling property, many efforts have been devoted to enhance membrane hydrophilicity, including material

modification, polymer blend and surface modification [51,52]. Among these approaches, blending with hydrophilic particles has been considered as an effective and convenient approach for preparation of antifouling membranes [53]. The presence of the inorganic particles in the polymeric membranes improves the membrane performance in terms of their flux and fouling resistance. The antibiofouling performance of the unfilled PES and the GO modified nanofiltration membranes was analyzed by measuring the water flux recovery after the membrane was fouled by 8000 ppm powder milk solution. Fig. 12 shows the pure water fluxes before and after protein filtration. Moreover, the results for protein solution flux are presented. Water fluxes of the fouled membranes were measured after washing with distilled water. For cleaning, the fouled membranes were removed from the dead-end module and rinsed with water for 2 min. Then, the membranes were immersed in 25 1C water for 20 min. Consequently, the flux recovery ratio (FRR) and the membrane resistance parameters were estimated to evaluate the antibiofouling properties. As shown in Fig. 12, the permeability of the membranes decreased greatly when pure water was substituted by protein solution in the filtration cell, which represents the membranes fouling. The result reveals that the highest value for flux reduction was for the unfilled PES membrane. After washing the membranes, the pure water flux of the unfilled PES membrane showed a large decrease. However, this flux reduction for the GO nanosheet embedded membranes was lower. The flux recovery ratios of the prepared membranes are depicted in Fig. 13. The higher FRR indicates a better antifouling property for the membrane. The FRR for the unmodified PES membrane (35%) was lower than the FRR for the membranes prepared by embedding nanoplates (more than 70%). This indicates the high antibiofouling property of the modified membranes induced by graphene oxide. In the best case, related to 0.5 wt% GO (M2) membrane, the flux recovery percentage of the membrane was 90.5%. The observed trend of flux recovery ratio is matched by hydrophilicity of the membranes (see Fig. 4). Hydrophilic surface can adsorb water molecules and form a water layer, which retards the adsorption of protein and other fouling agents. The observed FRR is quite comparable with other carbon nanofiller such as multiwalled carbon nanotubes [35] or inorganic nanofillers such as TiO2 [27], silica [39] and Fe3O4 [54] employed for the preparation of polyethersulfone membranes. In fact, membrane fouling can be classified as hydraulically reversible and irreversible. Hydraulically reversible fouling, in

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100

90.5

90 80

75

72

70

FRR(%)

60 50 40

35

30 20 10 0

Unfilled PES

M1

M2

M3

Fig. 13. Water flux recovery of the GO nanosheet blended PES membranes after protein fouling. (Average of three replicates was reported.)

80 Total Fouling

70

reversible fouling irreversible fouling

Fouling resistance (%)

60 50

the fabricated mixed matrix membranes are presented in Fig. 14. The results illustrate that the resistance factor in the modified membranes is lower while the flux recovery ratio of this membrane is higher. The results shown in Fig. 14 reveal that the total fouling resistance of the NF membranes prepared with the embedded GO nanoplates, which is the sum of Rr and Rir, were lower in comparison with that of the unfilled PES membrane. The irreversible resistance (Rir) of the membranes was considerably reduced from 66.1% for the unmodified membrane to 9.5% for the 0.5 wt% GO blended membrane (M2). These results demonstrate that the blended membranes show remarkable antifouling properties. In summary, the recovery ratios (FRR), reversible resistances (Rr), and irreversible resistances (Rir) of GO embedded membranes were improved i.e., the surface properties of the membrane were modified. In order to determine the reproducibility of the membrane performance and the durability of antibiofouling property, five cycles of protein fouling experiments were performed for the unfilled PES and 0.5 wt% GO membrane. Fig. 15 demonstrates that the flux recovery values in those five cycles were 90.5%, 88.1%, 86.2%, 85.8% and 84.1% for 0.5 wt% GO membrane. Therefore, reversible fouling was the dominant phenomenon in total fouling as hydraulic cleaning maintained high efficiency after five cycles.

40

4. Conclusion 30 20 10 0 Unfilled PES

M2

M3

M4

Fig. 14. Fouling resistance ratio of GO/PES nanofiltration membranes.

100 M2 membrane

Unfilled PES

90 80 70 60 50 40 30 20 10 0 First fouling

Second fouling Third fouling

Fourth fouling

The graphene oxide incorporated polyethersulfone membranes were prepared by direct addition of the nanoplates in the casting solution. The influences of blended GO on the morphology and performance of the fabricated nanocomposite membranes were examined by pure water flux, dye removal and fouling measurements. The results showed that the hydroxyl, carboxylic acid and other functional groups formed on the graphene are located on the surface of membrane by migration of the nanoplates to the surface. This enhanced the membrane hydrophilicity as well as the surface properties. A significant improvement was observed in fouling prevention in the prepared membrane. The results indicated that the membrane surface hydrophilicity was improved by blending of the GO nanoplates. The SEM images showed that the prepared mixed matrix membranes possessed finger-like structure. In the present study, the nanofiltration performance was examined by rejection of Direct Red 16. The results revealed a higher retention of the GO membranes compared with that of the unfilled PES. Regarding the obtained hydrophilicity, pure water flux and antifouling properties, the optimum concentration of 0.5 wt%. was found for the graphene oxide in the casting solution. The results from this study showed that the graphene oxide nanosheets are excellent antifouling material, which be promising for new applications for this type of membrane.

Fifth fouling

Fig. 15. Reproducible characteristic of unfilled PES and 0.5 wt% GO incorporated PES membranes during five protein filtrations.

which the fouling agents are loosely attached to the membrane, can be removed by backwashing. This leads to a reduction of membrane productivity and increase of operational costs [55]. Hydraulically irreversible fouling, in which the foulants are tightly bound to the membrane can only be removed by chemical cleaning. Therefore, this type of fouling not only causes an increase of operational complexity, but also reduces the membrane lifetime [56]. The total fouling ratio (Rt), hydraulically reversible fouling ratio (Rr), and hydraulically irreversible fouling ratio (Rir) values for

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