Fabrication of graphene oxide blended polyethersulfone membranes via phase inversion assisted by electric field for improved separation and antifouling performance

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Journal of Membrane Science

journal homepage:www.elsevier.com/locate/memsci

Fabrication of graphene oxide blended polyethersulfone membranes via phase inversion assisted by electric fi eld for improved separation and antifouling performance

Xiao Wang

^a

, Miao Feng

^a

, Yan Liu

^a

, Huining Deng

^a,⁎

, Jun Lu

^b

aSchool of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China

bSchool of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, PR China

A R T I C L E I N F O

Keywords:

Polyethersulfone Graphene oxide Phase inversion Electricfield Antifouling

A B S T R A C T

Novel antifouling blend polyethersulfoune (PES) membranes were prepared via phase inversion assisted by a direct current (DC) electricfield with the addition of extremely low concentration of graphene oxide (GO). The GO particles were observed migrating toward the anode and better distributed in the casting solution with a DC electricfield exerted. The effect of the applied DC electricfield intensity on the properties of blend membranes was systematically investigated. The enrichment of GO on the membrane surface assisted by the electricfield was confirmed by the enhanced hydrophilicity and negative charge density, and the XPS results. The blend PES membrane prepared under an electric field of 5000 V cm⁻¹ exhibits not only a high water flux (289.63L m⁻²h⁻¹), which is more than double of the pristine PES membrane (137.67L m⁻²h⁻¹), but also an improved methyl red rejection. The membrane fouling performance was studied with bovine serum albumin (BSA) as model protein. The PES/GO membranes prepared under a 5000 V cm⁻¹electricfield showed an ob- viously increasedflux recovery ratio (FRR) and a decreased irreversible fouling ratio (Rir) value, compared to ones without using an electricfield. The membrane fabrication assisted by an electricfield opens a new way to enhance hydrophilicity, charge density, separation performance and antifouling ability of mixed matrix mem- branes.

1. Introduction

Membrane technology has become a highly efficient industrial se- paration technology in the past few decades due to its adaptability, low cost, and low energy consumption [1]. However, membrane fouling caused by undesirable adsorption and deposition of foulants on the membrane surface or into the pores is the main limitation for utilization of this technology[2,3]. The main factors affecting membrane fouling are surface properties such as hydrophilicity, charges and roughness [4,5]. It is believed that the increasing surface hydrophilicity and ne- gative charge density, and reducing surface roughness would improve the fouling resistance, because most foulants are naturally hydrophobic and negatively charged[6–8].

Polyethersulfone (PES) is one of the most widely used materials for membrane fabrication, owing to its excellent thermal, mechanical, and dimensional stability. However, the intrinsic hydrophobic characteristic of PES membranes would result in severe membranes fouling, which is the major challenge for their utilization in industry [9,10]. To

overcome this challenge, many modification methods have been de- veloped to enhance the hydrophilicity of PES membranes, including chemical grafting[11,12], surface modification[13], and blending with hydrophilic materials[14,15].

A major current focus in the improvement of membrane perme- ability and antifouling performance is blending nanomaterials with excellent properties. Nanomaterials such as aluminum oxide (Al2O3) [16], silicon dioxide (SiO2)[17,18], titanium dioxide (TiO2)[19,20], carbon nanotubes (CNTs)[21]and graphene oxide (GO)[22–24]are added to fabricate blend membranes, enhancing waterflux, solute re- jection, as well as hydrophilicity and mechanical strength of these membranes. Among these nanoparticles, GO has shown great potential in modifying hydrophobic materials used in membrane fabrication, owing to its extremely high specific surface area and excellent physical and chemical stability. In addition, GO is also rich in oxygen-containing functional groups (carboxyl and hydroxyl). These hydrophilic groups have good dispersity in polar organic solvents or aqueous media, which is critical for GO as an additive for membrane fabrication. However,

https://doi.org/10.1016/j.memsci.2019.01.055

Received 10 September 2018; Received in revised form 31 January 2019; Accepted 31 January 2019

⁎Corresponding author.

E-mail address:huiningd@hebut.edu.cn(H. Deng).

Journal of Membrane Science 577 (2019) 41–50

Available online 01 February 2019

0376-7388/ © 2019 Elsevier B.V. All rights reserved.

T

compared to those without using an electric field. An improved gas permeability was also reported. Wu et al.[32]observed the accumu- lation of multi-walled carbon nanotubes (MWCNTs) toward the nega- tive electrode side in DC electricfield, which was ascribed to electro- phoresis of the charged nanomaterial. Since GO and CNTs have very similar properties as carbon nanomaterials, it should be feasible to use a DC electric field to form a modified layer with relatively high con- centration of GO on the membrane surface with a small amount of GO addition. Meanwhile, the charges and hydrophilicity of GO nano- particles are expected to be fully utilized to improve the antifouling ability of the membrane.

In this work, blend membranes were prepared by incorporating GO into PES matrix using the immersion precipitation phase inversion method assisted by a DC electricfield. The present of GO and roles of the electric field intensity on the microstructure, hydrophilicity, se- paration and antifouling performance of the fabricated blend mem- branes are systematically studied. It is expected that the application of a DC electricfield not only induces enrichment of GO on the membrane surface thus improving the separation and antifouling performance of the blend membranes, but also helps the good distribution of GO in the membrane. This means that only a very low concentration of GO is required for the blend membrane fabrication.

2. Experimental

2.1. Materials

Graphiteflakes were provided by Qingdao Guyu Graphite Co. Ltd.

NaNO3 and KMnO4 (analytical grade) were purchased from Tianjin University Kewei Company. H2O2, Congo red, methyl orange and me- thyl red (all in analytical grade) were supplied by Tianjin Sailboat Chemical Reagent Technology Co. Ltd. H2SO4(analytical grade) was provided by Tianjin Jiangtian Chemical Technology Co. Ltd.

Polyethersulfone (PES) was obtained from BASF Co. Ltd. (Germany).

Polyvinylpyrrolidone (PVP) was provided by Tianjin Guangfu Technology Development Co. Ltd. (China). N,N-Dimethylacetamide (DMAc) was obtained from Tianjin Kemiou Chemical Reagent Co. Ltd.

(China). Bovine serum albumin (BSA, MW= 67,000) was supplied by Beijing Dingguo Changsheng Biotechnology Co. Ltd. (China).

2.2. Preparation and characterization of GO

Graphene oxide (GO) was prepared by a modified Hummers method as reported previously[33]. Firstly, 100 mL of H2SO4was added in a three-neckedflask and was ice bathed below 4 °C for 15 min. Then 2 g of graphite powder together with 1 g of NaNO3were added with stir- ring. After 10 min's mixing, 6 g of KMnO4was slowly added and further stirred at 35 °C for 30 min. Subsequently, 100 mL of deionized water was slowly added to the mixture while the temperature was kept at 95 °C for 15 min. Finally, 200 mL of deionized water and 30 mL of 30%

H2O2were added to the mixture. The mixture was washed with 0.1 M hydrochloric acid repeatedly until no sulfate ion was detected and then

sion method, using DMAc as the solvent while deionized water as the non-solvent. Firstly, the 0.008 wt% of GO (relative to the weight of cast solution) nanocomposite were dispersed in DMAc and sonicated for 2 h.

Then 18% of PES and 1 wt% of PVP were added in the above mixture and was stirred at 60 °C for 10 h to obtain casting solution with well dispersed GO nanoparticles. The resulted homogenous casting solution was placed at 60 °C for 10 h to remove air bubbles. Then a membrane of 200 µm was casted on a clean glass plate. After pre-evaporated at room temperature for 5 min, it was immersed into the non-solvent bath and phase inversion happened. The prepared membrane was washed with deionized water for several times and then preserved in water before use.

The preparation of blend membranes assisted by electric field is similar to the above procedure but with electricfiled applied during pre-evaporation (Fig. 1). The DC electricfield was exerted by placing the glass plate with the casted solution between a pair of plate elec- trodes with a gap of 8 mm. The DC electricfield was exerted in a di- rection perpendicular to the membrane surface with the anode facing the surface of the prepared membrane. The pre-evaporation time was also controlled to 5 min to keep consistent with that without exerting electricfield.

2.4. Membrane characterization

The surface and cross-section morphologies of the prepared mem- branes were characterized by SEM (FEG450, USA). All the samples were sputtered with gold to ensure excellent conductivity prior to analysis. X- ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific Co., Ltd.) was used to analyze the membrane chemical composition on the top surface.

The surface roughness of the membrane samples was analyzed by atomic force microscopy (AFM，Agilent AFM 5500，USA). Membrane samples were observed to operate in a tapping mode in air at room

Fig. 1.Schematic diagram of fabrication process of PES/GO membranes by electricfield assisted phase inversion.

temperature.

The contact angle was tested by a contact angle analyzer (DSA100, Germany) and was used to characterize the hydrophilicity of the membranes. 2μL of deionized water was dropped onto the dry flat membrane surface and tested after 3 s. The contact angle offive dif- ferent locations for each membrane was investigated and the average was reported.

Membrane surface zeta potential was recorded on electrokinetic analyzer (SurPASS, Anton Paar, Austria) with 1 mM KCl solution as the background electrolyte. The pH of the solution was adjusted by NaOH and HCl solutions to obtain the zeta potential value at different pH.

Based on the results of the gravimetric method, the overall porosity

of the membranes (ε) was calculated through Eq.(1) [29]

= −

× ×

ε w w

A L dw

1 2

(1) wherew1 andw2are the weight of the wet and dry membrane (g), respectively;Ais the membrane effective area (m²),Lis the membrane thickness (m) anddwis the density of water (0.998 g cm⁻³).

Mean pore radium (rm) of the prepared membranes was evaluated by the Guerout–Elford–Ferry equation (Eq.(2)) based on the pure water flux and porosity results[29,34].

Fig. 2.(a) FT-IR, (b) XRD, (c) Raman spectra of the graphite and GO, and (d) SEM image and (e) size distribution of GO.

effective membrane area of 14.6 cm². To get steady water flux, the membranes werefirst operated at 0.6 MPa for 60 min with pure water.

Then the pure water permeability was tested at 0.4 MPa and the PWF was measured for 20 min. For every membrane sample, the PWF was tested three times to get an average value. The rejection experiments were performed using methyl red solution of 100 mg L⁻¹. The con- centration of the methyl red in the feed and permeate solution was measured by a UV spectrophotometer (TU-1810). The waterflux (Jw1) and rejection (R, %) were calculated by Eqs.(3)and(4)

= ∆

J V

W1 A t (3)

⎜ ⎟

= ⎛

⎝

− ⎞

⎠ C × R(%) 1 C 100

F P

(4) where V denotes the volume of permeated water (L), A is the membrane area (m²), and∆tpresents the permeation time (h). CPand CFare the concentration of dye in the permeation and feed solutions, respectively.

2.6. Anti-fouling test

Bovine serum albumin (BSA) solution was used as a model protein contaminant to test the antifouling property of membranes. The specific procedure is as follows: after the pure waterflux (Jw1) was tested, the separation of 1 g L⁻¹of BSA with pH of 7.0 prepared in a 0.1 M phos- phate buffer solution was performed at 0.4 MPa for 90 min. Then the fouled membrane was soaked in deionized water and placed in a shaker for 30 min. Finally, the pure water flux of the membrane (Jw2) was tested again. Theflux recovery ratio (FRR) value is used to evaluate the antifouling performance of the membranes and can be defined as

=J × FRR J^W 100%

W 2

1 (5)

The fouling of membranes could be separated to reversible fouling and irreversible fouling [36]. The reversible fouling caused by con- centration polarization can be easily removed by aqueous cleaning. The irreversible fouling refers to the fouling caused by deposition or ad- sorption of molecules on the membrane surface and the irreversible fouling ratio (Rir) value can be calculated by

Fig. 2b. The feature diffraction peak at 2θ= 26.7° was attributed to the 002 diffraction peak of graphite. However, this peak disappeared on the spectroscopy of GO. A characteristic peak of GO was shown at 2θ= 10.6°, which also verified the formation of GO.Fig. 2c shows that the characteristic diffraction peak of natural graphite is about 1598 cm⁻¹(G peak), which is the E2g vibration mode generated by the stretching vibration between sp² hybrid carbon atoms [38,39]. The spectra of GO has two characteristic peaks, one is the G peak at 1590 cm⁻¹and the other is D peak at 1345 cm⁻¹. The ratio of G peak intensity (IG) and D peak intensity (ID) represents the structural reg- ularity degree of graphite [40]. This IG/ID value of graphite is sig- nificantly reduced, indicating that the graphite is defective in the oxi- dation process due to destruction of the structural layer with oxygen- containing functional groups. The image of GO sheet (Fig. 2d) contains a layered and folded structure with wrinkled surface, which is similar to that shown in the literature[41]. The particle size distribution of GO particles varies from 400 to 825 nm, and the nominal effective diameter of particles is about 615 nm (Fig. 2e).

3.2. Response of GO in DC electricfield

The response of GO in DC electricfield was tested with a horizontal direction electricfield exerted and was observed by an optical micro- scope (CKX41, OLYMPUS, Japan). A pair of graphite electrodes was fixed on a glass plate and was place on the microscope stage. A drop of PES/GO casting solution was put between the electrodes and then the electricfield was applied. The migration of GO particles was recorded from the microscope.Fig. 3presents the images of GO status before and after the electricfield was exerted. Comparing the behavior of GO in the presence and absence of electricfield, we can notice obvious difference.

GO was disorderly distributed in the matrix before the electricfield was exerted (Fig. 3a). However, after applying the electricfield (Fig. 3b), GO moved toward the positive electrode side and accumulated near the electrode. This can be mainly ascribed to the negative charge from the oxygen-containing groups, so GO exhibits electrophoresis behavior in the DC electricfield. The accumulation of GO found in this work is similar to that reported by Wu et al. in their work distributing MWCNTs in polystyrene (PS) matrix with electricfield[32]. At the same time, it

Fig. 3.Micrographs of GO dispersed in casting solution before and after 5000 V cm⁻¹of DC electricfield applied.

can be seen fromFig. 3a that GO was distributed randomly with some small agglomerations in the casting solution, while the application of electric field contributed to disperse the GO nanoparticles better (Fig. 3b). This dispersion property is crucial for the fabrication of blend membranes with excellent performance with extreme low GO addition.

3.3. Characterization of PES/GO blend membranes 3.3.1. Morphological characterization

The surface and cross-section structure of the PES/GO blend mem- branes prepared under different electric field intensity was observed with SEM. As shown in Fig. 4, both PES/GO membranes shows a smooth surface without pores and GO agglomeration observed even under 200k times of magnification. This confirmed that the membrane has relatively tight porous structure with tiny pores with GO

nanoparticles well dispersed in the polymer matrix.Fig. 5presents the cross-section morphologies of the blend membranes with different electricfield intensity applied. The PES/GO blend membranes exhibited typical asymmetric porous structure, consisting of a dense top layer and afinger-like porous sub-layer. The pores in the electricfield assisted membranes became more uniform compared to that prepared without electricfield applied. This could be attributed to the better GO dis- tribution, and the hydrophilic nature of GO led to rapid exchange of the DMAc and non-solvents during phase inversion.

Fig. 6presents the three-dimensional AFM images of the pristine PES membrane and blended membranes with a scan area of 10 µm

× 10 µm. In these images, the brightest areas show the highest point of the membrane surface and the black areas display the valley of the membrane. It can be seen that the roughness of the membrane surface is affected by the addition of GO in the casting solution.Table 1shows Fig. 4.Surface-section SEM images of PES/GO membranes, the electricfield intensity: (a) 0 V cm⁻¹and (b) 5000 V cm⁻¹.

Fig. 5.Cross-section SEM images of PES/GO membranes, the electricfield intensity: (a) 0 V cm⁻¹, (b) 1000 V cm⁻¹, (c) 3000 V cm⁻¹and (d) 5000 V cm⁻¹.

that the surface roughness parameters of the membrane including the average roughness (Ra), the root mean square of the Z data (Rrms), and the height difference between the highest peak and the lowest valley (Rmax). The image shows that the large peaks and valleys in the pristine PES membrane are replaced by many small peaks in the PES/GO blended membranes, resulting in a decrease in membrane surface roughness. Similar behavior was reported by Zinadini et al.[42]and Ai et al.[43]After application of the electricfield, the surface roughness of the PES/GO blended membranes is further reduced due to the good alignment of GO in the membranes. For low concentrations of carbon based nanomaterials, they can be regularly collocated in the membrane.

Therefore, the membrane surface became smooth[44]”.

3.3.2. Membrane porosity and mean pore size

Table 2 lists the effect of GO and electricfield intensity on the porosity and mean pore size of the blend membranes. Compared to the pristine PES membrane, the porosity of the PES/GO membranes in- creased with GO added and exhibited further improvements with the increase of electricfield intensity during the membrane fabrication. The blend membrane prepared with 5000 V cm⁻¹of electricfield showed a porosity of 80 ± 1.9% and increased by 17.8% compared to that of the pristine PES membrane. The mean pore size of membranes with 0.008 wt% GO addition increased to 25.6 nm from 19.9 nm of the

pristine PES membrane. However, the application of electric field during phase inversion produced decreased mean pore size in the PES/

GO membranes. The mean pore size even showed slight shrink with higher electricfield intensity applied during preparation. These results can be explained by the influence of the kinetics during the impreg- nation of the precipitate phase. From a kinetic point of view, the ex- change of solvent and non-solvent during phase inversion is mainly affected by the hydrophilicity of the nanoparticles[45]. With the hy- drophilic GO nanoparticles added, larger pore channels is formed during the membrane fabrication, and thus the porosity and the mean pore diameter is increased. As discussed inSection 3.2, the GO nano- particles migrate toward the surface of the blend membranes and are prone to good dispersion in the polymer matrix driven by the DC electricfield (Fig. 3). The well dispersed GO particles would form more pores with small channel size during phase inversion, therefore higher porosity but slightly reduced mean pore size was detected (Fig. 7).

3.3.3. Contact angle

Water contact angle are used to analyze the hydrophilicity of the blend membranes prepared with different electricfield intensity and shown inFig. 8. The contact angle of the PES/GO blend membranes is lower than that of the pristine PES membrane, indicating improved hydrophilicity with GO addition. This is because the hydrophilic Fig. 6.AFM images of (a) Pristine PES, (b) PES/GO, and (c) PES/GO-5000 V cm⁻¹.

Table 1

Surface roughness of different membranes.

Membrane sample Surface roughness parameters

Ra(nm) Rrms(nm) Rmax(nm)

Pristine PES 10.923 13.519 106.475

PES/GO 9.079 11.469 102.104

PES/GO-5000 V cm⁻¹ 6.199 7.831 60.005

Table 2

Porosity of electricfield assisted PES/GO membranes.

Electricfield intensity (V cm⁻¹） Porosity (%) Mean pore size (nm)

Pristine PES 62.22 ± 2.1 19.9

0 69.08 ± 2.6 25.6

1000 75.00 ± 2.0 24.0

3000 77.18 ± 1.7 23.5

5000 80.00 ± 1.9 23.2

oxygen-containing groups on the GO are more likely to exposing on the membrane surface during phase inversion in water[46,47]. In case of low electric field intensity applied, the contact angle of the blend membranes presented only slight decrease. However, the contact angle of the blend membrane prepared with 5000 V cm⁻¹ of electricfield showed an obviously decrease to 50.7° ± 1.46, which was 15% lower than that of the blend membrane without electricfield. This indicates the enrichment of GO on the membrane surface caused by the migration of GO under the DC electricfield, and thus improved the hydrophilicity of the membrane. The blend membrane prepared with the DC electric field intensity of 5000 V cm⁻¹can achieve similar or even lower con- tact angles than that of membranes with higher GO concentration re- ported in the literature[42,43,48]. This showed that the application of electricfield made GO well utilized.

3.3.4. Zeta potential of the membrane surface

Fig. 9 shows the results of the zeta potential of PES/GO blend membranes prepared at different electric field intensity. The blend membranes prepared without electricfield exhibited the lowest nega- tive charge density, while the zeta potential of the DC electric field assisted PES/GO blend membranes was significantly decreased with the electric field intensity. The PES/GO membrane prepared at

5000 V cm⁻¹exhibited a zeta potential of−38 mV at pH of 7, while the blend membrane prepared without electric field applied was only

−26 mV at the same pH condition. This zeta potential result of PES/GO membranes also verified the migration of GO toward the membrane surface under DC electricfield.

3.3.5. Surface chemical composition

The chemical composition variations of the membrane surfaces were analyzed by XPS, as shown inFig. 10. The carbon and oxygen content of the blend membrane with the same composition prepared without electricfield are 70.71% and 21.91%, respectively. However, with the 5000 V cm⁻¹ of electric field applied during the pre-eva- poration, the content of carbon and oxygen on the membrane surface increased from to 71.32% and 22.71%, respectively. The result ob- viously demonstrated the acceleration of GO on the membrane surface with the DC electricfield exerted. As a result, this would contribute to the enhancement of the hydrophilicity and negative charge density of the blend membrane surface, and thus promoting the improvement of antifouling performance.

3.4. Separation performance of PES/GO membranes

Table 3presents the rejection of pristine PES membrane to sodium sulfate (Na2SO4), methyl red (MW 269.3 Da), methyl orange (MW Fig. 7.Schematic illustration of the change of membrane pore structure with

DC electricfield applied.

Fig. 8.Water contact angle of blend membranes prepared with different elec- tricfield intensity.

Fig. 9.Zeta potential of PES/GO membranes fabricated with different electric field intensity.

Fig. 10.XPS spectra of PES/GO membranes fabricated with and without elec- tricfield.

charge on methyl red compared to the divalent nature of Congo red.

Meanwhile, the rejection of methyl orange of the PES membrane is lower than that of methyl red, even with a larger molecular weight.

This is caused by the positive charge of methyl orange that mitigated the electrostatic repulsion effect from the negatively charged PES membrane. However, Congo red is not a good indicator to characterize the separation performance of the blend membranes, because the re- jection value would vary only between 95% and 100% when the re- jection performance is improved. Therefore, methyl red was selected as model dye to characterize the separation performance of the blend membranes.

The separation performance and flux recovery ratio of PES/GO blend membranes with different GO concentration prepared without the applied electricfield is shown inFig. 11. The rejection to methyl red of the membranes increased with the concentration of GO, while the pure water flux showed the maximum value at 0.008%. Further in- crease the concentration led to GO precipitation in the casting solution during the degasing process. The results of antifouling experiments (Fig. 11b) showed that theflux recovery ratio of the blend membranes gradually increases with the GO concentration, and theflux recovery ratio tends to be stable when the GO concentration exceeds 0.006 wt%.

Therefore, the GO concentration of 0.008 wt% was selected to fabricate composite membranes in our further study.

Fig. 12presents the pure waterflux of the PES/GO blend mem- branes prepared with different electricfield intensity. The pure water flux of the PES/GO blend membrane showed a sharp increase after adding 0.008 wt% of GO, which is mainly caused by the obviously enlarged mean pore size and increased porosity of the blend membrane (Table 2). The pure water flux of the electric field-assisted blend membranes increased with the electricfield intensity. The membrane permeability is closely related to membrane properties, including hy- drophilicity, porosity, and mean pore size. GO nanoparticles moved to the surface of the membrane driven by the DC electricfield and then the hydrophilicity of the membrane surface was improved. So the thickness

of the aqueous layer on the membrane surface also increased, leading to enhanced waterflux. The improved porosity of the blend membranes fabricated under higher electricfield intensity also resulted in the in- crease of theflux.

Fig. 13shows the methyl red rejection of membranes prepared with different DC electricfield intensity. The rejection to methyl red of the pristine PES membrane is only 67.23 ± 0.39%, while The PES/GO blend membranes exhibited obviously increased rejection with GO added. Furthermore, the blend membranes exhibited gradually in- creased rejection with the increase of exerted electric field intensity during the membrane preparation. The rejection of the blend mem- brane prepared with a DC electric field intensity of 5000 V cm⁻¹ reached maximum of 86.58 ± 0.35%, which was 29% higher than that of the pristine membrane. It is worthy to note that, the order of rejec- tion matched quite well with the order of zeta potential of the mem- branes. This confirmed that the addition of a very small amount of GO to the electricfield-assisted blend membranes produced relatively high negative charge density on the surface, leading to good rejection to the negatively charged methyl red.

The PES/GO (0.5 wt%) blended membranes prepared by Zinadini et al.[8]exhibited the pure waterflux of 20L m⁻²h⁻¹with direct red 16 rejection of 96%. From our results, the highest pure water flux reaches 289.86±6.25 L m⁻²h⁻¹ and showed methyl red rejection of 86.58% in our study. This electricfield-assisted PES/GO blend mem- brane shows good permeability, while the retention of methyl red is lower than the PES/GO (0.5 wt%) membrane. This is because the me- thyl red has lower molecular weight than that direct red 16, and the

Fig. 11.Effect of GO concentrations on (a) the separation performance and (b)flux recovery ratio of the membranes.

Fig. 12.Pure waterflux of membranes prepared with different electricfield intensity.

rejection of the pristine PES membrane to Congo red which has a si- milar molecular weight to direct red 16 is up to 95.38% in our study.

3.5. Antifouling performance

The antifouling ability of the blend membranes wasfirst evaluated with FRR. It is generally accepted that the membrane with higher FRR value have better antifouling performance. Fig. 14presents the anti- fouling results of the prepared membranes. As shown inFig. 14a, the pure waterflux of all the tested membranes cannot be fully restored to their initial values after water cleaning. This is caused by the strong adsorption of protein molecules on the surface, which is difficult to be completely removed by hydraulic cleaning. All the PES/GO blend membranes possessed a higher FRR value than the pristine PES mem- brane. The pristine PES membrane shows a FRR value of 46.43 ± 0.98%, while the FRR value of the PES/GO membrane fabri- cated at 5000 V cm⁻¹was increased to 89.18 ± 0.81%. The PES/GO membranes with 0.02–0.2 wt% of GO added prepared by Abdel-Karim et al. tested in similar conditions showed FRR values of 45–65%[23].

PES/GO blend membrane at 5000 V cm⁻¹ has higher or similar FRR values in literature[36,49,50]. The electricfield assisted PES/GO blend membranes exhibited excellent antifouling properties even with such an extremely low concentration of GO addition. This is because the hydrophilicity of the electricfield assisted blend membranes promoted the adsorption of water molecules and forming an aqueous layer, and thus prevented the adsorption of BSA. Moreover, the increased negative charge density of the electricfield assisted blend membranes improved the electrostatic repulsion interaction between the BSA molecules and the membrane surface. The FRR value of PES/GO blend membranes

hardly changed with low electricfield intensity applied and increase sharply at 5000 V cm⁻¹ as shown inFig. 14a, corresponding to the trend of the contact angle the membranes (Fig. 8). These results verified that the hydrophilicity of the membrane surface played a key role in improving antifouling performance[46,47].

To compare the precise antifouling ability of the prepared mem- branes, the irreversible fouling ratio (Rir) value, which indicats the strong interaction between the foulant and the membrane surface[36], is also calculated and presented inFig. 14b. Van der Waals force, hy- drophobic force, electrostatic force and hydrogen bonding force[29]

are the four types of forces responsible for adsorbing proteins onto the membrane surface. The Rir value of the pristine PES membrane is highest (53.57 ± 0.98%) among all the membranes due to the hy- drophobic interaction between the BSA molecules and membrane sur- face. The Rirvalues of PES/GO blend membranes with 0.008 wt% GO decreased sharply to 21.51 ± 0.66% even without electricfield ex- erted during fabrication. This is because BSA is negatively charged in the tested condition (pH 7) and the addition of GO increased the electrostatic repulsion between the BSA and the membrane surface. The PES/GO blend membranes prepared with low electricfield intensity showed hardly changed Rir, while the Rirvalue of the blend membrane prepared with the DC electricfield intensity of 5000 V cm⁻¹decreased to 10.82 ± 0.81%. The best antifouling ability corresponded to its best hydrophilicity and lowest surface zeta potential. This confirmed that the surface enrichment and good distribution of GO in the membranes under the DC electricfield remarkably improved the antifouling ability of the PES/GO membranes even with extremely low concentration of GO addition.

4. Conclusion

GO was fabricated by the modified Hummers method, and the FTIR and XRD characterization showed that the GO was successfully pre- pared. The effect of the DC electricfield intensity applied during the membrane fabrication on the properties of PES/GO blend membranes were investigated systematically. Comparative studies showed that electricfield assisted blend membranes have increased hydrophilicity and negative charge density on the surface, which is caused by the migration of GO under DC electricfield. The result of XPS also verified the enrichment of GO after DC electricfield exerted. The electricfield- assisted blend membranes had not only a high pure waterflux (289.63

±6.25 L m⁻²h⁻¹) compared to pristine PES membrane (137.67 ± 1.33L m⁻²h⁻¹), and also increased methyl red rejection.

The PES/GO membranes prepared with 5000 V cm⁻¹electricfield as- sisted showed a significantly increased flux recovery ratio and de- creased irreversible fouling ratio value compare to that without electric field, indicating well improved antifouling ability of the electricfield assisted blend membranes. The results suggest that electricfield assist is Fig. 13.Rejection of the prepared membranes to methyl red.

Fig. 14.(a) Theflux recovery ratios and (b) irreversible fouling ratio of the membranes fabricated with different electricfield intensity.

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