GOSiO2 hybrid microfiltration membrane towards enhanced perm-selectivity and anti-fouling property

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Preparation of PVDF/GO–SiO2 hybrid microfiltration membrane towards enhanced perm-selectivity and anti-fouling property Libin Yang, Liping Liu, Zhan Wang∗ Beijing Key Laboratory for Green Catalysis and Separation, Department of Chemistry and Chemical Engineering, Beijing University of Technology, Beijing 100124, PR China

a r t i c l e

i n f o

Article history: Received 4 May 2017 Revised 5 June 2017 Accepted 12 June 2017 Available online xxx Keywords: GO–SiO2 nanoparticles Polyvinylidene fluoride Antifouling property Microfiltration

a b s t r a c t Polyvinylidene fluoride (PVDF) microfiltration (MF) membranes with enhanced perm-selectivity and antifouling property were successfully prepared by incorporating GO–SiO2 nanoparticles into PVDF matrix. Under the optimal GO–SiO2 /PVP ratio of 0.5/1 (wt./wt.) in the casting solution, the prepared PVDF/GO–SiO2 hybrid membrane exhibited improved perm-selectivity with a water flux of 850 L/(m2 h) and HA rejection of 92%. Compared with the pristine PVDF membrane, the optimized hybrid membrane exhibited 89% higher water flux without sacrificing its rejection, increased porosity and hydrophilicity. In addition, the PVDF/GO–SiO2 hybrid membrane also showed satisfactory antifouling performance (the lower irreversible fouling ratio of 37.4% and high recovery ratio of 62%) and good stability in the longterm performance test. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Membranes are superior separation materials due to its small footprint, high separation efficiency and environmental friendliness [1–3]. Polyvinylidene fluoride (PVDF) microfiltration (MF) membranes have been widely used in various water treatment fields with the advantage of excellent chemical resistance, high thermalstability, good membrane-forming property and adequate mechanical strength [4]. However, the hydrophobic nature of PVDF material usually leads to severe membrane fouling during practical separation process, accompanying with serious blockage of membrane pores, decreased permeate flux and reduced lifespan. To address this problem, various methods have been explored to improve surface hydrophilicity and antifouling property of PVDF membranes, including (i) surface grafting with antifouling materials [5,6], (ii) incorporation of hydrophilic polymer material during membrane preparation [7–9] and (iii) embedding of inorganic nanoparticles in polymer matrixes [10–16]. Among these methods, the last method is most commonly used as it can combine the merits of both inorganic and polymer materials. Therefore, different hydrophilic nanoparticles, including carbon nanotubes (CNT) [10,11], titanium dioxide (TiO2 ) [12], zirconium dioxide (ZrO2 ) [13], alumina (Al2 O3 ) [14] and SiO2 [15], have been used to improve the antifouling property of PVDF membranes. However, these nanoparticles usually agglomerate together during membrane preparation process, ∗

Corresponding author. E-mail address: [email protected] (Z. Wang).

leading to a deteriorated membrane performance [16,17]. Therefore, the selection of nanoparticles with good polymer affinity is critical. Recently, graphene oxide (GO) has attracted broad attention due to its good dispersion in polymer matrix, excellent hydrophilicity and enhanced anti-fouling property [18–20]. GO is a superior separation material with characteristics of high specific surface area, good mechanical strength, low density and easily functionalized surface [21–23]. For example, incorporating a very small amount of GO into the polymer membrane can significantly enhance membrane mechanical and thermal properties [24,25], permeation performance [17,26] and anti-fouling properties [27]. However, the amphiphilic nature of GO hinders the ability to increase surface hydrophilicity of composite membrane or even induces membrane fouling due to the strong interaction between hydrophobic pollutant (proteins) and GO [28]. Fortunately, GO with abundant oxygen-containing functional groups can offer many reactive sites to synthesize various GO-based nanohybrid nanoparticles, such as GO–SiO2 [29–31], GO–TiO2 [32,33], GO–ZnO [34] and GO–CNT [35]. This GO-based nanohybrid nanoparticles have better hydrophilicity and good affinity with the polymer matrix, thereby increasing permeation performance of the prepared membrane [32,35]. For instance, the prepared rGO–TiO2 /PVDF membrane showed better permeability and antifouling property compared to the pristine PVDF membrane [32]. GO-multi-walled carbon nanotubes (MWCNTs)/ PVDF membrane exhibited uniform pore structure and enhanced permeability [30]. Furthermore, GO–SiO2 /PSf hybrid membrane were prepared by doping

http://dx.doi.org/10.1016/j.jtice.2017.06.018 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: L. Yang et al., Preparation of PVDF/GO-SiO2 hybrid microfiltration membrane towards enhanced perm-selectivity and anti-fouling property, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.018

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Fig. 1. The schematic illustration of synthesizing GO–SiO2 nanocomposite and the internal structure of GO–SiO2 .

GO–SiO2 hybrid nanoparticles into PSf matrix [16], which displayed excellent antifouling properties due to synergistic effects of GO and SiO2 [36,37]. Based on the above study, GO–SiO2 hybrid nanoparticles can also be used as a promising agent to improve the intrinsic hydrophobic nature and anti-fouling property of PVDF membrane. However, the use of GO–SiO2 nanoparticles for preparation of PVDF hybrid membrane still needs further investigation. In this study, GO–SiO2 nanoparticles were first synthesized by grafting SiO2 onto the surface of GO through hydrothermal treatment. Then the synthesized GO–SiO2 nanoparticles was incorporated into the casting solution by sonication, and novel GO–SiO2 /PVDF MF membrane was formed by a non-solvent induced phase separation process. The effects of GO–SiO2 loading and polyvinylpyrrolidone (PVP) concentration on membrane performance were studied. In addition, the physiochemical properties of membranes, including surface hydrophilicity, morphology, separation performance and anti-fouling property, were also systematically investigated. 2. Experimental 2.1. Materials PVDF (FR904, Mw = 2 × 106 Da, Mn = 4.7 × 106 Da) was obtained from Shanghai 3F New Materials Co., Ltd. Dimethylacetamide (DMAC ), ethanol, tetraethylorthosilicate (TEOS), NH3 ·H2 O and polyvinylpyrrolidone (PVP) were purchased from Tianjin Fuchen Chemical Reagent Factory. Raw graphene oxide sheet (GO: XF002-1) was purchased from Nanjing Nano Pioneer Co., Ltd. Nano-sized SiO2 with a particle size of 30 nm was supplied by Shanghai Maikun Chemical Reagent Factory. Humic Acid (HA) was received from Sigma-Aldrich Co. Llc (USA). All chemicals were of analytical grade and used without further purification. 2.2. Synthesis of GO–SiO2 nanocomposites GO–SiO2 was synthesized by the deposition of silica nanoparticles on GO surface through in situ hydrolysis of TEOS [37]. As shown in Fig. 1, 30 mg GO was first added into 400 mL ethanolwater (5:1, v/v) solution, followed by sonication for 1.0 h. Next the pH value of the solution was adjusted to 9.0 by NH3 ·H2 O and then 1.0 mL TEOS was added into the mixture, followed by sonication for 2 h. Then the obtained mixture was kept still overnight at room

temperature (25 ± 2 °C). Finally, the synthesized nanoparticles were separated from the mixture solution by centrifugation and rinsed thoroughly with ethanol and water. The final obtained nanoparticles were dried in a vacuum oven at 60 °C for 24 h before use. 2.3. Membrane preparation The PVDF/GO–SiO2 hybrid membranes were prepared by the non-solvent induced phase separation (NIPS) method. Firstly, a certain content of the synthesized GO–SiO2 nanocomposite were dispersed in DMAc by sonication for 2 h. In the case of the preparation of PVDF/SiO2 and PVDF/GO membranes, the same content as GO–SiO2 for the SiO2 or GO was used, respectively. Secondly, the 14 wt.% PVDF and 0.5–2.0 wt.% PVP were added in the above solutions followed by continuously stirring for 5 h at 50 °C. Then, the homogenous casting solutions were kept still in the oven at 50 °C for 24 h to remove air bubbles. After degassing, the casting solutions were casted on clean glass plates at a constant height of 150 μm. The as-cast membranes were then immediately immersed into a coagulation bath (DI-water at 25 °C). Finally, the prepared membranes were preserved in DI-water bath for 12 h before use. The hybrid PVDF membranes doping with SiO2 , GO and GO–SiO2 nanoparticles were referred to as M-SiO2 , M-GO and M-GO–SiO2 , respectively. 2.4. Characterization X-ray diffraction (XRD) analysis was characterized on a Broker D8 advance diffractometer (Germany). The FTIR spectra were collected by a Bruker Vector-22 spectrometer in the wavelength from 40 0 0 cm−1 to 500 cm−1 at a resolution of 2 cm−1 . The surface and cross-section morphologies of membranes were observed using a scanning electron microscopy (SEM) (NETHERLAND, FEI Quanta 200 ESEM). All samples were fractured in liquid nitrogen, posted on a stainless steel and sputtered with gold before SEM observation. Membrane hydrophilicity was measured by a water contact angle goniometer (Zwick/Roell BL-GRS500N) using 2 μL DI-water as the drop liquid each time. All samples were tested at five different positions to get an average value. The thermal stabilities of membranes were measured by thermal-gravimetrical (TGA) analysis (Q500 TA Instruments equipment) using nitrogen atmosphere at a flow rate of 20 mL/min with the temperature ranging from 25 °C to 600 °C and a constant heating rate of 10 °C/min. The

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mechanical properties of membranes were studied by an Instron testing machine (Zwick/Roell BL-GRS500N) at the tensile rate of 10 mm/min. The particle size distribution of HA solution was measured by a laser scattering particle size analyser (MAF-5001). XRay photoelectron spectroscopy (XPS, Thermo escalab 250Xi, USA) was used to characterize the chemical composition of nanoparticles (GO and GO–SiO2 ). Membrane porosity (P) was measured by the dry–wet method using the following equation [38]:

Mwater /ρwater P (% ) = , Mwater /ρwater + Mm /ρP

(1)

where Mm and Mwater represent the mass of the dry membrane (g) and the wet membrane (g), respectively. ρ water and ρ p denote the densities of water and PVDF (g/cm3 ), respectively. Mean pore radius rm (μm) of the membrane could be calculated by the Guerout–Elford–Ferry equation using filtration velocity method [39]:



rm =

(2.9 − 1.75P ) × 8ηlQ P × A × P

(2)

where P is the porosity of the membrane, η is the water viscosity (8.9 × 10−4 Pa s), l is the membrane thickness (m), Q is the permeate volume of the pure water per unit time (m3 /s), A is the effective area of membrane (m2 ) and P is trans-membrane pressure (0.1 MPa). 2.5. Measurements of permeate flux and the rejection The permeate flux and rejection of the membrane was tested by a dead-end system with an effective area of 19.6 cm2 under 0.1 MPa and 25 °C using 1.0 g/L HA as the feed solution. The permeate flux and rejection were calculated by Eqs. (3) and (4), respectively [40,41],

V J= A × t

(3)

where J is the permeate flux (L/m2 h), V is the volume increment of the permeate at the filtration time interval of t (h) with the effective area of A (m2 ),

3

Table 1 Surface tension properties (mJ/m2 ) of probe liquids at 20 °C.

DI-water Glycerol Diiodomethane

γ LW

γ+

γ−

γ AB

γ TOT

21.8 34.0 50.8

25.5 3.9 0.0

25.5 57.4 0.0

51.0 30.0 0.0

72.8 64.0 50.8

2.7. The calculation of surface energy The interfacial energies for aqueous systems including Lifshitz– van der Waals (LW) and acid–base (AB) interaction energy per unit area between two infinite planar surfaces were calculated using the following equations [44]:

GLW d0 = 2



γwLW −



γmLW

 

γaLW −



γwLW



(9)

     + − − GAB = 2 γ γ + γ − γw− m a w d0      + 2 γw− γm+ + γa+ − γw+    −2 γm+ γa− + γm− γa+

(10)

where γ LW and γ AB are the LW and AB component of surface tension (J/m2 ), respectively; γ + and γ − are electron-acceptor and electron-donor parameters of the polar component of the surface tension (J/m2 ), respectively; the subscript m denotes the membrane, w and a refer to the DI-water and HA, respectively. The free energy of interaction between two identical surfaces immersed in water (Gsws ) is used to evaluate the surface hydrophobicity/hydrophilicity of a substance,

 2  Gsws = − 2γsw = −2 γsLW − γwLW      −4 γs+ γs− + γw+ γw− − γs+ γw− − γs− γw+ .

(11)

where CP (g/L) and CF (g/L) are the concentrations of the permeate and the feed solution, respectively.

Surface tension properties of the membrane are related to contact angle measurements of three probe liquids (DI-water, glycerol and diiodomethane). For each sample, three kinds of contact angles were obtained. Then according to the known surface tension parameters of three probe liquids (shown in Table 1) and Eq. (14), a ternary system of linear equations was obtained,

2.6. Anti-fouling performance

γ T OT = γ LW + γ AB

(12)

 γ AB = 2 γ + γ −

(13)



R= 1−

CP CF



× 100%

(4)

Anti-fouling performances of membranes were investigated by typical foulant of humic acid (HA). Firstly, the pure water flux (Jw1 ) of the membrane was measured using DI-water as the feed at 0.1 MPa. Secondly, 1.0 g/L HA solution (pH = 7.4) were added into the filtration cell and filtrated for 80 min at 0.1 MPa, the flux for HA solution (Jp ) was measured. Thirdly, the fouled membrane was washed with DI-water for 25 min at a stirring speed of 400 rpm, and then the water flux (Jw2 ) of the cleaned membrane was measured again. The FRR was calculated by Eq. (5) [42],

F RR(% ) = (JW 2 /JW 1 ) × 100%.

(5)

Other anti-fouling parameters, including the total fouling ratio (Rt ), irreversible fouling ratio (Rir ) and reversible fouling ratio (Rr ), were also calculated [33,34,43],

Rt = (1 − JP /JW 1 ) × 100%

(6)

Rir = (JW 1 − JW 2 )/JW 1 × 100%

(7)

Rr = (JW 2 − JP )/JW 1 × 100%.

(8)

(1 + cos θ )γ1T OT = 2

property,

Journal

γsLW γlLW +



  γs+ γl− + γs− γl+ ,

(14)

where γ TOT is the total surface tension (J/m2 ); γ AB is the AB component of surface tension (J/m2 ); θ is the contact angle (°); the subscripts s and l correspond to the solid surface and the liquid, respectively. 3. Results and discussion 3.1. Characterization of GO–SiO2 synthetic nanocomposite 3.1.1. FT-IR spectroscopy The FTIR spectra of nanoparticles were illustrated in Fig. 2(a). Typical characteristic peaks of GO were observed at 1633 cm−1 and 1725 cm−1 , which was ascribed to C=C stretching vibration and C=O stretching vibration, respectively [45]. Compared with

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regular stacks of GO sheets by exfoliation. In addition, the chemical composition of synthesized GO–SiO2 nanoparticles was performed by XPS analysis (Fig. 3(b)–(d)). The C=O peak (286.8 eV) of GO–SiO2 was much lower than that of GO (Fig. 3(b) and (c)). Si2p peak at 104 eV was also detected in the XPS spectrum of GO–SiO2 (Fig. 3(d)), which was ascribed to Si–O–C groups [16,37]. This also proved that some C=O groups of GO were converted into Si–O–C groups by grafting SiO2 on GO surface. Therefore, the FTIR, XRD and XPS analyses proved the successful synthesis of GO–SiO2 nanoparticles. 3.2. Membrane morphology The surface and cross-section morphologies of membranes were shown in Fig. 4. All of the membranes displayed typical asymmetric structures with dense skin-layers and porous finger-like cross section structures. Comparing with the pure PVDF membrane, the membrane incorporating with nanoparticles (M-GO, M-SiO2 and M-GO–SiO2 ) showed more porous structures. This was attributed to the good hydrophilicity of SiO2 , GO and GO–SiO2 nanoparticles, which accelerated solvent and non-solvent exchange rate during the phase inversion process and promoted the formation of larger pore channels in the sub-layers [3]. Moreover, EDX mapping in Fig. 5 indicated that the element Si was uniformly dispersed in the membrane surface without visible agglomeration, demonstrating the well distribution of GO–SiO2 nanoparticles in the polymer matrix. In addition, the embedded hydrophilic GO–SiO2 nanoparticles in casting solution also accelerated solvent/non-solvent exchange rate during phase inversion process [38], leading to the formation of lager pores in the hybrid membrane. 3.3. The optimization of the additives concentrations in the casting solution In order to get an optimal membrane preparation condition, the effects of GO–SiO2 loadings and PVP contents on the membrane performance were investigated. Fig. 2. (a) The FTIR spectra of GO, SiO2 and synthesized GO–SiO2 nanoparticles, and (b) the FTIR spectra of the corresponding hybrid membranes doping with different nanoparticles (0.5 wt.%).

GO, new absorbance peaks at 801 cm−1 (Si–O–Si symmetric vibration), 1080 cm−1 (Si–O–Si asymmetric stretching vibration) and 961 cm−1 (Si–OH stretching vibration) was detected in FTIR spectrum of GO–SiO2 nanoparticles. In addition, the peak intensity of C=O stretching vibration at 1725 cm−1 is much higher on the pristine GO nanoparticles than on the modified GO–SiO2 nanoparticles. This can be explained that some C=O groups of GO was reacted with SiO2 during GO–SiO2 synthesis process [46], resulting in less C=O groups residual. The obtained nanoparticles were used to prepare PVDF MF membrane, and the FTIR spectra of the membrane were presented in Fig. 2(b). Typical peaks of GO, SiO2 and synthetic GO–SiO2 can be detected in the FTIR spectra of the hybrid membranes. This further demonstrated the existence of nanoparticles in the prepared hybrid membrane. 3.1.2. XRD measurement The X-ray diffraction (XRD) patterns of GO, SiO2 and GO–SiO2 hybrid nanoparticles were illustrated in Fig. 3. A strong diffraction peak at 2θ of 8.7° was observed for GO nanoparticles [47], and this peak disappeared in XRD pattern of GO–SiO2 hybrid nanoparticles. This further elucidated that SiO2 nanoparticles was successfully incorporated into GO surface [48,49], which destroyed the

3.3.1. GO–SiO2 concentration As presented in Fig. 6(a), when the GO–SiO2 content was increased from 0.1 wt.% to 1 wt.%, the pure water flux first increased from 300 L/(m2 h) to 500 L/(m2 h) and then decreased from 500 L/(m2 h) to 425 L/(m2 h), while the rejection for HA remained relatively constant. The high rejection was attributed to the smaller surface pore size of the membrane (Fig. 6(b)), which rejected most of HA particles during filtration process. Moreover, the prepared hybrid membrane with 0.5 wt.% GO–SiO2 achieved the maximum pure water flux of 500 L/(m2 h) and a rejection of 90%. The increased water flux was attributed to abundant hydrophilic groups (–COOH, –OH and Si–OH) and intrinsic porous structure of GOSiO2 nanoparticles [16,37], which enhanced pure water flux of the prepared membrane. However, further increasing of GO–SiO2 concentration (>0.5 wt.%) would lead to a decreased pure water flux and rejection due to the agglomeration of nanoparticles in the polymer matrix (Fig. S1). Therefore, the optimal GO–SiO2 content was 0.5 wt.%. 3.3.2. PVP concentration The effects of additive PVP contents on the membrane performance were also investigated. As shown in Table 2, the mean pore sizes, the pure water fluxes and the porosities of all membranes increased gradually with the increasing of PVP content. This can be explained that a higher PVP concentration accelerated solvent/nonsolvent exchange rate during immersion precipitation, resulting in the formation of a membrane with highly porous structure [50,51].

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Fig. 3. (a) XRD patterns of different nanoparticles, (b) narrow spectrum C1s of GO, (c) narrow spectrum C1s of GO–SiO2 (d) survey spectra of GO and GO–SiO2 .

Table 2 The performance of membranes at different PVP concentrations (nanoparticle content was 0.5 wt.%). PVP content

Membrane

Porosity (%)

Mean pore size (μm)

Pure water flux (L/(m2 h))

Flux recovery ratios (%)

0.5 wt.% PVP

Pure PVDF M-SiO2 M-GO M-GO–SiO2 Pure PVDF M-SiO2 M-GO M-GO–SiO2 Pure PVDF M-SiO2 M-GO M-GO–SiO2 Pure PVDF M-SiO2 M-GO M-GO–SiO2

65.0 ± 2.0 69.7 ± 2.5 71.2 ± 2.0 76.0 ± 2.0 72.0 ± 2.0 79.4 ± 2.0 82.6 ± 1.0 85.0 ± 2.0 80.0 ± 2.0 84.5 ± 2.0 85.4 ± 1.0 88.0 ± 3.0 83.0 ± 1.5 87.0 ± 2.5 88.5 ± 1.0 90.2 ± 2.0

0.08 0.09 0.10 0.11 0.09 0.10 0.10 0.11 0.14 0.12 0.13 0.12 0.17 0.12 0.14 0.13

230 ± 15 420 ± 30 465 ± 25 500 ± 20 450 ± 15 560 ± 30 665 ± 25 850 ± 20 1100 ± 20 930 ± 20 960 ± 15 1050 ± 18 1300 ± 20 980 ± 20 1190 ± 15 1450 ± 18

34.0 ± 1.0 45.0 ± 1.0 46.0 ± 1.0 59.0 ± 2.0 35.0 ± 2.0 49.0 ± 1.0 50.0 ± 2.0 64.0 ± 2.0 25.0 ± 2.0 32.0 ± 2.0 38.0 ± 2.0 49.0 ± 2.0 16.0 ± 1.0 20.0 ± 1.0 33.0 ± 1.0 41.0 ± 1.0

1 wt.% PVP

1.5 wt.% PVP

2 wt.% PVP

However, further increasing of PVP content would lead to the formation of large surface pore size with unfavorable anti-fouling property (low flux recovery ratio). Therefore, PVP content of 1 wt.% was optimal for preparation of PVDF MF membrane with high permeability and good anti-fouling property.

3.4. Effects of different nanoparticles on membrane property and performance With the optimal membrane preparation condition, i.e. nanoparticle contents of 0.5 wt.% and PVP concentration of 1.0 wt.%, PVDF hybrid membrane was prepared by doping with different nanoparticles (GO, SiO2 and GO–SiO2 ). The effect of different nanoparticles on membrane physicochemical property and separation performance were studied as follows.

3.4.1. Membrane hydrophilicity and separation performance The water contact angles (CA) and separation performances of membranes were shown in Fig. 7. The CA values of pure PVDF, M-SiO2 and M-GO membrane were 80.9°, 77.9° and 75.7°, respectively. Whereas the M-GO–SiO2 exhibited the lowest CA value of 68.3°, indicating its best hydrophilicity. This was owing to the synergistic effect of GO and SiO2 , which enhanced hydrophilicity of the synthesized GO–SiO2 nanoparticles [16]. In addition, the good compatibility between GO–SiO2 and polymer matrix also benefited the hydrophilicity of the prepared M-GO–SiO2 membrane. Hence, the M-GO–SiO2 exhibited 1.89 times water flux (850 L/(m2 h)) compared to that (450 L/(m2 h)) of pure PVDF membrane as a result of the enhanced hydrophilicity. And the water fluxes of MSiO2 and M-GO were 570 L/(m2 h) and 665 L/(m2 h), respectively. In addition, the rejection of all membranes remained at a relatively high level (about 90%). The improvement in the pure water flux of

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Fig. 4. SEM morphologies of the cross-section and top surfaces of different membrane samples. Membrane preparation conditions: 0.5 wt.% nanoparticles (GO, SiO2 and GO–SiO2 ) and 1.0 wt.% PVP in the casting solution.

Fig. 6. (a) The effect of GO–SiO2 content on the pure water flux and HA rejection of PVDF/GO–SiO2 hybrid membrane (0.1 MPa); (b) the particle size distribution of HA and pore size distribution of the membrane.

Fig. 7. Water contact angles, pure water fluxes and rejections of membranes doping with different inorganic nanoparticles. Membrane preparation conditions: 0.5 wt. % nanoparticles and 1.0 wt. % PVP and in the casting solution. Fig. 5. Elemental distribution of (a) C, (b) F, (c) O and (d) Si on the PVDF/GO–SiO2 hybrid membrane surface (0.5 wt.% GO–SiO2 and 1.0 wt.% PVP).

ity of M-GO–SiO2 (Table 2) decreased the mass transfer resistance during filtration process [54]. M-GO–SiO2 was attributed to the following reasons: (i) a lot of hydrophilic groups on the GO–SiO2 nanocomposite surface facilitated the water permeation through the membrane [43,52,53]; (ii) the interfacial gaps between GO–SiO2 and the PVDF polymer matrix provided additional water channels [44]; (iii) the increased poros-

3.4.2. Thermal and mechanical properties The thermal stability of pure PVDF membrane and nanohybrid membrane (M-SiO2 , M-GO and M-GO–SiO2 ) was evaluated by the TGA analysis (Fig. 8(a)). All the membranes exhibited stable weight

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Fig. 8. (a) TGA curves and (b) stress–strain curves of membranes. Membrane preparation conditions: the contents of nanoparticles and PVP were 0.5 wt.% and 1.0 wt.% in the casting solution.

below 400 °C due to the high thermal stability of PVDF materials [35]. Moreover, the M-GO–SiO2 membrane showed the highest residual weight about 40% at 600 °C, indicating its best thermal stability. This can be explained by the uniform dispersion of GO–SiO2 in the membrane matrix (Fig. 5) and their synergistic effect on membrane thermal stability [36,55]. The membrane mechanical properties were shown in Fig. 8(b) and Table 3. The membrane doping with nanoparticles exhibited better mechanical properties than that of pure PVDF membrane. Young’s modulus was enhanced from 18.9 MPa (PVDF membrane) to 20.0 MPa (M-PVDF/GO–SiO2 ). The tensile strength of MPVDF/GO–SiO2 was 0.85 MPa, which was higher than the PVDF hollow fiber membrane prepared with the same PVDF concentration (Table 3). This can be explained that the well-dispersed nanoparticles acted as reinforcing agents to enhance adhesion force of polymer chains, resulting in a dense chain packing and an increased mechanical property [56]. 3.5. Antifouling performance of membranes The anti-fouling behaviors of membranes were investigated using HA solution as the feed solution for three circles. Fig. 9(a) indicated that the fluxes of membranes showed a sharp decrease in

Fig. 9. (a) Permeate flux of membranes doping with different inorganic nanoparticles (0.5 wt.% content) in anti-fouling test; (b) flux recovery ratio and (c) fouling resistance of membranes. Filtration conditions: dead-end filtration using 1 g/L HA as the feed, with a , the stirring speed of 300 rpm, TMP of 0.1 MPa and filtration time of 80 min. Cleaning conditions: 20 °C DI-water with a stirring speed of 300 rpm and cleaning time of 25 min.

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L. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–10 Table 3. The mechanical properties of PVDF membranes. Membrane

Young’s modulus (MPa)

Maximum tensile stress (MPa)

Breaking elongation (%)

Refs.

Pure PVDFa M-PVDF/SiO2 a M-PVDF/GOa M-PVDF/GO–SiO2 a PVDF hollow fiberb PVDF hollow fiberc PVDF flat membraned

18.9 19.4 20.3 20.0 30.1 29.7 26.5

0.73 0.72 0.91 0.85 0.78 0.82 1.85

13.5 19.0 30.6 35.6 116 41.3 5.44

This This This This [57] [58] [59]

study study study study

Note: a PVDF concentration was 14 wt.% in the casting solution. b PVDF concentration was 12 wt.% and 10 wt.% in the outer-layer dope and inner-layer dope solution. c PVDF concentration was 15 wt.% in the dope solution. d PVDF concentration was 22 wt.% in the casting solution. Table 4 Surface tensions and interfacial free energy (mJ/m2 ) of membranes and HA foulant.

HA Original Pure PVDF M-SiO2 M-GO M-GO–SiO2 Fouled Pure PVDF M-SiO2 M-GO M-GO–SiO2

γ+

γ-

γ LW

γ AB

γ TOT

GLW sws

GAB sws

Gsws

0.04

14.93

40.09

1.47

41.56

−5.53

−23.05

−28.58

0.76 0.74 0.75 0.66

9.52 11.64 12.62 20.81

31.39 32.93 34.05 37.91

5.37 5.87 6.14 7.40

36.76 38.79 40.20 45.31

−1.74 −2.29 −2.72 −4.43

−32.84 −27.46 −25.06 −8.28

−34.58 −29.75 −27.78 −12.71

0.82 0.66 0.70 1.08

12.75 20.57 20.74 21.19

35.33 37.22 38.38 38.49

6.46 7.38 7.62 9.57

41.78 44.60 46.00 48.05

−3.25 −4.10 −4.66 −4.71

−24.53 −8.72 −8.35 −7.15

−27.78 −12.82 −13.01 −11.86

Table 5 Performance comparisons of different PVDF nanohybrid membranes. Membranes

Water flux (L/(m2 ·h))

Rejection (%)

Contact angle (°)

FRR (%)

Refs.

PVDF/SiO2 (3 wt.%) (UF) PVDF/GO (1 wt.%) (UF) PVDF/GO/MWCNTs (1 wt.%) (UF) PSF/SiO2 –GO (0.3 wt.%) (UF) PVDF/GO (2 wt.%) (UF) PVDF/rGO/TiO2 (0.05 wt.%) (UF) PVDF/GO (3 wt.%) (MF) Pure PVDF (0 wt.%) (MF) PVDF/GO–SiO2 (0.5 wt.%) (MF)

250 505 406 370 25 221 505 450 850

90 87 – 98 – 99 93 93 92

53 68 52 62 64 69 61 81 68

– 74 98 72 80 95 – 35 62

[62] [34] [64] [65] [51] [32] [21] This study This study

filtration of HA aqueous solutions and then recovered to some extent after rinsing with DI-water. To analysis anti-fouling properties of membranes in detail, the flux recovery ratio (FRR), total fouling ratio (Rt ), reversible fouling ratio (Rr ), and irreversible fouling ratio (Rir ) values were calculated and results were shown in Fig. 9(b) and (c). The pure PVDF membrane exhibited the lowest FRR of 35% and the highest Rir of 65%. This can be explained as followed: (i) the hydrophobic nature of PVDF membrane aggravating membrane fouling [60], (ii) the membrane displayed a more severe fouling in a dead-end unit than in cross-flow unit due to the compaction of cake layer [61]. In contrast, M-GO–SiO2 achieved the maximum FRR of 62% and lowest Rir of 37.4%, indicating its best anti-fouling property among four membranes. The M-GO–SiO2 also showed comparable anti-fouling property compared with literatures [28,59].This can be explained that the adsorption of hydrophobic HA foulant on the hydrophilic M-GO–SiO2 membrane surface was decreased, and attached HA foulants were easily removed during water rinsing process [32]. As proved by interfacial free energy (Gsws ) in Table 4, the M-GO–SiO2 showed the least negative charged Gsws value of −12.71 mJ/m2 , revealing the weakest interaction between hydrophilic M-GO–SiO2 surface and HA foulant [62,63]. Therefore, the adding of GO–SiO2 nanoparticles can effectively enhance the anti-fouling property of PVDF membranes. The performance comparisons between this study and literature were shown in Table 5. The prepared PVDF/GO–SiO2 hybrid

membrane exhibited 89% higher water flux, equivalent HA rejection, enhanced anti-fouling property compared with that of pure PVDF membrane. Moreover, the PVDF/GO–SiO2 hybrid membrane achieved high water flux and good rejection compare with other PVDF or PSF nanohybrid membranes and could be used as a potential candidate in MF applications.

4. Conclusions GO–SiO2 nanoparticles were first synthesized and then incorporated into PVDF matrix to prepare PVDF/GO–SiO2 hybrid MF membranes. The embedding of GO–SiO2 nanoparticles can effectively alter the morphology and improve the surface hydrophilicity, permeation performance and anti-fouling property of the prepared membrane. Due to the good compatibility and the synergistic effects of PVDF and GO–SiO2 , the nanoparticles were uniformly distributed in the PVDF membrane. Therefore, the prepared PVDF/GO–SiO2 membrane (0.5 wt.% GO–SiO2 contents) showed the lowest water contact angle of 68.3°, highest pure water flux of 850 L/(m2 h), best anti-fouling property (FRR 62%) and equivalent HA rejection of 92% among the four membranes (pure PVDF, MGO, M-SiO2 and M-GO–SiO2 ). Based on these potential advantages, this work provides a facile and promising solution to prepare hydrophilic PVDF MF membrane with enhanced perm-selectivity and anti-fouling property.

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Acknowledgments This work was supported by the Beijing Municipal Natural Science Foundation (Project No. 2122009), the National Natural Science Foundation of China (Project No. 21476006) and the State Key Laboratory of Separation Membranes and Membrane Processes of Tianjin Polytechnic University (Project No. M2-201608). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.06.018. References [1] Yang W, Wang Z, Zhou Y, Ye X, Shi L, Cheng L, et al. Study on the control of pore sizes of membranes using chemical methods Part IV. The role of organic acids. Desalination 2013;324:57–64. [2] Choi JH, Jegal J, Kim WN. Fabrication and characterization of multi-walled carbon nanotubes/polymer blend membranes. J Membr Sci 2006;284:406–15. [3] VatanpourV MSS, Moradian R, Zinadini S, Astinchap B. 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