- Email: [email protected]
water and oil-in-water emulsion separation
Composites Communications 7 (2018) 69–73
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
The poly(vinyl alcohol-co-ethylene) nanofiber/silica coated composite membranes for oil/water and oil-in-water emulsion separation
T
Mufang Lia,b,1, Yuqin Lia,1, Kangqi Changa, Pan Chenga,b, Ke Liua,b, Qiongzhen Liua,b, ⁎ Yuedang Wanga,b, Zhentan Lua,b, Dong Wanga,b, a b
College of Materials Science and Engineering, Wuhan Textile University, Wuhan 430073, China Hubei Key laboratory of Advanced Textile Materials & Application, Wuhan 430200, China
A R T I C L E I N F O
A B S T R A C T
Keywords: superhydrophilicity under water oleophobicity composite membrane nanofiber/SiO2 oil/water separation
In this study, the superhydrophilic and under water oleophobic nanofiber/SiO2 (N/S) coated composite membranes with two kinds substrates of polyamide mesh (PM) and poly(vinyl alcohol-co-ethylene) nanofiber membrane (NM) were prepared for the oil/water mixture and oil-in-water emulsion separation. The addition of nanofibers could enhance the film forming ability of SiO2 nanoparticles and the mechanical property of SiO2 coatings. Besides, the preparing processes of nanofibers, N/S coatings and composite membranes possessed advantages of high throughput, lower cost, and environmental friendness. The morphology, structure, wettability of the composite membranes were characterized. The composite N/S-PM membranes were used to separate the oil/water mixtures by gravity, while the N/S-NM membranes were used to separate the oil/water emulsions by pressure driven. The separation efficiency, flux, and reusability were analyzed. The results demonstrate the good oil/water separation performance of N/S coated composites membranes, and the great potential application of which in the industrial oil/water separation.
1. Introduction With the increasing damage of oily wastewater to health, environment and ecological balance, there are intense demands for the efficient oil/water separation technology [1,2]. The most common used methods for oil/water separation include adsorption, gravity or pressure-driven separation, coalescence and so on [3,4]. Among those methods, the gravity or pressure-driven separation based on superwetting membranes has become the most promising technology due to its high separation efficiency and relatively simple operational process, which can effectively separate the immiscible oil/water mixtures and oil/water emulsions [5,6]. To enhance the oil/water separation efficiency, many different superwetting membranes were prepared, such as the robust superhydrophobic-superoleophilic polytetrafluoroethylene (PTFE) nanofibrous membrane, modified Mg(OH)2 powders coated stainless steel mesh, poly(vinylidene fluoride) (PVDF)/graphene composite membrane and so on [7–9]. Compared with the superhydrophobic/superoleophilic membranes, the superhydrophilic/superoleophobic membranes exhibite advantages of antifouling and reusable properties because they can effectively avoid or reduce external oily fouling by the
⁎
1
formation of water barrier between the membranes and oil phase [10,11]. Moreover, as a base material, nanofiber membranes get more and more attention gradually due to their higher porosity, permeability, and versatility. In recent years, plenty of nanofibers and composite nanofiber membranes have been designed to separate the immiscible oil/ water mixtures and oil/water emulsions [12,13]. However, nearly all the nanofiber membranes are prepared by the electrospinning technology, which is difficult to realize mass produce due to the limitation of low production and high cost. Besides, the high skill requirements, high cost, environment unfriendliness and secondary pollution caused by various modification process also limit the industrial application of functional nanofiber membranes. In this study, the poly(vinyl alcohol-co-ethylene) (PVA-co-PE) nanofiber were prepared by an high throughput, environmental friendly method invented in our group [14,15]. Based on this method, the superhydrophilic and under water oleophobic nanofiber/SiO2 (N/S) coated composite membranes with two kinds substrates of polyamide meshes (PM) and PVA-co-PE nanofiber membranes (NM) were prepared for the oil/water and oil-in-water emulsion separation. The morphology, structure, wettability, oil/water separation efficiency, flux,
Corresponding author at: College of Materials Science and Engineering, Wuhan Textile University, Wuhan 430073, China. E-mail address: [email protected] (D. Wang). These authors contributed equally to this work.
https://doi.org/10.1016/j.coco.2018.01.001 Received 8 December 2017; Received in revised form 7 January 2018; Accepted 10 January 2018 2452-2139/ © 2018 Published by Elsevier Ltd.
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M. Li et al.
Fig. 1. SEM images and photographs of the (a1) PM, (a2) N/S-PM membrane, (b1) NM and (b2) N/S-NM membrane, (c) FTIR spectra of SiO2 nanoparticles, NM and N/S coating.
was measured using a total organic carbon analyzer (Element vario TOC select).
and reusability were analyzed. The results demonstrate the good oil/ water separation performance of N/S coated composites membranes, and the great potential application of which in the industrial oil/water separation.
2.3.2. Oil/water separation by gravity A series of surfactant-free oil/water mixtures were prepared to perform the oil/water separation test by gravity. The oils were dyed in red color using Sudan Ⅲ. The water was dyed in blue color using methylene blue. The dyed oils and water (v:v = 1:1) were mixed in a mixture to form the oil/water mixtures before testing. The N/S-PM membrane (effective area is 11.94 cm2) was fixed between two glass containers. The separation efficiency (S) was calculated according to the Eq. (1) [16].
2. Materials and Methods 2.1. Materials Poly(vinyl alcohol-co-ethylene) (PVA-co-PE; 44% ethylene) was obtained from Sigma-Aldrich (Milwaukee, WI, USA). Cellulose acetate butyrate (CAB) was obtained from Eastman Chemical Company. The hydrophilic nano-fumed silica (SiO2, 99.8%, 7–40 nm, the specific surface area is 150 m2/g) was obtained from Aladdin Industrial Co., Ltd. Acetone, alcohol, polyethylene glycol 4000 (PEG 4000) were obtained from Sinopharm Chemical Reagent Co., Ltd. The polyamide (PA) mesh (mesh number 150) was obtained from Hangzhou Daheng Filter Cloth Co., Ltd.
S = (m1/ m 0) * 100
(1)
where m0 and m1 are the mass of oil before and after the separation process, respectively. 2.3.3. Oil-in-water emulsion separation by pressure driven The surfactant-stabilized and surfactant-free oil-in-water emulsions were prepared to perform the oil/water emulsion separation test. The surfactant-stabilized emulsions were obtained by mixing 1 g of nonionic surfactant tween 80, 2.5 mL of oil and 497.5 mL of deionized water under intensively stirring for 3 h. While, the surfactant-free emulsions were prepared by directly mixing the oil and water together to avoid the effect of surfactant on the separation efficiency analysis. The separation efficiency (S) was calculated according to the Eq. (2).
2.2. Preparation of the PVA-co-PE nanofiber/SiO2 coated composite membranes The PVA-co-PE nanofibers and nanofiber membranes were prepared by our previously invented method [14,15]. The stable PVA-co-PE nanofiber/SiO2 (N/S) suspension was obtained by dispersing 1.1 g PVAco-PE nanofibers, 0.9 g SiO2 and 0.4 g PEG in 200 ml water based solution with a high speed shear mixer. Two kinds of N/S coated composite membranes were prepared by spraying the N/S suspension onto the PA mesh (PM) and PVA-co-PE nanofiber membrane (NM), which were defined as N/S-PM and N/S-NM respectively.
S = (1 − Cp/ C0) * 100
(2)
where Cp and C0 are the oil concentration of collected water and original oil-in-water emulsions, respectively.
2.3. Characterization
3. Results and discussion
2.3.1. Morphology and structure The morphologies of PA mesh, PVA-co-PE nanofiber membrane and corresponding N/S-PM, N/S-NM were analyzed by the scanning electron microscopy (SEM, JEOL JSM-5510LV, Japan). The structures were tested by the FTIR-ATR (FTIR, Vertex70, Bruker). The wettability of the N/S coated composite membranes were evaluated by the contact angle goniometry (KRUSS DSA30S, KRUSS Co., Germany). The droplet size distribution of oil-in-water emulsion was analyzed by the laser particle size analyzer (Malvern Zetasizer Nano-ZS). The solutions before and after the separation were observed by the COIC Biological Microscope H6000i (Chongqing COIC Industrial CO., Ltd). The concentration of oil
3.1. Morphology and structure To modify the wettability, the PVA-co-PE nanofiber/SiO2 (N/S, 50/ 50) suspension was prepared as the coating material. Then, the N/S suspension was sprayed onto the substrates of PA mesh (PM) and PVAco-PE nanofiber membrane (NF) to prepare the N/S-PM and N/S-NM composite membranes. Fig. 1 shows the morphologies of substrates and composite membranes. As shown in Fig. 1(a1), the mesh number of PM is 150. The network structure of PM ensures the formation of N/S coating on it. The nanofibers and SiO2 nanoparticles could be observed on the surface of N/S-PM membrane, as shown in Fig. 1(a2). Due to the 70
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Fig. 2. (a) The water contact angels in air of PM, N/S-PM, NM and N/S-NM, (b) Contact angles in water for different oils on the NS-NM.
Fig. 3. (a) The oil/water separation process by gravity (b) the separation efficiency of N/S-PM to various oils/water mixtures.
Fig. 4. (a) The droplet size distribution of surfactant-stabilized oil-in-water emulsion, (b) The oil/water emulsion separation process by press driven, (c) and (d) the optical microscope images of emulsions before and after the filtration.
addition of nanofibers increased the film forming ability and bondability of SiO2 nanoparticles, the surface of N/S-PM membrane was uniform and continuous. The NF membrane exhibited high porosity and small pore size in Fig. 1(b1). After the coating of N/S suspension, an uniform and continuous N/S layer formed on the surface of NF, as shown in Fig. 1(b2). Fig. 1(c) shows the FTIR spectra of SiO2 nanoparticles, PVA-co-PE nanofiber membrane and N/S coating. For the SiO2 nanoparticles, the obvious peak at 1078 cm-1 corresponded to the stretching vibration of Si-O-Si. For the PVA-co-PE nanofiber membrane, the absorption peak at 3300 cm-1 was assigned to the stretching vibration of hydroxyl groups. The peaks at 2847 cm-1 and 2925 cm-1 corresponded to the stretching vibrations of C-H bond. The absorption
peaks at 1329 cm-1 and 1451 cm-1 were assigned to the bending vibrations of C-H bond. After mixing the PVA-co-PE nanofibers and SiO2 together, the specific peaks of SiO2 and PVA-co-PE could be found in the spectrum of N/S coating. The wettability of membrane is very important to oil/water separation performance, which mainly depends on the surface roughness and chemical composition of the membrane [17]. Fig. 2(a) shows the water contact angels (CA) of all the samples in air. For the substrate of PM, its water CA was 122.4° after 6.0 S contact of water drop. The water CA of NM was 107.83° after 0.1 S, and it became 6.9° after 6 S, indicating the good hydrophilicity of nanofiber membrane. After coating the N/S layer onto the substrates, the hydrophilicity of both N/ 71
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Fig. 5. (a) The droplet size distribution of surfactant-free oil-in-water emulsion, (b) the separation efficiency and flux of N/S- NF membrane under different pressures (c) the reusability of N/S- NF membrane at the pressure of 0.5 MPa.
numbers of oil droplet dispersing in it. After passing through the N/SNM membrane, the emulsion turned to transparent and no oil droplet was observed by the optical microscope, indicating the high separation efficiency of N/S-NM membrane to the oil-in-water emulsion. This mainly benefited from the superhydrophilicity and underwater oleophobicity of N/S coatings and the smaller pore size of NF substrate. To analyze the separation efficiency quantitatively, the surfactantfree oil-in-water emulsions were prepared so that the oil content of emulsions could be calculated accurately by the TOC analyzer. As shown in Fig. 5(a), the droplet size distribution of surfactant-free oil-inwater emulsion was in the range of 60–450 nm, which was larger than that of the surfactant-stabilized oil-in-water emulsion. The separation efficiency and flux of N/S-NM membrane to the surfactant-free oil-inwater emulsions were tested under different operation pressures. As shown in Fig. 5(b), the separation efficiencies were higher than 99% at the pressure of 0.025–0.07 MPa, indicating the oil could be separated from the emulsions successfully by pressure driven. The Flux of N/S-NM membrane increased from 270 to 491 L m−2 h−1 with the pressure increased from 0.025 to 0.07 MPa. Moreover, the reusability of N/S-NM was also characterized by washing the same membrane in deionized water with an ultrasonic cleaner. As shown in Fig. 5(c), the separation efficiency was nearly unchanged after 6 times’ reuse. However, the flux decreased a lot in the process due to the membrane surface was polluted by the adherent oil in the filtration process.
S-PM and NS-NM composite membranes became better compared with the substrates. That mainly attributed to the excellent hydrophilicity of SiO2 nanoparticles and the increased roughness of surface after the N/S coating. The water CA of N/S-PM and NS-NM membranes were 13.75° after 0.1 S of water contacting, and they became 0° after 6 S, indicating the superhydrophilic property of N/S-PM and NS-NM membranes. Besides the hydrophilicity, the various oil CA in water of NS-NM membrane were also tested. Based on the density, the CA of the carbon tetrachloride and trichloromethane are downward, while the CA of edible oil, diesel and engine oil are upward. As shown in Fig. 2(b), the NS-NM membrane exhibited good underwater oleophobicity to various oils with the contact angles larger than 130°. 3.2. Oil/water separation by gravity The oil/water separation efficiency of N/S-PM membrane was evaluated by using a gravity-driven filtration system, the process was shown in Fig. 3(a). The colored oil/water mixture was poured into the upper container. Then the water with blue color penetrated through the N/S-PM membrane to the under container and the oil with red color still kept in the upper container due to the superhydrophilicity and under-water oleophobicity of N/S-PM membrane. This process realized the oil collection from the oil/water mixture and the N/S-PM membrane exhibited high separation efficiencies to various oil/water mixtures, as shown in Fig. 3(b).
4. Conclusion 3.3. Oil-in-water emulsion separation by pressure-driven The N/S-PM and N/S-NM composite membranes were prepared in this study. The photograph and SEM images exhibited the uniformity of composite membranes. The structure of N/S coating was demonstrated by the FTIR spectra. The water CA in air and oil CA in water of the composite membranes were tested. After the coating of N/S layer, both the N/S-PM and N/S-NM membranes exhibited superhydrophilicity and
The N/S-NM membrane was used to filtrate the oil-in-water emulsion by pressure driven. The droplet size distribution of surfactantstabilized oil-in-water emulsion was 40–200 nm, as shown in Fig. 4(a). The separation process and emulsions before and after the filtration were shown in Fig. 4(b-d). The original emulsion was milky with 72
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under water oleophobicity, ensuring their application in the oil/water separation. The N/S-PM membranes showed good separation efficiency to various oil/water mixtures by gravity. While the N/S-NM membranes could separate oils form the oil-in-water emulsions with efficiency of 99% above and large flux at the pressure of 0.025–0.07 MPa. Besides, the N/S-NM could keep equative separation efficiency, but lower flux after several times’ reuse. All of the results demonstrated the good oil/ water separation performance of N/S coated composite membranes.
[6] [7]
[8]
[9]
Acknowledgement [10]
The authors are thankful for the financial support of National Nature Science Foundation (51403166, 51473129), Science and Technology Innovation Major Projects of Hubei Province (2016AAA019), National Key Research and Development Program (2016YFC0400504), Excellent Innovative Team of Young Researchers from Hubei provincial department of education (T201408), and Creative research group of Hubei province (2015CFA028).
[11]
[12]
[13]
References [14] [1] S. Lei, Z. Shi, J. Ou, F. Wang, M. Xue, W. Li, G. Qiao, X. Guan, J. Zhang, Durable superhydrophobic cotton fabric for oil/water separation, Colloid. Surface. A 533 (2017) 249–254. [2] F. Sun, W. Liu, Z. Dong, Y. Deng, Underwater superoleophobicity cellulose nanofibril aerogel through regioselective sulfonation for oil/water separation, Chem. Eng. J. 330 (2017) 774–782. [3] D. Hu, X. Li, L. Li, C. Yang, Designing high-caliber nonwoven filter mats for coalescence filtration of oil/water emulsions, Sep. Purif. Technol. 149 (2015) 65–73. [4] M.M. Ghobashy, M.A. Elhady, Radiation crosslinked magnetized wax (PE/Fe3O4) nanocomposite for selective oil adsorption, Compos. Commun 3 (2017) 18–22. [5] G.M. Geise, H.S. Lee, D.J. Miller, B.D. Freeman, J.E. McGrath, D.R. Paul, Water
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Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
The poly(vinyl alcohol-co-ethylene) nanofiber/silica coated composite membranes for oil/water and oil-in-water emulsion separation
T
Mufang Lia,b,1, Yuqin Lia,1, Kangqi Changa, Pan Chenga,b, Ke Liua,b, Qiongzhen Liua,b, ⁎ Yuedang Wanga,b, Zhentan Lua,b, Dong Wanga,b, a b
College of Materials Science and Engineering, Wuhan Textile University, Wuhan 430073, China Hubei Key laboratory of Advanced Textile Materials & Application, Wuhan 430200, China
A R T I C L E I N F O
A B S T R A C T
Keywords: superhydrophilicity under water oleophobicity composite membrane nanofiber/SiO2 oil/water separation
In this study, the superhydrophilic and under water oleophobic nanofiber/SiO2 (N/S) coated composite membranes with two kinds substrates of polyamide mesh (PM) and poly(vinyl alcohol-co-ethylene) nanofiber membrane (NM) were prepared for the oil/water mixture and oil-in-water emulsion separation. The addition of nanofibers could enhance the film forming ability of SiO2 nanoparticles and the mechanical property of SiO2 coatings. Besides, the preparing processes of nanofibers, N/S coatings and composite membranes possessed advantages of high throughput, lower cost, and environmental friendness. The morphology, structure, wettability of the composite membranes were characterized. The composite N/S-PM membranes were used to separate the oil/water mixtures by gravity, while the N/S-NM membranes were used to separate the oil/water emulsions by pressure driven. The separation efficiency, flux, and reusability were analyzed. The results demonstrate the good oil/water separation performance of N/S coated composites membranes, and the great potential application of which in the industrial oil/water separation.
1. Introduction With the increasing damage of oily wastewater to health, environment and ecological balance, there are intense demands for the efficient oil/water separation technology [1,2]. The most common used methods for oil/water separation include adsorption, gravity or pressure-driven separation, coalescence and so on [3,4]. Among those methods, the gravity or pressure-driven separation based on superwetting membranes has become the most promising technology due to its high separation efficiency and relatively simple operational process, which can effectively separate the immiscible oil/water mixtures and oil/water emulsions [5,6]. To enhance the oil/water separation efficiency, many different superwetting membranes were prepared, such as the robust superhydrophobic-superoleophilic polytetrafluoroethylene (PTFE) nanofibrous membrane, modified Mg(OH)2 powders coated stainless steel mesh, poly(vinylidene fluoride) (PVDF)/graphene composite membrane and so on [7–9]. Compared with the superhydrophobic/superoleophilic membranes, the superhydrophilic/superoleophobic membranes exhibite advantages of antifouling and reusable properties because they can effectively avoid or reduce external oily fouling by the
⁎
1
formation of water barrier between the membranes and oil phase [10,11]. Moreover, as a base material, nanofiber membranes get more and more attention gradually due to their higher porosity, permeability, and versatility. In recent years, plenty of nanofibers and composite nanofiber membranes have been designed to separate the immiscible oil/ water mixtures and oil/water emulsions [12,13]. However, nearly all the nanofiber membranes are prepared by the electrospinning technology, which is difficult to realize mass produce due to the limitation of low production and high cost. Besides, the high skill requirements, high cost, environment unfriendliness and secondary pollution caused by various modification process also limit the industrial application of functional nanofiber membranes. In this study, the poly(vinyl alcohol-co-ethylene) (PVA-co-PE) nanofiber were prepared by an high throughput, environmental friendly method invented in our group [14,15]. Based on this method, the superhydrophilic and under water oleophobic nanofiber/SiO2 (N/S) coated composite membranes with two kinds substrates of polyamide meshes (PM) and PVA-co-PE nanofiber membranes (NM) were prepared for the oil/water and oil-in-water emulsion separation. The morphology, structure, wettability, oil/water separation efficiency, flux,
Corresponding author at: College of Materials Science and Engineering, Wuhan Textile University, Wuhan 430073, China. E-mail address: [email protected] (D. Wang). These authors contributed equally to this work.
https://doi.org/10.1016/j.coco.2018.01.001 Received 8 December 2017; Received in revised form 7 January 2018; Accepted 10 January 2018 2452-2139/ © 2018 Published by Elsevier Ltd.
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Fig. 1. SEM images and photographs of the (a1) PM, (a2) N/S-PM membrane, (b1) NM and (b2) N/S-NM membrane, (c) FTIR spectra of SiO2 nanoparticles, NM and N/S coating.
was measured using a total organic carbon analyzer (Element vario TOC select).
and reusability were analyzed. The results demonstrate the good oil/ water separation performance of N/S coated composites membranes, and the great potential application of which in the industrial oil/water separation.
2.3.2. Oil/water separation by gravity A series of surfactant-free oil/water mixtures were prepared to perform the oil/water separation test by gravity. The oils were dyed in red color using Sudan Ⅲ. The water was dyed in blue color using methylene blue. The dyed oils and water (v:v = 1:1) were mixed in a mixture to form the oil/water mixtures before testing. The N/S-PM membrane (effective area is 11.94 cm2) was fixed between two glass containers. The separation efficiency (S) was calculated according to the Eq. (1) [16].
2. Materials and Methods 2.1. Materials Poly(vinyl alcohol-co-ethylene) (PVA-co-PE; 44% ethylene) was obtained from Sigma-Aldrich (Milwaukee, WI, USA). Cellulose acetate butyrate (CAB) was obtained from Eastman Chemical Company. The hydrophilic nano-fumed silica (SiO2, 99.8%, 7–40 nm, the specific surface area is 150 m2/g) was obtained from Aladdin Industrial Co., Ltd. Acetone, alcohol, polyethylene glycol 4000 (PEG 4000) were obtained from Sinopharm Chemical Reagent Co., Ltd. The polyamide (PA) mesh (mesh number 150) was obtained from Hangzhou Daheng Filter Cloth Co., Ltd.
S = (m1/ m 0) * 100
(1)
where m0 and m1 are the mass of oil before and after the separation process, respectively. 2.3.3. Oil-in-water emulsion separation by pressure driven The surfactant-stabilized and surfactant-free oil-in-water emulsions were prepared to perform the oil/water emulsion separation test. The surfactant-stabilized emulsions were obtained by mixing 1 g of nonionic surfactant tween 80, 2.5 mL of oil and 497.5 mL of deionized water under intensively stirring for 3 h. While, the surfactant-free emulsions were prepared by directly mixing the oil and water together to avoid the effect of surfactant on the separation efficiency analysis. The separation efficiency (S) was calculated according to the Eq. (2).
2.2. Preparation of the PVA-co-PE nanofiber/SiO2 coated composite membranes The PVA-co-PE nanofibers and nanofiber membranes were prepared by our previously invented method [14,15]. The stable PVA-co-PE nanofiber/SiO2 (N/S) suspension was obtained by dispersing 1.1 g PVAco-PE nanofibers, 0.9 g SiO2 and 0.4 g PEG in 200 ml water based solution with a high speed shear mixer. Two kinds of N/S coated composite membranes were prepared by spraying the N/S suspension onto the PA mesh (PM) and PVA-co-PE nanofiber membrane (NM), which were defined as N/S-PM and N/S-NM respectively.
S = (1 − Cp/ C0) * 100
(2)
where Cp and C0 are the oil concentration of collected water and original oil-in-water emulsions, respectively.
2.3. Characterization
3. Results and discussion
2.3.1. Morphology and structure The morphologies of PA mesh, PVA-co-PE nanofiber membrane and corresponding N/S-PM, N/S-NM were analyzed by the scanning electron microscopy (SEM, JEOL JSM-5510LV, Japan). The structures were tested by the FTIR-ATR (FTIR, Vertex70, Bruker). The wettability of the N/S coated composite membranes were evaluated by the contact angle goniometry (KRUSS DSA30S, KRUSS Co., Germany). The droplet size distribution of oil-in-water emulsion was analyzed by the laser particle size analyzer (Malvern Zetasizer Nano-ZS). The solutions before and after the separation were observed by the COIC Biological Microscope H6000i (Chongqing COIC Industrial CO., Ltd). The concentration of oil
3.1. Morphology and structure To modify the wettability, the PVA-co-PE nanofiber/SiO2 (N/S, 50/ 50) suspension was prepared as the coating material. Then, the N/S suspension was sprayed onto the substrates of PA mesh (PM) and PVAco-PE nanofiber membrane (NF) to prepare the N/S-PM and N/S-NM composite membranes. Fig. 1 shows the morphologies of substrates and composite membranes. As shown in Fig. 1(a1), the mesh number of PM is 150. The network structure of PM ensures the formation of N/S coating on it. The nanofibers and SiO2 nanoparticles could be observed on the surface of N/S-PM membrane, as shown in Fig. 1(a2). Due to the 70
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Fig. 2. (a) The water contact angels in air of PM, N/S-PM, NM and N/S-NM, (b) Contact angles in water for different oils on the NS-NM.
Fig. 3. (a) The oil/water separation process by gravity (b) the separation efficiency of N/S-PM to various oils/water mixtures.
Fig. 4. (a) The droplet size distribution of surfactant-stabilized oil-in-water emulsion, (b) The oil/water emulsion separation process by press driven, (c) and (d) the optical microscope images of emulsions before and after the filtration.
addition of nanofibers increased the film forming ability and bondability of SiO2 nanoparticles, the surface of N/S-PM membrane was uniform and continuous. The NF membrane exhibited high porosity and small pore size in Fig. 1(b1). After the coating of N/S suspension, an uniform and continuous N/S layer formed on the surface of NF, as shown in Fig. 1(b2). Fig. 1(c) shows the FTIR spectra of SiO2 nanoparticles, PVA-co-PE nanofiber membrane and N/S coating. For the SiO2 nanoparticles, the obvious peak at 1078 cm-1 corresponded to the stretching vibration of Si-O-Si. For the PVA-co-PE nanofiber membrane, the absorption peak at 3300 cm-1 was assigned to the stretching vibration of hydroxyl groups. The peaks at 2847 cm-1 and 2925 cm-1 corresponded to the stretching vibrations of C-H bond. The absorption
peaks at 1329 cm-1 and 1451 cm-1 were assigned to the bending vibrations of C-H bond. After mixing the PVA-co-PE nanofibers and SiO2 together, the specific peaks of SiO2 and PVA-co-PE could be found in the spectrum of N/S coating. The wettability of membrane is very important to oil/water separation performance, which mainly depends on the surface roughness and chemical composition of the membrane [17]. Fig. 2(a) shows the water contact angels (CA) of all the samples in air. For the substrate of PM, its water CA was 122.4° after 6.0 S contact of water drop. The water CA of NM was 107.83° after 0.1 S, and it became 6.9° after 6 S, indicating the good hydrophilicity of nanofiber membrane. After coating the N/S layer onto the substrates, the hydrophilicity of both N/ 71
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Fig. 5. (a) The droplet size distribution of surfactant-free oil-in-water emulsion, (b) the separation efficiency and flux of N/S- NF membrane under different pressures (c) the reusability of N/S- NF membrane at the pressure of 0.5 MPa.
numbers of oil droplet dispersing in it. After passing through the N/SNM membrane, the emulsion turned to transparent and no oil droplet was observed by the optical microscope, indicating the high separation efficiency of N/S-NM membrane to the oil-in-water emulsion. This mainly benefited from the superhydrophilicity and underwater oleophobicity of N/S coatings and the smaller pore size of NF substrate. To analyze the separation efficiency quantitatively, the surfactantfree oil-in-water emulsions were prepared so that the oil content of emulsions could be calculated accurately by the TOC analyzer. As shown in Fig. 5(a), the droplet size distribution of surfactant-free oil-inwater emulsion was in the range of 60–450 nm, which was larger than that of the surfactant-stabilized oil-in-water emulsion. The separation efficiency and flux of N/S-NM membrane to the surfactant-free oil-inwater emulsions were tested under different operation pressures. As shown in Fig. 5(b), the separation efficiencies were higher than 99% at the pressure of 0.025–0.07 MPa, indicating the oil could be separated from the emulsions successfully by pressure driven. The Flux of N/S-NM membrane increased from 270 to 491 L m−2 h−1 with the pressure increased from 0.025 to 0.07 MPa. Moreover, the reusability of N/S-NM was also characterized by washing the same membrane in deionized water with an ultrasonic cleaner. As shown in Fig. 5(c), the separation efficiency was nearly unchanged after 6 times’ reuse. However, the flux decreased a lot in the process due to the membrane surface was polluted by the adherent oil in the filtration process.
S-PM and NS-NM composite membranes became better compared with the substrates. That mainly attributed to the excellent hydrophilicity of SiO2 nanoparticles and the increased roughness of surface after the N/S coating. The water CA of N/S-PM and NS-NM membranes were 13.75° after 0.1 S of water contacting, and they became 0° after 6 S, indicating the superhydrophilic property of N/S-PM and NS-NM membranes. Besides the hydrophilicity, the various oil CA in water of NS-NM membrane were also tested. Based on the density, the CA of the carbon tetrachloride and trichloromethane are downward, while the CA of edible oil, diesel and engine oil are upward. As shown in Fig. 2(b), the NS-NM membrane exhibited good underwater oleophobicity to various oils with the contact angles larger than 130°. 3.2. Oil/water separation by gravity The oil/water separation efficiency of N/S-PM membrane was evaluated by using a gravity-driven filtration system, the process was shown in Fig. 3(a). The colored oil/water mixture was poured into the upper container. Then the water with blue color penetrated through the N/S-PM membrane to the under container and the oil with red color still kept in the upper container due to the superhydrophilicity and under-water oleophobicity of N/S-PM membrane. This process realized the oil collection from the oil/water mixture and the N/S-PM membrane exhibited high separation efficiencies to various oil/water mixtures, as shown in Fig. 3(b).
4. Conclusion 3.3. Oil-in-water emulsion separation by pressure-driven The N/S-PM and N/S-NM composite membranes were prepared in this study. The photograph and SEM images exhibited the uniformity of composite membranes. The structure of N/S coating was demonstrated by the FTIR spectra. The water CA in air and oil CA in water of the composite membranes were tested. After the coating of N/S layer, both the N/S-PM and N/S-NM membranes exhibited superhydrophilicity and
The N/S-NM membrane was used to filtrate the oil-in-water emulsion by pressure driven. The droplet size distribution of surfactantstabilized oil-in-water emulsion was 40–200 nm, as shown in Fig. 4(a). The separation process and emulsions before and after the filtration were shown in Fig. 4(b-d). The original emulsion was milky with 72
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under water oleophobicity, ensuring their application in the oil/water separation. The N/S-PM membranes showed good separation efficiency to various oil/water mixtures by gravity. While the N/S-NM membranes could separate oils form the oil-in-water emulsions with efficiency of 99% above and large flux at the pressure of 0.025–0.07 MPa. Besides, the N/S-NM could keep equative separation efficiency, but lower flux after several times’ reuse. All of the results demonstrated the good oil/ water separation performance of N/S coated composite membranes.
[6] [7]
[8]
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Acknowledgement [10]
The authors are thankful for the financial support of National Nature Science Foundation (51403166, 51473129), Science and Technology Innovation Major Projects of Hubei Province (2016AAA019), National Key Research and Development Program (2016YFC0400504), Excellent Innovative Team of Young Researchers from Hubei provincial department of education (T201408), and Creative research group of Hubei province (2015CFA028).
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