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Surface modification to fabricate superhydrophobic and superoleophilic alumina membranes for oil/water separation Hongyan Tang, Liting Hao, Junchao Chen, Feng Wang, Huapeng Zhang, and Yuhai Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03344 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 20, 2018
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Graphical abstract 254x190mm (300 x 300 DPI)
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Figure 1 The schematic diagrams of the alumina membrane (a) and experimental setup for oil/water separation (b) 338x190mm (300 x 300 DPI)
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Figure 1 The schematic diagrams of the alumina membrane (a) and experimental setup for oil/water separation (b) 142x86mm (300 x 300 DPI)
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Figure 2 Images of liquid droplet on the surface of the modified alumina membrane (Mc). 48x45mm (300 x 300 DPI)
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Figure 2 Images of liquid droplet on the surface of the modified alumina membrane (Mc). 71x51mm (300 x 300 DPI)
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Figure 2 Images of liquid droplet on the surface of the modified alumina membrane (Mc). 40x37mm (300 x 300 DPI)
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Figure 3 The DSC curve of PTFE membranes 197x139mm (300 x 300 DPI)
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Figure 4 FTIR spectra of the membranes. (a) in the wavenumber range 3500-700 cm-1 197x139mm (300 x 300 DPI)
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Figure 4 FTIR spectra of the membranes.(b) in the wavenumber range 700-400 cm-1. 213x163mm (300 x 300 DPI)
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Figure 5 Elemental concentrations of the membranes. 90x44mm (300 x 300 DPI)
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Figure 5 Elemental concentrations of the membranes. 90x43mm (300 x 300 DPI)
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Figure 5 Elemental concentrations of the membranes. 90x43mm (300 x 300 DPI)
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Figure 5 Elemental concentrations of the membranes. 89x44mm (300 x 300 DPI)
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Figure 6 High-resolution C1s core spectra of modified alumina membranes and PTFE membranes. 297x213mm (300 x 300 DPI)
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Figure 6 High-resolution C1s core spectra of modified alumina membranes and PTFE membranes. 297x213mm (300 x 300 DPI)
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Figure 6 High-resolution C1s core spectra of modified alumina membranes and PTFE membranes. 297x213mm (300 x 300 DPI)
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Figure 6 High-resolution C1s core spectra of modified alumina membranes and PTFE membranes. 297x213mm (300 x 300 DPI)
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Figure 6 High-resolution C1s core spectra of modified alumina membranes and PTFE membranes. 297x213mm (300 x 300 DPI)
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Figure 7 TGA curves of the membranes. 297x209mm (300 x 300 DPI)
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Figure 8 SEM images of the membranes. 361x270mm (300 x 300 DPI)
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Figure 8 SEM images of the membranes. 361x270mm (300 x 300 DPI)
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Figure 8 SEM images of the membranes. 361x270mm (300 x 300 DPI)
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Figure 8 SEM images of the membranes. 361x270mm (300 x 300 DPI)
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Figure 9 Pore size distributions of the membranes. 297x206mm (300 x 300 DPI)
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Figure 9 Pore size distributions of the membranes. 297x206mm (300 x 300 DPI)
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Figure 10 Separation performance of the modified alumina membrane (Mc). 193x134mm (300 x 300 DPI)
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(a) water rejection of Mc
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Figure 10 Separation performance of the modified alumina membrane (Mc). (b) the permeation flux of Mc with different water volume contents over time (under 0.1 MPa), 279x193mm (300 x 300 DPI)
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Figure 10 Separation performance of the modified alumina membrane (Mc). (c) the permeation flux of Mc under different pressures over time (the water volume content is 1%) 213x163mm (300 x 300 DPI)
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Figure 10 Separation performance of the modified alumina membrane (Mc). (d) photos of the feeding and the permeation solutions. 254x190mm (300 x 300 DPI)
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Figure 11 Long-term oil/water separation performance of Mc over 48 h. 297x206mm (300 x 300 DPI)
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Surface modification to fabricate superhydrophobic and superoleophilic alumina membranes for oil/water separation
Hongyan Tang, Liting Hao, Junchao Chen, Feng Wang, Huapeng Zhang, Yuhai Guo* (The key laboratory of Fiber Materials and Processing Technology of Zhejiang Province, Zhejiang Sci-Tech University, Hangzhou 310018, China) Abstract: A facile approach to fabricate superhydrophobic and superoleophilic alumina membranes through surface modification is presented in this work. The modified alumina membranes were prepared through thermal decomposition of polytetrafluoroethylene (PTFE) materials on the surface of alumina membranes. Contact angle (CA) measurement shows that the modified alumina membranes exhibit superhydrophobicity (155º) and superoleophilicity (0º). Results of fourier transformed infrared spectroscopy, x-ray energy dispersive spectrometer and x-ray photoelectron spectroscopy indicate that the fluoric groups were formed on the surface of the modified alumina membranes, which may be due to partial decomposition of the PTFE polymer and deposition during sintering process. It is the key to hydrophobicity. FE-SEM images demonstrate the occurrence of the fluoric layer, which further induced that the pore size of the modified alumina membranes decreased. After sintering at 400 o C over 7 h under a nitrogen atmosphere, values of water rejection of the modified alumina membranes for oil/water separation are all higher than 97% over 4 h. A slight reduction in permeation flux can be found over 48 h. It indicates the fluoric layer is firmly linked with the alumina membranes. The asprepared membranes may have great potential for oil/water separation. Keywords: alumina membrane; polytetrafluoroethylene; superhydrophobic; oil/water separation 1. Introduction Oil/water separation is a worldwide industrial, environmental and health concern due to increasing industrial oily wastewater1-8. Many technologies have been used for oil/water separation, such as ultrasonic separation, coagulation, air flotation, heating, ozonation, flocculation and membrane separation9-13. Membrane separation has become an efficient technique for oil/water separation because of effective separation performance and relatively operational process. Among it, microfiltration (MF) and ultrafiltration (UF) are very effective treatments for oil/water separation2,14. Various substrates have been investigated for oil/water separation, such as polymer composites15-18, metal meshes19,20, filter paper21, manganese oxide *Corresponding author.
[email protected] (Y.H. Guo);Tel.:+8657186843255; Fax: +8657186843255.
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nanowire22, textiles23,24, silicon25 and plastics26. However, there are some drawbacks including low flexibility and poor mechanical stability27. Ceramic membranes, including alumina (Al2O3), zirconia (ZrO2), titania (TiO2), silica (SiO2) have attracted much more attention due to many advantages, such as superior mechanical strength, high chemical resistance, great thermal stability and biological stability2,14,28-31. Herein, the alumina membranes are chemical very alert, which can be employed in the pH-range from 1 to 14. Harsh chemical cleaning may not cause any problems. There is no limitation in temperature and pH. Coating and grafting are usually employed to modify ceramic membranes for oil/water separation. Membrane materials with special wettability can be appropriate for oil/water separation. Both superhydrophilic and underwater superoleophobic materials can provide a good alternative for penetrating water from light oil, which is named as ‘water removing’ type7,32. Lu et al. deposited different metal oxides on the surface of ceramic membrane and inducing influences on membrane fouling during ultrafiltration of oil/water emulsion33. Zhu et al. fabricated a low-cost multi-layer-structured mullite-titania composite ceramic hollow fiber microfiltration membrane, in which coal fly ash was effectively recycled34. Duan et al. patterned ZrO2 films displaying superhydrophilic /superhydrophobic properties with the groove and processus mastoideus structures using a photosensitive sol-gel method35. Zhou et al. modified alumina membranes by coating with nano-sized ZrO2, which improved the hydrophilic properties36. Chang et al. modified ceramic MF membranes by coating with nano-TiO237. Howarter et al. employed perfluorinated end-cappped polyethylene glycol surfactants to modify fritted glass membranes for oil/water separation38. Cui et al. fabricated zeolite MF membranes by in-situ hydrothermal technique on alumina tube for treating oily wastewater39. Graphene oxide was employed to modify alumina MF membranes to improve the hydrophilicity for oil/water separation through a facile method6. On the contrary, both superhydrophobic and underwater superhydroleophilic materials are totally different from water removing type of materials, which can penetrate oil and completely repel water when heavy oil accumulates on the material surface. It is named as ‘oil removing’ type, which is much more suitable for heavy oil/water separation40. Gao et al. modified the ZrO2 membrane with hexadecyltrimethoxysilane (HDTMS) to induce fouling during oil/water separation41. Wu et al. fabricated an inherently hydrophobic ceramic nanofiber membrane through pyrolysis of electrospun polycarbosilane nanofibers, in which trace amounts of palladium was introduced to adjust hydrophobicity42. Gao et al. improved the filtration performance of ZrO2 membrane in non-polar organic solvents30. Lu et al. improved the hydrophobic properties of alumina membranes by grafting fluoroalkylsilane on the surface43. Fluorosilane grafted zirconia membranes were fabricated and characterized in the air-gap membrane distillation process44. Razmjou et al. generated superhydrophobic membranes for membrane distillation via depositing TiO2 nanoparticles on PVDF membranes and then fluorosilanized usig H, 1H, 2H, 2H-perfluorododecyltrichlorosilane45.
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Those methods usually require relatively complicated processes or harsh modification conditions. PTFE materials are widely employed in membrane separation due to its intrinsic superior hydrophobicity, chemical resistance and high mechanical strength46,47. Belapurkar et al. successfully deposited a PTFE film on different substrates using a method based on radiationinduced polymerization of tetrafluoroethene monomer48. Li et al. switched hydrophobic PTFE membranes to hydrophilic ones through atomic layer deposition seeding and ZnO nanorod growth on the surface49. Herein, the objective of this study is to fabricate superhydrophobic and superoleophilic alumina membranes with PTFE materials for oil/water separation through simple surface modification. The modified alumina membranes were deeply characterized to evaluate surface hydrophobicity, surface chemical composition and surface morphologies. Finally, oil/water separation performance of the modified alumina membrane was investigated. The as-prepared membrane may have great potential for practical applications. It may provide a good alternative oil/water separation. 2. Experimental 2.1. Materials Alumina (Al2O3) membranes (flat-sheet) were supplied by Pingxiang Bocent Advanced Ceramic Co., Ltd. (Jiangxi, China). Polytetrafluoroethylene (PTFE) flat-sheet membranes (average pore diameter is 0.10 μm) were supplied by our lab. The lubricant oil（aliphatic saturated hydrocarbon）, viscosity, ~1.4 mPa●s-1, density, ~781 kg●m-3 at room temperature） was purchased from Beijing Tiancheng Meijia Lubricant Co. Ltd. (Beijing, China). DI water (treated by reverse osmosis) was provided by our lab. Sorbitan monooleate was purchased from Aladdin Industrial Inc. (Shanghai, China). 2.2. Modification of alumina membranes PTFE flat-sheet membranes were carefully spread on the whole surface of alumina membranes, which would be inserted into a furnace (HDXQ-4-8, Luoyang Hongda furnance Co., Ltd., Henan, China) for a sintering process under a nitrogen atmosphere. The sintering conditions were listed in Table 1. After that, the power of the furnace was turned off. The sample would be taken off until the temperature decreased to ~ 25 o C. The nitrogen atmosphere was always provided until the sample was taken off. The modified alumina membranes were ready for further characterization.
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Table 1 The sintering condition of the modified alumina membranes Sintering temperature, o C
No. Ma Mb Mc Md Me Mf
500 450 400 350 400 400
Sintering time, h 7 7 7 7 5 3
2.3. Oil/water separation Figure 1 the schematic diagrams of the alumina membrane (a) and the experimental setup for oil/water separation (b). There are upright collecting pipes inside of the alumina membrane on two sides (Side (β) and the opposite side), which were connected with the pump. There are no any holes on the two other sides (Side (α) and the opposite side). Alumina membrane (flat-sheet) was immersed in the feed solution. When the pump worked, the solution would be driven to permeate from the membrane surface, which induced that the filtrate entered into the collecting pipes. The modified alumina membrane (Mc) was chosen in this experiment. Experiments were carried out at 20.0±1 o C under ~ 0.10 MPa. Sorbitan monooleate was added into the lubricant oil, in which water was then added. Subsequently, the mixture was stirred by an emulsifier (FLUKO, Shanghai) at 2900 rpm for 15 min. The droplet size was ~ 2 µm. No demulsification or precipitation was observed over 1 month. The oil/water mixtures containing different contents of water were used as samples, in which the water volume contents were 1%, 2%, 4%, 6%, 8% (designated as 1#, 2#, 3#, 4#, 5#), respectively. The water rejection and permeation flux were calculated by Equations (1) and (2).
%
100
1
(1)
Herein, R is the water rejection. cf and cp are the water concentrations in the feeding solution and the permeation solution, respectively. The water content was determined by the Karl Fischer Coulometric method (831 KF coulometer metrohm).
(2)
Herein, J is the permeation flux (L·m-2·h-1). V is the permeation volume (L). A is the effective area (m2). t is the time (h).
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Figure 1 The schematic diagrams of the alumina membrane (a) and experimental setup for oil/water separation (b) 2.4. Characterization and Measurement The FTIR spectra were carried out by a Thermo Nicolet 5700 FTIR spectrophotometer with an ATR (attenuated total reflection) accessory. The wave number range was 400–4000 cm−1. The morphologies were examined by a FE-SEM (HITACHIE-1010). The element concentrations were observed with an X-ray energy dispersive spectrometer (EDS) (EVO MA 25, ZISS, Germany). Contact angle (CA) was measured by a tensiometer (DCAT21 Dataphysics, Germany). The reported data of CA were the average of five values to ensure the reproducibility of the results. The thermal properties were determined by TGA (Perkin-Elmer Pyris 1 series) under a nitrogen atmosphere. The sample was ～5 mg. The temperature range was 25-800 oC( 5 oC / min). The surface chemical compositional analyses were performed by XPS (VG ESCALAB 220iXL). The XPS spectra were collected by an Al Kα (1486.6 eV) X-ray source. The pore size was measured by a Capillary Flow Porometer (CFP-1500AE, Porous Materials, Inc., USA). The alumina membranes fully wetted by the Galwick (Its surface tension and density were 15.9 dyn/cm and 1.87 g/mL, respectively) was mounted on the sample chamber and then the chamber was sealed. Subsequently, pure nitrogen flowed into the chamber gradually. The increased nitrogen pressure would first reach the point that overcame the capillary flow of the fluid within the largest pore. Then, the pressure increased continuously until all pores were empty of the fluid. The measurements were described in the literature46,47. 2.5. Chemical stability test The modified alumina membrane was immersed into the lubricant oil over 96 h. Subsequently, the modified alumina membrane would be rinsed with isopropanol and alcohol and dried at 90 o C in an oven. CA was measured.
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3. Results and discussion 3.1. Surface hydrophobicity Table 2 shows values of CA of unmodified and modified alumina membranes. The corresponding videos are shown in Supplementary material. It can be seen that the value of water contact angle (WCA) of unmodified alumina membrane is 0°, which shows excellent hydrophilic. However, the WCA value of modified alumina membrane (Mc) is 155.3°, which indicates superhydrophobic. Furthermore, the shape of water droplet on the surface is almost globular (shown in Figure 2a). It indicates that sintering at 400 oC over 7 h could definitely improve surface hydrophobicity of the alumina membrane. Moreover, the hydrophobicity decreased (WCA value of Mb is 136.1°) if the sintering temperature increased. WCA value decreased to 0° when the temperature increased to 500 oC. With regard to oil contact angle (OCA), oil can absolutely wet the surface and penetrate through the pores as shown in Figure 2b. OCA values of unmodified and modified alumina membranes (Ma-Mc) are all 0°, which shows superoleophilicity. Furthermore, it can be seen ~3° reduction occurred to Mc after immersion in oil over 96 h, which indicates that the modified layer is firmly linked with alumina membranes and stable in oil. Therefore, the modified alumina membrane still remains good hydrophobicity. Gao et al. reported that 1° reduction of contact angle of the membranes occurred after immersion in oil30. In addition, when the sintering temperature decreased (Md), it can be observed that only a little change occurred to PTFE membranes on the surface of alumina membrane. The melting point of PTFE polymer is ~327 oC50. An obvious endothermal change begins at ~ 350 oC as shown in Figure 3. So, it showed that decomposition of the PTFE polymer hardly occurred when sintering at 350 oC. The similar phenomena could be observed when the sintering time decreased (Me and Mf). Therefore, values of WCA (Md, Me and Mf) were not reported in Table 2. Table 2 CA Values of unmodified and modified alumina membranes CA
Unmodified Ma
Water droplet (WCA) Oil droplet (OCA) Water droplet on the surface wetted by oil
Mb
Mc*
0° 0°
0° 0°
0°
39.3±1.1° 126.2±0.9° 143.2±1.0°
*, Mc immersed in oil over 96 h
136.1±1.0° 155.3±0.9° 0° 0°
152.1±1.0°
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Figure 2 Images of liquid droplet on the surface of the modified alumina membrane (Mc). (a) water droplet (WCA is 155.3°), (b) oil droplet (OCA is 0°), (c) water droplet on the surface wetted by oil (WCA is 143.2°).
Figure 3 The DSC curve of PTFE membranes 3.2. Chemical composition on the surface of the membranes Figure 4 presents FTIR spectra of the membranes. It can be seen from Figure 4(a) that the intense peaks at 1150 cm-1 and 1210 cm-1 were originated from the C-F groups of PTFE membranes (C-F stretching mode)47. Comparing to original PTFE membranes, the intensities of C-F groups of the modified alumina membranes (Mb and Mc) decreased, the C-F groups of Ma disappeared. It indicated that the contents of C-F groups of modified alumina membranes decreased after sintering process, which may be due to decomposition of the PTFE polymer. In combination with the data of Table 1 and Table 2, it tends to indicate that the C-F bonds would break down due to decomposition of the PTFE polymer after sintering at 400 oC or 450 oC over 7 h. It may accordingly induce that some new fluoric groups would engender and deposit on the surface of the alumina membranes (which may be further investigated). Therefore, values of CA (Mb and Mc) dramatically increased. It is the key to hydrophobicity of the modified alumina membranes. The C-F groups of Ma completely disappeared. However, a new peak at 2350 cm-1 appeared, which may be ascribed to —C=O of CO2. It indicates complete decomposition of the PTFE polymer after sintering at 500 oC over 7 h. Therefore, the value of CA was 0°, which was similar to that of unmodified alumina membrane. In addition, a lower sintering temperature (Md) or shorter sintering times (Me and Mf) would not bring great changes of the C-F groups because of almost no decomposition of the PTFE polymer. According to the literature51,52, there are 7 ACS Paragon Plus Environment
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several peaks in the lower wavenumber range, which are assigned to C-F wagging mode and C-F rocking mode. It can be observed from Figure 4(b) there are several peaks for PTFE membrane. However, they disappeared after the sintering process. So, almost no obvious peaks appeared for the samples of Ma, Mb and Mc. In a word, the key to hydrophobicity is that the C-F bonds partly break down due to decomposition of the PTFE polymer during sintering process.
Figure 4 FTIR spectra of the membranes. (a) in the wavenumber range 3500-700 cm-1,(b) in the wavenumber range 700-400 cm-1. Analyses of the elemental concentration of the membranes were performed using the FESEMEDS technique (Figure 5). Carbon (C) and fluorine (F) elements could be observed in the 8 ACS Paragon Plus Environment
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modified alumina membranes (Mb and Mc) besides aluminium (Al) and oxygen (O) elements. It indicates that the C-F bonds remained, which also indicates partial decomposition of the PTFE polymer. It can be observed that aluminium (Al), oxygen (O) and carbon (C) elements were seen in the modified alumina membrane (Ma). No fluorine (F) element occurred. It tends to show complete decomposition of the PTFE polymer. These results were in agreement with that of FTIR.
Figure 5 Elemental concentrations of the membranes. The C-F group of PTFE has one form of CF2 (the molecular structure of PTFE, CF2 CF 2 ). In this study, PTFE flat-sheet membranes spread on the surface of the alumina membrane. It would begin to decompose during the sintering process under a nitrogen atmosphere, which may be resolved into different components. Figure 6 presents high-resolution C1s core level spectra of modified alumina membranes and PTFE membranes. It can be seen that only one component appeared in the C1s spectrum of unsintered PTFE, namely, CF2 (291.3 eV). However, the corresponding C1s spectrum after sintering at 400 oC over 7 h would be resolved into five components, namely, CF3 (293.8 eV), CF2 (291.3 eV), CF-CFn (288.6 eV), C-CFn (286.5 eV) and C-C (284.78 eV)53-55. The spectra of Mb and Mc were also resolved into five components, which were similar to those of sintered PTFE. Only three components appeared in Ma, namely, CF-CFn (288.3 eV), C-CFn (286.6 eV) and C-C (284.78 eV). Surface energy is one of the most important factors of the hydrophobicity54, 55. In general, the PTFE polymer has superior hydrophobicity because of the critical surface tension of CF2 (18 dyn/cm). In addition, the surface tensions of other fluoric groups (CF3 and CF2H) are 6 dyn/cm and 15 dyn/cm, respectively56,57. Two fluoric groups (CF3 and CF2H) with low surface tensions 9 ACS Paragon Plus Environment
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did not appear in Ma. In addition, contact angle of PTFE materials (CF2) is usually ~130° 47. Therefore, the fluoric groups with lower surface tensions may be the reason of hydrophobicity of Mc (155.3°) and Mb (136.1°) and hydrophilicity of Ma (0°). Comparing with the spectra of Mb and Mc, the CF3 concentrations of Mc and Mb are 2.87% and 1.05%, respectively, which are higher than that of sintered PTFE membranes. Therefore, the hydrophobicity order of the modified alumina membranes is Mc > Mb >Ma, which is consistent with Table 2.
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Figure 6 High-resolution C1s core spectra of modified alumina membranes and PTFE membranes. TGA curves of unmodified alumina membranes, PTFE membranes and modified alumina membranes are shown in Figure 7. It can be seen that PTFE began to decompose at 500 oC, which would completely decompose until ~ 620 oC. The unmodified and modified alumina membranes (Ma) were very stable in the range of 40-800 oC. It tends to show that none of the PTFE polymer remained in the sample Ma due to complete decomposition of the PTFE polymer. Therefore, the value of WCA was 0°. The results are in agreement with those of FTIR, EDS and XPS. However, an apparent weight loss can be observed in the range of 400-620 oC in the modified alumina membrane (Mc). It may correspond to further decomposition of the PTFE polymer, which also indicates that only partial decomposition of the PTFE polymer occurred during sintering process of Mc. It is the key to hydrophobicity. Accordingly, the value of WCA reached 155.3°. It is consistent with the results of FTIR, EDS and XPS.
Figure 7 TGA curves of the membranes.
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3.3. Surface morphologies of the membranes The surface morphologies of the unmodified and modified alumina membranes are presented in Figure 8. PTFE membrane has node-fibril porous structure46. Differences happened to the alumina membranes after modification. PTFE membrane was not observed on the surface of the modified alumina membrane (Mc). However, some new particles deposited on its surface. It may be due to decomposition of the PTFE polymer under heat treatment, which is partial decomposition according to the results of FTIR and XPS. Consequently, the fluoric layer was formed on the surface of the modified alumina membrane (Mc). Accordingly, the surface hydrophobicity greatly increased, namely, the value of WCA of Mc reached 155.3°. In addition, the fluoric layer largely remained on the surface. Only a little part penetrated into the shallow inner alumina membrane according to the cross-section image. Figure 9 presents the pore size distributions of the unmodified and modified alumina membranes (Mc). It can be seen that the average pore size of the modified alumina membrane decreased (from ~ 0.31 μm to ~ 0.20 μm).
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Figure 8 SEM images of the membranes.
Figure 9 Pore size distributions of the membranes.
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3.4. Separation performance The Laplace-Young Equation (as shown in Equation (1)) can be used to calculate the critical entry pressure of the liquid when a liquid droplet is in contact with the membrane surface 30,58,59.
∆
(1)
Where, ∆ is the critical entry pressure of the liquid. is the surface tension of the liquid, N/m. is the contact angle of the liquid with the membrane surface. r is the membrane pore radius, m. The surface tension of oil/water mixture in this research is ~21.72×10-3 N/m. The critical entry pressure of Mc is ~0.21 MPa according to Equation (1). Therefore, the operating pressure (0.10 MPa) in the research is lower than the critical entry pressure. Figure 10 (a) presents the water rejection of the modified alumina membrane (Mc) during oil/water separation experiment. Herein, the oil/water mixtures were designated as 1#, 2#, 3#, 4#, 5#, in which the water volume contents were 1%, 2%, 4%, 6%, 8%, respectively. It can be seen that values of water rejection of the modified alumina membrane (Mc) are all higher than 97%, which means excellent hydrophobicity. It may be due to the fluoric groups deposited on the surface, which is consistent with the results of XPS and FE-SEM. Figure 10 (d) shows photos of the feeding and the permeation solutions. It can be found that the permeation solution is transparent, which shows that oil effectively penetrated through the modified alumina membrane. The corresponding permeation flux over time is shown in Figure 10 (b) and 10 (c). The permeation flux decreased at the beginning of separation process. It may be due to concentration polarization and membrane fouling, which further induces the flux decline 6. After ~ 40 min, a nearly stable flux is obtained. However, a slight reduction in permeation flux can be found over time, especially in the latter period. It can be observed from Figure S1 (surface morphology of the modified alumina membrane) that membrane fouling occurred after separation experiment over 4 h. Oil/water separation experiment on long-term operation (over 48 h) was performed. The corresponding result is presented in Figure 11. Herein, the fresh feeding solution would be provided every ~40 min. The modified alumina membrane was backwashed with isopropanol and ethanol. A slight reduction in permeation flux can be found. It indicates that the fluoric layer is firmly linked with alumina membrane, which is consistent with the result of CA (Table 2). Therefore, the as-prepared membranes may be a promising candidate for oil/water separation.
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Figure 10 Separation performance of the modified alumina membrane (Mc). (a) water rejection of Mc, (b) the permeation flux of Mc with different water volume contents over time (under 0.10 MPa), (c) the permeation flux of Mc under different pressures over time (the water volume content is 1%), (d) photos of the feeding and the permeation solutions.
Figure 11 Long-term oil/water separation performance of Mc over 48 h.
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4. Conclusion A facile approach to fabricate superhydrophobic and superoleophilic alumina membranes for oil/water separation through surface modification is presented. The alumina membranes were modified through partially thermal decomposition of PTFE materials and inducing deposition on the surface during sintering process. As a result, the fluoric layer with the fluoric groups was formed, which is the key to hydrophobicity. The fluoric layer on the surface caused the decline of the pore sizes of the modified alumina membrane. After sintering at 400 o C over 7 h under a nitrogen atmosphere, values of water rejection of the modified alumina membranes for oil/water separation are all higher than 97% over 4 h. A slight reduction in permeation flux can be found over 48 h. It indicates the fluoric layer is firmly linked with alumina membranes. The asprepared membranes may be a promising candidate for oil/water separation.
Supplementary material Supplementary material presents the videos of contact angle measurement. As shown in CA video, with regard to unmodified alumina membrane, water droplet was absorbed by the surface of over 2 s (from 1 s to 3 s). Oil droplet was absorbed by the surface over 6 s (from 1 s to 7 s). Water droplet on the surface wetted by oil was absorbed over 6 s (from 5 s to 11 s). With regard to Ma, water droplet was absorbed by the surface over 61 s (from 1 s to 62 s). Oil droplet was absorbed by the surface over 7s (from 5 s to 12 s). Water droplet on the surface wetted by oil was absorbed over 86 s (from 1 s to 87 s, CA over 60 s is ~ 39.3°). With regard to Mc, oil droplet was absorbed by the surface over 7 s (from 2 s to 9 s). Figure S1 presents surface morphology of the modified alumina membrane after separation experiment over 4 h. Acknowledgments The authors were financially supported by Zhejiang Natural Science Foundation (No. LY15B060010), China Scholarship Council (CSC), Zhejiang Ministry of Major scientific & Technological Project (No. 2013C01055), the training plan for ‘521’ talents of Zhejiang SciTech University. The authors were also grateful to Pingxiang Bocent Advanced Ceramic Co., Ltd. (Jiangxi, China) for supplying alumina (Al2O3) membranes. References (1) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G; Mariñas, B. J.; Mayes, A. M.
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Figure captions Figure 1 The schematic diagrams of the alumina membrane (a) and experimental setup for oil/water separation (b) Figure 2 Images of liquid droplet on the surface of the modified alumina membrane (Mc). Figure 3 The DSC curve of PTFE membranes Figure 4 FTIR spectra of the membranes. (a) in the wavenumber range 3500-700 cm-1, (b) in the wavenumber range 700-400 cm-1. Figure 5 Elemental concentrations of the membranes. Figure 6 High-resolution C1s core spectra of modified alumina membranes and PTFE membranes. Figure 7 TGA curves of the membranes. Figure 8 SEM images of the membranes. Figure 9 Pore size distributions of the membranes. Figure 10 Separation performance of the modified alumina membrane (Mc). (a) water rejection of Mc, (b) the permeation flux of Mc with different water volume contents over time (under 0.1 MPa), (c) the permeation flux of Mc under different pressures over time (the water volume content is 1%), (d) photos of the feeding and the permeation solutions. Figure 11 Long-term oil/water separation performance of Mc over 48 h.
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