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nanofiber composite superhydrophilic membrane with enhanced anti-adhesion property towards oil and anionic surfactant: Membrane fabrication and applications
Separation and Purification Technology 235 (2020) 116189
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Carbonaceous microsphere/nanofiber composite superhydrophilic membrane with enhanced anti-adhesion property towards oil and anionic surfactant: Membrane fabrication and applications ⁎
T
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Yuhuan Lva,b, Yajie Dingb,c, Jianqiang Wangb,c,d, , Bing Heb,c, Simin Yangb,c, Kai Pana, , ⁎ Fu Liub,c,d, a Beijing Key Laboratory of Advanced Functional Polymer Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China b Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China c Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China d University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Anti-adhesion Carbonaceous microsphere Nanofibrous membrane Oil/water separation Superhydrophilic
Fouling issue is the key obstacle that limited the practical applications of superwetting oil/water separation membrane. Dispersed oil phase and soluble surfactants in the emulsion are the primary foulants during separation of oil/water mixtures. Therefore, developing membranes with anti-adhesion property towards both dispersed phase and surfactant is of great importance. In this study, a superhydrophilic, underwater superoleophobic composite membrane with excellent antifouling property for separating oil-in-water emulsions stabilized by anionic surfactants was fabricated. Briefly, carboxylated carbonaceous microspheres were synthesized and loaded on nanofibrous membrane through a facile hydrothermal strategy. Acrylic acid was introduced to the hydrothermal process to manipulate the surface charge of carbonaceous microspheres. The fabricated membrane showed excellent anti-adhesion property towards dispersed phase (toluene) due to the improved surface wettability and hydration property. Underwater oil contact angle was up to 160 ± 1° (e.g., M8-2 membrane). Meanwhile, the fabricated membrane exhibited excellent anti-adhesion performance for oil-in-water emulsions stabilized by anionic surfactants (e.g. sodium laurylsulfonate) due to electrostatic repulsion. Therefore, the membrane can maintain the permeability as high as 41,020 ± 805 L m-2 h-1 bar−1 even after ten times of usage. The construction strategy and the antifouling analysis presented in the current study may give more opportunities for the understanding of superwetting oil/water separation process.
1. Introduction Recent years, oily wastewater treatment has attracted attentions from researchers’ due to the increased threat from industry discharges, domestic sewages and frequently marine oil spills [1,2]. Many strategies have been developed for separating of oil/water mixtures, including centrifugation, adsorption, flotation, coalesces, burning and membrane separation [3,4]. Due to the advantages of high selectivity, high energy efficiency, convenient operation, scalability and small footprint, etc. [5], membrane separation seems an attractive strategy for oil/water separation application. However, membrane fouling
significantly limited its practical applications, which will shorten the membrane lifespan and increase the operation cost [1,6]. Therefore, developing fouling resistant membranes towards both dispersed phase and surfactants is of highly importance in oil/water separation. Generally, fouling of the membrane (especially for oil/water separation membrane) was caused by the following three aspects: (1) low anti-adhesion property of the membrane interface towards dispersed phase (oil or water); (2) low interfacial stability of the hierarchical structure under longtime corrosion and (3) destruction of surface free energy due to the deposition of surfactant. Up to date, many strategies have been proposed for enhancing anti-adhesion property of the
⁎ Corresponding authors at: Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China (J. Wang, F. Liu). E-mail addresses: [email protected] (J. Wang), [email protected] (K. Pan), [email protected] (F. Liu).
https://doi.org/10.1016/j.seppur.2019.116189 Received 4 June 2019; Received in revised form 24 August 2019; Accepted 7 October 2019 Available online 08 October 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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The in situ generated negatively charged carboxylate carbonaceous microspheres played important roles for the construction of superwetting and negatively charged surface for fouling mitigation. Besides, carbonaceous microspheres with rigid featured core and hydrogel-like shell significantly improved the roughness and hydration capability of the membrane surface. The adhesion interactions between oil, anionic and cationic surfactants and the membrane surface were systematically investigated.
membranes by manipulation of the surface chemistry (e.g., surface grafting [7–11], surface coating [12–15] and plasma treatment [16,17], etc.) and hierarchical structure (e.g., surface loading [18–21], solvent/ heat induced roughening [22,23], controlled phase separation [24–26] and electrospray [25,27–30], etc.). Although anti-adhesion property of the membrane was increased, the longtime performance was limited. The reason was mainly attributed to the incompatible issue of surface chemistry and surface structure. Specifically, hierarchical structure can endow the membrane with enhanced wettability and anti-adhesion properties. However, the interfacial stability was usually weak due to the creep of polymers (especially in harsh environmental, e.g., acid/ base solutions or solutions containing organic solvents) and low interactions between loaded micro/nanoparticles and the substrate (particles were deposited on surface through adsorption or in situ grown, no covalent bonding existed). Moreover, the deposition of surfactants on membrane surface is the critical factor for the loss of effectiveness of superwetting property. The effect of surfactant deposition on the membrane separation performance was actually unneglectable [31–33]. Therefore, construction of superwetting interface with enhanced anti-adhesion to surfactants, interfacial stability and controlled surface chemistry (e.g., surface energy and surface charge) are of great importance for oil/water separation membrane. Carbonaceous microspheres prepared through low-temperature hydrothermal treatment of sugars (e.g., glucose or sucrose) have been known as a typical core-shell structure with rigid core and soft shell [34,35]. The core is composed of carbonaceous materials with higher degree of carbonization, while the existence of plentiful polar groups (resulting from the low degree of carbonization) endowing the shell layer with hydrogel-like properties [36]. In addition, size of the carbonaceous spheres can be well manipulated in a wide range from tens to thousands of nanometers [37]. Moreover, other functional components can be easily involved in the hydrolysis and polycondensation processes to tailor the surface functional properties during hydrothermal treatment. Therefore, attachment strength between loaded carbonaceous spheres and the substrate can be enhanced [38]. The unique properties of rigid core, hydrogel-like shell, macro/nano- size and active interface endowed the carbonaceous spheres potential application in construction of superwetting interface with controllable roughness, stability and chemistry. In this study, a superhydrophilic, underwater superoleophobic carbonaceous nanofibrous composite membrane with negative charged surface was fabricated through electrospun followed by hydrothermal treatment of sucrose. The surface carboxylation of carbonaceous microspheres was achieved by co-hydrothermal synthesis of acrylic acid.
2. Experimental section 2.1. Materials and chemicals Polyacrylonitrile (PAN, average Mw ~ 150,000) was purchased from Sigma-Aldrich (USA). N, N-dimethylformamide (DMF), toluene and sucrose were all analytical reagents and purchased from Sinopharm Chemical Reagent Co., Ltd (China). Acrylic acid (AR, > 99%), sodium laurylsulfonate (SLS, AR, 98.0%) and cetyltrimethylammonium bromide (CTAB, 99%) was purchased from Aladdin Industrial Corporation (China). All reagents were used without further purification. 2.2. Fabrication of the nanofibrous membranes PAN powder was added into DMF to prepare PAN/DMF solution with a concentration of 6.0 wt%. The obtained solution was heated at 80 °C for 6 h with continuous stirring. PAN nanofibrous membrane was prepared via electrospun according to our previous reported method with minor changes [39]. Specifically, 1.2 mL h−1 of feed rate was used during electrospun, a collection distance of 15.0 cm was used, and the applied voltage was 15 ± 3 kV. The electrospun nanofibers were deposited on a metal rotating drum to form the uniform nanofibrous membrane. The obtained nanofibrous membrane was heated at 100 °C for 2 h before further use. A hydrothermal treatment was used for the loading of carbonaceous spheres on PAN nanofibrous membrane. Typically, 8.0 g of sucrose and different amount of acrylic acid (0–3.0 g) were dissolved in 80 mL of deionized water. The solution was added into 200 mL Teflon-lined autoclave with a PAN membrane disk (5 cm × 5 cm). Then the autoclave was put into an oven and heated at 180 °C for 9 h. After that, the membrane was rinsed with deionized water to remove the loosely attached carbonaceous spheres. The obtained membrane was correspondingly denoted as M8-0, M8-1, M8-2, and M8-3, in accordance with the adding amount of acrylic acid (0, 1.0 g, 2.0 g and 3.0 g respectively). A schematic diagram of the fabrication process is shown in
Fig. 1. Schematic diagram of the fabrication process of nanofibrous membrane bearing carboxylate carbonaceous microspheres for emulsion separation. 2
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hydrothermal treatment of sucrose, large amounts of microspheres with relatively smooth surface appeared on the surface of nanofibers (Fig. 2b). Formation of microspheres was mainly due to the hydrolysis, polycondensation and nucleation growth processes of sucrose during hydrothermal treatment process [40–42]. The adding of acrylic acid has a great effect on morphology of the membrane. After adding acrylic acid in the hydrothermal process, size of carbonaceous spheres was decreased (Fig. 2c–e). It might be mainly due to the stabilization effect of acrylic acid for the primary nucleation, therefore their further growth was inhibited [38]. Besides the size change, surface of the carbonaceous spheres became rougher after adding acrylic acid. As the acrylic acid amount increased, primary formed small spheres assembled into large micrometer-sized spheres (Fig. 2e). In addition, surface of PAN nanofibers was also coated by an additional carbonaceous layer (Fig. 2c–e). The coated carbonaceous layer was closely linked to both the microspheres and the pristine PAN nanofibers, which made the carbonaceous layer as a uniform layer. Therefore, stability of the loaded carbonaceous microspheres was good enough during filtration. Moreover, no leaked microspheres can be found when a filtration of deionized water with a water head of 10 cm. The introduction of carbonaceous microspheres in nanofibrous membrane significantly enhanced the membrane surface roughness. RSa value of the membrane (obtained through LSCM analysis) increased from 0.9 μm to 2.3 μm after incorporation of carbonaceous microspheres (Fig. 3a–b). When acrylic acid was involved in the hydrothermal process, RSa value was further increased to 2.8 μm (Fig. 3e). These results were mainly due to the multidimensional integration of nanofibers and microspheres. When the adding amount of acrylic acid increased, the assembly of small spheres to large microspheres further improved the roughness of the membrane surface. Such a hierarchical structure will contribute the wetting properties of the membrane according to Wenzel theory [43,44].
Fig. 1. 2.3. Oil/water separation experiments Oil-in-water emulsions were used for evaluation of the separation performance of nanofibrous membranes. Typically, anionic surfactant SLS or cationic surfactant CTAB was dissolved in deionized water to get a 0.1 g L-1 aqueous solution. Then toluene was added in the prepared solution under constant stirring for 5 h to obtain the surfactant stabilized emulsions. Separation experiments were carried out using a custom-made filtration device under gravity driven. A membrane disk with a diameter of 4.0 cm was fixed between two glass tubes. Emulsions stabilized by different surfactants were filtrated through the membrane. During separation, an emulsion column height of 10 ± 0.5 cm was used, and membrane permeability was calculated in each 5 min. 2.4. Characterizations Field-emission scanning electron microscopy (FE-SEM, Hitachi S4800, Japan) was used to observe morphologies of the membranes. Roughness of the membranes was tested by laser scanning confocal microscope (LSCM, LSM700, Germany). Chemical composition of the membrane was analyzed with microscopic infrared spectrometer (Micro- FTIR, Cary660 + 620, America). Dynamic light scattering particle size analyzer (ZETA, Zeta-sizer Nano ZS, Britain) was used for analyzing the emulsion size and its distribution. Dynamic contact angle measuring instrument (DCAT21, China) was selected for surface contact angle (CA) and underwater oil contact angle (OCA) measurements. Toluene was used for OCA test. 2 μL of droplet was used for each test. The reported data of contact angles were the average value tested at five different points of each sample. Polarizing thermal stage microscope (BX51, Japan) was applied to observe the difference of feed emulsion solution and the filtrate solution.
3.2. Wetting properties of the membranes
3. Results and discussion
Wetting properties of the membranes were characterized using contact angle analysis. Results in Fig. 4 showed that all nanofibrous membrane exhibited fast wetting of water in air. The time for infiltration of 2 μL of water in the membrane was about 1 S. The intrinsic microstructure of nanofibrous membrane facilitate the enhancement of wettability due to the randomly deposition of nanofibers [45], which enhanced the interface roughness. Therefore, wettability of PAN nanofibrous membrane was significantly improved compared to a dense
3.1. Morphologies of the membranes Fig. 2 presents the morphologies of the nanofibrous membranes. The results indicated that pristine PAN nanofibrous membrane is composed of random deposited nanofibers. Uniform PAN nanofibers are randomly deposited to form a highly porous surface (Fig. 2a). After
Fig. 2. SEM images of PAN (a), M8-0 (b), M8-1 (c), M8-2 (d) and M8-3 (e). 3
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Fig. 3. LSCM morphology and roughness of PAN (a), M8-0 (b), M8-1 (c), M8-2 (d), M8-3 (e).
performance of the membranes. Fig. 5a showed that initial permeability of the membrane for anionic SLS stabilized toluene-in-water emulsion increased from 39387 ± 4076 Lm-2 h-1 bar−1 (M8-0) to 53265 ± 3823 Lm-2 h-1 bar−1 (M8-2 membrane) after carboxylate carbonaceous microsphere incorporation. As discussed above, hydrophilicity of the membrane was enhanced and affinity of the modified membrane towards water was also improved as the existence of plentiful of oxygen containing groups on the shell layer of carbonaceous microspheres. However, permeability of the membranes was quite low when cationic CTAB stabilized toluene-in-water emulsion was used. Permeability of M8-0 membrane was just 3348 ± 190 Lm-2 h-1 bar−1, suggesting that surfactant has a great effect on separation performance. Fig. 5b showed that the two emulsions have distinctly opposite charge properties. Due to the existence of sulfonate groups, anionic SLS stabilized emulsion exhibited negatively charged property with zeta potential of −51.7 mV at pH value of 5.9. Contrarily, zeta potential of CTAB stabilized emulsion was 51.5 mV at pH value of 5.2. The carbonaceous microsphere shell behaved as a hydrated and negatively charged soft layer endowed by carboxylic groups. When negatively charged SLS stabilized emulsions were filtrated through carbonaceous microsphere coated membrane, demulsification can be enhanced due to polarized distribution of SLS caused by electrostatic repulsion [33,54]. During demulsification process, SLS molecules were repelled from membrane surface to the opposite side of the emulsion droplet (Fig. 5c). The rearrangement of surfactants surrounding the oil droplet induced the deformation and demulsification. The resulted oil droplets were easily resisted to foul the membrane surface due to enhanced anti-
PAN membrane (e.g., water contact angle of PAN ultrafiltration membrane was about 40°–60°) [46,47]. A PAN ultrafiltration membrane was made for comparison with PAN nanofibrous membrane using the same materials through non-solvent induced phase separation (detailed preparation process can be found in supporting information). The fabricated PAN ultrafiltration membrane showed a water contact angle of 45° (see supporting information Fig. S1). In addition, enhanced capillarity effect of the nanofibrous membrane can facilitate the entering of water droplet into the membrane and therefore spontaneous wetting can be realized [48,49]. Although pristine PAN nanofibrous membrane showed a high underwater oil contact angle (150 ± 1°, Fig. 4d), its anti-adhesion property towards toluene was not satisfied [29,50]. This was mainly caused by the low anti-adhesion ability of PAN membrane and unstable hydration layer [50,51]. However, underwater oil contact angle of the membranes increased to 160 ± 1° after the introduction of carbonaceous microspheres (Fig. 4f), which might mitigate the membrane fouling. The enhanced wettability of the membrane was attributed to the enhanced surface roughness (Fig. 3) and large amount of oxygen containing groups at the surface of carbonaceous microspheres [52]. FTIR results in Fig. 4g confirmed the presence of oxygen containing groups, as new peaks appeared at about 1700 cm−1 and 3400 cm−1 in M8-0 and M8-2 which were attributed to hydroxyl groups and carbonyl esters respectively [53]. 3.3. Separation performance of the nanofibrous membranes Oil-in-water emulsions were used to evaluate the separation
Fig. 4. Water contact angle and underwater oil contact angle of PAN (a, d), M8-0 (b, e) and M8-2 (c, f); FTIR spectrum of the membranes (g). 4
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Fig. 5. Separation performance of the membranes (a); Zeta potential of two emulsions stabilized by SLS and CTAB (b); Proposed mechanism for separation of different emulsions (c).
physical extrusion and cutting effect of the nanofibers. Recycle performance is quite important for an oil/water separation membrane, as the oil and surfactant fouling issue is more severe compared to other separation processes (such as microfiltration or ultrafiltration). Fig. 7a showed that carboxylate carbonaceous microsphere loaded membrane exhibited excellent recycle performance for separation of SLS stabilized emulsion. Permeability can be easily recovered through a facile deionized water washing. Adhesion of demulsified oil droplet on membrane surface was inhibited, as oil droplet cannot be attached even under external force (Fig. 7b, corresponding video can be available in supporting information SV1). As comparison, oil droplet can be easily attached to the pristine PAN membrane (supporting information SV2). The existence of plentiful of oxygen containing groups (e.g., carboxylic groups and hydroxyl groups) endowed the carbonaceous microsphere layer with hydrogel-like properties. Therefore, hydration ability of the membrane can be significantly improved [29,50], which made the membrane exhibit excellent recycle performance. More importantly, the negatively charged membrane surface repelled demulsified oil and surfactant through electrostatic repulsion (as shown in Fig. 5c), which will benefit for the maintenance of surface properties. However, permeability of the carbonaceous microsphere loaded membrane rapidly decreased to near zero after 20 min of filtration when CTAB stabilized emulsion was used. Moreover, permeability
adhesion property of carbonaceous layer (as discussed above). However, membrane fouling was severe when positively charged emulsion stabilized by cationic CTAB was used due to the following two reasons: (1) enhanced surfactant adsorption through electrostatic attraction between negatively charged membrane surface and positively charged CTAB; and (2) enhanced oil adhesion attributed to decreased surface energy by surfactant adsorption (Fig. 5c). Therefore, it is significant to understand the fouling mechanism of surfactants on membrane surface and design the surface charge property according to the type of the surfactant. Separation efficiency of the membrane for both two emulsions was higher than 99.9% (Fig. 5a). Results in Fig. 6 showed that the feed solution was milky and the emulsion size was around 6000 nm. However, the solution turned to quite clear after filtration through the membrane. Light scattering results showed that there were still little number of particles with size of 344 nm existed in the filtrate (Fig. 6b3). These might be the dissolved surfactant micelles as similar results can be obtained if equivalent surfactant dissolved in deionized water (Fig. S2). Detailed study about the surfactant migration through the membrane during emulsion separation will be carried out in our following work. The high separation efficiency suggested that nanofibrous membrane has genetous advantage for emulsion separation, which might be mainly due to enhanced wettability, tailored surface charge,
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Fig. 6. Property of SLS stabilized emulsion before (a) and after (b) separation; (1: photo of the feed and filtrate solution, 2: polarizing photographs; 3: dynamic light scattering results).
stabilized emulsion was successfully fabricated through electrospun and subsequent carbonaceous microsphere loading. The loaded carbonaceous microspheres significantly enhanced membrane roughness and wettability. Introduction of acrylic acid into the hydrothermal process of sucrose can inspire a hydrophilic and negative charged carboxylate surface with improved anti-adhesion property to emulsions stabilized by anionic SDS, and the underwater oil contact angle increased from 150 ± 1° (M8-0 membrane) to 160 ± 1° (M8-2 membrane). Permeability of the membrane was also increased from 39387 ± 4076 L m-2 h-1 bar−1 (M8-0 membrane) to 53265 ± 3823 L m-2 h-1 bar−1 (M8-2 membrane). M8-2 membrane showed excellent recycle performance for separation of negatively charged SLS stabilized emulsion. However, permeability of the membrane was rapidly declined when positively charged CTAB emulsion was used. In general, carboxylated carbonaceous microspheres incorporated PAN nanofibrous membrane showed excellent anti-adhesion property towards oil and enhanced anti-adsorption property towards anionic surfactants. The current study might advance the practical
cannot be recovered through water washing. The main reason might be due to serious adsorption towards CTAB through electrostatic attraction (as shown in Fig. 5c). The adsorbed surfactant significantly decreased surface energy of the membrane [55,56], and further improved oil adhesion on membrane surface. Therefore, water wetting on the membrane became difficult after filtration of CTAB stabilized emulsions. Fig. 7c showed a distinct difference of wetting rate of the unfiltrated and filtrated (CTAB stabilized emulsion was used) membrane surface when a membrane disk was added into the water (corresponding video can be available in supporting information SV3). However, wetting property of the membrane was nearly not affected after filtration of SLS stabilized emulsion (Fig. 7d, supporting information SV4). 4. Conclusions A superhydrophilic, underwater superoleophobic nanofibrous membrane with excellent antifouling performance for anionic SLS
Fig. 7. Recycle performance of the M8-2 membrane for emulsion separation (a); Underwater oil adhesion properties of PAN and M8-2 membrane under external force (b); Water wetting process of the membrane after filtration of emulsions (c–d). 6
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applications of superwetting membranes for separating of oil-in-water emulsions stabilized by anionic surfactants.
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Acknowledgments This work is financially supported by National Key R&D Program of China (2017YFB0309600), National Nature Science Foundation of China (51703233, 5161101025), Youth Innovation Promotion Association of Chinese Academy of Science (2014258) and Nature Science Foundation of Ningbo (2018A610026). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116189. References [1] Y.Z. Zhu, D. Wang, L. Jiang, J. Jin, Recent progress in developing advanced membranes for emulsified oil/water separation, Npg Asia Mater. 6 (2014) e101. [2] M. Padaki, R. Surya Murali, M.S. Abdullah, N. Misdan, A. Moslehyani, M.A. Kassim, N. Hilal, A.F. Ismail, Membrane technology enhancement in oil–water separation. A review, Desalination 357 (2015) 197–207. [3] Z.L. Chu, Y.J. Feng, S. Seeger, Oil/water separation with selective superantiwetting/superwetting surface materials, Angew. Chem., Int. Ed. 54 (2015) 2328–2338. [4] B. Wang, W. Liang, Z. Guo, W. Liu, Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: A new strategy beyond nature, Chem. Soc. Rev. 44 (2015) 336–361. [5] L. Hou, N. Wang, J. Wu, Z. Cui, L. Jiang, Y. Zhao, Bioinspired superwettability electrospun micro/nanofibers and their applications, Adv. Funct. Mater. 28 (2018) 1801114. [6] H.J. Tanudjaja, J.W. Chew, In-situ characterization of cake layer fouling during crossflow microfiltration of oil-in-water emulsion, Sep. Purif. Technol. 218 (2019) 51–58. [7] P.-F. Ren, Y. Fang, L.-S. Wan, X.-Y. Ye, Z.-K. Xu, Surface modification of polypropylene microfiltration membrane by grafting poly(sulfobetaine methacrylate) and poly(ethylene glycol): Oxidative stability and antifouling capability, J. Membr. Sci. 492 (2015) 249–256. [8] X. Zhang, J. Tian, S. Gao, Z. Zhang, F. Cui, C.Y. Tang, In situ surface modification of thin film composite forward osmosis membranes with sulfonated poly(arylene ether sulfone) for anti-fouling in emulsified oil/water separation, J. Membr. Sci. 527 (2017) 26–34. [9] Y. Zhu, J. Wang, F. Zhang, S. Gao, A. Wang, W. Fang, J. Jin, Zwitterionic nanohydrogel grafted pvdf membranes with comprehensive antifouling property and superior cycle stability for oil-in-water emulsion separation, Adv. Funct. Mater. 28 (2018) 1804121. [10] Z. Zhu, Z. Li, L. Zhong, R. Zhang, F. Cui, W. Wang, Dual-biomimetic superwetting silica nanofibrous membrane for oily water purification, J. Membr. Sci. 572 (2019) 73–81. [11] Y. Ding, J. Wu, J. Wang, H. Lin, J. Wang, G. Liu, X. Pei, F. Liu, C.Y. Tang, Superhydrophilic and mechanical robust pvdf nanofibrous membrane through facile interfacial span 80 welding for excellent oil/water separation, Appl. Surf. Sci. 485 (2019) 179–187. [12] S. Kasemset, A. Lee, D.J. Miller, B.D. Freeman, M.M. Sharma, Effect of polydopamine deposition conditions on fouling resistance, physical properties, and permeation properties of reverse osmosis membranes in oil/water separation, J. Membr. Sci. 425 (2013) 208–216. [13] R. Zhou, P.-F. Ren, H.-C. Yang, Z.-K. Xu, Fabrication of antifouling membrane surface by poly(sulfobetaine methacrylate)/polydopamine co-deposition, J. Membr. Sci. 466 (2014) 18–25. [14] J. Wang, H. Guo, X. Shi, Z. Yao, W. Qing, F. Liu, C.Y. Tang, Fast polydopamine coating on reverse osmosis membrane: Process investigation and membrane performance study, J. Colloid Interface Sci. 535 (2019) 239–244. [15] N.W. Gao, Z.K. Xu, Ceramic membranes with mussel-inspired and nanostructured coatings for water-in-oil emulsions separation, Sep. Purif. Technol. 212 (2019) 737–746. [16] A. Venault, T.-C. Wei, H.-L. Shih, C.-C. Yeh, A. Chinnathambi, S.A. Alharbi, S. Carretier, P. Aimar, J.-Y. Lai, Y. Chang, Antifouling pseudo-zwitterionic poly (vinylidene fluoride) membranes with efficient mixed-charge surface grafting via glow dielectric barrier discharge plasma-induced copolymerization, J. Membr. Sci. 516 (2016) 13–25. [17] Z.F. Gao, G.M. Shi, Y. Cui, T.-S. Chung, Organic solvent nanofiltration (osn) membranes made from plasma grafting of polyethylene glycol on cross-linked polyimide ultrafiltration substrates, J. Membr. Sci. 565 (2018) 169–178. [18] Y. Liao, M. Tian, R. Wang, A high-performance and robust membrane with switchable super-wettability for oil/water separation under ultralow pressure, J. Membr. Sci. 543 (2017) 123–132. [19] J. Wang, Z. Wu, T. Li, J. Ye, L. Shen, Z. She, F. Liu, Catalytic pvdf membrane for continuous reduction and separation of p -nitrophenol and methylene blue in
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wastewater, ACS Appl. Mater. Interfaces 10 (2018) 16183–16192. [49] F. Li, Z. Wang, S. Huang, Y. Pan, X. Zhao, Flexible, durable, and unconditioned superoleophobic/superhydrophilic surfaces for controllable transport and oil-water separation, Adv. Funct. Mater. 28 (2018) 1706867. [50] G. Jiang, S. Zhang, Y. Zhu, S. Gao, H. Jin, L. Luo, F. Zhang, J. Jin, Hydrogel-embedded tight ultrafiltration membrane with superior anti-dye-fouling property for low-pressure driven molecule separation, J. Mater. Chem. A 6 (2018) 2927–2934. [51] S. Gao, Y. Zhu, J. Wang, F. Zhang, J. Li, J. Jin, Layer-by-layer construction of cu2+/alginate multilayer modified ultrafiltration membrane with bioinspired superwetting property for high-efficient crude-oil-in-water emulsion separation, Adv. Funct. Mater. 28 (2018) 1801944. [52] B. He, Y. Ding, J. Wang, Z. Yao, W. Qing, Y. Zhang, F. Liu, C.Y. Tang, Sustaining fouling resistant membranes: Membrane fabrication, characterization and mechanism understanding of demulsification and fouling-resistance, J. Membr. Sci.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Carbonaceous microsphere/nanofiber composite superhydrophilic membrane with enhanced anti-adhesion property towards oil and anionic surfactant: Membrane fabrication and applications ⁎
T
⁎
Yuhuan Lva,b, Yajie Dingb,c, Jianqiang Wangb,c,d, , Bing Heb,c, Simin Yangb,c, Kai Pana, , ⁎ Fu Liub,c,d, a Beijing Key Laboratory of Advanced Functional Polymer Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China b Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China c Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China d University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Anti-adhesion Carbonaceous microsphere Nanofibrous membrane Oil/water separation Superhydrophilic
Fouling issue is the key obstacle that limited the practical applications of superwetting oil/water separation membrane. Dispersed oil phase and soluble surfactants in the emulsion are the primary foulants during separation of oil/water mixtures. Therefore, developing membranes with anti-adhesion property towards both dispersed phase and surfactant is of great importance. In this study, a superhydrophilic, underwater superoleophobic composite membrane with excellent antifouling property for separating oil-in-water emulsions stabilized by anionic surfactants was fabricated. Briefly, carboxylated carbonaceous microspheres were synthesized and loaded on nanofibrous membrane through a facile hydrothermal strategy. Acrylic acid was introduced to the hydrothermal process to manipulate the surface charge of carbonaceous microspheres. The fabricated membrane showed excellent anti-adhesion property towards dispersed phase (toluene) due to the improved surface wettability and hydration property. Underwater oil contact angle was up to 160 ± 1° (e.g., M8-2 membrane). Meanwhile, the fabricated membrane exhibited excellent anti-adhesion performance for oil-in-water emulsions stabilized by anionic surfactants (e.g. sodium laurylsulfonate) due to electrostatic repulsion. Therefore, the membrane can maintain the permeability as high as 41,020 ± 805 L m-2 h-1 bar−1 even after ten times of usage. The construction strategy and the antifouling analysis presented in the current study may give more opportunities for the understanding of superwetting oil/water separation process.
1. Introduction Recent years, oily wastewater treatment has attracted attentions from researchers’ due to the increased threat from industry discharges, domestic sewages and frequently marine oil spills [1,2]. Many strategies have been developed for separating of oil/water mixtures, including centrifugation, adsorption, flotation, coalesces, burning and membrane separation [3,4]. Due to the advantages of high selectivity, high energy efficiency, convenient operation, scalability and small footprint, etc. [5], membrane separation seems an attractive strategy for oil/water separation application. However, membrane fouling
significantly limited its practical applications, which will shorten the membrane lifespan and increase the operation cost [1,6]. Therefore, developing fouling resistant membranes towards both dispersed phase and surfactants is of highly importance in oil/water separation. Generally, fouling of the membrane (especially for oil/water separation membrane) was caused by the following three aspects: (1) low anti-adhesion property of the membrane interface towards dispersed phase (oil or water); (2) low interfacial stability of the hierarchical structure under longtime corrosion and (3) destruction of surface free energy due to the deposition of surfactant. Up to date, many strategies have been proposed for enhancing anti-adhesion property of the
⁎ Corresponding authors at: Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China (J. Wang, F. Liu). E-mail addresses: [email protected] (J. Wang), [email protected] (K. Pan), [email protected] (F. Liu).
https://doi.org/10.1016/j.seppur.2019.116189 Received 4 June 2019; Received in revised form 24 August 2019; Accepted 7 October 2019 Available online 08 October 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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The in situ generated negatively charged carboxylate carbonaceous microspheres played important roles for the construction of superwetting and negatively charged surface for fouling mitigation. Besides, carbonaceous microspheres with rigid featured core and hydrogel-like shell significantly improved the roughness and hydration capability of the membrane surface. The adhesion interactions between oil, anionic and cationic surfactants and the membrane surface were systematically investigated.
membranes by manipulation of the surface chemistry (e.g., surface grafting [7–11], surface coating [12–15] and plasma treatment [16,17], etc.) and hierarchical structure (e.g., surface loading [18–21], solvent/ heat induced roughening [22,23], controlled phase separation [24–26] and electrospray [25,27–30], etc.). Although anti-adhesion property of the membrane was increased, the longtime performance was limited. The reason was mainly attributed to the incompatible issue of surface chemistry and surface structure. Specifically, hierarchical structure can endow the membrane with enhanced wettability and anti-adhesion properties. However, the interfacial stability was usually weak due to the creep of polymers (especially in harsh environmental, e.g., acid/ base solutions or solutions containing organic solvents) and low interactions between loaded micro/nanoparticles and the substrate (particles were deposited on surface through adsorption or in situ grown, no covalent bonding existed). Moreover, the deposition of surfactants on membrane surface is the critical factor for the loss of effectiveness of superwetting property. The effect of surfactant deposition on the membrane separation performance was actually unneglectable [31–33]. Therefore, construction of superwetting interface with enhanced anti-adhesion to surfactants, interfacial stability and controlled surface chemistry (e.g., surface energy and surface charge) are of great importance for oil/water separation membrane. Carbonaceous microspheres prepared through low-temperature hydrothermal treatment of sugars (e.g., glucose or sucrose) have been known as a typical core-shell structure with rigid core and soft shell [34,35]. The core is composed of carbonaceous materials with higher degree of carbonization, while the existence of plentiful polar groups (resulting from the low degree of carbonization) endowing the shell layer with hydrogel-like properties [36]. In addition, size of the carbonaceous spheres can be well manipulated in a wide range from tens to thousands of nanometers [37]. Moreover, other functional components can be easily involved in the hydrolysis and polycondensation processes to tailor the surface functional properties during hydrothermal treatment. Therefore, attachment strength between loaded carbonaceous spheres and the substrate can be enhanced [38]. The unique properties of rigid core, hydrogel-like shell, macro/nano- size and active interface endowed the carbonaceous spheres potential application in construction of superwetting interface with controllable roughness, stability and chemistry. In this study, a superhydrophilic, underwater superoleophobic carbonaceous nanofibrous composite membrane with negative charged surface was fabricated through electrospun followed by hydrothermal treatment of sucrose. The surface carboxylation of carbonaceous microspheres was achieved by co-hydrothermal synthesis of acrylic acid.
2. Experimental section 2.1. Materials and chemicals Polyacrylonitrile (PAN, average Mw ~ 150,000) was purchased from Sigma-Aldrich (USA). N, N-dimethylformamide (DMF), toluene and sucrose were all analytical reagents and purchased from Sinopharm Chemical Reagent Co., Ltd (China). Acrylic acid (AR, > 99%), sodium laurylsulfonate (SLS, AR, 98.0%) and cetyltrimethylammonium bromide (CTAB, 99%) was purchased from Aladdin Industrial Corporation (China). All reagents were used without further purification. 2.2. Fabrication of the nanofibrous membranes PAN powder was added into DMF to prepare PAN/DMF solution with a concentration of 6.0 wt%. The obtained solution was heated at 80 °C for 6 h with continuous stirring. PAN nanofibrous membrane was prepared via electrospun according to our previous reported method with minor changes [39]. Specifically, 1.2 mL h−1 of feed rate was used during electrospun, a collection distance of 15.0 cm was used, and the applied voltage was 15 ± 3 kV. The electrospun nanofibers were deposited on a metal rotating drum to form the uniform nanofibrous membrane. The obtained nanofibrous membrane was heated at 100 °C for 2 h before further use. A hydrothermal treatment was used for the loading of carbonaceous spheres on PAN nanofibrous membrane. Typically, 8.0 g of sucrose and different amount of acrylic acid (0–3.0 g) were dissolved in 80 mL of deionized water. The solution was added into 200 mL Teflon-lined autoclave with a PAN membrane disk (5 cm × 5 cm). Then the autoclave was put into an oven and heated at 180 °C for 9 h. After that, the membrane was rinsed with deionized water to remove the loosely attached carbonaceous spheres. The obtained membrane was correspondingly denoted as M8-0, M8-1, M8-2, and M8-3, in accordance with the adding amount of acrylic acid (0, 1.0 g, 2.0 g and 3.0 g respectively). A schematic diagram of the fabrication process is shown in
Fig. 1. Schematic diagram of the fabrication process of nanofibrous membrane bearing carboxylate carbonaceous microspheres for emulsion separation. 2
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hydrothermal treatment of sucrose, large amounts of microspheres with relatively smooth surface appeared on the surface of nanofibers (Fig. 2b). Formation of microspheres was mainly due to the hydrolysis, polycondensation and nucleation growth processes of sucrose during hydrothermal treatment process [40–42]. The adding of acrylic acid has a great effect on morphology of the membrane. After adding acrylic acid in the hydrothermal process, size of carbonaceous spheres was decreased (Fig. 2c–e). It might be mainly due to the stabilization effect of acrylic acid for the primary nucleation, therefore their further growth was inhibited [38]. Besides the size change, surface of the carbonaceous spheres became rougher after adding acrylic acid. As the acrylic acid amount increased, primary formed small spheres assembled into large micrometer-sized spheres (Fig. 2e). In addition, surface of PAN nanofibers was also coated by an additional carbonaceous layer (Fig. 2c–e). The coated carbonaceous layer was closely linked to both the microspheres and the pristine PAN nanofibers, which made the carbonaceous layer as a uniform layer. Therefore, stability of the loaded carbonaceous microspheres was good enough during filtration. Moreover, no leaked microspheres can be found when a filtration of deionized water with a water head of 10 cm. The introduction of carbonaceous microspheres in nanofibrous membrane significantly enhanced the membrane surface roughness. RSa value of the membrane (obtained through LSCM analysis) increased from 0.9 μm to 2.3 μm after incorporation of carbonaceous microspheres (Fig. 3a–b). When acrylic acid was involved in the hydrothermal process, RSa value was further increased to 2.8 μm (Fig. 3e). These results were mainly due to the multidimensional integration of nanofibers and microspheres. When the adding amount of acrylic acid increased, the assembly of small spheres to large microspheres further improved the roughness of the membrane surface. Such a hierarchical structure will contribute the wetting properties of the membrane according to Wenzel theory [43,44].
Fig. 1. 2.3. Oil/water separation experiments Oil-in-water emulsions were used for evaluation of the separation performance of nanofibrous membranes. Typically, anionic surfactant SLS or cationic surfactant CTAB was dissolved in deionized water to get a 0.1 g L-1 aqueous solution. Then toluene was added in the prepared solution under constant stirring for 5 h to obtain the surfactant stabilized emulsions. Separation experiments were carried out using a custom-made filtration device under gravity driven. A membrane disk with a diameter of 4.0 cm was fixed between two glass tubes. Emulsions stabilized by different surfactants were filtrated through the membrane. During separation, an emulsion column height of 10 ± 0.5 cm was used, and membrane permeability was calculated in each 5 min. 2.4. Characterizations Field-emission scanning electron microscopy (FE-SEM, Hitachi S4800, Japan) was used to observe morphologies of the membranes. Roughness of the membranes was tested by laser scanning confocal microscope (LSCM, LSM700, Germany). Chemical composition of the membrane was analyzed with microscopic infrared spectrometer (Micro- FTIR, Cary660 + 620, America). Dynamic light scattering particle size analyzer (ZETA, Zeta-sizer Nano ZS, Britain) was used for analyzing the emulsion size and its distribution. Dynamic contact angle measuring instrument (DCAT21, China) was selected for surface contact angle (CA) and underwater oil contact angle (OCA) measurements. Toluene was used for OCA test. 2 μL of droplet was used for each test. The reported data of contact angles were the average value tested at five different points of each sample. Polarizing thermal stage microscope (BX51, Japan) was applied to observe the difference of feed emulsion solution and the filtrate solution.
3.2. Wetting properties of the membranes
3. Results and discussion
Wetting properties of the membranes were characterized using contact angle analysis. Results in Fig. 4 showed that all nanofibrous membrane exhibited fast wetting of water in air. The time for infiltration of 2 μL of water in the membrane was about 1 S. The intrinsic microstructure of nanofibrous membrane facilitate the enhancement of wettability due to the randomly deposition of nanofibers [45], which enhanced the interface roughness. Therefore, wettability of PAN nanofibrous membrane was significantly improved compared to a dense
3.1. Morphologies of the membranes Fig. 2 presents the morphologies of the nanofibrous membranes. The results indicated that pristine PAN nanofibrous membrane is composed of random deposited nanofibers. Uniform PAN nanofibers are randomly deposited to form a highly porous surface (Fig. 2a). After
Fig. 2. SEM images of PAN (a), M8-0 (b), M8-1 (c), M8-2 (d) and M8-3 (e). 3
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Fig. 3. LSCM morphology and roughness of PAN (a), M8-0 (b), M8-1 (c), M8-2 (d), M8-3 (e).
performance of the membranes. Fig. 5a showed that initial permeability of the membrane for anionic SLS stabilized toluene-in-water emulsion increased from 39387 ± 4076 Lm-2 h-1 bar−1 (M8-0) to 53265 ± 3823 Lm-2 h-1 bar−1 (M8-2 membrane) after carboxylate carbonaceous microsphere incorporation. As discussed above, hydrophilicity of the membrane was enhanced and affinity of the modified membrane towards water was also improved as the existence of plentiful of oxygen containing groups on the shell layer of carbonaceous microspheres. However, permeability of the membranes was quite low when cationic CTAB stabilized toluene-in-water emulsion was used. Permeability of M8-0 membrane was just 3348 ± 190 Lm-2 h-1 bar−1, suggesting that surfactant has a great effect on separation performance. Fig. 5b showed that the two emulsions have distinctly opposite charge properties. Due to the existence of sulfonate groups, anionic SLS stabilized emulsion exhibited negatively charged property with zeta potential of −51.7 mV at pH value of 5.9. Contrarily, zeta potential of CTAB stabilized emulsion was 51.5 mV at pH value of 5.2. The carbonaceous microsphere shell behaved as a hydrated and negatively charged soft layer endowed by carboxylic groups. When negatively charged SLS stabilized emulsions were filtrated through carbonaceous microsphere coated membrane, demulsification can be enhanced due to polarized distribution of SLS caused by electrostatic repulsion [33,54]. During demulsification process, SLS molecules were repelled from membrane surface to the opposite side of the emulsion droplet (Fig. 5c). The rearrangement of surfactants surrounding the oil droplet induced the deformation and demulsification. The resulted oil droplets were easily resisted to foul the membrane surface due to enhanced anti-
PAN membrane (e.g., water contact angle of PAN ultrafiltration membrane was about 40°–60°) [46,47]. A PAN ultrafiltration membrane was made for comparison with PAN nanofibrous membrane using the same materials through non-solvent induced phase separation (detailed preparation process can be found in supporting information). The fabricated PAN ultrafiltration membrane showed a water contact angle of 45° (see supporting information Fig. S1). In addition, enhanced capillarity effect of the nanofibrous membrane can facilitate the entering of water droplet into the membrane and therefore spontaneous wetting can be realized [48,49]. Although pristine PAN nanofibrous membrane showed a high underwater oil contact angle (150 ± 1°, Fig. 4d), its anti-adhesion property towards toluene was not satisfied [29,50]. This was mainly caused by the low anti-adhesion ability of PAN membrane and unstable hydration layer [50,51]. However, underwater oil contact angle of the membranes increased to 160 ± 1° after the introduction of carbonaceous microspheres (Fig. 4f), which might mitigate the membrane fouling. The enhanced wettability of the membrane was attributed to the enhanced surface roughness (Fig. 3) and large amount of oxygen containing groups at the surface of carbonaceous microspheres [52]. FTIR results in Fig. 4g confirmed the presence of oxygen containing groups, as new peaks appeared at about 1700 cm−1 and 3400 cm−1 in M8-0 and M8-2 which were attributed to hydroxyl groups and carbonyl esters respectively [53]. 3.3. Separation performance of the nanofibrous membranes Oil-in-water emulsions were used to evaluate the separation
Fig. 4. Water contact angle and underwater oil contact angle of PAN (a, d), M8-0 (b, e) and M8-2 (c, f); FTIR spectrum of the membranes (g). 4
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Fig. 5. Separation performance of the membranes (a); Zeta potential of two emulsions stabilized by SLS and CTAB (b); Proposed mechanism for separation of different emulsions (c).
physical extrusion and cutting effect of the nanofibers. Recycle performance is quite important for an oil/water separation membrane, as the oil and surfactant fouling issue is more severe compared to other separation processes (such as microfiltration or ultrafiltration). Fig. 7a showed that carboxylate carbonaceous microsphere loaded membrane exhibited excellent recycle performance for separation of SLS stabilized emulsion. Permeability can be easily recovered through a facile deionized water washing. Adhesion of demulsified oil droplet on membrane surface was inhibited, as oil droplet cannot be attached even under external force (Fig. 7b, corresponding video can be available in supporting information SV1). As comparison, oil droplet can be easily attached to the pristine PAN membrane (supporting information SV2). The existence of plentiful of oxygen containing groups (e.g., carboxylic groups and hydroxyl groups) endowed the carbonaceous microsphere layer with hydrogel-like properties. Therefore, hydration ability of the membrane can be significantly improved [29,50], which made the membrane exhibit excellent recycle performance. More importantly, the negatively charged membrane surface repelled demulsified oil and surfactant through electrostatic repulsion (as shown in Fig. 5c), which will benefit for the maintenance of surface properties. However, permeability of the carbonaceous microsphere loaded membrane rapidly decreased to near zero after 20 min of filtration when CTAB stabilized emulsion was used. Moreover, permeability
adhesion property of carbonaceous layer (as discussed above). However, membrane fouling was severe when positively charged emulsion stabilized by cationic CTAB was used due to the following two reasons: (1) enhanced surfactant adsorption through electrostatic attraction between negatively charged membrane surface and positively charged CTAB; and (2) enhanced oil adhesion attributed to decreased surface energy by surfactant adsorption (Fig. 5c). Therefore, it is significant to understand the fouling mechanism of surfactants on membrane surface and design the surface charge property according to the type of the surfactant. Separation efficiency of the membrane for both two emulsions was higher than 99.9% (Fig. 5a). Results in Fig. 6 showed that the feed solution was milky and the emulsion size was around 6000 nm. However, the solution turned to quite clear after filtration through the membrane. Light scattering results showed that there were still little number of particles with size of 344 nm existed in the filtrate (Fig. 6b3). These might be the dissolved surfactant micelles as similar results can be obtained if equivalent surfactant dissolved in deionized water (Fig. S2). Detailed study about the surfactant migration through the membrane during emulsion separation will be carried out in our following work. The high separation efficiency suggested that nanofibrous membrane has genetous advantage for emulsion separation, which might be mainly due to enhanced wettability, tailored surface charge,
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Fig. 6. Property of SLS stabilized emulsion before (a) and after (b) separation; (1: photo of the feed and filtrate solution, 2: polarizing photographs; 3: dynamic light scattering results).
stabilized emulsion was successfully fabricated through electrospun and subsequent carbonaceous microsphere loading. The loaded carbonaceous microspheres significantly enhanced membrane roughness and wettability. Introduction of acrylic acid into the hydrothermal process of sucrose can inspire a hydrophilic and negative charged carboxylate surface with improved anti-adhesion property to emulsions stabilized by anionic SDS, and the underwater oil contact angle increased from 150 ± 1° (M8-0 membrane) to 160 ± 1° (M8-2 membrane). Permeability of the membrane was also increased from 39387 ± 4076 L m-2 h-1 bar−1 (M8-0 membrane) to 53265 ± 3823 L m-2 h-1 bar−1 (M8-2 membrane). M8-2 membrane showed excellent recycle performance for separation of negatively charged SLS stabilized emulsion. However, permeability of the membrane was rapidly declined when positively charged CTAB emulsion was used. In general, carboxylated carbonaceous microspheres incorporated PAN nanofibrous membrane showed excellent anti-adhesion property towards oil and enhanced anti-adsorption property towards anionic surfactants. The current study might advance the practical
cannot be recovered through water washing. The main reason might be due to serious adsorption towards CTAB through electrostatic attraction (as shown in Fig. 5c). The adsorbed surfactant significantly decreased surface energy of the membrane [55,56], and further improved oil adhesion on membrane surface. Therefore, water wetting on the membrane became difficult after filtration of CTAB stabilized emulsions. Fig. 7c showed a distinct difference of wetting rate of the unfiltrated and filtrated (CTAB stabilized emulsion was used) membrane surface when a membrane disk was added into the water (corresponding video can be available in supporting information SV3). However, wetting property of the membrane was nearly not affected after filtration of SLS stabilized emulsion (Fig. 7d, supporting information SV4). 4. Conclusions A superhydrophilic, underwater superoleophobic nanofibrous membrane with excellent antifouling performance for anionic SLS
Fig. 7. Recycle performance of the M8-2 membrane for emulsion separation (a); Underwater oil adhesion properties of PAN and M8-2 membrane under external force (b); Water wetting process of the membrane after filtration of emulsions (c–d). 6
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applications of superwetting membranes for separating of oil-in-water emulsions stabilized by anionic surfactants.
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