Biomimetic Multilayer Nanofibrous Membranes with Elaborated

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Surfaces, Interfaces, and Applications

Biomimetic multilayer nanofibrous membranes with elaborated superwettability for effective purification of emulsified oily wastewater Jianlong Ge, Qing Jin, Dingding Zong, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01952 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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ACS Applied Materials & Interfaces

Biomimetic multilayer nanofibrous membranes with elaborated superwettability for effective purification of emulsified oily wastewater Jianlong Ge†, Qing Jin‡, Dingding Zong‡, Jianyong Yu*,§, and Bin Ding*,†,§ †

Key Laboratory of Textile Science & Technology, Ministry of Education, College of

Textiles, Donghua University, Shanghai 201620, China ‡

State Key Laboratory for Modiﬁcation of Chemical Fibers and Polymer Materials, College

of Materials Science and Engineering, Donghua University, Shanghai, 201620, China §

Innovation Center for Textile Science and Technology, Donghua University, Shanghai

20051, China

*Correspondence to: [email protected], [email protected]

KEYWORDS:

multilayer

nanofibrous

membranes,

superhydrophilic,

superoleophobic, anti-oil-fouling, oil-in-water emulsions separation

1

ACS Paragon Plus Environment

underwater

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Creating a porous membrane to effectively separate the emulsified oil-in-water emulsions with energy-saving property is highly desired but remains a challenge. Herein, a multilayer nanofibrous membrane was developed with the inspiration of the natural architectures of earth for gravity driving water purification. As a result, the obtained biomimetic multilayer nanofibrous membranes exhibited three individual layers with designed functions, those were the inorganic nanofibrous layer to block the seriously intrusion of oil to prevent the destructive fouling of the polymeric matrix; the submicron porous layer with designed honeycomb-like cavities to catch the smaller oil droplets and ensures a satisfactory water permeability; the high porous fibrous substrate with larger pore size provided a template support and allows water to pass through quickly. Consequently, with the cooperation of three functional layers, the resultant composite membrane possessed superior anti-oil-fouling property and robust oil-in-water emulsions separation performance with good separation efficiency and competive permeation flux solely under the drive of gravity. The permeation flux of the membrane for emulsion was up to 5163 L m-2 h-1 with separation efficiency of 99.5%. We anticipate that our strategy could provide a facile route for developing a new generation of specific membranes for oily wastewater remediation. 1. INTRODUCTION With the rapidly development of industry and society, the critical issue of wastewater purification has become more and more urgent.1 Among the numerous water pollutants, oils or other insoluble organic solvents occupy a great part due to the frequent oil spill accidents and a large amount of discharged oily sewage, resulting in seriously damages to the ecological environment and human health.2 Thus, an effective treatment of the oily wastewater, especially the oil-in-water emulsions is highly desired. However, it remains a worldwide challenge for engineers due to the complex phase states, small size of the oil droplets (< 10 µm), and relative high viscosity.3-5 In recent years, the membrane separation technologies 2


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based on various membranes, e.g. microfiltration (MF) membranes or ultrafiltration (UF) membranes, have been progressively developed for the treatment of various wastewater owing to their advantages of simple operation process and high separation efficiency, which have been considered as promising candidates for the oily wastewater remediation.6 However, several critical limitations still remain because of the poor anti-oil-fouling property and the improper pore structures of the traditional polymeric membranes, causing low permeation fluxes (< 300 L m-2 h-1) even under high operating pressures (> 0.1 bar).7,8 Thus, the development of a new generation of specific oil-in-water emulsions separation membranes with good separation efficiency and high permeation flux under lower driven pressures are of significant importance to the oily wastewater remediation.4 Recently, a pioneer idea of constructing the superhydrophilic and underwater superoleophobic surface on porous membranes (e.g. metal mesh, fabrics) to endow them intriguing oil/water separation performance has attracted significant research attentions.9-11 With the excellent selective wettability and good anti-oil-fouling property, some of the obtained superwettable membranes exhibited extremely high permeation fluxes.9,11,12 However, the separation of emulsified oil water mixtures, especially separating the surfactant stabilized emulsions with submicron oil droplets is relatively difficult for those membranes due to their large pore size. Consequently, in order to obtain a good separation efficiency, the pore size of the membrane should be quite small to effectively block the intrusion of small oil droplets, which will result in a rapid decline of the permeation flux according to the Hagen-Poiseuille equation.13 Thus, developing a facile method to balance the permeation flux and separation efficiency of the membrane for oil-in-water emulsions is favorable. Inspired by the natural multilayer architecture of earth for water purification,14 e.g. the vegetation layer, the gravel layer, the sand layer, and soil layer for the step purification of surface water, much more attentions have been paid on the creation of multilayer or dual layer structured membranes with a combination of several unique advantages of individual layers to 3



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solve the abovementioned problems.14-17 Consequently, a variety of composite membranes derived from polymers or inorganics have been developed for the separation of oil-in-water emulsions, such as the porous polymeric membranes prepared by the phase separation method7,18 and the carbon nanotube (CNT) based films deposited on the traditional phase separation membranes,13,19-22 etc. Those new developed membranes exhibited outstanding anti-oil-fouling performance and high separation efficiency owing to their unique surface physical/chemical structures (e.g. the micro/nano-structured surface morphology and designed hydrophilic groups) and small pore size. However, without the relative expensive materials and complex processing, these membranes will not possess such elaborated surface and submicron pores. Moreover, the permeation flux leaves considerable room for improvement if the porosity of the membrane can be further increased. Nowadays, nanofibrous membranes have attracted significant research attentions in the area of membrane separation for their advantages of high porosity, small pore size, and easy to

be

functionalized.4,23,24

Among

various

synthesis

technologies

of

nanofibers,

electrospinning is considered as one of the most promising approaches for the large-scale fabrication of nanofibrous materials.25-28 As a result, several electrospun polymeric or inorganic membranes have been invented for the purpose of oil/water separation. With the merits of high porosity and tunable surface wettability, the electrospun oil/water separation membranes exhibited promising separation performance, especially the ultrahigh permeation flux for the immiscible oil/water mixtures or emulsions with large-sized oil droplets.29-37 However, most of these electrospun membranes still suffer from the low separation efficiency for emulsified oil-in-water emulsions with submicron oil droplets due to their large pore size of several micrometers. To solve this problem, some basic strategies, such as casting dense polymeric films on the surface of common electrospun membranes, were once taken to further enhance the separation efficiency of the membrane, but resulting in a rapidly decline of the permeation fluxes due to the low surface porosity of the film.6,38 4


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Most recently, we have developed a polymeric electrospun membrane with a ultrathin and submicron nanofibrous skin layer to further improve the oil-in-water emulsion separation efficiency on the basis of a relative high permeability.39 However, the hydrophilic modification of the polymeric membrane required a high alkalinity condition, whcih may result in a secondary pollution to water. Herein, we demonstrate a de novo strategy to fabricate a multilayer nanofibrous membrane via a facile integration of electrospinning and vacuum assisted deposition method. As shown in Figure 1, the membrane exhibit three individual layers: (i) the sedimentary SiO2 nanofibrous surface layer bonded by poly(ethylene glycol) diacrylate, exhibits intriguing superhydrophilicity and fine water-retaining property, serves as an anti-oil barrier to effectively block the seriously intrusion of oil droplets to avoid the destructive fouling of the polymeric matrix; (ii) the honeycomb-like polymeric nanofibrous layer with thin thickness and submicron pore size, is able to catch the smaller oil droplets and the numerous cavities may ensure a good water permeability to some extent; (iii) the high porous electrospun fibrous membrane with larger pore size allows water to pass through quickly and acts as the substrate to provide structural template for the honeycomb-like nanofibrous layer. With the cooperation of these individual functional layers, the composite membrane exhibited a superior anti-oil-fouling property and robust oil-in-water separation performance with good separation efficiency and high permeation flux solely under the driven of gravity.

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Figure 1.

Schematic illustration shows the pathway to fabricate the multilayer nanofibrous

membranes. 2. EXPERIMENTAL SECTION 2.1 Materials. Polyacrylonitrile (PAN, Mw = 90,000) used in this work was commercially obtained from Kunshan Hongyu Plastics Co., Ltd., China. Sodium dodecyl sulfate (SDS), oil red (sudan III), organic solvents (petroleum ether, n-hexane, dimethylformamide (DMF), and 1,2-dichloroethane), ammonium persulfate (APS), and poly(ethylene glycol) diacrylate (PEGDA) were purchased from Aladdin Chemistry Co. Ltd., China. Diesel oil (National IV Standard) was provided by the China National Petroleum Corporation. Pure water was obtained using a Heal-Force system. All chemicals were of analytical grade and used as received without further purification. 2.2 Preparation of the Electrospun Fibrous Substrates. Firstly, the precursor solution was prepared by dissolving the dried PAN powder in DMF at a concentration of 16 wt% and stirring for 12 h at room temperature. After that, the electrospinning experiment was carried out on a commercial electrospinning machine (DXES-3, SOF Nanotechnology Co., China) equipped with a group of syringes (5 of side by side) capped with 6-G metal, and the syringes periodically scanned in the horizontal direction with a width of 50 cm. During electrospinning 6


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process, the PAN solutions were pumped out at a fixed speed of 1.0 mL h-1. The applied work voltage was 25 kV to generate a stable and sufficient electric field intensity. The fibers were collected by a piece of polypropylene nonwoven fabric coated on a grounded, metallic rotating roller. The working distance was 20 cm. And the relative humidity and temperature during the electrospinning were 45 ± 5%, and 25 ± 5℃, respectively. 2.3 Fabrication of the Honeycomb-like Nanofibrous Layer. Firstly, the as-prepared electrospun fibrous membranes (EFM) were pre-treated by the isopropanol to remove the residual electric charge. Then the sufficiently dried membranes were coated on the grounded metallic rotating roller to be used as collector. The PAN solution with concentration of 5 wt% was prepared at first. Then the as-prepared solution was directly used for electrospinning by using the same electrospinning machine, but the feed rate of the solution were fixed at 0.2 mL h-1. The applied work voltage was fixed at 25 kV, meanwhile, the relative humidity was maintain at 20 ± 5%, the temperature was kept at 25 ± 2℃. The distance from needle tips to the surface of the collector was 20 cm. The whole spraying coating process was performed for 6 hours without breaking. Finally, a comopsite membrane with a honeycomb-like nanofibrous layer (HCNFM) could be obtained. 2.4 Construction of the Anti-oil Fouling Surface. Firstly, the SiO2 nanofibers (SNFs) were prepared according to our previous studies34 (see detail in the supporting information). Subsequently, the SNFs suspension was prepared. Typically, 0.3 g nanofibrous membranes were cut into 1 x 1 cm2 pieces, and were dispersed in 300 mL of water solution using a high-speed dispersion machine equipped with five blades (PHILIPS Viva Collection HR2166/00) for 5 min. After that, the PEGDA oligomer was added into the as-prepared SNFs suspension with a fixed concentration of 0.5 wt%, meanwhile, the APS was added into the dispersion with the weight ratio of 1:10 aspect to that of PEGDA oligomer, which acts as the initiator for the radical polymerization of PEGDA. The as-prepared HCNFM was used as the 7



substrate and a designed amount of the SNFs were filtrate on the surface of HCNFM via the vacuum filtration method. Finally, the composite membranes were heated in a hot air circulating oven at 80℃ for 2 h to ensure a well polymerization of PEGDA oligomer. 2.5 Evaluation of Emulsions Separation Performance. Generally, a few amount of oil and water were mixed in a volume ratio of 1:9, then the mixtures were under ultrasonic treatment (KUDOS SK3300HP, China) for 1 h at 25 ~ 30℃ to obtain the model surfactant-free emulsions. The diameters of oil droplets in the as-prepared model emulsions range from hundreds nanometers to several micrometers. Meanwhile, the surfactant-free emulsions could keep stable for about 4 h without obviously demusification. To prepare the corresponding surfactant-stabilized oil-in-water emulsions, the SDS was first dissolved in water with a concentration of 100 mg L-1 according to the previous studies,7,22 then the related oil was added into water with a volume ratio of 1: 99 aspect to water. After that, the SDS/oil/water mixtures were under ultrasonic treatment for 0.5 h at 25 ~ 30℃. The diameters of the oil droplets in the as-prepared SDS stabilized oil-in-water emulsions were hundreds nanometers. To evaluate the emulsion separation performance of the membranes, commercial dead-end filtration apparatus with different inner diameters and a homemade cross-flow module were used, respectively. For the dead-end filtration, the prewetted membranes were fixed between two vertical glass tubes of the dead-end filtration apparatus, then emulsions were directly poured onto the membranes and the height of the emulsions were maintained at ~ 10 cm. For the cross filtration, the prewetted membrane was fixed into the cell, the emulsions were pumped by a peristaltic pump with a fixed feeding flux of ~ 80 mL min-1, the separation area of the membrane is 289 mm2.The initial permeation fluxes were calculated with the volume of the collected filtrates within an initial time (1 min). The average total organic carbon (TOC) contents of the feed solutions and corresponding filtrates were measured to evaluate the separation efficiency.

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2.6 Apparatus and characterization. The scanning electron microscopy (Hitachi S-4800, Japan) was used to characterize the microstructures of the membranes. Nicolet 8700 Spectrometer (Thermo Fisher, USA) was employed to carry out the FT-IR spectroscopic analysis. The porous structures of the membranes were analyzed using a capillary flow porometer (Porous Materials Inc. CFP-1100AI, USA). The water contact angle (WCA), underwater oil contact angle (UWOCA), and sliding contact angle (SA) were measured using a contact angle goniometer (Kino SL200B, USA) equipped with a tilting base. A droplet of 3 µL liquid was used to measure the WCAs and UWOCAs, and for the testing of SAs, a droplet of 10 µL 1,2-dichloroethane was used. The underwater oil advancing and receding contact angles were measured based on the increment and decrement method, the maximum volume of oil used was 10 µL (1,2-dichloroethane). For each value, at least three measurements were carried out and the average value was obtained. The size distributions of oil droplet in the corresponding oil-in-water emulsions were characterized by optical microscopy (Olympus VHS3000, Japan). The TOC contents in the feed emulsions and corresponding filtrates were measured using a total organic carbon analyzer (Shimadzu TOC-L, Japan). 3. RESULTS AND DISCUSSION 3.1 Morphologies and Structure. We designed the composite nanofibrous membrane based on two principles: (i) the membrane must be highly porous to allow the fast permeation of water according to the Hagen-Poiseuille theory (ࡶ =

ઽ࢘૛ ∆࢖ ૡࣆࡸ

, in which the ۸ is the permeation

flux of liquid, ઽ is the porosity, ࢘ is the effective pore radius, ∆࢖ is the applied pressure,

ࣆ is the viscosity of liquid, and ࡸ is the thickness of the membrane),13,40 (ii) the pore size of the membrane must be submicron to effectively catch the highly emulsified oil droplets. To meet the first requirement, a highly porous EFM with porosity of ~ 90% was prepared via the common electrospinning method. As shown in Figure 2a and f, the obtained membrane exhibited typical morphology of electrospun fibrous materials,28,41 consisting of numerous 9



interpenetrating fibers with diameter around 1 µm, and the pore diameter of the membrane was around 4 µm (Figure 2f), which could ensure an ultrahigh water permeation flux but resulting in poor separation efficiency because of its large pore size. To solve this problem, a designed submicron porous nanofibrous layer was in-situ constructed on the surface of the EFM substrate. As shown in Figure 2b, the as-prepared nanofibrous layer uniformly covered the surface of EFM with a thickness of several micrometers (< 10 µm). More interestingly, as shown in Figure 2c and d, the nanofibrous layer exhibited a honeycomb-like structure with numerous cavities with depth of ~ 8 µm. And it should be mentioned that the bottom of each cavity was sealed by the associated ultrathin nanofibrous networks (Figure 2e), in which the diameter of the fibers was ~ 30 nm. As a result, the pore size of the honeycomb-like layer composited nanofibrous membrane (HCNFM) significantly decreased to ~ 0.7 µm (Figure 2f), which is crucial for filtrating the submicron oil droplets from water. In addition, we assumed that the formation of these unique honeycomb-like structures may be attributed to the competitive actions of solutions surface tension and electrostatic repulsion with the template effect of the electrospun fibrous substrates.42,43

Figure 2. (a) SEM image of the common EFM substrate. (b and c) Cross-section view and top view SEM images of the HCNFM. Inset in image (c) is a digital photo of a real honeycomb. (d and e) High magnified SEM images show the elaborated multilevel structures of a single 10


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cavity of in the honeycomb-like nanofibrous layer. (f) Pore size distribution curves of the EFM substrate and the corresponding HCNFM. Inspired by the gravel pack of the natural filter of earth, we intended to construct a similar functional layer on the surface of the obtained polymeric nanofibrous membrane, serves as a preliminary barrier for the oil droplets. SiO2 as a main component of soil,44 which is hydrophilic due to the hydroxy groups.45 Thus, we constructed a SiO2 nanofibrous layer on the surface of HCNFM via a facile vacuum filtration method (Figure 1). Figure 3a and Figure S1 demonstrates the digital photos of the obtained composite membranes, named as SNFs/HCNFM. Figure 3b and Figure S2 shows the cross section view SEM of the representative SNFs/HCNFM. It can be found that the composite membranes exhibited three-tier architectures with the cavities clearly be recognized. Figure 3c depicts the SEM images of the SNFs layer, demonstrating that the SNFs with diameter of ~ 450 nm and length of 10 ~ 40 µm interpenetrated with each other and deposited uniformly on the surface of HCNFM. Figure 3d indicates that there were no obviously change in the morphology of the honeycomb-like structures after coating with the x-PEGDA. Furthermore, the FT-IR characterizations were conducted to verify the composition of the SNFs/HCNFM. As shown in Figure 3e, the EFM substrate layer exhibited the specific peaks at 1726 cm-1 and 2243 cm-1, which was attributed to the groups of -C═O, and -C≡N of PAN,33 the specific peaks at 1100 cm-1 belongs to the Si-O-Si group of SiO2,45 and the 1726 cm-1 was assigned to the -C═O stretching of PEGDA. The barely visible specific peak at 1637 cm-1 (-C═C-) indicates that the PEGDA oligomer was well polymerized (Figure S4).35

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Figure 3. (a) The digital photo of the SNFs/HCNFM composite membrane. (b) The cross-section view SEM image of the representative SNFs/HCNFM composite membrane. (c and d) Top view SEM images of the SNFs layer and the HCNFM layer coated with x-PEGDA. (e) FT-IR spectra of the EFM substrate, pure SNFs, and the х-PEGDA coated SNFs layer. 3.2 Selective Wettability of the Membranes. As the selective wettability is a crucial factor for oil/water separation.8,23,46 We carefully studied the selective wetting performance of the obtained SNFs/HCNFM composite membranes with different loading amount of SNFs. As shown in Figure 4a, the completely wetting time (water, 3µL) of the membranes slightly increased with the presence of SNFs layer compare with the pristine HCNFM, which may be attributed to the reduction of the capillary effect due to the relative large pore size of the SNFs layer.47,48 However, the wetting time would not significant change since the SNFs fully covered the HCNFM (the loading amount of SNFs > 0.2 mg cm-2). Meanwhile, the underwater oil contact angles (UWOCAs) of the related membranes increased obviously with the presence of SNF layer, and the UWOCAs maintained around at 162° since the SNFs fully covered the HCNFM (Figure S5), similar high UWOCAs could also obtained from other oils, e.g. petroleum ether, n-hexane, diesel (Figure S6) on the surface of the membrane, indicating 12


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a good anti-oil performance of the selected SNFs/HCNFM composite membrane (the loading amount of SNFs

is 0.2 mg cm-2). Figure 4 b is the snapshot photos showing the dynamic

wetting process of water (top) and underwater oil adhesion behavior (bottom) on the surface of the pristine HCNFM. It can be found that when 3µL of water contact the surface of HCNFM, the water quickly spread and permeated into the membrane within 1 s, indicative of a good superhydrophilicity. Meanwhile, as shown in the bottom photos in Figure 4b and Movie S1, when a droplet of oil was pushed onto the surface of HCNFM underwater, the oil could not spread or permeated significantly, but the oil droplet was pinned on the surface of membrane and could not be removed anymore, which means that the anti-oil-fouling property of pristine HCNFM was poor. However, considering its relative high UWOCA, the lack of the anti-oil-fouling performance of the pristine HCNFM may be explained by the metastable Cassie effect.49-51 To further investigate the underwater interfacial property of oil and the membrane, the theoretical oil adhesion forces of the oil with different membranes were estimated according to the previous studies40,52 (see details in the supporting information). As shown in Figure 4c, the oil adhesion forces with the composite membranes obviously decreased with the increment of SNFs content, and a similar tendency was also observed from the corresponding sliding angles, a small sliding angle of 3° was obtained when the loading amount of SNFs was 0.2 mg cm-2. Besides, the photos in Figure 4d and Movie S2 vividly demonstrate the superhydrophilicity (top) and intriguing underwater anti-oil-adhesion property (bottom) of the related SNFs/HCNFM composite membrane, in which the oil droplet could completely detached from the surface of membrane without any change in morphology, confirming that the SNFs layer was powerful in enhancing the anti-oil-fouling performance of the membrane. Additionally, the Figure 4e, f and Movie S3 further verify the effect of SNFs layer for the robust anti-oil-fouling performance of the SNFs/HCNFM composite membrane in a practical way. As can be seen that for pristine HCNFM the continuous oil flow was blocked and pinned on the surface of the membrane. Surprisingly, after coated with SNFs 13



layer, the oil flow bounced off the surface of the membrane without any adhesion. This robust underwater superoleophobicity of the SNFs layer mainly resulted from the absorbed and sealed water layer on the surface of SNFs with the aid of PEGDA, which act as a cushion to prevent the contact of oil with the polymeric matrix of fibers,7,40,50 and it should be mentioned that the packed single SNFs could also provide a surface roughness, which enhanced the effect of Cassie state.53

Figure 4. (a) Wetting time of water and the UWOCAs on the surface of the composite membranes with different loading amount of SNFs. (b) Snapshot photos show the wetting process of water (top) and underwater oil adhesion property (bottom) on the surface of the pristine HCNFM. (c) The calculated underwater oil adhesion forces and the corresponding sliding angles on the surface of SNFs/HCNFMs with different loading amount of SNFs. (d) 14


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Snapshot photos show the wetting process of water (top) and underwater oil adhesion property (bottom) on the surface of the SNFs/HCNFM (0.2 mg cm-2 SNFs). (e and f) Photos demonstrate the adhesion and rejection behavior of oil on the surface of HCNFM and related SNFs/HCNFM (0.2 mg cm-2 SNFs), respectively. 3.3 Pore Structures and Water Permeability of the Membranes. Apart from the selective wettability, the pore structures and the water permeability are of great importance for the oil/water separation, especially for the oil-in-water emulsions.6,54,55 Herein, we also studied the influence of SNFs contents on the pore size and the thickness of SNFs layer of the comsposite membranes. As shown in Figure 5a, there was no obvious change in the average pore diameter of the SNFs/HCNFM membranes while the thickness of SNFs layer increased obviously. This result may be attributed the larger pore size of the SNFs layer than that of the HCNFM, and the tested pore diameters were assigned to the pore throat of the composite membranes, which were similiar with that of the HCNFM. In addition, the water permeabiltiy of the SNFs/HCNFMs were further evaluated by using a dead-end filtration device solely under the driving of gravity. As depicted in Figure 5b, the water permeation fluxes of the composite membranes decreased alomost linear to the thickness of the SNFs layer, which was consist with the Hagen-Poiseuille equation.13,40,56 The water permeation flux of the corresponding composite membranes with or without honeycomb-like structures were also compared (Figure S7), the result shows that the related water permeation fluxes of the SNFs/HCNFMs was higher than those of the SNFs coated membranes derived from polymeric nanofibrous composite membranes without honeycomb-like structures,39 confirming the effect of these cavities in enhacing the water permeability of the multilayer composite membranes. Comprehensively considering the selective wettability and water permeability, the SNFs/HCNFM with 0.2 mg cm-2 SNFs was chosen for the further oil/water separation performance studies.

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Figure 5. (a) Average pore diameter of the SNFs/HCNFMs with different loading amount of SNFs and the corresponding thickness of the SNFs layer. (b) Pure water permeation fluxes of the SNFs/HCNFMs with different thickness of the SNFs layer. 3.4 Effect of Surface Wettability on the Emulsions Separation. Figure 6a and Movie S4 demonstrates the change rate of permeation flux (J/J0) of the pristine HCNFM and the selected SNFs/HCNFM (0.2 mg cm-2 SNFs) for separating model surfactant-free oil-in-water emulsion (petroleum ether in water). It can be found that the J/J0 of HCNFM was sharply decreased with the increment of separation time, and only ~ 25% of the initial flux was remained after 10 min. In contrast, the J/J0 of the selected SNFs/HCNFM did not decrease anymore; and it should be mentioned that the tiny increment of the permeation flux may be attributed to the slightly coalescence of oil droplets during the separation.55 As shown in Figure 6b, the significant difference between the J/J0 of HCNFM and SNFs/HCNFM was mainly resulted from the difference of their surface anti-oil-adhesion performance. For the HCNFM, numerous oil droplets would be adhered on the surface of membrane during the separation process and the numerous oil droplets could not be removed due to their high adhesion force with membrane (Figure 4e). Thus, the surface pores were blocked soon, 16


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resulting in a sharply decline of the permeation flux. Whereas, for the SNFs/HCNFM, when the oil droplets came to the surface of membrane, they would not pinned on the membrane surface but freely move around for coalescence owing to the ultralow adhesion force with the SNFs layer. As a result, the free oil droplets could completely detach from the surface of membrane and floating upside when the diameter of droplets were big enough,55,57 leaving the membrane unpolluted (Figure 4f). Additionally, as shown in Figure 6c, the change rate of permeation fluxes for surfactant-stabilized emulsions of these two membranes were similar, which may be ascribed to that the surfactant surround the oil droplets acts as a barrier to protect the oil from contact the membranes;58 thus, the decline of the permeation flux was mainly attributed to the formed filter cake on the surface of membranes.59 Figure 6d demonstrates the photos of the feed emulsions and the relevant filtrates. It can be found that the original white milky emulsions became transparent after the separation. And the corresponding optical microscopic images show that there were numerous micro-scaled oil droplets presented in the entire view before the separation, whereas none droplets could be observed in the filtrate, this result could also be confirmed by the DLS curves of the oil droplet size distribution of emulsions before and after separation (Figure S8). All the above mentioned results demonstrate the high separation efficiency of the membranes both for surfactant-free and surfactant-stabilized emulsions.

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Figure 6. (a) The change rate of permeation fluxes of the pristine HCNFM and the selected SNFs/HCNFM (0.2 mg cm-2 SNFs) for the surfactant-free oil-in-water emulsion. (b) Schematic shows the assumed separation process of the HCNFM (left) and the selected SNFs/HCNFM (right). (c) The change rate of permeation fluxes of the pristine HCNFM and the selected SNFs/HCNFM for the surfactant-stabilized oil-in-water emulsion. (d) Digital photos and optical microscopic images of the surfactant-free emulsion (top) and surfactant-stabilized emulsion (bottom), respectively, before and after separation. 3.5 Oil-in-Water Emulsions Separation Performance. The separation performance for different emulsions derived from various oils were further evaluated, the model emulsions including surfactant-free emulsions, e.g. petroleum ether/H2O, n-hexane/H2O, diesel/H2O, and surfactant-stabilized

emulsions,

e.g.

SDS/petroleum

ether/H2O,

SDS/n-hexane/H2O,

SDS/diesel/ H2O. The separations were firstly carried out using a dead-end filtration device and solely driven by gravity. As shown in Figure 7a, the initial permeation fluxes of the selected SNFs/HCNFM composite membrane for petroleum ether/H2O, n-hexane/H2O, 18


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diesel/H2O, SDS/petroleum ether/H2O, SDS/n-hexane/ H2O, and SDS/diesel/ H2O, was 5163, 4773, 2063, 647, 433, and 267 L m-2 h-1, respectively. The variation of permeation flux for different emulsions may be attributed to the different viscosity and oil droplets content in the emulsions derived from different oils with various properties.5 It is noteworthy that despite the permeation flux of SDS/diesel/ H2O is the lowest here, considering the driving pressure of the operation was just ~1 kPa, the calculated permeation flux could be equal to 26700 L m-2 h-1 bar, which was comparable to the previous excellent works with similar separation conditions.7,13,20 Meanwhile, as shown in Figure 7b the corresponding TOC content in the filtrate was 2.0, 1.6, 29.0, 48.3, 49.7, and 46.0 mg L-1. The higher TOC value of the filtrate from the surfactant-stabilized emulsions may be due to the dissolved surfactant in water.7 The corresponding separation efficiency of the membrane for surfactant-free emulsions was > 99.5%, and for surfactant-stabilized emulsions, the separation efficiency could also be > 96.5% despite the existence of surfactant. Figure 7c demonstrate a comparison diagram showing the emulsions (surfactant-free and surfactant-stabilized) separation performance of the SNFs/HCNFM and other pioneer membranes reported before.7,13,60 It can be recognized that the obtained SNFs/HCNFM exhibited superior permeation fluxes even under a much lower driving pressure. This result was contributed by the merits of high porosity and robust anti-oil-fouling performance of the composite membrane. Furthermore, a cycling separation test for a surfactant stabilized emulsion was also conducted by using the same dead-end filtration device to evaluate the reusability of the membrane. After each testing cycle (10 min) the membrane was simply rinsed with pure water for several times, and then it was directly used for the next cycling test, the whole test process lasted for 50 min. Figure 7d demonstrates the initial and final permeation fluxes and the corresponding TOC content in the filtrate of each separation cycle. It can be found that the permeation flux decrease significantly with the increasing of separation time, and only ~ 22% of the initial permeation flux could be remained after 10 min continually separation, conforming to the abovementioned results in 19



Figure 6c. As expected, the permeation flux could completely recovered to the initial one after a simply wash with pure water, and the TOC content in the filtrate of each cycle kept at ~ 49 mg L-1, indicating that the separation efficiency was stable during the whole cycling test. As shown in Figure S8, there was almost no change in the surface morphology of the SNF layer after the cycling test, indicative of a robust reusability of the membrane. Moreover, the applicability of the membrane for different filtration devices was also evaluated. As shown in Figure S10a, Figure S10c, Movie S6, and Movie S7, the membrane exhibit a relative stable separation performance for both surfactant-free emulsion and surfactant-stabilized emulsions at a commercial filtration device with larger inner diameters (40 mm). Additionally, we also explored the operation of the membrane at a homemade cross-flow module, as shown in Figure S10b, Figure S10d, Movie S8 and Movie S9, the result demonstrate that the membrane could effectively separate the corresponding model emulsions. And it should be mentioned that the relative low permeation flux compare with those of the dead-end filtration may be due to the lower driven pressure.

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Figure 7. (a) Permeation flux of the selected SNFs/HCNFM for different oil-in-water emulsions with and without surfactant. (b) Separation efficiency and corresponding TOC content in the filtrate of different emulsions. (c) The relative separation flux and driving pressure of representative separation materials, the circled points are corresponded to the surfactant-free emulsions. (d) The cycling separation performance of the membrane for corresponding surfactant-stabilized emulsion. The SNFs/ HCNFM used here was obtained from the 0.2 mg cm-2 SNFs. 4. CONCLUSION In summary, we have developed a facile strategy for fabricating a biomimetic multilayer nanofibrous membrane with superior selective wettability, submicron pore structures, and robust anti-oil-fouling performance to effectively separate the highly emulsified oil-in-water emulsions. With synergistic effect of the elaborated individual functional layers, the as-prepared multilayer membranes were able to separate various surfactant-free/stabilized oil-in-water emulsions with high efficiency and much higher permeation fluxes compare with the conventional polymeric membranes. Moreover, the whole separation process was solely driven by gravity, which is expected to be highly energy efficient. The excellent anti-oil-fouling property endows the membrane with robust reusability for the long-term separations, which is crucial in the practical applications for oily wastewater remediation. We wish that this method could be useful for the design and fabrication of the next generation of specific membranes for oil-in-water emulsions separation. ASSOCIATED CONTENT Supporting Information Available: Supplementary Experiment; Cross-section and top view SEM images of the SNFs/HCNFMs with different loading amount of SNFs; Optical microscopy image of the as-prepared SNFs dispersion; Schematic shows the radical polymerization reaction of PEGDA; UWOCAs of the HCNFM and SNFs/HCNFMs with different loading amount of SNFs; UWOCAs of different oils on the surface of selected SNFs 21



/HCNFM; Pure water permeation flux of the composite membranes with and without honeycomb structures; DLS curves of the oil droplet size distribution of the surfactant free and surfactant stabilized emulsions and their corresponding filtrates after separation; SEM images of the corresponding SNF/HCNFM after a cycling test of emulsions separation; Permeation flux and corresponding separation efficiency of the membrane by using commercial dead-end filtration devices (diameter 40 mm), and by using a homemade cross-flow filtration system. Digital photos show the corresponding devices for dead-end filtration and cross-flow filtration. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (J. Yu), [email protected] (B. Ding) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no conflict of interest. ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (No. 51773033, 51673037 and 51473030), the Shanghai Committee of Science and Technology (No. 15JC1400500), the Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-03-E00024), the “111 Project” Biomedical Textile Materials Science and Technology, China (No. B07024).

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Biomimetic Multilayer Nanofibrous Membranes with Elaborated

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