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Breathable and asymmetrically superwettable Janus membrane with robust oil-fouling resistance for durable membrane distillation
Journal of Membrane Science 563 (2018) 602–609
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Breathable and asymmetrically superwettable Janus membrane with robust oil-fouling resistance for durable membrane distillation Zhigao Zhua, Zhiquan Liua, Lingling Zhonga, Chengjie Songa, Wenxin Shia, Fuyi Cuib, Wei Wanga, a b
T
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State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin 150090, PR China College of Urban Construction and Environmental Engineering, Chongqing University, Chongqing 400044, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrospinning Asymmetrically superwettable membrane Janus membrane Membrane distillation
A highly breathable membrane integrating an asymmetrically superwettable Janus skin and a hydrophobic nanofibrous membrane (NFM) was developed via sequential electrospinning and electrospraying for application in membrane distillation (MD). The electrosprayed asymmetrically superwettable Janus skin composed of lotusleaf-like nano/microstructured nanofilaments exhibited an intriguing underwater superoleophobicity of 164° and an in-air superhydrophobicity of 166°, thereby providing a robust resistance to membrane fouling with high flux. The newly developed membrane with an ultrathin Janus skin layer, high porosity and interconnected pore structure displayed a high water flux of over 25 L m−2 h−1, a robust oil resistance and an excellent durability in direct contact MD (ΔT = 40 °C) during the treatment of oil-in-saline water emulsion (1000 ppm oil). The present development can thus endow MD with the ability to desalinate challenging water matrices with complex compositions. Significantly, the sequential electrospraying process utilized in the construction of the Janus skin is expected to be applicable to a wide range of other selective separations.
1. Introduction Severe water pollution and freshwater scarcity are two global issues that threaten all human beings [1–3]. Therefore, it is of special importance to develop appropriate technologies for wastewater treatment and seek for alternative sources such as seawater and brackish water. Membrane distillation (MD) is a membrane based thermally driven process, in which water molecules in the hot feed vaporize at the liquid/membrane interface and diffuse across a hydrophobic porous membrane to the cold distillate [4–7]. Significantly, MD can make full use of waste heat to generate high-quality water and is not restricted by the water quality conditions, thus is emerged as a viable technology for wastewater treatment or the desalination of seawater and brackish water [8]. Durability and fouling are two critical factors that directly affect membrane durability during the treatment of challenging feedwaters such as wastewater or polluted seawater and brine [5,9,10]. Membrane durability is mainly induced by the destruction of the membrane hydrophobicity by the hot feedwater, resulting in the rapid infiltration of the feedwater into the hydrophobic membrane pores [11–13]. Another critical issue is membrane fouling, which is generally triggered by hydrophobic contaminants such as oil droplets [14–16]. Owing to strong hydrophobic-hydrophobic interactions, hydrophobic foulants can easily
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Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.memsci.2018.06.028 Received 18 May 2018; Received in revised form 14 June 2018; Accepted 16 June 2018
Available online 18 June 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.
attach to the hydrophobic membrane surface and wick into the membrane pores, preventing the transfer of vapor across the membrane [17]. To address these problems, novel membranes with unique wettabilities have been developed to maintain membrane durability and prevent membrane fouling. Superhydrophobic membrane was first created to maintain membrane durability and prevent membrane fouling, but their durability was still unsatisfactory due to the underwater oleophilicity of the superhydrophobic membranes [18–22]. Inspired by the features of marine creatures such as fish scales, clamshells and sharkskin, it has been found that a hydrophilic surface layer in combination with a hierarchical rough structure can effectively overcome oil fouling due to the preferential affinity for water [23–25]. Therefore, a Janus membrane with asymmetric superwettability may become a potential candidate for satisfying the contradictory requirements for simultaneous resistance to MD membrane fouling and maintainability of MD membrane durability. Recently, polymerization deposition and spray coating have become the two most common methods for fabricating asymmetrically wettable Janus membranes, but the breathability (water vapor transmission) of the MD membranes was sacrificed. For example, to ensure the hydrophilic monomer polymerized on one side of the hydrophobic membrane surface, the MD membrane must has narrow pore size to prevent
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hydrophobic membrane from being complete hydrophilization [26,27]. Therefore, this kind of membrane has seriously limited the transfer of vapor across the hydrophobic membrane. Meanwhile, the underwater oleophobic surfaces prepared via spray-coating chitosan-perfluorooctanoate/silica nanoparticles (CTS-PFO/SiNPs) on a hydrophobic microporous and nanofibrous polyvinylidene fluoride (PVDF) membrane were also reported [5,17]. However, the thick and dense hydrophilic skin layer extensively obstructed the membrane pores, causing the flux to drop sharply compared with that of the virgin hydrophobic membrane substrate. Moreover, the uneven thickness of the surface coating and the presence of defects may create breakthrough points for the entry of oily pollutants. Thus, a simple and universal method for constructing breathable and asymmetrically superwettable MD membranes for the efficient desalination of water matrices with complex compositions is required. In this work, we have designed and presented a sequential electrospraying method for fabricating a breathable and asymmetrically superwettable Janus skin layer integrated with electrospun hydrophobic PVDF nanofibrous membrane (NFM) to prevent membrane fouling and maintain membrane durability. The composite membrane has a hydrophobic PVDF fibrous membrane substrate with an interconnected pore structure, and the asymmetrically superwettable Janus skin layer exhibits underwater superoleophobicity and in-air superhydrophobicity respectively, which was achieved by simply tuning the concentration of the electrospraying solution containing hydrophilic or hydrophobic silica nanoparticles (SiO2 NPs, F-SiO2 NPs). The detailed procedures are shown in Fig. 1. It is worth noting that both the superhydrophobic and superhydrophilic composite layers have structures similar to that of lotus leaves with a hierarchical rough structure. Thus, the resultant Janus skin layer is able to form a water layer on the superhydrophilic surface and air pockets in the superhydrophobic surface deposited on the hydrophobic PVDF NFM [28,29]. Ultimately, the Janus membrane with excellent breathability displays a stable performance in the treatment of high-salinity water containing a high concentration of lubricating oil, which may be a potential candidate for oily wastewater treatment or the desalination of seawater and brackish water with complex compositions.
SpectrumChemical Co., Ltd., China. Polyvinylidene fluoride (PVDF, HSV 900) was purchased from Arkema Co., Ltd., China. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Mw: ~400,000), polystyrene (PS, Mw: ~350,000) and trimethoxy (heptadecafluorotetrahydrodecyl)-triethoxysilane (FAS, 97%) were purchased from Sigma-Aldrich. China. Silicon dioxide (SiO2, 7–40 nm) was purchased from Aladdin Chemical Regent Company. N,N-dimethylacetamide (DMAc), trichloromethane (CHCl3), sodium chloride (NaCl), sodium dodecylbenzenesulfonate (SDBS), and methylene blue (MeB) were purchased from Shanghai Runjie Chemical Reagent Company, China. Crude oil and soybean oil were obtained from the fifth Daqing Oil Production Factory and Jiusan Oils & Grains Industries Group Co., Ltd., respectively. Lubricating oil was purchased from Japan Mitsubishi Heavy Industries Co., Ltd. A commercial PVDF microporous membrane with an average pore diameter of 0.45 µm was purchased from Merck Millipore. All chemicals were used as received without further purification.
2. Materials and methods
Janus NFMs with asymmetric superwettability were prepared via electrospinning and sequential electrospraying techniques. First, necklace-structured PVDF membrane substrates were fabricated via electrospinning of a LiCl/PVDF solution with a solid content of 12 wt%. A high voltage of 30 kV and a feed rate of 3 mL h−1 in an enclosed space
2.2. Preparation of polymer solutions A PVDF solution was prepared by adding 6 g of PVDF powder into 44 g of LiCl/DMAc ionic liquid with vigorous stirring at 80 °C for 1 h followed by stirring at room temperature for another 12 h. The concentration of LiCl in the polymer solution was fixed at 0.1 wt%. A PVDF-HFP/PS hybrid polymer solution (4 wt%) containing superhydrophobic SiO2 NPs (6 wt%) was prepared as follows: First, 0.048 g of SiO2 NPs was dispersed in 19.2 g of DMAc with continuous stirring at room temperature for 30 min following by sonication at a power of 150 W for another 30 min. Second, 0.096 g of FAS was added to the above suspension with continuous stirring at 60 °C for 6 h. Finally, 0.4 g of PVDF-HFP and 0.4 g of PS were simultaneously added into the above prepared solution, which was stirred for 6 h. The procedures for preparing a PAN solution (4 wt%) containing 6 wt% SiO2 NPs were the same as that of the PVDF-HFP/PS solution (4 wt%) containing 6 wt% FSiO2 NPs. 2.3. Fabrication of nanofibrous Janus membranes
2.1. Materials Polyacrylonitrile
(PAN,
P1361)
was
obtained
from
Fig. 1. (a) Schematic representation of the fabrication of superhydrophobic SiO2 NPs. (b) Schematic illustration of the fabrication of the Janus skin layer with asymmetric superwettability on a necklace-structured PVDF NFM. (c) The proposed mechanism of the antifouling and antiwetting properties using an asymmetrically superwettable Janus skin layer and a plausible mechanism of vapor permeation across the porous membrane. 603
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affected the mechanical properties of the membrane substrate (2.74 MPa). The poor durability of this membrane substrate indicated that it cannot withstand the squeezing that occurs when the substrate is placed in the middle of the two membranous chambers. For the purpose of maintaining the rough fiber structure and improving the mechanical strength of the fibrous membrane, LiCl was added to the polymer solution to regulate the fiber structure because a polymer solution with a higher conductivity can induce the stretching effect and lead to more intense whipping during electrospinning [34,35]. As shown in Fig. 2b, the PVDF/LiCl fibrous membrane containing 0.1 wt% LiCl possessed thinner fibers with an average diameter of 385 nm (Fig. 2g), and no drops of solution were sprayed on the fibrous membrane surface. Significantly, the necklace-structured fibers with a rough surface can help the membrane substrate to improve the MD membrane stability. By carefully examining an enlarged FE-SEM image of a lotus leaf (inset of Fig. 2a), the unique nanostructures of the lotus leaf can also be obviously observed, demonstrating that the self-cleaning effect of the lotus leaf depends not only on the first-order rough structure but also on a secondary rough structure. To introduce unique nano/microstructures like those of the lotus leaf in the Janus skin layer, a sequential electrospraying technique was used to fabricate a functional Janus skin layer with asymmetric superwettability. Dilute PVDF-HFP/PS (4 wt%, 1/1) and PAN solutions (4 wt%) containing 6 wt% SiO2 NPs were prepared to fabricate the hierarchically structured Janus skin layer. The formation of this unique structure was attributed to competition between the electric force and the surface tension of the dilute solution in the presence of a strong electric field [36,37]. The details of screening the concentrations of polymers and SiO2 NPs are shown in Fig. S2-S6. FE-SEM images of the optimal superhydrophobic surface and superhydrophilic surface are shown in Fig. 2(c and d), revealing that all the microspheres (diameters ranging from 0.1 to 1.9 µm, Fig. 2g) are attached to ultrathin fibers (average diameter of 36 nm). Generally, the SiO2 NPs were used as a template to impart nanoscale roughness on the microsphere surface because the SiO2 NPs can lead to shrinkage of the polymer droplets during the evaporation of the solvent (Fig. S7) [28]. The multiscale rough structures of the Janus skin layer were consistent with a lotus leaf-like structure. A cross-sectional image of the Janus membrane and an enlarged image of the top layer are shown in Fig. 2(e and f) and demonstrate that the interconnected pore-structure of the membrane was assembled through the layer-by-layer accumulation of fibers. Meanwhile, the Janus skin layer with a thickness of 15 µm was strongly adhered to the membrane substrate owing to the unevaporated solvent in the microspheres and the strong electrostatic forces present during the electrospraying process [38,39]. In the meantime, the sequential electrospraying of a Janus skin layer with microsphere-on-string-structured fibers can affect the pore structure of the membrane substrate. As seen in Fig. 2h, the necklacestructured PVDF NFM exhibits an irregular pore structure with a pore size distribution ranging from 1.33 to 2.23 µm (average pore size of 1.87 µm), which is attributed to the accumulation of fibers with varying diameters. However, after the F-SiO2 @PVDF-HFP solution was sprayed onto the membrane substrate, the average pore size decreased substantially to 1.52 µm owing to the densely accumulated micropore-onstring-structured fibers deposited on the membrane substrate. Further electrospraying of the SiO2 @PAN solution onto the composite membrane slightly decreased the average pore size to 1.45 µm because of the overlapping coverage of the microspheres. It is worth noting that the microsphere-on-string-structured fibers did not plug the pores of the membrane substrate because the average pore size of the composite membrane was not significantly reduced, guaranteeing that vapor can diffuse across the membrane substrate without obstruction. The mechanical property, an important factor in determining the practical application of a membrane, is notably influenced by the membrane structure. Fig. 2i demonstrates typical tensile stress-strain curves of the PVDF-based fibrous membranes, where all the samples exhibited nonlinear elastic deformation in the first region (0–2.8%)
with a humidity of 65 ± 2% were used to generate the continuous jet stream. Second, the Janus skin layer with asymmetric superwettability was prepared via a sequential electrospraying technique using the above prepared F-SiO2 @PVDF-HFP/PS and SiO2 @PAN solutions. The generated superhydrophobic and superhydrophilic microsphere-onstring-structured fibers were continuously deposited on the surface of the as-prepared PVDF NFM. During the electrospraying process, a high voltage of 25 kV and a feed rate of 0.5 mL h−1 in an enclosed space with a humidity of 40 ± 2% were used to generate the microsphere-onstring-structured fibers. The work distance between the tip of the needle and the collector was fixed at 20 cm in an enclosed space with a constant temperature of 23 ± 1 °C. Finally, the resultant Janus membrane was dried at 60 °C under vacuum for 3 h to remove the residual solvent. 2.4. Characterization The morphology of the fibers was observed by a field emission scanning electron microscope (FE-SEM) (Zeiss, Sigma 500, Germany) and a transmission electron microscope (TEM) (JEM-2100F, JEOL Ltd.). The pore size distribution was characterized via a bubble point method using a capillary flow porometer (CFP-1100ai, Porous Materials Inc., USA). The mechanical properties of the relevant samples were examined with a tensile tester (XQ-1C, Shanghai New Fiber Instrument Co., Ltd., China). The liquid contact angles were measured using a contact angle goniometer (Kino SL200B). The liquid entry pressure was measured using a vacuum pump with different vacuum pressures. 2.5. MD Performance Tests The MD performance of relevant membranes was evaluated in a labscale direct contact membrane distillation (DCMD) system with an effective surface area of 16 cm2. The feed solution (3.5 wt% NaCl) fixed at 60 °C and deionized (DI) water fixed at 20 °C were circulated with peristaltic pumps. To evaluate the durability and fouling resistances of the relevant membranes, An oily saline feed solution was then prepared by mixing 1 g of lubricating oil and 1 L of NaCl (3.5 wt%) aqueous solution with vigorous stirring at 2200 rpm for 1 h. The resultant oil-insaline water emulsion was stable with no obvious demulsification for over 48 h. 3. Results and discussion 3.1. Surface and structural characterization of the Janus Membrane The objective of this paper was to obtain a breathable MD membrane with antiwetting and antifouling properties for application in the desalination of challenging saline water matrixes containing low-surface-tension contaminants. We designed an asymmetrically superwettable Janus membrane based on three criteria: (1) the asymmetrically superwettable Janus skin layer must be easily assembled on a composite membrane with an open-cell structure; (2) the Janus membrane must have the capacity to completely demulsify oil-in-saline water emulsions and repel hot water to simultaneously prevent membrane wetting and fouling; (3) the Janus membrane must have robust mechanical strength and durability for application. The first basic requirement was satisfied by the versatile, readily accessible electrospinning and sequential electrospraying methods applied in the fabrication of the fibrous membrane with an interconnected pore structure [30–33]; these methods involved the electrospinning of a PVDF/LiCl ionic liquid and sequential electrospraying of F-SiO2 @PVDF-HFP/PS and SiO2 @PAN solutions. First, inspired by the hierarchical structure of lotus leaves (Fig. 2a), a PVDF solution with a low concentration of 12 wt% was used to electrospun thin fibers with a rough surface imparted by the unstable whipping of the solution jet during electrospinning. As shown in Fig. S1, it is obviously observed that the entangled fibers with a microsized beads-on-string structure strongly 604
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Fig. 2. Morphology, pore structure and mechanical strength of the Janus membrane. (a) Photograph and enlarged FE-SEM images of a lotus leaf. (b) FE-SEM image of the necklace-structured PVDF NFM substrate. FE-SEM images of the Janus skin layer with asymmetric superwettability: (c) the superhydrophobic layer and (d) the superhydrophilic layer. (e) Cross-sectional image and (f) enlarged image of the top layer of the Janus membrane. (g) Fiber and hierarchical microsphere diameter distributions of the corresponding Janus membrane. (h) Pore size distribution and (i) mechanical strength of the Janus membrane at each stage of fabrication.
string-structured fibers due to the unstable stretching of the PVDF solutions during electrospinning (Fig. S2). In 2004, Jiang et al. reported that a dilute PS/DMF solution (5 wt%) was used to fabricate a microparticle film with particle diameters ranging from 2 to 7 µm, and this film exhibited a superhydrophobic surface with a contact angle of 160.4° [43]. Unfortunately, the poor durability of the microparticle film seriously limited its application. In addition, PS fibers with a low packing density that are fabricated from a high-concentration solution are also difficult to use as a free-standing membrane for separation [37]. Thus, hydrophobic PVDF-HFP was added into the PS solution to fabricate stabilized microsphere-on-string-structured fibers supported on the PVDF NFM with a certain wear resistance. To simulate the multiscale rough structure of a lotus leaf, various concentrations of SiO2 NPs were added to the PVDF-HFP/PS solution (4 wt%) to construct multiscale roughness in the membrane. As shown in Fig. 3(a and b), the water contact angle (WCA) of the F-SiO2 @PVDFHFP/PS skin layer significantly increased from 152° to 164°, demonstrating an obvious increase in the WCA with increasing SiO2 NP concentration. However, a reverse trend in the water wetting behavior occurred as the SiO2 NP concentration further increased to 8 wt% (156°) due to the aggregation of the SiO2 NPs [44]. This result was consistent with the experimental FE-SEM image results shown in Fig. S4. Furthermore, the construction of multiscale roughness on the hydrophilic membrane was also performed for the underwater oleophobic surface since the trapped air in the membrane pores was replaced by
under a small stress and then large elongation until breakage. Generally, the tensile strength is proportional to the thickness of a membrane [40]. Owing to the unique 3-dimensional web structure of fibrous membrane, the increasing thickness can decrease the maximum size of the inter-fiber space, thus more fibers can withstand higher mechanical strength. The Janus skin layer significantly increased the membrane thickness from 52 ± 3 to 66 ± 3 µm, but the microsphere-on-stringstructured fibers could not withstand the external stress, thus resulting in a continuous decrease in the membrane strength from 7.65 to 4.81 MPa. The relevant parameters of the fabricated Janus membrane and commercial PVDF membrane are listed in Table S1.
3.2. Wetting Properties of the Janus Membrane Generally, lotus plants, “which remain unsullied by mud and wash clean as if by magic” were praised by Chinese ancients as “gentlemen's flowers”. The lotus also symbolizes kind and honest people who faithfully fulfill their corporate responsibilities. Inspired by the hierarchically rough structure and self-cleaning performance of lotus leave (Fig. 2a), Wenzel and Cassie demonstrated that the establishment of nano/microscale structures was essential for improving the selectivity of a membrane [41,42]. The asymmetrically superwettable Janus skin layer was examined by contact angle and tensiometer-based oil probe experiments, as shown in Fig. 3. The electrospraying of a low concentration of PVDF was ineffective for the formation of microsphere-on605
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Fig. 3. Wetting properties of the Janus membrane. (a) WCAs of the superhydrophobic PVDF-HFP/PS skin layer and (b) UOCAs of the superhydrophilic PAN skin layer containing various concentrations of SiO2 NPs. The concentrations of both the PVDF-HFP/PS and PAN solutions employed for fabricating the Janus skin layers were fixed at 4 wt%. (c) Photographs of the dynamic measurements of water adsorption, underwater oil repulsion and in-air water repulsion. (d) Real-time-recorded force-distance curves for the optimal underwater superoleophobic surface (top Janus layer).
Fig. 3d. Based on the real-time-recorded force-distance curves, the top Janus skin layer displayed an extremely low adhesion force of approximately 22.92 μN, and no force drop was observed in the corresponding receding curve, demonstrating the extremely low adhesion of oil. The liquid entry pressure of Janus membrane was tested by hydrostatic pressure. According to Young-Laplace equation, the hydrostatic pressure was associated with the liquid surface tension (γliquid), liquid contact angle on the membrane surface (θadv) and shape of membrane pores, the equation is shown in below:
water. As shown in Fig. 3b, PAN microspheres (produced by electrospinning a 4 wt% PAN solution) containing 6 wt% SiO2 NPs displayed the optimal underwater superoleophobicity with an underwater oil contact angle (UOCA) of 164°. The change in the WCA of the hydrophobic membrane caused by changing the SiO2 NP concentration was consistent with the change in the UOCA of the hydrophilic membrane. To further investigate the dynamic wetting behavior of liquids on the asymmetrically superwettable Janus membrane surface, a highspeed camera was used to record the oil adhesion and water permeation processes on the top Janus skin layer and the water repelling ability of the bottom Janus skin layer. To avoid the measurement of superhydrophilic layer being affected by the bottom superhydrophobic skin layer of the Janus membrane, the superhydrophilic layer was sprayed on a hydrophilic PAN fibrous membrane for measurement of the dynamic wetting behavior. Fig. 3c (1) gives photographs of a water droplet (3 mL) in contact with the top skin layer of the Janus membrane. The water droplet spread quickly, and a WCA of 0° was achieved in a short period of 1.5 s, revealing the excellent water permeability of the superhydrophilic skin layer. Interestingly, after the membrane was wetted by water, the surface of the membrane suddenly exhibited underwater superoleophobicity because the infiltrated water with a high adhesion force of ~145 mN was difficult to replace with oil (CHCl3, ~27 mN) [45,46]. Significantly, despite the oil droplet being forced onto the membrane surface under strong pressure, the oil droplet left the membrane surface without undergoing deformation, demonstrating the robust antifouling performance (Fig. 3c (2)). Moreover, the bottom superhydrophobic layer of the Janus membrane also played a leading role in preventing the membrane from being wetted during the membrane distillation process, exhibiting a superior water-repelling ability, as shown in Fig. 3a. A water droplet was forced to come into sufficient contact with the superhydrophobic Janus skin layer and exhibited obvious deformation, and then the water droplet easily left the membrane surface without deformation owing to the air trapped in the hierarchically structured membrane and the low surface energies of PS (hydrophobic CH groups) and PVDF-HFP (hydrophobic CF2 groups), as shown in Fig. 3c (3). To better understand the oil repellency of the membrane in a complex underwater environment, the adhesion forces during the contact procedure were dynamically measured, as shown in
Hydrostatic pressure = −
4γwater . cosθadv dmax
Here, the γwater = 72.58 mN/m, θadv = 166°, dmax = 2.07 µm, the hydrostatic pressure of Janus membrane calculated from YoungLaplace equation was 136.08 kPa, which is far higher than that of the practical hydrostatic pressure of 95.52 kPa owing to the irregular aperture structure and asymmetrically hydrophobic membrane surface. The results demonstrate that the Janus membrane has excellent liquid entry pressure for durable and long-term membrane distillation. Saline water is generally polluted with domestic sewage and industrial wastewater; thus, the membrane should also possess the ability to isolate pollutants and prevent the membrane from being fouled. Comparing the wetting properties of the superhydrophobic and superhydrophilic skin layers of the Janus membrane, water droplets containing a wide range of contaminants with different surface tensions and different water qualities were measured to evaluate the membrane stability (Fig. 4a). It was exciting to observe that the bottom Janus skin layer was resistant to wetting by low-surface-tension water droplets and different water qualities (CA＞158°), as shown in Fig. 4b. Similarly, the necklace-structured PVDF NFM used as the membrane substrate also exhibited hydrophobicity toward various liquids. Even though the isolated top layer of the Janus skin exhibited superhydrophilicity with a WCA of 0°, as mentioned above (Fig. 3c (1)), when a superhydrophilic layer a few microns thick was deposited onto the superhydrophobic layer, the liquid contact angle sharply increased. This change could be attributed to the fact that the water droplet was comparatively more difficult to absorb by the ultrathin superhydrophilic layer. In contrast, 606
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Fig. 4. (a) Photographs of different liquid droplets deposited on the bottom and top layers of the Janus skin in air and under water, respectively. (b) In-air sessile drop CAs for three different surfaces with nine liquids (left of the dashed line) and UOCAs for three different surfaces with CHCl3 (right of the dashed line).
when the superhydrophilic or superhydrophobic side of the Janus skin was exposed to air, oil droplets tend to attach onto the hydrophobic membrane surface and possibly wick into the pores. After the membrane was immersed in water, the superhydrophilic skin layer immediately became underwater superoleophobic with an OCA (CHCl3) of 166°. This significant shift in the membrane wettability could be ascribed to the fact that the membrane surface was completely covered by the infiltrated water, thus avoiding direct contact between the oil droplets and the membrane surface. Conversely, the hydrophobic membrane substrate and the superhydrophobic layer of the Janus membrane displayed low UOCAs of 32 and 25°, respectively, due to the strong interactions between the oil droplet and the low-surface-energy materials that occurred under water [47,48]. As a result, the novel Janus membrane with asymmetric superwettable skin layer in combination with hydrophobic PVDF membrane substrate could prevent the membrane from being fouled and maintain membrane durability, thus providing robust stability for MD applications.
or the asymmetrically superwettable Janus skin layer was sprayed on the membrane substrate, both membranes were able to sustain a stable MD performance with a high water flux and perfect salt rejection. These results demonstrated that the upward capillary force of the superhydrophobic layer preserves a metastable Cassie-Baxter state (liquid-air interface) that prevents the membrane from being wetted [11,20,49]. In addition, it was clearly observed that the flux of the Janus membrane (28.16 L m−2 h−1) was slightly lower than those of the superhydrophobic PVDF NFM (29.01 L m−2 h−1) and the hydrophobic PVDF NFM (29.52 L m−2 h−1). This difference was attributed to the fact that the increase in the thickness of the MD membrane slightly increased the difficulty of vapor transmission [50]. The hydrophilic surface with an interconnected pore structure did not impede vapor transmission because the evaporation of water occurred on the hydrophobic membrane surface. In contrast, the commercial hydrophobic PVDF membrane exhibited a comparatively low water flux of 16.84 L m−2 h−1 due to its closed pore structure.
3.3. Durability of Relevant Membranes
3.4. Fouling resistance of relevant membranes
To better explain the importance of the lotus-leaf-structured membrane surface, the durability of membranes with different structures were evaluated under the same experimental conditions. Fig. 5(a and b) shows the time-dependent flux and durability of fabricated and commercial PVDF membranes. The hot feed side and cold side temperatures were fixed at 60 and 20 °C, respectively, cycling through a flat-sheet membrane cell. It was observed that the as-prepared PVDF membrane substrate maintained a high water flux of over 28 L m−2 h−1, but the conductivity continuously increased to 75 μS/cm after 48 h of operation. Though the unique necklace-structured PVDF nanofibers ensured that the membrane had a comparatively robust durability, a declining trend in the rejection of salt over time was clearly observed. In comparison, after the superhydrophobic F-SiO2 @PVDF-HFP/PS skin layer
To investigate the membrane fouling performance, a surfactantstabilized oil-in-saline water emulsion was used as the feedwater in MD experiments (inset of Fig. 6a). Oil droplets with an average particle size of 4.52 µm were dispersed in saline water, as shown in Fig. S9. The commercial hydrophobic PVDF microporous membrane and the electrospun superhydrophobic PVDF NFM were rapidly fouled in 30 min, resulting in a sharp decrease in the water flux and a substantial increase in the conductivity on the distilled water side, as shown in Fig. 6(a and b). The extensive membrane fouling was caused by the attachment and accumulation of oil droplets on the membrane surface, which ultimately blocked the hydrophobic membrane pores, as shown in Fig. 6c. In comparison, the Janus membrane exhibited a stable water flux of 25.42 L m−2 h−1 and a high salt rejection ratio of 100% after 30 h of
Fig. 5. Wetting resistance and durability of the Janus membrane compared with those of relevant membranes. (a) Water flux and (b) permeate conductivity. 607
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Fig. 6. Fouling resistance and durability of the Janus membrane compared with those of relevant membranes. (a) Water flux and (b) permeate conductivity. (c) Photographs of the relevant membranes after the fouling experiments. Optical microscopy images of the oil-in-saline water emulsion (d) before and (e) after distillation. (f) UV–vis spectra of the as-prepared oily saline water before and after distillation.
membrane with an underwater superoleophobic skin layer (164°) and in-air superhydrophobic skin layer (166°) is applicable as an MD membrane for desalinating challenging oil-in-saline water emulsions, which displayed a high water flux of over 25 L m−2 h−1, a robust oil resistance and an excellent durability in direct contact MD (ΔT = 40 °C) during the treatment of oil-in-saline water emulsion. This versatile method of constructing membranes with superwettable skins or asymmetrically superwettable Janus skins is expected to be applicable to a wide range of other selective separation fields.
operation. The outstanding antifouling properties are attributed to the underwater superoleophobic surface and the extremely low oil adhesion force of the superhydrophilic Janus skin layer. Meanwhile, the small oil droplets came into contact with the superhydrophilic membrane surface, they quickly aggregated to generate larger oil droplets, which then left the membrane surface and floated to the water surface according to Stokes' law of resistance [51]. Optical microscopy was also used to observe the changes in the oil-in-saline water before and after distillation, as shown in Fig. 6(d and e). The as-prepared oil-in-saline water emulsion exhibits numerous oil droplets in the image. However, after distillation, no oil droplets could be observed in the collected filtrate, implying the high efficiency of the hierarchically structured Janus membrane for separating oil droplets and preventing membrane from be fouled by oil droplets. Meanwhile, UV–vis spectroscopy was also used to measure the collected filtrate and the original milky white feed emulsion as shown in Fig. 6f. The collected filtrate shows flat line as compared with the oil-in-saline water emulsion with a strong adsorption peak at 303 nm, implying that the Janus membrane can totally isolate the oils from the oil-in-saline water emulsion.
Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (Grant no. 51573034), National Science and Technology Major Project (No. 2017ZX07501-002-01), the State Key Laboratory of Urban Water Resource and Environment in HIT of China (No. 2016DX02), Postdoctoral Science Foundation of Heilongjiang Prov. (No. LBH-TZ0606 and LBHQ16012), Scientific Research Foundation for Returned Scholars of Heilongjiang Prov. (No. LC2017023) and the Fundamental Research Funds for the Central Universities of China.
4. Conclusion
Appendix A. Supplementary material
The recent development of novel membranes with unique structure has great significance in the desalination of challenging oil-in-saline water emulsions. Thus, this process requires the fabrication of a breathable and asymmetrically superwettable Janus membrane to achieve concurrent breakthroughs in the membrane durability, fouling and water flux. Traditional methods such as polymerization deposition and spray coating are not feasible for conventional membranes due to their uncontrollable operation and extensive membrane pore blocking. This study showcases a versatile sequential electrospraying method that involves the simple tuning of the as-prepared solution to fabricate a breathable and asymmetrically superwettable Janus skin layer. By imitating the nano/microstructures of lotus leaves, the Janus
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Breathable and asymmetrically superwettable Janus membrane with robust oil-fouling resistance for durable membrane distillation Zhigao Zhua, Zhiquan Liua, Lingling Zhonga, Chengjie Songa, Wenxin Shia, Fuyi Cuib, Wei Wanga, a b
T
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State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin 150090, PR China College of Urban Construction and Environmental Engineering, Chongqing University, Chongqing 400044, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrospinning Asymmetrically superwettable membrane Janus membrane Membrane distillation
A highly breathable membrane integrating an asymmetrically superwettable Janus skin and a hydrophobic nanofibrous membrane (NFM) was developed via sequential electrospinning and electrospraying for application in membrane distillation (MD). The electrosprayed asymmetrically superwettable Janus skin composed of lotusleaf-like nano/microstructured nanofilaments exhibited an intriguing underwater superoleophobicity of 164° and an in-air superhydrophobicity of 166°, thereby providing a robust resistance to membrane fouling with high flux. The newly developed membrane with an ultrathin Janus skin layer, high porosity and interconnected pore structure displayed a high water flux of over 25 L m−2 h−1, a robust oil resistance and an excellent durability in direct contact MD (ΔT = 40 °C) during the treatment of oil-in-saline water emulsion (1000 ppm oil). The present development can thus endow MD with the ability to desalinate challenging water matrices with complex compositions. Significantly, the sequential electrospraying process utilized in the construction of the Janus skin is expected to be applicable to a wide range of other selective separations.
1. Introduction Severe water pollution and freshwater scarcity are two global issues that threaten all human beings [1–3]. Therefore, it is of special importance to develop appropriate technologies for wastewater treatment and seek for alternative sources such as seawater and brackish water. Membrane distillation (MD) is a membrane based thermally driven process, in which water molecules in the hot feed vaporize at the liquid/membrane interface and diffuse across a hydrophobic porous membrane to the cold distillate [4–7]. Significantly, MD can make full use of waste heat to generate high-quality water and is not restricted by the water quality conditions, thus is emerged as a viable technology for wastewater treatment or the desalination of seawater and brackish water [8]. Durability and fouling are two critical factors that directly affect membrane durability during the treatment of challenging feedwaters such as wastewater or polluted seawater and brine [5,9,10]. Membrane durability is mainly induced by the destruction of the membrane hydrophobicity by the hot feedwater, resulting in the rapid infiltration of the feedwater into the hydrophobic membrane pores [11–13]. Another critical issue is membrane fouling, which is generally triggered by hydrophobic contaminants such as oil droplets [14–16]. Owing to strong hydrophobic-hydrophobic interactions, hydrophobic foulants can easily
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Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.memsci.2018.06.028 Received 18 May 2018; Received in revised form 14 June 2018; Accepted 16 June 2018
Available online 18 June 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.
attach to the hydrophobic membrane surface and wick into the membrane pores, preventing the transfer of vapor across the membrane [17]. To address these problems, novel membranes with unique wettabilities have been developed to maintain membrane durability and prevent membrane fouling. Superhydrophobic membrane was first created to maintain membrane durability and prevent membrane fouling, but their durability was still unsatisfactory due to the underwater oleophilicity of the superhydrophobic membranes [18–22]. Inspired by the features of marine creatures such as fish scales, clamshells and sharkskin, it has been found that a hydrophilic surface layer in combination with a hierarchical rough structure can effectively overcome oil fouling due to the preferential affinity for water [23–25]. Therefore, a Janus membrane with asymmetric superwettability may become a potential candidate for satisfying the contradictory requirements for simultaneous resistance to MD membrane fouling and maintainability of MD membrane durability. Recently, polymerization deposition and spray coating have become the two most common methods for fabricating asymmetrically wettable Janus membranes, but the breathability (water vapor transmission) of the MD membranes was sacrificed. For example, to ensure the hydrophilic monomer polymerized on one side of the hydrophobic membrane surface, the MD membrane must has narrow pore size to prevent
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hydrophobic membrane from being complete hydrophilization [26,27]. Therefore, this kind of membrane has seriously limited the transfer of vapor across the hydrophobic membrane. Meanwhile, the underwater oleophobic surfaces prepared via spray-coating chitosan-perfluorooctanoate/silica nanoparticles (CTS-PFO/SiNPs) on a hydrophobic microporous and nanofibrous polyvinylidene fluoride (PVDF) membrane were also reported [5,17]. However, the thick and dense hydrophilic skin layer extensively obstructed the membrane pores, causing the flux to drop sharply compared with that of the virgin hydrophobic membrane substrate. Moreover, the uneven thickness of the surface coating and the presence of defects may create breakthrough points for the entry of oily pollutants. Thus, a simple and universal method for constructing breathable and asymmetrically superwettable MD membranes for the efficient desalination of water matrices with complex compositions is required. In this work, we have designed and presented a sequential electrospraying method for fabricating a breathable and asymmetrically superwettable Janus skin layer integrated with electrospun hydrophobic PVDF nanofibrous membrane (NFM) to prevent membrane fouling and maintain membrane durability. The composite membrane has a hydrophobic PVDF fibrous membrane substrate with an interconnected pore structure, and the asymmetrically superwettable Janus skin layer exhibits underwater superoleophobicity and in-air superhydrophobicity respectively, which was achieved by simply tuning the concentration of the electrospraying solution containing hydrophilic or hydrophobic silica nanoparticles (SiO2 NPs, F-SiO2 NPs). The detailed procedures are shown in Fig. 1. It is worth noting that both the superhydrophobic and superhydrophilic composite layers have structures similar to that of lotus leaves with a hierarchical rough structure. Thus, the resultant Janus skin layer is able to form a water layer on the superhydrophilic surface and air pockets in the superhydrophobic surface deposited on the hydrophobic PVDF NFM [28,29]. Ultimately, the Janus membrane with excellent breathability displays a stable performance in the treatment of high-salinity water containing a high concentration of lubricating oil, which may be a potential candidate for oily wastewater treatment or the desalination of seawater and brackish water with complex compositions.
SpectrumChemical Co., Ltd., China. Polyvinylidene fluoride (PVDF, HSV 900) was purchased from Arkema Co., Ltd., China. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Mw: ~400,000), polystyrene (PS, Mw: ~350,000) and trimethoxy (heptadecafluorotetrahydrodecyl)-triethoxysilane (FAS, 97%) were purchased from Sigma-Aldrich. China. Silicon dioxide (SiO2, 7–40 nm) was purchased from Aladdin Chemical Regent Company. N,N-dimethylacetamide (DMAc), trichloromethane (CHCl3), sodium chloride (NaCl), sodium dodecylbenzenesulfonate (SDBS), and methylene blue (MeB) were purchased from Shanghai Runjie Chemical Reagent Company, China. Crude oil and soybean oil were obtained from the fifth Daqing Oil Production Factory and Jiusan Oils & Grains Industries Group Co., Ltd., respectively. Lubricating oil was purchased from Japan Mitsubishi Heavy Industries Co., Ltd. A commercial PVDF microporous membrane with an average pore diameter of 0.45 µm was purchased from Merck Millipore. All chemicals were used as received without further purification.
2. Materials and methods
Janus NFMs with asymmetric superwettability were prepared via electrospinning and sequential electrospraying techniques. First, necklace-structured PVDF membrane substrates were fabricated via electrospinning of a LiCl/PVDF solution with a solid content of 12 wt%. A high voltage of 30 kV and a feed rate of 3 mL h−1 in an enclosed space
2.2. Preparation of polymer solutions A PVDF solution was prepared by adding 6 g of PVDF powder into 44 g of LiCl/DMAc ionic liquid with vigorous stirring at 80 °C for 1 h followed by stirring at room temperature for another 12 h. The concentration of LiCl in the polymer solution was fixed at 0.1 wt%. A PVDF-HFP/PS hybrid polymer solution (4 wt%) containing superhydrophobic SiO2 NPs (6 wt%) was prepared as follows: First, 0.048 g of SiO2 NPs was dispersed in 19.2 g of DMAc with continuous stirring at room temperature for 30 min following by sonication at a power of 150 W for another 30 min. Second, 0.096 g of FAS was added to the above suspension with continuous stirring at 60 °C for 6 h. Finally, 0.4 g of PVDF-HFP and 0.4 g of PS were simultaneously added into the above prepared solution, which was stirred for 6 h. The procedures for preparing a PAN solution (4 wt%) containing 6 wt% SiO2 NPs were the same as that of the PVDF-HFP/PS solution (4 wt%) containing 6 wt% FSiO2 NPs. 2.3. Fabrication of nanofibrous Janus membranes
2.1. Materials Polyacrylonitrile
(PAN,
P1361)
was
obtained
from
Fig. 1. (a) Schematic representation of the fabrication of superhydrophobic SiO2 NPs. (b) Schematic illustration of the fabrication of the Janus skin layer with asymmetric superwettability on a necklace-structured PVDF NFM. (c) The proposed mechanism of the antifouling and antiwetting properties using an asymmetrically superwettable Janus skin layer and a plausible mechanism of vapor permeation across the porous membrane. 603
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affected the mechanical properties of the membrane substrate (2.74 MPa). The poor durability of this membrane substrate indicated that it cannot withstand the squeezing that occurs when the substrate is placed in the middle of the two membranous chambers. For the purpose of maintaining the rough fiber structure and improving the mechanical strength of the fibrous membrane, LiCl was added to the polymer solution to regulate the fiber structure because a polymer solution with a higher conductivity can induce the stretching effect and lead to more intense whipping during electrospinning [34,35]. As shown in Fig. 2b, the PVDF/LiCl fibrous membrane containing 0.1 wt% LiCl possessed thinner fibers with an average diameter of 385 nm (Fig. 2g), and no drops of solution were sprayed on the fibrous membrane surface. Significantly, the necklace-structured fibers with a rough surface can help the membrane substrate to improve the MD membrane stability. By carefully examining an enlarged FE-SEM image of a lotus leaf (inset of Fig. 2a), the unique nanostructures of the lotus leaf can also be obviously observed, demonstrating that the self-cleaning effect of the lotus leaf depends not only on the first-order rough structure but also on a secondary rough structure. To introduce unique nano/microstructures like those of the lotus leaf in the Janus skin layer, a sequential electrospraying technique was used to fabricate a functional Janus skin layer with asymmetric superwettability. Dilute PVDF-HFP/PS (4 wt%, 1/1) and PAN solutions (4 wt%) containing 6 wt% SiO2 NPs were prepared to fabricate the hierarchically structured Janus skin layer. The formation of this unique structure was attributed to competition between the electric force and the surface tension of the dilute solution in the presence of a strong electric field [36,37]. The details of screening the concentrations of polymers and SiO2 NPs are shown in Fig. S2-S6. FE-SEM images of the optimal superhydrophobic surface and superhydrophilic surface are shown in Fig. 2(c and d), revealing that all the microspheres (diameters ranging from 0.1 to 1.9 µm, Fig. 2g) are attached to ultrathin fibers (average diameter of 36 nm). Generally, the SiO2 NPs were used as a template to impart nanoscale roughness on the microsphere surface because the SiO2 NPs can lead to shrinkage of the polymer droplets during the evaporation of the solvent (Fig. S7) [28]. The multiscale rough structures of the Janus skin layer were consistent with a lotus leaf-like structure. A cross-sectional image of the Janus membrane and an enlarged image of the top layer are shown in Fig. 2(e and f) and demonstrate that the interconnected pore-structure of the membrane was assembled through the layer-by-layer accumulation of fibers. Meanwhile, the Janus skin layer with a thickness of 15 µm was strongly adhered to the membrane substrate owing to the unevaporated solvent in the microspheres and the strong electrostatic forces present during the electrospraying process [38,39]. In the meantime, the sequential electrospraying of a Janus skin layer with microsphere-on-string-structured fibers can affect the pore structure of the membrane substrate. As seen in Fig. 2h, the necklacestructured PVDF NFM exhibits an irregular pore structure with a pore size distribution ranging from 1.33 to 2.23 µm (average pore size of 1.87 µm), which is attributed to the accumulation of fibers with varying diameters. However, after the F-SiO2 @PVDF-HFP solution was sprayed onto the membrane substrate, the average pore size decreased substantially to 1.52 µm owing to the densely accumulated micropore-onstring-structured fibers deposited on the membrane substrate. Further electrospraying of the SiO2 @PAN solution onto the composite membrane slightly decreased the average pore size to 1.45 µm because of the overlapping coverage of the microspheres. It is worth noting that the microsphere-on-string-structured fibers did not plug the pores of the membrane substrate because the average pore size of the composite membrane was not significantly reduced, guaranteeing that vapor can diffuse across the membrane substrate without obstruction. The mechanical property, an important factor in determining the practical application of a membrane, is notably influenced by the membrane structure. Fig. 2i demonstrates typical tensile stress-strain curves of the PVDF-based fibrous membranes, where all the samples exhibited nonlinear elastic deformation in the first region (0–2.8%)
with a humidity of 65 ± 2% were used to generate the continuous jet stream. Second, the Janus skin layer with asymmetric superwettability was prepared via a sequential electrospraying technique using the above prepared F-SiO2 @PVDF-HFP/PS and SiO2 @PAN solutions. The generated superhydrophobic and superhydrophilic microsphere-onstring-structured fibers were continuously deposited on the surface of the as-prepared PVDF NFM. During the electrospraying process, a high voltage of 25 kV and a feed rate of 0.5 mL h−1 in an enclosed space with a humidity of 40 ± 2% were used to generate the microsphere-onstring-structured fibers. The work distance between the tip of the needle and the collector was fixed at 20 cm in an enclosed space with a constant temperature of 23 ± 1 °C. Finally, the resultant Janus membrane was dried at 60 °C under vacuum for 3 h to remove the residual solvent. 2.4. Characterization The morphology of the fibers was observed by a field emission scanning electron microscope (FE-SEM) (Zeiss, Sigma 500, Germany) and a transmission electron microscope (TEM) (JEM-2100F, JEOL Ltd.). The pore size distribution was characterized via a bubble point method using a capillary flow porometer (CFP-1100ai, Porous Materials Inc., USA). The mechanical properties of the relevant samples were examined with a tensile tester (XQ-1C, Shanghai New Fiber Instrument Co., Ltd., China). The liquid contact angles were measured using a contact angle goniometer (Kino SL200B). The liquid entry pressure was measured using a vacuum pump with different vacuum pressures. 2.5. MD Performance Tests The MD performance of relevant membranes was evaluated in a labscale direct contact membrane distillation (DCMD) system with an effective surface area of 16 cm2. The feed solution (3.5 wt% NaCl) fixed at 60 °C and deionized (DI) water fixed at 20 °C were circulated with peristaltic pumps. To evaluate the durability and fouling resistances of the relevant membranes, An oily saline feed solution was then prepared by mixing 1 g of lubricating oil and 1 L of NaCl (3.5 wt%) aqueous solution with vigorous stirring at 2200 rpm for 1 h. The resultant oil-insaline water emulsion was stable with no obvious demulsification for over 48 h. 3. Results and discussion 3.1. Surface and structural characterization of the Janus Membrane The objective of this paper was to obtain a breathable MD membrane with antiwetting and antifouling properties for application in the desalination of challenging saline water matrixes containing low-surface-tension contaminants. We designed an asymmetrically superwettable Janus membrane based on three criteria: (1) the asymmetrically superwettable Janus skin layer must be easily assembled on a composite membrane with an open-cell structure; (2) the Janus membrane must have the capacity to completely demulsify oil-in-saline water emulsions and repel hot water to simultaneously prevent membrane wetting and fouling; (3) the Janus membrane must have robust mechanical strength and durability for application. The first basic requirement was satisfied by the versatile, readily accessible electrospinning and sequential electrospraying methods applied in the fabrication of the fibrous membrane with an interconnected pore structure [30–33]; these methods involved the electrospinning of a PVDF/LiCl ionic liquid and sequential electrospraying of F-SiO2 @PVDF-HFP/PS and SiO2 @PAN solutions. First, inspired by the hierarchical structure of lotus leaves (Fig. 2a), a PVDF solution with a low concentration of 12 wt% was used to electrospun thin fibers with a rough surface imparted by the unstable whipping of the solution jet during electrospinning. As shown in Fig. S1, it is obviously observed that the entangled fibers with a microsized beads-on-string structure strongly 604
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Fig. 2. Morphology, pore structure and mechanical strength of the Janus membrane. (a) Photograph and enlarged FE-SEM images of a lotus leaf. (b) FE-SEM image of the necklace-structured PVDF NFM substrate. FE-SEM images of the Janus skin layer with asymmetric superwettability: (c) the superhydrophobic layer and (d) the superhydrophilic layer. (e) Cross-sectional image and (f) enlarged image of the top layer of the Janus membrane. (g) Fiber and hierarchical microsphere diameter distributions of the corresponding Janus membrane. (h) Pore size distribution and (i) mechanical strength of the Janus membrane at each stage of fabrication.
string-structured fibers due to the unstable stretching of the PVDF solutions during electrospinning (Fig. S2). In 2004, Jiang et al. reported that a dilute PS/DMF solution (5 wt%) was used to fabricate a microparticle film with particle diameters ranging from 2 to 7 µm, and this film exhibited a superhydrophobic surface with a contact angle of 160.4° [43]. Unfortunately, the poor durability of the microparticle film seriously limited its application. In addition, PS fibers with a low packing density that are fabricated from a high-concentration solution are also difficult to use as a free-standing membrane for separation [37]. Thus, hydrophobic PVDF-HFP was added into the PS solution to fabricate stabilized microsphere-on-string-structured fibers supported on the PVDF NFM with a certain wear resistance. To simulate the multiscale rough structure of a lotus leaf, various concentrations of SiO2 NPs were added to the PVDF-HFP/PS solution (4 wt%) to construct multiscale roughness in the membrane. As shown in Fig. 3(a and b), the water contact angle (WCA) of the F-SiO2 @PVDFHFP/PS skin layer significantly increased from 152° to 164°, demonstrating an obvious increase in the WCA with increasing SiO2 NP concentration. However, a reverse trend in the water wetting behavior occurred as the SiO2 NP concentration further increased to 8 wt% (156°) due to the aggregation of the SiO2 NPs [44]. This result was consistent with the experimental FE-SEM image results shown in Fig. S4. Furthermore, the construction of multiscale roughness on the hydrophilic membrane was also performed for the underwater oleophobic surface since the trapped air in the membrane pores was replaced by
under a small stress and then large elongation until breakage. Generally, the tensile strength is proportional to the thickness of a membrane [40]. Owing to the unique 3-dimensional web structure of fibrous membrane, the increasing thickness can decrease the maximum size of the inter-fiber space, thus more fibers can withstand higher mechanical strength. The Janus skin layer significantly increased the membrane thickness from 52 ± 3 to 66 ± 3 µm, but the microsphere-on-stringstructured fibers could not withstand the external stress, thus resulting in a continuous decrease in the membrane strength from 7.65 to 4.81 MPa. The relevant parameters of the fabricated Janus membrane and commercial PVDF membrane are listed in Table S1.
3.2. Wetting Properties of the Janus Membrane Generally, lotus plants, “which remain unsullied by mud and wash clean as if by magic” were praised by Chinese ancients as “gentlemen's flowers”. The lotus also symbolizes kind and honest people who faithfully fulfill their corporate responsibilities. Inspired by the hierarchically rough structure and self-cleaning performance of lotus leave (Fig. 2a), Wenzel and Cassie demonstrated that the establishment of nano/microscale structures was essential for improving the selectivity of a membrane [41,42]. The asymmetrically superwettable Janus skin layer was examined by contact angle and tensiometer-based oil probe experiments, as shown in Fig. 3. The electrospraying of a low concentration of PVDF was ineffective for the formation of microsphere-on605
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Fig. 3. Wetting properties of the Janus membrane. (a) WCAs of the superhydrophobic PVDF-HFP/PS skin layer and (b) UOCAs of the superhydrophilic PAN skin layer containing various concentrations of SiO2 NPs. The concentrations of both the PVDF-HFP/PS and PAN solutions employed for fabricating the Janus skin layers were fixed at 4 wt%. (c) Photographs of the dynamic measurements of water adsorption, underwater oil repulsion and in-air water repulsion. (d) Real-time-recorded force-distance curves for the optimal underwater superoleophobic surface (top Janus layer).
Fig. 3d. Based on the real-time-recorded force-distance curves, the top Janus skin layer displayed an extremely low adhesion force of approximately 22.92 μN, and no force drop was observed in the corresponding receding curve, demonstrating the extremely low adhesion of oil. The liquid entry pressure of Janus membrane was tested by hydrostatic pressure. According to Young-Laplace equation, the hydrostatic pressure was associated with the liquid surface tension (γliquid), liquid contact angle on the membrane surface (θadv) and shape of membrane pores, the equation is shown in below:
water. As shown in Fig. 3b, PAN microspheres (produced by electrospinning a 4 wt% PAN solution) containing 6 wt% SiO2 NPs displayed the optimal underwater superoleophobicity with an underwater oil contact angle (UOCA) of 164°. The change in the WCA of the hydrophobic membrane caused by changing the SiO2 NP concentration was consistent with the change in the UOCA of the hydrophilic membrane. To further investigate the dynamic wetting behavior of liquids on the asymmetrically superwettable Janus membrane surface, a highspeed camera was used to record the oil adhesion and water permeation processes on the top Janus skin layer and the water repelling ability of the bottom Janus skin layer. To avoid the measurement of superhydrophilic layer being affected by the bottom superhydrophobic skin layer of the Janus membrane, the superhydrophilic layer was sprayed on a hydrophilic PAN fibrous membrane for measurement of the dynamic wetting behavior. Fig. 3c (1) gives photographs of a water droplet (3 mL) in contact with the top skin layer of the Janus membrane. The water droplet spread quickly, and a WCA of 0° was achieved in a short period of 1.5 s, revealing the excellent water permeability of the superhydrophilic skin layer. Interestingly, after the membrane was wetted by water, the surface of the membrane suddenly exhibited underwater superoleophobicity because the infiltrated water with a high adhesion force of ~145 mN was difficult to replace with oil (CHCl3, ~27 mN) [45,46]. Significantly, despite the oil droplet being forced onto the membrane surface under strong pressure, the oil droplet left the membrane surface without undergoing deformation, demonstrating the robust antifouling performance (Fig. 3c (2)). Moreover, the bottom superhydrophobic layer of the Janus membrane also played a leading role in preventing the membrane from being wetted during the membrane distillation process, exhibiting a superior water-repelling ability, as shown in Fig. 3a. A water droplet was forced to come into sufficient contact with the superhydrophobic Janus skin layer and exhibited obvious deformation, and then the water droplet easily left the membrane surface without deformation owing to the air trapped in the hierarchically structured membrane and the low surface energies of PS (hydrophobic CH groups) and PVDF-HFP (hydrophobic CF2 groups), as shown in Fig. 3c (3). To better understand the oil repellency of the membrane in a complex underwater environment, the adhesion forces during the contact procedure were dynamically measured, as shown in
Hydrostatic pressure = −
4γwater . cosθadv dmax
Here, the γwater = 72.58 mN/m, θadv = 166°, dmax = 2.07 µm, the hydrostatic pressure of Janus membrane calculated from YoungLaplace equation was 136.08 kPa, which is far higher than that of the practical hydrostatic pressure of 95.52 kPa owing to the irregular aperture structure and asymmetrically hydrophobic membrane surface. The results demonstrate that the Janus membrane has excellent liquid entry pressure for durable and long-term membrane distillation. Saline water is generally polluted with domestic sewage and industrial wastewater; thus, the membrane should also possess the ability to isolate pollutants and prevent the membrane from being fouled. Comparing the wetting properties of the superhydrophobic and superhydrophilic skin layers of the Janus membrane, water droplets containing a wide range of contaminants with different surface tensions and different water qualities were measured to evaluate the membrane stability (Fig. 4a). It was exciting to observe that the bottom Janus skin layer was resistant to wetting by low-surface-tension water droplets and different water qualities (CA＞158°), as shown in Fig. 4b. Similarly, the necklace-structured PVDF NFM used as the membrane substrate also exhibited hydrophobicity toward various liquids. Even though the isolated top layer of the Janus skin exhibited superhydrophilicity with a WCA of 0°, as mentioned above (Fig. 3c (1)), when a superhydrophilic layer a few microns thick was deposited onto the superhydrophobic layer, the liquid contact angle sharply increased. This change could be attributed to the fact that the water droplet was comparatively more difficult to absorb by the ultrathin superhydrophilic layer. In contrast, 606
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Fig. 4. (a) Photographs of different liquid droplets deposited on the bottom and top layers of the Janus skin in air and under water, respectively. (b) In-air sessile drop CAs for three different surfaces with nine liquids (left of the dashed line) and UOCAs for three different surfaces with CHCl3 (right of the dashed line).
when the superhydrophilic or superhydrophobic side of the Janus skin was exposed to air, oil droplets tend to attach onto the hydrophobic membrane surface and possibly wick into the pores. After the membrane was immersed in water, the superhydrophilic skin layer immediately became underwater superoleophobic with an OCA (CHCl3) of 166°. This significant shift in the membrane wettability could be ascribed to the fact that the membrane surface was completely covered by the infiltrated water, thus avoiding direct contact between the oil droplets and the membrane surface. Conversely, the hydrophobic membrane substrate and the superhydrophobic layer of the Janus membrane displayed low UOCAs of 32 and 25°, respectively, due to the strong interactions between the oil droplet and the low-surface-energy materials that occurred under water [47,48]. As a result, the novel Janus membrane with asymmetric superwettable skin layer in combination with hydrophobic PVDF membrane substrate could prevent the membrane from being fouled and maintain membrane durability, thus providing robust stability for MD applications.
or the asymmetrically superwettable Janus skin layer was sprayed on the membrane substrate, both membranes were able to sustain a stable MD performance with a high water flux and perfect salt rejection. These results demonstrated that the upward capillary force of the superhydrophobic layer preserves a metastable Cassie-Baxter state (liquid-air interface) that prevents the membrane from being wetted [11,20,49]. In addition, it was clearly observed that the flux of the Janus membrane (28.16 L m−2 h−1) was slightly lower than those of the superhydrophobic PVDF NFM (29.01 L m−2 h−1) and the hydrophobic PVDF NFM (29.52 L m−2 h−1). This difference was attributed to the fact that the increase in the thickness of the MD membrane slightly increased the difficulty of vapor transmission [50]. The hydrophilic surface with an interconnected pore structure did not impede vapor transmission because the evaporation of water occurred on the hydrophobic membrane surface. In contrast, the commercial hydrophobic PVDF membrane exhibited a comparatively low water flux of 16.84 L m−2 h−1 due to its closed pore structure.
3.3. Durability of Relevant Membranes
3.4. Fouling resistance of relevant membranes
To better explain the importance of the lotus-leaf-structured membrane surface, the durability of membranes with different structures were evaluated under the same experimental conditions. Fig. 5(a and b) shows the time-dependent flux and durability of fabricated and commercial PVDF membranes. The hot feed side and cold side temperatures were fixed at 60 and 20 °C, respectively, cycling through a flat-sheet membrane cell. It was observed that the as-prepared PVDF membrane substrate maintained a high water flux of over 28 L m−2 h−1, but the conductivity continuously increased to 75 μS/cm after 48 h of operation. Though the unique necklace-structured PVDF nanofibers ensured that the membrane had a comparatively robust durability, a declining trend in the rejection of salt over time was clearly observed. In comparison, after the superhydrophobic F-SiO2 @PVDF-HFP/PS skin layer
To investigate the membrane fouling performance, a surfactantstabilized oil-in-saline water emulsion was used as the feedwater in MD experiments (inset of Fig. 6a). Oil droplets with an average particle size of 4.52 µm were dispersed in saline water, as shown in Fig. S9. The commercial hydrophobic PVDF microporous membrane and the electrospun superhydrophobic PVDF NFM were rapidly fouled in 30 min, resulting in a sharp decrease in the water flux and a substantial increase in the conductivity on the distilled water side, as shown in Fig. 6(a and b). The extensive membrane fouling was caused by the attachment and accumulation of oil droplets on the membrane surface, which ultimately blocked the hydrophobic membrane pores, as shown in Fig. 6c. In comparison, the Janus membrane exhibited a stable water flux of 25.42 L m−2 h−1 and a high salt rejection ratio of 100% after 30 h of
Fig. 5. Wetting resistance and durability of the Janus membrane compared with those of relevant membranes. (a) Water flux and (b) permeate conductivity. 607
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Fig. 6. Fouling resistance and durability of the Janus membrane compared with those of relevant membranes. (a) Water flux and (b) permeate conductivity. (c) Photographs of the relevant membranes after the fouling experiments. Optical microscopy images of the oil-in-saline water emulsion (d) before and (e) after distillation. (f) UV–vis spectra of the as-prepared oily saline water before and after distillation.
membrane with an underwater superoleophobic skin layer (164°) and in-air superhydrophobic skin layer (166°) is applicable as an MD membrane for desalinating challenging oil-in-saline water emulsions, which displayed a high water flux of over 25 L m−2 h−1, a robust oil resistance and an excellent durability in direct contact MD (ΔT = 40 °C) during the treatment of oil-in-saline water emulsion. This versatile method of constructing membranes with superwettable skins or asymmetrically superwettable Janus skins is expected to be applicable to a wide range of other selective separation fields.
operation. The outstanding antifouling properties are attributed to the underwater superoleophobic surface and the extremely low oil adhesion force of the superhydrophilic Janus skin layer. Meanwhile, the small oil droplets came into contact with the superhydrophilic membrane surface, they quickly aggregated to generate larger oil droplets, which then left the membrane surface and floated to the water surface according to Stokes' law of resistance [51]. Optical microscopy was also used to observe the changes in the oil-in-saline water before and after distillation, as shown in Fig. 6(d and e). The as-prepared oil-in-saline water emulsion exhibits numerous oil droplets in the image. However, after distillation, no oil droplets could be observed in the collected filtrate, implying the high efficiency of the hierarchically structured Janus membrane for separating oil droplets and preventing membrane from be fouled by oil droplets. Meanwhile, UV–vis spectroscopy was also used to measure the collected filtrate and the original milky white feed emulsion as shown in Fig. 6f. The collected filtrate shows flat line as compared with the oil-in-saline water emulsion with a strong adsorption peak at 303 nm, implying that the Janus membrane can totally isolate the oils from the oil-in-saline water emulsion.
Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (Grant no. 51573034), National Science and Technology Major Project (No. 2017ZX07501-002-01), the State Key Laboratory of Urban Water Resource and Environment in HIT of China (No. 2016DX02), Postdoctoral Science Foundation of Heilongjiang Prov. (No. LBH-TZ0606 and LBHQ16012), Scientific Research Foundation for Returned Scholars of Heilongjiang Prov. (No. LC2017023) and the Fundamental Research Funds for the Central Universities of China.
4. Conclusion
Appendix A. Supplementary material
The recent development of novel membranes with unique structure has great significance in the desalination of challenging oil-in-saline water emulsions. Thus, this process requires the fabrication of a breathable and asymmetrically superwettable Janus membrane to achieve concurrent breakthroughs in the membrane durability, fouling and water flux. Traditional methods such as polymerization deposition and spray coating are not feasible for conventional membranes due to their uncontrollable operation and extensive membrane pore blocking. This study showcases a versatile sequential electrospraying method that involves the simple tuning of the as-prepared solution to fabricate a breathable and asymmetrically superwettable Janus skin layer. By imitating the nano/microstructures of lotus leaves, the Janus
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