English

• Aqueous oil separation is critical to modern society since the aqueous oil produced from domestic activities, modern chemical industrial process as well as marine oil spills cause a colossal waste of energy and take heavy tolls on the global environment [1-3]. Traditional methods for aqueous oil separation include absorption, air flotation, coalescence filtration, and vacuum heating dehydration, but most of them not only suffer from inferior positions of large equipment, low separation efficiency and time-consuming but also fail to treat the emulsified aqueous oil [4-7]. Membranebased for aqueous oil separation technology, working on the bases of pore interception such as microfiltration, ultrafiltration, and nanofiltration have been developed for oily water treatment, but the high drive force generally leads to severe membrane fouling, reducing membrane separation flux and impairing membrane durability [8-10].

  To address the issues mentioned above, macroporous membrane working on its selective wettability has been extensively studied for aqueous oil separation [11-14]. The characteristics of high efficiency, low cost as well as simple operation are pursuing goals that oil/water separation membranes required. Generally, the membrane separation flux is profoundly affected by membrane pore structure [15]. The traditional phase-inversion membrane with special surface wettability was first employed to fabricate selective superwetting membranes to separate aqueous oil. However, the closed pore structured membranes with high mass transfer resistance should operate at a comparatively high pressure, thus giving the water a chance to wet the membrane pores [3, 16, 17]. Electrospinning, a facile technology to fabricate 3-dimensional interconnected pore structured membrane by the layer-by-layer accumulation of fibers, has been demonstrated for various separation applications [18-21]. What is more, the membrane pore structure and density can also be precisely controlled by simply regulating the compositions of electrospinning solutions, machining parameters, and environmental conditions [22, 23].

  However, the electrospun fibrous membranes are generally suffered from low mechanical strength and extremely low bulk density owing to its simple accumulation of fibers without any interaction forces. The poor mechanical strength, to some extent, limited its practical applications [24]. Recently, chemical and physical crosslinking technologies were flourishing developed to obtain high strength electrospun fibrous membranes [25, 26]. On one hand, whether chemical or physical crosslinking, the crosslinked fibrous membrane becomes brittle that cannot even withstand the gravity of the aqueous oil; On the other hand, the crosslinking process, to some extent, can also block some membrane pores, which also greatly reduced the membrane permeability and increased the membrane mass transfer resistance [27-29].

  In this contribution, we designed a superhydrophobic and superoleophilic biomimetic fibrous membrane with robust mechanical strength for aqueous oil separation. The design criteria are based on three criteria: (1) The membrane must have robust mechanical strength for practical application; (2) the membrane must have fine fiber, adequate pore size and interconnected pores for low mass transfer resistance; (3) the membrane must have excellent selective wettability for high separation efficiency of aqueous oil. Based on the first two criteria, polysulfonamide (PSA), owing to its unique sulfone groups in the main chain that enhances the electron conjugation of benzene rings with NHCO groups, is considered as a specialized engineering plastic with superior thermal property, excellent chemical resistance and high mechanical properties [30, 31]. Although the single-component PSA fibrous membrane (FM) fabricated by electrospinning has the advantages of high porosity and interconnected pores, the PSA FM can be easily destroyed by the small droplets because of its poor spinnability and unstable Taylor corn during electrospinning process [32]. Polyacrylonitrile (PAN), one of the excellent spinnability polymers, was added as an assistant polymer to help the formation of PSA fibers [33, 34].

  The representative scanning electron microscope (SEM) images of PSA/PAN FMs fabricated from various weight ratios of PSA/PAN are shown in Figs. 1a-e, revealing the randomly oriented 3-dimensional non-woven structured fibrous membrane with high aspect ratio and interconnected pores for low mass transfer resistance. However, it is observed that elliptical beads with an average length of 5-10 μm have appeared along the PSA/PAN (9/1) fiber axis. This phenomenon could be attributed to that the low polymer concentration and weak interaction between PSA and PAN chains induced the instable whipping of solution jet during electrospinning [35, 36]. After the PAN ratio in the PSA/PAN solution was increased to 1/1, the beads on the fiber axis are gradually disappeared and the fiber diameter was increasingly improved from 252 nm to 388 nm as shown in Fig. S1 (Supporting information). Interestingly, it could be found that many welding points amongst the adjacent fibers could be observed from the dotted line and the inset, as shown in Fig. 1c. This unique welding pore structure is possibly caused by that the solvent in the dispersion phase (PSA/DMAC) encapsulated in the mother phase (PSA/DMAC) should overcome the double handicap to volatilize during the electro-stretching process. Thus, a small amount of residual solvent on the fiber surface facilitates the formation of welding pores amongst the adjacent fibers [32]. Further increasing the PAN ratio in the PSA/PAN hybrid solution from 1/1 to 0/1 would increase the PSA/PAN fiber diameter from 388 nm to 445 nm (Fig. S1). The membrane mechanical property is considered as one of the most important factors affecting the practical membrane application [37, 38]. The mechanical strength of PSA/PAN FMs would be notably influenced by PSA/PAN ratio as shown in Fig. 1f. It could be found that the PSA/PAN FM with an average thickness of 60 ± 2 μm fabricated from PSA/PAN ratio of 1/1 displayed optimized mechanical strength of about 13.50 MPa and elasticity modulus of about 1.06 MPa, which not only indicate the mechanical robustness of PSA/PAN FM but also indirectly verify the formation of welding points between the PSA/PAN fibers.

  Figure 1

  

  Figure 1.  SEM images of PSA/PAN FMs fabricated from various weight ratios of PSA/PAN (a) 9/1, (b) 3/1, (c) 1/1, (d) 1/3 and (e) 0/1 (the polymer concentration was fixed at 12 wt%). (f) Stress-strain curves of PSA/PAN FMs fabricated from various weight ratios.

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  To meet the third design criterion, the membrane must have excellent selective wettability for the high separation efficiency of aqueous oil. According to the classic Wenzel and Cassie-Baxter modes, the establishment of nano/microstructures on the membrane surface could effectively improve the membrane surface wettability from hydrophobicity to superhydrophobicity and hydrophilicity to superhydrophilicity depending on the membrane surface wettability [39-41]. As a result, the PSA/PSN FM with optimized mechanical strength was then decorated with α-Fe₂O₃ nanowires onto the fiber surface to create membrane surface roughness and grafted with 1H, 1H, 2H, 2H-perfluorododecyltrichlorosilane (FTCS) to lower membrane surface free energy. The biomimetic fibrous membrane with superhydrophobicity and superoleophilicity fabricated from various concentrations of Fe(NO₃)₃·9H₂O/urea (creating membrane roughness) and various concentrations of FTCS (lowering membrane surface free energy) was denoted as F_x-Fe_y@PSA/PAN, where x stands for the concentration of Fe(NO₃)₃·9H₂O/urea and y stands for the concentration of FTCS, respectively. The synthesis pathways and the surface morphology/optical photograph views of the fibrous membrane fabricated at each stage are schematically illustrated in Fig. 2. The as-prepared PSA/PAN FM demonstrated a smooth fiber surface.

  Figure 2

  

  Figure 2.  (a) Schematic illustrating the synthesis pathways of the superhydrophobic and superoleophilic fibrous membrane. SEM/optical photographs view of (b) the asprepared PSA/PAN FM, (c) the Fe_1.6@PSA/PAN FM and (d) the F₁-Fe_1.6@PSA/PAN FM.

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  After the PSA/PAN FM was synthesized at various concentrations of Fe(NO₃)₃·9H₂O/urea, all the PSA/PAN fiber surfaces displayed a microscale roughness, and the Fe based crystals were facilely positioned on the fiber surface as shown in Fig. S2 (Supporting information). Interestingly, when the PSA/PAN fiber was hydrothermally synthesized at a low Fe(NO₃)₃·9H₂O/urea concentration (0.8 wt% and 1.6 wt%), the α-Fe₂O₃ nanowire was successfully anchored along the PSA/PAN fiber axis (Figs. S2a and b). However, it is worth to note that when the Fe(NO₃)₃·9H₂O/urea concentration was increased to 2.4 wt% and 4.8 wt%, the membrane roughness was not always improved with increasing the mother liquid concentration. The reason could be attributed to that the nanowires synthesized at higher mother liquid are more easily to aggregate and form the nanoparticles on the fiber surface, as shown in Figs. S2c and d.

  The crystal structure, surface chemical composition and pore structure of the virgin and modified membrane were also characterized by XRD, XPS, and BET as shown in Fig. S3 (Supporting information). XRD patterns reveal the amorphous structure of PSA/PAN FM. With the attachment of Fe oxides, the relative intensities and positions of diffraction peaks are in good agreement with the standard XRD data for the α-Fe₂O₃ phase (JCPDS No. 33-0664) [42, 43]. After fluorination, the chemical compositions of the hierarchically rough fibrous membrane were also detected by XPS. The essential C, N, O, S and Fe were surveyed by scanning bonding energy from 0 to 1200 eV. An intensity peak strengthened at 688.56 eV for F₁-Fe_1.6@PSA/PAN FM was consisted with the F element. This result illustrated that the FTCS was successfully decorated on the hydroxylated Fe_1.6@PSA/PAN fiber surface via well-established silane chemistry [22, 44]. The α-Fe₂O₃ nanowire created the virgin PSA/PAN FM with a hierarchical rough fiber structure, thus significantly increasing the membrane specific surface area. N₂ ad/desorption isotherms and the calculated pore size distribution of relevant fibrous membranes are investigated in Figs. S3c and d. Both the isotherms could be categorized as typicalIV isotherms including monolayer adsorption, multilayer adsorption and capillary condensation, demonstrating the mesoporous structure of the relevant fibrous membranes [45, 46]. The narrow H1 hysteresis loop in the region of P/P₀ > 0.9 also indicated that the mesopores are open with a capillary condensation phenomenon. Significantly, the calculated specific surface area of F₁-Fe_1.6@PSA/PAN FM was dramatically increased from 3.42 m²/g to 32.2 m²/g, indicating the significant contribution of the α-Fe₂O₃ nanowires. Meanwhile, Fig. S3d also indicates that the F₁-Fe_1.6@PSA/PAN FM displayed a typical mesoporous feature of size in the range from 10 nm to 200 nm and centered at 125 nm.

  Water contact angle (WCA) of the fluorinated PSA/PAN FM with and without roughness construction is shown in Fig. 3a. The WCA of the Fe@PSA/PAN FM was sustainably increased as the FTCS concentration was increased from 0 to 1 wt%. However, further increasing the FTCS concentration would not affect the membrane hydrophobicity. The α-Fe₂O₃ nanowires decorated on the PSA/PAN FM surface exhibited superhydrophobicity with a WCA of 156°, which was much higher than that decorated by α-Fe₂O₃ nanoparticles with a WCA of 148°. The reason could be explained by that the air in the rough membrane pores would be maximized to avoid the direct contact between the membrane surface and the water droplet, as schematically demonstrated in Fig. 3b. Interestingly, both the membrane surfaces exhibited a reverse transition wettability with oil contact angles of 0° due to their advantages of the oleophilic and the rough membrane surface. To investigate the dynamic water-repelling and oil permeation performances of the F₁-Fe_1.6@PSA/PAN FM surface in air, a drop of water (3 μL) was placed and evaporated on the F₁@PSA/PAN and the F₁-Fe_1.6@PSA/PAN FM surfaces, respectively. The WCA of the F₁-Fe_1.6@PSA/PAN FM surfaces was slightly decreased from 156° to 152° as the contact time increased to 60 min, revealing the low pinning of the water droplet to the hierarchically structured membrane surface. Alternatively, the WCA of the fluorinated PSA/PAN FM surface without roughness construction was continuously decreased from 125° to 102° due to the incursion of water into the voids of fibrous membranes pores. The results and relevant photographs are shown in Fig. 3c. A high-speed camera also captured the adhesion ability of the water droplet and the oil droplet on the hierarchically rough membrane surface. As shown in Fig. 3d, a water droplet of 3 μL was forced to contact the membrane surface, and the water droplet could easily depart from the F₁-Fe_1.6@PSA/PAN FM surface without visible deformation even operated at a high preload pressure. On the contrary, when an oil droplet (trichloromethane, 3 μL) was contacted with the membrane surface, the oil droplet could be quickly absorbed by the porous membrane within 60 ms. The results illustrated that the F₁-Fe_1.6@PSA/PAN FM shows outstanding superhydrophobicity and superoleophilicity for potential application in aqueous oil separation.

  Figure 3

  

  Figure 3.  (a) WCA of the fluorinated PSA/PAN FM with and without roughness construction. (b) WCA and oil contact angle (OCA) of the F₁-Fe_1.6@PSA/PAN FM and the F₁-Fe_1.6@PSA/PAN FM and relevant schematic presentation. (c) WCA of the F₁-Fe_1.6@PSA/PAN FM and the F₁@PSA/PAN FM as a function of time. (d) Photographs of the dynamic water-repelling and oil permeation on the F₁-Fe_1.6@PSA/PAN FM surface in air.

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  The separation performances of the F₁-Fe_1.6@PSA/PAN FM for aqueous oil separation was conducted on a vacuum suction filter device, as shown in Fig. 4a. The F₁-Fe_1.6@PSA/PAN FM was fixed in the middle of the device, 200 mL of water-in-oil emulsion stabilized by Tween-80 (0.2 g of Tween-80, 2 g of water and 198 g of oil) was poured onto the filtration cell, the oil quickly permeated and passed through the membrane because of its superhydrophobicity and superoleophilicity as well as high porosity. After separation, the oil was reached to the conical flask by gravity, the dyed water was retained onto the membrane surface due to its superhydrophobicity. Herein, the methylene blue was used to dye the water, which can easily identify the separation effect of aqueous oil. No external driving force except gravity was used during the separation process. The optical microscopic image of the collected filtrate shown in Fig. 4b revealed that no droplet was observed in the full view of the image, indicating the high separation efficiency of our resultant F₁-Fe_1.6@PSA/PAN FM. Significantly, the membrane exhibited a promising flux of 16, 025 and 7323 L m^-2 h^-1 for oil/water mixture and water-inoil microemulsion separation without apparent flux decrease after 10 cycles separation (Figs. 4c and d), revealing the excellent antifouling performance for long term usage. The experimental results are competitive as compared with the literature results as shown in Table S1 (Supporting information). Moreover, benefiting from the high-temperature resistance of PSA, the hybrid fibrous membranes also exhibited remarkable thermal stability with a high WCA over 140° even annealing treatment at 250 ℃ for 10 min (Fig. 4e), implying the membrane also has the potential application in harsh conditions.

  Figure 4

  

  Figure 4.  (a) Schematic diagram showing the separation device for aqueous oil separation using F₁-Fe_1.6@PSA/PAN FM. (b) The microscopic image of water-in-oil microemulsion before and after separation. Changes of flux with increasing separation cycles for (c) oil/water mixture and (d) water-in-oil microemulsion separation. (e) The WCAs of F₁-Fe_1.6@PSA/PAN FM after annealing treatment in air at different temperatures.

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  In summary, a mechanically durable biomimetic fibrous membrane with superhydrophobicity and superoleophilicity was successfully developed for aqueous oil separation. The composite membrane was developed via emulsion electrospinning, in situ synthesis of α-Fe₂O₃ nanowires as well as fluorination processes. Benefiting from the prominent selectivity, high porosity and interconnected pores of the electrospinning fibrous membrane, the membrane after roughness construction and fluorination exhibited a high separation flux of 16, 025 and 7323 L m^-2 h^-1 for oil/water mixture and water-in-oil microemulsion separation as well as prominent antifouling performance. Meanwhile, during the electrospinning process, the synergistic effect between the polymers of PSA and PAN empowered the hybrid membrane with excellent mechanical strength of ~13.50 MPa and high thermal stability of over 200 ℃, matching well with the basic membrane requirement for treating the aqueous oil on a mass scale for long term usage.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  The authors gratefully acknowledge the National Natural Science Foundation of China (No. 51873047), the Fundamental Research Funds for the Central Universities (No. 30919011266), the Key Laboratory of Low-Carbon Conversion Science & Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences (No. KLLCCSE-201906) and the Open Founds for LargeScale Instruments and Equipment of Nanjing University of Science and Technology.

  Appendix A. Supplementary data

  Supplementary material relatedtothisarticle canbefound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.01.038.

  
  
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  Figure 1  SEM images of PSA/PAN FMs fabricated from various weight ratios of PSA/PAN (a) 9/1, (b) 3/1, (c) 1/1, (d) 1/3 and (e) 0/1 (the polymer concentration was fixed at 12 wt%). (f) Stress-strain curves of PSA/PAN FMs fabricated from various weight ratios.

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  Figure 2  (a) Schematic illustrating the synthesis pathways of the superhydrophobic and superoleophilic fibrous membrane. SEM/optical photographs view of (b) the asprepared PSA/PAN FM, (c) the Fe_1.6@PSA/PAN FM and (d) the F₁-Fe_1.6@PSA/PAN FM.

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  Figure 3  (a) WCA of the fluorinated PSA/PAN FM with and without roughness construction. (b) WCA and oil contact angle (OCA) of the F₁-Fe_1.6@PSA/PAN FM and the F₁-Fe_1.6@PSA/PAN FM and relevant schematic presentation. (c) WCA of the F₁-Fe_1.6@PSA/PAN FM and the F₁@PSA/PAN FM as a function of time. (d) Photographs of the dynamic water-repelling and oil permeation on the F₁-Fe_1.6@PSA/PAN FM surface in air.

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  Figure 4  (a) Schematic diagram showing the separation device for aqueous oil separation using F₁-Fe_1.6@PSA/PAN FM. (b) The microscopic image of water-in-oil microemulsion before and after separation. Changes of flux with increasing separation cycles for (c) oil/water mixture and (d) water-in-oil microemulsion separation. (e) The WCAs of F₁-Fe_1.6@PSA/PAN FM after annealing treatment in air at different temperatures.

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