UV-Driven Antifouling Paper Fiber Membranes for Efficient OilâWater

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UV-driven anti-fouling paper fibers membranes for efficient oil-water separation Yangyang Chen, Atian Xie, Jiuyun Cui, Jihui Lang, Yongsheng Yan, Chunxiang Li, and Jiangdong Dai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05930 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Industrial & Engineering Chemistry Research

UV-driven anti-fouling paper fibers membranes for efficient oil-water separation

Yangyang Chena, Atian Xiea, Jiuyun Cuib, Jihui Langc, Yongsheng Yana, Chunxiang Lia*, Jiangdong Daia*

aInstitute

of Green Chemistry and Chemical Technology, School of Chemistry and

Chemical Engineering, and bSchool of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China cCollege

of Physics, Jilin Normal University, Siping 136000, China

*Corresponding Author

E-mail: [email protected]; [email protected] Tel: +86 0511-88790683; Fax: +86 0511-88791800

1 ACS Paragon Plus Environment

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

Abstract Oil-water separation has received much attention worldwide due to the increasing oily wastewater and frequent oil leak events. Herein, a superhydrophilic/underwater superoleophobic membrane (PF@PDA@TiO2) is synthesized by polydopamine (PDA) anchoring TiO2 nanowires on the surface of paper fibers (PF) via vacuum assisted filtration technology for efficiently various oil-water separations. The oil contact angle (OCA) underwater of PF@PDA@TiO2 is about 156o revealing its favorable underwater superoleophobic property. Remarkably, the PF@PDA@TiO2 affords above high separation efficiency of 99%. More importantly, the PF@PDA@TiO2 displays ultraviolet-driven (UV-driven) superior anti-fouling performance even after 80 cycles. We believe that the low cost and superior anti-fouling PF@PDA@TiO2 will provide a striking application in oil-water separation. Keywords: TiO2 nanowires, superhydrophilicity, underwater superoleophobicity, UV-driven superior anti-fouling, oil-water separation


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1. Introduction Water pollution is becoming more and more serious1, 2. Especially, ever-growing oil leaks events and industrial oily wastewater have caused serious ecological crisis3, 4. The indecomposable waste oil may generate other toxic substances further polluting the biological environment and affecting human safety5, 6. Conventional methods, including centrifuge, settling tanks, depth filters and floatation have been limited due to low separation efficiency, high cost and secondary pollution7, 8. Therefore, it is an urgent task to design a new material and suitable method for solving oil-water separation problem. In recent years, super-wetting materials have been exhibited extremely important applications in oil-water separation mainly due to high separation efficiency9,10. In 2004, Jiang et al. firstly reported superhydrophobic/superoleophilic mesh membrane for oil-water separation11. Inspired by this idea, superhydrophobic materials have been broadly applied in oil-water separation12. For instance, Sehati et al. fabricated waterproofing coated brick by modification of oxalic acid, sodium silicate and hexadecyltrimethoxysilane for oil-water separation13. Shang et al. prepared superhydrophobic

surfaces

by

combination

of

polydopamine

and

1H,

1H,

2H,

2H-perfluorodecanethiol with preeminent absorption-separation and robust stability14. However, these superhydrophobic materials are easily polluted or blocked and even damaged by oils so that cannot

be

cycle15-17.

Considering

the

above,

it

is

worthwhile

mentioning

that

superhydrophilic/underwater superoleophobic materials are synthesized for oil-water separation. Inspired by fish scales, Jiang et al. developed a novel superhydrophilic/underwater superoleophobic

hydrogel-coated

membrane

for

oil-water

separation18.

Since

then,

superhydrophilic/underwater superoleophobic materials have been attracted considerable attention in oil-water separation19, 20. These materials allowed pass through of water but repelled oil on the surface, which can effectively prevent oil contamination because of formation water film on the material surfaces21,

22

. For example, Cheng et al. fabricated an underwater superoleophobic

separating film by modification of HS(CH2)11OH showing high separation efficiency and flux23. Zhu et al. fabricated a metallicfiber membrane by coating of Cu(OH)2 nanoneedles that have high separation efficiency24. Although superhydrophilic/underwater superoleophobic materials have shown anti-fouling performance, the drawbacks of cost-highly, toxicity limit their applications25. Meanwhile, the membranes surface will be inevitably polluted by oils shortening the service life 3 ACS Paragon Plus Environment


during the long-term oil-water separation process26. Therefore, it is of great significance to develop environment-friendly, non-toxicity, low-cost, superior anti-fouling membrane materials. Titanium dioxide (TiO2), an environmental-friendly, non-toxicity, low-cost material has been used in vast majority fields27. For example, photocatalytic splitting water28, 29, lithium ion battery30 and so on. Typically, TiO2 has been used as promising photocatalytic materials for degradation of organic pollutants under UV-driven31. Recently, TiO2-based membrane materials have exhibited very important application in oil-water separation due to its excellent UV-driven self-cleaning performance32,

33

. In this case, TiO2-based membrane can improve greatly the anti-fouling

performance of membrane materials34, 35. For instance, Xin et al. prepared TiO2 doped PVDF nanofiber membrane by electrospinning method, which has reversible oil-water separation with outstanding antifouling property36. Zhang et al. fabricated an underwater superoleophobic membrane by LbL assembly of sodium silicate and TiO2 nanoparticles with excellent self-cleaning ability37. Therefore, it is of much significant to explore more low-cost and facile prepared TiO2-based membrane materials with UV-driven anti-fouling property. In recent years, dopamine has been used more and more widely in the membranes synthesis38. Interestingly, polydopamine (PDA) acts as a surface-immobilized medium for generating and immobilizing nanoparticles on the membrane surface, these nanoparticles do not affect the surface membrane structure39. They impart superhydrophilicity and underwater superoleophobicity, high water flux and outstanding efficiency in oil-water mixture separation. Recently, it has been found that PDA can be used as an electron acceptor to promote electron transfer40, which will be conducive to photocatalytic reaction. The combination of dopamine, TiO2 nanowires and membrane not only enhances the membrane hydrophilicity but also improves the photocatalytic membrane effect. On the one hand, the coordination wires of the surface metal atoms in TiO2 nanoparticles that are smaller than 200 Å is incomplete and thus exhibits high affinity for oxygen-containing ligands41. Oxygen-rich enediol ligands, such as dopamine, form strongly coupled conjugated structures with surface Ti atoms by repairing undercoordination of the surface. So that TiO2 can be more firmly fixed on paper fibers. On the other hand, TiO2 has a large amount of hydroxyl groups on the surface, dopamine contains a large amount of amino groups, which form stable hydrogen bonds and immobilize on paper fibers containing a large amount of hydroxyl groups42. In this paper, a low-cost, facile prepared and UV-driven superior 4 ACS Paragon Plus Environment

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anti-fouling

superhydrophilic/underwater

superoleophobic

paper

fibers

membrane

(PF@PDA@TiO2) is fabricated TiO2 nanowires are first anchored the surface of paper fibers via polydopamine,

and

followed

by

facile

vacuum

assisted

filtration

technology.

The

PF@PDA@TiO2 shows high separation efficiency of above 99% for various oil-water mixtures. More importantly, the PF@PDA@TiO2 presents excellent stability and regenerative capacity. Moreover, the PF@PDA@TiO2 still maintains UV-driven superior self-cleaning property after being polluted by organic solvents. The preeminent performance material may have a better application prospects in oil-water separation. The comparison of easy preparation, recycling, number of cycles of use and costs to other reports was listed in Table S1. The synthetic procedure of PF@PDA@TiO2 is illustrated in scheme 1.

Scheme 1. Schematic diagram of the fabrication process of the PF@PDA@TiO2. 2. Experimental 2.1 Materials and chemicals The paper and soybean oil are purchased from local supermarket. P-25 (TiO2), dopamine hydrochloride (DA, 98%), tris (hydroxymethyl) aminomethane (Tris, 99.99%), methylene blue (MB), petroleum ether (AR, bp 90-120oC), Sudan III (AR) is obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). Toluene (≥99.5%), 1, 2-Dichloroethane (≥99.5%), hexane (>99%) are received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Diesel is purchased from China Petroleum & Chemical Corporation. 2.2 Characteristics X-ray photoelectron spectroscopy (XPS) is observed by a Kratos Axis Ultra DLD spectrometer equipped with an Al Kα X-ray source (1486.6 eV), and high resolution spectra is observed with a pass energy of 20 eV. Morphologies and compositions of the sample are 5 ACS Paragon Plus Environment


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investigated under the field-emission Scanning Electron Microscopy (SEM, JSM-7001F, JEOL, Japan). Crystal structures of the nanocomposites are conducted on a XRD-6100 diffractometer (Japan). Angle meter (KSV, CM200, Finland). UV-driven of nanocomposites is provided on photochemical

reaction

instrument

(GHX-2).

Thermogravimetry

is

measured

by

a

thermogravimetric analyzer (NETZSCH STA449F3, Germany). 2.3 Syntheses of TiO2 nanowires TiO2 nanowires are prepared according to previous work43. First, 3 g of P-25 power are dispersed 100 mL of NaOH solution (10 mol.L-1) under stirring for about 10 min. Then, the mixed solution (75 mL) is transferred into a Teflon-lined stainless autoclave (100 mL) and kept at 200oC for 24 h in an oven. After cooling naturally down to room temperature, the collected product is washed with deionized water to neutral. The sodium titanate product is soaked into 0.1 M nitric acid for 6 h to obtain hydrogen titanate. Finally, the TiO2 nanowires are obtained via heat-treatment of hydrogen titanate in air at 500oC for 3 h with a heating rate of 5 oC. min-1. 2.4 Preparation of PF@PDA@TiO2 PF@PDA@TiO2 is prepared by a facile vacuum assisted filtration process. First, TiO2 nanowires (0.04 g) are dispersed in deionized water (50 mL) under ultrasonic for 10 min, followed stirring for 30 min. Meanwhile, paper (0.3 g) is added to deionized water (50 mL) and stirred for 30 min. Then, the PF dispersion solution is poured into TiO2 suspension stirring for 30 min. After that, tris (hydroxymethyl) aminomethane (0.1211 g) and dopamine (0.2 g) are added to the mixture stirring for 6 h. Finally, the PF@PDA@TiO2 is obtained through a vacuum filter drying in vacuum oven at 60oC. The amount of the TiO2 nanowires is adjusted to be 0g, 0.02g, 0.04g, 0.06g, the samples are referred as PF@PDA@TiO2-X (For example, PF@PDA@TiO2-2 represents the amount of the TiO2 nanowires is 0.02g). 2.5 Oil-water separation experiments The PF@PDA@TiO2 is wetted by water and then sandwiched in two Teflon flanges having a diameter of 0.8 cm to form an oil-water separation device. The oil-water mixtures are prepared as followed: 15 mL oil colored with Sudan III is poured into 15 mL weighed water colored with methyl blue (MB). Finally, the oil-water mixtures are completely separated through separation device, followed the amount of the separated water is weighed. Hexane and 1, 2-dichloroethane are selected as the representatives. The separation efficiency (η) is calculated by the following 6 ACS Paragon Plus Environment

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equation：



m1 100% m0

(1)

Where m1 (g) and m0 (g) are the amount at time t and the initial amount, respectively. The liquid flux is calculated by the following formula: J

V St

(2)

Where J (L·m-2·h-1) is liquid flux, V (L) is the infiltrated volume of water, S (m2) is the effective area of contacting with oil-water mixture, Δt (h) is the penetrating time. The availability performance of PF@PDA@TiO2 is proven through oil-water separation experiments of 80 cycles. The anti-fouling properties of as-prepared membrane are demonstrated by oil-water separation after UV-driven of 80 cycles. 2.6 Porosity and pore size of membranes The porosity(Ɛ) of membranes is measured by gravimetric method according to the following formula44： ε=

W1 ― W2 ρwAL

(3)

× 100%

Where W1 (g) and W2 (g) are the wet and dry weights of the as-prepared membranes, respectively. And ρw represents the water density (0.998 g cm-3), A (cm2) is the effective area of membranes and L (cm) is the thickness of membranes. The pore size (rm) of membrane is calculated by the Guerout-Elford-Ferry equation45 𝑟𝑚 =

(2.9 ― 1.75𝜀) × 8𝜂𝑙𝑄 𝜀𝐴∆𝑃

(4)

Herein, ε is membrane porosity (%), η is water viscosity (8.9 × 10-4 Pa s), l signifies membrane thickness (m), Q is the volume of permeate water per unit time (m3 s-1), A denotes the membrane active area (m2), and ΔP is the operation pressure (0.1 MPa). 2.7 Antifouling performance of membrane The antifouling property of PF@PDA@TiO2 is demonstrated by following methods: Firstly, the CF/32Co-TiO2 was polluted by oil, then the polluted membrane was irradiated 1h under UV light (not wash them). Finally, the irradiated membrane was measured underwater OCA again. 3. Results and discussion As known, surface wettability, separation efficiency and water flux are important factors to



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evaluate the membrane performance. In order to optimize the condition, the OCA underwater, separation efficiency and water flux of membranes are tested and shown in Fig. 1. From Fig. 1a, the OCA underwater of original PF is 138o, but the OCA underwater of PF@PDA is 140o, indicating that polydopamine plays a certain role in the oleophobicity underwater and stains anti-fouling

of

the

membrane.

The

OCA

underwater

of

the

PF@PDA@TiO2-2,

PF@PDA@TiO2-4 and PF@PDA@TiO2-6 are 144o, 156o and 154o, respectively. The oleophobicity gradually increased with the increase of the amount of TiO2 nanowires, but the OCA underwater of the PF@PDA@TiO2-4 and PF@PDA@TiO2-6 are almost similar, which may be due to the roughness has no obvious change. Fig. 1b shows the separation efficiency and water flux of different membranes. With the increase of TiO2 nanowires, the separation efficiency gradually increases and the water flux gradually decreases, while the membrane separation efficiency of PF@PDA@TiO2-4 and PF@PDA@TiO2-6 are almost similar. On the basis of above results, the PF@PDA@TiO2-4 is chosen for following analysis and oil-water separation. Notes: If not otherwise specified, PF@PDA@TiO2 refers to the PF@PDA@TiO2-4.

Fig. 1. The OCAs underwater (a), separation efficiency and water flux (b) of PF and PF@PDA@TiO2-X. Morphology of samples is analyzed by SEM. The pure TiO2 nanowires have an average length of about 1.2-3 µm, and a diameter of about 220-300 nm, revealing an obvious network consisting of long nanowires (Fig. S1), proving that TiO2 nanowires have been synthesized successfully. The optical pictures of samples are displayed in Fig. S2. The PF shows 3D porous structure made up of paper fibers with smooth surface and the thicknesses about 200 µm (Fig. S3). The surface of PF@PDA has no obvious particles are observed in the magnified images, as


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depicted in Fig. 2a. And the thickness increased from 200 µm to 220 µm proved that polydopamine played a certain role in the thickness of the membrane. The structure of PF@PDA@TiO2-2 reveals that TiO2 nanowires are anchored on the membrane surface (Fig. 2b). It can be clearly seen that the surface was almost smooth, with only some small nanowires. The thickness of the PF@PDA@TiO2-2 is about 240 µm. The surface of PF@PDA@TiO2-4 is covered by TiO2 nanowires, as shown in Fig. 2c1. With the increase of TiO2 nanowires amount, the thickness of the membrane increased to 280 µm. A high-magnification SEM image shows that a large number of TiO2 nanowires bond on fibers surface of PF@PDA@TiO2-4 tightly (Fig. 2c2). It can be seen that the pristine paper fibers has been almost completely covered by TiO2 nanowires. And the TiO2 nanowires are strongly attached on the paper fibers due to the interaction between paper fibers, polydopamine and paper fibers. The SEM image of PF@PDA@TiO2-6 shows more aggregated TiO2 nanowires blocking the pores of membrane, which may result in low flux. The thickness of PF@PDA@TiO2-6 is about 285 µm, which is mainly due to the fact that the amount of TiO2 nanowires has not changed. The hydrophilic TiO2 nanowires are gathered on the whole membrane surface constructing micro-nano hierarchical structure, it is very crucial for hydrophilicity of membranes. Due to the creation of numerous hydrophilic hydroxyl groups with high surface energy on TiO2 surface, it has been confirmed that the photocatalytic degradation of low-energy hydrocarbon groups combined with the hydroxyl group on the surface of TiO2 is the main reason for photocatalytic induction of hydrophilicity. According to Wenzel's law46, the high-energy hydroxyl groups on the surface can induce hydrophilicity by photocatalysis, and the water molecules and the TiO2 can absorb water into the interior by capillary force, making the surface more hydrophilic. The combination of the inherent hydrophilicity and photocatalytic activity of TiO2 makes it easy to remove pollutants through UV irradiation and to recover separated materials. It is expected to achieve sustainable and efficient oil-water separation applications.



Fig. 2. SEM images of as-prepared membranes: (a) PF@PDA, (b) PF@PDA@TiO2-2, (c) PF@PDA@TiO2-4, (d) PF@PDA@TiO2-6 (insert: cross-sectional FE-SEM images of PF and PF@PDA@TiO2-X). To confirm the elements distribution, the PF@PDA@TiO2 is analyzed by EDS mapping. As shown in Fig. 3a-d, C, O, N and Ti elements are distributed on the surface of PF@PDA@TiO2, the C, O elements are characteristic of the cellulose paper, N, Ti elements are assignable to polydopamine and TiO2 nanowires, respectively. Elemental mapping graphs of C, O, N, and Ti reveal the homogenous distribution of PF, PDA and TiO2 nanowires in PF@PDA@TiO2. The chemical composition of samples is analyzed by XPS. The C 1s peak and O 1s peak are discovered on the surface of pure PF. N 1s peak is detected on the PF@PDA that proved which the paper fiber is modified by polydopamine successfully. The survey spectra also confirmed the existence of N 1s peak and Ti 2p peak in the PF@PDA@TiO2 (Fig. 3e). Meanwhile, the high resolution XPS spectrum of N 1s is shown in Fig. 3f, three different peaks located at 400.8, 399.1 and 398.0eV which can be assigned NH247, C-NH-C48 and C-N49, respectively. The Ti 2p spectrum (Fig. 3g) clearly demonstrated the presence of titanium in the PF@PDA@TiO2, the Ti 2p spectrum divided into two peaks, 463.9eV50 and 457.5eV51, respectively. Overall, the PF@PDA@TiO2 has been prepared successfully.


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Fig. 3. EDX mapping of PF@PDA@TiO2 (a-d); XPS of the PF, PF@PDA and PF@PDA@TiO2-4 (e); high resolution spectrum of N 1s (f) and Ti 2p (g) of PF@PDA@TiO2-4. Fig. 4 shows XRD pattern of TiO2 nanowires, PF and PF@PDA@TiO2-X. TiO2 nanowires show broad peaks at 28.2o, 48.0o, 55.1o, and 62.6o, which are characteristic of (101), (200), (211), (204) (JPCD no. 21-1272), the 15.1o, 29.9o, 44.5o belong to (200), (111), (-601) by XRD (JPCD no. 46-1238). The strong and sharp cellulose diffraction peak at 14.8o, 16.7o, 22.8o of the (110), (002), (120), corresponding to JPED no. 50-0926. PF and PF@PDA have same peaks showing polydopamine has no effect on the crystallinity of PF@PDA. Furthermore, partial characteristic diffraction peaks of TiO2 nanowires (101), (111), (200), (-601) and PF (110), (002), (120) are observed for PF@PDA@TiO2-2, PF@PDA@TiO2-4 and PF@PDA@TiO2-6. With the increase of the amount of TiO2 nanowires, the diffraction peak intensity of the (101), (111), (200), (-601) crystal plane is stronger, and the diffraction peaks of the (110), (002), (120) crystal plane are weaker, indicating the successful integration of the TiO2 nanowires and the paper fibers.

Fig. 4. XRD patterns of TiO2 nanowires, PF and PF@PDA@TiO2-X. 11 ACS Paragon Plus Environment


Fig. 5 shows wettability behaviors of PF@PDA@TiO2. Water can easily permeate the surface of PF@PDA@TiO2 and WCA of about 0o in the air, indicating superhydrophilicity of the PF@PDA@TiO2 (Fig. 5a). The PF@PDA@TiO2 presents underwater superoleophobicity with an OCA of about 156o (Fig. 5b). And underwater oil droplets (red) attain a quasi-spherical shape on the PF@PDA@TiO2 surface (Fig. 5c). In addition, Fig. 5d exhibits an oil droplet can completely detach from the superoleophobic surface even during severe deformation and repeated contact indicating low oil adhesion. These results indicate that the PF@PDA@TiO2 possessed outstanding superhydrophilic/underwater superoleophobic performance.

Fig. 5. (a) WCA and (b) OCA underwater of PF@PDA@TiO2; (c) A photograph of underwater oil droplets (red); (d) Dynamic oil adhesion process on the PF@PDA@TiO2 underwater. The mass ratio of PF and PF@PDA@TiO2-X was measured by thermogravimetric analysis (TGA) under a flowing air atmosphere with a heating rate of 10 °C min-1 from 200 to 800 °C. As shown in Fig. 6a, the results showed that the weight losses of PF and PF@PDA were about 93% from 350 to 600 °C. Hence, on account of the combustion of PF, PF@PDA@TiO2-X lost their weight rapidly from 250 to 600 °C. By calculating, the contents of TiO2 nanowires in PF@PDA@TiO2-X (X = 2, 4, 6) were 5.49, 6.14, and 12.70 wt%, respectively. The porosity and pore size of PF@PDA@TiO2-X are calculated and shown in Fig. 6b. The porosity of PF, PF@PDA, PF@PDA@TiO2-2, PF@PDA@TiO2-4 and PF@PDA@TiO2-6 are about 81.91%, 84.25%, 86.42%, 90.71% and 90.29%, and the pore size are about 200.40, 197.29, 195.19, 193.67, 193.61µm, respectively. It is clear observed that the porosity of PF@PDA@TiO2-4 is the largest, but the porosity of PF@PDA@TiO2-6 is almost unchanged. It shows the porosity increases despite increasing thickness-suggesting that the enhanced porosity may be due to the hydrophilic TiO2 nanowires. Based on the above results, it is proved again that PF@PDA@TiO2-4 is the 12 ACS Paragon Plus Environment

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optimal proportion.

Fig. 6. (a)TGA curves of PF and PF@PDA@TiO2-X, (b) Porosity and mean pore size of PF and PF@PDA@TiO2-X. Fig. 7 a, b shows a device for separating hexane-water and dichloroethane-water mixtures, the separation device with tilting of 15o for separating dichloroethane-water mixture (In order to prevent oil accumulation with a density higher than water, the oil-water mixture could not be separated effectively). After oil (red) and water (blue) mixture (Voil: Vwater=1:1) is poured into the device, water permeated quickly while the oil is repelled in the quartz tube under the effect of gravity. The separation efficiency and water flux are exhibited Fig. 7c, the results indicating PF@PDA@TiO2 has high separation efficiency for various oil-water mixtures of above 99%. Furthermore, the water flux for various oil-water mixtures is measured; the high viscosity and heavy oil-water mixture show the low flux with the values range from 7677-9265 L m-2 h-1, such as soybean oil, diesel and dichloroethane. The low viscosity oil-water mixtures show the high flux of 9597-10140L m-2 h-1. The separation efficiency and water flux for various oil-water mixtures of the PF@PDA@TiO2-2 and PF@PDA@TiO2-6 are showed in Fig. S4. The separation efficiency of PF@PDA@TiO2-2 is around 98%. The separation efficiency for various oil-water mixtures of PF@PDA@TiO2-6 is above 99% showing good separation performance but low flux. It is confirming again that PF@PDA@TiO2-4 is optimal for oil-water separation. Meanwhile, the cross-flow filtration for mass scale oil-water separation is conducted, as shown in Fig. S5a, b. When oil (colorless)-water (blue) mixture (Voil: Vwater=1:1) is poured into the cross-flow filtration device, water quickly flow through the membrane into the beaker but the oil remains in the feed port under 25 kPa pressure (note: the separated water becomes lighter in color because some of the



dye is adsorbed onto the membrane). Fig. S5c shows the separation efficiency of membranes for various oil-water mixtures is more than 99.9%, and the flux is also greater than 43 L m-2 h-1 bar-1, which indicates that these membranes can be used for large scale and effective cross-flow filtration at pressurized feeds below the intrusive pressure of the oil. The above results evidenced the as-prepared membranes with high separation efficiency show a promising application for the oil-water separation.

Fig. 7. (a1-b2) Photographs of oil (red)-water (blue) separation device; (c) Separation efficiency and water flux of PF@PDA@TiO2 for different oil-water mixtures (A-F represent dichloroethane, petroleum ether, toluene, soybean oil, diesel, hexane and water mixtures). Superhydrophilic/underwater superoleophobic membranes exhibit good anti-fouling ability but are easily contaminated by organic solutions during long-term use25. Thus, it is important to prepare a stable and superior anti-fouling membrane for oil-water separation. TiO2 nanowires are UV-driven material, which can decompose organic pollutants52. Therefore, the TiO2 nanowires modified membranes have superior anti-fouling ability after UV-driven. Fig. 8a shows separation efficiency and water flux of PF@PDA@TiO2 before 20 cycles. The separation efficiency and water flux have been high for the first ten times. However, after 10 cycles, the separation efficiency and water flux of PF@PDA@TiO2 decreasing gradually, which may be attributed to that the superhydrophilic membrane absorbs a large number oil blocking pores. Fortunately, underwater superoleophobic property of PF@PDA@TiO2 is restored after UV irradiation, as shown in Fig. 8b. The OCAs evolutions of the PF@PDA@TiO2 are depicted in Fig. 8b. During 80 oil-water separation cycles and once every 20 cycles by UV irradiation, the underwater OCA of the PF@PDA@TiO2 is measured and returned to underwater superoleophobic state. It is worthy to 14 ACS Paragon Plus Environment

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note that the OCAs do not changed obviously through 80 cycles. An intuitive process diagram of OCAs underwater is shown in Fig. S6. Especially, the PF@PDA@TiO2 still maintains excellent separation efficiency and water flux after 80 cycles (Fig. 8c). Additionally, the microstructure of PF@PDA@TiO2 is observed by SEM, there is no obvious change after 80 oil-water separation cycles, the 3D porous microstructure is not disrupted and TiO2 nanowires are still firmly combined with paper fibers (Fig. 8d). Significantly, the PF@PDA@TiO2 exhibits favorable flexibility (Fig. S7). Moreover, the PF@PDA@TiO2 still maintains original appearance after stirring in water for 45 min, demonstrating robust durability of PF@PDA@TiO2 (Fig. S8). This can be attribute to the strongly adhesion of polydopamine. These results illustrate that the PF@PDA@TiO2 membrane has superior anti-fouling property, excellent separation performance and outstanding regenerability.

Fig. 8. (a) Separation efficiency and water flux toward hexane and water mixtures for 20 cycles; (b) OCAs underwater of PF@PDA@TiO2 for 80 cycles; (c) Separation efficiency and water flux of PF@PDA@TiO2 for 80 cycles; (d) The SEM image of PF@PDA@TiO2 after 80 oil-water separation cycles. To better understand oil-water separation process, Fig. 9a shows a hypothetical



superhydrophilic/underwater superoleophobic mechanism device, the pores are completely filled with water forming continuous water layers with the adjacent surface. The affinity of water to the membrane material is stronger than that of oil, thus, oil can’t pass smoothly the membrane, oil is required different pressures to enter the pores53. The hexane oil column intrusion pressure is observed in Fig. 9b, consider the intrusion pressure as the static head to measure the maximum height of the oil column that the membrane is subjected to. Ppritical(liquid)=ρghmax54, where ρ (g/cm3) is the density of hexane, g (9.8N/kg) is the gravity acceleration, Ppritical(liquid) is the critical pressure calculated by the height of the measurement. The oil (red) maximum intrusion height (hmax) of PF@PDA@TiO2 about 0.5 m calculated which the maximum intrusion pressure (Ppritical(liquid)) value is 3239.9 Pa using above formula. When hydrostatic pressure of oil less than the value the oil will float on the membrane surface and stop flowing. Once the intrusion pressure is more than the value, the oil will penetrate into the underside of the membrane.

Fig. 9. The separation mechanism of PF@PDA@TiO2 (a); An oil (red) intrusion pressure of PF@PDA@TiO2 (b). 4. Conclusions 1 In conclusion, we report a paper fibers membrane anchored TiO2 nanowires using polydopamine via vacuum assisted filtration technology. The as-prepared PF@PDA@TiO2 membrane with superhydrophilicity/underwater superoleophobicity and superior anti-fouling properties shows high separation efficiency for various oil-water mixtures. Meanwhile, the separation efficiency of PF@PDA@TiO2 maintains above 99% after 80 cycles by UV-driven 16 ACS Paragon Plus Environment

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self-cleaning, revealing its excellent stability and regenerative capacity. We believe that the PF@PDA@TiO2 can be applied as a new membrane material for oil-water separation. Acknowledgments The work is grateful for financial support from the National Natural Science Foundation of China (21776110 and 51608226), Natural Science Foundation of Jiangsu Province (BK20170532 and BK20160501). References (1) Xie, A.; Dai, J.; Chen, X.; Ma, P.; He, J.; Li, C.; Zhou, Z.; Yan, Y. Ultrahigh Adsorption of Typical Antibiotics onto Novel Hierarchical Porous Carbons Derived from Renewable Lignin Via Halloysite Nanotubes-Template and in-Situ Activation. Chem. Eng. J. 2016, 304, 609-620. (2) Xie, A.; Dai, J.; Cui, J.; Lang, J.; Wei, M.; Dai, X.; Li, C.; Yan, Y. Novel Graphene Oxide– Confined Nanospace Directed Synthesis of Glucose-Based Porous Carbon Nanosheets with Enhanced Adsorption Performance. ACS Sustainable Chem. Eng. 2017, 5, 11566-11576. (3) Kintisch, E. An Audacious Decision in Crisis Gets Cautious Praise. Science 2010, 329, 735. (4) Aurell, J.; Gullett, B. K. Aerostat Sampling of PCDD/PCDF Emissions from the Gulf Oil Spill in Situ Burns. Environ. Sci. Technol. 2010, 44, 9431-9437. (5) Peterson, C. H.; Rice, S. D.; Short, J. W.; Esler, D.; Bodkin, J. L.; Ballachey, B. E.; Irons, D. B. Long-Term Ecosystem Response to the Exxon Valdez Oil Spill. Science 2003, 302, 2082. (6) Chu, Z.; Feng, Y.; Seeger, S. Oil/Water Separation with Selective Superantiwetting/Superwetting Surface Materials. Angew. Chem. Int. Ed. 2015, 54, 2328-2338. (7) Wang, X.; Yu, J.; Sun, G.; Ding, B. Electrospun Nanofibrous Materials: A Versatile Medium for Effective Oil/Water Separation. Mater. Today 2016, 19, 403-414. (8) Yu, Y.; Chen, H.; Liu, Y.; Craig, V. S. J.; Wang, C.; Li, L. H.; Chen, Y. Superhydrophobic and Superoleophilic Porous Boron Nitride Nanosheet/Polyvinylidene Fluoride Composite Material for Oil-Polluted Water Cleanup. Adv. Mater. Interfaces 2015, 2, 1400267. (9) Dai, J.; Chang, Z.; Xie, A.; Zhang, R.; Tian, S.; Ge, W.; Yan, Y.; Li, C.; Xu, W.; Shao, R. One-Step Assembly of Fe(III)-CMC Chelate Hydrogel onto Nanoneedle-Like CuO@Cu Membrane with Superhydrophilicity for Oil-Water Separation. Appl. Surf. Sci. 2018, 440, 560-569. (10) Xie, A.; Cui, J.; Yang, J.; Chen, Y.; Dai, J.; Lang, J.; Li, C.; Yan, Y. Photo-Fenton self-cleaning membranes with robust flux recovery for efficient oil/water emulsions separation. J. Mater. Chem. A 2019, DOI: 10.1039/C9TA00521H. (11) Feng, L.; Zhang, Z.; Mai, Z.; Ma, Y.; Liu, B.; Jiang, L.; Zhu, D. A Super-Hydrophobic and Super-Oleophilic Coating Mesh Film for the Separation of Oil and Water. Angew. Chem. Int. Ed. 2010, 43, 2012-2014. (12) Liu, Y.; Zhang, K.; Yao, W.; Zhang, C.; Han, Z.; Ren, L. A Facil Electrodeposition Process for Fabrication of Superhydrophobic and Superoleophilic Copper Mesh for Efficient Oil-Water Separation. Ind. Eng. Chem. Res. 2016, 55, 2704-2712. (13) Sehati, S.; Kouhi, M.; Mosayebi, J.; Rezaei, T.; Mosayebi, V.; Sehati, S.; Kouhi, M.; Mosayebi, J.; Rezaei, T.; Mosayebi, V. Fabrication of Superhydrophobic Nano Sol: Waterproofing of Coated Brick. J. Build. Engin. 2017, 13, 305-308. 17 ACS Paragon Plus Environment


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Graphical abstract

In this work, the PF@PDA@TiO2 membrane is prepared by vacuum filtration. The membrane still retains superhydrophilic/underwater superoleophobic properties after UV-driven (at least 80 times oil-water separations), indicating its superior anti-fouling and excellent regenerability. Highlights 

A low-cost PF@PDA@TiO2 membrane was fabricated via facile vacuum assisted filtration



The superhydrophilic/underwater superoleophobic membrane demonstrated high separation efficiency above 99%



The membrane exhibited outstanding regenerability and superior UV-driven anti-fouling property


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