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Separation and Purification Technology

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Superlyophobic anti-corrosive and self-cleaning titania robust meshmembrane with enhanced oil/water separation

Hongjun Kanga, Zhongjun Chengb,⁎, Hua Laia, Haoxiang Maa, Yuyan Liua,⁎, Xianmin Maic,Youshan Wangd, Qian Shaoe, Lichen Xiangf, Xingkui Guoe, Zhanhu Guof,⁎

aMIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute ofTechnology, Harbin, Heilongjiang 150001, PR ChinabNatural Science Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, Heilongjiang 150090, PR Chinac School of Urban Planning and Architecture, Southwest Minzu University, Chengdu 610041, PR ChinadNational Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin, Heilongjiang 150090, PRChinae College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, Chinaf Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, TN 34934 USA

A R T I C L E I N F O

Keywords:Oil/water separationTiO2 mesh membraneDual superlyophobicityRobust stabilityHigh efficiency

A B S T R A C T

Oil/water separation becomes a research hotspot because of the frequent oil spill accidents. Although lots ofseparating membranes with super-wettability have been fabricated to separate oil or water from the mixtures, toseparate both heavy (ρoil > ρwater) and light (ρoil < ρwater) oil/water mixtures with high efficiency and high fluxis still a challenge, especially to obtain a membrane with mechanically robust nanostructures, and simulta-neously anti-corrosive and self-cleaning ability. Herein, a novel titania nanostructured mesh membrane withsuperamphiphilicity in air, and dual superlyophobicity in liquid mediums were fabricated. Both high separationefficiency and flux of more than 99.2% and 16,954 Lm−2h−1 were obtained for both light and heavy oil/watermixtures. Importantly, the TiO2 mesh membrane showed mechanically robust nanostructures and anti-corrosiveability, which can remain dual superlyophobicity, i.e., both underwater superoleophobicity and underoil su-perhydrophobicity and high oil-water separating behavior even after immersion in solutions of strong acid,basic, high salty, various organic solvents, and even after being abrased and adhered by pure sands and adhesivetape, respectively. Meanwhile, TiO2 mesh membrane had shown photocatalytic decomposition of oil and organicmolecules under UV irradiation, demonstrating unique self-cleaning and anti-fouling ability of the membranes.Given so many merits of these unique membranes, it is believed to these membranes have great potentials to beused in wastewater treatment and oil spill recovery.

1. Introduction

The frequently happened oil spill accidents cause increasingly en-vironmental contamination [1–3]. With different interface interactionsexisting between oil and water, various membranes with special wet-ting performances have been designed to separate oil/water mixtures[4,5]. Generally, the existing separation membranes can be categorizedinto two categories. The first is for the membranes with super-hydrophilicity/underwater superoleophobicity, considered as “water-removing” separation membranes including porous nitrocellulosemembrane [6], hydrogel coated metal mesh [7], SiO2 nanoparticles(NPs) and waterborne polyurethane (PU) coated stainless steel mesh[8], and titania coated metal mesh [9–11], which allow only water to

penetrate through the membranes [12–20]. The other is those super-hydrophobic/superoleophilic separation membranes, considered as‘‘oil-removing” membranes, which allow oil to penetrate through themembranes [21–35], such as nanostructured layer steel mesh [36],superhydrophobic silica aerogels [37], polytetrafluoroethylene (PTFE)coated metal mesh [38], polydimethylsiloxane (PDMS) coated nano-wire membrane [39], fly ash coated superhydrophobic textile [40] andzeolite-coated mesh [41]. Noticeably, all these materials can separateeither light (ρoil < ρwater) or heavy (ρoil > ρwater) oil from the oil/water mixtures because of the barrier layer caused by the density dif-ference between oil and water. Briefly, for these “water-removing” typematerials, they are unsuitable for separating heavy oil/water mixture(ρoil > ρwater), because oil will accumulate on the material surface,

https://doi.org/10.1016/j.seppur.2018.03.002Received 19 December 2017; Received in revised form 21 February 2018; Accepted 2 March 2018

⁎ Corresponding authors.E-mail addresses: chengzhongjun@iccas.ac.cn (Z. Cheng), liuyy@hit.edu.cn (Y. Liu), zguo10@utk.edu (Z. Guo).

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forming the barrier layer and thus blocking the water permeation. Si-milarly, as to those ‘‘oil-removing” materials, separation of light oil/water mixture (ρoil < ρwater) is unfavorable because water can sinkdown on the membrane surface, and thus prevents oil from permeating.Therefore, novel functional materials that can separate both heavy(ρoil > ρwater) and light (ρoil < ρwater) oil/water mixtures with highefficiency are still necessary.

Great efforts have been made to solve the aforementioned problems.For example, stimuli-responsive separating membranes that can switchbetween the superhydrophobicity/superoleophilicity and super-hydrophilicity/underwater suproleophobicity have been reported toaddress the challenging separation of both heavy (ρoil > ρwater) andlight (ρoil < ρwater) oil/water mixtures [42–47]. However, all thesemembranes need external stimuli including pH, temperature, solvent,etc, which are complex for operation and time-consuming. For example,Liu et al. realized both “oil-in-water” and “water-in-oil” emulsions se-paration based on the special dual superlyophobicity (underoil super-hydrophobicity and underwater superoleophobicity) of the porousPVDF membranes [48]. Other membranes with similar wettability werealso reported, such as ZnO/Co3O4 overlapped copper mesh membrane[49] and TiO2 nanoclusters coated copper mesh membranes [50]. Allthese only demonstrated the separation of oil-in-water and water-in-oilemulsions. However, for immiscible oil/water mixtures, these mem-branes would be unfavorable for practical applications because the poresizes on these membranes are too small to realize high flux separation(small pore size is necessary for the separation of emulsions). To furtherovercome this imperfection, diatomite or corn cob powders (CCPs)coated stainless steel meshes were prepared by spraying the mixtures ofdiatomite/corn cob powders and waterborne polyurethane (PU), onwhich both heavy and light immiscible oil/water mixtures can be se-parated through “oil-removing” and “water-removing” separating pro-cesses [51,52]. Xiong et al [53] reported the separation of both heavyand light immiscible oil/water mixtures on a hierarchical ZnO nanos-tructured nonwoven membrane with similar dual superlyophobicity.However, both good antifouling ability and robust mechanical stabilityhave not been demonstrated on these membranes for separating heavyoil/water mixtures. Du et al [54] reported a TiO2-coated stainless steelmesh membrane with dual superlyophobicity, on which both heavy andlight immiscible oil/water mixtures can be separated. However, themembrane has not presented the anti-corrosive ability and robust na-nostructure stability.

In this study, a novel nanostructured TiO2 mesh membrane wasfabricated by a simple electrochemical anodization and heating process.Dual superlyophobicity, i.e., both underwater superoleophobicity andunderoil superhydrophobicity were observed and used to efficientlyseparate both light and heavy oil/water mixtures. Light or heavy oil/water mixtures could be directly separated with an ultrahigh flux ofmore than 16,954 Lm−2h−1. Without any troublesome step such aspre-wet and external stimuli, the separation efficiencies are above99.2% for all the used oil/water mixtures. Meanwhile, after physicalabrasion and immersion into water solutions including strong alkali,acid, salty, various organic solvents, the nanostructures of TiO2 meshmembrane have not been damaged and the TiO2 mesh membrane stillremains excellent separating property with high separation efficienciesabove 99.2%, demonstrating mechanically robust nanostructure stabi-lity and anti-corrosive ability. Besides, TiO2 mesh membrane has shownphotocatalytic decomposition of oil and organic molecules, which candecompose oil molecules under UV irradiation to make the fouledmembrane recover to original wetting performances, showing uniqueself-cleaning and anti-fouling ability. Finally, the separation mechanismof nanostructured TiO2 mesh membrane was carefully analyzed byYoung-Laplace equation.

2. Experimental

2.1. Materials

Pure Ti sheet with a thickness of 0.25mm was purchased fromQingyuan Metal Materials Co., Ltd. The diesel and gasoline were pro-vided by the local store. Hexane, petroleum ether, 1, 2-dichloroethaneand chloroform were purchased from Aladdin. All of other analyticalchemicals (ethylene glycol, ammonium fluoride, absolute ethanol andacetone, etc.) were supplied by Chemical Reagent Company of Harbin.

2.2. Fabrication of nanostructured titania mesh membrane

The nanostructured titania mesh membrane was prepared by ano-dizing the Ti mesh (40mm×40mm) in an ethylene glycol electrolyte(0.4 wt% NH4F and 2 vol% deionized water). Briefly, before anodiza-tion, Ti mesh was cleaned in acetone, alcohol, and distilled water, re-spectively. The anodization step was conducted at 45 V for 6 h (for thedetermination of experimental parameters, refer to Figs. S1–S3) in atwo-electrode cell with Ti foil and Pt foil (20mm×20mm) as theworking electrode and the counter electrode, respectively. The obtainedTiO2 mesh membrane was immediately rinsed with distilled water,dried in air, annealed at 350 °C for 1 h in air atmosphere to make TiO2

crystalline phase transform into anatase phase and enhance the na-nostructure robustness of TiO2 mesh membrane, and then cooled downto room temperature.

2.3. Oil/water separation experiment

The obtained titania mesh membrane was placed between two glasstubes and clamped with a cramp, above which, the burette with a valvewas fixed. The oil/water mixture (50% v/v) was firstly poured into theburette. When the valve on the burette was opened, the separation wasrealized with the action of gravity. The oil concentration of the originaloil/water mixture and the collected water after separation was mea-sured by using an infrared spectrometer oil content analyzer (CY2000,China). Through calculation with the absorbance and the correctioncoefficient, the oil content was obtained. The separation efficiency canbe calculated by Eq. (1) [10].

⎜ ⎟= ⎛⎝

− ⎞⎠

×R CC

(%) 1 100%O

P (1)

where Co and Cp are oil concentration of initial mixture and the filtratedwater after separation.

2.4. Anti-corrosive performance

The as-prepared titania mesh membrane was immersed in the cor-rosive solutions of 1mol L−1 HCl, 1 mol L−1 NaOH, 10 wt％NaCl andvarious oil mediums (1, 2-dichloromethane, chloroform and diesel),respectively for 24 h. After immersion, the water/oil wetting perfor-mances was evaluated by the contact angle and separation efficiencywas calculated by Eq. (1) to evaluate the anti-corrosive property andchemical stability.

2.5. Mechanical strength of nanostructured titania mesh membrane

To measure the mechanical strength of nanostructured TiO2 meshmembrane, the membrane was fixed with an inclination angle of 45°,and impacted by using the flowing sands (with an diameter of about450 μm) with an impact height of 25 cm (Fig. S9e). Meanwhile, themembrane was adhered using the adhesive tape with a pressure of5000 Pa. After impacting and adhering, the morphology and wettingperformances were examined.

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2.6. Self-cleaning performance

The TiO2 mesh fouled by a certain amount of diesel was irradiatedwith a 500W mercury xenon lamp under controlled light intensity witha wavelength at 365 nm (CHF-XM-500W, Beijing Chang Tuo tech-nology co., Ltd). The change of the contact angle under UV irradiationwas recorded to evaluate the self-cleaning performance of the TiO2

mesh membrane. The following heating treatment was carried out in anoven at 150 °C for 1 h.

2.7. Characterization.

The microstructures of TiO2 mesh membrane were observed on ascanning electron microscope (SEM, HITACHI, SU8010). The crystalphase of the membrane was identified through an X-ray diffractometer(XRD, X’Pert-Pro MRD, Philips). The contact angle (CA) was determinedby using a contact angle meter (JC 2000D5, Shanghai ZhongchenDigital Technology Apparatus Co., Ltd) with constant water/oil droplet(ca. 4 μL). The CA was performed for five points at different positions,and an average value was adopted. For underwater oil and underoilwater wettability measurements, the membrane was firstly fixed in atransparent container filled with water or oil, respectively. Then an oildroplet or water droplet was dripped upper or under the membrane tocontact the membrane, depending on the density difference between oiland water. The adhesive force is determined by using a micro-electro-mechanical balance system (Dataphysics DCAT 11, Germany). Briefly,an oil droplet (1, 2-dichloroethane, 4 μL) or a water droplet (4 μL) wassuspended with needle tip and controlled to contact with the surface ofthe TiO2 mesh membrane at a constant speed of 0.005mm s−1 and thento leave. The forces were recorded during the entire time.

3. Results and discussion

3.1. Surface morphology of nanostructured TiO2 mesh membrane.

Fig. 1a presents the SEM image of the original Ti mesh. It can beobserved that the Ti wires (average diameter 105 μm, Fig. 1b) werewoven together and formed “S” shaped pores. After the electrochemicalanodization for a certain time and heating process (Figs. S1–S3), anumber of nanoarrays grew vertically onto the Ti mesh substrate andthe average diameter of Ti wire increased to about 123 μm (Fig. 1c andd). From the magnified images (Fig. 1e), the nanowires were observedto bundle together and form lots of nanopores with an average pore sizeof about 200 nm. The height and diameter of nanowires were about18 μm and 20 nm, respectively (Figs. 1f and S4). The X-ray diffraction(XRD) result indicates that the obtained nanostructures are ascribed tothe anatase TiO2 (Fig. S5) [55], which is in agreement with the EDSresult (Fig. S6). From the above, it can be concluded that the nanos-tructured anatase TiO2 arrays can be produced on the Ti mesh substrateby a simple electrochemical anodization and heating process.

3.2. Wettability of nanostructured TiO2 mesh membrane

In air, the as-fabricated membrane exhibited superhydrophilicityand superoleophilicity with both water contact angle (WCA) and oilcontact angle (OCA) of almost 0° (Fig. 2a and b). After immersion inwater or oil (n-hexane), the membrane shows superoleophobicity andsuperhydrophobicity with an OCA and WCA of about 160° and 158°,Fig. 2c and d, respectively, indicating dual superlyophobicity of theobtained membrane in liquid medium. To further study the underwateroil-repellent and underoil water-repellent abilities of the membrane, anoil droplet (1, 2-dichloroethane) or a water droplet was used to contactthe membrane and then leave, respectively. From Fig. 2e and f, it can beseen that under a certain pressure, the oil or water droplet can stillleave the membrane easily without any residue, showing that themembrane possesses very low affinity to oil or water droplet in water or

oil. The underwater oil and underoil water adhesive forces for themembrane were further examined by a micro-electromechanical bal-ance system. It was too low to be examined (lower than 2 μN, Fig. 2gand h), further confirming the ultralow adhesion of the membrane. Inaddition, the membrane presents the underwater superoleophobicityfor various oils, and superhydrophobicity under various oils. As shownin Fig. 3, all underwater OCA and underoil WCA on the membrane arelarger than 150° for various oils including diesel, hexane, gasoline,petroleum ether, 1, 2-dichloroethane and chloroform. This indicates thedual superlyophobicity of membrane for a wider range of oils.

3.3. Separation performances of nanostructured TiO2 mesh membrane

The membrane with such a special superwetting performance isreported to be able to separate oil/water mixtures [56]. In order toevaluate the separating ability of the TiO2 mesh membranes, someproof-of-concept studies have been conducted. As mentioned in in-troduction, the traditional separating membranes with super-hydrophobicity/superoleophilicity and superhydrophilicity/under-water superoleophobicity can separate either light (ρoil < ρwater) orheavy (ρoil > ρwater) oil/water mixtures, because the barrier layercaused by the density difference between oil and water often arisesduring the separation process. Both heavy (ρoil > ρwater) and light(ρoil < ρwater) oil/water mixture were used to investigate the separ-ating property of the membrane. For light oil (hexane)/water mixture,the water and oil are dyed with methylene blue and oil red O, respec-tively, for clear observation (Fig. 4). Water filtrates form the membraneand oil is blocked (Fig. 4a and Video S1). However, for the heavy oil/water mixture, 1,2-dichloroethane/water mixture was used as an ex-ample, opposite phenomenon is observed that oil permeates throughthe membrane and water is blocked (Fig. 4b and Video S2). In addition,diesel, gasoline, petroleum ether, chloroform and water mixtures havealso been separated successfully. These results indicate that light oil(ρoil < ρwater) or heavy oil (ρoil > ρwater)/water mixtures could be se-parated under gravity-driven action, achieving “water-removing” or“oil-removing” type on our membrane.

Supplementary Video 1.

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(a) (b)

(d)(c)

(e) (f)

500 m

500 m

1 m

25 m

25 m

25 m

Fig. 1. (a) and (b) SEM images of pure Ti mesh substrate at low and high magnifications, respectively. (c) and (d) SEM images of TiO2 mesh membrane at low and high magnifications,respectively. (e) SEM images of the TiO2 nanostructure. (f) The cross-sectional SEM image of TiO2 nanostructures.

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Fig. 2. (a) and (b) A water droplet (4 μL) and an oil droplet (1, 2-dichloroethane, 4 μL) on the TiO2 mesh membrane in air with a CA of almost 0°; (c) and (d) Underwater oil droplet (1, 2-dichloroethane) and underoil water droplet on the membrane with OCA and WCA of about 160° and 158°, respectively; (e) and (f) Photographs of the dynamic underwater oil-adhesionand underoil water-adhesion measurement on the membrane. An oil droplet (1, 2-dichloroethane, 4 μL) (e) or a water droplet (4 μL) (f) was used as the detecting probe to contact themembrane and then leave. It can be seen that the mesh membrane exhibited an ultralow affinity to oil and water in water and oil, respectively. (g) and (h) Force-distance curves recordedbefore and after the underwater oil droplet (g) and underoil water droplet (h) came into contact with the membrane, respectively.

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Fig. 3. Statistic of OCA for various oils in water and WCA in various oils, respectively.

Fig. 4. Oil/water separation processes: (a) for light oil/water mixture (ρoil < ρwater), water would sink spontaneously due to the higher density (left Fig.) When the valve is opened, watercan permeate the membrane and the following oil be retained (right Fig.) (b) For heavy oil/water mixture (ρoil > ρwater), the oil would pass through the membrane and water wasblocked. The water is dyed with methylene blue and oil is dyed with oil red for clear observation.

Fig. 5. (a) The separation efficiency and flux of TiO2 mesh membrane for different lightand heavy oil/water mixtures (b) Experimental values of intrusion pressures for variousoils (water-removing type) and water (oil-removing type).

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Supplementary Video 2. To assess the separation ability for the TiO2 meshmembrane, the separation efficiencies were firstly investigated. Ac-cording to Eq. (1), the separation efficiencies were calculated for allused oil/water mixtures, which are higher than 99.4% (Fig. 5a), de-monstrating a good separating ability of our membrane. Besides theseparating efficiency, the liquid flux (F) is also measured, which iscalculated by using Eq. (2) [10].

=F VSt (2)

where V is the water or oil filtration volume, S is the membrane areaand t is the permeation time of liquids with a fixed volume. The resultsshow that all liquid fluxes were higher than 16,954 Lm−2h−1 (Fig. 5a),meaning that our membrane can realize fast separation. Furthermore,the intrusion pressure (an important performance during separationprocess) of the membrane was defined as the maximum height of liquidmediums (hexane, gasoline, petroleum ether, diesel, 1, 2-di-chloroethane and chloroform) and was also investigated. For “water-removing” separation type, various oil intrusion pressures are ex-amined, while for “oil-removing” separation type, water intrusionpressures are determined. Accordingly, the experimental intrusionpressures (Pexp) can be obtained by using Eq. (3) [7].

=P ρghexp max (3)

where Pexp is the experimental value, ρ is liquid density, g is gravityacceleration and hmax is liquid maximum height. The greater the valueof intrusion pressure, the better the stability of the TiO2 mesh mem-brane is. Fig. 5b indicates that the intrusion pressures for all liquids arehigher than 0.85 kPa, showing that TiO2 mesh membrane processes agood stability.

3.4. The anti-corrosive, mechanical stability and self-cleaning performances

For practical application, the anti-corrosive property of the mem-brane is of vital importance. Generally, inorganic separating materialssuch as oxide and metals are unstable in water solutions with strong

acid or basic [8,49], while organic membranes can be easily damagedby organic solvents (such as 1, 2-dichloromethane, chloroform anddiesel used in this work) [37,38], which seriously affect their realisticapplications. Noticeably, the as-prepared TiO2 mesh membranes pos-sess a unique anti-corrosive property. In order to estimate its anti-cor-rosive ability, the membrane is immersed in aqueous solutions con-taining 1M HCl, 10 wt% NaCl, 1M NaOH, and various oils used here (1,2-dichloromethane, chloroform and diesel), respectively for 24 h. Themicrostructures of the membranes had no apparent variation (Figs. S7and S8), and then the underwater oil and underoil water wetting per-formances were tested (for samples immersed in oils, treated by UVirradiation and heating process, we will discuss this in the later section).As shown in Fig. 6a and b, it can be seen that after 24-hour immersion,the underwater superoleophobicity and underoil superhydrophobicitycan still be observed with underwater OCA and underoil WCA of154°and 152°, respectively. Meanwhile, the separation efficiencies arestill higher than 99.2%, showing that the membrane possesses a betteranti-corrosive performance.

In addition to the anti-corrosive property, the mechanical stabilityof the surface microstructure is also important, since a lot of externalactions such as friction and pressing can also result in the damage ofsurface microstructures, which can often lead to the loss of separatingfunction of the membrane and is difficult to be avoided in realisticconditions. Herein, to demonstrate the good mechanical stability, theexperiments about the impact of flowing sands and the adhering ofadhesive tape were carried out. The membrane’s microstructures haveno apparent variation after these experiments, and the underwater su-peroleophobicity and underoil superhydrophobicity can still be ob-served, respectively (Fig. S9), indicating a good mechanical stability.

Another problem facing separating materials is the fouling and isoften inevitable during the practical applications. As is well known,

Fig. 6. (a) The contact angle and separation efficiency of TiO2 mesh membrane afterimmersion in HCl (1M), NaCl (10 wt%) and NaOH (1M) corrosive solutions for 24 h,respectively. (b) The contact angle and separation efficiency of the membrane after im-mersion in 1, 2-dichloromethane, diesel and chloroform mediums for 24 h, respectively.Taken hexane/water mixture and 1, 2-dichloroethane/water mixture as examples forlight and heavy oil/water mixtures, respectively.

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after continuous separation of complex oily wastewater, the membranemight be fouled by some oils or organic molecules, and then lose itsoriginal surface wettability and separating ability. With a strong photo-catalytic degradation activity of anatase TiO2 [57], the membrane isexpected to have a special self-cleaning property. To assess the self-cleaning performance, TiO2 mesh membrane has been fouled arbitrarilywith a certain amount of diesel. After fouling, the wettability of themembrane changes into hydrophobicity in air, underwater oleophobi-city and underoil superhydrophobicity (Fig. 7a–c). Noticeably, such anoil-fouled membrane could not separate light oil/water mixture, in-dicating that the oil fouling has seriously affected the separatingproperty of the membrane. After a simple UV irradiation and heatingprocess, it is worth to note that the wetting performance of the fouledTiO2 mesh membrane can restore to its original superhydrophilicity anddual superlyophobicity (Fig. 7d–f), demonstrating that the obtainedmembrane has a particular self-cleaning property (from here, it would

be easy to understand that why the membrane needs to be irradiated byUV and heating at 150°Cfor 1 h as mentioned in Fig. 6). As shown inFig. 7g, it is seen that the WCA of the membrane gradually decreaseswith the increase of UV illumination time. After UV illumination for240min, the membrane shows superhydrophilicity with a WCA of al-most 0°, which is ascribed to the direct decomposition of diesel mole-cules by photo-induced holes and hydroxyl radicals with strong oxi-dation [58].

3.5. Stability of separation performance

To further investigate the recyclability of nanostructured TiO2 meshmembrane, the separation efficiency over water/oil mixtures with thevolume ratio of 1:1 was repeated for 10 times. The overall separationefficiencies are shown in Fig. 8. The separation efficiencies of the na-nostructured TiO2 mesh membrane for “water-removing” type (hexane

Fig. 7. The contact angles of TiO2 mesh membrane fouled with diesel before (a–c) and after (d–f) UV illumination: (a) and (d) water contact angle in air, (b) and (e) underwater oil contactangle, (c) and (f) underoil water contact angle. (g) The time-dependent evolution of the water contact angles of TiO2 mesh membrane fouled with diesel under UV illumination in air.

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and water mixture, ρoil < ρwater) and “oil-removing” type (1, 2-di-chloroethane and water mixture, ρoil > ρwater) were higher than 99.4%and 99.2%, respectively (Fig. 8a and b). Based on the switchable abilitybetween underwater superoleophobicity and underoil super-hydrophobicity and the above-mentioned self-cleaning ability, the na-nostructured TiO2 mesh membrane can also be used to alternately se-parate light oil or heavy oil/water mixture. After 10 alternate cycles,the separation efficiency still remained higher than 99.2% (Fig. 8c).These results demonstrate that our membrane has a good recyclability.

3.6. Separation mechanism of nanostructured TiO2 mesh membrane

From the above, it can be found that the superamphiphilicity anddual superlyophobicity endow the membrane with special separatingability. For a better understanding of the phenomena, the inner me-chanism that affects the wettability of the membrane was carefullyanalyzed. TiO2 is known as a kind of hydrophilic and oleophilic ma-terial [59]. After introducing nanostructures, the capillary effect wouldenhance the surface wettability according to Wenzel Equation (cosθw= r cos θ) [60]. Thus, the membrane shows superhydrophilicity andsueproleophilicity in air (Fig. 2a and b). When such a superhydrophilicmembrane was placed in water, water could enter the nanostructureseasily. When an oil droplet contacts the membrane, a new solid/water/oil interface can be formed (Fig. S10a). The high OCA can be illustratedby Eq. (4) [61].

′ = + −θ f θ fcos cos 1ow ow (4)

where θ’ow and θow are the OCAs on the nanostructured membrane andflat substrate, respectively, f is the contact area fraction. 1, 2-di-chloroethane is chosen as the measured oil droplet, θ’ow=160 ± 1°(Fig. 2a), θow=117 ± 1° (Fig. S10b) and f=0.11, showing that thecontact area of oil/water interface is 89%. Thus, underwater super-oleophobic and low adhesive force can be viewed (Fig. 2g). Similarly, asthe membrane is placed into oil, a water droplet also resides in thecomposite state, and the solid/water contact area is as low as 16.5%.Therefore, our membrane also shows a low adhesive underoil super-hydrophobicity (Fig. 2h, Fig. S10 a, c and discussion).

To understand the separation process clearly, the water and oilwetting process was modelled in Fig. 9. As reported, ΔP is needed to beovercome before liquid wetting the pores because the advancing con-tact angle (θA) must be surpassed, which can be explained by Eq. (5)[62].

= = −Pγ

Rlγ θ A

2(cos )/A (5)

where γ is the interfacial tension, l is the perimeter of the pores, R isthe radius of the meniscus, θA is the advancing contact angle, and A iscross-sectional area of the pore. From Eq. (5), it can be easily under-stood that for the θA > 90°, the membrane could sustain a degree ofpressure because ΔP > 0. On the contrary, the membrane cannotwithstand any pressure for the θA < 90°, and liquids can permeatethrough the membrane spontaneously because ΔP < 0. Fig. 2a and bshow that in air, the membrane is superamphiphilicity with a θA ofalmost 0°, and the membrane cannot support any pressure sinceΔP < 0 (Fig. 9a and c). Therefore, as shown in Fig. 4, under gravity-driven action, the water and oil can permeate the membrane sponta-neously as they first contact the membrane. What needs to be stressed isthat, during the permeation process, some water or oil could be trappedin nanopores. When the following oil or water contacts the membrane,it would reside in the composite wetting states as discussed above.Accordingly, the underwater superoleophobicity and underoil super-hydrophobicity can be observed (Fig. 2c and d). Under these conditions,θA is certainly larger than 90°, showing that the TiO2 mesh membranecould support a degree of pressure because ΔP>0 (Fig. 9b and d).Therefore, after the permeation of water or oil, the following oil orwater would be retained, Fig. 4, thus, the separation of heavy or lightoil/water mixtures is successfully realized by using the TiO2 meshmembrane.

3.7. The performance comparison of reported separation membranes withspecial dual superlyophobicity.

As shown in Table 1, these separation membranes show special dualsuperlyophobicity, i.e., both underwater superoleophobicity and un-deroil superhydrophobicity, and high separating efficiency above98.0%. However, based on the special dual superlyophobicity, TiO2

Fig. 8. Recyclability test for 10 cycles on the membrane: (a) “water-removing” type se-paration (water/hexane oil mixtures); (b) “oil-removing” type separation (water/1, 2-dichloroethane mixture); (c) alternate separation between the above two types.

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nanoclusters coated copper mesh membrane [50] were only used toseparate “oil-in-water” and “water-in-oil” emulsions with highest fluxof 9860 Lm−2h−1 and 750 Lm−2h−1, respectively, which would beunfavorable for immiscible oil/water mixtures and significantly limit

their practical applications because the pore sizes on these membranesare too small to realize high flux separation (small pore size is necessaryfor the separation of emulsions). To further overcome this imperfection,a diatomite-coated stainless steel mesh [51] and ZnO nanostructuredPET nonwoven fabrics [53] have successively reported, which wereused to successfully separate both heavy and light immiscible oil/watermixtures on a single membrane with high flux of 25,200 Lm−2h−1 and6900 Lm−2h−1, respectively. However, the self-cleaning property andgood antifouling ability have not been demonstrated on these mem-branes for separating heavy oil/water mixtures due to preferential pe-netration of heavy oil. And then, TiO2-coated stainless steel meshmembrane [54] has been further reported, which show high flux ofmore than 7281 Lm−2h−1 and self-cleaning property due to TiO2

photocatalytic activity. However, the separation membrane has notpresented the anti-corrosive ability and robust nanostructure stability.Therefore, in this work, a novel titania nanostructural mesh membranewith dual superlyophobicity were fabricated to successfully separateboth heavy (ρoil > ρwater) and light (ρoil < ρwater) oil/water mixtureswith high separation efficiency and higher flux of more than 99.2% and16,954 Lm−2h−1, respectively. Most importantly, TiO2 nanostructuredmesh membrane can simultaneously possess anti-corrosive ability, self-cleaning property and robust nanostructure stability. Given so manymerits of the unique membrane, it could be believed to be potentiallyused in wastewater treatment and oil spill recovery.

4. Conclusion

In short, TiO2 mesh membrane has been fabricated by electro-chemical anodizing and heating processes, which shows super-amphiphilicity in air and dual super-lyophobicity. Based on the mem-brane with such special wettability of dual superlyophobicity, i.e., bothunderwater superoleophobicity and underoil superhydrophobicity,both light (ρoil < ρwater) and heavy (ρoil > ρwater) oil/water mixturescould be highly efficiently separated (separation efficiency higher than99.2%). Meanwhile, the membrane also demonstrates good mechanicalstability, anti-corrosive and self-cleaning ability, and remains excellent

Fig. 9. Schematic illustration of the liquid-wettingmodels: TiO2 mesh membrane shows super-hydrophilicity (a) and superoleophilicity (c) in air andcannot sustain any pressure because of △P < 0, thuswater or oil can permeate through the membranespontaneously. (b) and (d) water or oil first permeatesthrough the membrane and the nanostructures havebeen occupied by water or oil, showing underwatersuperoleophobicity or underoil superhydrophobicity.Some water or oils can be supported on the TiO2 meshmembrane because △P > 0. Thus, the separation ofall kinds of oil/water mixtures can be achieved re-gardless of density difference.

Table 1The performance comparison of reported separation membranes with dual super-lyophobicity.

Separation membrane Separationobject

Present performances

Lin et al.[50]TiO2 nanoclusters coatedcopper mesh membrane

Oil-in-wateremulsionWater-in-oilemulsion

Dual superlyophobicityHighest flux 750 L/m2 hSeparation efficiencyabove 99.97%Anti-corrosive ability

Li et al.[51]A diatomite coated coatedstainless steel mesh

Light oil/watermixtureHeavy oil/watermixture

Dual superlyophobicityThe flux above 27,000 L/m2 hSeparation efficiencyabove 99.2%Anti-corrosive ability

Xiong et al.[52]ZnO nanostructured PETnonwoven fabrics

Light oil/watermixtureHeavy oil/watermixture

Dual superlyophobicityHighest flux: 6900 L/m2 hSeparation efficiencyabove 99.0%

Du et al.[53]TiO2-coated stainless steelmesh membrane

Light oil/watermixtureHeavy oil/watermixture

Dual superlyophobicityThe flux above 7281 L/m2

hSeparation efficiencyabove 99.9%Self-cleaning property

This workTiO2 mesh membrane

Light oil/watermixtureHeavy oil/watermixture

Dual superlyophobicityThe flux above 16,954 L/m2 hSeparation efficiencyabove 99.4%Anti-corrosive abilitySelf-cleaning propertyRobust nanostructurestability

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separating property even after immersion into water solutions includingstrong alkali, acid, salty, various organic solvents, and even physicalabrasion. Considering high separating efficiency, anti-corrosion, andself-cleaning of the membrane, the membranes can be potentially ap-plied in wastewater disposing, microfluidic device, and oil spill re-covery.

Acknowledgements

This work was financially supported by the National ScienceFoundation of China (NSFC grant Nos. 21674030 and 51573035).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in theonline version, at https://doi.org/10.1016/j.seppur.2018.03.002.

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