Dually prewetted underwater superoleophobic and underoil

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Dually prewetted underwater superoleophobic and underoil superhydrophobic fabric for successive separation of light oil/water/heavy oil three-phase mixtures Guoliang Cao, Wenbo Zhang, Jia Zhen, Feng Liu, Haiyue Yang, Qianqian Yu, Yazhou Wang, Xin Di, Chengyu Wang, and Shih-Hsin Ho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08997 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Applied Materials & Interfaces

Dually prewetted underwater superoleophobic and underoil superhydrophobic fabric for successive separation of light oil/water/heavy oil three-phase mixtures Guoliang Cao,† Wenbo Zhang,† Jia Zhen,§ Feng Liu,† Haiyue Yang, † Qianqian Yu,† Yazhou Wang,† Xin Di,† Chengyu Wang*, † and Shih-Hsin Ho* ‡ †

Key Laboratory of Bio-Based Material Science and Technology of Ministry of Education,

Northeast Forestry University, Harbin, 150040, P. R. China ‡

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of

Technology, Harbin, 150040, P. R.China §

School of Technology, Harbin University, Harbin, 150040, P. R.China

KEYWORDS: underwater superoleophobicity, underoil superhydrophobicity, dually prewetted, successive separation, light oil/water/heavy oil mixtures

ABSTRACT: Remediation of oil spills requires new technologies to separate light oil/water/heavy oil mixtures. Low-cost, biological, and environmentally friendly materials are needed to treat water pollution caused by oils. In this study, a corn straw powder (CSP) coated

ACS Paragon Plus Environment

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fabric (CSPF) was fabricated by spraying waste CSP and polyurethane onto amphiphilic cotton fabric, and thus, the wettability of CSPF is enhanced by taking advantage of the hierarchical structure and increased surface roughness. Therefore, the CSPF could be dually prewetted (DCSPF) with both water and oil and showed underwater superoleophobic and underoil superhydrophobic properties without any further chemical modification. When applied to light oil/water/heavy oil separation, the DCSPF could be used to successively separate light oil/water/heavy oil three-phase mixtures under gravity with a high separation efficiency and flux. In addition, the DCSPF showed excellent structural and chemical stability according to repeated cycling and corrosive solution/oil separation experiments. The results of this study is of value in providing a simple, low-cost and environment-friendly approach for application in the field of successive separation of light oil/water/heavy oil three-phase mixtures.

INTRODUCTION

Pollution caused by oil spills has been recognized as a particularly severe environmental issue. There is an increasing demand for low-cost and efficient oil/water separation approaches.14

Recently, surfaces with modified surface properties have been used for oil/water separation.5-10

Such superhydrophobic/superoleophilic materials with porous structures have been extensively developed by designing the texture and chemical composition of surfaces to enable oil collection from oil/water mixtures.11-14 Generally, these materials can be divided into two categories, which are oil absorbents and strainers. Several commercial sponges/foams modified by hydrophobic nano particles,15-17 nanowire films,18,19 and microporous polymers20,21 have been shown to perform as effective oil absorbents with high selectivity, high efficiency, and good recyclability. However, many of these reported absorbents have a limited separation rate, poor continuous


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operation ability, and high operating costs, which limit their practical applications. Oil strainers are based on coating porous substrates with hydrophobic substances.22 Superhydrophobic and superoleophilic membranes containing polypropylene microfiltration film co-deposition of dopamine and low molecular weight polyethyleneimine,20 polyester textiles coated by silicone nanofilaments,23 and candle soot coated mesh films24 have been reported to enable separation of oil/water mixtures. However, most of these traditional superhydrophobic/superoleophilic materials are fabricated by chemically modifying the surfaces with fluoroalkyl silanes.17 These materials

might have adverse effects on both the habitat and living health. Underoil

superhydrophobic materials will ameliorate these problems, these could offer a substitution approach to fabricating oil-removing materials. Unlike superhydrophobicity in air, underoil superhydrophobicity is accomplished by adding oleophilic groups to a micro/nanoscale multistage rough surface without using low surface energy materials.25 Other approaches to oil removal include those based on materials with superhydrophilic and underwater superoleophobic surfaces,18,26 also known as water-removing materials. These systems are typically based on hydrogels,3 polymer films,27 silica,28 graphene oxide,29 alumina nanoparticles30 and nanocellulose.31,32 Among these materials, hydrophilic groups and polymers have been extensively used to accomplish superhydrophilic and underwater superoleophobic properties enabling highly efficient water collection from oil/water mixtures.33,34 Water generally has a higher density than that of light oils, and superhydrophilic and underwater superoleophobic films are widely used in these separations.35 However, underwater superoleophobic films are not appropriate candidates for separation of water/heavy oil mixtures because heavy oil forms a barrier between water and the film, thus preventing water outflow. Recently, underwater


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superoleophobic and underoil hydrophobic surfaces have been developed for water/heavy oil separation.36,37 However, to the best of our knowledge films with single wetting property can only be used to separate determinate oil/water mixtures. For example, superhydrophobic/superoleophilic films and underoil superhydrophobic films can only be used to separate water/heavy oil mixtures;25,38 and underwater superoleophobic films can only be used for separation of light oil/water mixtures.3,18, 27 Although there are many reports on separation of individual oil types, reports of surfaces that can be used to separate both oil types are extremely limited. More importantly, successive separation of light oil/water/heavy oil three-phase mixtures with underwater superoleophobic and underoil superhydrophobic films has yet to be achieved. Novel films with underwater superoleophobicity and underoil superhydrophobicity might enable successive separation of light oil/water/heavy oil three-phase mixtures by a simple, efficient, and economical synthetic approach. It has been suggested that, the films should be based on renewable materials, which do not contain fluoroalkyl silanes. Corn straw, is an abundant renewable resource, and has desirable attributes for a feedstock owing to its high yield, low cost, and environmental compatibility. The majority of corn straw is burned, contributing to environmental pollution and wasting a potentially useful resource.39 The treatment of waste with waste has many social, economic, and environmental benefits.40 Thus, corn straw powder (CSP) is considered to be a promising precursor material. We proposed that underwater superoleophobic and underoil superhydrophobic CSPs coated fabric might allow easy separation of light oil/water/heavy oil three-phase mixtures. Herein, we fabricated superamphiphilic waste CSP coated fabric (CSPF) by spraying waste CSP and polyurethane (PU) solutions on to cotton fabric. The dully prewetted CSPF (DCSPF)


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could be used for successive separation of light oil/water/heavy oil three-phase solution mixtures under gravity: half of the DCSPF was prewetted with water to form a water-containing region (WCR), which possessed underwater superoleophobicity; the other half of the DCSPF was prewetted with oil to form an oil-containing region (OCR), which possessed underoil superhydrophobicity. During the separation, oil and water were selectively passed through the OCR and WCR and separately collected in different containers. This is the first report of dually prewetted underwater superoleophobic and underoil superhydrophobic film used for successive gravity separation of light oil/water/heavy oil three-phase mixtures. Furthermore, the DCSPF exhibited high separation efficiency for multiple light oil/water/heavy oil three-phase mixtures and good chemical stability towards various corrosive solutions including 1 M solutions of HCl, NaOH, and NaCl. The information obtained in this study could greatly simplify successive separation of light oil/water/heavy oil three-phase mixtures. EXPERIMENTAL SECTION Materials. Waste corn straw was collected from a farmland in Harbin, Heilongjiang province,

China.

Hexane,

kerosene,

methyl

benzene,

chloroform,

dichloroethane,

dimethylbenzene, acetone, toluene, and absolute ethanol were purchased from Guangdong Guanghua Sci-Tech Co., Ltd. (Guangdong, China). Sodium hydroxide, sodium chloride, and hydrochloric acid were purchased from Tianjin Kaitong Chemical Reagent Co., Ltd. (Tianjin, China). Water from an Ultrapure Water System was provided by Beijing Zhongyang Yongkang Environmental Protection Technology Co., Ltd. (Beijing, China). Cotton fabric was purchased from a local market. All reagents were used as received without further purification. Pretreatment of the waste corn straw. First, the waste corn straw was placed in a pulverizer to obtain the crude CSP, which was then passed through a 400-mesh standard sieve to


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collect uniform powder. Before use, the CSP was ultrasonically cleaned with water, absolute ethanol, and water, in turn. The CSP was then submerged into sodium hydroxide aqueous solution and stirred at ambient temperature for 12 h. The pH of CSP was then adjusted to 6.5–7.0 with hydrochloric acid. The CSP finally was washed with water several times and dried at 40 °C. Preparation of CSP coated fabric (CSPF). First, the cotton fabric was ultrasonically cleaned with acetone, absolute ethanol and water, in turn. After washing, the pretreated cotton fabric was dried at 60 °C. Then, 1.0 g of the CSP was dispersed in 30 mL of absolute ethanol and stirred vigorously to make a mixture solution. After that 0.6 g of polyurethane was added in the mixture solution.36 Afterwards the mixture solution was sprayed onto a piece of cotton fabric substrate with a spray gun. Finally, the resulting CSPF was dried at room temperature to evaporate the absolute ethanol. Separation of light oil/water/heavy oil three-phase mixtures. The DCSPF was fixed between two frosted glass fixtures (Figure S1, Supporting Information): the upper frosted glass fixture was connected to a glass tube, and the lower frosted glass fixture had two outlets each attached to glass tubes. The two glass guide tubes were placed beneath the WCR and OCR (Figure S1e). Five types of light oil/water/heavy oil three-phase mixtures were investigated in this study, including: methylbenzene/water/chloroform (M1), dimethylbenzene/water/chloroform (M2),

hexane/water/chloroform

(M3),

kerosene/water/chloroform

(M4),

and

dimethylbenzene/water/dichloroethane (M5). The oils and water were tinted with Sudan III and methylene blue, respectively. A 300 mL of the light oil/water/heavy oil three-phase mixtures (volume ratio = 1:1:1) were added to an upper glass tube before coming into contact with the prewetted CSPF. The separation efficiency was measured according to η = (m1/m2) × 100,


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where m1 and m2 are the volume of the collected liquid phase after the separation process and the prepared liquid phase before the separation process, respectively. The flux was calculated according to F =V/St, where V is the volume of water or oils that permeated the film, was set to be 100 mL; t is the requisite time for infiltration of 100 mL liquid; and S is the area of the film. The separation process was driven only by gravity. The light oil/water/ heavy oil three phase mixtures were prepared in the upper glass tube in the first batch. The DCSPF initially came into contact with the heavy oil, which passed through the OCR and was collected in a designated beaker through the oil outlets. Next, the water layer passed though the WCR and was collected in a designated water beaker through the water outlet. Finally, the beaker containing heavy oil was replaced with a light oil beaker while water was removed from the remaining light oil/water twophase mixture. Finally, light oil flowed into the light oil beaker through the OCR. In this way heavy oil, water, and light oil were separated. In the second batch, the heavy oil, water, and light oil mixtures were sequentially dumped onto the upper glass tube. The separation process followed the same procedure but the collection sequence was changed to light oil then water followed by heavy oil. The structural stability of the DCSPF was evaluated through repeated cycling experiments of the light oil/water/heavy oil three-phase mixtures separation. Corrosive solution/oil separation. Mixtures of dimethylbenzene and a corrosive solution (1 M solutions of HCl, NaOH, and NaCl) were used to inspect the chemical stability of the CSPF. The dimethylbenzene and corrosive solutions were colored with Sudan III and methylene blue, respectively. The prewetted CSPF was fixed between two frosted glass fixtures. A 200 mL of the dimethylbenzene/corrosive solution mixtures (volume ratio = 1:1) were added to an upper glass tube before coming into contact with the prewetted CSPF. Gravity was the driving force in the course of the separation process. The chemical durability of the was assessed by Fourier


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transform infrared spectroscopy (FT-IR) (the DCSPF samples have been used for separating light oil (dimethylbenzene) and corrosive solutions mixtures for 10 cycles before testing). Characterizations. Scanning electron microscope (SEM) images were collected with a TM3030 tabletop microscope (accelerated voltage: 15.0 kV; Hitachi, Japan). The diameter distribution of CSP was measured by dynamic light scattering using a laser light scattering instrument (Masterizer 2000, Malvern Instruments Ltd., Malvern, UK). The average was obtained using software (Zetasizer software, Ver. 2.2 from Malvern, UK). Surface profile testing was done using a D-120 (KLA-Tencor Corporation, USA) surface profiler and a Contour GT-K 3D optical microscope (Bruker Corporation, USA). Photographs were taken with a digital camera (D 7000, Nikon, Japan). Static contact angle images of water or oil droplets (5 µL) were collected at room temperature on an OCA20 Contact angle measurement system (Data-physics, Germany) equipped with a video camera. At least five different positions in the sample were measured. The surface chemical composition was investigated by FT-IR, Nicolet 6700, Thermo Fisher Scientific, USA). RESULTS AND DISCUSSION The surface morphologies of the original and coated cotton fabric were characterized by SEM imaging and surface profile testing. As shown in Figure 1a, the SEM images of unmodified cotton fabric showed fiber bundles, which were weaved with glossy cotton fibers. The width of the bundles and the spacing between the bundles were approximately 200 µm each. After the cotton fabric was coated with the mixture of CSP and polyurethane, the surface became rough with many irregularly distributed micro-hoodoos (Figure 1b and Figure S2, Supporting Information). The distance between individual micro-hoodoos was irregularly and approximately ranging from tens to hundreds of microns, which produced a structure with numerous randomly


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oriented valleys (Figure 1c, Figure S2 and S3, Supporting Information). The high-magnification SEM image in Figure 1c shows that each micro-hoodoo was an agglomeration of crushed corn straw powders, featuring a rough surface with hole sizes of a few micrometers. The highmagnification SEM image in Figure 1d clearly shows a side view of the micro-hoodoo, which possessed a number of ~6 µm crushed corn straw fibers in diameter (Figure S4, Supporting Information) and hollow cavities of different sizes. Thus, the hierarchical structure of the was obtained from both the original cotton fabric surface roughness and coated corn straw powders.

Figure 1. (a) is the SEM image of the original cotton fabric. (b–c) are the SEM images of DCSPF surface at low and high magnifications, respectively.(d) is the SEM image of side view of the DCSPF surface.


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Generally, the wettability of a film surface is dependent on both its surface topography and chemical composition. Owing to the hierarchical surface structure accompanied by the amphiphilic chemical

composition,

the fabric showed

potential

for both

underoil

superhydrophobicity and underwater superoleophobicity, that is superamphiphilicity. When oil and water droplets (2 µL) contacted the fabric, they immediately spread out and permeated the fabric. The contact angles (CAs) of nearly 0° were obtained (Figure 2 a and b, left). The DCSPF became superoleophobic once submerged in water and superhydrophobic when submersed in oils, as shown in Figure 2 a and b (right). Both underwater oils droplets and the underoil water droplets accomplished a quasi-spherical shape on the DCSPF surface with CAs of 155° and 152°, respectively. Compared with the large CAs for underwater oils droplets and the underoil water droplets on the DCSPF surface, the original surfaces showed the underwater oils droplets with CAs of more than 125°, and the underoil water CAs of slightly more than 120° (Figure S5, supporting information). The wettability obtained in this study was primarily owed to the hierarchical surface structure, with hollowed-out micro-hoodoos irregularly distributed over the rough surface, resulting in superamphiphilicity of the DCSPF. In water, the DCSPF was completely infiltrated by water owing the superhydrophilicity, and a large amount of water became trapped in hollow microstructure forming an oil/water/solid composite interface. When the DCSPF contacted oils, the trapped water and hollow micro-hoodoos greatly reduced the contact area between the oils droplets and the DCSPF surface, bringing about CAs of more than 150° for oils droplets under water. Similarly, when the DCSPF was immersed in oils, the oils trapped in hollow micro-hoodoos greatly reduced the contact chance between the water droplet and the DCSPF surface. Therefore, underoil superhydrophobicity was obtained. In order to understand well the water- and oil-repelling properties of the DCSPF in the underwater or


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underoil environments, we measured the dynamic adhesion process during the contact procedure. In these experiments, oils and water droplets (5 µL) were pressed against the DCSPF with a preload and then relaxed. In the course of the relaxation procession, the shapes of the underwater oils droplets and the underoil water droplets remained spherical. Afterwards the oil and water droplets from the DCSPF surfaces were collected with very little adhesion force (Figure 2c and d). These results demonstrate that the DCSPF features excellent anti-adhesion and antifouling performance for both oil and water. In addition, the various underwater oils and underoil water CAs were measured (Figure S6, supporting information). The DCSPF exhibited stable underwater superoleophobicity for all oils, including chloroform and dichloroethane, with CAs of larger than 150°. The underoil superhydrophobicity (hexane, toluene, dimethylbenzene and kerosene) for all oils featured water CAs of greater than 150°.


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Figure 2. Wettability of the DCSPF. (a) is the chloroform contact angles of the DCSPF in air (left) and in water (right). (b) is the water contact angles of the DCSPF in air (left) and in nhexane (right). (c) Photographs of the dynamic underwater oil adhesion process on the DCSPF. (d) Photographs of the dynamic underoil water adhesion process on the DCSPF. The DCSPF was submerged in methylbenzene. The DCSPF wetting states are typical Cassie-Baxter-type composite interfaces where water or oil droplets are suspended by the composite interface of another immiscible liquid and the solid.41 In order to further understand the wettability of the underoil superhydrophobicity and underwater superoleophobicity surface,42 the Cassie’s equation was applied to evaluate the amphiphilicity of the polyurethane and CSP and the hierarchical structure of the DCSPF surface:


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cosθc = fs (cosθ+1) -1

(1)

where fs is the fraction of the solid surface contacting with liquid, the fraction of one liquid contacting with another immiscible liquid at the surface is 1− fs, θ and θc represent the CAs of the underwater oil droplets or the underoil water droplets on cotton fabric and DCSPF surfaces, respectively. Using Cassie’s equation, we can explain the wettability of the DCSPF underwater or underoil. Taking n-hexane as an example, when the n-hexane droplet was in contact with cotton fabric and DCSPF in water, the CA of θ and θc was measured to be 122° ± 2.5° and 155.5° ± 3.7°, respectively. Then, according to Cassie’s equation, we could calculate the fraction of the DCSPF and water contacting with oil droplet fs =0.24 and 1− fs = 0.76, respectively. A smaller area fraction of the solid surface means a lower area of the oils droplets contacting the DCSPF surface in water. Consequently, as shown in Figure 3a and b, the hierarchical microstructure filled with water or oil without trapped air layer when submerged in either of the two liquids.43,44 The submerged microstructure supported the oil/water interfaces when a second immiscible liquid was introduced, meaning that the second liquid was suspended by the texture rather than intruding into the texture.45,46 Furthermore, the under-liquid wettability of DCSPF is dependent on its surface topography and chemical composition with the presence of surrounding solvents.47 Consequently, the chemical composition of the DCSPF was assessed by Fourier transform infrared spectroscopy (FT-IR). As shown in figure 3 c, line A is the original DCSPF, in the high frequency region, the N-H stretching peak of PU was at 3360 cm-1,48 indicating that the CSP disturbed the hydrogen bonding between N-H and C=O due to the strong interaction between CSP and PU. And a shoulder peak appeared at 3290 cm-1, which was indicated the hydrogen-bonded O-H stretching.


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Peak at 2897 cm−1, which stem from C-H stretching vibrations. Peak at 1639 cm-1 represented an obvious C=O stretching vibration on account of CSP. The peak at 1429 cm-1 was designated as a crystalline of the cellulose in CSP and 896 cm-1 as an amorphous absorption peak.49 The absorption peak at 1029 cm-1 was C-O stretching vibration of the cellulose in CSP. Line B and C in figure 3 were the FT-IR spectroscopy of the DCSPF was prewetted by light oil (n-hexane) before and after separating the oil (n-hexane) /water mixtures. Peaks at 2962 cm−1, 2925 cm−1 and 2870 cm−1 represented obvious C-H stretching vibrations on account of being prewetted by n-hexane. A shoulder peak at 1453 cm−1, which stem from C-H flexural vibrations of n-hexane. From FT-IR spectroscopy the little change between the DCSPF before and after separating oil/water mixtures (figure 3 c, line B and C) has been observed, which indicates that the DCSPF trap an oil layer within the texture and the trapped oil layer is not displaced by the suspended water. The trapped oil layer is an extremely repellent medium which results in an ultrahigh underoil water contact angle. Similarly, as shown in figure 3 d line B and C, the N-H and O-H stretching vibration exhibits stronger absorption peaks centered at around 3335 cm−1 and 3290 cm−1 because of being prewetted by water. Peak at 1640 cm-1 represented an obvious O-H flexural vibration on account of water. The little change of the DCSPF spectra before and after separating oil/water mixtures (figure 3 d line B and C) has been found, which indicates that the DCSPF trap a water layer within the texture through the hydrogen bonds, and the trapped water layer is not displaced by the suspended oil. The trapped water layer is an extremely repellent medium which results in an ultrahigh underwater oil contact angle.50,51 This unique hierarchical microstructure and amphiphilic chemical composition provides a promising way to and create surfaces that combine underoil superhydrophobicity and underwater superoleophobicity. In addition, the various oils FT-IR spectroscopy were measured (Figure S7, supporting information).


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Figure 3. (a) and (b) are schematics of oil/water interfaces supported by unique microstructure surfaces. (c) FT-IR spectroscopy of the DCSPF before (A) and after (B) being prewetted by light oil (n-hexane), C is the DCSPF after separating n-hexane/water mixtures. (d) FT-IR spectroscopy of the DCSPF before (A) and after (B) being prewetted by water, C is the DCSPF after separating oil/water mixtures. Furthermore, the wettability of the DCSPF enabled selective collection of water and oils from three-phase mixtures of light oil/water/heavy oil after the DCSPF were prewetted by oil and water in different areas (Figure S1 e): half of the DCSPF was prewetted with water to form a WCR which featured underwater superoleophobicity. The other half of the DCSPF was prewetted with oil to form an OCR, which featured underoil superhydrophobicity. When separating the light oil/water/heavy oil three-phase mixtures, the DCSPF first came into contact with heavy oil (Figure 4a), which passed through the OCR but was blocked by the WCR, thus


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allowing the heavy oil to pass into the designated oil container (Figure 5c). When the DCSPF came in to contact with water (Figure 4b), the water passed through the WCR but was blocked by the OCR, and transferred to the designated water container (Figure 5d). Finally, after all the water was collected, the upper light oil phase passed through the OCR and flowed into another oil container (Figure 4c and 5c). Figure 4d-f shows schematics diagram of the separation of light oil/water/heavy oil three-phase mixtures. The effectiveness of the separation based on the DCSPF can be attributed to two properties: first, the unique microstructure with many irregularly distributed micro-hoodoos (Figure 5a) and chemical composition, produced novel wetting phenomena, trapping water and oil when submerged in the two liquids without the formation of a trapped air layer. This effect resulted in either a water or oil layer becoming trapped within the surface texture. The second property relates to the hierarchical submerged microstructure, which can support stable oil/water interfaces when a second immiscible liquid is introduced. This effect allows the second liquid to be suspended by the texture rather than intruding into the texture and prevented the trapped liquid layer from being displaced by the suspended liquid (Figure 5b). The unique wettability of the DCSPF allows collection of water and oils from light oil/water/heavy oil three-phase mixtures regardless of the relative densities of the water and oil phases.


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Figure 4. (a)-(c) are physical picture of separation of the light oil/water/heavy oil three-phase mixtures.(d)-(f) are schematics of separation of the light oil/water/heavy oil three-phase mixtures.


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Figure 5. (a) is the SEM image of side view of the DCSPF; (b) is cartoon schematics of separation process: left is CSPF unique microstructure, middile is the dully prewetted CSPF (DCSPF) unique microstructure prewetted by both of water and oil, right is the DCSPF was used to separate light oil/water/heavy oil three-phase mixtures; (c) is schematic of oil passed through the OCR but was blocked by the WCR; (d) is schematic of water passed through the WCR but was blocked by the OCR. To assess the ability of the DCSPF to separate three-phase mixtures of light oil/water/heavy oil, the M2 mixture (i.e., 30 mL of dimethylbenzene, 30 mL of water and 30 mL of chloroform) was selected as a representative example. The upper part of the mixture was dumped directly to the DCSPF. Initially, the dimethylbenzene and water phases came into contact with the DCSPF. The WCR of the DCSPF prevented the light oil from entering into water outlet and the light oil was initially collected in the light oil container under the OCR of the DCSPF; the OCR of the DCSPF simultaneously prevented water from entering the oil outlet, and water was rapidly collected in the water container under the WCR of the DCSPF (Figure S8 a-c, supporting information). After the light oil was collected, the light oil container was replaced with a heavy oil container (Figure S8 d, supporting information). The residual water and heavy oil mixture was then transferred to the DCSPF. As shown in Figure S8 e supporting information, the heavy oil was prevented from penetrating the WCR but rapidly collected in the heavy oil container under the OCR. In this way, we obtained separated light oil, water, and heavy oil, respectively (Figure S8 f, supporting information). The DCSPF maintained good successive separation ability after five cycles with an average separation efficiency of more than 98.0%. Moreover, the average water, light oil, and heavy oil fluxes were approximately 3.8, 8.9, and 13.3 L m−2 s−1, respectively (Figure S8 g, supporting information). Unlike traditional superwetting films, the DCSPF enabled the successive separation of light oil/water/heavy oil three-phase mixtures under the driving force of gravity.


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To further valuate the separation ability of the DCSPF, 300 mL of light oil/water/heavy oil three-phase mixtures (volume ratio = 1:1:1) were prepared namely M1, M2, M3, M4, and M5 mixtures. Figure 6a shows that the water, light oil, and heavy oil fluxes were approximately 4, 12, and 20 L m−2 s−1, respectively. Notably, the separation efficiency of the DCSPF was higher than 97% for all of the light oil/water/heavy oil three-phase mixtures (Figure 6b). Although previously reported superwetting films also had high fluxes and efficiency, the DCSPF realized efficient successive separation of light oil/water/heavy oil three-phase mixtures. To evaluate the stability of the as-prepared material, the light oil/water/heavy oil separation efficiency versus the number of separation cycles was also investigated. We used the M2 mixture as an example. The results in Figure 7 show that the DCSPF maintained excellent separation ability after 50 cycles with a separation efficiency greater than 96.0%, which indicated the reusability of the asprepared fabric. The separation efficiency and flux were unchanged as the number of cycles increased, suggesting that the DCSPF has excellent structural stability.

Figure 6. (a) The flux of different light oil/water/heavy oil mixtures. (b) The separation efficiency of different light oil/water/heavy oil mixtures.


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Figure 7. The light oil/water/heavy oil mixture separation efficiency versus the recycle numbers by taking the M2 mixture as an example:(a) chloroform, (b) water and (c) dimethylbenzene.

To further study the chemical durability, we confirmed that the DCSPF remained stable after contact with corrosive solutions (i.e., 1 M solutions of HCl, NaOH, and NaCl) in two phase mixtures with oil. Mixtures of light oil (dimethylbenzene) and corrosive solutions were tested. The DCSPF, was chemically stable and featured the same underwater superoleophobicity, enabling efficient separation of the oil/corrosive solutions mixtures. A 200 mL of the dimethylbenzene/1 M HCl solution mixtures (volume ratio = 1:1) were added to an upper glass tube before coming into contact with the prewetted DCSPF (Figure 8a). The DCSPF prevented the dimethylbenzene from entering into container owing to the underwater superoleophobicity of the DCSPF, while the 1 M HCl solution was rapidly collected in the container. Similarly, the mixtures of dimethylbenzene/1 M NaOH (Figure 8b) and dimethylbenzene/1 M NaCl (Figure 8c) were also successfully separated by the DCSPF. Consequently, the chemical durability of the DCSPF samples were assessed by Fourier transform infrared spectroscopy (FT-IR). From FT-IR spectroscopy we found little change of the DCSPF spectra after ten cycles of separating the corrosive solution/oil mixtures. These results indicate that the DCSPF is chemically resistant to extreme pH conditions and is a promising candidate for practical applications.


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Figure 8. Chemical durability experiments of the DCSPF for separation of dimethylbenzene and different corrosive solutions: (a) 1 M HCl, (b) 1M NaOH and (c) 1 M NaCl mixtures.(d) FT-IR spectroscopy of blank and after separating corrosive solutions/oil mixtures ten cycles.

CONCLUSION In summary, dually prewetted underwater superoleophobic and underoil superhydrophobic waste corn straw powder (CSP) coated fabric (DCSPF) was fabricated by a simple, low-cost and


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environmentally friendly spray-coating process. Multiple light oil/water/heavy oil three phase mixtures were separated with a high efficiency of more than 97%. In addition, the DCSPF maintained

excellent

separation

ability

after

50

cycles

of

separating

a

dimethylbenzene/water/chloroform three phase mixture with separation efficiency greater than 96.0%, suggesting that the DCSPF has excellent structural stability. Furthermore, after 10 cycles separation of dimethylbenzene and a corrosive solution (1 M HCl, NaOH, or NaCl) mixtures, the DCSPF showed good chemical stability which was confirmed by FTIR. This study provides a simple, low-cost and environment friendly approach for successive separation of light oil/water/heavy oil three-phase mixtures. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The apparatus used for abrasion tests; contact angle images; 3-D roughness profiles of the DCSPF; surface height profiles of the DCSPF; photographs of another batch, which the heavy oil, water, and light oil mixtures were sequentially dumped onto the DCSPF. AUTHOR INFORMATION Corresponding Author *E-mail: Chengyu Wang: [email protected] Shih-Hsin Ho: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was supported by the National Key Research and Development Program of China (2017YFD0600204) and the Special Fund for Forest Scientific Research in the Public Welfare (201504602) Notes The authors declare no competing financial interest. ABBREVIATIONS PU, polyurethane; SEM, scanning electron microscopy; CA, contact angle; OCR, oil-containing region; WCR, water-containing region; CSP, corn straw powder; CSPF, corn straw powder coated fabric; DCSPF, dually prewetted corn straw powder coated fabric.

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Table of Contents:


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Dually prewetted underwater superoleophobic and underoil

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