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A facile preparation of superhydrophobic halloysite-based meshes for efficient oil–water separation
Applied Clay Science 156 (2018) 195–201
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
A facile preparation of superhydrophobic halloysite-based meshes for efficient oil–water separation
T
Danyi Guoa, Jiahui Chena, Kun Houa, Shouping Xua, Jiang Chenga, Xiufang Wena, ⁎ ⁎ Shuangfeng Wanga, Chaoyun Huangb, , Pihui Pia, a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Nuclear and Radiation Safety Centre, MEP, Beijing 100082, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Superhydrophobic Halloysite Spray Stainless steel mesh Oil-water separation
A superhydrophobic halloysite-based mesh was facilely prepared by spraying epoxy/hexadecyltrimethoxysilanehalloysite nanotubes (HDTMS-HNTs) on stainless steel mesh. The as-prepared mesh was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical contact angle meter (OCA). The HNTs modified by HDTMS not only enhanced surface roughness, but also endowed hydrophobicity of the mesh. The mesh, with a static water contact angle of 154° and a sliding angle of 1.5°, was applied to separate a series of oil-water mixtures, such as n-hexane-water, isooctane-water and petroleum ether-water, with high separation efficiency of over 98%. The mesh still kept separation efficiency approximately 98.5% even after 25 separation cycles for n-hexane-water mixture separation. More importantly, the mesh is durable enough to withstand heat, chemical and mechanical challenges, such as hot water, strong alkaline, strong acid and sand abrasion, and high hydrostatic pressure. The as-prepared mesh will be a promising material in oil-water separation, because of the simple, economical and easily scalable preparation method and the excellent separation performance in radical oil-water separation.
1. Introduction
(Patowary et al., 2015), self-assemble (Shao et al., 2014), chemical vapor deposition (Hsieh et al., 2008) and so on. The limitations of these methods for large-scale fabrication are complex preparation, expensive equipment and long process. Besides, fluorinated acrylic resins were usually used to fabricate superhydrophobic surfaces (Wen et al., 2015; Pi et al., 2016). The long-chain fluorinated alkane can lower the surface energy greatly, and then makes the surface to be superhydrophobic, while its price is high and it will be accumulated in organisms which do harm to the environment and mankind. Therefore, it's highly warranted to find a simple, facile and low-energy method to fabricate a stable, economical and easily scalable superhydrophobic material in rapid oilwater separation. Halloysite clay is a kind of naturally deposited aluminosilicate material [Al2Si2O5(OH)4·nH2O], which is generally a hollow nanotubular structure as shown in Fig. S1. It is a two-layered (1:1) clay mineral with abundant Si-OH and Al-OH groups (Hou et al., 2017). In addition, the size of HNT varies from 50 to 70 nm in external diameter, 15 nm in lumen diameter and 1 ± 0.5 μm in length (Vergaro et al., 2010). Halloysite nanotubes (HNTs) are far less expensive, eco-friendly and abundant, compared with other nanosized materials, such as carbon nanotubes, boron nitride nanotubes and titanium dioxides
With the development of the industrial society, the problems of environment and pollution have been brought to the forefront. The ever-increasing oily sewages in the industrial production and frequent oil spills during oil exploitation and transportation not only lead to serious economic issues, but also pollute the ecosystem severely (Shannon et al., 2008; Ahmadun et al., 2009; Dubansky et al., 2013). Thus, it's urgently demanded to exploit a rapid and effective approach for oil sewage remediation. It is a facile and efficient methods to design new materials with special wettability for oil-water separation, since oil-water separation is affected by interface force. Recently, it has triggered increasing research interest to apply superhydrophobic surperoleophilic porous materials to oil-water separation (Wang et al., 2015; Pi et al., 2016; Qing et al., 2017; Zeng et al., 2017; Hou et al., 2018). Since the unique water repellency and oil affinity of these materials, they can separate oil from water effectively by means of filtration or absorption. In addition, a variety of methods to fabricate superhydrophobic and superoleophilic materials have been reported, for example, sol-gel processing (Yang et al., 2010), lithography(Gao et al., 2015), electrostatic spinning (Yohe et al., 2012), chemical etching
⁎
Corresponding authors. E-mail addresses: [email protected] (C. Huang), [email protected] (P. Pi).
https://doi.org/10.1016/j.clay.2018.01.034 Received 20 November 2017; Received in revised form 6 January 2018; Accepted 26 January 2018 Available online 13 February 2018 0169-1317/ © 2018 Published by Elsevier B.V.
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sample/200 mg of KBr) with 64 scans at 4 cm−1 resolution in the 4000–400 cm−1 region to characterized the chemical composition of samples. The structures of samples were characterized by X-ray diffraction (XRD) with an X-ray diffractometer (D8 Advance, Bruker) with Cu Kα radiation (λ = 1.5418 Å). The diffractograms were scanned in 2θ ranges from 10 to 70° at a rate of 8 °/min. The surface morphologies of the films were observed by a scanning electron microscopy (SEM, Merlin). In addition, contact angles and sliding angles of water and oil were measured using an optical contact angle measuring device (Data Physics OCA20, Germany) with 5 μL drop of deionized water or oil, respectively. The reported values of contact angles and sliding angles were the average of at least three measurements at different positions on the same mesh. The mechanical stability of the coating was evaluated by taking sand abrasion tests (Milionis et al., 2016; Hou et al., 2017) on the coated mesh surface using sand particles with diameters of 180–280 μm at a height of 18 cm.
nanotubes. With the introduction of HNTs, the coating is expected to have better thermal stability and/or fire resistance (Makaremi et al., 2015; Qin et al., 2016). Therefore, Halloysite clay with hydrophobic treatment for developing superhydrophobic materials (Wu et al., 2013; Ma et al., 2017) can open up a new avenue for the application of clay, which is also a promising candidate for oil-water separation. In this paper, a facile preparation of superhydrophobic meshes was reported by spraying epoxy and HNTs modified by hexadecyltrimethoxysilane (HDTMS) on stainless steel meshes. HNTs were transformed from hydrophilic to hydrophobic with modification of HDTMS. The introduction of epoxy resin could enhance the filmforming property and improve the stability of the coating. Besides, stainless steel meshes are widely applied to oil-water separation for a perfect combination of low cost, favorable mechanical property and easy reuse without squeezing, compared with other substrates, such as sponges, foams and filtration fabrics. Furthermore, spray is one of promising technique in practical production for its easy operation and low cost. The coated mesh has a perfect combination of robust superhydrophobicity, great reusability, excellent water pressure resistance and favorable stability to heat, chemistry and machinery, which is expected to be applied to large-scale oil-water separation, due to the easy, simple, cheap and scalable fabrication method.
2.4. Oil-water separation A series of oils, including n-hexane, isooctane, petroleum ether, vegetable oil and xylene, were mixed with equal volume of water to form oil-water mixtures. A superhydrophobic mesh was fixed in the miniature separator, of which the diameter is about 2 cm, and then 20 mL of oil-water mixtures were slowly poured into the separator at the speed of 1.2–1.3 mL/s. Oils penetrated the mesh into the beaker quickly while water was stuck in the separator. Gravity was the only driving force during the separation process. The water was collected in the container and weighed. The separation efficiency η was obtained according to η = (m1/m0) × 100%, where m0 and m1 are the mass of the water before and after the separation, respectively (Pan et al., 2008).
2. Experimental section 2.1. Material Natural HNTs were mined from Yichang, Hubei, China. Hexadecyltrimethoxysilane (HDTMS) was purchased from Aladdin Industrial Corporation. The epoxy resin E-44 was bought from Nantong Xingchen Synthetic Material Incorporation. Ethanolamine was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. Anhydrous ethanol were supplied from Tianjin Kemiou Chemical Reagent Co., Ltd. Hydrochloric acid (HCl) and acetone were purchased from Tianjin Hongda Chemical Reagent Co., Ltd. Stainless steel meshes were purchased from a local hardware store (Guangzhou, China), and purified by ultrasonic washing with water and acetone for 30 min, respectively, and then completely dried in an oven at 60 °C before use. HNTs were dispersed in 1 mol/L HCl (m/m = 1:20) with magnetic stirring for 24 h at room temperature and then washed with distilled water until the water was neutral and dried in vacuum drying oven at 60 °C. The other chemicals were used as received without any further purification.
3. Results and discussions The chemical composition and structure of pure HNTs and HDTMSHNTs were analyzed by FT-IR and XRD, which were shown in Fig. 2. In the FT-IR spectra (Fig. 2a), the peaks at 3693, 3624 and 911 cm−1 were due to the stretching vibration of the inner-surface hydroxyl groups, the stretching vibration of the inner hydroxyl groups and the deformation vibration of the inner-surface hydroxyl groups, respectively. The strong absorption band at 1034 cm−1 was associated with Si-O-Si stretching vibration. The peaks at 536 and 469 cm−1 were attributed to the deformations vibrations of Al-O-Si and Si-O-Si. Compared to the spectrum of pure HNTs, there were two new peaks appearing at 2922 and 2854 cm−1 in that of HDTMS-HNTs, which could be ascribed to the stretching vibrations of aliphatic CH groups. These two news peaks indicated that the HNTs was successfully modified with HDTMS. The powder XRD patterns of pure HNTs and HDTMS-HNTs, with the standard card of halloysite (JCPDS Card No. 29-1487) were given in Fig. 2b. The diffraction reflections of pure HNTs pattern were clearly observed at 2θ values of 12.35°, 20.03°, 24.95°, 35.05°, 38.14°, 54.41° and 62.23°, corresponding to the reflections of (001), (100), (002), (110), (003), (210) and (300) crystal planes of HNTs, respectively. Its diffraction reflections were consistent with those in the standard card of halloysite, which suggested a high purity of the halloysite clay. After modification of HDTMS, the diffraction reflections of HDTMS-HNTs remained much the same with that of pure HNTs, proving that HDTMSHNTs retained the crystalline phase of HNTs even after modification of HDTMS. The surface morphologies of an original mesh and a coated one were analyzed from their respective SEM images. It can be seen that surfaces of original mesh wires were smooth (Fig. 3a and b), whereas quantities of rod-like nanocrystals with 1 ± 0.5 μm in length and 50 to 70 nm in diameter piled up irregularly on surfaces of coated mesh wires (Fig. 3c and d), which constituted binary hierarchical rough structures on the surface of the coated mesh wires. Moreover, the average pore size of the
2.2. Fabrication of superhydrophobic hybrid epoxy/HDTMS-HNTs coated meshes Firstly, 1 g HNTs were dispersed in 50 mL ethanol-water (m/ m = 19:1) with vigorously stirring and then 2 mL HDTMS was added dropwise into the system. The reaction lasted at 78 °C for 4 h. The resultant HDTMS-HNTs were washed with ethanol and dried in vacuum drying oven at 60 °C. The powder was then milled through a 200-mesh screen. Secondly, stainless steel meshes soaked in ethanol solution with 0.5 wt% γ-(2,3-epoxypropoxy) propytrimethoxysilane for an hour and were dried at 80 °C for an hour. The fabrication process of superhydrophobic halloysite-based mesh is shown in Fig. 1. Epoxy resin E-44, HDTMS-HNTs and ethanolamine with a weight ratio of 1:4:0.16, were added in a mixed solvent of anhydrous ethanol and acetone (v/ v = 1:1), with magnetic stirring for 2 h to obtain a stable suspension. Subsequently, the suspension was sprayed onto the mesh by using a spray gun at a distance of 15 cm with the spraying pressure of 0.3 MPa. Finally, the as-prepared mesh was dried in an oven at 120 °C for 2 h. 2.3. Characterization FT-IR spectra were collected using a Fourier transform infrared spectrometer (PerkinElmer, USA) from standard KBr pellets (1 mg of 196
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Fig. 1. Schematic illustration of the fabrication of a superhydrophobic halloysite-based mesh.
out on it, which was shown in Fig. 4a. Fig. 4b showed that droplets of diffident liquids, namely, distilled water, tea, black ink, coffee, blue ink and milk, all kept highly spherical shapes on the coated mesh. In addition, the water contact angle (WCA) reached 154° on the coated mesh, and the water sliding angle (WSA) on it was merely 1.5°, which were shown in Fig. 4c and d. Therefore, the water is well repelled from the coated mesh surface. The superhydrophobicity of the coated mesh can be seen in Video 1. The wettability of the epoxy/HDTMS-HNTs coated mesh is also related to the pore size of the pristine mesh substrate. Fig. 5a displayed the relationship between the WCA, the WSA and the pore size of meshes (pore size varying from 20 μm to 200 μm). When the pore size was between 29 μm and 157 μm, the coated meshes exhibited superhydrophobicity with WCAs larger than 150° and WSAs less than 10°. When the pore size was above 38 μm, the WCA decreased roughly with the pore size increasing, and the WSA, however, appeared with an opposite trend. The reason of this phenomenon is that the pore size becomes too large to confine enough air within the structure, resulting in a poor hydrophobic force which can't support the water droplet. The WCA reached maximum when the pore size was 38 μm. Nevertheless, the WCA was 147.5° and the WSA was 5.7° when pore size was 23 μm.
mesh got smaller after being sprayed hybrid epoxy/HDTMS-HNTs, from 38 μm of the original mesh in Fig. 3a to 31.3 μm of the coated mesh in Fig. 3c. Even so, the mesh still kept clear pores, indicating the epoxy resin and HNTs didn't block off the mesh. The reason why the hybrid epoxy/HDTMS-HNTs coated mesh can achieve superhydrophobicity may be explained with the Cassie-Baxter's equation (Cassie and Baxter, 1944),
cos θA = f (cos θ + 1) − 1
(1)
where θA and θ are contact angles of the actual surface and the ideal surface, respectively, and f is the fractional geometrical areas of solid interfaces under the droplet. This equation only applies to a heterogeneous state where air is entrapped in gaps between the liquid droplet and the solid (Ahmad and Kan, 2016). The existence of the air pockets cuts down the contact area between the solid surface and the liquid droplet, and weakens the liquid/solid interaction (Quéré, 2005). Namely, f decreases, and θA increases, and therefore, the hydrophobicity of coating enhances since air is regarded as a very hydrophobic substance. The pristine mesh surface was both hydrophilic and superoleophilic since the water droplet was hemispherical and the oil droplet spread
Fig. 2. a) FT-IR spectra and b) XRD spectra of pure HNTs and HDTMS-HNTs.
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Fig. 3. SEM images of a-b) the original mesh, and c-d) the coated mesh.
Fig. 5b showed the WCAs and the WSAs on the superhydrophobic mesh with droplets at different pH values. The coated mesh exhibited stable superhydrophobicity with WCAs larger than 150° and WSAs less than 10° except for pH = 1 (WCA = 149°, WSA = 12°) and pH = 14 (WCA = 147.7°, WSA = 12.7°), suggesting that this superhydrophobic mesh can resist against not only pure water but also corrosive liquids. Moreover, Fig.6 shows the changes of water droplets with different pH placed on the coated mesh from 0 to 60 min. As time went by, water droplets became smaller gradually, while their spherical shape could be maintained and WCAs were always larger than 150° at different pH except pH = 1, 13 and 14 for at least one hour. The WCAs on the mesh with droplets at pH = 1 and 13 decreased obviously after 40 min. After 60 min, the WCA decreased from 149° to 121° at pH = 1, and it decreased from 150.3° to 134° at pH = 13.These results indicate that the coated mesh is durable enough to withstand chemical challenges. Fig. 7a–c showed the process of oil-water separation (more details in Video 2). Oil dyed with Sudan II permeated through the separator, but water dyed with methylene blue was blocked on the coated mesh. The separation process of 20 mL of the oil-water mixtures took about 16 s. Neither water nor oil could be seen in the collected oil or water respectively in Fig. 7c. This phenomenon exhibits the epoxy/HDTMSHNTs coated mesh can separate oil from water quickly. The epoxy/ HDTMS-HNTs coated meshes were applied to separate a series of oilwater mixtures, n-hexane-water, isooctane-water, petroleum etherwater, vegetable oil-water and xylene-water, among which separation efficiency were all at least 98% in Fig. 7d. These reflect the good performance of coated meshes in oil-water separation. What's more, a coated mesh was also used to separate n-hexane/ water mixture for 25 cycles. The separation efficiency was always over 98.5% from the first time to the twenty-fifth time (Fig. 8a). After the coated mesh was used for 25 times, it was evaluated by SEM and contact angle measurement again. The SEM image in Fig. 8a showed that the used coated mesh still kept nearly the same morphology as the unused one. In addition, the WCA on the used coated mesh was 147.5° in the insert of Fig. 8a, compared with the WCA of 154° on the unused one (Fig. 4c), which reflects that the used coated mesh still kept high hydrophobicity. In the meanwhile, the epoxy/HDTMS-HNTS coated mesh also kept stable superhydrophobicity when exposed to hot water. The WCA and the water-xylene separation efficiency of the coated mesh declined gradually with the temperature increasing, which can be seen in Fig. 8b. When the temperature of the water was between 30 and
Fig. 4. Optical images of a) oil and water (dyed with methylene blue) placed on the pristine mesh, and b) droplets of different solution on the coated mesh. Images of c) WCA and d) WSA on the mesh coated with epoxy/HDTMS-HNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The decrease of the WCA may results from the insufficient water/air interface (Zheng et al., 2005; Tian et al., 2011; Xiao et al., 2016). Therefore, the ideal pore size of stainless steel meshes is 38 μm (400 mesh size). And meshes with this size were chosen for various oilwater mixture separation and further experiments. The chemical durability of the superhydrophobic hybrid epoxy/ HDTMS-HNTs coated mesh in corrosive environments was tested. 198
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Fig. 5. Dependence of WCA and WSA on a) pore size of meshes and b) pH value.
pressure on the coated mesh could reach more than 4.90 kPa before water flew through the mesh. All the above-mentioned facts demonstrate that the coated mesh has a perfect combination of excellent separation performance, well heat resistance, good water pressure resistance and great reusability. Mechanical durability is another important property of the superhydrophobic mesh, and was tested by taking sand abrasion test, the process of which was shown in Fig. 9a. The coated mesh was tilted at 45°, and then sand particles with diameter of 180–280 μm impacted the coated mesh from a height of 18 cm. Fig. 9b showed the WCA and the nhexane-water separation efficiency of the coated mesh decreased gradually as the sand mass increased. The WCA was still greater than 150° when the sand mass was equal to 25 g. When the sand amount increased to 30 g, the WCA dropped to 147°. Furthermore, the n-hexane-water separation efficiency of the mesh was more than 98.4% when the sand mass was added from 0 to 30 g. The aforementioned facts demonstrate that the as-prepared mesh was fairly durable to withstand mechanical challenges.
Fig. 6. Optical images of water droplets with different pH placed on the coated mesh for different times.
60 °C, the WCAs on the epoxy/HDTMS-HNTS coated mesh were all above 150° and the separation efficiency of water-xylene mixture was above 97.5%. When it achieved 70 °C, the WCA and the separation efficiency decreased to 145.3° and 96.9%, respectively. Furthermore, the hydrostatic pressure property of the coated mesh was tested. The hydrostatic pressure was calculated by the equation,
P = ρghmax
4. Conclusions In conclusion, a superhydrophobic stainless steel mesh with binary hierarchical rough structures, was facilely fabricated by simple spraying with the mixtures of epoxy resin and HDTMS-HNTs. Experiments have proved the ideal pore size of stainless steel meshes is 38 μm (400 mesh size), and on meshes with this size the WCA and the WSA were 154° and 1.5°, respectively. In addition, the coated mesh exhibited stable superhydrophobicity with WCAs larger than 150° and WSAs less than 10° when pH varies from 2 to 13. The WCAs on the epoxy/HDTMS-HNTS coated mesh were all above 150° even when the water achieved 60 °C. What's more, the coated mesh showed high separation efficiency in
(2)
where P is the hydrostatic pressure, ρ is the density of the water, g is the acceleration of the gravity and hmax is the height of water that the coated mesh can brace. The height of the water column on the coated mesh was at least 50 cm in Fig. 8c. In other words, the hydrostatic
Fig. 7. a–c) Experimental setup of oil-water mixture separation. d) Efficiency of different oil-water mixture separation with the coated mesh.
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Fig. 8. a) Repetitive efficiency of n-hexane-water separation with a coated mesh, along with the SEM image and the WCA of the coated mesh after repetitive separation. b) Dependence of WCAs on the temperature of the water droplet placed on the coated mesh and separation efficiency of water-xylene mixture on different temperature. c) Optical image of the water column on the coated mesh with maximum height.
Fig. 9. a) Scheme of sand abrasion test. b) Variation of the water contact angle and the n-hexane-water separation efficiency of the coated mesh with sand mass.
various oil-water mixture separation. It still kept separation efficiency above 98.5% and the WCA 147.5° after twenty-five cycles of separation for n-hexane-water mixture. The separation efficiency of xylene-water mixture was still 96.9% even if the temperature was 70 °C. The epoxy/ HTDMS-HNTs coated mesh could bear hydrostatic pressure of at least 4.90 kPa. Furthermore, the WCA was still 147° and the separation efficiency for n-hexane-water mixture remained 98.4% after 30-gram sand abrasion on the coated mesh. This method to fabricate superhydrophobic meshes is simple, cheap and easy to scale up, and this obtained mesh can be a promising material in oil-water separation, which possesses great reusability, favorable stability and good superhydrophobicity. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2018.01.034.
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
A facile preparation of superhydrophobic halloysite-based meshes for efficient oil–water separation
T
Danyi Guoa, Jiahui Chena, Kun Houa, Shouping Xua, Jiang Chenga, Xiufang Wena, ⁎ ⁎ Shuangfeng Wanga, Chaoyun Huangb, , Pihui Pia, a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Nuclear and Radiation Safety Centre, MEP, Beijing 100082, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Superhydrophobic Halloysite Spray Stainless steel mesh Oil-water separation
A superhydrophobic halloysite-based mesh was facilely prepared by spraying epoxy/hexadecyltrimethoxysilanehalloysite nanotubes (HDTMS-HNTs) on stainless steel mesh. The as-prepared mesh was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical contact angle meter (OCA). The HNTs modified by HDTMS not only enhanced surface roughness, but also endowed hydrophobicity of the mesh. The mesh, with a static water contact angle of 154° and a sliding angle of 1.5°, was applied to separate a series of oil-water mixtures, such as n-hexane-water, isooctane-water and petroleum ether-water, with high separation efficiency of over 98%. The mesh still kept separation efficiency approximately 98.5% even after 25 separation cycles for n-hexane-water mixture separation. More importantly, the mesh is durable enough to withstand heat, chemical and mechanical challenges, such as hot water, strong alkaline, strong acid and sand abrasion, and high hydrostatic pressure. The as-prepared mesh will be a promising material in oil-water separation, because of the simple, economical and easily scalable preparation method and the excellent separation performance in radical oil-water separation.
1. Introduction
(Patowary et al., 2015), self-assemble (Shao et al., 2014), chemical vapor deposition (Hsieh et al., 2008) and so on. The limitations of these methods for large-scale fabrication are complex preparation, expensive equipment and long process. Besides, fluorinated acrylic resins were usually used to fabricate superhydrophobic surfaces (Wen et al., 2015; Pi et al., 2016). The long-chain fluorinated alkane can lower the surface energy greatly, and then makes the surface to be superhydrophobic, while its price is high and it will be accumulated in organisms which do harm to the environment and mankind. Therefore, it's highly warranted to find a simple, facile and low-energy method to fabricate a stable, economical and easily scalable superhydrophobic material in rapid oilwater separation. Halloysite clay is a kind of naturally deposited aluminosilicate material [Al2Si2O5(OH)4·nH2O], which is generally a hollow nanotubular structure as shown in Fig. S1. It is a two-layered (1:1) clay mineral with abundant Si-OH and Al-OH groups (Hou et al., 2017). In addition, the size of HNT varies from 50 to 70 nm in external diameter, 15 nm in lumen diameter and 1 ± 0.5 μm in length (Vergaro et al., 2010). Halloysite nanotubes (HNTs) are far less expensive, eco-friendly and abundant, compared with other nanosized materials, such as carbon nanotubes, boron nitride nanotubes and titanium dioxides
With the development of the industrial society, the problems of environment and pollution have been brought to the forefront. The ever-increasing oily sewages in the industrial production and frequent oil spills during oil exploitation and transportation not only lead to serious economic issues, but also pollute the ecosystem severely (Shannon et al., 2008; Ahmadun et al., 2009; Dubansky et al., 2013). Thus, it's urgently demanded to exploit a rapid and effective approach for oil sewage remediation. It is a facile and efficient methods to design new materials with special wettability for oil-water separation, since oil-water separation is affected by interface force. Recently, it has triggered increasing research interest to apply superhydrophobic surperoleophilic porous materials to oil-water separation (Wang et al., 2015; Pi et al., 2016; Qing et al., 2017; Zeng et al., 2017; Hou et al., 2018). Since the unique water repellency and oil affinity of these materials, they can separate oil from water effectively by means of filtration or absorption. In addition, a variety of methods to fabricate superhydrophobic and superoleophilic materials have been reported, for example, sol-gel processing (Yang et al., 2010), lithography(Gao et al., 2015), electrostatic spinning (Yohe et al., 2012), chemical etching
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Corresponding authors. E-mail addresses: [email protected] (C. Huang), [email protected] (P. Pi).
https://doi.org/10.1016/j.clay.2018.01.034 Received 20 November 2017; Received in revised form 6 January 2018; Accepted 26 January 2018 Available online 13 February 2018 0169-1317/ © 2018 Published by Elsevier B.V.
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sample/200 mg of KBr) with 64 scans at 4 cm−1 resolution in the 4000–400 cm−1 region to characterized the chemical composition of samples. The structures of samples were characterized by X-ray diffraction (XRD) with an X-ray diffractometer (D8 Advance, Bruker) with Cu Kα radiation (λ = 1.5418 Å). The diffractograms were scanned in 2θ ranges from 10 to 70° at a rate of 8 °/min. The surface morphologies of the films were observed by a scanning electron microscopy (SEM, Merlin). In addition, contact angles and sliding angles of water and oil were measured using an optical contact angle measuring device (Data Physics OCA20, Germany) with 5 μL drop of deionized water or oil, respectively. The reported values of contact angles and sliding angles were the average of at least three measurements at different positions on the same mesh. The mechanical stability of the coating was evaluated by taking sand abrasion tests (Milionis et al., 2016; Hou et al., 2017) on the coated mesh surface using sand particles with diameters of 180–280 μm at a height of 18 cm.
nanotubes. With the introduction of HNTs, the coating is expected to have better thermal stability and/or fire resistance (Makaremi et al., 2015; Qin et al., 2016). Therefore, Halloysite clay with hydrophobic treatment for developing superhydrophobic materials (Wu et al., 2013; Ma et al., 2017) can open up a new avenue for the application of clay, which is also a promising candidate for oil-water separation. In this paper, a facile preparation of superhydrophobic meshes was reported by spraying epoxy and HNTs modified by hexadecyltrimethoxysilane (HDTMS) on stainless steel meshes. HNTs were transformed from hydrophilic to hydrophobic with modification of HDTMS. The introduction of epoxy resin could enhance the filmforming property and improve the stability of the coating. Besides, stainless steel meshes are widely applied to oil-water separation for a perfect combination of low cost, favorable mechanical property and easy reuse without squeezing, compared with other substrates, such as sponges, foams and filtration fabrics. Furthermore, spray is one of promising technique in practical production for its easy operation and low cost. The coated mesh has a perfect combination of robust superhydrophobicity, great reusability, excellent water pressure resistance and favorable stability to heat, chemistry and machinery, which is expected to be applied to large-scale oil-water separation, due to the easy, simple, cheap and scalable fabrication method.
2.4. Oil-water separation A series of oils, including n-hexane, isooctane, petroleum ether, vegetable oil and xylene, were mixed with equal volume of water to form oil-water mixtures. A superhydrophobic mesh was fixed in the miniature separator, of which the diameter is about 2 cm, and then 20 mL of oil-water mixtures were slowly poured into the separator at the speed of 1.2–1.3 mL/s. Oils penetrated the mesh into the beaker quickly while water was stuck in the separator. Gravity was the only driving force during the separation process. The water was collected in the container and weighed. The separation efficiency η was obtained according to η = (m1/m0) × 100%, where m0 and m1 are the mass of the water before and after the separation, respectively (Pan et al., 2008).
2. Experimental section 2.1. Material Natural HNTs were mined from Yichang, Hubei, China. Hexadecyltrimethoxysilane (HDTMS) was purchased from Aladdin Industrial Corporation. The epoxy resin E-44 was bought from Nantong Xingchen Synthetic Material Incorporation. Ethanolamine was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. Anhydrous ethanol were supplied from Tianjin Kemiou Chemical Reagent Co., Ltd. Hydrochloric acid (HCl) and acetone were purchased from Tianjin Hongda Chemical Reagent Co., Ltd. Stainless steel meshes were purchased from a local hardware store (Guangzhou, China), and purified by ultrasonic washing with water and acetone for 30 min, respectively, and then completely dried in an oven at 60 °C before use. HNTs were dispersed in 1 mol/L HCl (m/m = 1:20) with magnetic stirring for 24 h at room temperature and then washed with distilled water until the water was neutral and dried in vacuum drying oven at 60 °C. The other chemicals were used as received without any further purification.
3. Results and discussions The chemical composition and structure of pure HNTs and HDTMSHNTs were analyzed by FT-IR and XRD, which were shown in Fig. 2. In the FT-IR spectra (Fig. 2a), the peaks at 3693, 3624 and 911 cm−1 were due to the stretching vibration of the inner-surface hydroxyl groups, the stretching vibration of the inner hydroxyl groups and the deformation vibration of the inner-surface hydroxyl groups, respectively. The strong absorption band at 1034 cm−1 was associated with Si-O-Si stretching vibration. The peaks at 536 and 469 cm−1 were attributed to the deformations vibrations of Al-O-Si and Si-O-Si. Compared to the spectrum of pure HNTs, there were two new peaks appearing at 2922 and 2854 cm−1 in that of HDTMS-HNTs, which could be ascribed to the stretching vibrations of aliphatic CH groups. These two news peaks indicated that the HNTs was successfully modified with HDTMS. The powder XRD patterns of pure HNTs and HDTMS-HNTs, with the standard card of halloysite (JCPDS Card No. 29-1487) were given in Fig. 2b. The diffraction reflections of pure HNTs pattern were clearly observed at 2θ values of 12.35°, 20.03°, 24.95°, 35.05°, 38.14°, 54.41° and 62.23°, corresponding to the reflections of (001), (100), (002), (110), (003), (210) and (300) crystal planes of HNTs, respectively. Its diffraction reflections were consistent with those in the standard card of halloysite, which suggested a high purity of the halloysite clay. After modification of HDTMS, the diffraction reflections of HDTMS-HNTs remained much the same with that of pure HNTs, proving that HDTMSHNTs retained the crystalline phase of HNTs even after modification of HDTMS. The surface morphologies of an original mesh and a coated one were analyzed from their respective SEM images. It can be seen that surfaces of original mesh wires were smooth (Fig. 3a and b), whereas quantities of rod-like nanocrystals with 1 ± 0.5 μm in length and 50 to 70 nm in diameter piled up irregularly on surfaces of coated mesh wires (Fig. 3c and d), which constituted binary hierarchical rough structures on the surface of the coated mesh wires. Moreover, the average pore size of the
2.2. Fabrication of superhydrophobic hybrid epoxy/HDTMS-HNTs coated meshes Firstly, 1 g HNTs were dispersed in 50 mL ethanol-water (m/ m = 19:1) with vigorously stirring and then 2 mL HDTMS was added dropwise into the system. The reaction lasted at 78 °C for 4 h. The resultant HDTMS-HNTs were washed with ethanol and dried in vacuum drying oven at 60 °C. The powder was then milled through a 200-mesh screen. Secondly, stainless steel meshes soaked in ethanol solution with 0.5 wt% γ-(2,3-epoxypropoxy) propytrimethoxysilane for an hour and were dried at 80 °C for an hour. The fabrication process of superhydrophobic halloysite-based mesh is shown in Fig. 1. Epoxy resin E-44, HDTMS-HNTs and ethanolamine with a weight ratio of 1:4:0.16, were added in a mixed solvent of anhydrous ethanol and acetone (v/ v = 1:1), with magnetic stirring for 2 h to obtain a stable suspension. Subsequently, the suspension was sprayed onto the mesh by using a spray gun at a distance of 15 cm with the spraying pressure of 0.3 MPa. Finally, the as-prepared mesh was dried in an oven at 120 °C for 2 h. 2.3. Characterization FT-IR spectra were collected using a Fourier transform infrared spectrometer (PerkinElmer, USA) from standard KBr pellets (1 mg of 196
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Fig. 1. Schematic illustration of the fabrication of a superhydrophobic halloysite-based mesh.
out on it, which was shown in Fig. 4a. Fig. 4b showed that droplets of diffident liquids, namely, distilled water, tea, black ink, coffee, blue ink and milk, all kept highly spherical shapes on the coated mesh. In addition, the water contact angle (WCA) reached 154° on the coated mesh, and the water sliding angle (WSA) on it was merely 1.5°, which were shown in Fig. 4c and d. Therefore, the water is well repelled from the coated mesh surface. The superhydrophobicity of the coated mesh can be seen in Video 1. The wettability of the epoxy/HDTMS-HNTs coated mesh is also related to the pore size of the pristine mesh substrate. Fig. 5a displayed the relationship between the WCA, the WSA and the pore size of meshes (pore size varying from 20 μm to 200 μm). When the pore size was between 29 μm and 157 μm, the coated meshes exhibited superhydrophobicity with WCAs larger than 150° and WSAs less than 10°. When the pore size was above 38 μm, the WCA decreased roughly with the pore size increasing, and the WSA, however, appeared with an opposite trend. The reason of this phenomenon is that the pore size becomes too large to confine enough air within the structure, resulting in a poor hydrophobic force which can't support the water droplet. The WCA reached maximum when the pore size was 38 μm. Nevertheless, the WCA was 147.5° and the WSA was 5.7° when pore size was 23 μm.
mesh got smaller after being sprayed hybrid epoxy/HDTMS-HNTs, from 38 μm of the original mesh in Fig. 3a to 31.3 μm of the coated mesh in Fig. 3c. Even so, the mesh still kept clear pores, indicating the epoxy resin and HNTs didn't block off the mesh. The reason why the hybrid epoxy/HDTMS-HNTs coated mesh can achieve superhydrophobicity may be explained with the Cassie-Baxter's equation (Cassie and Baxter, 1944),
cos θA = f (cos θ + 1) − 1
(1)
where θA and θ are contact angles of the actual surface and the ideal surface, respectively, and f is the fractional geometrical areas of solid interfaces under the droplet. This equation only applies to a heterogeneous state where air is entrapped in gaps between the liquid droplet and the solid (Ahmad and Kan, 2016). The existence of the air pockets cuts down the contact area between the solid surface and the liquid droplet, and weakens the liquid/solid interaction (Quéré, 2005). Namely, f decreases, and θA increases, and therefore, the hydrophobicity of coating enhances since air is regarded as a very hydrophobic substance. The pristine mesh surface was both hydrophilic and superoleophilic since the water droplet was hemispherical and the oil droplet spread
Fig. 2. a) FT-IR spectra and b) XRD spectra of pure HNTs and HDTMS-HNTs.
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Fig. 3. SEM images of a-b) the original mesh, and c-d) the coated mesh.
Fig. 5b showed the WCAs and the WSAs on the superhydrophobic mesh with droplets at different pH values. The coated mesh exhibited stable superhydrophobicity with WCAs larger than 150° and WSAs less than 10° except for pH = 1 (WCA = 149°, WSA = 12°) and pH = 14 (WCA = 147.7°, WSA = 12.7°), suggesting that this superhydrophobic mesh can resist against not only pure water but also corrosive liquids. Moreover, Fig.6 shows the changes of water droplets with different pH placed on the coated mesh from 0 to 60 min. As time went by, water droplets became smaller gradually, while their spherical shape could be maintained and WCAs were always larger than 150° at different pH except pH = 1, 13 and 14 for at least one hour. The WCAs on the mesh with droplets at pH = 1 and 13 decreased obviously after 40 min. After 60 min, the WCA decreased from 149° to 121° at pH = 1, and it decreased from 150.3° to 134° at pH = 13.These results indicate that the coated mesh is durable enough to withstand chemical challenges. Fig. 7a–c showed the process of oil-water separation (more details in Video 2). Oil dyed with Sudan II permeated through the separator, but water dyed with methylene blue was blocked on the coated mesh. The separation process of 20 mL of the oil-water mixtures took about 16 s. Neither water nor oil could be seen in the collected oil or water respectively in Fig. 7c. This phenomenon exhibits the epoxy/HDTMSHNTs coated mesh can separate oil from water quickly. The epoxy/ HDTMS-HNTs coated meshes were applied to separate a series of oilwater mixtures, n-hexane-water, isooctane-water, petroleum etherwater, vegetable oil-water and xylene-water, among which separation efficiency were all at least 98% in Fig. 7d. These reflect the good performance of coated meshes in oil-water separation. What's more, a coated mesh was also used to separate n-hexane/ water mixture for 25 cycles. The separation efficiency was always over 98.5% from the first time to the twenty-fifth time (Fig. 8a). After the coated mesh was used for 25 times, it was evaluated by SEM and contact angle measurement again. The SEM image in Fig. 8a showed that the used coated mesh still kept nearly the same morphology as the unused one. In addition, the WCA on the used coated mesh was 147.5° in the insert of Fig. 8a, compared with the WCA of 154° on the unused one (Fig. 4c), which reflects that the used coated mesh still kept high hydrophobicity. In the meanwhile, the epoxy/HDTMS-HNTS coated mesh also kept stable superhydrophobicity when exposed to hot water. The WCA and the water-xylene separation efficiency of the coated mesh declined gradually with the temperature increasing, which can be seen in Fig. 8b. When the temperature of the water was between 30 and
Fig. 4. Optical images of a) oil and water (dyed with methylene blue) placed on the pristine mesh, and b) droplets of different solution on the coated mesh. Images of c) WCA and d) WSA on the mesh coated with epoxy/HDTMS-HNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The decrease of the WCA may results from the insufficient water/air interface (Zheng et al., 2005; Tian et al., 2011; Xiao et al., 2016). Therefore, the ideal pore size of stainless steel meshes is 38 μm (400 mesh size). And meshes with this size were chosen for various oilwater mixture separation and further experiments. The chemical durability of the superhydrophobic hybrid epoxy/ HDTMS-HNTs coated mesh in corrosive environments was tested. 198
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Fig. 5. Dependence of WCA and WSA on a) pore size of meshes and b) pH value.
pressure on the coated mesh could reach more than 4.90 kPa before water flew through the mesh. All the above-mentioned facts demonstrate that the coated mesh has a perfect combination of excellent separation performance, well heat resistance, good water pressure resistance and great reusability. Mechanical durability is another important property of the superhydrophobic mesh, and was tested by taking sand abrasion test, the process of which was shown in Fig. 9a. The coated mesh was tilted at 45°, and then sand particles with diameter of 180–280 μm impacted the coated mesh from a height of 18 cm. Fig. 9b showed the WCA and the nhexane-water separation efficiency of the coated mesh decreased gradually as the sand mass increased. The WCA was still greater than 150° when the sand mass was equal to 25 g. When the sand amount increased to 30 g, the WCA dropped to 147°. Furthermore, the n-hexane-water separation efficiency of the mesh was more than 98.4% when the sand mass was added from 0 to 30 g. The aforementioned facts demonstrate that the as-prepared mesh was fairly durable to withstand mechanical challenges.
Fig. 6. Optical images of water droplets with different pH placed on the coated mesh for different times.
60 °C, the WCAs on the epoxy/HDTMS-HNTS coated mesh were all above 150° and the separation efficiency of water-xylene mixture was above 97.5%. When it achieved 70 °C, the WCA and the separation efficiency decreased to 145.3° and 96.9%, respectively. Furthermore, the hydrostatic pressure property of the coated mesh was tested. The hydrostatic pressure was calculated by the equation,
P = ρghmax
4. Conclusions In conclusion, a superhydrophobic stainless steel mesh with binary hierarchical rough structures, was facilely fabricated by simple spraying with the mixtures of epoxy resin and HDTMS-HNTs. Experiments have proved the ideal pore size of stainless steel meshes is 38 μm (400 mesh size), and on meshes with this size the WCA and the WSA were 154° and 1.5°, respectively. In addition, the coated mesh exhibited stable superhydrophobicity with WCAs larger than 150° and WSAs less than 10° when pH varies from 2 to 13. The WCAs on the epoxy/HDTMS-HNTS coated mesh were all above 150° even when the water achieved 60 °C. What's more, the coated mesh showed high separation efficiency in
(2)
where P is the hydrostatic pressure, ρ is the density of the water, g is the acceleration of the gravity and hmax is the height of water that the coated mesh can brace. The height of the water column on the coated mesh was at least 50 cm in Fig. 8c. In other words, the hydrostatic
Fig. 7. a–c) Experimental setup of oil-water mixture separation. d) Efficiency of different oil-water mixture separation with the coated mesh.
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Fig. 8. a) Repetitive efficiency of n-hexane-water separation with a coated mesh, along with the SEM image and the WCA of the coated mesh after repetitive separation. b) Dependence of WCAs on the temperature of the water droplet placed on the coated mesh and separation efficiency of water-xylene mixture on different temperature. c) Optical image of the water column on the coated mesh with maximum height.
Fig. 9. a) Scheme of sand abrasion test. b) Variation of the water contact angle and the n-hexane-water separation efficiency of the coated mesh with sand mass.
various oil-water mixture separation. It still kept separation efficiency above 98.5% and the WCA 147.5° after twenty-five cycles of separation for n-hexane-water mixture. The separation efficiency of xylene-water mixture was still 96.9% even if the temperature was 70 °C. The epoxy/ HTDMS-HNTs coated mesh could bear hydrostatic pressure of at least 4.90 kPa. Furthermore, the WCA was still 147° and the separation efficiency for n-hexane-water mixture remained 98.4% after 30-gram sand abrasion on the coated mesh. This method to fabricate superhydrophobic meshes is simple, cheap and easy to scale up, and this obtained mesh can be a promising material in oil-water separation, which possesses great reusability, favorable stability and good superhydrophobicity. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2018.01.034.
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