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Colloids and Surfaces A 583 (2019) 124010
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Environmentally-friendly fabrication of a recyclable oil-water separation material using copper mesh for immiscible oil/water mixtures
T
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Shuangshuang Xu, Qing Wang , Ning Wang, Xu Zheng, Lulu Lei Institute of NanoEngineering, College of Civil Engineering and Architecture, Shandong University of Science and Technology, 266590, Shandong, PR China
G R A P H I C A L A B S T R A C T
Superhydrophobic and superoleohobic surface fabricated with an environmentally-friendly modification by non-toxic stearic acid, exhibit self-cleaning, stability to long-term water jet exposure and recyclability to various oil–water mixtures.
A R T I C LE I N FO
A B S T R A C T
Keywords: Superhydrophobic Superoleophilic Environmentally-friendly Oil–water separation Recyclable Self-cleaning
Oily wastewater created by industrial disposal and oil spill disasters enormously affects human health and ecological systems. Environmentally-friendly methods for oil–water separation are highly desirable. We propose an environmentally-friendly approach for fabricating superwettable surfaces with recyclability to separate oilwater mixtures efficiently. A superwettable surface with a dendrite-like hierarchical structure for separating various oil–water mixtures was fabricated using copper mesh via modification by non-toxic stearic acid. The prepared surface was confirmed to be superhydrophobic and superoleophilic, in which the water contact angle, water slide angle and oil contact angle are 158°, 3° and 1°, respectively. These properties resulted in the surface achieving oil–water separation driven by gravity, and exhibiting efficient separation (> 98%) and high recyclability (throughout 22 consecutive cycles) of various oil–water mixtures. Moreover, the prepared surface also exhibited self-cleaning performance and stability to long-term water jet exposure. This efficient and environmentally-friendly method provides a green strategy for separating various oils from water.
1. Introduction Large volumes of oily wastewater are generated by industrial activity [1,2] and offshore oil spills during oil transportation [3–5]. These
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have led to increasingly serious pollution of water and oil resources in nature [6,7]. Oil-contaminated water contains toxic chemicals [8] that are harmful to human health and the ecosystem [9–11]. Measures should be taken to prevent this situation from deteriorating [12].
Corresponding author. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.colsurfa.2019.124010 Received 18 July 2019; Received in revised form 16 September 2019; Accepted 23 September 2019 Available online 24 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
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functional superwettable surface are imperative [42]. In this study, an environmentally-friendly method for fabricating superwettable surfaces with recyclability to efficiently separate oil–water mixture was proposed. A rough surface with dendrite-like hierarchical structures was fabricated on copper mesh by a replacement reaction. Subsequent modification with stearic acid yielded superhydrophobic/superoleophilic surfaces. Stearic acid contains 16 −CH2, 1 −CH3 and 1 −COOH groups, is often used as a low surface energy modifying material and is non-toxic [43,44]. The impact of different reaction times on wettability and morphologies were investigated. In order to study the separation efficiency of prepared copper mesh under different oil–water mixtures and separation cycles, the oil–water separation experiment was conducted. The stability of the mesh upon exposure to a water jet for an extended duration was studied and the self-cleaning performance was evaluated. The proposed method is efficient and pollution-free and the prepared surface has potential in oil–water separation application.
Conventional methods, including situ burning [13,14], gravity separation [15,16], air flotation [17,18] and biological treatment [19,20] are widely used to deal with oily wastewater. These technologies face several problems such as complexity, creation of secondary pollutants, low separation efficiencies and high operation costs [7,21,22]. An effective strategy for separating oil–water mixtures using the special wettability (superhydrophilic/superoleophobic or superoleophilic/ superhydrophobic) of solid surfaces was introduced [23,24]. Feng et al. reported a flexible oil absorbent material for potential application in oil–water separation, which was prepared by depositing rough graphene onto polydimethylsiloxane elastomeric matrices [3]. Cheng et al. prepared superhydrophobic coatings on a polyurethane kitchen sponge using high density polyethylene. The coated sponge exhibited excellent water–oil separation capability [25]. These superhydrophobic/superoleophilic materials can selectively remove oil from water or water from oil due to the wettability of oil and water on bionic surfaces is different [26]. Generally, the geometrical architecture (i.e., surface roughness) and chemical composition (i.e., surface energy) greatly determine the wettability of solid surfaces [27–29]. Superhydrophobic/superoleophilic surfaces with selective permeability to water and oil have been prepared on porous substrates by coordinating rough structures and low surface energies [10,30]. For example, Polymer substrates were used for fabricating superhydrophobic and superoleophilic surfaces to separation of oil-water mixture [31–33]. Metal substrates could improve the properties by fabricating superhydrophobic surface [34]. After a series of treatments on the metal mesh, the superhydrophobic and superoleophilic surfaces were obtained to achieve the separation of oil and water [35–38]. However, these reports face some drawbacks, such as low separation efficiency or harmful to the environment. Moreover, the mechanical properties of the fabricated surfaces are poor and easily damaged by some external force. Moreover, a nanostructured TiO2 mesh membrane with special wettability and capacity of 10 separation cycles was prepared by anodization at 45 V for 6 h and subsequent annealing at 350 °C for 1 h in air [23]. More efficient preparation methods and materials capable of more separation cycles are required for practical applications. Wen et al. fabricated a superhydrophilic in air and superoleophobic underwater surface by electrodepositing a layer of copper on the stainless steel surface. But its hydrophilicity will make it prone to corrosion in the air and separate oil-in-water emulsion only under gravity or a low applied pressure. Gao et al. prepared a superhydrophobic/superoleophilic filter paper by electrospraying with polyvinylidene fluoride/SiO2 microspheres [39]. Ke et al. fabricated superhydrophobic/superoleophilic sponges modified with octadecyltrichlorosilane and used the sponges to suck the oils out of water by dipping into the mixtures of oil/water [40]. These methods suffer from serious disadvantages such as requiring environmentally-harmful fluorine-containing modified materials, or octadecyltrichlorosilane which releases poisonous gas when exposed to water [41]. Environmentally-friendly methods for preparing
2. Experimental 2.1. Chemicals and materials Copper mesh (51 μm aperture, 90 μm wire diameter) was obtained from Shanghai Huaxin Metal Mesh Factory (Shanghai, China). Silver nitrate (AgNO3) and Stearic acid were purchased from the Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China) and Tianjin Beilian Fine Chemicals Development Co., Ltd. (Tianjin, China), respectively. Blend oil, methylene chloride, cyclohexane and ethyl alcohol were received from Qingdao Jingke Reagent Co., Ltd. (Qingdao, China). Deionized water was used in all experiments. All analytical grade chemicals are not further purification. 2.2. Sample preparation First, the copper mesh was cut to the desired size and polished with 600 mesh sandpaper to remove the surface oxide layer. Then, it was cleaned ultrasonically with anhydrous ethanol for 10 min to remove surface grease. After that, the copper mesh was immersed in a 0.01 M AgNO3 aqueous solution for different lengths of time at ambient temperature. Finally, the copper mesh was modified by placing it in 0.01 M ethanol solution of stearic acid for 1 h, as illustrated in Scheme 1. After each step was completed, the prepared surface was rinsed in deionized water, subsequently dried in the air. 2.3. Characterization The surface morphology was characterized by scanning electron microscopy (SEM, APERO, FEI Co., USA). The roughness was analyzed by atomic force microscopy (AFM, NanoManVS, Germany). Energy dispersion spectrum (EDS) attached to the SEM apparatus was used to
Scheme 1. Illustration of the preparation procedure of modified Ag-coated copper mesh. 2
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Fig. 1. SEM images of copper mesh at different replacement reaction times in a 0.01 M AgNO3 solution with different magnifications: (a, b) 0 min, (c, d) 0.5 min, (e, h) 2 min, (g, h) 10 min.
Scientific, USA). Chemical analysis was assessed via X-ray photoelectron spectroscopy (XPS, ThermoFischer, ESCALAB 250Xi, USA). The optical contact angle measurement device (DSA30, Kruss Co., Germany) was used to measure water contact angle (WCA) and oil
investigate the chemical compositions. The phase composition of prepared surface was analyzed by X-ray diffraction (XRD, Rigaku Ultima IV, Rigaku Co., Ltd., Japan). Fourier-transform infrared (FT-IR) spectrum was conducted by Nicolet 380 FT-IR spectrometer (Thermo Fisher 3
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modified copper mesh was glued to a glass slide and the slide was leaned against a culture dish. A thin layer of white chalk was spread over the superhydrophobic copper mesh. The modified Ag–coated copper mesh was then cleaned using water to observe its self-cleaning behavior. Digital photographs were obtained at different times. 2.5. Oil–water separation test The prepared superhydrophobic surface was first fixed in an oil–water separation device. The device was composed by a glass funnel and conical flask. Then, the oil–water mixture (1:1, v/v) was poured over the superhydrophobic copper mesh and was separated under gravity. The device was tested with five oil–water mixtures (blend oil, kerosene, diesel fuel, methylene chloride and cyclohexane). In the separation experiments, cyclohexane and methylene chloride were dyed red by Sudan III and deionized water was dyed blue by methyl blue. The separation efficiency (η) can be calculated with: η = m1/m2 ×100% [45,46], in which m1 and m2 denote oil mass of two separation cycles of oil-water begins and ends. The flux can be calculated with: Flux = V/St, where V is the volume of the permeated oil (L), S is the valid area (m2) of membrane, and t refers to the time (s) for the separation. The sample was washed with ethanol and deionized water and then dried at room temperature after separation. 3. Results and discussion 3.1. Morphology and composition Surface roughness and energy are two important factors determining the wettability of solid surfaces [47]. Here, we prepared a superhydrophobic/superoleophilic surface on copper mesh. A copper mesh surface with the ideal rough structure was prepared by a short replacement reaction with a 0.01 M AgNO3 solution at ambient temperature. Further modification was conducted using a 0.01 M ethanol solution of low-surface-energy stearic acid for 1 h. The surface morphologies of the original and the superhydrophobic surfaces with different replacement reaction times were demonstrated by SEM under different magnifications. The original copper mesh surface is relatively smooth, the average wire and pore diameters were about 45 μm and 100 μm, respectively (Fig. 1a and b). After the replacement reaction, the surface of the copper mesh became rough. With a short reaction time (0.5 min), flower-like clusters constituted by micro-sized ginkgo leaves began to appear on the copper mesh (Fig. 1c and d). After 2 min in AgNO3 solution, the surface of copper mesh (denoted as Ag-coated copper mesh) changed greatly (Fig. 1e and f). The enlarged images in Fig. 1g and h shows that the morphologies were transformed from micro flower-like clusters to micro-nano dendrite-like structures. The formed hierarchical structures are superhydrophobic because their rough surfaces trapped large amounts of air in the gaps [10,48]. As the reaction time increased, the amount of dendrite-like structures covering the copper mesh surface increased. At the same time, with the increasing of the dendrite-like structures, the aperture of the copper mesh decreased. When the immersion time was 10 min, the copper mesh aperture was completely covered by the dendrite-like structure (Fig. 1i and j). Figure S1 displays the AFM image of Ag-coated copper mesh, it can be shown that the surface is composed of many nano- and microsize protuberances. The composition of the original and Ag-coated copper mesh before and after modification was analyzed by EDS spectrum. The EDS spectrum of the original copper mesh only contained Cu (Fig. 2a). After the replacement reaction with the AgNO3 solution, elemental Ag was observed from EDS spectrum of Ag-coated copper mesh (Fig. 2b). This indicated that an Ag layer was deposited on copper mesh after replacement reaction. The presence of O attributed to copper oxides. After modification with stearic acid, an obvious C peak can be observed from superhydrophobic copper mesh (Fig. 2c). This demonstrated the
Fig. 2. EDS spectrum of the (a) original copper mesh, (b) Ag-coated copper mesh and (c) modified Ag-coated copper mesh.
contact angle (OCA). The water slide angle (WSA) was measured using a device constructed in our laboratory. The results were measured by calculating an average value using a 10-μL droplet from five different positions each time. The copper mesh was fixed to the glass slide during the measurement. 2.4. Self-cleaning experiments The white chalk was regarded as model contaminants to investigate the self–cleaning property of modified Ag-coated copper mesh. The 4
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Fig. 3. (a) XRD pattern of the original copper mesh and Ag-coated copper mesh. (b) FT-IR spectra of stearic acid and the modified Ag-coated copper mesh.
Fig. 4. XPS spectrum of the modified Ag-coated copper mesh, (a) survey spectrum; (b) Cu region; (c) Ag region; (d) C region, the C1 s peak was deconvoluted.
diffraction peaks of Cu2O (marked with square symbols in Fig. 3a) for the (110), (111), (200) and (220) planes were detected in the XRD pattern of Ag-coated copper mesh [50]. These results indicate that the composition of copper mesh changed after immersion into AgNO3 solution. The formation mechanisms of Ag and Cu2O on the copper mesh can be explained by Eqs. (1)–(3).
modification with low-surface-energy stearic acid was success. Fig. 3a indicated XRD pattern of untreated copper meshes and Agcoated copper meshes. It can be shown that three Cu diffraction peaks were observed for the (111), (200) and (220) crystal planes in the original copper mesh [49]. In addition to three Cu peaks, four Ag peaks (marked with heart symbols in Fig. 3a) were observed simultaneously in the Ag-coated copper mesh. The existent of Ag peaks indexed to Ag attributed to (111), (200), (220) and (311) planes in the XRD pattern of the Ag-coated copper mesh [10]. Combined with our earlier results (Fig. 2b), these results indicate that an Ag film covered the original copper mesh surface after replacement reaction. Additionally, four
Cu + 2Ag+ → Cu2+ + 2Ag Cu
2+
+ Cu → 2Cu
+
Cu+ +2 OH− → 2CuOH → Cu2O + H2O 5
(1) (2) (3)
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Fig. 7. The wetting behavior of tea, water and milk on the modified Ag-coated copper mesh.
3d and Cu 2p peaks on the modified Ag-coated copper mesh. The Cu 2p1/2 peak and Cu 2p3/2 at 952.42 and 932.43 eV are attributed to Cu2O (Fig. 4b). Fig. 4c shows that the two major peaks of Ag 3d XPS spectrum at 368.26 and 374.26 eV are correspond to Ag 3d5/2 and Ag 3d3/2, which confirm that sliver exists in the form of metallic Ag (Ag0). The three C 1s peaks at 288.14, 284.97 and 284.49 eV can be indexed to CeC, C–O and O=C–O groups, respectively (Fig. 4d).
Fig. 5. WCA and WSA results for Ag-coated copper mesh prepared in a 0.01 M AgNO3 solution with different replacement reaction times.
Eq. (1) shows Ag films covers the original copper mesh after replacement reaction. In Eq. (2), the Cu2+ generated in Eq. (1) is reduced to Cu+. Eq. (3) shows that Cu+ will exist in the form of Cu2O under nearly neutral or alkaline conditions. This is because Cu+ has strong polarization ability and is transformed into CuOH, which decomposes into Cu2O and H2O [51,52]. Stearic acid contains the −COOH active group, which can react with metals. Thus, −CH2 and −CH3 groups with low surface energies can be grafted onto the material surface to reduce the surface energy [53,54]. The FT-IR spectra of the modified Ag-coated copper mesh and stearic acid are shown in Fig. 3b. Compared with the stearic acid spectrum, the carboxyl band at 1701 cm−1 disappeared and carbonyl bands appeared at 1618 cm−1 and 1638 cm−1 for the carboxylate group resonance [55]. The peaks at 2920 cm−1 and 2850 cm−1 correspond in ascribed the stretching vibrations of −CH2 and −CH3 groups, respectively [56,57]. The band in 3450 cm−1 is caused by the stretching vibration of −OH [14]. Combined with our earlier results (Fig. 2c), these results imply that the surface of Ag-coated copper mesh was successfully modified by stearic acid. Furthermore, the composition and bonding states of superhydrophobic surface was examined by XPS. Fig. 4a shows C 1s, O 1s, Ag
3.2. Wettability measurement The replacement reaction time is a significant factor affecting surface roughness of copper mesh and it plays a crucial part in determining surface wettability behavior. We obtained WCA and WSA of surfaces fabricated on copper mesh under different reaction times and the same modification conditions (Fig. 5). Even after modification by stearic acid, WCAs of original copper mesh were only 133° and the WSA was as high as 29.5°, which is far from superhydrophobic. After immersion in an AgNO3 solution for 0.5 min, the WCA increased to 148° and the WSA decreased to 11°. However, these values still did not indicate superhydrophobicity. Along with the replacement reaction, the WCA of the modified Ag-coated copper mesh gradually increased while the WSA gradually decreased. When the reaction time was increased to 2 min, WCA reached to 158° and WSA was only 3°. These results indicated excellent superhydrophobicity. A further increase in the immersion time resulted in a slight decrease in the superhydrophobicity. However,
Fig. 6. WCA of the (a) original copper mesh, (b) Ag-coated copper mesh prepared with a 2 min replacement reaction and (c) modified Ag-coated copper mesh. (d) OCA of the modified Ag-coated copper mesh. (e) Silver mirror phenomenon of the modified Ag-coated copper mesh. (f) A water jet bouncing off the surface of the modified Ag-coated copper mesh. The wetting behavior of (g) water and (h) oil on the modified Ag-coated copper mesh. 6
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Fig. 9. Digital photographs of the self-cleaning process for the modified Agcoated copper mesh: (a) before, (b) during, and (c) after cleaning with water.
theory, for hydrophilic surfaces, the WCA decreases with the increase of surface roughness. Therefore, the WCA on Ag-coated copper mesh are less than WCA on a non-modified copper mesh. The WCA (Fig. 6c) and OCA (Fig. 6d) of the Ag-coated copper mesh after modification with stearic acid were 158° and 1°, respectively. This shows that a rough structure and surface modification were indispensable to achieve superhydrophobicity. We observed a silver mirror phenomenon (Fig. 6e) showing that a layer of air was trapped by the modified Ag-coated copper mesh surface. This layer reflected light with a silver mirror-like sheen from the surface after immersion in water and the surface remained completely dry after water extraction. These results show that air pockets could prevent water permeation. This also explains the water jet impingement (Fig. 6f). A jet of water still readily bounced off the modified copper mesh surface, even after exposure to the jet for 10 min, due to cavitation formed between the sliver film and water. We recorded digital images about the water droplet and oil droplet (methylene chloride) on modified copper mesh. The water droplets were perfectly spherical on modified copper mesh surface (Fig. 6g), which indicates that the surface had excellent water repellency. A methylene chloride droplet completely wetted the modified copper mesh and the contact angle was approximately 0° (Fig. 6h). Fig. 7 show that the modified Ag-coated copper mesh also had outstanding repellency to tea and milk. This indicates that the modified Ag-coated copper meshes were superhydrophobic as well as superoleophilic.
Fig. 8. Digital photographs of WSA measurements on the modified Ag-coated copper mesh.
the WCA was still larger than 150° and the WSA was below 5°. This wettability behavior is consistent with the results shown in Fig. 1 and indicated that the micro-nano hierarchical structure gradually became prominent as the replacement reaction time increases. An optimum surface roughness was obtained when immersed into AgNO3 solution for 2 min. As the reaction time continued to increase, the formation of micro-nano structures also increases, which in turn affected the pore size of copper mesh and inhibited superhydrophobicity. Form the above, the Ag-coated copper mesh prepared using a reaction time of 2 min and modified with stearic acid was used for further investigation. Moreover, the effect of AgNO3 concentration on WCA was shown in Figure S2, the WCA were all greater than 150°. The surface wettability of water and oil on copper mesh processed by different methods was assessed (Fig. 6a‒d). The WCA of untreated copper mesh was around 122° (Fig. 6a) and even after modification with stearic acid it was only 133° (Fig. 5). The WCA of the Ag-coated copper mesh after the replacement reaction in an AgNO3 solution for 2 min was about 109° (Fig. 6b). It can be explained with Wenzel's 7
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Fig. 10. Separation process of cyclohexane from water using a miniature boat made of modified Ag-coated copper mesh.
Fig. 11. Separation of (a–c) cyclohexane-water and (d–f) methylene chloride-water mixtures by the modified Ag-coated copper mesh.
results conclude the superhydrophobic copper mesh can be selfcleaning and resistant to contamination.
3.3. Self-cleaning properties The WSA of modified Ag-coated copper mesh was measured using a device constructed in our laboratory (Fig. 8 and Video S1). Before the measurements, the water droplet was stationary on modified Ag-coated copper mesh (Fig. 8a). When substrate was inclined to 2°, the water droplets began to slide (Fig. 8b and enlargement in Fig. 8c). In the process of oil–water separation, porous materials are easily contaminated with impurities, which can block holes and affect the separation efficiency. Therefore, when separating the oil from water or water from oil in practical application, the self-cleaning property is therefore important. We investigated self-cleaning of the modified Agcoated copper mesh (Fig. 9 and Video S2). Before the self-cleaning process (Fig. 9a), the glass slide with Ag-coated copper mesh was leaned against a culture dish and a sparse layer of chalk was uniformly coated on the surface. Water droplets dropped onto the copper mesh covered the layer of white chalk and slid down instantly (Fig. 9b). A distinct route was left on the superhydrophobic surface (Fig. 9c). These
3.4. Oil–water separation properties and recyclability Porous substrates possessing superhydrophobic and superoleophilic properties are ideal materials used as treating oily wastewater [10]. The modified Ag-coated copper mesh with hierarchical micro-nano structure fabricated in our study showed excellent superhydrophobicity and superoleophilicity. This suggested that it could be used to adsorb oil slicks on water (Fig. 10a‒c and Video S3). A small boat constructed with the modified Ag-coated surface was used to test deoiling ability in wastewater. The light oil cyclohexane floated on the surface of the water when mixed with water (Fig. 10b). Because of the superhydrophobicity and superhydrophilicity, the miniature boat floated on the water after it was placed in the beaker containing oil‒water mixtures. The oil slick on water surface collected in the miniature boat. The cyclohexane collected in the boat could be removed with a dropper to 8
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Fig. 12. Separation efficiencies for (a) different oil–water mixtures and with different numbers of separation cycles for (b) blend oil, (c) methylene chloride and (d) cyclohexane.
surface, and the superhydrophobic made water be left on the above of the copper mesh (Fig. 11b). No blue water can be shown from separated dichloromethane (Fig. 11c). The separation process was achieved using merely gravity. Separation of heavy oil methylene chloride from water was similar to that of cyclohexane. When a mixture of methylene chloride-water was poured into the vessel (Fig. 11e-f), oil easily penetrated through the modified Ag-coated copper mesh, which left water on the surface of the mesh. To investigate the separation efficiencies and flux of modified Agcoated surface towards different oil–water mixtures, we explored the separation efficiencies and flux for blend oil, kerosene, diesel fuel, methylene chloride and cyclohexane. The separation efficiencies towards as-prepared superhydrophobic surfaces were all nearly 99% to diverse oil–water mixtures (Fig. 12a). Because disparate oils have different densities, the separation efficiencies exhibit a little difference. Except for blend oil, the flux of methylene chloride, cyclohexane, diesel fuel and kerosene is all around 20,000 L· m−2· h-1. The flux of blend oil and diesel fuel is only 13,581 L· m−2· h-1. This is because the viscosity of these oil is relatively large (Figure S3).The influence of the number of cycles on the separation efficiencies of different oil–water mixtures is shown in Fig. 12b‒d. Although the separation efficiency of blend oil slightly decreased at higher viscosity, the separation efficiency remained above 96% (Fig. 12b). The separation efficiencies of methylene chloride and cyclohexane reached 98% after 22 cycles of separation, which indicate the modified Ag-coated copper mesh had stable superhydrophobicity and superoleophilicity. In the separation process, sample loss was mainly caused by oil droplets sticking to the device and
leave a clean water surface. In the following Wenzel equation, cos θ* = r cos θ,
(4)
in which θ* and θ represent contact angle of rough and smooth surfaces, respectively, and r is roughness. According to the Wenzel equation, surface wettability could increase with the increase of surface wettability (i.e., hydrophilic surfaces are more hydrophilic and hydrophobic surfaces are more hydrophobic). Therefore, oleophilicity can be strengthened with the increase of surface roughness because untreated copper mesh was oleophilic [22,58] and oil could penetrate the surface pores [59]. The Cassie-Baxter Eq. as follows,
cosθ * = f1 cosθ + f2
(5)
where f1 and f2 represent the area fraction of solid and droplet, respectively. When water droplets contact with superhydrophobic surfaces, the rough structure traps some of the air. Therefore, according to the Cassie-Baxter equation, the water contact angle increased with larger surface roughness. Next, we studied the separation performance of modified Ag-coated surface to diverse oil–water mixtures (Fig. 11). Fig. 11a‒c and Video S4 demonstrate the separation of cyclohexane-water mixtures (1:1, v/v) by the modified Ag-coated copper mesh. A superhydrophobic copper mesh was placed between glass funnel and flask (Fig. 11a). Then, oily wastewater was poured into the funnel. Cyclohexane (red) and water was separated quickly due to the superoleophility, the oil permeated to 9
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a small quantity remaining in the beaker. Additionally, the volatility of the oil also slightly reduced the measured separation efficiency. The prepared surface on copper mesh had high separation efficiency, good stability and good recyclability, so could be used for treating oily wastewater. Moreover, the effect of AgNO3 concentration on oil-water separation efficiency was shown in Figure S4. The oil-water separation efficiency of the prepared samples with different concentrations of AgNO3 all reached 98%. In addition, the flux of samples prepared at different AgNO3 concentrations to kerosene is also around 20,000 L· m−2· h-1 (Figure S5).
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Xiang, M.H.Z. Jiang, Y.Z. Zhang, Y. Liu, F. Shen, G. Yang, Y. He, L.L. Wang, X.H. Zhang, S.H. Deng, Fabrication of a novel superhydrophobic and superoleophilic surface by one-step electrodeposition method for continuous oil/water separation, Appl. Surf. Sci. 434 (2018) 1015–1020. [25] Y.Y. Cheng, B. Wu, X.F. Ma, S.X. Lu, W.G. Xu, S. Szunerits, R. Boukherroub, Facile preparation of high density polyethylene superhydrophobic/superoleophilic coatings on glass, copper and polyurethane sponge for self-cleaning, corrosion resistance and efficient oil/water separation, J. Colloid Interface Sci. 525 (2018) 76–85. [26] W.J. Ma, J.T. Zhao, O. Oderinde, J.Q. Han, Z.C. Liu, B.H. Gao, R.H. Xiong, Q.L. Zhang, S.H. Jiang, C.B. Huang, Durable superhydrophobic and superoleophilic electrospun nanofibrousmembrane for oil-water emulsion separation, J. Colloid Interface Sci. 532 (2018) 12–23. [27] G.R.T. Suyambulingam, K. Jeyasubramanian, V.K. Mariappan, P. Veluswamy, H. Ikeda, K. Krishnamoorthy, Excellent floating and load bearing properties of superhydrophobic ZnO/copper stearate nanocoating, Chem. Eng. J. 320 (2017) 468–477. [28] W.W. Yang, J. Li, P. Zhou, L.H. Zhu, H.Q. Tang, Superhydrophobic copper coating: switchable wettability, on-demand oil-water separation, and antifouling, Chem. Eng. J. 327 (2017) 849–854. [29] N. Wang, Q. Wang, S.S. Xu, X. Zheng, M.Y. Zhang, Facile Fabrication of amphiphobic surfaces on copper substrates with a mixed modified solution, RSC Adv. 9 (2019) 17366–17372. [30] M. Khosravi, S. Azizian, R. Boukherroub, Efficient oil/water separation by superhydrophobic CuxS coated on copper mesh, Sep. Purif. Technol. 215 (2019) 573–581. [31] X.Y. Yuan, W. Li, Z.G. Zhu, N. Han, X.X. Zhang, Thermo-responsive PVDF/PSMA composite membranes with micro/nanoscale hierarchical structures for oil/water emulsion separation, Colloids Surf. A Physicochem. Eng. Asp. 516 (2017) 305–316. [32] X.Y. Gao, G. Wen, Z.G. Guo, Durable superhydrophobic and underwater superoleophobic cotton fabricsgrowing zinc oxide nanoarrays for application in
4. Conclusions We fabricated a superwettable surface with a rough hierarchical structure on copper mesh by immersing in AgNO3 solution and stearic acid modification. The surfaces fabricated on the copper mesh at different replacement reaction times and under the same modification conditions were characterized in terms of morphology and wettability. The separation ability of the prepared surface was also studied. The WCA was as high as 158° and the WSA was less than 5°, which gave the stability to long-term water jet exposure and self-cleaning performance to white chalk. A variety of oil-water mixtures can be efficiently separated with the prepared superhydrophobic copper mesh. And the oily wastewater can be still highly separated even after 22 repeated cycles. The entire process was efficient and environmentally friendly. The prepared surface has great potential in applications of large-scale oil–water separation. Author contributions S.X., N.W. and X.Z. conducted the experiments and data analysis under the advising of Q.W.; S.X. and Q.W. wrote the manuscript. Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgments This work was supported by the Taishan Scholar Project of Shandong Province (No. TSHW20130956) and the Natural Science Foundation of Shandong Province, China (No. ZR2017MA013). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.124010. References [1] S.J. Yuan, C. Chen, A. Raza, R.X. Song, T.-J. Zhang, S.O. Pehkonen, B. Liang, Nanostructured TiO2/CuO dual-coated copper meshes with superhydrophilic, underwater superoleophobic and self-cleaning properties for highly efficient oil/water separation, Chem. Eng. J. 328 (2017) 497–510. [2] X.P. Li, M. Cao, H.T. Shan, F.H. Tezel, B.A. Li, Facile and scalable fabrication superhydrophobic and superoleophilic PDMS-co-PMHS coating on porous substrates for highly effective oil/water separation, Chem. Eng. J. 358 (2019) 1101–1113. [3] C.F. Feng, Z.F. Yi, F.H. She, W.M. Gao, Z. Peng, C.J. Garvey, L.F. Dumée, L.X. Kong, Superhydrophobic and superoleophilic micro-wrinkled reduced graphene oxide as a highly portable and recyclable oil sorbent, ACS Appl. Mater. Interfaces 8 (2016) 9977–9985. [4] M.Z. Ge, C.Y. Cao, J.Y. Huang, X.N. Zhang, Y.X. Tang, X.R. Zhou, K.Q. Zhang, Z. Chen, Y.K. Lai, Rational design of materials interface at nanoscale towards intelligent oil-water separation, Nanoscale Horiz 3 (2018) 235–260. [5] V. Singh, T.P. Nguyen, Y.-J. Sheng, H.-K. Tsao, Stress-Driven separation of surfactant stabilized emulsions and gel-emulsions by superhydrophobic/superoleophilic meshes, J. Phys. Chem. C 122 (2018) 24750–24759. [6] B. Lin, J. Chen, Z.-T. Li, F.-A. He, D.-H. Li, Superhydrophobic modification of polyurethane sponge for the oil-water separation, Surf. Coat. Technol. 359 (2018) 216–226.
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Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Environmentally-friendly fabrication of a recyclable oil-water separation material using copper mesh for immiscible oil/water mixtures
T
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Shuangshuang Xu, Qing Wang , Ning Wang, Xu Zheng, Lulu Lei Institute of NanoEngineering, College of Civil Engineering and Architecture, Shandong University of Science and Technology, 266590, Shandong, PR China
G R A P H I C A L A B S T R A C T
Superhydrophobic and superoleohobic surface fabricated with an environmentally-friendly modification by non-toxic stearic acid, exhibit self-cleaning, stability to long-term water jet exposure and recyclability to various oil–water mixtures.
A R T I C LE I N FO
A B S T R A C T
Keywords: Superhydrophobic Superoleophilic Environmentally-friendly Oil–water separation Recyclable Self-cleaning
Oily wastewater created by industrial disposal and oil spill disasters enormously affects human health and ecological systems. Environmentally-friendly methods for oil–water separation are highly desirable. We propose an environmentally-friendly approach for fabricating superwettable surfaces with recyclability to separate oilwater mixtures efficiently. A superwettable surface with a dendrite-like hierarchical structure for separating various oil–water mixtures was fabricated using copper mesh via modification by non-toxic stearic acid. The prepared surface was confirmed to be superhydrophobic and superoleophilic, in which the water contact angle, water slide angle and oil contact angle are 158°, 3° and 1°, respectively. These properties resulted in the surface achieving oil–water separation driven by gravity, and exhibiting efficient separation (> 98%) and high recyclability (throughout 22 consecutive cycles) of various oil–water mixtures. Moreover, the prepared surface also exhibited self-cleaning performance and stability to long-term water jet exposure. This efficient and environmentally-friendly method provides a green strategy for separating various oils from water.
1. Introduction Large volumes of oily wastewater are generated by industrial activity [1,2] and offshore oil spills during oil transportation [3–5]. These
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have led to increasingly serious pollution of water and oil resources in nature [6,7]. Oil-contaminated water contains toxic chemicals [8] that are harmful to human health and the ecosystem [9–11]. Measures should be taken to prevent this situation from deteriorating [12].
Corresponding author. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.colsurfa.2019.124010 Received 18 July 2019; Received in revised form 16 September 2019; Accepted 23 September 2019 Available online 24 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
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functional superwettable surface are imperative [42]. In this study, an environmentally-friendly method for fabricating superwettable surfaces with recyclability to efficiently separate oil–water mixture was proposed. A rough surface with dendrite-like hierarchical structures was fabricated on copper mesh by a replacement reaction. Subsequent modification with stearic acid yielded superhydrophobic/superoleophilic surfaces. Stearic acid contains 16 −CH2, 1 −CH3 and 1 −COOH groups, is often used as a low surface energy modifying material and is non-toxic [43,44]. The impact of different reaction times on wettability and morphologies were investigated. In order to study the separation efficiency of prepared copper mesh under different oil–water mixtures and separation cycles, the oil–water separation experiment was conducted. The stability of the mesh upon exposure to a water jet for an extended duration was studied and the self-cleaning performance was evaluated. The proposed method is efficient and pollution-free and the prepared surface has potential in oil–water separation application.
Conventional methods, including situ burning [13,14], gravity separation [15,16], air flotation [17,18] and biological treatment [19,20] are widely used to deal with oily wastewater. These technologies face several problems such as complexity, creation of secondary pollutants, low separation efficiencies and high operation costs [7,21,22]. An effective strategy for separating oil–water mixtures using the special wettability (superhydrophilic/superoleophobic or superoleophilic/ superhydrophobic) of solid surfaces was introduced [23,24]. Feng et al. reported a flexible oil absorbent material for potential application in oil–water separation, which was prepared by depositing rough graphene onto polydimethylsiloxane elastomeric matrices [3]. Cheng et al. prepared superhydrophobic coatings on a polyurethane kitchen sponge using high density polyethylene. The coated sponge exhibited excellent water–oil separation capability [25]. These superhydrophobic/superoleophilic materials can selectively remove oil from water or water from oil due to the wettability of oil and water on bionic surfaces is different [26]. Generally, the geometrical architecture (i.e., surface roughness) and chemical composition (i.e., surface energy) greatly determine the wettability of solid surfaces [27–29]. Superhydrophobic/superoleophilic surfaces with selective permeability to water and oil have been prepared on porous substrates by coordinating rough structures and low surface energies [10,30]. For example, Polymer substrates were used for fabricating superhydrophobic and superoleophilic surfaces to separation of oil-water mixture [31–33]. Metal substrates could improve the properties by fabricating superhydrophobic surface [34]. After a series of treatments on the metal mesh, the superhydrophobic and superoleophilic surfaces were obtained to achieve the separation of oil and water [35–38]. However, these reports face some drawbacks, such as low separation efficiency or harmful to the environment. Moreover, the mechanical properties of the fabricated surfaces are poor and easily damaged by some external force. Moreover, a nanostructured TiO2 mesh membrane with special wettability and capacity of 10 separation cycles was prepared by anodization at 45 V for 6 h and subsequent annealing at 350 °C for 1 h in air [23]. More efficient preparation methods and materials capable of more separation cycles are required for practical applications. Wen et al. fabricated a superhydrophilic in air and superoleophobic underwater surface by electrodepositing a layer of copper on the stainless steel surface. But its hydrophilicity will make it prone to corrosion in the air and separate oil-in-water emulsion only under gravity or a low applied pressure. Gao et al. prepared a superhydrophobic/superoleophilic filter paper by electrospraying with polyvinylidene fluoride/SiO2 microspheres [39]. Ke et al. fabricated superhydrophobic/superoleophilic sponges modified with octadecyltrichlorosilane and used the sponges to suck the oils out of water by dipping into the mixtures of oil/water [40]. These methods suffer from serious disadvantages such as requiring environmentally-harmful fluorine-containing modified materials, or octadecyltrichlorosilane which releases poisonous gas when exposed to water [41]. Environmentally-friendly methods for preparing
2. Experimental 2.1. Chemicals and materials Copper mesh (51 μm aperture, 90 μm wire diameter) was obtained from Shanghai Huaxin Metal Mesh Factory (Shanghai, China). Silver nitrate (AgNO3) and Stearic acid were purchased from the Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China) and Tianjin Beilian Fine Chemicals Development Co., Ltd. (Tianjin, China), respectively. Blend oil, methylene chloride, cyclohexane and ethyl alcohol were received from Qingdao Jingke Reagent Co., Ltd. (Qingdao, China). Deionized water was used in all experiments. All analytical grade chemicals are not further purification. 2.2. Sample preparation First, the copper mesh was cut to the desired size and polished with 600 mesh sandpaper to remove the surface oxide layer. Then, it was cleaned ultrasonically with anhydrous ethanol for 10 min to remove surface grease. After that, the copper mesh was immersed in a 0.01 M AgNO3 aqueous solution for different lengths of time at ambient temperature. Finally, the copper mesh was modified by placing it in 0.01 M ethanol solution of stearic acid for 1 h, as illustrated in Scheme 1. After each step was completed, the prepared surface was rinsed in deionized water, subsequently dried in the air. 2.3. Characterization The surface morphology was characterized by scanning electron microscopy (SEM, APERO, FEI Co., USA). The roughness was analyzed by atomic force microscopy (AFM, NanoManVS, Germany). Energy dispersion spectrum (EDS) attached to the SEM apparatus was used to
Scheme 1. Illustration of the preparation procedure of modified Ag-coated copper mesh. 2
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Fig. 1. SEM images of copper mesh at different replacement reaction times in a 0.01 M AgNO3 solution with different magnifications: (a, b) 0 min, (c, d) 0.5 min, (e, h) 2 min, (g, h) 10 min.
Scientific, USA). Chemical analysis was assessed via X-ray photoelectron spectroscopy (XPS, ThermoFischer, ESCALAB 250Xi, USA). The optical contact angle measurement device (DSA30, Kruss Co., Germany) was used to measure water contact angle (WCA) and oil
investigate the chemical compositions. The phase composition of prepared surface was analyzed by X-ray diffraction (XRD, Rigaku Ultima IV, Rigaku Co., Ltd., Japan). Fourier-transform infrared (FT-IR) spectrum was conducted by Nicolet 380 FT-IR spectrometer (Thermo Fisher 3
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modified copper mesh was glued to a glass slide and the slide was leaned against a culture dish. A thin layer of white chalk was spread over the superhydrophobic copper mesh. The modified Ag–coated copper mesh was then cleaned using water to observe its self-cleaning behavior. Digital photographs were obtained at different times. 2.5. Oil–water separation test The prepared superhydrophobic surface was first fixed in an oil–water separation device. The device was composed by a glass funnel and conical flask. Then, the oil–water mixture (1:1, v/v) was poured over the superhydrophobic copper mesh and was separated under gravity. The device was tested with five oil–water mixtures (blend oil, kerosene, diesel fuel, methylene chloride and cyclohexane). In the separation experiments, cyclohexane and methylene chloride were dyed red by Sudan III and deionized water was dyed blue by methyl blue. The separation efficiency (η) can be calculated with: η = m1/m2 ×100% [45,46], in which m1 and m2 denote oil mass of two separation cycles of oil-water begins and ends. The flux can be calculated with: Flux = V/St, where V is the volume of the permeated oil (L), S is the valid area (m2) of membrane, and t refers to the time (s) for the separation. The sample was washed with ethanol and deionized water and then dried at room temperature after separation. 3. Results and discussion 3.1. Morphology and composition Surface roughness and energy are two important factors determining the wettability of solid surfaces [47]. Here, we prepared a superhydrophobic/superoleophilic surface on copper mesh. A copper mesh surface with the ideal rough structure was prepared by a short replacement reaction with a 0.01 M AgNO3 solution at ambient temperature. Further modification was conducted using a 0.01 M ethanol solution of low-surface-energy stearic acid for 1 h. The surface morphologies of the original and the superhydrophobic surfaces with different replacement reaction times were demonstrated by SEM under different magnifications. The original copper mesh surface is relatively smooth, the average wire and pore diameters were about 45 μm and 100 μm, respectively (Fig. 1a and b). After the replacement reaction, the surface of the copper mesh became rough. With a short reaction time (0.5 min), flower-like clusters constituted by micro-sized ginkgo leaves began to appear on the copper mesh (Fig. 1c and d). After 2 min in AgNO3 solution, the surface of copper mesh (denoted as Ag-coated copper mesh) changed greatly (Fig. 1e and f). The enlarged images in Fig. 1g and h shows that the morphologies were transformed from micro flower-like clusters to micro-nano dendrite-like structures. The formed hierarchical structures are superhydrophobic because their rough surfaces trapped large amounts of air in the gaps [10,48]. As the reaction time increased, the amount of dendrite-like structures covering the copper mesh surface increased. At the same time, with the increasing of the dendrite-like structures, the aperture of the copper mesh decreased. When the immersion time was 10 min, the copper mesh aperture was completely covered by the dendrite-like structure (Fig. 1i and j). Figure S1 displays the AFM image of Ag-coated copper mesh, it can be shown that the surface is composed of many nano- and microsize protuberances. The composition of the original and Ag-coated copper mesh before and after modification was analyzed by EDS spectrum. The EDS spectrum of the original copper mesh only contained Cu (Fig. 2a). After the replacement reaction with the AgNO3 solution, elemental Ag was observed from EDS spectrum of Ag-coated copper mesh (Fig. 2b). This indicated that an Ag layer was deposited on copper mesh after replacement reaction. The presence of O attributed to copper oxides. After modification with stearic acid, an obvious C peak can be observed from superhydrophobic copper mesh (Fig. 2c). This demonstrated the
Fig. 2. EDS spectrum of the (a) original copper mesh, (b) Ag-coated copper mesh and (c) modified Ag-coated copper mesh.
contact angle (OCA). The water slide angle (WSA) was measured using a device constructed in our laboratory. The results were measured by calculating an average value using a 10-μL droplet from five different positions each time. The copper mesh was fixed to the glass slide during the measurement. 2.4. Self-cleaning experiments The white chalk was regarded as model contaminants to investigate the self–cleaning property of modified Ag-coated copper mesh. The 4
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Fig. 3. (a) XRD pattern of the original copper mesh and Ag-coated copper mesh. (b) FT-IR spectra of stearic acid and the modified Ag-coated copper mesh.
Fig. 4. XPS spectrum of the modified Ag-coated copper mesh, (a) survey spectrum; (b) Cu region; (c) Ag region; (d) C region, the C1 s peak was deconvoluted.
diffraction peaks of Cu2O (marked with square symbols in Fig. 3a) for the (110), (111), (200) and (220) planes were detected in the XRD pattern of Ag-coated copper mesh [50]. These results indicate that the composition of copper mesh changed after immersion into AgNO3 solution. The formation mechanisms of Ag and Cu2O on the copper mesh can be explained by Eqs. (1)–(3).
modification with low-surface-energy stearic acid was success. Fig. 3a indicated XRD pattern of untreated copper meshes and Agcoated copper meshes. It can be shown that three Cu diffraction peaks were observed for the (111), (200) and (220) crystal planes in the original copper mesh [49]. In addition to three Cu peaks, four Ag peaks (marked with heart symbols in Fig. 3a) were observed simultaneously in the Ag-coated copper mesh. The existent of Ag peaks indexed to Ag attributed to (111), (200), (220) and (311) planes in the XRD pattern of the Ag-coated copper mesh [10]. Combined with our earlier results (Fig. 2b), these results indicate that an Ag film covered the original copper mesh surface after replacement reaction. Additionally, four
Cu + 2Ag+ → Cu2+ + 2Ag Cu
2+
+ Cu → 2Cu
+
Cu+ +2 OH− → 2CuOH → Cu2O + H2O 5
(1) (2) (3)
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Fig. 7. The wetting behavior of tea, water and milk on the modified Ag-coated copper mesh.
3d and Cu 2p peaks on the modified Ag-coated copper mesh. The Cu 2p1/2 peak and Cu 2p3/2 at 952.42 and 932.43 eV are attributed to Cu2O (Fig. 4b). Fig. 4c shows that the two major peaks of Ag 3d XPS spectrum at 368.26 and 374.26 eV are correspond to Ag 3d5/2 and Ag 3d3/2, which confirm that sliver exists in the form of metallic Ag (Ag0). The three C 1s peaks at 288.14, 284.97 and 284.49 eV can be indexed to CeC, C–O and O=C–O groups, respectively (Fig. 4d).
Fig. 5. WCA and WSA results for Ag-coated copper mesh prepared in a 0.01 M AgNO3 solution with different replacement reaction times.
Eq. (1) shows Ag films covers the original copper mesh after replacement reaction. In Eq. (2), the Cu2+ generated in Eq. (1) is reduced to Cu+. Eq. (3) shows that Cu+ will exist in the form of Cu2O under nearly neutral or alkaline conditions. This is because Cu+ has strong polarization ability and is transformed into CuOH, which decomposes into Cu2O and H2O [51,52]. Stearic acid contains the −COOH active group, which can react with metals. Thus, −CH2 and −CH3 groups with low surface energies can be grafted onto the material surface to reduce the surface energy [53,54]. The FT-IR spectra of the modified Ag-coated copper mesh and stearic acid are shown in Fig. 3b. Compared with the stearic acid spectrum, the carboxyl band at 1701 cm−1 disappeared and carbonyl bands appeared at 1618 cm−1 and 1638 cm−1 for the carboxylate group resonance [55]. The peaks at 2920 cm−1 and 2850 cm−1 correspond in ascribed the stretching vibrations of −CH2 and −CH3 groups, respectively [56,57]. The band in 3450 cm−1 is caused by the stretching vibration of −OH [14]. Combined with our earlier results (Fig. 2c), these results imply that the surface of Ag-coated copper mesh was successfully modified by stearic acid. Furthermore, the composition and bonding states of superhydrophobic surface was examined by XPS. Fig. 4a shows C 1s, O 1s, Ag
3.2. Wettability measurement The replacement reaction time is a significant factor affecting surface roughness of copper mesh and it plays a crucial part in determining surface wettability behavior. We obtained WCA and WSA of surfaces fabricated on copper mesh under different reaction times and the same modification conditions (Fig. 5). Even after modification by stearic acid, WCAs of original copper mesh were only 133° and the WSA was as high as 29.5°, which is far from superhydrophobic. After immersion in an AgNO3 solution for 0.5 min, the WCA increased to 148° and the WSA decreased to 11°. However, these values still did not indicate superhydrophobicity. Along with the replacement reaction, the WCA of the modified Ag-coated copper mesh gradually increased while the WSA gradually decreased. When the reaction time was increased to 2 min, WCA reached to 158° and WSA was only 3°. These results indicated excellent superhydrophobicity. A further increase in the immersion time resulted in a slight decrease in the superhydrophobicity. However,
Fig. 6. WCA of the (a) original copper mesh, (b) Ag-coated copper mesh prepared with a 2 min replacement reaction and (c) modified Ag-coated copper mesh. (d) OCA of the modified Ag-coated copper mesh. (e) Silver mirror phenomenon of the modified Ag-coated copper mesh. (f) A water jet bouncing off the surface of the modified Ag-coated copper mesh. The wetting behavior of (g) water and (h) oil on the modified Ag-coated copper mesh. 6
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Fig. 9. Digital photographs of the self-cleaning process for the modified Agcoated copper mesh: (a) before, (b) during, and (c) after cleaning with water.
theory, for hydrophilic surfaces, the WCA decreases with the increase of surface roughness. Therefore, the WCA on Ag-coated copper mesh are less than WCA on a non-modified copper mesh. The WCA (Fig. 6c) and OCA (Fig. 6d) of the Ag-coated copper mesh after modification with stearic acid were 158° and 1°, respectively. This shows that a rough structure and surface modification were indispensable to achieve superhydrophobicity. We observed a silver mirror phenomenon (Fig. 6e) showing that a layer of air was trapped by the modified Ag-coated copper mesh surface. This layer reflected light with a silver mirror-like sheen from the surface after immersion in water and the surface remained completely dry after water extraction. These results show that air pockets could prevent water permeation. This also explains the water jet impingement (Fig. 6f). A jet of water still readily bounced off the modified copper mesh surface, even after exposure to the jet for 10 min, due to cavitation formed between the sliver film and water. We recorded digital images about the water droplet and oil droplet (methylene chloride) on modified copper mesh. The water droplets were perfectly spherical on modified copper mesh surface (Fig. 6g), which indicates that the surface had excellent water repellency. A methylene chloride droplet completely wetted the modified copper mesh and the contact angle was approximately 0° (Fig. 6h). Fig. 7 show that the modified Ag-coated copper mesh also had outstanding repellency to tea and milk. This indicates that the modified Ag-coated copper meshes were superhydrophobic as well as superoleophilic.
Fig. 8. Digital photographs of WSA measurements on the modified Ag-coated copper mesh.
the WCA was still larger than 150° and the WSA was below 5°. This wettability behavior is consistent with the results shown in Fig. 1 and indicated that the micro-nano hierarchical structure gradually became prominent as the replacement reaction time increases. An optimum surface roughness was obtained when immersed into AgNO3 solution for 2 min. As the reaction time continued to increase, the formation of micro-nano structures also increases, which in turn affected the pore size of copper mesh and inhibited superhydrophobicity. Form the above, the Ag-coated copper mesh prepared using a reaction time of 2 min and modified with stearic acid was used for further investigation. Moreover, the effect of AgNO3 concentration on WCA was shown in Figure S2, the WCA were all greater than 150°. The surface wettability of water and oil on copper mesh processed by different methods was assessed (Fig. 6a‒d). The WCA of untreated copper mesh was around 122° (Fig. 6a) and even after modification with stearic acid it was only 133° (Fig. 5). The WCA of the Ag-coated copper mesh after the replacement reaction in an AgNO3 solution for 2 min was about 109° (Fig. 6b). It can be explained with Wenzel's 7
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Fig. 10. Separation process of cyclohexane from water using a miniature boat made of modified Ag-coated copper mesh.
Fig. 11. Separation of (a–c) cyclohexane-water and (d–f) methylene chloride-water mixtures by the modified Ag-coated copper mesh.
results conclude the superhydrophobic copper mesh can be selfcleaning and resistant to contamination.
3.3. Self-cleaning properties The WSA of modified Ag-coated copper mesh was measured using a device constructed in our laboratory (Fig. 8 and Video S1). Before the measurements, the water droplet was stationary on modified Ag-coated copper mesh (Fig. 8a). When substrate was inclined to 2°, the water droplets began to slide (Fig. 8b and enlargement in Fig. 8c). In the process of oil–water separation, porous materials are easily contaminated with impurities, which can block holes and affect the separation efficiency. Therefore, when separating the oil from water or water from oil in practical application, the self-cleaning property is therefore important. We investigated self-cleaning of the modified Agcoated copper mesh (Fig. 9 and Video S2). Before the self-cleaning process (Fig. 9a), the glass slide with Ag-coated copper mesh was leaned against a culture dish and a sparse layer of chalk was uniformly coated on the surface. Water droplets dropped onto the copper mesh covered the layer of white chalk and slid down instantly (Fig. 9b). A distinct route was left on the superhydrophobic surface (Fig. 9c). These
3.4. Oil–water separation properties and recyclability Porous substrates possessing superhydrophobic and superoleophilic properties are ideal materials used as treating oily wastewater [10]. The modified Ag-coated copper mesh with hierarchical micro-nano structure fabricated in our study showed excellent superhydrophobicity and superoleophilicity. This suggested that it could be used to adsorb oil slicks on water (Fig. 10a‒c and Video S3). A small boat constructed with the modified Ag-coated surface was used to test deoiling ability in wastewater. The light oil cyclohexane floated on the surface of the water when mixed with water (Fig. 10b). Because of the superhydrophobicity and superhydrophilicity, the miniature boat floated on the water after it was placed in the beaker containing oil‒water mixtures. The oil slick on water surface collected in the miniature boat. The cyclohexane collected in the boat could be removed with a dropper to 8
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Fig. 12. Separation efficiencies for (a) different oil–water mixtures and with different numbers of separation cycles for (b) blend oil, (c) methylene chloride and (d) cyclohexane.
surface, and the superhydrophobic made water be left on the above of the copper mesh (Fig. 11b). No blue water can be shown from separated dichloromethane (Fig. 11c). The separation process was achieved using merely gravity. Separation of heavy oil methylene chloride from water was similar to that of cyclohexane. When a mixture of methylene chloride-water was poured into the vessel (Fig. 11e-f), oil easily penetrated through the modified Ag-coated copper mesh, which left water on the surface of the mesh. To investigate the separation efficiencies and flux of modified Agcoated surface towards different oil–water mixtures, we explored the separation efficiencies and flux for blend oil, kerosene, diesel fuel, methylene chloride and cyclohexane. The separation efficiencies towards as-prepared superhydrophobic surfaces were all nearly 99% to diverse oil–water mixtures (Fig. 12a). Because disparate oils have different densities, the separation efficiencies exhibit a little difference. Except for blend oil, the flux of methylene chloride, cyclohexane, diesel fuel and kerosene is all around 20,000 L· m−2· h-1. The flux of blend oil and diesel fuel is only 13,581 L· m−2· h-1. This is because the viscosity of these oil is relatively large (Figure S3).The influence of the number of cycles on the separation efficiencies of different oil–water mixtures is shown in Fig. 12b‒d. Although the separation efficiency of blend oil slightly decreased at higher viscosity, the separation efficiency remained above 96% (Fig. 12b). The separation efficiencies of methylene chloride and cyclohexane reached 98% after 22 cycles of separation, which indicate the modified Ag-coated copper mesh had stable superhydrophobicity and superoleophilicity. In the separation process, sample loss was mainly caused by oil droplets sticking to the device and
leave a clean water surface. In the following Wenzel equation, cos θ* = r cos θ,
(4)
in which θ* and θ represent contact angle of rough and smooth surfaces, respectively, and r is roughness. According to the Wenzel equation, surface wettability could increase with the increase of surface wettability (i.e., hydrophilic surfaces are more hydrophilic and hydrophobic surfaces are more hydrophobic). Therefore, oleophilicity can be strengthened with the increase of surface roughness because untreated copper mesh was oleophilic [22,58] and oil could penetrate the surface pores [59]. The Cassie-Baxter Eq. as follows,
cosθ * = f1 cosθ + f2
(5)
where f1 and f2 represent the area fraction of solid and droplet, respectively. When water droplets contact with superhydrophobic surfaces, the rough structure traps some of the air. Therefore, according to the Cassie-Baxter equation, the water contact angle increased with larger surface roughness. Next, we studied the separation performance of modified Ag-coated surface to diverse oil–water mixtures (Fig. 11). Fig. 11a‒c and Video S4 demonstrate the separation of cyclohexane-water mixtures (1:1, v/v) by the modified Ag-coated copper mesh. A superhydrophobic copper mesh was placed between glass funnel and flask (Fig. 11a). Then, oily wastewater was poured into the funnel. Cyclohexane (red) and water was separated quickly due to the superoleophility, the oil permeated to 9
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a small quantity remaining in the beaker. Additionally, the volatility of the oil also slightly reduced the measured separation efficiency. The prepared surface on copper mesh had high separation efficiency, good stability and good recyclability, so could be used for treating oily wastewater. Moreover, the effect of AgNO3 concentration on oil-water separation efficiency was shown in Figure S4. The oil-water separation efficiency of the prepared samples with different concentrations of AgNO3 all reached 98%. In addition, the flux of samples prepared at different AgNO3 concentrations to kerosene is also around 20,000 L· m−2· h-1 (Figure S5).
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4. Conclusions We fabricated a superwettable surface with a rough hierarchical structure on copper mesh by immersing in AgNO3 solution and stearic acid modification. The surfaces fabricated on the copper mesh at different replacement reaction times and under the same modification conditions were characterized in terms of morphology and wettability. The separation ability of the prepared surface was also studied. The WCA was as high as 158° and the WSA was less than 5°, which gave the stability to long-term water jet exposure and self-cleaning performance to white chalk. A variety of oil-water mixtures can be efficiently separated with the prepared superhydrophobic copper mesh. And the oily wastewater can be still highly separated even after 22 repeated cycles. The entire process was efficient and environmentally friendly. The prepared surface has great potential in applications of large-scale oil–water separation. Author contributions S.X., N.W. and X.Z. conducted the experiments and data analysis under the advising of Q.W.; S.X. and Q.W. wrote the manuscript. Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgments This work was supported by the Taishan Scholar Project of Shandong Province (No. TSHW20130956) and the Natural Science Foundation of Shandong Province, China (No. ZR2017MA013). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.124010. References [1] S.J. Yuan, C. Chen, A. Raza, R.X. Song, T.-J. Zhang, S.O. Pehkonen, B. Liang, Nanostructured TiO2/CuO dual-coated copper meshes with superhydrophilic, underwater superoleophobic and self-cleaning properties for highly efficient oil/water separation, Chem. Eng. J. 328 (2017) 497–510. [2] X.P. Li, M. Cao, H.T. Shan, F.H. Tezel, B.A. Li, Facile and scalable fabrication superhydrophobic and superoleophilic PDMS-co-PMHS coating on porous substrates for highly effective oil/water separation, Chem. Eng. J. 358 (2019) 1101–1113. [3] C.F. Feng, Z.F. Yi, F.H. She, W.M. Gao, Z. Peng, C.J. Garvey, L.F. Dumée, L.X. Kong, Superhydrophobic and superoleophilic micro-wrinkled reduced graphene oxide as a highly portable and recyclable oil sorbent, ACS Appl. Mater. Interfaces 8 (2016) 9977–9985. [4] M.Z. Ge, C.Y. Cao, J.Y. Huang, X.N. Zhang, Y.X. Tang, X.R. Zhou, K.Q. Zhang, Z. Chen, Y.K. Lai, Rational design of materials interface at nanoscale towards intelligent oil-water separation, Nanoscale Horiz 3 (2018) 235–260. [5] V. Singh, T.P. Nguyen, Y.-J. Sheng, H.-K. Tsao, Stress-Driven separation of surfactant stabilized emulsions and gel-emulsions by superhydrophobic/superoleophilic meshes, J. Phys. Chem. C 122 (2018) 24750–24759. [6] B. Lin, J. Chen, Z.-T. Li, F.-A. He, D.-H. Li, Superhydrophobic modification of polyurethane sponge for the oil-water separation, Surf. Coat. Technol. 359 (2018) 216–226.
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