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Bioinspired structured superhydrophobic and superoleophilic stainless steel mesh for efficient oil-water separation
Accepted Manuscript Title: Bioinspired structured superhydrophobic and superoleophilic stainless steel mesh for efficient oil-water separation Author: Yan Liu Kaiteng Zhang Wenguang Yao Jiaan Liu Zhiwu Han Luquan Ren PII: DOI: Reference:
S0927-7757(16)30236-9 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.04.011 COLSUA 20568
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
1-1-2016 27-3-2016 3-4-2016
Please cite this article as: Yan Liu, Kaiteng Zhang, Wenguang Yao, Jiaan Liu, Zhiwu Han, Luquan Ren, Bioinspired structured superhydrophobic and superoleophilic stainless steel mesh for efficient oil-water separation, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.04.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bioinspired structured superhydrophobic and superoleophilic stainless steel mesh for efficient oil-water separation Yan Liua, b, Kaiteng Zhanga, Wenguang Yaoa, Jiaan Liuc, Zhiwu Hana, Luquan Rena a.
Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun 130022, P.R. China; b. State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130022, China; c. Key Laboratory of Automobile Materials(Ministry of Education) and College of Materials Science and Engineering, Jilin University, Changchun, 130022, P. R. China.
Graphical abstract
Corresponding author. Tel.:+86 431 85095760; fax:+86 431 85095575 E-mail address:[email protected]
Highlights We have prepared a superhydrophobic and oleophilic stainless steel mesh by a facile immersion process. The superhydrophobic surface is described as low adhesion force with water. The wettability mechanism relies on micro-nano scale hierarchical structure and chemical component on surface. The as-prepared stainless steel mesh has an excellent oil-water separation property after being recycled ten times.
Abstract Oily wastewater have caused serious water pollution. The development of effective and cheap oil/water separation materials is urgent for treating this problem. Herein, inspired by superhydrophobic typical plant leaves such as lotus, red rose and marigold, superhydrophobic and superoleophilic stainless steel meshes with excellent oil-water separation efficiency were successfully fabricated by a facile immersion process and then surface modification with stearic acid. The simple immersion process, in which surface roughness and low-surface-energy coating are the two dominant factors of superhydrophobicity, could be accomplished sequentially. It was found that the as-prepared surface is both superhydrophobic and superoleophilic, with the static water angle (WCA) as high as 153 ± 3° and oil contact angle (OCA) of 0°. Furthermore, as-prepared meshes can be applied to separate an oil-and-water mixture bidirectionally and exhibited excellent oil-water separation efficiency including
petroleum, toluene, hexane, gasoline and diesel, even after being recycled ten times. Compared with previously reported strategies, this strategy is much easier to carry out and, in the meantime, allows the collection of oil in a continuous manner. This study provides a simple, fast, low cost and environmentally friendly route to fabricate oil-water separation materials as far as large scale manufacturing is considered, which has prospective application in industrial fields such as water treatment and petroleum refining. Key words: bioinspired; superhydrophobic; low adhesion; oil-water separation.
1. Introduction Due to the deteriorating condition of the oil pollution and increasing attention to the preservation of the environment, there is a increasing and urgent need to develop novel materials which can purify the polluted water effectively and quickly.[1-2] To resolve these water pollution issues, many approaches have been used for the treatment of oil-water pollution, including filtration, oil skimmers, centrifugal machine, magnetic separations, flotation technologies, oil-absorbing materials and combustion.[3-9] However, most of the methods involve harsh conditions, such as expensive equipment, complex device, complicated processing steps, high processing cost, long processing time and so on. Taking all factors into account, separation treatment stands out from the rest as a simple, universal, scalable approach for valid removal of oil from water. Many plants and insects exhibit excellent superhydrophobicity, such as lotus leaves, rose petals, marigold petals, water striders, butterfly wings, rice leaves, mosquito eyes and so on.[10-14] Inspired from nature, superhydrophobic surfaces are fabricated by simulating typical structures of plant surface and considering chemical composition simultaneously. Superhydrophobic surface can be widely used in fields such as fluidic drag
reduction[15-17],
self-cleaning
surface[18-19],
anti-fogging[22], battery and fuel cell applications[23-24].
corrosion
inhibition[20-21],
Designing and fabricating coatings with special wettability would be a facile and effective way to solve the problem of oil-water separation. A large number of oil/water separation materials have been explored to fabricate superhydrophobic surfaces. These include template[25] electrochemical deposition[26-27] chemical vapour deposition (CVD)[29-31], electro-spinning[32-33], sol–gel method[34-35], and many others[36-38]. In addition to fixed surface wetting properties, materials with switchable surface wettability have also been developed for controllable oil/water separation caused by pH value[39],electric field[40], thermo-treatment[41], or multiple stimuli[42]. Although it can achieve the control of the freedom of the wettability, but most of the production methods are complex and high cost. In most of the separation process, the wettability of the fixed material can meet the application requirements. So the most researchers focus on the study of fixed wettability. Zhang et al.[43] reported a method to fabricate a superhydrophobic and superoleophilic membrane by immersing porous polyurethane film into polystyrene colloidal solution. Wang et al.[44] fabricated such a film with hierarchical structure on copper mesh by a electrodeposition process. Inspired by selective wettability and hierarchical structure of papillae on lotus seeds, Xu et al.[45] fabricated papilla-like magnetic particles by thermal treatment of Fe microparticles and then modified by lauric acid. It exhibited superhydrophobicity, superoleophilicity and great oil removing capability from water. The above-mentioned materials can achieve high efficient oil-water separation. At present, a large amount of preparing methods are usually time-consuming and cost-ineffective and rely on specific equipment as well as chemicals including strong acid, strong alkali, and even toxic reagents, which greatly hinders their practical applications. This is why it is imperative to pursue facile, inexpensive, and environmentally friendly methods for fabricating functional interface materials. Stainless steel mesh is widely used for the separation of oil and water due to its porous structure constructed by microfibers and the price is cheap. Based on the above mentioned discussions, we reported a simple,one step processing method high efficient and low-cost route to prepare superhydrophobic stainless steel meshes. In this research, stainless steel meshes with superhydrophobic and superoleophilic
surface were fabricated by a simple immersion process and then surface modification with stearic acid. The whole process takes only 1 h under ambient condition. The immersion process takes place at atmospheric pressure, using a solution of 1.5 M copper chloride and 0.5 M hydrochloric acid that were cured in ambient circumstance. The superhydrophobic stainless steel meshes can be easily combined to an independent device which acts as a filter, forming a barrier that allows oils to pass through while repelling water.
2. Experimental methods 2.1. Materials The stainless steel mesh which were obtained from Changzhou Tengda Hardware Store. The mesh had pore diameters of 100 μm. Stearic acid (C18H36O2) was purchased from Tianjin Guangfu Fine Chemical Research Reagent Institute, copper chloride(CuCl2) was obtained from Tianjin Guangfu Technology Development Co., Ltd. Sudan III as a coloring agent (C22H12N4O) was purchased from Sinopharm Chemical Reagent Co., Ltd. Diesel and gasoline were products of SINOPEC. Silver nitrate(AgNO3) was purchased from Shanghai Chemical Reagent Co., LTD. All other chemicals were purchased from Beijing Chemical Works. This includes hydrochloric acid, anhydrous ethanol, chloroform, toluene, hexane, acetone, sodium chloride. All chemicals were used as received without further purification.
2.2. Sample preparation Stainless steel meshes were ultrasonically rinsed for 5 min in the solution of acetone, anhydrous ethanol and distilled water, respectively. After drying, the meshes were immersed in an aqueous solution of 1.5 M CuCl2 and 0.5 M HCl at room temperature for different immersion time(2, 5, 7, 10, 15, 20 and 30 s). After their timed reaction, the meshes were removed from the solution and rinsed again using distilled water, then drying in the ventilation heating furnace at 70 °C for 30 min. The prepared stainless steel meshes were immersed in a 0.1 M ethanol solution of stearic acid and keep it for 30 min at ambient condition. After modified by stearic acid, the sample was rinsed by sufficient anhydrous alcohol and then were dried in a vacuum
oven at 60 °C for 15 min. The after modified meshes showed hydrophobic or superhydrophobic properties.
2.3 Sample characterization The water/oil contact angles (CAs) were measured by contact angle meter (OCA 20 dataphysics, Germany) at room temperature so as to characterize the surface wettability. Water droplets (5 μL) were carefully dropped onto the surfaces, and the average static contact angle value was obtained by measuring five different positions of each surface. The surface morphologies of superamphiphobic samples were examined by a field emission scanning electron microscope (SEM, EVO 18, ZEISS). The chemical composition of the samples was characterized using an X-ray photoelectron spectroscopy (XPS, SPECS XR50, Japan) and Fourier transform infrared spectrophotometer (FT-IR, JACSCO, Japan).
2.4 Oil-water separation apparatus The device consists of peristaltic pump, retort stand. As shown in Fig. 8 the as-prepared stainless steel mesh was fixed on iron tricyclic by three binder clips. In order to guide the solution into the right side of the beaker smoothly, the prepared samples were folded into a pot mouth. There are two beakers under the mesh for collecting separated solution. On the left side of the beaker was used to collect oil, right beaker was used to collect water solution. In order to prepare oil water mixture, the 25 g of hexane (dyed with oil red) was poured into 25 g of water contained in a beaker, the hexane floated on the water, due to its lower density. The mixed solution is obtained through the peristaltic pump to the as-prepared mesh automatically The separation was achieved driven by gravity and this device can achieve continuous separation. Meantime, in order to distinguish between oil and water, oil was colored by Sudan III.
3. Results and Discussion 3.1 Surface morphology In order to investigate whether the reaction process provided enough surface
roughness, we characterized the surface morphology of the stainless steel mesh after immersion with scanning electron microscopy (SEM). Fig. 1a shows the surface morphology of the original stainless steel mesh, the mesh is composed of stainless steel wires with a diameter of 100 μm, and the stainless steel wires are interwined with each other to form a micrometer-sized rough surface. And Fig. 1b-d show the surface morphology of the as-prepared stainless steel mesh. It can be observed that not only the micro-scale stainless steel wires and pores, but also numerous as-coated Cu crystal existed on the stainless steel wires (Fig. 1b-f), forming the typically hierarchical structure. The stainless steel wires are uniformly covered by Cu crystal. The morphology of the stainless steel mesh was greatly influenced by the reaction time. The surface roughness of the meshes was changed obviously after immersing the solution of 1.5 M CuCl2 and 0.5 M HCl for a certain period of time. At a short response time, the effects of reaction is not obvious, hence the Cu particles are tiny and present relatively smooth surface, as shown in Fig. 1b. As shown in Fig. 1c, with the reaction time increasing to 5 s, the Cu crystal on the surface of the stainless steel mesh were grown up, and the number of the Cu crystal was increased. Longer immersion times encouraged formation of longer surface features that in some locations resembled needle-shaped structures. When the immersion time increased to 7 s, the needle-shaped surface structures were even more obvious and rough, with many being entangled with each other, as shown in Fig. 1d. Fig. 1e shows the SEM image of the stainless steel mesh after 15 s processing time, needle-shaped structures with sizes of several micrometers were uniformly dispersed on the surface of the wires. It can be seen that the original mesh wires has been completely covered by needle-shaped structures. There are a large amount of micro-nano scale needle-shaped laminae which can capture a large amount of air. Therefore, it provides the geometric condition for the formation of superhydrophobicity. But with the reaction time increasing to 30 s, although there is a deposition of Cu crystal, due to the stainless steel wires are severely etched, it will cause some loss of crystallization deposition. These findings show that an increase of immersion time leads to a change in morphology, resulting in the transition of wetting state from hydrophobicity to
superhydrophobicity. The microsized needle-shaped laminae in combination with the nanoscale granular protuberance endowed a hierarchical composite structure to the mesh that possessed superhydrophobic behaviors. Such micro-nano scale hierarchical structure can help support water droplets and is an essential necessity to obtain the superhydrophobic surfaces. From the surface morphology of the stainless steel meshes, we deduce that the ideal processing time is 15 s.
3.2 Chemical characterization XPS is used to test the chemical composition of the thin film of the stainless steel mesh surfaces modified after immersion processing. Fig. 2a shows the XPS spectrum of the as-prepared surface before and after modification with stearic acid and it reveals the presence of C, O, and Cu on the as-prepared surfaces. The elements of C, O, and Cu could all be found on both untreated and treated by stearic acid steel mesh surfaces. The element of the Cu is derived from a immersion process reaction between the stainless steel mesh and the CuCl2 solution. The elemental oxygen of the without modified sample is attributed to the oxidation of the mesh drying process in the ventilation heating furnace at 70 °C. Fig. 2b presents the Cu2p XPS spectrum of the as-prepared surface before modification with stearic acid. The Cu2p3/2 and Cu2p1/2 peaks centered at binding energy of 932.8 and 952.3 eV can be assigned to Cu0 and CuO species, respectively. But compared to the carbon content of the before modified sample, the sample after modified by stearic acid carbon content increased significantly. In the whole process, the carbon source mainly comes from the stearic acid. So it can be concluded that stearic acid has been combined with the mesh. But the combination mechanism is a simple adhesion to together or chemical bonding together need further confirmation. So that the stearic acid treatments could not be determined merely by surface element investigation. Thus, in order to confirm the combination mechanism of the stearic acid on the coated surface of the stainless steel mesh, FT-IR spectra was used. Fig. 3 shows FT-IR spectra of the stearic acid and as-prepared mesh surfaces modified by stearic acid. In
the stearic acid,the free stretching vibrations of the hydroxyl groups disappears at 1701 cm-1 and the new group appears at 1586 cm-1. This is due to the stretching vibration of C=O bond influenced by hydrogen bond, Cu2+ and COO- forming a new chemical structure (copper stearate), which leads to the absorption peak of C=O bond in the carboxyl group moving to the low wave position. However, the absorption peaks corresponding to the -CH3 and -CH2 are 2917 cm-1 and 2849 cm-1 respectively. The absorption peaks corresponding to -CH3 and -CH2 are 2850 cm-1 and 2956 cm-1 respectively. The corresponding absorption peak of these two functional groups have no obvious change, so the as-prepared coating contains functional groups of -CH3 and -CH2. It is known that the CH3 group with a surface energy of 24 mJ/m2 and the CH2 group with a surface energy of 31 mJ/m2, which were widely used to reduce the free energy. The FT-IR spectra indicated that the stearic acid was bound to the stainless steel mesh surfaces. The combination of the hierarchical structure and the low surface energy molecules made the mesh superhydrophobic and superoleophilic. Thus, the low-surface-energy coatings on the stainless steel mesh together with its hierarchical rough surface, microscale skeleton, and nanoscale structures endowed the substrate with superhydrophobic and superoleophlic properties.
3.3 Surface wettability 3.3.1 Formation of superhydrophobicity A combination of SEM, XPS and FT-IR characterizations show that the micro-nano scale hierarchical structures were created and that low surface energy materials were successfully prepared on the surface of stainless steel mesh through this method. It was expected that both the micro-nano scale hierarchical composite structure and chemical component made the as-prepared mesh possess superhydrophobicity, and the surface wettability was evaluated by contact angle measurement. Fig. 4a shows an image of a water droplet (5 μl) on the as-prepared stainless steel mesh and original mesh respectively. For the as-prepared mesh, it can be seen that the droplet can rest on the surface in a sphere shape and can roll off easily with slight tilt or shake, and have low adhesion to the mesh. Because of the increase of the surface
roughness, air can be trapped into the microstructures as a solid-liquid barrier. The air cushion decreases the contact area between the droplet and surface and thus the liquid droplets can roll off easily. But for the original mesh, the water droplets present a hemispherical shape and had high adhesion to the mesh. Stainless steel mesh after processing time of 15 s and then modified by stearic acid only tended to 153 ± 3°, indicating that the surface has very good superhydrophobicity as shown in Fig. 4b. Compared with original mesh, the resulting meshes turned red from their original silvery white color and showed hydrophilic properties as shown in Fig. 4a. Fig. 4c shows that the WCA (water contact angle) of the original stainless steel mesh tended to 84 ± 2°. In contrast, a drop of oil (hexane) on the as-prepared and original meshes quickly spread and diffused into its exhibiting highly oleophilic property. With the combination of hydrophobicity and oleophilicity in the substrate, this mesh may be used for oil/water separation. 3.3.2 Effect of the processing time on wettability A higher resolution SEM depicted in Figure 1d indicates that this mesh has both micro and nanoscale surface roughness and this surface roughness is crucial for controlling surface wettability. The micro/nanoscale hierarchical structures have a great influence on the wettability property, i.e., the micro/nanoscale structures can enhance both the hydrophobic and the oleophylic properties of the meshes. It is cheap and facile to fabricate nanostructures on stainless steel mesh by immersion process, but this method always needs to accurately control the processing time and solution concentration, which play important roles in
crystal growth. Since the
micro/nanoscale hierarchical structures have a great influence on the wettability property. Thus, we explored the effect of different processing times on wettability. The samples were prepared under 2, 5, 10, 15, 20 and 30 s of processing time. Fig. 5 shows the relationship between processing times and static contact angles on the as-prepared surface. The contact angle reached 130 ± 3° after 2 s of reaction time. With the time was extended to 15 s, the contact angle reached 153 ± 3° and sliding angle reached 10 ± 2°. Thus prepared samples have excellent hydrophobic properties and low adhesion properties. The combination of Fig. 1 and Fig. 5, we
found that there is a relationship between the processing time, roughness and contact angle. The longer treatment time, the sample surface is more rough, so the samples have better hydrophobicity. Combine the SEM and test results of wettability property, we generally estimates that the ideal processing time is 15s. 3.3.3 Low adhesion behavior For a further wettability evaluation, a as-prepared mesh with superhydrophobic surface was put on the testing platform and moved up and down slowly, making 5 μL droplet contact with the as-prepared mesh surface and then extrude, separate just as shown in Fig. 6(a-d). It can be seen that the water droplet can almost completely detach from the substrate even upon severe deformation. It is found that the droplet kept intact throughout the whole steps and low adhesion to the prepared surface was observed. Such an excellent superhydrophobicity should be closely related to the surface morphology and the special chemical composition of the as-prepared nanostructure. Toluene was absorbed immediately upon contact with the as-prepared mesh(Fig. 6e-f). The as-prepared stainless steel mesh exhibited superoleophilicity. We drop an water droplet on the as-prepared mesh surface, the water droplet does not spread through the fabricated mesh but rebounds several times before rolling off from the surface. This further shows that the samples have a low adhesion to water droplets. (video S1 in the Supporting Information shows the process).
3.4. Chemical Stability. The sample will be affected by many factors in the process of use, thereby reducing the service life. First of all, we explored the relationship between the static contact angle and the exposure time in the air. We exposed the samples directly to the air and measured the static contact angle of the samples at different time intervals. As shown in Fig. 7a, after the exposure time of seven weeks, the sample can keep a contact angle of 140 ± 3°. After seven weeks of exposure to the air, the samples were washed with alcohol and dried, The static contact angle of the sample can reach 148 ± 4°. Dust from the air may affect the wettability of the sample surface. Although the static contact angle has a slow downward trendand in the whole testing process, it has keep
a good hydrophobicity. Taking into account the oil-water separation device may be used in marine oil spill and ocean water is corrosive to the sample. To further illustrate the stability of samples prepared under corrosive environment, the sample was estimated by measuring the static WCA values for samples that were immersed in aqueous solutions of 3.5%w NaCl for different time. Fig. 7b shows the relationship between the soaking time and static contact angle. The measured static contact angle ranged from 143 ± 4° to 153 ± 3°. Due to the existence of hydrophobic structure, the contact between the corrosion solution and the sample surface can be prevented, so as to protect the surface is not damaged. The results indicate that the asprepared surface has good chemical stability in aqueous solutions of salts.
4. Separation of oil and water As
mentioned
above
the
as-prepared
meshes
showed
simultaneous
superhydrophobicity and superoleophilicity, which made it very promising as the material for oil-water separation. Compared with water, the oil has priority in penetrating the as-prepared mesh because of the capillarity effect and van der Waals attraction, which is the driving force for the penetration of the liquid into the prepared mesh. But the water droplet is not wetted and easily rolls off from the mesh due to the trapping of air in the rough structure of the mesh. To evaluate the possibility of a functional mesh to separate oil from water. we design the simple oil/water separation setup shown in Fig. 8 The oil-water interface is easily observed when the oil is dyed with Sudan III(A stain). When the mixture of hexane (dyed with Sudan III) and water was piped onto the as-prepared mesh, the hexane passed through the mesh into the beaker which used to collect the oil, directly and quickly. Water is prevented from wetting the mesh due to the mesh’s property of superhydrophobicity. But when too much water to store on the mesh, water will overflow into another beaker from the edge of the mesh. Therefore, the process of oil-water separation (see movie S2 in the Supporting Information)was realized. No
evidence indicated that there was oil residue on the water surface after collection. The separation was achieved driven by gravity and this device can achieve continuous separation. Considering the growing number of oil-spill accidents and the variable composition of oils, the collection device should be able to collect multiple types of oil from the oil-water mixture. Oil-water separation efficiency was used to quantitatively describe the oil-water separation ability of as-prepared mesh. This system was used for our oil–water separation tests as shown in Table 1. For every separation, a total 50 g oil/water mixture (25 g of water and 25 g of oil) was slowly delivered to the as-prepared mesh through pipe line and separated by the as-prepared mesh. The experiments were conducted in the continuous absorption-and-pumping manner described above. The oil separation efficiency, R (%), was then calculated based on the following equation: R(%)
herein,
GO
GP 100 GO
(25g) and G P are the oil weight in the pristine oil-water mixture and the
collected oil weight after separation. In order to give a more detailed description of the experimental results, we put the data in the Table 1. The oil–water separation data was collected from the 1 to 10 separation. According to Table 1 and Fig. 9 of the data analysis, for each type of organic solvent, the collection efficiency was over 93%, and even over 96% for some of them, as shown in Fig. 9a. The deficit in the collected amount of oil compared with the original amount was mostly absorbed by the device itself, and a small amount was left inside the beaker and pipeline. With the increase of the number of experiment, the separation efficiency of the samples decreased, but the separation efficiency was mostly higher than 93%. The collection efficiency of the device was very high and could be enhanced further if a larger amount of oil was to be collected. In addition, A small portion of the oil will be washed away by water, so there would be a small amount of red liquid on the right side of the beaker as shown in Fig. 8d. Therefore, we extended
the collection experiment to several other organic solvents, including toluene, gasoline, diesel and chloroform. It is note that, compared with No.1 experiment, the separation efficiency of the No.2 experiment had a leap stage, due to the mesh will absorb a portion of the oil in the No.1 experiment. As shown in Fig. 9b, different types of oils such as gasoline, diesel, hexane, toluene, and hexane can be separated by the above apparatus at an efficiency of more than 93%. The stability of the as-prepared mesh differs slightly even after 10 cycles, indicating its general suitability for various oil-water separation. For different types of oil, there are slight differences of separation efficiency, this may be due to the density of the oil itself and the adhesion properties of the sample. In order to further verify the experimental results, we will prove there is no water in the oil that is collected in the next test. After 10 cycles of steel mesh, we did the experiment again. But the mixture solution is no longer the oil and water mixture but the oil and 5% NaCl aqueous solution mixture. As shown in Fig. 10, on the left side of the beaker contained the collected oil. When the AgNO3 solution is poured into the the left side of the beaker. And then observe whether there is a white precipitate (Fig. 10b and Movie S3 of the Supporting Information). Once there is a small amount of NaCl aqueous solution penetrating the textile together with the oil, NaCl will react with AgNO3 to generate a white precipitate. No evidence indicated that there was NaCl aqueous on the oil collection vessel after collection, and indirect description that there is no water exist in the oil collection beaker. After 10 separate experiments, the sample was rinsed with sufficient ethanol to remove the surface of the oil, and then it was dried under ambient enviroment for 5 min. Next, we measured the WCA and OCA of the after 10 use of the as-prepared mesh. Fig. 10c shows that the WCA of the 10-used mesh tended to 145 ± 1°, The oil contact-angle of 10-used mesh about 0° (Fig. 10d), which indicates that the organic solvent will not damage the structure and chemical composition of the hydrophobic surface. The stearic acid and the rough surface are well integrated together; therefore, the stearic acid does not dissolve into the oil phase.
5. Conclusions In conclusion, we have achieved a functionally integrated device for oil-water separation through a facile immersion and modification process. Superhydrophobic and superoleophilic meshes were fabricated by an immersion process after modified by stearic acid. The as-prepared stainless steel mesh surface exhibits both superhydrophobicity with a WCA of 153 ± 3° possessing simultaneous a very low adhesion and superoleophilicity with an OCA of 0°. Besides, a series of oil/water mixtures, such as chloroform/water, hexane/water, toluene/water,gasoline/water and diesel/water, were observed to be separated by the mesh, the separation efficiency of more than 93%. And it remained an excellent efficiency even after 10 cycles. Furthermore, the as-prepared superhydrophobic mesh shows chemical stability in the air environment and corrosion environment. Thus, as-prepared functionalized meshes are expected to become sustainable and highly effective materials for oil-water separation, as the materials can be recycled many times with remarkable durability. We believe such selectively separating materials can be used in a lot of applications, such as waste water treatment. ACKNOWLEDGEMENTS The authors thank the National Natural Science Foundation of China (Nos. 51275555, 51475200 and 51325501) and Science and Technology Development Project of Jilin Province (No.20150519007JH).
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Figure Caption
Fig. 1 Surface morphology of the stainless steel mesh at different reaction time in an aqueous solution of 1.5 M CuCl2 and 0.5 M HCl. SEM images of newly prepared stainless steel samples after immersion times of (a)0 s, (b)2 s, (c)5 s, (d)7 s, (e)15s and (f) 30s. The insets show magnified images of corresponding surfaces.
Fig. 2 XPS spectrum of the stainless steel mesh surface before (black) and after (red) modified by stearic acid after immersion treatment of (a) full-spectrum and (b) Cu2p.
Fig. 3 FT-IR spectra of the stearic acid and as-prepared stainless steel mesh surfaces modified by stearic acid.
Fig. 4 Surface wettability of the as-prepared and original mesh: (a) the photograph of water droplet on the surface of as-prepared and original meshes respectively; (b) the CA of water droplet on the surface of as-prepared mesh; (c) the CA of water droplet on the surface of original stainless steel mesh.
Fig. 5 Contact angles of the different processing time on simple surfaces for 2 s, 5 s, 10 s, 15 s, 20 s, 30 s after modifying by stearic acid.
Fig. 6 The process of contact, deformation, and separate processes between 5μL water droplet and substrate (a-d). The process of contact and permeate processes between 5 μL toluene droplet and substrate (e, f). The arrows represent moving direction of the substrate.
Fig. 7 (a)Relationship between the exposure time and the static contact angle on the as-prepared copper mesh surface, (b) relationship between the soaking time and the static contact angle on the as-prepared copper mesh surface.
Fig. 8 (a) Five kinds of oil and NaCl solution for oil water separation, (b) top view of the reaction device, (c) before oil-water separation, (d) Separation process.
Fig. 9. (a) Separating efficiency of different types of oil and (b) recycled experiments for separating oil from water by using the above mentioned device.
Fig. 10. (a) Oil collected from above mentioned device and AgNO3 solution, (b) oil and AgNO3 solution mixture, (c) the WCA of after 10 times of recycling, (d) the OCA of after 10 times of recycling.
Table 1 Oil water separating tests of the as-prepared mesh. Here we chose same mass compositions of oil–water mixtures.
Round
1 2 3 4 5 6 7 8 9 10
Diesel
Gasoline
Chloroform
Hexane
Toluene
GP
R(%)
GP
R(%)
GP
R(%)
GP
R(%)
GP
R(%)
23.3 24.0 23.95 23.9 23.8 23.8 23.76 23.7 23.6 23.58
93.2 96.1 95.8 95.6 95.2 95.2 95.1 94.8 94.5 94.3
23.5 24.1 24.0 24.0 24.13 23.88 23.95 24.0 23.88 23.75
94 96.5 96.1 96.1 96.5 95.5 95.8 96.0 95.5 95.0
22.80 23.63 23.38 23.53 23.38 23.30 23.28 23.28 23.18 23.15
91.2 94.5 93.5 94.1 93.5 93.2 93.1 93.1 92.7 92.6
23.75 24.45 24.38 24.45 24.25 24.20 24.23 24.28 24.13 24.08
95.0 97.8 97.5 97.8 97.0 96.8 96.9 97.1 96.5 96.3
23.13 23.88 23.83 23.80 23.78 23.70 23.68 23.70 23.65 23.63
92.5 95.5 95.3 95.2 95.1 94.8 94.7 94.9 94.6 94.5
S0927-7757(16)30236-9 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.04.011 COLSUA 20568
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
1-1-2016 27-3-2016 3-4-2016
Please cite this article as: Yan Liu, Kaiteng Zhang, Wenguang Yao, Jiaan Liu, Zhiwu Han, Luquan Ren, Bioinspired structured superhydrophobic and superoleophilic stainless steel mesh for efficient oil-water separation, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.04.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bioinspired structured superhydrophobic and superoleophilic stainless steel mesh for efficient oil-water separation Yan Liua, b, Kaiteng Zhanga, Wenguang Yaoa, Jiaan Liuc, Zhiwu Hana, Luquan Rena a.
Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun 130022, P.R. China; b. State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130022, China; c. Key Laboratory of Automobile Materials(Ministry of Education) and College of Materials Science and Engineering, Jilin University, Changchun, 130022, P. R. China.
Graphical abstract
Corresponding author. Tel.:+86 431 85095760; fax:+86 431 85095575 E-mail address:[email protected]
Highlights We have prepared a superhydrophobic and oleophilic stainless steel mesh by a facile immersion process. The superhydrophobic surface is described as low adhesion force with water. The wettability mechanism relies on micro-nano scale hierarchical structure and chemical component on surface. The as-prepared stainless steel mesh has an excellent oil-water separation property after being recycled ten times.
Abstract Oily wastewater have caused serious water pollution. The development of effective and cheap oil/water separation materials is urgent for treating this problem. Herein, inspired by superhydrophobic typical plant leaves such as lotus, red rose and marigold, superhydrophobic and superoleophilic stainless steel meshes with excellent oil-water separation efficiency were successfully fabricated by a facile immersion process and then surface modification with stearic acid. The simple immersion process, in which surface roughness and low-surface-energy coating are the two dominant factors of superhydrophobicity, could be accomplished sequentially. It was found that the as-prepared surface is both superhydrophobic and superoleophilic, with the static water angle (WCA) as high as 153 ± 3° and oil contact angle (OCA) of 0°. Furthermore, as-prepared meshes can be applied to separate an oil-and-water mixture bidirectionally and exhibited excellent oil-water separation efficiency including
petroleum, toluene, hexane, gasoline and diesel, even after being recycled ten times. Compared with previously reported strategies, this strategy is much easier to carry out and, in the meantime, allows the collection of oil in a continuous manner. This study provides a simple, fast, low cost and environmentally friendly route to fabricate oil-water separation materials as far as large scale manufacturing is considered, which has prospective application in industrial fields such as water treatment and petroleum refining. Key words: bioinspired; superhydrophobic; low adhesion; oil-water separation.
1. Introduction Due to the deteriorating condition of the oil pollution and increasing attention to the preservation of the environment, there is a increasing and urgent need to develop novel materials which can purify the polluted water effectively and quickly.[1-2] To resolve these water pollution issues, many approaches have been used for the treatment of oil-water pollution, including filtration, oil skimmers, centrifugal machine, magnetic separations, flotation technologies, oil-absorbing materials and combustion.[3-9] However, most of the methods involve harsh conditions, such as expensive equipment, complex device, complicated processing steps, high processing cost, long processing time and so on. Taking all factors into account, separation treatment stands out from the rest as a simple, universal, scalable approach for valid removal of oil from water. Many plants and insects exhibit excellent superhydrophobicity, such as lotus leaves, rose petals, marigold petals, water striders, butterfly wings, rice leaves, mosquito eyes and so on.[10-14] Inspired from nature, superhydrophobic surfaces are fabricated by simulating typical structures of plant surface and considering chemical composition simultaneously. Superhydrophobic surface can be widely used in fields such as fluidic drag
reduction[15-17],
self-cleaning
surface[18-19],
anti-fogging[22], battery and fuel cell applications[23-24].
corrosion
inhibition[20-21],
Designing and fabricating coatings with special wettability would be a facile and effective way to solve the problem of oil-water separation. A large number of oil/water separation materials have been explored to fabricate superhydrophobic surfaces. These include template[25] electrochemical deposition[26-27] chemical vapour deposition (CVD)[29-31], electro-spinning[32-33], sol–gel method[34-35], and many others[36-38]. In addition to fixed surface wetting properties, materials with switchable surface wettability have also been developed for controllable oil/water separation caused by pH value[39],electric field[40], thermo-treatment[41], or multiple stimuli[42]. Although it can achieve the control of the freedom of the wettability, but most of the production methods are complex and high cost. In most of the separation process, the wettability of the fixed material can meet the application requirements. So the most researchers focus on the study of fixed wettability. Zhang et al.[43] reported a method to fabricate a superhydrophobic and superoleophilic membrane by immersing porous polyurethane film into polystyrene colloidal solution. Wang et al.[44] fabricated such a film with hierarchical structure on copper mesh by a electrodeposition process. Inspired by selective wettability and hierarchical structure of papillae on lotus seeds, Xu et al.[45] fabricated papilla-like magnetic particles by thermal treatment of Fe microparticles and then modified by lauric acid. It exhibited superhydrophobicity, superoleophilicity and great oil removing capability from water. The above-mentioned materials can achieve high efficient oil-water separation. At present, a large amount of preparing methods are usually time-consuming and cost-ineffective and rely on specific equipment as well as chemicals including strong acid, strong alkali, and even toxic reagents, which greatly hinders their practical applications. This is why it is imperative to pursue facile, inexpensive, and environmentally friendly methods for fabricating functional interface materials. Stainless steel mesh is widely used for the separation of oil and water due to its porous structure constructed by microfibers and the price is cheap. Based on the above mentioned discussions, we reported a simple,one step processing method high efficient and low-cost route to prepare superhydrophobic stainless steel meshes. In this research, stainless steel meshes with superhydrophobic and superoleophilic
surface were fabricated by a simple immersion process and then surface modification with stearic acid. The whole process takes only 1 h under ambient condition. The immersion process takes place at atmospheric pressure, using a solution of 1.5 M copper chloride and 0.5 M hydrochloric acid that were cured in ambient circumstance. The superhydrophobic stainless steel meshes can be easily combined to an independent device which acts as a filter, forming a barrier that allows oils to pass through while repelling water.
2. Experimental methods 2.1. Materials The stainless steel mesh which were obtained from Changzhou Tengda Hardware Store. The mesh had pore diameters of 100 μm. Stearic acid (C18H36O2) was purchased from Tianjin Guangfu Fine Chemical Research Reagent Institute, copper chloride(CuCl2) was obtained from Tianjin Guangfu Technology Development Co., Ltd. Sudan III as a coloring agent (C22H12N4O) was purchased from Sinopharm Chemical Reagent Co., Ltd. Diesel and gasoline were products of SINOPEC. Silver nitrate(AgNO3) was purchased from Shanghai Chemical Reagent Co., LTD. All other chemicals were purchased from Beijing Chemical Works. This includes hydrochloric acid, anhydrous ethanol, chloroform, toluene, hexane, acetone, sodium chloride. All chemicals were used as received without further purification.
2.2. Sample preparation Stainless steel meshes were ultrasonically rinsed for 5 min in the solution of acetone, anhydrous ethanol and distilled water, respectively. After drying, the meshes were immersed in an aqueous solution of 1.5 M CuCl2 and 0.5 M HCl at room temperature for different immersion time(2, 5, 7, 10, 15, 20 and 30 s). After their timed reaction, the meshes were removed from the solution and rinsed again using distilled water, then drying in the ventilation heating furnace at 70 °C for 30 min. The prepared stainless steel meshes were immersed in a 0.1 M ethanol solution of stearic acid and keep it for 30 min at ambient condition. After modified by stearic acid, the sample was rinsed by sufficient anhydrous alcohol and then were dried in a vacuum
oven at 60 °C for 15 min. The after modified meshes showed hydrophobic or superhydrophobic properties.
2.3 Sample characterization The water/oil contact angles (CAs) were measured by contact angle meter (OCA 20 dataphysics, Germany) at room temperature so as to characterize the surface wettability. Water droplets (5 μL) were carefully dropped onto the surfaces, and the average static contact angle value was obtained by measuring five different positions of each surface. The surface morphologies of superamphiphobic samples were examined by a field emission scanning electron microscope (SEM, EVO 18, ZEISS). The chemical composition of the samples was characterized using an X-ray photoelectron spectroscopy (XPS, SPECS XR50, Japan) and Fourier transform infrared spectrophotometer (FT-IR, JACSCO, Japan).
2.4 Oil-water separation apparatus The device consists of peristaltic pump, retort stand. As shown in Fig. 8 the as-prepared stainless steel mesh was fixed on iron tricyclic by three binder clips. In order to guide the solution into the right side of the beaker smoothly, the prepared samples were folded into a pot mouth. There are two beakers under the mesh for collecting separated solution. On the left side of the beaker was used to collect oil, right beaker was used to collect water solution. In order to prepare oil water mixture, the 25 g of hexane (dyed with oil red) was poured into 25 g of water contained in a beaker, the hexane floated on the water, due to its lower density. The mixed solution is obtained through the peristaltic pump to the as-prepared mesh automatically The separation was achieved driven by gravity and this device can achieve continuous separation. Meantime, in order to distinguish between oil and water, oil was colored by Sudan III.
3. Results and Discussion 3.1 Surface morphology In order to investigate whether the reaction process provided enough surface
roughness, we characterized the surface morphology of the stainless steel mesh after immersion with scanning electron microscopy (SEM). Fig. 1a shows the surface morphology of the original stainless steel mesh, the mesh is composed of stainless steel wires with a diameter of 100 μm, and the stainless steel wires are interwined with each other to form a micrometer-sized rough surface. And Fig. 1b-d show the surface morphology of the as-prepared stainless steel mesh. It can be observed that not only the micro-scale stainless steel wires and pores, but also numerous as-coated Cu crystal existed on the stainless steel wires (Fig. 1b-f), forming the typically hierarchical structure. The stainless steel wires are uniformly covered by Cu crystal. The morphology of the stainless steel mesh was greatly influenced by the reaction time. The surface roughness of the meshes was changed obviously after immersing the solution of 1.5 M CuCl2 and 0.5 M HCl for a certain period of time. At a short response time, the effects of reaction is not obvious, hence the Cu particles are tiny and present relatively smooth surface, as shown in Fig. 1b. As shown in Fig. 1c, with the reaction time increasing to 5 s, the Cu crystal on the surface of the stainless steel mesh were grown up, and the number of the Cu crystal was increased. Longer immersion times encouraged formation of longer surface features that in some locations resembled needle-shaped structures. When the immersion time increased to 7 s, the needle-shaped surface structures were even more obvious and rough, with many being entangled with each other, as shown in Fig. 1d. Fig. 1e shows the SEM image of the stainless steel mesh after 15 s processing time, needle-shaped structures with sizes of several micrometers were uniformly dispersed on the surface of the wires. It can be seen that the original mesh wires has been completely covered by needle-shaped structures. There are a large amount of micro-nano scale needle-shaped laminae which can capture a large amount of air. Therefore, it provides the geometric condition for the formation of superhydrophobicity. But with the reaction time increasing to 30 s, although there is a deposition of Cu crystal, due to the stainless steel wires are severely etched, it will cause some loss of crystallization deposition. These findings show that an increase of immersion time leads to a change in morphology, resulting in the transition of wetting state from hydrophobicity to
superhydrophobicity. The microsized needle-shaped laminae in combination with the nanoscale granular protuberance endowed a hierarchical composite structure to the mesh that possessed superhydrophobic behaviors. Such micro-nano scale hierarchical structure can help support water droplets and is an essential necessity to obtain the superhydrophobic surfaces. From the surface morphology of the stainless steel meshes, we deduce that the ideal processing time is 15 s.
3.2 Chemical characterization XPS is used to test the chemical composition of the thin film of the stainless steel mesh surfaces modified after immersion processing. Fig. 2a shows the XPS spectrum of the as-prepared surface before and after modification with stearic acid and it reveals the presence of C, O, and Cu on the as-prepared surfaces. The elements of C, O, and Cu could all be found on both untreated and treated by stearic acid steel mesh surfaces. The element of the Cu is derived from a immersion process reaction between the stainless steel mesh and the CuCl2 solution. The elemental oxygen of the without modified sample is attributed to the oxidation of the mesh drying process in the ventilation heating furnace at 70 °C. Fig. 2b presents the Cu2p XPS spectrum of the as-prepared surface before modification with stearic acid. The Cu2p3/2 and Cu2p1/2 peaks centered at binding energy of 932.8 and 952.3 eV can be assigned to Cu0 and CuO species, respectively. But compared to the carbon content of the before modified sample, the sample after modified by stearic acid carbon content increased significantly. In the whole process, the carbon source mainly comes from the stearic acid. So it can be concluded that stearic acid has been combined with the mesh. But the combination mechanism is a simple adhesion to together or chemical bonding together need further confirmation. So that the stearic acid treatments could not be determined merely by surface element investigation. Thus, in order to confirm the combination mechanism of the stearic acid on the coated surface of the stainless steel mesh, FT-IR spectra was used. Fig. 3 shows FT-IR spectra of the stearic acid and as-prepared mesh surfaces modified by stearic acid. In
the stearic acid,the free stretching vibrations of the hydroxyl groups disappears at 1701 cm-1 and the new group appears at 1586 cm-1. This is due to the stretching vibration of C=O bond influenced by hydrogen bond, Cu2+ and COO- forming a new chemical structure (copper stearate), which leads to the absorption peak of C=O bond in the carboxyl group moving to the low wave position. However, the absorption peaks corresponding to the -CH3 and -CH2 are 2917 cm-1 and 2849 cm-1 respectively. The absorption peaks corresponding to -CH3 and -CH2 are 2850 cm-1 and 2956 cm-1 respectively. The corresponding absorption peak of these two functional groups have no obvious change, so the as-prepared coating contains functional groups of -CH3 and -CH2. It is known that the CH3 group with a surface energy of 24 mJ/m2 and the CH2 group with a surface energy of 31 mJ/m2, which were widely used to reduce the free energy. The FT-IR spectra indicated that the stearic acid was bound to the stainless steel mesh surfaces. The combination of the hierarchical structure and the low surface energy molecules made the mesh superhydrophobic and superoleophilic. Thus, the low-surface-energy coatings on the stainless steel mesh together with its hierarchical rough surface, microscale skeleton, and nanoscale structures endowed the substrate with superhydrophobic and superoleophlic properties.
3.3 Surface wettability 3.3.1 Formation of superhydrophobicity A combination of SEM, XPS and FT-IR characterizations show that the micro-nano scale hierarchical structures were created and that low surface energy materials were successfully prepared on the surface of stainless steel mesh through this method. It was expected that both the micro-nano scale hierarchical composite structure and chemical component made the as-prepared mesh possess superhydrophobicity, and the surface wettability was evaluated by contact angle measurement. Fig. 4a shows an image of a water droplet (5 μl) on the as-prepared stainless steel mesh and original mesh respectively. For the as-prepared mesh, it can be seen that the droplet can rest on the surface in a sphere shape and can roll off easily with slight tilt or shake, and have low adhesion to the mesh. Because of the increase of the surface
roughness, air can be trapped into the microstructures as a solid-liquid barrier. The air cushion decreases the contact area between the droplet and surface and thus the liquid droplets can roll off easily. But for the original mesh, the water droplets present a hemispherical shape and had high adhesion to the mesh. Stainless steel mesh after processing time of 15 s and then modified by stearic acid only tended to 153 ± 3°, indicating that the surface has very good superhydrophobicity as shown in Fig. 4b. Compared with original mesh, the resulting meshes turned red from their original silvery white color and showed hydrophilic properties as shown in Fig. 4a. Fig. 4c shows that the WCA (water contact angle) of the original stainless steel mesh tended to 84 ± 2°. In contrast, a drop of oil (hexane) on the as-prepared and original meshes quickly spread and diffused into its exhibiting highly oleophilic property. With the combination of hydrophobicity and oleophilicity in the substrate, this mesh may be used for oil/water separation. 3.3.2 Effect of the processing time on wettability A higher resolution SEM depicted in Figure 1d indicates that this mesh has both micro and nanoscale surface roughness and this surface roughness is crucial for controlling surface wettability. The micro/nanoscale hierarchical structures have a great influence on the wettability property, i.e., the micro/nanoscale structures can enhance both the hydrophobic and the oleophylic properties of the meshes. It is cheap and facile to fabricate nanostructures on stainless steel mesh by immersion process, but this method always needs to accurately control the processing time and solution concentration, which play important roles in
crystal growth. Since the
micro/nanoscale hierarchical structures have a great influence on the wettability property. Thus, we explored the effect of different processing times on wettability. The samples were prepared under 2, 5, 10, 15, 20 and 30 s of processing time. Fig. 5 shows the relationship between processing times and static contact angles on the as-prepared surface. The contact angle reached 130 ± 3° after 2 s of reaction time. With the time was extended to 15 s, the contact angle reached 153 ± 3° and sliding angle reached 10 ± 2°. Thus prepared samples have excellent hydrophobic properties and low adhesion properties. The combination of Fig. 1 and Fig. 5, we
found that there is a relationship between the processing time, roughness and contact angle. The longer treatment time, the sample surface is more rough, so the samples have better hydrophobicity. Combine the SEM and test results of wettability property, we generally estimates that the ideal processing time is 15s. 3.3.3 Low adhesion behavior For a further wettability evaluation, a as-prepared mesh with superhydrophobic surface was put on the testing platform and moved up and down slowly, making 5 μL droplet contact with the as-prepared mesh surface and then extrude, separate just as shown in Fig. 6(a-d). It can be seen that the water droplet can almost completely detach from the substrate even upon severe deformation. It is found that the droplet kept intact throughout the whole steps and low adhesion to the prepared surface was observed. Such an excellent superhydrophobicity should be closely related to the surface morphology and the special chemical composition of the as-prepared nanostructure. Toluene was absorbed immediately upon contact with the as-prepared mesh(Fig. 6e-f). The as-prepared stainless steel mesh exhibited superoleophilicity. We drop an water droplet on the as-prepared mesh surface, the water droplet does not spread through the fabricated mesh but rebounds several times before rolling off from the surface. This further shows that the samples have a low adhesion to water droplets. (video S1 in the Supporting Information shows the process).
3.4. Chemical Stability. The sample will be affected by many factors in the process of use, thereby reducing the service life. First of all, we explored the relationship between the static contact angle and the exposure time in the air. We exposed the samples directly to the air and measured the static contact angle of the samples at different time intervals. As shown in Fig. 7a, after the exposure time of seven weeks, the sample can keep a contact angle of 140 ± 3°. After seven weeks of exposure to the air, the samples were washed with alcohol and dried, The static contact angle of the sample can reach 148 ± 4°. Dust from the air may affect the wettability of the sample surface. Although the static contact angle has a slow downward trendand in the whole testing process, it has keep
a good hydrophobicity. Taking into account the oil-water separation device may be used in marine oil spill and ocean water is corrosive to the sample. To further illustrate the stability of samples prepared under corrosive environment, the sample was estimated by measuring the static WCA values for samples that were immersed in aqueous solutions of 3.5%w NaCl for different time. Fig. 7b shows the relationship between the soaking time and static contact angle. The measured static contact angle ranged from 143 ± 4° to 153 ± 3°. Due to the existence of hydrophobic structure, the contact between the corrosion solution and the sample surface can be prevented, so as to protect the surface is not damaged. The results indicate that the asprepared surface has good chemical stability in aqueous solutions of salts.
4. Separation of oil and water As
mentioned
above
the
as-prepared
meshes
showed
simultaneous
superhydrophobicity and superoleophilicity, which made it very promising as the material for oil-water separation. Compared with water, the oil has priority in penetrating the as-prepared mesh because of the capillarity effect and van der Waals attraction, which is the driving force for the penetration of the liquid into the prepared mesh. But the water droplet is not wetted and easily rolls off from the mesh due to the trapping of air in the rough structure of the mesh. To evaluate the possibility of a functional mesh to separate oil from water. we design the simple oil/water separation setup shown in Fig. 8 The oil-water interface is easily observed when the oil is dyed with Sudan III(A stain). When the mixture of hexane (dyed with Sudan III) and water was piped onto the as-prepared mesh, the hexane passed through the mesh into the beaker which used to collect the oil, directly and quickly. Water is prevented from wetting the mesh due to the mesh’s property of superhydrophobicity. But when too much water to store on the mesh, water will overflow into another beaker from the edge of the mesh. Therefore, the process of oil-water separation (see movie S2 in the Supporting Information)was realized. No
evidence indicated that there was oil residue on the water surface after collection. The separation was achieved driven by gravity and this device can achieve continuous separation. Considering the growing number of oil-spill accidents and the variable composition of oils, the collection device should be able to collect multiple types of oil from the oil-water mixture. Oil-water separation efficiency was used to quantitatively describe the oil-water separation ability of as-prepared mesh. This system was used for our oil–water separation tests as shown in Table 1. For every separation, a total 50 g oil/water mixture (25 g of water and 25 g of oil) was slowly delivered to the as-prepared mesh through pipe line and separated by the as-prepared mesh. The experiments were conducted in the continuous absorption-and-pumping manner described above. The oil separation efficiency, R (%), was then calculated based on the following equation: R(%)
herein,
GO
GP 100 GO
(25g) and G P are the oil weight in the pristine oil-water mixture and the
collected oil weight after separation. In order to give a more detailed description of the experimental results, we put the data in the Table 1. The oil–water separation data was collected from the 1 to 10 separation. According to Table 1 and Fig. 9 of the data analysis, for each type of organic solvent, the collection efficiency was over 93%, and even over 96% for some of them, as shown in Fig. 9a. The deficit in the collected amount of oil compared with the original amount was mostly absorbed by the device itself, and a small amount was left inside the beaker and pipeline. With the increase of the number of experiment, the separation efficiency of the samples decreased, but the separation efficiency was mostly higher than 93%. The collection efficiency of the device was very high and could be enhanced further if a larger amount of oil was to be collected. In addition, A small portion of the oil will be washed away by water, so there would be a small amount of red liquid on the right side of the beaker as shown in Fig. 8d. Therefore, we extended
the collection experiment to several other organic solvents, including toluene, gasoline, diesel and chloroform. It is note that, compared with No.1 experiment, the separation efficiency of the No.2 experiment had a leap stage, due to the mesh will absorb a portion of the oil in the No.1 experiment. As shown in Fig. 9b, different types of oils such as gasoline, diesel, hexane, toluene, and hexane can be separated by the above apparatus at an efficiency of more than 93%. The stability of the as-prepared mesh differs slightly even after 10 cycles, indicating its general suitability for various oil-water separation. For different types of oil, there are slight differences of separation efficiency, this may be due to the density of the oil itself and the adhesion properties of the sample. In order to further verify the experimental results, we will prove there is no water in the oil that is collected in the next test. After 10 cycles of steel mesh, we did the experiment again. But the mixture solution is no longer the oil and water mixture but the oil and 5% NaCl aqueous solution mixture. As shown in Fig. 10, on the left side of the beaker contained the collected oil. When the AgNO3 solution is poured into the the left side of the beaker. And then observe whether there is a white precipitate (Fig. 10b and Movie S3 of the Supporting Information). Once there is a small amount of NaCl aqueous solution penetrating the textile together with the oil, NaCl will react with AgNO3 to generate a white precipitate. No evidence indicated that there was NaCl aqueous on the oil collection vessel after collection, and indirect description that there is no water exist in the oil collection beaker. After 10 separate experiments, the sample was rinsed with sufficient ethanol to remove the surface of the oil, and then it was dried under ambient enviroment for 5 min. Next, we measured the WCA and OCA of the after 10 use of the as-prepared mesh. Fig. 10c shows that the WCA of the 10-used mesh tended to 145 ± 1°, The oil contact-angle of 10-used mesh about 0° (Fig. 10d), which indicates that the organic solvent will not damage the structure and chemical composition of the hydrophobic surface. The stearic acid and the rough surface are well integrated together; therefore, the stearic acid does not dissolve into the oil phase.
5. Conclusions In conclusion, we have achieved a functionally integrated device for oil-water separation through a facile immersion and modification process. Superhydrophobic and superoleophilic meshes were fabricated by an immersion process after modified by stearic acid. The as-prepared stainless steel mesh surface exhibits both superhydrophobicity with a WCA of 153 ± 3° possessing simultaneous a very low adhesion and superoleophilicity with an OCA of 0°. Besides, a series of oil/water mixtures, such as chloroform/water, hexane/water, toluene/water,gasoline/water and diesel/water, were observed to be separated by the mesh, the separation efficiency of more than 93%. And it remained an excellent efficiency even after 10 cycles. Furthermore, the as-prepared superhydrophobic mesh shows chemical stability in the air environment and corrosion environment. Thus, as-prepared functionalized meshes are expected to become sustainable and highly effective materials for oil-water separation, as the materials can be recycled many times with remarkable durability. We believe such selectively separating materials can be used in a lot of applications, such as waste water treatment. ACKNOWLEDGEMENTS The authors thank the National Natural Science Foundation of China (Nos. 51275555, 51475200 and 51325501) and Science and Technology Development Project of Jilin Province (No.20150519007JH).
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Figure Caption
Fig. 1 Surface morphology of the stainless steel mesh at different reaction time in an aqueous solution of 1.5 M CuCl2 and 0.5 M HCl. SEM images of newly prepared stainless steel samples after immersion times of (a)0 s, (b)2 s, (c)5 s, (d)7 s, (e)15s and (f) 30s. The insets show magnified images of corresponding surfaces.
Fig. 2 XPS spectrum of the stainless steel mesh surface before (black) and after (red) modified by stearic acid after immersion treatment of (a) full-spectrum and (b) Cu2p.
Fig. 3 FT-IR spectra of the stearic acid and as-prepared stainless steel mesh surfaces modified by stearic acid.
Fig. 4 Surface wettability of the as-prepared and original mesh: (a) the photograph of water droplet on the surface of as-prepared and original meshes respectively; (b) the CA of water droplet on the surface of as-prepared mesh; (c) the CA of water droplet on the surface of original stainless steel mesh.
Fig. 5 Contact angles of the different processing time on simple surfaces for 2 s, 5 s, 10 s, 15 s, 20 s, 30 s after modifying by stearic acid.
Fig. 6 The process of contact, deformation, and separate processes between 5μL water droplet and substrate (a-d). The process of contact and permeate processes between 5 μL toluene droplet and substrate (e, f). The arrows represent moving direction of the substrate.
Fig. 7 (a)Relationship between the exposure time and the static contact angle on the as-prepared copper mesh surface, (b) relationship between the soaking time and the static contact angle on the as-prepared copper mesh surface.
Fig. 8 (a) Five kinds of oil and NaCl solution for oil water separation, (b) top view of the reaction device, (c) before oil-water separation, (d) Separation process.
Fig. 9. (a) Separating efficiency of different types of oil and (b) recycled experiments for separating oil from water by using the above mentioned device.
Fig. 10. (a) Oil collected from above mentioned device and AgNO3 solution, (b) oil and AgNO3 solution mixture, (c) the WCA of after 10 times of recycling, (d) the OCA of after 10 times of recycling.
Table 1 Oil water separating tests of the as-prepared mesh. Here we chose same mass compositions of oil–water mixtures.
Round
1 2 3 4 5 6 7 8 9 10
Diesel
Gasoline
Chloroform
Hexane
Toluene
GP
R(%)
GP
R(%)
GP
R(%)
GP
R(%)
GP
R(%)
23.3 24.0 23.95 23.9 23.8 23.8 23.76 23.7 23.6 23.58
93.2 96.1 95.8 95.6 95.2 95.2 95.1 94.8 94.5 94.3
23.5 24.1 24.0 24.0 24.13 23.88 23.95 24.0 23.88 23.75
94 96.5 96.1 96.1 96.5 95.5 95.8 96.0 95.5 95.0
22.80 23.63 23.38 23.53 23.38 23.30 23.28 23.28 23.18 23.15
91.2 94.5 93.5 94.1 93.5 93.2 93.1 93.1 92.7 92.6
23.75 24.45 24.38 24.45 24.25 24.20 24.23 24.28 24.13 24.08
95.0 97.8 97.5 97.8 97.0 96.8 96.9 97.1 96.5 96.3
23.13 23.88 23.83 23.80 23.78 23.70 23.68 23.70 23.65 23.63
92.5 95.5 95.3 95.2 95.1 94.8 94.7 94.9 94.6 94.5