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. 2021 Sep 23;6(39):25323–25328. doi: 10.1021/acsomega.1c03128

Fabrication of Superhydrophobic SA-CeO2@Cu Mesh and Its Application in Oil–Water Separation

Ting Liang †,‡,§,*, Biao Wang §, Zhenzhong Fan §,*, Qingwang Liu §
PMCID: PMC8495691  PMID: 34632190

Abstract

graphic file with name ao1c03128_0010.jpg

CeO2 was synthesized by the co-precipitation method on the Cu mesh substrate and modified the surface of CeO2@Cu mesh by stearic acid (SA). The superhydrophobic behavior was ascribed to the combination of hierarchical micro–nanostructure of CeO2 and the hydrophobic alkyl groups from SA. The SA-CeO2@Cu mesh had antiacid and base stability and excellent durability as well as high separation efficiency. The separation efficiency can be up to 98.0% after separating 30 times.

Introduction

It is pretty difficult to treat the increasing emissions of industrial oily wastewater because of the pressure of environmental protection and ecological balance. Therefore, the treatment of oily wastewater is one of the urgent problems in the field of environmental engineering. The traditional methods, such as flotation method,1,2 flocculation method,3 etc., have been unable to meet the current environmental requirements, and it is imperative to adopt new, efficient, and functional oil absorption or oil–water separation materials. Superhydrophobic and superhydrophilic materials4−7 have received great attention due to their potential application in oil–water separation. Only oil can pass through these materials due to their superhydrophilicity, while water is totally repelled due to their superhydrophobicity. Superhydrophobic–superhydrophilic surface has unique advantages, such as self-cleaning effect,8,9 superhydrophobic materials have properties of antifouling,10,11 anticorrosion,12,13 anti-icing,14,15 drag reduction,16,17 etc. Due to the large difference in surface tension between the oil phase and the water phase in the oil–water mixture, superhydrophobic materials can be used as the difference to selectively remove water or oil, thereby achieving efficient oil–water separation.

Many artificial superhydrophobic membrane materials, such as metallic meshes,18,19 sponge,20,21 and ceramic materials,22,23 etc, have become the focus of research on oil–water separation materials due to their high flux, low cost, simple operation, and high processing efficiency.

As we know, proper roughness and low surface energy are the key factors to prepare superhydrophobic surfaces.24−26 The proper roughness is useful to improve the performance of filtration membranes. The relationship between surface roughness and water repellency was worked out by Cassie and Baxter27 as well as Wenzel.28 Various methods are used to fabricate superhydrophobic surfaces, such as self-assembly29,30 electrospinning,31 sol–gel methods,32 etc. Many researchers have studied the practical applications of superhydrophobic materials. However, the poor stability and reusability of superhydrophobic materials seriously hindered their practical applications. It is urgently necessary to design superhydrophobic materials that have low cost and high separation efficiency and are environmentally friendly for oil–water separation. The membrane separation method is widely used in the treatment of oily wastewater because of its simple operation, good treatment effect, no secondary pollution, and low-energy consumption.

Recently, Azimi et al. reported that rare-earth oxide (REO) surfaces showed their thermally stable hydrophobicity after exposure to 1000 °C.33 Moreover, Azimi et al. also revealed that REOs become superhydrophobic with textured morphology. Compared with organic metals, these inorganic REOs are thermally and mechanically more stable, which allows us to use them as a novel durable hydrophobic coating. Zenkin et al.34 prepared Nd2O3, La2O3, and Y2O3 by the sputtering method and proved that the hydrophobicity was related to the nonpolar component of the surfaces of REOs. CeO2 is an inexpensive and widely used rare-earth compound with N-type semiconductor properties and a unique 4f electronic structure. It exhibits excellent oxygen storage and charge exchange capacity and is widely used in three-way catalysts, photocatalysis, wastewater treatment, electronic ceramics, etc. As a surface modifier, SA has nontoxic biocompatibility, and it is not easy to change the physical and chemical properties of the powder. The surfaces of metallic meshes have been modified by low-energy materials or nanoparticles to form superhydrophobic and superoleophilic for oil–water separation. According to the different mesh shapes and densities, the function of the copper mesh is different. The main role of copper mesh is screening, filtration, protection, etc. However, as far as we know, no works have reported the SA-CeO2@Cu mesh with superhydrophobic and superoleophilic properties for oil–water separation.

Herein, we report a superhydrophobic and superoleophilic CeO2-coated Cu mesh by the co-precipitation method and calcination and then used SA as a low-energy material. In addition, the SA-CeO2@Cu mesh showed desirable stability and high oil–water separation efficiency when suffered from acid and base. The prepared SA-CeO2@Cu mesh was also characterized. The oil–water separation experiments and the recyclability of SA-CeO2@Cu mesh were also investigated.

Results and Discussion

X-ray Diffraction (XRD) Analysis

As shown in Figure 1a,b, the XRD pattern of CeO2 and SA-CeO2 were consistent with the values in the standard card (JCPDS No. 34-0394). The diffraction peaks observed at 2θ values of 27.28, 33.58, 46.46, 57.08, 59.26, 68.88, 76.48, and 78.82° can be assigned to the (111), (200), (220), (311), (222), (400), (331), and (420) crystal planes of fluorite, respectively. The XRD pattern of SA-CeO2 was similar to that of CeO2 without any peaks attributed to the SA molecules, indicating that the crystalline phase of CeO2 did not change after the treatment with SA.

Figure 1.

Figure 1

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Image of XRD pattern of (a) CeO2 and (b) SA-CeO2.

FT-IR Analysis

As shown in Figure 2a, the broad peak around 3400 cm–1 was the bond stretching vibration absorption peak and the characteristic peak of the crystal water, which indicated that the sample contains crystal water. The surface modification process of CeO2 was a process in which the carboxyl group in SA reacted with the hydroxyl group on the surface of CeO2 to change the surface properties of CeO2. Compared with the unmodified CeO2, the broad peak of SA-CeO2 around 3400 cm–1 was the disappearance of the −OH stretching vibration absorption peak, indicating that the carboxyl group in SA was chemically bonded to the hydroxyl group on the surface of CeO2. As shown in Figure 2b, the characteristic absorption peak at 2924 and 2858 cm–1 were the −C–H stretching vibration peaks in −CH2 and −CH3. The carboxyl stretching vibration peak appeared at 1710 cm–1, which was consistent with the fatty long carbon chain group in SA, which indicated the presence of SA on the surface of the modified CeO2.

Figure 2.

Figure 2

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FT-IR spectrum of (a) CeO2 and (b) SA-CeO2.

Morphological Analysis of the CeO2@Cu Mesh

As shown in Figure 3, the microscopic morphology analysis of the surface of the untreated Cu mesh and the surface of the CeO2@Cu mesh. As shown in Figure 3a, the surface looked smooth at low magnification. After calcination, a uniform rod-shaped CeO2 with a size of about 100 nm was immobilized on the surface of CeO2@Cu mesh, which provided a nanolevel roughness to the surface of the Cu mesh. Also, the CeO2 nanoparticles exhibit immobilization with a resistance sonication in ethanol for 30 min with a significant amount of CeO2 nanoparticles still anchored on the Cu mesh surface. The CeO2@Cu mesh had a micro–nano-level layered structure, in which CeO2 nanoparticles were fixed on the surface of the copper mesh wire, which was the key factor for superhydrophobicity.35 The CeO2 nanoparticles can effectively reduce the contact area between the water drops and Cu mesh.

Figure 3.

Figure 3

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SEM images of (a) the untreated Cu mesh and (b)–(d) CeO2@Cu mesh.

Wettability Tests and Adhesion Performance Analysis

As shown in Figure 4a, the CA of water droplets on the untreated copper mesh was 82.1 ± 2°, and the oil droplets would spread out quickly on the untreated copper mesh. As shown in Figure 4b–d, the CA of water droplets on the CeO2@Cu mesh was 137.5 ± 2°, and the CAs of the SA-CeO2@Cu mesh immersed in SA for 1 h and 12 h were 150.1 ± 2° and 158.1 ± 2°, respectively. Due to the capillary effect produced by the microstructure, water was prevented from passing through the SA-CeO2@Cu mesh. The micro-/nanostructure can reduce the contact area between the solid–liquid and water and low surface energy SA, which reacted with the Cu mesh to generate copper stearate. The SA-CeO2@Cu mesh showed low adhesion to water.

Figure 4.

Figure 4

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CA on different Cu mesh surfaces water droplets on (a) untreated Cu mesh, (b) CeO2@Cu mesh, (c) CeO2@Cu mesh for 1 h, and (d) CeO2@Cu mesh for 12 h.

To have a better understanding of the superhydrophobic behavior of the SA-CeO2@Cu mesh, the wetting state and the oil–water separation performance were studied. Due to the modification of the low-energy material SA together with CeO2 nanoparticles, water droplets would be in the superhydrophobic Cassie state and high WCA can be explained by the Cassie–Baxter equation27,36 as follows.

graphic file with name ao1c03128_m001.jpg 1

where θ2 is the WCA on the SA-CeO2@Cu mesh, θ1 is the WCA on the original Cu mesh, and f1 and f2 (f1 + f2 = 1) are the area frictions of water connecting with the SA-CeO2@Cu mesh and air, respectively. According to eq 2, f2 can be calculated to be 0.937, indicating that 93.7% surface area was covered by air, which resulted in the superhydrophobicity of the SA-CeO2@Cu mesh. According to the Wenzel equation,28 the superoleophilic properties can be enhanced by the nanostructures. Therefore, when placed on the SA-CeO2@Cu mesh, the oil droplet would enter the nanostructures due to the capillary effect. The combination of the hydrophobic surface chemistry and roughness would lead to superhydrophobic and superoleophilic properties easily.

As shown in the low adhesion image of water droplets and the SA-CeO2@Cu mesh in Figure 5, 5 μL water droplets were suspended on the needle tube. By controlling the experimental platform, the water droplets contacted with the surface of the SA-CeO2@Cu mesh. Continuous rise in the experimental platform caused the water droplets and the surface of the SA-CeO2@Cu mesh to squeeze; then, as the experimental platform descended, the water droplets and the surface of the SA-CeO2@Cu mesh completely separated.

Figure 5.

Figure 5

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Adhesion images of the water droplets on the SA-CeO2@Cu mesh.

When the SA-CeO2@Cu mesh and the water droplets were completely separated, the water droplets recovered their original appearance and were fully suspended on the needle tube, and no liquid was adhered to the surface of the SA-CeO2@Cu mesh, indicating that the prepared SA-CeO2@Cu mesh surface had low adhesion to water droplets, and low adhesion performance was very important for superhydrophobic surfaces in the practical application of self-cleaning.

Oil–Water Separation

Toluene, cyclohexane, and kerosene were chosen as the oil phases. The separation efficiency of different oil–water mixtures when the mass of the oil phase was 10.0 g and the oil–water mass ratio was 1:1 is shown in Figure 6.

Figure 6.

Figure 6

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Separation efficiency with different separation times.

The separation efficiency of SA-CeO2@Cu mesh was calculated by eq 2. For the oil–water mixture with a mass ratio of 1:1, the first separation efficiency was as high as 99%. After several separation cycles, the separation efficiency can reach more than 98%, indicating that the SA-CeO2@Cu mesh still maintained the water–oil separation ability. In addition, the SA-CeO2@Cu mesh can separate different oil–water mixtures (water–cyclohexane, water–toluene, water–kerosene), indicating that the SA-CeO2@Cu mesh can generally separate oil–water mixture. After 30 separation cycles, the oil–water separation efficiency of the SA-CeO2@Cu mesh is still above 98%, indicating that it can be used as a filmlike material to efficiently separate an oil–water mixture, and the mechanical stability of the Cu mesh was excellent.

Chemical Stability

Chemical stability is important for practical applications. Therefore, to examine its chemical stability, the SA-CeO2@Cu mesh was immersed into an acid solution (HCl, pH = 1) and a base solution (NaOH, pH = 12) for 72 h; then, the wettabilities and oil–water separation experiment of the SA-CeO2@Cu mesh were investigated.

As shown in Figure 7, after immersing for 72 h, the CA in the hydrochloric acid solution with pH 1 was 156.3 ± 2° and that in the sodium hydroxide solution with pH 12 was 156.4 ± 2°; the oil–water separation efficiency remained above 98%, indicating that the SA-CeO2@Cu mesh still maintained superhydrophobicity, which proved the excellent acid and alkali resistance of the SA-CeO2@Cu mesh.

Figure 7.

Figure 7

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(a) CA at different immersion times. (b) Separation efficiency at different immersion times.

Conclusions

To summarize, the superhydrophobic and superoleophilic SA-CeO2@Cu mesh was prepared with the Cu mesh as the substrate and SA as the substance with low surface energy. The SA-CeO2@Cu mesh had chemical stability (e.g., immersed in acid, base) and excellent durability. The SA-CeO2@Cu mesh can separate different oils that are insoluble in water and had a separation efficiency above 98% after 30 separation cycles. The excellent reusability and the high separation efficiency result in the possibility of the practical application of the SA-CeO2@Cu mesh for oil–water separation.

Experimental Section

Materials

Cerium nitrate hexahydrate (Ce(NO3)3·6H2O), oxalic acid (H2C2O4), and SA were purchased from Aladdin (Shanghai China). Distilled water was made in our laboratory. Kerosene, cyclohexane, and toluene were purchased from Aladdin (Shanghai China). All reagents were analytical grade and used without further purification or modification. Cu mesh (copper content >99%, 3 cm × 3 cm) with a pore size of about 75 μm was obtained from Jiangxi Dali Mesh Co., Ltd., China.

Fabrication of Superhydrophobic Surfaces

The Cu mesh was rinsed with ethanol and deionized water respectively and then dried at 80 °C for 30 min. The cleaned Cu mesh was immersed into a mixed solution containing cerium nitrate and oxalic acid. The concentration of cerium nitrate and oxalic acid was 0.1 and 0.2 mol/L, respectively. The Cu mesh with the mixed solution was stirred at the speed of 1000 rpm for 1 h. Then, the CeO2@Cu mesh was calcined at a programmable muffle furnace at 450 °C for 4 h under a nitrogen atmosphere.

The coated Cu mesh was immersed in 1 wt % SA, which was dispersed in absolute ethanol for 1 h at 25 °C. Finally, the CeO2@Cu mesh modified with SA (SA-CeO2@Cu mesh) was washed thoroughly with absolute ethanol and then dried at 80 °C for 30 min in a vacuum drying oven.

Oil–Water Separation Experiment

In this experiment, the oil–water separation experiment can be carried out only by the gravity drive of the oil and water phase. After taking a large beaker and a small beaker, the prepared SA-CeO2@Cu mesh was put above the small beaker and the different proportions of kerosene, cyclohexane, and toluene with water were measured. The oil–water mixture was stirred with a glass rod before oil–water separation. During the continuous stirring process, the oil–water mixture is directly poured from the top of the separation device to observe the separation effect of the water–oil mixture. The oil–water separation device is shown in Figure 8.

Figure 8.

Figure 8

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Schematic diagram of the oil–water separation device.

The oil–water separation efficiency was calculated by eq 2

graphic file with name ao1c03128_m002.jpg 2

where m1 is the oil after oil–water separation, m is the volume of the initial oil, and η is the separation efficiency.

Characterization

X-ray diffraction (XRD) patterns of the samples were recorded using a Bruker D8 with Cu Kα radiation. The crystalline size of CeO2 and was calculated from the Scherrer equation. The micro- and nanoscale morphologies of the SA-CeO2@Cu mesh were analyzed by field emission scanning electron microscopy (FESEM, QUANTA FEG450). Water contact angle (WCA) measurements were carried out to investigate the superhydrophobicity and superoleophilicity of the CeO2@Cu mesh with a JY-PHB using a droplet (5 μL) of distilled water. All of the WCAs were measured at five different spots on each sample surface and then the average value was taken. The Fourier transform infrared spectroscopy (FT-IR) was performed using Thermo Nicolet with the scanning wavenumber range of 4500–400 cm–1.

Acknowledgments

The project is funded by the National Natural Science Foundation of Study on Mechanism of Magnetic Nanometer Fe3O4@SiO2 Composite Hyperbranched Macromolecule for Treating the Sewage Water of Oilfield (Fund No. 51774089).

Author Contributions

T.L. and B.W. contributed equally to the work and are co-first authors. T.L. and Z.F. initiated the project. T.L. and B.W. were supervised by Z.F. and Q.L. T.L. prepared the superhydrophobic surfaces. B.W. performed the oil–water separation experiment.

The authors declare no competing financial interest.

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