Facile fabrication of an underwater superoleophobic mesh for effective separation of oil/simulated seawater mixtures

Abstract

Recently, oil/water separation has become a global challenge due to frequent oil spills. Herein, we demonstrate an underwater superoleophobic Ag coated mesh, which is fabricated by a facile galvanic exchange reaction. The mesh was used to study gravity-driven oil/simulated seawater separation, in which a separation efficiency of above 94.0% could be achieved. In addition, the Ag coated mesh can maintain this high separation efficiency after 45 separation cycles. Furthermore, the Ag coated mesh exhibits excellent environmental stability under harsh conditions. We believe that the Ag coated mesh can be applied for the development of functional materials for oil/water separation.

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1. Introduction

Recently, with the development of the petroleum industry and increase in oil spills, oil/water separation has attracted worldwide attention.^1,2 To minimize the damage of oil pollution, many oil/water separation approaches have already been developed, including combustion, filtration, oil-absorption, etc.³ However, environmental and economic materials are urgently needed for the effective separation oil/water mixtures. Inspired by nature, materials with superhydrophobic–superoleophilic surfaces have recently attracted increasing attention to separate oil/water mixtures because of the different interface effects of water and oil.^4–6 These materials have been fabricated by many scientists via the construction of a hierarchal rough surface which has a low surface energy.^7–12 However, the materials involved in the aforementioned methods can be easily fouled by the adherent oil during the separation process, which thus limits their applications and decreases separation efficiency.¹³ Furthermore, such filter materials are not suitable for the gravity-driven removal of light oils with a density smaller than water from mixed solutions because the water settles below the oil, creating a layer of water between the oil and the mesh film. The water layer prevents the oil from maintaining contact with the mesh film. Unfortunately, most oils are lighter than water.

Inspired by fish scales, membranes with hydrophilic and underwater superoleophobic properties have shown promising application.^14–22 Since water commonly possesses a greater density than oils, these underwater superoleophobic materials have been developed to address the problems described above, and they have been proven extremely effective in oil/water separation. Xue et al. reported a novel superhydrophilic and underwater superoleophobic hydrogel-coated mesh for oil/water separation.¹⁴ Zhang et al. fabricated an underwater superoleophobic kapok membrane for the removal of oils and soluble dyes in water.²⁰ Dong et al. demonstrated an underwater superoleophobic graphene oxide coated mesh for the separation of oil and water.²¹ However, the main obstacle of these hydrogel and polymeric materials is their weak environmental adaptability due to their swelling characteristics under long-term scouring by water. In addition, there are few studies on the separation of corrosive and hot oil/water mixtures with high separation efficiency.^23–29 Moreover, the more complicated environment in practical oil/water separation, such as seawater, acid or alkali, or hot water, causes additional challenges to the separation. Therefore, it is of great significance to develop functional materials with stable underwater superoleophobicity for the separation oils from corrosive and hot water mixtures.

In this study, we report a simple route to prepare a type of Ag coated copper mesh with underwater superoleophobicity, which is fabricated via a facile galvanic exchange reaction.³⁰ Subsequently, the as-prepared mesh is applied in gravity-driven oil/simulated seawater separation experiments and shows a high separation efficiency of over 99.0% for a kerosene/simulated seawater mixture, which allows seawater to quickly permeate through the mesh, whereas the oil phase is retained above the mesh. Thus, the Ag coated mesh can be proven to become an energy efficient filter for oil/seawater separation. In addition, the Ag coated mesh possesses a high separation efficiency of above 94.0% for a series of oil/simulated seawater mixtures and still maintains the high separation efficiency of up to 99.0% after 45 kerosene/simulated seawater separation cycles. More importantly, the Ag coated mesh exhibits excellent environmental stability under a series of harsh conditions. In addition, we simulate the water and oil wetting process and deeply analyze the oil/water separation mechanism of the Ag coated mesh, thus proving the feasibility of the oil/water separation theoretically. Compared with previous reports, the preparation of the Ag coated mesh is extremely simple and fast, which is able to meet the needs of emergency conditions. At the same time, the mesh can overcome the shortcoming of polymer swelling, thus ensuring a greater flux in oil/water separation. We envision that our separation materials will be useful for a wide range of future practical applications including the clean-up of marine oil-spills.

2. Experimental

2.1. Materials

Copper mesh (200 mesh size) was purchased from a nearby metal shop. Waterborne polyurethane (PU) was purchased from Sinopharm Chemical Reagent Co., Ltd. Analytical grade silver nitrate was obtained from Tianjin, China Chemical Co., Ltd. Oils (hexane, kerosene, petroleum ether, etc.) and organic solvents (chloroform, absolute ethanol, toluene and acetone) were purchased from Guangdong Guanghua Sci-Tech Co., Ltd. Rapeseed oil was purchased from a local store. NaCl, HCl and NaOH were purchased from Shanghai Zhongqin Chemical Reagent Co., Ltd.

2.2. Preparation of underwater superoleophobic mesh

First, Cu mesh was washed with ethanol ultrasonically at ambient temperature. Second, the mesh was dipped in an aqueous solution of 1 M HCl to remove the metal oxides and then washed with deionized water. Subsequently, the Cu mesh was reacted with a 0.08 M AgNO₃ solution for about 30 seconds. Finally, the as-prepared mesh was rinsed with deionized water and dried.

2.3. Oil/water separation

Seven types of oils and organic solvents, including kerosene, hexane, petroleum ether, toluene, chloroform, tetrachloroethane and rapeseed oil were used in this study. They were colored with oil red O and mixed with water that was colored with methylene blue. The underwater superoleophobic silver coated mesh was fixed between two Teflon fixtures, both of which were attached with glass tubes. Before the separation process, the silver coated mesh was completely wetted with water. Mixtures of oil and water (50%, v/v) were slowly poured into a test tube through the Ag coated mesh, which was driven by its own gravity. The separation efficiency was calculated according to η = (m₁/m₀) × 100, where, m₀ and m₁ are the mass of the oil before and after the separation process, respectively.

2.4. Separation oil and corrosive solutions

Due to the fact that industrial oil-polluted water and marine oil spills generally contain highly corrosive and active ingredients, the mixtures of kerosene/corrosive solution (1 M NaOH and 0.1 M HCl) and kerosene/hot water (72 °C) were used to examine the stability of the silver coated mesh under harsh environments. As before, this mesh was also prewetted by water and the mixtures were poured slowly into a test tube through the as-prepared mesh.

2.5. Characterization

Field emission scanning electron microscopy (FE-SEM, Zeiss) was used to examine the morphological structures of the Ag coated surfaces. The contact angle (CA) of oil and water droplets was measured with an SL200KB apparatus under room temperature. The phase structure of the Ag and Cu samples was characterized with an X-ray diffractometer (XRD) (Rigaku Corp., D/max-2400) equipped with graphite monochromatized Cu Ka radiation. The water content of the oils was determined on a Karl Fischer Titrator (SN-WS200A).

3. Results and discussion

In this study, Cu mesh was applied as the separating substrate because of its widespread use in our routine life. Fig. 1 shows scanning electron microscopy (SEM) images of the surface morphologies and XRD patterns of the samples. As shown in Fig. 1a, the original mesh possessed an average pore diameter of 75 μm (200 mesh size), and its magnified view in the inset of Fig. 1a indicates that the original mesh had a smooth surface. After it was coated with silver, it can be seen apparently that the original mesh is thickly covered by silver trees on the microscale, which are randomly distributed (Fig. 1b). The high magnification FE-SEM image (Fig. 1c) reveals that hierarchical structures exist due to aggregation of the hydrophilic silver trees. The rough structure of the Ag coated mesh combined with its affinity for water is essential for its wettability.²² The chemical composition of the Cu mesh and the as-prepared mesh was further confirmed by XRD, as shown in Fig. 1d. The diffraction peaks located on the red line show the formation of the Ag phase (JCPDS card no. 04-0783). This means that micro/nano-structured Ag grew on the Cu mesh substrate (JCPDS card no. 04-0836). The reaction process can be formulated as follows: Cu + 2Ag⁺ → 2Ag + Cu²⁺.

Fig. 1 FE-SEM images of (a) the original copper mesh and (b and c) the as-prepared Ag coated meshes at low and high magnification, respectively. (d) XRD pattern of the original copper mesh and Ag coated mesh. Inset of (a) is a magnified image of the original copper mesh.

The wettability properties of the as-prepared meshes were characterized comprehensively. We estimated the wettability of the silver coated mesh by means of oil CA measurements in air/water surroundings. The as-prepared mesh surfaces were superamphiphilic obviously (Fig. 2a and b). After it was immersed in simulated seawater, the Ag coated mesh exhibits underwater superoleophobicity to a series of oils with oil CAs larger than 150° (Fig. 2c and d). When the Ag coated mesh contacts with oil droplets, water can be trapped in its rough microstructures and provides a strong repulsive force, which forms an oil/water/solid composite interface.^31–35 Thus, such underwater superoleophobicity can effectively prevent the Ag coated mesh from being polluted by oils during the oil/water separation processes.

Fig. 2 Wetting behavior of the Ag coated mesh towards (a) water and (b) oil in air. (c) Wetting behavior of the Ag coated mesh towards oil in simulated seawater. (d) Contact angle of oil droplets underwater on the as-prepared meshes.

Based on the wettability of underwater superoleophobicity, the as-prepared mesh can be utilized to remove water from oil/simulated seawater mixtures. The oil/simulated seawater separation experiment was performed as shown in Fig. 3a. The Ag coated mesh was clamped between two glass tubes and prewetted by simulated seawater before use. Mixtures of kerosene dyed with oil red O and simulated seawater dyed with methylene blue were poured into the upper glass tube. The kerosene was repelled and remained above the silver coated mesh, while the simulated seawater permeated through the mesh quickly. It can be seen that the driving force was only gravity during the whole process. In addition, a variety of other oil/simulated seawater mixtures have been separated efficiently by the same process. It is worth noting that, for the separation of heavy oil (trichloromethane and tetrachloroethane), we need to tilt the glass tube, making the simulated seawater contact with the Ag coated mesh. To quantitatively assess the effect of separation, the separation efficiency was calculated according to the formula η = (m₁/m₀) × 100. The separation efficiency can reach up to 99.0% for the kerosene/simulated seawater mixture and above 94.0% for other oils (Fig. 3b). Also, the silver coated mesh still retained its underwater superoleophobicity after 45 cycles with the separation efficiency remaining above 99.0% (Fig. 3c), which indicates favorable recyclability. To further study the separation ability of the Ag coated mesh, the intrusion pressures of different oils were measured, which indicate the maximum height (h_max) of oils that the Ag coated mesh can support. The intrusion pressures for diverse oils were measured by pouring oil onto the water prewetted mesh to acquire the maximum height. The intrusion pressure (ΔP) values were calculated using eqn (1):²

ΔP = ρgh_max (1)

where, ρ is the density of the oil, g is the acceleration of gravity, and h_max is the maximum height of oil that the mesh can support. Thus, h_max for a specific type of oil, which is an important parameter for practical operation, can be generally estimated. As shown in Fig. 3d, the intrusion pressures for a series of oils were determined by acquiring their maximum heights. The reason why the value for rapeseed oil was obviously lower than the others is possibly that the water content of rapeseed oil is higher than the others.

Fig. 3 Oil/simulated seawater separation studies of the Ag coated mesh: (a) the process of kerosene/simulated seawater separation; (b) the separation efficiency of oil/simulated seawater mixtures; (c) oil/simulated seawater separation efficiency versus recycle numbers by taking the oil/simulated seawater mixture as an example; and (d) the intrusion pressures for a series of oils.

To our surprise, as shown in Fig. 4, this Ag coated mesh can also separate corrosive oil/water mixtures and oil/hot water mixtures with high separation efficiency. A mixture of kerosene/hot water (72 °C), as a representative, was poured onto the prewetted mesh, and the hot water passed through the mesh quickly, while kerosene was kept in the top glass tube (Fig. 4a). Moreover, kerosene/1 M NaOH and kerosene/0.1 M HCl mixtures were successfully separated. In addition, the separation efficiencies of all the oil/water mixtures were still as high as 97.8% (Fig. 4b). The chemical inertness of the Ag coated mesh is a very vital consideration in harsh environmental applications, which will offer important opportunities in industry and everyday life, such as oil spill cleanup and separation of living waste oil.

Fig. 4 (a) Separation experiments for kerosene/hot water; and (b) separation efficiency of kerosene under different corrosive aqueous solutions: 1 M NaOH, 0.1 M HCl solutions and (72 °C) hot water.

To deeper analyze the oil/water separation mechanism of the Ag coated mesh, we simulated the water and oil wetting process in Fig. 5. Here, we assume that the pores are arranged approximately in a regular square array. The intrusion pressure (ΔP) is calculated using eqn (2):^36–38

where, γ_L1L2 is the water/oil interfacial tension, θ is the oil or water the contact angle on the Ag coated mesh, R is the radius of the meniscus, C is the circumference of the mesh pore, and A is the cross-sectional area of the pore. From eqn (2), the Ag coated mesh cannot withstand any pressure when the contact angle θ < 90°, because the intrusion pressure ΔP < 0 (capillary effect), and the water can permeate the mesh spontaneously. In contrast, the Ag coated mesh can sustain a certain pressure when the contact angle θ > 90°, because the intrusion pressure ΔP > 0 (negative capillary effect). In other words, the liquid cannot permeate the mesh spontaneously unless an external pressure is applied. As is shown in Fig. 5a, because the water contact angle θ is nearly 0°, the intrusion pressure ΔP < 0, that is, the mesh cannot support any pressure. When water contacts with the mesh, it would permeate the mesh spontaneously due to its gravity. However, before oil/water separation, the Ag coated mesh was prewetted by water, and the hierarchical structures are occupied, which can enrich the repellent force between water and oils. The oil would reside in a state of estrangement and the oil contact angle θ is obviously larger than 90°, thus the intrusion pressure ΔP > 0, which means that the mesh can sustain pressure to some extent (Fig. 5b). From the above, it is clear that the superhydrophilicity of the Ag coated mesh allows the coated mesh to absorb and retain water, whereas its property of underwater superoleophobicity retains oils on the mesh, therefore, oil/water mixtures can be separated successfully using this Ag coated mesh.

Fig. 5 Schematic of the wetting model of the Ag coated mesh. The coated mesh is permeable to water (a) in air, because ΔP < 0. (b) Oil cannot permeate the mesh after pre-wetting by water, because ΔP > 0. O₁ is the cross section center of the mesh and O₂ is the center of the spherical cap of the meniscus.

4. Conclusion

In summary, an underwater superoleophobic Ag coated mesh used for oil/simulated seawater separation has been demonstrated, which can be easily fabricated using a facile galvanic exchange reaction. This Ag coated mesh can separate water from oil/simulated seawater with a high separation efficiency of above 94.0%. In addition, the separation efficiency could still reach up to 99.0% after 45 separations. Moreover, the Ag coated mesh displays excellent environmental stability for kerosene/corrosive solutions and kerosene/hot water mixtures. This study has potential application to the rational design and development of functional materials for oil/water separation.

Acknowledgements

The work is supported by the Natural Science Foundation of China (Grants 21301141, 21261021) and the Nature Science Foundation of Gansu Province, China (Grant 145RJYA241).

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