Magnetically driven functionally integrated device for continuous and efficient collection of oil droplets from water

Abstract

To tackle the serious issue of the increasing number of oil spill accidents, many strategies have been proposed to design novel devices for oil–water separation. We have designed a magnetically driven functionally integrated device which can continuously clean up small amounts of floating oil spills located in different areas, collecting the oil into the interior of the device. This process can be recycled many times to realize a continuous “ON–OFF–ON” program. The separation efficiency is as high as 95%.

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The leakage of oil from industrial accidents or oil tankers is a severe catastrophe in the world, which will have a negative effect on ecosystems and further threatens every species along the marine food chain, from low-grade algae to higher mammals even including human beings. To tackle these deteriorating problems caused by oil pollution, numerous technologies and materials have been developed to recover spilt oil from polluted environments, such as physical diffusion, green bioremediation,^1,2 in situ combustion,³ oil skimmers,⁴ polymer nanoparticles⁵ and other absorbent materials.^6–8 The large-scale application of oil–water separating materials points toward high efficiency yet, in the meantime, being a low-cost system. Owing to the difference of interfacial effects for oil and water, utilizing the wetting behaviour of solid surfaces to design oil–water separating materials has attracted a great deal of attention in fundamental research and potential application in the field of oil–water separation. Utilizing different synthesis strategies, a series of superhydrophobic–superoleophilic materials have been used for oil–water mixture separation, which provides an important avenue for the development of advanced separation techniques. Since Jiang’s pioneering research on superhydrophobic and superoleophilic coatings on metal meshes through spraying polytetrafluoroethylene for oil–water separation,⁹ a broad spectrum of oil–water separating material has been fabricated through a variety of strategies.^10–16 In 2015, Jiang and co-workers have also summarized the recent progress of bioinspired surfaces with superwettability and some of them can be applied for oil–water separation.¹⁷ However, most of these materials or surfaces are focused on the pristine separation of oil or water from oil–water mixtures, ignoring the effective collection of spilt oil. Although they can separate oil contaminants from water very efficiently, they are not supposed to be applied in the place of the oil spills because they cannot subsequently selectively collect the separated oil. What is more, oil–water mixtures from a practical perspective contain a small amount of floating oil spills distributed in different areas, which cannot be separated and collected effectively through these methods. Simultaneously realizing continuous separation and controlled collection remains a great challenge in the oil–water separating process. Even in some continuous separation of oil–water mixtures, both water and oil are readily collected together, which results in a quick decrease in film life and secondary pollution. These drawbacks have heavily hampered their practical application in oil–water separation and make it difficult to achieve high continuity or high productivity.

Therefore, it is greatly imperative to design and fabricate a smart system,^18,19 which will realize the continuous, targeted and controlled separation and collection of spilt oil from water. A simple approach for the controlled movement of an object on a water surface is to introduce magnetically responsive materials into the system and then apply a magnetic field to direct the object to the targeted location.²⁰ Compared with conventional superhydrophobic–superoleophilic oil–water separating materials or surfaces, the smart system can be placed in the zone of polluted water and subsequently be moved by means of an external magnetic field. These studies have the merit of introducing a magnetic actuation; otherwise the small amount of floating oil spills distributed in different areas will seriously prohibit their large-scale application. Considering the practical applications, a smart switch for the device may be more suitable for continuous separation and collection of a small amount of floating oil spills.

To the best of our knowledge, there are few reports on the fabrication of functionally integrated devices with superhydrophobic–superoleophilic properties to directionally and continuously clean up small amounts of floating oil spills through a smart design. Owing to the smart design, the as-prepared device prefers to allow the rich oil layer to infiltrate while the additional water is prevented from passing through. Moving the device by applying a magnetic field, a small amount of floating oil droplets located in different areas on the water surface will return to the interior of the device to be separated and collected. After many cycles, most of the oil can be collected and pumped out. Therefore, in this communication, we have designed and developed a smart and simple strategy to prepare a functionally integrated device for highly efficient oil–water separation. At the beginning, this device can easily remove rich oil without any magnetic field, collecting the spilt oil into the interior of the device and pumping it away via a peristaltic pump, which exhibits a continuous operating feature. After most of the oil is separated and collected, this device stops collecting oil and oil around the device has already been cleaned up at this time. But there is still a thin layer of oil and some small oil droplets distributed in different areas in the water reservoir. In the presence of an external magnetic field, the device can move and collect the small amount of floating oil droplets located in different areas on the water surface. When the oil in the interior of the device stores enough, the device starts to recollect the oil and further pump it away. This process can be recycled many times to realize a continuous “ON–OFF–ON” program.²¹ More importantly, no water can be pumped out in the entire system, which also avoids excess water entering the oil phase to cause secondary pollution. This is a new attempt for the design of a next-generation functionally integrated device used in oil–water separation.

We obtained commercially available and poriferous copper foam with highly efficient oil–water separation capability using a facile and scalable fabrication technique based on inexpensive materials. To obtain an improved oil–water separating process by the functionally integrated device, we carried out the following design. The functionally integrated device was prepared according to the procedure as illustrated in Scheme 1. First, the copper foam was folded into two rectangular boxes (without a roof) possessing a designed dimension of 3 cm × 3 cm × 1.5 cm and 0.7 cm × 0.7 cm × 2 cm, and the as-prepared boxes were washed by immersing in ethanol/ethanol–deionized water (v/v 1 : 1)/deionized water sequentially for 15 min each time, followed by a drying process in an oven at 65 °C. Second, the as-prepared boxes were immersed in an aqueous solution of AgNO₃ at 50 °C for 15 min at room temperature and ambient pressure, followed by sufficient washing with deionized water and further drying in an oven at 65 °C; once immersed in the silver nitrate, Ag clusters would deposit quickly on the surface of the as-prepared boxes. Third, the as-prepared boxes were uniformly modified with a monolayer of methyl groups through immersion in an ethanol solution of SH(CH₂)₁₁CH₃ (5 mM) for 12 h. Then, they were rinsed with ethanol and dried in an oven at 65 °C. Superhydrophobic and magnetic nickel foam was fabricated according to our previous work.^18,19 Finally, the as-prepared superhydrophobic nickel foam was fixed to the tail of the bigger box and the smaller copper foam box was fixed in the interior of the bigger one to form the magnetic, durable, and superhydrophobic functionally integrated device. In order to easily observe the continuous oil absorption and collection process, the as-prepared device was designed to a rectangular box without a top surface. The copper foam used in our experiment was flexible and easy to fold and seal. During the folding process, we did have a small part of copper foam set aside, which could connect the device by using the irreversible deformation of copper foams to fix to each other. One important thing that should be pointed out is that the two assembled copper boxes have a ∼1 mm thick air gap. During the assembly process, the pump pipe should be inserted tightly to the bottom of the inner smaller copper foam box, leaving a small air gap between the two assembled copper foam boxes. This small air gap serves as the air switch which will regulate the separation and collection of the oil–water mixtures.

Scheme 1 Schematic illustration for the fabrication of the magnetically driven functionally integrated device.

Surface morphology and chemical composition are two important factors which significantly affect the wettability of a solid surface. The chemical composition determines whether the surface is hydrophilic or hydrophobic, while the morphological structure will amplify this surface property to superhydrophilicity or superhydrophobicity. In order to check whether the immersion process provided enough surface roughness, we characterized the surface morphology of the copper foam and nickel foam before and after electroless deposition followed by the consequent immersion in an ethanol solution of SH(CH₂)₁₁CH₃ with scanning electron microscopy (SEM). Before the immersion step, both the pristine copper foam and nickel foam substrates formed by continuous skeletal meshes possessed ample staggered holes with a mean diameter of about 500 µm, as shown in Fig. 1a and b. The insets of Fig. 1a and b show the magnified copper meshes and nickel meshes respectively, which have a relatively low surface roughness. Fig. 1c and d indicate that the porous foams were coated with a thick and hierarchical silver aggregates. The insets of Fig. 1c and d show close-packed and microscale structures with three-dimensional nanoscale branches. Moreover, the surface before and after modification with thiol molecules was characterized by energy dispersive X-ray spectroscopy (EDX, see Fig. S1 in the ESI†). The coverage of these silver aggregates on the porous foams depended largely on the deposition time. Such hierarchical aggregates with micro-nanostructures could substantially increase the surface roughness for the fabrication of a superhydrophobic and superoleophilic surface. Two methods were adopted to investigate the superhydrophobicity and superoleophilicity of the as-prepared device after modification in an ethanol solution of SH(CH₂)₁₁CH₃. From Fig. 1e, a spherical water droplet is observed to stand on the surface of the copper foam and could not penetrate through spontaneously, which indicated the superhydrophobicity of the as-prepared device. Static contact angle (CA) measurements were also carried out with water droplets and toluene droplets (the droplet volume is 4 µL) to clarify that the surface of the as-prepared device was superhydrophobic with a water contact angle (WCA) of 151.1° ± 0.5°, as shown in the inset of Fig. 1e. However, toluene easily spread and penetrated the surface when it contacted the surface, and then the oil droplet dropped down after more toluene was added, as shown in Fig. 1f. The surface was superoleophilic, with an oil contact angle (OCA) of about 0°, as presented in the inset of Fig. 1f. This phenomenon proved the superoleophilicity after modification by n-dodecanethiol (see Fig. S2 in the ESI†), which suggested that the treated copper foam and nickel foam could be used as an oil-sorption material for oil–water separation.

Fig. 1 SEM images of the copper foam and nickel foam (a and b) before and (c and d) after electroless deposition respectively, and the insets showing corresponding local amplified images; (e and f) schematic diagrams showing the wettability of oil and water droplets on the as-prepared device surface. (e) The shape of a water droplet on the device surface with a WCA of 151.1° ± 0.5°. (f) The toluene droplet spread and penetrated through the surface of the as-prepared device with an OCA ≈ 0°.

In order to investigate the continuous separation of the as-prepared device, the following experiment was carried out as shown in Fig. 2. To distinguish toluene from water, we dyed the toluene a blue colour with a solvent dye called Solvent Blue 78 which is widely used to dye resin, polyester, and other oil systems. The labelled toluene (60 mL) and plenty of water were added into a crystallizing dish (200 mm × 120 mm) forming a thin blue oil layer above the water owing to the fact that the density of toluene is lower than that of water. The continuous separation and collection process was realized via a facile combination of the two assembled copper foam boxes with a small air gap as the air switch and a peristaltic pump. The type of pump pipe was 19# and it had a certain thickness. A different thickness of the pump pipe would have different pump pressures. The pressure of the peristaltic pump (BT300-2J) used in our experiment was about 0.05 MPa and the speed was 30 rpm. In Fig. 2a, the dyed and rich toluene was rapidly absorbed into the interior of the as-prepared device as a result of the superhydrophobicity and superoleophylicity. Meanwhile the rich toluene would immerse the air gap and start the air switch afterwards. Adjusting the peristaltic pump, the toluene stored in the interior of the as-prepared device started to pump into the pipe and was collected into the outside wide-necked bottle. Furthermore, most of toluene around the as-prepared device was absorbed continuously into the interior while keeping the water blocked outside. When the height of the dyed toluene stored in the interior was lower than the bottom of the embedded superhydrophobic copper foam box, an air gap formed as a switch to terminate the toluene collection with air starting to pump into the pipe, as illustrated in Fig. 2b. The toluene within the pipe was further pumped away along the pipe into a wide-necked bottle and no more toluene or water could be pumped into the pipe at the original site because nearby toluene is consumed and the air gap prevented the permeation of water into the as-prepared device even under vigorous pumping as shown in Fig. 2c. From Movie S1 in the ESI,† some air was pumped into the collecting wide-necked bottle and formed continuous air bubbles in toluene at the moment. However, there was still a small quantity of residual floating oil droplets distributed in different areas on the water surface, which could not be separated and collected effectively through conventional methods. In order to remove the floating oil preferably, the as-prepared device should be driven to the area of pollution. The best strategy for the directional locomotion of the as-prepared device on a water surface was to introduce magnetically responsive materials into the system and then apply a magnetic field to drive it to the targeted position. To restart the separation and collection of the oil in this experiment, we introduced a magnetic field to drive the as-prepared device to absorb the floating oil spills on different sites. At this time, the oil droplets located in different areas would gradually be absorbed into the interior of the device. When the oil in the as-prepared device became enough and immersed the air gap again, the air switch restarted and toluene could be pumped into the pipe again, as shown in Fig. 2d. After continuous locomotion of the as-prepared device under an external magnetic field, no obvious floating oil was found on the water surface in the crystallizing dish as presented in Fig. 2e. When most of the oil was pumped out, the pipe pumped in continuous air bubbles instead of oil or water and oil collection stopped, which showed the high efficiency of oil–water separation. Moreover, no water could pass through the as-prepared device owing to the air gap even if the peristaltic pump kept running as in Fig. 2f (for a demonstration of the entire process, please see Movie S1 in the ESI†). This process had realized the continuous separation and collection of rich oil and floating oil droplets located in different areas. By taking advantage of the difference in wettability between oil and water, the oil–water mixtures were separated successfully without any external water in the collected oil, indicating the characteristics of facile operation and low energy consumption. To further realize the possible applications of oil–water separation, a well-connected seamless device was designed and fabricated (see Fig. S3 in the ESI†).

Fig. 2 Optical snapshots of continuous separation and controlled collection in the oil–water separation process. (a) The blue-dyed oil started to pump into the pipe and flowed into a wide-necked bottle after turning on the peristaltic pump. (b) The pipe started to pump in air instead of oil or water. (c) The as-prepared device stopped collecting oil. (d) It pumped in oil again from other areas along the water surface by the introduction of an external magnetic field. (e) The pipe pumped in air after cleaning up a small quantity of residual floating oil on the water surface. (f) Oil spills are cleaned up after the process. Insets are the corresponding local top-view photographs.

After completing the continuous separation and collection of rich oil and a small amount of residual floating oil, the device was evaluated with regard to its oil–water separation mechanism. Our simulation showed that the key to this smart design was a small air gap as a switch inside the as-prepared device to terminate toluene collection and keep the water outside at the same time. This small air gap as a switch will prevent the permeation of water into the as-prepared device even under vigorous pumping; owing to its superhydrophobicity and superoleophylicity, only the oil was absorbed and flowed along the pipe which was connected to a wide-necked bottle, making the device consistently gather oil spills on the water surface, as shown in Fig. 3a–c. When a suction was applied to the pipe of the as-prepared device by turning on the peristaltic pump, the stored oil started to pump into the pipe. From Fig. 3a, the as-prepared device started collecting oil when the peristaltic pump was turned on and the rich oil immersed the bottom of the embedded superhydrophobic copper foam box. After the oil around the as-prepared device began to vanish, the volume of the residual oil in the interior of the device decreased further until an air gap formed in the device. At this time, the height of the oil inside the as-prepared device was below the bottom of the embedded superhydrophobic copper foam box and some air would pump into the pipe instead of oil or water, as shown in Fig. 3b. Afterwards, the oil collection terminated in the original area with the pipe pumping in air instead of oil or water. To restart the oil collection, a magnetic field was introduced to induce the as-prepared device to move and absorb oils in other areas on the water surface. As we assumed, the small amount of floating oil was still absorbed quickly with the movement of the as-prepared device. When the volume of oil inside the device gradually increased, the oil would replace the air in the pump pipe and the pipe would pump in oil again, as shown in Fig. 3c. This self-regulating oil collection system will not only save a large amount of raw materials, but also make the collection of oil spills easier and smarter.

Fig. 3 Schematic for the oil–water separation mechanism in the smart design. (a) Oil separation and collection started when the as-prepared device was put in the oil–water mixture and the peristaltic pump was turned on. (b) Oil collection terminated at the original site with the pipe pumping in air instead of oil or water. (c) Oil collection restarted with pumping in oil again from other areas along the water surface after directed movement by the introduction of an external magnetic field.

To confirm our mechanism described above, the following experiment was designed and studied: we dropped dyed toluene oil droplets onto the water surface in different areas, which was followed by placing the as-prepared device on the water surface far from the oil zone as shown in Fig. 4a. In this case, the as-prepared device remained immobile and could not absorb the blue oil droplets automatically. To pull the as-prepared device directionally towards the dyed oil, we applied an external magnetic field above the beaker. Guided by the movement of the magnet, the as-prepared device could directionally clean up the dyed oil, as shown in Fig. 4b and c. When just contacting with the blue oil droplets, the device quickly absorbed them. However, the as-prepared device had porous three dimensional (3D) structures and could absorb a certain amount of oil, which would not collect oil automatically immediately (see Fig. S4 in the ESI†). Next we added some other oil droplets to the water surface as shown in Fig. 4d; the oil droplets located in different areas would be also absorbed into the interior of the device automatically by adjusting the external magnetic field. As the height of the oil stored in the interior of the as-prepared device rose above the bottom of the embedded superhydrophobic copper foam box, the air switch started under the driving force of the peristaltic pump and the oil could be pumped into the wide-necked bottle as shown in Fig. 4e. Once the oil surface was lower than the bottom of the embedded superhydrophobic copper foam box, some air would enter into the pump pipe and the air switch closed. Meanwhile the device stopped collecting oil and no water entered in the collected oil. No blue oil droplets were left on the surface as illustrated in Fig. 4f, indicating the excellent capability of the device to directionally clean up the small amount of floating oil on the water surface (for details, please see Movie S2 in the ESI†). Therefore the above results demonstrated that the as-prepared device performed well not only in directional oil separation and collection but also in controlled magnetically responsive oil absorption for a small amount of floating oil droplets located on different sites, exhibiting integrated functions. This smart design has broadened the application of the as-prepared device in oil–water separation by the addition of the magnetic field. Even though the as-prepared device had absorbed the oil, the hydrophobic property was still maintained and the water could be blocked outside (for details, see Fig. S5 and Movie S3 in the ESI†). To broaden the applications of the functionally integrated device from the water surface to underwater, we carried out the following experiment on the directional oil removal underwater with the control of the magnetic field, which indicated the excellent capability of the as-prepared device to directionally clean up oil spills underwater (for details, see Fig. S6 and Movie S4 in the ESI†).

Fig. 4 Optical snapshots of the continuous oil–water separation and collection using the as-prepared device for the small amount of floating oil droplets located in different areas. (a) The as-prepared device floated on the water surface keeping still. (b) Directed movement of the as-prepared device to absorb oil droplets by the introduction of an external magnetic field. (c) Directional removal of the oil droplets guided by the magnetic field. (d) Injection of another oil droplet into the system. (e) The as-prepared device started pumping in oil and storing oil in an oil collector. (f) Oil droplets cleaned up on the water surface.

As oil spill accidents occur frequently throughout the world, it is crucial to extend the application of the device for different oil–water mixtures, such as toluene, n-hexane, isobutanol and silicone oil. Separating efficiency is used to quantitatively characterize the ability of a device to perform oil–water separation. It can be defined according to the ratio of the volume of collected oil after separation versus that of the original. It is noteworthy that various oils have a specific viscosity (toluene: 0.59 mPa s; n-hexane: 0.33 mPa s; isobutanol: 4.7 mPa s; silicone oil: 4.5 mPa s), which can cause some very small differences in the oil–water separating efficiency. In order to better understand the separation efficiency of the as-prepared device, we took different kinds of oil–water mixtures as examples to separate oil from water. From Fig. 5a, we can observe that the flow velocity of the oil will decrease with the increase of the viscosity and then the oil–water separation efficiency will be different during the same time. This is mainly because the higher viscosity will lead to the decline of the absorption speed of the as-prepared device. Therefore the volume of the collecting oil has an obvious difference within the same time. However, the oil will be completely collected with the extension of the collecting time. We were surprised that the final separation efficiency was excellent, as shown in Fig. 5b. Furthermore, the device could be reused more than ten times and still had a high separation efficiency, which indicated the general suitability for various toluene–water separations, as shown in Fig. 5c. What is more, the as-prepared device could be recycled after it contacted one kind of oil. The recycling number and separation efficiency are shown in Fig. 5d (for details, see Fig. S7 and Movie S5 in the ESI†). After recycling ten times, the as-prepared device still had satisfactory superhydrophobicity (see Fig. S8, Movies S6 and S7 in the ESI†). These results indicate that this novel superhydrophobic and superoleophilic device can act as a potential candidate for oil–water separation, which will provide promising possibilities in industrial applications.

Fig. 5 (a) Comparison of the oil collecting rates of the as-prepared device for different oil–water mixtures with a specific viscosity at a constant flow pump speed of 30 rpm. (b) Separating efficiency of the as-prepared device for different oil–water mixtures. (c) Recycling experiments for separating toluene from water by the as-prepared device. (d) The separating efficiency and recycling number of different types of oil–water mixtures.

Conclusions

In summary, we have developed a smart system by fabricating a functionally integrated device through electroless deposition followed by surface modification with a self-assembled monolayer. In this smart system, the as-prepared device can collect oil continuously when the air switch has started. After most of the oil was separated from water and collected, the as-prepared device stopped collecting oil, namely due to the air switch closing. However there were still some floating oil spills located in different areas without further separation and collection. In order to separate and collect these floating oil droplets, we introduced an external magnetic field to drive the as-prepared device to the oil zone and realized re-separation and re-collection of oil spills. This process can be recycled many times and realized a continuous “ON–OFF–ON” program. By taking advantage of the functionally integrated device, we have realized the continuous separation of oil–water mixtures, which demonstrated a separation efficiency as high as 95%. What is more, it can be also reused more than ten times and still retains high separation efficiency. Meanwhile the collected oil will not contain any water, which realized an effective separation and collection of oil and avoided secondary pollution from oil. We believe that the continuous separation of oil–water mixtures may provide a novel strategy for handling oil spills under various conditions and situations and can be applied in other oil–water separating systems.

Acknowledgements

This work was supported by Beijing Natural Science Foundation (2131003).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Energy dispersive X-ray (EDX) patterns, optical snapshots and movies of the as-prepared device for oil–water separation. See DOI: 10.1039/c5ra27908a

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