Preparation of a stable superhydrophobic boat for efficient separation and removal of oil from water

Abstract

In this work, a Fe₂O₃@fabric composite was prepared via a process combining Fenton's and Schiff base reactions. The as-prepared surface exhibited superhydrophobic and superoleophilic properties simultaneously, with a high water contact angle of 160.2° and an oil contact angle of nearly 0°. The superhydrophobic surface possessed good mechanical stability, due to the fact that the surface could maintain superhydrophobicity after mechanical abrasion of 700 mm, and was torn by tape over 50 times. In addition, the composite fabric demonstrated highly efficient oil–water separation because of its extreme superhydrophobicity–superoleophilicity wettability. Inspired by water striders, a mini boat was made for self-driven, oil spill clean-up. We expect that this robust fabric will be widely adopted for application in oil–water separation and oil absorption.

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1. Introduction

The oil spill in the Gulf of Mexico, the largest offshore oil disaster in U.S. history, has aroused great attention in the area of oil–water separation and oil absorption. To date, many techniques have been put forward to solve the oil leakage problem, such as filtration,^1,2 mechanical extraction, chemical degradation,³ flotation^4,5 and absorption.^6,7 Among these methods, absorption is one of the most ideal choices, due to its convenience and low costs. Common oil absorbents are porous filtration materials such as textiles,⁸ foams,^9,10 and sponges.¹¹ However, they often suffer from low capacities/throughput. Besides, these porous materials absorb water when they absorb oil, which greatly lower the separation selectivity and efficiency.

Over the past decades, superhydrophobic surfaces have received wide spread attention in the application of oil–water separation for their reversible wettability (superhydrophobic–superoleophilic properties).^12–14 Unfortunately, the artificial superhydrophobic surfaces are easily destroyed by external stimulation, such as mechanical scratching and chemical degradation, which seriously restrict their practical applications. It is well-known that superhydrophobicity results from the synergistic effect of micro–nano hierarchical structure and low surface energy.^15,16 Establishing chemical bonding between the low surface energy and substrate is one of the effective approaches to improving their stability. Deng et al.¹⁷ reported a convenient radiation-induced graft polymerization method to prepare an extremely stable, superhydrophobic cotton fabric, which is chemically stable over the entire pH range (0–14), and durable for more than 250 commercial or domestic launderings. Another way to prolong the lifespan of superhydrophobic surfaces is introducing the bio-inspired self-healing function, including “structure repairing” and “surface energy repairing”.^18,19 Generally speaking, the self-repairing of the surface chemical composition demands that the substrate or coating have a porous structure and high specific surface area for storing hydrophobic components. Yu et al.²⁰ reported an approach using a facile in situ polymerization to coat fabrics with polydopamine@octadecylamine, thus endowing the fabrics with self-cleaning and multi-self-healing abilities. The treated fabric is also durable enough to withstand washing and mechanical abrasion, without apparently changing the hydrophobicity.

Cotton fabric is a renewable resource that has a porous structure and exhibits high absorption. It is also economical and environmentally friendly, and has been classed as the best candidate for oil–water separation. The high porosity provides superiority for storing hydrophobic components in fabricating self-healing superhydrophobic fabric. Moreover, cotton fabric has abundant functional groups, such as –NH₂ and –OH, which provide valid binding sites for chemical reactions between the coating and substrate. As a result, the durability of the superhydrophobic surface will be greatly improved. So far, superhydrophobic fabric has mainly been used in the area of oil–water separation and small area oil absorption.^21–23 Rare examples are currently available about their direct applications in dealing with large-scale off shore oil spills. It is desirable that good elasticity and easily scalable fabrication make it possible for the fabric to be used as the shell for large-scale oil absorption equipment.

In the present research, we have established a simple and low-cost method to prepare stable superhydrophobic fabric. Firstly, Fe₂O₃ is deposited on the fabric surface via a Fenton treatment to construct a rough structure. Then, lauraldehyde (LA) is chemically bonded to octadecylamine (ODA) by a Schiff base reaction to lower the surface energy. The as-prepared surface exhibits extreme wettability with a water contact angle of 160.2° and oil contact angle of nearly 0°. Besides, the resultant fabric can withstand mechanical abrasion, repeated tearing tests and corrosion of acid–base liquid, showing excellent mechanical and chemical stability. Furthermore, a superhydrophobic boat made from this robust fabric can be self-driven to clean up oil spills without extra power input, thus revealing the energy-saving and highly efficient potential application in dealing with large-scale off shore oil spills.

2. Experimental section

2.1. Materials

Lauraldehyde (C₁₂H₂₄O) and octadecylamine (CH₃(CH₂)₁₆CH₂NH₂) were purchased from Aldrich. FeSO₄·7H₂O, H₂O₂ and C₂H₅OH were obtained from Chongqing Chemical Reagent Factory, China. All reagents were of analytical grade, and Milli-Q water was used throughout the experiments. Cotton fabrics were obtained from a local fabric store. Prior to being treated, the cotton fabrics were washed with deionized water at 75 °C for 30 min to remove the impurities.

2.2. Preparation of superhydrophobic cotton fabric

The process of fabricating the superhydrophobic cotton fabric is as follows: 0.25 g FeSO₄·7H₂O were dissolved in 250 mL deionized water, followed by the addition of 12.5 mL H₂O₂ (30%) with magnetic stirring to prepare Fenton's reagent. The cleaned cotton pieces were immersed in Fenton's reagent and then put in a microwave oven for oxidation for 1 h. After Fenton treatment, the resulting fabrics were washed with deionized water to remove residue, and then dried in air. Superhydrophobicity was obtained by soaking the above-mentioned fabric in a mixed 0.01 M ethanolic solution of lauraldehyde (C₁₂H₂₄O) and octadecylamine (CH₃(CH₂)₁₆CH₂NH₂) for 2 h and subsequently drying in an oven at 60 °C.

2.3. Preparation of the superhydrophobic oil-absorption equipment

The cylindrical keel of the oil absorption equipment was firstly prepared using a wire mesh. The superhydrophobic fabric was then fixed onto the keel to obtain a large volume oil-absorbing piece of equipment with a superhydrophobic hull. For comparison, a common cloth apparatus was prepared from pristine fabric in the same way.

2.4. Characterization and tests

The water contact angle and slide angle were measured with a water drop volume of 10 μL using an optical contact angle meter (POWEREACH JC2000C1) at ambient temperature. The surface morphologies of the cotton fabrics were observed using a scanning electron microscope (SEM; HITACHI S-4800). The chemical composition of the surfaces was analyzed using Fourier transform infrared (FTIR; Bruker TENSOR-27) spectra and X-ray photoelectron spectra (XPS; Thermo ESCALAB 250). Digital photographs of the oil absorption test and oil–water separation were taken with a digital camera (Konica Minolta, Dimage Z20).

The stability of the resultant superhydrophobic fabric was evaluated by abrasion tests and repeated tear tests using adhesive tape. The abrasion test was carried out on a homemade scratch tester: SiC sandpaper (800 mesh) was used as the abrasion surface, with the superhydrophobic fabrics to be tested facing this abrasion material. Pressure of 20 kPa was applied on the surface, dragged forth with a speed of 1 cm s⁻¹.

3. Results and discussion

Fig. 1 shows the schematic illustration of the fabrication of superhydrophobic cotton fabrics through a Fenton treatment and Schiff's base reaction. Firstly, the cotton fabric was treated by Fenton's reagent in a microwave oven for 1 h, which resulted in large amount of Fe₂O₃ nanoparticles; the possible growth mechanism of Fe₂O₃ nanoparticles is considered as consisting of the following steps:^24,25

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ˙OH (1)

Fe³⁺ + 3OH⁻ → Fe(OH)₃ (2)

2Fe(OH)₃ → Fe₂O₃ + 3H₂O (3)

Fig. 1 Schematic of the process of fabrication of the superhydrophobic fabric via Fenton's and Schiff base reactions.

After Fenton treatment, more hydroxyl groups were exposed on the surface, which worked as anchoring points for the next graft of low surface energy materials. Lauraldehyde (C₁₂H₂₄O) was then reacted with octadecylamine (CH₃(CH₂)₁₆CH₂NH₂) by Schiff base reaction in ethanol, producing a large number of hydroxyl groups, which play an important role in the fabrication of the superhydrophobic fabric. Due to the large surface area and void space, the low surface energy materials can permeate the interspace and tightly adhere to the fabrics through the chemical bonding interaction generated between the hydroxyl groups of the cotton fabric and the Schiff base reaction;²⁶ thus, long chain hydrophobic alkyl groups were introduced onto the surface of the fabric.

The surface morphologies of pristine and Fenton treated fabric are shown in Fig. 2. According to Fig. 2a, the surface of the pristine fabric is very smooth. After being treated with Fenton's reagent, the weave structure of the cotton fabric was well covered by large amounts of random and close-packed nanoparticles (Fig. 2b). The high-resolution SEM image in Fig. 2c shows that these nanoparticles have an average size of 500 nm, and aggregated together, leading to the formation of a hierarchical micro–nano structure. In addition, based on the EDS spectrum (Fig. 2d) and the color of the treated fabric, these nanoparticles were speculated to be Fe₂O₃.

Fig. 2 SEM images of (a) the pristine fabric, (b and c) the superhydrophobic surface under different magnifications and EDS of (d) Fenton treated fabric.

A superhydrophobic character was obtained, due to the combination of the micro–nano structure and low surface energy. It is well known that aldehydes can react with amines via the Schiff base reaction; as shown in Fig. 3a, the XPS survey spectra show that the pristine fabric was mainly composed of the elements C and O. After treatment with Fenton's reagent, the peak of Fe 2p was observed, and this further confirmed that Fe₂O₃ was successfully coated onto the fabric surface. The new N 1s peak of the LA/ODA decorated fabric is derived from ODA. Furthermore, FTIR was applied to prove that the LA had chemically reacted with ODA. A peak appeared at 1645 cm⁻¹, which was assigned to the C=N stretching vibrations of the Schiff base reaction product, indicating the chemical bonding interaction between ODA and LA. The XPS and FTIR results confirmed that the ODA/LA was grafted onto the fabric, with long alkyl chains on the surface.

Fig. 3 (a) XPS spectra and (b) FTIR of the pristine fabric, Fenton treated fabric and superhydrophobic fabric.

The wettability properties of the fabrics were evaluated by water contact angle (WCA) measurements. The pristine cotton fabric showed superhydrophilic behavior, due to the capillary effect caused by hollow space and abundant hydroxyl groups within the fabric (Fig. 4a). After being treated with Fenton's reagent, followed by LA/ODA chemical modification, the as-prepared fabric exhibited high repellency to water, with a contact angle of about 160.2° (Fig. 4b and c). For comparison, the water contact angles (WCA) of Fenton treated and LA/ODA modified fabric were 0° and 135°, respectively, disclosing that both hierarchical structure and low surface energy are indispensable for realizing superhydrophobicity. Fig. 4d–f demonstrate the self-cleaning behavior of the modified fabric. The superhydrophobic fabric showed strong anti-contamination properties, even after being scoured by continuous water flow. No stain was left on the modified fabric. However, the pristine fabric was hydrophilic and was dyed by blue water flow.

Fig. 4 Water contact angles of (a) the pristine fabric and (b and c) the superhydrophobic surface; (d–f) comparison of the self-cleaning behavior of the modified fabric and the pristine fabric.

In practice, applications of superhydrophobic fabric are limited by the low robustness. In order to achieve its widespread use, improving the chemical and mechanical stability of superhydrophobic fabrics is crucial. Fig. 5 presents the variation of the water contact angle with pH of the aqueous solution. Satisfactorily, all contact angles of the fabrics were higher than 150° in the pH range from 1 to 14, indicating that the as-prepared surface has favorable acid-resistance and alkali-resistance.

Fig. 5 Water contact angles of the superhydrophobic fabric under different pH.

The mechanical stability of the resultant superhydrophobic fabric was evaluated by abrasion tests and repeated tear tests with an adhesive tape. The methodology for the scratch test is illustrated in Fig. 6a: 800 mesh SiC sand paper was used as the abrasion surface, with the superhydrophobic fabrics to be tested facing the abrasion materials. The change in CA with abrasion distance is displayed in Fig. 6b. The results show that the CA of the as-prepared superhydrophobic fabric decreased slightly and was still maintained above 150° after abrasion of 700 mm. Fig. 7a–e shows the process of the superhydrophobic fabric being peeled off the adhesive tape. After a 50-cycle tear test with adhesive tape, there was only a slight decrease in the water contact angle, and it remained at about 150°, exhibiting the excellent mechanical durability of the fabric. This kind of superhydrophobic fabric, which has high robustness, might find potential application in the area of protective clothing and oil-absorbing equipment.

Fig. 6 (a) Schematic of the scratch test; (b) the CA of the superhydrophobic surface as a function of the abrasion length.

Fig. 7 (a) Water contact angles of superhydrophobic fabric, repeatedly torn by adhesive tape. The inset depicts the tear process. (b) A piece of fabric; (c) the fabric pasted onto an adhesive tape, (d) fabric peeled off the adhesive tape; (e) water droplet on the fabric after peeling off the adhesive tape.

Pollution caused by oil spills is one of the most serious environmental problems worldwide. Superhydrophobic surfaces receive wide attention in waste oil treatment for their special hydrophobic–oleophilic wettability. However, most of them are used for oil–water separation, and rare examples are currently available on their direct applications in dealing with large-scale off shore oil spills. Inspired by water striders, we made two mini boats with pristine and superhydrophobic fabric, as shown in Fig. 8a and b. The mini boat made by pristine fabric started to sink as soon as it came in contact with water (Fig. 8c), while the mini boat made by the superhydrophobic fabric freely floated on the water and automatically captured oil in seawater (Fig. 8d), which may be ascribed to the water-repellant effect of captured air (Fig. 8e).

Fig. 8 Optical images of mini boats made from pristine fabric (a) and superhydrophobic fabric (b). (c) The pristine fabric boat sank underwater. (d) The superhydrophobic fabric boat floated freely on water. (e) Obvious air cushion beneath the superhydrophobic fabric boat.

In order to check the oil absorption capability of the superhydrophobic boat, an experiment simulating the self-driven oil spill elimination was conducted, as shown in Fig. 9. Firstly, 30 mL of toluene were poured into a beaker to simulate oil pollution (Fig. 9a–c). It can be seen that the superhydrophobic boat can float on the water, and the toluene continuously crosses through the fabric into the boat (Fig. 9d). With increasing time, the oil collected by the boat increases (Fig. 9e–g). During the whole process, not a drop of water was collected by the boat. In just one minute, almost all of the toluene was cleaned up by the mini boat, leaving a clear, bright water surface (Fig. 9h). Moreover, the whole clean-up process is self-driven and there is no extra power input, which greatly decrease the cost and enhance the efficiency in dealing with the large scale offshore oil spills. Fig. 9i is the schematic illustration of the superhydrophobic cloth boat made from the fabric for self-driven oil spill cleanup. In the first stage, the superhydrophobicity of the fabric prevents the hull bottom from being wetted by water, thus resulting in large buoyancy and the mini boat freely floating on the surface, providing the prerequisite for the self-driven clean up of large scale offshore oil spills. Because of the other superoleophilicity features of the fabric, the oil spill can permeate the boat hull and continuously enter into the superhydrophobic boat.

Fig. 9 (a–c) Simulation of an oil spill by pouring crude oil into water; (d–h) in situ collection of an oil spill by the superhydrophobic fabric boat; (i) schematic of the superhydrophobic boat made from the fabric for self-driven oil spill cleanup.

In addition to oil absorption ability, the superhydrophobic fabric can also be used in oil–water separation. The separation efficiency of the superhydrophobic fabric is determined by collecting and weighing the oil passing through the fabric, and it is defined as the mass ratio of the oil collected from the beaker to that in the original oil–water mixture. Fig. 10a declares the separation mechanism for the oil–water mixture. Since the as-prepared fabric shows superhydrophobic and superoleophilic properties simultaneously with a high water contact angle of 160.2° and an oil contact angle of nearly 0° (Fig. 10b), when a drop of water is placed onto the superhydrophobic surface, it will quickly roll away because of the self-cleaning effect of the fabric, while the oil will permeate through the fabric for its superoleophilicity. The separation process of the oil–water mixture is displayed in Fig. 10c. Firstly, 50 mL toluene and 50 mL water were mechanically mixed (water was dyed with methylene blue), then poured into the self-made separation device. It can be seen that the toluene infiltrated the fabric and quickly entered into the conical flask under the driving force of gravity, while the water was retained. About 47 mL toluene and 49 mL water were collected, indicating the high separation efficiency of the superhydrophobic fabric. The reusability of the superhydrophobic fabric for oil/water separation was carefully studied (Fig. 10d). The toluene–water mixture was applied in the experiment for reusability. The polluted superhydrophobic fabric was washed with ethanol and water after each separation experiment cycle to remove the absorbed oil. Subsequently, the clean fabric was dried in an oven. After measurement of the WCA, the clean fabric continued to be used in the next cycle. It can be seen that after 12 cycles, the separation efficiency of the superhydrophobic fabric varied from 98.6% to 95.4%, and the WCA decreased from 160.2° to 149.3°, demonstrating that fabric still displayed high separation efficiency and strong hydrophobicity.

Fig. 10 (a) Separation mechanism and (b) images of water (upper) and oil (lower) contact angles on the treated fabric, (c) separation process of toluene–water mixture, (d) variation of the WCA and the separation efficiency of the as-prepared fabrics versus the recycle numbers of oil–water separation.

Except for the toluene–water mixture, the mixtures of water and hexane, silicone oil, chloroform, and diesel were also successfully separated by the superhydrophobic fabric, and the results of the separation efficiency are shown in Fig. 11. All the separation efficiencies of the superhydrophobic fabric for above-mentioned oils were calculated up to 97%, indicating the broad adaptability and effective segregation of our self-made separation device.

Fig. 11 Separation efficiency of different types of oil or organic solvents.

4. Conclusions

In conclusion, a stable superhydrophobic fabric was successfully constructed through a process combining Fenton's and Schiff base reactions. The resultant fabric simultaneously showed superhydrophobic and superoleophilic properties, with a high water contact angle of 160.2° and an oil contact angle of nearly 0°. The coated fabric has remarkable chemical and mechanical stability against various harsh treatments, such as wide-range pH, scratch test and tear test using an adhesive tape. The as-prepared superhydrophobic–superoleophilic fabric can separate a series of oil–water mixtures with high separation efficiencies. Additionally, an energy-saving and highly efficient superhydrophobic boat was made from our treated fabric, for self-driven oil spill cleanup, which may have wide application in oil spills.

Acknowledgements

The authors specially thank for the financial support of this work from the National Natural Science Foundation of China (51103120).

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