Superhydrophobic Fe3O4/OA Magnetorheological Fluid for Removing Oil Slick from Water Surfaces Effectively and Quickly

Considering the severe impacts on the economic losses caused by oil spills, it is of great significance to develop an oil-absorbent material for removing the oil slick from the water surface effectively. As a new oil-absorbent material, magnetorheological fluid (MRF) has unsinkability, hydrophobicity, and lipophilicity, which could effectively remove the oil slick on the water surface while repelling water. Particularly, the prepared MRF shows a good response to external magnetic field. MRFs show high oil removal capacity in fresh water, deionized water, and salt water with efficiencies up to 94.39, 93.65, and 92.71%, respectively. Besides, Fe3O4/OA magnetic nanoparticles (MPs) could be reprepared into MRF by simple treatments. After the fifth cycle, the MRF prepared by the recovered Fe3O4/OA MPs still has high oil removal efficiency, and that means the Fe3O4/OA MPs has excellent reusability and stability. The method for preparing MRFs provided in this work is simple and effective, and the MRFs have a promising potential for cleaning oil slick.


INTRODUCTION
With the increase of oil explorations and sea transport activities, oil pollution has become a more and more severe problem year by year. Once oil spill occurs, oil films with low biodegradability will float on the water, which will damage the ecological environment seriously. Therefore, how to remove oil slick effectively has attracted worldwide attention. 1,2 The traditional methods used to solve this problem include mechanical collection, 3 chemical dispersants, 4 bioremediation, 5 in situ burning, 6 adsorption, 7 etc. On the basis of these methods, the removal of the oil slick can be realized, however, most of the methods are time-consuming and expensive processes, and even secondary pollutions will be generated. The impact of oil spills is tremendous, it has been reported that one ton of crude oil will spread on the water surface rapidly after spillage, forming an oil film of 12 km 2 in the area eventually. 8 It is important to increase the efficiency of the removal of oil slick, while first, it is necessary to prepare an oilabsorbing material with fast oil adsorption rate and high oil removal efficiency.
In recent years, the investigations of magnetic composite materials for oil removal have been increased significantly. Most of the magnetic composite materials concerned are hydrophobic and oleophilic. It can get rid of the oil slick quickly and efficiently without secondary pollution. The materials include, for example, modified magnetic nanoparticles (MPs), 9,10 modified hybrid sponges, 11,12 high oilabsorbing resins, 13,14 modified graphene aerogels, 15,16 etc.
It has been validated that the magnetic composite materials have the promising properties to remove the oil slick on the water surface. However, due to the inherent disadvantages, the available magnetic composite materials cannot be conveniently used in practice. For instances, the modified graphene aerogel is easy to collapse in the oil−water circulation operation, 17 because it is a brittle material. As for the modified hybrid sponges, although they have high absorption efficiency, the wide application in oil−water separation is limited due to their nonbiodegradability and nonreusability. 18 As a new type of a smart material, 19,20 magnetorheological fluids (MRFs) show good magnetic response to an external magnetic field. After modifications of the MPs in MRFs, MRFs become hydrophobic and oleophilic and also have unsinkable characteristics. Compared with other magnetic composite materials, MRFs are ecofriendly materials and show good oil removal performance. Rashin et al. 21 removed oil slick (motor oil) with the coconut oil-based MRF, and the oil removal efficiency was 91%. But coconut oil can cause secondary pollution and affect the reuse of oil slick. To avoid the secondary pollution, Tian et al. 22 used the lubricant oil-based MRF for removing oil slick (lubricant oil), and the efficiency of the lubricant oil-based MRF exceeds by 90%. However, these oil removal experiments of MRF are carried out on a fresh water surface, and the influence of water salinity was not investigated.
In this work, we developed a simple and practical approach to prepare Fe 3 O 4 /OA core−shell MPs. Fe 3 O 4 /OA MPs modified by oleic acid (OA) exhibit superhydrophobicity, superlipophilicity, and unsinkability. The morphology, surface, thermal stability, and magnetism of Fe 3 O 4 /OA MPs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), contact angle measurement instrument, thermogravimetric analysis (TGA), and vibrating sample magnetometer (VSM). The ultrasonic dispersion method was used to distribute the MPs in a carrier fluid (lubricant oil) uniformly to prepare an oil-based MRF.
Furthermore, two variables were selected as the influencing factors, namely the mass of the MRF (0.5, 1, and 2 g) and the type of water surfaces (fresh water, deionized water, and salt water). By carrying out the oil slick removal experiments, we obtained the oil removal efficiency of MRF under the influence of the variables mentioned above. Based on the multicycle experiments, the reusability of the Fe 3 O 4 /OA MPs was investigated.   (1) where D is particle diameter, λ is the X-ray wavelength of 0.154056 nm, k is the Scherrer constant, θ is the Bragg angle, and β is the peak width at half maximum of the diffraction peak. The average size of Fe 3 O 4 /OA MPs is about 16.4 nm. Figure 2 shows the SEM images of Fe 3 O 4 /OA MPs. It can be seen from Figure 2 that the size of Fe 3 O 4 /OA MPs is 18 nm approximately. The result is in good accordance with the result calculated by the Debye−Scherrer equation. Besides, the shape of the Fe 3 O 4 /OA MPs is spherical-like, which is mainly attributed to the use of NH 3 ·H 2 O as the alkali source to regulate the pH of the Fe 3+ /Fe 2+ mixture. 25 The oil removal capacity of Fe 3 O 4 /OA MPs depends on the size and shape of the particles, as well as the thickness of the OA coated on the surface of Fe 3 O 4 microspheres. Therefore, Fe 3 O 4 /OA MPs were further characterized by TEM. It can be seen from Figure 3 Figure 4 shows the weight loss of Fe 3 O 4 /OA MPs at different temperatures. As can be seen in Figure 4, the curve shows three steps of weight loss, and the reason is that the obtained Fe 3 O 4 /OA MPs are coated with bilayer OA-coated. 26,27 In the first step, Fe 3 O 4 /OA MPs show mild weight loss of about 0.58 wt % from 0 to 100°C, which can be attributed to the evaporation of the absorbed water. In the second step, it was found that the weight loss is about 10.35 wt % at a range from 100 to 500°C. In the third step, the significant weight loss of Fe 3 O 4 /OA MPs reached at 7.70 wt % from 500 to 780°C. The weight loss for the second and third steps is due to the thermal degradation of OA. The higher thermal degradation temperature shows that Fe 3 O 4 /OA MPs have excellent thermal stability, which indicates that Fe 3 O 4 /OA MPs have good adaptability in high-temperature environments. It should be noted that the OA in the inner layer and the OA in the outer layer are connected through the physical interaction, and the OA in the inner layer is coated on the surface of the Fe 3 O 4 microspheres through the chemical interaction. 28 Considering the presence of the absorbed water in the Fe 3 O 4 /OA MPs, the weight loss of the OA finally obtained was 18.05 wt %. Thus, it can be verified that the Fe 3 O 4 microspheres are well coated with OA.
Saber et al. 29 proposed that smaller MPs can remove oil slick on the water surface more effectively since the specific surface area of MPs in increased. Therefore, on the one hand, it is necessary to reduce the size of MPs, and on the other hand, the thickness of the OA coating should be increased as much as possible. In our previous work, 22      To our knowledge, when the water contact angles of the material are above a critical angle of 150°, the material can be considered as a superhydrophobic material. 31 Thereby, the Fe 3 O 4 /OA MPs we prepared possess the characteristic of superhydrophobicity. That means the Fe 3 O 4 /OA MPs with a superhydrophobic surface can be obtained by the simple OA coating method. Figure 6c shows the shape of the oil droplet on the Fe 3 O 4 /OA MPs bed. According to our observations, the oil droplet can merge with the bed of Fe 3 O 4 /OA MPs quickly and cannot maintain the shape of the droplets. In addition, it can be seen from Figure 6d that the left contact angle of lubricant oil is 24.89°, and the right contact angle is 24.70°. It demonstrates that Fe 3 O 4 /OA MPs are superoleophilic. The results shown in Figure 6 illustrate that Fe 3 O 4 /OA MPs exhibit superhydrophobicity and superoleophilicity, making it possible to remove oil slick on the water surface selectively.
(c,d) shape of an oil droplet on the surface of Fe 3 O 4 /OA MPs.
2.5. Sedimentation Stability Analysis. As shown in Figure 7, when the magnet contacts the bottle wall, a fraction of MRF in the bottle is attracted and shows a good magnetic response as the magnet moves.
As a magnetic suspension, the sedimentation stability of MRF was also investigated. The particle migration velocity of MRF was calculated by eq 2.
where V is the particle migration velocity, ρ p is the particle density, ρ c is the liquid density, ν is the kinematic viscosity of the liquid, g is the gravity constant, d represents the particle diameter, and φ is the volume fraction of MRF. The value of the particle migration velocity can be used to evaluate the sedimentation stability of MRF. 32 The kinematic viscosity of the MRF measured by the rheometer is 274.3 mPa· s. Besides, it was found that the particle−fluid density mismatch will affect the particle migration velocity. 33 The   In order to investigate the oil removal capacity of the MRF, the weight was measured to determine the efficiency of oil removal. As shown in Figure  8, the detailed experimental steps are as follows: first, 200 mL of fresh water was added to a 500 mL beaker, creating a water environment. In the next step, 2 g of lubricant oil was dropped into the middle of the water surface in the beaker to form oil slick (Figure 8a). The red dye (Sudan III) was used to label lubricant oil to observe the diffusion of oil on the water surface clearly and to confirm the position and shape of the oil slick. Then, a certain amount of MRF was added into the oil slick. It was found that when brought into contact with the oil slick, the MRF diffused in the oil phase and wrapped up the oil within a few seconds. Finally, oil−MRF mixtures floated on the water surface ( Figure 8b). As expected, the MRF completely repelled water and exhibited the property of hydrophobicity. Then, the oil−MRF mixture can be moved by the magnet easily ( Figure  8c), and it can be seen that most of the mixture was moved (Figure 8d). There still exists a small amount of oil on the water surface.
MRF can effectively remove oil slick on the water surface, which is mainly contributed by London dispersive forces. The London dispersive forces between the hydrophobic part of oil molecules and the carrier fluid are much stronger, whereas the interactions between the oil molecules and water molecules are much weaker. Therefore, the oil slick moves with the MRF because of the London dispersive forces. Figure 9 shows the oil removal efficiency of MRF in different types of water surfaces and demonstrates that MRF has excellent oil removal capacity. It can be found that on different types of water surfaces, the oil removal efficiency of the MRF is proportional to the mass of the MRF added in. After adding 0.5 g of MRF to the oil slick in the fresh water, the oil removal efficiency reached at 82.93%, indicating that a small amount of MRF already has good oil removal performance. Moreover,

ACS Omega
http://pubs.acs.org/journal/acsodf Article when the mass of the MRF is 2 g, the oil removal efficiency is 94.39%, suggesting that most of the oil slick can be removed from the water surface. Besides, it can be seen from Figure 9 that the oil removal efficiencies of the MRF (2.0 g) in salt water and deionized water are 92.71 and 93.63%, respectively. The high oil removal efficiency verifies that the MRF has promising applications in the treatment of oil slick. The oil removal efficiency of the MRF (1.0 g) on different types of water surface exceeds 90%. Comparing the results obtained when adding 0.5 g MRF, it was found that the oil removal efficiency (1.0 g MRF) increased rapidly. It is interesting that when the MRF is used to remove oil slick on three different types of water, the oil removal efficiency obtained by adding the same mass of the MRF is very close. Therefore, it can be asserted that water salinity has little effect on the oil removal efficiency of the MRF. 34 Figure 10 shows the oil removal capacity of the MRF prepared by the recovered Fe 3 O 4 /OA MPs after five cycles of oil−water separation. The efficiency of oil removal from different water surfaces only results in slight changes. The oil removal efficiency remains above 93%, which indicates that Fe 3 O 4 /OA MPs have good reusability. The excellent reusability of Fe 3 O 4 /OA MPs will play an important role in the removal of oil slick on the water surface.

CONCLUSIONS
In summary, Fe 3 O 4 /OA magnetic nanoparticles (MPs) were synthesized by the chemical coprecipitation method, which were coated with an oleic acid bilayer. Fe 3 O 4 /OA MPs has superhydrophobicity, superlipophilicity, unsinkability, superparamagnetism, thermal stability, and has been introduced into the preparation of oil-based MRF. In this study, an improved material was prepared for oil−water separation on the water surface. MRF can be used to recover oil slick on the water surface selectively and to realize the oil−water separation under the guidance of the magnet quickly and efficiently. The oil removal efficiencies of the MRF in salt water, fresh water, and deionized water were 92.71, 94.39, and 93.63%, respectively. It is worth noting that the efficiency of oil removal is still above 93% after five oil−water separation cycles, which indicates that Fe 3 O 4 /OA MPs have excellent reusability. Therefore, we believe that MRF will contribute to the removal of oil slick on the water surface in the future.   The oil removal capacity of the oil-based MRF can be determined by weight measurements. 35,36 First, oil was poured on the surface of water contained in a beaker. Then, the MRF was added to the oil slick surface. Then, the MRF spread and mixed with oil quickly. The final step was to separate the oil−MRF mixture from the water surface by a magnet. The oil removal rate of the MRF can be calculated according to eq 3.
where K is the oil removal rate, M 2 and M 1 are the total masses of oil, water, MRF, and beaker before and after oil removal, respectively. M 3 is the mass of oil slick added into the beaker. The weight of the oil removed was calculated from the difference between M 2 and M 1 .
Most of the contaminated oil in the oil−MRF mixture was separated by a high-speed centrifuge. After separating, the wet precipitate of Fe 3 O 4 /OA MPs was washed in anhydrous ethanol to remove the rest oil by ultrasonic treatment. The obtained wet precipitate of Fe 3 O 4 /OA MPs was used to reprepare MRF, which was used for the next oil removal step in order to investigate the reusability of Fe 3