Novel Coconut Oil Based Magnetite Nanofluid as an Ecofriendly Oil

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Novel Coconut Oil Based Magnetite Nanofluid as an Ecofriendly Oil Spill Remover M. Nabeel Rashin, R. Govindan Kutty, and J. Hemalatha* Advanced Materials Lab, Department of Physics, National Institute of Technology, Tiruchirappalli, Tamilnadu, India, 620 015 ABSTRACT: The removal of oil spills from a water surface has huge industrial importance, in view of ecological safety. A novel, ecofriendly coconut oil based magnetite nanofluid is prepared by the coprecipitation method, for removing oil contaminants from water. Magnetic particles with face centered cubic crystal structure and superparamagnetic behavior are prepared and used to get a stable suspension in coconut oil. The superhydrophobicity and superoleophilicity of the nanofluid are examined through contact angle measurements. The fluid offers a contact angle of 166° which is greater than the critical angle of 150°, indicating its superhydrophobic nature. This paper demonstrates the usage of the magnetite−coconut oil nanofluid in the removal of an oil spill from water with 91% efficiency. The mechanism is based on the selective absorbance property of superhydrophobic and superoleophilic nanofluids, which can collect contaminant oil while completely repelling the water molecules. When magnetic nanofluid is added to oil contaminated water, the nonpolar hydrophobic part of the contaminant oil gets attached to the carrier fluid (coconut oil) through London dispersive forces; this is then collected along with the magnetic nanofluid, by applying a magnetic field. The proposed method is an efficient biocompatible one that can be used on a large scale, for the cleanup of oil spills on water surfaces.

1. INTRODUCTION The oceans are polluted by oil every day from oil spills, routine shipping, drains, and dumping. The oil contaminant cannot dissolve in water and forms a thick sludge in the water, which can cause serious damage to the coastal environment and marine wildlife; also, it has long-term harmful impacts on aquatic ecosystems.1 A number of advanced response mechanisms, including mechanical, chemical, physical, and biological methods are available for controlling oil spills and minimizing their hazardous impacts on human health and the environment.2−6 There are simple techniques, such as skimming and burning, and also the advanced methods that use sophisticated dispersants, giant separators, natural or synthetic sorbents, and oil-eating microorganisms. Each of these techniques has its own merits and demerits.7 Thus, it is essential to promote complementary mechanisms, which can enable a quick and efficient clean up of the polluted water. It demands the synthesis of high performance smart materials having superhydrophobicity and superoleophilicity, which allow the proper disposal of oil without leaving a trace of secondary pollution.7,8 As per the available literature, there is a wide variety of oilremoval materials, based on carbon nanotubes, mesh films, sponges, etc.9−13 Although such materials exhibit an excellent absorption capacity, they have many limiting factors. It is difficult to remove the absorbed oil from them, which results in decreasing their oil-removal efficiency; 3 moreover, the adsorbent itself acts as a self-contaminant up to certain extent. Thus, it is a huge challenge to develop an excellent ecofriendly material and methodology to perform the oil removal efficiently. Recently, magnetic materials such as superhydrophobic core−shell Fe2O3@C nanoparticles14 and three-dimensional macroporous Fe/C nanocomposites15 have been found to provide a fast separation of oil from water by applying an appropriate external magnetic field. © 2014 American Chemical Society

Oil based magnetite magnetic nanofluid, a smart colloidal suspension of magnetite nanoparticles in an oil medium, possesses unique magnetic, thermal, and fluid properties.16−18 The superhydrophobic and superoleophilic nanofluids, with selective absorbance property, can collect contaminant oil while completely repelling the water molecules and can be used as an efficient tool to remove the oil contaminant. It can maintain good dispersion stability even under the influence of external fields, such as centrifugal and magnetic fields.2 Coconut oil has admirable biodegradability, environment friendliness, and nontoxicity,19,20 which makes it a suitable base fluid for ecofriendly oil removal. The present study encompasses the synthesis of stable magnetite−coconut oil nanofluid, and its utility as an ecofriendly oil spill remover.

2. MATERIALS AND METHODS 2.1. Materials. Single phase magnetite nanoparticles are synthesized by the coprecipitation method,21 using ferrous chloride (FeCl2) and ferric chloride (FeCl3) as the reactants. Double distilled water is used as the solvent, while oleic acid (C17H33COOH) is used as the surfactant. All the chemicals are of analytical grade, procured from Merck, India, and are used as purchased without further treatment. Pure coconut oil (Parachute brand from Marico Industries Ltd., Mumbai, India) and motor oil (Castrol India Ltd., Mumbai, India) are used for this study. 2.2. Methods. Aqueous solutions of 0.2 M ferrous chloride and 0.4 M ferric chloride are prepared independently. Then, both solutions are mixed together and stirred for 1 h to get a Received: Revised: Accepted: Published: 15725

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placed over a glass substrate which is kept horizontally in pure distilled water in a clean, transparent container. Five independent measurements are carried out, and the standard deviation is found to be within ±0.2 degrees. The efficiency of oil removal is realized qualitatively through digital photographs taken at every stage of the cleanup process, and it is further ensured, quantitatively, from the difference in weights of the contaminated water before and after employing the oil removal technique. The weight measurements of pure water, contaminated water, and water after the cleanup process are made by using a digital electronic balance. As the nonvolatile motor oil tends to have a lot of long-chain compounds and polycyclic aromatic hydrocarbons, it would be the most similar to typical crude oil and it is widely used23,24 as laboratory oil spill. Hence, it is used in the present work as the contaminant in the laboratory oil spill. A polythene covered permanent magnet of 0.35 T is used for the magnetic nanofluid assisted oil removal.

homogeneous mixture, for which the pH is found to be 2.6. Ammonia solution is added drop by drop to the mixture under mechanical agitation until its pH reaches 9. A black precipitate of Fe3O4, thus obtained, is washed five times with distilled water and with ethanol to eliminate impurities. 150 mL of water is then added to it, and the pH of the dispersion is adjusted to be 9 with the help of the ammonia solution. Eleven mL of oleic acid is added and stirred at 80 °C for 1 h to remove any excess ammonia. The oleic acid separates the Fe3O4 nanoparticles and keeps them in suspension in the water medium, preventing the agglomeration and forming an aqueous magnetite nanofluid. Coconut oil based magnetite nanofluid is obtained from the aqueous magnetite nanofluid by the phase-transfer method,22 the details of which are briefly discussed below. 100 mL of ethanol is added to the aqueous magnetite nanofluid and stirred for 5 min. After keeping the dispersion inert for 30 min, the Fe3O4 nanoparticles are precipitated and separated, using a permanent magnet. The treatment with ethanol is repeated persistently four times, to remove the outer layer of the surfactant, and consequently, single layered hydrophobic Fe3O4 nanoparticles are obtained. As a final step, 1 vol % of Fe3O4− coconut oil nanofluid has been prepared by ultrasonic treatment for 1 h. The response of the fluid to the external magnetic field is illustrated in Figure 1.

3. RESULTS AND DISCUSSION 3.1. Structural Morphological and Magnetic Studies. Figure 2a shows the XRD pattern21 of the uncoated Fe3O4 nanoparticles at room temperature; it exhibits typical reflections of (111), (220), (311), (222), (400), (422), (333), (440), and (533) planes, indicating the face centered cubic structure of Fe3O4. The strong and sharp reflection peaks show a high degree of crystallinity of the nanoparticles. All the peaks match well with the standard JCPDS 82-1533. No secondary peaks are detected in the XRD pattern, which ensures the phase purity of Fe3O4. The average crystallite size obtained using the Debye− Scherrer equation21,25 and the lattice constant (a) of Fe3O4 calculated using the inter planar spacing are 10 and 0.84 nm, respectively. The lattice constant is in good agreement with the values of nano Fe3O426 and the bulk Fe3O427 reported earlier. Figure 2b shows the TEM image of Fe3O4 nanoparticles synthesized through the coprecipitation route. The micrographs reveal that the magnetite particles are monodispersed and cubic in shape with an average size of 10 nm. This observation is in good agreement with the size estimated using the Debye− Scherrer formula from the XRD data. A magnetization study is made of nano Fe3O4 particles at room temperature, and the M−H plot is shown in Figure 2c. Because of their small size, the most favorable magnetic state of the nanoparticles is of a single domain. Therefore, the sample exhibits a saturation magnetization of 72 emu/g, with negligible remanent magnetization and coercivity, and an absence of hysteresis, with a typical superparamagnetic behavior. The critical diameter (dc) for monodomain formation in magnetic materials is estimated using the formula,21

Figure 1. Magnetite−coconut oil nanofluids in the (a) absence and (b) presence of a magnet.

dc ∼ 18( AK /μ0 MS2)

The prepared samples are characterized for phase identity by the X-ray diffraction method using Cu Kα radiation (λ = 1.541 Å) for 2θ values ranging from 10 to 80° in the X-ray diffractometer (Model Rigaku Ultima III). The magnetic properties of the nanoparticles are studied, using a vibrating sample magnetometer (Lake Shore, USA, Model 7404) with 15 kOe as the maximum applied magnetic field. The oil on water contact angle is measured with respect to the water surface, using a contact angle meter (Holmarc, India, Model no: HO-IAD-CAM-01A) by controlling the fluid flow at 0.07 μL per 30 s. For this measurement, using a microsyringe, a droplet of the sample (coconut oil, oleic acid, and ferrofluid) is

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where A is the exchange constant, K is the magnetic anisotropy, μ0 is permeability of free space, and MS is the saturation magnetization. For bulk magnetite, A = 1.3 × 10−11 J/m, K = 1.35 × 104 J/m3, and MS = 4.6 × 105 A/m, and hence, an estimated dc is about 28 nm. As the magnetite particles have a size of 10 nm, that is less than the critical diameter, 28 nm, it is obvious that all the particles are of single domain and should exhibit superparamagnetism. Also, the average diameter of the magnetite nanoparticles is measured by using the magnetization data in Langevin’s equation28,29 and is found to be 9.6 nm, 15726

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Figure 3. Optical image of the contact angles formed by sessile liquid drops of (a) oleic acid, (b) coconut oil, (c) coconut oil based Fe3O4 nanofluid (left), and (d) coconut oil based Fe3O4 nanofluid (right).

suggests the hydrophilic nature of magnetite. Figure 3c,d, shows that the Fe3O4−coconut nanofluid offers the contact angle of 166°, which is greater than that of the unmodified magnetite particle. This further confirms that the nanofluid also has strong superhydrophobicity and superoleophilicity and, therefore, can be utilized as a high performance smart material for ecofriendly oil removal. 3.3. Mechanism of Oil Removal. The removal of contaminant oil from a water surface is demonstrated in Figure 4. At first, the motor oil (contaminant) is poured into pure water in a beaker to replicate the oil spill. In the next step, superhydrophobic magnetic nanofluid is sprinkled on the surface of the contaminated water. As anticipated, when brought into contact with a layer of contaminant oil, the nanofluid quickly wraps up the oil while repelling the water. That is, the magnetic nanofluid molecules tend to associate with the molecules of the contaminant oil and form a pool of liquid, as seen in Figure 4c−e, within a few seconds or induced by mechanical agitation. The magnetic fluids (or surface coated nanomagnetic particles) neither sink nor attach with the water. This is due to the selective absorbent property of the superhydrophobic and superoleophilic nanofluid. The nanofluid bonded with the contaminant is then separated and collected from the water surface, with the help of a high field permanent magnet enclosed in a polythene cover.

Figure 2. (a) XRD pattern, (b) TEM image, and (c) M−H plot of magnetite nanoparticles.

which is in very close agreement with the value obtained by the Debye−Scherrer equation. 3.2. Surface Property Analysis. As the contact angle is the measure of hydrophobicity, the contact angle measurements of Fe3O4 nanofluid, oleic acid, and coconut oil drops are taken and shown in Figure 3. The fluids offer contact angles above the critical angle of 150°, indicating their superhydrophobic nature. Oleic acid with a contact angle of 172° and coconut oil with 170° prove their excellent superhydrophobicity, that makes them an unsinkable scaffold to nano-magnetite. The contact angle of the magnetite nanoparticle is reported2 as 32°, which 15727

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Figure 4. Optical images describing the removal of motor oil dispersed on the water surface, using coconut oil based magnetic nanofluid and a high field magnet: (a) Motor oil and pure water, (b) the contaminant dispersed on the water surface and magnetic nanofluid in a syringe, (c) after dropping the magnetic nanofluid onto the contaminated water, (d, e) oil and water mixture after agitation, top view and side view, (f, g) magnet dipped into the magnetized oil contaminant, (h) magnetized oil contaminant attaching to the magnet, and (i) cleaned water and collected contaminant.

magnetic fluid attaches with the contaminant oil and makes it magnetic in nature. Then, using a powerful magnet, the magnetic nanofluid along with the contaminant oil can be separated easily, as shown in Figure 5. 3.4. Efficiency of the Removal. The oil removal capacity of the magnetic nanofluid is determined quantitatively by

The mechanism of oil removal can be explained as follows. When magnetic nanofluid is added to the oil contaminated water, the nonpolar hydrophobic part of the contaminant oil gets attached with the carrier fluid (coconut oil) through London dispersive forces.30 The London dispersion forces arise from the instantaneous dipoles that occur from random, momentary shifts in charge, caused by the constant movement of the electrons. The instantaneous dipoles allow the attraction between two nonpolar molecules, as the positive instantaneous dipole from one molecule is attracted to an induced negative instantaneous dipole of the second molecule. The nonpolar molecules generally interact by London dispersion forces and are induced dipole−induced dipole interactions. These interactions become stronger as the molecules are larger and have similar polarizabilities. Moreover, the interaction becomes stronger as more molecules are bound together. In contrast, the interactions between the oil and the water molecules are weak, because the oil cannot make a hydrogen bond, as it is nonpolar, and water cannot form strong London forces with the oil due to the difference in polarizability. In addition, the stronger association among the polar entities leads to a tendency for the nonpolar materials to group together. By this means, the

Figure 5. Mechanism of contaminant removal: (a) Contaminant oil molecules attaching with the magnetic nanofluid, (b) uniform distribution of the magnetized contaminant oil over the water surface, and (c) collection of the magnetized fluid using a magnet. 15728

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weight measurements. The weights of pure water (mpw), water with the contaminant (mcw), and contaminant free water (mfw) after the cleanup process are measured and shown in Figure 6. The efficiency (η) of oil removal is estimated by eq 2 and is found to be 91%. m − mfw η = cw mcw − mpw

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-431-2503608. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the DST, Government of India, for the VSM facility under the FIST program sanctioned to Department of Physics, NIT, Tiruchirappalli.

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REFERENCES

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Figure 6. Weight change observed during the sequence of the experiment.

Thus, the coconut oil based magnetite nanofluid offers an efficient means for the fast removal of the contaminant oil. Another remarkable feature is its excellent biocompatibility and its capacity to avoid secondary contamination. It can be used for the ecofriendly removal of even thick layers of low, as well as, high viscosity hydrophobic oil spills on water surfaces on a large scale, by applying a strong external magnetic field. The surface modified magnetic particles can be removed from the collected oil, can be redispersed in coconut oil, and can be reused. The coconut oil and the contaminant oil can be separated to recover their values with the help of oil separators based on centrifugation and liquid chromatography. Even though the oil uptake capacity of the nanoparticles is low when compared to the solid based sorbent systems, they still possess the advantages of easy production and storage, fast distribution and collection, low cost, good recyclability, high resistance to corrosion, thermal stability, and environmental friendliness.2,14 Thus, this system offers a novel method for the ecofriendly removal of hydrophobic oil spills on water surfaces.

4. SUMMARY Biocompatible magnetic nanofluid of fcc magnetite has been primed and is found to be very stable even in strong magnetic fields. Exploring their stability, superhydrophobicity, and superoleophilicity, the authors have demonstrated an ecofriendly system, which when sprinkled on water makes a bond with the contaminant oil by selective absorbance. Further, the magnetized contaminant is collected by a high field magnetic bar. Thus, the present work provides an innovative insight into a highly efficient ecofriendly oil removal mechanism for water cleanup applications. 15729

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