Functionalized Magnetite Nanoparticles Using Two New Ionic Liquids for Efficient Oil Spill Cleanup

Recently, magnetite nanoparticles (MNPs) have gained great attention for oil spill cleanup due to their unique properties, e.g., high oil removal efciency, high surface area, and response to an external magnetic feld. Te efciency of MNPs for oil spill uptake can be enhanced by functionalizing their surface using diferent materials. Furthermore, the functionalization of MNP surface using these materials promotes their chemical stability. Tis study aims to functionalize the surface of magnetite nanoparticles (MNPs) using two newly synthesized hydrophobic ionic liquids (ILs) and apply them for oil spill cleanup. ILs were synthesized by the reaction of glycidyl-4-nonyl ether (GE) with fatty amines, either octadecylamine (OA) or dodecylamine (DA), to yield the corresponding amines, GEOA and GEDA, respectively. GEOA and GEDA were quaternized with acetic acid (AA) to produce the corresponding ILs, GEOA-IL and GEDA-IL. Te produced ILs, GEOA-IL and GEDA-IL, were applied for the surface modi-fcation of magnetite nanoparticles (MNPs), producing surface-modifed MNPs, GO-MNPs and GD-MNPs, respectively. GO-MNPs and GD-MNPs were characterized using Fourier transform infrared, X-ray difraction, transmission electron microscopy, dynamic light scattering, contact angle, and vibrating sample magnetometer. Teir oil removal efciency (ORE) was investigated at diferent MNP :crude oil ratios ranging from 1:1–1: 50. GO-MNPs and GD-MNPs showed high ORE even at low MNP ratios. Furthermore, GO-MNPs showed higher oil removal efciency than GD-MNPs, which may be explained using GEOA-IL containing a longer alkyl chain for MNP surface modifcation in comparison to GEDA-IL. Furthermore, GO-MNPs and GD-MNPs displayed excellent reusability in fve cycles, with a slight decrease in ORE with increasing cycles.


Introduction
Oil spills in aquatic environments impose a major threat on these environments, where a signifcant portion of oil is emulsifed, making its separation from the water phase extremely difcult.Oil spills afect the seaside fora, fauna such as birds and bivalve mollusks, and human health due to oil-contaminated food via the food chain.Some oil components in contaminated food can be considered a carcinogen [1,2].In addition, the contact area between water and the atmosphere is reduced as a result of oil spills afecting the chemical oxygen demand (COD) and biochemical oxygen demand (BOD), thereby hindering the respiration of aerobic marine organisms and photosynthesis in marine plants [3].Oil spills occur during oil extraction, transportation, and oil transfer between vessels, resulting in tremendous environmental impacts [3,4].From 2000 to 2011, over 224.000 tons of oil were released into the seas and oceans [5].Although a range of oil removal methods is available, including physical, chemical, and biochemical, these methods have some limitations, such as high cost, low efciency, and the fact that they contribute to other secondary pollutants [6].
Tere is an urgent need for a technique that has the potential to overcome all of these limitations of conventional techniques without increasing energy consumption.Tis technique does not contribute to pollution, is easy to use regardless of the weather conditions, and is efcient [7].
Recently, nanotechnology was applied signifcantly to combat the oil spill disaster.Several types of nanomaterials were reported for oil spill cleanup, including organic nanoclays, iron oxide (Fe 3 O 4 and Fe 2 O 3 ) nanoparticles, carbon-based nanoparticles (graphite, graphene, carbon nanotubes, etc.), and TiO 2 [7].In particular, magnetite nanoparticles (MNPs) are characterized by high oil removal efciency.Using MNPs, oil can be easily recovered from the water's surface.Furthermore, MNPs are characterized by their biodegradability, nonsinking properties, and reusability [8][9][10].Surface modifcation of MNPs improves their stability by creating a protective flm on their surface that prevents air oxidation.In addition, it prevents their agglomeration due to their magnetic nature and enhances their efciency in removing oil spills [6,11].Several organic and inorganic materials have been used to modify the surface of MNPs [12].Te use of hydrophobic materials, particularly for the surface modifcation of MNPs, often enhances their dispersity in oil more than in water.Tis increases their interaction with oil components and thereby increases their oil removal efciency [13].
Ionic liquids are organic salts having melting points lower than 100 °C.ILs have a variety of unique properties, e.g., low melting point and high mechanical and thermal stability, which make them suitable for MNP surface modifcation.Furthermore, due to their ionic nature, these materials can work efciently even in harsh saline conditions where conventional materials cannot work.Limited studies reported the application of surface-modifed MNPs using ionic liquids (ILs) for oil spill cleanup.Te surface-modifed MNPs using ILs exhibited excellent performance in oil spill cleanup compared to MNPs modifed using other organic materials [14,15].
Herein, two newly synthesized hydrophobic ILs were synthesized and used to modify MNP surfaces.First, glycidyl-4-nonyl ether (GE) was reacted with either octadecylamine (OA) or dodecylamine (DA) through a ringopening reaction to obtain the corresponding amines, GEOA and GEDA.Next, the produced amine, GEOA or GEDA, was quaternized with acetic acid (AA) to yield the corresponding ILs, GEOA-IL and GEDA-IL.Finally, the assynthesized ILs, GEOA-IL and GEDA-IL, were used to functionalize the surface of MNPs to obtain GO-MNPs and GD-MNPs, respectively.Te chemical structures, thermal stability, particle size, morphology, and magnetic properties of the as-synthesizedGO-MNPs and GD-MNPs were investigated using two diferent methods.In addition, the efciency of GO-MNPs and GD-MNPs for oil removal was evaluated using diferent MNP : crude oil ratios.
Crude oil was supplied by Aramco, Riyadh, Saudi Arabia.Its API and contents (weight%) of saturates, aromatics, resins, and asphaltenes (SARA) are 20.8 °, 16.3, 25.3, 48.1, and 8.3, respectively.Its full specifcation was reported in our earlier work [16].Brine solution (35 g/L) was prepared in our lab using double distilled water and sodium chloride.

2.2.
Synthesis of GEOA-IL and GEDA-IL.Glycidyl-4-nonyl ether (GE) (3 g, 10.85 mM) was stirred and heated with either OA (2.62 g, 10.85 mM) or DA (2.01 g, 10.85 mM) at 100 °C in a nitrogen atmosphere for 3 h to obtain the corresponding amines, GEDA and GEOA [17].Next, GEDA or GEOA was stirred and heated with equimolar AA at 100 °C for 5 h in a nitrogen atmosphere.Te obtained mixtures were cooled to room temperature and dissolved separately in isopropyl alcohol, and the unreacted AA was salted out using a supersaturated sodium chloride solution.Finally, the organic layer was separated, and isopropyl alcohol was evaporated under reduced pressure to obtain the corresponding ILs, GEOA-IL and GEDA-IL, as viscous liquids with yields of 98.8% and 98.4%, respectively.Te GEOA-IL and GEDA-IL synthesis route is presented in Scheme 1.

Synthesis of GO-MNPs and GD-MNPs.
For the synthesis of GO-MNPs and GD-MNPs, IL solution (4 g of either GEOA-IL or GEDA-IL dissolved in 200 mL of ethanol) was mixed and stirred with FeCl 3 and FeCl 2 solution (6.36 g of FeCl 3 and 3.9 g of FeCl 2 dissolved in 200 mL of distilled water) in a three-neck bottom fask supplied with a nitrogen inlet and thermometer.Te obtained mixture was heated at 50 °C, and ammonium hydroxide solution (28%) was added dropwise while stirring continuously, reaching pH 10.After adding NH 4 OH, the mixture was stirred for 1 h to ensure the reaction completeness.Te surface-modifed MNPs were separated using an external magnet, washed several times with ethanol, followed by distilled water, and dried at ambient temperature [18].Te overall reaction of the preparation of the surface-modifed MNPs can be carried out according to the following equation: 2 Journal of Chemistry Te hydrophobicity of GO-MNPs and GD-MNPs was examined using contact angle (CA) measurements as follows: 0.5 g of either GO-MNPs or GD-MNPs was dispersed in a small amount of chloroform and spread over the glass slide surface, and then chloroform was evaporated.A thin, dry flm was obtained on the glass slide surface by repeating this step many times.Te CA of seawater droplets on the prepared glass slide was evaluated using a drop-shape analyzer [19].

Efciency of GO-MNPs and GD-MNPs for Oil Spill
Removal.In a 100 mL beaker, 200 mg of heavy crude oil sample was injected on the surface of 70 mL of seawater.Over the surface of crude oil, diferent amounts of MNPs samples (ranging from 4 mg to 200 mg) were spread out and kept for 10 minutes.Ten, a block neodymium magnet (4300 Gauss) covered with a known weight plastic flm was used to remove MNPs with adsorbed oil on their surfaces.Both the MNPs with adsorbed oil on their surfaces and the plastic flm were recovered.Tey were frozen, lyophilized for 24 h to get rid of dragged water, and fnally weighed to calculate the mass of removed oil [3].All experiments were conducted in triplicate.Tis method was considered method A, and the oil removal efciency (ORE) was calculated using the following equation: ORE% � mass of collected oil mass of original oil × 100. ( Te ORE was also evaluated by a diferent method.First, 10 minutes after the dispersion of MNPs on the crude oil surface, the MNPs with adsorbed oil were collected on the inside wall of the beaker, where the external magnet was placed outside the beaker's wall.Terefore, the MNPs with adsorbed oil were collected on the inside wall of the beaker.Ten, the water and residual crude oil were poured into a separating funnel keeping external magnet attached to the wall of the beaker, and the residual oil was extracted with chloroform.In the following step, chloroform was evaporated under reduced pressure, and the mass of the residual crude oil was recorded.Tis method was considered method B, and the ORE was calculated using the following equation: ORE% � mass of original oil − mass of residu al oil mass of original oil × 100. (

Journal of Chemistry
For reusing GO-MNPs and GD-MNPs, the crude oil was removed from the surface of the collected MNPs by washing them using chloroform, followed by acetone.Finally, the obtained MNPs were dried in the air and reused in a new cycle.

Chemical Structures of the As-Synthesized ILs and MNPs.
Te chemical structures of the as-synthesized ILs, GEOA-IL and GEDA-IL, were confrmed using FTIR and 1 H-NMR as presented in Figures 1(a) and 2, respectively.In Figure 1, the stretching absorption bands of hydroxyl groups appear at 3316 cm −1 .C-H aliphatic's stretching and bending bonds appear at 2924, 2855 cm −1 , and 1465 cm −1 .Te stretching absorption bands of aromatic bonds, a =C-H and C=C, are assigned at 3050 cm −1 , 1608 cm −1 , and 1513 cm −1 , respectively.Te stretching absorption bands of aliphatic and aromatic C-O are observed at 1120 cm −1 and 1042 cm −1 , respectively.In the 1 H-NMR spectra (Figures 2(a) and 2(b)), the protons of the alkylamine and alkyl side chain of GE resonated between 0.7 and 3 ppm [17].Te protons of the acetate group, +NCH 2 and OCH, appear at 1.97 ppm, 3.95 ppm, and 4.1 ppm, respectively.Furthermore, the protons of the phenyl ring and +NH 2 are observed at 6.8 ppm, 7.2 ppm, and 8.8 ppm, respectively.
Te chemical structures of as-synthesized MNPs, GO-MNPs and GD-MNPs, were also investigated using FTIR and XRD.Te FTIR spectra of GO-MNPs and GD-MNPs (Figure 1(b)) show the same functional groups as GEOA-IL and GEDA-IL with low intensity, confrming the functionalization of MNP surfaces with these ILs.Furthermore, the appearance of new intensive bands at 568 cm −1 and 565 cm −1 for GO-MNPs and GD-MNPs, respectively (Fe-O), confrms the formation of pure magnetite without any other iron oxides.Figure 3 220), (311), (400), (422), (511), and (622) [20].Te data indicate that the crystal structures of GO-MNPs and GD-MNPs were not afected by their surface modifcation with the constituents of as-synthesized ILs.

Termal Stability of GO-MNPs and GD-MNPs.
Te TGA thermogram of GO-MNPs and GD-MNPs is presented in Figure 4.As shown in the fgure, there is no weight loss up to 200 °C.Te data also showed that the degradation of GO-MNPs and GD-MNPs occurs in two main steps.Te frst primary degradation step was between 200 °C and 420 °C which can be linked to the degradation of IL organic constituents.Te second degradation step was between 550 °C and 700 °C.Te degradation in this region is due to changes in the inorganic core where the MNPs at such temperature are converted to c-hematite or nonstoichiometric wustite (Fe (1 − x) O) [21].Te percentage of weight loss for GO-MNPs and GD-MNPs at 800 °C was 11% and 13%, respectively, which refects the relative decrease in the amount of IL on the MNP surface in the case of GO-MNPs as compared to that of GD-MNPs (Figure 4).

Particle Size of GO-MNPs and GD-MNPs.
Figures 5(a) and 5(b) represent the TEM images of GO-MNPs and GD-MNPs.As seen in the fgure, the MNPs are arranged in cluster form, which could be attributed to their magnetic nature, where these particles attract each other [22].Te TEM images also showed that the MNPs appeared in irregular shapes with average diameters of 10 nm and 8.5 nm for GO-MNPs and GD-MNPs, respectively.Te particle size (PS) of GO-MNPs and GD-MNPs was also measured in ethanol using the DLS technique, as depicted in Figures 6(a) and 6(b).Te average diameters determined by DLS for GO-MNPs and GD-MNPs are 690.7 nm and 193.2 nm, respectively.Te diference in the particle size values measured by TEM and DLS refects the behavior of GO-MNPs and GD-MNPs in ethanol, where they tend to agglomerate in clusters due to their magnetic nature.Furthermore, the increase in the PS of GO-MNPs measured by DLS can be attributed to an increased magnetization value that enhances their attraction to each other compared to that of GD-MNPs, as will be explained later.

Contact Angle Measurement. Te hydrophobicity of the
MNPs used for oil spill removal is crucial because it supports the dispersion of these nanoparticles in crude oil.It also reduces their water dispersion and thereby enhances their interaction with crude oil constituents.CA measurements are one of the most common methods of determining hydrophobicity.Te high CA values of water droplets on surfaces (>90 °) indicate the hydrophobicity of these surfaces, and as the hydrophobicity increases, the CA value increases.Herein, the CA of water droplets on the surface of GO-MNPs and GD-MNPs was measured as it was reported in the Experimental section.Figures 7(a) and 7(b) show the CA of seawater droplets on the surfaces of GO-MNPs and GD-MNPs, respectively.As shown in the fgure, the CA values of seawater droplets on the surfaces of GO-MNPs and GD-MNPs were 128.4 °and 120.3 °, respectively.Tese values indicated an increase in the hydrophobicity of GO-MNPs and GD-MNPs.Tis increase in hydrophobicity can be related to the functionalization of MNPs with hydrophobic ILs.In addition, GO-MNPs showed higher hydrophobicity than GD-MNPs due to the presence of longer alkyl chains in GEOA-IL used for the functionality of MNPs surfaces compared to those present in GEDA-IL.

Magnetic Characterization.
To retrieve the MNPs with oil adsorbed on their surfaces, they must respond to an external magnetic feld.GO-MNPs and GD-MNPs were successfully collected using an external magnetic feld during this study.Te response of these MNPs to an external magnetic feld was also investigated using VSM, as shown in Figure 8.As depicted in the fgure, the magnetization values are 56.7 emu/g and 49.4 emu/g for GO-MNPs and GD-MNPs, respectively.Tese data indicated the ability of 4 Journal of Chemistry GO-MNPs and GD-MNPs to respond to the external magnetic feld.

Oil Removal Efciency (ORE) of GO-MNPs and GD-MNPs.
As reported earlier, ILs used for MNP surface modifcation signifcantly enhance MNPs' performance for oil spill uptake compared to other organic materials [14,15].Terefore, in this work, the newly synthesized ILs, GEOA-IL and GEDA-IL, were employed for the MNPs' surface modifcation to enhance their performance for oil spill uptake.Te obtained surface-modifed MNPs, GO-MNPs and GD-MNPs, showed efective dispersion in low polar solvents with no dispersion in water and response to an external magnetic feld, indicating their ability to serve in crude oil removal.Herein, the ORE values of GO-MNPs and GD-MNPs were evaluated using two diferent methods, as reported in the Experimental section.Figures 9(a  Journal of Chemistry fgure, the ORE increased as the ratio of MNPs increased.In addition, GO-MNPs exhibited higher ORE than GD-MNPs at all ratios.Tis could be linked to an increase in hydrophobicity of GO-MNPs compared to GD-MNPs due to the presence of longer alkyl chains in GEOA-IL used for the functionality of MNP surface compared to those present in GEDA-IL.Te data also showed the diferences between the ORE values measured using methods A and B. At MNP : crude oil ratios of 1 : 1 and 1 : 2, the ORE values seem to be similar, while with a decrease in the MNP ratios, the difference increased signifcantly, whereas the ORE values in method B decreased with a decrease in the MNP ratios.In method B, with a decrease in the MNP ratio, the attraction between these nanoparticles and the external magnetic feld decreases where the magnet is placed outside the beaker.Tis leads to an increase in the distance between MNPs and the magnet.
Table 1 compares the MNPs' ORE values in the current work, the ORE values of untreated MNPs, and the ORE values of surface-modifed MNPs using diferent organic materials reported earlier.Te data showed that the surfacemodifed MNPs, GO-MNPs and GD-MNPs, showed higher

Journal of Chemistry
ORE values than the untreated MNPs.Tese data indicated the role of GEOA-IL and GEDA-IL in enhancing the efciency of the surface-modifed MNPs for oil spill uptake.Furthermore, the surface-modifed MNPs using ILs in this work and those reported earlier achieved higher ORE values than the surface-modifed MNPs using other organic materials.Tese data refect the role of ILs in improving the dispersity of MNPs in crude oil and their interaction with crude oil constituents, thus enhancing their performance for oil spill uptake.
Figures 10(a)-10(d) show the oil spilled over seawater, the dispersed GO-MNPs over the crude oil at MNP : crude oil ratio of 1 : 2, and the cleanness of seawater after the removal of oil with methods A and B, respectively.According to the fgure, in both methods, GO-MNPs were highly efective for oil removal and obtaining clean seawater.

Reusability of GO-MNPs and GD-MNPs.
Te reusability of MNPs is one of the most signifcant parameters since they can be easily retrieved with an external magnetic feld.Terefore, the reusability of GO-MNPs and GD-MNPs was investigated in fve cycles, as presented in Figure 11, using a ratio of 1 : 4 (MNP :crude oil).Te data indicated that the ORE of MNPs is reduced slightly with increasing cycles.For example, in the frst cycle, the ORE values were 96% and 93%     10 Journal of Chemistry for GO-MNPs and GD-MNPs, respectively, while they declined to 87% and 85% in the ffth cycle.Tis is possibly due to changes in the hydrophobicity of MNPs with an increasing number of cycles [14].

Conclusion
Tis work aimed to functionalize the surface of MNPs using two unprecedented ILs and apply them for oil spill removal.
For that, the epoxy ring of GE was opened with fatty amines, either OA or DA producing the corresponding amines, GEOA and GEDA, respectively.Following this, the produced amines, GEOA and GEDA, were quaternized with AA producing the corresponding ILs, GEOA-IL and GEDA-IL, respectively.GEOA-IL and GEDA-IL were used to functionalize MNP surfaces to generate GO-MNPs and GD-MNPs, respectively.Te synthesized MNPs, GO-MNPs and GD-MNPs, were characterized using diferent techniques, including FTIR, XRD, TGA, TEM, DLS, CA, and VSM.FTIR analysis confrmed the functionality of MNPs with ILs, GEDA-IL and GEOA-IL.Te TEM micrographs indicated the nanostructures of GO-MNPs and GD-MNPs with average PS diameters of 11 nm and 8 nm for GO-MNPs and GD-MNPs, respectively.Te DLS data exhibited higher PS than that measured with TEM due to the magnetic nature of MNPs in ethanol since they tend to agglomerate.Te CA data confrmed the hydrophobicity of GO-MNPs and GD-MNPs.GO-MNPs had higher hydrophobicity than GD-MNPs due to an increase in the hydrophobicity of IL (GEOA) used to functionalize the surface of MNPs when compared to GEDA-IL.Te VSM data indicated the response of GO-MNPs and GD-MNPs to the external magnetic felds.Due to the hydrophobicity of GD-MNPs and GO-MNPs and their response to the external magnet, their ORE values were investigated at diferent MNP : crude oil ratios using two methods.Te data exhibited increased ORE as the MNP ratio increased.In addition, GO-MNPs showed higher ORE than GD-MNPs, which could be linked to increased hydrophobicity due to the use of GEOA-IL containing longer alkyl chains (C18) for the functionality of MNPs than that of GEDA-IL (C12).However, the ORE values produced using the two methods seem similar at high MNP ratios, while the ORE values in method B decrease signifcantly as MNP ratios decreases, which could be linked to a decrease in the attraction between MNPs and the external magnetic feld, especially with the distance between them where the magnet was placed outside the beaker.Furthermore, GO-MNPs and GD-MNPs displayed efective reusability for crude oil removal in fve cycles with a limited decrease in their efciency with increasing cycles, which could be ascribed to the alteration in their hydrophobicity.Finally, the low cost of the synthesized MNPs due to the use of cheap raw materials in short routes, synthesis under mild conditions, their high oil removal efciency, and their reusability prompt their production in a commercial amount and use them for oil spill combat.

) MNPs IL 2 . 4 .
Characterization.Fourier transform infrared (FTIR) and proton nuclear magnetic resonance ( 1 H-NMR) were performed to confrm the chemical structures of the assynthesized ILs.In addition, FTIR and X-ray difraction (XRD) were conducted to confrm the chemical structures of the as-synthesizedGO-MNPs and GD-MNPs.Transmission electron microscopy (TEM) was used to investigate the morphology of GO-MNPs and GD-MNPs.Dynamic light scattering (DLS) was used to investigate the particle size (PS) of GO-MNPs and GD-MNPs in ethanol.A vibrating sample magnetometer (VSM) was employed to confrm the magnetic properties of GO-MNPs and GD-MNPs.
represents the XRD pattern of GD-MNPs and GO-MNPs.Te fgure shows apparent characteristic peaks at 2 theta values of 30.6 °, 36.2 °, 43.3 °, 54.4 °, 57.4 °, 62.8 °, and 75.1 °corresponding to the cubic spinel structure of Fe 3 O 4 as ( ) and9(b)   show the ORE at diferent ratios of GO-MNPs and GD-MNPs using methods A and B, respectively.As shown in the

Figure 10 :
Figure 10: Optical images of (a) oil spilled over seawater, (b) dispersed GO-MNPs over the crude oil at MNP : crude oil ratio of 1 : 2, (c) oil removal using method A, and (d) oil removal using method B.

Figure 11 :
Figure 11: Reusability of GO-MNPs and GD-MNPs using MNP : crude oil ratio of 1 : 4 using method A.

Table 1 :
Comparison between the ORE values of MNPs prepared in the current work, ORE values of untreated MNPs, and ORE values of surface-modifed MNPs using diferent organic materials.