Enhanced oil-spill removal and recovery from water bodies using diatomaceous earth and C18-silane-grafted polyurethane

This research focuses on the novel synthesis of polyurethane (PU) foam surface functionalized with diatomaceous earth (DE) particles and non-fluoro octadecylsilane (C18), for the use in enhanced clean-up of oil spill contaminants from water. The modified PU foam has improved hydrophobicity, wettability, water repellency, and biodegradability, which eliminates some of the drawbacks of PU such as its hygroscopic nature, limited hydrophobicity, ecotoxicity, and less biodegradability. The modified PU foam has been characterized by scanning electron microscopy to understand the microstructural changes during the surface modifications, Fourier transform infrared spectroscopy to track the integration of functional groups, X-ray crystallographic study to indicate the increase in the crystallinity of the resultant foam due to the incorporation of silane, and thermogravimetric analysis to understand the thermal stability and to calculate the thermal mass loss during the chemical modification. Furthermore, to test the enhanced hydrophobicity and oil spill clearance from water, the water contact angle has been measured and crude oil absorption capacity has been tested. The results show increased water repellency attributed to the strong hydrophobicity, and about 2.13 folds of increased crude oil absorption in comparison to the unmodified PU foam. Hence, the results collectively suggest the use of the synthesized surface-modified PU foam with superior hydrophobicity, water repellence, and surface wettability as a potential candidate for enhanced crude oil absorption from water bodies.


Introduction
Globally, oil spill disasters are rising to prominence as a potentially serious environmental issue as it imposes a significant negative impact on the marine or freshwater ecosystem [1]. Accidental oil spills occur all over the world during the critical stages of production, transportation, and storage, due to human errors, equipment failures, explosions or a natural disaster like an earthquake or storm, and result in the unintentional release of millions of gallons of crude petroleum oil or refined oil products, into the ocean or fresh water like streams or lakes. Some of the most serious oil spills of the past include the Torrey Canyon Oil Spill (UK) in 1967, Amoco Cadiz spill (France) in 1978, SS Atlantic Empress spill (Caribbean) in 1979, Ixtoc 1 oil well spill of Mexico in 1979, Exxon Valdez (Alaska) in 1989, MT Haven spill in Italy (1991), ABT Summer near Angola in 1991, and Gulf of Mexico oil spill in 2010. In response to these major oil spills, several national and international regulations were implemented. As a result, the number of major oil spill accidents, releasing greater than 700 tonnes of oil, reduced to 1.8 tonnes per year in 2010s in comparison to 1970s (24.5 tonnes per year) as per the International Tanker Owners Pollution Federation (ITOPF) report in 2019. Nevertheless, the minor accidental spills are still very common and act as a very serious environmental issue due to the detrimental effects of the hydrocarbons [2].
The United Arab Emirates is an important crude oil reserve of the world and one of the prominent oil development regions of the Arabian Gulf. Consequently, oil spill incidents in the UAE marine environment are a potential environmental threat. For instance, the major oil spill of Iranian crude oil into the Gulf of Oman, despite all efforts of remediation and containment, spread to 20 km of the shoreline of UAE waters resulting in serious environmental problems [3,4].
The spilled oil floats on the water surface that spreads up to form a thin layer of an oil slick that remains for a very long time, even decades or years, or forms a thick layer of heavy oil that reaches the ocean floor or reaches the beaches or shorelines, leading to air, water, and soil contamination. Consequently, the aquatic life, including the fishes, turtles, mammals, birds, plants, and corals, faces a major health threat. The prolonged exposure to the harsh polluted environment almost renders the organisms in a harmful habitat and almost impossible to survive [3][4][5].
Oil spill accidents require extensive remediation and cleaning due to long-term environmental destruction, leading to ecological risks and damages and economic losses faced by the marine and tourist industries [4]. Whenever there is an oil spill, it calls for the urgent requirement to control the spread of the oil, followed by the removal and disposal. In general, to mitigate the impact of oil spills, several physical, chemical, and biological remediation methods have been significantly developed and employed [6,7]. Some of the commonly used approaches include in situ burning [8], mechanical removal using booms [9], skimmers [10,11] or vacuum [12], chemical treatment using dispersants [13] or sorbents, and biological microbial remediation [14,15]. However, extensive research is still happening to identify the best material/ technique for effective oil spill clean-up and recovery.
One of the widely used ideal materials for active oil removal and recovery is the chemical porous absorbent materials, which are simple to use, inexpensive, non-toxic, and with better re-usability [16,17]. There are two broad categories of porous chemical absorbent materials: organic sorbent materials from natural fibers and synthetic sorbents.
The organic sorbents are environmentally friendly as they are based on natural fibers such as sugarcane [18], bamboo [19], peat [20], corn stalks [21], sawdust [22], vegetable fibers [23], or other natural sorbent materials like chitosan, bentonite, and activated carbon [24]. Organic sorbents are biologically degradable, renewable, and have a higher absorption capacity but have some practical limitations in applications. Such as a lack of hydrophobicity for selective oil absorption that leads to the absorption of both oil and water, resulting in the sinking of the sorbent. The natural fibers are modified with chemical moieties to overcome this limitation to enhance hydrophobicity and oleophilicity.
There are also several synthetic absorbent materials, particularly polymer-based materials such as polyurethane, polypropylene, polyethylene, and other cross-linked polymers, which are hydrophobic by nature and exhibit an excellent potential to capture oil-based pollutants due to the porous structural features [25][26][27]. Synthetic sorbents are considered one of the best agents to remediate oil pollution because most absorb up to 30 times of oil of their own weight [6].
Among the synthetic polymeric sorbents, PU foams are considered as one of the most preferred candidates, as they are very light due to low density and exhibit a high degree of porosity, which are excellent desirable features that make them ideal agents for oil removal [27][28][29]. One of the limitations of the use of PU is the presence of chemical moieties that absorbs water, which can be very easily tackled by chemical modification of PU for enhanced hydrophobicity as well as oleophilicity.
Several studies focus on different chemical modifications of PU for good oil removal. One of the approaches is copolymer grafting PU with oleophilic monomers to decrease water sorption and increase oil sorption. Hua Li et al. [27] have prepared PU foams with grafting copolymer with various oleophilic monomers (long-chain Lauryl Methacrylate (LMA)) with divinylbenzene and toluene as the initiator and solvent, respectively. The resulting modified PU form enhanced the sorption of oil and 50% decreased water sorption. A similar study by Li H in which the PU samples were grafted with LMA or coated with LMA microspheres resulted in the reduction of water sorption by 24-50% and an increase oleophilicity by 18-27% for diesel or kerosine oil [28].
Researchers also focus on PU composites and eco-friendly natural materials for enhanced oil removal. Oribayo et al. [30] developed a lignin-based PU foam modified with graphene oxide for enhanced oil spill clearance applications. Ren L et al. [31] successfully developed biomass carbon-based PU foam composites that exhibited oil absorption even after five consecutive cycles of absorption without any decreasing effect in absorbing the oil contaminants. Another interesting study based on PU natural material composite includes rice straw residues filled with PU matrix [11] and lignin-based PU with carbon nanotubes used as photothermal sorbents for highly viscous oils [32].
Incorporating natural superhydrophobic/hydrophobic silica-based materials into a PU foam matrix is similar to copolymer grafting PU with oleophilic monomers to achieve better oleophilicity and hydrophobicity. Sepiolite-coated PU foam [33], flexible PU foam with zeolitic imidazole framework [34], fluorinated DE-coated PU foam [35], and other PU silica composites [36,37] were found to present excellent oil absorption capacity and improved water repellence.
Our previous studies on the chemical modification of PU with alkyltrimethoxysilane and fluorosilane-modified DE showed improved surface roughness and a significant increase in the water contact angles, resulting in superhydrophobicity for efficient oil absorption applications [38][39][40][41]. Even though fluoro silanes provide significant oil removal benefits, they are associated with substantial health hazards in humans, animals, and birds. This research intends to use eco-friendly DE to modify the PU foam in addition to the fluorinated organic compounds, which considerably increases the hydrophobicity, yet still, decreases the ecotoxicity and enhances the biodegradation, making the modified PU better than the earlier PU modifications with only fluorinated organic compounds.
The current research focuses on the synthesis of PU foam grafted with DE particles and C18 for enhanced crude oil adsorption that overcomes the toxic impact of using organo fluoro compounds. The study mainly intends to analyze the deposition of DE and C18 on the PU surface from the results of scanning electron microscopic (SEM) analysis by comparing the surface roughness of the modified PU with that of the unmodified PU, and further characterize the surface functional groups on modified PU surface using Fourier transform infrared spectroscopy (FTIR). The acquired crystallinity due to the chemical modification of PU can be further characterized using X-ray diffraction analysis (XRD) and the thermal stability can be tested using thermogravimetric study (TGA). In addition to these characterization studies, the water contact angle can be measured to understand the differences in hydrophobicity due to the chemical modification, followed by the oil absorption studies. The outline of the research is represented in the following process flow chart:

Scanning electron microscopy (SEM) and elemental analysis
The surface features on DE, PU-DE, and PU-DE-C18 were characterized with SEM. Samples were then made conductive by coating with gold and palladium with a sputter coater for 80 s at 14 mA and examined under an FEI Quanta 250 SEM (FEI Company, Hillsboro, OR, USA). Additionally, the elemental analysis was performed for the DE sample using the energy dispersive X-ray analysis (EDX).

Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction analysis
The chemical moieties on the PU and PU-modified samples were tracked using the Perkin Elmer Frontier FTIR spectrometer (PerkinElmer Genetics Inc., Waltham, MA, USA), operating at a scanning range of 400-4000 cm −1 with 32 scans in a spectral resolution of 4 cm −1 . Samples of unmodified PU, PU-DE, and PU-DE-C18 were pressed against the crystal with a pressure applicator with a torque knob to ensure that the pressure was the same for all measurements taken to obtain the FTIR spectra for all samples. The XRD experiments were performed using the instrument X'Pert PRO powder diffractometer (Cu-Kα radiation 1.5406 Å, 45 kV, 40 mA) in the range of 5-80°, 2θ scale in order to study the crystallinity of the sample, using the diffraction pattern.

Thermogravimetric analysis
The thermal decomposition of DE, PU-DE, and PU-DE-C18 was studied by the thermogravimetric analysis (TGA) carried out at inert gas conditions using a TA instrument, model SDT 650, thermogravimetric analyzer (TA Instruments, New Castle, DE, USA). The samples were heated from 20 to 800 °C at a heating rate of 20 °C per minute in nitrogen to generate the thermal mass loss curve to study the thermal stability.

Water contact angle measurement
The measurement of water contact angle was done at room temperature by KRÜSS DSA25 Series (KRÜSS Scientific Instruments, Inc., Matthews, NC, USA) measurement analysis system with the help of the sessile drop standard method. The water contact angles were calculated by using the deionized water as a probe liquid, in a time interval of 3000 ms, with a liquid's dispense volume of 0.3 µL. At least five consecutive measurements were taken to determine the water contact angle.

Water repellency and oil absorption
The unmodified and the chemically modified PU samples were plunged into a distilled water column to test the underwater hydrophobicity of those materials. The oil absorption was experimented by immersing the 1 cm 3 PU foam into 50 ml of crude oil, which was allowed to stand for 30 min at room temperature, and then dried for 30 s. The oil absorption of the modified PU-DE-C18 in comparison to the unmodified PU was calculated by applying the following standard formula, wherein the variables m f and m i denote the mass of the PU/PU-DE-C18 before and after water/oil absorption, respectively:

Elemental analysis of DE and surface morphology of PU during chemical modification
The surface irregularities and microstructural changes on the surface of PU before and after the chemical modification was compared. Figure 1A represents the micrograph of DE possessing disk-shaped and irregularly shaped particles of 20 to 40 μm. The elemental analysis of DE (Fig. 1B) using EDX reveals 54.16 wt.% of silica and 40.60 wt.% of oxygen, indicating that the DE is mainly composed of SiO 2 with some minimal impurities noted. The micrograph of unmodified PU in Fig. 1C at 1 mm magnification shows the foam cell structures forming a smooth skeleton of a network of porous structures. Figure 1D at 500 μm magnification represents the irregular foam closed cell structure.
The SEM micrograph in Fig. 1F represents the PU coated with DE at 300 μm magnification, wherein the deposition of the disk-shaped and irregularly shaped particles of the DE on the surface of the PU matrix is seen. Figures 1E and F represent the DE-coated PU foam matrix at 500 μm and 300 μm magnification, in which the surface of the porous structures and their margins are thicker due to the deposition of the DE particles on the PU matrix. Figures 1G and 1H represent the SEM images of PU-DE-C18 at 500 µm and 400 μm magnifications, which can confirm the surface modification as there are a number of irregular microscale protrusions, ridges, and granular coatings all over the PU skeleton resulting in increased modified DE particle density, rigidness, and thickness.
In our previous study on PU coated with DE and fluorosilane, the modified PU possessed several microscale protrusions due to the deposition of DE and fluorosilane on the surface of PU [35]. Similar studies on PU deposited with surplus DE particles resulted in agglomeration on the surface of PU and PU coated with multiwalled carbon nanotubes exhibited surface roughness with superior hydrophobicity [42,43].

Study of FTIR spectra and XRD pattern for analyzing the chemical modification of PU
To ensure the surface chemical modification of the PU with DE and C18, FTIR is utilized to track the functional groups at the different stages of chemical modification. The FTIR spectra obtained for the unmodified PU, PU-DE, and PU-DE-C18 are plotted in Fig. 2. The significant peaks are represented in Table 1 to identify and compare the differences between the functional groups between the PU and the modified PU. The DE is composed mainly of SiO 2 exhibited peaks at 795 cm −1 due to Si-O-Si symmetric stretching and the sharp peaks at 1080 and 1200 cm −1 due to in plane Si-O stretching vibrations. In the FTIR spectra of the PU-DE-C18, the three peaks above are present, indicating the conjugation of DE-C18 to PU in the chemically modified PU. The FTIR peaks corresponding to the PU, mainly 2858 and 2919 cm −1 due to the CH 2 stretching, the peaks 1640 and 1715 cm −1 representing the C = O stretching of ester and urea, the peak 3280 cm −1 due to NH stretching, 1545 cm −1 due to NH deformation and 1100 cm −1 corresponding to the C-O stretching, can be seen both in the unmodified PU spectra as well as the PU-DE-C18 spectra.
The peaks observed in the PU-DE-C18 between 1000 to 1100 cm −1 are due to the appearance of R-Si-O siloxane bonds. The peaks at 1020/1080 cm −1 are due to the Si-O-Si vibration of the C18 silane. These unique peaks appear only in the PU-DE-C18 and are absent in the FTIR spectra of DE and PU. Our results are in agreement with the studies by Azhar et al. [44], who designed lignin and trichloromethyl silane-coated polystyrene sorbents for oil removal, who had emphasized the peaks between 1000 and 1100 cm −1 for the appearance of the siloxane bonds. In addition to these peaks, CH 2 stretching peaks intensity As shown in Fig. 3, the X-ray diffraction curves of unmodified PU and PU-DE did not exhibit any significant peaks due to the low degree of crystallinity. In contrast, the XRD curve of PU-DE-C18 exhibits a broad peak around 22°, indicating the increase in the degree of crystallinity due to the incorporation of silane into the PU. In response to the incorporation of silane compounds, most of the polymers exhibit an increase in crystallinity. For instance, Sabarinathan P et al. [45] report an increase in the crystallinity index when fishtail palm fiber is subjected to the silane treatment in their X-ray diffraction studies.

Thermal gravimetric analysis to determine the thermal stability
The residual mass loss and derivative mass loss of DE, PU-DE, and PU-DE-C18 are presented in Fig. 4. As evident from Fig. 4 results, both PU-DE and PU-DE-C18 exhibited multi-step degradation thermal behavior. Between the temperatures of 0 to 320 °C, both the PU-DE and the PU-DE-C18 exhibit two peaks around 100 °C and 280 °C, respectively, due to the physically adsorbed water and the  [46] compared the thermal behavior of PU and ceramic-embedded PU and they observed a similar major decomposition peak at around 420 °C due to the main decomposition of PU. Besides these common peaks, the chemically modified PU-DE-C18 exhibited a unique peak at around 491 °C due to the alkyl chain decomposition of the octadecyl moiety of the C18 [38,39]. From the comparative thermal mass loss curves, it is understood that the chemically modified PU-DE-C18 exhibited higher thermal stability compared to PU-DE. This indicates that the chemical modification of PU with DE and the C18 silane shifted the thermal degradation point higher temperature with respect to DE and PU-DE.
By analyzing the thermograms of DE, PU-DE, and PU-DE-C18 between 440 and 800 °C, the amount of C18 grafted on the PU foam was calculated. The mass loss of DE, PU-DE, and PU-DE-C18 was 0.2%, 1.2%, and 26.8%, respectively. The C18 grafted on the PU-DE-C18 was determined by the difference between the mass loss of PU-DE-C18 and the PU-DE, around 25.6%.

Measurement of water contact angle for wettability
The main objective of the chemical modification of PU is to improve the hydrophobicity of the PU foam, which can be assessed by measuring the contact angle of the PU before and after chemical modification. The contact angle between the surface of the PU and a water droplet is measured to get an idea about the wettability or hydrophobicity of the PU after chemical modification. A water contact angle of less than 90° signifies hydrophilicity, on the other hand, a water contact angle of greater than 90° will incline towards the hydrophobicity. The higher the water contact angle, the better the hydrophobicity and oleophilicity of a sorbent material for oil removal from water bodies [38][39][40][41]47]. The PU has an inherent water contact angle of 86°. In contrast, upon conjugation with DE, PU exhibited a shift in the water contact angle to 104° due to the surface roughness of the DE on PU, evident from the SEM micrographs of PU-DE ( Fig. 1E and F). Upon further chemical modification with the C18, PU exhibited an improved water contact angle of about 132°. The increase in the contact angle is due to lowered surface energy attributed to the rough surface due to the thicker coating of C18 silane-modified DE on the PU skeleton ( Fig. 1G and H). The water contact angles of PU, PU-DE, and PU-DE-C18 are represented in Table 2 and Fig. 5. Ambegoda et al. [48] have observed an increase in the contact angle with the introduction of hexadecyldimethoxy silane on DE particles, which is mostly in line with C18 modification on DE. Our results are consistent with similar studies on PU subjected to different types of chemical modification resulting in improved wettability and better use in oil absorption [32,34,43,49]. Figure 6 represents the progression in the surface hydrophobicity of PU-DE-C18 (Fig. 6C) compared to the unmodified PU (Fig. 6A) and PU-DE (Fig. 6B). Due to its high surface roughness and ability to repel water, PU-DE-C18 was able to float on the surface of the water. However, when forcedly submerged, PU-DE-C18 displayed the mirror effect as a result of the plastronic phenomenon, in which an air layer is locked between the PU surface and water [50][51][52][53]. The DE-PU also partly exhibited the water-repellent properties, but the unmodified PU was hydrophilic and had the tendency to be mixed with water. The chemically modified PU-DE-C18 exhibiting the strong hydrophobicity by floating in the water also expresses a strong adherence to oil, making it a suitable candidate for oil absorption and recovery. Previous studies by Martínez-Gómez et al. [54], Lei M et al. [55], and O'loughlin TE et al. [56] have demonstrated underwater superhydrophobicity and mirror effect of various materials used for oil removal.

Oil spill contamination removal from water bodies
In order to test the ability of the chemically modified PU foam to absorb oil, the foam was subjected to the absorption test with crude oil, in which the foam was immersed for a few minutes in crude oil. On, due to the surface wettability, as shown in Fig. 7, most of the viscous crude oil was absorbed by the novel foam very effectively. In contrast, the foam showed extreme water repellency when immersed in water, suggesting its potential for use in removing oil dispersed in any water body. The novel surface-modified PU (PU-DE-C18) exhibited 2.13 folds of increased crude oil absorption compared to the unmodified PU. The unmodified PU exhibited almost 350 folds of water absorption compared to the chemically modified PU (Fig. 8). The increased hydrophobicity and high surface roughness are the results of the chemical modification of PU with C18-silane and DE, which is responsible for the enhanced oil absorption and lack of water absorption. Our previous studies using PU grafted with DE and fluorosilane also showed a better oil absorption of about 1.67 folds than the unmodified PU [35]. Similar studies on PU grafted with non-fluorinated superhydrophobic coating, polydimethylsiloxane-coated carbon nanofiber, and carbon nanofiber/carbon foam composites also show better oil absorption from water [57][58][59].

Conclusion
In conclusion, we have successfully designed PU foam grafted with DE particles and C18 for oil absorption from water bodies. The SEM studies showed the high surface roughness of the modified foam attributed to the deposition of C18-DE particles on the surface of PU formed due to the micro-nano surface structures of DE, FTIR revealed the functional groups of DE and C18 on the PU surface, XRD showed the crystallinity of PU-DE-C18 by silane moieties, and TGA showed better thermal stability of the modified PU in comparison with the unmodified PU. The modified PU exhibited a high water contact angle and exhibits more than two folds of increased crude oil absorption than the unmodified PU. Consequently, the PU-DE-C18 foam can be considered as a potential candidate for oil absorption from water bodies due to its superior hydrophobicity, water repellency, and wettability.
Author contribution All authors contributed to the conceptualization and design of the study. The first author initiated the research and coordinated with the other authors in conceptualization, synthesis, characterization, and preparation of the manuscript. The second author was involved mainly in conceptualization and characterization. The third author focused on characterization, interpretation, and preparation of the manuscript. The fourth author also supported this project by involving in all of the above-mentioned critical areas.
Funding The research leading to these results received the funding from Abu Dhabi Department of Education and Knowledge (ADEK) under grant number AYIA19-008.
Data availability All data generated or analyzed during this study are included in this published article. The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request. Fig. 8 The oil and water absorption properties of the unmodified PU and the PU surface modified with DE and C18 silane were compared, in which the chemically modified PU exhibited 2.13 folds of increased oil absorption and almost 350 times decreased water absorption owing to the improved hydrophobicity and oleophilicity