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 fresh-water 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 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. These spilled oil floats on the water surface that spreads up to form a thin layer of oil slick that remains for a very long time, even decades or years, or form a thick layer of heavy oil that reaches the ocean floor or reaches the beaches or shore-lines, leading to air, water & soil contamination [2]. 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].
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 [5, 6]. Some of the commonly used approaches include in-situ burning [7], mechanical removal using booms [8], skimmers [9, 10] or vacuum [11], chemical treatment using dispersants [12] or sorbents and biological microbial remediation [13, 14]. 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 material 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 [15.16]. 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 [17], bamboo [18], peat [19], corn-stalks [20], saw dust [21], vegetable fibers [22] or other natural sorbent materials like chitosan, bentonite and activated carbon [23]. 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 absorption of both oil and water, resulting in 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 [24–26]. 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 [5].
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 [26–28]. One of the limitation with 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 approach is copolymer grafting PU with oleophilic monomers to decrease water sorption and increase oil sorption. Hua Li et al. has 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 [26]. 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 [27].
Researchers also focus on PU composites and eco-friendly natural materials for enhanced oil removal. Oribayo et al. developed a lignin-based PU foam modified with graphene oxide for enhanced oil spill clearance applications [29]. Ren L et al. 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 [30]. Another interesting study based on PU natural material composite includes rice straw residues filled with PU matrix [10] and lignin-based PU with carbon nanotubes used as photothermal sorbents for high viscous oils [31].
Incorporating natural superhydrophobic/hydrophobic silica-based materials into PU foam matrix is similar to copolymer grafting PU with oleophilic monomers to achieve better oleophilicity and hydrophobicity. Sepiolite coated PU foam [32], flexible PU foam with zeolitic imidazole framework [33], fluorinated DE coated PU foam [34] and other PU silica composites [35, 36] were found to present excellent oil absorption capacity and improved water repellence.
Our previous studies on 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 [37–40]. The fluoro silane modification of the PU, despite the significant benefits in the oil removal, has some serious environmental concerns, as most of the fluorinated organic compounds are associated with acute toxicity in humans, animals and birds who are in contact with the fluoro based materials for oil remediation from water bodies. The current research focuses on synthesis of DE and non-fluoro octadecylsilane modified PU foam for enhanced crude oil adsorption that overcomes the toxic impact of using organo fluoro compounds.