Graphene-Based Membranes in Oil/Water Separation: Comparison
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Oily wastewaters, oil spills from the petroleum industry, and oil-shipping accidents have caused detrimental effects on aquatic ecosystems. Several methods were proposed by various number of studies to tackle oil spillage cleanups; these methods include absorption, dispersants, solidifiers, and controlled burning. Absorption is one of the most commonly used methods in the separation and recovery of spilled oil from water thanks to its good efficiency, simple operation, and flexibility to combine with other methods. Graphene has a large surface area, with a high chemical and thermal stability. It is an excellent adsorption material and has several applications in the field of oil spill cleanup and water purification processes.

  • nanomaterials
  • graphene
  • water–oil separation
  • graphene oxide
  • reduced graphene oxide
  • foams
  • sponges
  • metal meshes

1. Graphene-Based Sponges for Oil/Water Separation

Based on several studies, three-dimensional (3D) porous sponges are very promising high-capacity sorbents owing to their large surface area. However, most of the commercially available sponges are poorly hydrophobic, which limits their application for oil spill cleanup and recovery [1]. Thus, surface modification is a necessity to increase the hydrophobicity and decrease the water-wettability of the sponges to obtain the desired superoleophilic and superhydrophobic oil sorbent. Sponges are usually coated with graphene, carbon nanotubes, polypyrrole, polyaniline nanofibers, and nanocrystals to improve their superoleophilicity and superhydrophobicity [2]. The graphene-based sponge exhibits high sorption capacities for several types of oils owing to its high surface area [3] (Figure 1). Typically, carbon materials have shown some attractive properties, such as mechanical integrity, high capacity, high porosity, and large pore volume [4]. Great efforts have been devoted to optimizing and developing carbon materials for oil absorption [5], with more hydrophobicity, oleophilicity, functional properties, and specific thermal property [6]. Table 1 shows the sorption capacities of several graphene-based modified sponges for various types of oils in water.
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Figure 1.  Crude oil derivatives’ absorption via graphene sponge in seawater.
 
Table 1. Sorption capacities of several graphene-based modified sponges for oil/water mixtures.
Sorbent Material Type of Oil Sorption Capacity (g/g) Reference
rGO@MF modified sponge Crude oil-in-water 2.1–5.6 [7]
Graphene-based sponge Oil and organic solvents 50–165 [8]
SiO2/GO-PU sponge Oil and organic solvents 80.0–180.0 [9]
Graphene-coated PU sponge lubricate oil 31.0 [10]
NMP/graphene PU sponge Oil and organic solvents 40.0–80.0 [11]
GO/PU sponge Oil and organic solvents 30.0–55.0 [12]
Thiolated graphene/PU sponge crude oil 29.5–90.0 [13]
RGO/PU sponge Oil and organic solvents 24.2–37.6 [14]
RGO/OAP/PU sponge Oil and organic solvents 24.7–80.3 [15]
G sponge Machine oil 35.5 [16]
Polyethylenimine/RGO decorated PU

sponge		 Bicycle chain oil 8.8 [1]
RGO and octadecylamine decorated PU

sponge		 Silicon oil 29.7 [17]
With the industrial revolution and the growth in industrial activities, large amounts of oil discharges are released into the aquatic environment. Thus, superhydrophobic and superoleophilic materials with reusability are needed.
Polyurethane (PU) sponge has attracted great attention in the oil/water separation field for its distinguishable three-dimensional (3D) porous structure. On the other hand, its oil-absorption efficiency is significantly hindered by its poor hydrophobicity. Rahmani, Samadi [18] investigated the application of graphene-coated polyurethane (PU) sponge for the oil spills’ selective absorption from water. The authors have integrated nanoporous graphene (NPG) and graphene oxide (GO) flakes with the porous structure of PU, leading to the preparation of NPG/PU and GO/PU, respectively. The results of the study show that the NPG/PU sponges had a high capacity in the removal of crude oil and organic liquid stains. The absorption capacity for NPG/PU sponge was greater than that of GO/PU sponge. Thus, in oil spill remediation, graphene-coated polyurethane (PU) sponge is an efficient and cost-effective method [18]. Xia et al. [14] used a one-pot solvothermal method to facilely fabricate a superhydrophobic reduced graphene oxide-coated polyurethane (RGO@PU) sponge in the presence of ethanol. The RGO@PU sponge with a superhydrophobic surface prepared by the authors exhibited a high oil-absorption capacity, 37 times greater than its original mass. The results of the study prove that the RGO@PU sponges have great potential as an absorbent for oil/water separation owing to their rapid oil-absorption rate and high oil/water separation efficiency (~99%) [14]. Qiang et al. [19] synthesized graphene oxide nanoribbons (GONRs) functionalized by silane molecules with several hydrophobic end-groups, and fabricated silane functionalized reduced GONR (silane-f-rGONR) coated polyurethane (PU) sponge composites using a facile dip-coating process. The results of the study show that the two types of silane modified sponges demonstrated distinct super-hydrophobicity and tunable oleophilicity/oleophobicity. The porous silane-f-rGONR@PU composites exhibited a superb selectivity for oil-water separation and high oil/solvent absorption capacity at static state. In addition at the dynamic shaking state, the porous silane-f-rGONR@PU composites have shown a superb oil/water separation efficiency compared with the rGONR@PU composite with poor oil/water separation. This study has shown a new method for synthesizing the super-hydrophobic, mechanically flexible and electrically conductive porous rGONR-based composites, with a promising application in oil pollution remediation from water [19]. Khalilifrad et al. [20] synthesized a modified magnetic superhydrophobic polyurethane sponge by connecting graphene oxide (GO), coated with functionalized oleic acid Fe3O4 nanoparticles on three-dimensional microstructure of commercial polyurethane sponge (Fe3O4@OA@GO-PU) using a low-cost and simple dip coating method. The results of the study showed that the eco-friendly modified sponge has efficiently adsorbed various oils from water with a high adsorption capacity. Consequently, the synthesized modified sponge in this study is a superb adsorbent for efficient applications in the separation of oil from water [20].
Crude oil spill cleanup and recycling oily wastewater are of great concern globally. Qin et al. [21] developed a simple approach to synthesize a solar-driven superhydrophobic/oleophilic polybenzoxazine/reduced graphene oxide wrapped-cellulose sponge (PBZRGOS) for an effective oil/water separation process and rapid crude oil remediation from water. Polybenzoxazine (PBZ) was introduced into the cellulose sponge as a binder to provide low energy for the surface. The results of the study show that the modified sponge exhibited a high oil/water separation efficiency, reaching 99.1%, which surpasses other reported modified sponges. Hence, it can be concluded that the synthesized superhydrophobic cellulose sponge can be successfully applied as a competitive functional sponge for the removal of spilled crude oil from water [21]. Pethsangave et al. [4] synthesized a novel superhydrophobic ultra-light graphene-based carrageenan sponge (GCS) absorbent via one pot hydrothermal method, in order to selectively adsorb oils and organic solvents from water mixtures. In the presence of formaldehyde, the graphene oxide (GO) nanosheets were reacted by the insertion of carrageenan, producing hydrophobic cross-linked structure between them. The results of the study show that the GCS had an excellent oil sorption capacity within the range of 25.2 to 50 g of oil per gram of adsorbent. The present study suggests that the synthesized carrageenan sponge (GCS) has great potentials in the removal of oil from water [4].
The development of porous sponge materials with surface super-hydrophobicity and high mechanical robustness is strongly required for several types of applications. The main drawback is the realization of the multiple functionalities simultaneously using a facile approach. Moa et al. [22] fabricated a super-hydrophobic, flame-retardant, and mechanically flexible functionalized silica/graphene oxide wide ribbon (GOWR) coated melamine sponge (M-GOWR@MF) via a simple two-step surface-modifying method. The results of the study show that the as-prepared MF sponge composites demonstrated super-hydrophobicity/super-oleophilicity and high mechanical robustness, leading to a superb absorption capacity for both heavy and floating oils from water and efficient performance of continuous oil/water separation. Thus, the hybrid functionalized silica/GOWR network in this study provides a new approach to develop advanced multi-functional polymer sponge composites for oil removal purposes from water [22]. Jamsaz et al. [15] investigated a novel recyclable, superoleophilic, superhydrophobic, and flame-retardant graphene-based polyurethane (PU) sponge fabricated by functionalizing polyurethane (PU) with reduced graphene oxide (RGO) and orthoaminophenol (OAP). The synthesized RGO/OAP/PU sponge in the study showed a high sorption capacity for a wide range of oils. Thus, the RGO/OAP/PU sponge can be successfully applied in the field of oil/water separation [15]. A superoleophilic/superhydrophobic material was prepared by coating hexadecyltrimethoxysilane-grafted reduced graphene oxide on a melamine sponge skeleton (HDTMS/rGO-MF) using a low-cost and simple method to absorb various oils and organic solvents from water [23]. The results of the study show that the absorption capacities of HDTMS/rGO-MF for several oils and organic solvents were 8.79–20.85 g/g. Several oil spill accidents have caused catastrophic environmental issues. Hence, hydrophobic sorbents have gained great attention for oil spill remediation. Zhou et al. [24] developed an oil/water separation material with robust mechanical properties by the modification of modifying melamine sponge with silk fibroin-graphene oxide (SGMS). The authors have used the silk fibroin as a molecular binder to combine melamine sponge and graphene oxide. After modification, the sponge surface became hydrophobic. The results of the study have shown that the SGMS exhibited an efficient oil/water separation, excellent oil adsorption capacity, good mechanical properties, and superior recyclability. These superb properties show that SGMS can be successfully applied in the adsorption of oil and organic pollutants under realistic conditions [24]. Wang et al. [25] developed a solar-driven self-heated sponge as a novel sorbent to achieve rapid collection of crude oil from spills using the advantage of light-to-heat conversion to reduce the oil viscosity significantly. The authors have fabricated the sorbent via facile dip-coating of reduced graphene oxide on a commercial melamine sponge. The sponge produced in this study has shown a distinguishable remediation of viscous crude oil spills [25].
The mechanical recovery of oils via oil sorbents is one of the most important and crucial methods in managing marine oil spills. However, the properties of the oil released into sea are affected by external environmental conditions. Shiu et al. [3] demonstrated a graphene-based (GB) sponge as a novel sorbent for the removal of crude oil and compared its performance with a commercial sorbent sheet under several environmental parameters. The results of this study showed that the GB sponge demonstrated excellent superoleophilic and superhydrophobic characteristics. Hence, the GB sponge is an efficient sorbent for crude oils, with high sorption capacity reaching 85–95 times its weight. Furthermore, the authors stated that the crude-oil-sorption capacity of the synthesized GB sponge was remarkably higher than that of the commercial sheet by almost five times. Hence, the GB sponge has a great potential in marine spilled-oil removal and hydrophobic solvent removal [3]. Yang et al. [6] reported a cost-effective, environmentally friendly, and mild approach to fabricate graphene-based sponge (GS) using in situ reduction-assembly of graphene sheets on the melamine sponge skeletons. The hydrophobic and oleophilic GS fabricated in this study exhibited high absorption capacities for oils and organic liquids with excellent recyclability. These superb performances make the GS an effective candidate for potential applications in oil/water separation [26]. Peng et al. [27] reported a cost-effective and facile method to synthesize a novel, superhydrophobic, and robust kaolinite modified graphene oxide-melamine sponge (K-GOMS). The graphene oxide (GO) sheets were used by the authors to enhance the roughness of the sponge surface. Several oils and organic solvents were selected by the authors to test the adsorption performance of the adsorbents. The results of the study demonstrated that the superhydrophobic K-GOMS exhibited a superior adsorption capacity for several types of oils and organic solvents. Hence, this study provides an effective approach for fabricating low-cost, facile, and large-scale production of superhydrophobic adsorbents for oil/water separation [27].
Nowadays, oil spills are considered a serious worldwide environmental crisis. To deal with this global issue, Meng et al. [28] synthesized a super-lipophilic and super-hydrophobic functionalized graphene oxide/polyurethane (FGP) sponge via a simple and cost-effective dip coating method. The synthesized FGP sponge by the authors demonstrated a great absorption capacity for a wide variety of oils and organic solvents from water, reaching 35 times its own mass. In addition, the FGP sponge demonstrated a good reusability. Therefore, this study provides a simple approach for the removal of oil spills and organic solvents from water [28]. Sun et al. [29] mentioned that modifying the commercial melamine sponge (MS) with graphene oxide (GO) or GO-based nanocomposites (NCs) is a facile, efficient, and low-cost method to synthesize three-dimensional (3D) porous materials with multifunctionality. Sun et al. [29] synthesized two types of 3D MSs that were separately functionalized via the reduced GO (RGO) sheets and Ag/RGO NC using combined methods of immersion process and chemical reduction. The results of the study demonstrated that, owing to the high porosity and unique surface topography, the RGO-MS exhibited a high absorption capacity, reaching 41 to 91 times its own weight for several types of oils and organic solvents. Furthermore, both the Ag/RGO-MS and RGO-MS showed high oil/water separation efficiencies. Thus, Ag/RGO-MS and RGO-MS are very promising candidates for potential applications in oil-spill removal and water disinfection [29]. Zhang [13] proposed a facile method to synthesize superhydrophobic polyurethane sponge for oil/water separation using a simple dipping-drying process. The results of the authors’ study showed that the GSH-based superhydrophobic sponge exhibited high absorption selectivity for a wide range of oils and organic solvents. This study demonstrated a new effective way to synthesize superhydrophobic materials for oil/water separation, with a great potential in large-scale industrial practical application [13]. Periasamy et al. [1] synthesized polyurethane dish-washing (PU-DW) sponges functionalized sequentially via polyethylenimine (PEI) and graphene oxide (GO) producing PEI/reduced graphene oxide (RGO) PU-DW sponges. The PEI/RGO PU-DW sponge was coated with 20% phenyltrimethoxysilane (PTMOS) to further enhance the absorption capacity and hydrophobicity of oil. The results of the authors’ study showed that the PTMOS/PEI/RGO PU-DW sponge absorbed a wide range of oils within 20 s and efficiently separated oil/water mixtures through a flowing system. Thus, with the merits of a fast absorption rate, low cost, and reusability, the PTMOS/PEI/RGO PU-DW sponge has a great potential as a superabsorbent for highly efficient removal and recovery of oil spills as well as for the separation of oil/water mixtures [1]. Zhou et al. [30] synthesized graphene/polyurethane (PU) sponges with superhydrophobicity via the one-pot solvothermal technique. The synthesized graphene/polyurethane (PU) sponge can act as a selective filter to effectively and continuously separate oil from water. Owing to the functionalized PU sponge’s structural integrity, the material was capable of separating oil up to 53,000 times its own mass with an oil–water separation efficiency greater than 99.5%. This study shows that the functionalized PU sponge and the integrated oil–water separation system provided have a great potential in industrial scale oil–water separation processes [30]. Zhao et al. [31] fabricated graphene-coated melamine sponges (GCMSs) via a one-step method. The GCMSs synthesized by the authors are hydrophobic, oleophilic, and demonstrated high absorption for oil liquids. The results of the study showed that the absorption capacities for diesel oil and gasoline were greater than 105 g g−1. Thus, GCMS has great application potential in the field of oil spillage cleanup [31]. Yang et al. [32] synthesized a hydrophobic and superoleophilic nitrogen-doped graphene (NG) sponge with a three-dimensional (3D) structure via a facile hydrothermal method without further surface modification or pretreatments. The as-prepared NG by the authors exhibited a good porosity, low density, and high specific surface area. The results of the study show that the NG sponges have strong absorption capacities, reaching 200 times its own weight in a short time for a wide range of oils and organic solvents. Thus, this study demonstrates that the NG sponge is a promising candidate in the field of spilled oil recovery owing to its feasibility, cost-effectiveness, and scalability [32]. In addition, Wu et al. [16] mentioned that the graphene sponge (GS) has many applications in oil removal owing to the hydrophobic nature of graphene sheets. However, current hydrothermal preparations of GS require the application of toxic reducing reagents that cause environmental pollution. Hence, several studies focused their work on finding an environmentally friendly methods to synthesize graphene sponge (GS). Wu et al. [16] reported that graphene oxide (GO) can be hydrothermally reduced by glucose forming GS for the adsorption of various oils and organic solvents. The authors have reduced the graphene sheets by glucose during the hydrothermal treatment and produced a 3D porous structure. The study results demonstrated that GSs have efficiently adsorbed oils and organic solvents with competitive adsorption capacities. Thus, GSs can be efficiently used for the remediation of oils from water [16].

2. Graphene-Based Hydrogels for Oil/Water Separation

Graphene-based hydrogel materials have gained great attention in several fields and applications, including oil/water separation. Various studies have focused their work on using graphene-based hydrogels for oil/water separation. Hu et al. [33] reported a novel, facile, cost-effective, and environmentally friendly method to synthesize fluorographene nanosheets via Michael’s addition reaction and the synthesis of amphiphobic LA/F/rGO hydrogel, based on a hydrothermal process of partially reduced graphene oxide (rGO) at low-temperature and 1H,1H,2H,2H-perfluorodecanethiol (PF) in the presence of l-ascorbic acid (LA) as a reducing agent. The results of the study show that the as-prepared LA/F/rGO hydrogel demonstrated oil bouncing and intriguing oil repellant behaviors underwater. The authors also added that pre-soaking with oil or water allows selective and efficient separation of oil or water from their mixtures. Thus, the new LA/F/rGO hydrogel synthesized in this work has proven to be a promising candidate in oil/water separation, wastewater treatment, and oil fence material. Hence, this study provides new approaches to the fabrication of novel graphene-based nanocomposite materials for oil/water separation processes and oil leaking, with various environmental and engineering applications [33]. The continuous increase in the production of produced water from oilfields is proven to cause detrimental environmental effects. Fong et al. [34] fabricated an efficient, environmentally friendly, and recyclable reduced graphene oxide immobilized κ-Carrageenan hydrogel composite (κCaGO) as an alternative sorbent for crude oil-in-water demulsification. Polyethyleneimine (PEI) was employed by the authors to produce a stable hydrogel composite. The immobilization conditions of graphene oxide (GO) on PEI-modified κ-Carrageenan (κC) beads were appropriately optimized by the authors. The results of this study show that the synthesized κCaGO could be efficiently used as a potential sorbent substitute for the separation of crude oil from produced water [34]. Sun et al. [35] presented a simple droplet microfluidic method for generating graphene oxide (GO) hydrogel composite particles for oil decontamination. The synthesized GO hydrogel composite particles provided in this study have proven to be an ideal candidate for oil decontamination [35]. The addition of graphene with its unique properties is shown to enhance the performance of traditional polymer hydrogels. Graphene in hydrogels acts as a gelator to self-assemble into the hydrogels. In addition, as a filler, it blends with small molecules and macromolecules, resulting in the production of multifunctional hydrogels. The addition of graphene to the hydrogels enhances its electrochemical performance and leaves the door open for a large number of applications in addition to oil/water separation. Such applications can extend to other water treatment applications (dye and metal removal), medical treatments, biomedical, tissue engineering, supercapacitor, and many others. Most of the published works on hydrogels are theoretical and applied at a small scale. A proper large-scale application and effective demonstration that exploits the full potential of graphene hydrogels is still lacking. The design and fabrication of new graphene hydrogels and the effect of formation mechanisms of the graphene and gels are not fully understood, nor researched.

3. Graphene-Based Aerogels for Oil/Water Separation

Graphene aerogels are the world’s lightest materials, with an unusual low density [36]. The combination of the hydrophobic properties of graphene sheets and low density makes the graphene aerogel a very promising candidate for oil absorption. The absorption capacity of the graphene aerogel approaches a level several hundred times greater than that of commercially available materials for environmental remediation purposes. Nowadays, using three-dimensional graphene aerogel materials for oily wastewater treatment with its complex compositions remains a great challenge owing to volume shrinkage, which results in low adsorption capacity and single-function. Ji et al. [37] introduced renewable Enteromorpha into the graphene aerogel using facile hydrothermal-freeze casting treatment, producing the ultralight, compression, and amphiphilic adsorbent for oil spill removal and water pollution cleanup. The results of this study show that, for clean-up of oil spills, the Enteromorpha modified graphene aerogel (EGA) showed superb adsorption capacity towards oil and organic solvents compared with pristine graphene aerogel (GA). Hence, EGA can be successfully used as an adsorbent for oil in oil/water separation processes [37]. Chatterjee et al. [38] developed a simple preparation method to synthesize aerogels with great shape retention for oil absorption application. To increase the selective oil absorption and hydrophobicity, the authors used a dip coating process to chemically modify the graphene sheets and attach it to the aerogel pore surface. Several examples were demonstrated by the authors to show the feasibility of applying the synthesized aerogel in oil spill control [38]. Zhan et al. [39] investigated the role of graphene oxide (GO) in increasing the oil absorption capacity by the aerogel materials and in decreasing the shrinkage in polyimide (PI) aerogels. The study includes grafting the GO particles with pyromellitic dianhydride (PMDA) forming GO-modified PMDA prior to reaction with 2,2′-dimethylbenzidine to form polyamic acid. The polyamic acid chains are then cross-linked using 1, 3, 5-tris (4-aminophenyl) benzene and chemically imidized using acetic anhydride and pyridine to obtain polyimide gel. The resultant aerogel specimens, obtained by supercritical drying of the gels, demonstrated great reduction of diameter shrinkage from 9.0% for unmodified monoliths to 0.8% in the presence of 5.2 wt% GO, a 9.1% reduction in surface energy compared with the unmodified aerogel, high surface area (>504 m2/g), high porosity (>93%), and low bulk density (<0.0905 g/cm3). These aerogels demonstrate a great increase in the hydrocarbon oil absorption capacity [39]. Meng et al. [40] synthesized a lignin-based carbon aerogel enhanced by graphene oxide (LCAGO) for the separation of oil from water. The synthesized aerogel by the authors demonstrated superhydrophobicity and good mechanical property, proving that it is capable of being used in oil/water separation. The results of the authors’ study reveal that the aerogel efficiently separated the light oil, heavy oil, and emulsified oil from water. The results of the study indicate that the material is highly effective and very practical as a water-cleaning material. In addition, the preparation process provided by the authors is energy-saving and is an efficient route for the separation and collection of oil from water with industry waste [40].
Recently, graphene aerogels have gained great research attention in oil/water separation processes based on their distinguishable properties. However, the over stacking of graphene oxide nanosheets (GO) results in a poor recyclability and low adsorption capacity. Kang, Cui [41] fabricated nitrogen-doped magnetic carbon nanospheres/graphene composite aerogels (MCNS/NGA) under weakly alkaline conditions via a one-step hydrothermal in situ electrostatic self-assembling strategy. The aerogels synthesized by the authors had super-elasticity, low density, high specific surface area, and good magnetic properties. Hence, the aerogels exhibited high adsorption capacity in the range of 187 g g−1 to 537 g g−1 towards various oils and organic solvents, surpassing most of the reported materials to date. Thus, the MCNS/NGA synthesized in this study are promising candidates in oil/water separation processes [41]. Cleaning up of crude-oil spills is considered a major environmental remediation issue and urges the fabrication of sorbents with high and stable hydrophobic and oleophilic properties. Thakkar, Pinna [42] synthesized aerogels with durable hydrophobicity via incorporating reduced graphene oxide (rGO) in highly porous silica matrices. The aerogels synthesized by the authors exhibited high oil selectivity and sorption. Hence, the aerogels fabricated in this study can be successfully implemented in oil/water remediation processes [42].
Low fluidity and high viscosity heavy crude oils frequently decrease their diffusion rate into the porous adsorbents, leading to a low efficiency in oil spill removal. Hence, the photothermal effect can be used to greatly decrease the viscosity of heavy crude oil via in situ solar light heating. Luo, Wang [43] synthesized photothermal carbon nanotube/reduced graphene oxide (CNT/RGO) microspherical aerogels by the fabrication of graphene oxide (GO)-based microspherical aerogels with several radially orientated microchannels, followed by high-temperature reduction of the GO components and growing CNTs inside the microchannels. The results of the study show that, under 1 sun irradiation, the aerogel surface temperature quickly rose to 83 °C in 1 min, causing a rapid decrease in the viscosity of crude oil. Moreover, the optimal microspherical aerogels exhibited an excellent adsorption capacity of heavy crude oil, up to 267 g g−1 in only 10 min, surpassing several other reported oil adsorbents [43]. Three-dimensional (3D) aerogels with lipophilic and hydrophobic properties have gained great attention in efficient oil spill clean-up. However, biodegradability, cost-effectiveness, and recycling are still great challenges in the application of aerogels for oil/water separation. Hu, Zhu [44] fabricated high biocompatibility, hydrophobic, and low cost composite aerogels via directional freezing-drying technology with the use of chitosan (CS) as the skeleton substrate, hydrophobic silicon particles/polydimethylsioxane (H-SiO2/PDMS) as the hydrophobic modifier, and reduced graphene oxide nanosheets (rGO) as enhancements. The composite aerogel synthesized in this study demonstrated high adsorption capacity for oil, as well as good thermal and chemical stability in a harsh environment. In addition, the adsorbed oils and organic solvents can be easily extruded from aerogels owing to its excellent compressive properties [44]. Cleaning, collecting, and recycling oil from oil spills, specifically high viscosity crude oil, are of worldwide concern. Sun et al. [45] synthesized a 3D macrostructure CuFeSe2-loaded graphene aerogel (GA-CuFeSe2) that shows an outstanding photothermal conversion capacity, surpassing various other materials. The synthesized 3D macrostructures supported by graphene holds a versatile pore structure that allows rapid diffusion of highly viscous oils and a high corrosion resistance. The high-performance oil sorption and photothermal conversion, driven by solar power, demonstrated that this material can be efficiently applied for oil spill clean-up [45]. Zhang et al. [23] developed a facile approach for a highly efficient oil/water separation process by incorporating dimethyldiallylammonium chloride acrylamide polymer (P(AM-DMDAAC)) into the graphene aerogels. The functionalized 3D graphene aerogel presented several outstanding physical properties, including low density, large specific surface area, and great hydrophobicity. The modified aerogel in this study demonstrated excellent adsorption capacity for a wide range of oils and organic solvents. The authors have also found a simple approach by changing the pH values to achieve controlled wettability transition of P(AM-DMDAAC)/graphene aerogels (PGAs). The hydrophobic PGA prepared at pH 2.03 demonstrated a superb oil/water separation performance. Thus, as an efficient and recyclable water purification material, the environmentally friendly and sustainable polymer-modified graphene aerogel has promising potential in oil/water separation [46].
In a severely damaged marine polluted environment, low-density and environmentally friendly aerogels have become potential materials for oil/water separation. However, several reported aerogels have the disadvantages of low oil absorption, poor flexibility, and compressibility, which hinders their application. Zhou et al. [47] reported an anisotropic, compressible lamellar lipophilic and hydrophobic graphene/polyvinyl alcohol/cellulose nanofiber carbon aerogel (a-GPCCA) prepared via carbonization and directional freeze-drying processes. The synthesized synthetic ultralight a-GPCCA had high porosity, reaching 99.61%, and low density (6.17 mg/cm3). In addition, the directional freeze-drying used by the authors produced a lamellar interpenetrated three-dimensional (3D) porous structure, with a high adsorption capacity, good compressibility, and recyclability. Moreover, carbonization provided it with a high thermal stability and hydrophobic properties, resulting in a high oil/water selectivity and combustion cyclicity. Thus, the a-GPCCA fabricated in this study has promising potential in the field of the offshore oil spill treatment and domestic industrial wastewater [47]. Song et al. [48] have successfully synthesized a grass-modified graphene aerogel (GGA) using an environmentally friendly, simple, and low-cost hydrothermal treatment. The graphene aerogel properties have been significantly enhanced by introducing the grass powder, leading to a highly efficient selective adsorption process. The results of this study show that the GGA can successfully be used in the oil/water separation of mixed solution via different manipulation methods. In addition, the GGA is capable of removing oils via adsorption, acting as a filter media dealing with oil/water mixtures, and collecting oils continuously using a peristaltic pump or gravity. This study demonstrates that the GGA is a very good and promising candidate in the oil/water separation field and spill oil clean-up for environmental remediation. Moreover, the addition of grass enhances the graphene aerogel’s oil adsorption efficiency, hydrophobicity, and mechanical property, providing a new route for high-value utilization of biomass waste [48]. Huang et al. [36] synthesized cork-like graphene aerogels via the EDA-ammonia double hydrothermal reduction method. The fabricated porous aerogels demonstrated high hydrophobicity, porosity, and excellent mechanical properties. In addition, the oil adsorption efficiency is significantly greater than that of the conventional adsorbents, with an oil adsorption capacity of 130.10 g/g for pure diesel and continuous treatment capacity of 71.67 g/g for emulsified oil in oily contaminated water. Hence, the continuous aerogel-based water treatment process is proposed in this study for graphene-based aerogels to have promising potential applications in oil spill remediation and water purification [36]. Chen, Li [49] have facilely modified an ultralight, compressive, and fire-resistant graphene aerogel via renewable lignin biomass with porous and hydrophobic skeletons. The lignin-modified graphene aerogel (LGA) in this study showed a highly efficient absorption of petroleum oils. Hence, LGA is a highly efficient, renewable, and recyclable potential candidate for applications in oil/water separation [49].
Low-density aerogels are very effective materials for oil absorption; however, they are limited by their low elasticity and compressibility. Mi et al. [50] prepared three-dimensional (3D), highly compressible, anisotropic, elastic, cellulose/graphene aerogels (CGAs) via bidirectional freeze-drying. The authors have grafted long carbon chains using the chemical vapor deposition method, forming modified cellulose/graphene aerogels (MCGA) with an enhanced superhydrophobicity. Thanks to its ultra-light weight and high surface area, MCGA holds an excellent absorption capacity of 80 to 197 times its weight, which surpasses most carbon-based aerogels and hydrophobic cellulose aerogels. Thus, MCGA is a very promising absorbent material for selective oil absorption and recovery [50]. In recent years, reduced graphene oxide (rGO) has attracted great attention from researchers around the world owing to its outstanding physicochemical properties and distinguishable surface chemistry. Cao et al. [51] developed a facile and environmentally friendly method to fabricate reduced graphene oxide/polydopamine composite aerogel modified by 1H,1H,2H,2H-perfluorodecanethiol (PFDT) and reinforced by chitosan. The aerogel synthesized by the author demonstrated high oil/water separation behavior, showing that the aerogels are very effective materials for the remediation of water from oil spill accidents [51]. Yu et al. [52] developed ultralight and nitrogen-doped graphene aerogels (UNGAS) via a hydrothermal method in the presence of graphene, L-arginine, and dopamine (DA). The synthesized UNGAS demonstrated high absorption capacity for several oils owing to low density, large surface area, and N-doping [52]. Cheng et al. [53] reported the preparation of a new type of ultralight 3D porous and highly hydrophobic graphene aerogel (GA) with melamine formaldehyde (MF) microspheres as a spacer and pore forming agent and graphene oxide as a precursor, followed by freeze-drying, hydrothermal reduction, and calcination process. The resultant calcined MF-GA (cMF-GA) exhibited a fast absorption rate and maximum sorption capacities up to 230 g/g for diesel oil. Thus, cMF-GA is a promising highly efficient and low-cost absorbent in practical applications of large-scale oil removal [53].

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