Recent progress and future prospects of oil-absorbing materials☆

Tao Zhang,Zhangdi Li,Yuanfei Lü,Yu Liu,Dongya Yang,Qiurong Li,Fengxian Qiu,*

1 Institute of Green Chemistry and Chemical Technology,School of Chemistry and Chemical Engineering,Jiangsu University,Zhenjiang 212013,China

2 School of Chemistry and Chemical Engineering,Jiangsu University,Zhenjiang 212013,China

3 School of Environmental and Chemical Engineering,Hebei Key Laboratory of Applied Chemistry,Yanshan University,Qinhuangdao 066004,China

Keywords:Oil/water separation Oil-absorbing material Superhydrophobic Hydrid absorbents

A B S T R A C T Oil and organic solvent contamination,derived from oil spills and organic solvent leakage,has been recognized as one of the major environmental issues imposing a serious threat to both human and ecosystem health.Among the various presented technologies applied for oil/water separation,oil absorption process has been explored widely and offers satisfactory results especially with surface modified oil-absorbing material and/or hybrid absorbents.In this review,we summarize the recent research activities involved in the designing strategies of oil-absorbing absorbents and their application in oil absorption.Then,an extensive list of various oil-absorbing materials from literature,including polymer materials,porous inorganic materials and biomass materials,has been compiled and the oil adsorption capacities toward various types of oils and organic solvents as available in the literature are presented along with highlighting and discussing the various factors involved in the designing of oil-absorbing absorbents tested so far for oil/water separation.Finally,some future trends and perspectives in oil-absorbing material are outlined.

1.Introduction

Petroleum or crude oil,originated from fossilized organic materials,are crucial resources for the economy of all countries[1].Oil reserves are distributed on the shelves of the ocean,on the land,and in the inland seas.Crude oil is pumped from the ground or the ocean bottom and is transported via pipelines or shipped with oil tankers to oil refineries for the production of diesel fuel, fuel oils, kerosene, jet fuel, ethane,and other petrochemicals [1]. However, frequent oil (or organic solvents)spillage and large amounts of oily wastewater were discharged from oil exploration, transportation and processing, which have a significant impact on the environment and wildlife. In addition, oily wastewater is daily produced from petroleum,food,textile,steel,leather and metal-finishing industries[2].This oily wastewater treatment has become of high importance over the last years worldwide.

Oily wastewater causes great damage to the coastal environment mainly in sensitive marine ecosystems and negative economic impacts on tourism and fisheries[3].Not only have numerous aquatic creatures(such as sea birds,mammals,seaweed and so on)been killed by contact with these spilled oils,but sand on seaside beaches has also been heavily contaminated[4].A typical example is oil spillage in Gulf of Mexico,the largest accidental crude oil in history,which released an estimated 4.9 million barrels of oil into the ocean, resulting in severe damage to marine life, coastal wetlands, the fishing industry, tourism, and so on[5].Keeping the view of toxic effects of oily wastewater,there is an urgent need to find out an effective and robust technology for oil/water separation.

2.Technologies for Oil/Water Separation

The environmental and economic demands emphasize the need for methods which can effectively separate oil/water mixtures.Over the past decade,researchers in many countries have done a lot of research,and many methods have been proposed to separate oil/water mixtures.Currently,several treatment processes for oil/water separation(including the remediation of the leakage of water-insoluble organic liquids hereafter) can be categorized into four different types: i) chemical methods(dispersion[6],solidification[7]),ii)in situ burning[8],iii)bioremediation[9],and iv)mechanical recovery[10](booms,barriers,and skimmers and absorbents[11]).A summary of the different methods of oil/water separation in this section,together with merits and shortcoming,can be found in Table 1.

Since petroleum is mainly composed of alkanes, it is difficult to participate in the chemical reaction.Therefore,the pollution problem of oil leakage is solved indirectly through the chemical method of adding coagulant to the oil[12].In situ burning solves the pollution of oil spill indirectly. However, both solidification and in situ burning have disadvantages,such as secondary pollution,low separation efficiency and waste of resources.With regard to biological method,it is a clean technology with the advantages of being environmentally safe,cost-effective,and does not generate secondary waste.However,the floating oils are hardly biodegraded by aquatic microbial communities;on the other hand, researchers use the special microorganism to degrade pollution,but in order to maintain the proliferation of microorganisms,it requires rigorous condition,because many variables restrict the use of microorganism,such as the accessibility of the pollution to the microorganism,the optimization of biological activity and the inherent biodegradability of the pollution[13,14].Hence,biological method has some disadvantages which limit its application on biodegradation of oils.Mechanical method commonly uses oil booms[15],barriers[16],and skimmers[17]to clean up oil spills or organic pollution.However,the oils or organic solvents cannot be recovered completely by those devices with the intrinsic hydrophilic property and low absorption selectivity.The oil separation efficiencies are not very high by using oil booms,barriers,and skimmers,especially for complex oil/water mixture systems.

Table 1Summary of the different methods of oil/water separation

Alternatively,oil/water separation process by the physical absorption has always been the subject of active research.Absorption method has been regarded as one of the most effective technologies because it is readily available,environmentally friendly,inexpensive and offers good recyclability[18,19].There has been an increasing amount of research on the fabrication of oil-absorbing materials with high oil absorption capacity and superior selectivity under different conditions,aiming at the separation of oils from water.A large number of traditional materials have been used widely to absorb oil spilled water,including oilabsorbing polymer materials, porous inorganic materials, biomass material and so on.However,traditional oil-absorbing materials have some disadvantages owing to the technical constrains and economic obstacles. In this review, various oil-absorbing materials have been assessed for oils and organic solvents absorption,as will be discussed in the following sections in this paper.

3.Oil Separation by Absorption

3.1.Oil-absorbing polymer materials

A variety of natural and low-cost oil-absorbing materials have been investigated for oil spill recovery, including activated carbon [20],wool fibers [21], zeolites [22], straw [23], fly ash [24], etc. However,these traditional absorbent materials when facing frequent occurrence of oil spill accidents generally have some shortcomings including environmental incompatibility,low absorption capacity,poor recyclability,etc.Many other oil-absorbing materials such as acrylic ester resin,polyurethane and Janus polymer and magnetic fluoropolymer sponges[25],which were synthesized by chemical method, were also used for oil absorption.

As a novel absorbent, oil-absorbing resin is a polymer with low crosslinking degree and a three-dimensional(3D)network structure.The polymer has a hydrophobic nature due to hydrophobic monomer[26]. Meanwhile, the oil-adsorbing resin also possess oil absorption variety, fast oil absorption rate, high oil retention capacity,recycling facilitates,and excellent reusability[27].What is more,the resin materials have a great advantage in dealing with the oil slick problem since it could float on the water surface for a long time.In the next sections,three kinds of oil-absorbing resins and the corresponding fabrication strategies for oil absorption at each stage will be discussed in more detail,referencing the literature in each segment as appropriate.

3.1.1.Acrylic ester resin adsorbents for oil adsorption

As a novel class of functional polymer,acrylic ester resins are considered one of the potentially efficient oil-absorbing materials for oil absorption due to their high oil retention ability, good reusability,rapid adsorption rates,relatively inexpensive,high adsorption capacity and thermal stability.Especially,given the outstanding performance,the acrylic ester resins as one of the exceptional oil/water separation strategies,have been widely developed over the last decades.However,low oil absorption capacity,high cost and complex synthesis process are still the main factors that hinder the practical applications in oil/water separation [28]. Acrylic ester resin composites with high absorption capacity and good reusability for highly efficient separation of oils from the water surface are urgently required to be developed[29].

One of the most employed strategies is to use porous oxides to modify the acrylic ester resin.In this strategy,the porous oxides can effectively reduce the mass transfer resistance of foams and accelerate the oil absorption rate[30].Furthermore,the oil absorption process is frequently regulated by the coalescent of nanometal oxide[19].Noticeably,Li and her research group have obtained abundant outstanding research results,for example,MgO fiber/acrylic ester resin composites[31],Layered Double Oxides (LDOs)/acrylic ester resin composites [32] and MnO2nanowires/acrylic ester resin composites[33]with oil absorption properties have been synthesized by a suspension polymerization with the existence of porous oxides.For example,Yan et al.[31]prepared acrylic ester resin composites via facile suspension polymerization using biomorphic hollow MgO fiber as inorganic components. The acrylic ester resin composites have a high absorbency,28.22 g·g-1for chloroform,25.23 g·g-1for carbon tetrachloride,15.13 g·g-1for toluene,and 10.44 g·g-1for gasoline.Additionally,the relevant results of absorption kinetics experiment disclosed that the absorption process was fitted to the pseudo-second-order kinetics model.In addition,the composite showed excellent oil absorbency and could be reused for at least six times while keeping high oil absorbency.

Deriving from special layered structure of layered double hydroxide(LDH)precursors,LDOs have attracted great interest by the researchers in theoretical and practical application fields in recent years.Wang et al.[32] described the synthesis characterization, and the oil-absorbing properties of biomorphic LDO/acrylic ester resin obtained by combining the biological template method and suspension polymerization method.The oils absorption by acrylic resin composites is illustrated in Fig.1.It is indicated that oil absorbing properties and thermal stability of polymer-matrix composites were improved significantly due to the nanocomposites with low particle content which the physicochemical properties were superior to the pristine polymer. The resin composites showed excellent stability over 5 cycles of use and regeneration without significant decrease in the oil absorption. The synthesized composites exhibit excellent oil-absorbing properties,offering the combined benefit of the oil absorption capacity of porous LDOs and good reusability of oil-absorbing resins.

Fig.1.(A)Schematic illustration of the synthesis of porous Al2O3/acrylic resin composites.Reproduced with permission from reference[26].Copyright 2017 Elsevier Inc.(B and C)the oils absorption by acrylic resin composites,the oils were labeled by Sudan II(C1,pre-oil absorption;C2,after oil absorption).Reproduced with permission from reference[32].Copyright 2016 Elsevier Inc.

Currently,our group have developed some strategies to synthesize acrylic ester resin composites. These composites overcome poor oil-water selectivity, low oil absorbency, and poor oil retention of pure acrylic ester resins'fatal disadvantages.Zhang et al.[33]successfully prepared MnO2hollow spheres/poly(n-butylacrylate-cobutyl methacrylate-co-methyl methacrylate)resin composite by suspension polymerization under microwave irradiation.Yue et al.[26]reported a novel strategy to prepare hierarchical porous Al2O3/acrylic resin composites by combined hydrothermal method and microwave polymerization route,as well as their application in oil absorption.Schematic illustration of the synthesis of Al2O3/acrylic resin composites is shown in Fig. 1. The experiment results indicate that the resin composites exhibit good thermal stability and excellent recyclability.In addition to mental oxides,biomass materials,as an important oil-bearing crop,are widely planted all over the world.Rong et al.developed an effective method for fabricating 3D hydrophobic composite resin(HCR)network containing ellipsoidal-like rapeseed flower carbon for oil recovery[2].The HCR demonstrates excellent oil/organic solvent absorption performance, selective separation performance on immiscible oil-water mixtures as well as a high absorption capacity of 58.8 g·g-1.

Xin et al. [34] used the modified sphere-like chitosan with vinyltriethoxysilane(A151)to synthesize the acrylate resin composite by butyl methacrylate(BMA),butyl acrylate(BA),poly vinyl alcohol(PVA),N,N′-methylene bisacrylamide(MBA),benzoyl peroxide(BPO),and ethyl acetate under microwave irradiation.Owing to the unique properties such as oil body membranes and cellulose components within the structures of rapeseed meals (RSMs), the novel rapeseed meal-grafted-poly(methyl methacrylate-cobutyl acrylate)(RSMs-g-P(MMA-co-BA))oil-absorbents were prepared and used for oil/water separation by Yang et al. in the present study [35]. Specifically, the RSMs-g-P(MMA-co-BA) were successfully synthesized through free radical graft copolymerization from RSMs,methyl methacrylate acrylamide (MBA)as crosslinker.The reusability for gasoline was further proved and could be reusable up to 8 cycles holding 83.42% of their initial uptake capacity, which exerted more excellent properties than P(MMA-co-BA). The outstanding selectivity of the RSMs-g-P(MMA-co-BA)in the mixture of diesel and water was also visually verified in the end.These findings of the study may provide a reference for the fabrication of other natural materials as oil-absorbents for oil/water treatment and purification.

3.1.2.Polyurethane material for oil absorption

Polyurethane(PU)materials also are widely used around the world due to the promising properties,such as,corrosion resistance,wearresistance,micro-structure,relatively small density.However,single polyurethane materials as absorbent could not meet the practical application due to the low adsorption capacity.Thus,the integration of other materials gradually begins a trend. Yuan et al. [18] reported a novel strategy of preparing composite foams with hierarchical porous structures through a one-step method for effective oil/water separation and selective oil adsorption.The PU composite foam is synthetized by a facile one-step foaming technology,which uses hierarchical hollow SiO2@MnO2cubes to modify the inner structures of composite polyurethane foam.With introduction of the hierarchical hollow SiO2@MnO2cubes,hierarchical hollow structure and large surface area of the sample are beneficial to the improved oil adsorption capability of composite PU foam.The synthetic route of the composite PU foam modified with hierarchical hollow MnO2@SiO2cubes is shown in Fig.2.The resulting composite PU foam exhibits high oil adsorption capacity and high selectivity, and oil adsorption capacity for carbon tetrachloride was measured to be 31.6 g·g-1.Also,the composite PU foam shows excellent elasticity, and the height of the composite PU foam decreased from 100%to 94%after 30 compressing-releasing cycles.

Fig.2.Schematic illustration of fabrication of the composite PU foam.Reproduced with permission from reference[36].Copyright 2017 Elsevier Inc.

Zhang et al.[37]fabricated a versatile oil-absorbing material composed of MnO2nanowires/PU foams,namely the superlong MnO2nanowires PU foam composites,using a PU sponge as a porous substrate and MnO2nanowires as modifiers.The hydrothermal method is employed to synthesize MnO2nanowires and then foaming technology is used to fabricate MnO2nanowires/PU foam composites.In order to enhance the hydrophobic and oleophilic properties,the surfaces of MnO2nanowires are chemically modified using silane coupling agent.The microstructures of these PU foams are affected by the content of MnO2nanowires. It was found that the sorbent is capable of scavenging 36.42 g·g-1of chloroform,14.66 g·g-1of toluene,and 4.54 g·g-1of edible oils.Besides,the foam composites are subjected to five adsorption-desorption cycles and the excellent reusability is demonstrated.The as-synthesized PU foam composites can work as an efficient and durable oil-absorbing material for the separation of oils and organic solvents.

Nanodiamonds(NDs)are among the most promising new carbonbased materials for various applications.Inspired by the remarkable adhesive ability of dopamine,Cao et al. reported a feasible approach for the preparation of superhydrophobic ND particles[38].The strategy is based on coating hydroxylated NDs with polydopamine(PDA)and subsequent reaction with 1H,1H,2H,2H-perfluorodecanethiol(PFDT).The resulting f-PDA modified NDs(NDs-fPDA)were firmly anchored onto the skeleton of commercial PU sponge due to the excellent adhesive ability of PDA.Meanwhile,the as-prepared sponge showed superhydrophobic property,oil/water separation behavior and high organic adsorption capacity. Graphene/PU sponges with superhydrophobicity have been one-pot synthesized by solvothermal technique[39].The surfaces of interconnected pores within the PU sponges were modified with(3-Mercaptopropyl)trimethoxysilane and graphite oxide via a solvothermal treatment,allowing craterlike functionalized graphene layers to be formed as a substructure and attached firmly to the polyurethane skeleton. By forming such characteristic nano-micro substructures on the backbones of PU sponges,the graphene/PU sponges possessed a superhydrophobicity with WCA exceeding 160°.When applied in conjunction with a simple vacuum system,this sponge could act as a selective filter to continuously and effectively separate the oil from water.Because the FGN/PU sponge retained original PU structural integrity, the material was chemically robust and capable of separating oil up to 53000 times of its own mass with high oil-water separation efficiency(＞99.5%).

Lu et al. [40] reported a facile strategy to synthesize silica nanoparticles-coated graphene oxide (SiO2/GO) nanohybrids using alkyl-rich silicane in GO ethanol-water solution at room temperature.The SiO2/GO nanohybrids are coated on the PU sponge surface,forming a roughness and hydrophobic surface, as well as the wellpreserved flexible, porous structure of PU sponge. The corresponding exploration also focused on constructing large surface area, microporous, and hydrophobic ternary sorbent by dipping the commercial PU sponge into SiO2/GO nanohybrids ethanol-water solution without damaging the intrinsic structure of polyurethane sponge and no extra processing procedure.The obtained M-PU sponge can be served as high and quick sorbent of organic solvents and oil,which is comparable to the reported sorbents in literature. More significantly, M-PU sponge can be recycled with imperceptible loss of sorption capability and hydrophobicity.

Li et al.[41]fabricated the superhydrophobic PU sponge by coating superhydrophobic attapulgite(APT)onto its skeleton surface via a facial ultrasonic dip-coating method.The coated PU sponges exhibit robust superhydrophobicity and high adsorption capability under a series of harsh conditions,which are used for the separation of mixtures of oil and various corrosive solutions and hot water.Moreover,the coated PU sponges can selectively adsorb oils from mixtures under extreme and harsh turbulent conditions. More importantly, the coated PU sponges can separate tiny oil droplets from surfactant-stabilized oilin-water emulsions with a separation efficiency of over 99.87% by employing a compression and agitation procedure.

Wang et al.[42]adapt a common and feasible approach to fabricate carbon nanotubes (CNTs) reinforced PU sponge that shows superhydrophobic and superoleophilic properties.The method involves the oxidative self-polymerization of dopamine and the reaction of hydrophilic polydopamine(PDA)with hydrophobic octadecylamine(ODA).The as-prepared sponge could quickly and selectively absorb various kinds of oils up to 34.9 times of its own weight,and the absorbed oils can be collected by a simple squeezing process. More interestingly,the mechanical strength of the as-prepared sponge is improved due to the structural reinforcement of CNTs anchored on the sponge skeleton.Furthermore, the recovered sponge could be reused to separate oilwater mixture 150 times while maintaining its high absorption capacity.In another study,a very hydrophobic and superoleophilic sponge was fabricated by Xi et al. via a homogeneous coating of soot on the polyurethane sponge (PUS) framework [43]. The composites have a large absorption capacity of up to 45 times its original weight for pump oil and 86 times its original weight for chloroform, along with high recyclability of more than 60 cycles.Due to the lower cost than other absorption materials,SPUS is considered to be a very promising absorbent for the treatment of oil spills or oil water separation.

Zhu et al.[44]utilized a robust superhydrophobic polysiloxane layer coated onto the surface of 3D porous polyurethane sponges through a one-step solution immersion method.The durability of the resulting sponges was investigated by repeated mechanical compressions,ultrasonication in polar solvents,and strong acid/alkali attacks.Results revealed that the superhydrophobic sponges showed excellent elasticity,high mechanical durability and good chemical stability.By combining the special wettability and high porosity,the sponges exhibited high oil absorption capacity and high selectivity when they were employed as absorptive materials for cleaning oils on the water surface. More importantly, the superhydrophobic sponges could be reused for oil-water separation for more than 300 cycles without losing their superhydrophobicity,exhibiting the highest reusability and durability among the reported counterparts.Therefore,the present study offers a simple and low-cost strategy for large-scale fabrication of robust superhydrophobic 3D porous materials that might be applied to the cleanup of oil spills on the water surface.

3.1.3.Other oil-absorbing polymer material

In addition to the abovementioned two types of polymer materials,there are many kinds of polymer materials for oil/water separation.The natural hydrophilicity of the cellulose ensured its excellent underwater superoleophobicity and antifouling properties.Thus,the composites based on cellulose materials have been broadly reported. Wang et al.[45]fabricated a structured cellulose sponge with stable superoleophobicity (water CA ＞150°) under water and superhydrophilic wettability(CA ≈0°)under oil without any further chemical modification for oil-water emulsion separation.Interestingly,the double layer construction of different pore sizes,which contains a top-layer with a pore size lower than 1 μm and a sub-layer with a pore size larger than 3 μm,ensures that the oil phase is resisted and the water phase easily and quickly permeates the sponge.The novel sponge can obtain high separation efficiency(＞99.94%),solely using gravity and has excellent antifouling properties.

Zheng et al.[46]reported the integration of cellulose and polymer.Cross-linked polyvinyl alcohol(PVA)-cellulose nanofibril(CNF)hybrid organic aerogels were prepared using an environmentally friendly freeze-drying process,then modified with methyltrichlorosilane via a simple thermal chemical vapor deposition process.The silane-treated,cross-linked PVA/CNF aerogels not only exhibited excellent absorption performance for various types of oils or organic solvents, but also showed a remarkable scavenging capability for several types of heavy metal ions tested(e.g.,Pb2+,Hg2+),making them versatile absorbents for various potential applications including water purification.

Chin et al.[47]developed a simple method for the preparation of highly porous and magnetic hydrophobic cellulose/TiO2aerogel by CO2supercritical point drying of magnetic cellulose gel and followed by surface coating with TiO2.Hydrophobic,magnetic and highly porous cellulose aerogel was prepared by a simple method for fast and selective absorption of oil from water surface.The aerogel was able to absorb oil up to about 28 times of its own weight within 10 min and could be easily removed and recovered from the water surface by an external magnet.It could be either reused after washing with ethanol or incinerated with the absorbed oil.The potential application of cellulose aerogel as an oil absorbent was demonstrated by its ease of preparation,low cost of precursor materials,magnetically retrievability,as well as high oil absorption capacity and efficiency.

Lin et al.[48]developed a hydrophobic,ultralight,porous and flexible cellulose aerogel with excellent adsorption properties and reusability in a considerably facile and rapid way.Cold plasma technology was employed for hydrophobic modification of the aerogel surface using trimethyl-chlorosilane(TMCS)as the plasma.Notably,the modification procedure was considerably more rapid(less than 3 min),more efficient and economically viable than traditional methods. The novel method not only modifies the surface chemical composition but it also alters the inner structure of the aerogels because of the high activity of the modifier.The absorption capacity of these cellulose aerogels is not only dramatically higher than commercial oil adsorbent sheets, but the capacity remains more or less constant even after 15 cycles.These modified cellulose aerogels are environment-friendly,green,reusable,abundant,and highly absorbent materials.Therefore,these cellulose aerogels can be used for separating organic contaminants.

Organic and inorganic hybrid materials gradually draw scientists'attention due to the promising microstructure,enhanced strength and larger specific surface area. Apart from the abovementioned three kinds of polymer,there are many polymers that are used for oil adsorption for oily wastewater treatment.Aydin[49]reported that organicinorganic hybrid gels were synthesized by the condensation of a linear aliphatic diol (1,8-octanediol) and altering the chain length of the alkyltriethoxysilanes through a bulk polymerization process without using any initiator,activator,catalyst,or solvent for the selective removal of oils from water. The following adsorption results showed that hybrid gels have high and fast absorption capacities and excellent reusability.Good selectivity, high thermal stability,low density,and excellent recyclability for the oil removal give the material potential applications.

3.2.Inorganic material for oil absorption

In the past few decades, some natural inorganic materials with adsorption properties have attracted attention of the world due to their preferable properties such as cost effectiveness,recyclability,facile processing and environmentally friendly nature. Large numbers of natural and low-cost inorganic materials,namely mineral clays,sand,carbon-based materials,porous membrane materials,inorganic polymer materials and metal based-materials, have been used to absorb oils and organic solvents from an oil/water mixture.Among the various sorbents used,carbon-based materials,in terms of cost,versatility and abundance,appears to be attractive material[50].

3.2.1.Traditional natural inorganic material

In the past report,most of the natural inorganic oil-absorbing materials are mineral clay,such as attapulgite[51,52],sand[53],zeolite[22],bentonite [54], palygorskite [55], modified or combined with other materials.

Attapulgite (ATP) as a kind of hydrated magnesium aluminum silicate usually presents in nature as fibrillate mineral. Liang et al.[52] used the natural, low-cost ATP nanocrystals for fabrication of three-dimensional porous ATP monolith employing CaCO3as a hard template. By treatment with polydimethylsiloxane using chemical vapor deposition(CVD)method,the resulting hydrophilic ATP monolith exhibits superhydrophobicity with a water CA of 151.4°and excellent oleophilicity,which makes the ATP monolith efficient absorbent for selective absorption of organics and oils from water.

Sand,a kind of inorganic natural resources and massively existed in the desert,in addition,the main component of sand is silicon dioxide.Li et al.[53]used sands as oil-absorption materials for selective separation of water and oil.The desert sands could serve as adsorbent material with underoil superhydrophilicity for efficient gravity-directed separation of various surfactant-stabilized and surfactant free water-in-oil emulsions with high flux,even the interspaces between sand particles are larger than that of emulsified droplets.

Shavandi[56]et al.reported adsorption of residue oil from palm oil mill effluent using natural zeolite and investigated the effect of different operational parameters such as pH, dose of adsorbent, stirring rate,contact time and initial oil concentration.The result showed that oil removal efficiencies by natural zeolite were up to 70%at a pH of 3.0 and 50 min of contact time. Zadaka-Amir et al. [57] investigated the adsorption of oil,spilled on a surface,by a variety of mineral sorbents.The result showed that edible oil adsorption was low on motmorillonite,high on sepiolite and extremely high on talc,suggesting that the magnitude of adsorption correlates with clay hydrophobicity.Despite the high adsorption on talc the efficiency of the clay to remove the oil was low,reaching 60% removal, while complete oil removal was achieved by sepiolite and only 45%by montmorillonite.Besides,oil adsorption by the high quality sepiolite was nearly complete even at an oil/sorbent ratio of 6 (w/w). However, although traditional natural inorganic material has lots of remarkable performance such as wide source,lowcost and facile preparation,it is lower absorbency and efficiency and unsuitable for specific occasions than synthetic composites.

3.2.2.Carbon-based materials

Carbon-based materials possess some unusual surface properties such as morphological,electrical,optical,and mechanical properties,which are always important research focus and gain widespread attention[58].Among them,carbon-based materials with intrinsic hydrophobic property have potential applications as a class of oil sorbents in the field of oil-spill cleanup.Reviewing the past few years of research,the types of carbon-based materials as oil sorbents have undergone great changes, form traditional carbon-based oil sorbents materials such as activated carbon, carbon black and graphite to recently hot carbon-based materials such as carbon nanoparticles,carbon nanofibers(CNFs),carbon nanotubes and graphene and so on.

Carbon nanotubes(CNTs)with a high surface area and superwetting properties are essential for applications in the oil recovery,which have a one-dimensional structure and inherent oleophilicity and hydrophobicity. 3D Carbon nanotube (CNT)-based aerogels/sponges with the light-weight,high porosity, and large surface area have proven their efficiency as super absorbents for oil/water separation[59].Recently,piperopoulos et al.[60].reported a synthesis of silicone foams(SFs)containing CNTs for oil-spill cleanup,increasing the mechanical properties and reusability of the foam by using of CNTs.By comparison of SFs filled with pristine CNTs and functionalized ones, demonstrating that SFs filled with functional CNTs reduce the foams'mechanical strength and the number of the recycling cycles,which are beneficial for economy and environment.However,the absorption capacity of the CNT-filled foams is relatively low but comparable with literature data and lacking of comparison with other carbon fillers.

Besides CNT-based aerogels/sponges with 3D network structure,carbon nanotube membrane has also attracted great attention for oilspill cleanup.Gui et al.[61].reported the fabrication of CNT sponges as efficient sorbent materials for oil and organic reagent sorption via CVD.The sorption capacities of many pollutants by these CNT sponges are over 100 g·g-1and up to 20-40 g·g-1after 10 cycles of absorption.Liu et al.[62]reported a facile approach to fabricating the polymer@CNT nanohybrid membranes with high water permeation,excellent antifouling properties and high separation efficiency.Modifying CNT with a series of polymers endowed the membranes with different surface charge and hydrophilicity.

As a kind of two-dimensional carbon-based materials,graphene and its derivatives have attracted great attention due to their unusual physicochemical properties.As we know,besides their intriguing physical and chemical properties,graphene-based materials have recently been reported to be provided with exciting hydrophobic properties [63].Moreover,three-dimensional(3D)graphene aerogels(GAs)/graphene sponges(GSs) generally exhibit macro-porous structures, extremely low densities, large internal surface areas, and high conductivities,marking a major step forward in the translation of the outstanding properties of individual graphene layers into macroscopic structures[64]. For example, Ge et al. [65] report a Joule-heated graphenewrapped sponge (GWS) for the high-speed absorbing viscous crude oil, which exhibiting an excellent potential in not only fundamental research but also practical applications in the field of crude-oil spills.As we all know, on the basis of the above concerns, individual CNT and graphene-based oil absorbents as popular carbon-based materials have shown promising results in selectively removing oil from water.There already were many task group reporting CNT/graphene composite materials which integrates the excellent performance of both,as oil absorbents in cleaning-up oil spills[66-68].Carbon nanotubes(CNTs)have good toughness and hydrophobicity,which could modify various properties of the graphene aerogel (GA) when CNTs are embed into the GA network. Wan et al. [69] fabricated graphene-CNTs aerogel(GCA)by one-step hydrothermal redox reaction which is a facile and green approach. The incorporation of CNTs into GA improves the morphologies,specific surface areas and hydrophobic properties and enhances the adsorption capacity and mechanical properties of GA.The prepared aerogels possess ultralight density ranging from approximately 6.2-12.8 mg·cm-3and the adsorption capacity reaching 100-270 times of its own mass.

Moreover,Sun et al.[70]prepared ultra-flyweight aerogels(UFAs)with extremely low density that is even smaller than the density of air at ambient conditions, which are made by freeze-drying process from the mix aqueous of CNTs and GGO.The obtained UFA with high adsorption capacity and reusability,besides,the preparation technology is environmentally friendly, facilitates and controllable, which can change and control UFAs with diverse shapes.These desirable multifunctional attributes would enable many UFA applications,including elastic and flexible conductors,high-performance conductive polymer composites,organic absorbents,environmental remediation materials,phase-change energy storage, sensors, supercapacitors, and catalyst beds.

3.2.3.Metal based-materials

Metal materials as one of the big family of inorganic materials have always been important research hot spots and focus due to a wide range of types and quantities,as well excellent physical and chemical properties.Among them,inorganic metal materials with low surface energy can be used as oil sorbents, such as metal-based particles,mesh and foam and so on[5].

Particles with hydrophobicity have been investigated over the past few decades due to their small size,high surface area and adsorption mechanism for oil-spill cleanup[71].Furthermore,particles have their special advantage that cannot be compete with films or monoliths in certain circumstances,such as when oil is spilled on ocean,initial bulk recovery treatment will leave a thin oil layer floating on the water surface. For some metal oxides that are not inherently hydrophobic,hydrophobilization can go along the in situ surface by a series of chemical reaction processes,such as CaCO3,SiO2and magnetic Fe3O4.

As one of the significant functional materials, metal-based foam materials have attracted intensive attentions in recent years,mainly due to their remarkable virtues such as ultralow density,excellent electrical conductivity,high specific surface area and porosity,microscopic porous structure and special chemical properties and so on. These kinds of materials have extensive applications in energy system,sensor,catalysis,and oil/water separation, etc. There were many researches mainly focused on simple mental such as Au,Ni,Cu,Ti foams,even alloy foams and metal-organic frameworks(MOFs)due to the combination of excellent properties and mature processing techniques.Rong et al.[72] reported the commercial copper foam coated with Cu3(PO4)2·H2O nanosheets through a facile and low-consumption in situ selfsacrificial template method and then modified with KH 570 vapor.The preparation process of superhydrophobic copper foam is illustrated in Fig.3.The as-prepared superhydrophobic copper foam had large pore structure,high water contact angle(153°),low sliding angle(4.5°).Further experimental results demonstrated that the superhydrophobic copper foam could capture the oil from the water quickly and selectively. Xu et al. [73] fabricated copper nanowire (CuNW) based aerogel/foam with ultralight flexible pressure sensing performance and hydrophobicity through a one-pot method and freeze-drying technology,which broadens their potential applications.

Compared with the difficult recycle of the metal-based particles and complex preparation process of metal-based foam oil sorbents,metalbased mesh has always arousing research interest due to its robust mechanical properties, simple fabrication and suitability for largescale occasions.The excellent separation performance of metal-based mesh achieved in the laboratory is now being applied to a large scale in practical applications[74].To the best of our knowledge,there were mainly several metal meshes used for oil/water separation, such as commercial stainless steel mesh and copper mesh[75],etc.After Jiang et al. [76] reported that a stainless-steel mesh was used to fabricate superhydrophobic/superoleophilic surfaces, inspired by the work,lots of mesh films with both superhydrophobic and superoleophilic properties have been developed for oil-spill clean-up. Li et al. [77]reported a facile,low-cost and efficient method to fabricate underwater superoleophobic SiO2coated surfaces via spraying mixtures of hydrophilic SiO2nanoparticles (NPs) and waterborne polyurethane (PU)onto stainless steel mesh substrates,which is suitable for separating polluted oil from corrosive and hot water.The separation efficiency is up to 99.0%.A novel all-inorganic Cu(OH)2nanowire-haired membrane has been fabricated by a facile surface oxidation of copper mesh and own extremely high separation efficiency for oil-water mixture[78].Gao et al. [79] prepared a superhydrophobic and oleophilic copper mesh film with hierarchical structures via facile electrodeposition and immersion processes.The WAC of modified mesh were up to 152.4°.Besides,the separation efficiency was above 90%with high oil flux.

3.2.4.Inorganic-based membrane materials

Membrane technology,a new and important separation technology,has been widely used in water treatment as the greatest demand for membrane products in the practical applications.Among them,inorganic membrane materials,consisting of inorganic composite materials[80,81]and organic-inorganic hybrid materials[82]except mentioned above carbon-based materials and metal-based materials,have always attracted enormous research attention to achieve separating behavior where the oil phase spreads and penetrates through materials easily while the water phase is simultaneously repelled.Recently,designing superhydrophilic/underwater superoleophobic membranes have been attracted more attention due to both the high-surface-energy material and surface roughness provide which can be effective in separating oil/water[83,84].

Fig.3.Schematic illustration of process of(A)the in situ self-sacrificial template method for preparation of superhydrophobic copper foam and(B)the steaming modification via KH570 vapor.Reproduced with permission from reference[72].Copyright 2018 Elsevier Inc.

Inorganic membranes have attracted attention on account of their higher mechanical strength and thermal stability under severe conditions [85]. He et al. [86] reported that a self-cleaning coating which derived from poly(2-methacryloyloxylethyl phosphorylcholine)was grafted on the inorganic-based membrane and both can oil repellency completely in underwater superoleophobicity and oil allows effective cleaned of fouled on dry surfaces by only water.Yuan et al.[87]constructed thermally stable and free-standing cryptomelane nanowire membranes with a porous network which exhibits controlled wetting behavior ranging from superhydrophilic to superhydrophobic via a self-assembly method.Zhang et al.[89]used HCl to chemically etch aluminum alloy substrates followed by spraying coating of hydrophobic nanoparticles to synthesize a highly robust superhydrophobic surface.Electrodeposited superhydrophobic conducting polythiophene coating had been reported on stainless steel on colloidal templation[90].

Two important factors should be considered to designing inorganic membranes,including possessing not only superhydrophilic and underwater superoleophobic property but also rough microstructure on the surface[88].There have been a number of techniques developed for the fabrication of superhydrophobic surface, such as dip coating,in situ chemical-reaction method(such as chemical etching,wet chemical reaction and electrodeposition),vapor deposition,spray coating,electrospinning and so on.To obtain the superhydrophobic surface,our group reported a low-cost and durable flexible membrane made of layered double hydroxide nanosheets on cellulose support based on construction of hierarchical rough structures and post hydrophobization[91].Fig.4 shows the schematic of the growth process for the fabrication of the LDH coating on cellulose.The obtained LDH membrane showed both superhydrophobic and superoleophilic propertied simultaneously.To optimize the fabrication process, we have developed a facile onestep strategy to fabricate superhydrophobic LDH/cellulose membranes in an open oil/water two-phase system[92].The surface morphology transformations of cellulose are being displayed in Fig.4B-D.In this system,the two dominant factors of superhydrophobic properties,surface roughness and low surface energy,could be obtained simultaneously to meet the demand of rapid construction of superhydrophobic membranes.Wang et al.[93]fabricated superhydrophobic nonwoven membrane by hydrothermal growth of ZnO hierarchical nanorods on the surface of atomic-layer-deposited ZnO ultrathin layer. However, oil absorption properties of membrane materials are limited by the insufficient pore volume,and most of membrane materials are used as filters for oil/water separation.

3.2.5.Inorganic polymer materials

As we all know, hierarchical roughness and low surface-energy on the surface of materials commonly endow surfaces with superhydrophobicity[94].Various materials have been selected as substrates for the preparation of superhydrophobic surface,such as textiles,metal mesh,porous sponges and polymer films[95,96].Hence,it is intensity expected that superhydrophobic surfaces are to be fabricated with a simple low-cost and environmentally friendly method and appropriate material.Modifying with inorganic polymer is a good choose.A novel high aspect ratio superhydrophobic film consist of vertically oriented titanate nanotubes after fluoroalkylsilane modification have been fabricated and possess intelligent response function by UV illumination[97]. Fang et al. [98] prepared a superhydrophobic membrane by electrospinning N-perfluorooctyl-substituted polyurethanes for oilwater separation,and the membrane had self-healing ability dependent on the migration of low-surface-energy fluorine-containing polymer[98].

Fig.4.Schematic of the growth process for the fabrication of the LDH coating on cellulose(A);SEM imagines of the AlOOH/cellulose membrane(B),the LDH/cellulose membrane(C)and the modified LDH/cellulose membrane(D).Reproduced with permission from reference[91,92].Copyright 2018 Elsevier Inc.

A facile method to fabricate superhydrophobic surfaces on steel substrates via electroless replacement deposition of copper sulfate and UV curing of vinyl-terminated polydimethylsiloxane,and possessed high separation efficiency and excellent reusability in oil-water separation[99].Meanwhile,a large amount of research about superhydrophobic textiles has been published due to the obvious advantages of low cost,flexibility,chemical stability and high absorption capability.A durable and robust superhydrophobic textile coating has fabricated by mixing polydimethylsiloxane with fluorinated alkyl silane and functionalized silica nanoparticles[100].Xue et al.[101]fabricated a superhydrophobic textile through three steps and possess durability against abrasion,including creation of rough structure by alkali etching, modified by mercapto-siloxane and hydrophobization with fluorine-containing materials.Zhang et al.[102]successfully prepared a superhydrophobic and superoleophilic textile by one-step growth of silicone nanofilaments via chemical vapor deposition of trichloromethylsilane and as a membrane for oil-water separation and selective oil absorption.Particularly,a simple vapor-liquid sol-gel approach to fabricating high stability polydimethylsiloxane-silica superhydrophobic surfaces on the cross-structure polyester textiles for oil adsorption which is very beneficial for environment safety and protection[103].

3.3.Biomass material for oil absorption

Biomass materials are the most abundant renewable resources on earth, with excellent performance and environmental friendliness.However,these biomass materials are not fully utilized.For example,agricultural and sideline products are often incinerated or decomposed after being discarded, which pollutes the environment and wastes resources to some extent. In the past, people used various biomass materials as adsorbents to remove oil pollutants from water,such as straw[3],wood chips[104],etc.However,due to the poor pore structure of these materials,the adsorption capacity of these materials is usually very low. In addition, these materials lack the selectivity of oil and water, which can absorb oil pollutants and water at the same time,which will further reduce the separation efficiency.Therefore,a great deal of research work is devoted to the development of biomass adsorbent with special wetting characteristics,which can selectively absorb oil pollutants, but can repel water. Through the deep processing of raw material,the development of functional cellulose matrix composites was found to be an effective way to utilize biomass materials[105,106]. Cellulose material mesh porous structure is conducive to the adsorption,with non-toxic,low price,good biodegradability,chemical stability and biocompatibility and wettability,as the ideal choice for preparation of oil/water separation materials.

3.3.1.Cellulose-based aerogel absorbents

Aerogel has good physical properties,such as low density,high pore structure,large specific surface area and excellent elastic properties.In recent years,various types of biomass aerogel have been reported in the literature.In the new development of aerogel,on the basis of the biomass fiber porous and ultralow density of biomass fiber aerogels due to its environmental and economic prospects,such as rich source,renewable,biodegradable,and easily surface modification,in the field of adsorption has attracted considerable research interest.

Sun et al. [107] fabricated a superhydrophilic and underwater superoleophobic robust sulfonated cellulose nanofibers(CNF)aerogel that was fabricated by using cellulose nanofiber through regioselective oxidative sulfonation treatment. The aerogel exhibits underwater superoleophobicity(θoil＞150°)characteristic for various oils,which could be used as a filter for oil/water separation.The sulfonated CNF aerogel maintained high oil-water separation efficiency by simply filtration. The oil content in filtrated water was always less than 300 ppm(0.03%),and kept a stable recyclability even after 20 cycles,which could be practically used as a filter for oil/water. Cheng et al.[108]investigated the pure cotton and cotton-cellulose aerogels are obtained using a cost-effective mixing-blending method with polyamideepichlorohydrin as strengthening additives.The obtained aerogels are silanized using methyltrimethoxysilane via a facile chemical vapor deposition to endow aerogels with hydrophobic surface. Effects of fiber concentrations and cotton-to-cellulose mass ratio on oil absorption performance in various solvents are also investigated.The cotton aerogel with an initial concentration of 0.25 wt%presents the highest oil absorption capacity over 100 g·g-1.Besides,the cotton/cellulose aerogels also demonstrate good absorption capacity in different pollutant organics.Ossi Laitinen et al.[109]demonstrates a straightforward method of producing a cellulose nanofibril aerogel that is low-cost,ultralight,highly porous, hydrophobic,and reusable superabsorbing cellulose nanofibril aerogel from recycled waste fibers using a simple,environmentally friendly nanofibrillation treatment involving deep eutectic solvent and freeze-drying.These sponges exhibited excellent absorption performances for various oils and organic solvents and were reusable.In particular,the nanofibril aerogels showed selectivity in absorbing marine diesel oil from an oil-water mixture and possessed ultrahigh absorption capacities of up to 142.9 g·g-1,much higher than those of the commercial absorbent materials(i.e.,polypropylene-based material)(8.1-24.6 g·g-1)that were used as references.The absorbed oil could easily be recovered by means of simple mechanical squeezing.In addition, the nanofibril sponges exhibited excellent reusability,maintaining a high capacity to absorb diesel oil for at least 30 cycles at 71.4%-81.0%of capacity compared to a fresh absorbent.

Li et al.[110]presents a robust salt-tolerant superoleophobic aerogel inspired by seaweed used without any further chemical modification for oil-seawater separation.The green aerogel is prepared by freezedrying of sodium alginate(SA)-nanofibrillated cellulose(NFC) using Ca2+ions as the crosslinking agent.The three-dimensional(3D)interconnected network structure of the developed aerogel ensures its high mechanical strength and good flexibility. The natural hydrophilicity of the polysaccharides contained in the aerogel ensures its excellent underwater superoleophobicity,antifouling and salt-tolerance properties.More impressively,the as-prepared aerogel can even keep its underwater superoleophobicity and high hydrophilicity after being immersed in seawater for 30 days,indicating its good stability in marine environments.Furthermore,the aerogel could separate oil-seawater mixtures with a high separation efficiency(of up to 99.65%)and good reusability(at least 40 cycles).The facile and green fabrication process combined with the excellent separation performance and good reusability makes it possible to develop engineering materials for oil-water separation in marine environments. The green, ultralight, and highly porous material was successfully prepared from paper waste cellulose fibers by Son T. Nguyen et al. [111]. The material was functionalized with methyltrimethoxysilane(MTMS) to enhance its hydrophobicity and oleophilicity.Water contact angles of 143°and 145°were obtained for the MTMS-coated recycled cellulose aerogel. The aerogel achieved high absorption capacities of 18.4,18.5,and 20.5 g·g-1for three different crude oils at 25 °C,respectively.In the investigated temperature range of 10,25,40,and 60°C for the absorption of the tested crude oil on the aerogel, a highest absorption capacity of 24.4 g·g-1was obtained.It was found that the viscosity of the crude oils is the main factor affecting their absorption onto the aerogel.The strong affinity of the MTMS-coated recycled cellulose aerogel to the oils makes the aerogel a good absorbent for crude oil spill cleaning.An novel superhydrophobic microfibrillated cellulose aerogels(HMFCAs)with high lipophilicity,ultralow density (≤5.08 mg·cm-3), superior porosity (≥99.68%) as well as extremely high mechanical stability were successfully prepared from microfibrillated cellulose aerogels(MFCAs)via a facile and environmentally friendly silanization reaction in liquid phase by Zhou et al.[112]. The superhydrophobicity of the as-prepared HMFCAs (water contact angle as high as 151.8°) was attributed to the formation of polysiloxane on the surface of HMFCAs by the silanization reaction.The HMFCAs exhibited excellent oil/water selective absorption capacity with oil absorption up to 159 g·g-1. The reusability experiment showed that the adsorption capacity still exceeded 92 g·g-1for pump oil after 30 absorption cycles,demonstrating its superior reusability.

A highly efficient absorbent (cellulose based aerogel) with a low density and high mechanical strength was fabricated via a novel physical-chemical foaming method,plasma treatment and subsequent silane modification process by Zhang et al. [113]. This aerogel has a perfect 3D skeleton and interconnected pores similar to honeycomb,which are favorable to oil adsorption and storage.More importantly,without introducing additional micro/nanoparticles,the rough micro/nano structure of the surface was directly constructed using plasma irradiation in this study.The low surface energy substrate was further introduced using a simple physical-soaking method and the resulting aerogel exhibited excellent superhydrophobicity (WCA ＞156°) and superoleophilicity (OCA = 0°), which can selectively and efficiently absorb various oils or organic solvents from polluted water.In addition,this aerogel has a high storage capacity and absorption capacity(up to 4300%and 99%of its mass and volume,respectively).More interestingly, this aerogel exhibits excellent mechanical abrasion resistance and corrosion resistance even in strong acid, alkali solution and salt marine environment.The aerogel could be reused more than 30 times after removal of the absorbed oil by rinsing with ethanol.

3.3.2.Cellulose-based membrane filter

Because most oils and water are intrinsically immiscible with each other,they can naturally divide into two layers.In the treatment of a large amount of oily wastewater, the simple use of adsorbents not only wastes time but also costs more, however the membrane filter can solve this problem very well[114].

Huang et al.[115]investigated the effect on the hydrophobic properties of cellulose-based filter paper by modifying the surface through grafting ESO with varying weight percents onto the cellulose fiber via an epoxide ring-opening polymerization process. This led to the replacement of the hydroxyl groups on the surface of the cellulose fibers with hydrophobic alkane groups, forming modified materials with different hydrophobic properties and surface morphologies.The water contact angle of Paper/PESO40 reached 145.1°,not only because of the changes in the surface composition but also because of the variation in the surface morphology.

Chenghong Ao et al. [116] fabricated a novel superhydrophilic graphene oxide (GO)@electrospun cellulose nanofiber (CNF) membrane that exhibited a high separation efficiency,excellent antifouling properties, as well as a high flux for the gravity-driven oil/water separation.Moreover,the GO@CNF membrane was capable to effectively separate oil/water mixtures in a broad pH range or with a high concentration of salt and the GO@CNF membrane exhibits underwater superoleophobicity with an underwater oil contact angle (OCA) of 155°when an oil (1,2-dichloroethane) droplet is placed on the membrane surface.

3.3.3.Biomass carbon-based absorbents

Carbon fibers retained porous network structure of biomass fibers,although the mechanical properties of biomass carbon fibers decreased,the biomass carbon fiber with good hydrophobic more rough surface of the porous in the field of oil/water separation has attracted the attention of a large number of researchers[117].

For example,Li et al.[118]report fabrication of pressure-sensitive and conductive biomass carbon aerogels by pyrolysis of cellulose aerogels composed of poplars catkin(PC)microfibers with a tubular structure. The resultant biomass carbon aerogels exhibit ultralow density (4.3 mg·cm-1), high compressibility (80%), high electrical conductivity(0.47 S·cm-1),and high absorbency(80-161 g·g-1)for oils and organic liquids. Distillation and combustion can be used for the recycling of the biomass carbon aerogels after absorption of oils owing to its good thermal stability and fire resistance.

Recently,our group adopted a simple alkalization,bleaching,freezedrying and carbonization route to prepare the ultralight, elastic,superhydrophobic and durable carbon fiber aerogels (CFAs)derived from sisal leaves.The fabrication process of CFAs is presented in Fig.5.The CFAs exhibited a superior absorption capacity in the range from 90 to 188 times of their own weight for different organic solvents and oils.After recycling 10 times,the CFAs still exhibit excellent oils absorption properties,as well as exhibit excellent stability under harsh conditions,showing that the CFAs has excellent reusability performance.

Direct conversion of biomass to porous aerogel provides a promising approach to developing oils absorbent materials for oils and organic solvents absorption.The winter melon carbon aerogel has been fabricated via a hydrothermal and post-pyrolysis process using winter melon as raw materials by Li et al.[120].This two-step process is a totally green,chemical-free,synthetic method with cheap and ubiquitous biomass as the only raw material.The winter melon carbon aerogel showed a low density of 0.048 g·cm-3and excellent hydrophobicity with a water contact angle of 135°.The absorption capacity of winter melon carbon aerogel can be 16-50 times its own weight for organic solvents and oils. Distillation was employed to regenerate of winter melon carbon aerogel and harvest the pollutants. After five absorptionharvesting cycles, the absorption capacity of winter melon carbon aerogel to organic solvents and low boiling point oils can recover to almost 100%of the starting value.The absorption capacity for crude oil decrease to 48%due to the solid residues.

Fig.5.The typical procedure for fabricating of CFAs.Reproduced with permission from reference[119].Copyright 2018 Springer Nature.

Bi et al. [121]fabricated twisted carbon fiber(TCF)aerogels via a facile method using an economic,environmentally friendly raw cotton.The TCF aerogels are three-dimensional network structures,which have excellent wettability.Importantly,the TCF aerogel can absorb a wide range of organic solvents and oils with a maximum sorption capacity up to 192 times the weight of the pristine TCF aerogel.Moreover,the TCF aerogel also exhibits the excellent recyclability, and maintains a high sorption capacity even after five cycles through distillation,burning or squeezing.A novel kind of carbon microbelt(CMB)aerogel with good selective sorption ability has been prepared using the waste paper as its precursor material by Bi et al. [122]. The waste paperproduced CMB aerogel possesses the high sorption capacity of 56-188 times its own mass.The CMB aerogel can be recycled and repeatably used via a simple method of distillation or squeezing.Most importantly,the abundant source and simple preparation method make the CMB aerogel cost-effective for possible industrial applications,such as barrier separation and water purification.

A facile method was successfully used for fabricating the magnetically carbonaceous fiber (MCF) aerogel by one-step pyrolysis of FeCl3-coated cotton by Liu et al. [123]. The MCF aerogel exhibited high hydrophobicity/superoleophilicity property without the modification of low surface-energy chemicals;especially it had good thermal stability and mechanical properties,high adsorption capacity of 22-87 times its own mass,and superior separation efficiency.The distillation,combustion and squeezing processes can be used for the recycling of the MCF aerogel.Most importantly,the natural material and facile fabrication method make the MCF aerogel cost-effective for oil-water separation.Therefore,the MCF aerogel is highly promising as a low-cost,potential, and environmentally friendly sorbent for environmental and ocean protection.

Yang et al.[124]reported the preparation of multi-functional carbon fiber (MCF) aerogel by a simple hydrothermal and carbonization process using disposable bamboo chopsticks.The developed material manifested dramatic multi-functionalities,including excellent flexibility under the mechanical compression,efficient capability to separate oily droplets from water,and high adsorption capacity for a variety of oils and organic solvents by up to 129 times of its own weight.Moreover,the MCF aerogel can be recycled for many times by distillation,combustion or squeezing,making the material satisfy the requirements for oil-water separation in practice.Coupled with economical,environmentally benign manufacturing process,sustainability of precursor and versatility of material.

The surface wetting properties of biomass carbon aerogel play a crucial role in oils and organics absorption.To improve the hydrophobicity of the biomass carbon aerogel, our group adopted a simple route to superhydrophobic,compressible and multifunctional hierarchical biomass carbon@SiO2@MnO2aerogel by in situ growth of MnO2nanosheets on the surface of biomass carbon[125].Schematic illustration of fabrication process is displayed in Fig.6.The obtained aerogel composites exhibit excellent superhydrophobicity with the water contact angle (WCA) of 155° and a very large absorption capacity(60-120 g·g-1)for different oils and organic solvents.What is more,the absorbed oils were collected from the aerogel composites by a simple mechanical squeezing process.

4.Conclusions and Future Perspectives

In the past decades,rapid economic development has increased the demand for oils and organic solvents,since they are major sources of energy and raw materials for so many products such as petrochemicals,materials,fuels and many other products.Oil pollutants,derived from the leakage of oils and organic solvents,are stable and very difficult to decompose and degrade in biological and environmental systems,which can cause health problems to animals and human beings. It is crucial to separate the oils and organic solvents from water in order to meet the discharge regulations set by the environmental authorities.Absorption method is believed to be one of the optimal processes for oil/water separation due to its low operational and capital cost as well as its high separation efficiency.

This review has attempted to cover a wide range of oil-absorbing materials which have been used so far for the separation of oils and organic solvents from the water or wastewater.Based on the literature reviewed, the following concluding remarks can be made: (1) Oilabsorbing polymer materials have high oil retention capacity,but in some cases these oil-absorbing materials have been found expensive,and the preparation processes are complicated. (2) Inorganic oilabsorbing materials,especially mineral clays,sand and carbon-based materials,are robust absorbent for oil/water separation,but their mechanical properties are however not always met and the oil absorption properties are affected by surface wettability of materials.(3)Biomass materials,especially cellulose-based aerogel after modifications and biomass carbon-based absorbents,offer satisfactory results in terms of oil absorption.The use of biomass materials and derivatives for absorbing the oils from water is of great interest as biomass is obtained from natural raw sources,and is an environmentally friendly material of low cost.However,there are some disadvantages also in the use of biomass materials including that surface modification or high-temperature carbonization,limiting the applications of biomass materials.

Fig.6.Schematic illustration of fabrication of the hierarchical biomass carbon@SiO2@MnO2 aerogel.Reproduced with permission from reference[125].Copyright 2018 Elsevier Inc.

It is worthwhile to note that a particular oil-absorbing material which shows high oil separation, properties, fast oil separation rate and good reusability in the laboratory under batch conditions,may fail in industrial applications.Cheap oil-absorbing material,in conjunction with functional surface and efficient synthetic strategies, hold great promise for the advancement of this field.

Last but not the least,it would be worthwhile to study the reusability of oil-absorbents and the regeneration studies need to be conducted in more detail to enhance the economic feasibility of the oils absorption process.Due to controlled wetting properties and excellent oil absorption properties,oil-absorbing materials will continue to become a thriving area of chemical separation,and will be a staple of the field for years to come.Continued investigation into this area will likely yield many new functional oil-absorbing materials,and as production costs are mitigated they may one day see common use in industrial applications.