RESEARCH ARTICLE

Oil spill cleanup using graphene

Muhammad Z. Iqbal & Ahmed A. Abdala

Received: 1 August 2012 /Accepted: 9 October 2012# Springer-Verlag Berlin Heidelberg 2012

Abstract In this article, we study the use of thermally re-duced graphene (TRG) for oil spill cleanup. TRG was synthe-sized by thermal exfoliation of graphite oxide andcharacterized by X-ray diffusion, Raman spectroscopy, SEM,TEM, elemental analysis, and Brunauer–Emmett–Teller(BET) surface area measurement. Various aspects of the sorp-tion process have been studied including the sorption capacity,the recovery of the adsorbed oil, and the recyclability of TRG.Our results shows that TRG has a higher sorption capacitythan any other carbon-based sorbents, with sorption capacityas high as 131 g of oil per gram TRG. With recovery of thesorbed oil via filtration and reuse of TRG for up to six cycles,1 g of TRG collectively removes approximately 300 g of crudeoil. Moreover, the effects of TRG bulk density, pore volume,and carbon/oxygen ratio and the oil viscosity on the sorptionprocess are also discussed.

Keywords Oil spill . Sorption . Graphene . Adsorbent .

Recycling . C/O ratio

Introduction

Water contamination due to oil spills causes serious environ-mental problems not only in oceans and the neighboringcoasts but also in subterranean water. Oil spills also result inthe loss of a valuable source of energy. Therefore, an efficientway to recover and reuse spilled oil remains of great researchinterest. Compared to onshore and land spills, offshore spills

are more hazardous by effect, but relatively easy to clean. Oilspills from tankers which transport 60 % of the world’s oil arestill a major threat to the environment because many trafficroutes cross the boundaries of different marine ecosystemsand marine biodiversity hot spots (Roberts et al. 2002). Ingeneral, there are three different methods for oil spill cleanup,i.e., mechanical, chemical, and biological. Mechanical clean-up includes methods such as skimming and the use of sorb-ents. Chemical cleanup includes methods such as dispersion,in situ burning, and the use of solidifiers. Biological methodsor bioremediation involves accelerating the microbiologicalbiodegradation of oil (Yang et al. 2009; Swannell et al. 1996;Margesin and Schinner 2001).

Sorbents used in mechanical cleanup are granular, loosechemophilic materials that are distributed on the chemicalslicks floating on the water surface by strewing to attract theliquid chemicals. The sorbent/chemical/water mixture isremoved mechanically from the water surface (Oebius1999). Like all other recovery principles, sorbents can onlyoperate at the water surface. Thus, the bulk density ofsorbents must be less than the density of water. Conversely,sorbents must also be able to sink into the chemicals to bewetted completely. Sorbents work through the mechanismof absorption, adsorption, or both (sorption). Organic sorb-ents have some associated problems like sinking by absorb-ing oil and water and producing particles like sawdust,which are difficult to collect after they have spread overthe water (Adebajao et al. 2003). Based on their sorptioncapacity (grams of oil removed by 1 g of sorbent), currentoil spill sorbents can be classified as low-capacity sorbents,mid-capacity sorbents, and high-capacity sorbents. Thesorption capacity of different sorbents in each category, theirbulk density, and oil density are provided in Table 1.

Among carbon-based sorbents, exfoliated graphite exhib-its the highest sorption capacity, approximately 86 g of oilper gram of sorbent. The sorption capacity of exfoliated

Responsible editor: Philippe Garrigues

M. Z. Iqbal :A. A. Abdala (*)Department of Chemical Engineering, The Petroleum Institute,P.O. Box 2533, Abu Dhabi, United Arab Emiratese-mail: aabdala@pi.ac.ae

Environ Sci Pollut ResDOI 10.1007/s11356-012-1257-6

graphite depends on the temperature, oil viscosity and den-sity, and bulk density of the sorbent (Toyoda and Inagaki2000; Toyoda et al. 2000). Different aspects of the sorptionprocess have been investigated, including exfoliated graph-ite recyclability and oil recovery (Inagaki et al. 2000), andthe sorption kinetics (Nishi et al. 2002a, b).

Graphene is a two-dimensional atomic thick sheet madeby a honeycomb lattice of carbon. Free-standing graphenewas first produced in 2004 using a micromechanical cleav-age technique (Novoselov et al. 2004). Currently, grapheneis produced by either bottom-up or top-down processes(Kim et al. 2010). Bottom-up methods produce smallamounts of large-sized graphene sheets, while top-downmethods produce large amounts of small-sized functional-ized graphene sheets. The extremely high surface area (the-oretical surface area of 2,630 m2/g) and the hydrophobicnature of graphene make it an excellent candidate for oilsorption applications.

Thermally reduced graphene (TRG) is produced by thethermal exfoliation of graphite oxide (Prud'homme et al.2010). TRG consists of single and few layers of function-alized graphene with lateral dimensions of 200 nm up to afew micrometers and a thickness of 1–3 nm. TRG sheetsare composed of aromatic islands separated by aliphaticregions containing hydroxyl and epoxy groups (McAllisteret al. 2007). The bulk density of TRG is extremely low (aslow as 3 g/L), depending upon the exfoliation conditions(Prud'homme et al. 2010).

In this article, TRG is used for the sorption of twodifferent oils (a 39° API crude oil and a 25° API oil fraction)from an oil–water interface. The effects of TRG bulk den-sity, total pore volume, and carbon/oxygen (C/O) ratio andthe oil viscosity on the sorption process parameters such asmaximum capacity, retention capacity, and the total recov-ery are investigated.

Experimental

Materials

Natural flake graphite (−10mesh, 99.9%, Alfa Aesar), sulfuricacid (95–97 %, J.T. Bakers), hydrochloric acid (37 %, Reidel-deHaen), hydrogen peroxide (30 % solution, BDH), potassiumpermanganate and sodium nitrate (Fisher Scientific), and filterpaper (Whatman no. 1, Qualitative) are used. Oil A is a 39°API crude oil (density, 828 kg/m3; viscosity, 43 cP) sample thatwas kindly supplied by the Abu Dhabi Onshore Company. OilB is the residual fraction of 280 °C atmospheric distillation ofthe 39° API crude oil (oil A). The density and viscosity of oil Bare 905 kg/m3 (25° API) and 112 cP, respectively.

Production of TRG

TRG was produced following the thermal exfoliation method(Prud'homme et al. 2010). In this method, graphite is oxidizedusing Staudenmaier’s (1898) method as follows: graphite (5 g)is placed in an ice-cooled flask containing a mixture of H2SO4

(90mL) andHNO3 (45mL). Potassium chlorate (55 g) is addedslowly to the cold reaction mixture. The reaction is stoppedafter 96 h by pouring the reaction mixture into water (4 L). AHCl solution (5 %) is used to wash the produced graphite oxide(GO) until no sulfite ions are detected. The mixture is thenwashed with water until no chloride ions are detected. GO isdried in a vacuum overnight. GO was exfoliated by rapidheating at 1,000 °C in a tube furnace (model 21100, BarnsteadThermolyne) under a flow of nitrogen for 30 s to 2 min.

Characterization of TRG

X-ray diffraction (XRD; X’Pert PRO MPD diffractometer,PANalytical) was used to confirm the oxidation of graphite

Table 1 Maximum sorption capacity and bulk density of different oil spill sorbents

Grade Sorbent Max. sorption(grams oil pergram sorbent)

Bulk density(kg/m3)

Oil density(deg API)

Reference

High capacity,>50 g/g

Silky floss fiber 85 620 25.8 Annunciado et al. (2005)

Exfoliated graphite (EG),carbonized fir fibers,vertically aligned CNT

69–86 5.6–6.0 >32 Toyoda and Inagaki (2002);Fan et al. (2010); Liang et al. (2012)

Carbon nanofibers (CNF) aerogel 48a 3.3 Gasoline Liang et al. (2012)

Mid-capacity,25–50 g/g

Natural fibers: milkweed fibers,silkworm cocoons, kapok,cotton fibers

33–50 20 34–39 Lim and Huang (2007); Annunciado et al.(2005); Lim and Huang (2007);Choi and Cloud (1992)

Low capacity,<25 g/g

PAN-based CNF, activated CNF,CNF fabric, polypropylenefiber, chrome shavings

7.4–14 52–150 18–34 Inagaki et al. (2002); Choi and Cloud(1992); Gammoun et al. (2007)

a Higher sorption capacities for vegetable oil (78 g/g) and organic solvents (115 g/g) were also reported

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and the complete exfoliation of graphite oxide. A scan wasconducted between 5° and 35° with a 0.02°/s step size at 40-kVvoltage with an intensity of 20 A using CuKα radiation ofwavelength 1.5406 Å. TEM images were obtained using FEITecnai G20 TEM with point-to-point resolution of 0.11 nm,coupled with energy-dispersive X-ray spectroscopy. The sam-ples were prepared by dispersing approximately ≈0.5 mg ofTRG in 25 mL of dimethyl formamide by sonication for10 min in a sonicating bath at room temperature. Two dropsof the suspension were deposited on a 400-mesh copper gridcovered with thin amorphous film (lacey carbon). SEM imageswere obtained using Philips FEI Quanta 200 SEM operated athigh-vacuum mode. A LabRAM HR (Horiba Scientific) wasused to obtain the Raman spectra. Typically, a ×50 objectivewas used with a 633-nm excitation line. The bulk density ofgraphene samples was measured by an AutoTap Tap densitymeter (model D-AT-3, Quantachrom Instruments). Each sam-ple was tapped 2,000 times before taking the measurement.The density values were recorded as averages of at least threereadings. The specific surface area, pore volume, and pore sizedistribution were determined using a QuantachromAutosorb-1(Quantachrom Instruments). Prior to the measurement, thesample was degassed for 16 h at 200 °C. TRG C/O ratio wasdetermined from CHN elemental analysis carried out by theMidwest MicroLab, LLC (Indiana, USA) using a combustionanalyzer at 990 °C under ultrapure oxygen; oxygen contentwas carried out using the Unterzaucher method at 1050 °C.

Sorption and recovery of oil

TRG samples were heated overnight in a vacuum oven at 70 °C prior to use in the sorption experiment. The maximumsorption capacity was estimated according to the methoddescribed by Toyoda and Inagaki for exfoliated graphite(Toyoda et al. 1998; Toyoda and Inagaki 2000). In this meth-od, 10 mL of oil Awas mixed with 50 mL of water in a 200-mL flask. The mixture was shaken for 1 min and allowed tosettle for 5 min. Of the TRG, 50–100 mg was introduced fromthe top onto the oil–water mixture. Oil-saturated TRG wasdrawn out using a wire gauze, drained for about 10 min, andtaken to a vacuum filtration setup and weighed.

Recovery of the oil was carried out by vacuum filtration(20mmHg) of the oil-saturated TRG; the mass of TRGwith theretained oil is determined. After oil recovery by filtration, TRGwas reused and the process was repeated for up to six cycles.

Results and discussion

Characterization of TRG

Rapid heating of graphite oxide leads to the simultaneousreduction and exfoliation of graphite oxide sheets. By

controlling the exfoliation condition, the bulk density, sur-face area, and the C/O contents of TRG can be varied, asdiscussed in detail elsewhere (Aksay et al. 2011). Directevidence of the exfoliation of GO is the substantial volumeexpansion of 100–300 times due to rapid heating, as shownin Fig. 1a. Generally, 1 g of GO with a bulk volume of about2 cm3 yields about 0.7 g TRG with a bulk volume of about150–180 cm3 depending on the density of the producedTRG. In addition, the XRD diffraction patterns providesupporting evidence for the complete exfoliation of GO toTRG. Figure 1b shows the XRD patterns for graphite, GO,and TRG. A strong diffraction peak (002) is observed at2θ026.5 for graphite, which corresponds to a d-spacing of3.37 Å. This peak is an intrinsic peak showing the stackedgraphene layers in pure graphite. The oxidation of graphiteleads to the introduction of polar oxygen functionalities onthe surface of GO. Due to the presence of these function-alities and the adsorbed water intercalation, the 002 peakshifts to 2θ011.4, indicating interlayer expansion to a d-spacing of 0.78 Å. In contrast to graphite and GO diffractionpatterns which indicate the presence of an ordered layeredstructure, the TRG diffraction pattern shows no noticeablediffraction peaks, confirming the complete exfoliation ofGO and the production of a non-stacked structure.

Raman spectroscopy is used to probe the structural andelectronic characteristics of graphitic materials. It providesuseful information on the defects (D band), in-plane vibra-tions of sp2 carbon atoms (G band), and the stacking order(2D or G' band) of graphene layers (Ni et al. 2008). In orderto confirm the production of TRG, the Raman spectra ofgraphite and TRG have been collected as shown in Fig. 2.The shape of the G' (or 2D) peak is used to distinguishbetween single-layer graphene, bilayer graphene, and thebulk graphite. Bilayer sheets or sheets with less than fivelayers have a broader and symmetrical G' peak, while bulkgraphite exhibits a distorted peak (Ni et al. 2008). TheRaman spectrum of TRG showed a broader and symmetricalG peak in the 2,500 to 2,800-cm−1 range, indicating that thegraphene sheets with less than five layers are obtained afterthermal exfoliation. A second prominent feature of theRaman spectra is the size, shape, and intensity of the Dand G bands. The intensity of the G band is known toincrease linearly with the thickness of the graphene layer(Wang et al. 2008). It can be observed in Fig. 2 that graphiteexhibits a strong and intense G band, showing bulk stackedsheets as compared to that of TRG, indicating a few layeredgraphene. The D band that appears as a result of induceddefects bears an inverse relation between its intensity andthe number of graphene layers. The intensity of the D banddecreases with increasing graphene thickness and is almostinvisible for defect-free bulk graphite (Gupta et al. 2006).The defects are more easily introduced into the thinnergraphene sheets, and the intensity of these defects is

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governed by the method of producing graphene (Ni et al.2008). The differences of the spectra of graphite and TRGclearly indicate that we have successfully produced gra-phene instead of multilayer graphite nanocrystallites.

The surface morphology of TRG samples having differ-ent bulk densities and C/O ratios is analyzed with SEM.Figure 3a, c shows the SEM images of TRG samples withbulk densities of 4.4 and 17 g/L, respectively. Though acrevice-like structure is prominent in the low-density TRGsample and the samples with higher densities appear to havefluffy agglomerated structures as produced, the TEM imagesof TRG after dispersing in dimethylformamide show gra-phene layers, as shown in Fig. 3b, d. The transparent graphenesheets are clearly visible, confirming that GO has been suc-cessfully exfoliated. However, the elastic corrugations and thescrolled or folded edges often result in different brightness inthe surface of graphene (Meyer et al. 2007). TRG sampleswith different bulk densities (3–17 g/L) and C/O ratios (8.7–17.9) are synthesized and used.

In addition to the bulk density, TRG samples were char-acterized with Brunauer–Emmett–Teller (BET) method for

surface area, pore volume, and pore size distribution. BETmeasurements indicate that the sorption/desorption behaviorof TRG follows a standard type II-b BET isotherm, asshown in the inset of Fig. 4. The cumulative pore volumewas determined using BJH cumulative desorption total porevolume method. The TRG sample with the highest C/O ratio(17:1) and the lowest bulk density (3.0 g/L) exhibits asurface area and pore volume of 1,260 m2/g and 3.7cm3/g, respectively. Both the surface area and the cumula-tive pore volume decrease with the decrease of the C/O ratioand the increase of bulk density.

Maximum sorption capacity

The maximum sorption capacity of TRG was determinedand confirmed through independent experiments. In the firstexperiment, Toyoda’s method (Toyoda and Inagaki 2000)was followed as described in “Experimental.” The amountof oil adsorbed by graphene is determined by the differencein the mass of the oil-saturated graphene picked up by thewire gauze and drained for 10 min and the mass of the addedgraphene. This amount is used to calculate the maximumsorption capacity. When TRG was added onto the oil–watersurface graphene, islands of oil-saturated graphene agglom-erates were formed with the boundary layer of free oil, asshown in Fig. 5.

The determined sorption capacity depends on the bulkdensity, total pore volume, and the C/O ratio of TRG and theviscosity of the oil, as will be discussed later. For example,1 g of TRG sample with a 17.9 C/O atomic ratio and 3.0 g/Ladsorbs 131 g of oil A and 108 g of oil B.

To reconfirm the sorption capacity determined from thefirst experiments, an exact amount of TRG is added suchthat the TRG/oil ratio is equivalent to the determined sorp-tion capacity. The brownish crude oil color disappears uponthe addition of TRG onto the oil–water surface. After re-moving the oil-saturated TRG, no oil can be visually

TRG

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Environ Sci Pollut Res

detected on the water surface. Oil-saturated TRG is takenout using a wire gauze, weighed, and taken to the filtrationsetup for the sorbed oil recovery. Typical appearance of theoil–water interface before TRG addition, after TRG addi-tion, and after removal of the oil–TRG mixture is shown bythe optical images taken using a small aluminum dish in-stead of a flask for better clarity of the images (Fig. 6).

To the best of our knowledge, the measured sorption ca-pacity of the low-bulk-density and high-C/O ratio TRG for oilA is higher than any previously reported sorption capacity forcarbon-based sorbents. This sorption capacity was confirmedusing various schemes of oil and TRG additions.

In addition to the high sorption capacity, the time for thesorption process is in the order of tens of seconds. Asobserved, floating crude oil from the oil–water interfacedisappears within 30 s of the addition of TRG without theneed to shake the beaker. This is an advantage compared toany sorbent reported previously. Nevertheless, the detailedkinetic process is beyond the scope of this article.

The effect of the bulk density of other sorbents on theirsorption capacity reveals a similar trend (Toyoda andInagaki 2000; Bastani et al. 2006). However, the effect ofsorbent C/O ratio on the sorption capacity has not beenreported. Therefore, a systematic investigation is carried

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Fig. 5 Oil-saturated graphene formed upon the addition of TRG to theoil–water surface

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Fig. 3 a, c SEM images ofTRG samples with bulk densityof 4.4 and C/O013. b, dHRTEM images of the samesamples

Environ Sci Pollut Res

out to determine the effect of these two factors on thesorption capacity of TRG. Two sets of samples are used inthis investigation, i.e., samples with the same C/O ratio buttheir bulk density has changed through compression andsamples with the same bulk density but different C/O ratios.Figure 7 shows the effect of these two structural parameterson the sorption capacity.

The results indicate that TRG sorption capacity is highlyaffected by both the bulk density and the C/O ratio. TRG C/Oratio, which can be used as indicative of TRG hydrophobicity,greatly affects its sorption capacity, as shown in Fig. 7a. As theTRG C/O ratio increases, TRG hydrophobicity increases,leading to an increase in the sorption capacity for both oil Aand oil B.

On the other hand, compression of TRG characterized byan increase in bulk density leads to a reduction of its sorp-tion capacity, as shown in Fig. 7b for two TRG samples. Thedecrease in TRG sorption capacity due to the increase of itsbulk density increase at a fixed C/O ratio is attributed to theagglomeration of TRG sheets induced by compressionwhich reduces the pore volume, resulting in less oil pickup.

Furthermore, the effect of oil viscosity can also be de-duced from Fig. 7a. TRG sorption capacity for the low-viscosity crude oil A is higher than the capacity for thehigh-viscosity oil B for the same TRG C/O ratio and bulk

density. This is attributed to the higher concentration oflong-chain hydrocarbons in oil B, which is the residue offractionating crude A. These long-chain hydrocarbons areexcluded from the majority of the micropores of TRG,reducing the effective pore volume of TRG. A similar de-crease in the sorption capacity of exfoliated graphite with oilviscosity was also attributed to the exclusion of long-chainhydrocarbons from the micropores of exfoliated graphite(Toyoda and Inagaki 2000).

In Fig. 8, the sorption capacity of TRG is plotted as afunction of the total pore volume of TRG. The sorptioncapacity increases with increasing pore volume for the twosamples. It is reasonable to assume that large pores (>50 nm)are effective for oil sorption along with the surface area ofgraphene. In the present study, the surface area of TRG wasmeasured by BET adsorption at 77 K. The as-measured sur-face areas of the TRG samples were 1260, 442, 570, and332 m2/g with pore volumes of 3.73, 0.67, 1.85, and1.08 cm3/g, respectively. There appears no conclusive relationbetween the surface area and the maximum sorption capacityin this case. This may be attributed to the fact that these TRGsamples have not only different bulk densities but also differ-ent C/O ratios. It is worth mentioning that the sorption capac-ity vs. the total pore volume data are collected from as-prepared samples without compaction or densification.

Fig. 6 Optical images of crude oil–water mixture (a) after the addition of TRG amount yielding an oil/TRG ratio equivalent to the sorptioncapacity (b) and after removal of the oil-saturated TRG (c)

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TRG recycling

The recovery of the sorbed oil was carried out by vacuumfiltration of the oil-saturated TRG, and TRG was reused forsorption for up to six cycles. It is observed that the requiredfiltration time increases with cycle number and oil viscosity.For oil B-saturated TRG, a long filtration time of about10 min was required, while oil A-saturated TRG was com-pletely filtered within 3 min, which increased with thenumber of times TRG was recycled. The effect of repeatedrecycling of TRG on the sorption capacity and the amountof oil recovered every cycle is shown in Fig. 9a, b, respec-tively. The results indicate a decrease in sorption with recy-cling of TRG. This decrease in sorption is attributed to twofactors: (1) the increase in the unrecovered/retained oil withcycle number and (2) possible compaction of TRG and adecrease of pore volume during the filtration/compressionrecovery process. As the cycle number increases, theamount of retained oil increases, which decreases fresh oilpickup in the next cycles and consequently reduces thesorption capacity. This is in agreement with previous find-ings on the sorption of heavy oils by exfoliated graphite(Toyoda and Inagaki 2000). Moreover, the sorption capacity

and the amount of oil recovered per cycle for oil A arealways higher than those for oil B due to the higher con-centration of long-chain hydrocarbons in oil B, which weattribute to the higher viscosity and average molecule size ofoil B, as discussed earlier. A mild gentle compression of theoil-saturated TRG using a spatula prior to filtration leads toan increase in the amount of recovered oil per cycle, but itdecreases the sorption capacity due to the reduction of thepore volume and the increase of the bulk density.

Only 36 % of sorbed oil A (131 g/g TRG) is recovered inthe first cycle using vacuum filtration. A small increase of7 % in the recovered amount was observed when mildcompression was applied, i.e., 43 % of the initially sorbedoil is recovered. Moreover, the amount of recovered oildecreases with cycle number. The results also indicateTRG’s high oil retention capacity.

TRG true sorption capacity, defined as the total oilretained from the previous cycle plus the fresh oil sorbedby 1 g of TRG, for both oil A and oil B as a function ofcycle number is shown in Fig. 10. TRG true sorption capac-ity for oil A and oil B did not show a significant decreaseduring the first few cycles when vacuum filtration wasapplied for the recovery. However, when vacuum filtrationwas combined with mild compression, the true sorptioncapacity decreased significantly after the second cycle.

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Fig. 10 True sorption capacity as a function of cycle number

Environ Sci Pollut Res

Regardless of the deterioration of the sorption capacity withthe number of cycles, TRG still sorbs 91 g of oil A and 81 gof oil B per gram, which remains higher than the highestreported sorption capacity for carbon-based sorbents(Toyoda and Inagaki 2000). Moreover, vacuum filtrationcombined with compression did not considerably affect thetrue sorption capacity as TRG sorbs 81 g of oil A per gramof TRG in the sixth cycle, which is comparable to the high-est sorption capacity previously reported by Toyoda andInagaki (2000).

The cumulative amount of oil removed from the oil–water interface is delineated in Fig. 11. One gram of TRGsorbed approximately 270 g of oil A and 185 g of oil Bfraction cumulatively in six cycles using vacuum filtrationas the recovery mean. A higher cumulative recovery,~300 g/g TRG, is obtained when simultaneous compressionand filtration is applied for recovering oil A. These resultsstrongly support the use of TRG in cleaning oil spills as only3.4 kg of TRG would be required to clean as much as 1 tonof the crude oil from the oil–water interface.

Effect of the hydrophobicity of TRG

Sorbent soaking in water before oil sorption usuallyaffects the maximum sorption capacity of the sorbents(Choi and Cloud 1992). The adsorbents containing highoxygen (less C/O) have been observed to attract watermolecules more than those with less oxygen (high C/O;Kaji et al. 1986; Beck et al. 2002). In order to determinethe selective oil/water sorption, we performed an oilsorption experiment by soaking high-C/O ratio (13.7)TRG (15 mg of 4.4 g/L) in water (200 mL) for10 min followed by adding different amounts of oil.Then, the oil/water-saturated TRG was removed, allowedto drain for about 10 min, and weighed. At all oil/TRGratios lower than the maximum sorption capacity for thatTRG sample (80 g/g), no free oil was visually detectedon the surface of the water, i.e., the oil sorption capacityis the same as the oil/TRG ratio. The mass of sorbedwater is calculated by the difference of the mass of thesorbed oil and water minus the mass of the added oil.When no oil is added, 1 g of TRG sorbs only 14 g ofwater. With the addition of oil at different oil/TRG ratios,the amount of water sorbed decreases to 2 g water pergram TRG when the oil/TRG ratio>sorption capacity,indicating the selective and oleophilic nature of graphene.We attribute the displacement of the pre-adsorbed waterby the added oil to the higher affinity of the hydrophobicTRG to oil than to water. To support this argument, asimple illustrative experiment was conducted, as shownin Fig. 12.

In this experiment, oil is added dropwise to water-saturated TRG. With continuing addition of the oil, thesorbed water is displaced by the oil and migrated away fromTRG as one large droplet, indicating the high affinity ofTRG to oil compared to water due to the hydrophobic natureof TRG (Fig. 12).

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Fig. 11 Cumulative oil removal by TRG for crude oil A and oilfraction B under different recovery options

Fig. 12 a TRG (C/O013.7) is saturated with water by dropwise addition of water. b Oil is added to the water-saturated TRG. c Sorbed water isdisplaced by the oil

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Conclusions

Testing TRG for the cleanup of oil spills has shown thatmaximum sorption capacities of 131 g oil per gram TRG for39° API crude oil and 108 g oil per gram TRG for heavy oilfraction are achieved. When used for multiple cycles, 1 g ofTRG adsorbs 295 g of oil A and 183 g of oil B collectivelyover six sorption/recovery cycles. The high sorption capac-ity of TRG is attributed to its very high surface area and porevolume and its hydrophobic nature. The sorption capacity ofTRG, however, was reduced in successive cycles. The sorp-tion capacity was found to depend on the bulk density, totalpore volume, and C/O ratio of TRG. Retention of 91 g oil Aand 81 g oil B per gram TRG was observed in the sixthcycle. In addition, the selective sorption of crude oil and theoleophilic nature of TRG were confirmed.

Acknowledgment The authors would like to thank Dr. Saeed Alhas-san and the Catalysis Group at the Department of Chemical Engineer-ing, the Petroleum Institute, Abu Dhabi, for assistance with thecharacterization of TRG.

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