Joule-heated graphene-wrapped sponge enables fast clean-up of viscous crude-oil spill.pdf

Joule-heated graphene-wrapped sponge enables fast clean-up of viscous crude-oil spill

Jin Ge 1?,Lu-An Shi 1?,Yong-Chao Wang 2?,Hao-Yu Zhao 1,Hong-Bin Yao 1,Yin-Bo Zhu 2,Ye Zhang 1,Hong-Wu Zhu 1,Heng-An Wu 2and Shu-Hong Yu 1*

The clean-up of viscous crude-oil spills is a global challenge.Hydrophobic and oleophilic oil sorbents have been demonstrated as promising candidates for oil-spill remediation.However,the sorption speeds of these oil sorbents for viscous crude oil are rather limited.Herein we report a Joule-heated graphene-wrapped sponge (GWS)to clean-up viscous crude oil at a high sorption speed.The Joule heat of the GWS reduced in situ the viscosity of the crude oil,which prominently increased the oil-diffusion coef ?cient in the pores of the GWS and thus speeded up the oil-sorption rate.The oil-sorption time was reduced by 94.6%compared with that of non-heated GWS.Besides,the oil-recovery speed was increased because of the vis-cosity decrease of crude oil.This in situ Joule self-heated sorbent design will promote the practical application of hydrophobic and oleophilic oil sorbents in the clean-up of viscous crude-oil spills.Frequent oil-spill accidents not only cause severe and long-term damage to marine ecosystems,but also lead to a great loss of valuable resources 1,2.To eliminate the environmental pollution of oil spills quickly,an ef ?cient and environment-friendly oil-recovery approach is in high demand.Here we summarize the criteria of an ideal method for oil-spill remediation as:(1)clean-up the oil spills at a high speed to alleviate the spreading and weathering of the oil,(2)be able to recover the oil spill with a high oil/water separation ef ?-ciency,(3)bring few adverse effects to the marine lives,(4)be compa-tible with a large-area application and easily manipulated and (5)work ef ?ciently under harsh marine conditions.Unfortunately,traditional oil-recovery technologies,such as dispersants 3,solidi ?ers 4,in situ burning 5and skimmers 6cannot meet all the above requirements.Recently,advanced sorbents with both hydrophobic and oleo-philic properties have become potential candidates to realize an ideal oil remediation that meets all the above criteria.For example,carbon-based aerogels 7–14,porous boron-nitride nanosheets 15,nanowire membranes 16,modi ?ed commercial sponges 17,18and organic silica aerogels 19have shown many advantages,such as a high ef ?ciency of oil/water separation,environment-friendly characteristics,easy manipulation and a self-adaption capability under harsh sea conditions.Especially,by taking advantage of the self-controlled interfaces of the oil sorbent under an external suction force,an in situ pumping device based on a hydrophobic sponge can collect the oil from a water surface in an ef ?cient and continuous way 20.

However,one crucial issue that hampers the practical application of these advanced absorbent materials is that they show a low capture performance for viscous crude oil.As is well known,about 40%of the world ’s oil reserve is heavy crude oil.Its viscosity

ranges from 103to 105mPa s at room temperature,which is much higher than that of the model oils (below 500mPa s)used in previous reports 7,11,14,18.Additionally,light crude-oil spills (1–100mPa s)will experience a remarkable increase in viscosity after the evaporation of its light components 2,21.The diffusion of such a viscous crude oil into the inner pores of the aforementioned sorbents is very slow,which results in a low oil-sorption speed and inef ?cient usage of materials.

Here we report a Joule-heated graphene-wrapped sponge (GWS)for the high-speed sorption of viscous crude oil from a water surface (Fig.1).The graphene coating uniformly wraps the skeletons of the sponge substrate,which endows the GWS with hydrophobic and conductive properties (Fig.1,top left).When GWS contacts the viscous crude oil on a water surface,the oil is absorbed selectively into the pores because of the hydrophobic property of graphene,but at a very limited oil-sorption speed.After applying a voltage to the GWS,current ?ows through the graphene coating and Joule heat is generated,which quickly heats up the GWS (Fig.1,middle).Then the hot GWS can heat up the surrounding crude oil and thus the oil viscosity decreases,which then increases the diffusion coef ?cient of the crude oil into the GWS.After cutting off the applied voltage,the absorbed oil localized in the pores of GWS is still hot and remains of low viscosity,which also facilitates the subsequent oil-recovery and transportation processes (Fig.1,top right).

Fabrication of GWS by centrifugation-assisted dip coating

As shown in Fig.2a,a porous substrate was ?rstly ?lled with gra-phene oxide (GO)solution.Then the porous substrate was centri-fuged to remove the excess GO solution away from the pores,to leave only a residual of the GO nanosheets on the skeleton ’s surface.The centrifugation not only recycles the GO solution,which reduces material consumption,but also makes a homo-geneous GO coating on the skeleton of the porous substrate without blocking the pores (Supplementary Fig.1).The GWS was obtained by reducing the GO coating with HI solution 22.Melamine sponge (MS)and mineral wool (MW)were selected as typical porous substrates to fabricate GWS (hereafter,MS@RGO and MW@RGO,respectively (RGO,reduced graphene oxide)).Scanning electron microscopy (SEM)images of MS@RGO (Supplementary Fig.2)and MW@RGO (Supplementary Fig.3)show that the open porous systems of the substrates remained.Some wrinkle stripes that appeared on the skeletons of MS and MW indicate the existence of the RGO coating.In the Raman spectra (Supplementary Figs 2and 3),the D peak (1,356cm ?1)

Division of Nanomaterials and Chemistry,Hefei National Laboratory for Physical Sciences at the Microscale,Collaborative Innovation Center of Suzhou Nano Science and Technology,CAS Center for Excellence in Nanoscience,Hefei Science Center of CAS,Department of Chemistry,University of Science and Technology of China,Hefei 230026,China.2CAS Key Laboratory of Mechanical Behavior and Design of Materials,Department of Modern Mechanics,CAS Center for Excellence in Nanoscience,University of Science and Technology of China,Hefei,Anhui 230027,China.?These authors contribute equally to this work.*e-mail:

shyu@https://www.360docs.net/doc/369259018.html,

and G peak (1,594cm ?1)of GO and RGO appeared after the coating processes,and the increased D/G intensity ratio demonstrated the reduction of GO 23.The GWS fabricated by the centrifugation-assisted dip-coating method consumed very little GO.For example,the weight content of graphene in MS@RGO was only 0.045mg cm –3,which is much less than that of ultralight graphene aerogels reported previously (0.16mg cm –3)(ref.7).The fabrication of the GWS can be easily scaled up by using industrial centrifuges (Supplementary Fig.4).Figure 2b shows a typical optical image of two pieces of the MS@RGO monolith.The spherical water droplets on their surfaces (inset shows a water contact angle of ～131°)and the light-emitting diode (LED)lamp between them indicate their good hydrophobic and electrical conduc-tivity properties,respectively.When a voltage was applied across the two sides of the MS@RGO monolith,the temperature of its surface increased very quickly because of the Joule-heating effect of RGO (Supplementary Movie 1).

Heat tolerance of the RGO coating

MW@RGO was selected as the model GWS because of the high thermostability of the mineral wool (over 1,000°C (Supplementary Fig.5)).The MW@RGO was gradually heated by step-increased voltages,and the resistance and surface temperature of the MW@RGO were recorded (Supplementary Fig.6illustrates the measurement set-up).Figure 2c shows the curve of the average surface temperatures (ASTs),maximum surface tempera-tures (MSTs)and the resistance change of MW@RGO versus time under different input voltages.The negative temperature coef ?cient of the resistance of RGO 24means the resistance decrease of MW@RGO at the initial stages of each voltage step resulted from the increase of temperature.When the voltage was less than 50V,the MST was less than 350°C (AST ≈285°C)and the decrease of R 0was caused by the reduction of residual oxygen-containing func-tional groups on the RGO nanosheets 25.At 55V,the MST exceeded 400°C,and R 0began to increase after the Joule-heating process,which indicates oxidation of the RGO coating.The oxidation might happen at the core of the MW@RGO because the inner temp-erature of the MW@RGO was higher than that of its surface and might have exceeded the breakdown temperature of graphene (T BD ≈600°C)(ref.26)in air.Therefore,the MW@RGO monolith might keep stable below the MST of 350°C (AST ≈285°C).In the

13.5hour test of Joule heating under a constant power supply,the AST of the MW@RGO monolith could be kept at around 285°C (Supplementary Fig.7).

The effect of Joule heating on the oil-sorption process is re ?ected clearly by the dynamic permeating behaviour of viscous crude-oil droplets into the MW@RGO monolith.Figure 2d is a photograph of a typical heavy crude oil used as the model viscous crude oil in the following test.When an oil droplet (～8μl)was placed onto the surface of MW@RGO (22°C),it took eight minutes for the oil droplet to penetrate entirely into the MW@RGO (Fig.2e,upper row).However,when the MW@RGO was heated to an AST of 90°C by an electric current,the whole process completed in only six seconds (Fig.2e,lower row),which demonstrates an extensive improvement in the oil-sorption speed under Joule heating of the RGO coating (Supplementary Movie 2).

In ?uence of oil temperature on the oil-sorption speed

A quantitative evaluation of the oil sorption into MS@RGO was per-formed based on the concept of the liquid sorption coef ?cient K s .The speci ?c value of K s can be obtained based on a wicking method 27–29(Supplementary Fig.8).The crude oil was heated to different temperatures by a hot plate and then the wicking test was performed.Figure 3a shows that the slopes of the curves at the initial stage becomes steeper with an increase in the oil temperature,whereas the curve at 91°C matches well with that at 97°C.Figure 3b (red squares)shows the K s values derived from the slopes of the curves at the initial stage,which reveals that the oil-sorption speed increases sharply after exceeding an oil temperature of 40°C,and keeps constant when the oil temperature exceeds 91°C.

To shed light on the contribution of the oil-viscosity decrease to the improvement of the oil-sorption speed,we evaluated all the par-ameters that in ?uence the liquid sorption coef ?cient K s ,which was theoretically derived as in equation (1)30:

K s =d l γμ

ε*λ r 0√ cos θ

2 (1)

where d l ,γand μare the density,surface tension and viscosity of

the oil,respectively,ε*is the effective sorption porosity of

the

Power supply

Sponge substrate

Current ?ow through graphene (to generate Joule heat)

Viscous crude-oil spill Viscous crude-oil spill

Improved oil-sorption speed

b s o r b e

A p p

l y v o

l t Figure 1|Schematic illustration of Joule-heated GWS used to clean-up a viscous crude-oil spill.The red colour applied to the graphene coating represents the temperature increase.The dark brown of the oil represents the original highly viscous crude oil,and the light brown of the oil represents the crude oil with a decreased viscosity.

sorbent,λis the average tortuosity factor of the capillaries (λ>1),r 0is the average pore radius and θis the contact angle of the interface between the oil and the pore wall of the sorbent.The second bracket represents the parameters of the sorbent ’s pore structure,which does not change during the Joule-heating process.The changes of d l ,θ,γand μwith an increase in oil temperature were measured separately.Figure 3c shows that both d l and θexperienced a small decrease during the heating process (only ～0.05kg m –3variation for d l and ～23°variation for θ).However,the value of d l

(cos θ)/2√was almost constant (Fig.3d).Therefore,K s is

mainly determined by the value of γ/μ .According to the measured results of the surface tension (γ)and the viscosity (μ)of the crude oil in the temperature range 10–97°C (Fig.3e),the values of γ/μ at different temperatures were calculated and plotted as a function of temperature (blue circle in Fig.3b).The results show that the values of γ/μ increase with an increase in the oil temperature and the trend of its variation with oil temperature is consistent with that of K s values obtained by the wicking method.As the surface tension of oil (γ)decreases with an increase of oil temperature (Fig.3e),the increase of K s is mainly caused by the decrease in oil viscosity,which is the basis of our Joule-heated sorbent design for the high-speed uptake of viscous crude oil.

To evaluate quantitatively the impact of Joule heating on the oil-sorption speed,the wicking method was combined with voltage-triggered Joule heating to measure the K s value for oil into GWS.MS@RGO was chosen as the study object because of its low cost,light weight,elasticity and mechanical robustness,which are impor-tant for the practical applications of Joule-heated sorbents.Electric-power densities of 0,0.27W cm –3(20V)and 0.58W cm –3(30V)were applied to MS@RGO,and the ASTs were 20,98and 148°C,respectively (Supplementary Fig.9).Figure 3f shows that with an increase of power density from 0to 0.58W cm –3,K s improved sixfold and the saturated oil-sorption time was reduced by 90%.To show clearly the signi ?cance of Joule heating on the oil-sorption speed of graphene-based sorbents,MW@RGO with an AST of ～250°C was used to absorb a crude-oil spill from a water surface (Supplementary Movie 3and Supplementary Fig.10).With the aid of Joule heating,the crude oil was quickly absorbed into MW@RGO rather than adhering to its surface.

Optimization of heat-transfer ef ?ciency

In a real scenario of oil clean-up by Joule-heated GWS (MS@RGO (Supplementary Fig.11)),the GWS will ?oat on the oil surface at the initial stage because of its light weight and the high viscosity of the crude oil.Then,with the aid of Joule heat,the crude oil is

gradually

Skeleton

Sponge substrate

GO solution T ake out, centrifuge Reduction

GO-wrapped sponge RGO-wrapped sponge

2004006008001,00055 V

40 V 35 V S u r f a c e t e m p e r a t u r e (°C )

Resistance (Ω)

Time (s)

100

200

300

Resistance of the MW@RGO monolith

?LED

MS@RGO Water droplet

MS@RGO

Viscous crude oil

Unheated (22 °C)

2 min

8 min

6 min

4 min

0 s

6 s

3 s 1 s Heated (90 °C)

Figure 2|Fabrication of the GWS and Joule-heating effect of the GWS.a ,Schematic illustration of the fabrication processes of GWS.b ,The photograph of two MS@RGO monoliths demonstrates their good hydrophobic and electric conductivity properties.Inset shows the water contact angle of GWS,～131°.c ,The resistance and surface-temperature changes of the MW@RGO monolith (2.6×2.3×0.8cm 3)applied with multivoltage steps from 30to 55V.Before the next voltage step,the former voltage was switched off and the MW@RGO monolith allowed to cool to room temperature.The resistance changes during the cooling processes were not recorded.R 0is the original resistance of the MW@RGO monolith.d ,Photograph of a typical viscous crude oil (provided by the China University of Petroleum).e ,Permeating behaviour of one oil droplet on the surface of MW@RGO monoliths with (lower row)or without (upper row)a voltage applied.

absorbed into the GWS.Meanwhile,the GWS gradually sinks into the oil until it reaches the oil –water interface.This clean-up pro-cedure is a complex dynamic process and its thermal transmission cannot be measured exactly in the experiment.Therefore,we performed simulations to investigate the heat transfer in the oil-sorption tests.As the core concept of our clean-up method is to reduce the viscosity of crude oil by Joule heating,the comparatively ideal hypothesis is that as much as possible of the quantity of Joule heat should be transferred to the oil.

The simulation of heat transfer from GWS to the oil (Supplementary Computational Methods 1and Supplementary Fig.12)indicates that the heat-utilization ef ?ciency can be improved by

con ?ning the heated region to the bottom part of GWS.Here,we propose two designs to con ?ne the heated region.As shown in Fig.4a,one is to coat only the bottom part of the GWS with a silver electrode (upper left,named as GWS-x,x denotes the thick-ness of the silver electrode).The other is to coat only the bottom part of the MS with RGO,with the GWS part coated with silver electrodes (lower left,named as GWS-MS-x ,x denotes the thickness of the silver electrode).The temperature distributions in GWS-18,GWS-9,GWS-5and GWS-MS-5under the same electric-power input were simulated (Supplementary Computational Methods 2).Figure 4a shows that the average temperature in the bottom part of the GWS-x (close to the oil –GWS interface)can be improved

m s (k g m ?2)

2030

t 1/2 (s 1/2)

100

1,000

10,000

V i s c o s i t y (m P a s )

γ (mN m ?1)

Oil temperature (°C)

0.90

0.92

0.940.96

0.98

O i l d e n s i t y (k g m ?3)

1520

2530θ°

Oil temperature (°C)

0.4

0.5

0.6

0.7

0.8

d l (c o s θ)/2 (k g m ?3)

Oil temperature (°C)

2030

0.0

2.5

5.0

7.5

10.0

m s (k g m ?2)

t 1/2 (s 1/2)

9791675946393125

Oil temperature (°C)

020*********

6810K s (k g m ?2 s ?1/2)

0.1

0.2

0.30.40.5

0.6b

√γ/μ (m 3/2 s ?1/2)Oil temperature (°C)

Figure 3|Effect of Joule heating on the oil-sorption kinetics.a ,Weight of viscous crude oil at different temperatures absorbed into the GWS per unit contact area (m s )versus the square root of the sorption time ( t √

).The measurements were performed at room temperature (20°C).b ,Liquid sorption

coef ?cient (K s )derived from a versus oil temperature,and the square root of the ratio of the oil surface tension to the oil viscosity (

γ/μ√),derived from e versus oil temperature.K s could be derived from the slope of the linear relation between the mass of oil absorbed per unit area (m s )and the square root of the absorption time (

t √).c ,Plots of oil density (d l ,purple squares)and θ(green circles)versus oil temperature.θis the contact angle of the oil droplet

(～1.6ul)on a RGO ?lm,which was approximated as the contact angle of the interface between the oil and pore wall.d ,d l

(cos θ)/2√,calculated according to the values in c ,versus oil temperature.e ,The change of oil viscosity and surface tension as a function of oil temperature.f ,The weight of viscous crude oil absorbed into MS@RGO (2×2×1.5cm ?3)with a power density per unit contact area versus the square root of sorption time.t s is the saturated oil-sorption time.The measurements were performed at room temperature (20°C).

when the heat region is concentrated to the bottom part of the GWS by reducing x .Particularly,for the cases of GWS-x and GWS-MS-x with the same electrode size of 5mm,the heated region in GWS-MS-5is focused more uniformly at the bottom,whereas that in GWS-5is focused more in the vicinity of the electrodes.Moreover,the average temperature of GWS-MS-5is higher than that of GWS-5.The corre-sponding experimental results (Fig.4b)match well with the simulated results in Fig.4a,which indicates that these two designs can improve the utilization ef ?ciency of Joule heating.

To optimize x for the highest energy-utilization ef ?ciency,we designed a homemade set-up (Fig.4c)to measure the sinking speed of GWS-x and GWS-MS-x under a very small hold force (<0.3mN,downward direction),as well as the temperature change of the water below the oil/water interface.First,we studied the relationship between the x and oil-sorption time under the same applied electric power (0.16W cm –3).Figure 4d shows that the oil-sorption times for both GWS-x and GWS-MS-x decrease with a decrease of x ,and the GWS-MS-x shows a faster oil adsorption than the corresponding GWS-x .Although GWS-MS-2.5shows the best performance,the oil-sorption time cannot be improved further by increasing the input power because of the limit to the stable temp-erature (～250°C)of the MS substrate.Figure 4e shows that the maximum electric power (P max )applied to GWS-x and GWS-MS-x increases with an increase of x ,and P max of GWS-x is higher than that of the corresponding GWS-MS-x .As the total electric energy (Q )depends on the product of electric power (P )and oil-sorption time (t ),it is possible to reduce t by increasing the electrode thickness or using GWS-x rather than GWS-MS-x ,without increas-ing Q dramatically.Figure 4f shows that the oil-sorption time of GWS-x -P max and GWS-MS-x -P max decreased with an increase of x ,and the oil-sorption time of GWS-x -P max is less than that of GWS-MS-x -P max when x =2.5and 5mm.When x increases to a certain value,the oil-sorption times of GWS-x -P max and GWS-MS-x -P max begin to increase,and the oil-sorption time of GWS-x -P max becomes longer than that of GWS-MS-x -P max when x >7.5mm.The energy consumption and oil-sorption time of GWS-x -P max and GWS-MS-x -P max are plotted in Fig.4g.GWS-MS-2.5-P max (166J)saved 65.6%of electric energy

compared

GWS-18

GWS-5a

°C

504030GWS-18GWS-9

GWS-MS-5GWS-x 200

300400500

Q (J )

Time (s)

100

20406080Water Air Oil

H e a t a b s o r b t i o n r a t i o (%)

Time (s)

100120140160180200Electrode thickness (mm)f

O i l -s o r p t i o n t i m e (s )

100

200300400500Electrode thickness (mm)d

O i l -s o r p t i o n t i m e (s )

0.1

0.20.30.40.5Electrode thickness (mm)

E l e c t r i c p o w e r (W c m ?3)

GWS-9

GWS-MS-5

Figure 4|Effects of heat distribution on the utilization ef ?ciency of heat energy.a ,Simulated temperature distribution on GWS-x and GWS-MS-x

(GWS here is MS@RGO)with different electrode structures.x denotes the height of the electrode.The sample thickness is 5mm and the input electric power is 0.094W cm –3.The room temperature was 24°C.b ,Experimental results that correspond to the simulations in a .c ,Schematic illustration of the set-up for the measurement of the sinking speed of oil sorbents and temperature change of water below the oil/water interface.d ,Oil-sorption time for GWS-x and GWS-MS-x under the same input electric power density (0.16W cm –3).x =2.5,5,7.5,10,15and 20mm.e ,Maximum electric power (P max )that could be applied to GWS-x and GWS-MS-x .The maximum electric power was obtained by increasing the electric power on GWS-x and GWS-MS-x gradually,until they emitted a scorched smell,which indicated the thermal degradation of the MS substrate.f ,Oil-sorption time for GWS-x and GWS-MS-x under their maximum electric power densities.g ,Electric energy and oil-sorption time for GWS-x -P max and GWS-MS-x -P max .h ,Simulated heat transfer of GWS-MS-10-P max during the oil-sorption process.As the temperatures of the GWS or GWS-MS change during the oil-absorption test,we used the resistances of GWS or GWS-MS at room temperature (R 0)to calculate the power densities in d –g by U 2/R 0.The real values are larger than the data we measured.

with GWS-20-P max (482J),which demonstrates the advantage of a heating region at the bottom part of the GWS.Among all the sorbents,GWS-MS-10-P max may be the best choice because of the shortest oil-sorption time and relatively low energy https://www.360docs.net/doc/369259018.html,pared with GWS-20,GWS-MS-10also saved 50%of the GO consumption.

To determine the energy utilization ef ?ciency of GWS-MS-10-P max during the whole oil-sorption process,the dynamic process of thermal diffusion was simulated according to the oil-sorption speed (Supplementary Fig.13,Supplementary Movie 4and Supplementary Computational Methods 3).Figure 4h shows that about 48.0,37.8and 4.8%of Joule heat transferred to the oil,air and water,respectively,during the oil-absorbing process.Although the oil temperature near the bottom surface of GWS-MS-10-P max only increased from 26.1to 34°C,the viscosity of the crude oil decreased by 41.3%and the oil-sorption time was reduced by 94.6%(Supplementary Fig.14).The heat power could be increased further if we use a sponge substrate with a higher thermal stability than MS,and then the sorption time of GWS-MS-10could be reduced further.The size of GWS-MS-10in the oil-absorbing experiment is very small,and considerable Joule heat would transfer to air from the side surfaces.However,for a larger GWS-MS-10,heat dissipation from the side surfaces would be decreased,and thus the utilization ef ?ciency of Joule heat would increase (Supplementary Fig.15).For the practical application of large GWS sorbents in the future,GWS could be embedded with dense electrodes and heated homogeneously under a low applied voltage (Supplementary Figs 16and 17,and Supplementary Movie 5).

Performance of Joule-heated GWS for crude-oil recovery

Oil-recovery speed is signi ?cant for the reuse of the oil and also to recycle the sorbent materials.Here mechanical compression

(Supplementary Fig.18)and in situ pumping (Supplementary Fig.19)were applied to collect the oil and recycle the sorbents.MS@RGO monoliths ?lled with the oil at 25and 75°C were com-pressed up to a strain of 75%(Supplementary Movie 6).Figure 5a shows that although the crude oil at 25°C extruded out of the surface of MS@RGO,the oil was too sticky to ?ow.By a compressive strain of 75%,the oil began to ?ow slowly.However,for the oil at 75°C,the oil began to ?ow quickly from the MS@RGO at a com-pressive strain of 45%(Fig.5b).The complete oil-recovery process took only ten seconds,which saves 81.8%of oil-sorption time com-pared with oil at 25°C (Supplementary Fig.20).In addition,the percentage of oil recovered from the MS@RGO also improved by 22.8%.Furthermore,the increase of the oil temperature lowered the pressure needed to compress the MS@RGO (Supplementary Fig.21).For the oil at 25°C,482kPa is needed to compress the MS@RGO to an 80%strain.However,the pressure decreased dramatically to 234kPa for the oil at 40°C,and to 74kPa for the oil at 60°C.Heating the oil in these experiments is to obtain a precise oil temperature.In an actual oil-spill clean-up,the oil will be heated by the Joule-heating effect.The elastic property of the MS substrate and the ?exibility of the RGO coating mean that MS@RGO can be recycled repeatedly though compression-recovery processes (Supplementary Fig.22).

The in situ pumping of oil-spill sorbents has been demonstrated as a promising approach to recover oil spills in an ef ?cient and con-tinuous way 20.Nevertheless,this method is suited only to oil with a low viscosity.By introducing Joule heating to decrease the oil vis-cosity,an in situ pumping method (Supplementary Fig.19)may be able to collect viscous oil spills.Figure 5c shows that the oil spill at room temperature was pumped out to the collecting vessel very slowly.After four minutes of pumping,the oil was still in the pipe.Finally,only ～1.58g of the oil spill was collected in eight

6 s 0%

45%

60%

75%

After 6 s

0%45%60%75%After 6 s

75%

25 s

40 s

4 min

8 min

3.87 g

1.58 g

Figure 5|In ?uence of Joule heat on the oil-recovery speed.a ,b ,Recovery of the viscous crude oil (25and 75°C,respectively)from GWS by compressing

GWS from 0to 75%strain.c ,d ,Continuously collecting the viscous crude oil from the surface of sea water by in situ pumping through GWS under applied voltages of 0and 17V,respectively.

minutes,which reveals that the ef?ciency of the in situ pumping method is too low for the collection of viscous crude-oil spills.In contrast,when an electric power of0.5W cm–3(17V)was applied to the MS@RGO to generate Joule heating,the in situ pumping oil-recovery process became very fast(Supplementary Movie7).As shown in Fig.5d,the crude oil can be pumped into the collecting vessel in only25seconds.After40seconds of pumping,3.87g of crude oil was collected.The calculated recovery rate(2,320kg h–1per kg of MS@RGO)was29times higher than that without applying an electric voltage to MS@RGO.

In conclusion,we demonstrate,for the?rst time,a Joule-heated sorbent design to realize the clean-up of viscous crude-oil spills at a high speed.This advance addresses the problem in oil-spill remedia-tion that the traditional porous oil sorbents cannot absorb viscous crude oil quickly.By concentrating the heated region of the sorbent at the oil surface,the electric energy for the oil clean-up can be decreased by65.6%and the consumption of RGO can be reduced by50%.The heat energy transferred to oil can be saved for oil recov-ery and,especially,oil transport by pipes.Although the fraction of heat transferred to the air is still high,there is a large potential to improve the energy ef?ciency further through air–GWS isolation. With the development of a thermally stable polymer sponge,the temperature of GWS could be improved further,which will lead to a faster oil-adsorbing speed.This Joule self-heated sorbent design will provide a new strategy for highly ef?cient remediation of viscous crude oil-spills.

Methods

Methods and any associated references are available in the online version of the paper.

Received8February2016;accepted2February2017; published online3April2017

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This work is supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China(Grant21521001),the National Natural Science Foundation of China(Grants21431006,21761132008and11525211),Key Research Program of Frontier Sciences,CAS(Grant QYZDJ-SSW-SLH036),the Chinese Academy of Sciences(Grants KFJ-EW-116,KJZD-EW-M01-1),the Strategic Priority Research Program of the Chinese Academy of Sciences(XDB22040402),the Ministry of Science and Technology of China(Grant2014CB931800),the Users with Excellence and Scienti?c Research Grant of Hefei Science Center of CAS(2015HSC-UE007)and the Fundamental Research Funds for the Central Universities(WK6030000018).We thank H.-L.Jiang for help in obtaining the crude oil provided by the China University of Petroleum(Huadong). Author contributions

J.G.and S.-H.Y.conceived the idea and designed the experiments.J.G.planned the experiments,performed the experiments,collected and analysed the data,and wrote the paper.S.-H.Y.supervised the project,analysed the results and wrote the paper.L.-A.S. designed and performed the experiments,and collected and analysed the data.H.-A.W. supervised the simulations of heat transfer.Y.-C.W.and Y.-B.Z.performed the simulations and wrote the paper,and contributed to the discussion of energy utilization and optimization design.H.-Y.Z.,Y.Z.and H.-W.Z.designed and performed the experiments.

H.-B.Y.analysed the results and wrote the paper.All authors discussed the results and commented on the manuscript.

Additional information

Supplementary information is available in the online version of the paper.Reprints and permissions information is available online at https://www.360docs.net/doc/369259018.html,/reprints.Correspondence and requests for materials should be addressed to S.H.Y.

Competing?nancial interests

The authors declare no competing?nancial interests.

Methods

Materials.All the chemicals were obtained commercially and used without further puri?cation.The MS was purchased from Runde Trading.GO was fabricated according to a modi?ed Hummers method31.Crude oil was provided by the China University of Petroleum.A self-priming pump(Mabuchi RS-360SH)was obtained from Wisdom Paradise Diy Studio.Sea salt was bought from Asia Marine Fantastic.Sea water was obtained by dissolving37g of sea salt into one litre of deionized water(DIW).

Instruments and characterization.The surface structures of the samples were investigated by SEM(Zeiss Supra40,Carl Zeiss).The viscosity of the oil was measured by a rotatory viscometer(NDJ-5S,Shanghai Ping Xuan Scienti?c Instruments).The density of the oil was measured by a digital densitometer (MDY-2,Shanghai Precision Instruments).Surface tension of the oil was measured by a Langmuir–Blodgett trough(KN2002,KSV NIMA).Water contact angles were measured by the CAST2.0contact-angle-analysis system(Solon Information Technology).The surface temperatures of the samples were measured by a thermal infrared camera(VarioCAM?hr head680,InfraTec).Raman spectra were performed on a LabRamHR with an excitation wavelength of514.5nm.Thermogravimetric analysis was conducted on a DTG-60H in air at a temperature range from

20to1,200°C.Heat conductivity and heat capacity were measured by a

Hot Dish2500(Hot Disk).The press–release cycle test was performed on a universal mechanical testing machine(Instron5565A).The weight of MS@GO during the wicking test was measured by an Instron5560A equipped with a10N load cell.

Preparation of GWS and MS-GWS.The hydrophobic and oleophilic MS@RGO was produced by a dip coating and reduction process.The MS was immersed into a GO solution(3mg ml–1)until fully wetted.Then,the MS fully?lled with GO solution was pulled out and centrifuged at3,000revolutions per minute for2min to remove unwanted GO solution.After this,the GO-coated MS(MS@GO)was dried at60°C for1h.MS@RGO was obtained by immersing MS@GO in HI(≥45.0wt%) at90°C for5s.Then the MS@RGO was pulled out and immediately rinsed with ethanol and DIW several times.Finally,the MS@RGO was dried in an oven at 200°C for2h.The fabrication method for MW@RGO was the same as that for MS@RGO,except the concentration of GO was1.5mg ml–1.For the fabrication of GWS-MS,the GO solution was?rst placed in a vessel with a?at bottom,and the depth of the GO solution could be tuned just by controlling the volume of it.Then the MS was put into the GO solution and contacted with the bottom of the vessel. The bottom surface should be kept parallel with the surface of the GO solution during the dipping process.After a time,the MS was taken out and put onto a plate, and turned upside down.Then the middle of the top surface of the MS was pressed gently a few times to ensure the GO solution was absorbed into the middle of the bottom part.The subsequent centrifugation and reduction processes were similar to those for fabricating the GWS.The thickness of the GWS in GWS-MS depends on the depth of the GO solution.

Measurement ofθ.First,3mg ml–1GO was sprayed onto a glass slide and dried in an oven at60°C for1h.Then the GO?lm on the glass slide was reduced to an RGO ?lm by HI(≥45.0wt%)at90°C.After this,the RGO?lm was connected to a power supply by silver paste and copper wires.By using the Joule-heating effect of the RGO ?lm,the surface temperature of it could be tuned by the amount of voltage applied. The surface temperature was measured by an infrared thermal camera.The oil contact angle was measured by a CAST2.0contact-angle-analysis system(Solon Information Technology).The volume of oil droplets was～1.6μl.Before each measurement,the oil droplets were left on the RGO?lm for14min(reduced to 4min for the RGO?lm at97°C).

Measurement of the sinking speeds of GWS-x and GWS-MS-x.During the oil-absorbing process,the weight of the oil absorbed by the sorbent cannot be measured directly.Fortunately,the oil-sorption speed can be re?ected by the sinking speed of the sorbent,which can be obtained by recording the displacement of the sorbent. However,because of the small size of the sorbents(2×2×2cm?3)we used for the oil-absorbing test,the posture of the sorbents tilts easily during the oil-absorbing process,which makes it dif?cult to measure the displacement of the sorbent.This posture-tilt mainly results from the inhomogeneous density and pore structure of the sorbent.To settle this problem,we put forward a homemade system to measure the sinking speed of the Joule-heated sponge(Fig.4c).The sorbent(GWS or GWS-MS)is?xed to the force sensor of a lifting arm(Instron5565A)by four iron wires, which avoids the sorbents tilting during the-oil-absorbing process.The movement of the lifting arm can be controlled precisely(1μm).As all the sorbents(GWS-x and GWS-MS-x)have electrodes of different weights,the in?uence of the weights of the electrodes is deducted.The force-sensor value is kept at a very small value

(<0.3mN),during the oil-absorbing process.Therefore,all the sorbents sank into the oil under the interaction force between the sorbents and the crude oil.

The displacement of the lifting arm was measured by a laser displacement sensor (CD33,OPTEX).The temperatures of water at the oil/water interface under the sorbent as well as that4and8mm below the oil/water interface were measured by a thermoelectric thermometer during the oil-absorbing test.

Pumping recovery method.Sea water(30ml)was added into a100ml beaker,and then～20ml of crude oil was added to the beaker to form a viscous crude-oil spill. Two opposing side walls of MS@RGO(2×2×2.9cm3)were coated with silver paste (in a square loop)and were connected to a power supply by copper wires.The MS@RGO was immersed into the water.Then one tip of a pipe was inserted into the MS@RGO,and the other tip was connected to the inlet of the self-priming pump. The pump’s outlet was connected to a collecting vessel by another pipe.An electric power of0.5W cm–3(17V)was applied to the MS@RGO.After heating it for1min, a voltage of5.76V was applied to the pump.Pumping through the unheated

MS@RGO was also performed at the same pump voltage.Before pumping,the MS@RGO monoliths were?lled with crude oil.

Data availability.The data that support the?ndings of this study are available within the article and in the Supplementary Information.

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