Reduced graphene oxide composites and its real-life application potential for in-situ crude oil removal

Journal Pre-proof Reduced graphene oxide composites and its real-life application potential for in-situ crude oil removal Xiaoxiao Wang, Guotao Peng, Mengmeng Chen, Mei Zhao, Yuan He, Yue Jiang, Xiaozhen Zhang, Yao Qin, Sijie Lin PII:

S0045-6535(20)30334-9

DOI:

https://doi.org/10.1016/j.chemosphere.2020.126141

Reference:

CHEM 126141

To appear in:

ECSN

Received Date: 18 December 2019 Revised Date:

4 February 2020

Accepted Date: 5 February 2020

Please cite this article as: Wang, X., Peng, G., Chen, M., Zhao, M., He, Y., Jiang, Y., Zhang, X., Qin, Y., Lin, S., Reduced graphene oxide composites and its real-life application potential for in-situ crude oil removal, Chemosphere (2020), doi: https://doi.org/10.1016/j.chemosphere.2020.126141. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

1

Reduced Graphene Oxide Composites and Its Real-Life Application

2

Potential for in-situ Crude Oil Removal

3

Xiaoxiao Wang,1 Guotao Peng,1 Mengmeng Chen,2 Mei Zhao,1 Yuan He,1 Yue Jiang,1

4

Xiaozhen Zhang,1 Yao Qin,3,* Sijie Lin1,*

5 1

6

College of Environmental Science and Engineering, Biomedical Multidisciplinary

7

Innovation Research Institute, Shanghai East Hospital, Shanghai Institute of Pollution

8

Control and Ecological Security, Key Laboratory of Yangtze River Water

9

Environment, Tongji University, 1239 Siping Road, Shanghai 200092, China. 2

10

11

College of Environmental and Chemical Engineering, Shanghai University of

Electric Power, Shanghai, 200090, China 3

12

The Institute for Translational Nanomedicine, Shanghai East Hospital, The Institute

13

for Biomedical Engineering & Nano Science, Tongji University School of Medicine,

14

1239 Siping Road, Shanghai 200092, China

15

16

*Corresponding authors:

17

Sijie Lin, [email protected]; Yao Qin, [email protected].

18

19

20

21

22

1

1

Abstract

2

Crude oil pollution can cause severe and long-term ecological damage and oil cleanup

3

has become a worldwide challenge. Conventional treatment strategies like in-situ

4

burning,

5

time-consuming. The high viscosity of crude oil also posed difficulty for traditional

6

absorbents. Herein, to address these limitations, we designed and fabricated a floating

7

absorbent that was comprised of reduced graphene oxide (RGO), melamine sponge

8

(MS), and a 3D-printed mounting platform. Through a facile one-pot hydrothermal

9

method, graphene oxide (GO) was simultaneously reduced to RGO and loaded in MS

manual

The

skimmer

resulted

and

bioremediation

RGO-MS

were

possess

and

10

(RGO-MS).

11

hydrophobicity/oleophilicity for oil absorption with a water contact angle of 122°.

12

The effective light-to-heat conversion allowed the RGO-MS composite to absorb

13

approximately 95 times its own weight of crude oil within 12 min under light

14

irradiation. A 3D-printed mounting platform for RGO-MS composites was further

15

fabricated to improve its applicability and allow easy retrieval. Taking advantages of

16

the RGO’s hydrophobicity/oleophilicity and photothermal property, the floating

17

ability of MS, this study demonstrated the real-life applicability of RGO-MS

18

composites for in-situ crude oil cleanup.

2

composites

labor-intensive

desirable

1

Introduction

2

In recent years, frequent oil spill has exerted great threat to the environment,

3

especially the sea and coastal places, therefore has become a worldwide

4

environmental issue.1-3 Conventional cleanup approaches, such as oil skimmer,4

5

bioremediation5, and in-situ burning6 have been widely used to remove the oil

6

contamination. But most of these approaches were either labor-intensive or

7

time-consuming. The negative impact on the air quality due to in-situ burning made it

8

not desirable for future oil pollution treatment. Since most of the oil density is lower

9

than water, floating absorbents offered an alternative cleanup approach by in-situ oil

10

absorption.7 Typical naturally-occurring absorbents (such as straw, zeolites, and wool

11

fibers)8 and common synthetic absorbents (such as polyurethane and polyethylene)9-11

12

were used for this purpose. However, these absorbents usually suffer from lack of

13

reusability or low absorption efficiency.12-14

14

15

Recently, novel adsorbing materials with improved performance have been

16

reported.15-21 Through silylation modification, a hydrophilic sponge could be turned

17

into a hydrophobic absorbent for oil-water separation.16 A superhydrophobic and

18

superoleophilic “sponge-like” aerogel could be obtained via sol–gel reaction, in which

19

methyltriethoxysilane (MTES) and dimethyldiethoxysilane (DMDES) were used as

20

co-precursors.19 Functional nanoparticles integrated into material matrix (such as

21

polymers) could provide additional surface roughness and increased hydrophobicity

22

for oil absorption.12 But most of these method was not applicable for crude oil 3

1

absorption due to its high viscosity (100-10000 mPa s at room temperature).22-24

2

Therefore, besides hydrophobicity/oleophilicity, the key for crude oil absorption is to

3

decrease oil viscosity for better fluidity.25 In this regard, Ge et al. recently

4

demonstrated a Joule-heated graphene-wrapped sponge that was able to convert

5

electricity into thermal energy to reduce the crude oil viscosity.24 However, since the

6

electro-thermal conversion would require additional electric power input, materials

7

with photothermal properties might be a better candidate for in field applications. In

8

fact, photothermal materials with the ability to convert light to heat efficiently have

9

been used in desalination, clean water production, catalysis and cancer therapy.26-32

10

Recently, photothermal oil absorbents were fabricated through coating photothermal

11

materials (such as polydopamine and polypyrrole) on the sponge.22, 23, 33 However, the

12

time-consuming preparation process and instability of polymers in heat continued to

13

be a challenge.34 Although graphene could be a suitable substitution due to its heat

14

stability, hydrophobic/oleophilic characteristic and photothermal property, its high

15

hydrophobicity could pose difficulty in applying in aqueous environment.35-40

16

17

Against this background, this study set out to explore the use of graphene-based

18

photothermal nanomaterials in combination with floating matrix to achieve in situ

19

crude oil absorption. A facile one-step hydrothermal synthesis method was used to

20

achieve graphene oxide (GO) to reduced graphene oxide (RGO) conversion and

21

loading to melamine sponge (MS) simultaneously. The resulted RGO-MS composite

22

possesses desirable hydrophobicity and oleophilicity. Upon light irradiation, the 4

1

photothermal property of RGO enabled fast temperature rise that lowered crude oil

2

viscosity. Moreover, by designing and fabricating a mounting platform through

3

3D-printing, multiple RGO-MS composites could be applied, retrieved, and replaced

4

after usage. The reusability of such composite was also evaluated.

5

6

Results and Discussion

7

Fabrication and Characterization of RGO-MS Composites

8

The commercially available MS was selected as the substrate material due to its

9

characteristics, including low cost, low density (< 10 mg/cm3), high porosity (> 99%),

10

high compressibility, and thermal stability.41-43 The schematic illustration of the

11

fabrication process of RGO-MS composites is shown in Figure S1. The ammonia

12

solution played an important role of promoting the stability of GO suspension through

13

electrostatic repulsion and L-ascorbic acid (L-AA) was used as a reductant.44 During

14

the process of reducing the GO into RGO, L-AA also served as a gelation agent which

15

facilitated the loading of RGO onto the framework of MS.45 Interestingly, the amount

16

of RGO loading was proportional to the concentrations of L-AA used, as shown in the

17

representative digital photographs and SEM images (Figure 1). The concentration of

18

L-AA (0, 0.5, 5, 10 mM) was used to label the corresponding composite as

19

RGO-MS-x.

20

three-dimensional porous structure with the pore sizes ranging from 30 to 150 µm

21

(Figure 1i, m). After loading with RGO, a thin layer of RGO sheet was covering the

22

MS framework as well as some pore areas (Figure 1j, k, n, o). When a relative high

Microstructurally,

MS

displayed

5

a

highly

interconnected

1

concentration of L-AA was used, severe agglomeration of RGO was observed (Figure

2

1l and p), which led to a collapse of overall macrostructure of MS, as evidenced in

3

Figure 1d and h. The cross-section of MS as shown in Figure 1e-h demonstrated that

4

the loading achieved the best uniformity when 5 mM of L-AA was used. Lower or

5

higher concentrations of L-AA led to heterogeneous distribution of RGO inside the

6

MS. These results suggested that the extend of GO to RGO conversion was crucial in

7

determining the quality of RGO loading. When GO was reduced to RGO, the

8

decreased hydrophilicity promoted the RGO particles to aggregate and self-assemble

9

on the framework of MS to minimize surface energy. However, too much of L-AA

10

would lead to severe aggregation of RGO that further collapsed the macrostructure of

11

MS. In the following discussions, we focused on RGO-MS-5 as the optimal RGO-MS

12

composite to evaluate its performances.

6

1

2

Figure1. Representative digital photographs and SEM images of MS before and

3

after loading with RGO. Representative digital photographs of the external surface

4

(a) and the cross-section (b) of MS and RGO-MS. The numbers of 0.5, 5 and 10

5

indicate the concentrations of L-ascorbic acid (mM) used for different loading

6

conditions. The microstructure of MS and RGO-MS was revealed by SEM (c-f, scale

7

bar: 200 µm). Higher magnification images (g-j, scale bar: 100 µm) show the

8

presence of RGO covering the framework and some pore areas of the MS.

9

10

Raman scattering was performed to further characterize the loading of RGO on MS.

11

Based on the characteristic D and G bands near 1355 and 1593 cm-1 (corresponding to

12

disordered structural defects and sp2 graphic domains46), it can be seen that the 7

1

conversion from GO to RGO using L-AA was successful, indicated by the ID/IG ratio

2

(Figure 2a). The ID/IG ratios increased from ~0.85 for GO to ~1.14 for RGO-5 and

3

RGO-MS-5, confirming that there were structural defects and disorder formation for

4

RGO during the hydrothermal treatment of GO.47, 48 RGO-5 was obtained using the

5

same method for comparison purpose. With the decrease of hydrophilic

6

oxygen-functional groups of GO, the extend of reduction increased. Using a confocal

7

Raman microscope, the microstructure of MS loaded with RGO was shown in Figure

8

2b and c. The microscopic image of a RGO-MS-5 composite and the corresponding

9

Raman mapping of the highlighted area confirmed that the RGO was evenly coated

10

on the framework of MS. The gradient color bar from black to red representing the

11

range of ID/IG ratios showed the coated graphene material were mostly in the form of

12

RGO. Moreover, the Raman spectrum of MS in Figure 2a exhibited a strong peak at

13

975 cm-1, which represented the triazine ring breathing vibration of melamine.43, 49

14

The same peak was also observed in the spectrum of RGO-MS-5, demonstrating RGO

15

was firmly loaded to MS framework. The loading of RGO on MS was also confirmed

16

using XRD and FTIR analyses (Figure S2 and S3).

8

1

2

Figure 2. Confocal Raman spectroscopy to confirm the loading of RGO on MS.

3

(a) Raman spectra of RGO-MS-5, RGO, GO, and MS (up-down). The characteristic D

4

and G bands near 1355 and 1593 cm-1 corresponded to disordered structural defects

5

and sp2 graphic domains, respectively. Comparing to the Raman spectrum of MS, the

6

presence of D and G bands in the Raman spectrum of RGO-MS-5 confirmed the

7

successful loading of reduced graphene oxide. The degree of reduction was reflected

8

by the ID/IG ratio, where the higher the ID/IG ratio the higher degree of reduction. (b)

9

Microscopic image showing the RGO-MS-5 framework. (c) Raman mapping of the

10

highlighted area in (b). The gradient color bar from black to red indicates the ID/IG

11

ratios from high to low. Most of the MS framework was in a red color proving the

12

RGO was evenly distributed in the MS.

13

9

1

RGO-MS composite possessed desirable oil absorption property

2

The loading of RGO altered the original surface property of MS and rendered it

3

hydrophobic and oleophilic, as shown in the wetting behavior of the RGO-MS using

4

both water and oil droplets (Figure 3). According to the dynamic contact angle (DCA)

5

measurements, the pristine MS (Figure 3a-d, i-l) exhibited an intrinsic hydrophilicity

6

and oleophilicity: water and oil (diesel) droplets were both quickly absorbed into the

7

sponge within 0.08s, thus the contact angles for both water and oil were not detectable

8

(ND). In contrast, the water contact angle displayed on RGO-MS-5 surface was ~122°

9

(Figure 3e-h), while oil droplet was quickly absorbed (Figure 3m-p). These results

10

showed that the loading of RGO rendered the MS not only desirable oil-absorbing but

11

also oil-water separation ability.

10

1

Figure 3. Surface characterization of MS and RGO-MS composite. The surface

2

hydrophilicity and oleophilicity was determined by dynamic contact angle (DCA)

3

measurements. Representative DCA images of MS (a-d, i-l) and RGO-MS-5 (e-h,

4

m-p), illustrating their wetting behavior with water and oil (diesel), respectively. The

5

digital photographs were captured every 0.04 s. Photographs in the red frame shows

6

the contact angles at static state (> 1 s). MS exhibited an intrinsic wettability of

7

hydrophilicity (a-d) and oleophilicity (i-l): water and oil droplets were quickly

8

absorbed into the sponge therefore the contact angles were not detectable (ND).

9

RGO-MS-5 exhibited clear hydrophobicity with a water contact angle of 122° (e-h)

10

and oleophilicity (m-p). 11

1

Figure 4 continued to show the selective oil-absorption feature of RGO-MS

2

composite in an oil-water mixture. After immersing in an oil-water mixture, the

3

RGO-MS composite was found to quickly absorb the oil component (diesel, stained

4

with oil red for easy viewing) and stayed afloat (Figure 4a and b). In contrast, the

5

pristine MS failed to stay afloat after 5 min likely due to water absorption (Figure 4c).

6

Interestingly, the absorbed content for RGO-MS composite was mostly oil while the

7

majority of absorbed content was water for the pristine MS (Figure 4d). To further

8

quantify the absorption capacity of RGO-MS composite, six different types of oil, i.e.

9

n-hexane, castor oil, soybean oil, diesel and silicone oil-10 and silicone oil-1000 were

10

tested. Figure 4e showed that the RGO-MS composite displayed outstanding

11

absorption capacity ranging from approximately 58 to 97 times of its original mass,

12

proving it a promising absorbent for light oil. These results demonstrated a successful

13

implementation of the idea of loading an oleophilic functional nanomaterials into a

14

floating substrate to form a composite for in-situ oil removal in aquatic environment.

15

Unlike most of the traditional absorbents that are both hydrophilic and oleophilic,

16

including the MS used as a substrate in this study, the hydrophobic surface

17

modification of the composite turned out to be a critical factor as water absorbing

18

would result in sinking of the absorbent, rendering it difficult for retrieval after usage.

19

12

1

2

Figure 4. Evaluation of in-situ oil absorption capability by RGO-MS composite.

3

(a-d) Digital photographs demonstrated a typical in-situ oil absorption processes using

4

RGO-MS and MS. Oil red was used to dye the diesel phase in red for easy viewing.

5

RGO-MS-5 and MS were simultaneously immersed in the oil-water mixture and

6

allowed for absorption for 5 min. The absorbed content by RGO-MS-5 was mostly

7

diesel while the one by MS was a mixture of diesel and water. (e) Quantification of oil

8

absorption capacity by RGO-MS-5 for six different types of oil, i.e. n-hexane, castor

9

oil, soybean oil, diesel and silicone oil (viscosity: 10 mPa s) (silicone oil-10) and

10

silicone oil (viscosity: 1000 mPa s) (silicone oil-1000). The absorption capacity (g/g)

11

was displayed as the total weight of oil absorbed per unit weight of RGO-MS

12

composite.

13

13

1

Photothermal property of the RGO-MS composite and its use on crude oil

2

absorption

3

In order to quantitatively confirm the excellent photothermal property of RGO was

4

successfully embedded in the RGO-MS composite, both absorbance and diffuse

5

reflectance spectra over the range of 200-2500 nm were measured (Figure 5a and S4).

6

The absorbance spectrum of RGO-MS composite exhibited distinctly higher intensity

7

in all wavelength than MS, confirming the increase of photo harvest capability.

8

Meanwhile, the RGO-MS composite had an average diffuse reflectance of 4.91%,

9

compared with 71.53% of the MS (Figure S4). The ability to convert photo energy to

10

thermal energy was further evaluated using a thermal infrared imager. As shown in

11

Figure 5b, the surface temperatures of RGO-MS composite and MS were compared

12

under light on (1.2 kW/m2 illumination) and off conditions. When light on, the surface

13

temperature of RGO-MS increased rapidly and went up to ~70℃within 60 s while the

14

surface temperature of MS barely moved and stayed ~28℃. Interestingly, when light

15

off, the surface temperature of RGO-MS composite dropped back to room

16

temperature with a minute, suggesting an effective heat dissipation. The

17

corresponding infrared thermal images of the pristine MS (Figure 5c) and RGO-MS

18

composite (Figure 5d) under illumination showed the distinct photothermal

19

properties before and after RGO loading. With the excellent light to heat conversion,

20

it is reasonable to speculate that the RGO-MS composite should possess a promising

21

potential to absorb oil with higher viscosity, i.e. crude oil.

22

14

1

2

Figure 5. Characterization of the photothermal property of RGO-MS composite.

3

(a) UV-vis-NIR absorption spectra of MS and RGO-MS-5 over the range of 200-2500

4

nm. RGO-MS-5 had a significant increase in the intensity in all wavelength range

5

compared to MS, demonstrating the improvement of photo-harvesting capability. (b)

6

Surface temperature of MS and RGO-MS-5 under the light on and off conditions

7

(simulated sunlight irradiation, power density: 1.2 kW/m2). The temperature of

8

RGO-MS-5 increased rapidly under light on and rose to over 70℃ within 1 min and

9

decreased quickly once light off, while the temperature remained at room temperature

10

for the MS. IR thermal images of MS (c) and RGO-MS-5 (d) under the light

11

irradiation showed clear difference in the photothermal conversion capability before

12

and after RGO loading. The color bar from purple to white indicates the temperature

13

changes from low to high. The outline of MS was marked with a dotted line for easy

14

viewing.

15

15

1

To quantify the crude oil absorption capacity of RGO-MS composite, a below-balance

2

model of an analytical balance was used to accommodate light irradiation and

3

measurement of crude oil absorption. As shown in the schematic diagram (Figure 6a),

4

an RGO-MS composite was attached to a wire connected to the notched hook

5

underneath the balance. A glass beaker containing crude oil was lifted until the oil

6

surface contacted the bottom of RGO-MS composite. The light was introduced by the

7

side to facilitate the crude oil absorption. To avoid interference from ambient light, the

8

whole setup was contained in a black acrylic box as shown in Figure 6b. Given the

9

high viscosity, most of the crude oil was found sticking to the surface of pristine MS

10

without diffusing into the structure. In the case of RGO-MS composite, with 12 min

11

of simulated sunlight irradiation, it was clear that the crude oil increased its fluidity

12

and the absorbed quantity was significantly higher than the pristine MS (Figure 6c).

13

14

15

Figure 6. Quantification of in-situ crude oil absorption by RGO-MS composite.

16

(a) Schematic illustration on the quantification method of crude oil absorption under

17

light irradiation. The below-balance mode of an analytical balance was used to 16

1

measure the quantity of crude oil adsorbed. RGO-MS-5 was attached to a wire

2

connected to the notched hook underneath an analytical balance. Light was introduced

3

by the side while RGO-MS-5 was laid on top of the crude oil in a glass beaker. (b)

4

Digital photograph of the setup for crude oil absorption. (c) Crude oil absorption

5

capacity (% w/w) of RGO-MS-5 vs. MS. Crude oil absorption capacity was calculated

6

based on the total weight of crude oil adsorbed per unit weight of RGO-MS-5 and

7

MS.

8

9

The photothermal conversion efficiency (η) can be defined by Equation (1)30: 

10


=

11

where Qs is the simulated sunlight energy (1.2 kW/m2), and Qe is the power consumed

12

by heating oil, which can be estimated by Equation (2)25:

13



 = ∭ −  

(1)

(2)

14

where T is the steady temperature of the composite, T0 is the initial temperature, Cp is

15

the heat capacity of the crude oil (~2.1 J/(g ℃)) and m is the mass of the crude oil

16

absorbed into the composite. The photothermal conversion efficiency of the RGO-MS

17

composite was estimated to be 66.9%, which is 8 times higher than MS, whose

18

efficiency was only 8.3%. The detailed calculation of photothermal conversion

19

efficiency can be found in the Supporting Information.

20

21

To improve the real-life applicability of the RGO-MS composite, a mounting platform

22

was designed and 3D printed with the aim to hold the composite in position while 17

1

allowing for easy retrieval after usage. The mounting platform, as shown in Figure

2

7a-d and S5, was comprise of two sitting slots for RGO-MS composites and in

3

between a handle to lift. The sizes of the sitting slot as well as the overall frame could

4

be adjusted according to specific usage. In this study, two RGO-MS composites were

5

installed for proof-of-principle. Under natural sunlight, the mounting platform along

6

with two RGO-MS composites was placed in a glass beaker containing synthetic

7

seawater and crude oil (Figure 7e). It took approximately 30 min for the crude oil to

8

be completely absorbed (Figure 7e-h). The duration was longer than the laboratory

9

setting due to a lower sunlight energy. Nonetheless, this result demonstrated the

10

applicability of the RGO-MS composite under a natural setting.

11

12

Figure 7. In-situ crude oil absorption by RGO-MS composite under sunlight. (a-d)

13

Floating mounting platform for RGO-MS composite made by three-dimensional

14

printing was design to hold the RGO-MS in place for easy retrieval. The mounting

15

platform was made of nylon in white color. (e-h) Digital photographs showed the

16

RGO-MS composite with mounting platform for practical use of in-situ crude oil 18

1

absorption under sunlight. RGO-MS composite imbedded with the mounting platform

2

was floating on simulated seawater with crude oil. The crude oil was completely

3

absorbed within 30 min under sunlight.

4

5

Moreover, the reusability of the RGO-MS composite was tested by subjecting it to

6

absorb/squeeze dry/solvent wash cycles. As shown in Figure 8a, the absorption

7

capacity of RGO-MS composite for crude oil remained relatively stable after 5 cycles.

8

And the photothermal property of the composite also stayed the same as shown in the

9

infrared thermal images (Figure 8b). One of the main reasons to implement a

10

composite strategy was to minimize the hazard potential of RGO to the living

11

organisms in the environment, as recent studies showed potential harmful effects of

12

graphene and graphene-based materials in vitro and in vivo.50, 51 Strategies like surface

13

functionalization and embedded 3D foam/sponge structures could increase the

14

biocompatibility.52, 53 In this study, the RGO-MS composite remained intact without

15

releasing RGO from the material matrix during the reusability test.

19

1

2

Figure 8. Characterization of the reusability of RGO-MS composite. (a) The

3

crude oil absorption capacity of RGO-MS-5 remained relatively unchanged after 5

4

cycles. And the composite continued to show excellent photothermal property as

5

indicated by the rise of temperature under simulated light irradiation. (b) The

6

corresponding IR images of RGO-MS-5 after each run of crude oil absorption. Crude

7

oil was rinsed off from RGO-MS-5 using n-hexane after each run.

8

9

Conclusion

10

In conclusion, the RGO-MS composite fabricated by simultaneously reducing GO to

11

RGO and loading to melamine sponge has excellent hydrophilicity/oleophilicity and

12

photothermal property that enabled effective crude oil absorption. Upon light 20

1

irradiation, the RGO-MS composite achieved in-situ crude oil absorption 95 times of

2

its own weight within 12 minutes. And 3D-printed mounting platform provided easy

3

retrieval of the composites for reuse purpose. Our study has demonstrated a facile

4

synthesis method for RGO-MS composite, its efficient oil absorption capability and

5

real-life applicability.

6

21

1

Experimental Section

2

Materials: Graphene oxide (GO, >99%, leaf size of 0.5-3 µm, thickness of 0.55-1.2

3

nm) was purchased from Aladdin Bio-Chem Co. Ltd. (Shanghai, China). L-Ascorbic

4

acid (L-AA) was obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai,

5

China). Ammonia solution (28.0-30.0%) was produced by InnoChem Science &

6

Technology Co. Ltd. (Beijing, China). The crude oil and diesel were both obtained

7

from China National Petroleum Corporation. Other light oils and organic solvents,

8

including castor oil, soybean oil, silicone oil (viscosity: 10 mPa s), silicone oil

9

(viscosity: 1000 mPa s) and n-Hexane were purchased from Aladdin Bio-Chem Co.

10

Ltd. (Shanghai, China) and used as supplied. The sea salt for simulating sea water and

11

melamine sponge (MS) are commercially available from the markets, obtained from

12

Yier BE Co. Ltd. (Guangzhou, China) and Nanjiren Co. Ltd. (Shanghai, China),

13

respectively.

14

15

Fabrication of RGO-MS composites: The RGO-MS composite was fabricated by a

16

facile one-pot hydrothermal method without adding any coupling agents. Firstly, the

17

MS was cleaned by ethanol and dried in an oven at 60 ℃ overnight. GO stock

18

suspension (3 mg/ml) was prepared in advance by ultrasonication (Ultrasonic

19

homogenizer, Ningbo Scientz Biotechnology Co. Ltd., 300 W) for 10 min. Then the

20

MS was immersed into a mixture of GO, L-AA and Ammonia solution. The detailed

21

procedure was as followed, GO stock solution was added to ultrapure water with 5

22

min of ultrasonic (Ultrasonic cleaner, Ningbo Scientz Biotechnology Co. Ltd., 40 22

1

kHz). Then the pH value of the solution was adjusted to ~10 by adding Ammonia

2

solution. Homogenize the mixed solution by ultrasonic for another 5 min. After that,

3

L-AA was added as a reducing agent into the above mixture under stirring at different

4

concentrations (CLAA = 0.5, 5 and 10 mM). The resulted mixture with MS were then

5

sealed in the Teflon-lined autoclave and heated at 95 °C for 1 h in the oven. After

6

cooling down to room temperature, the resulted composite was taken out and rinsed

7

by ultrapure water for several times. Finally, the composite was dried at 60 ℃ for 24

8

h.

9

10

Morphology and Structure Characterizations: The morphology of the as-prepared

11

composite was characterized using a field-emission SEM (Hitachi S4800, Japan).

12

Raman spectra and mapping were conducted using Renishaw inVia confocal Raman

13

microscope (UK). For DCA measurements, Powereach JC2000C1 instrument (China)

14

equipped with a CCD camera (25 FPS) was used. The optical absorbance and diffuse

15

reflection spectra were measured using Shimadzu UV-3600 spectrophotometer (Japan)

16

in the range of 200-2500 nm. The surface temperature and thermal images were

17

captured with Fluke TiS55 infrared camera (USA). FTIR spectra were characterized

18

by a Nicolet 5700 FTIR spectrometer (USA). XRD patterns were conducted by

19

Bruker D8 Advance X-ray diffractometer (Germany).

20

21

Evaluation of in-situ oil absorption capability: To assess the oil absorption property,

22

diesel (5 mL) dyed with oil red was added into simulated seawater. The MS and 23

1

RGO-MS composite were immersed into the oil-water mixture solution, respectively.

2

After absorption process reached equilibrium (5 min), the samples were taken out and

3

hand-squeezed. The liquids extruded were collected in the glass beakers. Photographs

4

were taken to record the full process of oil-water separation.

5

6

For quantification of different light oil absorption capacity, the RGO-MS composite

7

was immersed into six types of light oil separately, i.e. castor oil, soybean oil, diesel,

8

n-hexane and silicone oil (viscosity: 10 mPa s and 1000 mPa s) by using the same

9

method described above. The absorption capacity (Q) for light oil was calculated

10

according to the following equation: =

11

  

(3)

12

where m0 and m1 represent the weight of the sponge before and after oil absorption

13

(g), respectively.

14

15

Evaluation of the photothermal property: Simulated solar irradiation was provided

16

by EOS Xe-100 solar simulator (USA) with an AM 1.5 G filter. And the power

17

density was about 1.2 kW/m2. The MS and RGO-MS composite with 2 × 1.5 × 0.5 cm

18

shape were placed on the polystyrene foam with low heat conductivity. The real-time

19

temperature changes were recorded every 5 seconds using an infrared thermal camera

20

when the simulated sunlight was turned on and off.

21

22

Quantification of in-situ crude oil absorption: A customized setup including an 24

1

analytical balance (Sartorius, Germany), lifting stage and light source was used to

2

quantitively assess crude oil absorption capacity. The RGO-MS composite and MS (2

3

× 1.5 × 0.5 cm) were attached to the notched hook, connecting to the the analytical

4

balance. The beaker containing crude oil was placed on the moveable lifting stage.

5

After irradiation for 12 min, the stage was lowered, the sponge was taken out from the

6

crude oil, and the weight of the sponge which had absorbed crude oil was weighted by

7

the balance. The crude oil absorption capacity was also calculated according to the

8

Equation (3).

9

10

In-situ crude oil absorption under sunlight: The practical crude oil absorption

11

experiments were carried out under sunlight. Two RGO-MS composites were installed

12

in a 3D-printed mounting platform. The mounting platform was made of nylon in

13

white color. The detailed dimensions of the platform can be found in Figure S4.

14

RGO-MS composite imbedded with the mounting platform was floating on simulated

15

seawater with crude oil. After 30 min of sunlight, the platform was taken out to assess

16

the crude oil absorption.

17

18

Characterization of the reusability: The reusability was conducted by repeated 5

19

cycles of absorb/squeeze dry/solvent wash processes. And the infrared thermal images

20

were captured after each run to evaluate the photothermal property of RGO-MS

21

composites.

22

25

1

Acknowledgements

2

This work was financially supported by National Key R&D Program of

3

China(2018YFC1803100), NSFC grant #21777116 (Lin) and the Fundamental

4

Research Funds for the Central Universities (Y. Qin and S. Lin).

5

6

Conflict of Interest

7

The authors declare no conflict of interest.

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

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Graphene based composite provided an alternative strategy to achieve in situ crude oil removal.




The photo-thermal property offered by reduced graphene oxide converted sunlight energy into heat, which reduced the viscosity of crude oil drastically to allow easy adsorption.




A 3D-printed mounting platform was designed and fabricated to achieve easy retrieval of the composites after usage.




The strategy shown in this study was easy-to-apply, safe-to-use, and highly effective.

Author contributions SL and YQ conceived the project. XW, XZ, YH and YJ performed composite synthesis and characterization, XW, GP and MC conducted data analysis and graph presentation, GP, MZ and YQ provided inputs on the data and the manuscript. SL and XW wrote the manuscript.

Conflict of Interest The authors declare no conflict of interest.