Controlled synthesis of flexible graphene aerogels macroscopic monolith as versatile agents for wastewater treatment

Accepted Manuscript Full Length Article Controlled synthesis of flexible graphene aerogels macroscopic monolith as versatile agents for wastewater treatment Shuying Dong, Longji Xia, Teng Guo, Fangyuan Zhang, Lingfang Cui, Xianfa Su, Dong Wang, Wei Guo, Jianhui Sun PII: DOI: Reference:

S0169-4332(18)30817-1 https://doi.org/10.1016/j.apsusc.2018.03.132 APSUSC 38880

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

23 November 2017 6 March 2018 18 March 2018

Please cite this article as: S. Dong, L. Xia, T. Guo, F. Zhang, L. Cui, X. Su, D. Wang, W. Guo, J. Sun, Controlled synthesis of flexible graphene aerogels macroscopic monolith as versatile agents for wastewater treatment, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.03.132

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Controlled synthesis of flexible graphene aerogels macroscopic monolith as versatile agents for wastewater treatment Shuying Donga, Longji Xiaa, Teng Guoa, Fangyuan Zhanga, Lingfang Cuia, Xianfa Sua, Dong Wangb, Wei Guoc, Jianhui Suna,*

a

School of Environment, Henan Normal University, Key Laboratory for Yellow River

and Huai River Water Environmental and Pollution Control, Ministry of Education, Henan Key Laboratory for Environmental Pollution Control, Xinxiang, Henan 453007, P. R. China b

School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang,

Henan 453007, P. R. China c

Department of Chemistry, Xinxiang Medical University, East of JinSui Road,

Xinxiang, Henan 453003, P.R. China

*

Corresponding author. E-mail: [email protected] (J.H. Sun). Tel.: +86-373-3325971

1

Abstract: Emerging applications for environmental purification require agents that not only possess high decontamination efficiency, but also are capable of withstanding mechanical deformation without secondary pollution and degradation of performance. To this end, we have controlled synthesis of mechanically flexible graphene aerogel (GA) by vacuum freeze-drying of their hydrogel precursors obtained from heating the aqueous mixtures of graphene oxide (GO) and Vitamin C (VC) without stirring. Through the adaptable process conditions, such as the particle size of carbon, GO concentration, dosage of reducing agent and solution pH, the highly porous, ultralight and mechanically flexible GA are synthesized. Owing to the porous, robust and stable structure, the resulting GA show very promising performance in water purification including enrichment of organic liquid solvents (alcohols, oil and alkanes), removal of hexavalent chromium Cr(VI) and purified industrial wastewater, as well as flexible conductors. The successful creation of the GA may provide new insights into the design of carbon-based aerogels for various applications, as the GA can be prepared via a very simple procedure and available in macroscopic diverse morphologies with tunable porosity. Keywords: graphene aerogel; flexible; wastewater purification; adsorption; macroscopic

2

1. Introduction Due to the inherent hybrid electron orbital statuses, such as sp1, sp2 and sp3 hybridizations, carbon possesses diverse allotropes, which is one of the most ubiquitous elements in nature. Myriad carbonaceous materials have been developed in the

past

decades,

including

some

novel

carbon nanomaterials,

such as

zero-dimensional (0D) fullerenes [1], one-dimensional (1D) carbon nanotubes [2], and 2D graphene sheets [3], as well as 3D graphite and porous carbons [4]. Since the arrangement of carbon atoms and the hybrid electron orbital statuses will affect the solidity and the conductivity, respectively, different microstructures and components enable the carbon-based materials exhibit diverse physical and chemical properties [5]. Long-range π-conjugation in graphene yields extraordinary physical properties such as large specific surface area, extremely high electrical and thermal conductivity, and unique mechanical and chemical stability. Such unique properties qualify the graphene potential applications in various fields, such as energy storage [6], environmental purification [7,8], catalyst supports [9], and high performance electronics and sensors [10]. To keep pace with the growing prevalence of flexible wastewater treatment materials and many other emerging applications, there has been significant research efforts toward to explore profitable agents with mechanically robust under bending, stretching, or compression, and high recycle performance without secondary pollution [11-13]. Within this research, 3D graphene aerogel (GA), also called graphene foam, have received tremendous attention in various fields recently owing to its excellent 3

properties such as ultra-low density, low thermal conductivity, super compressibility and high electrical conductivity, etc [14]. GA is particularly promising for practical applications not only because of the above-mentioned merits but also attribute to the fact that it can be obtained in macroscopic monolith with tunable hierarchical nanoarchitectures [15]. Recently, several methods have been reported for the fabrication of GA, including sol-gel process [16], hydrothermal [17] and chemical reduction [18], and in-situ growth by chemical vapor deposition (CVD) [19]. However, some of the previously reported GA generally has intricate preparation methods or the disadvantages of mechanical performances, such as brittle or irreversibly damaged under mechanical deformations, all of which restricts its practical applications. Therefore, one important and effective route to employ GA with excellent physical properties for practical applications is to develop a simple and effective synthetic route. As we all known, pristine graphene with the disadvantage of poorly dispersion in the solvents and tends to restacking, primarily to limit the processability and applications [20]. However, the deep-oxidized graphene oxide (GO) sheets exhibit improved dispersibility due to the rich oxygen-containing groups. Consequently, GO can provide many reaction sites for further chemical modification to generate versatile composites and amplify the applications of graphene-based materials [21]. Herein, we design a green strategy for the fabrication of ultralight, robust and flexible graphene aerogel (GA) monoliths by vacuum freeze-drying of their hydrogel precursors obtained from heating the aqueous mixtures of graphene oxide (GO) and Vitamin C 4

(VC) without stirring. The effects of the particle size of carbon, GO concentration, dosage of reducing agent and solution pH on the microstructure and adsorption capacity of GA are clarified. Our results indicated that these ingredients and technological parameters have different effects on the performance of GA. The resulting ultralight GA not only maintains the physical properties of graphene, but also possessing outstanding mechanical properties, hydrophobicity, favourable electrical conductivity, and excellent recycle performances, thus holding great potential application in environmental purification. This macroscopic-assembled, all carbon aerogels exhibit excellent organic liquid solvents absorption capacity, purified industrial wastewater, and removal of Cr(VI) originating from the synergistic effect between GA and natural sunlight irradiation. 2. Experimental Section 2.1. Materials Natural graphite powders with an average particle size of 80 mesh, 325 mesh, 500 mesh, 1200 mesh and expandable graphite flakes with an average particle size of 325 mesh with a purity of >99 wt% were purchased from Aladdin. All other chemicals were supplied by Sinopharm Chemical Reagent Co., Ltd and used without additional purification and refinement. Deionized water used throughout this study was produced in our laboratory. 2.2. Synthesis of GA GO was synthesized by oxidizing natural graphite powders similar to the recipe described in our previous reports [22]. The GA was prepared by one-step reduction 5

and self-assembly of GO with VC without stirring, followed by vacuum freeze-drying. In a typical synthesis procedure, a specific amount of GO was well dispersed in deionized water and the dispersion was sonicated in an ultrasonicator surrounded by an ice bath to get exfoliated GO, followed by adding a certain ratio of VC to form a uniform suspension. Subsequently, 10 mL of the mixture was placed in a cylindrical glass vial (or other models ) and heating the aqueous mixtures without stirring at 90 °C for the duration of 1 h to obtain the graphene hydrogel. After that, the vial was cooled to room temperature and the as-formed hydrogel was poured out. Finally, the black cylinder-like or other shape graphene hydrogel was dialyzed with alcohol/water solution to remove the residual soluble impurities, and then freezed at -20 °C for 12 h and vacuum freeze-dried for 3 days to obtain the GA. The size and shape of the GA could be varied by adjusting the volume of GO-VC mixed solution and the reactor model according to the condition, respectively. 2.3. Instrumentals Scanning electron microscopy (SEM) micrographs were taken to inspect the morphologies of obtained GA by using a JSM-6390LV scanning electron microscope produced by JEOL. Powder X-ray diffraction (XRD) analysis was carried out on a Bruker-D8-AXS diffractometer system with a Cu Kα radiation (λ = 0.15406 Å). Transmission electron microscopy (TEM) micrographs were performed on a JEM-2100 system with an acceleration voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were collected in transmission mode on a FTIR Analyzer (Perkin-Elmer, Spectrum 400) with KBr as a reference sample. Raman spectra were 6

recorded at room temperature using a SPEX-1403 laser Raman spectrometer with 532 nm wavelength incident laser light. X-ray photoelectron spectra (XPS) were recorded by employing an Escalab-250Xi X-ray photoelectron spectrometer microprobe. The nitrogen adsorption-desorption isotherms were investigated by a Micromeritics ASAP 2010 apparatus, the specific surface area and pore size distribution were calculated according to the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods.

The three-dimensional excitation-emission matrix (3DEEM) fluorescence spectra of wastewater before and after adsorption treatment were detected using a Fluorescence Spectrophotometer (FP-6500, Japan). The 3DEEM spectra were collected with the scanning emission spectra 250-550 nm by varying the excitation wavelengths from 220 nm to 450 nm. The scanning speed was set at 1000 nm/min, while the excitation and emission slits were maintained at 10 nm. The TOC of the samples were analyzed by using a TOC/TNb Analyzer (VarioTOC, Elementar Analysensysteme GmbH, Germany).

3. Results and Discussion The GA preparation process is illustrated in Figure 1a. As we all known, graphene oxide (GO) nanosheets are hydrophilic and can be stable indefinitely in water. A certain amount GO was first ultrasonic dispersed in water forming homogeneous solution and mixed with vitamin C (VC) as a reducing agent. After heated in water bath, the GO sheets gradually self-assembled into graphene hydrogels due to the mild reduction and bridging effects of VC. Since the reduced graphene 7

oxide is hydrophobic, the formed graphene hydrogels can be easily separated from the water once successfully reduced. Then, the hydrogels were dialyzed with alcohol/water solution to remove any residues. After that, the hydrogels were prefrozen to create the desired pore structures using the ice templates formed in situ and then dried by vacuum freeze drying without volume shrinkage and structure cracking. In this preparation strategy, the particle size of carbon, GO concentration, dosage of reducing agent and solution pH are key parameters which would have impact on the microstructure and adsorption capacity of GA. As shown in Figure 1b and c, the optimal carbon size, GO concentration, GO/VC ratio and solution pH were 500 mesh, 2.5 mg/mL, ≤ 2.5 and natural pH, respectively, under which the as-formed GA possessed superior adsorption capacity for pump oil. Decreasing the solution pH, carbon size or GO concentration, or increasing the GO/VC ratio would result in severe decline of adsorption capacity for pump oil. Furthermore, the particle shape also had considerable impact on the microstructure and adsorption capacity of GA, for example, particle (325 meshes) better than that of flakes.

(Figure 1) The self-supporting GA has extremely low density, ~4 mg/cm3 (Figure 2a), which also exhibit excellent flexibility, as shown in Figure 2b, with no breaking or cracking observed over repeated bending. These excellent performance qualities are attributed to the highly porous and reticulate structures inherited from the stratiform graphene nanosheet. Figure 3c exhibits three differently shaped GA made from different reactor in such a way. More interestingly, the molding process might be utilized to manipulate the macroscopic feature of the GA and thus tailor the 8

microstructures and corresponding properties of the GA. In this regard, specific architecture of structure controllable graphene materials can be tailored through the unique reactor. The present approach provides an efficient way to prepare freestanding macroscopic graphene materials, capable of extending their applications to self-supporting scaffolds for adsorbent and catalysts, as well as binder-free electrodes for electronic devices. As can be seen in Figure 2d and e, the net region of GA porous scaffold exhibits a very interesting honeycomb-like architecture with an average pore size of 30~100 μm. This unique reticulate structure is induced by the outside-in ice crystal formation due to the prefreeze treatment and the inside-out solvent evaporation under vacuum freeze drying. The obtained GA was fully analyzed by TEM observations. As shown in Figure 2f and g, large graphene nanosheets (hundreds of square nanometers) were observed to be situated on the top of the copper grid, where they exhibited cascade structure and the number of layers can be visualized directly. Some of the graphene nanosheets were rippled and entangled with each other, resemble crumpled silk veil waves. As reported previously, corrugation and scrolling are intrinsic nature of graphene nanosheets due to that the thermodynamic stability of the 2D surface originates from microscopic crumpling via bending or buckling [23]. The stacked graphene nanosheets are transparent and exhibit a very stable nature under the electron beam. The corresponding selective area electron diffraction (SAED) pattern (inset in Figure 2g) yielded a spot pattern, showing the sixfold rotational symmetry expected for individual graphene sheets, indicative of graphene domains with different orientations [24]. The well-defined diffraction spots confirm the crystalline structure of the graphene nanosheets obtained via chemical reduction of graphene oxide. 9

(Figure 2) The wide-angle powder XRD patterns of the as-prepared GO and GA are displayed in Figure 3a. GO has a larger interlayer distance (0.78 nm, 2θ =11.6°) compared with that of graphite (0.34 nm), this expanded interlayer spacing confirms the presence of oxygenated functional groups on GO sheets [25]. After the reduction, the peak at 2θ=11.6° completely disappears, while a weak and broad peak centered at about 2θ=24° corresponding to the interlayer distance of about 0.37 nm emerges in the GA, indicating the effective reduction of GO and most oxygen-containing functional groups introduced into the interlayer spacing of graphite can be removed during the reduction. This fact can be further confirmed by the following FT-IR (Figure 3b), Raman (Figure 3c) and XPS spectra (Figure 3d) of prepared GO and GA. To study the chemical group of these graphene-based carbon materials, the as-prepared GO and GA was analyzed by FT-IR spectra. The GO synthesized with 80, 325, 325 flakes, 500 and 1200 mesh carbon were named as GO-80, GO-325, GO-325 flakes, GO-500 and GO-1200, respectively. Correspondingly the reduced graphene oxides were labeled as GA-80, GA-325, GA-325 flakes, GA-500 and GA-1200, respectively. As shown in Figure 3b, the FTIR spectra in GO-80, GO-325 and GO-500 present typical fingerprint groups of GO, including carboxylic species, hydroxyl species, and epoxy species. Several characteristic bands at about 1059, 1226, 1400, 1618 and 1719 cm−1 are assigned to the C-O-C stretching vibrations, the C-OH stretching peak, the O-H deformation of the C-OH groups, intrinsic C=C stretching mode and the C=O stretching vibrations of the -COOH group, respectively. The double band at 3158 and 3452 cm−1 are assigned to O-H stretching vibrations of adsorbed water molecules on GO. However, partial oxygen-containing groups in GO-325 flakes and GO-1200 were missing compared with the sample of GO-80, 10

GO-325 and GO-500. Especially after the reduction, the bands featuring oxygen-containing functional groups greatly disappear in the spectrum of the GA-500, which suggests the effective reduction of GO-500. While in other samples, the oxygen-containing groups scarcely changed before and after reduction, or partially reduced. This phenomenon indicates that the carbon species have crucial impact on the final quality of the GO and GA, which can better explain the various adsorption performances of GA synthesized from different carbon. Raman spectroscopy offers an efficacious tool to probe the ordered/disordered crystal structures and electronic properties of graphene and graphene-based materials. As shown in Figure 3c, the characteristic D-band (~1350 cm−1) and G-band (~1590 cm−1) can be observed for the GO and GA, which is generally assigned to local defects/disorders located at the edges of carbonaceous materials and the E2g phonon of sp2 bonds of carbon atoms, respectively. The increasing values of D-band/G-band intensity ratios (ID/IG) in the GA (1.23) compared with that of GO (0.99) suggested the increased defects and a decrease in the average size of sp2 domains after the reduction [26]. This phenomenon may be attributed to the restoration of numerous graphitic domains from amorphous regions of GO, which gives rise to stronger D band signal [27]. The increasing ID/IG ratio supported the reduction of GO happened, which is in accordance with the previous results [28]. The chemical composition of GO and GA were investigated by the XPS. As shown in Figure 3d, both the survey spectrum of GO and GA showed the distinguished peaks centered about 285.0 eV and 532.4 eV corresponding to the C 1s and O 1s, respectively. There is an obvious decline in the O 1s/ C 1s intensity ratios (IO/IC) of the GA compared with that of GO suggested the decrease amount of oxygen-containing functional groups during the reduction of GO, which can be 11

further confirmed by the high-resolution XPS C 1s spectra of these samples. Curve fitting of the XPS peaks can be conducted utilizing Gauss-Lorentzian peak shape after performing a Shirley background correction [29]. In the C1s XPS spectra, the main peak at 284.6 eV was attributed to the C-C and C=C bonds. The long tail observed at the higher binding energy can be fitted peaks at 286.0 eV and 288.1 eV suggest the existence of C-O and C=O functional groups, respectively, which indicates the formation of carbonated species. Compared with that of RGO, the amount of the main oxygen-containing functional groups in GO arising from C=O (288.1 eV) and C-O (286.0 eV) significantly decline, which is in accordance with the XRD, FT-IR and Raman spectra results. Figure 3e demonstrates that the GA can bear same size weight (50 g) without obvious deformation, as well as recover its original shape and size after unloading the stress, which obviously confirmed that the interpenetrating network structure endows GA with good tenacity and mechanical strength (Supplementary Movie 1). The good mechanical property is strongly desired for a qualified versatile agent in practical wastewater treatment whereas traditional agent is prone to collapse into pieces under small applied stress or hardly recycled. It is worth pointing out that according to the practical application for different wastewater treatment processes, the size of GA can be easily tailored by cutting or change reactor model.

(Figure 3) The porous characteristics of the prepared GA were characterized by nitrogen adsorption-desorption

test.

As

shown

in

Figure

4a,

typical

nitrogen

adsorption-desorption isotherms of the prepared GA belong to type IV with a clear hysteresis loop. The GA exhibits a Brunauer-Emmett-Teller (BET) specific surface 12

area of 148 m2/g. Rapid nitrogen uptake was observed at very low relative pressures in the isotherms, indicating the presence of mesoporosity and slit-shaped pores [30]. Moreover, the pore size distribution calculated by the BJH method was illustrated in Figure 4b, and the corresponding enlarged drawing of the pore size distribution ranging from 0-3 nm were inserting in Figure 4b. It can be seen that the pore size distribution of GA presents a relatively narrow distribution from micro to macro scale. This results benefit from the freeze-drying which utilizes the sublimation of a pre-frozen water to avoid pore size shrinkage and internal structure collapse in the GA. This kind porous structure can provide abundance of adsorption sites for pollution, which made the GA become an ideal adsorption material. The excellent adsorption performance of the GA (Supplementary Movie 2) was investigated by the adsorption of a wide range of organic liquids, including common pollutants such as alcohols, alkane, pump oil and cooking oil, as illustrated in Figure 4c and d. The adsorption capacity of the GA can reach up to 138-328 times its own weight. As shown in Figure 4c, the adsorption capacity of the GA for different alcohols was similar, while the alkane adsorption capacity declined as the increasing length of carbon chain (Figure 4d). This phenomenon can be ascribed to the fact that the practical adsorption capacity depends on the density, viscosity, surface tension of the pollutants, and the small pore size of GA, as well as the functional groups on the surface of the adsorbents. These adsorption capacities are nearly 1.5 times higher than those of GA obtained by reacting GO with ethylenediamine [31] and thermally treated GA prepared by water bath at 100 °C of GO suspension containing NaHCO3 and NaCl followed by freeze-drying and annealing at 800 °C for 1 h [32], 3 times higher than those of graphene foam synthesized by freeze-drying of GO followed by thermal reduction [33] and GA assembled with GO sheets by the hydrothermal method with 13

the assistance of thiourea [34], comparable to those of 3D graphene foams via CVD technique with the heating temperature of 1020 °C for 2 h under a constant mixed gas flow of argon (300 sccm), methane (10 sccm) and hydrogen (30 sccm) [19], GO–GNR aerogel prepared by freeze-drying the mixture of GO and graphene nanoribbons (GNRs) followed by chemically reducing using hydrazine vapor at 90 °C for 24 h [21] and N-doped GA prepared by extreme hydrothermal treatment of GO suspension incorporate with pyrrole followed by freeze-drying and annealing treatment at 1050 °C for 3 h [35]. This may have been caused by changes in the fabrication conditions, which would induce differences in the structural features and change their properties, such as the physical or chemical links between the graphene sheets, the arrangement and orientation of the graphene sheets, the porosity and the number of layers of graphene sheets. The remarkable adsorption capacity benefits predominantly from the low density and porous structure of the GA. In the adsorption experiments, oils and organic solvents attract to the hydrophobic GA based on their same nature, and the adsorption is a spontaneous and physical adsorption process [36]. It should be noted that the GA can be easily recycled by squeezing to release the oil from the pores of GA due to the excellent tenacity and mechanical strength (Supplementary Movie 1) of GA. The batch adsorption capacities of GA decreased scarcely and showed 8% reduction of adsorption capacities after 10 cycles, indicating the preferable recyclability of the GA, which has a potential for practical wastewater treatment.

(Figure 4) The satisfactory adsorption capacity demonstrates the potential application of the GA in the practical wastewater treatment. Consequently, The GA was used to remove 14

the dissolved organic matter (DOM) from actual industrial effluent from Xinxiang Patron Special Fabrics Co., Ltd. The flowchart of the wastewater treatment in one small-scale packing moving bed adsorbing column is shown in Figure 5a. The removal efficiencies of DOM using a macroscopic whole GA as adsorbent were evaluated by the three dimensional excitation-emission matrix (3DEEM) fluorescence spectra, and the spectra are depicted as contour maps in Figure 5 b and c. As shown in Figure 5b, the spectrum of wastewater before treatment has two peaks (A and B). As we all known, EXmax (Exitation) and EMmax (Emission) coordinates for each peak represent different substances [37]. The position of peaks A is indicative of visible fulvic acid, such as Tryptophan and protein-like, while the position of peak B is indicative of humic-like acid [38]. It is worth noting that the spectrum of wastewater after treatment without any peak, indicating that there is no notable compound is residual after the adsorption treatment. To further confirm the purifying effect, the TOC was conducted to evaluate the organic content, and the results indicated that the TOC could be completely removed after the adsorption treatment. The preferable purification performance can be attributing to the combination effect of adsorption and filtration. The removal efficiency of the small-scale packing moving bed adsorbing column could be recovered by squeezing to release the superfluous water and backwash with solvent. Hexavalent chromium [Cr(VI)] is one of the most frequently found heavy metal ion in wastewater and can cause serious dangerous to the environment and health. In this regard, it is imperative to devise effective remediation strategies to remove Cr(VI) from the environmental point of view. So far, several techniques such as solvent extraction, adsorption using adsorbents have been exploited to remove Cr(VI) from water [39-41]. Previous study has demonstrated that the UV-light irradiation is 15

beneficial for the removal efficiency using GO [42]. On the basis of this result and considering that free secondary pollution, in the present work, we choose GA to remove 10 mg/L Cr(VI) from water under natural sunlight irradiation. The results shown in Figure 5d indicate that the removal efficiency of Cr(VI) increase slightly with the dark adsorption test prolonged to 5 h, while there is no significant changes of the concentration of Cr(VI) occurred after 5 h sunlight irradiation in the absence of GA. It is worthy to note that with the presence of GA, the Cr(VI) could be completely removed after 5 h sunlight irradiation. This improvement maybe attributes to that the synergistic effect between GA and natural sunlight irradiation, and further investigation for the ultimate mechanism would be conducted in our future study. The recycling performance of the removal process is very important for potential applications in wastewater purification. The durability of as-synthesized GA was evaluated by reused the same sample for five consecutive cycles. GA showed high recycling performance and maintained removal efficiency of Cr(VI) about 86% after 5 cycles of testing. The slight decline of removal efficiency could be attributed to the adsorption of Cr on the surface of GA, and thus reduce the number of active sites. To elucidate the mechanism of the removal process we have carried out XPS and Raman analyses for the Cr-loaded GA. In the XPS analysis shown in Figure 5e, we can see that Cr 2p peaks centered at 577.2 and 586.8 eV corresponding to Cr 2p3/2 and Cr 2p1/2 could be curve-fitted with four components at binding energies of 577.2 eV, 579.9 eV, 586.8 eV and 589.2 eV. The peaks at binding energies of 577.2 and 586.8 eV indicate the existence of the Cr(III) oxidation state while the peaks at 579.9 and 589.2 eV represent the Cr(VI) oxidation state. Therefore, these results suggest the co-existence of both Cr(III) and Cr(VI) in the Cr-loaded GA. In the reduction of Cr(VI) to Cr(III), charge transfer is indispensable. This is analyzed using Raman 16

spectroscopy as shown in Figure 5f. The variations in the G band position for GA and Cr-loaded GA are observed, the G band is blue shifted from 1580 cm-1 to 1590 cm-1 after the Cr loaded, indicating that the charge transfer is from GA to Cr(VI) species. Similar observations on graphene were also reported in other studies [42,43]. Furthermore, due to its good mechanical strength and flexibility, GA has potential application in the flexible conductive material. As shown in Figure 5g and h, the current forms the return circuit when the GA connect to the circuit and the LED lamp can be illumined under 3V circuit, indicating the GA has preferable electrical conductivity and could be used as elastic conductors in various fields.

(Figure 5)

4. Conclusions In summary, we have successfully developed a simple green method to fabricate ultralight, compressible and multifunctional GA by vacuum freeze-drying of their hydrogel precursors obtained from heating the aqueous mixtures of GO and VC without stirring. The preparation parameters such as particle size of carbon, GO concentration, dosage of reducing agent and solution pH have different effects on the microstructure and adsorption capacity of GA. The unique porous structure makes the GA possess excellent elastic can recover after compression, as well as high adsorption capacity to organic solvents and oils (138-328 times its own weight), DOM in textile industrial wastewater, removal of Cr(VI), excellent electrical conductivity. These investigations have indicated that the synthesized GA reported here may have desirable potential application in many fields, including organic solvents absorbents, packing moving adsorbing bed, environmental remediation materials, and elastic and flexible conductors. 17

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants No. U1604137 and 21677047). The authors appreciate for the support from the Application research project of key research projects in henan higher education (Grant Nos. 17A610010 and 16A610012), Key Science and Technology Program of Henan Province (172102210449) and the Innovation Scientists and Technicians Troop Construction Projects of Henan Province, PR China. The authors also would like to thank the Youth Science Foundation (Grant No. 2015QK29) and Research Start-up Foundation (Grant No. 5101219170107) of Henan Normal University for the PhD, PR China. References [1] P. Ehrenfreund, B.H. Foing, Fullerenes and cosmic carbon, Science 329 (2010) 1159-1160. [2] C. Rutherglen, P. Burke, Carbon nanotube radio, Nano Lett. 7 (2007) 3296-3299. [3] X. Li, J. Yu, S. Wageh, A.A. Al-Ghamdi, J. Xie, Graphene in photocatalysis: a review, Small 12 (2016) 6640-6696. [4] M.J. Allen, V.C. Tung, R.B. Kaner, Honeycomb carbon: a review of graphene, Chem. Rev. 110 (2010) 132-145. [5] P. Hu, B. Tan, M. Long, Advanced nanoarchitectures of carbon aerogels for multifunctional environmental applications, Nanotechnol. Rev. 5 (2016) 23-39. [6] J. Zhu, D. Yang, Z. Yin, Q. Yan, H. Zhang, Graphene and graphene-based materials for energy storage applications, Small 10 (2014) 3480-3498. [7] G. Gorgolis, C. Galiotis, Graphene aerogels: a review, 2D Mater. 4 (2017). [8] S. Yu, X. Wang, W. Yao, J. Wang, Y. Ji, Y. Ai, A. Alsaedi, T. Hayat, X. Wang, 18

Macroscopic, spectroscopic, and theoretical investigation for the interaction of phenol and naphthol on reduced graphene oxide, Environ. Sci. Technol. 51 (2017) 3278-3286. [9] J. Yu, M. Jaroniec, Preface: Graphene and C3N4-based photocatalysts, Appl. Surf. Sci. 358 (2015) 1-1. [10] X. Li, T. Hua, B. Xu, Electromechanical properties of a yarn strain sensor with graphene-sheath/polyurethane-core, Carbon 118 (2017) 686-698. [11] M.Q. Yang, N. Zhang, Y. Wang, Y.-J. Xu, Metal-free, robust, and regenerable 3D graphene-organics aerogel with high and stable photosensitization efficiency, J. Catal. 346 (2017) 21-29. [12] C. Wu, X. Huang, X. Wu, R. Qian, P. Jiang, Mechanically flexible and multifunctional polymer-based graphene foams for elastic conductors and oil-water separators. Adv Mater 25 (2013) 5658-5662. [13] J. Zhang, B. Li, L. Li, A. Wang, Ultralight, compressible and multifunctional carbon aerogels based on natural tubular cellulose, J. Mater. Chem. A 4 (2016) 2069-2074. [14] M. Yang, N. Zhao, Y. Cui, W. Gao, Q. Zhao, C. Gao, H. Bai, T. Xie, Biomimetic architectured graphene aerogel with exceptional strength and resilience. Acs Nano; 11 (2017) 6817-6824. [15] Z. Xu, Y. Zhang, P. Li, C. Gao, Strong, Conductive, lightweight, neat graphene aerogel fibers with aligned pores, Acs Nano 6 (2012) 7103-7113. [16] M.B. Lim, M. Hu, S. Manandhar, A. Sakshaug, A. Strong, L. Riley, P.J. Pauzauskie, Ultrafast sol-gel synthesis of graphene aerogel materials, Carbon 95 (2015) 616-624. [17] H. Shan, D. Xiong, X. Li, Y. Sun, B. Yan, D. Li, S. Lawes, Y. Cui, X. Sun, 19

Tailored lithium storage performance of graphene aerogel anodes with controlled surface defects for lithium-ion batteries, Appl. Surf. Sci. 364 (2016) 651-659. [18] M. Chen, Z. Li, J. Li, J. Li, Q. Li, L. Zhang, The extraction of uranium using graphene aerogel loading organic solution, Talanta 166 (2017) 284-291. [19] L. Shi, K. Chen, R. Du, A. Bachmatiuk, M.H. Ruemmeli, K. Xie, Y. Huang, Y. Zhang, Z. Liu, Scalable seashell-based chemical vapor deposition growth of three-dimensional graphene foams for oil-water separation, J. Am. Chem. Soc. 138 (2016) 6360-6363. [20] M. Fan, Z.Q. Feng, C. Zhu, X. Chen, C. Chen, J. Yang, D. Sun, Recent progress in 2D or 3D N-doped graphene synthesis and the characterizations, properties, and modulations of N species, J. Mater. Sci. 51 (2016) 10323-10349. [21] C. Wang, X. He, Y. Shang, Q. Peng, Y. Qin, E. Shi, Y. Yang, S. Wu, W. Xu, S. Du, A. Cao, Y. Li, Multifunctional graphene sheet-nanoribbon hybrid aerogels, J. Mater. Chem. A 2 (2014) 14994-15000. [22] S. Dong, J. Sun, Y. Li, C. Yu, Y. Li, J. Sun, ZnSnO 3 hollow nanospheres/reduced graphene oxide nanocomposites as high-performance photocatalysts for degradation of metronidazole, Appl. Catal. B Environ. 144 (2014) 386-393. [23] G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu, J. Yao, Facile synthesis and characterization of graphene nanosheets, J. Phys. Chem. C 112 (2008) 8192-8195. [24] H. Varela-Rizo, I. Rodriguez-Pastor, C. Merino, I. Martin-Gullon, Highly crystalline graphene oxide nano-platelets produced from helical-ribbon carbon nanofibers, Carbon 48 (2010) 3640-3643. [25] Z.Y. Sui, Y.N. Meng, P.W. Xiao, Z.Q. Zhao, Z.X. Wei, B.H. Han, Nitrogen-doped graphene aerogels as efficient supercapacitor electrodes and gas 20

adsorbents, Acs Appl. Mater. Inter. 7 (2015) 1431-1438. [26] X. Bai, L. Wang, R. Zong, Y. Lv, Y. Sun, Y. Zhu, Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization, Langmuir 29 (2013) 3097-3105. [27] Y. Liu, Y. Hu, M. Zhou, H. Qian, X. Hu, Microwave-assisted non-aqueous route to deposit well-dispersed ZnO nanocrystals on reduced graphene oxide sheets with improved photoactivity for the decolorization of dyes under visible light, Appl. Catal. B Environ. 125 (2012) 425-431. [28] X. Zhou, T. Shi, H. Zhou, Hydrothermal preparation of ZnO-reduced graphene oxide hybrid with high performance in photocatalytic degradation, Appl. Surf. Sci. 258 (2012) 6204-6211. [29] D. Fu, G. Han, Y. Chang, J. Dong, The synthesis and properties of ZnO-graphene nano hybrid for photodegradation of organic pollutant in water, Mater. Chem. Phys. 132 (2012) 673-681. [30] Y. Zhuang, J.G. Seong, Y.S. Do, H.J. Jo, Z. Cui, J. Lee, Y.M. Lee, M.D. Guiver, Intrinsically microporous soluble polyimides incorporating troger's base for membrane gas separation, Macromolecules 47 (2014) 3254-3262. [31] J. Li, J. Li, H. Meng, S. Xie, B. Zhang, L. Li, H. Ma, J. Zhang, M. Yu, Ultra-light, compressible and fire-resistant graphene aerogel as a highly efficient and recyclable absorbent for organic liquids, J. Mater. Chem. A 2 (2014) 2934-2941. [32] S. Yang, L. Zhang, Q. Yang, Z. Zhang, B. Chen, P. Lv, W. Zhu, G. Wang, Graphene aerogel prepared by thermal evaporation of graphene oxide suspension containing sodium bicarbonate, J. Mater. Chem. A 3 (2015) 7950-7958. [33] Y. He, Y. Liu, T. Wu, J. Ma, X. Wang, Q. Gong, W. Kong, F. Xing, Y. Liu, J. Gao, An environmentally friendly method for the fabrication of reduced graphene 21

oxide foam with a super oil absorption capacity, J. Hazard. Mater. 260 (2013) 796-805. [34] J. Zhao, W. Ren, H.M. Cheng, Graphene sponge for efficient and repeatable adsorption and desorption of water contaminations, J. Mater. Chem. 22 (2012) 20197-20202. [35] Y. Zhao, C. Hu, Y. Hu, H. Cheng, G. Shi, L. Qu, A versatile, ultralight, nitrogen-doped graphene framework, Angew. Chem. Int. Ed. 51 (2012) 11371-11375. [36] J. Huang, Z. Yan, Adsorption mechanism of oil by resilient graphene aerogels from oil-water emulsion, Langmuir 34 (2018) 1890-1898. [37] C. Wang, Y.K. Huang, Q. Zhao, M. Ji, Treatment of secondary effluent using a three-dimensional electrode system: COD removal, biotoxicity assessment, and disinfection effects, Chem. Eng. J. 243 (2014) 1-6. [38] C. Jacquin, G. Lesage, J. Traber, W. Pronk, M. Heran, Three-dimensional excitation and emission matrix fluorescence (3DEEM) for quick and pseudo-quantitative determination of protein- and humic-like substances in full-scale membrane bioreactor (MBR), Water Res. 118 (2017) 82-92. [39] J. Wang, P. Wang, H. Wang, J. Dong, W. Chen, X. Wang, S. Wang, T. Hayat, A. Alsaedi, X. Wang, Preparation of molybdenum disulfide coated Mg/Al layered double hydroxide composites for efficient removal of chromium(VI), Acs Sustain. Chem. Eng. 5 (2017) 7165-7174. [40] J. Wang, Y. Liang, Q. Jin, J. Hou, B. Liu, X. Li, W. Chen, T. Hayat, A. Alsaedi, X. Wang, Simultaneous removal of graphene oxide and chromium(VI) on the rare earth doped titanium dioxide coated carbon sphere composites, Acs Sustain. Chem. Eng. 5 (2017) 5550-5561. 22

[41] T. Wen, J. Wang, S. Yu, Z. Chen, T. Hayat, X. Wang, Magnetic porous carbonaceous material produced from tea waste for efficient removal of As(V), Cr(VI), humic acid, and dyes, Acs Sustain. Chem. Eng. 5 (2017) 4371-4380. [42] D. Dinda, A. Gupta, S.K. Saha, Removal of toxic Cr(VI) by UV-active functionalized graphene oxide for water purification, J. Mater. Chem. A 1 (2013) 11221-11228. [43] H.L. Ma, Y. Zhang, Q.H. Hu, D. Yan, Z.Z. Yu, M. Zhai, Chemical reduction and removal of Cr(VI) from acidic aqueous solution by ethylenediamine-reduced graphene oxide, J. Mater. Chem. 22 (2012) 5914-5916.

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Figures List: Figure 1. (a) Legend of the synthesis process of GA. (b) GA with different experimental parameters: variety of carbon (Carbon), GO concentration (GO), weight ratio of GO/VC (GO/VC), and solution pH (pH). (c) Adsorption capacity of different GA synthesized under various preparing condition for pump oil. Labels 1-5 represent samples with pH from 1 to 12, GO/VC from 1 to 4, GO from 1 to 4 mg/mL, and carbon from 80 to 1200 mesh, respectively. Figure 2. Mechanical bendability and its origin. Photographs of (a) ultralight GA resting on a flower and (b) flexible GA. (c) GA with different shapes and sizes prepared under optimal parameters. (d and e) SEM image showing reticulate structure of GA. (f) Representative TEM micrographs and (g) high-resolution TEM images of the prepared GA; inset is the corresponding selected area electron diffraction pattern (SAED). Figure 3. (a) XRD patterns, (b) FT-IR spectra, (c) Raman spectra and (d) XPS spectra of prepared GO and GA. (e) Photographs of GA under pressure. Figure

4.

(a)

Typical

nitrogen

adsorption-desorption

isotherms

and

(b)

Barret-Joyner-Halenda (BJH) desorption pore size distribution profiles of prepared GA (Insets are the corresponding enlarged drawing of the pore size ranged from 0-3 nm). Adsorption capacity of different alcohols (c) and substances (d) using GA. Figure 5. (a) Schematic flowchart for the wastewater treatment process in an individual packing moving bed adsorbing column. Three dimensional excitation-emission matrix (3DEEM) fluorescence spectra of wastewater before (b) and after (c) adsorption treatment. (d) Removal efficiency of Cr(VI) at different conditions after 5 h reaction. (e) Cr 2p spectrum of the Cr-loaded GA. (f) Raman spectra of GA and Cr-loaded GA. The digital photographs of the GA disconnect (g) and connect (h) the circuit. 24

Figure 1. (a) Legend of the synthesis process of GA. (b) GA with different experimental parameters: variety of carbon (Carbon), GO concentration (GO), weight ratio of GO/VC (GO/VC), and solution pH (pH). (c) Adsorption capacity of different GA synthesized under various preparing condition for pump oil. Labels 1-5 represent samples with pH from 1 to 12, GO/VC from 1 to 4, GO from 1 to 4 mg/mL, and carbon from 80 to 1200 mesh, respectively.

25

Figure 2. Mechanical bendability and its origin. Photographs of (a) ultralight GA resting on a flower and (b) flexible GA. (c) GA with different shapes and sizes prepared under optimal parameters. (d and e) SEM image showing reticulate structure of GA. (f) Representative TEM micrographs and (g) high-resolution TEM images of the prepared GA; inset is the corresponding selected area electron diffraction pattern (SAED).

26

Figure 3. (a) XRD patterns, (b) FT-IR spectra, (c) Raman spectra and (d) XPS spectra of prepared GO and GA. (e) Photographs of GA under pressure.

27

Figure

4.

(a)

Typical

nitrogen

adsorption-desorption

isotherms

and

(b)

Barret-Joyner-Halenda (BJH) desorption pore size distribution profiles of prepared GA (Insets are the corresponding enlarged drawing of the pore size ranged from 0-3 nm). Adsorption capacity of different alcohols (c) and substances (d) using GA.

28

Figure 5. (a) Schematic flowchart for the wastewater treatment process in an individual packing moving bed adsorbing column. Three dimensional excitation-emission matrix (3DEEM) fluorescence spectra of wastewater before (b) and after (c) adsorption treatment. (d) Removal efficiency of Cr(VI) at different conditions after 5 h reaction. (e) Cr 2p spectrum of the Cr-loaded GA. (f) Raman spectra of GA and Cr-loaded GA. The digital photographs of the GA disconnect (g) and connect (h) the circuit.

29

Graphical Abstract

30



Green synthesis of mechanically flexible graphene aerogel (GA) was presented.



Diverse process conditions affect the microstructure and adsorption capacity of GA.



The GA possess excellent elastic can recover after compression without cracking.



The GA could adsorb organic solvents and oils 138-328 times its own weight.



The GA show promising performance in water purification and flexible conductors.

31