DOI:10.30919/esmm5f611

Received: 09 Dec 2019
Revised: 18 Dec 2019
Accepted: 18 Dec 2019
Published online: 18 Dec 2019

Amino Carbon Nanotube Modified Reduced Graphene Oxide Aerogel for Oil/Water Separation

Jingyi Cai,1# Jing Tian,1# Hongbo Gu,1* and Zhanhu Guo2

1Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China 

2Integrated Composites Lab (ICL), Department of Chemical & Biomolecular Engineering University of Tennessee, Knoxville, TN, 37966, USA

Corresponding author: hongbogu2014@tongji.edu.cn      

#There authors are contributed equally

Abstract

In this work, reduced graphene oxide/amino multi-walled carbon nanotube (rGO/MWCNTs-NH2) aerogel has been successfully fabricated through the chemical reaction between carboxyl groups on graphene oxide (GO) and amino groups on multi-walled carbon nanotubes (MWCNTs-NH2) in combination with freeze-drying and high-temperature annealing process. The optimal condition for fabrication of rGO/MWCNTs-NH2 aerogel is mass ratio of MWCNTs-NH2 to GO = 1:8 with GO concentration of 2 mg mL-1. In addition, owing to the strong chemical bonding between amino groups on MWCNTs and carboxyl groups on GO, the improved hydrophobic property and microstructures of rGO/MWCNTs-NH2 aerogel are obtained confirmed by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and contact angle measurement. After that, the adsorption tests and oil/water separation experiments prove that the rGO/MWCNTs-NH2 aerogel possesses an excellent adsorption performance for various organic solvents and oil including ethyl acetate (152.79 g g-1), cyclohexane (122.46 g g-1), acetone (219.54 g g-1), dichloromethane (242.31 g g-1), and sesame oil (156.35 g g-1), and achieves a high efficiency of oil/water separation within only 25 s. Our rGO/MWCNTs-NH2 aerogel is expected to be a promising material for treatment of oil spills.

Keywords: Graphene oxide; Amino multi-walled carbon nanotubes; Aerogel; Oil adsorption; Oil/water separation

1. Introduction

In recent years, oil spills and organic pollution caused by industrial accidents has brought serious environmental problems1 and endangered people's health due to its carcinogenic content.2 Adsorption,3-5 centrifugation,6 membrane separation,7 chemical demulsification,8 chemical oxidation,9 coagulation10 and electrochemical methods11 are normally exploited to remove oil from polluted water. Among them, adsorption has become a research hotspot with the characteristics of high oil removal efficiency, no secondary pollution, simple processing, low energy consumption and low operation cost.12 The adsorbents widely used in oily wastewater treatment include activated carbon,13 polyethylene,14 fly ash,15 bentonite,16 carbon fiber17 and super oil-adsorbing resin.18 However, they have some disadvantages, such as low oil/water separation efficiency and selectivity, difficult oil recovery and durable materials,19 which has severely limited their practical application. Therefore, it is important to find an efficient and fast material for treating oily wastewater.

Graphene is a material, comprising a layer of carbon atoms with unique electronic characteristics.20 Although it has excellent physical properties, its poor solubility greatly hinders its processing and application.21 Therefore, graphene oxide (GO) is often prepared for further usage because the introduction of polar oxygen-containing groups and defects could make it to exhibit a better processibility.22 Recently, aerogel has gradually attracted attentions due to its unique properties, such as high porosity, high specific surface area, low thermal conductivity and ultra-low density.12,23 Among them, reduced GO aerogel has a large specific surface area and extremely hydrophobic characteristics to ensure the efficiency for the separation of oil and organic solvents from water, so it has been widely studied in the removal of oil and organic solvents from sewage.24 However, due to the relatively poor mechanical properties of single rGO aerogel, the researchers commonly mix the GO with other materials to enhance its structure and properties. For example, Mi et al.25 prepared cellulose/graphene aerogel with outstanding compression and recoverability properties could adsorb 80-197 times its weights of oil and organic solvents. Wu et al.26 reported a graphene-based aerogel with Cu nanoparticles and found that this aerogel exhibited an excellent catalytic performance and good capability to remove the various oils and dyes from wastewater. Unfortunately, there are few reports on the reduced GO aerogels chemical bonded with multi-walled carbon nanotubes (MWCNTs) for oil/water separation.

In this work, we have chosen the amino group functionalized MWCNTs (MWCNTs-NH2) to fabricate the reduced GO/MWCNTs-NH2 aerogel in order to form the strong chemical bonding between amino groups of MWCNTs and carboxyl groups on GO to boost the mechanical property and hydrophobic property of GO aerogel, which further improves its oil adsorption properties. The scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) have been conducted to study the microstructures and chemical structures of aerogels. Furthermore, the oil adsorption capacity and oil/water separation capacity have been performed systematically.


2. Experimental

2.1 Materials

Graphite powder (~325 mesh, 99%), potassium persulfate (K2S2O4, 99%), amino multi-walled carbon nanotubes (MWCNTs-NH2), dichloromethane (CH2Cl2, 99.5%) and Sudan Red B (C24H20N4O, AR) were purchased from Alfa Aesar Chemical Co., Ltd., Beijing Inoke Technology Co., Ltd. from Beijing Deke Daojin Science and Technology Co. Ltd., Aladdin Reagent  Co., Ltd., and Maclean Biochemical Technology Co., Ltd., respectively. Phosphorus pentoxide (P2O5, 99%), sulfuric acid (H2SO4, 95~98%), potassium permanganate (KMnO4, 99.5%), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, 36~38%), L(+)-ascorbic acid (C6H8O6, 99.7%), ethanol (C2H5O, 95%), ethyl acetate (C6H12, 99.5%), cyclohexane (C4H8O2, 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as-received without any further treatment.

2.2 Synthesis of rGO aerogel

The GO was firstly fabricated via a modified Hummers method. Please see details in supplemetary materials. After that, GO solution (2 mg mL-1) and L(+)-ascorbic acid (LAA, mass ratio of GO : LAA = 1 : 2) were mixed for sonication of 10 min. Then the mixed solution was poured into a 25 mL of beaker with a sealing cap and put in a regular oven at temperature of 40 oC for 12 h to obtain the hydrogel. The obtained hydrogel was dialyzed in a 15% (v/v) ethanol solution for 6 h and then taken out for freeze of 12 h in a refrigerator. Thereafter, the frozen sample was placed in a freeze-dryer for 48 h to attain the aerogel. Finally, the rGO aerogel was acquired with annealing at 780 oC for 0.5 h in an infrared tube furnace (Beijing Huace Instruments Inc.).

2.3 Manufacture of rGO/MWCNTs-NH2 aerogel

MWCNTs-NH2 (mass ratio of MWCNTs-NH2 to GO = 1:8) was ultrasonically dispersed in deionized water at room temperature and mechanically stirred with a speed of 150 r min-1 for 1 h, and GO solution (2 mg mL-1) was added for additional ultrasonic and mechanical stiring of 1 h. After that, the temperature was raised to 60 oC and kept for 2 h in order to have a chemical recation between GO and MWCNTs-NH2. After cooling down to room temperature, LAA (mass ratio of GO : LAA = 1 : 2) was added into above solution for sonication of 10 min. Then, the mixed solution was poured into a 25 mL beaker with a sealing cap and maintained at 40 ° C for 12 h to obtain the hydrogel. The hydrogel was dialyzed in a 15% (v/v) ethanol solution for 6 h and then frozen in a refrigerator for 12 h. Afterward, the frozen sample was put in a freeze-dryer for 48 h to yield the aerogel. Finally, the aerogel was annealed at 780 oC for 0.5 h to acquire rGO/MWCNTs-NH2 aerogel. With the purpose of obtaining the optimal preparation conditions of rGO/MWCNTs-NH2 aerogel, the different GO concentrations (3, 4 and 5 mg mL-1) and different mass ratios of MWCNTs-NH2 to GO (1:6, 1:10, and 1:12) were applied to fabricate the rGO/MWCNTs-NH2 aerogel with the same procedure as mentioned above.

2.4 Adsorption test

2.4.1 Oil adsorption capacity test

Firstly, the weighing bottle filled with ethyl acetate was weighed on an electronic balance. Then, the aerogel was put into this weighing bottle filled with ethyl acetate for adsorption of 3 min and removed quickly with tweezers. The weighing bottle was weighed again on the electronic balance. The weight difference was the adsorbed ethyl acetate. The adsorption capacity of aerogel to cyclohexane, acetone, dichloromethane and sesame oil was done with the same procedure of ethyl acetate. The reported value was the average of three measurements with a deviation of 5%.

2.4.2 Oil/water separation test

The cyclohexane and Sudan Red B were poured into a small beaker and magnetically stirred for complete dissolution. Afterward, this solution was added into a beaker filled with deionized water. Then, the rGO/MWCNTs-NH2 aerogel was used to do oil/water separation test.

2.5 Characterizations

The surface morphology of rGO/MWCNTs-NH2 aerogel was observed on a field emission scanning electron microscope (FE-SEM, Hitachi S-4800 system) and a high resolution TEM (Tecnai G2 F20 S-TWIN). The chemical composition and elemental valence of the products were acquired by XPS (Kratos AXIS Ultra DLD spectrometer), FT-IR (Thermo Nicolet NEXUS spectrometer) and Raman spectra (Renishaw in Via Reflex Raman spectrometer). Adsorption-desorption isotherm curves of rGO aerogel and rGO/MWCNTs-NH2 aerogel were measured by Tristar3020 specific surface area and porosity analyzer with nitrogen as the adsorbent at liquid nitrogen temperature (-196 oC), and then combined with the BET (Brunauer-Emmett-Teller) equation and the BJH (Barrett-Joyner-Halenda) model to analyze the specific surface area and pore size distribution of samples. Water contact angles of rGO aerogel and rGO/MWCNTs-NH2 aerogel were measured on a contact angle measuring instrument (JC2000DS1, Shanghai Zhongchen Digital Technology Equipment Co., Ltd.).

3. Results and discussion

3.1 Optimal condition exploration for preparation of rGO/MWCNTs-NH2 aerogel

The rGO/MWCNTs-NH2 aerogels with different mass ratios of MWCNTs-NH2 to GO (including 1:6, 1:8, 1:10 and 1:12) and different GO concentrations (2, 3, 4 and 5 mg L-1) have been synthesized. The porous structure is observed in the rGO/MWCNTs-NH2 aerogel fabricated with different mass ratios of MWCNTs-NH2 to GO as depicted in the SEM images of Fig. S1. These samples were used to do the ethyl acetate adsorption test and the results are shown in Fig. S3(a). As the mass ratio of MWCNTs-NH2 to GO is 1:6, 1:8, 1:10, 1:12, ethyl acetate adsorption capacity of aerogels is 137, 152.8, 116.7, 88.046 g g-1, respectively, revealing that the best mass ratio of MWCNTs-NH2 to GO for producing rGO/MWCNTs-NH2 aerogel is 1:8.

After that, different GO concentrations (including 2, 3, 4 and 5 mg L-1) have been selected to produce the rGO/MWCNTs-NH2 aerogel under the MWCNTs-NH2 to GO mass ratio of 1:8. And the obtained results are listed in Fig. S2 and S3(b). The adsorption capacity for ethyl acetate of aerogels is 152.8, 88.4, 78.7, 61.8 g g-1, separately, for rGO/MWCNTs-NH2 aerogel synthesized with GO concentration of 2, 3, 4 and 5 mg L-1. Fig. S3(b), resulting from the decreased porous structure with the increasing GO concentration (as displayed in Fig. S2). This confirms that the best GO concentration for preparation of rGO/MWCNTs-NH2 aerogel is 2 mg L-1. In a word, the optimal GO concentration for synthesis of rGO/MWCNTs-NH2 aerogel is MWCNTs-NH2 to GO mass ratio of 8:1 with GO concentration of 2 mg L-1.

3.2 Structure and morphology of rGO/MWCNTs-NH2 aerogel

As shown in Fig. 1(a) and (b), it’s found that the gained rGO/MWCNTs-NH2 aerogel has a good cylindrical shape. Its diameter and height is respectively measured to be 1.95 and 2.37 cm. By weighting its weight on an electronic balance (42.9000 mg), its density is calculated to be 6.06 mg cm-3. It’s seen that this aerogel could be placed on a flower, Fig. 1(c) and (d), with no obvious deformation, indicating that the aerogel possesses a very low density.

Fig. 1 (a) Diameter and (b) height measurements of rGO/MWCNTs-NH2 aerogel; (c) and (d) digital photographs of rGO/MWCNTs-NH2 aerogel, showing that this aerogel could be placed on a flower.

Fig. 2 displays the SEM images of (a) GO aerogel, (b) rGO aerogel, (c) MWCNTs-NH2, (d) and (e) rGO/MWCNTs-NH2 aerogel and (f) TEM image of rGO/MWCNTs-NH2 aerogel. In Fig. 2(a), it’s noted that the graphene sheet is completely peeled off, proving that the graphite has been completely oxidized. There are many wrinkles on the rGO sheet after adding the reducing agent LAA and high temperature annealing, Fig. 1(b). In Fig. 1(c), the MWCNTs-MH2 presents an irregular entangled network structure. As depicted in Fig. 1(d) and (e), the MWCNTs-MH2 are noticed to be evenly dispersed within graphene sheet. Especially, in TEM image, Fig. 1(f), the MWCNTs-MH2 was partially embedded inside the graphene layer, demonstrating that the MWCNTs-NH2 is covalently bonded with GO because of chemical bonding between oxygen-containing functional groups on the GO and amino-group of MWCNTs-NH2, leading the uniform distribution of MWCNTs-MH2 within graphene layer. From the elemental mapping image of rGO/MWCNTs-NH2 aerogel (Fig. S4), the existence of N element (red) is attributed to the addition of MWCNTs-MH2. The uniform distribution of N element further demonstrates the possible chemical reaction between MWCNTs-NH2 and GO.

Fig. 2 SEM images of (a) GO aerogel; (b) rGO aerogel; (c) MWCNTs-NH2 (d) and (e) rGO/MWCNTs-NH2 aerogel, and (f) TEM image of rGO/MWCNTs-NH2 aerogel.

In the Raman spectra, Fig. 3(a), GO aerogel mainly has two characteristic peaks, containing G peak caused by the E2g vibration model of sp2 hybrid carbon atoms (near 1591 cm-1) and D peak (1354 cm-1) resulted from the defect structure of GO edge and amorphous structure.27 In comparison, the D peak and G peak of rGO aerogel respectively appear at 1348 and 1577 cm-1. The presence of peaks at 2686 and 2911 cm-1 attributed to the two phonon lattice vibrations of graphene, revealing that structural defects of rGO aerogel are reduced relative to GO aerogel. The shift in the D peak (1344 cm-1) of the rGO/MWCNTs-NH2 aerogel with its G peak at 1589 cm-1 compared with GO aerogel manifest that there is an interaction between GO and MWCNTs-NH2.

Fig. 3 (a) Raman spectra; (b) FT-IR spectra; (c) high resolution XPS spectrum of C1s; and (d) nitrogen adsorption-desorption isotherms and pore size distributions of rGO/MWCNTs-NH2 aerogel.

The FTIR spectrum (Fig. 3(b)) of GO aerogel reveals the existence of a large number of oxygen-containing groups in the chemical structure of GO aerogel, such as the stretching vibration peaks of C-O-C and C=O at 104528,29 and 1732 cm-1,30,31 respectively. These peaks are significantly reduced or disappear in FTIR spectrum of rGO aerogel, indicating that GO aerogel has been successfully reduced. The reduction of these hydrophilic oxygen-containing functional groups might improve the hydrophobic properties of rGO aerogel.

Fig. 3(c) shows the high resolution C 1s XPS spectrum of rGO/MWCNTs-NH2 aerogel, in which the C 1s XPS spectrum could be  deconvoluted into four peaks, i.e. 284.6, 286.4, 287.5 and 288.3 eV, corresponding to four functional groups on rGO/MWCNTs-NH2 aerogel including C=C, C-O, C=O and N-C=O.32 In addition, the appearance of N-C=O around at 288.3 eV might be from the chemical reaction between the amino group of MWCNTs-NH2 and the carboxyl group of GO by covalent bonding, which is consistent with SEM, TEM, and Raman results.

In the nitrogen adsorption-desorption isotherm curves, Fig. 3(d), the BET specific surface areas of rGO aerogel and rGO/MWCNTs-NH2 aerogel are estimated to be 261.05 and 233.44 m² g-1, respectively. The pore size distribution of rGO/MWCNTs-NH2 aerogel between 2.1 and 2.9 nm indicates that its internal structure owns a uniform pore size and belongs to mesoporous materials, whereas the pore size distribution curve of rGO aerogel has no obvious peak, meaning that its internal structure is non-uniform pore size.

3.3 Adsorption performance of rGO/MWCNTs-NH2 aerogel

Before doing adsorption test, the contact angle between aerogels and deionized water is measured and the results are displayed in Fig. 4. The water contact angle for rGO aerogel and rGO/MWCNTs aerogel is 83.54° (Fig. 4(a)) and 115.99° (Fig. 4(b)), respectively. This verifies that the addition of MWCNTs-NH2 could improve the hydrophobicity of rGO aerogel, which might be beneficial for the selective adsorption of grease. The adsorption capacity of rGO/MWCNTs aerogel for different oil and organic solvents, including ethyl acetate, cyclohexane, acetone, dichloromethane, and sesame oil, has been studied and the results are 152.79, 122.46, 219.52, 242.31 and 156.35 g g-1, respectively, as shown in Fig. 5. In comparison with the results in literature as listed in Table 1, it’s seen that the rGO/MWCNTs aerogel exhibits excellent adsorption capacity for various organic solvents, so that it has a prospective application in treatment of organic pollution or oil leakage. Subsequently, an oil/water separation experiment by rGO/MWCNTs aerogel is performed in a beaker with application of Sudan Red B to dye cyclohexane. The obtained results are listed in Fig. 6. It turns out that the rGO/MWCNTs aerogel could adsorb cyclohexane within 15 s and reach an oil-water separation completely within only 25 s. It could conclude that this material with fast oil/water separation capability and superior oil adsorption capacity is expected to be a potential candidate for large-scale oil removal from water.

Fig. 4 Water contact angle between (a) rGO aerogel and (b) rGO/MWCNTs-NH2 aerogel.

Fig. 5 Adsorption capacity of rGO/MWCNTs-NH2 aerogel for ethyl acetate, cyclohexane, acetone, dichloromethane and sesame oil.

Fig. 6 Oil/water separation experimental results of rGO/MWCNTs-NH2 aerogel for cyclohexane. 


4. Conclusions

In this work, we provide a method to synthesize the reduced graphene oxide/amino multi-walled carbon nanotube (rGO/MWCNTs-NH2) aerogel by chemical reaction between graphene oxide and amino multi-walled carbon nanotube in combination with freeze-drying and high-temperature annealing process. The SEM, TEM and XPS affirm the presence of strong chemical bonding between the amino groups of MWCNTs and carboxyl groups on GO, enhancing the hydrophobic property and microstructures of rGO/MWCNTs aerogel. The MWCNTs-NH2 is observed to be uniformly distributed within graphene layer verified by SEM and TEM. The optimal condition of rGO/MWCNTs-NH2 aerogel is the mass ratio of MWCNTs-NH2 to GO of 1:8 with GO concentration of 2 mg mL-1. Subsequently, the oil adsorption capacity of rGO/MWCNTs aerogel to ethyl acetate, cyclohexane, acetone, dichloromethane, and sesame oil is 152.79, 122.46, 219.52, 242.31 and 156.35 g g-1, accordingly. This rGO/MWCNTs aerogel could separate oil and water within 25 s, exhibiting a high oil/water separation efficiency.

 

Supporting Information

Additional Figs. S1-S4.


Acknowledgement

The authors are grateful for the support and funding from the Foundation of National Natural Science Foundation of China (No. 51703165), Shanghai Rising-Star Program (No. 19QA1409400). This work is supported by Shanghai Science and Technology Commission (14DZ2261100). 


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