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. 2020 Apr 24;5(17):9903–9911. doi: 10.1021/acsomega.0c00183

Preparation of Reduced Graphene Oxide Aerogel and Its Adsorption for Pb(II)

Chunjuan Gao 1,*, Zeliang Dong 1, Xiaocui Hao 1, Ying Yao 1, Shuyuan Guo 1
PMCID: PMC7203952  PMID: 32391477

Abstract

graphic file with name ao0c00183_0013.jpg

In this study, reduced graphene oxide (rGO) aerogels were successfully prepared using a facile hydrothermal method and determined with Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller surface area, and scanning electron microscopy. The rGO aerogels were used to remove Pb(II) from aqueous solution, and the adsorption performance of rGO aerogels was investigated. In addition, the adsorption–desorption cycle experiments were carried out to evaluate the recyclability and stability of rGO aerogels. The adsorption data were consistent with the pseudo-second-order kinetic model and Langmuir isotherm model. The experimental results showed that rGO aerogels have good adsorption capacity for Pb(II) and can be utilized for wastewater treatment.

Introduction

With the advancement of industrialization and economic development, water pollution became an increasingly serious problem.1,2 Heavy metal pollution in water attracted much public attention because heavy metal ions were extremely toxic even at low concentrations.3−5 As one of the major heavy metal pollutants, lead seriously harmed human health and especially had irreversible effects on children’s health and intelligence.6 It is worse that lead is gradually accumulated in the human body and is essentially nonbiodegradable.7,8 Hence, lead-containing wastewater must be treated before being discharged into the environment.

Lead-containing wastewater mainly came from the battery industry, petroleum industry, nonferrous metal smelting, and other industries. Because of its widespread use in industries, lead is a common pollutant in wastewater. There are many technologies and methods for heavy metal ions removal from wastewater, such as chemical precipitation, membrane filtration, ion-exchange, evaporation, concentration, and adsorption.9−14 Adsorption has the advantages of excellent separation efficiency, easy to operate, and low cost, and is still an effective and widely used method.5−18

It has been reported that various materials,19−25 including activated carbon, sewage sludge ash, clay minerals, zeolite, manganese oxides, peanut hulls, biomaterials, and polymeric materials had the adsorption ability for heavy metal ions. However, these materials had some common shortcomings such as complex preparation process and poor recyclability. Aerogel,26 a porous three-dimensional solid material with a continuous pore network of nanometer scale, has special properties such as high porosity, low density, and huge surface area. The charming characteristics of aerogel makes it widely applicable in the fields of adsorption, energy storage and conversion, drug carriers, photocatalysis, and so forth.27−30 As compared with the other carbonaceous materials, graphene had many outstanding properties such as huge surface area, excellent optical, electrical, and mechanical properties, and especially good adsorption capacity for heavy metal ions.31−36

In this work, the reduced graphene oxide (rGO) aerogels were synthesized using a simple one-step hydrothermal reduction assembly method. The rGO aerogels had three-dimensional network structures with nanoscale continuous pores and were used as adsorbents to remove Pb(II) in wastewater. The effects of pH, contact time, and Pb(II) concentration on the adsorption capacities of rGO aerogels were explored. The adsorption kinetic and isotherm studies were also carried out to investigate the adsorption behavior of rGO aerogel for Pb(II).

Results and Discussion

Density

GO was reduced to rGO, and a three dimensional network porous structure was formed by self-assembly. After freeze-drying, the water in rGO hydrogels was replaced by air, and the rGO aerogels were obtained. The densities of obtained rGO aerogels were calculated.

There were some factors that had an influence on the properties of rGO aerogels, including GO concentration, hydrothermal temperature, and hydrothermal time. When the concentration of GO was less than 1 mg/mL, the rGO hydrogel shrunk drastically during the freeze-drying process, and its volume decreased sharply. When the concentration of GO was higher than 3 mg/mL, the structure of the rGO aerogel remained almost constant during the freeze-drying process. Increasing hydrothermal temperature and prolonging hydrothermal reaction time might have made the densities of prepared rGO aerogels increased, resulting in the decrease of their adsorption capacity. The rGO aerogel with a minimum density of 12.2 mg/cm3 was prepared successfully under the optimal reaction condition (GO concentration was 3 mg/mL, hydrothermal temperature was 140 °C, and hydrothermal time was 8 h).

Fourier-Transform Infrared Spectra

Figure 1 shows the characteristic peaks of GO and rGO aerogels. The absorption peak of GO at 3390 cm–1 testified the stretching vibrational band of (−OH). The absorption peaks of GO at 1719, 1609, 1280, and 1040 cm–1 showed the stretching vibrational bands of (−C=O), (−C=C), (−C–O), and (−O−), respectively. The absorption peak of (−OH) disappeared in the rGO aerogel spectrum. Meanwhile, the intensities of the vibrational bands of (−C=O) and (−C–O) decreased in the rGO aerogel spectrum, indicating the reduction of the functional groups and the transformation from GO to rGO.

Figure 1.

Figure 1

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IR of GO and rGO aerogels (3 mg/mL, 140 °C, 8 h).

X-ray Diffraction

The phase structures of the samples were measured by X-ray diffraction (XRD), as shown in Figure 2. The sharp peak of GO at 2θ = 10.8° confirmed the presence of the oxygen-containing functional groups, which was identical with the interlayer spacing of 0.8 nm. In comparison, a broad diffraction peak at about 24.72° along with the interlayer spacing of 0.35 nm appeared, corresponding to the characteristic reflection peak of rGO aerogel. In addition, the peak at 2θ = 10.8° disappeared significantly after reduction by the hydrothermal synthesis method, suggesting that the oxygen-containing groups of GO were diminished.

Figure 2.

Figure 2

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XRD of GO and rGO aerogels (3 mg/mL, 140 °C, 8 h).

Raman Spectra

The Raman spectra of the samples were measured as shown in Figure 3. It was observed that two obvious peaks at 1335 and 1591 cm–1, which were clear indications toward the characteristic D and G bands of GO, respectively. As compared with GO, the D band at 1332 cm–1 and the G band at 1590 cm–1 became much stronger in the rGO aerogel, further proving the formation of the rGO aerogel after hydrothermal treatment.

Figure 3.

Figure 3

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Raman spectra of GO and rGO aerogels (3 mg/mL, 140 °C, 8 h).

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) of GO and rGO aerogels are shown in Figure 4. There were the O 1s peak and the C 1s peak, as shown in Figure 4a, proving that graphite has been oxidized to GO. Furthermore, the increase in the C/O ratio of rGO aerogel was attributed to the reduction of the oxygen-containing groups. Figure 4b shows the C 1s XPS spectra of GO in which the peaks at 284.8, 286.82, and 288.17 eV were attributed to (C–C), (C–O), and (C=O), respectively. As shown in Figure 4c, the intensities of peaks at 289.82 and 285.82 eV were significantly decreased because of the efficient reduction of GO.

Figure 4.

Figure 4

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XPS survey spectra overlay of GO and rGO aerogels (a) and separate detailed C 1s spectra of GO (b) and rGO aerogel (c).

Brunauer–Emmett–Teller

The N2 adsorption–desorption isotherm and the distribution of pore diameter of rGO aerogel were obtained through Brunauer–Emmett–Teller (BET), and the curves are shown in Figure 5. The rGO aerogel exhibited a typical IV isotherm accompanied by a hysteresis loop, which could give information on the presence of mesopores (Figure 5a). It is observed from Figure 5b that the pore diameter of rGO aerogel calculated through the Barrett, Joyner, and Halenda (BJH) model mainly was in the range below 100 nm. As compared with those of graphene-based materials, as shown in Table 1, the rGO aerogel showed a relatively high BET surface area (136.7 m2/g), which was favorable for the adsorption.

Figure 5.

Figure 5

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Nitrogen adsorption–desorption isotherm (a) and pore-size distribution curves based on BJH (b) of rGO aerogel.

Table 1. BET Surface Areas of Different Materials.

samples BET surface areas (m2/g) references
GO aerogel 84.7 (37)
GO@TiO2 74.9 (38)
rGO–MnO/NC-2 38.9 (39)
rGO@Fe3O4 MNPs 58.0 (40)
rGO aerogel 136.7 present study
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Scanning Electron Microscopy

The microstructure images of rGO aerogels were observed by scanning electron microscopy (SEM) (Figure 6). The SEM image in Figure 6a showed that the rGO aerogel had multilayered structure, which was formed with the rGO nanosheets through self-assembly. As shown in Figure 6b,c, the rGO nanosheets were interconnected continuously to form a three-dimensional porous network, providing support and maintaining its shape during freeze-drying.

Figure 6.

Figure 6

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SEM images of rGO aerogels with 10 mg/mL GO reduced (a–c) and with 2 mg/mL GO reduced (d–f).

With the decrease of the GO concentration, the overlapped morphology in the rGO aerogel disappeared (Figure 6d). At the same time, more micrometer-sized pores were distributed on the surface of the material, which provided more large surface area for the adsorption (Figure 6e,f). The continuous porous structure was beneficial for the diffusion and adsorption of Pb(II). It was virtually impossible to obtain the rGO aerogel when the GO concentration was less than 1 mg/mL, which revealed that the GO concentration played an important role in the formation of the rGO aerogel.

Effect of pH on the Adsorption Capacities of rGO Aerogels for Pb(II)

The pH value of solution was a critical parameter that greatly influenced the adsorption capacity of the adsorbents. Figure 7 reveals the effect of pH on the adsorption capacities of rGO aerogels for Pb(II). It could be obviously observed that the qe value was increased rapidly from 2.3 to 54.5 mg/g as pH variated from 1 to 5.5 because of the competitive adsorption of Pb(II) and H+ on the rGO aerogel. At a low pH (pH < 3.0), more H+ might prevent the adsorption of Pb(II) from the rGO aerogel, which would lead to the poor adsorption capacity of the rGO aerogel for Pb(II). As pH value increased, the adsorption efficiency of Pb(II) increased distinctly because H+ had lost its dominant position in the competition. The results suggested that the Pb(II) adsorption by rGO aerogel was sensitive to pH, and the most effective adsorption efficiency could be achieved at pH 5.5. Therefore, the pH of the solution was maintained at 5.5 in the further studies.

Figure 7.

Figure 7

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Effect of pH on the adsorption for Pb(II), conditions: Pb(II) = 50 mg/L (25 mL), sorbent = 10 mg.

Effect of Contact Time on Adsorption Capacities of rGO Aerogels for Pb(II)

The experiments with the different adsorption time were performed to investigate the influence of contact time on the adsorption efficiency for Pb(II). As shown in Figure 8, the qt for Pb(II) increased rapidly within the initial 5 min of starting adsorption. As the extension of contact time, the qt for Pb(II) increased gradually to a constant value at about 50 min, and the time was regarded as adsorption equilibrium time.

Figure 8.

Figure 8

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Effect of contact time on the adsorption for Pb(II), conditions: Pb(II) = 50 mg/L (25 mL), sorbent = 10 mg, pH = 5.5.

In order to further study how the contact time affected the adsorption process, the experimental data were analyzed by the pseudo-first-order model and the pseudo-second-order model. The pseudo-first- and second-order models were expressed by eqs 1 and 2

graphic file with name ao0c00183_m001.jpg 1
graphic file with name ao0c00183_m002.jpg 2

where qe was the equilibrium adsorption capacity for Pb(II) (mg/g), qt was the adsorption capacity for Pb(II) at t time, and k1 and k2 were rate constant of pseudo-first-order kinetic adsorption and pseudo-second-order kinetic adsorption, respectively.

The linear fitting curve of the pseudo-second-order kinetic model is shown in Figure 9. The corresponding fitted constants calculated using the pseudo-first- and second-order kinetic models are presented in Table 2. As seen from Table 2, the value of R2 of the pseudo-second-order kinetic model was higher than that of the pseudo-first-order kinetic model, indicating that the pseudo-second-order kinetic model was more suitable for the adsorption of rGO aerogels toward Pb(II).

Figure 9.

Figure 9

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Linear fitting curves of pseudo-second-order kinetics.

Table 2. Constants of Adsorption Kinetic Models.

kinetic models pseudo-first-order model
 pseudo-second-order model
constants R2 qe k1 R2 qe k2
  0.9371 34.62 0.1409 0.9996 37.45 0.0061
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Effect of Pb(II) Concentration on Adsorption Capacities of rGO Aerogels for Pb(II)

The effect of Pb(II) concentration on adsorption was investigated, and the results are shown in Figure 10. It was clearly observed that the value of q gradually increased as the Pb(II) concentration increased. The porous rGO aerogels could adsorb more ions at high Pb(II) concentration as a result of the adsorption–desorption equilibrium.

Figure 10.

Figure 10

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Effect of the Pb(II) concentration on the adsorption for Pb(II), conditions: sorbent = 10 mg, pH = 5.5.

In order to investigate precisely the isotherm adsorption of rGO aerogel for Pb(II), two most typical adsorption models, the Langmuir and Freundlich isotherm models were used to fit the adsorption data. The Langmuir and Freundlich isotherm models were expressed as the following equations

graphic file with name ao0c00183_m003.jpg 3
graphic file with name ao0c00183_m004.jpg 4

where Ce was the equilibrium concentration for Pb(II) (mg/L), qe was the equilibrium adsorption capacity for Pb(II) (mg/g), qm was the maximum adsorption capacity for Pb(II) (mg/g), and kl (L/mg), kf (L/mg), and 1/n were constants.

The fitted curves of two isotherm models are shown in Figure 11, and the isothermal constants which were calculated from the experiment data are mentioned in Table 3. The R2 value of Langmuir (Rl2 = 0.992) was higher than that of Freundlich (Rf2 = 0.986), indicating the adsorption isotherm of rGO aerogel for Pb(II) was more suitable for the Langmuir isotherm model.

Figure 11.

Figure 11

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Adsorption isotherms for Pb(II) onto rGO aerogel.

Table 3. Isothermal Constants for Pb(II) Adsorption.

isotherm models Langmuir
 Freundlich
constants Rl2 kl qm Rf2 kf 1/n
  0.998 0.02 58.04 0.986 2.513 0.614
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According to the Langmuir isotherm, the qm value of rGO aerogel was calculated to be 58.04 mg/g and was compared with other materials (Table 4). It was clear that the rGO aerogel showed a better adsorption capacity for Pb(II) than these materials such as magnetic graphene, rGO, rGO@Fe3O4 MNPs, chitosan-coated sand, amino-BC, Fe2O3–Al2O3, and rGO–Fe3O4. As compared to other adsorbents such as GO, NF–rGO, and Scenedesmus vacuolatus–rGO, the adsorption capacity of rGO aerogel was relatively lower. However, the synthesis of the aforementioned materials was more complicated than that of the rGO aerogel, and the rGO aerogel could be obtained using a facile hydrothermal method.

Table 4. Adsorption Capacities of Different Adsorbents for Pb(II).

adsorbents adsorption capacity (mg/g) references
rGO@Fe3O4 MNPs 49.00 (40)
magnetic graphene 6 (41)
GO 81.3 (42)
GT–rGO 6.945 (42)
rGO–Fe3O4 30.68 (43)
NF–rGO 121.6 (44)
Scenedesmus vacuolatus–rGO 95 (45)
chitosan-coated sand 12.32 (46)
amino-BC 55.65 (47)
Fe2O3–Al2O3 23.75 (48)
rGO aerogel 58.04 present study
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Recyclability and Stability Studies

It was necessary to test the adsorption ability of the regenerated rGO aerogel for the sake of evaluating the economics and stability of the adsorbent. The rGO aerogel was regenerated, and the adsorption experiments were carried out using the regenerated adsorbent. The aforementioned operation was repeated six times, and the data are shown in Figure 12. As shown in Figure 12, the adsorption capacity percentages of the regenerated adsorbent for Pb(II) were 100, 96.5, 94.2, and 90.1% in the first four cycle experiments, respectively. Subsequently, the adsorption capacity percentages declined obviously in the fifth and sixth cycle. The reduction of removal efficiency might be because of the destruction of active sites on the surface of the absorbent by hydrochloric acid during desorption. The performance of the rGO aerogel in the first four cycles revealed that the rGO aerogel has an excellent recyclability and stability.

Figure 12.

Figure 12

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Absorption recyclability and stability of rGO aerogel.

Conclusions

The rGO aerogels with porous structures were successfully prepared using a facile hydrothermal method without introducing any solid or aqueous activation agent, which were confirmed by the results of structural analysis and morphology test [i.e., Fourier-transform infrared (FT-IR) spectroscopy, XRD, XPS, SEM, and BET]. The results of adsorption experiments demonstrated that the rGO aerogels had shown good performance for the removal of Pb(II) and could be recycled at least four times. The adsorption data were in accordance with the pseudo-second-order kinetic model and Langmuir isotherm model, indicating that chemical adsorption played a dominant role in the adsorption process. Therefore, the rGO aerogels could be used as promising adsorbents for wastewater treatment.

Experimental Methods

Materials

Graphite powder was obtained from J & K Chemical Ltd. Lead nitrate, nitric acid, sulfuric acid, potassium permanganate, sodium nitrate, hydrochloric acid, hydrogen peroxide, and other chemicals were purchased from Tianjin Chemical Reagent Co. They were used without further purification.

Synthesis of GO and rGO Aerogel

GO was synthesized with graphite powder via a modified Hummer’s method.49 Concentrated H2SO4 (120 mL) was added into a 500 mL flask in an ice bath, followed by the addition of 5.0 g of graphite power. Then, 2.5 g of NaNO3 was slowly added under stirring, and the mixture was continuously stirred for 2 h. After that, 15 g of KMnO4 was slowly added to the mixture, and the mixture was stirred for another 2 h at <20 °C. Next, the ice bath was removed, and the mixture was stirred for 0.5 h at 35 °C. Deionized water (150 mL) was then poured into the mixture, and the solution was stirred for 0.5 h at 90 °C, and then, 30% H2O2 was gradually added until the solution turned bright yellow. After filtration while still warm, the product was washed with 5% HCl to remove metal ions, followed which the acid was washed off using deionized water. Finally, the product was dried and dispersed to obtain the GO suspension aqueous solution.

The effects of reaction parameters such as the concentration of GO (2–10 mg/L), reaction time (8–24 h), and reaction temperature (140–180 °C) on the preparation of rGO aerogel were studied. To synthesize the rGO aerogel, a certain amount of the GO suspension was weighed, diluted with purified water to different concentrations, and ultrasonicated for 0.5 h. Then, the dispersed homogeneous GO suspension was added into a 50 mL Teflon reactor. The Teflon reactor was sealed and heated in an electrical oven at a specific temperature for a certain time to obtain the rGO hydrogel. Next, the rGO hydrogel was removed from the Teflon reactor and was immersed in a 10% aqueous alcohol solution for more than 6 h. Finally, it was placed in a refrigerator for freezing overnight and was freeze-dried at −60 °C for 24 h under vacuum (<12 Pa) to obtain the rGO aerogel.

FT-IR Analysis

FT-IR measurement was performed on a Bruker VERTEX70 infrared analyzer.

XRD Analysis

The XRD studies of GO and rGO aerogels were characterized on a Philips PW1840 X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm), which operated at (40 kV, 40 mA).

Raman Analysis

Raman spectroscopy was conducted on Thermo Scientific DXR equipped with EZ OMNIC software to analyze the structure of GO and rGO aerogels.

XPS Analysis

Surface composition of GO and rGO aerogels were performed on a Thermo Fisher Scientific X-ray photoelectron spectrometer with monochromatic Al Kα radiation at 1486.6 eV.

BET Analysis

N2 adsorption and desorption isotherms of GO and rGO aerogels were determined by an ASAP2020 HD88 instrument at 78 K.

SEM Analysis

The SEM images of rGO aerogels were obtained by a Zeiss EM900 scanning electron micrography at 30 kV.

Adsorption Experiments

The effects, including contact time (10–120 min), pH value (1–5), and the initial concentration (6–100 mg/L) on the adsorption efficiency of rGO aerogels for Pb(II) were investigated. A 1000 mg/L Pb(II) stock solution was prepared using Pb(NO3)2 and diluted by adding deionized water to achieve the desired concentration. The initial pH value of each sample solution was adjusted by adding 0.001–2 mol/L of HNO3 or NaOH solution. The adsorption experiments of Pb(II) were carried out by adding absorbent (10 mg) to the experimental solution of the known concentration (25 mL) under shaking condition (100 rpm). After adsorption equilibrium, the adsorption solution was filtered to separate the supernatant. The concentration of Pb(II) in aqueous solution was analyzed by an inductive coupled plasma emission spectrometer (180–80, Hitachi, Japan).

The equilibrium adsorption capacity of Pb(II) [qe (mg/g)] was calculated using the following eq 5.

graphic file with name ao0c00183_m005.jpg 5

where Co was the initial concentration of Pb(II) (mg/L); Ce was the equilibrium concentration of Pb(II) (mg/L); V (L) was the volume of the adsorption solution; and W (g) was the weight of the adsorbent.

Acknowledgments

The authors are grateful for financial support from the National Natural Science Foundation of China (51602069), the Tianjin Science and Technology Supports Key Projects (18YFZCSF00350), the Guangdong Province Zhanjiang Bay Laboratory Project (012S19005-005), and the Haikou City’s Marine Economic Innovation and Development Demonstration City Project (HHCL201810).

The authors declare no competing financial interest.

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