Efficient removal of organic dyes using a three-dimensional graphene aerogel with excellent recycling stability

NEW CARBON MATERIALS Volume 34, Issue 4, Aug 2019 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2019, 34(4): 315-324

RESEARCH PAPER

Efficient removal of organic dyes using a three-dimensional graphene aerogel with excellent recycling stability Yue-ling Ding1, Zhen Tian1, Hui-jun Li1, Xiao-min Wang1,2,* 1

College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China;

2

Shanxi Key Laboratory of new Energy Materials and Devices, Taiyuan University of Technology, Taiyuan 030024, China

Abstract:

The development of a renewable absorbent with a high adsorption efficiency is of great importance for the purification of dye

wastewater. A nitrogen-doped graphene aerogel (N-GA) was prepared by a one-step hydrothermal and freeze-drying method using polyvinyl alcohol (PVA) as a cross-linking agent. Results indicated that the nitrogen doping provided more adsorption sites for the dyes. The PVA-N-GA showed excellent adsorption efficiencies for methylene blue (MB) (98.39% at pH=8) and methyl orange (MO) (78.78% at pH=2). The adsorption kinetic data of MB and MO on the PVA-N-GA were better fitted with a pseudo-second-order kinetic model than a pseudo-first-order kinetic model while the adsorption isotherms of MB and MO were better described by the monolayer Langmuir model and the multilayer Freundlich model, respectively. The PVA-N-GA showed excellent recycling stability with a negligible decay of adsorption efficiency after five cycles, which was ascribed to PVA making the three-dimensional (3D) pore structure of the N-GA more stable. The adsorption of the PVA-N-GA for dyes was mainly attributed to the π-π bonds, hydrogen bonds and electrostatic interactions. Key Words:

Three-dimensional porous structure; Graphene aerogel; Dyes; Adsorption; Recycle ability

1 Introduction Adsorption is a common method in dye polluted water treatment owing to its simplicity and efficiency [1]. Graphene is regarded as an ideal carbon material adsorbent with a remarkable mechanical stability and high theoretical specific surface area [2]. However, the aggregation of graphene sheets vastly hinders its application in the water treatment. Therefore, graphene aerogel (GA) with a three-dimensional (3D) porous structure is built to solve this problem and provide more adsorption surface and channels for dye molecules. However, the collapse of the 3D porous structure of the GA resulting from its fragility may lead to the second contamination for the water environment, which limits the recycle ability of the GA to a large degree [3, 4]. A promising strategy has been adopted to realize the repeatability by adding the cross-linking agent (PVA) to improve the mechanical stability of GA. For example, Xue et al. [5] prepared the polyvinyl alcohol graphene oxide (m-PVA-GO) hydrogel by the sol-gel method, which enhanced the mechanical property significantly. However, the maximum adsorption capacity for MB was only 7.5 mg g −1 for more than 5 h. Xiao et al. [6] synthesized a reduced graphene oxide/polyvinyl alcohol (RGO/PVA) aerogel with a stable porous structure via an in situ thermal reduction method,

while the cycle adsorption efficiency of acid brown (NR) and near infrared dyes (IR) were lower than 53 and 50%, respectively. It is worth noting that the adsorption efficiency of the GA for the organic dyes decreases by PVA, which may be attributed to the fact that PVA covers a part of graphene sheets [7]. In order to enhance the adsorption efficiency of organic dyes, N [8], B [9], S [10] and P [11] are used to chemically modify the GA. As noted, the doping of nitrogen can not only increase the activity of carbon [12] but also cause a large amount of defects to increase available active surface area. For instance, Yang et al. [13] found that nitrogen -doped porous carbon sheets (NPCS) could be used for adsorption of atrazine, and it had a high adsorption capacity (82.8 mg g−1) and adsorption rate (1.43 L (g h)−1). Ren et al. [14] prepared a nitrogen-doped graphene aerogel (N-GA) by the hydrothermal method. The adsorption efficiency of organic solvents kept 82.8%, and the adsorption capability of the N-GA maintained 79.9% after 10 cycles. Therefore, it is essential to develop a graphene-based aerogel with a high adsorption efficiency and good recycle ability in practical water treatment application. In this work, a kind of 3D polyvinyl alcohol nitrogen-doped graphene aerogels (PVA-N/GA) was fabricated through a one-pot hydrothermal and freeze-drying

Received date: 30 May 2019; Revised date: 29 Jul 2019 *Corresponding author. E-mail: [email protected] Copyright©2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(19)60017-X

Yue-ling Ding i et al. / New Carbon Materials, 2019, 34(4): 315-324

method. The influence of different adsorption conditions on adsorption performances of the PVA-N/GA towards methylene blue (MB) dye and methyl orange (MO) dye was systematically investigated. The kinetics and isotherm models were used to analyze adsorption mechanism of the PVA-N/GA. Moreover, the recycle ability of the PVA-N/GA was also studied.

2

Experimental

of the dye. 2.4 Cycle experiments To evaluate the recycle ability of the PVA-N/GA, the adsorbate-loaded aerogel was washed many times with DI water and then freeze-dried, and the adsorption process was repeated. The adsorption-desorption cycles were performed five times. The schematic illustration of the synthesis route and adsorption process of the PVA-N/GA is shown in Fig. 1.

2.1 Materials Graphite powder (350 mesh), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), hydrogen peroxide (H2O2, 30%), concentrated sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4), hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH), urea, polyvinyl alcohol (PVA), methylene blue (MB) and methyl orange (MO) were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without further treatment. Deionized (DI) water was used as the solvent throughout the experiments. 2.2 Preparation of the PVA-N/GA

Fig. 1 A schematic illustration of the synthesis route and adsorption

Graphene oxide (GO) was synthesized by the modified Hummers method [15-17]. Typically, 5 g of PVA was dissolved in 100 mL DI water and stirred at 90 oC. The GO solution (2 mg/mL, 20 mL) was mixed with 400 mg urea, then 100 μL PVA was added into the mixture and sonicated for 6 h. Subsequently, the mixture was transferred into a Teflon-lined autoclave at 180 oC for 12 h, then cooled to room temperature, followed by freeze-drying, and the PVA-N/GA was obtained.

process of the PVA-N/GA.

2.5 Characterization The microstructure of the samples was characterized by a scanning electron microscope (SEM, TESCAN, Czech). A X-ray photoelectron spectrometer (XPS, Amicus Budget, Japan) was used to verify element composition. Fourier transform infrared spectra were recorded using a Tensor 27 FTIR spectroscope (FTIR, Bruker, Germany). Raman spectra were obtained from a CRM200 Raman spectroscope (Raman, WITEC, Germany). The Brunauere-Emmette-Teller (BET, Quantachrome, America) specific surface area of the sample was estimated by nitrogen adsorption at 77 K. The concentrations of solutions was measured by a U-3900 ultraviolet-visible spectrophotometer (UV-vis, Hitachi, China).

2.3 Adsorption experiments MB (λmax = 664 nm) and MO (λmax = 464 nm) were selected to investigate the adsorption behavior of the PVA-N/GA. In adsorption experiments, 5 mg PVA-N/GA was added into 60 mL dye solutions at 25 oC, the solution pH values were adjusted with 0.1 M HCl and 0.1 M NaOH. The concentrations of the remaining solutions after the PVA-N/GA was removed by centrifugation were determined by a UV-vis spectrophotometer. The adsorption capacity (Qt) and removal rate (R) of the PVA-N/GA were calculated by the following equations (1) and (2): 𝑄t = 𝑅=

𝐶0 −𝐶t 𝑚

𝐶0 −𝐶t 𝐶0

×𝑉

(1)

× 100%

(2)

where C0 and Ct (mg L−1) are the concentrations of the solution at 0 and t h, respectively. Qt (mg g−1) is the amount adsorbed in t h, V (L) is the volume of the dye solution, and m (mg) is the weight of the adsorbent, R (%) is the removal rate

3

Results and discussion

3.1 Structural characterization The surface morphologies of the GA, N-GA, PVA-GA and PVA-N/GA were investigated by SEM. As illustrated in Fig. 2a, the surface of the GA exhibited an irregular 3D structure. Fig. 2b depicts massive laminar crumples of the N-GA due to the aggregation of graphene sheets. PVA-GA (Fig. 2c) was observed to have an obviously porous cross-linked structure on its surface, indicating that PVA was dispersed between graphene sheets and facilitated the formation of hydrogen bonds [18]. In addition, PVA could act as a supporting skeleton to build the porous structure [19]. As shown in Fig. 2d, the PVA-N/GA possessed a more tight

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porous structure with a large degree of cross-linked graphene sheets, which was related to the interactions between GO, urea and PVA. The FTIR spectra of the GA, N-GA, PVA-GA and PVA-N/GA are shown in Fig. 3a. GA showed characteristic peaks of -OH stretching vibration, the skeleton vibration of C=C bond graphene sheet and the deformation vibration of C-O-C at 3421, 1614, and 1388 cm−1, respectively. As for the PVA-N/GA, the intensities of peaks of -OH and epoxide C-O-C were weakened as compared with the GA, which testified that the oxygen functional groups were altered during the hydrothermal process. The peaks at 1635, 1558, 1540, 1494 cm−1 were respectively ascribed to C=C, -CONH, -NH2, stretching vibrations of the aromatic C-C bond，implying that nitrogen atoms had successfully been introduced into the graphene sheets [20, 21], as shown in Fig. 3b. The typical peaks at 2935, 2335, 1454, 1321 cm−1 were found in the PVA-N/GA, which could be ascribed to the bending vibration of C-H, the stretching vibration of -OH, C-O-C, CO-H, respectively, indicating that the introduction of PVA was successful. Simultaneously, the existences of the C-H and -OH were beneficial to form hydrogen bonds between PVA and the aerogel, promoting the adsorption of dyes [22]. The existence of nitrogen functional groups in the PVA-N/GA could offer more adsorption sites for dye adsorption [23]. The structural defects of the PVA-N/GA were investigated by Raman spectroscopy, as shown in Fig. 3b. The PVA-N/GA displayed two characteristic peaks, which were D

band (at 1350 cm−1) and G band (at 1585 cm−1). The intensity ratio (ID/IG) of the PVA-N/GA was about 1.02, indicating that the PVA-N/GA had more defects [24] as compared with that of the GA ( about 0.88), which further showed an average size decrease of sp2 domain [14] and an increase of the disorder degree [21] with the nitrogen incorporating into the GA. The XPS survey spectra were used to confirm the composition of the PVA-N/GA. Fig. 4a shows the survey spectra of the GA and PVA-N/GA, with a new peak of N 1s emerged in the PVA-N/GA, implying that nitrogen element was successfully introduced into the aerogel. The C 1s peak of the PVA-N/GA was deconvoluated into three peaks at 284.8, 286.3 and 288.8 eV, which were assigned to C-C, C-NH2 and C=O, respectively, suggesting that the oxygen functional groups had been partially reduced in Fig. 4b. The N 1s spectrum of the PVA-N/GA is shown in Fig. 4c, the deconvoluated peaks at 398.08, 399.75, and 400.11 eV could be ascribed to the pyridinic N, pyrrolic N and N-C=O, respectively, which reflected the existence of nitrogen atoms, thereby increased the surface active sites of the PVA-N/GA [21]. The O 1s peaks of the PVA-N/GA could be decomposed into three peaks, corresponding to C=O (531.08 eV), O-C=O (531.88 eV), C-O (532.68 eV), respectively (Fig. 4d) [19]. They were beneficial to the formation of the hydrogen bonds between PVA and the aerogel [25]. The XPS analysis revealed that nitrogen atoms and PVA had been successfully incorporated into the GA.

Fig. 2 SEM images of (a) GA, (b) N-GA, (c) PVA-GA and (d) PVA-N/GA.

Fig. 3 (a) FTIR spectra of the GA, N-GA, PVA-GA and PVA-N/GA, (b) FTIR spectra partial enlargement of the N-GA and PVA-N/GA and (c) Raman spectra of the GA and PVA-N/GA.

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Fig. 4 (a) XPS survey spectra of the GA and PVA-N/GA, the high resolution (b) C 1s, (c) N 1s, (d) O 1s spectra of the PVA-N/GA.

Fig. 5 (a) Nitrogen adsorption-desorption isotherms and (b) pore-size distribution of the PVA-N/GA.

Fig. 6 (a) Effect of the pH value on the adsorption capacities for MB and MO and (b) Effect of the adsorption time on MB and MO adsorption by the GA, N-GA, PVA-GA, PVA-N/GA.

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Table 1 Adsorption performance of different adsorbents for MB and MO. Adsorption capacity 4 h (mg

Adsorption efficiency

−1

(max)/%

Adsorbents

Adsorbates

Graphene/cysteamine aerogels

MB

50

49.87

(GCAs)

MO

20

17.95

MB

52

74.8

MO

30

31.72

3D rGO/ZIF-67 aerogel

MO

145

34

[32]

mGO/PVA-CG

MB

38

88

[33]

PGOH-0.5

MB

1.35

33.75

[34]

N/S-GHs

MB

637

60.66

[35]

MB

84.82

49.71

MO

53.94

47.77

MB

133.85

68.69

MO

94.21

59.32

This

MB

118.49

64.87

work

MO

76.11

53.42

MB

216.96

98.39

MO

165.1

78.78

GO/PEI aerogel（GP55）

GA

N-GA

PVA-GA

PVA-N/GA

g )

Ref.

[2]

[3]

Fig. 7 Adsorption kinetic analysis of MB and MO onto the PVA-N/GA: (a) the pseudo-first-order model and (b) the pseudo-second-order model.

Fig. 8 Linear fits of the Langmuir and Freundlich adsorption isotherms for (a) MB and (b) MO on the PVA-N/GA.

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Table 3 Adsorption isotherm parameters for the adsorption of MB and MO onto the PVA-N/GA. Adsorbates

Langmuir model −1

Freundlich model

Qmax (mg g )

kL

R

kF

1/n

R2

MB

236.14

1.5

0.9956

287.14

0.2417

0.9688

MO

189.07

0.6085

0.9498

327.01

0.3906

0.9962

The BET specific surface area and pore size distribution were analyzed by N2 adsorption as shown in Fig. 5. The adsorption- desorption isotherms (Fig. 5a) of the PVA-N/GA showed capillary condensation in the relative pressure range of 0.4-1.0, which featured a type IV isotherm, indicating the presence of mesopores in the sample. The hysteresis loop became steeper with the increase of pressure, demonstrating that the hysteresis ring belonged to a H3 type, and the slit holes existed in the PVA-N/GA of with a wide pore size distribution. The BET specific surface area of the PVA-N/GA was 210.37 m2 g−1. It could be seen from Fig. 5b that the PVA-N/GA had a wide pore distribution and the main peak is located at 41.1 nm, which indicated that the PVA-N/GA had mesopores. The PVA could support the 3D porous structure of the GA, which provided large diffusion channels for dye adsorption with the exposed cavities [26]. Defects caused by nitrogen doping increased the specific surface area of the GA and reduced the π-π restacking of graphene sheets, promoting the adsorption of dyes [21]. 3.2 Dye Adsorption The pH value of the solution is a crucial parameter for controlling adsorption [18, 27]. As depicted in Fig. 6a, the adsorption removal rate of MB increased with increasing the pH value from 2 to 8 and the maximum adsorption efficiency of MB was attained at pH=8 (98.39%). At a lower pH value, the hydroxyl group of PVA chains and amino group of urea were protonated, which made the surface of the adsorbent become positively charged, which was not beneficial for the adsorption of cationic dye (MB) through electrostatic attraction [28]. And the PVA-N/GA could also capture the dye molecules through interaction of π-π bond [29]. The adsorption of MB decreased with the pH value from 8 to 12, which might be ascribed to the competitive adsorption between H+ /H3O+ in the solution and MB, and to the deprotonation of nitrogen functional groups on the adsorbent. The adsorption efficiency of anionic dye (MO) declined with increasing the pH value, and the positive charge on the surface of the PVA-N/GA transformed to the negative charge [30, 31], so the negatively charged MO was hard to absorb on the PVA-N/GA for electrostatic repulsion. And the maximum adsorption efficiency of MO occurred at pH=2 (78.78%). The adjustment of the solution pH value had an effect on the protonation degree of functional groups on the adsorbent, changing the electrostatic interaction between the adsorbent and dyes [32]. Therefore, the optimum solution pH value was selected to study the adsorption of hybrid aerogels for dyes. Fig. 6b shows the dye adsorption efficiencies on the GA,

2

N-GA, PVA-GA and PVA-N/GA for MB and MO at different adsorption times. The adsorption efficiencies of the GA towards MB and MO were only 49.71 and 47.77%, respectively, suggesting that the GA had a little effect on the adsorption of dyes. And the N-GA and PVA-GA adsorption abilities became weaker. The PVA-N/GA could reach an adsorption equilibrium within 4 h under the optimum pH value, and the adsorption capacities of MB and MO were almost 216.96 and 165.1 mg g−1, respectively, which were much faster and more efficient than those of the GA, N-GA, PVA-GA. As can be seen from Table 1, the maximum adsorption efficiency of the PVA-N/GA was the highest among the adsorbents investigated, which was attributed to the electrostatic force, π-π, hydrogen bonds between the adsorbent and dyes [36]. Meanwhile, it also demonstrated that the formation of the 3D porous structure and more adsorption sites on the PVA-N/GA were favorable to increase the adsorption ability [37]. 3.3 Adsorption kinetics To further study the adsorption mechanism of the PVA-N/GA towards MB and MO, two types of the adsorption kinetic models, pseudo-first-order model and pseudo-second-order model, were used to analyze the adsorption rates according to the equations (3) and (4), respectively [38]: ln(𝑄𝑒 − 𝑄𝑡 ) = ln𝑄𝑒 − 𝑘1 𝑡 1 𝑄𝑒 −𝑄𝑡

=

1 𝑄𝑒

+ 𝑘2 𝑡

(3) (4)

Where k1 (h−1) and k2 (g (mg h)−1) are the kinetic rate constants of the pseudo-first-order and pseudo-second-order model, Qe (mg g−1) and Qt (mg g−1) are the adsorption amount of equilibrium and the adsorption amount of t (h), respectively.

Fig. 9 The comparison of the removal rate of different dyes by the PVA-N/GA.

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Fig. 10 (a, b) SEM images of the PVA-N/GA after adsorption, (c) EDS diagram of adsorbates, (d) FTIR spectra of the PVA-N/GA before and after adsorption and (e) The adsorption efficiency of MB and MO after five cycles (the inset shows the photographs of the GA and PVA-N/GA before and after cycles).

The adsorption related kinetic parameters of the models were provided in Table 2. The adsorption kinetic analyses of MB and MO were investigated at 25 oC and the optimum pH value as shown in Fig. 7a, b. The pseudo-second-order kinetic model (R2 > 0.98) was more suitable to describe the adsorption of MB and MO on the PVA-N/GA than the pseudo-first-order model, which suggested that the chemisorption between the dyes and active sites of the PVA-N/GA was a rate controlling step[39, 40]. The calculated Qe of MB (236.03 mg g−1) was higher than that of MO (188.9 mg g−1), which showed that the adsorption rate of the adsorbent to MB was faster than that to MO. The fast adsorption rate of dyes depended on the 3D connected pores and more adsorption sites on the PVA-N/GA[32]. 3.4 Adsorption isotherms Adsorption isotherms mainly analyzed the adsorption interactions between the PVA-N/GA and dyes, the Langmuir and Freundlich isotherm model parameters and fitting curves of MB and MO were displayed in Table 3 and Fig. 8a, b. And the two models were expressed as the equation (5) and (6). [41] 𝐶𝑒 𝑄𝑒

=

1 𝑄𝑚 𝑘𝐿

+ 𝐶𝑒 𝑄𝑚 1

ln𝑄𝑒 = lnk F + ln𝐶𝑒 𝑛

(5)

(6)

where Ce (mg L−1) is the concentration of the solution

when the adsorption reaches equilibrium, Qe (mg g−1) is the amount of dyes adsorbed at equilibrium, Qm (mg g−1) is the theoretical saturation capacity, kL (L mg−1) represents the Langmuir characteristic constant, kF (mg g−1) and n are Freundlich characteristic constants, which denote the factor that relates to the adsorption capacity and intensity. For the MB dye, the R2 value of the Langmuir model was higher than that of the Freundlich model, implying that adsorption of MB occurred in a monolayer on specific adsorption sites of the PVA-N/GA. The MO dye adsorption isotherm was better fitted to the Freundlich model, confirming the multilayer adsorption mechanism of MO on the PVA-N/GA. The fitted Qmax (MB) was 236.14 mg g−1, The higher Qm of MB than MO was ascribed to the higher electrostatic attraction [4]. The adsorption efficiency of the PVA-N/GA towards other organic dyes were displayed in Fig. 9. The results showed that the adsorption of the PVA-N/GA on cationic dyes (MB, RhB, CV) was slightly greater than that on anionic dyes (MO, CR, AB), which was caused by the strong electrostatic attraction between cationic dyes and the PVA-N/GA. And the π-π bond and hydrogen bond also promoted the adsorption of dyes. In addition, the PVA-N/GA also had a certain adsorption capacity for anionic dyes. Therefore, the PVA-N/GA could be regarded as a broad-spectrum dye adsorbent[42].

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3.5 Recycle ability evaluation Apart from the excellent adsorption efficiency, recycle ability of an adsorbent is also an important factor in the sustainable development. Fig. 10a displays the SEM image of the PVA-N/GA after adsorption, and the structure of the sample had no obvious change after adsorption compared with the PVA-N/GA before adsorption (Fig. 2d). Fig. 10b shows that there were many adsorbed particles between the sheets of the aerogel. Fig. 10c depicts that different chemical elements (C, O, Cl, S, N) were emerged on the adsorbate, which confirmed the existence of MB and MO. As could be seen in Fig. 10d, the peaks of the PVA-N/GA after adsorption were shifted to 3415, 2929, 2331, 1627, 1546, 1516, 1475, 1432, 1384 and 1282 cm−1 from 3421, 2912, 2335, 1635 1533, 1512 1494, 1388 and 1321 cm−1, respectively. The intensity of peaks of the PVA-N/GA was enhanced as compared with the sample before adsorption. The electrostatic force and π-π bond interactions between the PVA-N/GA and dyes occurred, resulting in the red shift of FTIR and increased peak strength [4, 7] . To further determine the recycle ability of the PVA-N/GA, the adsorption experiments were repeated for the recycled samples. As shown in Fig. 10e, after five cycles, the adsorption efficiency of MB and MO decreased lightly, which might be ascribed to incomplete desorption of the adsorbate-loaded aerogel [43]. However, the removal rate of MB and MO for the PVA-N/GA still maintained about 99.28 and 94.95% of the initial adsorption efficiency, respectively after five adsorption-desorption cycles due to its good 3D porous structure and strong interactions between the PVA-N/GA and dyes. As shown in the inset of Fig. 10e, the structure of the GA broke down after adsorption, and the structure of the PVA-N/GA remained good after five cycles, indicating that the PVA-N/GA had an excellent mechanical strength. It was attributed to the cross-linking coupling of PVA chains and GO nanosheets[22]. The existence of hydrogen bonds between PVA and GO aerogel also improved the stability of GA[34]. Therefore, the PVA-N/GA could be regarded as a reusable absorbent.

4

Conclusions

A polyvinyl alcohol cross-linked nitrogen-doped graphene aerogel (PVA-N/GA) with a 3D porous structure was prepared by a one-step hydrothermal and freeze-drying method. The results demonstrate that the maximum adsorption efficiency towards MB and MO on the PVA-N/GA reach 98.39 and 78.78%, respectively at the optimal pH values. Compared with the GA, the PVA-N/GA demonstrates a prominent dye elimination capacity. The adsorption kinetic data of MB and MO fit well the pseudo-second-order kinetic model, the adsorption isotherms are well described by the monolayer Langmuir model and multilayer Freundlich model, respectively. Moreover, the PVA-N/GA exhibits a favorable recyclable property. It is supposed that the electrostatic, π-π

bond, hydrogen bond interactions between the PVA-N/GA and dyes are the main adsorption mechanisms. And the porosity of the PVA-N/GA provides the large transfer channel for the entrapment of dyes at the adsorption sites, promoting adsorption process. Hence, the PVA-N/GA has a potential application for water purification.

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