Poly(acrylamide

Article pubs.acs.org/Langmuir

Preparation of Reduced Graphene Oxide/Poly(acrylamide) Nanocomposite and Its Adsorption of Pb(II) and Methylene Blue Yongfang Yang,*,† Yulei Xie,† Lichuan Pang,† Mao Li,† Xiaohui Song,‡ Jianguo Wen,† and Hanying Zhao*,‡ †

Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China Key Laboratory of Functional Polymer Materials, Ministry of Education, Department of Chemistry, Nankai University, Tianjin 300071, P. R. China

‡

S Supporting Information *

ABSTRACT: Carboxyl groups at the periphery of reduced graphene oxide (RGO) sheets are converted to amine groups by reaction with Nhydroxysuccinimide and 1,3-diaminopropane, and a free-radical polymerization initiator is anchored to the RGO sheets. Poly(acrylamide) (PAM) polymer brushes on RGO sheets (RGO/PAM) are synthesized by in situ free-radical polymerization. The heavy metals, Pb(II), and the benzenoid compounds, methylene blue, (MB) were selected and adsorbed by RGO/ PAM composites, and the adsorption capacity of RGO/PAM for Pb(II) and MB was measured. The experimental data of RGO/PAM isotherms for Pb(II) and MB followed the Langmuir isotherm model. The RGO/PAM displays adsorption capacities as high as 1000 and 1530 mg/g for Pb(II) and MB, respectively, indicating RGO/PAM is a good adsorbent for the adsorption of Pb(II) and MB. The adsorption kinetics of Pb(II) and MB onto RGO/PAM can be well fitted to the pseudo-second-order model. The adsorption processes of Pb(II) and MB onto RGO/ PAM are spontaneous at 298, 308, and 318 K.

1. INTRODUCTION With the development of the industry, environment pollution has increased seriously. Treatment of wastewater containing heavy metals, organic dyes, and aromatic compounds from metallurgical, mining, and dye manufacturing industries has been considered as a serious issue in the fields of environmentology. Heavy metals and dyes are important pollutants in wastewater due to their nonbiodegradability and strong toxicity to plants, animals, and human beings. Heavy metals such as lead can cause mental retardation, kidney disease, anemia, and so forth.1 Most dyes with a complex aromatic structure are more stable and difficult to be biodegraded. The presence of dyes in water can cause problems in aquatic life, and some dyes are toxic to human life. Because of the harmful effects of the heavy metals and organic dyes, they should be removed from wastewater before being released into the environment. Among the traditional technology for the removal of the heavy metals and organic dyes from the wastewater, adsorption is regarded as an easy and economic method.2,3 Recently, nanoadsorbents have a fast development for their large surface and “surface area to volume”.4 As a newly emerging carbon nanomaterial, graphene has attracted considerable attention for its particular properties such as excellent mechanical, optical, and electrical properties.5−7 For its super large surface area (2630 m2 g−1) and flat structure, graphene can be used as an excellent adsorbent.8−11 Machida et al. investigated Pb2+ adsorption onto a graphene layer of © 2013 American Chemical Society

activated carbon and charcoal, and found that acidic oxygen sites from carboxylic and lactonic groups and basic sites from πelectrons on graphene layer would be responsible for the Pb2+ adsorption.12 Huang et al. prepared graphene nanosheets by vacuum-promoted low-temperature exfoliation and used them to adsorb Pb2+ ions from an aqueous system. They found the adsorption against lead ion was enhanced by heat treatment because the lewis basicity of graphene sheets is improved after heat treatment under high vacuum.13 With a large delocalized π-electron system, graphene can interact with the benzene ring compounds through strong π−π stacking interaction, so it can serve as a good adsorbent for the benzenoid compounds such as cationic and anionic dyes.14−16 However, graphene sheets tend to aggregate through van der Waals interaction due to their high surface area, so graphene is often functionalized with inorganic nanoparticles, surfactants, hydrophilic groups, or polymer to improve the dispersion properties in aqueous solution and the adsorption capacity.17−19 Nguyen-Phan and co-workers prepared three-dimensional wormhole-like mesoporous titanosilicate/reduced graphene oxide (RGO) layered composites and applied them as photocatalytic adsorbents for the removal of dye components from wastewater.20 Magnetite nanoparticles were loaded onto graphene sheets to avoid the Received: May 27, 2013 Revised: July 25, 2013 Published: July 29, 2013 10727

dx.doi.org/10.1021/la401940z | Langmuir 2013, 29, 10727−10736

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Scheme 1. Synthesis of PAM Chains on RGO Sheets by Free Radical Polymerization and Adsorption of Pb(II) and MB

of RGO. Considering polymer grafted on the RGO basal planes may affect the benzenoid compounds adsorption capacity of RGO, we selected the carboxyl groups at the edge of RGO sheets as anchor sites to graft PAM chains. In a previous article, we reported the functionalization of RGO with poly(Nisopropylacrylamide) based on click chemistry and reversible addition−fragmentation chain transfer polymerization.43 In this study, the free-radical initiator was introduced to RGO boundaries and PAM chains at RGO plane boundaries were prepared by in situ free-radical polymerization (Scheme 1). The benzenoid compounds, MB, and the heavy metals, Pb(II), were selected and adsorbed by RGO/PAM composites, and the adsorption capacities were investigated.

possibility of agglomeration and to separate easily from wastewater for their remarkable magnetic properties.21−23 The Chen group prepared the graphene nanosheet (GNS)/ magnetite (Fe3O4) composite by a facile one-step solvothermal method. The resulting GNS/Fe3O4 shows extraordinary adsorption capacity and fast adsorption rates for removal of methylene blue (MB) from aqueous solution.24 Wang et al. synthesized sulfonated graphene nanosheets, and they used the nanosheets as sorbents to adsorb naphthalene and 1naphthol.25 Graphene oxide (GO) can be obtained from oxidation and exfoliation of graphite. During the process of the oxidation of graphite, oxygen functional groups such as epoxy, hydroxyl, and carboxylic acid groups are created on the graphene sheets.26−29 The presence of oxygen groups on the surface of GO provides a remarkable hydrophilic character and analogous chemical reactivity to make GO as an ideal adsorbent.30−36 For example, Yang and co-workers reported adsorption of Cu(II) by GO.37 Deng and co-workers reported that GO nanosheets can be used as sorbents for the removal of Cd(II) and Pd(II) ions from aqueous solutions.38 However, plenty of oxygen groups destroy the π−π conjugated srtucture of graphene and decrease the adsorption capacity of aromatic compounds which adsorbed onto graphene sheets through π−π stacking. Graphene can be functionalized with polymers containing heteroatoms to increase the adsorption capacity of graphene. Chandra and Kim synthesized the polypyrrole−RGO composite and found the composite presents a highly selective Hg2+ removal capacity for the functional groups of the polypyrrole.39 Poly(acrylamide) (PAM), a water-soluble polymer with large numbers of acetylamine groups on its macromolecular chains, has hydrogen bond interaction with aromatic structures and can interact with metal ions by chemical or physical adsorption.40−42 In this article, PAM chains were grafted onto RGO sheets (RGO/PAM) to enhance the dispersion property of RGO in aqueous solution and improve the adsorption capacity

2. EXPERIMENTAL SECTION 2.1. Materials. Natural flake graphite with an average particle size of 40 μm (99%) was purchased from Qingdao Guangyao Graphite Co. Ltd. Fuming nitric acid (>90%) was supplied by Tianjin FengChuan Chem. Co. Sulfuric acid (98%), potassium chlorate (98%), and hydrochloric acid (37%) were purchased from Tian Jin Institute of Chemical Agents and used as received. 1,3-Diaminopropane (98%) was provided by Beijing Chemical Reagents Co. N-Hydroxysuccinimide (NHS) and N-(3-(dimethylamino)propyl)-N-ethylcarbodiimide hydrochloride (EDC·HCl) were supplied by Shanghai Medpep Co. 4,4-Azobis(4-cyanovaleric acid) (ABCPA, 97%) was purchased from Aldrich. Before use, it was purified by recrystallization from methanol and dried in vacuum at room temperature. Acrylamide (98%) was provided by Alfa and was purified by recrystallization from chloroform and dried under vacuum at 45 °C for 24 h before use. All the solvents were distilled before use. 2.2. Preparation of RGO Sheets. GO was prepared according to the Staudenmaier method.44,45 Graphite (5 g) was reacted with concentrated nitric acid (45 mL) and sulfuric acid (87.5 mL) in the presence of potassium chlorate (55 g). On completion of the reaction, the mixture was added to excess water, and after filtration with GO was washed with HCl aqueous solution (5%) and water until the pH of the filtrate was neutral. The dried sample was stored in a vacuum oven at 60 °C before use. Thermal reduction of GO was conducted by placing 10728


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the sample in a Muffle furnace preheated to 950 °C under air atmosphere and held in the furnace for 30 s. 2.3. Preparation of RGO/PAM Nanocomposites. The acid chloride derivative of ABCPA (Cl-ABCPA) was prepared by a reaction of ABCPA and PCl5.46 ABCPA (3.0 g) was dissolved in dichloromethane (25 mL) and cooled to 0 °C. PCl5 (24 g) in 25 mL of CH2Cl2 was added into the above solution and stirred overnight. After the reaction, the excess PCl5 was removed by filtration. The clear solution was added into 5-fold of hexane at 0 °C, and 4,4-azo-bis(4cyanopentanoicchloride) was obtained after filtration. RGO sheets (0.600 g) were added to 80 mL of dry dimethylformamide. After 24 h of ultrasonication, NHS (2.05 g) and EDC·HCl (3.45 g) were added to the dispersion at 0 °C. After stirring at room temperature for 24 h, 2.28 mL of 1,3-diaminopropane was added, and the solution was stirred overnight at room temperature. RGO sheets modified by 1,3diaminopropane (RGO-NH2) were obtained after washing with water and ethanol. After drying in a vacuum oven at 40 °C, RGO-NH2 (0.60 g) was dispersed in a mixture of 80 mL of CH2Cl2 and 2 mL of triethylamine, and Cl-ABCPA (2.5 g) in 25 mL of dry CH2Cl2 was added to the dispersion. After stirring at 0 °C for 2 h, the dispersion was stirred at room temperature overnight. RGO-ABCPA was obtained after filtration and washing with methanol and dichloromethane. PAM on RGO sheets were prepared by free-radical polymerization. In a Schlenk flask, RGO-ABCPA (0.05 g) and acrylamide (1.2 g) monomer were dissolved in 7 mL of deionized water. After 20 min sonication, the dispersion was stirred at 90 °C for 9 h. The resulting product was dissolved in water and centrifuged to remove the free polymer chains which were not anchored to the nanosheets. The final product (RGO/PAM) was dried in vacuum at 50 °C. The free polymer was dissolved in water, precipitated in ethanol twice, and dried in vacuum at room temperature. Based on the viscometric method, the viscosity average molecular weight of the free polymer was determined to be about 1900 k. 2.4. Adsorption of Pb(CH3COO)2 and MB by RGO/PAM and RGO. RGO/PAM was dispersed in water and was ultrosonicated to form a homogeneous solution. The RGO/PAM aqueous solution (200 μL, 0.9 g/L) was added into a dialysis bag. The dialysis bag was put into a beaker containing 5 mL of Pb(CH3COO)2 (pH = 6, 80−1500 mg/L) or 5 mL of MB (pH = 6, 50−250 mg/L) aqueous solution. After it was stirred for 40 min, 1.5 mL of the aqueous solution was placed into a conical flask. Two drops of 0.1% xylenol orange aqueous solution were added to the aqueous solution of Pb(CH3COO)2 and the mixture was measured on a UV−vis spectrophotometer (SHIMADZU UV-2450 spectrophotometer) at a wavelength of 590 nm. The adsorption of MB was measured on a UV−vis spectrophotometer (SHIMADZU UV-2450 spectrophotometer) at a wavelength of 664 nm. In a controlled experiment, we used the RGO (200 μL, 1.5 g/L) to perform the adsorption experiment using the same procedure. The adsorption capacity was calculated using the following equation: adsorption capacity = (c0 − ce)V /m

of 300−700 μm, survey spectra were recorded with a pass energy of 160 eV, and high-resolution spectra were recorded with a pass energy of 40 eV. The apparent molecular weight of PAM was determined using an Ubbelohde viscosity meter with a diameter of 0.49 mm at 30 °C. Atomic force microscopy (AFM) images were collected on a Nanoscope IV atomic force microscope (Digital Instruments). The microscope was operated in tapping mode using Si cantilevers with a resonance frequency of 320 kHz. The voltage was between 2 and 3 V, and the tip radius was less than 10 nm. A drive amplitude of 1.2 V and a scan rate of 1.0 Hz were used. Fourier transform infrared (FTIR) absorption spectra were obtained on a Bio-Rad FTS 6000 system using diffuse reflectance sampling accessories.

3. RESULTS AND DISCUSSION 3.1. Morphological, Structural, and Chemical Characterization of RGO/PAM. AFM measurement provides a direct

Figure 1. AFM image and height profile of RGO. The sample was prepared by depositing a RGO aqueous dispersion (0.1 mg/mL) onto a new cleaved mica surface and dried under vacuum at room temperature.

(1)

where c0 (mg/L) and ce (mg/L) are the initial concentration and equilibrium concentration and V (mL) and m (g) are the total volume and the equivalent mass value of adsorbents used, respectively. 2.5. Characterization. The thermal properties of the composites were measured by thermogravimetric analysis (TGA). The samples were heated to 800 °C at a heating rate of 10 K/min under nitrogen atmosphere on a Netzsch TG209 instrument. High-resolution transmission electron microscopy (TEM) observations were carried out on a Tecnai G220S-TWIN electron microscope equipped with a model 794 CCD camera (512 × 512). TEM specimens were prepared by dipping copper grids into the dispersion of RGO/PAM and RGO, and drying in vacuum at room temperature. The RGO/PAM sample was stained in OsO4 atmosphere. The chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). XPS results were recorded on a Kratos Axis Ultra delay line detector (DLD) spectrometer employing a monochromated Al Kα X-ray source (hν = 1486.6 eV), hybrid (magnetic/electroatatic) optics, and a multichannel plate and DLD. All XPS spectra were recorded using an aperture slot

way to identify the thickness and the number of layers of RGO sheets. Figure 1 presents a tapping-mode AFM image of RGO sheets. The sample was prepared by depositing RGO aqueous dispersion onto a new cleaved mica surface and dried under vacuum at room temperature. The cross-sectional view of the AFM image indicates that the average height of the sheets is about 2.3 nm, which means the RGO flake has two or three layers. FTIR spectra of RGO, RGO-ABCPA, and RGO/PAM nanocomposites are shown in Figure 2. On the spectrum of RGO, the bands at 1622 and 1064 cm−1 are attributed to the stretches of aromatic CC bond and C−O stretch vibrations. After grafting of free-radical initiator to the RGO sheets, two peaks at 1446 and 1384 cm−1 corresponding to C−H asymmetric and symmetric bending vibrations of methyl groups in ABCPA are observed.47 The observation of the 10729


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Figure 2. FTIR spectra of RGO (a), RGO-ABCPA (b), and RGO/ PAM composites (c).

Figure 5. C1s XPS spectra of RGO (a), RGO-ABCPA (b), and RGO/ PAM nanocomposite (c).

Figure 3. TGA curves of RGO (a), RGO-ABCPA (b), RGO/PAM composites (c), and PAM (d).

Figure 6. TEM images of RGO (a) and RGO/PAM (b).

Figure 4. XPS spectra of RGO (a), RGO-ABCPA (b), and RGO/ PAM composites (c).

typical absorption peak at 2242 cm−1 representing the CN stretching vibrations of ABCPA also proves successful grafting of free-radical initiator to RGO sheets. On the spectrum of RGO/PAM, the absorption peaks at 2925 and 1647 cm−1 are attributed to C−H asymmetric stretching vibration and CO stretching vibration of the grafted PAM chains.48 RGO, RGO-ABCPA, and RGO/PAM nanocomposites were also characterized by TGA (Figure 3). RGO was found to have about 12 wt % weight loss in the range between 100 and 700

Figure 7. Photograph of RGO (a) and RGO/PAM (b) dispersed in water.

°C (curve a in Figure 3), which was attributed to the loss of the functional groups such as COOH and OH groups on the RGO 10730


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Figure 8. Pseudo-first-order kinetic plots for the adsorption of Pb(II) by RGO/PAM (A) and pseudo-second-order kinetics for adsorption of Pb(II) by RGO/PAM (B) (pH 6.0, 298 K).

Figure 9. Pseudo-first-order kinetic plots for the adsorption of MB by RGO/PAM (A) and pseudo-second-order kinetics for adsorption of MB by RGO/PAM (B) (pH 6.0, 298 K).

Table 1. Adsorption Kinetic Parameters of Pb(II) Adsorption by RGO/PAM pseudo-first-order −1

pseudo-second-order 2

C0 (mg/L)

K1 (min )

Qe,cal (mg/g)

R

80 240

0.096 0.037

1412 1679

0.85 0.76

Qe,cal (mg/g)

K2 g/(mg·min)

Qe,exp (mg/g)

R2

970 1000

0.0106 0.043

1000 1000

0.95 0.97

Table 2. Adsorption Kinetic Parameters of MB Adsorption by RGO/PAM pseudo-first-order −1

pseudo-second-order 2

C0 (mg/L)

K1 (min )

Qe,cal (mg/g)

R

50 100

0.029 0.023

831 954

0.912 0.96

Qe,cal (mg/g)

K2 g/(mg·min)

Qe,exp (mg/g)

R2

1250 1204

0.267 0.445

1310 1310

0.999 0.998

Figure 4 shows wide XPS spectra of RGO, RGO-ABCPA, and RGO/PAM nanocomposites. On the spectra of RGOABCPA, a peak at 399.1 eV corresponding to the N1s binding energy is observed; however, on the spectrum of original RGO no peak at 399.1 eV is detected, which implies that the surfaces of RGO sheets have been successfully modified with free-radical initiator. The weight percentage of nitrogen in RGO-ABCPA is 3.6%. On the RGO/PAM spectrum (curve c in Figure 4), we

sheets. After modification with free-radical initiator, RGOABCPA was found to have 20 wt % weight loss (curve b in Figure 3), and so approximately 8 wt % of the modified RGO was the free radical initiator. After in situ polymerization of acrylamide, RGO/PAM nanocomposite was found to have 42 wt % weight losses (curve c in Figure 3). At the same time, PAM sample displayed a 83 wt % loss from 100 to 700 °C (curve d in Figure 3). 10731


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PAM is a water-soluble macromolecule. After grafting of PAM chains, the dispersity of RGO sheets in water is significantly improved and only little precipitants can be observed. 3.2. Adsorption Kinetics of Pb(II) and MB Adsorption onto RGO/PAM. Kinetic study on adsorption can provide important information about the mechanism of the whole adsorption process. The adsorption kinetics of Pb(II) and MB adsorption onto RGO/PAM can be evaluated following the Lagergren pseudo-first-order (eq 2) and pseudo-second-order (eq 3) modes as expressed in the following equations.51 ln(qe − qt ) = ln qe − k1t

(2)

where qe and qt are the amount adsorbed in mg/g at equilibrium, time t in min, and k1 is the rate constant of adsorption (min−1). Figure 10. Adsorption isotherms of Pb(II) onto RGO and RGO/ PAM.

t /qt = 1/(k 2qe 2) + t /qe

(3)

where k2 is the rate constant of adsorption (min−1) for the pseudo-second-order adsorption process. As shown in Figures 8A and Figure 9A, our experimental data does not fit with the pseudo-first-order plots for the adsorptions of Pb(II) and MB. However, the linear plots of t/qt versus time indicate good agreement between experimental results and the calculated values obtained by the pseudo-second-order model at two different initial concentrations (Figures 8B and 9B). Tables 1 and 2 summarize the kinetic constants obtained by linear regression for the two models of Pb(II) and MB adsorption onto RGO/PAM, respectively. The values of correlation coefficients (R2) for the pseudo-second-order model are relatively higher than those for the pseudo-first-order adsorption, indicating that the pseudo-second-order adsorption mechanism is dominant in the adsorption processes of Pb(II) and MB by RGO/PAM. 3.3. Adsorption Isotherms of Pb(II) and MB Adsorption by RGO/PAM. Langmuir and Freundlich isotherms are often used to describe and understand the mechanism of the adsorption. The Langmuir adsorption isotherm is based on the assumption that adsorption takes place on homogeneous surface, while the Freundlich isotherm model assumes heterogeneity of adsorption surfaces. The equation can be expressed in the following equations52,53

calculated that the weight percentage of nitrogen increased to 7.4% after grafting of PAM onto RGO sheets, which indicates that PAM chains were successfully grafted onto the RGO sheets. Figure 5 shows C1s binding energy of RGO, RGOABCPA, and RGO/PAM nanocomposites. As shown in Figure 5a, the C 1s XPS spectrum of RGO shows four componential peaks at 284.6 eV (C−C), 286.6 eV (C−O), 288.4 eV (CO), and 289.2 eV (O−CO), respectively. After grafting of ABCPA, the peak at 285.7 eV corresponding to the C−N bond was observed in the spectrum of the RGO-ABCPA (Figure 5b).49,50 After PAM chains were introduced onto the RGO sheets, the percentage of the area of the peak corresponding to the C−N bond increased, which also demonstrates the grafting of PAM chains onto the surface of RGO sheets (Figure 5c). The morphology of RGO and RGO/PAM nanocomposite was studied by TEM. TEM specimens were prepared by dipping copper grids into the solution and drying in air. The RGO/PAM specimen was stained in OsO4 atmosphere. Figure 6a shows TEM image of RGO, where blank RGO surfaces were observed. Figure 6b is a TEM image of RGO/PAM nanocomposite on carbon film. The dark dots on RGO sheets represent PAM nanosized domains. It is observed that there is a higher grafting density of PAM at the edges of RGO sheets than that on the plane surface of graphene sheet. The sizes of PAM domains are in the range of 9−13 nm. Figure 7 shows a photograph of RGO and RGO/PAM nanocomposite in water after 2 h of sonication. For pure RGO, precipitated particles or aggregated structures can be observed.

ce/qe = 1/(KLqm) + ce/qm

(4)

log qe = log K F + 1/n(log ce)

(5)

Figure 11. Langmuir (a) and Freundlich (b) isotherms for the adsorption of Pb(II) onto RGO and RGO/PAM. 10732


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Table 3. Comparison of Results between Two Isotherm Models for Pb(II) Adsorption on RGO and RGO/PAM Langmuir model RGO RGO/PAM

Freundlich model

qm (mg/g)

KL (L/mg)

R2

RSD

KF ((mg1−nLn)/g)

n

R2

RSD

500 1000

0.00149 0.00137

0.989 0.987

0.0196 0.0173

13.14 7.42

1.84 1.88

0.960 0.950

0.013 0.011

R2 values of Langmuir isotherms are 0.989 and 0.987 for Pb(II) adsorption onto RGO and RGO/PAM, respectively (Table 3), which indicates that the adsorption behavior fit the Langmuir model well. In addition, based on the fitting result, the maximum adsorption capacity (qm) of Pb(II) by RGO was calculated to be 500 mg/g. After PAM was grafted onto RGO, qm reached 1000 mg/g, which is much higher than that of RGO. From the Langmuir isotherm, separation factors (RL) can be obtained according to the following equation:

Table 4. Maximum Adsorption Capacity (qm) of Various Adsorbents of Pb(II) adsorbent

qm (mg/g)

ref

GNSPF6 GNSC8P PDA-GH EDTA-GO EDTA-RGO RGO RGO/PAM

405.9 74.18 336.32 479 ± 46 204 ± 26 500 1000

33 33 55 56 56 this study this study

RL = 1/(1 + KLce)

(6)

The value of RL is related to the shape of the Langmuir isotherm and the nature of the adsorption process. KL is the Langmuir binding constant, which is related to the energy of adsorption. ce is the equilibrium concentration (mg/L) of the Pb(II) or MB. Based on eq 6, the values of RL for Pb(II) adsorption onto RGO/PAM were calculated to be in the range of 0.313−0.901, which indicates the adsorption of Pb(II) by RGO/PAM is a favorable process.34 The adsorption capacities of Pb(II) onto various GO or RGO, collected from the references, are listed in Table 4,33,55,56 and the comparative results show that the adsorption capacity of RGO/PAM is higher than those reported previously. Therefore, RGO/PAM is a good absorbent for removing Pb(II) from polluted water. R2 values of Freundlich isotherms for the adsorption of Pb(II) by RGO and RGO/PAM are found to be 0.960 and 0.950, respectively, which suggests that the adsorption of Pb(II) by RGO and RGO/PAM could also be explained reasonably by Freundlich model. The n value is an indicator of the favorite state of the absorption process. In the adsorptions of Pb(II) by RGO and RGO/PAM, the n values are 1.84 and 1.88 respectively, which also indicates that the adsorption of Pb(II) is favorable at this studied condition.55 MB With aromatic rings and cationic atoms (S+) can be adsorbed on RGO surface through π−π stacking and ionic interaction. The adsorption isotherms of MB by RGO and RGO/PAM are shown in Figure 12, and the Langmuir and Freundlich isotherms for the adsorptions of MB onto RGO and RGO/PAM are presented in Figure 13a and b, respectively. At

Figure 12. Adsorption isotherms of MB onto RGO and RGO/PAM.

where qe is the amount adsorbed at equilibrium (mg/g), ce is the equilibrium concentration of the MB (mg/L), constant KL is related to the energy of adsorption (L/mg), qm is the Langmuir monolayer adsorption capacity (mg/g), KF is roughly an indicator of the adsorption capacity, and 1/n is the adsorption intensity. RGO and RGO/PAM adsorption isotherms for Pb(II) are shown in Figure 10. Figure 11 shows the Langmuir (Figure 11a) and Freundlich (Figure 11b) isotherms for the adsorptions of Pb(II) onto RGO and RGO/PAM, respectively.

Figure 13. Langmuir (a) and Freundlich (b) isotherms for the adsorption of MB onto RGO and RGO/PAM. 10733


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Table 5. Comparison Results between Two Isotherm Models for MB Adsorption on RGO and RGO/PAM Langmuir model

Freundlich model

qm (mg/g)

KL (L/mg)

R2

RSD

KF ((mg1‑nLn)/g)

n

R2

RSD

833.3 1530.0

0.0358 0.0259

0.999 0.997

0.016 0.015

247.65 366.43

4.93 4.31

0.980 0.987

0.0071 0.0063

RGO RGO/PAM

ce) at different temperatures were treated according to the van’t Hoff equation:60

Table 6. Maximum Adsorption Capacity (qm) of Various Absorbents of MB adsorbent

qm (mg/g)

ref

GO GO sponge graphene GO RGO RGO/PAM

714 389.84 153.85 350 833.3 1530.0

56 57 58 59 this study this study

ln(qe /ce) = −ΔH /RT + ΔS /R

where R is the universal gas constant (8.3145 J mol−1 K−1) and T is the absolute temperature (K). Plotting ln(qe/ce) against 1/ T gives a straight line with slope and intercept equal to −ΔH/R and ΔS/R, respectively (Figures S1 and S2 in the Supporting Information). The related thermodynamic parameters were also calculated and are shown in Tables 7 and 8. The negative values of ΔH demonstrate the exothermic nature of the adsorption processes. The negative values of ΔS indicate that Pb(II) and MB molecules were orderly adsorbed onto the RGO/PAM. The Gibbs free energy of adsorption (ΔG) was calculated from the following relation:

Table 7. Thermodynamic Parameters for Pb(II) Adsorption onto RGO/PAM at Different Temperature T (K)

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (J/mol·K)

298 308 318

−0.51 −0.25 −0.17

−10.27

−32.78

ΔG = ΔH − T ΔS

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (J/mol·K)

298 308 318

−6.97 −6.1 −5.1

−34.8

−93.4

(8)

The negative values of ΔG (Tables 7 and 8) suggest the spontaneous nature of the adsorption process at 298, 308, and 318 K. In addition, the more negative value of ΔG for MB adsorption onto RGO/PAM indicates that MB is more easily adsorbed onto RGO/PAM than Pb(II).

Table 8. Thermodynamic Parameters for MB Adsorption onto RGO/PAM at Different Temperature T (K)

(7)

4. CONCLUSIONS Introduction of PAM to the RGO sheets can significantly enhance the dispersion property of RGO in aqueous solution and increase the adsorption capacity of RGO for heavy metal and MB. The PAM chains with functional groups grafting onto the RGO surface can make RGO/PAM an excellent adsorbent for removal of Pb(II) heavy metals in aqueous solution. The highest adsorption capacities of RGO/PAM for Pb(II) and MB are found to be 1000 and 1530 mg/g at 298 K, which is 2−3 times higher than that of RGO or functionalized GO. The adsorption kinetics of Pb(II) and MB onto RGO/PAM can be well fitted to the pseudo-second-order model. The Langmuir isotherm fits the equilibrium adsorption data for the adsorption of Pb(II) and MB onto RGO/PAM well. It is a favorable adsorption process. The negative values of ΔG and ΔH suggest the spontaneous and exothermic nature of the adsorption of Pb(II) and MB onto RGO/PAM at 298, 308, and 318 K.

low ce values, the adsorption capacities (qe) increase quickly. The qe values increase slowly after ce is above 150 mg/L. R2 values of Langmuir and Freundlich isotherms are found to be 0.999 and 0.980 for the adsorption of MB onto RGO, 0.997 and 0.987 for the adsorption of MB onto RGO/PAM, which suggests that the adsorption data of MB by RGO and RGO/ PAM fits the Langmuir and Freundlich model well (Table 5). The n value is an indicator of the favorite state of the adsorption process. Usually, when n is larger than 2, the absorbent is regarded as a good one. Here, the n value is 4.93 and 4.31 in the adsorption of MB by RGO and RGO/PAM. Therefore, both RGO and RGO/PAM are excellent adsorbents for MB. The value of RL for MB adsorption onto RGO/PAM calculated from eq 6 is 0.528−0.848, which also suggests the adsorption of MB by RGO/PAM is a favorable process. The maximum absorption capacity is 833.3 mg/g at 298 K. After PAM was grafted to RGO, the qm value of MB adsorption onto RGO/PAM is 1530.0 mg/g. The adsorption capacities of MB onto GO or RGO collected from the references are listed in Table 6,56−59 and the comparative results show the adsorption capacity of RGO/PAM is much higher than that of GO. Therefore, it can be concluded that RGO/PAM has much superior adsorption capacities for removing MB from water than RGO and GO. 3.4. Thermodynamic Analysis of Pb(II) and MB Adsorption by RGO/PAM. The thermodynamic studies provide in-depth information on inherent energetic changes during the process of adsorption. In this study, the effects of temperature on Pb(II) and MB adsorption onto RGO/PAM were investigated at 298, 308, and 318 K. The values of ln(qe/

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ASSOCIATED CONTENT

S Supporting Information *

van’t Hoff plots showing the adsorption of Pb(II) and MB onto RGO/PAM. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.Y.); [email protected]. cn (H.Z.). Notes

The authors declare no competing financial interest. 10734


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ACKNOWLEDGMENTS This project was supported by National Natural Science Foundation of China under Contract No. 20904009 and the open fund of Nankai University Education Ministry Key Laboratory of Functional Polymer Materials.

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