Adsorption of methylene blue from aqueous solution by graphene

Colloids and Surfaces B: Biointerfaces 90 (2012) 197–203

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Adsorption of methylene blue from aqueous solution by graphene Tonghao Liu a , Yanhui Li a,b,∗ , Qiuju Du a , Jiankun Sun a , Yuqin Jiao b , Guangming Yang a , Zonghua Wang a , Yanzhi Xia a,∗∗ , Wei Zhang c , Kunlin Wang c , Hongwei Zhu c , Dehai Wu c a b c

Laboratory of Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, 308 Ningxia Road, Qingdao 266071, China College of Electromechanical Engineering, Qingdao University, 308 Ningxia Road, Qingdao 266071, China Key Laboratory for Advanced Manufacturing by Material Processing Technology and Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China

a r t i c l e

i n f o

Article history: Received 24 May 2011 Received in revised form 8 October 2011 Accepted 12 October 2011 Available online 18 October 2011 Keywords: Graphene Methylene blue Adsorption Isotherm Kinetic Thermodynamic

a b s t r a c t Graphene was prepared using a modified Hummers’ method. The physico-chemical properties of graphene were characterized by TEM, BET specific surface area, FTIR, Raman and XRD measurements. The effect factors including pH, contact time, temperature and dosage on the adsorption properties of methylene blue onto graphene were investigated. The experimental data of isotherm followed the Langmuir isotherm model better than the Freundlich model. The maximum adsorption capacity obtained from Langmuir isotherm equation at 293 K was 153.85 mg/g, indicating graphene is a good adsorbent for the adsorption of MB. The kinetic study illustrated that the adsorption of methylene blue onto graphene fit the pseudo second-order model. The thermodynamic parameters indicated that the adsorption of methylene blue onto graphene was an endothermic and spontaneous process. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Dyes from the industries such as dye synthesis, printing, paper, textile, electroplating, pulp mill, food and cosmetic are the major source of water pollution. The complex aromatic structures and xenobiotic properties of dyes make them more difficult to degrade [1]. Dyes are harmful to flora and fauna, and some of the organic dyes and their products have a mutagenic or carcinogenic influence on human beings [2]. Consequently, much attention should be paid to treat dyes before discharge. Methylene blue (MB), an organic dye, has wide applications including cottons or wools dyeing, paper coloring, temporarily coloring hair, and coating for paper stock [3]. Due to its known strong adsorption onto solids, MB often serves as a model compound for removing dyes and organic contaminants from aqueous solutions [4]. Although not strongly poisonous, MB can have some harmful effects on human beings [5]. The dye can cause eye burns in humans and animals. It may stimulate the gastrointestinal tract and cause nausea, vomiting, and diarrhea if ingested. It may also cause dyspnea, tachycardia, cyanosis, methemoglobinemia and convul-

∗ Corresponding author at: Laboratory of Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, 308 Ningxia Road, Qingdao 266071, China. Fax: +86 532 85951842. ∗∗ Corresponding author. Fax: +86 532 85951842. E-mail addresses: [email protected] (Y. Li), [email protected] (Y. Xia). 0927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2011.10.019

sions if inhaled [6]. Owing to these harmful effects on humans, it is necessary to remove MB from aqueous solution. Many methods have been developed in the decoloration process, such as adsorption, reverse osmosis, precipitation and ion exchange [7]. Among these methods, adsorption is the most widely used method because of the ease of operation and comparable low cost of application. Various adsorbents, such as bamboo activated carbon [4], jute fiber carbon [6], cedar sawdust and crushed brick [8], unburned carbon [9], bentonite [10], garlic peel [11], modified expanded graphite power [12], and carbon nanotube [13] have been studied for adsorption of MB from aqueous solutions and some of them expressed good MB adsorption property. Graphene is a fascinating new member of carbon materials with honeycomb and one-atom-thick structure. It has excited enormous interest in recent years due to its exceptional properties such as extraordinary electronic properties [14], good thermal conductivity [15], high values of Young’s modulus and fracture strength [16] and fast mobility of charge carriers [17]. Owing to so many prominent properties, it shows great promise for potential applications such as sensor [18], solar cell [19], field-effect transistors [20], supercapacitors [21] and transparent electrodes [22]. Recently, experimental results showed that graphene is a promising adsorbent to remove heavy metals (Pb2+ , Cd2+ , Cu2+ , etc.) [23,24], dyes (methyl orange, methyl violet, rhodamine B, orange G, etc.) [25,26] and fluoride ions [27] from aqueous solutions. In this study, we chose MB as a typical adsorbate and used graphene as an adsorbent to remove MB from aqueous solution, the effects of pH,

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contact time, temperature and dosage on the adsorption capacity have been investigated in detail. 2. Experiments 2.1. Preparation of graphene

Morphology and structure of graphene were characterized by TEM (JEM-2100F). The Brunauer–Emmett–Teller (BET) surface area and pore diameter of graphene were determined from the N2 adsorption at −77 K using a Micrometric ASAP 2000 system. Functional groups of graphene were analyzed by a Perkin-Elmer-283B FTIR spectrometer within the wave range 400–4000 cm−1 . Raman spectroscopy was performed with a Renishaw RM2000 Raman microscope. XRD spectrum was recorded using a Bruker D8 diffrac˚ at a scanning rate of tometer with Cu K␣ radiation ( = 1.5418 A) 0.02◦ /s and time step of 2 s, 2 ranging from 5 to 70◦ . 2.3. Batch adsorption experiments A stock solution of MB (1000 mg/L) was prepared and further diluted to the required concentrations before used. The batch adsorption experiments were conducted in 250 mL conical flasks with 100 mL of standard solutions and equilibrated in a temperature-controlled water bath shaker (SHZ-82A). After adsorption equilibrium, the concentration of MB in the solution was measured using a UV–visible spectrophotometer (TU-1810, Beijing Purkinje General Instrument Co. Ltd., China). The specific adsorbed amount of MB was calculated according to the following equation: qe =

C − C  e 0 m

V

(1)

where C0 and Ce are the initial and equilibrium concentrations of MB in solution (mg/L), V is the volume of solution (L), m is the mass of adsorbent (g). The effect of initial pH on the adsorption of MB was studied in a pH range of 3.0–10.0 using 100 mL of solutions with MB concentrations of 60 mg/L. The pH of the MB concentration was adjusted to the required pH value by the addition of dilute HNO3 or NaOH. The effect of graphene dose on the adsorption of MB was studied by agitating 100 mL of 60 and 100 mg/L solutions of MB with different dosages of graphene (0.02–0.17 g).

10

Adsorption Desorption

8

3

Cumulative pore volume (cm /g)

2.2. Characterization of adsorbents

Fig. 1. TEM image of graphene.

Quantity adsorbed (mmol/g)

All chemicals and reagents used for experiments and analysis were of analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd., China. Graphene was synthesized from expandable graphite (Qingdao Henglide Graphite Co. Ltd., China) by a modified Hummers method [28]. Expandable graphite (5 g) was mixed with a mixture of 230 mL sulfuric acid (98 wt%), potassium permanganate (30 g) and sodium nitrate (5 g) in ice bath. The ice bath was then removed and the obtained mixture was kept at 273 K for 24 h. Later on the mixture was stirred at 308 K for 30 min and then slowly diluted with deionized water. The reaction temperature was rapidly increased to 371 K and kept for 15 min. Then, 30% H2 O2 was added to the mixture causing the color turning to yellow along with bubbling. Then, the mixture was centrifuged (centrifugation speed 4000 rpm) and washed with HCl (5%) and deionized water several times. After filtration and drying at 298 K for 3 days, graphene oxide (GO) was obtained. Graphene was synthesized by hydrazine reduction of GO [29,30]. The dispersion of graphene oxide was ultrasonicated until no visible particles existing. Then, hydrazine hydrate was added to the dispersion and heated at 383 K for 24 h under vacuum. The final product was washed thoroughly with deionized water, and dried at 373 K.

6

4

0.12

0.09

0.06

0.03

0.00 0

10

20

30

40

50

60

70

Pore diameter (nm)

2 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 2. Nitrogen adsorption–desorption isotherm and pore size distributions for graphene.

The effect of contact time on the adsorption of MB was studied by adding 1 g of adsorbents into 2 L of 20 and 40 mg/L MB solutions at room temperature. At predetermined time intervals, samples of these solutions were filtered and then measured by the UV–visible spectrophotometer. To evaluate the thermodynamic properties, 0.05 g adsorbents were added into 100 mL solutions with initial MB concentration ranging from 20 to 120 mg/L. These samples were shaken at 293, 313 and 333 K, respectively. 3. Results and discussion 3.1. Characterization of graphene The morphological structure of graphene was characterized by TEM and shown in Fig. 1. It is clearly that these graphene sheets are basically transparent. However, the elastic corrugations and the scrolled or folded edges often result in different brightness in the surface of the graphene [31]. The nitrogen adsorption–desorption isotherm and pore size distributions was shown in Fig. 2. The specific surface area and pore diameter of graphene are 295.56 m2 /g and 3.49 nm, respectively, which are higher than the reported values of 254 m2 /g and 2 nm [32]. The value of the specific surface area is much lower than the theoretical value (2600 m2 /g) may be due to the incomplete exfoliation of graphene oxide and the agglomerations of graphene layers during reduction process because of the unavoidable van der Waals

T. Liu et al. / Colloids and Surfaces B: Biointerfaces 90 (2012) 197–203

199

500

0.8

1100 400

26.46

0.4

472 3420 0.2

Intensity (a.u.)

Absorbance

0.6 300

10.8

200

100

642 793

1630 1410 0

0.0 600

1200

1800

2400

3000

3600

10

force between each single layer graphene [32,33]. The value of the pore diameter indicates that graphene is a mesoporous material. In order to investigate the functional groups of graphene, the FTIR spectra of graphene was analyzed and the result was shown in Fig. 3. The strong peak at 3420 cm−1 can be attributed to stretching vibration of OH groups. The bands at 1630 and 1410 cm−1 indicate the existence of asymmetric and symmetric stretching vibration of C O groups. The band at 1110 cm−1 is assigned to the appearance of C–N groups from hydrazine hydrate. The bands at 793 and 642 cm−1 reflect the C–H or N–H bending vibrations. The peak around at 472 cm−1 may correspond to C–O stretching vibration. Raman spectroscopy is a frequently used and nondestructive technique to study carbonaceous materials. It can be utilized to examine the ordered and disordered crystal structures and distinguish the single-, bi-, and multilayer characteristics of graphene layers [34]. Fig. 4 shows Raman spectrum of the graphene. The value of 1583 cm−1 for the G band can be primarily attributed to the single-layer graphene [34]. The 2D band is the most important feature in the Raman spectrum of graphene, and its position and shape are in accordance with the number of layers of graphene [35]. The 2D band at 2688 cm−1 corresponds to the graphene with a few layers [36]. The diffraction peak at 26.46◦ in the XRD pattern of graphene (Fig. 5) corresponds to the (0 0 2) planes of graphene layers which is similar to the reported value of graphene [37]. A small peak at

2D 2000

Intensity ( a. u.)

1800

G

1400

1200

1000 1500

40

50

60

70

Fig. 5. XRD pattern of graphene.

Fig. 3. FTIR spectrum of graphene.

1000

30

2θ (Degree)

Wavenumber (cm-1)

1600

20

2000

2500 -1

Raman shift ( cm ) Fig. 4. Raman spectrum of graphene.

3000

10.80◦ may be due to the existence of graphene oxide which comes from the uncomplete reduction of graphene oxide by hydrazine hydrate.

3.2. MB adsorption 3.2.1. Effect of pH The pH of aqueous solution is one of the most important factors to study the adsorption property of an adsorbent. Fig. 6a shows the effect of the initial pH on the adsorption of MB onto graphene. It can be seen that the MB removal percentage has no great change at the whole pH range. It can reach 85.95% at pH = 3.0 and increases to 99.68% at pH = 10.0. This may be due to the more functional groups formed on the surface of graphene which increase their surface complexation capability [38]. The adsorption mechanism of MB onto graphene at low pH and the insensitive to pH needs to be studied further.

3.2.2. Effect of adsorbent dosage The effect of different dosages on MB removal was carried out, and the result was shown in Fig. 6b. It is obviously that the MB removal percentage increases with increasing graphene dose. It is due to that increasing adsorbent dose serves to increase the surface area and the number of active sites for adsorption [39]. The MB adsorption capacity of graphene decreases as graphene dose increases. This may be due to that all active sites are entirely exposed and utilized at lower graphene dose and only part of active sites are exposed and occupied by MB at higher graphene dose. At the same graphene dosage, higher MB concentration acquires a higher equilibrium adsorption capacity, this is because that the larger MB concentration gradient increases the diffusion driving force of MB adsorbed by graphene [40].

3.2.3. Effect of temperature The temperature is one of the most important factors to determine the process of adsorption. The effect of temperature on adsorption of MB onto graphene was investigated at (293, 313 and 333 K), and the results were presented in Fig. 6c. It can be seen that the adsorption capacity increases as the temperature increases. The maximum adsorption capacity increases from 153.85 to 204.08 mg/g with the increase in temperature from 293 to 333 K. The experimental results demonstrate that the process of adsorption of MB onto graphene is endothermic.

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100

a b

180

100

80

40

60 mg/L

60 mg/L

80

100 mg/L

120

60

90 40

Removal (%)

60

q e(mg/g)

Removal (%)

150

60

20

20 30 0

0 4

6

8

0.00

10

0.03

PH

0.06

0.09

0.12

0.15

0.18

Dose (g)

c

d

80

200

120

q t (mg/g)

q e(mg/L)

160

293 K 313 K 333 K

80

60

20 (mg/L) 40 (mg/L)

40

20 40

0

10

20

30

40

0

300

600

900

1200

1500

Time (min)

Ce(mg/L)

Fig. 6. Effect of different experimental parameters on MB adsorbed by graphene: (a) pH effect, (b) dose effect, (c) temperature effect, (d) contact time effect. The error bar represents the standard deviations (n = 3).

3.2.4. Effect of contact time The effect of contact time on adsorption of MB onto graphene was carried out at two initial MB concentrations (20 and 40 mg/L) at 293 K and shown in Fig. 6d. It is apparent that the adsorption gradually increases with the increase in contact time for both concentrations. The time requires to reach the equilibrium is 255 and 855 min for 20 and 40 mg/L, respectively. The amount of dye adsorbed at equilibrium increases from 39.92 to 79.78 mg/g with the increase in dye concentration from 20 to 40 mg/L. Doubling the MB concentration from 20 to 40 mg/L leads to a doubling of the dye adsorption capacity, it is only a coincidence and there is no linear relationship between the adsorption capacities and concentrations. As different concentrations (20, 40, 60, 80, 100, and 120 mg/L) were used, the adsorption capacities are 39.92, 79.78, 115.66, 136.5, 155.18, 161.26 mg/g, respectively. It can be seen that there is only a little increase in the adsorption capacities as the dye concentration increases from 100 to 120 mg/L, so a saturable adsorption capacity will be obtained at high dye concentration. It is obviously that the adsorption of MB depends on the concentration of the dye. Similar results in MB removal has been reported in Refs. [11,41,42].

3.3. Adsorption isotherms Several mathematical models have been used for describing equilibrium studies for the adsorption of dyes on solid surfaces, and the Freundlich and Langmuir models are frequently utilized to

fit the experimental data. In this work, both models were applied to describe the experimental data obtained at three temperatures (293, 313 and 333 K). The Langmuir model assumes that the adsorption occurs on a homogenous surface and no interaction between adsorbates in the plane of the surface. The equation of the Langmuir isotherm is as follows [43]: Ce 1 Ce = + qe qmax qmax kL

(2)

where Ce is the equilibrium concentration of the solution (mg/L), qmax is the maximum adsorption capacity (mg/g), kL is a Langmuir constant related to the affinity of the binding sites and energy of adsorption (L/g). A straight line was obtained when Ce /qe was plotted against Ce (Fig. 7a), and qmax and kL could be evaluated from the slope and intercept (Table 1). The MB maximum adsorption capacity of graphene is 153.85 mg/g, which is higher than the reported values of pyrophylite (4.2 mg/g) [9] and carbon nanotube (46.2 mg/g) [14], indicating that graphene is a good adsorbent to remove dyes from aqueous solutions. The coefficients of determination R2 of the Langmuir equation demonstrate that the adsorption of MB onto graphene follows the Langmuir’s model. Another parameter RL , a dimensionless equilibrium parameter, which is defined as follows [44]: RL =

1 1 + kL C0

(3)

T. Liu et al. / Colloids and Surfaces B: Biointerfaces 90 (2012) 197–203

201

Table 1 Isotherm parameters for the adsorption of MB onto graphene. Temperature (K)

Langmuir

293 313 333

Freundlich

qmax (mg/g)

kL (L/g)

R2

RL

1/n

kF (L/g)

R2

153.85 185.19 204.08

1.44 2.00 3.27

0.9972 0.9961 0.9920

0.0058 0.0041 0.0025

0.1752 0.1891 0.1976

90.92 107.77 126.47

0.8689 0.8956 0.9108

where kL is the Langmuir constant (L/g) and C0 is the highest initial dye concentration (mg/L). This parameter indicates the isotherm is unfavorable (RL > 1), favorable (RL < 1), linear (RL = 1), or irreversible (RL = 0) [45]. Table 1 shows RL values between 0 and 1, which indicates the adsorption of MB onto graphene is favorable. The Freundlich equation is an empirical equation based on adsorption on a heterogeneous surface. The equation is commonly expressed as follows [46]: Lnqe = LnkF +

1 LnCe n

(4)

where kF is a Freundlich constant related to adsorption capacity (L/g), 1/n is an empirical parameter related to adsorption intensity. A straight line was obtained when Lnqe was plotted against LnCe (Fig. 7b) and n and kF could be evaluated from the slope and intercept (Table 1). According to the coefficients of determination, the Langmuir model fits better than Freundlich model. It is clear

that the values of kF and n increase as the temperature increases, indicating that adsorption is favorable at higher temperature [47]. 3.4. Kinetic studies In order to examine the controlling mechanism of the adsorption process, the pseudo-first-order Lagergren equation and the pseudo-second-order rate equation were applied to analyze the experimental data at two different initial MB concentrations (20 and 40 mg/L). The linearized-integral form of the pseudo first-order model is represented by [48]: log(qe − q1 ) = log qe −

(5)

where k1 is the Lagergren rate constant of adsorption (1/min), qe and qt are the amounts of MB adsorbed (mg/g) at equilibrium and at time t (min). k1 was calculated from the slope of the plot of log(qe − qt ) versus t (Fig. 8a) and the values of kinetic parameters

a

0.25

k t 2.303

a 4

20 (mg/L) 40 (mg/L)

log (qe - qt)

Ce/qe

0.20

0.15

0.10

293 K 313 K 333 K

0.05

0.00

2

0

-2 0

10

20

30

40

0

300

Ce (mg/L)

900

1200

1500

Time (min)

b

b

5.2

16

4.8

12

t/q t

Lnqe

600

4.4 293 K 313 K 333 K

4.0

8

20 (mg/L) 40 (mg/L)

4

0

3.6 -4

-2

0

2

4

LnC e Fig. 7. The equilibrium isotherm for MB adsorbed by graphene: (a) the Langmuir isotherm, (b) the Freundlich isotherm. The error bar represents the standard deviations (n = 3).

0

300

600

900

1200

1500

Time (min) Fig. 8. Adsorption kinetics of MB adsorbed by graphene: (a) pseudo-first-order model (b) pseudo-second-order model. The error bar represents the standard deviations (n = 3).

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Table 2 Parameters of pseudo-first-order and pseudo-second-order kinetic models. Concentration (mg/L)

Pseudo-first-order model

20 40

Pseudo-second-order model

qe,exp (mg/g)

k1 (1/min)

qe,cal (mg/g)

R2

qe,exp (mg/g)

k2 (1/min)

qe,cal (mg/g)

R2

39.92 79.28

0.0144 0.0096

12.21 67.29

0.8592 0.9844

39.92 79.28

0.0009 0.0001

40.98 84.75

0.9993 0.9924

Table 3 Thermodynamic parameters for MB adsorbed by graphene. Temperature (K)

G0 (kJ/mol)

H0 (kJ/mol)

S0 J/(K mol))

293 313 333

−0.89 −1.80 −3.28

16.54

59.25

at two initial concentrations were presented in Table 2. The results show that the experimental qe is not in agreement with calculated qe and the coefficients of determination R2 is low. Accordingly, the adsorption of MB onto graphene does not follow the pseudo-firstorder kinetic model. The linearized-integral form of the pseudo-second-order model is expressed as follows [49]: t 1 t = + qt qe 2k2 q2e

To evaluate the effect of temperature on adsorption process of MB onto graphene, the thermodynamic parameters such as Gibbs free energy (G0 ), enthalpy (H0 ) and entropy (S0 ) are calculated using the following equations [13]:

S 0 H 0 − R RT

This work was supported by the National Natural Science Foundation of China (50802045 and 20975056/B050902) SRF for ROCS, SEM, the Middle-aged and Youth Scientist Incentive Foundation of Shandong Province (BS09018), the Talisman Scholar Program of Shandong Province, and Program for Changjiang Scholars and Innovative Research Team in University (IRT0970), China. References

3.5. Thermodynamic study

ln(kL ) =

Acknowledgements

(6)

where k2 is the pseudo-second-order rate constant (g/mg min). k2 was determined from the intercept of the plot of t/qt versus t (Fig. 8b). The values of kinetic parameters were listed in Table 2. The coefficients of determination R2 are higher than 0.99 and the experimental qe is in accordance with calculated qe illustrate that the adsorption of MB onto graphene fits the pseudo-second-order model.

G0 = −RT ln kL

the Freundlich model. The pseudo-first-order Lagergren model and pseudo-second-order rate model were applied to study the kinetic of the adsorption. The results illustrated that the adsorption of MB onto graphene fit the pseudo-second-order model. The thermodynamic parameters indicated that the adsorption of MB onto graphene was an endothermic and spontaneous process.

(7) (8)

where R is the universal gas constant (8.314 J/mol K), kL is the Langmuir constant (L/g) and T is the absolute temperature (K). S0 and H0 were calculated from the intercept and slope of the plot of ln(kL ) versus 1/T. The thermodynamic parameters were listed in Table 3. The negative value of G0 suggests the feasibility and the spontaneous nature of the adsorption. In general, the values of G0 in between 0 and −20 kJ/mol indicate that the adsorption process is physisorption, while the values in between −80 and −400 kJ/mol correspond to chemisorption [50,51]. The values of G0 suggest the adsorption is a physisorption process. The positive value of H0 indicates that the adsorption reaction is endothermic. The positive value of S0 demonstrates the increased randomness at the solid–solute interface and the affinity of the graphene for the MB [52]. 4. Conclusions This study investigated the equilibrium and the dynamic adsorption of MB onto graphene. Batch adsorption experiments showed that the adsorption of MB onto graphene were dependent on adsorbent dosage, contact time and temperature. The equilibrium data followed the Langmuir isotherm model better than

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