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Adsorption of methylene blue from aqueous solutions by modified expanded graphite powder


Desalination 249 (2009) 331–336

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Desalination
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Adsorption of methylene blue from aqueous solutions by modi?ed expanded graphite powder
Mingfei Zhao, Peng Liu ?
Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China

a r t i c l e

i n f o

a b s t r a c t
The modi?ed expanded graphite (MEG) powder was used as a porous adsorbent for the removal of the cationic dye, methylene blue (MB), from aqueous solutions. The dye adsorption experiments were carried out with the bath procedure. Experimental results showed that the basic pH, increasing initial dye concentration and high temperature favored the adsorption. The dye adsorption equilibrium was attained rapidly after 5 min of contact time. Experimental data related to the adsorption of MB on the MEG under different conditions were applied to the pseudo-?rst-order equation, the pseudo-second-order equation and the intraparticle diffusion equation, and the rate constants of ?rst-order adsorption (k1), the rate constants of second-order adsorption (k2) and intraparticle diffusion rate constants (kint) were calculated, respectively. The experimental data ?tted very well in the pseudo-second-order kinetic model. The thermodynamic parameters of activation such as Gibbs free energy, enthalpy, and entropy were also evaluated. The results indicated that the MEG powder could be employed as an ef?cient adsorbent for the removal of textile dyes from ef?uents. ? 2009 Elsevier B.V. All rights reserved.

Article history: Accepted 28 January 2009 Available online 2 October 2009 Keywords: Adsorption Modi?ed expanded graphite Methylene blue Kinetics Isotherm Modeling

1. Introduction Graphite is the most stable allotrope carbon in which the carbon bonding involves sp2 (trigonal) hybridization. It consists of carbon layers (known as graphere layers) with covalent and metallic bonding within each layer and are linked by a weak van der Walls interaction produced by a delocalized π-orbital [1]. The special sandwich structure and long distance between the carbon layers make it easy to insert atoms or molecules between the carbon layers; heating the graphite intercalation compounds (GICs) induces the vaporization of the intercalated species and hence, a signi?cant expansion of the material along the crystallographic c-axis occurs, and porous expanded graphite (EG) is obtained. The morphology and pore structure of EG can be determined by altering preparation conditions, and there are many reports on preparation procedures, pore structure and application of EG [2–4]. EG is a highly porous worm-like and very light material with typical apparent densities of 0.002–0.01 g cm? 3, and is a material of growing importance because of numerous actual and potential applications in hydrogen storage [5], fuel cell [6], sensor [7], catalyst [8], biomedical materials [9] and adsorbent. The absorbing spaces of EG can be divided into two types. One is the wrapping absorption space (WAS) constructed by EG stacking on each other; the second is the pores in each EG worm-like segment [10]. EG is a good adsorbent due to the network pore structure, weak polarity, hydrophobic and lipophilic nature with extremely high selective sorption capacity to large organic compounds with weak polarity from water. EG has been used to remove
? Corresponding author. Tel.: +86 931 8912516; fax: +86 931 8912582. E-mail address: pliu@lzu.edu.cn (P. Liu). 0011-9164/$ – see front matter ? 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.01.037

heavy oil ?oating on water [11], solid-phase extraction sorbent for DDTs in water [12] and gas adsorbent [13]. In the present work, the adsorption properties of modi?ed expanded graphite (MEG), obtained by the oxidation of the expanded graphite, was ?rstly investigated with the cationic dye methylene blue (MB) as target pollutant from aqueous solution as a function of stirring speed, contact time, initial dye concentration, pH and temperature. The experimental data were analyzed using pseudo-?rst and second-order kinetic models, and intraparticle diffusion models. 2. Experimental 2.1. Materials Graphite powder (7500 mesh) with an average size of 500 μm used for preparing the modi?ed expanded graphite (MEG) are supplied by Shandong Qingdao Graphite Company (Qingdao, China). Concentrated sulfuric acid and concentrated nitric acid (Chemical Pure) from Chemical Company of Shanghai were used as chemical intercalant and oxidizer to prepare the modi?ed expanded graphite. Methylene blue (MB) (CI: 52,015; chemical formula: C16H18ClN3S; molecular weight: 319.86; maximum wavelength: 662 nm) supplied by Merck, was not puri?ed prior to use. 2.2. Modi?ed expanded graphite Graphite oxide (GO) was prepared according to the literature reports [14,15]. Brie?y, a mixture of concentrated sulfuric acid and

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nitric acid (4:1, v/v) was mixed with graphite powders at room temperature and stirred continuously for 16 h. The acid-treated natural graphite (i.e. GICs) was then washed thoroughly with water until the solution became neutral, and dried at 100 °C to remove the remaining moisture. The resulting GO was subsequently heated to 600 °C and kept at this temperature for 90 min in nitrogen atmosphere. The expansion ratio of the EG obtained was about 30, which was obtained based on the volume change of graphite before and after the expansion. The expanded graphite powders were treated with the mixture of sulfuric acid and nitric acid at room temperature for another 10 h. Then the products, modi?ed expanded graphite (MEG), were washed thoroughly with water until the solution became neutral, and dried at 100 °C. 2.3. Adsorption All of the methylene blue solution was prepared with distilled water. 0.35 g MEG was shaken with 100 mL of dye solution of known initial concentration at desired pH and temperature at 150 rpm for 180 min. The pH of the solution was adjusted with 0.1 N HCl or 0.1 N NaOH by using a Model 3C Digital pH-meter with a combined pH electrode. The pH-meter was standardized with NBS buffers before every measurement. A constant bath was used to keep the temperature constant. At the end of the adsorption period, the solution was centrifuged for 10 min at 5000 rpm. After centrifugation, the dye concentration in the supernatant solution was analyzed using a UV spectrophotometer (Shimadzu UV-260) by monitoring the absorbance changes at a wavelength of maximum absorbance (662 nm). The samples were pipetted from the medium reaction by the aid of a very thin point micropipette, which prevented the transition to the solution of the MEG samples. Preliminary experiments showed that the effect of the separation time on the amount of adsorbed dye was negligible. The amounts of dye adsorbed on the MEG at any time, t, were calculated from the concentrations in solutions before and after adsorption. At any times, the amount of MB adsorbed (mol/g) (qt), onto the MEG was calculated from the mass balance equation as follows: qt = V?C0 ? Ce ? = W ?1?

3. Results and discussion 3.1. Modi?ed expanded graphite The expanding of the graphite oxide obtained by being treated with the mixture of concentrated sulfuric acid and nitric acid was evidenced by X-ray diffractions (Fig. 1). The raw graphite showed a strong peak at 2θ = 26.40°, which was resulted from the diffraction of 002 planes with corresponding value of d-spacing of 0.338 nm, calculated with the Bragg equation. After the intercalation and thermal treatment, a sharp (002) peak with d-spacing of 0.347 nm (2θ = 25.66°) and another peak at 28.24° appeared. The morphologies of graphite powders before and after expansion were given in Fig. 2. The lamellar structure of graphite powders were transformed to a vermicular structure by expansion along the c-axis of graphite crystal. As shown in the SEM image of the expanded graphite, the furrows resulting from the parting of packed layers can provide channels to facilitate the transport of working ?uids. So it is expected that the expanded graphite might have a fast adsorption rate. The surface polar groups of the adsorbents favor the adsorption of the cationic dye. To introduce more polar groups to the surfaces of the expanded graphite, it was treated with the oxidative mixture. The FTIR technique was used for the characterization (Fig. 3). The absorbance peaks at about 3400, 1650, and 1160 cm? 1 which correspond to the ―OH stretching mode, ―C O vibration mode of the keto form and ―C―O vibration mode of the enol form were still presence in the FTIR spectrum of the expanded graphite after the thermal treatment. As being oxidized once more, these peaks increased in the FTIR spectrum of the MEG. It indicated that the anticipant polar groups had been introduced on the surfaces of the expanded graphite with the second oxidizing. Although there was no evident changes between the XRD and SEM analysis of the EG and MEG, it is clear that the surface oxygen element content increased and the surface carbon element content decreased after the second oxidation from the X-ray photoelectron spectroscopy (XPS) analysis (Fig. 4). It also indicated that the polar groups containing oxygen element were introduced in the second oxidation. The effects of pH value on the zeta potentials of the graphite powders are illustrated in Fig. 5. The MEG showed the negative zeta potential above pH 3 while the expanded graphite (EG) showed the negative zeta potential above pH 5. This indicated that their surfaces were negative charged in the high pH range (basic media). Continuously increasing the pH value to the basic condition, its absolute value increased and increased quickly with the increasing of the pH value in the basic solutions.

where qt is the amount of adsorbed dye on the MEG at any time (mmol/g); C0 and Ce are the initial and equilibrium liquid-phase concentrations of MB (mmol/l), respectively; V is the volume of MB solution, and W is the mass of the MEG used (g) [16,17]. The Δqt% of all the determined values was b3.0%. 2.4. Analysis and characterization Bruker IFS 66 v/s infrared spectrometer was used for the Fourier transform infrared (FTIR) spectroscopy analysis. The XRD patterns were recorded in the range of 2θ = 10–80? by step scanning with a Shimadzu XRD-6000 X-ray diffractometer. Nickel-?lter Cu Kα radiation (λ = 0.15418 nm) was used with a generator voltage of 40 kV and a current of 30 mA. X-ray photoelectron spectroscopy (XPS) was accomplished using a PHI-5702 multi-functional X-ray photoelectron spectrometer with pass energy of 29.35 eV and an Mg Kα line excitation source. The binding energy of C 1s (284.6 eV) was used as a reference. The surface morphologies of the graphite powders were characterization with a Philips XL-20 scanning electron microscope (SEM) (Philips Co., The Netherlands). The Zeta potentials of the graphite at different pH values were determined with Zetasizer Nano ZS (Malvern Instruments Ltd, UK). The concentration of the dye was determined using a UV–visible recording spectrophotometer-260 (UV-260, Shimadzu Co. Japan) at an absorbance wavelength of 662 nm, respectively.

Fig. 1. X-ray diffraction patterns.

M. Zhao, P. Liu / Desalination 249 (2009) 331–336

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Fig. 4. XPS survey spectra.

aqueous and solid phases. The effect of concentration on contact time was also investigated as a function of initial dye concentration. The effect of initial dye concentration and contact time on the removal rate of MB by the MEG is shown in Fig. 6. As shown, the adsorption increases with increasing initial dye concentration. The removal of dye depends on the concentration of the dye. The adsorption of MB onto the MEG was fast and the equilibrium was attained at 5 min. It was resulted from the furrow structure as shown in Fig. 5. The amount of MB adsorbed at equilibrium increases from less than 1.0 μg/g to 4.5 μg/g by increasing the initial MB concentration from 3.20 to 15.99 mg/L with the adsorption condition of initial pH 7 and 20 °C. 3.2.2. Effect of pH Effect of pH on the removal rate of MB by the MEG is shown in Fig. 7. As the pH increased, the removal rate decreased. The pH value of the dye solution plays an important role in the whole adsorption process and particularly on the adsorption capacity. It was found that the adsorption capacity increased with the increasing of pH values in the investigated range. As shown in Fig. 7, the negative charged surface revealed and the negative charge density increased with the increasing of pH value as shown in Fig. 5. So the adsorption capacity increased with the increasing of pH value. 3.2.3. Effect of temperature Fig. 8 exhibits contact time versus adsorbed amount graph at different temperatures. The equilibrium adsorption capacity of MB

Fig. 2. SEM images.

3.2. Adsorption rate 3.2.1. Effect of contact time and initial dye concentration The initial concentration provides an important driving force to overcome all mass transfer resistances of the dye between the

Fig. 3. FTIR spectra.

Fig. 5. Zeta potentials.

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M. Zhao, P. Liu / Desalination 249 (2009) 331–336

Fig. 6. Effect of contact time and initial dye concentration on adsorption of MB from aqueous solutions (20 °C, pH 7.0).

Fig. 8. Effect of contact time and temperature on the removal rate of MB from aqueous solutions (C0: 9.60 mg/L, pH 7.0).

onto the MEG was found to increase slightly with increasing temperature, decreasing from 2.70 μg/g at 20 °C to 2.72 μg/g at 70 °C indicating that the dye adsorption on the adsorbent was favored at higher temperatures. However, the adsorption equilibrium was attained slowly with the higher temperature. 3.3. Langmuir equilibrium isotherm The Langmuir equation can be represented as follows: Ce = ?x = m? = 1 = ?kb? + Ce = b ?2?

3.4. Adsorption kinetics 3.4.1. The pseudo-?rst-order kinetic model The pseudo-?rst-order kinetic model has been widely used to predict dye adsorption kinetics. A linear form of pseudo-?rst-order model was described by Lagergren [18]: log?qeq ? qt ? = logqeq ? kpf t = 2:303 ?3?

where Ce is the equilibrium concentration of MB remaining in the solution and x/m is the quantity of MB adsorbed per unit weight of the MEG. The Langmuir constants are called adsorption capacity (b) and bonding energy constant (k). The Langmuir line for MB is shown in Fig. 9 (points: experimental data; line: Langmuir model). The adsorption capacity (b) and bonding energy constant (k) were found to be 7.77 μg/g and 0.027 L/μg, calculated from the Langmuir model line, respectively. The Langmuir model effectively describes the sorption data with R2 values of 0.999. It showed that the adsorption of MB by the MEG powder ?ts the Langmuir equilibrium isotherm perfectly.

where qt is the amount adsorbed at time t (μg/g), and kpf is the equilibrium rate constant of pseudo-?rst-order adsorption (min? 1). The values of log (qeq?qt) were calculated from the kinetic data. The calculated qeq, kpf, and the corresponding linear regression 2 correlation coef?cient r1 values are shown in Table 1. It was observed that the rate constant kpf increased ?rst with an increase in temperature and then decreased. It was also observed that correlation coef?cients were lower for all temperatures. This shows no applicability of the pseudo-?rst-order model in predicting the kinetics of the methylene blue adsorption onto the MEG. 3.4.2. The pseudo-second-order kinetic model The kinetic data were further analyzed using Ho's pseudo-secondorder kinetics, represented by [19] t = qt = 1 = ?kps qeq ? + t = qeq
2

?4?

Fig. 7. Effect of contact time and initial pH on the removal rate of MB from aqueous solutions (20 °C, C0: 9.60 mg/L).

Fig. 9. Langmuir isotherm plots for the adsorption of MB (contact time, 24 h; temperature, 20 °C).

M. Zhao, P. Liu / Desalination 249 (2009) 331–336 Table 1 Adsorption kinetic parameters of methylene blue onto the MEG. T (°C) Pseudo-?rst-order kpf , min? 1 20 45 70
a b

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Pseudo-second-order qb, μg/g e 0.0054 0.0313 0.0292
2 r1 a qe, μg/g

Intraparticle diffusion qb, μg/g e 2.696 2.705 2.715
2 r2

kps, g/(μg min) 9.319 5.612 3.639

kid, μg/(g min1/2) 0.00053 0.00105 0.00183

C 2.689 2.692 2.694

2 r3

0.0074 0.0491 0.0345

0.939 0.899 0.905

2.696 2.705 2.716

0.999 0.999 0.999

0.961 0.976 0.971

Calculated. Experimental.

where kps is the rate constant of second-order adsorption (g/mg min). The values were calculated from the kinetic data (Fig. 8). A plot between t/q versus t gives the value of the constants kps (g/μg min) and also qeq (μg/g) can be calculated. The curves of the plots of t/qt versus t were given in Fig. 10 and the calculated qeq, kps, and the corresponding linear regression correlation 2 coef?cient r2 values are summarized in Table 1. The linear plots of t/qt versus t show good agreement between experimental and calculated qeq values. The correlation coef?cients for the second-order kinetics 2 model (r2 ) are greater than 0.999, indicating the applicability of this kinetics equation and the second-order nature of the adsorption process of methylene blue onto the MEG powder. 3.4.3. Intraparticle diffusion The adsorbate species are most probably transported from the bulk of the solution into the solid phase through an intraparticle diffusion process, which is often the rate-limiting step in many adsorption processes. The possibility of intraparticle diffusion was explored by using the intraparticle diffusion model [20]: qt = kid t
1=2

energy (Ea), activation free energy change (ΔG*), activation enthalpy change (ΔH*), and activation entropy change (ΔS*) can be calculated by using the following equations [21]: lnk2 = lnA ? Ea = RT lnk2 = kB TK * = h ΔG* = ? RTlnK * ΔH* = Ea ? RT ΔS* = ?ΔH* ? ΔG*? = T ?6? ?7? ?8? ?9? ?10?

+C

?5?

where C is the intercept and kid is the intraparticle diffusion rate constant (mol/g min1/2). The values kid, C, and the corresponding 2 linear regression correlation coef?cient r3 values are given in Table 1. The intraparticle rate constants calculated from Fig. 11 are 0.00053, 0.00105, and 0.00183 μg/g min1/2 at 20, 45, and 70 °C, respectively. It is observed that kid decreased slightly with the increasing of temperature, as shown in Table 1. 3.4.4. Activation the thermodynamic parameters The k2 values of the second-order kinetic equation for adsorption of MB onto the MEG at different temperatures (9.60 mg/L, pH 7.0) were calculated to be 3.43 × 105 (20 °C), 5.70 × 105 (45 °C), and 8.79 × 105 g/(mmol min) (70 °C), from the data in Fig. 10, respectively. The thermodynamic parameters including the Arrhenius activation

where A is the Arrhenius factor, kB and h are Boltzmann's constant (1.38 × 10? 23 J/K) and Planck's constant (6.626 × 10? 34 J s), R is the gas constant (8.3145 J/mol K), and K* is the equilibrium constant at temperature T. A linear plot of lnk2 versus 1/T for the adsorption of MB onto the MEG is constructed to generate the Ea value from the slope (Fig. 12). The Ea value was calculated to be 2.14 × 108 kJ/mol with a linear regression coef?cient of 0.993. Thus, the values of ΔG*, ΔH*, and ΔS* are 64.50 (69.00, 69.19) kJ/mol, 13.17 (13.00, 12.76) kJ/mol, and ?0.175 (? 0.179, ? 0.164) kJ/(K mol) at 20 (45, 70) °C, respectively. The low ΔH* values for the adsorption of MB with the MEG give a clear evidence that the interactions between MB and the surface polar groups such as carboxyl or hydroxyl groups of the MEG might be weak. Furthermore, the positive values of Ea, ΔG*, and ΔH* indicate the presence of an energy barrier in the adsorption process. The positive values for these parameters are quite common because the activated complex in the transition state is in an excited form. The negative values of ΔS* suggest decreased randomness at the solid/ solution interface and no signi?cant changes occur in the internal structure of the adsorbent through the adsorption of MB onto the MEG.

Fig. 10. Second-order kinetic equation for the adsorption of MB (20 °C, pH 7.0).

Fig. 11. Intraparticle diffusion plots for MB (C0: 9.60 mg/L, pH 7.0).

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Fig. 12. Arrhenius plots for adsorption of MB (C0: 9.60 mg/L, pH 7.0).

4. Conclusions The porous modi?ed expanded graphite (MEG) was prepared by the sequential intercalating, heating, and oxidizing treatment of the natural graphite powder. The product was characterized with FTIR, SEM, XPS and Zeta potential measurement. The adsorption experiments showed that the MEG is ef?cient adsorbent for the removal of the cationic dye from aqueous solution. 1. It had the fast adsorption rate and the equilibrium reached within only 5 min. 2. The adsorption of MB by the MEG ?ts the Langmuir equilibrium isotherm perfectly. 3. The kinetics of the cationic dye adsorption on the MEG follows the pseudo-second-order model. References
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