Adsorption behavior of methylene blue on carbon nanotubes

Adsorption behavior of methylene blue on carbon nanotubes

Bioresource Technology 101 (2010) 3040–3046 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/loca...

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Bioresource Technology 101 (2010) 3040–3046

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Adsorption behavior of methylene blue on carbon nanotubes Yunjin Yao *, Feifei Xu, Ming Chen, Zhongxiao Xu, Zhiwen Zhu School of Chemical Engineering, Hefei University of Technology, Hefei 230009, China

a r t i c l e

i n f o

Article history: Received 7 September 2009 Received in revised form 6 December 2009 Accepted 10 December 2009 Available online 8 January 2010 Keywords: Adsorption Carbon nanotube Isotherm Methylene blue Thermodynamics

a b s t r a c t The effect of temperature on the equilibrium adsorption of methylene blue dye from aqueous solution using carbon nanotubes was investigated. The equilibrium adsorption data were analyzed using two widely applied isotherms: Langmuir and Freundlich. The results revealed that Langmuir isotherm fit the experimental results well. Kinetic analyses were conducted using pseudo-first and second-order models and the intraparticle diffusion model. The regression results showed that the adsorption kinetics were more accurately represented by pseudo-second-order model. The activation energy of system (Ea) was calculated as 18.54 kJ/mol. Standard free energy changes (DG0 ), standard enthalpy change (DH0 ), and standard entropy change (DS0 ) were calculated using adsorption equilibrium constants obtained from the Langmuir isotherm at different temperatures. All DG0 values were negative; the DH0 values and DS0 values of CNTs were 7.29 kJ/mol and 64.6 J/mol K, respectively. Results suggested that the methylene blue adsorption on CNTs was a spontaneous and endothermic process. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Dyes are colour organic compounds which can colorize other substances. These substances are usually present in the effluent water of many industries, such as textiles, leather, paper, printing, and cosmetics. The complex aromatic structures of dyes make them more stable and more difficult to remove from the effluents discharged into water bodies (Crini, 2006). There are several methods to remove dyes such as physical and chemical processes to treat wastewaters including organic pollutant and dyes. The most commonly used methods for color removal are biological oxidation and chemical precipitation. However, these processes are effective and economic only in the case where the solute concentrations are relatively high. The adsorption technique has proven to be an effective and attractive process for the treatment of these dye-bearing wastewaters. The adsorption characteristics of dyes on various adsorbents have been extensively investigated for many purposes involving separation and purification. Carbon nanotubes (CNTs), with nano-sized diameter and tubular microstructure, have been the worldwide hotspot of study since their discovery because of their unique morphologies and various potential applications. Because of their relatively large specific surface areas and easily modified surfaces, much attention has been paid to the adsorption by CNTs of contaminants such as Zn2+ (Lu * Corresponding author. Tel.: +86 551 2901458; fax: +81 551 2901453. E-mail address: [email protected] (Y. Yao). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.12.042

and Chiu, 2006), Cd2+ (Li et al., 2003), Pb2+ (Kabbashi et al., 2009), Cu2+ (Wu, 2007), Cr6+ (Di et al., 2006), fluoride (Li et al., 2003), and dioxin (Long and Yang, 2001). Therefore, CNTs might be ideal sorbents for the removal of dyes from water. The reactive dye, methylene blue (MB) has wide applications, which include coloring paper, temporary hair colorant, dyeing cottons, wools, and coating for paper stock. Although MB is not extremely hazardous, it can cause some harmful effects. MB was employed as the organic pollutant to be treated by CNTs in this work. Earlier studies have obtained only equilibrium adsorption data and few works have measured the kinetics and thermodynamic parameters of adsorption on CNTs. Peng et al. (Peng et al., 2003) measured those of the adsorption of 1,2-dichlorobenzene and Li et al. (Li et al., 2005) examined the thermodynamics of the adsorption of Pb2+ on CNTs. Few investigations focused on the adsorption of organic pollutants on CNTs and simultaneously determined the kinetics equilibrium and thermodynamic parameters. Hence, this study elucidates the kinetics equilibrium and thermodynamics of the adsorption of MB onto CNTs. In this study CNTs were used and evaluated as a possible sorbent for the removal of a cationic dye (methylene blue, MB) from aqueous solution. The objective of this study is to investigate the effect of MB concentration, temperature, and contact time on the adsorption process. Kinetics and thermodynamics studies have been performed and the results have been analyzed by applying conventional theoretical methods. Thermodynamic parameters, such as DG0 , DH0 , and DS0 , were calculated.

Y. Yao et al. / Bioresource Technology 101 (2010) 3040–3046

2. Methods 2.1. Materials CNTs were produced by chemical vapor deposition using the carbon from acetylene cracking; iron nanoparticles embedded in mesoporous silica, prepared by mixing iron nitrate solution and tetraethoxysilane in a sol–gel process, were employed as the catalyst. The method of synthesis had been described before (Yao et al., 2008a,b). The raw product contains the aerogel support, catalyst particles, and a few amorphous carbons (mostly as a coating on the catalyst nanoparticles) as impurities. The as-grown CNTs were purified using a two-step purification procedure involving acid washing and oxidation in diluted air. The BET surface area was determined from adsorption isotherms using a Micromeritics ASAP 2020 Surface Area Analyzer. The BET surface area, average pore diameter, and pore volume was 160 m2/g, 20 nm, and 0.67 cm3/g, respectively. MB (C16H18ClN3S3H2O, molecular weight 373.9) were obtained from Shanghai Reagents Co. and used without further purification. All MB solutions used in this study were prepared by weighing and dissolving the required amounts MB in distilled water. 2.2. MB absorption Batch adsorption experiments were performed using 100 ml glass bottles with addition of 15 mg purified CNTs and 50 ml of MB solution of increased initial concentrations (C0) from 5 to 40 mg/l. The glass bottles were sealed and placed within a temperature control box to maintain water temperature. The pH of the samples was adjusted by adding 2 M HCl or 0.5 M NaOH to each 200 ml of the prepared solution to pH 7. The pH of solutions was measured with a pH meter. In the experiments on the effect of temperature, the temperature was held at 273, 298, and 333 K and the pH was fixed at 7. At the end of the equilibrium period, the suspensions were separated for later analysis of the dye concentration. The amount of MB adsorption at equilibrium qe (mg/ g) was calculated from the following equation:

qe ¼ VðC 0  C e Þ=W

ð1Þ

where C0 and Ce (mg/L) are the liquid-phase concentrations of dye at initial and equilibrium, respectively, V (L) the volume of the solution and W (g) is the mass of adsorbent used. The concentration of MB after and before adsorption was determined using a spectrophotometer (kmax = 665 nm). The procedures of kinetic experiments were identical with those of equilibrium tests. The effect of contact time on the amount of dye adsorbed was investigated at 20 mg/l initial concentration of dye and at different temperatures (273, 298, and 333 K).The amounts of dye adsorbed on CNTs at any time, t, were calculated from the concentrations in solutions before and after adsorption. At any times, the amount of MB adsorbed (mg/g) (qt) onto CNTs was calculated from the mass balance equation as follows:

qt ¼ VðC 0  C e Þ=W

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and the solid phases when the adsorption process reaches an equilibrium state. The adsorption isotherms of MB on CNTs at 273, 298, and 333 K are shown in Fig. 1, respectively. As seen from Fig. 1, equilibrium uptake increased with the increasing of equilibrium MB concentrations at the range of experimental concentration. This is a result of the increase in the driving force from the concentration gradient. In the same conditions, if the concentration of MB in solution was bigger, the active sites of CNTs were surrounded by much more MB ions, and the process of adsorption would carry out sufficient. Therefore, the values of qe increased with the increase of equilibrium MB concentrations. From Fig. 1, the adsorption capacity of MB onto CNTs 35.0 mg/g1 at 298 K, compared to other adsorbents, the quantity of MB adsorption onto CNTs is not high, but as natural materials, it is vast and cheap, so CNTs can be used to remove MB from solution. The increase of the equilibrium adsorption with increased temperature indicated that the adsorption of MB ions onto CNTs was endothermic in nature. The analysis of the isotherm data by seeing how well different models can accommodate them is an important step in establishing a model that can be successfully used for design purposes. Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb onto a surface is equal to the rate at which they desorb. At equilibrium, no change can be observed in the concentration of the solute on the solid surface or in the bulk solution, a situation characteristic of the entire system, including solute, adsorbent, solvent, temperature, pH, and so on. The equilibrium adsorption isotherm is vital to the design of adsorption systems, and its shape provides information about the homogeneity and heterogeneity of the adsorbent surface. Moreover, the correlation of the equilibrium data with either theoretical or empirical equations is essential to practical operation. The isotherm data were fitted to the Langmuir and Freundlich. The Langmuir model assumes that there is no interaction between the adsorbate molecules and the adsorption is localized in a monolayer. The Langmuir isotherm (Langmuir, 1918) is represented by the following linear equation:

Ce 1 1 ¼ þ Ce qe q0 K L q0

where Ce (mg/L) is the equilibrium concentration, qe (mg/g) the amount of adsorbate adsorbed per unit mass of adsorbate, and q0 and KL are the Langmuir constants related to adsorption capacity and rate of adsorption, respectively. When Ce/qe was plotted against Ce, straight line with slope 1/q0 was obtained (Fig. 2a), indicating that the adsorption of MB on CNTs follows the Langmuir isotherm.

ð2Þ

where qt is the amount of adsorbed dye on CNTs at any time (mg/g); C0 and Ct are the initial and liquid-phase concentrations of MB at any time (mg/L), respectively; V (L) is the volume of MB solution, and W (g) is the mass of CNTs sample used. 3. Results and discussion 3.1. Adsorption isotherms The adsorption equilibrium isotherm is important for describing how the adsorbate molecules distribute between the liquid

ð3Þ

Fig. 1. Adsorption isotherms of MB onto CNTs at different temperatures.

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a

Table 1 Isotherm parameters for removal of MB by CNTs different temperatures. Isotherms

Parameters

Temperatures (K)

Langmuir

q0 (mg/g) KL (L/mg) KL (L/mol) R2 RL S.D. (%)

Freundlich

KF (mg/g (L/mg)1/n) n R2 S.D. (%)

273

298

333

35.4 0.326 122 0.996 0.0712 2.50

46.2 0.213 79.6 0.995 0.105 7.10

64.7 0.564 211 0.995 0.0425 11.8

14.0 3.63 0.984 1.49

13.6 2.86 0.979 14.1

30.8 4.45 0.992 18.0

Table 2 Previously reported adsorption capacities of various adsorbents for methylene blue (Bulut and Aydin, 2006).

b

Adsorbent

q0, mg/g

Ref.

Indian rosewood sawdust Rice husk Neem leaf

11.8–51.4 40.58 8.76– 19.61 70.42 22.47 0.80–0.24

Garg et al. (2004) Vadivelan and Kumar (2005) Bhattacharyya and Sharma (2005) Gucek et al. (2005) Banerjee and Dastidar (2005) Tsai et al. (2006)

13.42 435 16.56– 21.50 35.4–64.7

Wang et al. (2005) Legrouri et al. (2005) Bulut and Aydin (2006)

Pyrophyllite Jute processing waste Eggshell and eggshell membrane Fly ash Activated carbon Wheat shells Carbon nanotubes

Fig. 2. Langmuir (a) and Freundlich (b) isotherms for MB dye adsorption onto CNTs at different temperatures.

The Langmuir constants KL and q0 were calculated from this isotherm and their values are listed in Table 1. Furthermore, the effect of the isotherm shape is considered with a view to predict whether an adsorption system is favorable or unfavorable. Another important parameter, RL, called the separation factor or equilibrium parameter, also evaluated in this study, is determined from the relation (Hall et al., 1966):

1 RL ¼ 1 þ bC 0

ð4Þ

where KL is the Langmuir constant and C0 (mg/L) is the highest dye concentration. The value of RL indicates the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0). RL values for MB adsorption onto CNTs were less than 1 and greater than zero indicating favorable adsorption (Table 1). The Freundlich isotherm model is an empirical relationship describing the adsorption of solutes from a liquid to a solid surface, and assumes that different sites with several adsorption energies are involved. The linear form of the Freundlich equation is

ln qe ¼ ln K F þ

1 ln C e n

ð5Þ

where qe is the amount adsorbed at equilibrium (mg/g) and Ce is the equilibrium concentration of the MB. KF and n are Freundlich constants, n giving an indication of how favorable the adsorption pro-

This study

cess and KF (mg/g (L/mg)1/n) is the adsorption capacity of the adsorbent. It is generally stated that values of n in the range 2–10 represent good, 1–2 moderately difficult, and less than 1 poor adsorption characteristics. The studied materials are good adsorbents for MB (n > 2). The slope 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to 0 (Haghseresht and Lu, 1998). The plot of ln qe versus ln Ce (Fig. 2b) gives straight lines with slope 1/n. Fig. 2b shows that the adsorption of MB also follows the Freundlich isotherm. Accordingly, Freundlich constants (KF and n) were calculated and listed in Table 1. The validity of models was determined by calculating the standard deviation (S.D., %) using

S:D: ¼

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ½ðqexp  qcal Þ=qexp  n1

ð6Þ

where the subscripts exp and cal refer to the experimental and the calculated data, and n is the number of data points. The S.D. values are smaller than 15% for both the Langmuir and the Freundlich models, suggesting that both models closely fitted the experimental results (Table 1). As also seen in Table 1, the adsorption capacities of CNTs for MB are varied in the range of 35.4–64.7 mg/g. Previously some researchers investigated several adsorbents such as wheat shells (Bulut and AydIn, 2006), rice husk (Vadivelan and Kumar, 2005), Indian rosewood sawdust (Garg et al., 2004), neem leaf powder (Bhattacharyya and Sharma, 2005), pyrophyllite (Gucek et al., 2005), jute processing wastes (Banerjee and Dastidar, 2005), eggshells (Tsai et al., 2006), fly ash (Wang et al., 2005), and activated carbon (Legrouri et al., 2005) for the removal of MB from aqueous solutions. By comparison of the results obtained in this study with those in the previously reported works (Table 2) on adsorption capacities of various adsorbent and activated carbon in aqueous

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Table 3 Coefficients of pseudo-first and second-order adsorption kinetic models and intraparticle diffusion model (dye concentration = 20 mg/l, CNTs = 15 mg and pH 7.0). Temperature (K)

qe,exp (mg/g)

Pseudo-first-order model 273 26.14 298 27.73 333 41.63 Temperature (K)

qe,exp (mg/g)

Pseudo-second-order model 273 26.14 298 27.73 333 41.63 Temperature (K)

ki (mg/g min0.5)

Intraparticle diffusion model 273 3.086 298 2.907 333 1.969

k1 (min1)

qe,cal (mg/g)

R2

0.03286 0.02938 0.05615

22.07 21.51 14.75

0.9830 0.9919 0.9910

k2 (min1)

qe,cal (mg/g)

R2

0.00182 0.00254 0.0078

31.37 31.09 43.02

0.99396 0.9951 0.99929

C (mg/g)

R2

1.935 4.723 27.30

0.9597 0.9758 0.9351

Fig. 3. Kinetic analysis of temperature effect (MB = 20 mg/l,CNTs = 15 mg and pH 7.0).

solution for MB it can be stated that our findings are extremely good. 3.2. Kinetic analyses It is important to be able to predict the rate at which contamination is removed from aqueous solutions in order to design an adsorption treatment plant. The effect of temperature on the adsorption rate of MB on CNTs was investigated at 273, 298, and 333 K. Increasing the temperature is known to increase the rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particle, owing to the decrease in the viscosity of the solution. In addition, changing temperature will change the equilibrium capacity of the adsorbent for a particular adsorbate. Fig. 3 shows the results of contact time experiments carried out at different temperatures for MB adsorption on CNTs. The 90 min adsorption capacity at 273, 298, and 333 K was 26.14, 27.73, and 41.63 mg/g, respectively, indicating the process to be endothermic. This kind of temperature dependence of the adsorbed amount of the dyes may reflect the increase in the case with which the dye penetrates into CNTs because of its larger diffusion coefficient. The adsorption is initially (contact time <45 min) rapid, and then slows (Fig. 3), perhaps because a large number of vacant surface sites were available for adsorption during the initial stage, and then, the remaining vacant surface sites were difficult to occupy because of the repulsive forces between the dye molecules on the CNTs and the bulk phase (ChungHsin, 2007). In order to investigate the adsorption processes of MB dyes on CNTs, kinetic analysis were conducted using pseudo-first and second-order models and the intraparticle diffusion model. These models are most commonly used to describe the sorption of dyes as well as other pollutants (heavy metals) on solid sorbents. The Lagergren rate equation is one of the most widely used adsorption rate equations for the adsorption of solute from a liquid solution. The pseudo-first-order kinetic model of Lagergren may be represented by

dqt ¼ k1 ðqe  qt Þ dt

ð7Þ

Integrating this equation for the boundary conditions t = 0 to t = t and q = 0 to q = qt, gives:

lnðqe  qÞ ¼ lnðqe Þ  k1 t

ð8Þ

where qe and qt are the amounts of MB adsorbed (mg/g) at equilibrium and at time t (min), respectively, and k1 is the rate constant of pseudo-first-order adsorption (min1). The validity of the model can be checked by linearized plot of ln(qe  qt) versus t. The rate constant of pseudo-first-order adsorption is determined from the slope of the plot. The values of k1 and qe at different temperatures are presented in Table 3. The pseudo-second-order equation based on adsorption equilibrium capacity can be expressed as (Wu, 2007)

dq ¼ k2 ðqe  qÞ2 dt

ð9Þ

Rearranging the variables in Eq. (9) gives

dq ðqe  qÞ2

¼ k2 dt

ð10Þ

Taking into account, the boundary conditions t = 0 to t = t and q = 0 to q = qt, the integrated linear form of Eq. (10) can be rearranged to obtain Eq. (11):

t 1 t þ ¼ q k2 q2e qe

ð11Þ

Since neither the pseudo-first-order nor the second-order model can identify the diffusion mechanism, the kinetic results were analyzed by the intraparticle diffusion model to elucidate the diffusion mechanism, which model is expressed as: (Kavitha and Namasivayam, 2007)

qt ¼ ki t 1=2 þ C

ð12Þ

where C is the intercept and ki is the intraparticle diffusion rate constant (mg/g min1/2), which can be evaluated from the slope of the linear plot of qt versus t1/2 . The results of Fig. 3 are fitted using pseudo-first- and second-order models and intraparticle diffusion model. Fig. 4(a–c) displays the linear regressions. Table 3 presented the coefficients of the pseudo-first and second-order adsorption kinetic models and the intraparticle diffusion model. The linear plots of t/qt versus t show good agreement between experimental (qe(exp)) and calculated (qe(cal)) values. Furthermore, the correlation coefficients for the second-order kinetics model (R2) are greater than 0.99. As a result, it can be said that MB adsorption onto CNTs takes place according to second-order kinetic model. Similar phenomena have been observed for MB adsorption on coir pith carbon (Kavitha and Namasivayam, 2007), perlite (Doan et al., 2004), and wheat shells (Bulut and AydIn, 2006). The values of k2, qe,exp and qe,cal all increased with the temperature. Özcan et al. (Özcan et al., 2006) proposed that the

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a

b

c

Fig. 4. Regressions of kinetic plots at different temperature: (a) pseudo-first-order model, (b) pseudo-second-order model and (c) intraparticle diffusion model.

adsorption of Acid Blue 93 by natural sepiolite proceeds by physisorption, in which increasing the temperature increases the adsorption rate but reduces adsorption capacity. However, this study suggested that the thermodynamic analyses were more appropriate for determining whether the adsorption was a physisorption or a chemisorption process, as would be discussed in the following section. The removal of MB by adsorption on CNTs was found to be rapid at the initial period and then to become slow and stagnate with the increase in contact time. The removal of MB by adsorption on surface of CNTs was due to MB as MB+ cationic form. Typically, various mechanisms control the adsorption kinetics; the most limiting were the diffusion mechanisms, including external diffusion, boundary layer diffusion and intraparticle diffusion (Özcan et al., 2006). Hence, the intraparticle diffusion model was utilized to determine the rate-limiting step of the adsorption process. If the regression of q versus t1/2 was linear and passes through the origin, then intraparticle diffusion was the sole rate-limiting step. The regression was linear, but the plot did not pass through the origin (Fig. 4(c)), suggesting that adsorption involved intraparticle diffusion, but that was not the only rate-controlling step. Other kinetic models may control the adsorption rate, which finding was similar to that made in previous works on adsorption. The values of C were helpful in determining the boundary thickness: a larger C value corresponded to a greater boundary layer diffusion effect (Kannan and Sundaram, 2001). The C values (1.935–27.296 mg/g) increased

with the temperature (273–333 K). The results of this study demonstrated increasing the temperature promoted the boundary layer diffusion effect. The ki Values calculated from Fig. 4 were 3.09, 2.91, and 1.97 mg/g min0.5 at 273, 298, and 333 K, respectively. From Table 3, it was observed that ki decreased with increasing temperature. 3.3. Thermodynamic analyses To estimate the effect of temperature on the adsorption of MB onto CNTs, the free energy change (DG0 ), enthalpy change (DH0 ), and entropy change (DS0 ) were determined. The Langmuir isotherm was used to calculate thermodynamic parameters using the following equations:

DG0 ¼ RT lnðK L Þ lnðK L Þ ¼

DS0 DH 0  R RT

ð13Þ ð14Þ

where KL is the Langmuir equilibrium constant (l/mol); R is the gas constant (8.314 J/mol K) and T is the temperature (K). Considering the relationship between DG0 and KL, DH0 and DS0 were determined from the slope and intercept of the van’t Hoff plots of ln(KL) versus 1/T. Table 4 presents the thermodynamic parameters at various temperatures. The negative values confirm the feasibility of the process and the spontaneous nature of the adsorption. The values

Y. Yao et al. / Bioresource Technology 101 (2010) 3040–3046 Table 4 Values of thermodynamic parameters (kJ/mol) for the adsorption of dyes onto CNTs. T, K

DG0 (kJ/mol)

DH0 (kJ/mol)

DS0 (J/mol K)

273 298 333

10.9 11.0 14.8

7.29

64.6

ofDG0 , were found to decrease 10.9 to 14.8 kJ/mol using the equilibrium constant, KL. The decrease in the negative value of DG0 with an increase in temperature indicates that the adsorption process of MB on CNTs becomes more favorable at higher temperatures. The values of DH0 and DS0 calculated from the plot of ln(KL) versus 1/T were given as 7.29 kJ/mol and 64.6 J/mol K, respectively. Kara et al. (Kara et al., 2003) suggested that the DH0 of physisorption is smaller than 40 kJ/mol. Based on DH0 , this study suggested that the adsorption of MB onto CNTs was a physisorption process. The value of DH0 was positive, indicating that the adsorption reaction was endothermic. The positive value of DS0 reflects the affinity of CNTs for MB and suggests some structural changes in dye and CNTs. The pseudo-second-order model was identified as the best kinetic model for the adsorption of MB onto CNTs. Accordingly, the rate constants (k2) of the pseudo-second-order model were adopted to calculate the activation energy of the adsorption process using the Arrhenius equation (Wu, 2007):

ln k2 ¼ lnðko Þ 

Ea RT

ð15Þ

where k2, ko, Ea, R, and T are the rate constant of the pseudo-secondorder model (g/mg min), the Arrhenius factor, the activation energy (kJ/mol), the gas constant (8.314 J/mol K) and the temperature (K), respectively. The activation energy could be determined from the slope of the plot of ln (k2) versus 1/T. The activation energy was 18.54 kJ/mol at pH 7.0. The magnitude of activation energy might give an idea about the type of sorption. There are two main types of adsorption: physical and chemical. Activated chemical adsorption means the rate varies with temperature according to finite activation energy (8.4–83.7 kJ mol1) in the Arrhenius equation. In nonactivated chemical adsorption, the activation energy is near zero (Han et al., 2009). Therefore, DH0 , DG0 , and Ea all suggested the same fact: the adsorption of MB onto CNTs was a physisorption process. The positive values of Ea suggested that rise in temperature favor the adsorption and adsorption process may be an endothermic in nature. Lazaridis and Asouhidou (Lazaridis and Asouhidou, 2003) stated that the activation energy of adsorption was less than 25– 30 kJ/mol in a diffusion-controlled process. Based on the results of activation energy and the intraparticle diffusion model (Table 4 and Fig. 4(c)), this study proposed that the adsorption involved intraparticle diffusion was not the only rate-controlling step, and the other kinetic models might control the adsorption rate. 4. Conclusion This investigation examined the equilibrium and the dynamic adsorption of MB onto CNTs. The results suggested the adsorption capacity increased with increasing temperature. The equilibrium data were analyzed using the Langmuir, and Freundlich isotherm models. Equilibrium data fitted very well with Langmuir isotherm equation. Adsorption data were modeled using the first and second-order kinetic equations and intraparticle diffusion models. The second-order kinetic equation could best describe the sorption kinetics. Thermodynamic analyses indicated that the adsorption of MB direct dyes onto CNTs was endothermic and spontaneous; additionally, the adsorption of direct dyes onto CNTs was via a physisorption process.

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