Adsorption of anionic and cationic dyes on activated carbon from aqueous solutions: Equilibrium and kinetics

Adsorption of anionic and cationic dyes on activated carbon from aqueous solutions: Equilibrium and kinetics

Journal of Hazardous Materials 172 (2009) 1311–1320 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.e...

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Journal of Hazardous Materials 172 (2009) 1311–1320

Contents lists available at ScienceDirect

Journal of Hazardous Materials journal homepage:

Adsorption of anionic and cationic dyes on activated carbon from aqueous solutions: Equilibrium and kinetics Araceli Rodríguez ∗ , Juan García, Gabriel Ovejero, María Mestanza Grupo de Catálisis y Procesos de Separación (CyPS), Departamento de Ingeniería Química, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 30 June 2009 Received in revised form 21 July 2009 Accepted 31 July 2009 Available online 8 August 2009 Keywords: Adsorption Activated carbon Cationic and anionic dyes Wastewater

a b s t r a c t Activated carbon was utilized as adsorbent to remove anionic dye, Orange II (OII), and cationic dye, Methylene blue (MB), from aqueous solutions by adsorption. Batch experiments were conducted to study the effects of temperature (30–65 ◦ C), initial concentration of adsorbate (300–500 mg L−1 ) and pH (3.0–9.0) on dyes adsorption. Equilibrium adsorption isotherms and kinetics were investigated. The equilibrium experimental data were analyzed by the Langmuir, Freundlich, Toth and Redlich–Peterson models. The kinetic data obtained with different carbon mass were analyzed using a pseudo-first order, pseudo-second order, intraparticle diffusion, Bangham and Chien–Clayton equations. The best results were achieved with the Langmuir isotherm equilibrium model and with the pseudo-second order kinetic model. The activated carbon was found to be very effective as adsorbent for MB and OII from aqueous solutions. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Synthetic dyes are indispensable to the textile, paper and dyeing industries. Many dyes and pigments are inert and non-toxic at the concentrations at which they are discharged into receiving waters. However, some are not so innocuous, and, in either case, the colour they impart is very undesirable to the water user. Dyes in wastewater undergo chemical as well as biological changes, consume dissolved oxygen, and destroy aquatic life. The dyestuffs are classified according to their solubility, colouring properties, and chemical structure. After colouring is completed, the waste ought not to be discharged into the environment without purification. Among several chemical and physical methods, the adsorption has been found to be superior compared to other techniques for wastewater treatment in terms of its capability for efficiently adsorbing a broad range of adsorbates and its simplicity of design [1,2]. Colour removal from industrial wastewaters by adsorption techniques has been of growing importance due to the chemical and biological stability of dyestuffs to conventional water treatment methods and the growing need for high quality treatment. Adsorption of dyes has been carried out by using adsorbents, such as pistachio sells, fruit stones, oil-palm stone, and bamboobased activated carbon [3–6]. Adsorption isotherms of Langmuir, Freundlich, Redlich–Peterson, Koble–Corrigan and Temkin equa-

∗ Corresponding author. Tel.: +34 91 394 4182; fax: +34 91 394 4114. E-mail address: [email protected] (A. Rodríguez). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.07.138

tions are often adopted to predict adsorption capacities. Besides several models, such as pseudo-second and Lagergren equation have been used to study the kinetics of the adsorptive processes [7–9]. Methylene blue (MB), cationic dye, and Orange II (OII), anionic dye, were selected as model compounds in order to evaluate the capacity of activated carbon for their removal from aqueous solutions in batch mode [6,7,10]. In this sense, the aim of the present study was to investigate the effect of initial adsorbate concentrations, temperature, pH, adsorbent particle size and stirring speed on the adsorption process of two different ionic character dyes on activated carbon. Activated carbon textural characterization was also carried out. The equilibrium, kinetic and thermodynamic data of the adsorption process were then evaluated to study the adsorption mechanism of MB and OII molecules on activated carbon. 2. Materials and methods 2.1. Materials The activated carbon (AC) F-400 was purchased from Calgon, France. Activated carbon was washed several times with water to remove surface impurities, followed by drying at 100 ◦ C for 48 h. The AC was ground and sieved through a five pieces sieve set pans with the following openings: 1.19, 1.00, 0.883, 0.589 and 0.500 mm. The sieved AC was then stored in a container. The basic dye, Methylene blue, and the acidic one, Orange II, were purchased from Sigma–Aldrich and used without further purification. The main characteristics of dyes used in this work and


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Nomenclature initial concentration of dye (mg L−1 ) concentration of dye in solution after equilibrium (mg L−1 ) V total volume of the solution (mL) W mass of the adsorbent (mg) qe:exp amounts of dye adsorbed at equilibrium time, experimental value (mg g−1 ) qe:calc amounts of dye adsorbed at equilibrium time, calculated value (mg g−1 ) qt amounts of dye adsorbed at time t (mg g−1 ) k1 pseudo-first order rate constant (L g−1 min−1 ) k2 pseudo-second order rate constant (g min−1 L−1 ) amount of solute adsorbed per weight of adsorbent qm in forming a complete monolayer on the surface (mg g−1 ) Langmuir constant related to the energy (L mg−1 ) kL kf and nf Freundlich constants N agitation speed

C0 Ce

meter Crison). The suspensions containing different doses of AC and the solutions of dyes were shaken with a magnetic stirrer. Samples were taken at different time intervals to carry out the kinetic study. The equilibrium concentrations of each solution were determined by means of the solution absorbance measured by a spectrophotometer (Shimadzu UV-2401PC) at 631 nm for MB and 485 nm for OII, respectively. These  values permit accurate concentration measurements so the concentration–absorbance ratio is linear for a wider experimental concentration range than using max . The amount of MB and OII adsorbed on the activated carbon surface was determined by a dye mass balance. The amount of adsorbed MB and OII at equilibrium, qe (mg g−1 ) was calculated by: qe =

(C0 − Ce )V W


where C0 and Ce (mg L−1 ) are the initial liquid-phase and equilibrium concentrations of the dye, respectively. V is the volume of the solution (L) and W is the mass of adsorbent used (g). The adsorption of MB and OII on the activated carbon was also evaluated at constant temperature of 30 ◦ C for the adsorption isotherms. The procedures for the kinetic experiments were basically identical to those of equilibrium tests.

the structure are shown in Table 1. The solutions of dyes were prepared by dissolving accurately weighed amounts of Methylene blue and Orange II respectively in distilled water.

3. Results and discussion

2.2. Characterization of adsorbent

Fig. 1 shows the nitrogen adsorption and desorption isotherms at 77 K. The properties of activated carbon determined in this study were listed in Table 2. The micropore volume is 0.26 (cm3 g−1 ), which was calculated applying the t-plot method to the experimental N2 adsorption data (Table 2). AC exhibited a narrow pore size distribution and was essentially microporous. It is noted that micropores area is often the major contribution to the adsorption capacity for adsorbate molecules small enough to penetrate. However, transport within these pores can be severely limited by steric effects [11]. For example, MB molecule has a minimum molecular diameter of about 0.8 nm and it has been estimated that the minimum diameter of pores that allow this molecule to enter in is 1.3 nm [12]. Therefore, MB molecules can only enter in the largest micropores, but most of it is likely to be adsorbed in mesopores. Fig. 2 shows the thermogravimetric (TG) and differential thermogravimetric (DTG) curves obtained from AC under inert conditions. The typical TG curves of AC heated in a helium atmosphere

Textural characterization of the activated carbon was done by using N2 adsorption–desorption at 77 K in a Micromeritics ASAP 2010 apparatus as described in a previous work. IR spectra were recorded in the range 400–4600 cm−1 using a Thermo Nicolet FTIR spectrophotometer. Thermogravimetric analysis (TGA) was performed with an EXTAR 6000 Seiko thermal analyzer at a heating rate of 10 ◦ C/min in helium flow (20 mL min−1 ). The morphology of the activated carbon was analyzed by scanning electron microscopy (SEM), with a JEOL JSM 6400 electron microscopy at 22 keV. 2.3. Adsorption of OII and MB Adsorption studies were performed in 250 mL glass vessels. The temperature was controlled at ±1 ◦ C. The pH was adjusted adding a small amount of dilute HCl or NaOH solution using a pH meter (pH

3.1. Characterization of AC

Table 1 Main characteristics and structures of dyes used. max (nm)

Chemical class

C.I. name

C.I. number

Molecular weight (g mol−1 )

Dye content (%)

Molecular formulae

Methylene blue (MB)

Cationic dye

Basic blue 9




C16 H18 ClN3 S·3H2 O 631

Orange II (OII)

Anionic dye

Acid Orange A




C16 H11 N2 NaO4 S




A. Rodríguez et al. / Journal of Hazardous Materials 172 (2009) 1311–1320


Fig. 1. Adsorption/desorption isotherms of N2 for the activated carbon. Fig. 3. FTIR of the activated carbon. Table 2 Physical and chemical characteristics of the adsorbent. 2



SBET (m g

Activated carbon F-400





SEXT (m g 384.0




Vmicro (cm g





Basicity (␮equiv. g−1 )

Acidity (␮equiv. g−1 )

Activated carbon F-400




The morphology of activated carbon was studied by SEM. Fig. 4 shows different views of activated carbon. Pores of different size and different shape could be observed in these figures. Table 2 also shows the point of zero charge (pHPZC ), defined as the pH below which the total surface of the carbon particles is positively charged, as well as the acidity and basicity of the activated carbon. The results indicated that the pHPZC was 7.63. The

consist of two stages corresponding to dehydration, and active pyrolysis, respectively. The weight loss of active carbon started around 475–500 ◦ C and continued to 900 ◦ C, probably related to the decomposition of more stable surface oxygen groups such as ketones, ethers and hydroxyls originally present in the structure of the activated carbon [13]. Fig. 3 shows FTIR spectra of activated carbon. The most significant bands are in the regions of 2920, 2850, 1629, 1558, 1461, 1118 and around 670 cm−1 . It can be noticed that for the activated carbon the peaks at 2920 and 2850 cm−1 are observed and their reason is the aliphatic groups (asymmetrical and symmetrical stretch of CH3 ). The bands located around 1629 and 1558 cm−1 corresponds to an aromatic carbon or carbonyls, respectively (stretch of C C in aromatic rings and stretch of C O). The band at 1461 cm−1 is characteristic of carboxylic groups. Hydroxyls absorbing at 1118 cm−1 are also present (OH stretch) [14].

Fig. 2. TGA and DTG curves of the activated carbon at 10 ◦ C/min under 20 sccm helium flow rate.

Fig. 4. SEM micrographs showing general appearance of activated carbon.


A. Rodríguez et al. / Journal of Hazardous Materials 172 (2009) 1311–1320

adsorption capacity of MB, showed the maximum value at pH 7. The reduction of MB removal at pH values higher than 7 may be ascribed to the increasing repulsive forces between surface functional groups of AC and MB, that mainly exists as anion form [15,16].

Fig. 5. Effect of initial concentration for the adsorption of Methylene blue on activated carbon (W/V = 2 mg mL−1 , T = 30 ◦ C, pH 7.0).

surface acidity or basicity of the activated carbon affects the pH of the solution and therefore the adsorption of dye (MB or OII) on the activated carbon. The amphoteric nature of carbon mainly depends on the surface functional groups. Cationic adsorption is favoured at pH > pHPZC and anionic adsorption is favoured at pH < pHPZC due to the surface net charge of the solid.

3.2.2. Effect of temperature on adsorption capacity Fig. 7a shows the equilibrium adsorption of MB (qe vs. Ce ) on activated carbon for three temperatures (30, 40 and 65 ◦ C). The shape of the isotherms indicates high affinity between adsorbent surface and dye molecules. The experimental equilibrium points rise sharply in the initial stages, low Ce and qe values, thus indicating that there are plenty of readily accessible sites and great affinity of the activated carbon for the MB molecules. The adsorbent is saturated when the plateau is reached. The decrease in the slope of the isotherm is due to the fewer active sites available at the end of the adsorption process. The AOII adsorption isotherms, Fig. 7b, show similar tendencies than MB ones. It was clear that the AC had a considerable affinity for the cationic dye with an approximate monolayer saturation capacity of 700 mg g−1 , and a lower affinity for the anionic one, leading to a saturation capacity of around 400 mg g−1 . Furthermore, it can be observed that the adsorption capacity increased with increasing temperature, thus indicating that the adsorption of these dyes on activated carbon was endothermic in nature.

3.2. Adsorption process of MB and OII molecules on activated carbon: equilibrium The adsorption isotherms of MB and OII (not shown) on AC were obtained plotting the amount of dye adsorbed on AC, qe vs. the equilibrium concentration of dye in liquid-phase, Ce . That was done for two initial concentration, C0 = 300 and 500 mg L−1 (Fig. 5) and it can be observed that initial concentration does not affect adsorption capacity. Adsorption equilibrium is well described by a monolayer Langmuir type isotherm for both dyes. 3.2.1. Effect of pH on MB and OII adsorption Fig. 6 shows the effect of pH on the removal of MB and OII by AC adsorption. The initial concentration of MB and OII was 500 mg L−1 . The adsorption of dyes was strongly pH dependent. The highest OII adsorption capacity was experimentally observed at pH 3.0, this capacity drastically decreased for higher pH values. However, the

Fig. 6. Effect of pH for the adsorption of Methylene blue and Orange II on activated carbon (C0 = 300 ppm, W/V = 2 mg mL−1 , T = 30 ◦ C).

Fig. 7. Effect of temperature for the adsorption of (a) Methylene blue and (b) Orange II on activated carbon.

A. Rodríguez et al. / Journal of Hazardous Materials 172 (2009) 1311–1320

3.2.3. Adsorption isotherms modelling Four common adsorption equilibrium models, Langmuir, Freundlich, Toth and Redlich–Peterson (RP), were used to fit the experimental data for MB and OII adsorption equilibrium in aqueous solution on AC at three different constant temperatures, 30, 40 and 65 ◦ C and a initial dye concentration, C0 = 500 mg L−1 . Langmuir isotherm theory is based on the assumption of adsorption on a homogeneous surface [17,18]. The Langmuir equation can be written in the following form: kL Ce qe = qm 1 + kL Ce


where qe is the solid phase equilibrium concentration (mg g−1 ), Ce is the liquid-phase equilibrium concentration of dye (mg L−1 ), kL is the equilibrium adsorption constant related to the affinity of binding sites (L mg−1 ), and qm is the maximum amount of dye per unit weight of adsorbent for complete monolayer coverage. The following Freundlich equation was also used to describe the equilibrium data:

 Ln qe = Ln kf +

1 nf

 Ln Ce


where kf and nf are characteristic constants. The Toth model is derived from potential theory and is applicable to adsorption on heterogeneous surfaces. This equation describes well many systems with sub-monolayer coverage and reduces to Langmuir equation when t = 1. The Toth equation can be written in the following form: kT Ce

qe =

[aT + Cet ]



The linear form of Eq. (4) is: Ln

q  e


= Ln Ce −

1 Ln(aT + C t ) t


The Redlich–Peterson (R–P) isotherm is more versatile than the Langmuir and Freundlich isotherms, because can be applied either in homogeneous or heterogeneous systems. R–P equation is represented by Eq. (6) where kR and aR are constants (L mg−1 ), and ˇ is an exponent which change between 0 and 1. This isotherm combines elements from both the Langmuir and Freundlich equations, and the mechanism of adsorption is a hybrid and does not follow ideal monolayer adsorption. The isotherm has a linear dependence on concentration in the numerator and an exponential function in


the denominator. qe =

kR Ce



1 + aR Ce

The linear form of Eq. (6) is:


k C R e qe

− 1 = Ln aR + ˇ Ln Ce


The characteristic parameters of the different models at several temperatures as well as the correlation coefficients R2 are listed in Table 3. As example, Fig. 8a and b shows Langmuir, Freundlich, Toth, and R–P isotherms and the experimental data obtained at 30 ◦ C for Methylene blue and Orange II, respectively. It can be seen that all models provided a good fit (Fig. 8). The correlation coefficients calculated for the Langmuir equation fitting are slightly lower than the ones obtained in the case Freundlich equation is used. However, we think that Langmuir describes better the experimental system, due to the experimental data reaches a saturation plateau at high Ce . This saturation tendency is not included in Freundlich model. 3.3. Adsorption kinetics of MB and OII on activated carbon Generally, four steps are involved during the adsorption process onto porous adsorbents: (a) external mass transfer in bulk liquidphase (b) boundary layer diffusion; (c) intraparticle mass transfer within particle; and (d) sorption on active site. Sorption may be considered instantaneous and boundary layer diffusion negligible, hence, the main resistances to mass transfer are external and intraparticle diffusions. These aspects were checked with experiments at two stirring speeds (Nlow and Nmedium ) and three adsorbent particle sizes (1.19–1.00, 0.883–0.589, 0.589–0.500 mm). Fig. 9a (MB) and b (OII) shows the effect of particle size and stirring speed on the MB and OII percentage uptake at different contact time, 30 ◦ C, an initial concentration of dye of 500 mg L−1 and W/V = 0.8 mg mL−1 . It can be observed that the percentage dye uptake increased with contact time, till an almost constant value is reached. In this point the amount of dye being removed from aqueous solution on the activated carbon is in a state of dynamic equilibrium with the amount of dye desorbed from the activated carbon. No differences in dye uptakes were observed between the two stirring speed experiments, indicating that the solutions were well mixed so external mass transport limitations were negligible. However, the smaller the particle size, the faster the adsorption processes were. Low particle size adsorbent presents a more acces-

Table 3 Langmuir, Freundlich, Toth and Redlich–Peterson constants related to the adsorption isotherms of Methylene blue and Orange II by activated carbon and correlation coefficients. T (◦ C)



Langmuir qm (mg g

Methylene blue

Orange II



kL (L mg


kf (L g−1 )






469.6 581.5 708.8

4.08 0.99 0.59

0.962 0.995 0.989

346.9 434.6 427.2

13.35 16.90 9.24

0.992 0.970 0.978

30 40 65


384.3 468.6 568.5

0.54 0.17 0.49

0.996 0.945 0.848

328.3 247.7 286.5

37.11 8.67 6.74

0.993 0.979 0.968

T (◦ C)

30 40 65

Freundlich −1

30 40 65


Toth kT (mg g

Methylene blue



684.6 569.1 769.5

Redlich–Peterson −1


aT (mg L 0.12 3.50 0.63





kR (L g−1 )

aR (L mg−1 )



0.16 1.63 0.57

0.995 0.998 0.997

777.0 403.9 620.3

21.27 0.59 1.06

0.94 1.04 0.96

0.995 0.999 0.996



A. Rodríguez et al. / Journal of Hazardous Materials 172 (2009) 1311–1320

Fig. 8. Comparison experimental and calculated equilibrium adsorption capacity using several isotherm models for the adsorption on activated carbon of (a) Methylene blue and (b) Orange II (V = 250 mL, T = 30 ◦ C, pH 7.0).

sible surface, which increases dye adsorption rate at the initial stages. As time proceeds, the concentration gradient is reduced due to accumulation of dye particles on the vacant sites, leading to decreasing adsorption rate at the later stage of the adsorption. The higher adsorption rate at the initial period may be due to a large number of vacant sites available that makes more intense the adsorbate concentration gradient between solution and adsorbent surface [19–21]. For both dyes, the adsorption was found to be extremely rapid at the initial stage and slows down as the adsorption proceeds. For the contact time range under which the experiments were conducted, the percentage of OII from solution was higher than for MB. At 100 min, OII removal percentage was 13.5% for particle size class 1.19–1.00 mm, 20.0% for particle size class 0.883–0.589 mm, and 25.5% for particle size class 0.589–0.50 mm (Nmedium ). The MB removal percentage was 12.0%, for particle size class 1.19–1.00 mm, 17.0% for 0.883–0.589 mm and 24%; for 0.589–0.50 mm. In conclusion, despite of the higher equilibrium capacities reached for the adsorption of MB on AC as compared with OII, the kinetic results revealed that the adsorption of OII on AC of different particle sizes was faster than MB adsorption from a solution of the same concentration. In order to investigate the adsorption kinetics of cationic and anionic dyes on activated carbon, five equations have been tested: pseudo-first order, pseudo-second order, intraparticle diffusion, Bangham and Chien–Clayton models.

Fig. 9. Effect of particle size on the percentage uptake rate at different contact time (C0 = 300 mg L−1 , V = 250 mL, T = 30 ◦ C). (a) Methylene blue at pH 7.0 and (b) Orange II at pH 3.0.

A simple kinetic analysis of adsorption is the first order rate expression given by Lagergren: log(qe − qt ) = log qe −

k1 t 2.303


where qe and qt are the amounts of adsorbate adsorbed (mg g−1 ) at equilibrium and at time t in min, respectively, and k1 is the rate constant of adsorption (min−1 ). Values of k1 were calculated from the plots of log(qe − qt ) vs. t. The experimental qe values did not agree with the calculated ones, obtained from the linear plots (Table 4), indicating that the first order model does not reproduce the adsorption kinetics of MB and OII on AC. The second order kinetic model is expressed as: 1 1 t = + t qt qe k2 q2e


where k2 (min g mg−1 ) is the rate constant of second order adsorption. The parameters k2 and qe can be obtained from the plot of (t/qt ) vs. t, that should show a linear relationship. The equilibrium adsorption capacity, qe , can be calculated from Eq. (8). Also, this model is more likely to predict the behaviour over the whole experimental range of adsorption than pseudo-first order model. The correlation coefficients are close to 1.0 for both dyes (Table 5). The calculated qe values fit quite well with the experimental data. The intraparticle diffusion model can be described by the following equation: qt = kp t 0.5 + C


Methylene blue Orange II

7.0 3.0

42.21 89.71

25.17 194.92

−35.5 −30.6

0.981 0.959 54.38 57.09

50.10 52.36




G (KJ mol−1 )


where kr is the rate constant of adsorption and 1/m is an indicator of the adsorption intensity. qt and 1/m can be determined from the linear plot of ln(qt ) vs. ln(t). Parameters of the Bangham model were listed in Table 4. The Elovich equation is generally expressed as follows: (12)

To simplify the Elovich Eq. (12), Chien and Clayton (C–C) assumed ˛ˇt  1 and by applying the boundary conditions qt = 0 at t = 0 and qt = qt at t = t Eq. (12) becomes:


4.59 1.00

S (J mol−1 K−1 )

0.0031 0.0041

0.004 0.047

R2 ˇ (g mg−1 ) ˛ (mg g−1 min−1 )

Chien–Clayton kinetic model

0.995 0.998

1.000 0.999 1.813 1.830 0.209 0.221 0.998 0.972 0.117 0.072

11.33 35.29

H (kJ mol−1 )

dqt = ˛ exp(−ˇqt ) dt


12.61 8.08


qt = kr t 1/m




where kP is the intraparticle diffusion constant (mg g−1 min−0.5 ). The slope of the linear part of the curve (qt vs. t0.5 ), gives the rate of adsorption, controlled by intraparticle diffusion. The extrapolation of the linear straight lines to the time axis gives intercepts C. If C is equal to zero, the only controlling step is intraparticle diffusion. However, if C = / 0, indicates that the adsorption process is rather complex and involves more than one diffusive resistance. The values of the diffusion rate constant, C and the regression coefficients R2 are given in Table 4. The Bangham kinetic model is expressed as


kP (mg g

376 384

Intraparticle diffusion kinetic model

104 342

272 174 315 407

0.975 0.997 0.0004 0.1237 39.20 35.33



0.972 0.975

0.927 0.987

R ) qe:calc (mg g )

Table 5 Thermodynamic parameters of Methylene blue and Orange II adsorption onto activated carbon at 30 ◦ C.

qt =

1 1 Ln(˛ · ˇ) + Ln t ˇ ˇ


where ˛ is the initial adsorption rate (mg g−1 min−1 ), ˇ is the desorption constant (g mg−1 ) for Chien–Clayton equation during any one experiment. The plots of qt vs. ln(t) for the C–C model for the adsorption of MB and OII on AC have been drawn to obtain the rate parameters. The correlation coefficients (R2 ), for the C–C kinetic model are between 0.995 and 0.998 in MB. Pseudo-first order, pseudo-second order, intraparticle diffusion, Bangham and Chien–Clayton kinetic models are illustrated in Figs. 10–14. As it can be seen, these adsorption systems follow a pseudo-second order kinetic model.


qe:exp (mg g

0.984 0.906

m kr (mg g−1 min−1 )

Bangham kinetic model


1.000 0.999 382 384

2.45 × 10 1.02 × 10−5

7.15 × 10−6 1.50 × 10−5

376 384

0.997 0.999 320 409 315 407

R2 qe:calc (mg g−1 ) qe:exp (mg g−1 )


k2 (g min−1 L−1 )

30 40

30 40 Orange II

30 40 Orange II

Methylene blue

30 40 Methylene blue


T (◦ C)


pH T (◦ C) Dye

30 40 Orange II


0.004 0.002 30 40 Methylene blue


0.0003 0.0013

min k1 (L g


Second order kinetic model


−1 −1


−1 −1

First order kinetic model pH T (◦ C) Dye

Table 4 Comparison of the first and second order, intraparticle diffusion, Bangham and Chien–Clayton rate constants obtained for Methylene blue and Orange II adsorption on activated carbon at several temperatures and pHs, W = 0.3 g.

A. Rodríguez et al. / Journal of Hazardous Materials 172 (2009) 1311–1320

Fig. 10. Pseudo-first order kinetic plot for the adsorption of Methylene blue and Orange II on activated carbon.


A. Rodríguez et al. / Journal of Hazardous Materials 172 (2009) 1311–1320

Fig. 11. Pseudo-second order kinetic plot for the adsorption of Methylene blue and Orange II on activated carbon.

Fig. 14. Chien–Clayton kinetic model plot for the adsorption of Methylene blue and Orange II on activated carbon.

Finally, in order to test the goodness of the pseudo-second order model, k2 evolutions were studied at different pH (Fig. 15a), particle size (Fig. 15b) and adsorbent dose (Fig. 14c) for MB and OII adsorption on AC. It can be observed that the rate constant decreased as particle size increases for both dyes (Fig. 15b) and increased with pH (Fig. 15a). Adsorbent dose has no effect on k2 in the case of OII adsorption. However in the MB system, k2 increases as W does (Fig. 15c). This can be due to steric considerations, MB molecules are bigger than OII ones. Finally it can be observed that temperature has a positive effect on k2 in OII adsorption (Fig. 15d). 3.4. Adsorption thermodynamics

Fig. 12. Intraparticle diffusion kinetic model plot for the adsorption of Methylene blue and Orange II on activated carbon.

The thermodynamic parameters that must be considered to determine the process are enthalpy of adsorption (H), free energy change (G) and entropy change (S) due to transfer of unit mole of solute from solution on the solid–liquid interface [22–24]. The positive value of H indicates the endothermic process and negative value indicates the exothermic process. The parameter S are used to identify the spontaneity in the adsorption process. The value of H and S were determined using the equation as follows: Ln kL =

S H − R RT


where kL is the Langmuir constant, L mol−1 , T, and R are temperature and universal gas constant, respectively. The calculated H and S at 30 ◦ C were shown in Table 5. The positive value of H (in MB and OII) indicates the endothermic nature of the adsorption process; as it was indicated when the effect of temperature on adsorption capacities was studied. Besides, H corresponds to chemisorption due to it is higher than 40 kJ mol−1 . This value indicates the formation of strong chemical bonds between the dye molecules and the adsorbents [25]. The positive value of S reflects the affinity of activated carbon particles for MB and OII. Another important thermodynamic parameter involved in the adsorption process is the free energy change and can be calculated using the relation: G = −RT Ln(kL )

Fig. 13. Bangham kinetic model plot for the adsorption of Methylene blue and Orange II on activated carbon.


The negative value of G (Table 5), indicating that the adsorption process leads to a decrease in Gibbs free energy, confirms the feasibility of the process and the spontaneous nature of the adsorption with a high preference of OII on AC. These values are consistent with the obtained by Karagozoglu et col. for the adsorption of a

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Fig. 15. Influence of (a) pH, (b), particle size, (c) adsorbent doses and on rate constant (k2 ), for Methylene blue and Orange II adsorption; (d) Influence of temperature on rate constant (k2 ) for Orange II adsorption.

basic dye, Astrazon Blue FGRL, onto different adsorbents, where the G value was between −12.73 and −17.76 kJ mol−1 , and −12.70 and −13.10, for sepiolite and AC, respectively [25]. Furthermore, the electrostatic attraction enhances the adsorption forces on the carbon surface. Consequently, the association, fixation, or immobilization of dye molecules on the carbon surface reduces the degree of freedom of the dye molecules. In the electrostatic repulsive force field, the enthalpy and entropy changes are positive but very little. In this state, the charged carbon surface repulses the ionic dye that has the same charge as the surface. It suggests that the system must draw some energy from environment to overcome the repulsive force to move ionic dye closer to the carbon surface. Therefore, these adsorption processes are endothermic in nature. The electrostatic repulsion weakens the adsorption force and increases the degree of freedom of the dye molecules on the carbon surface. 4. Conclusions The main conclusions are as follows: 1. The maximum adsorption of MB and OII at 65 ◦ C on the activated carbon was obtained at pH 7.0 and 3.0, respectively. The adsorption of dyes was found to be endothermic, that means adsorption capacities increase as temperatures do in the experimental range studied (30–65 ◦ C). 2. The behaviour of the isotherms might be described as type L, with a large increase in adsorption capacities at low adsorbate concentrations until the system reached a plateau. Several models (Langmuir, Freundlich, Toth and Redlich–Peterson) were used to fit the experimental data point. 3. The main controlling resistance in mass transfer during the adsorption of MB and OII on AC was intraparticle diffusion, as stirring speed and particle size experiments showed.

4. The kinetic modelling of the MB and OII adsorption on the activated carbon adsorbent indicate that adsorption process is pseudo-second order with the correlation coefficients higher than 0.999. 5. Thermodynamic constants were also evaluated using equilibrium constants at different temperatures. The positive values of H, S and G showed the endothermic and spontaneous nature of the adsorption process. The results of present work show that activated carbon F-400 is an effective adsorbent for cationic and anionic dyes as Methylene blue and Orange II, due to its high adsorption capacity. Acknowledgements The authors gratefully acknowledge the financial support from Ministerio de Educación y Ciencia by CONSOLIDER Program through TRAGUA Network CSD2006-44, CTQ2008-02728 and Ph D Grant (FPU AP2007-01198) and Comunidad de Madrid through REMTAVARES Network S-505/AMB/0395. References [1] G. Chen, L. Lei, X. Hu, P. Lock, Kinetic study into the wet air oxidation of printing and dyeing wastewater, Sep. Purif. Technol. 31 (2003) 71–76. [2] J.L. Sotelo, G. Ovejero, J.A. Delgado, I. Martinez, Adsorption of lindane from water on GAC: effect of carbon loading on kinetic behaviour, Chem. Eng. J. 87 (2002) 111–120. [3] A.A. Attia, B.S. Girgis, S.A. Khedr, Capacity of activated carbon derived from pistachio shells by H3 PO4 in the removal of dyes and phenolics, J. Chem. Technol. Biotechnol. 78 (2003) 611–619. [4] A. Aygun, S. Yenisoy-Karakas, I. Duman, Production of granular activated carbon from fruit stones and nutshells and evaluation of their physical, chemical and adsorption properties, Micropor. Mesopor. Mater. 66 (2003) 189– 195. [5] J. Guo, A.C. Lua, Textural and chemical characterizations of activated carbon prepared from oil-palm stone with H2 SO4 and KOH impregnation, Micropor. Mesopor. Mater. 32 (2007) 111–117.


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