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Evaluation of Adsorption Characteristics of Multiwalled Carbon Nanotubes Modified by a Poly(propylene imine) Dendrimer in Single and Multiple Dye Solutions: Isotherms, Kinetics, and Thermodynamics Ladan Eskandarian, Mokhtar Arami,* and Elmira Pajootan Textile Engineering Department, Amirkabir University of Technology, 424 Hafez Ave, Tehran, 15875-4413 Iran ABSTRACT: This research investigates the isotherm, kinetics, and thermodynamic parameters of the adsorption of two direct dyes in single and multiple dye solutions onto the modified multiwalled carbon nanotubes using poly(propylene imine) dendrimer (CNTDen). The surface morphology and functional groups of the prepared CNT-Den were studied using field emission scanning electron microscopy (FESEM) images and Fourier transform infrared (FTIR) spectra, respectively. The Langmuir, Freundlich, and Tempkin isotherms were studied, and the results indicated that the data followed the Langmuir isotherm. The kinetic studies revealed that the pseudo-second order model was best fitted to our results rather than pseudo-first and intraparticle diffusion model. The calculated thermodynamic parameters showed the endothermic and spontaneous nature of the adsorption process. The overall results illustrated that CNT-Den can be effectively used as an adsorbent for the removal of dyes from colored wastewaters especially textile effluents.

1. INTRODUCTION Water is the source of life, and still today, all around the world, far too many people spend their entire day searching for it. The Middle East is one of the most arid regions on the earth, in which water is often scarce, and over the last century its consumption has increased faster than the population. Water scarcity is either the lack of enough water (physical scarcity) or the lack of access to safe water (economic scarcity), which is a growing problem and a severe crisis that will only lead to political and social instability with its scarcity. Recommendations for its improvement will reveal the role of industries especially the textile industry which is one of the essential and high water consumer parts of every society. Recycling the wastewater and its reuse as an input to the system again will diminish the problem of the water consumption obviously. Also, the accurate and complete treatment of the discharged effluent could have the quality of fresh water that would be suitable enough to be used again for human consumption. Indiscriminate disposal of dyes and organic contaminants in wastewaters is a major worldwide concern due to their adverse effect to many forms of life. Therefore, wastewater treatment requires a high quality removal followed by a secure disposal for solving the ecological, biological, and industrial problems associated with dyes.1−3 The most common methods for dye removal include chemical precipitation, solvent extraction, ion exchange, nanofilteration, reverse osmosis, ozonation, and so forth. However, these techniques have certain disadvantages such as © 2014 American Chemical Society

high operational costs or inconvenience and secondary sludge disposal problem; also, it is rather difficult to treat the textile wastewater by conventional biological and physical-chemical processes because of the complex molecular structure of dyes. The adsorption technique has been found to be an excellent way for wastewater treatment in terms of its capability for efficiently adsorbing a broad range of adsorbates and its simplicity of design; so, it offers significant advantages among several physical and chemical processes, especially from an energetic and environmental point of view.4,5 The development of new methods to implement novel materials for a desirable nanostructure adsorbent has been the subject of intensive research. Carbon nanotubes (CNTs) are attractive nanoadsorbents because of their unique properties which, in most cases, arise from the combination of structural, electrical, and chemical properties and nanosized dimensions. These nanomaterials contain a great contact area; thus, they have a higher capacity for the adsorption and transportation of contaminants. Therefore, the chances of dyes to be absorbed by CNTs are increased especially when the long-term contact occurs. But this purpose will be effective and positive when the adsorbent is perfectly dispersed in adsorbate’s solution. One of the major points encountered in the investigation of raw CNT usage as an adsorbent is their hydrophobic structure and a Received: October 12, 2013 Accepted: January 7, 2014 Published: January 13, 2014 444

dx.doi.org/10.1021/je400913z | J. Chem. Eng. Data 2014, 59, 444−454

Journal of Chemical & Engineering Data

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Figure 1. Structure and pH stability of DR23 and DB86.

according to the obtained results. The adsorbent characteristics have been studied using field emission scanning electron microscopy (FESEM) images and Fourier transform infrared (FTIR) analysis. Wastewater treatment by CNT-Den is very fast and simple; therefore, in this study the adsorption isotherms, kinetics, and thermodynamic parameters for both single and multiple (binary) dye solutions were investigated. Also, the prepared CNT-Den and previously studied raw and/ or modified nanotubes from other studies were compared based on their enthalpy, entropy, and the amount of adsorbed pollutant at the equilibrium condition.

tendency to aggregate, which increases with the enlargement of the surface area that leads to inadequate dispersion in aqueous solutions. On the other hand, one of the promising functionalized molecules that could enhance the dispersion and separation of CNTs is dendrimer molecules.6−8 Dendrimers, a unique class of polymers, have been regarded as significant promising molecules in many fields because of their identical groups, highly branched molecular structure, and controllable size and shape. The initial studies are concerned about the synthetic aspects of dendrimers, but efforts are now mainly directed toward their applications. Their unique structural features hold out the promise of many applications in the fields of nanoelectronics and electrochemical (bio) sensors, carriers of active substances, and insulating materials or as in this study, they can be used as a modifying agent to cover the surface of CNTs as a powerful adsorbent.6,9−11 Adsorption processes are usually described through an isotherm that is the amount of adsorbate on the adsorbent as a function of its concentration at constant temperature. The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow the comparison of different materials. Also, the study of kinetics of the adsorption processes is necessary to identify the time needed for the treatment of certain amount of wastewater having a certain adsorbate concentration. Most of the sorption/desorption transformation processes of various solid phases are time-dependent. In order for us to understand the dynamic interactions of pollutants with solid phases and to predict their fate with time, knowledge of the kinetics of these processes is important. On the other hand, thermodynamic considerations of an adsorption process are essential to conclude whether the process is spontaneous or not. The most important application of thermodynamics for adsorption is the calculation of phase equilibrium between a liquid mixture and a solid adsorbent.12 The present study indicates the suitability of the low cost eco-friendly super adsorbent which has higher removal efficiency rather than CNTs for dye removal purposes

2. EXPERIMENTAL SECTION 2.1. Synthesis of Adsorbent. Carboxyl functionalized multiwalled carbon nanotubes (CNT-COOH) (purity > 95 %, length (10 to 20) μm and diameter (30 to 50) nm) was purchased from Neutrino and poly(propylene imine) (PPI) dendrimer (Generation 2, molecular weight: 770 g·mol−1, and molecular formula: C40H94N14) was supplied by BVSYMOChem. The CNT-Den adsorbent was prepared by the addition of 30 mL PPI dendrimer of generation 2 (500 mg·L−1) solution and 0.1 g of CNT-COOH to 70 mL of distilled water at pH 3. The resulted solution was sonicated for 60 min using Delta D68H Ultrasonic bath and then stirred at 200 rpm for 24 h. The solution was centrifuged at 5000 rpm for 10 min, and the adsorbent was dried in air at room temperature (25 °C). 2.2. Adsorption Experiments. C.I. Direct Blue 86 (DB86) and C.I. Direct Red 23 (DR23) were purchased from Ciba Ltd. Other chemicals of analytical grade were purchased from Merck. The pH of the solutions was adjusted by adding a small amount of H2SO4 (0.1 mol·kg−1) or NaOH (0.1 mol·kg−1). The structures of DR23 and DB86 and their absorbance spectra at different pH values are shown in Figure 1. It can be concluded that changing the pH of solutions does not have any effect on dyes structures, and their maximum absorbance wavelengths do not shift to higher or lower values, which means 445



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Figure 2. FESEM images of CNT-Den (a) before and (b) after dye adsorption.

electron microscope ((FESEM) JSM-6700F, JEOL, Japan) operated at the voltage of 20.0 kV.

that the dye molecules maintain their structures and do not degrade. Therefore, all of the obtained results are just affected by the removal of dyes during adsorption process, not by chemical degradation or hydrolysis relating to the pH of solutions. The adsorbent was added to the dye solutions from the prepared sonicated solution containing 1 g·L−1 as-produced adsorbent. The adsorption process using CNT-Den as adsorbent for the removal of DB86 and DR23 from single and binary systems was performed by the addition of the adsorbent [(0.015, 0.025, 0.05, 0.075, 0.1, and 0.125) g·L−1] into 200 mL dye solutions [(50, 75, 100, and 150) mg·L−1] at various pH values (3 to 11) for 60 min at 25 ± 1 °C. In case of binary solutions the initial dye concentration was adjusted by mixing the DR23−DB86 1:1. During the experiments, samples were collected at different time intervals [(2.5, 5, 10, 15, 20, 25, 30, 40, 50, and 60) min], centrifuged by Hettich EBA20 at 5000 rpm for 10 min and then analyzed for dye removal using vis spectrophotometer (Unico2100). The maximum wavelengths (λmax) used for DR23 and DB86 in solutions were 504 nm and 631 nm, respectively. The dye removal efficiency was calculated using eq 1: ⎡A − A⎤ dye removal(%) = ⎢ 0 ⎥ ·100 ⎣ A0 ⎦

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbent. FESEM images of CNT-Den and dye loaded CNT-Den indicated the adsorption of dye molecules on the adsorbent surface that possesses a welldefined structural variation in Figure 2a. The CNT-Den before adsorption showed a porous structure, but the case after adsorption (Figure 2b) showed that the surface of the nanotubes has been covered by the dye molecules. The FTIR spectra of CNT-Den before and after the adsorption of DB86 molecules are illustrated in Figure 3. The broadened band around 3420 cm−1 is observed in all curves, which can be assigned to the water adsorption of all samples. The peaks appearing at 2920 cm−1 and 2850 cm−1 in all spectra can be assigned to the aliphatic C−H stretching bond, the

(1)

where A0 and A are the absorbance of dye before and after the adsorption, respectively.13 The adsorption isotherms and kinetic experiments were performed in 200 mL of solution containing (50, 75, 100, and 150) mg·L−1 of DR23 and DB86 at pH 3 for 30 min. The experiments began by the addition of (0.09 and 0.1) g·L−1 CNT-Den for DB86 and DR23 in single solutions, respectively, and 0.1 g·L−1 CNT-Den in binary solutions. The thermodynamic parameters were investigated in 200 mL of solution of 0.075 g·L−1 CNT-Den and 50 mg·L−1 DR23 and DB86 at pH 3 for 30 min. The solution temperature was varied in the range of (298 to 328) K. 2.3. Adsorbent Characterization. The Fourier transform infrared (FTIR) spectroscopy of CNT-Den before and after the dye removal was measured using a Thermo Nicolet Avatar 360 FTIR spectrometer within the range of (500 to 4000) cm−1. The surface morphology of CNT-Den before and after the dye adsorption was observed by a field emission scanning

Figure 3. FTIR spectra of CNT-Den before and after dye adsorption. 446



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Figure 4. Effect of pH (dye concentration: 50 mg·L−1, time: 60 min, and adsorbent dosage: 0.09 g·L−1 for DB86 and 0.1 g·L−1 for DR23 and binary solutions) and adsorbent dosage (dye concentration: 50 mg·L−1, time: 60 min, and pH: 3) on dye removal (%).

Figure 5. Effect of dye concentration on dye removal (%) (adsorbent dosage: 0.09 g·L−1 for DB86 and 0.1 g·L−1 for DR23 and binary solutions, time: 60 min, and pH: 3) and pseudo-second order kinetic model for the adsorption of DR23 and DB86 from single and binary dye solutions.

the surface of CNT-Den were positively charged due to the protonation, and as a result, the electrostatic interactions between the anionic dye and CNT-Den surface were raised and more dye molecules were adsorbed on the CNT-Den adsorbent.14,15 3.3. Effect of Adsorbent Dosage. The adsorption of dyes on CNT-Den was studied by changing the adsorbent dosage of [(0.015 to 0.1) g·L−1] and [(0.015 to 0.125) g·L−1] for DB86 and DR23 for a single system, respectively, and [(0.015 to 0.125) g·L−1] for binary system, with the dye concentration of 50 mg·L−1 at room temperature and pH of 3. It was seen that, with increasing the adsorbent dosage, the dye removal efficiencies increased. The optimum CNT-Den concentration was selected as 0.09 g·L−1 for DB86 and 0.1 g·L−1 for DR23 in single and 0.1 g·L−1 in binary solutions, for 200 mL of 50 mg· L−1 dye solution. The increase in the adsorption with the increase in adsorbent dosage can be attributed to the increased CNT-Den surface area and the availability of more adsorption sites (Figure 4).

intensity of which is higher in CNT-Den after dye adsorption, probably due to the existence of dye molecules on the surface of the adsorbent. The corresponding peaks of the amide (−CO−NH−) bond appeared at 1640 cm−1 in the FTIR spectra indicates that PPI dendrimers have been successfully attached to the CNT-COOH by the formation of the amide bond. The peaks relating to SO and S−O in DB86 appeared at 1123 cm−1 and 617 cm−1, respectively, and the peaks at 1394 cm−1 and 1505 cm−1 are corresponding to the aromatic CC, which appear in the CNT-Den spectra, indicating the adsorption of dye molecules on the adsorbent. 3.2. Effect of pH. The pH of the solution significantly affects the adsorption process. As shown in Figure 4, when the pH value was in the acidic range, there was a significant increase in the adsorption capacity for all dye solutions, and it could be observed that the pH 3 was the optimum value for the adsorption process because the dye removal was more than 95 % in comparison with other pH values. Such results can be explained by the fact that, at low pH values, the NH2 groups on 447



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Table 1. Isotherm Parameters for DR23 and DB86 Adsorption onto the CNT-Den at (50, 75, 100, and 150) mg·L−1 Dye Concentrations in Single and Binary Dye Solutions, Time: 60 min, pH: 3, and Adsorbent Dosage: 0.09 g·L−1 for DB86 and 0.1 g·L−1 for DR23 and Binary Solutions (KL/L·mg−1; qm/mg·g−1; KT/L·mg−1; and BT/mg·g−1) Freundlich single binary

Langmuir

dye

1/n

log Kf

R12

DR23 DB86 DR23 DB86

0.259 0.009 0.206 0.241

2.518 2.749 2.47 2.148

0.988 0.161 0.960 0.921

3.4. Effect of Initial Dye Concentration. The effect of initial dye concentration was studied in the range of (50 to 150) mg·L−1 of dye solution at pH 3, which the maximum removal efficiency was 58.4 % and 46.9 % for DB86 and 66.8 % and 74.8 % for DR23 at single and binary solutions, respectively, at the initial dye concentration of 150 mg·L−1 (Figure 5, inner diagrams). The adsorption is initially (contact time < 7 min) rapid and then slow, perhaps because a large number of vacant surface sites were available for the 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 surface of the CNT-Den and in the bulk phase.16,17 In the case of direct red 23, removal efficiency was almost higher for all of the studied concentrations. This clearly suggests that CNT-Den has more affinity for the adsorption of DR23 compared to DB86. This difference may be due to the presence of two hydroxyl and two amine groups in the structure of DR23 dye molecules which made them more accessible to the adsorption sites of CNT-Den. As it observed from Figure 5 (inner diagrams), the decrease in the removal efficiency of DB86 in the range studied was more obvious than DR23, which could be the result of the spatial prohibition in the molecular structure of DB86. In addition, the curves are single, smooth, and continuous toward the saturation, indicating the formation of monolayer coverage of dye molecules on the CNT-Den surface.18 3.5. Adsorption Isotherm Studies. In this study, the Freundlich, Langmuir, extended Langmuir, and Tempkin models were employed to describe the equilibrium adsorption. The Freundlich isotherm is derived by assuming a heterogeneous surface with a nonuniform distribution of the heat of adsorption over the surface. The Freundlich isotherm can be expressed by eq 2:19 qe = KFCe1/ n

qm

KT

BT

1000 666.667 555.55 370.370

0.998 0.994 0.999 0.998

2.195 758.772 51.663 5.213

173.03 5.490 76.436 63.201

equilibrium), and all sites are identical and energetically equivalent. The general form of the Langmuir isotherm is written as eq 4: qe /qm = (CeKL)/(1 + CeKL)

(4)

where qe is the equilibrium concentration of dyes on the adsorbent (mg·g−1), Ce is the equilibrium concentration of dyes in solution (mg·L−1), qm is the monolayer capacity of adsorbent (mg·g−1), and KL is the Langmuir adsorption constant.20 After linearization of the Langmuir isotherm, eq 5 can be obtained: Ce/qe = (1/KLQ 0) + (Ce/qm)

(5)

The extended form of the Langmuir equation can be written as:21 qe, i = (qm, iKL, iCe, i)/(1 +

∑ KL,iCe,i)

(6)

where KL,i is the adsorption equilibrium constant of ith dye in the mixed dye system. In binary dye solutions, the amount of dye adsorbed is expressed as: qe,1 = (qm,1KL,1Ce,1)/(1 + KL,1Ce,1 + KL,2Ce,2)

(7)

qe,2 = (qm,2KL,2Ce,2)/(1 + KL,1Ce,1 + KL,2Ce,2)

(8)

According to eqs 6 and 7 we have: (KL,2Ce,2)/(KL,1Ce,1) = (qm,1qe,2)/(qe,1qm,2)

(9)

After rearrangement, a linear form of the extended Langmuir model in a binary dye system is obtained: (Ce,1/qe,1) = (1/qm,1KL,1) + (Ce,1/qm,1) + (Ce,1qe,2 /qe,1qm,2)

(2)

(10)

According to eq 10, the values of Ce,1/qe,1 have a linear correlation with Ce,1 and Ce,1qe,2/qe,1qm,2 if the adsorption obeys the extended Langmuir model. By using eq 10 as the fitting model, the R22 and isotherm parameters of an individual dye in the binary dye solutions were estimated and are listed in Table 1. To determine the favorability of DR23 and DB86 adsorption, a dimensionless constant (RL) called the separation factor or equilibrium parameter was given according to eq 11.22

where KF and 1/n are the adsorption capacity at the unit concentration and adsorption intensity, respectively. 1/n values indicate the type of isotherm to be irreversible (1/n = 0), favorable (0 < 1/n < 1), or unfavorable (1/n > 1). Equation 2 can be rearranged to a linear form: log qe = log KF + (1/n)log Ce

KL 0.400 0.333 1.2 0.221

Tempkin R22

(3)

The n, KF, and R22 values for the Freundlich adsorption isotherm are shown in Table 1. The adsorption isotherm data for the removal of DR23 and DB86 were fitted with the Langmuir (single solutions) and extended Langmuir (binary solutions) adsorption isotherms. According to the Langmuir model, the adsorption takes place at the specific homogeneous sites within the adsorbent with no interaction between the adsorbent molecules. Also, the adsorbent has a finite capacity for the adsorbate (at

RL = 1/(1 + KLC0)

(11)

−1

where KL (L·mg ) is the Langmuir constant and C0 (mg·L−1) is the initial dye concentration. The value of RL indicates the type of isotherm to be unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0).23 The Tempkin isotherm contains a factor that clearly takes into account the adsorbing species and adsorbent interactions. The Tempkin isotherm assumes that the heat of adsorption of 448



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all of the molecules in the layer decreases linearly with coverage due to adsorbent−adsorbate interactions. In addition, the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The Tempkin isotherm is given as:24

qe = RT /b ln(KTCe)

(12)

This can be written in the linear form as follows: qe = BT ln KT + BT ln Ce

(13)

BT = RT /b

(14)

where BT and KT are the Tempkin constants that can be determined by the plot of qe versus ln Ce. Also, T is the absolute temperature (K), and R is the universal gas constant (8.314 J· mol−1·K−1). The constant b is related to the heat of adsorption. The values of KT, BT, and R32 (coefficient of determination values of the Tempkin isotherm) are also shown in Table 1. It was observed that the Langmuir and extended Langmuir model had better described the adsorption of dyes (highest R2 values) in single and binary solutions, respectively. The linear plots of Ce/qe versus Ce obtained from different dye concentrations [(50 to 150) mg·L−1] are presented in Figure 6. It can be seen that the highest correlation coefficient (R22)

Figure 7. Plot of RL values versus initial dye concentration.

first 60 min. The time required to achieve the adsorption equilibrium was only 30 min. The adsorption kinetics of DR23 and DB86 onto CNT-Den are investigated using three kinetic models, namely, the pseudofirst order, pseudo-second order, and intraparticle diffusion model. The pseudo-first order 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 can be expressed by the following equation:27,28 dqt /dt = k1(qe − qt )

(15)

where qe and qt refer to the amount of dye adsorbed (mg.g−1) at equilibrium and at any time, t (min), respectively, and k1 is equilibrium rate constant of pseudo-first order sorption (min−1). Integrating eq 15 for the boundary conditions of t = 0 to t = t and qt = 0 to qt = qt gives eq 16: log(qe /(qe − qt )) = (k1/2.303)t

(16)

Equation 16 can be rearranged to obtain the following linear form (eq 17): log(qe − qt ) = log(qe) − (k1/2.303)t

Figure 6. Adsorption isotherm model for the removal of DR23 and DB86 from single and binary solutions.

(17)

The slope and intercept of the plot of log(qe − qt) versus t are used to determine the pseudo-first order rate constant, k1. The k1, qe, and coefficients of determination (R12) under different dye concentrations were calculated and given in Table 2. It was found that the correlation coefficient (R12) had low values for the adsorption of DR23 and DB86 at different concentrations studied and a very large difference existed between (qe)exp (experimental) and (qe)cal (calculated), indicating a poor pseudo-first order fit to the experimental data at both single and binary solutions. Another kinetic model is the pseudosecond order model, which is expressed by:29

values for both components were obtained, which indicated that the adsorbed dyes had formed monolayer coverage on the adsorbent surface, and showed that all adsorption sites were equal with uniform adsorption energies without any interaction between the adsorbed molecules. Also according to Figure 7, all RL values lay between 0 and 1, which signifies that the adsorption of both dyes onto CNT-Den in single and multiple dye solutions were favorable. A similar phenomenon has been observed in the adsorption of methyl orange, Procion Red MX5B, and methylene blue onto carbon nanotubes.18,25,26 3.6. Adsorption Kinetics. Adsorption kinetics is one of the most important characters which provide information concerning the mechanism of adsorption that are important to represent the adsorption efficiency of the adsorbent and, therefore, optimization of the parameters. The adsorption kinetics of DR23 and DB86 onto CNT-Den was studied to find out the rate constant of the adsorption process. According to the results, the adsorbent showed an excellent performance in the adsorption of DR23 and DB86 during the

t /qt = 1/k 2qe2 + (1/qe)t

(18)

where k2 is the equilibrium rate constant of pseudo-second order adsorption (g·mg−1·min−1). The slope and intercept of the plot of t/qt versus t were used to calculate the second-order rate constant, k2. The k2, qe, and coefficients of determination (R22) are given in Table 2. The possibility of intraparticle diffusion resistance affecting the adsorption was explored using the intraparticle diffusion model as:30 449



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Table 2. Kinetic Constants for DR23 and DB86 Adsorption onto the CNT-Den at (50, 75, 100, and 150) mg·L−1 Dye Concentrations in Single and Binary Dye Solutions, Time: 60 min, pH: 3, and Adsorbent Dosage: 0.09 g·L−1 for DB86 and 0.1 g·L−1 for DR23 and Binary Solutions (Dye Concentration/mg·L−1; (qe)exp/mg·g−1; (qe)cal/mg·g−1; k1/min−1; k2/g·mg−1·min−1, and kp/mg·g−1·s−0.5) pseudo-first order

pseudo-second order 2

dye concentration

(qe)exp

(qe)cal

k1

R

50 75 100 150

464.734 616.162 766.076 940.777

179.762 167.109 625.316 1186.042

0.458 0.465 0.465 0.401

0.834 0.811 0.923 0.777

50 75 100 150

554.6443 558.9431 623.9378 638.779

74.972 297.235 868.960 2584.639

0.332 0.464 0.476 0.5

0.824 0.804 0.892 0.846

50 75 100 150

246.993 354.139 448.344 527.081

27.759 215.625 349.793 805.563

0.341 0.450 0.432 0.492

0.769 0.874 0.781 0.789

50 75 100 150

197.979 270.695 292.355 336.048

83.656 146.083 192.930 507.692

0.205 0.357 0.343 0.471

0.906 0.871 0.825 0.860

single solution

binary solution

(qe)cal DR23 476.19 625 833.333 1000 DB86 555.556 588.235 625 666.667 DR23 250 357.143 454.454 555.556 DB86 204.082 285.714 312.5 357.143

intraparticle diffusion kp

I

R2

0.999 0.999 0.998 0.999

37.085 47.224 67.049 89.629

262.67 357.9 409.97 545.05

0.399 0.3671 0.4702 0.5512

4·10−2 1.3·10−3 8·10−4 1·10−4

1 0.999 0.989 0.998

41.637 44.757 56.57 67.542

323.45 317.63 257.67 249.78

0.356 0.398 0.610 0.676

4·10−2 1.1·10−2 6.0·10−3 4.6·10−3

1 0.999 0.999 0.999

18.649 28.088 36.048 44.398

144.11 199.77 250.79 291.22

0.358 0.397 0.408 0.434

7.5·10−3 5.3·10−3 4.1·10−3 4.1·10−3

0.999 0.999 0.999 0.999

17.018 23.085 24.645 29.382

102.05 144.98 160.06 175.16

0.480 0.458 0.441 0.482

k2

R

6.3·10−3 5.1·10−3 1.4·10−3 1·10−3

2

Figure 8. Plot of qt values versus t1/2 for the adsorption of DR23 and DB86 from single and binary dye solutions.

qt = k pt 1/2 + C

When the plots do not pass through the origin, this is indicative of some degree of boundary layer control and shows that the intraparticle diffusion is not the only rate-limiting step but also that other kinetic models may control the rate of adsorption, all of which may be operating simultaneously. As it observed from Figure 8 the plot did not pass through the origin, suggesting that the adsorption involved intraparticle diffusion; however, that was not the only rate-controlling step.32 Results showed that the correlation coefficient (R22) for the pseudo-second order adsorption model has the highest values (> 0.99). Figure 5 also indicates the plots of t/qt versus time at various dye concentrations. The values of (qe)exp and (qe)cal all increased with the dye concentration which might be attributed to an increase in the driving force of the concentration gradient with the increase in the initial dye concentration.25 The rate constants of the pseudo-second order model (k2) decreased as the initial concentration of dye in adsorption systems increased. Conversely, the intraparticle diffusion rate constants (kp) increased as the initial concen-

(19)

where kp (mg·g−1·min−1/2) is the intraparticle diffusion rate constant. C is a constant related to the thickness of the boundary layer, which is in direct ratio to the effect of the boundary layer. Generally, the plot of qt against t1/2 may show the multilinearity. This indicated that the adsorption processes contained two or more steps. The values of kp, C, and coefficients of determination (R32) are also shown in Table 2. The adsorption of a solute from solution by porous adsorbents is essentially relevant to three consecutive steps. The external surface adsorption or the instantaneous adsorption is the first step. The second step is a gradual adsorption stage where intraparticle diffusion is rate-limiting. The third step is the final equilibrium stage where intraparticle diffusion started to slow down due to the extremely low adsorbate concentrations left in the solutions (Figure 8, the three steps are shown with different colors).31 450



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(mg·g−1) and in the solution (mg·L−1), respectively. R is the universal gas constant (8.314 J·mol−1·K−1) and T is the absolute temperature (K). ΔH and ΔS parameters can be calculated from the slope and intercept of the plot ln KL vs 1/T, respectively (Figure 9). ΔG was calculated using the eq 23 for

tration of dye increased. The diverse effects of the initial concentration on k2 and kp have also been observed in other adsorption systems.33 Additionally, the C value varied like the kp values with dye concentration (Table 2). The values of C are helpful in determining the boundary thickness; a larger C value corresponds to a greater boundary layer diffusion effect.26 The results of this study demonstrated that increasing the dye concentration promoted the boundary layer diffusion effect in all conditions exceptionally in single solution of DB86, which is maybe because of the spatial prohibition in the molecular structure of DB86. The removal of direct dyes by the adsorption on CNT-Den was found to be rapid at the initial period of time and then to become slow and stagnate with the increase in contact time. A similar phenomenon has been observed in the adsorption of C.I. Direct Yellow 86 and C.I. Direct Red 224 from aqueous solutions using carbon nanotubes (CNTs).33 In addition, the optimum contact time for the adsorption of dyes appeared to be 30 min. This could be attributed to the large surface area, the sufficient exposure of active sites, and the high surface reactivity of the new adsorbent, CNT-Den. The sorption of DR23 and DB86 was rapid during the initial stages of the sorption process followed by a gradual process. However, in latter stages, the rate of DR23 and DB86 adsorption became slower. The dye had to first encounter the boundary layer effect and then to adsorb on the surface. Finally they had to diffuse into the structure of the adsorbent which took a longer time. As observed from Table 2, the amount of dye molecules adsorbed on the CNT-Den ((qe)exp) at binary system is half of the (qe)exp at single system, because in binary solutions the concentration of each dye has been diluted one time to reach 50 mg·L−1 of dye concentration totally, so the amount of dye molecules is diminished compared to single system. On the other hand, the adsorption of DR23 by CNT-Den is a little higher than the adsorption of DB86, which can be related to the linear structure and the more functional groups of DR23 dye molecules; hence, they can be adsorbed faster by the adsorbent, and the removal of DR23 in binary solution can be completed faster than DB86. 3.7. Thermodynamic Analysis. The effect of temperature on the adsorption of DR23 and DB86 using CNT-Den was studied in the range of (298 to 328) K. The dye removal increased with the raising of temperature from (298 to 328) K, demonstrating that the adsorption process was endothermic, because of the increasing interaction between the adsorbate and adsorbent. The thermodynamic parameters, namely, free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) have important roles to determine the spontaneity and heat change for the adsorption process. The thermodynamic parameters were calculated using the following equations:34 ΔG = −RT ln KL

(20)

ln KL = (ΔS /R ) − (ΔH /RT )

(21)

Figure 9. Plot of ln(qt/Ce) versus 1/T for the adsorption of DR23 and DB86 from single and binary dye solutions.

different temperatures. Results for both single and binary solutions are exhibited in Table 3. ΔG values at all temperatures were negative which demonstrated the spontaneous nature of the adsorption process. The positive values of ΔH and ΔS suggest the endothermic reaction and the increased randomness at the solid/solution interface during the adsorption of dyes by CNT-Den, respectively. Generally, the change in free energy for physisorption is between (−20 and 0) kJ·mol−1, but chemisorption is in the range of (−80 to 400) kJ· mol−1.35 The values of ΔG obtained in this study are within the ranges of the physisorption mechanisms. The value of ΔH for DB86 was lower than that for DR23, implying that the bond between DB86 and CNT-Den was weaker than that between DR23 and CNT-Den. Additionally, the KL and qm values for DR23 are greater than those for DB86, indicating that the affinity between DR23 and CNT-Den was stronger than the affinity between DB86 and CNT-Den. The reason of this phenomenon can be the linear structure of DR23 and also the more functional groups which it has.33 3.8. Comparison of CNTs and Modified CNTs as Adsorbents. Table 4 shows a comparison between qm values (obtained from Langmuir equations) and thermodynamic parameters of different CNT based adsorbents for the removal of dyes from solutions. According to Table 4, the prepared CNT-Den in this study has shown the best adsorption behavior and the highest qm values for dye removal by the addition of minimum amount of adsorbent. The data collected in Table 4 demonstrated that the modification of CNTs using PPI dendrimers increased the qm values the most; however, it should be considered that the experimental conditions employed in these researches are different from each other. Differences in dyes adsorption capacity are associated with the properties, such as structure, functional groups, pHzpc, and surface area, of each adsorbent. Most studies only investigated the adsorption capacity of an adsorbent, and few simultaneously determined the adsorption capacity and related thermodynamics parameters. Moreover, several investigations indicated that the adsorption of dyes exhibited positive values for ΔH and ΔS as shown in Table 4.16,18,25,26,28,32,33

where:

KL = qe /Ce

(22)

from eqs 20 and 22: (23)

ΔG = ΔH − T ΔS −1

where KL is the Langmuir equilibrium constant (L·mg ) and qe and Ce are the equilibrium dye concentration on CNT-Den 451



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Table 3. Thermodynamic Parameters for DR23 and DB86 Adsorption onto the CNT-Den at 50 mg·L−1 Dye Concentration in Single and Binary Dye Solutions, Time: 60 min, pH: 3, and Adsorbent Dosage: 0.075 g·L−1 (ΔH/KJ·mol−1; ΔS/J·mol−1, and ΔG/KJ·mol−1) ΔG at temperature single binary

dye

ΔH

ΔS

298 K

308 K

318 K

328 K

DB86 DR23 DB86 DR23

54.354 68.264 76.373 81.446

3.134 3.893 4.003 4.320

−10.349 −12.142 −6.782 −8.369

−11.736 −13.825 −8.379 −9.388

−15.354 −18.225 −10.244 −13.169

−16.350 −19.636 −15.540 −17.193

Table 4. qmax Values and Thermodynamic Parameters for the Adsorption of Various Dyes by CNTs and Modified CNT Adsorbents (qm/mg·g−1; Adsorbent Dosage/g·L−1; ΔH/KJ·mol−1 and ΔS/J·mol−1) adsorbent carbon nanotubes (CNTs) multiwalled carbon nanotubes (MWCNTs) MWCNTs MWCNTs MWCNTs magnetic MWCNT-Fe3C CNTs CNTs MWCNT CNTs MWCNTs filled with Fe2O3 particles MWCNTs filled with Fe2O3 particles MWCNT MWCNT MWCNT magnetic-modified Multi Walled Carbon Nanotubes (MMMWCNT) MMMWCNT MMMWCNT MMMWCNT MWCNT MWCNT modified carbon nanotubes with cationic surfactant (SF-CNT) SF-CNT CNT-Den CNT-Den

dye methylene blue direct Congo red reactive green HE4BD golden yellow MR methyl orange direct red 23 direct yellow 86 direct red 224 reactive red M2BE reactive red 2 neutral red methylene blue eriochrome cyanine R alizarin red S morin crystal violet

adsorbent dosage

ΔS

qm

pH 7 3 5

35.40 to 64.70 148.08 151.88

2.5 0.3 0.2 0.67 0.67 1.5

7 2.3 3.7 3 3 2

141.62 35.40 to 64.70 172.40 56.20 61.30 335.70

1.5 1 1 1.8

6.5 6 6 2

44.64 77.50 42.30 54.65 to 95.24

0.6 0.7 0.75

1 1 7

161.29 26.24 227.70

thionine Janus Green B methylene blue reactive blue 4 acid red 183 direct red 80

0.75 0.75 0.75 0.2 0.2 0.16

7 7 7 6 6 7.5

36.40 250 48.10 69 45 120.5

direct red 23 direct blue 86

0.16 0.09

7.5 3

188.7 666.667

3.134

54.354

direct red 23

0.1

3

1000

3.893

68.264

4. CONCLUSIONS In this study, the experiments were conducted to calculate the parameters at equilibrium condition for the adsorption of DR23 and DB86 in both single and multiple (binary) dye solutions using CNT-Den adsorbent. FESEM images and FTIR analysis showed the successful adsorption of both dyes onto the prepared adsorbent. The investigated equilibrium parameters were isotherm, kinetics, and thermodynamic parameters. For this purpose, Langmuir, Freundlich, and Tempkin isotherms were studied, and the results were in great agreement with the Langmuir model. The calculated parameters obtained at different temperatures indicated that the adsorption process using CNT-Den was spontaneous (ΔG < 0), endothermic (ΔH > 0), and in the range of the physisorption mechanisms. The pseudo-first order, pseudo-second order, and intraparticle diffusion models were selected to study the adsorption kinetics,

64.6

ΔH

0.3 2.5 2.5

0.1015

7.29

139.51 172.06 to 177.83

19.39 12.6 13.69 24.29

216.99

31.55

160.843

46.587

39.99 23.192

145.911 79.453

ref 25 36 36 36 18 16 33 33 37 26 38 38 32 28 28 20 20 20 20 39 39 40 40 this study this study

and the data followed pseudo-second order with high correlation coefficients. The maximum adsorption capacities (qm) were (1000 and 666.66) mg·g−1 for the removal of DR23 and DB86, respectively. Based on our experimental data, it is possible to propose a mechanism for the interactions of the DR23 and DB86 dye molecules with the CNT-Den adsorbent. In the first step, the CNT-Den is immersed in acidic solution (pH 3), so the NH2 functional groups of the adsorbent are protonated, which provides good affinity for the facile and fast adsorption of the anionic dye molecules. The second stage is the electrostatic attraction of the negatively charged dye molecules by the positively charged CNT-Den at pH 3. For the treatment of the simulated industrial colored effluents, a small amount of the synthesized adsorbent (0.1 g·L−1) could almost remove over 95 % of direct dyes (50 mg·L−1) after 60 min of contact time. Overall, it can be concluded that the 452



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modified CNTs with PPI dendrimer is a qualified adsorbent for the simple, uncomplicated, and rapid removal of dyes with high efficiencies.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +98 //2164542614. Fax: +98 2166400245. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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