Synthesis of magnetic multi-walled carbon nanotubes

Journal of Industrial and Engineering Chemistry 20 (2014) 3559–3567

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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Synthesis of magnetic multi-walled carbon nanotubes/magnetite/ chitin magnetic nanocomposite for the removal of Rose Bengal from real and model solution Mohamed Abdel Salam *, Reda M. El-Shishtawy, Abdullah Y. Obaid Chemistry Department, Faculty of Science, King Abdulaziz University, P.O Box 80200, Jeddah 21589, Saudi Arabia

A R T I C L E I N F O

Article history: Received 11 November 2013 Received in revised form 11 December 2013 Accepted 17 December 2013 Available online 24 December 2013 Keywords: Carbon nanotubes Chitin Magnetite Rose Bengal Adsorption Kinetics Thermodynamics

A B S T R A C T

Multiwalled carbon nanotubes (MWCNTs) were physically mixed with biopolymer chitin and magnetite to form a magnetic nanocomposite. The produced MWCNTS/chitin/magnetite (MCM) nanocomposite was characterized using scanning electron microscopy and surface-area analysis, and the magnetic properties were measured using a vibrating-sample magnetometer. The results revealed homogenous distribution of the chitin and magnetite nanoparticles within the MWCNTs in the MCM nanocomposite. The MCM nanocomposite was used to study the removal/adsorption of the well-known organic dye Rose Bengal (RB). The effect of different adsorption parameters was studied and optimized. The adsorption process was studied at different temperatures and the results were analyzed kinetically. Results revealed that the adsorption followed the pseudo-second-order kinetic model. Also, it was found that the adsorption of RB occurred in different steps including the diffusion of RB through the boundary layer to the external surface of the MCM nanocomposite, intraparticle diffusion, and adsorption of RB through the MCM nanocomposite particles. The adsorption was analyzed thermodynamically and results revealed spontaneity of the adsorption as the DG8 value was negative and endothermic in nature as the DH8 value was positive, and associated with the increase in the randomness as the DS8 value was positive. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Carbon nanotubes (CNTs) are a new emerging nanomaterial that has exceptional properties such as unique size distributions, novel hollow-tube structures, high specific surface areas, and electrical semiconductivity and conductivity. These properties allow CNTs to be used in a variety of applications such as catalysts [1], medicine [2], fuel cells [3], solar cells [4], nanoelectronics and photonics [5], sensors [6], composites for mechanical applications [7], and as adsorbents for different pollutants [8–11]. The adsorption ability of CNTs relative to other adsorbents is mainly due to their strong interactions with organic and inorganic pollutants. This interaction results from the delocalized electrons in hexagonal arrays of carbon atoms on the surface of CNTs. One of the strategies used to enhance the adsorption ability of carbon nanotubes is modification. There are two different approaches to the modification and enhancement of the adsorption capacity toward different adsorbates; chemical and physical modification. There are many studies that focus on the chemical modification of

* Corresponding author. Tel.: +966 541886660; fax: +966 2 6952292. E-mail address: [email protected] (M.A. Salam).

carbon nanotubes which occurs through the formation of chemical bonds at the CNTs’ surface with other chemical groups. Carbon nanotubes were functionalized with octadecylamine and polyethelene glycol and the modified carbon nanotubes showed higher efficiency for the removal and adsorption of different polyhalogenated organic pollutants [12,13]. Also, carbon nanotubes were functionalized with nitrogen-containing groups, which enhanced their adsorption ability towards CO2 and CH4 [14]. Physical modification is another approach to enhancing the ability of CNTs to adsorb different pollutants. For example, CNTs were physically modified with chitosan [15] and 8-hydroxyquinoline [11], and the produced nanocomposite was used efficiently for the removal of heavy-metal ions from an aqueous solution. Moreover, one of the problems associated with the application of CNTs for the removal/adsorption of environmental pollutants is the regeneration of the efficient but expensive CNTs. Carbon nanotubes can be separated from solution by centrifuge or filtration, which adds costs to environmental remediation. Magnetic-separation technology has gradually attracted the attention of many scientists as a rapid and effective technology for separating solid magnetic adsorbents. It has been used for many applications in medicine, diagnostics, cell biology, analytical

1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.12.049

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M.A. Salam et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 3559–3567

chemistry, mining, and environmental technology [16]. The main advantage of this technology is in its capability to treat a large amount of wastewater within a short time, producing no contaminants [17]. Magnetic-separation technology, combined with the adsorption process of adsorbent, has been widely used in environmental-purification applications. In general, magnetic adsorbents are often composed of magnetic nanoparticles dispersed in a cross-linked polymer matrix that includes a natural and synthetic polymer. Carbon nanotubes were modified with different magnetic materials and used for the removal, adsorption, and preconcentration of different pollutants in the environment [18–22]. This is because the introduction of magnetic properties into a multiwall carbon nanotube system combines the high adsorption capacity of CNTs and the separation convenience of magnetic materials. The modification of carbon nanotubes with a biopolymer, such as chitin, and magnetic materials, such as magnetite, and applications for environmental remediation from organic pollutants is still scarce in the literature [23,24]. In this research work, multiwalled carbon nanotubes were physically mixed with chitin, a well-known biopolymer, and magnetite as a magnetic material. The introduction of the magnetic properties into a multiwalled carbon nanotube/biopolymer, such as chitin, will combine the high adsorption capacity of both CNTs and chitin, and the separation convenience of magnetic materials. The morphological structure of the produced MWCNTs/chitin/magnetite was characterized using a scanning electron microscope (SEM), the magnetic properties of the magnetic nanocomposite was explored with vibrating-sample magnetometry (VSM) and specific surface area and pore analysis was performed. The MWCNTs/chitin/ magnetite magnetic nanocomposite was used for the adsorption of the well-known dye Rose Bengal (RB) as an example of organic pollutants from aqueous solution. The adsorption of RB from aqueous solution by the MCM magnetic nanocomposite was optimized and studied kinetically using different kinetic models to understand and reveal the possible adsorption mechanism for efficient removal.

2.3.1. Real water samples collection The wastewater sample (MBR 6000 STP) was collected from the membrane bio-reactor technology waste water treatment plant at King Abdulaziz University (KAUWW), Jeddah city (latitude deg. north 21.487954, longitude deg. east 39.236748). The waste water sample was then filtered through 0.45 mm Millipore filter paper and kept in Teflon1 bottles, at 5 8C, in the dark.

2. Experimental

3. Results and discussion

2.1. Preparation of the MWCNTs/chitin/magnetite magnetic nanocomposite

3.1. Characterization of MCM magnetic nanocomposite

MWCNTs with average diameters of 60–100 nm were obtained from Shenzhen Nano-Technologies, China, and were used as received. Spherical nanoparticles of magnetite (iron (II, III) oxide) with average diameter 50 nm, coarse flakes of chitin, and Rose Bengal dye were obtained from Sigma–Aldrich. All chemicals used in this study were obtained from Sigma–Aldrich (analytical grade), and all solutions were prepared using deionized water. The MCM magnetic nanocomposite was prepared by thoroughly mixing of MWCNTs, chitin flakes, and the magnetite nanoparticles (4:1:1 weight percent) using agate mortar till homogenous mixture was obtained. 2.2. Characterization techniques Scanning electron microscope (SEM) measurements were taken using an FEI-field emission scanning electron microscope (FISEM) (Quanta FEG 450, Netherlands). The specific surface area of the different MWCNTs was determined from nitrogen adsorption/ desorption isotherms measured at 77 K using a model NOVA 3200e automated gas sorption system (Quantachrome, USA). The magnetic properties were measured at room temperature by using the vibrating sample magnetometer (VSM) up to magnetic field of 5 kOe.

2.3. Adsorption experiment Adsorption experiments were performed to determine the effect of time and temperature on the adsorption process and to identify the adsorption rate. The experimental procedures were performed as follows: (1) a series of solutions of various RB concentrations were prepared; (2) the initial pH was measured, and a defined amount of the MWCNTs was then added to the solutions; (3) these solutions were agitated on a magnetic stirrer for a certain period of time, at room temperature; (4) at defined points in time, a certain volume of the solution was removed and immediately using ordinary magnet and the clear supernatant was collected using a glass pasture pipette; and (5) the residual RB concentration in the supernatant was determined using PerkinElmer Lambda 25 UV–vis spectrophotometer, USA. The amount of RB adsorbed was determined by measuring the difference in the concentrations of the samples that were obtained at two consecutive time intervals over the course of the adsorption experiment. The adsorption capacity of the MWCNTs—qt(mol g1), which represents the amount of RB adsorbed per amount of MWCNTs was calculated using a mass–balance relationship: qt ¼

ðC 0 C t ÞV m

(1)

where C0 and Ct are the concentrations of RB in solution (mol L1) at time t = 0 and t, respectively. V is the volume of the solution (L), and m is the mass of the dry adsorbent used (g). The kinetic curves obtained were analyzed using various-order kinetic equations to obtain the parameters for understanding the adsorption process.

Fig. 1 shows images from a field emission scanning electron microscope of the pristine MWCNTs (A), chitin (B), magnetite nanoparticles (C), and an MCM magnetic nanocomposite at different magnification power (Fig. 1D–F). It is clear from the images that pristine MWNTs had average outer diameter of 50 nm and an inner diameter in a range between 5–9 nm. MWCNTs of various sizes and directions were curved, forming an aggregated structure, due to the intermolecular force. The chitin (Fig. 1B) was in the form of flakes and there were not any nanoparticles in the image, whereas the magnetite nanoparticles (Fig. 1C) were spherical in shape with average particle size of 50 nm, in general. When the MWCNTs, chitin, and magnetite nanoparticles were mixed, the MCM magnetic nanocomposite was formed. The images show the coexistence of three different particles in the homogenous composite. The BET-specific surface areas of the MWCNTs, chitin, magnetite, and their magnetic nanocomposite were calculated from the nitrogen adsorption/desorption isotherms at 77 K, and were found to be 61.5, 5.2, 65.7, and 64.3 m2 g1, respectively. This showed that mixing of MWCNTs with chitin and magnetite nanoparticles did not greatly affect the specific surface area of the pristine MWCNTs. Due to the fact that magnetite nanoparticles were chosen to enhance the magnetic effect of the carbon nanotubes and chitin, it was important to investigate the field dependence of magnetization for MWCNTs, chitin, magnetite,


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Fig. 1. Scanning electron microscope images of pristine MWCNTs (A), chitin (B), magnetite nanoparticles (C), MWCNTs/chitin/magnetite nanocomposites at different magnification power (D–F).

and the MCM nanocomposite using a vibrating-sample magnetometer at room temperature with an applied field: 5 kOe H 5 kOe. The results are presented in Fig. 2. The figure shows that typical ferromagnetic hysteresis loops were observed for the pristine MWCNTs (A), magnetite nanoparticles (C), and the MCM nanocomposite, whereas this behavior was absent for the chitin. Magnetic parameters such as the saturation magnetization (Ms), remnant magnetization (Mr), and coercivity (Hc) were summarized in Table 1. From the table it is clear that the pristine MWCNTs possess weak magnetic properties, perhaps attributed to the catalyst residuals and/or the nanotubes used in the preparation. The saturation magnetization (Ms) of the MCM nanocomposite was found to be 9.42 emu/g, which is considerably smaller than that of pristine magnetite (72.5 emu/g). This was expected as a result of the presence of nonmagnetic chitin and the weakly magnetic MWCNTs with a high ratio in the composite compared to the strongly magnetic magnetite. The high value of Ms for the MCM nanocomposite compared with the pristine MWCNT suggests the successful modification of the carbon nanotubes/chitin with the addition of magnetite, which proposes a proper way for collecting MCM nanocomposite using magnets. From Table 1 it is also noted that an enhancement of coercivity of MCM nanocomposites compared to the magnetite nanoparticles: 144.3 and 106.5 G, respectively. This could be attributed to the slight enhancement in the surface area of the MCM nanocomposite compared to the pristine magnetite.

Table 1 Magnetic data for pristine MWCNTs, chitin, magnetite, and MCM nanocomposite. Materials

Ms*(emu. g1)

Hc*(G)

Mr*(emu.g1)

MWCNTs Chitin Magnetite MCM nanocomposite

0.676 0.0695 72.5 9.42

251.4 80.2 106.5 144.3

0.230 0.006 7.18 1.43

*

(Hc)

saturation magnetization (Ms), remenant magnetization (Mr) and coercivity

3.2. Adsorption study As was explained earlier, one of the main reasons for adding the magnetite is to change the adsorbent to a magnetic adsorbent, which facilitates the separation of the solid phase from the liquid phase for further determination of the concentration of pollutants and the recyclic/regeneration of both the adsorbate and the solid adsorbent. Fig. 3 shows the ease of separation of the MCM magnetic nanocomposite using a magnet, and the adsorption ability of the MCM nanocomposite to remove most of the MR dye from the solution, which turns its color from rose to clear. 3.2.1. The effect of adsorption parameters In general, the adsorption/removal of any adsorbate from aqueous solution using a solid adsorbent is greatly influenced by experimental/environmental conditions such as dosage of the solid adsorbent, solution temperature and pH, and the contact time between the adsorbate and the solid adsorbent. The effects of these conditions on the adsorption/removal of RB by a MCM magnetic nanocomposite from aqueous solution were investigated to understand and enhance adsorption process efficiency and to facilitate the removal of RB from solution. The adsorbent dosage is an essential parameter in the adsorption processes, as it determines the adsorbent’s capacity for a given initial concentration of adsorbate in a solution. Fig. 4 shows the effect of the MCM magnetic nanocomposite dosage on the adsorption of RB from an aqueous solution at solution pH 8.0, 120 min, solution temperature 298 K, and RB concentration 5.0 mg L1. It is clear from the figure that the amount of RB adsorbed increased gradually as the MCM nanocomposite dosage increased, until it reached 96.5% when 4.0 mg of MCM nanocomposite was used. The amount of RB adsorbed increased along with the MCM nanocomposite dosage mainly due to the increase in the number of active sites available for adsorption upon the increase in the adsorbent dosage. To evaluate the effect of the other parameters, a dosage of 1.0 mg MCM magnetic nanocomposite was employed. Fig. 5 shows the effect of contact time on the adsorption of RB from an aqueous solution by an MCM magnetic

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Fig. 4. Effect of the MCM magnetic nanocomposite dosage on the adsorption of RB from an aqueous solution. (Experimental conditions: 5.0 mL solution, pH 8.0, 120 min, 298 K, and RB concentration 5.0 mg L1.)

nanocomposite using 1.0 mg MCM nanocomposite, solution pH 8.0, solution temperature 298 K, and RB concentration 5.0 mg L1. As it is clear from the figure, the percent of RB removed from the solution after 1.0 min was 68.3%, and this amount increased gradually to equilibrium after 30 min with percentage of RB removed equal to 90%. Solution temperature is a factor that affects the adsorption/ removal process, which discloses the suitability of the adsorbent. To determine the effect of solution temperature on RB removal, four different temperatures were used: 283, 293, 308, and 323 K, and the results are presented in Fig. 6. It is clear from the figure that increasing the solution temperature significantly enhanced the adsorption of RB by the MCM nanocomposite. In general, raising the temperature from 283 to 293, 308, and 323 K was accompanied with a significant increase in the percentage of RB removed from 74.6 to 85.0, 91.7, and 99.9%, respectively. These results suggest that the removal of RB by MCM nanocomposite is endothermic in nature. Fig. 2. Vibrating sample magnetometer measurements pristine MWCNTs (A), chitin (B), magnetite nanoparticles (C), and MWCNTs/chitin/magnetite nanocomposites (D).

Fig. 3. Separation of the MCM magnetic nanocomposite from the solution using a magnet.

3.2.2. Kinetics studies To explore the potential of using a solid adsorbent such as MCM nanocomposite, it is very important to investigate the kinetics of the adsorption. This involves studying the adsorption rate to determine the effects of various factors on the process. This usually occurs through careful monitoring of the experimental conditions that affect the speed of the adsorption process, until it reaches

Fig. 5. Effect of time on the adsorption of RB from an aqueous solution by MCM magnetic nanocomposite. (Experimental conditions: 5.0 mL solution, pH 8.0, 1.0 mg MCM nanocomposite, 298 K, and RB concentration 5.0 mg L1.)


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temperature on the adsorption of RB by an MCM nanocomposite in terms of percentage of RB removed, and the amount adsorbed (qt) with time. It is clear from the figures that the percentage of RB removed, and the amount adsorbed, was increased by raising the temperature of the solution. This indicates the endothermic nature of the adsorption of RB by an MCM nanocomposite. The adsorption experimental data were treated kinetically using different models to understand the dynamics of the adsorption process. The kinetics of RB adsorption by MWCNTs were studied using the most well-known and used kinetic models: fraction-power function [25], Lagergren pseudo-first-order [26], pseudo-second-order [27], Elovich [28], liquid-film diffusion [29], and intraparticle diffusion models [30]. The fractional-power function model is a modified form of the Freundlich equation, and it can be written in its linearized form: ln qt ¼ ln a þ b ln t; Fig. 6. Effect of solution temperature on the adsorption of RB from an aqueous solution by MCM magnetic nanocomposite. (Experimental conditions: 5.0 mL solution, pH 8.0, 1.0 mg MCM nanocomposite, and RB concentration 5.0 mg L1.)

equilibrium. The kinetic data obtained is then used to develop appropriate mathematical models to describe the interactions between the adsorbate molecule and the solid adsorbent. Once the reaction rates and the dependent factors are unambiguously known, these results can be utilized to develop suitable adsorbent materials for industrial applications. Various kinetic models are usually applied to explore the complex dynamics of the adsorption process. The effect of solution temperature on the removal of RB from aqueous solution by an MCM nanocomposite was studied kinetically at 283, 293, 308, and 323 K. Fig. 7 shows the effect of solution

(2)

where qt (mg/g) is the amount of metal ion adsorbed per unit mass of MWCNTs at any time t, while a and b are coefficients with b < 1. The function ab is the specific sorption rate when t = 1 min. Fig. 8 presents the application of the fractional-power function equation to the adsorption of the RB by an MCM nanocomposite at different solution temperatures. Although a linear relationship exists between ln qt and ln t, as shown in Fig. 8, the correlation coefficients were very low, especially at low temperatures, as presented in Table 2. This may prove the unsuitability of the fractional-power function model for describing the adsorption of the RB by an MCM nanocomposite. The Lagergren pseudo-first-order kinetic model is another wellknown kinetic model that is usually used to emphasize the application of a solid adsorbent for the adsorption from an aqueous solution. The Lagergren pseudo-first-order kinetic model can be written as: lnðqe qt Þ ¼ ln qe k1 t;

(3)

where k1 (min1) is the pseudo-first-order adsorption rate coefficient and qe and qt are the values of the amount adsorbed per unit mass at equilibrium and at any time t, respectively. Plotting of ln (qe–qt) vs. t for the adsorption of RB by an MCM nanocomposite at different temperatures, the results did not converge well, without straight lines, as is shown in Fig. 9 and Table 2. Also, the calculated values for the amount adsorbed at equilibrium (qe, calc.) did not match with those measured experimentally (qe, exp.). This indicates that the pseudo-first-order Lagergren kinetic model is not suitable to describe the adsorption of RB by an MCM nanocomposite.

Fig. 7. Effect of solution temperature on the adsorption kinetics of RB by MCM nanocomposite. (Experimental conditions: 10.0 mL solution, pH 8.0, 2.5 mg MCM nanocomposite, and RB concentration 5.0 mg L1.)

Fig. 8. Fractional power kinetic model plots for the adsorption of RB by MCM nanocomposite at different temperatures.

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Table 2 Different kinetic models parameters for the adsorption for the RB on MCM magnetic nanocomposite at different temperatures. Fractional power model R2

Temperature

A

b

ab

283 K 293 K 308 K 323 K

3.74 4.16 4.28 4.54

0.096 0.080 0.075 0.066

0.361 0.332 0.321 0.300

0.834 0.875 0.939 0.933

Temperature

qe, exp (mg/g)

qe, calc (mg/g)

k1

R2

283 K 293 K 308 K 323 K

5.78 5.98 6.11 6.19

1.12 1.11 1.41 1.20

0.021 0.022 0.022 0.022

0.728 0.744 0.856 0.798

Temperature

k2 (g/mg.min)

qe, exp (mg/g)

qe, calc (mg/g)

h

R2

283 K 293 K 308 K 323 K

0.112 0.117 0.087 0.107

5.78 5.98 6.11 6.19

5.83 6.04 6.16 6.25

3.82 4.25 3.31 4.17

0.998 0.999 0.999 0.999

Pseudo-first-order kinetic model

Pseudo-second-order kinetic model

Elovich kinetic model Temperature 283 K 293 K 308 K 323 K

a (g/mg.min) 3

4.81 10 4.36 104 8.09 104 5.18 105

b (mg/g.min)

R2

0.472 0.416 0.403 0.367

0.861 0.898 0.951 0.941

Liquid film diffusion model Temperature

Temperature

Temperature

Temperature

283 K 293 K 308 K 323 K

283 K 293 K 308 K 323 K

283 K 293 K 308 K 323 K

283 K 293 K 308 K 323 K

Temperature

Temperature

Temperature

283 K 293 K 308 K 323 K

283 K 293 K 308 K 323 K

283 K 293 K 308 K 323 K

Intra-particle diffusion model

The pseudo-second-order kinetic model considered is also to describe the adsorption of the RB by an MCM nanocomposite. The linearized form of the pseudo-second-order rate equation is given as: t 1 t ¼ þ ; qt k2 q2e qe

(4)

Fig. 9. Pseudo-first-order kinetic model plots for the adsorption of RB by MCM nanocomposite at different temperatures.

where k2 (g/[mg min]) is the pseudo-second-order rate coefficient, and qe and qt are the values of the amount adsorbed per unit mass at equilibrium and at any time t, respectively. Plotting t/qt vs. t according to Eq. 4 must give a linear relationship, and based on this relationship, qe and k2 can be estimated from the plot’s slope and intercept, respectively. Applying the pseudo-second-order rate

Fig. 10. Pseudo-second-order kinetic model plots for the adsorption of RB by MCM nanocomposite at different temperatures.


equation to the adsorption of RB, experimental data converged very well with excellent regression coefficients (R2 > 0.99) and straight lines, as presented in Fig. 10. These findings verified that the pseudo-second-order kinetic model is the suitable kinetic model for the description of the adsorption of RB by an MCM nanocomposite from an aqueous solution. The pseudo-secondorder rate equation parameters, qe and k2 are calculated from the slope and intercept of the plot of t/qt vs. t, as shown in Table 2. The table shows that the amounts of RB adsorbed per unit mass of an MCM nanocomposite at equilibrium (qe, calc.), calculated from the slope of the pseudo-second-order plot, were in good agreement with the experimental values (qe, exp.). In general, it was found that the amount adsorbed at equilibrium (qe) increased by raising the solution temperature, indicating the endothermic nature of the adsorption of RB by an MCM nanocomposite from an aqueous solution: 5.78, 5.98, 6.11, and 6.19 mg g1 at 283, 293, 308, and 323 K, respectively. The Elovich equation is another kinetic model frequently used to describe the adsorption of adsorbates such as RB by solid adsorbent such as an MCM nanocomposite from an aqueous medium. The linear form of the Elovich equation could be written by the following equation: qt ¼ b lnðabÞ þ b ln t;

(5)

where a and b are the Elovich coefficients that represent the initial adsorption rate (g/[mg min]) and the desorption coefficient (mg/ g min), respectively. The Elovich coefficients a and b are calculated from the slope and intercept of the qt vs. ln t plots shown in Fig. 11, and their values are tabulated in Table 2. The correlation coefficients were satisfactory at high temperature, 308 and 323 K, but were not at low temperatures 278 and 293 K. This indicates that the Elovich model is not the suitable kinetic model to describe the adsorption of the RB by an MCM nanocomposite. According to the above results and depending on the correlation-coefficient values, it could be concluded that the pseudo-second-order kinetic model is the most appropriate kinetic model to describe the adsorption of RB by an MCM nanocomposite from an aqueous solution. This finding agrees with other studies that showed adsorption from aqueous solution by carbonnanotube-modified composites usually follow the pseudo second-order kinetic model. For example, the removal of the anionic dye Direct Red 23 by a magnetic multi walled carbon nanotubesFe3C nanocomposite [31], the removal of methylene blue with magnetite-loaded multiwall carbon nanotubes [32], the removal of lead by amino-modified multi walled carbon nanotubes [33], and

Fig. 12. Liquid-film diffusion kinetic model plots for the adsorption of RB by MCM nanocomposite at different temperatures.

the removal of methyl violet by a halloysite nanotube-Fe3O4 composite [34]. The mechanism for the removal of RB by adsorption usually occurred in different steps. The first step involves the migration of RB from the bulk of the solution to the external surface of the MCM nanocomposite, followed by the diffusion of RB through the boundary layer to the external surface of MCM nanocomposite, then the adsorption of RB at an active site on the surface of the MCM nanocomposite, and finally intra-particle diffusion and adsorption of RB through the MCM nanocomposite particles. Accordingly, the adsorption mechanism was examined with the liquid-film model and the intra-particle-diffusion model. The liquid-film diffusion model assumes that the flow of the adsorbate molecules through a liquid film surrounding the solid adsorbent is the slowest step in the adsorption process (i.e., the one that determines the kinetics of the rate processes). The liquid-film diffusion model is given by the following equation [29]: lnð1 FÞ ¼ kfd t;

(6)

where F is the fractional attainment of equilibrium (F = qt/qe), and kfd (min1) is the film-diffusion rate coefficient. A linear plot of ln(1 F) vs. t, with a zero intercept, suggests that the kinetics of the adsorption process are controlled by diffusion through the liquid film around the MCM nanocomposite. Fig. 12 showed that the application of the liquid-film diffusion model to the experimental adsorption data of RB by an MCM nanocomposite from an aqueous solution at different temperatures did not converge well, did not provide straight lines that passed through the origin, and had a correlation with very low correlation coefficients: 0.728, 0.744, 0.856, and 0.798 at 283, 293, 308, and 323 K, respectively. This indicates that the diffusion of the RB through the liquid film around the MCM nanocomposite was not the rate-determining step. It is noteworthy to mention that the liquid-film diffusion model was applied to the first few points within the first 10 min of adsorption, and the regression coefficients were slightly improved to 0.908, 0.810, 0.858, and 0.799. This may indicate that the liquid-film diffusion model may not be the rate-determining step, i.e., the slowest step, but may contribute to the adsorption process, especially at the beginning of the adsorption. The intra-particle-diffusion model can be expressed by the following equation [30]: qt ¼ kid t 1=2 þ C

Fig. 11. Elovich kinetic model plots for the adsorption of RB by MCM nanocomposite at different temperatures.

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(7)

where qt is the adsorption capacity at any time (t), kid is the intraparticle-diffusion rate constant (mg/g min1/2), and C (mg/g) is a


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that it is less than 42.0 kJ mole1 [36]. Also, it is noteworthy that the adsorption by pristine MWCNTs generally is exothermic in nature [8,10,37,38], but according to this study, the adsorption was endothermic in nature. According to previous studies, the reason could be either the presence of the metallic nanoparticles such as manganese oxide nanoparticles [39], zinc hydroxide nanoparticles [40], and silver nanoparticles [41], or the presence of chitin, which usually follows the endothermic adsorption [42–44]. The change in entropy was positive, which indicated an increase in the degree of freedom at the solid–liquid interface due to the immobilization of RB on the MCM nanocomposite surface. The DG values were calculated at 298 K from the relation:

DG ¼ DH T DS

Fig. 13. Intra-particle diffusion kinetic model plots for the adsorption of RB by MCM nanocomposite at different temperatures.

constant proportion to the thickness of the boundary layer. Usually, through the application of the intra-particle-diffusion model to the adsorption data, it is expected that a straight line will pass through the origin. Applying the intra-particle-diffusion model to the RB adsorption data at different temperatures yielded no straight lines that passed through the origin, as presented in Fig. 13. This indicates that the intra-particle-diffusion model is not the rate-determining step. Only the last part of the adsorption process converged with the intra-particle-diffusion model, providing satisfactory correlation coefficient values (Table 2), indicating that the last step of the RB adsorption by MCM nanocomposite from an aqueous solution was dependent on the inter-particle diffusion of the RB through the MCM particles, but was not the slowest step. Accordingly, it can be concluded that the adsorption mechanism of RB by an MCM nanocomposite was controlled by neither the liquid-film diffusion nor the intra-particle diffusion, but may be controlled by any of the other two steps: migration of RB from the bulk of the solution to the external surface of an MCM nanocomposite, and/or the adsorption of RB at an active site on the surface of the MCM nanocomposite.

(10)

The free energy change, DG, was found to be negative, 4.31 kJ mole1, as would be expected for a product-favored and spontaneous reaction. Generally, the negative value of DG, and the positive values of DH and DS, suggests that the adsorption of RB by an MCM nanocomposite is an entropy-driven process. 3.3. Environmental applications To study the applicability of MWCNTs for the removal of estrogenic compounds, real environmental samples must be investigated. A wastewater sample was taken from the King Abdulaziz University Wastewater treatment plant. The concentration of the RB was measured in sample and was below the detection limit of the UV–vis measurement. Then, the sample was spiked with RB and the final concentration was 5.0 mg L1. The solution pH was adjusted to 8.0 mg and then 3.0 mg of MCM nanocomposite was added to the solution, shaken for 2 h, and then the percentage of RB adsorbed was measured, calculated and found to be 98.8%. The used MCM nanocomposite was washed with acetone, dried and reused for the removal of RB from solution. Almost the same percentage of adsorption was obtained for five cycles. This is proof the ability to recycle and reuse an MCM nanocomposite for a number of adsorption cycles without losing adsorption efficiency. 4. Conclusions

3.2.3. Thermodynamic studies Thermodynamic parameters—Gibbs free-energy change (DG), enthalpy change (DH), and entropy change (DS)—were calculated for the adsorption of RB by the MCM magnetic nanocomposite from aqueous solution to evaluate thermodynamic feasibility and spontaneity. Thermodynamic parameters were calculated from the variation of the thermodynamic distribution coefficient D with a change in temperature according to the equation: D¼

qe Ce

(8)

where qe is the amount of RB adsorbed by the MCM magnetic nanocomposite (mg/g) at equilibrium and Ce is the equilibrium concentration of RB in solution (mg/L). The DH and DS can be calculated according to the following equation [35]: logD ¼

DS R

DH 2:303RT

(9)

Plotting log D vs. 1/T gives straight line, and the DH and DS values were calculated from the slope and the intercept of the straight line, respectively. The enthalpy value was found to be 38.7 kJ mole1, whereas the change in entropy value was fond to be 146.9 kJ mole1 K1. This may indicate that the adsorption of RB by an MCM magnetic nanocomposite is endothermic and physical in nature due to the positive value of enthalpy, as well as the fact

A magnetic nanocomposite composed of MWCNTs/chitin/ magnetite nanoparticles (MCM) was prepared through physical mixing (4:1:1 weight percent). The MCM magnetic nanocomposite was characterized using SEM, which showed the formation of the homogenous nanocomposite and the spreading of chitin and magnetite nanoparticles within the MWCNTs. Surface-area analysis showed that the mixing of the MWCNTs with chitin and magnetite nanoparticles did not affect greatly the specific surface area of the pristine MWCNTs. Also, magnetism measurements confirmed the formation of a strong magnetic nanocomposite with saturation magnetization of 9.42 emu/g. Different factors that affect the removal of RB by an MCM magnetic nanocomposite were studied, and the results showed that increasing the MCM nanocomposite’s mass enhanced the adsorption process, and most of the RB was removed from the solution using 2.0 mg of the MCM nanocomposite. Also, the effect of adsorption time was explored, and the results showed that the adsorption reached equilibrium within 30 min. The adsorption process was studied kinetically by fitting the experimental data with different kinetic models, and the results showed that the adsorption followed the pseudosecondorder kinetic model. The mechanism of the adsorption was explored and the results showed that the adsorption of RB occurred in different steps including the diffusion of RB through the boundary layer to the external surface of the MCM


nanocomposite, intraparticle diffusion, and adsorption of RB through the MCM nanocomposite particles. The adsorption was studied thermodynamically, and different thermodynamic parameters were calculated. Results showed that the removal of RB from aqueous solution by an MCM nanocomposite was physical in nature and endothermic as the adsorption capacity increased by raising the solution temperature, with an enthalpy (DH) value of 38.7 kJ mole1, a positive-entropy (DS) value of 146.9 kJ mole1 K1, and spontaneous with a negative free-energy (DG) value of 4.31 kJ mole1. Also, the negative value of DG, and the positive values of DH and DS, suggests that the adsorption of RB by an MCM nanocomposite is an entropy-driven process. The MCM nanocomposite was used for removing RB from a spikedwastewater sample, and the results showed that almost 100% of the RB was removed and the nanocomposite could be used for five consecutive cycles without losing its efficiency. Acknowledgment This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number (254/130/1432). The authors, therefore, acknowledge with thanks DSR technical and financial support. References [1] A.E. Shanahan, J.A. Sullivan, M. McNamara, H.J. Byrne, Preparation and characterization of a composite of gold nanoparticles and single-walled carbon nanotubes and its potential for heterogeneous catalysis, New Carbon Mater. 26 (2011) 347–355. [2] S. Peretz, O. Regev, Carbon nanotubes as nanocarriers in medicine, Curr. Opin. Colloid Interface Sci. 17 (2012) 360–368. [3] Y-C. Chiang, J-R. Ciou, Effects of surface chemical states of carbon nanotubes supported Pt nanoparticles on performance of proton exchange membrane fuel cells, Int. J. Hydrogen Energy 36 (2011) 6826–6831. [4] Y. Zemen, S.C. Schulz, H. Trommler, S.T. Buschhorn, W. Bauhofer, K. Schulte, Comparison of new conductive adhesives based on silver and carbon nanotubes for solar cells interconnection, Sol. Energy Mater. Sol. Cells 109 (2013) 155–159. [5] V. Sgobba, D.M. Guldi, Carbon nanotubes—electronic/electrochemical properties and application for nanoelectronics and photonics, Chem. Soc. Rev. 38 (2009) 165–184. [6] D.W.H. Fam, Al. Palaniappan, A.I.Y. Tok, B. Liedberg, S.M. Moochhala, A review on technological aspects influencing commercialization of carbon nanotube sensors, Sens. Actuators B 157 (2011) 1–7. [7] J.N. Coleman, U. Khan, W.J. Blau, Y.K., Small but strong: a review of the mechanical properties of carbon nanotube—polymer composites, Carbon 44 (2006) 1624–1652. [8] L.A. Al-Khateeb, A.Y. Obaid, N.A. Asiri, M. Abdel Salam, Adsorption behavior of estrogenic compounds on carbon nanotubes from aqueous solutions: kinetic and thermodynamic studies, J. Ind. Eng. Chem. (June) (2013), http://dx.doi.org/ 10.1016/j.jiec.2013.06.023. [9] S¸.S. Bayazit, I˙. I˙nci, Adsorption of Pb(II) ions from aqueous solutions by carbon nanotubes oxidized different methods, J. Ind. Eng. Chem. (March) (2013), http:// dx.doi.org/10.1016/j.jiec.2013.03.023. [10] H. Al-Johani, M. Abdel Salam, Kinetics and thermodynamic study of aniline adsorption by multi-walled carbon nanotubes from aqueous solution, J.Colloid Interface Sci. 360 (2011) 760–767. [11] M. Abdel Salam, S.A. Kosa, G. Al-Zhrani, Simultaneous removal of copper(II), lead(II), zinc(II), and cadmium(II) from aqueous solutions by multi-walled carbon nanotubes, C. R. Chim. 15 (2012) 398–408. [12] M. Abdel Salam, R.C. Burk, Solid phase extraction and determination of poly halogenated pollutants from freshwater using novel chemically modified multiwalled carbon nanotubes using gas chromatography, J. Sep. Sci. 32 (2009) 1060– 1068. [13] M. Abdel Salam, R.C. Burk, Novel application of modified multi-walled carbon nanotubes as a solid-phase extraction adsorbent for the determination of polyhalogenated organic pollutants in aqueous solution, Anal. Bioanal. Chem. 390 (2008) 2159–2170. [14] S. Fatemi, M. Vesali-Naseh, M. Cyrus, J. Hashemi, Improving CO2/CH4 adsorptive selectivity of carbon nanotubes by functionalization with nitrogen-containing groups, Chem. Eng. Res. Des. 89 (2011) 1669–1675. [15] M. Abdel Salam, M.S.I. Makki, M.Y. Abdelaal, Preparation and characterization of multi-walled carbon nanotubes/chitosan nanocomposite and their application as ion exchanging materials, J.Alloy Compd. 509 (2011) 2582–2587. [16] P. Liang, Y. Liu, L. Guo, J. Zeng, H. Lu, Multiwalled carbon nanotubes as solid-phase extraction adsorbent for the preconcentration of trace, J. Anal. At. Spectrom. 19 (2004) 1489–1492. [17] A. Stafiej, K. Pyrzynska, Adsorption of heavy metal ions with carbon nanotubes, Sep. Purif. Technol. 58 (2007) 49–52.

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