Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50http://www.jnanochem.com/content/3/1/50
ORIGINAL Open Access
Decoration of multi-walled carbon nanotubes(MWCNTs) with different ferrite nanoparticlesand its use as an adsorbentAhmed A Farghali1, Mohamed Bahgat2, Waleed M A ElRouby1* and Mohamed H Khedr1
Abstract
A simple and inexpensive synthesis route to produce multi-walled carbon nanotubes (MWCNTs) decorated withCoFe2O4 and Co0.5Ni0.5Fe2O4 nanoparticles using a simplified hydrothermal precipitation is reported. Transmissionelectron microscopy and X-ray diffraction confirm the formation of 14.9 nm CoFe2O4 and 26 nm Co0.5Ni0.5Fe2O4
nanoparticles on MWCNTs surface. The prepared composite was used for methyl green adsorption. The effects ofvarious parameters, such as temperature, initial dye concentration, and composite dosage, were investigated.Experimental results have shown that the amount of adsorbed dye increased with increasing initial dyeconcentration, composite dosage, and temperature. The adsorption kinetic data were analyzed using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. It was found that the pseudo-second-order kineticmodel was the most appropriate model in describing the adsorption kinetics. The adsorption isotherms of methylgreen onto MWCNTs decorated with CoFe2O4 and Co0.5Ni0.5Fe2O4 nanoparticles were determined at 298, 313, and323 K. Equilibrium data were fitted to the Langmuir and the Freundlich isotherm models, then the isothermconstants were determined. The equilibrium data were best represented by the Langmuir isotherm model.Thermodynamic parameters such as changes in the free energy of adsorption, enthalpy, entropy, and activationenergy were calculated.
BackgroundMany industries, such as textile, paper, plastic, anddyestuffs, consume substantial amount of water andalso use chemicals and dyes to color their productsduring the manufacturing process [1,2]. Color is thefirst contaminant recognized in wastewater, and thepresence of very small amounts of dyes in water ishighly visible and undesirable [3]. Most of these dyescontain aromatic rings, which make them carcinogenicand mutagenic [4,5]. Therefore, the removal of dyes iscurrently of high importance for environmental re-mediation. Adsorption technology is one of the mosteffective methods for dye and toxic removal due to itslow cost, high efficiency, simplicity, and insensitivityto toxic substances [6,7].
* Correspondence: [email protected] Department, Faculty of Postgraduate Studies for AdvancedSciences, Beni-Suef University, Beni-Suef 62111, EgyptFull list of author information is available at the end of the article
Due to their large specific surface area and small, hol-low, and layered structures, nanomaterials have recentlydrawn much attention for dye removal applications. Forinstance, carbon nanotubes (CNTs) are attracting in-creasing research interest as promising adsorbents forharmful cations, anions, and other organic and inorganicimpurities present in natural sources of water [8-10].However, it is difficult to separate CNTs from aqueoussolutions because of their small size. There are seriousconcerns over the health and environmental risks ofCNTs once they have been released into the environ-ment [11]. It should be noted that CNTs can enter cells,causing damage to plants, animals, and human beings[12]. Thus, there is potential for CNTs to become an-other source of environmental contaminant if the use ofCNTs is not responsibly managed. Compared with trad-itional centrifugation and filtration methods, the mag-netic separation method is considered as a rapid and
an Open Access article distributed under the terms of the Creative Commonsg/licenses/by/2.0), which permits unrestricted use, distribution, and reproductionroperly cited.
Figure 1 TEM images of pure MWCNT and MWCNT decoratedwith CoFe2O4 nanoparticles and with CO0.5Ni0.5Fe2O4
nanoparticles. Pure MWCNT (a), MWCNTs decorated with CoFe2O4
nanoparticles (b), and MWCNTs decorated with Co0.5Ni0.5Fe2O4
nanoparticles (c).
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 2 of 12http://www.jnanochem.com/content/3/1/50
effective technique for separating adsorbents from envir-onmental applications [13].Magnetic separation technology is a fast and easy
method for separating magnetic adsorbents from anaqueous solution. In recent years, magnetic separationtechnology, combined with the adsorption process, hasbeen widely used for dye removal from wastewaters[14-16]. To date, only a few studies have been conductedon the adsorption of dyes by magnetic CNTs. Qu et al.have prepared multi-walled carbon nanotubes (MWCNTs)filled with Fe2O3 for removal of methylene blue and neu-tral red from aqueous solution [17]. Gong et al. have syn-thesized magnetic MWCNT nanocomposites as adsorbentfor the removal of cationic dyes: methylene blue, neutralred, and brilliant cresyl from aqueous solution [18].Madrakian et al. used magnetic-modified MWCNTs forthe removal of cationic dye crystal violet, thionine, janusgreen B, and methylene blue from water samples [19].In the present work, the main purpose is to demon-
strate a simple and general procedure for the decorationof MWCNTs with magnetic ferrite (CoFe2O4 orCo0.5Ni0.5Fe2O4). The decorated MWCNTs were usedand evaluated as possible sorbents for the removal ofmethyl green from aqueous solution. The effect of me-thyl green concentration, temperature, and contact timeon the adsorption process was investigated. Kineticsand thermodynamics studies have been performed, andthe results have been analyzed. Thermodynamic param-eters, such as ΔG0, ΔH0, and ΔS0, were calculated.
Results and discussionCharacterizations of synthesized materialsFigure 1 shows TEM images of the pure and decoratedMWCNTs. Figure 1a shows the morphological structureof MWCNTs where the crystalline tubular structureof nanotubes are observed. It is observed that thenanotubes have clear inner channels with lengths ofsome microns. Figure 1b displays typical TEM imagesof MWCNTs decorated with CoFe2O4 nanoparticles. Itshows the morphology and the size distribution ofCoFe2O4 nanoparticles. It can be seen that the size ofnanoparticles is distributed from 8.3 to 18.9 nm, andthe mean particle size is about 14.9 nm. CoFe2O4
nanoparticles are seen as dense aggregates. The well-distributed nanoparticles deposited onto the carbonnanotubes demonstrate that the MWCNTs pretreat-ment processing was effective, which resulted in manyactive sites on the carbon nanotubes. Figure 1c showsthe typical TEM image of MWCNTs decorated withCo0.5Ni0.5Fe2O4 nanoparticles. Co0.5Ni0.5Fe2O4 nano-particles dispersed on the sidewalls of the carbonnanotubes and the inner cavity of the tube still clear.In X-ray diffraction (XRD) patterns of CoFe2O4 and
CoFe2O4/MWCNTs the diffraction peak at 18.4o,
30.09o, 35.55o, 43018o, 57.08o, and 62.66o are reflectionsof CoFe2O4. Nevertheless, in XRD patterns of CoFe2O4/MWCNTs, the additional peaks could be well seen at26.066o which is corresponding to graphite (Figure 2).
2- Theta - Scale
20 30 40 50 60 70 80
Co0.5Ni0.5Fe2O4/MWCNTs
Co0.5Ni0.5Fe2O4
CoFe2O4/MWCNTs
CoFe2O4
MWCNTs
1
1
11
1
1
2
2
22
2
22
2
2
33
33
3
3
3
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33
2- Theta - Scale
20 30 40 50 60 70 80
Co0.5Ni0.5Fe2O4/MWCNTs
Co0.5Ni0.5Fe2O4
CoFe2O4/MWCNTs
CoFe2O4
MWCNTs
1
1
11
1
1
2
2
22
2
22
2
2
33
33
3
3
3
3
3
3
33
Figure 2 XRD diffraction patterns of MWCNTs, CoFe2O4, CoFe2O4/MWCNTs, Co0.5Ni0.5Fe2O4 and Co0.5Ni0.5Fe2O4/MWCNTs. 1. Graphite, 2.CoFe2O4, 3. Co0.5Ni0.5Fe2O4.
5001000150020002500300035004000
%T
as prepared MWCNTs
Oxidized MWCNTs
Figure 3 FTIR spectra of the as-prepared and oxidized MWCNTs.
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 3 of 12http://www.jnanochem.com/content/3/1/50
The crystallite size of CoFe2O4 on MWCNTs surface wascalculated using the Scherer equation (Equation 1) [20].
D ¼ 0:89λ=B cosθ; ð1Þwhere λ = 0.154056 nm, Β is the full peak width at halfmaximum value, and θ is the diffraction angle.It was found that the average crystallite size is
about 14.9 nm. XRD patterns of Co0.5Ni0.5Fe2O4 andCo0.5Ni0.5Fe2O4/MWCNTs were also shown in Figure 2.The sample exhibits characteristic diffraction peaks ofCo0.5Ni0.5Fe2O4 at 2θ of 30.22°, 35.6°, 43.18°, 57.28°,and 62.84°, but in XRD of Co0.5Ni0.5Fe2O4/MWCNTs,there is an additional peak at 26.066° which is corre-sponding to graphite. The average crystallite size of theCo0.5Ni0.5Fe2O4 particles is determined using theScherer equation and found it to be about 26.2 nm.Fourier transform infrared spectroscopic analysis of
oxidized MWCNTs has been shown in Figure 3. TheFourier transform infrared spectroscopy (FTIR) study of
oxidized MWCNTs confirms the defective sites at thesurface of MWCNTs, and the presence of functiongroups > C = C (1,642/cm), > C =O (1,025/cm), = CH2
(2,857/cm, 2,925/cm), and -OH (3,439/cm). This leadsto the hydrophilic nature of MWCNTs. These functional
Table 1 Surface area measurements for MWCNTsdecorated with CoFe2O4 and MWCNTs decorated withCo0.5Ni0.5Fe2O4 nanoparticles
Measurement MWCNT decoratedwith CoFe2O4
MWCNT decorated withCo0.5Ni0.5Fe2O4
Surface area (m2/g) 109.54 176
Total porevolume (cm3/g)
0.0505 0.076
Average porediameter (nm)
18.96 17.33
Micro pore volume(cm3/g)
0.109 0.16
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 4 of 12http://www.jnanochem.com/content/3/1/50
groups may also act as anchoring sites for ferrite parti-cles in the decoration process and also in the adsorptionof dye molecules.N2 adsorption-desorption isotherm of MWCNTs dec-
orated with CoFe2O4 and MWCNTs decorated withCo0.5Ni0.5Fe2O4 nanoparticles are shown in Figure 4.There are hysteresis loops clearly visible in the isotherm,which is associated with capillary condensation inmesopores. These mesopores include mesopore-sizedinner cavities and aggregated pores resulted from aggre-gation of decorated MWCNTs. Surface area by theBrunauer-Emmett-Teller (BET) method, micropore vol-ume by the t-plot method, and pore size distributions bythe BJH equation are obtained for MWCNTs decoratedwith CoFe2O4 and Co0.5Ni0.5Fe2O4 as listed in Table 1.This adsorption isotherm exhibited a type-II shape. Itwas observed that there was a small closed adsorption-desorption hysteresis loop with a relative pressure above0.4, which is suggested to be due to the mesopores witha capillary condensation [21].
Adsorption of methyl green dyeEffect of adsorbent dosage on adsorption capacity ofmethyl greenThe mechanism of MWCNT adsorption for methylgreen dyes may be derived from two reasons: the firstreason might be based on the van der Waals interactionsoccurring between the hexagonally arrayed carbonatoms in the graphite sheet of MWCNTs and the aro-matic backbones of the dyes [22]; the second reasonmight be due to the electrostatic attraction between the
Relative pressure, P/Po
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
volu
me
[cc/
g]
0
50
100
150
200
250
300Adsorption (MWCNTs decorated with CoFe2O4)
Desorption (MWCNTs decorated with CoFe2O4)Adsorption (MWCNTs decorated with Co0.5Ni0.5Fe2O4)
Desorption (MWCNTs decorated with Co0.5Ni0.5Fe2O4)
Figure 4 Adsorption-desorption hysteresis loop of MWCNTsdecorated with CoFe2O4 and MWCNTs decorated withCo0.5Ni0.5Fe2O4.
positive cationic dyes and the negative chargedMWCNTs adsorbent surface.It was observed that the percentages of adsorbed dye
increased as the decorated MWCNT dosages wereincreased over the range 0.6 to 1.4 g/L as shown inFigure 5. The increase in the percentage of the removeddye with an adsorbent dosage can be attributed to anincrease in the adsorbent surface, which increased theavailability of the adsorption sites.
Effects of dye concentrationThe amount of adsorbed dye per unit of decoratedMWCNT mass increased as initial dye concentrationincreased due to the increase in the driving force of theconcentration gradient as shown in Figure 6. Theadsorption process at different concentrations was rapidin the initial 30 min then gradually decreased as adsorp-tion proceeded until equilibrium was reached. This pro-gression is expected to be based on the large number ofvacant surface sites available for adsorption during theinitial stage. After a certain time, the remaining vacantsurface sites are difficult to be occupied due to therepulsive forces between dye molecules on the decoratedMWCNTs and bulk phases [23].
Effect of temperatureTo study the effect of temperature on the adsorptionof methyl green dye onto MWCNTs decorated withCoFe2O4 or Co0.5Ni0.5Fe2O4 nanoparticles, the experi-ments were performed at temperatures of 298, 313,and 323 K as shown in Figure 7. It was observed thatthe equilibrium adsorption capacity of methyl greenonto both decorated MWCNTs increased with increas-ing the temperature due to increasing the mobility ofdye molecules.Equilibrium uptake increased with the increasing of
methylene blue concentrations at the present experi-mental range. This is a result of the increase in the driv-ing force from the concentration gradient. In the sameconditions, if the concentration of methyl green in the
Table 2 Isotherm parameters for removal of methyl green by the decorated MWCNTs at different temperatures
Isotherms Parameters Temperatures (K)
MWCNT decorated with CoFe2O4 MWCNT decorated with Co0.5Ni0.5Fe2O4
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 5 of 12http://www.jnanochem.com/content/3/1/50
solution was bigger, the active sites of MWCNTs deco-rated with CoFe2O4 or Co0.5Ni0.5Fe2O4 nanoparticleswere surrounded by much more methyl green ions, andthe process of adsorption would be carried out suffi-ciently. Therefore, the amount absorbed at equilibrium(qe) increased with the increase in equilibrium methylgreen concentrations. From Figure 7, the adsorptioncapacity of methyl green onto MWCNTs decorated withCoFe2O4 are 85.07, 89.41, and 96.24 mg/g at 298, 313,and 323 K, respectively, while the adsorption capacityfor MWCNTs decorated with Co0.5Ni0.5Fe2O4 are167.95, 288.40, and 243.89 mg/g at 298, 313, and 323 K,respectively. The increase of the equilibrium adsorption withthe increase in temperature indicated that the adsorption ofmethyl green ions onto CoFe2O4/MWCNT composite wasendothermic in nature. The higher adsorption capacity of
Time (min)
0 20 40 60 80 100 120 140
Ad
so
rp
tio
n %
0
20
40
60
0.6 g/L CoFe2O4/MWCNTs
1.0 g/L CoFe2O4/MWCNTs
1.4 g/L CoFe2O4/MWCNTs
0.6 g/L Co0.5Ni0.5Fe2O4/MWCNTs
1.0 g/L Co0.5Ni0.5Fe2O4/MWCNTs
1.4 g/L Co0.5
Ni0.5
Fe2
O4
/MWCNTs
0.6 g/L CoFe2O4/MWCNTs
1.0 g/L CoFe2O4/MWCNTs
1.4 g/L CoFe2O4/MWCNTs
0.6 g/L Co0.5Ni0.5Fe2O4/MWCNTs
1.0 g/L Co0.5Ni0.5Fe2O4/MWCNTs
1.4 g/L Co0.5
Ni0.5
Fe2
O4
/MWCNTs
Figure 5 Effects of decorated MWCNT dosages on theadsorption of methyl green dye. Dye concentration = 100 mg/Land T = 298 K.
MWCNTs decorated with Co0.5Ni0.5Fe2O4 than MWCNTsdecorated with CoFe2O4 may be attributed to the high sur-face area of MWCNTs decorated with Co0.5Ni0.5Fe2O4 ormay be due to the small crystallite size of CoFe2O4
nanoparticles on the MWCNTs surface which can act as ablocker for the active sites on the tube surfaces (Figure 1).
Adsorption isothermsThe quantity of the dye that could be adsorbed overMWCNTs decorated with CoFe2O4 or Co0.5Ni0.5Fe2O4
nanoparticles surface is a function of concentration, whichcould be explained by the adsorption isotherms. In thepresent study, the Langmuir [24] and Freundlich [25]isotherms are tested for methyl green dye adsorption.The Langmuir adsorption isotherm assumes that
adsorption takes place at specific homogeneous sites
Time (min)
0 20 40 60 80 100 120 140
Ads
orpt
ion
%
0
20
40
60
50 mg/L with CoFe2
O4
/MWCNTs
100 mg/L with CoFe2
O4
/MWCNTs
200 mg/L with CoFe2
O4
/MWCNTs
50 mg/L with Co0.5
Ni0.5
Fe2
O4
/MWCNTs
100 mg/L with Co0.5
Ni0.5
Fe2
O4
/MWCNTs
200 mg/L with Co0.5
Ni0.5
Fe2
O4
/MWCNTs
50 mg/L with CoFe2
O4
/MWCNTs
100 mg/L with CoFe2
O4
/MWCNTs
200 mg/L with CoFe2
O4
/MWCNTs
50 mg/L with Co0.5
Ni0.5
Fe2
O4
/MWCNTs
100 mg/L with Co0.5
Ni0.5
Fe2
O4
/MWCNTs
200 mg/L with Co0.5
Ni0.5
Fe2
O4
/MWCNTs
Figure 6 Effects of dye concentration on the adsorption ofmethyl green dye. CoFe2O4/MWCNTs and Co0.5Ni0.5Fe2O4/MWCNTs = 1 g/L and T = 298 K).
Ce (mg/L)
0 50 100 150 200 250
q e (m
g/g)
0
50
100
150
200
250
298 K (CoFe2O4/MWCNTs)
313 K (CoFe2O4/MWCNTs)323 K (CoFe2O4/MWCNTs)
298 K (Co0.5Ni0.5Fe2O4/MWCNTs)
313 K (Co0.5Ni0.5Fe2O4/MWCNTs)
323 K (Co0.5Ni0.5Fe2O4/MWCNTs)
Ce (mg/L)
0 50 100 150 200 250
q e (m
g/g)
0
50
100
150
200
250
298 K (CoFe2O4/MWCNTs)
313 K (CoFe2O4/MWCNTs)323 K (CoFe2O4/MWCNTs)
298 K (Co0.5Ni0.5Fe2O4/MWCNTs)
313 K (Co0.5Ni0.5Fe2O4/MWCNTs)
323 K (Co0.5Ni0.5Fe2O4/MWCNTs)
Figure 7 Adsorption isotherms of methyl green onto MWCNTsdecorated with CoFe2O4 and Co0.5Ni0.5Fe2O4 atdifferent temperatures.
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 6 of 12http://www.jnanochem.com/content/3/1/50
within the adsorbent. The Langmuir isotherm can bewritten in this form:
Ce=qe ¼ 1= q0KLð Þ þ 1=q0ð ÞCe; ð2Þ
where Ce (mg/L) is the equilibrium concentration, qe(mg/g) is the amount of adsorbate adsorbed per unitmass of adsorbate, and qo and KL are the Langmuir con-stants related to the adsorption capacity and the rateof adsorption, respectively. When Ce/qe was plottedagainst Ce, a straight line with slope 1/q0 was obtained(Figure 8a,c), indicating that the adsorption of methylgreen onto MWCNTs decorated with CoFe2O4 orCo0.5Ni0.5Fe2O4 nanoparticles follows the Langmuirisotherm. The Langmuir constants KL and qo werecalculated from this isotherm, and their values are listedin Table 2. Another important parameter, RL, is calledthe separation factor or the equilibrium parameter whichis determined from the relation [26]:
RL ¼ 1= 1þ KLCo½ � ð3Þ
where KL is the Langmuir constant (l/mg) and Co
(mg/L) is the highest dye concentration. The value ofRL indicates the type of the isotherm to be either un-favorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1),or irreversible (RL = 0). RL values for methyl green ad-sorption onto MWCNTs decorated with CoFe2O4 orCo0.5Ni0.5Fe2O4 nanoparticles were calculated andfound to be less than 1 and greater than zero indicat-ing the favorable adsorption (Table 2).
The Freundlich isotherm is an empirical equationemployed to describe the heterogeneous systems [27].The Freundlich equation is as follows:
ln qe ¼ ln KF þ 1=nð Þln Ce; ð4Þwhere qe is the amount adsorbed at equilibrium (mg/g)and Ce is the equilibrium concentration of methylgreen. n and KF are Freundlich constants, n giving anindication of how favorable the adsorption process andKF (mg/g (L/mg)1/n) is the adsorption capacity of theadsorbent. The slope 1/n ranging between 0 and 1 is ameasure of the adsorption intensity or the surfaceheterogeneity, becoming more heterogeneous as itsvalue gets closer to 0 [28]. The plot of ln qe versus lnCe (Figure 8b,c) gives straight lines with slope 1/nreflecting that the adsorption of methyl green also fol-lows the Freundlich isotherm. Accordingly, Freundlichconstants (KF and n) were calculated and listed inTable 2.
Kinetics analysesThe kinetic analysis of temperature effect was evaluated.The adsorption capacity increased with the temperature,indicating that the mobility of dye molecules increasedwith temperature, and the adsorption was endothermic.Additionally, increasing the temperature reduces the vis-cosity of the solution and increases the diffusion rate ofdye molecules.The pseudo-first-order, pseudo-second-order, and
intraparticle diffusion models were adopted to test theexperimental data and thereby elucidated the kineticadsorption process. The pseudo first-order model canbe expressed as follows:
ln qe−qð Þ ¼ ln qeð Þ−k1t; ð5Þwhere qe and q are the amounts of methyl greenadsorbed on MWCNTs decorated with CoFe2O4 orCo0.5Ni0.5Fe2O4 nanoparticles at equilibrium and atvarious times t (mg/g) and k1 is the rate constant of thepseudo-first-order model for the adsorption (per mi-nute) [29]. The values of qe and k1 can be determinedfrom the intercept and the slope of the linear plot of ln(qe − q) versus t (Figure 9a,b).The pseudo-second-order model is as follows:
t=q ¼ 1=k2q2e þ t=qe; ð6Þ
where qe and q are the amounts of dye adsorbed ontoMWCNTs decorated with CoFe2O4 or Co0.5Ni0.5Fe2O4
nanoparticles at equilibrium and at various times t (mg/g),and k2 is the rate constant of the pseudo-second-ordermodel for adsorption (g/mg/min) [29]. The slope andintercept of the linear plot of t/q as a function of t(Figure 9c,d) yielded the values of qe and k2. Additionally,
Ce (mg/L)
0 40 80 120 160 200 240
Ce/
q e
Ce/
q e
0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
298 K
313 K
323 K
298 K
313 K
323 K
Ce (mg/L)
0 50 100 150 200 2500.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
298 K
313 K
323 K
298 K
313 K
323 K
Ln Ce
1 2 3 4 5 6
Ln q
e
Ln q
e
3.3
3.6
3.9
4.2
4.5
4.8
298 K
313 K
323 K
298 K
313 K
323 K
Ln Ce
-1 0 1 2 3 4 5 63.5
4.0
4.5
5.0
5.5
298 K
313 K
323 K
298 K
313 K
323 K
(a) (b)
(c) (d)
Figure 8 Langmuir isotherms and Freundlich isotherms. Langmuir isotherms (a,c) for MWCNTs decorated with CoFe2O4 and Co0.5Ni0.5Fe2O4
nanoparticles, respectively, and Freundlich isotherms (b,d) for MWCNTs decorated with CoFe2O4 and Co0.5Ni0.5Fe2O4 nanoparticles, respectively,for methyl green dye adsorption at different temperatures.
Table 3 Coefficients of pseudo-first and second-order adsorption kinetic models and intraparticle diffusion model(methyl green = 100 mg/L, MWCNTs decorated with CoFe2O4 or Co0.5Ni0.5Fe2O4 = 1 g/L)
Orders model Parameters Temperatures (K)
MWCNT decorated with Co0.5Ni0.5Fe2O4 MWCNT decorated with CoFe2O4
298 K 323 K 313 K 298 K 323 K 313 K
Pseudo-first-order model qe Cal. (mg/g) 51.92 87.452 83.12 80.125 19.22 30.96
qe Exp. (mg/g) 44.96 91.59 85.40 82.39 47.58 46.43
Intraparticle diffusion model ki (mg/g min0.5) 3.214 4.98 4.061 3.4 2.05 2.958
C (mg/g) 0.591 −7.685 −8.66 −12.056 22.44 9.22
R2 0.981 0.991 0.999 0.997 0.90 0.958
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 7 of 12http://www.jnanochem.com/content/3/1/50
Time (min)
0 20 40 60 80 100 120 140
Ln (
q e-q t)
1.2
1.6
2.0
2.4
2.8
3.2
3.6
298 K
313 K
323 K
298 K
313 K
323 K
Time (min)
0 20 40 60 80 100 120 140
Ln (
q e-q t)
3.7
3.8
3.9
4.0
4.1
4.2
4.3
4.4
4.5
298 K
313 K
323 K
298 K
313 K
323 K
Time (min)
0 20 40 60 80 100 120
t/qt[m
in/(
mg/
g)]
0
1
2
3
4
298 K
313 K
323 K
Time (min)
0 20 40 60 80 100 120
t/qt[m
in/(
mg/
g)]
0
1
2
3
4
298 K
313 K
323 K
Time (min)
0 20 40 60 80 100 120
t/qt[m
in/(
mg/
g)]
1.0
1.5
2.0
2.5
3.0
3.5
4.0
298 K
313 K
323 K
t1/2 (min1/2)
3 4 5 6 7 8 9 10 11 12
q t (m
g/g)
0
10
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50
298 K
313 K
323 K
t1/2 (min1/2)
3 4 5 6 7 8 9 10 11 12
q t (m
g/g)
0
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298 K
313 K
323 K
t1/2 (min1/2)
3 4 5 6 7 8 9 10 11 12
q t (m
g/g)
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298 K
313 K
323 K
t1/2 (min1/2)
3 4 5 6 7 8 9 10 11 12
q t (m
g/g)
0
10
20
30
40
50
298 K
313 K
323 K
(a) (b)
(c) (d)
(e) (f)
Figure 9 Regressions of kinetic plots at different temperatures. Pseudo-first-order model (a) CoFe2O4/MWCNTs, (b) Co0.5Ni0.5Fe2O4/MWCNTs;pseudo-second-order model (c) CoFe2O4/MWCNTs, (d) Co0.5Ni0.5Fe2O4/MWCNTs; intraparticle diffusion model (e) CoFe2O4/MWCNTs,(f) Co0.5Ni0.5Fe2O4/MWCNTs.
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 8 of 12http://www.jnanochem.com/content/3/1/50
the initial adsorption rate h (mg/g/min) can be deter-mined using the equation h = k2q
2e . The adsorption
process on porous adsorbents generally has a four-stage bulk diffusion, film diffusion, intraparticle
diffusion, and finally, adsorption of the solute onto thesurface. Typically, bulk diffusion and adsorption areassumed to be rapid and therefore not rate determin-ing. Since the pseudo-second-order model cannot
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 9 of 12http://www.jnanochem.com/content/3/1/50
identify the diffusion mechanism, kinetic results wereanalyzed using the intraparticle diffusion model to elu-cidate the diffusion mechanism. Film diffusion wasnegligible, and intraparticle diffusion was the onlyrate-controlling step. The intraparticle diffusion modelis expressed as follows:
q ¼ kit1=2 þ C; ð7Þ
where C is the intercept and ki is the intraparticle dif-fusion rate constant (mg/g/min0.5), which can be deter-mined from the slope of the linear plot of q versus t1/2
[30] (Figure 9e,f ). Table 3 presents the kinetic parame-ters for the removal of methyl green by MWCNTs deco-rated with CoFe2O4 or Co0.5Ni0.5Fe2O4 nanoparticles. TheR2 value of the pseudo-second-order model exceeded 0.99;moreover, the q value (qe,cal) derived from the pseudo-second-order model was consistent with the experimentalq values (qenexp). Hence, this study showed that thepseudo-second-order model best represents adsorptionkinetics. A similar phenomenon has been observed inthe adsorption of Acid Blue 93 by natural sepiolite[31], and Acid Red 57 by surfactant-modified sepiolite[32]. If the regression of q versus t1/2 is linear andpasses through the origin, intraparticle diffusion isthen the sole rate-limiting step [32]. Although the re-gression was linear, the plot did not pass through theorigin (Table 3), indicating that adsorption involvedintraparticle diffusion that was not the only rate-controlling step. Other kinetic mechanisms may controlthe adsorption rate, which is a similar finding to thatobtained from other studies of adsorption [31]. Thevalues of C were helpful in determining the boundarythickness: a larger C value corresponds to a greaterboundary layer diffusion effect [33]. The C valuesincreased with the temperature (298 to 323 K), and so,increasing the temperature promoted the boundary-layer diffusion effect.
Adsorption thermodynamicsThe thermodynamic parameters, namely free energy(ΔG°), enthalpy (ΔH°), and entropy (ΔS°) have an import-ant role to determine spontaneity and heat change for the
Table 4 Thermodynamic parameters of methyl green dye ads
Temperature(K)
MWCNT decorated with CoFe2O4
ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (J/mol/K) Ea (kJ
298 −8.023 31.189 68.64 12.44
313 −9.019
323 −9.746
adsorption process. Thermodynamic parameters werecalculated using the following relations [34]:
ΔG� ¼ −RT ln KLð Þ; ð8Þ
ln KLð Þ ¼ ΔS�=RÞ− ΔH
�=RTÞ;��
ð9Þ
where KL is the Langmuir equilibrium constant (L/mol),R is the gas constant (8.314 J/mol/K), and T is thetemperature (K). ΔH° and ΔS° parameters can be calcu-lated from the slope and intercept of the plot ln KL ver-sus 1/T. From Equation 10, ΔG° was calculated using lnKL values for different temperatures. Results are sum-marized in Table 4. It can be seen that ΔG° values attemperatures 298, 313, and 323 K are negative. Hence,the adsorption process was a spontaneous process. Thedecrease in ΔG° value with the increase of temperatureindicates the efficient adsorption at higher temperature.The positive ΔH° value reflected that the adsorptionprocess is endothermic, and there is a strong interactionbetween MWCNTs decorated with CoFe2O4 orCo0.5Ni0.5Fe2O4 nanoparticles and methyl green. Sincemethyl green ions travel through solution and reach theadsorption sites, it is necessary for them to be stripped out(at least partially) of their hydration shell, that requires en-ergy input. Thus, the positive value of ΔH° indicates thatthe adsorption is increasing with temperature. Moreover,the positive value of ΔS° indicates that the degrees of free-dom increased at the solid–liquid interface during the ad-sorption of methyl green onto MWCNTs decorated withCoFe2O4 or Co0.5Ni0.5Fe2O4 nanoparticles and reflectedthe affinity of the composite toward methyl green ions inaqueous solutions [35].The pseudo-second-order model was identified as the
best kinetic model for the adsorption of methyl green ontoMWCNTs decorated with CoFe2O4 or Co0.5Ni0.5Fe2O4
nanoparticles. Accordingly, the rate constants (k2) of thepseudo-second-order model were adopted to calculate theactivation energy of the adsorption process using theArrhenius equation [36]:
ln k2ð Þ ¼ ln Að Þ− Ea=RTð Þ; ð10Þ
where k2, A, Ea, R, and T are the rate constant of the
orption onto decorated MWCNTs at various temperatures
MWCNT decorated with Co0.5Ni0.5Fe2O4
/mol) ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (J/mol/K) Ea (kJ/mol)
−21.7 17.38 35.9 11.075
−22.27
−22.59
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 10 of 12http://www.jnanochem.com/content/3/1/50
pseudo-second-order model (g/mg/min), the Arrheniusfactor, the activation energy (kJ/mol), the gas constant(8.314 J/mol/K), and the temperature (K), respectively.The activation energy could be determined from the slopeof the plot of ln(k2) versus 1/T (Table 4). Nollet et al. [37]mentioned that the physisorption process normally hadactivation energy of 5 to 40 kJ/mol, while chemisorptionhad higher activation energy (40 to 800 kJ/mol). There-fore, ΔH°, ΔG°, and Ea values confirm that the adsorptionof methyl green onto MWCNTs decorated with CoFe2O4
or Co0.5Ni0.5Fe2O4 nanoparticles was a physisorptionprocess. Lazaridis and Asouhidou [38] stated that in adiffusion-controlled process, the activation energy of ad-sorption was less than 25 to 30 kJ/mol. Based on the re-sults of activation energy and the intraparticle diffusionmodel (Table 4 and Figure 9e,f ), this study proposesthat the adsorption involved intraparticle diffusion, thatwas not the only rate-controlling step, and the otherkinetic models might control the adsorption rate.
ConclusionsMWCNTs decorated with CoFe2O4 and Co0.5Ni0.5Fe2O4
nanoparticles have been successfully prepared by a hydro-thermal precipitation method. The prepared compositeexhibits a homogeneous dispersion of MWCNTs in thematrix and coating of CoFe2O4 and Co0.5Ni0.5Fe2O4
nanoparticles on MWCNTs. This study investigated theremoval of methyl green from aqueous solution by theprepared composites. It was found that Co0.5Ni0.5Fe2O4/MWCNTs composite has a high adsorptive capacity thanCoFe2O4/MWCNTs for methyl green adsorption. Theequilibrium adsorption capacity of methyl green increasedwith the increase in temperature, methyl green concentra-tion, and adsorbent materials. The adsorption kineticscould be quite successfully fitted by a pseudo-second-order kinetic equation. The Langmuir and Freundlich ad-sorption isotherm models were used to express the adsorp-tion phenomenon of the methyl green. The equilibriumdata were well described by the Langmuir model. Thermo-dynamic analyses indicated that the adsorption of methylgreen onto MWCNTs decorated with CoFe2O4 and
Table 5 Structure and characteristics of Methyl green dye
Name C.I. number Formula Molecular we
Methyl green 42585 C26H33N3Cl2 458.5
Co0.5Ni0.5Fe2O4 nanoparticles was endothermic, spontan-eous, and physisorption process.
MethodsMaterial and methodsAll the reagents were of analytical grade and used asreceived without further purification. Cobalt nitrate[Co(NO3)2 · 6H2O], nickel nitrate [Ni(NO3)2 · 6H2O], andferric nitrate [Fe(NO3)3 · 9H2O] were obtained fromWINLAB, Baths, UK. Sodium hydroxide (NaOH) wasobtained from Bio Chem Laboratories, Inc., Grand Rapids,MI, USA and methyl green (Table 5) was obtained fromGurr microscopy materials, BDH chemicals Ltd., Poole,England. MWCNTs were produced by chemical vapor de-position using acetylene cracking over Fe-Co/CaCO3 cata-lyst/support. The method of synthesis had been describedbefore [39]. The raw product contains the support, catalystparticles, and a few amorphous carbons as impurities. Theas-grown MWCNTs were purified using a two-step purifi-cation procedure involving treatment with diluted HCland then with mixture of concentrated nitric acid/sulfuricacid (3:1 by volume, respectively).For the decoration of MWCNTs, a specific amount of
oxidized MWCNTs was first ultrasonicated in 100 ml ofdistilled water for 30 min. Afterward, this suspension wasmixed with a solution of analytical grade Co(NO3)2 · 6H2Oand Fe(NO3)3 · 9H2O in which the Co/Fe molar ratio wasmaintained at 1:2, in case of decoration of MWCNTs withCoFe2O4 nanoparticles. The mixture of Ni(NO3)2 · 6H2O,Co(NO3)2
. 6H2O and Fe(NO3)3 · 9H2O, in which the Ni/Co/Fe molar ratio was maintained at 1:1:4, was used in case ofdecoration of MWCNTs with Co0.5Ni0.5Fe2O4 nano-particles. Then, a sodium hydroxide solution (6 molar) wasadded dropwise into the above mixture with vigorous stir-ring until the pH value reaches 10. The mixture was stirredfor another 30 min for complete reaction then the solutionwas neutralized. The produced mixture was then placed ina Teflon-lined autoclave and maintained at 220°C for 10 h.The obtained precipitates were rinsed repeatedly with waterand ethanol, and then, dried at 100°C for 12 h in a vacuumoven. The prepared powder was denoted as CoFe2O4/MWCNTs and Co0.5Ni0.5Fe2O4/MWCNTs.
ight λmax Chemical structure
630 to 634, 420 nm
Farghali et al. Journal Of Nanostructure in Chemistry 2013, 3:50 Page 11 of 12http://www.jnanochem.com/content/3/1/50
The synthesized and decorated MWCNTs were char-acterized by powder XRD analysis (D8 Advance, BrukerAXS, Inc, Madison, WI, USA), transmission electron mi-croscopy (TEM, Jeol JEM-1230, Akishima-shi, Japan),and FTIR (JASCO 410, Mary‘s Court Easton, MD, USA).The BET surface area was determined from adsorptionisotherms using a Quantachrome NOVA automated gassorption system report.
Dye adsorption experimentsFor studying the effect of dye concentration on the ad-sorption process equal volumes, 50 ml of various methylgreen dye concentrations, were taken in number of 250-ml beakers. Definite weight of the adsorbent materials(CoFe2O4/MWCNTs or Co0.5Ni0.5Fe2O4/MWCNTs) wasadded in each beaker and shacked well. At differentinterval times, 5 ml of the sample solution was with-drawn and the change in characteristic absorption at thespecific beaks was measured using an ultraviolet–visible(UV–vis) spectrophotometer (Jasco 530), from whichthe concentration of dye was inferred.For evaluation, the effect of adsorbent materials con-
centration on the adsorption process equal volumes,50 ml of definite methyl green dye concentrations, weretaken in a number of 250-ml beakers. Different weightsof the adsorbent materials were added in each beakerand shacked well. At different interval times, 5 ml of thesample solution was withdrawn and the change in char-acteristic absorption at the specific peaks was measuredusing an UV–vis spectrophotometer (Jasco 530), fromwhich the concentration of dyes was inferred.For identifying the adsorption isotherm and kinetics
adsorption experiments were performed using 250-mlglass bottles, specific amount of the adsorbent materialswere inserted into the bottles along with 50 ml of themethyl green dye solutions. The initial concentration ofthe dye (C0) was varied from 50 to 400 mg/L. The glassbottles were sealed and placed within a temperaturecontrol box to maintain water temperature. Tempera-tures that were studied include 298, 313, and 333 K. Atthe end of the equilibrium period, the suspensions wereseparated for later analysis of the dye concentration. Theamount of dye adsorption at equilibrium qe (mg/g) wascalculated from the following equation:
qe ¼ V C0−Ceð Þ=W ; ð11Þ
where C0 and Ce (mg/L) are the liquid-phase concentra-tions of methyl green dye at initial and equilibrium,respectively. V (L) is the volume of the solution, and W(g) is the mass of the used adsorbent. The concentrationof the dye before and after adsorption was determinedusing a spectrophotometer (Jasco 530).
Kinetic experimental procedures were identical to theequilibrium tests. The effect of contact time on theamount of adsorbed methyl green dye was investigatedat specific initial dye concentration and at varying tem-peratures (298, 313, and 333 K). The amounts ofadsorbed dye on the adsorbent materials at any time, t,were calculated from the concentrations in solutionsbefore and after adsorption. At any time, the amount ofadsorbed dye (mg/g) onto the adsorbent materials wascalculated from the mass balance equation as follows:
qt ¼ V C0−Ctð Þ=W ; ð12Þ
where qt is the amount of adsorbed dye on the adsorbentmaterials at any time (mg/g); C0 and Ct are the initial andliquid-phase concentrations of dye at any time (mg/L),respectively; V (L) is the volume of dye solution, and W(g) is the mass of the adsorbent materials.
Competing interestsThe authors declare that they have no competing interests.
Authors‘ contributionsWR carried out the experimental part and wrote the manuscript forpublication. AF participated in the idea and the design of the study. MB, MK,and AF as supervisors, participated in this work. All authors read andapproved the final manuscript.
AcknowledgementsThe authors gratefully acknowledge the financial support of this work byBeni-Suef University nanotechnology research team.
Author details1Nanotechnology Department, Faculty of Postgraduate Studies for AdvancedSciences, Beni-Suef University, Beni-Suef 62111, Egypt. 2Minerals TechnologyDepartment Central Metallurgical R & D Institute (CMRDI), Helwan 11421,Egypt.
Received: 26 April 2013 Accepted: 17 May 2013Published: 15 July 2013
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doi:10.1186/2193-8865-3-50Cite this article as: Farghali et al.: Decoration of multi-walled carbonnanotubes (MWCNTs) with different ferrite nanoparticlesand its use as an adsorbent. Journal Of Nanostructure in Chemistry2013 3:50.
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