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Tian, G. Cui and S. Yan, RSC Adv., 2015, DOI: 10.1039/C5RA14748D.

http://dx.doi.org/10.1039/c5ra14748d

http://pubs.rsc.org/en/journals/journal/RA

http://crossmark.crossref.org/dialog/?doi=10.1039/C5RA14748D&domain=pdf&date_stamp=2015-09-14

Facile and cost-effective synthesis of CNT@MCo2O4 (M=Ni, Mn, Cu, Zn)

core-shell hybrid nanostructures for organic dye removal

Haizhen Li, Zebin Sun, Yaxi Tian, Guijia Cui, Shiqiang Yan

*

College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou

730000, P.R. China

* Corresponding author. Tel./fax: +86 931 8912582

E-mail address: [email protected] (S. Yan)

Page 1 of 27 RSC Advances

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View Article OnlineDOI: 10.1039/C5RA14748D


Abstract

This study presents four hybrid materials of CNTs@MCo2O4 (M=Ni, Mn, Cu, Zn)

which were prepared via a simple chemical bath deposition method at room

temperature followed by a post-annealing treatment. The resulting materials were

characterized by different techniques including SEM, EDS, XRD, FTIR, and BET

surface area measurement and were first used as adsorbents for removal of methyl

orange (MO). Experimental dates suggested that CNTs@MCo2O4 samples especially

CNTs@NiCo2O4 showed excellent adsorption properties (1188.3mg/g). The kinetic

adsorption of different materials could be accurately described by the

pseudo-second-order model and overall rate process was apparently influenced by

external mass transfer and intraparticle diffusion. Furthermore, Langmuir and

Freundlich isotherms are employed and both of them fitted the experimental data with

good correlation coefficients. Moreover, the thermodynamic parameters indicated that

the adsorption process was spontaneous and physical adsorption mechanisms

including electrostatic interaction might play a dominant role in the adsorption

mechanism between MO and adsorbents. Results of this work are of great

significance for environmental applications of CNTs@MCo2O4 as promising

adsorbents for organic pollutant from wastewater.

Keywords: CNTs, Spinel cobaltate, removal, methyl orange

1. Introduction

With the development of technology and industry, aqueous dye pollution,

commonly caused by the wastewater discharged from textile, leather and paper

Page 2 of 27RSC Advances

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industries, has emerged as one of most serious pollution in water especially in

developing countries.1 Unfortunately, the color wastewater is rather difficult to treat

because of the non-degradable synthetic origins and aromatic structures of the

pollutants.2 Meanwhile, the hazardous pollutants pose ecological risks and are toxic to

aquatic life and mankind if discharged directly into receiving waters, because they

may be mutagenic and carcinogenic and can cause a severe health hazard to human

beings, such as dysfunction of kidney, reproductive system, liver, brain and central

nervous system.3, 4

Thus, it is imperative to treat the dyes in their wastes to an

acceptable level before discharging them into the environment.

There are many physical, chemical and biological treatment techniques for

removing dyes from industrial effluent, such as chemical oxidation, photochemical

degradation, biological degradation, ultrasonic degradation, reverse osmosis, flotation

and adsorption procedures.5-10

Among those methods, adsorption is recognized to be

the most popular method because of its low cost, easy operation and high removal

efficiency.11-17

Among various available adsorbents, transition metal oxides have potential

applications in water treatment owing to their excellent characteristics such as

environmentally friendly synthesis methods and low price.18

In the large family of

metal oxides, spinel cobaltites, MCo2O4 (M=Ni, Mn, Cu, Zn) are very promising

multifunctional materials and have been widely used in several applications such as

catalysis, environmental remediation and electrochemical supercapacitors as well as

adsorption.19-21

However, the low specific area of spinel cobaltites micrometer powder


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could greatly reduce the contact area of the materials and dye molecules, which could

significantly decrease the adsorption capacity of spinel cobaltites. In addition, the

pure spinel cobaltites are easy to agglomerate. In order to alleviate the

above-mentioned problems, multi-walled CNTs are introduced due to their high

specific surface area, very high strength, chemical stability and low density.22-24

The

multi-walled CNTs could not only buffer the agglomeration of the active metal oxide

nanostructures, but also enhance the specific area of spinel cobaltites. And the active

sites of spinel cobaltites can be also further increased, which greatly increased the

adsorption capacity for dyes. Thus far, many studies focussing on CNT-supported

MCo2O4 have been reported,25-28

however, to the best of our knowledge, few of them

are selected to be adsorbents for removing dyes.

In this work, we grew MCo2O4 (M=Ni, Mn, Cu, Zn) on individual CNTs via a

simple and cost-effective chemical bath deposition method at room temperature

followed by a post-annealing treatment. The chemical co-precipitation method has

several advantages such as simplicity, low processing temperature and pressure, low

cost, high product purity and the ability to control the morphology compared with

most conventional methods. This prepared CNTs@MCo2O4 hybrids possess high

specific surface area and display a remarkable adsorption performance for removal

organic dyes. And their adsorption capability for organic pollutants were investigated

using methyl orange (MO) as a model. The CNTs@MCo2O4 were characterized by

field-emission scanning electron microscopy (FESEM), X-ray energy dispersive

spectrometry (EDS), X-ray diffraction (XRD), Fourier-transform infrared


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spectroscopy (FTIR) analysis, Brunauer, Emmett, and Teller surface area

measurement (BET) analyses. The adsorption capacities of different materials, effects

of pH, contact time at different initial concentrations were examined in bath

experiments. Moreover, the adsorption kinetics, rate-controlling mechanisms, and

thermodynamics of the adsorbents were also comprehensively investigated.

2. Experimental section

2.1. Materials

MWCNTs having diameter 20–30 nm and length of 5–20 µm, were purchased

from Alpha Nano Technology Co., Ltd. (China). Polyvinylpyrrolidone (PVP),

absolute ethanol and NH3·H2O (25-28 wt%) were purchased from the Sinopharm

Group Chemical Reagent Co., Ltd. CoCl2·6H2O, NiCl2·6H2O, MnCl2·4H2O,

CuCl2·2H2O and ZnCl2 were obtained from Kelong chemical company (Chengdu,

China). All chemicals are of analytical grade and used without further purification.

2.2. Preparation of CNT@MCo2O4

CNT@MCo2O4 nanomaterials were fabricated by a simple chemical bath

deposition method at room temperature followed by a post-annealing treatment. And

the detailed preparation procedure is illustrated in Scheme 1. Taking the preparation

of CNT@NiCo2O4 as an example, briefly, 35 mg MWCNT powder was uniformly

dispersed into 100ml 7 mg ml-1

PVP aqueous solution with ultrasonication for 1.5 h.

Then 0.9517g (4 mmol) of CoCl2·6H2O and 0.4754g (2 mmol) of NiCl2·6H2O were

dissolved into the MWCNT dispersion with vigorous stirring for 0.5 h to form a black

solution, followed by a drop-by-drop addition of 5% NH3·H2O solution until its pH


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reached 9. The mixture was kept still for 4 h at room temperature. The obtained

precipitate was filtered, washed with water and ethanol several times to remove the

surfactant and the residual ions, and dried at 60 ℃for 12 h under vacuum. In order to

obtain the CNT@NiCo2O4 nanostructure, the hybrid precursor was finally annealed in

air at 300 ℃ for 3 h with a slow heating rate of 1 ℃min-1

. With the same procedure

and amounts of CoCl2·6H2O and MWCNT, MnCl2·4H2O (0.396 g, 2mmol),

CuCl2·2H2O (0.341 g, 2mmol) and ZnCl2 (0.273 g, 2 mmol) were used instead of

NiCl2·6H2O for the syntheses of other CNT@MCo2O4 (M = Mn, Cu and Zn) hybrids,

respectively.

2.3 Material characterizations

The as-prepared CNT@MCo2O4 nanomaterials were characterized with

field-emission scanning electron microscopy (FESEM, JSM-6701F) and X-ray energy

dispersive spectrometry (EDS, Apollo40SDD). The crystallinity of the samples was

investigated by X-ray diffraction (XRD, XRD-6000, shimadzu, Japan). Fourier

transform infrared spectra (FTIR) were carried out using a NEXUS 670 FTIR

equipped with a pressed KBr pellets in wavenumber range of 400-4000 cm-1

. The

BET surface area, pore volume, and pore size were characterized using nitrogen

adsorption at liquid nitrogen temperatures (Sorptomatic 1990, Thermo, USA).

2.4. Adsorption experiments

Standard solutions of methyl orange (MO) in six different concentrations

(100-600 mg L-1

) were prepared by dissolving analytical grade methyl orange in

distilled water.


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To determine the adsorption capacities of different materials, the CNT@MCo2O4

were prepared as adsorbents for MO removal. 5 mg sample of the different adsorbents

was added to each 25 mL of MO solution with initial concentration of 100 mg L-1

to

600 mg L-1

(100 mg L-1

intervals). The mixture was sonicated for 5 minutes at room

temperature and stirred for 120 minutes and then filtered using a 0.45 µm

microporous membrane filter. The filtered MO solutions were diluted and measured

by a spectrophotometric method at 465 nm.

The effect of pH on MO adsorption was studied by varying the solution pH from

3.0 to 11.0, with the initial MO concentration of 100 mg L-1

. The pH of each solution

was adjusted using 1 M HCl or NaOH solution.

Adsorption kinetic experiments were conducted by adding 300 mg of different

CNT@MCo2O4 nanomaterials to each of 1500 ml MO solution, and the MO

concentration was 600 mg L-1

at 298 K. Samples were withdrawn at predetermined

time intervals and filtered through a 0.45 µm filter.

The amount of MO adsorption at equilibrium qe (mg/g) was calculated using the

following equation:

�� =�(��)

� (1)

Where V is the volume of the MO solution (L); C0 and Ce are the initial and

equilibrium concentration (mg L-1

) of MO respectively; and m is the weight of the

adsorbents (g).


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Scheme 1 Schematic presentation of the preparation procedure of CNT@MCo2O4

3. Results and discussion

3.1 Characterization of the adsorbents

The morphology of the CNT@NiCo2O4, CNT@MnCo2O4, CNT@CuCo2O4,

CNT@ZnCo2O4 and CNT were characterized by field-emission scanning electron

microscopy (FESEM) observation and transmission electron microscope (TEM). Fig.

1(a-d) shows the representative FESEM images of the as-prepared CNT@MCo2O4

hybrids. It can be found that the surface of CNT@MCo2O4 is flake shaped and

finegrained. And the surface of CNT (Fig.1e) is relatively smooth compared with

CNT@MCo2O4. Fig.1 (f) is the TEM of CNT@NiCo2O4. It can be seen that NiCo2O4

is uniformly coated on the CNT. However, the morphology of the CNT is not easy to

distinguish. The possible reason is that CNT has only left a skeleton through

annealing process in air in the process of our material preparation. In all cases,

MCo2O4 nanomaterials were found well-grown around individual CNTs to form a


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conformal coating on the surface, showing that the method of chemical bath

deposition at room temperature followed by a post-annealing treatment could be

generally used in the synthesis of various CNT@MCo2O4 hybrids.

Fig.1. SEM images of (a) CNT@NiCo2O4, (b) CNT@MnCo2O4, (c) CNT@CuCo2O4,

(d) CNT@ZnCo2O4 and (e) CNT. TEM image of (f) CNT@NiCo2O4

Fig.2. EDS patterns of (a) CNT@NiCo2O4, (b) CNT@MnCo2O4, (c) CNT@CuCo2O4

and (d) CNT@ZnCo2O4

To confirm the composition of the hybrids, the EDS attached to the FESEM was


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measured and the corresponding EDS spectra of the as-prepared CNT@MCo2O4

hybrids was displayed in Fig.2 (a-d). The EDS spectra of the CNT@MCo2O4 hybrids

revealed the presence of M (Ni, Mn, Cu, and Zn), Co, C and O with the atomic ratio

ratio of M/Co close to 1:2. The C mainly comes from the CNTs, while O from both

MCo2O4 and some oxygen-containing functional groups of CNTs. The results of EDS

confirmed that the MCo2O4 were successfully synthesized.

Fig.3. XRD patterns of (a) CNT@NiCo2O4, (b) CNT@MnCo2O4, (c)

CNT@CuCo2O4, (d) CNT@ZnCo2O4 and (e) CNT

The crystallographic phase of CNT@NiCo2O4, CNT@MnCo2O4,

CNT@CuCo2O4, CNT@ZnCo2O4 and CNT were identified by XRD, and the typical

wide-angle diffraction patterns are shown in Fig.3 (a-e). It can be noticed that

CNT@MCo2O4 consisted of seven well-defined diffraction peaks, which can be well

indexed to the (220), (311), (222), (400), (422), (511), and (440) crystal planes of

cubic spinel structural NiCo2O4 (JCPDS card no. 20-0781), MnCo2O4 (JCPDS card


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no.23-1237), CuCo2O4 (JCPDS card no. 78-2177), and ZnCo2O4 (JCPDS card no.

23-1390), respectively.29-32

It is worth mentioning that no other peaks are observed,

which effectively confirms the purity of the obtained MCo2O4 phase, which was

supported on CNTs. For CNT, two characteristic peaks can be clearly observed,

corresponding to the (002) and (100) crystal planes. However, the typical peaks (002)

of CNT are not observed in CNT@MCo2O4. The possible reason is that the typical

peaks of CNT are covered by the peaks of MCo2O4.

Fig.4. FTIR spectra of (a) CNT@NiCo2O4, (b) CNT@MnCo2O4, (c) CNT@CuCo2O4,

(d) CNT@ZnCo2O4 and (e) CNT

The FTIR spectra of CNT@NiCo2O4, CNT@MnCo2O4, CNT@CuCo2O4,

CNT@ZnCo2O4 and CNT are presented in Fig.4 (a-e). It is obvious that all spectra

exhibit adsorption bands at about 3400, and 1625 cm-1

, which can be associated to the

-OH stretching and bending vibration for the free or physically absorbed water

molecules.33, 34

The corresponding metal oxide characteristic peaks positioned at 650

and 555 cm-1

for the samples (a-d) further reveals the formation of the spinel MCo2O4


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nanostructure. These bands mainly correspond to the stretching vibrational modes of

M-O (M=Ni, Mn, Cu, Zn) in the tetrahedral positions and Co-O bands in octahedral

positions, respectively.34

Fig.5. Typical N2 adsorption–desorption isotherms of CNT@NiCo2O4,

CNT@MnCo2O4, CNT@CuCo2O4, CNT@ZnCo2O4 and CNT

As illustrated in Fig.5, the adsorption and desorption isotherms of N2 for the

CNT@NiCo2O4, CNT@MnCo2O4, CNT@CuCo2O4, CNT@ZnCo2O4 and CNT are

clearly of type IV, according to the IUPAC classification of adsorption isotherms.35

Type IV isotherms characteristically show the simultaneous presence of micro- and

mesopores.36

And the physical properties of the CNT@NiCo2O4, CNT@MnCo2O4,

CNT@CuCo2O4, CNT@ZnCo2O4 and CNT, such as surface areas, total pore volumes,

pore size are summarized in Table 1. It can be seen that the BET surface area, total

pore volume and average pore size of CNT@NiCo2O4 are 144.0 m2 g

-1, 0.8506 cm

3g

-1

and 23.63 nm, respectively, which were higher than that of other CNT@MCo2O4.

Generally, the BET surface area, total pore volume and average pore size have great


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influence on the material performance especially the adsorption capacity. Thus, the

CNT@MCo2O4 may have excellent adsorption performance, and this conclusion will

be confirmed in the part 3.2.1.

Table 1 Pore surface characteristics of different materials

Materials Surface area (m2

g-1

) Pore volume (cm3 g

-1) Pore size (nm)

CNT@NiCo2O4 144.0 0.8506 23.63

CNT@MnCo2O4 103.64 0.5877 22.68

CNT@CuCo2O4 73.45 0.2630 14.32

CNT@ZnCo2O4 120.58 1.0156 32.99

CNT 174.48 0.8532 19.56

3.2 Adsorption experiment studies

3.2.1. Adsorption capacities of different materials

The adsorption capacities of different materials including the CNT@NiCo2O4,

CNT@MnCo2O4, CNT@CuCo2O4, CNT@ZnCo2O4 and CNT were investigated, as

shown in Fig. 6. It is obvious that the adsorption capacities of CNT@MCo2O4 were

all increased with the initial concentration of MO increased in. And all of the samples

possess relatively high adsorption performance. Compared with CNT@MCo2O4, the

adsorption capacity of CNT is markedly lower. For CNT@MCo2O4, CNT is the

skeleton and MCo2O4 is coated on the CNT, and then MCo2O4 would be the mainly

adsorbent. Sulfonate groups of MO dyes would bind with the hydroxy-groups on the

surface of metal oxides through electrostatic interaction and hydrogen bond

interaction. However, the CNT surface possesses few functional groups so that the


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adsorption capacity of CNT for MO is much lower. In addition, CNT@NiCo2O4

shows the best adsorption capacity among the CNT@MCo2O4 nanomaterials,

indicating that CNT@NiCo2O4 has outstanding adsorption performance compared

with other adsorbents. It is well known that the adsorption properties for dyes depend

on the high specific surface area and the pore properties of the adsorbents.37

The

excellent adsorption capacity of CNT@NiCo2O4 may be ascribed to its large BET

surface area.

Fig.6. Adsorption capacities of different materials with regard to MO adsorption.

3.2.2. Effect of solution pH and Adsorption mechanism

The pH of the system exerts profound influence on the surface charge of the

adsorbent, the degree of ionization of different pollutants, the dissociation of

functional groups on the active sites of the adsorbent as well as the structure of the

dye.38

Thus, the adsorption experiments were performed at initial concentration of

100 mg L-1

and pH range between 3.0 and 11.0 to investigate the effect of the pH on

the adsorption capacities of the CNT@MCo2O4. As shown in Fig. 7, the adsorption


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capacity of CNT@NiCo2O4, CNT@MnCo2O4, CNT@CuCo2O4 and CNT@ZnCo2O4

were all decreased with the increase in pH from 3.0 to 11.0. At acidic pH, the surfaces

of CNT@MCo2O4 materials were positively charged due to protonation, which could

be expressed as MOH2+ (M represents Ni, Mn, Cu, Zn, Co), therefore, resulting in a

higher adsorption performance for MO because of the strong electrostatic attraction

between positively charged surfaces and the anionic MO species. This is mainly the

adsorption mechanism for CNT@MCo2O4 removing MO. However, with the pH

increased, the adsorption performance of CNT@MCo2O4 decreased. The possible

reason is that a growing number of OH- caused repulsion between the negatively

charged surface and the dye molecules.

Fig.7. Effect of solution pH on MO adsorbed onto CNT@MCo2O4

3.2.3. Effect of contact time

The adsorption equilibrium time is one of the most important parameters which

influenced the design of wastewater treatment systems.39

To determine the effect of


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contact time on the adsorption process, adsorption experiments of various materials

with initial MO concentration of 600 mg L-1

were conducted for different contact

times. The results are presented in Fig. 8. It can be easily observed that the adsorption

capacity of MO onto the adsorbents drastically increase during the initial stage and

then at a slow speed, which may be ascribed to the availability of a large number of

vacant active sites on the adsorbent surface 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 CNT@MCo2O4 and the bulk phase. Eventually, a

plateau is reached after a certain amount of time in all curves.40

Fig.8. Effect of contact time on MO adsorbed onto different materials at 600mg/L

3.2.4. Adsorption dynamics

In order to better evaluate the adsorption kinetics, two kinetic models including

pseudo-first-order and pseudo-second-order model were applied to analyze the

experimental data. These two kinetic models can be represented by the following


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equations: 9, 41

log(�� − ��) = log�� −��

�.��, (2)

�

��=

�

�� +

�

� (3)

Where qe and qt (mg g-1

) are the amounts of MO adsorbed at equilibrium and at time t

(min), respectively, while k1 (min-1

) and k2 (g mg-1

min-1

) represent the observed rate

constants for pseudo first order and pseudo second order models.

Table 2 Kinetic parameters for MO adsorption onto different adsorbents at 600 mg/L

Adsorbents

qe,exp

Pseudo-first-order model Pseudo-second-order model

k1 qe r2 k2×10

-4 qe r

2

CNT@NiCo2O4 1211 0.01621 374.7 0.9027 2.809 1206.9 0.9939

CNT@MnCo2O4 678.4 0.03871 291.6 0.9627 2.754 709.2 0.9996

CNT@CuCo2O4 804 0.01911 381.4 0.9796 2.20 819.7 0.9907

CNT@ZnCo2O4 906.9 0.02796 360.4 0.9152 2.018 925.9 0.9918

The values of qe and k1 for the pseudo-first-order kinetic model are determined

from the intercepts and the slopes of the plots of log (qe−qt) versus t, and plots of t/qt

versus t allows calculation of the rate constant k2 and qe for the pseudo-second-order

kinetic model. The straight line plots of them are presented in Fig.S1 and Fig.S2,

respectively. The calculated kinetic parameters and correlation coefficients for the

removal of MO with CNT@MCo2O4 are tabulated in Table 2. It is obvious that all the

experimental data showed better compliance with the pseudo-second-order kinetic

model in terms of higher correlation coefficient values (r2> 0.99). Furthermore, the qe

calculated from the pseudo-second-order kinetic model are rather approximate with


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those from the experimental results. These results suggest that the

pseudo-second-order kinetic model is more suitable to describe the adsorption

process.

3.2.5. Weber–Morris kinetic model

As the above two kinetic models were not able to highlight the diffusion

mechanisms and rate controlling steps that affect the adsorption process, we applied

the Weber–Morris kinetic model to study the steps of adsorption process. This

equation can be written as follow: 41

�� = �� ⁄ + (4)

Where qt is the adsorption capacity at time t, kd is the intra-particle diffusion rate

constant, C is a constant that gives idea about the thickness of the boundary layer, the

larger the value of C, the greater the boundary layer effect is. The values of kd and C

can be obtained by plotting qt against t1/2

.

The multilinear plots of intra-particle diffusion process of MO adsorption onto

CNT@MCo2O4 are presented in Fig.9. It suggests that two steps have taken place

during the adsorption process. The first linear portion is related to external diffusion

which is the diffusion of MO molecules from solution to the external surface of

CNT@MCo2O4. And the second regions can be attributed to the internal diffusion of

MO from the external surface to the internal pores of the adsorbent. According to the

Weber–Morris kinetic model, if the regression of qt versus t1/2

is linear and passes

through the origin, then intra-particle diffusion is the sole rate-limiting step. The

regression is linear, but the plot does not pass through the origin, suggesting that


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adsorption involve intra-particle diffusion, but that is not the only rate-controlling

step.42

From the Fig.9, it can be noticed that the plots did not pass through the origin,

indicating that both external diffusion and intra-particle diffusion are significant.

Fig.9. Weber–Morris plots for the kinetic modeling of MO adsorbed onto different

materials at 600mg/L

3.2.6. Adsorption isotherms

The equilibrium isotherms are very important for understanding the adsorption

systems. Adsorption isotherm provides information about the surface properties of

adsorbent, the adsorption behavior and the design of adsorption systems. It also

affords essential physiochemical data for assessing the applicability of the adsorption

process as a complete unit operation.43

In this work, two typical isotherm models,

such as Langmuir and Freundlich isotherm are applied to fit the equilibrium data. The

Langmuir model is based on the assumption that adsorption is localized on a

monolayer and all adsorption sites at the adsorbent are homogeneous.44

Whereas the


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Freundlich isotherm presumes that the multilayer of the adsorption process occurs on

a heterogeneous surface. These two models can be described by the following

equations: 10

�

�=

�

!�"+

�

�" (5)

ln�� = ln%& +�

'ln � (6)

Where qe (mg g-1

) is the amount of solution adsorbed per unit mass of the adsorbent,

Ce is the solute equilibrium concentration. And qm (mg g-1

) is the maximum adsorbate

amount, and b (L mg-1

) is the Langmuir constant related to adsorption affinity of the

binding sites. KF and n represent the Freundlich constants related to the adsorption

capacity and adsorption intensity, respectively.

The separation factor (RL) related to Langmuir isotherm is also used to evaluate

the feasibility of adsorption on adsorbent. It can be defined as follows:

() =�

�*!�� (7)

In this equation, C0 represents the initial MO concentration (mg L-1

) and b (L mg-1

)

refers to the Langmuir constant. And the values of RL suggest that the types of the

isotherm which can be irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) or

unfavorable (RL > 1)

The Langmuir and Freundlich isotherms are shown in Fig.S3 and Fig.S4,

respectively. The parameters such as b, qm, KF, n, RL and R2

values determined from

these two isotherms are displayed in Table 3. According to the determination

parameters, both Langmuir and Freundlich models closely fitted the experimental data

with good correlation coefficients (R2). And the RL values are lying in the 0-1 range,


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demonstrating that the isotherms are favorable for adsorption of MO onto

CNT@MCo2O4. The computed maximum MO adsorption capacities are 1257, 840.3,

869.6 and 980.4 mg g-1

at 293K for CNT@NiCo2O4, CNT@MnCo2O4,

CNT@CuCo2O4 and CNT@ZnCo2O4, respectively. It is clear that the CNT@MCo2O4

have excellent adsorption performance.

Table 3 Parameters of the Langmuir and Freundlich isotherms for MO adsorption

onto different adsorbents

Langmuir isotherm Freundlich isotherm

Adsorbents qm b r2 RL×10 1/n KF r

2

CNT@NiCo2O4 1257 0.0300 0.9900 0.525-2.497 0.2587 261.20 0.9954

CNT@MnCo2O4 840.3 0.0265 0.9939 0.591-2.737 0.2287 200.25 0.9823

CNT@CuCo2O4 869.6 0.0308 0.9955 0.513-2.452 0.2142 229.05 0.9949

CNT@ZnCo2O4 980.4 0.0322 0.9889 0.491-2.367 0.1794 310.7 0.9507

Thermodynamic parameters can improve our understanding regarding the

inherent energetic changes involved during the sorption process. The free energy

change (∆G0) was calculated from the variation of the thermodynamic equilibrium

constant (K0) to estimate the type of adsorption for the CNT@MCo2O4 toward MO,

which was determined by the following equations: 9

%� =+,

+=

-,

-

�

� (8)

∆.� = −(/ln%� (9)

In the equations, K0 is the thermodynamic equilibrium constant; as is the activity of

the adsorbed MO, ae is the activity of the MO in the solution at equilibrium, γs is the


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activity coefficient of the adsorbed MO, γe is the activity coefficient of MO in solution

at equilibrium; R is the gas constant (8.314 Jmol-1

K-1

); T is the sorption temperature in

Kelvin. As the MO concentration in the solution decreases and approaches zero, the

activity coefficient approaches unity, and reducing equation (8) to the following form:

%� =+,

+=

�

� (10)

Therefore, the values of K0 can be acquired by plotting ln(qe/Ce) versus qe and by

extrapolating qe to zero, as shown in Fig.S5.

Table 4 Thermodynamic parameters for MO adsorption onto different adsorbents

Adsorbents lnK0 r2 ∆G

0(kJ/mol)

CNT@NiCo2O4 5.4891 0.9724 -13.378

CNT@MnCo2O4 5.2259 0.9844 -12.737

CNT@CuCo2O4 5.7166 0.9995 -13.933

CNT@ZnCo2O4 6.5943 0.8801 -16.072

The acquired thermodynamic parameters such as lnK0, ∆G0 and R

2 are shown in

Table 4. The negative ∆G0 values confirm the feasibility of the process and the

spontaneous nature of the adsorption. Generally ∆G0 for physisorption is less than

that for chemisorption. The former is between -20 and 0 kJ/mol and the latter is

between −80 and -400 kJ/mol.45

Thus, the ∆G0 results implied that physisorption

might dominate the adsorption of MO onto CNT@MCo2O4.

3.3 Comparison of adsorbent performance with literature data

Previously some researchers investigated several adsorbents for the removal of

MO from aqueous solutions. Table 5 shows a comparison of the adsorption capacities


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of MO onto various adsorbents. It clearly can be seen that the MO removal capacities

of CNT@MCo2O4 (M = Ni, Mn, Cu, Zn) are higher than that of other adsorbents,38, 39,

46-51indicating that CNT@MCo2O4 nanomaterials have great potential as dye

adsorbents in wastewater treatment.

Table 5 Maximum adsorption capacities of MO onto various adsorbents

adsorbates adsorbents qmax(mg/g) Refs.

Methyl

orange

Carbon nanotubes 52.9 46

CNTs-A 149 38

NH2-MWCNTs 185.5 47

Activated carbon/Fe3O4 nanocomposites 303.0 48

Mesoporous MgO nanoplates 370 49

SiO2–Al2O3 mixed oxides 381 50

Powdered activated carbon modified by HNO3 384.6 48

H-δ-MnO2 nanoparticles 427 51

Phragmites australis activated carbon 437.9 39

CNT@NiCo2O4 1188.3 This work

CNT@MnCo2O4 790.5 This work

CNT@CuCo2O4 826.9 This work

CNT@ZnCo2O4 935.4 This work

4. Conclusions

In summary, CNT@MCo2O4 (M=Ni, Mn, Cu, Zn) nanomaterials were prepared

via a simple chemical bath deposition method at room temperature followed by a


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post-annealing treatment. Their adsorption properties for the removal of MO from

aqueous solution have been studied for the first time and the adsorption process were

systematically researched under different conditions including pH, contact time,

initial MO concentrations. The CNT@MCo2O4 samples especially CNTs@NiCo2O4

showed excellent adsorption properties. And the maximum adsorption capacity for

CNT@MCo2O4 (M=Ni, Mn, Cu, Zn) are 1188.3, 790.5, 826.9 and 935.4 mg/g,

respectively. The high adsorption capacity of CNT@MCo2O4 may be ascribed to their

high BET surface area. The kinetic data of the adsorption fitted well with the

pseudo-second-order kinetic model and the results of Weber–Morris kinetic model

indicated that overall process appeared to be jointly influenced by intraparticle

diffusion and external mass transfer. Moreover, Langmuir and Freundlich isotherms

are employed to describe the equilibrium adsorption and both of them fitted the

experimental data with good correlation coefficients. The adsorption mechanism was

also discussed, which could be attributed to physical adsorption including electrostatic

interactions and hydrogen bond interaction between MO and the adsorbents. The

results obtained in this work suggest that CNTs@MCo2O4 should be a promising

adsorbent for the removal of MO from wastewater.

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of 27 RSC Advances

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