Synthesis of multi-walled carbon nanotube–hydroxyapatite

Journal of Molecular Liquids 179 (2013) 46–53
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis of multi-walled carbon nanotube–hydroxyapatite composites and its
application in the sorption of Co(II) from aqueous solutions
Zhengjie Liu a, b, Lei Chen a,⁎, Zengchao Zhang a, Yueyun Li a, Yunhui Dong a, Yubing Sun b
a
b
School of Chemical Engineering, Shandong University of Technology, Zibo 255049, PR China
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, PR China
a r t i c l e
i n f o
Article history:
Received 3 November 2012
Received in revised form 6 December 2012
Accepted 6 December 2012
Available online 20 December 2012
Keywords:
Co(II)
MWCNT–HAP
Sorption
pH
FA/HA
a b s t r a c t
In this paper, multi-walled carbon nanotube–hydroxyapatite (MWCNT–HAP) composites were synthesized
and characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy
and X-ray diffraction (XRD) and used as an original adsorbent for Co(II) sorption from aqueous solutions.
The sorption of Co(II) on MWCNT–HAP composites was investigated as a function of contact time, pH, foreign
ions, fulvic acid (FA), humic acid (HA) and temperature. The results indicated that K+, Mg 2+ and Ca 2+ ions
restrained Co(II) sorption on MWCNT–HAP composites at low pH. In the whole pH ranges, anions (i.e., ClO4−,
NO3− and Br− herein) made no obvious effect on Co(II) sorption, while F− ions dramatically enhanced Co(II)
sorption. The presence of FA and HA enhanced Co(II) sorption on MWCNT–HAP composites at low pH values,
but suppressed Co(II) sorption at high pH values. The Freundlich and Langmuir models were used to imitate
the Co(II) sorption isotherms at three different temperatures. The thermodynamic data (ΔG 0, ΔS 0, and ΔH 0)
counted from the temperature dependent sorption isotherms suggested that the sorption of Co(II) on
MWCNT–HAP composites was a spontaneous and endothermic process. The high sorption capacity of
Co(II) on MWCNT–HAP composites suggested that the MWCNT–HAP composites were suitable materials in
heavy metal pollution cleanup.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Heavy metal pollution is a worldwide environmental problem because of its toxicity to living creatures and other negative effects on
the receiving soils and waters. Therefore, the removal of noxious
heavy metals is crucial in environmental pollution cleanup. Many
methods have been applied to remove noxious metals such as precipitation [1], ion exchange [2], coagulation [3], membrane processes [4]
and sorption [5,6]. Cobalt, one of the common toxic metals affecting
the environment, is widely present in industrious wastewater, cobaltbearing mineral mining and nuclear wastewater. Cobalt is suspected
to cause memory loss in humans and is reported to induce neurotoxicity
in animal experiments [7].
Carbon nanomaterials play a major role in environmental pollution
management. The interaction between metal ions and carbon
nanomaterials has been reported intensively [8,9]. Carbon nanotubes
(CNTs) are one of the most widely used materials in heavy metal pollution cleanup. Since the find of carbon nanotubes (CNTs) by Iijima in
1991 [10], the unique physicochemical properties of CNTs, such as
their particular structural, electrical, physical, electro-mechanical and
mechanical properties, have made them an important platform in
nanoscience [11]. These prominent properties make them promising
⁎ Corresponding author.
E-mail address: [email protected] (L. Chen).
0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2012.12.011
materials for various applications, such as energy storage [12], chemical
sensors [13] and nanoelectronic devices [14]. The CNTs possess high
sorption capacity for the removal of various pollutants from water due
to their large surface areas and their ability to build electrostatic interactions, and CNTs have aroused great interests in environmental pollution
management [8,15–17]. CNTs have shown exceptional sorption capabilities and high sorption efficiencies for a number of organic pollutants,
such as naphthol, phenol, aniline and their substitutes [18–20]. In addition, CNTs were also found to be superior adsorbents for heavy metal
ions [21]. Although the CNTs have high sorption capacity for different
organic and inorganic pollutants, the CNTs still do not have high selectivity for special pollutants. The surface modification of CNTs to graft
special functional groups can improve the high selectivity of CNTs in
the sorption of pollutants [22]. Shao et al. [23,24] modified the surface
properties of CNTs by using plasma technique, and the plasma surface
grafted CNTs improved the sorption of CNTs in the removal of Pb(II),
U(VI) and Cu(II) ions from aqueous solutions.
Hydroxyapatite (HAP, Ca10(PO4)6(OH)2), a major inorganic constituent of teeth and bones [25], has recently attracted considerable
attention in the field of environment for its use as an adsorbent for
the sorption of heavy metal ions from aqueous solutions because of
its superior ion exchange ability [26]. As HAP is easily synthesized
from wastes such as waste gypsum, waste shell and cow bone, it is
becoming a promising material for the remediation of soil and wastewater. However, HAP is usually supposed in powder and calcined
Z. Liu et al. / Journal of Molecular Liquids 179 (2013) 46–53
2. Experimental section
2.1. Chemicals
All chemicals used in this study were purchased as analytically pure
and used without any further depuration. Milli-Q water was used in
this study. Humic acid (HA) and fulvic acid (FA) were extracted from
the soil samples of Hua-Jia Ridge of Gansu province (China). The main
constituents of FA were: C: 50.15%, O: 39.56%, H: 4.42%, N: 5.38%, and
S: 0.49%; and those of HA were: O: 31.31%, C: 60.44%, H: 3.53%,
N: 4.22%, and S: 0.50% [29].
2.2. Synthesis of MWCNT–HAP composites
The MWCNTs in 3 mol/L nitric acid were sonicated for 20 min, and
then refluxed for 12 h at 90 °C to gain the carboxyl functional groups
(MWCNT–COOH). Then the compound was filtered under vacuum
through a 0.45 μm Millipore polycarbonate membrane and washed
with Milli-Q water until the pH value of the filtrate was ~ 6.0. The derived MWCNT–COOH was dried for 24 h at 100 °C under vacuum. The
MWCNT–HAP composites were prepared according to a research
study with some modification [30]. Typically, 20 mg of MWCNT–
COOH was reacted with 5 mL of 0.02 mol/L calcium chloride aqueous
solution and stirred for 1 h. Then, 5 mL of 0.02 mol/L aqueous solution of sodium phosphate dibasic was added dropwise with continuous stirring for 1 h. Thus derived samples were filtered and washed
several times with Milli-Q water to separate other unreacted reagents
and byproducts. Finally, the composites were dried under vacuum for
24 h at 50 °C, and used in the following experiments.
2.3. Characterization of MWCNT–HAP nanocomposites
The MWCNT–HAP sample was characterized with Fourier transform infrared (FT-IR) spectrometry (PE2000). The spectral resolution
was set to 1 cm −1, and 150 scans were collected for every spectrum.
The XRD pattern was determined from a D/Max-rB equipped with a
rotation anode using Cu Kα radiation (λ = 0.15406 nm). The XRD device was operated at 80 mA and 40 kV. The morphology of the material was observed using a field emission scanning electron microscope
(SEM, Sirion200, FEI Corp., Holland). The sample was grinded using
an agate mortar and pestle and ultrasonically dispersed in alcohol.
Then it was placed on a micro grid of silicon, and diverted to the analysis chamber in the SEM.
2.4. Batch sorption experiments
All the sorption experiments were conducted by using batch technique in polyethylene centrifuge tubes. The stock suspensions of
MWCNT–HAP and NaCl solution were pre-equilibrated for 48 h and
then a known volume of Co(II) solutions of varying initial concentrations was added. The pH was corrected to desired values with negligible
volumes of 1.0 or 0.1 M HCl or NaOH. After equilibrium, the liquid
and solid phases were separated by centrifugation at 9000 rpm for
20 min. The distribution coefficient (Kd) and sorption percentage (%)
were calculated from the following equations:
Kd ¼
C 0 −C e V
m
Ce
Sorption % ¼
ð1Þ
C 0 −C e
100%
C0
ð2Þ
where m (g) is the mass of MWCNT–HAP and V (mL) is the volume of
the solution.
For desorption experiments, the suspension of attapulgite was
centrifuged (9000 rpm, 20 min) at the end of the sorption experiments;
half of the supernatant was pipetted out and an equal volume of background electrolyte solution with the same pH value was added. Then
the mixture was shaken and centrifugation was done under the same
conditions as in the sorption experiments.
All measurements were the averages of triplicate determinations.
The relative standard deviations of the data were about 5%.
3. Results and discussion
3.1. Characterization of MWCNT–HA composites
The FTIR spectrum of the MWCNT–HAP sample is shown in Fig. 1.
The characteristic absorption peaks of the phosphate groups for HAP
are observed at 565 cm −1, 611 cm −1 and 1033 cm −1, which are attributed to the P\O bond of PO43− stretching vibration and the corresponding deformation vibration [31]. In addition, the FTIR spectrum
shows the characteristic peaks of the disordered structure of
MWCNTs at 1391 cm −1 and the band at 3427 cm −1 corresponds to
the characteristic band of \OH. The band at 1641 cm −1 is attributed
Transmittance
pellet forms, which is a disadvantage when recovering it after removing heavy metals from wastewater [25].
Several studies declared that polymer composites incorporating
adsorbents provide an emerging method to remove heavy metals
from aqueous solutions [26–28]. For instance, HAP composites with
cellulose, polyurethane and polyacrylamide were prepared and their
removal property of heavy metal ions was examined. However, the
MWCNT–HAP composites used as an adsorbent to remove heavy
metal ions from aqueous solutions have not been reported in early
studies. Herein, we synthesized MWCNT–HAP composites by self assembly method through an aqueous solution reaction and studied
their removal capacity of Co(II) from aqueous solutions. The MWCNTs
have enough hydrophilic groups, thus make access of heavy metal ions
in aqueous solutions easy while the MWCNT–HAP composites are
expected to give no influence on the ion exchange reaction of HAP.
The objectives of this study are: (1) to synthesize the MWCNT–HAP
composites and to characterize the synthesized composites in detail;
(2) to study the Co(II) sorption on MWCNT–HAP as a function of shaking time, pH, foreign ions and humic substances; (3) to study the Co(II)
sorption on MWCNT–HAP at three different temperatures and calculate
the thermodynamic data; (4) to discuss the mechanisms of Co(II) sorption on MWCNT–HAP; and (5) to evaluate the possible application of
the material in environmental pollution cleanup.
47
611
1391
565
1641
3427
1040
500
1000
1500
2000
2500
3000
Wavenumbers(cm-1)
Fig. 1. FTIR spectrum of the MWCNT–HAP sample.
3500
4000
48
Z. Liu et al. / Journal of Molecular Liquids 179 (2013) 46–53
75
Intensity
Sorption(%)
60
45
30
pH = 5.5 ± 0.1
pH = 6.0 ± 0.1
pH = 6.5 ± 0.1
15
10
20
30
40
50
60
0
70
5
10
Fig. 2. XRD pattern of the MWCNT–HAP sample.
to the C_O band [5,32]. These results indicate the formation of HAP
on the matrix of MWCNTs.
Fig. 2 shows the XRD pattern of the synthesized MWCNT–HAP composites. The diffraction peaks of the planes at 2θ = 25.86°, 31.78°, 32.9°
and 39.84 are the characteristic peaks of HAP [31]. In addition, the diffraction peak at 25.86° for MWCNTs attributing to hexagonal graphite
planes was consistent with the planes of HAP.
Fig. 3 displays the SEM image of the MWCNT–HAP composites.
The SEM image shows that an entangled reticulation of MWCNTs
with clusters of HAP attached to them, indicates that HAP is bounded
on the surfaces of MWCNTs and forms MWCNT–HAP composites.
3.2. Time-dependent sorption
The sorption of Co(II) on MWCNT–HAP composites as a function of
contact time was investigated at three different pH values (5.5± 0.1,
6.0 ± 0.1, and 6.5± 0.1). As one can see from Fig. 4, the sorption of
Co(II) on MWCNT–HAP occurs quickly and 5 h of contact time can
achieve the sorption equilibrium. The sorption of Co(II) on MWCNT–
HAP increases with increasing pH. The functional groups of MWCNT–
HAP composites contribute to the uptake of Co(II) and the properties
of these functional groups are affected by pH values. The surface properties of MWCNT–HAP are influenced by pH values, thus influence
Co(II) sorption. The whole sorption dynamic process can be divided
into two steps: an initial fast sorption, followed by a much slower sorption. The fast Co(II) sorption rate in the beginning of the contact time is
15
20
25
Time(h)
2Theta(degree)
Fig. 4. Effect of contact time on Co(II) sorption to MWCNT–HAP, T = 293 K, m / V =
0.6 g/L, C[Co(II)]initial = 1.69 × 10−4 mol/L, I = 0.01 M NaCl.
owing to the rapid diffusion of Co(II) from the solution to the outer surfaces of the MWCNT–HAP. The subsequent slow sorption process is due
to the longer diffusion range of Co(II) into the inner pores of MWCNT–
HAP composites or the exchange with cations in the inner surface of
MWCNT–HAP composites [33]. According to the above results, the contact time was fixed to 24 h in the following experiments to achieve
complete equilibrium.
A pseudo-second-order rate equation is used to simulate the kinetic sorption of Co(II) on MWCNT–HAP [5]:
t
1
1
¼
þ t
qt 2k′ qe 2 qe
ð3Þ
where qt (mg/g) is the amount of Co(II) ions adsorbed on the surface of
MWCNT–HAP at time t (h), and qe (mg/g) is the equilibrium sorption
capacity. k′ (g/(mg·h)) represents the rate constant of pseudosecond-order kinetics. Fig. 5 shows the linear plot of t / qt versus t. The
qe and k′ values counted from the intercept and slope of the linear
plot of t /qt versus t are shown in Table 1. The correlation coefficient
(R2) for the pseudo-second-order is very close to 1 (R2 = 0.999),
suggesting that the sorption system of Co(II) on MWCNT–HAP can be
described by the pseudo-second-order process well.
3.0
2.5
pH = 5.5 ± 0.1
pH = 6.0 ± 0.1
pH = 6.5 ± 0.1
t/qt(h g/mg)
2.0
1.5
1.0
0.5
0.0
0
5
10
15
20
25
Time(h)
Fig. 3. SEM image of the MWCNT–HAP sample.
Fig. 5. Pseudo-second-order rate simulation of Co(II) sorption on MWCNT–HAP. T =
293 K, m / V= 0.6 g/L, C[Co(II)]initial = 1.69 × 10−4 mol/L.
Z. Liu et al. / Journal of Molecular Liquids 179 (2013) 46–53
Table 1
Kinetic parameters of Co(II) sorption on MWCNT–HAP at different pH values.
pH
Pseudo-second-order
5.5
6.0
6.5
Cs (mg/g)
K (g·mg−1·min−1)
R2
8.9
9.6
10.5
0.107
0.131
0.230
0.999
0.999
0.999
3.3. Effect of anions
Fig. 6A shows the effect of monovalent anions on the sorption of
Co(II) from aqueous solution to MWCNT–HAP as a function of pH
values ranging from 3.0 to 11.0 in 0.01 M NaCl, NaNO3 and NaClO4 solutions, respectively. The results indicate that the sorption of Co(II) on
MWCNT–HAP is not influenced by the background electrolyte foreign
anions. The radius order of the three inorganic acid radicals is ClO4− >
NO3− > Cl − [5]. The negatively charged inorganic anions may form
complexes with the functional groups on the surfaces of MWCNT–
HAP. However, the effects of Cl −, NO3− or ClO4− on Co(II) sorption to
MWCNT–HAP are still very weak, suggesting that surface complexes
are formed on MWCNT–HAP surfaces. The effect of monovalent anions on Co(II) sorption from a solution to MWCNT–HAP can be negligible. The result is similar to Ni(II) sorption on GMZ bentonite [5].
However, the sorption of Th(IV) on Na-bentonite was influenced by
foreign anions [34]. The results demonstrate that the influence of
foreign anions on heavy metal ion sorption is influenced by many
factors, such as the properties of the adsorbent, the properties of
the metal ions and other environmental parameters such as pH and
temperature [35].
Fig. 6B shows the effect of halogen ions on the sorption of Co(II) to
MWCNT–HAP composites in 0.01 mol/L NaBr, NaF and NaCl solutions.
As can be seen from Fig. 6B the presence of F − promotes the sorption
of Co(II) dramatically, while no obvious difference is observed in the
presence of Br − and Cl − ions. A previous study reported that F − can
be adsorbed on an adsorbent surface easily in an acidic range via the
ion exchange reaction [36]. The crystal radius of Cl − (1.81 Å) and that
of Br − (1.95 Å) are much higher than that of F − (1.36 Å) [37], which
contributes partially to this result. Because of the great influence of
F − on the sorption, more researches and attentions are necessary to
study the mechanism of the mediate role of F − on Co(II) sorption
on MWCNT–HAP composites.
3.4. Effect of cations
The sorption of Co(II) on MWCNT–HAP as a function of pH in 0.01 M
LiCl, NaCl and KCl, is shown in Fig. 7A. One can see that the Co(II)
A
A
100
100
80
Sorption(%)
80
Sorption(%)
49
60
40
NaCl 0.01mol/L
NaNO3 0.01mol/L
20
60
40
LiCl 0.01mol/L
NaCl 0.01mol/L
KCl 0.01mol/L
20
NaClO4 0.01mol/L
0
0
2
4
6
8
10
12
2
4
6
pH
10
12
B
B
100
100
80
80
Sorption(%)
Sorption(%)
8
pH
60
40
NaF 0.01mol/L
NaCl 0.01mol/L
NaBr 0.01mol/L
20
0
60
40
0
2
4
6
8
10
NaCl 0.01mol/L
MgCl2 0.01mol/L
20
12
pH
Fig. 6. Effect of anions on Co(II) sorption to MWCNT–HAP. T = 293 K, m / V = 0.6 g/L,
C[Co(II)]initial = 1.69 × 10−4 mol/L.
CaCl2 0.01mol/L
2
4
6
8
10
12
pH
Fig. 7. Effect of cations on Co(II) sorption to MWCNT–HAP. T = 293 K, m / V= 0.6 g/L,
C[Co(II)]initial = 1.69 × 10−4 mol/L.
50
Z. Liu et al. / Journal of Molecular Liquids 179 (2013) 46–53
sorption is clearly influenced by the cations. The sorption percentage of
Co(II) on MWCNT–HAP at pH b 9.0 is in the sequence: Li+ > Na+ > K +,
which is similar to the order of their radii of hydrations: K + = 2.32,
Na+ = 2.76 and Li+ = 3.4 Å [38]. The results indicate that the cations
may alter the surface properties of MWCNT–HAP and thus affects the
sorption of Co(II) on MWCNT–HAP surfaces. Before the addition of
Co(II), Li+/Na+/K + ions have achieved sorption equilibration on
MWCNT–HAP surfaces. The sorption can be regarded as an exchange
of Co(II) with surface adsorbed Li+/Na+/K + ions. The radius of K + is
smallest in those of the three cations and therefore K + ion has the
highest affinity to the surface of MWCNT–HAP and the highest direction
for counter-ion exchange with the surface functional groups of
MWCNT–HAP, which reduces Co(II) sorption on MWCNT–HAP composites. However, at pH> 9.0, no obvious difference of Co(II) sorption
in LiCl, NaCl and KCl solutions is observed, which is attributed to the formation of inner-sphere surface complexes or surface precipitates at
high pH values.
Fig. 7B shows the effect of divalent cations (Ca 2+ and Mg 2+) on
the sorption of Co(II) on MWCNT–HAP composites. The sorption of
Co(II) on MWCNT–HAP composites in the presence of Ca 2+ and
Mg 2+ ions is lower than that of Co(II) in the presence of Na + and
no obvious difference is observed in the sorption curves of Co(II) in
A
100
Sorption(%)
80
60
40
FA 20mg/L
FA 10mg/L
no FA
20
0
2
3
4
5
6
7
8
9
10
the presence of Ca 2+ and Mg 2+ ions. Harter and Naidu [39] declared
that the sorption of cations decreased with the increasing valence of
competitive cations because the increasing of cation valence can
make the potential in the level of sorption less negative at pH b pHpzc
(point of zero charge). The higher valent ions are much more strongly
and easily adsorbed by MWCNT–HAP, and the divalent cations could
occupy two sites by forming complexes via ion charge reactions.
3.5. Effects of FA and HA
Humic substances (HS) are a main portion of dissolved natural
organic compounds which influence the transport and sorption of
metal ions and radionuclides significantly. Fig. 8A and B shows the
effects of FA and HA on Co(II) sorption on MWCNT–HAP composites.
As can be seen from Fig. 8A and B, the presence of FA/HA enhances
the Co(II) sorption on MWCNT–HAP at pH b 7.0 and then the sorption
decreases with the increasing of pH at pH > 7.0. The sorption curve of
Co(II) on MWCNT–HAP in the presence of 20 mg/L FA/HA is higher
than that of Co(II) in the presence of 10 mg/L FA/HA at pH b 7.0,
while the sorption curve of Co(II) in the presence of 20 mg/L FA/HA
is lower than that of Co(II) in the presence of 10 mg/L FA/HA at
pH > 7.0. At low pH values, the negatively charged FA/HA are easily
adsorbed on the MWCNT–HAP surface, which provides more functional groups to form complexes with Co(II) and therefore improves
Co(II) sorption. However, the sorption of Co(II) is reduced with increasing HA/FA concentration in high pH values. With increasing
pH, the negatively charged HA/FA become difficult to be adsorbed
on the negatively charged MWCNT–HAP. The residual free HA/FA
molecules in aqueous solutions can form soluble HS–Co complexes
in a solution and thereby reduce Co(II) sorption on MWCNT–HAP at
high pH values, and the relative strength between the complexes of
Co(II) with FA/HA in solution is larger than that of Co(II) with
MWCNT–HAP, and thus results in the reducing sorption of Co(II) at
high pH values. The results are very similar to that of Co(II) sorption
on MX-80 bentonite [40].
3.6. Effect of temperature and thermodynamic study
11
pH
B
The sorption isotherms of Co(II) on MWCNT–HAP composites at
T = 293, 313 and 333 K are shown in Fig. 9. One can see that the sorption process increases with increasing temperature, which indicates
that the sorption of Co(II) on MWCNT–HAP is favored at high temperature. The Langmuir and Freundlich equations are commonly used for
describing adsorption equilibrium for water and wastewater treatment applications [41].
100
2.5x10-4
80
60
Cs(mol/g)
Sorption(%)
2.0x10-4
40
HA 20mg/L
HA 10mg/L
no HA
20
3
4
5
6
7
8
9
10
1.0x10-4
333 K
313 K
293 K
5.0x10-5
0
2
1.5x10-4
11
pH
Fig. 8. Effect of FA (A) and HA (B) on Co(II) sorption to MWCNT–HAP as a function of
pH, T = 293 K, m / V= 0.6 g/L, C[Co(II)]initial = 1.69 × 10−4 mol/L, I = 0.01 M NaCl.
0.0
0.0
1.0x10-4
2.0x10-4
Ce(mol/L)
Fig. 9. Sorption isotherms of Co(II) on MWCNT–HAP at three different temperatures,
pH = 6.0 ± 0.1, m/ V = 0.6 g/L, I = 0.01 M NaCl.
Z. Liu et al. / Journal of Molecular Liquids 179 (2013) 46–53
The Langmuir sorption isotherm assumed that sorption occurred
at homogeneous and specific sites on the adsorbent, which can be
expressed by the following equation [42]:
Cs ¼
Table 2
The parameters for Langmuir and Freundlich at different temperatures.
T (K)
Langmuir
Cs
bC s max C e
:
1 þ bC e
ð4Þ
51
max
(mol/g)
−4
293
313
333
2.76 × 10
3.62 × 10−4
3.55 × 10−4
Freundlich
R2
b (L/mol)
3
7.142 × 10
5.66 × 103
6.99 × 103
0.990
0.994
0.997
kF (mol1−n Ln/g)
−2
6.4 × 10
7.6 × 10−2
3.7 × 10−2
n
R2
0.665
0.696
0.634
0.986
0.986
0.971
The linear form of the Langmuir equation can be expressed as:
Ce
1
Ce
¼
þ
C s bC s max C s max
ð5Þ
It can be expressed in the line form:
where Cs (mol/g) is the amount of Co(II) retained per unit weight of
MWCNT–HAP at equilibrium; Ce (mol/L) is the equilibrium solution
phase concentration of Co(II); Cs max (mol/g), is the maximum sorption capacity and b (L/mol) is a constant that relates the energy of
sorption.
The Freundlich sorption isotherm model is an empirical model and
it is represented as [5]:
n
Cs ¼ K F Ce :
ð6Þ
A
1.4
293 K
313 K
333 K
Ce/Cs(g/L)
1.2
1.0
0.8
logC s ¼ logK F þ n logC e
where kF (mol 1 − nL n/g) is the sorption capacity when the Co(II) equilibrium concentration equals to 1 and n is the Freundlich constants
related to the intensity of adsorption.
The experimental datum of Co(II) sorption (Fig. 9) is simulated
with Langmuir and Freundlich models and the results are displayed
in Fig. 10. Table 2 shows the relative values counted from the two
models. From the R 2 for Langmuir and Freundlich isotherms, one
can find that the Langmuir model fits the experimental datum better
than the Freundlich model, which indicates that the sorption sites
have equal energy [43]. What's more, MWCNT–HAP composites
have a limited sorption capacity, so the sorption process could be described by the Langmuir model better, since an exponentially increasing sorption was supposed in the Freundlich model [44]. The values of
Cs max calculated from the Langmuir model are the lowest at T =
293 K and the highest at T = 333 K, which indicates that Co(II) sorption is enhanced with high temperature.
Thermodynamic parameters such as the Gibbs free energy
change (ΔG 0), enthalpy change (ΔH 0), and entropy change (ΔS 0)
for Co(II) sorption on MWCNT–HAP composites are counted from
the sorption isotherms. The value of ΔG 0 is counted from the following equation:
0
0.6
ΔG ¼ −RT ln K
0.4
0.0
4.0x10-5 8.0x10-5 1.2x10-4 1.6x10-4 2.0x10-4 2.4x10-4
Ce(mol/L)
ð7Þ
0
ð8Þ
where K 0 is the equilibrium constant. The values of lnK 0 are gained
by plotting lnKd versus Ce (Fig. 11) and extrapolating Ce to zero. Constants of the linear fit of lnKd versus Ce for Co(II) sorption on
B
-3.6
-3.8
333 K
313 K
293 K
7.4
-3.9
LnKd(ml/g)
LogCs(mol/g)
7.6
293 K
313 K
333 K
-3.7
-4.0
-4.1
-4.2
7.2
7.0
6.8
-4.3
6.6
-4.4
-4.8
-4.6
-4.4
-4.2
-4.0
-3.8
-3.6
LogCe(mol/L)
Fig. 10. Langmuir (A) and Freundlich (B) isotherms of Co(II) sorption on MWCNT–HAP
at three different temperatures, pH = 6.0 ± 0.1, m / V= 0.6 g/L, I = 0.01 M NaCl.
0.0
4.0x10-5 8.0x10-5 1.2x10-4 1.6x10-4 2.0x10-4 2.4x10-4
Ce(mol/L)
Fig. 11. Linear plots of lnKd versus Ce of Co(II) sorption on MWCNT–HAP. pH = 6. 0 ±
0.1, m / V= 0.6 g/L, I = 0.01 M NaCl.
52
Z. Liu et al. / Journal of Molecular Liquids 179 (2013) 46–53
Table 3
Constants of linear fit of lnKd vs. Ce (lnKd =A+BCe) for Co(II) sorption on MWCNT–HAP.
Table 5
Comparison of Co(II) sorption by various sorbents.
T (K)
A
B
R2
Sorbents
Conditions
Cs max (mg/g)
References
293
313
333
7.45
7.54
7.72
−3795
−3458
−3795
0.994
0.995
0.988
Hydroxyapatite
Magnetic MWCNTs/IO
Palygorskite
Nedalco sludge
Eerbeek sludge
Magnetite/graphene oxide
MWCNT–HAP
Lemon peel
Al-pillared bentonite
T = 303
T = 293
T = 308
T = 293
T = 293
T = 303
T = 293
T = 298
T = 303
7.90
8.84
8.88
11.71
12.45
12.98
16.26
25.64
38.61
[31]
[46]
[47]
[48]
[48]
[49]
This work
[50]
[51]
Table 4
Values of thermodynamic parameters for Co(II) sorption on MWCNT–HAP.
T (K)
ΔG0 (kJ/mol)
ΔS0 (J/(mol·K))
ΔH0 (kJ/mol)
293
313
333
−18.15
−19.63
−21.38
80.75
5.52
5.66
5.52
3.7. Regeneration
MWCNT–HAP composites are listed in Table 3. The value of the standard entropy change (ΔS 0) is counted from the equation:
∂ΔG0
ΔS ¼ −
∂T
!
0
K, pH = 6.0
K, pH = 6.4
K, pH = 6.0
K, pH = 7.0
K, pH = 7.0
K, pH = 6.8
K, pH = 6.0
K, pH = 6.0
K, pH = 6.0
:
ð9Þ
P
The average standard enthalpy change (ΔH 0) is calculated from
the equation:
The repeated availability of MWCNT–HAP through many cycles of
desorption/sorption was investigated to evaluate the application potential of MWCNT–HAP in the removal of Co(II) from wastewater in
possible applications. Considering the loss of the MWCNT–HAP during each cycle, the amount of MWCNT–HAP and the volume of
Co(II) solution were adjusted to the comparable measurement. As
can be seen from Fig. 12, the sorption capacity of MWCNT–HAP to
Co(II) decreases slightly from 5.64 mg/g to 5.45 mg/g after five
rounds. The excellent regeneration capacity suggests that MWCNT–
HAP composites can be used repeatedly as an effective adsorbent
for the sorption of Co(II) from large volumes of aqueous solutions.
3.8. Comparison with other sorbents
0
0
0
ΔH ¼ ΔG þ TΔS :
ð10Þ
The values derived from Eqs. (8)–(10) are tabulated in Table 4.
The positive value of ΔH 0 demonstrates that the Co(II) sorption is
an endothermic process. In addition, sorption isotherms at different
temperatures can also prove it. The negative values of ΔG 0 show the
spontaneous sorption of Co(II) on MWCNT–HAP composites. The
value of ΔG 0 decreases with the increase of temperature, indicating
an increase in the sorption at high temperature. Cations are readily
desolvated at high temperature, thus the sorption becomes more
favorable. The positive values of ΔS 0 indicate the affinity of
MWCNT–HAP toward Co(II) in aqueous solutions and some structure
changes on the MWCNT–HAP [45]. The result of Co(II) sorption on
MWCNT–HAP is an endothermic and spontaneous process.
The maximum Co(II) sorption capacities of MWCNT–HAP calculated
from the Langmuir model equation were compared with other adsorbents reported in previous studies and were compiled in Table 5.
Although a direct comparison of MWCNT–HAP with other adsorbents
is difficult because of the different experimental conditions applied, it
has been found that the sorption capacity of MWCNT–HAP is higher
than that of other sorbents mentioned in Table 5. The high sorption
capacity of MWCNT–HAP makes MWCNT–HAP an attractive sorbent
for the removal of Co(II) from large volumes of aqueous solutions in
Co(II) pollution cleanup.
4. Conclusions
From the results of the Co(II) sorption on MWCNT–HAP composites, one can gain the following conclusions:
(1) The sorption of Co(II) on MWCNT–HAP is rather quick and the
kinetic sorption process can be described by the pseudo-secondorder model well;
(2) Foreign cations with different charges and radii influence the
sorption of Co(II) on MWCNT–HAP significantly;
(3) Anions (i.e., ClO4−, NO3− and Br−) make unobvious difference,
while F− dramatically enhances the sorption in the whole pH;
(4) The presence of FA and HA enhances Co(II) sorption on γ-Al2O3 at
low pH values, but suppresses Co(II) sorption on MWCNT–HAP at
high pH values;
(5) The thermodynamic data counted from temperature dependent
sorption isotherms indicates that the sorption reaction is an
endothermic and spontaneous process;
(6) The MWCNT–HAP composites are suitable materials in the
preconcentration and solidification of Co(II) ions and other
heavy metal ions from large volumes of aqueous solutions in
real environmental pollution cleanup.
6.0
qe (mg/g)
5.5
5.0
4.5
1
2
3
4
5
Round
Fig. 12. Recycling of MWCNT–HAP in the sorption of Co(II) from aqueous solutions.
pH = 6. 0 ± 0.1, m/ V = 0.6 g/L, I = 0.01 M NaCl.
Acknowledgment
Financial support from the Natural Science Foundation of Shandong
Province (ZR2009BM045) is acknowledged.
Z. Liu et al. / Journal of Molecular Liquids 179 (2013) 46–53
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