Research Journal of Applied Sciences, Engineering and Technology 3(9): 880-886, 2011
ISSN: 2040-7467
© Maxwell Scientific Organization, 2011
Submitted: June 14, 2011
Accepted: August 08, 2011
Published: September 20, 2011
Optimization and Modeling of Hexavalent Chromium Removal From Aqueous
Solution Via Adsorption on Multiwalled Carbon Nanotubes
1,2
Mina Gholipour, 2Hassan Hashemipour Rafsanjani and 2Ataollah Soltani Goharrizi
1
Department of Environmental Sciences, International Center for Science,
High Technology and Environmental Science, Kerman, Iran
2
Department of Chemical Engineering, Shahid Bahonar University of Kerman, Iran
Abstract: Hexavalent chromium and its derivatives are potential pollutant due to their mortal affects. Therefore,
It is essential to remove these components from wastewaters before disposal. Adsorption can be effective and
versatile method for removing of hexavalent chromium. In this article, removal of hexavalent chromium via
adsorption on multiwalled carbon nanotubes was investigated as a function of adsorbent dosage, initial solution
pH, initial Cr(VI) concentrations, contact time and temperature. The batch experiments were conducted at 3
different temperatures (17, 27 and 37ºC) and shows that Cr(VI) removal obeys pseudo-second order rate
equation. Rate constant (K) values in 3 temperatures, pre-exponential factor and adsorption activation energy
(E) was also obtained. The sorption data fitted well with Freundlich isotherm adsorption model.
Thermodynamic parameters such as Gibbs free energy ()Gº), enthalpy ()Hº) and entropy ()Sº) for Cr(VI)
adsorption were estimated and Results suggest that the adsorption process is a spontaneous and endothermic.
Key words: Adsorption, adsorption isotherm, adsorption kinetics, hexavalent chromium, multiwalled carbon
nanotubes, thermodynamic parameters
technology and adsorption. Except adsorption, other
processes have considerable disadvantages including
incomplete metal removal, requirements for expensive
equipment and monitoring system, high reagent and
energy requirements or generation of toxic sludge or other
waste products that require disposal (Aksu et al., 2002;
Baran et al., 2007). Adsorption is an effective and
versatile method for removing chromium particularly
when combined with appropriate regeneration steps. This
solves the problems of sludge disposal and renders the
system more economically viable, especially if low cost
adsorbents are used (Yavuz et al., 2006).
Several adsorbents which have been studied for
adsorption of metal ions such as activated carbon (Sekar
et al., 2004; Rao et al., 2006), fly ash (Ayala et al., 1998;
Weng et al., 2004), peat (Ho and Mc-Kay, 1999), sewage
sludge ash (Pan et al., 2003), zeolite (Biskup and Subotic,
2004), biomaterials (Li et al, 2004; Ekmekyapar et al.,
2006), recycled alum sludge (Chu, 1999), manganese
oxides (Sublet et al., 2003), peanut hulls (Brown et al.,
2000), kaolinite (Arias et al., 2002) and resins (Diniz
et al., 2005). However, these adsorbents suffer from low
adsorption capacities or removal efficiencies of the metal
ions. Therefore, researchers carried out investigation for
new promising adsorbents.
Carbon nanotubes are a relatively new form of carbon
first reported by Iijima (1991). Studies on this new
INTRODUCTION
Heavy metal pollution represents an important
environmental problem due to its toxic effects and
accumulation throughout the food chain and hence in the
human body. Water pollution by chromium ions is a
considerable concern, as this metal has found widespread
use in metal finishing, leather tanning, electroplating,
nuclear power plant, textile industries, and chromate
preparation (Kowalski, 1994). Chromium is an element of
6th group in the latest IUPAC Periodic Table, exists in
both hexavalent and trivalent forms. Hexavalent form is
more toxic than trivalent and requires more concern
(Mohanty et al., 2005). Strong exposure of Cr(VI) causes
cancer in the digestive tract and lungs (Kaufaman et al.,
1970) and may cause epigastric pain, nausea, vomiting,
severe diarrhea, and hemorrhage (Browning, 1969).
Cr(VI) concentrations in industrial wastewater range from
0.5 to 270.000 mg/L. The tolerance limit for Cr(VI) for
discharge into inland surface waters is 0.1 mg/L and in
potable water is 0.05 mg/L (Kobya, 2004). It is therefore,
essential to remove Cr(VI) from wastewater before
disposal.
Conventional methods for removing dissolved heavy
metal ions include chemical precipitation, chemical
oxidation or reduction, filtration, ion exchange,
electrochemical treatment, application of membrane
Corresponding Author: Mina Gholipour, Department of Environmental Sciences, International Center for Science, High
Technology and Environmental science, Kerman, Iran
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Res. J. Appl. Sci. Eng. Technol., 3(9): 880-886, 2011
also done at 17, 27 and 37ºC. The samples were taken and
analyzed at regular intervals.
The Cr(VI) uptake q, the amount of solute adsorbed
per unit weight of adsorbent (mg/g), was calculated by the
simple mass balance method as follow equation:
material has shown high adsorption capacity and removal
efficiency of various pollutants (Dai, 2002). The
environmental applications of carbon nanotubes that have
been investigated thus far include studies on the storage
of gases like hydrogen as well as the removal of metal
ions and organic contaminants (Li et al., 2002). Recently
multi-walled carbon nanotubes (MWCNTs) have also
been used as adsorbents for the removal of Pb2+, Cu2+,
Zn2+ and Cd2+ (Li et al., 2003; Lu, 2006). Di et al. (2006)
have also shown that ceria nanoparticles supported on
aligned carbon nanotubes are effective in the removal of
Cr(VI) (Di et al., 2006).
The present investigation deals with the application
of multiwalled carbon nanotube in the removal of
Chromium (VI) from aqueous solutions. The removal
amount of Cr(VI) was determined on the basis of different
effective parameters such as: adsorbent dosage, initial
Cr(VI) concentration, solution pH, contact time and
temperature. Adsorption kinetic, equilibrium isotherm and
thermodynamic parameters of the process were also
investigated.
q=
(Co − Ct )V
ms
(1)
where, C0 and Ct are the Cr(VI) concentrations (mg/L)
initially and at a given time t, respectively, V the volume
of the Cr(VI) solutions (L) and ms is the weight of
Adsorbent (g).
The removal percentage of Cr(VI) ions (R%) in
solution was calculated using Eq. (2):
R% =
Co − Ct
100
Co
(2)
All the experiments were conducted at Instrumental
Analysis Laboratory of International Center for Science,
High Technology and Environmental science during the
Autumn of 2010.
METHODOLOGY
MWCNTs, obtained by chemical vapor deposition
method, were purchased from Research Institute of
Petroleum Industry (RIPI) (Iran) and had a purity of
>95%. The stock solution of Cr (VI) was made by
dissolving a known amount of K2Cr2O7 (analytical grade)
in a known volume of double distilled water. Solutions of
0.1M H2SO4 and 0.1M NaOH were used for pH
adjustment.
Modeling:
Modeling of adsorption kinetic: The study of adsorption
kinetics is significant as it provides valuable insights into
the reaction pathways and the mechanism of the reactions.
Several kinetics models are used to explain the
mechanism of the adsorption processes. A simple pseudofirst order equation is given by Lagergren equation:
Batch adsorption experiments: Experiments were
conducted in 250 mL Erlenmeyer flasks containing known
Cr(VI) synthetic solutions. Known quantities of the
adsorbents were added to the solutions. The carbon
nanotubes were dispersed perfectly in the solution using
an ultrasonic bath. The removal percentage is measured at
three temperature (17, 27 and 37ºC) as the flasks content
is shaked at 190 rpm until equilibrium state is achieved.
The suspension was filtered through a 0.2 :m Biofil
syringe filter and the filtrate was analyzed to evaluate the
concentration of Cr(VI) metal in the solution. Metal
analysis was carried out by using a Cary 50 model UVvisible spectrophotometer.
The solution pH, CNT dosage and initial
concentration of Cr(VI) in the experiments were varied
from 1 to 10, 0.02 to 0.15 g/L and 20 to 80 mg/L
respectively to study the effect of these main parameter on
the removal percent.
The adsorption isotherm experiments were conducted
at constant temperature of 17, 27 and 37ºC for the
adsorbent dosage varying from 0.02 to 0.1 g. The contact
time effect on Cr (VI) removal and the kinetic study were
log(q e − q ) = log e −
K1
t
2.303
(3)
where, qe is the Cr(VI) uptake at equilibrium (mg/g), q is
the uptake at any time (t) and k1 is first order adsorption
rate constant.On the other hand, the pseudo-second order
equation based on equilibrium adsorption is expressed as:
t
1
1
=
+
t
q K2qe2 qe
(4)
where, k2 is the pseudo-second order rate constant
(g/mg.min) (Di et al., 2006). The linear regression
correlation coefficient value shows that which model can
justify the experimental data.
Modeling of adsorption isotherm: To examine the
relationship between solute adsorbed (qe) and the aqueous
concentration (Ce) at equilibrium, several isotherm models
are widely employed, of which the Langmuir and
881
Res. J. Appl. Sci. Eng. Technol., 3(9): 880-886, 2011
Freundlich equations are most widely used.According to
Langmuir model, adsorption occurs uniformly on the
active sites of the adsorbent, and once an adsorbate
occupies a site, no further adsorption can take place at this
site. Thus, the Langmuir model is given by the following
equation:
C = 1 + C
q
bQ0 Q0
(5)
where, Q0 and b are the Langmuir model parameters. C is
the equilibrium solution concentration (mg/L), q is the
Cr(VI) uptake at the equilibrium (mg/g) (Sharma and
Bhattacharyya, 2004).
The Freundlich isotherm is an empirical model that is
based on adsorption on heterogonous surface and is given
by the following equation (Oguz, 2005):
100
%Cr(VI)removal
⎛ 1⎞
log = log k + ⎜ ⎟ log C
⎝ n⎠
Fig. 2: XRD pattern of MWCNTs
(6)
80
60
40
20
0
where, q is the Cr(VI) uptake (mg/g), C is the equilibrium
concentration of the Cr(VI) solution (mg/L) and k and n
are Freundlich constants, which represent adsorption
capacity and adsorption intensity, respectively.
0.05
0.1
0.15
Adsorbent dosage(g)
0.2
Fig. 3: Effect of adsorbent dosage on %removal
Effect of adsorbent dosage:The amount of CNT
dosage used in the removal of Cr(VI) is critical for the
application of the adsorbent in Cr(VI) elimination.
Effect of this parameter on the removal percent is
investigated experimentally and the results presented in
Fig. 3. The results are shown that Cr(VI) removal
increases rapidly with increase in the dose of MWCNTs
due to the greater availability of the adsorbent (Fig. 2).
The increase in adsorbent dosage resulted in an increase
from 40 to 97% in adsorption of Cr(VI) ions.
RESULTS AND DISCUSSION
Adsorbent characterization: Figure 1 shows the TEM
image of the MWCNTs with ca. 10 nm i.d. and 30 nm
o.d., and hundreds of nm long. As the image shows the
nanotubes are curved. Figure 2 shows the XRD pattern of
the MWCNTs structure. The most intense peaks of
MWCNTs correspond to the (002) and (100) reflections.
Using BET surface area measurements, the specific
surface area of MWCNT was 102.5 m2/g.
Effect of initial Cr(VI) concentration: The initial
concentration of Cr(VI) provides an important driving
force to overcome all mass transfer resistances of metal
ions between the aqueous and solid phases.
Figure 4 shows that when the initial Cr(VI) ion
concentration increased from 20 to 80 mg/L, Cr(VI)
removal decreased from 100 to 40% and the uptake
capacity of MWCNTs increased. The decreasing in
removal percent was due to the saturation of the sorption
sites on adsorbents. In addition, the increase in uptake
capacity of MWCNTs with the increase of Cr(VI) ion
concentration is due to higher availability of Cr(VI) ions
in the solution, for the adsorption.
Effect of initial pH: The pH value of the solution is
another important factor that controls the sorption of
Cr(VI). Figure 5 shows the extent of removal of Cr(VI) as
a function of pH, and it shows that at lower pH, the
Cr(VI) removal efficiency was higher and at higher
Fig. 1: TEM image of MWCNTs
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Res. J. Appl. Sci. Eng. Technol., 3(9): 880-886, 2011
100
120
60
100
80
40
60
40
20
0
0
20
40
60
80
Initial concentration of Cr(VI)(mg/1)
O
90
O
80
%Cr(VI)removal
80
q(mg/g)
%Cr(VI)removal
O
37 C
27 C
17 C
100
140
70
60
50
40
20
30
0
20
100
0
Fig. 4: Effect of initial Cr(VI) concentration on %removal
20
40
60
80
Contact Time (hr)
100
120
Fig. 6: Effect of contact time and temperature on %removal
O
17 C
27 C
37 C
100
O
0.7
O
0.6
60
0.5
Log(qe-q)
%Cr(VI)removal
80
40
20
0.4
0.3
0.2
0
0
2
4
6
8
10
0.1
12
0
pH
0
20
40
Fig. 5: Effect of initial pH on %removal
60
80
t(hr)
Fig. 7: Pseudo-first order kinetic
pH the removal reduced considerably. With decreasing in
pH the amount of Cr(VI) removal increased Which the
best removal efficiency achieved the optimum pH 1-2.
The reason for the high removal percent of chromium
at lower pH range was explained below. The Cr(VI) exists
in different forms such as HCrO4G, Cr2O72G, CrO4G in
aqueous solution and the stability of these forms is
dependent on the pH of the system. The active form of
Cr(VI) adsorbed on the adsorbent is HCrO4G. This form
is stable at only lower pH range which leads to high
removal of chromium. But the concentration of this form
decreases when there is an increase in pH. Hence
chromium removal at higher pH decreases (Aksu,
2001; Hamadi et al., 2001).
14
O
17 C
27 C
10
37 C
t/q(hr,g/mg)
12
O
O
8
6
4
2
0
0
20
40
60
t(hr)
80
100
120
Fig. 8: Pseudo-second order kinetics plot
Kinetic of adsorption: In order to find the best model
correlate the adsorption process, the kinetic models (Eq.
3 and 4) are used with regression of the experimental data.
Results are presented in Fig. 7 and 8. The calculated
parameter of the models (rate constant) and correlation
regression coefficient (R2) are reported in Table 1. As
seen in Table 1, the value of R2 calculated from pseudosecond order kinetic is near 1 and higher than pseudofirst order kinetic model. These results indicate that the
adsorption of Cr(VI) on the MWCNTs follows pseudosecond order kinetics. Activation Energy of the adsorption
process is a criterion of the rate functionality with
temperature and it can be calculated using the rate
constant amounts in different temperature. The activation
energy was obtained using Arrhenius equation (Sarin and
Pant, 2006):
Effect of contact time and temperature: Figure 6 Shows
the effect of contact time and temperature on the
adsorption of Cr(VI) at 17, 27 and 37ºC by MWCNTs.
Temperature has two main effects on the adsorption
process. An increasing in temperature is known to
increase the diffusion rate of the adsorbate molecules
across the external boundary layer and within the
adsorbent. It could be the result of decreasing solution
viscosity. Furthermore, changing the temperature will
modify the equilibrium capacity of the adsorbent for a
particular adsorbate. For the contact time, removal
efficiency increased with an increasing in contact time
before equilibrium is reached and after equilibrium
removal efficiency would be constant. Results shows that
the amount of Cr(VI) adsorbed increased with increase in
contact time and temperature.
⎛ E ⎞
K2 = K0 exp⎜ −
⎟
⎝ RT ⎠
883
(7)
Res. J. Appl. Sci. Eng. Technol., 3(9): 880-886, 2011
Table 1: The adsorption kinetic model rate constants at different
temperatures
Model
290 K
300 K
310K
0.926
0.952
0.874
Pseudo first order
R2
0.0161
0.0173
0.018
K1 (1/1 h)
0.995
0.998
0.999
Pseudo second order
R2
0.022
0.042
K2 (g/mg.h) 0.011
Table 2: Isotherm model constants and correlation coefficients for
adsorption of Cr(VI) on MWCNTs
Model
290 K
300 K
310 K
0.985
0.995
0.986
R2
35.36
41.67
46.3
Langmuir
Q0 (mg/g)
b (L/mg)
3.76
1.25
0.95
0.994
0.996
0.991
R2
Freundlich
k
15.6
25.7
32.36
n
4.6
7.6
9
-2.5
Ln K
-3
0.9
37
0.8
-3.5
27
0.7
-4
17
C/q(g/1)
0.6
-4.5
-5
0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 0.0035
1/T(1/K)
0.5
0.4
0.3
0.2
0.1
0
Fig. 9: Determination of the adsorption activation energy, E
0
where, K0 is pre-exponential factor (g/mg.min), E the
sorption activation energy (J/mol), R the gas constant
(8.314 J/mol K) and T the solution temperature (K). The
Arrhenius plot of rate constant with temperature using
data reported in Table 1 was shown in Fig. 9. Activation
energy and pre-exponential factor were determined and
the amounts are K0 equal to 16 g/mg.hr and E equal to
50.07 KJ/mol. Hence, the relationship between K and T
can be represented in an Arrhenius form as
⎛ 50.07 ⎞
K = 16 exp⎜ −
⎟
⎝ 8.314T ⎠
5
10
C(mg/1)
15
20
Fig. 10: Langmuir plots for adsorption of Cr(VI) at different
temperatures
Kc =
qe
Ce
(9)
∆G ° = RT In K c
(8)
In k c =
Adsorption isotherms: Regression of equilibrium data of
the process using both Langmuir and Freundlich models
(Eq. 5 and 6) was carried out and shown in Fig. 10 and
11. The regression determination results presented in
Table 2, shown that Freundlich isotherm model has a
better fitting than Langmuir model. This shows that
adsorption occurs on heterogonous surface.
The Freundlich constants were determined from the
slope and intercept of a plot of log Q versus log C and
were reported in Table 2. There was a good agreement
between experimental and predicted behavior observed
from the values of the regression coefficient R2 close to 1.
Moreover, KF is large at higher temperatures showing that
the adsorption rate also increases with a rise in
temperature. The slope “1/n” is less than 1, which is
related to a favorable adsorption of chromium ions (McCabe et al., 2001).
(10)
∆S ° ∆H °
−
R
RT
(11)
where, Kc is the equilibrium constant, qe is the solid phase
concentration at equilibrium (mg/L) and Ce is the
equilibrium concentration in solution (mg/L). ∆ H ° and
∆ S ° were obtained from the slope and intercept of linear
plots of lnKc versus 1/T (Fig. 12) and The standard Gibbs
free energy ∆ G ° values (kJ/mol) were calculated from the
Eq. 10. Table 3 shows the calculated values of the
thermodynamic parameters for the adsorption of Cr(VI)
on MWCNT.
°
The positive value of enthalpy change ∆ H for the
process confirms the endothermic nature of the process,
°
the positive entropy of adsorption ∆ G reflects the
affinity of the adsorbent material toward Cr(VI) and the
negative free energy values ∆ G ° indicate the feasibility of
the process and its spontaneous nature. The amount of
∆ G° (<10 Kcal) suggest that the adsorption is physical
an also The increase in ∆ G ° with increasing temperature
shows that the adsorption is more favorable at high
temperature (Sharma and Bhattacharyya , 2004).
Thermodynamic parameters: Thermodynamic
parameters of the process such as Gibbs free energy
( ∆ G° ) , enthalpy ( ∆ H° ) and entropy ( ∆ S° ) were
calculated using the following:
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Res. J. Appl. Sci. Eng. Technol., 3(9): 880-886, 2011
parameters )Gº, )Hº and )Sº values of Cr(VI) adsorption
onto MWCNTs show that the adsorption process is a
spontaneous, physical and endothermic.
Thus, the results show that the MWCNTs can be
effectively applied for the removal of Cr(VI) from
wastewater.
Table 3: Thermodynamic parameters for the adsorption of Cr(VI) on
MWCNTs
)Gº (Kj/mol) )Hº (Kj/mol) )Sº (Kl/mol ºK)
Temp.(ºK)
290 K
-21.21
300 K
-23.7
52.1
0.257
310 K
-26.3
1.8
37
1.7
27
1.6
17
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The following major conclusions can be drawn based
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increase in Cr(VI) adsorption and increase in temperature
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pseudo-first order and pseudo-second order kinetics
equations. It was shown that the pseudo-second order
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results show that the Freundlich isotherms can be fitted
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