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Kinetic, thermodynamic and isotherm studies for acid blue 129 removal
from liquids using copper oxide nanoparticle-modified activated carbon
as a novel adsorbent
Farzin Nekouei
a,
⁎⁎, Shahram Nekouei
a
, Inderjeet Tyagi
b
, Vinod Kumar Gupta
b,c,
⁎
a
Young Researchers and Elite Club, Gachsaran Branch, Islamic AzadUniversity, Gachsaran, Iran
b
Chemistry Department, Indian Institute of Technology Roorkee, Roorkee 247667, India
c
King Fahd University of Petroleum and Minerals Dhahran, Saudi Arabia
abstractarticle info
Article history:
Received 21 August 2014
Received in revised form 2 September 2014
Accepted 15 September 2014
Available online 20 September 2014
Keywords:
Adsorption
Copper oxide nanoparticle loaded on activated
carbon
Thermodynamic and kinetic of adsorption
Acid blue 129
Liquid phase
A novel adsorbent, copper oxide nanoparticle loaded on activated carbon (CuO-NP-AC) was synthesized by a
simple, low cost and efficient procedure. Subsequently, this novel sorbent was characterized and identified
using different techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and laser light
scattering (LLS). The effects of some variables including pH, adsorbent dosage, initial dye concentration, contact
time and temperature were examined and optimized. The adsorption kinetic data were modeled using the
pseudo-first-order, pseudo-second order, intraparticle diffusion and Elovich models, respectively. The experi-
mental results indicatedthat the pseudo-second-order kinetic equation can better describe the adsorption kinet-
ics. Furthermore, Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich models were applied for analyzing
adsorption equilibrium data of acid blue 129 (AB 129) on the as-prepared adsorbent, which suggested that the
Langmuir model provides a better correlation of the experimental data. Also, thermodynamic parameters such
as ΔH, ΔS, E
a
,S*,andΔG were calculated. It was seen that the proposed adsorbent has hightendency and adsorp-
tion capacity for AB 129 adsorption in a feasible, spontaneous and endothermic way.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Nowadays, in such an industrial world one of the most important
concerns is securing the health of human race and environment. Dyes
and pigments present in wastewaters of manufacturing and textile in-
dustry contain dyes and auxiliary chemicals [1] lead to generation of haz-
ardous injuries to the animal and human health [2]. Because of their
complex (aromatic) molecular structures, dyes are stable towards heat
and oxidizing agent and are biodegradable with difficulty. In addition,
most dyes are toxic and harmful to some microorganisms and directly
destroy or inhibit from their catalytic activates [3]. Colored dyes are not
only esthetic, carcinogenic but also hinder light penetration and disturb
life processes of living organisms in water. Acid blue 129 (AB 129), an
acidic dye, is most widely used for the dyeing of cotton, wool, silk,
nylon, paper and leather (Table 1 and Scheme 1)[4]. This dye may be
harmful if there is contact to eyes, respiratory system and skin. Therefor e,
the removal of such colored agents from aqueous effluentsisnecessary.
Because of the importance of removal of dyes from solutions, researches
have tried to measure and remove dyes through various methods name-
ly coagulation, nanofiltration and ozonolysis, membrane filtration, oxi-
dation and adsorption process which are applied to remove color and
other contaminations from aqueous media [5–9]. Recently, adsorption
has become one of the most popular techniques because of some advan-
tagessuchashighefficiency and ability to use generable non-toxic and
cheap adsorbents [10–24]. Activated carbon appears to be the widely
used technique for dye removal because of its high porosity, large surface
area and high mechanical and chemical stability, with a least cost that
acts as mild reducing agent and catalyst [25–27]. Also, nanoparticles as
sorbents for separation, removal and or pre-concentration are applicable
for enrichment of trace elements as its effective protocol [28].
The objective of the presented work is to investigate the preparation
of a new and effective sorbent for the adsorption of AB 129 dye. The ef-
fects of adsorbent dosage, initial dye concentration, pH, contact time
and temperature on AB 129 adsorption onto CuO-NP-AC were studied.
Adsorption kinetics, isotherms and thermodynamic parameters were
also evaluated and reported.
Journal of Molecular Liquids 201 (2015) 124–133
⁎Correspondence to: V.K.Gupta, ChemistryDepartment, Indian Instituteof Technology
Roorkee, Roorkee 247667, India and Department of Applied Chemistry, University of
Johannesburg, Johannesburg, South Africa.
⁎⁎ Correspondence to: F. Nekouei, No.185, 7581796614, Beside Bonyade Shahid Org.
Valiye asr BLVD, Serahi, Gachsaran, Iran. Tel.: +989367012005; fax: +987424222431.
E-mail addresses: f.nekouei@hotmail.com,F.nekouei@iaug.ac.ir (F. Nekouei),
vinodfcy@gmail.com,vinodfcy@iitr.ac.in (V.K. Gupta).
http://dx.doi.org/10.1016/j.molliq.2014.09.027
0167-7322/© 2014 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
2. Materials and methods
2.1. Instrumentation
A double beam spectrophotometer (UV-1800, Shimadzu, Japan) was
used for determination of concentration of AB 129 at 629 nm. Theshape
and surface morphology of the obtained sample were investigated by a
field emission scanning electron microscope (FE-SEM, Hitachi S4160,
Japan) under an acceleration voltage of 20 kV. The X-ray diffraction
patterns of the products were recorded by employing INEL X-ray
diffractometer (model Equinox 3000). Particle size and size distribution
of the CuO nanoparticles were measured by laser light scattering
(Zetasizer Nanoseries, Malvern Instruments Co.). A Metrohm pH-
meter (model 691, Switzerland) was used in order to adjust the pH at
desirable values. Thermometer Metrohm, international ASTM sieves
and Stirrer model UKA are also used in this study.
2.2. Standard solutions and reagents
All the chemicals used in this study were of analytical grade and solu-
tions were prepared with distilled water. Applied reagents including cop-
per iodide (I), DMSO, oleic acid, ethylene-diamine, NaOH and HCl with
the analytical reagent grade were purchased from Merck (Darmstadt,
Germany). A stock solution of 200 mg L
−1
of AB 129 was prepared by dis-
solving 0.100 g of solid dye (Sigma-Aldrich, Germany) in water and dilut-
ing to 500 mL in a volumetric flask. All working solutions with desired
concentration were prepared by diluting the stock solution with distilled
water.
2.3. Adsorption studies
Concentrations of AB 129 were estimated using the linear regression
equations (obtained by plotting its calibration curve). The dye adsorp-
tion capacity of the adsorbent was determined at the time intervals in
the range of 1–30 min for 10 and 20 mg L
−1
at room temperature and
it was found that equilibrium was established after 20 and 25 min
for 10 and 20 mg L
−1
.Theinfluence of some variables namely pH,
adsorbentdosage, temperature, contact time and initial dye concentra-
tion on the adsorptive removalof AB 129 was examined by batch exper-
iments. To evaluate and calculate the kinetic, thermodynamic and
isotherm parameters of the adsorption process, 50 mL of 10 and
20 mg L
−1
of AB 129 in 100 mL Erlenmeyer flasks was agitated on a stir-
rer at 400 rpm at room temperature and obtained experimental data at
various times, temperatures and concentrations was fitted to different
models.
The percentage adsorption R was calculated as:
%Dye removal;RC0−Ct
C0
¼100 ð1Þ
where C
0
(mg L
−1
)andC
t
(mg L
−1
) are the dye concentration at initial
and after time t respectively and the amount of adsorbed AB 129 by
adsorbent (q
e
(mg g
−1
)) was calculated according to Eq. (2):
qe¼C0−Ce
ðÞ
V
Wð2Þ
where C
0
(mg L
−1
)andC
e
(mg L
−1
) are the initial and equilibrium dye
concentrations in solution, respectively, V is the volume of the solution
(L), and W is the mass (g) of the adsorbent used and the actual amount
of adsorbed dye at time t, q
t
(mg g
−1
), was calculated based on the
following equation:
qt¼C0–Ct
ðÞ
V
Wð3Þ
where C
0
(mg L
−1
)andC
t
(mg L
−1
) are the concentrations of dye at ini-
tial and any time t, respectively, V is the volume of thesolution (L),and
W is the mass (g) of the adsorbent.
2.4. Preparation of CuO nanoparticles by solvothermal method
Among the various chemical approaches for the synthesis of nano-
particles, the solvothermal method was chosen to synthesize CuO nano-
particles. In fact, solvothermal synthesis is a method for preparing a
variety of materials such as metals, semiconductors, ceramics, polymers
and nanocrystals. One of the most important characteristics of
solvothermal method is to allow for the precise control over the size,
shape distribution, and crystallinity of metal oxide nanoparticles or
nanostructures. These characteristics can be altered by changing certain
experimental parameters, including reaction temperature, reaction
time, solvent type, surfactant type, and precursor type. CuO nanoparti-
cles in DMSO were synthesized by the following method (Scheme 2):
After dissolving 3.1 g of CuI in 42.5 mL DMSO, the solution was heated
to 80 °C under a constant stirring rate. Then, 1.5 and 0.1 mL of
ethylenediamine andoleic acid were added to the solution, respectively.
The gray solution turned black and after a few minutes copper oxide
particles were precipitated at the bottom of the experiment dish. The
mixture was maintained at 80 °C for 2 h and the color of the reaction so-
lution became black completely. The resultant black products were sep-
arated from the reaction mixture and washed thoroughly with DMSO to
remove CuI crystals if remained and dried at ambient condition (in a
vacuum oven, 0.1 MPa) for 6 h prior to being characterized.
3. Results and discussion
3.1. Characterization of CuO nanoparticles
XRD analysis as powerful tools was used to study the crystal struc-
tures of the CuO nanoparticles. Fig. 1(a) displays the XRD spectrum of
CuO nanoparticles. In XRD pattern the sample indexes to tenorite,
synCuO (JCPDS number 00-045-0937) although has different intensities
of crystallinity. The two reflections at 2θ= 35.54 [002] and 2θ= 38.52
[111] were observed in the diffraction patterns, and are ascribed to the
Table 1
Properties of acid blue 129.
C.I. number Acid blue 129
Chemical formula C
23
H
19
N
2
NaO
5
S
Another name Brilliant Alizarine Sky Blue BS
Abbreviation name AB 129
Molecular weight 458.46
Name Sodium-1-amino-4-(2, 4,6-trimethylanilino)
anthraquinone-2-sulfonate
Maximum wavelength (λ
Max
) 629 (nm)
Application Cotton, wool, silk, nylon, paper and leather
Color Blue
Class Acid dye
Scheme 1. Chemical structure of acid blue 129.
125F. Nekouei et al. / Journal of Molecular Liquids 201 (2015) 124–133
formation of the CuO monoclinic crystal phase. The average size of
nanocrystallites (D) was estimated by Scherrer's formula [29].
D¼Kλ=βCos θð4Þ
where K (=0.89) is theshape factor, λis the X-ray wavelength of Cu K
α
radiation (0.154 nm), θis the Bragg angle and βis the experimental
full-width at half-maximum (FWHM) of the respective diffraction
peak (in units of radians). The average crystallite thickness of CuO is es-
timated at about 26.57 nm by Scherrer's formula.
The FE-SEM is the primary tool used for characterization of the sur-
face morphology and fundamental physical properties of photo-catalyst
surface. It is useful for determination of the particle size, shape, and
porosity. Morphology and microstructure of the CuO nanoparticles
according to FE-SEM studies (Fig. 1(b)) reveal that the CuO nanoparticles
have a considerable number of pores with irregular pores which are suit-
able for trapping and adsorption of dyes into these pores.
The volume average hydrodynamic diameter for the CuO nano-
particles, which is determined by the laser light scattering, was found
less than 25 nm with narrow size distribution of 0.15 polydispersity
(Fig. 1(c)).
3.2. Effect of initial solution pH
The pH of the solution is considered to be the most important con-
trolling parameter in the adsorption process [30].Fig. 2 shows the effect
of solution pH on the removal of AB 129 on the CuO-NP-AC. It was ob-
served that the AB 129 removal was highly dependent on the pH of
the solution which affected the surface charge of the sorbent. Subse-
quently, similar batch equilibrium experiments conducted in the pH
range of 1.0–8.0 and proposed procedure was performed. The maxi-
mum removalefficiency (around 100%) occurredat pH 2.0, then; the re-
moval percentage decreased dramatically as the initial solution pH
increased from 4.0 to 8.0. At lower pH, both activated carbon functional
groups and oxygen atoms were protonated and the adsorbent acquired
positive charge and finally AB 129 dye (anionic dye molecule) adsorbed
onto CuO-NP-AC through attraction forces. Dye adsorption via physical
CuI (3.1 g)
Ethylenediamine
(1.5 mL)
DMSO
(42.5 mL)
Heating and stirring
@ 80 °C for 2 h
Oleic acid (0.1 mL)
Separating and washing
several times by DMSO
Drying @ vacuum oven
0.1 MPafor 6 h
Scheme 2. Solvothermal method for preparation of CuO nanoparticles.
Fig. 1. (a).X-ray diffraction (XRD) pattern ofthe CuO nanoparticles. (b). FESEM images of
the CuO nanoparticles. (c). Histogram of the CuO nanoparticle size distribution.
126 F. Nekouei et al. / Journal of Molecular Liquids 201 (2015) 124–133
or chemical forces is another alternative possible adsorption mecha-
nism. At lower pH, probably the surface of the adsorbent has positive
charge due to protonation of functional group of CuO-AC in highly acidic
solution which favors the adsorption of the dye over CuO-AC. By
increasing pH, simultaneously, the number of negatively charged sites
of the adsorbent increases while the number of positively charged
sites of the adsorbent decreases and due to electrostatic repulsion be-
tween dye anions and the adsorbent the removal efficiency was de-
creased. It seems that in alkaline pH, number of OH
−
ions, on the
adsorbent surface significantly increased that led to a decrease in re-
moval efficiency [31,32].
3.3. Effect of contact time
Equilibrium time is one of the important parameters to design a low
cost wastewater treatment system [31]. The effect of the contact time on
the adsorption was investigated in the range of 1.0 to 30 min at room
temperature. The results are shown in Fig. 3. Sufficient contact time is
vital for the adsorption process to reach equilibrium for the maximum
dye adsorption on CuO-NP-AC. Fig. 3 shows the removal percentage of
AB 129 dye at initial concentrations of 10 and 20 mg L
−1
at pH 2.0.
The adsorption efficiency increases with increasing contact time and
reaches constant and maximum value after 20 and 25 min. Hence, the
optimum times of 20 and 25 min at 400 rpm stirring rate were selected
for quantitative adsorption of AB 129 dye at initial concentrations of 10
and20mgL
−1
for subsequent works. At higher concentration due to a
decrease in the ratio of AB129 to CuO-NP-AC surface area the rate of dif-
fusion and migration significantly decreased.
3.4. Effect of the amount of adsorbent
Amount of adsorbent is another important parameter in that it con-
trols the capacity of the adsorbent. The effect of CuO-NP-AC dose on the
adsorption of AB 129 from aqueous solutions was investigated using
various adsorbent doses (0.01–0.06 g) at constant AB 129 concentra-
tions of 10 and 20.0 mg L
−1
.AsshowninFig. 4,theremovalefficiency
of AB 129 by CuO-NP-AC increased sharply as the sorbent dose in-
creased from 0.01 to 0.045 g, then reached an almost constant value.
However, as expected, the adsorption capacities decreased with in-
creasing adsorbent mass, due to the reduction in both effective surface
area and adsorbate/adsorbent ratio [30]. The removal efficiency was
maximum when the CuO-NP-AC dose was 0.9 g L
−1
, after that the
removal was not significantly increased, so 0.9 g L
−1
was chosen as
the optimum adsorbent dose for further experiments.
3.5. Effect of initial dye concentration on adsorption of AB 129
In order to investigate the effectof initial concentration of AB 129 on
adsorption of AB 129, the effect of AB 129 concentration in the range of
10–80 mg L
−1
on its adsorption by CuO-NP-AC was examined and
the amount and percentage of AB 129 removal at different initial con-
centrations were presented in Fig. 5. From Fig. 5, it is observable that
an increase in initial dye concentration has positive correlation with re-
moval percentage and the percentage of dye removal was greater at
lower initial concentrations and smaller at higher initial concentrations.
This synergic correlation is assigned to the enhancement in the bulk and
film diffusion of target compounds to the external surface of adsorbent
and their subsequent pore diffusion [33]. Initial dye concentration at
afixed value of adsorbent has reverse correlation with equilibrium
time. The limiting factor for dye adsorption is the available site on the
adsorbent. It is obvious from Fig. 5 that at 0.045 g of adsorbent by in-
creasing the concentration from 10 to 60 mg L
−1
of AB 129, the removal
percentage decreases from 99.8% to 85.4%.
40
50
60
70
80
90
100
0510
Removal (%)
pH
10 ppm
20 ppm
Fig. 2. Effect of pH on the removal of AB 129 by CuO-NP-AC at room temperature,
adsorbent dosage of 0.045 g in 50 mL, contact times of 20 and 25 min for dye concentrations
of 10 and 20 mg L
−1
, respectively.
65
70
75
80
85
90
95
100
0 500 1000 1500 2000
Removal (%)
Time (S)
10 ppm
20 ppm
Fig. 3. Effectof contact time on the removal of AB 129at 0.045 g of CuO-NP-ACin 50 mL at
pH 2, at room temperature and AB 129 concentration of 10 and 20 mg L
−1
.
30
40
50
60
70
80
90
100
0 0.02 0.04 0.06 0.08
Removal (%)
amount of adsorbent (g)
10 ppm
20 ppm
Fig. 4. Effect of adsorbent dosage on the removal of AB 129 at pH 2, at room temperature
and AB 129 concentration of 10 and 20 mg L
−1
.
30
40
50
60
70
80
90
100
0 20406080100
Removal (%)
Initial dye concentration (mg/L)
Fig. 5. Effectof initial dye concentration on the removal of AB 129 at 0.045 g of CuO-NP-AC
in 50 mL at pH 2, at room temperature.
127F. Nekouei et al. / Journal of Molecular Liquids 201 (2015) 124–133
3.6. Effect of temperature
Temperature influence is an important factor in that it is necessary
to determine whether the ongoing adsorption process is endothermic
or exothermic naturally. For this aim, AB 129 adsorption studies
over CuO-NP-AC were carried out at various temperatures at 10 and
20 mg L
−1
of AB 129, pH 2.0, 0.9 g L
−1
of CuO-NP-AC and contact
times of 20 and 25 min, respectively. Fig. 6 shows that the removal per-
centage of AB 129 increases with increasing temperature and the max-
imum adsorption occurs at temperature of 333.15 K that shows the
endothermic nature of adsorption. It is observable in Fig. 6 that the
temperature has main effect on the adsorption process. An increase in
temperature is known to increase the diffusion rate of the adsorbate
molecules across the external boundary layer and within the pores. Fur-
thermore, changing the temperature will modify theequilibrium capac-
ity of the adsorbent for a particular adsorbate [34].
3.7. Equilibrium isotherms
Langmuir, Freundlich, Dubinin–Radushkevich and Tempkin isotherm
models give information about mechanism properties and tendency of
the adsorbent for target species by analysis of the experimental equilibri-
um data [35–37].
3.7.1. The Langmuir isotherm
The removal process by homogenous identical site surface with neg-
ligible interaction describes by the Langmuir equation. In the linear
form this model is presented as follows [38]:
Ce
qe
¼1
KLQm
þCe
Qm
:ð5Þ
The Langmuirplot (C
e
/q
e
vs. C
e
) for AB 129 adsorption at room tem-
peratures gives a straight line and the value of Q
m
and K
L
constants and
the correlation coefficients for this model are presented in Table 2 and
Fig. 7. The isotherm of AB 129 on CuO-NP-AC was found to be linear
over the whole concentration range studies with extremely high corre-
lation coefficients (R
2
N0.9985). Furthermore, the essential character-
istics of the Langmuir isotherm can be described by a separation
factor, which is defined by the following equation [34]:
RL¼1
1þKLC0
:ð6Þ
The value of R
L
indicates the shape of the Langmuir isotherm and na-
ture of the adsorption process. It is a favorable process whenthe value is
within the range of 0–1. In our study, Table 2 shows that the calculated
values of R
L
were found to be in the range of 0–1, indicating that the ad-
sorption process was favorable for CuO-NP-AC.
3.7.2. The Freundlich isotherm
Another well-known assumption for dye adsorption on heteroge-
neous surface along with interaction is the Freundlich isotherm model
that can be expressed in linear form as follows [39]:
log qe¼log KFþ1
nlog Ce:ð7Þ
From the intercept and slope of linear plot of log q
e
versus log C
e
,the
values of K
F
and 1 / n can be determined,respectively. K
F
approximately
shows adsorption capacity and 1 / n indicates the adsorption intensity.
The magnitude of the exponent (1 / n) represents the adsorption
70
75
80
85
90
95
100
280 300 320 340
Removal (%)
T (K)
10 ppm
20 ppm
Fig. 6. Effectof temperature on the removal of AB 129 at 0.045 g of CuO-NP-AC in 50 mL at
pH 2, and AB 129 concentration of 10 and 20 mg L
−1
.
Table 2
Isothermparameter correlation coefficientscalculated by variousadsorption models onto
0.045 g of CuO-NC-AC in 50 mL, pH 2,and room temperature.
Isotherm Equation Plot parameters Adsorbent
Langmuir Ce
qe¼1
KaQm
þCe
Qm
A plot: C
e
/q
e
versus C
e
Q
m
(mg g
−1
) 65.36
K
L
(L mg
−1
) 1.142
R
L
0.0096–0.081
R
2
0.9985
Freundlich log qe¼log KFþ1
n
log CePlot: lnq
e
versus ln C
e
1 / n 0.2461
K
F
(L mg
−1
) 31.35
R
2
0.9777
Tempkin q
e
=B
1
ln K
T
+B
1
ln C
e
Plot: q
e
versus ln C
e
B
1
7.970
K
T
(L mg
−1
) 108.92
R
2
0.9721
Dubinin and
Radushkevich
ln q
e
=lnQ
S
−βε
2
Plot: lnq
e
versus ε
2
Q
s
(mg g
−1
) 48.740
E (kJ mol
−1
) 5.0
R
2
0.755
0
0.1
0.2
0.3
0.4
0.5
0.6
010203040
Ce/qe
Ce(mg/L)
Fig. 7. Langmuir isothermfor adsorption of AB 129onto 0.045 g of CuO-NP-ACin 50 mL of
different initial dye concentrations, room temperature, pH 2.
0
0.5
1
1.5
2
-2 -1 0 1 2
Log qe
Log Ce
Fig. 8. Freundlich isothermfor adsorptionof AB 129 onto 0.045 g of CuO-NP-ACin 50 mL of
different initial dye concentrations, room temperature, pH 2.
128 F. Nekouei et al. / Journal of Molecular Liquids 201 (2015) 124–133
favorability. Values of n N1 represent a favorable adsorption [40].The
values of K
F
, n and the linear regression correlation (R
2
) for the
Freundlich model are given in Table 2 and Fig. 8. The value of 1 / n is
known as the heterogeneity factor and ranges between 0 and 1. For
more heterogeneous surface, the value of 1 / n is close to 0 [40],and
the values higher than unity of n show physical nature of the adsorption
process. According to Table 2,thelowR
2
value of this model shows its
inapplicability for interpretation of experimental data.
3.7.3. The Tempkin isotherm
The Tempkin and Pyzhev model is presented in linear form by the
following equation [41,42]:
qe¼B1ln KTþB1ln Ceð8Þ
where B
1
= RT / b, T is the absolute temperature in Kelvin, R is
the universal gas constant (8.314 J K
−1
mol
−1
), and values of B
1
and
K
T
were calculated from the plot of q
e
against lnC
e
(Table 2 and Fig. 9).
As shown in Table 2,thelowR
2
value of this model shows its inapplica-
bility for interpretation of experimental data.
3.7.4. The Dubinin–Radushkevich (D–R) isotherm
To estimate the porosity, free energy and the characteristics of
adsorbents, the D–R model was also applied [43,44]. The D–R model
has commonly been applied in the following equation (Eq. (9)) and its
linearformcanbeshowninEq.(10):
qe¼Qsexp −βε2
ð9Þ
ln qe¼ln Qs−βε2ð10Þ
where βis a constant related to the adsorption energy, Q
s
is the theoret-
ical saturation capacity, and εis the Polanyi potential, calculated from
Eq. (11).
ε¼RT ln 1 þ1
Ce
:ð11Þ
The slope of the plot of lnq
e
versus ε
2
gives β(mol
2
(kJ
2
)
−1
)andthe
interceptyields the adsorption capacity, Q
s
(mg g
−1
). The mean free en-
ergy of adsorption (E), for the transfer of onemole of target frominfinity
in the solution to the surface of the solid was calculated from the βvalue
using the following relation [45]:
E¼1
ffiffiffiffiffiffi
2β
p:ð12Þ
The calculated values of D–R are given in Table 2.ThevalueofEcal-
culated using Eq. (11) is 5.0 kJ mol
−1
corresponding to physio-sorption
process playing an important role in the adsorption of AB 129 onto
CuO-NP-AC.
3.8. Kinetic study
Every adsorption process may follow one of their combinations from
differentpatterns suchas chemical reaction, diffusion control and mass
transfer. Analysis of experimental data at various times makes it possi-
ble to calculate the kinetic parameters and take some information for
designing and modeling the adsorption processes [46].
3.8.1. The pseudo-first-order model
The linear form of the pseudo-first-order equation [47] can be
described as
log qe−qt
ðÞ¼log qe
ðÞ−k1
2:303t ð13Þ
where q
e
and q
t
(mg g
−1
) are the amounts of AB 129 adsorbed at equi-
librium and at time t ( min), respectively, and k
1
(min
−1
) is the rate con-
stant of the pseudo-first-order model. The values of q
e
and k
1
can be
obtained from the intercept and slope of a plot of ln(q
e
−q
t
) versus t.
The difference between experimental and theoretical q
e
values shows
inapplicability of the model even when this line has high correlation co-
efficient. The distance of experimental and theoretical (q
e
)values
(Table 3 and Fig. 10), indicates the inapplicability of this model for pre-
diction of adsorption data.
3.8.2. The pseudo-second-order model
Another well-known kinetic model is pseudo-second-order model
in which the linear form of the pseudo-second-order equation [48] is
presented as follows:
t
qt
¼1
k2q2
e
þ1
qe
tð14Þ
where k
2
(g/(mg min)) is the pseudo-second-order rate constant. The
parameters q
e
and k
2
can be estimated from the slope and the intercept
of the plot (t / q
t
)versust.BasedonTable 3 and Fig. 11 the highest R
2
value of this model and goodagreement of calculated and experimental
q
e
value under different conditions strongly confirm the applicability of
this model to interpret experimental data. The initial adsorption rates
can be calculated fromthe pseudo-second-order model by the following
equation [46]:
ho;2¼K2q2
e:ð15Þ
The results are shown in Table 3. The results notice that the initial
adsorption rate increases with elevation of the initialAB 129 concentra-
tion. The initial increase in h
o,2
probably attributed to enhancement in
the mass transport driving force emerged from a higher ratio of AB
129 molecules to reactive vacant adsorbent sites. At higher concentra-
tions due to apparent AB 129 dimerization and difficult diffusion of
large dimmers in small adsorbent pores that cause to kinetic parameters
worsened.
3.8.3. Intra-particle diffusion model
Adsorption of any dye from aqueous phase onto porous materials is
usually controlled by either the film diffusion (external mass transfer)
or the intra-particle diffusion rate or both [49]. The following equation
is used in order to identify the diffusion mechanism, the intra-particle
diffusion model [50]:
qt¼Kdi f t1
2þCð16Þ
where q
t
(mg g
−1
) is the amount of AB 129 adsorbed at time t (min),
k
dif
(mg/(g min
0.5
)) is the intra-particle diffusion rate constant and C
(mg g
−1
) is the intercept, which represents the thickness of the bound-
ary layer. A larger C value indicates a greater effect of the boundary layer
0
20
40
60
80
-6 -4 -2 0 2 4
qe
Ln Ce
Fig. 9. Tempkinisotherm for adsorption of AB 129 onto 0.045 g of CuO-NP-AC in 50 mL of
different initial dye concentrations, room temperature, pH 2.
129F. Nekouei et al. / Journal of Molecular Liquids 201 (2015) 124–133
[50]. The distance of R
2
values (Table 3 and Fig. 12) from unity indicates
the inapplicability of this model that rejects the rate-limiting step in the
intra-particle diffusion process.
3.8.4. Elovich model
Another rate equation based on the adsorption capacity is the
Elovich equation presented as follows [51,52]:
qt¼1
βln αβðÞþ
1
βln tðÞ:ð17Þ
Plot of q
t
versus ln(t) should yield a linear relationship if the Elovich
is applicable with a slope of (1 / β) and an intercept of (1 / β) ln(αβ).
The Elovich constants obtained from the slope and the intercept of the
straight line are reported in Table 3. As shown in Table 3, the low R
2
values of these models show their inapplicability for interpretation of
experimental data.
3.9. Thermodynamic study
Thermodynamic parameters including changes in the Gibbs free
energy (ΔG
0
, kJ mol
−1
), enthalpy (ΔH
0
, kJ mol
−1
), and entropy (ΔS
0
,
kJ mol
−1
K
−1
)) during the adsorption process are determined by the
following equations [53]:
ΔG0¼−RT ln K0:ð18Þ
ΔG0¼ΔH0−TΔS0:ð19Þ
where R (8.314 J mol
−1
K
−1
) is the gas constant, T (K) is the absolute
temperature, and K
C
(L/mol) is the Langmuir constant obtained from
the plot of C
e
/q
e
vs. Ce. The negative value of ΔG
0
and the decrease in
its value with rising temperature confirm the spontaneous nature and
feasibility of the adsorption via physical force (Table 4). The values of
other parameters such as enthalpy change (ΔH
0
), and entropy change
(ΔS
0
), may be determined from the Van't Hoff equation (Eq. (20)):
ln K0¼ΔS0
R−ΔH0
RT :ð20Þ
The slope and intercept of the Van't Hoff plot are equal to (−ΔH
0
/R)
and (ΔS
0
/R), respectively where R is the universal gas constant
(8.314 J mol
−1
K
−1
) and T is the absolute temperature (K). The data
(Fig. 13 and Table 4) show positive values of ΔH
0
that indicate the endo-
thermic nature of the adsorption. Furthermore, the positive values of ΔS
0
show the affinity of CuO-NP-AC for AB 129 and the increasing random-
ness at the solid–solution interface with some structural changes in the
adsorbates and adsorbents during the adsorption process. To further sup-
port the assertion that physical adsorption is the predominant mecha-
nism, the values of activation energy (E
a
) and sticking probability (S*)
were estimated from the experimental data using modified Arrhenius
type equation related to surface coverage (θ)asfollows[54]:
S¼1−θðÞe−Ea=RTðÞ ð21Þ
The sticking probability (S*) is a function of the adsorbate/adsorbent
system under investigation. The parameter S* shows the measure of the
potential of an adsorbate to remain on the adsorbent [55-71]. The values
of S* presented in Table 3 were found to be 3.59 × 10
−12
and 1.68 × 10
−10
Table 3
Adsorption kinetic parameters at different initial AB 129 on to 0.045 g of CuO-NC-AC in 50 mL at pH 2, room temperature and AB 129 concentration of 10 and 20mgL
−1
.
Model Equation Plot parameters C
o
(mg L
−1
)
10 20
First-order log(q
e
−q
t
) = log(q
e
)−k
1
t / 2.303 Plot: log (qe −qt) versus t
K
1
(min
−1
) 0.181 0.192
q
e
(calc) (mg g
−1
) 5.14 18.20
R
2
0.8668 0.9035
Second-order (t / q
t
)=1/(K
2
q
e
2
)+(1/q
e
)t Plot: (t/q
t
) versus t
K
2
(g/mg min) 0.0170 0.085
q
e
(calc) (mg g
−1
) 11.52 23.87
R
2
0.9957 0.9937
h (mg/g min) 2.26 48.43
Intraparticle q
t
=K
dif
t
1/2
+ C Plot: q
t
versus t
1/2
K
dif
(mg/(g min
1/2
) 0.7162 0.16
C 7.83 13.51
R
2
0.9466 0.9401
Elovich q
t
=1/βln(αβ)+1/βln(t) Plot: q
t
versus ln(t)
β1.299 0.462
dα4.9 E4 2.1 E3
R
2
0.8546 0.8501
Experimental adsorption capacity q
e
(exp) 11.08 22.16
-1.5
-1
-0.5
0
0.5
1
1.5
0102030
Log (qe
-qt)
t (min)
10 ppm
20 ppm
Fig. 10. Plots of pseudo-first-order kinetic for dye concentrations of 10 and 20 mg L
−1
.
0
0.5
1
1.5
2
0102030
t/qt
t (min)
10 ppm
20 ppm
Fig. 11. Plotsof pseudo-second-order kinetic for dye concentrations of 10 and 20 mg L
−1
.
130 F. Nekouei et al. / Journal of Molecular Liquids 201 (2015) 124–133
that lie in the range of 0 bS* b1 and are dependent on the temperature
of the system. The surface coverage (θ) can be calculated from the
following equation:
θ¼1−Ce
C0
:ð22Þ
The activation energy and sticking probability were estimated from
aplotofln(1−θ) versus 1 / T (Table 4).
4. Conclusions
The investigations showed the efficiency of CuO-NP-AC as a good,
green and low cost with satisfactory capacity adsorbent (65.36)
for the removal of AB 129 from aqueous solutions in a short time
(≤25 min). In Table 5, the performance of the proposed method has
been compared with other methods and some adsorbents reported for
removal of some acid blue dyes in literature. The Langmuir isotherm
could describe the adsorption isotherms compared to other models.
Also, the result of kinetic study of AB 129 on CuO-NP-AC indicated
that the adsorption follows the pseudo-second-order model after
performing based on pseudo-first-order, pseudo-second-order, Elovich
and intra-particle diffusion equations. Changes in activation enthalpy
(ΔH
0
), free energy of adsorption (ΔG
0
) and entropy (ΔS
0
)showthe
spontaneous and the endothermic feature of the adsorption process.
Nomenclature
C
0
initial dye concentration
C
t
dye concentration (mg L
−1
)attime(t)
q
e
equilibrium adsorption capacity (mg g
−1
)
C
e
dye concentration (mg L
−1
) at equilibrium
Vvolumeofsolution(L)
W weight of adsorbent (g)
K
1
rate constant of pseudo-first-order adsorption (min
−1
)
K
2
second-order rate constant of adsorption (mg g
−1
min
−1
)
h
o,2
second-order rate constants (mg/(g min
1/2
))
αinitial adsorption rate (mg g
−1
min
−1
)
βdesorption constant (mg g
−1
)
C intercept of intraparticle diffusion (related to the thickness of
the boundary layer)
K
dif
rate constant of intraparticle diffusion (mg/(g min
1/2
))
Q
m
maximumadsorption capacity reflected a complete monolay-
er (mg g
−1
) in the Langmuir isotherm model
K
L
Langmuir constant or adsorption equilibrium constant (L mg
−1
) that is related to the apparent energy of sorption
R
L
dimensionless equilibrium parameter (separation factor)
K
F
isotherm constant indicates the capacity parameter (mg g
−1
)
related to the intensity of the adsorption
n isotherm constant indicates the empirical parameter (g L
−1
)
related to the intensity of the adsorption
T absolute temperature in Kelvin
R universal gas constant (8.314 J K
−1
mol
−1
)
B
1
related to the heat of adsorption (B
1
=RT/b)
K
T
equilibrium binding constant
βconstant related to the adsorption energy at the D–R isotherm
(mol
2
kJ
−2
)
Q
s
theoretical saturation capacity at the D–R isotherm
εPolanyi potential at the D–R isotherm
E mean free energy of adsorption
q
e,exp
experimental data of the equilibrium capacity (mg g
−1
)
q
e,calc
equilibrium capacity obtained by calculating from the iso-
therm model (mg g
−1
)
R
2
correlation coefficient
D average size of nanocrystallites
ΔG
0
Gibbs free energy (kJ mol
−1
)
ΔH
0
enthalpy (kJ mol
−1
)
ΔS
0
entropy (kJ mol
−1
K
−1
)
E
a
activation energy (kJ mol
−1
)
S* sticking probability
θsurface coverage
0
5
10
15
20
25
0246
q
t
t0.5
10 ppm
20 ppm
Fig. 12. Plots of intraparticle diffusion model for dye concentrations of 10 and 20 mg L
−1
.
Table 4
Thermodynamic parameters for adsorption of AB 129onto 0.045 g of CuO-NP-AC at pH 2 at initial dye concentrations of 10 and 20 mg L
−1
.
C
o
(mg L
−1
) Temperature (K) K
e
ΔG
0
(kJ mol
−1
)ΔS
0
(J mol
−1
K
−1
)ΔH
0
(kJ mol
−1
)E
a
(kJ mol
−1
)S
⁎
10 283.15 10.17 −5.46 236.18 61.60 48.77 3.59 × 10
−12
293.15 22.55 −7.33
303.15 50.16 −9.87
313.15 110.30 −12.24
323.15 230.89 −14.62
333.15 471.25 −17.05
20 283.15 7.50 −4.74 195.67 50.94 44.44 1.68 × 10
−10
293.15 13.61 −6.36
303.15 25.55 −8.17
313.15 49.17 −10.14
323.15 93.78 −12.2
333.15 178.87 −14.37
0
1
2
3
4
5
6
7
0.0029 0.003 0.0031 0.0032 0.0033 0.0034 0.0035 0.0036
Ln Ke
1/T (K -1)
10 PPM
20 PPM
Fig. 13. Van't Hoff plots of AB 129 dye onto CuO-NP-AC for evaluating thermodynamic
parameters.
131F. Nekouei et al. / Journal of Molecular Liquids 201 (2015) 124–133
Acknowledgments
The authors state their gratitude to Young Researchers and Elite
Club, Gachsaran Branch, Islamic Azad University, Gachsaran, Iran, for
financial support of this work (Grant 1393). Inderjeet Tyagi thanks
DST, Govt. of India for supporting the work through Project No. DST/
WTI/2K11/352.
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Dye Adsorbent Adsorption capacity Contact time (min) Adsorbent dose (g) Reference
Acid blue 129 Activated carbon cloth 61.43 498 28 ± 0.1 [56]
Acid blue 129 Almond shell 11.95 14 0.4 [4]
Acid blue 129 Bentonite-CTAB 1.54 N100 –[57]
Acid blue 129 Raw bentonite 1.05 N1000 –[57]
Acid blue 92 Activated carbon cloth 68.75 1225 28 ± 0.1 [56]
Acid blue 120 Activated carbon cloth 28.04 1020 28 ± 0.1 [56]
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