Article

Application of Carbon Adsorbents Prepared from Brazilian-Pine Fruit Shell for the Removal of Reactive Orange 16 from Aqueous Solution: Kinetic, Equilibrium, and Thermodynamic Studies

Institute of Chemistry, Federal University of Rio Grande do Sul, UFRGS, Av. Bento Gonçalves 9500, Caixa Postal 15003, CEP 91501-970, Porto Alegre, RS, Brazil.
Journal of Environmental Management (Impact Factor: 2.72). 08/2010; 91(8):1695-706. DOI: 10.1016/j.jenvman.2010.03.013
Source: PubMed

ABSTRACT

Activated (AC-PW) and non-activated (C-PW) carbonaceous materials were prepared from the Brazilian-pine fruit shell (Araucaria angustifolia) and tested as adsorbents for the removal of reactive orange 16 dye (RO-16) from aqueous effluents. The effects of shaking time, adsorbent dosage and pH on the adsorption capacity were studied. RO-16 uptake was favorable at pH values ranging from 2.0 to 3.0 and from 2.0 to 7.0 for C-PW and AC-PW, respectively. The contact time required to obtain the equilibrium using C-PW and AC-PW as adsorbents was 5 and 4h at 298 K, respectively. The fractionary-order kinetic model provided the best fit to experimental data compared with other models. Equilibrium data were better fit to the Sips isotherm model using C-PW and AC-PW as adsorbents. The enthalpy and entropy of adsorption of RO-16 were obtained from adsorption experiments ranging from 298 to 323 K.

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Application of carbon adsorbents prepared from Brazilian-pine fruit shell for the
removal of reactive orange 16 from aqueous solution: Kinetic, equilibrium, and
thermodynamic studies
Tatiana Calvete
a
, Eder C. Lima
a
,
*
, Natali F. Cardoso
a
, Júlio C.P. Vaghetti
a
, Silvio L.P. Dias
a
, Flavio A. Pavan
b
a
Institute of Chemistry, Federal University of Rio Grande do Sul, UFRGS, Av. Bento Gonçalves 9500, Caixa Postal 15003, CEP 91501-970, Porto Alegre, RS, Brazil
b
Federal University of Pampa, UNIPAMPA, Bagé, RS, Brazil
article info
Article history:
Received 16 September 2009
Received in revised form
3 March 2010
Accepted 23 March 2010
Keywords:
Adsorption
Brazilian-pine fruit shell
Activated carbon
Reactive orange 16
Nonlinear isotherms
abstract
Activated (AC-PW) and non-activated (C-PW) carbonaceous materials were prepared from the Brazilian-
pine fruit shell (Araucaria angustifolia) and tested as adsorbents for the removal of reactive orange 16 dye
(RO-16) from aqueous efuents. The effects of shaking time, adsorbent dosage and pH on the adsorption
capacity were studied. RO-16 uptake was favorable at pH values ranging from 2.0 to 3.0 and from 2.0 to
7.0 for C-PW and AC-PW, respectively. The contact time required to obtain the equilibrium using C-PW
and AC-PW as adsorbents was 5 and 4 h at 298 K, respectively. The fractionary-order kinetic model
provided the best t to experimental data compared with other models. Equilibrium data were better t
to the Sips isotherm model using C-PW and AC-PW as adsorbents. The enthalpy and entropy of
adsorption of RO-16 were obtained from adsorption experiments ranging from 298 to 323 K.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Dyes are a kind of organic compound with a complex aromatic
molecular structure that can bring bright and rm color to other
substances. However, the complex aromatic molecular structures of
dyes make them more stable and more difcult to biodegrade
(Wang and Li, 2007). The extensive use of dyes in different kinds of
industries often poses pollution problems in the form of colored
wastewater discharged into environmental water bodies.
The most efcient method for the removal of synthetic dyes
from aqueous efuents is the adsorption procedure (Pavan et al.,
20 07, 2008a; Gupta and Suhas, 2009). This process transfers the
dye species from the water efuent to a solid phase thereby
keeping the efuent volume to a minimum (Wang and Zhu, 2006,
20 07). Subsequently, the adsorbent can be regenerated or stored
in a dry place without direct contact with the environment (Gupta
et al., 20 06; Pavan et al., 2008b; Wang and Li, 2007).
Activated carbon is the most employed adsorbent for toxic
species removal from aqueous efuents because of well-developed
pore structures and a high internal surface area that leads to its
excellent adsorption properties (Kavitha and Namasivayam, 2008;
Marsh and Reinoso, 2006). Besides these physical characteristics,
the adsorption capacity is also dependent on the source of organic
material employed for the production of the activated carbon
(Marsh and Reinoso, 2006; Olivares-Marín et al., 2006; Tan et al.,
20 08), as well as the experimental conditions employed in the
activation processes (Marsh and Reinoso, 2006).
Activated carbon can be prepared using a variety of chemical
(El-Hendawy, 2009) and physical (Yang et al., 2008) activation
methods and in some cases using a combination of both types of
methods (Albero et al., 2009). Chemical activation is the process
where the carbon precursor material is rstly treated with
aqueous solutions of dehydrating agents such as H
3
PO
4
, ZnCl
2
,
H
2
SO
4
, and KOH. Afterwards, the carbon material is dried at
373e393 K to eliminate the water. In a subsequent step, the
chemically treated carbon material is heated between 673 and
1073 K under nitrogen atmosphere (Marsh and Reinoso, 2006;
Faria et al., 2008 ). The physical activation consists of a thermal
treatment of previously carbonized material with suitable
oxidizing gases, such as air at temperatures in the 623e823 K
range or 1073e1373 K using steam and/or carbon dioxide (Marsh
and Reinoso, 2006; Li et al., 2008).
In recent years, a considerable number of studies have focused
on low cost alternative materials for the production of activated
*
Corresponding author. Tel.: þ55 51 3308 7175; fax: þ 55 51 3308 7304.
E-mail addresses: eder.lima@ufrgs.br, profederlima@gmail.com (E.C. Lima).
Contents lists available at ScienceDirect
Journal of Environmental Management
journal homepage: www.elsevier.com/locate/jenvman
0301-4797/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jenvman.2010.03.013
Journal of Environmental Management 91 (2010) 1695e1706
Page 2
Author's personal copy
carbons from agricultural wastes such as, coconut shell (Albero
et al., 2009; Mohan et al., 2005; Mohan et al., 2008; Vieira et al.,
20 06; Tan et al., 2008), coffee beans (Nunes et al., 2009), corn
grain (Balathanigaimani et al., 2009), bamboo (Ip et al., 2008; Chan
et al., 2008), gingelly (sesame), cotton, seed shells (Thinakaran
et al., 2008), cherry stones (Olivares-Marín et al., 20 06), apricot
stones (Demirbas et al., 2008), nutshell (Guo and Rockstraw, 2007),
oil-palm ber (Hameed et al., 2008), vine shoots (Corcho-Corral
et al., 2006), rice husk (Mohan et al., 2008), and coir pitch
(Kavitha and Namasivayam, 2008).
Alternatively to activated carbon, non-activated carbonized
materials also present some ability for the removal of dyes from
aqueous solutions (Elizalde-González et al., 2007; Elizalde-
González and Hernández-Montoya, 2009; Royer et al., 2009a).
Besides, these materials do not require an activation process at
higher temperatures in the presence of special gases.
In a previous paper (Calvete et al., 2009) it was proposed, the
use of Brazilian-pine fruit shell (Araucaria angustifolia syn.
Araucaria brasiliensis) as a precursor for the preparation of non-
activated carbonaceous materials (C-PW) as well as the activated
carbonaceous materials (AC-PW). These adsorbents were ef-
cient to remove the Procion Red MX 3B reactive dye from
aqueous solutions. Continuing the application of these adsor-
bents, in the present work, C-PW and AC-PW were employed for
the removal of reactive orange 16 (RO-16) from aqueous
efuents.
2. Materials and methods
2.1. Solutions and reagents
De-ionized water was used throughout the experiments for all
solution preparations. Reactive orange 16 (RO-16) (C.I. 17757;
C
20
H
17
N
3
O
10
S
3
Na
2
, 601.54 g mol
1
, see Scheme 1), was obtained
from Sigma Chemical Co., USA, (50% dye content) and used
without further purication. The stock solution was prepared by
dissolving dye in distilled water to the concentration of 5.00 g L
1
.
Working solutions were obtained by diluting the dye stock
solution to the required concentrations. To adjust the pH solu-
tions, 0.10 mol L
1
sodium hydroxide or hydrochloric acid solu-
tions was used. The pH of the solutions was measured using
a Hanna (HI 255) pH meter.
2.2. Adsorbents preparation and characterization
The Brazilian-pine fruit (piñon) shell was dried and milled as
previously reported (Brasil et al., 2006; Lima et al., 2007). The
carbonization of the Brazilian-pine fruit shells was achieved by
the procedure already reported in the literature (Royer et al.,
20 09a), obtaining the non-activated carbonaceous material
designed as C-PW.
To prepare the activated carbon adsorbent, an amount of
previously non-activated carbonized material (C-PW) was activated
as reported in the literature (Calvete et al., 2009). The activated
carbon obtained was assigned as AC-PW.
The adsorbents C-PW and AC-PW were characterized by FTIR
using a Nicolet FTIR, model 6700. Spectra were obtained with
a resolution of 4 cm
1
over 100 cumulative scans (Passos et al.,
20 06).
Adsorbent samples were also analyzed by scanning electron
microscopy (SEM) in a Jeol microscope, model JEOL JSM 6060, using
an acceleration voltage of 20 kV and magnication ranging from
100 to 20,000-fold (Lima et al., 2008).
Thermogravimetric (TGA) and derivative thermogravimetric
(DTG) curves were obtained on a TA Instruments model TGA
Q5000, with a heating rate of 10
C min
1
, under 100 mL min
1
of
an nitrogen ow, varying from room temperature to 700
C, with an
initial mass of approximately 3.0 0e20.00 mg of solid (Royer et al.,
2009b).
2.3. Adsorption studies
Adsorption studies for the evaluation of the C-PW and AC-
PW adsorbents for the removal of RO-16 dye from aqueous
solutions were carried out in triplicate using the batch contact
adsorption. For these experiments, xed amounts of adsorbents
(20.0e200.0 mg) were placed in a 50 mL glass Erlenmeyer asks
containing 20.0 mL of dye solutions (20.00e1500.0 mg L
1
),
which were agitated for a suitable time (0.25e8 h) from 298 to
323 K. The pH of the dye solutions ranged from 2.0 to 10.0.
Subsequently, in order to separate the adsorbents from the
Scheme 1. (A) Structural formulae of reactive orange 16. (B) Optimized three-
dimensional structural formulae of RO-16. The dimensions of the chemical molecule
were calculated using ChemBio 3D Ultra version 11.0.
Table 1
Kinetic adsorption models.
Kinetic model Nonlinear equation
Avrami q
t
¼q
e
{1 exp[(k
AV
t)]
nAV
}
Pseudo-rst-order q
t
¼q
e
[1 exp(k
f
t)]
Pseudo-second-order
q
t
¼
k
s
q
2
e
t
1þq
e
k
s
t
h
o
¼k
s
q
e
2
initial sorption rate
Elovich q
t
¼
1
b
lnð
a
b
Þþ
1
b
lnðtÞ
Intra-particle diffusion q
t
¼ k
id
ffiffi
t
p
þ C
T. Calvete et al. / Journal of Environmental Management 91 (2010) 1695e17061696
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aqueous solutions, the asks were centrifuged at 3600 rpm for
10 min, and aliquots of 1e10 mL of the supernatant were prop-
erly diluted with water. The nal concentrations of the dye
remaining in the solutions were determined by visible spectro-
photometry, using a Femto spectrophotometer provided with
optical-glass cells. Absorbance measurements were made at the
maximum wavelength of RO-16 which was 493 nm. The RO-16
detection limit using the spectrophotometric method, deter-
mined according to IUPAC (Lima et al., 1998), was 0.12 mg L
1
.
The amount of the dye uptake and percentage of the removal of
dye by the adsorbents was calculated by applying the Eqs. (1)
and (2), respectively:
q ¼
C
o
C
f
m
V (1)
% Removal ¼ 100
C
o
C
f
C
o
(2)
where q is the amount of dye taken up by the adsorbents (mg g
1
);
C
o
is the initial RO-16 concentration put in contact with the
adsorbent (mg L
1
), C
f
is the dye concentration (mg L
1
) after the
batch adsorption procedure, V is the volume of dye solution (L) put
in contact with the adsorbent, and m is the mass (g) of the
adsorbent.
2.4. Kinetic and equilibrium models
Avrami, pseudo-rst-order, pseudo-second-order, Elovich and
intra-particle diffusion model kinetic equations are given in Table 1
(Vaghetti et al., 2009a).
Langmuir, Freundlich, Sips and RedlichePeterson isotherm
equations are given in Table 2 (Vaghetti et al., 2009a).
2.5. Statistical evaluation of the kinetic and isotherm parameters
Kinetic and equilibrium models were t employing the nonlinear
tting method using the nonlinear tting facilities of the software
Microcal Origin 7.0. In addition, models were also evaluated by an
error function (Vaghetti et al., 2009b) that measures the differences
in the amount of dye taken up by the adsorbent predicted by the
models and the actual q measured experimentally.
Table 2
Isotherm models.
Isotherm model Equation
Langmuir
q
e
¼
Q
max
K
L
C
e
1þK
L
C
e
Freundlich q
e
¼ K
F
C
1=nF
e
Sips q
e
¼
Q
max
K
S
C
1=n
s
e
1þK
S
C
1=n
s
e
RedlichePeterson q
e
¼
K
RP
C
e
1þa
RP
C
g
e
where 0 g 1
Table 3
FTIR bands (cm
1
) of the adsorbents C-PW and AC-PW before and after the
adsorption of RO-16 dye.
C-PW AC-PW
Before After Before After
3432 3408 3432 3420
1707 1701 1641e1400 1634e1395
1165e1034 1161e1031 1094 1051
Fig. 1. Thermogravimetric (TGA) and derivative thermogravimetric (DTG) curves for:
A, RO-16; B, C-PW; C, C-PW þ RO-16; D, AC-PW; E, AC-PW þRO-16.
T. Calvete et al. / Journal of Environmental Management 91 (2010) 1695e1706 1697
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F
error
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
X
p
i
q
i;model
q
i;experimental
q
i;experimental
!
2
:
1
p 1
v
u
u
t
(3)
where q
i,model
is the value of q predicted by the tted model and
q
i
,
experimental
is the value of q measured experimentally, and p is the
number of experiments performed.
3. Results and discussion
3.1. Characterization of the adsorbents
FTIR technique was used to examine the surface groups of
adsorbents (C-PW and AC-PW) and to identify the groups respon-
sible for dye adsorption. Infrared spectra of the adsorbents and dye-
loaded adsorbent samples, before and after the adsorption process,
were recorded in the range 4000e400 cm
1
(Table 3). As previ-
ously observed for a y ash adsorbent (Kara et al., 2007) after the
adsorption procedure, the functional groups that interacted with
the dye suffered a shift to lower wavenumbers.
Table 3 presents the signicant changes of FTIR vibrational
spectra of C-PW and AC-PW before the adsorption and loaded
with the dye RO-16 after the adsorption. For C-PW, the intense
absorption bands at 3432 and 3408 cm
1
are assigned to OeH
bond stretching, before and after adsorption, respectively (Smith,
1999; Vaghetti et al., 2008; Pavan et al., 2006). Small bands at
1707 and 1701 cm
1
, before and after absorption, respectively, are
assigned to carbonyl groups of carboxylic acid. Small bands
ranging from 1165 to 1034 cm
1
and 1161 to 1031 cm
1
before and
after adsorption, respectively, are assigned to CeO stretching
vibrations of lignin (Smith, 1999). FTIR results indicate that the
dehydration of the Brazilian-pine fruit shell with sulfuric acid
producing a carbonized material (C-PW) does not completely
destroy all the chemical functions of the original biomaterial
without chemical treatment, as previously reported (Lima et al.,
2007; Vaghetti et al., 2008). Besides, the interaction of the dye
with the C-PW adsorbent should occur with the OeH bonds of
phenols and carboxylate groups.
The activation process to produce activated carbon from Bra-
zilian-pine fruit shell (AC-PW) decreased the amount and intensity
of vibrational bands when compared to C-PW adsorbent. This
indicates that the oxidation and activation processes for the
production of AC-PW were efcient and lead to the oxidation of the
functional groups of the starting material, as already observed by
the FTIR spectra of cherry stones activated by KOH (Olivares-Marín
et al., 200 6). Absorption bands at 3432 and 3420 cm
1
are assigned
to OeH bond stretching, before and after adsorption, respectively
(Smith, 1999), indicating that this group plays a role on the
adsorption of the RO-16 dye. Bands ranging from 1641 to
1400 cm
1
and 1634 to 1395 cm
1
before and after absorption,
respectively are assigned to C]C of aromatic rings. The shifts of
these bands to lower wavenumbers after the adsorption of the dye
indicate that the mechanism of interaction of the RO-16 dye with
the activated carbon AC-PW should also occur by the
p
e
p
inter-
action of the dye with the aromatic rings of the activated carbon
(Zhang et al., 2008), besides the interaction with its functional
groups (carboxylate, OH). In addition, strong bands of 1094 and
1051 cm
1
, before and after adsorption, respectively, conrm the
presence of CeO bond (Fig 1B) (Smith, 1999) reinforcing the
interaction of the dye with carboxylate groups.
Figure 1 shows the thermogravimetric (TGA) and derivative
thermogravimetric (DTG) curves of RO-16 (Fig 1A), C-PW (Fig 1B),
C-PW þRO-16 (Fig 1C), AC-PW (Fig 1D) and AC-PW þRO-16 (Fig 1E).
Fig. 2. (A) SEM for C-PW with magnication of 1.0 00. (B) SEM for C-PW with magnication of 2.500. (C) SEM for AC-PW with magnication of 1.000. (D) SEM for AC-PW with
magnication of 2.500.
T. Calvete et al. / Journal of Environmental Management 91 (2010) 1695e17061698
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The DTG curve of RO-16 (Fig 1A) shows basically three peaks
with several shoulders. These shoulders should correspond to the
isomers of RO-16, since their purity is about 50%. The rst mass
loss region (45e148
C) is consistent with the elimination of
moisture (5.7%). From 148 to 241
C the small mass loss (1.9%) is
consistent with the elimination of hydration water bound to the
solid dye. There is a mass loss of 14.2% and 19.7% from 241 to
380
C and from 380 to 675
C, respectively, leading to the
carbonization of the product. There is no further decomposition
above this temperature and the total mass losses were 43.8%.
Similar results of decomposition of dyes were reported in the
literature (Ma and Sun, 2004).
The DTG curve of C-PW (Fig 1B) shows basically three main
decomposition steps 87.1, 270.3 and 427.4
C and above this
temperature a gradual mass loss is observed up to 700
C. The rst
mass loss corresponds to the elimination of the moisture of the
material. The second and third decompositions of C-PW are
consistent with the degradation of partially carbonized leg-
ninocellulosic materials, as already reported by Macedo et al.
(2008). In general, the pyrolysis of lignocellulosics start with the
decomposition of hemicellulose (200e260
C), followed by the
decomposition of cellulose and lignin in the ranges 240e350
C and
280e500
C, respectively (Macedo et al., 2008). Therefore, the
second mass loss of C-PW may be the elimination of hemicellulose
plus cellulose and the third mass loss may be the elimination of
lignin.
The thermal curves of C-PW þRO-16 (Fig 1C) are very similar to
C-PW curves; however, due to the presence of RO-16 adsorbed, the
second DTG peak that corresponds to the elimination of hemi-
cellulose plus cellulose is mixed with the thermal decomposition of
the dye, shifting the peak from 270.3 to 292.7
C. The elimination of
the second part of lignin plus the remaining decomposed dye
occurs at 412.9
C. At 700
C, practically all the organic compounds
of C-PW plus of RO-16 dye were carbonized, leading to a residue
that corresponds to 49.1% of the original sample mass.
Thermal curves of AC-PW (Fig 1D) associated with the
previous results of FTIR depicted above (Table 3) show clearly
the difference between the activated carbon AC-PW and the
partially oxidized C-PW material, which keep its ligninocellulosic
structure of the starting material. From 45 to 197
C the mass
loss corresponds to only 4.4%, which may be associated with
moisture losses. From 197 to 435
C the AC-PW presents
a remarkable thermal stability (mass loss 1.9%), and above this
temperature, the activated carbon is destroyed. AC-PW þRO-16
thermal curves (Fig 1E) show clearly that the adsorbed dye is
decomposed at 305.6
C, which should be associated with the
thermal decomposition DTG peak of the dye that appears at
313.2
C(Fig 1A).
Scanning electron micrographs of C-PW (Fig. 2A and B) and
AC-PW (Fig. 2C and D) show the drastic differences of these
materials. C-PW is a more compact material presenting numerous
cavities (macropororous) that were originated from the treatment
Fig. 3. Effect of pH on the removal of RO-16 from aqueous solution. C-PW (A); AC-PW (B). The initial RO-16 concentrations and adsorbent masses were xed at 20 0.0 mg L
1
and
30.0 mg for both adsorbents. The contact time between adsorbent and adsorbate was xed at 4.0 h.
Fig. 4. Adsorbent dosage. --- % RO-16 removal; -C- amount adsorbed per gram (q).
C-PW (A); and AC-PW (B). Time of contact 4 h.
T. Calvete et al. / Journal of Environmental Management 91 (2010) 1695e1706 1699
Page 6
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Fig. 5. Kinetic models for the adsorption of RO-16. --- C-PW; -C- AC-PW. (A) C-PW 300 mg L
1
; (B) C-PW 600 mg L
1
; (C) AC-PW 300 mg L
1
; (D) AC-PW 600 mg L
1
; (E) C-PW
300 mg L
1
; (F) C-PW 600 mg L
1
; (G) AC-PW 300 mg L
1
; (H) AC-PW 600 mg L
1
.
T. Calvete et al. / Journal of Environmental Management 91 (2010) 1695e17061700
Page 7
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of the biomaterial with sulfuric acid; however, it also presents
similarities with the natural Brazilian-pine fruit shell ber (Lima
et al., 2008). On the other hand, AC-PW is a material that seems
to have a high surface area (subdivided pieces of carbon) that
could lead to an increase in the maximum amount adsorbed of
the RO-16 dye.
3.2. Effects of acidity on adsorption
One of the most important factors in adsorption studies is the
effect of the acidity of the medium (Royer et al., 2009b). Different
species may present divergent ranges of suitable pH depending on
which adsorbent is used. Effects of initial pH on percentage of
removal of RO-16 dye using C-PW and AC-PW were evaluated
within the pH range between 2 and 10 (Fig. 3A and B, respectively).
For C-PW as adsorbent, the percentage of removal of RO-16 dye was
kept constant in the pH range of 2.0e3.0. When the pH was
increased from 3.5 to 10.0 the percentage of dye removal decreased
by 54%. For AC-PW, the percentage of dye removal was constant for
pH solutions ranging from 2.0 up to 7.1. In the pH interval between
8.0 and 10.0, a 13% decrease in the percentage of adsorption was
observed. AC-PW adsorbent shows a larger optimal pH interval for
adsorption of RO-16 when compared with C-PW, which could be
associated with the remaining functional groups of C-PW materials
that present similarities with the unmodied natural material
(Lima et al., 2008).
The pH
PZC
values for C-PW and AC-PW, were determined in
a previous paper (Calvete et al., 2009), being these values 3.86 and
7.86, respectively. These values conrm the ranges of optimal pH
values for RO-16 removal from aqueous solutions (Fig 3). For pH
values lower than pH
PZC,
the adsorbent presents a positive surface
charge (Lima et al., 2008). The dissolved RO-16 dye is negatively
charged in water solutions (Roberts and Caserio, 1977). The
adsorption of the RO-16 dye takes place when the adsorbent
presents a positive surface charge. For C-PW, the electrostatic
interaction occurs for pH < 3.86, and for AC-PW this interaction
occurs for pH < 7.86. The initial pH values of the dye solutions were
xed at 2.5 and 6.0, for C-PW and AC-PW, respectively.
3.3. Adsorbent dosage
The study of adsorbent dosages for the removal of the dye
from aqueous solution was carried out using quantities of C-PW
and AC-PW adsorbents ranging from 20.0 to 200.0 mg and xing
the volume and initial dye concentration at 20.0 mL and
30 0.0 mg L
1
, respectively. For these experiments, the contact
time was xed at 4.0 h. The highest amount of dye removal was
attained for adsorbent masses of at least 50.0 mg of each adsor-
bent (Fig 4A and B, on the left). For adsorbent quantities higher
than these values, the dye removal remained almost constant.
Increases in the percentage of the dye removal with adsorbent
masses could be attributed to increases in the adsorbent surface
areas, augmenting the number of adsorption sites available for
adsorption, as already reported in several papers (Lima et al.,
20 08; Royer et al., 2009a, b; Vaghetti et al., 2008, 20 09a, b). On
the other hand, the increase in the adsorbent mass promotes
a remarkable decrease in the amount of dye uptake per gram of
adsorbent (q), (Fig. 4, on the right), an effect that can be math-
ematically explained by combining Eqs. (1) and (2):
q ¼
% Removal C
o
V
100 m
(4)
As observed in Eq. (4), the amount of dye uptake (q) and the mass of
adsorbent (m) are inversely proportional. For a xed dye
percentage removal, the increase of adsorbent mass leads to
a decrease in q values, since the volume (V) and initial dye
concentrations (C
o
) are always xed. These values clearly indicate
that the adsorbent mass must be xed at 50.0 mg, which is the
mass that corresponds to the minimum amount of adsorbent that
leads to constant dye removal. Adsorbent masses were therefore
xed at 50.0 mg for both C-PW and AC-PW.
3.4. Kinetic studies
Adsorption kinetic studies are important in the treatment of
aqueous efuents because they provide valuable information on
the mechanism of the adsorption process (Lima et al., 2008).
It is important to point out that the initial RO-16 concentra-
tions employed during the kinetic studies are relatively high
(300.0 and 600.0 mg L
1
) when compared with other studies
reported in the literature (Pavan et al., 2007, 2008b; Royer et al.,
20 09b). AC-PW has a very high adsorption capacity and adsorbs
practically all RO-16 when initial adsorbent concentrations are
lower than 200 mg L
1
. In order to study the mechanism of dye
adsorption, kinetic data were t using the four kinetic models
depicted in Table 1 (Fig. 5)AeD.
As can be seen, only the Avrami fractionary kinetic model
showed the best t, presenting low error function values and also
high R
2
values, for the two initial concentration levels of the dye
with both adsorbents. Table 4 shows the Avrami fractionary kinetic
order in separate, since the other kinetic models presented F
error
values at least 8.6 times higher than the Avrami fractionary kinetic
model. The lower the error function, the lower the difference of the
q calculated by the model from the experimentally measured q
(Vaghetti et al., 2009b). Additionally, it was veried that the q
e
values found in the fractionary-order were closer to the experi-
mental q
e
values, when compared with all other kinetic models.
These results indicate that the fractionary-order kinetic model
should explain the adsorption process of RO-16 taken up by the C-
PW and AC-PW adsorbents.
The Avrami kinetic equation has been successfully employed to
explain several kinetic processes of different adsorbents and
adsorbates (Cestari et al., 200 4; Lopes et al., 2003; Vieira et al.,
20 07). The Avrami exponent (n
AV
) is a fractionary number
related with the possible changes of the adsorption mechanism
that takes place during the adsorption process (Cestari et al., 2004;
Lopes et al., 2003). Instead of following only an integer-kinetic
order, the mechanism adsorption could follow multiple kinetic
orders that are changed during the contact of the adsorbate with
the adsorbent (Cestari et al., 2004; Lopes et al., 2003). n
AV
is
a resultant of the multiple kinetic order of the adsorption
procedure.
Table 4
Kinetic parameters for RO-16 removal using C-PW and AC-PW as adsorbents.
Conditions: temperature was xed at 298 K; pH 2.5 for C-PW and pH 6.0 for AC-PW;
adsorbent mass 50.0 mg.
C-PW AC-PW
300.0 mg L
1
600.0 mg L
1
300.0 mg L
1
600.0 mg L
1
Fractionary-order
k
AV
(h
1
) 0.457 0.485 0.601 0.600
q
e
(mg g
1
) 117 229 120 239
n
AV
1.63 1.65 1.32 1.31
R
2
0.9999 0.9999 0.9999 0.9999
F
error
3.67 10
3
1.10 10
2
2.61 10
3
2.55 10
3
Intra-particle diffusion
k
i,1
(mg g
1
h
0.5
)
a
76.5 155 7946.1 157
a
First stage.
T. Calvete et al. / Journal of Environmental Management 91 (2010) 1695e1706 1701
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Table 5
Isotherm parameters for RO-16 adsorption, using C-PW and AC-PW as adsorbents. Conditions: adsorbent mass of 50.0 mg; pH xed at 2.5 and 6.0 for C-PW and AC-PW, respectively; and using a contact time of 6 and 5 h for C-PW
and AC-PW, respectively.
C-PW AC-PW
298 K 303 K 308 K 313 K 318 K 323 K 298 K 303 K 308 K 313 K 318 K 323 K
Langmuir
Q
max
(mg g
1
)
258 272 283 288 302 314 425 426 427 448 445 456
K
L
(L g
1
) 0.844 0.874 0.943 1.04 1.11 1.20 0.986 1.20 1.46 1.36 1.74 1.81
R
2
0.9895 0.9915 0.9960 0.9992 0.9981 0.9947 0.9910 0.9841 0.9728 0.9915 0.9770 0.9868
F
error
4.98 10
2
4.97 10
2
2.63 10
2
1.19 10
2
2.34 10
2
1.96 10
2
3.89 10
2
5.16 10
2
5.22 10
2
3.77 10
2
6.68 10
2
3.3 10
2
Freudlich
K
F
((mg g
1
(mg L
1
)
1/n
F
)
151 156 164 174 179 202 242 250 260 271 278 294
n
F
10.0 9.52 9.73 10.6 9.71 11.5 8.80 8.99 9.76 9.71 9.80 10.4
R
2
0.6849 0.6850 0.7425 0.7380 0.7529 0.8106 0.8345 0.8370 0.8893 0.8276 0.8238 0.8438
F
error
0.280 0.311 0.219 0.217 0.272 0.119 0.183 0.168 0.110 0.166 0.187 0.118
Sips
Q
max
(mg g
1
)
253 267 279 287 305 320 436 443 450 458 465 472
K
S
((g L
1
)
1/n
s
) 0.801 0.862 0.942 1.03 1.09 1.20 0.963 1.09 1.23 1.29 1.45 1.57
n
S
0.786 0.793 0.843 0.944 1.10 1.21 1.25 1.37 1.58 1.23 1.40 1.36
R
2
0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9994 0.9999 1.000 0.9999 0.9999 1.000
F
error
4.21 10
3
5.73 10
3
4.79 10
3
3.91 10
3
2.85 10
3
2.01 10
3
9.63 10
3
3.05 10
3
2.01 10
3
3.06 10
3
2.65 10
3
1.72 10
3
RedlichePeterson
K
RP
(L g
1
) 218 238 265 299 358 418 496 625 845 715 1015 992
a
RP
(mg L
1
)
g
0.844 0.874 0.935 1.04 1.24 1.41 1.30 1.66 2.33 1.75 2.60 2.39
g 1.00 1.00 1.00 1.00 0.991 0.987 0.976 0.972 0.964 0.980 0.969 0.977
R
2
0.9898 0.9918 0.9962 0.9993 0.9994 0.9989 0.9988 0.9968 0.9970 0.9980 0.9957 0.9981
F
error
4.96 10
2
4.96 10
2
2.56 10
2
1.19 10
2
1.20 10
2
9.11 10
3
1.41 10
2
2.32 10
2
1.74 10
2
1.67 10
2
2.71 10
2
1.29 10
2
T. Calvete et al. / Journal of Environmental Management 91 (2010) 1695e17061702
Page 9
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Since kinetic results t very well to the fractionary kinetic
model (Avrami model) for the RO-16 dye using C-PW and AC-PW
adsorbents (Table 4, Fig. 5), the intra-particle diffusion model
(Vaghetti et al., 2009a) was used to verify the inuence of mass
transfer resistance on the binding of RO-16 to both adsorbents
(Table 4 and Fig 5EeH). The intra-particle diffusion constant, k
id
(mg g
1
h
0.5
)(Table 1), can be obtained from the slope of the plot
of q
t
(uptake at any time, mg g
1
) versus the square root of time.
Fig. 5EeH shows the plots of q
t
versus t
1/2
, with multi-linearity for
the RO-16 dye using C-PW and AC-PW adsorbents. These results
imply that the adsorption processes involve more than one single
kinetic stage (or adsorption rate) (Vaghetti et al., 2009a). For
instance, the adsorption process exhibits two stages, which can be
attributed to two linear parts (Fig. 5EeH). The rst linear part can
be attributed to intra-particle diffusion, which causes a delay in
the process (Vaghetti et al., 2009a). The second stage is the
diffusion through smaller pores, which is followed by the estab-
lishment of equilibrium (Vaghetti et al., 2009aAeD).
It was observed in Fig. 5 that the minimum contact time of
RO-16 with the adsorbents to reach the equilibration was about 5
and 4 h, using C-PW and AC-PW as adsorbents, respectively
(Fig. 5AeD). The longer required contact time to reach the
equilibrium for C-PW, in comparison with AC-PW, could be
attributed to the textural characteristics of the non-activated
carbon such as lower average pore volume and average pore
diameter, as already reported (Calvete et al., 2009). Extreme
distances between the atoms of RO-16 dye are 1.68 and 1.42 nm
(see Scheme 1B) while BJH average pore diameters of the
adsorbents are 3.65 and 5.32 nm for C-PW and AC-PW, respec-
tively (Calvete et al., 2009). Ratios of average pore diameter of the
adsorbents to the maximum distance among the atoms of the
molecule of RO-16 dye are 2.17 and 3.17, for C-PW and AC-PW,
respectively. Therefore, the diffusion of RO-16 dye from the bulk
adsorbate solution to the pores of C-PW adsorbent may have
been limited, thereby delaying the adsorption process. The
average pore diameter of the C-PW adsorbent could accommo-
date only two RO-16 molecules which were diffused from the
bulk of the adsorbate solution to the pores of the adsorbent.
When the AC-PW adsorbent was used, up to three RO-16 mole-
cules could be accommodated by each adsorbent pore. This
interpretation is also corroborated by the intra-particle diffusion
constant (k
id
) reported in Table 4, where the obtained values of
k
id
for C-PW were slightly lower than those obtained with AC-PW
(Lima et al., 2008).
3.5. Equilibrium studies
An adsorption isotherm describes the relationship between the
amount of adsorbate taken up by the adsorbent and the adsorbate
concentration remaining in solution. There are several equations
for analyzing experimental adsorption equilibrium data. The
equation parameters of these equilibrium models often provide
some insight into the adsorption mechanism, the surface properties
and afnity of the adsorbent. In this work, the Langmuir, Freund-
lich, Sips and RedlichePeterson isotherm models were tested
(Vaghetti et al., 2009a)
The isotherms of adsorption carried out from 298 to 323 K, of
RO-16 on the two adsorbents were performed using the best
experimental conditions described previously (Table 5, Fig 6).
Based on the F
error
, the Sips model is the best isotherm model for
both adsorbents at all the six temperatures studied. The Sips model
showed (Fig 6) the lowest F
error
values, which means that the q t
by the isotherm model was close to the q measured experimentally.
Although the Langmuir and the RedlichePeterson isotherm models
presented simulated the data very well for some adsorbents at
some temperatures, these models did not present a regular pattern
of suitable tting at all the temperatures for both adsorbents.
Similar results using different activated carbons for the adsorption
of RO-16 simulated by the Sips isotherm model have been reported
Fig. 6. The Sips isotherm for RO-16 adsorption on C-PW and AC-PW adsorbents, using
batch contact adsorption procedure. Conditions adsorbent mass of 50.0 mg; pH xed at
2.5 and 6.0 for C-PW and AC-PW, respectively; and using a contact time of 6 and 5 h for
C-PW and AC-PW, respectively.
Table 6
Comparison of maxima adsorption capacities for RO-16 taken up. The values were
obtained at the best experimental conditions of each work.
Adsorbent Q
max
(mg g
1
)
Reference
Powdered activated carbon 628 Lee et al., 2006a
Activated carbon from wood 367.5 Lee et al., 2006b
Activated carbon from coal 326.4 Lee et al., 2006b
Activated carbon from coconut 509.0 Lee et al., 2006b
Waterworks sludge 86.8 Won et al., 2006
Sewage sludge 47.0 Won et al., 2006
Digested sludge 159 Won et al., 2006
Landll sludge 114.7 Won et al., 2006
Polysulfone-immobilized protonated C.
glutamicum biomass
94.4 Vijayaraghavan and
Yun, 2008
Quaternary chitosan salt cross-linked 1060 Rosa et al., 2008
Chitosan cross-linked (beads) 30 Kimura et al., 2002
Chitosan cross-linked (beads) 5.6 Kimura et al., 2002
Corynebacterium glutamicum 156.6 Won et al., 2009
Chitosan 30.4 Crini and Badot, 2008
Carbonized Brazilian-pine fruit shell (C-PW) 320 This work
Activated carbon from Brazilian-pine fruit
shell (AC-PW)
472 This work
T. Calvete et al. / Journal of Environmental Management 91 (2010) 1695e1706 1703
Page 10
Author's personal copy
in the literature (Lee et al., 2006a, b). The worst isotherm model
was the Freundlich, which presented higher F
error
values, besides
presenting the lowest R
2
values.
The maximum amounts of RO-16 uptake were 320 and
472 mg g
1
for C-PW and AC-PW, respectively. These values indi-
cate that these adsorbents are good adsorbents for RO-16 removal
from aqueous solutions. As can be seen in Table 6, C-PW and AC-PW
adsorbents showed higher absorption when compared with several
adsorbents. Out of sixteen different adsorbents, only six presented
higher adsorption capacity than C-PW and just three presented
higher adsorption capacity than AC-PW. These outstanding
adsorption capacities for RO-16 place AC-PW as one of the best
adsorbents for the RO-16 removal from aqueous solutions.
3.6. Thermodynamic studies
Thermodynamic parameters related to the adsorption process, i.
e., Gibbs free energy change (
D
G
o
, kJ mol
1
), enthalpy change (
D
H
o
,
kJ mol
1
), and entropy change (
D
S
o
,J mol
1
K
1
) are determined by
the following equations:
D
G
o
¼
D
H
o
T
D
S
o
(5)
D
G
o
¼RTlnðKÞ (6)
The combination of Eqs. (5) and (6) gives:
lnðKÞ¼
D
S
o
R
D
H
o
R
1
T
(7)
where R is the universal gas constant (8.314 J K
1
mol
1
), T is the
absolute temperature (Kelvin) and K represents the equilibrium
adsorption constants of the isotherm ts (K
L
, Langmuir equilib-
rium constant and K
S
, Sips equilibrium constant, which must be
converted to SI units, by using the molecular mass of the dye)
obtained from the isotherm plots.
D
H
o
and
D
S
o
values can be
calculated from the slope and intercept of the linear plot of ln(K)
versus 1/T.
Thermodynamic results are depicted in Table 7. As can be seen for
both adsorbents, the enthalpyandentropy of adsorption values were
in fair agreement using both Langmuir (K
L
) and Sips (K
S
) equilibrium
constants, although results were slightly better t using the Sips
equilibrium constants (based on R
2
values), conrming the equilib-
rium studies described above. In addition, the magnitude of enthalpy
is consistent with a physical interaction of an adsorbent with an
adsorbate as already reported in the literature (Leechart et al., 2009;
Asouhidou et al., 2009). Enthalpy changes (
D
H
o
) indicate that
adsorption followed endothermic processes. Negative values of
D
G
indicate that the RO-16 reactive dye adsorption by C-PW and AC-PW
adsorbents is spontaneous and favorable processes for all studied
temperatures. The positive values of
D
S
o
conrm a high preference of
RO-16 molecules for the carbon surface of C-PW and AC-PW and also
suggest the possibility of some structural changes or readjustments
in the dye-carbon adsorption complex (Asouhidou et al., 20 09).
Besides, it is consistent with the dehydration of dye molecule before
its adsorption to carbon surface, and the releases of these water
molecules to the bulk solution.
The increase in the adsorption capacities of C-PW and AC-PW at
higher temperatures may be attributed to the enhanced mobility
and penetration of dye molecules within the adsorbent porous
structures by overcoming the activation energy barrier and
enhancing the rate of intra-particle diffusion (Leechart et al., 2009;
Asouhidou et al., 2009).
4. Conclusion
The carbonized Brazilian-pine fruit shell (C-PW) and the acti-
vated carbon prepared from Brazilian-pine fruit shell (AC-PW) are
Table 7
Thermodynamic parameters of the adsorption of RO-16 on C-PW and AC-PW adsorbents. Conditions: adsorbent mass of 50.0 mg; pH xed at 2.5 and 6.0 for C-PW and AC-PW,
respectively; and using a contact time of 6 and 5 h for C-PW and AC-PW, respectively.
Temperature (K)
298 303 308 313 318 323
Langmuir
C-PW
K
L
(L mol
1
) 5.08 10
5
5.26 10
5
5.67 10
5
6.23 10
5
6.68 10
5
7.20 10
5
D
G (kJ mol
1
) 32.5 33.2 33.9 34.7 35.5 36.2
D
H
o
(kJ mol
1
) 11.7 eeeee
D
S
o
(J K
1
mol
1
) 148 eeeee
R
2
0.9860 eeeee
AC-PW
K
L
(L mol
1
) 5.93 10
5
7.20 10
5
8.78 10
5
8.21 10
5
1.05 10
6
1.09 10
6
D
G (kJ mol
1
) 32.9 34.0 35.0 35.4 36.6 37.3
D
H
o
(kJ mol
1
) 18.7 eeeee
D
S
o
(J K
1
mol
1
) 174 eeeee
R
2
0.9137 eeeee
Sips
C-PW
K
S
((mol L
1
)
1/n
s
) 4.82 10
5
5.19 10
5
5.67 10
5
6.21 10
5
6.58 10
5
7.22 10
5
D
G (kJ mol
1
) 32.4 33.1 33.9 34.7 35.4 36.2
D
H
o
(kJ mol
1
) 12.9 eeeee
D
S
o
(J K
1
mol
1
) 152 eeeee
R
2
0.9973 eeeee
AC-PW
K
S
((mol L
1
)
1/n
s
) 5.79 10
5
6.54 10
5
7.41 10
5
7.75 10
5
8.72 10
5
9.42 10
5
D
G (kJ mol
1
) 32.9 33.7 34.6 35.3 36.2 36.9
D
H
o
(kJ mol
1
) 15.3 eeeee
D
S
o
(J K
1
mol
1
) 162 eeeee
R
2
0.9899 eeeee
T. Calvete et al. / Journal of Environmental Management 91 (2010) 1695e17061704
Page 11
Author's personal copy
good alternative adsorbents to remove reactive orange 16 (RO-16)
from aqueous solutions. Both adsorbents interact with the dye at
the solid/liquid interface when suspended in water. The best
conditions were established with respect to pH and contact time to
saturate the available sites located on the adsorbent surface. Five
kinetic models were used to adjust the adsorption and the best t
was the Avrami (fractionary-order) kinetic model; however, the
intra-particle diffusion model gave two linear regions, which sug-
gested that the adsorption can also be followed by multiple
adsorption rates. The maximum adsorption capacities were 320
and 472 mg g
1
for C-PW and AC-PW, respectively. The increased
adsorption capacity of AC-PW could be related to the improvement
on the textural characteristics (specic surface area, average pore
volume, average pore diameter) of the material after the activation
process with CO
2
. AC-PW is an activated carbon prepared from the
Brazilian-pine fruit shell. On the other hand, the carbonized
material C-PW presents some similarities with the unmodied
shell material, which was veried by the FTIR spectra, thermal
gravimetric curves and SEM images. These textural properties
explain the difference of maximum adsorption capacity of AC-PW
being much higher than C-PW adsorbent.
Thermodynamic parameters of adsorption (
D
H
o
,
D
S
o
and
D
G)
were calculated. Increases in the adsorption temperature lead to
increases in the amount adsorbed, indicating that the adsorption of
RO-16 on C-PW and AC-PW follows endothermic processes.
Acknowledgements
The authors are grateful to Ministério de Ciência e Tecnologia
(MCT), to Conselho Nacional de Desenvolvimento Cientíco e Tec-
nológico (CNPq), and to Coordenação de Aperfeiçoamento de Pes-
soal de Nível Superior (CAPES) for nancial support and
fellowships. We are also grateful to Centro de Microscopia Ele-
trônica (CME-UFRGS) for the use of the SEM microscope.
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