Bioresource Technology 97 (2006) 178–182
Potential of Agave lechuguilla biomass for Cr(III) removal
from aqueous solutions: Thermodynamic studies
J. Romero-González a, J.R. Peralta-Videa b, E. Rodrı́guez a,
M. Delgado a, J.L. Gardea-Torresdey a,b,*
a
Environmental Science and Engineering, University of Texas at El Paso, El Paso, TX 79968, United States
b
Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968, United States
Received 7 July 2004; received in revised form 2 January 2005; accepted 2 January 2005
Available online 29 March 2005
Abstract
Thermodynamic studies on the bioadsorption of Cr(III) onto Agave lechuguilla biomass were conduced. The experimental results
at different temperatures were modeled using the Langmuir and Freundlich isotherms to obtain the characteristic parameters of each
model. Both the Freundlich and Langmuir models were found to represent the bioadsorption process. The average adsorption
capacities calculated from Freundlich (4.7 mg/g) and Langmuir (14.2 mg/g) isotherms showed A. lechuguilla to be an effective biomass in the removal of Cr(III) from an aqueous solution. Thermodynamic parameters (DG0, DH0 and DS0) determined in the temperature range from 10 to 40 °C along with the parameters of the Dubinin–Radushkevick equation support the idea that the binding
of Cr(III) may be caused by interactions with functional groups such as carboxyl groups located on the outer surface of the cell
tissue of the bioadsorbent.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Chromium (III); Thermodynamic parameters; Adsorption; Agave lechuguilla
1. Introduction
Industrial wastewater effluents from metal-fishing
and mining-metallurgical sectors often contain high levels of heavy metal concentrations and thus create serious
environmental pollution hazards. Trivalent chromium is
an important pollutant introduced into natural waters
by a variety of industrial wastewaters including those
from the textile, leather tanning, electroplating, and metal finishing industries (Gaballah and Kilbertus, 1998).
Current technologies to remove trivalent chromium
such as precipitation and ionic exchange with synthetic
resins, not only incur operational costs but also create
*
Corresponding author. Address: Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968, United States. Tel.: +1
915 747 5359; fax: +1 915 747 5748.
E-mail address: [email protected] (J.L. Gardea-Torresdey).
0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2005.01.037
sludge disposal problems (Volesky, 2001). These processes also fail to meet the requirements of legislation
for its discharge which are from 0.1 to 3 mg/l of metal
concentration (World Health Organization, 1988). The
removal of heavy metals based on sorption on nonliving
biomass surfaces has been suggested as a low-cost substitute for the treatment of wasters containing heavy
metals (Bailey et al., 1998).
Agave lechuguilla, or lechuguilla, which is one of the
most characteristic plants of the Chihuahuan Desert,
represents a prospective biomass to be used in the removal of chromium. LechuguillaÕs central bud is an
excellent source of hard fibers, known as ‘‘ixtle’’, which
are used in making rope, sacks, mats, and brushes.
Recent research shows that lechuguilla fibers are comparable to glass fibers for their capacity to carry polishing
components. In addition, the ixtle is extremely strong
and durable, and is resistant to the effect of many
J. Romero-González et al. / Bioresource Technology 97 (2006) 178–182
179
chemical solutions and solvents (Gentry, 1982). Lechuguilla contains two steroidal saponins—yuccagenin
and ruizgenin (Blunden et al., 1980)—which can be used
as natural chelating agents to remove heavy metals such
as Cr, Cd, Cu, Pb and Zn from soil and waste (Hong et
al., 2000, 2002). These characteristics make lechuguilla a
potential biomaterial for the removal of chromium from
wastewater and to our knowledge no data has appeared
in the literature. In the present work, thermodynamic
parameters and isotherms models for adsorption of
Cr(III) onto lechuguilla biomass have been investigated
and reported herein.
ment sensitivity. Standards were prepared by diluting a
1000 mg/l Cr stock solution and linear calibration curves
were obtained with correlation coefficients of R2 = 0.99
or better.
Three replicates of each sample were analyzed and
the mean value and relative standard deviation given
by the instrument were recorded. In order to fit the linear calibration range, some samples were diluted using
5% HNO3. The final metal concentration was subtracted
from the initial metal concentration and the difference
was assumed to be the amount of chromium adsorbed
by lechuguilla biomass.
2. Methods
3. Results and discussion
Lechuguilla samples were collected from mountains
surrounding El Paso, Texas. The plants were washed
thoroughly using tap water in order to remove any soil
or debris. Only the leaves of the plants were utilized in
this study because they represent more than 90% of
lechuguilla plants. The washed samples were oven-dried
at 80 °C for three days and the dried samples were
ground using a blender (Wiley mill) and sieved to pass
through a 0.150 mm sieve in order to obtain uniform
particle size. The biomass preparation procedure followed laboratory techniques similar to those previously
reported by Gardea-Torresdey et al. (2000). In summary, a 250-mg sample of lechuguilla leaf biomass was
washed four times with 0.01 M HCl and three times with
deionized (DI) water in order to remove soluble material
or biomolecules that might interact with any sorbed metal ions. The washed biomass was resuspended in 50 ml
of deionized (DI) water to obtain a concentration of
5 mg of lechuguilla per ml of water. The suspension
was adjusted to pH 4 (using diluted solutions of HCl
and NaOH) due to previous batch studies showed that
Cr(III) bound better to lechuguilla at pH 4 (data not
shown). Aliquots of 5-ml of biomass solutions (5 mg/
ml) were taken and transferred to clean test tubes. The
samples were centrifuged (Marathon 6K Fisher Scientific centrifuge) at 3000 rpm and the supernatants were
discarded. The biomass pellets were reacted with 5 ml
of solutions of Cr(III) aliquots of 5, 10, 15, 20, 25, 30,
35, and 40 mg/l for 12 h and then equilibrated by
rocking (Speci-Mix M-26125) at their respective temperatures. These studies were performed at the temperatures of 10 °C in a refrigerator, 22 °C on the laboratory
bench, and 40 °C in an oven.
The final solutions of chromium concentrations within the test tube samples were determined by using a
flame atomic absorption spectrometer (FAAS) (Perkin–Elmer model 3110). The analytical wavelength used
was 359.4 nm with a slit width of 0.7 nm. The chromium
hollow-cathode lamp current was 30 mA. An impact
bead and a reducing flame were used to improve instru-
3.1. Sorption isotherms
The influence of temperature on Cr(III) adsorption
was investigated at different concentrations. These isotherms relate metal uptake per mass of adsorbent (qe)
to the adsorbate concentration at equilibrium (Ce).
Fig. 1 shows that the adsorption of Cr(III) onto lechuguilla was favored at high temperatures.
The adsorption capacity and affinity of lechuguilla
for Cr(III) was determined with two isotherms models
(Freundlich and Langmuir), using Cr(III) solutions at
10, 15, 20, 25, 30, 35 and 40 mg/l. The Freundlich isotherm is a nonlinear sorption model. This model proposes a monolayer sorption with a heterogeneous
energetic distribution of active sites, accompanied by
interactions between adsorbed molecules. The general
form of this model is
qe ¼ K F C e1=n ;
ð1Þ
where KF (mg/g) stands for adsorption capacity and n
for adsorption intensity.
The logarithmic form of Eq. (1) is:
qe (mg/g)
log qe ¼ log K F þ
1
log C e ;
n
ð2Þ
10
9
8
7
6
5
4
3
2
1
0
T = 10˚C
T = 22˚C
T = 40˚C
0
1
2
3
4
5
6
Ce (mg/l)
Fig. 1. Adsorption isotherms plots of Cr(III) on A. lechuguilla biomass
at pH 4.
180
J. Romero-González et al. / Bioresource Technology 97 (2006) 178–182
where KF and 1/n can be determined from the linear plot
of log(qe) versus log(Ce). Experimental values obtained
for the adsorption capacity experiments were used to
calculate the Freundlich model parameters at different
temperatures. Table 1 shows these results.
The Langmuir model represents one of the first theoretical treatments of nonlinear sorption and suggests
that uptake occurs on a homogeneous surface by monolayer sorption without interaction between adsorbed
molecules. In addition, the model assumes uniform energies of adsorption onto the surface and no transmigration of the adsorbate. The Langmuir isotherm is
represented in the following equation:
qe ¼
QL bC e
;
1 þ bC e
ð3Þ
where QL (mg/g) and b are Langmuir constants related
to adsorption capacity and the energy of adsorption,
respectively. Eq. (3) is usually linearized to obtain the
following form:
Ce Ce
1
¼
þ
:
qe QL bQL
ð4Þ
The linearized plot of Ce/qe versus Ce for Cr(III)
respectively were analyzed, and the results obtained at
the experimental temperatures are shown in Table 1.
In the Langmuir model the adsorption intensity (RL)
is expressed by the following equation:
RL ¼
1
;
1 þ bC 0
ð5Þ
where C0 (mg/l) is the initial concentration of the metal.
If the average of the RL values from the different initial
concentrations used is between 0 and 1, it indicates
favorable adsorption. The average values of RL for the
different initial Cr(III) concentrations at the respective
temperature are shown in Table 1.
The correlation coefficient values obtained from the
Freundlich and Langmuir isotherms are also presented
in Table 1. The values indicate that the adsorption pattern for Cr(III) on lechuguilla followed both the Freundlich isotherm (R2 > 0.9843) and the Langmuir
isotherm (R2 > 0.9821) at all experimental temperatures.
The values obtained for Cr(III) from the Freundlich
model at different temperatures showed a maximum
adsorption capacity (KF) of 5.99 mg/g at 40 °C with an
affinity value (n) equal to 2.84, which represents a favorable adsorption of Cr(III). According to the Langmuir
model, the maximum Cr(III) adsorption capacity was
obtained at 40 °C with a value of QL of 21.32 mg/g
and with an affinity (RL) of 0.07, which also represents
a favorable Cr(III) adsorption. As seen in Table 1, a decrease in the temperature produced a decrease in the
adsorption capacity in both models. The comparison
of the Cr(III) adsorption capacities of lechuguilla with
other biomass capacities was made at 22 °C (temperature at which the most of works reported these capacities). Therefore, even though the comparison of
adsorbents is difficult because their experimental set
can be different, the adsorption capacities of lechuguilla
biomass at 22 °C (4.5 mg/g and 11.31 mg/g for KF and
QL, respectively) are higher than the average values or
are in the range of obtained values of other biomasses
with similar experimental conditions. Typical values
found in the literature are 0.6 < KF < 3.8 mg/g and
1.4 < QL < 119 mg/g for Cr(III) (Bailey et al., 1998;
Machado et al., 2002; Yun et al., 2001).
The fact that the adsorption of Cr(III) onto lechuguilla obeyed both the Freundlich and Langmuir isotherms suggested the formation of an homogenous
monolayer of Cr(III) on the outer surface of the absorbent (Das et al., 2000). This further supports the idea
that the binding of Cr(III) may be caused by interactions
with functional groups such as carboxyl groups located
on the surface of the cell tissue of the bioadsorbent
(Gardea-Torresdey et al., 2002; Parsons et al., 2002).
3.2. Thermodynamic parameters
Thermodynamic parameters such as change in free
energy (DG0), enthalpy (DH0) and entropy (DS0) were
determined using the following equations:
Kc ¼
qe
Ce
ð6Þ
where Kc is the equilibrium constant, qe is the amount of
solute (mg) adsorbed on the adsorbent cubic decimeter
of the solution at equilibrium and Ce is the equilibrium
concentration (mg/dm3) of the solute in solution, T is
the temperature in Kelvin and R is the gas constant:
DG0 ¼ RT ln K c ;
ð7Þ
Table 1
Model parameters for the adsorption of Cr(III) on lechuguilla biomass at different temperatures and pH 4
Freundlich
Langmuir
2
T (°C)
KF (mg/g)
n
R
10
22
40
3.60
4.50
5.99
1.82
2.03
2.84
0.98
0.99
0.99
QL (mg/g)
b (l/mg)
RL
R2
9.92
11.31
21.32
1.41
0.49
0.66
0.06
0.12
0.07
0.99
0.98
0.99
J. Romero-González et al. / Bioresource Technology 97 (2006) 178–182
ln K c ¼
DH 0 1 DS 0
þ
:
T
R
R
ð8Þ
The equilibrium constants Kc were determined from the
intercept of Khan and Singh plots of ln qe/Ce versus qe
(Fig. 2) (Krishna et al., 2000). In addition, DH0 and
DS0 were obtained from the slope and intercept of
VanÕHoff plots of ln Kc versus 1/T (Fig. 3).
The values of the thermodynamic parameters at different temperatures in Eq. (8) are presented in Table 2.
The negative values of DG0 at all temperatures indicate the spontaneous nature of the adsorption of Cr(III)
on the adsorbent. The positive value of DH0 suggests the
endothermic nature of adsorption. The positive value of
DS0 shows the increased randomness at the solid/solution interface during the adsorption process, which suggests that Cr(III) ions replace some water molecules
from the solution previously adsorbed on the surface
of lechuguilla. These displaced molecules gain more
3.5
T = 10˚C
3.0
T = 22˚C
ln qe/Ce
2.5
2.0
1.5
1.0
0.5
0.0
2
4
6
8
translation entropy than is lost by the absorbate ions,
thus allowing the prevalence of randomness in the
system.
Another equation that has been used to determine
useful thermodynamic adsorption parameters is the
Dubinin–Radushkevick equation. The Dubinin–
Radushkevick equation does not assume a homogeneous surface or a constant sorption potential (Gemeay
et al., 2002). The linear presentation of this equation is
expressed by
2
ln qe ¼ ln qm K E e0 ;
1
0
e ¼ RT ln 1 þ
;
Ce
ð9Þ
ð10Þ
where e0 is the Polanyi potential, qm is the monolayer
capacity (mol/g), Ce is the equilibrium concentration
(mol/l), KE is the constant related to sorption energy
(mol2/kJ2). The parameters qm and KE can be obtained
from the intercept and slope of the plot shown in Fig.
4. The mean free energy of sorption, E, is calculated
by the following equation:
E ¼ ð2K E Þ1=2 :
T = 40˚C
0
181
10
qe (mg/g)
ð11Þ
The Dubinin–Radushkevick parameters and mean
free energy are given in Table 3. The magnitude of E
is useful for estimating the type of sorption reaction.
The E values obtained are between 10.20 and 14.74 kJ/
mol, which are in the energy range of an ion-exchange
reaction, i.e., 8–16 kJ/mol (Helfferich, 1962). This indicates and supports the idea that the sorption of trivalent
Fig. 2. Khan and Singh plots of ln qe/Ce versus qe for Cr(III)
adsorption on A. lechuguilla biomass.
-8.5
4.5
4.0
y = -6.5246x + 24.742
R2 = 0.9827
T = 10˚C
-8.9
T = 22˚C
T = 40˚C
-9.1
ln qe
ln Kc
3.5
-8.7
3.0
-9.3
-9.5
2.5
-9.7
-9.9
2.0
-10.1
400
1.5
3.1
3.2
3.3
3.4
3.5
3.6
550
700
850
1000 1150 1300
ε 02 (KJ/mol)2
1/T (10-3)
Fig. 3. VanÕHoff plot for the adsorption of Cr(III) on A. lechuguilla
biomass.
Fig. 4. Dubinin–Radushkevick plots for the adsorption of Cr(VI) on
A. lechuguilla biomass.
Table 2
Thermodynamic parameters for the adsorption of Cr(III) onto
lechuguilla at pH 4
Table 3
Dubinin–Radushkevick parameters for the adsorption of Cr(III) by
lechuguilla at pH 4
T (°C)
10
22
40
Kc
1.8493
1.3212
1.0563
DG0 (kJ/mol)
1.5999
0.6683
0.1288
DH0 (kJ/mol)
52.77
DS0 (J/mol/K)
153.36
T (°C)
10
22
40
KE (mol/kJ)2
3
4.8 (10 )
3.8 (103)
2.3 (103)
qm (mol/kg)
E (kJ/mol)
R2
0.72
1.30
1.61
10.20
11.47
14.74
0.99
0.99
0.99
182
J. Romero-González et al. / Bioresource Technology 97 (2006) 178–182
chromium onto lechuguilla biomass may proceed
through an ion exchange reaction, most likely via a carboxylic group.
4. Conclusions
In this study results showed that high temperatures
increased the bioadsorption capacity of Cr(III) by lechuguilla. In addition, the adsorption equilibrium data fitted well with the Freundlich and Langmuir models at
all temperatures. The fact that the adsorption of Cr(III)
onto lechuguilla obeyed both models showed the monolayer coverage of Cr(III) on the outer surface of the
absorbent.
A comparison of adsorption capacities calculated
from the Freundlich and Langmuir isotherms and those
obtained in the literature showed that A. lechuguilla biomass can be effective for the removal of Cr(III) from an
aqueous solution. On the other hand, the thermodynamic parameters (DG0, DH0 and DS0) for Cr(III)
adsorption and the parameters of the Dubinin–Radushkevick equation suggested that Cr(III) was bound by
functional groups in the external surface of the
adsorbent.
Acknowledgements
The authors would like to acknowledge financial support from the National Institutes of Health (NIH)
(Grant S06GM8012-33) and the University of Texas at
El PasoÕs Center for Environmental Resource Management (Cooperative agreement CR-819849-01-04)
through funding from the Office of Exploratory Research of the EPA. In addition, the authors acknowledge the financial assistance from HBCU/MI ETC that
is funded by the Department of Energy. Dr. GardeaTorresdey acknowledges the funding from the National
Institute of Environmental Health Sciences (Grant
R01ES11367-01) and the Dudley family for the Endowed Research Professorship in Chemistry. Jaime Romero-González also acknowledges the financial support
from the University of Guanajuato, Mexico.
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