Removal of Cd 2+ from Aqueous Solution using Eucalyptus Sawdust as a Bio-Adsorbent: Kinetic and Equilibrium Studies

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Journal of Environmental Treatment Techn iques 2020, Volume 8, Issue 1, Pages: 112-118
112
Removal of Cd2+ from Aqueous Solution using
Eucalyptus Sawdust as a Bio-Adsorbent: Kinetic
and Equilibrium Studies
Behzad Shamsi Zadeh1, Hossein Esmaeili2*, Rauf Foroutan2, Seyyed Mojtaba Mousavi3,
Seyyed Alireza Hashemi 3
1Department of chemical engineering, Omidieh Branch, Islamic Azad University, Omidieh, Iran
2Department of Chemical engineering, Bushehr Branch, Islamic Azad University, Bushehr, Iran
3Department of Medical Nanotechnology, School of Advanced Medical Sciences and Technologies, Shiraz University of Medical
Sciences, Shiraz, Iran
Received: 12/08/2019 Accepted: 28/09/2019 Published: 30/02/2020
Abstract
Heavy metals are not dissoluble in the environment and can be dangerous for many species. So, removal of heavy metals from the
water and wastewater is an important process. In this research, eucalyptus sawdust was prepared and employed for removal of
Cadmium ions from aqueous solution. For this purpose, several parameters such as pH of aqueous solution, adsorbent dosage, contact
time, initial concentration of cadmium ion and mixing rate were studied. The results showed that the best conditions for the removal of
Cd2+ were obtained at temperature of 30°C, mixing rate of 200 rpm, pH of 8, adsorbent dosage of 5 g/l, and initial concentration of
cadmium of 200 ppm which the removal efficiency was obtained 89.3 %. In order to study the kinetics of adsorbent, the pseudo-first
order and pseudo-second order kinetic models and intra-particle diffusion model were applied. According to the correlation coefficient
(R2), pseudo-second order kinetic model showed better correlation for kinetic behavior of the adsorbent. Furthermore, to study the
equilibrium behavior of adsorbent, Langmuir and Freundlich models used and results showed that the Langmuir isotherm model had
better matching with experimental data. So, this adsorbent can be used as natural and cheap adsorbent to remove Cd2+ ions from
aqueous solutions.
Keywords: Cadmium ions, Kinetic models, Isotherm models, Eucalyptus sawdust, Adsorption
1 Introduction
1
Heavy metals are not dissoluble in the environment and
can be dangerous for many species [1, 2]. Heavy metals can
also lead to changes in physical, chemical and biological
properties of water [3-5]. Rapid improvements in economical
industries like forgery, production of fertilizers, battery
manufacturing and etc. are leading to direct and indirect
increase in production rate of heavy metals into the
environments of developing countries [6]. There are other
industries which have heavy metals as byproducts including
automotive industry, dyes for the textile industry and mining
operations [7, 8]. As a result, recovery and removal of heavy
metals from the water and the waste water is a significant
process to maintain general and environmental health [1].
Cadmium is one of the most dangerous heavy metals and can
be hazardous for human health causing serious diseases like
kidney failures, hypertension, hepatitis and damage to the
lungs and bones cancer [9]. The amount of cadmium in swage
water and drinking water is reported to be equal to 0.1 and
0.05 mg/l, respectively [10].
Corresponding author: Hossein esmaeili, Department of
Chemical engineering, Bushehr Branch, Islamic Azad
University, Bushehr, Iran. E-mail:
esmaei li. hossein @iaubu shehr .ac.ir .
There are a number of different methods for recovery and
removal of heavy metals in soil and aqueous solutions like
electrochemical processes, ion exchange, chemical scaling,
osmosis, evaporation and surface adsorption [11]. Some of
these methods have disadvantages like high capital costs
and/or inapplicability in industrial scale [12]. Therefore, some
researches have focused on low cost and available adsorbent
with agricultural and biological origins [13]. Some of these
materials are sunflower wastes [14], orange peel [15], tea
factory wastes [16], sawdust [17-18], soybean straw [19],
olive stone [20, 21] and etc. These materials have been
utilized by researchers for recovery and removal of heavy
metals.
The main aim of this investigation is to assess the removal
of cadmium ion from aqueous solutions using eucalyptus
sawdust. To do so, the effect of several parameters such as
pH, adsorbent dosage, initial concentration of cadmium ion in
the solution, mixing rate and contact time on adsorption
process were investigated. In addition, kinetic and equilibrium
behavior of bio-adsorbent were examined.
2 Materials and Methods
2.1 Preparation of Cadmium solution
In order to prepare cadmium aqueous solution, 2.744 g of
cadmium nitrate Cd (NO3)2.4H2O is dissolved into one liter of
distilled water. This solution is used for preparation of
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Journal of Environmental Treatment Techn iques 2020, Volume 8, Issue 1, Pages: 112-118
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different concentrations of cadmium (3, 10, 15 and 20 ppm).
In all experiments, diluted solution and distilled water were
employed.
2.2 Preparation of adsorbent
Eucalyptus sawdust was gathered and washed with
distilled water till no soil particles were inside the material
and the rinsed water remained colorless. Thereafter, sawdust
was put inside the oven and heated up to 70°C for 24 hours to
remove humidity. The sawdust powder was prepared by
household milling machine (Moulinex) and passed sieve No.
25 (ASTM E 11) and stored in poly-ethylene containers.
2.3 Batch adsorption test and optimized conditions
The adsorption experiment was carried out as batch inside
250 ml Erlenmeyer flasks containing 100ml of synthetic
cadmium solution. Initial pH of the samples was set by 0.1
molar NaOH and HCl in the range of 2-10. Afterwards, 2g/l
of adsorbent was added up to the solution with cadmium
initial concentration of 10 ppm. The final solution was stirred
at 30°C and 200 rpm for 80 minutes. The solutes were filtered
through filter paper and 5ml of the solution was analyzed to
measure adsorbed cadmium ion concentration. The
optimization process repeated for other parameters as well as
pH. These parameters were adsorbent dosage (1-8 g/l), mixing
rate (0-200 rpm), and initial concentration of cadmium ion in
the solution (3-20 ppm). The pH of the solution was adjusted
on optimum condition and one of the parameter considered
variable while others were constant. The optimized condition
of each parameter was selected and investigation continued to
define the optimum condition of other parameters. The
cadmium ion concentration was identified by flame atomic
adsorption spectroscopy model SpectrAA-10 plus made by
Varian. The amount of adsorbed ions by bio-adsorbent for
each gram of adsorbent is identified by Eq. (1) [22]:
qe=(C0-Ce)V/W (1)
in Eq. (1), qe is the amount of adsorbed material per gram
of bio-adsorbent (mg/g) in equilibrium state, C0 and Ce are
initial and equilibrium concentrations of cadmium (mg/l), V is
the solution volume and W is the weight of adsorbent. In the
present study, the recovery of lead ion adsorption in different
conditions of the reaction is identified using Eq. (2) like
below:
% Adsorption=[(Ci-Co)/Ci]*100 (2)
Ci and Co are initial and final concentration of lead
concentration, respectively in aqueous solution after
equilibrium. In this research, all of the tests performed twice
and the best results were reported.
2.4 SEM Analysis
The change on the surface of eucalyptus sawdust was
studied before and after cadmium ion adsorption using SEM
apparatus (Hitachi type, S4160). For scanning the surface of
the adsorbent, these surfaces were covered with a thin layer of
gold both before and after adsorption. The SEM images
showed the apparent surface of the adsorbent. Figure 1 shows
the surfaces of eucalyptus sawdust before and after cadmium
ion adsorption.
3 Results and Discussions
3.1 Effect of pH
The capacity of ion metal adsorption and its mechanism
depends on initial pH of the solution [23]. Adsorption
recovery is greatly dependent on hydrogen cation in the
solution [1]. The effect of initial pH performed in the range of
pH = 2-10 and the percentage of removed cadmium ion is
shown in Figure 2. This figure shows that increasing initial
pH leads to increase in cadmium ion removal percentage and
optimum pH is 8 and removal recovery is also 80%. Low pH
means high hydrogen cation content and this cation
participates in reactions inside the aqueous solution. In fact,
there is a competition between hydrogen cation and cadmium
ions to occupy active sites of the adsorbent. If these sites were
occupied by hydrogen ions, there would be less active site for
cadmium ions and this reduces the recovery of adsorption.
High pH (pH > 7) means low hydrogen cation content and
high OH- content inside the solution. In this case, there is no
competition between hydrogen and cadmium ions and active
sites are occupied by cadmium ions, resulting in higher
recovery. In higher pH values (pH > 9) the adsorption
recovery is reduced again. In this case, hydroxide anions
compete with cadmium ions to lodge into active sites.
Besides, in high pH values, hydroxide anions make a complex
with cadmium ions and these ions deposit and accumulate in
the solution. Cadmium removal percentage in pH = 10 is
77.5%.
Figure 1: SEM images of the adsorbent a) eucalyptus sawdust before adsorption, b) sawdust after adsorption

Journal of Environmental Treatment Techn iques 2020, Volume 8, Issue 1, Pages: 112-118
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Figure 2: Effect of initial pH on efficiency of cadmium adsorption
(initial cadmium concentration 10 ppm, revolution rate 200 rpm,
temperature 30°C, contact time 80 minutes, adsorbent dosage 2 g/l)
3.2 Effect of mixing rate
The mixing rate and turbulence making inside aqueous
solution is an important parameter in adsorption process as
disturbance increases the possibility of contact between the
adsorbent and ions which results into higher recovery [24]. In
order to examine mixing rate on adsorption process, the
parameters were determined in laboratorial conditions as:
mixing rate 0 -200 rpm, initial cadmium ion 10 ppm, time 80
minutes, temperature 30°C, adsorbent dosage 2 g/l and pH 8.
Magnetic mixer was used for mixing process and this
parameter effect is shown in Figure 3. According to Fig. 3,
increasing the mixing rate in the range of 0-200 rpm,
increases the recovery of cadmium adsorption by the
adsorbent. Higher mixing rate means higher contact
possibility between active sites and cadmium ions. The
optimum recovery was in 200 rpm mixing rate equal to 80%.
Figure 3: Effect of mixing rate on cadmium adsorption (initial
cadmium concentration 10 ppm, temperature 30°C, contact time 80
min, adsorbent dosage 2 g/l, pH = 8)
3.3 Effect of adsorbent dosage
The amount of used adsorbent is a significant parameter in
adsorption process, as it determines the adsorption capacity in
a certain concentration of adsorbed material [25, 17]. The
testing parameters for the range of adsorbent dosage (1-8 g) to
remove cadmium ions are: initial cadmium concentration 10
ppm, temperature 30°C, contact time 80 minutes, mixing rate
200 rpm, pH = 8. The findings revealed that increasing the
amount of used adsorbent leads to increase in cadmium
adsorption, as higher dosage increases the number of active
sites for cadmium ion adsorption. Increase in the amount of
adsorbent up to 5 g/l, results into higher recovery of cadmium
removal that is 85%. Higher dosages of adsorbent in aqueous
solution, more than 5 g/l, show negligible increase in recovery
because of saturation in active sites. Even in some cases the
final recovery is reduced as there was higher possibility of
contact between adsorbent particles and active sites and this
factor makes flocculation on the sites and ultimately decreases
the number of active sites and adsorbent surfaces and final
recovery as a result.
Figure 4: Effect of adsorbent dosage on cadmium adsorption (initial
cadmium concentration 10 ppm, temperature 30°C, contact time 80
min, mixing rate 200 rpm, pH = 8)
3.4 Effect of initial concentration of cadmium ion and
contact time
In batch adsorption processes, initial concentration of
metals plays an important role in providing appropriate force
for transferring stable mass between solid and liquid phases
[26]. The testing conditions of different initial concentration
of lead ion (3-20 ppm) were: temperature 30°C, contact time
0-150 minutes, mixing rate 200 rpm, and pH = 8. Figure 5
shows the effect of initial cadmium ion (3, 10, 15, 20 ppm) in
different contact times (0-150 min) for removal of Cd2+ from
synthetic wastewater using eucalyptus sawdust. This figure
confirms that increasing both initial concentration of cadmium
ion and contact time, amplifies the recovery; due to
appropriate produced forces for mass transfer between solid
and liquid phases. In addition, Figure 5 shows that longer
contact time increases adsorption recovery. Cadmium ion
adsorption rate by the adsorbent is higher during initial time
intervals that because of activity of adsorption sites.
Equilibrium time of adsorption by adsorbent is 20 min. After
this period, the percentage of total adsorbed metal changes
negligibly, since active sites were occupied by cadmium ions
and adsorption continued through ions penetration into
adsorbent layers. The results of this stage of the experiments
were applied to investigate the kinetic behavior of prepared
adsorbent.
3.5 Kinetic studies
Adsorption kinetics is used for identification and control
mechanisms of surface adsorption processes. This mechanism
depends on physical and chemical properties of adsorbent. In
the present study, kinetics and cadmium adsorption
mechanism of the adsorbent were modeled by pseudo-first
and pseudo-second order kinetic models and intra-particle
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10 12
Adsorption %
pH
0
10
20
30
40
50
60
70
80
90
050 100 150 200 250
Adsorption %
Agitation speed(rpm)
0
10
20
30
40
50
60
70
80
90
0246810
Adsorption %
Adsorbent dosage(mg/l)

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diffusion model. The degree of this correlation was
determined by correlation coefficient (R2). The pseudo first
order kinetic model assumes that the rate of changes in the
solute is directly proportional to the changes in the saturation
concentration and the amount of consumed adsorbent versus
time. The linear form of the pseudo-first order model is as
equation 3 [25, 27]:
Ln (qe-qt) = Lnqe-K1t (3)
In this equation, qe is the amount of adsorbed ion in
equilibrium state (mg/g) per gram of the adsorbent, qt is the
amount of adsorbed ion (mg/g) per gram of adsorbent versus
time and k is the constant rate of adsorption (1/min).
Figure 5: Effect of initial concentration of cadmium ion on adsorption
using eucalyptus sawdust (temperature 30°C, mixing rate 200 rpm,
pH = 8, adsorbent dosage 5 g/l)
This constant value is obtained by plotting Ln (qe-qt)
versus t. Another kinetic model frequently used is pseudo-
second order model and the linear form of this equation is like
Eq. (4) [25]:



 

󰇛󰇜
In this equation, K2 is the constant rate of pseudo-second
order equation (g/mg.min), qe is the maximum adsorption
capacity (mg/g) and qt is the adsorbed amount during the time
t (min). Initial rate of adsorption is determined using Eq. (5)
[25]:
H = Kqe2 (5)
The values of qe and K2 are obtained by plotting t/qt
versus t. qe is the slope of the linear plot and K2 is the
intercept of the line. Another kinetic model is intra-particle
diffusion model which is as below:
qt=Kidt 1/2+C (6)
qt (mg/L) is the amount of adsorption time t (min) and kid
(mg/g.min) is the rate constant of intra-particle diffusion.
Table 1 list the parameters and constants of pseudo-first,
pseudo-second and intra-particle diffusion models for
eucalyptus sawdust in optimum conditions and for different
concentrations of cadmium ion (3, 10, 15, 20 ppm) and the
linear correlation between these parameters are shown in
Figures 6 to 8.
Figure 6: Kinetic diagram of pseudo-first order model using adsorbent
prepared by eucalyptus sawdust (temperature 30°C, pH = 8, mixing
rate 200 rpm, adsorbent dosage 5 g/l)
Figure 7: Kinetic diagram of pseudo-second order model using
adsorbent prepared by eucalyptus sawdust (temperature 30°C, pH = 8,
mixing rate 200 rpm, adsorbent dosage 5 g/l)
Based on the correlation coefficient for mentioned
adsorbent, the pseudo-second order kinetic model is more
-5
-4
-3
-2
-1
0
1
0 20 40 60 80 100
Ln(qe -qt)
Time(min)
3ppm 10ppm 15ppm 20ppm
-5
-4
-3
-2
-1
0
1
020 40 60 80 100
Ln(qe -qt)
Time(min)
3ppm 10ppm 15ppm 20ppm
0
50
100
150
200
250
300
350
050 100 150 200
t/qt
Time(min)
3ppm 10ppm
15ppm 20ppm

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appropriate in comparison with other models to describe the
kinetic behavior of adsorbent and best matches with
laboratorial data.
3.6 Isotherm models
The isotherm models are usually investigated for
description of adsorption process and related mechanisms
[28]. Langmuir and Freundlich are two isotherm models that
are widely used. Langmuir isotherm is widely used for
description of laboratorial data in previous studies. The linear
form of this model is like below [25, 27-29]:
1/qe=(1/kLqmax)1/Ce+1/qmax (7)
Ce is the concentration of metal ion in equilibrium state
(mg/l), qe is the adsorbed ion in equilibrium state per gram of
adsorbent. qmax and kL are the capacity of surface adsorption
(mg/g) and adsorption energy (l/g), respectively which are the
constants of Langmuir model.
The values are obtained by calculating the slope and
intercept of linear Langmuir equation in Ce/qe versus Ce
diagram. Another effective parameter in Langmuir equation is
RL that describes the properties of the equation. The value of
RL is the representative of the state and quality of adsorption
isotherm model. If RL >1, RL =0, RL =1 and 0 <RL< 1, the
process is non-desired, irrevocable, linear and desirable,
respectively [25, 29]. The value of RL is identified using Eq.
(8):
RL=1/(1+kLC0) (8)
C0 in Eq. (8) is the initial concentration of cadmium ion in
aqueous solution in mg/l. Another typical isotherm model
frequently used is Freundlich isotherm model. This model is
empirical and able to describe adsorption of organic and
inorganic compounds by different adsorbent. The non-linear
Freundlich isotherm model is like Eq. (9):
qe=KfCe1/n (9)
The linear form of this equation is like Eq. (10) which is
used in this study:
Lnqe= LnKf + 1/n LnCe (10)
qe is the capacity of equilibrium adsorption (mg/g), Ce is
the equilibrium concentration of cadmium ion in the solution
(mg/l), Kf and n are the constants of Freunlich model that
show the relationship between adsorption capacity and
adsorption intensity, respectively. In order to identify these
parameters (Kf and 1/n), Lnqe is plotted versus LnCe and the
slope of the line is 1/n and the intercept is Kf. The value of n
in many researches is in the range of 1-10. High values of n
represent the high interactions between the adsorbent and
metal ions and n = 1 shows the linear adsorption for all active
sites of the adsorbent [29, 30].
Figure 8: Kinetic diagram of intra-particle diffusion model using
adsorbent prepared by eucalyptus sawdust (temperature 30°C, pH = 8
and 9, mixing rate 200 rpm, adsorbent dosage 5 g/l)
To investigate the equilibrium behavior of the process, test
performed in the following conditions: initial cadmium ion
concentration 10 ppm, temperature 30°C, contact time 80
minutes, mixing rate 200 rpm, adsorbent dosage 1-5 g/l and
pH 8. Attained equilibrium data was examined for the
adsorbent in Langmuir and Freundlich models. Figure 9 and
10 shows the equilibrium diagram of Freundlich and
Langmuir for mentioned adsorbent. The constant values and
other parameters of the models are listed in Table 2.
Table 1: Constants and correlation coefficient of kinetic models for cadmium ion adsorption in different concentrations using
eucalyptus sawdust
Kinetic models
Adsorbate concentration(mg/L)
3 ppm
10 ppm
15 ppm
20 ppm
Pseudo-first order
qe cal
0.236
0.618
0.833
0.829
K1
0.0358
0.0362
0.0462
0.0313
qe.exp
0.456
1.62
2.61
3.572
R2
0.8763
0.9414
0.8778
0.7946
Pseudo-second order
qe.cal
0.481
1.666
2.671
3.627
K2
0.36
0.14
0.123
0.105
R2
0.999
0.9999
0.9998
0.9999
H
0.0833
0.388
0.877
1.381
qe.exp
0.456
1.62
2.61
3.572
Intraparticle diffusion
Kin
0.0253
0.0609
0.0883
0.0985
C
0.1981
0.9899
1.7351
2.567
R2
0.7002
0.7565
0.6362
0.6396
0
1
2
3
4
0 5 10 15
qt
t1/2
3ppm 10ppm 15ppm 20ppm

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The correlation coefficient for eucalyptus sawdust using
Langmuir and Freundlich model was 0.9693 and 0.8679,
respectively. This confirms that Langmuir model is a better
estimator of isotherm behavior of eucalyptus sawdust in
cadmium ion adsorption. It should be mentioned that the value
of RL is 0.273 indicating desirable adsorption.
Figure 9: Freundlich adsorption isotherm plots for the adsorption of
Cd2+ ion
Figure 10: Langmuir adsorption isotherm plots for the adsorption of
metal ion
Table 2: Constants and parameters of Langmuir and
Freundlich isotherm models
Adsorption isotherm
Isotherm constants
value
Langmuir
KL(L/g)
0.266
qmax(mg/g)
2.716
RL
0.273
R2
0.9693
Freundlich
Kf(mg/g)(L/mg)1/n
1.495
n
1.139
R2
0.8679
4 Conclusion
The findings of this research were promoted for
adsorption and removal of Cd2+ ion from aqueous solution
using bio-adsorbent prepared from eucalyptus sawdust.
Effective parameters in cadmium ion adsorption were pH, the
amount of used adsorbent, initial concentration of cadmium
ion in the solution, contact time and mixing rate. The
percentage of cadmium ion removal using eucalyptus sawdust
was 89.3%. The optimum conditions were: temperature 30°C,
mixing rate 200 rpm, adsorbent dosage 5 g/l, initial
concentration of cadmium in the aqueous solution 20 ppm and
pH of 8. Kinetic and isotherm behavior of the adsorbent were
investigated by different synthetic and isotherm models and
correlation coefficients of the adsorbent showed that pseudo-
second order kinetic model was better estimators for kinetic
behavior of adsorbent. Also, Langmuir isotherm model could
well describe adsorptive behavior in comparison with
Freundlich model. The value of RL for adsorbent was 0.273
indicating the adsorption process of cadmium ion by
eucalyptus sawdust is desirable. Therefore, the recovery of
ions removal and correlation coefficients of the models
confirm that eucalyptus sawdust can be considered as natural
and cheap adsorbent for cadmium ion removal.
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R² = 0.9693
0
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1/qe
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