Journal of Environmental Treatment Techniques  
2020, Volume 8, Issue 1, Pages: 112-118  
J. Environ. Treat. Tech.  
ISSN: 2309-1185  
Journal weblink: http://www.jett.dormaj.com  
2+  
Removal of Cd from Aqueous Solution using  
Eucalyptus Sawdust as a Bio-Adsorbent: Kinetic  
and Equilibrium Studies  
1
2*  
2
Seyyed Mojtaba Mousavi3,  
Behzad Shamsi Zadeh , Hossein Esmaeili , Rauf Foroutan  
,
Seyyed Alireza Hashemi 3  
1
Department of chemical engineering, Omidieh Branch, Islamic Azad University, Omidieh, Iran  
2
Department of Chemical engineering, Bushehr Branch, Islamic Azad University, Bushehr, Iran  
Department of Medical Nanotechnology, School of Advanced Medical Sciences and Technologies, Shiraz University of Medical  
Sciences, Shiraz, Iran  
3
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  
2+  
Cd 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  
2
(
R ), 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  
2+  
better matching with experimental data. So, this adsorbent can be used as natural and cheap adsorbent to remove Cd ions from  
aqueous solutions.  
Keywords: Cadmium ions, Kinetic models, Isotherm models, Eucalyptus sawdust, Adsorption  
1
There are a number of different methods for recovery and  
1
Introduction  
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.  
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].  
2
Materials and Methods  
2
.1 Preparation of Cadmium solution  
Corresponding author: Hossein esmaeili, Department of  
Chemical engineering, Bushehr Branch, Islamic Azad  
In order to prepare cadmium aqueous solution, 2.744 g of  
cadmium nitrate Cd (NO ) .4H O is dissolved into one liter of  
3 2 2  
distilled water. This solution is used for preparation of  
University,  
Bushehr,  
Iran.  
E-mail:  
esmaeili.hossein@iaubushehr.ac.ir.  
112  
Journal of Environmental Treatment Techniques  
2020, Volume 8, Issue 1, Pages: 112-118  
different concentrations of cadmium (3, 10, 15 and 20 ppm).  
In all experiments, diluted solution and distilled water were  
employed.  
% Adsorption=[(C  
i
-C  
o
)/C  
i
]*100  
(2)  
C
i
and C are initial and final concentration of lead  
o
concentration, respectively in aqueous solution after  
equilibrium. In this research, all of the tests performed twice  
and the best results were reported.  
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.  
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.  
2
2
2
5 (ASTM E 11) and stored in poly-ethylene containers.  
.3 Batch adsorption test and optimized conditions  
The adsorption experiment was carried out as batch inside  
50 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]:  
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  
q
e
=(C  
0
-C  
e
)V/W  
(1)  
in Eq. (1), qe is the amount of adsorbed material per gram  
of bio-adsorbent (mg/g) in equilibrium state, C and C are  
0
e
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:  
77.5%.  
Figure 1: SEM images of the adsorbent a) eucalyptus sawdust before adsorption, b) sawdust after adsorption  
113  
Journal of Environmental Treatment Techniques  
2020, Volume 8, Issue 1, Pages: 112-118  
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
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.  
9
0
0
2
4
6
8
10  
12  
80  
pH  
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
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%.  
0
2
4
6
8
10  
Adsorbent dosage(mg/l)  
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  
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
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  
2+  
different contact times (0-150 min) for removal of Cd 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.  
0
50  
100  
150  
200  
250  
Agitation speed(rpm)  
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  
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  
2
00 rpm, pH = 8. The findings revealed that increasing the  
amount of used adsorbent leads to increase in cadmium  
114  
Journal of Environmental Treatment Techniques  
2020, Volume 8, Issue 1, Pages: 112-118  
diffusion model. The degree of this correlation was  
2
determined by correlation coefficient (R ). The pseudo first  
1
0
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]:  
3
ppm  
10ppm  
40  
15ppm  
60  
20ppm  
0
20  
80  
100  
-1  
Ln (q  
e
-q  
t
) = Lnq  
e 1  
-K t  
(3)  
In this equation, q  
equilibrium state (mg/g) per gram of the adsorbent, q is the  
t
e
is the amount of adsorbed ion in  
-2  
amount of adsorbed ion (mg/g) per gram of adsorbent versus  
time and k is the constant rate of adsorption (1/min).  
-3  
-4  
-5  
Time(min)  
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)  
1
3
ppm  
20  
10ppm  
40  
15ppm  
60  
20ppm  
80  
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)  
0
1
0
100  
-
e t  
This constant value is obtained by plotting Ln (q -q )  
versus t. Another kinetic model frequently used is pseudo-  
second order model and the linear form of this equation is like  
Eq. (4) [25]:  
-2  
-
3
푡
1
푡
+
=
(4)  
-4  
푞ꢀ 푘2 푞푒2 푞푒  
-
5
In this equation, K  
2
is the constant rate of pseudo-second  
e
order equation (g/mg.min), q is the maximum adsorption  
capacity (mg/g) and q is the adsorbed amount during the time  
t (min). Initial rate of adsorption is determined using Eq. (5)  
25]:  
Time(min)  
t
3
3
50  
00  
[
2
H = Kq  
e
(5)  
250  
200  
The values of q  
versus t. q is the slope of the linear plot and K  
intercept of the line. Another kinetic model is intra-particle  
diffusion model which is as below:  
e
and K  
2
are obtained by plotting t/q  
is the  
t
3ppm  
10ppm  
20ppm  
e
2
1
1
50  
00  
15ppm  
q
t
=Kidt 1/2+C  
(6)  
5
0
0
q
t
(mg/L) is the amount of adsorption time t (min) and kid  
(
mg/g.min) is the rate constant of intra-particle diffusion.  
0
50  
100  
Time(min)  
150  
200  
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 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  
115  
Journal of Environmental Treatment Techniques  
2020, Volume 8, Issue 1, Pages: 112-118  
appropriate in comparison with other models to describe the  
kinetic behavior of adsorbent and best matches with  
laboratorial data.  
q
e
e
is the capacity of equilibrium adsorption (mg/g), C is  
the equilibrium concentration of cadmium ion in the solution  
(mg/l), K and n are the constants of Freunlich model that  
f
show the relationship between adsorption capacity and  
adsorption intensity, respectively. In order to identify these  
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]:  
parameters (K  
slope of the line is 1/n and the intercept is K  
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].  
f
and 1/n), Lnq  
e
e
is plotted versus LnC and the  
. The value of n  
f
[
1
/q  
e
=(1/k  
L e  
qmax)1/C +1/qmax  
(7)  
3
ppm  
10ppm  
15ppm  
20ppm  
C
e
is the concentration of metal ion in equilibrium state  
mg/l), q is the adsorbed ion in equilibrium state per gram of  
adsorbent. qmax and k are the capacity of surface adsorption  
mg/g) and adsorption energy (l/g), respectively which are the  
4
3
2
1
0
(
e
L
(
constants of Langmuir model.  
The values are obtained by calculating the slope and  
intercept of linear Langmuir equation in C /q versus C  
e e e  
diagram. Another effective parameter in Langmuir equation is  
R
R
L
that describes the properties of the equation. The value of  
is the representative of the state and quality of adsorption  
isotherm model. If R >1, R =0, R =1 and 0 <R < 1, the  
L L L L  
process is non-desired, irrevocable, linear and desirable,  
respectively [25, 29]. The value of R  
8):  
L
L
is identified using Eq.  
0
5
10  
15  
(
t1/2  
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)  
L L 0  
R =1/(1+k C )  
(8)  
0
C 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):  
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  
1/n  
q
=K C  
e f e  
(9)  
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.  
The linear form of this equation is like Eq. (10) which is  
used in this study:  
Lnq  
e
= LnK  
f
+ 1/n LnC  
e
(10)  
Table 1: Constants and correlation coefficient of kinetic models for cadmium ion adsorption in different concentrations using  
eucalyptus sawdust  
Adsorbate concentration(mg/L)  
Kinetic models  
3
ppm  
10 ppm  
15 ppm  
20 ppm  
Pseudo-first order  
q
K
q
R
e cal  
0.236  
0.0358  
0.456  
0.8763  
0.618  
0.0362  
1.62  
0.833  
0.0462  
2.61  
0.829  
0.0313  
3.572  
0.7946  
1
e.exp  
2
0.9414  
0.8778  
Pseudo-second order  
q
K
R
H
e.cal  
0.481  
0.36  
0.999  
0.0833  
0.456  
1.666  
0.14  
0.9999  
0.388  
1.62  
2.671  
0.123  
0.9998  
0.877  
2.61  
3.627  
0.105  
0.9999  
1.381  
3.572  
2
2
q
e.exp  
Intraparticle diffusion  
K
C
R
in  
0.0253  
0.1981  
0.7002  
0.0609  
0.9899  
0.7565  
0.0883  
1.7351  
0.6362  
0.0985  
2.567  
0.6396  
2
116  
Journal of Environmental Treatment Techniques  
2020, Volume 8, Issue 1, Pages: 112-118  
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  
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  
L
of R is 0.273 indicating desirable adsorption.  
L
Freundlich model. The value of R 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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