Computational Water, Energy, and Environmental Engineering, 2013, 2, 1-6
doi:10.4236/cweee.2013.23B001 Published Online July 2013 (http://www.scirp.org/journal/cweee)
Adsorption Characteristics of Zinc (Zn2+) from Aqueous
Solution by Natural Bentonite and Kaolin Clay Minerals:
A Comparative Study
Tushar Kanti Sen, Chi Khoo
Department of Chemical Engineering, GPO Box U1987, Curtin University, Perth, 6845 Western Australia, Australia
Received April, 2013
ABSTRACT
Clay minerals are one of the potential good adsorbent alternatives to activated carbon because of their large surface area
and high cation exchange capacity. In this work the adsorptive properties of natural bentonite and kaolin clay minerals
in the removal of zinc (Zn2+) from aqueous solution have been studied by laboratory batch adsorption kinetic and equi-
librium experiments. The result shows that the amount of adsorption of zinc metal ion increases with initial metal ion
concentration, contact time, but decreases with the amount of adsorbent and temperature of the system for both the ad-
sorbents. Kinetic experiments clearly indicate that adsorption of zinc metal ion (Zn2+) on bentonite and kaolin is a
two-step process: a very rapid adsorption of zinc metal ion to the external surface is followed by possible slow decreas-
ing intraparticle diffusion in the interior of the adsorbent. This has also been confirmed by an intraparticle diffusion
model. The equilibrium adsorption results are fitted better with the Langmuir isotherm compared to the Freundlich
model. The value of separation factor, RL from Langmuir equation give an indication of favourable adsorption. Finally
from thermodynamic studies, it has been found that the adsorption process is exothermic due to negative ∆H0 accompa-
nied by decrease in entropy change and Gibbs free energy change (∆G0). Overall bentonite is a better adsorbent than
kaolin in the the removal of Zn2+ from its aqueous solution.
Keywords: Metal Ion Adsorption; Clay Minerals; Kinetics; Isotherms
1. Introduction
Heavy metal ion pollution is currently of great concern
due to the increased awareness of the potentially hazard-
ous effects of elevated levels of these materials in the
environment [1,2]. There is an increasing and alarming
challenge to researchers and environmental control agen-
cies from the indiscriminate disposal of metals in the
environment. The main sources of zinc in the environ-
ment are the manufacturing of brass and bronze alloys
and galvanization [3,4]. It is also utilized in paints, rub-
ber, plastics, cosmetics and pharmaceuticals [4]. Zinc is
an essential element for life and acts as micronutrient
when present in trace amounts [3]. The WHO recom-
mended maximum acceptable concentration of zinc in
drinking water as 5.0 mg/L [5]. Beyond the permissible
limits, Zn2+ is toxic [6]. Precipitation, ion exchange, fil-
tration, solvent extraction and membrane technology and
adsorption on activated carbon are the conventional
method for the removal of heavy metal ions from aque-
ous solutions and all of which may be ineffective or ex-
tremely expensive, when the metals are dissolved in large
volumes of solution at relatively low concentration [3].
Adsorption on activated carbon is the conventional
methods for the removal of heavy metal ions from aque-
ous solutions but its high cost limits its use [7]. Therefore,
adsorption is used especially in the water treatment field
and the investigation has to be made to determine inex-
pensive and good adsorbent. Clay minerals such as kao-
lin, bentonite are the most wide-spread minerals of the
earth crust which are known to be good adsorbents/sor-
bents of various metal ions, inorganic anions and organic
ligands [8]. These clay minerals are good adsorbent al-
ternatives to activated carbon because of their large sur-
face area, high cation exchange capacity, chemical and
mechanical stability and layered structure Moreover,
oxides and clay minerals are important tropical soil sec-
ondary minerals, responsible for the low mobility and
bioavailability of heavy metals [9]. Although, there are
reported results on the adsorption capacity of clay miner-
als towards heavy metal ions [2] but a systematic studies
on zinc (Zn2+) adsorption characteristics on kaolin, ben-
tonite under various physicochemical parameters are
limited and also very scare. Therefore a study was con-
ducted in order to determine the influence of initial metal

T. K. SEN, C. KHOO 2
ion concentration, adsorbent dosages and temperature
changes on adsorption characteristics of natural bentonite
and kaolin. It is also essential to understand the mecha-
nism and kinetics of adsorption, because the studies of
adsorption kinetics and mechanism are ultimately a pre-
requisite for designing an adsorption column [3]. An-
other reason for this study is the importance of adsorp-
tion on solid surfaces in many industrial applications in
order to improve efficiency and economy. The kinetic
adsorption results have been analysed using both pseudo-
first-order and pseudo-second-order kinetics models. The
mechanism of the adsorption process has been explained
based on intra-particle diffusion model. The isotherm
equilibrium results are better fitted with Langmuir model.
Finally thermodynamic parameters are determined at
three different temperatures and it has been found that
the adsorption process is exothermic due to negative ∆H0
accompanied by decrease in entropy change and Gibbs
free energy change (∆G0).
2. Materials and Methods
2.1. Chemicals
All chemicals used were of analytical grade. Stock
standard solution of Zn2+ has been prepared by dissolving
the appropriate amount of its nitrate salt in deionised
water, acidified with small amount of nitric acid. This
stock solution was then diluting to specified concen-
trations. Kaolin BET surface area of 15.72 m2/g, mean
particle size of 17.94 μm) was obtained from Chem-
Supply pty Ltd, Perth WA. Bentonite (BET surface area
of 238.47 m2/g and mean particle size of 7.49 μm) was
obtained from Bronson & Jacobs Pty Ltd Australia. All
plastic sample bottles and glassware were cleaned, then
rinsed with deionised water and dried at 60˚C in a tem-
perature controlled oven. All measurements were con-
ducted at the room temperature (28  2, ℃). The con-
centration of Zn2+ was measured using a double beam
flame atomic absorption spectrophotometer. Sizes of
particles were measured by Malvern Master Seizer, Ver
1.2, UK. The pH was measured by Orion pH meter.
2.2. Adsorption Procedure
Adsorption measurements were determined by batch
experiments of known amount of the sample with 40 mL
of aqueous Zn2+ solutions as per Aries & Sen [3] in a
series of 60 ml plastic bottles. The mixture were shaken
in a constant temperature orbital shaker at 120 rpm at 30
℃ for a given time and then the suspensions were fil-
tered through a What man glass micro filter and the fil-
trates were analyzed using flame atomic absorption spec-
trophotometer with an air-acetylene flame. The experi-
ments were carried out by varying concentration of initial
Zn2+ solution, contact time, amount of adsorbent and
temperature of the system. Adsorption mechanisms were
studied according to predefined procedure with the Zn2+
concentration ranging from 1.0 to 40 mg /L. The Zn2+
concentration retained in the adsorbent phase was calcu-
lated according to Equation (1)
()
0
CCV
t
qtm

 (1)
where C0 (mg/L) and Ct (mg/L) are the concentration in
the solution at time t = 0 and at time t, V is the volume of
solution (L) and m is the amount of adsorbent (g) added.
The kinetics of adsorption of Zn (II) was carried out at
low and high initial metal ion concentration using the
same adsorption procedure started above. The only diffe-
rence was that samples were collected and analyzed at
regular time intervals during the adsorption process.
The transient behavior of the Zn (II) adsorption proc-
ess was analyzed using two adsorption kinetic models;
pseudo first and pseudo-second-order rate models. The
rate constant of adsorption was determined from the
pseudo-first-order rate model [10] as
1
log ()log2.303
et e
K
qq qt  (2)
where qt and qe represents the amount of metal ion ad-
sorbed (mg/g) at any time t and at equilibrium time re-
spectively and K1 represents the adsorption first-order
rate constant (min−1). Plot of Log (qe − qt) versus t gives
a straight line for pseudo first-order adsorption kinetics
which allow computation of the rate constant K1.
The pseudo-second-order model [3,10] based on equi-
librium adsorption is expressed as:
11
2
2
tt
qq
Kq e
te
 (3)
A plot between t/qt versus t gives the value of the con-
stants K2 (g/mg h) and also qe (mg/g) can be calculated.
The Constant K2 is used to calculate the initial sorption
rate h, at t → 0, as follows
2
2e
hKq (4)
Thus the rate constant K2, initial adsorption rate h and
predicted qe can be calculated from the plot of t/q versus
time t using Equation (3).
According to Weber & Morris (1963) [11] the intra-
particle diffusion model for most the uptake varies al-
most proportionately with t1/2 rather than with the contact
time and can be represented as follows:
0.5
tid
qK t (5)
where qt is the amount adsorbed at time t and t0.5 is the
square root of the time and Kid (mg/g.min0.5) is the rate
constant of intraparticle diffusion. When intra-particle
diffusion plays a significant role in controlling the kinet-

T. K. SEN, C. KHOO 3
ics of the adsorption processes, the plots of q
t vs. t0.5
yield straight lines passing through the origin and the
slope gives the rate constant Kid.
Thermodynamic parameters such as Gibb’s free en-
ergy (∆G0), enthalpy change (∆H0) and change in entropy
(∆S0) for the adsorption of zinc on aluminum oxide has
been determined by using the following equations [2]:
00
GHTS
0
(6)
00
log ()2.303 2.303
e
e
qS
CR


H
RT
(7)
where qe is the amount of zinc adsorbed per unit mass of
aluminium oxide (mg/g), Ce is equilibrium concentration
(mg/L) and T is temperature in K. qe /Ce is called the ad-
sorption affinity. The above equation is for unit mass of
adsorbent dose.
3. Results and Discussions
3.1. Characterization of Adsorbenst
The characterization of the structure and surface
chemistry of the adsorbent is of considerable interest for
the development of adsorption and separation processes.
FT-IR spectroscopy of bentonite which is not shown here
indicated the presence of hydroxyl, carboxyl and Si-O
which are important sorption sites. The particle size dis-
tribution of bentonite which is not shown here for which
mean particle size was 7.49 μm, whereas mean particle
size of kaolin was 17.95 μm. XRD analysis also indicates
that the main mineral of kaolin was kaolinite with trace
impurities of quartz, whereas in bentonite four different
mineral phases were present: mainly quartz (SiO2) and
muscovite.
3.2. Effect of Contact Time and Initial Metal Ion
Concentration on Zn(II) Metal Ion
Adsorption Kinetics
Figure 1 represents a plot of the amount of zinc metal ion
adsorbed (mg/g) versus contact time for Zn-kaolin and
Zn-bentonite system. Figure 2 represents a plot of the
amount of Zn (II) adsorbed at different initial metal ion
concentration range for both the system. From these plots,
it is found that the amount of adsorption i.e. mg of ad-
sorbate per gram of adsorbent increases with increasing
contact time at all initial metal ion concentrations and
equilibrium is attained within 80 minutes for both the
systems Further it was observed that the amount of metal
ion uptake, qt (mg/g) is increased with increase in initial
metal ion concentration (Figure 2). The increase in ad-
sorption is more pronounced for the Zn-bentonite system
compared to the Zn-Kaolin system. These kinetic ex-
periments clearly indicate that the adsorption of Zn (II) on
clay surface is a two-step process: a rapid adsorption of
Figure 1. Effect of contact time on Zn (II) metal ion adsorp-
tion by kaolin and bentonite. Initial metal ion concentrations
= 50 ppm, Initial solution pH = 6.65.
0
10
20
30
40
50
60
70
80
0 20406080
Initial Zn(II) concentration (mg/L)
Am ount Adsorb ed, q
e
(mg/g)
100
Zn - Kaolin
Zn - Benton ite
Figure 2. Effect of initial Zn (II) concentration on adsorp-
tion.
metal ions to the external surface is followed by possible
slow intraparticle diffusion in the interior of the particles
[10]. This has been confirmed by fitting experimental data
with diffusion model which is presented latter section.
This two-stage process is also due to presence of two
different types of binding sites on the adsorbents. More-
over, the amount of Zn(II) ions adsorbed per unit mass of
the biosorbent increases with the initial metal ion con-
centration (Figure 2) which might be due to the higher
availability of Zn(II) ions in solution. Further a higher
initial concentration provides increased driving force to
overcome all mass transfer resistance of metal ions be-
tween the aqueous and solid phases resulting in higher
probability of collision between Zn(II) ions and sorbents.
3.3. Effect of Adsorbent Dose on Zn(II)
Adsorption Kinetics
The results of the kinetic experiments with varying
adsorbent concentrations for which plots are not pre-
sented here. It has been found that the amount of Zn (II)
adsorbed per unit mass of adsorbent decreases as the
adsorbent mass increase for both systems. Several other
investigators have also reported the same trend of adsor-
bent concentration effect on metal ion adsorption [3].
Although the number of adsorption sites per unit mass of

T. K. SEN, C. KHOO 4
an adsorbent should remain constant, independent of the
total adsorbent mass, increasing the adsorbent amount in
a fixed volume reduces the number of available sites as
the effective surface area is likely to decrease
3.4. Effect of Temperature on Zn(II) Adsorption
Kinetics and Thermodynamics Study
To observe the effect of temperature on the adsorption
capacity, experiments were carried out in three different
temperatures of 30℃, 50℃ and 70℃ for a fixed initial
metal ion concentration of 50 ppm for which plots are
not presented here. It was found that with the increased
in temperature, adsorption capacity decreased for both
the systems. This is mainly because of decreased surface
activity suggesting that adsorption between metal ion and
clay minerals was an exothermic process. With increas-
ing temperature, the attractive forces between the clay
surfaces and metal ion are weakened and then sorption
decreases. This may be due to a tendency for metal ion to
excape from the solid phase of clay to the liquid phase
with an increase in temperature of the solution. The
values of Gibbs free energy (∆G0) have been calculated
by knowing the value of the enthalpy of adsorption (∆H0)
and the entropy of adsorption (∆S0) which are obtained
from the slope and intercept of a plot of log (qe/Ce)
versus 1/T (not shown here).
All these thermodynamic parameters are presented in
Table 1.
3.5. Zn(II) Adsorption Kinetic Models &
Isotherm
In this study, the two most widely used kinetic models;
pseudo-first-order and pseudo-second-order were em-
ployed which are described in earlier section. In the
pseudo-first-order model, the rate constant k1 and corre-
lation coefficient, R2, were determined by plotting log (qe
– qt) against time, t for both systems (not presented) with
very poor regression coefficient, R2 with range of 0.15 to
0.74 for various physicochemical parameters. Moreover,
the pseudo-first-order kinetic model predicts a much
lower value of the equilibrium adsorption capacity than
the experimental value for this system and hence it gives
the inapplicability of this model. But the pseudo-second-
Table 1. Thermodynamic parameters for zinc adsorption at
different temperatures.
System Temperature
(℃)
∆G0
(kJ·mol−1)
∆H0
(kJ·mol−1)
∆S0
(j·mol−1·K−1)
Zn-Kaolin
Zn-Bentonite
30
50
70
30
50
70
2.18
2.10
2.24
0.065
0.087
0.109
−0.228
−0.228
−0.228
−0.268
−0.268
−0.268
−0.0072
−0.0072
−0.0072
−0.0011
−0.0011
−0.0011
order kinetic model is fitted very well with very high
regression coefficient (R2) which is shown in Table 2.
The pseudo-second-order rate constant, k2, equilibrium
sorption capacity, qe and initial rate constant, h were cal-
culated for these systems from the fitted model equations
which are tabulated in Table 2. Higher correlation coef-
ficients (R2) with respect to fitted pseudo-first-order
model suggest that adsorption of zinc metal ion on clay
minerals follow Pseudo-second-order.
The most commonly used technique for identifying the
mechanism involved in the sorption process is by fitting
the experimental kinetic data with intraparticle diffusion
plot (Equation (5)). It has been found that the adsorption
plots (Which are not shown here) are not linear over the
whole time range and can be separated into two-three
linear regions which confirm the multi stages of adsorp-
tion.
The adsorption equilibrium data was fitted with Lang-
muir and Frenundlich isotherms within the metal ion
concentration range of 30 - 90 ppm respectively. Linear
regression was used to determine the most fitted isotherm.
The Freundlich adsorption isotherm can be expressed as
[10]
lnqe = ln Kf + 1/n (ln Ce) (8)
where qe is the amount of metal ion adsorbed atequili-
Table 2. Pseudo-second-orde r kinetic par ameters.
System System
parameters
k2
(g/mg.min) qe (mg/g) H (mg/g.min) R2
Zn-kaolin
Initial Zn(II)
concentration
30.00 ppm
50.00 ppm
70.00 ppm
Kaolin dosages
0.01 g
0.02 g
0.03 g
Temperature
30˚C
50˚C
70˚C
0.0143
0.0250
0.0071
0.0329
0.0535
0.0253
0.0358
0.0271
0.0066
25.4
28.4
37.3
28.4
13.0
9.09
27.5
24.9
21.3
9.25
20.0
9.80
26.0
9.61
2.05
27.7
16.2
2.88
0.999
0.999
0.998
0.998
0.996
0.9
0.999
0.999
0.9
Zn-bentonite
Initial Zn (II)
concentration
30.00 ppm
50.00 ppm
70.00 ppm
Bentonite
dosages
0.01 g
0.02 g
0.03 g
Temperature
30˚C
50˚C
70˚C
0.0090
0.010
0.002
0.013
0.440
0.022
0.013
0.0197
0.0194
30.9
41.6
57.1
45.2
19.4
9.85
36.2
35.9
30.0
8.69
18.3
7.63
27.7
166.62
2.19
17.1
24.9
17.5
0.9
0.995
0.999
0.9
0.9
0.990
0.990
0.997
0.9

T. K. SEN, C. KHOO 5
brium time, Ce is equilibrium concentration of nickel
metal ion in solution. Kf and n are isotherm constants
which indicates the capacity and the intensity of the
adsorption respectively and can be calculated from the
intercept and slope between ln q
e and lnCe which are
shown in Table 3 for both the systems. The Freundlich
plots are not shown here.
Also Langmuir isotherm equation was also fitted for
both the system with this same metal ion concentration
range. The linearized form of Langmuir can be written
Ce/qe = (1/Ka qm) + Ce/qm (9)
The Langmuir constants, qm (maximum adsorption
capacity) and Ka (values for Langmuir-2) can be obtained
from plots between Ce/qe versus Ce which are shown in
Figure 3 & Figure 4 respectively with fixed initial
conditions. The maximum adsorption capacity of Zn (II),
qm and constant related to the binding energy of the
sorption systems, Ka is calculated which are 56.49 mg/g
and 0.0437 for Zn-kaolin and 62.5 mg/g and 0.0648 for
Zn-bentonite respectively. Overall Langmuir isotherm
model had higher regression coefficient (R2) compared to
the Freundlich isotherm model for both the systems.
A further analysis of the Langmuir equation can be
Table 3. Freundlich parameters obtained from Freundlich
plots. Amount of iron oxide and also kaolin added = 8 mg;
pH 4.5; Temperature = 28℃; Shaker speed = 120 rpm.
Freundlich constants
Adsorbent KF n R2
Kaolin 9.58 2.92 0.874
Bentonite 7.69 1.75 0.994
Figure 3. Langmuir plot Zn-Kaolin system.
Figure 4. Langmuir plot for Zn-bentonite system.
made on the basis of a dimensionless equilibrium para-
meter, RL, also known as the separation factor, given by
(Sen & Gomez, 2011)
0
1
1
La
R
K
C

 (10)
where Ka is the Langmuir constant and C0 is the initial
metal ion concentration (mg/L). The separation factor, RL
has been calculated from Langmuir plot. It has been found
that the calculated range of RL values from 0.432 to 0.202
for Zn-kaolin and 0.339 to 0.146 for Zn-bentonite system
with the initial metal ion range of 30 to 90 ppm. These RL
values indicates favourable adsorption as it lie in the range
< 0 < R
L < 1 [2]. The maximum Langmuir adsorption
capacity of bentonite was more than kaolin.
4. Conclusions
The results obtained in this study demonstrated that kao-
lin and natural bentonite both can be used as an excellent
natural adsorbent to remove Zn (II) from wastewaters
with good efficiency and low cost. The amount of metal
ion Zn (II) adsorption on both clay minerals was found to
increase with increase in initial metal ion concentration
and contact time but found to decreases with an increase
in amount of adsorbent and temperature. The maximum
adsorption capacity of bentonite was found to 62.5 mg/g
with an initial Zn(II) concentration range of 30 to 90 ppm,
whereas for kaolin it was 56.49 mg/g with the same me-
tal ion concentration range. Kinetic experiments clearly
indicated that sorption of Zn (II) on both kaolin and ben-
tonite is a two steps process: a rapid adsorption of metal
ion to the external surface followed by intraparticle dif-
fusion into the interior of adsorbent which has also
been confirmed by intraparticle diffusion model. Overall
the kinetic studies revealed that adsorption process fol-
lowed the pseudo-second-order kinetics model. The
Langmuir isotherm model was applicable for both sys-
tems. The constant value, RL (low separation factor) in
Langmuir isotherms indicated that there was favourable
adsorption for both systems. Finally thermodynamic pa-
rameters were determined at three different temperatures.
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