Journal of Environmental Protection, 2011, 2, 817-827
doi:10.4236/jep.2011.26093 Published Online August 2011 (
Copyright © 2011 SciRes. JEP
Equilibrium Isotherms and Kinetic Studies of
Removal of Methylene Blue Dye by Adsorption
onto Miswak Leaves as a Natural Adsorbent
Taha M. Elmorsi1,2
1Chemistry Department, Faculty of Science, Jazan University, Jazan, KSA; 2Chemistry Department, Faculty of Science, Al-Azhar
University, Cairo, Egypt.
Received May 12th, 2011; revised June 24th, 2011; accepted July 29th, 2011.
In this research miswak leaves, agriculture wastes, available in large quantity in Saudi Arabia, was used as low-cost
adsorbent for removing methylene blue (MB) dye. Equilibrium behavior of miswak leaves was investigated by perform-
ing batch adsorption experiments. The effects of [MB] 0, pH, contact time and adsorbent dose were evaluated. An alka-
line pH (10.6) was favorable to the adsorption of MB dye. Adsorption isotherm models, Langmuir, Freundlich and
Temkin were used to simulate the equilibrium data. Langmuir equation was found to have the highest value of R2 com-
pared with other models. Furthermore, it was found that miswak leaves have a high adsorptive capacity towards MB
dye (200 mg/g) and show favorable adsorption of MB dye with separation factor (RL < 1). In addition, pseudo-first-
order, pseudo-second order and intra-particle diffusion were used to study the kinetics of MB adsorption onto miswak
leaves. Adsorption process undergoes pseudo-second order kinetic as proved by the high value of R2 and the low value
of sum of squared error (SSE percentage). Results indicated that intra-particle diffusion is not the limiting step, and the
adsorption process is spontaneous as indicated by the negative value of the G
Keywords: Miswak Leaves, Salvadora Persica, Methylene Blue, Adsorption Isotherms, Adsorption Kinetics
1. Introduction
Presence of many pollutants in water and wastewater
has increased recently due to high increase in various
industrial activities. Using dyes in many industries [1]
such as textile, paper, plastics, leather, food and cos-
metic, represent a large group of chemicals that get
mixed in wastewater among many aqueous pollutants.
In recent years, there is a dramatic increase in the an-
nual production of different synthetic dyes representing
more than 10,000 dyes [2]. Many azo dyes and their
intermediates have toxic effects on environment and
human health due to their carcinogenicity and visi-
bility [3]. It was reported that incomplete degradation
of dyes by bacteria in the sediment resulted in produc-
tion of some carcinogenic and harmful amines [4]. In
addition, presences of color substances in the water
body may decrease the light transmission which de-
creasing the photosynthesis activity, leading to de-
crease growth of bacteria and hence decreasing the bio-
degradation of impurities in water [5]. Methylene blue
dye (MB), is basic dye has been extensively used in
textiles and printing industry, and it has been found as
non-biodegradable dye. Therefore, it is essential that
wastewater contaminated with MB dye to be given
some treatments before discharge.
Many treatment techniques have been applied to a
broad range of water and wastewater contaminated
with dyes including physical- or chemical-treatment
processes [6]. These include chemical coagulation/flo-
cculation [7,8], ozonation, oxidation, photodegradation
[9], ion exchange, irradiation, precipitation and ad-
sorption. Several critical reviews on current treatment
technologies were reported [10]. Many of these tech-
niques are costly, required various tools and have limi-
tations. It has been reported that the adsorption onto
activated carbon, have proven to be the most efficient
and reliable method for the removal many pollutants,
including different dyes [7]. Although commercial acti-
vated carbon is very effective adsorbent, its high cost
requires the search for alternatives and low-cost adsor-
bents [11]. Several low-cost adsorbents have been
Equilibrium Isotherms and Kinetic Studies of Removal of Methylene Blue Dye by Adsorption onto Miswak Leaves
as a Natural Adsorbent
tested for removing dyes [12] including peat, pith, Or-
ange peel, Indian Rosewood [11], cellulose based
wastes, giant duckweed, banana pith and other agri-
cultural by-products [12]. On the other hand, Salvadora
persica (miswak or arak) is a common plant found
widely in different areas in Jazan, Saudi Arabia in ad-
dition to other countries.
Miswak has been used by many Islamic communi-
ties as toothbrushes, and has been scientifically proven
to be very useful in the prevention of tooth decay, even
when used without any other tooth-cleaning methods
[13,14]. However, roots of miswak only are used as a
toothbrush and the rest of plant such as leaves possibly
remain as an agriculture waste. To make further use of
the plant, the present study is an attempt to use miswak
tree leaves, as nonconventional low-cost adsorbent for
removal of MB dye from aqueous solution. The capac-
ity of adsorbent for adsorbate is obtained by adsorption
isotherm model, which is the equilibrium relationships
between adsorbent/adsorbate systems.
In this study, three models (Langmuir, Freundlich
[15-19,] and Temkin [20,21] have been used to de-
scribe the sorption process of MB onto miswak leaves.
Furthermore, kinetics of MB adsorption onto miswak
leaves will be investigated using a pseudo-first order
[15], a pseudo-second order [22,23] and an intraparti-
cle diffusion [15-24]. Linear regression analysis me-
thod will be used to determine the most fitted model
and finding its parameters [25].
2. Experimental
2.1. Chemicals
All chemicals used in this study were of analytical-grade
and used without further purification. Methylene blue
(MB) or basic blue-9 is a monovalent cationic dye with a
molecular formula of C16H18N3ClS (Mo. Wt. 319.85
g/mol), used as the model adsorbate in the present study
to evaluate the efficiency of leaves Salvadora persica
(miswak) as a natural adsorbent. The chemical structure
of MB is shown in Figure 1. HCl, NaOH used to adjust
the pH were purchased from BDH.
Figure 1. Chemical structure of methylene blue dye (MB).
2.2. Adsorbent
Salvadora persica (miswak or arak) was used as a natural
adsorbent. Salvadora persica plant is a member of the
Salvadoraceae family. Leaves of miswak were collected
from fields around El-Ardh area (Bathan) in Jazan, Saudi
Arabia. Adsorbent was air dried and washed several
times with distilled water, dried again then ground well
and sieved.
2.3. Preparation of Dye Solutions
Methylene blue (MB) was used in this study as an envi-
ronmental pollutant. Stock solutions (1000 mg/L) of MB
dye were prepared by dissolving the required amount in
distilled water. Batch experimental solutions were ob-
tained by diluting the dye stock solutions in accurate
different initial concentrations. Calibration curves were
prepared by serial dilutions (1.0 to 10.0 mg/L).
2.4. Adsorption Studies
In batch adsorption experiments, certain amounts of mis-
wak were added into several 10 mL bottles, each con-
taining 5.0 mL solution of MB dye with [MB]0 of 120
mg/L. Then the bottles were stirred at 800 rpm for 80
min using a magnetic stirrer at room temperature. Mis-
wak in the samples was separated by centrifugation and
the concentrations of dye at any time (t) were deter-
mined in the supernatant solutions. Adsorption isotherms
were determined by introducing 0.005 g (1.0 g/L)
miswak to respective 5.0 mL of different dye concentra-
tions (16 - 150 mg/L) at room temperature.
2.5. Effect of Adsorbent Mass
To investigate the effect of adsorbent mass, different
mass of miswak 0.005 to 0.015 g (1 - 3 g/L) was intro-
duced to a number of glass tubes containing a specific
volume of a fixed [MB]0 at the same pH and room tem-
perature. Concentrations of MB were measured at equi-
2.6. Effects of Initial Dye Concentration (0
C) and
valuate the effec
act Time
To et of both contact time and adsorp-
sed to obtain calibra-
tion kinetic, experiments were conducted at different
periods using previously described system.
2.7. Analytical Methods
Standard solution of MB dye was u
tion curves. UV–vis spectrophotometer (APEL) was used
for determining the concentrations of dye solutions. For
each adsorption experiment, samples were withdrawn at
interval times, and the adsorbate (miswak) was separated
Copyright © 2011 SciRes. JEP
Equilibrium Isotherms and Kinetic Studies of Removal of Methylene Blue Dye by Adsorption onto Miswak Leaves
as a Natural Adsorbent
Copyright © 2011 SciRes. JEP
by the centrifuge. Then concentrations of residual dye
solutions were measured by monitoring the absorbance
changes at a wavelength of maximum absorbance (λmax =
665 nm) for MB dye. The amount of dye sorbed at any
time, t
q, was calculated from;
At equilibrium, and ; therefore, the
e, e
ount of sorbed dy was c from
where , and are the initial concentration,
me concen any tiand equilibrium concentrations
of dye solution (mg/L), respectively, V is the volume of
the solution (L), and W is the mass of adsorbent (g) [26].
The dye removal percentage can be calculated as fol-
Removal C
2.8. Equilibrium Isotherm Modeling
between the
del is used to predict the
The experimental data at equilibrium
amount of adsorbed dye (e
q) on the adsorbent (miswak)
and the concentration of de in solution (e
C) at a con-
stant temperature and pH were used to describe the op-
timum isotherm model. The linear forms of Langmuir,
Freundlich [15-19] and Temkin [20,21] equations (Table
1) were used to describe the equilibrium data. Applica-
bility of these equations was compared by judging the
correlation coefficients (R2) [25].
2.8.1. Langmuir Isotherm
The Langmuir isotherm mo
sorption of aqueous compounds onto a solid phase [15,
19]. This mechanistic model assumes that a monolayer of
adsorbed material (in liquid, such as MB) is adsorbed
over a uniform adsorbent surface (a flat surface of solid
phase, such as miswak leaves) at a constant temperature
and that the distribution of the compound between the
two phases is controlled by equilibrium constant. Hence
at equilibrium both rates of adsorption and desorption are
equal. The Langmuir equation is derived as:
where m (the maximum capacity of adsorption, mg/g,)
(a constant related to the affinity of the binding
sites, L/mg,) are the Langmuir isotherm constants. Both
m and q
will greatly impact the conclusions made
about the experimental data and can be determined by a
simple method of equation optimization by linear regres-
sion [25]. That is to transform the isotherm variables to a
linear form and then to apply the linear regression analy-
sis of known e and e values as described by Line-
weaver-Burk (Langmuir-II).
C q
2.8.2. Fr eu ndlich Iso t h e r m
Freundlich isotherm model [15,19] is assuming that the
adsorption process takes place on a heterogeneous sur-
face. The Freundlich exponential equation is given as:
1/ n
qKC (4)
((L/mg) is an indicator of the adsorption
capacity and 1n is the adsorption intensity and indi-
cates both the relative distribution of energy and the het-
erogeneity of the adsorbent sites. The linear form is de-
rived by taking the log of the terms as shown in Table 1.
2.8.3. Temkin Isotherm
Temkin isotherm model (Equation (5)) was used also to
test the adsorption potential of miswak leaves to MB dye.
This model is taking into account the effects of indirect
adsorbate/adsorbate interactions on the adsorption proc-
ess. Furthermore, the model is assuming that the heat of
adsorption (Hads) of all molecules in the layer decreased
linearly by increase the coverage. The linear form of
Temkin is given as follows:
ln ln
Table 1. Different isotherm models used in this study and their linear forms.
Isotherm Nonlinear form Linear form Plot
log loglog
Freundlich e
logVS. log
Cln ln
eVS. ln
Equilibrium Isotherms and Kinetic Studies of Removal of Methylene Blue Dye by Adsorption onto Miswak Leaves
as a Natural Adsorbent
where, R is common gas constant (0.008314 kJ/mol K),
T is the absolutre (K), 1is the Temkin
onstant related to the heat of sorp(kJ/ml) which
e temperatu/T
tion co
indicates the adsorption potential (intensity) of the ad-
sorbent and T
(L/g) is Temkin constant related to ad-
sorption capacity. The liner plots of e
q versus lne
enable to determine the constants 1/T
b and T
the slope and ircept respectively.
3. Results and Discussion
3.1. Effects of Initial Dye Concenttion ()
g) of
MBents were conducted at a tem-
the increase in [MB]0 from 120 to 150 mg/L led to slight
improve in the adsorption capacity of MB onto miswak
leaves. Conseqently, 120 mg/L of MB was chosen as an
and decreased
The qe
and Contact Time
A miswak leaves dosage of 0.005 g (1.0 g/L) was ad
to 0.005 L of different concentrations (16 - 150 m/L
dye solution. Experim
perature of 303 K for 80 min to test the effect of initial
concentration and contact time on the adsorption process.
The results (Figure 2) indicated that the adsorption of
MB dye onto miswak increases as [MB]0 increased. At
the first 10 min of the adsorption process, as [MB]0 in-
creased by 9.4 times (from 16 to 150 mg/L), the ad-
sorbed amount (qt) onto miswak leaves increased 6.5
times (from 6.2 to 40.0 mg/g). Also, as the contact time
increased to 30 min, qt increased by about seven times
(from 9.01 to 62.13 mg/g). Therefore, the adsorption of
MB dye was very rapid during the first 10 min, and in-
creased gradually during the second 20 min until reached
equilibrium at 30 min. The results showed that the up-
take of MB dye by miswak leaves depends on [MB]0 and
contact time. This is because [MB]0 act as the driving
force that increases the mass transfer of MB dye from
aqueous solution onto the surface of miswak leaves.
During the adsorption process, solutions with different
initial concentrations possibly will reach equilibrium at
different times. This may be due to the time required for
the dye molecules to encounter the boundary layer effect,
then diffuse to the surface of the adsorbent and finally
diffuse to the porous structure of the adsorbent [27].
Therefore, solutions with low initial concentration (16
mg/L) reached equilibrium first at about 30 min, while
solutions with high initial concentration of 150 mg/L
takes longer time and reached equilibrium at 60 min. To
ensure complete equilibrium of the data, adsorption sam-
ples were collected at 80 min. It was noted that as [MB]0
dye increased from 16 to 150 mg/L, the removal % at
equilibrium decreased by 25% (from 55.78 to 41.79%).
This may be because the constant number of available
sites in miswak leaves is easily saturated by the increase
of [MB]0, which would lead to a decrease in the removal
percentage of MB dye. Other researchers reported simi-
lar trend [28,29]. Furthermore, Figure 2 indicated that
optimum [MB]0 for further experiments.
3.2. Effect of Solution pH on Dye Removal
Experiments were conducted at 120 mg/L [MB]0, 1.0g/L
miswak leaves dose, and 80 min contact time at 303 K,
to study the effect of solution pH on the equilibrium ad-
sorption capacity (qe) of MB dye onto miswak leaves as
shown in Figure 3. It is indicated that q of MB dye
reaching a maximum in a basic medium
by decreasing the pH values in the acidic medium.
of MB dye in a basic medium (at the pH range of 10.6 to
12.0) reached 60.9 mg/g and become about 13.54 at pH
2.8. Therefore, further adsorption experiments were per-
formed at pH 10.6 as an optimum pH value. The varia-
tions in the pH values from acidic to alkaline medium
would affect the adsorption rate because both the degree
of ionization of dye molecules and the surface properties
of the adsorbent (miswak leaves) would vary. It is pre-
viously reported that the adsorption process increased by
increasing the electrostatic attraction [2]. Thus anionic
dyes (dye) are favorably adsorbed by the adsorbent at
Figure 2. Effect of initial concentration and contact time on
the adsorption of MB dye (T = 303 K, pHi = 10.6 miswak
dosage = 1.0 g/L, [MB]i = 16 - 150mg/L, V = 0.005 L).
Figure 3. Effect of pH on the adsorption process (T = 303 K,
miswak dosage = 1.0g/L, [MB]i = 120mg/L, V = 0.005 L).
Copyright © 2011 SciRes. JEP
Equilibrium Isotherms and Kinetic Studies of Removal of Methylene Blue Dye by Adsorption onto Miswak Leaves 821
as a Natural Adsorbent
lower pH values due to the presence of H+ ions, while
adsorption of cationic dyes (dye+) are favorably adsorbed
at higher pH values, which led to increase the presence
of OH ions as a result of an increase the electrostatic
attraction in each case [2]. Therefore, performing the
adsorption in the basic medium, at pH 10.6, would in-
crease the negative charge on the adsorbent surface
causing an increase in the electrostatic attraction between
cationic dye molecules (MB dye) and the surface of
miswak leaves, hence increasing the adsorption rate of
resulted only in about
hus 1.0 g/L of
MB dye. On the other hand, the presence of high con-
centration of H+ ions in the acidic medium at pH 2.8
would make them compete effectively with cationic dye
molecules (MB dye) causing a decrease in the amount
of dye adsorbed. These results can be further proven by
opposite behaviour shown for the adsorption of anionic
dyes such as methyl orange (MO) onto Lapindo volcanic
mud (LVM) [30]. It was found that the highest adsorp-
tion capacity was obtained at pH 3. In addition, a similar
trend was shown by the adsorption of some metal cations
onto different adsorbents [31,32].
3.3. Effect of Adsorbed Amount
At constant [MB]0 (120 mg/L), different amounts of
miswak (0.5 to 3.0 g/L) were added to dye solutions
(0.005 L) to study the effect of miswak leaves amount on
MB dye adsorption. Results in Figure 4 shows that the
adsorption capacity of MB dye in the first stage in-
creased rapidly with the increase in the adsorbent dose
then increased slowly with the further increase in the
adsorbent dose. It can be seen that at 1.0 g/L of the ad-
sorbent dose, the adsorption capacity of the dye reached
the most at 60.90 mg/g. Then an increase in the dose of
miswak leaves from 1.0 to 3.0 g/L
0.7 mg/g more to reach 61.8 mg/g. T
miswak leaves was chosen as the optimum dose and used
in the further experiments. The increase in adsorption
capacity of MB dye with the increase in the amount of
miswak leaves up to 1.0 g can be assigned to the increase
in both the surface area and the adsorption sites to MB
dye molecules [23,33]. The adsorption of methylene blue
and indigo carmine dye onto different adsorbents such as
grass waste, rice husk ash, and bamboo-based activated
carbon respectively [23,29,33] was reported with a simi-
lar trend.
3.4. Isotherms for the Sorption Process
3.4.1. Langmuir isotherms
Linear fit of Langmuir-II for the adsorption of MB onto
miswak leaves at 30˚C is shown in Figure 5. The value
of qm, KL and R2 are presented in Table 2. The results
indicated that linear form of Langmuir-II model shows
Figure 4. Effect of adsorbent dose on the adsorption process
(T = 303 K, pHi = 10.6, [MB]i = 120 mg/L, V = 0.005 L).
Figure 5. Langmuir isotherm (T = 303 K, miswak dosage =
1.0 g/L, [MB]i = 16 - 150 mg/L, pHi = 10.6, V = 0.005 L).
Table 2. Langmuir, Freundlich and Temkin constants for
the adsorption process.
Langmuir constantsFreundlich constants Temkin constants
(L/mg) R2 KF
(L/mg) n R2 bT
mg) R2
ol (L/
200 0.00670.9991.7501.176 0.993 0.101 0.1610.950
the minimal deviation from the fitted equation as indi-
cated by the high value of R2 as 0.999. It was proposed
that when the value of R2 is greater than 0.89, the d-
by Langmuir isotherm indicates
ane at of monoyer coveB dye
ms utc[23]. Other resrs
po s
yes onto activated carbon prepared from various sources.
sorption data would follow the Langmuir model [2].
urthermore, the value of qm which is the measure of F
maximum adsorption capacity of miswak leaves for MB
dye was calculated as 200 mg/g (Table2). Representing
the experimental data
oth the homogenous nature of miswk leaves surfac
d th
form ion
rage of M
le at iter surfaearchere-
rtedimilar observations for the adsorption of different
Adsorption of acid orange 10 dye [34], direct dyes [35]
and Congo red dye [36] onto activated carbon prepared
from bagasse, sawdust and coir pith respectively.
Copyright © 2011 SciRes. JEP
Equilibrium Isotherms and Kinetic Studies of Removal of Methylene Blue Dye by Adsorption onto Miswak Leaves
as a Natural Adsorbent Separation Factor
Separation factor (
R), is a dimensionless constant [21,
23], and it is a good characteristic of the Langmuir iso-
R, can be expressed in the following equation:
where 0
C (mg/L) is the highest [MB]0 and
is Langmuir constant. The value of
R indicates the
shape of the isotherm to be either linear (
R= 1), unfa-
vourable (
R > 1), favourable (0 <
R < 1), or irre-
versible (
R = 0). Thus the
R values between 0 and 1
indicate favourable adsorption. Plot of
R versus
Figure 0
of MB at 30˚C is shown in 6. The
R val
adsorbent for MB dye.
3.4.2. F r e u ndlich Iso t h e r m
riu nto
o al
were in the range 0.500 to 0.904, which is less than
unity, indicating that the adsorption of MB onto miswak
leaves is a favourable process, and the data fits Langmuir
isothermodel. Accordingly, miswak leaves is a good
Equilibm adsorption data of MB dye o miswak
leaves was tested with Freundlich isothm model. The
linear plot of Freundlich isotherm at 303 Kigure 7) is
employed tdetermine the intercept vue of
the slope 1n along with R(Table 2). Although, the
value of 2
R (0.993) of Freundlich is sligy lower th
the value of 2
R (0.999) of Langmuir-II isotm. The
value of
htl an
1n (indicative of favorability) is 0.85, which
is close to the unity and indicates the favorability of the
adsorption process [5]. Therefore, Freundlich model is
still a good model to describe the adsorption data.
3.4.3. Temkin Isotherms
n isotherm was chosen to In addition, Temkin adsorptio
fit with the equilibrium adsorption data. The linear plot
of the Temkin isotherm at 303 K is illustrated in Figure
8. The parameters, T
and T
b of the Temkin equation
have been calculated for MB dye (Table2). Duo the
low value of both adsorptioapacity, T
e t
n c
, (0.161 L/g)
and the ve of 2
R (0.95), the data of equilibrium iso-
therms of MBnto miswak is poorly described by the
Temkin mel. On the other hand, comparison of maxi-
mum monolayer adsorption capacity (m
q) of MB onto
various adsorbents obtained in the literature is presented
in Table 3 in order to compare the efficiency of mi
iswak leaves are very effec-
leaves. It can be seen that m
tive adsorbent for cationic dyes such as MB with a rela-
tively large adsorption capacity of 200 mg/g when com-
pared with some other adsorbents.
3.5. Adsorption Kinetics
In order to study the adsorption of MB onto miswak
leaves and to interpret the results, experimntal data ob-
tained were fittedto different kinetic models such as
the pseudo-first-order [15], the pseudo-second order [22,
23] and an intraparticle diffusion [15,24].
Figure 6. Plot of separation factor versus [MB]i (T = 303 K,
miswak dosage = 1.0 g/L, [MB]i = 16 - 150 mg/L, pHi = 10.6,
= 0.005 L).
Figure 7. Freundlich isotherm (T = 303 K, miswak dosage =
.0 g/L, [MB]i = 16 - 150 mg/L, pHi = 10.6, V = 0.005 L).
Figure 8. Temkin isotherm (T = 303 K, miswak dosage = 1.0
g/L, [MB]i = 16 - 150 mg/L, pHi = 10.6, V = 0.005 L).
Copyright © 2011 SciRes. JEP
Equilibrium Isotherms and Kinetic Studies of Removal of Methylene Blue Dye by Adsorption onto Miswak Leaves 823
as a Natural Adsorbent
Table 3. Comparison of the maximum monolayer adsorp-
tion of MB onto various adsorbents.
Adsorbents qm (mg/g) References
Miswak leaves 200 This study
Natural Jordanian tripoli 16.6 [1]
Banana peel 20.8 [15]
Orange peel 18.6 [15]
Activated carbon prepared
from oil palm shell 243.90 [37]
Activated furniture (850˚C) 200 [38]
Activated tyres (850˚C) 130 [38]
Activated sewage char (800˚C) 120 [38]
Pyrolysed furniture 80 [38]
bamboo-based activated carbon 454.2 [23]
Pineapple leaf powder (PLP) 294.26 [39]
3.5.1. Ps e u do First-Order Equation
The rate cadsorption ie
prder equation given anged
Svehich expresse follows:
onstant of s determind from the
seudo first-oby Lrgren an
nska (1898) [15], wd as
loglog 2
et e
qq q t
 
wthe amounts of the MB aed
( time t (), respectively,
at adsorptioin1).
were calcu from tpe
alots of
s t
rffert conc he
rat the v were low
and valu
is showsrption of
.303 (
here q and t
q are
mg/g) at equilibri
um and at
nd k is the rate constan
n (m
Values of k and
nd the intercept of the p
latedhe slo
lo et
entrations (Figure
f R2
es do not agree well
espectively at di9). T
esults in Table 4 show thalues o
the experimental e
he calculated values. Th
hat the
e MB onto miswak is not first-order
3.5.2. Ps e udo-Second-Order Rate Equation
Equation of pseudo second-order based on equilibrium
adsorption [22-23] can be expressed as:
kq q (8)
 (9)
where, 2
k (g/mg·min) is the adsorponstant of
pseudo second-order adsorption rate. The value of e
and 2
k can be from the slope and intercept of the plot
tion rate c
q versus t respectively. The results in Figure 10
show linear plots for all different initial conce
Figure 9. Pseudo-first-order kinetics for the adsorption
proce (T = 303 K, miswak dosage =1.0 g/L, [MB]i = 16
-150 mg/L, pHi = 10.6, V = 0.005 L).
Figure 10. Pseudo-second order kinetics for the adsorptio
tudied with very high values of R (Table 4) in addition
to the good agreement between experimental and calcu-
lated values of . Therefore, the adsorption of MB
onto miswak isly represented by the pseudo sec-
ond-order kinetics. Moreover, adsorption process of MB
onto different natural adsorbents such as a pineapple leaf
powder (PLP) undergoes second-order kinetics [40].
3.5.3. Intraparticle Diffusion Study
In order to investigate the mechanism of the MB adsorp-
tion onto miswak, intra-particle diffusion based mecha-
nism was studied. It is proposed that the uptake of the
adsorbate (MB dye) by the adsorbent (miswak leaves)
r sorption system as:
tra-part diffusion rate constant (g/g·m). The
process (T = 303 K, miswak dosage = 1.0 g/L, [MB]i = 16
-150 mg/L, pHi = 10.6, V = 0.005 L).
varies almost proportionately with the square root of the
contact time (t1/2). Weber and Morris [15,24] proposed
the most-widely applied intra-particle diffusion equation
qkt (10)
where, t
q is the mount of MB dye adsorbed per unit
mass of adsorbent (mg/g) at a time t and id
k the in-
iclem in
Copyright © 2011 SciRes. JEP
Equilibrium Isotherms and Kinetic Studies of Removal of Methylene Blue Dye by Adsorption onto Miswak Leaves
as a Natural Adsorbent
Copyright © 2011 SciRes. JEP
d-order kinetic model
ss (miswak dosage = 1.0 g/L, [MB] = 16 - 150 mg/L, T = 303
Table 4. Adsorption kinetic parameters for the adsorption pr
K, pH 10.6 and V = 0.005 L).
First-order kinetic model
R2 SSE qe, cal k2 R2 SSE [MB] q, exp q, cal
0ee k1
16 8.93 7.18 –0.1059 0.956 1.87 9.524 0.025 0.997 0.54
30 16.40 13.18 –0.0852 0.99
60 31.68 28.97 –0.1175 0.97
90 44.66 86.90 –0.1819 0.9
12 31.48 50.00 0.0031 0.996 4.77
150 62.69 84.14 –0.1497 0.937 49.50 71.43 0.0025 0.994 7.82
3.22 17.86 0.0096 0.998 1.30
18.50 34.48 0.0062 0.996 2.51
120 60.90 62.23 –0.1128 0.901 47.71 66.67 0.0023 0.991 5.16
Units used for the above terms are a = mg/g, k1 = min-1, SSE = %, k2=g/mg·min-1.
rate parameter f stage i iained e slop
of thraight versus
If the intra-pffusione mech of th
adstion proceen the plvwi
be lar and iflot passe t the
the limitingss is onthtic
difn [41]. Oise, someon
e range.
s follow: [MB]0 = (mg/L), qe
id ok
line of
s obt
from the
e stt
is th
rticle danisme
orpss, thot of ersus 1/
er m
ine the ps throhe origin
rate procely duee intra-ple
fusiotherwe othchanismg
with the intra-particle diffusion is also involved [2].
igure 11 presented the intra-particle diffusion model. F
The results indicated that the plot of t
q versus 1/2
were not linear over the whole timFurthermore,
Figure 11. Intra-particle diffusion plot for the adsorption
process (miswak dosage = 1.0 g/L, [MB] = 16 - 150 mg/L, T
= 303 K, pH 10.6 and V = 0.005 L).
may be seen that the intra-particle diffusion of MB dye
occurred in 2 stages. The first straig portion is attrib-
uted to the macropore diffusion (phase I) and the second
linear portion is attributed to micro-pore diffusion (phase
II) [41]. The intra-particle diffusion constants for these 2
stages (1d
k and 2d
k) are given in Table 5. Results in-
dicated that the adsorption of MB dye onto miswak in-
volved more than one process, and the intra-parti
transport is not the rate-limiting step. Such finding is
similar to that made in previous works on adsorption [2].
In addition, the rate constants of the intra-particular dif-
fusion on miswak were slow and increased by the in-
crease in [MB]0, hence MB dye as a big molecule, dif-
fused slowly among the particles during the adsorption
process. Similarly Jadhav, D. N. et al. [41], reported
slow rate of sorption in case of big molecules such as
acidic, basic and disperse dyes on sawdust, polymerized
sawdust and sawdust carbon respectively.
On the other hand, the sorption rate was generally fast
and the intra-particle diffusion was the rate-limiting step
in case of sorption of metal ions [42] and sorption of
different dyes onto banana and orange peels [15].
3.6. Validity of Kinetic Models
The applicability of both pseudo-first order and pseudo-
second order models for the adsorption of MB onto
miswak were verified at different [MB] using the sum of
Table 5. Weber-Morris parameters (miswak dosage = 1.0
(mg/L) (mg/g·min ) (mg/g·min )
g/L, [MB] = 16 - 150 mg/L, T = 303 K, pH 10.6 and V =
0.005 L).
[MB]0, K1d –1/2 R2 K2d –1/2 R2
16 1.413 0.971 –0.027 0.884
30 2.872 0.989 0.327 0.884
60 5.944 0.973 0.276 0.884
90 9.208 0.972 0.051 0.884
120 11.200 0.958 –0.138 0.884
150 10.880 0.952 0.172 0.884
squared error (SSE, percentage ) equation [23] given by:
,exp ,eecal
where, N is the number of data po
ints used in the linear
lot ofh model. The validity of these models was
o models listed in Table 3. It is indicated that
p eac
compared by judging the low value of ,%SSE which
indicates the better fit. The values of (,%SSE ) obtained
for the tw
Equilibrium Isotherms and Kinetic Studies of Removal of Methylene Blue Dye by Adsorption onto Miswak Leaves 825
as a Natural Adsorbent
th-secorder kinc modellded the-
e35 to 7.81e first-
ordry hilues of (
49.agith tevious b2
and taearliethe psdr
(Ta fuprovepseudo-
sec dee the ps
reptrptioMB obood
ct [23].
energy (G˚) can be ca
follouations [2];
e pseudond-oeti yie low
st SSE,%
odel le
,ecal ob
3) to
of MB onto
ivated carbon
3.7. Standar
Standard free
wing eq
values (0.59). Whereas, th
er md to vegh va ,%SSE 1.87 to
50).rees whe pr values of
similar trend
oth R
qined r for eudo-s orde
ble rther the suitability of the
ond-okinetic to
iswak leaves. Also, a
scribadsorp roces
ortedhe adson of nto ba-base
d Free Energy Change (G˚)
ated from the
ln C
GRTK (12)
where, T is the temperature (K), R is gas constant
(kJ/mol·K), C
(L/g) is the standard thermodynamic
equi constant, e
q is the amount of adsorbed MB
dye per unit mass of miswak leaves at em (mg/g)
and e
C is the equilibrium aqueous concentration of MB.
The Gibbs free energy change (G˚) is negative indicat-
ing that the adsorption process of MB onto miswak is
4. Conclusions
The adsorption of MB
rent ads
Also, the eq
dlich isothe
red by mi
e rem
swak leaves wa
of RL less than unity.
KSA for thei
dye onto mis
els, the
g/g which is comarable with
diffeorbents used for the adsor
uilibrium data can be md by Fre-
unrm model. The adso
voswak leaves with value
asrahi, Biology nt, Fac
versity, Jazan, r help
studied. Among the three different isotherm mod
equilibrium data was best fitted with the Langm
equation. The equilibrium capacities based on the Lang-
muir analysis was 200 m
on of MB dye.
rption of MB was fa-
Furtherre, the adsorption process ipontaneous fol-
lows pseudo-second order kinetics and the mechanism
involved more than one process. The present work re-
vealed that the miswak leaves are a promising material
for thoval of MB dye from aqueous solutions.
5. Acknowledgements
The Author is grateful to Dr. Zarrag I. Al-Fif (Vice Dean)
and Dr. Yahya S. M
of Science, Jazan Uni
and support.
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