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Biosorption of basic dyes onto Azolla filiculoides equilibrium and kinetic modeling.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2008; 3: 368–373
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.157
Research Article
Biosorption of basic dyes onto Azolla filiculoides:
equilibrium and kinetic modeling
T. V. N. Padmesh,1 K. Vijayaraghavan,2 K. Anand1 and M. Velan1 *
1
2
Department of Chemical Engineering, Anna University, Chennai, India
Division of Environmental and Chemical Engineering, Chonbuk National University, Jeonbuk, South Korea
Received 25 October 2007; Revised 5 May 2008; Accepted 7 May 2008
ABSTRACT: The biosorption of basic dyes, rhodamine B (RMB) and methylene blue (MB), using Azolla filiculoides
was investigated in a batch reactor and in a fixed-bed column. Biosorption isotherms revealed that pH 5 was the
optimum condition for maximum biosorption of both basic dyes. Sorption isotherm data were fitted with Langmuir,
Freundlich, Redlich–Peterson and Sips models. A maximum uptake of 166.7 and 91.8 mg/g was observed for MB
and RMB, respectively, according to the Langmuir model. It was observed from the kinetic data that the biosorption
process using A. filiculoides followed pseudo-second order kinetics. The ability of A. filiculoides to biosorb MB in a
packed column was investigated, as well. Experiments conducted at different bed heights (15–25 cm) revealed that
decrease in bed height resulted in inferior biosorption performance. At 25 cm, the A. filiculoides bed was capable of
providing breakthrough at 76.2 h, with column uptake and removal efficiency of 80.2 mg/g and 84.9%, respectively.
The Thomas model was successfully used to analyze the experimental data and the model parameters were evaluated.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: Azolla filiculoides; nonlinear isotherms; kinetics; biosorption; packed-bed column
INTRODUCTION
Fresh-water algae are biological resources and are available in large quantities in many parts of the world.
However, at certain areas fresh-water algae are plentiful and fast growing and thereby threaten the tourism
industry by spoiling the environment. Alternative solutions, which utilize the potential of fresh-water algae,
are significant and beneficial to communities.
Pollution caused by industrial wastewater has become
a common problem in many countries. Especially,
organic, inorganic, and dye pollutions from industrial effluents disturb human health and ecological
equilibrium.[1] Among various industries, the textile
industry ranks first in the usage of dyes for coloration of
the fibers. The textile sector alone consumes about 60%
of the total dye production for coloration of various fabrics and out of it, around 10–15% of the dyes used for
coloration comes out through the effluents.[2] Color in
water body is not only aesthetically unpleasant but also
interferes light penetration and reduces photosynthetic
action. Many dyes or their metabolites have toxic as
*Correspondence to: M. Velan, Department of Chemical Engineering, Anna University, Chennai 600025, India.
E-mail: velan@annauniv.edu
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
well as carcinogenic, mutagenic, and teratogenic effects
on aquatic life and humans.[3,4]
The methods of color removal from industrial effluents include biological treatment, coagulation, flotation,
adsorption, oxidation, and hyper filtration.[5] Currently
research community is focused on technologies for the
treatment of polluted environments that are less costly
and eco-friendly. All physicochemical approaches are
often cost prohibitive while biological methods are relatively cheap as well as eco-friendly.[6] Biosorption is a
popular technique that utilizes inactive/dead biological
materials for the removal of dyes. Various biomaterials
have been examined for their biosorptive properties and
different types of biomass have shown different levels
of dye uptake. Among the most promising biomaterials
studied, fresh-water algae are found to be very efficient
on the uptake of dyes.[7,8] Fresh-water macroalgae have
been found to be potential biosorbents because of their
cheap availability, relatively high surface area, and high
binding affinity.[6] Its macroscopic structures present a
convenient basis for the production of biosorbent particles suitable for sorption process applications.[9]
Therefore, this study was aimed at utilization of freshwater macroalgae, Azolla filiculoides (A. filiculoides),
for the removal of rhodamine B (RMB) and methylene
blue (MB) from aqueous solutions.
Asia-Pacific Journal of Chemical Engineering
BIOSORPTION OF BASIC DYES ONTO AZOLLA FILICULOIDES
MATERIALS AND METHODS
Azolla filiculoides was collected from milk producers
union, Tirunelveli, India. It was then sun dried and
crushed to particle sizes in the range of 1–2 mm.
The crushed biomass (1 g) were then treated with 0.1
M HCl (100 ml) for 5 h followed by washing with
distilled water and finally dried in the oven overnight
at 60 ◦ C. The resultant biomass was subsequently used
in biosorption experiments.
MB and RMB were purchased from Ranbaxy Chemical (India) Ltd. The desired concentration of stock
solution was prepared with distilled water. The pH of
the solution was initially adjusted and controlled using
0.1 M HCl and 0.1 M NaOH. Biosorption experiments
were performed in a rotary shaker at 150 rpm using
250 ml Erlenmeyer flasks containing 0.2 g A. filiculoides in 50 ml of different dye concentrations. After
equilibrium was reached, the reaction mixture was centrifuged at 3000 rpm for 10 min. The supernatant was
analyzed using UV-spectrophotometer (Hitachi, Japan)
at a wavelength of 668 and 559 nm for MB and RMB,
respectively. The samples were diluted, whenever necessary, with distilled water to improve accurate estimation. The amount of dye biosorbed was calculated from
the difference between the dye quantity added to the
biomass and the dye content of the supernatant using
the following equation:
conditions. In this study, the applicability of the pseudofirst order and second-order model has been tested for
the sorption of basic dyes (RMB and MB) onto A.
filiculoides. The best-fit model was selected on the basis
of regression coefficient, R 2 , values.
Shown in Fig. 1 are the plots of RMB and MB
uptakes against time onto A. filiculoides at various initial concentrations. In general, the results revealed that
the removal of dyes was rapid at the initial stages of the
contact period, and thereafter, it became slower near the
equilibrium. This is obvious from the fact that a large
number of vacant binding sites are available for sorption during the initial stage, and after a lapse of time,
the remaining vacant sites are difficult to be occupied
due to repulsive forces between the solute molecules on
the solid and bulk phases. On comparing the two dyes,
A. filiculoides biosorbed more MB than RMB. However, MB biosorption rate was slow compared to RMB
biosorption rate. For both dyes, on increasing the initial
dye concentrations, the total dye uptake increased and
the total percent removal decreased. This is because at
lower concentration, the ratio of the initial moles of dye
molecules to the available surface area is low and subsequently the fractional sorption becomes independent
of initial concentration. However, at higher concentration the available sites of sorption becomes fewer
175
(1)
Uptake (mg/g)
where Q is the dye uptake (mg/g); C0 and Cf are the
initial and equilibrium dye concentrations in the solution (mg/l), respectively; V is the solution volume (l);
and M is the mass of biosorbent (g). For kinetic experiments, samples were taken at regular time intervals and
analyzed for dye concentration.
Continuous-flow sorption experiments were conducted in a glass column (2-cm internal diameter and
35-cm height). A known quantity of A. filiculoides was
packed in the column to yield the desired bed height
of the biosorbent. A known concentration of the dye
solution was pumped upward through the column at a
desired flow rate by a peristaltic pump (Miclins). The
concentration of dye at the vent of the column was measured at regular time intervals.
150
125
100
MB
75
50
25
0
0
200
400
Time (min)
600
800
100
Uptke (mg/g)
Q = V (C0 − Cf )/M
75
RMB
50
25
RESULTS AND DISCUSSION
0
0
200
400
Time (min)
600
800
Batch kinetic studies
The prediction of batch sorption kinetics is necessary for
the design of industrial sorption columns. The nature of
the sorption process depends on physical or chemical
characteristics of the biosorbent and also on the system
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 1. Effect of initial dye concentration
on uptake capacity of A. filiculoides (pH = 5
and temperature = 30 ◦ C). Initial dye concentration: (◊) 10 mg/l; () 50 mg/l; ( ) 100 mg/l;
(♦) 200 mg/l; () 500 mg/l; (ž) 700 mg/l; ()
1000 mg/l.
°
Asia-Pac. J. Chem. Eng. 2008; 3: 368–373
DOI: 10.1002/apj
369
T. V. N. PADMESH ET AL.
Asia-Pacific Journal of Chemical Engineering
compared to the moles of dye present and hence the
percentage dye removal is dependent upon the initial
dye concentration.[10] It can be concluded that the rate
of dye binding with algal biomass is more at initial
stages, which gradually decreases and remains almost
constant after an optimum period of 4 h.
To evaluate the differences in the biosorption rates
and uptakes, the kinetic data were described using
pseudo-first and pseudo-second order models. The linearized form of pseudo-first and pseudo-second order
models are shown in Eqns (2) and (3), respectively:
log(qe − qt ) = log(qe ) −
k1
t
2.303
t
1
1
=
+ t
2
qt
q
k2 qe
e
175
MB
Uptake (mg/g)
150
125
100
75
2 pH
4 pH
6 pH
8 pH
50
25
0
0
3 pH
5 pH
7 pH
200
400
600
Equilibrium Concentration (mg/L)
100
(2)
RMB
Uptake (mg/g)
370
(3)
where qt is the amount of dye sorbed at time t (mg/g);
k1 is the first order rate constant (1/min); and k2 is
the second order rate constant (g/mg min). The rate
constants, predicted equilibrium uptakes, and the corresponding correlation coefficients for all concentrations
tested have been calculated and summarized in Table 1.
In the case of pseudo-first order model, the correlation
coefficients were found to be above 0.926, but the calculated qe is not equal to experimental qe , suggesting
the insufficiency of the model to fit the kinetic data for
the initial concentrations examined. The reason for these
differences in the qe values is that there is a time lag,
possibly due to a boundary layer or external resistance
controlling at the beginning of the sorption process.
The pseudo-second order model is based on the
sorption capacity on the solid phase. Contrary to other
well-established models, it predicts the behavior over
the whole range of studies and it is in agreement with
75
50
2 pH
4 pH
6 pH
8 pH
25
0
0
200
400
600
3 pH
5 pH
7 pH
800
Equilibrium Concentration (mg/L)
Figure 2. Biosorption isotherms of MB and
RMB onto A. filiculoides at different pH conditions (temperature = 30 ◦ C, biomass dosage =
4 g/l and agitation rate = 150 rpm).
the binding mechanism being the rate-controlling step.
The pseudo-second order rate constant, experimental,
and predicted dye uptakes and the regression coefficient
are shown in Table 1. From Table 1, it was observed
that the correlation coefficients were better than those
of first order model. The values of predicted equilibrium
Table 1. Pseudo-first and pseudo-second order model constants at different initial dye concentrations.
Pseudo-first
order
Dye
MB
RMB
a
Pseudo-second
order
C0
(mg/l)
(qe )exp
(mg/g)
k1
(1/min)
qe
(mg/g)
R 2a
k2
(g/mg min)
qe
(mg/g)
R 2a
10
50
100
200
500
700
1000
10
50
100
200
500
700
1000
2.40
13.10
27.10
61.00
132.40
151.90
159.80
2.80
3.00
24.60
35.20
72.30
87.77
103.20
0.040
0.033
0.041
0.024
0.022
0.020
0.020
0.034
0.024
0.021
0.033
0.021
0.021
0.016
1.6
7.0
12.6
42.5
145.8
125.0
125.4
1.3
8.2
17.5
29.9
60.5
68.7
70.2
0.988
0.957
0.929
0.983
0.969
0.993
0.984
0.943
0.976
0.943
0.984
0.980
0.993
0.978
0.1110
0.0210
0.0180
0.0020
0.0003
0.0004
0.0004
0.1170
0.0090
0.0030
0.0040
0.0010
0.0010
0.0010
2.4
13.3
27.2
62.1
138.9
156.2
163.9
2.8
3.2
25.2
35.7
74.6
90.1
106.4
0.999
0.999
0.999
0.999
0.999
0.999
0.999
0.999
0.999
0.999
0.999
0.999
0.999
0.999
Correlation coefficient.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2008; 3: 368–373
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
BIOSORPTION OF BASIC DYES ONTO AZOLLA FILICULOIDES
sorption capacities showed good agreement with the
experimental equilibrium uptake values.
The equilibrium data were analyzed using the Langmuir, Freundlich, Redlich–Peterson and Sips isotherm
equations, which can be expressed as:
Biosorption isotherms
Langmuir model[11] :
Experimental biosorption isotherms of RMB and MB
onto A. filiculoides at different pH conditions (2–8) at
30 ◦ C are presented in Fig. 2. Solution pH played a relatively significant role in dye biosorption, with maximum
dye biosorption observed at pH 5 and decrease in uptake
with further increase in pH for the both dyes. This may
be due to the nature of the binding groups present in
the alga. For both dyes at all pH conditions, as the dye
concentration increased, the uptake also increased and
reached a plateau at higher concentrations resulting in a
favorable sorption isotherm. For instance, on changing
the initial MB concentration from 10 to 1000 mg/l, the
uptake increased from 2.37 to 161.58 mg/g at pH 5.
Freundlich model[12] :
Qmax bCeq
1 + bCeq
(4)
Q = KF Ceq 1/n
(5)
Q=
Q=
KRP Ceq
(6)
1 + aRP Ceq βRP
KS Ceq βS
1 + aS Ceq βS
(7)
Redlich-Peterson model[13] :
Sips model[14] :
Q=
where Qmax is the maximum dye uptake (mg/g), b
is the Langmuir equilibrium constant (l/g), KF and n
are Freundlich constants, KRP is the Redlich–Peterson
isotherm constant (l/g), aRP is the Redlich–Peterson
isotherm constant (l/mg); βRP is the Redlich–Peterson
Table 2. Biosorption isotherm model constants at different pH conditions.
Langmuir model
Freundlich model
Dye
pH
qmax
(mg/g)
b
(l/mg)
R2
KF
(l/g)
n
R2
MB
2
3
4
5
6
7
8
2
3
4
5
6
7
8
113.0
139.3
155.0
166.7
162.0
153.7
151.0
72.8
76.5
86.0
91.8
82.1
72.7
65.6
0.0137
0.0147
0.0153
0.0211
0.0202
0.0184
0.0145
0.0082
0.0205
0.0282
0.0345
0.0204
0.0197
0.0209
0.987
0.969
0.970
0.991
0.972
0.972
0.964
0.973
0.981
0.974
0.979
0.978
0.978
0.975
1.99
4.39
4.91
6.50
6.07
5.45
4.90
4.37
6.39
6.82
8.51
6.89
6.39
5.82
1.44
1.73
1.72
1.78
1.74
1.73
1.77
2.45
2.65
2.62
2.63
2.54
2.59
2.48
0.907
0.959
0.968
0.990
0.974
0.969
0.973
0.900
0.969
0.971
0.969
0.984
0.967
0.965
RMB
Redlich–Peterson
model
MB
RMB
2
3
4
5
6
7
8
2
3
4
5
6
7
8
KRP
(l/g)
aRP
(l/mg)
βRP
1.85
2.24
2.88
5.20
4.61
3.08
3.01
1.88
2.26
3.37
3.63
2.85
2.36
2.14
0.121
0.101
0.096
0.103
0.249
0.120
0.160
0.111
0.085
0.105
0.068
0.143
0.124
0.125
0.653
0.689
0.734
0.755
0.617
0.686
0.661
0.800
0.841
0.841
0.901
0.779
0.800
0.790
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Sips model
R2
KS
(l/g)
aS
(l/mg)
βS
R2
0.913
0.913
0.989
0.982
0.992
0.992
0.988
0.992
0.994
0.992
0.990
0.998
0.991
0.997
3.34
4.12
6.61
10.61
10.44
10.29
10.28
8.42
8.47
7.42
9.61
8.62
8.63
8.62
0.62
1.12
1.34
1.18
1.70
2.04
2.01
1.31
1.18
0.62
0.72
1.18
1.20
1.17
0.891
0.902
0.911
0.925
0.938
0.945
0.961
0.821
0.829
0.835
0.852
0.849
0.862
0.869
0.955
0.955
0.971
0.977
0.976
0.968
0.988
0.9901
0.977
0.980
0.965
0.989
0.977
0.991
Asia-Pac. J. Chem. Eng. 2008; 3: 368–373
DOI: 10.1002/apj
371
T. V. N. PADMESH ET AL.
Column studies
The batch experimental results revealed that A. filiculoides performed well in MB biosorption. Hence
packed column experiments were only performed to
examine the efficiency of A. filiculoides to continuously
biosorb MB.
Biosorption of MB by A. filiculoides was presented
in the form of breakthrough curves. Figure 4 shows
the breakthrough profile of MB biosorption at different
bed heights (15, 20, and 25 cm). In order to yield
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
250
MB
Uptake (mg/g)
model exponent, KS is the Sips model isotherm constant
(l/g); aS is the Sips model constant (l/mg) and βS is
the Sips model exponent. All the model parameters
were evaluated by nonlinear regression using Matlab
software.
The Langmuir adsorption isotherm has traditionally
been used to quantify and contrast the performance
of different biosorbents. It also served to estimate
the maximum dye-uptake values which could not be
reached by experiments. The constant b represents the
affinity between the sorbent and sorbate. The Langmuir
constant values (Table 2) revealed that A. filiculoides
performed well in MB biosorption. A maximum dye
uptake of 166.7 and 91.8 mg/g was obtained at optimum
pH 5 for MB and RMB, respectively. The correlation
coefficients were greater than 0.964.
To a minor extent, the Freundlich model also
described the equilibrium data. Both Freundlich constants (KF and n) also reached their maximum values at
pH = 5 for both the dyes (Table 2). This implies that
the binding capacity reaches the highest value and the
affinity between the alga and dye molecules was also
higher than other pH values investigated.
The Redlich–Peterson isotherm constants for biosorption of RMB and MB onto A. filiculoides are furnished
in Table 2. The isotherm constant, KRP , increases with
increasing pH and reaches maximum at pH 5. There
are two limiting behaviors for Redlich–Peterson model:
Langmuir form for βRP = 1 and Henry’s law form for
βRP = 0. In this study, the βRP values were close to
unity, i.e. the data can preferably be fitted with Langmuir model.
The Sips model constant KS was observed to be
maximum for MB biosorption. At low sorbate concentrations, Sips isotherm effectively reduces to the
Freundlich isotherm and thus does not obey Henry’s
law. At high sorbate concentrations, it predicts a monolayer sorption capacity characteristic of the Langmuir
isotherm. Similar to the Redlich–Peterson model, the
values of the Sips model exponent βS were also close to
unity. Also, Sips model described the equilibrium data
reasonably well at all conditions examined. A typical
example of batch biosorption isotherm fitted using four
examined models for both dyes are shown in Fig. 3.
Asia-Pacific Journal of Chemical Engineering
200
150
Experimental
Langmuir
Freundlich
Redlich Peterson
Sips
100
50
0
0
100
200
300
Equilibrium Concentration (mg/L)
400
125
RMB
Uptake (mg/g)
372
100
75
Experimental
Langmuir
Freundlich
Redlich Peterson
Sips
50
25
0
0
200
400
600
Equilibrium concentration (mg/L)
800
Figure 3. Application of isotherm models to
experimental isotherm data obtained during MB
and RMB biosorption onto A. filiculoides (pH = 5,
temperature = 30 ◦ C, agitation rate = 150 rpm,
biosorbent dosage = 4 g/l).
different bed heights, 7.4, 10.2, and 12.9 g of biomass
were added to produce 15, 20, and 25 cm, respectively.
The inlet dye concentration (100 mg/l) and the flow
rate (5 ml/min) were kept constant. The uptake of MB
increased with increase in the bed height. The increase
in MB uptake capacity with the increase in bed height of
the column was due to the increase in the surface area
of biosorption. At the optimum bed height of 25 cm,
the column breakthrough appeared at 76.2 h, thereafter,
A. filiculoides bed gets saturated with MB and column
exhaustion appeared at 129.1 h. The column MB uptake
and MB removal efficiency at 25 cm was recorded as
80.2 mg/g and 84.9%, respectively. The total volume
of MB solution treated during the column operation at
25 cm was 32 l.
Successful design of a column sorption process
required prediction of the concentration-time profile or
breakthrough curve for the effluent.[15] Various mathematical models can be used to describe fixed-bed
adsorption. Among these the Thomas model is simple and widely used by several investigators.[15,16] The
linearized form of Thomas model can be expressed as
follows[15] :
C0
kTh Q0 M
kTh C0
ln
−1 =
−
V
(8)
C
F
F
where kTh is the Thomas model constant (l/mg h), Q0
is the maximum solid-phase concentration of solute
Asia-Pac. J. Chem. Eng. 2008; 3: 368–373
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
BIOSORPTION OF BASIC DYES ONTO AZOLLA FILICULOIDES
• Batch experiments provided fundamental information
regarding optimum pH and maximum dye uptake. A.
filiculoides recorded maximum uptakes of 166.7 and
91.8 mg/g for MB and RMB, respectively, at pH 5
according to the Langmuir model.
• Kinetic studies showed that the maximum dye
biosorption occurred within 4 h. The pseudo-second
order kinetic equation represented and predicted the
experimental data well for both dyes with high correlation coefficients.
• Column experiments were performed in a packed
column, and the A. filiculoides bed exhibited column uptake and efficiency of 80.2 mg/g and 84.9%,
respectively, at bed height of 25 cm.
• Thus, A. filiculoides possesses all intrinsic characteristics to be employed for the treatment of basic
dye-bearing industrial effluents.
1
C/C0
0.75
0.5
0.25
0
0
50
100
150
Time (h)
Figure 4. Breakthrough curves for MB biosorption onto A. filiculoides biomass at different
bed heights (flow rate = 5 ml/min, initial MB
concentration = 100 mg/l, pH = 5). Bed height:
(♦) 15 cm; () 20 cm; (ž) 25 cm, (- - - - ) predicted
from Thomas model.
Table 3. Thomas model parameters at different bed
heights.
Bed
height
(cm)
15
20
25
Flow
rate
(ml/min)
C0
(mg/l)
Q0
(mg/g)
kTh
(l/mg h)
R2
5
5
5
100
100
100
73.30
75.27
79.85
0.0022
0.0020
0.0018
0.990
0.994
0.996
(mg/g), V is the throughput volume (l). The model
constants kTh and Q0 can be determined from a plot
of ln[(C0 /C ) − 1] against t at a given bed height. It
is clear from Fig. 4 that the model gave a good fit of
the experimental data at all bed heights examined with
high correlation coefficients greater than 0.99. Table 3
summarizes the Thomas model parameters obtained at
different bed heights.
CONCLUSIONS
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The present study investigated the following features
of dye biosorption on deactivated macro blue green
fresh-water alga A. filiculoides in a batch reactor and
packed-bed column.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2008; 3: 368–373
DOI: 10.1002/apj
373
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onto, dyes, biosorption, filiculoides, basic, modeling, kinetics, equilibrium, azolla
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