Environmental
Communication
Biosci. Biotech. Res. Comm. 9(3): 558-566 (2016)
Biosorption of reactive blue 19 dye using
Lemna minor
:
Equilibrium, kinetic and thermodynamic studies
Davoud Balarak
1
, Ferdos K. Mostafapour
1
and Hossein Azarpira*
2
1
Department of Environmental Health, Health Promotion Research Center, School of Public Health, Zahedan
University of Medical Sciences, Zahedan, Iran
2
Department of Environmental Health, Faculty of Heatlth School, Saveh University of Medical Sciences,
Saveh, Iran
ABSTRACT
Dyes are the main pollutants existing in wastewater of textile industries. This paper presents the sorption studies of
Reactive Blue 19 (RB19) dye by the Lemna minor. The effect of different parameters like pH, adsorbent dose, contact
time, temperature and initial dye concentrations were investigated. The biosorption data have been analysed using
Langmuir, Freundlich and Temkin isotherms. The equilibrium uptake capacity was increase from 7.12 mg/g to 46.51
mg/g, when increasing the dye concentration from 25 mg/L to 200 mg/L. The equilibrium data were best represented
by the Langmuir isotherm. The adsorption kinetics were found to follow the pseudo-second-order kinetic model.
Thermodynamic parameters such as ∆Go, ∆Ho and ∆So have also been evaluated and sorption process was feasible,
spontaneous and exothermic in nature. The results indicate that L. minor is a suitable adsorbent for the adsorption
of textile dyes.
KEY WORDS: ADSORPTION,
LEMNA MINOR
, REACTIVE BLUE 19, THERMODYNAMICS, ISOTHERMS, KINETICS.
558
ARTICLE INFORMATION:
*Corresponding Author: H.azarpira@savehums.ac.ir
Received 11
th
Aug, 2016
Accepted after revision 17
th
Sep, 2016
BBRC Print ISSN: 0974-6455
Online ISSN: 2321-4007
Thomson Reuters ISI ESC and Crossref Indexed Journal
NAAS Journal Score 2015: 3.48 Cosmos IF : 4.006
© A Society of Science and Nature Publication, 2016. All rights
reserved.
Online Contents Available at: http//www.bbrc.in/
INTRODUCTION
The one of important issues caused by industrialization
is the environmental problem which it is made by vari-
ous contaminants such as dyes, heavy metal, organic
pollutants, etc (Cengiz et al., 2012 and Balarak et al.,
2016). Various industries such as paper, textile, plas-
tic and food, etc apply the synthetic dyes and produce
colorful wastewater which can create problematic con-
dition to environment and human health (Li et al., 2011
and Robinson et al., 2002). Previous studies have indi-
cated that the textile industries are considered as the
Davoud Balarak et al.
largest dyes consumer (Ozcan et al., 2007 and Tan et al.,
2010). Aesthetic problems are the adverse effect of using
the dye which it can occur by discharging the dyes into
water body (even in low concentration). Also, they have
been shown the toxic to aquatic life The literature review
shows the dyes can be toxic due to presence of carcino-
gen compounds in their structure (Couto, 2009, Ali et al.,
2012, Yang et al., 2013 and Balarak et al., 2015).
Therefore the dye removal from water and waste-
water is important subject. There are many techniques
to remove the dyes from wastewater such as chemical
coagulation/ occulation, ozonation, oxidation pro-
cesses, chemical precipitation, adsorption, ion exchange,
reverse osmosis and ultra ltration, etc (Deniz et al.,
2011 and Oei et al., 2009). The adsorption and specially
adsorption onto activated carbon is promising and ef -
cient in dye removal, however the high cost of activated
carbon limits the use of it as adsorbent (Demirbas et
al., 2009). Today, various researches are performing to
discover the low-cost alternative adsorbent to remove
the dyes from wastewaters. Many natural adsorbent
such as Canola, Azolla, Wood, Coal, Rice straw have
been applied to remove the pollutants (Safa et al., 2011,
Zazouli et al., 2014, Ferdag et al., 2009). L. minor is one
of the plants which has been used for removal of dyes in
several earlier studies (Khataee et al., 2012 and Kiliç et
al., 2010). The characteristics of Lemna such as its small
size, high multiplication rates and vegetative propaga-
tion make it as a proper system to assess the aquatic
pollutants (Diyanati et al., 2013 and Zazouli et al., 2014).
Since L. minor can be found widely in Mazandaran
and as there are very few studies on L. minor in kinetic
analysis of pollution abatement, therefore, the present
study was carried out to investigate the RB19 dye removal
by dried L. minor. The effects of various parameters
including contact time, pH, adsorbent dose, temperature
and dye concentration were assessed along with also the
isotherm and kinetic studies which were also performed.
MATERIAL AND METHODS
Preparation of adsorbent: L. minor was collected from
rice elds of Sari, Iran. It was sun dried then crushed and
nally sieved to particle sizes in the range of 1–2 mm.
The biomass was treated with 0.1 M HCl for 5 h followed
by washing with distilled water and then dried in shade.
The resultant biomass was subsequently used in absorp-
tion experiments (Padmesh et al., 2006).
Instruments used for characterization: The surface images
of dried L. minor before and after adsorption process were
captured by scanning electron microscopy (SEM). The
SEM used was a Philips XL30. The speci c surface area of
dried L. minor before use was determined by the BET-N2
method using an ASAP 2000 apparatus based on nitrogen
adsorption–desorption isotherms at 77K.
The RB19 dye was obtained from Merck Company
(Germany). All chemicals used in this work were analyti-
cal reagent grade and used without further puri cation.
Stock solution was prepared by dissolving accurately
weighed dye in double distilled water. Experimental solu-
tions of the desired concentrations were obtained by
diluting the stock solution by using distilled water. RB19
(C
22
H
16
N
2
Na
2
O
11
S
3
) is a Azo dye, molecular weight of 626.5
g/mol, and its chemical structure is shown in Fig. 1.
FIGURE 1. The chemical structure of
Reactive Blue 19
ADSORPTION EXPERIMENTS
Various experimental conditions which may in uence
the biosorption of RB19 on dried L. minor including
contact time, pH, adsorbent dose, temperature and ini-
tial RB19 concentration, were tested using batch experi-
ments. Batch adsorption experiments were carried out
by adding a xed amount of sorbent (0.35 g) into 100
mL of different initial dye concentrations of solution.
To study the effect of adsorbent dose 25 mg/L dye
solution was prepared from the stock solution and the
different amount of sorbent was added (0.05- 0.5 g) to
the 100 mL of dye solution and the system is agitated in
a shaker for the equilibrium time of 75 min. To study the
effect of pH on equilibrium uptake capacity of L. minor
was measured by adding a xed amount of sorbent (0.35
g) into 100 mL of 25 mg/L RB19 dye solution having dif-
ferent pH such as 3-11. The pH of the dye solution was
varied by using 0.1 M HCL and 0.1M NaOH. The initial
and equilibrium dye concentrations were determined by
absorbance measurement using UV spectrophotometer
(DR 5000) at maximum 592 nm (Gök et al., 2007). All
batch experiments were carried out in triplicate. The
amount of adsorption at equilibrium, qe (mg/g), was
calculated by (Deniz et al., 2011):
(1)
Where C
0
and C
e
(mg/L) are the liquid-phase concentra-
tions of dye at initial and equilibrium respectively. V
is the volume of the solution (L) and M is the mass of
biomass used (g).
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Davoud Balarak et al.
FIGURE 2. The scanning electron microscopy (SEM) image of dried L. minor:
(a) before RB19 adsorption; (b) after RB19 adsorption (magni cation = 500).
FIGURE 3. Effect of contact time on RB19 biosorption (C
0
= 25 mg/L, adsorbent
dose 3.5 g/L, pH 3)
RESULTS AND DISCUSSION
CHARACTERIZATIONS OF DRIED
LEMNA MINOR
Dried L. minor was also examined before and after use
using environmental scanning electron microscopy. Fig.
2(a) clearly shows the pore textural structure of dried
L. minor before use. However, as show in Fig. 2(b) clear
pore textural structure is not observed on the surface
of dried L. minor after use which could be due to either
agglomeration on the surface or the incursion of RB19
into the pores of dried L. minor.
The speci c surface area is related to the number
of active adsorption sites of dried L. minor. For exam-
ple, Chandrasekhar and Vilar claimed that adsorption
increased with the speci c surface area and pore volume
of the sorbent (Chandrasekhar et al., 2006 and Vilar et al.,
2007). The surface area of dried L. minor was 30 m2/gr.
Effect of Contact Time and pH: The experiments were
performed to determine the optimum contact time in a
range of 10-150 min. As shown in Fig. 3, the adsorption
of RB19 on dried L. minor increased rapidly within the 30
min and then slowed from 45 min to 60 min and reach-
ing equilibrium after 75 min. The initial high adsorption
rate of RB19 on dried L. minor within the rst 30 min
was attributed to the high availability of binding sites
on the surface of dried L. minor, and the subsequent
lower biosorption rate after 30 min was decreased avail-
ability of binding sites on the surface of dried L. minor
due to absorption of initial RB19 molecules (Padmesh
et al., 2006). Similar results were observed for biosorp-
tion of reactive dyes on quaternized wood (Low et al.,
2000) and the biosorption of bromophenol blue dye
on Fungi (Zeroual et al., 2006). To ensure adsorption
reached equilibrium, contact time of 75 min was selected
for the remaining experiments.
Fig. 4. shows the plot of effect of solution pH on the
equilibrium uptake of RB19 using L. minor. The dye
uptake capacity was found to be more at pH 3. This
may be due to two sulfonate groups of RB19 are eas-
560 ADSORPTION OF REACTIVE BLUE 19 BY
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Davoud Balarak et al.
FIGURE 5. Effect of biomass dose on RB19 biosorption (C
0
= 25 mg/L, pH=3
Contact time=75 min)
ily dissociated at pH 3. Therefore pH 3 was taken as
optimum value. The dye uptake capacity is decreased
with increase the pH of the solution. This can be due
to the surface charge of the L. minor. At low pH the
active site on the sorbent is positively charged and can
absorb the RB19 dye, due to opposite charge attraction
between negatively charged dye anions and positively
charged adsorption sites. At high pH the surfaces are
probably negatively charged which repulse negatively
charged dye anions (Mane et al., 2007). The similar types
of results were obtained for sorption of Brilliant Green
dye from aqueous solutions onto rice husk and meth-
ylene blue onto Canola (Mane et al., 2007 and Balarak
et al., 2015).
Effect of Adsorbent Dose: The effect of adsorbent dose
on removal of RB19 was studied to determine an opti-
mum biosorbent dose. The tested biosorbent dosages
varied from 0.5 to 5 g/L using an initial RB19 concen-
tration of 25 mg/L and contact time of 75 min. As shown
in Fig. 5, the biosorption capacity of RB19 on the bio-
mass decreased from 19.6 to 4.97 mg/g, while the RB19
FIGURE 4. Effect of pH on RB19 removal ef ciency (C
0
= 25 mg/L, adsor-
bent dose of 3.5 g/L, contact time = 75 min)
562 ADSORPTION OF REACTIVE BLUE 19 BY
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removal percent increased from 39.2% to 99.4%, when
biosorbent dosage increased from 0.5 to 5 g/L. Lower
biosorption capacity of RB19 at a higher dosage of bio-
mass is probably due to the decrease of the surface area
of the biomass by the overlapping or aggregation dur-
ing the sorption (Dogan et al., 2008 and Mehmet et al.,
2004). However, the higher the dose of the biosorbent in
the solution, the greater the availability of active sites
for RB19, leading to the higher RB19 removal (Ho et al.,
2005 and Khattri et al., 2009). The removal of 99.4%
RB19 was observed when biomass dosage increased to
3.5 g/L and remained approximately constant with fur-
ther increases in biosorbent dosage. On the basis of both
biosorption capacity and the removal percentage, an
optimum biosorbent dosage of 3.5 g/L (0.35 g/100 mL)
was selected for all further experiments.
Isotherm studies: Isotherms are the equilibrium relations
between the concentration of the adsorbate on the solid
phase and its concentration in the liquid phase (Balarak
et al., 2015). The equilibrium biosorption data have been
analysed using Langmuir, Freundlich and Temkin iso-
therms. Analysis of such isotherms is important in order
to develop an equation that accurately represents the
results and could be used for design purposes.
The Langmuir isotherm model assumes the uniform
energies of adsorption onto the adsorbent surfaces. Fur-
thermore, the Langmuir equation is based on the assump-
tion of the existence of monolayer coverage of the adsorb-
ate at the outer surface of the adsorbent where all sorption
sites are identical. The Langmuir equation is given as fol-
lows (Garg et al., 2003 and Zazouli et al., 2014):
Where q
e
(mg/g) and C
e
(mg/L) are the amount of
adsorbed dye per unit mass of sorbent and unadsorbed
dye concentration in solution at equilibrium, respec-
tively. Q
max
is the maximum amount of the adsorbed dye
per unit mass of sorbent to form a complete monolayer
on the surface bound at high C
e
(mg/g), and K
L
(L/mg) is
a constant related to the af nity of the binding sites. The
plots of C
e
/q
e
vs. C
e
for the biosorption of RB19 onto L.
minor give a straight line of slope (1/q
max
) and intercept
(1/q
max
K
L
). The essential features of Langmuir can be
expressed in terms of dimensionless constant separation
factor RL which is calculated using the following equa-
tion equation (Gulnaz et al., 2011):
Values of R
L
indicate the shapes of isotherms to be either
unfavorable (R
L
> 1), linear (R
L
= 1), favorable (0 < R
L
< 1).
The Freundlich isotherm model equation is expressed
as (Diyanati et al., 2013 and Ong et al., 2007):
Where q
e
is the equilibrium dye concentration on the
adsorbent (mg/g); C
e
, the equilibrium dye concentration
in solution (mg/L); and K
F
is the Freundlich constant. In
this function, it is assumed that the sorbent has a surface
with a non-uniform distribution of sorption heat. This
equation was primarily proposed on a purely empirical
basis for adsorption phenomena occurring on gas–solid
interfaces, although it can be theoretically derived for an
adsorption model in which the heat of adsorption varies
exponentially with surface coverage. The slope of plot
1/n ranging 0 and 1, is a measure of adsorption intensity
or surface heterogeneity, becoming more heterogeneous
as its value gets closer to zero (Balarak et al., 2016).
The Temkin isotherm model suggests an equal dis-
tribution of binding energies over the number of the
exchanging sites on the surface. The linear form of the
Temkin isotherm equation is represented by the follow-
ing equation (Cicek et al., 2007 and Balarak et al., 2016):
(2)
Where B = RT/b, T is the absolute temperature in K, R
the universal gas constant (8.314 J/K mol), A the equi-
librium binding constant and the constant B is related to
the heat of adsorption.
The results of the isotherm constants are displayed in
Table 1. As shown in Table 1 that the correlation coef -
cients for the Langmuir isotherm model were close to 1.0
for all temperatures ( g. 6). The correlation coef cients
for Temkin isotherm were low and it can be said that the
experimental data was not tted better to the Temkin
isotherm model. The results indicated that the surface of
L. minor is homogeneous in nature and did not possess
equal distribution of binding energies on the available
binding sites. Finally the correlation coef cients for the
Freundlich and Temkin isotherm models were lower than
that of the Langmuir isotherm model.
Biosorption kinetics: In order to analyzed the biosorp-
tion kinetics of RB19 dye, the rst-order, pseudo-second-
order and intraparticle diffusion models were applied to
data obtained from the experiments. The rst-order rate
expression given as (Mittal et al., 2010):
Where q
e
and q
t
are the amounts of dye (mg/g) adsorbed
at equilibrium and time t, respectively, and k
1
is the rate
constant of adsorption (min
−1
) biosorption.
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Davoud Balarak et al.
FIGURE 6. Langmuir plots for the RB19 dye adsorption onto L. minor biomass
Table 1: Adsorption isotherm constants for the adsorption of RB19 onto L. minor at various temperatures
Tempkin modelFreundlich modelLangmuir model
R2ABR2KF (L/g)nR2RLKL(L/mg)qm
(mg/g)
qe exp
(mg/g)
Temp (K)
0.8390.6421.250.92114.252.740.9980.2710.02723.3122.44273
0.7990.4528.930.90410.442.170.9990.2080.03826.7224.95298
0.8240.2636.780.9366.791.650.9980.1630.05129.5327.38323
Table 2: Kinetic parameters for the adsorption of RB19 onto L. minor at various concentration
Con(mg/L) qe exp
(mg/g)
Pseudo- rst order Pseudo-second order Intraparticle diffusion
K1 qe R2 K2 qe R2 kdif C R2
25 7.12 0.0176 4.42 0.892 0.0098 7.76 0.999 0.471 4.46 0.812
50 13.75 0.0254 9.18 0.861 0.0071 12.95 0.998 0.583 3.24 0.798
100 25.54 0.0417 18.27 0.883 0.0052 24.84 0.999 0.772 2.97 0.826
200 46.51 0.0652 33.62 0.913 0.0027 48.17 0.998 0.942 1.78 0.845
The pseudo-second-order kinetic model is expressed
as (Pavan et al., 2008):
where q
e
is the biosorbed dye amount at equilibrium
(mg/g) for the pseudo-second-order biosorption, q
t
is the
amount of dye biosorbed at time t (mg/g) and k
2
is the
pseudo-second-order kinetic rate constant (g/mg min).
The intraparticle diffusion equation can be written as
follows (Dizge et al., 2008):
where C is the intercept, and kdif is the intraparticle dif-
fusion rate constant (mg g−1 min−1/2). The results of the
kinetic parameters for biosorption are given in Table 2.
The correlation coef cients for the pseudo-second-order
kinetic model were close to 1.0 for all concentration
564 ADSORPTION OF REACTIVE BLUE 19 BY
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Davoud Balarak et al.
( g. 7). The correlation coef cients for the rst-order
kinetics and intraparticle diffusion equation models were
lower than that the pseudo-second-order. These results
indicate that the biosorption of RB19 dye by L. minor
follows the pseudo-second-order kinetics model, which
relies on the assumption that the rate-limiting step may
be biosorption involving valence forces through the
sharing or exchange of electrons between biosorbent
and sorbate (Cicek et al., 2007). Waranusantigul sug-
gested that the removal of reactive dyes onto duck weed
obeyed the pseudo-second-order kinetic models (Wara-
nusantigul et al., 2003).
THERMODYNAMIC STUDIES
The feasibility of the adsorption process was evaluated
by the thermodynamic parameters including free energy
change (∆G), enthalpy (∆H), and entropy (∆S). (∆G)
was calculated from the following equation(Zazouli
et al., 2015 and Balarak et al., 2016):
where R is the universal gas constant (8.314 J/mol K), T is
the temperature (K), and K
L
is the distribution coef cient.
The K
d
value was calculated using following equation:
where q
e
and Ce are the equilibrium concentration
of RB19 dye on adsorbent (mg/L) and in the solution
(mg/L), respectively. enthalpy change (∆H), and entropy
change (∆S) of adsorption were estimated from the fol-
lowing equation:
The values of ∆G, ∆H, and ∆S for the adsorption of
RB19 dye onto L. minor at different temperatures are
given in Table 3. The negative values of ∆G in the tem-
perature range of 273–323 K indicated that the adsorp-
tion process was feasible and spontaneous. In addition,
the decrease in the values of ∆G with the increasing
temperature indicates the spontaneity of the process at
higher temperatures. Generally, the change of standard
free energy for physiosorption is in the range of -20 to
0 kJ/mol and for chemisorption varies between - 80 and
-400 kJ/mol. In the present study, the overall ∆G˚ has
values ranging from - 4.5 to - 8.28 kJ mol
-1
.
FIGURE 7. Pseudo-second-order kinetic plots for RB19 adsorption onto L. minor biomass
Table 4: Thermodynamic parameters for the adsorption
of AB19 on L. minor
T (K) ΔG0 (kJ/mol) ΔH0 (kJ/mol) ΔS0 (kJ/mol K)
273 −4.58
31.56 0.584298 −6.66
323 −8.28
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Davoud Balarak et al.
These results correspond to a spontaneous physi-
cal adsorption, indicates that this system does not gain
energy from external resource. The endothermic nature
was also con rmed from the positive values of enthalpy
change (∆H), while good af nity of RB19 towards the
adsorbent materials is revealed by the positive value of
∆S. This phenomenon had also been observed in the
adsorption of acid orang dye by cashew nut shell (Kumar
et al., 2010) and acidic dyes by Paenibacillus macerans
(Ferdag et al., 2009).
CONCLUSION
This study shows that L. minor is effective adsorbent for
the removal of RB19 dye from aqueous solution. The
adsorbent was the most effective at pH = 3. Adsorption
of RB19 onto L. minor increased with the increase in the
adsorbent dose and the optimum adsorbent dosage was
found to be 3.5 g/L. The equilibrium between the dye
and the adsorbent in the solution was established within
75 min. The best correlation was obtained using the
pseudo-second-order kinetic model. Equilibrium data
were also tted well to the Langmuir isotherm model.
Thermodynamic analyses indicated that the adsorption
of RB19 dyes onto L. minor was endothermic and spon-
taneous. The value of ∆H
o
was positive, indicating that
the adsorption reaction was endothermic. The positive
value of ∆S
o
re ects the af nity of L. minor for AB19
and suggests some structural changes in AB19 and L.
minor.
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