Accepted Manuscript Title: Biosorption of methylene blue onto Arthrospira platensis biomass: kinetic, equilibrium and thermodynamic studies Author: Dimitris Mitrogiannis Giorgos Markou Abuzer C¸elekli H¨useyin Bozkurt PII: DOI: Reference:
S2213-3437(15)00026-3 http://dx.doi.org/doi:10.1016/j.jece.2015.02.008 JECE 560
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Please cite this article as: Dimitris Mitrogiannis, Giorgos Markou, Abuzer C¸elekli, H¨useyin Bozkurt, Biosorption of methylene blue onto Arthrospira platensis biomass: kinetic, equilibrium and thermodynamic studies, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2015.02.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Biosorption of methylene blue onto Arthrospira platensis biomass: kinetic,
equilibrium and thermodynamic studies
Dimitris Mitrogiannisa*, Giorgos Markoua, Abuzer Çeleklic, Hüseyin Bozkurtd
Department of Natural Resources Management and Agricultural Engineering,
Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece b
Department of Biology, Faculty of Art and Science, University of Gaziantep, 27310
Gaziantep, Turkey c
Department of Food Engineering, Faculty of Engineering, University of Gaziantep,
27310 Gaziantep, Turkey *
Corresponding author: E-mail: [email protected]
Telephone: +30 6974876236
In this study, Arthrospira platensis biomass was employed as a biosorbent for the
removal of methylene blue (MB) dye from aqueous solutions. The kinetic data were
better described by the pseudo-second order model and equilibrium was established
within 60-120 min. The intra-particle diffusion was not the only rate-limiting step and
film diffusion might contribute to MB biosorption process. The increase of temperature
from 298 to 318 K caused a decrease of biosorption capacity. The Langmuir, Freundlich
and Dubinin-Radushkevich (D-R) isotherm models described well the experimental
equilibrium data at all studied temperatures. The maximum monolayer adsorption
capacity (qmax) was 312.5 mg MB/g at 298 K and pH 7.5. According to the results of the
thermodynamic analysis and the release of exchangeable cations from the biomass
surface, physical sorption and ion exchange were the dominant mechanisms of MB
biosorption at lower temperature. Methanol esterification of the dried biomass showed
the involvement of carboxyl functional groups in MB chemisorption. The thermodynamic
parameters indicated that MB biosorption onto A. platensis was a spontaneous, favorable
and exothermic process. The biosorption results showed that A. platensis could be
employed as an efficient and eco-friendly biosorbent for the removal of cationic dyes.
Keywords: Arthrospira platensis; methylene blue; cationic dye; thermodynamics;
biosorption mechanism; cation exchange
Synthetic dyes are hazardous pollutants which present toxic and aesthetic effects in
aquatic environments. Dye effluents, containing colored organic molecules, increase the
organic load of water bodies and reduce the sunlight penetration, affecting the
photosynthetic activity of phytoplankton and disturbing the ecological balance of the
aquatic environments. Moreover, some dyes display carcinogenic and mutagenic activity
[1, 2]. Potential sources of dyes are textile, leather, paper, printing, plastic, electroplating,
food and cosmetic industries.
Various physical, chemical and biological methods have been investigated for the
treatment of wastewaters contaminated with synthetic dyes . However, each of these
technologies has its disadvantages, such as high operational and initial capital costs, low
efficiency at low dye concentrations and production of undesirable sludge . Among
treatment technologies, adsorption is considered as an effective method for dye removal
using low-cost materials. Although activated carbon is the most commonly used
adsorbent and is very efficient to remove dyes from wastewater, it presents high costs of
production and regeneration . A number of studies have been made to find cost-
effective and eco-friendly methods for treatment of dye wastewaters using cheep
biomaterials as adsorbents .
Algae and cyanobacteria have gained interest as alternative biosorbents due to their
high binding affinity, their higher sorption selectivity for pollutants than commercial ion-
exchange resins and activated carbon, and due to their capability of growing using
wastewater as cultivation medium [3, 4, 6, 7]. The filamentous cyanobacterium
Arthrospira platensis is a potential biosorbent, having several advantages, such as relative
high growth rates, high biomass productivity, ease of cell harvesting and biomass
composition manipulation . The surface of A. platensis consists of various macro-
molecules with diverse functional groups such as carboxyl, hydroxyl, sulphate and
phosphate, which are responsible for dye binding . A. platensis has already been
studied for the removal of inorganic pollutants such as heavy metals [6, 10-12] and
organic pollutants such as anionic dyes [9, 13-15] and phenol [16, 17] from aqueous
solutions. To our knowledge, there is lack of published work about the adsorption of
cationic dyes onto A. platensis. The only related study to this, uses an artificial neural
network to predict the biosorption capacity of methylene blue onto Spirulina sp. .
However, there is no literature information about the biosorption kinetics and
thermodynamics of a cationic dye on this cyanobacterium and about the contribution of
the ion exchange mechanism on dye removal. Although the important role of the ion
exchange mechanism in MB removal by various biosorbents is mentioned very often, it
has not been widely investigated by detection measures .
Methylene blue (MB) is a common cationic dye used for dyeing paper, cotton, wool
and silk [7, 19]. The harmful effects of MB include: breathing difficulties, nausea,
vomiting, tissue necrosis, profuse sweating, mental confusion, cyanosis and
methemoglobinemia [5, 7]. MB has been widely employed as a model cationic dye in
adsorption studies, using low-cost adsorbents such as natural minerals (clays, zeolites,
perlite), activated carbon, dead or non-growing microbial biomass, agricultural and
industrial wastes .
The aim of the present study was to investigate the potential of A. platensis dry
biomass to remove MB dye from aqueous solutions. The effect of solution pH, initial MB
concentration, contact time, temperature and ionic strength on the biosorption capacity
was investigated. Kinetic, isotherm and thermodynamic parameters were estimated to
understand the biosorption rate and mechanisms of MB onto A. platensis.
2. Materials and methods
2.1. Biosorbent cultivation and preparation
The cyanobacterium A. platensis (SAG 21.99) used in this study was cultivated in
Zarrouk medium within 10 L plastic cubical photobioreactor, which were kept at 303 ± 2
K in semi-continuous cultivation mode with a dilution rate of 0.1 1/d . The A. platensis
biomass was harvested by filtration and rinsed with deionized (DI) water. The cultivation
medium salts were removed by washing the biomass twice by re-suspension in DI water.
After that the biomass was separated with centrifugation (5000 rpm for 5 min) and dried
overnight in an oven at 353 K. The dried biomass was milled (IKA Labortechnik, A10),
sieved through a metal sieve (100 mesh, particle diameter < 154 μm), and stored in a
plastic container inside an exsiccator containing silica gel to prevent moisture sorption by
the biomass. The chemical composition of the dried biomass consisted of 45-55%
proteins, 10-20% carbohydrates, and 5-7% lipids .
2.2. Preparation of dye solution
MB is a cationic dye with molecular formula C16H18N3SCl and molar weight of 319.85
g/mol. This cationic dye presents high water solubility at 293 K and is positively charged
on S atom . MB stock solution (1 g/L) was prepared by dissolving an appropriate
weighed amount of MB hydrate reagent (analytical grade, Sigma-Aldrich, India) in 1 L
DI water. The experimental solutions of desired initial concentrations were obtained by
dilution of MB stock solution with DI water.
2.3. Determination of pH zero point charge of A. platensis
To determine the zero point charge (pHzpc) of A. platensis biomass, the initial pH of 25
mL solutions containing 0.5 g/L of biosorbent and 0.1 M NaCl was adjusted at pH values
ranging from 3 to 9, using 0.1 M HNO3 and/or NaOH [19, 20]. The samples were agitated
for 24 h at 298 K, and the final pH values were measured using a pH-meter (Consort
P603, Belgium). Value of pHzpc was determined from the plot of final pH against initial
2.4. Batch biosorption experiments
The biosorption experiments were carried out in batch mode by mixing 12.5 mL
aqueous suspension containing 12.5 mg dried biomass with 12.5 mL MB dye solution of
known concentration. The final 25 mL solution was placed in a 50 mL plastic flask,
which was sealed and agitated with a rotary shaker at 140 rpm. The desired initial pH
(range 4-10) of the adsorbate and adsorbent solution was adjusted using 0.1 M HNO3
and/or NaOH before mixing them.
Biosorption kinetics were investigated with a biomass concentration of 0.5 g/L at three
initial dye concentrations (25, 50 and 100 mg/L) and pH 7.5±0.1. Samples were collected
at time intervals (2, 5, 10, 15, 30, 60, 90, 120, 180 and 240 min) and subjected to MB
concentration determination. The kinetic experiments were conducted in an air-
conditioned room with temperature of 298-300 K. Equilibrium experiments were carried
out at 298, 308 and 318 K, placing the flasks and shaker in a temperature controlled
incubator and using five different initial MB concentrations (6.25, 12.5, 25, 50, 100
mg/L), in order to estimate the parameters of isotherm models and thermodynamic
equations. The contact time of equilibrium experiments was chosen to be 24 h.
The amount of MB adsorbed per unit weight of A. platensis biomass at equilibrium, qe
(mg/g), and the percentage dye removal (R%), were calculated with the following
where Co (mg/L), C e (mg/L) and X (g/L) are the initial MB concentration, the MB
concentration at equilibrium, and the sorbent concentration in the solution, respectively. 6
The effect of ionic strength on the biosorption capacity was studied in solution
containing 0.5 g biosorbent/L, 50 mg MB/L and 0.0625-0.5 M NaCl at optimum pH (7.5).
For the investigation of the possible ion exchange mechanism involved in the biosorption
process, the concentration of cations Na+ and K+ released from the biomass after MB
sorption were determined. Biomass of 0.5 g/L was added in 50 mL solution containing
either DI water or 100 mg MB/L, which were shaken for 24 h at 298-318 K. The initial
pH of the dye solution was adjusted at 7.5±0.1 using dilute NH4OH and HCl solutions.
The cations released in the 0 mg MB/L solution containing only dried biomass were
considered as background concentration, which was subtracted from the cation amount
released after MB sorption in order to calculate the net cation release. Blank solution of
100 mg MB/L was also used to confirm no presence of cations.
2.5. Chemical modification of carboxyl groups on the biomass surface
The chemical modification of the dried biomass was applied to understand the role of
the surface carboxyl groups in MB sorption. The aim of the modification was to block the
carboxyl groups by esterification and then to determine the decrease of biosorption
capacity. The esterification of the dried biomass was carried out according to the method
described by Fang et al. . 1.0 g dried biomass of A. platensis was suspended in 50 mL
of 99.9% methanol solution and 0.6 mL concentrated HCl. The suspension was agitated
for 48 h at 333 K and allowed to cool at room temperature. The modified biomass was
washed three times by re-suspension in DI water. After that the biomass was separated
with centrifugation (5000 rpm for 5 min) and dried overnight in an oven at 323 K. For the
biosorption study, 100 mg of modified dried biomass were suspended in 100 mL DI water
and homogenized with a homogenizer (IKA-Labortechnick, Ultra Turrax T10, Germany).
Then 12.5 mL modified biomass suspension was mixed with 12.5 mL solution of 200 mg
MB/L. The final 25 mL solution containing 100 mg MB/L and 0.5 g/L of chemically
modified biosorbent was agitated for 24 h at 298 K and pH 7.5. The same procedure was
done for the untreated dried biomass of A. platensis for comparison purpose.
2.6. Analytical methods
For the determination of the unadsorbed MB concentration in each solution, 0.5 mL of
sample was withdrawn at the preselected time, t, and was placed in an Eppendorf type
centrifuge tube (1.5 mL), which contained 1 mL DI water. The diluted sample was
centrifuged for 2 min at 10000 rpm. The supernatant was collected, diluted with
appropriate DI water, and the MB concentrations were determined at the wavelength of
665 nm using a UV-vis spectrophotometer (Dr. Lange, Cadas 30, Germany). The
concentrations of Na+ and K+ were determined with a flame photometer (Sherwood
Scientific, model 400), followed by separation of the biomass from the sorption solution
by centrifugation at 10000 rpm for 5 min. All experiments were performed in triplicates
and the average values were recorded.
2.7. Mathematical models
2.7.1. Kinetic models
The biosorption kinetic experimental data were fitted with the following models:
The pseudo-first order model expressed by the following linearized form :
where q e (mg/g) and q t (mg/g) are the amount of adsorbed dye per gram of biomass at
equilibrium and at time t, respectively, and k1 (1/min) is the pseudo-first order rate
The pseudo-second order model expressed by the following linearized form :
where k2 (g/mg min) is the pseudo-second order rate constant.
197 198 199
The intra-particle diffusion model of Weber-Morris expressed by the following equation :
where kid (mg/g min0.5) is the intra-particle diffusion rate constant, and I (mg/g) is the y-
intercept which reflects the boundary layer thickness.
2.7.2. Equilibrium isotherm models
The biosorption equilibrium data were applied to the following isotherm models:
The Langmuir isotherm expressed by the following linearized form :
where q max (mg/g) is the maximum monolayer adsorption capacity, and KL (L/mg) is the
Langmuir isotherm constant related to the affinity and binding energy. The constant KL is
used for the prediction of the affinity between sorbate and biosorbent by the
dimensionless separation factor, RL, which is defined as :
where C o (mg/L) is the initial dye concentration.
The Freundlich isotherm expressed by the following linearized form :
where KF [(mg/g)(L/g)1/n] is the Freundlich isotherm constant representing the adsoption
capacity, and n is a dimensionless factor related to adsorption intensity and surface
224 225 226
The Dubinin-Radushkevich (D-R) isotherm expressed by the following linearized form :
where q s (mol/g) is the theoretical isotherm saturation capacity, KDR (mol2/kJ2) is the
Dubinin-Radushkevich isotherm constant, R (8.314 J/mol K) is the gas constant, and T
(K) the absolute temperature. 10
2.7.3. Goodness of model fit
The fit goodness of the applied mathematical models to the experimental data was
determined by the following three procedures: 1) The coefficient of determination (R2) to
the linearized data (linear regression), 2) The Composite Fractional Error Function
(CFEF) and 3) The Chi-square statistic (χ2). The last two non-linear functions, which
measure the difference between experimental and model predicted data, can be expressed
by the following equations :
where q e,exp (mg/g) and qe,cal (mg/g) are the experimental and model calculated values of
adsorption capacity, respectively, and n is the number of experimental samples. The
smaller the values of CFEF and χ2, the more similar are the calculated data to the
3. Results and discussion
3.1. Effect of initial solution pH
Fig. 1a shows the plot of initial pH versus final pH, wherein the pHzpc value (6.8) of A.
platensis was determined by the intersection point of both curves. This pHzpc value is
very similar with that reported in other studies [13, 15, 23] which found a pHzpc 7 for
Spirulina platensis using the method of the eleven points experiment [15, 23]. At pHzpc
the biosorbent surface is neutral.
The initial pH of the sorption solution is one of the most important factor of adsorption
process affecting the surface charge of the biosorbent and the ionization of the dye .
The surface charge distribution of a biosorbent depends on the kind and quantity of
functional groups, and the solution pH . Fig. 1b shows the effect of initial pH on the
MB biosorption onto A. platensis at equilibrium (24 h). It was observed that qe increased
as initial pH of the solution increased from 4 to 8, and then decreased at pH values of 9
and 10. Therefore, the initial pH of sorption solutions for the following experiments was
adjusted to 7.5±0.1.
At pH > pHzpc the biosorbent surface is negatively charged due to the deprotonation of
functional groups such as carboxyl, amino, phosphate and hydroxyl [13, 21], and thus
electrostatic attraction can occur between the negatively charged functional groups of
biosorbent surface and the positively charged cationic dye . In contrast, at pH < pHzpc
the biosorbent surface is positively charged and electrostatic repulsion occurs between
MB cations and A. platensis surface. At acidic pH, the H+ ions compete with MB cations
for available binding sites onto A. platensis . However, the remarkable qe at pH < pHzpc
where the most of the binding sites are protonated, suggests that hydrophobic interactions
also contributed to MB removal . In addition, based on typical deprotonation
constants for shortchained carboxylic groups (4 < pKa < 6), the increased MB binding in
the pH range of 4-6 may be also attributed to the deprotonation of carboxyl groups .
This was confirmed by the chemical modification of dried cells and the esterification of
surface carboxyl groups, which resulted to the decrease of the biosorption capacity (see
The decrease of qe at pH > 8 is difficult to be explained. Similar result was observed at
pH 9.5-11 for MB adsorption on cedar sawdust . Some of the reasons for the
biosorption decrease at high pH values might be the involvement of other adsorption
mechanisms such as ion exchange or chelation, or the hydrolysis of the biosorbent
surface which creates positively charged binding sites . In this study, it was observed
that the equilibrium pH (pHe) of the samples at initial pH 9 and 10 decreased by 0.85-
1.23 units, indicating that an exchange mechanism of H+ ions with MB cations occurred
(Fig. 1b). However, other dye-dye interactions such as an increased formation of MB
aggregates at higher pH, which are unable to enter into the pores of A. platensis, may be
responsible for the decreased q e at pH 9 and 10 .
3.2. Biosorption kinetics
Biosorption kinetic experiments were carried out at three initial MB concentrations
and at temperature of 298 K. As shown in Fig. 2a, the biosorption of MB onto A.
platensis was very rapid in the first 2-10 min for all studied concentrations. After the
rapid adsorption during the initial stage, the biosorption increased at a slower rate with
time and equilibrium was established within 60-120 minutes for all initial MB
concentrations. Equilibrium capacity did not changed significantly up to 24 h (data not
shown). The equilibrium time is in agreement with a previous work about MB
biosorption by Spirulina sp. .
The pseudo-first order model could not describe the kinetic data, because the plot of
log(qe-q t) versus t (Eq. 3) presented very low values for R2 (< 0.355) at all initial dye
concentrations investigated. Therefore, the kinetic parameters of this model are not
shown in Table 1.
The kinetic parameters qe and k2 of the pseudo-second order model, obtained from the
linear plots of t/q t versus t (Eq. 4), and the values of error functions are listed in Table 1.
Based on the linear regression analysis of the kinetic data (Fig. 2b), the pseudo-second
order model described very well the overall experimental data with R2 > 0.988. The
applicability of this model suggests that the biosorption rate was controlled by
chemisorption , involving exchange or sharing of electrons between the MB cations
and functional groups of the biomass surface . For the pseudo-second order kinetics,
the calculated q e values (qe,cal) agreed well with the experimental qe values (qe,exp) (Table
1). However, the nonlinear analysis of the kinetic data for the initial MB concentration of
50 mg/L showed relative high CFEF and χ2 values (Fig. 2a.), which are due to an
underestimation of the early time data (first 30 minutes) by the kinetic model .
The biosorption capacity (q e) at equilibrium, calculated from the pseudo-second order
model, increased with increasing initial MB concentration (Table 1). However, the
pseudo-second order rate constant (k2) decreased slightly when the initial MB
concentration increased from 25 to 100 mg/L, but its values [0.0134-0.0247 g/(mg min)]
demonstrated a same magnitude for all studied concentrations (Table 1). A decreasing
value of k2 suggests that the biosorption equilibrium capacity was established slower at
higher MB concentrations due to the limited quantity of binding sites at the biosorbent
surface . In addition, the nonlinear relationship between the rate constant values and
initial MB concentrations suggest that various mechanisms involved in the biosorption
process, such as ion exchange, chelation and physisorption .
The initial adsorption rate h (mg/g min) at 298 K was calculated from the pseudosecond order model parameters with the following equation :
and the values are shown in Table 1. It was found, that the initial adsorption rate h
increased from 18.52 to 138.89 mg/(g min) as the initial MB concentration increased
from 25 to 100 mg/L. This result suggests an increasing driving force between the liquid
and solid phase at higher dye concentrations and a decreasing diffusion time of MB
molecules from the solution to the binding sites . This observation is in agreement
with previous findings reported for MB adsorption on coconut bunch waste (Cocos
nucifera)  and marine algae Gelidium .
The half adsorption time or half-life, t0.5 (min), expresses the time required for the
biosorbent to remove the adsorbed amount of dye at equilibrium to its half, and is
calculated from the pseudo-second order model parameters with the following equation
As shown in Table 1, the estimated values of t 0.5 decreased from 1.479 to 0.581 min
when the initial MB concentration increased, indicating a faster biosorption . This
parameter is used as a measure of adsorption rate and to understand the operating time of
an adsorption system .
Fig. 3 shows the behaviour of the intra-particle diffusion model of Weber-Morris at
three initial MB concentrations and 298 K. This model was applied to the kinetic data in
order to determine the biosorption process mechanism and the rate controlling step. As
shown in Table 1, the values of R2 obtained from the linear regression plots of qt versus
t0.5 for the whole time data of the sorption process, were low (< 0.583). The low R2 values
suggest that the Weber-Morris model could not describe well the experimental data and
that the MB biosorption process was not limited by the intra-particle diffusion. However,
the calculated CFEF and χ2 values were very low (Table 1), suggesting that this model
fits well the experimental data for the overall time data. To the best of our knowledge,
there is no report known in literature about the intra-particle diffusion analysis of kinetic
data for cationic dyes onto A. platensis.
At all studied concentrations, the plot of q t versus t0.5 consists of three linear sections,
which do not pass through the origin (I ≠ 0). If I = 0, then the intra-particle diffusion is
the sole rate-limiting step. The multi-linearity of the plots suggests also that MB
biosorption onto A. platensis biomass took place in three phases. The first steeper section
represents the external mass transfer (film diffusion) of dye to biosorbent surface ,
which was completed very fast in the first 2-5 minutes of the process. The second linear
section (completed up to 90-120 min) describes a gradual sorption stage where intra-
particle diffusion is the rate-controlling step . The third linear section (starting after
120 min) represents the final equilibrium stage, where intra-particle diffusion starts to
slow down and an apparent saturation occurs [13, 34].
The high values of R2 (0.944 and 0.961 respectively) obtained from the second linear
sections of the intra-particle diffusion plot at initial dye concentrations of 50 and 100
mg/L, indicates that intra-particle diffusion occurred during this phase (Fig. 3, Table 1).
As shown in Table 1, the intra-particle diffusion rate constant, kid,,2, estimated from the
slope of the second linear section (Fig. 3), increased from 0.562 to 2.866 mg/(g min 0.5)
with the increasing initial dye concentration from 25 to 100 mg/L. This observation
shows a faster intra-particle diffusion at higher initial concentrations . For the same
linear section, the values of the y-intercept I increased from 22.05 to 58.94 mg/g when
the initial MB concentration increased. This result indicates an increasing boundary layer
effect and a greater involvement of the film diffusion at higher dye concentrations, for
this particular time range. Similar results for kid and I were observed for the biosorption
of phenol on Spirulina platensis nanoparticles .
3.3. Effect of initial MB concentration and temperature
Fig. 4 illustrates the effect of the initial MB concentration on the equilibrium
biosorption capacity of A. platensis at different temperatures. It was observed that qe
increased with the increase of initial MB concentration at all temperatures studied. At 298
K, the amount of MB adsorbed was 7.55 mg/g for the lowest initial MB concentration of
6.25 mg/L and increased to 89.56 mg/g for the highest initial MB concentration of 100
mg/L. This observation can be explained by the increasing driving force which overcome
the mass transfer resistance of MB dye between the aqueous and solid phase [1, 4].
Further, the number of collisions between MB cations and biosorbent can be increased
due to the increasing initial dye concentration, enhancing the sorption process . The
increasing driving force at higher dye concentrations is in agreement with the above
mentioned results for the initial adsorption rate h (at 298 K), which is estimated by the
parameters of the pseudo-second order kinetic model.
Although the enhancement of MB biosorption at higher initial dye concentrations was
also observed at 308 and 318 K, the values of qe for each initial concentration decreased
with the increasing solution temperature (Fig. 4). According to Dotto et al. , the
solubility of the dyes increases due to the temperature increase. As a result, the
interactions between MB molecules and the solvent are stronger than those between MB
and A. platensis. As shown in Fig. 4, the qe for the highest initial MB concentration of
100 mg/L, decreased from 89.56 mg/g at 298 K to 82.18 and 65.70 mg/g at 308 and 318
K, respectively. These results suggest the exothermic nature of MB sorption process and a
mechanism of physical sorption, dominant at lower temperatures . These findings are
further discussed by the thermodynamics analysis of isotherm experimental data in
The effect of the initial MB concentration on the percentage removal at different
temperatures is shown in Fig. 4. The percentage removal of MB at 298 K decreased from
60.4 to 44.8% when the initial dye concentration increased from 6.25 to 100 mg/L. The
same tendency of a decreasing percentage removal of MB was observed at 308 and 319
K. The only exception was the increase of percentage removal between the two lowest
initial MB concentrations of 6.25 and 12.5 mg/L at all temperatures studied. The negative
effect of the increasing initial dye concentration on the percentage removal may be due to
the saturation of the adsorption sites at higher MB concentrations . Similar results
were observed for the MB adsorption onto acid treated kenaf fibre char .
3.4. Biosorption isotherms
The relationship between the adsorbate (dye) concentration in the liquid phase and the
adsorbed dye amount per unit weight of biosorbent at equilibrium was analyzed using
three common isotherm models.
The calculated values of the adsorption isotherm parameters and error functions for
MB biosorption onto A. platensis are listed in Table 2. Based on the R2 values, the
Dubinin-Radushkevich model which was mainly used to investigate the MB sorption
mechanism, exhibited the best fit to the experimental data at all studied temperatures (R2
> 0.963). Although the Langmuir and Freundlich isotherm models presented satisfactory
and similar determination coefficients (R2 > 0.950 and 0.960, respectively), the
Freundlich model could better describe the experimental data than the Langmuir model
due to the lower CFEF and χ 2 values (Table 2).
Thus, the good and similar agreement of the three applied isotherm models with the
experimental data shows that the MB sorption was a complex process, involving more
than one mechanism . Both the monolayer biosorption and surface heterogeneity of
biosorbent affected the removal of MB from the solution , and no clear biosorption
saturation was occurred in the studied range of MB concentration .
The Langmuir model assumes a monolayer adsorption onto homogeneous surfaces
with finite number of binding sites and no interaction between adsorbate molecule [1, 4].
The constants qmax and KL were estimated from the intercept and slope of the linear plot
of experimental data of 1/q e versus 1/Ce (Fig. 5a).
The maximum monolayer adsorption capacity (qmax) decreased from 312.50 to 80.65
mg/g when the temperature increased from 298 to 318 K (Table 2). However, the
Langmuir constant K L increased with the increasing temperature (Table 2), indicating a
higher affinity (0.0414 L/mg) of A. platensis biomass for the MB molecules at 318 K.
The values of the dimensionless separation factor, RL, found to be less than unity and
greater than zero (0 < RL < 1) at all initial MB concentrations and temperatures,
confirming a favorable sorption process. If RL > 1 the adsorption is unfavorable. As
shown in Fig. 6, the higher the initial MB concentration, the lower the RL value and the
more favorable the MB biosoprtion.
A comparison of the maximum monolayer adsorption capacity (q max) for MB onto
various adsorbents [25, 26, 35-38] and that obtained onto A. platensis in this work, shows
that the cyanobacterium is an efficient biosorbent for the removal of MB from aqueous
solutions. According to recent studies, Spirulina platensis presented also a satisfactory
biosorption capacity for anionic dyes [9, 13, 23, 39].
The Freundlich model assumes a multilayer adsorption onto heterogeneous surfaces
with energetically non-equivalent binding sites and interactions between adsorbent
molecules . The constants KF and n were evaluated from the intercept and slope of the
linear plot of experimental data of ln(qe) versus ln(Ce) (Fig. 5b).
The values of the dimensionless Freundlich constant n related to the adsorption
intensity and surface heterogeneity, were higher than 1 and less than 10 (1 < n < 10) (see
Table 2), indicating a favorable sorption of MB onto A. platensis biomass at all studied
temperatures. No significant difference for n values was observed with respect to
temperature. The parameter ΚF represents a relative measure of adsorption capacity and
strength. When the equilibrium concentration Ce tends to be one, then ΚF reaches the
value of qe . As can be seen in Table 2, the values of ΚF increased slightly with the
rising temperature from 298 to 318 K, but decreased between 298 and 308 K. It shows
that the multilayer biosorption of MB was enhanced at higher solution temperature.
To distinguish between physical and chemical sorption, the mean free energy E (kJ/mol) of MB biosorption was calculated by the following equation:
where K DR (mol2/kJ2) is the constant of Dubinin-Radushkevich isotherm.
The parameter E is related to the mean free energy of sorption per molecule of sorbate,
assuming that the sorbate is transferred to the biosorbent surface from infinite distance in
the solution. Typical values of E for chemical sorption are in the range of 8–16 kJ/mol,
while E < 8 kJ/mol indicates physical sorption . As shown in Table 2, the mean free
energy E of MB biosorption onto A. platensis suggests a chemisorption mechanism,
because its values are in the range of 8-16 kJ/mol at all studied temperatures. The
increasing temperature caused a slight increase of E from 9.09 to 10.77 kJ/mol, indicating
an enhancement of the chemisorption at higher temperatures. The biosorption
mechanisms are further discussed in Section 3.7.
3.5. Biosorption thermodynamics
The thermodynamic behavior of MB biosorption onto A. platensis biomass was
investigated estimating the thermodynamic parameters of Gibbs free energy change
(ΔG°), enthalpy change (ΔΗ°) and entropy change (ΔS°). The values of these parameters
were estimated using the following equations :
ΔG° = -R T lnKc
ΔG° = ΔH° - TΔS°
485 486 487 488
where R is the universal gas constant [8.314 J/(mol K)], T the absolute solution
temperature (K), and Kc (Cad,e/Ce) is the adsorption equilibrium constant, which is the
ratio of the MB concentration adsorbed (Cad,e) to the MB concentration (Ce) in solution at
The negative values of ΔG° indicates a spontaneous and favorable adsorption process
at all studied temperatures and initial concentrations (see Table 3), suggesting that the
system required no energy input from outside . Similar thermodynamic behavior in
respect to negative ΔG° values has been found for Spirulina platensis dry biomass as a
biosorbent of anionic dyes [13, 23, 39]. For a given initial MB concentration in this work,
no significant change of ΔG° was observed with increasing temperature. However, the
ΔG° values decreased slightly as the initial MB concentration increased from 50 to 100
mg/L, indicating a more favorable adsorption of MB at lower dye concentration.
The values of enthalpy change (ΔΗ°) and entropy change (ΔS°) can be calculated from
the slope and intercept of the linear plot of lnKc versus 1/T, based on the Eq. (17). As 22
shown in Fig. 7, the determination coefficient (R2) of the plots was 0.939 and 0.940 for
the two highest initial MB concentrations, respectively, indicating that the estimated
values of ΔΗ° and ΔS° were confident. As can be seen in Table 3, the negative values of
ΔH° at all studied initial dye concentrations corresponds to an exothermic nature of MB
biosorption. Similar results for the cyanobacterium in respect to negative ΔH° values
obtained by other studies, which found an exothermic biosorption of anionic dyes [13, 23,
39] and phenol  onto Spirulina platensis dry biomass.
There are different results in the literature in respect to the exothermic or endothermic
nature of MB adsorption onto various materials, based on the estimated ΔH° values. An
exothermic adsorption of MB was found onto cyclodextrin/silica hybrid adsorbent 
and green algae Ulothrix sp. . On the other hand, an endothermic adsorption of MB
was found onto diatomite treated with sodium hydroxide , marble dust ,
montmorillonite clay , and acid treated kenaf fibre char .
The magnitude of enthalpy change can be used to classify the type of interaction
between sorbent and sorbate. Values of ΔH° < 30 kJ/mol indicates a physical sorption
such as hydrogen bonding . Other mechanisms of physical sorption such as Van der
Waals forces usually presents ΔH° values in the range 4-10 kJ/mol, hydrophobic bonds
forces about 5 kJ/mol, coordination exchange about 40 kJ/mol and dipole bond forces 2-
29 kJ/mol . In contrast, ΔH° > 80 kJ/mol indicates chemical bond forces and a
chemisorption process [13, 17, 20]. According to the ΔH° values (< 28.32 kJ/mol)
obtained in this study, the biosorption of MB dye onto A. platensis biomass was due to
physical adsorption, suggesting weak interactions between biomass and cationic dye .
Further, the negative effect of increasing temperature on qe (Fig. 4) and the applicability
of the pseudo-second order kinetic model showed that MB sorption process involved both
mainly physical and partly chemical sorption . The low negative values of ΔG° ranging
from -20 to 0 kJ/mol suggest that the dominant biosorption mechanism was physisorption
The weak binding and weak interactions between the biosorbent and the adsorbate
showed that the adsorbed MB molecules should be easily released . This point should
be further investigated in order to evaluate the regeneration and reuse ability of A.
platensis after dye desorption, in order to reduce the cost of the biosorption process.
The negative values of ΔS° for 50 and 100 mg MB/L were very low, indicating no
remarkable change on entropy  and a decreased disorder at the solid-liquid interface
during the MB biosorption onto A. platensis (see Table 3). This showed also that the
dispersion degree of the process decreased with increasing temperature . Based on the
Eq. (16) and the different magnitude of ΔH° and ΔS° values (Table 3), the enthalpy
change (ΔH°) contributed more than entropy change (ΔS°) to obtain the negative values
of ΔG° . This observation suggests that MB biosorption onto A. platensis was an
enthalpy-controlled process .
3.6. Effect of ionic strength
Dye effluents contain high concentrations of salts which affect the dye sorption onto
biosorbents. Fig. 8 presents the effect of ionic strength on the MB biosorption by A.
platensis at 298 K and pH 7.5. It was observed that qe decreased as the NaCl
concentration in sorption solution increased from 0.0625 to 0.5 M. The decrease of q e is
due to the competitive effect between Na+ and MB cations for the available surface
binding sites  and the electrostatically screening effect of salt . The latter
indicates that the electrostatic interactions should be one the main driving forces during
MB biosorption process . However, the remarkable biosorption capacity observed
even in the presence of much higher NaCl concentration (62.5 mmol/L) than the initial
MB concentration of 50 mg/L ( = 0.156 mmol/L) suggests that other interactions such as
hydrophobic interactions, π-π interactions and/or hydrogen bonding, contributed to MB
3.7. Biosorption mechanisms
The amounts of Na+ and K+ cations released from A. platensis surface into the solution
after MB sorption are listed in Table 4. Based on the total net cations release at 298 K, it
is evident that the cation exchange was one of the major biosorption mechanisms at this
temperature. In contrast, the net cations release at higher temperatures was negligible.
Besides, no significant change between initial and equilibrium pH was observed at all
studied temperatures (Table 4), suggesting that ion exchange between MB cations and
protons (H+) of surface functional groups did not take place at pH 7.5. A previous study
has confirmed the presence of Na+ and K+ on the cell wall surface of Spirulina sp. .
The total release of both cations measured in mg/L (data not shown) constituted up to
4.7% of the dried biomass weigth (500 mg/L), which agrees with the ash percentage (6.3-
7%) in the chemical composition of S. platensis dried biomass reported in the literature
[9, 39]. The mechanism of cation exchange between MB molecule and the exchangeable
cations of biomass surface can be described by the following equations :
S-O-K + CN+ → S-O-CN + K+
S-O-Na + CN+ → S-O-CN + Na+
where S is the surface of A. platensis biomass, Na+ and K+ are the exchangeable cations,
and CN+ is the positively charged nitrogen atom of the secondary amine group of MB
Fig. 9 shows the effect of the chemical modification of carboxyl groups on the
biosorption capacity. The esterified biomass of A. platensis presented a decrease in the
MB biosorption capacity (62.66 mg/g) by 25.5% compared to the biosorption capacity of
the untreated biomass (83.83 mg/g) (Fig. 9), due to the block of the surface carboxyl
groups. This result indicated the participation of carboxyl groups in the MB binding by
the untreated biomass, which is a chemisorption process. The cell wall of cyanobacteria
contains a thick structural layer of peptidoglycan and an extended layer of glycoproteins
and polysaccharides. These layers are the main source of reactive carboxyl groups on the
biosorbent surface . The reaction of the chemical esterification of surface carboxyl
groups is described by the following equation, where R are all the components in the
dried cells :
592 593 594
RCOOH + CH3OH → RCOOCH3 + H2O (20)
Recent studies for the removal of anionic dyes from aqueous solutions confirmed the
mesoporous structure of S. platensis dried microparticles which presented a particle size
in the range of 68-75 μm and an average pore radius of 2.25 nm (22.5 Å) [9, 13]. Note
that the average pore radius was not modified even in case of S. platensis nanoparticles
obtained from the microparticles through a mechanical method . Therefore, the A.
platensis microparticles (with particle diameter <154 μm) employed in this study might
have a mesoporous structure with a similar average pore diameter of around 4.5 nm. On
the other hand, the MB molecule has a parallelepiped shape with dimensions 1.7 × 0.76 ×
0.325 nm and its attachement on biomass surface may be done with different orientations
. Other workers have reported that the presence of mesopores (average pore diameter
of 2-50 nm) is favorable for MB adsorption by various adsorbents [5, 25]. Assuming that
the MB molecule lies flat on the biomass surface even on its largest face (1.7 nm) which
is smaller than the reported average pore radius of A. platensis (2.25 nm), the MB
biosorption in this study may also be due to the intraparticle diffusion of MB molecules
in the mesopores and due to the entrapment in intrafibrillar capillaries and spaces of the
structural exopolysaccharides . This assumption agrees with the diffusion analysis of
the kinetic data. Therefore, the mesoporous structure of A. platensis can facilitate the
accommodation of MB molecules in the biomass pores .
Dry biomass of A. platensis were used as biosorbent for methylene blue removal in
batch mode with respect to solution pH, contact time, initial dye concentration,
temperature and ionic strength. This study applied for the first time a kinetic and
thermodynamic analysis for the biosorption of a cationic dye onto A. platensis. In
addition, the role of ion exchange mechanism was directly investigated by detection
measures. The kinetic data were fitted very well by the pseudo-second order model, and
equilibrium was achieved within 60-120 min. It was found that the film and intra-particle
diffusion contributed to the MB biosorption process. The biosorption capacity of A.
platensis for MB increased with increasing initial dye concentration and decreased with
increasing temperature. At all studied temperatures, the Langmuir, Freundlich and
Dubinin-Radushkevich isotherm models fitted well the experimental equilibrium data,
indicating that MB biosorption was a complex process, involving more than one
mechanism. The carboxyl groups of biomass surface contributed to MB chemisorption.
The important role of hydrophobic interactions in MB removal was indicated by the
considerable biosorption capacity at low pH values and in the presence of NaCl in the
sorption solution. The release of Na+ and K+ cations from the biomass surface in the
solution after MB sorption confirmed the contribution of cation exchange mechanism.
Physical sorption and ion exchange were the dominant mechanisms of MB biosorption at
lower temperature. According to the thermodynamic analysis of equilibrium data, MB
biosorption onto A. platensis was a spontaneous, favorable and exothermic process. It
was concluded that A. platensis biomass has a great potential for removal of MB from
Professor D. Georgakakis of Agricultural University of Athens is kindly acknowledged
for his valuable support in respect of the availability of laboratory equipment.
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762 763 764 765 766 767 768 769 770
Fig. 1. (a) Plot of initial pH versus final pH for the determination of biomass pHzpc, and
(b) the effect of initial pH on MB biosorption onto A. platensis (pHe = equilibrium pH).
Fig. 2. (a) Effect of contact time on MB biosorption onto A. platensis at three different initial MB concentrations (biomass dosage = 0.5 g/L, pH 7.5, temperature = 298 K). Symbols and curves represent experimental data and fitted pseudo-second order kinetic model, respectively. (b) Pseudo-second order linear plots for MB biosorption onto A. platensis biomass. 776
Fig. 3. Intra-particle diffusion of MB cationic dye onto A. platensis at three different initial MB concentrations and 298 K. 777
Fig 4. Effect of initial MB concentration on the percentage removal of MB and the biosorption capacity of A. platensis at different temperatures. 778
Fig. 5. Linear plots of (a) Langmuir and (b) Freundlich isotherm model for the MB biosorption onto A. platensis at different temperatures. 779
Fig. 6. Relationship between initial MB concentration and dimensionless separation factor RL at different temperatures. 780
Fig. 7. Plots of lnKc versus 1/T for the estimation of thermodynamic parameters of MB biosorption onto A. platensis. 781
Fig. 8. Effect of ionic strength on MB biosorption onto A. platensis (C0 = 50 mg MB/L, pH = 7.5, temperature = 298 K). 782
Fig. 9. Biosorption of MB onto untreated and chemically modified biomass of A. platensis at 298 K (C0 = 100 mg MB/L, pH = 7.5). 783 784 785 786 787
Table 1. Kinetic and diffusion model parameters for MB biosorption onto A. platensis. Initial dye concentration (mg/L) 25
k2 (g/ mg min)
h (mg/g min)
kid (mg/ g min0.5)
kid,2 (mg/ g min0.5)
qe,exp (mg/g) Pseudo-first order model R2 Pseudo-second
intra-particle diffusion model: whole time data
intra-particle diffusion model: second linear section
Table 2. Isotherm parameters values of MB biosoprtion onto A. platensis at different temperatures. Isotherm models
Solution temperature (K) 298
6.05 × 10-9
5.85 × 10-9
4.31 × 10-9
4.98 × 10-6
4.73 × 10-6
4.93 × 10-6
5.42 × 10-6
4.46 × 10-6
5.40 × 10-6
qe,exp (mg/g) Langmuir qmax (mg/g) 1
Dubinin-Radushkevich qs (mol/g) BD (mol2/kJ2) E (kJ/mol) R
qe,cal corresponds to C0 = 100 mg/L.
Table 3. Thermodynamic parameters of MB biosorption onto A. platensis biomass. C0 (mg/L)
ΔG° (kJ/mol) 298 K
Table 4. Amount of cations released from A. platensis biomass (0.5 g/L) after MB biosorption (C0 = 100 mg/L, pH = 7.5). Cations released
Temperature (K) 298
After MB biosorption
After MB biosorption
Total net release (mmol/L)
qe (mmol MB/g)