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Title: Biosorption of methylene blue onto Arthrospiraplatensis biomass: kinetic, equilibrium and thermodynamicstudies
Author: Dimitris Mitrogiannis Giorgos Markou AbuzerCelekli Huseyin Bozkurt
PII: S2213-3437(15)00026-3DOI: http://dx.doi.org/doi:10.1016/j.jece.2015.02.008Reference: JECE 560
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Please cite this article as: Dimitris Mitrogiannis, Giorgos Markou, Abuzer Celekli,Huseyin Bozkurt, Biosorption of methylene blue onto Arthrospira platensis biomass:kinetic, equilibrium and thermodynamic studies, Journal of Environmental ChemicalEngineering 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 proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.
1
Biosorption of methylene blue onto Arthrospira platensis biomass: kinetic, 1
equilibrium and thermodynamic studies 2
3
Dimitris Mitrogiannisa*, Giorgos Markoua, Abuzer Çeleklic, Hüseyin Bozkurtd 4
a Department of Natural Resources Management and Agricultural Engineering, 5
Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece 6
b Department of Biology, Faculty of Art and Science, University of Gaziantep, 27310 7
Gaziantep, Turkey 8
c Department of Food Engineering, Faculty of Engineering, University of Gaziantep, 9
27310 Gaziantep, Turkey 10
* Corresponding author: E-mail: [email protected] 11
Telephone: +30 6974876236 12
13
Abstract 14
In this study, Arthrospira platensis biomass was employed as a biosorbent for the 15
removal of methylene blue (MB) dye from aqueous solutions. The kinetic data were 16
better described by the pseudo-second order model and equilibrium was established 17
within 60-120 min. The intra-particle diffusion was not the only rate-limiting step and 18
film diffusion might contribute to MB biosorption process. The increase of temperature 19
from 298 to 318 K caused a decrease of biosorption capacity. The Langmuir, Freundlich 20
and Dubinin-Radushkevich (D-R) isotherm models described well the experimental 21
equilibrium data at all studied temperatures. The maximum monolayer adsorption 22
capacity (qmax) was 312.5 mg MB/g at 298 K and pH 7.5. According to the results of the 23
2
thermodynamic analysis and the release of exchangeable cations from the biomass 24
surface, physical sorption and ion exchange were the dominant mechanisms of MB 25
biosorption at lower temperature. Methanol esterification of the dried biomass showed 26
the involvement of carboxyl functional groups in MB chemisorption. The thermodynamic 27
parameters indicated that MB biosorption onto A. platensis was a spontaneous, favorable 28
and exothermic process. The biosorption results showed that A. platensis could be 29
employed as an efficient and eco-friendly biosorbent for the removal of cationic dyes. 30
31
Keywords: Arthrospira platensis; methylene blue; cationic dye; thermodynamics; 32
biosorption mechanism; cation exchange 33
34
1. Introduction 35
Synthetic dyes are hazardous pollutants which present toxic and aesthetic effects in 36
aquatic environments. Dye effluents, containing colored organic molecules, increase the 37
organic load of water bodies and reduce the sunlight penetration, affecting the 38
photosynthetic activity of phytoplankton and disturbing the ecological balance of the 39
aquatic environments. Moreover, some dyes display carcinogenic and mutagenic activity 40
[1, 2]. Potential sources of dyes are textile, leather, paper, printing, plastic, electroplating, 41
food and cosmetic industries. 42
Various physical, chemical and biological methods have been investigated for the 43
treatment of wastewaters contaminated with synthetic dyes [3]. However, each of these 44
technologies has its disadvantages, such as high operational and initial capital costs, low 45
efficiency at low dye concentrations and production of undesirable sludge [4]. Among 46
3
treatment technologies, adsorption is considered as an effective method for dye removal 47
using low-cost materials. Although activated carbon is the most commonly used 48
adsorbent and is very efficient to remove dyes from wastewater, it presents high costs of 49
production and regeneration [5]. A number of studies have been made to find cost-50
effective and eco-friendly methods for treatment of dye wastewaters using cheep 51
biomaterials as adsorbents [3]. 52
Algae and cyanobacteria have gained interest as alternative biosorbents due to their 53
high binding affinity, their higher sorption selectivity for pollutants than commercial ion-54
exchange resins and activated carbon, and due to their capability of growing using 55
wastewater as cultivation medium [3, 4, 6, 7]. The filamentous cyanobacterium 56
Arthrospira platensis is a potential biosorbent, having several advantages, such as relative 57
high growth rates, high biomass productivity, ease of cell harvesting and biomass 58
composition manipulation [8]. The surface of A. platensis consists of various macro-59
molecules with diverse functional groups such as carboxyl, hydroxyl, sulphate and 60
phosphate, which are responsible for dye binding [9]. A. platensis has already been 61
studied for the removal of inorganic pollutants such as heavy metals [6, 10-12] and 62
organic pollutants such as anionic dyes [9, 13-15] and phenol [16, 17] from aqueous 63
solutions. To our knowledge, there is lack of published work about the adsorption of 64
cationic dyes onto A. platensis. The only related study to this, uses an artificial neural 65
network to predict the biosorption capacity of methylene blue onto Spirulina sp. [18]. 66
However, there is no literature information about the biosorption kinetics and 67
thermodynamics of a cationic dye on this cyanobacterium and about the contribution of 68
the ion exchange mechanism on dye removal. Although the important role of the ion 69
4
exchange mechanism in MB removal by various biosorbents is mentioned very often, it 70
has not been widely investigated by detection measures [7]. 71
Methylene blue (MB) is a common cationic dye used for dyeing paper, cotton, wool 72
and silk [7, 19]. The harmful effects of MB include: breathing difficulties, nausea, 73
vomiting, tissue necrosis, profuse sweating, mental confusion, cyanosis and 74
methemoglobinemia [5, 7]. MB has been widely employed as a model cationic dye in 75
adsorption studies, using low-cost adsorbents such as natural minerals (clays, zeolites, 76
perlite), activated carbon, dead or non-growing microbial biomass, agricultural and 77
industrial wastes [7]. 78
The aim of the present study was to investigate the potential of A. platensis dry 79
biomass to remove MB dye from aqueous solutions. The effect of solution pH, initial MB 80
concentration, contact time, temperature and ionic strength on the biosorption capacity 81
was investigated. Kinetic, isotherm and thermodynamic parameters were estimated to 82
understand the biosorption rate and mechanisms of MB onto A. platensis. 83
84
2. Materials and methods 85
2.1. Biosorbent cultivation and preparation 86
The cyanobacterium A. platensis (SAG 21.99) used in this study was cultivated in 87
Zarrouk medium within 10 L plastic cubical photobioreactor, which were kept at 303 ± 2 88
K in semi-continuous cultivation mode with a dilution rate of 0.1 1/d [6]. The A. platensis 89
biomass was harvested by filtration and rinsed with deionized (DI) water. The cultivation 90
medium salts were removed by washing the biomass twice by re-suspension in DI water. 91
After that the biomass was separated with centrifugation (5000 rpm for 5 min) and dried 92
5
overnight in an oven at 353 K. The dried biomass was milled (IKA Labortechnik, A10), 93
sieved through a metal sieve (100 mesh, particle diameter < 154 μm), and stored in a 94
plastic container inside an exsiccator containing silica gel to prevent moisture sorption by 95
the biomass. The chemical composition of the dried biomass consisted of 45-55% 96
proteins, 10-20% carbohydrates, and 5-7% lipids [6]. 97
98
2.2. Preparation of dye solution 99
MB is a cationic dye with molecular formula C16H18N3SCl and molar weight of 319.85 100
g/mol. This cationic dye presents high water solubility at 293 K and is positively charged 101
on S atom [20]. MB stock solution (1 g/L) was prepared by dissolving an appropriate 102
weighed amount of MB hydrate reagent (analytical grade, Sigma-Aldrich, India) in 1 L 103
DI water. The experimental solutions of desired initial concentrations were obtained by 104
dilution of MB stock solution with DI water. 105
106
2.3. Determination of pH zero point charge of A. platensis 107
To determine the zero point charge (pHzpc) of A. platensis biomass, the initial pH of 25 108
mL solutions containing 0.5 g/L of biosorbent and 0.1 M NaCl was adjusted at pH values 109
ranging from 3 to 9, using 0.1 M HNO3 and/or NaOH [19, 20]. The samples were agitated 110
for 24 h at 298 K, and the final pH values were measured using a pH-meter (Consort 111
P603, Belgium). Value of pHzpc was determined from the plot of final pH against initial 112
pH. 113
114
2.4. Batch biosorption experiments 115
6
The biosorption experiments were carried out in batch mode by mixing 12.5 mL 116
aqueous suspension containing 12.5 mg dried biomass with 12.5 mL MB dye solution of 117
known concentration. The final 25 mL solution was placed in a 50 mL plastic flask, 118
which was sealed and agitated with a rotary shaker at 140 rpm. The desired initial pH 119
(range 4-10) of the adsorbate and adsorbent solution was adjusted using 0.1 M HNO3 120
and/or NaOH before mixing them. 121
Biosorption kinetics were investigated with a biomass concentration of 0.5 g/L at three 122
initial dye concentrations (25, 50 and 100 mg/L) and pH 7.5±0.1. Samples were collected 123
at time intervals (2, 5, 10, 15, 30, 60, 90, 120, 180 and 240 min) and subjected to MB 124
concentration determination. The kinetic experiments were conducted in an air-125
conditioned room with temperature of 298-300 K. Equilibrium experiments were carried 126
out at 298, 308 and 318 K, placing the flasks and shaker in a temperature controlled 127
incubator and using five different initial MB concentrations (6.25, 12.5, 25, 50, 100 128
mg/L), in order to estimate the parameters of isotherm models and thermodynamic 129
equations. The contact time of equilibrium experiments was chosen to be 24 h. 130
The amount of MB adsorbed per unit weight of A. platensis biomass at equilibrium, qe 131
(mg/g), and the percentage dye removal (R%), were calculated with the following 132
equations: 133
(1) 134
135
(2) 136
137
where Co (mg/L), Ce (mg/L) and X (g/L) are the initial MB concentration, the MB 138
concentration at equilibrium, and the sorbent concentration in the solution, respectively. 139
7
The effect of ionic strength on the biosorption capacity was studied in solution 140
containing 0.5 g biosorbent/L, 50 mg MB/L and 0.0625-0.5 M NaCl at optimum pH (7.5). 141
For the investigation of the possible ion exchange mechanism involved in the biosorption 142
process, the concentration of cations Na+ and K+ released from the biomass after MB 143
sorption were determined. Biomass of 0.5 g/L was added in 50 mL solution containing 144
either DI water or 100 mg MB/L, which were shaken for 24 h at 298-318 K. The initial 145
pH of the dye solution was adjusted at 7.5±0.1 using dilute NH4OH and HCl solutions. 146
The cations released in the 0 mg MB/L solution containing only dried biomass were 147
considered as background concentration, which was subtracted from the cation amount 148
released after MB sorption in order to calculate the net cation release. Blank solution of 149
100 mg MB/L was also used to confirm no presence of cations. 150
151
2.5. Chemical modification of carboxyl groups on the biomass surface 152
153
The chemical modification of the dried biomass was applied to understand the role of 154
the surface carboxyl groups in MB sorption. The aim of the modification was to block the 155
carboxyl groups by esterification and then to determine the decrease of biosorption 156
capacity. The esterification of the dried biomass was carried out according to the method 157
described by Fang et al. [21]. 1.0 g dried biomass of A. platensis was suspended in 50 mL 158
of 99.9% methanol solution and 0.6 mL concentrated HCl. The suspension was agitated 159
for 48 h at 333 K and allowed to cool at room temperature. The modified biomass was 160
washed three times by re-suspension in DI water. After that the biomass was separated 161
with centrifugation (5000 rpm for 5 min) and dried overnight in an oven at 323 K. For the 162
8
biosorption study, 100 mg of modified dried biomass were suspended in 100 mL DI water 163
and homogenized with a homogenizer (IKA-Labortechnick, Ultra Turrax T10, Germany). 164
Then 12.5 mL modified biomass suspension was mixed with 12.5 mL solution of 200 mg 165
MB/L. The final 25 mL solution containing 100 mg MB/L and 0.5 g/L of chemically 166
modified biosorbent was agitated for 24 h at 298 K and pH 7.5. The same procedure was 167
done for the untreated dried biomass of A. platensis for comparison purpose. 168
169
2.6. Analytical methods 170
For the determination of the unadsorbed MB concentration in each solution, 0.5 mL of 171
sample was withdrawn at the preselected time, t, and was placed in an Eppendorf type 172
centrifuge tube (1.5 mL), which contained 1 mL DI water. The diluted sample was 173
centrifuged for 2 min at 10000 rpm. The supernatant was collected, diluted with 174
appropriate DI water, and the MB concentrations were determined at the wavelength of 175
665 nm using a UV-vis spectrophotometer (Dr. Lange, Cadas 30, Germany). The 176
concentrations of Na+ and K+ were determined with a flame photometer (Sherwood 177
Scientific, model 400), followed by separation of the biomass from the sorption solution 178
by centrifugation at 10000 rpm for 5 min. All experiments were performed in triplicates 179
and the average values were recorded. 180
181
2.7. Mathematical models 182
2.7.1. Kinetic models 183
The biosorption kinetic experimental data were fitted with the following models: 184
185
9
The pseudo-first order model expressed by the following linearized form [4]: 186
(3) 187
188
where qe (mg/g) and qt (mg/g) are the amount of adsorbed dye per gram of biomass at 189
equilibrium and at time t, respectively, and k1 (1/min) is the pseudo-first order rate 190
constant. 191
192
The pseudo-second order model expressed by the following linearized form [4]: 193
(4) 194
195
where k2 (g/mg min) is the pseudo-second order rate constant. 196
197
The intra-particle diffusion model of Weber-Morris expressed by the following 198
equation [6]: 199
(5) 200
201
where kid (mg/g min0.5) is the intra-particle diffusion rate constant, and I (mg/g) is the y- 202
intercept which reflects the boundary layer thickness. 203
204
2.7.2. Equilibrium isotherm models 205
The biosorption equilibrium data were applied to the following isotherm models: 206
The Langmuir isotherm expressed by the following linearized form [22]: 207
(6) 208
10
209
where qmax (mg/g) is the maximum monolayer adsorption capacity, and KL (L/mg) is the 210
Langmuir isotherm constant related to the affinity and binding energy. The constant KL is 211
used for the prediction of the affinity between sorbate and biosorbent by the 212
dimensionless separation factor, RL, which is defined as [23]: 213
(7) 214
215
where Co (mg/L) is the initial dye concentration. 216
217
The Freundlich isotherm expressed by the following linearized form [24]: 218
(8) 219
220
where KF [(mg/g)(L/g)1/n] is the Freundlich isotherm constant representing the adsoption 221
capacity, and n is a dimensionless factor related to adsorption intensity and surface 222
heterogeneity. 223
224
The Dubinin-Radushkevich (D-R) isotherm expressed by the following linearized 225
form [24]: 226
(9) 227
228
where qs (mol/g) is the theoretical isotherm saturation capacity, KDR (mol2/kJ2) is the 229
Dubinin-Radushkevich isotherm constant, R (8.314 J/mol K) is the gas constant, and T 230
(K) the absolute temperature. 231
11
232
2.7.3. Goodness of model fit 233
The fit goodness of the applied mathematical models to the experimental data was 234
determined by the following three procedures: 1) The coefficient of determination (R2) to 235
the linearized data (linear regression), 2) The Composite Fractional Error Function 236
(CFEF) and 3) The Chi-square statistic (χ2). The last two non-linear functions, which 237
measure the difference between experimental and model predicted data, can be expressed 238
by the following equations [6]: 239
(10) 240
241
(11) 242
243
where qe,exp (mg/g) and qe,cal (mg/g) are the experimental and model calculated values of 244
adsorption capacity, respectively, and n is the number of experimental samples. The 245
smaller the values of CFEF and χ2, the more similar are the calculated data to the 246
experimental one. 247
248
3. Results and discussion 249
3.1. Effect of initial solution pH 250
Fig. 1a shows the plot of initial pH versus final pH, wherein the pHzpc value (6.8) of A. 251
platensis was determined by the intersection point of both curves. This pHzpc value is 252
very similar with that reported in other studies [13, 15, 23] which found a pHzpc 7 for 253
12
Spirulina platensis using the method of the eleven points experiment [15, 23]. At pHzpc 254
the biosorbent surface is neutral. 255
The initial pH of the sorption solution is one of the most important factor of adsorption 256
process affecting the surface charge of the biosorbent and the ionization of the dye [3]. 257
The surface charge distribution of a biosorbent depends on the kind and quantity of 258
functional groups, and the solution pH [25]. Fig. 1b shows the effect of initial pH on the 259
MB biosorption onto A. platensis at equilibrium (24 h). It was observed that qe increased 260
as initial pH of the solution increased from 4 to 8, and then decreased at pH values of 9 261
and 10. Therefore, the initial pH of sorption solutions for the following experiments was 262
adjusted to 7.5±0.1. 263
At pH > pHzpc the biosorbent surface is negatively charged due to the deprotonation of 264
functional groups such as carboxyl, amino, phosphate and hydroxyl [13, 21], and thus 265
electrostatic attraction can occur between the negatively charged functional groups of 266
biosorbent surface and the positively charged cationic dye [11]. In contrast, at pH < pHzpc 267
the biosorbent surface is positively charged and electrostatic repulsion occurs between 268
MB cations and A. platensis surface. At acidic pH, the H+ ions compete with MB cations 269
for available binding sites onto A. platensis [3]. However, the remarkable qe at pH < pHzpc 270
where the most of the binding sites are protonated, suggests that hydrophobic interactions 271
also contributed to MB removal [26]. In addition, based on typical deprotonation 272
constants for shortchained carboxylic groups (4 < pKa < 6), the increased MB binding in 273
the pH range of 4-6 may be also attributed to the deprotonation of carboxyl groups [21]. 274
This was confirmed by the chemical modification of dried cells and the esterification of 275
13
surface carboxyl groups, which resulted to the decrease of the biosorption capacity (see 276
Section 3.6). 277
The decrease of qe at pH > 8 is difficult to be explained. Similar result was observed at 278
pH 9.5-11 for MB adsorption on cedar sawdust [27]. Some of the reasons for the 279
biosorption decrease at high pH values might be the involvement of other adsorption 280
mechanisms such as ion exchange or chelation, or the hydrolysis of the biosorbent 281
surface which creates positively charged binding sites [27]. In this study, it was observed 282
that the equilibrium pH (pHe) of the samples at initial pH 9 and 10 decreased by 0.85-283
1.23 units, indicating that an exchange mechanism of H+ ions with MB cations occurred 284
(Fig. 1b). However, other dye-dye interactions such as an increased formation of MB 285
aggregates at higher pH, which are unable to enter into the pores of A. platensis, may be 286
responsible for the decreased qe at pH 9 and 10 [28]. 287
288
3.2. Biosorption kinetics 289
Biosorption kinetic experiments were carried out at three initial MB concentrations 290
and at temperature of 298 K. As shown in Fig. 2a, the biosorption of MB onto A. 291
platensis was very rapid in the first 2-10 min for all studied concentrations. After the 292
rapid adsorption during the initial stage, the biosorption increased at a slower rate with 293
time and equilibrium was established within 60-120 minutes for all initial MB 294
concentrations. Equilibrium capacity did not changed significantly up to 24 h (data not 295
shown). The equilibrium time is in agreement with a previous work about MB 296
biosorption by Spirulina sp. [18]. 297
14
The pseudo-first order model could not describe the kinetic data, because the plot of 298
log(qe-qt) versus t (Eq. 3) presented very low values for R2 (< 0.355) at all initial dye 299
concentrations investigated. Therefore, the kinetic parameters of this model are not 300
shown in Table 1. 301
The kinetic parameters qe and k2 of the pseudo-second order model, obtained from the 302
linear plots of t/qt versus t (Eq. 4), and the values of error functions are listed in Table 1. 303
Based on the linear regression analysis of the kinetic data (Fig. 2b), the pseudo-second 304
order model described very well the overall experimental data with R2 > 0.988. The 305
applicability of this model suggests that the biosorption rate was controlled by 306
chemisorption [29], involving exchange or sharing of electrons between the MB cations 307
and functional groups of the biomass surface [30]. For the pseudo-second order kinetics, 308
the calculated qe values (qe,cal) agreed well with the experimental qe values (qe,exp) (Table 309
1). However, the nonlinear analysis of the kinetic data for the initial MB concentration of 310
50 mg/L showed relative high CFEF and χ2 values (Fig. 2a.), which are due to an 311
underestimation of the early time data (first 30 minutes) by the kinetic model [6]. 312
The biosorption capacity (qe) at equilibrium, calculated from the pseudo-second order 313
model, increased with increasing initial MB concentration (Table 1). However, the 314
pseudo-second order rate constant (k2) decreased slightly when the initial MB 315
concentration increased from 25 to 100 mg/L, but its values [0.0134-0.0247 g/(mg min)] 316
demonstrated a same magnitude for all studied concentrations (Table 1). A decreasing 317
value of k2 suggests that the biosorption equilibrium capacity was established slower at 318
higher MB concentrations due to the limited quantity of binding sites at the biosorbent 319
surface [25]. In addition, the nonlinear relationship between the rate constant values and 320
15
initial MB concentrations suggest that various mechanisms involved in the biosorption 321
process, such as ion exchange, chelation and physisorption [31]. 322
The initial adsorption rate h (mg/g min) at 298 K was calculated from the pseudo-323
second order model parameters with the following equation [32]: 324
325
(12) 326
327
and the values are shown in Table 1. It was found, that the initial adsorption rate h 328
increased from 18.52 to 138.89 mg/(g min) as the initial MB concentration increased 329
from 25 to 100 mg/L. This result suggests an increasing driving force between the liquid 330
and solid phase at higher dye concentrations and a decreasing diffusion time of MB 331
molecules from the solution to the binding sites [26]. This observation is in agreement 332
with previous findings reported for MB adsorption on coconut bunch waste (Cocos 333
nucifera) [32] and marine algae Gelidium [26]. 334
The half adsorption time or half-life, t0.5 (min), expresses the time required for the 335
biosorbent to remove the adsorbed amount of dye at equilibrium to its half, and is 336
calculated from the pseudo-second order model parameters with the following equation 337
[33]: 338
339
(13) 340
341
As shown in Table 1, the estimated values of t0.5 decreased from 1.479 to 0.581 min 342
when the initial MB concentration increased, indicating a faster biosorption [33]. This 343
16
parameter is used as a measure of adsorption rate and to understand the operating time of 344
an adsorption system [33]. 345
Fig. 3 shows the behaviour of the intra-particle diffusion model of Weber-Morris at 346
three initial MB concentrations and 298 K. This model was applied to the kinetic data in 347
order to determine the biosorption process mechanism and the rate controlling step. As 348
shown in Table 1, the values of R2 obtained from the linear regression plots of qt versus 349
t0.5 for the whole time data of the sorption process, were low (< 0.583). The low R2 values 350
suggest that the Weber-Morris model could not describe well the experimental data and 351
that the MB biosorption process was not limited by the intra-particle diffusion. However, 352
the calculated CFEF and χ2 values were very low (Table 1), suggesting that this model 353
fits well the experimental data for the overall time data. To the best of our knowledge, 354
there is no report known in literature about the intra-particle diffusion analysis of kinetic 355
data for cationic dyes onto A. platensis. 356
At all studied concentrations, the plot of qt versus t0.5 consists of three linear sections, 357
which do not pass through the origin (I ≠ 0). If I = 0, then the intra-particle diffusion is 358
the sole rate-limiting step. The multi-linearity of the plots suggests also that MB 359
biosorption onto A. platensis biomass took place in three phases. The first steeper section 360
represents the external mass transfer (film diffusion) of dye to biosorbent surface [13], 361
which was completed very fast in the first 2-5 minutes of the process. The second linear 362
section (completed up to 90-120 min) describes a gradual sorption stage where intra-363
particle diffusion is the rate-controlling step [34]. The third linear section (starting after 364
120 min) represents the final equilibrium stage, where intra-particle diffusion starts to 365
slow down and an apparent saturation occurs [13, 34]. 366
17
The high values of R2 (0.944 and 0.961 respectively) obtained from the second linear 367
sections of the intra-particle diffusion plot at initial dye concentrations of 50 and 100 368
mg/L, indicates that intra-particle diffusion occurred during this phase (Fig. 3, Table 1). 369
As shown in Table 1, the intra-particle diffusion rate constant, kid,,2, estimated from the 370
slope of the second linear section (Fig. 3), increased from 0.562 to 2.866 mg/(g min0.5) 371
with the increasing initial dye concentration from 25 to 100 mg/L. This observation 372
shows a faster intra-particle diffusion at higher initial concentrations [16]. For the same 373
linear section, the values of the y-intercept I increased from 22.05 to 58.94 mg/g when 374
the initial MB concentration increased. This result indicates an increasing boundary layer 375
effect and a greater involvement of the film diffusion at higher dye concentrations, for 376
this particular time range. Similar results for kid and I were observed for the biosorption 377
of phenol on Spirulina platensis nanoparticles [16]. 378
379
3.3. Effect of initial MB concentration and temperature 380
Fig. 4 illustrates the effect of the initial MB concentration on the equilibrium 381
biosorption capacity of A. platensis at different temperatures. It was observed that qe 382
increased with the increase of initial MB concentration at all temperatures studied. At 298 383
K, the amount of MB adsorbed was 7.55 mg/g for the lowest initial MB concentration of 384
6.25 mg/L and increased to 89.56 mg/g for the highest initial MB concentration of 100 385
mg/L. This observation can be explained by the increasing driving force which overcome 386
the mass transfer resistance of MB dye between the aqueous and solid phase [1, 4]. 387
Further, the number of collisions between MB cations and biosorbent can be increased 388
due to the increasing initial dye concentration, enhancing the sorption process [4]. The 389
18
increasing driving force at higher dye concentrations is in agreement with the above 390
mentioned results for the initial adsorption rate h (at 298 K), which is estimated by the 391
parameters of the pseudo-second order kinetic model. 392
Although the enhancement of MB biosorption at higher initial dye concentrations was 393
also observed at 308 and 318 K, the values of qe for each initial concentration decreased 394
with the increasing solution temperature (Fig. 4). According to Dotto et al. [23], the 395
solubility of the dyes increases due to the temperature increase. As a result, the 396
interactions between MB molecules and the solvent are stronger than those between MB 397
and A. platensis. As shown in Fig. 4, the qe for the highest initial MB concentration of 398
100 mg/L, decreased from 89.56 mg/g at 298 K to 82.18 and 65.70 mg/g at 308 and 318 399
K, respectively. These results suggest the exothermic nature of MB sorption process and a 400
mechanism of physical sorption, dominant at lower temperatures [4]. These findings are 401
further discussed by the thermodynamics analysis of isotherm experimental data in 402
Section 3.5. 403
The effect of the initial MB concentration on the percentage removal at different 404
temperatures is shown in Fig. 4. The percentage removal of MB at 298 K decreased from 405
60.4 to 44.8% when the initial dye concentration increased from 6.25 to 100 mg/L. The 406
same tendency of a decreasing percentage removal of MB was observed at 308 and 319 407
K. The only exception was the increase of percentage removal between the two lowest 408
initial MB concentrations of 6.25 and 12.5 mg/L at all temperatures studied. The negative 409
effect of the increasing initial dye concentration on the percentage removal may be due to 410
the saturation of the adsorption sites at higher MB concentrations [5]. Similar results 411
were observed for the MB adsorption onto acid treated kenaf fibre char [5]. 412
19
413
3.4. Biosorption isotherms 414
The relationship between the adsorbate (dye) concentration in the liquid phase and the 415
adsorbed dye amount per unit weight of biosorbent at equilibrium was analyzed using 416
three common isotherm models. 417
The calculated values of the adsorption isotherm parameters and error functions for 418
MB biosorption onto A. platensis are listed in Table 2. Based on the R2 values, the 419
Dubinin-Radushkevich model which was mainly used to investigate the MB sorption 420
mechanism, exhibited the best fit to the experimental data at all studied temperatures (R2 421
> 0.963). Although the Langmuir and Freundlich isotherm models presented satisfactory 422
and similar determination coefficients (R2 > 0.950 and 0.960, respectively), the 423
Freundlich model could better describe the experimental data than the Langmuir model 424
due to the lower CFEF and χ2 values (Table 2). 425
Thus, the good and similar agreement of the three applied isotherm models with the 426
experimental data shows that the MB sorption was a complex process, involving more 427
than one mechanism [4]. Both the monolayer biosorption and surface heterogeneity of 428
biosorbent affected the removal of MB from the solution [4], and no clear biosorption 429
saturation was occurred in the studied range of MB concentration [34]. 430
The Langmuir model assumes a monolayer adsorption onto homogeneous surfaces 431
with finite number of binding sites and no interaction between adsorbate molecule [1, 4]. 432
The constants qmax and KL were estimated from the intercept and slope of the linear plot 433
of experimental data of 1/qe versus 1/Ce (Fig. 5a). 434
20
The maximum monolayer adsorption capacity (qmax) decreased from 312.50 to 80.65 435
mg/g when the temperature increased from 298 to 318 K (Table 2). However, the 436
Langmuir constant KL increased with the increasing temperature (Table 2), indicating a 437
higher affinity (0.0414 L/mg) of A. platensis biomass for the MB molecules at 318 K. 438
The values of the dimensionless separation factor, RL, found to be less than unity and 439
greater than zero (0 < RL < 1) at all initial MB concentrations and temperatures, 440
confirming a favorable sorption process. If RL > 1 the adsorption is unfavorable. As 441
shown in Fig. 6, the higher the initial MB concentration, the lower the RL value and the 442
more favorable the MB biosoprtion. 443
A comparison of the maximum monolayer adsorption capacity (qmax) for MB onto 444
various adsorbents [25, 26, 35-38] and that obtained onto A. platensis in this work, shows 445
that the cyanobacterium is an efficient biosorbent for the removal of MB from aqueous 446
solutions. According to recent studies, Spirulina platensis presented also a satisfactory 447
biosorption capacity for anionic dyes [9, 13, 23, 39]. 448
The Freundlich model assumes a multilayer adsorption onto heterogeneous surfaces 449
with energetically non-equivalent binding sites and interactions between adsorbent 450
molecules [1]. The constants KF and n were evaluated from the intercept and slope of the 451
linear plot of experimental data of ln(qe) versus ln(Ce) (Fig. 5b). 452
The values of the dimensionless Freundlich constant n related to the adsorption 453
intensity and surface heterogeneity, were higher than 1 and less than 10 (1 < n < 10) (see 454
Table 2), indicating a favorable sorption of MB onto A. platensis biomass at all studied 455
temperatures. No significant difference for n values was observed with respect to 456
temperature. The parameter ΚF represents a relative measure of adsorption capacity and 457
21
strength. When the equilibrium concentration Ce tends to be one, then ΚF reaches the 458
value of qe [4]. As can be seen in Table 2, the values of ΚF increased slightly with the 459
rising temperature from 298 to 318 K, but decreased between 298 and 308 K. It shows 460
that the multilayer biosorption of MB was enhanced at higher solution temperature. 461
To distinguish between physical and chemical sorption, the mean free energy E 462
(kJ/mol) of MB biosorption was calculated by the following equation: 463
464
(14) 465
466
where KDR (mol2/kJ2) is the constant of Dubinin-Radushkevich isotherm. 467
The parameter E is related to the mean free energy of sorption per molecule of sorbate, 468
assuming that the sorbate is transferred to the biosorbent surface from infinite distance in 469
the solution. Typical values of E for chemical sorption are in the range of 8–16 kJ/mol, 470
while E < 8 kJ/mol indicates physical sorption [24]. As shown in Table 2, the mean free 471
energy E of MB biosorption onto A. platensis suggests a chemisorption mechanism, 472
because its values are in the range of 8-16 kJ/mol at all studied temperatures. The 473
increasing temperature caused a slight increase of E from 9.09 to 10.77 kJ/mol, indicating 474
an enhancement of the chemisorption at higher temperatures. The biosorption 475
mechanisms are further discussed in Section 3.7. 476
477
3.5. Biosorption thermodynamics 478
The thermodynamic behavior of MB biosorption onto A. platensis biomass was 479
investigated estimating the thermodynamic parameters of Gibbs free energy change 480
22
(ΔG°), enthalpy change (ΔΗ°) and entropy change (ΔS°). The values of these parameters 481
were estimated using the following equations [35]: 482
483
ΔG° = -R T lnKc (15) 484
485
ΔG° = ΔH° - TΔS° (16) 486
487
(17) 488
489
where R is the universal gas constant [8.314 J/(mol K)], T the absolute solution 490
temperature (K), and Kc (Cad,e/Ce) is the adsorption equilibrium constant, which is the 491
ratio of the MB concentration adsorbed (Cad,e) to the MB concentration (Ce) in solution at 492
equilibrium [38]. 493
The negative values of ΔG° indicates a spontaneous and favorable adsorption process 494
at all studied temperatures and initial concentrations (see Table 3), suggesting that the 495
system required no energy input from outside [23]. Similar thermodynamic behavior in 496
respect to negative ΔG° values has been found for Spirulina platensis dry biomass as a 497
biosorbent of anionic dyes [13, 23, 39]. For a given initial MB concentration in this work, 498
no significant change of ΔG° was observed with increasing temperature. However, the 499
ΔG° values decreased slightly as the initial MB concentration increased from 50 to 100 500
mg/L, indicating a more favorable adsorption of MB at lower dye concentration. 501
The values of enthalpy change (ΔΗ°) and entropy change (ΔS°) can be calculated from 502
the slope and intercept of the linear plot of lnKc versus 1/T, based on the Eq. (17). As 503
23
shown in Fig. 7, the determination coefficient (R2) of the plots was 0.939 and 0.940 for 504
the two highest initial MB concentrations, respectively, indicating that the estimated 505
values of ΔΗ° and ΔS° were confident. As can be seen in Table 3, the negative values of 506
ΔH° at all studied initial dye concentrations corresponds to an exothermic nature of MB 507
biosorption. Similar results for the cyanobacterium in respect to negative ΔH° values 508
obtained by other studies, which found an exothermic biosorption of anionic dyes [13, 23, 509
39] and phenol [17] onto Spirulina platensis dry biomass. 510
There are different results in the literature in respect to the exothermic or endothermic 511
nature of MB adsorption onto various materials, based on the estimated ΔH° values. An 512
exothermic adsorption of MB was found onto cyclodextrin/silica hybrid adsorbent [38] 513
and green algae Ulothrix sp. [31]. On the other hand, an endothermic adsorption of MB 514
was found onto diatomite treated with sodium hydroxide [29], marble dust [19], 515
montmorillonite clay [1], and acid treated kenaf fibre char [5]. 516
The magnitude of enthalpy change can be used to classify the type of interaction 517
between sorbent and sorbate. Values of ΔH° < 30 kJ/mol indicates a physical sorption 518
such as hydrogen bonding [13]. Other mechanisms of physical sorption such as Van der 519
Waals forces usually presents ΔH° values in the range 4-10 kJ/mol, hydrophobic bonds 520
forces about 5 kJ/mol, coordination exchange about 40 kJ/mol and dipole bond forces 2-521
29 kJ/mol [13]. In contrast, ΔH° > 80 kJ/mol indicates chemical bond forces and a 522
chemisorption process [13, 17, 20]. According to the ΔH° values (< 28.32 kJ/mol) 523
obtained in this study, the biosorption of MB dye onto A. platensis biomass was due to 524
physical adsorption, suggesting weak interactions between biomass and cationic dye [38]. 525
Further, the negative effect of increasing temperature on qe (Fig. 4) and the applicability 526
24
of the pseudo-second order kinetic model showed that MB sorption process involved both 527
mainly physical and partly chemical sorption [4]. The low negative values of ΔG° ranging 528
from -20 to 0 kJ/mol suggest that the dominant biosorption mechanism was physisorption 529
[1]. 530
The weak binding and weak interactions between the biosorbent and the adsorbate 531
showed that the adsorbed MB molecules should be easily released [38]. This point should 532
be further investigated in order to evaluate the regeneration and reuse ability of A. 533
platensis after dye desorption, in order to reduce the cost of the biosorption process. 534
The negative values of ΔS° for 50 and 100 mg MB/L were very low, indicating no 535
remarkable change on entropy [36] and a decreased disorder at the solid-liquid interface 536
during the MB biosorption onto A. platensis (see Table 3). This showed also that the 537
dispersion degree of the process decreased with increasing temperature [35]. Based on the 538
Eq. (16) and the different magnitude of ΔH° and ΔS° values (Table 3), the enthalpy 539
change (ΔH°) contributed more than entropy change (ΔS°) to obtain the negative values 540
of ΔG° [23]. This observation suggests that MB biosorption onto A. platensis was an 541
enthalpy-controlled process [39]. 542
543
3.6. Effect of ionic strength 544
545
Dye effluents contain high concentrations of salts which affect the dye sorption onto 546
biosorbents. Fig. 8 presents the effect of ionic strength on the MB biosorption by A. 547
platensis at 298 K and pH 7.5. It was observed that qe decreased as the NaCl 548
concentration in sorption solution increased from 0.0625 to 0.5 M. The decrease of qe is 549
25
due to the competitive effect between Na+ and MB cations for the available surface 550
binding sites [36] and the electrostatically screening effect of salt [40]. The latter 551
indicates that the electrostatic interactions should be one the main driving forces during 552
MB biosorption process [40]. However, the remarkable biosorption capacity observed 553
even in the presence of much higher NaCl concentration (62.5 mmol/L) than the initial 554
MB concentration of 50 mg/L ( = 0.156 mmol/L) suggests that other interactions such as 555
hydrophobic interactions, π-π interactions and/or hydrogen bonding, contributed to MB 556
removal [40]. 557
558
3.7. Biosorption mechanisms 559
560
The amounts of Na+ and K+ cations released from A. platensis surface into the solution 561
after MB sorption are listed in Table 4. Based on the total net cations release at 298 K, it 562
is evident that the cation exchange was one of the major biosorption mechanisms at this 563
temperature. In contrast, the net cations release at higher temperatures was negligible. 564
Besides, no significant change between initial and equilibrium pH was observed at all 565
studied temperatures (Table 4), suggesting that ion exchange between MB cations and 566
protons (H+) of surface functional groups did not take place at pH 7.5. A previous study 567
has confirmed the presence of Na+ and K+ on the cell wall surface of Spirulina sp. [41]. 568
The total release of both cations measured in mg/L (data not shown) constituted up to 569
4.7% of the dried biomass weigth (500 mg/L), which agrees with the ash percentage (6.3-570
7%) in the chemical composition of S. platensis dried biomass reported in the literature 571
26
[9, 39]. The mechanism of cation exchange between MB molecule and the exchangeable 572
cations of biomass surface can be described by the following equations [42]: 573
574
S-O-K + CN+ → S-O-CN + K+ (18) 575
S-O-Na + CN+ → S-O-CN + Na+ (19) 576
577
where S is the surface of A. platensis biomass, Na+ and K+ are the exchangeable cations, 578
and CN+ is the positively charged nitrogen atom of the secondary amine group of MB 579
molecule. 580
Fig. 9 shows the effect of the chemical modification of carboxyl groups on the 581
biosorption capacity. The esterified biomass of A. platensis presented a decrease in the 582
MB biosorption capacity (62.66 mg/g) by 25.5% compared to the biosorption capacity of 583
the untreated biomass (83.83 mg/g) (Fig. 9), due to the block of the surface carboxyl 584
groups. This result indicated the participation of carboxyl groups in the MB binding by 585
the untreated biomass, which is a chemisorption process. The cell wall of cyanobacteria 586
contains a thick structural layer of peptidoglycan and an extended layer of glycoproteins 587
and polysaccharides. These layers are the main source of reactive carboxyl groups on the 588
biosorbent surface [21]. The reaction of the chemical esterification of surface carboxyl 589
groups is described by the following equation, where R are all the components in the 590
dried cells [21]: 591
592
RCOOH + CH3OH → RCOOCH3 + H2O 593
(20) 594
27
595
Recent studies for the removal of anionic dyes from aqueous solutions confirmed the 596
mesoporous structure of S. platensis dried microparticles which presented a particle size 597
in the range of 68-75 μm and an average pore radius of 2.25 nm (22.5 Å) [9, 13]. Note 598
that the average pore radius was not modified even in case of S. platensis nanoparticles 599
obtained from the microparticles through a mechanical method [9]. Therefore, the A. 600
platensis microparticles (with particle diameter <154 μm) employed in this study might 601
have a mesoporous structure with a similar average pore diameter of around 4.5 nm. On 602
the other hand, the MB molecule has a parallelepiped shape with dimensions 1.7 × 0.76 × 603
0.325 nm and its attachement on biomass surface may be done with different orientations 604
[26]. Other workers have reported that the presence of mesopores (average pore diameter 605
of 2-50 nm) is favorable for MB adsorption by various adsorbents [5, 25]. Assuming that 606
the MB molecule lies flat on the biomass surface even on its largest face (1.7 nm) which 607
is smaller than the reported average pore radius of A. platensis (2.25 nm), the MB 608
biosorption in this study may also be due to the intraparticle diffusion of MB molecules 609
in the mesopores and due to the entrapment in intrafibrillar capillaries and spaces of the 610
structural exopolysaccharides [6]. This assumption agrees with the diffusion analysis of 611
the kinetic data. Therefore, the mesoporous structure of A. platensis can facilitate the 612
accommodation of MB molecules in the biomass pores [13]. 613
614
4. Conclusions 615
Dry biomass of A. platensis were used as biosorbent for methylene blue removal in 616
batch mode with respect to solution pH, contact time, initial dye concentration, 617
28
temperature and ionic strength. This study applied for the first time a kinetic and 618
thermodynamic analysis for the biosorption of a cationic dye onto A. platensis. In 619
addition, the role of ion exchange mechanism was directly investigated by detection 620
measures. The kinetic data were fitted very well by the pseudo-second order model, and 621
equilibrium was achieved within 60-120 min. It was found that the film and intra-particle 622
diffusion contributed to the MB biosorption process. The biosorption capacity of A. 623
platensis for MB increased with increasing initial dye concentration and decreased with 624
increasing temperature. At all studied temperatures, the Langmuir, Freundlich and 625
Dubinin-Radushkevich isotherm models fitted well the experimental equilibrium data, 626
indicating that MB biosorption was a complex process, involving more than one 627
mechanism. The carboxyl groups of biomass surface contributed to MB chemisorption. 628
The important role of hydrophobic interactions in MB removal was indicated by the 629
considerable biosorption capacity at low pH values and in the presence of NaCl in the 630
sorption solution. The release of Na+ and K+ cations from the biomass surface in the 631
solution after MB sorption confirmed the contribution of cation exchange mechanism. 632
Physical sorption and ion exchange were the dominant mechanisms of MB biosorption at 633
lower temperature. According to the thermodynamic analysis of equilibrium data, MB 634
biosorption onto A. platensis was a spontaneous, favorable and exothermic process. It 635
was concluded that A. platensis biomass has a great potential for removal of MB from 636
aqueous solutions. 637
638
Acknowledgement 639
29
Professor D. Georgakakis of Agricultural University of Athens is kindly acknowledged 640
for his valuable support in respect of the availability of laboratory equipment. 641
642
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644
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762
763
764
765
766
767
768
769
770
35
771
772
Fig. 1. (a) Plot of initial pH versus final pH for the determination of biomass pHzpc, and 773
(b) the effect of initial pH on MB biosorption onto A. platensis (pHe = equilibrium pH). 774
775
36
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
37
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
38
Fig. 5. Linear plots of (a) Langmuir and (b) Freundlich isotherm model for the MB
biosorption onto A. platensis at different temperatures.
779
39
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
40
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
41
Table 1. Kinetic and diffusion model parameters for MB biosorption onto A. platensis.
Initial dye concentration (mg/L)
25 50 100
qe,exp (mg/g) 29.48 54.94 82.95
Pseudo-first order model
R2 0.355 0.241 0.337
Pseudo-second order
model
qe,calc(mg/g) 27.40 55.56 80.65
k2 (g/ mg min) 0.0247 0.0134 0.0214 h (mg/g min) 18.52 41.32 138.89 t0.5 (min) 1.479 1.344 0.581 R2 0.998 0.998 0.988
CFEF 3.46 18.49 6.39 χ2 4.84 29.36 6.40 intra-particle diffusion model: whole time data
kid (mg/ g min0.5) 0.307 0.197 1.220 I (mg/g) 23.25 59.11 67.66 R2 0.583 0.269 0.517
CFEF 0.52 0.39 3.88 χ2 0.54 0.39 3.70 intra-particle diffusion model: second linear section
kid,2 (mg/ g min0.5) 0.562 1.024 2.866 I (mg/g) 22.05 54.59 58.94 R2 0.646 0.944 0.961
CFEF 0.48 0.15 0.94
χ2 0.50 0.15 0.91
788
42
Table 2. Isotherm parameters values of MB biosoprtion onto A. platensis at different
temperatures.
Isotherm models Solution temperature (K)
298 308 318 qe,exp (mg/g) 89.56 82.18 65.70
Langmuir
qmax (mg/g) 312.50 204.08 80.65 qe,cal (mg/g)1 117.42 86.94 59.31
KL (L/mg) 0.0109 0.0126 0.0414 RL (range) 0.478-0.936 0.442-0.927 0.195-0.794 R2 0.950 0.989 0.952 CFEF 10.82 4.02 3.71 χ2 8.96 3.24 3.62 Freundlich qe,cal (mg/g)1 99.75 82.55 64.95
KF ((mg/g)(L/mg)1/n) 4.766 3.512 5.003 n 1.319 1.291 1.641 R2 0.967 0.981 0.960
CFEF 2.86 1.50 1.42 χ2 2.89 1.50 1.59 Dubinin-Radushkevich qs (mol/g) 0.0048 0.0042 0.0017 BD (mol2/kJ2) 6.05 × 10-9 5.85 × 10-9 4.31 × 10-9 E (kJ/mol) 9.09 9.25 10.77 R2 0.974 0.986 0.963
CFEF 4.98 × 10-6 4.73 × 10-6 4.93 × 10-6
χ2 5.42 × 10-6 4.46 × 10-6 5.40 × 10-6 1 qe,cal corresponds to C0 = 100 mg/L.
789
43
Table 3. Thermodynamic parameters of MB biosorption onto A. platensis biomass.
C0 (mg/L) ΔH° (kJ/mol) ΔS° (kJ/mol/K) ΔG° (kJ/mol)
298 K 308 K 318 K
50 -28.32 -0.036 -17.65 -16.89 -16.94 100 -19.81 -0.011 -16.60 -16.77 -16.37
790
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)
298 308 318
Na+ (mmol/L) Background release 0.512 0.561 0.545 After MB biosorption 0.617 0.534 0.564 Net release 0.105 -0.027 0.019 K+ (mmol/L) Background release 0.112 0.206 0.171 After MB biosorption 0.237 0.169 0.189 Net release 0.125 -0.037 0.018
Total net release (mmol/L) 0.230 -0.064 0.037 Equilibrium pH 7.53 7.63 7.68 qe (mmol MB/g) 0.280 0.257 0.205
791
792