Biosorption of methylene blue onto Arthrospira platensis biomass: Kinetic, equilibrium and thermodynamic studies

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.

1

Biosorption of methylene blue onto Arthrospira platensis biomass: kinetic,

2

equilibrium and thermodynamic studies

3 4

Dimitris Mitrogiannisa*, Giorgos Markoua, Abuzer Çeleklic, Hüseyin Bozkurtd

5

a

Department of Natural Resources Management and Agricultural Engineering,

6 7

Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece b

Department of Biology, Faculty of Art and Science, University of Gaziantep, 27310

8 9

Gaziantep, Turkey c

Department of Food Engineering, Faculty of Engineering, University of Gaziantep,

10

27310 Gaziantep, Turkey *

11 12

Corresponding author: E-mail: [email protected] Telephone: +30 6974876236

13 14

Abstract

15

In this study, Arthrospira platensis biomass was employed as a biosorbent for the

16

removal of methylene blue (MB) dye from aqueous solutions. The kinetic data were

17

better described by the pseudo-second order model and equilibrium was established

18

within 60-120 min. The intra-particle diffusion was not the only rate-limiting step and

19

film diffusion might contribute to MB biosorption process. The increase of temperature

20

from 298 to 318 K caused a decrease of biosorption capacity. The Langmuir, Freundlich

21

and Dubinin-Radushkevich (D-R) isotherm models described well the experimental

22

equilibrium data at all studied temperatures. The maximum monolayer adsorption

23

capacity (qmax) was 312.5 mg MB/g at 298 K and pH 7.5. According to the results of the

1

24

thermodynamic analysis and the release of exchangeable cations from the biomass

25

surface, physical sorption and ion exchange were the dominant mechanisms of MB

26

biosorption at lower temperature. Methanol esterification of the dried biomass showed

27

the involvement of carboxyl functional groups in MB chemisorption. The thermodynamic

28

parameters indicated that MB biosorption onto A. platensis was a spontaneous, favorable

29

and exothermic process. The biosorption results showed that A. platensis could be

30

employed as an efficient and eco-friendly biosorbent for the removal of cationic dyes.

31 32

Keywords: Arthrospira platensis; methylene blue; cationic dye; thermodynamics;

33

biosorption mechanism; cation exchange

34 35

1. Introduction

36

Synthetic dyes are hazardous pollutants which present toxic and aesthetic effects in

37

aquatic environments. Dye effluents, containing colored organic molecules, increase the

38

organic load of water bodies and reduce the sunlight penetration, affecting the

39

photosynthetic activity of phytoplankton and disturbing the ecological balance of the

40

aquatic environments. Moreover, some dyes display carcinogenic and mutagenic activity

41

[1, 2]. Potential sources of dyes are textile, leather, paper, printing, plastic, electroplating,

42

food and cosmetic industries.

43

Various physical, chemical and biological methods have been investigated for the

44

treatment of wastewaters contaminated with synthetic dyes [3]. However, each of these

45

technologies has its disadvantages, such as high operational and initial capital costs, low

46

efficiency at low dye concentrations and production of undesirable sludge [4]. Among

2

47

treatment technologies, adsorption is considered as an effective method for dye removal

48

using low-cost materials. Although activated carbon is the most commonly used

49

adsorbent and is very efficient to remove dyes from wastewater, it presents high costs of

50

production and regeneration [5]. A number of studies have been made to find cost-

51

effective and eco-friendly methods for treatment of dye wastewaters using cheep

52

biomaterials as adsorbents [3].

53

Algae and cyanobacteria have gained interest as alternative biosorbents due to their

54

high binding affinity, their higher sorption selectivity for pollutants than commercial ion-

55

exchange resins and activated carbon, and due to their capability of growing using

56

wastewater as cultivation medium [3, 4, 6, 7]. The filamentous cyanobacterium

57

Arthrospira platensis is a potential biosorbent, having several advantages, such as relative

58

high growth rates, high biomass productivity, ease of cell harvesting and biomass

59

composition manipulation [8]. The surface of A. platensis consists of various macro-

60

molecules with diverse functional groups such as carboxyl, hydroxyl, sulphate and

61

phosphate, which are responsible for dye binding [9]. A. platensis has already been

62

studied for the removal of inorganic pollutants such as heavy metals [6, 10-12] and

63

organic pollutants such as anionic dyes [9, 13-15] and phenol [16, 17] from aqueous

64

solutions. To our knowledge, there is lack of published work about the adsorption of

65

cationic dyes onto A. platensis. The only related study to this, uses an artificial neural

66

network to predict the biosorption capacity of methylene blue onto Spirulina sp. [18].

67

However, there is no literature information about the biosorption kinetics and

68

thermodynamics of a cationic dye on this cyanobacterium and about the contribution of

69

the ion exchange mechanism on dye removal. Although the important role of the ion

3

70

exchange mechanism in MB removal by various biosorbents is mentioned very often, it

71

has not been widely investigated by detection measures [7].

72

Methylene blue (MB) is a common cationic dye used for dyeing paper, cotton, wool

73

and silk [7, 19]. The harmful effects of MB include: breathing difficulties, nausea,

74

vomiting, tissue necrosis, profuse sweating, mental confusion, cyanosis and

75

methemoglobinemia [5, 7]. MB has been widely employed as a model cationic dye in

76

adsorption studies, using low-cost adsorbents such as natural minerals (clays, zeolites,

77

perlite), activated carbon, dead or non-growing microbial biomass, agricultural and

78

industrial wastes [7].

79

The aim of the present study was to investigate the potential of A. platensis dry

80

biomass to remove MB dye from aqueous solutions. The effect of solution pH, initial MB

81

concentration, contact time, temperature and ionic strength on the biosorption capacity

82

was investigated. Kinetic, isotherm and thermodynamic parameters were estimated to

83

understand the biosorption rate and mechanisms of MB onto A. platensis.

84 85

2. Materials and methods

86

2.1. Biosorbent cultivation and preparation

87

The cyanobacterium A. platensis (SAG 21.99) used in this study was cultivated in

88

Zarrouk medium within 10 L plastic cubical photobioreactor, which were kept at 303 ± 2

89

K in semi-continuous cultivation mode with a dilution rate of 0.1 1/d [6]. The A. platensis

90

biomass was harvested by filtration and rinsed with deionized (DI) water. The cultivation

91

medium salts were removed by washing the biomass twice by re-suspension in DI water.

92

After that the biomass was separated with centrifugation (5000 rpm for 5 min) and dried

4

93

overnight in an oven at 353 K. The dried biomass was milled (IKA Labortechnik, A10),

94

sieved through a metal sieve (100 mesh, particle diameter < 154 μm), and stored in a

95

plastic container inside an exsiccator containing silica gel to prevent moisture sorption by

96

the biomass. The chemical composition of the dried biomass consisted of 45-55%

97

proteins, 10-20% carbohydrates, and 5-7% lipids [6].

98 99

2.2. Preparation of dye solution

100

MB is a cationic dye with molecular formula C16H18N3SCl and molar weight of 319.85

101

g/mol. This cationic dye presents high water solubility at 293 K and is positively charged

102

on S atom [20]. MB stock solution (1 g/L) was prepared by dissolving an appropriate

103

weighed amount of MB hydrate reagent (analytical grade, Sigma-Aldrich, India) in 1 L

104

DI water. The experimental solutions of desired initial concentrations were obtained by

105

dilution of MB stock solution with DI water.

106 107

2.3. Determination of pH zero point charge of A. platensis

108

To determine the zero point charge (pHzpc) of A. platensis biomass, the initial pH of 25

109

mL solutions containing 0.5 g/L of biosorbent and 0.1 M NaCl was adjusted at pH values

110

ranging from 3 to 9, using 0.1 M HNO3 and/or NaOH [19, 20]. The samples were agitated

111

for 24 h at 298 K, and the final pH values were measured using a pH-meter (Consort

112

P603, Belgium). Value of pHzpc was determined from the plot of final pH against initial

113

pH.

114 115

2.4. Batch biosorption experiments

5

116

The biosorption experiments were carried out in batch mode by mixing 12.5 mL

117

aqueous suspension containing 12.5 mg dried biomass with 12.5 mL MB dye solution of

118

known concentration. The final 25 mL solution was placed in a 50 mL plastic flask,

119

which was sealed and agitated with a rotary shaker at 140 rpm. The desired initial pH

120

(range 4-10) of the adsorbate and adsorbent solution was adjusted using 0.1 M HNO3

121

and/or NaOH before mixing them.

122

Biosorption kinetics were investigated with a biomass concentration of 0.5 g/L at three

123

initial dye concentrations (25, 50 and 100 mg/L) and pH 7.5±0.1. Samples were collected

124

at time intervals (2, 5, 10, 15, 30, 60, 90, 120, 180 and 240 min) and subjected to MB

125

concentration determination. The kinetic experiments were conducted in an air-

126

conditioned room with temperature of 298-300 K. Equilibrium experiments were carried

127

out at 298, 308 and 318 K, placing the flasks and shaker in a temperature controlled

128

incubator and using five different initial MB concentrations (6.25, 12.5, 25, 50, 100

129

mg/L), in order to estimate the parameters of isotherm models and thermodynamic

130

equations. The contact time of equilibrium experiments was chosen to be 24 h.

131

The amount of MB adsorbed per unit weight of A. platensis biomass at equilibrium, qe

132

(mg/g), and the percentage dye removal (R%), were calculated with the following

133

equations:

134

(1)

135 136

(2)

137 138

where Co (mg/L), C e (mg/L) and X (g/L) are the initial MB concentration, the MB

139

concentration at equilibrium, and the sorbent concentration in the solution, respectively. 6

140

The effect of ionic strength on the biosorption capacity was studied in solution

141

containing 0.5 g biosorbent/L, 50 mg MB/L and 0.0625-0.5 M NaCl at optimum pH (7.5).

142

For the investigation of the possible ion exchange mechanism involved in the biosorption

143

process, the concentration of cations Na+ and K+ released from the biomass after MB

144

sorption were determined. Biomass of 0.5 g/L was added in 50 mL solution containing

145

either DI water or 100 mg MB/L, which were shaken for 24 h at 298-318 K. The initial

146

pH of the dye solution was adjusted at 7.5±0.1 using dilute NH4OH and HCl solutions.

147

The cations released in the 0 mg MB/L solution containing only dried biomass were

148

considered as background concentration, which was subtracted from the cation amount

149

released after MB sorption in order to calculate the net cation release. Blank solution of

150

100 mg MB/L was also used to confirm no presence of cations.

151 152

2.5. Chemical modification of carboxyl groups on the biomass surface

153 154

The chemical modification of the dried biomass was applied to understand the role of

155

the surface carboxyl groups in MB sorption. The aim of the modification was to block the

156

carboxyl groups by esterification and then to determine the decrease of biosorption

157

capacity. The esterification of the dried biomass was carried out according to the method

158

described by Fang et al. [21]. 1.0 g dried biomass of A. platensis was suspended in 50 mL

159

of 99.9% methanol solution and 0.6 mL concentrated HCl. The suspension was agitated

160

for 48 h at 333 K and allowed to cool at room temperature. The modified biomass was

161

washed three times by re-suspension in DI water. After that the biomass was separated

162

with centrifugation (5000 rpm for 5 min) and dried overnight in an oven at 323 K. For the

7

163

biosorption study, 100 mg of modified dried biomass were suspended in 100 mL DI water

164

and homogenized with a homogenizer (IKA-Labortechnick, Ultra Turrax T10, Germany).

165

Then 12.5 mL modified biomass suspension was mixed with 12.5 mL solution of 200 mg

166

MB/L. The final 25 mL solution containing 100 mg MB/L and 0.5 g/L of chemically

167

modified biosorbent was agitated for 24 h at 298 K and pH 7.5. The same procedure was

168

done for the untreated dried biomass of A. platensis for comparison purpose.

169 170

2.6. Analytical methods

171

For the determination of the unadsorbed MB concentration in each solution, 0.5 mL of

172

sample was withdrawn at the preselected time, t, and was placed in an Eppendorf type

173

centrifuge tube (1.5 mL), which contained 1 mL DI water. The diluted sample was

174

centrifuged for 2 min at 10000 rpm. The supernatant was collected, diluted with

175

appropriate DI water, and the MB concentrations were determined at the wavelength of

176

665 nm using a UV-vis spectrophotometer (Dr. Lange, Cadas 30, Germany). The

177

concentrations of Na+ and K+ were determined with a flame photometer (Sherwood

178

Scientific, model 400), followed by separation of the biomass from the sorption solution

179

by centrifugation at 10000 rpm for 5 min. All experiments were performed in triplicates

180

and the average values were recorded.

181 182

2.7. Mathematical models

183

2.7.1. Kinetic models

184

The biosorption kinetic experimental data were fitted with the following models:

185

8

186

The pseudo-first order model expressed by the following linearized form [4]:

187

(3)

188 189

where q e (mg/g) and q t (mg/g) are the amount of adsorbed dye per gram of biomass at

190

equilibrium and at time t, respectively, and k1 (1/min) is the pseudo-first order rate

191

constant.

192 193

The pseudo-second order model expressed by the following linearized form [4]:

194

(4)

195 196

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 [6]:

200

(5)

201 202

where kid (mg/g min0.5) is the intra-particle diffusion rate constant, and I (mg/g) is the y-

203

intercept which reflects the boundary layer thickness.

204 205

2.7.2. Equilibrium isotherm models

206

The biosorption equilibrium data were applied to the following isotherm models:

207

The Langmuir isotherm expressed by the following linearized form [22]:

208

(6) 9

209 210

where q max (mg/g) is the maximum monolayer adsorption capacity, and KL (L/mg) is the

211

Langmuir isotherm constant related to the affinity and binding energy. The constant KL is

212

used for the prediction of the affinity between sorbate and biosorbent by the

213

dimensionless separation factor, RL, which is defined as [23]:

214

(7)

215 216

where C o (mg/L) is the initial dye concentration.

217 218

The Freundlich isotherm expressed by the following linearized form [24]:

219

(8)

220 221

where KF [(mg/g)(L/g)1/n] is the Freundlich isotherm constant representing the adsoption

222

capacity, and n is a dimensionless factor related to adsorption intensity and surface

223

heterogeneity.

224 225 226

The Dubinin-Radushkevich (D-R) isotherm expressed by the following linearized form [24]:

227

(9)

228 229

where q s (mol/g) is the theoretical isotherm saturation capacity, KDR (mol2/kJ2) is the

230

Dubinin-Radushkevich isotherm constant, R (8.314 J/mol K) is the gas constant, and T

231

(K) the absolute temperature. 10

232 233

2.7.3. Goodness of model fit

234

The fit goodness of the applied mathematical models to the experimental data was

235

determined by the following three procedures: 1) The coefficient of determination (R2) to

236

the linearized data (linear regression), 2) The Composite Fractional Error Function

237

(CFEF) and 3) The Chi-square statistic (χ2). The last two non-linear functions, which

238

measure the difference between experimental and model predicted data, can be expressed

239

by the following equations [6]:

240

(10)

241 242

(11)

243 244

where q e,exp (mg/g) and qe,cal (mg/g) are the experimental and model calculated values of

245

adsorption capacity, respectively, and n is the number of experimental samples. The

246

smaller the values of CFEF and χ2, the more similar are the calculated data to the

247

experimental one.

248 249

3. Results and discussion

250

3.1. Effect of initial solution pH

251

Fig. 1a shows the plot of initial pH versus final pH, wherein the pHzpc value (6.8) of A.

252

platensis was determined by the intersection point of both curves. This pHzpc value is

253

very similar with that reported in other studies [13, 15, 23] which found a pHzpc 7 for

11

254

Spirulina platensis using the method of the eleven points experiment [15, 23]. At pHzpc

255

the biosorbent surface is neutral.

256

The initial pH of the sorption solution is one of the most important factor of adsorption

257

process affecting the surface charge of the biosorbent and the ionization of the dye [3].

258

The surface charge distribution of a biosorbent depends on the kind and quantity of

259

functional groups, and the solution pH [25]. Fig. 1b shows the effect of initial pH on the

260

MB biosorption onto A. platensis at equilibrium (24 h). It was observed that qe increased

261

as initial pH of the solution increased from 4 to 8, and then decreased at pH values of 9

262

and 10. Therefore, the initial pH of sorption solutions for the following experiments was

263

adjusted to 7.5±0.1.

264

At pH > pHzpc the biosorbent surface is negatively charged due to the deprotonation of

265

functional groups such as carboxyl, amino, phosphate and hydroxyl [13, 21], and thus

266

electrostatic attraction can occur between the negatively charged functional groups of

267

biosorbent surface and the positively charged cationic dye [11]. In contrast, at pH < pHzpc

268

the biosorbent surface is positively charged and electrostatic repulsion occurs between

269

MB cations and A. platensis surface. At acidic pH, the H+ ions compete with MB cations

270

for available binding sites onto A. platensis [3]. However, the remarkable qe at pH < pHzpc

271

where the most of the binding sites are protonated, suggests that hydrophobic interactions

272

also contributed to MB removal [26]. In addition, based on typical deprotonation

273

constants for shortchained carboxylic groups (4 < pKa < 6), the increased MB binding in

274

the pH range of 4-6 may be also attributed to the deprotonation of carboxyl groups [21].

275

This was confirmed by the chemical modification of dried cells and the esterification of

12

276

surface carboxyl groups, which resulted to the decrease of the biosorption capacity (see

277

Section 3.6).

278

The decrease of qe at pH > 8 is difficult to be explained. Similar result was observed at

279

pH 9.5-11 for MB adsorption on cedar sawdust [27]. Some of the reasons for the

280

biosorption decrease at high pH values might be the involvement of other adsorption

281

mechanisms such as ion exchange or chelation, or the hydrolysis of the biosorbent

282

surface which creates positively charged binding sites [27]. In this study, it was observed

283

that the equilibrium pH (pHe) of the samples at initial pH 9 and 10 decreased by 0.85-

284

1.23 units, indicating that an exchange mechanism of H+ ions with MB cations occurred

285

(Fig. 1b). However, other dye-dye interactions such as an increased formation of MB

286

aggregates at higher pH, which are unable to enter into the pores of A. platensis, may be

287

responsible for the decreased q e at pH 9 and 10 [28].

288 289

3.2. Biosorption kinetics

290

Biosorption kinetic experiments were carried out at three initial MB concentrations

291

and at temperature of 298 K. As shown in Fig. 2a, the biosorption of MB onto A.

292

platensis was very rapid in the first 2-10 min for all studied concentrations. After the

293

rapid adsorption during the initial stage, the biosorption increased at a slower rate with

294

time and equilibrium was established within 60-120 minutes for all initial MB

295

concentrations. Equilibrium capacity did not changed significantly up to 24 h (data not

296

shown). The equilibrium time is in agreement with a previous work about MB

297

biosorption by Spirulina sp. [18].

13

298

The pseudo-first order model could not describe the kinetic data, because the plot of

299

log(qe-q t) versus t (Eq. 3) presented very low values for R2 (< 0.355) at all initial dye

300

concentrations investigated. Therefore, the kinetic parameters of this model are not

301

shown in Table 1.

302

The kinetic parameters qe and k2 of the pseudo-second order model, obtained from the

303

linear plots of t/q t versus t (Eq. 4), and the values of error functions are listed in Table 1.

304

Based on the linear regression analysis of the kinetic data (Fig. 2b), the pseudo-second

305

order model described very well the overall experimental data with R2 > 0.988. The

306

applicability of this model suggests that the biosorption rate was controlled by

307

chemisorption [29], involving exchange or sharing of electrons between the MB cations

308

and functional groups of the biomass surface [30]. For the pseudo-second order kinetics,

309

the calculated q e values (qe,cal) agreed well with the experimental qe values (qe,exp) (Table

310

1). However, the nonlinear analysis of the kinetic data for the initial MB concentration of

311

50 mg/L showed relative high CFEF and χ2 values (Fig. 2a.), which are due to an

312

underestimation of the early time data (first 30 minutes) by the kinetic model [6].

313

The biosorption capacity (q e) at equilibrium, calculated from the pseudo-second order

314

model, increased with increasing initial MB concentration (Table 1). However, the

315

pseudo-second order rate constant (k2) decreased slightly when the initial MB

316

concentration increased from 25 to 100 mg/L, but its values [0.0134-0.0247 g/(mg min)]

317

demonstrated a same magnitude for all studied concentrations (Table 1). A decreasing

318

value of k2 suggests that the biosorption equilibrium capacity was established slower at

319

higher MB concentrations due to the limited quantity of binding sites at the biosorbent

320

surface [25]. In addition, the nonlinear relationship between the rate constant values and

14

321

initial MB concentrations suggest that various mechanisms involved in the biosorption

322

process, such as ion exchange, chelation and physisorption [31].

323 324

The initial adsorption rate h (mg/g min) at 298 K was calculated from the pseudosecond order model parameters with the following equation [32]:

325 326

(12)

327 328

and the values are shown in Table 1. It was found, that the initial adsorption rate h

329

increased from 18.52 to 138.89 mg/(g min) as the initial MB concentration increased

330

from 25 to 100 mg/L. This result suggests an increasing driving force between the liquid

331

and solid phase at higher dye concentrations and a decreasing diffusion time of MB

332

molecules from the solution to the binding sites [26]. This observation is in agreement

333

with previous findings reported for MB adsorption on coconut bunch waste (Cocos

334

nucifera) [32] and marine algae Gelidium [26].

335

The half adsorption time or half-life, t0.5 (min), expresses the time required for the

336

biosorbent to remove the adsorbed amount of dye at equilibrium to its half, and is

337

calculated from the pseudo-second order model parameters with the following equation

338

[33]:

339 340

(13)

341 342

As shown in Table 1, the estimated values of t 0.5 decreased from 1.479 to 0.581 min

343

when the initial MB concentration increased, indicating a faster biosorption [33]. This

15

344

parameter is used as a measure of adsorption rate and to understand the operating time of

345

an adsorption system [33].

346

Fig. 3 shows the behaviour of the intra-particle diffusion model of Weber-Morris at

347

three initial MB concentrations and 298 K. This model was applied to the kinetic data in

348

order to determine the biosorption process mechanism and the rate controlling step. As

349

shown in Table 1, the values of R2 obtained from the linear regression plots of qt versus

350

t0.5 for the whole time data of the sorption process, were low (< 0.583). The low R2 values

351

suggest that the Weber-Morris model could not describe well the experimental data and

352

that the MB biosorption process was not limited by the intra-particle diffusion. However,

353

the calculated CFEF and χ2 values were very low (Table 1), suggesting that this model

354

fits well the experimental data for the overall time data. To the best of our knowledge,

355

there is no report known in literature about the intra-particle diffusion analysis of kinetic

356

data for cationic dyes onto A. platensis.

357

At all studied concentrations, the plot of q t versus t0.5 consists of three linear sections,

358

which do not pass through the origin (I ≠ 0). If I = 0, then the intra-particle diffusion is

359

the sole rate-limiting step. The multi-linearity of the plots suggests also that MB

360

biosorption onto A. platensis biomass took place in three phases. The first steeper section

361

represents the external mass transfer (film diffusion) of dye to biosorbent surface [13],

362

which was completed very fast in the first 2-5 minutes of the process. The second linear

363

section (completed up to 90-120 min) describes a gradual sorption stage where intra-

364

particle diffusion is the rate-controlling step [34]. The third linear section (starting after

365

120 min) represents the final equilibrium stage, where intra-particle diffusion starts to

366

slow down and an apparent saturation occurs [13, 34].

16

367

The high values of R2 (0.944 and 0.961 respectively) obtained from the second linear

368

sections of the intra-particle diffusion plot at initial dye concentrations of 50 and 100

369

mg/L, indicates that intra-particle diffusion occurred during this phase (Fig. 3, Table 1).

370

As shown in Table 1, the intra-particle diffusion rate constant, kid,,2, estimated from the

371

slope of the second linear section (Fig. 3), increased from 0.562 to 2.866 mg/(g min 0.5)

372

with the increasing initial dye concentration from 25 to 100 mg/L. This observation

373

shows a faster intra-particle diffusion at higher initial concentrations [16]. For the same

374

linear section, the values of the y-intercept I increased from 22.05 to 58.94 mg/g when

375

the initial MB concentration increased. This result indicates an increasing boundary layer

376

effect and a greater involvement of the film diffusion at higher dye concentrations, for

377

this particular time range. Similar results for kid and I were observed for the biosorption

378

of phenol on Spirulina platensis nanoparticles [16].

379 380

3.3. Effect of initial MB concentration and temperature

381

Fig. 4 illustrates the effect of the initial MB concentration on the equilibrium

382

biosorption capacity of A. platensis at different temperatures. It was observed that qe

383

increased with the increase of initial MB concentration at all temperatures studied. At 298

384

K, the amount of MB adsorbed was 7.55 mg/g for the lowest initial MB concentration of

385

6.25 mg/L and increased to 89.56 mg/g for the highest initial MB concentration of 100

386

mg/L. This observation can be explained by the increasing driving force which overcome

387

the mass transfer resistance of MB dye between the aqueous and solid phase [1, 4].

388

Further, the number of collisions between MB cations and biosorbent can be increased

389

due to the increasing initial dye concentration, enhancing the sorption process [4]. The

17

390

increasing driving force at higher dye concentrations is in agreement with the above

391

mentioned results for the initial adsorption rate h (at 298 K), which is estimated by the

392

parameters of the pseudo-second order kinetic model.

393

Although the enhancement of MB biosorption at higher initial dye concentrations was

394

also observed at 308 and 318 K, the values of qe for each initial concentration decreased

395

with the increasing solution temperature (Fig. 4). According to Dotto et al. [23], the

396

solubility of the dyes increases due to the temperature increase. As a result, the

397

interactions between MB molecules and the solvent are stronger than those between MB

398

and A. platensis. As shown in Fig. 4, the qe for the highest initial MB concentration of

399

100 mg/L, decreased from 89.56 mg/g at 298 K to 82.18 and 65.70 mg/g at 308 and 318

400

K, respectively. These results suggest the exothermic nature of MB sorption process and a

401

mechanism of physical sorption, dominant at lower temperatures [4]. These findings are

402

further discussed by the thermodynamics analysis of isotherm experimental data in

403

Section 3.5.

404

The effect of the initial MB concentration on the percentage removal at different

405

temperatures is shown in Fig. 4. The percentage removal of MB at 298 K decreased from

406

60.4 to 44.8% when the initial dye concentration increased from 6.25 to 100 mg/L. The

407

same tendency of a decreasing percentage removal of MB was observed at 308 and 319

408

K. The only exception was the increase of percentage removal between the two lowest

409

initial MB concentrations of 6.25 and 12.5 mg/L at all temperatures studied. The negative

410

effect of the increasing initial dye concentration on the percentage removal may be due to

411

the saturation of the adsorption sites at higher MB concentrations [5]. Similar results

412

were observed for the MB adsorption onto acid treated kenaf fibre char [5].

18

413 414

3.4. Biosorption isotherms

415

The relationship between the adsorbate (dye) concentration in the liquid phase and the

416

adsorbed dye amount per unit weight of biosorbent at equilibrium was analyzed using

417

three common isotherm models.

418

The calculated values of the adsorption isotherm parameters and error functions for

419

MB biosorption onto A. platensis are listed in Table 2. Based on the R2 values, the

420

Dubinin-Radushkevich model which was mainly used to investigate the MB sorption

421

mechanism, exhibited the best fit to the experimental data at all studied temperatures (R2

422

> 0.963). Although the Langmuir and Freundlich isotherm models presented satisfactory

423

and similar determination coefficients (R2 > 0.950 and 0.960, respectively), the

424

Freundlich model could better describe the experimental data than the Langmuir model

425

due to the lower CFEF and χ 2 values (Table 2).

426

Thus, the good and similar agreement of the three applied isotherm models with the

427

experimental data shows that the MB sorption was a complex process, involving more

428

than one mechanism [4]. Both the monolayer biosorption and surface heterogeneity of

429

biosorbent affected the removal of MB from the solution [4], and no clear biosorption

430

saturation was occurred in the studied range of MB concentration [34].

431

The Langmuir model assumes a monolayer adsorption onto homogeneous surfaces

432

with finite number of binding sites and no interaction between adsorbate molecule [1, 4].

433

The constants qmax and KL were estimated from the intercept and slope of the linear plot

434

of experimental data of 1/q e versus 1/Ce (Fig. 5a).

19

435

The maximum monolayer adsorption capacity (qmax) decreased from 312.50 to 80.65

436

mg/g when the temperature increased from 298 to 318 K (Table 2). However, the

437

Langmuir constant K L increased with the increasing temperature (Table 2), indicating a

438

higher affinity (0.0414 L/mg) of A. platensis biomass for the MB molecules at 318 K.

439

The values of the dimensionless separation factor, RL, found to be less than unity and

440

greater than zero (0 < RL < 1) at all initial MB concentrations and temperatures,

441

confirming a favorable sorption process. If RL > 1 the adsorption is unfavorable. As

442

shown in Fig. 6, the higher the initial MB concentration, the lower the RL value and the

443

more favorable the MB biosoprtion.

444

A comparison of the maximum monolayer adsorption capacity (q max) for MB onto

445

various adsorbents [25, 26, 35-38] and that obtained onto A. platensis in this work, shows

446

that the cyanobacterium is an efficient biosorbent for the removal of MB from aqueous

447

solutions. According to recent studies, Spirulina platensis presented also a satisfactory

448

biosorption capacity for anionic dyes [9, 13, 23, 39].

449

The Freundlich model assumes a multilayer adsorption onto heterogeneous surfaces

450

with energetically non-equivalent binding sites and interactions between adsorbent

451

molecules [1]. The constants KF and n were evaluated from the intercept and slope of the

452

linear plot of experimental data of ln(qe) versus ln(Ce) (Fig. 5b).

453

The values of the dimensionless Freundlich constant n related to the adsorption

454

intensity and surface heterogeneity, were higher than 1 and less than 10 (1 < n < 10) (see

455

Table 2), indicating a favorable sorption of MB onto A. platensis biomass at all studied

456

temperatures. No significant difference for n values was observed with respect to

457

temperature. The parameter ΚF represents a relative measure of adsorption capacity and

20

458

strength. When the equilibrium concentration Ce tends to be one, then ΚF reaches the

459

value of qe [4]. As can be seen in Table 2, the values of ΚF increased slightly with the

460

rising temperature from 298 to 318 K, but decreased between 298 and 308 K. It shows

461

that the multilayer biosorption of MB was enhanced at higher solution temperature.

462 463

To distinguish between physical and chemical sorption, the mean free energy E (kJ/mol) of MB biosorption was calculated by the following equation:

464 465

(14)

466 467

where K DR (mol2/kJ2) is the constant of Dubinin-Radushkevich isotherm.

468

The parameter E is related to the mean free energy of sorption per molecule of sorbate,

469

assuming that the sorbate is transferred to the biosorbent surface from infinite distance in

470

the solution. Typical values of E for chemical sorption are in the range of 8–16 kJ/mol,

471

while E < 8 kJ/mol indicates physical sorption [24]. As shown in Table 2, the mean free

472

energy E of MB biosorption onto A. platensis suggests a chemisorption mechanism,

473

because its values are in the range of 8-16 kJ/mol at all studied temperatures. The

474

increasing temperature caused a slight increase of E from 9.09 to 10.77 kJ/mol, indicating

475

an enhancement of the chemisorption at higher temperatures. The biosorption

476

mechanisms are further discussed in Section 3.7.

477 478

3.5. Biosorption thermodynamics

479

The thermodynamic behavior of MB biosorption onto A. platensis biomass was

480

investigated estimating the thermodynamic parameters of Gibbs free energy change

21

481

(ΔG°), enthalpy change (ΔΗ°) and entropy change (ΔS°). The values of these parameters

482

were estimated using the following equations [35]:

483 484

ΔG° = -R T lnKc

(15)

ΔG° = ΔH° - TΔS°

(16)

485 486 487 488

(17)

489 490

where R is the universal gas constant [8.314 J/(mol K)], T the absolute solution

491

temperature (K), and Kc (Cad,e/Ce) is the adsorption equilibrium constant, which is the

492

ratio of the MB concentration adsorbed (Cad,e) to the MB concentration (Ce) in solution at

493

equilibrium [38].

494

The negative values of ΔG° indicates a spontaneous and favorable adsorption process

495

at all studied temperatures and initial concentrations (see Table 3), suggesting that the

496

system required no energy input from outside [23]. Similar thermodynamic behavior in

497

respect to negative ΔG° values has been found for Spirulina platensis dry biomass as a

498

biosorbent of anionic dyes [13, 23, 39]. For a given initial MB concentration in this work,

499

no significant change of ΔG° was observed with increasing temperature. However, the

500

ΔG° values decreased slightly as the initial MB concentration increased from 50 to 100

501

mg/L, indicating a more favorable adsorption of MB at lower dye concentration.

502

The values of enthalpy change (ΔΗ°) and entropy change (ΔS°) can be calculated from

503

the slope and intercept of the linear plot of lnKc versus 1/T, based on the Eq. (17). As 22

504

shown in Fig. 7, the determination coefficient (R2) of the plots was 0.939 and 0.940 for

505

the two highest initial MB concentrations, respectively, indicating that the estimated

506

values of ΔΗ° and ΔS° were confident. As can be seen in Table 3, the negative values of

507

ΔH° at all studied initial dye concentrations corresponds to an exothermic nature of MB

508

biosorption. Similar results for the cyanobacterium in respect to negative ΔH° values

509

obtained by other studies, which found an exothermic biosorption of anionic dyes [13, 23,

510

39] and phenol [17] onto Spirulina platensis dry biomass.

511

There are different results in the literature in respect to the exothermic or endothermic

512

nature of MB adsorption onto various materials, based on the estimated ΔH° values. An

513

exothermic adsorption of MB was found onto cyclodextrin/silica hybrid adsorbent [38]

514

and green algae Ulothrix sp. [31]. On the other hand, an endothermic adsorption of MB

515

was found onto diatomite treated with sodium hydroxide [29], marble dust [19],

516

montmorillonite clay [1], and acid treated kenaf fibre char [5].

517

The magnitude of enthalpy change can be used to classify the type of interaction

518

between sorbent and sorbate. Values of ΔH° < 30 kJ/mol indicates a physical sorption

519

such as hydrogen bonding [13]. Other mechanisms of physical sorption such as Van der

520

Waals forces usually presents ΔH° values in the range 4-10 kJ/mol, hydrophobic bonds

521

forces about 5 kJ/mol, coordination exchange about 40 kJ/mol and dipole bond forces 2-

522

29 kJ/mol [13]. In contrast, ΔH° > 80 kJ/mol indicates chemical bond forces and a

523

chemisorption process [13, 17, 20]. According to the ΔH° values (< 28.32 kJ/mol)

524

obtained in this study, the biosorption of MB dye onto A. platensis biomass was due to

525

physical adsorption, suggesting weak interactions between biomass and cationic dye [38].

526

Further, the negative effect of increasing temperature on qe (Fig. 4) and the applicability

23

527

of the pseudo-second order kinetic model showed that MB sorption process involved both

528

mainly physical and partly chemical sorption [4]. The low negative values of ΔG° ranging

529

from -20 to 0 kJ/mol suggest that the dominant biosorption mechanism was physisorption

530

[1].

531

The weak binding and weak interactions between the biosorbent and the adsorbate

532

showed that the adsorbed MB molecules should be easily released [38]. This point should

533

be further investigated in order to evaluate the regeneration and reuse ability of A.

534

platensis after dye desorption, in order to reduce the cost of the biosorption process.

535

The negative values of ΔS° for 50 and 100 mg MB/L were very low, indicating no

536

remarkable change on entropy [36] and a decreased disorder at the solid-liquid interface

537

during the MB biosorption onto A. platensis (see Table 3). This showed also that the

538

dispersion degree of the process decreased with increasing temperature [35]. Based on the

539

Eq. (16) and the different magnitude of ΔH° and ΔS° values (Table 3), the enthalpy

540

change (ΔH°) contributed more than entropy change (ΔS°) to obtain the negative values

541

of ΔG° [23]. This observation suggests that MB biosorption onto A. platensis was an

542

enthalpy-controlled process [39].

543 544

3.6. Effect of ionic strength

545 546

Dye effluents contain high concentrations of salts which affect the dye sorption onto

547

biosorbents. Fig. 8 presents the effect of ionic strength on the MB biosorption by A.

548

platensis at 298 K and pH 7.5. It was observed that qe decreased as the NaCl

549

concentration in sorption solution increased from 0.0625 to 0.5 M. The decrease of q e is

24

550

due to the competitive effect between Na+ and MB cations for the available surface

551

binding sites [36] and the electrostatically screening effect of salt [40]. The latter

552

indicates that the electrostatic interactions should be one the main driving forces during

553

MB biosorption process [40]. However, the remarkable biosorption capacity observed

554

even in the presence of much higher NaCl concentration (62.5 mmol/L) than the initial

555

MB concentration of 50 mg/L ( = 0.156 mmol/L) suggests that other interactions such as

556

hydrophobic interactions, π-π interactions and/or hydrogen bonding, contributed to MB

557

removal [40].

558 559

3.7. Biosorption mechanisms

560 561

The amounts of Na+ and K+ cations released from A. platensis surface into the solution

562

after MB sorption are listed in Table 4. Based on the total net cations release at 298 K, it

563

is evident that the cation exchange was one of the major biosorption mechanisms at this

564

temperature. In contrast, the net cations release at higher temperatures was negligible.

565

Besides, no significant change between initial and equilibrium pH was observed at all

566

studied temperatures (Table 4), suggesting that ion exchange between MB cations and

567

protons (H+) of surface functional groups did not take place at pH 7.5. A previous study

568

has confirmed the presence of Na+ and K+ on the cell wall surface of Spirulina sp. [41].

569

The total release of both cations measured in mg/L (data not shown) constituted up to

570

4.7% of the dried biomass weigth (500 mg/L), which agrees with the ash percentage (6.3-

571

7%) in the chemical composition of S. platensis dried biomass reported in the literature

25

572

[9, 39]. The mechanism of cation exchange between MB molecule and the exchangeable

573

cations of biomass surface can be described by the following equations [42]:

574 575

S-O-K + CN+ → S-O-CN + K+

(18)

576

S-O-Na + CN+ → S-O-CN + Na+

(19)

577 578

where S is the surface of A. platensis biomass, Na+ and K+ are the exchangeable cations,

579

and CN+ is the positively charged nitrogen atom of the secondary amine group of MB

580

molecule.

581

Fig. 9 shows the effect of the chemical modification of carboxyl groups on the

582

biosorption capacity. The esterified biomass of A. platensis presented a decrease in the

583

MB biosorption capacity (62.66 mg/g) by 25.5% compared to the biosorption capacity of

584

the untreated biomass (83.83 mg/g) (Fig. 9), due to the block of the surface carboxyl

585

groups. This result indicated the participation of carboxyl groups in the MB binding by

586

the untreated biomass, which is a chemisorption process. The cell wall of cyanobacteria

587

contains a thick structural layer of peptidoglycan and an extended layer of glycoproteins

588

and polysaccharides. These layers are the main source of reactive carboxyl groups on the

589

biosorbent surface [21]. The reaction of the chemical esterification of surface carboxyl

590

groups is described by the following equation, where R are all the components in the

591

dried cells [21]:

592 593 594

RCOOH + CH3OH → RCOOCH3 + H2O (20)

26

595 596

Recent studies for the removal of anionic dyes from aqueous solutions confirmed the

597

mesoporous structure of S. platensis dried microparticles which presented a particle size

598

in the range of 68-75 μm and an average pore radius of 2.25 nm (22.5 Å) [9, 13]. Note

599

that the average pore radius was not modified even in case of S. platensis nanoparticles

600

obtained from the microparticles through a mechanical method [9]. Therefore, the A.

601

platensis microparticles (with particle diameter <154 μm) employed in this study might

602

have a mesoporous structure with a similar average pore diameter of around 4.5 nm. On

603

the other hand, the MB molecule has a parallelepiped shape with dimensions 1.7 × 0.76 ×

604

0.325 nm and its attachement on biomass surface may be done with different orientations

605

[26]. Other workers have reported that the presence of mesopores (average pore diameter

606

of 2-50 nm) is favorable for MB adsorption by various adsorbents [5, 25]. Assuming that

607

the MB molecule lies flat on the biomass surface even on its largest face (1.7 nm) which

608

is smaller than the reported average pore radius of A. platensis (2.25 nm), the MB

609

biosorption in this study may also be due to the intraparticle diffusion of MB molecules

610

in the mesopores and due to the entrapment in intrafibrillar capillaries and spaces of the

611

structural exopolysaccharides [6]. This assumption agrees with the diffusion analysis of

612

the kinetic data. Therefore, the mesoporous structure of A. platensis can facilitate the

613

accommodation of MB molecules in the biomass pores [13].

614 615

4. Conclusions

616

Dry biomass of A. platensis were used as biosorbent for methylene blue removal in

617

batch mode with respect to solution pH, contact time, initial dye concentration,

27

618

temperature and ionic strength. This study applied for the first time a kinetic and

619

thermodynamic analysis for the biosorption of a cationic dye onto A. platensis. In

620

addition, the role of ion exchange mechanism was directly investigated by detection

621

measures. The kinetic data were fitted very well by the pseudo-second order model, and

622

equilibrium was achieved within 60-120 min. It was found that the film and intra-particle

623

diffusion contributed to the MB biosorption process. The biosorption capacity of A.

624

platensis for MB increased with increasing initial dye concentration and decreased with

625

increasing temperature. At all studied temperatures, the Langmuir, Freundlich and

626

Dubinin-Radushkevich isotherm models fitted well the experimental equilibrium data,

627

indicating that MB biosorption was a complex process, involving more than one

628

mechanism. The carboxyl groups of biomass surface contributed to MB chemisorption.

629

The important role of hydrophobic interactions in MB removal was indicated by the

630

considerable biosorption capacity at low pH values and in the presence of NaCl in the

631

sorption solution. The release of Na+ and K+ cations from the biomass surface in the

632

solution after MB sorption confirmed the contribution of cation exchange mechanism.

633

Physical sorption and ion exchange were the dominant mechanisms of MB biosorption at

634

lower temperature. According to the thermodynamic analysis of equilibrium data, MB

635

biosorption onto A. platensis was a spontaneous, favorable and exothermic process. It

636

was concluded that A. platensis biomass has a great potential for removal of MB from

637

aqueous solutions.

638 639

Acknowledgement

28

640

Professor D. Georgakakis of Agricultural University of Athens is kindly acknowledged

641

for his valuable support in respect of the availability of laboratory equipment.

642 643

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644 645

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[38] L.B. Carvalho, T.G. Carvalho, Z.M. Magriotis, T.C. Ramalho, L.M.A. Pinto,

748

Cyclodextrin/silica hybrid adsorbent for removal of methylene blue in aqueous media, J.

749

Incl. Phenom. Macrocycl. Chem. 78 (2014) 77-87.

750

[39] G.L. Dotto, M.L.G. Vieira, V.M. Esquerdo, L.A.A. Pinto, Equilibrium and

751

thermodynamics of azo dyes biosorption onto Spirulina platensis, Braz. J. Chem. Eng. 30

752

(2013) 13-21.

33

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[40] W.-J. Luo, Q. Gao, X.-L. Wu, C.-G. Zhou, Removal of Cationic Dye (Methylene

754

Blue) from Aqueous Solution by Humic Acid-Modified Expanded Perlite: Experiment

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and Theory, Sep. Sci. Technol. 49 (2014) 2400-2411.

756

[41] K. Chojnacka, A. Chojnacki, H. Górecka, Biosorption of Cr3+, Cd2+ and Cu2+ ions

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by blue–green algae Spirulina sp.: kinetics, equilibrium and the mechanism of the

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process, Chemosphere 59 (2005) 75-84.

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[42] V. Hernández-Montoya, M.A. Pérez-Cruz, D.I. Mendoza-Castillo, M.R. Moreno-

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Virgen, A. Bonilla-Petriciolet, Competitive adsorption of dyes and heavy metals on

761

zeolitic structures, J. Environ. Manag. 116 (2013) 213-221.

762 763 764 765 766 767 768 769 770

34

771

772 773

Fig. 1. (a) Plot of initial pH versus final pH for the determination of biomass pHzpc, and

774

(b) the effect of initial pH on MB biosorption onto A. platensis (pHe = equilibrium pH).

775

35

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

36

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

37

Fig. 5. Linear plots of (a) Langmuir and (b) Freundlich isotherm model for the MB biosorption onto A. platensis at different temperatures. 779

38

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

39

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

40

Table 1. Kinetic and diffusion model parameters for MB biosorption onto A. platensis. Initial dye concentration (mg/L) 25

50

100

29.48

54.94

82.95

0.355

0.241

0.337

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

0.998

0.998

0.988

3.46

18.49

6.39

4.84

29.36

6.40

kid (mg/ g min0.5)

0.307

0.197

1.220

I (mg/g)

23.25

59.11

67.66

0.583

0.269

0.517

0.52

0.39

3.88

0.54

0.39

3.70

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

qe,exp (mg/g) Pseudo-first order model R2 Pseudo-second

order

model

R

2

CFEF χ

2

intra-particle diffusion model: whole time data

R

2

CFEF χ

2

intra-particle diffusion model: second linear section

788

41

Table 2. Isotherm parameters values of MB biosoprtion onto A. platensis at different temperatures. Isotherm models

Solution temperature (K) 298

308

318

89.56

82.18

65.70

312.50

204.08

80.65

qe,cal (mg/g)

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

0.950

0.989

0.952

10.82

4.02

3.71

8.96

3.24

3.62

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

0.0048

0.0042

0.0017

6.05 × 10-9

5.85 × 10-9

4.31 × 10-9

9.09

9.25

10.77

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

qe,exp (mg/g) Langmuir qmax (mg/g) 1

R

2

CFEF χ

2

Freundlich

Dubinin-Radushkevich qs (mol/g) BD (mol2/kJ2) E (kJ/mol) R

1

2

qe,cal corresponds to C0 = 100 mg/L.

789

42

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

Na+ (mmol/L)

+

K (mmol/L)

Temperature (K) 298

308

318

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

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

43