Accepted Manuscript Adsorption of anionic and cationic dyes on ferromagnetic ordered mesoporous carbon from aqueous solution: Equilibrium, thermodynamic and kinetics Xiaoming Peng, Depong Huang, Tareque Odoom-Wubah, Dafang Fu, Jiale Huang, Qingdong Qin PII: DOI: Reference:

S0021-9797(14)00341-5 http://dx.doi.org/10.1016/j.jcis.2014.05.035 YJCIS 19587

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

30 January 2014 20 May 2014

Please cite this article as: X. Peng, D. Huang, T. Odoom-Wubah, D. Fu, J. Huang, Q. Qin, Adsorption of anionic and cationic dyes on ferromagnetic ordered mesoporous carbon from aqueous solution: Equilibrium, thermodynamic and kinetics, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.05.035

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1

Adsorption of anionic and cationic dyes on ferromagnetic

2

ordered mesoporous carbon from aqueous solution:

3

Equilibrium, thermodynamic and kinetics

4

Xiaoming Peng1, Depong Huang2, Tareque Odoom-Wubah2,

5

Dafang Fu∗1, Jiale Huang∗2, Qingdong Qin1

6 7

( 1. School of Civil Engineering, Southeast University, Nanjing 210096, PR China. 2. Department of Chemical and Biochemical Engineering, College of Chemistry and

8

Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China.）

9

Abstaract: Ordered mesoporous carbon (Fe-CMK-3) with iron magnetic nanoparticles was

10

prepared by a casting process via SBA-15 silica as template and anthracene as carbon source,

11

was used as a magnetic adsorbent for the removal of anionic dye Orange II (O II) and cationic

12

dye methylene blue (MB) from aqueous solution. TEM and magnetometer images showed

13

that the iron magnetic nanoparticles were successfully embedded in the interior of the

14

mesoporous carbon. The effect of various process parameters such as temperature (25~45℃),

15

initial concentration (100~500mg L-1) and pH (2~12) were performed. Equilibrium adsorption

16

isotherms and kinetics were also studied. The equilibrium experimental data were analyzed by

17

the Langmuir, Freundlich, Temkin and Redlich-Peterson model. The equilibrium data for two

18

dyes adsorption was fitted to the Langmuir, and the maximum monolayer adsorption capacity

19

for O II and MB dyes were 269 and 316 mg g-1, respectively. Pseudo-first-order and

20

pseudo-second-order kinetic and intraparticle diffusion model were used to evaluate the

21

adsorption kinetic data. The kinetic data of two dyes could be better described by the pseudo

22

second-order model. Thermodynamic data of the adsorption process were also obtained. It

23

was found that the adsorption process of the two dyes were spontaneous and exothermic.

24

Keywords: Mesoporous carbon; magnetic nanoparticles; adsorption; dyes

25

1. Introdution

26

Synthetic organic dyes have emerged as important pollutants in the environment due to

1.∗ Corresponding author. E-mail: [email protected] 2.∗ Corresponding author. E-mail: [email protected] 1

27

their worldwide application in many industries, such as textile, paper, printing, food,

28

cosmetics, etc [1]. Many dyes and pigments are inert, toxic and difficult to biodegrade when

29

discharged into waste streams. In addition, the presence of dyes and pigment in water are

30

highly visible and affects the water transparency, resulting in reduction of light penetration,

31

and oxygen gas solubility in water. According to the literature, it is estimated that more than

32

over 7×105 t dyes are discharged annually to the aquatic ecosystem [2]. The great majority of

33

dye wastewater is colored effluent. The discharge of highly colored dye effluents into the

34

environment is currently one of the world’s major environmental problems for both

35

toxicological and esthetical reasons.

36

A variety of treatment methods, including chemical oxidation [3], biodegradation [4],

37

photocatalytic degradation [5] and adsorption [6] have been developed to remove dyes from

38

wastewater. Among these techniques, adsorption is the most favorable method for removal of

39

dyes because of its simple design, ease of operation, and high-performance removal efficiency

40

of toxic substances. Using various carbon materials as adsorbent for dyes has been widely

41

studied due to their large surface area and high adsorption capacity. Some researchers have

42

studied the production of activated carbon from bamboo [7], tire [8], mangosteen peel [9],

43

sawdust [10], oil palm [11], and sludge [12]. Activated carbons were also used as adsorbents

44

for molecules with large diameters. However, the applications of activated carbons are

45

restricted due to the presence of micropores. Mesoporous (2~50 nm) carbon, as carbon

46

material adsorbent, with open pore structure, high specific surface area, high thermal stability,

47

good chemical stability and large pore size could favor the dye adsorption of different

48

molecular structures. Some papers have reported the preparation of order mesoporous carbon

49

from rice husks [13], wood materials [14] and waste tires [8]. In addition, mesoporous carbon

50

can also be prepared using template method, mainly including soft and hard template. In the

51

soft template method, self-assembly of organic molecules which generates the nanostructures,

52

and molecules or moieties are manipulated at the molecular level and spatially organized in

53

nanospaces by the chemical interactions between templates and carbon precursors.

54

Mesoporous carbon with uniform pores can be synthesized using this method, but the pore

55

structure arrangements of these carbons are disordered. Hard template synthesis method using

56

post-synthesized templates served as formworks to synthesize ordered mesoporous carbon. 2

57

The porous structures of the mesopore carbon are predetermined by the templates resulting in

58

the mesopore carbons having well-defined nanostructures. In addition, the porous structure

59

and the pore diameters can be tuned by using this method. Some groups have synthesized

60

ordered mesoporous carbon using various mesoporous silica or aluminosilicate as a hard

61

template [15].

62

In most water treatment processes, the carbon powder materials dispersed in the treated

63

solutions are quite difficult to separate that even could result in secondary pollution.

64

Conventional separation technologies mainly include the process of filtration or centrifugation

65

procedure, which is rather complex, uneconomical and cannot be widely utilized. At present,

66

magnetic separation as a prospecting technology is an attractive alternative due to the fact that

67

it can be easily separated by means of an external magnetic field [16, 17].

68

In previous work, we used sucrose as the carbon precursor to prepare ordered

69

mesoporous carbon CMK-3, but the synthesized mesoporous carbon had little or no graphitic

70

character [18]. In this paper, we attempt to use ordered mesoporous SBA-15 silica as hard

71

template and anthracene serves as the carbon precursor to prepare ordered mesoporous carbon

72

CMK-3, then iron magnetic nanoparticles are loaded on the graphitic mesoporous carbon

73

framework via a simple wetness impregnation process, in which ferric chloride (FeCl3·6H2O)

74

was the iron source. The ordered mesoporous Fe-CMK-3 exhibited high surface area, large

75

pore volume, and narrow uniform pore size distribution. In the adsorption experiments, the

76

ordered mesoporous Fe-CMK-3 as an adsorbent for the removal of the anionic dye Orange II

77

and cationic dye methylene blue (MB) from aqueous solution. It indicated that the Fe-CMK-3

78

was of high performance in adsorption of the two dyes. The effects of contact time, dyes

79

initial concentration, pH, ionic concentration and temperature were investigated. Adsorption

80

isotherms and kinetics also were determined to elucidate the adsorption mechanism of MB

81

and OII molecules onto the Fe-CMK-3. Moreover, the experimental data were analyzed using

82

pseudo-first-order,

83

Thermodynamic data of the adsorption were also calculated.

84

2. Materials and methods

85

2.1. Materials

86

pseudo-second-order

and

intraparticle

diffusion

kinetic

models.

Triblock copolymer EO20PO70EO20 (Pluronic P123, Aldrich), tetraethyl orthosilicate 3

87

(TEOS, 98%, Aldrich), anthracene, HCl, NaOH, NaCl, H2SO4, FeCl3·6H2O were purchased

88

from Sigma-Aldrich. All other reagents were of analytical grade. Anionic dye Orange II (O II)

89

and cationic dye methylene blue (MB), were selected as the targeted adsorbates in this study.

90

Two dyes molecular characteristics and structure were illustrated in Fig. 1 and Table 1.

91

Table 1 General characteristics of MB and O II Dye

Chemical class

Molecular weight

Molecular formula

λmax(nm)

(g mol-1) Methylene blue(MB)

Cationic dye

373.9

C16H18CHN3·3H2O

665

Orange II (O II)

Anionic dye

350.3

C16H11N2NaO4S

485

92

N CH3 Cl

H3C S

N 93

+

-

3H2O

N

CH3

CH3

a. Methylene blue (MB)

94 95

O

N N

O 96

S O

Na

O -

97

b. Orange II (O II)

98

Fig.1 Chemical structures of MB and O II.

99

+

2.2. Synthesis of mesoporous carbon Fe-CMK-3

100

The Fe-CMK-3 samples were synthesized by a nanocasting process using SBA-15 silica

101

as template and anthracene as the carbon source. The synthesis procedure has slightly been

102

revised by Jun et al reported [19]. The SBA-15 template was prepared using the triblock

103

copolymer, EO20PO70EO20 (Pluronic P123, Aldrich) as the structure-directing agent and

104

tetraethyl orthosilicate (TEOS, 98%, Aldrich) as the silica source. The triblock copolymer 4

105

Pluronic P123 was dissolved in hydrochloride solution at 40 ℃. Then TEOS was added to

106

the above mixture solution under magnetic stirring. The molar composition ratio of the final

107

mixture was TEOS: P123: HCl: H2O=1:0.0165:5.755:191.61. The solution was magnetically

108

stirred at 40℃ for 24h and then transferred into an autoclave for 24h under static conditions

109

for hydrothermal treatment at 100℃. Subsequently, the mixture was filtrated, dried and

110

washed with distilled water and then calcined at 550℃ for 12h to remove the organic

111

template P123.

112

The Fe-CMK-3 samples were synthesized by the following the procedure (Scheme is

113

shown in Fig. 2): First of all, the mesoporous carbon CMK-3 products were obtained by

114

impregnating 2g of SBA-15 template with 5mL aliquots of 0.4g anthracene in acetone

115

containing 0.28g sulfuric acid. The solution was magnetically stirred until it was dried.

116

Then the mixture was heated at 160℃ for 8 h to initiate carbonization. The impregnation and

117

carbonization treatment processes were repeated under the same condition until the entire

118

anthracene precursor solution was consumed (reached 3g). Afterward, the mixtures were

119

placed in a quartz glass tube and heated at 400℃ for 4h, and then further carbonized under

120

nitrogen at 800℃ for another 4h (at a rate of 5℃ min-1). Finally, the product CMK-3 was

121

filtered and washed with 1M NaOH solution (Vdeionized water:Vethanol=1:1) for 1h at 100℃ to

122

dissolve the silica template, and then dried at 105℃ for 12 h. Subsequently, 0.5g of CMK-3

123

was dispersed in 0.2g FeCl3·6H2O and 5mL ethanol and then the mixture was stirred under

124

magnetic stirring for 6h at room temperature, The sample was evaporated at 50℃. The

125

obtained dried sample was then subjected to polymerization and carbonization at 900℃ for 4

126

h under nitrogen atmosphere [20].

127 5

128 129

Fig.2 Synthetic Procedure of Fe-CMK-3

130

The textural characterization of the CMK-3 and the Fe-CMK-3 were performed using

131

Powder X-ray diffraction (XRD), Transmission electron microscopy (TEM) and

132

Micrometitics ASAP 2020 surface area analyzer. The low-angle XRD pattern of sample

133

CMK-3 and Fe-CMK-3 are shown in Fig. 3. The sample Fe-CMK-3 showed three

134

well-resolved XRD peaks as well as CMK-3, which can be indexed as the (100), (110) and

135

(210) reflections of the 2-D hexagonal symmetry (p6mm), indicating that the sample still a

136

had good structural order after the modification. In addition, It was noted that the intensity of

137

the (100) reflection peak was weaker than that of the CMK-3, this indicates that the ordered

138

mesostructure system was slightly destroyed upon loading the iron nanoparticles.

Intensity (a.u.)

Fe-CMK-3 CMK-3

1.0

1.5

2.0

2.5

3.0

2 Theta (degree)

139 140

Fig. 3 XRD patterns of CMK-3 and Fe-CMK-3

141

Some of the changes that took place in the structure of the Fe-CMK-3 sample were

142

further confirmed by TEM images (Fig. 4). The TEM images of the Fe-CMK-3 showed that

143

the material still had well-organized uniform ordered pore mesostructure. Meanwhile some

144

well-dispersed dark spots in the carbon matrix also can be clearly observed, indicating that the

145

Ferrum nanoparticles were successfully grafted on the mesoporous carbon structure.

6

146

(a) CMK-3

147

148

(b) Fe-CMK-3

149 150

Fig.4 TEM images of CMK-3 and Fe-CMK-3 (a) CMK-3 (b) Fe-CMK-3

151

The specific surface area was calculated using the BET equation. The total pore volume

152

(Vp) was estimated from the amount of nitrogen adsorbed at a relative pressure of P/P0=0.98.

153

The micropores and mesopores volume of the Fe-CMK-3 were calculated using alpha-s

154

method and BJH method, respectively. Pore size distribution of the Fe-CMK-3 was estimated

155

using BJH method [21]. The pHPZC values of Fe-CMK-3 were determined by adjusting the pH

156

of 50 mL 0.01 mol L-1 NaCl solution to a value from 2 to 12 using 0.1 M HCl or 0.1 M NaOH.

157

0.1 g of Fe-CMK-3 was added and then shaked on a shaker at 120 rpm (25℃) for 48h. The

158

final pH was recorded. The pHPZC values can be determined from the plot ΔpH (pHinitial-pHfinal)

159

versus pHinitial is the point where pHinitial-pHfinal=0.

160

Table 2 Structural properties of CMK-3 and Fe-CMK-3 Sample

SBET (m2 g-1)

Vtotal (cm3 g-1)

Vmic (cm3 g-1)

CMK-3

858

1.03

0.47

3.2

Fe-CMK-3

731

0.91

0.24

3.1

Dmeso(nm) *

161

* The mesopore diameter corresponds to the most frequent value in the pore size distribution, Vmeo

162

=Vtotal-Vmic 7

163

The textural properties of the CMK-3 and Fe-CMK-3 samples are listed in Table 2. As

164

shown in Table 2, compared with CMK-3, there was a slight decrease in the BET surface area

165

and the total volume of Fe-CMK-3 after iron nanoparticles were incorporated into the carbon

166

structural under high treatment temperature. This can be attributed to the increased carbon

167

framework shrinkage with increase in the temperature [22, 23]. It can be observed that the

168

Fe-CMK-3 possesses a BET surface area of 731 m2 g-1, a micropores volume of 0.24 cm3 g-1

169

and a mesopore volume of 0.67 cm3 g-1. Fig. 5 displays the isotherms of N2

170

adsorption/desorption and the pore size distribution of the Fe-CMK-3 (CMK-3 not shown).

171

As it can be seen from Fig.5, Fe-CMK-3 shows that a typical uniform mesoporous structures

172

with an average pore diameter range equal to 3.1 nm. N2 adsorption/desorption isotherms of

173

the Fe-CMK-3 shows that the presence of a type H1 hysteresis loop which indicates a type IV

174

isotherm (Fig. 5), which is a typical mesoporous solid according to IUPAC classification [24].

175

The pore-size distribution curves (inset) calculated from the adsorption branches clearly

176

confirms narrow pore-size distributions. Fig. 6 showed that the pHPZC value of the Fe-CMK-3

177

was about 7.8.

800

600

-1

Volume( cm g )

700

3

0.10

)

cm 3/(gnm)

500

0.08

0.06

Desorption Dv(d)

(

400

300

0.04

0.02

0.00 0

5

10

15

20

25

30

35

Pore diameter(nm)

200 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure ( P/P0)

178 179 180

Fig. 5 N2 isotherms and pore size distributions of Fe-CMK-3. The inset is the pore size distribution of Fe-CMK-3

8

Δ

pH=( pHinitial-pHfinal)

3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5

2

4

6

8

10

12

-1.0 -1.5 -2.0 -2.5 -3.0

pHinitial

181 182

Fig. 6 PHpzc plot of Fe-CMK-3

183

The magnetic behavior of the Fe-CMK-3 was investigated using a magnetometer at room

184

temperature. Fig. 7 shows the magnetization curve measured for Fe-CMK-3. It revealed that

185

the Fe-CMK-3 exhibits strong ferromagnetic properties. A large magnetic hysteresis loop

186

could be observed, with the maximum value of saturation magnetization strengths (Ms)

187

reaching 4.77 emu g-1. From the inset of Fig. 7 also revealed that, the Fe-CMK-3 powders

188

moved to the wall of the glass bottle by an external magnetic field. This showed that, the

189

Fe-CMK-3 had high magnetization values and could be easily attracted to magnets by the

190

application of a magnetic field. This result proves that the Fe-CMK-3 can be easily separated

191

from the aqueous solution. It indicated that the porous carbon had magnetic characteristics and

192

removed the pollutants under the control of external magnetic fields. It is a simple way to

193

separate the magnetic adsorbent from waste water so as to avoid the second pollution.

194 195 196

Fig. 7 The magnetization curves measured at 300 K for the Fe-CMK-3. This inset shows that the Fe-CMK-3 could be separated from solution by an external magnet. 9

197

2.3. Batch adsorption experiments

198

Adsorption experiments were conducted to evaluate the adsorption ability of the

199

Fe-CMK-3 adsorbent for the removal of O II and MB dye from aqueous solutions. The first

200

experiment was to study the effect of pH on the two dyes adsorption process. The pH values

201

of the two dyes solutions were adjusted from 2 to 12 using 0.1 M HCl or 0.1 M NaOH

202

solutions. And then 50ml O II or MB solution (250mg L-1) and 0.03g Fe-CMK-3 was mixed

203

on a shaker (120rpm) until it reached equilibrium. The second experiment was to investigate

204

the effect of the dyes initial concentration. 0.03g Fe-CMK-3 were added into different initial

205

concentrations (100~500mg L-1) of O II or MB. The mixture was stirred at 120rpm until it

206

reach equilibrium at 25℃. The third experiment was to study the effect of the salt

207

concentration on adsorption, different concentrations of NaCl (0.1~1mol L-1) were added into

208

the mixture of 0.03g Fe-CMK-3 and dye concentration of 200mg L-1 O II or MB solutions

209

(50mL) , respectively. The fourth experiment was to investigate the effect of temperature.

210

0.03g of Fe-CMK-3 was added to 50mL of initial concentrations of 200mg L-1 O II or MB

211

solution. The experiments were conducted for 48 h at 25, 35 and 45℃. The concentrations of

212

O II and MB dye in the aqueous solution after and before adsorption experiment were

213

determined using a UV/Vis spectrometer at maximum wavelength of 631nm and 485nm,

214

respectively. The adsorbents were separated by an external magnet. Each experiment was

215

duplicated under identical conditions.

216 217

The amount of adsorbed dye qt (mg g-1) at different time and qe (mg g-1) at equilibrium, were calculated by:

qt =

218

(C0 − Ct )V W

(1)

219

qe =

220

(C0 − Ce )V W

(2)

221

where C0 (mg L-1) and Ce (mg L-1) are the initial liquid-phase and equilibrium

222

concentrations of the dye, respectively. Ct (mg L-1) is the liquid-phase concentration of dye at

223

any time t. V is the volume of the dye solution (L) and W is the mass of adsorbent used (g).

224

2.4. Kinetic models 10

225

Batch sorption kinetic experiments were carried out in 500mL flasks containing 300mL

226

of O II or MB dye solution (200mg L-1) with 0.1g Fe-CMK-3 adsorbent, respectively. The

227

flasks were agitated on a rotary shaker at 120rpm under constant temperature (25℃). The

228

samples were extracted at different time intervals, filtered and analyzed for the dye

229

concentrations. The sorption kinetics of the dyes was investigated using the pseudo-first-order,

230

pseudo-second-order and intraparticle diffusion model.

231

The pseudo-first-order equation is express as follows:

log ( q e − q t ) = logq e −

232

k1 t 2.303

(3)

233

where qe and qt are the amounts of O II or MB adsorbed (mg g-1) at equilibrium and at time t

234

(min), respectively and k1 (min−1) is the rate constant adsorption of pseudo-first-order

235

equation.

236

The pseudo-second-order equation can be expressed as:

t 1 t = + 2 qt k2 qe qe

237

(4)

238

where qe and qt are the amounts of O II or MB adsorbed (mg g-1) at equilibrium and at time t

239

(min), respectively. The k2 (g mg-1 min) is the rate constant of the second-order equation.

240

The dye sorption onto the solid surface is usually governed by either the liquid phase

241

mass transport rate or through the intraparticle mass transport rate [25]. The intraparticle

242

diffusion model is commonly used can be presented by the followed equation:

qt = ki t1/2 + C

243

(5)

244

where qt (mg/g) is the fraction of dye uptake at time t, ki is the intraparticle diffusion rate

245

constant (mg g−1min

246

relates to the thickness of the boundary layer.

247

2.5. Adsorption isotherms

−1/2

), t1/2 is the square root of the time, and C is the intercept, which

248

The adsorption isotherm reveals the distribution of the adsorption molecules between the

249

liquid phase and the solid phase when the adsorption process reaches an equilibrium state. In

250

current work, the batch experimental data were analyzed using Langmuir, Freundlich and

251

Temkin isotherm model equations.

252

The Langmuir model is based on the assumption of adsorption on a homogeneous 11

253

surface and once a sorbate molecule occupies a site, no further adsorption takes place at that

254

site [26]. The model is express by:

qe =

255

k L Ce Qm 1 + k LCe

(6)

256

where qe (mg g-1) is the solid-phase adsorbate concentration in equilibrium, Ce (mg L-1) is the

257

liquid-phase adsorbate concentration at equilibrium, kL (L mg-1) is Langmuir isotherm

258

constant related to the affinity of binding sites. Qm (mg g-1) is the maximum amount of dye

259

per unit weight of adsorbent for complete monolayer coverage.

260 261

The Freundlich model assumes that adsorption of dye molecular take place on heterogeneous surface by monolayer adsorption, was expressed as:

262

qe = K F Ce1/n

263

where qe (mg g-1) is the solid-phase adsorbate concentration in equilibrium, Ce (mg L-1) is the

264

liquid-phase adsorbate concentration at equilibrium. The KF (mg g-1) and n are Freundlich

265

isotherm constants and can be regarded as adsorption capacity and intensity, respectively.

(7)

266

The Temkin isotherm assumes that the heat of adsorption of all molecules in the phase

267

decreases linearly when the layer is covered and that the adsorption has a maximum energy

268

distribution of uniform bond [27].

269

The Temkin isotherm can be written as:

270

qe = B ln( K t Ce )

271

where B=RT/bt, bt(J mol-1), Kt(L g-1), R(8.314 J mol-1 K) and T(K) are the Temkin constant

272

related to heat of sorption, equilibrium binding constant, universal gas constant and absolute

273

solution temperature, respectively.

(8)

274

The Redlich-Peterson isotherm incorporates three parameters into a hybrid isotherm by

275

featuring both Langmuir and Freundlich isotherms. The mathematical expression of

276

Redlich-Peterson isotherm is defined as [28]:

277

qe =

K R Ce 1 + aLCeg

(9)

278

Where KR (L g-1) and αL (1 mg-1) are Redlich-Peterson isotherm parameters, and g is the

279

isotherm exponent, which lies between 1 and 0. 12

280

3. Results and discussion

281

3.1. Effect of solution pH

282

The pH in the solution plays an important role in the chemistry of both dye molecules

283

and the Fe-CMK-3 in aqueous solution. In addition, it has a significant effect on electrostatic

284

charges that are imparted by ionized dye molecules between adsorbent and adsorbate. 440

400

430

390

O II

380

420

370

410

-1

-1

qe( mg g )

360

qe ( mg g )

MB

350 340

400 390 380

330

370

320

360

310

350

300 2

4

6

pH

8

10

12

2

4

6

8

10

12

pH

285 286

Fig.8 Effect of solution pH on the adsorption equilibrium of O II and MB dye

287

As shown in Fig.8, the removal of the O II or MB from aqueous solution was highly

288

dependent on the pH of the solution. The pH of the solution affected the surface charge of the

289

Fe-CMK-3 and the degree of ionization of the dyes. For cationic dye MB, with an increase of

290

pH, the adsorption capacity of MB increased from 355 to 416 mg g-1, which primarily

291

attributes to the protonation of MB in the acidic solution, and the presence of excess hydrogen

292

ions as a result of competing with dye cations for the adsorption sites under low pH condition.

293

In the higher pH solution, the Fe-CMK-3 surface became negatively charged due to

294

deprotonation of the adsorbent surface and the formation of electrostatic double layer, which

295

changes its polarity, thus the MB dye sorption increases [29]. In addition, it may be due to the

296

neutralization of the negative sites at the surface of the Fe-CMK-3, and hence facilitates

297

diffusion and provides more active surface of the adsorbents resulting in enhanced uptake at

298

adsorbent surface [30]. With the gradual increasing of the pH of the solution, the number of

299

ionizable sites on the Fe-CMK-3 also increased. In addition, the maximum amount of

300

adsorbed MB dye solution concentrations (about pH 12) was above the zero point charge

301

(pHzpc=7.8), deducing that the surface negative charge density of Fe-CMK-3 increased which

302

favored the adsorption of the cationic dye [31]. However, the opposite trend was also

303

observed for anionic dye O II. The maximum adsorption capacity of O II was 390 mg g-1, 13

304

observed at pH 2.5. The uptake of O II drastically decreased with increasing solution pH from

305

2.5 to 11. The results also could be well explained by electrostatic interactions. The maximum

306

adsorption of O II at pH 2.5 due to the electrostatic attraction between the negatively charged

307

deprotonated O II dye and positively charged of the Fe-CMK-3 surface. Whereas, with an

308

increase in solution pH electrostatic repulsion between the negatively charged surface and O

309

II anionic dye molecules increased, thus decreasing the adsorption capacity. These results

310

showed that the solution pH＞pHpzc, the surface of the adsorbent is net negatively charged,

311

favoring the adsorption of O II in cationic species, while for a solution pH＜pHpzc, the surface

312

is positive charged, favoring adsorption of anionic species for a solution [32]. Similar result

313

was reported by Araceli et.al [33]. In conclusion, the maximum removal rate for O II and MB

314

was observed at pH 2.5 and pH 11.5 respectively.

315

3.2. Effect of contact time and initial concentration of dyes

316

As shown in Fig.9, the equilibrium adsorption capacity increased distinctly with

317

increasing the concentration of O II or MB dye in the initial stages and then the adsorption

318

rate became gradually slower. At some point in time, the adsorbed amount of O II or MB dye

319

onto the Fe-CMK-3 was in a state of dynamic equilibrium, and thereafter the rate of

320

adsorption reached a constant value. The equilibrium was established after one or more hours

321

of agitation time. The reason for this observation is thought to be the fact that, there was an

322

increase in the driving force of the concentration gradient between Fe-CMK-3 and the two

323

dyes, as an increase in the initial dye concentration. Moreover, it could be attributed to the

324

mass transfer driving force becoming larger as an increase of the initial concentration occurs,

325

resulting in higher uptake O II or MB from aqueous solution [29]. In summary, the whole

326

adsorption process of O II or MB dye onto Fe-CMK-3 mainly included three phases: The

327

initial rapid uptake phase, the slow uptake phase and the equilibrium phase. At the initial

328

stage, due to a large number of vacant sites available, there existed highly concentration

329

gradient between the adsorbate in solution and the adsorbate onto the adsorbent surface, thus

330

led to increase in dye sorption rate at initial stages. Afterward, as time proceed this

331

concentration gradient and adsorption sites were reduced due to the accumulation of O II or

332

MB dye molecules in the vacant sites, the adsorption rate of the two dyes were smoothly 14

333

increased and the finally reached equilibrium, As can be seen from Fig. 9, the adsorption

334

curves were single, smooth and continuous leading to saturation. It indicated the possible

335

monolayer coverage on the surface of the adsorbent by the dye molecules [30]. 400

300

O Ⅱ

MB

-1

Adsorption capacity ( mg g )

-1

Adsorption capacity ( mg g )

250

200

150

100 mg/L 200 mg/L 300 mg/L 400 mg/L 500 mg/L

100

50

0

300

200

100 mg/L 200 mg/L 300 mg/L 400 mg/L 500 mg/L

100

0 0

100

200

300

400

500

600

700

0

100

Time (min)

200

300

400

500

600

700

Time (min)

336 337

Fig.9 The effect of initial dye concentration and contact time on the adsorption capacity of O

338

II and MB dye （T=25℃, rpm=120, V=50 mL, pH=7）

339

The unit adsorption for O II increased from 166.4 to 287.01 mg g-1 with the increasing of

340

O II concentration from 100 to 500 mg L-1. And the adsorption amount for MB also increased

341

from 165.18 to 357.86 mg g-1 as the MB concentration increased from 100 to 500 mg L-1. It

342

may be ascribed to the driving force of the concentration gradient increased as an increase in

343

the initial dye concentration. The increase in initial dye concentration also enhances the

344

interaction between dye and adsorbent [34]. Additionally, it was observed that the contact

345

time needed for the O II or MB solutions with initial concentrations of 100~200 mg L-1 to

346

reach equilibrium was around 30 min, indicating the high affinity between the two dyes and

347

the porous carbon material. However, for MB or O II solutions with higher initial

348

concentrations, longer equilibrium time was required. This result might be attributed to the

349

condition that the dye molecules can quickly reach the boundary layer at initial adsorption

350

stage, after which they had to diffuse into the adsorbent surface, and then finally, MB or O II

351

molecules have to penetrate into the pores of Fe-CMK-3 via a longer diffusion length.

352

Therefore, it will take a relatively longer contact time to reach equilibrium, which shows that

353

the adsorption is dominated by diffusion [35, 36].

354

3.3. Effect of salt concentration

355

Textile industries usually use large amounts of salts for the dyeing of fabric. So textile

356

wastewater commonly contain dyes with higher salt concentration, thus the effect of salt 15

357

concentration on dye removal from aqueous solution is very important. Moreover, the salt

358

concentration of the solution is one of the factors that have influence on the hydrophobic and

359

electrostatic interaction between dye and surface functional adsorptive sites of the adsorbent

360

[37].

500

450

450

OⅡ

Adsorption capacity qe ( mg g-1)

e

Adsorption capacity q ( mg g-1)

500

400 350 300 250 200 150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

MB

400 350 300 250 200 150 100 50 0 0.0

0.2

-1

CNaCl ( mol L )

0.4

0.6

0.8

1.0

-1

CNaCl( mol L )

361 362

Fig.10 The effect of NaCl on the adsorption of O II and MB onto the Fe-CMK-3 （T=25℃,

363

rpm=120, V=50 mL, pH=7）

364

Fig. 10 showed that the effect of NaCl on the percentage removal of O II or MB. The

365

adsorption rate of the O II dye was significantly influenced by the salt concentration, while its

366

influence on MB was slight. It was observed that the adsorption equilibrium of O II obviously

367

increased from 274 to 333 mg g-1 with an increase in the NaCl concentration. The uptake of

368

MB also slightly increased from 322 to 336 mg g-1 with increasing NaCl concentration from 0

369

to 1 mol L-1. It became approximately constant from NaCl concentration 0.1 to 1 mol L-1. This

370

implies that salt concentration had an important influence on the adsorption of O II or MB

371

dyes on the Fe-CMK-3. In theory, an increase in salt will decrease the adsorption capacity due

372

to electrostatic forces between the adsorbent surface and adsorbate ions are attractive, but our

373

result was on the contrary before infers [38, 39]. This may be attributed to the aggregation of

374

O II or MB dyes molecules when the ionic strength of the solution increased thus increasing

375

the number of intermolecular forces. Other causes for this phenomenon maybe due to the

376

increase in ionic strength which enhances the adsorption of ionic compounds such as O II or

377

MB dyes because of the screening effect of the surface charge and the depressed electrostatic

378

repulsion by the added salt [40]. Furthermore, the process might have other interactions 16

379

besides the interaction between the dye molecules and the adsorbent surface to control the

380

adsorption process. In conclusion, Fe-CMK-3 could be used to efficiently remove O II or MB

381

from aqueous solution with higher salt concentration. But the result is not in good agreement

382

with the results of methylene blue adsorption onto natural zeolite [41].

383

3.4. Effect of temperature

384

The temperature used had a significant effect on the dye adsorption process. The rate of

385

diffusion of the dye molecules across the external boundary layer and in the internal pores of

386

the adsorbent particle increased with increasing the temperature. This could be ascribed to

387

decrease in the viscosity of the solution. Moreover, changing temperature will change the

388

equilibrium capacity of the adsorbent for the dye [42].

389

As shown in Fig. 11, the shape of the isotherms indicates high affinity between

390

Fe-CMK-3 surface and that of the dye molecules. The equilibrium adsorption capacity of O II

391

or MB adsorbed increased with the increase in the temperature of the adsorption process from

392

25 to 45℃. It indicated that the two dyes adsorption on the adsorbent were favored at higher

393

temperatures. This trend can be explained by the increase in the number of active sites on the

394

Fe-CMK-3 for the adsorption and accompanied by increasing in temperature, this observation

395

can be ascribed to the adsorbent polarity and hydrogen bonding which decreases as increase

396

of temperature, and thus more sites become available for O II or MB uptake [43]. Other

397

explanations could be that there is an increase in the mobility of O II or MB molecule in the

398

solution or the creation of new active sites on the adsorbent with an increase in the

399

temperature. The adsorption mechanism related to the removal of O II or MB on Fe-CMK-3

400

involves a physical process. The increasing number of O II or MB dye molecules may also

401

acquire sufficient energy to underground the interaction with active sites at the surface. This

402

kind of temperature dependence of the adsorption capacity of the adsorbed dye may reflect,

403

the increase in this case, with which the dye penetrates into the pores of the Fe-CMK-3 owing

404

to its larger diffusion coefficient [44]. The result indicates that the adsorption process of two

405

dyes on the Fe-CMK-3 was endothermic in nature. Similar result was observed by the

406

adsorption of methylene blue onto activated carbon [35].

17

300

400 350

Adsorption capacity ( mg g-1)

Adsorption capacity ( mg g-1)

250

200

25 ℃ 35 ℃ 45 ℃

150

100

50

OⅡ 0

300 250 200

25 ℃ 35 ℃ 45 ℃

150 100

MB

50 0

0

100

200

300

400

500

600

700

800

0

100

200

Time (min)

300

400

500

600

700

800

Time (min)

407 408

Fig.11 The effect of temperature on adsorption of O II and MB onto the Fe-CMK-3

409

（T=25℃, rpm=120, V=50 mL, pH=7）

410

3.5. Adsorption isotherms

411

The adsorption isotherm reveals the distribution of the adsorption molecules in liquid

412

phase and solid phase when the adsorption process reaches an equilibrium state. Equilibrium

413

adsorption experiments were carried out to evaluate the adsorption capacities of the

414

Fe-CMK-3 for O II and MB solution. The sorption equilibrium data of O II and MB dyes onto

415

the Fe-CMK-3 were analyzed with Langmuir, Freundlich, Temkin and Redlich-Peterson

416

isotherm models. Four isotherm models parameters were listed in Table 3.

417

Table 3 four isotherm parameters and correlation coefficients for the adsorption O II and MB

418

dyes on the Fe-CMK-3 at 25℃ Langmuir

dye

Freundlich R2

Temkin R2

Redlich-Peterson

Qm

KL

(mg g-1)

(L mg-1)

O II

269

0.302

0.998

14.92

190.17

0.726

15.44

117.54

MB

316

0.338

0.999

10.63

217.55

0.933

22.53

86.37

n

KF

B

(mg g-1)

Kt

R2

(L g-1)

aL

KR

R2

(L g-1)

(1 mg-1)

0.728

311.53

1.06

0.957

0.952

334.25

1.54

0.998

18

1.6

8

b 1.4

a 1.2

6 -1

Ce/qe ( g L )

1.0

ln qe

0.8 0.6

MB OⅡ

0.4

MB OⅡ

4

0.2 0.0 0

50

100

150

200

250

300

350

400

450

2

500

0

2

4

-1

Ce ( mg L )

6

8

ln Ce

419 500

3.0

c

d 2.5

ln[ KR( Ce/qe) -1]

300

-1

qe (mg g )

400

200

MB OⅡ

100

1.5

1.0

MB OⅡ

0.5

0.0

-0.5

0 0

420

2.0

1

2

3

4

ln Ce

5

6

7

8

-1

0

1

2

3

4

5

6

7

ln Ce

421

Fig.12 (a) Langmuir (b) Freundlich (c) Temkin and (d) Redlich-Peterson isotherm adsorption

422

model of the O II or MB dyes onto Fe-CMK-3 （T=25℃, rpm=120, V=50 mL, pH=7）

423

Fig. 12 showed the four model equilibrium adsorption linear of dye molecules onto the

424

Fe-CMK-3 at 25℃. As can be seen, the adsorption experimental data of O II or MB dyes onto

425

the Fe-CMK-3 was well described using Langmuir isotherm model with the best correlation

426

regression coefficients compared to Freundlich, Temkin and Redlich-Peterson model (Table 3

427

and Fig. 12). This can be attributed to the homogeneous distribution of active adsorption sites

428

on the surface of the Fe-CMK-3. According to the results of the dye adsorption isotherm

429

experiments, the maximum adsorption capacity of O II and MB dyes were 287 and 337 mg g-1,

430

respectively. Compared with O II, the MB dye showed the higher adsorption capacity onto the

431

Fe-CMK-3. This means that the mesoporous carbon Fe-CMK-3 had a strong affinity for

432

cationic MB in comparison with anionic O II. The result indicated that the Fe-CMK-3 has

433

high efficiency for cationic dye even under low concentrations [45]. In addition, a high value

434

of KL indicated higher affinity. The KF was one of the Freundlich constant which has been

435

used as a relative measure of the adsorption capacity and the nF value which has been used as

436

adsorption intensity [46]. The higher value of KF (MB) indicated that the Fe-CMK-3 19

437

possesses a higher adsorption capacity for MB compared to O II dyes. However, both dyes

438

have a 1/n value of less than 1 indicating a favourable adsorption of the dyes on the

439

Fe-CMK-3.

440

3.6. Adsorption kinetics

441

The adsorption rate is an important factor to consider when choosing a material to be

442

used as an adsorbent. In order to elucidate the rate of adsorption of O II or MB dyes onto

443

Fe-CMK-3, the kinetic data were investigated by pseudo-first-order, pseudo-second-order and

444

intraparticle diffusion models. The pseudo-first-order and the pseudo-second-order are shown

445

in Fig.13.

2.0 1.4

1.8

OII MB

OII MB

1.2

1.4 1.0 -1

t/qt( min.g mg )

log ( qe-qt)

1.6

1.2 1.0 0.8 0.6

0.8

0.6

0.4

0.4 0.2

0.2 0.0

0.0

0

50

100

150

200

250

300

350

400

0

50

100

150

Time (min)

200

250

300

350

400

Time (min)

446 447

(a) Pseudo-first-order

Fig.13 Kinetic plot of the adsorption of O II and MB dyes onto Fe-CMK-3 (a)

448

Pseudo-first-order (b) Pseudo-second-order

449 450

(b) Pseudo-second-order

Table 4 the kinetic parameters for the adsorption of O II and MB dyes onto Fe-CMK-3 at 25℃

Pseudo-first-order dye

Pseudo-second-order

qexp (mg g-1)

K

qe

(1 min-1)

(mg g-1)

R2

K (g (mg

qe

min)-1×10-2

(mg g-1)

R2

O II

280

0.0103

48.55

0.9802

0.1313

293.16

0.9999

MB

357

0.0087

44.88

0.9846

0.1352

337.14

0.9999

451

Table 4 showed that the calculated qe value is not consistent with experimental qe values,

452

thus the pseudo-first-order model did not fit well. However, it was clearly seen that the

453

correlation coefficient, the R2 values for the pseudo-second-order kinetics model has higher 20

454

value compare with the pseudo-first-order kinetics model, and its calculated equilibrium

455

adsorption capacity, qe,cal is in agreement with the experimental data. Thus, the

456

pseudo-second-order kinetics model is more suitable to describe the adsorption kinetics data

457

for the entire sorption period. This result suggests that the pseudo-second-order adsorption

458

mechanism is predominant, and that the overall rate of the dye adsorption process appears to

459

be controlled by the chemisorption or chemical adsorption process. The result also indicates

460

that the adsorption of O II or MB dye probably proceeds through surface exchange reactions

461

until the surface vacant sites were fully occupied; afterward, O II or MB dye molecules may

462

be able to diffuse into the sorbent inner area for further interactions. The wedging of the dye

463

into the particle is much slower than its movement from solution to the external solid surface

464

due to the greater mechanical obstruction presented by the surface molecules or surface layers

465

and the restraining chemical attractions between dye and adsorbent [47]. Similar phenomena

466

were observed by Wu [48].

467

Table 5 Intraparicle diffusion constants and correlation coefficients for adsorption of O

468

II and MB onto the Fe-CMK-3 at 25℃ Intraparticle diffusion model

469 470

dye

C (mg g-1)

ki (mg g-1 min1/2)

R2

O II

208.59

3.467

0.935

MB

331.58

1.563

0.952

The adsorption mechanism generally includes three steps: (I) film diffusion (II) intraparticle diffusion or pore diffusion on the surface (III) sorption onto interior sites.

471

For intraparticle diffusion model, the value of ki and C were obtained from the second

472

linear. According to the model, the intraparticle diffusion is the controlling step if the linear

473

plot passes through the origin. From the Fig.14 it can be seen, the first part, the plot of qt

474

versus t1/2 was a linear relationship in this period and the linear does not pass through the

475

origin, this result suggests that intraparticle diffusion is not the only rate controlling step in

476

the adsorption process and that some other mechanism might be involved in this process [49].

477

The second portion was the gradual adsorption stage, where the rates became slow. This could

478

be attributed to the extremely low dye concentration residual in the solution. The values of C 21

479

and its correlating coefficients R2 were listed in Table 5, R2 for the intraparticle diffusion

480

kinetic model are lower than that of the pseudo-second-order kinetic for the adsorption of O II

481

and MB onto the Fe-CMK-3, this result further indicates that the pseudo-second-order is

482

dominant and the overall rate of adsorption process should be controlled by several portions

483

[50].

400

300

350

Adsorption capacity ( mg g-1)

Adsorption capacity ( mg g-1)

250

200

150

100

50

OⅡ

300 250 200 150 100

MB

50 0

0 0

2

4

6

8

10

12

14

16

18

20

22

0

2

4

6

8

10

12

14

16

18

20

22

0.5

0.5

t (min)

t (min)

484 485 486

Fig.14 Intraparticle diffusion model of adsorption O II and MB onto the Fe-CMK-3 3.7. Thermodynamic studies

487

An increase in temperature resulted in a corresponding increase in the adsorption

488

capacity of the two dyes (Fig. 11). To better estimate the effect of temperature on the

489

adsorption of O II and MB onto the Fe-CMK-3, the thermodynamic parameters including

490

enthalpy(△H), entropy(△S) and Gibbs free energy(△G) were also determined, these equations

491

can be followed as:

492

△ G = − RT ln K L

(9)

△ S △ H − R RT

(10)

493

ln K L =

494

KL =

qe Ce

(11)

495

where △G is change of Gibbs free energy (kJ/mol), △S (J/(K mol)) and △H (kJ/mol) are

496

change of entropy and enthalpy of adsorption. KL represents the Langmuir constant, Qe is the

497

equilibrium concentration dye of O II or MB, qe is the equilibrium adsorption capacity of the

498

adsorbent, R is the universal gas constant (8.314 J mol K-1) and T is the absolute temperature. 22

499 500

Table 6 Thermodynamic parameters for the adsorption of the O II and MB onto the Fe-CMK-3 Dye

△G at temperature (℃) (kJ mol-1)

△H (kJ mol-1)

△S ( J(mol K)-1)

25℃

35℃

45℃

O II

-4.76

-5.37

-6.00

13.49

61.24

MB

-4.88

-5.26

-7.75

37.47

141.02

501

The calculation (△H, △S and △G) were listed in Table 6. The positive value for △H (O

502

II or MB) at the three temperatures (293K, 303K and 313K) indicates the adsorption process

503

is the endothermic in nature. The positive value of △S indicates the higher order of reaction

504

during the adsorption of O II or MB dye onto Fe-CMK-3, this result could be due to a

505

combination of the affinity of Fe-CMK-3 for O II or MB dye at the solid-solution interface.

506

The negative value of △G confirms that the adsorption of O II or MB dye onto Fe-CMK-3

507

was a very feasible the process and spontaneous in nature. The negative value for △G

508

increased with increasing temperature, implying that the spontaneity of the adsorption process

509

and the driving force for the adsorption of both O II and MB dye are proportional to the

510

temperature, which is consistent with in front of the experimental result [33]. In addition,

511

when the adsorption process is endothermic, there is electrostatic repulsion between the

512

adsorbent and adsorbate. The process needs to draw some energy from environment to

513

overcome the repulsion force to move the ionic dyes close onto the adsorbent. Thus, the

514

higher adsorption capacity was achieved at higher temperature [51].

515

4. Conclusion

516

This study confirmed that the magnetic nanoparticles were successfully loaded onto the

517

surface of the CMK-3 and also showed that, anthracene could be employed as the carbon

518

source for the production of CMK-3, which was a good adsorbent be utilized for the

519

adsorption of anionic dye (O II) and cationic dye (MB) from aqueous solution. The maximum

520

adsorption capacity of O II and MB onto the Fe-CMK-3 were 269 and 316 mg g-1,

521

respectively. The loaded iron magnetic nanoparticles of Fe-CMK-3 could be easily separated 23

522

from solution by the application of a simple magnetic process, facilitating separation and

523

reuse of the mesoporous carbon powder as adsorbents.

524

Acknowledgements

525

This work was supported by A Project Funded by the Priority Academic Program

526

Development of Jiangsu Higher Education Institutions (PAPD) (No. 1105007001), Research

527

Fund for Taihu Lake Pollution Control, Jiangsu Province, China (TH2012207), Project

528

supported by the natural science foundation of Jiangsu Province （BK20130626） and the

529

Fundamental Research Funds for Central Universities (No.2010121051).

530

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667 668 669 670 28

671 672 673 674 675 676 677 678 679

Highlights

680

·Ordered mesoporous carbon was prepared using anthracene as the carbon source.

681

·The Fe-CMK-3 could be easily separated from solution by the application of magnetic field.

682

·Fast adsorption and high adsorption capacity for acidic and alkaline dyes were observed.

683

·Adsorption of O II or MB dye onto Fe-CMK-3 was spontaneous and exothermic in nature.

684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 29

700 701 702 703 704 705 706 707 708 709 710

Graphical abstract

711 712 713

30