Quaternary ammonium β-cyclodextrin-conjugated magnetic nanoparticles as nano-adsorbents for the treatment of dyeing wastewater: Synthesis and adsorption studies

Accepted Manuscript Title: Quaternary ammonium ␤-cyclodextrin-conjugated magnetic nanoparticles as nano-adsorbents for the treatment of dyeing wastewater: Synthesis and adsorption studies Authors: Dan Cai, Tailiang Zhang, Fangjie Zhang, Xuemei Luo PII: DOI: Reference:

S2213-3437(17)30252-X http://dx.doi.org/doi:10.1016/j.jece.2017.06.001 JECE 1663

To appear in: Received date: Revised date: Accepted date:

8-4-2017 30-5-2017 2-6-2017

Please cite this article as: Dan Cai, Tailiang Zhang, Fangjie Zhang, Xuemei Luo, Quaternary ammonium ␤-cyclodextrin-conjugated magnetic nanoparticles as nano-adsorbents for the treatment of dyeing wastewater: Synthesis and adsorption studies, Journal of Environmental Chemical Engineeringhttp://dx.doi.org/10.1016/j.jece.2017.06.001 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

Quaternary ammonium β-cyclodextrin-conjugated magnetic nanoparticles as

2

nano-adsorbents for the treatment of dyeing wastewater: Synthesis and

3

adsorption studies

4

Dan Cai*, Tailiang Zhang, Fangjie Zhang, Xuemei Luo.

5

School of Chemistry and Chemical Engineering, Southwest Petroleum University,

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Chengdu 610000, People’s Republic of China

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Correspondence: D. Cai (E-mail:[email protected])

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Grphical abstract

9 10 11

ABSTRACT: The preparation of sorbents has been paid great attention on dye

12

wastewater treatment. In this article, a novel multi-layer magnetic adsorbent (GEPCD-

13

MNPs) was prepared by coating a multi-layer cationic polymer (GEP-CD) onto

14

magnetic nanoparticles (MNPs) via the formation of an ester bond. The physiochemical

15

characteristics of the GEPCD-MNPs were characterized by FTIR, X-ray diffraction,

16

contact angle, SEM, BET, VSM and zeta potential analysis. GEPCD-MNPs were used

17

to treat Congo red (CR) and hexavalent chromium (Cr (VI)) ions in simulated dye

18

wastewater. The adsorption capacities of the GEPCD-MNPs for CR and Cr(VI) reached

19

389.1 mg/g and 118 mg/g, respectively, which was due to the multi-cavity structure and

20

active functional groups (-COOH and -OH) contained in the GEPCD-MNPs. According

21

to the adsorption data, the dynamic and isothermal adsorption mode were studied, which

22

revealed that the adsorption process was consistent with the pseudo-second-order model

23

and Freundlich model. Meanwhile, the isothermal data for CR and Cr (VI) were analysed

24

thermodynamically, which showed that the adsorption processes were spontaneous and

25

that the adsorption of CR and Cr (VI) were endothermic. Moreover, the as-prepared

26

adsorbent is economic viable and easily controllable for pollutants adsorption in

27

industrial application.

28

Key words: Adsorption. Dye wastewater. Cr (VI). β-CD. Magnetic nanoparticles,

29

Multi-layer.

30

1. Introduction

31

Currently, there are large quantities of industrial wastewater containing toxic diazo dyes

32

and heavy metal ions that are produced by the plastic, printing, textile, paper, and

33

electroplating industries and thereafter discharged into the aquatic environment on a

34

large scale. These pollutants are non-biodegradable and represent a risk to human health

35

and ecosystems when accumulated. According to the ion types, dyes have been divided

36

into three categories [1], among which CR is an example of an anionic dye. Therefore,

37

exploring efficient and environmentally friendly methods to remove CR from dye

38

wastewater has attracted considerable attention from researchers. To date, various

39

traditional physicochemical methods have been developed for this purpose[1], including

40

adsorption[1-3],

41

coagulation[9], and ion exchange[10]. Among these conventional methods for dye

42

wastewater treatment, the adsorption technique has occupied a prominent place owing

43

to its economic value, high efficiency, and easy manipulability.

biological

degradation[4],

photocatalysis[5-8],

chemical

44

Over the last several decades, many researchers have studied adsorbents such as

45

activated alumina, activated carbon, molecular sieves, clays, and biopolymer materials

46

for the removal of heavy metals[11]. Natural biopolymer materials and their derivatives

47

have been widely used in the treatment of dye wastewater containing heavy metals[11]

48

because of their excellent characteristics of biodegradability and convenient fabrication.

49

Although many natural polymers have been reported, some disadvantages, such as low

50

surface area, poor dispersion and difficult separation, have limited their applications in

51

dye wastewater treatment. Various studies have been carried out on the development of

52

adsorbents with improved properties. Introducing magnetic particles into natural organic

53

polymers combines the advantages of the two. Magnetic sorbents possess better

54

purification effects and easier manipulability and are pollution-free, which helped

55

overcome the limitations of traditional sorbents. Therefore, magnetic adsorption is

56

considered to be an emerging technology for dyes wastewater treatment[12]. There have

57

been many studies on magnetic nanomaterials functionalized with biopolymers, such as

58

chitosan[13-15], alginate [16-18], gum arabic[19], and cellulose[20], that have been

59

evaluated for the removal of toxic metals and dyes from wastewater. Badruddoza et al.

60

synthesized a novel magnetic adsorbent (CMCD-MNPs) that combined magnetic

61

nanoparticles with carboxymethylated β-cyclodextrin[21]. These CMCD-MNPs

62

exhibited a better capacity to remove Cu2+ from wastewater due to their large number of

63

active functional groups (-OH and -COOH) and the cavity of β-cyclodextrin(β-CD)[21].

64

Meanwhile, Lan Yu et al. developed a cationic magnetic adsorbent (HP-β-CD/PEG400-

65

modified MNP) and studied its adsorption abilities for the removal of CR[22]. β-CD

66

possesses the capacity to complex with certain kinds of metal ions and CR and to be

67

modified with other organic molecules with suitable functional groups, forming new

68

compounds with improved quantities of active sites and cavities that are conducive for

69

wrapping and adsorbing species.

70

This work provides new insights into the design and fabrication of advanced

71

adsorption materials for the removal of water pollutants. In this work, a novel multi-

72

layer magnetic adsorbent (GEPCD-MNPs), which was synthesized from β-CD, succinic

73

anhydride (SA), 2, 3-glycidyltrimethylammonium chloride (GTA) and MNPs, was

74

evaluated for the treatment of CR and Cr (VI) at a range of pH values. In addition,

75

adsorption kinetic, isotherm and thermodynamic models were used to evaluate the

76

performance of the material for adsorbing CR and Cr (VI) ions. The adsorption

77

selectivity and mechanism of the GEPCD-MNPs were studied and are presented at the

78

end of the paper.

79

2. Experimental materials and methods

80

2.1 Experimental materials

81

β-CD (99%), succinic anhydride (SA, 99%), GTA (95%), FeCl3.6H2O (99%),

82

FeCl2·4H2O (98%) ,NH3·H2O (25%) ,K2CrO7 (>99.8%) and Congo Red (>95%) were

83

used for the preparation of the GEPCD-MNPs. All reagents were analytically pure and

84

provided by Kelong Chemical Corporation Ltd. in Sichuan. All water used in the

85

experiments was deionized water.

86

2.2 Methods

87

2.2.1 Preparation of MNPs

88

First, MNPs were synthesized by co-precipitation [23-25]. FeCl3·6H2O and FeCl2·4H2O

89

were reacted in a molar ratio of 2:1 under nitrogen to avoid the possibility of oxidization.

90

Then, excess ammonia solution was rapidly added to adjust the solution pH to 10-11

91

under ultrasonic agitation and was dispersed for 30 min. The following reaction equation

92

supports this method:

93

Fe

+ 2Fe

+ 8OH = Fe O (s) ↓ +4H O

94

2.2.2 Fabrication of GEPCD-MNPs

95

GEPCD-MNPs were synthesized via ring opening polymerization, and the detailed

96

experimental conditions can be found in previous literature reports[26]. Typically, β-CD

97

(10 g, 8.81 mmol) and SA (2.6 g, 26.4 mmol) were dissolved in DMF with magnetic

98

stirring for 10 h at 70°C under nitrogen. After the reaction went to completion, 10 wt%

99

NaOH solution was used to adjust the solution pH to 8-9. Next, 5.34 g (35.2 mmol) GTA

100

was added under same conditions for 6 h. When the reaction reached completion, the

101

solution pH was adjusted to 5-6 and allowed to react for 2 h for the ring open

102

polymerization of GEP-CD. As shown in scheme 1a showing, the grafting of SA and GTA

103

onto β-CD introduced -COOH and –OH groups into it, which may cause β-CD molecules to

104

cross-link with each other and form a multi-layer microsphere structure with many active

105

adsorption sites. Then, MNPs (0.681 g, 2.94 mmol) were introduced into the solution

106

(scheme 1b) under ultrasonic and atmospheric conditions and allowed to react for 20 min

107

under nitrogen. Finally, the precipitate was washed three times and dried in a vacuum

108

drying box at 333.15 K for twelve hours.

109 110

Scheme 1 a) Synthesis of GEP-CD and b) synthesis of GEPCD-MNPs

111

2.2.3 Characterization methods

112

Fourier transform infrared spectroscopy (WQF520, China) was performed to elucidate

113

the functionalization of the GEPCD-MNPs and the connections between GEP-CD and

114

the MNPs. A scanning electron microscope (Nova Nano SEM 450, Netherlands) was

115

used for determining the microscopic appearance and mean diameter. The size of the

116

GEPCD-MNPs was also determined by an X-ray diffractometer (X Pert PRO MPD,

117

Netherlands). The contact angle of CR and Cr(VI) on the surface of the GEPCD-MNPs

118

were characterized by an interface analyser (KRUSS DSA30S, Germany). The surface

119

area of GEPCD-MNPs and MNPs was investigated by surface area analyser (ST-MP-9,

120

America).The magnetization difference of MNPs between before and after modification

121

were analysed by vibrating sample magnetometer (DynaCool, America).

122

2.2.4 Adsorption experiments

123

To investigate the adsorption capacity of the GEPCD-MNPs, CR and Cr (VI) solutions

124

with different pH values and concentrations were prepared. A certain amount of

125

GEPCD-MNPs was mixed with those solutions while being shaken (110 rpm) in a

126

thermostatic bath at temperatures ranging from 303.15 K to 323.15 K. After an

127

equilibrium was reached, the concentrations of CR or Cr (VI) were evaluated by a UV-

128

via spectrophotometer at 497 nm and 540 nm, respectively, and the detail steps can be

129

found from the reported literature [27].The calculation method of adsorption capacity

130

(Q) was as follows:

131

Q=

132

Where C0 represents the initial concentration of Cr (VI) or CR, Ce represents the

133

equilibrium concentration of Cr (VI) or CR, V represents the volume of the test solution,

134

and m represents the mass of the GEPCD-MNPs (g).

(

)

(1)

135

To investigate the effect of shaking time on adsorption, solutions of Cr (VI) or CR

136

at certain concentrations (100 mg/L or 200 mg/L, respectively) were prepared at the

137

temperature of 293.15-323.15K, and their pH values were adjusted. Then, a certain

138

amount of GEPCD-MNPs was added.

139

3. Results and discussion

140

3.1 FT-IR analysis

141

The FTIR spectra in the 400–4000 cm−1 wavenumber range of the MNPs and GEPCD-

142

MNPs are shown in Fig. 1. The peaks at 3410 cm-1, 1630 cm-1, 1458 cm-1 and 582 cm-1

143

were confirmed to be the characteristic peaks of the MNPs[19]. The peaks at 945, 1025,

144

1161 and 1710 cm−1 shown in Fig. 1b were the characteristic peaks of GEPCD-MNPs.

145

The peak at 945 cm−1 was attributed to the vibration of the R-1,4-bond in β-CD, and the

146

peaks at 1025 and 1161 cm−1 were attributed to the antisymmetric glycosidic va(C–O–

147

C) vibrations and conjugated v(C–C/C–O) stretching vibrations[28] The adsorption peak

148

at 1459 cm-1 corresponded to the stretching vibration of the –CN moiety in the

149

quaternary ammonium group. The peak at 1710 cm−1 was attributed to the stretching of

150

the carbonyl groups (C=O)[29], which verified that the GEP-CD molecules were

151

conjugated through esterification. All the representative peaks of GEP-CD were present

152

in the FTIR spectrum with a small shift, which may be caused by the introduction of the

153

cationic quaternary ammonium group. The peaks at 1623 and 1401 cm−1 shown in Fig.

154

1b are due to the formation of –COO-Fe groups, indicating that the –COOH groups on

155

GEP-CD had reacted with the surface -OH groups of the MNPs, which resulted in the

156

formation of iron carboxylates[28]. Thus, it could be concluded that GEP-CD had been

157

successfully grafted onto the surface of the magnetic nano-adsorbents via chemical bond

158

formation. FTIR spectrum of CR consist of peaks at 3465, 2925, 1585, 1446, 1361, 1225

159

and 1062 cm− 1 attributed to N–H stretching vibrations of aromatic primary amine, O–H

160

stretching vibrations, N=N stretching vibrations, aromatic C=C stretching vibrations–N

161

bending vibrations, C–N stretching vibrations and S=O stretching vibrations of sulfonic

162

acid respectively as reported in literature[30]. FTIR spectrum of GEPCD-MNPs consists

163

of peaks due to both dye and adsorbent but at shifted position because of introduction of

164

the SO32- from CR and Cr2O72- ions, which confirms the interaction of between adsorbent

165

and pollutants. It’s worth notice that the peaks at 3595cm-1(-COOH) in Fig.1 (d) shows

166

higher intensity meanwhile a lower peak presents at 3260cm-1(-OH), which suggests the

167

Cr2O72- adsorbed on GEPCD-MNPs leads to the transition from –OH to –COOH so that

168

we can infer that β-CD has been destroyed since the oxidation of Cr2O72-.

169 170

Fig. 1 FTIR spectra of a) MNPs, b) GEPCD-MNPs, c) CR adsorbed on the surface of

171

GEPCD-MNPs and d) Cr (VI) adsorbed on the surface of GEPCD-MNPs

172

3.2 XRD patterns

173

Fig. 2 displays the XRD spectra of the MNPs, GEPCD-MNPs and GEP-CD materials.

174

The peaks in Fig. 2a coincided with the standard data of the Fe3O4 phase on the

175

diffraction PDF card 01-075-0449. The characteristic diffraction peaks at 2θ=11.98,

176

30.1, 35.4, 43, 56.9 and 62.5 corresponded to the six crystals in the cubic Fe3O4 on the

177

surface, revealing that the magnetic particles prepared by the chemical co-precipitation

178

method were single-phase cubic-structure Fe3O4[31]. The crystal C28 H58N2O4 detected

179

from GEP-CD pattern, which revels GEP-CD is a compound and exists various

180

polymers. The XRD patterns of the GEPCD-MNPs in Fig. 2b show that the positions of

181

the six characteristic diffraction peaks of the Fe3O4 crystal phase did not change and that

182

the intensity of the diffraction peaks changed minimally, which indicates that the layer

183

of GEP-CD covering the surface of the MNPs decreased the intensity of the

184

corresponding diffraction peaks but kept these phase peaks consistent. Additionally,

185

according to Debye–Scherrer’s formula[31], the mean crystal diameter of the MNPs and

186

GEPCD-MNPs were 24 nm and 66 nm, respectively, which indicates that GEP-CD

187

occupied the active sites of the MNPs, decreasing the high surface energy of the MNPs,

188

and the GEPCD-MNPs exhibited better dispersion properties than the MNPs.

189 190

Fig. 2 XRD spectra of a) MNPs, b) GEPCD-MNPs and c) GEP-CD

191

3.3 SEM images for GEPCD-MNPs

192

The SEM image of GEP-CD (Fig. 3b) clearly showed a multi-layer and globular

193

polymer, and the diameter of GEP-CD is micron level. The SEM image presented in Fig.

194

3a reveals that the MNPs existed in a spherical state, but aggregation and poor dispersion

195

could clearly be observed, which mainly due to the magnetic attraction and the high

196

surface energy of the MNPs[32]. As shown in Fig. 3c, the surface morphology of the

197

GEPCD-MNPs was also spherical but exhibits smaller diameters comparing with MNPs,

198

which indicated that the polymer (GEP-CD) occupying the active sites of the MNPs lead

199

to a reduction of the surface energy of the GEPCD-MNPs, resulting in GEPCD-MNPs

200

exhibiting better dispersion properties than the MNPs.

201 202

Fig. 3 SEM images of a) MNPs (200000X), b) GEP-CD (20000X) and c) GEPCD-

203

MNPs (250000X)

204

3.4 Contact angle

205

The wettability between a liquid and a solid is measured by contact angle (θ), which is

206

related to the free energy of the solid surface [33-35]. In other words, different θ values

207

are, to some degree, indicative of the dispersion properties of a solid in solution. θ values

208

greater than 90° indicate that a solid is hydrophobic, whereas θ values lower than 90°

209

indicate that a solid is hydrophilic[36]. The θ values for CR and Cr(VI) solution in

210

contact with the GEPCD-MNPs, both of which were less than 90°, are shown in Fig. 4.

211

This result indicated that the GEPCD-MNPs were hydrophilic and would be well

212

dispersed in CR and Cr(VI) solutions. Furthermore, comparing Fig. 4a with Fig. 4b

213

revealed that the hydrophilicity and dispersion of the GEPCD-MNPs in CR solution

214

should be better than those in Cr(VI) solution.

215 216

Fig. 4 Contact angle between the GEPCD-MNPs and a) CR solution and b) Cr(VI) ion

217

solution

218

3.5 VSM analysis

219

The magnetization curves (Fig.5) of GEPCD-MNPs and MNPs were obtained by VSM

220

at room temperature. As fig.4 shows, the maximal saturation magnetization curve of

221

MNPs was 62.3 emu.g-1, which is agreement with the reported literature[37]. However

222

the maximal saturation magnetization curve of GEPCD-MNPs decreased to 54.7 emu.g-

223

1

224

high magnetism and exhibits excellent manipulation while it was seprated from dyes

225

wastewater.

while polymer GEP-CD coated on MNPs. Still this, GEPCD-MNPs also maintains the

226 227

Fig.5 Magnetization of a) MNPs and b) GEPCD-MNPs at room temperature

228

3.5 Adsorption capacity of the GEPCD-MNPs for the removal of Cr(VI) and CR

229

3.5.1 Effect of pH value

230

The pH values of Cr(VI) and CR solutions were expected have a considerable impact on

231

adsorption efficiency; thus, the influence of the pH of CR and Cr(VI) solution on the

232

adsorption efficiency of the GEPCD-MNPs was explored. For CR solutions (Fig. 6a),

233

the removal rate decreased gradually as the pH value increased with the range from 5.25

234

to 7.25, by contrast, the removal rate of CR increased slowly as the pH value increased

235

from 7.25 to 9.25. The change in removal rate was attributed to the isoelectric point (IP)

236

of the GEPCD-MNPs (PhIP=7.48) and the cavity structure. As the schem.2 shows, On

237

the one hand, a pH lower than the IP of charge would cause the protonation of the MNPs

238

and the –COOH and –N (CH3)3Cl groups[26, 38, 39], which have a strong coordinative

239

affinity with CR. On the other hand, a pH higher than the IP of charge would cause the

240

deprotonation of the MNPs and the –COOH and –N+(CH3)3Cl groups[26, 38, 39], which,

241

along with the swelling of the cavity structure, would be disadvantageous to

242

adsorption[40]. Because the former effect had more influence than the latter, the removal

243

rate under alkaline conditions was lower than that under acidic conditions. For Cr(VI)

244

ion solutions (Fig. 6b), the removal rate decreased gradually as the pH value increased

245

within the range from 2.98 to 11.94. At a pH under the IP (PhIP=7.48), the positively

246

charged Fe-OH groups (Fe-OH2+), carboxylate ions (-COOH2 +) and quaternary

247

ammonium ions (–N+ (CH3)3) have a strong coordination affinity with Cr2O72- since

248

anion exchange at the quaternary ammonium N. Meanwhile, the strong oxidation presented

249

by Cr2O72- under acidic condition also aggravates consumption of Cr2O72-.When the ph

250

above the IP, the ion Cr2O72- was transformed into CrO2- under the alkaline condition,

251

thus, the electrostatic interactions between GEPCD-MNPs and Cr2O72- decreased with

252

the ph increasing. Therefore, the surface complexation between the adsorbent and

253

Cr2O72- mainly involve electrostatic interactions to form the chelate.

254

255

256 257

Fig. 6 a) Zeta potential of the GEPCD-MNPs and MNPs. b) Quantity of CR and Cr(VI)

258

adsorbed by the GEPCD-MNPs at different pH values. Temperature=303K, contact

259

time=20 min, shaking rate=110 rpm, initial concentration (C0, C0(CR)=200 mg/L,

260

C0(Cr(VI))=50 mg/L).

261

3.5.2 Adsorption kinetics of CR and Cr (VI)

262

The results of the adsorption kinetics studies, which are shown in Fig. 7a and Fig. 7c,

263

are presented as a function of shaking time for CR and Cr (VI) at temperatures of 303

264

K, 313 K, 323 K, and 333 K and were described by pseudo-first order and pseudo-second

265

order models. The two linear equations were presented as follows (Eqs. (2-3))[18, 38,

266

41, 42]::

267

log(

268

=

−

) = log

−

.

+

(2)

(3)

269

Where k1 (min-1) and k2 (g.mg-1.min-1) are the pseudo-first-order model and the pseudo-

270

second-order model rate constant, respectively. Instantaneous adsorption capacity and

271

equilibrium adsorption capacity in adsorption process are represented by parameters qt

272

(mg.g-1) and q e (mg.g-1), respectively.

273

The data listed in Table 1 are the kinetic parameters. The kinetics data were better fitted

274

by the pseudo-second order model. The pseudo-second order equation assumed that the

275

adsorption process involved a chemisorption mechanism and that the rate of site

276

occupation was proportional to the square of the number of unoccupied sites[28]. Fig.

277

7b and Fig. 7d show that the pseudo-second order equation was excellently applicable

278

to the adsorption of CR and Cr(VI) ions. The correlation coefficient (R2) for the pseudo-

279

second order were over 0.99. For CR and Cr(VI) ions, the q e values obtained by linear

280

regression were 306.75. 338.98, 371.75, and 389.11 mg·g-1 and 8.80, 8.82, 9.06, and 8.92

281

mg·g-1, respectively, which were in agreement with the experimental data. These

282

suggested that the adsorption process of GEPCD-MNPs was well described by the

283

pseudo-second-order kinetic model.

284 285

Fig. 7 a) and c) The adsorption process of CR and Cr(VI) ions on GEPCD-MNPs at

286

pH=6.81. b) Linear fitting of the pseudo-second order equation for 200 ppm (50ml) CR

287

and 20 mg of sample. d) Linear fitting of the pseudo-second order equation for 50 ppm

288

(20ml) Cr(VI) ions and (40mg)GEPCD-MNPs, and the whole adsorption process was

289

conducted at a shaker 110rpm.

290 291 292 293 294

295 296

Table 1. Adsorption kinetic parameters for Cr (VI) and CR Pseudo-first-order model Dyes

Pseudo-second-order model

T/K q e,cal(mg/g)

k1(min-1)

R12

qe,cal(mg/g)

k2(g.mg-1.min-1)

R22

303.15

154.80

-0.0693

0.9603

306.75

0.00326

0.9988

313.15

211.16

-0.0693

0.9554

338.98

0.00295

0.9987

323.15

202.69

-0.0647

0.9578

371.75

0.00269

0.9987

333.15

166.77

-0.0623

0.9626

389.11

0.00257

0.9997

303.15

2.899

-0.0400

0.8711

8.80

0.114

0.9996

313.15

1.68

-0.0334

0.8874

8.82

0.113

0.9998

323.15

1.52

-0.0304

0.6765

9.06

0.110

0.9999

0.5754

8.92

0.112

0.9995

CR

Cr(VI)

333.15

1.40

-0.0304

297 298

3.5.3 Isothermal adsorption model

299

The equilibrium isotherms presented in Fig. 8 were used to explore the adsorption

300

mechanisms of the GEPCD-MNPs for CR and Cr (VI) at temperatures of 303K, 313.15K

301

and 323.15K. The equilibrium data were well fitted by the adsorption isotherm models

302

of Langmuir and Freundlich, which are widely used to describe the relationship between

303

the sorption capability of an adsorbate and the equilibrium concentration of that

304

adsorbate in aqueous solution. The adsorption isotherm models are expressed in Eqs. (4)

305

and Eqs. (5)[41]:

306

=

+

(4)

307

lnq = lnC + lnK

308

Where Ce represent the concentration of adsorbate at equilibrium in dye solution (mg

309

L−1), qe and q m represent adsorption capacity of the adsorbate (mg·g−1) and the

310

monolayer adsorption capacity at equilibrium (mg·g−1), respectively, KL and KF are the

311

constant of Langmuir equilibrium and Freundlich, respectively. In the Freundlich

312

equation, 1/n is the index of adsorption intensity. The adsorption data for CR and Cr(VI)

313

were fitted by the Freundlich model and Langmuir isotherm model, the results were

314

shown in Fig. 8. The calculated fitting parameters are shown in Table 2:

(5)

315 316

Fig. 8 a) and c) The isothermal adsorption processes of CR and Cr (VI) and b) and d)

317

the Freundlich adsorption isotherm models of CR and Cr (VI)

318 319 320 321 322 323 324 325 326 327

328

Table 2. Parameters of isothermal adsorption model of CR and Cr(VI) Freundlich constants

Dyes

T/K

Langmuir constants

KF

KL R12

1/n

(mg/g)

CR

Cr(VI)

R22

q m(mg/g) (mg/L)

303.15

32.34

0.974

0.46

425.53

2.6*10 -4

0.921

313.15

32.78

0.979

0.51

877.19

0.00003

0.505

323.15

33.13

0.998

0.60

383.14

1.47*10 -4

0.994

303.15

3.20

0.949

0.98

-1363.07 -7.3*10-4

-0.062

313.15

2.42

0.998

1.00

196.08

0.0051

0.102

323.15

2.42

0.998

1.00

-377.36

-0.00265

-0.067

329 330

The correlation coefficients (R2) shown in Table 2 demonstrated that the isothermal

331

adsorption processes of CR and Cr(VI) were described better by the Freundlich model

332

than the Langmuir model. Based on the Freundlich isotherms, the values of 1/n were all

333

between 0 and 1. These data indicated that the adsorption processes of CR and Cr (VI)

334

are easily carried out. Additionally, the KF of CR is far better than that of Cr(VI) at

335

various temperatures, which confirms that GEPCD-MNPs favours CR over Cr(VI).

336

Moreover, the multi-cavity structure of GEPCD-MNPs with characteristic of hydrophobic

337

and good dispersion in CR solution also enhance CR adsorption. Table 3 contrasts this

338

study on the treatment process of CR and Cr(VI) to those from other researchers and

339

revealed that the adsorption capacity of the GEPCD-MNPs was equal to that of other

340

adsorbents; however, the mechanisms between the adsorbents reported by other

341

researchers and the adsorbate were unknown. In this study, the good adsorption ability

342

of the GEPCD-MNPs was contributed to their compatibility with the dye solutions, the

343

electrostatic neutralization between the positive surface charge of the GEPCD-MNPs

344

and the adsorbate, and the cavity complexation between the GEPCD-MNPs and the

345

adsorbate.

346 347 348

Table 3. Comparison of adsorption performance with those reported by other researchers BET surface area q max for CR q max for Cr(VI) Adsorbent sample Reference 2 −1 −1 −1 (m g ) (mg g ) (mg g ) GEPCD-MNPs Iron oxide (α-Fe2O3) nanoparticles and nanowhiskers

72.3

377.82

164.4

253.8

CDpoly-MNPs Nanocrystalline Fe3O4 spinel ferrites

17.0

27.70 -

Fe3O4/APTES particles

149.7

-

Flower-like α-Fe2O3

-

Activated carbon (laboratory grade)

492

[43]

[28] [44]

118.8

Fe3O4–GNs

Fe-crosslinked chitosan

This study

[45]

33.7

[46] 30.0

1.88

[47] [48]

295

[49]

349 350

3.5.4 Adsorption thermodynamics

351

Adsorption thermodynamics are based on the theory that the process of adsorption is a

352

driving force under circumstances where there is no energy transformation[19]. To

353

further explore the adsorption process, the changes in thermodynamic parameters, such

354

as standard Gibbs free energy change (ΔG ), standard enthalpy change (ΔH ), and

355

standard entropy change (ΔS ), were calculated using Eqs. (6-8) [50, 51], where R (8.314

356

J mol−1K−1) is the universal gas constant, T (K) is the absolute temperature, and n is the

357

Freundlich isotherm constant. The obtained ΔG0, ΔH0, and ΔS0 for the adsorption of CR

358

and Cr (VI) on samples are presented in Table 4. The ΔH0 of CR and Cr(VI) were

359

positive, which indicated that the adsorption by the GEPCD-MNPs of CR and Cr(VI)

360

was endothermic[52], this finding was consistent with the results. The values of ΔS0 was

361

found to be positive which indicates that the whole adsorption process is enthalpy driven.

362

The values of ΔG0 were negative, and the adsorption processes were spontaneous.

363

ln

364

∆

°

=−

365

∆

°

=

366

Table 4. Adsorption thermodynamic parameters of CR and Cr (VI) ions

=

∆ °

− ln

(6)

(7)

∆ ° ∆ °

(8)

△G(KJmol-1)

△H

△S

Dyes

(KJmol-1)

(JK-1)

303.15K

313.15K

323.15K

Congo red

0.98

32.13

-5.45

-5.16

-4.49

Cr(VI) ions

5.5

26.67

-2.56

-2.58

-2.66

367

3.5.5 Desorption study

368

The major cost of dye adsorption process depends on the recycle time and regeneration of

369

adsorbents. The GEPCD-MNPs after adsorbing pollutants were regenerated using 40ml

370

NaOH(1mol/L) under three minutes ulstrasound, and these adsorbents were resued for five

371

desorption times. The alkaline medium was used for desorption because hydroxyl ions and

372

pollutants exists ion exchange [53], and the desorption results was shown in Fig.9. It’s evident

373

that the adsorption capacity of GEPCD-MNPs for CR and Cr (VI) decreased with

374

regeneration increasing. Besides, the removal efficency of Cr (VI) shows a sharply decline

375

comparing with CR, which suggest that the structure of GEPCD-MNPs has been

376

destroyed by the oxidation of Cr (VI) though multiple recovery adsorption. Therefore,

377

the as-prepared adsorbent is economic viable for CR adsorption in industrial application.

378

379

Fig. 9 Desorption studies of CR and Cr (VI) from GEPCD-MNPs

380

Conclusions

381

In summary, a multi-layer cationic polymer was prepared through condensation at 70°C

382

and then was coated onto magnetic Fe3O4 via a 20 min esterification reaction under

383

ultrasonic conditions. The quaternary ammonium ions, cavity structure and hydrophilic

384

properties have different influences on the adsorption performance for CR and Cr(VI)

385

because of their effects on the electrostatic adsorption on the surface, the complexation

386

with anions and the dispersion of the adsorbent in treatment solutions. Therefore, the

387

GEPCD-MNP material exhibits a good adsorption performance for CR (389.1 mg/g) and

388

Cr(VI) (118 mg/g). The adsorption kinetics were demonstrated to follow the pseudo-

389

second-order model. The equilibrium adsorption isotherms were better fitted by the

390

Freundlich model than by the Langmuir model. The ΔH0 value of CR and Cr(VI) were

391

positive, which indicates that the adsorption by GEPCD-MNPs of CR and Cr(VI) were

392

endothermic. Because the values of ΔG0 were negative, we discovered the adsorption

393

processes were spontaneous. The as-prepared cationic layered polymer/magnetic

394

material GEPCD-MNPs were promising adsorbents for the removal of pollutants from

395

wastewater because of their unique cationic layered structure, cavity structure, low cost,

396

facile synthesis, and high efficiency.

397

Acknowledgements

398

This research was carried out smoothly thanks to the support of Southwest Petroleum

399

University and the co-authors.

400

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