EDTA-Cross-Linked Î²-Cyclodextrin: An Environmentally Friendly

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Environmental Science & Technology

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EDTA-Cross-Linked β-cyclodextrin: An

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Environmentally Friendly Bifunctional Adsorbent

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for Simultaneous Adsorption of Metals and Cationic

4

Dyes

5

Feiping Zhao,*,†,‡ Eveliina Repo,† Dulin Yin,*,‡ Yong Meng,‡ Shila Jafari,† Mika Sillanpää†

6

†

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Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland

8

‡

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Utilization of Resources, College of Chemistry and Chemical Engineering, Hunan Normal

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Laboratory of Green Chemistry, School of Engineering Science, Lappeenranta University of

National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine

University, 410081 Changsha, China

11 12 13 14 15 16 17

ACS Paragon Plus Environment

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ABSTRACT

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The discharge of metals and dyes poses a serious threat to public health and environment. What

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is worse, these two hazardous pollutants are often found to co-exist in industrial wastewaters,

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making the treatment more challenging. Herein, we report an EDTA-cross-linked β-cyclodextrin

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(EDTA-β-CD) bifunctional adsorbent, which was fabricated by an easy and green approach

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through the polycondensation reaction of β-cyclodextrin with EDTA as cross-linker, for

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simultaneous adsorption of metals and dyes. In this setting, cyclodextrin cavities are expected to

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capture dye molecules through the formation of inclusion complexes and EDTA units as the

26

adsorption sites for metals. The adsorbent was characterized by FT-IR, Elemental analysis, SEM,

27

EDX, Zeta potential, and TGA. In mono-component system, the adsorption behaviors showed a

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monolayer adsorption capacity of 1.241 and 1.106 mmol g-1 for Cu(II) and Cd(II), respectively,

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and a heterogeneous adsorption capacity of 0.262, 0.169, and 0.280 mmol g-1 for Methylene

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Blue, Safranin O, and Crystal Violet, respectively. Interestingly, the Cu(II)-dye binary

31

experiments showed adsorption enhancement of Cu(II), but no significant effect on dyes. The

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simultaneous adsorption mechanism was further confirmed by FT-IR, thermodynamic study, and

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elemental mapping. Overall, its facile and green fabrication, efficient sorption performance, and

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excellent reusability indicate that EDTA-β-CD has potential for practical applications in

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integrative and efficient treatment of coexistent toxic pollutants.

36 37 38


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INTRODUCTION

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Dyes and heavy metals commonly co-exist in the effluents of various industrial branches.1, 2 For

41

example, the wastewaters produced by dyes manufacturing and textile finishing industries

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contain heavy metals, which are used as mordant in the dyeing process.3 Both dyes and heavy

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metals are hazardous and have an important ecological impact on ecosystem due to their strong

44

toxicity, environmental persistence, and bioaccumulation.4, 5 A large number of conventional

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techniques have been used for the removal of a single class of contaminants individually (either

46

heavy metals or dyes).6-9 However, the different physical and chemical properties of the two

47

contaminant classes make the treatment of co-contaminated wastewaters more challenging.10

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Thus, the development of a facile and environmentally friendly approach to simultaneously

49

remove both the two contaminants from wastewaters is urgent and crucial.11 One suggested

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approach is to modify an existing successful method for a single class of contaminants so that it

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can be applied to simultaneous removal of both classes of contaminants.10

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Cyclodextrins (CDs), natural cyclic oligosaccharides produced from the enzymatic degradation

53

of starch by bacteria,12 have attracted significant attention as selective and highly efficient

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adsorbents in a wide range of medicinal and environmental processes.13-17 The most important

55

property of CDs is their geometrically well-defined cavities useful for host-guest inclusion

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interactions with a wide range of molecules with suitable size and polarity.18, 19 According to

57

their host-guest interactions feature,20 CDs are well-known to reliably form rapid and reversible

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inclusion complexes with various nonpolar organic molecules,21 especially aromatic

59

molecules.22, 23 It is worth noting that the high density of hydroxyl groups on the interior and

60

exterior of CDs are possible to be modified by various functional groups, endowing the CDs with

61

extra specific properties.19 M. L. Brusseau et al. have used Carboxymethyl-β-cyclodextrin


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(CMCD) to simultaneously remove organic (phenanthrene) and inorganic (Cd2+, Ni2+, Sr2+)

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contaminants from soil and aquifer materials.12 In that case, carboxymethyl groups on the

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outside of CMCD molecules were expected to complex metal ions. However, in comparison with

65

synthetic complexing agents, such as ethylenediaminetetraacetic acid (EDTA) and

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diethylenetriaminepentaacetic acid (DTPA), CMCD showed weaker metal complexing property,

67

and even its metal removal efficiency has been questioned.10 Moreover, due to the high water

68

solubility of CDs and their derivatives,20 they have to be immobilized on an insoluble support,22

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or to be cross-linked with cross-linkers such as glutaraldehyde (GLA) and epichlorohydrin (EPI)

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to obtain CD polymers (CDP) adsorbents.16, 24 However, both GLA and EPI have been reported

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to have high level of toxicity and carcinogenicity to human beings and animals.25, 26 More

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recently, our group has successfully synthesized EDTA-, EGTA-, and DTPA-functionalized

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chitosan biopolymers by using EDTA, EGTA or DTPA as cross-linkers.27-29 Especially, EDTA

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and DTPA are very powerful chelating agents that are able to form highly stable chelates with

75

metals. More significantly, it has been reported that EDTA could be degraded in a zerovalent

76

iron (ZVI)/air/water system at room temperature and 1 atm.30 Recently, Lee et al. reported a

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green and efficient approach for photocatalytic degradation of Cu(II)-EDTA by TiO2.31

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Therefore, these degradable aminopolycarboxylic acids have been widely used for example in

79

many industrial processes and products to chelate metals in order to inhibit the occurrence of

80

some undesired reactions.32, 33 The cross-linking method using aminopolycarboxylic acids has

81

greatly enhanced the metal adsorption abilities of the raw materials. More significantly, in

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comparison with GLA and EPI, EDTA is cheaper and less toxic.27 However, studies extending

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the proposed green synthesis method to prepare functional materials other than chitosan have not

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yet been published.


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Nowadays, a few researches have reported simultaneous removal of metals and dyes from

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multicomponent solutions by using single functional groups.2, 11, 34, 35 However, as the name

87

suggests, both metals and dyes load on the same and sole active site, resulting in inevitable

88

competitions for free adsorption sites.2 Here we report the green synthesis of a water-insoluble

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EDTA-cross-linked β-cyclodextrin (EDTA-β-CD) bifunctional material that can simultaneously

90

remove dyes as well as the metal ions from aqueous solution. The CD units are covalently joined

91

by repeating EDTA linkers (Figure 1). Each component of the EDTA-β-CD has a crucial role in

92

its functioning. The cavities of CDs are responsible for the capture of the dye molecules, forming

93

inclusion complexes. The EDTA-groups are expected to act not only as cross-linkers but also as

94

chelating sites for metal ions. As discussed above CDs have limited selectivity to nonpolar

95

organic molecules due to their nonpolar cavities while most of the dyes are polar (cationic or

96

anionic). A polar group modification has been reported to be able to render the CD molecule

97

more polar property.10 Therefore, the polar EDTA-groups on the outside of the CD cavities

98

might bring along another advantage: help the inclusion of polar pollutants, such as cationic

99

molecules. Moreover, the steric effect of EDTA moieties and CD cavities could endow the

100

network-structure polymer more advantage to bind organic compounds on the network by

101

physical sorption.16 A study in Mumbai India suggested that the highest content of metals in dyes

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and textile effluents were Cu(II) (33.3 mg L-1) and Cd(II) (31.0 mg L-1),36 which are often used

103

as the mordant in the dyeing process.3 Thus these two metals and three typical cationic dyes,

104

Methylene Blue (MB), Safranin O (SO), and Crystal Violet (CV), were chosen as model

105

pollutants in this study. Besides the adsorption kinetics and isotherms of each pollutant on

106

EDTA-β-CD in single systems, the mutual effects between the co-pollutants in binary systems


5


107

and the regeneration of the used adsorbent as well as the adsorption mechanisms were

108

systematically and extensively investigated.

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Figure 1. Synthesis of EDTA-cross-linked β-cyclodextrin (EDTA-β-CD) polymer.


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MATERIALS AND METHODS

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Materials. All reagents were purchased from Sigma-Aldrich (Finland) and were used without

114

further purification. β-cyclodextrin (β-CD) was 97+% pure and all other chemicals were

115

analytical grade. Stock solutions of 1000 mg L-1 were prepared via dissolving appropriate

116

amounts of metal nitrate salts and dyes powder in deionized water. Working solutions ranging

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from 10 to 500 mg L-1 of metals and dyes were prepared by diluting the stock solutions. The

118

chemical properties of dyes are presented in Table S1. Adjustment of pH was undertaken using

119

0.1 M NaOH/HNO3 for metals, and 0.1 M NaOH/HCl for dyes, respectively.

120

Synthesis of EDTA-β-CD. EDTA-β-CD polymers were synthesized by reacting β-CD with

121

EDTA as a cross-linker and sodium dihydrogen phosphate (MSP) as a catalyst, by reference to

122

the previous report 37 on the preparation of citric acid cross-linked β-CD polymers. Dried β-CD

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(4 g, 3.5 mmol), EDTA (6 g, 20.4 mmol), MSP (Na2HPO4·7H2O, 2.68 g, 10 mmol) and 20 mL

124

of deionized water were mixed in a round bottom flask and stirred in a 100 oC oil bath for 1 h.

125

Polyethylene glycol 200 (PEG-200, 0.5 g, 2.5 mmol) as dispersant was added dropwise to help to

126

dissolve β-CD in water. The mixture was transferred into a Petri dish (φ 160 mm) and heated in

127

an oven at 155 oC for 10 h. After cooling at room temperature, the resulting condensation

128

polymer product was ground and soaked with 500 mL of deionized water, and then suction

129

filtered and rinsed with a large amount of 0.1 M HCl, deionized water, 0.1 M NaOH, again

130

deionized water, and methanol, to remove the unreacted materials and catalyst. The final product

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was dried in vacuum at 60 oC overnight.


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Synthesis of EPI-β-CD. For the comparison purpose, an insoluble epichlorohydrin cross-linked

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β-CD (EPI-β-CD) polymer, which is the most commercial β-CD product and has been most

134

widely used in environmental applications,38 was synthesized according to a typical procedure16,

135

39

136

Characterization. Fourier transform infrared (FT-IR) spectroscopy of the type Nicolet Nexus

137

8700 (U.S.A.) was employed to qualitatively identify the functional groups of the prepared

138

adsorbent. Quantitative analyses of the contents of EDTA and β-CD groups in EDTA-β-CD

139

polymer were carried out with a 2400 Series II CHNS/O Analyzer (PerkinElmer, Inc., U.S.A.).

140

The scanning electron microscope (SEM) observations and the energy dispersive X-ray

141

spectroscopy (EDX) analysis were simultaneously performed with a Hitachi S-4100 Field

142

Emission Scanning Electron Microscope (Japan). Surface charge and a point of zero charge of

143

adsorbents were determined by isoelectric point titration as a function of pH by using a Zetasizer

144

Nano ZEN3500 (Malver, U.K.). Thermogravimetric analysis (TGA) was conducted using a

145

NETZSCH TG 209F1 (Germany) at a heating rate of 5 oC min-1 from 25 to 1000 oC.

146

Adsorption Experiments. All adsorption experiments were undertaken by mixing 10 mg of

147

adsorbents with 5 mL of metal solutions and dyes solutions with concentrations ranging from 10

148

to 500 mg L-1. The effect of pH was investigated at an initial concentration of 100 mg L-1 in the

149

pH range of 1-6 for metal solutions and pH of 1.5-10 for dye solutions, respectively. Alkaline

150

solutions were not used in the case of metals adsorption to avoid the hydroxide formation.29 The

151

kinetics experiments were studied at metal concentrations of 500 mg L-1 and dye concentrations

152

of 300 mg L-1. At designated contacted times, the adsorbents were separated from solutions using

153

0.45 µm polypropylene syringe filters. Dye concentrations were determined by UV-vis

154

spectrometry (PerkinElmer Lambda 45, U.S.A.) at the maximum absorbance of dyes (Table S1).

by using EPI as a cross-linker under an alkaline environment.


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After dilution with 2% HNO3, the metal concentrations were analyzed by an inductively coupled

156

plasma optical atomic emission spectrometer (ICP-OES) Model Icap 6300 (Thermo Electron

157

Corporation, U.S.A.). All the tests were conducted in triplicate, and the adsorption capacities

158

(mmol g-1) of adsorbents were calculated as:

159

=

160

Where C0 and Ct are the initial and residual concentrations (mmol L-1) of the analyte,

161

respectively, while M (g) and V (L) represent the weight of the adsorbent and volume of solution,

162

respectively.

163

Multi-Component Adsorption Studies of Cu(II) and dyes on EDTA-β-CD. Binary systems of

164

Cu(II)-MB, Cu(II)-SO and Cu(II)-CV were used in the adsorption experiments. Batch adsorption

165

tests were performed using initial concentration ranging from 10 to 200 mg L-1 for Cu(II) and

166

from 30 to 200 mg L-1 for dyes. In these experiments, we introduced a full factorial design

167

(Table S2) that the main factors of the adsorption process were the initial concentration of Cu(II)

168

and dyes (i.e., MB, SO or CV) in the binary solutions. The natural pH values of the binary

169

solutions, which ranged from 5.8 to 6.2, were used in these experiments without any further

170

adjustment. To reach the equilibrium adsorption of both pollutants (Cu(II) and dye), an excessive

171

contact time of 24 h was selected for these binary experiments. The adsorption capacities of

172

Cu(II), MB, SO, and CV were set as the response variables of this factorial design. For the

173

comparison purpose, the data of mono adsorption at the same operation conditions such as pH,

174

time and dosage as binary ones, were also involved in this factorial design experiments.

175

Regeneration. For the recovery and regeneration study, 0.05 g of dry adsorbent was firstly

176

mixed with 25 mL of 300 mg L-1 Cu(II) or MB solution. After saturation, the adsorbent was

( )

(1)


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collected and regenerated by using 10 mL of elution solution such as 1 M HNO3, ethanol, 1%

178

HCl in ethanol (v/v), or 5% HCl in ethanol (v/v). Then the adsorbent was washed by deionized

179

water and reconditioned for adsorption in succeeding cycles.

180

Evaluating the performance of EDTA-β-CD in model textile effluent. A synthetic sewage

181

was prepared according to OECD guideline for the testing of chemicals:40 a suitable amount of

182

biological and chemical medium, consisting of 160 mg peptone, 110 mg meat extract, 30 mg

183

urea, 28 mg K2HPO4, 7 mg NaCl, 4 mg CaCl2·2H2O, and 2 mg Mg2SO4·7H2O, were dissolved in

184

one liter of tap water. Then this OECD synthetic sewage was used as solvent to prepare the

185

model textile effluent containing 30 mg L-1 of Cu(II) and 300 mg L-1 of MB, which are close to

186

real textile sewage.36, 41 The adsorption was undertaken by mixing 0.10 or 0.25 g of adsorbent

187

with 50 mL of as-prepared model textile wastewater (dose 2 or 5 g L-1). After a 150 min contact

188

at room temperature, the adsorbent was separated and then successively treated by 10 mL of 5%

189

HCl in ethanol (v/v), 10 mL of 1 M HNO3 and 5 mL deionized water. Then the regenerated

190

EDTA-β-CD was reused once for the model textile effluent at the same experimental condition.

191

The concentrations of Cu(II) and MB in the effluents before and after adsorption were

192

determined by ICP-OES and UV-vis spectrometry, respectively.

193 194

RESULTS AND DISCUSSION

195

Characterizations and properties of EDTA-β-CD polymer. EDTA-β-CD polymer was

196

prepared by polycondensation reaction of β-CD with EDTA (Figure 1). In this reaction, the

197

primary hydroxyl groups of β-CD have priority to be esterified with the carboxyl groups of

198

EDTA, since the reaction was carried out in aqueous solution.37 However, when an excessive


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amount of EDTA is used, the secondary hydroxyl groups as well as the primary hydroxyl groups

200

of CDs will react with the carboxyl groups, even in the aqueous solution.15 Because Petri dish

201

was wide open and the reaction temperature was high, the water evaporated rapidly. Then the

202

water generated during the polycondensation reaction was instantly driven away, thus the

203

reaction equilibrium was pushed forward,37 resulting in a network EDTA-cross-linked β-CD

204

polymer. Figure S1 shows the FT-IR spectra of raw β-CD and β-CD polymers cross-linked by

205

EPI and EDTA. In comparison with β-CD and EPI-β-CD, two new vibration peaks at 1738 cm-1

206

and 1203 cm-1 in the EDTA-β-CD spectrum could be assigned to a C=O stretching vibration and

207

C-O characteristic peak for an ester bond respectively,15 indicating that the carboxylic acid of

208

EDTA forms ester bonds with the hydroxyl groups of CD successfully. The peak at 1679 cm-1

209

suggests the existence of free carboxylates.42 Thus, the polymer was considered as copolymer

210

(EDTA-β-CD) of β-CD unit and EDTA molecule (Figure 1).

211

The results of quantitative elemental analyses are presented in Table S3. The EDTA-groups

212

content in EDTA-β-CD could be calculated based on its N content because all the N element of

213

EDTA-β-CD comes from EDTA molecules (no N element in β-CD). Furthermore, the β-CD

214

content could be calculated by the exclusion of EDTA content from the composite. Thus, the

215

composition of EDTA-β-CD was defined as 64.53% of EDTA (2.21 mmol g-1) and 35.47% of β-

216

CD (0.31 mmol g-1). EDTA-β-CD has a little lower cross-linking degree, in comparison with EPI

217

cross-linked β-CD polymer (9.02) reported before.16 However, considered on the amount of

218

cross-linking terminals of the used cross-linkers (two for EPI and four for EDTA), the cross-

219

linking degree of EDTA-β-CD should indeed be two-fold when compared with EPI-β-CD.27 The

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high cross-linking degree endowed the water insolubility of β-CD polymers. Considering the fact

221

that the CD units and EDTA-groups were covalently joined via ester bonds, therefore, the


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amounts of active EDTA groups and active β-CD cavities remaining on the adsorbent were

223

quantified by back titration and photometric titration method, respectively. The back titration

224

result (Text S1) showed that the amount of free carboxylic acid groups on EDTA-β-CD is 5.84

225

mmol g-1. By assuming that one active EDTA group possesses four free COOH groups, the

226

amount of active EDTA groups on EDTA-β-CD is estimated to be 1.46 mmol g-1, which is less

227

than the total EDTA content (2.21 mmol g-1) obtained from elemental analyses. This could be

228

due to the fact that a part of EDTA groups act as cross-linkers. The photometric titration showed

229

that the absorbency of the alkaline phenolphthalein possesses a negative correlation with the

230

weight of active β-CD (Figure S2), and the amount of active β-CD cavities on EDTA-β-CD was

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calculated to be 0.293 mmol g-1 (Text S2), which is close to the total β-CD content (0.31 mmol g-

232

1

233

of β-CD is not significant. The amounts of active EDTA groups and active β-CD cavities

234

obtained here are well associated with the adsorption capacities of metals and dyes respectively,

235

which will be discussed in the adsorption isotherms section.

236

In addition, SEM (Figure S3a and b) images showed obvious differences of the morphologies

237

between EPI-β-CD and EDTA-β-CD, e.g., smooth EPI-β-CD and rough EDTA-β-CD. EDX

238

results (Figure S3c and d) were in a good agreement with those obtained from Elemental analysis

239

(Table S3). The much higher Na content of EDTA-β-CD than EPI-β-CD might be attributed to

240

the large amount of –COONa groups on its surface. Zeta potentials of CD polymers were

241

measured at different pH (Figure S4). The isoelectric point was determined as 2.23 for EDTA-β-

242

CD, which is much lower than 4.42 for EPI-β-CD.43, 44 This could be due to the introduction of

243

EDTA groups on the surface of EDTA-β-CD during the cross-linking process. The EDTA

244

species are H6EDTA2+ (0.54%), H5EDTA+ (28.43%), H4EDTA (44.27%), H3EDTA- (24.01%),

) defined by elemental analyses. This means that the effect of cross-linking on the active ability


12

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245

and H2EDTA2- (2.75%) at pH 1.82, where total charge is zero. The EDTA displays more positive

246

at pH < 1.82 and more negative at pH > 1.82 (MINEQL 3.0).27 Therefore, the introduction of

247

EDTA could significantly reduce the isoelectric point of the adsorbent. Similar phenomenon has

248

been obsearved in our previous studies on EDTA-modified silica and magnetic EDTA-

249

chitosan.27, 45

250

Moreover, compared to EPI-β-CD polymer, the stability of EDTA-β-CD decreased (see Figure

251

S5) due to the existing of EDTA groups. The curve of EDTA-β-CD gave three weight losses at

252

60-110 oC, 170-270 oC, and 270-930 oC, which corresponded to the water loss, EDTA

253

decomposition,27 and β-CD decomposition,46 respectively.

254

Effect of pH. It is apparent from the Figure 2 that the removal of metals and dyes by EDTA-β-

255

CD adsorbent was dependent on the pH. This could be attributed to the introduction of EDTA-

256

groups in the adsorbent. Similarly to several other EDTA-modified adsorbents,27, 45 EDTA-β-CD

257

functioned in a relatively low pHpzc value (2.23) and its zeta potential decreased along with

258

increasing pH (Figure S4). Thus, the effect of pH on metal ions and cationic dyes adsorption

259

onto EDTA-β-CD could be explained by electrostatic interaction between EDTA-β-CD surface

260

and charged adsorbates. As shown in Figure 2a, EDTA-β-CD had significantly higher adsorption

261

efficiency of metals than that of EPI-β-CD. This absolutely illuminates that EDTA-groups

262

played roles not only as cross-linkers but also as chelating sites for metal ions. As shown in

263

Figure 2b, EPI-β-CD adsorbed dyes moderately well in the order of MB>CV>SO. The CD

264

cavities could form inclusion complexes with suitable size organic molecules.18 Among these

265

three dyes, MB has the smallest molecular size (Table S1) and SO has the highest polarity (a

266

lone pair of electrons and two–NH2). Moreover, In comparison with EPI-β-CD, EDTA-β-CD

267

displayed much better adsorption performance for cationic dyes. This could be explained by two


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reasons: the electrostatic interactions between EDTA-groups and cationic dyes helped the

269

adsorption; more significantly, the polar EDTA-groups might be able to render the nonpolar CD

270

cavities more polar nature, which could form inclusion complexes with cationic dye molecules.10

271

272 273

Figure 2. Effects of pH on adsorption of metals (a) and dyes (b) by cross-linked β-CD

274

adsorbents.

275

Adsorption kinetics. The effect of contact time on the adsorption of mono-component pollutants

276

onto EDTA-β-CD is depicted in Figure S6. The adsorption was found to be very fast within the

277

first 60 min, and then gradually reached equilibrium after 360 min, 150 min, and 300 min for


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Cu(II), Cd(II), and dyes, respectively. Therefore, 10 h was selected for the subsequent adsorption

279

experiments as an excess contact time. Furthermore, the kinetic data was fitted with pseudo-

280

second-order kinetic model as follows47

281

=

+

(2)

282

Where qt and qe (mmol g-1) are the adsorption capacities at time t and at equilibrium,

283

respectively, while k (g mol-1 min-1) is the rate constant. The pseudo-second-order model was

284

perfectly applicable for the adsorption kinetics of EDTA-β-CD toward the pollutants, represented

285

by the high correlation coefficients R2 > 0.999 and fine consistencies between the calculated and

286

experimental qe values (Table S4 and insets of Figure S6). Note that the dyes had faster kinetics

287

compared with those of metals (higher k values, Table S4), suggesting that the modified CD

288

cavities inclusion process was faster than EDTA complexing.

289

Adsorption isotherms. Two different isotherms, i.e. Langmuir and Sips (Langmuir-Freundlich)

290

models, were employed to fit the experimental data of metals and dyes on EDTA-β-CD. The

291

Langmuir isotherm is based on homogeneous adsorption29

292

=

293

The Sips model is a combination of the Langmuir and Freundlich models and takes heterogeneity

294

into account32

295

=

296

where qe (mmol g-1) and Ce (mmol L-1) are the adsorption capacity and equilibrium concentration

297

of the adsorbate from experimental data, while qm (mmol g-1), KL/KS (L mmol-1), and nS present

( ) / ( ) /

(3)

(4)


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298

the maximum adsorption capacity of adsorbates, the energy constant, and the heterogeneity

299

factor obtained after nonlinear fitting, respectively.

300

According to the correlation coefficient R2 and the consistency between the calculated and

301

experimental qm values (Table S5) as well as the curve fitting to experimental data (Figure S7),

302

Langmuir model could describe better the metal adsorption isotherms, while the Sips model

303

fitted better with dyes. So these results might indicate homogeneous distribution of adsorption

304

active sites for metals (EDTA-groups) and heterogeneous active sites for dyes (CD cavities and

305

EDTA-groups). In addition, the resulting exponent nS values for dyes were still close to unity in

306

particular for MB (0.991) and SO (0.887). This means that the one kind of active site (might be

307

CD cavities) plays a dominant role and the other one (might be EDTA-groups) plays a minor role

308

in the adsorption of MB and SO onto EDTA-β-CD.43 The only exception seen for CV (nS 1.421)

309

might be attributed to its biggest branched structure, when EDTA-groups might have contributed

310

relatively more to its adsorption. The adsorption affinity (KL/KS) in this study showed that the

311

adsorbent appeared to exhibit stronger affinity toward dye molecules, as compared with metals

312

(Table S5).16, 47

313

Based on the active functional group amounts (EDTA 1.46 mmol g-1, CD 0.293 mmol g-1, Text

314

S1 and S2) in the adsorbent, it was calculated that 85.00% and 75.75% of active EDTA groups

315

were occupied by metal ions Cu(II) and Cd(II), while for dyes 89.42%, 57.68%, and 95.56% of

316

active CD cavities were occupied by MB, SO, and CV, respectively, if the EDTA contribution

317

for dyes was assumed negligible. Table 1 summarizes the maximum adsorption capacities of

318

metals and dyes on as-prepared and also some commonly used adsorbents. Obviously the higher

319

qm of EDTA-β-CD compared to those of most of the presented sorbents indicates that EDTA-β-

320

CD is efficient for the removal of metals and dyes in mono-component systems.


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Table 1. Comparison of the maximum adsorption capacities of metals and dyes by different

322

adsorbents Sorbents

Maximum metal adsorption capacity (mmol g-1) Ref. Cu(II)

Cd(II)

MB

SO

CV

0.035a

0.014a

-

-

-

48

0.738a

-

-

-

-

43

-

-

0.067a 0.151a -

49

palm kernel fiber

-

-

0.298a -

0.193a

50

magnetic graphene oxide

-

0.812a

0.201a -

-

11

clinoptilolite

-

0.033a

0.241a -

0.173a

2

erionite

-

0.085a

0.267a -

0.205a

2

EPI-β-CD

0.369

0.386

0.156

0.016

0.102

this study

EDTA-β-CD

1.241

1.106

0.262

0.169

0.280

this study

tannic acid immobilised activated carbon CMCD modified Fe3O4 nanoparticles nickel sulfide nanoparticleloaded activated carbon

323

a

Converted from the original unit of mg g-1 presented in the literatures.


17


Page 18 of 34

324

Simultaneous adsorption studies of Cu(II)-dye binary system. Figure S8 shows the results of

325

simultaneous adsorption of Cu(II) and dyes on EDTA-β-CD. For a comparison purpose, the

326

mono-component adsorption curves of each pollutants are also presented as references (black

327

balls). The effect of both Cu(II) and dyes in simultaneous adsorption on EDTA-β-CD was

328

determined by using the ratio of adsorption capacities (Rq)

329

=

330

Where qb,i and qm,i are the uptake of pollutant i in the binary system and mono-component

331

system with the same initial concentration, respectively. As the literature reports,2, 11, 51 there are

332

three possible cases in the simultaneous adsorption: i) if Rq > 1, synergism, i.e., the adsorption of

333

pollutant i is enhanced by the presence of co-pollutant; ii) if Rq < 1, antagonism, i.e., the sorption

334

of pollutant i is suppressed by the presence of co-pollutant; iii) if Rq = 1, non-interaction, i.e., the

335

adsorption of pollutant i is not affected by the presence of co-pollutant. The results of Rq are

336

plotted in Figure 3 as a function of the initial Cu(II) and dye concentration. Generally, the

337

sorption of Cu(II) was slightly reduced by the presence of dyes at low initial Cu(II)

338

concentrations (i.e., 10 and 50 mg L-1), while the adsorption capacity of Cu(II) was significantly

339

enhanced (Rq,Cu(II) > 1) by the presence of dyes at high Cu(II) concentrations (i.e., 100 and 200

340

mg L-1) in the binary systems. Higher dye concentration involved more effect on the adsorption

341

of Cu(II). Specifically, the adsorption capacity of Cu(II) at the initial concentration of 200 mg L-1

342

in mono-metal system (qe,[200, 0]) was 1.11 mmol g-1, while 1.36, 1.50, 1.47 mmol g-1 in Cu(II)-

343

MB, Cu(II)-SO, and Cu(II)-CV binary system (qe,[200,200], both the initial Cu(II) and dye

344

concentrations are 200 mg L-1), respectively. The results of Rq also confirmed this interactive

345

effect of dyes on Cu(II) adsorption. This synergic effect could be explained as follows: there are

, ,

(5)


18

Page 19 of 34


346

specific and different adsorption active sites for Cu(II) (EDTA-groups) and dyes (CD cavities)

347

on EDTA-β-CD according to the results of isotherms, and then the presence of dye on the

348

surface of EDTA-β-CD via CDs inclusion complexation would provide extra nitrogen-containing

349

groups, which might create new adsorption sites for Cu(II) ions. These nitrogen-containing

350

groups have been reported to be able to complex metals.29 Noticeably, SO showed the greatest

351

synergic effect on the adsorption of Cu(II). That is because among the three studied dyes, SO

352

molecular has the highest amount of nitrogen-containing groups (four), including two –NH2.

353

Some similar synergic effects have been observed by J. Deng et al. for the simultaneous removal

354

of Cd(II) and Orange G dye using a magnetic graphene oxide nanocomposite adsorbent.11

355

Interestingly, the presence of dyes enhanced the adsorption of Cu(II), in turn, however, the

356

presence of Cu(II) did not improve the adsorption of dyes (i.e., 0.833 < Rq,dye >1.029 for all

357

binary systems at tested conditions). This could be due to the divalent metal-EDTA complex

358

mechanism: metal ions were surrounded by EDTA groups, forming octahedral complex

359

structures.29, 32 Thus the presence of Cu(II) on the surface of EDTA-β-CD could not supply extra

360

sorption sites for dyes. The sorption of dyes were also slightly reduced at low initial dye

361

concentrations (i.e., 30 and 60 mg L-1) by the presence of Cu(II). The reason for this could be

362

that the EDTA-groups for metal binding can also play a minor role in cationic dye adsorption,

363

leading to the competition for the available adsorption sites. However, at high initial dye

364

concentrations (i.e., 100 and 200 mg L-1), the sorption capacities of dyes were not affected by the

365

presence of Cu(II) in binary solutions (0.959 < Rq,dye >1.029). The competitive sorption is not

366

apparent at high concentrations most likely because of the selectivity of the functional groups on

367

this bifunctional adsorbent.51


19


Page 20 of 34

368

369

370 371

Figure 3. Ratio of adsorption capacities (Rq) versus the initial concentration of binary solutions

372

for the simultaneous removal of Cu(II), MB, SO, and CV using EDTA-β-CD. The initial


20

Page 21 of 34


373

concentrations: for Cu(II) 10 mg L-1 (red), 50 mg L-1 (blue), 100 mg L-1 (pink), and 200 mg L-1

374

(olive); for dyes 30 mg L-1 (red), 60 mg L-1 (green), 100 mg L-1 (blue), and 200 mg L-1 (olive).

375

Mono-component (black) system as reference.

376

Adsorption mechanism. The elemental distribution for EDTA-β-CD after simultaneous

377

adsorption of Cu(II) and MB is illustrated in Figure S9. The bright signal spots of the elements

378

show that copper and sulphur are spread over the whole surface of EDTA-β-CD, indicating the

379

successful adsorption of Cu(II) and MB (sulphur is from MB) and the well-distributed adsorption

380

active sites. This result was further confirmed by EDX spectra. The high contents of Cu and S

381

are in a good agreement with the results of high adsorption capacities of Cu(II) and MB in binary

382

system (Figure S8).

383

To determine the adsorption nature, the adsorption thermodynamics were studied. The effect of

384

temperature on the sorption of Cu(II) and MB sorption onto EDTA-β-CD at 298 K, 308 K, and

385

318 K with an initial concentration of 100 mg L-1 were investigated. The enthalpies were

386

calculated by Van’t Hoff equations 52

387

=

(6)

388

∆ = −

(7)

389

=

390

where R is the gas constant (8.314 J mol-1 K-1), T (K) is the absolute temperature and Kc is the

391

adsorption equilibrium constant. The thermodynamic parameters were calculated from the plots

392

of lnKC versus 1/T (Figure S10) and the results were summarized in Table S6. The ∆G values

∆ !

∆"

− !

(8)


21


Page 22 of 34

393

were negative for both Cu(II) and MB at all the studied temperatures, indicating the spontaneous

394

adsorption nature for both these two pollutants.52 Importantly, the positive ∆H value for Cu(II)

395

indicated that Cu(II) sorption onto EDTA-β-CD is an endothermic process and the adsorption

396

would be enhanced when the temperature increased. This also suggested that Cu(II) adsorption

397

on EDTA-β-CD is mainly chemisorption mechanism,53 which might be attributed to the

398

chelation of EDTA groups and metal ions. On the contrary, the negative ∆H value for MB

399

indicated the exothermic nature and physisorption mechanism (host-guest inclusion of β-CD and

400

dye molecules) of MB sorption onto EDTA-β-CD.53 The inclusion complex of β-CD and dye

401

molecule dissociates with the increasing temperature. The positive ∆S values for both Cu(II) and

402

MB confirmed the increasing of randomness at solid/solution interface in the adsorption process

403

of the two species onto EDTA-β-CD.52

404

To further verify the adsorption mechanism, the FT-IR spectra of EDTA-β-CD before and after

405

Cu(II) and/or MB adsorption were compared in Figure S1. In the case of MB and MB-Cu(II)

406

adsorption, a weak bathochromic shift of the v(OH) from 3342 to 3307 cm-1 can be ascribed to

407

the weakening of the hydrogen bond in the heptameric host units.43 Moreover, one can explain

408

the new prominent appearance of the peak at 1660 cm-1 corresponding to the vibration of the

409

aromatic ring by the restriction of the aromatic groups of MB into the CD cavity.43 Similar FT-

410

IR results of CD cavity inclusion complexing with organic molecules have been reported

411

earlier.54 The shift of the v(OH) is not apparent for only Cu(II) adsorption, suggesting that there

412

might be no interaction between Cu(II) and CD cavity. Herein, it is convenient to discard the

413

inclusion complexation of CD with metals on EDTA-β-CD. In the case after Cu(II) adsorption,

414

the v(C=O) band at 1679 cm-1 obviously shifts to 1593 cm-1, reflecting the interaction between

415

EDTA carboxylate groups and copper ions.27 This behavior has also been reported by our group


22

Page 23 of 34


416

earlier for cobalt and nickel adsorption onto EDTA modified chitosan/silica.55 Noticeably, this

417

shift was also observed for only MB adsorption, confirming the interaction between EDTA-

418

groups and MB. All these results are in a good agreement with the adsorption studies in mono

419

and binary systems, and the thermodynamic study.

420

Based on these EDX, thermodynamics, and FT-IR results as well as adsorption experiments, a

421

probable adsorption mechanism for the simultaneous removal of metals and dyes is proposed and

422

described in Figure 4. It appears that each component of EDTA-β-CD has a crucial role in its

423

functioning: the EDTA-groups play the role not only as cross-linkers but also as adsorption sites

424

for metal ions (chemisorption). On the other hand, the modified cavities of CDs are responsible

425

for the capture of the cationic dye molecules by host-guest inclusion (physisorption). This result

426

could explain the kinetics that dyes have higher k values (0.679-0.882) than those of metals

427

(0.034-0.209) (Table S4) since physisorption is faster than chemisorption.56 Thus the presence of

428

Cu(II) ions might not significantly inhibit the adsorption of dyes in binary systems (Figure 3).

429

Moreover, the CD cavities are discarded to host metal ions but EDTA unit are also suggested to

430

be involved in MB adsorption. It is noticed that the full molecule sizes of SO and CV (Table S1,

431

Figure S11) are a little larger than the inner diameter of β-CD cavity reported as about 0.78 nm.57

432

Thus, it is possible that the branched parts (Figure S11b1, b2 and c1) of these two dyes inserted

433

into the CD cavities and the rest parts remained outside, providing possibility for metal binding.

434

Herein, the adsorption of dyes by CD cavities on EDTA-β-CD could create new specific

435

adsorption sites for metals, and partly improve the adsorption capacity of this novel bifunctional

436

adsorbent.


23


Page 24 of 34

437 438

Figure 4. Proposed mechanism for the simultaneous removal of metals and dyes by EDTA-β-

439

CD.

440

Desorption and regeneration. From a practical perspective, stability and reproducibility are

441

significant features of an advanced adsorbent. In this study, Cu(II)-loaded EDTA-β-CD was

442

regenerated using 1 M HNO3 for 5 times according to the methods for metal unloading from

443

other EDTA-modified materials.28, 45 In the case of dye-loaded EDTA-β-CD, owing to the

444

principal adsorption mechanism of CD inclusion complexes, organic solvents such as ethanol

445

could be good candidates for the regeneration of the adsorbent.23 In addition, since EDTA-

446

groups were partly involved in the cationic dyes adsorption, a low concentration of acid in


24

Page 25 of 34


447

ethanol solution was also further used for dyes desorption.58 Figure 5 illuminates that Cu(II)

448

loaded EDTA-β-CD could be easily regenerated by 1 M HNO3 and the regeneration efficiency

449

was almost above 95% for the first four cycles. Obviously the MB loaded adsorbent could not be

450

effectively regenerated by pure ethanol, but better using hydrochloric acid in ethanol solutions.

451

Along with increasing HCl concentration from 1 to 5% (v/v), the regeneration efficiency raised

452

to almost 100% at first 2 cycles and retained above 90% after the fifth cycle. All these results

453

suggested the stability and reusability of EDTA-β-CD in potential practical applications.

454 455

Figure 5. Regeneration of EDTA-β-CD for Cu(II) by 1 M HNO3 and for MB by ethanol, 1%

456

HCl/ethanol (v/v), and 5% HCl/ethanol (v/v), respectively (dose, 2 g L-1; pH 6; contact time, 10

457

h; initial concentration, 300 mg L-1).

458

Evaluating the performance of EDTA-β-CD in model textile effluent. To evaluate its

459

application in practice, EDTA-β-CD was tested to simultaneously remove Cu(II) and MB and

460

reuse in a model textile effluent. As shown in Figure S12, with the dosage of 2 g L-1, the EDTA-

461

β-CD achieved a relatively satisfactory removal of Cu(II) (96.06%) but very low removal of MB

462

(37.91%). This could be attributed to the high initial concentration of dye in the model textile


25


Page 26 of 34

463

effluent. Thus a dosage of 5 g L-1 was investigated in the basis of the adsorption capacity and the

464

adsorbent showed high removal efficiency for both the two target pollutants in model textile

465

effluent (96.87% Cu(II) and 91.82% MB). The adsorption capacity of MB was calculated to be

466

0.172 mmol g-1, which is relatively lower than those in mono-system (0.262 mmol g-1) and

467

binary-system (qe,[Cu50, MB200], 0.201 mmol g-1), might due to the presence of biological and

468

chemical medium such as dissolved organic nitrogen in the model textile effluent. More

469

importantly, the regenerated adsorbent could be successfully reused for the model textile effluent

470

without a significant efficiency loss. All these suggested EDTA-β-CD is qualified for practical

471

application in integrative and efficient treatment of coexistent toxic pollutants.

472

ASSOCIATED CONTENT

473

Supporting Information. Physico-Chemical characteristics of the studied dyes, detailed

474

experimental design for simultaneous adsorption, the results of characterizations (FT-IR,

475

elemental analysis, active groups quantitative analysis, SEM, EDX, zeta potential, and TGA) for

476

the as-prepared adsorbents, effect of time, the plots and parameters of the adsorption kinetics and

477

isotherms, simultaneous adsorption of copper and dyes in binary system, SEM and elemental

478

mapping of the adsorbent after copper and MB adsorption, thermodynamic study, and the 3D

479

dimensions of the studied dyes. This material is available free of charge via the Internet at

480

http://pubs.acs.org.

481

AUTHOR INFORMATION

482

Corresponding Author

483

* Tel.: +358-40-0205-215. Fax: +358-40-0205-215. E-mail: [email protected];

484

[email protected] (F. Z.).


26

Page 27 of 34


485

* Tel.: +86-731-88872-531. Fax: +86-731-88872-531. E-mail: [email protected] (D.Y.).

486

Notes

487

The authors declare no competing financial interest.

488

ACKNOWLEDGMENT

489

The authors are grateful to Finnish Funding Agency for Technology and Innovation (TEKES)

490

and EU Structural Funds for financial support. Authors also thank Sakari Modig (Aalto

491

University, Finland) for his contribution in performing part experiments in laboratory, Sara-

492

Maaria Alatalo for elemental analyses, and Dr. Michael Tam (University of Waterloo, Canada)

493

for helpful discussion and suggestion. We also thank the reviewers for their constructive

494

comments.

495

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EDTA-Cross-Linked Î²-Cyclodextrin: An Environmentally Friendly

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