Cyclodextrin Polymers as Highly Effective Adsorbents for Removal

Subscriber access provided by TU/e Library

Article

Cyclodextrin Polymers as Highly Effective Adsorbents for Removal and Recovery of Polychlorobiphenyls (PCBs) Contaminants in Insulating Oil Shintaro Kawano, Toshiyuki Kida, Kazuhiro Miyawaki, Yuki Noguchi, Eiichi Kato, Takeshi Nakano, and Mitsuru Akashi Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 19 Jun 2014 Downloaded from http://pubs.acs.org on June 20, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

Environmental Science & Technology

Cyclodextrin Polymers as Highly Effective Adsorbents for Removal and Recovery of Polychlorobiphenyls (PCBs) Contaminants in Insulating Oil Shintaro Kawano1, Toshiyuki Kida 1, Kazuhiro Miyawaki 2, Yuki Noguchi 2, Eiichi Kato2, Takeshi Nakano1, and Mitsuru Akashi1* 1

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1

Yamadaoka, Suita, Osaka, 565-0871, Japan; 2

NEOS Co. Ltd., 1-1 Ohike, Konan, Shiga, 520-3213, Japan

KEYWORDS. cyclodextrin-based polymer, adsorbent, polychlorobiphenyls, oil, recyclable material

ACS Paragon Plus Environment

1


Page 2 of 27

1

ABSTRACT

2

179 countries (parties) ratified the Stockholm Convention on Persistent Organic Pollutants

3

(POPs), and agreed to destroy polychlorobiphenyls (PCBs) and develop a sound management

4

plan by 2028. Currently still 3 million tons of PCB-contaminated oil and equipment need to be

5

managed under the Convention. Thus, the development of a facile and environmentally benign

6

method to treat large amounts of oil stockpiles contaminated with PCBs is a crucial subject.

7

Herein we report that cyclodextrin (CD) polymers, which are easily prepared by crosslinking the

8

renewable cyclic oligosaccharide γ-cyclodextrin (γ-CD) with dibasic acid dichlorides, are a new

9

selective and powerful adsorbent to remove PCBs contaminants in oil. When PCB (100 ppm)-

10

contaminated oil was passed through a column packed with the terephthaloyl-crosslinked γ-CD

11

polymer (TP-γ-CD polymer) at 80–110 °C, the PCB contaminants were completely removed

12

from the oil. Additionally, methyl esterification of the free carboxylic groups of the TP-γ-CD

13

polymer enabled the complete recovery of the PCBs adsorbed on the polymer (with >99.9%

14

recovery efficiency) by simply washing with acetone. The methyl-esterified TP-γ-CD polymer

15

could be recycled at least ten times for PCB adsorption without any loss in the adsorption

16

capability. These results revealed that the γ-CD polymers can function as highly effective and

17

powerful adsorbents for the removal and recovery of PCBs from PCB-contaminated oil and thus,

18

significantly contribute to the protection of the global environment.


2

Page 3 of 27

19


INTRODUCTION

20

The removal of persistent organic pollutants from the environment is an important subject

21

from the viewpoints of protecting the global environment and establishing a sustainable society.

22

Polychlorobiphenyls (PCBs) were widely used as insulating fluids in electric equipment, such as

23

capacitors and transformers,1 but their manufacture and commercial use have been prohibited in

24

many countries since the 1970s due to their strong toxicity, environmental persistence, and

25

bioaccumulation.2-5 However, vast quantities of insulating oils contaminated with PCBs are still

26

used or stored without the appropriate treatment in many countries, including Japan.6-8 This lack

27

of treatment is likely due to the high chemical stability of PCBs and the risk of generating highly

28

toxic dioxins upon incineration at low temperatures.

29

In the Stockholm Convention on Persistent Organic Pollutants (POPs), more than 179

30

countries (parties) ratified the Convention, and agreed to destroy PCBs and develop a sound

31

management plan by 2028.9 Currently still 3 million tons of PCB-contaminated oil and

32

equipment need to be managed under the Convention.10 Thus, the efficient and safe treatment of

33

large amounts of PCB-contaminated insulating oils is a crucial global issue. To date, chemical

34

decomposition of PCBs in insulating oils11-15 and incineration of PCB-contaminated oil at high

35

temperatures16,17 have been used to treat the contaminated oils. However, these methods have

36

serious drawbacks: harsh reaction conditions, specialized facilities, and non-recyclable oil after

37

treatment. In addition, they have the potential to generate polychlorodibenzo-p-dioxins

38

(PCDDs)/polychlorodibenzofurans (PCDFs).18 Thus, the development of a facile and

39

environmentally benign method to treat PCB-contaminated insulating oils is crucial.

40

Cyclodextrins (CDs) are renewable cyclic oligosaccharides, which are produced by enzymatic

41

degradation of starch. CDs have sub-nanometer sized cavities in which guest molecules with an


3


Page 4 of 27

42

appropriate size and shape can be incorporated. The inclusion ability of CDs has found many

43

applications in food, cosmetics, and pharmaceutical fields.19 CDs have also been used as efficient

44

and selective adsorbents to remove organic pollutants in aqueous environments.20-24 However, in

45

most cases, the application of CDs has been limited to aqueous media25 or several kinds of polar

46

organic media.26,27 Recently, we found that 6-O-modified β-cyclodextrins, such as heptakis(6-O-

47

tert-butyldimethylsilyl)-β-cyclodextrin (TBDMS-β-CD), effectively form inclusion complexes

48

with chlorinated benzenes,28 pyrene,29 and naphthalene derivatives30

49

including benzene and cyclohexane. We also reported the removal of chlorinated benzenes and

50

PCBs in insulating oil with TBDMS-β-CD28 and a channel-type γ-cyclodextrin (γ-CD)

51

assembly31 through the selective inclusion of these guest molecules within the CD cavities.

52

However, the removal of PCBs with these adsorbents has been limited to lower chlorinated

53

biphenyls (mono-, di-, and trichlorobiphenyls) due to the size restriction of the guest molecules

54

incorporated within the CD cavities. Actual PCB-contaminated insulating oils mainly contain

55

higher chlorinated biphenyls (tetra-, penta-, and hexachlorinated biphenyls).32 Thus, the

56

development of a new adsorbent which enables complete removal and recovery of all types of

57

PCBs, including the higher chlorinated biphenyls, in the insulating oils is necessary.

in nonpolar solvents

58

Considering the background, we designed and prepared CD polymers in which γ-CD is

59

crosslinked with dibasic acid dichlorides. These CD polymers are expected to show an

60

adsorption capability towards all types of PCBs in insulating oils through cooperative binding by

61

plural γ-CD cavities. Additionally, washing with the appropriate organic solvent should recover

62

the adsorbed PCBs on these polymers through the replacement of the PCBs incorporated within

63

the CD cavities by the organic solvent. Although there are a few reports on the adsorption of

64

PCBs in oil by activated carbon, the adsorption capability towards PCBs depends on the


4

Page 5 of 27


65

structure of PCB.33,34 The adsorption capability of activated carbon towards sterically bulky

66

ortho-substituted PCBs is very low. Herein we report the first example of the efficient removal

67

and recovery of a wide variety of PCB congeners, ranging from mono- to octachlorobiphenyls, in

68

oil (the PCB concentrations are 100 ppm or less) by using γ-CD polymers as adsorbents. These

69

γ-CD polymers can be recycled at least ten times without any loss in the adsorption capability

70

towards PCBs. The insulating oil cleaned by this method can be reused in electrical equipments

71

such as transformers and condensers. That is an advantage of the nondestructive removal

72

technology18,35,36 over the incineration method. Although we recently reported the removal of

73

PCBs in insulating oil by the β-CD polymers, which were prepared by the reaction of 2,6-di-O-

74

methyl-β-CD with diisocyanate crosslinkers, the recovery of PCBs adsorbed on these polymers

75

was poor, and the recycle of CD polymers was not examined.37

76 77

Materials and Methods

78

Materials. γ-Cyclodextrin (γ-CD) was purchased from Junsei Chemical Co., Ltd.(Japan). γ-

79

CD was vacuum dried at 80 °C overnight before use. Special grade pyridine, tetrahydrofuran

80

(THF), acetone, 2-propanol, and toluene were purchased from Wako Pure Chemical Industries,

81

Ltd. (Japan). Terephthaloyl dichloride (TPCl), isophthaloyl dichloride (IPCl), 4,4’-biphenyl

82

dicarbonyl dichloride (BPCl), and adipoyl dichloride (APCl) were purchased from Tokyo

83

Chemical Industry Co., Ltd. (Japan). Insulating oil was purchased from JX Nippon Oil and

84

Energy Co., Ltd. (Japan) (Table S1). 2-Chlorobiphenyl (2-MCB), 4-chlorobiphenyl (4-MCB),

85

4,4’-dichlorobiphenyl

86

trichlorobiphenyl (3,4,4’-TrCB), 3,3’,5,5’-tetrachlorobiphenyl (3,3’,5,5’-TeCB), 2,3’,4,5,5’-

87

pentachlorobiphenyl (2,3’,4,5,5’-PeCB), and

(4,4’-DiCB),

3,4’,5-trichlorobiphenyl

(3,4’,5-TrCB),

3,4,4’-

2,2’,4,4’5,5’-hexachlorobiphenyl (2,2’,4,4’5,5’-


5


Page 6 of 27

88

HeCB) were purchased from AccuStandard, Inc.(USA). Each PCB was dissolved in insulating

89

oil at a 100 ppm concentration to prepare the PCB-contaminated insulating oil. Kanechlor 500

90

(KC-500) was purchased from Wako Chemical Co., Ltd. (Japan).

91 92

Synthesis of γ-cyclodextrin (γ-CD) polymer. γ-CD polymers were synthesized by reacting γ-

93

CD with dibasic acid dichlorides, such as terephthaloyl dichloride (TPCl), isophthaloyl

94

dichloride (IPCl), 4,4’-biphenyl dicarbonyl dichloride (BPCl), and adipoyl dichloride (APCl), as

95

a crosslinker at various feed molar ratios (dibasic acid dichloride to γ-CD = 4, 6, 8, 10, and 14),

96

by reference to the previous reports38-40 on the preparation of terephthaloyl-crosslinked CD

97

polymers. Dried γ-CD (50 g, 0.044 mmol) and pyridine (660 mL) were added into a four-necked

98

round bottomed flask equipped with a mechanical stirrer, and stirred for 1 h at room temperature

99

to dissolve γ-CD in pyridine. The crosslinker dissolved in THF (230 mL) was added dropwise

100

into the pyridine solution over 1 h at 0 °C, and subsequently stirred for 4 h at 70 °C. After the

101

addition of water (100 mL) into the reaction mixture, the resulting solid was separated by suction

102

filtration, and washed with a large amount of water followed by acetone. The product was dried

103

in vacuo at 100 °C overnight to give the γ-CD polymer. Methyl esterification of the carboxylic

104

acid in the TP-γ-CD polymer was carried out by adding methanol (100 mL) instead of water into

105

the reaction mixture and stirring for 6 h at 60 °C. The resulting solid was filtrated and washed

106

with a large amount of methanol followed by acetone. The solid product was dried in vacuo at

107

100 °C overnight to give the methylated γ-CD polymer (denoted as Me-γ-CD polymer).

108 109

PCB adsorption experiments. Adsorption capability of the γ-CD polymers towards the PCBs

110

in insulating oil was examined by passing the PCB-contaminated insulating oils through a


6

Page 7 of 27


111

column packed with each γ-CD polymer. 200 mg of the γ-CD polymer was loaded into a

112

stainless column (diameter: 4.6 mm, length: 100 mm). A PCB-contaminated oil (100 ppm, 400

113

mg) was passed through the column at a prescribed temperature (25, 80, 110, or 130 °C) and at a

114

N2 gas pressure of 0.50 or 1.0 MPa. Here, the PCB concentration in the insulating oil was

115

adjusted to 100 ppm, by reference to the previous report.41 The flow rates estimated from the

116

amount of oil eluted after 4 h at a N2 gas pressure of 0.5 MPa and different temperatures are 18.5

117

mg/h (at 25 °C), 37.7 mg/h (at 80 °C), and 69.3 mg/h (at 110 °C). On the other hand, the flow

118

rates of oil at a N2 gas pressure of 1.0 MPa and different temperatures are 24.6 mg/h (at 25 °C),

119

46.1mg/h (at 80 °C), and 80.6 mg/h (at 110 °C). The concentration of PCBs in the oil emerged

120

from the column end was measured by GC/MS/MS (the detail is shown below). In this study, we

121

have chosen hexa- and less chlorinated biphenyls as test probes considering the main PCB

122

components in KC-500 are those chlorinated biphenyls. In the adsorption experiments for KC-

123

500, 10 ppm concentration of KC-500 in insulating oil was used as the sample solution.

124 125

PCB recovery experiments and regeneration of the γ-CD polymer. To recover the PCBs

126

from the PCB-adsorbed γ-CD polymer, the polymer was washed with various organic solvents (2

127

g) by passing them through a column packed with the PCB-adsorbed γ-CD polymer. The

128

amounts of PCBs in the resulting organic solvent were determined by GC/MS/MS (the detail is

129

shown below). The washed polymer in the column was dried for 12 h at ambient temperature

130

under N2 flow (0.5 MPa), and then used again for the adsorption of PCBs in insulating oil.

131 132 133

Characterization. Quantitative analyses of the PCBs in insulating oil were carried out with a gas

chromatography-tandem mass spectrometer [GC(450-GC)-MS/MS(320-MS, triple stage


7


Page 8 of 27

134

quadrupole spectrometer), Bruker Daltonics Inc. (USA)] equipped with a 30 m BGB172

135

capillary column (0.25 mm id), [BGB Analytik AG (Swizerland)]. The temperature program was

136

at 15 °C/min between 120–180 °C, at 2 °C/min up to 260 °C, at 8 °C/min up to 300 °C, and held

137

at 300 °C for 5 min. Helium and argon were used as the carrier gas and CID gas, respectively.

138

Both the injector and transfer line temperatures were maintained at 200 °C. The detection limit

139

for PCBs in insulating oils was below 0.1 ppb. The scanning electron microscope (SEM)

140

observations were performed with a JEOL JSM-6701F electron microscope [JEOL Ltd. (Japan)].

141

Fourier-transform infrared (FT-IR) spectra were recorded using a Perkin-Elmer Spectrum One

142

instrument [PerkinElmer, Inc. (USA)]. The 1H nuclear magnetic resonance (NMR) spectra were

143

measured with a JEOL JNMECS-400 spectrometer [JEOL Ltd. (Japan)]. Thermogravimetric

144

analysis (TGA) was conducted using a Seiko Instrument TG/DTA 6200 [Seiko Instruments Inc.

145

(Japan)]. at a heating rate of 5 °C/min with air-cooling unit (200 mL/min). The water content in

146

organic solvents was measured with HIRANUMA AQUACOUNTER (Karl Fischer Coulometric

147

Titrator) AQ-2200 [HIRANUMA SANGYO Co., Ltd. (Japan)]. The concentration of each PCB

148

in KC-500 was measured by analyzing the isooctane solution of KC-500 (22 ppm), which was

149

prepared by dissolving KC-500 in isooctane, with GC/MS/MS (see Table S2).

150 151

Results and discussion

152

γ-CD polymers were prepared by polycondensation reactions of γ-CD with varying amounts of

153

dibasic acid dichlorides, such as terephthaloyl dichloride (TPCl), isophthaloyl dichloride (IPCl),

154

4,4’-biphenyl dicarbonyl dichloride (BPCl), and adipoyl dichloride (APCl) (Figure 1a). In these

155

reactions, pyridine was chosen as a solvent for the selective esterification of the primary

156

hydroxyl groups of γ-CD with the dibasic acid dichlorides, by reference to the previous reports


8

Page 9 of 27


157

where selective reactions of the primary hydroxyl groups of CDs with aromatic disulfonyl

158

chlorides42 and tert-buytldimethylsilyl chloride43 were achieved in a pyridine solvent. However,

159

when excess amounts of dibasic acid dichlorides are used, the secondary hydroxyl groups as well

160

as the primary hydroxyl groups of CDs will react with the dibasic acid dichlorides even in the

161

pyridine solvent. Figure S1 shows the FT-IR spectra of the γ-CD polymers prepared by the

162

reactions of γ-CD with eight molar equivalents of dibasic acid dichlorides. The appearance of

163

C=O stretching bands characteristic of esters (around 1700 cm-1) as well as the disappearance of

164

C=O stretching bands of dibasic acid dichlorides (1760~1810 cm-1) provides support for the

165

formation of the γ-CD polymer through the esterification of the CD hydroxyl groups (see also

166

Table S3). The γ-CD polymers were stable in neutral water even after heated at 80 °C for 3 h

167

(Figure S2), but they were hydrolyzed in aqueous alkaline solutions. To determine the dibasic

168

acid linker/γ-CD ratio in the γ-CD polymers, they were hydrolyzed with a 1 M NaOD/D2O

169

solution, and the ratios of the resulting dibasic acids to γ-CD were analyzed by 1H NMR (Figure

170

S3 and Table S4). Scanning electron microscopy (SEM) observations (Figure S4) show that

171

these γ-CD polymers form sub-micrometer-sized particles. In particular, the particles derived

172

from the TP-γ-CD polymer exhibit a homogeneous size distribution with diameters ranging from

173

500 to 600 nm (Figure S4a). The particles from the IP-γ-CD polymer (diameter: 200–300 nm)

174

have a smaller size than those from the TP-γ-CD polymer, implying that the length of crosslinker

175

may affect particle size (Figure S4b). On the other hand, the BP- and AP-γ-CD polymers form

176

irregular aggregates composed of particulate structures (Figures S4c and S4d).

177

The adsorption capability of the γ-CD polymers bearing different crosslinkers (crosslinker/γ-

178

CD in feed = 8) towards 3,4,4’-trichlorobiphenyl (3,4,4’-TrCB) and 3,3’,5,5’-tetrachlorobiphenyl

179

(3,3’,5,5’-TeCB) in insulating oil (100 ppm) was examined at 25 °C (Figure 2a). The adsorption


9


Page 10 of 27

180

experiment was performed by passing the PCB-contaminated insulating oil through a column

181

packed with each γ-CD polymer (the oil/γ-CD polymer weight ratio = 2.0) under N2 flow (at 0.50

182

MPa). The PCB concentrations in the insulating oil emerged from the column end were analyzed

183

by GC/MS/MS to determine the removal efficiency of these PCBs, which was expressed as the

184

percentage of PCBs removed from the insulating oil. Figure 2b shows the removal efficiency of

185

these PCBs in insulating oil with various γ-CD polymers. The TP- and BP-γ-CD polymers show

186

higher adsorption capabilities towards these PCBs than the IP- and AP-γ-CD polymers. In

187

particular, the former and latter polymers display a remarkable difference in the adsorption

188

capability towards bulkier 3,3’,5,5’-TeCB. These results suggest that the choice of the

189

crosslinker is very important44,45 in designing a γ-CD polymer possessing a high PCB adsorption

190

capability; crosslinkers bearing aromatic groups as well as the appropriate molecular length can

191

induce a high PCBs adsorption. The γ-CD contents in the crosslinked polymers (crosslinker/γ-

192

CD in feed = 8) were calculated to be 52 wt% (TP-γ-CD polymer), 66 wt% (IP-γ-CD polymer),

193

40 wt% (BP-γ-CD polymer), and 65 wt% (AP-γ-CD polymer). Comparison of these values

194

supports that the nature of the crosslinker, such as the lipophilicity and affinity for PCBs, as well

195

as the γ-CD content in the polymer is related to the PCB adsorption.

196

In the following experiments, the TP-γ-CD polymer, which exhibited high adsorption

197

capability towards 3,4,4’-TrCB and 3,3’,5,5’-TeCB, was employed as the adsorbent.

198

Examination of the adsorption capability of the TP-γ-CD polymers with different crosslinker/γ-

199

CD feed ratios towards 3,3’,5,5’-TeCB at 25 and 80 °C indicates that the PCB adsorption is

200

remarkably affected by the degree of crosslinking of the γ-CD polymer (Figure 2c), as observed

201

in the previous report where the removal of aromatic compounds in water was carried out by

202

epichlorohydrin-crosslinked β-CD polymers.21 The adsorption capability of the γ-CD polymer


10

Page 11 of 27


203

towards 3,3’,5,5’-TeCB increases with an increase in the crosslinker/γ-CD feed ratio from 4:1 to

204

8:1. On the other hand, further increase in the ratio decreases the adsorption of 3,3’,5,5’-TeCB.

205

These results indicate that a moderate content of terephthaloyl crosslinker contributes to the

206

enhancement of the PCB adsorption capability of the γ-CD polymer possibly through the

207

interactions with PCBs and providing the γ-CD polymer enough lipophilicity for the PCB and oil

208

molecules to penetrate inside the γ-CD polymer network and to approach the γ-CD units present

209

therein. Thus, the balance of the γ-CD and crosslinker content in the polymer is considered to be

210

important for high PCB adsorption in oil.

211

Interestingly, raising the adsorption temperature from 25 °C to 80 °C significantly increases

212

the removal efficiency of 3,3’,5,5’-TeCB in all the cases. Especially, the complete removal of

213

3,3’,5,5’-TeCB from insulating oil (>99.9% removal efficiency) is realized when the TP-γ-CD

214

polymer with a crosslinker/γ-CD feed ratio of eight is used at 80 °C. As the temperature

215

increases, the viscosity of the insulating oil decreases and additionally, the intramolecular

216

hydrogen bonds of the TP-γ-CD polymer are weakened, which will enhance the mass transfer of

217

the insulating oil and PCB molecules through the TP-γ-CD polymer network. This increase in

218

the mass transfer may effectively induce the enhancement of the PCB removal efficiency by the

219

TP-γ-CD polymer.

220

Figure 2d shows the adsorption capability of the TP-γ-CD polymer towards various PCBs,

221

ranging from mono- to hexachlorinated biphenyls, in insulating oil. Lower chlorinated biphenyls,

222

such as 2-chlorobiphenyl (2-MCB), 4-chlorobiphenyl (4-MCB), 4,4’-dichlorobiphenyl (4,4’-

223

DiCB), and 3,4,4’-TrCB, are completely removed by the TP-γ-CD polymer at 25 °C. On the

224

other hand, the removal efficiency towards higher chlorinated biphenyls, such as 3,3’,5,5’-TeCB,

225

2,3’,4,5,5’-pentachlorobiphenyl (2,3’,4,5,5’-PeCB), and

2,2’,4,4’5,5’-hexachlorobiphenyl


11


Page 12 of 27

226

(2,2’,4,4’5,5’-HeCB), is lower at 25 °C, indicating that the removal efficiency decreases as the

227

PCBs become sterically bulkier. When the adsorption temperature is raised to 80 °C, all of these

228

PCBs are removed completely (with >99.9% removal efficiency) by the TP-γ-CD polymer. Here,

229

the weight ratio of adsorbed 2,2’,4,4’,5,5’-HeCB to the TP-γ-CD polymer was estimated to be

230

2.0 x 10-4(g/g). Comparison of the molecular size between the γ-CD cavity and these PCBs using

231

the Corey-Pauling-Koltun (CPK) model shows that the γ-CD cavity is not large enough to fully

232

accommodate tetra- and higher chlorinated biphenyls, suggesting that the plural γ-CD cavities

233

should cooperate in the capture of such PCBs. This cooperation may be promoted by an increase

234

in the penetration of insulating oil inside the TP-γ-CD polymer network via the cleavage of the

235

intramolecular hydrogen bonds between the γ-CD units as the temperature increases. When PCB

236

(10 ppm concentration of KC-500)-contaminated insulating oil (Table S2 and Figure S5) is

237

treated with this TP-γ-CD polymer at 80 °C, 98% of the PCBs are removed. Increasing the

238

temperature to 110 °C completely removes the PCBs from the insulating oil. These results show

239

that the TP-γ-CD polymer exhibits high adsorption capability towards a wide range of PCB

240

congeners including tri- and tetra-ortho PCBs in insulating oil at 110 °C. On the other hand,

241

because these adsorption experiments were conducted at a relatively low N2 pressure (0.50 MPa),

242

the yield of purified insulating oil (the percentage of the weight of purified insulating oil to the

243

original PCBs-contaminated insulating oil) at 80 °C is 30–40%. This low yield can be explained

244

by considering the high viscosity of the oil. However, increasing the temperature and N2 gas

245

pressure to 110 °C and 1.0 MPa, respectively, dramatically increases the yield of purified

246

insulating oil up to 81%, mainly due to a decrease in the viscosity of the oil.

247

We also examined the recovery of the PCBs adsorbed on the TP-γ-CD polymer by washing

248

with organic solvents. Examination of the recovery efficiency of 2,2’,4,4’5,5’-HeCB (40 µg)


12

Page 13 of 27


249

adsorbed on the TP-γ-CD polymer (200 mg) using acetone, 2-propanol, and toluene as the

250

washing solvent indicates that acetone is the most effective solvent for the recovery of the HeCB.

251

The water contents of these solvents are 1055 ppm (acetone), 214 ppm (2-propanol), and 50 ppm

252

(toluene). These results suggest that the water content in the solvent as well as the solvent

253

polarity is a factor governing the desorption of PCBs. For example, passing 2 g of acetone

254

through the column at ambient temperature and a N2 gas pressure of 0.50 MPa recovers 86% of

255

2,2’,4,4’5,5’-HeCB from the TP-γ-CD polymer. However, repeated washings with acetone (total

256

weight: 5 g) did not recover the residual 2,2’,4,4’5,5’-HeCB. The HeCB adsorption experiment

257

with the regenerated TP-γ-CD polymer reveals that the removal efficiency of the HeCB slightly

258

decreases from 100% to 97%. A decreased adsorption capability towards the HeCB is also

259

observed in the case of the fresh TP-γ-CD polymer which was not dried prior to the adsorption

260

experiment; the removal efficiency of 2,2’,4,4’5,5’-HeCB in insulating oil by the non-dried TP-

261

γ-CD polymer is 76%.

262

Thermal gravimetric analysis (TGA) of the TP-γ-CD polymer indicates that the water content

263

of the TP-γ-CD polymer before drying (9.6 wt%) is higher than that of the TP-γ-CD polymer

264

after drying overnight at 100 °C in vacuo (4.8 wt% water content)46 (Figures S6a and S6b).

265

These results suggest that an increase in the water content in the TP-γ-CD polymer can reduce

266

the PCB adsorption capability. Thus, the increase in the water content in the polymer, which is

267

possibly due to the washing process of the PCB-adsorbed TP-γ-CD polymer with acetone, may

268

explain the lower adsorption capability of the regenerated TP-γ-CD polymer. Here, the water

269

content in the original insulating oil is very low (0.002 wt%), and thus, the effect of the water in

270

the insulating oil on the adsorption behavior of the CD polymer is assumed to be negligibly low.


13


Page 14 of 27

271

To improve the recovery efficiency of PCBs adsorbed on the TP-γ-CD polymer and to

272

suppress the decrease in the adsorption capability of the regenerated TP-γ-CD polymer, the free

273

carboxylic acid residues in the TP-γ-CD polymer were converted to the methyl ester. 1H-NMR

274

analysis of the esterification product of the TP-γ-CD polymer shows that 13% of terepthaloyl

275

carboxylic acid is converted to the methyl ester (Figure S3b). We assumed that protection of the

276

free carboxylic acid residues by the methyl groups increases the flexibility of the polymer

277

network by weakening the intramolecular hydrogen bonds within the polymer and inhibits the

278

binding of the polymer with water. These will result in an increased recovery efficiency of PCBs

279

adsorbed on the polymer and an increased adsorption capability of the regenerated polymer. The

280

SEM observation of the methyl-esterified TP-γ-CD polymer (denoted as Me-TP-γ-CD polymer)

281

indicates the formation of nanoparticles with smaller diameters (~100 nm) than the particles

282

from the original TP-γ-CD polymer (Figure S4e).

283

Figure 3a shows the removal efficiency of 2,2’,4,4’,5,5’-HeCB (100 ppm) in insulating oil by

284

the Me-TP-γ-CD polymer and the yields of purified insulating oils at different temperatures (at a

285

N2 gas pressure of 1.0 MPa). Similar to the case of the TP-γ-CD polymer, both the adsorption

286

capability of the Me-TP-γ-CD polymer towards the HeCB and the yield of purified insulating oil

287

increase as the temperature increases. Raising the adsorption temperature from 25 to 130 °C

288

increases the yield of purified insulating oil from 34 to 82%. Even when the weight ratio of the

289

HeCB-contaminated oil to the Me-TP-γ-CD polymer is raised to 4.0, the HeCB is completely

290

removed from the insulating oil to give the pure oil in 80% yield. On the other hand, when the

291

weight ratio is raised further, coelution of small amounts of PCBs together with the oil is

292

observed. When KC-500 (10 ppm) in insulating oil is treated with the Me-TP-γ-CD polymer

293

above 80 °C, KC-500 is completely removed. It is noteworthy that the complete recovery of the


14

Page 15 of 27


294

HeCB adsorbed on the Me-TP-γ-CD polymer is achieved by washing with acetone at 25 °C. The

295

TGA of the Me-TP-γ-CD polymers before and after drying indicates that the water content is

296

almost identical (Figure S6c and S6d). This result implies that, in the case of Me-TP-γ-CD

297

polymer, the drying process of the polymer has a negligible effect on the adsorption capability

298

towards PCBs. In fact, the HeCB adsorption experiment with the regenerated Me-TP-γ-CD

299

polymer shows that this polymer completely removes the HeCB from insulating oil. These

300

results clearly indicate that the Me-TP-γ-CD polymer can be easily regenerated and recycled.

301

The recycling ability of the Me-TP-γ-CD polymer was examined by repeating PCB

302

adsorption with the polymer and PCB desorption from the polymer. Here, 2,2’,4,4’,5,5’-HeCB

303

was used as the PCB. Figure 3b shows the removal efficiency of HeCB along with the yield of

304

purified insulating oil in the adsorption process and the percentage of HeCB remaining in the γ-

305

CD polymer after the desorption process. As expected, HeCB is completely removed from the

306

insulating oil in the first cycle, and the subsequent washing with acetone (2 g) at ambient

307

temperature gave the >99.9% desorption of HeCB adsorbed on the Me-TP-γ-CD polymer,

308

indicating that HeCB adsorbed on the polymer is completely recovered. After the acetone

309

remaining in the Me-TP-γ-CD polymer was completely removed at ambient temperature for 12 h

310

under N2 flow, the adsorption and desorption processes were repeated. In every adsorption

311

process, HeCB is removed completely, and the adsorbed HeCB is completely desorbed upon

312

washing with acetone. However, the yield of purified insulating oil slightly decreases with

313

repetition. The Me-TP-γ-CD polymer can be recycled ten times without any loss in the HeCB

314

adsorption ability.

315

These results reveal that these γ-CD polymers can function as highly effective and powerful

316

adsorbents for removal and recovery of PCBs in insulating oil (this technology is not for pure


15


Page 16 of 27

317

PCB oils but for PCB-contaminated oils). Consequently, these polymers are expected to

318

significantly contribute to the protection of global environment. We believe that the removal and

319

recovery of PCBs in oil by the CD polymer may provide a new strategy for designing adsorbents

320

for the removal and recovery of harmful contaminants in various types of oils, including edible

321

oils. However, up-scale experiments using this technology are necessary for practical use.

322

In conclusion, we have demonstrated that γ-CD polymers prepared by crosslinking γ-CD with

323

terephthaloyl linkers (TP-γ-CD polymers) show a high adsorption capability towards PCB

324

congeners (mono- to octachlorinated biphenyls) in insulating oil. In particular, the methyl-

325

esterified γ-CD polymer (Me-TP-γ-CD polymer), in which the free carboxylic acid residues in

326

the TP-γ-CD polymer are converted to the methyl ester, enables the complete removal of PCBs

327

to afford pure insulating oil in more than 80% yield. Additionally, the PCB adsorbed on the Me-

328

TP-γ-CD polymer is completely recovered upon simply washing with acetone. The Me-TP-γ-CD

329

polymer can be recycled at least ten times for the PCB adsorption without any loss in the

330

adsorption capability.


16

Page 17 of 27


331

ASSOCIATED CONTENT

332

Supporting Information. This material is available free of charge via the Internet at

333

http://pubs.acs.org.

334 335

AUTHOR INFORMATION

336

Corresponding Author.

337

*Tel: +81-6-6879-7356; Fax: +81-6-6879-7359; E-mail: [email protected]

338 339

ACKNOWLEDGMENT

340

This work was partially supported by the Funding Program for Next Generation World-Leading

341

Researchers (GR067) and the Risk-Taking Fund for Technology Development from the Japan

342

Science and Technology Agency (JST).


17


Page 18 of 27

FIGURES.

Figure 1. Preparation of γ-CD polymers by crosslinking γ-CD with various dibasic acid dichlorides.


18

Page 19 of 27


Figure 2. (a) Schematic illustration of the PCBs adsorption system employed in this work. (b) Removal efficiency of 3,4,4’-TrCB and 3,3’,5,5’-TeCB by the γ-CD polymers with TP, IP, BP, and AP crosslinkers (feed molar ratio: crosslinker/γ-CD = 8) (at 25 ºC). (c) Removal efficiency of 3,3’,5,5’-TeCBs by the TP-γ-CD polymers with different crosslinker/γ-CD ratios at 25 and 80 ºC. (d) Removal efficiency of various PCB congeners from insulating oil by the TP-γ-CD polymer (feed molar ratio of the crosslinker to γ-CD is eight) at different temperatures (N2 pressure: 0.50 MPa).


19


Page 20 of 27

Figure 3. (a) Removal efficiency of 2,2’,4,4’,5,5’-HeCB and the yield of the resulting pure insulating oil as a function of temperature (n = 2). (b) Removal efficiency of 2,2’,4,4’,5,5’HeCB in insulating oil by the Me-TP-γ-CD polymer and the percentage of 2,2’,4,4’,5,5’HeCB remaining in the Me-TP-γ-CD polymer after the desorption process by washing with acetone for each cycle.


20

Page 21 of 27


REFERENCES

1.

Rouse, T. O. Mineral insulating oil in transformers. IEEE Electr. Insul. M. 1998, 14, 6-16.

2.

Gustafson, C. G. PCB’s-prevalent and persistent. Intensified research is needed to minimize their dangers. Environ. Sci. Technol. 1970, 4, 814-819.

3.

Risebrough, R. W.; Rieche, P. D.; Peakall, B.; Herman, S. G.; Kirven, M. N. Polychlorinated Biphenyls in the Global Ecosystem. Nature 1968, 220, 1098-1102.

4.

Safe, S. H. Polychlorinated biphenyls (PCBs): Environmental impact, biochemical and toxic responses, and implications for risk assessment. Crit. Rev. Toxicol. 1994, 4, 87-149.

5.

Kelly, B. C.; Ikonomou, M. G.; Blair, J. D.; Morin, A. E; Gobas, F. A. P. C. Food webspecific biomagnification of persistent organic pollutants. Science 2007, 317, 236-239.

6.

Kanbe, H.; Shibuya, M. Solvent cleaning of pole transformers containing PCB contaminated insulating oil. Waste Manage. 2001, 21, 371-380.

7.

Seok, J.; Hwang, K.-Y. Thermo-chemical destruction of polychlorinated biphenyls (PCBs) in waste insulating oil. J. Hazard. Mater. 2005, 124, 133-138.

8.

Kaya, D.; Imamoglu, I.; Sanin, F. D. Anaerobic mesophilic digestion of waste activated sludge in the presence of 2,3’,4,4’,5-pentachlorobiphenyl. Int. Biodeter. Biodegr. 2013, 83, 41-47.

9.

Stockholm

Convension

Website;

http://chm.pops.int/Implementation/PCBs/Overview/tabid/273/Default.aspx. 10.

Stockholm Convention (2010) PCB Elimination Club (PEN) magazine. Issue 1 12/2010; http://chmpopsint/tabid/738/Defaultaspx.

11.

Taniguchi, S.; Hosomi, M.; Murakami, A. Chemical decomposition of toxic organic compounds. Chemosphere 1996, 32, 199-202.


21


12.

Page 22 of 27

Hu, X.; Zhu, J.; Ding, Q. Environmental life-cycle comparisons of two polychlorinated biphenyl remediation technologies: Incineration and base catalyzed decomposition. J. Hazard. Mater. 2011, 191, 258-268.

13.

Izadifard, M.; Langford, C. H.; Achari, G. Photocatalytic dechlorination of PCB 138 using leuco-methylene blue and visible light; reaction conditions and mechanisms. J. Hazard. Mater. 2010, 181, 393-398.

14.

Ahmad, M.; Simon, M. A.; Sherrin, A.; Tuccillo, M. E.; Ullman, J. L.; Teel, A. L.; Watts, R. J. Treatment of polychlorinated biphenyls in two surface soils using catalyzed H2O2 propagations. Chemosphere 2011, 84, 855-862.

15.

Hatakeda, K.; Ikushima, Y.; Sato, O.; Aizawa, T.; Saito, N. Supercritical water oxidation of polychlorinated biphenyls using hydrogen peroxide. Chem. Eng. Sci. 1999, 54, 30793084.

16.

Tsuji, M.; Nakano, T.; Okuno, T. Measurement of combustion products from PCB waste incinerator. Chemosphere, 1987, 16, 1889-1894.

17.

Ishikawa, Y.; Noma, Y.; Yamamoto, T.; Mori, Y.; Sakai, S. PCB decomposition and formation in thermal treatment plant equipment. Chemosphere 2007, 67, 1383-1393.

18.

Weber R. Relevance of PCDD/PCDF formation for the evaluation of POPs destruction technologies – Review on current status and assessment gaps. Chemosphere, 2007, 67, 109-117.

19.

Szejtli, J. Introduction and general overview of cyclodextrin chemistry. Chem. Rev. 1998, 98, 1743-1754.


22

Page 23 of 27


20.

Murai, S.; Imajo, S.; Takasu, Y.; Takahashi, K.; Hattori, K. Removal of Phthalic acid esters from aqueous solution by inclusion and adsorption on β-cyclodextrin. Environ. Sci. Technol. 1998, 32, 782-787.

21.

Crini, G.; Bertini, S.; Torri, G.; Naggi, A.; Sforzini, D.; Vecchi, C.; Janus, L.; Lekchiri, Y. Sorption of aromatic compounds in water using insoluble cyclodextrin polymers. J. Appl. Polym. Sci. 1998, 68, 1973-1978.

22.

Crini, G.; Morcellet, M. Synthesis and applications of adsorbents containing cyclodextrins. J. Sep. Sci., 2002, 25, 789-813.

23.

Chen, C. Y.; Chen, C. C.; Chung, Y. C. Removal of phthalate esters by α-cyclodextrinlinked chitosan bead. Bioresour. Technol., 2007, 98, 2578-2583.

24.

Kawano, S.; Kida, T.; Takemine, S.; Matsumura, C.; Nakano, T.; Kuramitsu, M.; Adachi, K.; Akashi, M. Efficient removal and recovery of perfluorinated compounds from water by surface-tethered β-cyclodextrins on polystyrene particles Chem. Lett., 2013, 42, 392394.

25.

Rekharsky, M. V.; Inoue, Y. Complexation thermodynamics of cyclodextrins. Chem. Rev. 1998, 98, 1875-1917.

26.

Siegel, B.; Breslow, R. Lyophobic binding of substrates by cyclodextrins in nonaqueous solvents. J. Am. Chem. Soc. 1975, 97, 6869-6870.

27.

Danil de Namor A. F.; Traboulssi, R. D.; Lewis, F. V. Host properties of cyclodextrins towards anion constituents of antigenic determinants. A thermodynamic study in water and in N, N-dimethylformamide. J. Am. Chem. Soc. 1990, 112, 8442-8447.

28.

Kida, T.; Fujino, Y.; Miyawaki, K.; Kato, E.; Akashi, M. 6-O-modified β-cyclodextrin enabling inclusion complex formation in nonpolar media. Org. Lett. 2009, 11, 5282-5285.


23


29.

Page 24 of 27

Kida, T.; Iwamoto, T.; Fujino, Y.; Tohnai, N.; Miyata, M.; Akashi, M. Strong guest binding by cyclodextrin hosts in competing nonpolar solvents and the unique crystalline structure. Org. Lett. 2011, 13, 4570-4573.

30.

Kida, T.; Iwamoto, T.; Asahara, H.; Hinoue, T.; Akashi, M. Chiral recognition and kinetic resolution of aromatic amines via supramolecular chiral nanocapsules in nonpolar solvents. J. Am. Chem. Soc. 2013, 135, 3371−3374.

31.

Kida, T.; Nakano, T.; Fujino, Y.; Matsumura, C.; Miyawaki, K.; Kato, E.; Akashi, M. Complete removal of chlorinated aromatic compounds from oils by channel-type γcyclodextrin assembly. Anal. Chem. 2008, 80, 317-320.

32.

Takasuga, T.; Kumar, K. S.; Noma, Y.; Sakai, S. Chemical characterization of polychlorinated biphenyls, -dibenzo-p-dioxins, and -dibenzofurans in technical kanechlor PCB formulations in Japan. Arch. Environ. Contam. Toxicol. 2005, 49, 385-395.

33.

Kawashima, A.; Iwakiri, R.; Honda, K. Experimental study on the removal of dioxins and coplanar polychlorinated biphenyls (PCBs) from fish oil. J. Agric. Food Chem. 2006, 54, 10294-10299.

34.

Maes, J.; De Meulenaer, B.; Van Heerswynghels, P.; De Greyt, W.; Eppe, G.; De Pauw, E.; Huyghebaert, A. Removal of dioxins and PCB from fish oil by activated carbon and its influence on the nutritional quality of the oil. J. Am. Oil Chem. Soc. 2005, 82, 593-597.

35.

Miyoshi, K.; Nishio, T.; Yasuhara, A.; Morita, M. Dechlorination of hexachlorobiphenyl by using potassium–sodium alloy. Chemosphere, 2000, 41, 819–824.

36.

Noma, Y.; Mitsuhara, Y.; Matsuyama, K.; Sakai, S. Pathways and products of the degradation of PCBs by the sodium dispersion method. Chemosphere, 2007, 68, 871–879.


24

Page 25 of 27


37.

Kawano S.; Nakano T.; Miyawaki K.; Kato E.; Kida T.; Akashi M. Removal of polychlorobiphenyls in oils by dimethyl-β-cyclodextrin polymers Organohalogen Compounds, 2012, 74, 1194-1197 (Proceedings of Dioxin 2012: International Symposium on Halogenated Persistent Organic Pollutants in Cairns, Australia).

38.

Pariot, N.; Edwards-Levy, F.; Andry, M.-C.; Levy, M.-C. Cross-linked β-cyclodextrin microcapsules: preparation and properties. Int. J. Pharm. 2000, 211, 19-27.

39.

Pariot, N.; Edwards-Levy, F.; Andry, M.-C.; Levy, M.-C. Cross-linked β-cyclodextrin microcapsules. II. Retarding effect on drug release through semi-permeable membranes. Int. J. Pharm. 2002, 232, 175-181.

40.

Wilson, L. D.; Guo, R. Preparation and sorption studies of polyester microsphere copolymers containing β-cyclodextrin. J. Colloid Interf. Sci. 2012, 387, 250-261.

41.

Jones, C. G.; Silverman, J.; Al-Sheikhly, M.; Neta, P.; Poster, D. L. Dechlorination of polychlorinated biphenyls in industrial transformer oil by radiolytic and photolytic methods. Environ. Sci. Technol. 2003, 37, 5773-5777.

42.

Tabushi, I.; Yamamura, K.; Nabeshima, T. Characterization of Regiospecific A,C- and A,D-Disulfonate Capping of p-Cyclodextrin. Capping as an Efficient Production Technique. J. Am. Chem. Soc. 1984, 106, 5267-5270.

43.

Fügedi, P. Synthesis of heptakis (6-O-tert-butyldimethylsilyl) cyclomaltoheptaose and octakis (6-O-tert-butyldimethylsilyl) cyclomalto-octaose Carbohydrate Research 1989, 192, 366-369.

44.

Komiyama, M.; Hirai, H. Preparation of Immobilized β-Cyclodextrins by Use of Alkanediol Diglycidyl Ethers as Crosslinking Agents and Their Guest Binding Abilities. Polym. J. 1987, 19, 773-775.


25


45.

Page 26 of 27

Sugiura, I.; Komiyama, M.; Toshima, N.; Hirai, H. Immobilized β-Cyclodextrins. Preparation with Various Crosslinking Reagents and the Guest Binding Properties Bull. Chem. Soc. Jpn. 1989, 62, 1643-1651.

46.

Meier, M. M.; Luiz, M. T. B.; Szpoganicz, B.; Soldi, V. Thermal analysis behavior of βand γ-cyclodextrin inclusion complexes with capric and caprilic acid. Thermochim. Acta 2001, 375, 153–160.


26

Page 27 of 27


TOC Art 343


27

Cyclodextrin Polymers as Highly Effective Adsorbents for Removal

Recommend Documents