Reductive Debromination of Polybrominated Diphenyl Ethers

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Reductive debromination of polybrominated diphenyl ethers: Dependence on Br number of the Br-rich phenyl ring Shun Guo, Lihua Zhu, Tetsuro Majima, Ming Lei, and Heqing Tang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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Environmental Science & Technology

1

Reductive debromination of polybrominated diphenyl ethers:

2

Dependence on Br number of the Br-rich phenyl ring

3

Shun Guoa, Lihua Zhua*, Tetsuro Majimaa, Ming Leib, Heqing Tangb,*

4

a Key

5

of Education), School of Chemistry and Chemical Engineering, Huazhong University

6

of Science and Technology, Wuhan 430074, PR China

7

b

8

Nationalities, Wuhan 430074, PR China

Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry

College of Resources and Environmental Science, South-Central University for

9 10

Graphic abstract Reactive ring

Br m

m = 1~5 n = 0~5 m≥n

+

1

O

eBr m

Br n

Br n

··

- Br -

10-6

+H Br m-1

Br n

10-2

10-4

O

O

Relative rate constant (kR)

O

Br m-1

Br n

10-8

11 12

1

ACS Paragon Plus Environment

1

2

3

m

4

5


13

ABSTRACT: Reductive debromination has been widely studied for the degradation

14

of polybrominated diphenyl ethers (PBDEs), although the reaction mechanisms are

15

not so clear. In the present study, the photocatalytic degradation and debromination of

16

ten PBDEs were carried out with CuO/TiO2 nanocomposites as the photocatalyst

17

under anaerobic condition. The pseudo-first-order rate constants were obtained for the

18

photocatalytic debromination of PBDEs, and their relative rate constants (kR) were

19

evaluated against kR= 1 for BDE209. Unlike the generally accepted summary that kR

20

is dependent on total Br number (N) of PBDEs, kR is found to depend on Br number

21

on a phenyl ring with more Br atoms than the other one. In other word, a phenyl ring

22

substituted by more Br is more reactive for the reductive debromination. The

23

calculated LUMO energies (ELUMO) of all PBDEs are well correlated to more reactive

24

phenyl ring with more Br, compared with N of two phenyl rings. The result was

25

explained by LUMO localization on the Br-rich phenyl ring, suggesting the reductive

26

debromination occurs on the phenyl ring.

2


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28

INTRODUCTION

29

Polybrominated diphenyl ethers (PBDEs) are well known as brominated flame

30

retardants (BFRs) to be extensively used in various consumer products over the past

31

four decades.1 It is estimated that approximately 46 000, 25 000, and 380 000 tons of

32

commercial penta-BDE, octa-BDE, and deca-BDE, respectively, were and will be

33

consumed in the United States and Canada during 1970~2020.2 The widespread use of

34

PBDEs makes them ubiquitous in the environment all over the world.3-5 It is known

35

that PBDEs are bio-accumulative to cause thyroid dysfunctions and developmental

36

neurotoxicity.6,7 However, as a class of persistent organic pollutants (POPs), the

37

natural degradation of PBDEs is very slow. Therefore, much attention has been paid

38

to develop degradation of such pollutants.

39

The oxidative degradation of PBDEs is only possible under very extreme

40

conditions: prolonged UV light irradiation8 or concentrated sulfuric acid as solvent9.

41

On the other hand, many reductive processes such as zerovalent iron (ZVI)

42

reduction,10,11

43

biodegradation15 have been studied, and a stepwise debromination mechanism is

44

suggested. As PBDEs, penta-BDE, octa-BDE, and deca-BDE have been listed as

45

POPs by Stockholm Convention.16 The persistence of these POPs in natural

46

environment is resulted from their low reactivity. More seriously, during the reductive

47

debromination of highly brominated PBDEs (for example, deca-BDE, i.e. BDE209),

48

low-brominated PBDEs are gradually accumulated to be less reactive for the

49

subsequent reductive debromination,10,12,17 but such low-brominated PBDEs are more

photocatalytic

reduction,12,13

electrochemical

3


reduction,14

and


50

toxic than highly brominated PBDEs.18

51

The dependence of the reductive debromination rate on total Br number of

52

PBDEs has attracted much attention. It has been shown that the reactivity of the

53

debromination of PBDEs decreases with decreasing Br number in PBDEs.10,11,17,19

54

However, Granelli et al. studied the relative reduction rates of fifteen PBDEs by

55

sodium borohydride,17 and found that BDE181 (a hepta-BDE with total Br number

56

(N) of 7) was more reactive than two oct-BDEs, BDE201 and BDE202 (N= 8),

57

suggesting that the reactivity related to the substitution pattern. This experimental

58

result initiates the present study.

59

Generally, the electron-transfer initiated reductive dehalogenation of aromatic

60

halides includes two successive electron transfer processes.20-22 Firstly, electron is

61

transferred to the lowest unoccupied molecular orbital (LUMO) of aromatic halides,

62

giving the radical anion. Then, the electron of the radical anion transfers to σ

63

anti-bonding orbital of carbon-halogen bond (σC-X*), leading to the cleavage of a C-X

64

bond to give aromatic carbon radical and halide ion. The carbon radical abstracts a

65

hydrogen atom from solvent molecules to produce an aromatic compound with less

66

halogen atoms. If the reductive debromination of PBDEs precedes via this reaction

67

mechanism, their LUMO energies (ELUMO) can be a good descriptor for predicting the

68

reduction rates of PBDEs. Indeed, a good correlation has been observed for the

69

relation between the reaction rates and ELUMO of PBDEs.10,11 However, the relation

70

between ELUMO of PBDEs and their chemical structures is still unclear.

71

To clarify the reductive debromination mechanism, we obtained the relative rate 4


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72

constant of the photocatalytic reductive debromination of PBDEs. We confirmed that

73

the relative rate constant is mainly dependent on Br number of one phenyl ring with

74

more Br than the other ring. The dependence of Br number of one phenyl ring on the

75

reductive debromination is explained by LUMO distributions of PBDEs to be more

76

localized on it than on the other one.

77

EXPERIMENTAL SECTION

78

Chemicals and materials. BDE209 (purity >98%), BDE47 (>98.5%), and BDE15

79

(>98%) were purchased from J&K Chemical, China. BDE99 (>95%) was supplied by

80

Wuhan Kaymke Chemical, China. TiO2 powders (P25, ca. 80% anatase, 20% rutile;

81

surface area, ca. 50 m2 g-1) were purchased from the Degussa，Germany. All reagents

82

were analytical grade and used without further purification. In addition, six PBDEs

83

including

84

2-methyl-3,4,5,6-tetrabromo-diphenyl ether (2-CH3-BDE61, as an alternative of

85

BDE61) were synthesized and characterized in the present study. The details are

86

available in the Supporting Information.

87

Photocatalytic degradation of PBDEs. Photocatalytic debromination of PBDEs was

88

conducted in a quartz vessel under Ar atmosphere at 30 oC by using CuO/TiO2

89

nanocomposites as a photocatalyst. The photocatalyst was prepared according to the

90

reported method to show a good photocatalytic activity for the reductive

91

debromination of low-brominated PBDEs as reported in our recent work.23 Typically,

92

CuO/TiO2 (10 mg) was suspended in a 50 mL of a given PBDE solution (10-5 mol L-1)

93

in methanol. After the suspension was ultrasonically dispersed for 2 min, it was

BDE166,

BDE116,

BDE75,

5


BDE30,

BDE7,

and


94

purged with Ar for 30 min under magnetic stirring to remove dissolved O2. The

95

photocatalytic debromination was started by switching on the light, and during the

96

experiment the suspension was magnetically stirred under the protection of Ar

97

atmosphere. A PLS-SXE300 Xe lamp (Beijing Perfect Light, China) equipped with a

98

365 nm bandpass filter (365 ± 5 nm) was used as the light source. At given reaction

99

time intervals, aliquots (1 mL) were sampled, immediately centrifuged, and filtered

100

through a 0.22 μm membrane to remove the catalyst. The filtrates were analyzed by

101

HPLC (Agilent 1260 series, USA) equipped with a diode array and an SB-C18

102

column (4.6 × 150 mm). The mobile phase was 90% acetonitrile and 10% water at a

103

rate of 1.0 mL min-1, and the detection wavelength was set at 240 nm. Each of all the

104

experiments were usually conducted in triplicate, and the average values were

105

obtained throughout the present work. It should be noted that we do not need the same

106

irradiation intensity, because only relative debromination rate constant of PBDE is

107

used in the present work. Because the relative reactivity of PBDEs varies over 7

108

orders of magnitude as demonstrated in the present work, the irradiation intensity is

109

required to be adjusted in a wide range by changing irradiation path length and

110

irradiation area (light spot area). As a reference, the irradiation at 365 nm was

111

typically conducted with 4 mW cm-2 and an irradiation area of 1 cm-2. The irradiation

112

intensity in other cases was expressed by transforming it to an equivalent value with

113

an irradiation area of 1 cm-2.

114

GC-MS analysis method. The intermediate products were identificated by GC-MS

115

(TRACE 1300 GC coupled to ISQ system, Thermo Fisher Scientific, USA.) equipped 6


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116

with a TR-5MS capillary column (30 m × 0.25 mm × 0.25 μm). The initial oven

117

temperature was 40 oC. After maintaining for 1 min, the oven temperature was

118

increased by 26 oC min-1 to a final temperature 300 oC, and then held for 10 min. The

119

injector, transfer line and ion source temperatures were set at 280 oC. And the full

120

scan mode (m/z 50-700) was used for mass analysis.

121

Computation methods. All theoretical calculations were carried out with Gaussian09

122

program.24 The molecular geometries and molecular orbital properties of 209 PBDEs

123

were obtained with the aid of density functional theory (DFT) calculations using the

124

Becke’s

125

gradient-corrected correlation functional (B3LYP hybrid functional),25,26 which were

126

widely used in various studies27-29 with 6-31+G(d) basis sets.30 No symmetry

127

restriction was imposed during the optimization process. Because the reductive

128

debromination of PBDEs was carried in methanol, methanol was chosen as a model

129

solvent using the most widely used conductor polarized continuum model (CPCM).

three-parameter

hybrid

exchange

function

with

Lee-Yang-Parr

130

RESULTS AND DISCUSSION

131

Weak dependence of the reductive debromination of PBDEs on total Br

132

number. All PBDEs have a symmetric skeletal structure with two separated phenyl

133

groups connected by an ether bond (Figure 1a), in which a torsional angle exists

134

between two planes of phenyl rings. Five sites of each phenyl ring can be replaced by

135

Br. In the present study, the two phenyl rings were assigned to “m ring” and “n ring”,

136

representing the ring with more and less Br substitutions, respectively, m = 1~5, n =

137

0~5 and m ≥ n as shown in Figure 1a. 7



Page 8 of 28

138

To investigate the dependence of the reductive debromination of PBDEs on N,

139

we conducted the CuO/TiO2 photocatalytic degradation of nine PBDEs and

140

2-methyl-3,4,5,6-tetrabromo-diphenyl

141

atmosphere. Here, 2-CH3-BDE61 with m= 4 and a methyl group on m ring was

142

synthesized instead of BDE61 because we could not synthesize BDE61. Since the

143

ELUMO= -1.912 eV of 2-CH3-BDE61 is similar to ELUMO= -1.873 eV of BDE61, it is

144

rational to use 2-CH3-BDE61 as BDE61 in the present work. The ten PBDEs covered

145

N= 2, 3, 4, 5, 6, and 10, and also covered several groups of m ring having m = 1, 2, 3,

146

and 5. The photocatalytic reductive degradation of PBDEs was analyzed by a

147

pseudo-first-order rate equation (eq. 1), ln (c/c0) = - kt

148 149 150

ether

(2-CH3-BDE61)

under

anaerobic

(1)

where k is the pseudo-first-order rate constant (min-1), t is the reaction time (min), c0 and c are the concentrations of PBDE at 0 and t min, respectively.

151

Figure 1b showed the debromination of BDE166, BDE116, and BDE47 in a

152

quartz vessel under irradiation at 365 nm with 4 mW cm-2. After irradiation for 12.5

153

min, the removal of BDE166 (N= 6) and BDE116 (N= 5) were 85 and 70%,

154

respectively. k= 0.15 and 0.099 min-1 were obtained for BDE166 and BDE116,

155

respectively. When PBDE is changed from BDE166 to BDE116 and then to BDE47,

156

N decreases by 1 in each step. Both BDE166 and BDE116 have similar k, while

157

almost no debromination of BDE47 (N= 4) was observed under the same conditions.

158

The difference cannot be explained by N, although N is important for the

159

debromination of PBDEs. 8


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160

2

(a) Br m

3

3' Br n

6

4

(b)

O 1' 2'

1

6'

5

4' 5'

1.0

c/c0

BDE47 BDE116 BDE166

0.5

0.0

(c)

0

6

t/min

9

12

0 k= k=

-1

0 .4 9

-2 -3 -4

0.0 99 k min -1 = 0. 15 m in -1

k = 0.0 k=0

35 min -1

.0 5 5

min -1

-1 -1 in m in = 0 .7 3 m k

ln(c/c0)

3

0

5

10

15

20

25

161

t/min

162

Figure 1. (a) Structure of PBDEs with the positions. The two phenyl rings are defined

163

as m and n rings with m= 1~5, n= 0~5, and m≥ n. (b) Photodegradation of BDE166,

164

BDE116, and BDE47 in a quartz vessel under irradiation at 365 nm with 4 mW cm-2.

165

(c) A comparison between the degradation of BDE116 (dash line) and BDE166 (solid

166

line) under irradiation at 365 nm with three intensities: 2 (squares), 4 (triangles), and

167

25 mW cm-2 (circles). 9



168

169

Since it is not easy to perform photocatalytic debromination of all PBDEs under

170

the same reaction conditions, we obtained the relative k (kR) from the following

171

procedure. Firstly, BDE209 was selected as the reference with kR = 1, because

172

BDE209 is the most easily degradable PBDE. Then, some PBDEs such as BDE209

173

and BDE166 were used to simultaneously obtain k in one experiment under lower

174

irradiation intensity. The kR value of each PBDE was obtained from the ratio of k for

175

PBDE and BDE209. Secondly, BDE166 having smaller k was selected as the

176

reference and photocatalytic degradation of more difficult degradable PBDEs was

177

performed with higher irradiation intensity. This step was repeated several times, kR

178

for all BPDEs were obtained. To confirm independence of kR of PBDEs on the

179

irradiation intensity, the photocatalytic degradation of BDE166 and BDE116 was

180

examined at three irradiation intensities as shown in Figure 1c, where k were 0.055,

181

0.15, and 0.73 min-1 for BDE166 and 0.036, 0.099, and 0.49 min-1 for BDE116. The

182

ratios of k for BDE166 to BDE116 were constantly 1.5 at three irradiation intensities,

183

showing kR can be correctly obtained for PBDEs even at different irradiation

184

intensities. The photocatalytic reductive debromination of ten PBDEs was conducted

185

by dividing them into several pairs (Figure S2). The ratios of k were evaluated to be

186

kBDE209 = (3.4 ± 0.2)kBDE166, kBDE166 = (1.5 ± 0.1)kBDE116, kBDE166 = (11 ± 1)k2-CH3-BDE61,

187

k2-CH3-BDE61 = (47 ± 5)kBDE99, kBDE99 = (2.4 ± 0.2)kBDE30, kBDE75 = (2.3 ± 0.2)kBDE30,

188

kBDE30 = (11 ± 1)kBDE47, kBDE47 = (7.8 ± 0.8)kBDE7, and kBDE7 = (55 ± 10)kBDE15. By this

189

way, kR of ten PBDEs was obtained as illustrated in Table 1. 10


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190 191

Table 1. Relative rate constant (kR) of ten PBDEs. PBDEs

N

m, n

kR

BDE209

10

5, 5

1

BDE166

6

5, 1

(2.9 ± 0.2)×10-1

BDE116

5

5, 0

(2.0 ± 0.2)×10-1

2-CH3-BDE61

4

4(CH3), 0

(2.7 ± 0.3)×10-2

BDE99

5

3, 2

(5.7 ± 0.9)×10-4

BDE75

4

3, 1

(5.5 ± 1.1)×10-4

BDE30

3

3, 0

(2.4 ± 0.4)×10-4

BDE47

4

2, 2

(2.2 ± 0.4)×10-5

BDE7

2

2, 0

(2.5 ± 0.5)×10-6

BDE15

2

1, 1

(4.5 ± 1.3)×10-8

192 193

Among the ten PBDEs, BDE209 is the most highly brominated with N= 10 and

194

the most easily degradable with the largest kR, while BDE15 is the most lowly

195

brominated with N= 2 and has the smallest kR. As shown in Table 1, kR of BDE209 is

196

2×107 times larger than that of BDE15. This is in good agreement with reported

197

results showing that the reductive debromination of PBDEs becomes more difficult

198

with decreasing N.10,11,17,19 As shown in Table 1, kR of BDE7 is 55 times larger than

199

that of BDE15, although both BDE7 and BDE15 have N= 2; kR of BDE75 is 25 times

200

larger than that of BDE47, although both BDE75 and BDE47 have N= 4; kR of

201

BDE116 is 350 times larger than that of BDE99, although both BDE116 and BDE99

202

have N= 5. As shown in Figure 2a, no relation between logkR and N was observed,

203

suggesting that kR of PBDE is not dependent on N. Therefore, kR was compared for 11



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204

PBDEs with same m value. For m= 3, BDE99, BDE75 and BDE30 show similar kR in

205

the order of 10-4. For m= 5, BDE209 has the largest kR, while BDE116 has the

206

smallest kR. The kR difference was only 5 times, while Br number on n ring (n)

207

decreases from 5 to 0. This strongly suggested that kR of PBDE is mainly dependent

208

on m, which is confirmed by the clear correlation between log kR and m as shown in

209

Figure 2b. (b)

(a)

(c)

1

kR

10-2

10-4

10-8

210

log kR = -4.54ELUMO -10.63

log kR = 1.53m - 8.04

10-6

R2= 0.96

2

R = 0.97

2

4

6 N

8

10

1

2

3 m

4

5

-1.0

-1.5 -2.0 ELUMO/eV

-2.5

211

Figure 2. Plots of kR of PBDEs and (a) N, (b) m, and (c) ELUMO. The data represented

212

by open triangles and squares were from ref. 17 and 11, respectively.

213 214

The relation between kR and m is also supported by the experimental results of kR

215

for a series of PBDEs. Granelli et al. reported kR by sodium borohydride for fifteen

216

PBDEs with N= 6~10 such as BDE209, BDE208, BDE207, BDE206, BDE204,

217

BDE203, BDE202, BDE201, BDE198, BDE196, BDE184, BDE183, BDE181,

218

BDE154, and BDE153, showing two exceptions from the relation between kR and N

219

(Table S1).17 For N= 8, the congers are divided into two groups: the first group 12


Page 13 of 28


220

includes BDE204, BDE203, and BDE198, while the second group has BDE202,

221

BDE201, and BDE196. The kR values of the first group are 4 times larger than those

222

of the second group. BDE181 with N= 7 shows a much larger kR value than those of

223

PBDEs with N= 8 such as BDE202, BDE201, and BDE196. These exceptions from

224

the relation between kR and N are reasonably explained by the relation between kR and

225

m. For N= 8, the first and second groups have m= 5 and m=4, and the difference in

226

the m value causes that kR values of the first group are 4 times larger than those of the

227

second group. Similarly, BDE181 is a member of the N = 7 group with m = 5, while

228

BDE202, BDE201, and BDE196 have m= 4. It is certain that the kR value of an m = 5

229

member is considerably higher than that of the m = 4 members.

230

Weak dependence of ELUMO of PBDEs on the total Br number. From the

231

calculation as shown in the Experimental section, ELUMO of 209 PBDEs were obtained

232

(Table S2). For N= 10, deca-PBDE (BDE209) has the lowest ELUMO (about -2.3 eV).

233

Plots of ELUMO vs. N of PBDEs are shown in Figure 3a, indicating no clear relation

234

with a wide bell-mouth shape. As indicated by a dashed rectangle at the bottom of

235

Figure 3a, a narrow ELUMO range centered at -2.2 ± 0.1 eV covers twenty PBDEs with

236

N= 5~10. Similarly, 38 PBDEs with N= 4~8 are squeezed into a narrow ELUMO range

237

centered at -1.9 eV. Because of no clear correlation between ELUMO and N, we

238

consider m and n of PBDEs. Twenty PBDEs with ELUMO centered at -2.2 ± 0.1 eV

239

have m= 5, while 38 PBDEs with ELUMO centered at -1.9 eV have m= 4. This suggests

240

that ELUMO of PBDEs are dependent on m. As shown in Figure 3b, a plot of ELUMO vs.

241

m shows a clear relation. It should be noted that kR has a good correlation with ELUMO 13



Page 14 of 28

242

(Figure 2c), suggesting the relation between kR and m for all 209 PBDEs. This can be

243

explained by the frontier molecular orbital theory. The frontier orbitals of a PBDE

244

molecule include antibonding orbital of C-Br (σC-Br*) and antibonding orbital of C=C

245

(πC=C*). Since C-Br bond is very long (about 1.9Å) and the strong stabilization the

246

phenyl ring, the energy of σC-Br* might be lower than that of πC=C*. According to

247

Schrodinger’s equation, the molecular orbital is denoted by the linear combination of

248

atomic orbitals. Therefore, the phenyl ring with more Br substitutions has more

249

contribution of σC-Br* and has a lower energy. However, two phenyl rings of PBDE

250

are separated by an ether bond, the energy of antibonding orbital of C-O (σC-O*) is

251

much higher than σC-Br* and πC=C*, and therefore, σC-O* has very little contribution to

252

LUMO, to have no combination of “LUMO” of two phenyl rings. Thus, E“LUMO” of m

253

ring is extremely close to E“LUMO” of PBDE, while the “LUMO” of n ring becomes a

254

higher unoccupied molecular orbital. (b)

-0.8

-0.8

-1.2

-1.2

ELUMO/eV

ELUMO/eV

(a)

-1.6 -2.0 -2.4

255 256

-1.6 -2.0

1

2

3

4

5

6

7

8

9

10

-2.4

1

N

2

3

4

5

m

Figure 3. Dependence of ELUMO of 209 PBDEs on (a) N and (b) m.

257 258

The relation between kR and m. As discussed above, the Br-richer phenyl ring

259

is more reactive than the other ring for all PBDEs, because this ring mainly 14


Page 15 of 28


260

determines kR of the reductive debromination. To understand the relation between kR

261

and m, we analyzed the LUMO distributions of 23 PBDEs with m= 1~5 as shown in

262

Figure 4. A large change of LUMO from πC=C* to σC-Br* is observed as m increases

263

from 1, 2 to 4, 5, consistent with above demonstrated explanation that more σC-Br*

264

character would be involved in the LUMO as the Br substitution increases.

265

Furthermore, BDE209 has two phenyl rings fully substituted by Br (N= 10, m= n= 5),

266

and LUMO is distributed on both rings. All other PBDEs with m= 5 have LUMO

267

distribution on m ring. A similar LUMO distribution on m ring is observed for PBDEs

268

with m= 4, while LUMO is distributed on both rings for BDE202 and BDE201 with

269

m= n= 4. For m= 3, LUMO is mainly distributed on m ring, but also on n ring with

270

increasing n for 0 ~ 3. When m= 2 or 1, LUMO is distributed on two rings, consistent

271

with our results as mentioned above.

15



209 (5, 5)

203 (5, 3)

202 (4, 4)

154 (3, 3)

47 (2, 2)

272

208 (5, 4)

207 (5, 4)

206 (5, 4)

181 (5, 2)

198 (5, 3)

201 (4, 4)

153 (3, 3)

166 (5, 1)

184 (4, 3)

99 (3, 2)

7 (2, 0)

75 (3, 1)

15 (1, 1)

Page 16 of 28

204 (5, 3)

116 (5, 0)

183 (4, 3)

30 (3, 0)

3 (1, 0)

273

Figure 4. LUMO distributions of 23 PBDEs. The number is the serial number of

274

PBDEs (for example, 209 denotes BDE209), and the numbers in the parenthesis is m,

275

n.

276 277

During the electron-transfer initiated reductive debromination of PBDEs, an

278

electron is firstly entered to the LUMO of PBDEs. To quantify the selectivity of

279

LUMO, the atomic dipole moments corrected Hirshfeld population (ADCH) on three

280

components such as m ring, O, and n ring of PBDEs and PBDEs.- with the same

281

geometry as the neutral PBDEs are calculated by using Multiwfn software. 31,32 The 16


Page 17 of 28


282

ADCH differences of three components between PBDE and PBDE.- were used to

283

simulate the distribution of LUMO. As shown in Table S3, LUMO is mainly localized

284

on m ring for all PBDEs with m> n. When an electron is attached to a neutral

285

molecule, the charge increase on m ring is in the range from 0.776 (BDE99) to 0.917

286

(BDE116), but that on n ring is less than 0.1. For PBDEs with m= n such as BDE209,

287

BDE153, BDE47, and BDE15, the charge increases on two rings are similar. Both

288

LUMO localization and ADCH differences show preferential reduction of m ring,

289

which is responsible for the relation between kR and m.

290

Because both kR and LUMO distribution of PBDEs show clear dependences on

291

m, we conclude that m is important but n is not for reductive debromination. As

292

shown in Table 2, polybrominated phenols (C6HxBr5-xOH), polybrominated phenyl

293

methyl ethers (C6HxBr5-xOCH3), and polybrominated phenyl trifluoromethyl ethers

294

(C6HxBr5-xOCF3), with the same chromophore (i.e. C6Br5O-), have similar ELUMO.

295

ELUMO of BDE116 (5, 0) is calculated to be -2.209 eV, while ELUMO increases to

296

-2.151 eV for phenyl ring (n= 0) change to an electron-donating methyl group (CH3),

297

and decreases to -2.314 eV for phenyl ring (n= 0) change to an electron-withdrawing

298

trifluoromethyl group (CF3). The same tendency is observed for all PBDEs containing

299

different m=1~5. Increasing electron-withdrawing group leads to lower ELUMO and

300

larger kR according to Figure 2c. However, the influence of the substituted group on

301

ELUMO is often less than 0.2 eV. In contrast, ELUMO increases by 0.2~0.5 eV for each

302

addition of a Br on m ring. Therefore, m has stronger influence on ELUMO than the

303

substituted groups on polybrominated phenoxyl analogues where ELUMO are mainly 17



Page 18 of 28

304

dependent on m ring. With increasing m, the eletrophilicity of m ring increases

305

through inductive and conjugative effects. The substituted group influences slightly

306

the eletrophilicity of m ring by indirect inductive effect via ether bond. As the result,

307

even CF3 has little influence, smaller than 0.2 eV, on ELUMO. It should be mentioned

308

that the change of ELUMO for PBDEs with m= 5 induced by Br number substituted on

309

n ring (n= 1~4) is less than 0.1 eV. Similarly, the change of ELUMO for PBDEs with

310

m= 4 induced by Br number substituted on n ring (n= 1~3) is small, and so on.

311

Table 2. ELUMO of bromophenoxyl analogues with different substituted groups

312

(bromophenoxyl-Y) Bromophenoxyl

ΔELUMO, 1 b /eV

ELUMO of bromophenoxyl-Y/eV

ΔELUMO,maxa

-C6H5

-CF3

-H

-CH3

/eV

-2.209

-2.314

-2.161

-2.151

0.163

-1.876

-1.985

-1.870

-1.791

0.194

-1.436

-1.565

-1.372

-1.360

0.205

-1.123

-1.312

-1.107

-1.051

0.261

-0.977

-1.093

-0.859

-0.845

0.248

0.31 ~ 0.44

0.25 ~ 0.42

0.27 ~ 0.50

0.31 ~ 0.42

The maximum difference between the different columns in the same row.

313

a

314

b

315

row.

The value differences between two adjacent upper and down rows except for the last

316

As mentioned above, m of PBDEs is important for kR and ELUMO, while n has

317

less influence. As shown in Table S3, the m ring acts as the electron acceptor in the 18


Page 19 of 28


318

reduction process, causing the debromination on m ring. To prove this mechanism, we

319

analyzed

320

debromination of BDE99 (m, n= 3, 2) and BDE166 (5, 1). As shown in Figure 5a, the

321

debromination of parent BDE99 produces five tetra-BDEs as the intermediates, three

322

of which (BDE47, BDE49, and BDE66) are resulted from the debromination on m

323

ring, while other two (BDE48 and BDE74) are from the debromination on n ring.

324

According to their relative abundance, the average concentrations of former three

325

intermediates are about three times higher than those of latter two as shown in Figure

326

S3. This means that total amount of intermediates from m-ring-debromination are

327

more than 80% for the debromination of BDE99, demonstrating clearly that the

328

debromination occurs mainly on m ring.

the

intermediates

generated

the

CuO/TiO2

photocatalytic

(b) 74 Br

66 Br

49 Br

Br 48 Br 47

20

329

48 49

47 74 66

3 Br

22

24

26

114 Br

117

99

O

Relative abundance

Relative abundance

(a)

from

24

28

115 Br

O

117 Br

Br

Br 116

Br

115

4 Br 114

26

28

166

30

32

Retention time/min

Retention time/min

330

Figure 5. GC-MS chromatograms of the solutions after the photodegradation of (a)

331

BDE99 and (b) BDE166 on CuO/TiO2 under Ar atmosphere. The numbers represent

332

the serial number of the corresponding PBDE. For example, the number 47 means

333

BDE47. The numbers beside the chemical structures of BDE99 and BDE166

334

represent the serial number of the corresponding PBDE generated from eliminating Br

335

at the position. 19



336

The m-ring debromination process becomes much clear for BDE166. The two

337

phenyl rings of BDE166 have a large difference of Br number (5 against 1). As shown

338

in Figure 5b, the debromination of BDE166 gives three penta-BDE products

339

(BDE114, BDE115, and BDE117) together with several tetra-BDE products. If the

340

debromination of the n ring (n= 1) could occur competitively, BDE116 would be

341

obtained. However, it was not detected even after a more careful analysis to eliminate

342

any possible interference from BDE115 (Figure S3b).

343

Environmental Implications. PBDEs are ubiquitous pollutants being considered as

344

the first generation of “emerging contaminants”33. The present study provides an

345

understanding of basic environmental chemistry of these pollutants in terms of the

346

relation between kR and m. When m > n, the m ring is more reactive than the n ring of

347

a PBDE in the electron-transfer initiated reductive debromination. Because the

348

debromination preferably occurs on the Br-rich phenyl ring, the debromination of

349

BDE209 proceeds in a stepwise mechanism, i.e., N= 10 to 9, 9 to 8, and then 8 to 7,

350

and so on. This indicates that the reductive debromination proceeds in a “cross

351

debromination” mechanism in which two phenyl rings in BDE209 cause the

352

debromination to give intermediates with two phenyl rings having same Br ((m, n)=

353

(5, 5) to (4, 5), (4, 5) to (4, 4), then (4, 4) to (3, 4), and so on). The present work

354

experimentally confirms that kR of BDE209 (5, 5) decreases by a magnitude of 107

355

compared with kR of BDE15 (1, 1). Such a large decrease in the reactivity for PBDEs

356

with small N indicates that the reductive debromination is not suitable for the

357

complete debromination of PBDEs when their debromination is governed by 20


Page 20 of 28

Page 21 of 28


358

reductive debromination involving electron transfer. This makes us easily understand

359

that reductive debromination of highly brominated PBDEs was observed to be

360

stopped at the accumulation of low-brominated PBDEs in the anaerobic

361

biotransformation34 and ZVI transformation35. Therefore, we have to develop

362

alternative methods for achieving complete debromination of PBDEs. Indeed,

363

metal-induced catalytic debromination of PBDEs in the presence of hydrogen sources

364

have been reported recently,36-38 which could be a candidate for the complete

365

debromination of PBDEs.

366

ASSOCIATED CONTENT

367

Supporting Information. Synthesis of PBDEs (Text S1 and Scheme S1); 1H NMR

368

spectrums of synthesized PBDEs (Figure S1); Photocatalytic reductive degradation of

369

10 congeners of PBDEs (Figure S2); GC-MS chromatograms of the solutions

370

obtained from the photodegradation of (a) BDE99 and (b) BDE 166 on CuO/TiO2

371

under Ar atmosphere (Figure S3); Relative rate of reductive debromination of PBDEs

372

by sodium borohydride (Table S1); ELUMO values and substituted patterns (m and n)

373

of all 209 PBDE congeners (Table S2); ADCH populations of 23 PBDEs and their

374

vertical anion radicals who are divided into three fragments: m ring, O atom and n

375

ring ( Table S3).

376

AUTHOR INFORMATION

377

Corresponding Authors

378

* Tel/Fax: +86 27 87543632 (L. Zhu), +86 27 67843990 (H. Tang); E-mail: 21



379

[email protected] (L. Zhu), [email protected] (H. Tang).

380

Notes

381

The authors declare no competing financial interest.

382

ACKNOWLEDGMENTS

383

The authors acknowledge the financial supports from the National Natural Science

384

Foundation of China (Grants Nos. 21707170 and 21777194). The authors would

385

thank all the reviewers for their greatly helpful suggestions and discussions.

386

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387

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Reductive Debromination of Polybrominated Diphenyl Ethers

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