Photochemical Formation of Hydroxylated Polybrominated Diphenyl

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Photochemical Formation of Hydroxylated Polybrominated Diphenyl Ethers (OH-PBDEs) from Polybrominated Diphenyl Ethers (PBDEs) in Aqueous Solution under Simulated Solar Light Irradiation Qian Zhao, Huimin Zhao, Xie Quan, Xin He, and Shuo Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b01240 • Publication Date (Web): 02 Jul 2015 Downloaded from http://pubs.acs.org on July 4, 2015

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Environmental Science & Technology

1

Photochemical Formation of Hydroxylated

2

Polybrominated Diphenyl Ethers (OH-PBDEs) from

3

Polybrominated Diphenyl Ethers (PBDEs) in

4

Aqueous Solution under Simulated Solar Light

5

Irradiation

6

Qian Zhao, Huimin Zhao*, Xie Quan, Xin He, Shuo Chen

7

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education,

8

China), School of Environmental Science and Technology, Dalian University of Technology,

9

Dalian 116024, China.

10

Corresponding author E-mail: [email protected]

11 12

TABLE OF CONTENTS (TOC)

Br

Fe(Ⅲ Ⅲ) Dissolved oxygen FA, Fe(III)-FA

Br O

hv

ortho-tetra-BDE radical

Br

Br

BDE 47

hv •OH

Br

Br

Br

Br

HO

OH Br

O

O Br

+

Br Br

6-OH-BDE 47

2'-OH-BDE 68

13 1 ACS Paragon Plus Environment


14 15

ABSTRACT: Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) are of great concern

16

due to their higher toxicity compared to PBDEs. However, the abiologic process whereby

17

PBDEs are converted to OH-PBDEs in the aquatic environment is not well understood. To

18

explore the possibility of OH-PBDEs photoformation in natural water, the photo-hydroxylation

19

of BDE-47 has been investigated in aqueous Fe(III) and/or fulvic acid (FA) solutions and in

20

natural lake water under simulated solar light irradiation. The results showed that 6-OH-BDE-47

21

and 2'-OH-BDE-68 were generated from BDE-47 under these conditions. Based on the

22

identification of derivatives and reactive radicals, OH-PBDEs formation can be ascribed to an

23

addition reaction of ortho-tetra-BDE radical and hydroxyl radical (•OH), with or without a

24

subsequent Smiles rearrangement reaction. Since the ortho-tetra-BDE radical could be readily

25

produced by the photolysis of BDE-47, even in pure water, •OH production was considered as

26

critical for the photoformation of OH-PBDEs. Thus, it is reasonable to deduce that the

27

photoreactive components (Fe(III), FA) in aqueous solution played an important role through

28

influencing •OH generation. Although the yields of OH-PBDEs did not increase regularly with

29

increasing concentration of these photoreactive components in solution, this study suggests a

30

possible abiotic origin of OH-PBDEs formation in the aquatic environment.

31 32

KEYWORDS: OH-PBDEs; PBDEs; photochemical formation; ortho-PBDEs radical.

33 34

INTRODUCTION

35

Polybrominated diphenyl ethers (PBDEs), a series of brominated flame retardants (BFRs), have

36

been incorporated into electronic products and everyday objects since the 1970s.1 As added

37

flame retardants, PBDEs can easily enter the environment during production processes or when 2 ACS Paragon Plus Environment

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the products are in use. They have become ubiquitous contaminants because of their persistence,

39

bioaccumulation, and toxicity.2,3 Due to these characteristics, PBDEs have been considered as

40

persistent organic pollutants (POPs) over the last few decades.4 The occurrence of hydroxylated

41

PBDEs (OH-PBDEs) as PBDEs metabolites has been reported in various biological and

42

abiological samples.5,6 Notably, OH-PBDEs have even been detected in wastewater effluents.7-9

43

The concern over OH-PBDEs has risen steadily since they elicit various harmful effects in

44

exposed organisms, including neurotoxicity and disruption to thyroid hormone homeostasis and

45

sex hormone steroidogenesis.4,10

46

It is worth noting that OH-PBDEs are not entirely of manmade origin; therefore, their origin

47

has attracted a great deal of attention. Recent reports have suggested that OH-PBDEs may be

48

produced naturally in biotic and abiotic environments.5,11-14 For example, some studies have

49

revealed that PBDEs or methoxylated PBDEs (MeO-PBDEs) accumulated in organisms could be

50

metabolized to OH-PBDEs,11,13,15 other researchers have suggested that OH-PBDEs are most

51

likely synthesized by marine organism, such as marine bacteria and red algae.16-18 Notably, in

52

marine organism such as red algae and blue mussels,11,13 the concentration ratios of OH-PBDEs

53

and PBDEs were relatively high. Moreover, some OH-PBDEs congeners have been detected in

54

salmon blood, algae, and seashell despite the relevant precursor PBDEs not being present.13,19

55

These findings implied that the metabolic process is not the only pathway for OH-PBDEs

56

formation in the environment. Raff et al. suggested that the atmospheric oxidation of PBDEs by

57

hydroxyl radical (•OH) was another source of OH-PBDEs,14 and the mechanism of this process

58

has been further deduced based on experimental results using isopropyl nitrite as a source of •OH

59

in He/air mixtures14 as well as calculation methods.20-22 It may explain why certain OH-PBDEs

60

are detected in surface water samples despite their corresponding PBDEs precursors being 3 ACS Paragon Plus Environment


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nonexistent.5 However, it is still unclear as to whether OH-PBDEs could be formed in actual

62

surface water, since the components of surface water are complex and very different from those

63

in air (especially in the absence of an •OH source such as isopropyl nitrite).

64

Photolysis is one of the most important processes for the transformation of POPs in the

65

aqueous environment, during which •OH plays an important role as an active species generated

66

by various photoactive compounds (e.g., Fe(III) species, dissolved organic matter).23-25

67

Moreover, •OH can further combine with many aromatic compounds (such as benzene, 4-

68

chlorobiphenyl, etc.), forming the corresponding hydroxylated products in aquatic systems, as

69

has previously been proved by laboratory studies.26,27 Therefore, it is possible that the oxidation

70

of PBDEs with •OH forming OH-PBDEs may occur in surface water under irradiation by

71

sunlight. However, since the photolysis process can be influenced by certain components,

72

namely important photoreactive species such as Fe(III) and fulvic acid (FA), there is little direct

73

experimental evidence to explain the photochemical processes and mechanisms of OH-PBDEs

74

formation in natural surface water.20

75

Herein, we present the results of photo-hydroxylation studies of PBDEs in aqueous solution,

76

using 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) as a model compound. BDE-47 is one of the

77

abundant PBDEs congeners detected in ground and surface waters,28 and the ecological risk of

78

its hydroxylated products is much higher than that of the parent molecule.4 The photo-

79

hydroxylation of PBDEs has also been investigated in the presence of important environmental

80

factors, such as Fe(III) (at different pH conditions) and fulvic acid (FA). The objectives of this

81

study were to ascertain whether OH-PBDEs can be produced by the phototransformation of

82

PBDEs and to explore the reaction pathways in natural surface water.


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MATERIALS AND METHODS

85

Chemicals. Standards of BDE-47, 6-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (6-OH-BDE-

86

47), and 2'-hydroxy-2,3',4,5'-tetrabromodiphenyl ether (2'-OH-BDE-68) were purchased from

87

Accustandard (New Haven, CT, USA). Standard stock solutions of these at concentrations

88

ranging from 2 mg/L to 20 mg/L were prepared in dichloromethane. They were stored in a

89

refrigerator at 4 °C in the dark and used within one month of preparation.

90

The derivatization reagent, pentafluorobenzoyl chloride (PFBCl), of analytical grade, was

91

acquired from Acros Organics (Geel, Belgium). 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was

92

purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Oxygen-18 water (H218O,

93

97.5 atom%

94

(FeCl3·6H2O, analytical grade) was obtained from Beijing Chemical Reagent Factory (China).

95

Suwannee River fulvic acid (FA) was purchased from the International Humic Substance Society

96

(IHSS). All of the solutions used in the experiments were prepared using Milli-Q water (18

97

MΩ·cm). Other chemicals were of analytical, pesticide, or high-performance liquid

98

chromatography (HPLC) grade, as detailed in Text S1 of the Supporting Information (SI).

18

O) was purchased from ICON Services Inc. (New Jersey, USA). Ferric chloride

99

Photochemical Experiments and Analysis of Intermediates. All of the photochemical

100

experiments were carried out in a photochemical reactor with a xenon lamp equipped with a 290

101

nm cutoff filter (average light intensity 125 mW/cm2). Details of the photochemical experiments

102

are given in Text S2.1 of the SI.

103

Extraction and isolation of hydroxylated intermediates and BDE-47 were based on a method

104

described by other researchers.5,11,13,29 The hydroxylated intermediates obtained after

105

derivatization with PFBCl and BDE-47 were identified and quantified by means of a gas

106

chromatograph with an electron capture detector (GC-ECD 6890, Agilent, USA) and a gas 5 ACS Paragon Plus Environment


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chromatograph coupled to a mass spectrometer with an electron ionization ion source (GC-MS

108

6890/5973, Agilent, USA). Details of the sample pretreatment processes and instrument

109

conditions are provided in Text S2 of the SI, and GC/MS traces of the standard OH-PBDEs are

110

shown in Figure S1 in the SI.

111

Analysis of Radicals. To deduce the mechanism of the photoformation of the OH-PBDEs, the

112

important radicals (tetrabromodiphenyl ether radical (tetra-BDE radical) and •OH radical)

113

produced during the photolysis of BDE-47 were trapped by DMPO and subsequently determined

114

by chromatography and electron spin resonance spectrometry.

115

For determination of the tetra-BDE radical, BDE-47 was reacted with DMPO (final

116

concentration 100 mmol/L (mM)) in the absence and presence of Fe(III) solution. The reaction

117

solutions were extracted by means of solid-phase extraction (SPE) cartridges (C18, 500 mg, 6 cc,

118

Waters, USA) and the extractives were detected by high performance liquid chromatography

119

(HPLC, Waters 2695, photodiode array detector (PDA) 2996, Waters, USA). Subsequently, the

120

sample from BDE-47 photolysis with DMPO and Fe(III) in acetonitrile/water (AcCN/water)

121

solution was identified by means of an Agilent 1200 Series HPLC-electrospray ionization-triple

122

quadrupole 6410 mass spectrometer (LC-ESI-MS/MS, Agilent, USA) operating in negative ion

123

mode.30,31 In order to obtain satisfactory signals from tetra-BDE-DMPO, the experiment was

124

carried out using acetonitrile as a co-solvent (AcCN/water = 6:4) under nitrogen-saturated

125

conditions. AcCN is considered to be an effective co-solvent that does not have any obvious

126

impact on the reactive intermediates produced by the photolysis of PBDEs or the

127

chromatographic detection.

128

Source of Hydroxyl Groups in OH-PBDEs. An oxygen-18 (18O) isotopic tracer method was

129

used to determine the source of hydroxyl groups in the OH-PBDEs. Under nitrogen-saturated 6 ACS Paragon Plus Environment

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and oxygen-saturated conditions, BDE-47 was irradiated in oxygen-18 water with 5 µM Fe(III).

131

The irradiated aqueous samples were extracted by means of SPE, and the collected materials

132

were detected by LC-ESI-MS/MS. Further details of this procedure are provided in Text S3.3 of

133

the SI.

134 135

RESULTS AND DISCUSSION

136

Photochemical Formation of OH-PBDEs. Previous studies have proved that the production

137

of •OH is essential for the photochemical formation of OH-PBDEs in He/air mixtures,14,32 and

138

iron is considered as a ubiquitous and crucial inducer of •OH in natural water.33,34 Therefore, we

139

tried to detect and identify the intermediates in the photolysis of BDE-47 (200 ng/L) in 2 µM

140

Fe(III) solution (pH 5.6 ± 0.1). These experimental conditions were adopted based on the

141

following considerations: (1) acidic conditions are benefit for •OH production by photolysis of

142

the photoreactive Fe(III)-hydroxy complex, a common Fe(III) species; (2) the actual pH value

143

varies from 5 to 8 and the iron concentration ranges from 10-7 to 10-4 M in natural surface water.

144

After 2 h of irradiation, extracts with and without PFBCl derivatization were analyzed by GC-

145

MS to detect the intermediates from the phototransformation of BDE-47. The intermediates were

146

identified based on comparison of the retention times, mass spectra, and assignments of major

147

fragment ions of the compounds presented in Figure 1 with the characteristic parameters of

148

authentic standards of 6-OH-BDE-47 and 2'-OH-BDE-68 (Figure S1). The GC-MS trace for the

149

non-derivatized extract showed two apparent peaks due to molecular ion clusters with m/z ratios

150

of 502 and 500 at 22.852 and 23.996 min (Figures 1A and 1C). Because of the defined

151

abundance ratio of the two natural bromine isotopes (79Br and 81Br; 50.69% to 49.31%), based on

152

the abundance ratios of the molecular ion clusters (M) centered at m/z 502 (100% abundance) 7 ACS Paragon Plus Environment


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and m/z 500 (68% abundance), it is reasonable to infer that the two intermediates were structural

154

isomers containing four bromine atoms. The major fragment ions at m/z 422 and 342 were

155

consistent with [M−Br] and [M−Br2] and further supported the above inference. The mass

156

fragmentation patterns of the peaks at 23.352 and 24.500 min of the PFBCl-derivatized extract,

157

which featured intense fragment ions at m/z 696 and 694 as the dominant feature of C6F5-CO-O-

158

tetra-BDE, confirmed that the two intermediates each had one hydroxyl group (-OH). All of the

159

GC-MS traces of the intermediates with and without PFBCl derivatization showed good matches

160

in terms of retention times and assignments of major fragment ions with authentic standards of 6-

161

OH-BDE-47 and 2'-OH-BDE-68 (see Figure S1 in the SI), confirming these as the two products

162

formed during the phototransformation of BDE-47.

163 100

(A) Intermediate 1 (RT 22.852 min) Non-derivatization (M-Br2) 342

100

Br

(M) 502

Br O

Br

%

HO

(B) Intermediate 1 (RT 23.352min) Derivatization

F F

F

F

(M-Br-C7F5O) 422

Br

(M) 696

F O

O O

Br

%

MW: 502

Br

406

Br

Br

338 207

281 0 200

313

368

300

500 m/z

400

100 (C) Intermediate 2 (RT 23.996 min) Non-derivatization (M-Br2) 342

MW: 696

467

(M-Br) 422

236

0 300

100 Br

OH Br

O

(M) 502

400

500

(D) Intermediate 2 (RT 24.500 min) Derivatization

F F

(M-Br-C7F5O) 422

313

F

F Br

Br Br

%

%

(M) 696

F O

O

Br

O

MW: 502

700 m/z

600

Br Br

207

236

(M-Br) 422

259

MW: 696 379

313

164

0 200

300

400

500 m/z

0 300

450 400

536

483 500

616 600

700 m/z

165

Figure 1. Mass spectra of the non-derivatized (A, C) and derivatized (B, D) intermediates.

166

Intermediates 1 and 2 were 6-OH-BDE-47 and 2'-OH-BDE-68, respectively.

167 168

Mechanism of the Formation of OH-PBDEs. In many articles, it has been speculated that

169

photo-hydroxylation of aromatic compounds in aqueous solution is initiated by light excitation 8 ACS Paragon Plus Environment

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and the generation of aryl radicals, based on indirect experimental results.26,27 Significantly,

171

however, the previously reported mechanisms of OH-PBDEs formation, based on calculation

172

results and final products from gas-phase experiments, did not provide direct evidence for the

173

formation of reactive aryl radicals.14, 20 To completely explain the process of the photoformation

174

of OH-PBDEs, we sought to identify reactive aryl radicals by using DMPO as an effective

175

trapping agent for carbon-centered radical species.30

176

A new intermediate was detected during the photochemical reaction of BDE-47 in

177

DMPO/Fe(III) solutions under simulated sunlight irradiation (Figure 2 and Figure S2), whereas

178

no signal was detected in dark experiments. Notably, no similar intermediate was found in the

179

photochemical reaction of BDE-47 without DMPO, implying that the new intermediate was a

180

kind of radical adduct produced under irradiation and trapped by DMPO, that could not be easily

181

detected without a trapping agent due to its short lifetime. The mass spectrum of this DMPO-

182

radical adduct suggested that it was a tetrabromo compound with a molecular weight of 596.6, as

183

it gave a deprotonated molecular ion ([M−H]-, M is molecular weight of the intermediate) at m/z

184

595.6 (100% abundance) and a peak at m/z 594.6 with approximately 68% abundance (Figure

185

2A). In addition, another major fragment ion at m/z 420.7 could be attributed to

186

[M−H−DMPO−Br+17], arising from the loss of DMPO and one bromine atom from the DMPO-

187

radical adduct, with the added 17 mass units possibly being due to the ion source of the LC-

188

MS/MS and the mobile phase (ammonium acetate). This fragment ion (m/z 420.7) further

189

supported the inference of this intermediate being a DMPO-adduct containing four bromine

190

atoms (abbreviated as tetra-BDE-DMPO adduct). To identify the structure of the tetra-BDE-

191

DMPO adduct, the product ions from its deprotonated molecular ion ([M−H]-, m/z 595.6,

192

fragmented by collision-induced dissociation) were analyzed (Figure 2B), which included a Br9 ACS Paragon Plus Environment


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ion. Because ortho-substituted OH-PBDEs or MeO-PBDEs can only give a characteristic Br- ion

194

as a product ion of [M−H]- or [M−Br+O]-,35,36 the results of product ion (Q3) scans suggested

195

ortho-substituted DMPO in the tetra-BDE-DMPO adduct. Details of the tetra-BDE-DMPO

196

adduct are given in Table S1 in the SI. Additionally, the same tetra-BDE-DMPO adduct could be

197

detected in photochemical reaction of BDE-47 even in pure water using DMPO as a trapping

198

agent. The results suggested that the formation of the tetra-BDE radical at the ortho position

199

(removal of a hydrogen atom at the ortho position, abbreviated as ortho-tetra-BDE radical) was a

200

feasible process during the photochemical formation of OH-PBDEs in natural surface water.

201 (B) Product ion scan: Farg = 135.0 V [email protected]

100

100 [M-H]−

Relative abundance (%)


(A) Scan (12.596 - 12.761 min)

595.6 420.7

%

[M-H]−

%

Br−

0 400

202

0

450

500

550 m/z

600

650 m/z

100

200

300 400 m/z

500

600 m/z

203

Figure 2. Precursor ion (Q1, A) and product ion (Q3, B) scans for BDE-47-DMPO adducts

204

under nitrogen-saturated conditions. (1 mg/L BDE-47, 100 mM DMPO, and 5 µM Fe(III)

205

dissolved in AcCN/H2O (6:4) with 5 min irradiation).

206 207

Another important radical for the formation of OH-PBDEs was identified as •OH, because the

208

characteristic quartet peaks of the DMPO adduct with a 1:2:2:1 intensity ratio were observed in

209

ESR spectra of radicals trapped by DMPO after in situ irradiation (Figure S3 in the SI),

210

consistent with similar spectra reported by others for the •OH-DMPO adduct.37 The generation of

211

•

OH was considered as crucial for the photoformation of OH-PBDEs in He/air mixtures14 and 10 ACS Paragon Plus Environment

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212


this was corroborated by calculation methods.20-22 To explore the origin of •OH, the photochemical reaction of BDE-47 was performed in

213

18

O-

214

water with Fe(III) (AcCN/H218O = 6:4, 5 µM Fe(III)) and the products were analyzed by LC-

215

MS/MS (Figure 3). Under nitrogen-saturated conditions (Figure 3A), the major fragment of the

216

intermediate deprotonated molecular ion ([M−H]-) with m/z 503 was 2 mass units higher than

217

that of the 6-OH-BDE-47 standard (Figure S4), indicating that the hydroxyl group of the

218

intermediate (containing oxygen-18) came from

219

(Figure 3B), the major fragment of the intermediate deprotonated molecular ion ([M−H]-) was

220

also at m/z 503, and the fragments were complex with ion peaks ranging from m/z 497 to 507 and

221

the abundance of that at m/z 501 up to 83%. This result suggested that the OH-PBDEs

222

intermediate was a mixture of

223

water is another source of hydroxyl groups in the intermediate. Therefore, •OH was produced

224

from water and oxygen dissolved therein, and it can be surmised that the components of surface

225

water can influence the formation of OH-PBDEs by affecting the yield of •OH. Furthermore, the

226

product ions of [M−H]- at both m/z 503 and 501 showed a characteristic Br- ion, implying that

227

•

228

intermediate.

18

OH- and

16

18

O-water. Under oxygen-saturated conditions

OH-tetra-BDE, indicating that dissolved oxygen in

OH reacted with the ortho-tetra-BDE radical to form an ortho-substituted OH-PBDEs

229

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(A) Nitrogen-saturated condition: Scan (10.32 - 10.527 min)

Product ion scan: Farg = 135.0V [email protected]

100

100 Relative abundance (%)


[M-H]− 502.6 500.6 504.6

[M-H]− 502.6

%

% 498.6

506.6

Br−

0

0 400

450

500 m/z

550

600 m/z

100

200

300 m/z

400

500 m/z

(B) Oxygen-saturated condition: Product ion scan: Farg = 135.0 V [email protected] 504.6

498.6 496.6

%

0 400

500.6

100

490

506.6

500

510



[M-H]− 502.6

502.6

100 [M-H]− 502.6

450

500 m/z

550

600 m/z

[M-H]− 500.6

%

%

Br−

0

230


Scan (10.348 - 10.767 min)

100

Br−

0 100

200

300 m/z

400

500 m/z

100

200

300 m/z

400

500 m/z

231

Figure 3. Precursor ion (Q1) and product ion (Q3) scans for (A) under nitrogen-saturated

232

conditions, and (B) under oxygen-saturated conditions and the magnified region between m/z

233

490 and 510. Conditions: 1 mg/L BDE-47 and 5 µM Fe(III) dissolved in AcCN/H218O (6:4), with

234

5 min irradiation.

235 236

The formation of 6-OH-BDE-47 can be easily explained based on the addition reaction of •OH

237

and the ortho-tetra-BDE radical, whereas the formation of the other product, 2'-OH-BDE-68,

238

cannot be deduced directly. Considering the molecular structure characterization, we speculated

239

that 2'-OH-BDE-68 might be formed by a Smiles rearrangement reaction, an intramolecular

240

nucleophilic aromatic substitution that involves hydroxyl ionization, nucleophilic attack by O- at

241

the 1-position forming an intermediate spiro complex, ring-opening reaction, and hydrogenation

242

(Scheme 1).38-40 We further verified that 2'-OH-BDE-68 could indeed be derived from 6-OH-

243

BDE-47 under simulated solar light irradiation (Figure S5 in the SI), and that this reaction did

244

not occur in the dark (data not shown). Therefore, the following pathways are suggested to be 12 ACS Paragon Plus Environment

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245

responsible for the photochemical production of OH-PBDEs in aqueous solution: generation of

246

ortho-PBDEs radical and •OH under irradiation, addition reaction at the ortho-position between

247

the ortho-PBDEs radical and •OH directly forming the final OH-PBDEs products or PBDEs-OH

248

adduct, the latter being further stabilized to form the final OH-PBDEs products, and Smiles

249

rearrangement reaction of OH-PBDEs forming various OH-PBDEs products (Scheme 1). Br

Br O

Br

Br

BDE-47

hv Br

Br O

Br

Br

ortho-tetra-BDE radical •OH Addition reaction Br

Br O

Br

Inactivation

HO

PBDEs radical-OH* adducts

Br

6-OH-BDE-47 -H+ Br

1O

Br

O

Br O

Br

Br

Br

Br Br

O

Br

Br

Br

1 O

1 O O

Br

OH Br

Br Br

2'-OH-BDE-68

250 251

Scheme 1. Proposed pathway for the formation of 6-OH-BDE-47 and 2'-OH-BDE-68 from

252

BDE-47.

253 254

Effects of Important Photoreactive Components on the Photoformation of OH-PBDEs.

255

In this section, we describe investigations of the effects of important photoreactive components

256

(Fe(III), FA) as well as pH on the photoformation of OH-PBDEs in water, since they play

257

important roles in the generation of •OH in natural surface water.

258

Effect of Iron. 6-OH-BDE-47 and 2'-OH-BDE-68 were verified as the photoproducts of the

259

phototransformation of BDE-47 in solution containing Fe(III) at pH 5.6 ± 0.1. This confirmed

260

the photo-hydroxylation of BDE-47 in aqueous solution containing iron, which has been reported

261

as a ubiquitous and crucial inducer of •OH in natural water. From Figure 4A, it is clear that at 13 ACS Paragon Plus Environment


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262

Fe(III) concentrations ([Fe(III)]) < 5 µM, the formation yields of 6-OH-BDE-47 and 2'-OH-

263

BDE-68 were enhanced with increasing [Fe(III)].

264

Considering that 6-OH-BDE-47 and 2'-OH-BDE-68 were photolabile compounds (Table S2),

265

their net yields were thought be related to their generation and photodegradation rates and the

266

photodegradation rate of BDE-47. According to the method presented by Erickson et al.41 for

267

OH-PBDEs, the phototransformations of 6-OH-BDE-47, 2'-OH-BDE-68, and BDE-47 were

268

fitted to Equation (1).

269

d k Pd ⋅ α [P0 ] −kPd ⋅t [Int t ] = d d ⋅ e − e −kInt ⋅t k Int − k P

(

)

(1)

270

where [Intt] is the concentration of intermediate photoproduct (6-OH-BDE-47 or 2'-OH-BDE-68)

271

at time t, [P0] is the starting concentration of the initial reactant (BDE-47), and kpd and kIntd are

272

the degradation rate constants of BDE-47 and OH-PBDEs, respectively. The term α[P0] refers to

273

the fraction of BDE-47 that goes to form OH-PBDEs, and the conversion percentage is defined

274

as α multiplied by 100. Based on Equation (1) for the photodegradation kinetics of BDE-47, 6-

275

OH-BDE-47, and 2'-OH-BDE-68, the photoformation percentage yields (α) of 6-OH-BDE-47

276

and 2'-OH-BDE-68 under different conditions were calculated, and these are listed in Table S2 in

277

the SI. As shown in Table S2, the conversion percentages to form 6-OH-BDE-47 and 2'-OH-

278

BDE-68 were the highest (up to 14% and 6%, respectively) with [Fe(III)] increased to 10 µM,

279

and the degradation rate constants of BDE-47, 6-OH-BDE-47 and 2'-OH-BDE-68 were also

280

maximized under these conditions. Thus, the formation yields of 6-OH-BDE-47 and 2'-OH-

281

BDE-68 were a balance between their formation and degradation.


Page 15 of 27


(B)

6-OH-BDE 47, ng/L

4

0 µM Fe(III) 2 µM Fe(III) 5 µM Fe(III) 10 µM Fe(III)

2

4 6-OH-BDE 47, ng/L

(A)

2

0 1

2 3 Irradiation time (h)

4

5

0

1


4

5

(D)

8

0.2 mg/L FA 1 mg/L FA 5 mg/L FA 10 mg/L FA

4

8 6-OH-BDE-47, ng/L

(C) 6-OH-BDE-47, ng/L

2

0 0

0.2 mg/L FA + Fe(III) 1 mg/L FA + Fe(III) 5 mg/L FA + Fe(III) 10 mg/L FA + Fe(III)

4

0 8 2′ -OH-BDE-68, ng/L

0 8 2′ -OH-BDE-68, ng/L

2

0 4 2′ -OH-BDE 68, ng/L

2′ -OH-BDE 68, ng/L

0 4

pH = 5.6 ± 0.1 pH = 7.1 ± 0.1 pH = 8.2 ± 0.1

4

0

4

0 0

1


4

5

0

1


4

5

283

Figure 4. Photoformation of 6-OH-BDE-47 and 2'-OH-BDE-68 vs. irradiation time (A) with

284

different Fe(III) concentrations at pH 5.6 ± 0.1, (B) at different pH (5 µM Fe(III)), (C) with

285

different FA concentrations (pH 5.6 ± 0.1), and (D) with different FA concentrations in 5 µM

286

Fe(III) solution (pH 5.6 ± 0.1). Other condition: 200 ng/L BDE-47.

287 288

Significantly, the formations of 6-OH-BDE-47 and 2'-OH-BDE-68 were not obvious in the

289

absence of Fe(III), indicating the importance of Fe(III) during the photoformation of OH-PBDEs.

290

ESR spectra of radicals trapped with DMPO in the presence and absence of Fe(III) indicated the

291

impact of this cation on the formation of OH-PBDEs, mainly reflecting its influence on the

292

formation of •OH (Figure S3 in the SI).33,34,42 Notably, as an important oxidant in this system,

293

•

OH could not only enhance the generation of OH-PBDEs, but also promoted the 15 ACS Paragon Plus Environment


Page 16 of 27

294

photodegradation of PBDEs and OH-PBDEs. Since the resistances to photodegradation of the

295

two OH-PBDEs (Table S2 in the SI) and their formation mechanisms were different, the impacts

296

of Fe(III) on the yields of 6-OH-BDE-47 and 2'-OH-BDE-68 were also different. Nevertheless, it

297

was confirmed that the photochemical formation of OH-PBDEs occurred in the presence of

298

Fe(III).

299

Effect of pH. Figure 4B presents the effect of pH on the photoformation of 6-OH-BDE-47 and

300

2'-OH-BDE-68 over the typical range for natural surface water. At pH 5.6 ± 0.1 and 7.1 ± 0.1,

301

there were only small changes in the concentrations of the OH-PBDEs. The concentrations of 6-

302

OH-BDE-47 and 2'-OH-BDE-68 at pH 8.2 ± 0.1 were decreased, but still reached 1.3 ng/L and

303

1.1 ng/L, respectively. These results suggested that OH-PBDEs could be formed universally in

304

natural waters. There were three possible reasons for the pH effect on the formation of 6-OH-

305

BDE-47 and 2'-OH-BDE-68: (1) different concentrations of dominant iron species at different

306

pH values lead to different yields of •OH (Text S3.4); (2) the different acidities of 6-OH-BDE-47

307

(pKa = 7.27) and 2'-OH-BDE-68 (pKa = 6.90) and different light absorbances43,44 resulted in

308

different photodegradation susceptibilities at different pH values (Table S2); (3) 6-OH-BDE-47

309

needs to be deprotonated for the occurrence of Smiles rearrangement, making the process pH-

310

dependent.

311

Effects of FA and Fe(III)-FA Complex. The effect of FA on the photoformation of OH-PBDEs

312

in simulated natural water was investigated. FA is one of the most photoreactive organic

313

components in natural water, and it is considered to be an important photosensitizer for the

314

formation of •OH45. Moreover, FA also has a strong affinity for Fe(III), forming an Fe(III)-FA

315

complex, leading to the production of Fe(II) and an facile cycle of Fe(III)/Fe(II), herein a

316

continuous production of •OH.46 16 ACS Paragon Plus Environment

Page 17 of 27


317

Figures 4C and 4D show the formation of 6-OH-BDE-47 and 2'-OH-BDE-68 at FA

318

concentrations ([FA]) of 0.2, 1.0, 5.0, and 10 mg/L (as total organic carbon (TOC)) and with or

319

without 5 µM Fe(III). As anticipated, the formation of 6-OH-BDE-47 and 2'-OH-BDE-68

320

increased with increasing [FA]. However, the photoformation of OH-PBDEs did not increase

321

regularly with increasing [FA]. It is interesting to note that at [FA] < 1.0 mg/L, the

322

photoformation of OH-PBDEs was enhanced with increasing [FA], whereas at high

323

concentration (10 mg/L) it slowed down markedly. This phenomenon could be principally

324

attributed to: (1) the balance between the enhancement of •OH generation by FA as a

325

photosensitizer and the competition between FA and PBDEs for •OH;22,46,47 (2) the acceleration

326

of the photodegradation of OH-PBDEs and BDE-47 induced by •OH (Figure S6). Similarly, in

327

the simultaneous presence of 5 µM Fe(III) and FA (forming an Fe(III)-FA complex), a lower

328

concentration of FA accelerated the photoformation of OH-PBDEs, whereas a higher

329

concentration of FA retarded the process, which was consistent with other findings.47,48 Less 6-

330

OH-BDE-47 and 2'-OH-BDE-68 were photogenerated in the simultaneous presence of Fe(III)

331

and FA than with FA alone. This was mostly caused by the continuous and high production of

332

•

333

photodegradation of OH-PBDEs and BDE-47 (Table S2), leading to a reduction in the

334

concentration of photogenerated OH-PBDEs. Nevertheless, the percentage yields of OH-PBDEs

335

in the simultaneous presence of 5 µM Fe(III) and 1 mg/L FA were as high as 8 - 9% (Table S2).

OH facilitated by the coexistence of Fe(III) and FA (Figure S3), which could accelerate the

336

Photochemical Formation of OH-PBDEs in Actual Reservoir Water. In order to verify the

337

photoformation of OH-PBDEs from PBDEs in a real water sample, a reservoir sample was

338

collected from Dalian, Liaoning, China. BDE-47 concentrations adopted in the experiments

339

ranged from 50 to 200 ng/L. The concentrations of 6-OH-BDE-47 and 2'-OH-BDE-68 increased 17 ACS Paragon Plus Environment


Page 18 of 27

340

with increasing BDE-47 concentration, but reached their maxima after different irradiation times

341

(Figure 5). The results indicated that OH-PBDEs could indeed be photogenerated in this surface

342

water sample. The different time evolutions of 6-OH-BDE-47 and 2'-OH-BDE-68 production

343

may have been related to their different photoformation mechanisms. Hence, the results further

344

illustrated the photoformation of OH-PBDEs and explained the possible photochemical sources

345

of OH-PBDEs.

346 15 6-OH-BDE 47, ng/L

50 ng/L BDE 47 100 ng/L BDE 47 200 ng/L BDE 47

10

5

0

(B)

50 ng/L BDE 47 100 ng/L BDE 47 200 ng/L BDE 47

10

5

0

0

347

2′ -OH-BDE 68, ng/L

15

(A)

1

2 3 4 Irradiation time (h)

5

0

1

2 3 4 Irradiation time (h)

5

348

Figure 5. Time evolutions of 6-OH-BDE-47 (A) and 2'-OH-BDE-68 (B) production in actual

349

reservoir water with different BDE-47 concentrations.

350 351

Environmental Significance. This work has shown that the photochemical formation of OH-

352

PBDEs from PBDEs could occur under simulated sunlight irradiation in aqueous solution

353

containing Fe(III) and/or FA, and in natural reservoir water. Under such irradiation, 6-OH-BDE-

354

47 and 2'-OH-BDE-68 were identified as the photochemical products of BDE-47 photolysis. A

355

possible photoformation process of OH-PBDEs in aqueous solution has been deduced, which

356

involves three steps: (1) generation of ortho-PBDEs radical and •OH under irradiation; (2)

357

addition reaction at the ortho-position between the ortho-PBDEs radical and •OH, directly

358

forming the final OH-PBDEs products or PBDEs-OH adduct; (3) further stabilization of the 18 ACS Paragon Plus Environment

Page 19 of 27


359

PBDEs-OH adduct and Smiles rearrangement reaction of the OH-PBDEs forming the final

360

products. Since the ortho-PBDEs radical could be easily generated in aqueous solution, even in

361

pure water, formation of the •OH radical was considered as a controlling step during this process.

362

It has been suggested that photoreactive components (such as Fe(III) and FA) in natural surface

363

water may play an important role in the photoformation of OH-PBDEs from PBDEs. Therefore,

364

it is reasonable to propose that the phototransformation of PBDEs to OH-PBDEs indeed occurs

365

in natural surface water due to the widespread occurrence of photoreactive components for •OH

366

generation in aquatic environments. Such formation of OH-PBDEs from PBDEs may be low in

367

natural water because the concentration of PBDEs is low and radical species may also react with

368

other natural species. For example, according to the only two literature reports on the

369

simultaneous concentrations of OH-PBDEs and PBDEs in surface water,5,49 the average

370

concentrations of these compounds were as high as 109 pg/L and 6.3 - 87 pg/L in surface water

371

from Busan, Korea,49 and up to 2.2 - 70 pg/L and 17 - 250 pg/L in surface water from Ontario,

372

Canada.5 We can only give clues to help interpret the abiotic sources of OH-BDEs arising from

373

the transformation of PBDEs. However, considering that the total peak yields of 6-OH-BDE-47

374

and 2'-OH-BDE-68 during the phototransformation of BDE-47 (50 ng/L) in actual reservoir

375

water were up to 7.8% (the peak concentrations of 6-OH-BDE-47 and 2'-OH-BDE-68 were 2.5

376

ng/L and 1.4 ng/L, respectively), their universality and harmful effects on aquatic ecosystems

377

should not be ignored. The experimental results support the hypothesis of a natural abiotic

378

chemical process for the formation of OH-PBDEs and provide useful information for better

379

understanding the mechanism of natural formation of OH-PBDEs in surface water.

380 381

ASSOCIATED CONTENT 19 ACS Paragon Plus Environment


Page 20 of 27

382

Supporting Information.

383

Analysis of OH-PBDEs, ortho-tetra-BDE radical and •OH identification (Text S1-3, Scheme S1,

384

Figure S1-S4, and Table S1); degradation kinetics of BDE-47 and OH-PBDEs (Text S4, Figure

385

S5-S6 and Table S2). This material is available free of charge via the Internet at

386

http://pubs.acs.org.

387 388

AUTHOR INFORMATION

389

Corresponding Author

390

*Phone: +86-411-84706263; Fax: +86-411-84706263; E-mail: [email protected]

391

Notes

392

The authors declare no competing financial interest.

393 394

ACKNOWLEDGMENT

395

This work was supported by the National Basic Research Program of China (No.

396

2013CB430403).

397

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Photochemical Formation of Hydroxylated Polybrominated Diphenyl

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