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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
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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,
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Dalian 116024, China.
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Corresponding author E-mail: [email protected]
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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
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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,
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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
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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
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in He/air mixtures14 as well as calculation methods.20-22 It may explain why certain OH-PBDEs
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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
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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
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PBDEs and to explore the reaction pathways in natural surface water.
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MATERIALS AND METHODS
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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 (%)
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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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 (%)
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
Relative abundance (%)
Relative abundance (%)
[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
Relative abundance (%)
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.
282 14 ACS Paragon Plus Environment
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(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
2 3 Irradiation time (h)
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
2 3 Irradiation time (h)
4
5
0
1
2 3 Irradiation time (h)
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
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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
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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
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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
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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
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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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10.1139/v78-368.
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(38) Gerasimova, T. N.; Kolchina, E. F. The Smiles rearrangement of o-aminophenyl phenyl 25 ACS Paragon Plus Environment
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ethers. Russ. Chem. Rev. 1995, 64 (2), 133-140, doi: 10.1070/RC1995v064n02ABEH000141.
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(39) Haglund, P.; Malmvärn, A.; Bergek, S.; Bignert, A.; Kautsky, L.; Akano, T.; Wiberg, K.;
521
Asplund, L. Brominated dibenzo-p-dioxins: a new class of marine toxins? Environ. Sci. Technol.
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2007, 41 (9), 3069-3074, doi: 10.1021/es0624725.
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(40) Arnoldsson, K.; Andersson, P. L.; Haglund, P. Formation of environmentally relevant
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brominated dioxins from 2,4,6,-tribromophenol via bromoperoxidase-catalyzed dimerization.
525
Environ. Sci. Technol. 2012, 46 (13), 7239-7244, doi: 10.1021/es301255e.
526
(41) Erickson, P. R.; Grandbois, M.; Arnold, W. A.; McNeill, K. Photochemical formation of
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brominated dioxins and other products of concern from hydroxylated polybrominated diphenyl
528
ethers (OH-PBDEs). Environ. Sci. Technol. 2012, 46 (12), 8174-8180, doi: 10.1021/es3016183.
529 530
(42) Flynn, C. M. Hydrolysis of inorganic iron(III) salts. Chem. Rev. 1984, 84 (1), 31-41, doi: 10.1021/cr00059a003.
531
(43) Rayne, S.; Forest, K. pKa values of the monohydroxylated polychlorinated biphenyls (OH-
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PCBs), polybrominated biphenyls (OH-PBBs), polychlorinated diphenyl ethers (OH-PCDEs),
533
and polybrominated diphenyl ethers (OH-PBDEs). J. Environ. Sci. Health Part A. 2010, 45 (11),
534
1322-1346, doi: 10.1080/10934529.2010.500885.
535
(44) Steen, P. O.; Grandbois, M.; Mcneill, K.; Arnold, W. A. Photochemical formation of
536
halogenated dioxins from hydroxylated polybrominated diphenyl ethers (OH-PBDEs) and
537
chlorinated derivatives (OH-PBCDEs). Environ. Sci. Technol. 2009, 43 (12), 4405-4411, doi:
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10.1021/es9003679.
539
(45) Miller, C. J.; Rose, A. L.; Waite, T. D. Hydroxyl radical production by H2O2-mediated
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oxidation of Fe(II) complexed by Suwannee river fulvic acid under circumneutral freshwater
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conditions. Environ. Sci. Technol. 2013, 47 (2), 829-835, doi: 10.1021/es303876h. 26 ACS Paragon Plus Environment
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(46) Ou, X.; Chen, S.; Quan, X.; Zhao, H. M. Photochemical activity and characterization of the
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complex of humic acids with iron(III). J. Geochem. Explor. 2009, 102 (2), 49-55, doi:
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10.1016/j.gexplo.2009.02.003.
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(47) Liu, H.; Zhao, H. M.; Quan, X.; Zhang, Y. B.; Chen, S. Formation of chlorinated
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intermediate from bisphenol A in surface saline water under simulated solar light irradiation.
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Environ. Sci. Technol. 2009, 43 (20), 7712-7717, doi: 10.1021/es900811c.
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(48) Yu, C. Y.; Zhang, Y. B.; Quan, X.; Chen, S.; Han, J. B.; Ou, X. X.; Zhao, J. C.
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Photochemical effect of humic acid components separated using molecular imprinting method
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applying porphyrin-like substances as templates in aqueous solution. Environ. Sci. Technol.
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(49) Kim, U. J., Yen, N.T.H., Oh, J. E. Hydroxylated, methoxylated, and parent polybrominated
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source [J]. Environ. Sci. Technol., 2014, 48(13), 7245-7253, doi: 10.1021/es5006972.
27 ACS Paragon Plus Environment
Article
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
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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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38
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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61
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.
83 4 ACS Paragon Plus Environment
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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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107
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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130
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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153
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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170
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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193
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 (%)
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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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 (%)
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
Relative abundance (%)
Relative abundance (%)
[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
Relative abundance (%)
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.
282 14 ACS Paragon Plus Environment
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(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
2 3 Irradiation time (h)
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
2 3 Irradiation time (h)
4
5
0
1
2 3 Irradiation time (h)
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
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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
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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
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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
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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
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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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