Available online at www.sciencedirect.com
Chemosphere 71 (2008) 258–267
www.elsevier.com/locate/chemosphere
Photochemical degradation of six polybrominated diphenyl ether
congeners under ultraviolet irradiation in hexane
Lei Fang, Jun Huang, Gang Yu *, Lining Wang
POPs Research Centre, Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, PR China
Received 25 July 2007; received in revised form 17 September 2007; accepted 18 September 2007
Available online 5 November 2007
Abstract
The photodegradation of six individual PBDE congeners (BDE-28, 47, 99, 100, 153, 183) in hexane was investigated under UV light in
the sunlight region, employing a mercury lamp filtered with Pyrex glass. All photodegradation reactions followed the pseudo-first-order
kinetics, with the half-lives ranging from 0.26 h for BDE-183 to 6.46 h for BDE-100. The photochemical reaction rates of PBDEs
decreased with decreasing number of bromine substituents in the molecule, also in some cases were influenced by the PBDE substitution
pattern. Principal photoproducts detected were less brominated PBDEs, and no PBDE-solvent adducts were found. Consecutive reductive debromination was confirmed as the main mechanism for the photodegradation of PBDEs in hexane. In general, debromination
firstly occurred on the more substituted rings, when the numbers of bromine atoms on the two phenyl rings were unequal. For less brominated PBDEs, the photoreactivity of bromines at various positions of phenyl rings decreased in the order: ortho > para; while for
higher brominated PBDEs, the difference became not significant.
2007 Elsevier Ltd. All rights reserved.
Keywords: Polybrominated diphenyl ethers; Photodegradation; Hexane; Kinetics; Reductive debromination
1. Introduction
Polybrominated diphenyl ethers (PBDEs) are important
chemical flame retardants, but also environmental pollutants. Because of their persistence and probable toxicity
and carcinogenic/mutagenic human health effects (Cetin
and Odabasi, 2007), PBDEs as well as their potential risks
to human health have received more concerns from both
environmental chemists and biologists (de Wit, 2002; Birnbaum and Staskal, 2004; Wu et al., 2007). Furthermore,
high toxic products, such as polybrominated dibenzofurans
(PBDFs) and polybrominated dibenzo-p-dioxins (PBDDs),
might to be generated by the combustion of PBDEs (Sakai
et al., 2001; McDonald, 2002).
*
Corresponding author. Tel.: +86 10 6278 7137; fax: +86 10 6279 4006.
E-mail address: [email protected] (G. Yu).
0045-6535/$ - see front matter 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2007.09.041
Since PBDEs are used as additives in consumer products, they can migrate into the environment easily during
the process of manufacture, use and disposal. As the ubiquitous environmental pollutants, PBDEs have been
detected in various environmental media, including freshwater and marine sediments (Lacorte et al., 2003; Rayne
et al., 2003; Zegers et al., 2003), sewage sludge (Knoth
et al., 2007), soil (Hassanin et al., 2004), air (Strandberg
et al., 2001; Gouin et al., 2002) and biota (Law et al.,
2002; She et al., 2002; Zennegg et al., 2003). Although only
decaBDE is the major technical product currently in use,
lower brominated PBDEs are more often found in the environment, especially those congeners with four or five bromine substituents, e.g., 2,2 0 ,4,4 0 - tetraBDE (BDE-47),
2,2 0 ,4,4 0 ,5-pentaBDE (BDE-99) and 2,2 0 ,4,4 0 ,6-pentaBDE
(BDE-100). In addition, some studies (de Wit, 2002; Norstrom et al., 2002) demonstrated that PBDE levels had
been continuously increasing since the mid 1970s, posing
a long-term environmental problem.
L. Fang et al. / Chemosphere 71 (2008) 258–267
2. Experimental
2.1. Reagents and materials
Six individual PBDE congeners, each at 50 lg ml1 in
isooctane were purchased from AccuStandard Inc.,
USA. The polybrominated diphenyl ether analytical standard solution EO-5099 was purchased from Cambridge
Isotope Labs. (USA). The components of EO-5099 solution were: three mono-BDEs, seven di-BDEs, eight triBDEs, six tetra-BDEs, seven penta-BDEs, five hexa-BDEs
and three hepta-BDEs. The concentrations of each compound ranged from 100 ng ml1 to 250 ng ml1. Hexane
and methanol were purchased from Dima Technology
Inc. (USA) and acetone was from Fisher Chemical Co.
(USA). All solvents were of analytical grade. Standard
stock solutions were prepared by diluting the target compound in hexane.
The numbering system for PBDE congeners used in this
study is in analogy to the PCB numbering system as suggested by Ballschmiter and Zell, 1980.
2.2. Photodegradation experiments
Photodegradation experiments were performed in a
photo reactor developed by the Department of Electrical
Engineering, Tsinghua University (Fig. 1). It consisted of
a 500 W mercury lamp, quartz tubes, Pyrex glass tubes
and a water cooling system. The distance between the
Water
30mm
Lamp
320mm
In order to evaluate the risk of PBDEs to the humans, it
is very important to have abundant data to elucidate their
transportation, transformation and fate in the environment. So far, studies regarding biotic and abiotic transformation of PBDEs are far fewer than the better studied
PCBs. Limited data available suggest that photochemical
degradation is the main transformation process for PBDEs
in the environment. Thus it is important to understand the
photochemical behavior of PBDEs, including both photodegradation kinetics and photoproducts. Recently several
studies on the photodegradation of decabromodiphenyl
ether (decaBDE or BDE-209) have been reported using different light sources and matrices (Watanabe and Tatsukawa, 1987; Hua et al., 2003; Bezares-Cruz et al., 2004;
Söderström et al., 2004; Ahn et al., 2006). Watanabe and
Tatsukawa (1987) studied the photolysis of decaBDE in a
mixture of hexane, benzene, and acetone (8:1:1) in both
sunlight and artificial UV light. Their results indicated that
photoproducts of decaBDE are lower brominated PBDEs
and PBDFs. Söderström et al. (2004) presented a study
on the photodegradation of decaBDE in different matrices
(toluene, silica gel, sand, soil and sediment), showing that
different matrices affect the photolysis rate of PBDEs
strongly but have little effect on the products. Other studies
on photolysis of decaBDE in hexane and adsorbed to
hydrated surfaces, clay minerals, metal oxides and sediment, also showed similar results of photolytic debromination (Hua et al., 2003; Bezares-Cruz et al., 2004; Ahn et al.,
2006).
Unlike decaBDE, studies on the photodegradation of
lower brominated PBDEs are rather few, which are actually more persistent, bioaccumulative and toxic. Peterman
et al. (2003) briefly reported on sunlight photolysis of a
mixture of 39 PBDE congeners in triolein. Eriksson et al.
(2004) investigated photodegradation rate of 15 individual
PBDE congeners in methanol/water (8:2), nine congeners
in methanol and four congeners in tetrahydrofuran. In
their study, the photochemical reaction rate increased with
the increase of bromination degree. They also studied the
photoproducts of decaBDE, and the generated compounds
were identified as PBDEs and PBDFs. Another study on
photolysis of five individual PBDE congeners was performed using solid-phase microextraction fibres as photolytic support, indicating that reductive debromination
was the main photodegradation way (Sanchez-Prado
et al., 2005). However, rare studies were reported on the
photolysis of less brominated PBDEs in hexane, which is
a good hydrogen donator and has been widely used as
the model solvent of lipophilic environmental phases.
The primary objectives of our study were to determine
the photodegradation kinetics of six individual PBDE
congeners in hexane under UV light in the sunlight region,
to identify the photolysis intermediates and products, and
further to discuss the mechanism of the photodegradation.
The selected six PBDE congeners, BDE-28, BDE-47, BDE99, BDE-100, BDE-153 and BDE-183, were representative
for those frequently detected in the environment.
259
66mm
Fig. 1. Apparatus used in the photochemical experiments.
260
L. Fang et al. / Chemosphere 71 (2008) 258–267
UV lamp and tubes was 5 cm. Pyrex glass was used to filter
the part of ultraviolet light with wavelengths less than
290 nm. Thirty milliliter hexane solutions of different individual PBDE congeners (10 ng ml1) were irradiated in
Pyrex glass tubes sealed with glass stoppers. The experiments were performed in triplicate and corresponding dark
control tests were conducted. As for the identification of
photoproducts of PBDEs, the initial solution volumes were
changed to 10 ml and initial concentrations were
150 ng ml1.
2.3. Analytical methods
Irradiated samples were analyzed by using an Agilent
6890 gas chromatography equipped with a micro electron
capture detector (lECD). A HP-5 capillary column
(30 m · 0.32 mm i.d., 0.25 lm film thickness, Agilent Technologies) was used with nitrogen carrier gas. The split–
splitless injector was kept at a temperature of 280 C and
the oven temperature program was as follows: initial temperature hold at 80 C for 2 min, then increased to
200 C at a rate of 20 C min1 and hold for 2 min, then
increased to 280 C at a rate of 5 C min1 and hold for
15 min.
The identification of degradation products was confirmed by GC–MS–NCI analysis on a Trace gas chromatography connected to a Trace DSQ quadrupole mass
spectrometry (Thermo Finnigan, USA) with a DB-5 MS
capillary column (30 m · 0.25 mm i.d., 0.25 lm film thickness, Agilent Technologies). Helium was used as the carrier gas at a flow rate of 1 ml min1. The temperature
program was from 80 C (held for 2 min) to 180 C (held
for 1 min) at 10 C min1, 3 C min1 to 210 C,
2 C min1 to 260 C, 3 C min1 to 280 C (held for
15 min), 10 C min1 to 300 C (held for 2 min). The
injector, ion source and transfer line temperatures were
280, 260 and 260 C, respectively. The mass spectrometry
was used in the negative chemical ionization mode with
methane as reagent gas. The samples were both analyzed
in full scan mode (m/z = 50–750) and SIM mode (m/
z = 79, 81).
3. Results and discussion
3.1. Photochemical degradation kinetics
Dark control experiments for each PBDE congener
showed no disappearance of the congener or appearance
of degradation products. All photodegradation experiments were performed in triplicates, and three congeners
were randomly selected for the comparison of triplicate
experiments. The standard deviations of calculated halflives for BDE-28, BDE-99 and BDE-153 were 1.15%,
7.21% and 4.24%, respectively. Fig. 2 shows the results
from three parallel experiments on photolysis of BDE-28,
together with corresponding rate constants and half-lives,
indicating a fairly good reproducibility.
Fig. 2. Triplicate experiments on photochemical degradation of BDE-28
in hexane, together with corresponding rate constants and half lives.
It was found that photochemical reactions of six individual PBDE congeners in hexane followed pseudo-first-order
kinetics, which was in agreement with the photochemistry
behavior of PBDE congeners in methanol/water (8:2) and
pure methanol (Eriksson et al., 2004). Table 1 displays
the photodegradation rate constants and half-lives of these
six irradiated PBDE congeners in hexane. The observed
rate difference for PBDE congeners studied here was up
to 27 times between the slowest photolysis congener
(BDE-100) and the fastest one (BDE-183). The photolysis
half-lives of PBDE congeners in this study were different
from previous studies. Take example for BDE-99, its
half-life was 0.32 h in our experiment, while the photodegradation half-lives were 64 h in methanol/water (8:2) (Eriksson et al., 2004) and less than 2 min in solid-phase
microextraction polydimethylsiloxane fiber (Sanchez-Prado et al., 2005), respectively. The difference in photochemical reaction rates could be attributed to the different
reaction conditions, including UV light sources and photolytic matrices.
It was indicated from Table 1 that higher brominated
diphenyl ethers degraded faster than the lower brominated
congeners, which was similar to the results of previous
study (Eriksson et al., 2004). Thus, the degree of bromination in PBDE congeners seemed to be an important factor
for their photoreactivity. There may be two reasons. First,
Table 1
Photodegradation rate constants, half-lives and HOMO energies of PBDE
congeners in hexane upon exposure to UV irradiation
PBDE
congener
Photodegradation rate
constant k (h1)
Half-life t1/2
(h)
HOMO
energy (eV)
BDE-28
BDE-47
BDE-99
BDE-100
BDE-153
BDE-183
0.14
0.30
1.83
0.10
2.30
2.64
4.97
2.53
0.32
6.46
0.29
0.26
9.199
9.329
9.338
9.748
9.400
9.534
L. Fang et al. / Chemosphere 71 (2008) 258–267
2006). HOMO energies of PBDE congeners were calculated by PM3 Hamiltonian contained in MOPAC 2000,
which was implemented in Chem3D Ultra (Ver.9.0, CambridgeSoft). As showed in Table 1, HOMO energies
increased with increasing number of bromine substituents,
except for BDE-100. Thus, increasing the number of bromine atoms led the increase of photodegradation rate of
PBDEs.
However, the photodegradation of BDE-100 showed the
different result. BDE-99 and BDE-100 were both pentabromodiphenyl ethers, and only one substituted position for
higher brominated diphenyl ethers absorbed at longer
wavelength, which was presented by Eriksson et al.
(2004). Second, different number of bromine atoms in
PBDE congeners resulted into different molecular structure
and property. The energy of the highest occupied molecular orbital (HOMO energy), which accounts for the electron donor ability, is one of the important influencing
factors for the photodegradation of PBDEs. The higher
this donor property is, the respective photodegradtion
property is higher. Previous study found that increasing
HOMO energy resulted in higher reactivity (Niu et al.,
a
b
30
BDE 15
BDE 28
BDE 15
BDE 28
BDE 47
20
Concentration (nmol/l)
Concentration (nmol/l)
261
20
10
15
10
5
0
0
0
5
10
15
0
4
Time (hour)
20
d
BDE 28
BDE 49
BDE 47
BDE 66
BDE 99
15
Concentration (nmol/l)
Concentration (nmol/l)
c
10
5
20
BDE 28
BDE 75
BDE 47
BDE 100
15
10
5
0
0
0
4
0
8
10
Time (hour)
Concentration (nmol/l)
16
BDE 28
BDE 47
BDE 77
BDE 118
12
BDE 49
BDE 66
BDE 99
BDE 153
8
6
8
4
4
2
0
0
0
1
2
3
Time (hour)
20
30
40
Time (hour)
4
5
f
Concentration (nmol/l)
e
8
Time (hour)
BDE 28
BDE 47
BDE 99
BDE 154
BDE 138
12
BDE 49
BDE 66
BDE 118
BDE 153
BDE 183
2.0
1.5
8
1.0
4
0.5
0
0.0
0
1
2
3
4
Time (hour)
Fig. 3. The concentration of PBDE congeners and their photoproducts at different irradiation times. (a) BDE-28; (b) BDE-47; (c) BDE-99; (d) BDE-100;
(e) BDE-153; (f) BDE-183.
262
L. Fang et al. / Chemosphere 71 (2008) 258–267
the five bromine atoms was different. But the observed rate
difference between them was up to 19 times. Possible explanation was that BDE-100 had one specific brominated phenyl ring with 2,4,6 substitution pattern which made it more
stable and difficult to photodegrade. This presume was
confirmed by the photodegradation of BDE-75, which
was one photoproduct of BDE-100 and also had the specific brominated phenyl ring with 2,4,6 substitution pattern. As shown in Fig. 3d, BDE-75 was degraded more
slowly than another tetra-brominated product BDE-47.
Hence, the photoreactivity of PBDE congeners was also
affected by the positions of substituted bromines, and more
PBDE congeners should be considered in further study.
3.2. Photochemical degradation products
Previous studies showed that reductive debromination
was the main photodegradation mechanism for BDE-209
(Bezares-Cruz et al., 2004; Söderström et al., 2004). In
order to identify as many of photoproducts in this study
as possible, GC retention times and mass spectra of the
products were compared to those of standards and previous literatures (Rayne and Ikonomou, 2003; Korytar
et al., 2005). Those products found in this study whose
GC retention times and mass spectra matched one of the
known standards, were identified as PBDE congeners and
summarized in Table 2. Because not all standards were
available, other products who possessed similar GC reten-
tion times and distinctive fragments, were identified as
structural isomer groups. This process is consistent with
that used by some researchers to identify PBDE congeners
in environmental samples.
The observed photoproducts were mostly less brominated diphenyl ethers. Fig. 3 showed the concentration
profiles of initial PBDE congeners and their photoproducts
at different irradiation times. Therefore, it could be concluded that the main decomposition mechanism about
photolysis of the six PBDE congeners was reductive debromination, which was in agreement with the results by Sanchez-Prado et al. (2005) for photolysis of five individual
PBDE congeners on solid-phase microextraction fibres.
Besides PBDE congeners, few products whose mass spectra
contained distinctive fragments with m/z ratios of 79 and
81 were tentatively identified as PBDFs. However, mass
balance calculations and peak areas both showed that generated PBDFs amount was rather few, which was different
from the results reported by Eriksson et al. (2004) for photodegradation of BDE-209 in methanol/water (8:2). Contrary to the photolysis of PCBs in hexane (Miao et al.,
1999), no brominated benzene was detected in this study.
Additionally, PBDE-solvent adducts were also not detected
by GC–MS in our experiment. So cleavage of inter-ring
bonds to produce brominated benzenes and replacement
reactions to form PBDE-solvent adducts may not be significant process for the photolysis of PBDEs under our experimental conditions.
Table 2
Relative retention times of photoproducts
Br atoms
BDE number
IUPAC name
RT GC–ECD (min)
RT GC–MS (min)
1
BDE-1
BDE-3
2-Bromodiphenyl ether
4-Bromodiphenyl ether
0.976
1.000
0.968
1.000
2
BDE-8
BDE-15
2,4 0 -Dibromodiphenyl ether
4,4 0 -Dibromodiphenyl ether
1.302
1.353
1.361
1.429
3
BDE-30
BDE-32
BDE-17
BDE-25
BDE-28
BDE-33
BDE-37
2,4,6-Tribromodiphenyl ether
2,4 0 ,6-Tribromodiphenyl ether
2,2 0 ,4-Tribromodiphenyl ether
2,3 0 ,4-Tribromodiphenyl ether
2,4,4 0 -Tribromodiphenyl ether
2 0 ,3,4,-Tribromodiphenyl ether
3,4,4 0 -Tribromodiphenyl ether
1.518
1.629
1.667
1.667
1.718
1.718
1.795
1.611
1.739
1.782
1.789
1.848
1.848
1.950
4
BDE-52
BDE-75
BDE-49
BDE-47
BDE-66
BDE-77
2,2 0 ,5,5 0 -Tetrabromodiphenyl ether
2,4,4 0 ,6-Tetrabromodiphenyl ether
2,2 0 ,4,5 0 -Tetrabromodiphenyl ether
2,2 0 ,4,4 0 -Tetrabromodiphenyl ether
2,3 0 ,4,4 0 -Tetrabromodiphenyl ether
3,3 0 ,4,4 0 -Tetrabromodiphenyl ether
1.987
2.021
2.052
2.121
2.177
2.268
2.190
2.234
2.279
2.372
2.456
2.588
5
BDE-100
BDE-99
BDE-118
2,2 0 ,4,4 0 ,6-Pentabromodiphenyl ether
2,2 0 ,4,4 0 ,5-Pentabromodiphenyl ether
2,3 0 ,4,4 0 ,5-Pentabromodiphenyl ether
2.431
2.519
2.591
2.822
2.959
3.073
6
BDE-154
BDE-153
BDE-138
2,2 0 ,4,4 0 ,5,6 0 -Hexabromodiphenyl ether
2,2 0 ,4,4 0 ,5,5 0 -Hexabromodiphenyl ether
2,2 0 ,3,4,4 0 ,5 0 -Hexabromodiphenyl ether
2.770
2.892
3.082
3.350
3.545
3.801
7
BDE-183
2,2 0 ,3,4,4 0 ,5 0 ,6-Heptabromodiphenyl ether
3.367
4.077
RT is relative retention time, normalized to retention time of BDE-3.
L. Fang et al. / Chemosphere 71 (2008) 258–267
263
Fig. 4. GC–MS chromatograms showing the photoproducts of BDE-153 at different irradiation times.
The consecutive debromination of PBDE congeners,
including the formation and subsequent photolysis of generated congeners, is evident in Fig. 4, where GC–MS chromatograms for photolysis of BDE-153 in hexane are
displayed that correspond to increasing time of irradiation.
The first step in the photolysis of BDE-153 was loss of one
bromine atom to produce three pentaBDEs, as shown in
the chromatogram after 0.25 h of irradiation (Fig. 4). The
three pentaBDEs were identified as BDE-101, BDE-99
and BDE-118, confirmed by matching mass spectra and
GC–MS retention time to standards. Less substituted
congeners appeared in succession as the irradiation time
increased. There was a clear, inverse correlation between
irradiation time and number of bromine atoms on the
remaining congeners. Similar trends were observed during
the photolysis of other five PBDE congeners.
Based on the identification of produced photoproducts,
major photodegradation pathways of BDE-28, BDE-47,
BDE-99, BDE-100 and BDE-153 were proposed in Fig. 5.
Bold arrows represented the dominating pathways, while
dashed arrows represented the presumed pathways. The
photoproducts of BDE-28, BDE-47, BDE-99 and BDE153 observed in this study were all shown in Fig. 5, except
for two diBDEs from BDE-99 and four triBDEs from
BDE-153. The photodegradation of BDE-100 produced
three tetraBDEs. The peaks at 29.76 and 31.65 min were
confirmed as BDE-75 and BDE-47. The peak at 28.82 min
was undetermined, and it may be BDE-50 (2,2 0 ,4,6-tetrabromodiphenyl ether) or BDE-51 (2,2 0 ,4,6 0 -tetrabromodiphenyl ether) according to the structure of BDE-100.
Considering BDE-183, the photochemical reaction was
more complicated. The observed photoproducts included
six hexaBDEs, six pentaBDEs and five tetraBDEs. All five
tetraBDEs were identified as BDE-75, BDE-49, BDE-47,
BDE-66 and BDE-77, while several hexaBDEs and pentaBDEs were undetermined due to the limit of standards.
Among these photoproducts, only three hexaBDEs were
identified as BDE-138, BDE-153 and BDE-154, and three
pentaBDEs as BDE-99, BDE-100 and BDE-118. Because
of the indetermination of some products, it was difficult to
describe the whole photodegradation pathway of BDE-183.
The unsymmetrical substituted diphenyl ethers, such as
BDE-28 and BDE-99, were usually photodegraded by
debromination on the more substituted ring. For example,
BDE-49, BDE-47 and BDE-66 were detected as the photoproducts of BDE-99, which were generated by the lose of
an ortho, meta, or para bromine atom from the more substituted ring, respectively. Consequently, the possible product
BDE-74 (2,4,4 0 ,5-tetrabromodiphenyl ether) formed by the
lose of an ortho bromine atom from the less substituted
ring was not detected. Contrarily, BDE-75 (2,4,4 0 ,6-tetrabromodiphenyl ether) was detected as the photoproduct
of BDE-100. Compared with the photolysis of BDE-99,
the presence of BDE-75 during the photolysis of
BDE-100 indicated that both the steric effect of three adjacent ortho bromine atoms (2,2 0 ,6) and the stable structure
of the specific brominated phenyl ring with 2,4,6 substitution pattern favored the debromination on the less
substituted ring of BDE-100. According to the previous
study about the photodegradation of PCBs in hexane
264
L. Fang et al. / Chemosphere 71 (2008) 258–267
Br
a
O
Br
Br
BDE28 (2,4,4'-)
Br
O
O
Br
Br
Br
BDE15 (4,4'-)
BDE8 (2,4'-)
Br
O
O
Br
BDE1 (2-)
BDE3 (4-)
Br
b
O
Br
Br
Br
BDE47 (2,2',4,4'-)
Br
Br
O
O
Br
Br
Br
Br
BDE28 (2,4,4'-)
BDE17 (2,2',4-)
Br
Br
O
O
O
Br
Br
Br
Br
BDE15 (4,4'-)
BDE8 (2,4'-)
BDE4 (2,2'-)
Br
O
O
BDE1 (2-)
Br
BDE3 (4-)
Fig. 5. Proposed major photodegradation pathways of PBDE congeners. (a) BDE-28; (b) BDE-47; (c) BDE-99; (d) BDE-100; (e) BDE-153.
(Miao et al., 1999), the photoproducts by the lose of an
ortho chlorine atom from the less substituted ring were also
observed, but no photoproducts arising from the lose of a
meta or para chlorine atom from the less substituted ring
was detected. Hence, the undetermined tetra-brominated
product of BDE-100 was presumed to be BDE-51, rather
than BDE-50 which was formed by the lose of a para bromine atom from the less substituted ring.
L. Fang et al. / Chemosphere 71 (2008) 258–267
265
Br
c
O
Br
O
Br
Br
BDE17 (2,2',4-)
Br
O
Br
BDE49 (2,2',4,5'-) Br
Br
Br
BDE25 (2,3',4-) Br
Br
Br
O
Br
Br
O
Br
Br
Br
O
Br
Br
Br
Br
Br
BDE28 (2,4,4'-)
BDE47 (2,2',4,4'-)
Br
BDE99 (2,2',4,4',5-)
O
Br
O
Br
BDE33 (2',3,4-)
Br
Br
Br
Br
BDE66 (2,3',4,4'-)
O
Br
Br
BDE37 (3,4,4'-)
Br
Br
d
O
Br
Br
BDE30 (2,4,6-)
Br
Br
O
O
Br
Br
Br
Br
Br
Br
Br
BDE32 (2,4',6-)
BDE75 (2,4,4',6-)
Br
O
O
Br
Br
Br
Br
BDE100 (2,2',4,4',6-)
O
Br
Br
BDE28 (2,4,4'-)
Br
Br
Br
Br
BDE47 (2,2',4,4'-)
O
Br
Br
BDE17 (2,2',4-)
Fig. 5 (continued)
In this study, both the ortho-debromination products
and para-debromination products were observed in the
photodegradation of BDE-28 and BDE-47. For BDE-28,
the concentration of ortho-debromination product, BDE15, was much higher than the para-debromination one,
BDE-8. Similar result was obtained in the photodegradation of BDE-47. Thus, bromines at the ortho positions of
BDE-28 and BDE-47 showed remarkable higher reactivity
than those at para positions, which was in agreement with
the conclusion from the photolysis of PCBs. As shown in
266
L. Fang et al. / Chemosphere 71 (2008) 258–267
Br
e
Br
O
Br
Br
Br
O
BDE52 (2,2',5,5'-)
Br
Br
Br
Br
O
Br
BDE101 (2,2',4,5,5'-)
Br
Br
O
Br
Br
Br
Br
Br
BDE49 (2,2',4,5'-) Br
O
Br
O
Br
Br
Br
Br
Br
Br
Br
BDE99 (2,2',4,4',5-)
BDE153 (2,2',4,4',5,5'-)
Br
Br
BDE47 (2,2',4,4'-)
Br
Br
O
Br
Br
O
Br
Br
Br
Br
Br
BDE118 (2,3',4,4',5-)
BDE66 (2,3',4,4'-)
O
Br
BDE77 (3,3',4,4'-)
Br
Br
Br
Fig. 5 (continued)
previous reports on the degradation of PCBs (Bunce, 1982;
Miao et al., 1999), the chlorine atoms at ortho positions
usually show much higher elimination efficiency than the
chlorine atoms at meta and para position, and the ortho
chlorines are lost preferentially when ortho and other positions are substituted by chlorines.
Regarding higher brominated PBDE congeners, e.g.,
BDE-99 and BDE-153, photoproducts formed by debromination from ortho-, meta- or para-substituted position
were all detected, and the difference of their concentrations
was small. Therefore, as the degrees of bromination
become higher, the molecular structures of PBDE congeners were more complicated and it was more important for
the bromine substituted pattern to account for the effect
on the photodegradation. In order to better understand
the influence of bromination degree and substitution pattern on photodegradation of PBDEs, further studies about
more individual PBDE congeners are being accomplished.
4. Conclusions
Six PBDE congeners (BDE-28, 47, 99, 100, 153, 183)
have been shown to undergo sequential reductive debromination under the irradiation of a mercury lamp filtered
with Pyrex glass, following the pseudo-first-order kinetics.
No evidence of other photochemical degradation products
resulting from aryl-oxygen bond cleavage or replacement
reaction was observed. The degradation rates of PBDEs
in hexane were remarkably dependent on the degree of bromination. Among these PBDE congeners, the slowest photolysis congener was not BDE-28 but BDE-100, which
indicated bromine substitution pattern was another factor
for their photodegradation rates. Photo-reactivity of bromine atoms at different substituted positions was also
investigated. For less brominated PBDE congeners, bromines at the ortho positions showed higher photo-reactivity
than those at para positions, while for higher brominated
PBDE congeners, debromination was influenced by more
factors and the observed difference on photoreactivities of
bromines at various positions (ortho, meta, para) was limited. These findings may help to elucidate the environmental fate of PBDEs.
Acknowledgments
This research was supported by the National Natural
Science Foundation of PR China (No. 20507010).
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