Photoreductive debromination of decabromodiphenyl ether by pyruvate

Article abstract of DOI:10.1016/j.cattod.2014.01.002

Polybrominated diphenyl ethers (PBDEs) have aroused highly environmental concerns because of their toxicity and ubiquitousness in the biological and environmental system. Here, we report that decabromodiphenyl ether (BDE209) undergoes efficient reductive debromination reaction by pyruvate under UV irradiation (>360 nm). The photoreductive degradation kinetics of BDE209 has been further investigated under different reaction conditions. The debromination reactions occur in a stepwise process, producing a series of lower brominated congeners. The debromination shows unconspicuous position-selective property. The possible photoreductive mechanism has been proposed by UV-vis and kinetic isotope effect (KIE). This study not only provides a potential application of removal of PBDEs contaminations but also provides some information for the fate of PBDEs in environment.

Full text of DOI:10.1016/j.cattod.2014.01.002

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Catalysis Today 224 (2014) 89–93
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Photoreductive debromination of decabromodiphenyl ether by
pyruvate
Zhaowu Jiang, Wensheng Linghu, Yimin Li, Chunyan Sun∗
Department of Chemistry, Shaoxing University, Shaoxing 312000, Zhejiang, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Polybrominated diphenyl ethers (PBDEs) have aroused highly environmental concerns because of their
toxicity and ubiquitousness in the biological and environmental system. Here, we report that decabro-
modiphenyl ether (BDE209) undergoes efficient reductive debromination reaction by pyruvate under UV
irradiation (>360 nm). The photoreductive degradation kinetics of BDE209 has been further investigated
under different reaction conditions. The debromination reactions occur in a stepwise process, produc-
ing a series of lower brominated congeners. The debromination shows unconspicuous position-selective
property. The possible photoreductive mechanism has been proposed by UV–vis and kinetic isotope effect
(KIE). This study not only provides a potential application of removal of PBDEs contaminations but also
provides some information for the fate of PBDEs in environment.
Received 1 August 2013
Received in revised form
27 December 2013
Accepted 6 January 2014
Available online 28 January 2014
Keywords:
Polybrominated diphenyl ethers
Pyruvate
Photoreductive
Debromination
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
method for debromination of PBDEs [18–21]. Photocatalysis has
also been studied for degradation of PBDEs [22–25]. For example,
Persistent organic pollutants (POPs) arouse significant scien-
tific concern because they show persistent and bioaccumulative
in the environment [1]. Polybrominated diphenyl ethers (PBDEs)
have attracted particular attention in this field due to extensive
use as flame retardants and global distribution. In recent years,
PBDEs have been detected in sediments [2,3], marine organisms
[4,5], food samples [6], and human mother’s milk [7], even in Arctic
animals [8,9]. Some researches indicated that PBDEs accumulated
in the body could influence liver enzyme activity and neurological
development at a key period of brain growth [10,11].
During the past few years, research on PBDEs has been greatly
spread and mighty endeavors have been devoted to study the
transformation of PBDEs in the environment. For example, pho-
tolytic debromination of PBDEs has been observed on the surface
of clay minerals, metal oxides, silica gel, sand, soil and sediment
[12,13], and in toluene, hexane or methanol/water [14–16]. The
biotic reductive debromination has also been reported by anaer-
obic bacteria [17], juvenile rainbow trout and common carp [4,5].
Besides the studies on the environmental transformation of PBDEs,
the development of potential methods for PBDEs removal in con-
taminated environmental system is another urgent and significant
issue. Zerovalent iron reduction has been proven an effective
BDE209 could be a rapid photocatalytic debromination by TiO2.
Recently, it was found that BDE209 could be an efficient reduc-
tive debrominate under visible-light irradiation (≥420 nm) in the
presence of various carboxylate anions [26]. This implies that some
carbonylic species common in the environmental media may use
to remediate PBDEs.
Pyruvic acid (2-oxopropanoic, PA) is widespread environmen-
tal carbonylic species, as ubiquitous components of surface water
and the atmospheric aerosol. PA has been globally produced in
the photochemical degradation of colored organic matter tint-
ing rivers, lakes, and oceans and in the atmospheric oxidation of
organic gas [27–29]. PA may undergo efficient photodecarboxyla-
tion via the excited triplet state, and excitation reaction by the n-␲*
band in aqueous PA induces its efficient photodecarboxylation.
With R-cleavage of 3PA* into geminate radical pairs, the release of
CO2 follows during the subsequent reduction of PA [27,29]. These
results imply that the n-␲* excitation of PA might induce long-
range electron transfer from the promoted carbonyl chromophore
into neighboring carbonyl acceptors [29]. Considering that PBDEs
can undergo reductive debromination, we realize that the long-
range electron transfer from PA* might effectively reduce PBDEs to
their lower bromo congeners under appropriate conditions.
In this paper, for the first time the photoreductive degrada-
tion of PBDEs by pyruvate has been investigated. BDE209 as the
major product of PBDEs [1], was selected as a target PBDE. The
photoreductive degradation kinetics of BDE209 has been exam-
ined. The photoreductive mechanism has been proposed on the
∗
Corresponding author at: No.508 Huancheng, West Road, Shaoxing, China.
Tel.: +86 575 8834 1521; fax: +86 0575 8834 1521.
E-mail address: sunchunyan@usx.edu.cn (C. Sun).
0920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2014.01.002

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Z. Jiang et al. / Catalysis Today 224 (2014) 89–93
basis of experimental data. It not only provides a new method for
degradation of PBDEs but also puts a new insight into the photo-
chemistry of pyruvic acid and the fate of PBDEs in the environment
due to pyruvic acid co-exist widely with the PBDEs in many envi-
ronmental situations, such as the atmospheric aerosol.
2. Materials and methods
2.1. Materials
BDE209 was purchased from Aldrich Chemical Company (USA).
BDE203, BDE204 and a standard solution of PBDEs (EO5103) were
purchased from Cambridge Isotope Laboratories (CIL, Andover,
MA). Pyruvic acid was purchased from Aldrich Chemical Company
(USA). CH3OD and CD3OH were purchased from Cambridge Isotope
Laboratories. HCl, NaOH, tetrahydrofuran, hexane, methanol were
analytical reagents (Chemical Co., Shanghai). They were used with-
out further purification. Deionized and doubly distilled water was
used throughout the study.
Fig. 1. Temporal curves of the photodegradation of BDE209 under different con-
ditions. BDE209: 1.0 × 10−5 mol/L, sodium pyruvate: 5.0 × 10−5 mol/L, solvent:
methanol/THF/H2O (V/V: 100/1/0.5); wavelength >360 nm; (a) BDE209/sodium
pyruvate/Ar/UV, (b) BDE209/sodium pyruvate/air/UV, (c) BDE209/sodium pyru-
vate/Ar/heat (50 ◦C by water bath), (d) BDE209/Ar/UV.
2.2. Methods
3. Results and discussion
2.2.1. Experimental setup
3.1. Degradation kinetics
BDE209 stock solution (1 × 10−3 mol/L) in tetrahydrofuran was
diluted with methanol to 1 × 10−5 mol/L. 0.05 mL pyruvic acid
aqueous solution (1 × 10−2 mol/L) was added to 10 mL BDE209
methanol solution in a Pyrex vessel. Reaction solutions were mag-
netically stirred during the irradiation. The Pyrex vessel was purged
with argon for 30 min to remove O2 and protected under argon
atmosphere during the irradiation. A PLS-SXE300 Xe lamp (Beijing
Trusttech Co. Ltd.) was used as the light source. To eliminate the
direct photolysis of BDE209, a cutoff filter was used to cut the irra-
diation below 360 nm. To investigate the effect of acids/bases and
H2O on the reaction kinetics, a given amount acid/base and H2O
were added under otherwise identical conditions. At given time
intervals, 1 mL aliquots were used for analysis.
To eliminate direct photolysis of BDE209 [15,16], the irradi-
ation below 360 nm was filtered completely. The degradation of
BDE209 was scarcely observed in methanol solution without pyru-
vate under the UV irradiation (Fig. 1d and Fig S1). BDE209 exhibited
little degradation when the reactions were carried out in the pres-
ence of pyruvate but under the air-saturated condition (Fig. 1b).
However, rapid degradation of BDE209 occurred in the anoxic
BDE209/sodium pyruvate solution under UV irradiation, and more
than 60% of BDE209 disappeared after 1 h of irradiation (Fig. 1a).
The kinetics was fitted by pseudo-first-order process, giving a rate
constant of 0.54 0.02 h−1 (t1/2 = 0.8 h). By contrast, the reaction
system containing sodium pyruvate in the dark heating in water
bath under anoxic condition showed no disappearance of BDE209
(Fig. 1c). It indicated that the degradation was initiated by pyruvate-
based photoreductive reaction and oxygen would decrease the
degradation rate.
2.2.2. HPLC analysis
BDE209 in samples was quantified with a SHIMADZU HPLC sys-
tem (LC-20AT pump and UV–vis SPD-20A detector) with a DIKMA
Platisil ODS C-18 column (250 × 4.6 mm, 5 ␮m film thickness).
The mobile phase was 2% water in acetonitrile at 1 mL/min and
the detector wavelength was set at 240 nm. The quantification of
BDE209 used a calibration cure with BDE209 standard.
3.2. Product analysis
The product analysis by GC-␮ECD showed that the degrada-
tion of BDE209 by sodium pyruvate led to the formation of its
lower brominated congeners in a stepwise way (Fig. 2). Before the
irradiation, the only dominant GC peak was from BDE209, with
trace nona-BDEs due to impurity. After 0.5 h, nona-BDEs appeared
as main intermediates. Nona-BDEs and octa-BDEs appeared as
main intermediates, concomitantly with the significant decrease of
BDE209 after 1 h. After irradiation of 14 h, the octa-BDEs and hepta-
BDEs were measured as the dominant intermediates. All PBDE
congeners with more than octa-BDEs vanished, and transformed
to hexa-BDEs penta-BDEs at further prolonged irradiation.
The degradation products were identified with chemical stan-
dards (Fig. 3). The first step was the loss of one bromine atom to
form three nona-BDEs. After 0.5 h, all nona-BDEs appeared, which
was identified to be BDE208, 207, and 206, respectively, accord-
ing to their well-established GC elution orders [30,31]. There were
five octa-BDEs observed after 2 h. BDE203 and BDE204 were iden-
tified by the chemical standards. The peak with a retention time
slightly longer than that of BDE203 was BDE1 9 6, according to
the relative retention times and the orders of GC elution obtained
from the literatures [23,30,31]. Six hepta-BDEs were detected after
14 h of irradiation. BDE183, BDE181 and BDE190 were identified
2.2.3. GC-ꢀECD analysis
Products were detected by GC-␮ECD analysis. The products
were extracted by hexane and the internal standards pen-
tachlorophenol (PCP) were added to all samples before the analysis
with gas chromatograph (GC) (Agilent 7890A) equipped with an
electron capture detector (ECD) (Agilent Technologies Co. USA) and
a programmable pressure on-column injection port and a DB-5 cap-
illary column (30 m × 50 ␮m, i.d. × 0.1 ␮m film thickness). Splitless
10 ␮L injection was performed manually at 300 ◦C. The carrier gas
was helium at a constant flow rate of 1.0 mL/min. The oven tem-
perature was kept at 100 ◦C for 2 min, increased at 15 ◦C/min to
230 ◦C, then increased at 5 ◦C/min to 270 ◦C, and finally increased
at 10 ◦C/min to 320 ◦C for 10 min. The standard samples of BDE203,
BDE204 and PBDEs (EO5113) were used to identify the degradation
products.
2.2.4. UV–vis absorption spectra
UV–vis absorption spectra were conducted with 10 × 10 mm
quartz cuvettes by a Hitachi U-3010 spectrophotometer (Hitachi
Co. Japan) under anoxic conditions.

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Z. Jiang et al. / Catalysis Today 224 (2014) 89–93
10Br
91
0h
9Br
0.5h
8Br
7Br
1h
2h
6Br
20
5Br
Fig. 4. Effect of acid/base on the photoreductive degradation of BDE209 by pyru-
vate in CH3OH under UV irradiation. BDE209, 1.0 × 10−5 mol/L, sodium pyruvate:
5.0 × 10−5 mol/L, solvent: methanol/THF/H2O (V/V: 100/1/0.5); (a) 0.01 M NaOH, (b)
no HCl/NaOH, (c) 0.06MHCl, (d) 0.12MHCl.
14h
15
25
30
35
Retention time (min)
with the chemical standards. The peak, whose retention time was
slightly shorter than that of BDE183, could be assigned as BDE184
based on the retention time of literature [30]. Four hexa-BDEs
peaks were identified to be BDE155, BDE154, BDE153 and BDE166,
respectively, with the chemical standards. Three penta-BDEs were
identified and they were BDE119, BDE99 and BDE116, respectively.
Among all intermediates, the products of first debromi-
nation step (nona-BDEs) provide direct information of the
debromination pathway of BDE209. All the three nona-BDE inter-
mediates were identified during the photocatalytic reductive
reaction. They were BDE206 BDE207, and BDE208, correspond-
ing to ortho-, meta-, and para-debrominated intermediates of
BDE209, respectively. Their relative peak areas were in the
orders for BDE207 > BDE208 > BDE206 (1:0.87:0.72) after 1 h. It
indicated that the debromination by pyruvate showed uncon-
spicuous position-selective property, which was different from
our previous studies in photocatalytic TiO2 system [23]. In
TiO2 system, ortho-debromination was much easier than those
from others, and debromination from the para position was the
most unfavorable pathway with peak areas in the orders for
BDE206 » BDE207 > BDE208 (1:0.25:0.16). This showed that the
debromination pattern of BDE209 varied greatly in the different
reducing system. Identification as other lower brominated interme-
diate showed that the meta-debromination was slightly easier than
those from other positions, and the debromination mainly showed
no obvious position-selective in pyruvate system.
Fig. 2. GC-␮ECD chromatograms of degradation products of BDE209 in methanol at
different irradiation times. Reaction conditions: BDE209: 1.0 × 10−5 mol/L; sodium
pyruvate: 5.0 × 10−5 mol/L, wavelength >360 nm, solvent: methanol/THF/H2O (V/V:
100/1/0.5).
a)
207
208
206
500
209
0
16
18
20
22
24
26
28
30
32
34
184
b)
183
2000
3.3. Effect of acid and base
203
The degradation experiments were performed under the addi-
tion acid and base into the solution. As illustrated in Fig. 4, when
HCl added the reaction suppressed markedly and addition of NaOH
accelerated the degradation of BDE209. It is well known that pyru-
vic acid appears ionization balance in the solution due to weak acid.
The concentration of pyruvate anions changed with the addition
of acid and base in the solution. Accordingly, the concentration of
pyruvate anions should be much lower in the solution after addition
of HCl and higher in the solution after addition of NaOH. It implied
that the photochemical reactivity was closely related to the con-
centration of pyruvate anions. In other words, pyruvate anion was
the key factor in the reaction for degradation of BDE209.
196
204
153
1000
181
190
119
99
116
154
155
166
0
16
18
20
22
24
26
Retention time (min)
3.4. Effect of H2O
Fig. 3. Products identification with GC-␮ECD chromatograms by the reten-
tion time of standard samples. (a) 1 h, (b) 14 h. Reaction conditions: BDE209:
1.0 × 10−5 mol/L; sodium pyruvate: 5.0 × 10−5 mol/L wavelength >360 nm, solvent:
methanol/THF/H2O (V/V: 100/1/0.5).
To investigate the effect of H2O in the photoreductive degrada-
tion of PBDEs by pyruvate, the reactions were further carried out

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Z. Jiang et al. / Catalysis Today 224 (2014) 89–93
of BDE209. The attempt to estimate the quantum efficiency
of the photochemical reaction was unsuccessful because the
enhanced absorption was rather weak. On the other hand, the weak
absorption but rapid photochemical reaction implied that the pho-
toefficiency of the halogen-bond system might be quite high.
3.6. Kinetic isotope effect
Kinetic isotope effect (KIE) is the ratio of the rate constants
of a reactant, which has an isotopic substitution. KIE will help
understand the mechanisms of a reaction and indicate the rate-
determining step in the degradation reaction. KIE basically owns
two types [32]: Primary KIE and Secondary KIE. Primary KIE occurs
when substitution is done at the bond which reacts. Secondary
KIE happens when the substitution is at a bond other than the
one which reacts. In the degradation reaction, BDE209 was firstly
reduced by getting electron from pyruvate to generate BDE209•−
radical and then followed by abstracting hydrogen from solvent.
So the KIE experiments may reveal more information about the
mechanism of degradation BDE209 by pyruvate. We performed
degradation experiments under the same condition with CH3OD
and CD3OH as solvent respectively, replacing of CH3OH. As illus-
trated in Fig. S2, the results showed that the degradation rate of
BDE209 was unconspicuous varied by deuterium substituted sol-
vent. The KIE was 1.01 0.1 for CH3OD and 1.03 0.2 for CD3OH.
There was no obvious KIE in the reaction. The results indicated that
abstracting hydrogen from solvent is not the rate determining step
in the degradation reaction of BDE209 by pyruvate.
Fig. 5. Temporal curves of the photodegradation of BDE209 under different
conditions. BDE209: 1.0 × 10−5 mol/L, sodium pyruvate: 5.0 × 10−5 mol/L, wave-
length >360 nm, purged with argon. (a) methanol/THF/H2O (V/V:100/1/0.5), (b)
methanol/THF/H2O (V/V: 100/1/1), (c) methanol/THF/H2O (V/V: 100/1/2).
with different ratios of H2O. As seen from Fig. 5, the rate of degrada-
tion was depressed with the ratio of H2O increasing. When the ratio
of H2O was 0.5%, about 60% of BDE209 disappeared after 1 h. Nev-
ertheless, the rate of degradation was depressed on some extent
when the ratio of H2O was 1%. The rate of BDE209 degradation
was markedly depressed in menthol with 2% of H2O. The results
indicated that the degradation of BDE209 would markedly be sup-
pressed in the presence of water. BDE209 is strongly hydrophobic,
but pyruvate is quite hydrophilic. The pyruvate would be pref-
erentially dissolved in water, which would block the interaction
between pyruvate and BDE209, and further inhibit the electron
transfer from pyruvate in its excited state to BDE209 molecu-
lar. Thus water may produce adverse effect on the degradation of
BDE209 by pyruvate.
3.7. A possible reaction mechanism
Based on the above experiments, a possible reaction mechanism
has been proposed. Halogen atoms in aromatic compounds are
known to possess positive surface region along their covalent bond,
which makes the halogen atom easy to interact with the electron
donor [26]. Such noncovalent interactions are referred to halo-
gen bonds by analogy with the better-known hydrogen bonds. The
halogen-bond interaction between the bromine atom of BDE209
and the O atom of pyruvate anion induced the stronger absorp-
tion than BDE209 and pyruvate alone respectively. The enhanced
absorption led to pyruvate excitation by irradiation (wavelength
>360 nm) (Eq. (1)). Then the n-␲* excitation of PA ion [29] induced
long-range electron transfer from the promoted carbonyl chro-
mophore into neighboring BDE209 molecular. Subsequently, PA
lost electron and changed to CO2 and CH3CHO [28,29] (Eq. (2)).
BDE209 obtained electron and formed C10Br9O• and Br− (Eq. (3)).
The C10Br9O• radical abstracted hydrogen atom from methanol and
yielded lower bromo congeners C10Br9OH and CH3OH transformed
to HCHO with losing hydrogen atom (Eq. (4)).
3.5. UV–vis absorption spectra
The measurement of UV–vis absorption spectra indicated that
neither sodium pyruvate nor BDE209 had absorption beyond
360 nm (Fig. 6 spectra a and b, respectively), which is consistent
with the above observation that BDE209 alone is stable under
irradiation beyond 360 nm (Fig. 1d). However, a new absorption
band appeared in the pyruvate/BDE209 solution in the wave-
length region of 350–600 nm (Fig. 6 spectra c). It indicated that
the enhanced absorption led to pyruvate excitation by irradiation
(wavelength >360 nm) and at last resulted in the debromination
hv
CH3COCOOH−→ CH3COCOOH∗
(1)
(2)
(3)
(4)
CH3COCOOH ∗ + C12Br10O → CH3CHO + C12Br10
O
•− + CO2
C12Br10
O
•− → C12Br9O• + Br−
C12Br9O• + CH3OH → C12Br9OH + HCHO
The theoretical calculations indicated that the strength of halo-
gen bond greatly depended on the existence state of pyruvate [26].
The strength of the halogen bond became stronger with the con-
centration of pyruvate anion increasing. Thus the addition of HCl
(NaOH) suppressed (accelerated) the degradation of BDE209 due
to the concentration of pyruvate anion changing.
Fig. 6. UV–vis absorption spectra in CH3OH solution. (a): sodium pyruvate, (b):
BDE209, (c): BDE209/sodium pyruvate. BDE209: 1.0 × 10−5 mol/L, sodium pyruvate:
5.0 × 10−5 mol/L. Solvent: methanol/THF/H2O (V/V:100/1/0.5).