Synthesis of SiO2 coated zero-valent iron/palladium bimetallic nanoparticles and their application in a nano-biological combined system for 2,2′,4,4′-tetrabromodiphenyl ether degradation

Article abstract of DOI:10.1039/c5ra22388a

Polybrominated diphenyl ethers (PBDEs) are emerging persistent organic pollutants and the degradation of PBDEs is still a significant challenge owing to their extreme persistence and toxicity. In this study, the remediation of 2,2′,4,4′-tetrabromodiphenyl ether (BDE47) was investigated by employing a nano-biological combined system with SiO₂-coated zero-valent iron/palladium bimetallic nanoparticles (SiO₂-nZVI/Pd) as a reductant and Pseudomonas putida as a biocatalyst. The SiO₂-nZVI/Pd exhibited much lower toxicity to the P. putida strain and higher reactivity in debromination than nZVI/Pd. The strain could grow well when the dosage was up to 1.0 g L^-1. During the combined process, BDE47 (5 mg L^-1) was completely debrominated to diphenyl ether (DE) within 2 h by SiO₂-nZVI/Pd (1.0 g L^-1) and then DE was completely degraded by P. putida after 4 days in sequential aerobic biodegradation. All the possible intermediates in the whole process were identified by ultra performance liquid chromatography (UPLC) and gas chromatography-mass spectrometer (GC-MS) analyses. The detection of BDE17, BDE7, BDE1 and DE indicated that rapidly stepwise debromination preferentially occurred at para positions in the anaerobic stage. Moreover, during aerobic biodegradation by P. putida, a number of phenolic compounds, such as phenol, catechol and hydroquinone were generated via ring opening by dioxygenation and further mineralized through the tricarboxylic acid cycle (TCA). Importantly, this combined process achieved rapid mineralization of PBDEs and avoided the generation of some highly toxic products like bromophenols and HO-PBDEs, which might have promising application prospects in the remediation of halogenated POPs.

Full text of DOI:10.1039/c5ra22388a

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PAPER
Synthesis of SiO coated zero-valent iron/
2
palladium bimetallic nanoparticles and their
Cite this: RSC Adv., 2016, 6, 20357
application in a nano-biological combined system
0
0
for 2,2 ,4,4 -tetrabromodiphenyl ether
degradation†
a
b
ab
ab
Yuancai Lv, Zhuyu Niu, Yuancai Chen* and Yongyou Hu
Polybrominated diphenyl ethers (PBDEs) are emerging persistent organic pollutants and the degradation of
PBDEs is still a significant challenge owing to their extreme persistence and toxicity. In this study, the
0
0
remediation of 2,2 ,4,4 -tetrabromodiphenyl ether (BDE47) was investigated by employing a nano-
biological combined system with SiO -coated zero-valent iron/palladium bimetallic nanoparticles (SiO
nZVI/Pd) as a reductant and Pseudomonas putida as a biocatalyst. The SiO -nZVI/Pd exhibited much
lower toxicity to the P. putida strain and higher reactivity in debromination than nZVI/Pd. The strain
2
2
-
2
ꢀ1
ꢀ1
could grow well when the dosage was up to 1.0 g L . During the combined process, BDE47 (5 mg L
)
ꢀ1
2
was completely debrominated to diphenyl ether (DE) within 2 h by SiO -nZVI/Pd (1.0 g L ) and then DE
was completely degraded by P. putida after 4 days in sequential aerobic biodegradation. All the possible
intermediates in the whole process were identified by ultra performance liquid chromatography (UPLC)
and gas chromatography-mass spectrometer (GC-MS) analyses. The detection of BDE17, BDE7, BDE1
and DE indicated that rapidly stepwise debromination preferentially occurred at para positions in the
anaerobic stage. Moreover, during aerobic biodegradation by P. putida,
a number of phenolic
compounds, such as phenol, catechol and hydroquinone were generated via ring opening by
dioxygenation and further mineralized through the tricarboxylic acid cycle (TCA). Importantly, this
combined process achieved rapid mineralization of PBDEs and avoided the generation of some highly
toxic products like bromophenols and HO–PBDEs, which might have promising application prospects in
the remediation of halogenated POPs.
Received 26th October 2015
Accepted 13th February 2016
DOI: 10.1039/c5ra22388a
www.rsc.org/advances
thyroid hormone disruption, neurotoxicity and abnormal
behaviors. Thus, it is highly urgent to explore effective strate-
1
Introduction
6
As one kind of effective and economical ame retardant, poly- gies to deal with BDE47.
brominated diphenyl ethers (PBDEs) have been widely used in
Biodegradation exhibited environmentally friendly sights for
a variety of industrial products. At present, PBDEs have been persistent organic pollutants (POPs) remediation and was
1
detected frequently in air, soil, sediment and sewage sludge deemed as an effective, economic and safe way. Meanwhile,
1
–4
samples, even in biological samples.
Among the PBDEs a considerable number of microorganisms have been reported
0
0
3,7,8
congeners, 2,2 ,4,4 -tetrabromodiphenyl ether (BDE47) is the to be able to degrade PBDEs.
most widely distributed congener in environment and organism PBDEs oen needed a rather long operation time and generated
8
However, biodegradation of
3,5
9
samples and has some serious health side-effects such as some more toxic products such as bromophenols, HO–PBDEs
10
and MeO–PBDEs.
Recently, owing to its high reactivity, nano technology has
a
State Key Laboratory of Pulp and Paper Engineering, College of Light Industry and
become an efficient treatment in the elimination of contami-
Food Science, South China University of Technology, No. 281 Wushan Road, Tianhe
District, Guangzhou, 510640, China. E-mail: donkey1204@hotmail.com; ppyyhu@
11,12
nants.
Based on this, the nano-biological integrated system
scut.edu.cn; chenyc@scut.edu.cn; Tel: +86 13672458060
has attracted great attention and is considered as a potential
b
Ministry of Education Key Laboratory of Pollution Control and Ecological strategy for the remediation of halogenated POPs.13–15 In which,
Remediation for Industrial Agglomeration Area, College of Environment and Energy,
as an effective pretreatment process. nZVI and its composite
South China University of Technology, Guangzhou, 510006, China. E-mail:
materials could rapidly and greatly reduce the resistance of the
niuzhuyu11@163.com
contaminants and greatly improved the efficiency in biological
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra22388a
13
process. Murugesan et al. found that an integrated redox
1
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process consisting of nZVI/Pd mediated reductive dehalogena- 2.2 Synthesis and characterization of SiO -coated zero-valent
2
tion followed by oxidative degradation by Sphingomonas sp. pH- iron/palladium bimetallic nanoparticles
0
7 was an efficient strategy for complete degradation of TCS.
Synthesis of SiO
nanoparticles (SiO
7H O with NaBH
information could be seen in ESI.†
The morphology of SiO -nZVI/Pd particles was observed with
2
-coated zero-valent iron/palladium bimetallic
-nZVI/Pd) was carried by reducing FeSO
under nitrogen atmosphere. The detailed
16
Similarly, Kim et al. also found the combined system of nZVI
and aerobic biodegradation could transform BDE-209 into
2
4
-
$
2
4
17
small molecular substances. According to Shih et al.’s study, it
could immensely improve the degradation of deca-BDE when
combining ZVI with the anaerobic sludge.
However, two problems limited the application of nZVI/
biological process. Due to the strong magnetic effects between
the nanoparticles, a severe agglomeration occurred, which
greatly decreased the activities of nZVI or nZVI composite
2
transmission electron microscopy (TEM, JEM 2100F, Japan).
The N Brunnaer–Emmett–Teller (BET) specic surface areas of
2
the particles were measured by ASAP 2020. X-ray diffraction
(XRD) spectra were obtained using Cu Ka radiation at 40 kV and
40 mA (MAC Science Co., M18XHF).
18–20
materials.
On the other hand, it has been proved that nZVI
particles could greatly inactivate the bacterial cells through
2
.3 Toxicity of nZVI/Pd and SiO -nZVI/Pd particles to P.
2
21–23
physical coating, membrane disruption and oxidative stress.
The strong antibacterial activities of nZVI or nZVI/Pd particles
signicantly limited application. According to the previous The toxicity evaluation was based on the effects of nZVI/Pd and
studies, some attempts have veried that surface coated nZVI SiO -nZVI/Pd particles on the growth and inactivation of P.
particles with polymer and natural organic matter (NOM) or putida strain. Experiments were conducted using different
sodium–oleate decreased the toxicity of nZVI particles to E. coli amounts of nZVI/Pd and SiO -nZVI/Pd particles (0, 0.5 and 1.0 g
) in 250 mL Erlenmeyer asks containing 50 mL of sterilized
putida
2
2
ꢀ
1
24,25
when compared with non-coated nZVI particles,
which was
L
attributed to the fact that the coating prevented the adhesion of MSM medium. The initial cell concentration was about 2.0 ꢂ
7
ꢀ1
nZVI particles onto cell membrane.
0.6 ꢃ 10 CFU mL (about 0.025 of OD600). Flasks were incu-
ꢁ
In this study, a nano-biological hybrid process was employed bated in a shaking incubator at 200 rpm at 30 C. For growth
to completely mineralize BDE47 with SiO
2
-nZVI/Pd particles test, 2 g L of glucose was added as carbon source while no
ꢀ
1
and Pseudomonas putida strain. In order to improve the debro- glucose was supplied in inactivation test. At the sampling
mination efficiency and reduce the toxicity to P. putida strain, points, 100 mL of culture was aseptically drawn out and plated
ꢁ
the nZVI/Pd particles were coated with SiO lm by using tet- onto nutrient agar media (incubated at 37 C for 24 h) aer the
2
raethyl orthosilicate (TEOS) as silicon source and their toxic proper dilution to monitor the number of P. putida cells and
effects on P. putida and reactivity were both evaluated. BDE47 evaluate the toxic effect of particles. To nd out whether nZVI/
was rstly completely debrominated to DE by SiO
2
-nZVI/Pd Pd and SiO -nZVI/Pd particles affect cell membrane and
2
particles under anaerobic condition, making the effluent morphology of P. putida cells, TEM analysis was performed. The
much easier for subsequent biological mineralization. Then DE samples for TEM analysis were prepared as followed standard
was subsequently treated with P. putida strain. Here, the protocol (see ESI†).
degradation kinetics of the reduction debromination was
examined. The intermediates during the whole process were
monitored and the degradation pathways of PBDEs by this
treatment were explored.
2
.4 Sequential aerobic treatment followed anaerobic
debromination
Aer anaerobic debromination process, all the rubber caps were
removed. Then 2.0 mL of prepared resting cells (seen in ESI†) of
P. putida in MSM were poured into the bottles to make a cell
number of about 1.5 ꢂ 0.3 ꢃ 10 CFU mL . All test bottles were
incubated for further 4 days under identical conditions, while
the bottles were capped with trilaminar sterile gauze to supply
fresh air. The test bottles that contained debrominated prod-
ucts without resting cells were set as controls. At preselected
time intervals, three bottles were drew out and the samples were
analyzed aer extraction.
2
Material and methods
8
ꢀ1
2.1 Materials
0
0
2
,2 ,4,4 -Tetrabromodiphenyl ether (BDE47, 99%) and diphenyl
ether (DE, 99%) were purchased from J&K Scientic Ltd and
Sigma-Aldrich, respectively. BDE-1, BDE-3, BDE-7, BDE-15, BDE-
ꢀ1
1
7, BDE-28 and BDE-47 at 50 mg mL in isooctane were from
AccuStandard. HPLC grade methanol and dichloromethane
were separately supplied by Merck (Germany) and CNW (Ger-
many). FeSO
4 2
$7H O (99%), NaBH4 (98%) and ethyl alcohol
2
2.5 Anaerobic debromination by SiO -nZVI/Pd particles
(99.7%) were purchased from Aladdin (Shanghai, China).
Deionized water was prepared by the Milli-Q water purication Anaerobic debromination of BDE-47 was conducted under
system (Millipore). The strain, Pseudomonas putida, was isolated nitrogen atmosphere in serum bottles (100 mL). 200 mL of BDE-
ꢀ
1
from the sludge of pulping wastewater and kept in our labora- 47 stock solution (dissolved in toluene, 1000 mg L ) was added
tory. Minimal salt medium (MSM) and phosphate buffer (pH to the bottle. Aer gentle vaporization of the solvent through N2,
26
7
.00) were prepared by the method reported previously. Before 19.0 mL deoxygenated MSM and 1.0 mL SiO -nZVI/Pd slurry
2
use, the MSM and phosphate buffer were previously sterilized in were separately poured into the bottle in a glove box (LM1000S,
ꢁ
an autoclave at 121 C for 30 min.
2
Dellix Industry Co. Ltd, China). N was kept in the headspace of
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the test tube before sealing the Teon coated rubber cap with which was similar with Zhuang et al.’s study. Furthermore,
aluminium crimp. These bottles were incubated in a shaking Inductively Coupled Plasma Optical Emission Spectrometry
ꢁ
incubator at 200 rpm at 30 C. The controls were operated in (ICP-OES) was employed to determine the percentage of Fe, Pd
same process but without SiO -nZVI/Pd. At each sampling and Si in SiO -nZVI–Pd. The results showed that the percent-
2
2
point, three bottles were collected and analyzed aer extraction ages of Fe, Pd and Si were 87.61%, 0.26% and 5.78%, respec-
(see ESI†).
tively, indicating the successful synthesis of SiO
valent iron/palladium bimetallic nanoparticles.
2
-coated zero-
The BET specic surface area of the synthesized nZVI/Pd
2
.6 Analytical methods
2
ꢀ1
particles was 32.68 m g , which agreed with data reported in
BDE47 and DE concentrations were detected by ultra perfor-
mance liquid chromatography (UPLC, Waters Acquity UPLC
system, America). The intermediates and products during the
whole treatment were identied by Thermo Trace GC Ultra
instrument coupled to a Thermo DSQ II mass spectrometer
27
studies that employed similar synthesis methods. Similarly,
the BET specic surface areas of SiO -nZVI/Pd and SiO aer
2
2
2
ꢀ1
vacuum drying were 111.76 and 268.54 m g , respectively. This
could be ascribed to two reasons. On one hand, which was more
2
important, the presence of corrugated SiO lm on the surface of
(Thermo Electron Corporation, Waltham, USA). The concen-
nZVI/Pd greatly expand the BET specic surface area of nZVI/Pd.
On the other hand, the reducing of agglomeration also could
partly improve the particles' BET specic surface area.
ꢀ
tration of bromide ions (Br ) were determined by an ion-
chromatography system (ICS-90, Dionex). The detailed infor-
mation could be seen in ESI.†
TOC was determined by LiquiTOC trace (Elementar, Ger-
many). Before determination, all the samples were with 0.22 mm
glass ber lters.
3
.2 Toxicity and reactivity of SiO -nZVI/Pd and nZVI/Pd
2
3.2.1 Toxicity of SiO
2
-nZVI/Pd and nZVI/Pd to P. putida
cells. It was well known that the nanoparticles were toxic to
2.7 Statistical analysis
21–23
bacterial cells.
Thus, it was important to evaluate the toxicity
All experiments were performed in triplicate and the data were of the nanoparticles to the cells when a nano-bio treatment
expressed as mean ꢂ standard deviation. The statistical anal- involving nanoparticles and bacteria was employed for efficient
yses were performed with one-way analysis of variance (ANOVA) degradation of pollutants. Here, in order to evaluate the anti-
using Minitab 16 Soware. A multiple comparison Tukey test bacterial activity of SiO -nZVI/Pd and nZVI/Pd, the growth and
2
was applied to assay the differences among treatments.
inactivation of P. putida cells under different dosage of nano-
ꢀ1
particles (with ranges from 0.0 to 1.0 g L ) was conducted
Fig. 2). Considering the interference of the dispersed nZVI
(
3
Results and discussion
particles on the accurate spectrophotometric measurement of
the cell suspensions, the cell growth was assessed with colony
3.1 Particle characterization
Fig. 1 and S1† showed TEM images and size distribution of forming unit (CFU) counting. As shown in Fig. 2a, P. putida
SiO -nZVI/Pd and nZVI/Pd particles. Similar to the previous strain was able to grow well when the dosage of nZVI/Pd was 0.5
reports,
caused server agglomeration of nZVI/Pd particles without SiO
Fig. S1a and b†). However, once nZVI/Pd particles were coated strain could still grow well even SiO -nZVI/Pd dosage was up to
2
18,19
ꢀ1
the magnetic effects between the nanoparticles g L in MSM whereas bacterial growth was drastically inhibited
ꢀ
1
2
with high nZVI/Pd amount (1.0 g L ). However, the P. putida
(
2
ꢀ
1
with a transparent and corrugated SiO lm, it can be clearly 1.0 g L , suggesting that SiO -nZVI/Pd was less toxic than nZVI/
2
2
observed an evident decrease on the aggregation between nZVI/ Pd. Compared with control, the lag phases of P. putida cell were
ꢀ
1
ꢀ1
Pd particles and the synthesized nZVI/Pd particles formed 8 h (0.5 g L of SiO
2
-nZVI/Pd), 16 h (1.0 g L of SiO
2
-nZVI/Pd),
ꢀ
1
ꢀ1
linear chains in space (Fig. 1a–d). The particle size of nZVI/Pd 12 h (0.5 g L of nZVI/Pd) and 24 h (1.0 g L of nZVI/Pd),
without SiO
2
was mainly (92.1%, Fig. S1c†) in the range of 20– respectively, which were longer than that of control culture (4
0 nm while the main particle size (75.0%, Fig. 1e) increased to h), indicating the nanoparticles prolonged the exponential
0–50 nm when coated with SiO . All the results indicated that phase of cell growth.
To evaluate the toxicity of nZVI/Pd in quantitative manner,
4
3
2
the coated SiO2 lm on nZVI/Pd surface prevented the
agglomeration of nZVI/Pd particles and slightly increased their Logistic model was employed to obtain the relative parame-
28
2
sizes.
Fig. 1f exhibited the XRD patterns of SiO
ters. The high R values (0.9904 and 0.9939, Fig. 2b) suggested
2
, Pd, fresh Logistic model could well describe the effects of various particle
synthesized nZVI/Pd, nZVI and SiO
the pattern of fresh synthesized SiO
2
-nZVI/Pd, respectively. In concentrations on the inactivation of P. putida. The EC20, EC50
-nZVI/Pd, it clearly depicted and EC80 of SiO -nZVI/Pd were 0.53, 0.75 and 1.05 g L ,
2
ꢀ1
2
a weak characteristic diffraction peak of SiO
2
, located at 2q ¼ respectively, which was 1–2 times larger than that of nZVI/Pd
ꢁ
ꢀ1
2
7.36 , and three characteristic diffraction peaks appeared at 2q (0.31, 0.50 and 0.83 g L ). The results indicated that the
ꢁ ꢁ ꢁ
44.69 , 65.02 and 82.35 , which were in accordance with coating of SiO on nZVI/Pd could greatly reduce the toxicity to P.
2
¼
nZVI/Pd and nZVI. The three peaks can be indexed as the (110), putida cells.
0
(200) and (211) plane of a-Fe (JCPDS card no. 06-0696).
To check the effects of nZVI/Pd and SiO -nZVI/Pd on
2
However, due to the low dosage, no characteristic diffraction morphology of P. putida cells, both the control and particles
ꢁ
ꢁ
ꢁ
peaks (2q ¼ 39.8 , 46.2 and 67.8 ) of palladium were observed, treated cells were analyzed by TEM (Fig. 3). Compared with
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2 2 2
Fig. 1 TEM images (a–d) and size distribution (e) of SiO -nZVI/Pd and XRD (f) spectra of SiO -nZVI/Pd, nZVI/Pd, nZVI, Pd and SiO .
untreated cells (Fig. 3a), the TEM images (Fig. 3b and c) clearly to the control as incubation progressed (Fig. 2), indicating that
revealed a large amount of nZVI/Pd particles adhered to the P. putida strain could tolerate SiO -nZVI/Pd with the range of 0–
-nZVI/Pd could be employed for
Fig. 3b, some cells were broken while no ruptured cells were degradation of PBDEs in the nano-bio treatment process.
observed in Fig. 3c, suggesting the coating of SiO greatly 3.2.2 Reactivity of SiO -nZVI/Pd and nZVI/Pd on BDE47
2
ꢀ1
surface of cells once nZVI/Pd particles were added. As shown in 1.0 g L . Therefore, SiO
2
2
2
decreased the toxicity of nZVI/Pd particles. As reported in remediation. Although SiO2 played an important role in
previous studies, surface coated nZVI with polymer or NOM reducing the biotoxicity, whether it could enhance PBDEs
prevented the adhesion of nZVI particle onto cell membrane, degradation was indistinct. Fig. 4 illustrated the degradation
2
4
leading to the decrease of toxicity of nZVI to E. coli. Similarly, efficiencies and rates of BDE47 by SiO
2
-nZVI/Pd and nZVI/Pd
-nZVI/Pd
the toxicity of nZVI particles greatly declined when combined with different dosages. As shown in Fig. 4a, both SiO
with sodium–oleate. Furthermore, the TEM images of bio- and nZVI/Pd exhibited excellent elimination of BDE47. It took
2
25
logical slices (Fig. 3e and f) clearly exhibited the rupture of the 120 and 180 min to completely degrade BDE47 for 1.0 and 0.5 g
ꢀ
1
cell membrane caused by adhered nZVI/Pd particles, leading to
L
of SiO
2
-nZVI/Pd, which were lower than that of nZVI/Pd (150
23
ꢀ1
the inactivation of cells. Kim et al. also observed severe and >210 min for 1.0 and 0.5 g L ). Meanwhile, the degrada-
damages caused by nZVI particles to the cell membranes of tion of BDE47 obeyed the pseudo-rst-order model with 0.0466
ꢀ1
ꢀ1
ꢀ1
Escherichia coli.
(1.0 g L ) and 0.0247 min (0.5 g L ) of rate constant for SiO2-
Although an initial retardation in growth were observed in nZVI/Pd, which were 1.3–1.5 times larger than those (0.0359 and
ꢀ1
2
the presence of SiO -nZVI/Pd, the cell number was almost equal 0.0183 min ) of nZVI/Pd particles. All the results manifested
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Fig. 2 Effect of nZVI/Pd and SiO -nZVI/Pd nanoparticles on the
growth (a) and inactivation (b) of P. putida in MSM medium. Initial cell Fig. 4 Effects of particle dosage on degradation of BDE47.
7
ꢀ1
concentration: 2.0 ꢂ 0.6 ꢃ 10 CFU mL and initial pH: 7.00.
Fig. 3 TEM images of P. putida cells (a), nZVI/Pd treated cells (b) and SiO
and nZVI/Pd treated cells (e). Initial cell concentration: 2.0 ꢂ 0.6 ꢃ 10 CFU mL , nZVI/Pd dosage: 0.5 g L and SiO
2
-nZVI/Pd treated cells (c) and the biological slices of P. putida cells (d)
7
ꢀ1
ꢀ1
ꢀ1
2
-nZVI/Pd dosage: 1.0 g L .
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2
that the coating of SiO greatly improved the reductive activity
of the nanoparticles, causing a signicant improvement on
degradation of BDE47. According to Wan et al.’s results, coating
of SiO could effectively reduce the particle surface passivation
2
and severe agglomeration between the particles and enhance
29
the degradation of 2,4-dichlorophenol. The modication of
polyvinyl pyrrolidone (PVP) on nZVI/Ni surface was also
observed to enhance the aqueous dispersions and catalytic
20
activity to debrominate BDE209.
3
.3 Debromination of BDE47 by SiO -nZVI/Pd
2
The debromination of BDE47 was conducted under anoxic
ꢀ
1
conditions with 5 mg L (10.28 mM) of initial BDE47 concen-
ꢀ
1
tration, 1.0 g L
of SiO -nZVI/Pd dosage and 6.00 of pH.
2
Fig. S2† showed the GC-MS chromatographs of BDE47 and the
intermediate compounds formed during 120 min debromina-
tion. All of PBDEs congeners were identied by matching with
the standards (Fig. S3†). As shown in Fig. S2,† with a decline of
BDE47, DE to tri-BDE congeners appeared in a sequential
manner, suggesting that BDE47 underwent stepwise debromi-
nation to form a series of congeners during the anaerobic
reduction by SiO -nZVI/Pd. Aer 120 min incubation, DE was
2
dominant in the system.
The variation of BDE47 and by-products and the carbon
mass balance were shown in Fig. 5. As shown in Fig. 5a, BDE47
was almost eliminated in 60 min. Tri-BDEs rst appeared in
considerable amounts and remained as dominant products in
the rst 20 min and then declined and nally disappeared aer
60 min. From 25 to 35 min, di-BDEs were found to be the main
product in the system and almost disappeared aer 90 min. The
amounts of mono-BDEs increased and reached a peak at 40 min
and then faded away in later stages. Meanwhile, with the reac-
tion proceeded, the concentration of DE gradually increased
and reached 8.3 mM at the end of anaerobic process. Further-
more, the reductive debromination was intuitively reected by
the release of bromide ion (Fig. 5c). The concentration of
bromide ion increased continually during the reaction and

nally reached 39.5 ꢂ 0.9 mM.
In order to elucidate the rate-limiting step during the
complete debromination of BDE47, degradation kinetics of
BDE47 and three major products (BDE17, BDE7 and BDE1) were
studied under the same condition (Fig. 5b). The loss of BDE47
and three main congeners during the debromination reaction
were also in accord with the pseudo-rst-order model. The rate
constants was 0.0466, 0.03, 0.0333 and 0.0194 min for BDE47,
BDE17, BDE 7 and BDE1, respectively. The relative half-life
periods of four congeners was 15.1, 23.1, 20.8 and 35.7 min,
Fig. 5 Variation of BDE47 and by-products (a), degradation kinetics of
ꢀ
PBDEs (b) and release of Br (c) during debromination. SiO
dosage: 1.0 g L , Pd content (wt%): 0.3%, initial PBDEs concentration:
2
-nZVI/Pd
ꢀ1
ꢀ
1
1
0.28 mM and initial pH: 6.00.
19
indicating that the debromination rate was proportional to the phenyl ring than para-Br, leading to the higher resistance.
number of bromine on DE. Thus, the debromination of mono- Meanwhile, the formation of bromide ion (Fig. 5c) also followed
BDE was rate-limiting step during anaerobic reductive process. the rst-order kinetic model and its rate constant was 0.0215
ꢀ
1
Interestingly, the rate constant of BDE17 was little lower than min . Similar results have been reported in the reductive
31
that of BDE7, indicating that debromination rate at ortho dechlorination of lindane and TCS by nFe/Pd.
position was lower than that at para position. This might be Based on the results above mentioned, it could clear be
related to the steric hindrance of the bromine atom at the conrmed that stepwise debromination was the degradation
3
0
different positions on the benzene ring. The ortho-Br endured mechanism of BDE47 by SiO
2
-nZVI/Pd nanoparticles (Fig. S4†).
19,32
more hindrance from neighboring oxygen and the neighboring The pathways were similar to those reported for nZVI/Pd,
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demonstrating that SiO had no effects on degradation mech-
anism of BDE47 by nZVI/Pd. BDE17 and BDE1 were the main
products in the debromination pathways for BDE47 and BDE7
resulting from debromination of one para-Br, indicating the
persistence of ortho-Br on the same benzene ring. Furthermore,
the appearance of BDE7 followed BDE17 suggested that the
debromination preferentially proceeded at the benzene ring
with fewer bromine atom. The appearance of BDE28, BDE15
and BDE3 manifested the debromination at ortho positions
simultaneously occurred, but it was just the minor pathway
considering the low amount. Owing to the high reductive
activity of nZVI particles, reductive dehalogenation was the
main reaction under anaerobic condition, with C–X (X ¼ Cl or
Br) bonds sequentially broke and halogen atoms replaced by
5,33
33
hydrogen. According to Wang et al.’s study, regioselectivity
of the debromination reaction was related to length of the C–Br
bonds and an elongation of C–Br bonds was conducive to their
rupture. They found that an additional electron from nZVI/Pd
combined with the benzene ring and lengthened C–Br bonds
at para positions, leading to their prior dissociation. Moreover,
H atom transfer was found to control PBDE debromination by
Fig. 6 The variation of DE, TOC and viable cell number during the
subsequent biodegradation. Initial DE concentration: 30 ꢂ 2 mM, initial
ꢀ1
DE concentration: 4.3 ꢂ 0.4 mg L and initial pH: 7.00.
2
the reduction by SiO -nZVI/Pd particles was able to the removal
of bromine, hence reducing the resistance and making the
products more susceptible to further biodegradation.
19,34
palladized ZVI.
The steric hindrance played a vital role in
inhibiting the formation of a precursor complex between the H
The P. putida strain showed good removal of DE when it was
provided as sole source of carbon and energy. However, the
degradation pathways of PBDEs by P. putida had yet not been
reported. To obtain sufficient quantity of intermediates at
specic time interval, ve bottles were collected and then sub-
jected to triple extraction by dichloromethane. All the extracts
were combined and concentrated for the GC-MS analysis of
metabolites. The detailed chromatogram and mass spectra of
the intermediates at 12 h was presented in Fig. 7 and S5.†
Approximately 7 kinds of possible byproducts were identied
through the National Institute of Standards and Technology
30
atom and palladium. The ortho-Br bore the most hindrance
from neighboring oxygen, while para-bromines were least
hindered by the oxygen atom and the other phenyl ring,
resulting in a general debromination preference of para-Br >
19
meta-Br > ortho-Br for palladized ZVI.
3.4 Biodegradation of DE by P. putida strain
Due the efficient reductive debromination, BDE47 was eventu-
ally transformed into non-brominated compound, DE. In this
study, an aromatic compounds degrader, P. putida strain, was
employed for the degradation of DE. For better monitor of TOC
and the byproducts during biodegradation, the initial DE was
(NIST11) library and comparison with previous reports.
Peak 1 was the target contaminant, DE (RT ¼ 17.68 min),
with molecular ion at m/z 170 and major fragment ions at m/z
ꢀ
1
adjusted to 30 ꢂ 2 mM (4.3 ꢂ 0.4 mg L of TOC) by adding
1
41, 77, 51. The peak 2 (RT ¼ 20.73) with and 3 (RT ¼ 23.06)
a measured dose of DE.
which gave molecular ion at m/z 186 and 202 were identied as
During aerobic biodegradation, the variation of DE, TOC and
cell number were supervised and the results were shown in
Fig. 6. As shown in Fig. 6, no signicant variation of DE and
TOC were observed over time in the control experiments,
implying that the possible abiotic degradation during the
process was basically negligible. However, in the system inoc-
ulated P. putida cells, the DE was depleted in 3 days, indicating
that DE could be rapidly removed by P. putida cells. Similarly,
ꢀ
1
the TOC also decreased from 4.3 ꢂ 0.4 to 1.4 ꢂ 0.5 mg L aer
4
days, suggesting DE and the intermediates could be easily
mineralized by P. putida through tricarboxylic acid cycle (TCA).
Meanwhile, an evident cell growth (Fig. 5) was observed coupled
with the DE consumed and the cell number increased to 3.2 ꢂ
8
ꢀ1
0
.3 ꢃ 10 CFU mL , indicating that the substrates could be
3,7,8
well utilized for cell growth. According to previous studies,
the PBDEs was rather difficult to be mineralized by the micro-
organisms even when they were treated for several days or
certain months. The great shortening in treating time and
Fig. 7 GC/MS analysis of intermediates during biodegradation of DE
improvement in biodegradability were ascribed to the fact that by P. putida.
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32,35
mono-hydroxylated and di-hydroxylated DE products, respec- 2-hydroxydiphenyl ether.
Meanwhile, identication of
tively. Peak 2 was conrmed as 2-hydroxy DE (major fragment several phenolic compounds, such as phenol, catechol and
ions at m/z 158, 109, 77), but the specic structure of peak 3 hydroquinone conrmed that the angular dioxygenation
could not be fully conrmed with certainty through NIST11. simultaneously occurred to DE and 2-hydroxydiphenyl ether
35
According to Yamazoe et al.’s results, 2,3-dioxygenation was during the biodegradation, which led to cleavage of ether bond.
a common enzyme during the metabolic process of P. putida Subsequently, further oxidization and ring cleavage resulted in
strain, which could add a hydroxyl group at the 2,3-positions of generation of a series of carboxylic acids like maleic acid and
the benzene ring. Thus, peak 3 was speculated to be 2,3-dihy- trans,trans-2,4-hexadienedioic acid. Finally, the carboxylic acids
droxydiphenol ether. The peaks (4–6) with retention time at were mineralized into CO
2 2
and H O via the tricarboxylic acid
6.70, 9.21 and 10.07 min were a series of phenolic compounds cycle (TCA). Similarly, both angular dioxygenation and lateral
formed via the cleavage of ether bond, which were identied as dioxygenation were veried in the degradation of diphenyl ether
8
phenol, catechol and p-hydroquinone, respectively. The peak 7 by Janibacter sp. strain YY-1 (ref. 35) and Bacillus cereus JP12. In
(
RT ¼ 12.51 min) and 8 (RT ¼ 13.72 min) with molecular ion at previous study, the angular dioxygenation was mostly
m/z 116 and 142 were ring-opening products, which were maleic conrmed during the biodegradation of PBDEs by Sphingomo-
acid and trans,trans-2,4-hexadienedioic acid, respectively.
According to Bressler and Fedorak's results, aerobic genation occurred when the Pseudomonas cepacia Et4 was
biodegradation of polycyclic aromatic compounds by bacteria employed for the degradation of DE. However, lateral dioxyge-
was a process which involved a complex oxygenation enzyme nation pathway would result in the formation of some toxic
system. Generally, oxygenation systems are divided into three products such as dibenzo-p-dioxins, dioxin and hydroxylated
9,37,38
39
nas sp.
Meanwhile, Pfeifer et al. found that lateral dioxy-
36
8,35
different types: (a) lateral dioxygenation, in which one of the PBDEs.
To the best of our knowledge, there is little infor-
aromatic rings is hydroxylated; (b) angular dioxygenation, in mation in the literature on the PBDEs degradation pathway by
which the carbon atom bonded to the heteroatom and the Pseudomonas putida strain. In this study, PBDEs degradation
adjacent carbon in the aromatic ring are both oxidized to pathway utilized by this strain was similar to the pathway re-
hemiacetal; and (c) ve-membered ring monooxygenation, in ported for Janibacter sp. strain and Bacillus cereus JP12.
which the methylene carbon atom is oxidized. Based on the
identied intermediates, a degradation pathway of DE by P.
putida strain was proposed in Fig. 8. In this study, both lateral
4
Conclusions
dioxygenation and angular dioxygenation occurred during the The results of this study indicated that SiO -nZVI/Pd was
2
biodegradation of DE by P. putida in. The formation of a promising pretreatment process for remediation of PBDEs
2
,3-dihydroxydiphenol ether suggested that lateral dioxygena- particularly when employed in combination with biological
0
tion occurred at 2,3 -positions of benzene ring. Due to the treatment.
dehydration, 2,3-dihydroxydiphenol ether would transform into
2
(1) Compared with nZVI/Pd, SiO -nZVI/Pd particles exhibited
much lower toxicity to the growth of P. putida strain and higher
reactivity on reductive debromination.
2
(2) During the pretreatment by SiO -nZVI/Pd, BDE47 could
be completely transformed into DE through step-by-step
debromination, reducing the toxicity and resistance of the
products. This made the treated effluent compatible for further
biological treatment whilst avoiding the formation of highly
toxic intermediates such as bromophenols and OH–PBDEs.
(3) Aer debromination, DE could be fully utilized and
mineralized by P. putida strain and 67% of TOC was removed
aer 4 days of aerobic biodegradation.
The results indicated that SiO -nZVI/Pd-microorganisms
2
combined process could be a potential strategy for complete
remediation of refractory POPs such as PCB and PBDEs, espe-
cially in the highly polluted districts.
Acknowledgements
The research was nancially supported by grants from
Guangdong Province Science and Technology Project
(No: 2013B090200016, 2013B021000008, 2015B020215007,
2015B020235009), Joint fund of Guangdong Province (No:
U1401235) and Major Science and Technology Program for the
Industry-Academia-Research Collaborative Innovation of
Guangzhou.
Fig. 8 Aerobic degradation mechanism of DE by P. putida strain.
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0 Z. Fang, X. Qiu, J. Chen and X. Qiu, J. Hazard. Mater., 2011,
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