The UV/Fenton degradation of tetrabromobisphenol a catalyzed by nanocrystalline chromium substituted magnetite

Article abstract of DOI:10.1166/jnn.2014.8967

The heterogeneous UV/Fenton degradation of tetrabromobisphenol A (TBBPA) catalyzed by nanocrystalline Fe₃O₄and Fe_2.04Cr_0.96O₄was investigated, with focus on the influence of UV light and initial pH, degradation pathways and effect of Cr substation. The catalysts were prepared by a precipitation-oxidation method and characterized by chemical analysis. XRD. XAFS. TG-DSC, BET surface area and magnetometer. At pH 6.7 and under UV irradiation, almost complete degradation of TBBPA by Fe_2.04Cr_0.96O₄was accomplished within 240 min. and the leaching Fe ions were negligible. The substitution of chromium greatly increased the BET specific surface area and surface hydroxyl amount, which improved the heterogeneous UV/Fenton catalytic activity of magnetite. Moreover, Cr³⁺on the octahedral sites enhanced the electron transfer process in the magnetite structure to accelerate the ?OH generation. The produced ?OH radicals preferentially attacked the C-Br bonds of TBBPA and then β-cleavaged the C-C bonds between benzene rings and isopropyl groups. The above results are of great significance for well understanding the effect of transition metal substitution on the UV/Fenton catalytic activity of magnetite and prospecting the application of magnetite minerals in environmental purification.

Full text of DOI:10.1166/jnn.2014.8967

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 7307–7314, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
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Printed in the United States of America
The UV/Fenton Degradation of Tetrabromobisphenol A
Catalyzed by Nanocrystalline Chromium
Substituted Magnetite
Yuanhong Zhong1ꢀ3, Xiaoliang Liang1, Zisen He1ꢀ3, Wei Tan1ꢀ3, Hongping He1ꢀ∗,
Runliang Zhu1, Yin Zhong2, Jianxi Zhu1, Peng Yuan1, and Zheng Jiang4
1Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry,
Chinese Academy of Sciences, 511 Kehua Street, Guangzhou 510640, China
2Pearl River Delta Research Center of Environment Pollution and Control, Guangzhou Institute of Geochemistry,
Chinese Academy of Sciences, Guangzhou 510640, China
3University of Chinese Academy of Sciences, Beijing 100049, China
4SSRF, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
The heterogeneous UV/Fenton degradation of tetrabromobisphenol A (TBBPA) catalyzed by
nanocrystalline Fe3O4 and Fe2ꢀ04Cr0ꢀ96O4 was investigated, with focus on the influence of UV light
and initial pH, degradation pathways and effect of Cr substation. The catalysts were prepared by a
Delivered by Publishing Technology to: Purdue University Libraries
precipitation-oxidation method and characterized by chemical analysis, XRD, XAFS, TG-DSC, BET
IP: 62.204.117.89 On: Wed, 28 Oct 2015 14:11:47
surface area and magnetometer. At pH 6.7 and under UV irradiation, almost complete degrada-
Copyright: American Scientific Publishers
tion of TBBPA by Fe2ꢀ04Cr0ꢀ96O4 was accomplished within 240 min, and the leaching Fe ions were
negligible. The substitution of chromium greatly increased the BET specific surface area and sur-
face hydroxyl amount, which improved the heterogeneous UV/Fenton catalytic activity of magnetite.
Moreover, Cr3+ on the octahedral sites enhanced the electron transfer process in the magnetite
•
•
structure to accelerate the OH generation. The produced OH radicals preferentially attacked the
Br bonds of TBBPA and then ꢁ-cleavaged the C C bonds between benzene rings and iso-
C
propyl groups. The above results are of great significance for well understanding the effect of
transition metal substitution on the UV/Fenton catalytic activity of magnetite and prospecting the
application of magnetite minerals in environmental purification.
Keywords: Magnetite,
Isomorphous
Tetrabromobisphenol A.
Substitution,
Catalytic
Activity,
UV/Fenton,
1. INTRODUCTION
metals. Costa et al. indicated the remarkable effect of
Co and Mn substitution on enhancing the reactivity of
magnetite towards H2O2 decomposition and methylene
blue degradation while Ni showed an inhibitive effect.21
Recently, the Fenton catalytic activity of Cr substituted
magnetite also has been investigated in the degradation of
aqueous cationic and anionic dyes.23 It has been proved
that the improvement extent varied with the substitution
level and the degradation mechanism was related to dye
species. In addition, the photo-catalysis of UV/Fenton pro-
cess by substituted magnetites also has attracted great
attention, because the introduction of ultraviolet light into
Fenton system obviously accelerates the pollutant degra-
dation and mineralization.24 For instance, Ti4+ substitution
During the last decade, growing interest has been evoked
in the application of iron oxides in industry and envi-
ronmental processes,1–6 especially for the transition metal
substituted magnetites as catalysts with nano-size.7–14 One
of the most promising applications should be as heteroge-
neous Fenton catalysts in the degradation of organic pollu-
tants, e.g., dyes, phenol and hydroquinone,15–18 due to the
merits of easy handling, low cost, nontoxicity and envi-
ronmental harmony.19–22 It has been widely acknowledged
that the catalytic activity of magnetite is greatly dependent
on the species, valence and occupancy of substituting
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 9
1533-4880/2014/14/7307/008
doi:10.1166/jnn.2014.8967
7307

The UV/Fenton Degradation of TBBPA Catalyzed by Nanocrystalline Chromium Substituted Magnetite
Zhong et al.
in magnetite greatly improved the UV/Fenton degradation
of tetrabromobisphenol A (TBBPA) under neutral pH con-
dition, and the degradation efficiency increased with the
increment of titanium content.25 TBBPA is a widely used
brominated flame retardant, one of persistent organic pol-
lutants (POPs) with negative effect on various aspects of
mammalian and human physiology.26–29 However, to the
best of our knowledge, the application of other substituted
magnetites in the UV/Fenton degradation of TBBPA has
been still quite limited, especially in terms of the process
conditions, degradation pathways and mechanism.
In this study, the catalytic activity of nanocrystalline Cr
substituted magnetite (Fe3−xCrxO4) has been investigated
in the heterogeneous UV/Fenton degradation of TBBPA.
Effect of initial pH and UV irradiation, degradation path-
ways and reaction mechanism were discussed in detail.
The obtained results are of high importance for well under-
standing the role of substitution on the catalytic activity
of magnetite towards UV/Fenton reaction and developing
novel and efficient catalysts for POPs degradation.
(ARM) were measured on a JR6A spinner magnetometer.
The leaching Fe ion concentration was analyzed on a PE-
3100 flame atomic absorption spectrophotometer (FAAS).
2.2. TBBPA Degradation
TBBPA degradation experiment was carried out in a home-
made photo-reactor with or without UV light source.
Methanol was chosen as the hydrotropic agent to increase
the solubility of TBBPA, and the volume ratio of methanol
to water was optimized at 2:3 in the preliminary experi-
ment. The volume of TBBPA solution was 500 mL. The
dosage of catalyst was 0.5 g L−1 while the initial con-
centrations of TBBPA and H2O2 were 20 mg L−1 and
10 mmol L−1, respectively. The initial pH of solution was
adjusted by HCl (0.1 mol L−1ꢄ and NaOH (0.1 mol L−1
ꢄ
solution. Before the TBBPA degradation experiment, the
suspension containing catalyst and TBBPA was firstly
stirred for 60 min in dark to achieve adsorption equilib-
rium. Then 5.0 mL of suspension was filtered through a
0.22 ꢅm nylon filter to separate the magnetite, and the
equilibrium solution was sampled for TBBPA concentra-
tion analysis. The degradation was initiated by simultane-
ously adding H2O2 and exposing to natural light or UV
light (ꢆ = 365 nm). At given intervals, the reaction solu-
tion was sampled and filtered for TBBPA concentration
analysis.
2. MATERIALS AND METHODS
2.1. Magnetite Synthesis and Characterization
All the chemicals and reagents used in this study were
of analytical grade and used as received. The mag-
netite samples Fe3O4 and Fe2ꢀ04Cr0ꢀ96O4 was synthe-
The TBBPA concentration was analyzed on
a
Delivered by Publishing Technology to: Purdue University Libraries
sized by a precipitation-oxidation method as described
Shimadzu LC-20A high performance liquid chromatogra-
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in literatures.25ꢁ30 From previous researches, all the mag-
netite samples prepared by this method were in the nano-
size range of 20 to 100 nm.13ꢁ24 The chemical analysis
results showed that the molar ratio between Fe and Cr in
Fe2ꢀ04Cr0ꢀ96O4 was about 2:1.
Copyright: American Scientific Publishers
phy (HPLC), equipped with an Inertisil ODS-SP column
(150 mm × 4.6 mm, 5 ꢅm particles). The mobile phase
was a mixture of methanol and 0.2% glacial-acetic-acid
solution at a volumetric ratio of 4:1. The degradation prod-
ucts of TBBPA were analyzed on a gas chromatographic
mass spectrometry (GC-MS), equipped with a Rxi-5ms
fused silica capillary column (DB-5MS, 30 m×0.25 mm×
0.25 ꢅm).
Powder X-ray diffraction (PXRD) patterns were
recorded between 10ꢀ and 80ꢀ (2ꢂ) at a step of 1ꢀ min−1 on
a Bruker D8 advance diffractometer with Cu Kꢃ radiation
(40 kV and 40 mA). BET specific surface area was mea-
sured on the basis of N2 physisorption capacity at 77 K
on an ASAP 2020 instrument, after degassed at 433 K
for 12 h. X-ray absorption fine structure (XAFS) spectrum
of Fe2ꢀ04Cr0ꢀ96O4 was obtained on the new Wiggler beam-
line BL14W1 of Shanghai Synchrotron Radiation Facility
(SSRF). The storage ring was operated at 3.5 GeV elec-
tron energy and 150–300 mA beam current. The BL14W1
was a focused X-ray beamline, using a Si (111) dou-
ble crystal monochromator. The Cr K-edge absorption
spectrum was recorded in the transmission mode. Data
analyses were performed using IFEFFIT software pack-
age. Thermogravimetric and differential scaning calorime-
try (TG-DSC) analyses were synchronously performed on
a Netzsch STA 409 PC instrument. Approximately 10 mg
of finely ground sample was heated in a corundum cru-
3. RESULTS AND DISCUSSION
3.1. Magnetite Samples Characterization
The PXRD patterns (Fig. 1) of magnetite samples
well correspond to the standard card of magnetite
(JCPDS: 19-0629), indicating that the synthetic samples
have well crystallized spinel structure. The lattice parame-
ters (a0) for Fe3O4 and Fe2ꢀ04Cr0ꢀ96O4 (Table I) are 0.8392
and 0.8403 nm, and the average crystal sizes are 25.9
and 25.4 nm, which shows that the prepared samples are
nanocrystalline magnetites. Cr substitution does not obvi-
ously impact on the crystal structure of magnetite.
XAFS characterization was carried out to probe the
valence and occupancy of chromium in the substituted
magnetite samples. The detailed discussion for X-ray
absorption near-edge spectrum (XANSE) of Fe2ꢀ04Cr0ꢀ96O4
has been presented elsewhere.32 The Cr atoms in
Fe2ꢀ04Cr0ꢀ96O4 are in valence of +3 and occupy the octahe-
dral sites. Herein, extended X-ray absorption fine structure
ꢀ
cible with heating rate 10 C min−1 under N2 atmosphere.
Point of zero charge (pHpzcꢄ was determined by acid-
base potentiometric titration.31 Isothermal remanent mag-
netization (IRM) and anhysteretic remanent magnetization
7308
J. Nanosci. Nanotechnol. 14, 7307–7314, 2014

Zhong et al.
The UV/Fenton Degradation of TBBPA Catalyzed by Nanocrystalline Chromium Substituted Magnetite
Figure 2. Fourier transform curve (full line) from the Cr K-edge
EXAFS spectrum of Fe2ꢀ04Cr0ꢀ96O4 and the best fitting spectrum (dashed
line).
Figure 1. X-ray diffraction patterns of Fe3O4 and Fe2ꢀ04Cr0ꢀ96O4.
(EXAFS) analysis was performed to determine the local
atomic structure of Cr in Fe2ꢀ04Cr0ꢀ96O4. Figure 2 shows
the Fourier Transform (FT) curve and the best fitting char-
acteristic of Fe2ꢀ04Cr0ꢀ96O4. On the FT curve, the first peak
is ascribed to the first-nearest Cr–O shell while the second
one is ascribed to the first-nearest Cr–Cr shell.33 From the
fitted crystal structure of Fe2ꢀ04Cr0ꢀ96O4, Cr atom is coordi-
nated with six nearest oxygen atoms and the Cr–O distance
The magnetic properties of two magnetite samples were
induced by the alternating field to investigate the step-
wise IRM acquisition and the demagnetization curve at
room temperature. The obtained results show that both the
nanocrystalline magnetites are magnetic materials (Fig. 4
and Table I), which are benefit for their recycle in future
application. Compared to Fe3O4, the SIRM decreases
about 61% after Cr substitution. The SIRM value of
is 2.055 Å (Table II). ThDe efiltitviengrecdurbvyePwueblllicsohrirnegspToencdhs ntoology to: Purdue University Lib−ra1ries
Fe2ꢀ04Cr0ꢀ96O4 is 6.1 emu g at 1000 mT while that of
−1
IP: 62.204.117.89 On: Wed, 28 Oct 2015 14:11:47
the obtained curve of Fe2ꢀ04Cr0ꢀ96O4, indicating that the Cr
Fe3O is 15.7 emu g . Fe2ꢀ04Cr0ꢀ96O4 with lower SIRM
Copyright: American Scient4ific Publishers
atoms in Fe2ꢀ04Cr0ꢀ96O4 are in the center of octahedrons.
possesses the higher ARM. These observed variations in
magnetic parameters might be ascribed to the substitution
of Cr3+ for Fe3+ in magnetite structure and the decrease
in grain size by Cr substitution.36
As shown in Figure 3 and Table I, the BET specific
surface area of Fe2ꢀ04Cr0ꢀ96O4 (110.0 m2 g−1ꢄ is about six
times larger than that of Fe3O4 (18.9 m2 g−1ꢄ. The isomor-
phous substitution of chromium obviously increases the
specific surface area of magnetite.
3.2. TBBPA Degradation
The surface hydroxyl (M–OH) are the functional groups
on iron oxide surface and have vital influence on surface
properties, e.g., surface acidity (Brønsted acid and Lewis
acid), adsorption34 and photocatalysis.35 The amount of
surface hydroxyl in two magnetite samples was determined
by TG-DSC analysis (Table I).23 The surface hydroxyl
amount of Fe2ꢀ04Cr0ꢀ96O4 (6.60%) is much higher than
that of Fe3O4 (0.48%), demonstrating that Cr substitution
greatly increases the surface hydroxyl amount.
3.2.1. Effect of UV Radiation
The degradation processes of TBBPA through heteroge-
neous Fenton reaction catalyzed by Fe2ꢀ04Cr0ꢀ96O4 at near
neutral pH were investigated with or without UV-light
source (Fig. 5). Only about 4% of TBBPA was removed
under natural light. For the system with UV radiation in
wavelength of 365 nm, the TBBPA removal was rather
efficient. 20 mg L−1 of TBBPA was almost degraded after
240 min reaction.
•
In the Fenton degradation without UV radiation, OH
radical was produced by the reaction between
FeII
Table I. The characterization results of Fe3O4 and Fe2ꢀ04Cr0ꢀ96O4.
on magnetite surface and H2O2 through the Haber-Weiss
mechanism (Eq. (1)). But it was obvious that the amount
Parameters
Fe3O4
Fe2ꢀ04Cr0ꢀ96O4
Lattice parameter, a0/nm
Average crystal size/nm
BET specific surface/m2 g−1
Surface hydroxyl amount/%
pHapzc
SIRMb/emu g−1
ARMb/A m−1
0ꢀ8392
25ꢀ9
18ꢀ9
0ꢀ48
6ꢀ96
15ꢀ7
0ꢀ044
−32ꢀ3
0ꢀ8403
25ꢀ4
110ꢀ2
6ꢀ60
6ꢀ85
6ꢀ1
0ꢀ070
−23ꢀ7
Table II. Fitting parameters of Cr3+ EXAFS in magnetite.
Shell
Na
Rb (Å)
ꢇ2 (Å2)c
Cr–O
Cr–Cr
6
6
2.055 (0.011)
2.961 (0.026)
0.002
0.008
Hc/mT
Notes: aN: the number of backscattering atoms around the absorbing Cr atom; bR:
the absorber-backscatterer distance; cꢇ2: the Debye-Waller value.
Notes: apHpzc: point of zero charge; broom temperature.
J. Nanosci. Nanotechnol. 14, 7307–7314, 2014
7309

The UV/Fenton Degradation of TBBPA Catalyzed by Nanocrystalline Chromium Substituted Magnetite
Zhong et al.
Figure 5. Effect of UV light on TBBPA degradation.
Figure 3. Nitrogen adsorption–desorption curves of Fe3O4 and
Fe2ꢀ04Cr0ꢀ96O4.
pKa2.38 The aqueous solubility and degradation efficiency
of TBBPA should be pH dependent. The effect of pH on
the TBBPA adsorption and degradation by Fe2ꢀ04Cr0ꢀ96O4
was investigated at different initial pH (Fig. 6). Obvi-
ously, the adsorption efficiency was strongly dependent
on the initial pH. TBBPA was hardly adsorbed on the
Fe2ꢀ04Cr0ꢀ96O4 surface at pH above 7.7, but the adsorp-
tion efficiency gradually increased with the decrease of
pH under 7.7. The adsorption was more favorable in
acid condition. In the tested pH range, TBBPA existed
•
of produced OH radicals was not enough for complete
degradation of TBBPA. But the introduction of UV irradi-
ation greatly accelerated the degradation of TBBPA. From
previous research, the vital role of UV light in UV/Fenton
•
reaction may originate from two aspects. The OH rad-
icals were generated by photolysis of H2O2 (Eq. (2)).37
•OH radical was the most active oxidant with a quite high
redox potential and could destroy the TBBPA molecule.
Moreover, under UV radiation FeIII could be effectively
reduced to FeII with stronger catalytic activity for Fen-
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as molecule and anion. When the initial pH was below
ton reaction (Eq. (3)).
IP: 62.204.117.89 On: Wed, 28 Oct 2015 14:11:47
the pKa of TBBPA (pKa = 7ꢀ5), TBBPA was adsorbed
Copyright−: American Scientific1Publishers
1
FeII +H2O2 → FeIII + OH+ OH
(1)
(2)
(3)
•
•
onto the magnetite surface in form of molecular. But
when the solution pH was above pKa1 of TBBPA, TBBPA
was mainly anionic and the magnetite surface was nega-
tively charged. The electrostatic repulsion between TBBPA
and magnetite surface was dominant, resulting in the
decrease of TBBPA adsorption. Similar effect of initial
pH was previously reported in TBBPA adsorption on
soil.39
H2O2 +hꢈ → 2•OH
FeIII +H2O2 +hꢈ → FeII +2•OH
3.2.2. Effect of pH
TBBPA is an ionizable hydrophobic organic compound
and its ionized constant pKa was 7.5 for pKa1 and 8.5 for
Figure 4. The magnetization curves of Fe3O4 and Fe2ꢀ04Cr0ꢀ96O4 at room temperature. ((a) The stepwise IRM acquisition until achieving saturation;
(b) The alternating field demagnetization curves for determination of remanent coercivity).
7310
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Zhong et al.
The UV/Fenton Degradation of TBBPA Catalyzed by Nanocrystalline Chromium Substituted Magnetite
Table III. The kinetic parameters of TBBPA UV/Fenton degradation in
different pH solution.
pH
102kapp/min−1
t1/2/min
R2
10.8
10.0
9.2
8.3
7.7
6.7
4.8
3.8
3.5
3.2
1.37
1.19
1.59
1.44
1.08
1.79
1.19
0.68
0.46
0.20
42ꢀ9
56ꢀ4
37ꢀ0
43ꢀ6
53ꢀ6
16ꢀ1
32ꢀ8
57ꢀ5
84ꢀ2
176ꢀ6
0.993
0.993
0.986
0.986
0.983
0.962
0.951
0.948
0.931
0.982
Figure 6. Effect of intial pH on TBBPA adsorption and degradation by
Fe2ꢀ04Cr0ꢀ96O4.
Table III shows the rate constant kapp and half-life t1/2
for TBBPA degradation in Figure 7. Among these pro-
cesses, the maximum degradation efficiency of TBBPA
catalyzed by Fe2ꢀ04Cr0ꢀ96O4 was obtained at initial pH 6.7,
with the highest kapp (0.0179 min−1ꢄ and 16.1 min needed
for 50% TBBPA removal. The kapp slightly decreased
along with the initial pH increased to the alkaline range,
and significantly decreased when the initial pH decreased
to 3.2. At initial pH 3.2, the t1/2 for TBBPA degrada-
tion was about 176.6 min, over 10 times more than that
at pH 6.7. The comparative study between adsorption and
degradation showed that higher adsorption capacity did not
But for TBBPA degradation, the pH effect was com-
pletely different (Fig. 6). The removal efficiency was about
56% at the pH of 3.2. With the pH increase from 3.2 to
6.7, close to the pHpzc of Fe2ꢀ04Cr0ꢀ96O4 (pHpzc = 6ꢀ85), the
TBBPA removal efficiency gradually increased. However,
in the pH range of 6.7 to 10.8, the TBBPA removal did not
increase obviously. The results suggested that the degrada-
tion efficiency of TBBPA were dependent to the pHpzc of
magnetite rather than the pKa of TBBPA.
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result in higher degradation efficiency, suggesting that the
The effect of pH on degradation kinetics of TBBPA
were also investigated (Fig. 7). All the degradation pro-
cesses were well fitted by the pseudo-first-order rate
equation (Eq. (4)) with correlation coefficients higher
than 0.93.
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role of magnetite in UV/Fenton degradation was mainly
Copyright: American Scientific Publishers
•
to decompose H2O2 for OH radical generation but not to
adsorb TBBPA for degradation.
The initial pH not only influenced the photo-catalytic
performance of catalysts, but also affected the Fe leach-
ing from catalyst surface.40 From previous research, when
the solution pH was below 4.0, the iron dissolution
became obvious and the degradation would be homo-
geneous process.41 Thus, to optimize the initial pH for
−lnꢉCt/C0ꢄ = kapp
t
(4)
where C0 and Ct are the TBBPA concentrations at the
initial time and different reaction time t, mg L−1. kapp is
the apparent pseudo-first-order rate constant, min−1
.
Figure 7. Kinetic process of TBBPA degradation through UV/Fenton reaction catalyzed by Fe2ꢀ04Cr0ꢀ96O4 (a) and fitted by pseudo-first-order equation
(b) (TBBPA: 20 mg L−1, H2O2: 10 mmol L−1, catalyst dosage: 0.50 g L−1; 500 mL, 25 ꢀC).
J. Nanosci. Nanotechnol. 14, 7307–7314, 2014
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The UV/Fenton Degradation of TBBPA Catalyzed by Nanocrystalline Chromium Substituted Magnetite
Zhong et al.
Figure 9. Comparison in the TBBPA degradation efficiency through
UV/Fenton reaction between Fe3O4 and Fe2ꢀ04Cr0ꢀ96O4. (TBBPA:
20 mg L−1, H2O2: 10 mmol L−1, catalyst dosage: 0.50 g L−1; 500 mL,
pH: 6.5, 25 ꢀC).
Figure 8. The pH variety before and after UV/Fenton reaction cat-
alyzed by Fe2ꢀ04Cr0ꢀ96O4.
maximum removal efficiency of TBBPA, it was necessary
to simultaneously track the pH variety during degradation
to avoid the Fe leaching. As shown in Figure 8, when
the initial pH varied from 3.2 to 7.7, all the final pH
after 240 min reaction increased to the value close to the
pHpzc of Fe2ꢀ04Cr0ꢀ96O4, ascribed to the buffering effect of
hydroxyl groups on magnetite surface. But for the systems
with initial pH higher than 7.7, the final pH decreased but
was still higher than 6.0, which was ascribed to the appear-
surface directly accepted photo-generated holes under
•
UV irradiation.43ꢁ44 and adsorbed H2O2 to produce OH
radicals.45ꢁ46
Besides, the catalytic activity of magnetite was mainly
dependent on the octahedral cations in spinel structure, as
the octahedral sites were almost exclusively exposed at
the surface of the spinel structure.16 As Cr3+ almost occu-
pied the octahedral site, it improved the electron transfer
between Cr3+ and Fe2+, and the reduced Cr2+ could partic-
ipate in the heterogeneous Fenton reaction on the thermo-
dynamic view (Eq. (5)). It should be another reason for the
improvement of UV/Fenton catalytic activity of magnetite
by Cr substitution.
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ance of several medium acidic products during TBBPA
degradation. Based on the degradation efficiency, pH vari-
IP: 62.204.117.89 On: Wed, 28 Oct 2015 14:11:47
Copyright: American Scientific Publishers
ety and further application potential, the optimum initial
pH should be neutral. The concentration of leaching iron
in the system with initial pH 6.7 was also tracked to see
the reusability of Fe2ꢀ04Cr0ꢀ96O4. All the dissolved iron con-
centrations during the degradation were below detection
limit (DL) of FAAS, indicating that TBBPA degradation
by Fe2ꢀ04Cr0ꢀ96O4 at initial pH 6.7 was the heterogeneous
process.
•
•
CrII +H2O2 → CrIII + OH+ OH−ꢁ
(5)
ꢊE = 2ꢀ184 V
Additionally, the reused Fe2ꢀ04Cr0ꢀ96O4 still retained its
catalytic activity after three cycles (data not showed).
Based on the stronger activity and good reusability of
Fe2ꢀ04Cr0ꢀ96O4, it should be a promising catalyst for
TBBPA degradation.
3.3. Improvement Mechanism of
Cr Substitution
From previous researches, Fenton and UV/Fenton catalytic
activity of magnetite were improved by Cr3+ 23 and Ti4+
substitution,25 and the improvement extent depended on
the substitution level. Additionally, with larger specific
surface area and more surface hydroxyl amount, mag-
netite exhibited better catalytic performance in organic
pollutant degradation. As seen in Figure 9, the pres-
ence of Cr3+ in magnetite structure obviously improved
the UV/Fenton catalytic activity of magnetite in TBBPA
degradation. It should be related to the increased spe-
cific surface area and surface hydroxyl amount of mag-
netite by Cr substitution (Table I).23ꢁ25 With the increase
of specific surface area of magnetite, the contact fre-
quency between magnetite surface and reactant molecules
was enhanced,42 which increased the reaction rate.
Moreover, the superficial hydroxyl groups on magnetite
3.4. Degradation Pathways
To further investigate the catalytic activity of Fe3O4 and
Fe2ꢀ04Cr0ꢀ96O4 in heterogeneous UV/Fenton degradation of
TBBPA, the degradation products in their existing sys-
tems were identified by GC-MS. The concentrations of
detected degradation products were determined by com-
paring their relative peak area to that of TBBPA. In the
system containing Fe3O4, eight intermediate products were
found after 240 min degradation. About 40% (relative
percentage composition) of the detected compounds was
TBBPA and approximate 45% were tribromobisphenol
A (TriBBPA, Product 1), suggesting that the dominant
degradation stage in this system was the release of first
bromine from TBBPA. Another main residual product
7312
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Zhong et al.
The UV/Fenton Degradation of TBBPA Catalyzed by Nanocrystalline Chromium Substituted Magnetite
Figure 10. Proposed reaction schemes for TBBPA degradation in UV/Fenton reaction catalyzed by Fe2ꢀ04Cr0ꢀ96O4. (The uncertain substituent position
of bromines was displayed in dotted boxes).
was identified as phenol (2), about 8.6%. The individual
UV radiation enhanced the TBBPA degradation. All the
degradation processes at different initial pH were well
fitted by the pseudo-first-order equation in kinetics. The
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ratio of other six residual products was less than 2.0%.
IP: 62.204.117.89 On: Wed, 28 Oct 2015 14:11:47
They were 4-isopropylene-2,6-debromophenol (3), 4-
(2-hydroxyisopropyl)-2,6-dibromophenol (4), 4-isopropyl-
2,6-dibromophenol (5), monobromobisphenol A (6) and
two isomers of dibromobisphenol A (7 and 8).25ꢁ28ꢁ47–49
Due to the stronger catalytic activity of Fe2ꢀ04Cr0ꢀ96O4,
TBBPA in its system was almost completely degraded,
and only six residual products were found after 240 min
reaction. 63% of the products were identified as phenol
(Products 2 in Fe3O4 containing system), and the relative
percentage of Products 1 and 4–6 were 11.2%, 6.2%, 2.0%
and 9.3%, respectively. Besides, Product 3 was not found,
but one isomer of dibromobisphenol A (7) with percentage
of 8.3% was detected. Therefore, both of the debromina-
tion and ꢋ-scission reaction were the dominant pathways
for TBBPA degradation in UV/Fenton reaction catalyzed
Copyright: American Scientific Publishers
chromium substitution obviously increased the BET sur-
face area and superficial hydroxyl amount, and accel-
•
erated OH radical generation, both of which improved
the TBBPA degradation. In the UV/Fenton system cat-
•
alyzed by Fe2ꢀ04Cr0ꢀ96O4, the generated OH radicals pref-
erentially attacked the C Br bonds of TBBPA and then
ꢋ-cleavaged the C C bonds between benzene rings and
isopropyl groups. The above results are of high importance
for well understanding the effect of Cr substitution on the
UV/Fenton catalytic activity of magnetite, and prospect-
ing the application of magnetite minerals for environmental
purification.
Acknowledgments: This is contribution No. IS-1594
from GIGCAS. We would like to thank Shanghai Syn-
chrotron Radiation Facility (SSRF) for providing us the
beam time for the XAFS measurement. This work was
financially supported by the National Natural Science
Foundation of China (Grant Nos. 41172045 and 41103056)
and Shanghai Tongji Gao Tingyao Environmental Science
and Technology Development Foundation (STGEF).
•
by Fe2ꢀ04Cr0ꢀ96O4 (Fig. 10). Therefore, the OH radicals
may preferentially attack the C Br bonds of TBBPA, and
then ꢋ-cleavage the C C bonds between benzene rings
and isopropyl groups. It was worth mentioning that all
these products in this system were not detected finally
and TBBPA was mineralized through the heterogeneous
UV/Fenton reaction catalyzed by Fe2ꢀ04Cr0ꢀ96O4.
References and Notes
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