Efficient H2O2 oxidation of organic pollutants catalyzed by supported iron sulfophenylporphyrin under visible light irradiation

Article abstract of DOI:10.1021/jp036054n

An efficient method has been developed to degrade toxic and nonbiodegradable organic pollutants such as Sulforhodamine B (SRB) and 2,4-dichlorophenol (DCP). It is based on the photocatalytic power of iron tetrasulfophenylporphyrin supported on a commercial anionic ion-exchange resin (Amberlite IRA 900) (FePR) to activate an oxidant H₂O₂ in aqueous media. Visible light irradiation (?? > 450 nm) significantly accelerates the degradation process. The new catalyst is effective over a wide pH range, and can be easily recycled by filtration. The SRB and DCP were mineralized with yields of 56% and 68% at a catalyst/substrate molar ratio of 1:33 and 1:535, respectively. Moreover, the supported-catalyst would suppress greatly the undesirable side-reaction of H₂O₂: conversion to O₂. UV-vis spectroscopy, high-performance liquid chromatography, ion chromatography, IR, spin-trapping electron paramagnetic resonance, surface photovoltage spectroscopy, and total organic carbon measurements were used to examine the photoreaction processes. The photocatalytic degradation pathways mainly involve the formation and reaction of ?·OH radicals. On the basis of the experimental results, a possible reaction mechanism is proposed.

Full text of DOI:10.1021/jp036054n

J. Phys. Chem. B 2004, 108, 7263-7270
7263
Efficient H2O2 Oxidation of Organic Pollutants Catalyzed by Supported Iron
Sulfophenylporphyrin under Visible Light Irradiation
Yingping Huang, Jing Li, Wanhong Ma, Mingming Cheng, and Jincai Zhao*
Laboratory of Photochemistry, Center for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, China
Jimmy C. Yu
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, N.T., Hong Kong, China
ReceiVed: July 15, 2003; In Final Form: December 25, 2003
An efficient method has been developed to degrade toxic and nonbiodegradable organic pollutants such as
Sulforhodamine B (SRB) and 2,4-dichlorophenol (DCP). It is based on the photocatalytic power of iron
tetrasulfophenylporphyrin supported on a commercial anionic ion-exchange resin (Amberlite IRA 900) (FePR)
2 2
to activate an oxidant H O in aqueous media. Visible light irradiation (λ > 450 nm) significantly accelerates
the degradation process. The new catalyst is effective over a wide pH range, and can be easily recycled by
filtration. The SRB and DCP were mineralized with yields of 56% and 68% at a catalyst/substrate molar
ratio of 1:33 and 1:535, respectively. Moreover, the supported-catalyst would suppress greatly the undesirable
2 2 2
side-reaction of H O conversion to O . UV-vis spectroscopy, high-performance liquid chromatography, ion
chromatography, IR, spin-trapping electron paramagnetic resonance, surface photovoltage spectroscopy, and
total organic carbon measurements were used to examine the photoreaction processes. The photocatalytic
degradation pathways mainly involve the formation and reaction of ‚OH radicals. On the basis of the
experimental results, a possible reaction mechanism is proposed.
Introduction
Degradation of persistent and nonbiodegradable organic
show that the iron tetrasulfophthcyline (FePcS) supported on a
resin can catalyze H2O2 to degrade Orange-II and salicylic acid
1-4
under visible irradiation without the addition of CH3CN.
However, FePcS supported on the resin exhibits scarce photo-
catalytic activity for the degradation of some organic pollutants
such as 2,4-dichlorophenol and also gives much low activity at
neutral pH values. Clearly, there is a demand for new supported
metal complexes that possess excellent catalytic activity, high
pollutants by oxidation with H2O2 has been studied extensively.
-
5
6
It has also been found that oxidants including HSO5 , O2,
and H2O27
-10
could be activated catalytically by metal com-
10
7,9
plexes such as metalloporphyrin, metallphathcyline, and
tetraamido macrocyclic ligand. In fact, these metal complexes
8
can mimic the action of natural enzymes such as peroxidase
18
1
1,12
stability, and ease of posttreatment separation. Also, it is very
important to further investigate the photocatalytic reaction
mechanism for the degradation of organic pollutants in water
using supported metal complexes as photocatalysts.
and P450 in activating H2O2 and O2 in aqueous media.
Despite their potential, these synthetic catalysts have a serious
flaw. They have fewer binding sites than their natural enzyme
counterparts, and thus exhibit lower catalytic activity and poorer
stability than peroxidase and P450.13 Two approaches for
improvement have been proposed by using (1) a heterogeneous
Herein, we describe the development of a new catalytic
system consisting of iron tetrasulfophenylporphyrin supported
on a commercial anionic ion-exchange resin (Amberlite IRA
supported catalyst system, and (2) a homogeneous supra-
molecular metal complex system.1
4,15
Appropriate support
900) (FePR). This system is highly effective in activating H O2
2
materials, such as neutral organic polymers, ion exchange
membranes or resins, and inorganic materials (clay, zeolites,
for the photooxidation of an organic dye (sulforhodamine B,
SRB) and a small molecular compound (2,4-dichlorophenol,
DCP) under visible light irradiation (λ > 450 nm). The
effectiveness of this system is compared with an analogous
process involving a homogeneous FeTPPS4 catalyst in solution.
The results indicate that both organic dyes and small organic
compounds can be effectively degraded under visible light
irradiation. The mineralization yields of SRB and DCP are 56%
and 68%, respectively. In particular, FePR shows 3.5 times
higher activity for the degradation of SRB than FeTPPS4 does
under the otherwise identical experimental conditions. The
stability of FePR is also much higher than that of FeTPPS4 under
the same illumination intensity. Interestingly, the FePR catalyst
can actually inhibit the direct decomposition of H2O2 to O2 in
the photocatalytic system. In addition, the supported catalyst
etc.), would provide binding sites that allow the catalytic
oxidation reaction to occur more efficiently.1
6,17
Another
advantage of a supported catalyst system is the ease of separation
from the reaction solution by filtration.18 Meunier el al.
reported a novel system in which iron tetrasulfophthalocyanine
19
(FePcS) supported on an anionic resin was used as catalyst to
degrade chlorophenols in an acetonitrile/H2O solvent mixture
using KHSO5 as an oxidant. However, the relatively low
catalytic activity and a need for an organic cosolvent render
this method impractical for pollution treatment. Recently,
20
preliminary results reported by our group in a communication
*
Author to whom correspondence should be addressed. Fax: (+86)-
1
0-8261-6495. E-mail: jczhao@iccas.ac.cn.
1
0.1021/jp036054n CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/04/2004

7264 J. Phys. Chem. B, Vol. 108, No. 22, 2004
Huang et al.
FePR can be separated easily from the reaction system and be
reused. It is a significant advantage in the environmental
treatment of toxic nonbiodegradable organic pollutants.
(10 mM), and an eluent composed of Na2CO3 (1.8 mM) and
NaHCO3 (1.7 mM), respectively. The photodegradation of DCP
(no absorption in the visible region) was analyzed by high-
performance liquid chromatography (HPLC) on an inersil
ODS-3 5-µm column (250 × 4.6 mm) at room temperature.
The HPLC system consisted of a Dionex P580 pump and a built-
in UVD 340S diode array detector. DCP was detected at 304
nm by using an eluent composed of methanol/water (70%/30%
v/v) at a flow rate of 1.0 mL/min. An Apollo 9000 TOC
instrument was used for measurements of total organic carbon
Experimental Section
Materials. Sulforhodamine B (SRB, Acros) (see the structure
below), Organe II (Acros), 2,4-dichlorophenol (DCP), and H2O2
were of analytical reagent grade and were used without further
purification. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO, Sigma)
was used as the ESR spin-trapping reagent. Tetraphenylporphine
tetrasulfonic acid (TPPS4) was purchased from Tokyo Kasei
Kogyo Co., Ltd. The anionic exchange resin containing am-
monium groups (macroreticular resin of moderately high
porosity with benzyltrialkylammonium functionality, Amberlite
IRA-900) was obtained from Aldrich. Horseradish peroxidase
(TOC) values of the degraded solutions. The concentration of
H2O2 was measured by the POD method in which the DPD is
oxidized by H2O2 based on the POD-catalyzed reaction (ꢀ )
4
-1
-1 21
2+
3+
2
.1 × 10 M cm ). The concentrations of Fe or Fe
after reduction by hydroxylammonium chloride were measured
spectrophotometrically by a 1,10-phenanthroline method with
(POD) was purchased from Humei Biologic Engineering Co.,
-
7
2+
4
a detection limit of 1.8 × 10 mol/L for Fe (ꢀ ) 1.1 × 10
cm ).
Surface photovoltage spectroscopy of photocatalysts was
and N,N-dimethyl-p- phenylenediamine (DPD) reagent was
purchased from Merck. NaOH and HClO4 solutions were used
to adjust the pH of the solution. Deionized and doubly distilled
water was used throughout this study. FeTTPS4 was prepared
-
1
-1 22
M
measured by surface photovoltage Stanford research systems
equipped with a model SR 830 DSP lock-in amplifier. Infrared
analysis was carried out with a TENSOR 27 (Bruker) FTIR
spectrophotometer. The samples for the photodegradation of
SRB were prepared as follows. The reacted solution was filtered,
and the filtrate was evaporated (temperature below 323 K) under
reduced pressure until water was mostly removed. Finally, the
dried samples were further kept in a vessel containing P2O5 for
more than 24 h. The samples for IR were supported on
anhydrous KBr. A Brucker model EPR 300E spectrometer
equipped with a Quanta-Ray Nd:YAG laser (355 and 532 nm)
was used for measurements of the electron paramagnetic
resonance (EPR) signals of radicals spin-trapped by DMPO.
The settings were the following: center field ) 3486.7 G, sweep
width ) 100.0 G, microwave frequency ) 9.82 GHz, and power
-
3
by dissolving 40 mg of TPPS4 and 10 mL of 4.3 × 10 mol/L
Fe(ClO4)3 with 10 mL of water into a rockered flask. The
solution was held at 100 °C with refluence for 10 h until the
Sort band of TPPS4 (420 nm) disappeared and characteristic
3
+
band of FeTPPS4 (380 nm) appeared. Excessive Fe was
eliminated by cationic ion-exchange resin (Amberlite IR-120B).
The Amberlite IRA 900 resin, after being ground and sieved
by 200 meshes, was pretreated by alcohol, HCl, NaOH solution,
and water in turn to remove the impurities. FePR with different
exchange quantities were prepared by the addition of 500 mg
of the pretreated Amberlite IR A 900 to an aqueous solution of
FeTPPS4 at different concentrations after 24 h of gentle magnetic
stirring. The exchange amount of FeTPPS4 onto resin also was
calculated by measuring the concentration of FeTPPS4 in the
solution by a spectrophotometric method. The FePR catalyst
with a different exchange amount of FeTPPS4 onto resin was
separated by filtration, and washed with water until no FeTPPS4
could be detected in the washing solution. The catalysts were
dried in air at room temperature and then at 65 °C for 48 h.
The amount of saturated exchange for FeTPPS4 by 1 g of resin
is 33 µmol. The catalysts with exchange amounts of 33, 15,
and 8 µmol/g of resin, respectively, were prepared. The FePR
catalyst of 15 µmol/g resin exchange amount exhibited the
highest activity compared to the other two catalysts with
different exchange amounts, and therefore was used for all
experiments.
)
5.05 mW. To minimize experimental errors, the same quartz
capillary tube was used for all EPR measurements. A Trio-2000
model gas chromatography/mass spectroscopy (GC/MS) equipped
with a BPX5 column, size 30 m × 0.25 mm, was used to
analyze the photocatalytic reaction intermediates. Samples were
prepared as follows: several dispersions (40 mL) containing
-4
SRB (1.25 × 10 M), FePR (10 mg, 15 µmol FeTPPS4 per
gram of catalyst), were prepared. The absorption/desorption
equilibrium was established between SRB and the resin with
-2
stirring for about 8 h. After H2O2 (1.0 × 10 M) was added,
samples were irradiated with visible light at different time
intervals. Then, the supported catalyst (FePR) was removed by
filtration. Subsequently, the water in the filtrate was removed
under reduced pressure (below 60 °C). The remaining residue
was dissolved in methanol.
Photoreactor and Light Source. A 500-W halogen lamp
(Institute of Electric Light Source, Beijing) used as the visible
light source was positioned inside a cylindrical Pyrex vessel
surrounded by a jacket with circulating water (Pyrex) to cool
the lamp. A cutoff filter (diameter ) 3 cm) was used to
completely remove wavelengths less than 450 nm and to ensure
irradiation only by visible light (λ > 450 nm). The distance
between the reaction vessel and light source was 10 cm.
Procedures and Analyses. Unless otherwise noted, all the
experiments were carried out in a Pyrex vessel (60 mL) in
aerated solutions. At given irradiation time intervals, 3-mL
samples were collected and analyzed immediately by observa-
tion of variations in the UV/vis spectra using a Hitachi 3010
spectrophotometer. Also, UV-vis diffuse reflectance spectra
of the FePR and resin blank were determined using the Hitachi
For reference, the structures of SRB and FeTPPS4 are shown
below:
CHART 1: Structures of Sulforhodamine B and
FeTPPS4
3
010 spectrophotometer equipped with integraph (Φ 150 mm).
2-
-
The amounts of SO4 and Cl ions were analyzed by a DX-
20 ion chromatograph (DIONEX) using an eluent of NaOH
1

Efficient H2O2 Oxidation of Organic Pollutants
J. Phys. Chem. B, Vol. 108, No. 22, 2004 7265
Figure 2. Photodegradation of DCP under different conditions: (a)
Figure 1. The degradation of SRB under different conditions: (a) SRB/
DCP/FePR, visible light; (b) DCP/H
2
O
2
, visible light; (c) DCP/H
2 2
O /
FePR, visible light; (b) SRB/H
visible light; (d) SRB/H /FePR, visible light. The reactions a-d were
done at pH 9.0; [SRB] ) 1.25 × 10 M; [FePR] ) 10 mg/40 mL (15
2
O
2
/FePR, in the dark; (c) SRB/H
2
O
2
,
FePR, dark; (d) DCP/H
2 2
O /FePR, visible light. Reactions a-d were
-
3
2
O
2
done at pH 9.0; [DCP] ) 1.0 × 10 M; [FePR] ) 5 mg/40 mL (15
-
4
-2
µmol FeTPPS
4
per g resin); [H
was 48 mW/cm .
O
2 2
] ) 4.0 × 10 M. The light intensity
-
2
2
µmol FeTPPS
4
/g resin); [H
O
2 2
] ) 1.0 × 10 M. The light intensity
2
was 48 mW/cm .
Results and Discussion
Photodegradation of Organic Compounds. SRB and DCP
were employed as model pollutants to examine their degradation
and materialization with FePR as catalyst and H2O2 as oxidant
under visible irradiation (λ > 450 nm). The photodegradation
of organic compounds was carried out after the adsorption/
desorption equilibrium had been established between the resin
and organic compounds. During the degradation of SRB, the
characteristic absorption peak of SRB at 565 nm in UV-vis
spectra diminished and disappeared completely after visible
irradiation for 160 min (Figure 1). The degradation rates of SRB
with and without visible irradiation are shown in the inset of
Figure 1.
In the absence of FePR (curve c) or H2O2 (curve a), the
degradation of SRB was scarcely observable under the visible
irradiation. No obvious degradation of SRB was found in the
dark (curve b). However, very significant degradation of SRB
occurred under visible irradiation for the SRB/FePR/H2O2
system (curve d). It indicated that visible irradiation and the
presence of both photocatalyst and H2O2 are necessary for the
degradation reaction of substrates. The first-order kinetic
constant of the photodegradation of SRB for SRB/FePR/H2O2
-
4
Figure 3. TOC removal of SRB (curve a, [SRB] ) 1.25 × 10 M,
-
2
-3
[H
H
2
O
O
2
] ) 2.0 × 10 M) and DCP (curve b, [DCP] ) 1.0 × 10 M,
-2
2 2
[
2
2
] ) 4.0 × 10 M) in the presence of FePR/H
O as a function
of irradiation time. Initial conditions: The amount of FePR: 10 mg
for SRB, 5 mg for DCP; V ) 40 mL; pH 9.2.
or FePR alone (curve a) under visible irradiation. Visible
irradiation could effectively accelerate the degradation reaction
of DCP in the DCP/FePR/H2O2 system (curve d). This rapid
degradation of DCP that has no absorption above a wavelength
of 310 nm illustrates that the photoexcitation of FePR by visible
light initiates the photocatalytic degradation process and the
photosensitization process induced by the photoexcitation of
substrates does not play a major role.
-
2
-1
was obtained (khν ) 1.3 × 10 min ). However, under the
-
3
otherwise identical experimental conditions, khν was 4.3 × 10
-
1
min for the SRB/FeTPPS4/H2O2 system. FePR exhibited
higher catalytic activity than FeTPPS4. During the photocatalytic
degradation of SRB in the FePR/H2O2 system, the catalyst
surface before irradiation was red-brown in color, due to the
adsorption of SRB. After exposure to visible irradiation for about
The mineralization degree of the organic compounds was
evaluated by determination of the changes in the total organic
carbon (TOC) in the photodegradation of SRB and DCP. After
the photocatalytic reaction proceeded for 300 min and 450 min
for SRB and DCP in the presence of FePR and H2O2 under
visible irradiation, the TOC removal yields for SRB and DCP
were 56% and 68%, respectively (seen Figure 3).
The free SO42 and Cl- ions were also determined in the
degradation of SRB and DCP, respectively, under visible
irradiation by an ion chromatograph (see Figure 4). About 65%
of the SO4 free ion from SRB and 70% of the Cl ion from
DCP were released in the reaction solution after 160 min and
380 min of photocatalytic reaction SRB and DCP of in the
presence of FePR and H2O2 under visible irradiation, respec-
tively. There results indicate that both SRB and DCP underwent
160 min, the surface of the catalyst renewed to the blue (inherent
color of FePR). This suggests that the substrate molecules both
on the surface of the support catalyst and in the bulk solution
can be efficiently degraded, and the catalyst is stable for the
photocatalytic degradation of organic compounds with H2O2 as
oxidant. Meanwhile, no free Fe2 or Fe and FeTPPS4 were
+
3+
-
detected in the degraded solution using the spectrophotometric
method.22
Under similar experimental conditions, the photodegradation
of DCP under visible irradiation was monitored by HPLC using
an eluent of methanol/water (70%:30%) with ultraviolet detec-
tion at 304 nm; the degradation kinetics of DCP is displayed in
Figure 2. The degradation of DCP hardly took place both in
the dark (curve c) and in the presence of H2O2 alone (curve b)
2
-
-

7266 J. Phys. Chem. B, Vol. 108, No. 22, 2004
Huang et al.
Figure 6. Decomposition of H
2
O
2
for the SRB/FePR system under
-4
Figure 4. Evolution of SO 2-
-
visible light irradiation. Initial: [SRB] ) 2.0 × 10 M 40 mL; [H
2 2
O ]
O /FePR,
2 2
4
(curve a) and Cl ions (curve b) during
-
2
)
1.2 × 10 M; [FePR] ) 10 mg/40 mL; pH ) 9.1. (a) H
visible light; (b) SRB/H , visible light, (c) SRB/H /FePR in the
dark; (d) SRB/H /FePR, visible light.
the photodegradation of SRB and DCP, respectively. For curve a, [SRB]
-
5
2
O
2
2 2
O
)
7.5 × 10 M; [FePR] ) 10 mg/40 mL (15 µmol FeTPPS
4
per 1 g
] ) 1.0 × 10 M. pH ) 9.0, for curve b, [DCP] ) 4.0
per 1 g resin);
-
2
2 2
O
resin); [H
2
O
2
-
4
×
10 M; [FePR] ) 5 mg/40 mL (15 µmol FeTPPS
4
-
2
[H
2
O
2
] ) 4.0 × 10 M; pH ) 9.2.
Figure 7. Catalyst recycling in repetitive degradation of SRB (1.25
-
4
Figure 5. Effect of pH on the degradation of SRB (1.25 × 10 M)
-4
-2
×
10 M/run) by H
O
2 2
(1.0 × 10 M) in the presence of FePR (10
-
2
in the presence of FePR (10 mg/40 mL) and H
under visible irradiation.
O
2 2
(1.0 × 10 M)
mg/40 mL).
during the photocatalytic degradation of substrates using FePR
as catalyst under visible irradiation. The decomposition of H2O2
relies on degradation of substrates (see Figure 6). No H2O2
decomposition was observed either when the SRB was absent
under visible irradiation in the presence of catalyst or when the
reaction was performed in the dark in the presence of both
substrates and catalyst. This is significantly different from the
not only a simple discoloration or dechloronation but also an
irreversible decomposition in the photocatalytic reaction.
In addition, FePR as catalyst can also efficiently degrade the
others anionic compounds, such as Orange-II ([Orange-II] )
-
4
-4
1
.25 × 10 M, FePR ) 10 mg/40 mL, [H2O2] ) 5 .5 × 10
-
2
M, pH ) 9.2, the first-order kinetic constant was 1.2 × 10
-
1
-3
min ), salicylic acid (SA) ([SA] ) 1.2 × 10 M, FePR ) 5
mg/40 mL, [H2O2] ) 2 × 10 M, pH ) 9.0, the first-order
3+
-
2
homogeneous photoreaction catalyzed by FeTPPS4 (or Fe /
2
+
-
3
-1
Fe ) in which H2O2 suffers rapid decomposition regardless of
the organic substrates. This rigorous correlation suggests that
present system provides a more economical H2O2 usage for the
photooxidation of organic pollutants.
kinetic constant was 6.2 × 10 min ). Because of the
properties of the resin, which can easily adsorb anionic
compounds, types of the cationic compounds such as Rhodamine
B (RhB) and Malachite Green (MG) were found not obviously
degraded under the same experimental conditions, because they
are hardly adsorbed on the catalyst. However, FeTPPS4 in the
solution can degrade both the cationic and anionic compounds
such as RhB, MG, SRB, and orange-II with H2O2 as oxidant
under visible irradiation, indicating that FePR is a kind of
selective photocatalyst for the degradation of anionic and
nonionic compounds.
The FePR as a kind of heterogeneous photocatalyst can be
easily recycled by a simple filtration. After 5 recycles for the
photodegradation of SRB (0.125 mM SRB/each cycle, 10 mg
catalyst/40 mL), the catalyst did not exhibit any significant loss
of activity (seen Figure 7). Confirming that the FeTPPS4
supported on the resin cannot be obviously degraded during
the photocatalytic oxidation of the pollutant molecules. How-
ever, the FeTPPS4 in homogeneous solution was not stable using
H2O2 as oxidant under visible irradiation. The catalytic activity
of FeTPPS4 in the solution in the second cycle was about 40%
FePR catalyst at a wide pH range from 1 to 11.0 exhibits
excellent catalytic activity for the degradation of SRB. The
greatest degradation rates in 60 min of reaction were found at
pH < 4.0 and pH > 7.8 (see Figure 5). FePR also has very
high catalytic activity, even at natural pH values (pH 6-8). It
seems that the optimal pH value depends on the both the
adsorption property of substrate on the resin and the catalytic
activity of the catalyst. The FeTPPS4 in solution has better
catalytic activity only at pH >11.0 for the degradation of SRB.
More interesting was the behavior of H2O2 decomposition
2
+
3+
of that of the first run. In addition, no Fe or Fe ions as well
as FeTPPS4 in the reaction bulk solution were detected using
2
2
the spectrophotometric method for FePR/SRB/H2O2 system.
An important advantage of this photocatalyst is that the
photocatalyst can be removed easily from the reaction solution
by simple filtrates and reused for the photocatalytic experiments
without significant loss of activity.

Efficient H2O2 Oxidation of Organic Pollutants
J. Phys. Chem. B, Vol. 108, No. 22, 2004 7267
-
1
at 1648 cm is attributed to vibrations of the carbon-nitrogen
2
3
bond. As the photocatalytic reaction preceded, bands of
characteristic vibrations of the carbon-nitrogen bond (1648
-
1
-1
cm ), the C-aryl bond (1342 cm ), and the aromatic ring
-1
(
1594 cm ) decreased with the irradiation time and disappeared
after about 4 h of photoreaction. In addition, bands of the -SO3
-1
-1
group (1182 cm , 676 cm ) also completely disappeared. With
the photocatalytic reaction process, two strong new IR bonds
-
1
-1
at 1440 cm and 1616 cm attributable to carboxylic acid
2
4
and primary amines intermediates appeared. The IR results
indicate that the large conjugated chromophore structure of SRB
was destroyed and further decomposed to smaller organic
species under visible irradiation.
The GC-MS results of the photooxidation intermediates for
SRB/FePR/H2O2 system are presented in Figure 9.
The results of gas chromatography in Figure 9 show that
obviously six different peaks appeared. Among the intermedi-
ates, N,N-dimethylformamide (A), 2-hydroxybenzoic acid (B),
4
-(ethylamino)benzoic acid (C), and 1,3-isobenzofurandione (D),
Figure 8. IR spectrum of the intermediates during the degradation of
-
4
-2
SRB (1.25 × 10 M) in H
2
O
2
(1.0 × 10 M)/FePR (10 mg/40 mL)
respectively, were identified. These products of mainly small
organic acids and amines are all biodegradable. No signals of
the products were observed for the photodegradation of SRB
in the absence of H2O2 under the same experimental conditions
as above or in the presence of FePR and H2O2 in the dark.
system under visible light irradiation at pH 9.0.
The information about active radicals involved for the
photodegradation of SRB with H2O2 as oxidant and FePR as
catalyst was obtained using spin-trapping ESR (Figure 10). No
ESR signals were observed either when the organic substrate
was absent under visible irradiation in the presence of catalyst,
or when the reaction was performed in the dark for the aqueous
SRB/FePR/H2O2 system. Under visible irradiation the charac-
•
teristic quartet peaks of DMPO- OH adducts appeared gradu-
ally in the aqueous SRB/FePR/H2O2 system, and the intensity
increased with irradiation time. The ESR signals of DMPO-
•
-•
OOH/O2 adducts with characteristic six peaks were observed
•
under visible irradiation in the methanol media, since the OOH/
O2 radicals are very unstable and undergo facile dispropor-
tionation rather than slow reaction with DMPO in aqueous
solution, and the OOH/O2 radicals can be trapped by DMPO
and detected in the organic media such as CH3OH even
containing a part of water. However, the intensity is much
weaker (see Figure 10), and its intensity scarcely increased with
irradiation time. It indicated that photocatalytic reaction for SRB/
Figure 9. GC spectra of the degradation products formed in the visible
light-assisted photodegradation of SRB in the presence of FePR and
•
-
H
2
O
2
with retention time. Column: BPX5, 30 m × 0.25 mm.
•
•-
Analysis of the Intermediates in the Photocatalytic Deg-
radation of SRB. The IR spectroscopy was used to monitor
the temporal course of the photodegradation conversion of SRB
2
5
(
Figure 8).
Before irradiation, the band at 1594 cm- corresponds to
1
FePR/H2O2 mainly involved the OH radical. For comparison,
•
-
1
aromatic ring vibrations, whereas the 1342 cm band is due
to C-aryl bond vibrations. The bands at 1182 cm and 676
cm are caused by vibrations of the -SO3 groups. The band
the ESR also was measured for the homogeneous SRB/FeTPPS4/
-
1
•
H2O2 system. The signals of DMPO- OH adducts increased
-
1
with irradiation time (Figure 12).
•
•
Figure 10. The ESR signals of the DMPO- OH (left, aqueous solution) and DMPO- OOH (right, methanol media) adducts for SRB/FePR/H
2
O
2
-
4
-2
system in the dark and under visible irradiation [FePR] ) 5 mg/40 mL; [SRB] ) 1.25 × 10 M; [H
2
O
2
] ) 1.0 × 10 M; [DMPO] ) 0.04 M.

7268 J. Phys. Chem. B, Vol. 108, No. 22, 2004
Huang et al.
•
•
Figure 11. The ESR signals of the DMPO- OH (left, aqueous solution) and DMPO- OOH (right, methanol media) adducts for SRB/FeTPPS
4
/
-
6
-4
-2
H
2
O
2
system in the dark and under visible irradiation [FeTPPS
4
] ) 1.87 × 10 M; [SRB] ) 1.25 × 10 M; [H
2
O
2
] ) 1.0 × 10 M; [DMPO]
)
0.04 M.
•
-•
Figure 12. Comparison of radical intensity of OH and O
2
for the SRB/FePR/H
O
2 2
system (left) and the SRB/FeTPPS
4
/H
2 2
O system (right)
-
4
-2
irradiated by laser (l ) 532 nm). Conditions: [FePR] ) 5 mg/40 mL; [SRB] ) 1.25 × 10 M; [H
2 2
O
] ) 1.0 × 10 M; [DMPO] ) 0.04 M;
-
6
[
FeTPPS
4
] ) 1.87 × 10 M.
SCHEME 1: Proposed Photodegradation Mechanism of Organic Pollutants in the Aqueous H2O2/FePR System under
Visible Light Irradiation
•
Under the same experiment conditions, the intensity of OH
radicals was less than that of the FePR system. The intensity
H2O2 as oxidant under visible irradiation is proposed as shown
in Scheme 1.
•
-•
of OOH/O2 obviously increased with irradiation time for the
FeTPPS4 system (Figure 11). The relative intensity for OH and
OOH/O2 is displayed in Figure 12. Because OOH/O2
III
III
First of all, [Fe PR] forms [HOFe PR] complex in aqueous
•
26
solution. Then, H2O2 as nucleophilie addition reagent com-
•
-•
•
-•
plexes to the axial site of the iron center of the [HOFePR] to
radicals in aqueous solution can easily undergo further dispro-
portionation into O2 that is a side-reaction of H2O2, we can find
from Figure 12 that the supported-FeTPPS4 photocatalyst utilizes
H2O2 more economically than the homogeneous FeTPPS4
system. EPR signals were also measured for the DCP/FePR/
H2O2 system with 532 nm laser irradiation. The EPR signal
intensity of the DMPO- OH adducts was enhanced gradually
with increasing illumination time. The case is the same as that
for the FePR/SRB/H2O2 system under our experimental condi-
III
27
III
give [HOOFe PR] species. Upon visible irradiation, [HOOFe -
III
PR] is converted to [HOOFe PR]* excited-state transition
species, which may undergo intramolecular electron transfer
to generate [Fe PR] and ‚OOH intermediates. However, this
reaction is not significent in our system as evidenced by ESR
experiments. Alternatively, the O-O band cleavage of [HOOFe -
PR]* results in generation of [PRFe dO] and OH radicals
species. The HO radicals are much more active than that of
PRFe dO] species. So, the photodegradation reaction with HO
28
II
29
III
•
Vb
•
30
•
V
•
[
•
tions. No signals of DMPO- OH adducts were observed in the
radicals is predominant in the organics/FePR/H2O2 system under
visible irradiation. To evidence this notion, the influence of
dark under otherwise identical conditions as those in the
photoreaction. It further clarifies that the degradation of organic
compounds is derived mainly by the excitation of catalyst by
visible light irradiation for supported catalyst systems.
•
addition of methanol, a known scavenge of OH radicals, on
the degradation of SRB was investigated under visible light
irradiation for the SRB/FePR/H2O2 system. The results indicated
that addition of methanol decreased significantly the degradation
of SRB at various concentrations of methanol. It further
suggested that active species for the photocatalytic degradation
On the basis of the experimental results and literature
information, a possible photocatalytic reaction pathway for the
degradation of organic compounds with FePR as catalyst and

Efficient H2O2 Oxidation of Organic Pollutants
J. Phys. Chem. B, Vol. 108, No. 22, 2004 7269
Figure 13. UV-vis diffuse reflectance spectra of FePR (curve a) and
Figure 15. Relation between initial rate for the degradation of SRB
the blank resin (curve b). Inset: the absorption spectra of FeTPPS
4
-
4
3
+
(1.25 × 10 M) with FePR (5 mg/40 mL) (a) or FeTPPS (1.87 ×
(
curve c) and Fe (curve d) in aqueous solution at pH ) 9.0 and pH )
4
-
6
-
6
3+
-3
2 2
10 M) (b) and the initial concentration of H O .
2
.6, respectively. [FeTPPS
4
] ) 1.0 × 10 M, [Fe ] ) 4.3 × 10 M.
TABLE 1: Catalytic Characteristics of FePR and FeTPPS4
in Photodegradation of SRB with H
2
O
2
as Oxidant at pH )
9.0
K
m
V
(min)
max
[C]
(10 M)
K
cat
3
-6
(10 M min-1
6
-1
catalyst
(10- mol/L)
)
FePR
FeTPPS
19.14
9.28
0.99
0.27
1.87
1.82
0.53
0.15
4
3
+
FeTPPS4, or Fe -resin alone, suggesting that photochemistry
properties of FePR are more advantageous than those of
FeTPPS4.
We further compared the photodegradation of organic com-
pounds (SRB) catalyzed by FePR and that by FeTPPS4.
Photocatalytic reaction kinetic measurements (Figure 15) were
made to reveal the relation between the degradation rate of SRB
and the initial concentration of H2O2 in the presence of FePR
or FeTPPS4, respectively. The photodegradation rate of SRB
increased with the increasing the initial concentration of H2O2
and then saturated. This phenomenon is similar to the enzyme
Figure 14. Surface photovoltage spectroscopy of FePR (curve a),
3
+
FeTPPS (curve b), and Fe -resin blank (curve c).
4
•
•
by FePR mainly are OH radicals. HO radicals generated will
react immediately with organic pollutants and degrade them
effectively. Without organic pollutants in the [FePR]/H2O2
31
catalytic reaction. So, the kinetics of photodegradation of SRB
with H2O2 using FePR and FeTPPS4 as catalysts were analyzed
in a format similar to that used in enzymatic catalysis.
To set the experimental conditions, the initial concentrations
of H2O2 and SRB were kept in great excess over the concentra-
tion of the catalyst concentration. Then, the Michaelis-Menten
equation is as follows:
•
system, the HO radicals would rapidly recombine with the
V
[
PRFe dO] to renew H2O2. The detailed reaction mechanism
needs further study.
Photochemical Characteristics of FePR. Figure 13 presents
the UV-vis diffuse reflectance spectra of the FePR (curve a)
and the blank resin (curve b). The absorption spectra of FeTPPS4
3
+
1
V
1
1
1
(
curve c) and Fe (curve d) in aqueous solution were also
)
+
shown in the inset of Figure 13.
Vmax [S] Km
The maximum absorbance of FeTPPS4 in aqueous solution
was located at 383 nm and 525 nm. The FePR displays a very
broad range absorbance in 200-700 nm regions with maxima
located at 421 nm, 560 nm, and 620 nm. Such an extended
coverage in the visible spectrum makes FePR possible to utilize
most of the energy from sunlight. The difference in the
absorption spectra between FePR and FeTPPS4 also indicates
the strong interaction between the sulfonate groups of the iron
sulfoporphyrin and ammonium groups of the resin in the FePR
catalyst.
where V represents velocity of degradation of SRB, [S] is the
concentration of H2O2, Km represents the affinity between the
active site of catalyst and substrate, Vmax represents maximal
catalytic reaction velocity of catalyst, Kcat is the catalytic
constant of photocatalytic reaction, and Kcat ) Vmax/[C]. Figure
1
5 shows the relation between the 1/V and the 1/[S].
The results in Table 1 indicated that the activity of FePR is
about 3.5 times that of FeTPPS4 in aqueous solution for the
photodegradation of SRB under visible irradiation. It proves
that the supported catalyst exhibits excellent catalytic activity
in the photocatalytic reaction process.
Surface photovoltage spectroscopy (SPS) is also a useful
method to evaluate illumination-induced charge transfer on the
catalyst surface. The illumination-induced surface photovoltage
Conclusion
3
+
spectra of FePR (curve a), FeTPPS4 (curve b), and Fe resin
blank (curve c) are displayed in Figure 14. It indicated that FePR
could be excited much more effectively to cause charge transfer
by visible irradiation (λmax 421 nm and 480-520 nm) than
Toxic and nonbiodegradable organic pollutants, such as
Sulforhodamine B (SRB) and 2,4-dichlorophenol (DCP), can
be efficiently photodegraded in an aqueous solution by using a

7270 J. Phys. Chem. B, Vol. 108, No. 22, 2004
Huang et al.
novel photocatalytic oxidation system consisting of H2O2 and
FeTPPS4 supported on a resin. The FePR catalyst exhibits both
much higher catalytic activity and much better stability than
FeTPPS4 in the homogeneous system for the photodegradation
of organic pollutants, and the FePR can be recycled easily by
filtration. It is interesting that the FePR catalyst can suppress
the undesirable decomposition of H2O2 to O2. This makes the
FePR system more economical than the one with FeTPPS4. The
reaction mechanism involves the formation and reaction of ‚
OH radicals in the degradation pathways. This FePR/H2O2
photocatalytic system provides new possibilities for the oxidative
removal of persistent organic pollutants in aquatic environment
under visible light irradiation.
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Acknowledgment. The generous financial support by 973
project (No. 2003CB415006), the National Science Foundation
of China (Grant No. 20133010, No. 20373074, No. 50221201,
and No.20371048), the Chinese Academy of Sciences, Ministry
of Science and Technology of China (No. 2002AA601250) is
gratefully acknowledged.
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