Photo-Fenton degradation of malachite green catalyzed by aromatic compounds under visible light irradiation

Article abstract of DOI:10.1039/b107404k

The photo-Fenton degradation of malachite green (MG) catalyzed by aromatic derivatives under visible light irradiation was examined. Aromatic derivatives exhibit significant catalytic action in the photo-Fenton reaction. The degradation rates of MG with various kinds of aromatic derivatives obey the following order: hydroquinone > salicylic acid > quinone > carboxylic aromatic derivatives > amido aromatic derivatives. GC-MS detection of the products formed in the MG Fenton degradation in the presence of aromatic derivatives shows that the addition of the latter does not change the MG degradation pathways, although it greatly alters the reaction rate. The MG degradation mainly involves two different pathways: cleavage with a [MG-OH]^· transition intermediate and N-demethylation.

Full text of DOI:10.1039/b107404k

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Photo-Fenton degradation of malachite green catalyzed by aromatic
compounds under visible light irradiation
a
Feng Chen, Jianjun He, Jincai Zhao* and Jimmy C. Yu
a
a
b
a
Laboratory of Photochemistry, Center for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, China. E-mail: jczhao@ipc.ac.cn;
Fax: þ86 10 6487 9375
b
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong, China
Received (in Montpellier, France) 13th August 2001, Accepted 14th November 2001
First published as an Advance Article on the web 19th February2002
The photo-Fenton degradation of malachite green (MG) catalyzed by aromatic derivatives under visible light
irradiation was examined.Aromatic derivatives exhibit significant catalytic action in the photo-Fenton reaction.
The degradation rates of MG with various kinds of aromatic derivatives obey the following order:
hydroquinone > salicylic acid > quinone > carboxylic aromatic derivatives > amido aromatic derivatives.GC-
MS detection of the products formed in the MG Fenton degradation in the presence of aromatic derivatives
shows that the addition of the latter does not change the MG degradation pathways, although it greatly alters
ꢀ
the reaction rate.The MG degradation mainly involves two different pathways: cleavage with a [MG–OH]
transition intermediate and N-demethylation.
The Fenton reaction has been widely studied to treat various
1
excites the dye molecules, which then transfer electrons to
–4
kinds of organic pollutants in wastewater and was found to
effectively degrade almost all kinds of organic compounds
through the attack by OH radicals.Recently, the introduction
ferric ions to produce ferrous ions, hence promoting the
Fenton reaction.
Addition of other compounds has been found to greatly
ꢀ
of visible light to dye=Fenton systems has been found to greatly
influence the Fenton degradation of target organic com-
12–15
5
–7
accelerate the Fenton degradation of dye compounds. The
reaction mechanism of the visible light assisted photo-Fenton
process involves the formation of hydroxyl and hydroperoxyl
radicals and electron transfer between the excited dyes and the
pounds in the dark.
Aromatic derivatives, such as
hydroquinone and hydroquinone-like compounds, were found
to have a catalytic action, to greatly promoting the Fenton
1
6–18
degradation of organic compounds in the dark.
Con-
5
–11
ferric ions:
sidering that aromatic derivatives are the main pollutants in
wastewater and most dye-contaminated wastewaters contain
various kinds of aromatic derivatives such as phenol and
hydroquinone, a detailed study on their effect on the Fenton
and photo-Fenton reactions would be of great importance.
However, little work has been done on the behavior of aro-
matic derivatives in the visible light assisted photo-Fenton
degradation of dyes.
Fe þ H2O2 À! Fe3þ þ OH þ HOÀ
2þ
ꢀ
À1
3
À1
k1 ¼ 58 mol dm s
ð1Þ
ð2Þ
3þ
2þ
ꢀ
þ
Fe þ H2O À! Fe þ HO þ H
2
2
À1
3
À1
k2 ¼ 0:02 mol dm s
This paper shows that aromatic derivatives with different
kinds of substituent groups can catalyze the visible light
assisted photo-Fenton reaction, and hence promote dye
degradation.The effect of the aromatic derivatives depends
highly on their different molecular structures.In order to
reveal the details of the influence of these aromatic deriva-
tives on the visible light assisted photo-Fenton degradation of
dyes, the variations in the concentrations of MG and H O in
Fe þ OH À! Fe þ HOÀ
2
þ
ꢀ
3þ
ð3Þ
ð4Þ
ð5Þ
Fe þ HO À! Fe þ O2 þ Hþ
3þ
ꢀ
2þ
2
ꢀ
ꢀ
OH þ H2O2 À! HO þ H2O
2
2
2
hn
the reaction process were measured and the EPR technique
was used to detect the active radicals generated in the reac-
tion solution.To better understand the effect of the aromatic
derivatives on the visible light assisted photo-Fenton reac-
tion, addition of inorganic and aliphatic organic compounds
was also examined and found to suppress the degradation of
MG.The degraded products formed in the visible light
assisted photo-Fenton degradation of MG in the presence of
quinone were also determined by GC-MS measurements to
reveal more details about the mechanism of Fenton degra-
dation of dyes.
Dye À! Dye*
ð6Þ
ð7Þ
Dye* þ Fe þ À! Dye þ þ Fe2þ
3
ꢀ
Cycling of ferrous and ferric ions proceeds through reac-
tions (1) and (2), which continually produce the active radicals
OH, HO ).The slow reaction (2) is the rate-determining step
2
of this Fenton process.Any action that promotes the trans-
formation of ferric ions to ferrous ions would help accelerate
the Fenton reaction.In the dye =Fenton solution, visible light
ꢀ
(
ꢀ
3
36
New J. Chem., 2002, 26, 336–341
DOI: 10.1039/b107404k
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002

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Experimental
Materials
Malachite green (MG), salicylic acid, 1,4-hydroquinone, 1,2-
hydroquinone, resorcinol, quinone, aniline, o-phthalic acid,
potassium fluoride, ethylenediaminetetraacetic acid (EDTA),
propandioic acid, KH
2
PO
4
, Fe(ClO
4
)
3
Á9H
2
O and hydrogen
peroxide were of laboratory reagent grade and used without
further purification.The spin trap, DMPO, was purchased
from Sigma Chemical Co.Deionized and doubly distilled
water was used throughout this study.
3þ
Fig. 1 Degradation of MG under various conditions: (a) in Fe
solution under visible light irradiation, (b) in H solution under
visible light irradiation, (c) in the presence of Fe and H in the
under visible light irra-
and quinone in the dark, (f)
and quinone under visible light
2
O
2
3
þ
2
O
2
3
þ
þ
2 2
dark, (d) in the presence of Fe and H O
3
2 2
diation, (e) in the presence of Fe , H O
3
þ
Photoreactor and light source
in the presence of Fe , H
2
O
2
À4
À3
À2
À3
irradiation.MG (1 Â 10 mol dm ), H
2
O
2
(2 Â 10 mol dm ),
3þ
À4
À3
À5
À3
A 500 W halogen lamp (Institute of Electric Light Source,
Beijing) was positioned inside a cylindrical Pyrex jacket and
was cooled by recirculating water.A cutoff filter was placed
outside the Pyrex jacket to completely remove wavelengths
shorter than 450 nm to ensure that the irradiation only con-
sisted of visible light.The reactor was a cylindrical Pyrex vessel
with a volume of 70 ml and was placed at a fixed distance from
the light source.
Fe (2 Â 10 mol dm ), quinone (5 Â 10 mol dm ).The insert
shows the UV-vis absorbance spectrum of MG for (f).
light and addition of quinone greatly promoted the Fenton
degradation of MG (curves d and e), where under both con-
ditions almost all of the MG molecules disappeared in about
80 min.The photo-Fenton degradation of MG under visible
light irradiation in the presence of quinone showed a quicker
rate, revealing that addition of quinone and visible light irra-
diation cooperatively promote the Fenton process.Since only
MG absorbs the visible light in the solution, the promotion of
the visible light assisted photo-Fenton reaction is attributed to
an electron transfer between the excited MG and the ferric ions
[as shown in eqn.(7)].
Analytical methods
Reaction solutions were freshly prepared from stock solutions
À3
of Fe(ClO
À3
4
)
dm ) at pH 4.8 and their pH was adjusted to the desired
3
(5 mmol dm ) at pH 1.9 and MG (1 mmol
values with HClO
4
and NaOH solutions.Then H
2
O
2
was
The excited MG molecules transfer electrons to the ferric
ions to produce ferrous ions, which promote the regeneration
of ferrous ions [as does reaction (2)] and accelerates the Fenton
reaction.Addition of quinone to the reaction showed similar
effects as those observed with visible light irradiation.In a
recent study, it was found that addition of aromatic derivatives
to the dye=Fenton system enhances the concentration of fer-
added to the solution to initiate the reaction.At given time
intervals, 3.5 ml samples were taken out and their absorbance
values at 617 nm (absorption maximum for MG) were recorded
with a Lambda Bio20 UV-VIS spectrophotometer (Perkin–
Elmer Co.). The GC-MS data were obtained with a Trio-2000
spectrometer, equipped with an SPB5 column, 30 m Â
1
8
0
.32 mm. The sample was prepared as follows: the solvent of
rous ions in the dark. The reaction mechanism of the cata-
lysis of quinone is shown below (the redox potentials of
the reacted solution was vaporized directly (below 323 K)
under reduced pressure.The remaining residue was dissolved
in methanol.Changes in the H O concentration were deter-
ꢀ
quinone and HO (aq) are also indicated to reveal the ther-
):
2
2
1,22
2
2
modynamic basis
1
9,20
mined by a photometric method described in the literature.
ꢀ
þ
HO ðaqÞ þ H þ e ! H2O2ðaqÞ 1:44 V
ð8Þ
ð9Þ
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.The settings were: center field ¼ 3486.7 G, sweep
width ¼ 100.0 G, microwave frequency ¼ 9.82 GHz, power ¼
2
þ
ꢀ
O2 þ H þ e ! HO ðaqÞ À 0:046 V
2
p-quinone þ 2e þ 2Hþ ! p-hydroquinone 0:6992 V ð10Þ
5
.05 mW. To minimize experimental errors, the same quartz
capillary tube was used for all EPR measurements.
ꢀ
HO þ quinone ! semiquinone þ O2
2
9
À1 À1
22;23
k11 ¼ 1 Â 10 M
s
ð11Þ
ð12Þ
Results and discussion
ꢀ
HO þ semiquinone ! hydroquinone þ O2
2
Degradation of MG
À1
À1
s
9
22;23
k12 ¼ 1 Â 10 M
À4
À3
Fig.1 displays the degradation of MG (1 Â 10 mol dm ) in
3
þ
À4
À3
the presence of the Fe
(
(2 Â 10
mol dm ) and H
2
O
2
À2
À3
2 Â 10 mol dm ) with and without visible light irradiation.
þ
3
Although MG was scarcely decomposed in the Fe homo-
geneous (curve a) and H O homogeneous (curve b) solutions,
2
2
3þ
the coexistence of Fe and H
the MG to some extent (curve c).Both irradiation by visible
2 2
O (Fenton reagent) degraded
ð13Þ
New J. Chem., 2002, 26, 336–341
337

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different kinds of inorganic compounds.Compared to the
control reaction, the degradation rate of MG in the presence of
inorganic compounds was slower.This is mainly attributed to
complexation of the inorganic compounds with the iron
ions, which makes the reaction between iron and H
2
O
2
less
À5
probable.The photo-Fenton degradation of MG (2 Â 10
ð14Þ
À3
mol dm ) was also suppressed in the presence of different
kinds of aliphatic compounds [as shown in Fig.2(B)].Besides
forming complexes with iron, these aliphatic compounds may
also interact with OH radicals.The photo-Fenton degradation
Although addition of quinone (curve e) promoted the MG
degradation more efficiently than visible light irradiation
ꢀ
(
curve d) in the initial stages of the reaction, its effect abated
of MG in the presence of EDTA is an interesting example.
Initially, it is very slow due to the strong complexation
between iron and EDTA.However, the degradation rate picks
up as more free iron ions are released due to the decomposition
with continued degradation of MG and was weaker than that
of visible light irradiation in the later stages of the MG
degradation process.This is due to decomposition of the
quinone, lowering its catalytic effect.Accompanying by the
decrease in the absorbance of the MG, the maximum of
the characteristic absorption peak also moved from 617 nm to
ꢀ
of EDTA by OH radicals.The suppression effects of inorganic
and aliphatic compounds that we observed are quite similar to
12,18
those reported for reactions in the dark.
Fig.3 shows the visible light assisted photo-Fenton degra-
5
99 nm, which indicates N-demethylation of the MG.
The Fenton degradation of rhodamine 6G was also exam-
À5
À3
dation of MG (2 Â 10 mol dm ) in the presence of different
kinds of aromatic derivatives.Unlike their inorganic and ali-
phatic counterparts, these aromatic species generally have a
positive influence on the degradation of MG.It has been
reported that hydroquinone and related aromatic compounds
could catalyze the Fenton degradation of target compounds by
directly reducing Fe(III) to Fe(II), while other aromatic deri-
vatives catalyzed the Fenton reaction through the hydro-
ined under various conditions.The results were similar to
those of MG: the Fenton degradation of rhodamine 6G was
accelerated by quinone both in the dark and under visible light
irradiation.This indicates that the promotion of the Fenton
reaction by quinone is insensitive to the nature of the dye in
solution.
1
4,15,18
MG degradation in the presence of different kinds of compounds
quinone-like degraded products.
Our results show that
most aromatic derivatives are also effective catalysts in visible
light assisted photo-Fenton reactions.Among the compounds
studied, 1,4-hydroquinone and 1,2-hydroquinone are the most
effective.Since salicylic acid must be converted to hydro-
quinone or hydroquinone-like compounds before it catalyzes
the photo-Fenton reaction, the catalytic power of salicylic acid
is weaker than that of hydroquinone.This was also observed in
the presence of resorcinol, which cannot be oxidized directly to
quinone because of its structure and hence exhibits a lower
catalytic effect than 1,4-hydroquinone and 1,2-hydroquinone.
As shown in Fig.3, the effectiveness of the aromatic deri-
vatives is in the order: hydroquinones > salicylic acid >
quinone > resorcinol > carboxylic aromatic derivative > amido
aromatic derivative.This series correlates well with the ability
to transform to hydroquinone or hydroquinone-like com-
pounds.Since amido aromatic derivatives tend to react with
protons to generate ammonium in the acidic solution under the
Fenton reaction conditions, they are less likely to undergo
electrophilic attack by hydroxyl radicals to transform to
Fig.2(A) a shows the visible light assisted photo-Fenton
À5
À3
degradation of MG (2 Â 10 mol dm ) in the presence of
Fig. 2 Fenton degradation of MG: (a) control reaction and in the
Fig. 3 Fenton degradation of MG: (a) control reaction and in the
presence of: (b) aniline, (c) o-phthalic acid, (d) resorcinol, (e) quinone,
(f) salicylic acid, (g) 1,2-hydroquinone, (h) 1,4-hydroquinone.Visible
À
À
À
presence of: (b) H
2
PO
4
, (c) F , (d) NO
3
, (e) EDTA, (f) propane
À5
À3
À3
diacid.Visible light irradiation, MG (2 Â 10
mol dm ), H O
2 2
À3
À3
3 þ
À4
À5
À3
À3
À3
(
(
2 Â 10 mol dm ), Fe
(2 Â 10 mol dm ), other compounds
light irradiation, MG (2 Â 10 mol dm ), H
O
2 2
(2 Â 10 mol dm ),
À5 À3
À4
À3
3 þ
À4
À3
2 Â 10 mol dm ).
Fe
(2 Â 10 mol dm ), other compounds (2 Â 10 mol dm ).
3
38
New J. Chem., 2002, 26, 336–341

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ꢀ
hydroquinone-like compounds.As a result, amido aromatic
derivatives do not exhibit catalytic effects in the visible light
assisted photo-Fenton reaction.In fact, the MG photo-
degradation rate in the presence of aniline is even slower than
that of the control reaction.
EPR intensity of DMPO– OH.On the other hand, the pre-
sence of quinone and salicylic acid promotes the generation of
OH radicals.The EPR results are also in good agreement with
ꢀ
the MG Fenton degradation rates and the extent of H
decomposition.Addition of aromatic compounds promotes
regeneration of ferrous ions, accelerates the reaction between
the iron ions and H O , and facilitates the generation of OH
2 2
2 2
radicals from decomposition of H O .
2 2
O
ꢀ
2
H O
2
determination and EPR measurements
The change of H O concentration during the Fenton reaction
2
2
gives some important information about the process.In order
to enhance the concentration effects, a relatively low initial
GC-MS analysis
À4
À3
2
H O
2
concentration of 4 Â 10 mol dm was used.Fig.4
Fig.6 displays the GC-MS chromatograms of the products
formed in the Fenton degradation of MG in the presence of
quinone under visible light irradiation.The GC peak of MG
does not appear since it is too non-volatile for GC separation.
Identified degradation products are listed in Table 1 together
with their retention times and areas.The most abundant
intermediate (peak I, retention time 24.62 min) is 4-dimethyl-
aminobenzophenone, which is generated through the cleavage
of the bond between the central carbon atom and the dime-
thylaminophenyl group.The MS data of 4-dimethylamino-
benzophenone are listed in Table 2.Because of the influence of
the dimethylamino group, MG tends to fragment at the bond
between the central carbon atom and dimethylaminophenyl.
shows the H decomposition rate in the presence of EDTA,
2 2
O
quinone and salicylic acid.EPR experiments have confirmed
that a large amount of hydroxyl radicals are generated in the
6
,24
Fenton reaction.
Fig.5 shows the DMPO spin-trapping
EPR spectra in the presence of various kinds of compounds.
The intensity of the EPR signals correlates well with the
Fenton reaction rate.Addition of EDTA, which significantly
suppresses the degradation of MG, results in a much weaker
0
Although 4,4 -dimethylaminobenzophenone does not appear
in the GC spectra, some analogous degradation products
0
such as 4-dimethylamino-4 -methylaminobenzophenone and
0
4
-dimethylamino-4 -aminobenzophenone are present, indicat-
ing that MG also fragments at the bond between the central
carbon atom and the phenyl group.Some chlorinated com-
pounds were also detected in the degradation products, sug-
gesting that chlorine radicals are also involved in the
degradation of MG.Table 2 lists the MS data of the degra-
dation products.
MG Fenton degradation experiments were also carried out
in the presence of quinone but without visible light irradiation.
GC-MS measurements show similar degradation products
being generated.We believe that the irradiation by visible light
does not alter the major MG Fenton degradation pathways.
Small amounts of products other than those listed in Table 1
Fig. 4
in the presence of: (b) EDTA, (c) quinone, (d) salicylic acid.Visible
H
2
O
2
concentration variation in (a) the control reaction and
were also detected.They were HCON(CH
3 2 3
) , CH OCON-
(
change in TOC value during the photo-Fenton degradation of
CH , and 4-hydroxylbenzoic acid.We also monitored the
3 2
)
À5
À3
À4
À3
light irradiation, MG (2 Â 10 mol dm ), H
2
O
2
(4 Â 10 mol dm ),
À5 À3
3
Fe
þ
À4
À3
(2 Â 10 mol dm ), other compounds (2 Â 10 mol dm ).
Fig. 6 GC chromatograms of MG Fenton degradation products in
the presence of quinone under visible light irradiation: (a) 30% of MG
degraded; (b) 90% of MG degraded.The intensity scale in (b) is two
Fig. 5 DMPO spin-trapping EPR spectra of MG/Fenton solution in
(a) the control reaction and in the presence of: (b) EDTA, (c) quinone,
À4
À3
À4
(
H
d) salicylic acid.Visible light irradiation, MG (1 Â 10 mol dm ),
times larger than that in (a).Initial concentrations: MG (1 Â 10 mol
À2
À3
3 þ
À4
À3
À3
À3
À3
À3
3 þ
À4
À3
2
O
2
(2 Â 10 mol dm ), Fe
(2 Â 10 mol dm ), other com-
dm ), H
2
O
2
(2 Â 10 mol dm ), Fe
(2 Â 10 mol dm ), qui-
À4
À3
À5
pounds (1 Â 10 mol dm ).
none (5 Â 10 mol dm ).
New J. Chem., 2002, 26, 336–341
339

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Table 1 Retention times and peak areas of the identified degradation
products in the visible light assisted photo-Fenton degradation of MG
Table 2 MS data of the degradation products formed in the visible
light assisted photo-Fenton degradation of MG
Peak area (%)
Retention
time=min
Peak m=z (rel.int.of predominant ions in fragmentation pattern, %)
Peak Product
a
b
A
B
C
D
E
59(79); 45(37); 29(80); 15(100)
136(5); 105(23); 77(78); 51(100)
122(6); 105(15); 77(58); 51(100)
179(38); 148(70); 132(22); 118(25); 104(72); 77(75)
211(80); 210(765); 194(13);165(79); 152(33); 134(93);
118(57); 91(100)
A
B
C
D
E
F
HCONHCH
4-Dimethylaminophenol
3
4.37
9.32
10.66
17.65
21.15
22.07
25.6
0.90
1.25
0
22.8
1.24
0
2.50
4.60
0.24
1.33
8.98
C
6
H
5
COONH
NC
MW ¼ 211
4
3
(CH )
2
6
H
5
COOCH
3
F
G
H
I
223(8); 146(26); 118(8); 105(43); 90(45); 77(100)
241(9); 136(60); 120(5); 105(82); 77(100)
211(16); 134(67); 105(21); 91(15); 77(100)
225(37); 148(100); 105(32); 91(12); 77(97)
247(3); 245(8); 170(8); 168(26); 105(28); 77(100)
241(12); 164(14); 121(20); 105(28); 77(100)
241(10); 148(11); 121(100); 120(38); 105(14)
255(13); 148(16); 134(100); 120(52); 105(17)
3-Oxo-4-methylimino-1,5-
cyclohexadienyl phenone
3-Hydroxyl-4-dimethylamino-
benzophenone
G
23.60
24.28
5.30
3.11
J
H
I
4-Methylaminobenzophenone
15.2
500
0
13.2
100
1.85
K
L
M
4-Dimethylaminobenzophenone 24.61
J
Chloric-4-methylamino-
benzophenone
25.16
25.4
K
L
2-Hydroxyl-4-dimethylamino-
benzophenone
4-Dimethylamino-4 -amino-
benzophenone
10.1
2.23
1.24
0.34
0
many more degradation products and hence more detailed
degradation pathways can be postulated.Interestingly, we
have found a new pathway in which MG molecules are cleaved
at the bond between the central carbon atom and the phenyl
group.
25.84
27.3
0
0
0
M
4-Dimethylamino-4 -
methylaminobenzophenone
Mechanism of the MG degradation
MG in the presence of quinone.It changed from 12 3. 8 ppm to
.58 ppm in 240 min, indicating mineralization of organic
compounds.
In a recent study, the degradation products formed in the
visible light assisted photo-Fenton degradation of MG without
aromatic additives were analyzed by GC-MS.Only five
products (N,N-dimethylaniline, 4-dimethylaminophenol, 4-
methylaminobenzophenone, 4-dimethylaminobenzophenone,
Aromatic derivatives accelerate the photo-Fenton reaction
under visible light irradiation but do not alter the degradation
pathways for MG degradation.The visible light assisted
photo-Fenton degradation of MG in the presence of quinone
mainly involves two different pathways as summarized in
7
2
5
ꢀ
Scheme 1: cleavage with a [MG–OH] transition intermediate
and N-demethylation.All of the proposed degradation pro-
ducts have been identified by our GC-MS measurements
except those that are highly unstable or too non-volatile for
4
-hydroxybenzoic acid) were detected.This work identifies
Scheme 1 MG photo-Fenton degradation pathways in the presence of quinone under visible light irradiation.
New J. Chem., 2002, 26, 336–341
3
40

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GC separation.Because of the influence of the dimethylamino
group, the [MG–OH] transition intermediates are mainly
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1
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Aromatic derivatives can effectively catalyze the photo-Fenton
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ꢀ
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Acknowledgments
This work was supported by the National Natural Science
Foundation of China (No.29877026, No.20077027, and No.
K.B.Patel and R.L.Willson,
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341