Fenton Degradation of Organic
Compounds Promoted by Dyes under
J I A H A I M A , W E N J I N G S O N G ,
C H U N C H E N G C H E N , W A N H O N G M A ,
J I N C A I Z H A O , * A N D Y A L I N T A N G
Key Laboratory of Photochemistry, Center for Molecular
Science, Institute of Chemistry, The Chinese Academy of
Sciences, Beijing 100080, China
The influence of dyes on the Fenton reaction of organic
compounds under visible irradiation (λ > 450 nm) was
examined. It was found that the presence of dyes could
such as salicylic acid, sodium benzenesulfonate, benzyl-
trimethylammonium chloride, and trichloroacetic acid under
visible irradiation and that a complete mineralization of
those compounds could also be achieved. The dyes such
unit showed much more significant effect on the reaction
than the dyes such as malachite green without the
quinone unit. A reaction mechanism of dye AV as a
under visible irradiation is proposed based on the cycle
of Fe3+/Fe2+catalyzed by quinone species and an electron
transfer from the excited dye molecule to Fe3+.
Fenton and photo-Fenton reactions have been proven
effective methods to treat organic pollutants in wastewater,
mechanism for the Fenton reaction is shown below (5, 11,
12), and the slow reaction (eq 2) is rate-determining step of
In recent years, many studies have shown that organic
pollutants could be degraded much more rapidly under UV
illumination of the Fe2+/H2O2or Fe3+/H2O2system than in
the dark Fenton reactions (13-16). For such photo-Fenton
reactions, the effect of UV light is attributed to the direct
HO• formation and regeneration of Fe2+from the photolysis
of the complex Fe(OH)2+in solution as follows (14, 17, 18).
are also important. The HO• quantum yield for reaction 6 is
0.14 at 313 nm and 0.017 at 360 nm (13). Visible light
irradiation cannot lead to this reaction and hence cannot
accelerate the Fenton reaction since the absorption wave-
organic pollutants cannot absorb visible light.
Dyes represent the principal pollutants in the textile and
photographic industry (19). In China, more than 1.6 × 109
m3of dye-containing wastewater per year drains into
environmental water system without treatment. The dyes
are usually difficult to biodegrade; and other conventional
treatment methods such as activated carbon adsorption,
coagulation and reverse osmosis are also not effective for
treatment of those dye pollutants. During recent years, it
was found that due to effective electron transfer from the
visible light-excited dyes into Fe3+, which leads to regenera-
tion of Fe2+and an easy cycle of Fe3+/Fe2+, much faster
degradation and mineralization of various dyes have been
achieved in the photo-Fenton reaction under visible light
irradiation (20-23), comparing with the Fenton reaction in
the dark. The mechanism is shown below.
However, could such a dye photosensitization principle
promote Fenton degradation of other organic compounds
coexisted in the solution? The objective of this study is to
investigate how the presence of dyes influences the Fenton
reaction of the other organic compounds under visible
Hydroquinone and catechol have been found to greatly
catalyze the Fenton degradation of organic compounds (24,
25), which is attributed to the fact that they could reduce
Fe3+to Fe2+rapidly at low pHs and hence make a rapid
regeneration of Fe2+bypassing eq 2, the slow step of the
Fenton reaction (26).
The resulting quinone or semiquinone can rapidly react
thus the quinone cycle is built up.
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dye + visible light f dye*(7)
dye* + Fe3+f Fe2++ dye+•
Fe2++ H2O2f Fe3++ HO• + OH-
HO• + dye (or dye+•) f degraded products(9)
Fe2++ H2O2f Fe3++ HO• + OH-k1) 76 M-1s-1
Fe3++ H2O2f Fe2++ HO2•+ H+
k2) 0.02 M-1s-1
Fe2++ HO• f Fe3++ OH-
k3) 3 × 108M-1s-1(3)
HO• + H2O2f HO2•+ H2O k4) 2.7 × 107M-1s-1
HO2•+ HO2•f H2O2+ O2
k5) 8.5 × 105M-1s-1
FeIII(OH)2++ UV f Fe2++ HO•
More surprisingly, various aromatic compounds could
under visible irradiation or in the dark depending on their
ability to be transformed into hydroquinone-like intermedi-
ates (27, 28).
structure unit (see the molecular structures below) on the
photo-Fenton reaction of organic compounds under visible
irradiation was examined. It was found that the dyes such
much more effective catalytic effect on the Fenton reaction
can accelerate significantly the Fenton degradation and
of the dyes without the quinone structure unit. On the basis
of the cycle of Fe3+/Fe2+catalyzed by the quinone structure
unit and the electron transfer of excited dye molecules by
visible light, a reaction mechanism is proposed. Better
understanding of these reactions is very important for
type technologies for the treatment of practical wastewaters
containing complicated components.
Materials and Reagents. Malachite green (MG) was from
Sigma. Alizarin Violet 3B (AV) was of analytical grade. Fe-
(ClO4)3and HClO4were from Aldrich. Salicylic acid, sodium
B, Acridine Orange, Alizarin Red, and hydrogen peroxide
(30%) were of analytical grade and were used as received
purified in a Barnstead Nanopure system to a resistivity >
Procedures. All experiments were conducted in aerated
solution, and the initial pH of 2.90 was adjusted with HClO4.
All solutions were freshly prepared daily. Stock solutions of
were carried out in a cylindrical Pyrex vial (50 or 100 mL)
with a small magnetic stir bar.
The light source was a 500-W halogen lamp (Institute of
Pyrex jacket and cooled by circulating water. A cutoff filter
was placed outside the Pyrex jacket to completely remove
wavelengths shorter than 450 nm to ensure that the irradia-
tion only consisted of visible light.
Analyses. At the given reaction time intervals, samples
were taken out and immediately analyzed by measuring the
UV-vis spectra of the dyes with a Lambda Bio20 UV-vis
nm for MG and 561 nm for AV, respectively. Salicylic acid,
Diamonsil C-18 column and detected with a Dionex P580
The mobile phases were 70% methanol/30% water for the
analyses of salicylic acid (monitored at 298 nm; retention
time, tR, 2.8min), 30% methanol/70% water for benzene-
60% methanol/40% water for BTAC (monitored at 215 nm;
tR, 2.0 min). All the mobile phases were at 1.0 mL/min and
water effluent contained 0.1% H3PO4. All samples were
analyzed immediately to avoid further reactions.
The TOC (total organic carbon) was measured with a
Tekmar Dohrmann Apollo 9000 TOC analyzer.
ped with an in situ irradiation source of Quanta-Ray ND:
tube was used for all measurements to minimize errors.
Cyclic voltammetry experiments were performed on a
283 (EG&G Instrucments) using a platinum electrode as
working electrode with a reference electrode (SCE) and an
auxiliary platinum electrode between -1.0 and +1.0 V. The
scan rate was 50 mV/s.
Results and Discussion
of Dyes. Figure 1 shows the (photo-)Fenton degradation of
three organic compounds (salicylic acid, BTAC, and sodium
benzenesulfonate) in the presence of two different dyes of
AV and MG, respectively. The two dyes were selected for
their clearly different molecular structures with or without
a quinone structure unit. Because those substrates could
not absorb visible light, visible irradiation has no effect on
their Fenton degradation. Compared with the control reac-
tion, the presence of MG decreased the degradation rates of
those compounds (except BTAC, see discussion below) in
the dark while accelerated their degradation under visible
of the three target compounds, especially under visible
acid still existed in the control reaction, but only about 10%
salicylic acid remained in the system in the presence of AV
replaced by BTAC or sodium benzenesulfonate, the similar
results were obtained (parts B and C of Figure 1). The two
compounds under visible irradiation. In the presence of AV
under visible irradiation, the degradation induction period
of the two compounds declined very much compared with
in the Fe3+homogeneous or H2O2homogeneous solutions.
can effectively inject electrons into Fe3+which leads to
regeneration of Fe2+and an easy cycle of Fe3+/Fe2+in the
presence of H2O2and thus a continuous production of HO•
(eqs 7, 8, and 1). So dyes could be degraded much faster in
the Fenton reaction under visible light irradiation than in
of target compounds under visible irradiation (curves d and
degradation of other organics that could not absorb visible
light under visible irradiation.
AV showed much more effective acceleration effects on
the degradation of the target organic compounds than MG
did under visible irradiation and worked even in the dark.
It has been reported that quinone/hydroquinone analogues
could greatly catalyze the Fenton degradation of organic
compounds (24-27). Because of its quinone structure unit,
the dye AV could play a role like hydroquinone to recycle
Fe2+from Fe3+in the Fenton reaction, and thus the
degradation of target organic compounds was greatly pro-
moted in the presence of AV.
The HO2•produced in the initial Fenton reaction would
quickly react with AV and gives a dye semiquinone radical
of AVH•. Then the AVH• reduces Fe3+to regenerate Fe2+,
MG without the quinone structure unit did not show this
effect in the dark and performed just as a substrate. So the
dye AV has two catalysis effects on regenerating Fe2+from
Fe3+; one is by quinone unit cycle, another is by the dye*/
of the coexisted organics.
The effect of other dyes, Rhodamine B, Acridine Orange,
and Alizarin Red, on the Fenton degradation of sodium
benzenesulfonate was also investigated; the similar results
were observed (Table 1). All the dyes significantly promoted
structure showed much greater effect than the other two
Mineralization of Substrates. Figure 2 shows the min-
eralization of sodium benzenesulfonate in Fenton reaction.
The control reaction could reach about 40% TOC removal
within 4 h. Without visible irradiation, addition of AV into
the sodium benzenesulfonate solution only got about 32%
of the system value very slightly), though the degradation of
results were consistent with those reported previously that
the dark Fenton reaction always could not reach deep
addition of AV nearly led to complete mineralization of the
total system at the same time scale. We have reported that
complete mineralization of various dyes could be achieved
And here due to photosensitization of dye AV under visible
irradiation, deep mineralization of coexisted small organic
molecules such as sodium benzenesulfonate was also
in experiments for Fenton degradation of salicylic acid in
the presence or absence of AV. So, as a cocatalyst in the
FIGURE 1. Fenton degradation of organic compounds: (a) in the
presence of MG in the dark, (b) control reaction in the dark, (c) in
the presence of AV in the dark, (d) in the presence of MG under
Initial concentrations: 2 × 10-5M MG, 2 × 10-5M AV; (A) 1 × 10-4
M Fe3+, 2 × 10-3M H2O2, 4 × 10-4M salicylic acid; (B) 2 × 10-4
M Fe3+, 4 × 10-3M H2O2, 2 × 10-4M BTAC; (C) 2 × 10-4M Fe3+,
4 × 10-3M H2O2, 2 × 10-4M sodium benzenesulfonate.
reactions: (a) control reaction, (b) in the presence of AV in the
dark, (c) in the presence of AV under visible irradiation. Initial
concentrations: 2 × 10-4M Fe3+, 2 × 10-2M H2O2, 4 × 10-4M
sodium benzenesulfonate, 2 × 10-5M AV.
TABLE 1. Degradation of Sodium Benzenesulfonate (2 × 10-4
M) in the Presence of Different Dyes (2 × 10-5M) in the
Fenton Reaction (2 × 10-4M Fe3+, 4 × 10-3M H2O2) under
have changed the reaction pathway of organic molecules in
favor of deep mineralization. Noting that this mechanism is
distinct from other photo-Fenton methods to achieve total
mineralization of organics such as direct photolysis of
(14) or Fe ligand complexes to give reactive ROO• (15, 16).
We also performed the experiments of trichloroacetic acid
as an example of aliphatic compounds; the presence of AV
under visible irradiation also significantly enhanceded the
evidence the catalysis effect of AV on the Fenton reaction
under visible irradiation, cyclic voltammetry and EPR
experiments were performed. Figure 3 gives the cyclic
voltammetry results of an Fe3+solution in the presence of
AV under visible irradiation. As irradiation time increased,
the cathodic peak current (ipc) appeared and increased and
finally kept constant after 4 min of irradiation. It meant that
Fe2+was produced and accumulated in the solution.
to Fe3+under visible irradiation, Fe2+could be easily
regenerated, and this available pathway of recycling Fe ions
is very important in the Fenton reaction system. The redox
potential of the AV*/AV+•molecule which has a same
chromophore with quinizarin is about -0.68 V against the
Ag/AgCl electrode (30). The redox potentials of the Fe3+/
Fe2+couple is 0.771 V vs NHE, i.e., 0.549 V against the Ag/
AgCl electrode. So the electron injection from AV* into Fe3+
is thermodynamically probable. Figure 4 shows the EPR
spectra of 4-fold characteristic peaks of HO• radicals in the
Fenton reaction in the presence of AV using DMPO as the
radical scavenger (23, 31). Weak HO• and HO2•signals were
with the control reaction, the DMPO-HO• signals became
stronger in the presence of AV in the dark and increased
more under visible irradiation. Further, due to the rapid
in the presence of AV (panels A and B of Figure 4). So the
addition of AV to Fenton system increased generation of
HO• radicals significantly due to regeneration of Fe2+from
Fe3+. By cyclic voltammetry and EPR experiments, we
confirmed the catalysis effect of AV in the Fenton reaction.
Degradation of Dyes Concomitantly with the Degrada-
HO• attack. Therefore, at the same time to catalyze the
visible irradiation or in the dark. What is interesting is the
case in the presence of salicylic acid. Figure 5 shows the
acid in the dark. The presence of phenol derivatives such as
salicylic acid greatly catalyzed Fenton degradation of MG
(Figure 5A). However, from Figure 5B, we found that the
presence of salicylic acid could not accelerate the AV
by AV (Figure 1 A). So the promotion of the Fenton reaction
by quinone structure unit is sensitive to the nature of
coexisting compounds. When a dye molecule and a small
aromatic molecule coexist in the Fenton reaction system,
the one for which it is easier to become the hydroquinone
of the other substrate. Under visible irradiation, the dyes
degraded much faster than in the corresponding dark
FIGURE 3. Cyclic voltammetry of 2 × 10-4M Fe3+in the presence of 2 × 10-5M AV under visible irradiation. The scan rate was 50 mV/s
from -1.0 to +1.0 V.
FIGURE 4. DMPO spin-trapping EPR spectra of Fenton reaction of
2 × 10-4M Fe3+and 4 × 10-3M H2O2: (A) in the presence of 2 ×
10-5M AV under visible irradiation; (B) in the presence of 2 × 10-5
M AV in the dark reaction; (c) without AV in the dark.
the presence of salicylic acid delayed the degradation of AV
and accelerated the MG degradation. In Figure 1B, even the
addition of MG also quickened up the degradation of BTAC
to some extent in the dark and the result was different from
that of salicylic acid and sodium benzenesulfonate. It may
under HO• attack than BTAC, so the degradation of BTAC
could even be accelerated a little by MG in the dark.
Discussions on the Induction Period. The Fenton
an induction period followed by a fast decomposition. A
similar result was also reported by Pignatello (26), in which
an induction period existed in the Fenton degradation of
phenol and was found to be sensitive to the presence of
hydroquinone and quinone. We performed the cyclic deg-
in the dark in the same reactor. In Figure 6, the first cycle
presented a long induction period at the beginning of the
cycle nearly became a linear reaction process and showed
no lag period. The shape of the reaction curve of the third
cycle was distinct from the former two; the initial reaction
process became the fastest period. Compared with the
corresponding AV experiments (see Figure 1, curve c) and
others’ works (26-28), we suggested that the production of
hydroquinone/quinone analogues controlled the reaction
induction period. In the absence of AV, no hydroquinone/
quinone structure unit existed in the system, so the dark
Fenton reaction was very slow. Once some hydroquinone/
quinone analogues are present, the quinone cycle works, so
the reaction became dramatically fast. In Fenton reactions,
HO• addition to the aromatic substrates to give mono- or
multihydroxyl intermediates or products (hydroquinone/
quinone analogues) has been firmly established (7, 10, 26,
28). As the cycles increased, the hydroquinone/quinone
analogues accumulated more and more in the reaction
sodium benzenesulfonate degradation was increasingly
A possible reaction mechanism is proposed in Scheme 1
based on all the information obtained above.
The presence of dyes greatly promoted the Fenton
The dyes with a quinone structure showed a much more
significant effect. We choose dye AV as an example to
elucidate the reaction mechanism. The presence of AV dye
analogue cycle, another cycle of Fe3+/Fe2+induced by the
greatly promote the HO• production and thus accelerate
Concomitantly with the degradation of organics, the AV as
a coexisting organic compound would also be decomposed.
Therefore, by driving the cycle of iron and continuous HO•
production, dye AV played a role of sacrificial cocatalyst in
mechanism needs further study. Better understanding of
these reactions is helpful for practical wastewater treatment
and revelation of Fenton-type reaction mechanism.
This work was financially supported under grants from the
5006), the National Science Foundation of China (Nos.
FIGURE 5. Fenton degradation of dyes in the dark: (a) control
reaction, (b) in the presence of salicylic acid. Initial concentra-
tions: (A) 2 × 10-4M Fe3+, 2 × 10-3M H2O2, 4 × 10-4M salicylic
acid, 2 × 10-5M MG; (B) 4 × 10-5M Fe3+, 2 × 10-2M H2O2, 4 ×
10-4M salicylic acid, 2 × 10-4M AV.
FIGURE 6. Cyclic Fenton degradation of sodium benzenesulfonate
in the presence of MG in the dark. Initial concentrations: 2 × 10-4
M Fe3+, 5 × 10-3M H2O2, 2 × 10-4M sodium benzenesulfonate, 2
Compounds in the Presence of AV under Visible Irradiation
Mechanism of Fenton Degradation of Aromatic
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Received for review January 1, 2005. Revised manuscript
received May 30, 2005. Accepted June 2, 2005.