Photo-Fenton and photo-Fenton-like processes for the degradation of methyl orange in aqueous medium: Influence of oxidation states of iron

Article abstract of DOI:10.2478/s11696-010-0011-0

Degradation of methyl orange (MO) was carried out by the photo-Fenton process (Fe²⁺/H₂O₂/UV) and photo-Fenton-like processes (Fe³⁺/H₂O₂/UV, Fe²⁺/S₂O₈^2-/UV, and Fe³⁺/S₂O₈^2-/UV) at the acidic pH of 3 using hydrogen peroxide and ammonium persulfate (APS) as oxidants. Oxidation state of iron had a significant influence on the efficiency of photo-Fenton/photo-Fenton-like processes. It was found that a process with a source of Fe³⁺ ions as the catalyst showed higher efficiency compared to a process with the Fe²⁺ ion as the catalyst. H₂O₂ served as a better oxidant for both oxidation states of iron compared to APS. The lower efficiency of APS is attributed to the generation of excess protons which scavenges the hydroxyl radicals necessary for degradation. Further, the sulfate ions produced from S₂O₈^2- form a complex with Fe²⁺/Fe³⁺ ions thereby reducing the concentration of free iron ions in the solution. This process can also reduce the concentration of hydroxyl radicals in the solution. Efficiency of the various MO degradation processes follows the order: Fe³⁺/H₂O₂/UV, Fe³⁺/APS/UV, Fe²⁺/H₂O₂/UV, Fe²⁺/APS/UV.

Full text of DOI:10.2478/s11696-010-0011-0

Chemical Papers 64 (3) 378–385 (2010)
DOI: 10.2478/s11696-010-0011-0
ORIGINAL PAPER
Photo-Fenton and photo-Fenton-like processes
for the degradation of methyl orange in aqueous medium:
Influence of oxidation states of iron
L. Gomathi Devi*, S. Girish Kumar, K. S. Anantha Raju, K. Eraiah Rajashekhar
Department of Post Graduate Studies in Chemistry, Central College Campus, Dr. B.R. Ambedkar Veedi, Bangalore University,
Bangalore-560 001, India
Received 22 May 2009; Revised 5 August 2009; Accepted 12 August 2009
Degradation of methyl orange (MO) was carried out by the photo-Fenton process (Fe2+/H2O2/UV)
and photo-Fenton-like processes (Fe3+/H2O2/UV, Fe2+/S2O28−/UV, and Fe3+/S2O28−/UV) at the
acidic pH of 3 using hydrogen peroxide and ammonium persulfate (APS) as oxidants. Oxidation state
of iron had a significant influence on the efficiency of photo-Fenton/photo-Fenton-like processes. It
was found that a process with a source of Fe3+ ions as the catalyst showed higher efficiency com-
pared to a process with the Fe2+ ion as the catalyst. H2O2 served as a better oxidant for both
oxidation states of iron compared to APS. The lower efficiency of APS is attributed to the gener-
ation of excess protons which scavenges the hydroxyl radicals necessary for degradation. Further,
the sulfate ions produced from S2O82−form a complex with Fe2+/Fe3+ ions thereby reducing the
concentration of free iron ions in the solution. This process can also reduce the concentration of
hydroxyl radicals in the solution. Efficiency of the various MO degradation processes follows the
order: Fe3+/H2O2/UV, Fe3+/APS/UV, Fe2+/H2O2/UV, Fe2+/APS/UV.
c
ꢀ 2009 Institute of Chemistry, Slovak Academy of Sciences
Keywords: photo-Fenton process, photo-Fenton-like process, ammonium persulfate, methyl or-
ange, degradation pathway
Introduction
The reason for the utilization of AOPs was mainly
due to the inability of biological processes to treat
highly contaminated toxic waste water. In this regard,
Fenton and photo-Fenton processes are quite efficient
methods for the degradation of dye molecules (Ghiselli
et al., 2004; Ntampegliotis et al., 2006). This process
is quite simple, inexpensive, effective and promising
in the water treatment industry. The Fenton reaction
involves the in situ production of hydroxyl radicals
which are non selective in their attack on pollutant
molecules. The main advantages of the photo-Fenton
process can be summarized as follows: (i) depth of
light penetration is high since the solution is trans-
parent; (ii) contact between the pollutant and the
oxidizing agent is high; (iii) no adsorption is needed
since the process is homogeneous, (iv) cost effective
and simple. Methyl orange (MO) is an acid dye be-
The industrial waste water released from leather,
paper and textile units usually contains various color-
ing substances causing a significant threat to the envi-
ronment. Due to the large scale production and exten-
sive application, large quantities of unused dyes are re-
leased damaging the aquatic environment. These dyes
absorb sunlight and initiate various chemical reactions
leading to the formation of intermediates which are
much more harmful and carcinogenic than the par-
ent dye molecule (Guetta¨ı & Ait Amar, 2005). The
usual conventional techniques such as adsorption on
activated carbon, flocculation, reverse osmosis, etc.,
transfer the contaminants from one phase to another.
Advanced oxidation processes (AOPs) have drawn sig-
nificant attention to the mineralization of these dyes.
*Corresponding author, e-mail: gomatidevi naik@yahoo.co.in

L. G. Devi et al./Chemical Papers 64 (3) 378–385 (2010)
379
III, gold orange, tropaeoline D), 4-dimethylaminoazo-
benzene-4ꢀ-sulfonic acid sodium salt (Fig. 1), was sup-
plied by BDH (Bangalore, India) and was used as re-
ceived.
(CH3)2N
N
N
SO3
Methyl orange (MO)
HO
Irradiation procedure
OH
OH
The photocatalytic degradation experiment was
carried out in a circular glass reactor (150 mm × 75
mm) whose surface area was 176 cm2. Irradiation of
the reactor was made by direct exposure of the solu-
tion with a 125 W medium pressure mercury vapor
lamp (Philips) at a distance from 29 cm. Photon flux
of the light source was found to be 7.75 mW cm−2 as
determined by the ferrioxalate actinometry, the peak
maximum was found at around 370 nm. The lamp was
warmed for 10 min to reach a constant output. In a
typical experiment, 200 mL of the 10 mg L−1 dye so-
lution with calculated amounts of the Fe2+/Fe3+ solu-
tion along with the oxidizing agent (H2O2/APS) were
added. The pH value of the dye solution was 6.6 and
it dropped to 2.2 on addition of the Fe2+/Fe3+ ions.
pH of the solution was adjusted by either adding di-
lute NaOH or H2SO4. Most of the experiments were
carried out thrice under identical conditions and the
error between the consecutive experiments was in the
range of approximately 2.4 %.
NH4
NO3
(CH3)2N
SO3H
OH
OH
OH
O
– H2O
CO2
H2O
O
Analytical methods
CO2
H2O
The samples (5 mL) were taken from the reactor at
definite time intervals. They were analyzed by the UV-
VIS spectroscopic technique using a Shimadzu UV-
1700 Pharmaspec UV-VIS spectrophotometer. The
samples were extracted into a non-aqueous ether
medium and 1 µL of each was subjected to the GC-
MS analysis (using a GC-MS-QP-5000 Shimadzu) and
a Thermo Electron Trace GC ultra coupled to a DSQ
mass spectrometer equipped with an Alltech ECONO-
CAP-EC-5 capillary column (30 m × 0.25 mm i.d. ×
0.25 mm film thickness) was used. Pure helium was
used as the carrier gas at the flow rate of 1.2 mL
min−1. The injector, transfer line, and trap temper-
atures were 220◦C, 250◦C, and 200◦C, respectively.
Electron impact ionization was carried out at 70 eV.
Fig. 1. Probable degradation pathway for MO.
longing to the mono azo series used as a pH indi-
cator and also as a silk and wool dye. Azo dyes are
resistant to aerobic degradation (Pagga & Brown,
1986), but they can be decolorized by anaerobic treat-
ment (Brown & Hamburger, 1987) leading to the pro-
duction of toxic by-product such as aromatic amines
(Baughman & Weber, 1994).Therefore their degra-
dation process in aqueous media becomes very im-
portant. The present research work investigates the
influence of oxidation states of iron, Fe2+/Fe3+, in
the presence of symmetrical peroxides like H2O2 and
APS for the decolorization of MO under UV light. In
particular, direct comparison of the influence of var-
ious processes like Fe2+/H2O2/UV, Fe3+/H2O2/UV,
Fe2+/APS/UV, and Fe3+/APS/UV on the decoloriza-
tion kinetics was made.
Results and discussion
The various reactions between Fe2+/Fe3+ with ox-
idants H2O2/S2O28− along with their apparent rate
constants are shown in Table 1 (Kuši´c et al., 2006;
Truong et al., 2004). Degradation in the Fenton pro-
cess (dark) was ∼ 5 % for all the oxidation processes.
This indicates that oxidants and oxidation states of
iron have equivalent effect in dark. Complete decol-
orization for the above processes was achieved when
the system was amended with UV light. The higher ef-
ficiency of photo-Fenton/photo-Fenton-like processes
Experimental
Hydrogen peroxide (30 mass %), ferrous ammo-
nium sulfate hexahydrate, anhydrous ferric chloride,
ammonium persulfate, and methanol were supplied
from NICE chemicals (Bangalore, India). Methyl or-
ange (MO) (synonymous names: helanthine B, orange

380
L. G. Devi et al./Chemical Papers 64 (3) 378–385 (2010)
Table 1. Reactions involved in photo-Fenton/photo-Fenton-like processes
Reaction
Eq.
ka
.
Fe2+ + H2O2 → Fe3+ + HO + HO−
(1)
(2)
(3)
(4)
(5)
(6)
63–76 M−1 s−1
0.01–0.02 M−1 s−1
(0.1–3.1) × 105 M−1 s−1
3.33 × 10−6 M−1 s−1
(3.0–4.3) × 108 M−1 s−1
–
.
Fe3+ + H2O2 → Fe2+ + H+ + HOO
.
Fe3+ + HOO → Fe2+ + O2 + H+
.
Fe3+ + H2O → Fe2+ + HO + H+
.
Fe2+ + HO → Fe3+ + HO−
.
Fe2+ + S2O28− → Fe3+ + SO42− + SO4−
.
.
SO−4 +H2O → SO2− + HO + H+
(7)
(8)
6.6 × 102 s−1
4
Fe2+ + SO24− → FeSO4
22.9 M−1
Fe3+ + SO42− → FeSO+4
(9)
389 M−1
4470 M−2
Fe3+ + 2SO42− → Fe(SO4)−
(10)
2
.
Fe3+ + S2O28− → Fe2+ + 2SO4−
(11)
(12)
(13)
(14)
–
–
.
H+ + HO + e− → H2O
.
.
HO + HO → H2O2
(4.2–5.3) × 109 M−1 s−1
.
.
SO24− + HO → SO−4 + HO−
–
a) Values of rate constants were taken as reported by Kuši´c et al. (2006) and Truong et al. (2004).
can be ascribed to the huge production of hydroxyl
100
radicals under UV light. Oxidation and reduction of
iron ions continuously take place in the photo-Fenton
process whereas this cycle of reactions is rather slow
80
in the dark Fenton process.
60
Effect of pH
40
pH of the solution plays a significant role in the
effective action of oxidants on the pollutants. Kang
et al. (2002) reported that the photo-Fenton process
20
can remove pollutants only in acidic condition since
higher pH values are reported to be unsatisfactory.
0
1
2
3
4
5
6
7
8
9
10
Degradation of the dye was carried out in the pH
range of 1.2–9.5 by maintaining the concentration of
Fe3+ ions and H2O2 constant. At the lower pH, 1.2,
the degradation rate is reduced due to the excess H+
ions present in the solution acting as hydroxyl radi-
cal scavengers according to Eq. (12) in Table 1 (Bar-
busin´ski & Majewski, 2003). When pH of the medium
was increased from 1.2 to 3.0, complete decoloriza-
tion of the dye was achieved. At pH 3, half of the
iron species exists as Fe3+ ions and half as com-
pH
Fig. 2. Influence of pH on the decolorization under experimen-
tal conditions: c[Fe3+] = 6 mg L−1, c[H2O2] = 10
mg L−1, and c[MO] = 10 mg L−1
.
almost resisted degradation as shown in Fig. 2. This
inefficiency at higher pH values may be caused by the
instability of Fe2+/Fe3+ ions since they precipitate as
iron oxyhydroxide which reduces the concentration of
hydroxyl radicals in the solution affecting thus the de-
colorization rate.
plexes of [Fe (OH) (H2O) ]2+ which are the dominant
5
photo active species with highest light absorption co-
efficient and quantum yield for hydroxyl radicals pro-
duction along with Fe2+ ions regeneration in the wave-
length range of 280–370 nm. A change in this opti-
mum pH leads to the decrease in the concentration of
[Fe (OH)]2+ complexes and it can also result in pre-
cipitation of ferrous ions as oxyhydroxides. However,
the exact ratio of these photoactive species is uncer-
tain because the measurements generally suffer from
uncertainty (Barnum, 1983). The various photoactive
species of iron formed under different pH conditions
Effect of oxidizing agents
The influence of oxidant’s concentration on the
degradation rate was monitored by maintaining other
reaction parameters constant. The present study in-
vestigates the application of H2O2 (hydrogen perox-
ide) and S2O28− (peroxydisulfate) which are symmet-
rical peroxides and can be used as potential oxidants
in light induced reaction processes. Persulfate gener-
ates free radicals like sulfate and hydroxyl radicals
which provide free radical mechanism similar to the
hydroxyl radical pathways generated in the Fenton
are: [Fe (H2O) ]3+ (pH 1–2), [Fe (OH) (H2O) ]2+ (pH
5
2–3), and [Fe (6OH) (H2O) ]+ (pH 3–4) (Neamtu et
2
4
al., 2003). Beyond the optimum pH 3, degradation rate
slightly decreased and in the alkaline medium, the dye

L. G. Devi et al./Chemical Papers 64 (3) 378–385 (2010)
381
5
4
3
2
1
0
in the concentration of oxidant leads to a decrease in
the efficiency of the process. This can be due to the
recombination process of the excess hydroxyl radicals
generated as shown in Eq. (13). Similar experiments
were carried out with APS and its optimum concen-
tration was found to be 20 mg L−1 for both oxidation
states of iron (Fig. 3). The decolorization time was
longer when using APS for both oxidation states of
iron compared to H2O2. This can be caused by the
fact that APS generates protons along with hydroxyl
radicals, which lowers pH of the reaction medium. The
excess protons act as hydroxyl radical scavengers and
lower the degradation efficiency. In addition, the sul-
fate anion forms a strong complex with Fe2+/Fe3+
ions thereby preventing these ions to take part in the
cyclic Fenton process (Eqs. (8)–(10)). Sulfate radi-
cals can also scavenge the generated hydroxyl radi-
cals thereby affecting the decolorization kinetics (Eq.
(14)). Final pH of the solution remained at the level
of 3.3–3.5 for H2O2. While for APS, final pH of the
solution was in the range of 2.7–2.8.
0
10
20
30
40
50
-1
c /(mg L )
Fig. 3. Apparent rate constant
k versus concentration of
oxidants (H2O2/APS) on the decolorization kinet-
ics: Fe2+/H2O2/UV ( ), Fe3+/H2O2/UV ( ), Fe2+
/
,
◦
APS/UV (ꢀ), Fe3+/APS/UV ( ); c[Fe3+] = 6 mg L−1
•
c[Fe2+] = 10 mg L−1 for H2O2, c[Fe2+/Fe3+] = 10
mg L−1 for APS. Standard deviations for all the ex-
periments varied in the range of
0.009.
Effect of Fe2+/Fe3+ ions
5
4
3
2
1
Optimization of the catalyst is a necessary step
in the photo-Fenton reaction mechanism. In the pres-
ence of oxidants alone, 47 % and 25 % of the dye
was decolorized after three hours of UV illumination
for H2O2 and APS, respectively. Higher efficiency of
H2O2 is caused by the fact that direct photolysis of
H2O2 yields two hydroxyl radicals, while APS results
in the formation of one sulfate radical. This sulfate
radical, in a subsequent reaction with water, gener-
ates one hydroxyl radical (Eq. (7)). The production
of hydroxyl radicals is a single step for H2O2 as an
oxidant while two steps are needed for APS. However,
complete decolorization at a higher rate was achieved
in the presence of Fe2+/Fe3+ ions along with the ox-
idants. The Fe2+/Fe3+ ions decompose the oxidants
to the respective free radicals under UV light at a
higher rate thus increasing the concentration of hy-
droxyl radicals in the solution, which enhances the
degradation rate. The apparent rate constant calcu-
lated for the Fe3+/H2O2/UV process is 4.6 fold higher
than that of the Fe2+/H2O2/UV process, despite the
fact that the rate constant of the reaction between
Fe3+ and H2O2 is lower than that of the reaction be-
tween Fe2+ and H2O2 (Fig. 4). Higher efficiency of
Fe3+ ions can be ascribed to the formation of a hy-
droperoxyl radical which reduces the Fe3+ ion to the
Fe2+ ion at a much higher rate (Eq. (3)). The Fe2+
ions so formed react with H2O2 generating hydroxyl
radicals (Eq. (1)). Fe3+ ions can also react with water
forming hydroxo complexes which on UV irradiation
generate Fe2+ ions and hydroxyl radicals (Eq. (4)).
Though this sequence of reactions can also take place
in the process with Fe2+ ions as the catalyst, it is
still vital that these reactions depend mainly on pro-
0
3
6
9
12
15
18
21
2+
3+
-1
c [Fe /Fe ]/(mg L )
Fig. 4. Apparent rate constant
k versus concentration of
Fe2+/Fe3+ ions on the decolorization kinetics: Fe2+
/
H2O2/UV ( ), Fe3+/H2O2/UV ( ), Fe2+/APS/UV
◦
(ꢀ), Fe3+/APS/UV
(
); c[H2O2]
=
10 mg L−1
;
•
c[APS] = 20 mg L−1. Standard deviations for all the
experiments varied in the range of
0.009.
process. The sulfate radical is one of the strongest oxi-
dizing species in aqueous media with the redox poten-
tial of 2.6 V. It is next only to the hydroxyl free radical
whose redox potential is 2.8 V. With an increase in the
H2O2 concentration from 2 mg L−1 to 10 mg L−1, the
decolorization efficiency of the Fe2+/H2O2/UV and
Fe3+/H2O2/UV processes increased due to excess gen-
eration of hydroxyl radicals. The apparent rate con-
stant of the photo-Fenton-like process is high com-
pared to the photo-Fenton process. A further increase

382
L. G. Devi et al./Chemical Papers 64 (3) 378–385 (2010)
Table 2. Decolorization at different initial dye concentrations for photo-Fenton and photo-Fenton-like processes
Decolorization/%
c[MO]/(mg L−1
)
Fe2+/APS/UV
Fe3+/APS/UV
Fe2+/H2O2/UV
Fe3+/H2O2/UV
10
15
20
100
54
31
100
66
40
100
60
40
100
76
60
Table 3. Decolorization, apparent rate constant (k), and process efficiency (Φ) in the absence (A) and in the presence (B) of
methanol under optimized experimental conditions
Decolorization/%
k · 10−2/min−1
Φ · 10−12/(mg L−1 Einstein−1
)
Advanced Fenton process
A
B
A
B
A
B
Fe2+/H2O2/UV
Fe3+/H2O2/UV
Fe2+/APS/UV
Fe3+/APS/UV
100
100
100
100
32
15
54
73
1.01
4.65
0.86
1.17
006
008
005
007
0.18
006
004
003
002
16.2
36.5
10.8
13.9
5.82
5.4
5.8
0.23
0.24
0.56
10.1
portional concentration of the Fe2+/Fe3+ ions. The
process with Fe3+ ions as the catalyst has higher con-
centration of these ions, which accounts for its higher
efficiency. The concentration of Fe2+ was optimized
to 10 mg L−1 and Fe3+ was optimized to 6 mg L−1
with H2O2, while both Fe2+/Fe3+ were optimized to
10 mg L−1 with APS. The apparent rate constant cal-
culated for the Fe3+/APS/UV process was 1.36 fold
higher than that of the Fe2+/APS/UV. Fe2+ on re-
action with APS generates one sulfate radical (Eq.
(6)) which on reaction with water generates one hy-
droxyl radical. In contrast, Fe3+ generates two sulfate
radicals which on reaction with water generate two
hydroxyl radicals (Eq. (11)). Hence, a higher number
of hydroxyl radicals is generated, which accounts for
higher efficiency of the Fe3+/APS/UV process. Higher
concentration of the Fe2+/Fe3+ ions reduces the ap-
parent rate constant for both the oxidants due to the
scavenging effect of hydroxyl radicals by the Fe2+ ions
in the solution (Devi et al., 2009) (Eq. (5)).
agent; moreover, the dye solution can serve as an in-
ner filter reducing the number of photons entering
the solution (Dutta et al., 2001). This process de-
creases the concentration of hydroxyl radicals in the
solution. It is therefore preferential to have a lower
concentration of the pollutant to achieve higher rates
in the photo-Fenton/photo-Fenton-like processes. The
Fe3+/H2O2/UV process retained its efficiency even at
higher dye concentrations probably due to the excess
generation of hydroxyl radicals.
Effect of scavenger
The role of hydroxyl radicals in the degradation of
the MO dye was confirmed by carrying out the degra-
dation in the presence of a free radical scavenger like
methanol. Methanol is known to deactivate the hy-
droxyl radical and its derivatives (Devi et al., 2009).
Methanol reacts with hydroxyl radicals and to a lower
extent with hydrogen radicals whose second order rate
constants are 9.7 × 108 mol−1 s−1 and 2.6 × 106
Effect of initial concentration of dye
mol−1 −1, respectively (Eqs. (15) and (16)).
s
.
.
CH3OH + OH → CH3O + H2O
(15)
(16)
Efficiency of the dye degradation depends on the
initial concentration of the dye. The degradation of
the dye at various initial concentrations was investi-
gated by maintaining the constant concentration of
Fe2+/Fe3+ ions and H2O2/APS. When the dye con-
centration was 10 mg L−1, complete decolorization
was achieved in the desired time by all oxidation pro-
cesses studied. With the increase in the initial con-
centration to 15 mg L−1, the decolorization efficiency
decreased for all oxidation processes. Table 2 shows
the percentage decolorization efficiency of all oxida-
tion processes for different initial concentrations of
the dye. Higher initial concentration lowers the effi-
ciency due to insufficient availability of the oxidizing
.
.
CH3OH + H → CH3O + H2
c0 − c
Φ =
(17)
tIS
Though the initial step of the hydroxyl radicals
quenching is the same for both the oxidants, the ex-
tent of quenching depends not only on the nature of
the oxidants but also on the oxidation process. The
percentage decolorization, apparent rate constant and
the process efficiency calculated in the presence and in
the absence of alcohol are shown in Table 3. Process
efficiency can be defined as the concentration of the
pollutant degraded divided by the amount of energy

L. G. Devi et al./Chemical Papers 64 (3) 378–385 (2010)
383
1.2
5
4
3
2
1
0
A
0.8
0.4
B
C
0.0
190
400
500
300
Wavelength/nm
0.00
0.05
0.10
0.15
0.20
-1
c [MeOH] /(mol L )
Fig. 6. UV-VIS spectra of the MO dye before irradiation (A),
at 30 min (B), and at 4 h (C) under experimental con-
ditions: c[Fe3+] = 6 mg L−1, c[H2O2] = 10 mg L−1
pH 3.0.
;
Fig. 5. Effect of methanol concentration on the degradation ki-
netics of Fe3+/ H2O2/UV ( ) and Fe3+/APS/UV (
)
•
processes. Standard deviations for all the experiments
varied in the range of 0.009.
Table 4. Major photoproducts of the degradation process iden-
tified by the GC-MS technique
Degradation product
m/za
in terms of intensity and exposure surface area per
time (Eq. (17), where c0 is the initial concentration of
the dye substrate and c is the concentration at time
t and (c0 − c) denotes the concentration of the dye
degraded in mg L−1, I is the irradiance [I/(Einstein
4-Dimethylaminophenol
4-Hydroxybenzenesulfonic acid
Phenol
1,4-Dihydroxybenzene
Hydroquinone
137
174
94
110
108
78
m−2
s
−1) = 8.36 × λ (nm) × power (W)], where λ is
Benzene
370 nm and the power is 125 W, S denotes the solution
irradiated plane surface area in cm2 and t represents
the irradiation time in min). The decrease in the ap-
parent rate constant is larger for the oxidant H2O2
compared to APS (Fig. 5). This is due to the inability
of methanol to quench sulfate radicals which mediate
the degradation even in the absence of hydroxyl rad-
icals. Possible reactions of sulfate radical anions with
the organic pollutants are: (i) abstraction of a hydro-
gen atom from the saturated carbon; (ii) addition to
unsaturated compounds; (iii) removal of an electron
from anions and neutral molecules (Neta et al., 1977).
The process with Fe3+ ions as the catalyst generates a
higher number of hydroxyl radicals compared to Fe2+
ions. Hence, methanol quenches hydroxyl radicals to a
greater extent and thus lower apparent rate constant
is observed for the Fe3+ ion compared to Fe2+ ion me-
diated processes. The largest decrease in the apparent
rate constant was observed for the Fe3+/H2O2/UV
process. This provides evidence for the importance of
hydroxyl radicals in the photo-Fenton/photo-Fenton-
like degradation mechanisms.
a) Values given for parent peaks [M]+
.
lution changes from orange yellow to red due to the
formation of monoprotonated form of MO which ex-
ists as a resonance hybrid between its quinine diimine
and azonium structures. In an acidic solution, more in-
tense absorption band shift to higher wavelengths (at
506 nm). The visible region band can be attributed
to azonium ions. Two new bands at 317 nm and 226
nm in the UV region appeared due to the modifica-
tion of the π system delocalization. On irradiation,
the band at 506 nm reduced its intensity and com-
pletely disappeared at 30 min and no new band ap-
peared in the UV-VIS region. This confirms that pho-
todegradation of the azo chromophore is responsible
for the color of the MO dye molecule. The band at 229
nm decreased slowly indicating that the intermediates
formed during the degradation contain a benzene ring.
At 4 hours, the band completely disappeared confirm-
ing the complete degradation of the dye molecule as
shown in Fig. 6.
UV-VIS spectroscopic analysis
GC-MS analysis
MO in the pH range of 4.1–9.5 exhibits a band at
465–485 nm in the visible region attributed to the azo
form of the dye. The bands at 276 nm and 197 nm
may be caused by the presence of benzene rings in
MO. Under progressive protonation, color of the so-
Mass spectrum recorded at the time of decol-
orization (for the experiment using 10 mg L−1 of
Fe3+, H2O2, and MO, respectively, at pH 3) showed
two peaks at m/z 137 and 174 corresponding to the

384
L. G. Devi et al./Chemical Papers 64 (3) 378–385 (2010)
Fig. 7. Mass spectra of various intermediates formed during the reaction after the irradiation time of 30 min (A), 90 min (B), 2.5
h (C); c[Fe3+] = 10 mg L−1, c[H2O2] = 10 mg L−1; pH 3.0.
formation of 4-hydroxy-N,N-dimethylaniline and 4-
hydroxybenzenesulfonic acid, respectively (Table 4,
Fig. 7). This shows that the degradation of azo dye
by the homogeneous Fenton process proceeds through
a rupture of azo bonds by the attack of hydroxyl
radicals. Low intensity peaks were not accounted for.
The solution after 90 min of irradiation showed in-
tense peaks at m/z 94 and 110 and a low intensity
peak at m/z 108, which may be attributed to the for-
mation of phenol, 1,4-dihydroxybenzene, and hydro-
quinone, respectively. Formation of these intermedi-
ates can be explained as follows: (i) phenol is formed
by the loss of the N,N-dimethyl substituent as a sec-
ondary amine; (ii) 4-hydroxybenzenesulfonic acid, on
the loss of the sulfonate group followed by the subse-
quent hydroxylation, can give 1,4-dihydroxybenzene;
(iii) 1,4-dihydroxybenzene can be oxidized to hydro-
quinone in the presence of atmospheric oxygen.
and the loss of the sulfo group (SO3H) in benzene-
sulfonic acid might result in the formation of benzene.
Complete degradation of MO was confirmed by a peak
at m/z 44 corresponding to the M+ ion of formed CO2.
Based on the intermediates analyzed by the UV-VIS
spectroscopic analysis and the GC-MS technique, a
probable degradation pathway was proposed (Fig. 1).
Conclusions
Photo-Fenton and photo-Fenton-like processes
were used as potential techniques for the degrada-
tion of MO. The oxidation state of iron ions had
a significant effect on the decolorization kinetics. It
was found that processes with Fe3+ ions as the cat-
alyst show higher process efficiency compared to the
processes with Fe2+ as the catalyst. H2O2 served as
a better oxidant for both oxidation states of iron
compared to APS. A decrease in the apparent rate
constant in the presence of methanol confirmed the
role of hydroxyl radicals in the degradation process.
The solution, after 2.5 h of irradiation, showed a
peak at m/z 78 indicating the formation of benzene.
The dehydroxylation of phenol, 1,4-dihydroxybenzene

L. G. Devi et al./Chemical Papers 64 (3) 378–385 (2010)
385
Efficiency of the studied processes for the degra-
dation of MO follows the order: Fe3+/H2O2/UV,
Fe3+/APS/UV, Fe2+/H2O2/UV, Fe2+/APS/UV. The
degradation was monitored by the UV-VIS spec-
troscopy and GC-MS techniques. Based on the inter-
mediates analyzed, a probable degradation mechanism
has been proposed.
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Acknowledgements. Financial support from the UGC Major
Research Project (2007–2010), India is acknowledged.
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