Effect of chloride and sulfate ions on the advanced photo Fenton and modified photo Fenton degradation process of Alizarin Red S

Article abstract of DOI:10.1016/j.molcata.2013.03.023

The degradation of Alizarin Red S (ARS) was carried out by heterogeneous advanced photo Fenton processes (HAPFP) of the type Fe⁰/H ₂O₂/UV and heterogeneous modified photo Fenton process (HMPFP) of the type Fe⁰/ammonium persulfate (APS)/UV. The influence of various reaction parameters like pH, catalyst loading, concentration of the oxidants and the influence of inorganic anions such as Chloride (Cl^-) and Sulfate (SO₄^2-) ions on processes were investigated. Quenching of the hydroxyl radical by inorganic anions was confirmed by the decrease in the degradation rate constant for the HAPFP from 3.33 × 10^-2 to 0.19 × 10^-2 (for Cl^- (1 M)) and 0.29 × 10^-2 min^-1 (for SO₄^2- (1 M)). Similar decrease in rate constant for HMPFP is from 4.67 × 10 ^-2 to 0.41 × 10^-2(for Cl^- (1 M)) and 0.51 × 10^-2 min^-1 (for SO₄^2- (1 M)) process. The combined effect of concentration of sulfate and chloride ions on the rate constant for the degradation of ARS with APS and H₂O ₂ as oxidants is investigated. The initial degradation mechanism involves the cleavage of a quinone group to catechol as detected by UV-visible and GC-MS analysis.

Full text of DOI:10.1016/j.molcata.2013.03.023

Journal of Molecular Catalysis A: Chemical 374–375 (2013) 125–131
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Effect of chloride and sulfate ions on the advanced photo Fenton and modified
photo Fenton degradation process of Alizarin Red S
L. Gomathi Devi∗, C. Munikrishnappa, B. Nagaraj, K. Eraiah Rajashekhar
Department of Post Graduate Studies in Chemistry, Bangalore University, Central College City Campus, Dr. B. R. Ambedkar Street, Bangalore 560001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The degradation of Alizarin Red S (ARS) was carried out by heterogeneous advanced photo Fenton pro-
cesses (HAPFP) of the type Fe0/H2O2/UV and heterogeneous modified photo Fenton process (HMPFP) of
the type Fe0/ammonium persulfate (APS)/UV. The influence of various reaction parameters like pH, cat-
alyst loading, concentration of the oxidants and the influence of inorganic anions such as Chloride (Cl−)
and Sulfate (SO42−) ions on processes were investigated. Quenching of the hydroxyl radical by inorganic
anions was confirmed by the decrease in the degradation rate constant for the HAPFP from 3.33 × 10−2 to
Received 17 August 2012
Received in revised form 15 February 2013
Accepted 20 March 2013
Available online xxx
Keywords:
Zero valent metallic iron powder
Alizarin Red S
Advance Fenton process
Combined ionic effect
Modified Fenton process
0.19 × 10−2 (for Cl− (1 M)) and 0.29 × 10−2 min−1 (for SO4 (1 M)). Similar decrease in rate constant for
2−
HMPFP is from 4.67 × 10−2 to 0.41 × 10−2(for Cl− (1 M)) and 0.51 × 10−2 min−1 (for SO4 (1 M)) process.
2−
The combined effect of concentration of sulfate and chloride ions on the rate constant for the degradation
of ARS with APS and H2O2 as oxidants is investigated. The initial degradation mechanism involves the
cleavage of a quinone group to catechol as detected by UV–visible and GC–MS analysis.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
radical generation [6–9]. Lucking et al. tested iron powder, graphite
and activated carbon as active catalysts for the oxidation of 4-
Anthroquinone dyes represent the most important class of
commercial dyes. Alizarin Red S (ARS) belongs to the class of
Anthroquinone dyes. Pollution of water by these dyes is a seri-
ous problem and the removal of these dyes from wastewater is
a challenge to the related industries. Such stable compounds are
difficult to destroy by common treatment methods because of its
ability to chelate with specific cations. For the treatment of dye
containing wastewater, advanced oxidation process (AOPs) is gen-
erally accepted as potential technique to treat organic pollutant in
the industrial waste water. AOPs involve the generation of hydroxyl
radicals in situ which is a powerful oxidant and can effectively min-
eralize almost all the recalcitrant compounds. The most common
AOP techniques include photodecomposition of hydrogen peroxide
(H2O2/UV), photolysis of Ozone (O3/UV), photocatalysis (semicon-
ductor/UV), Fenton and photo Fenton’s reactions (Fe2+/H2O2/UV)
[1–4]. Compared to all the oxidation processes, Fenton’s process
(both homogenous and heterogeneous) is relatively economic and
simple. The disadvantages of this methodology are sludge genera-
tion due to iron, slow regeneration of Fe2+ ions in the solution and
also non reusability of the catalyst limits the efficiency of the reac-
tion especially in homogenous process [5]. Iron and its oxides are
widely explored in the degradation of the pollutants by hydroxyl
chloropenol in the presence of H2O2 [10]. They concluded that
metallic iron powder can replace iron salts as a catalyst in the photo
Fenton process. The reduction of iron ions (from Fe3+ to Fe2+) on the
iron surface proceeds at a faster rate in the heterogeneous process
compared to the same reaction in the homogeneous process.
2Fe3+ + Fe0 → 3Fe2+
(1)
The advantage of using solid metallic Iron powder is that it
can be easily removed through filtration or centrifugation after the
treatment. Contamination of Iron after the treatment is minimized.
Whereas in the case of classical Fenton process Fe2+ ions are used as
the catalyst and removal of these ions from the effluent is difficult
task. Hence it is more advantageous to use Fe0 metallic powder as
the catalyst from the environmental point of view. Moreover, use
of Fe0 as catalyst instead of solution of iron salts prevents the addi-
tional anion loading in the treatment of waste water. The remaining
iron powder can be easily removed and recycled after the treat-
ment. The main aim of the present research work is to use Zero
valent metallic iron (ZVMI) of 300 mesh size as a source to gener-
ate Fe2+ ions in the photo Fenton process. Industrial effluents are
usually contaminated with various inorganic ions. The influence of
these ions on the degradation rate is studied in detail. Individual
and combined effects of Cl− (Chloride) and SO4 (sulfate) ions on
2−
the degradation rates were investigated in detail. Effect of inorganic
anions on the photo degradation of ARS was studied. The efflu-
ents containing dyes are very difficult to treat because they contain
∗
Corresponding author. Tel.: +91 080 22961336.
E-mail address: gomatidevi naik@yahoo.co.in (L.G. Devi).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.03.023

126
L.G. Devi et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 125–131
Fe0 surface + H2O2−→ Fe3+ + 2OH−
(4)
various other ingredients like detergents, oils, inorganic salts etc.
The present research work mainly aims at studying the influence
of inorganic anions like Chloride (Cl−) and Sulfate (SO42−) on the
heterogeneous advanced photo Fenton processes (HAPFP) of the
type Fe0/H2O2/UV and Heterogeneous Modified Photo Fenton Pro-
cess (HMPFP) of the type Fe0/ammonium persulfate (APS)/UV on
the degradation rate.
Alternatively iron powder can also react with oxidants like H2O2
and APS leading to the generation of Fe2+ ions in the bulk of the
solution.
Fe0 + H2O2 → Fe2+ + 2OH−
(5)
2−
Fe0 + S2O8 → 2SO42− + Fe2+
(6)
2. Materials and methods
These Fe2+ ions so formed can react with oxidants leading to
the generation of active free radicals. During this process Fe2+ is
2.1. Materials
oxidized to Fe3+
.
Fe2+ + H2O2 → Fe3+ + 2• OH
(7)
(8)
Alizarin Red S (ARS), iron powder (300 mesh size, electrolytic),
ammonium per sulfate ((NH4)2S2O8 (APS), and hydrogen perox-
ide H2O2 (HP) (50% w/v), sodium chloride (NaCl), sodium sulfate
(Na2SO4) were supplied from S D Fine Chemicals, Bombay, India
and were used as received. The molecular formula, formula weight
and ꢀmax of ARS are C14H6 Na2O7S H2O, 360.28 and 514–520 nm
respectively.
2−
−
Fe2+ + S2O8 → Fe3+ + SO42− + SO4
Ferric ions (Fe3+) so formed can either react with water or with
H2O2 in the following two ways:
i) Ferric ions (Fe3+) can react with water molecules to form aqua
complex, which on UV-irradiation generates Fe2+ ions and hydroxyl
radicals.
2.2. Irradiation procedure
Fe3+ + H2O ⇔ [Fe(OH)2+] + H+
(9)
Advanced photo Fenton experiments were carried out at room
temperature using a circular glass reactor whose surface area is
176.6 cm2. 125 W medium pressure mercury vapor lamp is used
as the light source. Photon flux was found to be 7.92 mW/cm2 by
ferrioxalate actinometry [11,12]. The irradiation was carried out by
direct focusing the light into the reaction mixture in open air condi-
tion at a distance of 29 cm. The reaction mixture was continuously
stirred. All the experiments were performed using double distilled
water. The pH of the solution was adjusted either by adding dilute
NaOH or dilute H2SO4 and it is measured by using Systronics Digital
pH meter.
[Fe(OH)2+] + hv → Fe2+ + • OH
(10)
ii) Alternatively Fe3+ ions can also react with H2O2 leading to the
generation of Fe2+ ions and hydroperoxyl radicals. This hydroper-
oxyl radical has the ability to reduce Fe3+ ions and simultaneously
generate hydroxyl radicals. Fe2+ ions thus formed in the reaction
can actively participate in the cyclic Fenton reactions as described
by all the above equations.
Fe3+ + H2O2 → Fe2+ + • O2H + H+
(11)
Fe3+ + • O2H → Fe2+ + • OH
(12)
2.3. Analytical methods
Fe3+ ions can also react with persulfate anion. Two molecules of
sulfate radicals are produced which can react with water molecule
leading to the generation of hydroxyl radicals.
•
The samples collected at different time intervals were cen-
trifuged and filtered through 0.45 ␮m Millipore filter to remove the
catalyst particles. The centrifugates were analyzed by UV–visible
and GC–MS spectroscopic techniques using Schimadzu UV-1700
Pharmaspec spectrophotometer and GC–MS-QP 5000 Schimadzu
mass spectrometer. GC–MS analysis (using GC-MS-QP-5000 Shi-
madzu) and Thermo Electron Trace GC ultra coupled to a DSQ mass
spectrometer equipped with an Alltech ECONO-CAP-EC-5 capillary
column (30 m × 0.25mmi.d. ×0.25 mm film thickness) was used.
Pure helium was used as the carrier gas at a flow rate of 1.2 ml/min.
The injector/transfer line/trap temperature was at 220/250/200 ◦C
respectively. Electron impact ionization was carried out at 70 eV.
GC–MS-QP 5000 Schimatzu mass spectrometer was used to identify
the intermediates formed during the degradation process.
2−
−
Fe3+ + S2O8 → Fe2+ + 2SO4
(13)
SO4 • +H2O → SO42− + OH + H+
(14)
•
−
3.1. Effect of pH
Kang et al. had reported that the pollutants can be decolorized
efficiently in photo Fenton’s process only under acidic conditions
[13].
In the present study the experimentally observed degrada-
tion rate constants at pH 3 are found to be 3.33 × 10−2 and
4.67 × 10−2 min−1 with H2O2 and APS as oxidants respectively
2+
(Fig. 1). At pH 3, the concentration of Fe3+ ions and Fe [OH]
3. Results and discussions
complexes are the dominating photo active species which exist in
almost equal proportions. The decrease in this optimum pH leads
to decrease in the concentration of Fe [OH] 2+ complexes and it can
also result in the precipitation of ferrous ions as oxy hydroxides.
The various photoactive species of iron formed at different pH con-
The various possible reactions taking place on the Fe0 surface
with the oxidants are illustrated as follows:
Under acidic conditions iron powder undergoes oxidation to
give ferrous ions (Fe2+).
3+
2+
+
ditions are: Fe [H2O]6
(pH 1–2), Fe [OH] [H2O]5
(pH 2–3) and
H
Fe0−→ Fe2+
(2)
+
Fe [OH]2[H2O]4 (pH 3–4) [14,15]. The lower rate constant at pH
1.5 is mainly due to the excess H+ ions in the solution acting as
hydroxyl radical scavenger [16] (Eq. (15)).
Iron powder in the presence of H2O2 generates Fe2+ ions
which are partially adsorbed on iron surface. Further, these surface
adsorbed Fe2+ ions can react with H2O2 leading to the generation
of hydroxyl radicals.
H+ + • OH + e− → H2O
(15)
Feng et al. had reported that the degradation efficiency
decreases beyond the optimum pH conditions (pH 3) [17]. Dye
Fe0 + H2O2 → Fe2+ surface + 2OH−
(3)

L.G. Devi et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 125–131
127
Table 1
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
H2O2
APS
Percentage degradation, rate constant and process efficiency values for the degra-
dation of ARS at different experimental conditions.
Oxidation
processes
Percentage
Rate constant
Process efficiency (Ф)
(×10−12 ppm Einstien−1
degradation (k) × 10−2 (min−1
)
)
UV/H2O2
UV/APS
15
26
18
26
38
37
74
100
0.08
0.14
0.01
0.18
0.21
0.27
3.33
4.67
12.61
18.83
14.27a
14.14a
15.59a
16.42
52.41
70.12
Fe0/dark
Fe0/H2O2/dark
Fe0/APS/dark
Fe0/UV
HAPFP
HMPFP
a
Process efficiency in dark process is expressed in ppm.
0
20
40
60
80
Similar experiments were carried out with APS and its opti-
mum concentration was found to be 40 ppm with a rate constant of
4.67 × 10−2 min−1 (Fig. 1). The rate constant values calculated for
the HMPFP is almost 1.5 times higher than HAPFP (Fig. 1). This is
because of the influence of oxidants on the final pH of the reaction
medium. The final pH of the solution was 3.6 for 20 ppm H2O2 and
2.9 for 40 ppm APS. Since APS provides better acidic pH condition
which is most essential for the photo Fenton’s process, it accel-
erates the rate of the reaction compared to H2O2. This favorable
acidity conditions results in the higher concentration of the Fe2+
ions in the bulk of the solution due to the dissolution of iron pow-
der. This further enhances the generation of hydroxyl free radicals.
The pH of the solution was 2.12 after one hour of UV irradiation for
higher concentration of APS (80 ppm). Hence rate constant drasti-
cally decreases to 0.68 × 10−2 min−1. The excess acidity increases
the concentration of protons in the solution which acts as hydroxyl
radical scavenger. The observed higher pH in the case of H2O2
reduces the concentration of photo active species leading to the
decrease in the rate constant.
Concentration of Oxidants in ppm
Fig. 1. Plot of rate constant versus concentration of oxidants (H2O2 and APS) for the
experimental conditions of: [Fe0] = 10 mg, [ARS] = 40 ppm at pH 3.0 0.04. The rate
constants were calculated from the slope of the linear plot of log C/C0 versus time,
where C0 and C are the initial and residual concentration of ARS at different time
intervals.
resists degradation when the solution pH is greater than 7. This
is due to the coagulation of Fe3+ complex molecules in the reaction
medium which inhibits the catalytic reaction of Fe2+ ions with the
oxidants. At high pH conditions, iron oxy hydroxide precipitates
and gets deposited on the surface of iron powder preventing the
electron transfer between the catalyst and the oxidant. This hin-
ders the process of formation of the hydroxyl radicals. Therefore
both HAPFP and HMPFP experiments were carried out at optimized
condition of pH 3.
3.2. Effect of oxidants
3.3. Effect of catalyst loading
The present study investigates the application of H2O2 and APS
which are symmetrical peroxides and can be potential oxidants in
the light induced reaction processes. Persulfate ions can generate
sulfate and hydroxyl free radicals and they can provide free radical
mechanism similar to hydroxyl radical pathways as generated in
the Fenton’s chemistry. Sulfate radical is one of the strongest oxi-
dizing species in aqueous media with a redox potential of 2.6 V,
which is next only to the hydroxyl free radical whose redox poten-
tial is 2.8 V. The sulfate radical anions produced in the case of APS
shows various possible reaction mechanism in the process of min-
eralization which are as follows: (i) abstraction of hydrogen atom
from the saturated carbon, (ii) The ions are capable of adding to
the unsaturated compounds, (iii) It can remove an electron from
the anions and neutral molecules [18,19]. These attributes com-
bine to make the persulfate ions a viable option for the chemical
oxidation of a broad range of contaminants. The influence of oxi-
dants on the degradation rate was investigated by maintaining the
other reaction parameters constant (Iron powder dosage = 10 mg,
dye concentration = 40 ppm at pH 3.0 0.04). When the concentra-
tion of H2O2 was 20 ppm, the rate constant for the degradation
is 3.33 × 10−2 min−1 as shown in Fig. 1. This is the maximum
rate observed for H2O2. Higher or lower concentration of H2O2
decreases the rate constant. This can be due to the recombination
of excessively generated hydroxyl radicals or these hydroxyl radi-
cals may get involved in the unwanted reaction pathways as shown
below:
The optimization of the catalyst concentration is a necessary
step in the photo Fenton reaction mechanism. The degradation effi-
ciency in the absence of Fe0 catalyst was found to be 15 and 26
percent for H2O2/UV and APS/UV process for two hours of irradi-
ation (Table 1) respectively. This is due to the direct photolysis of
oxidants in the presence of UV light. However complete degrada-
tion and higher efficiency was achieved by the use of iron surface
in the presence of oxidants. The iron surface catalytically decom-
poses the oxidants to respective free radicals/ions at a faster rate to
generate more hydroxyl radicals under UV light. In contrast, over-
loading of the catalyst hinders the degradation efficiency. This may
be due to: (i) higher concentration of the catalyst results in turbidity
which hinders the UV light penetration [20]. (ii) Higher dosage of
iron powder increases the concentration of Fe2+ ions in the solution
which can also act as hydroxyl radical scavenger [16].
Fe2+ + OH• → Fe3+ + HO−
Additionally photo reduction of Fe3+ to Fe2+ ions takes place at a
faster rate on the iron surface in HAPFP as shown by the following
equation:
(18)
2Fe3+ + Fe0 → 3Fe2+ surface
(19)
The increase in the dosage of iron powder shifts the reaction pH
from acidic to alkaline condition which decreases the degradation
rate. The shift in the pH mainly depends on the nature of the oxi-
dants which can be explained as follows: When the catalyst loading
was varied from 10–40 mg, the final pH of the reaction mixture was
found to be 3.6 with H2O2 as the oxidant. The increase in the cat-
alyst concentration to 80 mg the pH of the solution changes to 4.8.
O2H• + OH• → H2O + O2
(16)
(17)
2OH• → H2O2

128
L.G. Devi et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 125–131
Turbidity in the reaction mixture is observed at this pH condition.
With further increase in the catalyst loading to 150 mg, excess iron
precipitates as iron oxy hydroxide which gets separated from the
true solution and the pH of the solution changes to 5.6. But in the
case of APS as the oxidant the final pH of the reaction medium was
found to be 3.9 for all higher concentration of iron powder. Hence
it can be concluded that APS effectively inhibits the precipitation of
iron powder by providing excess acidity to the reaction medium.
0.5
0.4
0.3
0.2
0.1
H2O2
APS
3.4. Effect of initial dye concentration
Concentration of the substrate dye molecule affects the rate
of degradation significantly in a photo Fenton process. Therefore
experiments were performed at different initial dye concentra-
tions by maintaining the other reaction parameters constant. The
percentage degradation decreases to almost fifty percent for the
100 ppm initial dye concentration. This may be due to the fact
that, as the dye concentration increased, a proportional increase of
hydroxyl radicals does not take place. High dye concentration pre-
vents the UV light penetration into the depth of the solution, there
by decreasing the extent of generation of hydroxyl radicals [21]. As
reported by Reis et al. the active centers on the iron surface will be
completely occupied by the dye molecules at high concentrations
of the dye, which are capable of reducing the catalyst surface itself
thereby hindering the process of generation of hydroxyl radicals
[22]. The HMPFP shows higher efficiency compared to HAPFP at
higher initial dye concentrations.
0.0
0.2
0.4
0.6
0.8
1.0
Concentration of Na 2SO 4 salt (M)
Fig. 2. Plot of rate constant versus concentration of Na2SO4 ([Fe0] = 10 mg,
[ARS] = 40 ppm, [H2O2] = 20 ppm/[APS] = 40 ppm at pH 3.0 0.04).
Further the generated ferrous ions reacts with hydrogen peroxide
forming ferric ions and hydroxyl radicals as discussed previously
(Eqs. (7) and (10)) [24,25]. The various possible reactions that can
take place between chloride and Fe3+ ions are shown below:
Fe3+ + Cl− ↔ [Fe(Cl)2+
]
(21)
(22)
(23)
[Fe(Cl)2+] + hꢁ → Fe2+ + Cl•
Cl• + Cl− → Cl2
•
−
3.5. Process efficiency (Ф)
In presence of sodium sulfate, the degradation reaction rate con-
stant decreases drastically for both HAPFP and HMPFP. Sulfate ions
The process efficiency (Ф) implies the effectiveness of the
present method. It can be defined as the ratio of residual concentra-
tion of the pollutant to the amount of energy in terms of intensity
and exposure surface area per time.
in aqueous solution can form complex with both ferrous and ferric
2+
ions of the type: FeSO4, Fe (OH)
and FeSO4+. This reduces the
concentration of the ferrous and ferric ions in the solution whose
concentration is most essential in Fenton Chemistry. At pH 3.0,
C0 − C
˚ =
(20)
+
2+
t.I.S
FeSO4 and Fe (OH)
are the major predominant ferric species
+
present. On irradiation with UV light FeSO4 produces ferrous ions
and sulfate anions whose quantum yield is quite low [23,24].
where C0 is the initial concentration of the dye substrate, C is the
concentration at time ‘t’. (C0 − C) denotes the residual pollutant
concentration in mg L−1 or ppm. ‘I’ is the irradiation intensity. ‘S’
denotes the solution irradiated plane surface area in cm2 and ‘t’ rep-
resents the irradiation time in minutes. Process efficiency under UV
illumination is expressed in ppm Einstein−1. Process efficiency for
the process in the absence of light is expressed as ppm. The results
show that the HMPFP shows higher efficiency compared to all the
other process (Table 1).
−
FeSO4+ + hꢁ → Fe2+ + SO4
(24)
The influence of these anions on the degradation rate can be
summed up as follows:
1) Both Fe (II) and Fe (III) ions form strong complexes with these
anions, which effect the distribution and reactivity of the iron
species present in the solution.
2) Both chloride and sulfate ions scavenge the hydroxyl radicals
leading to the generation of the less reactive inorganic species
like Cl−•
3.6. Effect of inorganic salts (chloride and sulfate) on the
degradation rate of ARS
3) Dichloride anion and sulfate radical anion are the major pre-
dominant contributors for the free radical degradation reaction
mechanism in presence of chloride and sulfate ions.
Presence of inorganic anions like chloride and sulfate ions in
the waste water has a significant effect on the overall Fenton
degradation reaction rate [23]. The presence of these anions in the
reaction medium has decreased the degradation rate drastically.
The first order rate constant (k) value for HAPFP has decreased from
3.33 × 10−2 to 0.28 × 10−2 min−1 in presence of chloride ions and
0.13 × 10−2 min−1 with sulfate ions. Similar decrease in k value for
HMPFP process is from 4.672 × 10−2 to 0.22 × 10−2 min−1 in pres-
ence of chloride ions and 0.33 × 10−2 min−1 with sulfate ions as
shown in Fig. 1 (Table 1). In both HAPFP and HMPFP, the rate con-
stant decreases drastically when sodium chloride is added. The
addition of sodium chloride to an aqueous solution of ferric ions
•−
−•
are oxidant species formed and they are
The Cl2
and SO4
less reactive compared to the hydroxyl radicals [26,27].
4) In the presence of UV light the quinone group in the ARS
molecule may interact more effectively with sulfate and chloride
ions leading to a different degradation mechanisms.
3.6.1. Effect of sulfate on degradation rate of ARS in HAPFP and
HMPFP
In case of HAPFP and HMPFP when the concentration of sodium
sulfate is increased from 0.01 to 0.1 M, the rate constant increases
slightly as shown in Fig. 2. The rate of degradation increases with
further increase of sodium sulfate concentration to 1 M. The excess
results in the formation of less reactive [Fe (Cl) 2+] and [Fe (Cl2) +
]
complexes, although [Fe (OH) 2+] is still the predominant species
at pH 3.0. In the absence of any added salt at pH 3.0 the reaction
is dominated by the photo reactive [Fe (OH)2+] complex (Eq. (10)).

L.G. Devi et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 125–131
129
H2O2
APS
0.6
0.5
0.40
0.35
0.30
0.25
0.20
0.15
0.10
H2O2
APS
0.4
0.3
0.2
0.1
0.0
0.0
0.2
0.4
0.6
0.8
1.0
0.02
0.04
0.06
0.08
0.10
Molar Concentration of NaCl
Molar Concentration of NaCl
Fig. 3. Plot of rate constant versus concentration of NaCl ([Fe0] = 10 mg,
[ARS] = 40 ppm, [H2O2] = 20 ppm/[APS] = 40 ppm at pH 3.0 0.04).
Fig. 4. Plot of rate constant versus concentration of NaCl. ([Fe0] = 10 mg,
[ARS] = 40 ppm, [H2O2] = 20 ppm/[APS] = 40 ppm and [Na2SO4] = 1 M at pH
3.0 0.04).
sulfate ions react with water molecule leading to the generation of
the hydroxyl radicals as shown below.
ClOH• + Fe2+ → Cl− + OH− + Fe3+
(34)
−
Other possible reactions at pH 3.0 are:
In presence of chloride ions various inorganic radicals like Cl•,
HClOH•, ClOH•−, Cl2•−are formed among which, the dichloride
anion radical is predominant and hence a greater trend in the
decrease of the degradation rate is observed and it inhibits the
production of hydroxyl radicals [31,32].
2−
−
H
+ + SO4 ↔ HSO4
(25)
(26)
(27)
(28)
(29)
(30)
(31)
(32)
HSO4− + OH• → SO4 − + H2O
•
SO4 − + H2O → H+ + SO42− + OH•
•
•
•
3.7. Combined effect of chloride and sulfate ions
SO4 − + H2O2 → H+ + SO42− + OH2
Combined effect of concentration of chloride and sulfate ions
on the degradation of ARS using hydrogen peroxide and APS as the
oxidants was studied. The concentration of sodium sulfate (1 M) is
kept constant and concentration of sodium chloride is varied from
0.02 to 0.10 M for both HAPFP and HMPFP as shown in the Fig. 4.
In the case of HAPFP the rate constant increases from 0.2 × 10−2
to 0.39 × 10−2 when the concentration of NaCl was varied from 0.02
to 0.1 M. Similar increase in the rate constant from 0.41 × 10−2 to
0.61 × 10−2 was observed for HMPFP.
•
•
•
SO4 − + 2OH2 → 4H+ + SO4 − + O2
•
•
SO4 − + SO4 → S2O8
−
2−
•
−
2−
Fe2+ + SO4 → Fe3+ + SO4
•
•
SO42− + Cl → SO4 − + Cl−
The rate of degradation in HMPFP is high compared to HAPFP,
due to the higher concentration of the hydroxyl radicals generated
in this process.
The maximum retardation of the rate is observed in case of
HAPFP compared to HMPFP. The maximum quenching of hydroxyl
free radicals by chloride free radicals takes place in the case of
HAPFP, where hydrogen peroxide is used as the oxidant. In pres-
ence of UV light more number of chloride free radicals are produced
as the concentration of sodium chloride is increased. These free
radicals react with sulfate ions leading to the formation of sulfate
anion radical (Eq. (32)) which further generates hydroxyl radicals
according to Eq. (27).
3.6.2. Effect of chloride ions
In HAPFP, the rate constant decreases slightly when sodium
chloride is added. In case of HMPFP process when the concentra-
tion of sodium chloride is increased from 0.01 to 1 M rate constant
increases slightly as shown in Fig. 3.
Some of the sulfate radical anions (from APS) react with water
molecule to generate hydroxyl radicals (Eq. (27)). Sulfate ion can
react with chloride radical leading to the formation of sulfate anion
radical (Eq. (32)) which later yields hydroxyl radicals. Sulfate ions
can also react with chloride radical to form hydroxyl radical. There-
fore the concentrations of hydroxyl radicals are sustained to some
extent. In case of HAPFP the rate of degradation decreases slightly
when concentration of sodium chloride is increased from 0.01 to
1 M (Fig. 3). When the chloride ion concentration increases in the
photo assisted Fenton reactions at pH 3 in situ formation of the
FeCl2+ complex takes place and this complex competes with the Fe
(OH)2+ complex for UV light photons and hence the rate of pro-
duction of hydroxyl radical decreases. Cl− acts as a strong free
radical scavenger and it reacts with OH• under acidic pH conditions
[28–30].
In the other experiment the concentration of sodium chloride
(1 M) is kept constant and concentration of sodium sulfate is varied
from 0.02 to 0.10 M for both HAPFP and HMPFP. The results are
shown in the Fig. 5.
In both the cases the degradation rate increases gradually with
increase in concentration of sodium sulfate. The degradation rate
is slightly high in HMPFP compared to HAPFP. The concentration of
sulfate anions are more in case of HMPFP compared to HAPFP.
The sulfate anion radical further participates in the degradation
reaction mechanism as shown in the (Eqs. (25)–(32)). Hydroxyl and
hydroperoxyl radicals are further generated. The extent of produc-
tion of these radicals and the rate at which they are produced will
dictate the degradation reaction rate. However one can conclude
that the observed rate constants in the presence of sulfate and chlo-
ride ions are very low compared to the degradation reaction in the
absence of these ions.
Cl− + OH → ClOH•
(33)
•
−

130
L.G. Devi et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 125–131
O
-
H2O2
APS
SO3
0.5
0.4
0.3
0.2
0.1
0.0
OH
O
O
OH
SO3H
OH
COOH
+
+
COOH
COOH
1
2
3
0.02
0.04
0.06
0.08
0.10
Molar concentration of Na2SO4
OH
OH
Fig. 5. Plot of rate constant versus concentration of Na2SO4. ([Fe0] = 10 mg,
[ARS] = 40 ppm, [H2O2] = 20 ppm/[APS] = 40 ppm and [NaCl] = 1 M at pH 3.0 0.04).
4
3.8. UV–visible and GC–MS analysis
The ꢀmax of the ARS at pH 3 is found to be 422 nm before
the initiation of the reaction. In the absence of inorganic salts,
there was continuous decrease in the absorbance of this peak
in the photo Fenton process. However, in the presence of inor-
ganic salt, the peak corresponding to the ꢀmax showed red shift
to 525–536 nm. This shift was observed over the entire concen-
tration range of NaCl/Na2SO4 and the intensity of absorbance
increased with increase in the concentration. The GC–MS analy-
sis has been performed for the experiment Fe0/APS/UV (HMPFP)
([ARS] = 40 ppm, [Fe0] = 10 mg and [APS] = 40 ppm at pH 3.0 0.04
exposed to UV irradiation) which shows higher efficiency. During
the degradation process about 4 ml of the sample was withdrawn at
regular time intervals and this solution was centrifuged and filtered
to remove the catalyst particles present in the solution. The samples
were further extracted into a non aqueous ether medium and 1.0 ␮l
of each of this solution is injected into the GC–MS instrument. The
various major reaction intermediates formed during the process of
degradation were analyzed by GC–MS technique and the results are
presented in Table 2. Several low intensity m/z peaks corresponding
to the minor intermediate products were left unidentified. A prob-
able degradation reaction mechanism has been proposed based
on the UV–visible and GC–MS spectroscopic analysis is shown in
Fig. 6.
O
SO2-
4
CO2
+
H2O
+
O
5
Fig. 6. Probable degradation reaction mechanism pathway of ARS.
4. Conclusion
The influence of various inorganic anions that are commonly
found in industrial effluents on the degradation mechanism is stud-
ied. Both sulfate and chloride ions retard the efficiency of the
process by scavenging the hydroxyl radical and also by forming
the complexes with iron ions. These inorganic ions reduce the rate
of generation of hydroxyl radicals from Fenton reagents. The maxi-
mum retardation of the rate is observed in case of HAPFP compared
to HMPFP. However the degradation process was not completely
quenched since some of the less reactive radicals like Cl•, HClOH•,
ClOH•, Cl2•−, SO4•−, OH2• are formed and the degradation reaction
is sustained. The extent of formation of higher reactive ions among
these ions will decide the fate of the reaction. However one could
conclude that the most reactive radical is hydroxyl free radical,
higher reaction rate in the absence of salts confirms this fact.
Acknowledgements
Table 2
List of the major intermediate products identified by GC–MS technique at various
time intervals during the degradation of ARS.
Financial support from DST-SERC major research project, DST-
IDP major research project and UGC major research project is
acknowledged.
Irradiation time (min)
30
m/z values
165
Intermediate products
Pthalic acid (1)
138
174
Salicylic acid (2)
o-Hydroxy sulfonic acid (3)
References
[1] A. Rathi, H.K. Rajor, R.K. Sharma, J. Hazard. Mater. 102 (2003) 231–241.
[2] M.Y. Ghaly, G. Hartel, R. Mayer, R. Haseneder, Waste Manage. 21 (2001) 41–47.
[3] S. Kim, W. Choi, Environ. Sci. Technol. 36 (2002) 2019–2027.
[4] Y. Sun, J.J. Pignatello, Environ. Sci. Technol. 27 (1993) 304–310.
[5] I. Muthuvel, M. Swaminathan, Catal. Commun. 8 (2007) 981–986.
[6] J. Yu, X. Yu, B. Huang, X. Zhang, Y. Dai, Cryst. Growth Des. 9 (3) (2009)
1474–1480.
60
90
110
108
Catechol (1, 2 dihydroxybenzene) (4)
Cyclo 3, 5- hexadiene 1, 2-dienone (5)
96
44
18
Sulfate anion
Carbon dioxide
Water molecules
The number in the parenthesis represents the intermediate products in the proposed
mechanism (Fig. 6).
[7] X. Zhou, J. Lan, G. Liu, K. Deng, Y. Yang, G. Nie, J. Yu, L. Zhi, Angew. Chem. Int.
Ed. 51 (2012) 178–182.

L.G. Devi et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 125–131
131
[8] Q. Xiang, J. Yu, P.K. Wong, J. Colloid. Interface Sci. 357 (2011) 163–167.
[9] J. Zhou, Z. Zhang, B. Cheng, J. Yu, Chem. Eng. J. 211–212 (15) (2012) 153–160.
[10] F. Lucking, H. Koser, M. Jank, A. Ritter, Water. Res. 32 (1998) 2607–2614.
[11] L. Gomathi Devi, G.M. Krishnaiah, J. Photochem. Photobiol.: Chem. 121 (1999)
141–145.
[12] J. Lee, H.H. Seliger, J. Chem. Phys. 40 (1964) 519–523.
[13] S.F. Kang, C.H. Liao, M.C. Chen, Chemosphere 46 (2002) 923–928.
[14] C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, John-Wiley and sons, New
York, 1976.
[20] O.S.N. Sum, J.Y. Feng, X.J. Hu, P.L. Yue, Top. Catal. 33 (2005) 233–242.
[21] K. Dutta, S. Mukhopadhyay, S. Bhattacharjee, B. Chaudhuri, J. Hazard. Mater. 84
(2001) 57–71.
[22] F.A.P. Reis, E.M. Azeved, J.C.R. Nozaki, Sol. Energy 77 (2004) 29–35.
[23] D.L. Joseph, T. Le Giang, L. Bernard, Chemosphere 55 (2004) 715–723.
[24] Amilcar Machulek Jr., J.E. Moraes, L.T. Okano, C.A. Silvério, F.H. Quina, Pho-
tochem. Photobiol. Sci. 8 (2009) 985–991.
[25] J. Kiwi, A. Lopez, V. Nadtochenko, Environ. Sci. Technol. 34 (2000)
2162–2168.
[15] M. Neamtu, A. Yediler, I. Siminiceanu, A. Kettrup, J. Photochem. Photobiol. A
161 (2003) 87–93.
[16] K. Barbusinski, J. Majewski, Pol. J. Environ. Stud. 12 (2003) 151–155.
[17] H.E. Feng, Le-cheng, Lei, New. J. Chem. 5 (2004) 198–205.
[18] P. Neta, V. Madhavan, H. Zemel, R.W. Fessemdem, J. Am. Chem. Soc. 99 (1977)
163–164.
[26] P. Neta, R.E. Huie, A.B. Ross, J. Phys. Chem. 17 (1988) 1027–1247.
[27] Y. Dong, J. Chen, C. Li, H. Zu, Dyes Pigm. 73 (2007) 261–268.
[28] C. Chiing-Chang, W. Ren-Jang, T. Yi-You, L. Chung-Shin, J. Chin. Chem. Soc. Taip.
56 (2009) 1147–1155.
[29] H. Chen, T. Zhang, J. Zhang, J. Hazard. Mater. 161 (2009) 1473–1477.
[30] S.S. Ashraf, A.M. Rauf, S. Alhadrami, Dyes Pigm. 69 (2006) 74–78.
[31] Q. Yinan, C. Shang, M.C. Lo Irene, Water. Res. 38 (2004) 2375–2383.
[32] E.M. Siedlecka, P. Stepnowski, Pol. J. Environ. Stud 14 (2005) 823–828.
[19] L. Gomathi Devi, S. Girish Kumar, K. Mohan Reddy, C. Munikrishnappa, J. Hazard.
Mater. 164 (2009) 459–467.