Photocatalytic degradation of lindane by polyoxometalates: Intermediates and mechanistic aspects

Article abstract of DOI:10.1016/j.cattod.2010.02.017

The photocatalytic degradation of lindane (γ-1,2,3,4,5,6-hexachlorocyclohexane) has been studied in the presence of the polyoxometalate PW₁₂O₄₀^3- in aqueous solutions. Lindane is fully decomposed to CO₂, Cl^- and H₂O, while a great variety of intermediates has been detected using GC-MS, including aromatic compounds (dichlorophenol, trichlorophenols, tetrachlorophenol, hexachlorobenzene, di- and trichloro-benzenodiol), non-aromatic cyclic compounds (penta-, tetrachlorocyclohexene, heptachlorocyclohexane), aliphatic compounds (tetrachloroethane) and condensation products (polychlorinated biphenyls). The number and nature of the intermediates implies that the mechanism of decomposition of lindane is based on both oxidative and reductive processes. Common intermediates have been reported during photolysis of lindane in the presence of titanium dioxide. A similar overall mechanism of polyoxometalates and TiO₂ photocatalysis through the formation of common reactive species is suggested.

Full text of DOI:10.1016/j.cattod.2010.02.017

Catalysis Today 151 (2010) 119–124
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Photocatalytic degradation of lindane by polyoxometalates: Intermediates and
mechanistic aspects
S. Antonaraki, T.M. Triantis, E. Papaconstantinou, A. Hiskia∗
Institute of Physical Chemistry, National Center for Scientific Research “Demokritos”, 15310 Agia Paraskevi, Attiki, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 12 March 2010
The photocatalytic degradation of lindane (␥-1,2,3,4,5,6-hexachlorocyclohexane) has been studied in
the presence of the polyoxometalate PW12O40 in aqueous solutions. Lindane is fully decomposed to
3−
CO2, Cl− and H2O, while a great variety of intermediates has been detected using GC–MS, including
aromatic compounds (dichlorophenol, trichlorophenols, tetrachlorophenol, hexachlorobenzene, di- and
trichloro-benzenodiol), non-aromatic cyclic compounds (penta-, tetrachlorocyclohexene, heptachloro-
cyclohexane), aliphatic compounds (tetrachloroethane) and condensation products (polychlorinated
biphenyls). The number and nature of the intermediates implies that the mechanism of decomposition of
lindane is based on both oxidative and reductive processes. Common intermediates have been reported
during photolysis of lindane in the presence of titanium dioxide. A similar overall mechanism of poly-
oxometalates and TiO2 photocatalysis through the formation of common reactive species is suggested.
Keywords:
Polyoxometalates
Photocatalysis
Lindane
Photodegradation mechanism
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
has been applied to many organic pollutants, for example phenol,
chlorophenols and chloroacetic acids [5,6,14], diversified pesticides
Water detoxification has become an environmental issue of ever
increasing importance. In addition to conventional technologies
used for purification of water, a new type of treatment process,
commonly referred as advanced oxidation processes (AOPs), is
emerging. AOPs are increasingly being considered as alternatives
to more conventional technologies because they destroy haz-
ardous organic compounds rather than transferring them to other
media. AOPs are generally characterized by their ability to generate
hydroxyl radicals, some examples being UV light in the presence
of hydrogen peroxide or ozone and UV–near-visible light in the
presence of TiO2 [1–3].
Recently the photocatalytic mineralization of a great variety of
chemicals has been demonstrated using polyoxometalates (POM),
providing an interesting route to the destruction of toxic and haz-
ardous pollutants even in the ppb or few ppm levels [4–8]. POM are
acid condensation products, mainly of molybdenum and tungsten
[9–11] that become powerful oxidizing reagents [12] upon excita-
tion with near-visible and UV light, capable of destroying a great
variety of pollutants in the aquatic environment [4–8]. POM are at
least as effective as the widely publicized TiO2, representing a sim-
ilar photocatalytic performance in terms of the overall mechanism,
the intermediate species involved and the final degradation prod-
ucts (i.e., CO2, H2O and inorganic anions) [13]. The above method
[15], insecticides such as lindane [4] and fenitrothion [16], herbi-
cides such as atrazine [17], bentazone, etc. [18]. In all cases final
degradation products were CO2, H2O and inorganic anions, with the
exception of atrazine, which is degraded into the less toxic cyanuric
acid.
Lindane (1,2,3,4,5,6-hexachlorocyclohexane; predominately ␥-
isomer) is an organochlorine insecticide which has been used on
a wide range of soil-dwelling and plant-eating insects [19]. It has
been found in groundwater samples, as well as in the marine envi-
ronment where organisms tend to concentrate this highly lipophilic
compound in the fatty tissues [20]. Due to its high toxicity, it is
classified as Restricted Use Pesticide purchased only to certified
applicators, it is no longer manufactured in the US and most of its
uses, i.e., in agriculture and in the diary industry have been canceled
by EPA.
Despite its persistence, lindane biodegradation has been
reported under various aerobic and anaerobic conditions [21].
Unfortunately, the process proceeds rather slowly [22]. Alterna-
tively, lindane can be converted by decomposition via sonochem-
ical induced destruction [23]. Removal of lindane and its isomers
can also be established by dehydrochlorination either by thermal or
base-assisted processes, but these can form toxic trichlorobenzenes
(TCBs) [24,25].
Another possible degradation method for hexachlorocyclohex-
anes (HCH) is the catalytic reduction over metal catalysts. The main
disadvantage in this case is the partially decomposition of lindane
to the toxic benzene [26].
∗
Corresponding author. Tel.: +30 210 6503643, fax: +30 210 6511766.
E-mail address: hiskia@chem.demokritos.gr (A. Hiskia).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.02.017

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S. Antonaraki et al. / Catalysis Today 151 (2010) 119–124
In this paper, we report a detailed examination of the interme-
ric cell (1 cm path). After 20 min of oxygenation, the cell was closed
with a serum cap. For identification of intermediates products 60 ml
of the above solution were added to a cell and after 30 min of oxy-
genation was closed with an airtight cap. Photolysis was performed
at 20 ◦C under constant stirring.
diates formed during the photocatalytic degradation of lindane in
the presence of POM, of which the complete mineralization to CO2
and Cl− has already been reported [4]. Lindane was selected as a
target compound in this study, not only due to its high environ-
mental impact but also due to its well-studied photodegradation in
the presence of TiO2, which indicated the formation of a plethora
of intermediates through both oxidative and reductive pathways.
The aim of this work was focused on (i) the careful control of all
transformation steps of the process, (ii) the definition of the pho-
todegradation mechanism in the presence of POM, comprising both
oxidative and reductive pathways and (iii) comparison with TiO2
literature data, in order to define possible similarities of POM and
TiO2 on the degradation mechanism.
2.4. Analysis of the photolyzed solutions
When the GC–MS was used for the identification of the inter-
mediate products of lindane, the photolyzed solution of lindane
was extracted with dichloromethane three times (60 ml each), and
the organic layers were combined, dried over sodium sulfate, con-
centrated in a rotary evaporator to ∼1 ml and evaporated under a
gentle stream of nitrogen to dryness. The residue was redissolved
in 200 ␮l of dichloromethane for GC–MS analysis. For the construc-
tion of the lindane disappearance curve, photolyzed solutions of
lindane (4 ml) after being extracted and dried as above, were con-
centrated and adjusted to a final volume of 4 ml dichloromethane
and injected to the GC–MS.
2. Experimental
2.1. Materials
H3PW12O40, dichloromethane 99.8% and sodium sulfate were
obtained from Panreac (Barcelona, Spain). Lindane 99.5% was
purchased from Dr. Ehrenstorfer (Augsburg, Germany) and hex-
achlorobenzene 99.5% from Riedel de Haën (Seeize, Germany).
Analytical reagent grade perchloric acid (HClO4 70%) was product
of Riedel de Haën (Seeize, Germany). Aqueous solution containing
lindane at 2.4 × 10−5 M were prepared by stirring the solid in water
for 10 h followed by filtration to remove any undissolved substrate.
Water was purified with a Millipore Milli-Q Plus System and extra
pure dioxygen was used for oxygenation of solutions.
3. Results
3.1. Photocatalytic degradation of lindane in the presence of
polyoxometalates
Photocatalytic degradation of aqueous, dioxygen saturated
solutions of lindane (2.4 × 10−5 M) took place effectively in the
(7 × 10−4 M). Irradiation was performed
3−
presence of PW12O40
at ꢀ > 320 nm to avoid direct photolysis of substrate. The disap-
pearance of lindane (Fig. 1) was monitored by GC-MS analysis of
extracted solutions at various photolysis intervals. Lindane shows
a chromatographic peak at tR = 29.31 min (Table 1) which, under the
experimental conditions used, disappeared after about 3 h of pho-
tolysis, whereas complete mineralization to CO2 and Cl− required
a much longer time (ca. 10 h) [6]. When the same experiment was
performed in the absence of catalyst, lindane showed high stability
for more than 5 h of illumination.
2.2. Instrumentation
GC–MS analysis was performed using an Agilent 6890 Series
gas chromatograph interfaced to an Agilent 5973 mass selective
detector (Wilmington, DE, USA). Data acquisition, processing and
instrument control were performed by the Agilent MSD Chem-
Station software. The analytical column was a HP-5MS (5% diphenyl
and 95% dimethylsiloxan) capillary column, 30 m × 0.25 mm i.d.,
0.25 mm film thickness. A split-splitless injector was used in pulsed
splitless mode. The injector temperature was 210 ◦C and the injec-
tion volume 2.0 ␮l. When concentrated extracts were analyzed for
intermediates identification, the mass spectrometer detector was
set to be off at the time of lindane elution, to avoid filament dam-
age. Non-concentrated extracts were also analyzed for monitoring
lindane. In that case the mass spectrometer detector was set to be
on during sample analysis. Flow rate of helium was 1 ml/min. The
oven was programmed as follows: isothermal at 50 ◦C for 4 min,
from 50 to 150 ◦C at 5 ◦C/min, then from 150 to 230 ◦C at 10 ◦C/min
and held for 15 min. The electron energy was set at 70 eV, the ion
source temperature was maintained at 230 ◦C and the quadrapole
temperature at 150 ◦C.
3.2. Identification of Intermediates
A
detailed study of the photodegradation of lindane (I)
(Scheme 1) with POM reveals the formation of numerous
2.3. Photolysis experiments
Photolysis experiments were performed with Oriel 1000 W Xe
arc lamp (Stratford, CT, USA), equipped with a cool water circu-
lating filter to absorb the near IR radiation. This lamp gives a flat
response from ca. 320 to 750 nm corresponding to irradiance at
0.5 m, ca. 200 mW m−2 nm−1 according to the supplier. The inci-
dent radiation was reduced to about 40% with a slit diaphragm in
order to obtain reasonable photolysis times. A 320 nm cut off filter
was used to avoid direct photolysis of substrate.
Aqueous solution of lindane (2.4 × 10−5 M) in the presence of
PW12O403− (7 × 10−4 M) was adjusted to pH 1 using perchloric acid,
because this POM is perfectly stable at this pH value. An amount of
4 ml of the above solution was added to an 8 ml spectrophotomet-
Fig. 1. Photodegradation of oxygenated aqueous solution of lindane (2.4 × 10−5 M)
in the absence (᭹) and presence (ꢀ) of H3PW12O40 (7 × 10−4 M), at pH 1 (HClO4
0.1 M).

S. Antonaraki et al. / Catalysis Today 151 (2010) 119–124
121
Table 1
Intermediates with their retention time and spectral characteristics.
Intermediates
tR (min)
M+
m/z
Lindane (I) (target compound)
1,1,2,2-Tetrachloroethane (XVII)
2,4-Dichlorophenol (VII)
4,5-Dichloro-1,2-benzenodiol (XIII)
2,4,5-Trichlorophenol (X)
4,6-Dichloro-1,3-benzenodiol (XII)
2,3,6-Trichlorophenol (VIII)
2,3,5-Trichlorophenol (IX)
2,3,4,5,-Tetrachlorocyclohexene (IV)
2,3,4,5,6-Pentachlorocyclohexene (III)
2,3,4,6-Tetrachlorophenol (XI)
3,4,6-Trichloro-1,2-benzenediol (XIV)
Hexachlorobenzene (V)
1,1,2,3,4,5,6-Heptachlorocyclohexane (II)
Tetrachloro-biphenyl (XV)
Tetrachloro-biphenyl (XV)
29.31
7.52
290
166
162
178
196
178
196
196
218
252
230
212
282
322
290
290
358
219, 183, 111
131, 95, 83, 61
16.75
20.69
21.22
21.32
21.76
23.31
23.63
23.68
25.80
25.89
28.37
30.36
30.82
31.39
35.68
135, 126, 98, 63
132, 115, 97, 51
160, 132, 97, 62
149, 115, 86, 51
160, 132, 97, 62
160, 132, 97, 62
183, 147, 122, 111
219, 181, 146, 111
194, 168, 131, 96
178, 148, 113, 85
249, 214, 177, 142
289, 253, 217, 180
220, 185, 150, 110
255, 220, 184, 150
290, 218, 181, 145
Hexachloro-biphenyl (XVI)
species. Several chromatographic peaks not present in the
non-irradiated solution appeared in the photolyzed one. The
identification of the species detected was performed on the
basis of comparison with mass spectra libraries HP Pest and
Wiley 275, authentic standards when they were available, litera-
ture data and EI fragmentation patterns. For all library matched
species the degree of match was mostly more than 95%. The
intermediates together with their retention times and spec-
tral characteristics are given in Table 1. They are compiled in
the following categories: aromatic compounds (chlorophenols,
chlorodiols and hexachlorobenzene), non-aromatic cyclic com-
pounds (chlorocyclohexenes, heptachlorocyclohexane), aliphatic
compounds (tetrachloroethane) and condensation products (poly-
chlorinated biphenyls). The formation of all intermediates is
followed by their decay, during the photocatalytic process, coming
finally to total photodecomposition to CO2 and HCl [6].
In more details, among the aromatic intermediates iden-
tified were chlorophenols, specifically 2,4-dichlorophenol (VII),
three isomers of trichlorophenol (2,3,6-trichlorophenol (VIII),
2,4,5-trichlorophenol (X), 2,3,5-trichlorophenol (IX)) and 2,3,4,6-
tetrachlorophenol (XI). Their formation and decay with pho-
tolysis time is depicted in Fig. 2A. There were also identified
two benzenediols-dichloro (1,3-benzenediol 4,6-dichloro (XII),
1,2-benzenediol 4,5-dichloro (XIII)) and 1,2-benzenediol 3,4,6-
trichloro (XIV), see Fig. 2B. Hexachlorobenzene (V) was also formed
and decayed, Fig. 2C. It can be seen that the highest concentration
of the aromatic intermediates is formed after 0.5–2 h of photolysis.
The non-aromatic cyclic intermediates identified were 2,3,4,5,6-
pentachlorocyclohexene (III), 2,3,4,5-tetrachlorocyclohexene (IV)
and 1,1,2,3,4,5,6-heptachlorocyclohexane (II). The formation and
decay of II and III are shown in Fig. 2C, with a higher concentration
formed after 2 h and 15 min of photolysis, respectively. The IV was
only identified at the 1-h photolyzed solution.
and decay of these intermediates, at various photolysis intervals, is
shown in Fig. 2D.
4. Discussion
Absorption of light by POM (ε ∼ 104 M−1 cm−1) at the oxygen to
metal charge-transfer (O → M CT) band below 400 nm, enhances
their oxidizing ability (∼3 eV), and makes them powerful oxidiz-
ing reagent able to oxidize a great variety of pollutants in the
aquatic environment through a hole–electron mechanism similar
to semiconductor photocatalysis. Thus continuous photolysis in the
presence of POM results in complete mineralization of substrates
with final products CO2, H2O and inorganic ions [13].
In most of the proposed mechanisms concerning the photocat-
alytic degradation of organic pollutants (S), in the presence of POM,
the oxidation is supposed to be performed by highly oxidizing •OH
radicals produced indirectly [27], through the reaction of excited
POM (POM*) with water. The overall reactions that take place in
the photocatalytic circle in the presence of oxygen are:
hv
POM−→ POM∗ i.e., POM (h+ + e−)
(1)
(2)
POM(h+ + e−) + H2O → POM(e−) + •OH + H+
•OH + S → oxidation products
→ → → CO2, H2O and inorganic ions
(3)
POM(h+ + e−) + S → POM(e−) + S+
→ → → CO2, H2O and inorganic ions
(4)
Dioxygen is a very effective oxidant for reduced POM [28–30],
thus its main action is the regeneration of the catalyst.
−
POM(e−) + O2 → POM + O2
(5)
•
The only linear compound that could be identified was 1,1,2,2-
tetrachloroethane (XVII), after 2 h of photolysis. The formation and
decay of this compound is also shown in Fig. 2C.
This superoxide radical anion O2•− (H+ + O2•− ↔ HO2• pKa = 4.8)
[31] may participate further in oxidative processes [5].
Furthermore, condensation products are being formed, i.e., two
polychlorinated biphenyls with two (XV) and three chlorine atoms
(XVI) to each ring. These products were detected from the begin-
ning of photolysis. These by-products can be easily recognized as
they are eluted at long retention times and they are well separated
from the other degradation intermediates. From comparison of the
spectra with Wiley database, it is possible to extract data on the
molecular structure of the condensation products, e.g., the num-
ber of Cl atoms. However, without comparison with standards, the
mass spectra of XV and XVI do not give further information about
the position of Cl atoms in the two aromatic rings. The formation
O2 −/HO2 + S → oxidation products
(6)
•
•
as well as reductive processes [32].
Thus, upon photolysis of an aqueous solution of POM both oxida-
tive and reductive species are formed. The oxidative species are
•OH radicals, the excited POM and the superoxide radical anions,
while the reductive species are reduced POM, POM (e−), and the
superoxide radical anions. It should be noticed that POM, for exam-
ple H3PW12O40, being at their highest oxidation state (all tungsten
are W6+) participate in the photoredox process, at the beginning,
exclusively as oxidant.

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S. Antonaraki et al. / Catalysis Today 151 (2010) 119–124
Scheme 1. Proposed pathways for the photocatalytic degradation of lindane by H3PW12O40
.
The detailed study of the photocatalytic degradation of lindane
solution and lasts about 3 h. This, however, is reasonable since the
species that originates from, i.e., lindane (I) seem to disappear at
about that time. Polychlorinated biphenyls also start to form from
the beginning of photolysis. They last, though, to about 6 h, whereas
one of them (hexachloro-biphenyl, XVI), persists up to 12 h of pho-
tolysis, i.e., long after all intermediates have disappeared. As shown
earlier [4] CO2 and Cl− build up during the photocatalytic process
and present the final disintegrating products of lindane (I).
It appears, then, that too many processes, i.e., chlorination,
dechlorination, hydrogenation, dehydrogenation, dimerization,
etc. take place at about the same time, right from the beginning
of photocatalytic process.
in the presence of POM reveals the formation of a great variety of
intermediates prior to mineralization, suggesting a complex mech-
anism of degradation (Scheme 1).
Fig. 2(A–C) shows that too many diversified intermediates
appear, with the exception of 1,1,2,2-tetrachloroethane (XVII),
right from the beginning of photolysis. However, their peak appear-
ance in the formation and decay curves varies considerably.
For instance, most of the intermediates start to form from
the first minute of photolysis, as mentioned, and last for about
1 h. However, for what is worth, 2,3,6-triclorophenol (VIII) and
2,3,5-trichlorophenol (IX) appear to be resilient to photocatalytic
decomposition being present in the photolyzed solution for about
3 h. Heptachlorocyclohexane (II) is also present in the photolyzed
We will attempt, below, to provide reasonable mechanistic
aspects that lead to the formation of the intermediates identified in

S. Antonaraki et al. / Catalysis Today 151 (2010) 119–124
123
Fig. 2. Formation and decay of (A) chlorophenols, (B) di- and trichloro-benzenodiols, (C) heptachlorocyclohexane, hexachlorobenzene, pentachlorocyclohexene, tetra-
chloroethane and (D) polychlorinated biphenyls upon photolysis of aqueous solution of lindane (2.4 × 10−5 M) in the presence of H3PW12O40 (7 × 10−4 M), at pH 1 (HClO4
0.1 M).
the study. Firstly, chlorine radicals must form either from homolytic
scissions of lindane (I) and their chlorinated products or oxida-
tion of chloride ions by hole trapping or through •OH radicals [33].
Superoxide radicals (O2•−) are known to form in photolyzed oxy-
genated aqueous POM solutions (reaction (5)). The existence of H
atoms can be attributed to the known reaction
of pentachlorocyclohexene (III) by the known mechanism involv-
ing alkaline hydrolysis followed by rapid dehydration [36] must be
excluded given our acidic conditions. Zaleska et al. [37] reported the
formation of pentachlorocyclohexene (III) as by-product of lindane
with TiO2 in the form of powdered anatase, rutile and supported
on glass hollow microspheres as well. The same intermediate was
detected in TiO2 photocatalysis by Guillard et al. [35] and Vidal
[38]. It was also identified as intermediate product of lindane, due
to temperature effect under subcritical conditions without the use
of catalysts or other additive, by Kubatova et al. [39].
There are two possible pathways for the formation of 2,3,4,5-
tetrachlorocyclohexene (IV), either from pentachlorocyclohexene
(III) or directly from lindane through reductive dehalogenation by
superoxide radical anions, according to the above. Guillard et al.
[35] also found tetrachlorocyclohexene during photocatalysis of
lindane with TiO2. This intermediate makes a brief appearance after
60 min of photolysis.
Chlorobenzenes (VI) could be formed by further Cl
elimination from 2,3,4,5,6-pentachlorocyclohexene (III), 2,3,3,5-
tetrachlorocyclohexene (IV) [39], or hexachlorobenzene (V)
through reductive dechlorination [40], resulting in less substituted
chlorobenzenes. Dechlorination of hexachlorobenzene by direct
electron transfer and/or hydrogen atom addition is proposed as
the likely mechanism of hexachlorobenzene degradation on alu-
mina nanoparticles [41]. Trichlorobenzenes are also known to be
formed upon photolysis of a lindane/TiO2 solution [37]. Although
chlorobenzenes besides hexachlorobenzene were not directly
detected in our case, their presence was indirectly suggested by
the formation of their hydroxylation products (chlorophenols)
and dimerization products (polychlorinated biphenyls). The lat-
ter procedure of dimerization was also reported in the case of
chlorophenols as substrates, which were subjected to reductive
dechlorination (elimination of one Cl atom) upon photolysis in
the presence of TiO2 to result in the corresponding phenyl radical
POM(e−) + H+ ↔ POM + 1/2H2
(7)
and quite reasonably through the knocking out of H atoms from
“aliphatic” and aromatic rings during the photocatalytic process
via attacks by excited POM* and/or Cl atoms [13].
Thus, chlorine atoms must attack lindane (I) to form hep-
tachlorocyclohexane (II) and this process seems to last to the point
where all lindane (I) has decomposed, about 3 h, as mentioned ear-
lier.
Alternatively, heptachlorocyclohexane (II) can be produced by
the addition of chloride ion to the lindane radical cation. This cation
could be formed during photolysis of lindane aqueous solution with
POM through oxidation directly by POM (h+ + e−) or indirectly via
attack of •OH radical. A similar explanation has been reported by
Chen et al. [34] on the formation of 2,4,6-trichlorophenol as Cl
adduct upon photolysis of 2,4-dichlorophenol in the presence of
POM/TiO2 photocatalyst. Guillard et al. [35] identified the same
intermediate during photolysis of aqueous solution of lindane with
TiO2. The formation of CCl2 groups has also been observed in
1,1,2,2-tetrachloroethane (XVII), an intermediate product detected
upon photolysis of POM/lindane. This appeared after 1 h of pho-
tolysis by which time breaking of cyclic intermediates has taken
place.
The formation of 2,3,4,5,6-pentachlorocyclohexene (III) directly
from lindane can be attributed to the dehalogenation of lindane
by superoxide radical anions. The latter have been reported to
dehalogenate aliphatic halocarbon substrates to the correspond-
ing alkenes via a reductive pathway [32]. The alternative formation

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S. Antonaraki et al. / Catalysis Today 151 (2010) 119–124
which was finally converted to dimerization products of one Cl
atom less in each ring compared to the parent substrate [34,42].
In an analogous way, the formation of polychlorinated biphenyls
with two and three Cl atoms to each ring could be attributed
to the reductive dechlorination of tri- and tetrachlorobenzenes,
respectively.
Reaction of hydroxyl radicals with organic compounds is known
to include both addition to the aromatic or double bond and
H abstraction. Thus, the electrophilic hydroxyl radicals abstract
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H
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