Homogeneous photodegradation study of 2-mercaptobenzothiazole photocatalysed by sodium decatungstate salts: Kinetics and mechanistic pathways

Article abstract of DOI:10.1016/j.jphotochem.2010.04.010

The photochemical degradation of a benzothiazole derivative, 2-mercaptobenzothiazole (MBT) has been studied in aqueous solution in the presence of a polyoxometalate (POM): sodium decatungstate salts Na₄W₁₀O₃₂ (DTA). In aerated conditions, the photodegradation rate of MBT clearly increased in the presence of DTA by a factor six when compared with the direct photolysis with k_MBT = 0.25 h^-1 and t_1/2(MBT) = 2.8 h. For the total comprehension of the degradation mechanism, the oxygen influence has been investigated. Oxygen appeared essential for DTA regeneration, its absence induced a three times inhibition of MBT disappearance and completely stopped the photocatalytic cycle. The main photoproducts were identified with LC-ESI-MS and LC-DAD techniques and using some calculations obtained by B3LYP/6-21G method in Gaussian 4.1 software. All the results allowed to propose a mechanistic pathway. Electron transfer and H atom abstraction processes involving W₁₀O₃₂^4-* excited state species were involved in the degradation. In the primary step of the degradation, the hydroxylation of the aromatic ring leading to four OH-MBT isomers and the formation of disulfide form of MBT were observed. For longer irradiation time, a secondary electron transfer permitted the oxidation of OH-MBT isomers and the formation of sulfoxide derivatives. For prolonged exposure (around 100 h), the complete mineralization was noticed in the presence of sodium decatungstate salts.

Full text of DOI:10.1016/j.jphotochem.2010.04.010

Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 153–160
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Homogeneous photodegradation study of 2-mercaptobenzothiazole
photocatalysed by sodium decatungstate salts:
Kinetics and mechanistic pathways
A. Allaouia,b, M.A. Maloukia, P. Wong-Wah-Chungc,∗
a Laboratoire des Sciences et Technologies de l’Environnement, Université Mentouri, 25000 Constantine, Algeria
b Clermont Université, Laboratoire de Photochimie Moléculaire et Macromoléculaire, BP 10448, F-63000 Clermont-Ferrand, France
c Clermont Université, ENSCCF, Laboratoire de Photochimie Moléculaire et Macromoléculaire, BP 10448, F-63000 Clermont-Ferrand, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The photochemical degradation of a benzothiazole derivative, 2-mercaptobenzothiazole (MBT) has been
studied in aqueous solution in the presence of a polyoxometalate (POM): sodium decatungstate salts
Na4W10O32 (DTA). In aerated conditions, the photodegradation rate of MBT clearly increased in the
presence of DTA by a factor six when compared with the direct photolysis with kMBT = 0.25 h−1 and
t1/2(MBT) = 2.8 h. For the total comprehension of the degradation mechanism, the oxygen influence has
been investigated. Oxygen appeared essential for DTA regeneration, its absence induced a three times
inhibition of MBT disappearance and completely stopped the photocatalytic cycle. The main photoprod-
ucts were identified with LC–ESI-MS and LC-DAD techniques and using some calculations obtained by
Received 6 January 2010
Received in revised form 12 April 2010
Accepted 15 April 2010
Available online 22 April 2010
Keywords:
2-Mercaptobenzothiazole
Decatungstate salts
Photodegradation
UV light
B3LYP/6–21G method in Gaussian 4.1 software. All the results allowed to propose a mechanistic path-
4−*
way. Electron transfer and H atom abstraction processes involving W10O32
excited state species were
involved in the degradation. In the primary step of the degradation, the hydroxylation of the aromatic ring
leading to four OH-MBT isomers and the formation of disulfide form of MBT were observed. For longer
irradiation time, a secondary electron transfer permitted the oxidation of OH-MBT isomers and the for-
mation of sulfoxide derivatives. For prolonged exposure (around 100 h), the complete mineralization was
noticed in the presence of sodium decatungstate salts.
Oxygen
Photocatalysis
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
States (EPA) to estimate that over 450 tons of MBT (3% of the
US production) may be lost annually into the environment [3].
The pollution of the environment by organic compounds is
always of great concern because the occurrence of these com-
pounds and their increasing concentration pointed out the toxicity
risks for aquatic organisms and human. As a consequence, many
researchers have focused their studies on different purposes: (i)
the pollution assessment in a large wide of compartments (rivers,
soils, lakes, . . .), (ii) the organic pollutants fate in natural environ-
ment, their impacts on different populations, (iii) the elimination
processes’ or advanced oxidized processes’ (AOPs) improvement.
Among the organic pollutants, the benzothiazole derivatives and
particularly 2-mercaptobenzothiazole (MBT) is on great interest
regarding all previous studies.
As a consequence, MBT has been measured in different com-
partments and was found in surface waters, in roads dusts and
in wastewater effluents from tanneries, sewage treatment plants
and rubber additive manufactories. This obviously confirms the
prospective pollution of environmental compartments [4]. More-
over, many previous studies have proved that MBT is a strong
pollutant of aquatic compartments, allergen and potentially muta-
gen for human that means a non-negligible impact of MBT on
environment and on human health [5,6].
Some studies on the environmental behavior and the biodegra-
dation of MBT showed that it is rather recalcitrant and it
is not completely mineralized [7]. The biodegradation studies
put in evidence the formation of different metabolites. Among
them, 6-hydroxybenzothiazole (6-OH-MBT), dihydroxybenzoth-
iazole, demethylated MBT (2-methylthiobenzothiazole) and the
diacid derivative of MBT have been identified [8]. Other studies
focused on the direct phototransformation proved that MBT is pho-
toreactive under irradiation at 313 nm with a quantum yield equal
to 0.02 and photostable under sunlight irradiation with a quan-
tum yield equal to 0.002 [9,10]. The main degradation products
MBT is widely used in industries as rubber vulcanization accel-
erator, fungicide, bactericide, conservator or corrosion inhibitor
[1,2]; this leads the Environmental Protection Agency of United
∗
Corresponding author. Tel.: +33 473407172.
E-mail address: pascal.wong-wah-chung@univ-bpclermont.fr
(P. Wong-Wah-Chung).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.04.010

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A. Allaoui et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 153–160
observed were benzothiazole (BT) and 2-hydroxybenzothiazole
(OBT) and the process occurred from the triplet and singlet excited
states through the formation of BTS• radical.
solar irradiation showed the potential applications of DTA in the
degradation of many compounds among them chlorinated phenol,
imidachloprid or atrazine [19,26].
All these results demonstrating that MBT was very stable in
environmental conditions, lead many researchers to work on its
elimination. For example, the ozonation process and the photoin-
duced degradation (in the presence of iron oxides/UV irradiation,
TiO2, . . .) were tested to reach the total elimination of MBT. In
the presence of O3, MBT disappearance was effective in less than
10 min, the mineralization was partial and the formation of BT,
benzothiazole-2-sulfite (BTOSO2) and OBT was observed [11]. In
addition, systems combining iron-oxalate complexes and UVA irra-
diation showed that MBT is adsorbed on the support and photo
Fenton reactions are responsible of MBT induced degradation. The
authors put into evidence that the degradation rate depends on dif-
ferent parameters: the oxalate and iron oxides concentrations and
the photonic flux intensity [12]. In more recent studies, titanium
dioxide doped with neodymium, cerium or lanthanum allowed the
efficient elimination of MBT under irradiation by OH• radical attack
with the formation of BT, OBT, BTOSO2 and aniline sulfonate (ASA)
as degradation products [13,14].
Regarding the decatungstate salts properties and the environ-
mental stability of MBT, the present work is dealing with its
photodegradability in the presence of DTA under UV irradiation and
in aqueous solution. Our purpose was to evaluate the photodegra-
dation of 2-mercaptobenzothiazole through the determination of
disappearance kinetics, the initial rate constants and the quan-
tum yields considering the influence of oxygen. In addition, with
reference to the photoproducts’ identification, the mechanistic
pathways involved in the degradation will be proposed in order
to have a better insight into the MBT degradation scheme.
2. Materials and methods
2.1. Materials
Sodium decatungstate Na4W10O32 was prepared and purified
according to the published procedure [27]. The decatungstate ion
obtained after recrystallisation gave an UV–vis absorption maxi-
mum at 323 nm in acetonitrile and the IR spectrum, realized by
diffuse reflexion, showed main vibration bands at 1004, 962, 912
and 800 cm−1 in agreement with the literature values [17,27].
Regarding the remaining of MBT in natural compartments, its
established toxicity and its low biodegradability, we focused our
study on the potential elimination of this benzothiazole deriva-
tive under UV irradiation in the presence of a polyoxometalate
(POM): the decatungstate anion W10O324− (DTA). The system using
decatungstate anion and UV irradiation was considered because
of DTA photocatalytic activity and its efficient induced degrada-
tion of many organic pollutants in water (chlorinated phenols,
pesticides, aromatic and aliphatic alcohols, alkenes, . . .) [15,16].
It is well known that the DTA reactivity under irradiation comes
from the formation of a very oxidative specie in the excited state
2-Mercaptobenzothiazole
(benzothiazolethiol,
m.p.
178–181 ◦C) was purchased from Fluka (99% purity) and was
used without further purification. Titanium dioxide TiO2 Degussa
P25 was 80:30 anatase:rutile with a surface area of 44 m2/g.
Methanol was from Carlo Erba (HPLC grade) and the other chemi-
cals were of purest grade commercially available and used without
further purification.
Solutions were prepared with water purified by a Millipore
Milli-Q device and the pH was adjusted with NaOH or HCl and con-
trolled with an ORION pHmeter to 0.1 pH unit. The ionic strength
was not controlled.
The deaeration and the oxygenation of the solutions were
accomplished respectively by bubbling N2 and O2 for a maximum
of 30 min at room temperature prior to irradiation and all along the
irradiation period.
W
10O324−*, which allows the degradation of organic pollutants by
H atom abstraction or by electron transfer. DTA reduced species
(W10O345−) is then regenerated in the presence of oxygen and
the obtained catalytic cycle could also lead to the continuous
degradation of organic molecules and to their total elimination
[17–20]. Papaconstantinou and Mylanos put into evidence, for the
first time, the total decomposition of chlorinated phenols into CO2
and chlorine in the presence of inorganic POMs under irradia-
tion [15]. The total mineralization was reached in 600 min under
irradiation in aerated aqueous solution. During the photocatalytic
degradation of chlorinated phenol, Mylanos et al. supposed that
the disappearance occurs through OH• radical formation, dechlo-
rination, hydroxylated compounds oxidation, open ring reaction
regarding the photoproducts detected [21]. All the mechanistic
pathways described by Mylanos and coworkers have also been
confirmed in different studies realized on the organic pollutants’
degradation in the presence of DTA (chlorinated phenol, atrazine,
metsulfuron methyl, . . .) but some authors described also demethy-
lation reaction and bound scission [19,20]. Moreover, the same
authors, who worked on 4-nitrophenol degradation in the presence
of POMs, showed that the decatungstate anion leads to a higher 4-
2.2. Procedures
Two irradiation setups with emission at 365 nm were used all
along the study. The quantum yields were calculated from the
results obtained after monochromatic irradiation at 365 nm. These
irradiations were conducted with a xenon lamp (1000 W) equipped
with a LOT Oriel grating monochromator. The beam was paral-
lel and the reactor was a quartz cell of 1 cm or generally 2 cm
path length. The photonic flux at 365 nm measured by ferrioxalate
actinometry was equal to 2.4 × 1014 0.2 × 1014 photons cm−2 s−1
[28].
The kinetics’ establishment and the photoproducts’ identifica-
tion were realized by using a cylinder reactor made in stainless
steel. The container was equipped with three high-pressure mer-
cury lamps (black lamp, Mazda MAW 125 W) emitted selectively
at 365 nm (97% of the emission) and symmetrically installed in the
down part of the cylinder. The reactor, a water-jacketed Pyrex tube
(Ø = 2.8 cm) containing a maximum of 100 mL solution, was located
in the centre. The solutions were continuously stirred during irra-
diation experiments by a magnetic stirrer. This irradiation set up
allowed us to determine the initial rate constants and the half-
life times; these values were exclusively determined in aerated
and oxygen saturated conditions in the presence of DTA because
in these conditions the kinetics were corresponding to first order
kinetics.
3−
4−
nitrophenol disappearance than with PW10O40 and SiW12O32
[21]. Nevertheless, in almost all studies realized on the photoin-
duced degradation of organic pollutants, the authors established
that titanium dioxide catalyst is generally a more powerful can-
didate than POMs [22,23]. Only a few studies demonstrated that
4−
POMs and particularly W10O32
is more capable to mineralize
organic pollutants and more precisely small molecules like acetic
acid, 1,1,2-trichlorinated ethane or formulated compounds such as
the pesticide imidachloprid [24,19]. Moreover, like DTA polyoxoan-
ions absorbs in the UV region, it has been considered with great
interest in water detoxification processes because of the large over-
lap of its absorption spectrum with that of the solar UV emission
spectrum and its low toxicity [25]. In fact, some experiments with

A. Allaoui et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 153–160
155
The concentration evolutions of the initial and the degradation
products were determined by HPLC experiments using a Waters liq-
uid chromatography system equipped with two Waters 510 pumps,
an automatic injector model Waters 717 and a Waters 996 diode
array detector. An Agilent reverse phase column (Eclipse XDB-C8,
5 ␮m, 250 mm × 4.6 mm) and mixtures of water with 0.1% of H3PO4
and methanol (45/55, v/v) were used to obtain a correct elution of
MBT. The flow rate was 1 mL min−1 and the injected volume was
30 ␮L.
Absorption spectra were recorded on a Cary 3 double beam
spectrophotometer. Ions were measured by ionic chromatogra-
phy (Dionex DX320, column AS11 for anions, eluent KOH; Dionex
ICS1500, column CS16 for cations, eluent hydroxymethanesul-
fonate), while the mineralization was monitored by total organic
carbon measurements with a Shimadzu TOC 5050 equipped with
an automatic sample injector. The concentration in total organic
carbon was obtained from three injections and accordingly to a
calibration curve established the day of the measurements.
The photoproducts’ identification were carried out by LC–MS
studies realized with a Waters HPLC system (Alliance 2695) cou-
pled to a Q-ToF mass spectrometer equipped with a pneumatically
assisted electro spray ionization source (ESI). The HPLC system was
also coupled with a Waters 996 diode array detector. The chro-
matographic separation was reached by using an elution program
from 95% water (with 0.2% acetic acid) and 5% methanol to 5% water
(with 0.2% acetic acid) and 95% methanol after 15 min; the obtained
isocratic conditions were maintained during 10 min. The flow rate
was 0.2 mL min−1, the injected volume 30 ␮L and the column was
distributed by Waters (Xterra MS C18, 3.5 ␮m, 100 mm × 2.1 mm).
The electro spray source parameters were in positive mode: capil-
lary voltage 3000 V (2100 V in negative mode), cone voltage 35 V,
extraction cone voltage 2 V, desolvation temperature 250 ◦C, source
temperature 100 ◦C, ion energy 2 V and collision energy 10 eV (7 eV
in negative mode).
Fig. 1. Evolution of MBT (1.0 × 10−4 M) concentration in water under irradiation
4−
at 365 nm: (*) without W10O324−, in the presence of W10O32 (2.0 × 10−4 M): (ꢀ)
aerated solution, (♦) deoxygenated solution, (ꢁ) oxygen saturated solution.
quence, the MBT direct degradation was evaluated and the kinetics
obtained are represented in Fig. 1. We observed a weak disappear-
ance equal to 25% for MBT after 8 h under direct irradiation.
The kinetics obtained for the direct degradation of MBT under
irradiation at 365 nm is in agreement with the previous quite low
quantum yield determined when studying the MBT direct photol-
ysis under irradiation at 313 nm [9]. The initial rate constants were
exclusively determined in aerated and oxygen saturated conditions
in the presence of DTA because in these conditions the kinetics were
corresponding to first order kinetics. In addition, the low rate con-
stant and the high half-life time value, determined in this study
under direct irradiation at 365 nm, confirm the low direct pho-
todegradability of MBT and it suggests that MBT could be present
for long period of time in the environmental water compartments.
3. Results and discussions
3.2. Degradation kinetics of 2-mercaptobenzothiazole in the
presence of decatungstate salts
3.1. Degradation of 2-mercaptobenzothiazole under direct
irradiation
The degradation of MBT (1.0 × 10−4 M) was studied in the pres-
4−
First, the degradation of MBT (1.0 × 10−4 M) was studied in the
ence of W10O32
(2.0 × 10−4 M) under irradiation at 365 nm in
4−
different conditions: aerated, oxygenated and deoxygenated solu-
tions. During the study, the degradation of MBT was evaluated by
the initial quantum yields determination in all experimental condi-
tions and under monochromatic irradiation at 365 nm. The results
are gathered in Table 1.
absence of W10O32 upon irradiation at 365 nm in order to eval-
uate the direct photodegradation process. This contribution has to
be considered for the precise determination of the decatungstate
photocatalytic efficiency and mechanism.
2-Mercaptobenzothiazole may exist under three different forms
in the environmental pH range: two molecular tautomer forms
(thiol or thione) and an anionic form (S−). Their presence depends
on the pH (pKa = 6.95 0.05). Studies on the tautomeric equilib-
rium of MBT gave the evidence that the thione form of MBT is the
prevailing species [29]. Moreover, differences have been evidenced
previously in the behavior of MBT forms in terms of photodegra-
dation and primary species [9]. This points out the main role of the
pH in the study. In our experimental conditions, the molecular form
is the dominant form present in the solutions because the pH was
around 4.4.
The pH was controlled to confirm that the main species in
solution is W10O324−. In fact, the stability and the speciation of
polytungstates in aqueous solution is very pH dependent. It has
been showed for a pH between 6.9 and 4.2 that the decatungstate
ions photoactivity was preserved and entirely inhibited at higher
pH. This has been attributed to the formation of species like Keggin-
type POMs (H2W12O466−) with an absorption at lower wavelength
(ꢀ = 260–265 nm) and also a weak photoactivity under our irradia-
Table 1
Initial rate constants, half-life times and quantum yields of MBT (1 × 10−4 M) degra-
dation in the presence of DTA (2.0 × 10−4 M) under irradiation at 365 nm in different
experimental conditions.
At very low pH, the UV-visible absorption spectrum
of the molecular form presents
a maximum at 320 nm
and since the molecular form is
(ε = 26,500 200 M−1 cm−1
)
Conditions
k (h−1
)
t1/2 (h)
˚
the main form present in the solutions, this implies a minor
recovery with the emission spectrum of the solar light.
Nevertheless, the direct elimination could be assumed with
the irradiation device because of the MBT basic form tail beyond
365 nm and because of the two incompletely filtered rays of the
mercury lamps at lower wavelengths (313 and 334 nm). As a conse-
Direct photolysis
Aerated
Oxygen saturated
Deoxygenated
0.03
0.25
0.47
–
23
2.8
1.5
–
0.02a
0.032
0.047
0.022
a
Under irradiation at 313 nm [9].

156
A. Allaoui et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 153–160
Table 2
tion conditions [20,30]. The pH measured for solutions of MBT-DTA
was equal to 4.4 implying that the conditions involved optimal
photocatalyst efficiency.
In aerated conditions (Fig. 1), MBT differently disappeared with
a conversion of 90% after 8 h irradiation. This demonstrates that the
photocatalysis process increases by a factor of 4 the MBT degrada-
tion when compared to the direct photodegradation.
Maximum of absorbance and dipole moment calculated with B3LYP/6–21G method
in Gaussian 4.1 software for MBT and hydroxylated MBT.
Name
MBT
4-OH
297
5-OH
310.9
6-OH
305.2
7-OH
298.5
ꢀmax (nm)
ꢁ (D)
299.3
5.11
6.57
5.79
6.20
6.47
Moreover, it is worth noting that the initial degradation rate
is 6 times faster in the presence of DTA (Table 1). These results
put into evidence the major catalystic role in the primary step of
the degradation and also suppose a minor involvement of direct
photolysis in the 2-mercaptobenzothiazole elimination under our
experimental conditions. The remarkable decrease of the half-life
times clearly illustrated the influence of DTA. In fact, the MBT half-
life time is divided by a factor of 8 in aerated conditions (Table 1). In
addition, the photocatalyst efficiency is evidenced by the quantum
yields determined in aerated solutions compared to those mea-
sured in the absence of decatungstate and cited in the literature:
negative modes after convenient irradiation times. The degradation
products are listed in Table 3.
4.1. Identification of 2-mercaptobenzothiazole photoproducts
Aerated solutions of MBT and decatungstate salts irradiated
2 and 4 h correspond to a conversion extent of 40% and 60%,
respectively have been analyzed by LC–ES-MS in positive and neg-
ative modes. The obtained photoproducts have been considered
as degradation products essentially resulting from MBT induced
degradation in the presence of W10O324−. This process is predom-
inant because of a negligible MBT direct photodegradation (10% of
conversion after 4 h) and particularly because of the competitive
absorbance between MBT and DTA at 365 nm favorable for DTA
(εDTA = 1800 100 M−1 cm−1 and εMBT = 10 1 M−1 cm−1).
Some calculated results obtained by B3LYP/6–21G method in
Gaussian 4.1 software are gathered in Table 2. It is worth noting
that the presence of an OH group on the aromatic ring generally
modified the maximum of absorbance in the UV region and the
polarity when compared to MBT.
the quantum yields are almost 2 times higher in the presence of
4−
W10O32
whatever the experimental conditions are [9]. Never-
theless, the efficient photocatalysis reaction with DTA appeared
to be very dependent on several parameters among them oxygen
concentration.
3.2.1. Effect of oxygen on 2-mercaptobenzothiazole degradation
As shown in Fig. 1, 2-mercaptobenzothiazole degradation was
significantly inhibited in deaerated conditions (around 50%) while
in oxygen saturated solution the total degradation was reached in
8 h. The presence of oxygen increased the photocatalytic effect of
decatungstate salts.
A bathochromic effect was observed for the hydroxylated MBT
*
attributed to the decrease of the ␲ excited state energy due to
the electronic delocalization growth favored by the presence of an
electron doublet on the oxygen atom. This is clearly noted for 5 and
6-OH-MBT. Nevertheless, for 4 and 7-OH-MBT isomers, almost the
same maximum of absorbance is observed compared to the one of
MBT. This is probably due to the presence of intramolecular hydro-
gen bound between the hydroxyl function and S atom or between
the amine and the hydroxyl group.
In addition, the presence of the OH group on the aromatic ring
confers an additional polarity to the initial molecule. The dipole
moment increase after the hydroxylation of MBT clearly indicates
that the elution order should be controlled by the hydroxyla-
tion level. Moreover, considering the monohydroxylated MBT, it
appears that the dipole moment increases as following: 5-OH < 6-
OH < 7-OH < 4-OH. Nevertheless, it is worth noted that 4-OH and
7-OH-MBT present quite similar calculated polarities.
During the degradation of MBT in the presence of DTA, six prod-
ucts have been clearly detected and identified (Fig. 2). Among them,
four hydroxylated MBT (1, 2, 3 and 4) have been put in evidence
(Table 3). These compounds evidently correspond to the differ-
ent isomers expected for OH-MBT. The OH-MBT assignment was
based on the LC–MS-ESI analysis in negative and positive mode
with a ratio m/z respectively equal to 182 and 184 which match
with the molecular anion ([M-H]−) and cation ([M+H]+). In addition,
hydroxylated MBT have already been observed in the biological
degradation of MBT in the presence of Rhodococcus rhodochrous by
Haroune and coworkers and the results obtained for the identifica-
tion of those compounds confirm our hypothesis [7]. Correlations
between calculated and experimental retention times allowed us
to attribute the photoproducts 1, 2, 3 and 4 to respectively 5-OH,
6-OH, 7-OH and 4-OH-MBT.
In the absence of oxygen, the degradation inhibition could be
explained by the limited reoxidation of the reduced catalyst. This
well known behavior occurs by the accumulation of the reduced
5−
species W10O32
in the solution leading to the catalytic cycle
opening and justifies the observation of the blue coloration during
the irradiation of the mixture. Nevertheless, in the first step of the
reaction, the availability of DTA salts could explain the initial fast
degradation of MBT until a plateau is reached. This plateau value
is reached after the total consumption of initial DTA and residual
oxygen and it is also corresponding to the remaining MBT con-
centration noted with the direct photolysis process. Concerning
the degradation in the oxygen saturated conditions, the oxygen
5−
concentration increase is in favor of W10O32
reoxidation and
also improves the photocatalytic degradation. The half-life time is
divided by a factor of 2 and the initial rate constant is multiplied by
a factor of 2 with higher oxygen concentration (Table 1).
Moreover, the oxygen effect is underlined by the con-
siderable increase of the quantum yields as
a function of
oxygen concentration according to the decrease order: deoxy-
genated < aerated < oxygen saturated. These results confirm the
major role of oxygen in the reoxidation process of reduced
decatungstate W10O325− and also its significant involvement in the
photocatalytic reaction.
4. Kinetics and identification of 2-mercaptobenzothiazole
photodegradation products in the presence of
decatungstate salts
Solutions of decatungstate salts and 2-mercaptobenzothiazole
were analyzed by HPLC all along the irradiation period in order
to detect the degradation products formation, to establish their
kinetics and to allow their identification. The study has been real-
ized in the three experimental conditions previously cited (aerated,
deoxygenated and oxygen saturated). The main degradation prod-
ucts have been identified by LC–ESI-MS analysis in positive and
An additional oxidized compound (5) have been observed with
a lower retention time and a ratio m/z equal to 200 and to 198
respectively in positive and negative modes. The compound 5 was
considered to be a sulfoxide derivative of OH-MBT. The hypothe-
sis of such a structure was done according to the retention time
and to the UV-spectrum shape (located at 336 nm) which imply an

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157
to 333 in positive mode, has been attributed to the MBT dimer. The
low polarity of the product is in agreement with a retention time
equal to 19.1 min (isocratic elution mode with 95% of MeOH) and
the exclusive detection of this product in positive mode confirms
also the attribution.
4.2. Kinetic of 2-mercaptobenzothiazole photoproducts
The photoproducts occurrence and kinetics clearly showed the
major consequence of experimental conditions and put in evidence
the main involvement of induced degradation.
In oxygenated conditions, four main products are detected
during the irradiation of MBT and they have been attributed to
hydroxylated MBT and to product 5 according to their retention
time as well as their UV-spectrum. All the kinetics are represented
in Fig. 3. No more than three kinetics of OH-MBT isomers have been
established because of the really close polarity of 7 and 4-OH-MBT
which did not permit their correct separation by HPLC. It is worth
noting that the hydroxylation of MBT takes place at the beginning of
the irradiation. The formation of OH-MBT isomers are followed by
their disappearance after different irradiation times as a function of
OH-MBT isomers. 5-OH-MBT is degraded after 1 h under irradiation
and the degradation of the other isomers occurs later (2 or 3 h). The
product 5 appears after 1 h under irradiation implying that it is a
secondary product and is obviously formed from OH-MBT isomers
degradation.
Fig. 2. Chromatogram of MBT and DTA irradiated 2 h at 365 nm in aerated solution,
ꢀdet = 340 nm, [MBT] = 1.0 × 10−4 M, [W10O324−] = 2.0 × 10−4 M, LC–MS-DAD analysis
conditions.
increase of the polarity and a monohydroxylation of the aromatic
ring. The presence of an O atom on the benzothiazole part has been
considered because we rejected the possibility of two OH groups
on the aromatic ring. In fact, it has been shown that compounds
with two OH groups on the aromatic ring could rapidly lead to an
open ring derivative which is not detected in our experiments [8].
Another product, with a high retention time and a ratio m/z equal
In aerated solutions, the same kinetics have been observed for
hydroxylated MBT but on a higher time scale. Their disappearance
Table 3
Proposed structures for MBT photoproducts by LC–ESI-MS analysis.
Retention
time (min)
Molecular weight
(g/mol)
Observed ion m/z (abundance
%) in positive mode
Observed ion m/z (abundance
%) in negative mode
Proposed structure
19.1
332
333 (100): [M+H]+
11.8
10.8
167
183
168 (100): [M+H]+
184 (100): [M+H]+
166 (100): [M–H]−
134 (10): 166-32 [–S]−
182 (100): [M–H]−
182 (100): [M–H]−
182 (100): [M–H]−
182 (100): [M–H]−
10.4
9.9
183
183
183
184 (100): [M+H]+
184 (100): [M+H]+
184 (100): [M+H]+
9.6
8.9
199
200 (100): [M+H]+
198 (100): [M–H]−

158
A. Allaoui et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 153–160
in the first step of the irradiation and disappears for higher time in
aerated conditions (Fig. 3). No other photoproduct was observed
within the irradiation time implying that the direct degradation
was negligible.
According to MBT photoproducts identification and kinetics and
the oxygen role, a mechanism described in Fig. 4 has been pro-
posed. The mechanism described below implies, in a first step,
two concomitant processes. First, an electron transfer process
4−*
with W10O32
excited state to give rise to a MBT radical cation
located on the aromatic ring and second, a H atom abstraction with
4−*
W10O32
excited state leads to another MBT radical located on
the S atom of the thiol group (–S•).
In a second step, the formation of OH-MBT isomers can be
attributed to a successive reaction of MBT radical cation with oxy-
gen present in the solution. Such OH-MBT formations have been
previously put in evidence in different previous studies [8,13,14].
Simultaneously, the MBT dimer formation could occur by the rad-
ical recombination of MBT radicals (–S•). This result is relevant
considering previous studies on the oxidation of mercaptothiazoles
in the presence of hydrogen peroxide and on the MBT photolysis in
organic solutions [31,32].
Fig. 3. Kinetics of MBT (1.0 × 10−4 M) photoproducts in the presence of DTA
(2.0 × 10−4 M) and under irradiation at 365 nm: (ꢀ) 7 and 4-OH-MBT, (᭹) 5-OH-
MBT, (♦) 6-OH-MBT, (ꢁ) product 5 in oxygenated and LC-DAD analysis conditions,
and (ꢀ) MBT dimer in aerated and LC–MS analysis conditions.
occurred two times later than in oxygen saturated conditions. This
can be explained by the continuous reoxidation of reduced DTA
in oxygenated conditions which permits the more efficient oxida-
tion of MBT and of its primary photoproducts. As a consequence,
the formation of product 5 was negligible in aerated conditions.
In deoxygenated conditions, product 5 was not detected and the
appearance of OH-MBT isomers took place on longer time. After 6 h
under irradiation, their concentrations are always increasing prob-
ably because of residual diffused oxygen. The results obtained by
LC–MS analysis showed that the disulfide form seems to be present
In a third step, similar reactions have been expected. An electron
transfer process involving DTA excited state and an electron dou-
blet of S atom on benzothiazole ring takes place and a radical cation
(S+•) is formed. A further reaction with superoxide or hydroxide
ion in aqueous solution gives then rise to the sulfoxide derivative
(product 5) as described in the literature [33]. In our conditions,
•−
the reaction with O2 could be favored because of its formation
through the reoxidation reaction involving the reduced DTA and
dissolved oxygen.
Fig. 4. Degradation mechanism of MBT in the presence of DTA under irradiation at 365 nm.

A. Allaoui et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 153–160
159
Fig. 5. (A) TOC evolution and degradation of MBT (insert) in the presence of DTA (ꢂ) and in the presence of TiO2 (ꢀ); (B) formation of inorganic ions upon irradiation of MBT
in the presence of DTA: sulphate (ꢀ), nitrate (ꢂ) and ammonium ions (ꢀ) [MBT] = 1.0 × 10−4 M, [DTA] = 2.0 × 10−4 M, [TiO2] = 0.2 g/L; aerated conditions, ꢀirr = 365 nm.
5. Mineralization
recent studies realized with doped titanium dioxides like photo-
catalyst put in evidence that around 50% of MBT are mineralized
in less than 80 min [34]. In order to compare the efficiency of DTA
and TiO2 in the same conditions, we also realized some degradation
and mineralization experiments with Degussa P25, a conventional
TiO2 photocatalyst, at a concentration equal to 0.2 g/L. The results
presented in Fig. 5A show that the degradation of MBT in the pres-
ence of titanium dioxide is five times more efficient than with DTA
(insert Fig. 5A) and the mineralization is reached in less than 70 h
also about two times more efficiently than with DTA.
The mineralization of MBT has been determined by dissolved
organic carbon (or total organic carbon, TOC) measurements and
the formation of inorganic ions has been quantified as a function of
time in aerated solutions and under irradiation at 365 nm. This set
of experiments was done in order to put into evidence the photo-
catalyst efficiency and also the potential application of W10O324− in
the elimination of this benzothiazole derivative. The mineralization
and the degradation kinetics are represented in Fig. 5A.
As shown in Fig. 5A, the organic compounds degradation takes
place on a shorter time scale than the mineralization. The total dis-
appearance of MBT is effective in 20 h when its total mineralization
is observed after 100 h under irradiation. This result indicates that
the mineralization is initiated by the MBT photoproducts’ elimi-
nation and the presence of a short induction period during the
Nevertheless, we can also conclude that DTA seems to be a good
candidate for the induced elimination of 2-mercaptobenzothiazole
in aqueous homogeneous solutions.
6. Conclusion
+
formation of inorganic species (SO42−, NH4 and NO3−) confirms
this hypothesis (Fig. 5B). The correlation between the decrease in
organic dissolved carbon concentration and the formation of inor-
ganic ions is evidenced by the corresponding kinetics presented
in Fig. 5A and B. After 90 h upon irradiation, the mineralization
of MBT is almost total (92% of disappearance) and the concentra-
tion of sulphate ions is equal to 1.9 × 10−4 M, which corresponds
to 95% of the sulphur atom present in the molecule also very simi-
lar to the expected value of 1.84 × 10−4 M. For the same irradiation
time, the concentration of ammonium ions reaches a value around
1.1 × 10−4 M which is comparable to the nitrogen atoms expected
value (0.92 × 10−4 M); the nitrate ions were detected in very low
concentration (maximum equal to 5 × 10−4 M). All the results are
in complete agreement with the total mineralization of MBT in the
presence of DTA.
All along the study, we demonstrated the decatungstate salts
were efficient in the degradation of 2-mercaptobenzothiazole in
aqueous homogeneous conditions under UV irradiation. In fact, the
MBT disappearance and TOC kinetics, the rate constants and also
the quantum yields clearly illustrated the more important degra-
dation and the possible total elimination of MBT when compared
to direct and/or biological degradations.
The whole process revealed to be oxygen dependent because
5−
of the restricted W10O32 reoxidation step. The degradation of 2-
mercaptobenzothiazole occurred by electron transfer and H atom
abstraction processes with the very oxidative specie W10O324−* and
the main photoproducts formed were monohydroxylated products,
sulfoxide derivatives and the dimer of MBT. The calculations by
Gaussian 4.01 software allowed us to confirm MBT photoproducts
identification considering their dipole moment and their UV–vis
absorption spectrum. The total MBT mineralization was reached
in the presence of DTA attesting the potential applications for the
elimination of different organic pollutants and particularly ben-
zothiazole derivatives.
Moreover, compared with the results obtained during the
biodegradation of MBT in the presence of R. rhodochrous, the pres-
ence of DTA enhanced the elimination of MBT. In fact, the authors
put into evidence that only 30% of MBT is mineralized after 128 h of
incubation, this is four times less efficient than with DTA [7]. More

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A. Allaoui et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 153–160
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Financial support (Programme National Exceptionnel (PNE)
no. 148) for the PhD work by Algerian government is gratefully
acknowledged.
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