Thiobencarb degradation by TiO2 photocatalysis: Parameter and reaction pathway investigations

Article abstract of DOI:10.1002/jccs.201100251

The present study deals with the photocatalytic degradation of the thiocarbamate herbicide, thiobencarb (TBC), in the presence of TiO₂ particles and UV-A (λ = 365 nm) radiation. Results show rapid and complete oxidation of TBC after 90 min, and slightly over 70% of TBC was mineralized after 32-h treatment. Factors such as solution pH, TiO₂ dosage, and the presence of anions are found to influence the degradation rate. The establishment of the reaction pathway is made possible by a thorough analysis of the reaction mixture identifying the main intermediate products generated. Results suggest that possible transformation pathways may include hydroxylation, dealkylation and C-S bond cleavage processes. The possible degradation pathways are proposed and discussed on the basis of the evidence of oxidative intermediate formation.

Full text of DOI:10.1002/jccs.201100251

JOURNAL OF THE CHINESE
.
CHEMICAL SOCIETY
Article
Thiobencarb Degradation by TiO2 Photocatalysis: Parameter and Reaction Pathway
Investigations
Hsiao-Fang Lai,b Chiing-Chang Chen,c Ren-Jang Wub and Chung-Shin Lua*
aDepartment of General Education, National Taichung Nursing College, Taichung 403, Taiwan, R.O.C.
bDepartment of Applied Chemistry, Providence University, Taichung 433, Taiwan, R.O.C.
cDepartment of Science Application and Dissemination, National Taichung University, Taichung 403, Taiwan, R.O.C.
(Received May 9, 2011; Accepted August 15, 2011; Published Online August 26, 2011; DOI: 10.1002/jccs.201100251)
The present study deals with the photocatalytic degradation of the thiocarbamate herbicide, thiobencarb
(TBC), in the presence of TiO2 particles and UV-A (l = 365 nm) radiation. Results show rapid and com-
plete oxidation of TBC after 90 min, and slightly over 70% of TBC was mineralized after 32-h treatment.
Factors such as solution pH, TiO2 dosage, and the presence of anions are found to influence the degrada-
tion rate. The establishment of the reaction pathway is made possible by a thorough analysis of the reac-
tion mixture identifying the main intermediate products generated. Results suggest that possible transfor-
mation pathways may include hydroxylation, dealkylation and C–S bond cleavage processes. The possi-
ble degradation pathways are proposed and discussed on the basis of the evidence of oxidative intermedi-
ate formation.
Keywords: Photocatalysis; Titanium dioxide; Thiocarbamate herbicide; Thiobencarb; Intermediate;
Reaction pathway.
INTRODUCTION
Thiobencarb (TBC, S-4-chlorobenzyl diethylthiocar-
various adsorbents,5,6 chemical oxidation,7 biodegrada-
tion,8,9 photolysis10,11 and photocatalysis.12-16 However
each has limitations and disadvantages. The adsorption
method involves only a phase transfer of pollutants without
degradation; the chemical oxidation method is unable to
mineralize all organic substances; and in biological treat-
ment methods, the slow reaction rates and the disposal of
activated sludge are the drawbacks.17,18 The photocatalytic
processes have received increasing attention in recent
years because they are not burdened by the above disad-
vantages. Since organic pollutants can be completely de-
graded into harmless matter by photocatalysis under ambi-
ent temperature and pressure, scientists predict that it will
soon be recognized as one of the most effective means of
dealing with various kinds of wastewater.19
bamate) is a thiocarbamate herbicide used in rice produc-
tion, at a worldwide (40 countries) rate of approximately
18,000 tons per year.1 It may contaminate surface waters as
a result of releases of rice paddy water soon after field ap-
plication, or from spray drift associated with aerial or
ground spray application. TBC is resistant to degradation
by hydrolysis and has been detected in river waters. It is
moderately toxic to aquatic invertebrates and fish in acute
toxicity tests.2,3 The US Environmental Protection Agency
(USEPA) researchers have reported the reference doses in
publicly accessible databases such as the Integrated Risk
Information System, which contained 0.01 mg/kg/d of
TBC. The Ministry of the Environment in Japan released
the Environmental Quality Standards for Water Pollutants,
which included 0.02 mg/L of TBC.4 Because of the impor-
tance of this compound, an urgent need for more enhanced
technologies to reduce its presence in the environment has
become evident.
Titanium dioxide (TiO2)-based photocatalytic oxida-
tion is a promising technology in water and wastewater
treatment because TiO2 is a cheap, stable, and nontoxic cat-
alyst.20 The initial step in TiO2-mediated photocatalysis
degradation is proposed to involve the generation of an
Degradation and purification processes for the or-
ganic compound in polluted water include adsorption on
(e-/h+) pair, leading mainly to the formation of hydroxyl
radicals (·OH), as well as superoxide radical anions (O2
)
·-
* Corresponding author. Tel: +886-4-2219-6999; Fax: +886-4-2219-4990; E-mail: cslu6@ntcnc.edu.tw
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and hydroperoxyl radicals (·OOH), and these radicals are
the oxidizing species in the photocatalytic oxidation pro-
cesses.21–23 Among them hydroxyl radicals are the most
powerful oxidizing species that TiO2 photocatalysis pro-
duces and can attack organic contaminants present at or
near the surface of TiO2.24
amount of TBC in the aqueous solution. Solid-phase mi-
croextraction (SPME) and gas chromatography/mass spec-
trometry (GC/MS) were utilized for the analysis of inter-
mediate products resulting from the photocatalytic degra-
dation process. SPME holder and fiber-coating divinylben-
zene-carboxen-polydimethylsiloxane (DVB-CAR-PDMS
50/30 mm) were supplied from Supelco (Bellefonte, PA).
GC/MS analyses were run on a Perkin-Elmer AutoSystem-
XL gas chromatograph interfaced to a TurboMass selective
mass detector. The mineralization of TBC was monitored
by measuring the total organic carbon (TOC) content with a
Dohrmann Phoenix 8000 Carbon Analyzer, which em-
ployed a u.v./persulfate oxidation method by directly in-
jecting the aqueous solution.
Sturini et al.25 investigated the degradation of thio-
carbamate herbicides in a TiO2 heterogeneous aqueous sys-
tem. Their results indicated that thiobencarb degraded to
4-chlorobenzaldehyde and S-4-chlorobenzyl acetyl(eth-
yl)thiocarbamate. However, only two photodegradation in-
termediates of TBC have been separated and identified,
and the detailed mechanisms are still unclear. Considering
the environmental concerns regarding TBC and very lim-
ited information in the literature with regard to the photo-
degradation intermediates and reaction mechanisms, a
thorough intermediate study on the degradation of TBC in
dilute aqueous solution using the TiO2/UV process was un-
dertaken. Moreover, some potential factors likely to influ-
ence the photodegradability of TBC have not been well
documented. In this study, various parameters that may af-
fect the photodegradation of TBC in the presence of TiO2
suspensions are also analyzed to obtain a more complete
knowledge of TiO2 photocatalytic efficiency.
Procedures and analyses
TBC solution (5 mg L-1) with the appropriate amount
of photocatalyst was mixed and used in photocatalytic ex-
periments. For reactions in different pH media, the initial
pH of the suspensions was adjusted by the addition of a few
drops of either 0.1 N NaOH or 0.1 N HNO3 solutions. Prior
to irradiation, the dispersions were magnetically stirred in
the dark for 30 min to ensure the establishment of the ad-
sorption/desorption equilibrium. Irradiations were carried
out using two UV-365 nm lamps (15 watt). An average irra-
diation intensity of 0.1 mW/cm2 was maintained through-
out the experiments and was measured by internal radiome-
ter. At given irradiation time intervals, the aqueous TiO2
dispersion was sampled (5 mL) and centrifuged to separate
the TiO2 particles. After each irradiation cycle, the amount
of residual TBC was determined by HPLC. Two different
kinds of solvents were prepared in this study namely, sol-
vent A, 25 mM aqueous ammonium acetate buffer (pH 6.9)
and solvent B, methanol. LC was carried out on an Atlantis
dC18 column (250 mm × 4.6 mm i.d., dp = 5 mm). The flow
rate of the mobile phase was set at 1.0 mL/min. A linear
gradient was run as follows: t = 0, A = 95, B = 5; t = 20, A =
50, B = 50; t = 35-40, A = 10, B = 90; t = 45, A = 95, B = 5.
The elution was monitored at 220 nm.
EXPERIMENTAL
Materials
Thiobencarb (99.8%) was obtained from Sigma-
Aldrich. Standard solutions containing 5 mg L-1 of TBC in
water were prepared, protected from light, and stored at 4
oC. Other chemicals were of reagent grade and were used as
such without further purification. The TiO2 nanoparticles
(P25, ca. 80% anatase, 20% rutile; particle size, ca. 20-30
nm; BET area, ca. 55 m2g-1) were supplied by Degussa.
De-ionized water was used throughout this study. The wa-
ter was purified with a Milli-Q water ion-exchange system
(Millipore Co.) to give a resistivity of 1.8 × 107 W-cm.
Apparatus and instruments
The apparatus for studying the photocatalytic degra-
dation of TBC has been described elsewhere.26 The C-75
Chromato-Vue cabinet of UVP provided a wide area of il-
lumination from the 15-Watt UV-365 nm tubes positioned
on two sides of the cabinet interior. A Waters LC system,
equipped with a binary pump, an autosampler, and a photo-
diode array detector, was used for the determination of the
The analysis of organic intermediates was accom-
plished by SPME-GC/MS. The SPME fiber was directly
immersed into the sample solution to extract TBC and its
intermediates for 30 min at room temperature, with mag-
netic stirring at 550 ± 10 rpm on the Corning stirrer/plate
(Corning, USA). Finally, the compounds were thermally
desorbed from the fiber to the GC injector for 45 min. Sepa-
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TiO2/UV Photocatalytic Degradation of Thiobencarb
ration was carried out in a DB-5 capillary column (5% di-
phenyl/95% dimethyl-siloxane), 60 m, 0.25-mm i.d., and
1.0-mm thick film. A split-splitless injector was used under
the following conditions: injector temperature 250 °C, split
flow 10 mL/min. The helium carrier gas flow was 1.5 mL/
min. The oven temperature program was 1.0 min at 60 °C
then it was increased at 8 °C min-1 until reaching 240 °C. It
was kept at this temperature for 21.5 min, the total run time
being 45 min.
Next, blank experiments conducted in the presence of UV
radiation, but in the absence of photocatalyst, did not result
in any measurable degradation of TBC. Photolysis was
found to be negligible in overall degradation process.
Third, the results of the photocatalytic experiments showed
that TBC could be degraded efficiently in aqueous TiO2
dispersions by UV light irradiation. After UV irradiation
for 90 min, ca. 99.8% of TBC was degraded.
Mass spectrometric detection was performed in full-
scan conditions for both electron impact (EI) and chemical
ionization (CI) using isobutane as reagent gas. The ion
source and inlet line temperatures were set at 220 and 250
°C, respectively. Electron impact mass spectra were ob-
tained at 70 eV of electron energy, and monitored from 20
to 350 m/z. The mass spectrometer was tuned regularly
with perfluorotributylamine using the fragment ions at m/z
69, 131, 219 and 502. EI mass spectra were identified using
the NIST 2008 Library and the analytes were automatically
identified by the NIST MS-Search 2.0 software. Chemical
ionization mass spectrometry was operated in the positive
ionization mode, isobutane was used as reagent gas at an
apparent pressure of 4.4 × 10-4 Torr in the ionization source.
The full scan mode with a mass range of m/z 40-350 was
used to confirm the analytes. The autotuning software per-
formed the reagent gas flow adjustment, and the lens and
electronic tuning.
pH effect
According to previous studies, pH appears to play an
important role in the photocatalytic process of various pol-
lutants.27,28 This is because pH influences the surface
charge of the semiconductor, thereby affecting the interfa-
Fig. 1. TBC degradation under control conditions
(TiO2 only and UV only) and photocatalytic
conditions (experimental conditions: TBC = 5
mg L-1, TiO2 = 0 g L-1 in photolysis, 0.5 g L-1 in
photocatalysis, UV-365 nm = 0.1 mW/cm2 in
photolysis and photocatalysis conditions).
RESULTS AND DISCUSSION
Blank experiments
To confirm the role of TiO2 in the photocatalysis reac-
tion, three sets of experiments were performed to compare
TBC degradation rates with and without catalysts. One set
was performed with TBC (5 mg L-1) exposed to TiO2 (0.5 g
L-1) but no UV (the TiO2-only condition). The second set
was performed by exposing TBC (5 mg L-1) to UV without
TiO2 (the photolysis condition). Then, the third set was per-
formed by exposing TBC (5 mg L-1) to TiO2 (0.5 g L-1) in
the presence of UV illumination (the photocatalysis condi-
tion). The results are presented in Fig. 1. First, blank exper-
iments in the dark (without exposure to UV light) revealed
that the change of initial TBC concentrations after 90 min
of mixing with TiO2 was less than 4% that the adsorption of
TBC on TiO2 was insignificant and could be neglected.
Fig. 2. pH effect on the photocatalytic degradation rate
of TBC. Experimental conditions: TBC con-
centration 5 mg L-1; TiO2 concentration 0.5 g
L-1.
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cial electron transfer and the photoredox process.29 The in-
fluence of the initial pH value on the photodegradation rate
of TBC for the TiO2 suspensions is demonstrated in Fig. 2.
The results indicated that the degradation rate increased
with an increase in pH in the studied range of 3-10. The ef-
fect of pH on a photocatalytic reaction was generally as-
cribed to the surface charge of the photocatalyst and its re-
lation to the ionic form of the organic compound (anionic
or cationic). Electrostatic attraction or repulsion between
the photocatalyst’s surface and the organic molecule was
taking place, and these events consequently enhanced or
inhibited, respectively, the photodegradation rate.30 Due to
the non-ionic property of TBC, the observed increase in the
degradation rate under an alkaline pH can be ascribed to the
high hydroxylation of the photocatalyst’s surface due to the
presence of a large quantity of OH- ions. Consequently, a
higher concentration of ·OH species was formed, and the
overall rate was enhanced.24,31
dosed, the intensity of incident UV light was attenuated be-
cause of the decreased light penetration and increased light
scattering, which counteracted the positive effect coming
from the dosage increment and reduced the overall perfor-
mance.32
Effects of anions
Studying the effects of anions on the photocatalytic
degradation of TBC is important because anions are often
present in natural water and industrial wastewater. The ef-
fects of the presence of various anions such as chloride, bi-
carbonate, carbonate and sulfate were studied using 0.05 M
solutions of their sodium salts and an initial concentration
of 5 mg L-1 of TBC with a 0.5 g L-1 of TiO2. The results
showed that all these anions inhibited the degradation sig-
nificantly (see Fig. 4). These substances might compete for
the active sites on the TiO2 surface or deactivate the photo-
catalyst and, subsequently, decrease the degradation rate of
TBC. Also, inhibition effects of anions could be explained
as the reaction of positive holes (h+) and hydroxyl radicals
(·OH) with anions that behaved as h+ and ·OH scavengers
resulting in prolonged TBC removal.33 A major drawback
resulting from the high reactivity and non-selectivity of
·OH was that it also reacted with non-target compounds
present in the background water matrix, i.e. inorganic an-
ions present in water. This resulted in a higher ·OH demand
to accomplish the desired degree of degradation.15 Similar
results were reported previously by Chen et al.,34 suggest-
ing that inorganic anions were capable of inhibiting the
photocatalytic degradation of dichloroethane in an aqueous
Effect of TiO2 dosage
In photocatalytic processes, the amount of photocat-
alyst is an important parameter that can affect the degrada-
tion rate of organic compounds. In order to obtain the opti-
mum catalyst dosage, the relationship between the dosage
and degradation rate was investigated as shown in Fig. 3.
The photocatalytic degradation rate was found to increase
with increasing TiO2 dosages, but the reaction was retarded
at high TiO2 dosages. The increase in the rate was likely
due to the increase in the total surface area (or number of
active sites) available for photocatalytic reaction as the
dosage of TiO2 increased. However, when TiO2 was over-
Fig. 4. Effect of anions on the photocatalytic degrada-
tion rate of TBC. Experimental conditions:
TBC concentration 5 mg L-1; TiO2 concentra-
tion 0.5 g L-1; pH 10.
Fig. 3. Effect of TiO2 dosage on the photocatalytic
degradation rate of TBC. Experimental condi-
tions: TBC concentration 5 mg L-1; pH 7.
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TiO2/UV Photocatalytic Degradation of Thiobencarb
concentration of reaction intermediates was low and thus
the intermediates had to be preconcentrated before the ap-
plication of an appropriate analytical procedure. Prior to
GC/MS analysis; the samples were preconcentrated using a
solid-phase microextraction (SPME) method, a selective
tool for the trace analysis of organic compound in water
samples. Therefore, the intermediates generated in the
TBC solution during the photocatalytic degradation proc-
ess with UV irradiation were examined with SPME-GC/
MS.
suspension of TiO2.
Evolution of TOC
The complete degradation of an organic molecule by
photocatalysis normally leads to the conversion of all its
carbon atoms to gaseous CO2 and of the heteroatoms into
inorganic anions that remain in solution. In order to study
the mineralization during TBC photocatalysis, total or-
ganic carbon (TOC) measurement was performed. The re-
sults are shown in Fig. 5. It was found that approximately
71% of the TBC was mineralized within 32 h by the photo-
catalytic reaction. Complete mineralization of TBC was
not achieved after 32 h of oxidation although TBC disap-
peared after 90 min. The great difference between degrada-
tion efficiency and mineralization efficiency implied that
the products of TBC oxidation mostly stayed at the inter-
mediate product stage under the present experimental con-
ditions.
Fig. 6 displays the chromatogram of the reacted solu-
tion after irradiation for 280 min in the presence of TiO2. At
least eight compounds were identified at retention times of
less than 45 min. One of the peaks was the initial TBC; the
other seven (new) peaks were the intermediates formed.
We denoted the related intermediates as species I-VII. Ex-
cept for the initial TBC, the other peaks increased at first
and subsequently decreased, indicating formation and sub-
sequent transformation of the intermediates. Some other
minor peaks were present, but mass fragment information
did not allow elucidation of their structures.
Separation and identification of the intermediates
Since thiobencarb is sparingly soluble in water, it is
therefore a challenge to study its photodegradation in dilute
aqueous solutions in the absence of any organic solvents,
and to identify its degradation products. A relatively low
intensity UV-365 lamp (15 watt) was used in this study to
identify organic intermediates. This made it possible to ob-
tain slower degradation rates and provide favorable condi-
tions for the determination of intermediates. Additionally,
the initial TBC concentration (100 mg L-1) was selected to
be high enough to facilitate the identification of intermedi-
ate products. In photocatalytic degradation process, the
Intermediate identification is usually carried out on
the basis of EI mass spectra, mainly because their structure
can be elucidated by comparing the unknown compound
spectrum with published spectra from spectra databases or
research papers. Otherwise, from the knowledge of the mo-
lecular weight and the interpretation of fragmentation pat-
terns according to well-known mass spectroscopy rules, it
is possible to hypothesize a molecular structure.35 Table 1
summarizes the identified intermediates of TBC along with
their retention time and the characteristic ions of the mass
spectra. Four intermediates (compounds I-IV) were identi-
fied by the molecular ion and mass fragment ions and also
through comparison with NIST library data. The similari-
ties of these compounds to the NIST library data were more
than 89%. Three intermediates (compounds V-VII) not in-
cluded in the library were identified by the molecular ion
and the interpretation of the mass spectra. The molecular
mass of these intermediates was determined using positive
ion chemical ionization (CI) mass spectrometry through
the abundant protonated molecules, and then structural
data was obtained from the electron impact (EI) fragmenta-
tion patterns. The presence and number of chlorine atoms
in the suspected intermediates can be easily attained by tak-
ing into account both the relative intensity of the 35Cl/37Cl
Fig. 5. Depletion in TOC measured as a function of ir-
radiation time for an aqueous solution of TBC
(5 mg L-1) in the presence of TiO2 (0.5 g L-1).
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Table 1. Identification of the intermediates from the photodegradation of thiobencarb by GC/MS
Peaks
Photodegradation intermediates
4-chlorobenzaldehyde
R.T. (min)
MS peaks (m/z)
I
16.21
17.89
18.31
19.39
31.60
34.40
36.47
41.45
140,139,111,75,50
156,155,99,63,39
II
4-chloro-2/3-hydroxybenzaldehyde
4-chlorobenzyl alcohol
III
IV
V
142,107,79,77,51
4-chlorobenzyl mercaptan
158,125,89,63,45
S-4-chlorobenzyl ethylthiocarbamate
S-4-chlorobenzyl diethylthiocarbamate
S-4-chlorobenzyl acetyl(ethyl)thiocarbamate
S-4-chloro-2/3-hydroxybenzyl diethylthiocarbamate
229,158,125,89,72
257,125,100,72,44,29
271,156,146,125,89,43
273,141,100,72,44,29
TBC
VI
VII
signals and the mass differences between the two masses.
The EI and CI mass spectra of the identified interme-
diates are shown in Fig. 7 and Fig. 8, respectively. Com-
pound I was identified as 4-chlorobenzaldehyde with a fit
value of 99%, found by searching the mass spectra library.
The molecular mass was determined from the CI mass
spectrum to be m/z = 140 by the observation of an [M + H]+
ion of 141. The double signal of protonated molecular ion
141/143 appeared to be in a ratio of 3:1 due to the natural
ratio of the chlorine isotope. Based on this, the intermediate
product can be confirmed to contain one chlorine atom.
The peak eluting at 17.89 min (compound II) during
GC/MS analysis of TBC solution was identified as 4-
chloro-2/3-hydroxybenzaldehyde with the mass spectra li-
brary. The molecular mass was determined from the CI
mass spectrum to be m/z = 156 that corresponded to the hy-
droxylated derivative of 4-chlorobenzaldehyde (addition
of OH group, m/z = [M + 16]+). However, even an EI mass
spectrum did not provide secure information about the po-
sition of the hydroxyl group in this hydroxylated deriva-
tive.35 As the pure compound II is not commercially avail-
able, a more conclusive identification is difficult.
A search of the mass spectra library selected 4-chlo-
robenzyl alcohol with a fit value of 89% for compound III,
Fig. 6. GC/MS chromatogram obtained for TBC solution (100 mg L-1) after 280 min of irradiation with UV light in the pres-
ence of TiO2.
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whose retention time on GC/MS was 18.31 min. The mo-
lecular mass was determined from the CI mass spectrum to
be m/z = 142 by the observation of [M + H]+ ion of 143. The
peak eluting at 19.39 min (compound IV) during GC/MS
analysis of TBC solution was identified as 4-chlorobenzyl
mercaptan with a fit value of 99%, with the mass spectra li-
brary. The protonated molecular ion (m/z = 159) for com-
pound IV was not obviously detected by the GC/MS, but
two characteristic fragments were found in high abundance
for the CI mass spectrum. The mass spectrum showed the
characteristic ions at m/z = 125 and m/z = 47 that corre-
spond to the groups [ClC6H4CH2]+ and [CH2SH]+, respec-
tively. The fragment ion 125/127 appears to be in a ratio of
3:1 due to the natural ratio of the chlorine isotope. It can be
Fig. 8. CI mass spectra of intermediates formed during
the photodegradation of TBC after they were
separated by GC/MS method. Spectra are de-
noted I-VII and correspond to the I-VII species
in Fig. 6, respectively.
Fig. 7. EI mass spectra of intermediates formed during
the photodegradation of TBC after they were
separated by GC/MS method. Spectra are de-
noted I-VII and correspond to the I-VII species
in Fig. 6, respectively.
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deduced that a chlorine atom is present in the molecular ion
of this intermediate.
termediates detected during photocatalysis. By describing
the typical bell-shaped profiles in most of the cases, we
clearly observed the change in the distribution of each in-
termediate during the photocatalytic degradation of TBC (5
mg L-1). Compounds I, VI and VII may be considered as
the main intermediates, due to their high concentration
compared with other intermediates detected. Their concen-
trations increase with time of irradiation and go through
maximum at 30 min and then decline gradually. Com-
pounds II~V were present at lower concentrations, but they
were considered as main intermediates, because they in-
creased at first and subsequently decreased.
Compound V had a molecular ion at m/z = 229 and
major fragments at m/z = 125 and 72, which were common
ions with thiobencarb. The molecular mass difference of 28
amu, compared to the initial thiobencarb molecule, indi-
cated that a de-ethylated product was formed. According to
mass spectrometric analyses, compound V was tentatively
identified as S-4-chlorobenzyl ethylthiocarbamate. N-
Dealkylation of thiobencarb to compound V was previ-
ously reported as a process mediated by hydroxyl radi-
cals.36
The molecular mass of compound VI was determined
from the CI mass spectrum to be m/z = 271 by the observa-
tion of an [M + H]+ ion of 272. The EI mass spectrum
showed the characteristic ions at m/z = 125 and m/z = 43
that correspond to the groups [ClC6H4CH2]+ and [CH3CO]+,
respectively. According to mass spectrometric analyses,
compound VI was tentatively identified as S-4-chloro-
benzyl acetyl(ethyl)thiocarbamate. This compound was
further identified by comparing the mass spectrum with
previous reported spectrum. The fragments of the studied
compound match with the previous spectrum of the litera-
ture.25
Initial photooxidation pathway
Irradiation of TiO2 suspension with photons, the en-
ergy of which is equal to or higher than that of the band gap
energy of the semiconductor (e.g. 3.2 eV for TiO2), results
in the formation of conduction band electrons (e-) and va-
lence band holes (h+). Hydroxyl radicals can be produced
by the oxidation of water by these holes and are recognized
as one of the most powerful oxidants.24 Nakamura and
Compound VII was attributed as the ring-hydroxy-
lated derivative of thiobencarb. It exhibited the same char-
acteristic ions (m/z = 100, 72, 44) with thiobencarb and its
molecular ion M+ at m/z = 273 corresponded to the addition
of –OH group to the phenyl ring. The main difference was
the presence of an m/z = 141 ion which corresponded to the
[ClC6H3(OH)CH2]+ group. This means that the hydroxyla-
tion takes place in the aromatic moiety because the mass
spectra shows the peak at m/z = 141 that correspond to the
ring-hydroxylated analog of [ClC6H4CH2]+ (m/z = 125), a
fragment common to thiobencarb (addition of OH group,
m/z = [M + 16]+). The double signal of protonated molecu-
lar ion 274/276 appeared to be in a ratio of 3:1 due to the
natural ratio of the chlorine isotope. It can be deduced that a
chlorine atom is present in the molecular ion of this inter-
mediate. According to mass spectrometric analyses, com-
pound VII was tentatively identified as S-4-chloro-2/3-
hydroxybenzyl diethylthiocarbamate. As the pure com-
pounds V-VII are not commercially available, a more con-
clusive identification is difficult.
Fig. 9. Evolution profiles of the reaction intermediates
detected during the photocatalytic degradation
of TBC (5 mg L-1).
Fig. 9 shows the evolution profiles of the reaction in-
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94

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
TiO2/UV Photocatalytic Degradation of Thiobencarb
Nakato37 proposed a mechanism of water oxidation that in-
volved a nucleophilic attack on a surface-trapped hole at a
bridged O site. Murakami et al.38 reported that when the
O-O bond in Ti-O-OH broke, the hydroxyl radicals could
be formed from the bridge OH groups.
Proposed photodegradation pathway of
TBC under UV irradiation in aqueous TiO2
dispersions
Scheme I
Various aromatic intermediates were identified by
GC/MS techniques during the treatment, which were in-
volved into three tentative degradation routes. One was
based on the initial attack of the aromatic ring by ·OH lead-
ing to the formation of hydroxylated derivative. The sec-
ond route was based on the abstraction of hydrogen atoms
of the N-alkyl group followed by the addition of oxygen re-
sulting in dealkylated derivative. Finally, the third possible
degradation route was based on the oxidative cleavage of
the C–S bond of the substrate molecule. The molecular
structure of the degradation intermediates and the tentative
photocatalytic degradation pathway of TBC are presented
in Scheme I.
Aromatic hydroxylation is a typical reaction of hy-
droxyl free radicals (·OH) and involves hydroxycyclo-
hexadienyl radical intermediates that are oxidized by mo-
lecular oxygen to the phenolic products.39 Hydroxyl radi-
cals attacked preferentially the aromatic moiety due to their
electrophilic character to form the ring-hydroxylated prod-
uct (compound VII) but also attacked the N-alkyl chain to
form the de-alkylated product (compound V). It is well
known that the ·OH radical is an electrophile and that C-H
bonds adjacent to nitrogen are responsible for a pro-
nounced stereoelectronic effect that produces high rates of
H-atom abstraction. Therefore, the a-hydrogen atoms in
the N-ethyl group of TBC molecule were the most prone to
radical attack. Hydroxyl radicals yielded carbon-centered
radicals upon the H-atom abstraction from the N-ethyl
group, or they reacted with the lone-pair electron on the N
atom to generate cationic radicals, which subsequently
converted into carbon-centered radicals.40 The carbon-cen-
tered radical generated after addition of oxygen forming
the peroxyl radical decomposed to different intermediates
(compounds V and VI).
catalytic degradation of organophosphorus pesticides41 and
thioethers.42
The interfacial transfer of a single electron from the
sulfur atom led to the formation of thiobencarb cation radi-
cal. The scission of the C-S bond in this cation radical led to
the formation of a ClC6H4CH2S· radical that was the pre-
cursor of 4-chlorobenzyl mercaptan (compound IV). The
H atom necessary for S-H bond formation was proposed to
have originated from the proton reduction by photogener-
ated electron H+ + e- ® H· as already observed in the deg-
radation of the insecticide fenitrothion by Kerzhentsev et
al.43 Similarly, the scission of the C-S bond in the TBC cat-
ion radical led to the formation of (4-chlorophenyl)meth-
ylium ion. The carbocation was extremely unstable and un-
In addition to the degradation routes of hydroxylation
and N-de-ethylation, an alternative pathway was also iden-
tified. First, oxidation of the thiobencarb molecule to form
the thiobencarb cation radical took place when positive
holes attacked it, initiating a series of reactions. The forma-
tion of cation radicals had also been observed in the photo-
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95

Article
Lai et al.
1.
derwent rapid hydrolysis. Hydrolysis of (4-chlorophenyl)-
methylium ion yielded 4-chlorobenzyl alcohol (compound
III). Subsequently, 4-chlorobenzyl alcohol underwent oxi-
dation to 4-chlorobenzaldehyde (compound I) and 4-chlo-
ro-2/3-hydroxybenzaldehyde (compound II).
10. Chen, B.; Lee, W.; Westerhoff, P. K.; Krasner, S. W.;
Herckes, P. Water Res. 2010, 44, 3401.
11. Segal-Rosenheimer, M.; Dubowski, Y. J. Photochem. Photo-
biol., A 2010, 209, 193.
12. Lu, C. S.; Chen, C. C.; Mai, F. D.; Li, H. K. J. Hazard. Mater.
2009, 165, 306.
CONCLUSION
13. Wu, R. J.; Chen, C. C.; Chen, M. H.; Lu, C. S. J. Hazard. Ma-
ter. 2009, 162, 945.
Photodegradation using TiO2 as a catalyst is an effi-
cient method of degrading TBC. After a 0.1 mW/cm2 light
intensity (365 nm UV) irradiation for 90 min, ca. 99.8% of
TBC is degraded. The photodegradation rate of TBC is
found to increase along with increasing pH and to increase,
and then decrease, along with increasing catalyst concen-
tration. In addition, the presence of inorganic ions such as
chloride, bicarbonate, carbonate and sulfate, which are of-
ten present in natural water and industrial wastewater, de-
crease the photocatalytic degradation rate of TBC. Seven
intermediate products are detected by the solid-phase mi-
croextraction (SPME) and gas chromatography/mass spec-
trometry (GC/MS) technique, giving insight into the early
steps of the degradation process. The analysis of the inter-
mediate products reveals the presence of hydroxylation,
dealkylation and C–S bond cleavage pathways. The reac-
tion pathway investigation provides a better understanding
and new insights on the mechanism of degradation of thio-
bencarb with TiO2.
14. Chen, C. C.; Wu, R. J.; Yao, I. C.; Lu, C. S. J. Hazard. Mater.
2009, 172, 1021.
15. Konstantinou, I. K.; Albanis, T. A. Appl. Catal., B 2004, 49,
1.
16. Watanabe, N.; Horikoshi, S.; Kawasaki, A.; Hidaka, H.;
Serpone, N. Environ. Sci. Technol. 2005, 39, 2320.
17. Pelizzetti, E.; Maurino, V.; Minero, C.; Carlin, V.; Pramauro,
E.; Zerbinati, O.; Tosato, M. Environ. Sci. Technol. 1990, 24,
1559.
18. Wang, K. H.; Hsieh, Y. H.; Chou, M. Y.; Chang, C. Y. Appl.
Catal., B 1999, 21, 1.
19. Chen, S.; Cao, G. Chemosphere 2005, 60, 1308.
20. Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
Chem. Rev. 1995, 95, 69.
21. Daneshvar, N.; Salari, D.; Khataee, A. R. J. Photochem.
Photobiol., A 2003, 157, 111.
22. Dionysiou, D. D.; Suidan, M. T.; Bekou, E.; Baudin, I.;
Laîné, J. M. Appl. Catal., B 2000, 26, 153.
23. Da Silva, C. G.; Faria, J. L. J. Photochem. Photobiol., A
2003, 155, 133.
ACKNOWLEDGMENT
24. Bianco, P. A.; Vincenti, M.; Banciotto, A.; Pramauro, E.
Appl. Catal., B 1999, 22, 149.
This research was supported by the National Science
Council of Taiwan (NSC 98-2113-M-438-001; NSC 99-
2113-M-438-001-MY2).
25. Sturini, M.; Fasani, E.; Prandi, C.; Casaschi, A.; Albini, A. J.
Photochem. Photobiol., A 1996, 101, 251.
26. Chen, C. C.; Lu, C. S.; Mai, F. D.; Weng, C. S. J. Hazard.
Mater. 2006, 137, 1600.
REFERENCES
27. Evgenidou, E.; Fytianos, K.; Poulios, I. J. Photochem.
Photobiol., A 2005, 175, 29.
1. Pentyala, S. N.; Chetty, C. S. J. Appl. Toxicol. 1993, 13, 39.
2. Jinno, H.; Hanioka, N.; Takahashi, A.; Nishimura, T.;
Toyo’oka, T.; Ando, M. Toxicol. in Vitro. 1997, 11, 731.
3. Kodama, S.; Yamamoto, A.; Matsunaga, A. J. Agric. Food
Chem. 1997, 45, 990.
28. Piscopo, A.; Robert, D.; Weber, J. V. Appl. Catal., B 2001,
35, 117.
29. Zhu, X.; Yuan, C.; Bao, Y.; Yang, J.; Wu, Y. J. Mol. Catal., A.
2005, 229, 95.
4. Amin, Md. N.; Kaneco, S.; Kato, T.; Katsumata, H.; Suzuki,
T.; Ohta, K. Chemosphere 2008, 70, 511.
30. Evgenidou, E.; Fytianos, K.; Poulios, I. Appl. Catal., B 2005,
59, 81.
5. Lin, S. H.; Juang, R. S. J. Environ. Manage. 2009, 90, 1336.
6. Wang, S.; Peng, Y. Chem. Eng. J. 2010, 156, 11.
7. Crimi, M.; Ko, S. Chemosphere 2009, 74, 847.
8. Schaefer, C. E.; Yang, X.; Pelz, O.; Tsao, D. T.; Streger, S.
H.; Steffan, R. J. Chemosphere 2010, 81, 1104.
31. Cao, Y.; Chen, J.; Huang, L.; Wang, Y.; Hou, Y.; Lu, Y. J.
Mol. Catal., A 2005, 233, 61.
32. Wong, C. C.; Chou, W. Chemosphere 2003, 50, 981.
33. Mahmoodi, N. M.; Arami, M.; Limaee, N. Y.; Gharanjig, K.
J. Hazard. Mater. 2007, 145, 65.
9. Haritash, A. K.; Kaushik, C. P. J. Hazard. Mater. 2009, 169,
www.jccs.wiley-vch.de
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 87-97
96

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
TiO2/UV Photocatalytic Degradation of Thiobencarb
34. Chen, H. Y.; Zahraa, O.; Bouchy, M. J. Photochem. Photo-
39. Draper, W. M.; Crosby, D. G. J. Agric. Food Chem. 1984, 32,
728.
biol., A 1997, 108, 37.
35. Konstantinou, I. K.; Albanis, T. A. Appl. Catal., B 2003, 42,
40. Lee, J.; Choi, W. Environ. Sci. Technol. 2004, 38, 4026.
41. Evgenidou, E.; Konstantinou, I.; Fytianos, K.; Albanis, T. J.
Hazard. Mater. B. 2006, 137, 1056.
319.
36. Ruzo, L. O.; Casida, J. E. J. Agric. Food Chem. 1985, 33,
272.
42. Fox, M. A.; Abdel-Wahad, A. A. Tetrahedron Lett. 1990, 31,
4533.
37. Nakamura, R.; Nakato, Y. J. Am. Chem. Soc. 2004, 126,
1290.
43. Kerzhentsev, M.; Guillard, C.; Herrmann, J.-M.; Pichat, P.
Catal. Today 1996, 27, 215.
38. Murakami, Y.; Kenji, E.; Nosaka, A. Y.; Nosaka, Y. J. Phys.
Chem. B. 2006, 110, 16808.
J. Chin. Chem. Soc. 2012, 59, 87-97
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