Heterogeneous photocatalytic degradation of disulfoton in aqueous TiO 2 suspensions: Parameter and reaction pathway investigations

Article abstract of DOI:10.1002/jccs.201200027

The photocatalytic degradation of organophosphorus insecticide disulfoton is investigated by having titanium dioxide (TiO₂) as a photocatalyst. About 99% of disulfoton is degraded after UV irradiation for 90 min. The effects of the solution pH, catalyst dosage, light intensity, and inorganic ions on the photocatalytic degradation of disulfoton are also investigated, as well as the reaction intermediates which are formed during the treatment. Eight intermediates have been identified and characterized through a mass spectra analysis, giving insight into the early steps of the degradation process. To the best of our knowledge, this is the first study reporting the degradation pathways of disulfoton. The results suggest that possible transformation pathways may involve in either direct electron or hole transfer to the organic substrate. The photodegradation of disulfoton by UV/TiO₂ exhibits pseudo-first-order reaction kinetics and a reaction quantum yield of 0.267. The electrical energy consumption per order of magnitude for photocatalytic degradation of disulfoton is 85 kWh/(m3 order).

Full text of DOI:10.1002/jccs.201200027

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Heterogeneous Photocatalytic Degradation of Disulfoton in Aqueous TiO2
Suspensions: Parameter and Reaction Pathway Investigations
a
Ming-Hung Chen, Chiing-Chang Chen, Ren-Jang Wu and Chung-Shin Lu *
b
a
c
a
Department of Applied Chemistry, Providence University, Taichung 433, Taiwan, ROC
Department of Science Application and Dissemination, National Taichung University of Education, Taichung 403,
b
Taiwan, ROC
c
Department of General Education, National Taichung University of Science and Technology, Taichung 403, Taiwan,
ROC
(Received: Jan. 17, 2012; Accepted: Jul. 31, 2012; Published Online: Jan. 17, 2013; DOI: 10.1002/jccs.201200027)
The photocatalytic degradation of organophosphorus insecticide disulfoton is investigated by having tita-
2
nium dioxide (TiO ) as a photocatalyst. About 99% of disulfoton is degraded after UV irradiation for 90
min. The effects of the solution pH, catalyst dosage, light intensity, and inorganic ions on the photocata-
lytic degradation of disulfoton are also investigated, as well as the reaction intermediates which are
formed during the treatment. Eight intermediates have been identified and characterized through a mass
spectra analysis, giving insight into the early steps of the degradation process. To the best of our knowl-
edge, this is the first study reporting the degradation pathways of disulfoton. The results suggest that pos-
sible transformation pathways may involve in either direct electron or hole transfer to the organic sub-
2
strate. The photodegradation of disulfoton by UV/TiO exhibits pseudo-first-order reaction kinetics and a
reaction quantum yield of 0.267. The electrical energy consumption per order of magnitude for photocat-
3
alytic degradation of disulfoton is 85 kWh/(m order).
Keywords: Photocatalysis; Titanium dioxide; Disulfoton; Intermediate.
INTRODUCTION
Organophosphorus pesticides (OPs) are popular can-
result, effective purification methods for removing these
pollutants from water are in urgent demand.
didates to replace the more persistent organochlorine com-
pounds which are suspected to be bioaccumulated up the
food chain. OPs represent more than one-third of the total
There are several methods to remove OPs from water,
11
9 10
including adsorption, chemical oxidation, hydrolysis
1
2
and biodegradation. Each method has its own limitations
and disadvantages in applications. Adsorption is one of the
most recognized methods used in the removal of such haz-
1
,2
insecticides used in the world. The primary acute toxic
effect caused by organophosphorus pesticides is usually
the inhibition of enzyme acetylcholinesterase, which plays
1
3
ardous substances from polluted water. Adsorption in-
3
,4
a critical role in the proper functioning of nerve cells.
volves only in phase transfer of pollutants and decreasing
1
4
OPs act as the cholinesterase inhibitors in insects and mam-
mals and bring about a non-reversible phosphorylation of
esterases in the organisms’central nervous system. This ac-
their concentration from water without degradation; the
chemical oxidation method is unable to mineralize all or-
ganic substances; hydrolysis processes are typically very
slow and sometimes inefficiency; and in biological treat-
ment methods, the slow reaction rates and the disposal of
5
,6
tivity is also responsible for the toxicity in humans. Due
to their extensive use, they are the chemicals most fre-
quently associated with toxicity to aquatic fauna as well as
1
5,16
activated sludge are the drawbacks to be considered.
7
humans. Disulfoton is a thioether-containing organophos-
phate pesticide that is found widely used in agriculture, pri-
marily on crops such as corn, potatoes, cotton and grains.
Its chemical structure is shown in Fig. 1. About 1.2 million
8
pounds of disulfoton are used annually in the United States.
Contamination of the environment by disulfoton is primar-
ily due to the release during its use as insecticide by con-
stant leaching and superficial runoff during rainfall. As a
Fig. 1. Chemical structure of disulfoton.
*
Corresponding author. Tel: +886-4-2219-6999; Fax: +886-4-2219-6825; E-mail: cslu6@nutc.edu.tw
380
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
2
TiO /UV Photocatalytic Degradation of Disulfoton
The processes involving in light and homogeneous photo-
chemistry have been interested in the last years because
they overcame the above disadvantage. Advanced oxida-
tron.
Little has been reported on the degradation of disul-
2
4
foton under UV irradiation. Zamy et al. investigated the
photochemical transformation of four selected organophos-
phorus pesticides (disulfoton, isofenfos, isazofos and pro-
fenofos) in dilute aqueous solutions. They analyzed the
degradation products of disulfoton upon polychromatic ir-
radiation (l > 285 nm) in aqueous solution. Their results
showed the formation of disulfoton–oxon, disulfoton–
sulfoxide and dithiophosphoric acid-O, S, O-triethylester.
tion processes (AOPs), based on H
2 2 3
O /UV, O /UV, and
H
2
O
2
/O /UV combinations, employing photolysis of H O
2 2
3
·
and ozone which produce hydroxyl radicals ( OH) have
0
grown success. Hydroxyl radical is strongly oxidative (E =
1
.8 V) that it can destroy hazardous contaminants. The
5
2
end products are carbon dioxide, water and inorganic min-
7-19
1
eral salts.
Homogeneous photochemistry, in general,
2
5
requires high energy photons and rarely enhances the total
degradation of the pollutant.
Elsewhere, Hebert and Miller studied the photolysis of
disulfoton on soil. They attributed the degradation of disul-
foton to the generation of singlet oxygen, but no informa-
tion concerning degradation products was being made
available.
Photocatalytic oxidation using semiconductor mate-
rials, such as TiO
2
, has been demonstrated as the effective
means for organic degradation in a very short time and with
1
7,20
high efficiency.
TiO
2
is broadly used as a photocatalyst
To the best of our knowledge, no studies on the photo-
catalyic degradation of disulfoton have been published yet.
In this paper, the photocatalytic degradation of disulfoton
is reported for the first time. Various parameters which
might affect the photodegradation of disulfoton in the pres-
because of its non-toxicity, photochemical stability and
2
1
low cost. When activation of the semiconductor photo-
catalyst for reaction is achieved through the absorption of
photon of ultra-bandgap energy (Eg), electron is promoted
from the lowest unoccupied energy band (the valence band,
+
ence of TiO
2
suspensions are investigated in order to obtain
photocatalysis. This
h
VB) to the conduction band, with the concomitant genera-
a more complete knowledge of TiO
2
tion of hole in the highest occupied band (the conduction
-
band, e CB), as set forth in equation (1). OH radicals were
study also focuses on the identification of the reaction in-
termediates and the understanding of the mechanistic de-
formed directly by the oxidation of H
2
O with holes in the
2
tails of the photodegradation of disulfoton in the TiO /UV
valence band of TiO , which are shown in equation (2)-(3).
2
light process as a foundation for future application of this
energy saving technology.
The processes are described by equation (4)-(5), the sur-
face trapped CB electron could reduce the organic substrate
or react with the adsorbed molecular O
2
, generation of the
EXPERIMENTAL SECTION
-
superoxide radical anion, O2· . The hydroxyl radical is a
Materials and reagents
powerful oxidizing agent and attacks organic pollutants
Disulfoton (98.2%) was obtained from Sigma-Aldrich.
-
Standard solution containing 5 mg L of disulfoton in water was
o
prepared, protected from light, and stored at 4 C. 1,2-diethyldi-
1
presented at or near the surface of TiO
1
tion (6).
2
, as shown in equa-
5,21-23
sulfane (Sigma-Aldrich) was used for intermediate confirmation.
Other chemicals were of reagent grade and used as such without
further purification. Titanium dioxide (P25)—a known mixture of
80% anatase and 20% rutile, with an average particle size of 30
+
+ hn (l < 388 nm) ®h VB + e CB
-
TiO
2
(1)
(2)
(3)
(4)
(5)
+
O + h VB ®Ti(IV)-OH + H
+
Ti(IV)-H
2
+
Ti(IV)-OH + h VB ®{>Ti(IV)-OH }
·
+
-
Ti(IV)-OH + e CB ®{>Ti(III)-OH}
2 -1
nm, nonporous, and a reactive surface area of 50 ± 10 m g —was
used as received for all degradation experiments and supplied by
Degussa Co. De-ionized water was used throughout this study.
-
{
{
2
>Ti(III)-OH} + O ®Ti(IV)-OH + O2·
·
+
>Ti(IV)-OH } + organic pollutants ®
Ti(IV)-OH + oxidized pollutants
(6)
The water was purified with a Milli-Q water ion-exchange system
7
Millipore Co.) to give a resistivity of 1.8 × 10 W-cm.
(
where >TiOH represents the primary hydrated surface
-
, e CB is a conduction band electron,
Apparatus and instruments
functionality of TiO
+
2
The schematic diagram of experimental apparatus was
26
shown in our early report. The C-75 Chromato-Vue cabinet of
·
+
VB is the valance band hole, {>Ti(IV)-OH } is the sur-
h
face trapped VB hole (surface bound hydroxyl radical,
·
OH), and {>Ti(III)-OH} is the surface trapped CB elec-
UVP provided a wide area of illumination from the 15-Watt
UV-365 nm tubes positioned on two sides of the cabinet interior.
J. Chin. Chem. Soc. 2013, 60, 380-390
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381

Article
Chen et al.
Solid-phase microextraction (SPME) was utilized for the analysis
of disulfoton and intermediates resulting from the photocatalytic
degradation process. SPME holder and fiber-coating divinylben-
zene-carboxen-polydimethylsiloxane (DVB-CAR-PDMS 50/30
mm) were supplied from Supelco (Bellefonte, PA). GC/MS analy-
ses were run on a Perkin-Elmer AutoSystem-XL gas chromato-
graph interfaced to a TurboMass selective mass detector. The
analysis of ionic byproducts was performed by ion chromatogra-
phy (IC) with a Dionex ICS-90 instrument.
a mass range of m/z 40–350 was used to confirm the analytes. The
autotuning software performed the reagent gas flow adjustment
and the lens and electronic tuning.
The ionic byproducts from disulfoton degradation were an-
alyzed using a Dionex ICS-90 ion chromatograph. The column
3
-
4
was an IonPacÒ AS4A-SC (4 ´ 250 mm) for phosphate (PO )
2
-
and sulfate (SO
the injection volume was 100 mL for the filtered reaction samples.
The eluent consisted of a mixture of 3.5 mM Na CO and 1 mM
NaHCO for the anion analysis. For these operating conditions,
4 4
the retention time for PO and SO was 5.4 and 6.7 min, respec-
4
) analyses. The flow rate was 1.0 mL/min, and
2
3
Procedures and analysis
3
-
1
3-
2-
Disulfoton solution (5 mg L ) with the appropriate amount
of photocatalyst was mixed and used in photocatalytic experi-
ments. For reactions in different pH media, the initial pH of the
suspensions was adjusted by adding a few drops of either 0.1 N
tively. For quantitative studies, standard solutions and calibration
curves for each ion were prepared in the range from 1 to 10 mg
-
1
L . In order to avoid adsorbed processes of the anions on the sur-
face of the catalyst during the reaction progress, a desorption pro-
cedure was applied. This consisted of the addition of NaOH (1 N)
to the reaction mixture after irradiation until it reached a pH of 12.
3
NaOH or 0.1 N HNO solutions. Prior to irradiation, the disper-
sion was magnetically stirred in the dark for 30 min to ensure the
establishment of the adsorption/desorption equilibrium. Irradia-
tion was carried out using two UV-365 nm lamps (15 watt). After
each irradiation cycle, the amount of disulfoton was thus deter-
RESULTS AND DISCUSSION
mined by SPME-GC/MS. The aqueous TiO
2
dispersion was sam-
particles. The
Photodegradation kinetics
pled (5 mL) and centrifuged to separate the TiO
2
Regression analysis based on the pseudo-first-order
reaction kinetics for the degradation of disulfoton in photo-
catalytic process was conducted and the results are shown
in Fig. 2. Degradation rate constants (kapp) were determined
clear solution was then transferred into 4 mL sample vial. The
SPME fiber was directly immersed into the sample solution to ex-
tract disulfoton and its intermediates for 30 min at room tempera-
ture, with magnetic stirring at 550 ± 10 rpm on the Corning stir-
rer/plate (Corning, USA). Finally, the compounds were thermally
desorbed from the fiber to the GC injector for 40 min. Separation
was carried out in a DB-5 capillary column (5% diphenyl/95%
dimethyl-siloxane), 60 m, 0.25-mm i.d., and 1.0-mm thick film. A
split-splitless injector was used under the conditions of injector
temperature 250 °C, a split ratio 1:10, the helium carrier gas flow
0
from the slope of ln C /C = kappt plots. About 99% of the
disulfoton was degraded after UV irradiation for 90 min.
Table 1 lists the calculated values of kapp and the linear re-
gression coefficients for the pseudo-first-order kinetics of
the photodegradation of the studied compound. It seemed
that the photocatalytic degradation of disulfoton followed
pseudo-first-order kinetics satisfactorily. Similar results
were reported by others for the photocatalytic degradation
of organophosphorus pesticides (Table 2). The disulfoton
reaction rate is similar to other organophosporus com-
0
.6 mL/min, the oven temperature program 1.0 min at 60 °C then
-
being increased at 8 °C min until reaching 240 °C, which was
1
kept for 16.5 min. The total run time was 40 min.
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 obtained at 70 eV of electron energy
and monitored from 10 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 ioniza-
tion mass spectrometry was operated in the positive ionization
mode, isobutane was used as reagent gas at an apparent pressure
Fig. 2. Photodegradation kinetics of disulfoton in the
2
presence of TiO particles. Experimental con-
-
dition: disulfoton concentration 5 mg L , TiO
1
2
-
of 4.4 × 10 Torr in the ionization source. The full scan mode with
4
-1
concentration 0.5 g L , pH 6.
382
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
2
TiO /UV Photocatalytic Degradation of Disulfoton
Table 1. Photodegradation kinetics parameters (rate constants
2
and linear regression coefficients R ) for disulfoton in
generally ascribed to the surface charge of the photocata-
lyst and its relation to the ionic form of the organic com-
pound (anionic or cationic). Electrostatic attraction or re-
pulsion between the photocatalyst’s surface and the or-
ganic molecule took place, and these events consequently
enhanced or inhibited, respectively, the photodegradation
-
the presence of TiO (0.5 g L )
1
2
-
1
2
R
pH
Kapp (min )
4
6
9
0.0309
0.0674
0.0799
0.9963
0.9917
0.9939
2
7,28
rate.
Due to the non-ionic property of disulfoton, the
observed increase of the degradation rate with an increase
in pH could be ascribed to the generation of hydroxyl radi-
cals through photo-oxidation by photocatalyst’s surface
2
pounds treated by TiO photocatalysis.
pH effect
-
holes due to the presence of a large quantity of OH ions.
·
Consequently, a higher concentration of OH species was
Control experiments were performed to guarantee
that the results obtained during the photocatalytic tests
were consistent and not due to hydrolysis. The results indi-
cated that only slight hydrolysis was detected after 3 h in
the solution kept in the dark at pH 6 (see Fig. 3). Photo-
formed, and the overall rate was enhanced, as indicated in
equation (3).
2
Effect of TiO dosage
For economic removal of pesticide effluent from waste-
water, it is necessary to find the optimum amount of cata-
2
catalytic degradation of disulfoton by TiO obtained in this
study was much faster than it by hydrolysis. The contribu-
tion of hydrolysis is negligible.
lyst for efficient degradation. Hence, the effect of TiO
age on the photodegradation rate of disulfoton was investi-
gated using TiO at different concentrations, keeping all
other parameters identical. The optimum amount expressed
2
dos-
The real effluent stuff of pesticide could be dis-
2
7
2
charged at a different pH, and therefore the effect of pH
on the rate of photocatalytic degradation of disulfoton was
studied. The results indicated that the degradation rate in-
creased with an increase in pH in the studied range of 4–11
-
1
in gcat per L of solution was found equal to 0.5 g L , and the
result is shown in Fig. 4. The degradation rate was found to
2
increase with increasing TiO concentration up to a certain
(
Fig. 3). The effect of pH on a photocatalytic reaction is
Fig. 3. pH effect on the photodegradation rate of di-
-
2
Fig. 4. Effect of TiO dosage on the photodegradation
1
sulfoton with concentration of TiO
2
at 0.5 g L
rate of disulfoton. Experimental condition: di-
-1
sulfoton concentration 5 mg L , pH 6.
-
1
and disulfoton at 5 mg L .
Table 2. Photodegradation kinetics parameters (rate constants and linear regression
2
coefficients R ) for other organophosphorus pesticides in the presence of TiO2
TiO dosage
2
pH
K
app
-1
(min )
2
R
OPs concentration
Ref
-
g L )
1
(
value
-
Parathion (5 mg L )
1
1.0
0.5
0.5
0.1
0.1
0.5
6.5
6
0.167
0.998
0.996
0.985
0.981
0.976
0.9917
18
19
-
Phorate (12 mg L )
1
0.1020
0.0814
0.103
-
Terbufos (5 mg L )
1
6
26
-
Dichlorvos (20 mg L )
1
-
40
-
Dimethoate (20 mg L )
1
-
0.048
40
-
1
Disulfoton (5 mg L )
6
0.0674
this study
J. Chin. Chem. Soc. 2013, 60, 380-390
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383

Article
Chen et al.
-
water. The effects of Cl and NO
-
value and then obviously decrease at high TiO
2
dosages.
3
ions on the degradation
The activity of photocatalyst could be attributed to the
availability of active sites on catalyst surface when the light
penetration being in the suspension. The active sites of cat-
alyst surface increased with the catalyst concentration in
rate of disulfoton were examined individually by adding
NaCl and NaNO to the system until the resultant solution
contained 0.05 M of Cl and NO
tion began. The degradation rate was inhibited in the pres-
3
-
-
3
ions before the irradia-
-
-
the suspension. However, when TiO
intensity of incident UV light was attenuated because of the
shielded of incident light by TiO particle and increased
2
was overdosed, the
3
ence of Cl and NO ions as shown in Fig. 6. To investigate
the effect of variation of the concentration of anion on the
degradation rate of disulfoton, nitrate was also added at a
different concentration (0.1 M) in the initial solution. It can
be seen that the photodegradation efficiency of disulfoton
decreases further with increasing the concentration of an-
ion. This might be because of the following two reasons. (i)
The adsorbed anions might compete with disulfoton for the
2
turbidity of the suspension reducing the light transmission.
Furthermore, the increase of light scattering which coun-
teracted the positive effect came from the dosage increment
and therefore reduced the overall performance.
The solution turbidity measured using HACH 2100P
Turbidimeter was ca. 425, 1320, 2385 and 4908 NTU for
2
active sites on the TiO surface or deactivate the photocata-
the solution with different concentrations of TiO
2
(0.1,
lyst and, subsequently, slow down the degradation rate of
+
disulfoton molecule. (ii) The reaction of positive holes (h )
0
.25, 0.5 and 1.0 g/L), respectively. The excess of catalyst
·
and hydroxyl radical ( OH) with anions that behaved as h
+
concentration might increase the turbidity of the solutions
leading to a shielding effect on the penetration of UV light
and inducing an unfavorable condition for the photocata-
lytic degradation of disulfoton. The kinetics of disulfoton
disappearance in the presence of different concentrations
·
and OH scavengers resulted in prolonged disulfoton re-
moval. Amajor drawback resulting from the high reactivity
·
and non-selectivity of OH was that it also reacted with
non-target substances present in the background water ma-
trix, i.e. inorganic anions present in water. This result in a
2
of TiO (0.1, 0.25, 0.5 and 1.0 g/L) followed an apparent
first-order degradation curve which is consistent to the
Langmuir–Hinshelwood model. The kapp values in the pres-
ence of 0.1, 0.25, 0.5 and 1.0 g/L of TiO
2
were 0.016,
.0189, 0.0674 and 0.0285 min , respectively, confirming
the positive influence of the increased number of TiO ac-
tive sites on the process kinetics. However, this beneficial
effect tends to level off and then to decrease for TiO con-
centrations higher than 1.0 g/L depending on the reactor,
-
1
0
2
2
2
9
due to relevant scattering of the incident light. Similar ob-
3
0,31
servations were reported by other authors as well.
All
-
Fig. 5. Effect of light intensity on the photocatalytic
degradation of disulfoton in water. Experimen-
tal condition: disulfoton concentration 5 mg
1
2
further studies were made using a TiO dosage of 0.5 g L .
Effect of light intensity
-
1
-1
2
L , TiO concentration 0.5 g L , pH 6.
UV light intensity is an important factor in the pro-
cess of photocatalytic degradation. The effect of light in-
tensity was studied, keeping the wavelength (365 nm) unal-
tered. The effect of the UV light intensity on the degrada-
tion of disulfoton is illustrated in Fig. 5. The results indi-
cated that the degradation rate increases with increase in
the light intensity. It might be because a higher UV inten-
2
sity provided more photons to activate TiO thus generat-
·
ing more OH for disulfoton degradation.
Effects of anions
Fig. 6. Effect of anions on the photocatalytic degrada-
tion of disulfoton in water. Experimental condi-
The study of the effects of anions on the photocata-
lytic degradation of disulfoton is important because anions
are rather common in natural water and industrial waste-
-
1
tion: disulfoton concentration 5 mg L , TiO
2
-
concentration 0.5 g L , pH 6.
1
384
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
TiO
2
/UV Photocatalytic Degradation of Disulfoton
·
higher OH demanded to accomplish the desired degree of
fur and phosphorus atoms were oxidized to sulfate and
phosphate ions, respectively, after they were released from
the disulfoton molecule. The stoichiometric concentration
3
2-34
degradation.
presence of NO
The decrease in rate was higher in the
-
than that of Cl ions. It might be because
-
3
2
-
3-
nitrate had a larger anionic size than chloride did, which
5
could block the active sites more effectively.
of SO
4
and PO
4
reached about 95% of the initial organic
3
sulfur and 99% of the initial organic phosphorus in a irradi-
ation time of 6 hours, respectively. The ion chromato-
graphic data confirmed that the complete degradation of
disulfoton molecule by photocatalysis led to the conver-
sion of the heteroatoms into inorganic anions.
Effects of cations
The degradation of disulfoton was studied in the pres-
ence of 0.05 M solutions of Fe(NO and Cu(NO . It was
found that the degradation of disulfoton in the presence of
these concentrations of Fe(NO and Cu(NO decreased.
That is, the degradations of disulfoton in the presence of
Fe(NO and Cu(NO were 87.6% and 57.2%, respec-
3
)
3
3 2
)
3
)
3
3
)
2
Separation and identification of the intermediates
After searching the cited literature and to the best of
our knowledge, we have come to the conclusion that no
prior study has investigated the photocatalytic transforma-
tion of disulfoton and very little is known about the use of
3
)
3
3 2
)
tively, as against 99% in their absence, as shown in Fig. 7.
Formation of ionic by-products
Since disulfoton is a phosphorodithioate pesticide
-
2
TiO in the treatment of disulfoton in aqueous solution. In
2
containing sulfur and phosphorus atoms, the sulfate (SO
3
4
)
this paper, the photocatalytic degradation mechanism of
disulfoton is reported for the first time. Since disulfoton is
-
and phosphate (PO
4
) are potential ionic degradation by-
-
1
products. Measurements of phosphate and sulfate ions
gave valuable information about how this process was
achieved. Therefore, the ionic by-products produced dur-
ing photocatalysis were measured. Preliminary results
sparingly soluble in water (water solubility: 12.0 mg L ), it
is therefore a challenge to study its photodegradation in di-
lute aqueous solutions in the absence of any organic sol-
vents, and to identify its degradation products. A relatively
low intensity UV-365 lamp (15W) was used in this study
for the identification of organic intermediates. This al-
lowed us to obtain slower degradation rates and provide fa-
vorable conditions for the determination of intermediates.
3
-
showed that PO
4
2
was retained on the surface of TiO in
the degradative process and this ion was not easily detected
in the irradiated solutions. The adsorption of phosphate
ions onto the surface of the photocatalyst has already been
-
1
3
6-38
Additionally, the initial disulfoton concentration (5 mg L )
was selected high enough to facilitate the identification of
intermediate products.
reported.
Therefore, a procedure to desorb these com-
pounds was applied (see Section 2 for the correct evalua-
tion of the formation of these ions).
In photocatalytic degradation process, the concentra-
tion of reaction intermediates was low and thus the inter-
mediates had to be preconcentrated before the application
of an appropriate analytical procedure. Prior to GC–MS
analysis, the samples were preconcentrated using a solid-
As shown in Fig. 8, photocatalytic treatment of the
disulfoton resulted in the destruction of the parent mole-
cule as evidenced by evolution of inorganic anions. Ion
chromatography analysis showed the increase of sulfate
and phosphate concentration in the reaction mixture as the
photocatalysis progressed. The increase indicated that sul-
Fig. 8. Evolution of sulfate and phosphate anions orig-
inating from disulfoton photocatalytic degrada-
Fig. 7. Effect of cations on the photocatalytic degrada-
tion of disulfoton in water. Experimental condi-
tion. Experimental condition: disulfoton con-
-1
-
tion: disulfoton concentration 5 mg L , TiO
1
centration 5 mg L , TiO
-1
L , pH 6.
2
concentration 0.5 g
2
-
concentration 0.5 g L , pH 6.
1
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385

Article
Chen et al.
phase microextraction method, a selective tool for the trace
analysis of organic compound in water samples. Therefore,
the intermediate products generated in the disulfoton solu-
tion during the photocatalytic degradation process were an-
alyzed by SPME-GC/MS and identified (a) by using the
identification program of NIST library with a fit value
higher than 70%, (b) by comparing the spectra with previ-
ously reported spectra and (c) by the interpretation of the
mass spectra and the investigation of their characteristic
ions.
Fig. 9 displays the chromatogram of the reacted solu-
tion after irradiation for 20 min. At least eight components
were identified at retention times less than 40 min. One of
the peaks was the initial disulfoton (peak I); the other seven
(
new) peaks were the intermediates formed. The disulfoton
and its related intermediates were denoted as species I-
VIII. Except for the initial disulfoton, the other peaks in-
creased at first and subsequently decreased, indicating the
formation and subsequent transformation of the intermedi-
ates. Table 3 summarizes the identified intermediates of
disulfoton along with their retention time and the charac-
teristic ions of the mass spectra. Some other minor peaks
were presented, but mass fragment information did not al-
low elucidation of their structures.
the NIST library data were more than 72%. Two intermedi-
ates (II and IV) not included in the library were identified
by the molecular ion and the interpretation of the mass
spectra.
The molecular mass of these compounds was deter-
mined using positive ion chemical ionization (CI) mass
Fig. 1S shows the EI mass spectra of intermediates
spectrometry through the abundant protonated molecule
+
M+H] , and the structural data were obtained from the
formed during the photocatalytic degradation of disulfoton
-1
(0.5 g L ) particles. The molecular
[
in the presence of TiO
2
electron impact (EI) fragmentation patterns. Five interme-
diates (III, V-VIII) were identified by the molecular ion and
mass fragment ions and also through the comparisons with
NIST library data. The similarities of these compounds to
mass of compound II was determined from the EI mass
spectrum to be m/z = 214, which was confirmed by the ob-
+
servation of an [M+H] ion of 215 using positive ion CI
mass spectrometry (Fig. 2S). The EI mass spectrum
showed the characteristic ions at m/z = 153, m/z =125 and
+
P(S)] ,
m/z = 97 corresponding to the groups, [(C
+
2
H
5
O)
2
+
[(C
2 5 2
H O)P(S)(OH)] and [(HO) P(S)] , respectively. Based
on these data, compound II was identified as dithiophos-
2
4
phoric acid-O,S,O-triethylester. Zamy et al. investigated
the analysis of disulfoton aqueous solutions after irradia-
tion (l > 285 nm) and showed the formation of dithiophos-
phoric acid-O,S,O-triethylester.
The molecular mass of compound III was determined
from the CI mass spectrum to be m/z = 150 by the observa-
+
tion of an [M+H] ion of 151 (Fig. 3S). A search of the EI
mass spectra library selected 1, 2-bis(ethylthio)ethane as a
good match (98%) for compound III. It exhibited the char-
acteristic ions at m/z = 89, m/z = 75, and m/z = 61 corre-
Fig. 9. GC/MS chromatogram obtained for disulfoton
solution after 20 min of irradiation with UV
2
light in the presence of TiO .
386
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J. Chin. Chem. Soc. 2013, 60, 380-390

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
2
TiO /UV Photocatalytic Degradation of Disulfoton
+
sponding to the groups [CH
2
CH
S] , respectively. This compound
was also identified, in a previous study, as a by-product of
2
SCH
2
CH
3
] , [CH
2
=
Scheme I Proposed photodegradation pathway of
+
+
disulfoton under UV irradiation in aqueous
2 3 3 2
SCH CH ] and [CH CH
2
TiO dispersions
3
9
disulfoton hydrolysis.
The molecular mass of compound IV was determined
from the CI mass spectrum to be m/z = 122 by the observa-
+
tion of an [M+H] ion of 123 (Fig. 4S). The EI mass spec-
trum showed the characteristic ions at m/z = 75, m/z = 61
2
and m/z = 47 corresponding to the groups, [CH =
+
+
+
2 3 2 2 2
SCH CH ] , [HSCH CH ] and [HS=CH ] , respectively.
Based on these data, compound IV was identified as 2-(eth-
ylthio)ethanethiol.
Compound V was identified as 1,2-diethyldisulfane
by a library search with a fit value of 93%. This intermedi-
ate was confirmed by the comparison of MS spectrum and
GC retention time with an authentic compound obtained
from Aldrich Chemical Co. Neat 1,2-diethyldisulfane, dis-
solved in methanol, yielded the same GC retention time and
mass spectrum as that of the suspected 1,2-diethyldisulfane
found in the irradiated samples of disulfoton photocatalytic
experiment. 1,2-Diethyldisulfane is an interesting interme-
diate because it is most likely to form from the dimerization
of ethanethiol (a molecular segment of disulfoton’s side
chain).
The mass spectrum of compound VI exhibited a peak
+
at m/z = 88 which corresponded to the molecular ion [M] .
A search of the mass spectra library revealed ethyl(vi-
nyl)sulfane to be a good match (90%) for compound VI,
the retention time of which on GC was 9.31 min.
1
5,40-42
organophosphorus pesticides.
Compound VII was identified as ethanethiol by a li-
brary search with a fit value of 72%. The mass spectrum of
this compound showed a peak at m/z = 62 which corre-
In path A, the interfacial transfer of a single electron
from the sulfur atom, which was located far from the phos-
phorus atom, led to the formation of disulfoton cation radi-
+
sponded to the molecular ion [M] , and the characteristic
+
=SH] .
cal 1a (Scheme I, path A). The scission of the C-S bond in
·
the cation radical 1a led to the formation of SC
ion at m/z = 47 corresponding to the group [CH
2
2
H
5
radicals
Compound VIII was identified as ethanol with a fit
value of 92% found by searching the mass spectra library.
This compound was further identified by matching its re-
tention time and mass spectrum with that of an authentic
standard. They exhibited the exact retention time and simi-
lar mass spectra.
which were the precursor of ethanethiol (compound VII).
The H atoms necessary for S-H bond formation were pro-
posed to be originated from the proton reduction by photo-
+
generated electrons H + e ® H as already observed in the
-
·
degradation of the insecticide fenitrothion by Kerzhentsev
3
et al.. The recombination of SC H radicals could form
2 5
4
·
Initial photooxidation pathway
dimers such as 1,2-diethyldisulfane (compound V). The
dimerization of thiol radical was also reported in the case of
the photocatalytic degradation of thiocarbamate pesti-
Scheme I shows the proposed mechanism for the gen-
eration of the primary detected intermediates. Oxidation of
the disulfoton molecule to form the disulfoton cation radi-
cal took place when positive holes attacked it, initiating a
series of reactions. The formation of cation radicals were
also observed in the photocatalytic degradation of other
1
cides. Similarly, the scission of the C-S bond in the cation
7
radical 1a led to the formation of ethanylium cation
+
] . This carbocation was extremely unstable and
[
CH
3
CH
2
J. Chin. Chem. Soc. 2013, 60, 380-390
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387

Article
Chen et al.
underwent rapid hydrolysis to yield ethanol (compound
VIII).
actinometer studies resulted in a reaction quantum yield for
disulfoton of 0.267.
The interfacial transfer of a single electron from the
sulfur atom, located near the phosphorus atom, led to the
formation of disulfoton cation radical 1b (Scheme I, path
Electrical energy efficiency
2
UV/TiO photodegradation of aqueous organic pol-
lutant is an electric-energy intensive process, and electric
energy can represent a major fraction of the operating
costs, simple figure-of-merit based on electric energy con-
sumption can be very useful. In the case of low pollutant
concentrations, the appropriate figure-of-merit was the
electrical energy per order (EEO), defined as the number of
kilowatt hours of electrical energy required to reduce the
concentration of a pollutant by one order of magnitude
B). The scission of the C-S bond in this cation radical led to
·
P(S)S radicals and 2-(ethyl-
the formation of (C
2
H
5
O)
2
thio)ethan-1-ylium ions. The carbocation was extremely
unstable and underwent rapid hydrolysis and dehydrogena-
tion. Hydrolysis of this carbocation yielded 2-(ethylthio)-
ethanol while the dehydrogenation led to the formation of
ethyl(vinyl)sulfane (compound VI). Additionally, the scis-
sion of the P-S bond in the disulfoton cation radical 1b led
to the formation of 2-(ethylthio)ethanethiol (compound
IV).
3
3
(90%) in 1 m of contaminated water. The EEO (kWh/(m
order)) could be calculated from the following equa-
4
tions:
8
In path C, dithiophosphoric acid-O,S,O-triethylester
P´ t ´1000
(
compound II) and 2-(ethylthio)ethanethiolate were formed
·
by electron transfer followed by SC H radical addition-
2 5
E
E
EO
EO
=
=
(8)
(9)
V ´ 60´ log(C / C )
0
3
8.4´ P
4
0
elemination reaction. 2-(ethylthio)ethanethiolate was the
precursor of 1,2-bis(ethylthio)ethane (compound III),
which acted as the nucleophile and attacked disulfoton at
a-carbon of ethoxy group. The thiophosphate esters might
react like alkyl halides by nucleophilic displacement at the
V ´ kapp
where P is the power input (kW) from the wall to drive the
UV lamp, t is the irradiation time (min), V is the volume (L)
of the water in the reactor, C
o
and C are the initial and final
4
4
carbon bound to the oxygen of an alcohol moiety.
pollutant concentrations, and kapp is the pseudo-first-order
-
1
Relative photonic efficiency and quantum yield
Quantum yield is an important parameter in photo-
chemistry. Its precise measurement is often difficult in a
heterogeneous dispersion owing to non-negligible scatter-
rate constant (min ) for the decay of the pollutant concen-
tration. The degradation of disulfoton was found to follow
pseudo-first-order kinetics, and hence the figure-of-merit
electrical energy per order (EEO) was appropriate for esti-
mating the electrical energy efficiency. This study showed
4
5
ing of the light irradiance impinging on the reactor. Con-
sidering the difficulty of quantum yield measurements,
phenol was selected as a standard secondary actinometer to
assess the relative photonic efficiency (xrel) of the photo-
degradation of disulfoton against phenol according to the
that disulfoton could be treated easily and effectively with
3
/UV process with EEO value of 85 kWh/(m or-
the TiO
der).
2
4
6
protocol reported by Serpone et al..
CONCLUSION
Disulfoton could be successfully degraded by TiO
2
rate of disappearance of substrate
rel = ¾¾¾¾¾¾¾¾¾¾¾¾¾¾
rate of disappearance of phenol
x
(7)
under UV irradiation for 90 min and ca. 99% of disulfoton
was degraded. The photodegradation rate of disulfoton was
found to increase along with increasing pH and to increase,
and then decrease along with increasing catalyst concentra-
In this study, the relative photonic efficiency xrel was
2
.43 determined under identical experimental conditions
-
(
initial concentration of reactant, 0.0182 mM [100 mL];
tion. In addition, the presence of inorganic ions such as Cl
-
-
loading, 0.5 g L [P25]; pH 6). The quantum yield
1
TiO
2
and NO
3
that were often presented in natural water sys-
could subsequently be determined from xrel, as Fdisulfoton
=
tems decreased the photocatalytic degradation rate of di-
sulfoton.
x
rel
F
phenol, where Fphenol was the quantum yield for the
photocatalyzed oxidative degradation of phenol using
Degussa P25 TiO as the standard catalyst material. The
quantum yield of photodegradation of phenol was deter-
Establishment of the reaction pathway was made
possible by a thorough analysis of the reaction mixture
identifying the main intermediate products generated.
Eight intermediates were identified and characterized
2
4
7
mined (Fphenol = 0.11 ± 0.01) by Serpone et al.. Therefore,
388
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J. Chin. Chem. Soc. 2013, 60, 380-390

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
2
TiO /UV Photocatalytic Degradation of Disulfoton
through a mass spectra analysis. The reaction pathway
could proceed via the formation of disulfoton cation radi-
cals by attacking positive holes to initiate the series of re-
actions. The interfacial transfer of a single electron from
the sulfur atom initiated C-S bond cleavage and led to the
formation of the detected intermediates. Another reaction
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