Reduction of dichlorvos and omethoate residues by O2 plasma treatment

Article abstract of DOI:10.1021/jf900995d

A practical, inexpensive, and green chemical process is greatly needed for degrading pesticides in food and environmental water. In this work, the impact of O₂ plasma treatment on reduction of dichlorvos (DDVP) and omethoate in maize was determ

Full text of DOI:10.1021/jf900995d

6238 J. Agric. Food Chem. 2009, 57, 6238–6245
DOI:10.1021/jf900995d
Reduction of Dichlorvos and Omethoate Residues by O2
Plasma Treatment
,
†,‡
,§
HEN,* HUI
‡
§
‡
Y
ANHONG
†
B
AI,*
J
IERONG
C
M
U
, CHUNHONG
Z
HANG
,
AND
B
AOPING
L
I
‡
Department of Chemistry, School of Science and School of Life Science and Technology and
Department of Environmental Science and Engineering, School of Energy and Power Engineering, Xi’an
Jiaotong University, Xi’an 710049, China
§
A practical, inexpensive, and green chemical process is greatly needed for degrading pesticides in
food and environmental water. In this work, the impact of O2 plasma treatment on reduction of
dichlorvos (DDVP) and omethoate in maize was determined by gas chromatography (GC). The main
plasma-induced degradation mechanisms were investigated through identification of intermedi-
ates or products during O
2
plasma treatment for DDVP and omethoate on solid surfaces by gas
plasma
chromatography/mass spectrometry (GC/MS). The results clearly demonstrate that O
2
treatment is significantly effective in the degradation of original DDVP and omethoate, and the
degradation efficiency mainly depends upon related operating parameters and chemical structures
of pesticides. Moreover, GC/MS analyses show that DDVP and omethoate molecules are degraded
into less-toxic compounds, and the plasma degradation mechanisms for pesticides can be
dominated by a free-radical reaction. It is concluded that O2 plasma has the potential to reduce
pesticide residues in agricultural products.
KEYWORDS: Reduction; dichlorvos residues; omethoate residues; degradation mechanisms; O
treatment
2
plasma
INTRODUCTION
reduction methods for food and environmental water polluted
with OP pesticides.
Organophosphorus (OP) pesticides, a group of cholinesterase-
inhibiting insecticides, have been used extensively as an alter-
native to organochlorine compounds in both agricultural and
residential environments, and their use is expected to increase
at least in the near future because of their broad spectrum of
insecticidal activity and effectiveness. However, their extensive
and indiscriminate use on agricultural production, postharvest,
storage, and transportation may result in the presence of such
pesticides ubiquitous in the diet (as residues on treated raw
agricultural commodities that can be ultimately found in the
human diet) and drinking water (as residues found in environ-
mental water, such as groundwater, that can be used for drinking
water sources) (1-4). Overexposure to OP pesticides had the
potential to pose a health risk to adults, particularly in children,
along the food-chain transfer. The toxic effects of OP pesticides
including alterations in metabolism, reproduction, mutagenicity,
carcinogenesis, neurotoxicity, and endocrine-disrupting effects
have become a serious environmental concern as well as a public
health priority (5-7). Furthermore, the fact cannot disregarded
that “cocktail effects” of pesticides can lead to higher adverse
effects on human health through the combined effects of multisite
use of several pesticides (8, 9). For these reasons, there is urgent
demand in research and development of the efficient and effective
The chemical oxidation processes (such as photocatalysis,
ozonation, and hydrolysis), physical method (such as ultrasonic
irradiation and ionizing radiation), and biological method appear
to be the most popular degradation methods for OP pesti-
cides (10-15), but their efficiency is somewhat limited or un-
desired because toxic compounds are sometimes formed as well.
In addition, the lack of chemistry control and the failure of
expeditious and complete decomposition and detoxification of
trace pesticides have been a major hurdle to overcome. To resolve
these challenges, nonthermal plasmas (NTPs) as an innovative
tool have been proven to be effective for the degradation of OP
molecules in some literature (16-21). The main characteristic of
5
NTPs is its high electron temperatures (10 K); i.e., the electrons
are preferentially excited with the energies of 1-10 eV, whereas
the bulk gas (contained the more massive reactive species)
temperature remains at ambient temperature. Therefore, a sig-
nificant energy savings can be realized. In general, most covalent
bond energy equals 3-6 eV. As such, this energy-savings poten-
tial is a primary reason why most organic molecules can easily be
destroyed by NTPs, in which high-energy electrons collide with
the molecules of the background gas or pollutants, and secondary
electrons and highly reactive species were produced by mecha-
nisms of ionization, excitation, and dissociation (22). As a result,
NTPs are good sources of highly reactive species and plasma
electrons that are capable of reacting with and decomposing
chemical pollutants to yield safer products. Similarly, under
selective experimental conditions when OP pesticide molecules
*
To whom correspondence should be addressed. Telephone: +86-
2
9-82655169 (Y.B.); +86-29-82664818 (J.C.). Fax: +86-29-82655489
(
(
Y.B.); +86-29-82664818 (J.C.). E-mail: yhbai7@mail.xjtu.edu.cn
Y.B.); jrchen@mail.xjtu.edu.cn (J.C.).
pubs.acs.org/JAFC
Published on Web 06/17/2009
© 2009 American Chemical Society

Article
J. Agric. Food Chem., Vol. 57, No. 14, 2009 6239
Figure 1. Chemical structures of the (a) general structure of OP pesticide,
(b) DDVP (phosphate ester pesticide), and (c) omethoate (phosphorothioic
acid ester pesticide).
are exposed to O plasma, high-energy electrons generated from
2
plasma provide sufficient energy to dissociate the molecules of the
feed gas or OP pesticide, produce free radicals (e.g., O and OH)
3
3
Figure 2. Schematic diagram of the plasma reactor: (1) gas bottle,
and excited-species-initiated chemical reactions, which are nor-
mally not able to occur at low temperatures, and then promote
various desired conversions of OP pesticides to liberate unharm-
ful or less hazardous compounds. In addition, UV and irradiate
light derived from NTPs may also enhance the degradation
efficiency of OP pesticides. When the OP pesticide absorbs UV
light, it can be degraded through bond scissions or oxidations that
take place in its electronically excited states. Therefore, a combi-
nation of physical and chemical unit processes of NTPs is very
attractive and actually employed to ensure the reduction of OP
pesticide residues. However, no literature was found in examining
the degradation efficiency of OP pesticides in agricultural pro-
(
(
2) valve, (3) mass flow meter, (4) inductance coil, (5) reaction chamber,
6) sample, (7) vacuum gauge, (8) electromagnetism valve, (9) vacuum
pump, (10) grounding protection, (11) rf generator, and (12) matching
system.
MATERIALS AND METHODS
Reagents and Materials. DDVP and omethoate standard (99.0-
9.5% purity) were provided by the Institute of Environmental Protection
9
and Monitoring, Ministry of Agriculture of China. The stock solutions of
DDVP and omethoate were prepared in acetone at a concentration of
100 μg/mL. The solutions, stored in the dark and in the refrigerator at 4 °C,
were used no more than 12 h after they were made to ensure that no
ducts by O plasma.
2
2
degradation occurred prior to O plasma treatment. DDVP purchased
OP pesticides are still widely used in China and the rest of the
world, where the conditions of use have raised concerns.
Around 10 000 tons of DDVP and 8000 tons of omethoate are
applied annually to agricultural crops in China. DDVP is the
common name of an O-2,2-dichlorovinyl-O,O-dimethyl phos-
phate, with the molecule formula of C H Cl O P, molecule
from the Hubei Shalongda Chemical Co. with a purity of 80% and
omethoate purchased from Hebei Chemical Co. with a purity of 40% were
employed for simulation to maize. All organic solvents were purchased
from Xi’an Chemical Co. and redistilled before using. Maize was pur-
chased from a local market.
4
7
2
4
Plasma Generator. Figure 2 shows the schematic diagram of the
plasma treatment system employed in this research. It consists of four
parts: gas inlet, reaction chamber, vacuum pump, and power supply
(SY-500W, 13.56 MHz) and matching network (SP-II matcher), which
are made by the Science Academy of China. The reaction chamber is a
cylindrical Pyrex glass tube (1000 mm in length and 45 mm in diameter),
where inductively coupled radio-frequency (rf) discharge is initiated.
Pesticide Degradation Procedures. In the plasma degradation of
the pesticide test series, the central design was applied to determine the
degradation efficiency of pesticide residues and to evaluate the effects
of plasma factors on the reduction of pesticides in maize. First, maize
purchased from market was randomly selected and fortified with DDVP
and omethoate aqueous solution at normal farm concentration by spray.
Then, the maize samples were dried at room temperature for 24 h and
positioned onto a glass plate (100 ꢀ 35 mm) at the distance of 20 cm
(discharge zone), 45 cm (afterglow zone), and 70 cm (remote zone) from
the center of the induction coil in the Pyrex glass tube of the reactor.
-1
mass of M=220.98 g mol , and CAS registry number 62-73-7.
In animal studies, the acute oral LD50 in rats is between 56 and
-
1
8
0 mg/kg and its acceptable daily intake (ADI) is 0.004 mg kg
-
1
day . Recently, it has been reported that exposure to DDVP
affects glucose metabolism and produces hyperglycemia (7).
Omethoate is a phosphorothioic acid ester, with the Interna-
tional Union of Pure and Applied Chemistry (IUPAC) name
of O,O-dimethyl-S-methyl-carbamoyl-methylthiophosphate,
the molecule formula of C H NO PS, molecule mass of M=
5
12
4
-1
2
13.2 g mol , and CAS registry number 1113-02-6. The oral
LD50 of omethoate in rats is approximately 50 mg/kg of body
-
1
weight, and its acceptable daily intake (ADI) is 0.0003 mg kg
-
1
day . As a metabolite of dimethoate (the use of dimethoate on
crops can lead to residues of omethoate in treated produce),
omethoate has clear mutagenic potential and allergic disor-
ders (9). The chemical structures of DDVP and omethoate are
listed in Figure 1.
Afterward, the maize samples were exposed to O
treatment times of 30, 60, 90, and 120 s at discharge power levels of 30, 60,
0, and 120 W with different O flux. Each maize sample was withdrawn
2
oxygen plasma for
9
2
The objective of this study was to investigate whether DDVP
and omethoate could be degraded by O2 plasma treatment.
DDVP and omethoate were employed as the target pesticides
because of their extensive use in China. This work focused on
examining the degradation effectiveness ofDDVP and omethoate
from the plasma reaction chamber immediately after plasma treatment.
The extraction of pesticide residues were carried out according to a
standard method of grains established by the Ministry of Agriculture of
China (GB/T19649-2005), with some modifications. DDVP and ometho-
ate residues in maize with different plasma treatment conditions were
detected and quantified using GC analyses.
in maize subject to the influence of various O plasma-operating
2
parameter factors by gas chromatography (GC) analysis. To
clarify the major plasma degradation reactions as well as degra-
dation mechanisms, O2 plasma was applied to perform the
degradation of DDVP and omethoate on solid substrates. Iden-
tification of the formation of intermediates and products after
plasma treatment was also analyzed by gas chromatography/
mass spectrometry (GC/MS). To the best of our knowledge, this
is the first time to study the reduction of OP pesticides by O2
plasma and reveal the chemical reaction and plasma degradation
mechanisms for OP pesticides. As such, this work has scientific
significance for crop protection and other practical applications
involving agricultural and food chemistry.
In the second series, to clarify the plasma degradation mechanisms for
pesticides, the DDVP and omethoate standard solutions at a concentra-
tion of 100 μg/mL were spin-coated onto glass slides (20 ꢀ 20 mm). The
glass slides were positioned on a glass plate (100ꢀ35 mm) at the distance of
2
0 cm (discharge zone), 45 cm (afterglow zone), and 70 cm (remote zone)
from the center of the induction coil in the Pyrex glass tube of the reactor.
Then, the samples were exposed to O oxygen plasma at the optimal
2
plasma degradation condition based on the previous investigation. After
plasma treatment, each sample was withdrawn from the plasma reaction
chamber immediately and dissolved in an ultrasonic bath of acetone for
5
min, with a metered volume at 2.0 mL for GC/MS analyses. Addition-
ally, the well-known radical scavenger, tert-butanol, was used to evaluate
the role of radicals derived from O2 plasma on the degradation of

6240 J. Agric. Food Chem., Vol. 57, No. 14, 2009
Bai et al.
Table 1. Mean Recovery and Relative Standard Deviation (RSD) for Pesti-
cides Fortified in the Maize Sample at Various Fortification Levels (n = 6)
at a 95% confidence level (p<0.05), the least significant difference (LSD)
test was performed among means.
pesticides
concentration (μg/mL)
recovery (%)
RSD (%)
RESULTS AND DISCUSSION
0
1
.1
.0
98
102
103
91
2.8
3.3
3.1
3.9
2.7
1.5
Degradation of DDVP Residues in Maize at Different O2
DDVP
1
0.0
.1
Plasma Treatment Conditions. To evaluate reduction efficiency
of DDVP residues in maize under different O plasma treatment
conditions, the samples were positioned at the distance of 20 cm
0
2
omethoate
1.0
0.0
92
1
89
(
dischargezone), 45cm (afterglowzone), and 70cm (remote zone)
from the center of the induction coil in the Pyrex glass tube of the
pesticides. At the same conditions, 100 μL of tert-butanol was coated onto
the glass slides (20ꢀ20 mm), which was parallel to the corresponding glass
slides, with pesticide samples in the same glass plate. They were then
reactor. Then, they were exposed to the O plasma for treatment
2
times of 30, 60, 90, and 120 s at discharge power levels of 30, 60,
9
0, and 120 W with difference O flux. The reduction efficiency of
exposed to O
treatment alone was compared to tert-butanol+plasma treatment to
estimate the contribution of the radicals generated in O plasma. Each
2
oxygen plasma. The degradation efficiency of plasma
2
DDVP by the O plasma was investigated from the remaining
2
fraction as a function of the treatment time, discharge power, and
2
sample was manipulated 3 times to ensure the reproducibility. Identifica-
tion of intermediates and products as well as the pesticide residues were
detected by GC/MS and GC, respectively.
O flux, respectively.
2
Effects of the Treatment Time on the Reduction of DDVP in
Maize. The diagram of the remaining fraction of DDVP residues
in maize at the distance of 20 cm (discharge zone), 45 cm
Chromatographic Analyses. DDVP and omethoate residues were
detected and quantified using a shimadzu-2010 gas chromatograph (GC)
equipped with a flame photometric detector (FPD). GC conditions: RTX-
(
afterglow zone), and 70 cm (remote zone) as a function of the
treatment time is plotted in Figure 3a. It can be noticed that the
remaining fraction of DDVP decreases sharply with an increasing
plasma treatment time up to 120 s, regardless of the sample
position. This indicates that the treatment time is the most
influential parameter and the longer treatment time can speed
up the reduction efficiency of pesticide. Almost 90% of the initial
amounts of DDVP is removed after 120 s in the discharge zone.
Furthermore, treatment in the discharge zone shows noticeable
effects compared to those in the afterglow and remote zones with
statistical significance (p < 0.05). It is anticipated that DDVP
residue levels would be reduced considerably by O2 plasma
treatment and the interaction between the pesticide molecule
and active species has almost completed within 120 s.
Effects of the Discharge Power on the Reduction of DDVP
in Maize. The remaining fraction of DDVP residues versus
discharge power at the above-mentioned distances is shown in
Figure 3b. It is interesting to mention the diminution of its
degradation as the discharge power varies from 30 to 60 W.
Further increasing the discharge power enhances degradation of
DDVP, and then it tends to remain stable beyond 90 W; i.e., the
degradation efficiency of 120 W is the greatest in the discharge
zone, whereas the degradation efficiencyof 60W was the lowest in
the remote zone. Therefore, the degradation efficiency appears to
undulate with an increasing discharge power. ANOVA analysis
also indicates that removal efficiency of DDVP is significantly
2
25 high-performance capillary columns of 30 m ꢀ 0.32 mm ꢀ 0.25 μm.
The rate of the temperature rise is 70 °C (1 min) f 30 °C/min f 170 °C
(
3 min) f 4 °C/min f 225 °C f 25 °C/min f 250 °C (1 min). Injector
and detector temperatures were 250 and 280 °C, respectively. Nitrogen
was used as the GC carrier gas, and gas flow through the column was
8
0 mL/min. Sample solution (2.0 μL) was injected in a split ratio of 1:10,
and the quantification of pesticide was performed using an external
standard. Each sample was injected twice during GC analysis to monitor
the reproducibility. The degradation efficiency of OP pesticides was
described with the remaining fraction by calculating the ratio of the
pesticide concentration at a given time (C
plasma treatment. The remaining fraction η was calculated from the
t 0
) to the initial one (C ) prior to
C
t
equation as follow:η ¼ ꢀ 100%.
C
0
The intermediates or products formed during plasma degradation of
OP pesticides are determined by GC/MS. GC/MS analyses were per-
formed on an Agilent Technologies 6890 gas chromatograph, equipped
with a HP-5MS column (30 mꢀ0.25 mm inner diameterꢀ0.25 μm) coated
with 5% phenyl/95% methylpoly siloxane, coupled to a MSD 5973
selective mass detector (Agilent Technologies). The oven temperature
program is 60 °C (held for 2 min) ramped at 20 °C/min to 270 °C (held for
5
min); the injector port temperature was 150 °C. Helium was used as a
carrier gas at a flow of 1 mL/min. The temperatures of the ion source and
interface were set at 230 and 280 °C, respectively. A 1.0 μL sample was
injected for detection, and a split ratio of 1:10 was used. MS conditions:
electron energy, 70 eV; collect current, 300 μA; source temperature,
1
50 °C. Identification and confirmation of the pesticides were based on
their GC retention times and a comparison of their sample mass spectrum
to the characteristic ions in the standard mass spectra.
effective in discharge power by O plasma treatment conducted
2
Recovery Experiments and Detection Limits. Maize samples
were fortified at 0.1, 1.0, and 10.0 μg/mL, respectively, by adding DDVP
and omethoate pesticides. The recovery assays were replicated 6 times,
and the data are presented in Table 1. The limits of detection (LODs)
for DDVP and omethoate, by considering a signal-to-noise (S/N) ratio
of 3, were determined to be 0.005 and 0.011 mg/L, respectively. The
quantity of each OP was calculated with the use of the external standard
method.
at 30, 90, and 120 W than at 60 W with statistical significance (p<
0
1
.05) but not in the case of 30 and 90 W, 30 and 120 W, and 90 and
20 W (p>0.05). At this point, the result agrees well with our
previous experimental results about the distribution of electrons,
ions, and oxygen radicals in O plasma. In our previous experi-
2
ment (23), across all power levels, we observed a similar trend for
all created active species between density and power. When the
discharge power ranges from 0 to 30 W, the electron energy is
increased to remain in the peak values of 11.0 eV in the discharge
zone, whereas it declined from 30 to 60 W because of colliding
with each other, and then it is augmented rapidly above 60 W. As
a result, the discharge power ranges from 0 to 30 W and leads to
the enhancement of the possibility for the pesticide molecule to
experience the dissociation by collision with electrons and the
probability of reactions between them is also promoted. When the
discharge power varies from 30 to 60 W, electrons cannot carry
the necessary energy to perform a dissociation collision because
of their relatively high recombination rate; consequently, the
Statistical Analysis and Quality Assurance. Quality control and
quality assurance measures were incorporated in the analytical scheme. In
this study, the effects of variable plasma treatment conditions including
different discharge power, plasma treatment time, and variance distance
from the induction coil on the degradation of DDVP and omethoate were
designed on the basis of the preliminary orthogonal experiment. Each
sample was analyzed 3 times to ensure reproducibility. To determine
whether the differences between plasma treatment conditions influenced
the degradation efficiency and the differences between DDVP and
omethoate were statistically significant, a one-way analysis of variance
(ANOVA) test was performed by Statistical Package for the Social
Sciences (SPSS, version 13.0). When significant differences were found

Article
J. Agric. Food Chem., Vol. 57, No. 14, 2009 6241
Figure 3. Effects of (a) treatment time, (b) discharge power, and (c) O flux on the reduction of DDVP in maize at the (9) discharge zone, (2) afterglow zone,
and (b) remote zone. Error bars are standard deviations of three replicates.
2
average energy of mass of active species (secondary electrons and
radicals) is declined as well as the probability of degradation
reactions. As the discharge power further increased, the average
energy of electrons is increased because the increase of the
discharge power can extend the action sphere of the plasma inner
electronic field. Besides high-energy electrons directly providing
energy to pesticide molecules, they can provide energy to feed gas
there was no significant difference with the O flux in the case of
2
20 cm /min and 60, 80, and 100 cm /min (p>0.05). For the same
3
3
reason, at low O flux, the number of active species colliding with
2
the pesticide molecule is less than that at high flux, whereas the
average energy of reaction species is higher than that at high flux.
As a result, the probability of each reaction species colliding with
the pesticide molecule increases; therefore, degradation increases.
3
(
O ) to form a large number of free radicals, such as oxygen
Similarly, at high O
2
flux exceeding 40 cm /min, the concentra-
2
radicals, which are considered highly active and instable and can
also react with pesticide molecules through the free-radical
tion of reaction species is large but the average energy and
residence time are relatively low; therefore, the action on pesticide
reaction. The initial concentration of O radicals is the highest
molecules is comparatively weak. Consequently, when O flux is
2
3
in the discharge zone and decreased with the increase of the
distance to the remote zone. It implies that the contact area
between pesticide- and plasma-active species increases as well as
the reaction probability. According to the above results, it can be
concluded that the treatment time and discharge power are both
important factors for the reduction of DDVP and the optimum
O2 plasma treatment conditions are 120 W after 120 s of
treatment time to obtain the best reduction of DDVP residues
in maize. Therefore, for the subsequent investigation of effects of
changed, while other conditions are fixed, the action on pesticide
molecules is the combining effects of the average energy and the
amount of reaction species and their residence time.
Degradation of Omethoate Residues in Maize at Different O
2
Plasma Treatment Conditions. The second phase of this work was
to evaluate reduction efficiency of omethoate residues in maize
under different O plasma treatment conditions and compare the
2
differences with DDVP. The samples were positioned at the
distance of 20 cm (discharge zone), 45 cm (afterglow zone), and
70 cm (remote zone) from the center of the induction coil in the
Pyrex glass tube of the reactor, and they were also exposed to the
O flux on the removal of DDVP, two plasma treatment condi-
2
tions are fixed.
Effects of O Flux on the Reduction of DDVP in Maize.
O
2
plasma for treatment times of 30, 60, 90, and 120 s at discharge
power levels of 30, 60, 90, and 120 W with different O flux.
Similarly, the reduction efficiency of omethoate by the O plasma
2
Figure 3c displays the remaining fraction of DDVP as a function
2
of O flux. As shown in Figure 3c, it is obvious that the reduction
2
2
efficiency increased initially but decreased later with the increase
was investigated from the remaining fraction as a function of the
above-mentioned operating parameters.
of O flux. The results can be explained by the fact that the
2
constant energy is obtained in the reaction system, when the
discharge power and treatment time are fixed. Thus, the different
effects can be related to the average energy of reaction species,
Effects of the Treatment Time on the Reduction of Ometho-
ate in Maize. The disappearance of omethoate residues in maize
at the above-mentioned distance versus time is plotted in
Figure 4a. A similar trend is observed with omethoate to those
found in DDVP in this study. The reduction efficiency of
omethoate was increased as the treatment time increased at all
distances. In O2 plasma treatment at the distance of 20 cm
(discharge zone), almost 95% of omethoate is removed after
which increases with the enhancement of the O flux in the range
2
3
of 20-40 cm /min, and subsequently, different drawdown
3
3
appears (40-100 cm /min). O flux at 40 cm /min was shown
2
to be a more noticeable plasma degradation efficiency of DDVP
than other flux with statistical significance (p<0.05); however,

6242 J. Agric. Food Chem., Vol. 57, No. 14, 2009
Bai et al.
Figure 4. Effects of (a) treatment time, (b) discharge power, and (c) O flux on the reduction of omethoate in maize at the (9) discharge zone, (2) afterglow
zone, and (b) remote zone. Error bars are standard deviations of three replicates.
2
1
7
20 s, whereas it is reduced only about 19% at the distance of
0 cm (remote zone) after 30 s. Clearly, the longer plasma
Effects of O Flux on the Reduction of Omethoate in Maize.
2
The remaining fraction of omethoate residues versus O flux at
2
treatment time can also help improve reduction efficiency of
omethoate, and the interaction between pesticide and active
species has almost completed within 120 s. However, treatment
in the discharge zone is notably effective compared to those in
the afterglow and remote zones with statistical significance (p<
the above-mentioned distances is presented in Figure 4c. The
degradation increases initially but decreases later with increasing
O flux. As the same reason in the previous discussion, at low O
flux, the average energy and residence time of reaction species are
higher than that at high flux; thus, the probability of each reaction
2
2
0
.05). This is most likely due to a higher intensity of reactive
species colliding with pesticide enhances, and therefore, degrada-
3
species (electrons, ions, radicals, etc.) in the discharge zone than
that in the afterglow and remote zones.
tion increases. At higher O flux over 40 cm /min, the number of
2
reaction species is large but the average energy and residence time
is lower; therefore, the action on pesticide is relatively small.
Effects of the Discharge Power on the Reduction of Ometho-
ate in Maize. Figure 4b displays the variation in the remaining
fraction of omethoate in maize over the discharge power at the
mentioned distance. It can be seen that increased discharge power
cause a degradation of omethoate over 60 W, whereas a different
trend is observed between 0 and 30 W regardless of the sample
position. The degradation of omethoate of 120 W discharge
power in the discharge zone is the greatest, whereas 60 W in the
remote zone is the least effective. The different effects can be
related to the average energy of reaction species increasing with
the enhancement of the discharge power in the range for 0-30 W,
and subsequently, a different fall trend appears (30-60 W). Over
The result shows more noticeable plasma reduction efficiency of
3
omethoate at O flux of 40 cm /min compared to others (p<0.05)
2
3
but no significant difference in the case of 20 cm /min and 60, 80,
and 100 cm /min of O flux (p>0.05).
3
2
Comparison of the Effects of Distance on the Degradation
of DDVP and Omethoate in Maize. The degradation fraction of
DDVP and omethoate residues in maize versus the distance is
plotted in Figure 5. The results indicate that O plasma degrada-
2
tion of DDVP and omethoate in the discharge zone are more
effective with 88 and 94% degradation fraction, respectively,
compared to those in afterglow and remote zones (p<0.05). This
can be explained by the fact that the intensities of various species
including electrons, ions, and free radicals are decreased with the
increase of the distance far from the discharge zone. Furthermore,
the degradation behavior of DDVP and omethoate appears to
differ. Besides plasma treatment parameters, the chemical struc-
ture of pesticide is the dominant factor for the persistence because
it influences the chemical stability during the degradation reac-
tion. The results also indicate that the reduction of omethoate
shows higher effects than that of DDVP during O2 plasma
treatment in all distances with statistical significance (p<0.05).
This is probably due to the fact that S of the P-S bond in the
omethoate molecule possessing smaller electronegativity than O
of P-O in the DDVP molecule results in a difference activity of
90 W, the probability of each reaction species colliding with
pesticide enhances; therefore, degradation increases. It is antici-
pated that residue levels would be reduced considerably by the O2
plasma treatment if the discharge power is increased beyond 60
W. Moreover, ANOVA results indicate that reduction efficiency
of omethoate by the O plasma process is significantly effective in
discharge power treatments conducted at 30, 90, and 120 W than
2
60 W with statistical significance (p<0.05) but not in the case
of 30 and 90 W, 30 and 120 W, and 90 and 120 W (p>0.05).
Therefore, the optimum plasma treatment conditions for the
reduction of omethoate is also at 120 W discharge power and
120 s treatment time. The subsequent investigation should be
taken under the optimum plasma treatment conditions.

Article
J. Agric. Food Chem., Vol. 57, No. 14, 2009 6243
reaction initiated by the electron attacking the P atom. Moreover,
the bond energy of P-S is less than that of the P-O bond.
According to the above results, it is clearly demonstrated that O2
plasma treatment is significantly effective in the degradation of
original DDVP as well as omethoate in maize. Furthermore, the
interaction between active species and the omethoate molecule is
more effect than that of the DDVP molecule. Notwithstanding,
we cannot ignore the fact that, besides degrading pesticides, a
certain amount of DDVP and omothoate on the surface of maize
The ions at m/z 109 and 79 are characteristic of the phosphate
+
esters and belong to the groups [(CH O) P(O)] and [CH O-P-
3
2
3
+
OH] , respectively. C was identified as 2,2-dichlorovinyl O-
2
methylphosphate with more than 70% matching and exhibited
+
the [M] ion at m/z 207 that corresponds to the loss of a methyl
group (M-14) from DDVP and the characteristic ions of [CH OP-
3
+
+
+
+
(OH) ] , [CH OP(O)OH] , [CH O-P-OH] , [P(OH) ] , and
2
3
3
2
+
[OP-OH] at m/z 96, 95, 79, 65, and 64, respectively. C was
3
identified as O,O,O-trimethyl phosphoric ester with more than
+
can be reduced by evaporation during O plasma treatment.
70% matching and exhibited the [M] ion at m/z 140, 110, 95, and
79 that belong to the groups [(CH O) P(O)] , [(CH O) P-
(OH)] , [CH OP(O)OH] , and [CH O-P-OH] , respectively.
As shown in Table 2, omethoate was mainly converted into C1
2
+
Identification of Intermediates and Possible Degradation Mecha-
nisms. When a new treatment process is considered, not only the
reduction of the target compound is of interest but also the
formation of reaction byproducts and degradation of mechan-
isms are of great importance. The optimum pesticide degradation
of plasma treatment conditions (120 W discharge power, 120 s
3
3
3
2
+
+
+
3
3
and C , just like in the case of DDVP. C and C were also
3
5
7
detected in the O plasma degradation of omethoate. C was
2
5
identified as O,O,S-trimethylphosphorothiate and exhibited the
3
+
treatment time, and 40 cm /min oxygen flux) was taken to
[M] ion at m/z 156, 141, 126, 110, 95, and 79. C was identified as
7
provide a favorable condition for identification and evolution
of degradation intermediates. DDVP and omethoate spin-coated
N-methyl-2-sulfanylacetamide and exhibits a peak at m/z 105,
which corresponds to the molecular ion [M] , and the character-
+
on glass slides (20ꢀ20 mm) were exposed to the O oxygen plasma
istic ions at m/z 73 and 58 that correspond to the loss of HS- and
2
at the optimal plasma degradation conditions. After plasma
treatment, identification of intermediates and products of de-
tailed GC/MS analysis was performed in this research. Table 2
displays GC/MS data of the identification of intermediates and
products of DDVP and omethoate along with their retention
times and the characteristic ions of the mass spectra. Three
byproducts for DDVP and four byproducts for omethoate
have been identified as possible degradation intermediates using
GC/MS.
HSCH - fragments, respectively.
2
On the basis of the distribution of intermediates by the aid of
GC/MS analyses, theory of organic chemical mechanisms, and
bond energies (24) (Table 3), we propose the possible O plasma
2
degradation mechanisms for DDVP and omethoate. It can be
seenfrompanels a and b of Figure6 that the dominated O plasma
2
degradation mechanisms for DDVP and omethoate are free-
radical-reaction-initiated through (i) one-electron transfers to the
π bond within either PdO or CdC (pathway I) and (ii) free-
radical attacks to pesticide molecules (pathway II). It is interest-
ing to notice that C and C are identified as intermediates during
From Table 2, C , C , and C were detected as O plasma
1
2
3
2
degradation intermediates of DDVP and C , C , C , and C
1
3
5
7
1
3
were detected as those of omethoate. C1 was identified as
O plasma degradation of DDVP and omethoate, and they are
2
O,O-dimethyl phosphonic ester, with more than 90% matching.
also detected as identified intermediates during advanced oxida-
tion of various OP pesticides (25-28). The formations of C and
1
C because of the cleavage of either the P-O or P-S bonds
3
resulting in several intermediates are proposed to arise by attack
of one electron, followed by various radicals, such as hydroxyl
and methyl radicals. C as another intermediate of O plasma
2
2
degradation of the DDVP molecule is formed through the carbon
atom of the methoxy group attacked by the hydroxyl radical,
followed by the elimination of the methoxy group. The complete
mineralization of DDVP degradation by O plasmas produces
2
3-
-
PO4 , CO , Cl , etc. Consider the omethoate molecule, the
2
scission of the P-S bond leads to the formation of SCH C(O)-
3
2
NHCH radicals that are the precursor of C and C . Cleavage of
3
1
7
the C-S bonds followed by the rearrangement of methyl radical
is evident from the detections of C . Continuous cleavage of the
5
P-S bond within the C5 molecule, followed by adding the
Figure 5. Comparison of effects of the distance on degradation of DDVP
and omethoate in maize. Applied treatment time, 120 s; discharge power,
methoxy group could lead to the formation of C and C . C
1
3
5
and C are also identified as main intermediates of the degrada-
7
3
20 W; O flux, 40 cm /min. Error bars are standard deviations of three
1
replicates.
tion of dimethoate (25). Hydrogen sulphide, although not detect-
able for omethoate, is proposed as one of the possible
2
t
Table 2. GC/MS Retention Times (R ) and Special Characteristics of DDVP and Omethoate Identified Intermediates
pesticide/intermediate
R
t
(min)
characteristic ions (m/z)
109, 95, 80, 79
C
1
, O,O-dimethyl phosphonic ester
3.82
4.73
5.12
8.02
C
C
C
2
, 2,2-dichlorovinyl O-methylphosphate
, O,O,O-trimethyl phosphoric ester
, DDVP
207, 96, 95, 79, 65, 64
140, 110, 95, 79
220, 185, 145, 109, 79
DDVP
3
4
C
1
C
3
C
5
C
6
C
7
, O,O-dimethyl phosphonic ester
, O,O,O-trimethyl phosphoric ester
, O,O,S-trimethylphosphorothiate
, omethoate
3.78
4.64
5.60
9.62
9.87
109, 95, 80, 79
140, 110, 95, 79
omethoate
156, 141, 126, 110, 95, 79
213, 156, 141, 126, 110, 79, 58
105, 73, 58
, N-methyl-2-sulfanylacetamide

6244 J. Agric. Food Chem., Vol. 57, No. 14, 2009
Bai et al.
Table 3. Average Bond Energies (KJ/mol, 298.15 K, 100 kPa)
bond
bond energy
bond
bond energy
C-H
C-O
C-C
CdC
C-Cl
O-H
414
351
347
620
331
460
P-O
PdO
P-H
P-S
P-C
S-C
502
755
318
230
264
289
Figure 7. Comparison of effects on the degradation of DDVP and
2 2
omethoate by O plasma alone and O plasma + tert-butanol. Applied
3
treatment time, 120 s; discharge power, 120 W; O flux, 40 cm /min. Error
bars are standard deviations of three replicates.
2
compared to that in the absence of it during O plasma treatment
2
(
p < 0.05) and the O plasma degradation efficiency is greatly
2
retarded with the distance far from the discharge zone. This is
probably due to tert-butanol molecules scavenging the radicals
formed in O plasma when OP pesticides are exposed to O
2
2
plasma in the presence of tert-butanol. Accordingly, this radical
scavenger could diminish the degradation reaction between the
radicals and OP pesticide molecules. In this case, the result is
similar to that in our previous study, in which the intensities of
electrons decreased quickly, but those of radicals declined slowly
from the discharge zone to the remote zone. When tert-butanol
molecules are added to the OP pesticides treated by O plasma
2
processing, they can scavenge radicals derived from O plasma
2
and lead to a slow down of the degradation reaction between
these radicals and OP pesticide molecules. These results can be
reasonably inferred that O plasma degradation mechanisms of
2
OP pesticides are dominated by the free-radical mechanisms.
Therefore, it is believed that O plasma degradation of DDVP
2
and omethoate through the formation of intermediates, followed
by the generation of radicals, induces pesticide molecules to
decompose into small ones. However, to draw a more detailed
plasma degradation mechanism for these pesticides, further
points are still needed to be clarified.
From what has been mentioned above, we can come to the
conclusion that the plasma-induced degradation reactions of
DDVP and omethoate molecules are demonstrated in this study.
Our results indicate that O2 plasma is capable of degrading
DDVP and omethoate residues completely in maize in a very
short exposure time and the optimum plasma treatment condi-
tions for the reduction of these two widely used OP pesticides are
Figure 6. Potential degradation pathways of (a) DDVP and (b) omethoate
during O plasma treatment.
2
intermediates that can be further transformed to sulfate. Finally,
3-
2-
-
this can proceed to form PO4 , H S, SO4 , CO , Cl , etc.
2
2
3
Therefore, the relative significance of the C-O or C-S cleavage
versus P-O or P-S cleavage depends upon not only energies
120 W of discharge power, 120 s of treatment time, and 40 cm /
min of O
lower thanthat of omethoate because of their chemical structures.
Identification of intermediates during O plasma treatment
flux. Moreover, the removal effectiveness of DDVP is
2
provided by reactive species derived from O plasma but also the
2
nature of the leaving group during plasma degradation proces-
sing. In short, the key step for plasma degradation is that high-
energy electrons can provide enough energy to cleave chemical
2
appears to be a necessary process to best understand the mechan-
isms of plasma degradation. Therefore, the major degradation
bonds by reacting with feed O gas or OP pesticide molecules to
products formed during O plasma treatment of DDVP and
2
2
form high intensities of various free radicals initiated by the free-
radical chain reaction, and then OP pesticides are decomposed
ultimately to form a series of less toxic molecules than parent
pesticide molecules.
omethoate were identified, and the reaction pathways were
examined. It has been verified that most of the identified inter-
mediates are less toxic compounds than the parent compounds
and O plasma degradation mechanisms for OP pesticides can be
2
To verify the important role of the free radicals in O plasma
dominated by the free-radical reaction, which is confirmed by
adding radical scavenger. Because the degradation reactions are
2
degradation of DDVP and omethoate, a well-known radical
scavenger, i.e., tert-butanol, was employed (29). Figure 7 shows
the comparison of effects on the degradation of OP pesticides
based on reactive species derived from O
atoms, radicals, etc.), a wide range of chemical contaminants in
agricultural products can be reduced by O plasma. As such, the
application of O plasma challenges the conventional methods for
plasma (such as excited
2
(
DDVP and omethoate) by O plasma alone and O plasma+
2
2
2
tert-butanol. Decreased degradation of DDVP and omethoate
are demonstrated by the presence of tert-butanol. As shown in
Figure 7, a significant decrease of the degradation fractions of
DDVP and omethoate is observed in the presence of tert-butanol
2
the degradation of pesticides because of its crucial advantages,
such as high efficiencies, low cost, and without causing secondary
pollution. Further work needs to be carried out to elucidate

Article
J. Agric. Food Chem., Vol. 57, No. 14, 2009 6245
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´