Sonolytic degradation of parathion and the formation of byproducts

Article abstract of DOI:10.1016/j.ultsonch.2010.01.016

Ultrasonic degradation of parathion has been investigated in this study. At a neutral condition, 99.7% of 2.9 μM parathion could be decomposed within 30 min under 600 kHz ultrasonic irradiation at ultrasonic intensity of 0.69 W/cm². The degrada

Full text of DOI:10.1016/j.ultsonch.2010.01.016

Ultrasonics Sonochemistry 17 (2010) 802–809
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Sonolytic degradation of parathion and the formation of byproducts
a
b
a
c
a
a
Juan-Juan Yao a, , Nai-Yun Gao , Yang Deng , Yan Ma , Hai-Jun Li , Bin Xu , Lei Li
*
a State Key Laboratory of Pollution Control and Resources Reuse, Tongji University, Shanghai 200092, China
b Department of Civil Engineering and Surveying, University of Puerto Rico, P.O. Box 9041, Mayagüez, PR 00681-9041, USA
c Faculty of Material Science and Chemistry Engineering, China University of Geosciences, Wuhan 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 August 2009
Received in revised form 30 December 2009
Accepted 31 January 2010
Available online 4 February 2010
Ultrasonic degradation of parathion has been investigated in this study. At a neutral condition, 99.7% of
2.9 M parathion could be decomposed within 30 min under 600 kHz ultrasonic irradiation at ultrasonic
l
intensity of 0.69 W/cm2. The degradation rate increased proportionally with the increasing ultrasonic
intensity from 0.10 to 0.69 W/cm2. The parathion degradation was enhanced in the presence of dissolved
oxygen due to formation of more ÅOH, but was inhibited in the presence of nitrogen gas owning to the free
radical scavenging effect in vapor phase within the cavitational bubbles. CO23ꢀ, HCO3ꢀ, and Clꢀ exhibited
the inhibiting effects on parathion degradation, and their inhibition degrees followed the order of
CO23ꢀ > HCO3ꢀ > Clꢀ. But Brꢀ had a promoting effect on parathion degradation, and the effect increased
with the increasing Brꢀ level. Moreover, both the hydrophobic and hydrophilic natural organic matters
(NOM) could slow the parathion degradation, but the inhibiting effect caused by hydrophobic component
was greater, especially the strongly hydrophobic NOM. The three reaction pathways of parathion sonol-
ysis were proposed, including formation of paraoxon, formation of 4-nitrophenol, and unknown species
products. The kinetics tests showed that anyone of these pathways could not be overlooked, and the frac-
tions of the parathion decomposed in the three pathways were 28.19%, 32.92% and 38.89%, respectively.
In addition, 66.61% of paraoxon produced was degraded into 4-nitrophenol. Finally, kinetics models were
established to adequately predict the concentrations of parathion, paraoxon and 4-nitrophenol as a func-
tion of time.
Keywords:
Parathion
Ultrasonic irradiation
Paraoxon
4-Nitrophenol
Kinetics model
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
diation processes, the transformation from parathion to paraoxon
is more rapid and completes [8–10].
Massive applications of organophosphorus pesticides (Ops) in
agriculture and forestry have been causing great concerns about
the environment pollution due to their non-specific inhibiting ef-
fects on enzyme acetylcholinesterase (AChE) in human nervous
system. The pollution may greatly degrade the quality of surface
water and groundwater [1–3]. As one of the representative Ops,
parathion has been listed as ‘‘extremely hazardous” by the World
Health Organization. Although, parathion has been banned in
many countries (e.g. USA) [4], it is still being used in other coun-
tries. For example, in China, parathion is legally used for forestry
and crops other than vegetables, fruits, teas or herbal medicines
[5]. The chemical structure and physical properties of parathion
containing phosphorus–sulfur double bound (P@S) is shown in
Table 1. When parathion is applied in agricultural activities or
transports into surface water, it is readily oxidized to paraoxon
(P@O), a more potent AChE inhibitor, by the oxidants naturally
present [6,7]. During water chlorination, ozonization and UV irra-
Recently, ultrasonic irradiation, an advanced oxidation process,
has received increasing attention for the degradation of various or-
ganic pollutants in water [11–15]. Application of ultrasound to
aqueous solutions forms cavitation bubbles, which will undergo
transient collapse events. Quasi-adiabatic compression of transient
bubbles generates average vapor temperatures near 5000 K and
pressures up to 10,000 bar. In aqueous solution, three different
reaction zones have been postulated: (i) interiors of bubbles where
water vapor is pyrolyzed to hydroxyl radicals (ÅOH) and hydrogen
atoms (ÅH), as shown in Eq. (1), and where gas-phase pyrolysis
and/or combustion reactions of volatile substances dissolved in
water occur.
ÞÞÞ
H2O ! HÅ þ ÅOH
ð1Þ
(ii) Water-bubble interfaces where 600–1000 K temperature with a
high gradient exists, and where locally condensed ÅOH has been re-
ported. A thin shell of transiently supercritical water exists in this
region. (iii) Bulk solution at ambient temperature where reactions
* Corresponding author. Tel.: +86 21 65982691; fax: +86 21 65986313.
E-mail address: yao_juanjuan@yahoo.cn (J.-J. Yao).
Å
Å
with OH or H that migrates from the interface occur.
1350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2010.01.016

J.-J. Yao et al. / Ultrasonics Sonochemistry 17 (2010) 802–809
803
Table 1
Chemical structure and physical properties of ethyl parathion [10,42].
Compound
Parathion
Structure
MW
Log KOH
Vp
Ws
11
291.26
3.83
3.78 ꢁ 10ꢀ5
Log KOH: octanol–water partition coefficient.
Ws: water solubility, mg Lꢀ1 (20 °C).
Vp: vapor pressure, mm Hg (20 °C).
Sonolysis of parathion was reported first by Kotronarou et al.
[16] who identified the final products and proposed a simple deg-
radation pathway. They proposed that the hydrolysis of parathion
by supercritical water at the water-bubble interface is dominant
pathway for sonolytic degradation of parathion. However, the
quantitative of byproducts was not done to prove their supposi-
tion. Wang et al. [43,44] reported the sonocatalytic degradation
of parathion in the presence of heterogeneous sonocatalysts TiO2.
However, little literature has been available to explore the effect
of environmental factors on parathion degradation and the forma-
tion model of byproducts by ultrasonic irradiation alone.
The objectives of this study are to: (i) evaluate the effect of irra-
diation intensity, dissolved gasses, anions, and natural organic
matters, on parathion sonolysis; and (ii) establish kinetic models
of the degradation of parathion and the formation of paraoxon
and 4-nitrophenol (two major highly toxic byproducts of para-
thion) based on our proposed reaction pathways.
(SF600, Shanghai Acoustics Laboratory, Institute of Acoustics, Chi-
nese Academy of Sciences, China). A cylindrical stainless steel reac-
tion vessel (i.d. 100 mm, and volume 600 mL) was directly
connected to an ultrasonic transducer (SF600, Shanghai Acoustics
Laboratory, Institute of Acoustics, Chinese Academy of Sciences,
China) with flanges and a flexible Teflon O-ring for sealing. The
vessel was immersed in a water bath to control the reaction tem-
perature at a constant level. In the tests to investigate the effects of
gases, the vessel was sealed with a special stainless steel lid via
flanges, and the gases were introduced into the solution through
a Teflon tube, as shown in Fig. 1(a). In all the other tests, the vessel
was open to the air, as shown in Fig. 1(b).
All tests were performed at 25.0 1.0 °C and under atmospheric
pressure. In all experiments, 300 mL of parathion solution was pre-
pared daily and then stored into the reaction vessel. The solution
pH was adjusted to 7.0 0.05 with 1.0 M HCl and 1.0 M NaOH,
and remained uncontrolled during the ultrasonic irradiation, ex-
cept in the experiments to investigate the effect of anions in which
the initial pH was not adjusted. The reactions were initiated by
turning on the ultrasonic transducer. At each sampling time,
2. Materials and methods
5 mL sample was pipetted to a 10 mL glass bottle with 200
lg/L
2.1. Chemicals
internal standard addition and extracted by 1 mL dichloromethane
immediately. The extract was collected and subsequently analyzed
by GC–MS. All the experiments were carried out at least in dupli-
cates. The standard deviations of duplicate experiments were less
than 10%. The symbols in Figs. 2–5 showed the mean values of
the duplicate experiments. In the experiments to investigate effect
of dissolved gases, the gas investigated was introduced into the
solution for 30 min with the flow rate of 0.5 L/min prior to the
reaction. In the experiments to investigate effect of anions, a cer-
tain amount of salt (NaCl, NaHCO3, NaBr or Na2CO3) was added
to form designated initial concentration of the corresponding anion
before the reaction.
Parathion (99%, purity) and paraoxon (99%, purity) were pur-
chased from Dr. Ehrenstorfer GmbH (German). Acetonitrile (HPLC
grade), dichloromethane (PESTANAL grade), 4-nitrophenol stan-
dard solution, triphenylphosphate (>99%) as internal standard
(I.S.), Supelite DAX-8 resin, Amberlite XAD-4 resin, Amberlystra-
26(OH) anion-exchange resin and DOWEX 50WX4-50 cation-ex-
change resin were obtained from Sigma–Aldrich (USA). All the
other reagents are analytical grade except as noted. All the solu-
tions were prepared using the water purified by a Milli-Q Gradient
water purification system. Natural organic matters (NOMs) were
extracted from a natural surface water (Yangtze River, Shanghai,
China) by 0.45 lm polyether sulfone membrane filtration and sub-
sequent nanofiltration membrane (NF-90, Toray Co., Japan) in the
state key laboratory of pollution control and resources reuse labo-
ratory at Tongji University (Shanghai, China). The different compo-
nents of NOM were fractionated by Supelite DAX-8 and Amberlite
XAD-4 [17]. All the component solution passed through the DOW-
EX 50WX4-50 cation-exchange resin and Amberlystra-26(OH) an-
ion-exchange resin subsequently before use.
2.3. Analysis
Parathion and paraoxon in the samples were extracted by
dichloromethane, and quantified by a Shimadzu GC/MS-QP2010s
gas chromatograph–mass spectrometer equipped with a 30 m
RTX-5MX column by RESTEK (film thickness: 0.25 lm; i.d.
0.25 mm). Helium (purity >99.999%) was used as the carrier gas,
with a flow rate of 1.5 mL/min. The injection volume of each ex-
2.2. Experiments
tract was 1.0 lL. SIM mode was used with a dwell time of 50 ms
for each ion. The GC oven temperature program was: initial tem-
perature at 40 °C, hold for 1 min, 25.0 °C minꢀ1 gradient until
200.0 °C, 10.0 °C minꢀ1 until 220.0 °C, 30 °C minꢀ1 until 270.0 °C,
and hold for 3 min. 4-Nitrophenol was analyzed by a Shimadzu
All the experiments were conducted in the sonochemical reac-
tors shown in Fig. 1. The ultrasonic generator provided a fixed fre-
quency of 600 kHz with an electric power output up to 100 W

804
J.-J. Yao et al. / Ultrasonics Sonochemistry 17 (2010) 802–809
Fig. 1. Schematic diagram of the sonochemical reactors: (a) for the tests to investigate the effects of gases; and (b) for all the other tests (1: ultrasonic transducer; 2: stainless
steel reaction vessel; 3: sample solution; 4: water bath; 5: cooling water inlet; 6: cooling water outlet; 7: ultrasonic generator; 8: gas inlet; 9: gas outlet; 10: sealed but
operable sampling point).
0
0
O2-Saturation
-1
-2
-3
-4
-5
-6
-7
-1
-2
-3
-4
-5
-6
N2-Saturation
None gas addition
Air-Saturation
2
0.69 W/cm
2
0.48 w/cm
2
0.24 w/cm
2
0.10 w/cm
0
10
20
30
40
50
60
0
10
20
30
40
50
Time (min)
Time (min)
Fig. 2. Effect of ultrasonic intensity on parathion sonolysis (initial para-
thion = 2.9 M; and ultrasonic frequency = 600 kHz).
Fig. 3. Effect of dissolved gas on parathion degradation (initial parathion = 2.9 lM;
l
ultrasonic frequency = 600 kHz; and ultrasonic intensity = 0.69 W/cm2).

J.-J. Yao et al. / Ultrasonics Sonochemistry 17 (2010) 802–809
805
bon (DOC) was measured by Shimadzu TOC-VCPH. In order to
calculate the actual ultrasonic intensity, the energy dissipated by
the transducer of ultrasonic device was determined by calorimetry
[18].
(a)
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
3. Results and discussion
3.1. Effect of ultrasonic intensity on parathion degradation
none addtion
Cl-1=0.01M
-
The effect of ultrasonic intensity on parathion degradation with
time is shown in Fig. 2. The degradation was pseudo 1st-order a
reaction at various actual ultrasonic intensities (0.10–0.69 W/
cm2). The degradation rate of parathion proportionally increased
from 0.020 to 0.192 minꢀ1 with the increasing ultrasonic intensity
from 0.10 to 0.69 W/cm2 (as shown in Table 2). The positive effect
was primarily due to the enhanced cavitational activity at higher
ultrasonic intensity that increased the number and violence of
bubble collapse. In addition, the mixing intensity was also en-
hanced with the increasing ultrasonic intensity due to the acoustic
streaming generated by cavitational processes, in agreement with
the previous findings about other Ops [19–21].
HCO =0.01M
3
-
Br =0.01 M
-
Br =0.001 M
2-
CO =0.01M
3
0
5
10
15
20
25
30
Time (min)
2.5
2.0
1.5
1.0
0.5
0.0
(b)
Br-=0.001M
Br-=0.01M
None anion addition
Cl-=0.01 M
3.2. Effects of different dissolved gases on parathion degradation
The effects of different dissolved gases, O2, N2, and air, on para-
thion degradation are shown in Fig. 3. The degradation in the pres-
ence of O2, N2, or air was a pseudo 1st-order reaction. Among them,
O2 could achieve the highest rate constant of 0.288 minꢀ1, followed
by air (0.211 minꢀ1), no gas addition (0.186 minꢀ1
)
and N2
(0.123 minꢀ1) (as shown in Table 3). Owning to the little difference
between their specific heats (O2, = 1.39; N2, = 1.40), the dissim-
c
c
ilarity of the two gases in the impacts on parathion degradation is
not related the well-known gas property that significantly influ-
ences the final collapse temperatures and pressures within the
bubbles [22]. Under ultrasonic irradiation, water vapor can be
pyrolyzed to ÅOH and ÅH, as shown in Eq. (1). After O2 enters the va-
por phase of the bubble, it can undergo pyrolysis (Eq. (2)), because
the dissociative bond energies in the gas-phase for water and oxy-
gen are both 119 kcal molꢀ1 [23]. The formed highly active oxygen
atom (ÅOÅ) can react with water molecules to form ÅOH, as shown in
Eq. (3). Moreover, O2 can also combine abundant hydrogen atom
(ÅH) inside the bubbles (Eq. (5)), or at the relatively cool interfacial
0
10
20
30
40
50
60
Time (min)
Fig. 4. Effect of anions on parathion degradation and the paraoxon formation: (a)
parathion degradation; (b) paraoxon formation) (initial parathion = 2.9
l
M; ultra-
sonic frequency = 600 kHz; ultrasonic intensity = 0.69 W/cm2).
0
-1
-2
-3
Å
Å
region (Eq. (6)), to form additional OH [24]. Some OH can form
H2O2 through Eqs. (7) and (8). Wakeford et al. [22] reported that
the formation rate of H2O2 was much higher in O2- or air-saturated
Table 2
Pseudo 1st-order rate constants for the sonochemical degradation of parathion under
different ultrasonic intensities.
-4
Ultrasonic intensity (W/cm2)
kapp (minꢀ1
)
R2
None
-5
-6
strongly hydrophobic NOM =10 mg/L
weekly hydrophobic NOM =10 mg/L
hydrophilic NOM =10 mg/L
0.69
0.48
0.24
0.10
0.199
0.108
0.049
0.020
0.992
0.987
0.997
0.990
0
5
10
15
20
25
30
Time (min)
Table 3
Fig. 5. Effects of different components of dissolved natural organic matters on
Pseudo 1st-order rate constants for the sonochemical degradation of parathion under
different dissolved gas conditions.
parathion degradation (initial parathion = 2.9
lM; ultrasonic frequency = 600 kHz;
ultrasonic intensity = 0.69 W/cm2).
Dissolved gas
kapp (minꢀ1
)
R2
O2
N2
Air
0.288
0.123
0.211
0.186
0.997
0.993
0.998
0.998
LC-2010AHT HPLC equipped with a VP-ODS column (250 mm ꢁ
4.6 mm i.d.) and ultraviolet detector (UV wavelength = 318 nm).
Elution was performed with a mobile phase composed of acetoni-
trile/water/acetic acid (50:49.6:0.4, v/v/v). Dissolved organic car-
None gas addition

806
J.-J. Yao et al. / Ultrasonics Sonochemistry 17 (2010) 802–809
Å
water than in N2-saturated water. Therefore, the positive impact of
O2 on parathion sonolysis is due to the increase in ÅOH production.
On the other side, N2 present in bubble can scavenge free radicals
[25], as shown in Eqs. (9)–(14), and thus inhibit the parathion deg-
air–water interface and therefore react with OH to yield stable
and long-lived radical anion, BrÅ2ꢀ and inhibit the self-recombina-
tion of ÅOH at the air–water interface (Eqs. (15)–(20)) [29].
Although the OH is consumed in the presence of Brꢀ, the total
Å
Å
Å
radation. Yasui et al. [26] reported that OH and OÅ were rapidly
consumed inside bubbles by oxidizing nitrogen when the temper-
ature during the bubble collapse was higher than 7000 K.
amount of radical increased at the interface region. Secondly, com-
Åꢀ
Å
pared with OH, the non-recombined Br2 is more easy to diffuse
out of the water cage into the bulk solution to react with parathion
[30]. Moreover, BrÅ2ꢀ has been demonstrated to be relatively more
ÞÞÞ
O2 ! 2ÅOÅ
ð2Þ
ð3Þ
Å
selective than OH in its reaction with organic sulfur compounds,
ÅOÅ þ H2O ! 2ÅOH
2ÅOH ! ÅOÅ þ H2O
O2 þ HÅ ! ÅOH þ ÅOÅ
O2 þ HÅ ! ÅO2H
2HOÅ ! H2O2
including sulfides [31,32]. The reaction between parathion and
BrÅ2ꢀ is shown in Eq. (21). BrÅ2ꢀ attacks P@S bond to finally form
paraoxon (P@O) and re-release Brꢀ.
ð4Þ
ð5Þ
ð6Þ
Brꢀ þ ÅOH ! ÅBrOHꢀ; k ¼ 1:06 ꢁ 1010 Mꢀ1 sꢀ1
ÅBrOHꢀ ! BrÅ þ OHꢀ; k ¼ 4:2 ꢁ 106 Mꢀ1 sꢀ1
ÅBrOHꢀ ! Brꢀ þ ÅOH; k ¼ ð3:3 ꢂ 0:4Þ ꢁ 107 Mꢀ1 sꢀ1
ÅBrOHꢀ þ Brꢀ ! BrÅ2ꢀ þ OHꢀ; k ¼ 1:9 ꢁ 108 Mꢀ1 sꢀ1
ð15Þ
ð16Þ
ð17Þ
ð18Þ
ð19Þ
ð20Þ
ð7Þ
2HOÅ ! O2 þ H2O2
ð8Þ
2
N2 þ ÅOH ! NO2 þ HÅ
N2 þ ÅOÅ ! NOÅ þ NÅ
O2 þ NÅ ! NOÅ þ ÅOÅ
NOÅ þ OÅ ! ÅNO2
ð9Þ
ð10Þ
ð11Þ
ð12Þ
ð13Þ
ð14Þ
BrÅ þ Brꢀ ! BrÅ2ꢀ
;
k ꢃ ꢁ1010 Mꢀ1 sꢀ1
NOÅ þ ÅOH ! HNO2
ÅNO2 þ ÅOH ! HNO3
ÅOH þ ÅOH ! H2O2; k ¼ 1:2 ꢁ 1010 Mꢀ1 sꢀ1
ð21Þ
3.3. Effects of various anions on parathion degradation
The formations of paraoxon in the presence of different anions
under ultrasonic irradiation of parathion are shown in Fig. 4(b).
The produced paraoxon had the highest level when Brꢀ was pres-
ent thus supporting the enhanced hypothesis mentioned above.
Effects of different anions on parathion sonolysis are shown in
Fig. 4(a). The pseudo 1st-order rate constants of parathion degra-
dation with different anions additions are summarized in Table
4. The CO23ꢀ, HCOꢀ3 , and Clꢀ slowed the parathion degradation,
and the degrees of their inhibiting effects followed the order of
CO23ꢀ > HCOꢀ3 > Clꢀ. However, Brꢀ exhibited a promoting effect to
enhance the decomposition, and the promoting effect was in-
creased with the increasing Brꢀ concentration. CO32ꢀ and HCOꢀ3
Table 4
Pseudo 1st-order rate constants for the sonochemical degradation of parathion under
different anion additions.
Anion
kapp (minꢀ1
0.138
)
R2
Å
0.010 M CO23ꢀ
0.010 M HCOꢀ3
0.010 M Clꢀ
0.010 M Brꢀ
0.001 M Brꢀ
None
0.996
have a well-known OH scavenging effect due to their high rate
constants with ÅOH in bulk solution (1.5 ꢁ 107 and
0.166
0.186
1.080
0.388
0.199
0.999
0.999
0.949
0.975
0.992
3.8 ꢁ 108 Mꢀ1 sꢀ1, respectively) [27,28].
The promoting effect of Brꢀ can be explained as follows: Firstly,
Brꢀ at sufficiently high concentration could be able to reach the

J.-J. Yao et al. / Ultrasonics Sonochemistry 17 (2010) 802–809
807
To our knowledge, this is the first finding on the enhanced effect
of Brꢀ on sonolytic degradation. Kotronarou et al. reported a sim-
ilar finding that another halogen ion Iꢀ could accelerate the son-
olytic degradation of both carbon tetrachloride and H2S with a 3–
10-fold enhancement in overall reaction rate [33]. Among all an-
ions studied, Clꢀ only slightly slowed down the degradation of
parathion, as shown in Fig. 4(a). Although Clꢀ, as another halogen
hydrophobic properties may have a stronger competition with
parathion for reactive oxygen species, especially at the interface re-
gion of the cavitational bubbles [35].
3.5. The models of parathion degradation and the formation of
paraoxon and 4-nitrophenol
ion, can also react with OH to finally yield ClÅ2ꢀ, the primary inter-
A proposed mechanism of parathion sonolysis is showed in
Fig. 6. The first step of parathion degradation under ultrasonic irra-
Å
mediate radical ClOHꢀ itself is unstable so that formation ClÅ2ꢀ is
Å
more difficult than the production of BrÅ2ꢀ (Eqs. (22) and (23))
[27,34].
diation included: (1) Route I: the oxidative attack by OH on the
Å
P@S bond, which results in the formation of paraoxon; [36,37]
(2) Route II: the decomposition of p-nitrophenoxy mainly caused
by hydrolysis, which results in the formation of 4-nitrophenol
and O,O-diethyl phosphorothioate [38,39]. (4) Route III indicates
that the remainder of parathion is sonolyzed into some unknown
products (e.g. small organic molecules formed by directly pyrolysis
of parathion, a semi-volatile compound, inside the bubbles).
The continuous attack of ÅOH on P@O bond of paraoxon (Routes
I–I) and the hydrolysis of paraoxon (Route I–II) results in the for-
mation of the 4-nitrophenol and O,O-diethyl phosphonic ester.
Route I–III indicates that the remainder of paraoxon is sonolyzed
into some unknown products. In each pathway, the sonolysis is a
pseudo 1st-order degradation reaction [28]. Therefore, the overall
parathion degradation is expressed as Eq. (24).
Clꢀ þ ÅOH ! ÅClOHꢀ; k ¼ 4:3 ꢁ 109 Mꢀ1 sꢀ1
ð22Þ
ð23Þ
ÅClOHꢀ ! Clꢀ þ ÅOH; k ¼ 6:1 ꢁ 109 Mꢀ1 sꢀ1
Thus, little difference in parathion degradation was observed
with no addition of any anion and in the presence of Clꢀ, as shown
in Fig. 4(a). Moreover, the introduction of Clꢀ did not significantly
influence the formation rate of paraoxon, compared with that in
the control (no additional anions), as shown in Fig. 4(b).
3.4. Effects of different components of dissolved natural organic
matters on parathion degradation
The effects of different components of dissolved NOM on para-
thion are shown in Fig. 5. The pseudo 1st-order rate constants of
parathion degradation with different components of dissolved nat-
ural organic matters are summarized in Table 5. Hydrophobic and
hydrophilic NOM both slowed down the parathion degradation,
but the inhibiting effect caused by hydrophobic component was
more significant, particularly the strongly hydrophobic compo-
nent. Since parathion is a strongly hydrophobic organic compound
itself, the strongly hydrophobic NOM component with the similar
d½parathionꢄ
¼ ꢀðkI þ kII þ kIIIÞ½parathionꢄ ¼ ꢀk0½parathionꢄ ð24Þ
dt
where, kI, kII and kIII are the pseudo 1st-order rate constants for each
pathway of parathion degradation in Fig. 6, and k0 is the overall rate
constant. Also, the formation of paraoxon and 4-nitrophenol can be
expressed as Eqs. (24) and (25), respectively:
d½paraoxon ꢄ
¼
¼
g1k0½parathion ꢄ ꢀ ðkI—I þ kI—II þ kI—IIIÞ½paraoxon ꢄ
g1k0½parathion ꢄ ꢀ k1½paraoxon ꢄ ð25Þ
dt
Table 5
Pseudo 1st-order rate constants for the sonochemical degradation of parathion under
different components of dissolved natural organic matters additions.
where, kI–I, kI–II and kI–III are the pseudo 1st-order rate constants for
each pathway of paraoxon degradation in Fig. 6, and k1 is the overall
rate constant for sonolytic degradation of paraoxon.
Components
kapp (minꢀ1
)
R2
Strongly hydrophobic NOM
Weekly hydrophobic NOM
Hydrophilic NOM
None
0.127
0.156
0.180
0.199
0.998
0.998
0.996
0.998
d½4 ꢀ nitrophenol ꢄ
¼
g2k0½parathion ꢄ þ g3k1½paraoxon ꢄ
ꢀ k2½4-nitrophenol ꢄ
dt
ð26Þ
Fig. 6. The proposed reaction pathways for parathion sonolysis.

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J.-J. Yao et al. / Ultrasonics Sonochemistry 17 (2010) 802–809
3.5
3
0.8
0.7
Parathion
Fitting curve for Parathion
Paraoxon
Fitting curve for Paraoxon
0.6
0.5
0.4
0.3
0.2
0.1
0
2.5
2
4-NItrophenol
Fitting curve for 4-nitrophenol
1.5
1
0.5
0
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70
Time (min)
Time (min)
Fig. 7. The model of parathion degradation and the formation of paraoxon and 4-nitrophenol (ultrasonic frequency = 600 kHz; and ultrasonic intensity = 0.69 W/cm2, none
gas.).
(Route III), among which any one cannot be overlooked. Moreover,
hydrolysis contribute the transformation of parathion to 4-nitro-
phenel in Route II. Several previous studies [20,40,41] showed that
ultrasonic irradiation can accelerate the hydrolysis of ester due to
formation of a hot water shell at the cavitation bubble interface.
Table 6
Estimated values of rate constants and fractions (ultrasonic frequency = 600 kHz; and
ultrasonic intensity = 0.69 W/cm2, none gas).
Constant
Value (Mꢀ1 sꢀ1
)
Fraction coefficient
Value
k0
k1
k2
0.1988
0.1458
0.0972
g1
g2
g3
0.2819
0.3292
0.6661
4. Conclusions
This paper provides a full picture of parathion degradation un-
der ultrasonic irradiation. In this study, 99.6% of 2.9 lM parathion
where
g1 and
g
2 are the fraction coefficients (
g
1 = kI/k0,
g2 = kII/k0) of
was decomposed at neutral conditions within 30 min under
600 kHz ultrasonic irradiation. Dissolved oxygen greatly enhanced
the degradation, while nitrogen gas slowed down the reactions due
to its free radical scavenging effect in vapor phase within the cavi-
tational bubbles. Thus, air bubbling of parathion-containing solu-
tion may be a method to increase the tꢀreatment efficiency of
sonolysis in practice. Moreover, CO23ꢀ,HCO3 , and Clꢀ were found
to have inhibiting effects on parathion degradation, and their inhi-
bition degrees followed the order of CO23ꢀ > HCO3ꢀ > Clꢀ, implying
that removal of alkalinity prior to sonolysis is another potential
pre-treatment process. Differently, Brꢀ enhanced the parathion
degradation, and the promoting effect was further enhanced with
the increasing Brꢀ concentration. Both the hydrophobic and hydro-
philic natural organic matters slowed the parathion degradation
rate, but the inhibiting effect caused by hydrophobic component
was greater. Based on the proposed pathways of parathion sonoly-
sis, kinetics models could be established to well predict the con-
parathion degradation via Routes I and II, respectively. g3 is the
fraction coefficient ( 3 = (kI–I + kI–II)/k1) of paraoxon degraded via
Route I–I and I–II. k2 is the overall rate pseudo 1st-order rate con-
g
stant for sonolytic degradation of 4-nitrophenol.
Integration and rearrangement of Eqs. (24)–(26) give Eqs. (27)–
(29), which can be used to predict the concentrations of parathion,
paraoxon and 4-nitrophenol as a function of time:
0t
½parathionꢄt ¼ ½parathionꢄ0eꢀk
ð27Þ
ð28Þ
ꢀ
ꢁꢂ
ꢃ
g1k0½parathionꢄ0 eðk
k1 ꢀ k0
ꢀ 1
1ꢀk0Þt
½paraoxonꢄt ¼
1t
ek
ꢀ
ꢁꢄꢀ
ꢁ
k0½parathionꢄ0
k1 ꢀ k0
g2k1 þ g1g3k1 ꢀ g2k0
k2 ꢀ k0
½4-nitrophenol ꢄt ¼
ꢂ
ꢃ
ꢀ
ꢁꢂ
ꢃꢅ
ð29Þ
2ꢀk0Þt
2ꢀk1Þt
eðk
ꢀ 1
g1g3k1 eðk
ꢀ 1
ꢁ
ꢀ
2t
ek
k2 ꢀ k1
ek
2t
centrations of parathion, paraoxon and 4-nitrophenol as
a
function of time. Under ultrasonic irradiation, the first step of para-
thion degradation includes three parallel pathways: (1) formation
of paraoxon; (2) formation of 4-nitrophenol; and (3) unknown spe-
cies production. The fractions of the parathion decomposed via the
three pathways were 28.19%, 32.92%, and 38.89%, respectively. This
observation indicates that the primary mechanism of parathion
sonolysis is different from photo-degradation of parathion, in
which the first pathway is the dominant one. In addition, 66.61%
of paraoxon produced was degraded into 4-nitrophenol. To sum
up, our results demonstrate that ultrasonic irradiation is an effec-
tive treatment for control of aqueous parathion.
The rate constants k0, k1, k2 and the fraction coefficients g1, g2, g3
were determined by the least-square method via the Matlab
2008a program (MathWorks, Inc.), as shown in Fig. 7.
The observed data well fit the modeled data from Eqs. (27)–(29)
as shown in Table 6. Under ultrasonic irradiation, 28.19% of the
parathion was degraded into paraoxon (Route I), and 32.92% of
parathion was transformed to 4-nitrophenol (Route II). In addition,
66.61% of paraoxon produced was degraded into 4-nitrophenol
(Route I–I and Route I–II).Of note, the degradation of parathion into
paraoxon (Route I) was regarded as the predominant pathway for
photolysis or photocatalysis of parathion [36,37]. However, our
observation indicates that parathion is decomposed by sonolysis
in a different fashion. Three parallel pathways explain the para-
thion degradation, including paraoxon formation (Route I), 4-nitro-
phenel formation (Route II), and production of unknown species
Acknowledgements
Support for the research has been provided by the National Ma-
jor Project of Science and Technology Ministry of China (Grant No.

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