Ultrasonic irradiation of dichlorvos: Decomposition mechanism

Article abstract of DOI:10.1016/S0043-1354(00)00304-3

The sonochemical degradation of dichlorvos in a batch reactor is investigated. Dichlorvos was irradiated with 500kHz ultrasound at input powers ranging from 86 to 161W. Acoustic power and sparge gas are two factors which greatly affect sonochemical degradation efficiency. Increasing total acoustic power input from 86 to 161W resulted in a change in the rate constant from 0.018±0.001min^-1 to 0.037±0.002min^-1. The change in rate constant due to sparge gas (Argon, Oxygen, and Argon/Oxygen (60/40%v/v) mixture) at a power of 161W is also investigated, with the Argon/Oxygen mixture giving the highest rate constant (0.079±0.005min^-1). Total organic carbon and ion chromatographic analyses are employed to determine and quantify major degradation products, including dimethyl phosphate, formate, carbon dioxide, chloride, and phosphate. The extent of mineralization, indicated by a decrease in the total organic carbon, and the formation of the various intermediates and products, varies with saturating gas. A pathway for dichlorvos decomposition is proposed, based upon formation rates of the various intermediates and products and the rate of decrease of the total organic carbon in the system. The limiting steps in the mineralization pathway appear to be transformation of dimethyl phosphate and formate. Copyright (C) 2001 Elsevier Science Ltd. The sonochemical degradation of dichlorvos in a batch reactor is investigated. Dichlorvos was irradiated with 500 kHz ultrasound at input powers ranging from 86 to 161 W. Acoustic power and sparge gas are two factors which greatly affect sonochemical degradation efficiency. Increasing total acoustic power input from 86 to 161 W resulted in a change in the rate constant from 0.018±0.001 min^-1 to 0.037±0.002 min^-1. The change in rate constant due to sparge gas (Argon, Oxygen, and Argon/Oxygen (60/40% v/v) mixture) at a power of 161 W is also investigated, with the Argon/Oxygen mixture giving the highest rate constant (0.079±0.005 min^-1). Total organic carbon and ion chromatographic analyses are employed to determine and quantify major degradation products, including dimethyl phosphate, formate, carbon dioxide, chloride, and phosphate. The extent of mineralization, indicated by a decrease in the total organic carbon, and the formation of the various intermediates and products, varies with saturating gas. A pathway for dichlorvos decomposition is proposed, based upon formation rates of the various intermediates and products and the rate of decrease of the total organic carbon in the system. The limiting steps in the mineralization pathway appear to be transformation of dimethyl phosphate and formate.

Full text of DOI:10.1016/S0043-1354(00)00304-3

Wat. Res. Vol. 35, No. 3, pp. 665–674, 2001
# 2001 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
PII: S0043-1354(00)00304-3
0043-1354/01/$ - see front matter
ULTRASONIC IRRADIATION OF DICHLORVOS:
DECOMPOSITION MECHANISM
JENNIFER D. SCHRAMM and INEZ HUA*
School of Civil Engineering, 1284 Civil Engineering Building, Purdue University, West Lafayette,
IN 47907, USA
(First received 1 September 1999; accepted in revised form 1 May 2000)
Abstract}The sonochemical degradation of dichlorvos in a batch reactor is investigated. Dichlorvos was
irradiated with 500 kHz ultrasound at input powers ranging from 86 to 161 W. Acoustic power and sparge
gas are two factors which greatly affect sonochemical degradation efficiency. Increasing total acoustic
1
power input from 86 to 161 W resulted in a change in the rate constant from 0.018 Æ 0.001 min
to
0.037 Æ 0.002 min 1. The change in rate constant due to sparge gas (Argon, Oxygen, and Argon/Oxygen
(60/40% v/v) mixture) at a power of 161 W is also investigated, with the Argon/Oxygen mixture giving the
highest rate constant (0.079 Æ 0.005 min 1). Total organic carbon and ion chromatographic analyses are
employed to determine and quantify major degradation products, including dimethyl phosphate, formate,
carbon dioxide, chloride, and phosphate. The extent of mineralization, indicated by a decrease in the total
organic carbon, and the formation of the various intermediates and products, varies with saturating gas. A
pathway for dichlorvos decomposition is proposed, based upon formation rates of the various
intermediates and products and the rate of decrease of the total organic carbon in the system. The
limiting steps in the mineralization pathway appear to be transformation of dimethyl phosphate and
formate. # 2001 Elsevier Science Ltd. All rights reserved
Key words}dichlorvos, insecticide, sonolysis, ultrasonic, water pollution
NOMENCLATURE
ture and use (Howard, 1991). Worldwide production
1
in 1989 was estimated as 4 million kg yr
(Interna-
cf
Final concentration [M]
ci
Cp
Cv
Initial concentration [M]
tional Programme on Chemical Safety, 1989), and in
1994, over 2250 kg of dichlorvos were applied in the
state of California (University of California
Statewide Integrated Pest Management Project,
1997). Dichlorvos is a mutagen and a suspected
carcinogen (US Environmental Protection Agency,
1988), and the United States Environmental Protec-
tion Agency (US EPA) lists dichlorvos as a toxic
chemical in the Toxics Release Inventory (TRI). In
1995, reported releases of dichlorvos under the TRI
totaled 118 kg (US Environmental Protection
Agency, 1997).
1
Heat capacity at constant pressure [J mol 1 K
Heat capacity at constant volume [J mol 1 K
Electrical energy per order
]
1
]
EE/O
GC/ECD Gas chromatograph/electron capture detector
IC
InC
k
MDL
P
Ion chromatograph
Inorganic carbon
Polytropic index
Method detection limit
Power [W]
Treatment time [min]
Total carbon
Total organic carbon
Toxics release inventory
Volume treated [L]
t
TC
TOC
TRI
V
Adsorption to soil and volatilization of dichlorvos
do not occur significantly (Howard, 1991). The
compound can possibly leach into the groundwater
due to a water solubility of 7.2 Â 10 2 M and a log
INTRODUCTION
Kow of 1.16 (Howard, 1991). The Henry’s Law
constant (9.58 Â 10 7 atm m3 mol
at 258C) indi-
1
Dichlorvos (O-2,2-dichlorovinyl-O,O-dimethyl phos-
phate}C4H7Cl2O4P}CAS Registry No. 62-3-7) is a
chlorinated organophosphate insecticide which is
released into the environment through its manufac-
cates that dichlorvos will not volatilize appreciably
from the aqueous phase (Howard, 1991). In all
environments, hydrolysis is the main pathway for
degradation of dichlorvos. In environmental waters,
the half-life of dichlorvos is between 20 and 80 h at
pH values between 4 and 9; faster degradation occurs
at higher pH values (Howard, 1991). Biodegradation
*Author to whom all correspondence should be addressed.
Tel.: +1-765-494-2409; fax: +1-765-496-1988; e-mail:
hua@ecn.purdue.edu
665

666
Jennifer D. Schramm and Inez Hua
has also proven to be important (Lamoreaux and dichlorvos, and to propose a reaction pathway for
Newland, 1978; Lieberman and Alexander, 1983). dichlorvos decomposition.
Dichlorvos degrades in the atmosphere by vapor-
phase reactions with hydroxyl radical and possibly
ozone. The half-life in the atmosphere is as low as 2 d
and as high as 320 d (Howard, 1991). Direct
photolysis of dichlorvos in the atmosphere does not
occur (Howard, 1991). Treatment methods such as
biodegradation (Lamoreaux and Newland, 1978;
Lieberman and Alexander, 1983), ozonolysis (Koga
et al., 1992), Fenton’s reagent (Lu et al., 1997; Lu
et al., 1999), and photocatalysis (Harada et al., 1990;
Lu et al., 1993; Lu et al., 1994; Lu et al., 1995;
Mengyue et al., 1995) have been applied to the
MATERIALS AND METHODS
Dichlorvos (98%, Supelco, Bellefonte, PA) and hexanes
(pesticide grade, Fisher Scientific, Pittsburgh, PA) were used
as received. Potassium hydrogen phthalate, sodium hydro-
gen carbonate, and sodium carbonate were dried and
desiccated as directed by Standard Methods (APHA,
1995). Sodium chloride, potassium dihydrogen phosphate,
formic acid (88%) and dimethyl phosphate (ACROS
Organics, NJ) were used as received.
Solutions of dichlorvos were made with reagent grade
water (R ¼18 MO cm 1) from a NANOpure Ultrapure
decomposition of dichlorvos. This study introduces Water System (Barnstead, Dubuque, IA). These solutions,
stored in the dark until just prior to sonication, were utilized
sonolysis as a possible complement to the aforemen-
no more than 12 h after they were made to ensure that no
tioned treatment methods.
degradation occurred prior to sonication. Each experiment
was performed with 800 mL of 5 Â 10 4 M dichlorvos
Ultrasonic irradiation has been investigated in the
transformation of various environmentally important
organic compounds, including carbon tetrachloride
(Francony and Petrier, 1996; Hua and Hoffmann,
1996; Hung and Hoffman, 1998), humic acids
(Nagata et al., 1996), methyl tert-butyl ether (Kang
solution. A UES1.5-660 Pulsar (Ultrasonic Energy Systems,
Panama City, FL) generator and ultrasound transducer
were used with a water-jacketed glass vessel (The Custom
Glass Shop, Vineland, NJ) for a batch system as shown in
Fig. 1. Cooling water at 128C was supplied by a refrigerated
circulator (Fisher Scientific). Solution temperature was
and Hoffman, 1998; Kang et al., 1999), atrazine monitored by a thermocouple thermometer with a stainless
steel probe (Cole Parmer, Niles, IL). The temperature of the
solution was held at 20 Æ 68C during the experiments. Total
(Koskinen et al., 1994; Petrier et al., 1996), parathion
(Kotronarou et al., 1992), and carbofuran (Pfalzer,
1998). Sonochemistry results from acoustic cavita-
acoustic power output was determined by a calorimetry
method (Mason, 1991). Argon, Oxygen, or a mixture of the
tion, which is the formation and collapse of cavita-
tion bubbles in response to ultrasonic waves.
Extreme temperatures and pressures exist within the
collapsing gas bubbles (Mason and Lorimer, 1991),
leading to pyrolytic reactions and the formation of
two gases (60/40 Ar/O2 (v/v)) was dispersed into the system
by a glass-fritted diffuser. The sparge gas flow rate was
100 mL min 1. Gas was sparged into the system for at least
20 min prior to and during the experiment to allow for
saturation with gas. The pH of the samples was measured
with a Corning 320 pH meter during experiments. The pH
free-radical species. The conditions within the bubble decreased from ꢀ 3.3 to ꢀ 2.7 during the experiments.
Once the solution was saturated with gas, a zero sample
are so extreme that the bubbles emit light, a
(5 mL) was taken, and sonication commenced. During
sonication, samples were taken every 15 min (or every
10 min with the Argon/Oxygen mixture sparge gas) until at
phenomenon known as sonoluminescence (Young,
1976; Crum, 1994; Putterman, 1995). An important
reaction is the pyrolysis of water vapor to form
hydroxyl radical and hydrogen atoms. Pyrolysis and
radical reactions dominate inside the bubble and at
the liquid–bubble interface. Any radicals that have
not been scavenged in these regions as well as
least 90% degradation was achieved. Note that the
transducer was designed by UES to continuously cycle
through ‘‘on-off’’ periods. For every one minute of
operation time, the solution was irradiated for 48.9 s. Thus,
the effective sonication time appears in all figures and
calculations. Each sample was placed in an 8 mL glass vial
hydrogen peroxide formed from the combination of and extracted with 2 mL of hexane immediately after
sampling. The extraction mixture was agitated for at least
two hydroxyl radicals will be available for reactions
2 min to ensure contact between the water and hexane
in the bulk liquid phase (Henglein, 1987; Misik et al.,
1995). Hydrolysis reactions can also be enhanced,
layers. Three replicate experiments were performed for each
reactor variable (e.g., sparge gases and power). Three
particularly for compounds that partition to the
bubble interface (Hua et al., 1995b; Tuulmets and
Raik, 1999).
Previous studies of the decomposition of dichlor-
vos by advanced oxidation technologies have iden-
replicate experiments were also executed for the study of
reaction intermediates as well as total organic carbon
(TOC). All samples were analyzed at least in duplicate.
The analysis of dichlorvos was performed with a Hewlett-
Packard Model 5890 Series II Gas Chromatograph/Electron
Capture Detector (GC/ECD) equipped with an HP-5
tified the mineralization products chloride and (30 m  0.32 mm  25 mm, 5% phenylmethylsilicone) col-
umn and operated in the split mode (split ratio=50 : 1).
phosphate but have not identified reaction inter-
One microliter of the hexane layer was injected into the GC
with a temperature program of 1208C for 3 min, 108C per
mediates (Harada et al., 1990; Lu et al., 1993; Lu
et al., 1995; Mengyue et al., 1995; Lu et al., 1997). A
minute to 1508C, and 1508C for 3 min. The detector
primary objective of this study was to identify and
quantify key intermediates in the transformation of
dichlorvos during sonolysis. Other objectives were to
ascertain the impact of acoustic output power and
sparge gas on the sonolytic decomposition kinetics,
temperature was 2808C, and the injector temperature was
2508C. The retention time of dichlorvos at these conditions
was 8.9 minutes with a method detection limit (MDL) of
5.0 Â 10 6 M. Calibration standards ranging from
1.0 Â 10 5 M to 5.0 Â 10 4 M were prepared with reagent
grade water, extracted with hexane in the same ratio as the
to demonstrate the partial mineralization of samples, and analyzed with the GC so that a calibration

Ultrasonic irradiation of dichlorvos
667
Fig. 1. Experimental apparatus. Diameter of emitting surface=3 cm. Volume of solution=800 mL.
Frequency=500 kHz, and power ranges from 0 to 161 W. Transducer and generator manufactured by
Ultrasonic Energy Systems. Glass vessel manufactured by The Custom Glass Shop.
curve could be obtained each time the GC was operated. phosphate were prepared according to Standard Methods
Once a calibration curve was obtained, the areas of the (1995). Stock solutions of 2.8 Â 10 2 M for chloride ion,
peaks for each sample were correlated to a concentration.
The TOC was analyzed with a Shimadzu model TOC-
5000A Total Organic Carbon Analyzer. Total carbon (TC)
and inorganic carbon (InC) stock solutions (0.083 M
1.1 Â 10 2 M for phosphate ion, 2.2 Â 10 2 M for formate
ion, 8.0 Â 10 3 M for dimethyl phosphate ion were pre-
pared in reagent grade water. The MDL was 5.2 Â 10 5 M
for chloride, 5.0 Â 10 6 M for phosphate, 2.5 Â 10 5 M for
carbon) were prepared according to Standard Methods formate, and 9.8 Â 10 6 M for dimethyl phosphate. The
(APHA, 1995). A TC calibration curve was obtained by calibration standards were then prepared from dilutions of
analyzing solutions with concentrations of 0, 8.3 Â 10
4
,
the stock solution, and a calibration curve for each ion was
obtained for each set of new samples. The retention times of
3
2.08 Â 10
,
and 4.17 Â 10 3 M. Concentrations of 0,
8.3 Â 10 5, 4.16 Â 10 4, and 8.3 Â 10 4 M comprised the each ion were as follows: 2.3 min for dimethyl phosphate,
InC calibration curve. The MDL for TOC was 4.0 Â 10–5 M.
2.5 min for formate, 3.1 min for chloride, and 10.0 min for
Samples of 5 mL were placed into autosampler vials without phosphate.
any extraction and analyzed for both TC and InC. The TOC
was determined by calculating the difference between the TC
and the InC.
RESULTS
Ions were analyzed with an ion chromatograph (IC)
which consisted of the following (all components were
Results from the first series of experiments revealed
the influence of the total acoustic power output and
the dissolved gas composition on the decomposition
of dichlorvos. The kinetics of dichlorvos decomposi-
tion were best described by a first-order decay. A plot
of ln(C/Co) vs. sonication time, Fig. 2, shows that the
manufactured by Dionex): an Automated Sampler (Model
ASM-2), a 4400 Integrator, a CD20 Conductivity Detector,
a GP40 Gradient Pump, an IonPac AS4A 4 mm column,
AG4A 4 mm Guard Column, and an ASRS-1 4 mm self-
regenerating suppressor. The eluent (80% 10 mM (NaH-
CO3+Na2CO3) solution and 20% reagent grade water) was
pumped at a flow rate of 2.0 mL min 1. Five mL samples
were extracted with 2 mL of hexane in order to remove first order approximation of the kinetics fits quite
organics that could foul the column. The aqueous layer was
well. Table 1 lists the first-order rate constants
then placed in an autosampler vial for analysis. The
(mean Æ standard deviation) obtained from the ki-
following compounds were identified and quantified by
netic experiments. The rate at which dichlorvos was
IC: chloride, phosphate, dimethyl phosphate, and formate.
destroyed during sonication at 500 kHz increased
when the total acoustic power output was increased
Since the pKa of the dimethyl phosphate is 1.25 (Edmund-
son, 1988), partitioning of dimethyl phosphate into the
organic phase was not a concern because the compound will
exist primarily as an ion. The ions were identified by spiking
a sample with a known amount of standard ion solution and
comparing the peak areas of the unspiked and spiked
1
from 86 W (k ¼0.018 Æ 0.001 min
) to 161 W
(k ¼0.037 Æ 0.002 min 1). There is also a significant
enhancement of the first order decomposition rate
samples. Sodium chloride and potassium dihydrogen constant when a mixture of Ar and O2 gas is used to

668
Jennifer D. Schramm and Inez Hua
saturate the solution (k ¼0.079 Æ 0.005 min 1). This
The TOC of the reaction solution was monitored
is consistent with previous investigators’ results during sonication to determine the impact of
(Hua et al., 1995a; Drijvers et al., 1998).
dissolved gas on dichlorvos mineralization. Figure 5
The anionic reaction intermediates and products shows the decrease in the TOC concentration with
(formate, dimethyl phosphate, and chloride) were sonication time. The decrease in the TOC concentra-
determined by IC. Figures 3(a) and (b) presents the tion can be assumed to be equal to the concentration
accumulation of formate and dimethyl phosphate, of CO2 produced. The production of CO2 ranges
4
respectively. Dimethyl phosphate is of particular from 4.0 Â 10
to 6.5 Â 10 4 M after 48.9 min of
interest, because its rate of accumulation reflects the sonication, depending on the sparge gas. The
rate of the initial transformation step(s). With O2 and conversion of the initial TOC to CO2 at 161 W and
Ar/O2 sparge gases, the dimethyl phosphate ion 48.9 min of sonication was 19.5% for Ar, 21.5% for
concentration reached a plateau of approximately O2, and 31% for the Ar/O2 sparge gas.
3.6 Â 10 4 M before declining. The concentration of
formate ion increased quite rapidly in the case of O2
and Ar/O2 sparges to approximately 3.1 Â 10 4 M,
DISCUSSSION
while the Ar sparge did not exhibit the rapid
accumulation of formate ion.
Based upon our experimental results, and the
results of other investigations, a pathway is proposed
The mineralization products phosphate and chlor-
for dichlorvos mineralization (Fig. 6) during sonica-
ide were also quantified, as shown in Fig. 4(a) and
tion. The complete mineralization would result in
(b). Chloride ion, in the presence of O2 and Ar/O2,
production of carbon dioxide, phosphate, chloride,
and water by the following reaction:
rapidly approached the maximum theoretical con-
centration of 1.0 Â 10 3 M. However, the concentra-
tion of chloride did not reach the theoretical
maximum with the Ar sparge. Phosphate ion
accumulated slowly, regardless of the sparge gas.
C4H7Cl2O4P þ 9=2O2! 4 CO2þH2O
ð1Þ
þ2 Cl þPO34 þ5 Hþ
Products of dichlorvos sonolysis are defined as
anything on the right hand side of equation (1).
Any other compounds identified during decomposi-
tion of dichlorvos are considered reaction intermedi-
ates. The reaction intermediates and products that
are boxed in Fig. 6 were identified and quantified in
this study by total organic carbon analysis for CO2
and ion chromatography for dimethyl phosphate,
phosphate, formate, and chloride.
The bond most susceptible to hydrolysis is the
vinyl phosphate bond (Pearson and Songstad, 1967;
Mabey and Mill, 1978). Hydrolysis results in
dimethyl phosphate (C2H4PO4 ) and dichlorovinyl
alcohol (C2H2Cl2O) (Hodgson and Casida, 1962;
Patil and Shingare, 1994; Khandelwal and Wedzicha,
1998). This is of particular interest because hydrolysis
reactions are accelerated during sonolysis (Hua et al.,
1995b; Tuulmets and Raik, 1999). It has been
postulated that a layer of supercritical water exists
at the cavitation bubble interface and contributes to
the accelerated hydrolysis rates observed during
sonolysis (Hua et al., 1995b).
Fig. 2. Impact of saturating gas and power on the
Dimethyl phosphate can further react to form
disappearance of dichlorvos at 500 kHz. Lines through
monomethyl phosphate (CH4PO4 ) and methanol
points are first-order approximations. Initial dichlorvos
concentration=5.0 Â 10 4 M, and sparge gas flow rate is
(CH3OH) (Bunton et al., 1960; Rapp et al., 1997).
1
.
100 mL min
Monomethyl phosphate can hydrolyze to phosphate
Table 1. Observed first order rate constants for sonication of dichlorvos
1
)
Gas sparge
Acoustic power (W)
Rate constant (min
R2 value
Argon
Argon
Argon
Oxygen
Argon/oxygen (60/40 v/v)
86
124
161
161
161
0.018 Æ 0.001
0.025 Æ 0.002
0.037 Æ 0.002
0.031 Æ 0.003
0.079 Æ 0.005
0.99
0.99
0.99
0.97
0.98

Ultrasonic irradiation of dichlorvos
669
Fig. 3. Impact of saturating gas on the accumulation of (a) formate and (b) dimethyl phosphate during
sonication of dichlorvos at 161 W and 500 kHz. Initial dichlorvos concentration=5.0 Â 10 4 M, and
1
sparge gas flow rate is 100 mL min
.
Fig. 4. Impact of saturating gas on the accumulation of (a) phosphate ion and (b) chloride ion during
sonication of dichlorvos at 161 W and 500 kHz. Initial dichlorvos concentration=5.0 Â 10 4 M, and
1
sparge gas flow rate is 100 mL min
.
(PO34 ) and methanol (Bunton et al., 1958; Rapp formic acid (CH2O2), which then ionizes to formate
et al., 1997; Florian and Warshel, 1998). The (CHO2 (Downes and Sutton, 1973; Toy and
)
methanol formed from hydrolysis of dimethyl Stringham, 1993; O’Shea et al., 1998). Both dimethyl
phosphate and monomethyl phosphate can react phosphate and phosphate are identified and
with two hydroxyl radicals in multiple steps to create quantified in this study. The dimethyl phosphate

670
Jennifer D. Schramm and Inez Hua
accumulates to ꢀ 3.8 Â 10 4 M with the O2 and Ar/ dimethyl phosphate with the Ar sparge did not reach
O2 sparges before reaching a plateau and decreasing. this plateau. Phosphate ion accumulates very slowly,
At the plateau, the rate of dimethyl phosphate indicating that demethylation of the dimethyl- and
formation is equal to its degradation rate. The monomethyl-phosphate ions limits the rate of com-
plete mineralization of dichlorvos.
Dichlorovinyl alcohol is unstable and will quickly
tautomerize to dichloroacetaldehyde (C2H2Cl2O)
(Hodgson and Casida, 1962; Patil and Shingare,
1994; Khandelwal and Wedzicha, 1998). Dichloro-
acetaldehyde reacts with hydroxyl radical to form
dichloroacetic acid (C2H2Cl2O2). This is similar to
the sonolytic reaction of acetaldehyde to form acetic
acid (Toy and Stringham, 1993). The pKa of
dichloroacetic acid is 1.3 (Bowden et al., 1998),
indicating that during the sonication experiments
performed, it would be present primarily as the
conjugate base, dichloroacetate ion (C2H2Cl2O ).
This ion will not partition into the bubble, so
pyrolysis reactions are unlikely. Dichloroacetate
most likely hydrolyzes to a chlorohydroxyacetate
intermediate (C2H2ClO3 ), which is very unstable
(Diefallah et al., 1981; Diefallah et al., 1982). This
intermediate eliminates hydrochloric acid to form
glyoxylate (C2HO3 ) (Diefallah et al., 1981; Diefallah
et al., 1982). The glyoxylate can then hydrolyze to
formate and carbon dioxide (Carraway et al., 1994).
Furthermore, glyoxylic acid decomposes rapidly
during ozonolysis (Caprio et al., 1987). Because of
the similarity between ozonolysis pathways and
sonolysis pathways, glyoxylic acid/glyoxylate is not
expected to accumulate. Small amounts of formic
acid are observed during ozonolysis of glyoxylate
(Caprio et al., 1987), and because some free-radical
Fig. 5. Conversion of organic carbon to CO2 during
sonolysis of dichlorvos at 161 W and 500 kHz. Initial
dichlorvos concentration=5.0 Â 10 4 M, and sparge gas
flow rate is 100 mL min 1. Note that the most rapid
conversion occurs with a mixture of dissolved gases.
Fig. 6. Proposed pathway for the sonolytic mineralization of dichlorvos.

Ultrasonic irradiation of dichlorvos
671
pathways present during ozonation are similar to of sonication at the bench scale to that of photo-
those of sonolysis, it is possible that oxidation plays catalysis. These calculations do not necessarily reflect
some role in transforming glyoxylate to formic acid the energy required for scale-up of either technology.
during sonolysis.
The equation for EE/O is as follows:
Chloride and formate ions as well as carbon
dioxide are identified. The chloride ion rapidly
accumulates for the O2 and Ar/O2 sparge gases,
nearly reaching the maximum theoretical concentra-
tion of 1.0 Â 10 3 M after 1 h of sonication. Again,
the Ar sparge lags behind, indicating that a barrier to
dechlorination exists. Very little formate accumulates
when Ar is the sole sparge gas. In fact, for all
conditions of sparge gas composition and sonication
time investigated, a decrease in formate concentra-
tion was not observed. Formate either accumulates
(Ar) or accumulates and levels off (Ar/O2 and O2).
Thus, the decomposition rate of formate is slower
than its production rate from larger, precursor
organic compounds. Formate and other short-chain
organic acids are generally accepted as ‘‘refractory’’
compounds which limit complete mineralization
during oxidative processes (Bjerre and Sorensen,
1992; Shende and Mahajani, 1997; Shende and
Levec, 1999).
PÂtÂ1000
EE=O ¼
ð2Þ
VÂ60Âlogðci=cf Þ
where P is the power in kW, t is the treatment time in
min, V is the volume treated in L, ci is the initial
concentration in M, and cf is the final concentration
in M. The EE/O determined for sonication at the
optimum conditions from this study (Ar/O2 sparge,
wall power=660 W) was 400 kWh order–1 m–3. Lu
et al. (1994) studied the decomposition of dichlorvos
in a fixed film illuminated TiO2 reactor with the
addition of 1 Â 10 4 M hydrogen peroxide (pH=4,
T ¼308C,
initial
dichlorvos
concentration=
2.6 Â 10 5 M, volume=1 L, power=20 W). Harada
et al. (1990) added 1.2 Â 10 2 M hydrogen peroxide
to a suspension of TiO2 (initial pH=3.1, initial
dichlorvos
me=0.25 L, power=500 W). The EE/O for the
concentration=1.0 Â 10 3 M,
volu-
photocatalysis studies was 83.5 kWh order 1 m
3
for Lu et al. (1994) and 996.6 kWh order 1 m
for
3
The observed differences in the results of all
compounds with varying sparge gas occur due to
differences in the properties of the gases and their
behavior in the cavitation bubble. The temperature
of bubble collapse is dependent on the specific heat,
thermal conductivity, and solubility of the sparge
gas. One factor affecting the temperature of bubble
collapse is the polytropic index, k, which is the ratio
of the two heat capacities, Cp=Cv (Mason and
Lorimer, 1991). The polytropic index reflects the
amount of heat released when a gas is compressed.
Thus, for an imploding bubble, a larger value of k
indicates greater heat release and, therefore, higher
temperatures within the bubble. The k of Ar is higher
than that of O2, so the temperature of bubble
collapse will be higher for Ar than it is for O2. This
indicates that thermolysis reactions will dominate
with the Ar sparge more than with the O2 sparge.
With O2, even though the temperature of bubble
collapse is lower, there are additional oxidants and
radicals that can be formed from thermolysis of O2,
such as oxygen atom, hydroperoxyl radical, and ozone
(Hua et al., 1995a). Thus, oxidizing radical reactions
usually dominate near the bubble of oxygenated
solutions. The mixture of the two sparge gases
increases the k from that of pure O2, thus increasing
the temperature of bubble collapse while still adding
the oxidizing radicals from O2 thermolysis.
Harada et al. (1990). Thus, sonication exhibits an
intermediate EE/O value with respect to the other
bench scale advanced oxidation process (AOP)
systems discussed in this article.
The quantification of reaction intermediates and
products allowed for the calculation of mass balances
for carbon, chlorine, and phosphate. The equation
for carbon recovery was
%C recovered ¼
DDVPCt þ DMPtC þ FormatetC þ CO2;t
"
#
ð3Þ
100
DDVPC0
where DDVPCt is the concentration of dichlorvos at
time t (M as C), DMPCt is the concentration of
dimethyl phosphate at time t (M as C), FormateCt is
the concentration of formate at time t (M as C),
CO2;t is the concentration of carbon dioxide at time t
(M as C, based on TOC measurements), and DDVP0C
is the initial concentration of dichlorvos (M as C).
Fig. 7(a) shows the change in carbon recovery as
sonication proceeds.
The chlorine recovery was determined as
"
#
DDVPCt I þ Clt
DDVPC0 I þ Cl0
%Cl recovery
100
ð4Þ
where DDVPCt l is the concentration of dichlorvos at
time t (M as chloride), Clt is the concentration of
chloride ion (M) at time t, DDVPC0 l is the initial
dichlorvos concentration (M as chloride), and Cl0 is
the initial concentration of chloride ion (M). This is a
chlorine recovery because it includes all of the
chlorine in solution at each time. The change in
chlorine recovery with sonication time is shown in
Fig. 7(b).
The observed results for degradation kinetics
compare well with other technologies that have been
investigated for aqueous dichlorvos degradation in
small-scale batch systems. An electrical energy per
order (EE/O) calculation (Bolton et al., 1996) was
performed for the comparison of sonication with
photocatalysis. The purpose of performing these
calculations was to compare the energy requirement

672
Jennifer D. Schramm and Inez Hua
where DDVPPt O4is the concentration of dichlorvos at
time t (M as phosphate), DMPPt O4 is the concentra-
tion of dimethyl phosphate at time (M as
t
phosphate), PO34;t is the concentration of phosphate
at time t (M), DDVPP0O4 is the initial dichlorvos
concentration (M as phosphate), DMPP0O4 is the
initial concentration of dimethyl phosphate (M as
phosphate), and PO34;0 is the initial concentration of
phosphate (M). Figure 7(c) shows the phosphate
recovery as a function of elapsed time.
The O2 sparge gas resulted in the best recovery of
carbon, chlorine, and phosphate while the Ar and
Ar/O2 sparges resulted in much lower recoveries for
each element. The products and intermediates from
Fig. 6 that were identified account for at least 50% of
the mass balances on carbon, chlorine and phosphate
regardless of gas sparge. Thus, the mass balances
demonstrate that dichlorvos is primarily transformed
into more innocuous compounds. The trend for each
recovery is that it decreases with time. This trend is
most likely due to the fact that other compounds are
proposed in the pathway that were not identified and
quantified. Thus, in the mass balance, these com-
pounds account for part of the element that is
unrecovered. Furthermore, there is a certain amount
of analytical error (variation) associated with the
measurements, which propagates through the calcu-
lations. It is likely that analytical error accounts for
some of the calculated recoveries greater than 100%.
Fig. 7. Impact of saturating gas on the recovery of (a)
carbon, (b) chlorine, and (c) phosphate during the sonica-
tion of dichlorvos at 500 kHz and 161 W. Initial dichlorvos
concentration=5.0 Â 10 4 M, and sparge gas flow rate is
1
.
100 mL min
The recovery of total phosphate was calculated by
the following equation:
CONCLUSION
%PO34 recovery ¼
"
#
DDVPPt O4 þ DMPPt O4 þ PO43;t
DDVPP0O4 þ DMPP0O4 þ PO43;0
Dichlorvos quickly decomposes when exposed to
ultrasonic irradiation. Increasing the output power
increases the rate of dichlorvos decomposition. The
ð5Þ
100

Ultrasonic irradiation of dichlorvos
673
Ar/O2 sparge gas with a power of 161 W gives Drijvers D., Langenhove H. V. and Vervaet K. (1998)
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oxidation of dimethyl phosphate and of formate.
However, the presence of oxygen in solution facil-
itates the mineralization of dichlorvos and its
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Acknowledgements}The authors gratefully acknowledge
financial support from the US Department of Energy
(DOE Grant Number DE-FG07-96ER14710). Jennifer
Schramm was supported by the US Department of
Education (GAANN Fellowship) for part of this project.
Rebeca Marquez performed some of the ion chromato-
graphy analyses.
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