Sonolysis of 2,4-dichlorophenoxyacetic acid in aqueous solutions. Evidence for 'OH-radical-mediated degradation

Article abstract of DOI:10.1021/jp003478y

2,4-dichlorophenoxyacetic acid (2,4-D) undergoes efficient degradation when an U2- or Ar-saturated aqueous solution is subjected to high-frequency (640 kHz) sonolysis. 2,4-dichlorophenol, hydroquinone, and catechol are major reaction intermediates common

Full text of DOI:10.1021/jp003478y

3176
J. Phys. Chem. A 2001, 105, 3176-3181
Sonolysis of 2,4-Dichlorophenoxyacetic Acid in Aqueous Solutions. Evidence for
•
OH-Radical-Mediated Degradation
Julie Peller,†,‡ Olaf Wiest,* and Prashant V. Kamat*
,‡
,†
Radiation Laboratory and Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556
ReceiVed: September 25, 2000; In Final Form: December 7, 2000
2
2,4-dichlorophenoxyacetic acid (2,4-D) undergoes efficient degradation when an O - or Ar-saturated aqueous
solution is subjected to high-frequency (640 kHz) sonolysis. 2,4-dichlorophenol, hydroquinone, and catechol
are major reaction intermediates common to various experimental conditions. The similarities between the
•
reaction intermediates of sonolytic and radiolytic reactions indicate OH radical as the primary reactive species
responsible for 2,4-D degradation. Very little 2,4-D degradation occurs if the sonolysis is carried out in the
•
presence of the OH radical scavenger tert-butyl alcohol, also indicating that little or no pyrolysis of the
•
compound occurs. Most of the 2,4-D eventually forms oxalic acid, which, unlike other OH-mediated oxidation
methods, is not easily mineralized with high-frequency ultrasound.
•
Introduction
,4-Dichlorophenoxyacetic acid (2,4-D) is a common herbi-
and a sparging gas. To substantiate the role of the OH radical
in the sonolytic degradation of 2,4-D, we also employed γ
radiolysis in our investigation, which allows for oxidative
conditions in which the OH radical is the exclusive reactive
2
cide. The fate of this compound in the environment is of great
interest because of its high-quantity usage and its chlorinated
aromatic nature. Recent experiments have shown that 2,4-D
residues from lawn applications are present in homes and that
the quantities and locations of the herbicide are dependent on
•
species.
When ultrasound waves are introduced into an aqueous
solution, cavitation occurs, as bubbles rapidly form and subse-
quently collapse inward from the buildup of pressure through
rarefaction/compression cycles. Under these sonolytic condi-
tions, high temperatures and pressures accompany the implosion
of the cavitation bubbles. The specific conditions created during
implosion depend on a few variables, namely, the frequency
and power of the sound waves, the dissolved gas, and the
1
traffic patterns in the homes. Although herbicide applications
are designed to deposit on plants or grass, inadvertent trans-
2
location occurs to nearby areas or indoor residential areas.
Because herbicides are used in large quantities on farms and
lawns, they contaminate water supplies through runoff and other
means. Unfortunately, 2,4-D breaks down slowly in water, with
half-lives reported from 6 to 170 days, depending on environ-
mental conditions.3-5 The toxicity data on 2,4-D is limited, but
2
8-31
solute(s) present in the medium.
Sonolysis can induce the degradation of organic compounds
along two different pathways. Upon collapse of the cavitation
bubble, volatile compounds are vaporized and destroyed via
pyrolytic or combustive reactions. Hydrocarbons and other
volatile compounds form intermediates and products that mirror
6
it is considered moderately toxic toward aquatic life and the
LD50 for mice is 368 mg/kg.
3
2
Advanced oxidation processes, namely, photocatalysis, radi-
olysis, and UV/H2O2, have been used to degrade a variety of
environmentally hazardous compounds (see, for example, refs
3
3-36
those of pyrolysis or combustion.
The second reaction
7
-17). Several recent studies have used some of these methods
pathway is a chemical process at the interface of the bubble,
induced by hydrogen atoms and hydroxyl radicals formed from
the homolysis of water. Solute molecules that do not diffuse
to break down 2,4-D into nontoxic substances and to eventually
mineralize the herbicide.18-23 Often these breakdown sequences
were found to introduce new chemical intermediates/products
that are more toxic than the parent compound. For example,
one of the main intermediates in the breakdown of the herbicide
•
into the bubble are likely to undergo radical attack by OH or
•
37
H. The concentration of hydroxyl radicals at the interface is
-
2
believed to be as high as 1 × 10 M upon bubble collapse.
2
,4-D is 2,4-dichlorophenol (2,4-DCP). This intermediate
Hydrogen radicals are present in about a 1:4 ratio to hydroxyl
3
8
exhibits higher toxicity than 2,4-D as determined from the
microtox bioassay.
aromatic compounds have also been reported using photo-
radicals.
2
2,24
Along with 2,4-DCP, other chlorinated
In oxygen-saturated aqueous solutions, oxidizing radicals are
formed according to the following series of reactions:39
19,21
18,25
catalysis,
the Fenton-type oxidation,
and UV/hydrogen
peroxide oxidation systems.2 Whereas most of these studies
employ UV light as an activation source to initiate the oxidation,
the advanced oxidation process of sonolysis utilizes sound waves
6,27
•
•
H O f H + OH
(1)
(2)
2
•
•
•
1
2
H + O f HO f OH + O
2
2
2
*
Author to whom correspondence should be addressed. Tel.:
2•
OH f H O
(3)
(4)
(
219) 631-5411. Fax: (219) 631-8068. E-mail: pkamat1@nd.edu. Web:
2
2
http://www.nd.edu/∼pkamat.
†
Notre Dame Radiation Laboratory.
Department of Chemistry and Biochemistry.
•
‡
2HO f H O + O
2
2
2
2
1
0.1021/jp003478y CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/17/2001

Sonolysis of 2,4-Dichlorophenoxyacetic Acid
J. Phys. Chem. A, Vol. 105, No. 13, 2001 3177
Argon and molecular oxygen are the gases often used in
aqueous sonochemistry because they have high specific heat
ratios and are conducive to radical formation. When Ar is used
as the sparging gas, OH radicals are produced with greater
3
8
efficiency. The main source of H2O2 in the aqueous system
is from the recombination of hydroxyl radicals (reaction 3).
Solute present at the interface of the bubble will scavenge the
hydroxyl radicals and decrease the formation of hydrogen
peroxide.38 The Ar-sparged sonolytic system is never anoxic
32
because of the thermolysis of water vapor.
As previously mentioned, hydrogen atoms are present in about
a 1:4 ratio to hydroxyl radicals. The participation of hydrogen
atoms in reaction 5 accounts for some of the difference, and
notably, hydroxyl radicals are produced. Equation 6 shows
another fate for hydrogen atoms.40
Figure 1. Ultrasound sample cell. The transducer is separated from
the sample solution by a polyethylene film. The lower portion of the
glass holder and the transducer are submerged in ice water during
sonolysis experiments.
•
•
Analysis. The samples were taken out at different time
intervals during γ irradiation or sonolysis. The progress of the
degradation was monitored using a Shimadzu 3101PC UV/vis
spectrometer. Intermediates were identified by comparison with
authentic samples using a Waters HPLC system (Milennium
H + H O f H + OH
(5)
(6)
2
2
•
•
H + H f H2
In the present study, we demonstrate the feasibility of
sonolysis as an effective method for the destruction of 2,4-D.
By comparing intermediates and products of the sonolytic
degradation with those determined in γ-radiolysis experiments,
we elucidate the role of OH radical in the breakdown of 2,4-D
in aqueous solutions.
2
010, Waters 717 plus Autosampler, Waters 600 Controller
Solvent Pump) with three separate columns: Alltech Econosil
C18, 10 µm (250 × 4.6 mm) and Alltech Econosphere C8, 5
µm (150 and 250 × 4.6 mm). A solvent gradient consisting of
methanol, water, and water containing 1% acetic acid was
utilized with all three columns. A photodiode array detector
monitored the 200-400 nm range. Intermediates were identified
by comparison of their retention times and spectra with those
of authentic samples. Detection of short-chain carboxylic acids
was achieved using a Phenomenex AQUA, 5µ, C18 column.
The mobile phase was 20 mM potassium phosphate at a pH of
•
Experimental Section
Materials and Reagents. 2,4-Dichlorophenoxyacetic acid
Aldrich, 99%), 2,4-dichlorophenol (Aldrich, 99%), catechol
Aldrich, 99%), chlorohydroquinone (Aldrich, 85%), sodium
hydroxide (Fisher, ACS), hydrochloric acid (Fisher, ACS),
-chlorocatechol (TCI, 99%), tert-butyl alcohol (Fisher, ACS),
and hydroquinone (Aldrich, 99%) were used as supplied.
Chlorohydroquinone (Aldrich) was recrystallized from methanol/
water. Unbuffered aqueous solutions were prepared using
Milli-Q purified water. The pH of the solutions was adjusted
by adding either HCl or NaOH. A small drop in pH (0.5-2.0
pH units) was noted at the end of the sonolysis experiments.
High-purity N2O, O2, and Ar gases were supplied by Mittler
Supply Co., South Bend, IN.
(
(
2.5. Chloride ion concentrations were monitored using a Dionex
ion chromatographer fitted with an Ionpac AS12A column
preceded by an AG12A guard column. An eluent solution
consisting of 2.7 mM NaCO3/0.3 mM NaHCO3 was set at an
isocratic flow rate of 1.5 mL/min. Mineralized carbon was
measured using a Shimadzu total organic carbon analyzer, model
TOC-5050, equipped with an ASI-500A autosampler. All
experiments were carried out at least in duplicate. The reported
values are within the experimental error of 5%.
4
Results and Discussion
γ-Radiolysis. γ-Radiolysis experiments were conducted in
60
a 109 Shepherd, cobalt-60 ( Co) source. The aqueous solutions
were sparged with N2O(g) for 45 min in glass vials that were
capped with a rubber septum before γ irradiation. Samples were
taken at the indicated times. Fricke dosimetry was performed
to determine the dose rate. A dose rate of 5.3 krad/min was
determined using the following dosimetry solution: 0.40 mM
H2SO4, 10.0 mM FeNH4SO4.
Sonolysis. Sonolysis experiments were performed using an
ultrasound of 640-kHz frequency (Ultrasonics Energy Systems,
Panama City, FL). The cell has a volume capacity of ap-
proximately 600 mL and consists of a polyethylene base (PE
window) that is mounted about 4 cm above the transducer
Sonochemistry of 2,4-D in Aqueous Solutions. High-
frequency sonolytic systems have proven to be effective in the
2
9,30,36,41,42
degradation of organic compounds.
In the present
study, we were able to degrade 2,4-D in both Ar- and
O -saturated solutions using high-frequency (660 kHz) sonolysis.
2
Figure 2A shows the absorption spectra recorded during the
sonolysis of 0.20 mM 2,4-D in an Ar-sparged aqueous solution.
Figure 2B shows the decay of 2,4-D as monitored from the
absorption band at 285 nm under different sparging gas
conditions.
In the case of Ar(g) as the sparging gas, nearly 90%
degradation was achieved in less than 100 min. When oxygen
was used as the sparging gas, the degradation occurred at a
similar rate (lifetime of 29 min). The use of Ar(g) as the sparging
gas is expected to allow for the creation of higher temperatures
upon collapse of the cavitation bubbles as the temperature
attained inside the bubble depends on the average specific heat
ratio and the thermal conductivity of the dissolved gas.2 These
higher temperatures, in turn, create a more efficient system for
the production of hydroxyl radicals as measured by the
(
Figure 1). This configuration allows for the separation of the
transducer from the reaction mixture. The power output as
determined by the calorimetric method was approximately 50
W. Throughout the experiments, the transducer and the lower
portion of the cell remained submerged in an ice water bath.
The solutions were initially saturated with either oxygen or argon
and were continuously sparged with either gas throughout the
experiments. The average temperature of the solution during
sonolysis was 303 K.
8,29
3
8
terephthalate dosimetry. In our experiments, the sparging gas

3178 J. Phys. Chem. A, Vol. 105, No. 13, 2001
Peller et al.
TABLE 1: Sonolysis of 2,4-D and 2,4-DCP under Various
Conditions of pH and Sparging Gas
a
compound sparging gas
pH
lifetime (min)
2
2
2
2
2
2
2
,4-D
,4-D
,4-D
,4-D
,4-DCP
,4-DCP
,4-DCP
O
2
3.8
3.8
2.2
11.8
5.7
5.7
29
29
21
38
15
13
12
Ar
Ar
Ar
O
2
Ar
Ar
3.1
a
Values obtained from the first-order kinetic analysis of the decay
of the parent compound. The standard deviation in these measurements
is (2 min.
Figure 3. Sonolytic degradation of 0.20 mM 2,4-D in various pH
solutions using Ar as the sparging gas. The degradation was monitored
over a 2-h period: a ) pH 3.5, b ) pH 11.8, c ) pH 2.2.
Figure 2. (A) UV absorbance spectra of 0.20 mM 2,4-D recorded at
different times during 2-h sonolysis of the Ar(g)-sparged aqueous
solution: a ) 0, b ) 5, c ) 10, d ) 20, e ) 40, f ) 60, g ) 120 min.
(B) Sonolytic degradation of 0.20 mM 2,4-D in aqueous solution using
different sparging gases, Ar (0), O (b), and N O (4).
2
2
(viz., Ar and O2) did not have a significant effect on the rate of
degradation or on the intermediates that were formed.
The sonolytic degradation was quite effective at lower pH.
When an aqueous 2,4-D solution at pH 2.2 was subjected to
high-frequency ultrasound waves, the degradation occurred with
a lifetime of 21 min. Under alkaline conditions (pH ) 11.8),
2
,4-D remained in the solution for much longer, with a
degradation lifetime of 38 min. The difference in reactivity at
these two pH values arises from the difference in absorptivity
of the protonated and unprotonated forms of 2,4-D to the
ultrasonic bubbles.
Figure 4. Sonolytic breakdown of 0.20 mM 2,4-D (0) and the growth
and decay of the reaction intermediate 2,4-DCP (b) in aqueous solution
using Ar as the sparging gas. Inset shows the growth of the end product,
oxalic acid, during the sonolysis.
Because 2,4-D is a polar, nonvolatile compound, the sonochem-
ical reaction with the hydroxyl radical is expected to take place
at the cavitation bubble interface where the hydroxyl radical
32
concentration is a maximum. Recently, Greiser and co-
than that of the lower-pH solutions (Table 1 and Figure 3),
supports such an argument.
4
3
•
workers proposed that OH radicals are scavenged by solute
molecules at the bubble surface when the volatility of the solute
is low. They performed extensive studies on phenol in various
pH solutions. When phenol is in its ionized form, the rate of
degradation is lowest. This observation is in agreement with
earlier work reported by Currell et al.44 We found the same
type of reactivity with 2,4-D. The hydrophobic character of
One of the prominent intermediates in the sonolysis of 2,4-D
was 2,4-dichlorophenol (2,4-DCP). This primary intermediate
was detected as soon as we initiated the sonolysis of 2,4-D in
aqueous solution. The chlorinated phenol as the major reaction
intermediate has also been detected in the degradation of 2,4-D
1
9-23,27
using other advanced oxidation processes.
An important
2
,4-D is highest at very low pH conditions where the compound
observation concerning the chlorinated phenol intermediate in
these experiments was its short existence under ultrasonic
conditions. The concentration of 2,4-DCP never rose above 7%
of the 2,4-D concentration (Figure 4). The half-lives of 2,4-D
and 2,4-DCP under the various experimental conditions are
summarized in Table 1.
In addition to 2,4-DCP, we were also able to identify several
other aromatic intermediates using HPLC with comparison to
standards. The aromatic intermediates identified in the sonolytic
degradations of 2,4-D include 4-chlorocatechol, 2-chloro-
hydroquinone, 4-chlorophenol, catechol, hydroquinone and
is in its protonated form (pKa ) 2.9), and the degradation rate
of 2,4-D is highest at the low pH. In its protonated form, 2,4-D
exists in higher concentrations at the bubble surface and thus,
is more readily subjected to the OH radical attack. High-pH
solutions undergo sonolytic degradation at a much slower rate.
In higher-pH solutions, 2,4-D is present in its ionized form and
does not readily accumulate at the bubble interface. The ionized
form of the 2,4-D molecules does not adsorb as well to the
air/water interface as the protonated form. The observed lifetime
of 2,4-D at the high-pH conditions, which is considerably longer

Sonolysis of 2,4-Dichlorophenoxyacetic Acid
J. Phys. Chem. A, Vol. 105, No. 13, 2001 3179
Figure 6. Degradation of 0.20 mM 2,4-D in the presence of tert-butyl
alcohol (0.01-0.5%). The data show the percent degradation following
1 and 2 h of sonolysis using Ar as the sparging gas.
Figure 5. Radiolytic breakdown of 2,4-D (0) and intermediate
formation of 2,4-DCP (b), 4-chlorocatechol (4), and chloride ion (1)
6
0
using a Co source. The initial concentration of 2,4-D was 47 ppm,
and the solution was saturated with N O gas.
2
a free-radical-induced pyrolysis reaction mechanism. The tert-
•
butyl alcohol is able to scavenge OH radicals in the bubble
1
,2,4-benzenetriol. The nature of the chemical intermediates
•
and prevent the accumulation of OH radicals at the interface
formed during the ultrasonic destruction of the 2,4-D solutions
indicates a hydroxyl-radical-induced oxidation process.
γ-Radiolysis of 2,4-D Solutions. To independently assess
the role of hydroxyl radical in the oxidation process, we carried
out γ radiolysis of 2,4-D solutions in aqueous solution (pH )
of the bubble. In our experiments, varying concentrations of
tert-butyl alcohol were added to 0.20 mM 2,4-D solutions. The
results are summarized in Figure 6. In the presence of adequate
•
OH radical scavenger (0.05%), very little degradation of 2,4-D
took place. The small decrease in the concentration of 2,4-D
3
.5). In the radiolysis experiments, aqueous electrons were
•
with adequate scavenger was accompanied by the presence of
scavenged using dissolved N2O to achieve an exclusive OH-
radical reactive environment to study the oxidative degradation
processes.
•
2
,4-DCP and catechol, both products of OH-mediated reactions.
At lower concentrations of the tert-butyl alcohol scavenger
(
<0.05%), 9-24% of the 2,4-D was degraded during 60 min
-
•
H O + N O + e f N + OH + OH
(7)
of sonolysis. At these low tert-butyl alcohol concentrations,
2
2
2
aq
2
•
,4-D was able to compete for the OH radicals at the bubble
The G value for the disappearance of 2,4-D was calculated
for the initial 5 min of radiolysis and was determined to be 4.4.
This value is comparable to the G values of other aromatic
compounds under similar radiolytic conditions and suggests
surface. As the tert-butyl alcohol continued to degrade, more
hydroxyl radicals were available at the bubble interface for the
2,4-D to react. Another factor that also affects the rate of 2,4-D
degradation is the formation of volatile products from the tert-
butyl alcohol degradation that accumulate inside the bubble.
Such volatile products decrease the temperature inside the
•
45
direct interaction with OH, which has a G value of 6.0.
Once again, 2,4-DCP was the prominent intermediate in the
hydroxyl-radical attack on 2,4-D using γ radiolysis. The
concentration of 2,4-DCP reached a maximum of 9.7 ppm at
32
bubble, which, in turn, slow the sonolytic reactions. From these
studies, we can infer that 2,4-D does not undergo direct pyrolysis
in the sonolytic bubbles but rather becomes oxidized at the
1
0 min into the radiolysis and was almost completely degraded
in 60 min. At 40 min into the radiolysis, the parent compound,
,4-D, was no longer detected. 4-Chlorocatechol was also a
•
interface by reacting with OH radicals.
2
Analysis of 2,4-Dichlorophenol. A closer study of the
readily detected intermediate.
intermediates and products formed in the sonolytic degradation
of 2,4-D provided some additional insights into the OH-
•
The chloride ion concentration was followed during the
radiolysis of 2,4-D. After 60 min of radiolysis, close to 90% of
the chloride ion was detected via ion chromatography, corre-
sponding to the absence of 2,4-D, 2,4-DCP, and 4-chloro-
catechol. Only trace amounts of smaller chlorinated components
were possibly still present. These results are summarized in
Figure 5. The similarity between the reaction intermediates
identified in the γ radiolysis and sonolysis experiments further
mediated reactions. As mentioned earlier, the first stable
•
intermediate formed in the OH attack on 2,4-D is the corre-
sponding phenolic form, 2,4-DCP. 2,4-Dichlorophenol has been
reported to be a more toxic and problematic intermediate in
•
22
OH-mediated processes than the parent compound, 2,4-D. In
the present sonolysis work, the concentration of 2,4-DCP never
increased above the level of 0.015 mM when 2,4-D (0.20 mM)
was subjected to high-frequency ultrasound waves, indicating
that it is readily degraded.
•
supports our argument that OH-mediated reactions dominate
the destruction of 2,4-D and 2,4DCP.
Effect of tert-Butyl Alcohol on the Sonolytic Degradation
of 2,4-D. If indeed the hydroxyl radical is a major participant
in the sonolytic degradation of 2,4-D, we should be able to
The low-level accumulation of 2,4-DCP is indicative of the
fact that it also competes well for the OH radicals at the bubble
surface. Separate sonolysis experiments were performed using
0.18 mM 2,4-dichlorophenol solution (pH 5.7). Under Ar-
saturated conditions, the lifetime of 2,4-DCP in the solutions
at pH 5.7 was 13 min. When oxygen was used as the sparging
gas, the lifetime was slightly increased to 15 min. A 2,4-DCP
solution adjusted to pH 3 was tested under the same experi-
mental conditions. The rate of reaction was slightly greater at
the lower pH, with a calculated lifetime of 12 min (See Table
1). The rate of degradation of the chlorinated phenol was faster
than that of the organic acid 2,4-D under all conditions tested.
The greater hydrophobicity of 2,4-DCP is likely to be the reason
•
suppress this reaction by using a known OH radical scavenger
in the solution. Sonolysis experiments were performed using
•
tert-butyl alcohol, which is a common OH radical scavenger
in the radiolysis of water (reaction 8).
•
•
(
CH ) COH + OH f (CH ) CH COH + H O (8)
3 3 3 2 2 2
Extensive work on the sonochemistry of tert-butyl alcohol
3
2
has recently been reported by von Sonntag and co-workers.
They determined that the alcohol is pyrolized in the bubble via

3180 J. Phys. Chem. A, Vol. 105, No. 13, 2001
Peller et al.
SCHEME 1
SCHEME 2
SCHEME 3
Figure 7. Change in total organic carbon (TOC) of 2,4-D in aqueous
solution during 5 h of sonolysis and γ radiolysis. Sparging gases: Ar
2 2
(0), O (b), and N O(g) with γ radiolysis (4).
for the increased reactivity as it will accumulate preferentially
at the air/water interface over the 2,4-D.
Extent of Mineralization. In the sonolytic degradation of
2,4-D, the final product before mineralization to carbon dioxide
is oxalic acid. When solutions of oxalic acid were subjected to
high-frequency ultrasound waves under O2-saturated and Ar-
saturated conditions for 2 h, no change in concentration was
detected under either atmosphere. Furthermore, the ultrasonic
degradation of 2,4-D showed very little change in overall carbon
content after 3 h, where oxalic acid accounted for 95% of the
remaining carbon. This suggests that oxalic acid constitutes an
endpoint in the mechanistic pathways for the sonolytic degrada-
tion of 2,4-D. Solutions of glyoxylic acid and glycolic acid were
also subjected to high-frequency ultrasonic waves. Both com-
pounds underwent chemical transformations to yield oxalic acid,
at which point little or no further change took place. Other
papers have reported similar findings of low mineralization in
of 2,4-D is the release of chloride ion. The chloride ion
concentration in the sonolysis and radiolysis of 2,4-D was
monitored using ion chromatography. Figure 5 shows the
buildup of the chloride ion concentration following the radiolysis
of a 2,4-D solution. The release of chloride ion in the first few
minutes is rather slow, and its growth rate is independent of
2
,4-D decay. A similar pattern of chloride ion accumulation
was also noted in the sonolysis experiments. The slow chloride
ion buildup in these experiments correlated with the formation
of chlorinated aromatic intermediates, viz., 2,4-DCP. The
delayed appearance of chloride ions along with the clear
presence of 2,4-DCP suggests that the initial step in the
sonolytic degradation of organic compounds.4
6,47
•
mechanism is the OH attack ipso to the ether substituent
The accumulation of oxalic acid might arise from either the
•
(Scheme 1). The next steps in the mechanism involve hydroxyl
radical attacks on 2,4-DCP at the carbons containing the
chlorines (ipso attacks), which corresponds to the steady increase
in chloride ions. The hydroxyl attack is followed by loss of
HCl, which restores the aromatic ring and affords the mono-
chlorinated intermediates 4-chlorocatechol and 2-chlorohydro-
quinone (Scheme 2). Further radical reactions with hydroxyl
or hydrogen atoms (sonolysis) leads to the dechlorinated
aromatics (hydroquinone, catechol, and 1,2,4-benzenetriol),
which were detected readily via HPLC throughout the degrada-
tion process (Scheme 3). The oxidative degradation of these
primary intermediates proceeds with a ring-opening reaction via
hydroxyl radical attack. In sonolysis, the degradation of 2,4-D
yields oxalic acid as the end product. However, with a proper
lack of reactivity of oxalic acid toward OH radicals or its
inability to accumulate at the bubble (air/water) interface. To
•
examine the reactivity of OH radicals, we analyzed the oxalic
acid concentration and total organic carbon (TOC) in the γ
radiolysis of N2O-saturated aqueous solutions of 2,4-D. The
mineralization of 2,4-D proceeds at a much higher rate under
radiolytic conditions than in sonolysis (Figure 7). After 4 h of
radiolysis, 60% of the 2,4-D was mineralized, whereas less than
2
5% of the 2,4-D was mineralized after 4 h using high-frequency
•
ultrasound. Thus, it is evident that lack of reactivity toward OH
radicals is not the major cause for oxalic acid accumulation in
the sonolysis of 2,4-D solutions.
Moreover, oxalic acid is known to degrade via hydroxyl
radicals.48,49
Therefore, the origin of the slow mineralization of
•
strategy of OH radical attack, it can be further mineralized (e.g.,
oxalic acid is not an inherently slow reaction with hydroxyl
radicals. γ-Radiolysis experiments conducted using a N2O-
saturated oxalic acid solution revealed a lifetime of ap-
proximately 35 min. However, the concentration of oxalic acid
was not affected during 2 h of sonolysis. The extremely high
polarity of oxalic acid makes its adsorption at the bubble inter-
face less probable. Because most of the hydroxyl radical attack
occurs at the bubble interface, we see little chemical transforma-
tion of oxalic acid. These studies, as well as those showing the
difference in reactivity of protonated and deprotonated forms
of other compounds, lead us to believe that the extent of
hydroxyl radical attack in the bulk solution is extremely small
compared to that of the reaction pathways at the bubble interface.
Mechanism of 2,4-D Degradation. Another important aspect
via radiolysis) to carbon dioxide and water. Studies indicate
that the intermediate steps from oxidized aromatics to highly
oxidized short-chain acids involve many competing path-
50,51
ways,
thus adding further complexity to the simple degrada-
tion mechanism presented here. Further modeling work is being
done in our laboratory to better understand the mechanism of
the hydroxyl-radical-mediated degradation.
Conclusion
In this investigation, high-frequency sonolysis has been shown
to be effective in the degradation of the herbicide 2,4-D. The
technique is extremely useful for minimizing the accumulation
of the toxic intermediate 2,4-dichlorophenol. The major end
product of the degradation of 2,4-D is oxalic acid, which is
•
that can shed light into the OH-radical-mediated transformations

Sonolysis of 2,4-Dichlorophenoxyacetic Acid
J. Phys. Chem. A, Vol. 105, No. 13, 2001 3181
resistant to sonochemical degradation. Comparisons with the
γ-radiolysis experiments confirm that the hydroxyl radical is
the main reactive species in the ultrasonic destruction of 2,4-
dichlorophenoxyacetic acid and 2,4-dichlorophenol. The hy-
droxyl radical attack is limited mainly to the organics accumu-
lated near the cavitation bubble interface. Efforts are underway
to couple this technique with another advanced oxidation process
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Acknowledgment. We thank I. Duncanson for the design
2
,4-dichlorophenoxyethanoic acid (2,4-D) in water by TiO2-UV, H2O2-
UV or direct photolysis. In Photocatalytic Purification and Treatment of
Water and Air; Ollis, D. F., Al-Ekabi, H., Ed.; Elsevier: Amsterdam, 1993.
of the sonolysis cell; Dr. K. Vinodgopal for many useful
discussions; and the Department of Chemistry, Indiana Uni-
versity Northwest, for providing the facilities to perform HPLC
and TOC analysis. The work described herein was supported
by the Office of the Basic Energy Sciences of the U.S.
Department of Energy. This is Contribution No. 4151 from the
Notre Dame Radiation Laboratory.
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