Page 1
th
ga
ir
Isr
chnio
b Department of Civil Engineering, The Grand Water Research Institute, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel
Received 24 April 2003; accepted 12 October 2003
clude chemical oxidation, advanced oxidation processes,
photochemical and photocatalytic reactions, sono-
chemical and sono-electrochemical processes. These de-
gradation mechanisms were suited to several specific
ranges from 370 to 666 mg/kg in small animals), it can
be converted into highly toxic chloroorganic products
[2]. Therefore, a maximum concentration level of 70 ppb
is recommended in drinking water by the World Health
Organization [4]. 2,4-D is a poorly biodegradable pol-
lutant [5]. In fresh water it can be mineralized (i.e.
converted to CO and chloride ions) by various micro-
ry 11*Available online 24 January 2004
Abstract
A new method for detoxification of hydrophilic chloroorganic pollutants in effluent water was developed, using a combination of
ultrasound waves, electrochemistry and Fenton’s reagent. The advantages of the method are exemplified using two target com-
pounds: the common herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and its derivative 2,4-dichlorophenol (2,4-DCP). The high
degradation power of this process is due to the large production of oxidizing hydroxyl radicals and high mass transfer due to
sonication. Application of this sono-electrochemical Fenton process (SEF) treatment (at 20 kHz) with quite a small current density,
accomplished almost 50% oxidation of 2,4-D solution (300 ppm, 1.2 mM) in just 60 s. Similar treatments ran for 600 s resulted in
practically full degradation of the herbicide; sizable oxidation of 2,4-DCP also occurs. The main intermediate compounds produced
in the SEF process were identified. Their kinetic profile was measured and a chemical reaction scheme was suggested. The efficiency
of the SEF process is tentatively much higher than the reference degradation methods and the time required for full degradation is
considerably shorter. The SEF process maintains high performance up to concentrations which are higher than reference methods.
The optimum concentration of Fe2þ ions required for this process was found to be of about 2 mM, which is lower than that in
reference techniques. These findings indicate that SEF process may be an effective method for detoxification of environmental
water.
� 2003 Elsevier B.V. All rights reserved.
Keywords: Sonoelectrochemistry; Fenton reagent; Ultrasound wave
1. Introduction
Halogenated organic pollutants are toxic materials
that are often present in industrial effluents and some-
times in drinking water. Many of these materials
are very stable and resist traditional biodegradation
treatments. Several different approaches are known
for the decomposition of these compounds. These in-
compounds, since their efficiency is compound-depen-
dent. Therefore, we shall restrict our discussion to our
target material, which is 2,4-dichlorophenoxyacetic acid
(2,4-D).
This herbicide is used in controlling broadleaf weed in
cereal crops, sugarcane, turf, and pastures. After 50
years of use, 2,4-D is the most widely used herbicide
worldwide [1]. Although its mild toxicity, (oral LD50A new sono-electrochemical me
of hydrophilic chloroor
Yakov Yasman a, Valery Bulatov a, Vladim
Robert Armon b,
a Department of Chemistry, The Grand Water Research Institute, Te
Ultrasonics SonochemistCorresponding author. Fax: +972-4-8292579.
E-mail address: israel@techunix.technion.ac.il (I. Schechter).
1350-4177/$ - see front matter � 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2003.10.004od for enhanced detoxification
nic pollutants in water
V. Gridin a, Sabina Agur a, Noah Galil b,
ael Schechter a,*
n-Israel Institute of Technology, Technion City, Haifa 32000, Israel
(2004) 365–372
www.elsevier.com/locate/ultsonch2
biological schemes only at concentration levels that do
Page 2
[24–27], chlorinated aromatic compounds, biphenyls
and pesticides [28]. A complete sonolytic degradation
Sononot exceed 10 ppb. Therefore, alternative methods must
be considered for this compound in groundwater.
Many oxidation schemes were applied in attempts to
mineralize 2,4-D in aqueous solutions [3–11]. Some of
the processes were based on the chemical [3,4] and
photochemical [4–11] degradation, mediated by hydro-
xyl radicals. Significant accumulation of chloride ions
was reported on the time scale of 4 h; [10]. 2,4-
Dichlorophenol (2,4-DCP), chlorohydroquinone and
hydroquinone were detected as the intermediates of the
processes involved.
Complete degradation of a 0.1 mM herbicide solution
at pH approximately 3 was reported using a chemical
degradation approach [4]. Similar results were achieved
by using the Fenton reagent, where considerable deg-
radation of 2,4-D was detected after 10 min of the oxi-
dative process. Note, however, that these results were
achieved in pure solutions of relatively low initial her-
bicide concentrations. At high concentrations and in the
presence of various salts, the classical Fenton reaction is
not so effective anymore. As the pollutant concentration
approaches the solubility limit, the inorganic salt effect
results in formation of suspension, which is actually the
case for many effluents. Therefore, an alternative deg-
radation method is required for effluent water at rather
high pollutant concentrations.
Effective electrochemical methods were also developed
for the treatment of wastewater polluted by toxic and
stable organic compounds [12–17]. Direct electrochem-
ical techniques [12] for water purification involve anodic
or cathodic decomposition; however, these are com-
monly associated with anodic oxidation of water (release
of O2). Therefore, only low current yields can be
achieved this way. Additional problems, which are re-
lated to the low miscibility of most organics in water and
to the hindered mass transfer to the anode, are respon-
sible for rather low space-time yields.
Considerably better results may be achieved using
indirect electrochemical oxidation, where the pollutants
are oxidized in the bulk by a mediator in a high oxida-
tion state. For example, the application of the electro-
Fenton process to degradation of 2.4-D resulted in 67%
TOC removal, while only 16% were achieved by direct
anodic oxidation (100 mA for 4 h) [17].
In addition to the above chemical, photochemical and
electrochemical approaches, sono-chemical techniques
have also been developed. Since these are closely related
to our target technique, they are more elaborated in the
following.
Sono-chemical reactions involving chloroorganic sol-
utes in aqueous solutions have received intensive atten-
tion since 1950 due to the work of Weissler et al., Parke
and Taylor [18,19]. Studies of sono-chemical modifica-
tions of carbon tetrachloride and chloroform in aqueous
environment [20] then followed. Since 1990, there has
366 Y. Yasman et al. / Ultrasonicsbeen increasing interest in the ultrasound-mediatedof 2,4-D 0.2 mM, performed at high frequency ultra-
sound radiation of 640 kHz and at a pH of 2.2, has oc-
curred within 21 min [29]. Under alkaline conditions
(pH¼ 11.8), a longer degradation time was required (38
min) [29].
The chemical aspects of continuous and pulsed
ultrasound treatment of various aqueous solutions are
believed to be associated with acoustic cavitations.
There are three regions of importance in respect of the
aqueous sono-chemical processes. The first region is the
interior of the collapsing cavitation bubbles in which
extreme thermodynamic conditions, due to high local
temperature and pressure, rapidly set in [30–33]. In this
region, a fast pyrolysis of volatile solutes takes place;
water molecules also undergo thermal decomposition to
produce H atoms and �OH radicals [34]. The second
region is the interfacial boundary between the gaseous
and the liquid phases where the temperature is lower
than inside the bubbles, yet still high enough to cause
thermal decomposition of organic solutes. It is believed
that the reactive radicals formed from water decompo-
sition in gas bubbles are localized in this region. The
third region is the bulk of solution (usually at ambient
temperature) where various reactions of organic solutes
with either �OH radicals or H atoms, which escape from
the bubbles’ interface, may occur [35].
Sono-electrochemistry was also successful in waste-
water detoxification [15]. However, as far as we know,
the combination of ultrasonic waves, electrical field and
Fenton’s process, has never been examined for water
treatment. This approach seems promising, due to the
power of combining its individual components which
may enhance the overall performance.
In this study we addressed the sono-electrochemical
Fenton (SEF) process and evaluated its effect upon
degradation of chlorinated aromatic compounds in
environmental water. We studied the intermediate
compounds formed in this process, as well as the various
factors affecting its efficiency. Kinetic investigation
provided insight to the chemical reactions that take
place and to the fate of the various intermediates.
2. Experimental procedures
2.1. Materials and reagents
Both target compounds: 2,4-dichlorophenoxyaceticdegradation of various organic contaminants present
both in natural and industrial water resources.
Ultrasound was applied to decomposition of such
chlororganic compounds as: chloroform [21], carbon tet-
rachloride [22], chlorofluorocarbons [23], chlorophenols
chemistry 11 (2004) 365–372acid (Aldrich, 98%) and 2,4-dichlorophenol (Fluka,
Page 3
95%) were tested by HPLC and used as supplied with
no further purification. Spectroscopy purity dichlo-
rometane (Carlo Erba, 99.8%) was used after testing
by GLC. Analytical grade reagents of sodium sulfate
(Agan, Israel), ferrous sulfate (Mallinckrodt, USA) and
hydrogen peroxide (Carlo Erba, 30% ) were used.
2.2. Sono-electrochemical facility
The schematic diagram of our sono-electro-oxidation
facility is shown in Fig. 1. The reactor was a glass cyl-
inder vessel (internal diameter 25 mm, effective sample
volume of 10 ml). The solution temperature was main-
tained at 25± 1 �C by circulating water in a double-
jacket cooling array.
Sonication was achieved at low frequencies (20 kHz)
using an ultrasonic generator Sonic @ Materials Model
2020, fitted with a horn that emits ultrasound via a
titanium alloy tip (13 mm in diameter) dipped in the
in our diagrams.
Classical Fenton experiments were also carried out for
Y. Yasman et al. / Ultrasonics SonoFig. 1. Schematic diagram of sono-electro-oxidation facility: (1) po-
tentiostat, (2) electrodes, (3) 20 kHz transducer, (4) water jacketedstudied solutions from the top of the reactor. The aver-
age output electric power of the generator was 75 W.
Both cathode and anode were made of nickel foil
(0.125 mm thin) in the form of cylindrical segments of 11
mm radius and 20 mm height. They were placed around a
horn (11 mm radius). The support electrolyte was
Na2SO4 (0.5 g/l). Sono-electro-oxidation was carried out
in the galvanostatic mode at current intensities not
exceeding 100 mA. Similar to recent reports [17], the SEF
scheme used here appeared to be insensitive to the cur-
rent density, within the experimentally tested range of
10–100 mA/cm2. The dissolution of the electrodes in the
acidic media, under the combined application of elec-
trolysis and sonication, was checked. The maximum
concentration on Ni ions was less than 1 mg/l, which was
negligible compared to the electrolyte concentration.reactor, (5) frequency generator, (6) water criostat.comparison, under the following conditions: 2,4-D
concentration of 1,2 mM, Fe2þ––3.0 mM and H2O2––
3.0 mM. All experiments were performed under inten-
sive stirring.
2.3. Analytical cross-referencing
Variations of 2,4-D and 2,4-DCP concentration
occurring under either sono-Fenton’s or sonoelectro-
Fenton’s processes were readily monitored using HP
8453 UV–Visible photodiode array Hewlett Packard
spectrophotometer.
In order to identify stable intermediates formed
during degradation of 2,4-D and 2,4-DCP, the organic
components present in small samples (5 ml) of the tested
solution were further extracted using 5 ml of dichlo-
rometane. The solvent was allowed to evaporate and the
resulting analyte was then tested by GC and GC-MS.
Hewlett-Packard gas chromatograph (model 5890 with
FID detector) and Finnigan TSQ-70B mass-spectrome-
ter were used. The chloride ion concentration in the
sampled solutions was determined by potentiometric
titration using AgNO3.
3. Results and discussion
In this study we intend to evaluate the performance of
sono-electrochemical Fenton process for decomposition
of 2,4-D and 2,4-DCP in water. As a reference, we
should consider traditional oxidation processes. We
examined several such processes for degradation of
chloroorganic pollutants in aqueous solutions. First, we
present the performance of the classical Fenton oxida-
tion process, which takes place in the presence of Fe2þ
ions. (Hereafter we shell use the term ‘‘degradation’’ for
decomposition of the toxic compounds and their deriv-
atives, such as 2,4-D and 2,4-DCP, chlorinated phenols
and quinones. Full degradation means full removal ofAll experiments were carried out under initial con-
centration of 0.25–1.5 mM 2,4-D and 0.35–1.5 mM 2,4-
DCP and pH 3. In order to address the effect of variable
concentration of Fenton’s reagent upon the decomposi-
tion of 2,4-D and 2,4-DCP, the experiments were carried
out by using Fe2þ in the range from 0.5 to 50 mM, while
keeping H2O2 unchanged at a level of 30 mM.
Stock solutions (250 ml each) at various reagent
concentrations were prepared, and samples were in-
serted into the reaction cell and left for given durations.
They were taken out of the process at the specified time
and analyzed. In order to ensure that the solutions left
in the reactor are not affected by the analytical sampling,
a fresh sample was used for each time-duration point
chemistry 11 (2004) 365–372 367these compounds from the reaction mixture; we assume
Page 4
of the electrochemical oxidation of hydrophilic phenols
[12] and the homolytical reactions of phenoxyacetic acid
[3] (reaction (3) and (4)):
ArOH ! ArO� þHþ þ e� ð3Þ
ArOCH2COOH ! ArOCH2COO� þHþ þ e� ð4Þ
We believe that the oxidative and reductive electro-
chemical processes and the reactions involving HO� are
significantly more effective under the SEF process.
In all cases, degradation of halogenated organic
compounds takes place via several intermediate mole-
cules. These side-products may be further decomposed
until degradation is accomplished, or may remain in the
solution. Regarding the decomposition of 2,4-D and 2,4-
DCP, recent reports indicate that the intermediates
fied for 2,4-D in Fig. 3, where UV spectra were acquired
Sonochemistry 11 (2004) 365–372that the remaining chloroorganic non-aromatic com-
pounds offer no significant problem for standard meth-
ods of effluent water purification.)
The results for classical Fenton oxidation of 2,4-D
are shown in Fig. 2, where the initial spectrum and that
obtained after application of Fenton process for 7 h, are
provided. Clearly, sizeable changes are observed only
after a rather long time. The changes are mainly
attributed to the Fenton oxidation, since the half life-
time of 2,4-D in ambient water is much longer (6–170
days, depending on the environmental conditions it is
kept under) [36].
In contrast to the above results, dramatic increase in
the degradation rate for both 2,4-D and 2,4-DCP occurs
with either sono-Fenton (SF) or sono-electrochemical
Fenton (SEF) schemes. Typical results obtained after
just 60 s of application of SF and SEF to aqueous
solutions of 2,4-D are shown in Fig. 2. A considerably
much faster decomposition of the starting compound is
observed in both processes. The faster degradation of
2,4-D and 2,4-DCP within SF and SEF schemes (reac-
250 275 300 325
0.0
0.5
1.0
1.5
2.0
2.5
3.0 1.2 mM 2.4-D Initial concentration
Fenton (60s)
SF (60s)
SEF (60s)
Ab
so
rb
an
ce
Wavelength / nm
Fig. 2. UV absorbance spectra of as-prepared 1.2 mM of 2,4-D; and
following 60 s of Fenton, SF and SEF processes. [Fe2þ]¼ 3.0 mM,
[H2O2]¼ 3.0 mM.
368 Y. Yasman et al. / Ultrasonicstions (1) and (2)) is believed to be due to the high effi-
ciency for the production of �OH radicals as well as to
the ultrasonic cleaning of electrode’s active surfaces
during these processes.
Fenton reaction : H2O2 þ Fe2þ ! OH� þHO� þ Fe3þ
ð1Þ
Ultrasonic reaction : H2O ! HþHO� ð2Þ
In this context, note that among very reactive species
suitable for SE oxidation treatments (such as O2��, OH�,
HO�2, ROO
�) the hydroxyl radical is certainly the most
reactive [37]. It is a very strong one-electron oxidizing
agent, which seldom reacts as an electron transfer re-
agent; it is very reactive in hydrogen atom abstraction
and in electrophilic addition processes [37].
Probably phenoxy––ArO� and phenoxyacetic
ArOCH2COO� radicals are formed in the initial stagesat various times after the initiation of the SEF process
on 2,4-D contaminated water. The presence of inter-
mediate products is evident from the modifications of
the original UV absorption band at 250–270 nm. The
absorption at these wavelengths increases in correlation
with the rapid decrease of the 2,4-D peaks. GC/MS
identification patterns of the intermediates produced in
250 275 300 325
0.5
1.0
1.5
2.0
2.5
3.0
SEF process
2.4-D, 2 mM
180sec
120sec
60sec
30sec
0 sec
Ab
so
rb
an
ce
Wavelength / nm
Fig. 3. UV absorbance spectra taken during SEF treatment of 2 mM ofmight be even more harmful than the parent compounds
themselves [38]. In order to understand the SF and the
SEF processes and the variations in their efficiencies,
and in order to evaluate the usefulness of these pro-
cesses, we need to identify and analyze the intermediates
formed. Then we can carry out a kinetic investigation of
the parent molecules and intermediate compounds.
3.1. Study of the intermediates
The intermediates produced in the SEF process were
detectable soon after the initiation of the oxidation. The
highest concentrations of such compounds were gener-
ally observed within the first 30–180 s. This is exempli-2,4-D after various times. [Fe2þ]¼ 3.0 mM, [H2O2]¼ 3.0 mM.
Page 5
Table 1
GC/MS identification of intermediates obtained in SEF process of 2,4-D
(M+1)þ Structure (
221
OCH2COOH
CI
CI
1
2.4-D
191
CI
CI
OCHO
1
Formate
219
CI
O
O
O
3
Y. Yasman et al. / Ultrasonics Sonochemistry 11 (2004) 365–372 369CI
Lactoneboth SF and SEF degradation of 2,4-D were studied.
The corresponding molecules are summarized in Table
1. A suggested chemical scheme for the first stages of
Cl
ClCl
Cl
OCH2CH2O
Cl
Cl
O
O
O
OCHO
Cl
Cl
OH
Cl
Cl
OH
Cl
Cl OH
OCH2COOH
Cl
Cl
2,4-D
2,4-DCP
Fig. 4. Schematic presentation of the proposed first chemical stages in
SEF decomposition of 2,4-D.M+1)þ Structure
62 (Mþ)
CI
CI
OH
2.4-DCP
78 (Mþ)
CI
CI
OH
OH
Quinone
51
CI
CI
OCH2CH2O
C I
CI
Trace: diphenoxyethaneSEF decomposition of 2,4-D, which may be responsible
for these intermediate compounds, is shown in Fig. 4.
Worth noting is a close similarity of the intermediate
compounds obtained with SEF treatments to those re-
ported for the UV-photolysis tests [26]. The latter,
however, are by far slower processes (days) than the
former ones (minutes). Moreover, a dimerization stage,
which is quite common in UV-photolysis [34], seems
absent in both SF and SEF processes. This could be
attributed to instability of dimers within a double-
charge layer situated either nearby the electrodes (as for
SEF processes) or at the cavitation-bubble interfaces
(as for both SF and SEF).
3.2. Kinetic investigation
Kinetic study revealed the rate constants associated
with the studied decomposition. The corresponding rate
constant for the decomposition of 2,4-D is 0.01 s�1 ; it is
about 10 times smaller for 2,4-DCP. This finding is
somewhat surprising, since analysis of the intermediate
products obtained during SEF decomposition of 2,4-D
shows that 2,4-DCP is formed. Therefore, one might
have anticipated SEF decomposition of such intermedi-
ates to be the rate limiting process for the decomposition
of 2,4-D herbicide. According to our observations,
however, this was not the case. This finding could be
attributed to the partial contribution of 2,4-DCP to the
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