The mechanism of sonophotocatalytic degradation of methyl orange and its products in aqueous solutions

Article abstract of DOI:10.1016/j.ultsonch.2011.03.017

In this study, it was found that a hybrid technique, sonophotocatalysis, is able to degrade a parent organic pollutant (methyl orange) as well as its by-products. The analysis of products formed during the whole degradation has demonstrated that the pH or the selection of oxidation process (sonolysis/photocatalysis/sonophotocatalysis) is able to control the degradation pathway. It was shown in the by-products analysis that the solution pH can alter the amount of each product generated during the sonophotocatalytic degradation. It was revealed that the different degradation rates of methyl orange and its products result from the solution pH and the nature of the organic products. Furthermore, a comparison of the data obtained from the oxidation processes on the degradation of the reaction intermediates identified the advantages of the combined system. It is concluded that sonophotocatalysis is capable of yielding a more complete and faster mineralization of organic pollutants than the individual processes. However, as in the degradation of the parent compound, the overall mineralization is lower than an additive effect (negative synergistic effect).

Full text of DOI:10.1016/j.ultsonch.2011.03.017

Ultrasonics Sonochemistry 18 (2011) 974–980
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
The mechanism of sonophotocatalytic degradation of methyl orange
and its products in aqueous solutions
⇑
Yuanhua He, Franz Grieser, Muthupandian Ashokkumar
Particulate Fluid Processing Center, School of Chemistry, University of Melbourne, Parkville, VIC. 3010, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, it was found that a hybrid technique, sonophotocatalysis, is able to degrade a parent organic
pollutant (methyl orange) as well as its by-products. The analysis of products formed during the whole
degradation has demonstrated that the pH or the selection of oxidation process (sonolysis/photocataly-
sis/sonophotocatalysis) is able to control the degradation pathway. It was shown in the by-products anal-
ysis that the solution pH can alter the amount of each product generated during the sonophotocatalytic
degradation. It was revealed that the different degradation rates of methyl orange and its products result
from the solution pH and the nature of the organic products. Furthermore, a comparison of the data
obtained from the oxidation processes on the degradation of the reaction intermediates identified the
advantages of the combined system. It is concluded that sonophotocatalysis is capable of yielding a more
complete and faster mineralization of organic pollutants than the individual processes. However, as in the
degradation of the parent compound, the overall mineralization is lower than an additive effect (negative
synergistic effect).
Received 29 December 2010
Received in revised form 10 March 2011
Accepted 17 March 2011
Available online 9 April 2011
Keywords:
Acoustic cavitation
Methyl orange
Photocatalysis
Sonolysis
Sonophotocatalysis
Ó 2011 Elsevier B.V. All rights reserved.
1
. Introduction
Although both photocatalytic [1–4] and sonolytic [5–8] degra-
ble of accelerating the adsorption activity of reactant on the
photocatalyst; and (vi) sonolysis is likely to decompose the
hydrophobic part of the pollutant compound, which is unlikely
to occur on the surface of the photocatalyst.
dation of organic pollutants have been found to be useful in the
remediation of environmental contaminants, the critical limit for
these individual processes is their relatively low efficiency for the
complete mineralization of organic pollutants. Some considerable
effort has been directed at enhancing the efficacy of advanced oxi-
dation processes (AOPs) by forming hybrid systems from two or
more techniques [7,9–12].
However, the combined use of photocatalysis and sonolysis for
the treatment of pollutants is rather limited, mostly due to the lack
of a detailed mechanism for this hybrid process. Most publications
in this field deal with the degradation kinetics of the parent organic
pollutants as a function of operation conditions. Relatively little is
known about the reaction of by-products and pathway associated
sonophotocatalytic treatment, since the product analysis plays a
critical role in exploring and understanding the mechanism of this
combined system and establishing a framework for its further
application.
Methyl orange (MO), a typical azo dye, was selected as an or-
ganic pollutant in this study. Several recent studies [16–20] on
the sonophotocatalytic treatment of MO showed it to be a very
promising potential application of the hybrid system. However,
all of these publications have mainly focused on the influence
of various catalysts on the primary degradation of the parent or-
ganic pollutants. To the best of our knowledge, the identification
and further degradation of the products during sonophotocatalyt-
ic degradation of methyl orange have not been studied to date.
In thepresentwork, thedegradation of bothMO and its derivative
products were systematically examined. A key aspect of this study is
the evaluation of the degradation behavior of subsequent products
formed during photocatalytic, sonochemical and sonophotocatalytic
A number of research groups have examined the combination
of ultrasound and photocatalysis, known as sonophotocatalysis
(
SPC), for environmental remediation [7,10–15]. The effective-
ness of SPC can be highlighted as follows: (i) ultrasound, through
acoustic cavitation, provides an extra source of OHÅ from cavita-
tion; (ii) acoustic cavitation can remove intermediates from the
photocatalytic active sites. Acoustic cavitation generates a num-
ber of physical effects, such as shear forces, turbulence and mi-
cro-streaming, that help regenerate the active catalytic surface;
(
iii) when the catalyst or the pollutant is in the form of a pow-
der or an agglomerate, acoustic cavitation increases the unifor-
mity of the dispersion, thereby increasing the available surface
area; (iv) acoustic cavitation is able to enhance mass transfer to-
wards the liquid–solid interface; (v) acoustic cavitation is capa-
⇑
Corresponding author. Tel.: +61 3 93475180; fax: +61 3 83447090.
E-mail address: masho@unimelb.edu.au (M. Ashokkumar).
1
350-4177/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2011.03.017

Y. He et al. / Ultrasonics Sonochemistry 18 (2011) 974–980
975
degradation of MO. We have found that there is a synergistic degra-
dation of the intermediate products formed leading to a higher min-
eralization efficiency during the combined process.
150 mm ꢁ 4.6 mm HPLC column. The mobile phase consisted of
methanol and 10 mM ammonium acetate (30/70 v/v). The flow
ꢀ1
rate of the mobile phase was 1.0 mL min
.
The mineralized carbon was measured using a Shimadzu Total
Organic Carbon (TOC) Analyzer, model TOC-VCSH equipped with
an ASI-V autosampler. All the experiments were performed in
duplicate, unless the maximum standard deviation or maximum
coefficient of variation was exceeded, in which case additional
measurements were conducted. A maximum standard deviation
of 0.1 and a maximum coefficient of variation of 2% were set as
acceptable criteria.
2
. Experimental details
2.1. Chemicals
All chemicals were purchased from commercial sources and
used without further purification. MO was purchased from Tokyo
Chemical Company. For each degradation experiment, a 250 mL
Mass spectrometry (MS) and LC/MS were performed on an Agi-
lent Technologies 6510 Accurate-Mass Q-TOF coupled with an Agi-
lent 1100 Series HPLC system. Chromatographic separation of the
degradation products was performed using an Eclipse Plus C18 col-
umn (Agilent Technologies, 50 mm ꢁ 4.6 mm). The instrument was
calibrated using an automatic tuning procedure with respective
standard compounds. The mass spectra were recorded between m/
z 25 and 100 in Auto MS/MS mode. The gas temperature was set at
aqueous solution containing 100
lM MO was used as the initial
solution. Perchloric acid (HClO ) and sodium hydroxide (NaOH),
4
purchased from Ajax FineChem, were used to adjust the solution
pH. A series of different concentrations of acid and alkaline solu-
tions were made to control the solution pH. Degussa P25 TiO
2
with
2
ꢀ1
a specific surface area of 57 m g was used as the photocatalyst.
ꢀ1
The loading of the photocatalyst was 1 mg mL for all experi-
ments. The ultra pure water (>18.6 M
the preparation of the aqueous solutions was sourced from a
Milli-Q system (Millipore).
ꢀ1
X cm at 25 °C) used in
3
25 °C and the collision energy was set to increase linearly with a
2
V/100 Da and 10 V offset. Mass profiles were acquired by Aiglent
MassHunter Workstation Software Acquisition and processed using
Agilent MassHunter Workstation Software Qualitative Analysis.
2.2. Degradation experiments
Sonophotocatalytic experiments were conducted in a 250 mL
3. Results and discussion
cylindrical Pyrex vessel with a quartz plate on one side. The soni-
cation vessel contained a jacket, allowing the solution to be cooled
3.1. Photocatalytic, sonolytic and sonophotocatalytic degradation
(
23 ± 3 °C) with cold water. An ultrasound transducer (213 kHz)
was placed at the bottom of the vessel and the power absorbed
The chromatograms of MO during the sonophotocatalysis
changes were observed as shown in Fig. 1. Similar chromatograms
were collected for the sonolytic and photocatalytic experiments.
Fig. 1 shows a clear decrease in the concentration of MO with
increasing irradiation time.
A comparison of the degradation rate constants of MO observed
for the different AOPs and for different solution pH values is shown
in Fig. 2. A pseudo-first order kinetics model (Eq. (1)) was applied
to compare the degradation performance of the three advanced
oxidation processes under the chosen experimental conditions.
ꢀ1
by the solution was fixed at 55 mW mL for all experiments, as
measured by calorimetry. All experiments were carried out in an
air atmosphere. It is known that sonication of water in air atmo-
sphere generates nitric acid. However, we deliberately added per-
chloric acid and sodium hydroxide in order to maintain the
solution pH at 2 and pH 10, respectively. Hence, the pH change
due to nitric acid formation was not an issue in our experiments.
A 450 W Oriel Model 66021 xenon-arc lamp was used as the
light source for the photocatalytic degradation experiments. An
optical filter (cut-off wavelength 320 nm) was installed between
the vessel and the lamp. During photocatalysis, the sonicator was
switched off. Sonolysis experiments were carried out without light
C
C
0
ln ¼ kt
ð1Þ
t
2
irradiation, but in the presence of the TiO photocatalyst. During
sonophotocatalysis, the solution was irradiated with both light
and ultrasound.
Before each experiment was conducted, the solution was stirred
with an overhead stirrer for 30 min to disperse the photocatalyst
and to allow adsorption equilibrium of the dye to be reached on
Where, C
tant; C
0
is the initial concentration of the degradation reac-
is the concentration at time t; k is the rate constant. The
t
pseudo-first order kinetics is based on the assumption that the
2
the TiO surface. During the whole degradation experiment, the
stirring speed was kept constant at 400 rpm and a bench top pH
meter was used to monitor the solution pH.
Prior to analysis, all samples were filtered through a 0.45
ter to remove the suspended TiO particles.
lm fil-
2
2.3. Analytical determinations
The progress of MO degradation was regularly monitored by
UV–visible absorption using a Varian spectrophotometer (Cary
Bio50). A computer controlled high performance liquid chromatog-
raphy (HPLC) system (Shimadzu LC-10 AT VP system) comprising a
solvent delivery pump, UV–visible diode array detector and an
auto-sampler was used to characterize the organic pollutants and
the by-product concentration–time profiles. The detector wave-
length range was set from 190 to 640 nm. To qualitatively and
quantitatively analyze the formation of the by-products, fractions
Fig. 1. 3D chromatographs observed during 60 min sonophotocatalytic degradation
ꢀ1
of 100
lM MO at pH 2. The TiO
2
was 1 mg mL . The ultrasonic frequency was
ꢀ1
213 kHz and the power was 55 mW mL . The detection wavelength was 464 nm
were separated on an Alltech Econosphere C18
5
l
m,
for monitoring the MO concentration.

9
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Y. He et al. / Ultrasonics Sonochemistry 18 (2011) 974–980
Table 1
The pseudo first-order rate constants of sonolysis, photocatalysis and sonophotoc-
atalysis of 100
l
M MO in aqueous solution at pH 2 and pH 10.
pH Value
Process
pH 2
pH 10
Sono Photo Sonophoto Sono Photo Sonophoto
k (10 2 min
ꢀ
ꢀ1
)
1.2
2.4
0.9
3.4
1.2
1.2
1.1
2.7
Synergistic index
all the samples at both pH 2 and pH 10 were adjusted to above
2 prior to analysis. A separate adsorption experiment showed that
the minimum adsorption of MO occurred on the TiO above pH 12.
1
2
It is clearly seen that there is no significant influence of pH on the
sonolysis of MO. Generally, due to the different ionic forms of
acid–base organic molecules, and consequently different surface
activities at different pH values, the sonochemical degradation
exhibits a pH-dependent rate constant [22,23]. The slight difference
in the rate constants obtained at the two different solution pH val-
ues implies there is not a large difference in the bubble/solution
surface activity between the anionic and zwitterionic forms of MO.
From Table 1, the rate constant for the photocatalytic degrada-
tion of MO at pH 2 is almost twice that obtained at pH 10. Accord-
ing to the generally accepted mechanism of photocatalysis, the
overall degradation efficiency is determined by the competition
between the photo-generated recombination of free charge–carri-
ers and trapping of the charged carriers by the surface adsorbed or-
ganic pollutants. At pH 2, due to the electrostatic attraction of
2
positively charged TiO and the negative charge of the zwitterionic
dye, the adsorption of the dye at the semiconductor/solution inter-
face is facilitated [24].
The faster sonophotocatalytic degradation at pH 2 is most likely
due to the same mechanism that is responsible for the photocata-
lytic component of the process. Moreover, at both solution pH val-
ues, it is clearly seen from Table 1 that there is no apparent
synergistic effect in the combined system. At both pH values, the
sonophotocatalytic system shows an additive effect. As heteroge-
neous photocatalysis and ultrasonic irradiation initialize degrada-
tion of organic pollutants by reaction with OHÅ radicals [2,22], it
therefore appears that under the conditions of the experiments
both AOPs generate equivalent quantities of OHÅ radicals, irrespec-
tive of the presence of each other.
Fig. 2. Influence of pH on the degradation kinetics during the sonolysis, photoca-
talysis and sonophotocatalysis of an aqueous solution containing 100
l
M MO. The
ꢀ
1
TiO
2
loading in the oxidation system was 1 mg mL
.
The applied ultrasonic
ꢀ
1
frequency was 213 kHz and the power was 55 mW mL . The detection wavelength
used with HPLC was 464 nm.
initial degradation reaction step is OHÅ attack on the dye molecule
[
2,22] and during the whole degradation process, the production of
OHÅ radicals remains constant and reacts only with the organic pol-
lutant. However, as the degradation products build up in solution,
they could compete with MO for OHÅ radicals [21].
Here, in order to investigate the influence of pH and individual
oxidation process, the data for the first 90 min was selected and
used to calculate the rate constants as listed in Table 1.
At both pH 2 and pH 10, it is obvious that the sonophotocatalyt-
ic process is faster than the non-hybrid AOP degradation. Table 1
lists the experimentally obtained rate constants based on a pseu-
do-first order treatment of the data (Eq. (1)). The ratio of the
sonophotocatalytic rate constant to the sum of the rate constants
of the individual processes was used to evaluate the synergistic ef-
fect of the combined system as shown in Eq. (2).
k
sonophoto
Synergistic Index ¼
ð2Þ
k
sono þ kphoto
Fig. 3. Changes in TOC observed as a function of irradiation time during the
photocatalytic, sonochemical and sonophotocatalytic degradation of 100
l
M MO at
where, ksonophoto, ksono and kphoto, are the rate constants for sonop-
hotocatalysis, sonolysis and photocatalysis, respectively. In order
pH 2 in aqueous solution. The TiO
2
loading in photocatalytic and sonophotocatalytic
ꢀ1
process was 1 mg mL . The ultrasonic frequency was 213 kHz and the power was
ꢀ
1
to reduce the influence of pH on the adsorption of MO onto TiO
2
,
55 mW mL .

Y. He et al. / Ultrasonics Sonochemistry 18 (2011) 974–980
977
Fig. 3 shows the TOC content of aqueous solutions containing
MO during the sonochemical, photocatalytic and sonophotocata-
lytic degradation processes at pH 2. It is obvious that the TOC in
all systems gradually decreases with an increase in irradiation
time, but not quite obeying first-order kinetics over the whole time
range examined. The photocatalytic and sonochemical oxidation
processes can effectively mineralize the azo dye. It can be seen that
the combined system has a higher mineralization efficiency com-
pared to the individual processes. However, as in the degradation
of the parent compound, the overall mineralization is lower than
an additive effect of the individual processes (negative synergistic
effect).
3.2. Proposed sonophotocatalytic degradation pathway
The chromatograms of MO after 60 min sonolysis, photocatal-
ysis and sonophotocatalysis at pH 2 are presented in Fig. 4. The
chromatograms show clearly the appearance of peaks corre-
sponding to the by-products from the degradation of the parent
MO. It is evident that the same by-products were formed during
all three AOPs. After 60 min degradation, nearly 70% of the MO
was removed by photocatalysis (Fig. 4a), 50% by sonolysis
(Fig. 4b) and 80% by sonophotocatalysis (Fig. 4c). However,
according to Fig. 4, after 60 min, many intermediate species were
still present in the solution. There are seven main products (A–G)
observed in the chromatographs of all three AOPs. The peak cor-
responding to the remaining MO decreases in the following order:
sonophotocatalysis < photocatalysis < sonolysis. These observa-
tions indicate that the photocatalytic and sonophotocatalytic deg-
radation processes are the more efficient processes in
decomposing the parent azo dye. This observation corresponds
to the UV–vis spectra, kinetics results and TOC analysis. Addition-
ally, the by-products include hydrophobic as well as hydrophilic
intermediate compounds. As mentioned previously, the solvo-
phyllic nature of the compound favours differing processes during
sonophotocatalytic degradation. The hydrophobic products tend
to be surface active at the bubble/solution interface and therefore
more readily degraded by acoustic cavitation. However, photoca-
talysis has a greater capability to decompose the hydrophilic
products.
The molecular structures of the by-products were deduced by
analyzing the samples with MS, MS/MS and comparing the results
from the literature [25–29]. The parent molecule, MO (retention
time: 8.4 min), exhibits a clear MS signal corresponding to a nega-
tive ion m/z 304. It is worth noting that one intermediate among
these seven products, G, exhibited a longer retention time (m/z
3
20, retention time: 12.8 min) indicating greater hydrophobicity.
Normally, hydroxylated products show a more hydrophillic char-
acter. Despite this contradiction, it was confirmed that product G
was a monohydroxylated MO. The same hydrophobic hydroxylated
product has been observed in photocatalysis by a number of re-
search groups [25–27,29].
Product D (m/z 290, retention time: 3.6 min) is a demethylated
intermediate formed by cleaving a methyl group from the MO mol-
ecule. The remaining intermediate compounds are by-products
formed by the addition of one or two hydroxyl groups or cleavage
of one or two methylene groups. For example, Product B (m/z 292,
retention time: 2.4 min) is a derivative formed by addition of one
hydroxyl group and extraction of two methyl groups from the
MO molecule.
Based on the structures of the by-products formed during the
degradation, all the identified intermediates contain both the azo
group and the sulfonic group. The structures of these intermediates
were further confirmed by UV–vis spectroscopy.
Fig. 4. High performance liquid chromatographs observed after 60 min of photo-
catalytic, sonolytic and sonophotocatalytic degradation of 100 lM MO at pH 2. The
ꢀ
1
TiO
2
loading in all three oxidation systems was 1 mg mL . In sonolysis and
sonophotocatalysis, the ultrasonic frequency was 213 kHz and the power was
ꢀ
1
5
6
5 mW mL . The detection wavelength range of 3D chromatographs was 190–
40 nm and 464 nm for monitoring MO concentration.
ic degradation of MO originated from the hydroxylation and/or
demethylation of MO. Fig. 5 shows a proposed sonophotocatalytic
degradation pathway scheme. The degradation pathway mainly
As shown in the list of by-products (Fig. 4), the molecules
formed during the sonolytic, photocatalytic and sonophotocatalyt-

9
78
Y. He et al. / Ultrasonics Sonochemistry 18 (2011) 974–980
Fig. 5. Schematic illustration of the proposed degradation pathway and the events taking place during the sonophotocatalytic degradation of MO.
focuses on the transformation of the intermediate species, which
precedes the aromatic ring-opening.
crease, demonstrating that they undergo two processes:
formation and decomposition.
Products D and G are intermediate compounds formed through
the demethylation and hydroxylation of MO, respectively (Fig. 5).
The demethylation of MO is a process which involves the homo-
lytic rupture of the nitrogen-carbon bond of the amine group lead-
ing to the substitution of a methyl group with a hydrogen atom.
Consecutive demethylation is the likely pathway to the formation
of Products E and B. Whichever decomposition process MO mole-
cules undergo, sonolysis or photocatalysis, OHÅ radical attack is
the main cause of degradation [2,22]. Products A, B, E and G are
formed by multiple substitutions of hydroxyl groups. Evidently,
due to simultaneous and independent occurrences, both demethyl-
ation and hydroxylation can be expected to couple with each other
or take place alternatively. Consequently, the by-products of MO
degradation are determined by the individual demethylation and
hydroxylation processes, and by their combinations.
Fig. 6a shows the formation trend of Product D as a function of
irradiation time during sonolysis, photocatalysis and sonophotoca-
talysis. The peak areas in both photocatalysis and sonophotocatal-
ysis show a bell shape trend with irradiation time. The amount of
Product D formed during both processes reaches its maximum
within the first 30 min. However, the amount of Product D existing
in solution gradually increases over the whole sonochemical deg-
radation process. This observation demonstrates that the decom-
position is faster than the formation during photocatalytic and
sonophotocatalytic degradation, and the hybrid technique is better
for decomposing the intermediate D. The destruction of the inter-
mediate by sonolysis seems to be the slowest among these three
advanced oxidation processes. Compared to pH 2, the amount of
Product D formed at pH 10 is considerably larger (shown in
Fig. 6b), indicating that in spite of the formation of the same main
active species, the pH environment is capable of changing the pro-
portion of products formed. Consequently, it is possible to choose
the right process and pH conditions in order to inhibit the forma-
tion of certain products, which may also be harmful to the environ-
3.3. Degradation of by-products
Two primary intermediate by-products, Product D (a demethy-
ment. At pH 10, the amount of Product D during the
lated product) and Product G (a monohydroxylated product) were
monitored in order to investigate the sonochemical, photocatalytic
and sonophotocatalytic degradation of the intermediate products.
Fig. 6 shows the HPLC peak areas for these two intermediates as
a function of the irradiation time. The peak areas of both by-prod-
ucts at different pH values first increase and subsequently de-
sonophotocatalytic process reaches a maximum at 30 min, but in
photocatalysis it is at 90 min. At pH 10, the hybrid system exhibits
a greater synergistic effect for the degradation of Product D than
that at pH 2.
Fig. 6c and d show the change in amount of Product G with irra-
diation time under the three AOPs at pH 2 and pH 10, respectively.

Y. He et al. / Ultrasonics Sonochemistry 18 (2011) 974–980
979
Fig. 6. The concentration changes of Products D and G during 120 min sonophotocatalytic degradation of 100
was 1 mg mL . In the sonolysis and sonophotocatalysis processes, the frequency of ultrasound was 213 kHz and the power was 55 mW mL
l
M MO at pH 2 and pH 10. The TiO
2
loading in each experiment
ꢀ
1
ꢀ1
.
The hydroxylation of MO in photocatalytic and sonophotocatalytic
processes is faster than under sonolysis. During the first 30 min at
pH 2, MO was hydroxylated by photocatalysis at the same rate as
sonophotocatalysis. However, after 30 min, the amount of mono-
hydroxylated product in sonophotocatalysis decreased while that
in photocatalysis increased until 60 min. The reason for the faster
sonophotocatalytic decomposition of Product G may lie in the
sonochemical pathway. Compared to Product D, Product G exhibits
a greater hydrophobicity (HPLC chromatograph in Fig. 4), leading
to a higher bubble/solution surface activity. This property allows
these molecules to accumulate at the liquid/bubble interface and
thus increases their exposure to the primary radicals generated
within the cavitation bubbles.
dation of MO in aqueous solution. The pH and the physico-chemi-
cal properties of the organic species affect their hydrophobicity
and therefore their surface active properties. These changes con-
tribute to changes in the overall sonophotocatalysis rate. Further-
more, it has been demonstrated that the solution pH can affect
the build-up rate of each by-product generated during sonolysis,
photocatalysis and sonophotocatalysis. It was found that sonop-
hotocatalytic degradation of the parent organic pollutant, as well
as the derivative by-products, was much faster than the individual
oxidation processes combined. In particular, it has been demon-
strated that sonophotocatalysis was significantly more effective
at degrading the intermediate by-products. Therefore, the sonop-
hotocatalytic technique is likely to lead to both a more complete
and a faster mineralization of organic pollutants in aqueous solu-
tions than the individual processes. It was observed that sonolysis
is effective in decomposing hydrophobic organic compounds and
photocatalysis is better for hydrophilic compounds. The hydropho-
bic quality of a solute leads to a higher surface activity of the mol-
ecules and results in enhancing the encounters between the
organic molecules and the highly reactive primary radicals at the
liquid/bubble interface. Hydrophilic molecules on the other hand
The maximum amounts of Products D and G are higher at pH 10
than that at pH 2. One possible reason is that the alkaline environ-
ment favours the formation of OHÅ radicals and consequently facil-
itates the hydroxylation process.
4
. Conclusions
The results reported here demonstrate that sonophotocatalysis
2
are more readily adsorbed onto the surface of TiO and decom-
is an effective technique for environmental remediation. A lower
solution pH environment facilitates the sonophotocatalytic degra-
posed by photocatalysis. The results shown in this paper are also
significant in understanding the sonophotocatalytic degradation

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Y. He et al. / Ultrasonics Sonochemistry 18 (2011) 974–980
activities and provide information related to the decomposition
pathway of organic dye molecules.
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treatment: A critical review, AIChE J. 50 (2004) 1051–1079.
[
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The financial support and the infrastructural facilities provided
by Particulate Fluids Processing Centre (PFPC), a special research
centre funded by the Australian Research Council (ARC), are greatly
acknowledged. Particular acknowledgements are extended to our
colleagues, Dr. Adam Brotchie and Dr. Glenna Drisko, for the help
in preparation of manuscript. Y. He thanks the University of Mel-
bourne for the award of an International Postgraduate Research
Scholarship (IPRS) and the David Hay Memorial Fund.
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