Article
pubs.acs.org/JPCB

Inﬂuence of Microbubbles on Free Radical Generation by Ultrasoundin Aqueous Solution: Dependence of Ultrasound Frequency
Nanae Masuda,† Aya Maruyama,† Toshihiko Eguchi,‡ Tsutomu Hirakawa,§ and Yoshinori Murakami*,††Department of Materials Engineering, Nagaoka National College of Technology, Nishi-Katakai, Nagaoka, 940-2188, Japan
Aura-tec Co., Ltd. 1725-2 Tsubuku-honmachi, Kurume, Fukuoka 830-0047, Japan§
Environment Management Research Institute, AIST, Tsukuba, Ibaraki 305-8569, Japan‡

ABSTRACT: The inﬂuence of microbubbles on sonochemical eﬃciencies has been investigated under 28, 45, and 100
kHz ultrasound irradiation. For the 28 and 100 kHz ultrasoundfrequencies, microbubbles suppressed the I3− formation fromKI solution as well as the 7-hydroxycoumarin formation fromcoumarin solution caused by the ultrasonic irradiation. On theother hand, for the 45 kHz ultrasound frequency, microbubbles enhanced the I3− formation from KI solution as well as
7-hydroxycoumarin formation from coumarin solution causedby the ultrasonic irradiation. Detection of H2O2 after theirradiation of ultrasound in the presence or absence of
microbubbles was also performed, and it was found that H2O2
formation was enhanced only when microbubbles wereintroduced under the 45 kHz ultrasonic irradiation, which
was in good agreement with the results of KI oxidation
dosimetry measurements and of coumarin ﬂuorescent probe
measurements. Based on these present results, plausible mechanisms that explain the dependence of the ultrasound frequency on
the enhancement and suppression of free radical formation in the presence of MBs were proposed.

1. INTRODUCTION
In a recent years, the application of sonochemical processes as
an advanced oxidation process have intrigued by a lot ofengineers because of the high eﬃciencies as well as the easy
operations.1−3 Ultrasonic irradiation into a liquid is known togenerate acoustic cavitation, and then the collapse of thesecavitation bubbles results in a hot spot, which has an extremely
high temperature and pressure.4 As a consequence of the hightemperature and pressure in a hot spot, water in a solution wasthermally decomposed to form reactive hydroxyl radicals,followed by the sonochemcal reactions as well as sonoluminescence via the free radical reaction mechanism.
It has been reported that the addition of alcohol to an
aqueous solution greatly reduces sonoluminescence relative tothat in pure water.5−8 They suggested that alcohol adsorbs on
the surface of bubbles and stabilizes the pulsation of bubbles.
On the other hand, Sunartio et al.9 reported that, at relatively
low ultrasonic powers, adding a certain concentration of alcoholincreased the sonoluminescence intensity relative to that forpure water. Tuziuti et al. 10 also observed a higher
sonoluminescent intensity with relative lower power in asolution of lower ethanol concentration. They attributed thereason for the increase of the sonoluminescent intensity as adecrease of bubble−bubble coalescence under low solubility.
Thus, the important roles of the stability of cavitation bubbles
on the intensity of sonoluminescence have been suggested from© 2015 American Chemical Society

these works. It can also be expected that the stability ofcavitation bubbles has an inﬂuence not only on thesonoluminescence intensity but also on the overall reaction
rates of sonochemical reactions.
Recently, microbubbles (MBs) with diameters of 10−50 μm
have been explored for various applications for water
treatment.11 MBs are so tiny, that they are stable in an aqueous
solution for longer periods of time than normal macrobubbles.
Therefore, MBs are well-known to have excellent gas
dissolution ability, and thus various applications of MBs areavailable now.12 It has also been reported that ζ-potentials of
MBs in aqueous solutions were negative for a wide range of pH
conditions13 and that hydroxyl radicals were also formed bycollapsing MBs under acidic conditions less than pH < 3.14,15
Although numerical works have been done for the investigationof the physical and chemical properties of MBs, relatively fewer
works have been done for the investigation on the interactionof ultrasound with MBs. Kobayashi et al. 16 reported
agglomeration and rapid ascent of microbubbles using
ultrasound with frequency of 2.4 MHz and concluded that
aggregation of microbubbles was caused by the secondary
Bjerknes force17 on the bubble in the liquid phase. They also
Received: June 15, 2015
Revised: August 27, 2015
Published: September 17, 2015
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DOI: 10.1021/acs.jpcb.5b05707
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The Journal of Physical Chemistry B

the air into the sample solution using a high pressure systemand then by reducing the pressure abruptly at the exit nozzles.
Figure 2 shows the bubble diameter distributions measured via

reported that addition of surfactants to the MBs in a solution
decreased the aggregation eﬃciencies using ultrasound.18 Since
ultrasound accelerates the aggregation process of the microbubbles in an aqueous solution, there might be some eﬀects on
the behaviors and stabilities of the cavitation bubbles formed by
the irradiation of ultrasound to a solution. Because stabilities of
the cavitation bubbles are strongly related to the eﬃciencies ofthe free radical formation by the thermal decomposition ofwater in a hot spot, the free radical formation by ultrasoundmight be inﬂuenced by the existence of MBs in a solution.
However, as far as we know, there are no reports that theamount of free radicals formed by ultrasound cavitation isinﬂuenced by the MBs introduced into an aqueous solution.
In the present work, we have investigated the inﬂuence ofexistence of MBs on generation of free radical by ultrasoundirradiation using several analytical techniques such as KI
method, coumarin ﬂuorescence probe method, and a titanium
sulfonate reagent titration method to detect H2O2 formed by
the recombination reaction of OH + OH. With three diﬀerent
ultrasound frequencies (25, 64, and 100 kHz), the diﬀerent
eﬀects of the free radical formation on MBs were investigated.

Figure 2. Bubble diameter distributions of the microbubble generator(OM4-MDG-045, AURA Tec, Co. Ltd.) measured via the laser
particle size analyzer.

Figure 1. Schematic ﬁgure of the experimental setup.

the laser diﬀraction particle size analyzer (SALD-7000,
Shimazu). For these bubble size measurements, the measurements were performed using the ﬂowing system with ﬂow rateof 1 L/min to avoid the inﬂuence of the coalescence of
microbubbles during the bubble size measurements. The
diameter and length of the connecting tube between the
microbubble generator and the laser diﬀraction particle size
analyzer were about 4 mm and 50 cm, respectively. As shown in
Figure 2, it is clearly seen that relatively uniform bubble size
distributions having diameters between 0.6−1.2 μm with the
maximum of 1 μm (full width of half-maximum is within 0.4μm) were obtained for this microbubble generator. The
number of microbubbles generated in the microbubble
generator is estimated to be around 1.0 × 109 bubbles cm−3
s−1 based on the assumption that all of the gas introduced into
the MB generator becomes microbubbles with 1 μm diameter(5 × 10−19 m3 in volume).

Electronics), which has a maximum output of 300 W, wasattached to the bottom of the water tank, and the temperatureof the water tank was kept to around 10 °C by circulating thewater of the tank to the cooling bath (LTC-S300C, AS ONE
Inc.). A 500 mL glass container with external diameter of about
80 mm and a ﬂat thin bottom of about a few millimeters wasplaced in the center of the water bath and was exposed toultrasound continuously for a certain time from the ultrasoundtransducer attached at the bottom of the water bath after 300
mL of sample solution was ﬁlled into a glass container in thewater tank. During the ultrasonic irradiation, the temperature of
the sample solution was carefully checked to be constant via athermometer in a sample solution. The ultrasound transducer
can generate three diﬀerent ultrasound frequencies of 28, 45,
and 100 kHz, and then the sonochemical eﬃciency of eachfrequency was checked by the KI oxidation dosimetry method,which was previously reported by Koda et al.19 as the best
method to calibrate sonochemical eﬃciencies compared withthe other methods.
To introduce MBs into the sample solution, the microbubble
generator (OM4-MDG-045, AURA Tec Co.Ltd.) was connected to the sample solution. The microbubble generator
circulates the sample solution with the ﬂow speed of 1.3−1.7
L/min and produces very uniform microbubbles by dissolving

3. RESULTS AND DISCUSSIONS
3.1. Measurement of I3− Absorption Formed by
Ultrasound Irradiation to Ki Solution in the Presence
and Absence of MBs. It has been reported by Koda et al.19
that KI oxidation dosimetry using 0.1 mol dm−3 KI solution was
the simple and the easiest method to calibrate the sonochemicaleﬃciency. Therefore, we have ﬁrst attempted to use the same
KI oxidation dosimetry techniques to investigate the eﬀects of
microbubbles on the sonochemical eﬃciencies. Since cavitation
formed by the diﬀerent ultrasound frequencies might possiblyinteract with the microbubbles in a diﬀerent way, we have
performed the KI oxidation dosimetry experiments with three
diﬀerent ultrasound frequencies (28, 45, and 100 kHz) in boththe presence and absence of microbubbles.
Figure 3 illustrates examples of the absorption spectra in thepresence and absence of microbubbles after 30 min ultrasoundirradiation. All of the results measured at three diﬀerent
ultrasound frequencies (28, 45, and 100 kHz) were also given.
Figure 3 clearly shows that the inﬂuence of microbubbles on
the sonochemical eﬃciencies diﬀered with three diﬀerent
ultrasound frequencies. That is, I3− absorption at 350 nm wasenhanced by the presence of microbubbles under the 45 kHz
ultrasonic irradiation, while I3− absorption was suppressed by
the presence of microbubbles under irradiation of 28 and 100

2. EXPERIMENTAL METHODS
Figure 1 shows a schematic of the experimental setup. As
shown in Figure 1, the ultrasonic transducer (W-115T, Honda

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DOI: 10.1021/acs.jpcb.5b05707
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speculated that microbubbles inﬂuenced the formation ofcavitation bubbles. To conﬁrm that microbubbles have diﬀerent
eﬀects on the I3− formation by the irradiation of three diﬀerent
ultrasound frequencies (28, 45, and 100 kHz) to 0.1 mol dm−3
KI solution, a diﬀerent size of glass container (volume 1000
mL, external diameter of about 120 mm with a ﬂat bottom) wasused and 400 mL of sample solution was ﬁlled into the glass
container for the KI oxidation dosimetry experiments. Figure 4

Figure 4. Comparison of the eﬀect of MBs on the ﬁnal absorbance forI3− at 350 nm formed by the 30 min ultrasound irradiation to 0.1 M KI
solution. The ultrasound frequencies were (a) 28 kHz, (b) 45 kHz,and (c) 100 kHz.

shows the comparisons of the I3− absorbance between in theabsence of MBs and in the presence of MBs after 30 min
ultrasound irradiation for each ultrasound frequency. In Figure
4, the results of the 500 mL glass container are also given. All
these experiments were repeated more than three times, andthe average value is used in Figure 4. As shown in Figure 4,
irreversible to the type of glass containers, microbubbles
enhanced I3− formation by ultrasonic irradiation with thefrequency of 45 kHz. On the other hand, microbubbles
suppressed I3− formation by ultrasonic irradiation with thefrequencies of 28 and 100 kHz. It was also found that the 1000
mL glass container had larger enhancements of I3− formation
than the 500 mL container with the frequency of 45 kHz, butthat the eﬀect of the suppression for I3− formation with thefrequencies of 28 and 100 kHz was almost similar between
these two containers.
3.2. Measurement of the Fluorescence Spectra for 7Hydroxycoumarin Formed by the Sonolysis of Coumarin in the Presence or Absence of MBs. It has been
demonstrated in the previous section that inﬂuence of the
microbubbles to the ultrasonic oxidation reactions of I− ion in
KI solution is strongly dependent on the ultrasound frequencies(28, 45, and 100 kHz). To conﬁrm such ultrasound frequency
dependence, coumarin ﬂuorescence probe methods were also
applied in the present study. Since coumarin is easily oxidized
by OH radicals to form 7-hydroxycoumarin by the following
reaction scheme

Figure 3. Absorption spectra of I3− after the 30 min irradiation ofultrasound to 0.1 M KI solution in the presence (solid line) andabsence (dashed line) of MBs with frequency of (a) 28 kHz, (b) 45
kHz, and (c) 100 kHz.

kHz ultrasounds. It was also conﬁrmed that I3− absorbance at
350 nm after 30 min ultrasonic irradiation in the absence of
microbubbles was nearly the same value of around 0.10
irreversible to the ultrasound frequencies, suggesting that thesonochemical eﬃciencies were nearly the same with three
diﬀerent ultrasound frequencies in our experimental setups.
Since I3− was formed by the oxidation of I− ion followed by thesubsequent reaction with the excess I−,−

2I → I 2 (oxidation by ultrasound)−I2 + I→ I3−

coumarin + OH → 7‐hydroxycoumarin(3)

and 7-hydroxycoumarin is much more ﬂuorescent thancoumarin, the amount of OH radicals can be easily monitored
via the ﬂuorescence of 7-hydroycoumarin around 450 nm
formed by reaction 3 in a coumarin solution. So far, therefore,
this coumarin probe method is now accepted as one of thestandard methods used as the probe of OH radicals formed invarious environments.20,21 Ultrasound irradiation is also known
to form OH radicals by the collapse of the cavitation

(1)
(2)

the present results indicated that microbubbles had positive or
negative eﬀects on the oxidization reaction 1, depending on theultrasound frequencies. Oxidation of I− ion by the ultrasonicirradiation was due to OH radicals formed by the collapse of
these cavitation bubbles in a hot spot, and therefore, it was
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Figure 5. Fluorescent spectra of 0.1 mM coumarin solution after 30 min ultrasound irradiation in the (a) absence and (b) presence of MBs. The
solid and dashed lines are the absorption spectra taken at 30 and 0 min after the ultrasound irradiation, respectively. Excitation wavelength was 332
nm, and the ultrasound frequencies were (1) 28 kHz, (2) 45 kHz, and (3) 100 kHz.

the temporal change of the ﬂuorescence intensity at 400 nm
during the circulation of the sample solution into the MB
generator without irradiating ultrasound and it was found thatthe ﬂuorescence intensity started decreased immediately after
the circulation into MB generator and then the ﬂuorescenceintensity at 400 nm became nonzero constant value even after
the long-term circulation, which was typical example of thetemporal proﬁle of the adsorption equilibrium. Thus, it was
conﬁrmed that the decrease was due to the adsorption to thewall of the MB generator or otherwise the interface between air
and water of bubbles, not due to the reaction caused by MBs,
which is consistent with the previous results with Takahashi etal. that at neutral pH condition no phenol decomposition wasobserved by the collapse of MBs.15 To conﬁrm these results ofthe coumarin ﬂuorescent probe experiments were not causedby the special condition caused by the 500 mL glass container,
the 500 mL glass container with 300 mL of 0.1 mM coumarinsolution was replaced with a 1000 mL glass container with 400
mL of 0.1 mM coumarin solution and the same coumarin
ﬂuorescene probe experiments were performed. Although
diﬀerent size of glass container was used, similar frequency
dependence was also observed.
Increase of the sonochemical eﬃciencies by the introductionof MBs under the 45 kHz ultrasound irradiation was suggested
from both KI oxidation dosimetry and coumarin ﬂuorescentprobe measurements in the present study. However, to conﬁrm
the important roles of MBs on the enhancement of theformation of ultrasonic cavitation bubbles, information about
the absolute concentration of OH radicals increased by the
introduction of MBs under the 45 kHz ultrasound irradiation
was important. Since the reaction products and the yield of OH
radical for the reaction of coumarin with OH radical has alreadybeen reported by Zhang et al.21 using the same ﬂuorescencemeasurements, we can estimate the absolute concentration of
OH radicals formed by the ultrasonic irradiation to the 0.1 mM
coumarin solution. Absolute concentration of 7-hydoroxycoumarin was determined by measuring the ﬂuorescence intensity
at 450 nm with various concentrations of 7-hydroxycoumarin.
From these ﬂuorescence calibration experiments, the amount of

bubbles,22,23 and thus, the coumarin probe method isappropriate for monitoring the inﬂuence of microbubbles on
the sonochemical reactions.
Figure 5 shows typical examples of the ﬂuorescence spectraof 0.1 mM coumarin solution before and after the 30 min
ultrasound irradiation in the absence and presence of MBs. In
the absence of MBs, only the ultrasound with the frequency of
100 kHz gave a small shoulder around 450 nm, indicating that
7-hydroxycoumarin was formed by the irradiation of 100 kHz
ultrasound in 0.1 mM coumarin solution. On the other hand,
there was no clear evidence for the formation of 7hydroxycoumarin when the ultrasound frequencies were 28
and 45 kHz. This is consistent with the previous study of Koda
et al.6 that sonochemical eﬃciencies abruptly increase around
100 kHz. In other words, sonochemical reactions only occurred
for the ultrasound frequency above 100 kHz, but fewer
sonochemical reactions occurred for the ultrasound frequencies
of below 100 kHz, such as 28 and 45 kHz.
However, when MBs were introduced into the sonochemical
reactors, a small shoulder peak around 450 nm, which appeared
when the 100 kHz frequency of ultrasound was irradiated to 0.1
mM coumarin solution in the absence of MBs, was completely
disappeared. On the other hand, a large ﬂuorescence peak of 7hydroxycoumarin around 450 nm appeared when the 45 kHz
frequency of ultrasound was irradiated to 0.1 mM coumarinsolution in the presence of MBs. Thus, we could conclude thatmicrobubbles enhances 7-hydroxycoumarin formation only
when ultrasound of 45 kHz frequencies were irradiated to the
0.1 mM coumarin solution and that formation of 7hydroxycoumarin was suppressed when MBs were introduced
into the solution with ultrasound irradiation of 28 or 100 kHz
frequencies. These results are in good agreement with theresults of KI oxidation dosimetry experiments as was describedin the previous section. Decrease of the ﬂuorescent intensity
around 400 nm was also observed for the ultrasound of all
frequencies (28, 45 and 100 kHz) when the MBs are
introduced into the sonochemical reactors as shown in (1b),(2b), and (3b) of Figures 5. To understand the reason for suchdecrease of the ﬂuorescence around 400 nm, we have measured
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around 400 nm with H2O2, the increased amount of H2O2 byintroducing MBs under the 45 kHz ultrasound irradiation wasdetermined to 2 × 1016 molecules cm−3. Dividing the numberof H2O2 molecules with time, the formation rate of H2O2
became 2 × 1012 molecules cm−3 s−1. This value is about 100
times larger than the estimated value of the rate of OH radical
formation derived by the coumarin probe method. The reason
is not yet well understood, but one plausible explanation is that,for coumarin ﬂuorescent probe measurements, a large part of
OH radicals were lost with other side reactions originating fromthe existence of coumarin in a solution.
3.4. Mechanism for the Enhancement and Suppression of Sonochemical Reactions with MBs. The present
experiments support the conclusions that MBs play importantroles on the sonochemical eﬃciencies. Ogi et al.25 have alreadypointed out the important roles of the dissolved gas insonochemical degradations. However, the eﬀect of ultrasoundfrequencies on the free radical enhancement or suppression of
MBs as presented in this paper have not been reported before.
Since the sonochemical eﬃciencies were strongly related to the
hot spots generated by the ultrasonic cavitation, it is speculated
that microbubbles inﬂuenced the stabilities of ultrasoniccavitation. According to the previous experimental studies,26
stable cavitation is produced in a standing wave ﬁeld. Since air
in microbubbles strongly reﬂects the ultrasound wave that havetransversed in water due to the large diﬀerence of acoustic
impedance between air and water, microbubbles disturb thestanding wave formed by the ultrasonic transducer attached to
the water tank. Thus, microbubbles reduces the hot spot
created by the collapse of cavitation bubbles and then reduced
the yield of OH radical formation. This might be the reason forthe suppression of OH radical formation by microbubbles forthe 28 and 100 kHz ultrasound irradiations.
For 45 kHz ultrasound irradiation, OH radical formation wasenhanced by microbubbles. This phenomena could not be
explained only by the disturbance of standing wave of
ultrasound wave by microbubbles. Makuta et al.27 have recently
reported the bubble collapse by the ultrasonic irradiation
followed by the microbubble generations. If 45 kHz ultrasoundcould interact with MBs with diameter of around 1 μm used in
the present study and new small cavitation bubbles are formed
from them, it is probable that OH radical formation under theirradiation of 45 kHz ultrasound was enhanced by microbubbles. The schematic ﬁgure of the mechanism was given in
Figure 7.
Although the bubble collapse by the irradiation of ultrasoundwas experimentally conﬁrmed by Makuta et al.,27 fundamental
understanding of the interaction between bubble and ultrasound is not well understood. It is well-known that primary andsecondary Bjerknes forces are the main forces that act to a
bubble in the acoustic ﬁeld.4 However, these forces are
dependent on the acoustic force ﬁeld, and it is not probablethat these primary and secondary Bjerknes forces are the reason
why only the ultrasound frequency of 45 kHz has the ability toenhance the radical formation. According to the recent
experimental studies of the direct observation of a single
bubble under the ultrasound ﬁeld,28 the Minnaert bubble
resonant frequency of bubble is the key factor on the collapse of
bubbles under the ultrasound ﬁeld. The Minnaert bubble
resonance frequency f, which is given in the following
expression29

7-hydroxycoumarin increased by the 30 min ultrasoundirradiation of 45 kHz ultrasound frequency in the presence ofMBs was estimated to be around 8.3 × 1013 molecules cm−3.
Since around 8.3 × 1013 molecules cm−3 of 7-hydroxycoumarin
were formed by the 30 min ultrasound irradiation, theformation rate of 7-hydroxycoumarin was calculated to bearound 1.5 × 108 molecules cm−3 s−1. Using literature value(6.1%)21 of the reaction yield of 7-hydroxycoumarin by thereaction of OH with coumarin, about 2.5 × 109 molecules cm−3
s−1 of OH radicals were increased by introducing MBs in the
500 mL glass container under the 45 kHz ultrasoundirradiation.
3.3. Measurement of Hydroxyperoxide (H2O2) Formedby the Sonolysis of Water in the Presence and Absenceof MBs. In both of the measurements of the KI method andthe coumarin probe method, it was conﬁrmed that microbubbles enhanced the OH radical formation with 45 kHz
ultrasonic irradiation, while microbubbles suppressed the OH
radical formation with 28 and 100 kHz ultrasonic irradiation.
Since OH radicals formed by the sololysis of water are expected
to recombine with each other to form H2O2 by the following
reaction,
OH + OH → H 2O2(4)

It is speculated that H2O2 formation will also be enhanced by
MBs under 45 kHz ultrasound irradiation and that H2O2
formation will be suppressed by MBs under 28 and 100 kHz
ultrasound irradiation. In the present study, H2O2 were titrated
by adding 2 mL of titanium sulfate (Ti2(SO4)3) reagent to theultrasound irradiated water sample (400 mL) and measured theappearance of new absorption band around 400 nm.24 For
these H2O2 measurements after 3 h ultrasound irradiation topure water, titanium sulfate reagent was added to titrate H2O2
formed by the ultrasound irradiation. As expected, newabsorption band around 400 nm by reaction of H2O2 with
Ti(SO4)2 was observed only with the experiments of the 45
kHz ultrasonic irradiation in the presence of MBs for 3 h. But
when the ultrasound frequencies became 28 and 100 kHz, no
H2O2 formation was observed using this Ti2(SO4)3 titration
reaction method. The results of the ultrasound frequency of 45
kHz are given in Figure 6. Calibrating the absorption band

Figure 6. Absorption spectra between 300 and 600 nm after 3 hirradiation of ultrasound to water after adding 2 mL of Ti2(SO4)3 in
the presence (solid line) and absence (dashed line) of MBs withfrequency of 45 kHz.
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Figure 7. Schematic of the inﬂuence of the MBs on the formation to the cavitation babbles.
1/2⎛ 1 ⎞⎛ 3γP0 ⎞⎟⎜f=⎜⎟⎝ 2πR ⎠⎝ ρ ⎠(2) Dai, Y.; Li, F.; Ge, L.; Zhu, F.; Wu, L.; Yang, X. Mechanism of theenhanced degradation of pentachlorophenol by ultrasound in thepresence of elemental iron. J. Hazard. Mater. 2006, 137, 1424−1429.
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where R is the bubble radius (m), γ is the speciﬁc heat ratio, ρ isthe density of water (kg/m3), and P0 is the pressure of the
bubble (Pa), was calculated for all of the ultrasound frequencies(28, 45, and 100 kHz) used in the present study. However, thecalculated bubble radii R that were resonant to the ultrasoundfrequencies of 28, 45, and 100 kHz were around 120, 70, and 30
μm, respectively, and thus the resonant bubble radius is muchlarger than the bubble radius (around 1 μm) generated by the
microbubbles used in the present study. According to therecent numerical simulation of sonochemical reactions forvarious ultrasonic frequencies and various acoustic amplitude
inside an isolated spherical air bubble,30 dissociation of H2O2 toform OH radical preferentially proceeds with lower ultrasoundfrequencies due to the longer duration of high temperature at
the bubble collapse, although ultrasound with higher frequency
has higher energy densities inside the bubble. Thus, thechemistry of the microbubble under the ultrasound irradiation
are complicated phenomena. So far, the mechanism ofenhanced radical formation by the 45 kHz ultrasoundirradiation obtained in the present study is not clearly given.
The direct visualization of the collapse of microbubbles as wellas the measurements of the bubble size reduction with theirradiation of 45 kHz ultrasound is another way to make clear
the radical-enhancement mechanism with the ultrasound of 45
kHz, but it was not successful because microbubbles
disappeared immediately after the irradiation of ultrasound as
was previously reported by Kobayashi et al.16 Further work isneeded to clarify the reasons, and now studies are underway toclarify the mechanism.

■

AUTHOR INFORMATION

Corresponding Author*E-mail: murakami_mb@nagaoka-ct.ac.jp.
Notes

The authors declare no competing ﬁnancial interest.

■

REFERENCES(1) Chowdhury, P.; Viraraghavan, T. Sonochemical degradation ofchlorinated organic compounds, phenolic compounds and organicdyes − A Review Article. Sci. Total Environ. 2009, 407, 2474−2492.
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method to calibrate sonochemical efficiency of an individual reaction
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DOI: 10.1021/acs.jpcb.5b05707
J. Phys. Chem. B 2015, 119, 12887−12893