Mechanism of enhancement of sonochemical-reaction efficiency by pulsed ultrasound

Article abstract of DOI:10.1021/jp802640x

The enhancement of sonochemical-reaction efficiency by pulsed ultrasound at 152 kHz has been studied experimentally through absorbance measurements of triiodide ions from sonochemical oxidation of potassium iodide at different liquid volumes to determine

Full text of DOI:10.1021/jp802640x

4875
2008, 112, 4875–4878
Published on Web 05/10/2008
Mechanism of Enhancement of Sonochemical-Reaction Efficiency by Pulsed Ultrasound
Toru Tuziuti,* Kyuichi Yasui, Judy Lee, Teruyuki Kozuka, Atsuya Towata, and Yasuo Iida
National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98 Shimoshidami,
Moriyama-ku, Nagoya 463-8560, Japan
ReceiVed: March 27, 2008; ReVised Manuscript ReceiVed: April 23, 2008
The enhancement of sonochemical-reaction efficiency by pulsed ultrasound at 152 kHz has been studied
experimentally through absorbance measurements of triiodide ions from sonochemical oxidation of potassium
iodide at different liquid volumes to determine sonochemical efficiency defined by reacted molecules per
input ultrasonic energy. The mechanism for enhancement of the reaction efficiency by pulsed ultrasound is
discussed using captured images of sonochemiluminescence (SCL), and measured time-resolved signals of
the SCL pulses and pressure amplitudes. The high sonochemical-reaction efficiency by pulsed ultrasound,
compared with that by continuous-wave ultrasound, is attributed both to the residual pressure amplitude during
the pulse-off time and to the spatial enlargement of active reaction sites.
Intense ultrasound radiated into a liquid creates collapsing
cavitation bubbles,1 which have uniqueness of high temperature,
high atmospheric pressure, and rapid heating and cooling rates.2
Some oxidants of hydroxyl radicals, hydrogen peroxide, ozone
etc. are created inside the bubbles.3 The oxidants react with
chemicals such as luminol and emit light, called sonochemilu-
minescence (SCL).4 Chemical reaction with acoustic bubbles
is referred to as sonochemical reaction,2,5 which is expected to
be a useful tool for novel material synthesis,6 environmental
treatment,7,8 biological application,9 etc.
pulsed ultrasound. An appropriate pulse-off time that is specific
to the ultrasound pulsing operation suppresses the generation
of degassing bubbles formed by the coalescence of bubbles,
thereby promoting a high amplitude of sound propagation that
leads to the expansion and contraction of the bubbles. Therefore,
the number of bubbles that are effective for sonochemical
reaction is increased.
In this study, the chemical effects of pulsed ultrasound are
investigated by absorbance measurements of triiodide ions from
the sonochemical oxidation of potassium iodide13 at different
liquid volumes, to determine the sonochemical efficiency defined
by reacted molecules per input ultrasonic energy. The mecha-
nism of highly efficient sonochemical reaction by pulsed
ultrasound is examined together with captured images of SCL
and measured time-resolved signals of SCL and pressure
amplitude.
An irradiation of pulsed ultrasound is useful to obtain highly
efficient sonochemical reaction.10–12 Casadonte et al.11 showed
that both a high yield of H2O2 production and SCL patterns
enlarged toward the side wall of the vessel were obtained using
a modulated mode sonication, compared with those from a
continuous-wave (CW) mode. Henglein et al.12 suggested that
the chemical yield is closely connected to the lifetime of the
bubbles. As the pulse-off time becomes longer, smaller bubbles
dissolve into the liquid more rapidly, because of the higher
excess internal pressure 2σ/r (σ, surface energy; r, radius).
However, the mechanism for the effective change of the spatial
region in sonochemical reactions with a liquid volume under
pulsed ultrasound has not been sufficiently investigated.
The effective region of sonochemical reaction is spatially
restricted when using CW ultrasound, because many degassing
bubbles1 are created under such conditions of sonication. These
degassing bubbles are ineffective for sonochemical reaction, due
to their small volumetric oscillation, and they prevent the
propagation of sound by absorption and scattering of the sound,
which leads to a decrease in the sound pressure amplitude. As
a result, the spatial region for those bubbles that expand and
contract violently, leading to sonochemical reaction, is limited.
Therefore, it is important to optimize the spatial distribution of
cavitation bubbles and the sound field to accomplish high
sonochemical efficiency in a large-scale container for industrial
applications, and this can be effectively implemented using
In this experiment, a continuous or pulsed wave sinusoidal
signal of 152 kHz is generated by a function generator (NF
Electronic Instruments, 1942), and amplified by a power
amplifier (ENI, 2100L, or Honda Electronics, L-400BM-L) to
drive a Langevin-type transducer (Honda Electronics, o.d. 45
mm) attached to a transducer plate (2 mm thick, o.d. 100 mm).
The transducer plate is located at the bottom of a slender liquid
container made of stainless steel. The container is slender with
a rectangular shaped inner volume (100 × 100 × 1510 mm3)
and has six observation windows. The container was filled with
an aqueous solution of 0.1 M potassium iodide (Wako) prepared
from distilled water, and the solution was saturated by air.
Bubbling was performed during sonication at the corner of the
container interior, in order not to disturb the sound field. The
oxidation of I- provides I2, and I3- is produced by the reaction
of I2 and I- in the presence of excess I-.13 The absorbance of
-
I3 was measured with a spectrophotometer (Jasco, V-530)
within the range 280-600 nm, including the peak at 352 nm.14
The peak absorbance value was measured under different
conditions of liquid volume and the way of sonication that was
CW or pulsing operation. The liquid volume was varied from
* Corresponding author. E-mail: tuziuti.ni@aist.go.jp.
10.1021/jp802640x CCC: $40.75
2008 American Chemical Society

4876 J. Phys. Chem. A, Vol. 112, No. 22, 2008
Letters
Figure 1. Dependence of sonochemical-reaction efficiency (SE) on
the liquid volume.
Figure 2. Liquid-volume dependence of the sonochemical-reaction
efficiency (SE) ratio of pulsed ultrasound to continuous-wave (CW)
ultrasound.
0.45 to 5.00 dm3, with a corresponding liquid height ranging
from 44.0 to 477 mm. The liquid temperature inside the
container was maintained at 25 ( 1 °C using a water circulation
cooling jacket attached to the outside of the container. At each
liquid volume, the ultrasonic power was estimated calori-
metrically13 using a thermocouple set at an intermediate position
of the liquid volume. The calorimetrically determined ultrasonic
power was 29 ( 4.5 W, and was relatively constant within the
range of liquid volume used. The net ultrasound irradiation time
was varied from 450 to 3600 s for both CW and pulsed
ultrasonication, depending on the liquid volume. The pulsing
operation condition was a repetition of n-acoustic cycles-ON
and n-acoustic cycles-OFF (variable n ) 100, 1000, or, 10000).
From the measured absorbance of I3- and the ultrasonic power,
the sonochemical-reaction efficiency (SE) was calculated by the
following equation,15
the sonochemical-reaction yield, Henglein showed a periodic
increase and decrease in the yield at an ultrasonic frequency of
10 kHz as the volume increased (height increase);17 the
periodicity has the successive maxima separated by half a
wavelength of sound. This appears to be due to the sound field
resonance sensitivity to the specific liquid height, especially at
low frequency where the sound source is located at the bottom
of the vessel. The magnitude of the maxima decreases mono-
tonically as the liquid volume increases (height increases).
Figure 2 shows the liquid-volume dependence on the ratio
of the pulsed ultrasound SE to the CW ultrasound SE, where
most of the ratios are greater than 1. In this study, pulsing
operation was found to be the most efficient compared with
CW operation. It is remarkable that at 3 dm3 and 100 cycle
ON-100 cycle OFF, the ratio is the highest with an efficiency
that is 5.1 times higher than that under CW operation. A sudden
decrease in the ratio was observed for liquid volumes greater
than 3 dm3. Further investigation is required to clarify the sudden
decrease; however, the origin of the high ratio obtained at 3
dm3 is discussed below.
Figure 3 shows a comparison between photographic images
of luminol-SCL at 3 dm3 under different conditions of pulsed
ultrasound. The SCL images were captured with a digital camera
(Nikon, D70). Luminol (3-aminophthalhydrazide) reacts with
OH radicals generated in the cavitation bubbles to yield
aminophthalate anions and a blue fluorescence when intense
ultrasound propagates through the luminol solution,18 that is,
the luminol exhibits SCL. A solution consisting of 0.33 M
NaOH (Wako) and 1.9 mM luminol (Wako) was prepared using
distilled water. The solution was saturated by air. The temper-
ature of the solution in the container was set at 25 °C. From
Figure 3, the region of luminescence, both around the center
and near the liquid surface, becomes fainter in the order from
(a) to (d). This tendency is consistent with the order of
magnitude of the pulsed to CW SE ratio shown in Figure 2. As
the pulse-ON time increases, the sound field during the pulse-
ON time comes close to that during CW sonication. It is
proposed that under CW sonication, sound propagation is
prevented by the significant production of degassing bubbles,
which results in a decrease in the sound pressure amplitude.
Therefore, the luminescent region under CW sonication is
narrow compared with that under pulsed mode sonication, where
SE ) CI/(Pt/VI)
(1)
for CW and pulsed ultrasound at different n and different liquid
volumes, where CI denotes the I3- ion concentration in the 0.1
mol dm-3 KI solution, VI is the solution volume, P is the
ultrasonic power, and t is the net time of irradiation. Note that
the ultrasonic power was calorimetrically determined at each
liquid volume under CW ultrasound. The ultrasonic power for
the pulsed mode was regarded as the same as that for CW if
the net irradiation time between the CW and the pulsed modes
was common.
Figure 1 shows the dependence of the sonochemical-reaction
efficiency (SE) on the liquid volume. As the liquid volume
increases, each SE value at 152 kHz decreases monotonically,16
because the region that is greater than the cavitation threshold
decreases as the liquid volume increases. A maximum SE of
1.2 × 10-9 mol J-1 was obtained for 1000 cycle ON-1000
cycle OFF at 152 kHz, which is 1.9 times higher than the
reported maximum SE (6.4 × 10-10 mol J-1) at 128.9 kHz CW
from the literature.15 The results reported in the literature15 were
obtained with operating parameters of ultrasonic frequency at
45.0, 128.9, 231.0, and 490.0 kHz, and at different liquid heights
up to 700 mm. Compared with the reported maximum SE (6.4
× 10-10 mol J-1) at 128.9 kHz CW ultrasound in a liquid
volume of 0.585 dm3 from the literature,15 the SE (8.0 × 10-10
mol J-1) obtained at 152 kHz CW and in a liquid volume of
0.587 dm3 in the present experiment was higher.
With regard to the influence of volume change in the
longitudinal direction (change in the height of the liquid) on

Letters
J. Phys. Chem. A, Vol. 112, No. 22, 2008 4877
Figure 3. Comparison of luminol-sonochemiluminescence (SCL)
photographic images for a liquid volume of 3 dm3 under different
conditions of pulsed ultrasound; pulsed ultrasound repetition of (a) 100
cycle ON-100 cycle OFF, (b) 1000 cycle ON-1000 cycle OFF, (c)
10000 cycle ON-10000 cycle OFF, and (d) CW ultrasound. Note that
the exposure time for capturing the images was 3 min for (a), (b), and
(c), and 1.5 min for (d). The exposure times were selected to equalize
the net irradiation time between different conditions. Note that the top
of each image corresponds to the liquid surface and the bottom
corresponds to the position of 60 mm from the transducer plate.
Figure 4. Measured waveforms of sound-pressure amplitude under
different conditions of pulsed ultrasound; pulsed ultrasound repetition
of (a) 100 cycle ON-100 cycle OFF, (b) 1000 cycle ON-1000 cycle
OFF, and (c) 10000 cycle ON-10000 cycle OFF. Note that each
vertical axis is an arbitrary unit and each time scale is different.
Figure 5. Measured waveforms of SCL intensity under different
condition of pulsed ultrasound; pulsed ultrasound repetition of (a) 100
cycle ON-100 cycle OFF for seven times, (b) 1000 cycle ON-1000
cycle OFF for three times, and (c) 10000 cycle ON-10000 cycle OFF
for one time. The inset of (a) shows a magnified view of one time.
the influence of degassing bubbles is suppressed. It was found
that the high efficiency in the sonochemical reaction comes
partly from spatial enlargement of the active region. It is
noteworthy that the active region can be extended in the
longitudinal direction using pulsed ultrasound.
Figure 4 shows measured waveforms of sound-pressure
amplitude under different conditions of pulsed ultrasound. The
measurement was performed at a pressure antinode nearest to
the liquid surface when the liquid volume was 3 dm3, using a
hydrophone (Bru¨el and Kjær, 8103) and a digital oscilloscope
(Yokogawa, DL1540C). Figure 4a shows that the acoustic
amplitude increases and decreases periodically but barely
reaches zero. The residual acoustic amplitude is effective for
reuse of the seed bubbles for pulsation.12 From Figure 4b,c, a
contrast in the rest time is apparent; as the rest time proceeds,
tiny bubbles dissolve in the liquid and the number of bubbles
for effective sonochemical reaction decreases.19
Figure 5 shows measured waveforms of the SCL intensity at
an antinode nearest to the liquid surface under conditions of

4878 J. Phys. Chem. A, Vol. 112, No. 22, 2008
Letters
pulsed ultrasound for the luminol solution prepared similarly
to that in Figure 3. SCL was detected using a photomultiplier
sensor (Hamamatsu Photonics, H7732-10), and the waveform
was observed using a digital oscilloscope. The time-averaged
intensity of SCL was compared between Figure 5a-c. A
magnitude of intensity in the order of (a) > (b) > (c) was
obtained. This is consistent with the order of magnitude found
for the SE ratio shown in Figure 2. The inset of Figure 5a (100
cycle ON-100 cycle OFF) shows that the SCL intensity has a
long tail relative to the pulse-off time. This is attributed to the
sound pressure amplitude that barely reaches zero during the
rest time, as seen in Figure 4a. In Figure 5b (1000 cycle
ON-1000 cycle OFF) and Figure 5c (10000 cycle ON-10000
cycle OFF), the intensity decays and reaches zero during the
rest time, according to the pressure variation shown in Figure
4b,c. Dekerckheer et al.20 studied SCL pulses at different pulse-
lengths and on/off pulse ratios, and results of SCL pulse similar
to those shown in Figure 5 were reported. Ashokkumar et al.21
showed that a sonoluminescence pulse different from that of
SCL develops, and the intensity plateaus with a pulse length
corresponding to the change in the waveform of the pressure
amplitude. However, so far, a mechanism for the enhancement
of SE by pulsed ultrasound has not been identified from both
the measured pressure amplitude and the SCL pulse. The present
results may be evidence that the population of sonochemically
active bubbles is closely related with the residual acoustic
pressure. The residual pressure amplitude maintains a certain
number of pulsating bubbles due to the suppression of dissolution.
Thus, high SE by pulsed ultrasound, compared with that by
CW ultrasound, is attributed both to the residual pressure
amplitude during the pulse-off time and to the spatial enlarge-
ment of active reaction sites. The effect of spatial enlargement
is also obtained using a partially degassed solution, even under
CW ultrasound. The authors have previously studied the
sonochemical oxidation of potassium iodide at different dis-
solved-gas concentrations22 and showed that the relative SE at
intermediate concentrations is at most 2 times higher compared
with that at saturation, and the oxidation could not be observed
for a completely degassed solution. Accordingly, it is proposed
that the effect of residual pressure amplitude is superior to that
of spatial enlargement.
Acknowledgment. Part of this study was supported by
funding from the Ministry of Education, Culture, Sports, Science
and Technology of Japan.
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JP802640X