A novel liquid plasma AOP device integrating microwaves and ultrasounds and its evaluation in defluorinating perfluorooctanoic acid in aqueous media

Article abstract of DOI:10.1016/j.ultsonch.2011.01.003

A simplified and energy-saving integrated device consisting of a microwave applicator and an ultrasonic homogenizer has been fabricated to generate liquid plasma in a medium possessing high dielectric factors, for example water. The microwave waveguide and the ultrasonic transducer were interconnected through a tungsten/titanium alloy stick acting both as the microwave antenna and as the horn of the ultrasonic homogenizer. Both microwaves and ultrasonic waves are simultaneously transmitted to the aqueous media through the tungsten tip of the antenna. The microwave discharge liquid plasma was easily generated in solution during ultrasonic cavitation. The simple device was evaluated by carrying out the degradation of the perfluorooctanoic acid (PFOA), a system highly recalcitrant to degradation by conventional advanced oxidation processes (AOPs). PFOA is 59% degraded in an aqueous medium after only 90 s of irradiation by the plasma. Intermediates were identified by electrospray mass spectral techniques in the negative ion mode.

Full text of DOI:10.1016/j.ultsonch.2011.01.003

Ultrasonics Sonochemistry 18 (2011) 938–942
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Short Communication
A novel liquid plasma AOP device integrating microwaves and ultrasounds
and its evaluation in defluorinating perfluorooctanoic acid in aqueous media
Satoshi Horikoshi a,b, , Susumu Sato c, Masahiko Abe b,d, Nick Serpone e,
⇑
⇑⇑
a Department of Material and Life Science, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo 102-8554, Japan
b Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
c Sensor Photonics Research Center, Toyo University, 2100 Kujirai, Kawagoe, Saitama 350-8585, Japan
d Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
e Gruppo Fotochimico, Dipartimento di Chimica Universita di Pavia, Via Taramelli 10, Pavia 27100, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A simplified and energy-saving integrated device consisting of a microwave applicator and an ultrasonic
homogenizer has been fabricated to generate liquid plasma in a medium possessing high dielectric fac-
tors, for example water. The microwave waveguide and the ultrasonic transducer were interconnected
through a tungsten/titanium alloy stick acting both as the microwave antenna and as the horn of the
ultrasonic homogenizer. Both microwaves and ultrasonic waves are simultaneously transmitted to the
aqueous media through the tungsten tip of the antenna. The microwave discharge liquid plasma was eas-
ily generated in solution during ultrasonic cavitation. The simple device was evaluated by carrying out
the degradation of the perfluorooctanoic acid (PFOA), a system highly recalcitrant to degradation by con-
ventional advanced oxidation processes (AOPs). PFOA is 59% degraded in an aqueous medium after only
90 s of irradiation by the plasma. Intermediates were identified by electrospray mass spectral techniques
in the negative ion mode.
Received 5 November 2010
Received in revised form 30 December 2010
Accepted 12 January 2011
Available online 19 January 2011
Keywords:
Microwaves
Ultrasonic cavitation
Microwave discharged liquid plasma
Wastewater treatment
Perfluorinated acid
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
tion by this pulsed discharge plasma system were reported using
two direct current electrodes (e.g. discoloration of dyes [3], photo-
In physics and chemistry, plasma is a substance similar to gas in
which a certain portion of the particles are ionized. Plasma is typ-
ically defined as an electrically conducting medium generally con-
sisting of negatively charged electrons, positively charged ions, and
neutral atoms or molecules or both. Plasma gas applications are
readily found in various industrial fields such as, surface treat-
ments and etching in microelectronics [1], among others. In envi-
ronmental remediation, plasma is widely used in certain air
cleaners to sterilize and to decompose various volatile organic con-
taminants (VOCs) [2]. By contrast, reports of plasma generated in
liquids have been rather scarce compared to gas-phase plasma.
In addition, research in the use of plasma in liquids has been rather
limited in environmental remediation. For instance, Sugiartoa et al.
[3] reported an early study on the decomposition of a dye by the
streamer discharge plasma generated with high-voltage pulses be-
tween two electrodes. Later, some studies on wastewater purifica-
decomposition of phenol on activated carbon [4], and sterilization
of microorganisms [5]).
Such plasma techniques have been recently diverted to the syn-
thesis of nanotubes [6] and diamonds [7] by plasma generated in
non-polar solvents. However, although these studies are interesting
they have not led to an increase in the number of reports. Perhaps,
devices needed to generate the plasma necessitate applied voltages
at the kilowatt scale and an optimal arrangement of the electrodes.
In addition, clear advantages have not been delineated relative to
other advanced oxidation processes (AOPs). Accordingly, plasma-
related studies carried out in the last 20 years have not placed such
technique among the major techniques in wastewater treatments.
Sergiienko et al. [8] developed a method using copper electrodes
and a titanium ultrasonic horn that allowed an electric plasma
discharge to occur with relatively low electric power in insulating
organic solutions (low dielectric factors) as a result of the presence
of an ultrasonic cavitation. They succeeded in producing stable elec-
tric plasma in an ultrasonic cavitation field containing a thousand
tiny activated bubbles, in which the electrical conductivity was
improved because of the formation of radicals and free electrons.
In the study reported herein, an energy-saving and simple
liquid plasma device was manufactured using a novel integrated
design that coupled microwaves and an ultrasonic homogenizer.
⇑
Corresponding author at: Department of Material and Life Science, Faculty of
Science and Technology, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo 102-
8554, Japan.
⇑⇑
Co-corresponding author.
E-mail addresses: horikosi@rs.noda.tus.ac.jp (S. Horikoshi), nick.serpone@unipv.
it (N. Serpone).
1350-4177/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2011.01.003

S. Horikoshi et al. / Ultrasonics Sonochemistry 18 (2011) 938–942
939
The plasma source was herein changed from a pulsed direct
current voltage power unit to utilization of microwave radiation.
In this regard, formation of plasma in a liquid medium was
observed by Ishijima et al. [9] on injecting intense microwave
radiation (frequency, 2.45 GHz) in a metal vessel evacuated to
saturated vapor pressure of water; the microwave breakdown gave
rise to plasmas in many bubbles in the boiling water.
In the fabrication of our novel device, the plasma generator in
liquid media was simplified by integrating the microwave antenna
with the ultrasonic horn. Preliminary results with this novel device
were presented elsewhere [10]. The aqueous media plasma gener-
ator is evaluated by examining the degradation of perfluoroocta-
noic acid (PFOA), a system recalcitrant to decomposition by
traditional AOP processes. Such perfluorinated acids are widely
used as industrial surfactants, as fire retardants, as carpet cleaners,
and in paper coatings [11]. PFOA does not undergo natural decom-
position. Even with the use of advanced oxidation processes, the
decomposition of PFOA is rather difficult. Pyrolysis of PFOA neces-
sitates temperatures over 1200 °C [12].
system are displayed in Fig. 1a and b. The microwave generator
was constructed using a Micro Denshi Co., Ltd. 2.45-GHz micro-
wave generator (maximal power, 1500 W), which was coupled to
an isolator (air cooling device), a power monitor, and a three-stub
tuner. The microwaves continuously irradiated through the tung-
sten/titanium alloy (Ti-6Al-4V) antenna (diameter: 10 mm; length:
300 mm). In a preliminary study, the integrity of the antenna tip
was examined by plasma generation in aqueous media. The tita-
nium tip of the antenna was easily melted by the heat of the plas-
ma in aqueous media (melting point of the titanium alloy: 1540 °C
as per the data sheet of the material). Accordingly, the antenna
material was changed to the more heat resistant tungsten metal
whose melting point is 3422 °C. Unfortunately, however, an anten-
na consisting solely of tungsten did not easily generate ultrasonic
cavitation. Thus, a hybrid antenna was designed in which only
the tip of the antenna was made of tungsten. Ultrasonic cavitation
was generated from the tungsten tip of the microwave antenna
(Fig. 1a) that was subsequently used as the ultrasonic horn in the
ultrasonic homogenizer (Mitsui electric Co. Ltd., ultrasonic homog-
enizer UX300; frequency: 20.0 0.5 kHz). The optimized frequency
was adjusted to 19.5 kHz with the automatic tuner of the ultra-
sonic homogenizer.
2. Experimental setup
The hybrid titanium/tungsten antenna also plays the role of the
horn in the ultrasonic homogenizer. Fig. 1c shows that a loose fit of
the horn in the microwave metallic cone can lead to microwave
leakages. However, a tight fit of the antenna with the metallic cone,
2.1. Integrated microwaves/ultrasonic system
A schematic illustration and a photograph of the microwave
discharge liquid plasma with the supported ultrasonic cavitation
a
b
Ultrasonic
transducer
Three-stub tuner
Short-circuit
Metal cone
plunger
MW
Ultrasonic
MW
wave
Wave guide
Titanium alloy antenna
Tungsten antenna
Microwave
Water
discharged
level
plasma
e
d
c
Leakage
No leakage
No leakage
MW
MW
MW
Node
Fig. 1. (a) Schematic illustration of the novel device used to generate microwave discharge liquid plasma with simultaneous ultrasonic cavitation; (b) an actual photograph of
the system; (c) cartoon showing that a loose fit of the antenna/horn in the metallic cone can lead to microwave leakages; (d) cartoon illustrating that a tight fit of the antenna
in the microwave metallic cone affects the generation of ultrasounds, while preventing microwaves to leak out; (e) cartoon illustrating the proper fit of the microwave
antenna in the metallic cone allowing for microwave irradiation and ultrasonic cavitation, but simultaneously preventing microwave leakages.

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S. Horikoshi et al. / Ultrasonics Sonochemistry 18 (2011) 938–942
as shown in Fig. 1d, affects the ultrasonic vibrations and only
microwaves can be delivered to the aqueous medium. To prevent
microwave leakages from occurring through the antenna and the
ultrasonic transducer the metallic cone in the waveguide was mod-
ified and the tight fit was relaxed such that the node of the ultra-
sounds on the antenna was made to touch the cone through the
insert (see Fig. 1e). The position of the node was calculated by
the equation: k/2 = c/2f, where k/2 is the node length; c is the speed
of sound; and f is the frequency. The length of the node was esti-
mated at 129.9 mm for a speed of sound of the titanium alloy of
5068 m sÀ1 [13]. Note that the position of the node becomes an
important criterion for the metallic horn to touch the cone. As
such, the node was fine-tuned to the position of maximum cavita-
tion before the experiments.
simultaneous microwave irradiation (power, 220 W; ignition
power, 500 W) and ultrasonic cavitation (power, 90 W). Evidently,
a decrease in the temperature around the tip of the antenna ap-
pears to control the deterioration of the antenna.
Tsochatzidis et al. [14] measured the size of the ultrasonic cav-
itation bubbles under 20-kHz ultrasonic irradiations by the phase-
Doppler analytical technique and reported that such bubbles were
distributed in the range of ca. 5–10 lm. The ultrasonic cavitation
bubbles will become a positive factor in plasma generation as they
will define the domain where plasma is easily generated. Plasma
extended into the aqueous medium from the antenna tip by about
20 mm on irradiation for 10 s (Fig. 2b), and extended downward to
40 mm on irradiation for 30 s (Fig. 2c). By contrast, the microwave
discharge water plasma did not diffuse to 20 mm under microwave
irradiation alone, requiring simultaneous ultrasonic cavitation.
Accordingly, ultrasonic cavitation not only caused a decrease of
the microwave power needed for plasma generation but also
caused an increase of the diffusion length of the plasma so
generated.
2.2. Evaluation of the device by the degradation of perfluorooctanoic
acid (PFOA)
High-purity perfluorooctanoic acid (PFOA: C7F15COOH) was
purchased from Wako Pure Chemical Industries, Ltd. Aqueous
PFOA solutions (50 mL; 0.010 mM) were introduced into a Pyrex
glass cylindrical reactor (length: 60 mm; internal diameter:
55 mm); the length of the hybrid antenna tip bathing the aqueous
solution was 8 mm. The aqueous PFOA solution was oxygenated by
bubbling oxygen gas for 5 min prior to irradiation/cavitation. The
defluorination of PFOA was assayed by ion chromatography using
a JASCO LC-2000 Plus HPLC chromatograph equipped with a con-
ductivity detector and an I-524 anionic column. The intermediates
were identified in the negative ion mode with a mass spectral
detector of an Agilent Technologies 6100 LC–MS (ESI mode) sys-
tem. The eluent was a solution of acetonitrile/water (1:1 v/v).
The measured UV/vis spectrum of the light plasma in water
using simultaneous microwave irradiation (power, 220 W) and
ultrasonic cavitation (power, 90 W) is displayed in Fig. 3. Measure-
ment of the UV/vis spectral wavelengths was carried out with an
Opto Sirius Co. USB-Iss-UV–VIS spectrophotometer through the fi-
ber probe. The emitted wavelengths of the water plasma were
433 nm (OH), 486 nm (Hb), 456 nm (H ), 777 nm (O) and 845 nm
a
(O), each assigned according to the MIT database of wavelength
tables [15]. On the other hand, no wavelength due to nitrogen
(main peak: 411 nm) was observed, which is consistent with the
notion that the generated plasma is produced entirely by the
degradation of water. The spectrum of the water plasma under
microwave irradiation alone was similar in shape. Thus, no
changes in the spectrum occurred by ultrasonic cavitation.
3. Results and discussion
3.2. Evaluation of the novel device by the degradation of
perfluorooctanoic acid (PFOA)
3.1. Generation of the microwave discharge plasma under ultrasonic
cavitation in water
The degradation of the model pollutant PFOA in aqueous media
was examined to evaluate the liquid plasma method consisting of
the microwave discharge plasma and ultrasonic cavitation (MW
power, 220 W; power for cavitation, ca. 90 W). Fig. 4 displays the
degradation of PFOA by the liquid plasma method as % defluorina-
tion and compares it to the degradation of PFOA in water by ultra-
sonic cavitation alone. Note that the expected theoretical yield of
defluorination of PFOA (0.010 mM) was 0.15 mM. The concentra-
tion of fluoride ion increased with time of irradiation by the plas-
ma, reaching 59% defluorination for 90 s of plasma irradiation. On
the other hand, no defluorination occurred when the PFOA solution
was treated by ultrasonic cavitation alone. The temperature of the
PFOA aqueous solution reached 51 °C from an initial 23 °C after
irradiation for 90 s. However, in the vicinity where plasma was
generated the temperature was far greater, with the tungsten tip
of the antenna reaching temperatures over 1000 °C. Moreover,
the plasma generated in water produced such active species as
The microwave discharge water plasma was generated in
300 mL of water in a beaker under simultaneous microwave irradi-
ation and ultrasonic cavitation. Initially, we confirmed generation
of ultrasonic cavitation at a power level of ca. 90 W without micro-
wave irradiation and found that cavitation was generated most
efficiently near the top of the aqueous medium close to the micro-
wave antenna. Ultrasonic cavitation was not affected when the
metal cone was connected to the waveguide (Fig. 2a). The pink col-
ored light water plasma was continuously produced by microwave
irradiation alone at a power level of ca. 1000 W (power consump-
tion) for the ignition in water under normal atmospheric condi-
tions. However, under simultaneous microwave irradiation and
ultrasonic cavitation at a power level of ca. 90 W, the pink colored
water plasma could be generated at a microwave power level of
500 W (see Fig. 2b), i.e. at 50% power by simultaneous ultrasonic
cavitation. The microwave power was decreased to 700 W when
microwave irradiation alone was used and to 220 W for microwave
irradiation/ultrasonic cavitation while maintaining the plasma.
Thus, the microwave power could be decreased by 69% for contin-
ued plasma generation. Consequently, the decrease in microwave
power can lower the temperature around the tip of the titanium/
tungsten antenna. The generation of the water plasma became
unstable under microwave irradiation for ca. 20 s without ultra-
sonic cavitation. The tungsten tip of the hybrid antenna was worn
out (corroded) by the heat of the plasma at a microwave power le-
vel of 700 W; the photograph of the damaged hybrid antenna after
microwave irradiation for only 20 s is illustrated in Fig. 2d. How-
ever, the damage to the hybrid antenna could be minimized under
the ÅOH, ÅH, HO2 , and ÅO1(D) radicals [4,16]. It is these active radical
Å
species that likely promote the plasma pyrolytic decomposition of
PFOA. Note that water plasma produced by microwave irradiation
alone could not be sustained for a few minutes owing to the dete-
rioration of the antenna tip. Therefore, decomposition of PFOA was
rendered difficult under microwave irradiation alone. The degrada-
tion/defluorination of PFOA required simultaneous microwave
irradiation and ultrasonic cavitation.
Intermediates from the degradation of PFOA were identified by
electrospray mass spectral techniques in the negative (M–) ion
mode. The initial PFOA substrate was seen at m/z = 413. The LC–
MS spectral intermediates were seen at m/z = 363, 313, and 263

S. Horikoshi et al. / Ultrasonics Sonochemistry 18 (2011) 938–942
941
Fig. 2. Photographs showing (a) non-irradiation of water by the microwaves, (b) water plasma when water was subjected to microwave discharge (power, 220 W) under
ultrasonic cavitation (power, 90 W) for 10 s of irradiation, and (c) after 30 s of irradiation. (d) Photograph of the tungsten tip of the hybrid antenna after microwave irradiation
for only 20 s (MW power, 700 W).
100
15000
80
10000
Liquid plasma
60
40
20
5000
0
Ultrasonic cavitation
40 60
350
450
550
650
750
850
Wavelength (nm)
0
20
80
100
Time (sec)
Fig. 3. Ultraviolet and visible wavelengths emitted by the microwave discharge
water plasma under ultrasonic cavitation (MW power, 220 W; power for cavitation,
90 W).
Fig. 4. Defluorination yields (%) from the degradation of aqueous PFOA solutions
(0.010 mM) by ultrasonic cavitation alone and by the microwave discharge liquid
plasma under simultaneous ultrasonic cavitation (MW power, 220 W; power for
cavitation, 90 W).
(Fig. 5) subsequent to a 90-s plasma irradiation period (MW power,
220 W; power for cavitation, 90 W). These m/z peaks correspond to
species obtained from the sequential loss of the perfluoromethyl
unit (–CF2–) through decarboxylation–oxidation–decarboxylation
steps of the PFOA substrate (see Scheme 1). Additional intermedi-
ates were seen at m/z = 369, 319, 269, 235, and 219.
which also led to significant diffusion of the plasma into a larger
volume of the aqueous medium. Moreover, the lifetime of the an-
tenna was extended by a decrease in the microwave power. The
device was evaluated by examining the extent of degradation of
the perfluorooctanoic acid (PFOA) pollutant in aqueous media
which was significant (ca. 59%) after only 90 s of plasma irradia-
tion. It is generally difficult to produce a reactive space of several
thousands of degrees in a large volume of water, yet the proposed
novel device shows that although only the tip of the allowed an-
tenna is immersed in the polluted water, it easily yields a signifi-
cant reactive space in the aqueous medium. Accordingly, we
suggest that the novel liquid plasma device may have potential
applications in wastewater treatment processes even in flow-
through systems.
4. Summary
A simple, novel and energy-saving liquid plasma device oper-
ated under a normal atmosphere has been fabricated that leads
to the simultaneous formation of microwave discharge plasma
and ultrasonic cavitation from a tungsten/titanium alloy antenna
system. The microwave applied power to generate the plasma de-
creased when microwaves were coupled to ultrasonic cavitation,

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S. Horikoshi et al. / Ultrasonics Sonochemistry 18 (2011) 938–942
m/z=263
PFOA
m/z=413
m/z=363
m/z=269
m/z=313
m/z=319
m/z=369
200
250
300
350
400
(m/z)
Fig. 5. Electrospray mass spectra (negative-ion mode) recorded during the decomposition of PFOA by liquid plasma methods after 90-s of plasma irradiation (MW power,
220 W; power cavitation, 90 W).
References
decarboxylation
C F H + CO
C F COOH
7
15
2
7
15
[1] National Research Council, Plasma Processing of Materials: Scientific
Opportunities and Technological Challenges, National Academies Press,
Washington, DC, 1991.
(m/z=369)
(m/z=413)
oxidation
[2] K. Urashima, J.-S. Chang, Removal of volatile organic compounds from air
streams, industrial flue gases by non-thermal plasma technology, IEEE Trans.
Dielectr. Electr. Insul. 7 (2000) 602–614.
C F H + CO
C F COOH
6
13
2
6
13
(m/z=363)
(m/z=319)
[3] T. Sugiartoa, S. Ito, T. Ohshima, M. Sato, J.D. Skalny, Oxidative decoloration of
dyes by pulsed discharge plasma in water, J. Electrostatics 58 (2003) 135–145.
[4] D.R. Grymonpré, W.C. Finney, B.R. Locke, Aqueous-phase pulsed streamer
corona reactor using suspended activated carbon particles for phenol
oxidation: model-data comparison, Chem. Eng. Sci. 54 (1999) 3095–3105.
[5] M. Sato, T. Ohgiyama, J.S. Clements, Formation of chemical species and their
effects on microorganisms using a pulsed high-voltage discharge in water, IEEE
Trans. Ind. Appl. 32 (1996) 32106–32112.
[6] N. Sano, M. Naito, M. Chhowalla, T. Kikuchi, S. Matsuda, K. Iimura, H. Wang, T.
Kanki, G.A.J. Amaratunga, Pressure effects on nanotubes formation using the
submerged arc in water method, Chem. Phys. Lett. 378 (2003) 29–34.
[7] H. Toyota, S. Nomura, Y. Takahashi, S. Mukasa, Submerged synthesis of
diamond in liquid alcohol plasma, Diamond, Related Mater. 17 (2008) 1902–
1904.
C F H + CO
2
5
11
C F COOH
(m/z=313)
5
11
(m/z=269)
C F H + CO
2
C F COOH
4
9
4
9
(m/z=219)
(m/z=263)
-
C F OH
F ions + CO gas
4
9
2
(m/z=235)
[8] R. Sergiienko, E. Shibata, H. Suwa, T. Nakamura, Z. Akase, Y. Murakami, D.
Shindo, Synthesis of amorphous carbon nanoparticles and carbon
encapsulated metal nanoparticles in liquid benzene by an electric plasma
discharge in ultrasonic cavitation field, Ultrasonics Sonochem. 13 (2006) 6S–
12S.
[9] T. Ishijima, H. Hotta, H. Sugai, M. Sato, Multibubble plasma production and
solvent decomposition in water by slot-excited microwave discharge, Appl.
Phys. Lett. 91 (2007) 121501–121503.
Scheme 1. Sequential loss of CF2 units from the PFOA perfluorinated acid and
decarboxylated intermediates produced during the degradation of PFOA by plasma
irradiation for 90 s. Note that loss of CF2 units after the first decarboxylation step
likely occurs by oxidation of the protonated terminal carbon and formation of F–
ions and CO2 and so on in the Z-type pathway shown above.
[10] S. Horikoshi, S. Sato, Y. Mitsui, N. Serpone, M. Abe, Novel Chemical Reaction
Development with Microwave Discharge Plasma under Ultrasonic Cavitation
in Solution, Proceedings 1st Global Congress on Microwave Energy
Applications, Otsu, Japan 2008.
Acknowledgments
[11] R. Renner, Growing concern over perfluorinated chemicals, Environ. Sci.
Technol. 35 (2001) 154A–160A.
[12] M.M. Schultz, D.F. Barofsky, J.A. Field, Fluorinated alkyl surfactants, Environ.
Eng. Sci. 20 (2003) 487–501.
[13] See Japanese Website: <http://exile.itc.pref.tokushima.jp/report/usonic/basic/
default.htm> (accessed October 2010).
[14] N.A. Tsochatzidis, P. Guiraud, A.M. Wilhelm, H. Delmas, Determination of
velocity, size and concentration of ultrasonic cavitation bubbles by the phase-
Doppler technique, Chem. Eng. Sci. 56 (2001) 1831–1840.
[15] MIT wavelength tables, vol. 2 wavelength element, MIT press, Boston. MA,
1991.
[16] A.A. Joshi, B.R. Locke, P. Arce, W.C. Finney, Formation of hydroxyl radicals,
hydrogen peroxide and aqueous electrons by pulsed streamer corona
discharge in aqueous solution, J. Hazard. Mater. 41 (1995) 3–30.
Financial support to S.H. from the Japan Society for the Promo-
tion of Science (JSPS) through a Grant-in-aid for young scientists
(No. B-21750210) and from a Grant-in-Aid for Scientific Research
on Priority Areas (No. 18070006, ‘‘Science and Technology of
Microwave-Induced Thermally Non- Equilibrium Reaction Fields’’)
from the Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan, is gratefully appreciated. One of us (N.S.) thanks
Prof. Albini and his group at the Universita di Pavia, Italy, for their
continued kind hospitality during the many semesters spent in
their laboratory since 2002. We are also grateful to the personnel
of Mitsui electric Co. Ltd. for technical assistance.