22P009-O
Radio Frequency Plasmas in Water under Pressurized Condition
圧力負荷を伴う水中高周波プラズマ
Fumiya Haraguchi, Tsunehiro Maehara, Shinobu Mukasa, Hiromichi Toyota, Shinfuku Nomura andAyato Kawashima
原口史也, 前原常弘, 向笠忍, 豊田洋通, 野村信福, 川嶋文人*
Graduate School of Science and Engineering, Ehime University,
3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
愛媛大学大学院理工学研究科 〒790-8577 松山市文京町3*Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan*愛媛大学農学部 〒790-8566 松山市樽味3-5-7
Radio frequency plasmas in water under pressurized condition were investigated. In spectroscopic
measurements, the intensity ratio between OH and Ha, IOH/IHa, decreases with increasing in pressure.
Methylene blue was employed for degradation study. As compared with exposure at 1 atm, larger
degradation was achieved at 3 atm. Concentration of hydrogen peroxide was 65 mg/L after 3 min

exposure at 1 atm while it was 180 mg/L after exposure at 3 atm.
1. Introduction
Plasma in water can be used in advanced
oxidation processes (AOPs) since it produces active
species such as OH radicals. A number of studies
have been conducted on discharges generated inwater [1-3].
Recently, the generation of radio frequency (RF)plasmas in water has been reported [4]. RF power
is injected into water through an electrode. RF
plasma surrounded by a bubble is generated andmaintained on the electrode. In ref [4], plasmas
are utilized for the degradation of methylene blue
dissolved in water. This degradation may be due toOH radical produced by RF plasma. In ref [5], thetemperature of OH, electron density, and behaviorof bubbles generated by a 27.12MHz in-liquid
plasma are investigated in water under pressures
ranging from 0.1 to 0.4MPa. The temperature of
OH and the electron density increase withincreasing in pressure. These changes following
after pressurization may influence the generation
and activity of OH radicals. Here, in order todiscuss the influence of the pressure, we carried out
the degradation studies and observation of theformation of hydrogen peroxide under pressurizedcondition.
2. Experimental Apparatus
RF plasma surrounded with bubbles in water wasgenerated on the electrode in a pressure vessel (Fig.
1). The pressure vessel (ID: 60 mm, height: 100mm) was made of stainless steel and it can be usedbelow 10 atm. Since the vessel was connected tothe ground line, it functioned as a counter electrode.
Water or methylene blue solution (250 mL) was

stored in the vessel. Water flowing through astainless pipe was used to control the temperatureof water.
The hot electrode was made of copper; its
diameter was 3 mm. Its top was covered with aceramic pipe (OD: 5 mm, ID: 3 mm). The
electrodes were connected to a resonator, and RF
power was supplied to the resonator through thetuner from a power supply (T161-6013H, Thamway,
f = 13.56 MHz). Here, the net input power wasfixed at 400 W.

Fig. 1 Schematic view of experimental setup.

3. Experimental Results and Discussions
The plasmas were initiated as follows. Keeping
the RF power low, the reflected RF power wasreduced by adjusting the tuner. Then, the RF power
was increased until breakdown. The breakdown wasobserved around 600W at an atmospheric pressure.
Just after breakdown, the RF power was decreased
to 400 W. Pressure was gradually increased to 3atm for one minute after breakdown.

(a) 1 atm
OHCuHaOI100
HbIntensity (a.u.)

0200

400

600

800
Ha(b) 3 atm

OIOH100
Hb
0400600

800

Wavelength (nm)Fig.2 Emission spectra of RF plasma
maintained at (a) 1 atm and (b) 3 atm.

Emission spectra of RF plasma generated in waterat (a) 1 atm and (b) 3 atm are shown in Fig. 2. The
spectral lines, generated due to the radicals, OH, O,and H, were observed. This shows that watermolecules are decomposed into OH, O, and Hradicals by the RF plasma. In Fig. 2(a), strong
continuous spectrum is observed. This is theradiation from the electrode and shows that theelectrode is heated by large RF power. Copper lines
are also observed. In Fig.2 (b), continuous spectrum
is not observed. This suggests that the electrode iscooled enough in high pressure case.
Theintensity ratio between OH and Ha, IOH/IHa,
decreases with increasing in pressure. Reference [5]shows that temperature of RF plasma in water
increases with increasing in pressure. Increasing intemperature leads to decomposition of molecules.
The change of the intensity ratio may be induced
from increasing in temperature.
RF plasma in water can degrade organic matter.
Here, we employed methylene blue solution (5mg/L). As compared with the solution before
exposure, the solution after exposure lightened andits color tone varied. This variation is obvious fromthe absorbance spectrum. A double-beamspectrophotometer (U-2810, Hitachi) was used forthe measurement of absorbance spectra. Typicalvariation of absorbance spectra with exposure timeis shown in Fig. 3. As shown in Fig.3 larger
degradation was achieved at 3 atm.
Hydrogen peroxide is also produced by thegeneration of RF plasma in water through a
recombination process: 2OH· → H2O2. After pure

4. Summary
Radio frequency plasmas in water can be
maintained even under pressurized condition.
Spectroscopic measurements show that the intensity
ratio between OH and Ha, IOH/IHa, decreases withincreasing in pressure. In degradation study,
plasmas at 3 atm can degrade more than those at 1atm. Concentration of hydrogen peroxide was 65
mg/L after 3 min exposure at 1 atm while it was
180 mg/L after exposure at 3 atm.

AbsorbanceIntensity (a.u.)

200water (250 ml) was exposed to the RF plasma for 3min, the concentration of hydrogen peroxide wasdetermined using acolorimetricmethod(DPM-H2O2, Kyoritsu Chemical-Check Lab). It was
65 mg/L after exposure at 1 atm while it was 180
mg/L after exposure at 3 atm. Degradation ofmethylene blue and generation of hydrogen peroxidesuggest that pressurized condition enhances theformation of OH radical. This is in contrast with thefact that the intensity ratio decreases with increasing
in pressure.

10.5

0

Before Exposure
1 atm
3 atm

400600800

Wavelength (nm)Fig.3 Absorbance spectra of methylene blue solution
before (solid line), after exposure at 1 atm (dashedline) and 3 atm (bold line), respectively. The
exposure time is 3 min.

Acknowledgments
The authors thank Mr. H. Okumura for technicalsupport.
References[1] J. S. Clements, M. Sato, R. H. Davis: IEEE Trans.
Ind. Appl. 23 (1987) 224.
[2] K. Kitano, H. Aoki, S. Hamaguchi: Jpn. J. Appl.
Phys. 45 (2006) 8294.
[3] T. Ishijima, H. Hotta, H.Sugai, M. Sato: Appl. Phys.
Lett. 91 (2007) 121501.
[4] T. Maehara, I. Miyamoto, K. Kurokawa, et al.:Plasma Chem. Plasma Process. 28 (2008) 467.
[5] S. Nomura, S. Mukasa, H. Toyota, et al.: Plasma
Sources Sci. Technol. 20 (2011) 034012