SVI-3
気液混相プラズマとその応用
Gas-liquid mixed phase plasma and its application
安藤 晃
Akira Ando
東北大学大学院工学研究科
Graduate school of Engineering, Tohoku University
1. Introduction
Water pollution is one of the most serious
environmental problems. Recently, plasma
discharge in water has attracted much attention
for water treatments [1-5]. Discharge in water
enhances active species, such like ozone (O 3) andOH radicals, and ultraviolet (UV) rays also
irradiated. Strong electrical field and shock waves
are also generated in the discharge and they work
effectively for water treatment [6].
In this study, we have investigated a gas-liquid
mixed phase plasma using pulsed high voltage withthe pulse width of several tens of nanoseconds. We
utilized it to decompose organics and to sterilize
bacteria in water. Active species, O3 and OH radials
are generated in the discharge. In order toinvestigate the effect of water purification,
decolorization of indigo carmine (organic dye withblue color) solution and sterilization rate of Bacillus
Subtillis (spore-forming bacteria) were used asindicator.

phase) through multiple holes drilled in theseparator. Electrodes were placed below the holes
and in liquid. Pulsed high voltage was applied
between them. The discharge was formed between
the electrode placed in the lower part and surface of
bubbles above the small holes in water [9, 10].

Fig. 1. The electric circuit of the high voltage pulse
generator

2. A high voltage pulsed power supply and theplasma reactor
2.1 High voltage pulsed power supply
Figure 1 shows an electric circuit of thehigh-voltage pulsed power supply. The circuit
consists of a capacitor C, a gap switch GS, aninductor L, and SOS (semiconductor openingswitch) diodes. When the gap switch was shorted
after the capacitor was charged by DC power
supply, current started to oscillate in the LC circuit.
When the current was interrupted by the SOS
diodes, the sudden interruption of current resulted
in the generation of high voltage pulse Vo with ashort pulse width of 40ns typically [7-10]. The
pulse repetition rate was variable.
2.2 Gas-liquid mixed phase plasma reactor
Figure 2 shows the plasma reactor for gas-liquidmixed phase discharge. The upper part (liquidphase) of the reactor was filled with water, andworking gas was fed from the lower part (gasFig. 2. Schematic diagram of the reactor

3. Water treatment in a gas-liquid mixed phaseplasma
3.1 Observation of gas-liquid plasma
Figure 3 shows the photos of a bubble formed
above a hole of the separator and lightning indischarge in the bubble by using a high-speed
camera. Discharge extended along the surface of thebubble. The shapes of bubbles and discharges weredifferent according to working gas species.
Figure 4 shows the production rate of activespecies (O3 and OH radical) in various gases. O3production rate was the highest when oxygen wasused as a working gas. OH radical production rate
was evaluated from H2O2 concentration according
to recombination of OH radicals [11], and it washighest when He was used as a working gas.

Decolorization Rate [%]10080airO60

2N , He
240
20001020 30 40
time [s]5060

Fig.5. Decolorization rate in various gas species

Fig. 3. A bubble and a discharge in the bubble.

Fig. 4. O3 and H2O2 concentration in various gas species

3.3 Effect of sterilization
Sterilization effect using this plasma treatment
was observed in various gas species as shown inFig.6. Active species generated with the discharge
worked effectively for inactivation. When air wasused as working gas, the sterilization effect washigh and the decrease of the pH value was large.
The dependence on pH value of the solution isshown in Fig.7.
References[1] B. R. Locke, et al. : Ind. and Eng. Chemistry Res.,45, 882 (2006).
[2] P.Bruggeman, et al. : J.Phys.D, Appl.Phys 42 (2009).
[3] S. Kawano, et al. : IEEJ. PPT-11-5, ED-11-28 (2011)[4] K. Yasuoka, et al: Plasma Source Sci. Technol. 20,034009 (2011).

Fig.6. Sterilization effect appeared on the number of thecolonies (a) untreated, (b) 5 min, (c) 10 min and (d) 15min.
0.15

Survival Ratio [-]3.2 Effect of organics decomposition
Figure 5 shows decolorization rate of indigo
carmine solution in various gas species.
Decolorization rate was high when air and O2 wereused as working gas. This result was caused by highO3 production shown in Fig.4 and high reaction rate
between O3 and indigo carmine. On the other hand,a small change of decolorization rate by OH radical
was observed in N2 and He. Further experimentswith persistent organics are performed.

0.1

0.05

3.5

4

4.5

5

5.5

6

6.5

7

7.5

pHFig. 7. Survival ratio as a function of pH value of thesolution.

[5] T. Watanabe, J. Plasma Fusion Res. 75 (1999).
[6] M. Sato, Plasma Source Sci. Technol. 17 (2008).
[7] T. Namihira, et al.: J. Plasma Fusion Res. 81 (2005).
[8] K. Takaki, et al.: IEEE Tran Die Ele Ins. 14 (2007).
[9] R. Shimokawa, et al. : IEEJ. PPT-11-4, ED-11-27(2011).
[10] T. Togashi, et al. : IEEJ. PST-12-67, PPT-12-73(2012).
[11] T. Miichi, IEEJ Trans. FM. 126 (2006).