Nomura A. et al.: 3D integrated micro-solution plasma for the treatment of water

3D Integrated Micro-solution Plasma for The Treatment of Water- Effects of Discharge Gases Nomura A, Himeno Y, Tanaka K, and Shirafuji T.
Osaka City University, Department of Physical Electronics and Informatics
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 Japan
nomura.ayano@t-shirafuji.jp
Methylene blue molecules in aqueous solution have been decomposed by using a novel 3D integrated
micro-solution plasma reactor operated with Ar and He gases. Energy efficiency for methylene-blue decomposition in the case of Ar is relatively higher than that in the case of He. This result suggests thatcheaper Ar gas has brought about superior performance in water purification. In both cases of Ar and He,
methylene-blue decomposition efficiency is one order of magnitude higher than that of conventional solution plasma.
Keywords: helium, argon, bubble, plasma, liquid, water, purification

1Introduction
Recently, electrical discharges in aqueous solution (solution plasma (SP)) are examined as
methods for eliminating toxic organics inaqueous solution [1]. One of the reasons isthat it can generate OH radicals. These radicals are highly reactive and can decompose
organic materials in water.
In our previous works, we have developed 3D
integrated micro-solution plasma (3D IMSP)[2], in which a large amount of tiny plasma(microplasma [3]) is generated in a porous
dielectric material being filled with agas/liquid mixed medium. Although 3D IMSP
requires external gas (Ar) supply, the gas canbe recycled by using an appropriate reactor
combined with a gas circulation system. 3DIMSP can be applied to gold nanoparticlessynthesis [4] and decomposition methylene
blue (MB) molecules in water [2]. Moreover,
3D IMSP has shown superior performance inthe case of decomposition of MB molecules incomparison to conventional SP.
However, in our previous reports, we haveexamined only Ar gas for operating 3D IMSP
because its metastable excited species (Ar 3P2,3P0) have a capability to decompose H2O toproduce reactive OH radicals. On the otherhand, He gas is also commonly used for various plasma treatments. He atoms can be excited to metastable excited states (1S0, 3S1),
which are at higher-energy levels than those of
Ar atoms. Thus, we have investigated characteristics of 3D IMSP with He gas and compared its performance in MB decomposition
Plasma Physics and Technology
2014, 1, 3, 147-149

Liquid out
Insulator
PlasmaHV metalelectrode

PlasmaHV metalelectrode

OES

Gas

Grounded
electrode

PorousdielectricSide
crosssectionalview

Liquid in(a) 3D IMSP
Metal
electrode
Tungsten
φ1 mm

Insulator
φ 6 mm (Outer)φ 1 mm (Inner)

Front
crosssectionalview

Liquid out
Insulator
Gap1 mm

PlasmaLiquid inFrontcrosssectionalview(b) Conventional SP

Fig. 1: Schematic illustration of (a) a 3D IMSP
reactor and (b) a conventional SP reactor

with that of 3D IMSP with Ar gas. We havealso compared both with the performance ofconventional SP [2, 3].
2
EXPERIMENTAL
Figures 1(a) and 1(b) show schematic illustrations of 3D IMSP and conventional SP reactors. The porous dielectric material is a rodshaped silica pumice (diameter 20 mm), whichis inserted in a glass tube (inner diameter
30 mm). The average diameter of the pores isapproximately 0.5 mm. A metal electrode(SUS 304: outer diameter, 1/4 in.), to which ahigh-voltage pulse is applied, is inserted in thecentral axis of the pumice rod. It has five holes(diameter, 2 mm) for feeding gas into the po-Nomura A. et al.: 3D integrated micro-solution plasma for the treatment of water

Intensity (arb .unit)

2.5w Filter

Absorption coefficiet (cm-1)

w/o Filter(a) ArArOH

N2 SPBOHβ

w/o filter w/ filter

Intensity (arb. unit)(b) HeHα

1.0
0.5

10 min0 min
10 min20 min20 min30 min30 min0 min10 min
20 min30 minHeFig. 4: UV-VIS spectra of MB aqueous solution
treated with (a) 3D IMSP (with Ar), (b) 3D IMSP(with He), and (c) conventional SPHeN2 SPB

HαHβ400

500600
Wavelength (nm)O700800

Fig. 2: OES of 3D IMSP operated with (a) Ar and(b) He gases

ity is 7.4±0.9 μS/cm. The liquid temperature iscontrolled to 30°C by circulating and coolingthe liquid in order to prevent the condensationof MB due to vaporization of water. The maximum treatment time is 30 min.
3201.5Conv. SPHe0 min0.0400600800 400
600800 400600800
Wavelength (nm)Wavelength (nm)Wavelength (nm)

OH300

2.0Ar

20.61 eV 21S0

RESULTS AND DISCUSSION

3.1OES
Before performing MB decomposition exper12.7 eV 4d
13iments, we have measured OES of 3D IMSP
3486 nm Hβ12.1 eV 3d
12 11.75 eV 4 P2
3operated with Ar and He in order to investi656 nm Hα11 11.55 eV 4 P0gate differences in the chemical species in the10.2 eV 2p
10microplasma in the 3D IMSP reactor. Figure
94.0 eV A2Σ+2(a) and 2(b) show OES of 3D IMSP operated
4with Ar and He gases, respectively. In both3309 nm
2cases, emission lines from H, O, and OH are
1observed, which means that H2O vapor is sup31S0 11S01s X2Π
0plied into gas bubbles from the inner surface
OHArHe
Hof the gas bubbles and are dissociated in theplasma in both cases. Appearance of the 2ndFig. 3: Energy diagram of Ar, He, H and OH
positive system band of N2 is due to parasitic
rous dielectric material filled with the aqueous discharge ignited in isolated pores, which aresolution to be treated. The grounded electrode included in the porous dielectric material dur(metal mesh, SUS 304; wire diameter, ing its fabrication process, and/or due to air
0.29 mm; aperture, 0.98 mm) is attached to the discharge in a small gap between the metal
outer surface of the glass tube. The voltage is mesh and glass tube.
supplied from a bipolar high voltage power A difference in the OES is found in the intensource (Haiden, SBP-5K-HK2). Voltage form sity of Hα and Hβ relative to OH intensity. In
is a square-wave pulse with amplitude of 5 kV, the case of He, Hα and Hβ intensity is higher
a frequency of 20 kHz, and a pulse width of than those in the case of Ar. This is due to the2.0 µs. The gas supplied through the central fact that the energy levels (approx. 20 eV) ofelectrode is He or Ar, and their flow rate is metastable He atoms are high enough to have
1.1 L/min. No external gas is supplied in the H (3d; 12.1 eV) atoms, which are able to emitcase of conventional SP. Sample to be treated 656 nm photons as shown in Fig. 3. Thisis aqueous solution (150 mL) containing MB means that, in the case of He, higher-energy(0.65±0.08 mg), of which electrical conductiv- electrons and/or metastable excited species
19.82 eV 23S1Energy (eV)19148

Nomura A. et al.: 3D integrated micro-solution plasma for the treatment of water

Decomposed MB (mg)1.00.8

Table I: Energy efficiency of the decomposition ofMB molecules for the first 10 min treatment with
3D IMSP using Ar and He, and conventional SPArHeConv. SP
ΔMB10 min (mg)0.40
0.230.048E (Wh)
5.4
5.615
η (mg/Wh)
0.073 0.042
0.0032

Initial amount of MB
0.65±0.08mg in 150 mL

0.6Ar
0.4

He0.2
0.0

Conv. SP

0510 15 20Time (min)2530

Fig. 5: Amount of decomposed MB molecules as afunction of duration of treatment by 3D IMSPs
operated using Ar and He, and conventional SP(He 1S0, 3S1) are working for decomposing
H2O molecules to produce H and OH. Thus,
we have investigated effects of this difference
on the performance of MB decomposition.
3.2
MB DECOLORIZATION
Figures 4(a)-4(c) show UV-VIS spectra of MB
aqueous solution as a function of time oftreatment with 3D IMSP using Ar and He gases, and with conventional SP, respectively. In
all cases, we can see a reduction in the absorption band centered at 665 nm, which corresponds to a MB absorption band. To investigate reduction rate quantitatively, we have
converted the reduction in the absorption coefficient to the amount of decomposed MB molecules. The results are shown in Fig. 5. We
can see almost 100% of MB molecules aredecomposed for 30 min in the case of Ar,
while slightly longer time is required to accomplish the decomposition in the case of He.
Since this is not so marked difference, we arenow examining reproducibility of this difference. However, the both results obtained with
3D IMSP show superior performance than thatwith conventional SP.
To investigate power efficiency for the MB
decomposition, we have calculated energy efficiency η using following equation,h (mg/Wh) =DMB10 min (mg)
,E (Wh)(1)where ΔMB10min is the quantity of decom-

posed MB for the first 10 min, and E is consumed electrical energy which is calculated
from voltage and current waveform. Calculated results are summarized in Table I together
with the result for conventional SP. In bothcases of Ar and He, their energy efficiency issuperior than that of conventional SP. Since
He gas is more expensive and is poor in its
availability, we can conclude that 3D IMSP
with Ar gas is better than that with He gas.
Regarding the result that 3D IMSP with Argas shows higher decomposition rate than thatwith He, we are now examining its reproducibility.
4CONCLUSION
We have demonstrated MB decompositionusing a novel 3D IMSP reactor with Ar andHe gases, and compared their energy efficiency to conventional SP. In both cases of 3DIMSP treatments of MB aqueous solution withAr and He gases, their energy efficiencies areapproximately one order of magnitude higher
than that of conventional SP treatment. Comparing 3D IMSP with Ar and He gases, 3D
IMSP with Ar gas has shown slightly superior
performance, although we must check its reproducibility.
REFERENCES[1] Parvulescu V I, Magureanu N, Lukes P, Plasma Chemistry and Catalysis in Gases and Liquids,Wiley-VCH, Wienheim, 2012.
[2] Shirafuji T, Himeno Y, Jpn. J. Appl. Phys. 52(2013) 11NE03 (5pp).
[3] Tachibana K, IEEJ Trans. Electr. Electron. Eng.
1 (2006) 145-155.
[4] Shirafuji T, Ueda J, Nakamura A, Cho S-P,
Saito N, Takai O, Jpn. J. Appl. Phys. 52 (2013)
126202 (5pp).

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