Plasma and Fusion Research: Regular Articles

Volume 11, 2406025 (2016)

Degradation of Acetic Acid in Water Using Gas-Liquid Plasma
with SPG Membrane∗)
Guanyang TANG, Atsushi KOMURO, Kazunori TAKAHASHI and Akira ANDO
Department of Electrical Engineering, Tohoku University, Sendai 980-8579, Japan(Received 30 November 2015 / Accepted 9 February 2016)
The gas-liquid mixed phase plasma generated by a nanoseconds-pulsed discharge in bubbles was used for
degradation of acetic acid in water. A Shirasu porous glass (SPG) membrane tube was adopted as micro-bubblegenerator and part of a discharge reactor. A large number of tiny bubbles are generated from dense micro-pores(average diameter of φ50 µm) of the SPG wall and a discharge through SPG membrane was initiated between
high voltage electrode and bubble surface. Comparing with a resin tube reactor which has six mechanical holes(diameter of φ1 mm), the surface area of bubbles increased with the same gas flow rate. The hydrogen peroxide(H2 O2 ) concentration in treated water using SPG membrane reactor increased by about 71% compared with thatusing the resin tube reactor, and the degradation amount of acetic acid was also promoted by about 82% when
Ar gas was used with the flow rate of 2 L/min. Meanwhile the H2 O2 production and degradation of acetic acidproportionally increased with the growth of gas flow rate in the SPG membrane reactor.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: gas-liquid plasma, degradation of acetic acid, bubbles, SPG membrane
DOI: 10.1585/pfr.11.2406025

1. Introduction
Water pollution caused by persistent organic pollutants (POPs) has been one of the serious environmental
issues. As one of advanced oxidation processes (AOPs),
non-thermal plasma has excellent performance in termsof organic degradation [1]. There have been reported
some experiments in degradation of organic pollutant, suchas methylene blue for dyeing, phenol for pharmaceuticalmanufacture. As the advantages of structure and energy
eﬃciency, discharge in bubble and with short pulsed voltage have been applied in some researches [2–6].
In the process of plasma reaction in liquid, various active species such as oxygen radicals (· O), hydroxyl radicals (· OH), and ozone (O3 ) are generated to react with organic pollutants and to initiate degradation processes. Especially, OH radicals play the key role on the degradation
process because of its very high oxidation potential (2.8 V)
which is higher than most of chemical bonding energy inorganic pollutants. Therefore, OH radicals could decompose POPs into CO2 or the other inorganics [7].
In the previous researches we found that the interfacial area of plasma and water was one of the key macroscopic parameters for the generation of OH radicals, andthis was the reason why a bubble discharge had a better
performance on degradation of organic than a water surface discharge [8]. When the gas flow rate and reactor volume is fixed due to cost considerations in practical applications, a larger interfacial surface area is feasible and canauthor’s e-mail: tang@ecei.tohoku.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

be obtained by generating more and smaller bubbles [9].
The SPG (Shirasu Porous Glass) membrane is a kindof new material which has dense holes of uniform micronsize (50 nm ∼ 50 µm). These pores are much smaller thanthose used in the previous gas-liquid mixing devices [10].
Actually, a SPG membrane has already used for the water purification as a micro-bubble generator [11, 12]. In
this research an SPG membrane tube was adopted as bubble generating structure to produce more and smaller bubbles, and it also played as a part of discharge reactor. During the bubbles generation, 360-degrees discharge plasma
was initiated. Plasma diﬀused along the surface of bubbles through the SPG membrane. Through the dependenceof H2 O2 generation and organic degradation eﬃciency on
bubble generating structure and gas flow rate, we discussed
the advantage of adopting SPG in raising organics degradation eﬃciency and reducing gas consumption.

2. Experimental Apparatus andMethods
A nanosecond pulsed high voltage which is conduciveto the non-thermal plasma generation was employed toproduce a non-thermal plasma in this organic degradation reactor [13]. The schematic diagram of a pulse highvoltage circuit is shown in Fig. 1. When a charging voltage and pulsed repetition frequency were set to 11 kV and30 Hz, respectively, a pulsed high voltage (peak voltage≈ −20 kV, pulse width ≈ 40 ns) were generated. This kindof nanosecond high-voltage pulse power source was developed in [14]. A 1 kΩ parallel resistor was used to stabilize

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c 2016 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 11, 2406025 (2016)

Fig. 1 Schematic diagram of the pulse power supply.
Fig. 3 Schematic of a gas-liquid mixed plasma reactor.

Fig. 2 Typical time evolutions of pulse discharge voltage andcurrent.

the output voltage. Typical temporal evolution of an output
voltage and a discharge current are shown in Fig. 2 and it
is considered that resistive current is the main componentof discharge current.
A Schematic of the gas-liquid mixed phase plasma reactor is shown in Fig. 3. A cylindrical tube, where an SPG
membrane or a resin tube is used, is immersed in liquid solution and various species introduced into the tube. A highvoltage wire electrode (tungsten wire of φ1 mm in diameter) placed in its center axis position. A mesh grounded(GND) electrode was immersed in the solution as well.
Figures 4 (a) and (b) show an SPG membrane tube and normal resin tube (six holes of φ1 mm in diameter), respectively. Figure 4 (c) shows a photo of discharge occurredat the six holes in the resin tube. Figure 5 shows a SEM
image of the SPG membrane which has porous structure,
where each hole diameter is around φ50 µm. Argon gas
was injected into liquid via the holes, and a number of bubbles were formed in the liquid phase. By applying pulsed
high voltage, streamer-like discharge was occurred in thetube and also appeared in the bubbles through the holes asshown in Fig. 4 (c).
Acetic acid has a C-C bond which has a relatively highbinding energy (2.4 V). This binding energy was higher
than the oxidation potential of usual active species inconventional water treatment method such as chlorine orozone [15]. Then, acetic acid is hardly degradated intoinorganics. As OH radicals has higher oxidation potential, a gas-liquid plasma can be used for the degradation ofacetic acid, that is substitute of POPs in this experiment.
Acetic acid solution of 100 mL with the concenration ofFig. 4 Photographs of bubble generation structures using (a) an
SPG membrane and (b) a resin tube, and (c) a discharge
photo taken in the resin tube (exposure time 0.3 s).

Fig. 5 SEM image of SPG membrane (φ50 µm) used in this experiment.

10 mgTOC /L was used as the treatment target, the initia conductivity and pH were about 70 µS/cm and 3.5, repectivityly. Argon gas was fed to the bubble generating structurewith various flow rates.
Due to the extremely short lifetime of OH radicals inwater, the production of H2 O2 was currently used to evaluate the generation of OH radicals as described in the reaction (1). H2 O2 are widely used as an indicator of OH radical production because the recombination reaction of OH
radical into H2 O2 is a main quenching reaction [16]. OH
radicals could react with acetic acid and decompose it toCO2 as described in the reaction (2) [17]. We evaluated thedegradation eﬀect of acetic acid by the TOC (total organiccarbon) measurement. The H2 O2 concentration of the so-2406025-2

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lution was measured with H2 O2 test kits (Kyoritsu WAKH2 O2 ) and the TOC concentration was measured with TOCanalyzer (Shimadazu TOC-L).
· OH + · OH → H2 O2 ,(1)

CH3 COOH + · OH → CH3 CO2 + H2 O
→ CO2 ↑ + · CH3 + H2 O.

(2)

3. Experimental Results and Discussion
When gas was injected to the SPG membrane tube, thesurface was covered by tiny bubbles. The number of bubbles decreased with the decrease of gas flow rate (2 L/min,1.5 L/min and 1.0 L/min) as shown in Fig. 6. Assumingthat bubbles are formed uniformly on the SPG surface andits shape is sphere, about 300 bubbles can be appeared on
the SPG surface within a circle of 1mm in diameter with2 L/min Ar.
The number of bubbles generated by the SPG membrane tube was estimate by counting bubbles generated on
the surface of SPG membrane in the high-speed photography (exposure time 1 ms). The equivalent volume of onesecond was about 30 mL close to the measured gas flowrate (33 mL). The gas-liquid interface area almost doubled
by using the SPG membrane compared with that by using
the resin tube at the same gas flow rate.
The concentration of H2 O2 produced in the solution
with the SPG membrane was higher than that with theresin tube in the same discharge frequency (30 Hz), charging voltage (11 kV) and gas flow rate (2 L/min) as shownin Fig. 7. The concentration of TOC was also measured
as a function of the treated time as shown in Fig. 8. TheH2 O2 production rates, acetic acid degradation rate andacetic acid degradation eﬃciency were shown in Table 1.
It was considered that SPG membrane reactor had a better
performance than the resin tube.
The discharge power was calculated by equation (3).

P=V · Idt · f.
(3)
Where P is discharge power (W), V and I are pulsed voltage and current, f is discharge frequency (Hz). Though theoutput voltage was maintained at about −20 kV, the current(peak value −40 A) in resin tube reactor was larger thanthat (peak value −25 A) in SPG membrane reactor. Therefore the discharge power (0.45 W) of the resin tube reactor
was higher than that (0.38 W) of SPG membrane reactor.
When the gas flowrate was changed, the H2 O2 concentration and TOC concentration were measured as a function of treated time and are shown in Figs. 9 and 10, respectively. These treatment eﬃciencies are summarized inTable 2. The production of H2 O2 , degradation amount anddegradation eﬃciency were approximately in direct proportion to the gas flow rate.
The discharge power did not obviously change withthe variation of gas flow rate. As the inside of SPG mem-Fig. 6 Photographs of bubble generation using the SPG membrane tube with various gas flow rate. (a) 2 L/min, (b)1.5 L/min and (c) 1 L/min.

Fig. 7 H2 O2 concentration as a function of treated time in theSPG membrane and the resin tube.

Fig. 8 TOC concentrations of acetic acid solution as a functionof treated time in the SPG membrane and resin tube.

brane tube was filled with Ar gas and the outside was water, there was a large relative dielectric permittivity gap between the two phase regions (1 in Ar gas and 80 in water).
Also the resistance of the solution was low. The high voltage was mainly applied inside of the tube and the discharge
type in the SPG membrane reactor was considered to be awire-cylinder.
DBD discharge. As the electric-field was strong in thevicinity of H.V. wire electrode, electric power was mainly

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Table 1 Numerical results of resin tube reactor and SPG membrane reactor with 2 L/min Ar.

Table 2 Numerical results of SPG membrane reactor with various Ar flow rates.

4. Conclusion

Fig. 9 H2 O2 concentration as a function of treated time with various gas flow rate in the SPG reactor.

Fig. 10 TOC concentrations of acetic acid solution as a functionof treated time with various gas flow rate in the SPG reactor.

consumed here. Plasma was generated near the surfaceof the wire electrode and expanded toward the wall as afilament-like streamer [18]. A part of plasma streamer diffused into the bubbles via the holes and creeping discharge
occurred on the surface of the bubbles. OH radicals wereproduced near the interface surface between gas and liquidand degradation reaction of organic was occurred. Therefore, the more bubbles were generated on the surface, themore production of OH and degradation of POPs were occurred.

The gas-liquid mixed phase plasma utilizing pulsed
high voltage was used for degradation of acetic acid in water. The influence of bubble generating structure and gas
flow rate were discussed. By using SPG membrane instead of normal resin tube with φ1 mm mechanical holes,
the gas-liquid interfacial area was enlarged and both OH
radical generation and degradation of acetic acid were enhanced. The productions of OH radical and degradation
amount of acetic acid were approximately proportional tothe gas flow rate in the SPG membrane reactor. The degradation eﬃciency of 1 L/min Ar using SPG membrane wasalmost equal to that of 2 L/min Ar with resin tube. The
SPG membrane tube worked as an admirable bubble generating structure to provide larger gas-liquid interfacial area
in a plasma degradation device. It could eﬀectively improve the organic degradation and reduce gas consumption.
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