Generation of Three-Dimensionally Integrated Micro Solution Plasmas and Its Application to Decomposition of Organic Contaminants in Water

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1 Journal of Photopolymer Science and Technology Volume 26, Number 4 (2013) SPST Generation of Three-Dimensionally Integrated Micro Solution s and Its Application to Decomposition of Organic Contaminants in Water Tatsuru Shirafuji 1, Yuta Himeno 1, Nagahiro Saito 2 and Osamu Takai 3 1 Department of Physical Electronics and Informatics, Osaka City University, Sugimoto, Sumiyoshi-ku,Osaka , Japan 2 Materials, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku,Nagoya , Japan 3 Materials and Surface Engineering Research Institute, Kanto Gakuin University, Fukuura, Kanazawa-ku, Yokohama-shi, Kanagawa , Japan Three-dimensionally integrated micro solution plasmas were generated in gas bubbles in porous dielectric materials filled with gas/liquid mixed medium. The methylene blue molecules in aqueous solution were decomposed by the treatment with the three-dimensionally integrated micro solution plasmas. Decomposition efficiency was 16 times higher than conventional solution plasma in the case of applied voltage of 5 kv. With decreasing applied voltage from 5 to 2 kv, the decomposition efficiency was improved up to 360% in comparison to that of 5 kv, while reduction in the decomposition rate was 40%. Keyword: microplasma, solution plasma, porous dielectric, methylene blue, decomposition 1. Introduction s in and in contact with liquid have attracted much attention because of their possible application fields such as nano materials synthesis, surface modification, water treatment, sterilization, recycling of rare materials, and decomposition of toxic compounds [1-3]. Among them, various types of plasmas are used for these purposes, which include pulsed streamer discharges in water [1, 4, 5], dielectric barrier discharges [6-8], atmospheric dc glow discharges [9, 10], low-pressure dc glow discharges [11, 12], RF (13.56 MHz) discharges [13], arc discharges [14], and microwave discharges [15]. In our previous work, we have successfully obtained glow discharges in aqueous solution, which have been named as "solution plasmas (SPs)", and applied this technique to nanoparticles synthesis and modification of the surface of nano-materials [16, 17]. Our SP, however, is ignited in a small volume between two stylus s, actual treatment area or volume should be enlarged for practical industrial applications. In the case of gas phase processes, large area processing is realized by producing large area plasmas. In the case of solution plasma processing, however, preparation of one large volume plasma in liquid is meaningless, because the most important region is gas-liquid or plasma-liquid interface. Preparation of large number of tiny plasmas (microplasmas [18]), which may be called as "integrated micro solution plasmas (IMSPs)", is rather important in the case of solution plasma processing, because we can expect much larger area of the interface Received April 5, 2013 Accepted May 10,

2 HV metal OES Grounded SUS304 Wire diameter 0.29 mm Aperture 0.98 mm Side cross Metal Tungsten φ1 mm Insulator Borosilicate Glass (Duran) t 2 mm Porous dielectric Silicate glass Pore diameter 1 mm (average) Insulator Al2O3 φ 6 mm (Out. Diam.) φ 1 mm (Inn. Diam.) (a) Liquid out HV metal SUS304 1/4 inch (OD) Liquid in Front cross Liquid in Front cross t 3 mm φ2 mm x 6 Liquid out Insulator Borosilicate Glass (Duran) t 2 mm Gap 1 mm Fig. 1 Schematic illustration of (a) 3D-IMSP reactor, and conventional SP reactor. Gas between gas (or plasma) and a liquid medium. We have already confirmed feasibility of three-dimensionally integrated micro solution plasmas () through numerical simulation [19], and also through experiments [20]. In this work, we report capability of decomposing organic contaminants, methylene blue, in water by using the 3D IMSPs. We also show that the show higher decomposition efficiency than the conventional SP, and that further improvement may be possible according to the applied voltage dependence of the decomposition rate and efficiency. 2.Experimental 2.1. Experimental setup Figure 1(a) shows a schematic illustration of the experimental setup for treatment of aqueous solution with the. It consists of a central, porous dielectric material, a cross-shaped glass tube, and outer. The central, on which high voltage pulse is applied, is a metal tube (SUS 304, outer diam mm, thickness 1.24 mm), and has five holes (diam. 5 mm) to feed argon gas (1.1 L/min) into the porous dielectric material filled with aqueous solution to be treated. The porous dielectric material inserted in the cross-shaped glass tube is the rod-shaped pumice made of SiO 2 (20 mm diam.), in which average pore diameter is approximately 0.5 mm. Most of the pores are not isolated but connected to each other. Thus, we can flow liquid and gas in the porous dielectric material. Small amount of isolated pores is also contained in the porous dielectric material used in this study, of which the influence on the process performance is discussed in the section of results and discussion. The outer, which is grounded, is a metal mesh (SUS 304, wire diam mm, aperture 0.98 mm), which is attached on the outer surface of the cross-shaped glass tube. High voltage bipolar square wave pulse (amplitude 2-5 kv, frequency 20 khz, pulse width 2.3 us) is applied between these two s from a power source (Haiden, SBP-5K-HF2). Typical voltage amplitude is 5 kv, and varied down to 2 kv for the purpose of exploring possibility of improving process performance. Figure 1 shows a schematic illustration of the experimental setup for treatment of aqueous solution with conventional solution plasma (SP) for comparison. It consists of two s facing each other, insulators covering the s, and cross-shaped glass tube. The s are tungsten wires (diam. 1 mm), and their gap is 1 mm. The insulators are Al 2 O 3 (outer diam. 6 mm, inner diam. 1 mm). High voltage pulse is applied between these s from a power source (KURITA Seisakusho Co., Ltd, MPP-HV01). The reason why we have selected the different power source in the case of the conventional SP is that it requires generation of bubbles between the s through Joule heating, and the power supply should have higher current capacity than that of Haiden power source used in the case of the, in which Joule heating is not 508

3 Absorption Coefficient (cm -1 ) Absorption coefficient (cm -1 ) 4 5 kv, 20 khz, Ton=2.3 us (a) Ar 1.1 L/min 3 Methylene Blue 3.8 mg/l, 150 ml, 30 o C min 10 min 20 min 30 min Conventional SP 0.6 kv, 30 khz, Ton=2 us Methylene Blue 3.8 mg/l, 150 ml, 30 o C 0 min 10 min 20 min 30 min Wavelength (nm) 800 Decomposition rate (x10-3 cm -1 s -1 ) Decomposition efficiency (cm -1 W -1 h -1 ) 3.0 (a) khz, Ton=2.3 us, Ar 1.1 L/min 2.0 Methylene Blue 3.8 mg/l, 150 ml, 30 o C khz, Ton=2.3 us, Ar 1.1 L/min Methylene Blue 3.8 mg/l, 150 ml, 30 o C Pulse voltage amplitude (kv) Fig. 2 Absorption coefficient spectra around 665 nm being taken after the treatment of the methylene blue aqueous solution with (a) 3D-IMSPs and required because the bubbles are generated by external gas supply. Since we utilized different power sources when we compare the process performance of the and conventional SP, we cannot compare their process performances by means of apparent decomposition rate. Thus, we focused on the energy efficiency, which is the decomposition rate normalized with consumed electrical energy during the process. We also paid attentions for experimental conditions for the and conventional SP to become as the same as possible. The liquid to be treated is methylene blue aqueous solution of 150 ml with concentration of 3.8 mg/l. The liquid is circulated and cooled to keep its temperature at 30 o C. Table 1. Energy efficiency for decomposing methylene blue molecules with the and Type Δα ± Pdt +P d t Δα/ Pdt ± /+P d t (cm -1 ) (Wh) (cm -1 /Wh) Conv. SP x x10-1 Fig. 3 Methylene blue decomposition rate and energy efficiency as a function of applied pulse voltage amplitude in the case of. 3. Results and Discussion 3.1 Characterization of decolorization Methylene blue aqueous solution is often utilized for characterizing performance of decomposing organic substances in aqueous solution, because its decolorization can be a good indicator of the decomposition of methylene blue molecules. In this study, however, in order to characterize decolorization quantitatively, we have measured optical absorption coefficient of the methylene blue aqueous solution as a function of treatment time. Figure 2 (a) show spectra of optical absorption coefficient of methylene blue aqueous solution treated with the, and figure 2 is those with A broad peak at 665 nm corresponds to the methylene blue molecules, and its magnitude is proportional to concentration of the methylene blue molecules in the aqueous solution. As can be understood by comparing Fig. 2(a) for the and Fig. 2 for the conventional SP, the rate of reduction in the optical absorption coefficient for the is larger than that for the However, in order to perform proper comparison of decomposition performance for these two methods, we must compare decomposition 509

4 efficiency η defined by following equation; η= Δα/ P dt (1) where ± is reduction in absorption coefficient during a certain treatment time, P is power calculated by multiplying voltage and current, and +P dt is energy consumed during the treatment. Table 1 shows the results for the first 10-min treatment with the and As can be understood from this table, the 3D IMSPs show 16 times higher energy efficiency than the This result means that the 3D IMPSs is more cost effective than the 3.2 Improvement of efficiency of For the purpose of exploring possibility to improve the efficiency of the, we have studied applied-voltage dependence of the energy efficiency. Figure 3(a) and 3 shows applied-voltage dependence of the methylene-blue decomposition rate and energy efficiency, respectively. The decomposition rate in this work is defined by ±/ t, where t is the time required for reducing absorption coefficient by ±. In this study, the t is the first 10 min after starting the treatment. Figure 3(a) indicates that the decomposition rate decreases from to cm -1 s -1 with decreasing applied voltage from 5 to 2 kv. While the decomposition rate decreases by 40% with decreasing the applied voltage, the energy efficiency shown in Fig. 3 is improved up to 360%, namely, from 1.6 to 5.7 cm -1 W -1 h -1. This result means that excess energies are consumed for the other processes than plasma generation in gas bubbles in liquid when applying the higher voltage. In addition, the decomposition efficiency increases steeply when the voltage decreases, which suggests that the processes for the excess energy consumption occur when the voltage exceeds a certain threshold. One of the possible causes of the excess energy consumption is parasitic discharge in isolated pores, which are formed when the pumice is fabricated and contain air, namely 80% of N 2 and 20% of O 2, although their concentration is smaller than connected pores. The electrical discharge is ignited when the electric field applied in the pores exceeds a certain threshold, and the threshold for the air, N 2 /O 2 gas mixture, is higher than Ar [21] from the view point of electron collision cross section of them. Thus, the reason why the energy efficiency increases steeply with decreasing the voltage may be explained by disappearance of the parasitic discharge of the air in the isolated pores. In order to verify this hypothesis, we are now designing the reactor with the dielectric material which contains fully connected pores. 4. Conclusion We have proposed a reactor for the 3D IMSPs, which is expected to have effective interaction between plasma and liquid because of employment of much larger number of micro plasmas in the liquid in comparison to conventional reactor for the SP. We have successfully obtained the 3D IMSPs, and demonstrated capability of decomposition of an organic substance, methylene blue in this study, in water, and have shown that the has 15.7 times higher treatment efficiency than conventional SP. The results of voltage dependence of the has suggested that parasitic air discharge is also ignited in unintentionally incorporated isolated pores in the dielectric material, and consume excess energy to reduce energy efficiency when higher voltage is applied. This problem can be solved technically by preparing and utilizing the dielectric material which contains only fully connected pores. References 1. B. R. Locke, M. Sato, P. Sunka, M. R. Hoffmannm, and J. -S. Chang, Ind. Eng. Chem. Res., 45 (2006), O. Takai, Pure Appl. Chem., 80 (2008), P. Bruggeman, and C. Leys, J. Phys. D: Appl. Phys., 42 (2009), T. Namihira, S. Sakai, T. Yamaguchi, K. Yamamoto, C. Yamada, T. Kiyan, T. Sakugawa, S. Katsuki and H. Akiyama, IEEE Trans. Sci., 35 (2007), K. Tachibana and T. Shirafuji, Trans. Mater. Res. Soc. Jpn., 35 (2010), O. Sakai, M. Kimura, T. Shirafuji and K. Tachibana, Appl. Phys. Lett., 93 (2008), 510

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