Methylene blue can be decomposed by OH radicals. Decoloration characteristics of organic dye in an aqueous solution by a pulsed-discharge plasma Shin Ikoma 1, Yasushi Miyazaki 1, Kohki Satoh 1,2, and Hidenori Itoh 1 1 Department of Electrical and Electronic Engineering, Muroran Institute of Technology, 27-1 Mizumoto, Muroran , Japan 2 Center of Environmental Science and Disaster Mitigation for Advanced Research, Muroran Institute of Technology, 27-1 Mizumoto, Muroran , Japan INTRODUCTION APPARATUS & CONDITIONS  Background Various methods and techniques to reduce environmental pollutants like dioxin, toluene, trichloroethylene, etc. have been developed. Advanced Oxidation Processes (AOPs) [1], such as ozone-UV oxidation, photo catalytic oxidation, pulsed-discharge etc., have been used as effective methods for purifying polluted water. Electrode : A multi-needle electrode and a water bath electrode A multi-needle electrode: 1, 4, 37 and 55 stainless-steel needles are arranged on a stainless-steel plate. A water bath electrode: A water bath made of stainless-steel, which has the dimensions of 119mm inner diameter and 12mm depth, is placed under the multi-needle electrode. The water bath electrode is earthed. Gap length: A distance between the tip of multi-needle electrode and the surface of the test liquid is fixed at 4mm. Discharge chamber : A cylindical discharge chamber is made of acrylic resin (140mm inner diameter, 100mm height and 1.5L volume). H.V. pulse generator: A high-voltage pulse, generated by a blumlein pulse-generator, is applied to the multi-needle electrode. The coaxial transmission lines are charged at kV. Pulse repetition rate is 20pps (pulses per second). Concentration measurements of methylene blue : The test liquid is exposed to the pulsed discharge for 60min, and the temporal variation in concentration of methylene blue, O 3 and NO x are measured. Concentration of methylene blue mesured by absorption photometry using 650nm laser and Photonic Multi-Channel Analyzer (PMA). The hazardous substances can be directly decomposed by the collision with high energy electrons [1]. The species which have high oxidation potential, such as OH, O, O 3 etc., are produced by a pulsed-discharge plasma generated above a water surface [2], and that the species can dissolve into the water, reacting with substances in the water. UV radiation may contribute to the decomposition of the hazardous substances [1][2].  Objective We generated a pulsed-discharge plasma above test liquid (containing low concentration of methylene blue) and investigated the decomposition characteristics of methylene blue by the plasma exposure. RESULTS & DISCUSSION (2) Influence of electrode configuration on methylene blue decomposition We investigated the decomposition characteristics of aqueous methylene blue solution, by pulsed discharge, with the different electrode configuration and composition of the background gas. 55needles 1needle  Filamental discharges from the needle tip reach the water surface, and the discharges split into several branches, then some of them reach the water bath electrode.  Methylene blue can be decomposed by OH radicals generated by the reaction between electrons and H 2 O vapour. 55 and 37 needles 1 and 4 needles A mixture of effects caus- ing decomposition of the hazardous substances is expected by using pulsed- discharge plasma. Pulsed discharge [5] C.Mavroyannis et al.:Can. J. Chem., 39 (1961) [6] O.Eichwald et al.:J. Appl. Phys. 82 (1997) Pawlat et al. [3] investigated the effects of the pulse repetition rate and gas flow rate on the decomposition characteristics of methylene blue. Georgescu et al. [4] reported that the O 3 contributes to the decomposition of methylene blue. The number of needles (1, 4, 37 and 55) Influence of electrode configuration on the decomposition characteristics Influences of background-gas composition on the decomposition characteristics The decomposition rate ＞ ＞＞ Decomposition rate of methylene blue increases monotonously with the increase of O 3 concentration except for 1needle. The O 3 contributes to the decomposition of methylene blue. O 2 + e → 2O + e O 2 + O → O 3 O3O3O3O3 OH O3O3O3O3 OH OH OH O3O3O3O3 O3O3O3O3  O 3 produced with the high concentration, O 3 contributes to the decomposition of methylene blue. Mesurement of voltage and current Applied voltage and discharge current are measured by high voltage probe (Tektronix, P6015A) and digital oscilloscope (Yokogawa, DL1620). Background gas : N 2 (99.99%) ， O 2 (99.5%) and Ar(99.99%) Mixture ratios are N 2 (or Ar) : O 2 =100:0, 90:10, 80:20, 60:40, 40:60, 20:80, 10:90 and 0:100 (3) Influence of the background-gas composition (N 2 -O 2 ) on methylene blue decomposition O 2 + e → 2O + e O 2 + O → O 3 H 2 O + e → OH + H N 2 (A) +H 2 O → OH + N 2 +H H 2 O + e → OH + H O 2 + e → 2O + e O 2 + O → O 3 N 2 (A) +H 2 O → OH + N 2 +H H 2 O + e → OH + H N + O 3 → NO + O 2 ････ (6) N + O 3 → NO + O 2 ････ (6) NO + O 3 → NO 2 + O 2 ････ (9) N + O → NO ････ (5) NO + O → NO 2 ････ (8) Methylene blue decomposition rate tends to increase when the number of needle-electrodes increases, except for the case of 1needle. In pure O 2, O 3 produced with the highest concentration (120ppm), and the O 3 contributes to the decomposition of methylene blue. In pure N 2, slight increase in the decomposition rate can be made by the contribution of OH radicals, produced by the reaction between N 2 molecules exited in metastable state and H 2 O vapour. In N 2 -O 2 mixture, the decomposition rate of methylene blue decreases due to the decomposition of O 3 by NO and N and the inhibition of O 3 generation by NO x production. (4) Influence of the background-gas composition (Ar- O 2 ) on methylene blue decomposition In N 2 -O 2 mixture, NO x production inhibits O 3 generation. OH radicals produced by the reaction between Ar exited in metastable state and H 2 O vapour. H 2 O + Ar* → OH + H + Ar Exited molecules of Ar (Ar*) Exited molecules of Ar can decompose the benzen ringsExited molecules of Ar can decompose the benzen rings [7]. ＞ ＞ in pure N 2 68% in N 2 -O 2 mixture 40 ～ 65% in pure O 2 98% OH radicals produced by the reaction between N 2 exited in metastable state and H 2 O. The decomposition rate decreases due to the production of NO x. Excited molecules of Ar may contribute Methylene blue decomposition. Substitution of polluted water : A aqueous methylene blue solution Weight : 70g Initial concentration : 10ppm  Input energy per pulse is 0.7J.  Maximum values of discharge-voltage, discharge-current and electrical power are respectively -20kV, -70A and 1.5MW. (1) The characteristics of an applied HV pulse O and O 3 contribute to the decomposition of methylene blue. NO x production processes In the case of 1needle Methylene blue decomposition rate after 15min plasma exposure Following reactions can be induced by the pulsed-discharge plasma with the background gas consisting of N 2 and O 2 [1,2,5,6]. N 2 + e → N + N + e ････ (4) N 2 (A) + O → NO + N ････ (7) O 2 + e → 2O + e ････ (1) O 2 + O → O 3 ････ (2) N 2 (A) +H 2 O → OH + N 2 +H ････ (3) Low oxygenHigh oxygen Low oxygen concentration （ NO production ）････ High oxygen concentration (NO 2 production) ････ N produced by the reaction (4), leads NO production reactions in (5) and (6). NO produced by the reaction between N 2 molecules exited in metastable state and O, as shown in (7). NO production reactions shown in (5), (6) and (7) are activated, and they are consumed for NO 2 productions shown in (8) and (9). The decomposition rate decreases due to the consumption of O, O 3 and N 2 (A) by NO x production. N 2 (or Ar):O 2 =100:0 ， 90:10 ， 80:20 ， 60:40 ， 40:60 ， 20:80 ， 10:90 and 0:100 [3] J.Pawlat et al.:Acta Physica Slovaca 55 (2005) 479. [4] N.Georgescu et al.:Proceedings of GD 2006 F05 (2006) 497.  The decomposition rate of methylene blue is nearly-constant with O 3 concentration decreases at the same time.  O 3 concentration increases in proportional to the increase of the O 2 concentration. C 16 H 18 ClN 3 S ･ 3H 2 O  Recent works (Decomposition of methylene blue by using pulsed-discharge plasma) [1] M.A.Malik et al.:Plasma Sourse Sci. Technol. 10 (2001) 82. [2] P.Lukes et al.:J. Phys. D: Appl. Phys. 38 (2005) 409. [7] N.Goto et al.: IEEJ Trans., FM, 126 (2006) 321. H 2 O + e → OH + H [needles]  Pulse width is 500ns. CONCLUSIONS