Decomposition of Methylene Blue in an Aqueous Solution using a Pulsed-discharge Plasma at Atmospheric Pressure

Abstract

Decomposition characteristics of methylene blue in an aqueous solution by the exposure of pulsed-discharge plasma generated above a water surface are investigated for different background-gas composition (N2-O2 mixture and Ar-O2 mixture) and electrode configuration (1 needle and 55 needles). It is likely that OH radicals, produced by the reaction between H2O vapour and high energy electrons in the pulsed plasma expanding on the water surface, contribute to the decomposition of methylene blue. Further, O3 is found to contribute to methylene-blue decomposition at high oxygen concentration, and slight contribution of OH radicals, produced by the reaction between N2 molecules excited in metastable state (N2(A3Σu+)) and H2O vapour, is observed in pure N2. In N2-O2 mixture, O3 concentration decreases significantly and simultaneously NOx is produced by the pulsed plasma. Therefore, NOx probably destroys O3 and inhibits O3 generation. In Ar-O2 mixture, the high decomposition rate of methylene blue is obtained independent of O3 concentration. It is likely that Ar atoms in metastable states, such as 3P2 and 3P0, contribute to decomposition of methylene blue. Higher decomposition rate of methylene blue and higher O3 concentration are observed when streamer-like discharge is generated with 55 needle-electrodes, as compared to the single needle-electrode.