DISCHARGE DUTY CYCLES EFFECTS OF 20 KHZ AIR ATMOSPHERIC PRESSURE PLASMA JET ON METHYLENE BLUE SOLUTION DEGRADATION Chakkaphan Wattanawikkam 1,2 , Atipong Bootchanont 1,2 , Naris Barnthip 1,2 , Sorapong Pavasupree 3 , Pinit Jitjing 4 , Dheerawan Boonyawan 5 , Keandtisak Kachayut 6 and Porramain Porjai 1,2,* Received: December 20, 2021; Revised: April 21, 2022; Accepted: April 24, 2022 Abstract The degradation of methylene blue (MB) using air atmospheric pressure plasma jet (air APPJ) in contact with liquid was proposed. In this study, an AC-driven air APPJ using three discharge duty cycles (30%, 50%, and 80%) of a neon transformer was investigated. In each discharge duty cycle, the ignition and extinguishment phases were characterized. The constituent particles of the air APPJ in the gas phase were observed by an optical emission spectrophotometer (OES). The OES results of all discharge duty cycles found the NO (A-X) bands, • OH radical, N2 second positive system, N2 first negative system, N+, N, and O. The emission spectra produced by the discharge duty cycle of 80% were the highest relative intensity. The effects of the different discharge duty cycles on the acidity and conductivity of water with and without MB were also determined. The complete dye degradation within 30 min was obtained in the discharge duty cycle 80%, which played an essential role in carrying reactive species from the gas phase to the liquid phase, thus promoting the degradation of MB. The best reaction kinetic rate k is 0.1692 min -1 , which is six times compared to discharge duty cycle 50%. Keywords: Air atmospheric pressure plasma jet, duty cycle, Methylene blue, reactive oxygen/nitrogen species, wastewater treatment 1 Division of Physics, Faculty of Science and Technology, Rajamangala University of Technology Thanyaburi, Pathum Thani, Thailand. E-mail: porramain_p@rmutt.ac.th 2 Smart Materials Research Unit, Rajamagala University of Technology Thanyaburi 12110, Pathum Thani, Thailand. 3 Department of Materials and Metallurgical Engineering, Faculty of Engineering, Rajamangala University of Technology Thanyaburi, Pathum Thani, Thailand. 4 Electrical Engineering Department, Rajamangala University of Technology Thanyaburi, Pathum Thani, Thailand. 5 Plasma and Beam Physics Research Facility, Chiang Mai University, 239 Huay Kaew Rd., Chiang Mai 50200, Thailand. 6 Genetron Technology (Thailand) Co., Ltd 51/3 Vibhavadi Tower Room 4 Floor 14 Ngamwongwan Road Ladyao Chathuchak Bangkok 10900, Thailand. * Corresponding author Suranaree J. Sci. Technol. 30(2):010204(1-12)