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Through using a direct-current driven plasma jet operated underwater,degradation of methylene blue (MB) is investigated with air and oxygen used as working gases.With a low power voltage,a plasma plume extends from the needle electrode,which is purple in air.It ts pink after it bridges the two electrodes.During the process,oxygen plasma remains white.Discharge operates in a pulsed mode or a continuous one,which depends on the magnitude of power voltage.For the pulsed mode,oxygen discharge has a shorter plume and a higher pulse frequency than air discharge under the same power voltage.For the same current of the continuous mode,both power and gap voltages of oxygen discharge are higher than those of air discharge.Moreover,MB degradation efficiency increases with increasing power voltage or initial concentration of MB solution.Compared with air discharge,oxygen discharge has a higher degradation efficiency with the same power voltage and treatment time.The pulsed oxygen discharge with power voltage of about 6.5 kV has the highest efficiency in degrading MB dye,reaching approximately 85.8％ after 10 min treatment.As a comparison,after 10 min treatment in air discharge,the highest degradation efficiency is 63.7％,which appears in the continuous mode at a power voltage of 10.6 kV.Besides,optical spectra from the discharges are also compared for the two types of working gases.