Experimental Investigation On Degradation Of Methylene Blue By A Plasma Jet Excited By A DC Voltage

Non-thermal plasmas generated by atmospheric pressure discharge have the ability to generate various reactive species?OH,H₂O₂,O,O₃,etc?and UV radiation in solution.Therefore,atmospheric pressure discharge in solution has been used in a variety of fields,such as water purification,chemical treatment and sterilization.In this thesis,a needle-to-mesh discharge device is designed to produce a large number of reactive species in aqueous solution for the degradation of methylene blue dye.By using a needle-to-mesh discharge device,an air plasma jet excited by a direct current voltage is investigated underwater.Through electrical and optical measurements,it is found that the discharge presents three stable modes including an intermittently-pulsed discharge,a periodically-pulsed discharge and a continuous discharge with increasing the power voltage.Moreover,the plume length and the discharge frequency are investigated in the pulsed discharge as functions of the power voltage or the gas flow rate.In addition,the continuous mode of discharge mechanism is a normal glow discharge through analyzing the voltage-current characteristics.By optical emission spectroscopy,it is found that the emission intensities of OH radical and oxygen atom increase with increasing the power voltage or the gas flow rate.Using this plasma jet,the degradation of methylene blue is investigated in aqueous solution.By analyzing the absorbance of the methylene blue solution,it can be found that the degradation efficiency of methylene blue dye increases with increasing the power voltage,the gas flow rate,or the treatment time.Compared with the degradation efficiency in the intermittently-pulsed mode or the periodically-pulsed one,it is more efficient in the continuous mode,reaching 98%after 21 minutes treatment.Chemical method is used to verify the existence of Cl^-,SO₄^2-,and NO₃^-in the product of the treated methylene blue solution.Based on this,the chemical reaction process of methylene blue degradation is deduced at the last.