Diagnosis Of Nanosecond Pulse Excited Bubble Discharge Characteristics And Its Application In Removing Humic Acid In Water

Atmospheric pressure low-temperature discharge plasma-liquid interaction is a research hotpot in the fields of biomedicine,environmental remediation,green agriculture,etc.The bubble discharge has a good application prospect as it can significantly reduce the breakdown voltage and effectively increase the reaction volume and promote the mass transfer of reactive species.However,there are still some problems for bubble discharge,such as unstable discharge,insufficient research on the generation and mass transfer process of reactive spices,and lack of systematic research on the regulation of reactive species and plasma parameters,which seriously restricts the development of applications.In this thesis,the bubble discharge dominated by argon gas was excited by bipolar nanosecond pulse to realize a low-temperature plasma with wide adaptability to solution conductivity and stable operation for a long time.The optical emission spectra and chemical probe method were used to diagnose the bubble discharge characteristics with bubbling one or more mixture gases of nitrogen,oxygen,and argon,revealing the generation rules of reactive species and the variation rules of plasma temperatures.The bubble discharge was also applied to the removal of humic acid which is a refractory organic pollutant in water.The main results are as follows:1.The effects of sinusoidal alternating current,microsecond pulse,and nanosecond pulse excitation modes on the stability of bubble discharge were compared,and the discharge morphology,electrical characteristics,optical emission spectra characteristics,and aqueous reactive species characteristics of bubble discharge with three different gases of nitrogen,oxygen,and argon were studied.It was found that a bubble discharge with long-term low temperature and stable operation can be achieved in the solution with a conductivity of the order of hundred m S/cm using bipolar nanosecond pulse to excite argon bubble discharge.While it cannot be broken down in the solution with a conductivity of 40μS/cm using microsecond pulse excitation,and the discharge is unstable in deionized water using alternating current excitation.When excited by nanosecond pulse,the argon bubble discharge can significantly enhance the discharge luminescence intensity,the discharge area,and the concentration of aqueous reactive species compared with nitrogen and oxygen bubble discharge.Particularly,the energy yield of hydrogen peroxide H₂O₂ can be achieved as high as 3.13 g/k Wh.Using Fe I（3d⁷4p-3d⁷4s）and Ar II（3s²3p⁴4p-3s²3p⁴4s）spectral lines,the electronic excitation temperatures of nitrogen,oxygen,and argon bubble discharge are calculated to be 0.581 e V,0.647 e V,and 1.324 e V,respectively.In nitrogen and oxygen bubble discharges,the gas temperature of discharge plasma can be obtained by fitting the N₂（C³Π_u-B³Π_g）band,which are 350-380 K and 430-520K,respectively.While in argon bubble discharge,the gas temperature is calculated by the OH（A²Σ-X²Π）band,which is 560 K.According to the variation of the emission intensity,the optimized experimental parameters are determined as pulse peak voltage of 30 k V and gas flow rate of 200 m L/min when the pulse repetition frequency is fixed at 100 Hz.2.The discharge morphology,electrical characteristics,gaseous reactive species characteristics,and aqueous reactive species characteristics of nitrogen/oxygen,argon/nitrogen,and argon/oxygen mixture bubble discharges were studied.It was found that the addition of oxygen to nitrogen or argon bubble discharge enhances the emission intensity of O I（2s²2p³3p-2s²2p³3s）and reduces the emission intensity of OH（A²Σ-X²Π）.In nitrogen/oxygen mixture bubble discharge,the variation of oxygen proportion can change the discharge morphology and regulate the plasma temperatures,but the generated aqueous reactive species concentration is relatively low.Argon bubble discharge can significantly increase the concentrations of aqueous reactive species.When the proportions of nitrogen or oxygen added to argon are not higher than 20%,the discharge remains a stable glow-like morphology.The variation of nitrogen or oxygen proportion can effectively regulate the emission intensities of Ar I（3s²3p⁵4p-3s²3p⁵4s）,H（n=3→n=2）,and OH（A²Σ-X²Π）,and the plasma temperatures,as well as the concentrations of aqueous reactive species of hydrogen peroxide H₂O₂,nitrate NO ₃^–,and nitrite NO ₂^–.Plasma activated water prepared by bubble discharge with argon gas and adding 20%of nitrogen or oxygen into argon was used for the germination of Nelumbo nucifera gaertn.fruct.et semin.Seed.It was found that all plasma activated water can inhibit seed mildew and promote seed germination,but shows a dual effect on promoting and inhibiting the rooting and germination of seed.3.D-mannitol was selected as the quencher of aqueous hydroxyl radical·OH.The contents of gaseous reactive species under different oxygen/nitrogen proportion were studied in argon/nitrogen/oxygen mixture bubble discharges,and the concentrations of aqueous reactive species in deionized water and D-mannitol solution were compared,to analyze and reveal the generation and mass transfer rules of reactive species in the bubble discharge.It was found that the aqueous reactive species of hydrogen peroxide H₂O₂,nitrate NO ₃^–and nitrite NO ₂^–are mainly derived from the dissolutions of hydrogen peroxide H₂O₂,nitric acid HNO₃ and nitrous acid HNO₂ in the gas phase in the bubble discharge.The metastable argon atoms in the discharge promote the generation of gaseous excited hydroxyl radical OH（A）.When the discharge system does not contain oxygen,the hydrogen peroxide is mainly produced by the recombination of hydroxyl radicals·OH,and the proportion of aqueous hydrogen peroxide derived from gaseous hydrogen peroxide is 67%.The aqueous hydroxyl radicals have dual effects of generation and consumption on hydrogen peroxide.The addition of oxygen in the discharge provides an additional pathway for the generation of hydrogen peroxide.A small amount of oxygen added in the discharge significantly promotes the generation of nitrogen oxides NO_x to inhibit the generation of aqueous hydroxyl radicals·OH and to increase the aqueous concentration of nitrate and nitrite.While nitrite has a consumption effect on aqueous hydrogen peroxide.4.The nanosecond pulse excited bubble discharge-activated carbon adsorbent synergetic system was used to efficiently remove humic acid in water.The activated carbon was characterized by electron scanning electron microscopy-energy dispersive X-ray spectroscopy,Fourier transform infrared spectroscopy,and Raman spectroscopy,and the reactive species were measured,to analyze the synergistic mechanism between bubble discharge and activated carbon.The humic acid solution was characterized by the measurements of three-dimensional fluorescence spectra and dissolved organic carbon and the calculation of specific UV absorbance,to analyze the removal mechanism of humic acid.It was found that the humic acid removal efficiency reaches 98.80%in bubble discharge-activated carbon synergistic treatment system.The synergistic efficiency factor is as high as 651.52%,and the energy efficiency is improved from 0.04-0.25 g/k Wh to 0.12-1.01 g/k Wh,compared with the solo discharge treatment system.The residual amount of humic acid adsorbed on the surface of activated carbon is only 4.52%.The discharge plasma has a modification effect on the activated carbon in the synergistic system,enhancing the adsorption capacity of activated carbon.Meanwhile,the activated carbon promotes the conversion of reactive species of hydrogen peroxide H₂O₂and ozone O₃ into strong oxidizing radicals like hydroxyl radical·OH and hydrogen superoxide radical HO₂·,accelerating the breakup of humic acid macromolecular substances into small molecular fragments,the decomposition of condensed aromatic structures,and the elimination of functional groups of hydroxyl,carbonyl,and amino groups.In addition,plasma activated carbon-contained water has the ability of in situ purification of polluted water sources.