The Degradation Of Methylene Blue Solution By DBD Nonthermal Plasma And The Chemical Behavior Of The Radical Species Generated In The Gas And Liquid Phases

This essay is focused on the degradation of the high toxic organic wastewater with darkcolor. Methylene blue solution was used as the research subject. Dielectric Barrier Discharge(DBD) plasma was employed to treat methylene blue solution and the treatment process wasstudied in detail. The radical species in the gas phase and the liquid phase of the DBDdegradation process were studied.The main contents are shown as below:(1) Took air as the working gas of the DBD process, the intermediate products and thefinal products in the methylene blue degradation process by DBD plasma were detected. Thedegradation path of the methylene blue molecule was inferred based on the analysis of themolecular structure. The degradation effect of methylene blue solution was studied whenhydrogen, argon or air was employed as the different working gas of the DBD process,respectively. The liquid phase in the DBD system was designed to analyze the occurringposition of the degradation reactions. The results indicate that: the degradation percentage ofmethylene blue solution reached the maximum value when air was used as the working gasand the degradation percentage of methylene blue solution was84.8%when it was treated for30min. when different working gases were employed, the variation trends and scales of thepH value, Total Organic Carbon (TOC) value and conductivity during the DBD processeswere studied. The influence mechanism of the working gas on the DBD degradation processwas inferred as that: different radical species were generated when different working gaseswere employed as the working gases of the DBD processes. The main reason caused thedifferent degradation effect was that different radical species and discharge products weregenerated when different working gases were employed. The experimental results alsoindicate that the degradation reactions occurred not only at the interface of the gas-liqud phase,but in the entire liquid phase of the DBD system.(2) The radical species generated in the gas phase during the three different DBDprocesses were studied using Optical Emission Spectroscopy (OES) method. The energybalance of the DBD process was studied when different working gas was employed. Thespecies of the radical and their energy states were analyzed when argon, hydrogen or air wasemployed as the working gas, respectively. Boltzman-plot method was used to calculate theelectron temperature when argon was employed as the working gas of the DBD process. Thedischarge products (ozone and nitrogen oxidize) were quantitatively detected when air wasemployed as the working gas of the DBD process. (3) Hydroxyl radical and hydrogen peroxide in the liquid phase were quantitativelydetermined. Different quenchers were added into methylene blue solution to analyze thecontributions of the hydroxyl radical and hydrogen peroxide to the methylene bluedegradation process, respectively. The results indicate that: when air was employed as theworking gas of the DBD process, the concentration of hydroxyl radical in the liquid phasereached the maximum value. When air was employed as the working gas, the concentratio n ofhydroxyl radical in the liquid phase was2.27×10-4mol/L when it was treated for30min.When argon was employed as the working gas, the concentration of hydrogen peroxide in theliquid phase reached the maximum value. When argon was employed as the working gas, theconcentration of hydrogen peroxide in the liquid phase was1.27×10-5(w/w) when it wastreated for30min. When hydrogen was employed as the working gas of the DBD process, theconcentrations of hydrogen peroxide and hydroxyl radical were both the lowest and they were6.05×10-5mol/L and2.32×10-6(w/w) when it was treated for30min. When argon, air orhydrogen was employed as the working gas, the contribution values of hydroxyl radical andhydrogen peroxide were47.8%and21.8%,29.0%and18.8%,44.4%and16.6%,respectively.(4) It was suggested that the methylene blue solution DBD degradation process wasdivided into three stages:The first stage: the excitation, dissociation and ionization process in the gas phase. It wasalso the generation processes of the radical species and high energy electrons in the gas phase;The second stage: the interaction process between the radical species and the watermolecules at the gas-liquid interface. It was also the radical species (hydroxyl radical,hydrogen peroxide et al.) generation process in the liquid phase. Hydroxyl radical andhydrogen peroxide are the products of the interactions between radical species, high energyelectrons and the water molecules.The third stage: the reaction processes between radical species (hydroxyl radical,hydrogen peroxide et al.) and methylene blue molecules in the liquid phase.