Study On The Degradation Of 4-chlorophenol By Pulsed High Voltage Gas-liquid Hybrid Discharge System

Pulsed high voltage gas-liquid hybrid discharge could combine gas phase and liquid phase discharge to produce plenty of active species,such as ozone,hydrogen peroxide,hydroxyl radical etc.,which enhanced the removal efficiency of pollutant and energy yield of electric source.In this paper,4-chlorophenol(4-CP)is selected as a typical organic pollutant for degradation.During this study,three discharge reactors were designed and compared with each other;the electric parameters and operation parameters were optimized;the degradation mechanisms of 4-CP were deduced;and the kinetic model was also established.Firstly,in order to investigate the synthetic effect of liquid phase and gas phase discharge,three gas-liquid hybrid discharge reactors:HDBW reactor with both electrodes submerged into liquid phase,HDAW reactor with high voltage hollow electrodes immersed in water whereas the ground electrode placed over water,and GD reactor with pulsed high voltage electrodes in the gas phase above water level and the ground electrode submerged into liquid,were compared with each other.In this study,degradation of 4-CP,mineralization of TOC,energy yield,discharge mode,and the change of ozone and hydrogen peroxide in these three reactors were investigated. During the comparison,it is considered that HDAW reactor could combine discharge in both liquid phase and gas phase to enhance 4-CP removal.It appears that HDAW reactor is promising as a high-performance gas-liquid hybrid discharge reactor.In addition,special attention was given to some important operation parameters affecting 4-CP removal and energy yield.At the pulsed voltage of 16 kV,pulse repetition frequency of 100 Hz,electrode distance of 20 mm,oxygen flow rate of 0.3 m³/h,and initial concentration of 50 mg/L,the 4-CP removal efficiency was 100%after 7 min spark discharge.The effects of discharge modes and active species on 4-CP removal were also investigated to obtain that the hydroxyl radical oxidation played a main role in 4-CP degradation.Second,a novel continuous gas-liquid hybrid discharge reactor was designed.The effects of pulsed high voltage,repetition frequency,initial concentration of 4-CP,gas flow rate of oxygen and discharge unit on 4-CP removal and energy yield were investigated.At the pulsed voltage of 16 kV,pulse repetition frequency of 100 Hz, liquid flow rate of 40 ml/min,oxygen flow rate of 0.4 m³/h,the 4-CP removal efficiency was 100%and energy yield was 5.39×10^-9mol/J.Ozone and hydrogen peroxide was produced in this reactor and was utilized for 4-CP removal.An improved model was estimated by classical basic chemical engineering principles to predict 4-CP degradation and intermediates formations.In addition,the reaction mechanisms of 4-CP with oxygen,nitrogen,or air bubbling were explored.In the both case of oxygen bubbling and nitrogen bubbling, plenty of hydrogen peroxide was produced,while only ozone was formed in oxygen atmosphere and nitrate and nitrite ions were formed in nitrogen atmosphere.Except for the main similar intermediates,such as 4-chlorocatechol,hydroquinone, 1,4-benzoquinone,2,4-dichlorophenol,formic acid,acetic acid,oxalic acid,etc., produced in the both cases of oxygen and nitrogen bubbling,special intermediates (5-chloro-3-nitropyrocatechol and 4-chloro-2-nitrophenol)were produced in nitrogen atmosphere.The intermediates of 4-CP degradation with air bubbling were similar with that nitrogen bubbling.The reaction pathway of 4-CP in the case of oxygen bubbling was oxygen/ozone attack on the radical hydroxylated derivatives of 4-CP.In the case of nitrogen bubbling,hydroxylation was the main reaction pathway with effect of N atom on degradation of 4-CP.In the case of air bubbling,the reaction pathway was hydroxylation,ozone oxidation and N atom attack.Finally,the kinetic model for 4-CP degradation in gas-liquid hybrid discharge system was established.The effect of pulsed voltage,repetition frequency,liquid conductivity and oxygen flow rate on the reaction rate constants of active species (H₂O₂,O₃,OH,e_aq^-)was investigated.The correlation coefficient(r)and ANOVA system(F,p)between the experimental and predicted model was evaluated.During the model calculation,it was validated that the effect of hydroxylation on 4-CP removal was more than 88.9%,which played the main role in 4-CP degradation.The model could acceptably predict the 4-CP degradation in both batch reactor and continuous reactor for further large-scale applications.