The Degradation Of Perfluorooctanoic Acid In Simulated Waste Water By Non-thermal Plasma At Atmospheric Pressure

Perfluorooctanoic acid(PFOA)is frequently used in fire control,cleaning,lubrication and other production and living fields as well as aerospace,electroplating and other scientific and technological fields because of its excellent thermal stability,chemical stability and hydrophobic and oleophobic properties.If PFOA is remained in the natural environment the environmental will be definitely polluted which certainly causes the biological health problems.Therefore,it is required to treat PFOA polluted water efficiently and reasonably before draining off them.However,the recent techniques in PFOA treatment have the problems in secondary pollution,high energy consumption,low degradation rate and poor mineralization effect and so an.In this thesis,the simulated wastewater with PFOA was treated by nano-second dielectric barrier discharge(DBD)non-thermal plasma technique at atmospheric pressure.During plasma treatment,high-energy particles,various kinds of long-lived active species and short-lived free radicals,as well as ultraviolet radiation and shock wave,etc.are generated,which can effectively degrade pollutants in water.The highlight works are as follows:(1)Confirmation the effect of liquid parameters in simulated wastewater on degradation,including initial solution concentration,liquid flow rate,initial solution p H and electrical conductivity,etc.The results demonstrated that the wastewater in low initial concentration,the high electrical conductivity and the acidic conditions at the flow rate of 280 m L/min will benefit to the degradation of PFOA.(2)The effects of different discharge gases on the degradation of PFOA.The degradation of PFOA was carried out in air,oxygen and argon plasma,respectively.The results showed that the degradation rate of PFOA in argon plasma was the highest.The reaction kinetic constant was 0.86 times higher than that in oxygen atmosphere.After treatment for 50 min,the degradation rate reached 86%.In mixed gases,air mixed argon,it was found that the degradation rate was further increased,which indicated the effect of nitrogen derivatives on the degradation of PFOA.(3)The effects of parameters of power supply on the degradation of PFOA.The results showed that the pulse frequency,the pulse width and the applied power had an impact on the degradation.The optimal parameters were then obtained.The advantage of the nanosecond pulses discharge was attained by comparison with AC discharge.When the average discharge power was 24 W,the initial concentration of PFOA was 75mg/L,and treated period was 50 min,the degradation rates were 99%and 66%,and the corresponding energy yields were 204.5?g/k J and 136.4?g/k J,respectively.(4)The mechanism of plasma degradation on PFOA.The p H and the conductivity of the treated solution were measured in liquid.In gas,the active species in the gas phase of air,argon and oxygen were diagnosed by OES.The results showed that hydroxyl radical(·OH)scavenger and superoxide anion radical(·O₂^-)scavenger could significantly reduce the degradation effect of PFOA,rather than hydrated electrons.(5)The mineralization of PFOA was measured.The total organic carbon(TOC)concentration was decreased by 62.8%after 50 min treatment.The calculated defluorination rate was 49%.The degradation products were analyzed by LC-MS,and six possible intermediate products were inferred.The possible degradation pathway of PFOA in plasma system was then speculated.Besides,the toxicity of the possible intermediate products was evaluated from simulation.