Mechanism Study On Formation Of PCDD/Fs From C₂H_2x As Precursors And Removal In Waste Incineration

Dioxins are chemically stable and highly toxic which cause soil and water contaminated easily and aren't easily degradable in the environment.Dioxins are a severe threat to the health of animals,plants and human beings which are mainly transmitted through the food chain.Under the current incineration conditions,some rubbish can't be completely combusted and it leads to a certain amount of small molecule alkanes generated.The generated small molecule alkanes form dioxin precursors through complex physico-chemical reactions via catalytic oxidation at high temperature and generate dioxins finally.Dioxins formation mechanism and control methods have always been the hotspot of research,however,due to the limitations of laboratory equitment and detection methods,the research contents mainly focused on toxicity and spatial distribution etc.Therefore,it's a primary task to study the dioxins formation mechanism and control methods.This dissertation applied the quantum chemistry method to study the dioxin gas phase formed by C₂H_2x?x=1³?at B3LYP/6-311+G?d,p?level based on the density functional theory?DFT?.Four chlorophenoxy radicals,22 reaction pathways and 17PCDD/Fs are obtained.The molecular structures of intermediates and transition states in the process of dioxin fromation were explored.Thermodynamic and kinetic anlysis of elementary reactions were investigated.Corresponding chemical reaction rate constants were calculated based on the variational transition state theory?VTST?and the pre-exponential factor and activation energy parameters of all elementary reactions were acquired with the temperature range from 300 to 1300K.The adsortption properties of four PCDD/Fs effected by pure activated carbon,functional oxygen groups including hydroxy and carboxyl modified activated carbon were studied and 32adsorption configurations were obtained.Results indicated that:?1?The reaction barrier of cyclization and intramolecular elimination of H is high in the process of chlorophenoxy radical formation by C₂H_2x?x=1³?,which may be more difficult to react in the conventional natural environment.It'more likely for free radical OH combining chlorophenol to generate chlorophenol than chlorobenzene directly oxidation to forming chlorophenol.?2?The substitution position and number of chlorine atoms have a great influence on the reaction potential barrier and heat of elementary reaction.Furthermore,the electron effect and steric hindrance effect also have a significant effect on the formation mechanism of PCDDs.The ortho-postion Cl substitution could increase the extraction barrier of ring H,increase the reaction barrier of ortho-postion OH and finally reduce the molecular reactivity.Intramolecular H elimination is the determining step in the PCDDs formation process;ring closure is the determining step in the PCDFs formation process.?3?Activated carbon has strong chemical adsorption capacity for dioxins.The electrostatic potential?ESP?and average local ionization energy?ALIE?of activated carbon surface being modified by functional oxygen groups including hydroxy and carboxyl haved changed,the distribution of electrostatic potential broadens and negative electrostatic potential becomes more negative.Different surface functional groups have effects on the adsorption capacity of activated carbon.In general,the carboxyl modified activated carbon is more stable and adsorptive than the hydroxyl modified activated caron.?4?The adsorption energy of the same kind of dioxin molecules is related to the adsorption configuration.The adsorption energy of different molecules related to the number of intramolecular oxygen and chlorine atoms,the calculation results demonstrated that the adsortption energy is positively correlated with the number of intramolecular oxygen and chlorine atoms.PCDDs's adsorption capacity of is greater than PCDFs's on the same activated carbon surface.The behavior of PCDD/Fs adsorbed on activated carbon is competitive.The results of this paper can provide important theoretical parameters for establishing the mathematic model of environmental supervision and risk decision-making anlysis.At the same time,it can provide directional theoretical guidance for the study of dioxin adsorption by activated carbon applying experimental methods.