A Theoretical Study On The Formation And Degradation Of Dioxins

Known as the typical persistent organic pollutants.dioxin is lised in the Stockholm convention.And as the high toxic man-made compounds undesignedly, dioxin is paid more and more attention to by environmental chemist nowadays. Researches on the cause,path and mechanism of formation and degradation are increasing throughout the world.Chlorophenols are major precursors of PCDD/Fs' formation.In pyrogenation reactors,incinerators and photolysis process,to ascertain the role which chlorophenols act is the key to find out the formation mechanism of PCDD/Fs.The reactions of chlorophenols with H,Cl,O(³P)and OH initiate the formation of PCDD/Fs.The reasonable mechanism is important to cut off emission of PCDD/Fs.A series quantum chemistical calculations based on density functional theory have been carried out in the study on the formation and degradation of PCDD/Fs.Major valuable results are listed as follows:1.The most direct route to the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans(PCDD/Fs)in combustion and thermal processes is the gas-phase reaction of chemical precursors such as chlorinated phenols.Reactions of chlorophenols with atomic H are important initial steps for the formation of PCDD/Fs.Detailed insight into the mechanism and kinetical properties of crucial elementary steps is a prerequisite for understanding the formation of PCDD/Fs.In this study,the reactions of all 19 chlorophenol congeners with H,which initiated the formation of PCDD/Fs,have been studied theoretically using molecular orbital theory for the first time.Studies show that the Cl substitution has an effect on the strength and reactivity of the O-H bond in chlorophenols.The phenoxyl-hydrogens in 4-chlorophenol(4-CP),2,3-chlorophenol(2,3-DCP)and 2,4,5-chlorophenol (2,4,5-TCP)are most reactive.The formation of PCDD/Fs shows a strong dependency on the substitution pattern of chlorophenol.2,4,5-TCP,which has a Cl atom substituted at its ortho site,has the minimum number of Cl atoms to form 2,3,7,8-TeCDD.2.The 2-chlorophenol(2-CP)is a suitable kind of model compound to study the dioxin formation reaction.In this study,the mechanisms the reactions of 2-chlorophenol with H atom,Cl atom,OH radical and O(³P)were studied by ab initio techniques.The rate constants are deduced using canonical variational transition-state theory(CVT)with small curvature tunneling corrections(SCT)over a wide temperature range 400～1200 K.The reaction rate expressions are K_H(T)= k(T)=(2.22×10^-12)exp(-2011.15/T),K_Cl(T)=4.46×10^-11exp(-68.95/T),K_OH(T)= 1.97×10^-12exp(-1609.66/T)and k_O(³P)(T)=7.48×10^-14exp(-544.34/T),respectively.3.High-level quantum chemical method is adopted to elucidate the degradation mechanism of 2,3,7,8-TeCDD with the photo-Feston reagent.Several possible reaction pathways are proposed.The information of the stationary points including the reactants,intermediates,transition states and products are calculated at the B3LYP/6-31G(d)level.Two degradation mechanisms are proposed:ring-opening and adducting mechanism,adducting and ring-opening mechanism.The equilibrium distribution of the stable geometries and the equilibrium constants in aqueous phase indicating the reaction trend are performed according to the difference of Gibbs free energy.The main products obtained are 4,5-dichlorinate-o-dihydroxybenzene and 4,5-dichlorinate-o-quinone.The degradation mechanism proposed in the experiment is testified and improved.