Theoretical Study On The Ring-opening Degradation Mechanism Of2,3,7,8-TCDF

Dioxin is a kind of hypertoxic Persistent Organic Pollutants (POPs), includingPolychlorinated dibenzo-p-dioxins (PCDDs) and Polychlorinated dibenzofurans(PCDFs), which are easy to accumulate in biological tissue and show evidentteratogenicity, carcinogenicity and mutagenicity, thus dioxin has been listed in theStockholm Convention on Persistent Organic Pollutants as one of the twelvepersistent organic pollutants under priority control. The main emission sources ofdioxin are waste incineration and some industrial manufacture processes. Onceemitted from original sources, dioxin may be widely spread in the environmentthrough the long-range atmospheric transport (LRAT) and deposition, and dioxin ishard to be decomposed in the environment due to its chemical stability, which hascaused serious environmental pollution. Thus, it is necessary to investigate thedegradation mechanism of dioxin, by which we may find some feasible methods toeliminate dioxin.In this paper, the quantum chemistry computational software package Gaussian09was used to perform the calculation. By applying density functional theory (DFT),the ring-opening degradation mechanism of2,3,7,8-Tetra-chlorinated dibenzofuran(2,3,7,8-TCDF) initiated by OH radical was studied. Firstly, the geometry structuresof reactants, transition states, intermediates, and products involved in the degradationwere optimized at B3LYP/6-311+G(d,p) level, and the corresponding vibrationalfrequencies were also calculated at the same level. On the base of transition states,the minimum energy path (MEP) were obtained by intrinsic reaction coordinatesmethod (IRC), and the potential energy surface of the degradation reaction wereestablished. Secondly, the potential energy surface information were refined atMPWB1K/6-311+G(3df,2p) level. Finally, combining Transition State Theory (TST)and RRKM theory, the rate constants were calculated by means of the VARIFLEX code for all the reaction channels of the ring-opening degradation of2,3,7,8-TCDF,and the temperature dependence of the reaction rate constants were given.The study results indicate that the adducts TCDF-OH can continue toexperience a series of subsequent unimolecular ring-opening degradation after theOH radical was added to2,3,7,8-TCDF. At the low temperature, the OH radicaladdition reactions are much faster than the subsequent unimolecular ring-openingdegradations. Hence, the subsequent unimolecular ring-opening reactions are therate-determining steps. While at high temperature, the subsequent unimolecularring-opening reactions gradually surpass the OH radical addition reactions, thus theOH radical addition reactions are the rate-determining steps of the global reactions.According the reaction rate constants calculated, the ring-opening degradation of2,3,7,8-TCDF may execute smoothly when the reaction temperature reaches to500~850K. The results of this study can serve as the theoretical foundation for thetechnology development of dioxin elimination, especially， the decompositiontemperature can be used as the process parameter of the dioxin elimination.