Theoretical study on the radical anions and reductive dechlorination of selected polychlorinated dibenzo-p-dioxins

Abstract

For the effective use of remediation technologies for PCDDs contamination, it is essential to study the reactivity and dechlorination pathways of these compounds. In this study, density functional theory (DFT) calculations (B3LYP/6-31+G(d), B3LYP/6-311+G(d,p)) were performed to investigate the neutrals and different anionic states of selected PCDD congeners. The calculated adiabatic electron affinities and frontier orbital energies of the PCDD congeners (in gas-phase and in solution) are significantly correlated with the reported dechlorination rate constants, showing that this kind of reductive cleavage reaction is kinetically controlled by the electron transfer step. The predicted major dechlorination pathways of 1,2,3,4-TeCDD and its daughter products based on the energies of the anionic states were found to be satisfactorily consistent with the reported experimental results. Simulation of the 1,2,3,4-TeCDD dechlorination process showed that not only the dechlorination regioselectivity but also the reactivity of the PCDDs played an important role in the distribution of dechlorinated products. An exponential correlation was found between the sum of the concentration of the PCDD congeners and the reaction time in the simulation, indicating that the time required for the conversion of the PCDD congeners to the fully dechlorinated product (dibenzo-p-dioxin) might not be significantly dependent on the initial concentration of 1,2,3,4-TeCDD.