Modeling the reductive dechlorination of polychlorinated dibenzo-p-dioxins: kinetics, pathway, and equivalent toxicity.

Reductive dechlorination of polychlorinated dibenzo-p-dioxins (PCDDs) involves 256 reactions linking 76 congeners with highly variable toxicities, so it is challenging to assess the overall effect of this process on the environmental impact of PCDD contamination. This study describes a quantitative solution to this problem that allows calculation of the toxic equivalent quantity (TEQ) of the mixture of PCDD congeners predicted by a linear free-energy relationship (LFER) that relates rate constants of dechlorination to calculated reduction potentials. The reduction potentials were derived from Gibbs free energies, calculated using density functional theory (DFT) and vapor pressures and solubilities for individual congeners. The LFER was calibrated with rate constants that we recently reported for PCDD dechlorination by nano-zerovalent iron (nZVI). Simulation done with this model predicts that more than 100 years would be required for complete dechlorination of octachlorinated dibenzo-p-dioxin (OCDD) to dibenzo-p-dioxin (DD) under conditions representative of treatment with nZVI and that the TEQ of the mixture of intermediates during this process increases 10-fold, peaking at around 3-6 years, mainly because of the higher toxicity of 2,3,7,8-substituted congeners.