Relative rates of formation of polychlorinated dioxins and furans from precursor and de novo reactions

Though it is generally agreed that there are two major pathways for the formation of polychlorinated dibenzo-p-dioxins and furans (PCDD/F), namely the formation from chemically similar precursors (chlorobenzenes and chlorophenols) and the so-called de novo synthesis, some difficulties remain. Extrapolation from laboratory based studies to incinerators has not been straightforward. The question 0f relative rates is explored in this paper. It is concluded that the precursor-based reactions tend t0 be faster, when computed on a per carbon basis. Some observed incinerator PCDD/PCDF-ratios can be reconciled with this observation, if PCDD and PCDF formation occur at different rates as a function 0f temperature.