Mechanistic aspects of the de-novo synthesis of PCDD/PCDF on model mixtures and MSWI fly ashes using amorphous 12C- and 13C-labeled carbon.

The formation of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) from amorphous 12C- and 13C-labeled carbon was studied on model mixtures and real fly ashes. PCDD/F can either be formed directly (de-novo) from carbon already present in fly ash or step-by-step via condensation of two aromatic rings. Using model mixtures containing 12C- and 13C-labeled carbon in various ratios we observed the formation of the following compound classes: 12C6-PCPh, -PCBz, 13C6-PCPh, -PCBz, 12C12-PCDD/ F, 13C12-PCDD/F, and 12C6 13C6-PCDD/F. By examining the fraction of the mixed PCDD/F (one of the two aromatic ring is composed solely of 12C-atoms while the other contains only 13C-atoms) in the total concentration of PCDD/F, conclusions on the formation of these three ring structures are possible. From the experimental results, it can be concluded that both reaction mechanisms are operative in the formation of PCDD/F from carbon. On fly ashes approximately half of the total amount of PCDD is formed via condensation of de-novo created C6-precursors e.g. chlorophenols, while the remainder is directly released (de-novo) from the carbon i.e., formed from a related C12-structure. However, the condensation of intermediate aromatic C6-precursors is of minor importance in the formation of PCDF. With increasing temperature the relative amount of the 12C6 13C6-PCDD formed by condensation decreases due to the faster evaporation of chlorophenols. At a constant reaction temperature, the ratio of both reaction pathways is hardly influenced by reaction time. In experiments with fly ashes doped with 13C-labeled carbon, this carbon isotope shows a similar reactivity as the native carbon present on the fly ash. Thus, the used amorphous carbons are suitable models for this investigation.