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

The formation of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) from amorphous C-12- and C-13-labeled carbon was studied on model m ixtures 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 C-12- and C-13-labeled carbon in various ratios we observed the formation of the following compoun d classes: C-12(6)-PCPh, -PCBz, C-13(6)-PCPh, -PCBz, C-12(12)-PCDD/F, C-13( 12)-PCDD/F, and (C6C6)-C-12-C-13-PCDD/F. By examining the fraction of the m ixed PCDD/F (one of the two aromatic ring is composed solely of C-12-atoms while the other contains only C-13-atoms) in the total concentration of PCD D/F, conclusions on the formation of these three ring structures are possib le. From the experimental results, it can be concluded that both reaction m echanisms are operative in the formation of PCDD/F from carbon. On fly ashe s approximately half of the total amount of PCDD is formed via condensation of de-novo created C-6-precursors e.g. chlorophenols, while the remainder is directly released (denovo) from the carbon i.e., formed from a related C -12-structure. However, the condensation of intermediate aromatic C-6-precu rsors is of minor importance in the formation of PCDF. With increasing temp erature the relative amount of the (C6C6)-C-12-C-13-PCDD formed by condensa tion decreases due to the faster evaporation of chlorophenols. At a constan t reaction temperature, the ratio of both reaction pathways is hardly influ enced by reaction time. In experiments with fly ashes doped with C-13-label ed carbon, this carbon isotope shows a similar reactivity as the native car bon present on the fly ash. Thus, the used amorphous carbons are suitable m odels for this investigation.