Mechanistic studies on the role of PAHs and related compounds in PCDD/F formation on model fly ashes

Model fly ashes containing Florisil (C), CuCl2. 2H(2)O and PAHs with struct ures similar to dibenzo-p-dioxin or dibenzofuran were heated at 250 degrees C in He/O-2 with regard to a supposed intramolecular reaction mechanism for oxygen incorporation. Highest reactivities in PCDF formation could be foun d for model compounds containing a biphenyl structure, while condensed Ir-s ystems lead to a decrease in reactivity for such compounds. Biphenyl is alm ost completely converted to dibenzofuran. PCDD formation from six-membered rings like xanthme/9,10-dihydroanthracene is of minor importance. O-18-labe ling of gaseous oxygen reveals no common reaction step for oxygen incorpora tion using 9-fluorenone, xanthene, diphenyl ether and diphenyl-2-carboxylic acid as model compounds. Pre-existing oxygen in reactants is a major sourc e for ether groups in PCDD and PCDF. Determination of labeled and unlabeled CO and CO2 besides He/O-2 reflects higher reactivities towards oxidation f or model compounds containing ether groups than for compounds with carbonyl groups. (C) 2001 Elsevier Science Ltd. All rights reserved.