| Since their discovery in combustion and incineration processes in the late 1970's, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) have received increasing attention as toxic organic pollutants. Commonly referred to as “dioxins”, these highly stable bio-accumulating compounds are considered immunotoxins, mutagens and carcinogens; specific isomers are among the most toxic compounds.; The work discussed in this thesis stems from the investigation of the potential for PCDD/F formation in an alternative method of waste reduction/recycling known as the Catalytic Extraction Process (CEP). Similar to PCDD/F formation in incineration and combustion processes, formation on the dust by-product of the CEP process in the cooler, post combustion region was the overall focus of this work. For incineration, two predominate heterogeneous mechanisms have been proposed to explain the drastic increase in concentration in this region: (i) formation from chemical similar precursors to PCDD/F, especially chlorophenols, catalyzed by metals in fly ash and (ii) formation directly from the particulate carbon in fly ash (de novo synthesis).; Iron chlorides were investigated as promoters for the de novo synthesis of PCDD/F. A model CEP dust system, containing a support matrix of alumina/calcium oxide/silica (typical of the components of actual CEP dusts), was used to investigate the PCDD/F formation as a function of carbon black and reaction conditions in a fixed bed reactor. This matrix was prepared via a chemical vapor deposition process to produce a glassy powder and believed to be similar to the matrix of actual CEP dusts. An effort was made to determine the role(s) of various iron compounds in the de novo synthesis that were formed by the thermal decomposition of iron(II) chloride. Predominately one of the possible 210 tetra-octa-CDD/F congeners (namely, the fully chlorinated O8CDF) was formed in this system, with only minor amounts of PCDD and lower than H7CDF chlorinated PCDF. This pattern was investigated in order to establish a plausible mechanism of formation that was useful in the kinetic modeling of the de novo synthesis pathway of PCDD/F formation. |
