Doctor of Philosophy (PhD)
Transition metal oxides present in waste incineration systems have the ability to catalyze the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) through surface mediated condensation of organic dioxin precursors. Current studies have concentrated on the catalytic effects of individual transition metal oxides, while the complex composition of fly ash introduces the possibility of synergistic or antagonistic effects between multiple, catalytically active components. In addition, there is at present hardly a quantitative link between fly ash physical properties/chemical reactivity and PCDD/Fs yield.
We have tested fly ash surrogates containing different ratios of iron (III) oxide and copper (II) oxide to study the cooperative effects between two transition metals. The presence of both iron and copper oxides increased the oxidative power of the fly ash surrogates in oxygen rich conditions and led to extremely high PCDD/F yields under pyrolytic conditions (up to >5% yield) from 2-monochlorophenol precursor. PCDD/F congener profiles from the mixed oxide samples are similar to results obtained from only CuO, however the total PCDD/F yield increases with increasing Fe2O3 content. Careful analysis of the reaction products and changes to the oxidation states of active metals indicate the CuO surface sites are condensation reaction centers while the Fe2O3 is affecting the bond energy in CuO and increasing the ability of copper centers to form surface-bound radicals that are precursors to PCDD/Fs.
Three fly ash surrogates containing 1%, 2.5%, and 4% of Fe2O3 were prepared and their effects on the PCDD/F formation were investigated and compared to typical 5% iron oxide sample. The results showed that under pyrolysis conditions, the total dioxin yields increased with the increasing content of iron in the fly ash surrogates. To understand the effects of the content and morphology of iron and the formation of PCDD/F, the surrogates were characterized using transmission electron microscopy (TEM) and X-ray diffraction (XRD). TEM and XRD analysis confirmed that 1% of Fe2O3 fly ash surrogate was amorphous, and crystallinity increased with the increasing iron content. Our results demonstrate the amount of iron in the fly ash influences the morphology and size of the Fe2O3 nanoclusters, determining the furan congeners’ distribution and total PCDD/F yield.
Guan, Xia, "The Effects of Metal Speciation, Content and Cluster Size in Fly Ash on PCDD/F Formation" (2018). LSU Doctoral Dissertations. 4552.