Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F, dioxin) are byproducts of industrial and thermal processes. As one of the predominant transition metals in fly ash, the catalytic activity of iron for the formation of dioxins has been extensively investigated. However, the concentration of iron in fly ashes from municipal waste incinerators vary depending on facilities and sampling time. Little information exists regarding the relationship between the content of iron and the yields of dioxins in fly ash. 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. Our results showed that under pyrolysis conditions, the total dioxin yields increased with the increasing content of iron in the fly ash surrogates. The yields of mono-chlorodibenzo-p-dioxin and dichlorodibenzofuran followed the same trend; dibenzofuran and dibenzo-p-dioxin were only formed on 5% of Fe2O3. The formation mechanism of PCDD/F was proposed that chlorinated phenols chemisorb on Fe2O3/ fly ash to form phenoxy-type, environmental persistent free radicals (EPFRs), which subsequently react to form PCDD/F. 1%, 2.5%, 4% and 5% of Fe2O3 were dosed with 2-chlorophenol vapor for the study of EPFRs formation. The results showed concentration of the EPFRs increased with the increasing amount of iron, which is consistent with the yields of PCDD/F. To understand the effects of the content and morphology of iron present in fly ash and the formation of PCDD/F, the surrogates were characterized using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The size of the Fe2O3 particles was within 3-9 nm. Particle size distribution histograms and TEM diffraction pattern indicated that with higher content of iron in the fly ash surrogates, more crystallized and larger Fe2O3 particles were formed. XRD analysis further confirmed that 1% of Fe2O3 fly ash surrogate was amorphous, crystallinity of Fe2O3 appeared in 2.5% of Fe2O3, and more crystalized particles were formed on 4% and 5% of Fe2O3. Our results demonstrate the amount of iron in the fly ash influences the morphology and structure of the Fe2O3 nanoparticles, thus EPFRs and PCDD/F formation. Our study will provide a guide in developing predictive models for controlling the emissions of dioxins for iron rich fly ashes.

Formation of dioxins from 2-chlorophenol over fly ash: Role of Fe2O3

255th National Meeting (2018)

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