Since the discovery of the polychlorinated dibenzo-p-dioxins and polychlorinated dibonzofurans as by-products of chemical synthesis and in the exhaust of combustion processes, researchers have concentrated their attention on the understanding of their origin and mitigation. Detection of those extremely toxic compounds in the exhaust of incinerators was a shocking discovery as it threatened the well being of the population at large. From the very beginning it was understood that the key for the control of dioxins emission is the understanding of their formation mechanism followed by engineering and regulatory efforts to minimize it.  The theory of PCDD/F formation in combustion systems evolved over the years with advances in analytical methods and better understanding of the dioxin chemistry. The gas phase precursor model assumed formation of PCDD/Fs at temperatures between 1000-600oC in the post flame zone of combustion and included both radical-radical and radical-molecule interactions with Smiley rearrangement. Detailed kinetic studies of this model created an accurate picture of dioxins formation at this temperature regime, however the results were far from the observed dioxins yields. Discovery of increased concentration of PCDD/Fs in cool zone of combustors compared to post flame zones have indicated a much higher contribution of lower temperature mechanism. Surface catalyzed processes have been identified as a source of dioxins and two models were proposed: denovo synthesis from the polyaromatic carbon deposits with the assistance of metal chlorides and surface mediated precursor mechanism. With time particularly the last model have been proven to be a major contributor to PCDD/F emissions.  At present a new questions arise. Do we know everything about the dioxins formation? Do we know how to prevent their formation/emission? Does the dioxins research is still important? Unfortunately, the users are no, no and yes. Despite many years of research some mechanistic questions are still unanswered such as for example the dioxis/furans ratio. No predictive model of PCDD/F emissions is available. New sources of dioxins in fuels have emerged.

Award Address  (ACS Award for Creative Advances in Environmental Science and Technology sponsored by the ACS Division of Environmental Chemistry and ACS Publications journal Environmental Science & Technology). Dioxins � a long research journey but are we there yet?

247th National Meeting (2014)

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