Degradation of Dioxins in Chelated Municipal Solid Waste Incineration Fly Ash by Low-Temperature Pyrolysis

In this study, low-temperature pyrolysis is applied to raw and chelated municipal solid waste incinerator fly ash (FA) to destroy polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) and international toxic equivalency (I-TEQ) values and to identify destruction/reformation routes based on PCDD/F-signature. In addition, different temperatures and reaction times were also studied for dioxin destruction to identify optimum test conditions in the nitrogen atmosphere. Afterward, samples were analyzed by High-Resolution Gas Chromatography-Mass Spectrometry to obtain the signal intensity of dioxin isomers. The removal rate of PCDD/F was observed between 98.96 and 99.90% and the I-TEQ decomposition rate was 89.97 and 99.80% across all FAs under different temperatures and reaction times. A chelating agent (EDTANa2) enhanced thermal decomposition, whereas adding an inhibitor (Na2HPO4) promoted PCDD/F reformation. There is a variable output of dioxins under different temperatures and reaction times; however, longer reaction times and temperatures proved highly effective. The overall toxicity of all thermally tested FAs was lower than the limit of 50 ng I-TEQ/kg and observed in the range of 1.84 ± 0.11 to 19.45 ± 0.89 ng I-TEQ/kg. The investigation of the PCDD/F signature suggests thermal decomposition as a major pathway of destruction; chlorination/dechlorination also exists to some extent, as observed by the high percentage contribution of hepta- and octa-chlorinated congeners (H7CDD/F and O8CDD/F). The precursor pathway is the dominant formation mechanism, whereas the de novo route also exists and dominates in certain FAs. Overall, low-temperature pyrolysis is an auspicious disposal technology and must receive additional studies for large-scale applications.