Insights into fate and mechanism of polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) during sequential washing-thermal treatment of incineration fly ash

Washing and thermal treatment are commonly employed to control hazardous components such as chlorides and heavy metals in incineration fly ash (FA). However, the fate and transformation of polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) during these processes have often been assumed rather than explicitly elucidated. In this study, the occurrence, migration, and degradation mechanisms of PCDD/Fs during sequential washing and thermal treatment were systematically investigated. The results revealed that PCDD/Fs in FA are predominantly formed via de novo synthesis pathways, with TeCDFs and OCDD as the dominant congeners. During the washing process, PCDD/Fs were physically transferred and enriched in the solid phase, resulting in a toxic equivalency (I-TEQ) of up to 650 ng-TEQ/kg, with 2,3,4,7,8-PeCDF identified as the major contributor. Thermal treatment effectively degraded PCDD/Fs and reduced TEQs in both RFA (91 %) and WFA (98.6 %), meeting the regulatory limits for resource utilization in China. The detoxification mechanisms were found to involve structural destruction as well as dechlorination/chlorination reactions. However, the relative contributions of structural breakdown and chlorination to the overall degradation process remain uncertain. Interestingly, dechlorination during washing not only enhanced the degradation efficiency but also influenced the degradation mechanisms of PCDD/Fs. By clarifying the occurrence, migration, and degradation mechanisms of PCDD/Fs during combined treatment processes, valuable insights can be provided for the safe disposal of FA.