Decomposition and reformation pathways of PCDD/Fs during thermal treatment of municipal solid waste incineration fly ash

Thermal treatment of municipal solid waste incineration (MSWI) fly ash (FA) allows heavy metals solidification, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) decomposition, and environmentally stable materials production, but lacking advanced insights into PCDD/Fs dramatically limits its development. In this study, the decomposition and reformation of PCDD/Fs during thermal treatment of two typical fly ashes (loading 0.294 and 0.594 ng I-TEQ/g PCDD/Fs, respectively) are systematically investigated, under conditions of three heating temperature (500, 800, and 1100 °C) and two atmospheres (oxidative and inert). Over 95 % of PCDD/Fs in FA are effectively decomposed for all tests mainly via cyclic skeleton destruction accompanied by dechlorination, but reformation predominantly through de novo synthesis in off-gases significantly reduces the overall elimination efficiency. Moreover, both de novo synthesis and chlorination are enhanced as temperature increase promoting migrations of catalytic metals and chlorine, yet are weakened at the absence of oxygen, both of which are revealed by PCDD/F-signatures evolution. Catalytic metal is identified as the most critical factor accounting for PCDD/Fs reformation, which is also evidenced by kinetic models of de novo synthesis. Finally, the decomposition and reformation pathways during thermal treatment of fly ashes are proposed. The results pave the way for controlling PCDD/Fs reformation and improving the thermal treatment of fly ashes.