Mechanochemical degradation of PCDD/Fs in fly ash within different milling systems

Two distinct mechanochemical degradation (MCD) methods are adopted to eliminate the polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) from fly ash in municipal solid waste incinerators. First, experiments are conducted in a planetary ball mill for selecting suitable additives, and an additive system of SiO₂-Al is chosen for its high-efficiency, low-price, and good practicability. The I-TEQ value of PCDD/Fs in washed fly ash decreases dramatically from 6.75 to 0.64 ng I-TEQ/g, after 14 h of milling with 10 wt % SiO₂-Al, and dechlorination is identified as the major degradation pathway. Then, this additive is applied in a horizontal ball mill, and the results indicate that the degradation of PCDD/Fs follows the kinetic model established in planetary ball mills. However, longer milling time is required for the same supplied-energy because of the lower energy density of horizontal ball mills, resulting in partial loss of Al reactivity and a lower degradation efficiency of PCDD/Fs. During MCD, the evolution of PCDD/F-signatures is analogous, indicating a similar acting mechanism of all additives in both the two milling systems. Finally, a major dechlorination pathway of PCDD-congeners is proposed based on the signature analysis of congeners synthesized from chlorophenols.