Effect of low-temperature thermal dechlorination on polychlorinated naphthalenes in municipal solid waste...

Polychlorinated naphthalenes (PCNs) are dioxin-like contaminants unintentionally generated during municipal solid waste incineration (MSWI). This study evaluated the concentrations, congener profiles, and dioxin-like toxicities of PCNs and PCDD/Fs in MSWI fly ash before and after low-temperature thermal dechlorination (LTD) at five full-scale facilities. LTD consistently induced stepwise dechlorination, reducing the average chlorination degree of PCNs by 1.1-1.5 points. Total PCN concentrations, however, exhibited facility-dependent behavior: decreases of 98-99 % were observed at some facilities, whereas others showed increases of 230-370 % relative to pre-LTD levels. This mass increase is plausibly attributed to the reformation of low-chlorinated PCNs on the fly ash carbon surfaces during the cooling phase. Despite these contrasting changes in mass concentrations, PCN-derived dioxin-like toxicity (PCN-TEQ) decreased substantially at all facilities (35-99 % reduction). This robust detoxification occurred because LTD preferentially degrades the highly chlorinated congeners (e.g., 1,2,3,6,7,8-hexaCN and 1,2,3,4,6,7-hexaCN) that dominate PCN-TEQ. The contribution of PCNs to total toxicity consequently declined from 0.18 to 0.45 % before LTD to 0.03-0.30 % afterward. For PCDD/Fs, LTD generally lowered mass concentrations, although PCDD/F-TEQ reductions varied among facilities due to differences in residual high-TEF congeners. These findings unequivocally demonstrate that LTD is a highly effective strategy for mitigating the dioxin-like toxicity associated with PCNs in MSWI fly ash. The results enhance understanding of chlorinated pollutant transformation during full-scale LTD operation and support its optimization for safer MSWI fly ash management.