Desalination and then silica fume-assisted sintering render MSWI fly ash reliably safe: Heavy metal stabilization and mechanistic insights

The safe handling of municipal solid waste incineration fly ash (MSWI-FA) is imperative due to its leaching of heavy metals (HMs), particularly after extensive carbonation. Taking advantage of a "waste-treating-waste" strategy, this study systematically screened the coupled process of silica fume addition, water washing, and low-temperature sintering, and pioneered the conversion of MSWI-FA to a safe and friendly level. The function of silica fume during co-sintering was contingent upon pre-desalination, which determined whether to stabilize or solubilize the HMs. Without pre-desalination, the sintering of MSWI-FA with silica fume exacerbated the leachability and environmental risk of HMs compared to the untreated MSWI-FA. Interestingly, desalination and then silica-assisted sintering (W/Si-Sint) at 700°C reduced the leachability of HMs by 99.9 %, with Cd leaching less than one-fifth of the GB16889 limit and Pb and Zn being undetectable. Heavy metal stability was maintained over a broad pH range (2.64–13.5) and 50 days of natural exposure. Risk assessments confirmed the HMs in treated MSWI-FA at safe levels and posed low ecological risks in leachates. Additionally, the W/Si-Sint process achieved a 2.77-fold mass reduction in MSWI-FA and enabled a favorable recovery of Pb and Zn. Advanced analyses like spherical-aberration-corrected scanning transmission electron microscopy (Cs-STEM) provided new insights into the stabilization mechanisms.