Investigating the migration mechanisms of heavy metals under silicate polymerization in gasification slag from landfilled municipal solid waste with rice husk addition
Landfilled municipal solid waste (MSW) in China exceeds 8 billion tons, with high moisture (30–50%) and ash content (>50%), complicating conventional treatment. Slag gasification offers a clean and resource-oriented route, but heavy metal leaching from the resulting slag poses environmental risks. This study investigates the effect of rice husk addition (5–15%) on the vitrification of landfilled-waste slag and the immobilization of heavy metals (Cr, Zn, Cu). Results show that adding 5–10% rice husk lowers the slag flow temperature to a minimum of 1213 °C, attributed to active SiO2 reacting with CaO and Fe2O3 to form low-melting eutectics like anorthite. Leaching concentrations of Cr and Zn decrease from 41.60 and 108.00 mg/L to 5.89 and 7.10 mg/L, respectively, with 10–15% rice husk. The amorphous SiO2 enhances silicate polymerization (Q3, Q4 networks), promoting physical encapsulation and chemical incorporation of heavy metals into stable phases such as Zn2SiO4 and CuFe2O4, increasing the residual fraction and reducing bioavailability. At temperatures >1400 °C, volatilization of Cu and Zn increases, with residual rates dropping to 33–60% and 31–55%, respectively, while Cr remains stable (70–123%). This work elucidates the mechanistic role of rice husk in slag structure modulation and heavy metal immobilization, providing a theoretical basis for the co-treatment of landfilled waste and biomass via a 'treating waste with waste' strategy.