Heavy metal leaching and landfill gas emissions from a recirculated MSW landfill mixed with stabilized MSW incineration fly ash: Behavior and interactive effects

In China, the co-landfill of stabilized MSW incineration fly ash (FA) and raw MSW is a historical legacy issue, which can affect the stability of heavy metals in FA and the stabilization process of MSW. This study investigates a recirculating semi-aerobic co-landfilling system of MSW and stabilized FA, focusing on landfill gas emissions and heavy metal leaching. Three landfill columns with varying recirculation frequencies were established for simulated experiments. Results indicate that leachate recirculation enhances microbial activity but causes accumulation of NH4+-N and COD, inhibiting organic matter decomposition. Recirculation frequency does not significantly affect CO2 production, with peak concentrations in columns S1 with natural rainfall, S2 with weekly leachate recirculation, and S3 with bi-daily leachate recirculation at 245 days being 60.95 %, 58.42 %, and 61.86 %, respectively. Compared with simulated rainfall, leachate recirculation delayed the stabilization process but exerted a positive effect on mitigating Cd and Pb leaching. Heavy metal leaching is driven by pH fluctuations, carbonation, and recirculation frequency. Early acidic conditions increase heavy metal release, while certain degree of carbonation stabilizes Pb, Ni, and Cr. However, Zn and Cu mobility depends on the balance between acid leachate leaching and CO2 carbonation. Heavy metal contamination reduces microbial metabolic activity and CO2 production by altering microbial communities. These findings highlight the complexity of pollutant dynamics in co-landfilling and offer insights for optimizing strategies to mitigate environmental risks of disposing stabilized FA and MSW.