Influence of intruding organic acids on the leaching dynamics of heavy metals in stabilized MSWI fly ash from landfills under various percolation conditions

In the unregulated co-landfill disposal of stabilized fly ash and municipal solid waste (MSW), the organic acid in MSW leachate poses a significant threat to the environmental safety of stabilized fly ash. Bulk bags (FIBCs)used for packaging stabilized fly ash may experience varying degrees of damage in landfill environments, which directly influence the percolation pathways of the invading liquid, subsequently affecting the leaching behavior of heavy metals in fly ash. A column leaching experiment was conducted to simulate the impact of organic acid on the leaching behavior and environmental risk associated with six heavy metals (Pb, Zn, Cu, Cd, Cr, Ni) in fly ash under multiple percolation paths. The findings suggest that the complex percolation paths can more effectively prevent heavy metals leaching from fly ash. Furthermore, the leaching of heavy metals is the lowest when the percolation path does not go through the bottom, with Pb, Zn, Cu, Cd, Cr and Ni being only 4.85 %, 27.36 %, 0.11 %, 2.22 %, 13.04 % and 0.42 % of the highest leaching amount under other paths (except Zn, the highest leaching of heavy metals is under the "top in and bottom out" path). Indicate that the integrity of FIBCs bottom significantly reduces the leaching of heavy metals, and favors environmental risk control of heavy metal, with the RI value did not reach the high-risk level (>= 600) in all stages. The percolation path has the most significant influence on the cumulative leaching amount of Cu, while it has the least influence on Zn. This influence is due to factors such as the contact degree of fly ash and organic acid, the transfer rate of heavy metals in liquid phase, and the properties of heavy metals. Heavy metal leaching was typically higher towards the end of the experiment, especially under the "top in and bottom out" path.