A critical review of heavy metal characteristics and risk assessment approaches of municipal solid waste incineration bottom ash in subgrade construction

The global generation of municipal solid waste (MSW) has significantly escalated and it is nowadays an urgent environmental problem. Incineration has emerged as a prevalent MSW treatment method, with the resulting MSW incineration bottom ash (MSWI BA) offering potential for beneficial reuse, particularly in subgrade construction. However, concerns persist in terms of long-term leaching and ecotoxicity of MSWI BA heavy metals. This critical review consolidates the existing research on the physicochemical properties of MSWI BA, its heavy metal characteristics, and the associated risk management strategies for its reuse in road construction applications. The review systematically analyzes the physical and chemical properties of MSWI BA, confirming its suitability as a construction aggregate. It further examines the typical heavy metal content, including Pb, Zn, Cu, and Cr, and the influential factors such as waste composition, incineration temperature, and flue gas treatment. The leaching behavior of heavy metals under different environmental conditions is investigated through the synthesis of laboratory test data and field monitoring results. Common risk assessment methods at various stages, from BA pretreatment to subgrade design, are evaluated. Additionally, the review summarizes the regulatory standards for MSWI BA application in road construction across different regions, identifies current research limitations, and provides suggestions for future research directions to promote the sustainable utilization of this valuable resource. The findings of this comprehensive review contribute to a better understanding of the feasibility and environmental implications of MSWI BA reuse, supporting informed decision-making and the development of effective management strategies for this growing waste stream. The insights gained can aid in the design of appropriate treatment and application protocols to mitigate the risks associated with heavy metal leaching, thereby facilitating the circular economy in the subgrade construction.