Impact and recoverability of metals from waste: a case study on bottom ash from municipal solid waste incineration plants

Introduction: Municipal Solid Waste Incineration (MSWI) plants generate significant amounts of solid end-products, such as bottom ash (BA), containing potentially toxic elements like Cr, Ni, As, Cd, and Pb, base elements (e.g., Si, Al, Fe, Ti, Cu, and Zn), and other technology-critical elements (TCE), such as Co, Ga, Mg, Nb, P, Sb, Sc, V, Li, Sr, and REE. The accurate determination of these elements in anthropogenic wastes and the assessment of their removal are crucial for the circular economy.Methods: This paper aims to characterize BA samples from two Italian MSWI plants (named FE and FC) by X-ray fluorescence (XRF) and comparatively assess the removal of a selection of elements using the aqua regia digestion (ARD) method, followed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analytical determination.Results and discussion: According to the XRF analysis, Ca, Fe, Al, Mg, and Na had high concentrations in BA, and their contents increased with decreasing particle size in both FE and FC samples. The Enrichment Factor (EF) based on the upper continental crust's average values of Zn, Cu, and Pb was high (EF > 30), while Cr, Ni, and As were scarcely enriched (EF > 1), and REE enrichment was very low (EF < 1). In both FE and FC plants, the Degree of Elements Extractability (DE) was high (>80%), especially in the fine-grained fractions of MSWI bottom ash. The Enrichment Factor (EF) based on the upper continental crust average values of Zn, Cu, and Pb was high (EF > 30), while Cr, Ni, and As were scarcely enriched (EF > 1), and REE enrichment was very low (EF < 1). The bibliometric analysis helped highlight research trends in the assessment and treatment of MSWI-BA, discriminating the literature impact on environment/health issues and recovery/recycling strategies for the circular economy associated with the MSWI-BA material.Conclusion: Although higher data coverage is needed, the present study suggests ARD as an effective method for better understanding the environmental impact and recoverability of useful elements from anthropogenic materials like MSWI bottom ash.