From waste to resource: multi-technique metal speciation and mineralogical characterisation of incinerator bottom ash for circular economy applications

In the framework of the European Green Deal and circular economy strategies, the sustainable management of Municipal Solid Waste Incineration (MSWI) residues has emerged as a key challenge. Bottom Ash (BA) represents a significant volume of these residues and is a valuable source of secondary raw materials (SRMs) and Critical Raw Materials (CRMs). Conventional regulatory assessments often focus on bulk chemical composition, which can overestimate environmental risks, such as the HP14-ecotoxic property, by assuming that heavy metals are present in their most reactive and bioavailable oxidised forms.

This study presents an integrated, multi-technique analytical workflow designed to bridge the gap between total elemental concentration and actual environmental risk through speciation-based assessment. By employing a controlled-density separation procedure using LST Fastfloat, distinct density classes were successfully fractionated: a light fraction containing plastics and organic matter; a medium fraction containing glassy blebs and silicates; and a heavy fraction containing metallic alloys and ferrous materials.

To achieve a rigorous scientific characterisation of these fractions, a synergistic multi-technique approach was employed. While SEM-EDX provided high-resolution morphological data and localised chemical speciation – revealing that metals like Zn, Cu and Pb are frequently hosted within metallic alloys rather than oxides – X-Ray Powder Diffraction (XRPD) was crucial for identifying the crystalline mineralogical assemblages. XRPD analysis allowed to confirm the incorporation of heavy metals into stable metallic phases or inert silicate matrices, significantly limiting their mobility and environmental impact under standard conditions.

The quantitative chemical framework was further refined by integrating Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) with Total Reflection X-Ray Fluorescence (T-XRF). The utilisation of ICP-OES was instrumental to ensuring regulatory-grade accuracy in the bulk chemical characterisation; by providing a precise total elemental inventory, it facilitated a direct comparison between the total concentration of heavy metals and their actual mineralogical sequestration as identified by SEM-EDX and XRPD. Simultaneously, T-XRF – characterized by its high sensitivity and minimal sample volume requirements – provided precise quantification of trace elements and a fundamental cross-validation of the wet-chemical results obtained via ICP-OES after microwave-assisted mineralisation. These integrate analyses demonstrates that the heavy metal content is predominantly sequestered in stable, non-reactive phases – such as metal alloys and glassy blebs. These findings have significant implications for the reclassification of BA from hazardous to non-hazardous waste. This research provides a scientifically robust workflow method to characterize BA precisely, thereby reducing their disposal cost and enabling reuse as secondary raw materials.