In-depth mineralogical quantification of MSWI bottom ash phases and their association with potentially toxic elements

Municipal solid waste incineration bottom ash fractions ≤4 mm are the most contaminated ones in terms of potentially toxic elements (PTEs). In order to estimate potential environmental impacts, it is important to understand the association of the PTEs with the mineral phases. Large area phase mapping (SEM/EDX) using "PhAse Recognition and Characterization - PARC" software in combination with quantitative X-ray powder diffraction has been used to characterize amorphous and crystalline BA phases for the first time. The results show that one of the main incineration products was melilite and an amorphous phase with a melilitic composition. The ratio of crystalline to amorphous melilite was 1:2. They formed an inhomogeneous layer around BA particles and contained a high percentage of the PTEs, i.e., Cu, Zn, Ni and Cr. Other major sources of PTEs (especially Ni and Cu) were iron oxides produced during incineration and the weathering products, such as calcite and ettringite (Cu and Zn). After extensive characterization of BA, a sequential extraction procedure (SEP) was performed, which exposed bottom ash to different chemical environments designed to dissolve specific phases and release their PTEs into solution. The extracted solutions and solid residues generated from the extraction procedure were analyzed to identify the association between PTEs and dissolved phases of BA. By combining SEP results with information obtained via large area phase mapping it is shown that SEP can be used for studying the association of PTEs with the phase that cannot be investigated with XRD/EDX, such as organic matter and Fe-Mn-hydrous oxides. Furthermore, according to SEP results a high percentage (40-80 wt%) of each investigated PTE can be considered immobile and not susceptible to leaching in the environment.