Physicochemical characteristics and speciation of potentially toxic elements in municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash is classified as hazardous waste, mainly due to the enrichment of potentially toxic elements (PTEs). Conventional bulk characterization of homogenized samples has limitations for evaluating the environmental behavior of MSWI fly ash. This study combined X-ray fluorescence (XRF), X-ray diffraction (XRD), SEM–EDS, digestion–based ICP–MS, BCR sequential extraction, and Ca–normalized LA–ICP–MS imaging across a 2.34 × 1.56 mm mapped region to characterize bulk operational speciation fractions of target elements. BCR results indicated that Cr and Ni were dominated by the residual fraction (F4), Cu by the oxidizable fraction (F3), and Zn, As, Cd, and Pb by the reducible fraction (F2). Cd had a 38.16 % acid-soluble fraction (F1) and exhibited the highest RAC and ICF risk indices. Element–specific hotspot masks were generated using 256–bin Otsu thresholding. Overlap proportions and the area–normalized preference index were applied to evaluate the spatial association between target-element hotspots and three indicator–element domains (Si–Al, Fe/Mn, Na/K). Pb and Cd showed the highest preference for the Na/K domain, Cr, Ni, and Cu for the Fe/Mn domain, and Zn for the Si–Al domain, whereas As showed moderate multi–domain occurrence. Correlation analysis of microscale phase data and macroscopic risk indicators revealed that environmental risk was related to chemical fractionation and microscale occurrence rather than total concentration alone. This study provides a scientific foundation for environmental–risk discrimination and the development of PTE stabilization technologies for MSWI fly ash.