Heavy metal speciation and risk assessment of cremation fly ash based on particle size fractionation: A case study of funeral home in Shandong, China

Environmental pollution resulting from body cremation has become increasingly concerning. However, there were still significant knowledge gaps in the industry regarding the unclear physicochemical properties and heavy metal occurrence characteristics of cremation fly ash, as well as the ambiguous environmental risks. This study systematically investigated the micro-morphology, elemental composition and mineral phase characteristics of cremation fly ash with different particle sizes. The modified BCR sequential extraction method was used to explore the distribution patterns of heavy metal species and their correlation with physicochemical properties. The risk assessment index method RAC and the comprehensive heavy metal toxicity index model method STI were employed to quantify the environmental risks. The results indicated that the average particle size of cremation fly ash is 42.67 μm, with mineralogical phases including halite, potassium salts, calcite, portlandite, and trace amounts of ZnS and Sr3NiNb2O9. Heavy metals such as Cr, Cu, Pb, and Hg mainly existed in stable forms, while Zn and Cd were predominantly present in labile forms. The proportion of stable Mn and As increased with particle size. Among heavy metals, Zn exhibited the highest concentration (2845.48—4854.91 mg·kg−1), with fine fly ash (−1）. Environmental risk assessment revealed that fine fly ash (<50 μm) is the primary risk source, with Zn and Cd posing extremely high risks across all particle sizes. These findings revealed a clear relationship between the physicochemical properties and heavy metal speciation of cremation fly ash. Targeted control of Zn and Cd in fine particulate ash (<50 μm) was recommended to support enhanced hazardous waste management in the funeral industry.