Elemental content and radionuclide activity of bottom and fly ashes from Municipal Solid Waste Incineration: a time series analysis

Municipal Solid Waste Incineration (MSWI) plants pose significant environmental concerns, generating solid by-products, namely Fly Ash (FA) and Bottom Ash (BA). These MSWI residues have received attention due to the presence of valuable elements, Potentially Toxic Elements (PTE), and other contaminants. Radionuclide detection is also critical because they can concentrate in incineration ashes to pose a radiological hazard. Therefore, multi-element and radionuclide analysis was performed on BA and FA, including samples from bag filters containing lime (FAL) and soda (FAS) additives collected from two MSWI plants in northern Italy. BA and FA were sampled in 2013, 2020, 2021, and 2022 for a multi-year assessment, including during the COVID-19 pandemic. Our objectives were to evaluate the potential of elemental flows and radiological impact of the two different MSWI plants. The chemical concentration of 70 elements and the activity of 8 radionuclides were determined using sector Field Inductively Coupled Plasma Mass Spectrometry (ICP-SFMS) and alpha and gamma spectrometry, respectively. Regarding major elements (Fe, Al, Mg, Ti, and P), high mean concentrations were found in BA, followed by FAL and FAS. Notably, in BA samples, Fe, Al, Zn, and Cu averaged 47600, 35300, 4100, and 3500 mg kg(-1), respectively, and critical raw materials, namely elements of economic importance such as Mg, P, Ti, and Ba, were concentrated at 16100, 6800, 3500, and 1400 mg kg(-1), respectively. The annual flows of elements from MSWI residue streams ranged in the order of 10(3)-10(4) kg a(-1) for Fe, Al, Zn, Cu, Mg and Ti, and the sum of Rare Earth Elements (& sum;REE) was about hundreds of kg per year. Chondrite-normalized patterns of REE and normalized patterns of selected elements over crustal averages helped to evaluate anthropogenic signals, which enabled us to hypothesise elemental sources related to the input MSW. BA and FA showed a higher content of natural radionuclides than artificial ones. In BA, natural radionuclides, K-40 and Pb-210, ranged from 666 to 693 Bq kg(-1) and 23.3-48.1 Bq kg(-1), respectively. In FA, K-40 ranged from 308 to 2198 Bq kg(-1) and Pb-210 from 17.1 to 534 Bq kg(-1). Activity concentration index (ACI) results in all-natural radionuclides below the permissible limit (<1). Still, the significant abundance of Pb-210 and K-40, coupled with their complex behaviour, calls for new and continuous evaluation of long-term emissions and the radiological hazard related to MSWI systems.