Grain size and mineralogical constraints on leaching in the bottom ashes from municipal solid waste incineration: a comparison of five plants in northern Italy

Abstract

Introduction: Bottom ashes (BA) from municipal solid waste incinerators (MSWIs) are currently classified by the European Waste Catalogue as industrial non-hazardous waste. To promote their reuse, identification and characterization of the heavy metal-bearing phases (both glass and minerals), as well as their weathering behavior, must be addressed for what concern the chemical composition, mineralogical phases, and in high concentrations and pollutants’ mobility. An important point to be noted is whether the results from a given plant can be generalized.Materials and methods: In this work, BA from five waste-to-energy (WtE) plants in northern Italy were sorted based on different grain sizes. The area showed similar MSW production, collected from a culturally homogeneous area, and similar collection management. For each grain size, a mineralogical, chemical, and physical characterization was carried out using XRF, XRD, TGA, and the leaching test.Results and discussion: We found that for major elements, the average chemical composition of the incinerators is similar, with some differences in minor elements. Ferrara (FE) and Forlì-Cesena (FC) BA show portlandite, higher ettringite, and less amorphous than the Torino (TO), Parma (PR), and Piacenza (PC) BA. This affects the pH, the release, and toxicity of the leachates. In FE and FC ashes, ettringite is insoluble, and we not only have low sulfate but also Ni and Ba beyond reglementary limits, suggesting that Ni and Ba are present as hydroxides with portlandite, which are soluble. In TO, PR, and PC, Cr and sulfates are beyond limits, suggesting that Cr comes from dissolution in ettringite. Cu and Cl are always beyond limits; the dissolution of chlorides accounts for only 30%–35% of the global Cl leachate. We observe that in the assessment of the potential toxicity of the ashes, mineralogy has a higher effect than the bulk chemical composition. Grain size sorting, although useful together with other techniques, is not by itself able to comply with the PTE risk level.Conclusion: The findings contribute to the development of efficient treatment strategies for BA, highlighting the need for a more thorough investigation to understand composition and properties and to find innovative ways to reuse, promoting the circular economy and sustainable waste management practices.

Sequential extraction procedure of municipal solid waste incineration (MSWI) bottom ash targeting grain size and the amorphous fraction

“…After the first step, there was a mild but significant weight loss, ranging from 3% in the larger grain size to 9% in the smaller one. As reported in Mantovani et al (2023), the higher weight loss in major elements occurred for Ca, Na, and K, together with S and Cl. These elements were found in the leachate as they are present in highly soluble phases such as chlorides, anhydrite, and hydrocalumite.…”

“…At the end of the procedure, the remaining phases ideally consist of an amorphous fraction, silicates, and well-crystallized oxides, which are expected to be abundant in BA (Mantovani et al, 2023). In agreement with previous literature (Bruder-Hubscher et al, 2002;Alam et al, 2019;Haberl and Schuster, 2019;Pérez-Martínez et al, 2019), a final dedicated step for the complete removal of the more resistant phases has been included in our modified SEP.…”

“…The recycling of BA is linked to the results of leaching tests, where the release of PTEs from partially or completely watersoluble phases such as ettringite, hydrocalumite, and salts is evaluated. Notably, these phases become more significant in aged BA, resulting in increased leachability (Mantovani et al, 2023). By including a dedicated step for water-soluble phases in the extraction procedure, we allow for a more comprehensive investigation of their leaching behavior.…”

“…Recently, Alam et al (2019) suggested a new method to examine not only the chemical composition of eluates but also the mineralogical composition of the BA residues, including the quantitative evaluation of the amorphous phases that make up between 30% and 80% of the ashes (Mantovani et al, 2023). The Alam et al (2019) investigation showed that the standard BCR procedure of dissolving the carbonate fraction with a 0.11 M acetic acid extraction was not able to provide complete carbonate dissolution.…”

“…The SEP was done on BA of small and medium grain sizes (<4 mm), fractions that are more reactive and host a higher concentration of PTEs (Mantovani et al, 2023).…”