Particle Size and Potential Toxic Element Speciation in Municipal Solid Waste Incineration (MSWI) Bottom Ash

Abstract

The speciation of potentially toxic elements (PTE) in bottom ashes from municipal solid waste incineration (MSWI) and their relationship with grain size is investigated. The proposed enrichment of several potential toxic elements in lower sized grains is discussed, comparing the literature and new data on Parma’s waste incinerator. Results from X-ray fluorescence spectrometry (XRF), SEM-EDS, and XRD analyses on different grain size show (1) a positive Si-trend, correlated with grain size and few lithophile elements, such as Zr and Rb. In Parma, Al, K, Mg, and Fe also correlate with Si for the portion below 2 mm; (2) a Ca-trend, with a strong negative correlation with Si and a positive correlation with loss on ignition (LOI), S, Cl, Ti, Zn, Pb, and Sn. Mineralogical composition shows a little change in grain size, as in previous investigations, but with substantial differences in amorphous content. SEM-EDS analysis shows that the amorphous portion is highly heterogeneous, with portions coming from melting during incineration, residual glass, and unburnt loss on ignition (LOI). The above results show that PTE elements are either present as metals (such as Cu and Ni, or Zn, Pb and Sn) in carbonate, sulfate, and amorphous residual LOI portions.

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

“…The starting amorphous composition is different from size to size, with higher silica for the larger grains, suggesting an Frontiers in Environmental Science frontiersin.org enrichment of silica-based glass in this granulometric fraction in accordance with Mantovani et al (2021). After the first step, the glass composition appears like the starting one, although in the two finer grain sizes (i.e., the sizes with larger weight loss), Na 2 O and K 2 O are diluted in the residual amorphous phase, suggesting that some amorphous Na/K-bearing soluble gel is present at the initial stage, as found in Mantovani et al (2023).…”

“…earlier study (Mantovani et al, 2021;De Matteis, 2023). The waste input (tons/year) is represented by undifferentiated MSW of 126.317 t/y and special waste (construction and demolition waste, sewage sludge, cemetery waste, biomedical waste, etc.)…”

“…XANES analysis found Zn to be typically associated with carbonates, such as smithsonite (ZnCO 3 ) or hydrozincite [Zn 5 (CO 3 ) 2 (OH) 6 ], or with hydroxides, such as Zn(OH) 2 , that are leached by acid attack (Rissler et al, 2020;De Matteis, 2023). Moreover, Zn was found in melilite solid solution, that is, hardystonite (Mantovani et al, 2021); this could account for some leached Zn, as melilite showed a major decrease in this step. Frontiers in Environmental Science frontiersin.org…”

“…For Cu, it was about 50% of the bulk composition. SEM-EDS and XANES analyses showed that both elements were commonly found as metal droplets within a silicate matrix that are oxidized in a soluble ionic species (Mantovani et al, 2021;De Matteis, 2023).…”

“…In contrast, Pb, Zn, and Cu were only marginally present in the residual fraction (Figure 4). Microprobe analysis showed that Zn, Cu, and Pb are present in silicate glasses as impurities (Mantovani et al, 2021); moreover, Zn was found in spinel oxides. Also, Zn was observed in silicates like willemite or in solid solution in feldspars.…”