The immobilization of potentially toxic elements due to incineration and weathering of bottom ash fines

Incineration bottom ash fines (≤ 125 μm) are known to contain potentially toxic elements (PTEs) and inorganic salts. The most abundant PTEs in the fines were Zn (0.5%), Cu (0.25%), Pb (0.12%), Mn (0.08%) and Cr (0.03%). The systematic quantification of the mineral phases and PTEs associated with them was performed with a multimethod approach using quantitative XRD, phase mapping with PhAse Recognition and Characterization (PARC) software and microprobe analysis. The mineral phases in the fines can be categorized as follows: 1) residual phases (e.g., quartz), 2) incineration phases (e.g., melilitic slag and iron oxides) and 3) quenching/weathering phases (e.g., calcite, ettringite, gypsum, hydrous Fe- and Al-oxides). Among the incineration phases, the melilitic slag was observed to contain Cr, Cu and Zn with 0.02%, 0.13% and 0.19%, respectively. In order of predominance, the weathering phases containing the most PTEs were: calcite < ettringite < hydrous Al-oxides < hydrous Fe-oxides. More than 70% of the phases in the BA fines were formed during incineration and weathering processes that explain the enrichment of PTEs in the smaller particles. During the one-batch leaching test, dissolution of weathering phases, especially ettringite, was observed (total mass loss: 7.2%).