Zinc speciation in fly ash from MSWI using XAS - novel insights and implications

The chemical forms of zinc in fly ash from municipal solid waste incineration (MSWI) crucially affect ash management, influencing both material recovery options and the risk of unwanted leaching into ecosystems. The zinc speciation was investigated in fly ash samples sourced from full-scale MSWI plants, including four grate fired boilers (GB) and one fluidized bed boiler (FB). We applied X-ray Absorption Spectroscopy (XAS), and the spectra were analyzed against a unique library of over 30 relevant compounds, tailored to the nuances of zinc chemistry of fly ash. Nano-XANES and sequential leaching were employed as complementary analytical methods. Multiple chemical forms of zinc were found in the ash, whereof potassium zinc chloride salts (K₂ZnCl₄) emerged as the predominant form in GB fly ash representing 41-64 % of the zinc content, while less for FB fly ash (19 %). The mere exposure to humidity in the air during storage resulted in hydroxylation of the alkali zinc chlorides into Zn₅(OH)₈Cl₂·H₂O. Other forms of zinc in the ash were Zn₄Si₂O₇(OH)₂·H₂O, ZnFe₂O₄, ZnAl₂O₄, surface adsorbed zinc, and Zn₅(CO₃)₂(OH)₆. Notably, the proportion of zinc in spinel forms (ZnFe₂O₄ and ZnAl₂O₄) increased threefold in FB ash compared to GB ash, representing ∼60 % and ∼10-20 % of the zinc, respectively.