Speciation of Zn in WtE fly ash residues after wet treatment

Waste-to-energy fly ash can be repurposed if treated to limit the leaching of potentially toxic elements such as Zn. Ashes with lime additives can be stabilised via gypsum formation, while those without additives are better suited for acid-leaching treatment (and material recovery). In this study, ashes from grate-fired boilers - three without lime addition and one ash with lime addition - were treated by acid-leaching and gypsum stabilisation, respectively. By combining X-ray absorption near-edge structure (XANES), surface-sensitive X-ray photoelectron spectroscopy (XPS), and sequential leaching of untreated and treated ash samples, we aimed to improve understanding of Zn chemical forms and leaching risks from the treated ashes. In untreated ashes, Zn was enriched at particle surfaces and was most commonly found in the form of easily soluble K_xZn_yCl_z. Acid-leaching decreased the Zn content, specifically the K_xZn_yCl_z form, resulting in a higher proportion of spinels and silicates. Correspondingly, the concentration of Ca, Fe, S, and Si increased at the particle surfaces. In gypsum-stabilised ash, the Zn content remained similar to that in untreated ash, but the K_xZn_yCl_z was transformed into other Zn compounds. Both treatment methods resulted in more stable Zn compounds in the residues than in the untreated ashes. This work supports the transition towards safe and efficient use of fly ash residues by understanding how treatment affects Zn speciation, leachability and surface enrichment.