Comparative assessment of natural and synthetic zeolites for heavy metal removal from real steam-washing MSWI fly ash wastewaters

This study comparatively evaluates the performance of natural and synthetic zeolites for the removal of dissolved heavy metals from real steam-washing (SW) wastewaters generated during Municipal Solid Waste Incineration (MSWI) fly ash treatment. Steam washing represents an effective pre-treatment strategy for fly ash dechlorination and stabilisation; however, it generates secondary effluents characterized by high ionic strength and strong ion competition, that challenge conventional wastewater treatment approaches. Batch adsorption experiments were performed using four zeolites with different framework topologies, Si/Al ratios, pore architectures, and exchangeable cations: synthetic LTA-type zeolite 5 A, FAU-type zeolites 13× and Y720, and a natural CHA-type chabazite-rich zeolite. The experiments were conducted using real SW wastewaters collected after different steam-washing exposure times and containing environmentally relevant metals such as Pb, Zn, Cd, Cu, Mn, Fe, Tl, and Sb over several orders of magnitude in concentration. Among the investigated materials, FAU-type zeolite 13× exhibited the highest overall operational uptake performance, particularly for Pb, Zn, Tl, and Cu, while zeolite 5 A also demonstrated high removal efficiencies for Zn, Pb, Cd and Cu. Zeolite Y720 showed distinct uptake behavior for selected metals, including Mn and Fe, whereas natural chabazite effective Pb and Zn removal but lower overall performance under strong multi-ion competition. Structural and thermal analyses, including XRD/Rietveld refinement and TG-DTA, confirmed the preservation of zeolite crystallinity after treatment and provided evidence of significant zeolite–wastewater interactions without detectable formation of secondary crystalline phases. This work provides a comparative screening of zeolite behavior under realistic, highly competitive environmental matrices. The results highlight the importance of matching zeolite framework topology and cation-exchange environment in determining operational metal uptake from complex SW effluents and support the future development of integrated treatment strategies for hazardous MSWI-derived wastewaters.