Upscaling MSWI fly ash into multifunctional foamed geopolymers: synergistic water filtration and intrinsic heavy metal immobilization via a dual-stabilization mechanism

Hazardous municipal solid waste incineration (MSWI) fly ash poses a global environmental challenge, with millions of tons annually ending up in landfills and risking heavy metal contamination of stormwater. Herein, for the first time in unwashed, high‑chlorine MSWI fly ash, we demonstrate a metal‑selective dual stabilization mechanism: lead (Pb) is chemically immobilized via covalent Pb–O–Si/Al bonding (XPS shift: − 0.6 eV), while cadmium and zinc are physically encapsulated. This mechanism enables a foamed geopolymer filter with four synergistic functions: (i) high Pb 2 ⁺ adsorption capacity (200.6 mg·g⁻ 1 at pH 6.0), (ii) steady water flux (0.082 mL·cm⁻ 2 ·s⁻ 1 ) under continuous flow, (iii) low thermal conductivity (0.092 W·m⁻ 1 ·K⁻ 1 ), and (iv) intrinsic immobilization of the fly ash’s own heavy metals. The filter maintained effluent Pb concentrations as low as 0.003 mg·L⁻ 1 over five filtration cycles, well below national drinking water thresholds. This waste‑to‑value strategy provides a robust framework for upcycling hazardous byproducts into high‑performance environmental materials, advancing circular economy and sustainable water management.