Green synthesis of NaP1 zeolite from MSWI fly ash and silica-rich solid waste with superior Cu(II) removal efficiency

Microwave hydrothermal treatment provides a promising strategy for detoxifying and valorizing municipal solid waste solid waste incineration fly ash (MSWI-FA) into zeolite materials. However, the intrinsically low Si/Al ratio of MSWI-FA often results in reduced zeolite purity. To address this issue, an alkali fusion-assisted microwave hydrothermal method was developed to synthesize NaP1 zeolite directly from MSWI-FA, employing silica-rich solid waste as an eco-friendly alternative to conventional sodium silicate (Na₂SiO₃). The resultant MSWI-FA-based zeolite (IFA-NaP1) was comprehensively evaluated for its Cu(II) adsorption characteristics. Among the silica-rich solid waste investigated (i.e., silica fume, rice husk ash, and waste glass), silica fume-derived IFA-NaP1 exhibits the most favorable properties, achieving a cation exchange capacity of 2.46 meq∙g⁻¹, 11% higher than that of its Na₂SiO₃-based counterpart. Mechanistic analysis reveals distinctive silica source-dependent pathways: Na₂SiO₃ induces multinuclear competition and sequential transformations, whereas silica fume enables a direct sequential transformation. Leaching assessments of IFA-NaP1 confirm its effective immobilization of Pb, Zn, and Cd, with Cr and Cu levels remaining within regulatory thresholds. The silica fume-derived IFA-NaP1 synthesized at 140 °C exhibits superior Cu(II) removal, achieving a maximum adsorption capacity of 123 mg∙g⁻¹, 13% higher than Na₂SiO₃-derived material. Thermodynamic analysis shows Cu(II) adsorption is endothermic and entropy-driven, involving mechanisms such as electrostatic attraction, ion exchange, silyl radical reduction, surface complexation, and precipitation. This work highlights a sustainable strategy for converting MSWI-FA into efficient zeolite adsorbents, advancing hazardous waste detoxification and heavy metal remediation.