Valorization of municipal solid waste incineration fly ash into hierarchical ZIF-8@MCM-41 adsorbents for removal of methylene blue

The removal of methylene blue (MB) from wastewater poses a significant environmental challenge due to its toxicity and persistence. Hierarchical ZIF-8@MCM-41 composites were developed as efficient adsorbents for MB removal. Increasing MCM-41 content enhanced mesoporosity, resulting in surface areas of 643.91–704.50 m²/g and pore volumes of 0.48–0.53 cm³/g. This compositional tuning influenced adsorption behavior, shifting from the Freundlich isotherm characteristic of ZIF-8 toward Langmuir-dominated behavior typical of MCM-41, while the composites showed intermediate behavior described by the Temkin and Redlich–Peterson isotherm models. The incorporation of MCM-41 into ZIF-8 significantly improved adsorption performance, increasing the equilibrium adsorption capacity (qe) from ∼28 mg/g for pristine ZIF-8 to 67.78 mg/g for the M20Z composite at 40 ppm. Although pure MCM-41 exhibited the highest adsorption capacity at elevated concentrations (up to 396.88 mg/g at 200 ppm), the composites demonstrate a synergistic effect by enhancing mesoporosity and improving accessibility to active sites. Adsorption occurred via multilayer interactions on heterogeneous micro- and mesoporous surfaces, driven by electrostatic attraction and π–π interactions. Kinetic analysis indicated that the pseudo‑second‑order (PSO) model provided the best fit for all adsorbents, with R² values ranging from ≈ 0.982 to 1.000 and experimental qe values increasing with initial MB concentration, suggesting a chemisorption‑dominant mechanism. These ZIF-8@MCM-41 composites derived from municipal solid waste incineration fly ash (MSWI FA) represent promising, sustainable, and cost-effective adsorbents contributing to waste valorization and circular economy strategies.