Recyclable fly ash-derived zeolite without acid pretreatment as a green material for combined adsorption and catalytic treatment of dye-contaminated water

Fly ash (FA), a byproduct of coal combustion, poses major environmental disposal challenges. This study aimed to develop a green, one-step hydrothermal strategy to valorize Indonesian FA into zeolite without toxic acid pretreatment, and to evaluate its performance for methylene blue (MB) removal from aqueous solution. The obtained fly ash-derived zeolite (FADZ) was characterized using FTIR, XRD, and SEM, and exhibited a high surface area (226.25 m2/g). The adsorption of MB followed pseudo-second-order kinetics with a maximum capacity of 92.44 mg/g (Langmuir model). By utilizing the natural iron oxides in FA as catalytic sites, the material enabled a synergistic adsorption–Fenton-like degradation process, enhancing MB removal efficiency from 34.28% (adsorption only) to 98.11%. Thermodynamic analysis confirmed that the adsorption was endothermic and spontaneous. Reusability tests demonstrated stable performance with > 95% removal efficiency over four cycles. Overall, the study demonstrates that FADZ is a stable, dual-functional, and cost-effective material for sustainable dye-contaminated wastewater treatment.