Asynchronous and synchronous adsorption oxidation on α-Fe₂O₃/ZrO₂ derived from MIL-53(Fe)/mixed-linker UiO-66 synthesized by an ultrasonication method generating degradation of methylene blue

Industrial discharge of synthetic dyes, particularly methylene blue (MB), continues to pose a major environmental challenge due to their persistence, toxicity, and resistance to natural degradation. This study reports the development of a MOF-derived α-Fe₂O₃/ZrO₂ (F/Zr-M-S2/U(AA–NH₂)) heterostructured composite synthesized through an ultrasonication-assisted route using MIL-53(Fe)/mixed-linker UiO-66 precursors, followed by calcination at 600 °C. The composite was evaluated in both asynchronous and synchronous adsorption–oxidation systems to elucidate the interplay between surface adsorption and H₂O₂ activation. The asynchronous mode exhibited superior performance, achieving 99.23 % MB removal at neutral pH, governed by first order kinetics with oxidation as the dominant removal pathway. Comprehensive characterization (XRD, FTIR, SEM–TEM, BET, TGA, XPS) confirmed the formation of a stable F/Zr-M-S2/U(AA–NH₂) with well-dispersed redox-active sites and suitable surface and textural properties that support efficient H₂O₂ activation. Radical scavenging tests showed that hydroxyl radicals (·OH) play the primary role in MB degradation, followed by superoxide species (O₂·⁻). LC–MS analysis validated a multistep degradation pathway involving N-demethylation, sulfur oxidation, aromatic ring cleavage, and subsequent mineralization. Phytotoxicity tests revealed significantly reduced toxicity of the degradation products compared to the parent dye. The catalyst maintained over 85 % efficiency after four reuse cycles with minimal iron leaching. These findings demonstrate that the ultrasonically synthesized (F/Zr-M-S2/U(AA–NH₂) composite offers a robust, efficient, and environmentally safe platform for heterogeneous Fenton-based treatment of dye-contaminated wastewater.