Impact of varying the core–shell structural sequence on the efficiency of cascade reagent-free Fenton-like oxidation: the case of magnetically recycling resorcinol–formaldehyde resin/magnetite composite microspheres†

Iron-based inorganic–organic hybrid Fenton catalysts, which recently emerged, are recognized as among the novel materials for the deep mineralization of inert pollutants for environmental remediation in photo-assisted Fenton-like reactions, which are one of the typical advanced oxidation processes (AOPs). In this work, magnetically recycling catalysts of resorcinol–formaldehyde resin/magnetite (RF/Fe₃O₄) core–shell microspheres were rationally designed by tuning the core–shell sequence for visible-light-driven reagent-free Fenton-like oxidation of organic dyes. It is noted that the impact of the core–shell sequence on the nano-structure and reactivity of such spherical catalysts is rarely reported. Here, although the apparent degradation efficiencies of the organic dye methylene blue (MeB) by these two core–shell catalysts were similar (97.4% by RF@Fe₃O₄ and 98.9% by Fe₃O₄@RF within 20 min), the intrinsic activity of Fe₃O₄@RF was revealed to be superior to the RF@Fe₃O₄ catalyst, including a better total content of organic carbon (TOC) removal rate (56% vs. 42.6%), a much larger normalized reaction rate constant k (0.46 vs. 0.27 min⁻¹), an improved degradation rate on anti-interference capacity against foreign ions (Cl⁻, 98.3% vs. 80%) and its enhanced stability under acidic reaction conditions. We confirmed that the core–shell sequence imposed a significant impact on regulating the surface properties and active sites of the composite catalysts. The degradation of organic dyes followed a cascade Fenton-like process. Besides, the participation of Fe₃O₄ endowed the catalysts with a profitable magnetic recovery property. This work sheds light on the rational construction of organic–inorganic hybrid catalysts with magnetic recycling features for potential large-scale application in photo-involved AOPs.