Remediation of organic pollutants by Fenton-like reactions with spinel catalysts: pathways, transformation products, and reactor perspectives

Spinel ferrite-catalyzed Fenton and Fenton-like advanced oxidation processes are emerging as a practical route for removing persistent organic pollutants from water, yet its selectivity, by-product formation, and scale-up behavior remain insufficiently resolved. This review consolidates recent advances in the activation of oxidants using hydrogen peroxide, peroxymonosulfate, and peroxydisulfate, and links operating conditions to the dominant oxidation pathways. It distinguishes regimes in which hydroxyl or sulfate radicals prevail from conditions that favor singlet oxygen generation or direct surface electron transfer, emphasizing the roles of pH, background anions, and matrix scavengers in controlling the kinetics and transformation profiles. Progress in catalyst development is centered on iron-rich spinels of the general composition MFe 2 O 4 , in which defect engineering, lattice symmetry modulation, and targeted doping tune Fe 2+ to Fe 3+ cycling and oxygen vacancy density, thereby regulating oxidant activation. Hybrid designs that couple spinels with carbon supports or semiconductor junctions enhance charge transport and enable visible-light-driven photo-Fenton operation. Durability is addressed through operando characterization of leaching, phase evolution, and surface restructuring, as well as magnetic recovery strategies that support reuse. Reactor perspectives highlight the shift from slurry testing to continuous configurations, including fixed beds, monoliths, and adsorption oxidation composites, supported by optimized oxidant dosing and hydrodynamics to mitigate mass-transfer limits and suppress halogenated by-products in chloride-rich waters. Key transformation patterns and ecotoxicity concerns are summarized, and priorities are proposed for standardized by-product reporting, durability metrics, and pilot-scale validation across variable influent chemistries.