Heterogeneous Fenton catalyst based on clay decorated with nano-sized amorphous iron oxides prevents oxidant scavenging through surface complexation

Heterogeneous Fenton catalysts based on iron-oxides are of great interest due to their low cost and high reactivity. Herein, the activity of nano-sized amorphous iron-oxide particles deposited on montmorillonite clay (MMT) was compared to crystalline hematite and magnetite-coated MMT. Hematite, magnetite, and their respective clay composites showed little catalytic activity and almost no phenanthrene (PHE) oxidation at circumneutral pH - in part, due to the decomposition of hydrogen peroxide. In contrast, the amorphous iron oxide (Fe)-MMT, showed very high catalytic activity (over 60% PHE degradation in 60 min) with only minimal consumption of H₂O₂ (3.3%). In-depth characterization of the Fe-MMT coupled with kinetic and mechanistic experiments was performed to understand the reaction pathway and mechanism. The results suggest that initially H₂O₂ is complexed to the nano-sized iron oxide catalytic sites on the surface, forming a stable Fe-MMT-H₂O₂ complex that is activated primarily when the pollutant is introduced. The reactive species, ∙OH and ¹O₂, were detected upon the diffusion of PHE to the surface - leading to oxidation and mineralization with a minimal decomposition of H₂O₂. Quenching experiments further revealed that ¹O₂ played an important role in the extensive mineralization of PHE. The amorphous Fe-MMT also exhibited high stability and performance in semi-continuous cycle batch experiments. Thus, this low-cost and simple Fe-MMT catalyst was found to be highly efficient towards the activation of H₂O₂, oxidizing PHE rapidly while reducing radical scavenging and unwanted side reactions. These findings, regarding the role of amorphous phases in the reactivity of iron-oxides, have the potential to improve the design and implementation of solid Fenton catalysts for pollutant remediation in engineered and natural systems.