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

Abstract

• Crystalline and amorphous Fe-oxide-clay composites were compared as Fenton catalysts. • Nano-sized amorphous iron-oxide-clay best-promoted phenanthrene oxidation. • A stable H 2 O 2 complex was formed with the amorphous iron-oxide-clay surface. • Single oxygen was primarily responsible for phenanthrene degradation. • Oxidation of phenanthrene was achieved with minimal H 2 O 2 waste. 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 H 2 O 2 (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 H 2 O 2 is complexed to the nano-sized iron oxide catalytic sites on the surface, forming a stable Fe-MMT-H 2 O 2 complex that is activated primarily when the pollutant is introduced. The reactive species, ∙OH and 1 O 2 , were detected upon the diffusion of PHE to the surface - leading to oxidation and mineralization with a minimal decomposition of H 2 O 2 . Quenching experiments further revealed that 1 O 2 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 H 2 O 2 , 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.