Ferrate meets carbon nanotubes: Unraveling the catalytic pathways for enhanced oxidation efficiency of polycyclic aromatic hydrocarbons

Carbon nanotubes (CNT) have been extensively explored for oxidant activation. However, the intrinsic mechanism of CNT-driven ferrate (Fe(VI)) activation remains unclear. This study investigates the catalytic activation of Fe(VI) by CNT and Fe₂O₃-loaded CNT (FeCNT) using phenanthrene (PHE) as a model to elucidate general oxidation mechanism for organic contaminants. Compared to Fe(VI) alone, Fe(VI)/CNT and Fe(VI)/FeCNT systems significantly increased the degradation rate of PHE by factors of 4.64 and 5.99, respectively. Mechanistic investigations revealed that CNT enhanced the oxidizing ability of high-valent iron and facilitated electron transfer from PHE to high-valent iron, while FeCNT further promoted oxidation through ¹O₂ generation via Fe₂O₃-mediated H₂O₂ decomposition. Kinetic modeling quantified the contributions of different reactive species: in the Fe(VI)/CNT system, high-valent iron accounted for 64.05 % of PHE degradation, while CNT catalysis contributed 35.95 %. In the Fe(VI)/FeCNT system, high-valent iron contributed 61.91 %, FeCNT catalysis 31.28 %, and ¹O₂ 6.82 % as extra. Both Fe(VI)/CNT and Fe(VI)/FeCNT systems significantly reduced the toxicity of PHE-contaminated water. Additionally, Fe(VI)/FeCNT retained 93.45 % PHE removal efficiency after five cycles, demonstrating superior reusability and greater resistance. These findings provide new insights into Fe(VI)-based oxidation catalysis and highlight FeCNT as an efficient and durable catalyst for pollutant degradation in complex water matrices.