Ultralow Dose Protoporphyrin IX-Mediated Ferrate(VI) System for Rapid Degradation of Phenolic Pollutants: Key Roles of Fe(V/IV/III) Complexes

Ferrate(VI) (Fe(VI)) is an ecofriendly oxidant for sustainable water remediation. Developing efficient activators that function at low dosages to enhance the oxidative performance is a key objective. This study first demonstrates that ultralow dosages (1 μM) of protoporphyrin IX (PPIX), at least an order of magnitude lower than conventional activators, markedly enhance Fe(VI)-mediated selective oxidation of phenolic pollutants. Mechanistic studies revealed that PPIX readily complexed with Fe(V/IV), enabling the formation of the Fe(V/IV)-PPIX complexes. These complexes served as the primary reactive species that significantly reduced the energy barrier for the oxidation of phenolic pollutants and accelerated electron transfer compared to unchelated Fe(V/IV). This electron-transfer efficiency is further modulated by the binding strength between phenolics and Fe(V/IV)-PPIX, with stronger binding facilitating faster electron transfer. Crucially, PPIX remained stable and was not consumed in the reaction. As a result, Fe(III)-PPIX could be efficiently reoxidized by Fe(VI/V/IV) to regenerate the reactive Fe(V/IV)-PPIX, establishing a sustainable catalytic cycle for pollutant degradation at ultralow PPIX dosages. The robustness of the redox cycling was supported by cyclic experiments, where the phenol removal efficiency remained stable after five consecutive runs. These findings provide new insights into developing low-dose catalysts to strengthen the Fe(VI) system for pollutant degradation.