Role of Fe(IV) and Fe(V) in micropollutant abatement by visible-light-activated ferrate(VI) via ligand-to-metal charge transfer

Although light-assisted ferrate (Fe(VI)) activation offers an activator-free strategy for micropollutant abatement, how visible light generates Fe(V)/Fe(IV) remains unclear at the molecular level. Here, five narrow-band LEDs (440–525 nm) were used to link Fe(VI) photochemistry with pollutant oxidation kinetics and iron-species contributions. At 440 nm, Vis-LED/Fe(VI) degraded 79.6–99.3% of six electron-rich micropollutants within 3 min, increasing rate constants by up to 4.3-fold relative to Fe(VI). Scavenging experiments and kinetic modeling identified Fe(V) and Fe(IV) as the dominant reactive species, together accounting for over 96% of diclofenac sodium (DCF) degradation. Time-dependent density functional theory calculations showed that visible excitation induced ligand-to-metal charge transfer (LMCT), which elongated a Fe-O bond from 1.654 to 2.025 Å and generated an Fe(V)-oxyl excited state that abstracted a hydrogen atom from water to give Fe(V). The apparent quantum yield of Fe(VI) decomposition decreased from 0.140 to 0.104 mol einstein −1 as wavelength increased from 440 to 525 nm, consistent with the charge-transfer component being photochemically productive. The system performance improved with light intensities and Fe(VI) concentrations, but declined under alkaline conditions and in humic-acid-rich water. These results establish visible-light activation of Fe(VI) via LMCT and identify Fe(V)/Fe(IV) as the oxidants responsible for micropollutant abatement.