Ferryl Ion in the Photo-Fenton Process at Acidic pH: Occurrence, Fate, and Implications

Fenton processes produce reactive species that can oxidize organic compounds in natural and engineered systems. While it is well-documented that Fenton reactions produce hydroxyl radical (HO^•) under acidic conditions, we demonstrated the generation of ferryl ion (Fe^IVO²⁺) in the UV/Fe(III) and UV/Fe(III)/H₂O₂ systems at pH 2.8 using methyl phenyl sulfoxide (PMSO) as the probe compound. Moreover, we clarified that Fe^IVO²⁺ is parallelly formed via the oxidation of Fe(III) by HO^• and the O-O homolysis of [Fe^III-OOH]²⁺ in the photo-Fenton process. The rate constant for the reaction between HO^• and Fe³⁺ measured by laser flash photolysis was 4.41 × 10⁷ M^-1 s^-1. The rate constant and quantum yield for thermal and photo O-O homolysis of [Fe^III-OOH]²⁺ complex were 1.4 × 10^-2 s^-1 and 0.3, respectively, which were determined by fitting PMSO₂ formation. While Fe^IVO²⁺ forms predominantly through the reaction between HO^• and Fe³⁺ in the absence of H₂O₂, the relative contribution of [Fe^III-OOH]²⁺ O-O homolysis to Fe^IVO²⁺ formation highly depends on the molar ratio of [H₂O₂]₀/[Fe(III)]₀, the level of HO^• scavenging, and incident irradiance in the UV/Fe(III)/H₂O₂ system. Accordingly, an optimized kinetic model was developed by incorporating Fe^IVO²⁺-involved reactions into the conventional photo-Fenton model, which can accurately predict Fe(II) formation and contaminant decay in the UV/Fe(III) and UV/Fe(III)/H₂O₂ systems. Our study illuminated the underlying formation mechanism of reactive oxidative species in the photo-Fenton process and highlighted the role of Fe^IVO²⁺ evolution in modulating the iron cycle and pollutant abatement therein.