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 (FeO) in the UV/Fe(III) and UV/Fe(III)/HO systems at pH 2.8 using methyl phenyl sulfoxide (PMSO) as the probe compound. Moreover, we clarified that FeO is parallelly formed via the oxidation of Fe(III) by HO and the O-O homolysis of [Fe-OOH] in the photo-Fenton process. The rate constant for the reaction between HO and Fe measured by laser flash photolysis was 4.41 × 10 M s. The rate constant and quantum yield for thermal and photo O-O homolysis of [Fe-OOH] complex were 1.4 × 10 s and 0.3, respectively, which were determined by fitting PMSO formation. While FeO forms predominantly through the reaction between HO and Fe in the absence of HO, the relative contribution of [Fe-OOH] O-O homolysis to FeO formation highly depends on the molar ratio of [HO]/[Fe(III)], the level of HO scavenging, and incident irradiance in the UV/Fe(III)/HO system. Accordingly, an optimized kinetic model was developed by incorporating FeO-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)/HO systems. Our study illuminated the underlying formation mechanism of reactive oxidative species in the photo-Fenton process and highlighted the role of FeO evolution in modulating the iron cycle and pollutant abatement therein.