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 IV O 2+ ) in the UV/Fe(III) and UV/Fe(III)/H 2 O 2 systems at pH 2.8 using methyl phenyl sulfoxide (PMSO) as the probe compound. Moreover, we clarified that Fe IV O 2+ 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 Fe 3+ measured by laser flash photolysis was 4.41 × 10 7 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 PMSO 2 formation. While Fe IV O 2+ forms predominantly through the reaction between HO • and Fe 3+ in the absence of H 2 O 2 , the relative contribution of [Fe III -OOH] 2+ O-O homolysis to Fe IV O 2+ formation highly depends on the molar ratio of [H 2 O 2 ] 0 /[Fe(III)] 0 , the level of HO • scavenging, and incident irradiance in the UV/Fe(III)/H 2 O 2 system. Accordingly, an optimized kinetic model was developed by incorporating Fe IV O 2+ -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)/H 2 O 2 systems. Our study illuminated the underlying formation mechanism of reactive oxidative species in the photo-Fenton process and highlighted the role of Fe IV O 2+ evolution in modulating the iron cycle and pollutant abatement therein.