Ferryl Ion in the Photo-Fenton Process at Acidic pH: Occurrence, Fate, and Implications
Guowei Deng, Zhen Wang, Jinxing Ma, Jin Jiang, Di He, Xianhui Li, Aleksandra Szczuka, Zhong Zhang
Abstract
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.