Role of Quinone Intermediates as Electron Shuttles in Fenton and Photoassisted Fenton Oxidations of Aromatic Compounds

Fenton and related reactions are potentially useful oxidation processes for destroying toxic organic compounds in water. In these reactions, H₂O₂ is combined with Fe(II) or Fe(III) in the presence or absence of light to generate hydroxyl radicals (HO^•). A relatively neglected area of research is the influence of organic species on the reactivity of iron, and hence on the rate or course of the reaction. This study examined the oxidation of phenol (2 mM) by Fenton systems in the dark (Fe³⁺/H₂O₂, Fe²⁺/H₂O₂, and mixed Fe³⁺/Fe²⁺/H₂O₂ systems) and under UV/visible light (Fe³⁺/H₂O₂/hν). Reactions were conducted at an initial pH of 2.8, with H₂O₂ in excess and iron in catalytic concentrations. In all cases, the reactions display autocatalysis. In dark reactions, the lag phase decreases (a) with increasing total iron concentration, [Fe]_T; (b) as initial [Fe²⁺] increases when [Fe]_T is held constant; (c) in proportion to the amount of hydroquinone (1,4- or 1,2-) added; and (d) in proportion to the amount of quinones (1,4-benzoquinone or 5-hydroxy-1,4-naphthoquinone [juglone]) added. The hydroquinones reduce Fe³⁺ to Fe²⁺. An important result is the finding that quinones serve as electron-transfer catalysts between dihydroxycyclohexadienyl radicalthe HO^• adduct of phenoland Fe³⁺ by way of a semiquinone radical. This conclusion is supported by simulations with a kinetic model employing known and proposed steps. In irradiated solutions, the lag phase decreases with decreasing wavelength (480−300 nm; 10 nm band-pass). A cause of light initiation, especially above ∼410 nm where photolysis of Fe³⁺ and/or H₂O₂ is negligible, is direct photolysis of quinones to HO^• and semiquinone radicals, which subsequently reduce Fe³⁺ and re-form the quinone. Thus, quinones play an important catalytic role in Fenton oxidation of aromatic compounds.