OH radical-initiated oxidation of organic compounds in atmospheric water phases: part 2. Reactions of peroxyl radicals with transition metals

Abstract

The aim of this series of two publications is to characterise radical reactions of organic molecules, occurring in atmospheric water phases. In this second study, the reactions occurring in a mixed peroxyl radical system generated from the OH radical attack on a model compound, 2-butoxyethanol, in oxygenated aqueous solution are investigated in the presence of O 2 −/HO 2 radicals, H 2O 2, and of copper (I and II)- and iron (II and III) species. A general mechanism of the reactions occurring in such systems is discussed and compared to the formation of organic hydroperoxides and hydrogen peroxide from this oxidation reactions. The main features of the mechanism are as follows: (i) production of peroxyl radicals following OH-attack on the model compound; (ii) cycling of the transition metals between the oxidation states given above, arising from a reduction reaction with superoxide, O 2 −, and oxidation reactions by organic peroxyl radicals, organic hydroperoxides, superoxide, and oxygen; (iii) reactions of organic peroxyl radicals, in contrast to superoxide radicals, occur only with the reduced forms of the transition metals and lead to the formation of organic hydroperoxides; (iv) the reaction of organic peroxyl radicals with Cu (I) is fast ( k>10 8 M −1 s −1); (v) the reaction of organic hydroperoxides with Cu (I) can be much faster (2 orders of magnitude) than the analogous reaction of hydrogen peroxide and leads to the production of oxyl radicals; and (vi) oxalate, a strong ligand for Cu (II), reduces the activity of copper, owing to its stabilisation of the copper (II) complex. The mechanism is discussed in relation to atmospheric conditions.