Mechanism of Fenton Oxidation of Levoglucosan in Water

Abstract

Levoglucosan (Levo) is a major saccharide formed by the combustion of cellulosic materials. Levo was once considered an inert tracer of biomass-burning aerosols; however, recent studies have indicated that Levo in atmospheric condensed phases does indeed react with atmospheric reactants. Here, we report the results of a time-resolved mass spectrometric study of the oxidation of Levo in aqueous solutions with ferrous ion (Fe2+)/hydrogen peroxide (H2O2) (i.e., Fenton's reagent). The major products of the Fenton oxidation of Levo were oxygen-atom-incorporated species (Levo+nO, n = 1-3). Experiments using Levo-d7 (all C-H bonds replaced by C-D) and D2O or H218O as the solvent revealed that OH predominantly (∼85% of all C-H bonds) abstracts H atoms attached to the carbon atoms possessing a hydroxyl moiety (-OH), which is followed by the formation of a carbonyl moiety (-C═O). Subsequent hydration of these products results in the formation of geminal diols (detected as Levo+1O species). Our results also suggest the formation of α-hydroxy-hydroperoxides (detected as Levo+2O species) that exist in equilibrium, with the compounds possessing a -C═O moiety and with H2O2. H-abstractions from -O-H were found to be a minor reaction pathway (≤5% of all H-abstractions). The present proposed oxidation mechanisms improve our understanding of how the chemical components of atmospheric condensed phases change by metal-catalyzed aging processes without sunlight.