Non-Hydroxyl Radical Species Production during Dark Air Oxidation of Alluvial Soils

Abstract

Natural environments subjected to hydrologically driven redox fluctuations are regarded as propitious to contaminant degradation since they favor the cyclic oxygenation of Fe(II) minerals, which produces oxygen reactive species (ROS), such as the hydroxyl radical, OH^•. However, the identity of these reactive species may vary as a function of physicochemical conditions and remains a matter of research. Here, using spin-trapping electron paramagnetic resonance (EPR) with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap, we show that a non-hydroxyl reactive species is produced in significant amounts upon air-oxidation of alluvial soil suspensions (Seine River Basin, France). Indeed, among the DMPO–OH^•, DMPO–CO₂^•–, and DMPO–alkyl^• adducts observed, the latter dramatically increases with the addition of ethanol or, to a lesser extent, tert-butanol, especially with phosphate buffer. This result reveals a dominant non-hydroxyl species, which we interpret as Fe(IV) since it is known to oxidize alcohols to alkyl^• radicals and is favored by phosphate ligands. With phosphate buffer and ethanol, the DMPO–alkyl^• production correlates with the initial reduced-state iron pool in the samples, as determined using Fe K-edge X-ray absorption spectroscopy (XAS). Fe(II) phyllosilicates, Fe(0) in one soil core and, to a lesser extent, vivianite, are found to be the most significantly oxidized iron phases upon soil oxygenation, and pyrite appears less reactive. Hence, we show that a significant reactive species, differing from OH^•, forms upon oxygenation of soil Fe(II) minerals, especially in the presence of soil-sourced phosphate. Our results may therefore call to further directly identify this putative Fe(IV) species and to investigate its ability to degrade organic contaminants in natural environments.