Phosphate Boosts Nonhydroxyl Radical Species Production upon Air Oxidation of Magnetite and Iron Sulfides at Neutral pH

Heterogeneous Fenton-like reactions using O₂ as the sole oxidant are gaining interest for designing organic contaminant degradation in soils and groundwaters since they may provide alternatives to current processes involving strong oxidants. Indeed, several Fe(II)-bearing oxide and sulfide mineral phases have been proven to generate reactive species upon air oxidation. However, the mechanisms of these reactions and the identity of the reactive species produced upon oxygenation may deserve further research. Here, we show using electron paramagnetic resonance (EPR) spectroscopy combined with the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap that air oxidation of pH 7 phosphate-buffered aqueous suspensions of magnetite (Fe₃O₄) or mackinawite (FeS) nanoparticles produces a reactive species that is distinct from the hydroxyl radical (^•OH) and abstracts a hydrogen atom from ethanol. This reactive species grows dramatically with phosphate concentration, and ultrafiltration reveals that it occurs as both aqueous and surface species. Based on these evidence and from extant reports, we hypothesize that the nonhydroxyl reactive species produced is Fe(IV), whose formation is enhanced in the presence of phosphate ligands. Fe₃O₄ magnetite (∼14 nm) generates about 30% more of this putative ferryl species than FeS mackinawite (∼9 nm), while FeS₂ (pyrite ∼12 nm + marcasite ∼4 nm) appears unreactive under these conditions.