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

Heterogeneous Fenton-like reactions using O-2 as thesole oxidant are gaining interest for designing organic contaminantdegradation in soils and groundwaters since they may provide alternativesto current processes involving strong oxidants. Indeed, several Fe-(II)-bearingoxide and sulfide mineral phases have been proven to generate reactivespecies upon air oxidation. However, the mechanisms of these reactionsand the identity of the reactive species produced upon oxygenationmay deserve further research. Here, we show using electron paramagneticresonance (EPR) spectroscopy combined with the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap that air oxidation of pH 7 phosphate-bufferedaqueous suspensions of magnetite (Fe3O4) ormackinawite (FeS) nanoparticles produces a reactive species that isdistinct from the hydroxyl radical ((OH)-O-center dot) and abstractsa hydrogen atom from ethanol. This reactive species grows dramaticallywith phosphate concentration, and ultrafiltration reveals that itoccurs as both aqueous and surface species. Based on these evidenceand from extant reports, we hypothesize that the nonhydroxyl reactivespecies produced is Fe-(IV), whose formation is enhanced in the presenceof phosphate ligands. Fe3O4 magnetite (similar to 14nm) generates about 30% more of this putative ferryl species thanFeS mackinawite (similar to 9 nm), while FeS2 (pyrite similar to 12nm + marcasite similar to 4 nm) appears unreactive under these conditions.