Phosphate ligand-mediated production of reactive oxygen species during oxygenation of Fe(II)-phosphate complexes

Reactive oxygen species (ROS) production upon the oxygenation of reduced iron minerals is of critical importance to redox cycles of Fe and the fate of refractory organic contaminants. The environmental impact factors during this process, however, have been underappreciated. In this study, prominently enhanced production of hydroxyl radicals (•OH) was observed by oxygenation of Fe(II) with 5-50 mM phosphate. The results of spin trap electron spin resonance (ESR) experiment showed that Fe(II)-phosphate complexes facilitated the generation of •OH. The degradation experiment of p-nitrophenol (PNP) confirmed that •OH formation was dominated by a consecutive one-electron O₂ reduction (90.2-96.9 %), and the quantification of PNP degradation products revealed that Fe(II)/phosphate molar ratios regulated the O₂ activation pathways for O₂^•- or •OH production. The further experimental and theoretical investigation demonstrated that the coordination of phosphate with Fe(II) plays a dual role in ROS generation that facilitated O₂^•- formation by lowering the energy barrier for Fe(II) oxidation and altered the reaction pathway of •OH formation due to its occupation of sites for electron transfer. The present work highlights an important role of natural oxyanions in O₂ activation by Fe(II) and raises the possibility of in situ degradation of contaminants in subsurface environment.