Insights into phenanthrene attenuation by hydroxyl radicals from reduced iron-bearing mineral oxygenation

The oxygenation of Fe(II)-bearing minerals for hydroxyl radicals (HO^•) formation and contaminant attenuation receive increasing attention, while the mechanisms for specific Fe(II) species in manipulating HO^• formation and contaminant attenuation are unclear. Herein, a total of four Fe(III)-bearing minerals were applied in the reduction-oxygenation processes to produce HO^•. Results showed that the total HO^• generated from the Fe-(oxyhydr)oxides were significantly higher than those from the Fe-silicates, with the order of goethite and hematite (~1500 μmol kg^-1) > Fe-montmorillonite (~550 μmol kg^-1) > chlorite (~120 μmol kg^-1). The HO^• formation was largely hinged on the reactive Fe(II) species, i.e., the surface-adsorbed/low-crystalline Fe(II) in the Fe-bearing minerals. For the co-incubation of minerals and phenanthrene, the concentrations of phenanthrene decreased from the initial 3.0 mg L^-1 to 0.7 mg L^-1 and 1.9 mg L^-1 for Fe-montmorillonite and goethite, respectively, suggesting the HO^• mediated by the Fe-montmorillonite was more conducive for phenanthrene attenuation. The goethite tended to promote the formation of free HO^•, while the Fe-montmorillonite with interlayer structure can provide attachment sites for the surface-adsorbed/low-crystalline Fe(II), resulting in high potential for surface-bound HO^• formation and phenanthrene attenuation. This study highlights the importance of Fe-bearing minerals in manipulating HO^• formation, providing new insight into the removal of contaminants in ecosystems.