Effect of in situ generated iron oxyhydroxide coatings on FeS oxygenation and resultant hydroxyl radical production for contaminant degradation

Interaction of mackinawite (FeS) with O2 has attracted much attention recently for contaminant transformation due to reactive oxidant production. However, the effect of in situ generated Fe (oxyhydr)oxide coatings on FeS reactivity and the resultant reactive oxidant production remains poorly understood. To clarify this effect, we specially prepared partially oxidized FeS with in situ generated Fe (oxyhydr)oxide coatings, and then stored it in the anoxic environment to explore the interactions between FeS and Fe (oxyhydr)oxide coatings, and characterized the variation of morphology and Fe(II) species by STEM, XRD, Mössbauer spectroscopy and XPS. We found that the reactivity of FeS upon oxygenation and the resultant [rad]OH production for phenol oxidation were prominently accelerated after anoxic settling. XRD patterns exhibited the formation of more crystalline Fe (oxyhydr)oxide coatings, Mössbauer spectra identified the generation of a new Fe(II) species, and XPS results revealed the increase in Fe(II) fraction on the surface during anoxic settling. These variations were most probably attributed to the electron transfer between FeS core and Fe (oxyhydr)oxide coatings driven by the redox potential gradient. Our findings deepened the mechanistic understanding of FeS reactivity and the resultant oxidizing impact on contaminant transformation under O2-perturbed conditions. © 2020 Elsevier B.V.