Critical Role of Mineral Fe(IV) Formation in Low Hydroxyl Radical Yields during Fe(II)-Bearing Clay Mineral Oxygenation

In subsurface environments, Fe(II)-bearing clay minerals can serve as crucial electron sources for O₂ activation, leading to the sequential production of O₂^•-, H₂O₂, and ^•OH. However, the observed ^•OH yields are notably low, and the underlying mechanism remains unclear. In this study, we investigated the production of oxidants from oxygenation of reduced Fe-rich nontronite NAu-2 and Fe-poor montmorillonite SWy-3. Our results indicated that the ^•OH yields are dependent on mineral Fe(II) species, with edge-surface Fe(II) exhibiting significantly lower ^•OH yields compared to those of interior Fe(II). Evidence from in situ Raman and Mössbauer spectra and chemical probe experiments substantiated the formation of structural Fe(IV). Modeling results elucidate that the pathways of Fe(IV) and ^•OH formation respectively consume 85.9-97.0 and 14.1-3.0% of electrons for H₂O₂ decomposition during oxygenation, with the Fe(II)_edge/Fe(II)_total ratio varying from 10 to 90%. Consequently, these findings provide novel insights into the low ^•OH yields of different Fe(II)-bearing clay minerals. Since Fe(IV) can selectively degrade contaminants (e.g., phenol), the generation of mineral Fe(IV) and ^•OH should be taken into consideration carefully when assessing the natural attenuation of contaminants in redox-fluctuating environments.