The Role of Oxides in Reduction Reactions at the Metal-Water Interface

The oxide layer that lies at the iron-water interface under environmental conditions can influence the reduction of solutes by acting as a passive film, semiconductor, or coordinating surface. As a passive film, oxides may inhibit reaction by providing a physical barrier between the underlying metal and dissolved oxidants. Sustained reduction of solutes requires localized defects in the passive film (e.g., pits), or some mechanism for transferring electrons through the oxide. In the semiconductor model, conduction band electrons from the oxide may contribute to solute reduction, but electron hopping (resonance tunneling) appears to be more important due to the high population of localized states in oxides formed under environmental conditions. Ultimately, electron transfer to the solute must occur via a precursor complex at the oxide-water interface. For dehalogenation of chlorinated aliphatic compounds on an iron oxide surface, a surface complexation model suggests that the outer-sphere precursor complex is weak and partially displaced by common environmental ligands.