Diffusion- and pH-Dependent Reactivity of Layer-Type MnO₂: Reactions at Particle Edges versus Vacancy Sites

Layer-type manganese oxides are among the strongest solid-phase oxidants in surface environments and readily oxidize a range of chemical species. However, knowledge of the role played by different surface sites in contaminant oxidation is scarce. In this study, we investigate the reactivity of particle edges versus vacancy sites in δ-MnO₂ by combining Co sorption kinetic experiments with quick X-ray absorption spectroscopy. During the fast kinetic phase (t < 10 min), Co sorption and oxidation occurred dominantly at edge sites at pH 8; at pH 6 and pH 4, reactions also occurred at vacancy sites but were limited in extent. At longer reaction times (t > 10 min), continuous accumulation of Co at vacancy sites was observed, while the amount of Co at particle edges decreased or remained constant depending on the absence or presence of aqueous Co(II), respectively. These data are consistent with the diffusion-limited transport of metal cations to vacancy sites. In addition, at pH 8, the kinetics and extent of reaction at particle edges are greater than at pH 4-6. These results suggest that, although particle edges will be the first to react, layer vacancies will serve as the long-term sorption and oxidation sites for contaminant metals in MnO₂-rich systems.