Surface-induced oxidation of Mn(II) and crystallization of manganese (hydr)oxides on clay minerals

The crystallization induced by foreign surfaces is crucial for various geological and geochemical processes on Earth's surface, but the underlying mechanisms remain poorly understood. In this study, we investigated the oxidation of Mn(II) and crystallization of manganese (hydr)oxides (MnOx) on kaolinite (Kln) and montmorillonite (Mnt), two prevalent clay minerals in the nature with different structures and properties. Our results reveal that both Kln and Mnt surfaces can catalyze the heterogeneous oxidation of Mn(II) and crystallization of MnOx, but the crystallization pathways, as well as the morphologies and phases of the products, are distinct. The Mn(II) adsorbed by Kln's outer surfaces were oxidized to Mn(III) and crystallized into MnOx, which were identified as hausmannite and manganite nanoaggregates. In contrast, the adsorption of Mn(II) and its oxidation to Mn(III) occurred both on the outer surfaces and interlayer spaces of Mnt, while the crystallization of MnOx only occurred on the outer surfaces, indicating that the confined volume of Mnt's interlayer spaces can inhibit the crystallization of Mn(II/III) cations. Hausmannite emerged as the first MnOx product on Mnt, followed by groutite and manganite, all of which were well-dispersed fine nanoparticles. Additionally, the Si(IV) ions dissolved from Mnt participated in the formation of rhodonite with Mn(II), but this mineral phase was absent on Kln. Notably, MnOx nanoaggregates on Kln were assembled by a group of MnOx nanoparticles with slight misorientations among them, and the misorientations of these nanoparticles can gradually recover over time, indicating the contribution of crystallization by particle attachment (CPA). On the other hand, the strong electrostatic interactions between MnOx nanoparticles and Mnt surfaces reduced the mobility of nanoparticles, inhibiting their growth via CPA. Above results demonstrate the significant influence of the confinement effect and surface interactions on the surface-induced crystallization process of MnOx. These findings can also advance our knowledge about the role of clay minerals in Mn(II) oxidation and MnOx formation, and offer a possible explanation for the presence of various MnOx with diverse structures in the nature.