Coupled effects of iron (hydr)oxides and clay minerals on the heterogeneous oxidation of aqueous Mn(II) and crystallization of manganese (hydr)oxides

The formation of nanominerals and mineral nanoparticles (NMMNs) has drawn broad attention due to their high reactivity and omnipresence in the environment. While the heterogeneous formation of NMMNs on surfaces of various minerals has been extensively studied, there is limited understanding of how mineral heteroaggregates influence this process. In this study, we investigated how heteroaggregates of iron (hydr)oxides and clay minerals affect the heterogeneous oxidation of aqueous Mn(II) and crystallization of manganese (hydr)oxides (MnOx). Our results revealed that iron (hydr)oxides (ferrihydrite) and clay minerals (kaolinite or montmorillonite) in heteroaggregates exerted coupled effects on these processes, dictating the distribution of Mn and the morphology of MnOx. Specifically, ferrihydrite catalyzed gradual oxidative removal of Mn(II) and triggered MnOx nucleation; in contrast, kaolinite/montmorillonite rapidly adsorbed Mn(II) but hardly catalyzed its oxidation. These reactions collectively resulted in fast adsorption and gradual oxidation of Mn(II) on the heteroaggregates. Further, MnOx nanoparticles formed on ferrihydrite surfaces migrated to kaolinite/montmorillonite surfaces, leading to interactions between MnOx and various component minerals within the heteroaggregates. This significantly altered the subsequent growth pathways and the eventual morphology of MnOx. Consequently, while MnOx nanoparticles in the ferrihydrite-only system aggregated freely and formed well-extended nanowires, those in the ferrihydrite-kaolinite system predominantly became short nanorods due to the immobilization by kaolinite surfaces; in the ferrihydrite-montmorillonite system, considerable MnOx nanoparticles attached to montmorillonite surfaces due to strong electrostatic attraction, and subsequently grew into blocky particles via particle attachment. These findings illustrate that surface reactivities of heteroaggregated ferrihydrite and kaolinite/montmorillonite are coupled when they interact with aqueous Mn(II) or MnOx. Our work exemplifies, for the first time, the cooperation between surfaces of various minerals during the heterogeneous formation of NMMNs. Findings from this study also enhance our understanding of MnOx formation on surfaces with diverse atomic structures, and contribute to the knowledge of Mn cycling in the environment.