Atomic Insights into the Heterogeneous Crystallization of Manganese (Oxyhydr)oxides on Typical Iron (Oxyhydr)oxides: from Adsorption to Oxidation to Crystallization

Heterogeneous crystallization of manganese (oxyhydr)oxides (MnO_x) on iron (oxyhydr)oxides (FeO_x) is crucial for the biogeochemical cycling of Mn, yet atomic-level insights into this process are important but relatively limited. Herein, we revealed the distinct adsorption, oxidation, and crystallization mechanisms of Mn on hematite (Hem), ferrihydrite (Fhy), and goethite (Gth). Gth exhibited highest ability in Mn(II) removal and oxidation, followed by Hem and Fhy. Manganite and hausmannite were the main MnO_x products with distinct proportions, and morphologies cross the systems. MnO_x growth mechanisms involve surface-induced nucleation, crystallization by particle attachment (CPA), and self-catalyzed growth. On Fhy, self-catalyzed growth was dominant; for Gth, surface-induced nucleation was prevalent, supplemented by CPA; and Hem combined all three mechanisms. These distinct mechanisms led to nanoparticles primarily of hausmannite on Gth and nanowires of manganite and hausmannite on Hem and Fhy, with those on Hem displaying lower aspect ratios. Differences in MnO_x structure and morphology were attributed to Mn(II)-FeO_x complexation, FeO_x electronic band structure, and crystal structure mismatch between MnO_x and FeO_x, which respectively influenced the direct and indirect electron transfer and heterogeneous nucleation efficiency. This work advances our understanding of MnO_x crystallization on FeO_x at the nanoscale, explaining the diverse morphology and structure of MnO_x in different environments.