Hematite-mediated Mn(II) abiotic oxidation under oxic conditions: pH effect and mineralization

Interactions between manganese (Mn) and iron (Fe) are widespread processes in soils and sediments, however, the abiotic transformation mechanisms are not fully understood. Herein, Mn(II) oxidation on hematite were investigated at various pH under oxic condition. Mn(II) oxidation rates increased from 3 × 10^-4 to 8 × 10^-2 h^-1 as pH increased from 7.0 to 9.0, whereas hematite enhanced Mn(II) oxidation rates to 1 h^-1. During oxidation process, high pH could promote the oxidation of Mn(II) into Mn minerals, resulting in the rapid consumption of the newly-formed H⁺, and high pH facilitated Mn(II) adsorption and oxidation by altering Mn(II) reactivity and speciation. Only granule-like hausmannite was found on the hematite surface at pH 7.0, whereas hausmannite particles and feitknechtite and manganite nanowires were formed at pH from 7.5 to 9.0. Moreover, a co-shell structured nanowire composed of manganite and feitknechtite was observed owing to autocatalytic reactions. Specifically, electron transfers between Mn(II) and O₂ occurred on the surface or through bulk phase of hematite, and direct electron transfers in the O₂-Mn(II) complex and indirect electron transfers in the O₂-Fe(II/III)-Mn(II) complex may both have contribution to the overall reactions. The findings provide a comprehensive interpretation of Fe-Mn interaction and have implications for the formation of soil Fe-Mn oxyhydroxides with unique properties in controlling element cycling.