Photochemical Formation and Transformation of Birnessite: Effects of Cations on Micromorphology and Crystal Structure

As important components with excellent oxidation and adsorption activity in soils and sediments, manganese oxides affect the transportation and fate of nutrients and pollutants in natural environments. In this work, birnessite was formed by photocatalytic oxidation of Mn²⁺_aq in the presence of nitrate under solar irradiation. The effects of concentrations and species of interlayer cations (Na⁺, Mg²⁺, and K⁺) on birnessite crystal structure and micromorphology were investigated. The roles of adsorbed Mn²⁺ and pH in the transformation of the photosynthetic birnessite were further studied. The results indicated that Mn²⁺_aq was oxidized to birnessite by superoxide radicals (O₂^•–) generated from the photolysis of NO₃^– under UV irradiation. The particle size and thickness of birnessite decreased with increasing cation concentration. The birnessite showed a plate-like morphology in the presence of K⁺, while exhibited a rumpled sheet-like morphology when Na⁺ or Mg²⁺ was used. The different micromorphologies of birnessites could be ascribed to the position of cations in the interlayer. The adsorbed Mn²⁺ and high pH facilitated the reduction of birnessite to low-valence manganese oxides including hausmannite, feitknechtite, and manganite. This study suggests that interlayer cations and Mn²⁺ play essential roles in the photochemical formation and transformation of birnessite in aqueous environments