REDOX-DRIVEN CRYSTALLIZATION OF MANGANESE OXIDES

Tetravalent manganese (Mn) oxides are ubiquitous and redox-active minerals in soils and other geological environments. Freshly precipitated Mn(IV) oxides in the environment are often poorly crystalline layered structures which, however, can transform to well-crystallized layered structures or tunneled Mn(IV) oxides. It had long been believed that the transformation was a slow process at room temperature. However, recent studies showed that the structural crystallization and transformation can be rapid, driven by redox reactions between Mn(II) and Mn(IV) of layered Mn(IV) oxides. Since Mn(II) and Mn(IV) oxides often coexist during Mn redox cycling, such redox-driven transformation by Mn(II) could be significant for crystallization and transformation of Mn(IV) oxides in the environment. In this presentation, I will give an overview of such reactions, and discuss how they are affected by solution chemistry, including pH, Mn(II) to Mn(IV) ratios, and the type of co-existing metal cations and oxyanions.