Metal Adsorption Controls Stability of Layered Manganese Oxides.

Hexagonal birnessite, a typical layered Mn oxide (LMO), can adsorb and oxidize Mn(II) and thereby transform to Mn(III)-rich hexagonal birnessite, triclinic birnessite, or tunneled Mn oxides (TMOs), remarkably changing the environmental behavior of Mn oxides. We have determined the effects of coexisting cations on the transformation by incubating Mn(II)-bearing δ-MnO at pH 8 under anoxic conditions for 25 d (dissolved Mn < 11 μM). In the Li, Na, and K chloride solutions, the Mn(II)-bearing δ-MnO first transforms to Mn(III)-rich δ-MnO or triclinic birnessite (T-bir) due to the Mn(II)-Mn(IV) comproportionation, most of which eventually transform to a 4 × 4 TMO.