ADSORPTION OF METAL CATIONS AND OXYANIONS 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 metal cations and oxyanions on the transformation by incubating Mn(II)-bearing δ-MnO₂ at pH 8 under anoxic conditions for 25 d. In the Li⁺, Na⁺, and K⁺ chloride solutions, the Mn(II)-bearing δ-MnO₂ first transforms to Mn(III)-rich δ-MnO₂ or triclinic birnessite (T-bir) due to the Mn(II)-Mn(IV) comproportionation, most of which eventually transform to a 4 × 4 TMO. In contrast, Mn(III)-rich δ-MnO₂ and T-bir form and persist in the Mg²⁺ and Ca²⁺ chloride solutions. However, the presence of surface-adsorbed Cu(II) and phosphate strongly inhibit or slow down the transformation of Mn(II)-bearing δ-MnO₂ to T-bir and TMOs. The stabilizing power of cations and oxyanions on the δ-MnO₂ structure positively correlates with their binding strength to δ-MnO₂ (Li⁺, Na⁺, and K⁺ < Mg²⁺ and Ca²⁺ < Cu(II) and phosphate). Since the adsorption of metals and oxyanions decrease the surface energy of minerals, our finding suggests that the surface energy largely controls the thermodynamic stability of LMOs. Our study indicates that the adsorption of divalent metal cations, particularly transition metals, and strongly adsorptive oxyanions can be an important cause of the high abundance of LMOs, rather than the more stable TMO phases, in the environment.