Manganese Oxide Surface Chemistry and Adsorption of Cr(III)

            Manganese oxides are prevalent in geological settings and play a significant role in the geochemical cycling of heavy metals due to their high adsorption capacity.  Of particular interest is their ability to oxidize Cr(III) to toxic Cr(IV).  While much work has focused on the mechanism of oxidation, several questions remain about the specific manganese-chromium interactions prior to and during oxidation.  An understanding of the structure, chemical properties, and redox reactivity of manganese oxides surfaces is an essential foundation for determining possible Cr(III) oxidation mechanisms.

            Periodic density functional theory calculations have been combined with ab initio thermodynamics to identify stable beta‑MnO₂ (110) and gamma‑MnOOH (010) surface terminations and to probe their redox behavior in response to varying oxygen and water chemical potentials.  Under ambient conditions, oxidation of the beta‑MnO₂ (110) surface is not likely, while oxidation of the gamma‑MnOOH (010) surface is predicted to be favorable.  Reduction of the clean surfaces leads to significant surface reconstructions that result from the competition between optimal d‑orbital occupation and manganese coordination geometry and lattice constraints of the bulk.  Jahn‑Teller effects allow multiple oxidation states to exist at the surface.  While these surfaces are not predicted to be stable at ambient conditions, they may be important in the oxidation mechanism.  Hydration of the clean beta‑MnO₂ (110) and gamma‑MnOOH (010) surfaces generally lowers the surface free energies significantly with hydrogen bonding networks formed at monolayer coverage.  Adsorption of Cr(III) has been investigated on the hydrated surfaces.  The effect of manganese oxidation state on adsorption geometries and electron transfer will be discussed.