Mn oxides are known to oxidize Cr(III) to the highly toxic Cr(VI), but the mechanism or even the form of manganese involved is not understood.  Here we present periodic density-functional theory calculations of Cr(III) adsorption on the hydrated �-MnO2(110) (pyrolusite) and ?-MnOOH(010) (manganite) surfaces and identify the most important binding sites and determine the relative importance of Mn(IV) and Mn(III) centers in the oxidation reaction.  On the pyrolusite surface, chromium adsorption was found to highly favorable, especially when accompanied by a transfer of two electrons into the slab.  Two types of pyrolusite adsorption sites were found to promote oxidation: one involving singly coordinated oxygen atoms, and the other doubly coordinated bridging oxygen atoms.  Oxidation by Mn(III) on manganite was found to be less favorable.  In the absence of oxygen, disproprtionation of the Mn(III) centers preceded Cr(III) oxidation on manganite, indicating that Mn(IV) plays a primary role in the oxidation process.

Cr(III) adsorption and oxidation on hydrated  �-MnO<sub>2</sub>(110) and ?-MnOOH(010) surfaces

247th National Meeting (2014)

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Cr(III) adsorption and oxidation on hydrated �-MnO2(110) and ?-MnOOH(010) surfaces

Cr(III) adsorption and oxidation on hydrated �-MnO₂(110) and ?-MnOOH(010) surfaces