Oxidation Mechanism of Ce(III) to Ce(IV) by δ-MnO2: Insights from EXAFS Spectroscopy and Density Functional Theory

The oxidation of cerium (Ce) from the soluble trivalent state (Ce(III)) to the insoluble tetravalent state (Ce(IV)) on manganese (Mn) oxides critically influences its environmental fate and geochemical cycle, and is also of interest in water treatment. However, a comprehensive mechanistic understanding of how Ce is immobilized upon interaction with Mn oxides in soils and marine sediments is still lacking. The bonding structure of Ce on δ-MnO2, the most abundant Mn oxide, was investigated by X-ray absorption spectroscopy at environmentally relevant pH and Ce concentration, and the oxidation reaction was modeled by atomistic calculation. Ce(III) is adsorbed as a six-coordinate complex at particle edges and a nine-coordinate complex at Mn(IV) vacancy sites of the MnO2 phyllomanganate layer. Ce(III) oxidation is nonspontaneous and requires hydrolysis of the sorption complexes to proceed. Gibbs free energy calculations of possible oxidation pathways show that electron transfers from Ce(III) to Mn(IV) at edge sites, and from Ce(III) to interlayer Mn(III) at vacancy sites, are thermodynamically favorable. Thus, the redox reactivity of δ-MnO2 depends on its crystallographic structure and the Mn valence. Our findings show that Mn(IV) and Mn(III) are kinetically more effective oxidants of Ce(III) than dissolved oxygen, and therefore, that cerium can be immobilized by Mn oxides even under suboxic conditions. The new mechanistic insights from this study improve understanding of the oxidative uptake of Ce by Mn oxides and its relevance to natural and engineered systems.