Density Functional Theory Modeling of the Oxidation Mechanism of Co(II) by Birnessite

Abstract

Phyllomanganates of the birnessite family are the most abundant manganese oxides on Earth and the strongest inorganic oxidants in the environment. Birnessite controls the oxidative scavenging of cobalt in soils, lake and marine sediments, and ferromanganese crusts and nodules, leading to enrichments of the order of one billion times the concentration in solution. However, a detailed mechanistic understanding of the enrichment processes is lacking. Here, we perform density functional theory (DFT) calculations to explore the mechanisms of Co(II) to Co(III) oxidation on the layer edge and surface of birnessite nanoparticles. We show that Co(II) sorption on a layer edge is an unlikely oxidation pathway. In contrast, Co(II) sorbed on a Mn(IV) vacancy site exposed on the layer surface as an octahedral triple-corner sharing (TCS) complex enters the vacancy where it is oxidized to Co(III) by a layer Mn(IV) cation, which is reduced to Mn(III). The stepwise reaction proceeds as follows. The octahedral TCS complex is transformed to a smaller tetrahedral TCS complex, allowing Co(II) to cross the surface oxygen layer and to fill the empty octahedral Mn(IV) site. When in the octahedral vacancy, Co(II) is converted from the high-spin (t2g 5 eg 2 ) to the low-spin (t2g 6 eg 1 ) state and the Co(II) octahedron becomes strongly distorted by the Jahn-Teller effect. Afterward, the electron exchange reaction between Mn(IV) (t2g 3 eg 0 ) and Co(II) (t2g 6 eg 1 ) takes place, resulting in the formation of a regular lowspin Co(III) (t2g 6 eg 0 ) octahedron and a Jahn-Teller distorted high-spin Mn(III) (t2g 3 eg 1 ) octahedron.These findings refine previously proposed mechanisms of Co(II) oxidation by birnessite and fill gaps in our understanding of global Co sequestration in natural systems.

Oxidation and Nanoparticle Formation during Ce(III) Sorption onto Minerals

“…The oxidation of various elements by sorption on birnessite was shown earlier for Co­(II), − I 2 and I – , , Cr­(III), As­(III), and TcO 2 . Actually, birnessite demonstrates the highest oxidative ability compared with other common minerals, including manganese-containing minerals .…”

Density Functional Theory Modeling of the Oxidation Mechanism of Tl(I) by Birnessite

“…Spin-unrestricted DFT calculations were performed on cluster models with ORCA 5.0.3 and a computational methodology similar to that used previously to study the complexation energetics of Co­(II)/Co­(III), Ni­(II), Cu­(II), Zn­(II), and Pb­(II) on birnessite. , In ref , we investigated the effect of the size of the birnessite model on DFT results. Using protons to yield (nearly) charge-neutral, symmetric systems, we found that a cluster model with 18 Mn­(IV) octahedra provides robust results.…”

“…The accuracy of prediction of Δ G obtained with the afore-described computational methodology has been benchmarked previously , with the first hydrolysis constants of the hexahydrated Mn 2+ , Co 2+ , Ni 2+ , and Zn 2+ ions . Δ G values for these ions are calculated reliably within chemical accuracy (1–2 kcal/mol).…”

“…The second hypothesis is supported experimentally by the observation of Wick et al that the oxidative uptake of Tl­(I) on nanoparticulate birnessite (δ-MnO 2 ) is constrained by the availability of unblocked vacant sites. Tl­(I) does not appear to be able to displace interlayer Mn­(III) complexed on the layer vacancies of birnessite, in contrast to Co­(II). − A likely reason is the weak acidity of Tl­(I) with a hydrolysis constant of p K a = 11.7 . This, in turn, helps to explain the high stability of Tl­(I) in surficial environments and its co-occurrence with Tl­(III) on birnessite, despite the high oxidative properties of Mn­(III) and Mn­(IV).…”

“…Here, we use density functional theory (DFT) to gain atomic-level insight into the molecular mechanism of the Tl­(I) → Tl­(III) oxidation on the vacant layer sites and edge sites of birnessite. The computational approach follows the modeling scheme used previously for the Co­(II) → Co­(III) oxidation . The thermodynamic feasibility of a number of reaction pathways is evaluated from the calculation of the Gibbs free energies (Δ G ) of complexation and oxidation of Tl­(I) on vacancy-containing and vacancy-free MnO 2 nanolayers.…”