Structural Chemistry of Co(II) Sequestration in Biogenic Mn Oxides

S. M. Webb,1 B. M. Tebo,2 J. R. Bargar1

2Scripps Institute of Oceanography, Marine Biology Research Division

Natural Mn oxide colloids and grain coatings are ubiquitous in the environment and profoundly impact the ground water quality via their ability to degrade and sequester contaminants. These oxides are dominantly generated via bacterial oxidation of Mn(II) and have extremely high sorptive capacities for heavy metals. In addition, Mn oxides readily oxidize a variety of recalcitrant organic and inorganic compounds. The mechanis(s) and products of bacterial Mn(II) oxidation are therefore believed to have deep and wide-ranging roles in the environmental cycling of trace metals, organics, and carbon.

We have utilized XAS and synchrotron-based in-situ transmission XRD to probe the structures of fully hydrated biogenic Mn oxides produced by spores of the marine bacterium, Bacillus sp., strain SG-1. Biooxides were produced in the absence and presence of Co(II), which has a high affinity for Mn oxides and can oxidize to Mn(III). The primary biogenic reaction product is a poorly crystalline solid similar to d-MnO2 in which Mn has an oxidation state close to 4+. The structure of the oxide is analogous to acid birnessite, but with fewer layer vacancies and greater local order around Mn(IV). Co(II) is oxidized to Co(III) and incorporated at Mn(IV) sites in the octahedral layer. The presence of Co(III) alters the stability and structure of the host Mn oxides. This observation, combined with the very high sorptive capacity of the Mn oxides, imply that bacteria can exert tremendous influence over Co cycling in impacted waters via Mn oxide biomineralization.

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