Manganese oxides: Parallels between abiotic and biotic structures

A large number of microorganisms are responsible for the oxidation of Mn²⁺_(aq) to insoluble Mn^3+/4+ oxides (MnO _x) in natural aquatic systems. This paper reports the structure of the biogenic MnO_x, including a quantitative analysis of cation vacancies, formed by the freshwater bacterium Leptothrix discophora SP6 (SP6-MnO_x). The structure and the morphology of SP6-MnO_x were characterized by transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), including full multiple-scattering analysis, and powder X-ray diffraction (XRD). The biogenic precipitate consists of nanoparticles that are approximately 10 nm by 100 nm in dimension with a fibrillar morphology that resembles twisted sheets. The results demonstrate that this biogenic MnO _x is composed of sheets of edge-sharing of Mn⁴⁺O ₆ octahedra that form layers. The detailed analysis of the EXAFS spectra indicate that 12 ± 4% of the Mn⁴⁺ layer cation sites in SPG-MnO_x are vacant, whereas the analysis of the XANES suggests that the average oxidation state of Mn is 3.8 ± 0.3. Therefore, the average chemical formula of SP6-MnO_x is Mⁿ⁺ _yMn³⁺_0.12[□_0.12Mn ⁴⁺_0.88]O₂· zH₂O, where M ⁿ⁺_y represents hydrated interlayer cations, □_0.12 represents Mn⁴⁺ cation vacancies within the layer, and Mn³⁺_0.12 represents hydrated cations that occupy sites above/below these cation vacancies.