Nanometer Scale Morphology of Bacteriogenic Mn Oxides

Manganese oxides, which are ubiquitous in surface waters, soils, and sediments, are believed to result from the bacterially catalyzed oxidation of Mn(II) to Mn(IV). Bacteriogenic Mn oxides exhibit high surface areas and degrade or oxidize a variety of organic and inorganic compounds. Via these reactions, Mn oxides influence the biogeochemical cycling of micronutrients and help to control the composition of natural waters. Recent EXAFS (extended X-ray absorption fine structure) and WAXS (wide angle X-ray scattering) investigations of bacteriogenic Mn oxides indirectly suggest that particles are as small as 10 or 20 nm , . However, no detailed systematic quantitative measurements of bacteriogenic Mn oxide particle sizes and morphology have been reported. We have characterized manganese oxides produced by the marine sporeforming bacterium Bacillus sp., strain SG-1 using SAXS (small angle X-ray scattering) and TEM (transmission electron microscopy). TEM measurements indicate that the particles are layered structures having basal-plane widths of several hundred nm. WAXS measurements indicate that particles are extremely thin, ~1nm thick, which corresponds to a single Mn oxide layer. The particles appear to have particularly high aspect ratios, with the majority of all structural MnO6 octahedral units exposed to solution. Furthermore, it follows that basal plane sites are the overwhelmingly dominant surface binding sites. Particles with such small dimensions often have properties that are intermediate between those of molecular clusters and bulk materials. Therefore, the reactivity of natural manganese oxides is expected to differ substantially from that of synthetic crystalline analogs.