Abstract:
In natural waters, manganese oxides (MnOx) are important in mediating the bioavailability of tracemetals such as Ni, Cu, Zn, Cd, and Pb, as these metals readily adsorb to the MnOx surface. Manganese from a variety of anthropogenic sources usually enters the aquatic environment in dissolved form as Mn2+. It is subsequently oxidized under oxic and neutral (pH = 6-7) conditions. Often this oxidation is catalyzed by bacteria, such as Leptothrix discophora, as part of their natural metabolic process. Mn K-edge X-ray Absorption Fine Structure Spectroscopy (XAFS) was used to investigate the local structure of manganese oxide on the sheath produced by the bacterium Leptothrix discophora SP-6. The features observed in the near edge region of the Mn K-edge spectrum indicate the presence of three oxidation state of manganese: Mn2+, Mn3+, and Mn4+. Fitting the experimental XAFS data identifies the bacterial MnOx, as being composed of single-layer microcrystals with layers similar to those occurring in Na-birnessite. Some MnO6 octahedra might lie outside the layer plane, sharing corners with those in the layer plane. X-ray diffraction results for the same samples are consistent with the single-layer structure.
