The structure of the manganese oxide on the sheath of the bacterium Leptothrix discophora : An XAFS study

In natural waters, manganese oxides (MnOx) are important in mediating the bioavailability of trace metals such as Ni, Cu, Zn, Cd, and Pb, as these metals readily adsorb to the MnO x surface. Manganese from a variety of anthropogenic sources usually enters the aquatic environment in dissolved form as Mn 2+ . 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 states of manganese: Mn 2+ , Mn 3+ , and Mn 4+ . Fitting the experimental XAFS data identifies the bacterial MnO x as being composed of single-layer microcrystals with layers similar to those occurring in Na-birnessite. Some MnO 6 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.