Manganese Oxides:Parallels between Abiotic and Biotic Structures

摘要

A large number of microorganisms are responsible for the oxidation of Mn_(aq)~(2+)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 Mn~(4+)O_6 octahedra that form layers.The detailed analysis of the EXAFS spectra indicate that 12 +-4% of the Mn~(4+)layer cation sites in SP6-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 M_y~(n+)Mn_(0.12)~(3+)[square_(0.12)Mn_(0.88)~(4+)]O_2 centre dot zH_2O,where M_y~(n+)represents hydrated interlayer cations,square_(0.12)represents Mn~(4+)cation vacancies within the layer,and Mn_(0.12)~(3+)represents hydrated cations that occupy sites above/below these cation vacancies.