EXAFS, XANES and in-situ SR-XRD characterization of biogenic manganese oxides produced in sea water

Abstract

Natural manganese oxide nanoparticles and grain coatings are ubiquitous in the environment and profoundly impact the quality of sediments via their ability to degrade and sequester contaminants. These oxides are believed to form dominantly via oxidation of Mn(II) by marine and freshwater bacteria and have extremely high sorptive capacities for heavy metals. We have used XANES, EXAFS, and synchrotron (SR)-XRD techniques to study biogenic manganese oxides produced by spores of the marine Bacillus sp., strain SG-1 in sea water as a function of reaction time under fully in-situ conditions. The primary biogenic product is a nanocrystalline solid with an oxidation state and layered phyllomanganate local structure similar to that in δ-MnO2. XRD data show the biooxides to have a phyllomanganate 10 Å basal plane spacing, suggesting the interlayer is hydrated and contains calcium. Fits to EXAFS spectra suggest the octahedral layers of the biooxides to be relatively flat (out-of-plane bend <10°) and to have relatively low octahedral layer Mn site vacancies (12 to 14%). These results suggest that aqueous Ca2+ is inserted into the biogenic oxide structure after completion of the enzymatic oxidation process. The biooxides observed in this study may be the most abundant manganese oxide phase suspended in the oxic and sub-oxic zones of the oceanic water column.