STRUCTURAL TRANSFORMATIONS OF BIRNESSITE (δ-MnO2) DURING BIOLOGICAL AND ABIOLOGICAL REDUCTION

Open Access
Author:
Fischer, Timothy B
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 13, 2010
Committee Members:
  • Peter J Heaney, Dissertation Advisor
  • Peter J Heaney, Committee Chair
  • Maryann Victoria Bruns, Committee Chair
  • James David Kubicki, Committee Member
  • Christopher Howard House, Committee Member
  • William D Burgos, Committee Member
Keywords:
  • birnessite
  • bioreduction
  • manganese oxides
  • chromium
  • shewanella
  • time-resolved XRD
  • siderophores
Abstract:
Time-resolved structural analyses of synthetic birnessite (δ-MnO2) during reaction with both biological and abiological reactants confirmed that the evolution of the mineral’s crystal structure during reduction and dissolution depended upon the nature of the reactant and the fate of the reduced Mn. The first-ever real-time X-ray diffraction (XRD) analysis of a biological-mineral reaction demonstrated that the reductive dissolution of birnessite during direct electron transfer by total-membrane fractions of the facultative anaerobe Shewanella oneidensis was characterized by a collapse of the birnessite structure due to a decrease in the c unit-cell parameter. The observed structural collapse was verified by analysis of the reaction products of batch reactions between whole-cell cultures of S. oneidensis and birnessite. A combined XRD and X-ray absorption spectroscopy (XAS) examination indicated that the unit-cell collapse was caused by reduction of structural Mn4+ to Mn3+, which increased the net negative charge on birnessite’s octahedral sheets, followed by an inferred increase in the interlayer Na:H2O ratio. The reduced Mn2+ precipitated as rhodochrosite (MnCO3). The reduction, chelation, and removal of Mn from the birnessite crystal structure by bacterial siderophores did not result in a structural collapse of the mineral, despite the large amount of Mn removed from the MnO6 octahedral sheets (up to 20 mol%). Rather, the unit-cell parameters remained constant throughout the complete dissolution of birnessite. A third structural pathway was revealed during the reduction of birnessite in the presence of dissolved transition metals. When aqueous Cr3+ was oxidized to Cr6+ by reduction of octahedral Mn, the birnessite crystal structure experienced a phase transformation from triclinic to hexagonal. During this abiological reduction, the reduced Mn remained part of the hexagonal crystal structure, occupying positions above or below octahedral vacancies. The specialized nature of birnessite’s transformations in response to alternative reduction/dissolution mechanisms presents the possibility that crystallographic analysis of birnessite may serve as a useful biomarker in studies of environments where it is desirable to know whether life forms participated in mineral redox processes.