Structural characterization of terrestrial microbial Mn oxides from Pinal Creek, AZ Page: 1 of 22
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Structural characterization of terrestrial microbial Mn oxides
from Pinal Creek, AZ
John R. Bargar a,*, Christopher C. Fuller b, Matthew A. Marcus C Adrian J. Brearley d,
M. Perez De la Rosa a, Samuel M. Webb a, Wendel A. Caldwell
a Stanford Synchrotron Radiation Lightsource, 2575 Sand Hill Road, Building 137, MS 69, Menlo Park, CA 94025, USA
b US Geological Survey, Water Resources Division, 345 Middlefield Road, MS 465, Menlo Park, CA 94025, USA
cAdvanced Light Source, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720, USA
d Department of Earth and Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, NM 87131, USA
Received 19 March 2008; accepted in revised form 15 October 2008; available online 25 November 2008
Abstract
The microbial catalysis of Mn(II) oxidation is believed to be a dominant source of abundant sorption- and redox-active
Mn oxides in marine, freshwater, and subsurface aquatic environments. In spite of their importance, environmental oxides
of known biogenic origin have generally not been characterized in detail from a structural perspective. Hyporheic zone
Mn oxide grain coatings at Pinal Creek, Arizona, a metals-contaminated stream, have been identified as being dominantly
microbial in origin and are well studied from bulk chemistry and contaminant hydrology perspectives. This site thus presents
an excellent opportunity to study the structures of terrestrial microbial Mn oxides in detail. XRD and EXAFS measurements
performed in this study indicate that the hydrated Pinal Creek Mn oxide grain coatings are layer-type Mn oxides with dom-
inantly hexagonal or pseudo-hexagonal layer symmetry. XRD and TEM measurements suggest the oxides to be nanopartic-
ulate plates with average dimensions on the order of 11 nm thick x 35 nm diameter, but with individual particles exhibiting
thickness as small as a single layer and sheets as wide as 500 nm. The hydrated oxides exhibit a 10-A basal-plane spacing and
turbostratic disorder. EXAFS analyses suggest the oxides contain layer Mn(IV) site vacancy defects, and layer Mn(III) is
inferred to be present, as deduced from Jahn-Teller distortion of the local structure. The physical geometry and structural
details of the coatings suggest formation within microbial biofilms. The biogenic Mn oxides are stable with respect to trans-
formation into thermodynamically more stable phases over a time scale of at least 5 months. The nanoparticulate layered
structural motif, also observed in pure culture laboratory studies, appears to be characteristic of biogenic Mn oxides and
may explain the common occurrence of this mineral habit in soils and sediments.1. INTRODUCTION
Manganese oxides, reactive high surface area particles,
play innumerable roles in the transformation and cycling
of major and trace elements in freshwater and marine envi-
ronments (Murray, 1975; O'Reilly and Hochella, 2003;
Tebo et al., 2004; Villalobos et al., 2005b; Webb et al.,
2005b,c). Mn oxides are among the strongest of naturally
Corresponding author. Fax: +1 650 926 4100.
E-mail address: bargar@slac.stanford.edu (JR. Bargar).occurring metal sorbents and oxidatively degrade or trans-
form numerous organic and inorganic compounds, includ-
ing humic and fulvic acids, aromatic hydrocarbons, Cr(III),
Co(II), and hydrogen sulfide (Jenne, 1967; Huang, 1991).
Oxidation of Mn(II) in surface and near-surface environ-
ments is believed to be dominantly mediated by microbes
(Tebo et al., 2004). This belief is based primarily upon the
ubiquity of Mn(II)-oxidizing microorganisms in the envi-
ronment (Tebo et al., 2004) and upon the much faster rates
of Mn(II) oxidation at circum-neutral pH when catalyzed
by microorganisms as compared to rates of homogeneous
or surface-mediated abiotic mechanisms (Morgan, 2005).
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Bargar, John; Fuller, Christopher; Marcus, Matthew A.; Brearley, Adrian J.; Perez De la Rosa, M.; Webb, Samuel M. et al. Structural characterization of terrestrial microbial Mn oxides from Pinal Creek, AZ, article, March 19, 2008; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc927715/m1/1/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.