『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
structure 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
dominantly hexagonal or pseudo-hexagonal layers symmetry. XRD
and TEM measurements suggest the oxides to be nanoparticulate
plates with average dimensions on the order of 11 nm thick × 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-Å 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 transformation 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
1.1. Background
1.1.1. Pinal Creek site description
1.1.2. Pinal Creek sediment mineralogy
2. Materials and methods
2.1. Sample collection and preparation
2.1.1. Preparation for bulk analysis
2.1.2. Preparation for microbeam analysis
2.1.3. Mn oxide coatings formed in-situ on stream-incubated
quartz (SIQ site)
2.2. Electron microscopy
2.3. X-ray imaging and structure techniques
2.3.1. Imaging
2.3.2. X-ray diffraction
2.3.3. XANES and EXAFS spectroscopy
3. Results
3.1. X-ray diffraction
3.1.1. Bulk XRD data
3.1.2. μ-XRD data
3.2. Electron microscopy and μ-SXRF measurements of selected
grains
3.3. XANES and EXAFS spectroscopy
3.3.1. XANES
3.3.2. EXAFS
3.3.2.1. Mn-O shell
3.3.2.2. Mn-Mn shell
3.3.2.3. Out-of-plane bending parameter
3.3.2.4. Fractional occupancy fOCC parameter
4. Discussion
4.1. Particle size
4.2. Structure of Pinal Creek biogenic Mn oxides
4.2.1. Bulk data
4.2.2. Micron-scale locations within individual coatings
4.3. Stability of Pinal Creek biogenic Mn oxides: constraints
on biological oxidation mechanisms
4.4. Evidence in support of biofilm formation environment for
Mn oxide coatings
4.5. Comparison to previous studies at Pinal Creek
4.6. Comparison to laboratory bacteriogenic Mn oxides
4.7. Implications for natural attenuation of contaminant metals
in the hyporheic zone at Pinal Creek
Acknowledgments
Appendix A. Supplementary data
References