Biogenic UO2 - Characterization and Surface Reactivity

Journal Article · Fri Feb 02 00:00:00 EST 2007 · AIP Conference Proceedings

DOI:https://doi.org/10.1063/1.2644500· OSTI ID:21054612

Singer, David M ^[1]; Farges, Francois ^[1]; Brown, Jr, Gordon E ^[1]

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1. Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California, 94305-2115 (United States)

Nano-scale biogenic UO2 is easier to oxidize and more reactive to aqueous metal ions than bulk UO2. In an attempt to understand these differences in properties, we have used a suite of bulk and surface characterization techniques to examine differences in the reactivity of biogenic UO2 versus bulk UO2 with respect to aqueous Zn(II). Precipitation of biogenic UO2 was mediated by Shewanella putrefaciens CN32, and the precipitates were washed using two protocols: (1) 5% NaOH, followed by 4 mM KHCO3/KCl (NA-wash; 'NAUO2', to remove surface organic matter), and (2) 4 mM KHCO3-KCl (BI-wash; 'BIUO2', to remove soluble uranyl species). BET surface areas of biogenic-UO2 prepared using the two protocols are 128.63 m2g-1 and 92.56 m2g-1, respectively; particle sizes range from 2-10 nm as determined by FEG-SEM. Surface composition was probed using XPS, which showed a strong carbon 1s signal for the BI-washed samples; surface uranium is > 90% U(IV) for both washing protocols. U LIII-edge XANES spectra also indicate that U(IV) is the dominant oxidation state in the biogenic UO2 samples. Fits of the EXAFS spectra of these samples yielded half the number of uranium second-shell neighbors relative to bulk UO2, and no detectable oxygen neighbors beyond the first shell. At pH 7, the sorption of Zn(II) onto both biogenic and bulk UO2 is independent of electrolyte concentration, suggesting that Zn(II) sorption complexes are dominantly inner-sphere. Fits of Zn K-edge EXAFS spectra for biogenic UO2 indicate that Zn(II) sorption is dependent on the washing protocol. Zn-U pair correlations are observed for the NA-washed samples, but not for the BI-washed ones, suggesting that Zn(II) sorbs directly to the UO2 surface in the first case, and possibly to organic matter in the latter. Further work is required to elucidate the binding mechanism of Zn(II) to bulk UO2.

• OSTI ID: 21054612
• Journal Information: AIP Conference Proceedings, Vol. 882, Issue 1; Conference: XAFS13: 13. international conference on X-ray absorption fine structure, Stanford, CA (United States), 9-14 Jul 2006; Other Information: DOI: 10.1063/1.2644500; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-243X
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ABSORPTION SPECTROSCOPY
BIOCHEMISTRY
CORRELATIONS
ELECTROLYTES
FINE STRUCTURE
ORGANIC MATTER
PARTICLE SIZE
PH VALUE
POTASSIUM CHLORIDES
PRECIPITATION
REACTIVITY
SCANNING ELECTRON MICROSCOPY
SODIUM HYDROXIDES
SORPTION
SURFACE AREA
URANIUM
URANIUM DIOXIDE
VALENCE
X-RAY PHOTOELECTRON SPECTROSCOPY
X-RAY SPECTROSCOPY