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Title: Effects of calcium and phosphate on uranium(IV) oxidation: Comparison between nanoparticulate uraninite and amorphous UIV–phosphate

Abstract

The mobility of uranium in subsurface environments depends strongly on its redox state, with UIV phases being significantly less soluble than UVI minerals. This study compares the oxidation kinetics and mechanisms of two potential products of UVI reduction in natural systems, a nanoparticulate UO2 phase and an amorphous UIV–Ca–PO4 analog to ningyoite (CaUIV(PO4)2·1–2H2O). The valence of U was tracked by X-ray absorption near-edge spectroscopy (XANES), showing similar oxidation rate constants for UIVO2 and UIV–phosphate in solutions equilibrated with atmospheric O2 and CO2 at pH 7.0 (kobs,UO2 = 0.17 ± 0.075 h-1 vs. kobs,UIVPO4 = 0.30 ± 0.25 h-1). Addition of up to 400 μM Ca and PO4 decreased the oxidation rate constant by an order of magnitude for both UO2 and UIV–phosphate. The intermediates and products of oxidation were tracked by electron microscopy, powder X-ray diffraction (pXRD), and extended X-ray absorption fine-structure spectroscopy (EXAFS). In the absence of Ca or PO4, the product of UO2 oxidation is Na–uranyl oxyhydroxide (under environmentally relevant concentrations of sodium, 15 mM NaClO4 and low carbonate concentration), resulting in low concentrations of dissolved UVI (<2.5 × 10-7 M). Oxidation of UIV–phosphate produced a Na-autunite phase (Na2(UO2)PO4·xH2O), resulting in similarly low dissolved U concentrations (<7.3 ×more » 10-8 M). When Ca and PO4 are present in the solution, the EXAFS data and the solubility of the UVI phase resulting from oxidation of UO2 and UIV–phosphate are consistent with the precipitation of Na-autunite. Bicarbonate extractions and Ca K-edge X-ray absorption spectroscopy of oxidized solids indicate the formation of a Ca–UVI–PO4 layer on the UO2 surface and suggest a passivation layer mechanism for the decreased rate of UO2 oxidation in the presence of Ca and PO4. Interestingly, the extractions were unable to remove all of the oxidized U from partially oxidized UO2 solids, suggesting that oxidized U is distributed between the interior of the UO2 nanoparticles and the labile surface layer. Accounting for the entire pool of oxidized U by XANES is the likely reason for the higher UO2 oxidation rate constants determined here relative to prior studies. In conclusion, our results suggest that the natural presence or addition of Ca and PO4 in groundwater could slow the rates of UIV oxidation, but that the rates are still fast enough to cause complete oxidation of UIV within days under fully oxygenated conditions.« less

Authors:
ORCiD logo [1];  [1];  [2];  [3]
  1. Argonne National Lab. (ANL), Argonne, IL (United States)
  2. Argonne National Lab. (ANL), Argonne, IL (United States); Illinois Inst. of Technology, Chicago, IL (United States)
  3. Argonne National Lab. (ANL), Argonne, IL (United States); Bulgarian Academy of Sciences, Sofia (Bulgaria)
Publication Date:
Research Org.:
Argonne National Laboratory (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Biological and Environmental Research (BER); USDOE Office of Science (SC), Basic Energy Sciences (BES)
OSTI Identifier:
1248730
Alternate Identifier(s):
OSTI ID: 1397565
Grant/Contract Number:  
AC02-06CH11357
Resource Type:
Accepted Manuscript
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 174; Journal Issue: C; Journal ID: ISSN 0016-7037
Publisher:
The Geochemical Society; The Meteoritical Society
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; EXAFS; redox; uranium

Citation Formats

Latta, Drew E., Kemner, Kenneth M., Mishra, Bhoopesh, and Boyanov, Maxim I. Effects of calcium and phosphate on uranium(IV) oxidation: Comparison between nanoparticulate uraninite and amorphous UIV–phosphate. United States: N. p., 2015. Web. doi:10.1016/j.gca.2015.11.010.
Latta, Drew E., Kemner, Kenneth M., Mishra, Bhoopesh, & Boyanov, Maxim I. Effects of calcium and phosphate on uranium(IV) oxidation: Comparison between nanoparticulate uraninite and amorphous UIV–phosphate. United States. https://doi.org/10.1016/j.gca.2015.11.010
Latta, Drew E., Kemner, Kenneth M., Mishra, Bhoopesh, and Boyanov, Maxim I. Tue . "Effects of calcium and phosphate on uranium(IV) oxidation: Comparison between nanoparticulate uraninite and amorphous UIV–phosphate". United States. https://doi.org/10.1016/j.gca.2015.11.010. https://www.osti.gov/servlets/purl/1248730.
@article{osti_1248730,
title = {Effects of calcium and phosphate on uranium(IV) oxidation: Comparison between nanoparticulate uraninite and amorphous UIV–phosphate},
author = {Latta, Drew E. and Kemner, Kenneth M. and Mishra, Bhoopesh and Boyanov, Maxim I.},
abstractNote = {The mobility of uranium in subsurface environments depends strongly on its redox state, with UIV phases being significantly less soluble than UVI minerals. This study compares the oxidation kinetics and mechanisms of two potential products of UVI reduction in natural systems, a nanoparticulate UO2 phase and an amorphous UIV–Ca–PO4 analog to ningyoite (CaUIV(PO4)2·1–2H2O). The valence of U was tracked by X-ray absorption near-edge spectroscopy (XANES), showing similar oxidation rate constants for UIVO2 and UIV–phosphate in solutions equilibrated with atmospheric O2 and CO2 at pH 7.0 (kobs,UO2 = 0.17 ± 0.075 h-1 vs. kobs,UIVPO4 = 0.30 ± 0.25 h-1). Addition of up to 400 μM Ca and PO4 decreased the oxidation rate constant by an order of magnitude for both UO2 and UIV–phosphate. The intermediates and products of oxidation were tracked by electron microscopy, powder X-ray diffraction (pXRD), and extended X-ray absorption fine-structure spectroscopy (EXAFS). In the absence of Ca or PO4, the product of UO2 oxidation is Na–uranyl oxyhydroxide (under environmentally relevant concentrations of sodium, 15 mM NaClO4 and low carbonate concentration), resulting in low concentrations of dissolved UVI (<2.5 × 10-7 M). Oxidation of UIV–phosphate produced a Na-autunite phase (Na2(UO2)PO4·xH2O), resulting in similarly low dissolved U concentrations (<7.3 × 10-8 M). When Ca and PO4 are present in the solution, the EXAFS data and the solubility of the UVI phase resulting from oxidation of UO2 and UIV–phosphate are consistent with the precipitation of Na-autunite. Bicarbonate extractions and Ca K-edge X-ray absorption spectroscopy of oxidized solids indicate the formation of a Ca–UVI–PO4 layer on the UO2 surface and suggest a passivation layer mechanism for the decreased rate of UO2 oxidation in the presence of Ca and PO4. Interestingly, the extractions were unable to remove all of the oxidized U from partially oxidized UO2 solids, suggesting that oxidized U is distributed between the interior of the UO2 nanoparticles and the labile surface layer. Accounting for the entire pool of oxidized U by XANES is the likely reason for the higher UO2 oxidation rate constants determined here relative to prior studies. In conclusion, our results suggest that the natural presence or addition of Ca and PO4 in groundwater could slow the rates of UIV oxidation, but that the rates are still fast enough to cause complete oxidation of UIV within days under fully oxygenated conditions.},
doi = {10.1016/j.gca.2015.11.010},
journal = {Geochimica et Cosmochimica Acta},
number = C,
volume = 174,
place = {United States},
year = {Tue Nov 17 00:00:00 EST 2015},
month = {Tue Nov 17 00:00:00 EST 2015}
}

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Works referencing / citing this record:

On the determination of transportation, range and distribution characteristics of Uranium-238, Thorium-232 and Potassium-40: a critical review
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