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

The mobility of uranium in subsurface environments depends strongly on its redox state, with U IV phases being significantly less soluble than U VI minerals. This study compares the oxidation kinetics and mechanisms of two potential products of U VI reduction in natural systems, a nanoparticulate UO 2 phase and an amorphous U IV–Ca–PO 4 analog to ningyoite (CaU IV(PO 4) 2·1–2H 2O). The valence of U was tracked by X-ray absorption near-edge spectroscopy (XANES), showing similar oxidation rate constants for U IVO 2 and U IV–phosphate in solutions equilibrated with atmospheric O 2 and CO 2 at pH 7.0 (k obs,UO2 = 0.17 ± 0.075 h -1 vs. k obs,U IV PO4 = 0.30 ± 0.25 h -1). Addition of up to 400 μM Ca and PO 4 decreased the oxidation rate constant by an order of magnitude for both UO 2 and U IV–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 PO 4, the product of UO 2 oxidation is Na–uranyl oxyhydroxide (under environmentally relevant concentrations of sodium, 15 mM NaClO 4 and low carbonate concentration), resultingmore » in low concentrations of dissolved U VI (<2.5 × 10 -7 M). Oxidation of U IV–phosphate produced a Na-autunite phase (Na 2(UO 2)PO 4·xH 2O), resulting in similarly low dissolved U concentrations (<7.3 × 10 -8 M). When Ca and PO 4 are present in the solution, the EXAFS data and the solubility of the UVI phase resulting from oxidation of UO 2 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–PO 4 layer on the UO 2 surface and suggest a passivation layer mechanism for the decreased rate of UO 2 oxidation in the presence of Ca and PO 4. Interestingly, the extractions were unable to remove all of the oxidized U from partially oxidized UO 2 solids, suggesting that oxidized U is distributed between the interior of the UO 2 nanoparticles and the labile surface layer. Accounting for the entire pool of oxidized U by XANES is the likely reason for the higher UO 2 oxidation rate constants determined here relative to prior studies. In conclusion, our results suggest that the natural presence or addition of Ca and PO 4 in groundwater could slow the rates of U IV oxidation, but that the rates are still fast enough to cause complete oxidation of U IV 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:
OSTI Identifier:
1248730
Grant/Contract Number:
AC02-06CH11357
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
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; EXAFS; redox; uranium