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

Journal Article · Tue Nov 17 00:00:00 EST 2015 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2015.11.010· OSTI ID:1248730

Latta, Drew E.  ^[1]; Kemner, Kenneth M. ^[1]; Mishra, Bhoopesh ^[2]; Boyanov, Maxim I. ^[3]

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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)

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 UO₂ phase and an amorphous U^IV–Ca–PO₄ analog to ningyoite (CaU^IV(PO₄)₂·1–2H₂O). The valence of U was tracked by X-ray absorption near-edge spectroscopy (XANES), showing similar oxidation rate constants for U^IVO₂ and U^IV–phosphate in solutions equilibrated with atmospheric O₂ and CO₂ 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 PO₄ decreased the oxidation rate constant by an order of magnitude for both UO₂ 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 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 U^VI (<2.5 × 10^-7 M). Oxidation of U^IV–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 U^IV oxidation, but that the rates are still fast enough to cause complete oxidation of U^IV within days under fully oxygenated conditions.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER); USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC02-06CH11357
• OSTI ID: 1248730
• Alternate ID(s): OSTI ID: 1397565
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 174, Issue C; ISSN 0016-7037
• Publisher: The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
EXAFS
redox
uranium