Effects of calcium and phosphate on uranium(IV) oxidation: Comparison between nanoparticulate uraninite and amorphous UIV–phosphate.
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| Title: | Effects of calcium and phosphate on uranium(IV) oxidation: Comparison between nanoparticulate uraninite and amorphous UIV–phosphate. |
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| Authors: | Latta, Drew E.1 drew-latta@uiowa.edu, Kemner, Kenneth M.1 Kemner@anl.gov, Mishra, Bhoopesh1,2 bmishra3@iit.edu, Boyanov, Maxim I.1,3 mboyanov@ice.bas.bg |
| Source: | Geochimica et Cosmochimica Acta. Feb2016, Vol. 174, p122-142. 21p. |
| Subjects: | Calcium, Phosphates, Uranium, Oxidation kinetics, Oxidation-reduction reaction, Comparative studies, X-ray spectroscopy |
| Abstract: | 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 2 O). The valence of U was tracked by X-ray absorption near-edge spectroscopy (XANES), showing similar oxidation rate constants for U IV O 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), resulting 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 · x H 2 O), 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 U VI phase resulting from oxidation of UO 2 and U IV –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–U VI –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. 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. [ABSTRACT FROM AUTHOR] |
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| Database: | Engineering Source |
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