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] |
| Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.) | |
| Database: | Engineering Source |
| FullText | Text: Availability: 0 |
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| Header | DbId: egs DbLabel: Engineering Source An: 112552025 AccessLevel: 6 PubType: Academic Journal PubTypeId: academicJournal PreciseRelevancyScore: 0 |
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| Items | – Name: Title Label: Title Group: Ti Data: Effects of calcium and phosphate on uranium(IV) oxidation: Comparison between nanoparticulate uraninite and amorphous UIV–phosphate. – Name: Author Label: Authors Group: Au Data: <searchLink fieldCode="AR" term="%22Latta%2C+Drew+E%2E%22">Latta, Drew E.</searchLink><relatesTo>1</relatesTo><i> drew-latta@uiowa.edu</i><br /><searchLink fieldCode="AR" term="%22Kemner%2C+Kenneth+M%2E%22">Kemner, Kenneth M.</searchLink><relatesTo>1</relatesTo><i> Kemner@anl.gov</i><br /><searchLink fieldCode="AR" term="%22Mishra%2C+Bhoopesh%22">Mishra, Bhoopesh</searchLink><relatesTo>1,2</relatesTo><i> bmishra3@iit.edu</i><br /><searchLink fieldCode="AR" term="%22Boyanov%2C+Maxim+I%2E%22">Boyanov, Maxim I.</searchLink><relatesTo>1,3</relatesTo><i> mboyanov@ice.bas.bg</i> – Name: TitleSource Label: Source Group: Src Data: <searchLink fieldCode="JN" term="%22Geochimica+et+Cosmochimica+Acta%22">Geochimica et Cosmochimica Acta</searchLink>. Feb2016, Vol. 174, p122-142. 21p. – Name: Subject Label: Subjects Group: Su Data: <searchLink fieldCode="DE" term="%22Calcium%22">Calcium</searchLink><br /><searchLink fieldCode="DE" term="%22Phosphates%22">Phosphates</searchLink><br /><searchLink fieldCode="DE" term="%22Uranium%22">Uranium</searchLink><br /><searchLink fieldCode="DE" term="%22Oxidation+kinetics%22">Oxidation kinetics</searchLink><br /><searchLink fieldCode="DE" term="%22Oxidation-reduction+reaction%22">Oxidation-reduction reaction</searchLink><br /><searchLink fieldCode="DE" term="%22Comparative+studies%22">Comparative studies</searchLink><br /><searchLink fieldCode="DE" term="%22X-ray+spectroscopy%22">X-ray spectroscopy</searchLink> – Name: Abstract Label: Abstract Group: Ab Data: 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] – Name: AbstractSuppliedCopyright Label: Group: Ab Data: <i>Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.) |
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| RecordInfo | BibRecord: BibEntity: Identifiers: – Type: doi Value: 10.1016/j.gca.2015.11.010 Languages: – Code: eng Text: English PhysicalDescription: Pagination: PageCount: 21 StartPage: 122 Subjects: – SubjectFull: Calcium Type: general – SubjectFull: Phosphates Type: general – SubjectFull: Uranium Type: general – SubjectFull: Oxidation kinetics Type: general – SubjectFull: Oxidation-reduction reaction Type: general – SubjectFull: Comparative studies Type: general – SubjectFull: X-ray spectroscopy Type: general Titles: – TitleFull: Effects of calcium and phosphate on uranium(IV) oxidation: Comparison between nanoparticulate uraninite and amorphous UIV–phosphate. Type: main BibRelationships: HasContributorRelationships: – PersonEntity: Name: NameFull: Latta, Drew E. – PersonEntity: Name: NameFull: Kemner, Kenneth M. – PersonEntity: Name: NameFull: Mishra, Bhoopesh – PersonEntity: Name: NameFull: Boyanov, Maxim I. IsPartOfRelationships: – BibEntity: Dates: – D: 01 M: 02 Text: Feb2016 Type: published Y: 2016 Identifiers: – Type: issn-print Value: 00167037 Numbering: – Type: volume Value: 174 Titles: – TitleFull: Geochimica et Cosmochimica Acta Type: main |
| ResultId | 1 |
