Redox chemistry of uranium in reducing, dilute to concentrated NaCl solutions

Çevirim-Papaioannou, Neşe; Yalçıntaş, E.; Gaona, Xavier; Dardenne, Kathy; Altmaier, M.; Geckeis, Horst

doi:10.1016/j.apgeochem.2018.07.006

Cited by 14 publications

(7 citation statements)

References 55 publications

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“…No clear trend is observed for the latter as a function of time or pH. The values of hydration water determined for the aged solid phases are in line with data reported by Cevirim-Papaioannou for a U(IV) hydrous oxide phase aged at T = 22°C for up to 798 days ( n = 1.0 ± 0.5) ( Cevirim-Papaioannou, 2018 ). Figure 2 shows the evolution of the number of hydration waters in M(IV)O 2 (s, hyd) as a function of time, with M = Th (this work), U ( Cevirim-Papaioannou, 2018 ) and Tc ( Yalcintas et al, 2016 ; Grenthe et al, 2020 ).…”

Section: Resultssupporting

confidence: 89%

“…The values of hydration water determined for the aged solid phases are in line with data reported by Cevirim-Papaioannou for a U(IV) hydrous oxide phase aged at T = 22°C for up to 798 days ( n = 1.0 ± 0.5) ( Cevirim-Papaioannou, 2018 ). Figure 2 shows the evolution of the number of hydration waters in M(IV)O 2 (s, hyd) as a function of time, with M = Th (this work), U ( Cevirim-Papaioannou, 2018 ) and Tc ( Yalcintas et al, 2016 ; Grenthe et al, 2020 ). The figure shows a clear qualitative trend to decrease the number of hydration waters with ageing time, thus supporting the transformation of hydroxide/hydrated phases into the corresponding, thermodynamically stable, oxides.…”

Section: Resultssupporting

confidence: 89%

“…The number of hydration waters in An (IV) hydrous oxides has been often assumed as 2 ( i.e. AnO 2 ⋅2H 2 O(s) ≅ An(OH) 4 (s)), although a few previous studies report clearly lower values of hydration waters (0.6–1) for M(IV) solid phases aged for 1–3 years at room temperature ( Yalcintas et al, 2016 ; Cevirim-Papaioannou, 2018 ). TG-DTA measurements were performed to determine the number of hydration waters of freshly precipitated ThO 2 (am, hyd) as well as of solid phases aged at T = 80°C for t = 1, 2, 4.5 and 5.5 months.…”

Section: Resultsmentioning

confidence: 99%

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Interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system

Kiefer

Neill²,

Çevirim-Papaioannou³

et al. 2022

Front. Chem.

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The impact of temperature on a freshly precipitated ThO2(am, hyd) solid phase was investigated using a combination of undersaturation solubility experiments and a multi-method approach for the characterization of the solid phase. XRD and EXAFS confirm that ageing of ThO2(am, hyd) at T = 80°C promotes a significant increase of the particle size and crystallinity. TG-DTA and XPS support that the ageing process is accompanied by an important decrease in the number of hydration waters/hydroxide groups in the original amorphous Th(IV) hydrous oxide. However, while clear differences between the structure of freshly precipitated ThO2(am, hyd) and aged samples were observed, the characterization methods used in this work are unable to resolve clear differences between solid phases aged for different time periods or at different pH values. Solubility experiments conducted at T = 22°C with fresh and aged Th(IV) solid phases show a systematic decrease in the solubility of the solid phases aged at T = 80°C. In contrast to the observations gained by solid phase characterization, the ageing time and ageing pH significantly affect the solubility measured at T = 22°C. These observations can be consistently explained considering a solubility control by the outermost surface of the ThO2(s, hyd) solid, which cannot be properly probed by any of the techniques considered in this work. Solubility data are used to derive the thermodynamic properties (log *K°s,0, ΔfG°m) of the investigated solid phases, and discussed in terms of particle size using the Schindler equation. These results provide new insights on the interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system, with special emphasis on the transformation of the amorphous hydrous/hydroxide solid phases into the thermodynamically stable crystalline oxides.

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Section: Resultssupporting

confidence: 89%

Section: Resultssupporting

confidence: 89%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system

Kiefer

Neill²,

Çevirim-Papaioannou³

et al. 2022

Front. Chem.

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“…Whilst signal contribution from a calcium uranate type environment was also indicated by XANES linear combination fitting, it is likely that this would form in this system as a hydrous analogue [e.g. CaU 2 O 7 ÁxH 2 O (cr) ] (C ¸evirim- Papaioannou et al, 2018). Calcium uranate phases have previously been found to form in cementitious systems (Sutton et al, 2003;Felipe-Sotelo et al, 2017).…”

Section: U VI -Ettringite Systemsmentioning

confidence: 78%

Spectroscopic evaluation of U^VI–cement mineral interactions: ettringite and hydrotalcite

Yorkshire¹,

Stennett²,

Walkley³

et al. 2022

J Synchrotron Rad

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Portland cement based grouts used for radioactive waste immobilization contain high replacement levels of supplementary cementitious materials, including blast-furnace slag and fly ash. The minerals formed upon hydration of these cements may have capacity for binding actinide elements present in radioactive waste. In this work, the minerals ettringite (Ca6Al2(SO4)3(OH)12·26H2O) and hydrotalcite (Mg6Al2(OH)16CO3·4H2O) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing UVI from radioactive waste streams. U L III-edge X-ray absorption spectroscopy (XAS) was used to probe the UVI coordination environment in contact with these minerals. For the first time, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was applied to probe the Al coordination environment in these UVI-contacted minerals and make inferences on the UVI coordination, in conjunction with the X-ray spectroscopy analyses. The U L III-edge XAS analysis of the UVI-contacted ettringite phases found them to be similar (>∼70%) to the uranyl oxyhydroxides present in a mixed becquerelite/metaschoepite mineral. Fitting of the EXAFS region, in combination with 27Al NMR analysis, indicated that a disordered Ca- or Al-bearing UVI secondary phase also formed. For the UVI-contacted hydrotalcite phases, the XAS and 27Al NMR data were interpreted as being similar to uranyl carbonate, that was likely Mg-containing.

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“…Under such conditions, the first technical barrier consisting of stainless-steel casks will corrode over the geological time scales to be considered. Depending on the technical backfill material (clays like bentonite), the natural bedrock (e.g., clay, granite, or salt), and the nature of the eventually penetrating groundwater, a range of Fe(II)-bearing minerals may form during the corrosion processes, including oxides like magnetite (Fe 3 O 4 ), sulfides like mackinawite (FeS), and (hydroxo) carbonates like siderite (FeCO 3 ) and chukanovite (Fe 2 (OH) 2 (CO 3 )), as well as Fe-rich clays like nontronite. − The low redox potential along with the catalytic effect of these Fe(II)-bearing minerals has been shown to greatly reduce the mobility of redox-reactive actinides (U, Np) and fission products (Se, Tc) by sorption and reduction processes producing solid compounds with very low solubility like U IV O 2 , − Np IV O 2 , − FeSe, − and Tc IV O 2 . ,, While formation of PuO 2 with similarly low solubility has also been observed during mineral sorption reactions under both oxic and anoxic conditions, − Pu may also be further reduced to the oxidation state III, where it occurs as aquo ion with high solubility in acidic to weakly alkaline solutions. Note the similar geochemical behavior of Pu(III) with its next actinide neighbors, Am and Cm, which are prevalently trivalent.…”

Section: Introductionmentioning

confidence: 99%

Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption

Dumas¹,

Fellhauer²,

Schild³

et al. 2019

ACS Earth Space Chem.

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The reliable prediction of possible plutonium migration into the geological environment is crucial for the safety assessment of radioactive waste repositories. Fe(II)-bearing corrosion products like magnetite, which form on the surface of steel waste containers, can effectively contribute to the retardation of the potential radionuclide release by sorption and redox reactions, eventually followed by formation of secondary precipitates. A retardation process even more efficientespecially when considering the required long time scales for nuclear waste repositionis structural incorporation by magnetite, as has been demonstrated for Tc and U. Here we show that this mechanism might not be as relevant for Pu retention: after a rapid reduction of Pu(V) to Pu(III) in acidic Fe(II)/Fe(III) solution, base-induced magnetite precipitation (pHexp ≈ 12.5) leads only to a partial (∼50%) incorporation, while the other half remains at the surface by forming tridentate sorption complexes. Neither solid nor sorbed Pu(IV) species were observed in the starting solution and after precipitation. With Fe(II)-enforced recrystallization at pHexp = 6.5, a process potentially mimicking long-term, thermodynamically controlled aging, the equilibrium between both Pu species is even further shifted toward the sorption complex. A detailed analysis of the incorporated species by Pu LIII-edge X-ray absorption fine-structure (XAFS) spectroscopy shows a pyrochlore-like coordination environment (split 8-fold oxygen coordination shell with Pu–O distances of 2.22 and 2.45 Å and an edge-sharing linkage to Fe-octahedra with Pu–Fe distances of 3.68 Å), which is embedded in the magnetite matrix (Pu–Fe distances of 3.93, 5.17, and 5.47 Å). This suggests that the reason for the partial incorporation is the structural incompatibility of the large Pu(III) ion for the octahedral Fe site in magnetite. The adoption of a pyrochlore-like local environment within the magnetite long-range structure might be induced by the rapid coprecipitation rather than being a thermodynamically stable state (kinetic entrapment).

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Redox chemistry of uranium in reducing, dilute to concentrated NaCl solutions

Cited by 14 publications

References 55 publications

Interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system

Interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system

Spectroscopic evaluation of U^VI–cement mineral interactions: ettringite and hydrotalcite

Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption

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Redox chemistry of uranium in reducing, dilute to concentrated NaCl solutions

Cited by 14 publications

References 55 publications

Interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system

Interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system

Spectroscopic evaluation of UVI–cement mineral interactions: ettringite and hydrotalcite

Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption

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Spectroscopic evaluation of U^VI–cement mineral interactions: ettringite and hydrotalcite