Intrinsic formation of nanocrystalline neptunium dioxide under neutral aqueous conditions relevant to deep geological repositories

Husar, Richard; Hübner, René; Hennig, Christoph; Martin, Philippe; Chollet, M.; Weiß, Stephan; Stumpf, Thorsten; Zänker, Harald; Ikeda‐Ohno, Atsushi

doi:10.1039/c4cc08103j

Cited by 19 publications

(29 citation statements)

References 30 publications

(38 reference statements)

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“…Interestingly, in the current study, there was no strong evidence for a significant Np-Np interaction in the EXAFS at approximately 3.8 Å (although it is possible this is a consequence of the limited data range). Its absence (within the data range constraints for these samples) suggests that nano-crystalline NpO 2 may not be a dominant reaction product in this system and in contrast to recent studies [7,46]. In U(VI)O 2…”

Section: Resultscontrasting

confidence: 49%

Neptunium Reactivity During Co-Precipitation and Oxidation of Fe(II)/Fe(III) (Oxyhydr)oxides

et al. 2019

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Fe(II) bearing iron (oxyhydr)oxides were directly co-precipitated with Np(V)O2+ under anaerobic conditions to form Np doped magnetite and green rust. These environmentally relevant mineral phases were then characterised using geochemical and spectroscopic analyses. The Np doped mineral phases were then oxidised in air over 224 days with solution chemistry and end-point oxidation solid samples collected for further characterisation. Analysis using chemical extractions and X-ray absorption spectroscopy (XAS) techniques confirmed that Np(V) was initially reduced to Np(IV) during co-precipitation of both magnetite and green rust. Extended X-Ray Absorption Fine Structure (EXAFS) modelling suggested the Np(IV) formed a bidentate binuclear sorption complex to both minerals. Furthermore, following oxidation in air over several months, the sorbed Np(IV) was partially oxidised to Np(V), but very little remobilisation to solution occurred during oxidation. Here, linear combination fitting of the X-Ray Absorption Near Edge Structure (XANES) for the end-point oxidation samples for both mineral phases suggested approximately 50% oxidation to Np(V) had occurred over 7 months of oxidation in air. Both the reduction of Np(V) to Np(IV) and inner sphere sorption in association with iron (oxyhydr)oxides, and the strong retention of Np(IV) and Np(V) species with these phases under robust oxidation conditions, have important implications in understanding the mobility of neptunium in a range of engineered and natural environments.

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Section: Resultscontrasting

confidence: 49%

Neptunium Reactivity During Co-Precipitation and Oxidation of Fe(II)/Fe(III) (Oxyhydr)oxides

et al. 2019

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“…[4a-c, e, f, 9-11, 26] Four distinct uranium atoms (U1, U2, U3, U4) are involved in this {U 6 } subunit.U 3a nd U4 are nine-fold coordinated by two m 3 -oxo groups,t wo m 3 -hydroxo groups,f our oxygen atoms from benzoate ligands, and one water molecule. In the resulting monocapped square antiprismatic environment, the UÀO oxo bond lengths are in the range 2.20(3)-2.300 (17) , whereas the UÀO hydroxo bond lengths range from 2.48(2)-2.512 (15) . The UÀO carboxylate bond lengthsa re in the range 2.37(2)-2.55(2) and the H 2 OÀUb ond lengths range from 2.54(3)-2.59(2).The same coordination geometry (mono-capped square antiprism) is observed for the uranium atoms U1 andU 2, which are bondedt ot wo m 3 -hydroxog roups, three m 3 -oxog roups, and four oxygen atoms from carboxylate arms of oxalate and glycolate species.…”

Section: Structural Descriptionoft He Unknown Poly-oxo Cluster U 12mentioning

confidence: 99%

“…The corresponding UÀO oxo bond lengths are observed to be between2 . 15(2) and 2.183(9) ; the UÀO oxalate bond lengths are in the range 2.56(2)-2.756 (17).T he U5,U 6, and U7 are nine-fold coordinated to one m 1 -chlorine atom and one m 2 -chlorine atom, one oxo group, one water molecule, two oxygen atoms from glycolate groups,t wo oxygen atoms from the oxalate groups, and one oxygen atom from the benzoate group. The UÀCl bond lengthsa re between 2.636(9) and 2.871 (6) .…”

Section: Structural Descriptionoft He Unknown Poly-oxo Cluster U 12mentioning

confidence: 99%

“…The consequence is the modification of the mobility of the actinidesd uring their transport in environmental systems. [13] In some reducing conditions, nanosized crystalline metallico xideso ff luorite type (AnO 2 )a re produced with uranium [14] under microbial activity, or neptunium [15] in aqueous carbonate solution.H owever,u nderstanding of the reaction mechanisms for the formation of such AnO 2 particles is still underi nvestigation. It is assumed that the pre-nucleation hydroxo-oxo clusters may play ac riticalr oleb efore the formation of the final phases, [16] and therefore, the knowledge of the occurrenceo fw ell-defined species might be crucial in this context.…”

Section: Introductionmentioning

confidence: 99%

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Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U₃₈

Falaise

Volkringer

Hennig

et al. 2015

Chemistry A European J

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The ex-situ qualitative study of the kinetic formation of the poly-oxo cluster U38 , has been investigated after the solvothermal reaction. The resulting products have been characterized by means of powder XRD and scanning electron microscopy (SEM) for the solid phase and UV/Vis, X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and NMR spectroscopies for the supernatant liquid phase. The analysis of the different synthesis batches, stopped at different reaction times, revealed the formation of spherical crystallites of UO2 from t=3 h, after the formation of unknown solid phases at an early stage. The crystallization of U38 occurred from t=4 h at the expense of UO2 , and is completed after t=8 h. Starting from pure uranium(IV) species in solution (t=0-1 h), oxidation reactions are observed with a U(IV) /U(VI) ratio of 70:30 for t=1-3 h. Then, the ratio is inversed with a U(IV) /U(VI) ratio of 25/75, when the precipitation of UO2 occurs. Thorough SEM observations of the U38 crystallites showed that the UO2 aggregates are embedded within. This may indicate that UO2 acts as reservoir of uranium(IV), for the formation of U38 , stabilized by benzoate and THF ligands. During the early stages of the U38 crystallization, a transient crystallized phase appeared at t=4 h. Its crystal structure revealed a new dodecanuclear moiety (U12 ), based on the inner hexanuclear core of {U6 O8 } type, decorated by three additional pairs of dinuclear U2 units. The U12 motif is stabilized by benzoate, oxalates, and glycolate ligands.

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“…No statistical validity could be achieved by adding Np(V)-O ax scattering paths at 1.8-1.9 Å confirming no significant dioxygenyl Np(V) (NpO 2 + ) was present on the magnetite crystals. Additionally, due to the data quality of the EXAFS spectra, no further backscatterers (Np or Fe) could be fit to the spectra and thus it was not possible to distinguish between nanoparticulate NpO 2 or surface bound Np 4+ [16,35,38,65]. The fits to the neptunium 4f 7/2 and 5/2 transitions of the XPS spectra from both magnetite crystals (Figure 3e) could be modelled as a single Np(IV) species with binding energies for the neptunium 4f 7/2 and 5/2 transitions of 403.2-403.4 and 415.0-415.2 eV (Table S2), respectively.…”

Section: Neptuniummentioning

confidence: 99%

Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

et al. 2019

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Neptunium and uranium are important radionuclides in many aspects of the nuclear fuel cycle and are often present in radioactive wastes which require long term management. Understanding the environmental behaviour and mobility of these actinides is essential in underpinning remediation strategies and safety assessments for wastes containing these radionuclides. By combining state-of-the-art X-ray techniques (synchrotron-based Grazing Incidence XAS, and XPS) with wet chemistry techniques (ICP-MS, liquid scintillation counting and UV-Vis spectroscopy), we determined that contrary to uranium(VI), neptunium(V) interaction with magnetite is not significantly affected by the presence of bicarbonate. Uranium interactions with a magnetite surface resulted in XAS and XPS signals dominated by surface complexes of U(VI), while neptunium on the surface of magnetite was dominated by Np(IV) species. UV-Vis spectroscopy on the aqueous Np(V) species before and after interaction with magnetite showed different speciation due to the presence of carbonate. Interestingly, in the presence of bicarbonate after equilibration with magnetite, an unknown aqueous NpO2+ species was detected using UV-Vis spectroscopy, which we postulate is a ternary complex of Np(V) with carbonate and (likely) an iron species. Regardless, the Np speciation in the aqueous phase (Np(V)) and on the magnetite (111) surfaces (Np(IV)) indicate that with and without bicarbonate the interaction of Np(V) with magnetite proceeds via a surface mediated reduction mechanism. Overall, the results presented highlight the differences between uranium and neptunium interaction with magnetite, and reaffirm the potential importance of bicarbonate present in the aqueous phase.

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Intrinsic formation of nanocrystalline neptunium dioxide under neutral aqueous conditions relevant to deep geological repositories

Cited by 19 publications

References 30 publications

Neptunium Reactivity During Co-Precipitation and Oxidation of Fe(II)/Fe(III) (Oxyhydr)oxides

Neptunium Reactivity During Co-Precipitation and Oxidation of Fe(II)/Fe(III) (Oxyhydr)oxides

Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U₃₈

Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

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Intrinsic formation of nanocrystalline neptunium dioxide under neutral aqueous conditions relevant to deep geological repositories

Cited by 19 publications

References 30 publications

Neptunium Reactivity During Co-Precipitation and Oxidation of Fe(II)/Fe(III) (Oxyhydr)oxides

Neptunium Reactivity During Co-Precipitation and Oxidation of Fe(II)/Fe(III) (Oxyhydr)oxides

Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U38

Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

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Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U₃₈