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

Abstract

The dilution of aqueous neptunium carbonate complexes induces the intrinsic formation of nanocrystalline neptunium dioxide (NpO2) particles, which are characterised by UV/Vis and X-ray absorption spectroscopies and transmission electron microscopy. This new route of nanocrystalline NpO2 formation could be a potential scenario for the environmental transport of radionuclides from the waste repository (i.e. under near-field alkaline conditions) to the geological environment (i.e. under far-field neutral conditions).

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

“…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…”

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

“…[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.…”

“…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) .…”

“…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.…”

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

“…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.…”