Neptunium and Uranium Interactions with Environmentally and Industrially Relevant Iron Minerals

Abstract

Neptunium (237Np) is an important radionuclide in the nuclear fuel cycle in areas such as effluent treatment and the geodisposal of radioactive waste. Due to neptunium’s redox sensitivity and its tendency to adsorb strongly to mineral phases, such as iron oxides/sulfides, the environmental mobility of Np can be altered significantly by a wide variety of chemical processes. Here, Np interactions with key iron minerals, ferrihydrite (Fe5O8H·4H2O), goethite (α-FeOOH), and mackinawite (FeS), are investigated using X-ray Absorption Spectroscopy (XAS) in order to explore the mobility of neptunyl(V) (Np(V)O2+) moiety in environmental (radioactive waste disposal) and industrial (effluent treatment plant) scenarios. Analysis of the Np LIII-edge X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) showed that upon exposure to goethite and ferrihydrite, Np(V) adsorbed to the surface, likely as an inner-sphere complex. Interestingly, analysis showed that only the first two shells (Oax and Oeq) of the EXAFS could be modelled with a high degree of confidence, and there was no clear indication of Fe or carbonate in the fits. When Np(V)O2+ was added to a mackinawite-containing system, Np(V) was reduced to Np(IV) and formed a nanocrystalline Np(IV)O2 solid. An analogous experiment was also performed with U(VI)O22+, and a similar reduction was observed, with U(VI) being reduced to nanocrystalline uraninite (U(IV)O2). These results highlight that Np(V) may undergo a variety of speciation changes in environmental and engineered systems whilst also highlighting the need for multi-technique approaches to speciation determination for actinyl (for example, Np(V)O2+) species.

Sulfidation and Reoxidation of U(VI)-Incorporated Goethite: Implications for U Retention during Sub-Surface Redox Cycling

“…Sulfidation of iron (oxyhydr)­oxides is a complex multi-step process, which typically results in reductive dissolution at the mineral surface, forming elemental sulfur (S 8 0 ) and releasing Fe­(II) into solution. , The resultant aqueous Fe­(II) may then react with aqueous HS – to form secondary iron sulfide phases, such as mackinawite (FeS). , Aqueous HS – may also react with S 8 0 to form surface-associated polysulfide (S n 2– ) species . Consequently, these complex processes may influence the behavior and fate of radionuclides, including U. ,− …”

“… 46 Consequently, these complex processes may influence the behavior and fate of radionuclides, including U. 26 , 47 − 50 …”

Computational study of the interactions of tetravalent actinides (An = Th–Pu) with the α-Fe13 Keggin cluster

“…The mechanism by which actinides are sequestered during Fh formation is unknown and of significant interest. 2–4…”

“…The mechanism by which actinides are sequestered during Fh formation is unknown and of significant interest. [2][3][4] Studies have shown that Fh formation can potentially occur through several competing pathways. One proposed route proceeds via polymerisation of small molecular Fe(III) species such as dimers and trimers.…”