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Neptunium and uranium interactions with environmentally and industrially relevant iron minerals

DOI: 10.3390/min12020165 DOI Help

Authors: Luke T. Townsend (The University of Manchester; University of Sheffield) , Kurt F. Smith (The University of Manchester) , Ellen H. Winstanley (The University of Manchester) , Katherine Morris (University of Manchester) , Olwen Stagg (The University of Manchester) , J. Frederick W. Mosselmans (Diamond Light Source) , Francis R. Livens (The University of Manchester) , Liam Abrahamsen-Mills (National Nuclear Laboratory) , Richard Blackham (Sellafield Ltd) , Samuel Shaw (The University of Manchester)
Co-authored by industrial partner: Yes

Type: Journal Paper
Journal: Minerals , VOL 12

State: Published (Approved)
Published: January 2022
Diamond Proposal Number(s): 24074 , 21441 , 13559

Open Access Open Access

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.

Journal Keywords: neptunium; uranium; iron (oxyhydr) oxide; iron sulfide; geological disposal of radioactive waste; contaminated land; EARP

Subject Areas: Materials, Earth Science, Environment

Instruments: B18-Core EXAFS , I20-Scanning-X-ray spectroscopy (XAS/XES)

Added On: 07/02/2022 09:25


Discipline Tags:

Earth Sciences & Environment Radioactive Materials Mineralogy Materials Science Nuclear Waste Geology Geochemistry

Technical Tags:

Spectroscopy X-ray Absorption Spectroscopy (XAS) Extended X-ray Absorption Fine Structure (EXAFS) X-ray Absorption Near Edge Structure (XANES)