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

1
Research Centre for Radwaste Disposal, School of Earth and Environmental Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
2
Department of Civil and Environmental Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, UK
3
Diamond Light Source Ltd., Diamond House, Harwell Science & Innovation Campus, Didcot OX11 0DE, UK
4
Photon Science Institute, School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
*
Author to whom correspondence should be addressed.
Geosciences 2019, 9(2), 81; https://doi.org/10.3390/geosciences9020081
Received: 23 November 2018 / Revised: 9 January 2019 / Accepted: 1 February 2019 / Published: 8 February 2019
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Abstract

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. View Full-Text
Keywords: uranium; neptunium; magnetite; surface; synchrotron; XAS; XPS; geodisposal; reduction uranium; neptunium; magnetite; surface; synchrotron; XAS; XPS; geodisposal; reduction
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Bots, P.; van Veelen, A.; Mosselmans, J.F.W.; Muryn, C.; Wogelius, R.A.; Morris, K. Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface. Geosciences 2019, 9, 81.

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