May 15 – 20, 2022
Casino Conference Centre
Europe/Prague timezone

Uptake of Np(V) by zirconia: a combined batch, spectroscopic, and surface complexation modeling study

May 17, 2022, 2:20 PM
20m
Marble Hall

Marble Hall

Verbal Radionuclides in the Environment, Radioecology Environmental Radioactivity

Speaker

Isabelle Jessat (Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology)

Description

The interactions of long-lived actinides, such as the transuranium element neptunium, with corrosion products in the near-field of a repository are important processes that have to be considered when assessing the safety of a nuclear waste repository. As a main corrosion product of the zircaloy cladding material of spent nuclear fuel rods, zirconia (ZrO$_{2}$) constitutes a first possible barrier against the release of radionuclides.
To gain a detailed understanding of the surface processes in the Np(V)−zirconia system, a comprehensive, multi-method approach was applied. The Np(V)−ZrO$_{2}$ system has been studied on the macroscopic level by conducting pH-dependent batch sorption experiments under varying conditions (ionic strength, Np(V) concentration, and solid-to-liquid ratio (m/V)). In addition, a Np(V) sorption isotherm at pH 6 was collected. The results revealed that Np(V) sorption onto ZrO$_{2}$ was affected by pH, Np(V) concentration, and solid-to-liquid ratio. Uptake of Np(V) increased with pH, starting around pH 3 with maximum sorption reached from pH 6. The shift of the sorption edge towards lower pH with increase of the m/V ratio points to the presence of different kinds of sorption sites. This is supported by the Np(V) sorption isotherm results, where the shape suggests strong and weak binding sites. Furthermore, Np(V) uptake was found to be independent of ionic strength and zeta potential measurements revealed a shift towards higher pH values of the isoelectric point of the neat ZrO$_{2}$ in the presence of Np(V). Hence, the formation of Np(V) inner-sphere surface complexes is indicated.
Molecular information about the surface species were obtained by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) and in situ Infrared (IR) Spectroscopy, revealing the predominant formation of inner-sphere Np(V) surface complexes. A short Np−Zr distance derived from EXAFS suggests the presence of Np(V) bidentate inner-sphere complexes on the ZrO$_{2}$ surface.
These information obtained on a macroscopic and a molecular level can be used to restrict the number of surface species as well as their denticity in a surface complexation model. The thermodynamic surface complexation parameters of the Np(V)−zirconia system derived in this study will help to make more reliable predictions about the fate of Np(V) in the environment.

Primary author

Isabelle Jessat (Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology)

Co-authors

Dr André Rossberg (Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology) Dr Andreas Scheinost (Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology) Dr Johannes Lützenkirchen (Karlsruhe Institute of Technology (KIT), Institute for Nuclear Waste Disposal (INE)) Dr Harald Foerstendorf (Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology) Dr Norbert Jordan (Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology)

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