Motivation

Scientific Background

The disposal of high-level nuclear waste in deep geological formations poses major scientific and social challenges to be met in the next decades. One of the key issues is the long-term safety of a waste repository system over extended periods of time, up to 10⁶ years. Demonstrating the repository safety over such geological time spans requires a sound understanding of the geochemical behavior of long-lived and radiotoxic radionuclides such as the actinides. The actinide elements U, Np, and Pu form oxo-cations (‘actinyl-cations’) in oxidizing aqueous environments. The environmental behavior of the actinyl ions U(VI)O₂²⁺, Np(V)O₂⁺ and Pu(V,VI)O₂^(+,2+) is to a large extent controlled by sorption reactions (inner- and outer-sphere surface complexation, ion-exchange, co-precipitation/structural incorporation) with minerals. Calcite is a common mineral in most rock types which are currently considered for deep geological waste disposal. It is almost ubiquitous in the geosphere and is a possible alteration product of concrete bound materials as used in the technical barriers around nuclear waste disposal systems. Its reactivity could make calcite an important sink for actinides as well in the near field as in the far field around waste disposals.
The goal of this study is to elucidate the mechanisms of actinide uptake by calcite.

Methods
* For the co-precipitation studies mixed flow reactor (MFR) experiments are used to synthesize actinide containing calcite samples and
to measure macroscopic partition coefficients and precipitation rates.
* Surface complexation is studied by batch sorption experiments.
* Microscopic growth rates and the influence of actinide incorporation on these growth rates are investigated by atomic force
microscopy (AFM) via in-situ crystal growth observation experiments.
* Along with all these experiments XAFS and XRD methods are applied for the structural investigation of the reaction products.

Status quo

At first work was focused on the structural incorporation of the actinyl oxo-cation Np(V)O₂⁺into calcite which was compared to the structural incorporation of U(VI)O₂²⁺.
Four neptunyl and one uranyl containing calcite samples have been synthesized. And the structural environment around the actinyl ions in the calcite host has been investigated by EXAFS analyses. Neptunyl shows a much higher affinity to calcite than uranyl does. This is reflected in the partition coefficients which range from 0.5 to 10.3 for neptunyl in calcite while the one for uranyl in calcite is 0.02. A reason for this difference can be found in the structural environment of the incorporated actinyl ions. The linear neptunyl, (O=Np=O)⁺, ion incorporated into calcite shows an equatorial coordination by four monodentate bound carbonate ions with an Np-O distance of 2.40 Å. This coordination is well compatible to the calcite structure. The very similar uranyl, (O=U=O)²⁺, ion incorporated into calcite is equatorially coordinated by about five oxygen atoms, belonging to three or four carbonate ions of which one or two are bound in bidentate fashion. This structural environment is not compatible to the calcite structure. A detailed description of the experiments and discussion of the results will soon be available in: F. Heberling, M.A. Denecke, D. Bosbach; Neptunium(V) co-precipitation with calcite, Environmental Science & Technology, (submitted). EXAFS spectra and pictures of the MFR and the coordination structures can be found on the corresponding poster contribution to the Goldschmidt conference 2007.

In the meanwhile six further MFR experiments on NpO₂⁺ co-precipitation with calcite have been conducted and a thermodynamic model to describe the macroscopically observed dependence of the uptake on the solution composition is under construction. A new highly doped neptunyl-calcite sample containing 1.2 % (mol) neptunium will soon be investigated by XRD and low temperature EXAFS. Hopefully these measurements will further elucidate the substitution mechanisms operating during co-precipitation that provide the charge balance in the solid-solution.
Batch sorption experiments on NpO₂⁺ adsorption to the calcite surface under varying pH, p(CO₂) and concentration conditions are ongoing. First results show that adsorption at atmospheric p(CO₂) at pH 8.3 in a concentration range between 1.2 ∙ 10^-7 mol/L and 3.7 ∙ 10^-5 mol/L can be described by a Freundlich-isotherm: q = 2.42 ∙10^-5 ∙ c^0.4640. Where q is the surface loading in mol/g and c is the concentration in the solution after 72 hours in mol/L. The structure of the adsorption surface complexes will be investigated by EXAFS during the next beamtime.
The flow through cell for the AFM in situ crystal crystal growth observation is working and first successes in producing and observing growth spirals on a calcite (104) surface have been made (see video concerning crystal growth).

Contact

If you have any suggestions, critics, or questions you are welcome to contact me under:
Frank Heberling ∂does-not-exist.ine fzk de.