Structural analysis of ternary actinyl(V/VI) sorption complexes on gibbsite – A combined quantum chemical and spectroscopic approach

For the safety assessment of high-level nuclear waste repositories, it is mandatory to know the transportation paths of contaminants, e.g. actinyl ions (UO22+, NpO2+), in the geological barrier. The most attention needs to be focused on the transport in aquifers, because water contamination, depending on retention and migration processes of radionuclides in the geosphere, is of primary environmental concern. The migration behavior of actinides in ground water is mainly controlled by aquatic speciations and sorption processes at water-mineral interfaces. Hence, the investigation of complex species in aqueous solutions and at mineral surfaces becomes essential for the safety assessment in the near and far field of nuclear repositories.

For deep ground repositories, clay and clay minerals are considered as possible host rocks, because they show a low permeability and are expected to have a high retention capacity towards actinyl ions. But the complexity of naturally occurring minerals in particular their surface often hampers the unequivocal interpretation of results obtained from sorption experiments. The use of model phases only showing one particular functional group at the surfaces with a well defined surface topology is an appropriate approach for the understanding of the basic sorption processes. Aluminum oxide and hydroxides are of special interest because they represent main components in clays and clay minerals. In particular, gibbsite is widely used as a model system because it represents not only the most common crystalline aluminum hydroxide but also a ubiquitous weathering product of alumosilicates. Furthermore, the elemental structural unit of gibbsite, that is the Al(OH)6 octahedron, occurs ubiquitously as part of the structure of common clay minerals like kaolinite.

In the present study, the sorption processes of U(VI) and Np(V) on gibbsite were studied under consideration of the aqueous speciation. First, the structural data of an aqueous dimeric U(VI)-carbonato species were revisited and refined by a combined approach of quantum chemical calculations and vibrational spectroscopy. The combination of these techniques is expected to provide progress in the identification of the molecular structures of the aqueous uranyl species, which in turn are needed for a reliable prediction of surface complexes. The results show that an isomer with a carbonate ligand bridging the two uranyl units is most likely the predominant structure. Second, the sorption processes and the influence of atmospherically derived carbonate on them were analyzed by a combined spectroscopic approach using vibrational and X ray absorption spectroscopy. From results provided, complementary molecular information can be obtained because of the different molecular scales probed by each technique.

In the absence of atmospherically derived CO2, the relevant interface processes can be described by the formation of stable U(VI) surface complexes at trace concentrations which continuously change to surface precipitates with ongoing U(VI) accumulation at the surface. In contrast, in the presence of carbonate ions the surface speciation on gibbsite is significantly changed due to the formation of dimeric uranyl carbonato surface complexes inhibiting the formation of insoluble polymeric species in the micromolar concentration range. The interatomic distances and coordination numbers obtained by EXAFS spectroscopy are concordant with the values calculated for the aquatic dimeric U(VI)-carbonato species.

From the in situ sorption experiments of Np(V) on gibbsite probed by vibrational spectroscopy, the formation of only monomeric inner sphere complexes is derived from experiments in inert gas and in the presence of atmospheric equivalent added carbonate. These findings are supported by results from EXAFS spectroscopy providing evidence for Np–C and Np–Al interactions. Additionally, the values of interatomic distances and coordination numbers are concordant with values for an inner-sphere complex.

From the results of this study, it can be proposed that Al-hydroxides effectively retard the dissemination of actinyl ions at micromolar concentrations in water-bearing host rocks at near neutral pH values, where gibbsite shows the lowest solubility. Furthermore, this work contributes to a better understanding of the geochemical interactions of actinides, in particular U(VI) and Np(V), in the environment and are of relevance for the assessment of the migration behavior of actinyl ions in groundwater systems. The multiplicity of spectroscopic experiments and quantum chemical calculations carried out within this study yields a profound collection of data which can be used as reference for future radioecological investigations of more complex sorption systems in aqueous solution.