Daniel R. Fröhlich

SORPTION OF NEPTUNIUM ON CLAYS AND CLAY MINERALS – A REVIEW

During recent decades, the search for possible repositories for high-level nuclear waste has yielded large amounts of sorption data for actinides on different minerals. Clays and clay minerals are of special interest as potential host-rock formations and backfill materials, by virtue of their good retardation properties. Neptunium (Np) is one of the actinides which is considered in long-term scenarios due to its long-lived nuclide 237Np (t1/2 = 2.1 × 106 y). Because neptunium sorption is heavily dependent on the experimental conditions, comparison of sorption data from different experiments is challenging. Normalizing reported data with respect to the surface area of the sorbent enables conversion of conventional distribution coefficients (Kd) to normalized (Ka) values, which improves comparability among the results of different experiments. The present review gives a detailed summary of sorption data of Np on clays and clay minerals and examines critically the applicability of the Ka approach.

The pH dependence of Am(III) complexation with acetate: an EXAFS study

The complexation of acetate with Am(III) is studied as a function of the pH (1-6) by extended X-ray absorption fine-structure (EXAFS) spectroscopy. The molecular structure of the Am(III)-acetate complexes (coordination numbers, oxygen and carbon distances) is determined from the raw k(3)-weighted Am LIII-edge EXAFS spectra. The results show a continuous shift of Am(III) speciation with increasing pH value towards the complexed species. Furthermore, it is verified that acetate coordinates in a bidentate coordination mode to Am(III) (Am-C distance: 2.82 ± 0.03 Å). The EXAFS data are analyzed by iterative transformation factor analysis to further verify the chemical speciation, which is calculated on the basis of thermodynamic constants, and the used structural model. The experimental results are in very good agreement with the thermodynamic modelling.

The Complexation of Cm(III) with Succinate Studied by Time-Resolved Laser Fluorescence Spectroscopy and Quantum Chemical Calculations

The complexation of Cm(III) with succinate in an aqueous NaCl solution was studied as a function of ionic strength, ligand concentration, and temperature using time-resolved laser fluorescence spectroscopy (TRLFS). After the Cm(III) speciation was determined by peak deconvolution, the temperature-dependent thermodynamic stability constants (log Kn0(T)) were determined for the stepwise formation of [CmSucn]3–2n (n = 1–3) in the temperature range 20–80 °C (n = 3 only when T ≥ 50 °C) using the specific ion interaction theory (SIT). The first and second complexation steps show an endothermic behavior, as the respective standard reaction enthalpies (ΔrHm0) and entropies (ΔrSm0) derived from the integrated van’t Hoff equation are positive. These TRLFS results are complemented by quantum chemical calculations to resolve the molecular structure of the formed Cm(III) complexes. The results show that the formation of a seven-membered chelate ring is the favored conformation of [CmSucn]3−2n (n = 1−3).

Combined EXAFS Spectroscopic and Quantum Chemical Study on the Complex Formation of Am(III) with Formate

The complexation of Am(III) with formate in aqueous solution is studied as a function of the pH value using a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy, iterative transformation factor analysis (ITFA), and quantum chemical calculations. The Am LIII-edge EXAFS spectra are analyzed to determine the molecular structure (coordination numbers; Am-O and Am-C distances) of the formed Am(III)-formate species and to track the shift of the Am(III) speciation with increasing pH. The experimental data are compared to predictions from density functional calculations. The results indicate that formate binds to Am(III) in a monodentate fashion, in agreement with crystal structures of lanthanide formates. Furthermore, the investigations are complemented by thermodynamic speciation calculations to verify further the results obtained.