Adsorption reactions on mineral surfaces are influenced by the overall concentration of the adsorbing metal cation. Different site types (strong vs. weak ones) are often included to describe the complexation reactions in the various concentration regimes. More specifically, strong sites are presumed to retain metal ions at low sorbate concentrations, while weak sites contribute to metal ion retention when the sorbate concentration increases. The involvement of different sites in the sorption reaction may, thereby, also be influenced by competing cations, which increase the overall metal ion concentration in the system. To date, very little is known about the complex structures and metal ion speciation in these hypothetical strong- and weak-site regimes, especially in competing scenarios. In the present study, we have investigated the uptake of the actinide americium on corundum (α–Al2O3) in the absence and presence of yttrium as competing metal by combining extended X-ray absorption fine structure spectroscopy (EXAFS) with density functional theory (DFT) calculations. Isotherm studies using the radioactive 152Eu tracer were used to identify the sorption regimes where strong sites and weak sites contribute to the sorption reaction. The overall americium concentration, as well as the presence of yttrium could be seen to influence both the amount of americium uptake by corundum, but also the speciation at the surface. More specifically, increasing the Am3+ or Y3+ concentrations from the strong site to the weak site concentration regimes in the mineral suspensions resulted in a decrease in the overall Am–O coordination number from nine to eight, with a subsequent shortening of the average Am–O bond length. DFT calculations suggest a reduction of the surface coordination with increasing metal–ion loading, postulating the formation of tetradentate and tridentate Am3+ complexes at low and high surface coverages, respectively.

The coordination chemistry of Cm(III) with aqueous phosphates was investigated by means of laser-induced luminescence spectroscopy and ab initio simulations. For the first time, in addition to the presence of Cm(H2PO4)2+, the formation of Cm(H2PO4)2+ was unambiguously established from the luminescence spectroscopic data collected at various H+ concentrations (-log10 [H+] = 2.52, 3.44, and 3.65), ionic strengths (0.5-3.0 mol·L-1 NaClO4), and temperatures (25-90 °C). Complexation constants for both species were derived and extrapolated to standard conditions using the specific ion interaction theory. The molal enthalpy ΔRHm0 and molal entropy ΔRSm0 of both complexation reactions were derived using the integrated van't Hoff equation and indicated an endothermic and entropy-driven complexation. For the Cm(H2PO4)2+ complex, a more satisfactory description could be obtained when including the molal heat capacity term. While monodentate binding of the H2PO4- ligand(s) to the central curium ion was found to be the most stable configuration for both complexes in our ab initio simulations and luminescence lifetime analyses, a different temperature-dependent coordination to hydration water molecules could be deduced from the electronic structure of the Cm(III)-phosphate complexes. More precisely, where the Cm(H2PO4)2+ complex could be shown to retain an overall coordination number of 9 over the entire investigated temperature range, a coordination change from 9 to 8 was established for the Cm(H2PO4)2+ species with increasing temperature.

Thermodynamic databases are essential for the safety assessments of radioactive waste repositories. They have to be reliable, comprehensive, and describe the key mechanisms controlling the mobility of contaminants in the environment. However, in many cases these prerequisites are not fulfilled. An important example is the complexation of actinides with aqueous phosphates, for which this work provides complexation constants for spectroscopically identified species at 25 °C and at elevated temperature. The complexation of Cm(III) was studied at sub micromolar concentrations by laser induced luminescence spectroscopy as a function of total phosphate concentration (0-0.06 M ΣPO4) in the temperature range 25-90°C, using NaClO4 as a background electrolyte at –log[H+] ranging from from 2.5 to 3.6. The formation of both CmH2PO42+ and Cm(H2PO4)2+ complexes was revealed, the latter being spectroscopically evidenced for the first time. Complexation constants were found to increase when raising the ionic strength from 0.5 to 3.0 M. Temperature-dependent (25 to 90 °C) complexation constants for the identified species were derived, and were recalculated to standard conditions with the van´t Hoff equation and the Specific Ion Interaction Theory. Endothermic and entropy driven reactions were established for both complexes. In addition, relativistic quantum chemical investigations were performed to study the complexation strength of Cm(III) with aqueous phosphates and to provide insight in potential changes of the coordination number with increasing temperature and to probe the character of the Cm water and Cm phosphate bonds.

This study investigates the retention of trivalent actinides (Cm3+) and their lanthanide analogues (Eu3+) on monoclinic zirconia (ZrO2), a solid phase known to form on the zircaloy cladding material surrounding spent nuclear fuel (SNF) rods. Two zirconia solids with varying carbon content were utilized. The influence of carbon impurities on the ZrO2 surface charge was investigated via zeta-potential measurements. Batch data was collected for various Eu3+ concentrations, while the Cm3+ surface speciation was studied on the molecular level with laser spectroscopy (TRLFS). The spectroscopic sorption data was modelled using the Diffuse Double Layer (DDL) model. The ZrO2 surface charge measurements yielded a pH(IEP) of 6 which was influenced by the presence of inorganic and organic carbon species. The pH-dependent uptake of Eu3+ showed a maximum sorption above pH 5.5, with no impact of the carbon concentration. The speciation of the trivalent metal, however, was different in the presence of intrinsic organic carbon in the sample, resulting in the formation of an organic Cm3+-complex on the surface. This ternary complex was absent on the ZrO2 material with low carbon content. Here, the surface speciation was dominated by Cm3+ and Cm3+-hydrolysis complexes which could be well described by our DDL model.