Am(III) and Eu(III) Complexation with Small Organic Molecules at Elevated Temperatures – Studied by Spectroscopy, Calorimetry and Density Functional Theory

Argillaceous rocks, like the Opalinus clay formation (OPA) in Switzerland are potential host rocks for nuclear waste repositories. The OPA can contain up to 1% dissolved organic matter. It consists of small organic molecules like formiate, citrate or lactate. Such small organic molecules can like the ubiquitous humic acid influence the migration behaviour of radionuclides.

The understanding of the complex behaviour of radionuclides with such natural organic matter and the thermodynamic quantification of the interaction is of great importance to simulate and predict their migration behaviour in the environment. Additionally, it is crucial to study the complex behaviour of radionuclides at elevated temperatures, because especially in the near field of nuclear waste disposals higher temperatures are prevailing.

We investigated the complex behaviour of Am(III) complexes with lactate and substituted benzoic acids like pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid, BTC) which serve as model compounds for humic substances at ambient and elevated temperatures with time-resolved laser-induced fluorescence spectroscopy (TRLFS).

TRLFS has been extensively used as a sensitive and selective technique to analyze actinide complexation with inorganic and organic ligands in trace metal concentrations. However, the application of TRLFS onto Am(III) complex systems was up to now limited because of the much lower luminescence intensity and much shorter lifetime in comparison to U(VI) or Cm(III).

Using the emission of the 5D1-7F1 transition at around 690 nm, spectral data like luminescence lifetimes and maxima and complex stability constants were calculated. Temperature dependent stability constants were determined to estimate thermodynamic data (enthalpy, entropy).
The Am(III) aquo ion shows at pH 4-5 a luminescence lifetime of 23 ns, corresponding to approximately 9 coordinating water molecules. Complexation with BTC shows no change of the emission maximum but an increase of the luminescence intensity and lifetime. The luminescence lifetime was prolonged to 27 ns, corresponding to 8 remaining water molecules in the first coordination shell. This indicates an exchange of 1 water molecule with 1 coordination site of the ligand, resulting in an Am-BTC 1:1 complex. In contrast, complexation with lactate causes additionally a red shift of the luminescence maximum of about 5 nm. The luminescence lifetime is prolonged up to 37 ns which corresponds to 5-6 remaining water molecules, indicating an exchange of about 3-4 water molecules with coordination sites of ligand molecules which implies the formation of 1:1, 1:2 and 1:3 complexes. The stability constants increase with rising temperature which is consistent with an endothermic complexation reaction.

TRLFS investigations with the inactive lanthanide analogue Eu(III) and the same ligands resulted in similar complex behaviour of Am(III) and Eu(III). This fact permits performing further investigations concerning the complex behaviour of trivalent actinides at elevated temperatures exemplarily with Eu(III). Temperature-dependent isothermal titration calorimetry (ITC) shows with rising temperature a stepwise polymerization between Eu(III) and BTC. ATR FT-IR (attenuated total reflection Fourier transform infrared spectroscopy) in combination with DFT (density functional theory) calculations was used to determine main structural features of the monomer and the polymer. In principle, we found monodentate coordination of one carboxylate group of BTC to one Eu(III) with a small fraction of chelating coordination mode of two neighbouring carboxylate groups.

The results suggest that the migration of actinides will be strongly influenced by organic matter. Small organic molecules can enhance the mobility with rising temperature due to higher complexation ability. In contrast, complex organic materials like humic substances are able to immobilize radionuclides under certain conditions due to the possibility of polymerization.