UV-vis spectroscopy of Eu(III) and Am(III) complexation with small organic acids at low concentrations and variable temperatures

The long-term safety analysis of nuclear waste disposals in clay formations requires detailed knowledge of the interaction of actinides at relevant temperatures (up to 100 °C). It is well known that the migration behaviour of the actinides is strongly influenced by natural organic matter (NOM) from the clay. However, the current thermodynamic database is lacking fundamental data about the temperature-dependent complexation of americium (and its analogue europium) with small organic acids, which are structural similar components of natural organic matter in the clay.

We present our results for the complexation of both Am(III) and Eu(III) with 2-hydroxybenzoic acid (salicylic acid), 1,2,4,5-benzentetracarboxylic acid (pyromellitic acid) and 2,3-dihydroxbutanedioic acid (L-tartaric acid) in the temperature range from 20 to 50°C, at ionic strength of 0.1 mol/l NaClO4 and pH below 6.

For the first time, temperature-dependent UV-Vis spectroscopic measurements with a Liquid Waveguide Capillary Cell were used for complexation investigations. Due to the long optical path length of the capillary cell (up to 2 m) very low americium and europium concentrations can be measured. Thus, the presented absorption measurements were carried out at Am(III) concentrations below 5e-7 mol/l and Eu(III) concentration below 5e-4 mol/l. For all investigated model ligands we observed a slight shift of the absorption maxima of Eu(III) (394 nm) and Am(III) (504 nm) spectra of 0.5 to 2 nm to higher wavelengths due to the complexation.
We first validated the method with europium: at 25°C the determined constants for the 1:1 complexes of europium salicylate, europium pyromellitate and europium L tartrate with logß111=15.4+/-0.06, logß112 = 14.3 +/-0.2, logß111=7.9+/-0.3 respectively, are in good agreement with the literature values [1], [2].
For the americium salicylate system, both a 1:1 complex and a 1:2 complex were observed at pH 4. The stability constants of these complexes are logß111= 15.42+/-0.06 and logß122=30.98+/-0.05 and comparable to the reported ones for the analogous complexes with europium [1]. With increasing temperature, the stability constants for both complex systems increase, indicating an endothermic complexation reaction. The same behaviour is expected for the americium pyromellitate system, where just a 1:1 complex with log ß112=15.03+/-0.1 can be reported until now.

The temperature dependence and the interaction of americium with L-tartrate is presently under investigation. The stability constants and thermodynamic parameters (enthalpy, entropy, Gibbs energy) of all above investigated systems are reported.

[1] Aoyagi, N. et al. Radiochemica Acta, Vol. 92, 2004, 589-593
[2] R. Kulshrestha, S. Sengar, M. Singh, Indian J. of Chemistry 26A, 1987, 940-943