Time resolved fluorescence spectroscopy of Eu(III) complexes with benzoic acid derivatives at different temperatures

Humic acids have a great influence on the migration of actinides and heavy metals. To get a quantitative understanding of the humic acid / actinide interaction on a molecular level, we investigate the complexation behavior of trivalent lanthanides and actinides with different functionalized benzoic acids as model compounds for humic acids. Our primary objective is to determine the temperature dependence of the complexation behavior, because in radioactive waste repositories one can observe elevated temperatures and concerning this matter only few investigations are known.

In this study, we present the reaction of Eu(III) with 2-hydroxybenzoate (salicylate), 2,5-dihydroxybenzoate and 1,2,4,5-benzenetetracarboxylate (pyromellitate), investigated with time-resolved laser-induced fluorescence spectroscopy (TRLFS) at different temperatures (10-50°C).

The Eu pyromellitate complex system shows strong luminescence behavior. The intensity of the 5D0 7F2 transition band increases with increasing ligand concentration. With increasing temperature, the total emission intensity decreases slightly. At lower pH (around 3) a MHL complex species with a stability constant of logK = 3.92 (25°C) could be determined. The lifetime of about 125 µs indicates, according to the equation of Kimura et al. (n(H2O) = 1.07 x (1/t -0.62) [1]), the exclusion of 1 water molecule from the first shell, implying an inner sphere complex with monodentate coordination. At pH around 5, a ML complex species (logK = 4.74, 25°C) was identified. The lifetime of about 135 µs stands for the substitution of 2 water molecules, according to an inner sphere complex with bidentate coordination. The lifetimes remain constant over the determined temperature range, as well as the lifetime of the free Eu3+(aq) ion. With increasing temperature, the stability constants increase, indicating an endothermic complexation reaction. The thermodynamic parameters of the Eu pyromellitate complexes were calculated to be ΔH = 9.4 kJ mol-1, ΔG = 22.3 kJ mol-1, ΔS = 106 J mol-1 K-1 (MHL; 25 °C) and ΔH = 15.2 kJ mol-1, ΔG = 28.1 kJ mol-1, ΔS = 145 J mol-1 K-1 (ML; 25 °C); these values are in good agreement with the literature [2]. The Eu complexes with salicylate and 2,5-dihydroxybenzoate show no luminescence behavior; only static and dynamic quenching is observable. For both ligands, the complex stability constants for a 1 : 1 complex species each could be determined. With increasing temperature, the stability constant also increases in both cases, indicating again an endothermic complexation reaction. The stability constants and thermodynamic parameters of these complexes are reported and compared with those determined by UV-vis spectroscopy with a liquid waveguide capillary cell (LWCC).

[1] T. Kimura, G.R. Choppin, Y. Kato, Z. Yoshida, Radiochim. Acta 72 (1996) 61-64.
[2] G.R. Choppin, E.N. Rizkalla, T.A. El-Ansi, A. Dadgar, J. Coord. Chem. 31 (1994) 297-304.