Joint Project: Interaction and transport of actinides in natural clay rock with consideration of humic substances and clay organics - Investigations of temperature dependence of complexation and sorption of trivalent actinides (Am(III)) in the system actinide-NOM-natural clay rock-aquifer (Final Report BMWi Project No.: 02 E 10417)


Joint Project: Interaction and transport of actinides in natural clay rock with consideration of humic substances and clay organics - Investigations of temperature dependence of complexation and sorption of trivalent actinides (Am(III)) in the system actinide-NOM-natural clay rock-aquifer (Final Report BMWi Project No.: 02 E 10417)

Acker, M.; Barkleit, A.; Müller, M.; Schott, J.; Bernhard, G.

The main objective of this project was to study the interaction processes between An(III)/Ln(III), (representated by Am(III) and Eu(III), respectively), organic model ligands, and Opalinus Clay at elevated temperature (until 80°C). The thermodynamic data (log, ΔG, ΔH and ΔS) for the complexation and sorption processes have been derived and led to an enhancement of the thermodynamic database by data at elevated temperatures.

The complexation of Am(III)/Eu(III) with small organic ligands (pyromellitic, salicylic, lactic, acetic, citric, and tartaric acid) that serve as model ligands for natural organic material, like humic substances or clay organic was investigated by temperature dependent UV-Vis and time-resolved laser-induced fluorescence spectroscopy (TRLFS).
For the first time, the UV-Vis absorption spectroscopy utilizing a Long Path Flow Cell (LPFC) has been established for speciation analysis of Am(III) at trace concentrations. A detection limit of 5*10-9 mol/l-1 Am(III) was determined with an 2 m LPFC.
Several Am(III)/Eu(III)-organic ligand complexes were spectroscopically and thermodynamically characterized. General, all studied complexation reactions are endothermic and driven by entropy.
Furthermore, the interaction of Eu(III) with pyromellitc acid (1,2,4,5-benzene-tetracarboxylic acid) had been studied in detail with additional methods like isothermal titration calorimetry (ITC) and Fourier-transform infrared spectroscopy in combination with density function theory (DFT) calculations. At elevated temperature and higher concentration (> 5 mM Eu(III) and pyromellitic acid) a temperature-dependent polymerization was observed. It had been shown that predominantly chelating coordination with two carboxylic groups in the monomeric complex and monodentate binding of a single carboxylic group in the polymeric complex of the polycarboxylate with Eu(III) occurs.

The sorption of Eu(III) on Opalinus Clay was investigated in absence and presence of the small organic ligands citric and tartaric acid at different temperatures and under synthetic pore water conditions (I = 0.4 M, pH = 7.6) by batch experiments. Time-resolved laser-induced fluorescence spectroscopy was used for analysis of the Eu(III) speciation in the binary system Eu(III)-Opalinus Clay and ternary system Eu(III)-Opalinus Clay-small organic ligand under pore water conditions.
The Eu(III) sorption was found to increase generally with temperature in a considerably endothermic reactions with enthalpies of about 50 kJ/mol 1. In presence of tartrate or citrate the Eu(III) sorption decreases with increasing ligand concentration due to a complex formation of Eu(III) in solution. This complex formation was verified by TRLFS investigations. The detected Eu(III) surface species on Opalinus Clay with a luminescence lifetime of (201  9) µs is no influenced by the presence of the studied organic ligands.

Keywords: Americium; Europium; Opalinus clay; organic model ligands; complexation; sorption; elevated temperature; thermodynamics

  • Other report
    Dresden: TU Dresden, 2011
    49 Seiten

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Publ.-Id: 16462