Eu(III) and Cm(III) complexation by nitrilotriacetic acid to further evaluate its impact on the radionuclide retention by cementitious phases

Aminopolycarboxylates (APCs) show great complexation potential towards (lanthanide and actinide) metal ions. As such they are often used as decontamination or decorporation agents. Especially trivalent actinides are of great interest, due to their prevalence in spent nuclear fuel. Accurate thermodynamic data on this complexation behavior is key for safety assessments of nuclear waste repositories. In a worst-case scenario – a groundwater intrusion into the repository a (re-)mobilization of radionuclides (RNs) is to be avoided. Low molecular weight organic ligands may however alter the retention potential of the repository relevant solid phases towards the RNs. A ligand of interest is nitrilotriacetic acid (NTA), which is a typical representative of the APCs. It has been previously shown that it forms complexes with trivalent RNs such as Eu(III)[1] and Am(III)[2] and is used as a decontaminating agent. This work focuses on Eu(III) as a nonradioactive analog to some trivalent actinides with outstanding luminescence properties which make it a great probe for time-resolved laser-induced fluorescence spectroscopy (TRLFS) study.
This work utilizes a multi-method approach with nuclear magnetic resonance (NMR) spectroscopy, TRLFS and isothermal titration calorimetry (ITC) to gain accurate and reliable thermodynamic and spectroscopic data on the Eu(III)-NTA system. NMR spectroscopic experiments showed three distinct complexes, which could be attributed to a 1:1, a 1:2 and a 2:2 Eu(III)-NTA complex, the latter of which existing only at increased concentrations. This observation could be confirmed by TRLFS[3]. Complex formation constants were determined from pH and concentration series applying TRLFS. TRLFS data were evaluated using parallel factor analysis as described elsewhere[4]. Verification of those log β values as well as information about the reaction enthalpy ΔH, the reaction entropy ΔS and the Gibbs free energy ΔG were obtained via ITC measurements.
To confirm the proposed similarities in thermodynamic data for complex formation, similar experiments have been conducted with Cm(III). The formation of the 1:1 and the 1:2 complex could be confirmed with log β values similar to Eu(III).
The retention of Eu(III) on calcium aluminum silicate hydrate (C-A-S-H) phases was observed using batch experiments. Preliminary results have shown little to no impact of NTA on the Eu(III) retention. This may be explained by the high concentration of Al(III) and Ca(II) ions in the supernatants of the samples, as NTA readily complexes these ions as well.
Acknowledgement: The German Federal Ministry for Economic Affairs and Energy (BMWi) is thanked for financial support within the GRaZ II project, no. 02E11860B.

[1] Choppin, G. R. et al. (1977). The complexation of lanthanides by aminopolycarboxylate ligands - II. J. Inorg. Nucl. Chem. 39: 2025-2030
[2] Akram, N. and Bourbon, X. (1995). Analyse critique de donnees thermodynamiques: pouvoir complexant de l'EDTA, du NTA, du citrate et de l'oxalate vis a vis de cations metalliques. Etudes Experimentations Calculs. Andra.
[3] Sieber, C. et al. (2023). Eu(III) and Cm(III) complexation of NTA, EDTA, and EGTA studied by means of NMR, TRLFS, and ITC – an improved approach to more robust thermodynamics. in preparation
[4] Drobot, B. et al. (2015). Combining luminescence spectroscopy, parallel factor analysis and quantum chemistry to reveal metal speciation–a case study of uranyl (VI) hydrolysis. Chem. Sci. 6: 964-972