Impact of nitrilotriacetic acid on Eu(III) retention by repository relevant solid phases

Radionuclide speciation inside long-term radioactive waste repositories needs to be
understood in order to ensure effective containment of the waste. Organic ligands
originating from the degradation of organic components inside such a repository can
possibly affect the mobility of radionuclides in solution. The present study focuses on
nitrilotriacetic acid, NTA, as a model molecule and europium, Eu(III), as a
nonradioactive analog with outstanding luminescence and magnetic properties.
The complexation of NTA with Eu(III) as a function of pH was studied using nuclear
magnetic resonance (NMR) spectroscopy. Samples were prepared with Eu(III) to NTA
ratios of 1:1 and 1:2 in D₂O with 1 M NaCl as background electrolyte to simulate high
ionic strength. The ¹H and ¹³C NMR spectra of the NTA solutions with Eu(III) show
clearly distinguishable signals for the free NTA and two Eu-NTA complexes, which is
indicative of a 1:1 and a 1:2 Eu-NTA complex. The interaction of Eu(III) with NTA is
relatively strong and favors the 1:2 Eu-NTA complex even in solution containing 1:1
Eu-NTA ratio, except in very acidic solutions. Time-resolved laser-induced
fluorescence spectroscopy (TRLFS) measurements yield quantitative information on
the complexation behavior.
As repository relevant cationic groundwater components, the influence of Ca(II) and
Al(III) on Eu(III) complexation with NTA is studied in detail with NMR spectroscopy.
This combination of NMR spectroscopy and TRLFS yields qualitative and quantitative
information on the coordination environment from the ligand’s and the metal ion’s
perspective, respectively. In subsequent studies focusing on ternary systems
comprising repository relevant solid phases (e.g. calcium aluminum silicate hydrate
(C-A-S-H) phases), radionuclides and organic ligands, this will allow the identification
of radionuclide speciation in solution and on solid phases.