EDTA is a strong complexing ligand used for decontamination purposes in nuclear reactors and clean-up operations that is eventually co-disposed together with radioactive waste and often exists as a subsurface co-contaminant. Complexation of actinides(An) with strong organic ligands like EDTA forms stable aqueous species which can significantly affect the potential retention/mobilization processes in geochemical environments by enhancing the solubility limit and altering the sorption behavior on mineral surfaces. An accurate knowledge of this aquatic chemistry and, more generally, the thermodynamics of actinides are fundamental needs that support nuclear waste disposal concepts. The availability of complete and correct chemical and thermodynamic models helps provide reliable geochemical model calculations and source-term estimations for the safety assessment of nuclear waste repositories. The present work focusses on An(IV) complexation with EDTA and the effect of these complexes on the long-term solubility and the redox distribution of actinides under repository relevant environments. The solubility of An(IV) in the presence of EDTA was investigated from both undersaturation and oversaturation conditions. Experiments were performed in an inert gas atmosphere with &lt; 2 ppm O2 at T = (22 ± 2) °C. Oversaturation solubility experiments were conducted with [An]0 = 10–3 M, whereas undersaturation solubility experiments were performed with approximately 3 mg of AnO2(am,hyd). Strongly reducing conditions (pH + pe &lt; 2) were fixed in each independent solubility sample with SnCl2. Background electrolyte solutions were prepared with [EDTA] = 0.01 M, constant ionic strength I = 0.5 and 5.0 M Na4EDTA–NaCl–HCl–NaOH, and covering the pHm range 0–13 (with pHm = –log [H+]). An concentration, pHm and Eh values were monitored at regular time intervals. The results obtained on the impact of EDTA on the aqueous speciation and long-term solubility of An(IV) will be presented in this contribution and discussed in the context of the presence/absence of ternary An(IV)-EDTA-OH species.

Effect of EDTA on An(IV) chemistry under repository relevant conditions

256th National Meeting (2018)

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