Impact of formate, citrate and gluconate on the uptake of An(III)/Ln(III) and An(IV) by cement

In the context of low and intermediate-level nuclear waste (L/ILW), cementitious materials are widely used for the conditioning and storage of the waste, as well as for the construction of engineered barrier systems. The main solid phases in hydrated cements can retain radionuclides present in the waste, thus preventing or slowing down their release from the disposal site. Soluble small organic molecules are expected in the repository as a component of the emplaced waste, as degradation products of organic components disposed of, but also as cement additives. The potential interaction of these organic ligands with actinides may importantly alter both their solubility and sorption properties. This study aims to provide a quantitative description of the interaction of low molecular weight organics (formate, citrate and gluconate) with selected cement phases, i.e., AFm, ettringite, and C-S-H phases. The impact of these organic ligands on the uptake of Eu(III), Cm(III), and Pu(III/IV) by cement and individual cement phases is investigated using a combination of sorption experiments and advanced spectroscopic techniques.
Sorption experiments were performed with the synthesized cement phases in the presence of formate, citrate, or gluconate at 10^-4 M ≤ [L] ≤ 0.1 M and at pH ≈ 13.3. ... mehrThe samples were filtered with 0.45 μm nylon filters and dried in a desiccator (37% relative humidity) for 14 days. The separated solid phases were characterized by XRD, TGA, and FT-IR analyses. Supernatant solutions were characterized after filtration by ICP-OES (Ca, Al, Si, Na, sulfate) and NPOC (total organic content). Sorption experiments with 152^Eu + nat^Eu and 242^Pu were carried out in the absence and presence of organic ligands by varying the S:L ratio, order of addition, and [L]_tot. In the sorption experiments with plutonium, redox conditions were buffered with hydroquinone (HQ, pe + pH ≈ 9) or Sn(II)Cl2 (pe + pH ≈ 1.5). Phase separation was achieved by ultrafiltration via 10 kDa filters. The measurement of 242^Pu and 152^Eu was carried out using ICP-MS and gamma counter analysis, respectively. TRLFS investigation was conducted exclusively on the Cm(III)-C-S-H-GLU system.
Formate uptake was found to be very weak (R_d ≈ 10^-3 m^3∙kg^-1) for all investigated cement phases (AFm, ettringite, and C-S-H) [1]. Significantly higher Rd values were quantified for the uptake of citrate and gluconate. The uptake of citrate and gluconate by C-S-H phases with Ca/Si = 0.8-1.4 was found to be relatively high with R_d ≈ 10^-3 - 10^-2 m^3∙kg^-1. Furthermore, both gluconate and citrate show a systematic increase in log R_d values with increasing Ca/Si ratio. This observation hints towards a Ca complexation-mediated process at the surface of C-S-H phases, as also validated in the study of Androniuk et al. [2]. Formate and citrate have a negligible impact on the uptake of Pu(III/IV) and Eu(III) by C-S-H phases for both redox-buffer systems. In line with previous studies with Th(IV) and Eu(III) [3], gluconate significantly decreases the uptake of Pu(III/IV) and Eu(III) by C-S-H phases. These observations are explained by the possible formation of stable ternary / quaternary complexes in the aqueous phase, i.e. Ca-An(IV)-OH-GLU and Ca-An(III)/Ln(III)-OH-GLU [4]. TRLFS confirmed the key role of Ca in the formation of quaternary aqueous complexes Ca-An(III)/Ln(III)-OH-GLU, whereas at low ligand concentrations, the incorporation of Cm(III) in the C-S-H structure was observed. This experimental study was complemented with molecular dynamics calculations on the binary and ternary systems defined by C-S-H / Eu(III) / GLU, which provide key insights for the mechanistic understanding of the retention processes in cementitious systems.
References
[1] E. Wieland, A. Jakob, J. Tits, B. Lothenbach, D. Kunz, Applied Geochemistry, 67 (2016) 101-117.
[2] I. Androniuk, C. Landesman, P. Henocq, A.G. Kalinichev, Physics and Chemistry of the Earth, Parts A/B/C, 99 (2017) 194-203.
[3] E. Wieland, Paul Scherrer Institute (PSI), 2014.
[4] A. Tasi, X. Gaona, T. Rabung, D. Fellhauer, J. Rothe, K. Dardenne, J. Lützenkirchen, M. Grivé, E. Colàs, J. Bruno, Applied Geochemistry, 126 (2021) 104862.
Acknowledgement
The EURAD-CORI project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 847593.