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The retention of actinides in different oxidation states (An(X), X = III, IV, VI) by a calcium-silicate-hydrate (C-S-H) phase with a Ca/Si (C/S) ratio of 0.8 was investigated in the presence of gluconate (GLU). The actinides considered were Am(III), Th(IV), Pu(IV), and U(VI). Eu(III) was investigated as chemical analogue for Am(III) and Cm(III). In addition to the ternary systems An(X)/GLU/C-S-H, also binary systems An(X)/C-S-H, GLU/C-S-H, and An(X)/GLU were studied. Complementary analytical techniques were applied to address the different specific aspects of the binary and ternary systems. Time-resolved laser-induced luminescence spectroscopy (TRLFS) was applied in combination with parallel factor analysis (PARAFAC) to identify retained species and to monitor species-selective sorption kinetics. 13C and 29Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were applied to determine the bulk structure and the composition of the C-S-H surface, respectively, in the absence and presence of GLU. The interaction of Th(IV) with GLU in different electrolytes was studied by capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS). The influence of GLU on An(X) retention was investigated for a large concentration range up to 10−2 M. The results showed that GLU had little to no effect on the overall An(X) retention by C-S-H with C/S of 0.8, regardless of the oxidation state of the actinides. For Eu(III), the TRLFS investigations additionally implied the formation of a Eu(III)-bearing precipitate with dissolved constituents of the C-S-H phase, which becomes structurally altered by the presence of GLU. For U(VI) sorption on the C-S-H phase, only a small influence of GLU could be established in the luminescence spectroscopic investigations, and no precipitation of U(VI)-containing secondary phases could be identified.

The uptake of Pu(IV) by hardened cement paste (HCP) at degradation stage I was investigated in the absence and presence of gluconate (GLU). Furthermore, the influence of the ionic strength was examined by different background electrolytes. For low ionic strength (I = 0.3 M) artificial cement pore water (ACW, pH = 13) and for high ionic strength (I = 2.5 M) cement pore water based on the diluted caprock solution (ACW-VGL, pH = 12.5) were used. Sorption experiments were performed under Ar atmosphere using HCP in the binary system HCP / GLU ([GLU]0/M = 1×10-1 - 1×10-8) and the ternary system HCP / Pu(IV) / GLU ([239Pu(IV)]0/M = 1×10-8, [GLU]0/M = 1×10-2) with S/L of 0.1 – 50 g L-1 within a contact time of 72 h. GLU sorbs strongly on HCP, a saturation of the sorption sites of HCP with GLU was observed at [GLU] ≥ 1×10-4 M at S/L = 5 g L-1. The effects of the order of addition of the components Pu(IV) and GLU showed a low sorption of Pu(IV) to HCP, indicating strong kinetic effects. In the absence of GLU, quantitative uptake (S% = 99%) of Pu(IV) by HCP was observed. GLU had a significant effect on the sorption of Pu(IV) to HCP in both background electrolytes. Powder samples from batch experiments were investigated by X-ray absorption spectroscopy to determine the structural parameters of the near-neighbour environment of sorbed Pu ([Pu(III)]0/M = 5×10-6) on HCP (S/L = 2.5 g L-1) in dependence of the ionic strength of the background electrolyte (ACW and ACW-VGL) and in presence of GLU ([GLU]0/M = 1×10 2). Pu LIII-edge X-ray absorption near-edge structure spectra confirm the presence of Pu(IV) as the dominant species on HCP. Plutonium was incorporated mainly into the calcium silicate hydrates phases (C-S-H) of HCP. Also, the ionic strength of the background electrolytes, as well as the presence of GLU do not influence the speciation of Pu on HCP during the degradation state I in cement-based repository.

In deep geological repositories for radioactive waste, interactions of radionuclides with mineral surfaces occur under complex geochemical conditions involving complex solution compositions and high pH resulting from degradation of cementitious geo-engineered barriers. Ca2+ cations have been hypothesized to play an important role as mediators for the retention of U(VI) on Ca-bentonite at (hyper)alkaline conditions, despite the anionic character of both the mineral surface and the aqueous uranyl species. To gain deeper insight into this sorption process, the effect of Ca2+ on U(VI) and Np(VI) retention on alumosilicate minerals has been comprehensively evaluated, using batch sorption experiments and time-resolved laser-induced luminescence spectroscopy (TRLFS). Sorption experiments with Ca2+ or Sr2+ and zeta potential measurements showed that the alkaline earth metals sorb strongly onto Ca bentonite at pH 8–13, leading to a partial compensation of the negative surface charge, thereby generating potential sorption sites for anionic actinyl species. U(VI) and Np(VI) sorption experiments in the absence and presence of Ca2+ or Sr2+ confirmed that these cations strongly enhance radionuclide retention on kaolinite and muscovite at pH ≥ 10. Concerning the underlying retention mechanisms, site-selective TRLFS provided spectroscopic proof for two dominating U(VI) species at the alumosilicate surfaces: (i) A ternary U(VI) complex, where U(VI) is bound to the surface via bridging Ca cations with the configuration surface ≡ Ca – OH – U(VI) and, (ii) U(VI) sorption into the interlayer space of calcium (aluminum) silicate hydrates (C-(A-)S-H), which form as secondary phases in the presence of Ca due to partial dissolution of alumosilicates under hyperalkaline conditions. Consequently, the present study confirms that alkaline earth elements, which are ubiquitous in geologic systems, enable strong retention of hexavalent actinides on clay minerals under hyperalkaline repository cond

Plutonium plays an important role within nuclear waste materials because of its long half-life and high radiotoxicity. The aim of this study was to investigate with high spatial resolution the reactivity of the more oxidized forms of Pu(V,VI) within Opalinus Clay (OPA) rock, a heterogeneous, natural argillaceous rock considered as a potential repository host. A combination of synchrotron based X-ray microprobe and bulk techniques was used to study the spatial distribution and molecular speciation of Pu within OPA after diffusion and sorption processes. Microscopic chemical images revealed a pronounced impact of geochemical heterogeneities concerning the reactivity of the natural barrier material. Spatially resolved X-ray absorption spectroscopy documented a reduction of the highly soluble Pu(V,VI) to the less mobile Pu(IV) within the argillaceous rock material, while bulk investigations showed second-shell scattering contributions, indicating an inner-sphere sorption of Pu on OPA components. Microdiffraction imaging identified the clay mineral kaolinite to play a key role in the immobilization of the reduced Pu. The findings provide strong evidence that reduction and immobilization do not occur as linked processes on a single reactive phase but as decoupled, subsequent, and spatially separated reactions involving different phases of the OPA.