The An(III)-An(IV)-H2O(l) system: solid phase, interface and aquatic chemistry

Due to the electronic configurations, several oxidation states of the early actinides (+III to +VII; An: Th, U, Np, Pu, Am) can exist in aqueous solution, which results in unique chemical behaviour as a function of the redox boundary conditions. In underground repositories for nuclear waste disposal, the corrosion of iron occurring after the closure of the repository and possible water access will promote reducing conditions, for which the oxidation states +III and +IV are expected to control the solution chemistry of the actinides. In aqueous systems, An(IV) behavior is dominated by the formation of sparingly soluble, nanoparticulate and amorphous hydrous oxides, AnO2(am, hyd), and a strong tendency towards hydrolysis. The transition of these amorphous oxy-hydroxides into the thermodynamically stable crystalline oxides AnO2(cr), is kinetically hindered due to their low solubility, and is generally not observed in aqueous systems. However, ageing processes induced by time or temperature may facilitate this transition with the consequent decrease of the overall solubility.
The impact of temperature on a freshly precipitated ThO2(am, hyd) solid phase was investigated using a combination of undersaturation solubility experiments and a multi-method approach for the characterization of the solid phase, including XRD, XAFS, TG-DTA and XPS. ... mehrThe results presented in this contribution will elaborate on the interlink between solubility, structure, surface chemistry and thermodynamics in the ThO2(s, hyd)–H2O(l) system, with special emphasis on the transformation of the amorphous hydrous / hydroxide solid phases into the thermodynamically stable more crystalline oxides. Analogies and differences with the PuO2(s, hyd)–H2O(l) system will also be discussed.
The stability of the Pu(OH)3(s) solid phase in alkaline reducing systems has been controversially discussed in the literature. The current thermodynamic selection in the NEA-TDB Update volume supports the stability of this Pu(III) solid phase above the border of water reduction (Grenthe et al., 2020), although the co-existence of PuO2(s) and Pu(OH)3(s), or alternatively the formation of a mixed valence PuO2-x(s) phase, have been proposed at (pe + pH) ≈ 2 based on advanced spectroscopic methods (Tasi et al., 2018). This contribution will elaborate on the interplay between PuO2(s) and Pu(OH)3(s), and further on the coupling with Fe(0) chemistry and corresponding corrosion products under conditions potentially relevant for nuclear waste disposal.
From a experimentalist’s perspective, this contribution will emphasize the key role of theory in the field of actinide chemistry, and further elaborate on the importance of the interplay and continuous communication between theoretical and experimental groups.
References:
Grenthe, I., Gaona, X., Plyasunov, A.V., Rao, L., Runde, W.H., Grambow, B., Konings, R. J. M., Smith, A.L., Moore, E.E. (2020). Second Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium, Chemical Thermodynamics. OECD Nuclear Energy Agency, Boulogne-Billancourt, France.
Tasi, A., Gaona, X., Fellhauer, D., Böttle, M., Rothe, J., Dardenne, K., Schild, D., Grivé, M., Colàs, E., Bruno, J., Källström, K., Altmaier, M., Geckeis, H. (2018). Radiochim. Acta, 106, 259–279.