Cerium(III/IV) aquatic geochemistry: what can be learnt from actinide analogues?

International audienceCerium (Ce) is the most abundant rare earth element (REE) in the Earth’s crust and is used for many applications (technical, medical, catalytic, etc, [1]). By contrast with its REE neighbors that are most stable under the +III oxidation state, Ce can be found as Ce(III) or Ce(IV) under environmentally relevant condition, which has drastic consequence on Ce aqueous chemistry, solubility and sorption to natural particles and colloids, mobility and bioavailability. Geochemical speciation modeling might be a helpful tool to understand Ce speciation. Unfortunately, huge gaps in Ce aqueous thermodynamic databases (especially for Ce(IV) hydrolysis and solubility) prevent us from using accurately such approach. f-elements (REE (5f) and actinides (An) (6f)) show rather consistent chemistry under the same oxidation state, and are often considered as chemical analogues. Many studies where dedicated to An(IV) (An = Th, U, Np, Pu) hydrolysis and solubility because they are key elements in the field of nuclear waste disposal. In particular, Pu might be a relevant actinide analogue for Ce aqueous chemistry. Indeed, effective ionic radii of Ce4+ and Pu4+ are comparable and we found that solubility product of CeO2(cr) is highly consistent with that of AnO2(cr) [2].In this study, we estimated Ce(III/IV) aqueous (including redox) speciation on the basis of REE(III), An(IV) and Pu(III/IV) behavior. Moreover, using a recently developed approach to predict sorption and redox speciation of Pu at mineral surfaces [3], it was possible to predict available laboratory data of Ce sorption to manganese oxide, which is known as efficient sorbent and oxidant for Ce(III) [4]. Although preliminary, these results are very promising and might help to predict Ce speciation in naturally relevant conditions.References:[1] Dahle, J.T., Arai, Y., (2015): Environmental Geochemistry of Cerium: Applications and Toxicology of Cerium Oxide Nanoparticles, Int. J. Environ. Res. Public Health 12, 1253-1278.[2] Neck, V., Altmaier, M., Müller, R., Bauer, A., Fanghänel, Th., Kim, J.I., (2003): Solubility of crystalline thorium dioxide, Radiochim. Acta 91, 253–262. [3] Banik, N.L., Marsac, R., Lützenkirchen, J., Diascorn, A., Bender, K., Marquardt, C.M., Geckeis, H., (2016): Sorption and Redox Speciation of Plutonium at the Illite Surface. Environ. Sci. Technol. 50, 2092−2098.[4] Ohta, A., Kawabe, I., (2001): REE(III) adsorption onto Mn dioxide (δ-MnO2) and Fe oxyhydroxide: Ce(III) oxidation by δ-MnO2, Geochim. Cosmochim. Acta 65, 695–703