Interaction of trivalent lanthanides and actinides with aluminium oxides and hydroxides

The subject of the present work is the interaction of trivalent actinides Cm(III) and lanthanides (Eu(III) and Gd(III)) with aluminium oxides and -hydroxides, namely corundum/sapphire (α-Al₂O₃), gibbsite (α-Al(OH)₃) and bayerite (β-Al(OH)₃), as well as the polyoxocations ''Al₁₃'' (Al₁₃O₄(OH)₂₄(H₂O)₁₂⁷⁺) and ''GaAl₁₂'' (GaO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺). These investigations should contribute to a sound understanding of the solution-mineral-interface reactions relevant for the migration of radionuclides (in particular the actinides) in the near and far field of a repository for high-level radioactive waste in deep geological formations, e.g. adsorption and structural incorporation. Batch sorption studies show similar logK_d values for Eu(III) sorption onto corundum and bayerite at low metal ion concentrations ([M(III)] ∝ 7.10^-7 M). These similarities can be related to the formation of Al-OH groups on the aluminium oxide surfaces. Comparable logK_d values could be found in earlier investigations for Gd(III) sorbed onto γ-Al₂O₃. In agreement with the batch sorption results at trace metal ion concentrations, time resolved laser fluorescence spectroscopy (TRLFS) studies on the interaction of Cm(III) with corundum and bayerite suggest a similar Cm(III) speciation between pH = 3 and 13. Three different surface sorbed Cm(III) species can be distinguished for these mineral phases ([surface..Cm(OH)_x(H₂O)_5-x]^3-x with x = 0, 1, 2). Comparable TRLFS results can also be found for Cm(III) sorbed on the sapphire (001) single crystal surface and on ''Al₁₃'' and ''GaAl₁₂''. In the Cm(III)/gibbsite system, spectroscopic results show clear differences in comparison to bayerite, corundum and γ-Al₂O₃, e.g. a Cm(III) species of the type gibbsite-Cm(H₂O)₅ is absent in the TRLFS spectra. This result can be related to a higher IEP for gibbsite, leading to a sorption edge, which is shifted to higher pH values. As a result, the first hydrolysed Cm(III) surface species is formed directly. In addition to the surface sorbed Cm(III) species, the existence of an incorporated Cm(III) species can be verified by TRLFS, which is formed as a consequence of Al(OH)₃ precipitation due to a high Al(III) concentration at intermediate pH values. For the sapphire (110) single crystal surface, the TRLFS results indicate a different Cm(III) coordination compared to the (001) surface. Except for Cm(III) bound to the sapphire (110) crystal surface, the batch and TRLFS results point to a similar sorption coordination of M(III) on the aluminium(hydr)oxide surfaces. In agreement with these investigations, EXAFS results indicate the presence of comparable binding sites on the basal or edge surfaces of gibbsite and bayerite. In these sorption environments, the cation is coordinated by two or three aluminol groups of the mineral surface (bi- and tridentate coordination). In the case of corundum, the spherical particle structure and the absence of well-established crystal planes inhibits an analogue assignment. In addition, the EXAFS studies show the principle binding of M(III) to different sorption sites, leading to several M(III) species existing simultaneously in different coordination environments. In summary, the results show that a combination of batch experiments and different spectroscopic methods is necessary to investigate processes occurring at the water-mineral-surface. pH-dependent Cm(III) TRLFS investigation show different surface sorbed M(III) species on the aluminium oxides/-hydroxides, while the Gd(III)-EXAFS data indicate the presence of M(III) bound to different sorption sites. Besides M(III) sorption onto the mineral surface, also incorporation processes are possible.