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Sorption of trivalent Cm onto γ-Al 2 O 3  is investigated by time-resolved laser fluorescence spectroscopy (TRLFS). The pH dependent sorption of Cm(III) is followed at constant ionic strength of 0.1 M NaClO 4  under argon atmosphere in the pH range between 3.5 and 13.2 at a total Cm(III) concentration of 2.5×10 -7  mol/L. Three different surface-sorbed Cm(III) species appearing at different pH are identified by their emission spectra with peak maxima at 600.6 nm, 602.5 nm, and 605.7 nm using an excitation wavelength of λ ex =396.6 nm. No variation of the emission decay is observed for the three sorbed species (constant lifetimes of 110 μs). The pH dependent species distribution obtained from peak deconvolution of emission spectra is in agreement with earlier Eu/Cm sorption studies and sorbed species are assigned to &gt;Al−O−Cm(OH)  x   (2- x ) (H 2 O) 5- x   inner-sphere complexes. Indirect Cm excitation  via  energy transfer from the solid to the metal ion at λ ex =355 nm yields emission bands significantly shifted compared to spectra obtained at λ ex =396.6 nm. We interpret this observation as a hint for the existence of Cm inner-sphere complexes bound to different γ-Al 2 O 3  surface sites. The exact nature of those species is presently not known, but they likely involve Cm interaction with Al(OH) 3  phases at the solid surface which may even dominate the solid surface chemistry. Complementary EXAFS measurements are performed with Lu and Gd, taken as chemical homologues of the trivalent actinides. EXAFS for Gd/Lu(III) sorbed onto γ-Al 2 O 3  at pH=6 identifies a single oxygen coordination shell with a high degree of disorder. For Gd this shell consists of 9±1.5 atoms at a 2.45±0.02 Å distance. A coordination number of 7±1.5 and a Lu−O distance of 2.28±0.02 Å is found in the Lu-samples. Those values are quite close to what is known for the respective aquo ions. The large asymmetry in the coordination shell, however, suggests inner-sphere complexation. This is in agreement with TRLFS results showing clearly the existence of inner-sphere surface complexes where the first coordination shell of the trivalent metal ions consists of 5H 2 O/OH -  entities. Together with EXAFS data, a metal ion coordination to four oxygen atoms belonging to the γ-alumina surface is concluded. Both spectroscopic studies indicate a certain variability of surface binding sites for trivalent metal ions.

radiochimica acta

Cm(III) sorption onto γ-Al 2 O 3 : New insight into sorption mechanisms by time-resolved laser fluorescence spectroscopy and extended X-ray absorption fine structure