Cm(III) sorption onto sapphire (α-Al2O3) single crystals

In the present study, Cm(III) sorption onto different crystal planes (018), (104), (012), (110), and (001) of sapphire single crystals (area 1 cm(2)) is investigated at low Cm(III) concentrations and at pH 4.5 and 5.1 by time-resolved laser fluorescence spectroscopy (TRLFS), X-ray photoelectron spectroscopy (XPS), alpha-spectrometry, and autoradiography. A homogeneous Cm(III) distribution on the sapphire surfaces is demonstrated by autoradiography for all samples. Sorbed Cm(III) concentrations are in the range 2 x 10(-12) - 7 x 10(-11) mol/cm(2) or 0.02-0.4 atoms/nm(2). TRLFS and XPS spectra of sufficient intensity are obtained for all samples, proving the sensitivity of these techniques and their applicability at very low Cm(III) concentrations. TRLFS spectra of Cm(III) sorbed onto the (001) surface show distinct differences with regard to peak position and fluorescence lifetime compared to those obtained for the other four crystal planes. Similar information is available from XPS. The extent of Cm(III) sorption differs for the individual crystal planes, with the highest sorption taking place at the (001) orientation and the lowest at the (018) orientation. Similar TRLFS spectra for Cm(III) sorbed at the (001) plane and onto colloidal gamma-Al2O3 suggest the presence of similar surface species. Variations in the TRLFS and XPS spectra for Cm(III) sorbed at the (001) plane after high vacuum treatment and after water contact suggest an important influence of surface relaxation processes on the surface properties. Such an effect is much less pronounced for the other sapphire crystal planes.