Formation of colloid-borne An(III): speciation of ligand competition in the An(III)-Al-Si-humic acid system by sensitized fluorescence spectroscopy

Full text of publication follows: Colloid-borne actinides undergo migration with little geo-chemical hindrance, thus increasing the mobility of actinides in natural aquifer systems. As kernels of aquatic colloids, hydroxyl-aluminosilicate (HAS) composites incorporate trace actinides during their heterogeneous nucleation into the oxo-bridged structure to form stable colloid-borne actinides [1]. The formation of colloid-borne Cm(III) species was studied in the presence of humic acid, by time-resolved laser fluorescence spectroscopy (TRLFS). In the absence of humic acid, spectroscopic speciation proves the formation of three different colloid-borne species: Cm-HAS(I), Cm-HAS(II) and Cm-HAS(III) [2]. In the Cm-HAS(III) species formed at pH > 6.0, Cm(III) is structurally incorporated into HAS colloids. In presence of humic acid, various colloid borne Cm(III) species are formed at pH > 6, consisting of Cm-humate, Cm-HAS(III) and a Cm-humate-aluminosilicate species, depending on the order of addition of different components concerned. The Cm(III)-borne species were characterized by fluorescence emission and their lifetime at different excitation energies (370 - 405 nm), i.e. directly at the absorption maxima of Cm(III) or indirectly by energy transfer from humic acid. Hence, the coordination of Cm to humic acid was ascertained by intra-molecular energy transfer process, which facilitates differentiating Cm-humate, Cm-humate-HAS and Cm-HAS species. 2-D LIBD (2 dimensional laser-induced breakdown detection) validates the existence of colloids of 15 to 30 nm in diameter and thus supports indirectly the TRLFS results. Addition of humic acid after the Cm incorporation into the HAS colloid structure (formation of Cm-HAS(III)) did not show direct interaction of humic acid with Cm at first. By aging, different initial components convert into a stable Cm-humate-aluminosilicate species with a characteristic emission band at 604 nm. The unique final species formed in the course of time prevails after 20 days in all samples, not depending on the composition of the initial solutions and the order of component addition. [1] Kim, M. A., Panak, P. J., Yun, J. I., Kim, J. I., Klenze, R., Koehler, K., Colloids and Surfaces A, Physiochem. Eng. Aspects 216, 97-108 (2003). [2] Kim, M. A., Panak, P. J., Yun, J. I., Priemyshev, A., Kim, J. I, Colloids and Surfaces A, Physiochem. Eng. Aspects 254, 137-145 (2003). (authors)