Free and silica-gel bound tetraazamacrocycles as complexing agents of actinide cations: a XAFS investigation of the solid-state coordination scheme

There is a continuous need for processes aimed at the selective uptake of toxic metal ions from dilute wastewaters and industrial process streams. Insofar, solid-liquid extraction processes using chelating agents grafted onto hydrophilic supports are appealing. The Institut de Chimie Moléculaire de l'Université de Bourgogne has designed a series of hybrid organic-inorganic materials for the selective uptake of toxic metal ions, including actinides [1]. These extracting agents are obtained by grafting functionalized tetraazamacrocyclic ligands (cyclen and cyclam derivatives) on the surface of silica-gel beads. In collaboration with the Commissariat à l'Energie Atomique (CEA Center of Valduc), one of these modified gel has been prepared on a semi-industrial scale and used in the final decontamination step of real effluents for removing the residual -emitters (U, Pu, Am) contained at the sub-g/L level. This process allowed the successful decontamination of more than 50 m³ wastewaters, with a residual radioactivity level of less than 5 Bq/m³.
We have used XAFS to investigate the coordination scheme of UO22+ and Pu4+ cations at the surface of the organic-inorganic hybrid materials. The results show that surface complexes are formed through a combined action between the reactive solid support, namely amorphous silica, and the chelating groups appended on the immobilized ligands [2].

[1] F. Barbette, F. Rascalou, H. Chollet, J. L. Babouhot, F. Denat, R. Guilard, Anal. Chim. Acta, 502, 179-187, 2004.
[2] L. Giachini, S. Faure, M. Meyer, L.V. Nguyen, B. Batifol, H. Chollet, R. Guilard, A.C. Scheinost, C. Hennig, Proceedings of the 5th Workshop on Speciation, Techniques, and Facilities for Radioactive Materials at Synchrotron Light Sources (Actinide XAS 2008), in press.