Sorption of Am(III) onto 6-line-Ferrihydrite and its Alteration Products:

Investigations by EXAFS

Silvia Stumpf¹, Thorsten Stumpf¹, Thomas Fanghänel¹, Kathy Dardenne¹, Christoph Hennig², Reinhardt Klenze¹

¹Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, P.O. Box 3640, 76021

Karlsruhe, Germany

²Forschungszentrum Rossendorf, ROBL-CRG, European Synchroton Radiation Facility, BP

220, F-38043, Grenoble, France

ABSTRACT

For the long-term performance assessment of nuclear waste repositories, knowledge concerning the interactions of actinide ions with mineral surfaces is imperative. The mobility of released radionuclides is strongly dependent on the sorption/desorption processes at mineral surfaces and their incorporation into the mineral frame.

Ferrihydrite is able to dominate the retardation of radionuclides in particular the actinides in the near-field as well as in the far-field of a nuclear waste repository. Upon heating pure ferrihydrite converts to crystalline goethite and hematite. In the presence of di- or trivalent metal ions the transformation is affected leading to the promotion of one crystal formation over the other and incorporation of these ions. This study deals with the question if released Am(III) can be immobilized by sorption onto 6-line-ferrihydrite. Moreover it is studied what happens if ferrihydrite transforms to a more crystalline phase. Therefore the sorption of Am(III) onto ferrihydrite at different pH values and its reaction with the mineral surface after its transformation were investigated by EXAFS. EXAFS measurements show the sorption of Am(III) by bidentate bonding onto 6-line-ferrihydrite at pH 5.5 (Am-Fe 3.7 Å; Am-O 2.48 Å ) and pH 8 (Am-Fe 3.69 Å; Am-O 2.48 Å ). The alteration of 6-line-ferrihydrite leads to the formation of goethite and hematite at both pH values. While Am(III) is released during the transformation at pH 5.5 it is incorporated in the goethite like structure at pH 8 (Am-Fe 3.59; Am-Fe 3.32 Å; Am-O 2.46 Å ).