AMERICIUM CO-PRECIPITATION WITH THE SMECTITE
HECTORITE: POWDER AND POLARIZED EXAFS INSIGHTS
1 Institute for Nuclear Waste Disposal (INE),
Karlsruhe Institute of Technology, Campus North, P.O. Box 3640, D-76021
Karlsruhe, Germany. (Correspondence: Nicolas.finck@kit.edu)
2 Helmholtz Virtual Institute “Advanced
Solid-Aqueous RadioGeochemistry”,
KEYWORDS: Actinides, Smectite, EXAFS
Clay
minerals are major sorbing solids in deep nuclear waste repositories. They may
also form as secondary phase upon alteration of the waste matrix over
geological time scale in the presence of ground water. The precipitation of
such alteration phases, like hectorite for example [1], represents a significant retention potential
for radionuclides, including actinides (An) [2]. The immobilization of An(III) may occur by
surface adsorption and by incorporation in the bulk structure. Recent
investigations [3] showed the possibility to incorporate Lu(III) in octahedral
sites of hectorite by co-precipitation.
Hectorite was co-precipitated in
presence of Am(III). After characterization of the bulk structure (XRD) of the
Am(III)-containing samples associated with the synthesis protocol [3,4], the
Am(III) local chemical environment was probed by X-ray absorption spectroscopy.
In the precursor phase (brucite) prepared as oriented sample, powder EXAFS data
point to Am(III) located in an octahedral environment and polarized EXAFS data
suggest an anisotropic environment around Am and thus a structural association
with the solid phase. Powder EXAFS data indicate that Am(III) is six-fold
coordinated by oxygen in the doped hectorite and the detection of next nearest
Mg/Si backscatterers strongly corroborate with a clay-related environment. In
contrast, higher number of oxygen backscatterers were detected for surface
adsorbed Am(III), as well as lower number of Mg/Si neighbors.
[1] H.U. Zwicky, et al., Mater. Res. Soc. Symp. Proc. 127, 129-136 (1989).
[2] J.I. Kim, et al., Engineer. Geol. 52, 221-230 (1999).
[3] N.
Finck, et al., Environ. Sci. Technol. 43, 8807-8812 (2009).
[4] K.A. Carrado, et al., Clay. Miner. 32, 29-40 (1997).