Institute
for Nuclear Waste Disposal (INE), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen,
Clay minerals may play a major
role in deep nuclear waste repository sites. Over geologic time scales, nuclear
waste glass may corrode upon contact with groundwater. Studies showed the
presence clay minerals, such as the smectite hectorite [1], in the glass
alteration layer. The formation of such alteration phases represents a high
retention potential for RN, including the actinides (An). The immobilization of
An(III) may occur by several distinct molecular-scale binding mechanisms. The most
effective retention may very likely occur by incorporation in the bulk
structure. Recent investigations [2] showed the possibility to incorporate
Lu(III) in the hectorite octahedral sites by coprecipitation.
In the present study, hectorite
was synthesized in the presence of Am(III) (AmCopHec).
The samples associated with the multi-step synthesis protocol (Am-doped
brucite: AmBru; AmCopHec;
Am adsorbed on hectorite: AmAdsHec) were
characterized by X-ray diffraction and the local Am chemical environment was
probed by EXAFS spectroscopy. The presence of Am(III) did not significantly
influence the synthesis procedure. EXAFS data indicated that Am is six-fold
coordinated by O atoms in the precursor phase (AmBru).
Furthermore, polarized data collected for AmBru evidenced
an anisotropic environment surrounding Am and thus a structural association
with brucite. The fit results indicate an in-plane orientation of the O shell,
but the next nearest detected Mg shell did not show a clear angular dependence.
Powder EXAFS data collected for AmCopHec indicated
the presence of Mg and Si cationic neighbors. Furthermore, data showed a
shortening in the Am-Mg distance from 3.22(2) Å in AmBru
to 3.17(2) Å in AmCopHec. The data may be interpreted
as Am(III) substituting for Mg in the octahedral sheet of AmBru
and AmCopHec. This substitution may be very limited
and induce high structural strain. Powder data collected for surface sorbed Am(III) evidenced a chemical environment different
from coprecipitated Am(III). Specifically, the Mg and
Si shells are located at significantly higher distances (d(Am-Mg) = 3.22(2) Å;
d(Am-Si) = 3.80(2) Å). Data may be interpreted as Am(III) binding to the clay
layer edges.
Acknowledgements
Anka is acknowledged for provision of synchrotron radiation beam time.
References
[1] Thien,
B.; Godon, N.; Hubert, F. et al. (2010). Appl. Clay
Sci. 49, 135-141.
[2] Finck,
N.; Schlegel, M.L.; Bosbach, D. (2009). Environ. Sci. Technol. 43, 8807-8812.