Lu(III)
coprecipitation with hectorite: a P-EXAFS approach
N. Finck1,4, M.L.
Schlegel2, and D. Bosbach3,4
1Institute for Nuclear Waste Disposal,
(*correspondence:
nicolas.finck@ine.fzk.de)
2CEA
Saclay, DEN/DPC/SCP/LRSI, F-91191
3Institute
for Energy Research 6, Jülich Research Centre, D-52425
4Helmholtz Virtual Institute „Advanced Solid Aqueous Radiogeochemistry“,
Various
secondary phases may form upon alteration of the High Level nuclear Waste (HLW)
glass over geological time scales, including hectorite [1] (a magnesian
smectite). Such secondary phases represent a significant retention potential
for radionuclides (RNs), including actinides. In addition to (surface)
adsorption reactions, effective RN incorporation into the bulk structure may
occur by coprecipitation. Recently, hectorite was coprecipitated in the
presence of the trivalent lanthanide Eu(III) [2], as non-radioactive chemical
homologue for trivalent actinides. Time-resolved laser fluorescence
spectroscopy data suggested that Eu(III) substitutes for cations at octahedral position.
Polarized EXAFS
(P-EXAFS) experiments [3] were performed on Lu(III)-containing samples
associated with the hectorite multi-step synthesis protocol [4]. P-EXAFS spectra
were collected for the (Mg/Lu) hydroxide precursor and the Lu(III)-coprecipitated
hectorite at different angles a between the electric field vector of the X-ray
beam and the mineral layer plane. The modeling results for the oxygen shell (d(Lu-O)
= 2.27 Å) strongly suggest that Lu(III) is located in an octahedral brucite-like
environment in the precursor. For this sample, an additional Mg shell is
detected at 3.30 Å. The apparent coordination numbers for the O and Mg shells decrease with
increasing a, supporting
the Lu(III) incorporation in flattened brucite layers. In hectorite, the short
Lu-O distance (2.19 Å) and the detection of Mg (3.12 Å) and Si (3.37 Å) shells strongly
suggest that Lu(III) is located in a strained octahedral clay-like environment.
Finally, no surface complex was detected, as evident by comparison with the
Lu(III) sorbed smectite.
[1 Zwicky et al. (1989) Mater. Res. Soc. Symp. Proc. 127,
129-136. [2] Finck et
al. (2008) J. Contam. Hydrol. 152 253-262. [3] Manceau et al. (1988) Phys. Chem. Miner. 16, 180-185. [4] Carrado et al. (1997) Clay Miner. 32, 29-40.