MECHANISMS OF Ln(III) UPTAKE
BY the clay mineral hectorite: a
Polarized EXAFS approach
1 Institute
for Nuclear Waste Disposal,
2 CEA Saclay - DEN/DPC/SCP/LRSI,
3 Institute for Energy Research 6, Jülich Research Centre, D-52425
* Helmholtz Virtual Institute « Advanced
Solid-Aqueous RadioGeochemistry »,
Clay minerals are major sorbing solids in geological and engineered barriers of
High-Level nuclear Waste (HLW) repositories. They may form as secondary phases
upon alteration of the HLW matrix over geological time scales in the presence
of ground water. The precipitation of such alteration phases represents a
significant retention potential for radionuclides
(RN), including actinides [1]. Various distinct molecular-level binding
mechanisms may account for this retention. Specifically, RN incorporation into
the bulk structure of clay minerals may occur by co-precipitation, resulting in
long-term optimal immobilization, especially if a stable solid solution forms.
However, the incorporation of actinides, and their chemical surrogates, the
lanthanides, in an octahedral lattice site may be hindered by their large ionic
radii. Yet, Time-Resolved Laser Fluorescence Spectroscopy (TRLFS)
data collected for Eu(III) [2] or Cm(III) [3]
co-precipitated at the trace level with hectorite, a smectite frequently observed
in HLW glass corrosion experiments [4], suggested
an Eu or Cm substitution for cations at octahedral
sites.
EXAFS
experiments were performed on Lu(III)- or Eu(III)-containing samples associated with the hectorite multi-step
synthesis protocol [5]. Polarized EXAFS (P-EXAFS) spectra were collected for the
(Mg/Lu) hydroxide precursor and the Lu(III)-doped
hectorite at different angle a between the
mineral layer plane and the electric field vector of the X-ray beam. For
(Mg/Lu) hydroxide an oxygen shell was detected at 2.27 Å, pointing to six-fold coordination
by O, and Mg as next neighbors at 3.30 Å. These results, together with the decreasing
O and Mg coordination numbers with increasing a values, may support the
assumption of Lu(III) incorporation in a flattened brucite layer. An oxygen shell was detected at significantly short distance (2.19 Å) for the Lu(III)
co-precipitated hectorite, and Mg (3.12 Å) and Si (3.36 Å) as next nearest
neighbors. These data are consistent with Lu(III)
located in a strained clay-like octahedral environment. The Lu(III)
retention mechanism by surface adsorption was investigated by collecting
P-EXAFS spectra for Lu(III) ions sorbed onto hectorite.
The split in two oxygen subshells was consistent with
Lu(III) binding to the clay particle edges, as was
reported for Y sorbed onto hectorite [6]. The 5 O
neighbors detected at 2.35 Å may represent the bound water molecules, as is typical
for Ln(III)
forming inner-sphere surface complexes [7], and the O shell located at 2.23 Å can
be attributed to the sorbent surface. Finally, powder EXAFS data collected for Lu(III) sorbed onto silica
indicated the presence of an O shell at 2.22 Å, and two Si shells at ~ 3.0 and
~ 3.8 Å, suggesting that Lu polyhedra share edges and
corners with Si tetrahedra. The formation of such
surface-sorbed species during the hectorite
co-precipitation experiments thus can be ruled out.
The ability
of the hectorite structure to accommodate the larger Eu(III) ions at octahedral
sites at concentrations higher than the trace level was investigated by
performing EXAFS experiments and compared to TRLFS data. EXAFS data collected
for the Eu(III)
co-precipitated hectorite, for the (Mg/Eu) hydroxide precursor and for the Eu(III) sorbed
silica revealed a distinct crystallochemical environment
in each sample.
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Kim and B. Grambow, Engineer. Geol. 52, 221-230 (1999).
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102, 253-262
(2008).
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Brandt et al., Geochim. Cosmochim. Acta 71, 145-154 (2007).
[4] H.U.
Zwicky et al., Mater.
Res. Soc. Symp. Proc. 127,
129-136 (1989).
[5] K.A.
Carrado et al., Clay
Miner. 32,
29-40 (1997).
[6] M.L. Schlegel, Radiochim. Acta 96, 667-672 (2008).
[7] T. Stumpf et al.,
Radiochim. Acta 90, 345-349 (2002).