MECHANISMS OF Ln(III) UPTAKE BY the clay mineral hectorite: a Polarized EXAFS approach
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 . 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)  or Cm(III)  co-precipitated at the trace level with hectorite, a smectite frequently observed in HLW glass corrosion experiments , 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 . 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 . 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 , 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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