Lu(III) coprecipitation with hectorite: A P-Exafs approach

Complete text of publication follows: 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 α 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 Angstroms) 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 Angstroms. The apparent coordination numbers for the O and Mg shells decrease with increasing α, supporting the Lu(III) incorporation in flattened brucite layers. In hectorite, the short Lu-O distance (2.19 Angstroms) and the detection of Mg (3.12 Angstroms) and Si (3.37 Angstroms) 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