Yttrium co-precipitation with smectite: A polarized XAS and AsFlFFF study

Abstract

The Y(III) binding mechanism(s) by coprecipitation with or by adsorption on hectorite, a mineral phase frequently detected in nuclear waste glass alteration experiments, was investigated by polarized EXAFS (P-EXAFS) spectroscopy. The novelty in this study is the use of yttrium to take advantage of the higher angular dependence of the absorption coefficient at the K-edge for P-EXAFS measurements. In the coprecipitation experiment, a brucite precursor was prepared in the presence of Y and subsequently aged to produce hectorite. In the adsorption experiment, Y(III) ions contacted pre-formed hectorite in dispersion. The coprecipitated hectorite and brucite and the hectorite from adsorption experiment were each prepared as textured samples and the Y(III) local environment was probed by P-EXAFS spectroscopy. P-EXAFS analysis indicated that Y(III) is 6-fold coordinated by O atoms in both the coprecipitated brucite and the coprecipitated hectorite, and surrounded by next-nearest Mg/Si shells. The angular dependences of the coordination numbers strongly point to Y(III) substituting for Mg(II) in brucite layers. Upon hectorite crystallization the local environment evolved. Mg and Si shells were detected at distances suggesting an octahedral clay-like environment in the coprecipitated hectorite, and this finding was corroborated by the angular dependence of the coordination numbers. In the adsorption sample, Y(III) forms inner-sphere surface complexes at the platelet edges (i.e., (0 1 0) plane), slightly tilted off the median clay plane. The presence of such surface complexes in the coprecipitation sample could not be evidenced. Finally, the supernatant of the dispersion containing the coprecipitated hectorite was analyzed by the asymmetrical flow field-flow fractionation (AsFlFFF) technique coupled to ICP-MS to obtain information on the smallest sized particles. The AsFlFFF data indicate that nanoparticulate hectorite of various sizes (50–75nm, 125–140nm and >450nm) can be separated from the bulk dispersion and this finding was corroborated by TEM experiments on the same supernatant. Furthermore, AsFlFFF data also indicate that Y(III) behaves like Mg, used as fingerprint of the presence of hectorite. This finding suggests random substitution for octahedral cation within hectorite nanoparticles. Trivalent yttrium was used as proxy for trivalent actinides (An(III)). Consequently, this study supports the incorporation of An(III) into hectorite forming in the nuclear waste glass alteration layer in deep disposal sites, as already suggested in previous studies.