Hectorite was synthesized from a Eu(III)-bearing brucite precursor in a multistep procedure. In a separate experiment, Eu(III) ions were adsorbed onto hectorite in suspension. Colloids were extracted from both samples. The size distributions in the colloidal fractions were characterized by application of the asymmetrical flow field-flow fractionation (AsFlFFF) method and the corresponding elemental compositions were obtained by ICP-MS. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize the local chemical environment surrounding Eu in the bulk samples and in the colloidal fractions.

The EXAFS results show that Eu is associated with hectorite upon co-precipitation or adsorption. Results from AsFlFFF suggest that Eu is structurally associated with the colloidal fraction extracted from bulk Eu-bearing co-precipitated hectorite. The AsFlFFF data are different for the colloidal fraction containing Eu(III) adsorbed on hectorite; in this sample they are consistent with a surface retention mechanism. These small but significant differences enable surface sorbed Eu to be distinguished from co-precipitated Eu. Eu is very likely located in a clay-like environment in the co-precipitation experiment, and it forms inner-sphere surface complexes in the adsorption experiment. The results obtained using the different experimental techniques agree, and show the benefits of using multiple methods of analysis.

Trivalent europium was used as non-radioactive chemical homologue for trivalent actinides. Similar retention mechanisms are expected for the trivalent actinides if they are co-precipitating with or adsorbing onto sheet silicates. The present study provides information which can be usefully added to the safety assessments required for deeply buried nuclear waste disposal sites.

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