Local structure of Y and Ho in calcite and its relevance to Y fractionation from Ho in partitioning between calcite and aqueous solution

Among rare earth elements (REEs), the behavior of Y and Ho in most igneous activities is very close due to the similarity in their ionic radii, while Y fractionates from Ho in marine systems. In this study, in order to elucidate Y–Ho fractionation observed in marine systems in terms of structural chemistry, we examined Ho LIII-edge and Y K-edge EXAFS study for two partitioning systems, namely, 1) calcite-aqueous solution (Y–Ho fractionation system) and 2) strong acid cation exchange resin-aqueous solution (non-Y–Ho fractionation system). The results of the EXAFS analysis did not show significant differences in interatomic distances to the most neighboring O atoms between Ho and Y for all the samples. However, it was found that the first shell Ho–O and Y–O distances in the Y–Ho doped calcite were shorter than those in the aqua ion. In contrast, the first shell Ho–O and Y–O distances in the Y–Ho doped resin were similar to those in the aqua ion. Previous studies have suggested that lanthanide (Ln) is more covalent due to 4f orbital participation in bonding relative to Y. Spectroscopic studies on various Ln3+ complexes show that Racah parameters for 4f electron repulsion in Ln3+ ions decrease with an increase in covalency of bonding of Ln3+ ions with anionic ligands. Therefore, our EXAFS results suggest that Y–Ho fractionation in partitioning is possibly attributed to the difference of change in covalency associated with the ligand exchange between Y and Ho, which we have observed as differences in partition coefficients between calcite and aqueous solution.