Uptake of Trivalent Actinides (Cm(III)) and Lanthanides (Eu(III)) by Calcium Silicate Hydrates: A Wet Chemistry and Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) Study

The interaction of the two chemical homologues (Cm (III) and Eu(III)) with calcium silicate hydrates (CSH phases) at pH 13.3 has been investigated in batch-type sorption studies using Eu(III) and complemented with time-resolved laser fluorescence spectroscopy (TRLFS) using Cm(III). The sorption data for Eu(III) reveal fast sorption kinetics and a strong uptake by CSH phases with distribution ratios of (6±3)·105 L kg-1. Three different Cm(III) species have been identified: A non-fluorescing species, which was identified as a curium hydroxide (surface) precipitate, and two fluorescing Cm(III)/CSH sorbed species. The fluorescing sorbed species have characteristic emission spectra with main peak maxima at 618.9 nm and 620.9 nm and fluorescence emission lifetimes of 289 ± 11 µs and 1482 ± 200 µs, respectively. From the fluorescence lifetimes it was calculated that the two fluorescing Cm(III) species have one to two and no water molecules left in their first coordination sphere suggesting that these species are incorporated into the CSH structure. A structural model for Cm(III) and Eu(III) incorporation into CSH phases is proposed based on the substitution for Ca at two different types of sites in the CSH structure.
Key words : curium, europium, CSH, sorption, incorporation, TRLFS