A fluorescence spectroscopic study on the speciation of Cm(III) and Eu(III) in the presence of organic chelates in highly basic solutions

The speciation of Eu(III) and Cm(III) was investigated by time resolved laser fluorescence spectroscopy (TRLFS) over a range of base concentrations from 0.01 M NaOH to 7.5 M NaOH and in the presence of several organic chelates including EDTA, HEDTA, NTA, and oxalate. The results of this work suggest that both Eu(III) and Cm(III) form strong mixed ligand complexes with organic chelates and the hydroxyl groups(s) in dilute NaOH solutions. However, in concentrated NaOH solutions, Eu(III)-/Cm(III)-containing colloidal nanoparticles are the primary cause for the measured Eu(III)/Cm(III) in the aqueous solutions. Therefore, the interpretation of these data solely in terms of the formation of amphoteric hydroxyl species (e.g. Eu(OH)(4)(-)) would appear to be inappropriate. The organic chelating ligands form strong complexes with surface Cm(III)/Eu(III) sites of the colloidal nanoparticles. For Cm(III), such surface complexes show largely red-shifted fluorescence spectra as compared with the aqueous complexes and unusually short fluorescence lifetimes. The decreased fluorescence lifetimes are likely due to the presence of transition metal impurities in the nanoparticle and reduced inter-nuclear distance between neighboring Cm(III) centers.