Multiple investigations of aqueous Eu(iii)–oxalate complexes: the reduction in coordination number and validation of spectral linear correlation

Abstract

Detailed information on the An(III)/Ln(III) complexation properties in solution is essential for separation chemistry and the prediction of their potential for radionuclide migration from nuclear waste repositories into natural aquifers. In the present study, to better reveal and confirm the structural information of [Eu(Ox)_x (H₂O)_h−2x]^3−2x (h = 8, 9; x = 0–3) aqueous species, especially the variable coordination number (CN), and explore the validity of the spectral linear correlation between the luminescence lifetime and the residual hydration number in the first coordination sphere of Eu(III) compounds in solution, a comparison between the spectral results and the theoretical calculations in a wide parametric space in terms of the pH value and oxalate concentration was carried out by combining time-resolved luminescence spectroscopy (TRLS) with speciation modelling and density functional theory (DFT) calculations. We have found direct and clear evidence for the 9-fold to 8-fold coordination number reduction of Eu(III) atoms upon coordination with more than one oxalate in an aqueous medium, and as well systematically validated the applicability of the spectral linear correlation in an aqueous system (otherwise solid state) involving multiple species with the support of relatively reliable and clear speciation modelling.