Formation of mixed Eu(III)-CO3-fulvic acid complex: Spectroscopic evidence and NICA-Donnan modeling

Abstract

Humic substances impact the fate of radionuclides in the environment. The formation of mixed complexes between lanthanides(III), carbonates, and humic substances is still a matter of debate and further studies are needed. Time-resolved luminescence spectroscopy (TRLS) is used to evidence the presence of Eu(III)-CO3-SRFA ternary complexes in media representative of environmental conditions. The evolutions of the Eu(III) luminescence spectra from the 5D0 → 7F0 to the 5D0 → 7F2 transitions are showing modifications of the asymmetry ratio — area ratio of the 5D0 → 7F2 over the 5D0 → 7F1 transition — that indicates the presence of several species in addition to the EuCO3+ and Eu-SRFA complexes. The occurrence of bi-exponential decay times, with one decay time faster than the one from Eu3+, is typical of the occurrence of fulvic complex, even in the presence of carbonate. The analyses of the indirect excitation of the Eu-CO3-SRFA system are used to build complexation isotherms varying either SRFA or total carbonate concentrations in the framework of the NICA-Donnan modeling of the Eu(III)-SRFA system. In view of the restricted pH range, no strict attribution to the different possible types of sites can be done. Different complex(es) formations are considered and discussed.