Influence of humic acid size on actinide complexation

The purpose of this research was to investigate whether the molecular size of humic fractions has an effect on the nature and the stability of the complexes formed. Ultrafiltration was used as the method of fractionation to obtain five different size fractions of nominal molecular size. Each fraction was chemically characterized. Samples of large and small size fractions were placed for three months in basic solution (pH 10.0) and in acidic solution (pH 4.0). The complete analytical characterization of these two size fractions was repeated. Rearrangement of the size fraction was observed over the range of size of the unfractionated macromolecule due to recombination and/or dissociation of the fractionated macromolecules. The rearrangement was greater under basic conditions. It was also shown that, in high sodium concentration, a sodium effect takes place on the protonation constant of humic acid.; In studies for evaluation of the nuclear waste management, numerous models are used. In this study, two models representing quite different approaches in the analysis of metal-humic interaction were evaluated. The Charge Neutralization Model (CNM) is used for macroscopic metal concentrations leading to a binding constant which is independent of pH, metal concentration, and ionic strength whereas the Polyelectrolyte Model (PM) is used for tracer concentrations and the binding constant is dependent on pH and ionic strength. In the Polyelectrolyte Model conditions, the binding constants obtained between 241Am and two size fractions of humic acid (LBHA>300kD and LBHA>10kD) are greater for the larger size fraction and for pH 6.0. A significant difference, one or two orders of magnitude, is found in the binding constants obtained for trivalent actinides and the same fractions of humic acids at pH 4.0 and 6.0, using these two different models. The binding constants obtained with tracer amount of metal cations by the polyelectrolyte model are larger than by the charge neutralization model using macromolar amount of metal cations.; The study of Eu 5D₀ - 7F ₀ excitation spectra as a function of time and loading of humic was conducted to determine the type of sites bound to the metal. The peak maximum obtained after equilibrium in the two different situations (PM and CNM) appeared at different wavenumbers on the spectrum. Therefore, Eu ion has a different coordination environment in these two model conditions, which can be explained by the difference in numbers of functional groups of the humic acid bound to the metal ion. The mode of binding between trivalent actinides and humic acid in the Polyelectrolyte Model conditions differs with that of the Charge Neutralization Model conditions.