Studies on interactions of actinides with organic ligands using femtosecond-laser-induced spectroscopy

Polyelectrolytic organic macromolecules, like humic substances, are important complexing agents towards actinide metal ions. Due to these properties humic substances are possible carriers for the migration of actinides in the environment. The study of the high molecular compounds is very complex. Therefore the investigation of simple model ligands would give a more detailed description of the binding behavior of humic substances. Especially the different bonding of carboxylic and phenolic group and the discrimination between them is of interest to get more detailed information about the complex formation. The various hydroxy benzoic acids allow the investigation of these differences.
To study the complex formation of actinides with such organic ligands we used the new, recently developed tool of femtosecond laser-induced time-resolved fluorescence spectroscopy. The method uses the fluorescence properties of organic ligands with p-electron systems and their change by interaction with metal ions to study the complex formation of these systems.
The method was validated by study of the deprotonation of 2,3 dihydroxybenzoic acid. Within the error limits a very good agreement of the third protonation constant was found.
The studies of the complex formation of uranium(VI) with 2,3 dihydroxybenzoic acid in the pH range from 3.0 to 4.5 showed an one to one complex. The dependence of the complex formation on pH leads to the conclusion that besides the carboxylic group one of the two phenolic OH-groups contributes to the complex formation. The formation constant was determined to be log K(I=0.1M) = - 3.02 ± 0.09.
Studies of the complex formation in the system neptunium(V)-2,3 dihydroxybenzoic acid showed a completely different behavior. At low pH values also an one to one complex formation was found. This complex formation was not dependent on pH. That means complexes with the deprotonated carboxylic group were formed.
The formation constant was determined to be log K(I=0.1M) = 0.11 ± 0.04. With increasing pH the complex reaction changes. An intermediate complex can be observed by its own fluorescence properties. At pH values higher then six an one to two complex will be formed. The complex formation constants and the stoichiometry of the complex reaction will presented.