Laser Induced Spectroscopy of Actinides - A Tool to Study Interaction with Various Ligands at Low Concentrations

Heavy metals like actinides in the aquatic environment are normally transported as a complexed species. Knowledge about the complex formation is therefore essential for prediction of the migration of these elements.
Spectroscopic methods have the advantage to be non-invasive and non-destructive. The high intensity of laser light sources allows the excitation of all species in the illuminated volume. Therefore laser-based methods reach low detection limits.
Mainly Laser-induced Photoacoustic Spectroscopy (LIPAS) and Time-resolved Laser-induced Fluorescence Spectroscopy (TRLFS) were used for such speciation studies. The introduction of lasers with pulse durations in the pico- and femtosecond range in combination with short gated CCD-cameras enables studies of fluorescing species with fluorescence lifetimes of some nanoseconds.
As uranium, americium and curium show fluorescence properties time-resolved laser-induced fluorescence spectroscopy enables the determination of species up to detection limits of 10-8 M and less. Besides the study of the complex formation of uranium (sulfate, phosphate, arsenate) we determined the formed species in several mining related waters as function of pH. The change in the speciation of uranium(VI) is shown to be in agreement with calculations.
Absorption spectroscopy can be used as tool for direct determination of actinide species. However the concentration of actinides in the natural environment is much lower than the detection limit of conventional UV-VIS spectroscopy. Using laser-induced photoacoustic spectroscopy the detection limit can be decreased by about three orders of magnitude. Studies of the complex formation of uranium(IV) with phosphate and arsenate will be presented, demonstrating the advantage of direct speciation methods.
Polyelectrolytic organic macromolecules, like humic substances, are important complexing agents towards actinide metal ions. 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 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 actinide ions to study the complex formation of these systems.