Bulg. J. Phys. vol.34 no.s1 (2007), pp. 083-084

Speciation of Actinides

G. Geipel
Forschungszentrum Rossendorf, Institute of Radiochemistry, P.O.Box 510 119, D-01324 Dresden, Germany
Abstract. Actinides under environmental conditions can be found in a sub-μ-molar to the lower millimolar concentration range. However, the direct detection of species under these conditions with conventional spectroscopic techniques is not possible.
By application of very intense light sources, as are available with the different laser systems the excitation of traces of actinides ions becomes possible. Several ions of actinides possess outstanding spectroscopic properties which allow the detection of species also in the lowest concentration range.
For the non-invasive detection of species in solution especially the laser-induced photoacoustic spectroscopy (LIPAS) and in the case of fluorescing species the time-resolved laser-induced fluorescence spectroscopy (TRLFS) are method of first choice. LIPAS is a very sensitive method for absorption spectroscopy and can be applied therefore for all non-fluorescent systems. Compared to conventional UV-VIS spectroscopy up to three orders of magnitude lower detection limits can be achieved. TRLFS is only applicable in systems where the deexcitation takes place via radiative processes with emission of photons. In the series of the lower actinides here the systems containing protactinium(IV), uranium(IV) and uranium(VI) as well as americium(III) and curium(III) have to be named. Due to the extremely high quantum efficiency of curium(III) ions concentrations down to 10-11 mol/L can be observed be TRLFS. On the other hand organic ligands as humic substances are ubiquitous in the nature. The interaction of such compounds with actinides can also studied using the fluorescence properties of the organic ligand. A brief overview for these methods will be given.
Examples for the application of the laser-induced spectroscopic methods we be given. At first examples for the spectroscopy and detection of uranium species in natural waters from mining related to drinking waters will be shown. As special case the determination pure uranyl carbonate species, which are non-fluorescent at room temperature, will be shown. Also the interaction with sulfate, phosphate and arsenate will be touched on. The application of LIPAS will be shown especially for the uranium(IV) phosphate and arsenate systems. As uranium(IV) shows also fluorescence properties some problems with the detection of these species will be summarized. Studies of the interaction of curium and americium with several ligands will also shortly summarized.
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. To study the complex formation of actinides with such organic ligands the TRLFS with ultrashort excitation pulses is a very suitable method.

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