Test site Gessenwiese: Results in TRLFS investigations on surface water and pore waters, and preliminary results on plant saps concerning uranium speciation

Plants may take up uranium from contaminated sites and thus may represent a potential risk. To evaluate the risk of uranium being transferred from the environment into the human food chain knowledge concerning the uranium speciation is essential since reactivity and toxicity of uranium largely depend on its speciation.
Plants take up their nutrients from the soil in dissolved form and thus uranium may enter plants as dissolved species. To study this process the speciation of uranium in a uranium contaminated site, i.e. Gessenheap near Ronneburg in Saxony/Germany was studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) in the aqueous phase, i.e. in surface water (drainage channel on the Gessenheap) and in pore water (approximately 0.5 – 1 m below surface of test site Gessenwiese). In both waters uranium sulfate species dominate the uranium speciation and clearly show that uranium sulfate species are in contact with plant components.
In a second step we determined the uranium concentration in the plants obtained from the test field Gessenheap by ICP-MS. For these analyses cell saps of birch leaves, of a birch boletus, and of compartments of butterbur (leaves, roots, and sprouts) were obtained by ultra centrifugation. The highest uranium concentration was found in sap samples from roots of butterbur with 39.3 ppb uranium (approximately 1.5 • 10-7 M uranium); in the sap samples of birch leaves 18.1 and 16.6 ppb uranium, respectively were detected. In all other sap samples the uranium contents were below 4 ppb uranium. The uranium content in the studied dried plant materials were less than 2.2 ppb, and dried birch leaves contains less than 0.1 ppb uranium.
The above listed uranium concentrations turned out to be too low to be successfully analyzed by TRLFS. In addition, we have to mention that plant sap may contain substances which quench the fluorescence signal of uranium (VI), e. g. some heavy metal ions or organic compounds. For future studies plant samples containing higher uranium concentrations are used for TRLFS investigations, possibly mosses or lichens.