Uranium transfer into plants

To protect the health of the population, knowledge is necessary about the transfer of radioactive heavy metals, like uranium, via the food chain soil-water-plant-animal-human. The overall process of uptake of heavy elements into plant tissues from contaminated soils is quantified using the soil-to-plant transfer factor (TF). The transfer factor of uranium for lupine and dandelion is in the range of 10-2 to 10-3. To obtain a more mechanistic understanding of the uptake of heavy metals in plants knowledge about the individual physical-chemical processes is needed. This study focuses on the determination of uranium speciation after uptake of uranium by plants (lupine, dandelions, lamb¢s lettuce). For the first time, laser-induced fluorescence spectroscopy and X-ray absorption spectroscopy were used to determine the chemical speciation of uranium in plants. Information on the spatial distribution of uranium in the plant was achieved by scanning electron microscopy, coupled with energy-dispersive X-ray microanalysis. Differences were detected between the uranium speciation in the initial solutions (hydrophonic and pore water of soil) and inside the plants. The oxidation state of uranium remained hexavalent after the uptake. The chemical speciation of uranium was identical in the roots, shoot axis, and leaves and was independent of the various investigated plants. First results are reported concerning the speciation of uranium in cell sap and solid cell components after separation. Spectroscopic data of relevant model compounds (e.g. sugar phosphates, inorganic phosphates, carboxylic acids, proteins) are compared with the uranium complexes in the different compartments of the plants.The results indicate that the uranium is predominantly bound to phosphoryl groups but in dependence on the experimental conditions the formation of uranium carboxylate complexes cannot be excluded.