Influence of plants on speciation and mobility of U(VI) and Eu(III)

For a safety assessment of repositories, possible accident scenarios must be considered. If radionuclides (RNs) are released from a repository into the groundwater, they can migrate and interact with the biosphere. In addition to microorganisms, higher organisms such as plants also play a role in this context. For this reason, it is necessary that a process understanding on RN plant interactions on the molecular level is elucidated by suitable biochemical and spectroscopic methods to improve biogeochemical models for a reliable modeling of the transfer behavior of RNs in the environment up to the food chain. Interaction with plant cells can lead to immobilization and speciation changes of the RNs. Uranium as the main component (by mass) of spent nuclear fuel rods is of interest for our investigations. Furthermore, we studied europium as a non-radioactive analogue for the trivalent actinides Am(III) and Cm(III), whereby the former in particular will dominate the radiotoxicity in a potential repository. We investigated the time and concentration-dependent immobilization of these two metals to the biomass of different plant cell cultures. Biochemical methods were accompanied by spectroscopic (time-resolved laser-induced fluorescence spectroscopy, TRLFS) and microscopic techniques for determination of speciation and localization of the metals within the plant cells, respectively. In addition, the combination of luminescence spectroscopy with microscopy in the form of chemical microscopy revealed for the first time the spatially resolved localization of different Eu(III) species in plant root tissues. We observed bioassociation (immobilization) of U(VI)/Eu(III), but in some cases also re-mobilization of U(VI), which temporally coincided with a change in the metal speciation in solution. Moreover, we successfully identified metabolites released by the plant cells that can complex with U(VI)/Eu(III) and influence their mobility and thus bioavailability in the environment. Our studies can help to integrate RN plant interactions in assessment strategies of nuclear waste repositories.