On the uptake and binding of uranium (VI) by the green alga Chlorella Vulgaris

Uranium could be released into the environment from geogenic deposits and from former mining and milling areas by weathering and anthropogenic activities. The elucidation of uranium behavior in geo- and biosphere is necessary for a reliable risk assessment of radionuclide migration in the environment. Algae are widespread in nature and the most important group of organisms in the aquatic habitat. Because of their ubiquitous occurrence in nature the influence of algae on the migration process of uranium in the environment is of fundamental interest e.g. for the development of effective and economical remediation strategies for contaminated waters. Besides, algae are standing at the beginning of the food chain and play an economically relevant role as food and food additive. Therefore the transfer of algae-bound uranium along the food chain could arise to a serious threat to human health. Aim of this work was the quantitative and structural characterization of the interaction between U(VI) and the green alga Chlorella vulgaris in environmental relevant concentration and pH range with special emphasis on metabolic activity. Therefore a defined medium was created which assures the survival/growth of the algae as well as the possibility to predict the uranium speciation. The speciation of uranium in the mineral medium was calculated and experimentally verified by time-resolved laser-induced fluorescence spectroscopy (TRLFS). The results of the sorption experiments showed that both metabolic active and inactive algal cells bind uranium in significant amounts of around 14 mg U/g dry biomass and 28 mg U/g dry biomass, respectively. Another interesting observation was made during the growth of Chlorella cells in mineral medium at the environmental relevant uranium concentration of 5 μM. Under these conditions and during ongoing cultivation a mobilization of the algae-bound uranium occurred. At higher uranium concentrations this effect was not observed due to the die off of the algae cells during the sorption experiment and a behavior analogue to initially heat killed cells. Differences in the contribution of functional groups to the uranium binding depending on the metabolic activity of the algal biomass were also shown by the difference spectra of the ATR-FTIR measurements calculated between uranium contaminated and untreated biomass. All together the spectroscopic investigations showed in case of Chlorella vulgaris a mixture of functional groups is responsible for the coordination of U(VI). The metabolic activity of the cells influences the binding of uranium under the given experimental conditions. Models of the structure of the formed U(VI)-algae complexes in dependence of the metabolic activity were created. For living algal cells, mainly carboxylic and organic phosphate groups are involved in the binding of uranium, whereas in dead algal biomass additionally inorganic phosphates are involved to some extend in the interaction of uranium with algal cells. The transferability of the obtained findings concerning the experiments in mineral medium to natural occurring mining related waters was verified exemplary with the surface water of ''Gessenwiese'' (former ''Gessenhalde'' at Ronneburg). The obtained results of this study contribute to the prediction of the migration behavior of uranium under environmental conditions, the radiological risk assessment of geogenic and anthropogenic appearing uranium and a reliable estimation of the accumulation of uranium in the food chain.