U(VI) species sorbed on the green algae Chlorella vulgaris - a TRLFS and EXAFS study

Algae in soil and water can have a significant influence on the transport of actinides in the biological and geological environment in particular on the load pathway soil - plant - animal - human. The chemical characterization of the radionuclide species is essential for a reliable assessment of the migration. Due to the relative simple structure of mono-cellular green algal cells and their properties to bind high amounts of heavy metals and actinides they are used as model organisms for the investigations of different biosorption and bioaccumulation processes.
Sorption studies with Chlorella vulgaris cells have shown that up to 40 % of the initial uranium is bound by the algal cells at pH 3. The uranium removal is almost complete at pH 5 and 6 in our experiments. The use of the time-resolved laser-induced fluorescence spectroscopy (TRLFS) and X-ray absorption spectroscopy (XAS) is an effort to obtain information about the possible structure and the spectroscopic properties of the sorbed U(VI) species on the algal cells in comparison to those of the initial uranyl species in the contact solution.
The luminescence spectroscopic investigations of formed algal uranyl complexes indicate that the binding of U(VI) to carboxyl groups plays a dominating role at pH 3, whereas a minor impact of organic phosphate compounds on the U(VI) sorption can not be excluded. In contrast, at pH 5 and 6 the phosphate groups are mainly responsible for the removal and binding of U(VI) by formation of organic and/or inorganic uranyl phosphates. The results of the uranium LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopic measurements support these evidence.