Uptake and binding of U(VI) by the green alga Chlorella vulgaris

For the risk assessment of radionuclides in the environment a comprehensive elucidation of the migration behavior of U(VI) in biosphere is necessary. Aim of this study was the quantitative and structural characterization of the interaction between U(VI) and the green alga Chlorella vulgaris at U(VI) concentrations from 5 μM to 1 mM, and in the pH range of 4.4 to 7.0 with special focus on metabolic activity. The obtained findings of the sorption experiments demonstrate clearly, the interactions with U(VI) are heavily influenced by the U(VI) concentration and the status of the Chlorella cells. Living cells bind in a 0.1 mM U(VI) solution at pH 4.4 within 5 min 14.3±5.5 mg U/g dry biomass and dead cells 28.3±0.6 mg U/g dry biomass. Furthermore, Chlorella cells die at concentration higher than 0.1 mM within 48 h. Interestingly, at lower U(VI) concentration of 5 μM, living cells firstly bind almost all U(VI) within the first 5 min of incubation. But then algal cells mobilize up to 80% of the bound U(VI) during ongoing incubation. The release of metabolism related substances is suggested to cause this mobilization of U(VI). As potential leachates for algal-bound U(VI) oxalate, citrate and ATP were tested and found to be able to mobilize more than 50% of the algal-bound U(VI).
The formed U(VI)-algae-complexes were structurally characterized by TRLF, EXAFS and ATR-FTIR spectroscopy. Obtained results demonstrate carboxylic and organic/inorganic phosphate groups were involved in the U(VI) complexation with varying contributions dependent on cell status, U(VI) concentration and pH.
With the help of TEM U(VI) was detected in form of 30-70 nm needle-like deposits in the cell wall of living algae.