Expérimental Study on the 134Cs Accumulation and Retention by a Planktonic Microalgae, Selenastrum capricornutum Printz
For a better understanding of the radiocesium transfer in a freshwater ecosystem, a study concerning a simplified trophic chain from the Fratel dam, in the River Tejo, is being developed under the CEC contract n° B16-0245-P and also the CEC contract n° B16-B-198-P. In this paper is presented the study of the interaction water - primary producer (Selenastrum tapricornutum Printz, an unicellular planktonic algae) in the presence of 134Cs. The accumulation experiments were carried out in confined medium, during stabilization of the culture development. When the microalgae were cultivated in labelled river water, without any chemical change except for the addition of 134Cs, the evaluated concentration factor was CF = (1,6 ± 0,2)103, referred to dry weight. Otherwise, when the water was enriched by several cations by the addition of nutrients, the concentration factor was lower, CF = (1,6 ± 0,2)103. In every case, equilibrium was reached within the first 24 h. The retention was also carried out in river water and was studied through three parallel experiments : the labelled microalgae were maintained respectively in a confined medium, in a renewed medium with one single renewal made after 24 h's experiment, and in a renewed medium with renewals made at each sampling time. In every case, the 134Cs loss obeyed a two-differential equation and the results obtained were similar, namely concerning the biological half-lives (Tb1 = 0.015 days in every case and Tb2 respectively 0.53, 0.42 and 0.54 days); therefore a total model could be deduced : R (t) = 76,7 e-45,0 t + 20,1 e-128 t (t in days) showing two biological half-lives, which were Tb1 = 0.015 and Tb2 = 0.54 days. Taking into account the biological dilution due to the biomass increase during the experiments, the elimination kinetics suggests that 134Cs is on the one hand adsorbed on the cell surface and on the other band is absorbed intracellularly.