The kinetics of cadmium uptake by Selenastrum capricornutum Printz was studied at 21 ~ and 0 ~ and at four different concentrations of free cadmium (10 -9, 10 -8, 10 -7 and 10 -3 M). The free cadmium concentrations in the medium were kept constant through buffering by a chelating agent (NTA).
Initial uptake occurred within 10 min at both temperatures, indicating a fast process. At 21 ~ the cadmium uptake increased during the experimental sampling times from 0.12 to 50 h, whereas at 0 ~ the cadmium uptake remained constant after the first sampling time. Apparently, two different processes were involved in cadmium uptake, a first fast, probably passive process and a second slower, possibly physiological process.
The cadmium uptake processes are described with a generalized metalligand complex formation model. The experimental dissociation constants for the fast (Kn,F) and the slow process (KR,s) were reproducible for different experiments and they were both in the order of 5 10 -6 mol L -1. For the slow process, the uptake-and release rate constants (kl and k2) were quantified; for two experiments, the mean value for kl was 111 4-28 L mol-l h-l and the mean value for k2 was (5.1 + 1.8) 10 -4 h -1. These values were calculated using the fitted value of the cadmium uptake in the fast process.