KINETICS OF CADMIUM UPTAKE BY GREEN-ALGAE

The kinetics of cadmium uptake by Selenastrum capricornutum Printz was studied at 21 degrees C and 0 degrees C 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 degrees C, the cadmium uptake increased during the experimental sampling times from 0.12 to 50 h, whereas at 0 degrees C, 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 (K-R,K-F) and the slow process (K-R,K-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 (k(1) and k(2)) were quantified; for two experiments, the mean value for k(1) was 111 +/- 28 L mol(-1) h(-1) and the mean value for k(2) 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.