Uptake kinetics of Tc-99 in common duckweed

The uptake of the nuclear waste product technetium-99 was studied in common duckweed (Lemna minor). In addition to measurements, a model involving two compartments in duckweed with different chemical forms of technetium was d erived. The model was tested by chemical speciation, i.e. differentiating b etween reduced Tc-compounds and (TcO4)-O-VII. The TcO4- concentrations meas ured were in good agreement with those predicted by the model. Two processe s determine technetium uptake: (1) transport of (TcO4)-O-VII- across the ce ll membrane, and (2) reduction of Tc-VII. The TcO4 concentration in duckwee d reaches a steady state within 2 h while reduced Tc-compounds are stored, as a result of absence of release or re-oxidation processes. Bioaccumulatio n kinetic properties were derived by varying Tc-99 concentration, temperatu re, nutrient concentrations, and light intensity. The reduction of techneti um in duckweed was highly correlated with light intensity and temperature. At 25 degrees C the maximum reduction rate was observed at light intensitie s above 200 mu mol m(-2) s(-1) while half of the maximum transformation rat e was reached at 41 mu mol m(-2) s(-1). Transport of TcO4 over the cell mem brane requires about 9.4 kJ mol(-1), indicating an active transport mechani sm. However, this mechanism behaved as first-order kinetics instead of Mich aelis-Menten kinetics between 1 x 10(-14) and 2.5 x 10(-5) mol l(-1) TcO4. Tc uptake could not be inhibited by 10(-3) mol l(-1) nitrate, phosphate, su lphate or chloride. (C) 2000 Elsevier Science B.V. All rights reserved.