ABSTRACT
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 derived. The model was tested by chemical speciation, i.e. differentiating between reduced Tc-compounds and Tc(VII)O(4)(-). The TcO(4)(-) concentrations measured were in good agreement with those predicted by the model. Two processes determine technetium uptake: (1) transport of Tc(VII)O(4)(-) across the cell membrane, and (2) reduction of Tc(VII). The TcO(4)(-) concentration in duckweed reaches a steady state within 2 h while reduced Tc-compounds are stored, as a result of absence of release or re-oxidation processes. Bioaccumulation kinetic properties were derived by varying 99Tc concentration, temperature, nutrient concentrations, and light intensity. The reduction of technetium in duckweed was highly correlated with light intensity and temperature. At 25 degrees C the maximum reduction rate was observed at light intensities above 200 µmol m(-2) s(-1) while half of the maximum transformation rate was reached at 41 µmol m(-2) s(-1). Transport of TcO(4)(-) over the cell membrane requires about 9.4 kJ mol(-1), indicating an active transport mechanism. However, this mechanism behaved as first-order kinetics instead of Michaelis-Menten kinetics between 1x10(-14) and 2.5x10(-5) mol l(-1) TcO(4)(-). Tc uptake could not be inhibited by 10(-3) mol l(-1) nitrate, phosphate, sulphate or chloride.
ABSTRACT
A novel chromatographic technique is described which has prospects for studying the lability of a metal complex in an aqueous system. It is based on interactions of metal species with an ion-exchange column under steady-state conditions. For this purpose, the column is equilibrated with the sample itself by using it as the mobile phase. With the aid of a high specific activity radiotracer, the characteristics of the metal species interaction with the ion-exchange column can be visualized in a radiochromatogram. Under particular experimental conditions, information regarding the dissociation kinetics of the metal complex can be extracted from this radiochromatogram. Erroneous results due to undesirable interactions of metal species with the chromatographic system are prevented, since the system remains in constant chemical equilibrium with the sample to be analyzed. The potential of this technique was investigated with copper complexes, using high specific activity 64Cu for labeling. Preliminary results obtained with four complexes (Cu-EDTA, Cu-NTA, Cu-citrate, and Cu-glycine) are discussed. For three complexes, dissociation rate constants could be determined: (6.2 +/- 0.3) x 10(-3) s(-1) for Cu-EDTA, (1.0 +/- 0.04) x 10(-2) s(-1) for Cu-NTA, and (3.1 +/- 0.2) x 10(-2) s(-1) for Cu-citrate. No dissociation rate constant could be determined for Cu-glycine owing to incompatible experimental conditions.
