Influence of salinity and eutrophication on bioaccumulation of 99technetium in duckweed

This study concerns the bioaccumulation of the long-lived nuclear waste product 99Tc in duckweed (Lemna minor L.). 99Tc was present as the oxyanion TcO4-, being the main chemical form of technetium in aerobic water systems. In contrast with terrestrial plants, bioaccumulation in duckweed proved to be independent of the nitrate concentration in the medium. However, uptake is controlled by electrostatic effects in the cell wall, which affects the bioaccumulation of 99Tc in duckweed in natural environments. These waters are characterized by a range of salinity and hardness, and this study suggests that this may result in up to a threefold difference in 99Tc accumulation. Because of screening of negative charges in the cell wall, the highest accumulation may be expected in hard, brackish water. This behavior can be described by a general model, which includes electrostatic effects and binding of cations at the cell wall. The model also explains why cationic radionuclides are preferably taken up in soft, fresh water while anionic species are concomitantly taken up in hard, chlorine-rich waters.