Uptake, biotransformation, and elimination of 99Tc in duckweed.

Aquatic plants may play an important role in the environmental fate of the long-lived radioactive waste product 99Tc. Aquatic plants show a strong accumulation and retention of Tc, even after they have died. This study focuses on possible bio-organic Tc compounds formed in the water dwelling plant duckweed to possibly explain the accumulation and retention. Moreover, a change in chemical speciation often implies a different fate and behaviour in the biosphere. A mild separation technique was used to distinguish between reduced Tc species and TcO(4)(-). Accumulation experiments suggested that reduction of Tc(VII)O(4)(-) and subsequent complexation are responsible for the accumulation of Tc in duckweed. A steady state concentration of TcO(4)(-) in duckweed was reached within 24 h, but the total concentration of Tc increased continuously. Only a small part (</=5%) of Tc was present as TcO(4)(-). Elimination experiments showed that TcO(4)(-) is the only mobile species. Other Tc species are responsible for the retention of Tc in duckweed. It is known that these species are not bio-available and only slowly re-oxidise to pertechnetate, resulting in a longer residence time in ecosystems.