Technetium-99 Behavior in the Terrestrial Environment

Field Observations and Radiotracer Experiments

抄録

Obtaining data on ⁹⁹Tc in the rice paddy field environment is important because Tc is a redox sensitive element. The behavior of Tc is expected to be different under upland field and rice paddy field conditions since the redox conditions in the soil environment differ. However, most of the data on the nuclide behavior in soil were obtained under upland field conditions. To understand the global fallout ⁹⁹Tc distributions in soil samples collected in Japan, a simple and rapid separation method was developed in order to determine low-levels of ⁹⁹Tc in soil samples by an inductively coupled plasma mass spectrometry. Also, radiotracer experiments using soils under aerobic and anaerobic conditions were carried out to clarify the Tc behavior under paddy field conditions. The results of determination of global fallout ⁹⁹Tc in Japanese soils indicated that the radionuclide had been accumu-lating in rice paddy fields. The mechanisms can be explained by the immobilization of Tc in soil under anaerobic conditions. From the radiotracer experiments, it was clear that under waterlogged conditions, the highly mobile TcO₄^- in soil was readily changed to other immobilized forms, such as TcO₂, TcS₂ and organically bound forms. To this immobilization, the microbial activity seemed to have an important role in Tc sorption reactions. When the soil, which was once kept in anaerobic conditions, was air-dried again and kept in aerobic conditions, the chemical forms of immobilized Tc did not change remarkably. Interestingly, the similar Tc behavior was observed in a real wet forest near the Chernobyl Reactor.