Chemical transformation of technetium in soil during the change of soil water conditions

The most stable form of Tc in the surface environment is TcO4−, which is known to have high mobility and availability to plants in the environment. The form can be readily changed to insoluble forms in soil under anaerobic conditions, such as in a rice paddy field. Because the anaerobic and aerobic conditions occur repeatedly in the paddy field, it was necessary to determine whether or not the immobilized Tc would re-oxidize and be transformed to TcO4− as the field is being air-dried. In this study, radiotracer experiments using a selective extraction method were carried out to investigate chemical transformation of Tc in soil during the change of soil water conditions. During 52-day waterlogging period, the percentage of Tc that was removed by 0.05M CaCl2 solution decreased gradually from 0.85 to 0.25. That is, TcO4− was transformed to insoluble chemical forms and adsorbed on to soil components. After the waterlogging period, the soil was air-dried for 43 days to measure the change of Tc extractability. The results showed that even though the soil was re-air-dried and kept in aerobic conditions, Tc extractability was almost the same as the initial value obtained at just the end of the waterlogging period. From these results, in a paddy field ecosystem, Tc would adsorb on to soil materials under a waterlogged condition and the Tc would hardly re-mobile. Further, Tc in soil became less available with time due to a shift to less readily oxidizable forms. This suggests that Tc would be accumulating in the soil under such conditions.