Mobility and bioavailability of technetium in rice paddy fields

Field observations and radiotracer experiments were carried out to investigate the behavior of 99Tc in paddy fields. The concentrations of global fallout 99Tc in soil collected from paddy fields in Japan were 6 - 88 milli-bequerels per kilogram (mBq/kg)-dry and activity ratios of 99Tc to 137Cs ranged from 1.1 x 10-3 to 7.0 x 10-3 with an average of (4.8 +/- 2.1) x 10-3. (The theoretical activity ratio from nuclear fission yield is presently calculated as 3.3 x 10-4 with correction for radioactive decay.) This result implies that there was a tendency for 99Tc to accumulate in rice paddy fields. To understand the mechanisms of the accumulation, a sequential extraction method was applied over a 6-month period to waterlogged soils contaminated with TcO4-. From the results, it was clear that the TcO4- in soil underwaterlogged conditions was readily changed to other insoluble physicochemical forms, such as TcO2, TcS2 and organically bound forms