Physico-chemical fate of technetium-99 and selenium-79 in soils - comparison of long term soil/plant transfers

The fate of technetium-99 and selenium-79 is guided by the soil characteristics (pH, organic matter, redox process, etc.). It would appear, however, that the fate of the isotope itself conditions migration in the soil and its availability for plants. As an oxidizer, technetium destroys certain organic molecules, among them cysteine, and becomes insoluble in the sulphur state; in the presence of iron hydrates acting as the bond cation, it is fixed in the same way as a molybdate, but can still be taken up by plants; the reduced form seems to be only a transitory one, the affinity of which for organic matter and hydroxyl groups represents a temporary form of immobilization. Losses through the percolation of isotopes in the soil as a function of the method of application, as compared with other radioisotopes, are fairly high. In the case of the soil/plant transfer, there are two types of value to be considered: long-term and short-term (2-3 year) values. Long-term values are recommended for technetium. In the short term and in a case of accident these values are higher, on average, by an order of magnitude. In the case of selenium, the initial form of application or the redox stage predominating in the soil are the two overriding factors: the type of plant organ considered also affects the value to be considered. (author)