Use of Rhenium as a Chemical Analogue of Technetium in Soil-to-Plant Transfer Systems

The long-lived radionuclide Tc-99 is thought to be highly mobile in biogeochemical cycles, however, the behavior in the natural environment is not clear because Tc is a redox sensitive element. Since the geochemical behaviors of Re and Tc are similar, Re transfer factors from soil to plant can be a Tc surrogate in the natural environment. However, in soil solution to plant systems, it was not clear whether or not Tc and Re were taken up by plants similarly. Thus, a radiotracer experiment was carried out to compare the plant uptake ratios of Tc-95m and Re-183 from nutrient solutions. Radish plants were transplanted to nutrient solution culture containers including radiotracers, separately. At 1-3 d after the transplanting, the nutrient solutions were collected and the plants were separated into three parts (leaves, fleshy and fine roots) then each sample was oven-dried at 60 ̊C. The average concentration ratios of Tc-95m and Re-183 in the nutrient solutions after/before the exposure were almost 1. The results suggested that Tc and Re absorption rates by the plant were almost the same as water uptake. The Tc and Re absorbed through the fine roots were distributed to the leaves and fleshy roots. The distribution ratios for elements, defined as “activity in a plant part” divided by “total absorbed activity”, were depended on the plant species, however, the distribution ratios for Tc and Re were almost the same in a plant sample. Possibly, Re can be used as the chemical analogue of Tc in soil solution to plant uptake.