Determination of bioavailable rhenium fraction in Japanese agricultural soils

Rhenium (Re) is a trace element for which the concentration level in the earth\u27s upper crust is low, e.g, 0.4 ng/g. Due to the behavioral similarities of Re and technetium (Tc) in the environment, Re is of great interest from the viewpoint of long-term environmental dose assessment; Re can be studied as a chemical analogue for fissiogenic Tc-99, the behavior of which is not well known. Tc-99 is of potential long-term importance in the environmental dose assessment because it has a long half-life of 211000 y and it is produced in the fissions of U-235 and Pu-239 at relatively high ratios (ca. 6%). As a result of fallout from nuclear weapons testing and discharges from nuclear facilities, the nuclide is widely distributed in the environment. However, Tc-99 has no stable isotopes and its amount in the natural environment is at ultra-trace levels even in soil. Interestingly, although Tc and Re are non-essential elements for plants, they are considered to be highly mobile in biogeochemical cycles; indeed, for Tc, a transfer factor of 5 (on wet weight basis) in the edible parts of common plants was recommended by an IAEA publication in 1982. It is well known that Tc and Re can be easily absorbed by plants as water soluble anionic forms, pertechnetate and perrhenate, respectively. Thus, in this study, water soluble Re in soil was determined. The water soluble (bioavailable) Re fraction in various soils under natural conditions was verified using 55 Japanese agricultural soils, that is, 21 paddy soils, 19 upland soils and 15 other land use soils, e.g., orchards.18th World Congress of Soil Scienc