Saturday, 15 July 2006

Determination of Bioavailable Rhenium Fraction in Japanese Agricultural Soils.

Keiko Tagami and Shigeo Uchida. National Institute of Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba, 263-8555, Japan

Rhenium (Re) is a trace element for which the concentration level in the earth's 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. The soil samples were air-dried and passed through a 2-mm mesh sieve. For total Re determination, an alkaline fusion method was used, and for water soluble Re fraction determination, the soil sample was kept contact with deionized water for 7 days to extract the fraction. Both water soluble and total Re concentrations were determined by inductively coupled plasma mass spectrometry (Yokogawa analytical systems, 7500a). Since Re concentrations in soil samples were expected to be very low, before ICP-MS analysis, separation and concentration procedures for the element were necessary from matrices using TEVA resin (Eichrom Industries). Chemical recovery was determined with enriched Re-185 tracer. The results showed that the concentrations of total Re were almost the same among the different land uses with a geometric mean of 0.28 ng/g: the total Re was given as a probabilistic value with a log-normal distribution. Interestingly, Re was retained in the paddy soils although it is thought to be highly soluble in water as perrhenate. For all soil uses <0.1-40% of total Re is water soluble in Japanese soils. This lower than expected Re solubility from soil than expected is the limitation factor for Re absorption by plants. In the paddy field soils, the average water soluble Re ratio was slightly higher than the other two uses (16%). However, there was no relation between the amounts of total Re and water soluble Re. Acknowledgement: This work has been partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry (METI), Japan.

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