Aging effect on technetium behaviour in soil under aerobic and anaerobic conditions

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Abstract
Andosol and Gray lowland soil (GLS) are the two most common agricultural soils in Japan; in this study one fraction of each soil was kept under oxidizing conditions, while anaerobic conditions (simulating conditions in rice paddies) were allowed to develop in the other fraction. At various times over a 6‐month period, portions of the four combinations were contacted with selective extractants to determine the amount of Tc retained in soil solution and ion exchange sites of the soil, by acetic acid‐extractable sites, by soil organic solids, and by sesquioxides. The amount of dissolved and ion exchangeable Tc decreased to 20% of the initial concentration within 60 days for both soils under saturated (anaerobic) conditions. In the case of the GLS, the fraction of exchangeable Tc continued to decrease to less than 10% of the initial Tc at the end of 6 months. Under aerobic conditions, the amount of the ion exchangeable Tc in the Andosol rapidly decreased to 80% of the initial input, and then remained constant. In the aerobic GLS tests, however, the amount of Tc in this fraction decreased to 50% of the initial input over the 6‐month period. This decrease may result from the development of localized reducing conditions, despite the open contact with the atmosphere. In both anaerobic soils, and in the aerobic GLS, most of the non‐exchangeable Tc was approximately evenly divided between Tc released by the treatment used to assess sorption to solid organics, and Tc that was not released by any of the leaching agents. In both soils, 137Cs was rapidly converted to non‐exchangeable forms within 10 days of its addition. It is inferred that elements that are not in solution or retained on ion exchange sites are not readily available for plant uptake.