Influence of Microbial Activity on Technetium Behaviour in Soil Under Waterlogged Condition
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INFLUENCE OF MICROBIAL ACTIVITY ON TECHNETIUM BEHAVIOUR IN SOIL UNDER WATERLOGGED CONDITION
K. TAGAMI AND S. UCHIDA Division of Radioecology, National Institute of Radiological Sciences, 3609 Isozaki, Nakaminato-shi, Ibaraki 311-12, Japan
ABSTRACT
The behaviour of Tc in soil depends upon its chemical forms. Under aerobic conditions, Tc is present as pertechnetate Tc+7 (TcO4 -), which has a high geochemical mobility and bioavailability. However, the form changes through a combination of factors such as redox conditions and microbial activity in soils. The anaerobic condition canbe supplied in waterlogged rice paddy soil, which is common to Japan and other Southeast Asian countries. In this study, we focused on the influence of microbial activity on Tc behaviour in soil under the waterlogged condition. The microbial activity is expected to be controlled by addition of glucose and the activity causes a change in the soil redox condition. Air-dried soil with increasing glucose contents and sterile soil were used to compare the effect of microbial activity. The soils were waterlogged with 99Tc solution. Although the concentrations of 99Tc in the surface and bottom solutions of air-dried soils decreased over time with increasing glucose concentration, those of sterile soils were almost constant during the experiment. From these results, it was assumed that the activity of microorganisms influenced Tc adsorption onto the soil.
INTRODUCTION
Technetium-99 is a pure R -emitting radionuclide, which has a long half-life (2.1 X 10' y) and relatively high fission yield from 235U and 239Pu (roughly 6%). The sources of environmental 99Tc are nuclear weapon tests and nuclear industries such as nuclear fuel reprocessing plants, nuclear power plants and waste disposal facilities. Besides these, the increasing use of 99'Tc in nuclear medicine has introduced another environmental source, since 99mTc directly decays into 99Tc with a 6.02 h half-life. With more 99Tc in the environment, it is important to determine its behaviour for dose assessment. Technetium is known to exist in all valence states from +7 to -1. The most stable form of Tc in natural aqueous solutions in equilibrium with the atmosphere is the pertechnetate form, TcO4 [1]. It has been reported that Tc present in anionic species could migrate freely through soils and clays [2], and also could be readily taken up by plant roots [3]. The migrations of Tc in soil and in root uptake by plants, however, are related to the sorption behaviour of Tc in the Mat. Res. Soc. Symp. Proc. Vol. 353 01995 Materials Research Society
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soil-water system. For example, under relatively low redox conditions, pertechnetate in solution is reduced to insoluble dioxide, TcO2 [4, 5, 6]. In the natural environment, the chemically reducing condition occurs in deep underground; and it has been found that Tc has a high adsorbability onto minerals [7]. Even in surface soil, the reducing condition is generated by microbial activity and/or chemical reactions. According to Stalmans et al. [5], Tc
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