From the aforementioned results, the behavior of 14C in rice paddy fields could be considered as follows (a conceptual diagram appears in Fig. 26.5). When irrigation water is contaminated by 14C-bearing sodium acetate, the 14C compound is taken up and metabolized by indigenous bacteria. A part of the 14C is assimilated by the bacterial cells, and the rest of the 14C is released as gaseous compounds from the cells as a result of dissimilation. The dominant chemical species of 14C in gas forms is carbon dioxide, and thus some of the released 14CO2 is dissolved in soil solution depending on pH. For example, when the pH of the soil solution is less than 6.5, most of 14C in gas forms is released into the air. The released 14CO2 is eventually taken up by rice plants during photosynthesis. When the pH of the soil solution is between 6.5 and 10.5,14C-bearing bicarbonate ion dominates in the soil solution. In addition, once 14CO2 has been released into the air, a part of the 14CO2 gas may be redissolved in the soil solution again as bicarbonate ion. When the pH of the soil solution is greater than 10.5, although this is not probable in paddy fields, 14C — bearing carbonate ion dominates in the soil solution. Carbonate ion is thermally unstable and thus precipitates as carbonate minerals such as CaCO3. In these alkaline situations, the ratio of 14C in the solid phase may increase as a result of the precipitation of 14C. Because the root uptake of 14C by rice plants is negligible, gasification of 14C is an important environmental transfer pathway for the safety assessment of TRU wastes, and bacteria are responsible for driving this pathway.

Acknowledgments 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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