Presentation The bioavailability of Pu in Japanese rice paddy soils

Ni, Youyi  ,  Men, Wu  ,  Zheng, jian  ,  Tagami, Keiko  ,  Uchida, Shigeo

Plutonium in the environment has caused public concern due to its radiotoxicity and long half-lives of its main isotopes 239Pu (T¬1/2=24110 yeas) and 240Pu (T1/2=6563 years). The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident once more highlighted the necessity to study the transfer of Pu in the biosphere for the purpose of radiological assessment. Since rice is the staple food in Asian countries, it is important to investigate the bioavailability of Pu in paddy soil to rice for which can control soil-plant transfer. Transfer factor (TF) is defined as the ratio between radioactivity concentration of a radionuclide in the dried plant tissue (Bq/kg-dry) and that in the dry mass of the soil in rooting layer (Bq/kg-dry), which might corresponds to bioavailability of the radionuclide in the soil. However, this traditional definition has been criticized to be oversimplified since only a certain fraction of the total radionuclide in the soil is available for plant uptake. In this study, we employed available transfer factor (ATF) for the evaluation of bioavailability of Pu in rice paddy soils to rice in Japan. The ATF of Pu is defined as the ratio between Pu concentration in rice and the bioavailable Pu concentration in the corresponding paddy soil [1]. We estimated that the bioavailable fraction of Pu in soil was the sum of water soluble fraction and exchangeable fractions of Pu which were extracted by H2O and 1M NH4OAc, respectively. The whole analytical method for the bioavailable Pu determination consists of H2O/NH4OAc extraction, Fe(OH)3 co-precipitation,two-stage ion exchange chromatography (AG 1X8 + AG MP-1M) and then Pu was finally measured with SF-ICP-MS.The bioavailable fraction of Pu in rice paddy soils ranged from 0.03 to 0.24%; thus, only a very small fraction of Pu would be available for rice uptake. The bioavailable fraction of Pu in rice paddy soil samples were one order of magnitude lower than the values in upland soils reported previously [2,3], indicating the possible influences of soil types and Pu loss from paddy soils during irrigation. The ATF of Pu from paddy soil to rice ranged from 6.7×10-3 to 8.0×10-2, about three orders of magnitude higher than the traditional TF (10-5) due to the low bioavailable fraction of Pu. Slight differences were found between ATF values of different sampling sites for rice, probably as a result of different rice types. References [1] Baeza A et al. J.Environ. Radioact. 2005, 81(1): 89-106. [2] Sokolik G A et al. Environ. Int. 2004, 30(7): 939-947. [3] Guillén J et al. J. Environ. Radioact. 2015, 144: 96-102.Acknowledgements: This work was partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, trade and Industry (METI), Japan. Youyi Ni thanks the China Scholarship Council for offering a scholarship (201606010056) to support his Ph.D. study.

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