||Simultaneous determination of radiocesium and Pu isotopes in river suspended particles by ICP-MS/MS and SF-ICP-MS
曹, 立国 ,
鄭, 建 ,
Tsukada, Hirofumi ,
田上, 恵子内田, 滋夫
Due to the Fukushima nuclear accident in March 2011, great amounts of radionuclides were released into the environment. Among the released various radionuclides, radiocesium and Pu isotopes are important for radiological assessment due to their relatively long half-lives and radioactive and chemical toxicity. Because of their expected persistent presence in the environment, it is essential to have a better understanding of their behavior in a series of environmental processes. For this purpose, powerful geochemical tracers which are capable of differentiating Fukushima accident-derived radiocesium and Pu from the global fallout source are highly required. Fukushima accident-released radiocesium has been characterized by a 135Cs/137Cs isotopic ratio of 0.34-0.39, which is different from those of global fallout (ca. 2.7) and the Chernobyl accident (0.5). Similarly, Fukushima accident-released Pu has its unique isotopic composition. Thus, 135Cs/137Cs and 240Pu/239Pu isotopic ratios have been considered as useful tracers for long-term estimation of environmental behaviors of the released radionuclides. Considering the fact that these radionuclides could be transported from forest to the ocean via inland river runoff, it is extremely necessary to quantify the Fukushima accident derived radioceium and Pu in river suspended particles, which is important for understanding the migration mechanism and fate of the released radionuclides. However, collection of large amount river suspended particles is a very difficult task, the sample size available for analysis is normally limited. In this study, we report a new analytical method for simultaneous determination of radiocesium (135Cs and 137Cs) and Pu isotopes (239Pu and 240Pu) in small sample size of 1-2 g river suspended particles. River suspended particles were collected from Kokuni, Minami Soma, Okuma and Nagadoro, in Fukushima prefecture from June 2013 to August 2014, using a solids-retaining centrifuge. After nitric acid digestion of the river suspended particles, radiocesium and Pu were sequentially pre-concentrated by using ammonium molybdophosphate and ferric hydroxide co-precipitation, respectively. After two-stage ion-exchange chromatography separation, radiocesium and Pu were measured by ICP-MS/MS and SF-ICP-MS, respectively. The developed analytical method was validated by the analysis of soil reference materials (IAEA 375, IAEA soil-6 and JSAC-0471). The results showed that 135Cs/137Cs isotopes in the investigated river suspended particles ranged from 0.339 to 0.39, with an average of 0.36 ± 0.06, indicating that the radiocesium primarily derived from the accident. Additionally, the detected 240Pu/239Pu isotopic ratios ranged from 0.182-0.208, suggesting that global fallout was the main source of Pu contamination in suspended particles. Acknowledgements: This work was partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, trade and Industry (METI), Japan.