Soil-soil solution distribution coefficients of global fallout 239Pu and 237Np in Japanese paddy soils

Abstract

239Pu and 237Np are long-lived radionuclides that emit alpha rays, and once released into the environment, they are present there for a long period of time. Therefore,239Pu and 237Np are important radionuclides in the safety assessment related to geological disposal of radioactive waste because of the possibility of long-term exposure to humans. Mobilities of these radionuclides in the environment are of particular interest; therefore, in this study, we have made the first-time determination of the soil-soil solution distribution coefficient (Kd, L/kg) using global fallout 239Pu and 237Np in soil. The Kd values were determined by extracting these radionuclides from 23 soil samples using a laboratory batch method. The desorption Kd values of 239Pu were found to vary from 3.2 × 103 to 1.4 × 105 L/kg, and their geometric mean (GM) and arithmetic mean (AM) were 2.3 × 104 L/kg and 3.2 × 104 L/kg, respectively. The desorption Kd values of 237Np ranged from 8.9 × 102 to 2.1 × 104 L/kg, and their GM and AM were 4.1 × 103 L/kg and 6.4 × 103 L/kg, respectively. A comparison between the obtained Kd values of 239Pu and 237Np indicated that the former were about 6-fold higher than the latter in Japanese paddy field soils. Pearson's correlation analysis suggested that the main factors contributing to the sorption and desorption of Pu isotopes in Japanese paddy soils were related to the amounts of Fe and Mn oxides, while the sorption and desorption of 237Np in them would be controlled by an ion-exchange reaction, and/or complex-formation with organic matter, especially in Fluvisols.