Effect of ashing temperature on accurate determination of plutonium in soil samples

Abstract

An acidic leaching method using HNO 3 is widely employed to release the global fallout Pu from soil samples for further chemical separations in radioecology and toxicology studies and in many applications using Pu as a useful tracer. In the methods sample ash treatment step to decompose organic matter in soil, various ashing temperatures (400-900 °C) are used; however, the effect of ashing temperature on the accurate Pu analysis has not been well investigated. In this study, two standard reference soils (IAEA-soil-6 and IAEA-375) were used to determine the ashing temperature effect (from 375 to 600 °C) on the HNO 3 leaching method. The Pu analytical results of both standard reference materials showed that lower 239+240 Pu activity was observed when the ashing temperature exceeded 450 °C, and the 239+240 Pu activity continued to decrease as the ashing temperature was raised. Approximately 40% of the Pu content could not be leached out by concentrated HNO 3 after ashing for 4 h at 600 °C. The Pu loss was attributed to the formation of refractory materials, which are insoluble in HNO 3 solution. This hypothesis was confirmed by the XRD analysis of soil samples, which revealed that plagioclase-like silicate materials were formed after high-temperature ashing. To ensure Pu release efficiency in HNO 3 leaching, we recommend 450 °C as the ideal ashing temperature. This recommendation is also useful for analysis of other important artificial radionuclides (e.g., 137 Cs, 90 Sr, 241 Am) for which an ashing process is needed to decompose the organic content in soil samples.