Fractionation of some natural radionuclides in Albic Retisol

In the present work, the fractionation of heavy natural radionuclides was investigated in Albic Retisol in the Moscow region of Russia. Fractionation was performed using the Pavlotskaya method. ²³⁸U and ²³²Тh were measured via inductively coupled plasma mass spectrometry and ²²⁶Ra was measured using an alpha-radiometer that considered the decay of Ra isotopes and accumulation of the resulting decay-related products. Results showed that the potential mobility of radionuclides during migration to adjacent environments and biological availability to plants follows the order ²²⁶Ra > ²³⁸U > ²³²Тh. The depletion of ²²⁶Ra in the humus horizon A was manifested as a decrease in the ²²⁶Ra/²³⁸U activity ratio to below 1 in fraction F4.1 (associated with organic matter) and fraction F4.2 (acid-soluble compounds). The profile distribution of ²²⁶Ra and ²³⁸U in the residual fraction revealed the genetic relationship between these radionuclides. Mobile compounds of ²³⁸U and ²³²Th leached into the lower part of the profile where they became fixed, mainly as the components of fractions associated with organic matter and crystallized iron oxides. A similar profile distribution was found for ²³⁸U, ²³²Тh, and Fe in the acid-soluble fraction associated with crystallized iron oxides. The results obtained herein can form the basis for regulating the mobility and bioavailability of radionuclides to ameliorate their adverse effects on plants and soil ecosystems.