Distribution and geochemical association of actinides in a contaminated soil as a function of grain size

Abstract

Summary The distribution of actinides in a contaminated soil as a function of grain size was investigated by determining 239,240Pu, 241Am, and 238U after size fractionation. The soil was contaminated as the result of a fire at a nuclear missile site in the US. In all cases, the activity of each isotope increased with decreasing particle size, although differences were observed for 238U. A simple spherical model was used to represent surface area as a function of grain size. Fitted values for the distributions of 239,240Pu and 241Am by this model correlated well with experimental values, suggesting that their association is a surface sorption phenomenon. Fitted values for 238U did not correlate with measured values, indicating that U association was not strictly a surface mechanism. To further investigate the partitioning of these isotopes within each size fraction, sequential extractions were performed on the various size fractions. 239,240Pu, 241Am, and 238U are associated with primarily reducible, acid sulfide soluble, and refractory phases, with more total activity associated with the refractory phase for the smaller size fractions. Considering only the final refractory phase for each size fraction, the activity of 238U was greater than 239,240Pu and 241Am. In the context of the behavior of U as a function of grain size, this result suggests that a significant fraction of the U is primordial and incorporated into the refractory matrix, most likely the silicates and refractory clay minerals.