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

Abstract

The distribution of actinides in a contaminated soil as a function of grain size was investigated by determining 239,240 Pu, 241 Am, and 238 U 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 238 U. A simple spherical model was used to represent surface area as a function of grain size. Fitted values for the distributions of 239,240 Pu and 241 Am by this model correlated well with experimental values, suggesting that their association is a surface sorption phenomenon. Fitted values for 238 U 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. 239,240 Pu, 241 Am, and 238 U 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 238 U was greater than 239,240 Pu and 241 Am. 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.

Assessment of a sequential phase extraction procedure for uranium-series isotope analysis of soils and sediments

“…The U concentration increased by 6.6% after removal of Fe and Mn oxides,376showing that there is little or no U in the removed phase. A similar observation was reported 377 byLee et al (2004), while leaching soil samples with ammonium acetate. When the leaching 378 procedure was complete 0.9% of U was removed from the sample.379When the clay-size fraction was subsequently removed from the leached sample, U380 concentration decreased by 17% compared with the previous stage, indicating the presence of 381 a large proportion of U in the size fraction below 2 µm.…”

“…Removal of a fraction with a 450higher ( 230 Th/ 238 U) than the equilibrium value will leave the residue with a ( 230 Th/ 238 U) of < 451 1, and the same was observed here. Removing the clay-sized fraction from the residue again 452 increased the ( 230 Th/ 238 U), indicating that the ratio in the removed fraction is lower than the 453 residue from the previous step.454In summary, when the sequential extraction procedure for removal of different phases 455 is applied to a soil sample with little or no secondary phases, the U-series isotope ratios vary 456 in a similar way to that observed with soil/sediment samples containing secondary phases 457(Blanco et al, 2004;Lee et al, 2004;Schultz et al, 1998;Plater et al, 1992).…”

Characterization of Pu concentration and its isotopic composition in soils of Gansu in northwestern China

“…Lee et al (2004) indicated that Pu tended to partition to specific soil particles rather than to sorb homogeneously to all particles. As shown in Table 1 and Fig.…”

Activities of Pu and Am Isotopes and Isotopic Ratios in a Soil Contaminated by Weapons-Grade Plutonium

“…However, for the various size fractions of the BOMARC soil, the calculated value of 241 Am (AAm ) 1.81 Bq/g) was a little higher than the measured activity concentration of 241 Am (1.08 Bq/g) ( Figure 3). These results suggest that the 241 Am produced from 241 Pu decay is less refractory (19) …”