The impact of Pu speciation on distribution coefficients in Mayak soil

To assess the long-term consequences when radionuclides are released into t he environment, information on the source term, transport and transformatio n processes, interaction with soils (K-D) and biological uptake (CF) is nee ded. Among the artificial radionuclides released to the environment by nucl ear activities, the transuranium elements are a major concern, due to very long half-lives and their accumulation in bone as well as high radiotoxicit y. Plutonium has been produced in greater quantity than other transuranic e lements, however, environmental assessments are complicated by the complex environmental behaviour. Physico-chemical forms of Pu will determine the in teractions with soils and, thus, the degree to which soils can act as a sin k or a potential diffuse source of contaminants. In the present work, dynam ic tracer experiments have been performed where different Pu-species are ad ded to a 'Mayak soil-rainwater system' to obtain information on K-D values. After a defined contact time, the samples where then sequentially extracte d and results are used in a dynamic box model to estimate interaction and f ixation rates. The interaction of all Pu-species with soils seems to be rap id and follows a two-step reaction. Up to contact times of a few weeks, the K-D for Pu(III,IV) (730 +/- 240 1/kg) is approximately one order of magnit ude higher than for Pu(V,VI) (90 +/- 20 1/kg) and Pu(III,IV)-organic (40-60 1/kg). After 3 months contact time, the K-D in only the two organic-bound Pu-species were significantly lower. This shows that the initial associatio n with the soil is dependent on the Pu-species in the rainwater. After only 1 h of contact, between 33 and 40% of the plutonium was strongly bound to the soil components, i.e. only extractable with strong HNO3. The extraction of soil-bound Pu followed a similar pattern for all the original species, suggesting that the next step of Pu interaction mechanism with soil was rat her independent of the original species. For both the Pu(V,VI) and Pu-organ ic species, the rainwater-desorption extract gave consistently higher K-D v alues than that calculated from the rainwater-sorption data; whereas for Pu (III,IV), desorption K-D values were more similar to sorption K-D values. T his supports the suggestion that the observed difference in Pu adsorption t o soils reflects Pu-speciation in the water soluble phase, and that actual soil-Pu interactions are rather independent of the original speciation. Mod elling of the extraction data show a different in association rate for the different Pu species, where the Pu(III,IV) has the fastest association rate as expected. (C) 2000 Elsevier Science B.V. All rights reserved.