Effect of Humic Substances on Plutonium and Americium Speciation in Soils and Soil Solutions

The molecular-weight distribution of humic substances (HSs) of the organic and mineral soils and corresponding soil solutions was studied by gel chromatography, and their distribution over fractions of humus acids was evaluated by the Ponomareva-Plotnikova chemical fractionation technique. Distribution of ^239,240Pu and ²⁴¹Am over HS fractions of different dispersity was studied. In these fractions, HSs are associated with different mineral components and have different solubility and mobility. Plutonium and americium are more strongly associated with humic substances of organic soils as compared to those of mineral soils. Therefore, in passing from mineral to organic soils, the migration ability of radionuclides decreases. These effects are caused by qualitative difference in the HS chemical composition of the HSs of organic and mineral soils. As compared to plutonium, americium enters the composition of poorly soluble organomineral complexes to a smaller extent, forming to a greater extent mobile anionic complexes in the soil solutions. This difference can be responsible for enhanced americium mobility and its easier availability to plants.