308 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 2 Estimated Concentrations and Molecular Weights of ^Mobile Plutonium 

 in Soils from Measured Diffusion Coefficients* 



*From Garland and Wildung (1977). 



fPlutonium added at a level of 620,000 pg/g of soil. 



have a molecular weight of between 10,000 and 25,000, which approximates the 

 molecular weights of the most mobile plutonium species as determined from diffusion 

 coefficients. This fraction, therefore, likely consisted of small particles of Pu(0H)4 or 

 hydrated oxide. 



The estimated diffusion coefficients for the least mobile plutonium components 

 ranged from 2.3 to 3.1 x 10"^ cm' /sec with corresponding soil concentrations of 150 to 

 1200 pg/g (Table 2). This concentration of plutonium in soil approximated the quantity 

 of water-soluble plutonium passing the 0.001 5-jum ultrafiltration membrane (Table 1). 

 Hypothetical globular proteins in this size range would have average molecular weights of 

 < 10,000. Particles of Pu(0H)4 or hydrated oxides would have molecular weights of 

 200,000 to 500,000. Estimated molecular weights for these least mobile species 

 calculated from diffusion coefficients were between 600,000 and 900.000. Thus it would 

 appear, as in the case of the most mobile species, that the least mobile species of 

 plutonium were particulate Pu(0H)4 or hydrated oxides. 



The comparison of filtration and diffusion data indicates that the mobile plutonium 

 in incubated soils was in the form of hydrated oxide or hydroxide in a continuum of 

 sizes. If it can be assumed that plutonium in particulate form was not available to plants, 

 it is possible that the small fraction of plutonium taken up by plants was present in soil as 

 reaction or dissolution products with insufficient stability and/or concentration to be 

 detected by the methods used. Insight into this possibility was not provided by 

 comparison of plutonium behavior in different soils, as might be expected, because the 

 estimated concentrations and molecular weights of the mobile species were not related to 

 the soil properties measured. 



Several conclusions can be drawn from studies of the soil chemistry of plutonium 

 which have important implications in terms of the potential role of the soil microbiota in 

 influencing plutonium behavior in soil. The definition of plutonium solubility by 

 filtration or diffusion alone is compHcated by plutonium chemistry, but, in conjunction, 

 the measurements suggest that mobile plutonium is largely particulate. However, a 

 fraction of the mobile plutonium is available to plants. 



