presumptive evidence for the presence of a dispersible ligand in higher 138 pa/g * 1,700 Least Mobile Species Molecular Diffusicn Weight, Coefficient g/moale 1077 x Hesson Salkum Muscateen Ritzville Quillayute 3 .0 2. 5 2 4 1 5 1 3 1,790 5,000 7,200 8,100 21,000 13,000 24 47 9 55 36 pg/g* 139 pa/g of soll. *Plutonium added at a ievel of 620,000 53 centration, _ Soils Several conclusions may be drawn from studies of the soil chemistry of Pu which have important implications in terms of the potential role of the soil microbiota in influencing Pu behavior in soil. Definition of Pu solubility by filtration or diffusion alone is complicated by Pu chemistry, but, in conjunction, the measurements suggest that mobile Pu is largely particulate. However, a fraction of the mobile Pu is available to plants. mmr a4 eos ea . NAAN 5 8 methods employed. Insight into this possibility was not provided by comparison of Pu 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. Pug (DTPA) 4 with insufficient stability and/or concentration to be detected by the Control The comparison of filtration and diffusion data indicates that the mobile Pu in incubated soils was in the form of hydrated oxide of hydroxide ina continuum of sizes. If it can be assumed that Pu in particulate form was not available to plants, it is possible that the small fraction of Pu taken up by plants was present in soil as reaction or dissolution products Weight, g/mole This concentration of Pu in soil approximated the quantity of water-soluble Pu passing the 0.0015 uw ultrafiltration membrane (Table 1). Hypothetical globular proteins in this size range would have average molecular weights < 10,000. Particies of Pu(OH}, 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 Pu were particulate Pu(OH)}, or hydrated oxides. Coefficient x 1076 of 150 - 1200 pg/g (Table 2). Treatment The estimated diffusion coefficients for the least mobile Pu components ranged from 2,3 - 3.1 x 107? cm’/sec with corresponding soil concentrations Soil Con- the molecule was a Molecular if piffusion However, Most Mobile Species through this membrane. Plutonium in Soils from Estimated Concentrations and Molecular Weights of Mobi Je 1977). Measured Diffusion Coefficients (Garland and Wildung, than 500 would pass hydrated PuO; sphere of similar dimensions, it would have a molecular weight between 10,000 and 25,000 approximating the molecular weights of the most mobile Pu species as determined from diffusion coefficients. This fraction, therefore, likely consisted of small particles of Pu(OH),y or hydrated oxide. Table 2. Applying diffusion principles to characterization of mobile Pu spectes in soila, Garland and Wildung (1977) estimated the concentrations and molecular weight of mobile Pu in five surface soils representing a range in particle size distributions, pH (4.4-6.2) organic C (0.7 to 12.5%) and cation exchange capacities (14 to 45 meq/100 g). The diffusion coefficients calculated for the most mobile species in the five soils varied From 1.5 to 3.0 x 10°* cm’/sec (Table 2}. Estimated concentrations and molecular weights of the most mobile Pu components in the five soils ranged from 93 to 55 pe/g and from 5000 to 21,000, respectively. Thus, estimated concentrations of the most mobile Pu species were of the same order of magnitude as those observed by water extraction and subsequent ultrafiltration through the 0.0010 pp membrane (Table 1). This membrane retained Pu-DTPA (molecular weight 1700). Hypothetical globular peptides of molecular welghts less Soil Concentration, concentration in the Muscatine soil.