Table 1. Solubilities of Plutonium in Water Extracts of a Ritzville Silt Loam as Determined by Filtration with Membranes of Different Pore Sizes (Garland and Wildung, 1977). Membrane Pore Size, Plutonium Solubility, vm pg/g* 5 0.45 0.01 0.0015 6.0012 0.9010 60,009 20,000 4,000 1,000 300 50 “Plutonium added at a level of 620,000 pg/q of soil, | | from particulate matter, it is evident that Pu in these filtrates may be in colleodial forms. The Pu in the 0.0010 vy filtrate appeared soluble, was stable in solution, and approximated the quantity of Pu taken up by plants (Wildung and Garland, 1974). Of the soluble Pu forms likely to enter soils (previous section), Pu({NO3), and Pu-DTPA likely represent, in their respective chemistries, the Tange in soil behavior likely to occur. The water solubility (< 0.01 p) of 7?%pu and 73%Pu amended to a Ritzville silt loam (organic C content 0.7%, pH 6.2) in the Pu(NO3),y and Pu-DTPA forms differs markedly (Wildung and Garland, 1975). The DTPA complexes of both isotopes were water-soluble in soil and appeared to be etable over the first 40 days of incubation (Fig. 4). After 7 days of incubation, the 7°*Pu-DTPA appeared to be slightly less soluble then the 7**pu-DIPA. After 95 days of incubation, both isotopes, initially added as the complex, appeared to decrease in solubility; perhaps as a result of microbial degradation of the organic mofety and the development of new chemical equilibria. Equilibrium concentrations of soluble Pu added as the nitrate were not obtained until 7-10 days. The solubility of 7?*Pu and 7°°Pu added to the soil as nitrates was much lower than the DTPA complexes, likely reflecting hydrolysis to the largely insoluble hydrated oxide, It is clear that organic ligands may have a pronounced effect on Pu solubility in soil. The tate of decrease in solubility of each isotope added as the nitrate was similar. However, in contrast to the slightly lower solubility of the ? 98 Ppu-DTPA compared to the 735 pu-DIPA, 24%pu added ag the nitrate was a consistent factor of 2-3 times more soluble than *?°Pu initially added as the nitrate. This difference probably resulted from the formation of larger hydrated oxide particles at the higher Pu concentration Cpu), but it may also have reflected the presence of soil components such as organic ligands, which stabilized Pu in solution but were present in Limited concentrations and became important only at lower Pu concentrations. The water solubility of 738pu, when incorporated in relatively large Pu oxide particles (*1 yy), would be expected to be greater than the solubility ef 73°pu oxide particles of similar size due to crystal damage and radiolysis arising from the greater specific activity of the 239pu (an approximate factor of 270). However, the behavior of the two isotopes in soil on solubilization of the oxide might be expected to follow a course similar to that exhibited by the nitrates (Fig. 4). Equilibrium solubility after 6 days of incubation (Garland et al., 1976) of Pu, added as Pu(NO3)}4, in soils of different properties occurred after approximately 20 hours (Fig. 5). The quantities of Pu soluble at equilibrium in water and 0.01 HMCaCl, differed with soil type. In the CaCl, solution, Bolubility was lowest in the Muscatine soil which exhibited higher silt and clay content than the other soils. Importantly, at equilibrium there was more Pu extracted by water than 0,01 M CaCl. in the Muscatine soil, The Hesson and Ritzville soils did not exhibit this property. This may be related to a difference in the dispersibility of fine colloids in this soil and/or the presence of higher concentrations of stabilizing ligands. However, the lack of a proportional dilution effect (not shown in Fig. 5) in the water extractability of Pu at lower solution to soil ratios in thia soil as compared to the Ritzville and Hesson soils, provided 135