RELATIONSHIP OF MICROBIAL PROCESSES 305 



A reasonable approach to the study of the chemistry of plutonium in soil is to direct 

 initial attention to the factors influencing its solubility in soil. However, plutonium 

 solubility in soil is difficult to define because solubility will depend on the method of 

 measurement and because solubility must be arbitrarily evaluated because of the sorption 

 of plutonium on submicron clay particles and the formation of submicron particles of 

 hydrated plutonium oxide, which are difficult to centrifuge and may pass membrane 

 filters. These effects can be illustrated by comparison of the differences in the solubility 

 of plutonium in soils [100 days after amendment as Pu(N03)4] as determined by water 

 extraction and subsequent membrane filtration with the use of membranes of different 

 average pore sizes (Table 1). The major fraction of the plutonium added was sorbed on 



TABLE 1 Solubilities of Plutonium in Water Extracts 



of a Ritzville Silt Loam as Determined by Filtration 



with Membranes of Different Pore Sizes* 



*I'rom Garland and Wildung (1977). 



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



the soil since a maximum of 10% of the extracted plutonium passed through the 5-/jm 

 membrane. Successive filtration through membranes with decreasing pore size resulted in 

 decreases in plutonium concentration in the filtrate. Thus plutonium in the aqueous 

 extract appeared to-be in a wide range of particle sizes. Although membranes with pore 

 sizes of 0.45 ;um are commonly used to separate soluble matter from particulate matter, 

 plutonium in these filtrates may be in colloidal forms. The plutonium in the 0.001 0-/im 

 filtrate appeared soluble, was stable in solution, and approximated the quantity of 

 plutonium taken up by plants (Wildung and Garland, 1974). Of the soluble plutonium 

 forms likely to enter soils (previous section), Pu(N03)4 and plutonium-diethylenetri- 

 aminepentaacetic acid (DTPA) represent, in their respective chemistries, the range in soil 

 behavior likely to occur. The water solubility (<0.01 /jm) of ^^^Pu and ^^^Pu amended 

 to a Ritzville silt loam (organic C content. 0.7%: pH 6.2) in the Pu(N03 )4 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 stable over the tlrst 40 days of 

 incubation (Fig. 2). After 7 days of incubation, the ^-^ ^Pu-DTPA appeared to be slightly 

 less soluble than the ^^"^Pu-DTPA. 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 moiety and the development of new chemical equilibria. 



Equilibrium concentrations of soluble plutonium added as the nitrate were not 

 obtained until 7 to 10 days. The solubility of ^''^Pu and ^^^Pu added to the soil as 

 nitrates was much lower than the DTPA complexes, which likely reflects hydrolysis to 



