RELATIONSHIP OF MICROBIAL PROCESSES 313 



Changes in the soil respiration rate and plutonium solubility during the 65-day 

 incubation period are shown in Fig. 4(a). The lack of CO2 evolution from the 

 gamma-irradiated soil verified its sterility. The increased CO2 evolution rates in the 

 nonsterile soil over the 4- to 12-day period reflected logarithmic growth for all classes of 

 organisms. The concentration of plutonium in the 0.01 -/um tlltrate during the incubation 

 period ranged from approximately 0.04 to 0.14% of the plutonium initially applied. 

 Solubility of plutonium was essentially identical in the sterile and nonsterile soils, 

 decreasing with time. 



In a subsequent experiment, the plutonium-containing sterile soil was inoculated with 

 the plutonium-treated nonsterile soil (1 g) which had been previously incubated for 65 

 days [Fig. 4(a)] , and the respiration rate and solubility of plutonium in the inoculated soil 

 were measured for a period of 30 days [Fig. 4(b)] . When the sterile soils were inoculated 

 with nonsterile soil, CO2 evolution increased at a much more rapid rate without a lag 

 phase, and this was followed by a factor of 2 increase in water solubility (<0.01 jum) 0/ 

 plutonium beginning after 5 days of incubation, which suggests the development of a 

 microbial population in the plutonium-containing nonsterile soil that was particularly 

 capable of alteration of plutonium solubility. 



An analogous set of experiments was conducted with amended (carbon and nitrogen) 

 sterile and nonsterile soils. The carbon and nitrogen were added to increase microbial 

 activity and assess the effect of increased activity on plutonium solubility. The 

 amendments markedly increased microbial activity (respiration rate, microbial types/ 

 numbers) in the nonsterile soil but did not increase solubility in the <0.01 fim fraction 

 compared to unamended soils. However, there was a significant increase in plutonium 

 solubility in the <0.45-/im fraction of the nonsterile soil on initial incubation. As in the 

 case of the soil that was not amended with carbon and nitrogen, reinoculation of the 

 sterile soil with the nonsterile soil markedly increased solubility in the <0.01-/um fraction. 



At least under the conditions of this study, the evidence strongly suggested that the 

 solubility of plutonium in soil was influenced by the activity of the soil microflora. The 

 potential mechanisms affecting the change in solubility include mechanisms (1) and (2) 

 described on page 312, i.e., indirectly through the production of organic acids that may 

 complex plutonium or the alteration of the solution pH and/or Eh near the soil colloid 

 without measurable effects on the overall soil pH or directly through the reduction of 

 plutonium to Pu(III). Oxidation to Pu(VI) would likely increase solubility, but recent 

 evidence suggests that this does not occur in these systems. Of course, a combination of 

 the mechanisms is possible, i.e., alteration of affinity for organic ligands through a change 

 in valence state. If the mechanism of solubiHzation was indirect, the results might be 

 applicable to other transuranic elements; e.g., from consideration of the aqueous 

 chemistry, a reduction in pH would be expected to increase the solubility of the other 

 transuranic elements as well as that of plutonium. 



Increased water solubility of plutonium on incubation under optimum conditions for 

 microbial activity may increase plutonium uptake by plants provided that the limiting 

 factor is not discrimination at the root membrane. To determine if the increased 

 solubility on incubation resuhed in increased plutonium uptake by plants, the incubated 

 soils were planted with barley and cultured by a spht-root technique that allowed 

 measurement of the uptake, sites of deposition, and chemical forms of plutonium in plant 

 shoots and roots (Wildung and Garland, 1974). The resuhs were compared with the 

 results of similar plant studies in which the soils had not been incubated. 



