RELATIONSHIP OF MICROBIAL PROCESSES 309 



This material is obviously not particulate but is present in insufficient concentration 

 for characterization with current methods. The question remains, "What is the form of 

 the small quantity of plutonium available to plants?" This information is essential to 

 understanding the mechanisms whereby plutonium can be resupplied to solution from the 

 solid phase in a range of soils and to predictions of the long-term availability of 

 plutonium to plants. From investigations of plutonium valence state in a neutral, 

 0.0004M NH4HCO3 solution equilibrated with PUO2 microspheres and in burial-ground 

 leachates, Bondietti and Reynolds (1976) concluded that Pu(VI) may be stable in 

 significant quantities in solution and suggested that monomeric Pu(VI) and its complexes 

 may be important in plutonium mobilization. In the present studies, evidence was 

 presented which suggested that plutonium ions are stabilized in soil solution by inorganic 

 or organic ligands for subsequent uptake by the plant. Furthermore, equilibration of 

 weathered plutonium-contaminated soil with chelating resins has been shown (Bondietti, 

 Reynolds, and Shanks, 1976) to result in significant desorption of plutonium from the 

 solid phase. It is known that organic ligands result in the most stable plutonium 

 complexes. Soluble organic ligands in soil are generally derived from microbial processes. 



Chemical Reactions Influencing the Behavior of Other Transuranic Elements 



Other transuranic isotopes of concern in the nuclear fuel cycle include ^^^ Am, ^'*^Am, 

 ^'^^Cm, ^"^^Cm, ^'*'*Cm, and ^^''Np. Althougli detailed studies of their interaction with 

 soils are lacking, some information has become available in recent years. Furthermore, the 

 aqueous chemistries of these elements have been fairly well established (Katz and 

 Seaborg, 1957). The most stable ions of americium and curium in aqueous solutions are 

 the cations (III); neptunium is most stable as the oxyion (Np02). Disproportionation is 

 not common with these elements. Thus major differences in their environmental behavior 

 as compared with that of plutonium would be expected. Hydrolysis reactions may still be 

 a primary factor governing the environmental behavior of americium and curium, but 

 greater mobility and plant availability in soils might be predicted on the basis of greater 

 solubility of the hydroxides in comparison with Pu(0H)4. The neptunium oxycation 

 should not be subject to significant hydrolysis at environmental pH values (Burney and 

 Harbour, 1974). Of the transuranic elements, neptunium has been the least studied, but, 

 because of its chemical characteristics, it may be the most available to the biota. A 

 comparison of plutonium, americium, and neptunium sorption in several soils (Routson, 

 Jansen, and Robinson, 1975) indicated sorption in the order Pu > Am > Np. The 

 chemistry of curium should be very similar to that of americium if present at equal mass 

 concentrations. 



Organic Complexation Reactions 



Research to date on the chemistry of the transuranic elements in soil has pointed to the 

 importance of understanding transuranic-element organic complexation reactions in soil, 

 particularly in surface soils and aquatic sediments where organic-matter content is 

 generally highest or in subsoils where the transuranic elements may be dispersed in 

 conjunction with synthetic complexing agents. Very little information is available 

 concerning the interaction of the transuranic elements with the soil organic fraction. 

 However, despite the difficulties in characterization of soil organic complexes, much is 

 known both theoretically and experimentally regarding the interactions of metals with 

 functional groups of soil organic matter (Keeney and Wildung, 1977). Much of this 



