320 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Kennedy, and Rosen, 1968; McBride and Wolfe, 1971; Taylor and Hanna, 1977). The 

 rr^thyl derivative of mercury may be present in significant quantities in soils (Beckert 

 et al., 1974). Wood (1974) suggested that methylated derivates of mercury and arsenic 

 are important in governing their behavior in the environment. McBride and Edwards 

 (1977) also suggested that these reactions occur abiotically. The process of biochemical 

 methylation of metals can be described as an overlap between the chemistry of methyl 

 cobalamine (an intermediate in methane synthesis by anaerobic bacteria and methionine 

 synthesis in aerobic bacteria) and the chemistry of the metals. In the case of the 

 transuranic elements, particularly plutonium, it is the complexity of the aqueous 

 chemistry that has limited research into alkylation phenomena. 



It is unknown whether an ionic species of plutonium is capable of reacting in vitro 

 with an alkyl cobalamine. Further, if a mechanism for biological alkylation of plutonium, 

 similar to the mercury— arsenic— platinum alkylation reaction, did exist, it would be of 

 importance in influencing environmental behavior only if the alkylated molecule 

 exhibited stability (Wood, 1974), i.e., a half-life in soils and sediments of hours rather 

 than seconds. Considering the coordination chemistry of the actinides, Marks (1976) 

 noted that U— C and Th— C linkages are formed in organic solvents and that the 

 complexes are relatively stable thermally, although they are highly sensitive to oxygen. 

 Meaningful microbial studies await the development of an understanding of the chemical 

 speciation of transuranic elements in aqueous solutions at environmental concentration 

 levels. 



Indirect Transformations. The potential for indirect transformation of the transuranic 

 elements may be greater than that for direct transformation. The potential for plutonium 

 interaction with microbial cells and metabolites has been demonstrated, and many of the 

 other transuranic elements form stable complexes with Icnown microbial metabolites. 



Plutonium is taken up directly by microorganisms. Beckert and Au (1976) 

 demonstrated the uptake of ^^^Pu, applied initially to malt agar in nitrate, citrate, and 

 dioxide forms, by a common soil fungus, Aspergillus niger. By a specialized spore 

 collection method, the plutonium was shown to be present in the fruiting bodies. 

 Subsequent washing to remove external contamination indicated that the major portion 

 of the ^^^Pu was incorporated into the spores. The order of uptake (10^'') was related 

 to pH and expected solubility of the plutonium added; plutonium in the initially soluble 

 nitrate and citrate forms exhibited a factor of 2 to 3 greater uptake than the dioxide. The 

 availability to microorganisms of the plutonium in citrate and nitrate might be expected 

 to be considerably higher than that of the oxide from solubility considerations at the 

 picocurie per milliliter level. The relatively high microbial availability of plutonium as the 

 oxide is highly significant, and further studies are warranted to determine the mechanisms 

 of solubilization and uptake and the significance of microorganisms in recycling 

 processes. 



The amount of literature on organic acids and bases, capable of complexing heavy 

 elements, which are produced directly or by secondary syntheses by a variety of 

 microorganisms, is increasing. Their concentration and form in soils will be dependent on 

 the environmental factors influencing microbial metabolism, such as carbon source, and 

 their residence time will be dependent on subsequent chemical and microbiological 

 stability. 



In preliminary (unpublished) studies by ourselves and others, mixed cultures of soil 

 organisms, isolated from soil on the basis of carbon requirements and plutonium 

 resistance, were analyzed as to their ability to transport plutonium into cells and to alter 



