The Relationship of Microbial Processes 

 to the Fate and Behavior 

 of Transuranic Elements in Soils, Plants, 

 and Animals 



R. E. WILDUNG and T. R. GARLAND 



Soil physico chemical and microbial processes will influence the long-term solubility, 

 form, and bioavailability of plutonium and other transuranic elements important in the 

 nuclear fuel cycle. Consideration is given to the chemistry /microbiology of the 

 transuranic elements in soil, emphasizing possible organic complexation reactions in soils 

 and plants and the relationship of these phenomena to gastrointestinal absorption. 



Initial solubility of the transuranic elements in soil is governed largely by hydrolysis 

 with soil sorption in the order Pu > Am — Cm > Np. Soluble (<0.01 jim), diffusible 

 plutonium in soils (usually less than 0.1% of total) appears to be largely present as 

 particulates of hydrated oxide, but several lines of evidence indicate that microorganisms 

 influence the solubility and plant availability of plutonium and that the nonparticulate 

 plant-available fraction is stabilized in solution by inorganic or organic ligands of limited 

 concentration in soil. Vie possible role of soil microorganisms in influencing the 

 solubility, form, and plant availability of the transuranic elements is discussed on the 

 basis of the (1 J known chemistry of organic ligands in soils: (2) effects on the soil 

 microflora; and (3) principal microbial transformation mechanisms, including direct 

 alteration (valence state and alkylation), indirect alteration (metabolite interactions and 

 influence on the physico chemical environment), and cycling processes (biological uptake 

 and release on decomposition of tissues). 



The toxicity of plutonium to microorganisms depends on plutonium solubility in soil. 

 However, soil microorganisms are generally resistant to plutonium; toxicity is due 

 principally to radiation rather than to chemical effects. Highly resistant bacteria, fungi, 

 and actinomycetes have been isolated from soil, and these organisms liave been shown to 

 be capable of transporting plutonium into the cell and altering its form in the cell and in 

 solution. Vie resulting soluble plutonium complexes exhibit a range of mobilities in soil 

 and tend to be of higher molecular weight than simple complexes (plutonium- 

 diethylenetriaminepentaacetic acid) and negatively charged. Vie forms of plutonium 

 complexes, although not well defined, are dependent on organism type, carbon source, 

 and time of plutonium exposure during growth. Viese factors, in turn, are a function of 

 plutonium source, soil properties, and soil environmental conditions. Knowledge of the 

 relative influence of these factors serves as a valuable basis for predicting the long-term 

 behavior of plutonium and other transuranic elements in soils. There is growing evidence 

 that these phenomena also markedly influence the availability of plutonium to plants and 

 animals. 



Plutonium present in solution as an organic complex is readily assimilated by the 

 plant in the Pu(IV) state. Evidence to date indicates that soil sorption rather than plant 

 discrimination limits plant uptake of plutonium and that organometal complexes serve 

 mainly to deliver plutonium to the root membrane; i.e., the ligands are not taken up by 



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