312 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



appeared to predominate. Basic amino acids were not detected, although two acidic 

 amino acids (aspartic and glutamic acids) were present. 



Stevenson and Ardakani (1972) concluded that organic acids and amino acids, 

 although present only in small quantities in soil, were present in sufficient quantities in 

 water-soluble forms to play a significant role in the solubilization of mineral matter in 

 soil. Small quantities of a number of other complexing agents, such as nucleotide 

 phosphates, polyphenols, phytic acid, porphyrins, and auxins, also exist in soil (pertinent 

 references summarized by Mortensen, 1963). Complexation with biochemicals of recent 

 origin would likely be the principal mechanism for microbial mobilization of the 

 transuranic elements in soil. 



Microbial Transformation of the Transuranic Elements in Soil 



Potential Mechanisms of Transformation 



From the resuhs of limited studies of soil chemistry, microbiology, and plant availability 

 of transuranic elements in soils and by inference from studies of complexation of other 

 trace metals in soils, it can be concluded that the soil microflora will play a significant 

 role in transformations governing the form, and, ultimately, the long-term solubility and 

 behavior of transuranic elements in soil. Four general mechanisms whereby micro- 

 organisms may alter the form of trace metals in soil (Alexander, 1961; Wood, 1974) are 

 (1) indirect mechanisms resulting from metal interactions with microbial metabolites or 

 changes in pH and Eh; (2) direct transformations, such as alkylation and aUeration of the 

 valence state through microbial oxidation (use of the metal as an energy source) or 

 microbial reduction (use of the metal as an electron acceptor in the absence of oxygen); 

 (3) immobihzation by incorporation into microbial tissues; and (4) release of metals on 

 decomposition of organic residues. 



All these mechanisms may be operational in transformations of transuranic elements 

 in soils. However, on the basis of present knowledge, it is not possible to draw 

 conclusions as to their relative importance in affecting the long-term behavior of the 

 transuranic elements. Since there is a paucity of information available, these mechanisms 

 will be addressed around a framework of current information that is limited principally to 

 plutonium. 



Microbial Alteration of Solubility in Soil 



To provide a preliminary assessment of the potential for microbial alteration of 

 plutonium solubility in soil under aerobic conditions, Wildung, Garland, and Drucker 

 (1973; 1974) measured soil respiration rate (an index of soil microbial activity), microbial 

 types and numbers, and plutonium water solubility in sterile (gamma irradiation) and 

 nonsterile soils that contained 10 ^Ci (Pu)/g (soil) [added as Pu(N03)4] . Carbon dioxide 

 evolution was used as a measure of soil respiration rate. For a measure of plutonium 

 solubility, the soil was subsampled at intervals during incubation over a 65-day period, 

 and the subsamples (1 g) were suspended in 1 liter of distilled water. After a 4-hr 

 equilibration period, an aUquot of the soil suspension was filtered through 5-, 0.45-, and 

 0.01-jum filters. The plutonium in the 0.45- and 0.01 -^tm filtrates was designated water 

 soluble, although it was recognized that plutonium likely was present as fine colloids 

 (previous section). 



