RELATIONSHIP OF MICROBIAL PROCESSES 303 



or Np02) can occur through cation-exchange reactions with particulate surfaces. 

 CompHcating this situation, disproportionation and complexation reactions may occur 



concurrently. 



Transuranic elements entering the soil as stable organocomplexes. such as might occur 

 in the vicinity of a spent -fuel separation facility, may be initially highly soluble (Wildung 

 and Garland, 1975). The duration of solubility and mobility in the soil will be a function 

 of the stability of the complex to substitution by major competing ions, such as calcium 

 and hydrogen (Lahav and Hochberg, 1976; Lindsay, 1972; Norvell, 1972), and the 

 stability of the organic ligand to microbial decomposition (Wildung and Garland, 1975). 

 The disruption of the complex may lead to a marked reduction in transuranic-element 

 solubility through hydrolysis and precipitation reactions, as described for acid solutions 

 on dilution. A portion of the ion released may react with other, perhaps more stable, 

 ligands in soil. The mobility of the intact complexes, in turn, will be principally a 

 function of their chemical and microbiological stability and the charge on the complex, 

 which will govern the degree of sorption on soil particulates. 



Further generalizations of transuranic-element behavior on the basis of source terms 

 are complicated by the overwhelming importance of soil properties and processes in 

 influencing transuranic-element behavior on a regional and local basis. This chapter 

 considers, in detail, the influence of soil properties and abiotic and biotic processes on the 

 long-term solubility of the transuranic elements entering soils. Consideration is also given 

 to the implications of these processes in terms of transuranic-element plant and animal 

 availability. Principal emphasis is directed toward the role of soil microorganisms in this 

 phenomenon. Microorganisms, in intimate association with soil particles, are known to 

 play an important role in effecting solubilization of elements considered insoluble in soils 

 strictly on the basis of their inorganic chemistry. To date studies of the microbiology of 

 the transuranic elements have been limited principally to plutonium. This chapter will 

 emphasize plutonium. but, where possible, the available information is used as a 

 framework for broader discussions encompassing the long-term behavior of other 

 transuranic elements. 



Chemical Reactions Influencing Plutonium Behavior 



The principal chemical reactions likely influencing plutonium behavior in soil are: 



• Four oxidation states 



Pu3\ Pu'\ PuOr. PuOr (Pu'\ Pu^') 



• Disproportionation 



Pu''' + PuOr^Pu'' + PuOr 



• Hydrolysis 



Pu^' + 4H2 0^Pu(OH)4 +4H^(Ksp ^10^'^ 



• Complex formation 



2Pu^' + 3 DTPA^Pu.DTPAj (logK~ 18) 



