14 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



DIVERSE SOURCES OF PLUTONIUM 



BIOLOGICAL TISSUES 



SOIL 



PARTICULATES 



DEPOSITION 



Pu(IV)L,,L2 



Pu(IV)L, 



TRANS- 

 LOCATION 



ORGANIC 



LIGANDS 



IL,I \ 



Pu(IV)L, 



CELL 

 MEMBRANE 



Pu"* 



TRANS- 

 PORT 



DIFFUSION 



EXUDATION 



DECOMPOSTION 



SOIL 

 SOLUTION 



oh; HCO3. CO^" 

 ORGANIC LIGANDS 



MICROBIAL ACTIVITY 



SORPTION 



DESORPTION 



ORGANIC AND 



INORGANIC SURFACE 



REACTIONS 



Fig. 4 Model for plutonium chemistry in the ingestion pathway. Regardless of the form 

 of plutonium entering soils, sediments, or water, plutonium is predominantly converted 

 (exceptions given in aquatic section) to Pu(IV), which is largely insoluble and associated 

 with the solid phase of soils and sediments. Soluble plutonium is also present primarily 

 as Pu(IV) stabilized by complexation with inorganic and organic ligands. The Pu(IV) 

 complexes are largely dissociated at the cell surface with Pu"* ^ ion transport across the 

 cell membrane. Mobility in biological tissues is facilitated by formation of secondary 

 complexes. 



organic materials. There is direct evidence that plutonium forms complexes with 

 microbial metabolites and considerable indirect evidence supporting microbial influence 

 on plutonium solubility in soil (Wildung and Garland, 1977; Wildung, Gariand, and 

 Cataldo, 1979). The complexed Pu(IV) is probably the only plutonium that is available 

 for plant uptake (Fig. 4). The formation and the delivery of these complexes to roots are 

 the rate-limiting processes in the ingestion pathway. 



Chemical properties of other transuranic elements (americium, curium, and neptu- 

 nium) in the environment have not been established. Laboratory studies have been 

 limited to studies of (1) the soil sorption of americium and neptunium, which indicate 

 sorption in the order Pu> Am> Np (Routson, Jansen, and Robinson, 1977); (2) the 

 sorption of Cm(III) and Np(V) on soil clay, which indicate sorption in the order 

 Cm> Np with an apparent association of neptunium with organic matter and amorphous 

 iron (Bondietti and Tainura, this volume); and (3) the solution behavior of ^^"^Cm in a 

 freshwater lake, which shows that soluble curium (50% of total) was largely anionic 

 (Dahlman, Bondietti, and Eyman, 1976). Field studies have been limited by relatively 

 low concentrations in the environment and lack of sensitive analytical methods for 

 certain nuclides of importance (e.g., ^'*''^'*^Am, ^^"^Cm, and ^^^Np). The aqueous 

 chemistry of these elements has been fairly well established (Katz and Seaborg, 1957; 

 Keller, 1971) and allows some predictions of behavior in soils and sediments. Major 

 differences in their environmental behavior as compared with plutonium would be 

 expected, and sorption on solid surfaces may be a function of the predominant valence 

 state and its tendency to hydrolyze (Dahlman, Bondietti, and Eyman, 1976). The only 

 stable ions of americium and curium in aqueous solutions are the trivalent cations. Their 

 chemistry in soUs and sediments is simOar if they are present in similar mass 

 concentrations. Hydrolysis reactions may be a primary factor governing the behavior of 

 americium and curium, but greater mobility and biological availability can be predicted 

 because of greater solubility of their hydroxides in comparison with Pu(0H)4. For 

 neptunium, Np02 is the most stable species in aqueous solution and should not be 

 subject to significant hydrolysis at environmental pH values (Burney and Harbour, 1974). 

 Of the transuranic elements, the environmental behavior of neptunium has been least 

 studied, but, because of its chemical characteristics, it is the most soluble in soils and may 



