SYNTHESIS OF THE RESEARCH LITERATURE 13 



10~^ (Garland and Wildung, 1977). Diffusion coefficients for total plutonium in soil are 

 on the order of 10"^ (Relyea and Brown, 1975), which indicates httle mobility "by this 

 mechanism. However, field studies have shown that plutonium may penetrate up to 

 30 cm in arid soil (Nyhan, Miera, and Neher, 1976) and that plutonium is more mobile 

 through biological (e.g., root uptake and transport) and physicochemical mechanisms 

 tlian would be predicted on the basis of diffusion alone. Furthermore, a fraction of 

 plutonium in •soils is readily dissolved. Studies of soils that had contained plutonium for 

 over 30 yr indicated that up to 13% of the plutonium was extractable with chelating 

 resins (Bondietti, Reynolds, and Shanks, 1976). This plutonium was probably available 

 for plant uptake and, in the case of perennials, may continue to be available for several 

 croppings, as demonstrated for clover (Romney, Mork, and Larson, 1970) and alfalfa 

 (Wildung et al., 1977). The chemical/biological phenomena controlling the quantity and 

 form of mobile plutonium are the key to predicting long-term implications of plutonium 

 in the environment. 



Under aerobic conditions the ultimate behavior of plutonium in soils and sediments, 

 regardless of the chemical form of the source material, will be governed by processes that 

 influence hydrolysis and sorption on the solid phase and formation of soluble complexes 

 with organic or inorganic Ugands (Fig. 4). Initially, sorption and precipitation processes 

 predominate when Pu(IV) is added as the soluble nitrate and account for 98% of the total 

 plutonium a few hours after Pu(IV) has been added to a neutral silt loam soil (Wildung 

 and Garland, 1975). The addition of Pu(IV) as the DTPA complex results in nearly 

 100% plutonium solubility before a gradual reduction in solubility occurs by processes 

 described earlier. From a thermodynamic standpoint, the formation of Pu(V and VI) in 

 soil solution is theoretically possible. However, studies of the interactions of Pu(VI) with 

 organic ligands representing a range of common soil metaboUtes (Wildung, Garland, and 

 Cataldo, 1979), humic substances, and reducing sugars (Bondietti, Reynolds, and Shanks, 

 1976) suggest that Pu(VI) will be readily reduced to Pu(III) + (IV) in aerobic surface 

 soils. The presence of Fe(II), a reductant, may further promote the formation of reduced 

 plutonium under most soil conditions except highly alkaline soils. 



Because Pu(IV) readily forms insoluble hydrolysis products, the interaction of these 

 species with mineral and organic surfaces results in the relative immobihty of plutonium 

 in soils and sediments. Hydrolysis products sorb on the sofid phase by mechanisms other 

 than ion exchange, and attempts to extract exchangeable plutonium from soils using 

 MgCl2 (Muller, 1978) and resins (Bondietti, Reynolds, and Shanks, 1976) resulted in the 

 removal of relatively small quantities (<13%) of the total plutonium. The major portion 

 of plutonium associated with the soUd phase in soils and sediments (Muller, 1978; 

 Edgington, Wahlgren, and Marshall, 1976) was extractable with citrate-dithionite, but 

 with citrate alone much less was extracted, which suggests the association of plutonium 

 with the reductant-soluble iron on the surfaces of soil/sediment particles (Wildung, 

 Schmidt, and Routson, 1977) or with iron in the original particles that were deposited. 



The importance of hydrolysis in governing plutonium behavior extends to the soluble 

 fraction, at least over the short term (months). Almost all soluble and diffusible 

 plutonium on soil has been shown to be Pu(OH)n (Wildung et al., 1977). The 

 small quantity remaining in soil/sediment solutions is probably present as the Pu'*''" ion 

 stabilized against hydrolysis by interaction with a predominant anion (CO3" or HCO^, 

 depending on pH and ionic composition) and organic ligands (Wildung, Garland, and 

 Cataldo, 1979). Concentration of low-molecular-weight organic ligands, bicarbonate ion, 

 and carbonate ion are directly related to microbial metabolism and decomposition of 



