SYNTHESIS OF THE RESEARCH LITERATURE 19 



The Eh— pH diagram (Fig. 3) shows that under normal environmental conditions 

 Pu(III) and Pu(VI) can coexist and that the ratio of the two states will depend on the 

 oxidizing conditions and pH in the system. Therefore the relatively high concentrations 

 of plutonium in ELA lakes (other than 885) and lakes in the southeastern United States 

 could be due to the reduction of Pu(IV) to Pu(III) as well as to complexation of Pu(IV) 

 by organic ligands. The high concentration of plutonium and the very low fraction of 

 Pu(VT) in Little Manitou Lake, wliich contains high sulfate concentrations, could also be 

 due to the formation of sulfate complexes, which stabilize the Pu(IV) state. 



Similar measurements have evaluated the relative concentration of Pu(III + IV) and 

 Pu(V + VI) in White Oak Lake water (Bondietti and Sweeton, 1977) and indicated that 

 Pu(IV) was the dominant oxidation state present. Plutonium(IV) rather than Pu(III) was 

 suggested because another study indicated that Pu(III) was unstable toward oxidation to 

 Pu(IV) at pH >5 (Bondietti, 1977). 



Charge characteristics of plutonium in Lake Michigan water indicate that the element 

 is not associated with colloidal matter in the size range 0.003 < x < 0.45 /jm and that it is 

 almost quantitatively absorbed by anion exchange resins. In water samples from acidic 

 lakes, the majority of the plutonium behaves like cationic or uncharged species. These 

 results and the differences in oxidation state discussed earlier strongly suggest that the 

 solubility of plutonium is governed by different complexing agents. In waters of high pH, 

 the concentration of COl" and HCOi" ions is relatively high, and carbonate complexes 

 can form. In waters of low pH. such complexes cannot exist, and the solubility must be 

 due to complexing with other ligands, such as natural organic compounds. 



In addition to measuring concentrations in water columns, most investigators have 

 measured the concentration of plutonium in surficial sediments. In a few cases 

 measurements have been made of plutonium in suspended particulate material. Table 6 

 sliows that there is some relationship between concentrations in water and concentrations 

 in surface sediments. 



If there is mixing of the surficial sediment with the water column and a true 

 equilibrium between the water and particulate matter or sediment, the distribution 

 constant, Kp, for the reaction between filtered water (<0.45 jum) and sediment is 



_ Concentration per kilogram of sediment 

 Concentration per Uter of water 



Values of Kq vary from lO'* to 5 x 10^, but most values do not vary more than 

 fivefold. Considering the wide variety of sediment types involved and the differences in 

 source terms and sizes of aquatic environments, the small range in values strongly suggests 

 a commonahty in the behavior of plutonium in these systems. 



Sediment characteristics affect the uptake of radionuclides, and fivefold to tenfold 

 variations in distribution coefficients can be explained solely in terms of differences in 

 distributions of particle sizes in sediments (Duursma and Bosch, 1970). Little information 

 is given on sediment characteristics, but sediments from small ponds and rivers probably 

 are generally coarser than those from deep waters of the Great Lakes. 



More recent experiments have shown the distribution of plutonium between solution 

 and solid phases to be a true equilibrium. Sediments labeled with ^^^Pu from the Miami 

 River were equilibrated with Lake Michigan water. Kinetic studies indicated that 

 equilibrium was reached in 1 day or less. Moreover, the ratio of oxidation states in water 

 from this experiment is the same as that observed for ^^'Pu in Lake Michigan (D. M. 



