Th are still cationic, while in 8 M HNO; or in the strong carbonate solutions, both cations would form negatively-charged complexes which do not readily adsorb, The extraction conditions can change the Pu oxidation state present and therefore, these results only support the plant uptake data which indicated that Pu and Th are likely present in similar forms, TABLE 5. EXTRACTION OF U, Th, AND Pu FROM FLOODPLAIN SOIL USING MILD EXTRACTANTS U Extractant Th Pu While neptunium has not been studied extensively, pliant uptake (Price 1973) and soil adsorption (Routson et al,, 1975) studfes have indicated higher mobility relative to the rest of the actinides. Bonditetti (1976) noted evidence for reduction of Np(V) to Np(IV) in an acid soil. Thus, while Np may be very available to plants if it remains oxidized (Np(V)), if Fe? or other reductants are present, Np(IV) may be stabilized to some extent. Tetravalent Np would be expected to behave like Pu(I¥) (low plant uptake) and attention should be paid to instances where Np uptake is similar to Pu since this may indicate that Np(IV) is being stabilized. a* SUMMARY 1 M HNO, 73 10% NaC0, - 5% NaHCO, 71 7.9 4s 7.7 54 *Z of & M HNO, extractable U (8.18 ug/g), Th (16.8 ug/g} and Pu (144 dpm/g). The question of which Pu oxidation states dominate in the environment has been discussed, using thermodynamic approaches and environmental measurements to illustrate the complexity of the problem, When thermochemical calculations are considered, trivalent Pu appears to be a likely soluble state, particularly under reducing conditions, Penta- and hexavalent states have also been calculated to be important. The oxidation states of Pu in White Oak Lake water appear to be III or IV. While Pu(II1) might be present (based on thermodynamic considerations), the observed concentrations are consistent with the presence of Pu(IV). Observed concentrations of soluble Pu in various natural waters do not remotely approach current permissible limits. Concentrations of soluble Pu exceeding predicted solubilities have been observed in Pu0. dissolution studies. This phenomenon appears related to complex oxidation state changes as the result of radiolysis; however, the absolute dissolution rate of Pu. is very small and some evidence is available to raise the question of the relative importance of chemical solubility vs. physical fragmentation as the important "dissolution" mechanism. The relative availability of transuranium elements to plants appears related to the oxidation state present in the soil. From the limited data available, the pentavalent oxidation state appears to be more available than the other valences. Plutonium uptake by plants appears to be less than reported for Am, Cm, or U. In one case, the behavior of Pu and Th appeared similar. This would suggest that plant assimilation of these elements should follow the valence order 5 > 6 = 3 > 4. This order is the inverse of the relative tendency of adsorption to soil. The information presented exemplifies the complexities of evaluating Pu behavior in natural solutions and identifies how further research should be structured to answer important questions of environmental mobility and biogeochemistry of 471