However, the thermochemical approach provides the necessary boundary parameters upon which experimental efforts can be based. Thus, while it is difficult to imagine natural environments in which Pu species are at equilibrium, the delineation of speciation domains through such calculations provides valuable information not readily obtained experimentally. FIELD OBSERVATIONS ON Pu SPECIATION To evaluate the oxidation states of Pu in environmental solutions, a method which selectively separates one or more oxidation states is necessary. CARBONATE COMPLEXES Both Polzer (1971) and Andelman and Rozzell (1970) considered the possibility of carbonate complexation of Pu(IV). The only Pu(IV) carbonate complex which has been reported is PuC0 3? » having a stability constant of 10%7 (Gelman et al., 1962). The validity of this very high constant has been questioned however (Cleveland, 1970); thus, the use of this constant by Polzer q971) and Andelman and Rozzell (1970) may be questionable. Rai and Serne (1976) chose not to include it in their analysis. Substantial research on Th(IV) and U(IV) indicates that the carbonate complexes of tetravalent actinides are important only at high carbonate concentrations. Thus U(C03)5° is stable only over a narrow range of p's (McClaine et al 1956). Desorption studies using 0.025 M HCO; showed that U(VI) bound to Proteins was more effectively removed than UCIV) (Dounce and Flagg, 1949) The same authors also reported oxidation-reduction potential studies which indicated that HCO; was a better complexer for U(VI) than U(IV). A similar rutv). eee. for re Moore and Dam (cited in Connick, 1954) estimated the conplexat - uon u ay PeCv) otentia in 45%fo K2CO3 to be near - -0.2 volt, indicating strong The potential in 1 M OH is about -0.4 volt (Connick, 1954 that carbonate interacts with Pu(VI) more strongly than PUG)ake coapetin tion with OH . Under environmental conditions, the greater tendency of tetravalent actinides to hydrolyze may thus make carbonat e complexation more important for the less hydrolytic hexavalent state. This effect is recognized in the geochemistry of Th(IV) and U(VI); carbonates Playing a role in the mobility of U(VI) but not Th(IV) (e.g., 23°Th produced from radiodec ay of ?34y 1s rapidly removed from seawater ). Bondietti et al. (1976) observed a greater tendenc for PU(VI) to remain in solution than Pu(IV) in dilute bicarbonate solutions. The available information suggests that the tend ency of carbonates, at typical environmental concentrations, to complex with Pu(IV) in an environmentally importante manner _be viewed with some reservati ons. While carbonates do complex Pu(Iv), the C032 /OH ratio appears critical. Because of the dilute concentrations of Pu which are found in near-neutral solutions, the method(s) would have to rely on the coprecipitation of Pu (or any other actinide) with a carrier. Bismuth phosphate, for example, is an excellent carrier of tri- and tetravalent actinides. The BiPO, precipitation of PU(III) + Pu(IV) has been used for nuclear fuel reprocessing and to determine Pu in water samples (Kooi et al., 1958). Bismuth phosphate precipitates do not carry significant amounts of the penta- and/or hexavalent species of Pu, Np, and U. Therefore this approach was taken to determine the oxidation state of Pu in water and will be discussed briefly. The samples of natural water assayed for Pu oxidation states were taken from White Oak Lake (WOL), a freshwater impoundment on the Oak Ridge National Laboratory reservation. The analytical methods employed for the natural water samples were essentially as outlined by Scott and Reynolds (1975), except that treatment with NaNO. at 70 C. was omitted. The water samples were collected and filtered (Whatman 50, then Millipore 0.45 um). Eighteen liters were acidified, 236Ppu tracer added (for recovery efficiency), and the solution made 0.1 M in NH20H and 10-3 M in FeClj. This sample was used to determine total Pu (the NH)OH and Fe(II) reduce Pu(VI) and Pu(V)). An additional 18 liters of filtered water were tagged 1 hour before acidification with a mixture (27% Pu(IV), 73% Pu(V) + Pu(VI)) of 242Pu. The BiPO, procedure (without valence adjustment) was used to selectively determine Pu(III) + PU(IV). Table 1 summarizes the results on Pu behavior in filtered WOL water. When determined, the Pu appeared anionic rather than cationic. That is, it was quantitatively retained by Dowex-1 anion exchange resin but not by Dowex-50 cation resign (NA form). One sample (September, 1975) also showed that the Pu which passed the 0.45 um filter also passed a 10,000 MW Amicon membrane filter, suggesting low molecular size. Measurements of redox potential and pH immediately after sample collection and at the time of analysis showed only slight changes (the redox potentials (Eh) ranged from 0.32 to 0.42 volts; pH from 7.9 to 8.9). The oxidation states of the indigenous Pu appeared to be largely Pu(III) or Pu(1IV), in that there was no difference found between the amount of Pu carried by the BiPO, precipitates (reduced and nonreduced). The recovery of 2"2pu wag 25% of the total added, which was about what was expected 1f only Pu(III) and Pu(IV) was carried. Thus, while added 242Py was fractionated based on known oxidation state differences, the indigenous Pu was not. The results suggest that for these water samples, the dominant oxidation states present were (IIL) or (IV). For calculation purposes, the 2397240py activity was used to obtain the molar concentrations of Pu, assuming that 233py was dominant. The exact nature of the solution-phase Pu species which exist in natural waters are not known. To the extent that species characteristics in WOL water are understood, i.e., low molecular weight, Pu(1I1) or (IV), and anionic, it is of interest to compare the observed characteristics of Pu in filtered water 456 457