with a simple model for Pu speciation in solution.
This model would involve
the most stable oxidation state, (IV), and OH ton.
TABLE 1. CHARACTERISTICS OF Pu IN THE < 0.45 um FRACTION OF
WHITE OAK LAKE WATER
Study and Date
Moles/Liter*
Pu, x 10!
Comments
Solution Characteristics
Oct. 1974
0.8
Sept. 1975
2.)
100% antonic
100% anionic,
< 10,000 MW
Baes and Mesmer (1976) have evaluated the hydrolytic properties of Pu(1V) and
calculated hydrolysis quotients for the stepwise hydrolysis of Pu’ .
These
hydrolytic stability constants have been used to calculate the concentrations
of five Pu{1¥) species which could theoretically exist in equilibrium with
crystalline Pu09. The results of these calculations are presented in Figure 4.
It_can be poted that as the solution pH increases, the successive addition of
OH to Pu"
results in a neutral species, Pu(OH),, dominating the soluble
species from about pH 4.2 to 5.5.
Sy analogy to U(IV), Baes and Mesmer calculated the theoretical stability constant for a Pu(OH),5 species. This hypothetical species dominates the Pu species present in solution above pH 5.5. At
pH 8, about the pH of WOL water, the calculated equilibrium concentation of
this species would be pi3.5. The observed concentrations of the negativelycharged, Pu(III) or Pu(IV) species in WOL was ~ p15.
Thus, the observed
concentrations of Pu did not exceed this simple model for Pu(IV) speciation in
water.
In light of the above observations, a number of reported concentrations of Pu
in filtered natural water from different environments have been compared to
this estimated Pu0z solubility. Table 2 summarizes the references used for
this evaluation. Generally, these investigators reported Pu results in radloactivity unites (1.e., pCi/l). The reported activity values for 239*240py
Valence State Characterization
April 1976
Total
1.34 0.7
BiPO, (reduced)
Pu (111) + (IV)
1.1 4 0.1
BiPO,
Total
1.5 + 0.2
BiPO, (reduced)
Pu (III) + (1¥)
1.6 + 0.2
BiPO,
Oct. 1976
#10715 y 239py = 14.7 FCA.
have been converted to moles/1 739pu.
Where water pH's were not given, pH_8 was assumed.
The maximum values reported
by each investigator and the Pu(OQH),
line are plotted in Figure 5. The
radiation concentration guide (RCG} value* for soluble 729pu in unrestricted
areas is also plotted.
Fitting observed data to a theoretical solubility Limit was not done to suggest
that crystalline Pu0. controls the soluble Pu in natural solutions.
It is
generally the case that observed concentrations of trace metals in natural
solutions (for example, seawater) are not controlled by the solubility product
of the least soluble salt (Turekian, 1969).
For an element like Pu, it is not
likely that solubility will control solution concentrations because of the
trace levels released to the, environment. However, to the extent that an
upper limit on solution concentrations can be defined, it is important to
evaluate what concentrations of Pu could be expected in contaminated sites.
If reported values of soluble Pu remain at or below a predicted value, confidence in our understanding of the solution behavior of Pu is enhanced.
It is important to note that only Pu(IV) was considered in the data-fitting
model,
The previous discussion on theoretical speciation ftndicated that Pu(ILII)
could be important. However, those analyses considered Pu(IV) to be so insoluble that it could be neglected relative to the solubility of Pu(III). We
have chosen to consider only Pu(IV) because it may be the most stable oxidation
state, although not necessarily the most soluble. Plutonium (V) and (VI) were
not considered solely because they do not appear to dominate the soluble Pu in
WOL.
*USAEC Rules and Regulations, Title 10, Part 20, Appendix B, Table II, Dec. 10,
1969,
458
459