UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

T = temperature in degrees kelvin
F = Faraday constant (96,487 coulombs/equivalent).
At 25.0 " C (298 K),
Eh (mV) ' 59.2 pe. (5.3)
Using Equations 5.1 and 5.3, an Eh value was calculated for each pH value used as an input for
the MINTEQA2 calculations of plutonium aqueous speciation. The plutonium aqueous species
that were included in the computation scheme are tabulated in Table 5.10. Thermodynamic data
for these species were taken primarily from Lemire and Tremaine (1980) and other secondary
sources and database modifications described by Krupka and Serne (1996).
Results are plotted as a species distribution diagram (Figure 5.3). The data show that, under very
2+ +
low pH (~3 - 3.5) conditions, PuF2 and PuO2 are the dominant species of plutonium. The free
+
ionic species, PuO2 appears to be the dominant form within the pH range of 4 to 5. Within the
+ 2-
pH range of 5.5 to 6.5, the main species of plutonium appear to be PuO2, and Pu(OH)2(CO3) 2 ,
"
with minor species being the neutral hydrolytic species Pu(OH) 4(aq)
and the phosphate complex
4-
Pu(HPO4) 4 . At pH values exceeding 6.5, the bulk of the dissolved plutonium (~90 percent)
2- "
would be comprised of the Pu(OH) 2(CO3) 2 species with a minor percentage of Pu(OH)4(aq).
These illustrative computations indicate that, under pH conditions that typically exist in surface
and groundwaters (>6.5), the dominant form of dissolved plutonium would be the tetravalent
2-
complex species, Pu(OH) 2(CO3) 2 .
Polymeric species of plutonium may not occur under environmental conditions because the total
plutonium concentrations in nature are at least 7 orders of magnitude less than the concentrations
required for the formation of such species (Choppin, 1983). It is important to note that the
speciation of plutonium would change significantly with changing redox conditions, pH, the types
and total concentrations of complexing ligands and major cationic constituents.
5.6.4 Dissolution/Precipitation/Coprecipitation
Allard and Rydberg (1983) calculated that the aqueous concentrations of plutonium in nature may
be controlled by the solubility of the solid phase PuO 2@xH 2O. Many observations show that
plutonium associated with soils and particulate organic matter is present in tetravalent oxidation
state (Nelson and Lovett, 1980; Nelson et al., 1987; Silver, 1983). Calculations by Allard and
Rydberg (1983) based on available thermodynamic data show that, under reducing conditions, the
solubility of dissolved plutonium would be limited by the solid phase PuO 2 at pH values greater
than 8, and by the solid phase Pu 2(CO 3) 3 of trivalent plutonium at lower pH values.
5.37