One of the most complex problems concerning nuclear waste management and the
restoration of plutonium production sites is the treatment and disposition
of mixed and TRU wastes. Hydrothermal oxidation, which has been shown to be
effective in oxidizing a wide variety of organic material to CO2,
water, salts and other nonhazardous oxides, is a promising new technology
for the treatment and volume reduction of actinide-containing waste.
Information on the speciation and solubility of plutonium under process
effluent conditions will facilitate the development of separation techniques
for removing it from the treated solutions. Such a strongly oxidizing
environment will generate plutonium(VT); and upon the destruction of
organics, hydrothermal reactor solutions will contain carbonate. We are
investigating the solubility and speciation of the plutonium(VI) carbonate
system as a function of ionic strength (0.1 to 5.0 M). Formation constants
for the tris- and biscarbonato complexes of plutonium(IV) were determined to
be, log β130 = 17.7 and log β120 = 13.6, respectively,
by spectrophotometry. These formation constants indicate that
PuO2CO3(aq) is the plutonium (VI) carbonate
solution species with the largest relevant stability range. We prepared and
characterized the corresponding solid using XRD, EXAFS, and diffuse
reflectance, and initiated solubility experiments in 0.1, 0.2, 0.5, 1,2, and
5 M NaCl at 22±1°C under 100% CO2. Data collected thus far yield
the solubility products, log Ksp mol2/kg2 =
-12.9 (0.1 m NaCl), -12.4 (0.2 m NaCl), -12.5 (0.5 m NaCl), -12.3 (1 m
NaCl), -12.2 (2 m NaCl), -12.3 (5 m NaCl).