UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

The distribution of thorium aqueous species (Figure 5.5) was also calculated as a function of pH
using the MINTEQA2 for a concentration of 1 µg/l total dissolved thorium and the water
composition in Table 5.1. The thermodynamic data were principally from Langmuir and Herman
- 6-
(1980). The thermodynamic constants for the aqueous species Th(OH) 3CO3 and Th(CO3) 5 from
Östhols et al. (1994) were also included in these speciation calculations. Below pH 5, dissolved
thorium is dominated by thorium fluoride complexes. Between pH 5 and 7, dissolved thorium is
predicted to be dominated by thorium phosphate complexes. Although phosphate complexation is
expected to have a role in the mobility of thorium in this range of pH values, the adequacy of the
thermodynamic constants tabulated for thorium phosphate complexes in Langmuir and Herman
(1980) are suspect, and may over predict the stability of these complexes. At pH values greater
than 7.5, more than 95 percent of the dissolved thorium is predicted to be present as
-
Th(OH) 3CO3. The species distribution illustrated in Figure 5.5 changes slightly in the pH range
from 5 to 7 if the concentration of total dissolved thorium is increased from 1 to 1,000 µg/l. At
-
the higher concentration of dissolved thorium, the stability of Th(OH) 3CO3 extends to a pH of
-
approximately 5, the hydrolytic species Th(OH) 3 becomes an important species (about 30 percent
of the dissolved thorium), and the thorium phosphate species are no longer dominant.
Thorium organic complexes likely have an important effect on the mobility of thorium in
3- 2-
soil/water systems. Langmuir and Herman (1980) used citrate (C6H5O7 ), oxalate (C2O4 ), and
4-
ethylenediamine tetra-acetic acid (EDTA) (C10H12O8N2 ) to show the possible role of organic
complexes in the mobility of thorium in natural waters. Based on the stability constants available
for thorium citrate, oxalate, and ethylenediamine complexes, calculations by Langmuir and
Herman (1980) indicate that thorium organic complexes likely predominate over inorganic
complexes in organic-rich waters and soils. For the concentrations considered by Langmuir and
Herman (1980), the ThEDTA " (aq) complex dominates all other thorium aqueous species over the
pH range from 2 to 8. This would in turn have an important effect on the solubility and
adsorption of thorium in such waters.
5.9.4 Dissolution/Precipitation/Coprecipitation
The main thorium-containing minerals, thorite [(Th,U,Ce,Fe,etc.)SiO 4], thorianite (crystalline
ThO 2), monazite [(Ce,La,Th)PO 4) and zircon (ZrSiO 4), are resistant to chemical weathering and
do not dissolve readily at low-temperature in surface and groundwaters. Because these minerals
form at temperature and pressure conditions associated with igneous and metamorphic rocks, it is
unlikely that the thermodynamic equilibrium solubilities (where the rate of precipitation equals the
rate of dissolution) of these minerals will control the concentration of dissolved thorium in lowtemperature
soil/water environments. The rate at which thorium is released to the environment,
as might be needed in a source-term component of a performance assessment model, may
however be controlled by the kinetic rates of aqueous dissolution (i.e., non-equilibrium
conditions) of 1 or more of these phases.
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