UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

from about 10 -8.5 mol/l (0.0007 mg/l) to less than 10 -9 mol/l (0.0002 mg/l) in the pH range from 5
to 10 (Ryan and Rai, 1987). The concentration of dissolved thorium increases to approximately
10 -2.6 mol/l (600 mg/l) as pH decreases from 5.0 to 3.2.
Felmy et al. (1991) determined that the solubility of hydrous thorium oxide increases with
increasing ionic strength. At pH values above 7 in 3.0 M NaCl solutions, the solubility of hydrous
thorium oxide increased by approximately 2 to 3 orders of magnitude compared to that
determined in 0.1 M NaClO 4 solutions. Moreover, the pH at which hydrous thorium oxide
exhibits rapid increases in solubility with decreasing pH changes from pH 5 in 0.1 M NaClO 4 to
approximately pH 7 in 3.0 M NaCl. In studies conducted at high hydroxide and carbonate
concentrations, Rai et al. (1995) determined that the solubility of hydrous thorium oxide increases
dramatically in high carbonate solutions and decreases with increases in hydroxide concentration
at fixed carbonate concentrations. This supports the assertion that soluble thorium-carbonate
complexes likely dominate the aqueous speciation of thorium dissolved in natural waters having
basic pH values.
5.9.5 Adsorption/Desorption
Thorium concentrations in surface- and groundwaters may also be controlled to very low levels
(# few µg/l) by adsorption processes. Humic substances are considered particularly important in
the adsorption of thorium (Gascoyne, 1982). Thibault et al. (1990) conducted a critical
compilation and review of published K d data by soil type needed to model radionuclide migration
from a nuclear waste geological disposal vault to the biosphere. Thibault et al. list K d values for
thorium that range from 207 to 13,000,000 ml/g. The range of thorium K d values listed for
organic soil was 1,579 to 1.3 x 10 7 ml/g. Based on our experience, the very high K d values
reported for thorium should be viewed with caution. The studies resulting in these values should
be examined to determine if the initial concentrations of thorium used for these K d measurements
were too great and precipitation of a thorium solid (e.g., hydrous thorium oxide) occurred during
the equilibration of the thorium-spiked soil/water mixtures. As noted in the letter report for
Subtask 1B, precipitation of solids containing the contaminant of interest results in K d values that
are erroneously too high.
The adsorption of thorium on pure metal-oxide phases has also been studied experimentally in
conjunction with surface complexation models. 1 Östhols (1995) studied the adsorption of thorium
on amorphous colloidal particles of silica (SiO 2). Their results indicate that the adsorption of
thorium on silica will only be important in the pH range from 3 to 6. In neutral and alkaline pH
values, silica surface sites are not expected to be efficient adsorbents for thorium.
Iron and manganese oxides are expected to be more important adsorbents of thorium than silica.
Hunter et al. (1988) studied the adsorption of thorium on goethite ("-FeOOH) and nsutite
((-MnO 2) in marine electrolyte solutions. Their experiments indicate that adsorption of thorium
1 Surface complexation models are discussed in Volume I of this report.
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