UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

although mg/l concentrations of 232 Th have been detected in high-acid groundwaters beneath
uranium tailings sites (Langmuir and Herman, 1980).
Although the normal ranges of thorium concentrations in igneous, metamorphic, and sedimentary
rocks are less than 50 ppm, thorium concentrations can be as high as 30 and 300 ppm,
respectively, in oceanic sand/clays and marine manganese nodules (Gascoyne, 1982). These
anomalously high concentrations of thorium have been explained by the tendency of thorium to
strongly adsorb on clay and oxyhydroxide phases (Langmuir and Herman, 1980).
The mineralogy of thorium-containing minerals is described by Frondel (1958). Most thoriumcontaining
minerals are considered fairly insoluble and resistant to erosion. There are few
minerals in which thorium is an essential structural constituent. Important thorium minerals
include thorite [(Th,U,Ce,Fe,etc.)SiO 4] and thorianite (crystalline ThO 2). Thorite is found in
pegmatites, gneisses, granites, and hydrothermal deposits. Thorianite is chiefly found in
pegmatitic rocks, but is best known as a detrital mineral. 1 Thorium also occurs, however, as
variable, trace concentrations in solid solution in many rare-earth, zirconium, and uranium
minerals. The 2 most important minerals of this type include monazite [(Ce,La,Th)PO 4] and
zircon (ZrSiO 4). Monazite and zircon are widely disseminated as accessory minerals in igneous
and metamorphic rocks. They also occur in commercial quantities in detrital sands derived from
regions of these rocks due to their resistance to erosion (Deer et al., 1967; Frondel, 1958).
Concentrations of thorium can be several weight percent in these deposits.
Because of their long half lives, 228 Th (t ½ = 1.913 y), 230 Th (t ½ = 8.0 x 10 4 y), and 232 Th (t ½ =
1.41 x 10 10 y), which are all alpha-particle emitters, pose long-term health risks and are therefore
environmentally important. Contamination includes thorium-containing soils and thorium
dissolved in surface- and groundwaters. Of the contaminated sites considered in EPA/DOE/NRC
(1993), radioactive contamination of soil, surface water, and/or groundwater by 228 Th, 230 Th,
and/or 232 Th has been identified at 21 of the 45 Superfund National Priorities List (NPL) sites and
23 of the 38 NRC Site Decommissioning Management Plan (SDMP) sites. Some of the
contamination resulted from the separation and processing of uranium and from the use of
monazite and zircon sands as source materials for metallurgical processes.
5.9.3 Aqueous Speciation
Thorium occurs only in the +4 oxidation state in natural soil/water environments. Dissolved
thorium forms a variety of hydrolytic species, and, as a small, highly charged ion, undergoes
extensive chemical interaction with water and most anions. The available thermodynamic data for
thorium-containing aqueous species and solids have been compiled and critically reviewed by
Langmuir and Herman (1980) for an analysis of the mobility of thorium in low-temperature,
natural waters.
1 A detrital mineral is defined as “any mineral grain resulting from mechanical disintegration of
parent rock” (Bates and Jackson, 1980).
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