UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

J.1.0 Background
Appendix J
Partition Coefficients For Uranium
The review of uranium K d values obtained for a number of soils, crushed rock material, and
single-mineral phases (Table J.5) indicated that pH and dissolved carbonate concentrations are the
2 most important factors influencing the adsorption behavior of U(VI). These factors and their
effects on uranium adsorption on soils are discussed below. The solution pH was also used as the
basis for generating a look-up table of the range of estimated minimum and maximum K d values
for uranium.
Several of the studies identified in this review demonstrate the importance dissolved carbonate
through the formation of strong anionic carbonato complexes on the adsorption and solubility of
dissolved U(VI). This complexation especially affects the adsorption behavior of U(VI) at
alkaline pH conditions. Given the complexity of these reaction processes, it is recommended that
the reader consider the application of geochemical reaction codes, and surface complexation
models in particular, as the best approach to predicting the role of dissolved carbonate in the
adsorption behavior of uranium and derivation of K d values when site-specific K d values are not
available for U(VI).
J.2.0 Availability of K d Values for Uranium
More than 20 references were identified that reported the results of K d measurements for the
sorption of uranium onto soils, crushed rock material, and single mineral phases. These studies
were typically conducted to support uranium migration investigations and safety assessments
associated with the genesis of uranium ore deposits, remediation of uranium mill tailings,
agriculture practices, and the near-surface and deep geologic disposal of low-level and high-level
radioactive wastes (including spent nuclear fuel).
A large number of laboratory uranium adsorption/desorption and computer modeling studies have
been conducted in the application of surface complexation models (see Chapter 5 and Volume I)
to the adsorption of uranium to important mineral adsorbates in soils. These studies are also
noted below.
Several published compilations of K d values for uranium and other radionuclides and inorganic
elements were also identified during the course of this review. These compilations are also briefly
described below for the sake of completeness because the reported values may have applicability
to sites of interest to the reader. Some of the K d values in these compilations are tabulated below,
when it was not practical to obtain the original sources references.
J.2