UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

5.6.6.2.2 Limits of K d Values with Respect to Dissolved Carbonate Concentrations
The dissolved carbonate content of the soils used for the regression relationships ranged from
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about 0.1 to 6 meq/l (0.1 to 6 mmol/l of HCO3 ). The dissolved carbonate values were measured
on saturation extracts obtained from these soils. The standard procedure for obtaining saturation
extracts from soils has been described by Rhoades (1996). The saturation extracts are obtained by
saturating and equilibrating the soil with distilled water followed by vacuum filtration to collect
the extract. Saturation extracts are usually used to determine the pH, the electrical conductivity,
and dissolved salts in soils. For soils with pH values less than 8.5, the saturation extracts typically
contain less than 8 mmol/l of dissolved carbonate (Richards, 1954).
The regression relationship indicates that within the range of 0.1 to 6 mmol/l of dissolved
carbonate, the K d values increase with increasing dissolved carbonate values. Adsorption
experiments conducted by Sanchez et al. (1985) showed however that very high concentrations
(100 to 1,000 meq/l) of dissolved carbonate in matrix solution decreases Pu adsorption on
goethite. The dissolved carbonates in soil saturation extracts are 3 to 4 orders of magnitude less
than the concentrations used in experiments by Sanchez et al. (1985). The data by Glover et al.
(1976) show that within very low concentration range of dissolved carbonate (0.1 to 6 mmol/l )
found soil saturation extracts, K d values for Pu increase as a function of dissolved carbonate. This
correlation may be strictly serendipitous and a more likely variable that would lead to an increased
K d would be increasing pH.
5.7 Radon Geochemistry and K d Values
5.7.1 Overview: Important Aqueous- and Solid-Phase Parameters
Controlling Retardation
The migration of radon, an inert gas, in soil/water systems is not affected itself by aqueous
speciation, precipitation/dissolution, or adsorption/desorption processes. Therefore, the mobility
of radon is not affected by issues associated with the selection of appropriate “adsorption” K d
values for modeling contaminant transport and risks in soil /water systems. Radon is soluble in
water, and the hydrostatic pressure on ground water below the water table is sufficient to keep
dissolved radon in solution.
The generation of radon is however affected by the concentrations of its parent elements which,
along with radon’s decay products, are of regulatory concern. Because aqueous speciation,
precipitation/dissolution, or adsorption/desorption processes can affect the movement of radon’s
parents and decay products in soils, these processes should be considered when modeling
contaminant transport in a total environmental system, including air transport pathways.
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