Understanding Variation in Partition Coefficient, Kd, Values Volume II

Further, complexation of cesium by common industrial chelates (e.g., EDTA) is believed to be poor
due to their low stabilities and the presence of competing cations (e.g., Ca 2+ ) at appreciably higher
concentrations than that of cesium. Therefore, aqueous complexation is not thought to greatly influence
cesium behavior in most groundwater systems.
5.3.4 Dissolution/Precipitation/Coprecipitation
Neither precipitation nor coprecipitation are expected to affect the geochemistry of cesium in
groundwater. The solubility of most cesium compounds in water is very high.
5.3.5 Sorption/Desorption
In general, most soils sorb cesium rather strongly (Ames and Rai, 1978). Some mica-like minerals,
such as illite {(K,H 3O)(Al,Mg,Fe) 2(Si,Al) 4O 10[(OH) 2,H 2O]} and vermiculite
[(Mg,Fe,Al) 3(Si,Al) 4O 10(OH) 2A4H 2O], tend to intercalate (fix) cesium between their structural layers
(Bruggenwert and Kamphorst, 1979; Douglas, 1989; Smith and Comans, 1996). These silicate
minerals can be thought of as having a crystal lattice composed of continuous sheet structures. The
distance between the silicate layers is controlled by the type of cation associated with the adsorption
sites on the layers. Large hydrated cations, such as Na + , Li + , Ca 2+ , and Mg 2+ , tend to pry the layers
further apart, whereas small hydrated cations, such as K + , have the opposite effect. The interlayer
distance between the sheets of mica-like minerals excludes the absorption of the majority of cations by
size, while permitting the Cs + ion to fit perfectly between the layers. Consequently, these mica-like
minerals commonly exhibit a very high selectivity for Cs + over other cations, including cations existing at
much higher concentrations. Even a small amount (e.g., 1-2 weight percent) of these mica-like minerals
in a soil may strongly absorb a large amount of dissolved cesium (Coleman et al., 1963; Douglas,
1989). Some researchers have considered the exchange of trace cesium on these mica-like minerals to
be nearly irreversible (Douglas, 1989; Routson, 1973), meaning that cesium absorbs at a much faster
rate than it desorbs.
The effect of cesium concentration and pH on cesium adsorption by a calcareous soil containing micalike
minerals has been studied by McHenry (1954). The data indicate that trace cesium concentrations
are essentially completely adsorbed above pH 4.0. When placed in a high-salt solution, 4 M NaCl,
only up to 75 percent of the trace cesium was adsorbed, and the adsorption was essentially
independent of pH over a wide range. At cesium loadings on the soil of less than 1 percent of the soil
CEC, the effect of competing cations on cesium adsorption was slight. Low concentrations of
dissolved cesium are typical of cesium-contaminated areas. Thus competition may not play an
important role in controlling cesium adsorption in most natural groundwater environments. The results
of McHenry (1954) also indicate that trace concentrations of cesium were adsorbed to a greater
degree and were more difficult to displace from the soil by competing cations than when the cesium was
adsorbed at higher loadings.
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