UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

Because strontium generally exists in nature at much lower concentration than calcium, it
commonly does not form pure phases (Faure and Powell, 1972). Instead it forms coprecipitates
(solid solutions) with calcite and anhydrite. Calcite can allow the substitution of several hundred
parts per million strontium before there is any tendency for strontianite to form. Strontium can
also coprecipitate with barium to form (Ba (1-x),Sr x)SO 4 in more-alkaline environments (Ainsworth
and Rai, 1987; Felmy et al., 1993).
5.8.5 Adsorption/Desorption
A great deal of research has been directed at understanding and measuring the extent to which
strontium adsorbs to soils [reviewed by Ames and Rai (1978) and Strenge and Peterson (1989)].
The primary motivation for this research is the need to understand the environmental fate and
mobility of 90 Sr, particularly as it relates to site remediation and risk assessment. The mechanism
by which strontium partitions from the dissolved phase to the solid phase at pH values less than 9
is commonly believed to be cation exchange 1 (Ames and Rai, 1978; Lefevre et al., 1993;
McHenry, 1958).
Among the most important environmental parameters affecting the magnitude of a strontium K d
value is the soil CEC (Ames and Rai, 1978; Lefevre et al., 1993; McHenry, 1958). This finding is
consistent with cation exchange proposed as the mechanism generally controlling strontium
adsorption. The results of Serne and LeGore (1996) also indicate that strontium adsorption is
largely controlled by cation exchange. They reported that 90 Sr adsorption was reversible; that is,
strontium could be easily desorbed (exchanged) from the surfaces of soils. Natural soils that had
been in contact with 90 Sr for approximate 27 y could be leached of adsorbed 90 Sr as readily as
similar soils containing recently adsorbed strontium, indicating that 90 Sr does not become more
recalcitrant to leaching with time. Furthermore, these studies suggested that cation exchange, and
not (co)precipitation, was responsible for 90 Sr sorption because the latter would leach at a much
slower rate.
Some studies indicate that a fraction of some 90 Sr sorbed to soil components may not be readily
exchanged [see review in Brady et al. (1999)]. For example, Schulz and Riedel (1961) studied
the influence of aging on the sorption of carrier-free 90 Sr into nonexchangeable forms by three
soils. They observed that less than 10% of the total applied carrier-free 90 Sr was not easily
1 Cation exchange is a reversible adsorption reaction in which an aqueous species exchanges with
an adsorbed species. Cation exchange reactions are approximately stoichiometric and can be
written, for example, as
CaX(s) + 90 Sr 2% (aq) = 90 SrX(s) + Ca 2% (aq)
where X designates an exchange surface site. Adsorption phenomena are discussed in more detail
in Volume I of this report.
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