Transport and Sorption - Environmental Modeling - page 110

with the solid phase and are called tracers . Chloride is such a tracer in most
environments.
K d not only depends on the component but also on the solid material. In clays
sorption can be expected to be high due to the high surface area per volume and due
to the high electric potential. Clay minerals have an excess of imbalanced negative
charges, thus favoring the adsorption of cations. Divalent cations are usually more
strongly adsorbed than monovalent ions (Fetter 1994 ).
In the literature, K d values are treated extensively for different kinds of chemical
species, for inorganic and organic components, for chemicals of the natural envi-
ronment and for contaminants. K d values extend over several orders of magnitude,
from low values as 2 10 4 m 3 /kg for sodium (H aetal . 1997 ) up to high values

like 400 m 3 /kg for protactinium (Geibert 2001 ). For tracer-like components even
lower values may be found and for strongly fixed components even higher values.
Often ad- and desorption are not taking place at the surfaces of the porous matrix
directly but on organic material that itself is fixed at the solid matrix. Especially in
aquatic sediments near the sediment-water surface this type of connection may be
dominant. Synthetic organic chemicals tend to adsorb on organic carbon. If c org
denotes the concentration of organic material, K org denotes the distribution coeffi-
cient on organic carbon, and the distribution coefficients are related by the formula:

€ oltt €

K d ¼ K org c org =r s (6.3)

where the ratio c org =r s represents the weight fraction of organic matter in the solid
phase (Karickhoff et al. 1979 ; Karickhoff 1984 ). For pure sand, which does not
contain any organics, the adsorption is thus zero. As an alternative to K org , the
octanol-water-distribution coefficient K ow can be taken. For several chemical
components a relation between K ow and K org is given in the form

log ðK org Þ¼a log ðK ow Þþb (6.4)

where are empirical constants. A mathematically similar relation often can

be stated for K org and solubility S of a component:

a and b

ðSÞþb

log ðK org Þ¼a log (6.5)

and b

a

with empirical constants (Karickhoff et al. 1979 ). While K org and K ow

are correlated positively, the correlation between K org and S is negative. Highly
soluble chemicals can be expected to interact only marginally with the porous
material. Vica versa, chemicals with low solubility show a strong tendency of
interaction with the solid matrix. This is illustrated in Table 6.1 showing a classifi-
cation concerning mobility using K ow . Mobile components have low distribution
coefficients, low K ow , and a high solubility. Immobile, strongly sorbing components
have high distribution coefficients and low solubility.