Page 68 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 51

maximum adsorbed concentration. It should be noted that, in this book, both
symbols S and X are used for COC concentration in soil. Symbol S means
“mass of COC/mass of dry soil,” while X means “mass of COC/mass of soil
plus moisture.”
On the other hand, the Freundlich isotherm can be expressed in the fol-
lowing form:

S = KC 1/n (2.23)

where both K and 1/n are empirical constants. These constants are different
for different adsorbates, adsorbents, and solvents. For a given compound,
the values will also be different for different temperatures. When using the
isotherms, we should ensure that the units among the parameters and the
empirical constants are consistent.
Both isotherms are nonlinear. Incorporating the nonlinear Langmuir or
Freundlich isotherm into the mass balance equation to evaluate the COC’s
fate and transport will make the computer simulation more difficult or more
time consuming. Fortunately, it was found that, in many environmental
applications, the linear form of the Freundlich isotherm applies. It is called
the linear adsorption isotherm, since 1/n = 1, thus

S = KC (2.24)

which simplifies the mass balance equation in a fate and transport model.
Partition Coefficient. For soil–water systems, the linear adsorption iso-
therm is often written in the following form:
S = K C
p
thus (2.25)

S
K = /C
p
where K is called the partition coefficient that measures the tendency of a
p
compound to be adsorbed onto the surface of soil or sediment from a liquid
phase. It describes how a COC distributes (partitions) itself between the two
media (i.e., solid and liquid). Henry’s constant, which was discussed previ-
ously, can be viewed as the vapor–liquid partition coefficient.
For a given organic chemical compound, the partition coefficient is not the
same for every soil. The dominant mechanism of organic adsorption is the
hydrophobic bonding between the compound and the natural organics asso-
ciated with the soil. It was found that K increases linearly with the fraction
p
of organic carbon (f ) in soil, thus
oc
K = f K
p oc oc (2.26)