Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 140

the contaminant. In the literature, K d is sometimes represented as K p , the mass-specii c partition
(or distribution) coefi cient. The retardation factor assumes that sorption is reversible, instanta-
neous, and linear (i.e., not proportional to solute concentration or organic matter) (Freeze and
Cherry, 1979).

3.3.2.4 Organic Carbon Partition Coeffi cient, K oc
The sorption of hydrophobic organic compounds is strongly controlled by the presence of soil
organic material. K oc is the partitioning coefi cient for a solute between the aqueous and organic
phases and is dei ned as the ratio of the amount of chemical adsorbed (in micrograms) per unit mass
of organic carbon (oc, in grams) in the soil or sediment to the concentration (in micrograms per
milliliter of solution) of the chemical in solution at equilibrium (Lyman et al., 1990). K oc is unitless *
and is represented as

K d

___

K oc =

f oc ,

(3.33)

where f oc is the fraction of organic carbon in soil (mass ratio).
K oc is not a primary physicochemical compound property because the degree to which a com-
pound is adsorbed is in relation to its K oc and dependent upon the soil organic matter content,
organic matter properties, and groundwater chemistry. Measured values of K oc are obtained for
particular conditions of the measured soil and solution; however, because K oc is the K d value
normalized for an aquifer's organic carbon content, f oc , use of K oc values to contrast contaminant
mobility reduces the variability of partition-coefi cient measurements in different soils (Dragun,
1988). K oc values derived from linear isotherms are therefore considered to be largely independent
of the properties of soil or sediment. Nevertheless, variability in soil properties including clay con-
tent, the particular clay mineral(s), surface area, soil pH, soil temperature, and type of soil organic
matter may contribute to different measured organic carbon partition coefi cients. Values of K oc for
the same compound measured on different soils have uncertainties ranging from 10% to 140%
(Lyman, 1990). When estimating the distribution coefi cient K d from the organic carbon partition
coefi cient K oc and soil organic carbon content f oc (i.e., K d

= K oc × f oc ), the following assumptions are

made (Suthersan, 2002):

• Sorption is exclusively to the organic component of the soil.

• All soil organic matter has the same sorption capacity per unit mass.

• The sorption-desorption process is in equilibrium.

• The sorption-desorption isotherms are identical, that is, hysteresis does not occur.

Of course, soil organic matter varies in type and sorption capacity among different soils. A
signii cant portion of soil organic matter is composed of humic and fulvic acids. Fulvic acid is com-
posed of organic polymers with molecular weight greater than 2000, whereas humic acid is
composed of organic polymers with higher molecula r weights, up to 30 0,0 0 0 daltons. † Some sources
report that compounds have a greater propensity to sorb to one type of organic matter than another
(Dragun, 1988), whereas others i nd that there is only a small variability of the sorption properties
of soil organic matter (Nguyen et al., 2005). For humic acid, absorption of organic compounds into
the organic matrix plays a greater role than adsorption onto its surfaces (Niederer et al., 2006b).
Sorption by amorphous soil organic matter involves ab sorption into instead of ad sorption onto the

* Note that K oc is unitless, which assumes that 1 mL of solution = 1 g of solution, that is, organic compounds are present at
low concentrations. Adsorption isotherms become nonlinear at high concentrations.
† Dalton (Da) is an atomic mass unit equal to one-twelfth of the mass of an unbound atom of carbon-12, that is, the approxi-
mate mass of a hydrogen atom.