Contaminants and Land Environment Sustainability Indicators - Sustainable Practices in Geoenvironmental Engineering - page 328

of Cl − than in the presence of ClO 4 − . This is the result of competition of Cl - with OH - for
formation of complexes with the Cd 2+ that were not sorbed by the kaolinite soil.
9.5 Surface Complexation and Partitioning
As opposed to speciation and complexation in the aqueous phase (porewater), surface
complexation refers to the complexes formed by the inorganic contaminants and the reac-
tive sites on the soil particle surfaces. Surface complexation includes several mechanisms
of solute-particle surface interaction, described broadly as sorption mechanisms. These
have been discussed previously as nonspeciic adsorption, speciic adsorption, and che-
misorption. The processes involved have been described as coulombic molecular interac-
tions, with bonds formed that include ionic, covalent, and van der Waals.
The result of surface complexation is partitioning. We describe partitioning of contami-
nants as the transfer of contaminants in the porewater to the soil solids as a result of sorp-
tion mechanisms between the two. This is also called mass transfer (of contaminants).
Section 2.5.3 of Chapter 2 has considered the partitioning of contaminants in a very general
manner. Partitioning, as a phenomenon, includes the transfer of both inorganic and
organic chemical contaminants. This is an important phenomenon since this is the out-
come of one of the fundamental processes that determines the persistence and fate of con-
taminants. In the case of organic chemicals, the processes involved include London-van
der Waals forces, hydrophobic reactions, hydrogen bonding and charge transfer, ligand
and ion exchange, and chemisorption. In this section, we will be examining partitioning of
both inorganic contaminants and organic chemicals in greater detail. Although there may
be some similarity in mass transfer mechanisms in the partitioning of inorganic solutes
and organic chemicals, it is generally more convenient to consider these separately.
9.5.1 Partitioning of Inorganic Contaminants
A popular measure of partitioning of inorganic and organic chemical contaminants is the
partition coeficient k p . Chapter 2 has given a very brief description of the general types
of partition coeficients. In brief, partition coeficients describe the relationship between
the amount of contaminants transferred onto soil particles (sorbed by the soil particles)
and the equilibrium concentration of the same contaminants remaining in the porewa-
ter (Figure 9.13). The popular relationships such as Langmuir and Freundlich are shown
in Figure 2.15 in Chapter 2. To distinguish between partition coeficients obtained using
different laboratory techniques, the term distribution coeficient k d is often used to denote
partitioning of contaminants obtained with batch equilibrium adsorption isotherm proce-
dures. By and large, the distribution coeficient is the partition coeficient most commonly
used to describe partitioning of heavy metals and other inorganic solutes.
What is the signiicance of partitioning? It is necessary to bear in mind that partitioning
is the result of mass transfer of contaminants from the porewater onto the soil particles.
How one determines the results of mass transfer (i.e., partitioning) can be a contentious
issue, and can severely affect one's prediction of the transport and fate of contaminants
under consideration. A quantitative determination of partitioning, such as the distribution
coeficient k d , is needed in many mathematical relationships structured to evaluate the fate
of contaminants transporting in a soil system. For example, Equation 2.2 in Chapter 2 uses