Contaminants and Land Environment Sustainability Indicators - Sustainable Practices in Geoenvironmental Engineering - page 329

Infinite capacity—high sorption
Common type of adsorption
isotherm
Infinite capacity—low sorption
Equilibrium concentration of target
contaminants in porewater, c
FIGURE 9.13
Illustration of partitioning of target contaminants between contaminants in porewater and contaminants
sorbed by soil particles. Note that unless limits are placed on the maximum sorption capacity of the soils,
speciication of constant k d values means ininite sorption capacity by the soil particles in the soil-water system.
the distribution coeficient k d as a key parameter in the relationship used to predict the
transport of contaminants in a saturated soil.
There are at least two broad issues regarding the determination and use of the distribu-
tion coeficient k d : (1) the types of tests used to provide information for determination of
k , and (2) range of applicability of k d in transport and fate predictions. We will discuss the
former in this section and leave the latter discussion for a later section where the problem
of prediction of transport and fate of contaminants is addressed. Laboratory tests used to
provide information on the mass transfer of contaminants from the porewater onto soil
solids are the most expedient means to provide one with information on the partitioning
of contaminants. By and large, these tests provide only the end result of the mass transfer,
and not direct information on the basic mechanisms responsible for partitioning.
The distribution coeficient k d is determined from information gained using batch equi-
librium tests on soil solutions. Ratios of 10 or 20 parts solution to 1 part soil are generally
used, and the candidate or target contaminant is part of the aqueous phase of the soil
solution. In many laboratory test procedures, the candidate soil is used in the soil solution,
and the candidate or target contaminant is generally a laboratory-prepared contaminant,
e.g., PbNO 3 for assessment of sorption of Pb as a contaminant heavy metal. Since the soil
particles are in a highly dispersed state in the soil solution, one would expect that all the
surfaces of all the particles are available for interaction with the target contaminant in
the aqueous phase of the soil solution. Using multiple batches of soil solution where the
concentration of the target contaminant is varied, and by determining the concentration
of contaminants sorbed onto the soil solids and remaining in the aqueous phase, one will
obtain characteristic adsorption isotherm curves such as those shown in Figures 9.13 and
2.15. The slope of the adsorption isotherm deines k d .
Consider the illustration shown in Figure 9.14. This schematic compares the loose
structure or dispersed state of soil particles in a soil suspension (bottom illustration)
with an aggregation of a multitude of particles constituting a microstructural unit that