Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 137

A key determinant of whether a contaminant will be mobile in the subsurface is the kinetics of
the sorptive process compared to the rate of chemical transport by advection and dispersion.
Chemicals that are adsorbed more quickly than they are advected will be retained in the aquifer
matrix (Schwarzenbach and Westall, 1981).
Adsorption of chemicals to organic and mineral surfaces controls retardation of contaminant
migration in surface water and groundwater (Brusseau and Reid, 1991). * Compounds may also sorb
onto micrometer-sized colloids that are advected with groundwater l ow. Low-solubility hydro-
phobic compounds such as chlorinated solvents can be substantially more mobile when sorbed to
colloids than they are in the aqueous phase (Huling, 1989). Adsorption of contaminants to carbon
surfaces also has bearing on treatability using liquid or granular activated-carbon i lters.
The forces governing sorption include electrostatic interactions, van der Waal's interactions,
hydrogen bonding, charge transfer, ligand exchange, direct and induced dipole-dipole interactions,
chemisorption, and hydrophobic bonding (Suthersan, 2002). Organic compounds can adsorb to both
organic matter and mineral surfaces. The degree to which sorption occurs depends on both soil
properties such as soil organic matter content, clay type, and properties of the organic compound,
including water solubility, molecular volume, the octanol/water partition coefi cient ( K ow ), and the
organic carbon partition coefi cient ( K oc ). These terms and their role in adsorption of stabilizer com-
pounds to aquifer solids and suspended solids are described further in this section.
Contaminant residence time in the subsurface also plays an important role in sorption processes,
as contaminants tend to become more strongly bound to soil over time. The age of a release is
important as it has bearing on enhanced sorption of organic compounds to inorganic soil particles.
Generic fate and transport models rely on published K oc values and assumed total organic carbon
content to estimate contaminant partitioning between sorbed and aqueous phases, as well as to
predict retardation (see Section 3.3.2.4 ). This approach can signii cantly underestimate the sorption
potential of a contaminant and overpredict its mobility in the soil or groundwater, as discussed in
Section 3.5 .
3.3.2.1 Soil Properties Affecting Adsorption
Organic matter in soil or sediment is the primary factor in the sorption of nonionic organic contami-
nants from water by soils. Organic compounds also adsorb to soil minerals but only weakly, at least
initially, owing to the strong competitive adsorption of water on polar mineral surfaces. Sorption to
soil organic matter has an approximately inverse proportionality to a compound's aqueous solubility
(Chiou and Kile, 1994).
Adsorption is a surface phenomenon: it increases with higher surface area, which is in cubic
proportion to particle size and is also related to pore size. Micropores increase the adsorption capac-
ity. The distribution of pore sizes in organic matter is relevant to sorption processes, but it is difi cult
to quantify and is not usually measured. Large molecules whose hydrated radii exceed the diameter
of pore sizes will be excluded from absorption into soil organic matter (“steric exclusion”) and will
be limited to surface adsorption (Bennett, 2004b).
Most organic matter in soil is bound to clay as a clay-organic complex. Two major types of
adsorbing surfaces are available to an organic compound: clay-organic and clay alone. The relative
contribution of organic and inorganic surface areas to adsorption will depend upon the extent to
which the clay is coated with organic matter. The inl uence of clay on organic chemical adsorption
is signii cant in soils with organic matter contents below 1% (Dragun, 1988). Active agents govern-
ing adsorption of organic compounds onto mineral surfaces include silicates and aluminum and
iron oxides and hydroxides. Oxide surfaces may be amphoteric hydroxyl sites † capable of both
donating and accepting a proton, providing anionic, neutral, and cationic sites, depending on pH and
* Retardation is the rate of chemical migration in a saturated aquifer relative to groundwater velocity; see Section 3.3.2.3
for further discussion of retardation.
† Amphoteric substances can react either as acids or bases.