Environmental Engineering Reference
In-Depth Information
in the soil water) to the vapor phase in soil air and then to the atmosphere is dependent
on many physical and chemical properties of both the chemical and the soil, and on the
process involved in moving from one phase to another. The three main distribution or
transport processes involved are
1. Compound in soil solids ↔ compound in solution
2. Compound in solution ↔ compound in vapor phase in soil air
3. Compound in vapor phase in soil air → compound in atmosphere
Partitioning of a chemical among the three phases can be estimated from either vapor
or solution phase desorption isotherms. The process by which a compound evaporates in
the vapor phase to the atmosphere from another environmental compartment is deined as
volatilization. This process is responsible for the loss of chemicals from the soil to the air
and is one of the factors involved in the persistence of an organic chemical. Determination
of volatilization of a chemical from the soil to the air is most often achieved using theoreti-
cal descriptions of the physical process of volatilization based on Raoult's law and Henry's
law. The rate at which a chemical volatilizes from soil is affected by soil and chemical
properties, and environmental conditions. Some of the properties of a chemical involved in
volatilization are its vapor pressure, solubility in water, basic structural type, and the num-
ber, nature and position of its basic functional groups. Other factors affecting volatiliza-
tion rate include adsorption, vapor density, and water content of the soil in the subsurface.
Adsorption impacts directly on the chemical activity by reducing it to values below that
of the pure compound. In turn, this affects the vapor density and the volatilization rate
since vapor density is directly related to the volatilization rate. Vapor density is the con-
centration of a chemical in the air, the maximum concentration being a saturated vapor.
The role of water content is seen in terms of competition for adsorption sites on the soil.
Displacement of nonpolar and weakly polar compounds by water molecules can occur
because of preferential sorption (of water). Hydrates—i.e., hydration layer on the soil par-
ticle surfaces—will increase the vapor density of weakly polar compounds. If dehydration
occurs, the compound sorbs onto the dry soil particles. This means that the chances for
volatilization of the organic chemical compound are better when hydrates are present.
When a vapor is in equilibrium with its solution in some other solvent, the equilibrium
partial pressure of a constituent is directly related to the mole fraction of the constituent
in the aqueous phase. Once again, designating
P
i
as the partial pressure of the constituent,
X
i
as the mole fraction of the constituent in the aqueous phase, and
H
i
as Henry's constant
for the constituent, Henry's law states that:
P
i
= H
i
X
i
. By and large, so long as the activity
coeficients remain relatively constant, the concentrations of any single molecular species
in two phases in equilibrium with each other will show a constant ratio to each other. This
assumes ideal behavior in water and the absence of signiicant solute-solute interactions
and also absence of strong speciic solute-solvent interactions.
Partitioning of organic chemicals is most often described by the partition coeficient
k
ow
. This is the octanol-water partition coeficient and has been widely adopted in studies
of the environmental fate of organic chemicals. The octanol-water partition coeficient is
sometimes known as the
equilibrium partition coeficient
, i.e., coeficient pertaining to the
ratio of the concentration of a speciic organic contaminant in other solvents to that in
water. Results of countless studies have shown that this coeficient is well correlated to
water solubilities of most organic chemicals. Since
n
-octanol is part lipophilic and part
hydrophilic (i.e., it is amphiphilic), it has the capability to accommodate organic chemicals
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