Geoscience Reference
In-Depth Information
Chapter 7
Partitioning of Volatile Compounds
Volatilization (also referred to as vaporization or evaporation) is the conversion of
a chemical from the solid or liquid phase to a gas or vapor phase. The partitioning
of a volatile compound in the subsurface environment comprises two distinct
patterns: volatilization of contaminant molecules (from the liquid, solid, or
adsorbed phase) and dispersion of the resulting vapors in the subsurface gas phase
or the overlying atmosphere by diffusive and turbulent mixing. Even though the
two processes are fundamentally different and controlled by different chemical and
environmental factors, they are not wholly independent under natural conditions:
only by integrating their effects can volatilization be characterized.
The volatilization process changes the contaminant from a solid or liquid state,
where the molecules are held together by intermolecular forces, into a vapor phase.
The molar heats of fusion (DH
f
), volatilization (DH
v
), and sublimation (DH
s
) are
related according to the Born-Haber cycle by
DH
s
¼ DH
f
þ
DH
v
:
ð
7
:
1
Þ
Even at low temperature, some molecules may overcome the energy barrier of
the cohesive forces and escape from the solid or liquid state into the gaseous phase.
The vapor dispersion process is described mathematically as a vapor flux
(I) through any plane at a height z and is expressed by
dp
dz
I ¼ K
z
;
ð
7
:
2
Þ
z
where K
z
is the transfer coefficient and dp/dz is the vapor pressure gradient at
height z. Air flow just above the land surface may be turbulent and can be
described by eddy diffusion. In the boundary layer along the soil surface and in the
subsurface matrix, vapor dispersion may be described by a molecular diffusion
process.
Taylor and Spencer (
1990
) pointed out that a laminar layer can be regarded as
the limiting distance above the soil surface to which the smallest eddies of the
overland turbulent flow can penetrate. Therefore, above this layer, transport takes
