Handbook of Solvents - George Wypych - ChemTech - Ventech!

1152 William R. Roy
where:
Q diffusive flux (mass/area-time)
Df diffusion coefficient (area/time)
∇a concentration gradient (mass/volume/distance)
Compared with the relatively unobstructed path for the diffusion of solvents in the atmosphere,
diffusion coefficients for solvents in soil air will be less because of the tortuosity
of the soil matrix pathways. Several functional relationships have been developed that relate
the soil diffusion coefficient (Ds) to various soil properties (see Roy and Griffin 11 ), such
as the Millington Equation 12
33 . 2
D = D η / η
[17.1.5]
s f a t
where:
ηa the air-filled porosity, and
ηt total soil porosity
17.1.3.2 Adsorption
As discussed in 17.1.2.4., adsorption by soil components can remove solvents dissolved in
water. Furthermore, the rate of movement of dissolved solvents through soil may be retarded
by adsorption-desorption reactions between the solvents and the solid phases. The
partitioning of solvents between the liquid phase and soil is usually described by an adsorption
isotherm. The adsorption of solvents may be described by the Freundlich Equation:
l/ n
x / m = K C
[17.1.6]
f
where:
x the mass adsorbed
m mass of sorbent
Kf the Freundlich constant, a soil-specific term
C the equilibrium concentration of the solvent in water, and
n the Freundlich exponent which describes the degree of non-linearity of the isotherm
When n is equal to one, the Freundlich Equation becomes a relatively simple partition function:
x/m = KC [17.1.7]
where K is an adsorption or distribution coefficient which is sometimes written as K d.Ithas
been known since the 1960s that the extent of adsorption of hydrophobic (sparingly soluble
in water) solvents often correlates with the amount of organic matter in the soil. 13 When K d
is divided by the amount of organic carbon in the soil, the resulting coefficient is the organic
carbon-water partition coefficient (K oc):
K d x 100/organic carbon(%) = K oc [17.1.8]
The organic carbon-water partition coefficient is a compound-specific term that allows the
user to estimate the mobility of a solvent in saturated-soil water systems if the amount or organic
carbon is known. For hydrophilic solvents, K oc values have been measured for many
compounds. Other values were derived from empirical relationships drawn between water
solubility or octanol-water partition coefficients. 13