Geoscience Reference
In-Depth Information
homogeneous solid comprising many components, which is known as a solid
solution. The fugacity approach for the interpretation of environmental behavior of
a chemical contaminant is described in detail by Mackay and Paterson (
1981
).
The BET equation describes the phenomenon of multilayer adsorption, which is
characteristic of physical or van der Waals interactions. In the case of gas
adsorption, for example, multilayer adsorption merges directly into capillary
condensation when the vapor pressure approaches its saturation value and often
proceeds with no apparent limit. The BET equation has the form
P
V
ð
P
0
P
Þ
¼
1
V
m
C
h
þ
ð
C
h
1
Þ
P
V
m
C
h
P
0
ð
5
:
7
Þ
where P is the equilibrium pressure at which a volume V of gas is adsorbed, P
0
is
the saturation pressure of the gas, V
m
is the volume of gas corresponding to an
adsorbed monomolecular layer, and C
h
is a constant related to the heat of
adsorption of the gas on the solid in question. If a plot of P/(P
0
- P) against P/P
0
results in a straight line, the effective surface area of the solid can be calculated
after V
m
has been determined, either from the slope of the line (C
h
- 1)/V
m
C
h
or
from the intercept 1/V
m
C
h
.
It is interesting to note the effect of laboratory-scale variability on the nonlinear
sorption behavior of contaminants in a porous medium, composed of various
particles that are characterized individually by randomly distributed sorptive
capacities and selectivity coefficients. A discrepancy is observed between the
results obtained for an individual particle and for an ensemble of particles. As the
variability in underlying sorptive properties increases, the Langmuir isotherm
ceases to describe the behavior of the aggregates of individual particles, under
either static or dynamic conditions. Assessment of the pollution hazard from
parameters obtained in the laboratory therefore should consider the variability
among the individual particles making up the analyzed geosorbent sample. Sch-
warzenbach et al. (
2003
) introduced the concept of ''the complex nature of the
distribution coefficient,'' showing that this parameter may lump together many
chemical species. The solute-geosorbent exchange should describe an appropriate
equilibrium expression that incorporates properties of the various geosorbent
components. The resulting K
d
parameter is weighted by the availability of sorbent
properties in the total solid phase of the sample measured. Despite this limitation,
the distribution coefficient gives an effective representation of the solute-geosor-
bent relationship with regard to contaminant adsorption-desorption behavior.
In conclusion, the different shapes of isotherms describing equilibrium distri-
butions of a contaminant, between geosorbents and aqueous or gaseous phases,
depend on the sorption mechanism involved and the associated sorption energy. At
low contaminant concentration, all models reduce to essentially linear correlation.
At higher contaminant concentration, when sorption isotherms deviate from lin-
earity, an appropriate isotherm model should be used to describe the retention
process.
