Geoscience Reference
In-Depth Information
5.2 Quantifying Adsorption
Quantifying adsorption of contaminants from gaseous or liquid phases onto the
solid phase should be considered valid only when an equilibrium state has been
achieved, under controlled environmental conditions. Determination of contami-
nant adsorption on surfaces, that is, interpretation of adsorption isotherms and the
resulting coefficients, helps to quantify and predict the extent of adsorption. The
accuracy of the measurements is important in relation to the heterogeneity of
geosorbents at a particular site. The spatial variability of the solid phase is not
confined only to field conditions; variability is present at all scales, and its effects
are apparent even in well-controlled laboratory-scale experiments.
5.2.1 Adsorption-Desorption Coefficients
Adsorption-desorption coefficients are determined by various experimental tech-
niques related to the status of a contaminant (solute or gas) under static or con-
tinuous conditions. Solute adsorption-desorption is determined mainly by batch or
column equilibration procedures. A comprehensive description of various exper-
imental techniques for determining the kinetics of soil chemical processes,
including adsorption-desorption, may be found in the topic by Sparks (
1989
) and
in many papers (e.g., Nielsen and Biggar
1961
; Bowman
1979
; Boyd and King
1984
; Peterson et al.
1988
; Podoll et al.
1989
; Abdul et al.
1990
; Brusseau et al.
1990
; Hermosin and Carnejo
1992
; Farrell and Reinhard
1994
; Schrap et al.
1994
;
Petersen et al.
1995
).
Application of analytical techniques from molecular geochemistry can be used
to study reactions at the molecular level. Such studies can elucidate the parti-
tioning and interactions of contaminant species in aqueous, solid, and gas phases.
While spectroscopic methods provide information on chemical reactions on the
contaminant-solid interface, other techniques may provide additional spatial
information at an atomic level. In an extensive review on molecular geochemistry,
O'Day (
1999
) summarizes common analytical methods (Table
5.2
) and discusses
their benefits in understanding contaminant-solid interactions at the molecular
level.
5.2.2 Adsorption Isotherms
The sorption process generally is studied by plotting the equilibrium concentration
of a compound on the adsorbent, as a function of equilibrium concentration in the
gas or solution at a given temperature. Adsorption isotherms are graphs obtained
