Geoscience Reference
In-Depth Information
5.3 Kinetics of Adsorption
Adsorption kinetics involve a time-dependent process that describes the rate of
adsorption of chemical contaminants on the solid phase. The ''standard'' chemical
meaning of kinetics usually covers the study of the rate of reactions and molecular
processes when transport is not a limiting factor; however, this definition is not
applicable to subsurface conditions. In the ''real'' subsurface environment, many
kinetic processes are a blend of chemical- and transport-controlled kinetics.
Understanding the kinetics of contaminant adsorption on the subsurface solid
phase requires knowledge of both the differential rate law, explaining the reaction
system, and the apparent rate law, which includes both chemical kinetics and
transport-controlled processes. By studying the rates of chemical processes in the
subsurface, we can predict the time necessary to reach equilibrium or quasi-state
equilibrium and understand the reaction mechanism. The interested reader can find
detailed
explanations
of
subsurface
kinetic
processes
in
Sparks
( 1989 )
and
Pignatello ( 1989 ).
The mechanistic rate law is not applicable to processes in the subsurface, if we
assume only that chemically controlled kinetics occur and neglect the transport
kinetics. Instead, apparent rate laws, which comprise both chemical- and
transport-controlled processes, are the proper tool to describe reaction kinetics on
subsurface soil constituents. Apparent rate laws indicate that diffusion and other
microscopic transport phenomena, as well as the structure of the subsurface and
the flow rate, affect the kinetic behavior.
Based on these rate laws, various equations have been developed to describe
kinetics of soil chemical processes. As a function of the adsorbent and adsorbate
properties, the equations describe mainly first-order, second-order, or zero-order
reactions. For example, Sparks and Jardine ( 1984 ) studied the kinetics of potas-
sium adsorption on kaolinite, montmorillonite (a smectite mineral), and vermic-
ulite (Fig. 5.3 ), finding that a single-order reaction describes the data for kaolinite
and smectite, while two first-order reactions describe adsorption on vermiculite.
The Elovich equation was developed to determine the kinetics of heterogeneous
chemisorption of gases on solid surfaces. This equation assumes a heterogeneous
distribution of adsorption energies, where the energy of activation (E) increases
linearly with surface coverage (Rao et al. 1989 ). A simplified Elovich equation
used to study the rate of soil chemical processes is given by
q ¼ 1
Y ln ð XY Þþ 1
Y ln ð t þ t 0 Þ;
ð 5 : 8 Þ
where q is the amount sorbed at time t, X and Y are constants, and t 0 is an
integration constant. An application of Eq. ( 5.8 ) for the case of PO 4 sorption on
soils is shown in Fig. 5.4 . In this particular case, a linear relationship is observed.
Chien and Clayton ( 1980 ) found that the Elovich equation was best based on the
highest values of the simple correlation coefficient. Polysopoulos et al. ( 1986 ),
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