Environmental Engineering Reference
In-Depth Information
Chapter 6
Transport and Sorption
6.1
Interphase Exchange
Aside from advection, diffusion and dispersion, which can be formulated separately
for the solid and the fluid phases, the interaction between the phases is another
process which is relevant in many environmental systems. Under certain
conditions, particles in the pore space are attracted by the surface of the porous
medium where some chemical processes tend to bind them in various ways.
Different processes like electrical attraction and repulsion, complexation or chemical
reaction can be distinguished in a detailed look, gathered under the general term
sorption .
In multiphase environments, sorption denotes processes that affect an exchange
of components between phases. One can speak of interphase exchange. In porous
media there is the exchange between solid and fluid phase, i.e. between the water in
the pore space and the solid matrix. Adsorption denotes fluxes from the fluid to the
solid phase, while desorption is the opposite process, in which there is a flux from
the solid to the fluid phase. In the following the term mobilization is used fre-
quently. A pollutant particle which has been fixed at the surface of the solid matrix
in a first time period, may be re-mobilized and freed for dispersion and advection
processes in a second time period.
A schematic view on adsorption and desorption in the pore space of a porous
medium is given in Fig. 6.1 . In the remainder of this chapter, we will mostly refer to
that multi-phase set-up as it is the standard concept for environmental models
concerning groundwater, seepage water and pore water in aquatic sediments.
In a first fundamental distinction, the speed of the processes governing inter-
phase exchange in relation to the transport processes is of concern. One speaks of
fast sorption if sorption is faster than the transport processes; and of slow sorption if
sorption is slower than transport. Thus the characterization of sorption depends on
the specific situation.
Let c denote the fluid phase concentration and c s the solid phase concentration.
Normally c and c s are not independent but connected. High or low concentrations in
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