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in solution; weak surface interactions via van der Waals, dipole-dipole, induced
dipole, and other weak intermolecular forces; and surface reactions where the sorbate
actually binds with the solid. 106
For charged pollutants, the additional mechanism of ion exchange can occur to
promote sorption. It should also be mentioned that, aside from organically rich
materials, solids with little or no organic content also sorb neutral organic chemicals.
For such cases, the sorbent consists of inorganic matter, such as clay. Such inorganic
solid sorption is usually significant only when the organic carbon content of the
solids is quite low. 45
Adsorption-desorption in natural waters is in some cases a reversible reaction.
The chemical is initially dissolved (e.g., discharged into) in water in the presence of
various concentrations of suspended solids. After an initial kinetic reaction, a dynamic
equilibrium is established in which the rate of the forward reaction (sorption) is
exactly equal to the rate of the reverse reaction (desorption). After equilibrium is
attained, samples taken from the filtered water and/or solids and analyzed for chemical
concentration can be used to obtain the partition coefficient. Sorption reactions usually
reach equilibrium quickly, and the kinetic relationships can often be assumed to be
at steady state. This is sometimes referred to as the “local equilibrium” assumption,
when the kinetics of adsorption and desorption are rapid relative to other kinetic and
transport processes in the system. 45 The rate expressions of these sorption-desorption
processes are given in Section 4.2.4.
Adsorption and ion exchange are significant processes in the environmental
context. Whenever water is in contact with suspended matter (organic matter or clay
particles), sediment, or biota, a significant transfer of chemicals by adsorption and
ion exchange can take place. Pure adsorption and ion exchange are rarely observed
in nature. A mixture of the two processes is most often observed. 143 Adsorption is
the transfer of components from the liquid phase onto the surface of a solid phase.
It is often explained as an electrical attraction to the solid surface of chemicals
possessing a (minor) electrical charge. Adsorption results in the formation of a
molecular layer of adsorbate on the solid surface. Often, an equilibrium adsorbate
concentration is rapidly formed on the solid surface that is sometimes followed by
a slow diffusion of the adsorbate into the particles of the adsorbent. In contrast, the
ion exchange process is an actual exchange of ions between a liquid in contact with
the solid phase. The ion exchange process can be explained in the same way as any
other chemical process. That is, the chemical energy at equilibrium after the process
has occurred is lower than it was before the process was initiated. If pure ion
exchange occurs, the number of ions released is equivalent to the number of ions
taken up by the process.
Adsorption and ion exchange are fast processes, and both have great importance
in water quality modeling whenever the concentration of suspended matter in the
water is sufficiently high so that significant amounts of the modeled chemical
compounds are adsorbed or present on the exchange complex. The normal model
simulation time interval is weeks, days, or hours. This implies that these two pro-
cesses can be simulated using equilibrium equations. Modeling of surfactants, pes-
ticides, and heavy metals often emphasizes adsorption and ion exchange processes
as these materials represent easily sorbed chemicals. 120
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