Geoscience Reference
In-Depth Information
5.8 Reversible and Irreversible Retention
Reversible and irreversible retention of contaminants on the subsurface solid phase
is a major process in determining pollutant concentrations and controlling their
redistribution from the land surface to groundwater. After being retained in the
solid, contaminants may be released into the subsurface liquid phase, displaced as
water-immiscible liquids, or transported into the subsurface gaseous phase or from
the near surface into the atmosphere. The form and the rate of release are governed
by the properties of both contaminant and solid phase, as well as by the subsurface
environmental conditions. We consider here contaminants adsorbed on the solid
phase.
Release through reversible retention can be assessed on the basis of physico-
chemical and biological processes. In the case of the former, release is caused by a
change in the properties of the fluid surrounding the retaining solid phase. Low-
ering of the pollutant concentration in the liquid phase, for example, may cause a
change in the established equilibrium, and as a consequence, enhanced transfer of
the adsorbed compound to the liquid phase occurs. Also, contaminants that pre-
viously entered living organisms by an uptake process may subsequently be
released. In many cases, the release isotherms do not coincide with the retention
isotherms, indicating the phenomenon of hysteresis. This means that not all
adsorbed molecules can be transferred back into the solution phase. In the sub-
surface, where a multicomponent solid phase is present, and where phenomena
other than adsorption-desorption may occur, it is better to use the term retention
hysteresis rather than adsorption hysteresis. Retention hysteresis may vary
according to the nature of the contaminant and the solid phase, the site and sample
history (e.g., wetting-dry cycles), and the experimental procedure used.
Genuine (true) and apparent hysteresis may be considered to explain contam-
inant release from the subsurface solid phase. Genuine hysteresis assumes that
observed data are real and the equilibrium results can be explained on the basis of
well-identified phenomena. Apparent hysteresis results from an experimental
artifact due, for example, to a failure to reach retention or release equilibrium.
5.8.1 Genuine Hysteresis
Genuine hysteresis is considered when contaminant release results only from
desorption. Experimental data can be interpreted in terms of genuine desorption
only when the system is at equilibrium and released molecules are those adsorbed
onto the solid-phase surface. Molecules brought back into the solution as result of
dissolution, diffusion out of the solid matrix, or biotic/abiotic transformation
cannot be considered desorbed molecules. In the subsurface, it is almost impos-
sible to distinguish between desorbed molecules and molecules that were not
subjected to adsorption and desorption.
