Geoscience Reference
In-Depth Information
explanations (1) and (2), the contaminant release always exhibits a hysteresis
pattern. When dealing with chemical reactions and biochemical transformations
(explanation
3),
contaminant
retention,
in
some
situations,
may
reach
total
irreversibility.
5.8.2 Apparent Hysteresis
Apparent hysteresis occurs mainly when complete equilibrium is not reached.
Diffusion into the solid matrix or into micropores of aggregates is considered a
main cause of apparent hysteresis. In a transitory state, sorption occurs concur-
rently with desorption and the concentration of contaminant in the liquid phase is
erroneously low because some fraction is associated with sorption.
Apparent hysteresis also may be caused by other phenomena. During the
consecutive extractions and dilution steps used as a common technique in
desorption studies, weathering of the sorbent may occur, resulting in a possible
increase of contaminant sorption and decrease in its release. Degradation of the
contaminant induced by physicochemical or biological factors, or a volatilization
process leading to a decreased contaminant concentration in solution, are addi-
tional factors affecting a true hysteresis result.
The moisture status of the subsurface solid phase also may lead to an apparent
hysteresis in the adsorption-desorption process. It is known that clay materials,
mainly smectites, and humic substances can hydrate and swell or dehydrate and
shrink. The physicochemical state of many molecules sorbed in wet conditions
may be modified on drying, making the substances more difficult to desorb. The
retention of sorbate molecules during drying or slow swelling of organic surfaces
may be the reason for the decrease in their desorption. On rewetting, when mol-
ecules are sorbed at polar surface sites, they orient their hydrophobic part toward
the solution phase, reducing considerably the access of water, and thus slowing
down the swelling and desorption (Mingelgrin and Gerstl 1993 ).
Drying of the subsurface solid phase can cause an increase in the rate of
desorption. If penetration of a sorbate toward inner surfaces does not reach its
equilibrium by the time drying commences, a fraction of the sorbate may remain
localized at more accessible outer surfaces in an amount greater than that corre-
sponding to the equilibrium level. Under these conditions, the drying of the system
may increase the rate of desorption during successive rewetting.
The history of the surface is an additional factor affecting the release of con-
taminants adsorbed on solid phases into the liquid or gaseous phase. For example,
the effect of drying on contaminant desorption is influenced by the time allowed
for its transport into the aqueous phase. In sorbing systems, like sediments that are
permanently wet, the history of the system determines the fate of sorbed molecules
(Pignatello 1989 ).
Release methodology may lead to incorrect desorption parameters, which in
turn may be (erroneously) interpreted as hysteresis. For example, Hodges and
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