Geoscience Reference
In-Depth Information
Fig. 5.7 Illustration of a
water-immiscible liquid
trapped in the vadose zone
immediately after a spill. The
dashed line represents the
water table region (Schwille
1984 )
K sp ¼½A b þ ½B a b ;
ð 5 : 14 Þ
where K sp is the solubility product, or the equilibrium constant, between an ionic
solid and its saturated solution. When IAP \ K sp , the solution is below saturation
and minerals dissolve on contact with the solution. When IAP [ K sp , the solution
is ''supersaturated'' and precipitation occurs.
The IAP can be larger than the corresponding solubility product constant for the
solid if the active shell of the solid is of radius \1 lm. This behavior may be
explained by the fact that the surface energy of these very small particles con-
tributes to the Gibbs energy of the precipitate, increasing the activity relative to
that in the standard state, where the interfacial energy component is negligible.
Additional precipitate formation processes may occur in the presence of nucleating
agents.
It should be noted that, in the natural subsurface solid phase, differentiation
between adsorption and precipitation can be very difficult, because the new solid
phase may precipitate homogeneously onto the surface of an existing solid phase.
Weathering may provide host surfaces for the more stable phase into which they
transform chemically.
5.7.2 Liquid Trapping
Trapping is an important form of nonsorptive retention of contaminants in the
subsurface. Trapping may occur, for example, when spills of water-immiscible
fluid compounds (e.g., petroleum products) leave residual ganglia or bulb con-
figurations in the subsurface.
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