Geoscience Reference
In-Depth Information
Fig. 5.6
Forces affecting sorption of nonpolar organic contaminants
Entropy-Related Adsorption Force
Entropy-related adsorption, denoted hydrophobic sorption (or solvophobic inter-
action), is the partitioning of nonpolar organics out of the polar aqueous phase
onto hydrophobic surfaces. Figure
5.6
shows a schematic model of forces that
contribute to the sorption of hydrophobic organics, relevant to the subsurface
environment.
A major feature of hydrophobic sorption is the weak interaction between the
solute and the solvent. The primary force in hydrophobic sorption appears to be the
large entropy change resulting from the removal of a solute from solution. The
entropy change is due largely to the destruction of the cavity occupied by the
solute in the solvent and the destruction of the structured water shell surrounding
the solvated organic. Hydrophobic interfaces may be found mainly on organic
matter and on organically coated minerals.
Hydrophobic sorption, being an entropy-driven process, provides the major
contribution to sorption of hydrophobic contaminants on subsurface solid phases.
When a hydrophobic organic compound is adsorbed on a solid phase, the partitioning
of the compound and its adsorption by the surface directly from the water phase
should be considered. These processes occur in partially saturated and saturated
subsurface regimes, where water is likely to be the wetting phase. In such cases, the
wetting phase completely or partially coats the solid-phase surface, thus increasing
the retention capacity because the wetting phase serves as an additional sink.
Rao et al. (
1989
) suggested that at least four mechanisms of adsorption should
be considered for hydrophobic organic compounds. The first mechanism involves
the sorption of the neutral molecular species from the aqueous phase, which is
