Geoscience Reference
In-Depth Information
5. London-van der Waals forces, which are multipole interactions produced by
correlation between fluctuation-induced multipole moments in two nearly
uncharged polar molecules. These forces also include dispersion forces that
arise from the correlation between the movement of electrons in one molecule
and those of neighboring molecules. The van der Waals dispersion interaction
between two molecules is generally very weak, but when many groups of atoms
in a polymeric structure act simultaneously, the van der Waals components are
additive.
6. Chemisorption, denoting the situation when an actual chemical bond is formed
between the molecules and surface atoms.
7. Atoms that are rearranged, forming new compounds at the demand of the
unsatisfied valences of the surface atoms.
Entropy-related adsorption, known as hydrophobic sorption, involves the
partitioning of nonpolar organics from a polar aqueous phase onto hydrophobic
surfaces, where they are retained by dispersion forces. The major feature of
hydrophobic sorption is the weak interaction between the solute and the solvent.
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.
2.3.3 Kinetic Considerations
A perspective based on kinetics leads to a better understanding of the adsorption
mechanism of both ionic and nonionic compounds. Boyd et al. (
1947
) stated that
the ion exchange process is diffusion controlled and the reaction rate is limited by
mass transfer phenomena that are either film diffusion (FD) or particle diffusion
(PD) controlled. Sparks (
1988
) and Pignatello (
1989
) provide a comprehensive
overview on this topic.
In the case of subsurface cation exchange, charge compensation cations are held
in the solid phase within crystals in interlayer positions, structural holes, or surface
cleavages and faults of the crystals as well as on the external surfaces of clay
minerals. Cations held on external surfaces are immediately accessible to the
aqueous phase. On reaching this phase, they move by diffusion to regions of
smaller concentration, the diffusion being affected by the tortuosity of the porous
medium. An additional restriction affecting the rate of exchange is given by the
fact that the arrival rate of incoming cations to the exchange sites is much slower
than the release rate of the outgoing cations. The characteristic period of ion
exchange in the subsurface ranges from a few seconds to days, being due to
various constituents of the solid phase and the properties of the adsorbate.
In the case of nonionic compounds, the driving forces for adsorption consist of
entropy changes and weak enthalpic (bonding) forces. The sorption of these
compounds is characterized by an initial rapid rate followed by a much slower
