Geoscience Reference
In-Depth Information
The properties of organic and inorganic constituents in the subsurface solid
phase, as well as the properties of the contaminants (e.g., ion charge and radius),
define the time span of ion exchange, which may range from a few seconds to days
(Yaron et al. 1996 ). The slowly exchangeable cations are situated on exchange
sites in interlayer spaces of the minerals (e.g., smectites) or in cages and channels
of organic matter, and exchange and move into solution by diffusive flux.
In general, when a charged solid surface faces an ion of similar charge in an
aqueous suspension, the ion is repelled from the surface by Coulomb forces. The
Coulomb repulsion produces a region in the aqueous solution that is relatively
depleted of the anion and an equivalent region far from the surface that is rela-
tively enriched.
Anionic negative adsorption may occur in the subsurface when negatively
charged clay minerals repel anions from the mineral surface. If, for example, a
dilute neutral solution of KCl is added to dry clay, the Cl - equilibrium concen-
tration in the bulk solution will be greater than the Cl - concentration in the
solution originally added to the clay. Anionic negative adsorption is affected by the
anion charge, concentration, pH, the presence of other anions, and the nature and
charge of the surface. Negative adsorption may decrease as the subsurface pH
decreases and when anions can be adsorbed by positively charged surfaces. The
larger negative charge of the surface results in a greater anion negative adsorption.
Acidic organic contaminants in their anionic form are expected to be repelled by
negatively charged clay surfaces.
5.5 Adsorption of Nonionic Contaminants
The sorption of a nonpolar organic contaminant on a solid phase is derived by
enthalpy- and entropy-related forces. Hassett and Banwart ( 1989 ) suggested that
sorption occurs when the free energy of the reaction is negative due to enthalpy or
entropy. The enthalpy is primarily a function of the changes in the bonding
between the adsorbing surface and the sorbate (solute) and between the solvent
(water) and the solute. The entropy is related to the increase or decrease in the
order of the system on sorption.
The forces that control adsorption of nonionic contaminants on the solid phase
were summarized by Yaron et al. ( 1996 ) in terms of enthalpy and entropy
adsorption forces. These are discussed next.
Enthalpy-Related Adsorption Forces
Hydrogen bonding refers to the electrostatic interaction between a hydrogen atom
covalently bound to one electronegative atom (e.g., oxygen) and another elec-
tronegative atom or group of atoms in a neighboring molecule. The hydrogen atom
may be regarded as a bridge between electronegative atoms; this bonding is
conceived
of
as
an
induced
dipole
phenomenon.
The
H
bond
generally
is
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