Geoscience Reference
In-Depth Information
enriched. Sposito (
1984
) characterized this macroscopic phenomenon through the
definition of the relative surface excess of an anion in a suspension, by
¼
n
i
M
w
m
i
S
C
ð
w
Þ
i
ð
2
:
47
Þ
where n
i
is total moles of ion i in the suspension per unit mass of solid, M
w
is the
molecular mass of water in the suspension per unit mass of solid, m
i
is the molality
of ion i in the supernatant solution, and S is the specific surface area of the
suspended solid. This C
i
(w)
is the excess moles of ion (per unit area of suspended
solid) relative to an aqueous solution containing M
w
kilograms of water and the ion
of molality, m
i
.
If an anion approaches a charged surface, it is subject to attraction by positively
charged surface sites on the surface or repulsion by negatively charged ones.
Because clay materials in the subsurface normally are negatively charged, anions
tend to be repelled from mineral surfaces. Negative adsorption of anions is affected
by the anion charge, concentration, pH, the presence of other anions, and the
nature and charge of the surface.
2.3.2 Adsorption of Nonionic Compounds
Adsorption of nonionic compounds on subsurface solid phases is subject to a series
of mechanisms such as protonation, water bridging, cation bridging, ligand
exchange, hydrogen bonding, and van der Waals interactions. Hasset and Banwart
(
1989
) consider that the sorption of nonpolar organics by soils is due to enthalpy-
related and entropy-related adsorption forces.
Enthalpy-related adsorption forces include the following processes:
1. Hydrogen bonding, which refers to the electrostatic interaction between a
hydrogen atom covalently bound to one electronegative atom and to another
electronegative atom or group of atoms in a molecule. The hydrogen atom may
be regarded as a bridge between electronegative atoms. In general, this bonding
is conceptualized as a dipole phenomenon, where the hydrogen bond exhibits
an asymmetrical distribution of the first electron of the H atom induced by
various electronegative atoms.
2. Ligand exchange processes, involving replacement of one or more ligands by
the adsorbing species and, in some cases, can be considered a condensation
reaction.
3. Protonation mechanisms, including a Coulomb electrostatic force resulting
from charged surfaces. Due to the surface acidity, solutes having proton-
selective organic functional groups can be adsorbed through a protonation
reaction.
4. Pi (p) bonds, occurring as a result of overlapping of p orbitals when they are
perpendicular to aromatic rings.
