Geoscience Reference
In-Depth Information
Chapter 5
Sorption, Retention, and Release
of Contaminants
Contaminant retention on geosorbents is controlled by their physicochemical
properties, their structural pattern as well as the properties of the contaminants
themselves. The properties of these adsorbents control their capacity to retain
release contaminants in the subsurface environment.
Contaminants may be adsorbed on the solid phase or on suspended particles in
the liquid phase. Environmental factors, such as temperature, pH, and water
content in the subsurface prior to contamination, also affect the nature of con-
taminant adsorption. Other physical processes of retention include precipitation,
deposition, and trapping. Under natural conditions, pollutants often consist of more
than a single contaminant, comprising a mixture of organic and inorganic toxic
compounds. Each of these compounds can react differently with the existing
minerals and chemicals in the subsurface.
5.1 Surface Properties of Adsorbents
Clay minerals, oxides, and humic substances are the major natural subsurface
adsorbents of contaminants. Under natural conditions, when humic substances are
present, humate-mineral complexes are formed with surface properties different
from those of their constituents. Natural clays may serve also as a basic material
for engineering novel organo-clay products with an increased adsorption capacity,
which can be used for various reclamation purposes.
Clay minerals are characterized by a high surface charge and a very small
particle size. A detailed presentation of two types of layered silicate clay (kaolinite
and smectite) is given in
Chap. 1
.
Clay minerals have a permanent negative charge due to isomorphous substi-
tutions or vacancies in their structure. This charge can vary from zero to
[200 cmol
c
/kg (centimoles/kg) and must be balanced by cations (counter-ions) at
or near the mineral surface (Table
5.1
), which greatly affect the interfacial prop-
erties. Low counter-ion charge, low electrolyte concentration, or high dielectric
constant of the solvent lead to an increase in interparticle electrostatic repulsion
