Geoscience Reference
In-Depth Information
The main properties affecting adsorptive capacity of clay are considered to be
the available surface and the CEC, as well as the nature of the saturating cation, the
hydration status, and the surface acidity. Although amorphous oxides and
hydroxides of iron, aluminum, and silica can adsorb organic molecules, only
limited information exists in this direction. It is known, however, that Al and Fe
hydroxides can adsorb organic contaminants, and therefore, their presence leads to
an increase in the adsorption capacity of montmorillonite (Terce and Calvet
1977
).
For example, after removing Al and Fe oxides from soil particles, the adsorptive
capacity of soils for atrazine (an organic herbicide) decreased significantly and the
adsorption kinetics were affected (Huang et al.
1984
).
The adsorption of contaminants on geosorbents also is affected by climatic
conditions reflected in the subsurface temperature and moisture status. Calvet
(
1984
) showed how the soil moisture content may affect adsorption of contami-
nants originating from agricultural practices. The moisture content determines the
accessibility of the adsorption sites, and water affects the surface properties of the
adsorbent. The competition for adsorption sites between water and, say, insecti-
cides may explain this behavior. Preferential adsorption of the more polar water
molecules by soil hinders insecticide adsorption at high moisture content; reduced
competition is found at low moisture content, leading to an increase in adsorption.
A negative relationship between pesticide adsorption and soil moisture content has
been known for a long time (e.g., Ashton and Sheets
1959
; Yaron and Saltzman
1978
).
It is important to examine the effect of moisture content on the surface prop-
erties of clays and organic matter in relation to the adsorption of organic con-
taminants. In general, it is accepted that water molecules are attracted to clay
surfaces mainly by the exchangeable cations, forming hydration shells. Adsorbed
water provides adsorption sites for organic contaminants. An important feature of
water associated with clay surfaces is its increased dissociation, giving the surface
a slightly acidic character. A negative relationship usually exists between the
surface acidity of clays and their water content.
Adsorption usually increases as the temperature decreases, while desorption is
favored by temperature increases. The temperature may indirectly affect adsorp-
tion by its effect on organic-water interactions. The complex relationship among
adsorbent, adsorbate, and solvent as affected by temperature was described by
Mills and Biggar (
1969
) for the case of an organic insecticide. The adsorption of
lindane (1,2,3,4,5,6-hexachlorocyclohexane) and its beta-isomer by a peat (high
organic content), a clay soil, a Ca-bentonite, and silica gel decreased as the
temperature of the system increased. The authors suggested that this adsorption-
temperature relationship reflects not only the influence of energy on the adsorption
process but also the change in solubility of the adsorbate. They considered that the
change in activity in solution with temperature is related to the change in reduced
concentration, which is the ratio between the actual concentration of the solute at a
given temperature and its solubility at the same temperature. Adsorption isotherms
obtained by using the reduced concentration, in contrast to normal adsorption
isotherms, showed an increase in adsorption with increasing temperature. This
