Geoscience Reference
In-Depth Information
stabilize. With the application of alum, the soil structure changed from dispersive to
more flocculated and nondispersive. The effect of aluminum sulfate on the dis-
persivity of clay soils could be due to a pH effect, which favors the interaction
between clay particles and electrolyte, causing a decrease in soil dispersivity.
Alum treatment also affects the physicomechanical properties of a dispersive
soil. Ouhadi and Goodarzi (
2006
) showed that under constant stress, the density of
dispersed soil bentonite increased as the percentage of alum increased
(Fig.
18.13
b). This behavior can be attributed to a decrease in the double-layer
thickness with an increase in added alum, due to the replacement of sodium ions
by aluminum ions with a higher valence.
18.1.4 Pesticides
Pesticides (recall
Sect. 4.1
) comprise a large number of organic compounds,
mainly with toxic properties, that may reach the land surface following agricultural
practices or industrial accidents. According to their molecular properties, pesti-
cides partition between solid and aqueous phases of the soil-subsurface geosystem
(recall Part III), and under the influence of environmental conditions, pesticides
may be redistributed among the land surface, vadose zone, and groundwater (recall
Part IV). Pesticides may be retained on the soil and subsurface mineral phases by
sorption. Sorption is a dynamic process for both the sorbing compound, which
transfers from the fluid state to a solid state, and for the sorbent itself, which
undergoes physical changes upon sorption (Pignatello
2012
). When the sorption is
irreversible, two interrelated processes are involved: desorption and binding. In
general, cationic pesticides are adsorbed by clay minerals, while nonionic pesti-
cides are adsorbed by the organic fraction of the soil-subsurface solid phase.
Adsorption of charged pesticides on the organic fraction of mineral phases (e.g.,
humic macromolecules) decreases according to the order Ca
2+
-humate, H
+
-humic
acid, and Na
+
-humate. This can be explained not only by the preference for
exchange of ions of higher valence, but also by the effect of macromolecules,
which is more important than consideration of valence. Adsorbed organo-cation
complexes bridge negative charges with strands, and the interstrand binding causes
macromolecule shrinkage, exclusion of water from the matrix, and precipitation of
organoclay-humate complexes. Adsorption of nonionic pesticides on the soil-
subsurface solid phase is controlled by enthalpy and entropy. Enthalpy-related
adsorption forces include hydrogen bonding, ligand exchange processes, proton-
ation mechanisms, p-p bonds, van der Waals forces, and chemisorption. Entropy-
related adsorption forces, also referred to as hydrophobic sorption, involve the
partitioning of nonpolar pesticides from the polar aqueous phase onto hydrophobic
surfaces. Nonadsorptive retention of pesticides in the subsurface involves their
accumulation to form a new bulk solid phase within pores and along solid phase
boundaries. In some cases, bound pesticide residues may irreversibly alter the
natural solid phase matrix (Yaron et al.
2010
and references therein).
