Geoscience Reference
In-Depth Information
water with a soil surface include the water electrolyte concentration, pH, and
sodium
Na
+
adsorption
ratio
(SAR)
expressed
by
the
ratio
between
and
Ca
2+
+ Mg
2+
concentrations (SAR = [Na
+
] /[(Ca
2+
+ Mg
2+
)/2]
1/2
).
Clay dispersion is governed by the attractive and repulsive forces in the elec-
trical double layer at the surface of charged colloids; the balance between the
forces is determined by factors such as exchangeable cation concentration and
ionic strength (Rengasamy and Olsson
1991
). The critical ion concentration for
clay dispersion from soil aggregates is lower than the flocculation values (see
below) observed in the separated clay soils. Chorum et al. (
1994
) attributed this
behavior to the high negative charge of separated soil clays, due to exposure of
surfaces that were originally bound in aggregates.
Dispersion and release of clay particles from the soil solid matrix are consid-
ered to be the major source of mobile colloidal particles in the subsurface (Ryan
and Elimelech
1996
). Release of colloidal particles is controlled by their surface
interaction with the solid matrix and by hydrodynamics of the kinetics of water
flow. Summarizing the existing information in this field, Grolimund and Borkovec
(
2006
) showed that (1) particle release is usually enhanced by lowering the ionic
strength (2) the release process depends on the solution composition whereby
monovalent counterions promote this process most effectively (3) the release is
influenced by the pH (4) release kinetics are exponential, and (5) aging effects
should be considered.
Flocculation, or coagulation, of Na-clay dispersion systems occurs in most
cases between the negative edges and the negative faces of the clay particle. The
flocculation value of a clay suspension in a sodic water is defined as the minimum
concentration of Na
+
in the aqueous electrolyte solution necessary to flocculate a
suspension of clay in a given time, under specific conditions of exchangeable
cation concentration, pH, and suspension concentration. Based on a number of
experiments on coagulation of Na-montmorillonite dispersions, Lagaly and Zies-
mer (
2003
) found that the anisometric shape and charge distribution on mont-
morillonite particles caused very low critical coagulation concentration of
inorganic salts. Adsorption of multivalent anions increased the density of the
negative edge charges. As a consequence, the edge/face coagulation became a
face/face coagulation with a high critical salt concentration. Lagaly and Ziesmer
(
2003
) pointed out that in dispersion of high montmorillonite contents, the floc-
culation may be initiated by edge/edge aggregation.
Flocculation between the positive edge and the negative face occurs only at
pH \ 6. The critical flocculation concentrations of the clay fraction extracted from
three strongly structured soils from Natal (two dominated by c and one by kao-
linite) were compared with those of smectite from Wyoming and kaolinite from
Georgia in Na-saturated forms (Frenkel et al.
1992
). They found that the soil clays
had much higher critical flocculation concentrations than their Na-saturated clay
reference counterparts. This difference was attributed to the existence of humic
acid coating on soil-clay surfaces.
