Geoscience Reference
In-Depth Information
so-called adsorbed cations are so numerous and so close to the particle surfaces that
they do not move outside of this domain even if the outside solution fl ows. The
negative charge of the particle surfaces is completely compensated even if the water
solution fl ows through the pores. The particles together with the adsorbed cations
form a unit without any charge, i.e., each unit behaves as if it were neutral. When
two such neutral units meet, they are not repulsed - on the contrary, they are mutu-
ally attracted, form micro-fl occules, and coagulate. A typical example of the
coagulation-causing cation is calcium, Ca
2+
.
A completely different behavior is manifested when monovalent cations like Na
+
are adsorbed on the negatively charged surfaces of solid clay particles and humic
substances. If they have to balance the same negative charge, their number should
be twice that of bivalent cations. Let us give an oversimplifi ed example by consider-
ing that a very small solid surface has 100 negative charges. To balance these nega-
tive charges to obtain a charge of zero, the condition for fl occulation, we have to
supply the adsorption zone with 50 calcium cations (Ca
2+
) or with 100 sodium cat-
ions (Na
+
). In reality, because there is usually not enough space in proximity to the
solid surface to accommodate 100 monovalent cations, a certain percentage of those
cations remains outside of the unit (particle+adsorbed cations) when soil water
fl ows. As a result, the unit (particle + adsorbed cations) retains a negative charge.
Again with oversimplifi cation: if there is only space for 80-monovalent-cation jux-
taposition to the surface of the solid particle while water is fl owing, 20 cations move
away with the water leaving 20 unbalanced negative charges. Therefore the unit
(particle + adsorbed cations) maintains a portion of the negative charge of the solid
particle. Although it is less than the 100 negative charges of the solid surface with-
out adsorbed ions, the unit nevertheless sustains its negative charge behavior. Hence,
when two such negatively charged units meet, they repulse each other. Without
being mutually attracted, they cannot fl occulate to form the originating nucleus of a
future microaggregate. In addition to the above-simplifi ed counting of charges,
there is another factor why the majority of monovalent cations cannot be pushed
into the fi lm around the particle and coexist with it. That factor is the motion of
water molecules rotating and orienting themselves in the vicinity of an ion in such
a way to form an envelope of water continually surrounding the ion. Because the
thickness of water envelopes around monovalent cations is generally much greater
than those around bivalent cations, the size of a sodium cation together with its
larger water envelope is much bigger than a calcium cation with its smaller water
envelope. Hence, there are two primary factors that prevent soil particles with
adsorbed monovalent cations from coagulating and forming a nucleus of microag-
gregates. The most active in this negative activity is the sodium cation, Na
+
. If the
water solution is changed and Na
+
starts to prevail, the earlier stability of microag-
gregates is lost since an opposite process to coagulation starts, and the clay particles
are repelled.
As we have already mentioned, cations attracted to soil particles from solutions
of mineral salts in soil are readily exchanged when the composition of the soil solu-
tion changes. With this change of solution composition, basic soil properties also
change. One of the sources of dynamics in microaggregation is the direct
consequence of this inevitable change in soil properties.
