Geoscience Reference
In-Depth Information
Between pH 5 and 8, that is in ordinary soils, Al is very slightly
soluble. Thus in natural waters, its concentration is less than 10
-8
moles
l
-1
or, if preferred, 10
-2
µ
moles l
-1
. Much higher concentrations are seen
though, as in Table 3.2.
Table 3.2
Mean concentrations of aluminium (as Al) in spring waters on igne-
ous rock (Tardy 1969).
m
moles l
-1
m
g l
-1
Min. (Africa)
0.55
5
Max. (French Massif Central)
6.6
60
The phenomenon is explained by the formation of relatively soluble
polymers containing 13 Al ions (G. Bourrié, pers. comm.). All the same
it is still low. In order for aluminium to be lost in large quantities
from soil profiles, long periods of pedogenesis are necessary, lasting
hundreds of thousands of years. There is one major exception however:
the involvement of complexation phenomena (Chap. 11).
Solubility of ions is influenced by ionic charge, ionic size and, above all,
the ratio of the two. Large ions with low charge are soluble. The small
highly charged ions are much less soluble. At the pH values usually
found in soil (5 to 8), the following approximate scale of decreasing
solubility can be kept in mind:
Na
+
> K
+
> Ca
++
> Mg
++
> (Si
4+
) > Fe
++
> Fe
3+
> Al
3+
Thus, the monovalent ions are the most soluble in pure water. Following
them are the divalents, then the trivalents. In soils, retention by the clay
minerals first affects the trivalents (completely fixed), then the divalents
(strongly retained), and then the monovalents (easily exchangeable).
Thus the hierarchy is not reversed. Overall, the monovalents will be
exported, the divalents too but more slowly. The trivalents will only
be eliminated in small amounts except if pedogenesis acts over long
periods. Iron, which easily passes from the trivalent state to divalent
state, is exported in the latter form.
In the list, Si
4+
seems to be in the wrong place, because it is small
and at the same time highly charged. Actually it reacts with water to
be associated with O
2-
and yields oxyanions of various kinds that are
relatively soluble (see Fig. 3.6). The oxyanion of sulphur (SO
4-
) behaves
similarly.
The elements transported in solution are not necessarily eliminated.
They may interact and give new minerals (neoformation).
Summary
