Agriculture Reference
In-Depth Information
general, in water systems rates of diffusion are much greater than the rates of
reaction and are rarely rate limiting. However in soils diffusion may be much
slower, for example where a reaction is catalysed by sorption on inaccessible
surfaces, and so diffusion is often rate limiting. The formation of the solid phase
then has three stages. First a critical cluster of the constituent solutes must form
from a supersaturated solution. Spontaneous crystal growth may then occur. In
effect the solubility of the nuclei that are initially formed is greater than that
of the larger crystallites that grow from them. The difference arises from the
much greater surface energy of the nuclei compared with the crystallites. In
the third stage large crystals form slowly from crystallites through the process
known as ripening. Nucleation may occur
homogeneously
from a pure solution
that is sufficiently supersaturated. But in soil systems it is more likely to occur
heterogeneously
with the surfaces of soil particles acting as catalysts. If the
surface matches the precipitating phase well, the interfacial energy between the
two solids is less than that between the precipitate and the solution, and the
energy barrier for nucleation is therefore decreased. Consequently the degree
of supersaturation necessary for precipitation to start is smaller. For example,
soil solutions are often highly supersaturated with respect to gibbsite, but it will
precipitate from similar solutions containing smectite.
The particular combinations of ions and molecules that will form precipitates
in a given solution can be predicted from equilibrium thermodynamics. However,
this often gives a misleading picture because there are kinetic limitations or there
is inhibition, particularly in soil solutions. There may also be isomorphous sub-
stitution of one cation for another in the precipitate, resulting in a solid solution
with a different solubility to the pure compound.
3.7.3 CO-PRECIPITATION IN SOLID SOLUTIONS
In general the solid phases formed under natural conditions are not simple pure
compounds but contain foreign ions isomorphously substituted in crystal lattices.
The activity of the solid phase is thereby decreased. This may have little effect
on the solubility of the major component, but it may greatly decrease the solu-
bility of the minor component compared with its pure form. This process differs
from adsorption or occlusion in that it represents the equilibrium state of the sys-
tem. A requirement is that the foreign ion can diffuse freely during precipitation
and that there is a high structural compatibility between the two pure phases.
Hence the radii of the ions involved must be similar and the minor compo-
nent should become uniformly distributed through the solid. As discussed earlier
rates of precipitation tend to be slower than rates of adsorption; rates of solid
solution formation are intermediate between rates of adsorption and precipita-
tion (Table 3.14). Solid solutions seem likely to form in submerged soils because
reductive dissolution reactions generate large concentrations of dissolved ions
over short times, and so there are opportunities for mixed precipitates to form.
