Agriculture Reference
In-Depth Information
essential metals may become buried in forms largely inaccessible to desorption,
so they may accumulate over time.
Examples in Soil Systems
Solid solutions will only form between ions with similar radii
(
±
15%
)
.
Table 3.15 shows the radii in crystal lattices of divalent cations that might form
solid solutions in soils. Hence, for example Mn
2
+
,
Fe
2
+
and Cd
2
+
might be
expected to form solid solutions in CaCO
3
, but Cu
2
+
and Zn
2
+
would not.
However, soils do not necessarily behave the same as pure systems. Thus there is
little evidence for strong association of Cd
2
+
or Pb
2
+
with calcite
(
CaCO
3
)
in soil
systems, despite having similar radii to Ca
2
+
(McBride, 1994). However Cd
2
+
and Pb
2
+
are both commonly associated with hydroxyapatite
(
Ca
10
(
PO
4
)
6
(OH)
2
)
,
and hence may contaminate phosphate fertilizers. Also Cu
2
+
,
Zn
2
+
,
Ni
2
+
and
Co
2
+
are excluded from haematite during its crystallization but substitute into
magnetite
(
Fe(III)
2
Fe(II)O
4
)
. Small divalent cations such as Zn
2
+
,
Cu
2
+
and
Mg
2
+
often react with Al hydroxide to form solid solutions called hydrotalcites
of general formula [M
2
+
1
−
x
Al
x
(OH)
2
]
x
+
X
n
−
x/n
,whereX
n
−
is an anion. These
are noted for their permanent positive charge and hence anion exchange capacity.
Features of metal sorption in soils that indicate—but do not prove—solid
solution formation include (McBride, 1994): sorption capacities that increase
over time; decreasing reversibility of sorption with time; selectivity for metals
based largely on ionic radii; and solubilities well below those predicted from the
solubility products of pure systems.
The cycles of reduction and oxidation of Fe and Mn oxides in intermittently
submerged soils provide opportunities for co-precipitation with trace metals. In
most natural systems it is the rate of dissolution of the solid phase that limits
solid solution formation rather than thermodynamics, so conditions in submerged
soils are highly conducive to formation of solid solutions.
Table 3.15
Radii of divalent cations
in crystal lattices
Cation
a
Ionic radius (nm)
Mg
2
+
0.072
Ca
2
+
0.100
Mn
2
+
0.083
Fe
2
+
0.078
Co
2
+
0.075
Ni
2
+
0.069
Cu
2
+
0.073
Zn
2
+
0.074
Cd
2
+
0.075
Pb
2
+
0.119
a
In six-fold coordination.
