Agriculture Reference
In-Depth Information
70
60
50
Cr(VI) not added
40
30
Cr(VI) added
20
10
Abiotic control
0
0
20
40
60
80
Time (h)
Start of Cr(VI)
addition
Figure 7.7
Abiotic reduction of toxic Cr(VI) to Cr(III) by Fe(III) (Wielinga
et al
., 2001).
Reproduced by permission of the American Chemical Society
at pH
<
2 and remains stable to pH
>
10. The mobility of Tl
3
+
in aerobic soil
is therefore expected to be low. Under moderate reducing conditions Tl
3
+
is
reduced to Tl
+
(pe
0
=
21
.
3, independent of pH). The reduced Tl
+
behaves very
differently, acting more like an exchangeable alkali metal cation. However incor-
poration into sulfide minerals may limit its solubility and mobility.
Lead
Lead occurs mainly in the
+
2 oxidation state in soils, but it may be oxidized to
Pb
4
+
. It is the least mobile heavy metal in soils. In aerobic soils it is chemisorbed
on clays and oxides; forms complexes with organic matter, especially with S-
containing functional groups; and forms insoluble hydroxides, carbonates and
phosphates. All of these increase with pH, so solubility is greatest under acid
conditions. In anaerobic soils it is precipitated as the highly insoluble sulfide
galena (PbS, p
K
=
27
.
5). It may also be methylated into volatile forms.
Arsenic and Antimony
Arsenic and antimony are Group VB elements and both occur in soils predomi-
nantly in
+
3and
+
5 oxidation states and they have similar redox and sorption
behaviour. The oxidized forms are rather insoluble in soils and the reduced forms
much more soluble.
