Agriculture Reference
In-Depth Information
plants and animals, whereas Cr(III) is more strongly sorbed, forms complexes
with organic matter and precipitates as insoluble Cr(OH)
3
at high pH, and is far
less toxic. Oxidation of Cr(III) to Cr(VI) by O
2
is slow, but oxidation by Mn
oxides is thermodynamically favourable under acid conditions: in Table 7.8, pe
0
∗
values at pH 5 for reduction of Mn(III,IV) oxides are greater than for reduction
of Cr(VI). This process is catalysed by sorption of Cr(III) onto Mn oxide
surfaces (Eary and Rai, 1987). However it is slower than analogous reactions
with other species that have been studied (As(III)
→
As(V) and Se(IV)
→
Se (VI)) because oxidation of Cr(III) is less thermodynamically favourable and
because Cr
3
+
is less strongly sorbed on positively-charged Mn oxides at low
pH (Scott and Morgan, 1996).
Chromate is reduced to Cr(III) in dissimilatory microbial reactions, but this pro-
cess is inhibited at moderate concentrations of C(VI) and so is probably of limited
value in detoxifying soils contaminated with Cr(VI) (Lovley, 1993). However,
Cr(VI) can also be reduced to Cr(III) abiotically by oxidation of Fe(II): Fe(III)
in ferric oxide is reduced to Fe(II) biotically:
4Fe
(
OH
)
3
+
8H
+
−−−→
4Fe
2
+
+
11H
2
O
subsequently Cr(VI) is reduced abiotically to Cr(III) as Fe(II) is re-oxidized
to Fe(III):
3Fe
2
+
+
HCrO
4
−
+
8H
2
O
−−−→
3Fe
(
OH
)
3
+
Cr
(
OH
)
3
+
5H
+
Wielinga
et al
. (2001) demonstrated this process by incubating goethite anaer-
obically at pH 7 with lactate and an iron-reducing bacterium, and introducing
Cr(VI) after commencement of Fe(III) reduction (Figure 7.7). In treatments with-
out Cr(VI), accumulation of Fe(II) in solution continued, but in the treatments
with Cr(VI) it was reversed; in abiotic controls there was no accumulation of
Fe(II). Chromate can also be reduced abiotically by sulfide.
CH
2
O
+
CO
2
+
Boron
Boron occurs in soil predominantly as the uncharged B
(
OH
)
3
, though at high pH
B
(
OH
)
3
is converted to B
(
OH
)
4
−
(
p
K
=
9
.
0
)
, which forms insoluble Ca salts. It
may therefore be deficient to plants in acid soils in humid regions, as a result
of B
(
OH
)
3
leaching, or in calcareous soils as a result of precipitation in Ca
salts. By contrast, in alkaline soils in arid regions, soluble Na borate salts may
accumulate. Boron toxicity in rice is quite commonly reported where irrigation
water is obtained from deep groundwater in dry seasons (Ponnamperuma and
Yuan, 1966; Cayton, 1985; Ayers and Westcot, 1989).
Thallium
Thallium occurs in soils in both
+
3and
+
1 oxidation states. Tl
3
+
behaves much
like Al
3
+
, but hydrolyses even more readily and insoluble Tl
(
OH
)
3
is formed
