Agriculture Reference
In-Depth Information
anaerobic conditions As
(
OH
)
3
is absorbed passively in the transpiration stream,
and so because of the far greater solubility in soil of As
(
OH
)
3
than HAsO
4
2
−
,As
should accumulate in plants more rapidly under flooded conditions than under
drained aerobic conditions. In rice, the disease 'straighthead' is repeatedly linked
to moderate As concentrations in the plant (Horton
et al
., 1983) and this may
limit the accumulation of As in rice grain.
The redox and sorption behaviour of Sb is similar, but no volatile forms are
produced. The oxidized form of Sb, antimonite, Sb(V), has the anionic form
Sb
(
OH
)
6
−
at pH
>
4, and is sorbed on oxides and silicate clays. The reduced
form, antimonite, Sb(III), is present as the uncharged Sb
(
OH
)
3
molecule except
at very low or very high pH where the Sb
(
OH
)
2
+
cation and Sb
(
OH
)
4
−
anion
form, respectively. The uncharged Sb
(
OH
)
3
is little sorbed on soil surfaces.
Selenium
Selenium has a complex chemistry in the environment because of its multiple
oxidation states and variable surface adsorption properties. Qualitatively it is
analogous to sulfur occurring in the oxidation states
+
6 (selenate, SeO
4
2
−
),
+
4
(selenite, SeO
3
2
−
), 0 (elemental selenium) and
−
2(Se
2
−
, selenide) The Se
2
−
anion closely resembles S
2
−
(radii 0.20 and 0.185 nm, respectively) and is often
associated with sulfide minerals. Also, like S, Se is subject to volatilization
through biological methylation.
Under oxidizing conditions SeO
4
2
−
is the thermodynamically favoured form
(for the reduction SeO
4
2
−
→
SeO
3
2
−
,pe
0
∗
=
7
.
9 at pH 7—Table 7.8) and under
mildly reducing conditions SeO
3
2
−
is the favoured form (for SeO
3
2
−
→
Se,
pe
0
∗
=
3
.
0atpH7).SinceSeO
3
2
−
is strongly sorbed on oxides and precipi-
tates as Fe
2
(
SeO
3
)
3
,whereasSeO
4
2
−
is only weakly sorbed, especially at high
pH, this leads to large changes in solubility. Hence toxic concentrations of Se
tend to occur in alkaline soils in arid and semi-arid regions, and irrigation of
such soils may move SeO
4
2
−
into groundwater. Microbially mediated reductions
of SeO
4
2
−
to SeO
3
2
−
and of SeO
3
2
−
to elemental Se have been documented
in a wide variety of soils and aquatic sediments (Lovley, 1993). Reduction is
inhibited by NO
3
−
and Mn(IV), which are preferred electron acceptors. In pure
systems rates of transformation of SeO
3
2
−
to SeO
4
2
−
and vice versa are slow
but, as for oxidation of Cr(III) and As(III), rapid oxidation of SeO
3
2
−
sorbed on
Mn oxides can occur, with Mn(IV) acting as oxidant (Scott and Morgan, 1996).
In reducing environments Se is present as Se
2
−
which forms insoluble com-
pounds with metals, especially Fe(II), or, if metal concentrations are insufficient,
the foul-smelling poisonous gas H
2
Se may be formed. Reduction of SeO
3
2
−
to
Se
2
−
(pe
0
∗
=
0
.
3atpH5and
−
1
.
7 at pH 7) is microbially mediated at low pH
(Lovley, 1993).
Though large concentrations of Se can develop in poorly drained soils as
a result of accumulation of insoluble Se
2
−
compounds, Se is also lost under
