Agriculture Reference
In-Depth Information
The chemistry of Hg in aerobic soils is also complicated and so it is difficult to
make general predictions about its mobility. The Hg
2
+
cation is B-type and forms
strong bonds with soft ligands such as the sulfhydryl group (-SH) and S
2
−
anion,
but not with the main functional groups in organic matter (Section 3.1). In aerobic
soil it is therefore likely to be immobile at trace concentrations but moderately
mobile at greater concentrations (McBride, 1994). However the mobility also
depends on pH. At pH
>
4, the predominant form in solution is Hg
(
OH
)
2
0
,
which is little sorbed on soil surfaces, but at pH
>
7Hg
2
+
is precipitated as
Hg
(
OH
)
2
and HgCO
3
.
Concentrations of Hg in the global atmosphere (Slemr
et al
., 1985; Slemr and
Langer, 1992; Mason
et al
., 1994) and deposited in ice and lake sediments (Weiss
et al
., 1971; Swain
et al
., 1992) are increasing, probably due to industrial activity.
Accumulations of Hg in soils and sediments tend to correlate with soil organic
matter content, and the greatest natural accumulations are in peaty and submerged
soils. Though submerged soils are sinks for Hg as HgS, they are also the main
source of methyl mercury in the environment (St Louis
et al
., 1994).
Vanadium
5 vanadate species (VO
(
OH
)
3
0
,
VO
2
(
OH
)
2
−
and VO
3
(
OH
)
2
−
) and under reducing conditions as the
+
Vanadium occurs in soils predominantly as the
4 vanadyl
cation
(
VO
2
+
)
. Less commonly V
3
+
may also form and substitute for Fe
3
+
in minerals. Interchange between these oxidation states with redox conditions
greatly alters the solubility of V in soils.
The vanadate equilibria are given in Table 7.7. The VO
2
(
OH
)
2
−
and
VO
3
(
OH
)
2
−
anions are sorbed on positively charged sites on oxides and sili-
cates at low pH, but sorption decreases with pH as the surface positive charge
decreases. Consequently V is quite soluble at high pH and less soluble at low pH.
Reduced V(IV) is much less soluble. The VO
2
+
cation behaves like Cu
2
+
and forms strong complexes with organic ligands and is chemisorbed on oxides
and silicate clays. The V
4
+
ion is isomorphously substituted for Si
4
+
and Al
3
+
in kaolinite (Gehring
et al
., 1993). Hence the mobility of V under reducing or
acid conditions is expected to be low. Reduction of VO
2
+
to VO
2
+
occurs at
pe
0
∗
=
6
.
9 pH 5 and 2.9 at pH 7 (Table 7.7) and so requires only weakly reduc-
ing conditions. A wide range of heterotrophic bacteria and fungi is capable of
reducing V(V) (Bautista and Alexander, 1972).
+
Chromium
Chromium occurs in soils predominantly as the immobile
+
3 chromic
cation
(
Cr
3
+
)
, but may be oxidized to or added as
+
6 chromate species
(
CrO
4
2
−
,
HCrO
4
−
)
. Chromate is weakly sorbed on soils and is highly toxic to
