Agriculture Reference
In-Depth Information
that contain toxic concentrations of Cd when aerobic may be entirely suitable for
wetland rice cultivation (Takijima and Katsumi, 1973; Bingham
et al
., 1976). The
Cd content of rice grain in Cd-contaminated soil has been found to be correlated
with the number of days the soil is drained prior to harvest (Page
et al
., 1981).
Cobalt, Nickel and Copper
Cobalt and nickel are Group VIIIA and copper Group IB elements. They occur
predominantly in the
2 oxidation state in soils as divalent cations, though Co
2
+
may be oxidized to Co
3
+
forming very insoluble compounds with Mn oxides, and
Cu
2
+
may be reduced to Cu
+
, especially if soft bases such as halides and S
2
−
are
present to stabilize the Cu
+
ion. All are chalcophiles and tend to form insoluble
sulfides in anaerobic conditions (p
K
s
=
21
.
3-25
.
6, 19.4-26.6 and 36.1, respec-
tively). They therefore tend to have low mobilities in submerged soils, especially
Cu
2
+
, and accumulate.
All are strongly sorbed on soil surfaces, increasingly as the pH increases, and
are more strongly bound to functional groups in organic matter than Cd
2
+
or
Zn
2
+
. They therefore tend to show the minimum in solubility at near neutral
pH discussed for Zn. Ni
2
+
and Cu
2
+
form highly stable complexes with organic
matter, especially with ligands containing N and S, and they therefore tend to
accumulate in organic soils.
+
Mercury
Mercury occurs in soils predominantly in the
2 oxidation state. Elemental Hg
in the atmosphere is oxidized to Hg
2
+
and deposited in rainfall. It is a strong
chalcophile and under anaerobic conditions forms the extremely insoluble sul-
fide cinnabar (HgS, p
K
=
52
.
7). Nonetheless it is not entirely immobilized under
anaerobic conditions because it is reduced to volatile Hg
0
or methylated to volatile
methyl mercury compounds by microbial action, and so returned to the atmo-
sphere. The methylation is mediated by various bacteria, especially methanogens,
through the reactions:
+
methylation
−−−−−→
Hg
2
+
CH
3
Hg
+
methylation
−−−−−→
CH
3
Hg
+
(
CH
3
)
2
Hg
↑
Other volatile methyl mercury compounds, such as
(
C
6
H
5
)
2
Hg, are also formed.
The CH
3
Hg
+
unit is very inert with respect to decomposition. Therefore, once
formed, methyl mercury compounds are not readily demethylated. The bio-
geochemistry of Hg in the environment is reviewed by Ridley
et al
. (1977)
and Mason
et al
. (1993).
