Agriculture Reference
In-Depth Information
(Na
2
CH
3
, AsO
3
), and diethylarsenic acid [(CH
3
)
2
AsO(OH)] are widely used as
agricultural insecticides, larvicides, and herbicides. Sodium arsenite (NaH
2
AsO
4
) is
used for aquatic weed control and for sheep and cattle dips. Arsenic acid (H
3
AsO
4
)
is used to defoliate cotton balls prior to harvesting and as a wood preservative.
As
2
O
3
is used in the manufacture of pharmaceuticals. Elemental As is mainly used
in Pb, Cu, Sb, Sn, Al, and Ga alloys.
Mining, smelting and are beneficiation, pesticides, fertilizers and chemical in-
dustries, thermal power plants using coal, wood preservation industries using CCA
and incineration of preserved wood wastes contribute to significant influx of As to
the environment. Global emission of As in the atmosphere has been estimated to
be 0.019 Gg (0.012-0.026 Gg), but in soil and aquatic environment, the estimated
figures are 0.082 and 0.042 Gg, respectively. However, there has been a substantial
decrease in the atmospheric emission of As in Europe, from Circa 0.005 Gg in 1986
to 0.00031 Gg in 1995.
Mining and Ore Beneficiation
Elevated concentration of As, as well as other metals such as cadmium copper,
nickel and zinc are commonly encountered in the acid mine effluents. The principal
source of As in mine tailings is the oxidation of arsenopyrite (FeAsS) following the
reaction.
FeAsS(s) + 13 Fe
3+
+ 8H
2
O = 14 Fe
2+
+ SO
2
4
+ 13H
+
+ H
3
AsO
4
Arsenopyrite can be oxidized by both O
2
and Fe, but the rate of oxidation by Fe
III
is
faster than pyrite. The rate of this reaction was reported as 1.7 µ mal/m
2
, a reaction
faster than a similar oxidation reaction for pyrite. Under extremely acidic environ-
ment, with a pH of about 1.5 and an aqueous As concentration at > 10 m mol/l,
As precipitates as scordite (FeAsO
4
.2H
2
O). Under acidic conditions (pH < 3), As
V
may substitute SO
4
in the structure of Jarosite [KFe
3
(SO
4
)
2
(OH)
6
] in different mine
wastes. Adsorption of As on Fe(OH)
3
surfaces was found to be the principal sink
for As in studies of acid mine drainage. However, the adsorption of As by Fe(OH)
3
may be only transient as changes in redone conditions (Eh) and pH may result in
dissolution of Fe(OH)
3
with consequent mobilization of As. Effluents and water in
tailings ponds are often treated with lime to increase pH levels to stabilize the dis-
solved As and other metals as precipitates.
Agriculture
Over hundreds of years, inorganic arsenicals (arsenic trioxide, arsenic-acid, arse-
nates of calcium, Cu, Pb, and Na and arsenites of Na and K) have been widely used
in pigments, pesticides, insecticides, herbicides and fungicides. At present, As is
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