Agriculture Reference
In-Depth Information
The oxidized form of As, arsenate, As(V), which is present as HAsO
4
2
−
at
neutral pH (p
K
values in Table 7.8), is sorbed on soil surfaces in a similar way to
orthophosphate. The reduced form arsenite, As(III), which is present in solution
largely as H
3
AsO
3
(
p
K
1
=
9
.
29
)
, is only weakly sorbed, hence mobility tends to
increase under reducing conditions. Mobility will also increase without reduction
of As(V) because, as for phosphate, reductive dissolution of iron oxides results
in desorption of HAsO
4
2
−
into the soil solution. Under prolonged submergence
As(III) may be co-precipitated with sulfides.
Additionally, like Hg, As is converted into volatile compounds in microbially
catalysed reactions that are sensitive to pH and redox conditions (Fowler, 1983).
Under strongly reducing conditions methyl- and alkyl-arsine compounds may
form, resulting in loss of As to the atmosphere. The resulting cycle of reactions
is complex but involves in anaerobic soil:
reduction
−−−−→
AsH
3
↑
H
3
AsO
3
methylation
−−−−−→
↑
H
3
AsO
3
(
CH
3
)
3
As
andondiffusionof
(
CH
3
)
3
As into oxic zones,
methylation
−−−−−→
(
CH
3
)
3
As
(
CH
3
)
3
HAsO
↑
Also in oxic zones,
methylation
−−−−−→
CH
3
HAsO
3
methylation
−−−−−→
AsO
4
3
−
(
CH
3
)
3
HAsO
↑
Re-oxidation of As(III) to As(V) under oxidizing conditions is fast and is catal-
ysed by sorption onto oxides with the oxide metal acting as oxidant (Scott and
Morgan, 1995).
Reductive dissolution of Fe oxyhydroxides holding sorbed As appears to
explain the very large concentrations of As in water from wells drilled into
alluvial sediments of the Brahmaputra and Ganges Rivers in Bangladesh and
West Begal (Nickson
et al
., 1998, 2000). Dissolved As has accumulated from
the reduction of As-rich Fe oxyhydroxides formed upstream of the contaminated
areas by weathering of As-rich base metal sulfides. The reduction is driven by
sedimentary organic matter in the deposits. Release of As from oxidation of pyrite
in shallow wells contributes little to the water contamination because any As(IV)
released would be re-sorbed on Fe oxides formed in pyrite oxidation.
The HAsO
4
2
−
ion is taken up by plant roots by the same transport systems
as H
2
PO
4
−
, leading to excessive uptake and toxicity in plants growing on soils
with high arsenate levels (Meharg and Hartley-Whitaker, 2002). However the
extent of uptake varies greatly between soil conditions and plant species (Marin
et al
., 1993; Onken and Hossner, 1995; Schmoger
et al
., 2000). Presumably under
