Agriculture Reference
In-Depth Information
no longer used in agriculture in the west, but persistence of the residue of the inor-
ganic arsenicals in soil is an issue of environmental concerns. Studies by Kenyon
et al.
1979
and Aten et al.
1980
have indicated elevated concentrations of As in
vegetables grown in soils contaminated by lead arsenate used as an insecticide in
apple orchards. The recalcitrant nature of arsenical herbicides has, however, been
observed in agricultural soils particularly around old orchards. Biomethylation of
As is a mechanism through which a significant quantity of methylarsines may be
released into the atmosphere following the application of As compounds to the soil.
A relatively faster production of dimethyl and trimethylarsinics has been reported
from grasslands treated with methylarsenic compounds while grass treated with
sodium arsenite indicated slow release of methylorsene into the atmosphere.
Wood Preservation
The extensive contamination of soils and aquatic environments has been due to the
use of CCA and other As-based chemicals in wood preservation industries. The wood
preservative chemical like CCA has attained wide scale industrial application due
to biocidic characteristics of Cu
II
and As
v
. The preservative chemical used for pres-
sure impregnation comprises a water-based mineral of diachronic acid (H
2
Cr
2
O
7
)
arsenic acid (H
3
AsO
4
) and Cu
II
as divalent cation at variable proportions. Cr is
used to bind As and Cu into the cellular structure of the wood. Fixation of CCA is
dependent on the transformation of Cr
III
to Cr
III
, a reaction that is dependent on the
temperature and water content of the wood. Cr
III
forms insoluble complexes with
both As and Cu. Further stabilization of these complexes takes place after complete
fixation of the As and Cu in the wood tissues and minimizes the risk of leaching of
the CCA components from processed wood. Among the active ingredients of CCA
wood preservatives. As is most mobile and toxic to a broad range of organisms
including human beings.
Chemistry of Arsenic in Soil
The natural content of As in soils varies significantly but is mostly in a range below
10 mg/kg. The background concentration of As in soils is governed by the lithalogy
of the parent rocks. As concentration in Swedish tills (< 0.06 mm) range between < 5
and 175 mg/kg with a medium value of 8 mg 1 kg (Selenics, Personal Communica-
tion 2000). Availability and dispersal of As in the soil environment are influenced
by several factors. Rainfall, surface runoff, rate of infiltration, and the groundwater
level like climatic and geographic characteristics and their fluctuations affect the
mobility and distribution of As. The speciation and mobility of As in soils also
depends upon the soil physical characteristics like grain size and mineralogy and
chemical characteristics such as redox potential (Eh) and pH conditions of the soils.
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