Biogeochemical chemical weapons, burining of Arsenic rich fossil fuels,

Biogeochemical
cycle of arsenic:
Several  biological and physico- chemical
processes play an important role in the biogeochemical cycle of arsenic. The hydrogen
potential (pH) and the redox potential are some of the important factors
determining the occurrence and concentration of Arsenite (III) and Arsenate (V)
species in the environment.  In nature,
arsenic is associated with sulphurous compounds, sulphur-containing minerals,
iron and other metals lke Ni, Cu, Ag, Au, and Co. Arsenic is a widespread
element found in around 200 different minerals , with the most common ones
include orpiment (As2S3), arsenopyrite (FeAsS), and
realgar (AsS). Ancient or recent volcanic activities results in the inclusion
of Arsenic in the hydrothermal environment. The earth’s atmosphere also has
significant presence of arsenic species owing to wind erosion processes, sea
spray, volcanic emissions, forest fires and volatilization (in cold climates).
Human activities like pharmaceutical manufacturing, glassmaking
industry, wood processing, chemical weapons, burining of Arsenic rich fossil
fuels, electronics industry etc. also contribute to
the addition of arsenic compounds into the environment. The retention of
arsenic in the atmosphere has been reported to be less than 10 days. After this
retention time period, Arsenic is released in the form of dust followed by precipitation
in the form of rainfall. A higher pH (~9.2) results in a slightly reductive
environment which favours the formation of 
As(OH)3  (arsenous acid). The
solubility and the bioavailability of Arsenic in the environment depends on its
speciation. Reduction of Arsenate into Arsenite increases its solubility in
water. However, Arsenate has a tendency to co-precipitate with sulphur or iron,
or  adsorbed by calcite, clay
etc.

Microbial
metabolisms like Arsenate reduction, Arsenite Oxidation and methylation
processes are also a determining factor of the occurrence of the various
arsenic oxidation states in the environment. 
Reduction of Arsenate to Arsenite by Arsenate reductase enzymes is a
common feature in the microbial world with incidences of oxidation of Arsenite
to Arsenate has also been reported in contaminated environments. These
reactions also contribute to the protective and/or energy metabolisms of the
bacteria from various Arsenic-induced stress condition. 

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