Increasing human activities have modified the global cycle of heavy metals and metalloids, including the toxic non-essential elements like Arsenic, Mercury, Cadmium, and Lead1. Air pollution, water pollution, soil pollution has become a threat to the plant and animal communities, including the human race due to these four’ metals. Among these four metals, arsenic exhibits a complex metabolism and is perhaps the most abundant pollutant as well as an impending human carcinogen. Henceforth, its origin and mobilization in the nature, biochemistry and bioavailability should be well implied to monitor our arsenic resources, since it has substantial use in the present-day world.
Arsenic is a trace element found in the earth’s crust at an average concentration of ?5 ?g/g (ppm) (1). Although its comparative copiousness in the earth’s crust, due to natural mineralization arsenic can become intense in some parts of the world. Arsenic is a component of 245 minerals, associated most frequently with other metals such as copper, gold, lead, and zinc in sulfidic ores.1?3
Due to natural weathering of rocks, biological activity, and volcanic eruption, arsenic may be released into the environment (Figure 1). Anthropogenic activities like using of arsenical pesticides/herbicides, industrial by-products and wastes, burning of fossil fuels and woods, mineral mining, ore smelting, and well drilling, can also mobilize and intensify arsenic into the environment. (References from reading material 1)———————————expand
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