About one third of the arsenic in the atmosphere comes from natural sources. Volcanic action is the most important natural source of arsenic. The next most important natural source is the arsenic-containing vapor that is generated from solid or liquid forms of arsenic salts at low temperatures. The rest two third comes from man-made sources. Mining
, metal smelting, burning of fossil fuels and coal-fired power plants are the major industrial processes that contribute arsenic contamination to air, water and soil. Elemental arsenic is produced commercially from arsenictrioxide. Arsenic trioxide is a by-product of metal smelting operations.
Environmental contamination also occurs from pesticides used in agriculture and from chemicals used for timber preservation. About 70% of the world production of arsenic is used in timber treatment, 22% in agricultural chemicals, and the remainder in glass, pharmaceuticals and metallic alloys
Thus, the sources of arsenic can be categorized as: (i) geological (geogenic), (ii) anthropogenic (human activities), and (iii) biological (biogenic). Figure-1 depicts a schematic diagram of major sources and routes of arsenic in soil and aquatic ecosystem. Important arsenic bearing minerals are given in Table-1, in which, the most common is Arsenopyrite(FeAsS).
The transport and distribution of arsenic in the environment is complex due to the many chemical forms in which it may be present and because there is continuous cycling of different forms of arsenic through air, soil and water. Arsenic dissolved in water can be present in several different forms. In well-oxygenated water and sediments, nearly all arsenic is present in the stable form of arsenate (V). Some arsenite(III) and arsenate(V) forms are less stable and are interchangeable, depending on the chemical and biological conditions.
Some chemical forms of arsenic adhere strongly to clay and organic matter, which can affect their behavior in the environment. Weathered rock and soil, containing arsenic, may be transported by wind or water erosion. Arsenic releases into the atmosphere by industrial processes or volcanic activity and attaches to particles that are dispersed by the wind and fall back to the ground.
Major Sources of Arsenic in Soils and Aquatic Ecosystems
Arsenic is widely distributed in all geological materials at varying concentrations. An average concentration of 1.5 to 2.0 mg As kg-1 is expected in the continental crust of the earth. The mean concentrations of As in igneous rocks range from 1.5 to 3.0 mg kg -1, whereas in sedimentary rocks range from 1.7 to 400 mg kg -1 Arsenic ranks52nd in crustal abundance and it is a major constituent in more than 245minerals. These are mostly sulfide-containing ores of copper (Cu), nickel (Ni), lead (Pb), cobalt (Co), zinc (Zn), gold (Au), or other base metals. The most important ores of As include pyrites, realgar, and orpiment.
Arsenic is introduced into soil and water during the weathering of rocks and minerals followed by subsequent leaching and runoff. Therefore, the primary source of As in soil is the parent (or rock) materials from which it is derived.
Arsenic is also being introduced into the environment through various anthropogenic activities. These sources release As compounds that differ greatly in chemical nature (speciation) and bioavailability. Major sources of As discharged onto land originate from commercial wastes (_40%), coal ash (_22%), mining industry (_16%), and the atmospheric fallout from the steel industry (_13%). Arsenic trioxide (As2O3) is used extensively in the manufacturing of ceramic and glass, electronics, pigments and antifouling agents, cosmetics, fireworks, and Cu based alloys. Arsenic is also used for wood preservation in conjunction with Cu and chromium (Cr), i.e., copper–chromium–arsenate (CCA).
Industries that manufacture As-containing pesticides and herbicides release
As-laden liquid and solid wastes that, upon disposal, is likely to contaminate soil and water bodies. For example, indiscriminate discharge of industrial effluents from the manufacturing of Paris green (copper acetoarsenite, an arsenical pesticide) resulted in the contamination of soil and groundwater.
Biological sources contribute only small amounts of As into soil and water ecosystems. However, plants and micro- and macro-organisms affect there distribution of As through their bioaccumulation (e.g., biosorption), biotransformation (e.g., bio-methylation), and transfer (e.g., volatilization).
Arsenic accumulates readily in living tissues because of its strong affinity for proteins, lipids, and other cellular components aquatic organisms are particularly known to accumulate As, resulting inconsiderably higher concentrations than in the water in which they live (i.e., bio-magnification). Upon disposal or consumption they subsequently become a source of environmental contamination. Arsenic could be transferred from soil to plants and then to animals and humans, involving terrestrial and aquatic food chains.