Arsenic (As) is a toxic metalloid found in rocks, soil, water, sediments, sand air. It enters into the terrestrial and aquatic ecosystems through a combination of natural processes such as weathering reactions, biological activity, and volcanic emissions, as well as a result of anthropogenic activities. Excessive use of As
-based pesticides and indiscriminate disposal of domestic (sewage) and industrial (timber, tannery, paints, electroplating, etc). Wastes, as well as mining activities, have resulted in widespread arsenic contamination of soils and waterways.
Of the various sources of As in the environment, drinking water probably poses the greatest threat to human health (Smedley and Kinniburgh, 2002).People drinking As-contaminated water over prolonged periods often show typical arsenical lesions, which are a late manifestation of As toxicity. Arsenic has been unequivocally demonstrated to be both toxic and carcinogenic to humans and animals.
Arsenic contamination of surface and groundwater occurs worldwide and has become a sociopolitical issue in several parts of the globe.
There has been no comprehensive review on the biogeochemistry and transformation of As in relation to its remediation. The present review, therefore, aims to integrate fundamental aspects of As transformation and recent developments on As speciation in relation to remediation strategies for the risk management of As-contaminated terrestrial and aquatic ecosystems. The review first discusses the various sources and distribution of As in soil, sediments, and water. The transformation of As in these systems is examined in relation to As speciation and bioavailability.
ORIGIN, GEOLOGY AND GEOCHEMISTRY OF ARSENIC
Arsenic has the symbol 'As', and atomic number 33, atomic weight 74.92 is often referred to as a metal but, it is classified chemically as a nonmetal or metalloid belonging to Group-15 of the periodic table.
The most common oxidation states for arsenic are: -3 (arsenides: usually alloy-like intermetallic compounds), +3 (arsenites (As(III)), and most organo-arsenic compounds), and +5 (arsenates(As(V)): the most stable inorganic arsenic oxy-compounds). Arsenic is always present as compounds with oxygen, chlorine, sulphur, carbon and hydrogen on one hand, and with lead, gold and iron on the other. It can exist in inorganic or organic form; inorganic arsenic is generally more toxic than organic arsenic.
Inorganic arsenic occurs naturally in many kinds of rocks and it is most commonly found with sulfide ores as arsenopyrite. Inorganic arsenic compounds are known to be human carcinogens.
Arsenic combined with carbon and hydrogen is classified as organic arsenic. Both inorganic and organic compounds are typically white to colorless powders. Arsenic in element form is insoluble in water.
It is soluble in oxidized form. Arsenic is found in the natural environment in abundance in the Earth's crust and in small quantities in rock, soil, water and air. The average concentration of arsenic in the continental crust is 1-2 mg/kg. The mean concentration of arsenic in igneous rocks ranges from 1.5 to 3.0 mg/kg, whereas in sedimentary rocks it ranges from 1.7 to 400 mg/kg.
Note: As= Arsenic