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Arsenic: King of Poison

Arsenic is a crystal-shape metalloid element, which is brittle in nature, and gray or tin-white in color. Arsenic is a natural part of the earth's crust in some parts of the world and may be found in water that has flowed through arsenic-rich rocks. Arsenic is also emitted into the atmosphere by high-temperature processes such as coal-fired power generation plants, burning vegetation and volcanic action. High concentrations of arsenic in drinking-water are found in various parts of the world including Argentina, Bangladesh, Chile, China, Hungary, India (West Bengal), Mexico, Nepal, Pakistan, Thailand, USA, and Viet Nam. A variety of instrumental techniques available for the determination of arsenic in water and air. (WHO Environmental Health Criteria, No. 224: Arsenic)

Arsenic is a Group-V element in the periodic chart . Its atomic number is 33, and the atomic weight is 74.92. Arsenic cannot be found in nature as a free element. It exhibits a broad range of chemical reactivity with an ability to form alloys with other elements and covalent bonds. Arsenic precipitates readily in oxidation-reduction, methylation-demethylation and acidbase reactions.

Arsenic can not only create compounds with oxygen, chlorine, sulfur, carbon and hydrogen, but also with lead, mercury, gold and iron. It mainly remains as oxide, hydrate, sulfide, arsenete, and arsenite. Gold, silver, copper, zinc, iron, and other metals exist with arsenic in these chemical compounds.

As many as 150 species of arsenic bearing minerals exist in the earth. But only three of them are considered as arsenic ore. Because, the amount of arsenic is higher in these three compounds. Moreover, they are more available than other arsenic compounds. These three compounds are Realgar or arsenic disulfide, orpiment or arsenic trisulfide, and arsenopyrite or ferus arsenic sulfide. There are differences in the physical properties of these three types of arsenic minerals. But a common feature of them is a garlic-like odour. Arsenopyrite has been primarily identified as the main source of arsenic pollution in the groundwater of Bangladesh and West Bengal. When arsenopyrite oxidises and mixes with water, no smell is found in the water. Arsenic burns in air giving off an odour of garlic and dense white fumes of arsenic trioxide.

Chemically arsenic compounds are two types- inorganic and organic arsenic. Inorganic arsenic are divided into two types- trivalent and pentavalent arsenic. Inorganic arsenic are more toxic than the organic ones. The trivalent arsenic is 60 times more toxic than pentavalent arsenic. Trivalent arsenic compounds are known as arsenite and the pentavalents are as arsenate. Usually there are four types of arsenic compounds existing in water. These are arsenite, arsenate, monomethyl arsonic acid and dimethyl arsinic acid. But in the groundwater arsenic predominantly occurs in the tri- and pentavalent form. These two were found in the groundwater of Bangladesh and West Bengal.

The name of some inorganic arsenic compounds can be mentioned here- arsenous oxide or arsenic trioxide, sodium arsenite, potassium arsenite, copper arsenite, copper acetoarsenite, arsenic acid, sodium arsenate, potassium arsenate, arsenic disulfide or realgar, orpiment or arsenic trisulfide, arsenopyrite, arsine, arsenic triiodide, etc.. Organic arsenic compounds include monomethyl arsonic acid, dimethyl arsinic acid or cacodylic acid, sodium cacodylet, atoxyl, stovarsol, sulpharsinamine etc..

Arsenic is a ubiquitous element being widely distributed in the earth. Nature itself is an adequate source of arsenic. According to availability of natural elements in environment, the position of arsenic is 20th.

Arsenic is found in atmosphere, in the aquatic environment, in soils and sediments and in organisms. Different minerals of arsenic are mixed with underground rock. The arsenic contents of the earth's crust is 1.5-2 mg/kg. However, arsenic up to 550 mg. can prevail in per kg. polluted soil. Arsenic poison can spread in air from the industrial wastes. Generally, nominal amount of arsenic also exists in water. Per liter natural water contains 1-2 microgram of arsenic.

Trace quantity of arsenic is also found in most of the fruits, vegetables, fish and meat. But high level of arsenic exists in sea water. Food of marine origin are much richer in arsenic than other foods. Arsenic up to 5 milligram can be found in per kilogram of sea fish. It means human beings are getting some amount of arsenic through air, food and drinks everyday. But the arsenic does not cause damage of human bodies as its extent is very minimum. The arsenic which are received through food is generally organic. Its poisonous effect is low in comparison to inorganic arsenic. Usually a man can receive arsenic up to 150 microgram for per kilogram weight of his body without any effect. But sensitive individual may however fall sick for only 20 microgram arsenic.

There are a lot of arsenic in earth's bio-mass. Arsenic also exists in lava of volcano. Geothermal system and uranium and gold mines are sources of arsenic. It occurs in the air in areas where coal is burnt, particularly near smelter and refineries.

Arsenic compounds are mainly used in agriculture, forestry, and industrial processes. Arsenic tri-oxide is used in manufacturing of agricultural chemicals (pesticides), glass and glassware, industrial chemicals, copper and lead alloys and pharmaceuticals.

In agriculture , arsenic compounds such as lead arsenate, copperacetoarsenite, sodium arsenite, calcium arsenate and organic arsenic compounds are used as pesticides. Substantial amount of methyl arsonic acid and dimethyl arsinic acid are used as selective herbicides.

Chromated copper arsenite, sodium arsenate and zinc arsenate are used as wood preservatives. Some phenylarsenic compounds such as asranilic acid are used as feed additives for poultry and swine.

Small amount of arsenic continue to be used as drugs in some countries. As medicine arsenic is used since the 5th century BC when Hippocrates recommended the use of an arsenic sulfide for the treatment of abscess. Arsenic preparations were used for the treatment of skin disorder, tuberculosis, leukemia, asthma, leprosy, syphilis, amoebic dysentery etc.. Homeopaths are also using arsenic as drug.

Besides, arsenic is used in the preparation of dyes, poisonous gas, transistor, as a component of semiconductor, as a preservative in tanning and in the industry of textile, paper etc..

The arsenic pollution problem took a severe shape in the present world. The pollution has been spread more or less in every continent. It occurs as results of geological processes and different development activities- agriculture, exploration of mineral resources and their processing-by human being. Arsenic pollution took place in different states of the world due to various reasons.

(Source: Dhaka Community Hospital: Hosted: May, 2001)