ARSENIC

 

            The word arsenic has become almost synonymous with the word poison. One of the earliest documented cases of arsenic poisoning was Nero's poisoning of Britannicus to secure the Roman throne in 55 A.D. In Seventeenth-Century France, white arsenic, As2O3, acquired the name poudre de succession, inheritance powder. The widespread use of arsenic as a poison resulted from its common availability, low cost, and the fact that As2O3 is odorless and tasteless. Use of arsenic as a poison declined dramatically in the nineteenth century after the development of a reliable and sensitive chemical test for it.

 

            In spite of the well-known toxic effects of arsenic, Fowler's solution, a 1% solution of potassium arsenite (K3AsO3), discovered in 1786, was the most widely used medication for a variety of illnesses for 150 years. Donovan's solution, arsenic triiodide (AsI3), and de Valagin's solution, arsenic trichloride (AsCl3), were also recommended to treat rheumatism, arthritis, malaria, trypanosome infections, tuberculosis, and diabetes. The drug Salvarsan, whose chemical name is arsphenamine, was discovered in 1909, and was the main treatment for syphilis until it was replaced by penicillin in the 1940s. The use of arsenic in contemporary medicine has been severely curtailed, but it is still used in the treatment of severe parasitic diseases.

 

            Arsenic occurs naturally in soils, rocks, water, air, plants, and animals. Minerals that contain arsenic include arsenopyrite (FeAsS), realgar (AsS), orpiment (As2S3), and arsenolite (As2O3). Although elemental arsenic has several allotropic forms - gray, yellow, and black arsenic - only gray arsenic is ordinarily stable. Gray arsenic is a brittle, crystalline, semi-metallic solid that sublimes at 615C and 1 atmosphere. As much as 100,000 tons of arsenic is produced worldwide, with most of it is obtained as a by-product of the smelting of copper, lead, cobalt, and gold ores. The quantity of arsenic associated with lead and copper ores may range from 2-3%, whereas gold ores may contain up to 11% arsenic. Arsenic trioxide, As2O3, is the chief commercial compound. Elemental arsenic is produced by reducing arsenic trioxide with carbon.

 

            Approximately 90% of all arsenic used in the United States is used as a wood preservative. Smaller quantities are used for the production of agricultural chemicals (insecticides, herbicides, algicides, and growth stimulants for plants and animals), the production of glass and nonferrous alloys, and in the electronics industry. Extremely high-purity arsenic (99.999%) is used to make the gallium arsenide or indium arsenide which are employed in the manufacture of semiconductors. Gallium arsenide is used in light-emitting diodes (LEDs) and solar cells. Indium arsenide is used to produce infrared devices and lasers. Prior to the introduction of DDT in the 1940s, most pesticides were made from inorganic arsenic compounds. As a result, some agricultural soils have arsenic concentrations in excess of 100 ppm, but most studies suggest that this arsenic is confined to the topsoil. In comparison, most soils unaffected by human activities contain less than 10 ppm arsenic.

 

            Arsenic is linked to a number of adverse health effects, including skin, bladder, and lung cancer. Arsenic forms stable complexes with oxygen and sulfur, and can replace phosphorus in the backbone of DNA, resulting in conformational changes and strand breakage. For most people living in the United States, arsenic exposure comes primarily from drinking water. A U.S. Geological Survey study using data collected from 18,850 wells in 595 counties over the past 20 years provides information on where elevated concentrations of arsenic are likely to occur in ground water resources in the United States (Welch et al. 2000). Wells sampled in the study were used for public water supply, research, agriculture, industry, and domestic water supply. Eight percent of the 2,262 public water supply systems tested had arsenic concentrations greater than the 10 ppb. At a regional scale, arsenic concentrations exceeding 10 ppb were more frequently observed in the western United States, although higher concentrations were laced throughout the Great Plains, New England, and California.

 

            Contamination of a drinking water source can result from natural or human activities. The most common cause of arsenic concentrations exceeding 10 ppb in ground water is the release of naturally occurring arsenic from iron oxides. This occurs when iron oxide reacts with natural or anthropogenic organic carbon. Arsenic is a “contaminant of concern” in 30 % of all Superfund sites; however, arsenic at these sites does not appear to come directly from waste disposal, but from naturally occurring arsenic associated with iron oxide that is mobilized in sites contaminated with volatile organic compounds. Other major sources of arsenic include geothermal waters and sulfide minerals. Most rivers and streams contain arsenic concentrations less than 1 ppb; however, geothermal springs in Yellowstone National Park often have arsenic concentrations that exceed 1000 ppb. Sulfide minerals act as both a source and sink for arsenic. Oxidation of pyrite (FeS2), which may contain up to 6.5 % arsenic, and arsenopyrite (FeAsS), releases arsenic to the aqueous phase resulting in elevated concentrations of arsenic in surface waters and ground water. However, in sulfide-rich waters, precipitation of sulfide minerals removes arsenic from solution.

 

References

www.epa.gov/safewater/arsenic.html

Welch, A.H., Westjohn, D.B., Helsel, D.R. and Wanty, R.B., 2000. Arsenic in ground water of the United States: Occurrence and geochemistry. Ground Water 38(4) 589-604.