Arsenic

Arsenic (pronEng|ˈɑrs^ənɪk) is a chemical element that has the symbol As and atomic number of 33. Arsenic was first documented by Albertus Magnus in 1250cite book |last=Emsley |first=John |title=Nature's Building Blocks: An A-Z Guide to the Elements |year=2001 |id=ISBN 0-19-850341-5 |pages=43,513,529 |publisher=Oxford University Press |location=Oxford] . Its Atomic Mass is 74.92. Its position in the periodic table is shown at right. This is a notoriously poisonous metalloid that has many allotropic forms: yellow (molecular non-metallic) and several black and grey forms (metalloids) are a few that are seen. Three metalloidal forms of arsenic with different crystal structures are found free in nature (the minerals arsenic "sensu stricto" and the much rarer arsenolamprite and pararsenolamprite), but it is more commonly found as arsenide and arsenate compounds. Several hundred such mineral species are known. Arsenic and its compounds are used as pesticides, herbicides, insecticides and various alloys.

The most common oxidation states for arsenic are -3 (arsenides: usually alloy-like intermetallic compounds), +3 (arsenates(III) or arsenites, and most organoarsenic compounds), and +5 (arsenates(V): the most stable inorganic arsenic oxycompounds). Arsenic also bonds readily to itself, forming, for instance, As-As pairs in the red sulfide realgar and square As₄^3- ions in the arsenide skutterudite. In the +3 oxidation state, the stereochemistry of arsenic is affected by possession of a lone pair of electrons.

Notable characteristics

Arsenic is very similar chemically to its predecessor, phosphorus. Like phosphorus, it forms colourless, odourless, crystalline oxides As₂O₃ and As₂O₅ which are hygroscopic and readily soluble in water to form acidic solutions. Arsenic (V) acid, like phosphorous acid, is a weak acid. Like phosphorus, arsenic forms an unstable, gaseous hydride: arsine (AsH₃). The similarity is so great that arsenic will partly substitute for phosphorus in biochemical reactions and is thus poisonous. However, in subtoxic doses, soluble arsenic compounds act as stimulants, and were once popular in small doses as medicinals by people in the mid 18th century.

When heated in air it oxidizes to arsenic trioxide; the fumes from this reaction have an odor resembling garlic. This odor can be detected on striking arsenide minerals such as arsenopyrite with a hammer. Arsenic (and some arsenic compounds) sublimes upon heating at atmospheric pressure, converting directly to a gaseous form without an intervening liquid state. The liquid state appears at 20 atmospheres and above, which explains why the melting point is higher than the boiling point [ [|http://www.corrosionsource.com/handbook/periodic/33.htm] ] . Elemental arsenic is found in many solid forms: the yellow form is soft, waxy and unstable, and is made of tetrahedral As₄ molecules similar to the molecules of white phosphorus. The grey, black or 'metallic' forms have somewhat layered crystal structures with bonds extending throughout the crystal. They are brittle semiconductors with a metallic luster. The density of the yellow form is 1.97 g/cm³; rhombohedral "grey arsenic" is much denser with a density of 5.73 g/cm³; the other metalloidal forms are similarly dense.

Applications

Lead hydrogen arsenate was used well into the 20th century as an insecticide on fruit trees. Its use sometimes resulted in brain damage to those working the sprayers. In the last half century, monosodium methyl arsenate (MSMA), a less toxic organic form of arsenic, has replaced lead arsenate's role in agriculture.

Scheele's Green, a copper arsenate, was used in the 19th century as a coloring agent in sweets.Fact|date=September 2008

The application of most concern to the general public is probably that of wood treated with chromated copper arsenate, also known as CCA or Tanalith. The vast majority of older pressure-treated wood was treated with CCA. CCA lumber is still in widespread use in many countries, and was heavily used during the latter half of the 20th century as a structural and outdoor building material. According to the EPA's website, CCA lumber was discontinued for residential and general consumer construction on December 31, 2003. Alternatives to the discontinued CCA lumber are ACQ, Borates, Copper Azole, Cyproconazole, and Propiconazole. These are all arsenic-free wood pressure treatments. It was commonly used in situations where rot or insect infestation was a possibility. Although the use of CCA lumber was banned in many areas after studies showed that arsenic could leach out of the wood into the surrounding soil (from playground equipment, for instance), a risk is also presented by the burning of older CCA timber. The direct or indirect ingestion of wood ash from burnt CCA lumber has caused fatalities in animals and serious poisonings in humans; the lethal human dose is approximately 20 grams of ash. Scrap CCA lumber from construction and demolition sites may be inadvertently used in commercial and domestic fires. Protocols for safe disposal of CCA lumber do not exist evenly throughout the world; there is also concern in some quarters about the widespread landfill disposal of such timber.

During the 18th, 19th, and 20th centuries, a number of arsenic compounds have been used as medicines, including arsphenamine (by Paul Ehrlich) and arsenic trioxide (by Thomas Fowler).Arsphenamine as well as Neosalvarsan was indicated for syphilis and trypanosomiasis, but has been superseded by modern antibiotics.Arsenic trioxide has been used in a variety of ways over the past 200 years, but most commonly in the treatment of cancer. The US Food and Drug Administration in 2000 approved this compound for the treatment of patients with acute promyelocytic leukemia that is resistant to ATRA. [Antman, Karen H. (2001). [http://theoncologist.alphamedpress.org/cgi/content/full/6/suppl_2/1 The History of Arsenic Trioxide in Cancer Therapy] . Introduction to a supplement to "The Oncologist". 6 (Suppl 2), 1-2. PMID 11331433.] It was also used as Fowler's solution in psoriasis. [Huet "et.al". Noncirrhotic presinusoidal portal hypertension associated with chronic arsenical intoxication. "Gastroenterology" 1975;68(5 Pt 1):1270-7.PMID 1126603 ]

Copper acetoarsenite was used as a green pigment known under many different names, including 'Paris Green' and 'Emerald Green'. It caused numerous arsenic poisonings.

Other uses;
* Various agricultural insecticides, termination and poisons.
* Used in animal feed, particularly in the US as a method of disease prevention [http://www.atsdr.cdc.gov/toxprofiles/tp2-c5.pdf (section 5.3, pg. 310)] and growth stimulation.Fact|date=May 2008
* Gallium arsenide is an important semiconductor material, used in integrated circuits. Circuits made using the compound are much faster (but also much more expensive) than those made in silicon. Unlike silicon it is direct bandgap, and so can be used in laser diodes and LEDs to directly convert electricity into light.
* Also used in bronzing and pyrotechny.

Recently new research has been done in locating tumours using arsenic-74 (a positron emitter). The advantages of using this isotope instead of the previously used iodine-124 is that the signal in the PET scan is clearer as the iodine tends to transport iodine to the thyroid gland producing a lot of noise. [cite journal | journal = Journal of Clinical Cancer | year = 2008 | volume = 14 | pages =1377–1385 | doi = 10.1158/1078-0432.CCR-07-1516 | title = Vascular Imaging of Solid Tumors in Rats with a Radioactive Arsenic-Labeled Antibody that Binds Exposed Phosphatidylserine
last = Jennewein | first =Marc | coauthors= Lewis,Matthew A.; Zhao, Dawen; Tsyganov, Edward; Slavine, Nikolai; He, Jin; Watkins, Linda; Kodibagkar, Vikram D.; O'Kelly, Sean; Kulkarni, Padmakar; Antich, Peter P.M; Hermanne, Alex; Rösch, Frank; Mason Ralph P.; Thorpe, Philip E. ]

History

The word "arsenic" is borrowed from the Persian word زرنيخ "Zarnikh" meaning "yellow orpiment". "Zarnikh" was borrowed by Greek as "arsenikon", which means masculine or potent. Arsenic has been known and used in Persia and elsewhere since ancient times. As the symptoms of arsenic poisoning were somewhat ill-defined, it was frequently used for murder until the advent of the Marsh test, a sensitive chemical test for its presence. (Another less sensitive but more general test is the Reinsch test.) Due to its use by the ruling class to murder one another and its potency and discreetness, arsenic has been called the "Poison of Kings" and the "King of Poisons".Fact|date=September 2008

During the Bronze Age, arsenic was often included in bronze, which made the alloy harder (so-called "arsenical bronze").

Arsenic was first isolated by Geber (721-815), an Arabian alchemist. [George Sarton, "Introduction to the History of Science" (cf. Dr. A. Zahoor and Dr. Z. Haq (1997), [http://www.cyberistan.org/islamic/Introl1.html "Quotations From Famous Historians of Science"] , [http://www.cyberistan.org Cyberistan] )] Albertus Magnus (Albert the Great, 1193-1280) is believed to have been the first European to isolate the element in 1250.cite book |last=Emsley |first=John |title=Nature's Building Blocks: An A-Z Guide to the Elements |year=2001 |id=ISBN 0-19-850341-5 |pages=43,513,529 |publisher=Oxford University Press |location=Oxford] In 1649, Johann Schröder published two ways of preparing arsenic.

In the Victorian era, "arsenic" (colourless, crystalline, soluble "white arsenic") was mixed with vinegar and chalk and eaten by women to improve the complexion of their faces, making their skin paler to show they did not work in the fields. Arsenic was also rubbed into the faces and arms of women to "improve their complexion". The accidental use of arsenic in the adulteration of foodstuffs led to the Bradford sweet poisoning in 1858, which resulted in approximately 20 deaths and 200 people taken ill with arsenic poisoning.Fact|date=September 2008

Occurrence

In 2005, China was the top producer of white arsenic with almost 50% world share, followed by Chile and Peru, reports the British Geological Survey.

Arsenopyrite, also unofficially called mispickel, (FeAsS) is the most common arsenic-bearing mineral. On roasting in air, the arsenic sublimes as arsenic (III) oxide leaving iron oxides.

The most important compounds of arsenic are arsenic (III) oxide, As₂O₃, ("white arsenic"), the yellow sulfide orpiment (As₂S₃) and red realgar (As₄S₄), Paris Green, calcium arsenate, and lead hydrogen arsenate. The latter three have been used as agricultural insecticides and poisons. Orpiment and realgar were formerly used as painting pigments, though they have fallen out of use due to their toxicity and reactivity. Although arsenic is sometimes found native in nature, its main economic source is the mineral arsenopyrite mentioned above; it is also found in arsenides of metals such as silver, cobalt (cobaltite: CoAsS and skutterudite: CoAs₃) and nickel, as sulfides, and when oxidised as arsenate minerals such as mimetite, Pb₅(AsO₄)₃Cl and erythrite, Co₃(AsO₄)₂. 8H₂O, and more rarely arsenites ('arsenite' = arsenate(III), AsO₃^3- as opposed to arsenate (V), AsO₄^3-).In addition to the inorganic forms mentioned above, arsenic also occurs in various organic forms in the environment. Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolised to a less toxic form of arsenic through a process of methylation. For example certain molds produce significant amounts of trimethylarsine if inorganic arsenic is present. The organic compound arsenobetaine is found in some marine foods such as fish and algae, and also in mushrooms in larger concentrations. The average person's intake is about 10-50 µg/day. Values about 1000 µg are not unusual following consumption of fish or mushrooms. But there is little danger in eating fish because this arsenic compound is nearly non-toxic.

"See also , ."

Toxicity

Arsenic and many of its compounds are especially potent poisons. Arsenic disrupts ATP production through several mechanisms. At the level of the citric acid cycle, arsenic inhibits pyruvate dehydrogenase and by competing with phosphate it uncouples oxidative phosphorylation, thus inhibiting energy-linked reduction of NAD+, mitochondrial respiration, and ATP synthesis. Hydrogen peroxide production is also increased, which might form reactive oxygen species and oxidative stress. These metabolic interferences lead to death from multi-system organ failure (see arsenic poisoning) probably from necrotic cell death, not apoptosis. A post mortem reveals brick red colored mucosa, due to severe hemorrhage. Although arsenic causes toxicity, it can also play a protective role. [Casarett and Doull's Essentials of Toxicology 2003] .

Elemental arsenic and arsenic compounds are classified as "toxic" and "dangerous for the environment" in the European Union under directive 67/548/EEC.

The IARC recognizes arsenic and arsenic compounds as group 1 carcinogens, and the EU lists arsenic trioxide, arsenic pentoxide and arsenate salts as category 1 carcinogens.

Exposure to lower levels of arsenic can cause nausea and vomiting, decreased production of red and white blood cells, abnormal heart rhythm, damage to blood vessels, and a sensation of “pins and needles” in hands and feet.

Arsenic is known to cause due to its manifestation in drinking water, “the most common species being arsenate [HAsO₄^2- ; As(V)] and arsenite [H₃AsO₃ ; As(III)] ”. The ability of arsenic to undergo redox conversion between As(III) and As(V) makes its availability in the environment more abundant. According to Croal, Gralnick, Malasarn, and Newman, “ [the] understanding [of] what stimulates As(III) oxidation and/or limits As(V) reduction is relevant for bioremediation of contaminated sites (Croal). The study of chemolithoautotrophic As(III) oxidizers and the heterotrophic As(V) reducers can help the understanding of the oxidation and/or reduction of arsenic. [Croal, Laura R., Jeffrey A. Gralnick, Davin Malasarn, and Dianne K. Newman. "The Genetics of Geochemisty." Annual Review of Genetics 38.1 (2004): 175-206. 24 April, 2006. ] Treatment of chronic arsenic poisoning is easily accomplished. British antilewisite (dimercaprol) is prescribed in dosages of 5mg/kg up to 300mg each 4 hours for the first day. Then administer the same dosage each 6 hours for the second day. Then prescribe this dogae each 8 hours for eight additional days. [ AJ Giannini. The Psychiatric, Psychogenic ans Somatopsychic Disorders Handbook New Hyde Park, NY. Medical Examination Publishing Co.1978, pp.pp.81-82.]

Arsenic in drinking water

Arsenic contamination of groundwater has led to a massive epidemic of arsenic poisoning in Bangladesh [Andrew Meharg, Venomous Earth - How Arsenic Caused The World's Worst Mass Poisoning, [http://www.macmillanscience.com/1403944997.htm Macmillan Science] , 2005.] and neighbouring countries. Presently 42 major incidents round the world have been reported on groundwater arsenic contamination. It is estimated that approximately 57 million people are drinking groundwater with arsenic concentrations elevated above the World Health Organization's standard of 10 parts per billion. The arsenic in the groundwater is of natural origin, and is released from the sediment into the groundwater due to the anoxic conditions of the subsurface. This groundwater began to be used after western NGOs instigated a massive tube well drinking-water program in the late twentieth century. This program was designed to prevent drinking of bacterially contaminated surface waters, but failed to test for arsenic in the groundwater.(2) Many other countries and districts in South East Asia, such as Vietnam, Cambodia, and Tibet, China, are thought to have geological environments similarly conducive to generation of high-arsenic groundwaters. was reported in Nakhon Si Thammarat, Thailand in 1987, and the dissolved arsenic in the Chao Phraya River is suspected of containing high levels of naturally occurring arsenic, but has not been a public health problem due to the use of bottled water. [http://wedc.lboro.ac.uk/conferences/pdfs/28/Kohnhorst.pdf]

The northern United States, including parts of Michigan, Wisconsin, Minnesota and the Dakotas are known to have significant concentrations of arsenic in ground water. Increased levels of skin cancer have been associated with arsenic exposure in Wisconsin, even at levels below the 10 part per billion drinking water standard. [cite journal |author=Knobeloch LM, Zierold KM, Anderson HA |title=Association of arsenic-contaminated drinking-water with prevalence of skin cancer in Wisconsin's Fox River Valley |journal=J Health Popul Nutr |volume=24 |issue=2 |pages=206–13 |year=2006 |pmid=17195561 |doi=]

Epidemiological evidence from Chile shows a dose dependent connection between chronic arsenic exposure and various forms of cancer, particularly when other risk factors, such as cigarette smoking, are present. These effects have been demonstrated to persist below 50 parts per billion. [cite journal |author=Ferreccio C, Sancha AM |title=Arsenic exposure and its impact on health in Chile |journal=J Health Popul Nutr |volume=24 |issue=2 |pages=164–75 |year=2006 |pmid=17195557 |doi=]

A study of cancer rates in Taiwan [cite journal |author=Lamm SH, Engel A, Penn CA, Chen R, Feinleib M |title=Arsenic cancer risk confounder in southwest Taiwan data set |journal=Environ. Health Perspect. |volume=114 |issue=7 |pages=1077–82 |year=2006 |pmid=16835062 |doi=] suggested that significant increases in cancer mortality appear only at levels above 150 parts per billion.

Analyzing multiple epidemiological studies on inorganic arsenic exposure suggests a small but measurable risk increase for bladder cancer at 10 parts per billion. [cite journal |author=Chu HA, Crawford-Brown DJ |title=Inorganic arsenic in drinking water and bladder cancer: a meta-analysis for dose-response assessment |journal=Int J Environ Res Public Health |volume=3 |issue=4 |pages=316–22 |year=2006 |pmid=17159272 |doi=] According to Peter Ravenscroft of the Department of Geography at the University of Cambridge [cite web |url=http://www.usatoday.com/news/world/2007-08-30-553404631_x.htm |title=Arsenic in drinking water seen as threat - USATODAY.com |accessdate=2008-01-01 |format= |work=] roughly 80 million people worldwide consume between 10 and 50 parts per billion arsenic in their drinking water. If they all consumed exactly 10 parts per billion arsenic in their drinking water, the previously cited multiple epidemiological study analysis would predict an additional 2,000 cases of bladder cancer alone. This represents a clear underestimate of the overall impact, since it does not include lung or skin cancer, and explicitly underestimates the exposure. Those exposed to levels of arsenic above the current WHO standard should weigh the costs and benefits of arsenic remediation.

Arsenic can be removed from drinking water through coprecipitation of iron minerals by oxidation and filtering. When this treatment fails to produce acceptable results, adsorptive arsenic removal media may be utilized. Several adsorptive media systems have been approved for point-of-service use in a study funded by the United States Environmental Protection Agency (U.S.EPA) and the National Science Foundation (NSF).

Magnetic separations of arsenic at very low magnetic field gradients have been demonstrated in point-of-use water purification with high-surface-area and monodisperse magnetite (Fe₃O₄) nanocrystals. Using the high specific surface area of Fe₃O₄ nanocrystals the mass of waste associated with arsenic removal from water has been dramatically reduced. [Yavuz et al., Low-Field Magnetic Separation of Monodisperse Fe₃O₄ Nanocrystals [http://www.sciencemag.org/cgi/content/full/314/5801/964 Science] , 2005.]

Occupational Exposures

Industries that use inorganic arsenic and its compounds include wood preservation, glass production, nonferrous metal alloys, and electronic semiconductor manufacturing. Inorganic arsenic is also found in coke oven emissions associated with the smelter industry. [cite web|url= http://www.osha.gov/SLTC/arsenic/index.html |title=OSHA Arsenic |accessdate=2007-10-08|publisher=United States Occupational Safety and Health Administration]

Occupational exposure to arsenic may occur with copper or lead smelting and wood treatment, among workers involved in the production or application of pesticides containing organic arsenicals. Human exposed to arsenic through air, drinking water, and food (meat, fish, and poultry); of this food is usually the largest source of arsenic. Arsenic was also found in wine if arsenic pesticides are used in the vineyard. Arsenic is well absorbed by oral and inhalation routes, widely distributed and excreted in urine; most of a single, low-level dose is excreted within a few days after consuming any form of inorganic arsenic. Remains of arsenic in nails and hair can be detected even after years and years after the exposure.

Testing for Exposure in Humans to Arsenic

There are tests available to measure arsenic in your blood, urine, hair, and fingernails. The urine test is the most reliable test for arsenic exposure within the last few days. Urine testing needs to be done within 24-48 hours for an accurate analysis of an acute exposure. Tests on hair and fingernails can measure exposure to high levels of arsenic over the past 6-12 months. These tests can determine if you have been exposed to above-average levels of arsenic. They cannot predict whether the arsenic levels in your body will affect your health.

[http://www.atsdr.cdc.gov/tfacts2.html]

[http://www.atsdr.cdc.gov/tfacts2.html]

Arsenic in the metabolism of bacteria

Some species of bacteria obtain their energy by oxidizing various fuels while reducing arsenate to arsenite. The enzymes involved are known as arsenate reductases (Arr).

In 2008, bacteria were discovered that employ a version of photosynthesis in the absence of oxygen with arsenites as electron donors, producing arsenates (just like ordinary photosynthesis uses water as electron donor, producing molecular oxygen). Researchers conjecture that historically these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive. One strain PHS-1 has been isolated and is related to the γ-Proteobacterium "Ectothiorhodospira shaposhnikovii". The mechanism is unknown, but an encoded Arr enzyme may function in reverse to its known homologues. [cite journal
author=Kulp, T. R. et al
year=2008
title=Arsenic(III) fuels anoxygenic photosynthesis in hot spring biofilms from Mono Lake, California.
journal=Science
volume=321
issue=5891
pages=967–970
doi=10.1126/science.1160799
laysummary=http://www.rsc.org/chemistryworld/News/2008/August/15080802.asp
laysource=Chemistry World, 15 August 2008]

Compounds

Arsenic compounds resemble in many respects phosphorus compounds, as arsenic and phosphorus occur in the same group (column) of the periodic table.

*Arsenic acid (H₃AsO₄)
*Arsenous acid (H₃AsO₃)
*Arsenic trioxide (As₂O₃)
*Arsine (Arsenic Trihydride AsH₃)
*Cadmium arsenide (Cd₃As₂)
*Gallium arsenide (GaAs)
*Lead hydrogen arsenate (PbHAsO₄)

Arsenic also occurs in the II oxidation state, but only in the As₂⁴⁺ cation, As(II) is never found otherwise. [cite web|url=http://www.webelements.com/webelements/compounds/text/As/As4S4-12279902.html|title=Arsenic: arsenic(II) sulfide compound data|accessdate=2007-12-10|publisher=WebElements.com]

"See also ."

Isotopes

Arsenic has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of ⁷⁵As, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope ⁷⁶As with a half-life of 1.0778 days and produce approximately 1.13 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several hours.Fact|date=February 2007 Such a weapon is not known to have ever been built, tested, or used.

ee also

* Aqua Tofana
* Arsenic poisoning
* Fowler's solution
* Grainger challenge
* White arsenic

Notes

References

* [http://periodic.lanl.gov/elements/33.html Los Alamos National Laboratory – Arsenic]

External links

* [http://ctd.mdibl.org/detail.go?type=chem&acc=D001151&queryTerms=arsenic&queryType=contains CTD's Arsenic page] and [http://ctd.mdibl.org/detail.go?type=chem&acc=D001152&queryTerms=arsenicals&queryType=contains CTD's Arsenicals page] from the Comparative Toxicogenomics Database
* [http://www.asmalldoseof.org/ A Small Dose of Toxicology]
* [http://www.atsdr.cdc.gov/csem/arsenic/ ATSDR - Case Studies in Environmental Medicine: Arsenic Toxicity]
* [http://www.clu-in.org/contaminantfocus/default.focus/sec/arsenic/cat/Overview/ Contaminant Focus: Arsenic] by the EPA.
* [http://www.inchem.org/documents/ehc/ehc/ehc224.htm Environmental Health Criteria for Arsenic and Arsenic Compounds, 2001] by the WHO.
* [http://www-cie.iarc.fr/htdocs/monographs/vol23/arsenic.html Evaluation of the carcinogenicity of arsenic and arsenic compounds] by the IARC.
* [http://www.cdc.gov/niosh/topics/arsenic/ National Institute for Occupational Safety and Health - Arsenic Page]
* [http://www.npi.gov.au/database/substance-info/profiles/11.html National Pollutant Inventory - Arsenic]
* [http://www.origen.net/arsenic.html origen.net – CCA wood and arsenic: toxicological effects of arsenic]
* [http://www.webelements.com/webelements/elements/text/As/index.html WebElements.com – Arsenic]
* [http://www.kfunigraz.ac.at/achwww/Seiten/englische%20Seiten/e_default.htm Institute of Chemistry Karl Franzens University Austria]