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Arsenic
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Arsenic ( or [in compounds or as adjective]) is a well-known chemical element that has the symbol As and atomic number 33. Arsenic was first documented by Albertus Magnus in 1250. Its atomic mass is 74.92. 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.
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Arsenic ( or [in compounds or as adjective]) is a well-known chemical element that has the symbol As and atomic number 33. Arsenic was first documented by Albertus Magnus in 1250. Its atomic mass is 74.92. 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 square As43- ions in the arsenide skutterudite. In the +3 oxidation state, the stereochemistry of arsenic is affected by possession of a lone pair of electrons.
History
The word arsenic is borrowed from the Persian word Zarnikh meaning "yellow orpiment". Zarnikh was borrowed by Greek as arsenikos, 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.
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. Albertus Magnus (Albert the Great, 1193–1280) is believed to have been the first European to isolate the element in 1250. In 1649, Johann Schröder published two ways of preparing arsenic.
Cadet's fuming liquid (impure cacodyl), the first organometallic compound, was synthesized in 1760 by Louis Claude Cadet de Gassicourt by the reaction of potassium acetate with arsenic trioxide.
In the Victorian era, "arsenic" (colourless, crystalline, soluble "white arsenic" trioxide) 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.
Characteristics
Isotopes
Naturally occurring arsenic is composed of one stable isotope, 75As. As of 2003, at least 33 radioisotopes have also been synthesized, ranging in atomic mass from 60 to 92. The most stable of these is 73As with a half-life of 80.3 days. Isotopes that are lighter than the stable 75As tend to decay by ß+ decay, and those that are heavier tend to decay by ß- decay, with some exceptions.
At least 10 nuclear isomers have been described, ranging in atomic mass from 66 to 84. The most stable of arsenic's isomers is 68mAs with a half-life of 111 seconds.
Allotropes
Like phosphorus, arsenic is an excellent example of an element that exhibits allotropy, as its various allotropes have strikingly different properties. The three most common allotropes are metallic grey, yellow and black arsenic.
The most common allotrope of arsenic is grey arsenic. It has a similar structure to black phosphorus (ß-metallic phosphorus) and has a layered crystal structure somewhat resembling that of graphite. It consists of many six-membered rings which are interlinked. Each atom is bound to three other atoms in the layer and is coordinated by each 3 arsenic atoms in the upper and lower layer. This relatively close packing leads to a high density of 5.73 g/cm3.
Yellow arsenic is soft and waxy, not unlike . Both have four atoms arranged in a tetrahedral structure in which each atom is bound to the other three atoms by a single bond, resulting in very high ring strain and instability. This form of the elements are the least stable, most reactive, more volatile, less dense, and more toxic than the other allotropes. Yellow arsenic is produced by rapid cooling of arsenic vapour with liquid nitrogen. It is rapidly transformed into the grey arsenic by light. The yellow form has a density of 1.97 g/cm³.
Black arsenic is similar in structure to red phosphorus.
Chemical
Arsenic is very similar chemically to its predecessor in the Periodic Table, 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 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, arsenic oxidizes to arsenic trioxide; the fumes from this reaction have an odour resembling garlic. This odour 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.
Compounds
Arsenic compounds resemble in many respects those of phosphorus as both arsenic and phosphorus occur in the same group (column) of the periodic table.
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.
Whilst arsenic trioxide forms during oxidation of arsenic, arsenic pentoxide is formed by the dehydration of arsenic acid. Both oxides dissolve in strong alkaline solution, with the formation of arsenite AsO33- and arsenate AsO43- respectively. The protonation steps between the arsenate and arsenic acid are similar to those between phosphate and phosphoric acid. However, arsenite and arsenous acid contain arsenic bonded to three oxygens and no hydrogen, in contrast to phosphite and phosphorous acid (more accurately termed 'phosphonic acid'), which contain non-acidic P-H bonds. Arsenous acid is genuinely tribasic, whereas phosphonic acid is not.
A broad variety of sulfur compounds of arsenic are known, As4S3, As4S4, As2S3 and As4S10. All arsenic(III) halogen compounds (except with astatine) are known and stable. For the arsenic(VI) compounds the situation is different: only the arsenic pentafluoride is stable at room temperature. Arsenic pentachloride is only stable at temperatures below -50 °C and the pentabromide and pentaiodide are unknown.
Arsenic is used as group 5 element as part of the III-V semiconducting compounds. Gallium arsenide, indium arsenide and aluminium arsenide are used as semiconductor material when the properties of silicon are not suitable for the application and the higher price of the compounds is acceptable.
Arsenic also has a formal oxidation state of +2 in As4S4, realgar. This is achieved by pairing As atoms to produce dimeric cations [As-As]2+, so the total covalency of As is still in fact three.
Occurrence
Arsenopyrite, also unofficially called mispickel, (FeAsS) is the most common arsenic-bearing mineral.
In the litosphere the minerals of the formula M(II)AsS, with M(II) being mostly Fe, Ni and Co, are the dominat arsenic minerals.
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), AsO33- as opposed to arsenate (V), AsO43-).
In addition to the inorganic forms mentioned above, arsenic also occurs in various organic forms in the environment.
Production
In 2005, China was the top producer of white arsenic with almost 50% world share, followed by Chile, Peru and Morocco, reports the British Geological Survey and the United States Geological Survey. The arsenic was recovered mostly during mining operations, for example the production from Peru comes mostly from copper mining and the production in China is due to gold mining. The arsenic is there part of the smelter dust from copper, gold, and lead smelters.
On roasting in air of arsenopyrite, arsenic sublimes as arsenic (III) oxide leaving iron oxides, while roasting without air results in the production of metallic arsenic. For further purification of the arsenic from sulfur and other chacogenes is sublimed in vacuum or in a hydrogen atmosphere or by distilling it from molten lead-arsenic mixture.
Applications
Wood preservation
The toxicity of arsenic to insects, bacteria and fungi makes it an ideal component for the preservation of wood. The worldwide treatment with chromated copper arsenate, also known as CCA or Tanalith was the largest consumer of arsenic since the introduction of the process in the 1950s. Due to the environmental problems caused by the arsenic most countries banned the use of chromated copper arsenate on consumer products. The ban began in the European Union and in the United States in 2004.
In 2002 in the United States 90% of the 19,600 metric tons of arsenic compounds were used to preserve wood, in 2007 still 50% of the 5,280 metric tons of consumption was used for this purpose. In the European Union the use of arsenic in consumer products According to the USEPA's website, CCA lumber was discontinued for residential and general consumer construction on December 31, 2003 and alternative methods are now used like ACQ, Borates, Copper Azole, Cyproconazole, and Propiconazole.
Although discontinued, this application is also one of the most concern to the general public. 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. 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.
Medical
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 500 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. It was also used as Fowler's solution in psoriasis. 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.
Pigments
Copper acetoarsenite was used as a green pigment known under many different names, including 'Paris Green' and 'Emerald Green'. It caused numerous arsenic poisonings. Scheele's Green, a copper arsenate, was used in the 19th century as a coloring agent in sweets.
Military
After World War I the United States built up a stockpile of 20,000 tons of Lewisite; a chemical weapon, acting as a vesicant (blister agent) and lung irritant. The stockpile was neutralized with bleach and dumped into the Gulf of Mexico after the 1950s. During the Vietnam War the United States used Agent Blue (a mixture of sodium cacodylate) and dimethyl arsinic acid (cacodylic acid) as one of the rainbow herbicides to deprive the Vietnamese of valuable crops.
Other uses
Biological role
Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolised to less toxic forms of arsenic through a process of methylation. For example, the mold Scopulariopsis brevicaulis 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.
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 ?-Proteobacterium Ectothiorhodospira shaposhnikovii. The mechanism is unknown, but an encoded Arr enzyme may function in reverse to its known homologues.
Safety
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, 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.
Elemental arsenic and arsenic compounds are classified as "toxic" and "dangerous for the environment" in the European Union under directive 67/548/EEC.
The International Agency for Research on Cancer (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.
Arsenic is known to cause arsenicosis due to its manifestation in drinking water, “the most common species being arsenate [HAsO42- ; 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.
Treatment of chronic arsenic poisoning is easily accomplished. British anti-Lewisite (dimercaprol) is prescribed in dosages of 5 mg/kg up to 300 mg each 4 hours for the first day. Then administer the same dosage each 6 hours for the second day. Then prescribe this dosage each 8 hours for eight additional days.
Arsenic in drinking water
Arsenic contamination of groundwater has led to a massive epidemic of arsenic poisoning in Bangladesh and neighbouring countries. Presently 42 major incidents around 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. However,
a study of cancer rates in Taiwan suggested that significant increases in cancer mortality appear only at levels above 150 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 local and western NGOs and the Bangladeshi government undertook a massive shallow 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. 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. Arsenicosis 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.
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.
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.
Analyzing multiple epidemiological studies on inorganic arsenic exposure suggests a small but measurable risk increase for bladder cancer at 10 parts per billion. According to Peter Ravenscroft of the Department of Geography at the University of Cambridge, 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.
Epidemiological studies have suggested a correlation between chronic consumption of drinking water contaminated with arsenic and the incidence of type 2 diabetes. However, the literature provides insufficient scientific evidence to show cause and effect between arsenic and the onset of diabetes mellitus type 2.
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. Occupational exposure and poisoning may occur in persons working in these industries.
See also
External links
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