Arsenic (As)
Arsenic (As) is a metalloid with atomic number 33, known for its occurrence both in elemental form and as a compound with sulfur or other metals like copper and iron. Primarily produced in China, arsenic has significant applications, particularly in wood preservatives, herbicides, and insecticides. Approximately 90% of arsenic usage in the U.S. is for pressure-treated wood, with the electronics industry utilizing high-purity forms in semiconductor technologies. Despite its utilitarian roles, arsenic is highly toxic, with its dangers varying based on its chemical form; arsenious oxide, for example, can be lethal in small doses. Chronic exposure can lead to severe health issues, including skin conditions and potential carcinogenic effects. Historically, arsenic has been recognized since ancient times and has been involved in notorious poisoning cases. While U.S. production ceased in 1985, the regulations surrounding arsenic use continue to tighten, reflecting growing awareness of its health risks and environmental impact.
Arsenic (As)
Where Found
Elemental arsenic is occasionally found in minerals, but more frequently it is combined chemically with sulfur, either alone or with metals such as copper, nickel, cobalt, or iron. China is the world’s largest producer of elemental arsenic. Because of the health risks of arsenic, there has been no U.S. production of arsenic trioxide or arsenic metal since 1985.
Primary Uses
Wood preservatives, herbicides, and insecticides are major uses of arsenic chemicals. Arsenic is used to harden lead alloys for battery plates, solder, and lead shot, while arsenides of gallium and indium have uses in lasers, light-emitting diodes, and transistors. In the United States, approximately 4 percent of arsenic (measured in metric tons of arsenic content) goes to agricultural chemicals, 3 percent to glass products, 3 percent to nonferrous alloys and electronics, and 90 percent to pressure-treated wood. The electronics industry uses a very pure form of arsenic in gallium-arsenide semiconductors for solar cells, space research, and telecommunications.
Technical Definition
Arsenic (abbreviated As), atomic number 33, belongs to Group V of the periodic table of the elements and is classified as a metalloid, rather than a metal or nonmetal. There is only one naturally occurring isotope, with an atomic weight of 74.93. Elemental arsenic exhibits gray, yellow, and black forms with densities of 5.73, 1.97, and 4.73 grams per cubic centimeter. The common gray form sublimes when heated to 613° Celsius and melts under a pressure of 28 atmospheres (2.8 million pascals) at 817° Celsius.
Description, Distribution, and Forms
Arsenic is widely distributed and is found in soils and seawater in trace amounts: 1 to 40 parts per million in soil, and 2 to 5 parts per billion in seawater. It averages about 1.8 parts per million by weight. The most abundant arsenic mineral is arsenical pyrite (also called arsenopyrite or mispickel), a sulfide of iron and arsenic. Other significant ores are arsenolite (arsenious oxide), orpiment (As2S3), and realgar (As4S4). Seawater averages 0.5 to 2 parts per billion of arsenate, but lakes and streams often have higher concentrations; these vary from one body of water to another. Lake Michigan, for example, has levels of 0.5 to 2.4 parts per billion. Fish and shellfish have arsenic levels about one thousand times greater than seawater and much higher than federal drinking water standards (0.05 part per million). Arsenobetaine, with a formula of (+)(CH3)3As-CH2-CO2(-), is common in fish, and many other methylated compounds are found in marine organisms.
Arsenic toxicity is highly dependent on the state of chemical combination of the element. Elemental arsenic is less toxic than combined forms; the most dangerous forms are arsine, arsenites, and arsenious oxide. Ingestion of as little as 0.1 gram of arsenious oxide has caused death. Methylated compounds such as arsenobetaine are much less toxic.
Arsenic intoxication symptoms include skin rashes, anemia, gastrointestinal distress, internal bleeding, and shock. Chronic poisoning can result in a gangrenous condition of the feet (“blackfoot disease”), and the action of arsenic as a carcinogen and teratogen has been established. Paradoxically, arsenic, like selenium, is an essential trace nutrient for some species. The toxicity of arsenic is only partly understood, particularly its carcenogenicity. Arsenate, AsO4(3-), because of its similarity to phosphate, can react with adenosine, leading to uncoupling of oxidative phosphorylation, an important energy-producing system in plants and animals. Arsenite, AsO3(3-), inhibits many enzymes by binding to thiol (-SH) groups that exist, for example, in pyruvate oxidase. The vital tricarboxylic acid cycle is thereby disrupted. Administration of antidotes such as penicillamine or dimercaptopropanol that contain -SH groups will bind preferentially to the arsenite and keep it from the enzymes. Ingested arsenic tends to accumulate in the hair and can be detected by neutron activation analysis. Atomic absorption spectroscopy can also detect and measure trace amounts of arsenic.
History
Arsenic was known in early times in India, Persia, and Mesopotamia, and it is mentioned in the writings of Aristotle, Hippocrates, and Pliny the Elder. These ancient writings are often vague and do not allow the modern reader to decide exactly whether elemental arsenic or some compound such as an oxide or sulfide is described. European alchemists such as Albertus Magnus (thirteenth century) and Johannes Schröder (seventeenth century) published procedures for preparing arsenic from orpiment (As2S3) or arsenious oxide (As2O3), but priority in discovery is considered uncertain.
The toxic characteristics of arsenious oxide were noticed long ago, and the substance became notorious in various homicidal poisoning cases. The Roman emperor Nero, for example, poisoned his own brother. Accidental arsenic poisoning is exemplified by the events reported in 1973 in Pelham, Minnesota, where a well had been drilled on land that had received heavy dosages of arsenic insecticides.
In spite of the danger, pigments containing arsenic were still used in cakes and candy in the nineteenth century, and arsenic-containing medicines of dubious value were used in the early twentieth century. Arsenicals are still occasionally used to treat stubborn parasitic diseases (trypanosomiasis, amoebiasis).
Obtaining Arsenic
There are more than two hundred recognized arsenic-containing minerals. Arsenic is leached into water by weathering of rocks and is distributed by volcanic action. In the soil, microorganisms can metabolize arsenate or arsenite, producing a variety of organic methylated compounds that find their way into water and into the bodies of all sorts of marine creatures. An estimated 40,000 metric tons of arsenic are added to the world’s oceans annually by weathering of rocks, as compared with world industrial production of 55,000 to 70,000 metric tons per year.
Human activity accounts for significant releases of arsenic into the air, water, and soil. Smelters emit arsenic oxide dust, and herbicides and insecticides remain in the soil. One of the large smelters in the United States (no longer operating) emitted 181 metric tons of arsenious oxide per year into the area surrounding Tacoma, Washington.
Uses of Arsenic
Production of arsenic has ceased in the United States, and uses are subject to increasingly severe regulation. Nevertheless, the United States still imports about several thousand metric tons per year, mainly for wood preservation and pesticide uses.
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