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The arsenic (As) cycle is the biogeochemical cycle of natural and anthropogenic exchanges of arsenic terms through the atmosphere, lithosphere, pedosphere, hydrosphere, and biosphere. Although arsenic is naturally abundant in the Earth's crust, long-term exposure and high concentrations of arsenic can be detrimental to human health. [1] [2]
Arsenic's largest reservoir on Earth is the lithosphere. [2] Earth's crust contains more than 200 mineral types containing As, including many sulfide minerals. [3] Arsenic is abundant in ore deposits containing arsenopyrite (FeAsS) and tennantite. [4] Sedimentary rocks bearing coal and shale may also contain high As. [5] Major fluxes of As from the lithosphere to the atmosphere are volcanic emissions. [2]
Soil is the second largest global reservoir of As [2] [3] Under oxic conditions, As is present in soils as arsenate (As(III)), which can bind to Fe(III) hydroxides. [2] The speciation of As in soil depends on soil pH and other factors. Acidic soils may contain arsenate bound to aluminium and iron, while basic soils may contain calcium-bound arsenate. [6] The residence time for As in soils depends on the climate type, ranging from 1,000 to 3,000 years for moderate climates. [7]
Freshwater and groundwaters commonly contain <1 ppb of As. [2] The concentration of As is pH dependent; acidic conditions mobilize As at pH <5. [2] Oxic seawater contains As(III) as arsenate (average of 1.7 ppb). [8] Major sinks include sedimentation and subduction. [2]
Arsenic is naturally present in the biosphere, with highest concentration in plant roots. Terrestrial plants can contain up to 200 ppm (parts per million) As. [6] [7] [9] Marine organisms (e.g. Annelida and Echinodermata) contain 6-8 ppm. [7] The human body also contains trace As with highest concentrations in the kidneys and liver (up to ~1.5 ppm). [7]
Human use arsenic in pesticides, wood preservatives, metal treatment, paint, and coal-based power plants. [2] Anthropogenic residues and discharges from coal-based power plants, mining, and smelting can contaminate rivers, lakes, streams and soil. [1] Anthropogenic As emissions originate from steel and glass production, and forest and grassland burning. [2] In the atmosphere, As is mainly present in particulates such as dust, with a residence time of 7 to 10 days. [10] [11]
Arsenic is a metalloid with an atomic number of 33, and its common oxidation states are +3 or +5, as arsenate(As III) and arsenite(As V). [12] Arsenic is primarily found as organic arsenic compounds, inorganic arsenic compounds, and arsine gas. Arsenic toxicity is dependent on its oxidation state; As(III) is more toxic than As(V) because of its ability to bind to thiol groups on proteins and enzymes, and its slower excretion rate from the body. [13] The World Health Organization recognizes that inorganic arsenic is extremely toxic for humans (EPA maximum of 10 ppb in water) and detrimental to aquatic life. [14]
Arsenic is a chemical element; it has symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a notoriously toxic metalloid. It has various allotropes, but only the grey form, which has a metallic appearance, is important to industry.
Arsenic poisoning is a medical condition that occurs due to elevated levels of arsenic in the body. If arsenic poisoning occurs over a brief period of time, symptoms may include vomiting, abdominal pain, encephalopathy, and watery diarrhea that contains blood. Long-term exposure can result in thickening of the skin, darker skin, abdominal pain, diarrhea, heart disease, numbness, and cancer.
A pnictogen is any of the chemical elements in group 15 of the periodic table. Group 15 is also known as the nitrogen group or nitrogen family. Group 15 consists of the elements nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), and moscovium (Mc).
Bromoform is an organic compound with the chemical formula CHBr3. It is a colorless liquid at room temperature, with a high refractive index and a very high density. Its sweet odor is similar to that of chloroform. It is one of the four haloforms, the others being fluoroform, chloroform, and iodoform. It is a brominated organic solvent. Currently its main use is as a laboratory reagent. It is very slightly soluble in water and is miscible with alcohol, benzene, chloroform, ether, petroleum ether, acetone and oils.
1,4-Dioxane is a heterocyclic organic compound, classified as an ether. It is a colorless liquid with a faint sweet odor similar to that of diethyl ether. The compound is often called simply dioxane because the other dioxane isomers are rarely encountered.
Ethylbenzene is an organic compound with the formula C6H5CH2CH3. It is a highly flammable, colorless liquid with an odor similar to that of gasoline. This monocyclic aromatic hydrocarbon is important in the petrochemical industry as a reaction intermediate in the production of styrene, the precursor to polystyrene, a common plastic material. In 2012, more than 99% of ethylbenzene produced was consumed in the production of styrene.
The arsenate is an ion with the chemical formula AsO3−4. Bonding in arsenate consists of a central arsenic atom, with oxidation state +5, double bonded to one oxygen atom and single bonded to a further three oxygen atoms. The four oxygen atoms orient around the arsenic atom in a tetrahedral geometry. Resonance disperses the ion's −3 charge across all four oxygen atoms.
Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). The hexavalent form rarely occurs naturally.
Arsenic pentoxide is the inorganic compound with the formula As2O5. This glassy, white, deliquescent solid is relatively unstable, consistent with the rarity of the As(V) oxidation state. More common, and far more important commercially, is arsenic(III) oxide (As2O3). All inorganic arsenic compounds are highly toxic and thus find only limited commercial applications.
Monosodium methyl arsenate (MSMA) is an arsenic-based herbicide. It is an organo-arsenate; less toxic than the inorganic form of arsenates. However, the EPA states that all forms of arsenic are a serious risk to human health and the United States' Agency for Toxic Substances and Disease Registry ranked arsenic as number 1 in its 2001 Priority List of Hazardous Substances at Superfund sites.
Chromated copper arsenate (CCA) is a wood preservative containing compounds of chromium, copper, and arsenic, in various proportions. It is used to impregnate timber and other wood products, especially those intended for outdoor use, in order to protect them from attack by microbes and insects. Like other copper-based wood preservatives, it imparts a greenish tint to treated timber.
Disodium methyl arsenate (DSMA) is the organoarsenic compound with the formula CH3AsO3Na2. It is a colorless, water-soluble solid derived from methanearsonic acid. It is used as a herbicide. Tradenames include Metharsinat, Arrhenal, Disomear, Metharsan, Stenosine, Tonarsan, Tonarsin, Arsinyl, Arsynal, and Diarsen.
Ammonium arsenate is the inorganic compound with the formula (NH4)3AsO4. It is prepared by treating a concentrated solution of arsenic acid with ammonia, resulting in precipitation of colorless crystals of the trihydrate. Upon heating, it releases ammonia.
Arsenic biochemistry refers to biochemical processes that can use arsenic or its compounds, such as arsenate. Arsenic is a moderately abundant element in Earth's crust, and although many arsenic compounds are often considered highly toxic to most life, a wide variety of organoarsenic compounds are produced biologically and various organic and inorganic arsenic compounds are metabolized by numerous organisms. This pattern is general for other related elements, including selenium, which can exhibit both beneficial and deleterious effects. Arsenic biochemistry has become topical since many toxic arsenic compounds are found in some aquifers, potentially affecting many millions of people via biochemical processes.
Aluminum is the third most abundant element in the lithosphere at 82,000 ppm. It occurs in low levels, 0.9 ppm, in humans. Aluminum is known to be an ecotoxicant and expected to be a health risk to people. Global primary production (GPP) of aluminum was about 52 million tons in 2013 and remains one of the world's most important metals. It is used for infrastructure, vehicles, aviation, energy and more due to its lightweight, ductility, and cheap cost. Aluminum is harvested from gibbsite, boehmite, and diaspore which make up bauxite. The aluminum cycle is the biogeochemical cycle by which aluminum is moved through the environment by natural and anthropogenic processes. The biogeochemical cycle of aluminum is integral with silicon and phosphorus. For example, phosphates store aluminum that has been sedimented and aluminum is found in diatoms. Aluminum has been found to prevent growth in organisms by making phosphates less available. The humans/lithosphere ratio (B/L) is very low at 0.000011. This level shows that aluminum is more essential in the lithospheric cycle than in the biotic cycle.
The gold cycle is the biogeochemical cycling of gold through the lithosphere, hydrosphere, atmosphere, and biosphere. Gold is a noble transition metal that is highly mobile in the environment and subject to biogeochemical cycling, driven largely by microorganisms. Gold undergoes processes of solubilization, stabilization, bioreduction, biomineralization, aggregation, and ligand utilization throughout its cycle. These processes are influenced by various microbial populations and cycling of other elements such as carbon, nitrogen, and sulfur. Gold exists in several forms in the Earth's surface environment including Au(I/III)-complexes, nanoparticles, and placer gold particles. The gold biogeochemical cycle is highly complex and strongly intertwined with cycling of other metals including silver, copper, iron, manganese, arsenic, and mercury. Gold is important in the biotech field for applications such as mineral exploration, processing and remediation, development of biosensors and drug delivery systems, industrial catalysts, and for recovery of gold from electronic waste.
The iodine cycle is a biogeochemical cycle that primarily consists of natural and biological processes that exchange iodine through the lithosphere, hydrosphere, and atmosphere. Iodine exists in many forms, but in the environment, it generally has an oxidation state of -1, 0, or +5.
The lead cycle is the biogeochemical cycle of lead through the atmosphere, lithosphere, biosphere, and hydrosphere, which has been influenced by anthropogenic activities.
The fluorine cycle is the series of biogeochemical processes through which fluorine moves through the lithosphere, hydrosphere, atmosphere, and biosphere. Fluorine originates from the Earth’s crust, and its cycling between various sources and sinks is modulated by a variety of natural and anthropogenic processes.
The zinc cycle is a biogeochemical cycle that transports zinc through the lithosphere, hydrosphere, and biosphere.
