Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable. The name of the element is derived from the Greek word "chrōma" (χρώμα), meaning colour, because many of its compounds are intensely colouredSony VPCCA3SFX/R battery. Chromium oxide was used by the Chinese in the Qin dynasty over 2,000 years ago to coat weapons such as bronze crossbow bolts and steel swords found at the Terracotta Army. It later came to the attention of the West when it was discovered by Louis Nicolas Vauquelin in the mineral crocoite (lead(II) chromate) in 1797. Crocoite was used as a pigmentSony VPCCA3S1E battery, and after the discovery that the mineral chromite also contains chromium, this mineral was used to produce pigments as well.
Chromium was regarded with great interest because of its high corrosion resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding chromium to form stainless steelSony VPCCA3E1E battery. This application, along with chrome plating (electroplating with chromium) are currently the highest-volume uses of the metal. Chromium and ferrochromium are produced from the single commercially viable ore, chromite, by silicothermic or aluminothermic reaction or by roasting and leaching processesSony VPCCA38EC/R battery.
Although trivalent chromium (Cr(III)) is required in trace amounts for sugar and lipid metabolism, few cases have been reported where its complete removal from the diet has caused chromium deficiency. In larger amounts and in different forms, chromium can be toxic and carcinogenic. The most prominent example of toxic chromium is hexavalent chromium (Cr(VI)). Abandoned chromium production sites often require environmental cleanupSony VPCCA38EC battery.
Characteristics
Physical
Chromium is remarkable for its magnetic properties: it is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, it transforms into a paramagnetic state.[1]
Passivation
Chromium metal left standing in air is passivated by oxygen, forming a thin protective oxide surface layer. This layer is a spinel structure only a few atoms thick. It is very dense, and prevents the diffusion of oxygen into the underlying materialSony VPCCA37EC/B battery. This barrier is in contrast to iron or plain carbon steels, where the oxygen migrates into the underlying material and causes rusting.[3] The passivation can be enhanced by short contact with oxidizing acids like nitric acid. Passivated chromium is stable against acids. The opposite effect can be achieved by treatment with a strong reducing agent that destroys the protective oxide layer on the metal. Chromium metal treated in this way readily dissolves in weak acids. Sony VPCCA37EC battery
Chromium, unlike metals such as iron and nickel, does not suffer from hydrogen embrittlement. However, it does suffer from nitrogen embrittlement, reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.[5]
Occurrence
Crocoite (PbCrO4)
Chromite ore
Chromium is the 21st most abundant element in Earth's crust with an average concentration of 100 ppm. Sony VPCCA36FW/W battery Chromium compounds are found in the environment, due to erosion of chromium-containing rocks and can be distributed by volcanic eruptions. The concentrations range in soil is between 1 and 3000 mg/kg, in sea water 5 to 800 µg/liter, and in rivers and lakes 26 µg/liter to 5.2 mg/liter.[7] Chromium is mined as chromite (FeCr2O4) oreSony VPCCA36FW/B battery.[8] About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, while Kazakhstan, India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.[9]
Although rare, deposits of native chromium exist.[10][11] The Udachnaya Pipe in Russia produces samples of the native metal. This mine is a kimberlite pipe, rich in diamonds, and the reducing environment helped produce both elemental chromium and diamond. Sony VPCCA36FH/W battery
The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location, but in most cases, the Cr(III) is the dominating species,[7] although in some areas the ground water can contain up to 39 µg/liter of total chromium of which 30 µg/liter is present as Cr(VI) Sony VPCCA36FG/B battery.
Isotopes
Main article: Isotopes of chromium
Naturally occurring chromium is composed of three stable isotopes; 52Cr, 53Cr and 54Cr with 52Cr being the most abundant (83.789% natural abundance). 19 radioisotopes have been characterized with the most stable being 50Cr with a half-life of (more than) 1.8×1017 years, and 51Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minuteSony VPCCA36FA/B battery. This element also has 2 meta states.[14]
53Cr is the radiogenic decay product of 53Mn. Chromium isotopic contents are typically combined with manganese isotopic contents and have found application in isotope geology. Mn-Cr isotope ratios reinforce the evidence from 26Al and 107Pd for the early history of the solar system. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites indicate an initial 53Mn/55Mn ratio that suggests Mn-Cr isotopic composition must result from in-situ decay of 53Mn in differentiated planetary bodiesSony VPCCA36EC/W battery. Hence 53Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the solar system.[15]
The isotopes of chromium range in atomic mass from 43 u (43Cr) to 67 u (67Cr). The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.[14] 53Cr has been posited as a proxy for atmospheric oxygen concentration. Sony VPCCA36EC battery
Compounds
Chromium is a member of the transition metals, in group 6. Chromium(0) has an electronic configuration of 4s13d5, owing to the lower energy of the high spin configuration. Chromium exhibits a wide range of possible oxidation states, where the +3 state is most stable energetically; the +3 and +6 states are most commonly observed in chromium compounds, whereas the +1, +4 and +5 states are rare. Sony VPCCA35FW/W battery
The following is the Pourbaix diagram for chromium in pure water, perchloric acid or sodium hydroxide:
Chromium(III)
Chromium(III) chloride hexahydrate ([CrCl2(H2O)4]Cl·2H2O)
Anhydrous chromium(III) chloride (CrCl3)
A large number of chromium(III) compounds are known. Chromium(III) can be obtained by dissolving elemental chromium in acids like hydrochloric acid or sulfuric acid. The Cr3+ ion has a similar radius (63 pm) to the Al3+ ion (radius 50 pm), so they can replace each other in some compounds, such as in chrome alum and alumSony VPCCA35FW/B battery. When a trace amount of Cr3+ replaces Al3+ in corundum (aluminium oxide, Al2O3), the red-colored ruby is formed.
Chromium(III) ions tend to form octahedral complexes. The colors of these complexes is determined by the ligands attached to the Cr centre. The commercially available chromium(III) chloride hydrate is the dark green complex [CrCl2(H2O)4]Cl. Closely related compounds have different colours: pale green [CrCl(H2O)5]Cl2 and the violet [Cr(H2O)6]Cl3Sony VPCCA35FN/R battery. If water-free green chromium(III) chloride is dissolved in water then the green solution turns violet after some time, due to the substitution of water by chloride in the inner coordination sphere. This kind of reaction is also observed with solutions of chrome alum and other water-soluble chromium(III) saltsSony VPCCA35FH/D battery.
Chromium(III) hydroxide (Cr(OH)3) is amphoteric, dissolving in acidic solutions to form [Cr(H2O)6]3+, and in basic solutions to form [Cr(OH)6]3−. It is dehydrated by heating to form the green chromium(III) oxide (Cr2O3), which is the stable oxide with a crystal structure identical to that of corundum. Sony VPCCA35FG/PI battery
Chromium(VI)
Chromium(VI) oxide
Chromium(VI) compounds are powerful oxidants at low or neutral pH. Most important are chromate anion (CrO2−
4) and dichromate (Cr2O72-) anions, which exist in equilibrium:
2 [CrO4]2- + 2 H+ [Cr2O7]2- + 2 H2O
Chromium(VI) halides are known also and include hexafluoride CrF6 and chromyl chloride (CrO2Cl2).[4]
Sodium chromate is produced industrially by the oxidative roasting of chromite ore with calcium or sodium carbonate. The dominant species is therefore, by the law of mass action, determined by the pH of the solutionSony VPCCA35FF/P battery. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of potassium chromate. At yet lower pH values, further condensation to more complex oxyanions of chromium is possible.
Both the chromate and dichromate anions are strong oxidizing reagents at low pH:[4]
Sodium chromate (Na2CrO4)
Cr2O2−
7 + 14 H3O+ + 6 e− → 2 Cr3+ + 21 H2O (ε0 = 1.33 V)
They are, however, only moderately oxidizing at high pH:[4]
CrO2−
4 + 4 H2O + 3 e− → Cr(OH)3 + 5 OH− (ε0 = −0.13 V)
Chromium(VI) compounds in solution can be detected by adding an acidic hydrogen peroxide solution. The unstable dark blue chromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adduct CrO5·OR2. Sony VPCCA35FA/PI battery
Chromic acid has the hypothetical formula H2CrO4. It is a vaguely described chemical, despite many well-defined chromates and dichromates are known. The dark red chromium(VI) oxide CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid".[4] It can be produced by mixing sulfuric acid with dichromate, and is a strong oxidizing agentSony VPCCA2Z0E battery.
Chromium(V) and chromium(IV)
The oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate. The only binary compound is the volatile chromium(V) fluoride (CrF5). This red solid has a melting point of 30 °C and a boiling point of 117 °C. It can be synthesized by treating chromium metal with fluorine at 400 °C and 200 bar pressureSony VPCCA2SFX/R battery. The peroxochromate(V) is another example of the +5 oxidation state. Potassium peroxochromate (K3[Cr(O2)4]) is made by reacting potassium chromate with hydrogen peroxide at low temperatures. This red brown compound is stable at room temperature but decomposes spontaneously at 150–170 °C.[19]
Compounds of chromium(IV) (in the +4 oxidation state) are slightly more common than those of chromium(V) Sony VPCCA2S1E battery. The tetrahalides, CrF4, CrCl4, and CrBr4, can be produced by treating the trihalides (CrX3) with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water.
Chromium(II) and chromium(I)
Many chromium(II) compounds are known, including the water-stable chromium(II) chloride, CrCl2, which can be made by reduction of chromium(III) chloride with zinc. The resulting bright blue solution is only stable at neutral pH. Sony VPCCA2S0E battery Many chromous carboxylates are also known, most famously, the red chromous acetate (Cr2(O2CCH3)4), which features a quadruple bond.
Chromium(I)
Most Cr(I) compounds are obtained by oxidation of electron-rich, octahedral Cr(0) complexes. Other Cr(I) complexes contain cyclopentadienyl ligands. As verified by X-ray diffraction, a Cr-Cr quintuple bond (length 183.51(4) pm) has also been described. Sony VPCCA2AJ battery Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions.
Chromium compound determined experimentally to contain a Cr-Cr quintuple bond
Chromium(0) and chromium(I)
Main article: organochromium chemistry
Many chromium(0) compounds are known. Most are derivatives of chromium hexacarbonyl or bis(benzene)chromium.
History
Weapons found in burial pits dating from the late 3rd century BC Qin Dynasty of the Terracotta Army near Xi'an, China have been analyzed by archaeologists. Although buried more than 2,000 years ago, the ancient bronze tips of crossbow bolts and swords found at the site showed no sign of corrosion, because the bronze was coated with chromium. Sony VPCCA1S3C/CN1 battery
Chromium later came to the attention of westerners in the 18th century. On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Beryozovskoye mines in the Ural Mountains which he named Siberian red lead. Though misidentified as a lead compound with selenium and iron components, the mineral was Crocoite (lead chromate) with a formula of PbCrO4.Sony VPCCA1S2C/CN1 battery
In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that had useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. A bright yellow pigment made from crocoite also became fashionable.[22]
The red colour of rubies is from a small amount of chromium(III) Sony VPCCA1S1E/W battery.
In 1797, Louis Nicolas Vauquelin received samples of crocoite ore. He produced chromium trioxide (CrO3) by mixing crocoite with hydrochloric acid. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven.[23] He was also able to detect traces of chromium in precious gemstones, such as ruby or emerald. Sony VPCCA1S1E/P battery
During the 1800s, chromium was primarily used as a component of paints and in tanning salts. At first, crocoite from Russia was the main source, but in 1827, a larger chromite deposit was discovered near Baltimore, United States. This made the United States the largest producer of chromium products till 1848 when large deposits of chromite where found near BursaSony VPCCA1S1E/G battery, Turkey.[8]
Chromium is also known for its luster when polished. It is used as a protective and decorative coating on car parts, plumbing fixtures, furniture parts and many other items, usually applied by electroplating. Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924. Sony VPCCA1S1E/D battery
Metal alloys now account for 85% of the use of chromium. The remainder is used in the chemical industry and refractory and foundry industries.
Production
Approximately 4.4 million metric tons of marketable chromite ore were produced in 2000, and converted into ~3.3 million tons of ferro-chrome with an approximate market value of 2.5 billion United States dollars.[26] The largest producers of chromium ore have been South Africa (44%) India (18%), Kazakhstan (16%) Zimbabwe (5%), Finland (4%) Iran (4%) and Brazil (2%) with several other countries producing the rest of less than 10% of the world production. Sony VPCCA1S1E/B battery
The two main products of chromium ore refining are ferrochromium and metallic chromium. For those products the ore smelter process differs considerably. For the production of ferrochromium, the chromite ore (FeCr2O4) is reduced in large scale in electric arc furnace or in smaller smelters with either aluminium or silicon in an aluminothermic reaction. Sony VPCCA1S1E battery
Chromium ore output in 2002[26]
For the production of pure chromium, the iron has to be separated from the chromium in a two step roasting and leaching process. The chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms the stable Fe2O3. The subsequent leaching at higher elevated temperatures dissolves the chromates and leaves the insoluble iron oxideSony VPCCA1S1C/CN1 battery. The chromate is converted by sulfuric acid into the dichromate.[27]
4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2
2 Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O
The dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.[27]
Na2Cr2O7 + 2 C → Cr2O3 + Na2CO3 + CO
Cr2O3 + 2 Al → Al2O3 + 2 Cr
Applications
Metallurgy
Decorative chrome plating on a motorcycle.
The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The high speed tool steels contain between 3 and 5% chromiumSony VPCCA1C5E battery. Stainless steel, the main corrosion-proof metal alloy, is formed when chromium is added to iron in sufficient concentrations, usually above 11%. For its formation, ferrochromium is added to the molten iron. Also nickel-based alloys increase in strength due to the formation of discrete, stable metal carbide particles at the grain boundaries. For example, Inconel 718 contains 18.6% chromiumSony VPCCA1AHJ battery. Because of the excellent high temperature properties of these nickel superalloys, they are used in jet engines and gas turbines in lieu of common structural materials.[28]
The relative high hardness and corrosion resistance of unalloyed chromium makes it a good surface coating, being still the most "popular" metal coating with unparalleled combined durability. A thin layer of chromium is deposited on pretreated metallic surfaces by electroplating techniques. There are two deposition methods: Thin, below 1 µm thicknessSony VPCCA1AGJ battery, layers are deposited by chrome plating, and are used for decorative surfaces. If wear-resistant surfaces are needed then thicker chromium layers are deposited. Both methods normally use acidic chromate or dichromate solutions. To prevent the energy consuming change in oxidation state, the use of Chromium(III) sulfate is under development, but for most applications, the established process is used. Sony VPCCA1AFJ battery
In the chromate conversion coating process, the strong oxidative properties of chromates are used to deposit a protective oxide layer on metals like aluminium, zinc and cadmium. This passivation and the self healing properties by the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating method. Sony VPCCA190 battery Because of environmental and health regulations on chromates, alternative coating method are under development.[30]
Anodizing of aluminium is another electrochemical process, which does not lead to the deposition of chromium, but uses chromic acid as electrolyte in the solution. During anodization, an oxide layer is formed on the aluminium. The use of chromic acid, instead of the normally used sulfuric acid, leads to a slight difference of these oxide layers. Sony VPCCA18EC/D battery The high toxicity of Cr(VI) compounds, used in the established chromium electroplating process, and the strengthening of safety and environmental regulations demand a search for substitutes for chromium or at least a change to less toxic chromium(III) compounds.[25]
Dye and pigment
School bus painted in chrome yellow[32]
The mineral crocoite (lead chromate PbCrO4) was used as a yellow pigment shortly after its discovery. After a synthesis method became available starting from the more abundant chromite, chrome yellow was, together with cadmium yellowSony VPCCA18EC battery, one of the most used yellow pigments. The pigment does not photo degrade but it tends to darken due to the formation of chromium(III) oxide. It has a strong color, and was used for school buses in the US and for Postal Service (for example Deutsche Post) in Europe. The use of chrome yellow declined due to environmental and safety concerns and was replaced by organic pigments or alternatives free from lead and chromiumSony VPCCA17FX/G battery. Other pigments based on chromium are, for example, the bright red pigment chrome red, which is a basic lead chromate (PbCrO4·Pb(OH)2). A very important chromate pigment, which was used widely in metal primer formulations was zinc chromate, now replaced by zinc phosphate. A wash primer was formulated to replace the dangerous practice of pretreating aluminium aircraft bodies with a phosphoric acid solutionSony VPCCA17FX/D battery. This used zinc tetroxychromate dispersed in a solution of polyvinyl butyral. An 8% solution of phosphoric acid in solvent was added just before application. It was found that an easily oxidized alcohol was an essential ingredient. A thin layer of about 10–15 microns was applied, which turned from yellow to dark green when it was cured. There is still a question as to the correct mechanism. Chrome green is a mixture of Prussian blue and chrome yellow, while the chrome oxide green is Chromium(III) oxide. Sony VPCCA17EC/W battery
A red color is achieved by doping chromium(III) into the crystals of corundum, which are then called ruby. Therefore, chromium is used in producing synthetic rubies.[34]
Chromium oxides are also used as a green color in glassmaking and as a glaze in ceramics.[35] Green chromium oxide is extremely light-fast and as such is used in cladding coatings. It is also the main ingredient in IR reflecting paints, used by the armed forces, to paint vehicles, to give them the same IR reflectance as green leaves. Sony VPCCA17EC battery
[edit]Wood preservative
Because of their toxicity, chromium(VI) salts are used for the preservation of wood. For example, chromated copper arsenate (CCA) is used in timber treatment to protect wood from decay fungi, wood attacking insects, including termites, and marine borers.[37] The formulations contain chromium based on the oxide CrO3 between 35.3% and 65.5%. In the United States, 65,300 metric tons of CCA solution have been used in 1996. Sony VPCCA16FW/W battery
Tanning
Main article: Tanning
Chromium(III) salts, especially chrome alum and chromium(III) sulfate, are used in the tanning of leather. The chromium(III) stabilizes the leather by cross linking the collagen fibers.[38] Chromium tanned leather can contain between 4 and 5% of chromium, which is tightly bound to the proteins.[8] Although the form of chromium used for tanning is not the toxic hexavalent varietySony VPCCA16FH/B battery, there remains interest in management of chromium in the tanning industry such as recovery and reuse, direct/indirect recycling,[39] use of less chromium or "chrome-less" tanning are practiced to better manage chromium in tanning.
[edit]Refractory material
The high heat resistivity and high melting point makes chromite and chromium(III) oxide a material for high temperature refractory applications, like blast furnaces, cement kilns, molds for the firing of bricks and as foundry sands for the casting of metalsSony VPCCA16FG/W battery. In these applications, the refractory materials are made from mixtures of chromite and magnesite. The use is declining because of the environmental regulations due to the possibility of the formation of chromium(VI).[27]
Catalysts
Several chromium compounds are used as catalysts for processing hydrocarbons. For example the Phillips catalysts for the production of polyethylene are mixtures of chromium and silicon dioxide or mixtures of chromium and titanium and aluminium oxide. Sony VPCCA16FG/B battery Fe-Cr mixed oxides are employed as high-temperature catalysts for the water gas shift reaction.[41][42] Copper chromite is a useful hydrogenation catalyst.[43]
[edit]Other use
Chromium(IV) oxide (CrO2) is a magnetic compound. Its ideal shape anisotropy, which imparts high coercivity and remnant magnetization, made it a compound superior to the γ-Fe2O3. Chromium(IV) oxide is used to manufacture magnetic tape used in high-performance audio tape and standard audio cassettes. Sony VPCCA16EC/P batteryChromates can prevent corrosion of steel under wet conditions, and therefore chromates are added to drilling muds.[45]
Chromium(III) oxide is a metal polish known as green rouge.
Chromic acid is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds. It is prepared in situ by dissolving potassium dichromate in concentrated sulfuric acid, which is then used to wash the apparatusSony VPCCA16EC battery. Sodium dichromate is sometimes used because of its higher solubility (50 g/L versus 200 g/L respectively). Potassium dichromate is a chemical reagent, used in cleaning laboratory glassware and as a titrating agent. It is also used as a mordant (i.e., a fixing agent) for dyes in fabric.
Biological role
Chromium has no verified biological role and has been classified as not essential for mammals.[46] (Cr(III) or Cr3+) occurs in trace amounts and appears to be benign. Sony VPCCA15FX/W batteryChromium deficiency is controversial or is at least extremely rare. It has been attributed to only three people on parenteral nutrition, which is when a patient is fed a liquid diet through intravenous drips.[48] In contrast, hexavalent chromium (Cr(VI) or Cr6+) is very toxic and mutagenic when inhaled. Cr(VI) has not been established as a carcinogen when in solution, although it may cause allergic contact dermatitis (ACD). Sony VPCCA15FW/W battery Although no biological role for chromium has ever been demonstrated, dietary supplements for chromium include chromium(III) picolinate, chromium(III) polynicotinate, and related materials. The benefit of those supplements is questioned by some studies.[50]
The use of chromium-containing dietary supplements is controversial owing to the absence of any verified biological role, the expense of these supplements, and the complex effects of their use. Sony VPCCA15FW/P battery The popular dietary supplement chromium picolinate complex generates chromosome damage in hamster cells (due to the picolinate ligand).[52] In the United States the dietary guidelines for daily chromium uptake were lowered from 50–200 µg for an adult to 35 µg (adult male) and to 25 µg (adult female).[53]
Precautions
Main article: Chromium toxicity
Water insoluble chromium(III) compounds and chromium metal are not considered a health hazard, while the toxicity and carcinogenic properties of chromium(VI) have been known for a long time. Sony VPCCA15FW/D battery Because of the specific transport mechanisms, only limited amounts of chromium(III) enter the cells. Several in vitro studies indicated that high concentrations of chromium(III) in the cell can lead to DNA damage.[55] Acute oral toxicity ranges between 1.5 and 3.3 mg/kg.[56] The proposed beneficial effects of chromium(III) and the use as dietary supplements yielded some controversial results, but recent reviews suggest that moderate uptake of chromium(III) through dietary supplements poses no risk. Sony VPCCA15FW/B battery
Cr(VI)
The acute oral toxicity for chromium(VI) ranges between 50 and 150 µg/kg.[56] In the body, chromium(VI) is reduced by several mechanisms to chromium(III) already in the blood before it enters the cells. The chromium(III) is excreted from the body, whereas the chromate ion is transferred into the cell by a transport mechanismSony VPCCA15FH/W battery, by which also sulfate and phosphate ions enter the cell. The acute toxicity of chromium(VI) is due to its strong oxidational properties. After it reaches the blood stream, it damages the kidneys, the liver and blood cells through oxidation reactions. Hemolysis, renal and liver failure are the results of these damages. Aggressive dialysis can improve the situation. Sony VPCCA15FH/P battery
The carcinogenity of chromate dust is known for a long time, and in 1890 the first publication described the elevated cancer risk of workers in a chromate dye company.[58][59] Three mechanisms have been proposed to describe the genotoxicity of chromium(VI). The first mechanism includes highly reactive hydroxyl radicals and other reactive radicals which are by products of the reduction of chromium(VI) to chromium(III) Sony VPCCA15FH/L battery. The second process includes the direct binding of chromium(V), produced by reduction in the cell, and chromium(IV) compounds to the DNA. The last mechanism attributed the genotoxicity to the binding to the DNA of the end product of the chromium(III) reduction.[60]
Chromium salts (chromates) are also the cause of allergic reactions in some people. Chromates are often used to manufacture, amongst other things, leather products, paints, cement, mortar and anti-corrosivesSony VPCCA15FH/G battery. Contact with products containing chromates can lead to allergic contact dermatitis and irritant dermatitis, resulting in ulceration of the skin, sometimes referred to as "chrome ulcers". This condition is often found in workers that have been exposed to strong chromate solutions in electroplating, tanning and chrome-producing manufacturers. Sony VPCCA15FH/B battery
Environmental issues
As chromium compounds were used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial sites, now needing environmental cleanup and remediation per the treatment of brownfield land. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications. Sony VPCCA15FG/W battery
In 2010, the Environmental Working Group studied the drinking water in 35 American cities. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of listSony VPCCA15FG/R battery; 25 cities had levels that exceeded California's proposed limit.[64] Note: Concentrations of Cr VI in US municipal drinking water supplies reported by EWG are within likely, natural background levels for the areas tested and not necessarily indicative of industrial pollution (CalEPA Fact Sheet), as asserted by EWG. This factor was not taken into consideration in their reportSony VPCCA15FG/P battery.
Arsenic ( /ˈɑrsɨnɪk/ ar-sə-nik) is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.[5] Arsenic is a metalloid. It can exist in various allotropes, although only the grey form has important use in industrySony VPCCA15FG/G battery.
The main use of metallic arsenic is for strengthening alloys of copper and especially lead (for example, in car batteries). Arsenic is a common n-type dopant in semiconductor electronic devices, and the optoelectronic compound gallium arsenide is the most common semiconductor in use after doped silicon. Arsenic and its compounds, especially the trioxide, are used in the production of pesticides (treated wood products), herbicides, and insecticidesSony VPCCA15FG/D battery. These applications are declining, however.[6]
Arsenic is notoriously poisonous to multicellular life, although a few species of bacteria are able to use arsenic compounds as respiratory metabolites. Arsenic contamination of groundwater is a problem that affects millions of people across the world.
Characteristics
Physical characteristics
Crystal structure common to Sb, AsSb and grey As
The three most common allotropes are metallic grey, yellow and black arsenic, with grey being the most common. Sony VPCCA15FG/B batteryGrey arsenic (α-As, space group R3m No. 166) adopts a double-layered structure consisting of many interlocked ruffled six-membered rings. Because of weak bonding between the layers, grey arsenic is brittle and has a relatively low Mohs hardness of 3.5. Nearest and next-nearest neighbors form a distorted octahedral complex, with the three atoms in the same double-layer being slightly closer than the three atoms in the next.[8] This relatively close packing leads to a high density of 5.73 g/cm3. Sony VPCCA15FG battery Grey arsenic is a semimetal, but becomes a semiconductor with a bandgap of 1.2–1.4 eV if amorphized.[10] Yellow arsenic is soft and waxy, and somewhat similar to tetraphosphorus (P4). Both have four atoms arranged in a tetrahedral structure in which each atom is bound to each of the other three atoms by a single bond. This unstable allotrope, being molecularSony VPCCA15FF/W battery, is the most volatile, least dense and most toxic. Solid yellow arsenic is produced by rapid cooling of arsenic vapour, As4. It is rapidly transformed into the grey arsenic by light. The yellow form has a density of 1.97 g/cm3.[9] Black arsenic is similar in structure to red phosphorus.[9]
Isotopes
Main article: Isotopes of arsenic
Naturally occurring arsenic is composed of one stable isotope, 75As.[11] 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 daysSony VPCCA15FF/P battery. Isotopes that are lighter than the stable 75As tend to decay by β+ decay, and those that are heavier tend to decay by β- 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.[11]
Chemistry
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 hammerSony VPCCA15FF/L battery. Arsenic (and some arsenic compounds) sublimes upon heating at atmospheric pressure, converting directly to a gaseous form without an intervening liquid state at 887 K (614 °C).[2] The triple point is 3.63 MPa and 1,090 K (820 °C).[9][2] Arsenic makes arsenic acid with concentrated nitric acid, arsenious acid with dilute nitric acid, and arsenic trioxide with concentrated sulfuric acidSony VPCCA15FF/G battery.
Arsenic burning in the air
Compounds
See also Category: Arsenic compounds
Arsenic compounds resemble in some respects those of phosphorus, which occupies the same group (column) of the periodic table. Arsenic is less commonly observed in the pentavalent state, however. The most common oxidation states for arsenic are: −3 in the arsenides, such as alloy-like intermetallic compounds; and +3 in the arsenites, arsenates(III), and most organoarsenic compoundsSony VPCCA15FF/B battery. Arsenic also bonds readily to itself as seen in the square As3−
4 ions in the mineral skutterudite. In the +3 oxidation state, arsenic is typically pyramidal, owing to the influence of the lone pair of electrons.
[edit]Inorganic
Arsenic forms colorless, odorless, crystalline oxides As2O3 ("white arsenic") and As2O5, which are hygroscopic and readily soluble in water to form acidic solutions. Arsenic(V) acid is a weak acid. Its salts are called arsenates, which is the basis of arsenic contamination of groundwater, a problem that affects many peopleSony VPCCA15FA/W battery. Man-made arsenates include Paris Green (copper(II) acetoarsenite), calcium arsenate, and lead hydrogen arsenate. The latter three have been used as agricultural insecticides and poisons.
The protonation steps between the arsenate and arsenic acid are similar to those between phosphate and phosphoric acid. Unlike phosphorus acid, arsenous acid is genuinely tribasic, with the formula As(OH)3Sony VPCCA15FA/P battery.
A broad variety of sulfur compounds of arsenic are known. Orpiment (As2S3) and realgar (As4S4) are somewhat abundant and were formerly used as painting pigments. In As4S10, arsenic has a formal oxidation state of +2 in As4S4, which features As-As bonds so that the total covalency of As is still in fact three. Sony VPCCA15FA/L battery
The trifluoride, trichloride, tribromide, and triiodide of arsenic(III) are well known, whereas only Arsenic pentafluoride (AsF5) is the only important pentahalide. Again reflecting the lower stability of the 5+ oxidation state, the pentachloride is stable only below −50 °C.[9]
Organoarsenic compounds
Main article: organoarsenic chemistry
Trimethylarsine
A large variety of organoarsenic compounds are known. Several were developed as chemical warfare agents during World War I, including vesicants such as lewisite and vomiting agents such as adamsite. Cacodylic acidSony VPCCA15FA/G battery, which is of historic and practical interest, arises from the methylation of arsenic trioxide, a reaction that has no analogy in phosphorus chemistry.
Alloys
Arsenic is used as the group 5 element in the III-V semiconductors gallium arsenide, indium arsenide, and aluminium arsenide. The valence electron count of GaAs is the same as a pair of Si atoms, but the band structure is completely different, which results distinct bulk properties. Other arsenic alloys include the II-IV semiconductor cadmium arsenideSony VPCCA15FA/B battery.
Occurrence and production
See also: Arsenide minerals and Arsenate minerals
A large sample of native arsenic
Minerals with the formula MAsS and MAs2 (M = Fe, Ni, Co) are the dominant commercial sources of arsenic, together with realgar (an arsenic sulfide mineral) and native arsenic. An illustrative mineral is arsenopyrite (FeAsS), which is structurally related to iron pyrite. Many minor As-containing minerals are knownSony VAIO VPCF135Z1E/B battery. Arsenic also occurs in various organic forms in the environment.[17] Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolized to a less toxic form of arsenic through a process of methylation.[18]
Other naturally occurring pathways of exposure include volcanic ash, weathering of arsenic-containing minerals and ores, and dissolved in groundwater. It is also found in food, water, soil, and air. Sony VAIO VPCF137HG/BI batteryArsenic is absorbed by all plants, but is more concentrated in leafy vegetables, rice, apple and grape juice, and seafood.[20] An additional route of exposure is through inhalation.
Arsenic output in 2006
In 2005, China was the top producer of white arsenic with almost 50% world share, followed by Chile, Peru, and Morocco, according to the British Geological Survey and the United States Geological Survey.[22] Most operations in the US and Europe have closed for environmental reasons. The arsenic is recovered mainly as a side product from the purification of copperSony VAIO VPCF136FG/BI battery. Arsenic is part of the smelter dust from copper, gold, and lead smelters.[23]
On roasting in air of arsenopyrite, arsenic sublimes as arsenic(III) oxide leaving iron oxides,[17] while roasting without air results in the production of metallic arsenic. Further purification from sulfur and other chalcogens is achieved by sublimation in vacuum or in a hydrogen atmosphere or by distillation from molten lead-arsenic mixture. Sony VAIO VPCF135FG/B battery
History
Realgar
Alchemical symbol for arsenic
The word arsenic was borrowed from the Syriac word ܠܐ ܙܐܦܢܝܐ (al) zarniqa [25] and the Persian word زرنيخ Zarnikh, meaning "yellow orpiment", into Greek as arsenikon (Αρσενικόν). It is also related to the similar Greek word arsenikos (Αρσενικός), meaning "masculine" or "potent". The word was adopted in Latin arsenicum and Old French arsenicSony VAIO VPCF127HG/BI battery, from which the English word arsenic is derived.[25] Arsenic sulfides (orpiment, realgar) and oxides have been known and used since ancient times.[26] Zosimos (circa 300 AD) describes roasting sandarach (realgar) to obtain cloud of arsenic (arsenious oxide), which he then reduces to metallic arsenic.[27] 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 presenceSony VAIO VPCF11Z1E battery. (Another less sensitive but more general test is the Reinsch test.) Owing 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.[28]
During the Bronze Age, arsenic was often included in bronze, which made the alloy harder (so-called "arsenical bronze")Sony VAIO VPCF11S1E battery.Albertus Magnus (Albert the Great, 1193–1280) is believed to have been the first to isolate the element in 1250 by heating soap together with arsenic trisulfide.[5] In 1649, Johann Schröder published two ways of preparing arsenic.[31]
Cadet's fuming liquid (impure cacodyl), often claimed as the first synthetic organometallic compound, was synthesized in 1760 by Louis Claude Cadet de Gassicourt by the reaction of potassium acetate with arsenic trioxide. Sony VAIO VPCF11M1E/H battery
In the Victorian era, "arsenic" ("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. Sony VAIO VPCF11M1E battery
Applications
Agricultural
Roxarsone is a controversial arsenic compound used as a nutritional supplement for chickens
The toxicity of arsenic to insects, bacteria and fungi led to its use as a wood preservative.[34] In the 1950s a process of treating wood with chromated copper arsenate (also known as CCA or Tanalith) was invented, and for decades this treatment was the most extensive industrial use of arsenicSony VAIO VPCF11JFX/B battery. An increased appreciation of the toxicity of arsenic resulted in a ban for the use of CCA in consumer products; the European Union and United States initiated this process in 2004.[35][36] CCA remains in heavy use in other countries however, e.g. Malaysian rubber plantations.[6]
Arsenic was also used in various agricultural insecticides, termination and poisons. For example, lead hydrogen arsenate was a common insecticide on fruit treesSony VAIO VPCF119FJ battery,[37] but contact with the compound sometimes resulted in brain damage among those working the sprayers. In the second half of the 20th century, monosodium methyl arsenate (MSMA) and disodium methyl arsenate (DSMA) – less toxic organic forms of arsenic – have replaced lead arsenate in agriculture.
Arsenic is still added to animal food, in particular in the U.S. as a method of disease prevention and growth stimulationSony VAIO VPCF119FC/BI battery. One example is roxarsone, which is used as a broiler starter by about 70% of the broiler growers since 1995.[40] The Poison-Free Poultry Act of 2009 proposes to ban the use of roxarsone in industrial swine and poultry production.[41]
[edit]Medical use
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 trypanosomiasisSony VAIO VPCF119FC battery, 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.[42] It was also used as Fowler's solution in psoriasis.[43] Recently new research has been done in locating tumours using arsenic-74 (a positron emitter) Sony VAIO VPCF117HG/BI battery. 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 body tends to transport iodine to the thyroid gland producing a lot of noise.[44]
In subtoxic doses, soluble arsenic compounds act as stimulants, and were once popular in small doses as medicine by people in the mid-18th century. Sony VAIO VPCF116FG/BI battery
Alloys
The main use of metallic arsenic is for alloying with lead. Lead components in car batteries are strengthened by the presence of a few percent of arsenic.[6] The dezincification can be strongly reduced by adding arsenic to brass a copper zinc alloy.<ref<"Dealloying".</ref> Gallium arsenide is an important semiconductor material, used in integrated circuits. Circuits made from GaAs are much faster (but also much more expensive) Sony VAIO VPCF115FG/B battery 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.[6]
Military
After World War I, the United States built up a stockpile of 20000tons of lewisite (ClCH=CHAsCl2), a chemical weapon that is a vesicant (blister agent) and lung irritant. The stockpile was neutralized with bleach and dumped into the Gulf of Mexico after the 1950s.[45] During the Vietnam War the United States used Agent Blue, a mixture of sodium cacodylate and its acid form, as one of the rainbow herbicides to deprive the Vietnamese of valuable crops. Sony VAIO VPCF21ZHJ battery
[edit]Other uses
Copper acetoarsenite was used as a green pigment known under many 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.[48]
Also used in bronzing and pyrotechnics.
Up to 2% of arsenic is used in lead alloys for lead shots and bullets.[49]
Arsenic is added in small quantities to alpha-brass to make it dezincification resistant. This grade of brass is used to make plumbing fittings or other items that are in constant contact with water. Sony VAIO VPCF21Z1E/BI battery
Arsenic is also used for taxonomic sample preservation.
Until recently arsenic was used in optical glass. Modern glass manufacturers, under pressure from environmentalists, have removed it, along with lead.[51]
Biological role
Bacteria
Arsenobetaine
Some species of bacteria obtain their energy by oxidizing various fuels while reducing arsenate to arsenite. Under oxidative environmental conditions some bacteria use arsenite, which is oxidized to arsenate as fuel for their metabolism.[52] The enzymes involved are known as arsenate reductases (Arr) Sony VAIO VPCF21AHJ battery.
In 2008, bacteria were discovered that employ a version of photosynthesis in the absence of oxygen with arsenites as electron donors, producing arsenates (just as ordinary photosynthesis uses water as electron donor, producing molecular oxygen). Researchers conjecture that, over the course of history, these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thriveSony VAIO VPCF21AGJ battery. 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.[53]
[edit]Heredity
Arsenic has been linked to epigenetic changes that are heritable changes in gene expression that occur without changes in DNA sequence and include DNA methylation, histone modification, and RNA interferenceSony VAIO VPCF21AFJ battery. Toxic levels of arsenic cause significant DNA hypermethylation of tumour suppressor genes p16 and p53, thus increasing risk of carcinogenesis. These epigenetic events have been observed in in vitro studies with human kidney cells and in vivo tests with rat liver cells and peripheral blood leukocytes in humans. Sony VAIO VPCF219FJ/BI battery Inductive coupled plasma mass spectrometry (ICP-MS) is used to detect precise levels of intracellular arsenic and its other bases involved in epigenetic modification of DNA.[55] Studies investigating arsenic as an epigenetic factor will help in developing precise biomarkers of exposure and susceptibility.
The Chinese brake fern (Pteris vittata) hyperaccumulates arsenic present in the soil into its leaves and has a proposed use in phytoremediation. Sony VAIO VPCF217HG/BI battery
[edit]As a building block of life
A NASA-funded astrobiology research team claimed on December 2, 2010 that the microbe strain GFAJ-1 of the Gammaproteobacteria (designated Halomonadaceae) group has the ability to substitute arsenic for at least part of the phosphorus in the molecules of its cells, including DNA and ATP. Bacteria from Mono Lake, a naturally arsenic-rich site in CaliforniaSony VAIO VPCF217HG battery, were cultured in an environment high in arsenic but low in phosphorus. This finding has faced strong criticism from the scientific community; many scientists have argued that there is no evidence that arsenic is actually incorporated into biomolecules.[59][60]
[edit]Biomethylation
Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolised through a process of methylation.[61] For example, the mold Scopulariopsis brevicaulis produce significant amounts of trimethylarsine if inorganic arsenic is present. Sony VAIO VPCF14ZHJ batteryThe 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. Sony VAIO VPCF14AHJ battery
Environmental issues
Occurence in drinking water
Main article: Arsenic contamination of groundwater
Widespread arsenic contamination of groundwater has led to a massive epidemic of arsenic poisoning in Bangladesh[64] and neighbouring countries. As of this writing, 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 billionSony VAIO VPCF14AGJ battery. However, a study of cancer rates in Taiwan[65] 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, owing 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 centurySony VAIO VPCF14AFJ battery. This program was designed to prevent drinking of bacteria-contaminated surface waters, but failed to test for arsenic in the groundwater. Many other countries and districts in Southeast Asia, such as Vietnam and Cambodia have geological environments conducive to generation of high-arsenic groundwaters. Arsenicosis was reported in Nakhon Si Thammarat, Thailand in 1987Sony VAIO VPCF149FJ/BI battery, 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 owing to the use of bottled water.[66]
In the United States, arsenic is most commonly found in the ground waters of the southwest.[67] Parts of New England, Michigan, Wisconsin, Minnesota and the Dakotas are also known to have significant[clarification needed] concentrations of arsenic in ground water. Increased levels of skin cancer have been associated with arsenic exposure in WisconsinSony VAIO VPCF148FJ/B battery, even at levels below the 10 part per billion drinking water standard.[68] According to a recent film funded by the US Superfund, millions of private wells have unknown arsenic levels, and in some areas of the US, over 20% of wells may contain levels that exceed established limits.[69]
Low-level exposure to arsenic at concentrations found commonly in US drinking water compromises the initial immune response to H1N1 or swine flu infection according to NIEHS-supported scientistsSony VAIO VPCF13ZHJ battery. The study, conducted in laboratory mice, suggests that people exposed to arsenic in their drinking water may be at increased risk for more serious illness or death in response to infection from the virus.[70]
Epidemiological evidence from Chile shows a dose-dependent connection between chronic arsenic exposure and various forms of cancer, in particular when other risk factors, such as cigarette smoking, are presentSony VAIO VPCF13Z0E/B battery. These effects have been demonstrated to persist below 50 parts per billion.[71]
Analyzing multiple epidemiological studies on inorganic arsenic exposure suggests a small but measurable risk increase for bladder cancer at 10 parts per billion.[72] According to Peter Ravenscroft of the Department of Geography at the University of Cambridge,[73] roughly 80 million people worldwide consume between 10 and 50 parts per billion arsenic in their drinking waterSony VAIO VPCF13M8E/B battery. 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 remediationSony VAIO VPCF13AHJ battery.
Early (1973) evaluations of the removal of dissolved arsenic by drinking water treatment processes demonstrated that arsenic is very effectively removed by co-precipitation with either iron or aluminum oxides. The use of iron as a coagulant, in particular, was found to remove arsenic with efficiencies exceeding 90%. 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) Sony VAIO VPCF13AGJ battery. A team of European and Indian scientists and engineers have set up six arsenic treatment plants in West Bengal based on in-situ remediation method (SAR Technology). This technology does not use any chemicals and arsenic is left as an insoluble form (+5 state) in the subterranean zone by recharging aerated water into the aquifer and thus developing an oxidation zone to support arsenic oxidizing micro-organismsSony VAIO VPCF13AFJ battery. This process does not produce any waste stream or sludge and is relatively cheap.[76]
Another effective and inexpensive method to remove arsenic from contaminated well water is to sink wells 500 feet or deeper to reach purer waters. A recent 2011 study funded by the U.S. National Institute of Environmental Health Sciences' Superfund Research Program shows that deep sediments can remove arsenic and take it out of circulationSony VAIO VPCF138FJ/BI battery. Through this process called adsorption in which arsenic sticks to the surfaces of deep sediment articles, arsenic can be naturally removed from well water.[77]
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. Sony VAIO VPCF138FJ/B battery
Epidemiological studies have suggested a correlation between chronic consumption of drinking water contaminated with arsenic and the incidence of all leading causes of mortality.[citation needed] The literature provides reason to believe arsenic exposure is causative in the pathogenesis of diabetes. Sony VAIO VPCF138FC/BI battery
Hungarian engineer László Schremmer has recently discovered that by the use of chaff-based filters it is possible to reduce the arsenic content of water to 3 microgram/litre. This is especially important in areas where the potable water is provided by filtering the water extracted from the underground aquifer. Sony VAIO VPCF135FG battery
Wood preservation in the US
As of 2002, US-based industries consumed 19,600 metric tons of arsenic. Ninety percent of this was used for treatment of wood with chromated copper arsenate (CCA). In 2007, 50% of the 5,280 metric tons of consumption was still used for this purpose. In the United States, the use of arsenic in consumer products was discontinued for residentialSony VAIO VPCF12AHJ battery, and general consumer construction on December 31, 2003 and alternative chemicals are now used, such as Alkaline Copper Quaternary, borates, copper azole, cyproconazole, and propiconazole.[81]
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 countriesSony VAIO VPCF12AGJ battery, 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 timberSony VAIO VPCF12AFJ battery. 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 worldSony VAIO VPCF129FJ/BI battery; there is also concern in some quarters about the widespread landfill disposal of such timber.
[edit]Mapping of industrial releases in the US
One tool that maps releases of arsenic to particular locations in the United States[82] and also provides additional information about such releases is TOXMAP. TOXMAP is a Geographic Information System (GIS) from the Division of Specialized Information Services of the United States National Library of Medicine (NLM) that uses maps of the United States to help users visually explore data from the United States Environmental Protection Agency'sSony VAIO VPCF128FJ/B battery (EPA) Toxics Release Inventory and Superfund Basic Research Programs. TOXMAP is a resource funded by the US Federal Government. TOXMAP's chemical and environmental health information is taken from NLM's Toxicology Data Network (TOXNET)[83] and PubMed, and from other authoritative sources.
Toxicity and precautions
Main articles: Arsenic poisoning and Arsenic toxicity
Arsenic and many of its compounds are especially potent poisons. Many water supplies close to mines are contaminated by these poisonsSony VAIO VPCF127FJ/W battery. In the United States, the maximum allowed concentration in drinking water is 10 ppb, 5 ppb for bottled water.[84] There is no standard for food, though in 2012 press coverage prompted calls for developing such.[84] The People's Republic of China does have a food standard.[84]
Elemental arsenic and arsenic compounds are classified as "toxic" and "dangerous for the environment" in the European Union under directive 67/548/EECSony VAIO VPCF11ZHJ battery. 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 owing to its manifestation in drinking water, "the most common species being arsenate [HAsO42-; As(V)] and arsenite [H3AsO3 ; As(III)]" Sony VAIO VPCF11AHJ battery.
[edit]Treatment
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. Sony VAIO VPCF11AGJ batteryHowever the Agency for Toxic Substances and Disease Registry (ATSDR) states that the long-term effects of arsenic exposure cannot be predicted.[21] Blood, urine, hair, and nails may be tested for arsenic; however, these tests cannot foresee possible health outcomes from the exposure.[21] Excretion occurs in the urine and long-term exposure to arsenic has been linked to bladder and kidney cancer in addition to cancer of the liver, prostate, skin, lungs and nasal cavity. Sony VAIO VPCF11AFJ battery
Exposure risks and remediation
Occupational exposure and arsenic poisoning may occur in persons working in industries involving the use of inorganic arsenic and its compounds, such as 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. Sony VAIO VPCF118FJ battery
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.Sony VAIO VPCF117FJ battery
Biological mechanism
Arsenic disrupts ATP production through several mechanisms. At the level of the citric acid cycle, arsenic inhibits lipoic acid, which is a cofactor for 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, whichSony VAIO VGN-CS33H battery, it is speculated, has potential to form reactive oxygen species and oxidative stress. These metabolic interferences lead to death from multi-system organ failure, it is presumed from necrotic cell death, not apoptosis. A post mortem reveals brick-red-coloured mucosa, owing to severe haemorrhage. Although arsenic causes toxicity, it can also play a protective role. Sony VAIO VGN-CS33H/Z battery
The high affinity of arsenic(III) oxides for thiols is usually assigned as the cause of the high toxicity. Thiols, in the form of cysteine residues, are situated at the active sites of many important enzymes.
Chromium was regarded with great interest because of its high corrosion resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding chromium to form stainless steelSony VPCCA3E1E battery. This application, along with chrome plating (electroplating with chromium) are currently the highest-volume uses of the metal. Chromium and ferrochromium are produced from the single commercially viable ore, chromite, by silicothermic or aluminothermic reaction or by roasting and leaching processesSony VPCCA38EC/R battery.
Although trivalent chromium (Cr(III)) is required in trace amounts for sugar and lipid metabolism, few cases have been reported where its complete removal from the diet has caused chromium deficiency. In larger amounts and in different forms, chromium can be toxic and carcinogenic. The most prominent example of toxic chromium is hexavalent chromium (Cr(VI)). Abandoned chromium production sites often require environmental cleanupSony VPCCA38EC battery.
Characteristics
Physical
Chromium is remarkable for its magnetic properties: it is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, it transforms into a paramagnetic state.[1]
Passivation
Chromium metal left standing in air is passivated by oxygen, forming a thin protective oxide surface layer. This layer is a spinel structure only a few atoms thick. It is very dense, and prevents the diffusion of oxygen into the underlying materialSony VPCCA37EC/B battery. This barrier is in contrast to iron or plain carbon steels, where the oxygen migrates into the underlying material and causes rusting.[3] The passivation can be enhanced by short contact with oxidizing acids like nitric acid. Passivated chromium is stable against acids. The opposite effect can be achieved by treatment with a strong reducing agent that destroys the protective oxide layer on the metal. Chromium metal treated in this way readily dissolves in weak acids. Sony VPCCA37EC battery
Chromium, unlike metals such as iron and nickel, does not suffer from hydrogen embrittlement. However, it does suffer from nitrogen embrittlement, reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.[5]
Occurrence
Crocoite (PbCrO4)
Chromite ore
Chromium is the 21st most abundant element in Earth's crust with an average concentration of 100 ppm. Sony VPCCA36FW/W battery Chromium compounds are found in the environment, due to erosion of chromium-containing rocks and can be distributed by volcanic eruptions. The concentrations range in soil is between 1 and 3000 mg/kg, in sea water 5 to 800 µg/liter, and in rivers and lakes 26 µg/liter to 5.2 mg/liter.[7] Chromium is mined as chromite (FeCr2O4) oreSony VPCCA36FW/B battery.[8] About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, while Kazakhstan, India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.[9]
Although rare, deposits of native chromium exist.[10][11] The Udachnaya Pipe in Russia produces samples of the native metal. This mine is a kimberlite pipe, rich in diamonds, and the reducing environment helped produce both elemental chromium and diamond. Sony VPCCA36FH/W battery
The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location, but in most cases, the Cr(III) is the dominating species,[7] although in some areas the ground water can contain up to 39 µg/liter of total chromium of which 30 µg/liter is present as Cr(VI) Sony VPCCA36FG/B battery.
Isotopes
Main article: Isotopes of chromium
Naturally occurring chromium is composed of three stable isotopes; 52Cr, 53Cr and 54Cr with 52Cr being the most abundant (83.789% natural abundance). 19 radioisotopes have been characterized with the most stable being 50Cr with a half-life of (more than) 1.8×1017 years, and 51Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minuteSony VPCCA36FA/B battery. This element also has 2 meta states.[14]
53Cr is the radiogenic decay product of 53Mn. Chromium isotopic contents are typically combined with manganese isotopic contents and have found application in isotope geology. Mn-Cr isotope ratios reinforce the evidence from 26Al and 107Pd for the early history of the solar system. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites indicate an initial 53Mn/55Mn ratio that suggests Mn-Cr isotopic composition must result from in-situ decay of 53Mn in differentiated planetary bodiesSony VPCCA36EC/W battery. Hence 53Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the solar system.[15]
The isotopes of chromium range in atomic mass from 43 u (43Cr) to 67 u (67Cr). The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.[14] 53Cr has been posited as a proxy for atmospheric oxygen concentration. Sony VPCCA36EC battery
Compounds
Chromium is a member of the transition metals, in group 6. Chromium(0) has an electronic configuration of 4s13d5, owing to the lower energy of the high spin configuration. Chromium exhibits a wide range of possible oxidation states, where the +3 state is most stable energetically; the +3 and +6 states are most commonly observed in chromium compounds, whereas the +1, +4 and +5 states are rare. Sony VPCCA35FW/W battery
The following is the Pourbaix diagram for chromium in pure water, perchloric acid or sodium hydroxide:
Chromium(III)
Chromium(III) chloride hexahydrate ([CrCl2(H2O)4]Cl·2H2O)
Anhydrous chromium(III) chloride (CrCl3)
A large number of chromium(III) compounds are known. Chromium(III) can be obtained by dissolving elemental chromium in acids like hydrochloric acid or sulfuric acid. The Cr3+ ion has a similar radius (63 pm) to the Al3+ ion (radius 50 pm), so they can replace each other in some compounds, such as in chrome alum and alumSony VPCCA35FW/B battery. When a trace amount of Cr3+ replaces Al3+ in corundum (aluminium oxide, Al2O3), the red-colored ruby is formed.
Chromium(III) ions tend to form octahedral complexes. The colors of these complexes is determined by the ligands attached to the Cr centre. The commercially available chromium(III) chloride hydrate is the dark green complex [CrCl2(H2O)4]Cl. Closely related compounds have different colours: pale green [CrCl(H2O)5]Cl2 and the violet [Cr(H2O)6]Cl3Sony VPCCA35FN/R battery. If water-free green chromium(III) chloride is dissolved in water then the green solution turns violet after some time, due to the substitution of water by chloride in the inner coordination sphere. This kind of reaction is also observed with solutions of chrome alum and other water-soluble chromium(III) saltsSony VPCCA35FH/D battery.
Chromium(III) hydroxide (Cr(OH)3) is amphoteric, dissolving in acidic solutions to form [Cr(H2O)6]3+, and in basic solutions to form [Cr(OH)6]3−. It is dehydrated by heating to form the green chromium(III) oxide (Cr2O3), which is the stable oxide with a crystal structure identical to that of corundum. Sony VPCCA35FG/PI battery
Chromium(VI)
Chromium(VI) oxide
Chromium(VI) compounds are powerful oxidants at low or neutral pH. Most important are chromate anion (CrO2−
4) and dichromate (Cr2O72-) anions, which exist in equilibrium:
2 [CrO4]2- + 2 H+ [Cr2O7]2- + 2 H2O
Chromium(VI) halides are known also and include hexafluoride CrF6 and chromyl chloride (CrO2Cl2).[4]
Sodium chromate is produced industrially by the oxidative roasting of chromite ore with calcium or sodium carbonate. The dominant species is therefore, by the law of mass action, determined by the pH of the solutionSony VPCCA35FF/P battery. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of potassium chromate. At yet lower pH values, further condensation to more complex oxyanions of chromium is possible.
Both the chromate and dichromate anions are strong oxidizing reagents at low pH:[4]
Sodium chromate (Na2CrO4)
Cr2O2−
7 + 14 H3O+ + 6 e− → 2 Cr3+ + 21 H2O (ε0 = 1.33 V)
They are, however, only moderately oxidizing at high pH:[4]
CrO2−
4 + 4 H2O + 3 e− → Cr(OH)3 + 5 OH− (ε0 = −0.13 V)
Chromium(VI) compounds in solution can be detected by adding an acidic hydrogen peroxide solution. The unstable dark blue chromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adduct CrO5·OR2. Sony VPCCA35FA/PI battery
Chromic acid has the hypothetical formula H2CrO4. It is a vaguely described chemical, despite many well-defined chromates and dichromates are known. The dark red chromium(VI) oxide CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid".[4] It can be produced by mixing sulfuric acid with dichromate, and is a strong oxidizing agentSony VPCCA2Z0E battery.
Chromium(V) and chromium(IV)
The oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate. The only binary compound is the volatile chromium(V) fluoride (CrF5). This red solid has a melting point of 30 °C and a boiling point of 117 °C. It can be synthesized by treating chromium metal with fluorine at 400 °C and 200 bar pressureSony VPCCA2SFX/R battery. The peroxochromate(V) is another example of the +5 oxidation state. Potassium peroxochromate (K3[Cr(O2)4]) is made by reacting potassium chromate with hydrogen peroxide at low temperatures. This red brown compound is stable at room temperature but decomposes spontaneously at 150–170 °C.[19]
Compounds of chromium(IV) (in the +4 oxidation state) are slightly more common than those of chromium(V) Sony VPCCA2S1E battery. The tetrahalides, CrF4, CrCl4, and CrBr4, can be produced by treating the trihalides (CrX3) with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water.
Chromium(II) and chromium(I)
Many chromium(II) compounds are known, including the water-stable chromium(II) chloride, CrCl2, which can be made by reduction of chromium(III) chloride with zinc. The resulting bright blue solution is only stable at neutral pH. Sony VPCCA2S0E battery Many chromous carboxylates are also known, most famously, the red chromous acetate (Cr2(O2CCH3)4), which features a quadruple bond.
Chromium(I)
Most Cr(I) compounds are obtained by oxidation of electron-rich, octahedral Cr(0) complexes. Other Cr(I) complexes contain cyclopentadienyl ligands. As verified by X-ray diffraction, a Cr-Cr quintuple bond (length 183.51(4) pm) has also been described. Sony VPCCA2AJ battery Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions.
Chromium compound determined experimentally to contain a Cr-Cr quintuple bond
Chromium(0) and chromium(I)
Main article: organochromium chemistry
Many chromium(0) compounds are known. Most are derivatives of chromium hexacarbonyl or bis(benzene)chromium.
History
Weapons found in burial pits dating from the late 3rd century BC Qin Dynasty of the Terracotta Army near Xi'an, China have been analyzed by archaeologists. Although buried more than 2,000 years ago, the ancient bronze tips of crossbow bolts and swords found at the site showed no sign of corrosion, because the bronze was coated with chromium. Sony VPCCA1S3C/CN1 battery
Chromium later came to the attention of westerners in the 18th century. On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Beryozovskoye mines in the Ural Mountains which he named Siberian red lead. Though misidentified as a lead compound with selenium and iron components, the mineral was Crocoite (lead chromate) with a formula of PbCrO4.Sony VPCCA1S2C/CN1 battery
In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that had useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. A bright yellow pigment made from crocoite also became fashionable.[22]
The red colour of rubies is from a small amount of chromium(III) Sony VPCCA1S1E/W battery.
In 1797, Louis Nicolas Vauquelin received samples of crocoite ore. He produced chromium trioxide (CrO3) by mixing crocoite with hydrochloric acid. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven.[23] He was also able to detect traces of chromium in precious gemstones, such as ruby or emerald. Sony VPCCA1S1E/P battery
During the 1800s, chromium was primarily used as a component of paints and in tanning salts. At first, crocoite from Russia was the main source, but in 1827, a larger chromite deposit was discovered near Baltimore, United States. This made the United States the largest producer of chromium products till 1848 when large deposits of chromite where found near BursaSony VPCCA1S1E/G battery, Turkey.[8]
Chromium is also known for its luster when polished. It is used as a protective and decorative coating on car parts, plumbing fixtures, furniture parts and many other items, usually applied by electroplating. Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924. Sony VPCCA1S1E/D battery
Metal alloys now account for 85% of the use of chromium. The remainder is used in the chemical industry and refractory and foundry industries.
Production
Approximately 4.4 million metric tons of marketable chromite ore were produced in 2000, and converted into ~3.3 million tons of ferro-chrome with an approximate market value of 2.5 billion United States dollars.[26] The largest producers of chromium ore have been South Africa (44%) India (18%), Kazakhstan (16%) Zimbabwe (5%), Finland (4%) Iran (4%) and Brazil (2%) with several other countries producing the rest of less than 10% of the world production. Sony VPCCA1S1E/B battery
The two main products of chromium ore refining are ferrochromium and metallic chromium. For those products the ore smelter process differs considerably. For the production of ferrochromium, the chromite ore (FeCr2O4) is reduced in large scale in electric arc furnace or in smaller smelters with either aluminium or silicon in an aluminothermic reaction. Sony VPCCA1S1E battery
Chromium ore output in 2002[26]
For the production of pure chromium, the iron has to be separated from the chromium in a two step roasting and leaching process. The chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms the stable Fe2O3. The subsequent leaching at higher elevated temperatures dissolves the chromates and leaves the insoluble iron oxideSony VPCCA1S1C/CN1 battery. The chromate is converted by sulfuric acid into the dichromate.[27]
4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2
2 Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O
The dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.[27]
Na2Cr2O7 + 2 C → Cr2O3 + Na2CO3 + CO
Cr2O3 + 2 Al → Al2O3 + 2 Cr
Applications
Metallurgy
Decorative chrome plating on a motorcycle.
The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The high speed tool steels contain between 3 and 5% chromiumSony VPCCA1C5E battery. Stainless steel, the main corrosion-proof metal alloy, is formed when chromium is added to iron in sufficient concentrations, usually above 11%. For its formation, ferrochromium is added to the molten iron. Also nickel-based alloys increase in strength due to the formation of discrete, stable metal carbide particles at the grain boundaries. For example, Inconel 718 contains 18.6% chromiumSony VPCCA1AHJ battery. Because of the excellent high temperature properties of these nickel superalloys, they are used in jet engines and gas turbines in lieu of common structural materials.[28]
The relative high hardness and corrosion resistance of unalloyed chromium makes it a good surface coating, being still the most "popular" metal coating with unparalleled combined durability. A thin layer of chromium is deposited on pretreated metallic surfaces by electroplating techniques. There are two deposition methods: Thin, below 1 µm thicknessSony VPCCA1AGJ battery, layers are deposited by chrome plating, and are used for decorative surfaces. If wear-resistant surfaces are needed then thicker chromium layers are deposited. Both methods normally use acidic chromate or dichromate solutions. To prevent the energy consuming change in oxidation state, the use of Chromium(III) sulfate is under development, but for most applications, the established process is used. Sony VPCCA1AFJ battery
In the chromate conversion coating process, the strong oxidative properties of chromates are used to deposit a protective oxide layer on metals like aluminium, zinc and cadmium. This passivation and the self healing properties by the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating method. Sony VPCCA190 battery Because of environmental and health regulations on chromates, alternative coating method are under development.[30]
Anodizing of aluminium is another electrochemical process, which does not lead to the deposition of chromium, but uses chromic acid as electrolyte in the solution. During anodization, an oxide layer is formed on the aluminium. The use of chromic acid, instead of the normally used sulfuric acid, leads to a slight difference of these oxide layers. Sony VPCCA18EC/D battery The high toxicity of Cr(VI) compounds, used in the established chromium electroplating process, and the strengthening of safety and environmental regulations demand a search for substitutes for chromium or at least a change to less toxic chromium(III) compounds.[25]
Dye and pigment
School bus painted in chrome yellow[32]
The mineral crocoite (lead chromate PbCrO4) was used as a yellow pigment shortly after its discovery. After a synthesis method became available starting from the more abundant chromite, chrome yellow was, together with cadmium yellowSony VPCCA18EC battery, one of the most used yellow pigments. The pigment does not photo degrade but it tends to darken due to the formation of chromium(III) oxide. It has a strong color, and was used for school buses in the US and for Postal Service (for example Deutsche Post) in Europe. The use of chrome yellow declined due to environmental and safety concerns and was replaced by organic pigments or alternatives free from lead and chromiumSony VPCCA17FX/G battery. Other pigments based on chromium are, for example, the bright red pigment chrome red, which is a basic lead chromate (PbCrO4·Pb(OH)2). A very important chromate pigment, which was used widely in metal primer formulations was zinc chromate, now replaced by zinc phosphate. A wash primer was formulated to replace the dangerous practice of pretreating aluminium aircraft bodies with a phosphoric acid solutionSony VPCCA17FX/D battery. This used zinc tetroxychromate dispersed in a solution of polyvinyl butyral. An 8% solution of phosphoric acid in solvent was added just before application. It was found that an easily oxidized alcohol was an essential ingredient. A thin layer of about 10–15 microns was applied, which turned from yellow to dark green when it was cured. There is still a question as to the correct mechanism. Chrome green is a mixture of Prussian blue and chrome yellow, while the chrome oxide green is Chromium(III) oxide. Sony VPCCA17EC/W battery
A red color is achieved by doping chromium(III) into the crystals of corundum, which are then called ruby. Therefore, chromium is used in producing synthetic rubies.[34]
Chromium oxides are also used as a green color in glassmaking and as a glaze in ceramics.[35] Green chromium oxide is extremely light-fast and as such is used in cladding coatings. It is also the main ingredient in IR reflecting paints, used by the armed forces, to paint vehicles, to give them the same IR reflectance as green leaves. Sony VPCCA17EC battery
[edit]Wood preservative
Because of their toxicity, chromium(VI) salts are used for the preservation of wood. For example, chromated copper arsenate (CCA) is used in timber treatment to protect wood from decay fungi, wood attacking insects, including termites, and marine borers.[37] The formulations contain chromium based on the oxide CrO3 between 35.3% and 65.5%. In the United States, 65,300 metric tons of CCA solution have been used in 1996. Sony VPCCA16FW/W battery
Tanning
Main article: Tanning
Chromium(III) salts, especially chrome alum and chromium(III) sulfate, are used in the tanning of leather. The chromium(III) stabilizes the leather by cross linking the collagen fibers.[38] Chromium tanned leather can contain between 4 and 5% of chromium, which is tightly bound to the proteins.[8] Although the form of chromium used for tanning is not the toxic hexavalent varietySony VPCCA16FH/B battery, there remains interest in management of chromium in the tanning industry such as recovery and reuse, direct/indirect recycling,[39] use of less chromium or "chrome-less" tanning are practiced to better manage chromium in tanning.
[edit]Refractory material
The high heat resistivity and high melting point makes chromite and chromium(III) oxide a material for high temperature refractory applications, like blast furnaces, cement kilns, molds for the firing of bricks and as foundry sands for the casting of metalsSony VPCCA16FG/W battery. In these applications, the refractory materials are made from mixtures of chromite and magnesite. The use is declining because of the environmental regulations due to the possibility of the formation of chromium(VI).[27]
Catalysts
Several chromium compounds are used as catalysts for processing hydrocarbons. For example the Phillips catalysts for the production of polyethylene are mixtures of chromium and silicon dioxide or mixtures of chromium and titanium and aluminium oxide. Sony VPCCA16FG/B battery Fe-Cr mixed oxides are employed as high-temperature catalysts for the water gas shift reaction.[41][42] Copper chromite is a useful hydrogenation catalyst.[43]
[edit]Other use
Chromium(IV) oxide (CrO2) is a magnetic compound. Its ideal shape anisotropy, which imparts high coercivity and remnant magnetization, made it a compound superior to the γ-Fe2O3. Chromium(IV) oxide is used to manufacture magnetic tape used in high-performance audio tape and standard audio cassettes. Sony VPCCA16EC/P batteryChromates can prevent corrosion of steel under wet conditions, and therefore chromates are added to drilling muds.[45]
Chromium(III) oxide is a metal polish known as green rouge.
Chromic acid is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds. It is prepared in situ by dissolving potassium dichromate in concentrated sulfuric acid, which is then used to wash the apparatusSony VPCCA16EC battery. Sodium dichromate is sometimes used because of its higher solubility (50 g/L versus 200 g/L respectively). Potassium dichromate is a chemical reagent, used in cleaning laboratory glassware and as a titrating agent. It is also used as a mordant (i.e., a fixing agent) for dyes in fabric.
Biological role
Chromium has no verified biological role and has been classified as not essential for mammals.[46] (Cr(III) or Cr3+) occurs in trace amounts and appears to be benign. Sony VPCCA15FX/W batteryChromium deficiency is controversial or is at least extremely rare. It has been attributed to only three people on parenteral nutrition, which is when a patient is fed a liquid diet through intravenous drips.[48] In contrast, hexavalent chromium (Cr(VI) or Cr6+) is very toxic and mutagenic when inhaled. Cr(VI) has not been established as a carcinogen when in solution, although it may cause allergic contact dermatitis (ACD). Sony VPCCA15FW/W battery Although no biological role for chromium has ever been demonstrated, dietary supplements for chromium include chromium(III) picolinate, chromium(III) polynicotinate, and related materials. The benefit of those supplements is questioned by some studies.[50]
The use of chromium-containing dietary supplements is controversial owing to the absence of any verified biological role, the expense of these supplements, and the complex effects of their use. Sony VPCCA15FW/P battery The popular dietary supplement chromium picolinate complex generates chromosome damage in hamster cells (due to the picolinate ligand).[52] In the United States the dietary guidelines for daily chromium uptake were lowered from 50–200 µg for an adult to 35 µg (adult male) and to 25 µg (adult female).[53]
Precautions
Main article: Chromium toxicity
Water insoluble chromium(III) compounds and chromium metal are not considered a health hazard, while the toxicity and carcinogenic properties of chromium(VI) have been known for a long time. Sony VPCCA15FW/D battery Because of the specific transport mechanisms, only limited amounts of chromium(III) enter the cells. Several in vitro studies indicated that high concentrations of chromium(III) in the cell can lead to DNA damage.[55] Acute oral toxicity ranges between 1.5 and 3.3 mg/kg.[56] The proposed beneficial effects of chromium(III) and the use as dietary supplements yielded some controversial results, but recent reviews suggest that moderate uptake of chromium(III) through dietary supplements poses no risk. Sony VPCCA15FW/B battery
Cr(VI)
The acute oral toxicity for chromium(VI) ranges between 50 and 150 µg/kg.[56] In the body, chromium(VI) is reduced by several mechanisms to chromium(III) already in the blood before it enters the cells. The chromium(III) is excreted from the body, whereas the chromate ion is transferred into the cell by a transport mechanismSony VPCCA15FH/W battery, by which also sulfate and phosphate ions enter the cell. The acute toxicity of chromium(VI) is due to its strong oxidational properties. After it reaches the blood stream, it damages the kidneys, the liver and blood cells through oxidation reactions. Hemolysis, renal and liver failure are the results of these damages. Aggressive dialysis can improve the situation. Sony VPCCA15FH/P battery
The carcinogenity of chromate dust is known for a long time, and in 1890 the first publication described the elevated cancer risk of workers in a chromate dye company.[58][59] Three mechanisms have been proposed to describe the genotoxicity of chromium(VI). The first mechanism includes highly reactive hydroxyl radicals and other reactive radicals which are by products of the reduction of chromium(VI) to chromium(III) Sony VPCCA15FH/L battery. The second process includes the direct binding of chromium(V), produced by reduction in the cell, and chromium(IV) compounds to the DNA. The last mechanism attributed the genotoxicity to the binding to the DNA of the end product of the chromium(III) reduction.[60]
Chromium salts (chromates) are also the cause of allergic reactions in some people. Chromates are often used to manufacture, amongst other things, leather products, paints, cement, mortar and anti-corrosivesSony VPCCA15FH/G battery. Contact with products containing chromates can lead to allergic contact dermatitis and irritant dermatitis, resulting in ulceration of the skin, sometimes referred to as "chrome ulcers". This condition is often found in workers that have been exposed to strong chromate solutions in electroplating, tanning and chrome-producing manufacturers. Sony VPCCA15FH/B battery
Environmental issues
As chromium compounds were used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial sites, now needing environmental cleanup and remediation per the treatment of brownfield land. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications. Sony VPCCA15FG/W battery
In 2010, the Environmental Working Group studied the drinking water in 35 American cities. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of listSony VPCCA15FG/R battery; 25 cities had levels that exceeded California's proposed limit.[64] Note: Concentrations of Cr VI in US municipal drinking water supplies reported by EWG are within likely, natural background levels for the areas tested and not necessarily indicative of industrial pollution (CalEPA Fact Sheet), as asserted by EWG. This factor was not taken into consideration in their reportSony VPCCA15FG/P battery.
Arsenic ( /ˈɑrsɨnɪk/ ar-sə-nik) is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.[5] Arsenic is a metalloid. It can exist in various allotropes, although only the grey form has important use in industrySony VPCCA15FG/G battery.
The main use of metallic arsenic is for strengthening alloys of copper and especially lead (for example, in car batteries). Arsenic is a common n-type dopant in semiconductor electronic devices, and the optoelectronic compound gallium arsenide is the most common semiconductor in use after doped silicon. Arsenic and its compounds, especially the trioxide, are used in the production of pesticides (treated wood products), herbicides, and insecticidesSony VPCCA15FG/D battery. These applications are declining, however.[6]
Arsenic is notoriously poisonous to multicellular life, although a few species of bacteria are able to use arsenic compounds as respiratory metabolites. Arsenic contamination of groundwater is a problem that affects millions of people across the world.
Characteristics
Physical characteristics
Crystal structure common to Sb, AsSb and grey As
The three most common allotropes are metallic grey, yellow and black arsenic, with grey being the most common. Sony VPCCA15FG/B batteryGrey arsenic (α-As, space group R3m No. 166) adopts a double-layered structure consisting of many interlocked ruffled six-membered rings. Because of weak bonding between the layers, grey arsenic is brittle and has a relatively low Mohs hardness of 3.5. Nearest and next-nearest neighbors form a distorted octahedral complex, with the three atoms in the same double-layer being slightly closer than the three atoms in the next.[8] This relatively close packing leads to a high density of 5.73 g/cm3. Sony VPCCA15FG battery Grey arsenic is a semimetal, but becomes a semiconductor with a bandgap of 1.2–1.4 eV if amorphized.[10] Yellow arsenic is soft and waxy, and somewhat similar to tetraphosphorus (P4). Both have four atoms arranged in a tetrahedral structure in which each atom is bound to each of the other three atoms by a single bond. This unstable allotrope, being molecularSony VPCCA15FF/W battery, is the most volatile, least dense and most toxic. Solid yellow arsenic is produced by rapid cooling of arsenic vapour, As4. It is rapidly transformed into the grey arsenic by light. The yellow form has a density of 1.97 g/cm3.[9] Black arsenic is similar in structure to red phosphorus.[9]
Isotopes
Main article: Isotopes of arsenic
Naturally occurring arsenic is composed of one stable isotope, 75As.[11] 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 daysSony VPCCA15FF/P battery. Isotopes that are lighter than the stable 75As tend to decay by β+ decay, and those that are heavier tend to decay by β- 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.[11]
Chemistry
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 hammerSony VPCCA15FF/L battery. Arsenic (and some arsenic compounds) sublimes upon heating at atmospheric pressure, converting directly to a gaseous form without an intervening liquid state at 887 K (614 °C).[2] The triple point is 3.63 MPa and 1,090 K (820 °C).[9][2] Arsenic makes arsenic acid with concentrated nitric acid, arsenious acid with dilute nitric acid, and arsenic trioxide with concentrated sulfuric acidSony VPCCA15FF/G battery.
Arsenic burning in the air
Compounds
See also Category: Arsenic compounds
Arsenic compounds resemble in some respects those of phosphorus, which occupies the same group (column) of the periodic table. Arsenic is less commonly observed in the pentavalent state, however. The most common oxidation states for arsenic are: −3 in the arsenides, such as alloy-like intermetallic compounds; and +3 in the arsenites, arsenates(III), and most organoarsenic compoundsSony VPCCA15FF/B battery. Arsenic also bonds readily to itself as seen in the square As3−
4 ions in the mineral skutterudite. In the +3 oxidation state, arsenic is typically pyramidal, owing to the influence of the lone pair of electrons.
[edit]Inorganic
Arsenic forms colorless, odorless, crystalline oxides As2O3 ("white arsenic") and As2O5, which are hygroscopic and readily soluble in water to form acidic solutions. Arsenic(V) acid is a weak acid. Its salts are called arsenates, which is the basis of arsenic contamination of groundwater, a problem that affects many peopleSony VPCCA15FA/W battery. Man-made arsenates include Paris Green (copper(II) acetoarsenite), calcium arsenate, and lead hydrogen arsenate. The latter three have been used as agricultural insecticides and poisons.
The protonation steps between the arsenate and arsenic acid are similar to those between phosphate and phosphoric acid. Unlike phosphorus acid, arsenous acid is genuinely tribasic, with the formula As(OH)3Sony VPCCA15FA/P battery.
A broad variety of sulfur compounds of arsenic are known. Orpiment (As2S3) and realgar (As4S4) are somewhat abundant and were formerly used as painting pigments. In As4S10, arsenic has a formal oxidation state of +2 in As4S4, which features As-As bonds so that the total covalency of As is still in fact three. Sony VPCCA15FA/L battery
The trifluoride, trichloride, tribromide, and triiodide of arsenic(III) are well known, whereas only Arsenic pentafluoride (AsF5) is the only important pentahalide. Again reflecting the lower stability of the 5+ oxidation state, the pentachloride is stable only below −50 °C.[9]
Organoarsenic compounds
Main article: organoarsenic chemistry
Trimethylarsine
A large variety of organoarsenic compounds are known. Several were developed as chemical warfare agents during World War I, including vesicants such as lewisite and vomiting agents such as adamsite. Cacodylic acidSony VPCCA15FA/G battery, which is of historic and practical interest, arises from the methylation of arsenic trioxide, a reaction that has no analogy in phosphorus chemistry.
Alloys
Arsenic is used as the group 5 element in the III-V semiconductors gallium arsenide, indium arsenide, and aluminium arsenide. The valence electron count of GaAs is the same as a pair of Si atoms, but the band structure is completely different, which results distinct bulk properties. Other arsenic alloys include the II-IV semiconductor cadmium arsenideSony VPCCA15FA/B battery.
Occurrence and production
See also: Arsenide minerals and Arsenate minerals
A large sample of native arsenic
Minerals with the formula MAsS and MAs2 (M = Fe, Ni, Co) are the dominant commercial sources of arsenic, together with realgar (an arsenic sulfide mineral) and native arsenic. An illustrative mineral is arsenopyrite (FeAsS), which is structurally related to iron pyrite. Many minor As-containing minerals are knownSony VAIO VPCF135Z1E/B battery. Arsenic also occurs in various organic forms in the environment.[17] Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolized to a less toxic form of arsenic through a process of methylation.[18]
Other naturally occurring pathways of exposure include volcanic ash, weathering of arsenic-containing minerals and ores, and dissolved in groundwater. It is also found in food, water, soil, and air. Sony VAIO VPCF137HG/BI batteryArsenic is absorbed by all plants, but is more concentrated in leafy vegetables, rice, apple and grape juice, and seafood.[20] An additional route of exposure is through inhalation.
Arsenic output in 2006
In 2005, China was the top producer of white arsenic with almost 50% world share, followed by Chile, Peru, and Morocco, according to the British Geological Survey and the United States Geological Survey.[22] Most operations in the US and Europe have closed for environmental reasons. The arsenic is recovered mainly as a side product from the purification of copperSony VAIO VPCF136FG/BI battery. Arsenic is part of the smelter dust from copper, gold, and lead smelters.[23]
On roasting in air of arsenopyrite, arsenic sublimes as arsenic(III) oxide leaving iron oxides,[17] while roasting without air results in the production of metallic arsenic. Further purification from sulfur and other chalcogens is achieved by sublimation in vacuum or in a hydrogen atmosphere or by distillation from molten lead-arsenic mixture. Sony VAIO VPCF135FG/B battery
History
Realgar
Alchemical symbol for arsenic
The word arsenic was borrowed from the Syriac word ܠܐ ܙܐܦܢܝܐ (al) zarniqa [25] and the Persian word زرنيخ Zarnikh, meaning "yellow orpiment", into Greek as arsenikon (Αρσενικόν). It is also related to the similar Greek word arsenikos (Αρσενικός), meaning "masculine" or "potent". The word was adopted in Latin arsenicum and Old French arsenicSony VAIO VPCF127HG/BI battery, from which the English word arsenic is derived.[25] Arsenic sulfides (orpiment, realgar) and oxides have been known and used since ancient times.[26] Zosimos (circa 300 AD) describes roasting sandarach (realgar) to obtain cloud of arsenic (arsenious oxide), which he then reduces to metallic arsenic.[27] 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 presenceSony VAIO VPCF11Z1E battery. (Another less sensitive but more general test is the Reinsch test.) Owing 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.[28]
During the Bronze Age, arsenic was often included in bronze, which made the alloy harder (so-called "arsenical bronze")Sony VAIO VPCF11S1E battery.Albertus Magnus (Albert the Great, 1193–1280) is believed to have been the first to isolate the element in 1250 by heating soap together with arsenic trisulfide.[5] In 1649, Johann Schröder published two ways of preparing arsenic.[31]
Cadet's fuming liquid (impure cacodyl), often claimed as the first synthetic organometallic compound, was synthesized in 1760 by Louis Claude Cadet de Gassicourt by the reaction of potassium acetate with arsenic trioxide. Sony VAIO VPCF11M1E/H battery
In the Victorian era, "arsenic" ("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. Sony VAIO VPCF11M1E battery
Applications
Agricultural
Roxarsone is a controversial arsenic compound used as a nutritional supplement for chickens
The toxicity of arsenic to insects, bacteria and fungi led to its use as a wood preservative.[34] In the 1950s a process of treating wood with chromated copper arsenate (also known as CCA or Tanalith) was invented, and for decades this treatment was the most extensive industrial use of arsenicSony VAIO VPCF11JFX/B battery. An increased appreciation of the toxicity of arsenic resulted in a ban for the use of CCA in consumer products; the European Union and United States initiated this process in 2004.[35][36] CCA remains in heavy use in other countries however, e.g. Malaysian rubber plantations.[6]
Arsenic was also used in various agricultural insecticides, termination and poisons. For example, lead hydrogen arsenate was a common insecticide on fruit treesSony VAIO VPCF119FJ battery,[37] but contact with the compound sometimes resulted in brain damage among those working the sprayers. In the second half of the 20th century, monosodium methyl arsenate (MSMA) and disodium methyl arsenate (DSMA) – less toxic organic forms of arsenic – have replaced lead arsenate in agriculture.
Arsenic is still added to animal food, in particular in the U.S. as a method of disease prevention and growth stimulationSony VAIO VPCF119FC/BI battery. One example is roxarsone, which is used as a broiler starter by about 70% of the broiler growers since 1995.[40] The Poison-Free Poultry Act of 2009 proposes to ban the use of roxarsone in industrial swine and poultry production.[41]
[edit]Medical use
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 trypanosomiasisSony VAIO VPCF119FC battery, 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.[42] It was also used as Fowler's solution in psoriasis.[43] Recently new research has been done in locating tumours using arsenic-74 (a positron emitter) Sony VAIO VPCF117HG/BI battery. 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 body tends to transport iodine to the thyroid gland producing a lot of noise.[44]
In subtoxic doses, soluble arsenic compounds act as stimulants, and were once popular in small doses as medicine by people in the mid-18th century. Sony VAIO VPCF116FG/BI battery
Alloys
The main use of metallic arsenic is for alloying with lead. Lead components in car batteries are strengthened by the presence of a few percent of arsenic.[6] The dezincification can be strongly reduced by adding arsenic to brass a copper zinc alloy.<ref<"Dealloying".</ref> Gallium arsenide is an important semiconductor material, used in integrated circuits. Circuits made from GaAs are much faster (but also much more expensive) Sony VAIO VPCF115FG/B battery 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.[6]
Military
After World War I, the United States built up a stockpile of 20000tons of lewisite (ClCH=CHAsCl2), a chemical weapon that is a vesicant (blister agent) and lung irritant. The stockpile was neutralized with bleach and dumped into the Gulf of Mexico after the 1950s.[45] During the Vietnam War the United States used Agent Blue, a mixture of sodium cacodylate and its acid form, as one of the rainbow herbicides to deprive the Vietnamese of valuable crops. Sony VAIO VPCF21ZHJ battery
[edit]Other uses
Copper acetoarsenite was used as a green pigment known under many 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.[48]
Also used in bronzing and pyrotechnics.
Up to 2% of arsenic is used in lead alloys for lead shots and bullets.[49]
Arsenic is added in small quantities to alpha-brass to make it dezincification resistant. This grade of brass is used to make plumbing fittings or other items that are in constant contact with water. Sony VAIO VPCF21Z1E/BI battery
Arsenic is also used for taxonomic sample preservation.
Until recently arsenic was used in optical glass. Modern glass manufacturers, under pressure from environmentalists, have removed it, along with lead.[51]
Biological role
Bacteria
Arsenobetaine
Some species of bacteria obtain their energy by oxidizing various fuels while reducing arsenate to arsenite. Under oxidative environmental conditions some bacteria use arsenite, which is oxidized to arsenate as fuel for their metabolism.[52] The enzymes involved are known as arsenate reductases (Arr) Sony VAIO VPCF21AHJ battery.
In 2008, bacteria were discovered that employ a version of photosynthesis in the absence of oxygen with arsenites as electron donors, producing arsenates (just as ordinary photosynthesis uses water as electron donor, producing molecular oxygen). Researchers conjecture that, over the course of history, these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thriveSony VAIO VPCF21AGJ battery. 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.[53]
[edit]Heredity
Arsenic has been linked to epigenetic changes that are heritable changes in gene expression that occur without changes in DNA sequence and include DNA methylation, histone modification, and RNA interferenceSony VAIO VPCF21AFJ battery. Toxic levels of arsenic cause significant DNA hypermethylation of tumour suppressor genes p16 and p53, thus increasing risk of carcinogenesis. These epigenetic events have been observed in in vitro studies with human kidney cells and in vivo tests with rat liver cells and peripheral blood leukocytes in humans. Sony VAIO VPCF219FJ/BI battery Inductive coupled plasma mass spectrometry (ICP-MS) is used to detect precise levels of intracellular arsenic and its other bases involved in epigenetic modification of DNA.[55] Studies investigating arsenic as an epigenetic factor will help in developing precise biomarkers of exposure and susceptibility.
The Chinese brake fern (Pteris vittata) hyperaccumulates arsenic present in the soil into its leaves and has a proposed use in phytoremediation. Sony VAIO VPCF217HG/BI battery
[edit]As a building block of life
A NASA-funded astrobiology research team claimed on December 2, 2010 that the microbe strain GFAJ-1 of the Gammaproteobacteria (designated Halomonadaceae) group has the ability to substitute arsenic for at least part of the phosphorus in the molecules of its cells, including DNA and ATP. Bacteria from Mono Lake, a naturally arsenic-rich site in CaliforniaSony VAIO VPCF217HG battery, were cultured in an environment high in arsenic but low in phosphorus. This finding has faced strong criticism from the scientific community; many scientists have argued that there is no evidence that arsenic is actually incorporated into biomolecules.[59][60]
[edit]Biomethylation
Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolised through a process of methylation.[61] For example, the mold Scopulariopsis brevicaulis produce significant amounts of trimethylarsine if inorganic arsenic is present. Sony VAIO VPCF14ZHJ batteryThe 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. Sony VAIO VPCF14AHJ battery
Environmental issues
Occurence in drinking water
Main article: Arsenic contamination of groundwater
Widespread arsenic contamination of groundwater has led to a massive epidemic of arsenic poisoning in Bangladesh[64] and neighbouring countries. As of this writing, 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 billionSony VAIO VPCF14AGJ battery. However, a study of cancer rates in Taiwan[65] 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, owing 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 centurySony VAIO VPCF14AFJ battery. This program was designed to prevent drinking of bacteria-contaminated surface waters, but failed to test for arsenic in the groundwater. Many other countries and districts in Southeast Asia, such as Vietnam and Cambodia have geological environments conducive to generation of high-arsenic groundwaters. Arsenicosis was reported in Nakhon Si Thammarat, Thailand in 1987Sony VAIO VPCF149FJ/BI battery, 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 owing to the use of bottled water.[66]
In the United States, arsenic is most commonly found in the ground waters of the southwest.[67] Parts of New England, Michigan, Wisconsin, Minnesota and the Dakotas are also known to have significant[clarification needed] concentrations of arsenic in ground water. Increased levels of skin cancer have been associated with arsenic exposure in WisconsinSony VAIO VPCF148FJ/B battery, even at levels below the 10 part per billion drinking water standard.[68] According to a recent film funded by the US Superfund, millions of private wells have unknown arsenic levels, and in some areas of the US, over 20% of wells may contain levels that exceed established limits.[69]
Low-level exposure to arsenic at concentrations found commonly in US drinking water compromises the initial immune response to H1N1 or swine flu infection according to NIEHS-supported scientistsSony VAIO VPCF13ZHJ battery. The study, conducted in laboratory mice, suggests that people exposed to arsenic in their drinking water may be at increased risk for more serious illness or death in response to infection from the virus.[70]
Epidemiological evidence from Chile shows a dose-dependent connection between chronic arsenic exposure and various forms of cancer, in particular when other risk factors, such as cigarette smoking, are presentSony VAIO VPCF13Z0E/B battery. These effects have been demonstrated to persist below 50 parts per billion.[71]
Analyzing multiple epidemiological studies on inorganic arsenic exposure suggests a small but measurable risk increase for bladder cancer at 10 parts per billion.[72] According to Peter Ravenscroft of the Department of Geography at the University of Cambridge,[73] roughly 80 million people worldwide consume between 10 and 50 parts per billion arsenic in their drinking waterSony VAIO VPCF13M8E/B battery. 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 remediationSony VAIO VPCF13AHJ battery.
Early (1973) evaluations of the removal of dissolved arsenic by drinking water treatment processes demonstrated that arsenic is very effectively removed by co-precipitation with either iron or aluminum oxides. The use of iron as a coagulant, in particular, was found to remove arsenic with efficiencies exceeding 90%. 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) Sony VAIO VPCF13AGJ battery. A team of European and Indian scientists and engineers have set up six arsenic treatment plants in West Bengal based on in-situ remediation method (SAR Technology). This technology does not use any chemicals and arsenic is left as an insoluble form (+5 state) in the subterranean zone by recharging aerated water into the aquifer and thus developing an oxidation zone to support arsenic oxidizing micro-organismsSony VAIO VPCF13AFJ battery. This process does not produce any waste stream or sludge and is relatively cheap.[76]
Another effective and inexpensive method to remove arsenic from contaminated well water is to sink wells 500 feet or deeper to reach purer waters. A recent 2011 study funded by the U.S. National Institute of Environmental Health Sciences' Superfund Research Program shows that deep sediments can remove arsenic and take it out of circulationSony VAIO VPCF138FJ/BI battery. Through this process called adsorption in which arsenic sticks to the surfaces of deep sediment articles, arsenic can be naturally removed from well water.[77]
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. Sony VAIO VPCF138FJ/B battery
Epidemiological studies have suggested a correlation between chronic consumption of drinking water contaminated with arsenic and the incidence of all leading causes of mortality.[citation needed] The literature provides reason to believe arsenic exposure is causative in the pathogenesis of diabetes. Sony VAIO VPCF138FC/BI battery
Hungarian engineer László Schremmer has recently discovered that by the use of chaff-based filters it is possible to reduce the arsenic content of water to 3 microgram/litre. This is especially important in areas where the potable water is provided by filtering the water extracted from the underground aquifer. Sony VAIO VPCF135FG battery
Wood preservation in the US
As of 2002, US-based industries consumed 19,600 metric tons of arsenic. Ninety percent of this was used for treatment of wood with chromated copper arsenate (CCA). In 2007, 50% of the 5,280 metric tons of consumption was still used for this purpose. In the United States, the use of arsenic in consumer products was discontinued for residentialSony VAIO VPCF12AHJ battery, and general consumer construction on December 31, 2003 and alternative chemicals are now used, such as Alkaline Copper Quaternary, borates, copper azole, cyproconazole, and propiconazole.[81]
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 countriesSony VAIO VPCF12AGJ battery, 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 timberSony VAIO VPCF12AFJ battery. 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 worldSony VAIO VPCF129FJ/BI battery; there is also concern in some quarters about the widespread landfill disposal of such timber.
[edit]Mapping of industrial releases in the US
One tool that maps releases of arsenic to particular locations in the United States[82] and also provides additional information about such releases is TOXMAP. TOXMAP is a Geographic Information System (GIS) from the Division of Specialized Information Services of the United States National Library of Medicine (NLM) that uses maps of the United States to help users visually explore data from the United States Environmental Protection Agency'sSony VAIO VPCF128FJ/B battery (EPA) Toxics Release Inventory and Superfund Basic Research Programs. TOXMAP is a resource funded by the US Federal Government. TOXMAP's chemical and environmental health information is taken from NLM's Toxicology Data Network (TOXNET)[83] and PubMed, and from other authoritative sources.
Toxicity and precautions
Main articles: Arsenic poisoning and Arsenic toxicity
Arsenic and many of its compounds are especially potent poisons. Many water supplies close to mines are contaminated by these poisonsSony VAIO VPCF127FJ/W battery. In the United States, the maximum allowed concentration in drinking water is 10 ppb, 5 ppb for bottled water.[84] There is no standard for food, though in 2012 press coverage prompted calls for developing such.[84] The People's Republic of China does have a food standard.[84]
Elemental arsenic and arsenic compounds are classified as "toxic" and "dangerous for the environment" in the European Union under directive 67/548/EECSony VAIO VPCF11ZHJ battery. 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 owing to its manifestation in drinking water, "the most common species being arsenate [HAsO42-; As(V)] and arsenite [H3AsO3 ; As(III)]" Sony VAIO VPCF11AHJ battery.
[edit]Treatment
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. Sony VAIO VPCF11AGJ batteryHowever the Agency for Toxic Substances and Disease Registry (ATSDR) states that the long-term effects of arsenic exposure cannot be predicted.[21] Blood, urine, hair, and nails may be tested for arsenic; however, these tests cannot foresee possible health outcomes from the exposure.[21] Excretion occurs in the urine and long-term exposure to arsenic has been linked to bladder and kidney cancer in addition to cancer of the liver, prostate, skin, lungs and nasal cavity. Sony VAIO VPCF11AFJ battery
Exposure risks and remediation
Occupational exposure and arsenic poisoning may occur in persons working in industries involving the use of inorganic arsenic and its compounds, such as 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. Sony VAIO VPCF118FJ battery
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.Sony VAIO VPCF117FJ battery
Biological mechanism
Arsenic disrupts ATP production through several mechanisms. At the level of the citric acid cycle, arsenic inhibits lipoic acid, which is a cofactor for 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, whichSony VAIO VGN-CS33H battery, it is speculated, has potential to form reactive oxygen species and oxidative stress. These metabolic interferences lead to death from multi-system organ failure, it is presumed from necrotic cell death, not apoptosis. A post mortem reveals brick-red-coloured mucosa, owing to severe haemorrhage. Although arsenic causes toxicity, it can also play a protective role. Sony VAIO VGN-CS33H/Z battery
The high affinity of arsenic(III) oxides for thiols is usually assigned as the cause of the high toxicity. Thiols, in the form of cysteine residues, are situated at the active sites of many important enzymes.
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