Chromium is valued for its highcorrosion resistance andhardness. A major development in steel production was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to formstainless steel. Stainless steel andchrome plating (electroplating with chromium) together comprise 85% of the commercial use. Chromium is also greatly valued as ametal that is able to be highlypolished while resistingtarnishing. Polished chromium reflects almost 70% of thevisible spectrum, and almost 90% ofinfrared light.[11] The name of the element is derived from theGreek word χρῶμα,chrōma, meaningcolor,[12] because many chromium compounds are intensely colored.
Industrial production of chromium proceeds fromchromite ore (mostly FeCr2O4) to produceferrochromium, an iron-chromium alloy, by means ofaluminothermic orsilicothermic reactions. Ferrochromium is then used to produce alloys such as stainless steel. Pure chromium metal is produced by a different process:roasting andleaching of chromite to separate it from iron, followed by reduction withcarbon and thenaluminium.
Trivalent chromium (Cr(III)) occurs naturally in many foods and is sold as adietary supplement, although there is insufficient evidence that dietary chromium provides nutritional benefit to people.[13][14] In 2014, theEuropean Food Safety Authority concluded that research on dietary chromium did not justify it to be recognized as an essentialnutrient.[15]
While chromium metal and Cr(III) ions are considered non-toxic, chromate and its derivatives, often called "hexavalent chromium", is toxic andcarcinogenic. According to the European Chemicals Agency (ECHA),chromium trioxide that is used in industrial electroplating processes is a "substance of very high concern" (SVHC).[16]
Gaseous chromium has a ground-stateelectron configuration of [Ar] 3d5 4s1. It is the first element in the periodic table whose configuration violates theAufbau principle. Exceptions to the principle also occur later in the periodic table for elements such ascopper,niobium andmolybdenum.[17]
Chromium is the first element in the 3d series where the 3d electrons start to sink into the core; they thus contribute less tometallic bonding, and hence the melting and boiling points and theenthalpy of atomisation of chromium are lower than those of the preceding elementvanadium. Chromium(VI) is a strongoxidising agent in contrast to themolybdenum(VI) andtungsten(VI) oxides.[18]
Chromium has a highspecular reflection in comparison to other transition metals. Ininfrared, at 425 μm, chromium has a maximum reflectance of about 72%, reducing to a minimum of 62% at 750 μm before rising again to 90% at 4000 μm.[11] When chromium is used instainless steel alloys andpolished, the specular reflection decreases with the inclusion of additional metals, yet is still high in comparison with other alloys. Between 40% and 60% of the visible spectrum is reflected from polished stainless steel.[11] The explanation on why chromium displays such a high turnout of reflectedphoton waves in general, especially the 90% in infrared, can be attributed to chromium's magnetic properties.[19] Chromium has unique magnetic properties; it is the only elemental solid that showsantiferromagnetic ordering at room temperature and below. Above 38 °C, its magnetic ordering becomesparamagnetic.[7] The antiferromagnetic properties, which cause the chromium atoms to temporarilyionize and bond with themselves, are present because the body-centric cubic's magnetic properties are disproportionate to thelattice periodicity. This is due to the magnetic moments at the cube's corners and the unequal, but antiparallel, cube centers.[19] From here, the frequency-dependentrelative permittivity of chromium, deriving fromMaxwell's equations and chromium'santiferromagnetism, leaves chromium with a high infrared and visible light reflectance.[20]
Chromium metal in air ispassivated: it forms a thin, protective surface layer of chromium oxide with thecorundum structure. Passivation can be enhanced by short contact withoxidizing acids likenitric acid. Passivated chromium is stable against acids. Passivation can be removed with a strongreducing agent that destroys the protective oxide layer on the metal. Chromium metal treated in this way readily dissolves in weak acids.[21]
The surfacechromiaCr2O3 scale, is adherent to the metal. In contrast, iron forms a more porous oxide which is weak and flakes easily and exposes fresh metal to the air, causing continuedrusting. At room temperature, the chromia scale is a few atomic layers thick, growing in thickness by outwarddiffusion of metal ions across the scale. Above 950 °C volatilechromium trioxideCrO3 forms from the chromia scale, limiting the scale thickness and oxidation protection.[22]
Chromium, unlike iron and nickel, does not suffer fromhydrogen embrittlement. However, it does suffer from nitrogenembrittlement, reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.[23]
Naturally occurring chromium is composed of four stableisotopes;50Cr,52Cr,53Cr and54Cr, with52Cr being the most abundant (83.789%natural abundance).50Cr isobservationally stable, as it is theoretically capable ofdecaying to50Ti viadouble electron capture with ahalf-life of no less than 1.3×1018 years. Twenty-fiveradioisotopes have been characterized, ranging from42Cr to70Cr; the most stable radioisotope is51Cr with a half-life of 27.7 days. All of the remainingradioactive isotopes have half-lives that are less than 24 hours and the majority less than 1 minute. Chromium also has twometastablenuclear isomers.[9] The primarydecay mode before the most abundant stable isotope,52Cr, iselectron capture and the primary mode after isbeta decay.[9]
53Cr is theradiogenic decay product of53Mn (half-life 3.74 million years).[24] Chromium isotopes are typically collocated (and compounded) withmanganese isotopes. This circumstance is useful inisotope geology. Manganese-chromium isotope ratios reinforce the evidence from26Al and107Pd concerning the early history of theSolar System. Variations in53Cr/52Cr and Mn/Cr ratios from several meteorites indicate an initial53Mn/55Mn ratio that suggests Mn-Cr isotopic composition must result from in-situ decay of53Mn in differentiated planetary bodies. Hence53Cr provides additional evidence fornucleosynthetic processes immediately before coalescence of the Solar System.[25]53Cr has been posited as a proxy for atmospheric oxygen concentration.[26]
Chromium is a member ofgroup 6, of thetransition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.[29][30]
Chromium(II) compounds are uncommon, in part because they readily oxidize to chromium(III) derivatives in air. Water-stablechromium(II) chlorideCrCl 2 that can be made by reducing chromium(III) chloride with zinc. The resulting bright blue solution created from dissolving chromium(II) chloride is stable at neutralpH.[21] Some other notable chromium(II) compounds includechromium(II) oxideCrO, andchromium(II) sulfateCrSO 4. Many chromium(II) carboxylates are known. The redchromium(II) acetate (Cr2(O2CCH3)4) is somewhat famous. It features a Cr-Crquadruple bond.[32]
Chromium(III) tends to formoctahedral complexes. Commercially availablechromium(III) chloride hydrate is the dark green complex [CrCl2(H2O)4]Cl. Closely related compounds are the pale green [CrCl(H2O)5]Cl2 and violet [Cr(H2O)6]Cl3. If anhydrous violet[35]chromium(III) chloride is dissolved in water, the violet solution turns green after some time as the chloride in the innercoordination sphere is replaced by water. This kind of reaction is also observed with solutions ofchrome alum and other water-soluble chromium(III) salts. Atetrahedral coordination ofchromium(III) has been reported for the Cr-centeredKeggin anion [α-CrW12O40]5–.[36]
Chromium(III) hydroxide (Cr(OH)3) isamphoteric, 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), a stable oxide with a crystal structure identical to that ofcorundum.[21]
Chromium(VI) compounds are oxidants at low or neutral pH.Chromate anions (CrO2− 4) anddichromate (Cr2O72−) anions are the principal ions at this oxidation state. They exist at an equilibrium, determined by pH:
2 [CrO4]2− + 2 H+ ⇌ [Cr2O7]2− + H2O
Chromium(VI) oxyhalides are known also and includechromyl fluoride (CrO2F2) andchromyl chloride (CrO 2Cl 2).[21] However, despite several erroneous claims,chromium hexafluoride (as well as all higher hexahalides) remains unknown, as of 2020.[37]
Chromium(VI) oxide
Sodium chromate is produced industrially by the oxidative roasting ofchromite ore withsodium carbonate. 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 ofpotassium chromate. At yet lower pH values, further condensation to more complexoxyanions of chromium is possible.
Chromium(VI) compounds in solution can be detected by adding an acidichydrogen peroxide solution. The unstable dark bluechromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adductCrO 5·OR 2.[21]
Chromic acid has the hypothetical formulaH 2CrO 4. It is a vaguely described chemical, despite many well-defined chromates and dichromates being known. The dark redchromium(VI) oxideCrO 3, the acidanhydride of chromic acid, is sold industrially as "chromic acid".[21] It can be produced by mixing sulfuric acid with dichromate and is a strong oxidizing agent.
Compounds of chromium(V) are rather rare; 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 volatilechromium(V) fluoride (CrF5). This red solid has a melting point of 30 °C and a boiling point of 117 °C. It can be prepared by treating chromium metal with fluorine at 400 °C and 200 bar pressure. 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.[38]
Compounds of chromium(IV) are slightly more common than those of chromium(V). The tetrahalides,CrF4,CrCl4, and CrBr4, can be produced by treating the trihalides (CrX 3) with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water. Organic compounds containing Cr(IV) state such as chromium tetrat-butoxide are also known.[39]
Most chromium(I) compounds are obtained solely by oxidation of electron-rich,octahedral chromium(0) complexes. Other chromium(I) complexes containcyclopentadienyl ligands. As verified byX-ray diffraction, a Cr-Crquintuple bond (length 183.51(4) pm) has also been described.[40] 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 is the 21st mostabundant element in Earth's crust[41] with an average concentration of 100 ppm. Chromium compounds are found in the environment from theerosion of chromium-containing rocks, and can be redistributed by volcanic eruptions. Typical background concentrations of chromium in environmental media are: atmosphere <10 ng/m3; soil <500 mg/kg; vegetation <0.5 mg/kg; freshwater <10 μg/L; seawater <1 μg/L; sediment <80 mg/kg.[42] Chromium is mined as chromite (FeCr2O4) ore.[43]
About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, about a third in Kazakhstan,[44] while India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.[45] Although rare, deposits ofnative chromium exist.[46][47] TheUdachnaya Pipe in Russia produces samples of the native metal. This mine is akimberlite pipe, rich indiamonds, and thereducing environment helped produce both elemental chromium and diamonds.[48]
The relation between Cr(III) and Cr(VI) strongly depends onpH andoxidative properties of the location. In most cases, Cr(III) is the dominating species,[27] but in some areas, the ground water can contain up to 39 μg/L of total chromium, of which 30 μg/L is Cr(VI).[49]
Theancient Chinese are credited with the first ever use of chromium to preventrusting. Modern archaeologists discovered that bronze-tippedcrossbow bolts at thetomb of Qin Shi Huang showed no sign of corrosion after more than 2,000 years, because they had been coated in chromium.[50][51] In multipleWarring States period tombs, sharpjians and other weapons were also found to be coated with 10 to 15 micrometers of chromium oxide, which left them in pristine condition to this day.[52] Chromium was not used anywhere else until the experiments of French pharmacist and chemistLouis Nicolas Vauquelin (1763–1829) in the late 1790s.[53]
Chromium minerals as pigments came to the attention of the west in the eighteenth century. On 26 July 1761,Johann Gottlob Lehmann found an orange-red mineral in theBeryozovskoye mines in theUral Mountains which he namedSiberian red lead.[54][55] Though misidentified as alead compound withselenium andiron components, the mineral was in factcrocoite with a formula of PbCrO4.[56] In 1770,Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that was discovered to possess useful properties as apigment inpaints. After Pallas, the use of Siberian red lead as a paint pigment began to develop rapidly throughout the region.[57] Crocoite would be the principal source of chromium in pigments until the discovery ofchromite many years later.[58]
The red color of rubies is due to trace amounts of chromium within thecorundum.
In 1794,Louis Nicolas Vauquelin received samples of crocoiteore. He producedchromium trioxide (CrO3) by mixing crocoite withhydrochloric acid.[56] In 1797, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven, for which he is credited as the one who truly discovered the element.[59][60] Vauquelin was also able to detect traces of chromium in preciousgemstones, such asruby andemerald.[56][61]
During the nineteenth century, chromium was primarily used not only as a component of paints, but intanning salts as well. For quite some time, the crocoite found in Russia was the main source for such tanning materials. In 1827, a larger chromite deposit was discovered nearBaltimore, United States, which quickly met the demand for tanning salts much more adequately than the crocoite that had been used previously.[62] This made the United States the largest producer of chromium products until the year 1848, when larger deposits of chromite were uncovered near the city ofBursa, Turkey.[43] With the development of metallurgy and chemical industries in the Western world, the need for chromium increased.[63]
Chromium is also famous for its reflective, metallic 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 byelectroplating. Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924.[64]
Piece of chromium produced withaluminothermic reactionWorld production trend of chromiumChromium, remelted in a horizontal arczone-refiner, showing large visible crystal grains
Approximately 28.8 million metric tons (Mt) of marketable chromite ore was produced in 2013, and converted into 7.5 Mt of ferrochromium.[45] According to John F. Papp, writing for the USGS, "Ferrochromium is the leading end use of chromite ore, [and] stainless steel is the leading end use of ferrochromium."[45]
The largest producers of chromium ore in 2013 have been South Africa (48%), Kazakhstan (13%), Turkey (11%), and India (10%), with several other countries producing the rest of about 18% of the world production.[45]
The two main products of chromium ore refining areferrochromium 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 inelectric arc furnace or in smaller smelters with eitheraluminium orsilicon in analuminothermic reaction.[65]
For the production of pure chromium, the iron must be separated from the chromium in a two step roasting and leaching process. The chromite ore is heated with a mixture ofcalcium carbonate andsodium 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 thechromates and leaves the insoluble iron oxide. The chromate is converted bysulfuric acid into the dichromate.[65]
The creation of metal alloys account for 85% of the available chromium's usage. The remainder of chromium is used in thechemical,refractory, andfoundry industries.[67]
The strengthening effect of forming stable metal carbides at grain boundaries, and the strong increase in corrosion resistance made chromium an important alloying material for steel.High-speed tool steels contain 3–5% chromium.Stainless steel, the primary corrosion-resistant metal alloy, is formed when chromium is introduced toiron in concentrations above 11%.[68] For stainless steel's formation, ferrochromium is added to the molten iron. Also, nickel-based alloys have increased strength due to the formation of discrete, stable, metal, carbide particles at the grain boundaries. For example,Inconel 718 contains 18.6% chromium. Because of the excellent high-temperature properties of these nickelsuperalloys, they are used injet engines andgas turbines in lieu of common structural materials.[69]ASTM B163 relies on chromium for condenser and heat-exchanger tubes, whilecastings with high strength at elevated temperatures that contain chromium are standardised with ASTM A567.[70]AISI type 332 is used where high temperature would normally causecarburization,oxidation orcorrosion.[71]Incoloy 800 "is capable of remaining stable and maintaining itsaustenitic structure even after long time exposures to high temperatures".[72]Nichrome is used as resistance wire for heating elements in things liketoasters and space heaters. These uses make chromium astrategic material. Consequently, during World War II, U.S. road engineers were instructed to avoid chromium in yellow road paint, as it "may become a critical material during the emergency".[73] The United States likewise considered chromium "essential for the German war industry" and made intense diplomatic efforts to keep it out of the hands ofNazi Germany.[74]
Decorative chrome plating on a motorcycle
The high hardness and corrosion resistance of unalloyed chromium makes it a reliable metal for surface coating; it is still the most popular metal for sheet coating, with its above-average durability, compared to other coating metals.[75] A layer of chromium is deposited on pretreated metallic surfaces byelectroplating techniques. There are two deposition methods: thin, and thick. Thin deposition involves a layer of chromium below 1 μm thickness deposited bychrome plating, and is used for decorative surfaces. Thicker chromium layers are deposited if wear-resistant surfaces are needed. Both methods use acidic chromate ordichromate solutions. To prevent the energy-consuming change in oxidation state, the use of chromium(III) sulfate is under development; for most applications of chromium, the previously established process is used.[64]
In thechromate conversion coating process, the strong oxidative properties of chromates are used to deposit a protective oxide layer on metals like aluminium, zinc, and cadmium. Thispassivation and the self-healing properties of the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating method.[76] Because of environmental and health regulations on chromates, alternative coating methods are under development.[77]
Chromic acidanodizing (or Type I anodizing) of aluminium is another electrochemical process that does not lead to the deposition of chromium, but useschromic acid as an 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.[78]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.[64]
The mineralcrocoite (which is alsolead 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 withcadmium yellow, one of the most used yellow pigments. The pigment does not photodegrade, 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 United States and for the postal services (for example, theDeutsche Post) in Europe. The use of chrome yellow has since declined due to environmental and safety concerns and was replaced by organic pigments or other alternatives that are free from lead and chromium. Other pigments that are based around chromium are, for example, the deep shade of red pigmentchrome red, which is simply lead chromate withlead(II) hydroxide (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 pre-treating aluminium aircraft bodies with a phosphoric acid solution. This used zinc tetroxychromate dispersed in a solution ofpolyvinyl 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 μm 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 ofPrussian blue andchrome yellow, while the chrome oxide green ischromium(III) oxide.[79]
Chromium oxides are also used as a green pigment in the field of glassmaking and also as a glaze for ceramics.[80] Green chromium oxide is extremelylightfast and as such is used in cladding coatings. It is also the main ingredient ininfrared reflecting paints, used by the armed forces to paint vehicles and to give them the same infrared reflectance as green leaves.[81]
Chromium(III) ions present incorundum crystals (aluminium oxide) cause them to be colored red; when corundum appears as such, it is known as aruby. If the corundum is lacking in chromium(III) ions, it is known as asapphire.[note 3] A red-colored artificial ruby may also be achieved by doping chromium(III) into artificial corundum crystals, thus making chromium a requirement for making synthetic rubies.[note 4][82] Such a synthetic ruby crystal was the basis for the firstlaser, produced in 1960, which relied onstimulated emission of light from the chromium atoms in such a crystal. Ruby has a laser transition at 694.3 nanometers, in a deep red color.[83]
Chromium(VI) salts are used for the preservation of wood. For example,chromated copper arsenate (CCA) is used intimber treatment to protect wood from decay fungi, wood-attacking insects, includingtermites, and marine borers.[84] 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 were used in 1996.[84]
Chromium(III) salts, especiallychrome alum andchromium(III) sulfate, are used in the tanning ofleather. The chromium(III) stabilizes the leather by cross linking thecollagen fibers.[85] Chromium tanned leather can contain 4–5% of chromium, which is tightly bound to the proteins.[43] Although the form of chromium used for tanning is not the toxic hexavalent variety, there remains interest in management of chromium in the tanning industry. Recovery and reuse, direct/indirect recycling,[86] and "chrome-less" or "chrome-free" tanning are practiced to better manage chromium usage.[87]
The high heat resistivity and high melting point makeschromite and chromium(III) oxide a material for high temperature refractory applications, likeblast furnaces, cementkilns, molds for the firing ofbricks and as foundry sands for thecasting of metals. 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).[65][88]
Chromic acid is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds.[96] It is prepared by dissolvingpotassium dichromate in concentrated sulfuric acid, which is then used to wash the apparatus.Sodium dichromate is sometimes used because of its higher solubility (50 g/L versus 200 g/L respectively). The use of dichromate cleaning solutions is now phased out due to the high toxicity and environmental concerns. Modern cleaning solutions are highly effective and chromium free.[97]
The possible nutritional value of chromium(III) is unproven.[101][102] Although chromium is regarded as a trace element anddietary mineral, its suspected roles in the action ofinsulin – a hormone that mediates the metabolism and storage of carbohydrate, fat, and protein – have not been adequately established.[13][14] Themechanism of its actions in the body is undefined, leaving in doubt whether chromium has a biological role in healthy people.[13][14][15]
"Chromium deficiency", involving a lack of Cr(III) in the body, or perhaps some complex of it, such asglucose tolerance factor, is not accepted as a medical condition, as it has no symptoms and healthy people do not require chromium supplementation.[13][14] Some studies suggest that the biologically active form of chromium(III) is transported in the body via an oligopeptide calledlow-molecular-weight chromium-binding substance (chromodulin), which might play a role in the insulin signaling pathway.[13][105]
The chromium content of common foods is generally low (1–13 micrograms per serving).[13][106] The chromium content of food varies widely, due to differences in soil mineral content, growing season, plantcultivar, and contamination during processing.[14][106] Chromium (andnickel) leach into food cooked in stainless steel, with the effect being largest when the cookware is new. Acidic foods that are cooked for many hours also exacerbate this effect.[107][108]
There is disagreement on chromium's status as an essential nutrient. Governmental departments from Australia, New Zealand, India, and Japan consider chromium as essential,[109][110][111] while the United States and European Food Safety Authority of the European Union do not.[13][15]
The U.S.National Academy of Medicine (NAM) updated theEstimated Average Requirements (EARs) and theRecommended Dietary Allowances (RDAs) for chromium in 2001. For chromium, there was insufficient information to set EARs and RDAs, so its needs are described as estimates forAdequate Intake (AI). From a 2001 assessment, AI of chromium for women ages 14 through 50 is 25 μg/day, and the AI for women ages 50 and above is 20 μg/day. The AIs for women who are pregnant are 30 μg/day, and for women who are lactating, the set AI is 45 μg/day. The AI for men ages 14 through 50 is 35 μg/day, and the AI for men ages 50 and above is 30 μg/day. For children ages 1 through 13, the AI increases with age from 0.2 μg/day up to 25 μg/day.[13] As for safety, the NAM setsTolerable Upper Intake Levels (ULs) for vitamins and minerals when the evidence is sufficient. In the case of chromium, there is not yet enough information, hence no UL has been established. Collectively, the EARs, RDAs, AIs, and ULs are the parameters for the nutrition recommendation system known asDietary Reference Intake (DRI).[112]
Australia and New Zealand consider chromium to be an essential nutrient, with an AI of 35 μg/day for men, 25 μg/day for women, 30 μg/day for women who are pregnant, and 45 μg/day for women who are lactating. A UL has not been set due to the lack of sufficient data.[109] India considers chromium to be an essential nutrient, with an adult recommended intake of 33 μg/day.[110] Japan also considers chromium to be an essential nutrient, with an AI of 10 μg/day for adults, including women who are pregnant or lactating. A UL has not been set.[111]
The EFSA does not consider chromium to be an essential nutrient.[15][113][114]
For U.S. food and dietary supplement labeling purposes, the amount of the substance in a serving is expressed as a percent of theDaily Value (%DV). For chromium labeling purposes, 100% of the Daily Value was 120 μg. As of 27 May 2016, the percentage of daily value was revised to 35 μg to bring the chromium intake into a consensus with the officialRecommended Dietary Allowance.[115][116] A table of the old and new adult daily values in the United States is provided atReference Daily Intake.
After evaluation of research on the potential nutritional value of chromium, the European Food Safety Authority concluded that there was no evidence of benefit by dietary chromium in healthy people, thereby declining to establish recommendations in Europe for dietary intake of chromium.[15]
Food composition databases such as those maintained by the U.S. Department of Agriculture do not contain information on the chromium content of foods.[117] A wide variety of animal and vegetable foods contain chromium.[13][112] Content per serving is influenced by the chromium content of the soil in which the plants are grown, by foodstuffs fed to animals, and by processing methods, as chromium is leached into foods if processed or cooked in stainless steel equipment.[118] One diet analysis study conducted in Mexico reported an average daily chromium intake of 30 micrograms.[119] An estimated 31% of adults in the United States consume multi-vitamin/mineral dietary supplements,[120] which often contain 25 to 60 micrograms of chromium.
Chromium is an ingredient intotal parenteral nutrition (TPN), because deficiency can occur after months of intravenous feeding with chromium-free TPN.[121] It is also added to nutritional products forpreterm infants.[122] Although the mechanism of action in biological roles for chromium is unclear, in the United States chromium-containing products are sold as non-prescription dietary supplements in amounts ranging from 50 to 1,000 μg. Lower amounts of chromium are also often incorporated into multi-vitamin/mineral supplements consumed by an estimated 31% of adults in the United States.[120] Chemical compounds used in dietary supplements include chromium chloride, chromium citrate,chromium(III) picolinate,chromium(III) polynicotinate, and other chemical compositions.[13] The benefit of supplements has not been proven.[13][123]
The notion of chromium as a potential regulator of glucose metabolism began in the 1950s when scientists performed a series of experiments controlling the diet of rats.[124] The experimenters subjected the rats to a chromium deficient diet, and witnessed an inability to respond effectively to increased levels of blood glucose. A chromium-richBrewer's yeast was provided in the diet, enabling the rats to effectively metabolize glucose, and so giving evidence that chromium may have a role in glucose management.[124]
In 2005, the U.S. Food and Drug Administration had approved a qualified health claim for chromium picolinate with a requirement for specific label wording:
"One small study suggests that chromium picolinate may reduce the risk of insulin resistance, and therefore possibly may reduce the risk of type 2 diabetes. FDA concludes, however, that the existence of such a relationship between chromium picolinate and either insulin resistance or type 2 diabetes is highly uncertain."
In other parts of the petition, the FDA rejected claims for chromium picolinate and cardiovascular disease, retinopathy or kidney disease caused by abnormally high blood sugar levels.[125] As of March 2024, this ruling on chromium remains in effect.[126]
In 2010, chromium(III) picolinate was approved by Health Canada to be used in dietary supplements. Approved labeling statements include: a factor in the maintenance of good health, provides support for healthy glucose metabolism, helps the body to metabolize carbohydrates and helps the body to metabolize fats.[127] The European Food Safety Authority approved claims in 2010 that chromium contributed to normal macronutrient metabolism and maintenance of normal blood glucose concentration, but rejected claims for maintenance or achievement of a normal body weight, or reduction of tiredness or fatigue.[128]
However, in a 2014 reassessment of studies to determine whether a Dietary Reference Intake value could be established for chromium, EFSA stated:[15]
"The Panel concludes that no Average Requirement and no Population Reference Intake for chromium for the performance of physiological functions can be defined." and
"The Panel considered that there is no evidence of beneficial effects associated with chromium intake in healthy subjects. The Panel concludes that the setting of an Adequate Intake for chromium is also not appropriate."
Given the evidence for chromium deficiency causing problems with glucose management in the context of intravenous nutrition products formulated without chromium,[121] research interest turned to whether chromium supplementation would benefit people who have type 2 diabetes but are not chromium deficient. Looking at the results from four meta-analyses, one reported a statistically significant decrease in fastingplasma glucose levels and a non-significant trend in lowerhemoglobin A1C.[129] A second reported the same,[130] a third reported significant decreases for both measures,[131] while a fourth reported no benefit for either.[132] A review published in 2016 listed 53randomized clinical trials that were included in one or more of sixmeta-analyses. It concluded that whereas there may be modest decreases in fasting blood glucose and/or HbA1C that achieve statistical significance in some of these meta-analyses, few of the trials achieved decreases large enough to be expected to be relevant to clinical outcome.[133]
Twosystematic reviews looked at chromium supplements as a mean of managing body weight in overweight and obese people. One, limited tochromium picolinate, a common supplement ingredient, reported a statistically significant −1.1 kg (2.4 lb) weight loss in trials longer than 12 weeks.[134] The other included all chromium compounds and reported a statistically significant −0.50 kg (1.1 lb) weight change.[135] Change in percent body fat did not reach statistical significance. Authors of both reviews considered the clinical relevance of this modest weight loss as uncertain/unreliable.[134][135] The European Food Safety Authority reviewed the literature and concluded that there was insufficient evidence to support a claim.[15]
Chromium is promoted as a sports performance dietary supplement, based on the theory that it potentiates insulin activity, with anticipated results of increased muscle mass, and faster recovery of glycogen storage during post-exercise recovery.[123][136][137] A review of clinical trials reported that chromium supplementation did not improve exercise performance or increase muscle strength.[138] The International Olympic Committee reviewed dietary supplements for high-performance athletes in 2018 and concluded there was no need to increase chromium intake for athletes, nor support for claims of losing body fat.[139]
Irrigation water standards for chromium are 0.1 mg/L, but some rivers inBangladesh are more than five times that amount. The standard for fish for human consumption is less than 1 mg/kg, but many tested samples were more than five times that amount.[140] Chromium, especially hexavalent chromium, is highly toxic to fish because it is easily absorbed across the gills, readily enters blood circulation, crosses cell membranes and bioconcentrates up the food chain. In contrast, the toxicity of trivalent chromium is very low, attributed to poor membrane permeability and little biomagnification.[141]
Acute and chronic exposure to chromium(VI) affects fish behavior, physiology, reproduction and survival. Hyperactivity and erratic swimming have been reported in contaminated environments. Egg hatching and fingerling survival are affected. In adult fish there are reports of histopathological damage to liver, kidney, muscle, intestines, and gills. Mechanisms include mutagenic gene damage and disruptions of enzyme functions.[141]
There is evidence that fish may not require chromium, but benefit from a measured amount in diet. In one study, juvenile fish gained weight on a zero chromium diet, but the addition of 500 μg of chromium in the form of chromium chloride or other supplement types, per kilogram of food (dry weight), increased weight gain. At 2,000 μg/kg the weight gain was no better than with the zero chromium diet, and there were increased DNA strand breaks.[142]
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.[143] Because of the specifictransport mechanisms, only limited amounts of chromium(III) enter the cells. Acute oral toxicity ranges between 50 and 150 mg/kg.[144] A 2008 review suggested that moderate uptake of chromium(III) through dietary supplements poses no genetic-toxic risk.[145] In the US, theOccupational Safety and Health Administration (OSHA) has designated an airpermissible exposure limit (PEL) in the workplace as a time-weighted average (TWA) of 1 mg/m3. TheNational Institute for Occupational Safety and Health (NIOSH) has set arecommended exposure limit (REL) of 0.5 mg/m3, time-weighted average. TheIDLH (immediately dangerous to life and health) value is 250 mg/m3.[146]
The acuteoraltoxicity forchromium(VI) ranges between 1.5 and 3.3 mg/kg.[144] 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 mechanism, by which alsosulfate andphosphate ions enter the cell. The acute toxicity of chromium(VI) is due to its strongoxidant properties. After it reaches the blood stream, it damages the kidneys, the liver and blood cells through oxidation reactions.Hemolysis,renal, and liver failure result. Aggressive dialysis can be therapeutic.[147]
Thecarcinogenity of chromate dust has been known for a long time, and in 1890 the first publication described the elevated cancer risk of workers in a chromate dye company.[148][149] Three mechanisms have been proposed to describe thegenotoxicity of chromium(VI). The first mechanism includes highly reactivehydroxyl radicals and other reactive radicals which are by products of the reduction of chromium(VI) to chromium(III). The second process includes the direct binding of chromium(V), produced by reduction in the cell, and chromium(IV) compounds to theDNA. The last mechanism attributed the genotoxicity to the binding to the DNA of the end product of the chromium(III) reduction.[150][151]
Chromium salts (chromates) are also the cause ofallergic reactions in some people. Chromates are often used to manufacture, amongst other things, leather products, paints, cement, mortar and anti-corrosives. Contact with products containing chromates can lead to allergiccontact 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.[152][153]
In 2010, theEnvironmental Working Group studied the drinking water in 35 American cities in the first nationwide study. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, withNorman, Oklahoma, at the top of list; 25 cities had levels that exceeded California's proposed limit.[155]
The more toxic hexavalent chromium form can be reduced to the less soluble trivalent oxidation state in soils by organic matter, ferrous iron, sulfides, and other reducing agents, with the rates of such reduction being faster under more acidic conditions than under more alkaline ones. In contrast, trivalent chromium can be oxidized to hexavalent chromium in soils by manganese oxides, such as Mn(III) and Mn(IV) compounds. Since the solubility and toxicity of chromium (VI) are greater that those of chromium (III), the oxidation-reduction conversions between the two oxidation states have implications for movement and bioavailability of chromium in soils, groundwater, and plants.[156]
^The melting/boiling point of transition metals are usually higher compared to the alkali metals, alkaline earth metals, and nonmetals, which is why the range of elements compared to chromium differed between comparisons
^Most common oxidation states of chromium are in bold. The right column lists a representative compound for each oxidation state.
^Any color of corundum (disregarding red) is known as a sapphire. If the corundum is red, then it is a ruby. Sapphires are not required to be blue corundum crystals, as sapphires can be other colors such as yellow and purple
^WhenCr3+ replacesAl3+ incorundum (aluminium oxide, Al2O3),pink sapphire orruby is formed, depending on the amount of chromium.
^abcArblaster, John W. (2018).Selected Values of the Crystallographic Properties of Elements. Materials Park, Ohio: ASM International.ISBN978-1-62708-155-9.
^Cr(–4) is known inNa4Cr(CO)4; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8).doi:10.1021/ic052110i.
^Cr(0) is known inCr(CO)6; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8).doi:10.1021/ic052110i.
^χρῶμαArchived 22 April 2021 at theWayback Machine, Henry George Liddell, Robert Scott,A Greek-English Lexicon, on Perseus
^abcdefghijk"Chromium". Office of Dietary Supplements, US National Institutes of Health. 2 June 2022. Retrieved17 October 2024.
^abcde"Chromium". Micronutrient Information Center, Linus Pauling Institute, Oregon State University, Corvallis. January 2024. Retrieved17 October 2024.
^abcdefghHolleman, Arnold F; Wiber, Egon; Wiberg, Nils (1985). "Chromium".Lehrbuch der Anorganischen Chemie (in German) (91–100 ed.). Walter de Gruyter. pp. 1081–1095.ISBN978-3-11-007511-3.
^"Live Chart of Nuclides".International Atomic Energy Agency – Nuclear Data Section.Archived from the original on 23 March 2019. Retrieved18 October 2018.
^abKotaś, J.; Stasicka, Z. (2000). "Chromium occurrence in the environment and methods of its speciation".Environmental Pollution.107 (3):263–283.doi:10.1016/S0269-7491(99)00168-2.PMID15092973.
^Theopold, Klaus H.; Kucharczyk, Robin R. (15 December 2011), "Chromium: Organometallic Chemistry", in Scott, Robert A. (ed.),Encyclopedia of Inorganic and Bioinorganic Chemistry, John Wiley & Sons, Ltd, pp. eibc0042,doi:10.1002/9781119951438.eibc0042,ISBN978-1-119-95143-8.
^"Chromium(III) compounds".National Pollutant Inventory. Commonwealth of Australia.Archived from the original on 22 April 2021. Retrieved8 November 2018.
^Luther, George W. (2016)."Introduction to Transition Metals".Inorganic Chemistry for Geochemistry & Environmental Sciences: Fundamentals & Applications. Hydrate (Solvate) Isomers. John Wiley & Sons. p. 244.ISBN978-1-118-85137-1.Archived from the original on 10 June 2024. Retrieved7 August 2019.
^Thaler, Eric G.; Rypdal, Kristin; Haaland, Arne; Caulton, Kenneth G. (1 June 1989). "Structure and reactivity of chromium(4+) tert-butoxide".Inorganic Chemistry.28 (12):2431–2434.doi:10.1021/ic00311a035.ISSN0020-1669.
^Emsley, John (2001)."Chromium".Nature's Building Blocks: An A–Z Guide to the Elements. Oxford, England, UK: Oxford University Press. pp. 495–498.ISBN978-0-19-850340-8.
^abcNational Research Council (U.S.). Committee on Biologic Effects of Atmospheric Pollutants (1974).Chromium. National Academy of Sciences.ISBN978-0-309-02217-0.Archived from the original on 10 June 2024. Retrieved11 March 2019.
^Fleischer, Michael (1982)."New Mineral Names"(PDF).American Mineralogist.67:854–860.Archived(PDF) from the original on 26 September 2021. Retrieved16 February 2009.
^Gonzalez, A. R.; Ndung'u, K.; Flegal, A. R. (2005). "Natural Occurrence of Hexavalent Chromium in the Aromas Red Sands Aquifer, California".Environmental Science and Technology.39 (15):5505–5511.Bibcode:2005EnST...39.5505G.doi:10.1021/es048835n.PMID16124280.
^Cotterell, Maurice (2004).The Terracotta Warriors: The Secret Codes of the Emperor's Army. Inner Traditions / Bear & Co. p. 102.ISBN1-59143-033-X.
^Luo, Zhewen (1993).China's Imperial Tombs and Mausoleums. Foreign Languages Press. p. 44.ISBN7-119-01619-9.
^Jacques Guertin; James A. Jacobs; Cynthia P. Avakian (2005).Chromium(VI) Handbook. CRC Press.ISBN978-1-56670-608-7.
^Meyer, RJ (1962).Chrom: Teil A – Lieferung 1. Geschichtliches · Vorkommen · Technologie · Element bis Physikalische Eigenschaften (in German). Berlin, Heidelberg: Springer Berlin Heidelberg Imprint Springer.ISBN978-3-662-11865-8.OCLC913810356.
^Casteran, Rene."Chromite mining".Oregon Encyclopedia. Portland State University and the Oregon Historical Society.Archived from the original on 26 September 2021. Retrieved1 October 2018.
^Glenn, William (1895)."Chrome in the Southern Appalachian Region".Transactions of the American Institute of Mining, Metallurgical and Petroleum Engineers.25: 482.Archived from the original on 26 September 2021. Retrieved8 January 2019.
^van der Krogt, Peter."Chromium".Archived from the original on 26 September 2021. Retrieved24 August 2008.
^Ortt, Richard A Jr."Soldier's Delight, Baltimore Country".Maryland Department of Natural Resources. Maryland Geological Survey.Archived from the original on 3 May 2021. Retrieved13 May 2019.
^abcdPapp, John F. & Lipin, Bruce R. (2006)."Chromite".Industrial Minerals & Rocks: Commodities, Markets, and Uses (7th ed.). SME.ISBN978-0-87335-233-8.Archived from the original on 10 June 2024. Retrieved5 June 2020.
^Edwards, J (1997).Coating and Surface Treatment Systems for Metals. Finishing Publications Ltd. and ASMy International. pp. 66–71.ISBN978-0-904477-16-0.
^Zhao J, Xia L, Sehgal A, Lu D, McCreery RL, Frankel GS (2001). "Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3".Surface and Coatings Technology.140 (1):51–57.doi:10.1016/S0257-8972(01)01003-9.hdl:1811/36519.
^Gettens, Rutherford John (1966)."Chrome yellow".Painting Materials: A Short Encyclopaedia. Courier Dover Publications. pp. 105–106.ISBN978-0-486-21597-6.
^Gerd Anger et al. "Chromium Compounds" Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim.doi:10.1002/14356007.a07_067
^abHingston, J; Collins, CD; Murphy, RJ; Lester, JN (2001). "Leaching of chromated copper arsenate wood preservatives: a review".Environmental Pollution.111 (1):53–66.doi:10.1016/S0269-7491(00)00030-0.PMID11202715.
^Brown, EM (1997). "A Conformational Study of Collagen as Affected by Tanning Procedures".Journal of the American Leather Chemists Association.92:225–233.
^Sreeram, K.; Ramasami, T. (2003). "Sustaining tanning process through conservation, recovery and better utilization of chromium".Resources, Conservation and Recycling.38 (3):185–212.Bibcode:2003RCR....38..185S.doi:10.1016/S0921-3449(02)00151-9.
^Qiang, Taotao; Gao, Xin; Ren, Jing; Chen, Xiaoke; Wang, Xuechuan (9 December 2015). "A Chrome-Free and Chrome-Less Tanning System Based on the Hyperbranched Polymer".ACS Sustainable Chemistry & Engineering.4 (3):701–707.doi:10.1021/acssuschemeng.5b00917.
^Vincent, JB (2013). "Chromium: Is It Essential, Pharmacologically Relevant, or Toxic?". In Astrid Sigel; Helmut Sigel; Roland KO Sigel (eds.).Interrelations between Essential Metal Ions and Human Diseases. Metal Ions in Life Sciences. Vol. 13. Springer. pp. 171–198.doi:10.1007/978-94-007-7500-8_6.ISBN978-94-007-7499-5.PMID24470092.
^Maret, Wolfgang (2019). "Chapter 9. Chromium Supplementation in Human Health, Metabolic Syndrome, and Diabetes". In Sigel, Astrid; Freisinger, Eva; Sigel, Roland K. O.; Carver, Peggy L. (eds.).Essential Metals in Medicine:Therapeutic Use and Toxicity of Metal Ions in the Clinic; In: Metal Ions in Life Sciences. Vol. 19. Berlin: de Gruyter GmbH. pp. 231–251.doi:10.1515/9783110527872-015.ISBN978-3-11-052691-2.PMID30855110.{{cite book}}:|journal= ignored (help)
^Flint GN; Packirisamy S (1997). "Purity of food cooked in stainless steel utensils".Food Additives and Contaminants.14 (2):115–126.doi:10.1080/02652039709374506.PMID9102344.
^ab"Chromium".Nutrient Reference Values for Australia and New Zealand. 2014.Archived from the original on 7 October 2021. Retrieved4 October 2018.
^ab"DRIs for Chromium (μg/day)"(PDF).Overview of Dietary Reference Intakes for Japanese. 2015. p. 41.Archived(PDF) from the original on 23 April 2021. Retrieved4 October 2018.
^"USDA Food Composition Databases".United States Department of Agriculture Agricultural Research Service. April 2018.Archived from the original on 3 December 2019. Retrieved4 October 2018.
^Grijalva Haro, MI; Ballesteros Vázquez, MN; Cabrera Pacheco, RM (2001). "Chromium content in foods and dietary intake estimation in the Northwest of Mexico".Arch Latinoam Nutr (in Spanish).51 (1):105–110.PMID11515227.
^Finch, Carolyn Weiglein (February 2015). "Review of trace mineral requirements for preterm infants: What are the current recommendations for clinical practice?".Nutrition in Clinical Practice.30 (1):44–58.doi:10.1177/0884533614563353.PMID25527182.
^abVincent, John B (2010). "Chromium: Celebrating 50 years as an essential element?".Dalton Transactions.39 (16):3787–3794.doi:10.1039/B920480F.PMID20372701.
^San Mauro-Martin I, Ruiz-León AM, Camina-Martín MA, et al. (2016). "[Chromium supplementation in patients with type 2 diabetes and high risk of type 2 diabetes: a meta-analysis of randomized controlled trials]".Nutr Hosp (in Spanish).33 (1): 27.doi:10.20960/nh.v33i1.27.PMID27019254.
^abOnakpoya, I; Posadzki, P; Ernst, E (2013). "Chromium supplementation in overweight and obesity: a systematic review and meta-analysis of randomized clinical trials".Obes Rev.14 (6):496–507.doi:10.1111/obr.12026.PMID23495911.S2CID21832321.
^Lefavi RG, Anderson RA, Keith RE, Wilson GD, McMillan JL, Stone MH (1992). "Efficacy of chromium supplementation in athletes: emphasis on anabolism".International Journal of Sport Nutrition.2 (2):111–122.doi:10.1123/ijsn.2.2.111.PMID1299487.
^Vincent JB (2003). "The potential value and toxicity of chromium picolinate as a nutritional supplement, weight loss agent and muscle development agent".Sports Med.33 (3):213–230.doi:10.2165/00007256-200333030-00004.PMID12656641.S2CID9981172.
^Jenkinson DM, Harbert AJ (2008). "Supplements and sports".Am Fam Physician.78 (9):1039–1046.PMID19007050.
^abKatz, SA; Salem, H (1992). "The toxicology of chromium with respect to its chemical speciation: A review".Journal of Applied Toxicology.13 (3):217–224.doi:10.1002/jat.2550130314.PMID8326093.S2CID31117557.
^Eastmond, DA; MacGregor, JT; Slesinski, RS (2008). "Trivalent Chromium: Assessing the Genotoxic Risk of an Essential Trace Element and Widely Used Human and Animal Nutritional Supplement".Critical Reviews in Toxicology.38 (3):173–190.doi:10.1080/10408440701845401.PMID18324515.S2CID21033504.
^Newman, D. (1890). "A case of adeno-carcinoma of the left inferior turbinated body, and perforation of the nasal septum, in the person of a worker in chrome pigments".Glasgow Medical Journal.33:469–470.
^Langard, S (1990). "One Hundred Years of Chromium and Cancer: A Review of Epidemiological Evidence and Selected Case Reports".American Journal of Industrial Medicine.17 (2):189–214.doi:10.1002/ajim.4700170205.PMID2405656.
^Cohen, MD; Kargacin, B; Klein, CB; Costa, M (1993). "Mechanisms of chromium carcinogenicity and toxicity".Critical Reviews in Toxicology.23 (3):255–281.doi:10.3109/10408449309105012.PMID8260068.
^Ngan, V (2002)."Chrome Allergy". DermNet NZ.Archived from the original on 7 July 2016. Retrieved7 June 2008.
^Basketter, David; Horev, L; Slodovnik, D; Merimes, S; Trattner, A; Ingber, A (2000). "Investigation of the threshold for allergic reactivity to chromium".Contact Dermatitis.44 (2):70–74.doi:10.1034/j.1600-0536.2001.440202.x.PMID11205406.S2CID45426346.
^Baselt, Randall C (2008).Disposition of Toxic Drugs and Chemicals in Man (8th ed.). Foster City: Biomedical Publications. pp. 305–307.ISBN978-0-9626523-7-0.
^James, Bruce (1996). "The challenge of remediating chromium-contaminated soil".Environmental Science and Technology.30 (6):248A –251A.doi:10.1021/es962269h.PMID21648723.
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