Chromium

Typical wastewaters

• electroplating and metal finishing Cr(VI) as chromate and dichromate from chromic acid baths in plating rinse water; reduced then precipitated as Cr(OH)₃
• leather tanning Cr(III) from basic chromium sulfate tanning floats, precipitated as Cr(OH)₃ at pH 8 or above and recoverable
• textile dyeing with chromium mordant or chromium containing dyes Cr(III) from chrome mordant and metal complex dyeing of wool
• wool scouring total chromium, mass limited per 1000 kg of wool
• cooling tower blowdown (once chromate treated) chromate CrO₄²⁻ from chromate corrosion inhibitors; 0.2 mg/L total chromium limit
• chemical sector effluent (chromium compound production) total chromium 5.0 to 25 µg/L after treatment; may not apply where the load comes from chromium-organic compound production
• steel and pulp mills total chromium, the US EPA source list

1 · Identity

Symbol, number

Cr, 24

Oxidation states in water

+3 as Cr³⁺ and its hydroxy complexes Cr(OH)n^(3-n)+, precipitating as Cr(OH)₃ between about pH 6 and 8 (the element entry) and redissolving only in strong alkali; +6 as chromate CrO₄²⁻, hydrogen chromate HCrO₄⁻ and, at high concentration, dichromate Cr₂O₇²⁻, soluble at every pH and adsorbed poorly.

Note

The element entry already sets out the two states and the reduce-then-precipitate rule. This chapter gives the numbers: the limits, the occurrence, the analytical splits and the reactions.

2 · Occurrence in water

Natural sources

Weathering and erosion of chromium bearing rocks; high natural chromium, including Cr(VI), in groundwater from mafic and ultramafic volcanic and metamorphic rocks, ophiolite complexes and serpentine rich units (WHO background document). Uncontaminated water is usually below 1 µg/L.

Anthropogenic sources

Metal finishing and chromium plating (chromic acid baths), leather tanning with basic chromium sulfate, chrome mordant and metal complex dyeing of wool and textiles, chromate pigments and corrosion inhibitors, cooling tower blowdown (once chromate treated), steel and pulp mills (US EPA source list), landfill leachate, and chromium chemical plants in the ledger's chemical chapter. Pre-chlorination or pre-ozonation in a treatment plant can oxidise Cr(III) to Cr(VI) (WHO).

3 · Speciation

Chromium(VI) is an oxyanion: hydrogen chromate HCrO₄⁻ below about pH 6.5, chromate CrO₄²⁻ above it, dichromate only in concentrated acid solutions. It is soluble across the whole pH range, mobile in groundwater, poorly adsorbed except on iron oxides at low pH, and it is what plating and chromate baths discharge. Chromium(III) is a cation that hydrolyses, precipitates as Cr(OH)₃ between about pH 6 and 8 and is adsorbed at relatively high pH (WHO background document), so it leaves water with the sludge. The distribution between the two depends on redox potential, pH, oxidants (chlorine, ozone, manganese oxides) and reductants (organic matter, iron(II), sulfide); Cr(VI) is easily reduced by organic matter in soil (WHO). Treatment therefore reduces Cr(VI) first, or adsorbs it on iron, and precipitates Cr(III); oxidative pre-treatment in a drinking water plant can do the opposite.

Solubility

Cr(OH)₃ is the controlling solid for Cr(III), least soluble around pH 8 to 9 and redissolving in strong alkali; Cr(VI) has no controlling solid in ordinary water except barium and lead chromate at high concentration. Constants are not quoted because the sources read print none.

Hydrolysis

Cr³⁺ hydrolyses to Cr(OH)n^(3-n)+ (WHO notation), so chromium(III) salts are acidic and precipitation consumes alkalinity; HCrO₄⁻ is a weak acid with pKa about 6.5 (Stumm and Morgan chapter 3, from the chapter, not re-read).

Complexation

Cr(III) forms inert complexes with organic ligands and adsorbs on iron oxides at higher pH; Cr(VI) adsorbs on ferrihydrite, goethite and iron oxide coated sand, more strongly at low pH (WHO treatment section).

Precipitates

Cr(OH)₃ (hydroxide precipitation), chromium in ferric hydroxide sludge after reduction with iron(II), chromite and Cr₂O₃ in aged sludge, barium and lead chromate pigments.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidifying a sample for total chromium destroys the speciation: Cr(VI) samples need the alkaline buffer of EPA 218.7 and prompt analysis, and chlorine residual must be quenched or it keeps oxidising Cr(III). Cr(III) adsorbs on container walls and particles, so total chromium needs digestion of the unfiltered sample.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Chromium(VI) is more toxic than chromium(III) and is a human carcinogen by inhalation (IARC Group 1); orally it is rapidly and efficiently reduced to Cr(III) in the gut, though a proportion may remain available. In two year drinking water studies tumours of the oral cavity in rats and small intestine in mice appeared at 0.77 and 0.38 mg/kg body weight per day, attributed to a threshold mode of action; environmental Cr(VI) levels are orders of magnitude below the doses tested, so WHO kept 50 µg/L total chromium (fact sheet, 2020). Chromium(III) is an essential trace element (the element entry).

Bioaccumulation

Cr(VI) is reduced in gastric fluid (70 percent within a minute at pH 2) and Cr(III) is poorly absorbed; no bioaccumulation concern is raised in the sources read.

Ecotoxicity

US EPA aquatic life criteria (1995): chromium(VI) freshwater 16 µg/L acute and 11 µg/L chronic, saltwater 1,100 and 50 µg/L; chromium(III) freshwater 570 µg/L acute and 74 µg/L chronic, expressed as a function of hardness.

Flags

• The EU 25 µg/L value applies only from 12 January 2036; 50 µg/L until then.
• The California Cr(VI) MCL is a state standard effective 1 October 2024; no federal Cr(VI) MCL exists.
• The HCrO₄⁻ pKa and the dichromate condensation are cited to Stumm and Morgan chapter 3 from memory.
• The reduction equations are electron balances; the CWW BREF names ferrous sulfate and sulfite without printing them.
• The occurrence figures are 1980s to 2010s surveys compiled by WHO; the UCMR 3 Cr(VI) range is quoted through WHO, not from the EPA data files.
• The textile BAT-AEL footnote 8 on chromium was not read.
• The tannery pretreatment exemption and the wool scouring mass based limit are not comparable with concentration limits.
• Abu Dhabi values cover two media (marine 0.2 and 0.15 mg/L; sewer 5 mg/L); other GCC states not read.
• Tannery and plating raw effluent chromium concentrations were not read; only the limits are quoted.

Gaps

• No raw effluent chromium concentrations for tanneries, plating shops or dye houses were read; the ledger's chapters hold them.
• No solubility product for Cr(OH)₃ or adsorption constants on iron oxides were read.
• The WHO 80 to 96 percent ion exchange figure is quoted as printed without the matrix it refers to.
• EPA 218.6 and ISO 18412 are cited by number from the ZDHC methods table; their texts were not opened.
• Other GCC discharge standards (Saudi, Oman, Qatar) were not read.
• The dichromate half reaction potential, the Cr(III) hydrolysis step and the lime stoichiometry are from Stumm and Morgan chapters 3, 6 and 8 and Metcalf and Eddy chapter 6, from the chapter, not re-read.
• The manganese oxide oxidation, the chlorine oxidation of Cr(III) and the chromate ligand exchange on ferrihydrite are electron and mass balances written from mechanisms the WHO background document describes in words; no rate constants, surface constants or yields were read.

Sources

Identity

Name and symbol

Chromium, Cr

Atomic number

24 protons

Position

group 6 · period 4 · d-block · transition metal

CAS number

7440-47-3

Atomic structure

Atomic mass

51.9961 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵
[Ar] 3d⁵⁴s¹

Electrons per shell

2, 8, 13, 1

Valence electrons

6 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

2,180 K (1,906.85 °C)

Boiling point

2,944 K (2,670.85 °C)

Density

7.15 g/cm3

Appearance

silvery metallic

Thermal conductivity

93.9 W/(m·K)

Electrical resistivity

125 nΩ·m at 20 °C

Electrical conductivity

8 MS/m

Crystal structure

body-centered cubic

Molar heat capacity

23.35 J/(mol·K)

Chemical properties

Oxidation states

+6, +3, +2

Electronegativity

1.66 (Pauling Scale)

Ionisation energy

6.767 eV
1st 652.9, 2nd 1,590.6, 3rd 2,987 kJ/mol

Electron affinity

0.666 eV

Atomic radius

empirical 139, covalent 139, van der Waals 189 pm

Ionic radius

Cr²⁺ 73 low spin; Cr³⁺ 62 low spin; Cr⁴⁺ 55 low spin; Cr⁵⁺ 49 low spin; Cr⁶⁺ 44 low spin; Cr²⁺ 80 high spin pm

Reactivity

A group 6 transition metal ([Ar] 3d5 4s1) whose thin, corundum-structured oxide film passivates it against air and acids, which is why it is plated onto steel and makes steel stainless; its compounds are all coloured, in the +3 and +6 states above all.

with water

Does not react; the passivating oxide film keeps the metal bright and untarnished in water and moist air.

with oxygen, air

Passivated by a thin chromium(III) oxide layer at room temperature; on strong heating the oxide scale grows, and above 950 C volatile chromium trioxide is lost from it: $\ce{4 Cr + 3 O2 -> 2 Cr2O3}$

with acids

Passivated chromium is stable against acids, and nitric acid strengthens the passivation; once the film is removed the metal dissolves readily in hydrochloric or sulfuric acid to chromium(III) salts and hydrogen: $\ce{2 Cr + 6 HCl -> 2 CrCl3 + 3 H2}$

with halogens

Combines with fluorine at 400 C and 200 bar to the volatile pentafluoride; the trihalides CrX3 are the common halides and give tetrahalides with more halogen at elevated temperature: $\ce{2 Cr + 5 F2 -> 2 CrF5}$

Typical compounds

• Cr₂O₃ chromium(III) oxide green pigment; corundum-structured oxide, reduced by aluminium to the metal
• FeCr₂O₄ chromite the ore; refractory bricks, ferrochromium
• K₂Cr₂O₇ potassium dichromate orange oxidiser for analysis and tanning
• Na₂CrO₄ sodium chromate from roasting chromite with soda ash
• CrO₃ chromium trioxide chromic acid of electroplating; substance of very high concern
• PbCrO₄ lead chromate chrome yellow pigment

Occurrence, production and use

Crustal abundance

1.02×102 milligrams per kilogram

Oceanic abundance

3×10-4 milligrams per liter

Occurrence and sources

The principal ore is chromite, which is found in Zimbabwe, Russia, New Zealand, Turkey, Iran, Albania, Finland, Democratic Republic of Madagascar, and the Phillippines. The metal is usually produced by reducing the oxide with aluminum.

• chromite (iron chromium oxide, FeCr2O4 spinel) layered igneous complexes and podiform deposits; 95 percent of world resources in Kazakhstan and southern Africa, mined in South Africa, Kazakhstan, Turkey, India, Finland, Brazil and Zimbabwe; United States resources in the Stillwater Complex, Montana
• crocoite (lead chromate) the Siberian red lead of the 1798 discovery; a mineralogical curiosity, not an ore
• trace chromium(III) in silicates the colour of emerald and ruby; chromium is a substance discharged from base metal, iron ore and industrial mineral extraction

Extraction, production

Carbothermic smelting of chromite to ferrochromium

Chromite is reduced with carbon in an electric arc furnace to high carbon ferrochromium, the form in which chromium enters stainless steel; China is the leading ferrochromium producer. Chromium metal is made by reducing chromium(III) oxide with aluminium or silicon. Reductants are named but not the product set, so no equation is written. World chromite production about 47 million tonnes gross weight in 2024 (estimate).

Chromium(III) oxide pigment from sodium dichromate

In the SIC BREF's solid state process sodium dichromate (Na2Cr2O7) is mixed with sulphur and calcined in a rotary kiln to chromium(III) oxide, then washed and milled; the sulphur co product is not stated, so no equation. The wash water carries chromate, and the BREF lists chromium removal from waste water for the pigment sector.

Uses

Chromium is a blue-white metal that is hard, brittle and very corrosion resistant. Chromium can be polished to form a very shiny surface and is often plated to other metals to form a protective and attractive covering. Chromium is added to steel to harden it and to form stainless steel, a steel alloy that contains at least 10% chromium. Other chromium-steel alloys are used to make armor plate, safes, ball bearings and cutting tools.

Chromium forms many colorful compounds that have industrial uses. Lead chromate (PbCrO4), also known as chrome yellow, has been used as a yellow pigment in paints. Chromic oxide (Cr2O3), also known as chrome green, is the ninth most abundant compound in the earth's crust and is a widely used green pigment. Rubies and emeralds also owe their colors to chromium compounds. Potassium dichromate (K2Cr2O7) is used in the tanning of leather while other chromium compounds are used as mordants, materials which permanently fix dyes to fabrics. Chromium compounds are also used to anodize aluminum, a process which coats aluminum with a thick, protective layer of oxide. Chromite, chromium's primary ore, is used to make molds for the firing of bricks because of its high melting point, moderate thermal expansion and stable crystal structure.

Chromium is used to harden steel, manufacture stainless steel, and form many useful alloys. It is mostly used in plating to produce a hard, beautiful surface and to prevent corrosion. Chromium gives glass an emerald green color and is widely used as a catalyst.

The refractory industry uses chromite for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stability of crystalline structure.

• Stainless steel and alloys: ferrochromium for stainless and heat resisting steel, which contains at least 10.5 percent chromium; superalloys, the major strategic use; chromium plating for a mirror finish on steel and plastics chromium has no substitute in stainless steel, the leading end use; recycled stainless scrap supplied 23 percent of United States apparent consumption in 2024 (USGS)
• Textiles: chrome (mordant) dyeing of wool: the dye is applied from an acidic bath and the fibre is then chromed with dichromate in the afterchrome process, so exhausted baths carry chromium(VI) and chromium(III); 1:1 and 1:2 metal complex dyes with chromium bound inside the dye molecule; the BREF answers with low chrome afterchroming and reactive dyeing of wool without chrome (BAT 42a)
• Chemicals: chromium(III) oxide green pigments from sodium dichromate; Phillips chromium catalysts for high density polyethylene; iron oxide and chromium oxide shift catalyst in ammonia plants; chromium discharged from pigment and phosphoric acid plants
• Pharmaceuticals and fine chemicals: metallisation of azo dyes with chromium(III) oxide or chloride to form 1:1 and 1:2 chelated complexes; dichromate as an inorganic oxidant
• Leather: about 90 percent of all leather is tanned with chromium(III) salts; the effluent is toxic
• Mining: chromite extraction is grouped with iron ore in the MWEI BREF; chromium is among the substances discharged from ore processing

Safety, toxicity

Chromium compounds are toxic and should be handled with proper safeguards.

GHS classification, signal word Danger

• H317 May cause an allergic skin reaction Sensitization, Skin
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H334 May cause allergy or asthma symptoms or breathing difficulties if inhaled Sensitization, respiratory
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
• H320 Causes eye irritation Serious eye damage/eye irritation
• H341 Suspected of causing genetic defects Germ cell mutagenicity
• H371 May cause damage to organs Specific target organ toxicity, single exposure
• H400 Very toxic to aquatic life Hazardous to the aquatic environment, acute hazard
• H410 Very toxic to aquatic life with long lasting effects Hazardous to the aquatic environment, long-term hazard

Discovery and name

Discovered by

Louis Nicolas Vauquelin

Discovered

1794

First isolated

not in sources

Named by

not in sources

Origin of the name

from the Greek χρῶμα, "color", because many chromium compounds are intensely colored

Chromium is used extensively in automobile trim as chromium metal because of its shiny finish and corrosion resistance.