Cobalt

Typical wastewaters

• textile dyeing (metal complex dyes) 1:1 and 1:2 cobalt azo dye complexes leaving with the dyebath, not broken by pH adjustment; 0.05 to 0.01 mg/L by ZDHC level
• battery manufacture (cadmium subcategory, electrodeposited anodes) cobalt as a co-pollutant with cadmium, nickel and zinc; mass limited per kg of cadmium used
• nuclear reactor and fuel cycle liquid effluent cobalt-60, the activation product of steel; removed by ion exchange and reverse osmosis
• chemical sector effluent with chelating agents Co(II) and Co(III) held in solution by ammonia, cyanide, EDTA and dye ligands, escaping hydroxide precipitation
• base metal ore processing cobalt as a trace metal of ore processing effluent (MWEI BREF via the element entry) no concentration read

1 · Identity

Symbol, number

Co, 27

Oxidation states in water

+2, Co²⁺, the pink aquo ion and the state of every simple salt in water; +3 only when held in complexes (ammine, cyanide, cobalamin) or in the oxide and oxyhydroxide solids that form on manganese oxide surfaces. Cobalt-60 is a gamma emitting activation product of reactor steel, half-life about 5.3 years, and the cobalt radionuclide of drinking water regulation.

Note

The element entry has the ores, the battery and superalloy uses and the complex chemistry in outline; this chapter is the trace metal in supplies and effluents.

2 · Occurrence in water

Natural sources

Weathering of cobalt bearing nickel and copper sulfides and laterites; cobalt travels with nickel and is sorbed and oxidised on manganese oxide coatings, so dissolved cobalt in natural water is low. In US public water systems under UCMR 3 cobalt was found above the 1 µg/L reporting level in only 833 of 62,982 samples (1.3 percent) and in 247 of 4,922 systems.

Anthropogenic sources

Lithium ion battery cathode manufacture and recycling; hydroformylation and hydrodesulfurisation catalysts; cobalt blue and other pigments; 1:1 and 1:2 cobalt complexed azo dyes in textile dyeing (the reason ZDHC lists it); cobalt and nickel electroplating; cobalt as a co-pollutant of cadmium battery manufacture, where 40 CFR 461.12 sets a cobalt mass limit; base metal ore processing (MWEI BREF via the element entry). Cobalt-60 from nuclear fuel cycle facilities (WHO chapter 9).

3 · Speciation

Cobalt is Co²⁺ in oxic and anoxic water alike; it hydrolyses only at high pH and precipitates as Co(OH)₂, CoCO₃ or CoS. What distinguishes it from zinc is the strength of its complexes: ammonia, cyanide, EDTA and dye ligands hold cobalt in solution where the free ion would precipitate, and Co(III) complexes once formed are kinetically inert. On manganese oxide surfaces Co(II) is oxidised and fixed as Co(III), which is why manganese nodules and MnO₂ filter media scavenge it.

Solubility

Cobalt chloride, sulfate, nitrate and acetate are freely soluble (element entry); Co(OH)₂, CoCO₃ and CoS control it in treatment. No solubility products are printed in the sources read.

Hydrolysis

Stepwise to CoOH⁺ and Co(OH)₂ at high pH only; constants not printed in the sources read (Stumm and Morgan chapter 6 has them).

Complexation

Ammonia, cyanide, EDTA and azo dye ligands form strong Co(II) and inert Co(III) complexes; the CWW BREF notes that chelating agents make hydroxide precipitation incomplete. Constants not quoted.

Precipitates

Co(OH)₂, CoCO₃, CoS; Co(III) oxyhydroxide on manganese oxides.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidify for total cobalt; filter 0.45 µm in the field for dissolved cobalt. Complexed cobalt (cyanide, EDTA) is dissolved but not exchangeable, so speciation matters for treatability tests. Cobalt-60 samples need no preservation beyond acidification but the count needs volume.

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

Essential as the centre of vitamin B₁₂ (about 1 mg in the body, element entry); no WHO evaluation for drinking water. The EPA used 70 µg/L as the reference concentration for the UCMR 3 data summary; the health basis of that figure was not read.

Bioaccumulation

Not addressed in the sources read.

Ecotoxicity

No US EPA national recommended aquatic life criterion for cobalt in the table read; no EU EQS. Not addressed further in the sources read.

Flags

• No WHO document on cobalt exists; the absence of a fact sheet is inferred from the WHO 2022 fact sheet address returning nothing for cobalt.
• The 70 µg/L UCMR 3 reference concentration is a screening value whose derivation was not read.
• The hydroxide, sulfide and ammine equations are textbook chemistry written here; no source read prints them for cobalt.
• Cobalt sorption and oxidation on manganese oxides is cited to Stumm and Morgan from memory of the surface chemistry chapters, not re-read.
• Abu Dhabi values cover two media (marine 0.2 mg/L, sewer 5 mg/L); other GCC states not read.
• The 40 CFR 461.12 cobalt limit is a mass per kg of cadmium, not a concentration.

Gaps

• No natural water concentration range for cobalt was read; only the US UCMR 3 screening data and the ocean abundance figure.
• No cobalt specific removal efficiency, optimum precipitation pH, solubility product or complex formation constant was read.
• No cobalt aquatic life criterion or ecotoxicity data was read.
• The health basis of the EPA 70 µg/L reference concentration and any cobalt entry on the EPA Contaminant Candidate List were not read (the CCL 5 list page did not load).
• Cobalt in battery and catalyst plant effluents as concentrations is in the ledger's chapters, not here.
• ISO 8288 was not read; its scope is taken from the WHO zinc document reference list and no catalogue url is given.
• Other GCC discharge standards were not read.
• The cobalt carbonate and manganese oxide equations are cited to Metcalf and Eddy chapter 6 and to Stumm and Morgan by chapter, from the chapters, not re-read; no constants are quoted.
• No source read prints a stoichiometry for the destruction of the cobalt cyanide or cobalt EDTA complexes, so none is written.
• The hexaammine air oxidation is standard inorganic chemistry written here; the element entry supports only the statement that cobalt(III) survives in complexes.

Sources

Identity

Name and symbol

Cobalt, Co

Atomic number

27 protons

Position

group 9 · period 4 · d-block · transition metal

CAS number

7440-48-4

Atomic structure

Atomic mass

58.933 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷
[Ar] 4s²³d⁷

Electrons per shell

2, 8, 15, 2

Valence electrons

9 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

1,768 K (1,494.85 °C)

Boiling point

3,200 K (2,926.85 °C)

Density

8.86 g/cm3

Appearance

Hard lustrous bluish gray metal

Thermal conductivity

100 W/(m·K)

Electrical resistivity

62.4 nΩ·m at 20 °C

Electrical conductivity

16.03 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

24.81 J/(mol·K)

Chemical properties

Oxidation states

+3, +2

Electronegativity

1.88 (Pauling Scale)

Ionisation energy

7.881 eV
1st 760.4, 2nd 1,648, 3rd 3,232 kJ/mol

Electron affinity

0.661 eV

Atomic radius

van der Waals 192 pm

Ionic radius

Co²⁺ 65 low spin; Co³⁺ 55 low spin; Co²⁺ 75 high spin; Co³⁺ 61 high spin; Co⁴⁺ 53 high spin pm

Reactivity

A group 9 transition metal ([Ar] 3d7 4s2) of the iron triad, hard and less easily oxidised than iron; its simple compounds are mostly cobalt(II), and cobalt(III) survives only in complexes and in the fluoride.

with water

Does not react with water at room temperature.

with oxygen, air

Combines slowly with oxygen in air but resists oxidation well enough to be used for plating; it does not catch fire unless finely powdered: $\ce{2 Co + O2 -> 2 CoO}$

with acids

Reacts with most acids to give hydrogen and pink cobalt(II) salts: $\ce{Co + 2 HCl -> CoCl2 + H2}$

with halogens

Forms the four dihalides CoF2 (pink), CoCl2 (blue), CoBr2 (green) and CoI2 (blue-black); only fluorine, with its very high reduction potential, gives a stable cobalt(III) halide: $\ce{Co + Cl2 -> CoCl2}$

Typical compounds

• CoCl₂ cobalt(II) chloride blue when dry, pink hydrated; sympathetic ink
• CoO cobalt(II) oxide green rock-salt oxide; blue glass and ceramics
• Co₃O₄ cobalt(II,III) oxide blue spinel from CoO heated in air
• CoAl₂O₄ cobalt aluminate cobalt blue pigment
• CoSO₄ cobalt(II) sulfate soluble salt; animal mineral supplement
• Co₂(CO)₈ dicobalt octacarbonyl carbonylation and hydrosilylation catalyst

Occurrence, production and use

Crustal abundance

2.5×101 milligrams per kilogram

Oceanic abundance

2×10-5 milligrams per liter

Occurrence and sources

Cobalt occurs in the minerals cobaltite, smaltite, and erythrite, and is often associated with nickel, silver, lead, copper, and iron ores, from which it is most frequently obtained as a by-product. It is also present in meteorites.

Important ore deposits are found in Zaire, Morocco, and Canada. The U.S. Geological Survey has announced that the bottom of the north central Pacific Ocean may have cobalt-rich deposits at relatively shallow depths in water close to the the Hawaiian Islands and other U.S. Pacific territories.

• cobaltite, skutterudite (smaltite), erythrite the cobalt minerals of the classic deposits, usually associated with nickel, silver, lead, copper and iron ores from which cobalt is obtained as a by product; ore in DR Congo, Canada, Australia, Zambia, Morocco and Brazil
• cobalt in copper and nickel ores sediment hosted stratiform copper deposits of Congo (Kinshasa) and Zambia; nickel laterites of Australia, Indonesia, the Philippines and Cuba; magmatic nickel copper sulphides of Australia, Canada, Russia and the United States; identified land resources about 25 million tonnes
• manganese nodules the deep ocean floor, holding cobalt with nickel and copper

Extraction, production

By product recovery from copper and nickel refining

Most cobalt is a by product of nickel refining and of Congolese copper mining; concentrates and partially refined intermediates are shipped to refiners, above all in China, which expanded metal refining capacity through 2024. Refined cobalt is sold as cathode (an electrolytic product), powder and salts. The sources name the host processes but no reaction, so no equation is written. World mine production about 290,000 tonnes in 2024 (estimate), a record; the resulting surplus pushed the LME price to about 12 dollars a pound.

Uses

Although cobalt is used in electroplating to give objects an attractive surface that resists oxidation, it is more widely used to form alloys. Alnico, an alloy consisting of aluminum, nickel and cobalt is used to make powerful permanent magnets. Stellite alloys, which contain cobalt, chromium and tungsten, are used to make high-speed and high temperature cutting tools and dyes. Cobalt is also used to make alloys for jet engines and gas turbines, magnetic steels and some types of stainless steels.

Cobalt-60, a radioactive isotope of cobalt, is an important source of gamma rays and is used to treat some forms of cancer and as a medical tracer. Cobalt-60 has a half-life of 5.27 years and decays into nickel-60 through beta decay.

Cobalt compounds have been used for centuries to color porcelain, glass, pottery, tile and enamel. Some of these compounds are known as: cobalt blue, ceruleum, new blue, smalt, cobalt yellow and cobalt green. In addition to being used as a dye, cobalt is also important to human nutrition as it is an essential part of vitamin B12.

It is alloyed with iron, nickel and other metals to make Alnico, an alloy of unusual magnetic strength with many important uses. Stellite alloys, containing cobalt, chromium, and tungsten, are used for high-speed, heavy-duty, high temperature cutting tools, and for dies.

Cobalt is also used in other magnetic steels and stainless steels, and in alloys used in jet turbines and gas turbine generators. The metal is used in electroplating because of its appearance, hardness, and resistance to oxidation.

Cobalt salts have been used for centuries to produce brilliant and permanent blue colors in porcelain, glass, pottery, tiles, and enamels. It is the principal ingredient in Sevre's and Thenard's blue. A solution of the chloride is used as a sympathetic ink. Cobalt carefully used in the form of the chloride, sulfate, acetate, or nitrate has been found effective in correcting a certain mineral deficiency disease in animals.

Soils should contain 0.13 to 0.30 ppm of cobalt for proper animal nutrition.

• Batteries: lithium ion battery cathodes, the leading global use, though cobalt content is being reduced and cobalt free iron phosphate cathodes hold significant share in China leading global use in 2024 (USGS, ranking only)
• Superalloys and hard metals: superalloys for aircraft gas turbine engines; cemented carbides for cutting and wear resistant tools; magnets alloyed with aluminium and nickel; electroplating United States consumption 2024: superalloys 51 percent, chemical applications 25 percent, other metallic 15 percent, cemented carbides 9 percent (USGS, US figures)
• Chemicals: cobalt salts as accelerators for room temperature peroxide curing of unsaturated polyester resins; cobalt molybdenum catalyst for hydrodesulphurisation of ammonia plant feed gas; cobalt catalysts for hydroformylation (carbonylation) of olefins; cobalt blue and other pigments; cobalt is among the substances discharged from pigment plants
• Pharmaceuticals and fine chemicals: metallisation of azo dyes with cobalt to form 1:1 and 1:2 chelated complexes, listed by the OFC BREF among processes involving heavy metals
• Agriculture and medicine: cobalt salts for livestock mineral deficiency; cobalt-60 for cancer radiotherapy and food irradiation
• Mining: cobalt ore is grouped with iron and other metalliferous ores in the MWEI BREF and is listed among the substances of base metal ore processing

Safety, toxicity

Exposure to cobalt (metal fumes and dust) should be limited to 0.05 mg/m3 (8-hour time-weighted average 40-hour week).

GHS classification, signal word Danger

• H317 May cause an allergic skin reaction Sensitization, Skin
• H334 May cause allergy or asthma symptoms or breathing difficulties if inhaled Sensitization, respiratory
• H341 Suspected of causing genetic defects Germ cell mutagenicity
• H350 May cause cancer Carcinogenicity
• H360F May damage fertility Reproductive toxicity
• H413 May cause long lasting harmful effects to aquatic life to the aquatic environment, long-term hazard
• H302 Harmful if swallowed Acute toxicity, oral
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H330 Fatal if inhaled Acute toxicity, inhalation
• H360Fd May damage fertility; Suspected of damaging the unborn child Reproductive toxicity
• 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
• H411 Toxic to aquatic life with long lasting effects to the aquatic environment, long-term hazard
• H320 Causes eye irritation Serious eye damage/eye irritation
• H351 Suspected of causing cancer Carcinogenicity
• H360 May damage fertility or the unborn child Reproductive toxicity
• H370 Causes damage to organs Specific target organ toxicity, single exposure
• H372 Causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
• H361 Suspected of damaging fertility or the unborn child Reproductive toxicity
• H350i May cause cancer by inhalation Carcinogenicity
• H373 May causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
• H228 Flammable solid Flammable solids

Discovery and name

Discovered by

Georg Brandt

Discovered

1735

First isolated

not in sources

Named by

not in sources

Origin of the name

from the kobelt ore, possibly named after Kobolds

Cobalt is a brittle, hard metal, resembling iron and nickel in appearance. It has a metallic permeability of about two thirds that of iron. Cobalt tends to exist as a mixture of two allotropes over a wide temperature range. The transformation is sluggish and accounts in part for the wide variation in reported data on physical properties of cobalt.