Cobalt - Co

Cobalt (Co) is a magnetic transition metal used in rechargeable batteries, high-temperature alloys, hardmetals, catalysts and pigments. Its role in vitamin B₁₂ is distinct from exposure to cobalt metal or inorganic salts. Chemical form also matters when assessing cobalt in water and selecting treatment.

Chemical properties · Applications · Cobalt in water and treatment · Health effects · Environmental effects

Selected properties of cobalt metal
Symbol / atomic number	Co / 27
Standard atomic weight	58.933 194(3)
Group / period / block	9 / 4 / d
Electron configuration	[Ar] 3d⁷ 4s²
Electronegativity (Pauling)	1.88
Density	8.86 g cm⁻³ at room temperature
Melting point	1495 °C
Boiling point	2927 °C
Covalent radius	0.118 nm
Non-bonded radius	0.200 nm
Common oxidation states in compounds	+2 and +3
Stable natural isotope	⁵⁹Co
First ionisation energy	760.4 kJ mol⁻¹
Discovery	Georg Brandt, 1735

Physical and atomic data follow the Royal Society of Chemistry; atomic weight, stable isotope and discovery date follow CIAAW. Atomic weight is dimensionless; the corresponding molar mass is approximately 58.933194 g mol⁻¹. The number in parentheses indicates uncertainty in the final digit. Ionic radii depend on oxidation state, coordination and spin state.

Pieces of cobalt metal on a blue background — Cobalt metal.

Cobalt: properties and occurrence

Cobalt is a hard, lustrous metal that, like iron and nickel, can be magnetised. Important cobalt-bearing minerals include cobaltite, skutterudite and erythrite. Much commercial cobalt is recovered alongside copper or nickel rather than from a separate cobalt-only operation.

Georg Brandt identified cobalt in the eighteenth century; CIAAW gives the discovery date as 1735. Cobalt compounds had been used to colour glass and ceramics long before the element was recognised.

Applications of cobalt

Batteries: cobalt is used in the electrodes of some rechargeable battery chemistries.
Alloys and tools: applications include gas-turbine superalloys, wear-resistant materials, cemented carbides and magnets.
Chemicals: cobalt compounds serve as catalysts and as colourants in glass, ceramics and pigments.
Radiation sources: cobalt-60 is used in controlled medical and industrial applications. It is radioactive and must be distinguished from stable cobalt-59.

Cobalt in water and water treatment

See Lenntech's cobalt removal from water page for treatment options and an ion-exchange application.

Sources and chemical forms

Rock weathering supplies cobalt naturally. Mining, metal refining, chemical manufacture and waste streams can add further inputs. Dissolved cobalt commonly occurs in the +2 oxidation state, either as ions or complexes. Cobalt can also bind to particles and sediments.

Mobility depends on pH, oxidation–reduction conditions, ligands and solids. Acidic conditions can increase mobility, but cobalt is not mobile only in acidic water. A total-cobalt result does not by itself distinguish dissolved, particle-bound or complexed cobalt.

Potential removal processes

Precipitation and solids separation: pH adjustment can form cobalt-containing precipitates, followed by clarification or filtration. Complexing agents and other metals affect the result.
Ion exchange: suitable cation-exchange or chelating resins can capture dissolved cobalt. Selectivity, competing ions, breakthrough and regeneration need evaluation.
Adsorption: selected media bind cobalt at their surfaces. Capacity depends on the medium and water chemistry.
Membranes: reverse osmosis or nanofiltration may be suitable, depending on cobalt speciation, membrane performance and pretreatment. Rejected cobalt remains in the concentrate.

Define the treatment target and characterise the feed before choosing a process. Bench or pilot tests can assess residual concentration, media capacity and solids separation. Removal transfers cobalt into sludge, spent media, regenerant or concentrate; those streams need appropriate management.

If radioactive cobalt is suspected, chemical concentration alone is insufficient: radionuclide-specific analysis and radiological controls are also needed. Ordinary cobalt-removal performance should not be assumed to demonstrate compliance with a radiological requirement.

Health effects of cobalt

Vitamin B₁₂ and other cobalt forms

Cobalt is part of vitamin B₁₂ (cobalamin), which is essential for human health. This nutritional role does not make cobalt metal or inorganic cobalt salts substitutes for vitamin B₁₂. Food, water, air and occupational activities can contribute to cobalt exposure.

Cobalt dust and fumes can affect the respiratory system, causing sensitisation, asthma and other lung effects. Skin contact can cause allergic dermatitis. Other effects depend on the compound, dose and route of exposure; ATSDR's assessment identifies blood effects from oral exposure as presumed and thyroid effects as suspected. Exposure controls should address the actual material and process.

Cancer-hazard classifications

IARC's Volume 131 distinguishes between cobalt forms:

Cobalt metal without tungsten carbide or other metal alloys, and soluble cobalt(II) salts: probably carcinogenic to humans (Group 2A).
Cobalt(II) oxide: possibly carcinogenic to humans (Group 2B).
Cobalt(II,III) oxide, cobalt(II) sulfide and other cobalt(II) compounds evaluated in that volume: not classifiable as to carcinogenicity to humans (Group 3).

Group 3 does not mean proven safe. These classifications identify hazards and do not quantify risk from a particular exposure. Cobalt-containing mixtures need assessment for their own composition.

Radioactive cobalt

Cobalt-60 emits beta particles and gamma radiation and has a half-life of about 5.27 years. Half-life alone does not establish safety: activity, exposure route, shielding and time all influence radiological risk. Stable cobalt-59 does not present this radioactive-decay hazard.

Environmental effects of cobalt

Cobalt is an element and is not destroyed by conventional water treatment. It changes chemical form or moves between water, solids and organisms. Dissolved, bioavailable cobalt can harm aquatic and sediment-dwelling organisms, affecting survival, growth or reproduction. Water hardness and other site conditions influence toxicity.

Low potential for bioaccumulation does not mean that an elevated discharge is harmless. Environmental assessment should consider the receiving water, cobalt species and exposure, together with the composition of treatment residuals. For related aquatic background, see algae and cyanobacteria.

Cobalt page last reviewed: 24 October 2026 by Lenntech.