Gallium

Typical wastewaters

• semiconductor and photovoltaic manufacture (GaAs and GaN wafer fabrication) dissolved Ga(III) with arsenic in rinse and etch water; Ga³⁺ and its first hydroxo complexes in acid process water at pH 2 to 3 the arsenic, not the gallium, is the regulated parameter; the gallium is recovered by siderophore complexation; no concentration given in the abstract read
• municipal sewage (urban wastewater reaching a coastal stream, Taiwan) hydroxo-complexed Ga(III), Ga(OH)₃^0 and Ga(OH)₄⁻, 11,000 to 19,000 pmol/L in the receiving stream the review quotes the wastewater impacted stream, not the effluent itself

1 · Identity

Symbol, number

Ga, 31

Oxidation states in water

+3 only; hydrolysed to Ga(OH)₃^0 and Ga(OH)₄⁻ in natural water (the review's thermodynamic statement for Ga and In)

Note

The element entry covers the amphoteric metal and its hydroxide. In water gallium behaves as a smaller, more hydrolysed aluminium: no redox chemistry, solubility controlled by hydroxide.

2 · Occurrence in water

Natural sources

Weathering of aluminium minerals in which gallium substitutes for Al (bauxite, sphalerite in the element entry); scavenged in the ocean, with elevated surface and deep concentrations and a minimum at intermediate depth.

Anthropogenic sources

Semiconductor and optoelectronic manufacture (GaAs, GaN wafers); wafer fabrication process water; urban wastewater in Taiwan raised a coastal stream to 11,000 to 19,000 pM.

3 · Speciation

Dissolved inorganic gallium is the neutral trihydroxide and the gallate anion Ga(OH)₄⁻; the free ion matters only in acid water. Because hydrolysis is so strong, gallium is scavenged onto particles and its ocean profile is that of a scavenged element.

Solubility

Controlled by gallium hydroxide, amphoteric like aluminium hydroxide; no solubility product was read this session.

Hydrolysis

Ga³⁺ hydrolyses at lower pH than Al³⁺ and passes to the tetrahydroxo anion in neutral water (qualitative, from the review's species list).

Complexation

Hydroxamate siderophores (desferrioxamine B and E) bind Ga³⁺ so strongly that 100 percent complexation was reached in fab process water; EDTA in six fold excess at pH 3.5 strips it again.

Precipitates

Ga(OH)₃ or GaOOH on neutralising acid gallium solutions; amphoteric, so it redissolves as gallate in strong alkali. None forms in natural water at natural concentrations.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Gallium is scavenged: filter at once and acidify, and use trace-metal clean handling, since pmol/kg levels are far below any laboratory blank from glass or aluminium contact.

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

No human health guideline anywhere read. Gallium salts are used therapeutically and diagnostically; no drinking water toxicology was read.

Bioaccumulation

Not biomagnified or biodiluted through the food web in Arctic pelagic systems, distributed proportionally in tissues (review). Medaka in a soil-water-fish system accumulated Ga dose dependently with growth inhibition and altered swimming at sublethal levels.

Ecotoxicity

96 h LC₅₀ for the freshwater shrimp Macrobrachium nipponense 2.77 mg/L Ga(III) (indium 6.89, cadmium 0.054, copper 0.031 mg/L in the same tests). In acidic aluminium rich paddy soil, gallium above 1 mmol/kg killed medaka largely by displacing aluminium into the water.

Flags

• The wafer fabrication process water gallium concentration is not given in the abstract read.
• The Taiwan stream value is a single wastewater impacted site quoted in the review.
• The equation is a stoichiometric statement of the species named in the review; no hydrolysis constant was read.

Gaps

• No river, groundwater or municipal wastewater concentration for gallium was read; the review covers marine and coastal water only.
• No hydrolysis constants or solubility product were read; speciation is qualitative.
• Arsenic removal from GaAs fab wastewater is the arsenic entry's story and is not repeated here.
• No GCC discharge standard was read; no GCC row is written.
• No standard analytical method text for gallium was read.

Sources

Identity

Name and symbol

Gallium, Ga

Atomic number

31 protons

Position

group 13 · period 4 · p-block · post-transition metal

CAS number

7440-55-3

Atomic structure

Atomic mass

69.723 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p¹
[Ar] 4s²³d¹⁰⁴p¹

Electrons per shell

2, 8, 18, 3

Valence electrons

3 outer shell

Physical properties

State at room temperature

Solid

Melting point

302.91 K (29.76 °C)

Boiling point

2,477 K (2,203.85 °C)

Density

5.91 g/cm3

Appearance

silvery blue

Thermal conductivity

40.6 W/(m·K)

Electrical resistivity

270 nΩ·m at 20 °C

Electrical conductivity

3.7 MS/m

Crystal structure

base-centered orthorhombic

Molar heat capacity

25.86 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.81 (Pauling Scale)

Ionisation energy

5.999 eV
1st 578.8, 2nd 1,979.3, 3rd 2,963 kJ/mol

Electron affinity

0.3 eV

Atomic radius

empirical 122, covalent 122, van der Waals 187 pm

Ionic radius

Ga³⁺ 62 pm

Reactivity

A moderately reactive group 13 metal that behaves much like aluminium: a thin oxide skin protects it at room temperature, and it is amphoteric, dissolving in both strong acids and strong alkalis.

with water

Does not react with water at ordinary temperatures; the oxide film holds.

with oxygen, air

Stable in air at room temperature under a protective oxide film; on strong heating it oxidises to gallium(III) oxide: $\ce{4 Ga + 3 O2 -> 2 Ga2O3}$.

with acids

Attacked only slowly by mineral acids, dissolving to gallium(III) salts with hydrogen: $\ce{2 Ga + 6 HCl -> 2 GaCl3 + 3 H2}$; it also dissolves in hot alkali to gallates.

with halogens

Combines with the halogens to gallium(III) halides; with chlorine the product is the dimer $\ce{Ga2Cl6: 2 Ga + 3 Cl2 -> Ga2Cl6}$.

Typical compounds

• Ga₂O₃ gallium(III) oxide the stable oxide, amphoteric
• GaCl₃ gallium trichloride Lewis acid, dimeric Ga2Cl6 as the solid
• GaAs gallium arsenide III-V semiconductor for LEDs, lasers and solar cells
• GaN gallium nitride blue LED and power electronics semiconductor
• Ga(OH)₃ gallium hydroxide amphoteric; dissolved in potassium hydroxide for electrolysis

Occurrence, production and use

Crustal abundance

1.9×101 milligrams per kilogram

Oceanic abundance

3×10-5 milligrams per liter

Occurrence and sources

Gallium is often found as a trace element in diaspore, sphalerite, germanite, bauxite, and coal. Some flue dusts from burning coal have been shown to contain as much 1.5 percent gallium.

• trace gallium in bauxite about 50 parts per million; world bauxite resources hold more than 1 million tonnes of gallium, of which less than 10 percent is recoverable
• trace gallium in sphalerite, germanite, diaspore and coal some United States zinc ores carry up to 50 parts per million; coal flue dusts have held up to 1.5 percent gallium

Extraction, production

By product recovery from alumina refining and zinc residues

Primary low purity (99.99 percent) gallium is extracted from the sodium aluminate liquor of the Bayer process and from zinc processing residues, then refined to 99.999 percent and higher for wafers; new scrap from gallium arsenide device manufacture is reprocessed in Canada, China, Japan, Slovakia and the United States. Production about 760,000 kilograms primary and 320,000 kilograms refined in 2024 (estimate); capacity 1.1 million kilograms primary. No reaction is stated by the sources.

Electrolysis of gallium hydroxide in potassium hydroxide

Lecoq de Boisbaudran's original method and still the basis of alkaline electrowinning; the sources name the electrolyte but not the anode product, so no equation is written.

Uses

Gallium melts near room temperature and has one of the largest liquid ranges of any metal, so it has found use in high temperature thermometers. Gallium easily forms alloys with most metals and has been used to create low melting alloys. Gallium is used as a doping material for semiconductors and has been used to produce solid-state items like transistors and light emitting diodes. Gallium arsenide (GaAs) can produce laser light directly from electricity. Large amounts of gallium trichloride (GaCl3) have been gathered to build the Gallium Neutrino Observatory, an observatory located in Italy built to study particles called neutrinos which are produced inside the sun during the process of nuclear fusion.

Gallium wets glass or porcelain and forms a brilliant mirror when it is painted on glass. It is widely used in doping semiconductors and producing solid-state devices such as transistors.

Magnesium gallate containing divalent impurities, such as Mn+2, is finding use in commercial ultraviolet-activated powder phosphors. Gallium arsenide is capable of converting electricity directly into coherent light. Gallium readily alloys with most metals, and has been used as a component in low-melting alloys.

• Electronics: gallium arsenide and gallium nitride wafers for integrated circuits, laser diodes, LEDs, photodetectors and solar cells; trimethyl and triethyl gallium for epitaxial layers; gallium phosphide wafers United States 2024: integrated circuits 79 percent, optoelectronic devices 20 percent, research 1 percent; about 83 percent of gallium consumed was in GaAs, GaN and GaP wafers (USGS, US figures)
• Alloys and instruments: low melting alloys; high temperature thermometers exploiting the long liquid range; mirrors, since gallium wets glass
• Mining: gallium leaves the ground inside bauxite and zinc ore, so its extraction footprint is that of the bauxite and alumina refining and base metal sectors of the MWEI BREF; the Middle Tennessee zinc mines drilled to define gallium and germanium resources in 2024

Safety, toxicity

Its toxicity appears to be of a low order, but should be handled with care until more data is available.

GHS classification, signal word Danger

• H290 May be corrosive to metals Corrosive to Metals
• H302 Harmful if swallowed Acute toxicity, oral
• H314 Causes severe skin burns and eye damage Skin corrosion/irritation
• H412 Harmful to aquatic life with long lasting effects to the aquatic environment, long-term hazard

Discovery and name

Discovered by

Lecoq de Boisbaudran

Discovered

1875

First isolated

not in sources

Named by

not in sources

Origin of the name

after Gallia (Latin for: France), homeland of the discoverer

It is one of four metals mercury, cesium, and rubidium which can be liquid near room temperature and, thus, can be used in high-temperature thermometers. It has one of the longest liquid ranges of any metal and has a low vapor pressure even at high temperatures.

There is a strong tendency for gallium to supercool below its freezing point. Therefore, seeding may be necessary to initiate solidification.

Ultra-pure gallium has a beautiful, silvery appearance, and the solid metal exhibits a conchoidal fracture similar to glass. The metal expands 3.1 percent on solidifying; therefore, it should not be stored in glass or metal containers, because they may break as the metal solidifies.

High-purity gallium is attacked only slowly by mineral acids.