Antimony

Typical wastewaters

• textile dyeing and finishing (polyester) Sb(III) and Sb(V) released from the antimony trioxide polyester catalyst in the dyeing liquor at pH 4 to 5 and 130 C; antimony trioxide flame retardant synergist from finishing ZDHC limits do not yet apply to polyester wet processing, but antimony must be sampled and reported
• mine drainage (antimony mining and smelting districts) Sb(OH)₃ (aq) and thioantimonite complexes in reducing, sulfidic mine water, oxidising slowly to Sb(OH)₆⁻ on aeration US EPA source list for the antimony MCL
• petroleum refining, ceramics, electronics and lead-acid battery manufacture antimonate Sb(OH)₆⁻ in oxic effluent US EPA source list for the antimony MCL; no effluent figure read

1 · Identity

Symbol, number

Sb, 51

Oxidation states in water

+5 as the antimonate anion Sb(OH)₆⁻ in oxic water, the form WHO identifies for antimony leached from materials and the less toxic one; +3 as the neutral Sb(OH)₃ in reducing water and as the soluble potassium antimony tartrate of the toxicology studies, more toxic and genotoxic in vitro and in vivo; -3 (stibine) has no water chemistry. 0 as the metal in lead alloys and solders.

Note

The element entry covers stibnite, the trioxide and the halides. In water antimony is the arsenic analogue one row down: an anion in the +5 state, a neutral hydroxide in the +3 state, both poorly adsorbed at neutral pH.

2 · Occurrence in water

Natural sources

Weathering of stibnite and other antimony minerals; hydrothermal and mining districts. WHO: groundwater below 0.001 µg/L and surface water below 0.2 µg/L in general; drinking water appears to be below 5 µg/L. Antimony is not normally a raw water contaminant.

Anthropogenic sources

Dissolution from metal plumbing and fittings, the most common source in drinking water (WHO), with antimony solder replacing lead; leaching from PET bottles, which hold antimony trioxide catalyst residues (213 mg Sb per kg plastic in one brand); antimony trioxide in polyester manufacture and hence polyester dyeing effluent (ZDHC); antimony trioxide flame retardant synergist in textile finishing (element entry); antimony mining and smelting, petroleum refineries, ceramics, electronics and lead-acid battery plants (US EPA source list); the ledger's textile, chemical and mining chapters carry the plant figures.

3 · Speciation

Antimony(V) exists as the antimonate anion Sb(OH)₆⁻ across the whole natural pH range because antimonic acid is a fairly strong acid; antimony(III) is the neutral Sb(OH)₃ up to high pH. Neither is a cation, so neither hydrolyses to a filterable hydroxide, and neither adsorbs well on aluminium hydroxide. Ferric hydroxide adsorbs both: Sb(V) best at pH 4.5 to 5.5 and progressively worse as pH rises, Sb(III) well from pH 4 to 10 (Guo and others 2009). Phosphate and humic acid compete with Sb(V) for the iron surface and barely affect Sb(III). Oxic water holds Sb(V); reducing water and stibnite districts give Sb(III), and the sulfide complexes of Sb(III) keep antimony mobile in sulfidic water. Antimony leached from materials is the Sb(V) oxo-anion (WHO).

Solubility

No controlling solid in ordinary water; Sb₂O₃ and Sb₂S₃ are the sparingly soluble source minerals and iron antimonates the proposed treatment solids. No solubility products quoted; the sources read print none.

Hydrolysis

Both states are fully hydrolysed hydroxo species at natural pH; Sb(OH)₅ is a stronger acid than Sb(OH)₃, hence the anion for Sb(V) and the neutral molecule for Sb(III). The pKa is not printed in the sources read and is not quoted.

Complexation

Sulfide (thioantimonites), tartrate (potassium antimony tartrate), organic matter; phosphate and humic acid compete at the iron surface rather than complexing antimony (Guo and others 2009).

Precipitates

Antimony adsorbed on and co-precipitated with ferric hydroxide; Sb₂S₃ in sulfidic sediments; iron antimonate proposed in the coagulation literature.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Preserve with hydrochloric acid, not nitric alone: antimony is unstable in nitric acid solutions and adsorbs out (EPA 200.8). Speciation (Sb(III) against Sb(V)) needs prompt analysis; oxic storage converts Sb(III). PET sample bottles are an antimony source; use glass or polyethylene. Bottled water results depend on storage temperature and age.

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

TDI 6 µg/kg body weight from a 90 day rat study with the soluble potassium antimony tartrate; the form decides the toxicity: Sb(V) leached from materials is the less toxic, antimony trioxide has low bioavailability, soluble Sb(III) salts are genotoxic in vitro and in vivo. Antimony trioxide is possibly carcinogenic by inhalation (IARC 2B), the trisulfide not classifiable (Group 3); there are no data indicating carcinogenicity by the oral route (WHO fact sheet). US health effects listed: increased blood cholesterol, decreased blood sugar.

Bioaccumulation

Not addressed in the sources read; total exposure from environment, food and drinking water is very low compared with occupational exposure (WHO).

Ecotoxicity

No US EPA national recommended aquatic life criterion for antimony and no EU EQS; not addressed in the sources read.

Flags

• The Sb(OH)₆⁻ and Sb(OH)₃ formulas and the antimonic acid dissociation are textbook speciation, not printed in the WHO text, which names only the antimony(V) oxo-anion; cited to Stumm and Morgan chapter 6 from memory, not re-read.
• The WHO groundwater figure of below 0.001 µg/L is below the detection limits WHO itself lists and is quoted as printed.
• Guo and others 2009 was read as the abstract only; removal percentages are in the paper, not the abstract.
• The pre-oxidation and neutral pH iron removal item is a consequence drawn from the pH dependence, not a design rule read.
• Westerhoff and others 2008 was read as the abstract; the temperature model figures are from it.
• EU law was read on legislation.gov.uk mirrors because eur-lex did not respond; eur-lex urls kept for consistency.
• Abu Dhabi marine value 0.1 mg/L; the sewer regulation has no antimony row; other GCC states not read.
• Standard Methods and ISO method numbers other than those in the sources read (EPA 200.8, ISO 17294-2 and the methods the WHO documents cite) are quoted from memory and were not confirmed this session.

Gaps

• No seawater, municipal wastewater, mine water or polyester dyehouse antimony concentration was read.
• No pKa, solubility product or adsorption constant is quoted.
• Adsorbents (iron oxide media, activated alumina), reverse osmosis and ion exchange for antimony are not covered because no read source gives figures.
• The WHO 2003 background document on antimony was not read; the fact sheet was.
• The EU textiles BAT conclusions (2022/2508) were not read for an antimony BAT-AEL.
• EU law was read on legislation.gov.uk mirrors; other GCC discharge standards were not read.
• No surface complexation constants or surface species formulas for antimony on ferric hydroxide were read; the ligand exchange equation is written by analogy with arsenate.

Sources

Identity

Name and symbol

Antimony, Sb

Atomic number

51 protons

Position

group 15 · period 5 · p-block · metalloid

CAS number

7440-36-0

Atomic structure

Atomic mass

121.76 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p³
[Kr] 5s²⁴d¹⁰⁵p³

Electrons per shell

2, 8, 18, 18, 5

Valence electrons

5 outer shell

Physical properties

State at room temperature

Solid

Melting point

903.78 K (630.63 °C)

Boiling point

1,860 K (1,586.85 °C)

Density

6.685 g/cm3

Appearance

silvery lustrous gray

Thermal conductivity

24.4 W/(m·K)

Electrical resistivity

417 nΩ·m at 20 °C

Electrical conductivity

2.4 MS/m

Crystal structure

rhombohedral

Molar heat capacity

25.23 J/(mol·K)

Chemical properties

Oxidation states

+5, +3, -3

Electronegativity

2.05 (Pauling Scale)

Ionisation energy

8.64 eV
1st 834, 2nd 1,594.9, 3rd 2,440 kJ/mol

Electron affinity

1.07 eV

Atomic radius

empirical 139, covalent 139, van der Waals 206 pm

Ionic radius

Sb³⁺ 76; Sb⁵⁺ 60 pm

Reactivity

A brittle group 15 metalloid, a poor conductor, stable in air at room temperature and attacked only by oxidising acids; it shows the +3 and +5 states and, like arsenic, is toxic.

with water

Does not react with water.

with oxygen, air

Stable in air at room temperature; on heating it burns to antimony trioxide, which forms as Sb4O6 in the vapour: $\ce{4 Sb + 3 O2 -> 2 Sb2O3}$.

with acids

No reaction with non-oxidising acids; oxidising acids such as nitric acid attack it, and concentrated nitric acid takes the trioxide on to the pentoxide.

with halogens

Combines with the halogens to the trihalides: $\ce{2 Sb + 3 Cl2 -> 2 SbCl3}$; with excess chlorine or fluorine the pentahalides form: $\ce{2 Sb + 5 Cl2 -> 2 SbCl5}$.

Typical compounds

• Sb₂O₃ antimony trioxide flame-retardant synergist, glass fining agent
• Sb₂S₃ stibnite the principal ore; kohl, safety-match heads
• SbCl₃ antimony trichloride from stibnite in hydrochloric acid; Lewis acid
• SbF₅ antimony pentafluoride with HF gives fluoroantimonic superacid
• SbH₃ stibine unstable toxic hydride, semiconductor dopant
• InSb indium antimonide infrared detector semiconductor

Occurrence, production and use

Crustal abundance

2×10-1 milligrams per kilogram

Oceanic abundance

2.4×10-4 milligrams per liter

Occurrence and sources

Antimony is not abundant, but is found in over 100 mineral species. It is sometimes found natively, but more frequently it is found as the sulfide stibnite.

• stibnite (Sb2S3) China (Hunan), Tajikistan, Russia, Myanmar, Bolivia, Turkey, Australia; Stibnite district, Idaho
• native antimony and more than 100 other minerals minor
• crustal and oceanic abundance about 0.2 ppm (BGS figure via RSC); 0.2 mg/kg crust and 0.00024 mg/L seawater (PubChem)

Extraction, production

Roasting of stibnite to antimony(III) oxide

$\ce{2 Sb2S3 (s) + 9 O2 (g) -> 2 Sb2O3 (s) + 6 SO2 (g)}$

RSC states the sulfide is roasted to the oxide; sulfur dioxide as the sulfur product is implied by roasting in air, not named

Carbon reduction of the oxide to metal

$\ce{2 Sb2O3 (s) + 3 C (s) -> 4 Sb (l) + 3 CO2 (g)}$

RSC states the oxide is then reduced with carbon; The added secondary source prints the carbon dioxide form, which is the equation given.

Secondary antimony as antimonial lead

recovered at secondary lead smelters from spent lead-acid batteries and returned to battery grids; about 3,500 t in the United States in 2024

Uses

Antimony is a brittle metal and is a poor conductor of heat and electricity. Very pure antimony is used to make certain types of semiconductor devices, such as diodes and infrared detectors. Antimony is alloyed with lead to increase lead's durability. Antimony alloys are also used in batteries, low friction metals, type metal and cable sheathing, among other products. Antimony compounds are used to make flame-proofing materials, paints, ceramic enamels, glass and pottery. The ancient Egyptians used antimony, in the form of stibnite, for black eye make-up.

Antimony is finding use in semiconductor technology for making infrared detectors, diodes and Hall-effect devices. It greatly increases the hardness and mechanical strength of lead. Batteries, antifriction alloys, type metal, small arms and tracer bullets, cable sheathing, and minor products use about half the metal produced. Compounds taking up the other half are oxides, sulfides, sodium antimonate, and antimony trichloride. These are used in manufacturing flame-proofing compounds, paints ceramic enamels, glass, and pottery.

• Lead alloys and batteries: antimonial lead for lead-acid battery grids; ammunition, cable sheathing, type metal, bearing alloys metal products 40 percent of US antimony use in 2024 (usgs-mcs2025-antimony)
• Textiles (flame-retardant finishing, polyester dyeing): antimony trioxide synergist in flame-retardant finishes; antimony BAT-AEL applies; antimony released from polyester in pretreatment and disperse dyeing (BAT-AEL 0.1 to 0.2 mg/l, up to 1.2) flame retardants 39 percent of US antimony use in 2024, across all substrates (usgs-mcs2025-antimony)
• Chemicals (polyester catalyst, pigments, glass): antimony trioxide polycondensation catalyst for PET; antimony in lead-chromate and complex inorganic colour pigment effluent (Pb, Sb pretreatment by precipitation); enamels, paints, glass, pottery nonmetal products including ceramics, glass and rubber 21 percent of US use in 2024 (usgs-mcs2025-antimony)
• Electronics: indium antimonide and other semiconductors for infrared detectors and diodes
• Mining: stibnite mining and gold-antimony deposits; antimony reported in extractive-waste water at base-metal and precious-metal sites

Safety, toxicity

GHS classification, signal word Danger

• H301 Toxic if swallowed Acute toxicity, oral
• H302 Harmful if swallowed Acute toxicity, oral
• H332 Harmful if inhaled Acute toxicity, inhalation
• H351 Suspected of causing cancer Carcinogenicity
• H373 May causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
• H411 Toxic to aquatic life with long lasting effects to the aquatic environment, long-term hazard
• H315 Causes skin irritation Skin corrosion/irritation
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
• 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
• H401 Toxic to aquatic life to the aquatic environment, acute hazard

Discovery and name

Discovered by

Arabic alchemists

Discovered

before AD 815

First isolated

not in sources

Named by

not in sources

Origin of the name

Uncertain. Possibly from Greek anti ("not") + monos ("alone"), for not occurring alone in nature

Antimony is a poor conductor of heat and electricity. Antimony and many of its compounds are toxic.