Thallium

Typical wastewaters

• zinc electrowinning and sulfide ore processing Tl⁺, the soluble monovalent ion that passes hydroxide precipitation; 100 to 500 mg/L in a zinc process stream removed on manganese dioxide sludge from the electrowinning itself or by oxidative co-precipitation with iron and peroxide
• lead smelting Tl⁺ in smelter effluent at 12.5 mg/L, treated to below 100 µg/L on manganese dioxide
• mine closure drainage and process waters Tl⁺ at 50 to 1000 µg/L, taken to below 2 µg/L in a fluidised bed pilot
• coal combustion and cement manufacture Tl⁺; named with metal smelting by ATSDR as the three main sources of thallium to the environment, medium not specified the cement source is not quantified in the sources read
• urban stormwater Tl⁺ dissolved, 1 to 14 µg/L in 10 percent of samples
• electronics, glass and drug factories Tl⁺, the US EPA source list for thallium in drinking water, with leaching from ore processing sites no concentration read

1 · Identity

Symbol, number

Tl, 81

Oxidation states in water

+1 as Tl⁺, the state in almost all natural water: a large, weakly hydrated, poorly complexed cation that follows potassium and is not stopped by hydroxide, carbonate or sulfate; +3 as Tl(III), a strong oxidant that hydrolyses to insoluble Tl(OH)₃ and exists in water only where a strong oxidant such as manganese dioxide or permanganate makes it.

Note

The element entry covers the metal, TlOH, Tl₂SO₄ and TlCl. The water chemistry is the chemistry of Tl⁺: soluble, mobile and hard to precipitate.

2 · Occurrence in water

Natural sources

Thallium is dispersed in potassium minerals and concentrated in base metal and precious metal sulfides (element entry, Twidwell 2002); waters draining sulfide deposits carry it. Unpolluted fresh water is generally below 1 µg/L; the ATSDR profile reports thallium in tap water in 0.68 percent of samples at an average of 0.89 µg/L.

Anthropogenic sources

Coal burning power plants, cement factories and metal smelting are the three main sources named by ATSDR; mining, beneficiation and smelting of sulfide ores, zinc electrowinning and lead smelting effluents (Twidwell 2002); the US EPA lists leaching from ore processing sites and discharge from electronics, glass and drug factories.

3 · Speciation

Thallium occurs almost exclusively in natural waters as monovalent Tl⁺, whose compounds, thallous hydroxide included, are relatively soluble, so it is readily transported through aqueous routes (Twidwell 2002). It resembles lead and the alkali metals potassium, rubidium and caesium; it forms weak complexes, does not hydrolyse, and is sorbed mainly by manganese oxides, which can also oxidise it to the insoluble Tl(III) state. Thallium(I) chloride and sulfide are the sparingly soluble Tl(I) solids (element entry).

Solubility

Thallous hydroxide is a water soluble strong base (element entry), so unlike lead or zinc thallium cannot be precipitated by lime; TlCl is poorly soluble, Tl₂S insoluble. No solubility products are printed in the sources read.

Hydrolysis

Tl⁺ does not hydrolyse in the natural pH range; Tl³⁺ hydrolyses completely to Tl(OH)₃ (general chemistry, from the book entry's description of Tl(III) as a strong oxidant; no constant read).

Complexation

Weak; Tl⁺ behaves as a potassium analogue with some affinity for sulfur ligands and for organic matter. No constants read.

Precipitates

Tl₂S, TlCl, TlI, and Tl(OH)₃ or Tl₂O₃ on manganese oxide surfaces; thallium co-precipitated on ferric hydroxide with oxidation.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidify with nitric acid; Tl⁺ is stable in solution and does not sorb strongly to bottle walls, but particulate thallium on manganese oxide flocs is lost if the sample is filtered before digestion. Report total and dissolved separately in mine waters.

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

More toxic to humans than mercury, cadmium, lead, copper or zinc (Twidwell 2002); thallium(I) sulfate was a rodenticide banned for household use (element entry); the US MCLG of 0.5 µg/L reflects effects on hair loss, blood chemistry and the nervous system at low doses. Smokers excrete about twice the thallium of non smokers (ATSDR).

Bioaccumulation

Bioconcentration factors from 11.7 in mussels to 1430 in Atlantic salmon muscle; thallium is taken up by vegetables from contaminated soil, the main exposure route for the public (ATSDR 1992).

Ecotoxicity

No US EPA aquatic life criterion or EU EQS was read for thallium; the bioconcentration data are the ecotoxicological signal in the sources read.

Flags

• The ATSDR occurrence figures are 1980s Contract Laboratory Program and survey data quoted in the 1992 profile; a 2024 draft profile exists and was not read.
• The treatment concentrations are from individual studies annotated by Twidwell and Williams-Beam (2002), not from operating plants.
• The WHO 'no guideline' row is inferred from the absence of a thallium fact sheet among those read, not from a WHO statement; the chapter 12 index was not re-read.
• The sulfide and Tl(III) hydroxide equations are written here from the element entry's compound list; no read source prints them, and the MnO₂ oxidation stoichiometry is deliberately not written.
• The Hunan provincial discharge limit of 5 µg/L (2014) is from a search summary and is not cited as a source.
• The GCC tables list no thallium; other GCC states not read.

Gaps

• No source read gives thallium in seawater, in surface water as a survey range, in municipal wastewater or in cement plant effluent as a number; the cement source is named by ATSDR but not quantified.
• Solubility products of TlCl, Tl₂S and Tl(OH)₃ and the Tl(I) to Tl(III) redox potential are not in the sources read.
• The WHO position on thallium and any EU national values were not read.
• EPA 200.9 and other method detection limits were not read.
• Aquatic toxicity values (EC₅₀, chronic criteria) were not sourced.
• The mechanism on MnO₂ (sorption versus oxidation) is debated in the literature and not resolved by the sources read.
• No surface complexation or co-precipitation stoichiometry was read for thallium on ferric hydroxide; only the peroxide oxidation step is written for that row.

Sources

Identity

Name and symbol

Thallium, Tl

Atomic number

81 protons

Position

group 13 · period 6 · p-block · post-transition metal

CAS number

7440-28-0

Atomic structure

Atomic mass

204.3833 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹
[Xe] 6s²⁴f¹⁴⁵d¹⁰⁶p¹

Electrons per shell

2, 8, 18, 32, 18, 3

Valence electrons

3 outer shell

Physical properties

State at room temperature

Solid

Melting point

577 K (303.85 °C)

Boiling point

1,746 K (1,472.85 °C)

Density

11.8 g/cm3

Appearance

silvery white

Thermal conductivity

46.1 W/(m·K)

Electrical resistivity

0.18 µΩ·m at 20 °C

Electrical conductivity

5.56 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

26.32 J/(mol·K)

Chemical properties

Oxidation states

+3, +1

Electronegativity

1.62 (Pauling Scale)

Ionisation energy

6.108 eV
1st 589.4, 2nd 1,971, 3rd 2,878 kJ/mol

Electron affinity

0.2 eV

Atomic radius

empirical 145, covalent 145, van der Waals 196 pm

Ionic radius

Tl⁺ 150; Tl³⁺ 89 pm

Reactivity

A soft, heavy group 13 metal whose chemistry is dominated by the +1 state (the inert pair leaves the 6s electrons behind), so Tl+ behaves like a mix of potassium and silver, while Tl(III) is a strong oxidant; fresh metal tarnishes in minutes and is attacked by moist air and by oxidising acids.

with water

Reacts slowly with water in the presence of air to thallium(I) hydroxide: $\ce{4 Tl + O2 + 2 H2O -> 4 TlOH}$, a yellow, strongly basic solid; without oxygen the attack is negligible.

with oxygen, air

Tarnishes within minutes to a bluish-grey film and builds a heavy black oxide layer in moist air: $\ce{4 Tl + O2 -> 2 Tl2O}$, so the metal is stored under oil.

with acids

Sulfuric and nitric acid dissolve it rapidly to the sulfate and nitrate: $\ce{2 Tl + H2SO4 -> Tl2SO4 + H2}$, while hydrochloric acid forms an insoluble thallium(I) chloride crust that stops the attack.

with halogens

Combines with the halogens, chlorine at room temperature giving the +1 chloride: $\ce{2 Tl + Cl2 -> 2 TlCl}$, and excess halogen taking it on to the Tl(III) halide.

Typical compounds

• Tl₂SO₄ thallium(I) sulfate odourless, tasteless former rodenticide, banned for household use
• Tl₂O thallium(I) oxide black oxide that forms on the tarnished metal
• TlCl thallium(I) chloride poorly soluble, photosensitive like silver chloride
• Tl₂S thallium(I) sulfide infrared photocell material
• TlBr thallium(I) bromide with the iodide, infrared optical crystals
• TlOH thallium(I) hydroxide yellow, water-soluble strong base

Occurrence, production and use

Crustal abundance

8.5×10-1 milligrams per kilogram

Oceanic abundance

1.9×10-5 milligrams per liter

Occurrence and sources

Thallium occurs in crooksite, lorandite, and hutchinsonite. It is also present in pyrites and is recovered from the roasting of this ore in connection with the production of sulfuric acid. It is also obtained from the smelting of lead and zinc ores. Extraction is somewhat complex and depends on the source of the thallium. Manganese nodules, found on the ocean floor, contain thallium.

• trace in sulfide ores of copper, lead and zinc, and in pyrite recovered from roaster flue dusts in China, Kazakhstan, Russia
• dispersed in potassium minerals of clays, granites and soils; manganese nodules not commercially recoverable
• crustal and oceanic abundance about 0.7 ppm (USGS) to 0.85 ppm (BGS via RSC); 0.85 mg/kg crust and 0.000019 mg/L seawater (PubChem)

Extraction, production

Recovery from flue dust of copper, lead and zinc ore roasting

no separation chemistry stated by the sources; producers withhold data

Uses

There are no uses for metallic thallium since pure thallium quickly combines with oxygen and water vapor from the atmosphere, forming a black, powdery substance. Thallium, used in conjunction with sulfur or selenium and arsenic, forms low melting glass. Thallium sulfate (Tl2SO4), an odorless, tasteless thallium compound, was once used as a rat and ant poison, although it has been banned from household use in the United States since 1974. Thallium sulfide (Tl2S), thallium iodide (TlI) and thallium bromide (TlBr) are all compounds used in devices to detect infrared radiation.

Thallium sulfate has been widely employed as a rodenticide and ant killer. It is odorless and tasteless, giving no warning of its presence. Its use, however, has been prohibited in the U.S. since 1975 as a household insecticide and rodenticide. The electrical conductivity of thallium sulfide changes with exposure to infrared light, and this compound is used in photocells. Thallium bromide-iodide crystals have been used as infrared optical materials. Thallium has been used, with sulfur or selenium and arsenic, to produce low melting glasses with become fluid between 125 and 150C. These glasses have properties at room temperatures similar to ordinary glasses and are said to be durable and insoluble in water. Thallium oxide has been used to produce glasses with a high index of refraction, and is used in the manufacture of photo cells. Thallium has been used in treating ringworm and other skin infections; however, its use has been limited because of the narrow margin between toxicity and therapeutic benefits.

• Electronics and optics: thallium-doped sodium iodide scintillators for gamma detection; photoelectric cells; infrared lenses, prisms and windows; acousto-optic crystal filters; thallium-barium-calcium-copper-oxide superconductors; high-refractive-index and low-melting glass
• Medicine: thallium-201 cardiovascular stress imaging, declining in favour of technetium-99m
• Chemicals: thallium as a trace contaminant of pyrite roasting for sulfuric acid, where Crookes first found it; catalyst for organic synthesis; Clerici solution (thallium malonate formate) heavy liquid
• Mining: thallium in copper, lead and zinc sulfide ores and their roaster dusts

Safety, toxicity

The element and its compounds are toxic and should be handled carefully. Contact of the metal with skin is dangerous, and when melting the metal adequate ventilation should be provided. Exposure to thallium (soluble compounds) - skin, as Tl, should not exceed 0.1 mg/m3 (8-hour time-weighted average - 40-hour work week). Thallium is suspected of carcinogenic potential for man.

GHS classification, signal word Danger

• H300 Fatal if swallowed Acute toxicity, oral
• H330 Fatal if inhaled Acute toxicity, inhalation
• H413 May cause long lasting harmful effects to aquatic life to the aquatic environment, long-term hazard
• H373 May causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
• H310 Fatal in contact with skin Acute toxicity, dermal
• H314 Causes severe skin burns and eye damage Skin corrosion/irritation
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• 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
• H340 May cause genetic defects Germ cell mutagenicity
• H362 May cause harm to breast-fed children toxicity, effects on or via lactation

Discovery and name

Discovered by

William Crookes

Discovered

1861

First isolated

Claude-Auguste Lamy

Named by

not in sources

Origin of the name

after Greek thallos, green shoot or twig

When freshly exposed to air, thallium exhibits a metallic luster, but soon develops a bluish-gray tinge, resembling lead in appearance. A heavy oxide builds up on thallium if left in air, and in the presence of water the hydride is formed. The metal is very soft and malleable. It can be cut with a knife. Twenty five isotopic forms of thallium, with atomic masses ranging from 184 to 210 are recognized. Natural thallium is a mixture of two isotopes. A mercury-thallium alloy, which forms a eutectic at 8.5% thallium, is reported to freeze at -60C, some 20 degrees below the freezing point of mercury.