Thorium

Typical wastewaters

• uranium and thorium mining, milling and tailings (acid leachate) Th⁴⁺ with ThSO₄²⁺ and fluoro complexes at low pH; up to 38 mg/L thorium, probably thorium-230, in acidic uranium tailings water and milligrams per litre of thorium-232 in high acid groundwater beneath tailings thorium-230, not thorium-232, is the isotope to measure near uranium mills
• phosphate rock processing and phosphate fertiliser manufacture Th(IV), hydrolysed and particulate at neutral pH; named by ATSDR among the major industrial releases to surface water no concentration read
• tin processing Th(IV), hydrolysed and particulate; named by ATSDR among the major industrial releases to surface water no concentration read
• monazite and zircon sand processing (legacy sites) particulate and sorbed Th(IV) in surface water and groundwater at Superfund and NRC decommissioning sites thorium contamination identified at 21 of 45 Superfund NPL sites and 23 of 38 NRC decommissioning sites
• drinking water treatment residuals (NORM scale, softening and iron removal sludge) thorium and its daughters trapped in solids depositing on sediment and pipe walls, building up over time; radium-228 and thorium-228 join radium-226 in the sludge on the solids rather than in a liquid effluent

1 · Identity

Symbol, number

Th, 90

Oxidation states in water

+4 only, as Th⁴⁺ below pH 3.5, then the hydroxo complexes Th(OH)₂²⁺, Th(OH)₃⁺ and Th(OH)₄ (aq), and above pH 7.5 the carbonate complex Th(OH)₃CO₃⁻ (EPA Kd volume II). Natural thorium is essentially thorium-232 (half life 1.41 x 10^10 years); thorium-230 (8.0 x 10^4 years) and thorium-234 (24.1 days) come from the uranium-238 chain and thorium-228 (1.913 years) from the thorium-232 chain.

Note

The element entry covers monazite, thoria and the reactor hopes. This chapter is about why Th⁴⁺ stays in the sediment and what the radiological limits are.

2 · Occurrence in water

Natural sources

Thorite, thorianite, monazite and zircon resist weathering and do not dissolve at low temperature; what dissolves is capped by hydrous thorium oxide and by sorption (EPA Kd volume II). Fresh water rarely exceeds 1 µg/L; the US population weighted averages in community supplies were below 0.01 pCi/L thorium-232 and below 0.04 pCi/L thorium-230 (ATSDR, citing Cothern). Thorium-232 rarely exceeds 0.1 pCi/L in natural waters and thorium-230 can reach 0.4 pCi/L; Austrian surface water carried 1.24 to 2.90 µg/L and Austrian groundwater 0.5 to 2.90 µg/L; alkaline Mono Lake reached 1.02, 1.41 and 0.7 pCi/L of thorium-228, -230 and -232 (ATSDR).

Anthropogenic sources

Uranium and thorium mining, milling and processing, tin processing, phosphate rock processing and phosphate fertiliser production are the major industrial releases to surface water (ATSDR); acidic leaching of uranium tailings mobilises thorium-230 and thorium (probably thorium-230) has reached 38 mg/L in low pH tailings water (ATSDR), with milligram per litre thorium-232 in high acid groundwater beneath tailings (EPA Kd volume II). Thorium contamination of soil, surface water or groundwater was identified at 21 of 45 Superfund NPL sites and 23 of 38 NRC decommissioning sites, some from monazite and zircon sand processing (EPA Kd volume II). The MWEI BREF data collection had no operator monitoring thorium in extractive waste water (element entry).

3 · Speciation

Th⁴⁺ is the largest tetravalent cation and hydrolyses above pH 3: in pure water the uncomplexed ion dominates below pH 3.5 and then Th(OH)₂²⁺, Th(OH)₃⁺ and Th(OH)₄ (aq) in turn, the last two over the widest pH range. In a typical river water fluoride complexes dominate below pH 5, phosphate complexes between 5 and 7 (constants suspect), and above pH 7.5 more than 95 percent of dissolved thorium is Th(OH)₃CO₃⁻ (EPA Kd volume II). Hydrous thorium oxide caps dissolved thorium at about 10⁻8.5 to below 10⁻9 mol/L (0.0007 to 0.0002 mg/L) between pH 5 and 10 in 0.1 M perchlorate, rising to about 600 mg/L as pH falls from 5.0 to 3.2, and its solubility rises by two to three orders of magnitude in 3 M chloride and dramatically in high carbonate (EPA Kd volume II). Goethite and manganese dioxide sorb thorium from 0 percent at pH 2.5 to 3.5 to 90 to 100 percent at pH 5 to 6.5; sulfate lowers sorption and 100 meq/L carbonate alkalinity or EDTA lifts it off again, so carbonate rich alkaline water and organic rich water are where thorium moves (EPA Kd volume II). Humic and citrate, oxalate and EDTA complexes can dominate in organic rich water; ThEDTA (aq) dominates from pH 2 to 8 at the concentrations Langmuir and Herman considered.

Solubility

Hydrous thorium oxide about 10⁻8.5 to below 10⁻9 mol/L at pH 5 to 10 in 0.1 M NaClO₄, up to 10⁻2.6 mol/L (600 mg/L) at pH 3.2; higher in brines and in carbonate; crystalline ThO₂, thorite and monazite far lower and kinetically inert (EPA Kd volume II).

Hydrolysis

Begins above pH 3; polynuclear species Th₂(OH)₂⁶⁺, Th₄(OH)₈⁸⁺ and Th₆(OH)₁₅⁹⁺ are believed to matter at higher concentration (EPA Kd volume II).

Complexation

Fluoride, phosphate, sulfate, chloride weak, carbonate strong at high pH, humics and chelants strongest (EPA Kd volume II).

Precipitates

Hydrous thorium oxide, ageing to ThO₂; thorium phosphate in phosphate rich water; thorium carried in Fe and Mn oxide sludge.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Thorium sorbs to bottle walls and particles within minutes at neutral pH: acidify at collection, and filter first only if dissolved thorium is the question. Old thorium Kd values were inflated by precipitation of hydrous thorium oxide during the test (EPA), and the same happens in a spiked sample.

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

Radiological: bone seeking alpha emitters with ingestion dose coefficients of 7.2 x 10⁻8 to 2.3 x 10⁻7 Sv/Bq (WHO Table 9.2); chemical toxicity is not the basis of any limit. Thorium compounds are not readily absorbed by the body and intakes from water are very low (ATSDR).

Bioaccumulation

Plant to soil transfer ratio below 0.01 (ATSDR); thorium concentrates in sediment (0.5 to 31 pCi/g of the three isotopes) rather than in water.

Ecotoxicity

Not addressed in the sources read.

Flags

• Most concentration data are 1980s compilations quoted by ATSDR and EPA; no modern survey of thorium in water was read.
• The thorium phosphate constants are suspect and may over predict those complexes, as EPA itself notes.
• The hydrolysis equations are net reactions written from the species distribution, with constants from Stumm and Morgan chapter 6 not re-read.
• The Euratom values were read from the retained UK copy of the directive on legislation.gov.uk.
• EPA 200.8 gives an instrument detection limit for thorium, not a method detection limit; Table 7 was not located in the text read.

Gaps

• No measured thorium concentration in municipal wastewater, rare earth processing effluent or monazite plant effluent was read; the ledger's mining chapter and the MWEI BREF note that no operator monitored it.
• No thorium specific removal efficiency was read; WHO Table 9.4 has no thorium row.
• Alpha spectrometry method numbers for isotopic thorium were not read.
• No aquatic toxicity value was read.
• No GCC discharge standard was read; radioactive waste is prohibited to sewer in Abu Dhabi per the Ra chapter, not re-read here.
• No surface species or surface complexation constants for thorium on goethite or manganese dioxide were read, so the coagulation row carries no equation.

Sources

Identity

Name and symbol

Thorium, Th

Atomic number

90 protons

Position

no group (f-block) · period 7 · f-block · actinide

CAS number

7440-29-1

Atomic structure

Atomic mass

232.0377 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d²
[Rn] 7s²⁶d²

Electrons per shell

2, 8, 18, 32, 18, 10, 2

Valence electrons

4 ns, (n-1)d and (n-2)f

Physical properties

State at room temperature

Solid

Melting point

2,023 K (1,749.85 °C)

Boiling point

5,061 K (4,787.85 °C)

Density

11.72 g/cm3

Appearance

silvery

Thermal conductivity

54.0 W/(m·K)

Electrical resistivity

157 nΩ·m at 0 °C

Electrical conductivity

6.37 MS/m

Crystal structure

facecentredcubic

Molar heat capacity

26.23 J/(mol·K)

Chemical properties

Oxidation states

+4

Electronegativity

1.3 (Pauling Scale)

Ionisation energy

6.08 eV
1st 587, 2nd 1,110, 3rd 1,930 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 206, covalent 206, van der Waals 237 pm

Ionic radius

Th⁴⁺ 94 pm

Reactivity

A highly reactive, electropositive actinide (Th4+/Th at -1.90 V, more electropositive than zirconium) that behaves like a heavier zirconium or hafnium: +4 in essentially all its chemistry, colourless diamagnetic Th4+, and a metal that is slow to corrode when pure and bulk but pyrophoric when powdered.

with water

Slowly attacked by water at ordinary temperature, and more quickly by steam, giving the dioxide and hydrogen: $\ce{Th + 2 H2O -> ThO2 + 2 H2}$, the oxide film then slowing further attack.

with oxygen, air

Pure bulk thorium keeps its lustre for months in air, but oxide-contaminated metal tarnishes grey then black, turnings ignite when heated and burn with a brilliant white light: $\ce{Th + O2 -> ThO2}$, and the powder is pyrophoric.

with acids

Does not dissolve readily in most common acids; hydrochloric acid is the exception and dissolves it: $\ce{Th + 4 HCl -> ThCl4 + 2 H2}$, while nitric acid passivates it unless a little fluoride is present.

with halogens

Reacts with the halogens on heating to the tetrahalides: $\ce{Th + 2 Cl2 -> ThCl4}$, the fluoride ThF4 being the compound reduced with calcium to make the metal.

Typical compounds

• ThO₂ thorium dioxide thoria, highest melting oxide (3300 C), gas mantles and crucibles
• ThF₄ thorium tetrafluoride reduced with calcium to give the metal
• ThCl₄ thorium tetrachloride from the metal and chlorine or hydrochloric acid
• Th(NO₃)₄ thorium nitrate soluble salt of the monazite processing route
• ThSiO₄ thorium silicate the mineral thorite

Occurrence, production and use

Crustal abundance

9.6 milligrams per kilogram

Oceanic abundance

1×10-6 milligrams per liter

Occurrence and sources

Thorium-232 is a primordial nuclide, having existed in its current form for over 4.5 billion years, a half-life is comparable to the age of the Universe and thus predating the formation of the Earth. Thorium was forged in the cores of dying stars through the r-process and scattered across the galaxy by supernovas. Thorium is found in small amounts in most rocks and soils. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals including thorite (ThSiO4), thorianite (ThO2 + UO2) and monazite. Thorianite is a rare mineral and may contain up to about 12% thorium oxide. Monazite contains 2.5% thorium, allanite has 0.1 to 2% thorium and zircon can have up to 0.4% thorium.[66] Thorium-containing minerals occur on all continents. Thorium is now thought to be about three times as abundant as uranium and about as abundant as lead or molybdenum. Thorium is recovered commercially from the mineral monazite, which contains from 3 to 9% ThO2 along with rare-earth minerals.

• monazite, a rare earth and thorium phosphate the world's primary source of thorium, in heavy-mineral-sand placers, carbonatites and veins; recovered for rare earths, with thorium a by-product mostly left unrecovered; China was the leading importer of monazite, supplied by Nigeria, Madagascar, Thailand and Indonesia (usgs-mcs2025, PDF p186 to 187, printed p182 to 183)
• thorite (thorium silicate), uranothorite and thorianite primary thorium minerals; thorite was the mineral of discovery from Brevig
• trace thorium in phosphate rock reporting to phosphogypsum phosphogypsum from wet-process phosphoric acid is stored indefinitely because of weak radioactivity from naturally occurring uranium and thorium and their daughters radium, radon and polonium; the ledger's chemical chapter holds phosphogypsum under the phosphoric-acid hub
• thorium in mine water no operator reported the monitoring of thorium in extractive waste influenced water in the MWEI BREF data collection (PDF p184, printed p156)

Extraction, production

By-product recovery from monazite concentrates; the source does not print the leach chemistry

world production and reserves are associated with monazite recovery in heavy-mineral-sand deposits; thorium consumption worldwide is small compared with most mineral commodities (usgs-mcs2025)

Thorium metal by reducing thorium oxide with calcium, or by electrolysis of the fluoride

$\ce{ThO2 + 2 Ca -> Th + 2 CaO}$

no balanced equation printed by the source

Uses

Thorium is used as an alloying agent to improve magnesium's strength at high temperatures. Thorium is also used to coat tungsten filaments used in electronic devices, such at television sets. When bombarded with neutrons, thorium-232 becomes thorium-233, which eventually decays into uranium-233 through a series of beta decays. Uranium-233 is a fissionable material and can be used as a nuclear fuel.

Thorium oxide (ThO2), one of thorium's compounds, has many uses. It is primarily used in a type of lantern mantel known as a Welsbach mantle. This mantle, which also contains about 1% cerium oxide, glows with a bright white light when it is heated in a gas flame. Thorium oxide has a very high melting point, about 3300°C, and is used to make high temperature crucibles. Thorium oxide is also used to make glass with a high index of refraction that is used to make high quality camera lenses. Thorium oxide is used as a catalyst in the production of sulfuric acid (H2SO4), in the cracking of petroleum products and in the conversion of ammonia (NH3) to nitric acid (HNO3).

Thorium's most stable isotope, thorium-232, has a half-life of about 14,050,000,000 years. It decays into radium-228 through alpha decay or decays through spontaneous fission.

The principal historic use of thorium has been in the preparation of the Welsbach mantle, used for portable gaslights. These mantles, consisting of thorium oxide with about 1% cerium oxide and other ingredients, glow with a dazzling light when heated in a gas flame. Thorium is an important alloying element in magnesium, imparting high strength and creep resistance at elevated temperatures. Because thorium has a low work-function and high electron emission, it is used to coat tungsten wire used in electronic equipment. The oxide is also used to control the grain size of tungsten used for electric lamps; it is also used for high-temperature laboratory crucibles. Glasses containing thorium oxide have a high refractive index and low dispersion. Consequently, they find application in high quality lenses for cameras and scientific instruments. Thorium oxide has also found use as a catalyst in the conversion of ammonia to nitric acid, in petroleum cracking, and in producing sulfuric acid. Thorium metal is a source of nuclear power. There is probably more energy available for use from thorium in the minerals of the earth's crust than from both uranium and fossil fuels. Any sizable demand from thorium as a nuclear fuel is still several years in the future. Work has been done in developing thorium cycle converter-reactor systems. Several prototypes, including the HTGR (high-temperature gas-cooled reactor) and MSRE (molten salt converter reactor experiment), have operated. While the HTGR reactors are efficient, they are not expected to become important commercially for many years because of certain operating difficulties.

• Nuclear: candidate fuel for a new generation of reactors; research and development programmes reported in Australia, Belgium, Brazil, Canada, China, Czechia, Denmark, Finland, France, Germany, India, Israel, Italy, Japan, the Republic of Korea, the Netherlands, Norway, Russia, the United Kingdom and the United States; nuclear medicine India and China are developing thorium reactor plants but the technology is still very new (rsc-element-90)
• Electrical and lighting products: magnetrons in microwave ovens; metal-halide lamps; tungsten filaments; welding electrodes domestic (United States) demand for thorium alloys, compounds and metals was limited in 2024 (usgs-mcs2025, PDF p187, printed p183)
• Catalysts, ceramics and coatings: thorium oxide as an industrial catalyst; high-temperature ceramics; optical coatings (thorium fluoride, now largely replaced by yttrium fluoride and proprietary materials); formerly thoriated glass for high-quality camera lenses
• Metallurgy: alloying agent in magnesium for strength and creep resistance at high temperature (a magnesium alloy with lanthanides, yttrium and zirconium now substitutes in aerospace)
• Heavy-mineral-sand and rare earth mining: none as a product: thorium is a by-product of monazite recovered for rare earths and is the reason monazite residues are radioactive exports of unspecified thorium compounds from the United States were 44,100 kg through August 2024 at a unit value of 73 dollars per kilogram (usgs-mcs2025)
• Phosphoric acid and phosphate fertilisers: none as a product: thorium is a trace constituent of phosphate rock that reports to phosphogypsum, listed for the ledger link

Safety, toxicity

GHS classification, signal word Danger

• H272 May intensify fire; oxidizer Oxidizing liquids; Oxidizing solids
• H302 Harmful if swallowed Acute toxicity, oral
• 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
• H373 May causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure

Discovery and name

Discovered by

Jöns Jakob Berzelius

Discovered

1829

First isolated

not in sources

Named by

not in sources

Origin of the name

after Thor, the Norse god of thunder

When pure, thorium is a silvery-white metal that is air-stable and retains its luster for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming gray and finally black. The physical properties of thorium are greatly influenced by the degree of contamination with the oxide. The purest specimens often contain several tenths of a percent of the oxide. High-purity thorium has been made. Pure thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn. Thorium is dimorphic, changing at 1400°C from a cubic to a body-centered cubic structure. Thorium oxide has a melting point of 3300°C, which is the highest of all oxides. Only a few elements, such as tungsten, and a few compounds, such as tantalum carbide, have higher melting points. Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric. Powdered thorium metal is often pyrophoric and should be handled carefully. When heated in air, thorium turnings ignite and burn brilliantly with a white light.