Uranium

Typical wastewaters

• uranium mine drainage and mill effluent uranyl carbonate anions UO₂(CO₃)₂²⁻ and UO₂(CO₃)₃⁴⁻ in oxic water, limited in the US at 2 mg/L monthly and 4 mg/L daily limits, not measured concentrations; raw tailings pore water is in the ledger's mining chapter
• uranium mill tailings uranyl carbonate complexes leaching from the tailings; named by WHO among the anthropogenic sources of uranium in drinking water no concentration read
• phosphate fertiliser manufacture and use uranium carried in phosphate fertilisers, named by WHO; phosphogypsum and phosphoric acid figures are in the ledger's fertiliser chapter no concentration read
• drinking water treatment residuals (spent anion exchange brine, coagulation and softening sludge, reverse osmosis concentrate) uranyl carbonate concentrated in the sodium chloride regenerant brine, about 80,000 pCi/L from a 40 µg/L feed over 30,000 bed volumes; 800 pCi/g in dry ferric hydroxide at 50 percent removal and 135 pCi/g in calcium carbonate sludge at 90 percent removal every effective process concentrates uranium and its daughters

1 · Identity

Symbol, number

U, 92

Oxidation states in water

+6 as the uranyl ion UO₂²⁺ and, at natural pH, its carbonate complexes UO₂CO₃, UO₂(CO₃)₂²⁻, UO₂(CO₃)₃⁴⁻ and the calcium uranyl carbonates Ca₂UO₂(CO₃)₃ and CaUO₂(CO₃)₃²⁻, the mobile form of oxic water; +4 as insoluble UO₂ and U(OH)₄ in reducing water and sediments, the immobile form; +5 and +3 are transient. Natural uranium is 99.27 percent uranium-238 with 0.72 percent uranium-235 and 0.0054 percent uranium-234 (WHO background document); 1 µg of natural uranium is 0.67 pCi (Clifford).

Note

The element entry covers the metal, the oxides, yellowcake and the fluorides. This chapter is about the uranyl carbonate anion in groundwater and how to take it out.

2 · Occurrence in water

Natural sources

Leaching from granites and other uranium bearing deposits; drinking water uranium is most commonly natural (WHO). It is mobile in oxic, bicarbonate rich groundwater as the uranyl carbonate anions and immobile in reducing aquifers, so small supplies on crystalline bedrock (Finland, Norway, Canada, New Mexico, central Australia) are the classic problem wells.

Anthropogenic sources

Release in mill tailings, emissions from the nuclear industry, combustion of coal and other fuels, and phosphate fertilisers that contain uranium (WHO); uranium mine drainage and mill effluents, regulated in the US at 2 mg/L monthly; in situ leach operations; phosphogypsum and phosphoric acid in the ledger's fertiliser chapter.

3 · Speciation

Below pH 5 uranium(VI) is the uranyl cation UO₂²⁺; between pH 5 and 6.5 the neutral UO₂CO₃ dominates, between 6.5 and 7.6 the divalent anion UO₂(CO₃)₂²⁻, and above 7.6 the tetravalent anion UO₂(CO₃)₃⁴⁻ (Clifford, at 10⁻2 atm CO₂, 2.38 mg/L uranium, 25 C). In calcium rich water the neutral Ca₂UO₂(CO₃)₃ and the anion CaUO₂(CO₃)₃²⁻ take over, which lowers both toxicity to cells and the affinity for anion resin (WHO background document). Under reducing conditions U(VI) is reduced to U(IV), which hydrolyses to black oxyhydroxide precipitates (element entry) and stays in the sediment.

Solubility

Uranyl salts (nitrate, chloride, ethanoate) are freely soluble; UO₂ and the metal are insoluble (WHO background document Table). Uranium(VI) is limited in natural water not by a solid but by sorption; U(IV) oxide keeps dissolved uranium very low in reducing water.

Hydrolysis

UO₂²⁺ hydrolyses to UO₂OH⁺ and polymeric hydroxo species above about pH 5, but in real water carbonate complexation outcompetes hydrolysis until pH 10.5 (Clifford); U(IV) hydrolyses near neutral pH to black oxyhydroxide precipitates (element entry).

Complexation

Carbonate is the master ligand; calcium and magnesium form ternary uranyl carbonates; phosphate, sulfate and fluoride complexes matter in acid mine and mill water; uranyl binds phosphate, carboxyl and hydroxyl groups of proteins (WHO background document). Constants not printed in the sources read.

Precipitates

UO₂ (uraninite) and U(OH)₄ on reduction; uranyl hydroxide and calcium uranate in lime softening sludge; uranyl phosphates (autunite family) where phosphate is dosed; uranium co-precipitated on Fe(OH)₃ in coagulation; yellowcake U₃O₈ and ammonium diuranate in the mill (element entry).

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidify with nitric acid to pH below 2 after filtration if dissolved uranium is wanted; unfiltered acidified samples give total uranium including particulate U(IV). Anoxic samples oxidise and release sorbed uranium on standing. Report both µg/L and Bq/L when the radiological guidance levels are in play; the conversion depends on the isotope ratio and is 0.67 pCi/µg only for natural abundance.

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

Nephritis is the primary chemically induced effect in humans; epidemiological studies of exposed populations show correlations with urinary alkaline phosphatase and beta microglobulin and modest proximal tubular changes, within the normal range and not consistent; no clear evidence of effects below 30 µg/L; carcinogenicity data insufficient (WHO). Gastrointestinal absorption is 1 to 2 percent; the uranyl ion replaces calcium in bone (WHO background document).

Bioaccumulation

Uranium accumulates in the skeleton and kidney; highest food concentrations in shellfish (WHO background document); not a food chain biomagnifier.

Ecotoxicity

Not addressed in the sources read; no US EPA aquatic life criterion or EU EQS exists for uranium.

Flags

• Occurrence figures are national surveys of the 1980s to 2000s compiled by WHO; the 700 µg/L maximum is one Canadian private supply.
• The speciation pH boundaries are Clifford's for one CO₂ pressure and uranium concentration; they shift with alkalinity and calcium.
• The uranyl hydroxide precipitation equation and the hydrolysis statement are cited to Stumm and Morgan from memory of the text, not re-read.
• The removal percentages come from three reviews and one bench study quoted by WHO (Aieta 1987, Lowry and Lowry 1988, Sorg 1988) and from Clifford's slides; they disagree at the margins and are all quoted.
• The residual activities (80,000 pCi/L brine, 800 pCi/g, 135 pCi/g) are Clifford's worked examples for a 40 µg/L feed, not plant data.
• The 30 µg/L to 20 pCi/L conversion uses Clifford's 0.67 pCi per µg for natural abundance.
• The GCC tables list no uranium; other GCC states not read.

Gaps

• No source read gives uranium in seawater, surface water as a survey, municipal wastewater or measured mill effluent; the ledger's mining and fertiliser chapters hold the tailings and phosphogypsum figures.
• In situ leach restoration, bioreduction and zero valent iron barriers for uranium plumes were not sourced.
• EPA 908.0 and alpha spectrometry detection limits in the drinking water methods were not read; the Euratom performance characteristic stands in.
• Aquatic toxicity of uranium was not sourced.
• Other GCC discharge standards (Saudi, Oman, Qatar) were not read.
• Stability constants of the uranyl carbonate and calcium uranyl carbonate complexes, the U(VI) to U(IV) reduction potential and the surface complexation constants on ferric hydroxide are not printed in the sources read; the reduction, hydrolysis and sorption equations are written from Stumm and Morgan chapters 6, 8 and 9 and the book's own uranium entry, from the chapter, not re-read.
• No equation is written for calcium uranate in lime softening sludge: the sources name the solid without a composition that can be balanced with confidence.

Sources

Identity

Name and symbol

Uranium, U

Atomic number

92 protons

Position

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

CAS number

7440-61-1

Atomic structure

Atomic mass

238.028 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f³ 6d¹
[Rn] 7s²⁵f³⁶d¹

Electrons per shell

2, 8, 18, 32, 21, 9, 2

Valence electrons

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

Physical properties

State at room temperature

Solid

Melting point

1,408 K (1,134.85 °C)

Boiling point

4,404 K (4,130.85 °C)

Density

18.95 g/cm3

Appearance

silvery gray metallic; corrodes to a spalling black oxide coat in air

Thermal conductivity

27.5 W/(m·K)

Electrical resistivity

0.280 µΩ·m at 0 °C

Electrical conductivity

3.57 MS/m

Crystal structure

orthorhombic

Molar heat capacity

27.665 J/(mol·K)

Chemical properties

Oxidation states

+6, +5, +4, +3

Electronegativity

1.38 (Pauling Scale)

Ionisation energy

6.194 eV
1st 597.6, 2nd 1,420 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 196, covalent 196, van der Waals 240 pm

Ionic radius

U³⁺ 103; U⁴⁺ 89; U⁵⁺ 76; U⁶⁺ 73 pm

Reactivity

A silvery-white, strongly electropositive actinide that reacts with almost all non-metals, faster as temperature rises; the +4 and +6 states dominate its chemistry, with +3 and +5 also known in solution.

with water

Finely divided uranium reacts with cold water, and U3+ ions liberate hydrogen from water, so the +3 state is unstable in solution; the +4 ion hydrolyzes near neutral pH to black oxy-hydroxide precipitates.

with oxygen, air

Tarnishes in air to a dark oxide film and is pyrophoric as a powder; the important oxides are UO2 (reactor fuel), UO3 and U3O8, the most stable oxide and the form of yellowcake, to which UO2 slowly converts in air.

with acids

Dissolves in hydrochloric and nitric acid; other non-oxidizing acids attack it only slowly, the metal first giving U3+ which water and air oxidize to U4+ and then to the yellow uranyl ion UO2^2+; unaffected by alkalis.

with halogens

Combines directly with chlorine, bromine and iodine to the tetrahalides and lower halides; fluorides are made from UF4, and fluorination of UF4 gives the volatile hexafluoride: $\ce{UF4 + F2 -> UF6}$.

Typical compounds

• UO₂ uranium dioxide black, the ceramic fuel of most power reactors
• U₃O₈ triuranium octoxide the most stable oxide, the yellowcake reporting standard
• UF₆ uranium hexafluoride volatile white solid used for isotope enrichment
• UF₄ uranium tetrafluoride green salt, the intermediate to UF6 and the metal
• UCl₄ uranium tetrachloride from the metal and chlorine, hydrolyzes in water
• UO₂(NO₃)₂ uranyl nitrate yellow, the soluble uranyl salt of nuclear reprocessing

Occurrence, production and use

Crustal abundance

2.7 milligrams per kilogram

Oceanic abundance

3.2×10-3 milligrams per liter

Occurrence and sources

Uranium is the heaviest naturally-occurring element available in large quantities. The heavier “transuranic” elements are either man-made or they exist only as trace quantities in uranium ore deposits as activation products. Uranium occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals. Uranium, not as rare as once thought, is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as molybdenum or arsenic. It occurs in numerous natural minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, and tobernite. It is also found in phosphate rocks, lignite, monazite sands, and is recovered commercially from these sources. The United States Department of Energy purchases uranium in the form of acceptable U3O8 concentrates. This incentive program has greatly increased the known uranium reserves.

• uraninite (pitchblende, uranium oxide), brannerite and carnotite the uranium ore minerals; pitchblende from silver mines was the mineral of discovery
• uranium in phosphate rock and monazite sands phosphate rock carries natural radioactivity into wet-process phosphoric acid (aaf-bref-2007, PDF p33, printed p5), and phosphogypsum is stored indefinitely because of its uranium and thorium and their daughters (wikipedia-phosphogypsum); the chemical chapter holds this under the phosphoric-acid hub
• uranium mine water and tailings uranium ore extraction contributed 3 questionnaires to the MWEI data collection (PDF p130, printed p102; the Czech Republic reported 2 sites in 2012, PDF p37, printed p9); one underground site reported tank leaching with tailings in ponds (PDF p119, printed p91); covers on uranium extractive waste control radon emissions (PDF p575, printed p547)
• uranium tailings the radioactive waste of conventional uranium mining and milling, holding the decay products of the uranium-238 chain and heavy metals
• naturally occurring radioactive elements (radium, uranium) in produced water produced water from oil and gas formations carries naturally occurring radioactive elements such as radium and uranium (MWEI glossary, PDF p651, printed p623)

Extraction, production

Mining, milling and leaching of uranium ore to yellowcake, a powdered concentrate of about 80 percent uranium oxide obtained from leach solutions

almost 50,000 t of uranium produced in 2022; Kazakhstan, Canada and Namibia 69 percent of world production; Australia, Niger, Russia, Uzbekistan and China above 1,000 t each (wikipedia-uranium-mining); no leach equation is printed by the sources

Enrichment of the purified oxide in uranium-235 for reactor fuel

physical isotope separation, not a chemical reaction; the sources do not describe the conversion chemistry

Uranium metal by reducing uranium halides with Group 1 or Group 2 metals, or uranium oxides with calcium or aluminium

no balanced equation printed by the source; Peligot's 1841 isolation heated uranium tetrachloride with potassium

Breeding: uranium-238 captures a neutron and undergoes beta decay to plutonium-239

a nuclear transformation stated in words by the source; it makes uranium the feed for plutonium

Uses

Since it is naturally radioactive, uranium, usually in the form of uranium dioxide (UO2), is most commonly used in the nuclear power industry to generate electricity. Naturally occurring uranium consists of three isotopes: uranium-234, uranium-235 and uranium-238. Although all three isotopes are radioactive, only uranium-235 is a fissionable material that can be used for nuclear power.

When a fissionable material is struck by a neutron, its nucleus can release energy by splitting into smaller fragments. If some of the fragments are other neutrons, they can strike other atoms and cause them to split as well. A fissionable material, such as uranium-235, is a material capable of producing enough free neutrons to sustain a nuclear chain reaction.

Only 0.7204% of naturally occurring uranium is uranium-235. This is too low a concentration to sustain a nuclear chain reaction without the help of a material known as a moderator. A moderator is a material that can slow down a neutron without absorbing it. Slow neutrons are more likely to react with uranium-235 and reactors using natural uranium can be made using graphite or heavy water as a moderator. Methods also exist for concentrating uranium-235. Once the levels of uranium-235 have been increased to about 3%, normal water can be used as a moderator.

Uranium-238, uranium's most common isotope, can be converted into plutonium-239, a fissionable material that can also be used as a fuel in nuclear reactors. To produce plutonium-239, atoms of uranium-238 are exposed to neutrons. Uranium-239 forms when uranium-238 absorbs a neutron. Uranium-239 has a half-life of about 23 minutes and decays into neptunium-239 through beta decay. Neptunium-239 has a half-life of about 2.4 days and decays into plutonium-239, also through beta decay.

Although it does not occur naturally, uranium-233 is also a fissionable material that can be used as a fuel in nuclear reactors. To produce uranium-233, atoms of thorium-232 are exposed to neutrons. Thorium-233 forms when thorium-232 absorbs a neutron. Thorium-233 has a half-life of about 22 minutes and decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of about 27 days and decays into uranium-233, also through beta decay. If completely fissioned, one pound (0.45 kilograms) of uranium-233 will provide the same amount of energy as burning 1,500 tons (1,350,000 kilograms) of coal.

Uranium is a dense metal that has uses outside of the nuclear power industry. It is used as a target for X-ray production, as ammunition for some types of military weaponry, as a shield against radiation, as a counterweight for aircraft control surfaces and in the gyroscopes of inertial guidance systems.

Uranium compounds have been used for centuries to color glass. A 2,000 year old sample of yellow glass found near Naples, Italy contains uranium oxide. Uranium trioxide (UO3) is an orange powder and has been used in the manufacture of Fiestaware plates. Other uranium compounds have also been used to make vaseline glass and glazes. The uranium within these items is radioactive and should be treated with care.

Uranium's most stable isotope, uranium-238, has a half-life of about 4,468,000,000 years. It decays into thorium-234 through alpha decay or decays through spontaneous fission.

Uranium was used in as coloring agents in ceramic glazes and glass in ancient Rome and in the Middle Ages producing orange-red to lemon yellow hues. More recently it was used as an orange glaze in contemporary Fiestaware© dishware but was later discontinued for health reasons. Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 has the distinction of being the only naturally occurring fissileisotope. This means it can be split into two or three fragments (fission products) by thermal neutrons. Uranium-238 is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. Another fissile isotope, uranium-233, can be produced from natural thorium and is also important in nuclear technology. While uranium-238 has a small probability for spontaneous fission or even induced fission with fast neutrons, uranium-235 and to a lesser degree uranium-233 have a much higher fission cross-section for slow neutrons. In sufficient concentration, these isotopes maintain a sustained nuclear chain reaction. This generates the heat in nuclear power reactors, and produces the fissile material for nuclear weapons. This nuclear conversion can be brought about in breeder reactors where it is possible to produce more new fissionable material than the fissionable material used in maintaining the chain reaction. Depleted uranium (238U) (depleted of uranium-235) is used in balistic armor penetration and as armor plating.

Uranium-238 is not fissile, but is a fertile isotope, because after neutron activation it can produce plutonium-239, another fissile isotope. Indeed, the238U nucleus can absorb one neutron to produce the radioactive isotope uranium-239. 239U decays by beta emission to neptunium-239, also a beta-emitter, that decays in its turn, within a few days into plutonium-239. 239Pu was used as fissile material in the first atomic bomb detonated in the "Trinity test" on 15 July 1945 in New Mexico.

Uranium-235 is of even greater importance because it is the key to utilizing uranium. 235U, while occurring in natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone.

Uranium-235 can be concentrated by gaseous diffusion and other physical processes, if desired, and used directly as a nuclear fuel, instead of natural uranium, or used as an explosive.

Natural uranium, slightly enriched with 235U by a small percentage, is used to fuel nuclear power reactors to generate electricity. Natural thorium can be irradiated with neutrons to produce the important isotope 233U as follows: 232Th(n, gamma) >233Th(beta) >233Pa(beta) >233U. While thorium itself is not fissionable, 233U is, and in this way may be used as a nuclear fuel. One pound of completely fissioned uranium has the fuel value of over 1500 tons of coal.

The uses of nuclear fuels to generate electrical power, to make isotopes for peaceful purposes, and to make explosives are well known. Uranium in the U.S.A. is controlled by the U.S. Nuclear Regulatory Commission. New uses are being found for depleted uranium, i.e., uranium with the percentage of 235U lowered to about 0.2%. Uranium is used in inertial guidance devices, in gyro compasses, as counterweights for aircraft control surfaces, as ballast for missile reentry vehicles, and as a shielding material. Uranium metal is used for X-ray targets for production of high-energy X-rays; the nitrate was once used as a photographic toner, and the acetate was once used in analytical chemistry. Crystals of uranium nitrate are triboluminescent. Uranium salts have also been used for producing yellow "Vaseline" glass and glazes.

• Nuclear power: enriched uranium fuel for electricity generation; breeder feed for plutonium-239; source material for the synthetic transuranium elements nearly all of the world's mined uranium is used to power nuclear power plants (wikipedia-uranium-mining, 2022 context)
• Defence: naval reactor fuel for submarines; nuclear weapons; depleted uranium in ammunition, armour, ship ballast and aircraft counterweights
• Uranium mining and oil and gas produced water: the product of the uranium ore sector; a contaminant with radium-226 in extractive waste water and tailings, removed by membranes and ion exchange (0.035 to 0.145 mg/l after ion exchange, PDF p473, printed p445); a natural radioactive constituent of produced water uranium in discharged extractive waste water below 0.1 mg/l yearly average at the one reporting operator (mwei-bref-2018, PDF p184, printed p156)
• Phosphoric acid and phosphate fertilisers: none as a product: uranium is a trace constituent of phosphate rock that reports to the acid and to phosphogypsum, listed for the ledger link

Safety, toxicity

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
• H411 Toxic to aquatic life with long lasting effects to the aquatic environment, long-term hazard
• H250 Catches fire spontaneously if exposed to air Pyrophoric liquids
• H350 May cause cancer Carcinogenicity
• 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

Discovery and name

Discovered by

Martin Heinrich Klaproth

Discovered

1789

First isolated

Eugène-Melchior Péligot

Named by

not in sources

Origin of the name

after planet Uranus, itself named after Greek god of the sky Uranus

Pure uranium is a silvery white, weakly radioactive metal, which is harder than most elements. It is malleable, ductile, slightly paramagnetic, strongly electropositive and is a poor electrical conductor. Uranium metal has very high density, being approximately 70% denser than lead, but slightly less dense than gold. Uranium metal exhibits in three crystallographic modifications: alpha > (688°C) > beta > (776°C) > gamma. Uranium is pyrophoric when finely divided. It is a little softer than steel and is attacked by cold water in a finely divided state.In air, uranium metal becomes coated with a layer of oxide. Acids dissolve the metal, forming the +3 oxidation state which oxidizes rapidly by water and air to form higher oxidation states. Uranium metal is unaffected by alkalis. Uranium metal can be prepared by reducing uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures. The metal can also be produced by electrolysis of KUF5 or UF4, dissolved in a molten salt mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament.

Uranium metal reacts with almost all nonmetallic elements and their compounds, with reactivity increasing with temperature. Hydrochloric and nitric acids dissolve uranium, but non-oxidizing acids other than hydrochloric acid attack the element very slowly. When finely divided, it can react with cold water. In air, uranium metal oxidizes and becomes coated with a dark layer of uranium oxide. Uranium forms a variety of alloys and compounds with the most important oxidation states being uranium(IV) and uranium(VI), and their two corresponding oxides are, respectively, uranium dioxide, UO2 and uranium trioxide, UO3. Besides the oxides, other Important uranium compounds include fluorides, chlorides, bromides, iodides, carbonates, hydrides, carbides, nitrides, phosphates, etc. At room temperatures, uranium hexafluoride, UF6, has a high vapor pressure, making it useful in the gaseous diffusion process used to separate the rare U-235 from the common U-238 isotope. Uranium hydrides, nitrides and carbides are relatively inertsemimetallic compounds that are minimally soluble in acids and have been used as stable fuel pellets in nuclear power reactor technology.

Uranium exists in aqueous solutions in the +3, +4, +5, and +6 oxidation states. Oxidation state +6 as the UO22+ ion (yellow in color) is the most stable state in solution. Uranium in the +5 state as the UO2+ ion is colorless, quite unstable and disproportionates (reacts with itself) to form the +6 and +4 states. The +4 state (green) is reasonably stable in solution, but the +3 state (dark green or dark red depending on the illumination source - daylight vs fluorescent light) is unstable and easily oxidizes to +4. The +4 state in near-neutral pH solutions readily hydrolyzes to form black oxy-hydroxide precipitates.