Barium

Typical wastewaters

• oil and gas produced water and flowback Ba²⁺ at 815 mg/L average in shale gas water and 25 mg/L in coal bed methane water, kept dissolved by the absence of sulfate; BaSO₄ with radium precipitates on mixing with sulfate water downstream of a discharge dissolved barium fell from 13.4 to 0.93 mg/L over 20 to 300 m
• drilling wastes (barite mud) and metal refineries barite BaSO₄ solids in drilling waste; Ba²⁺ in refinery effluent, capped by BaSO₄ where sulfate is present US EPA source list for the barium MCL
• radium removal residuals (barium chloride dosing, barium sulfate impregnated media) Ba(Ra)SO₄ filter solids and spent media carrying radium every barium removal step concentrates the radium
• water softening residuals (cation exchange brine, lime softening sludge) Ba²⁺ in the spent regenerant brine; BaCO₃ in lime sludge; both carry any radium
• textile wet processing total barium, a sample and report only ZDHC parameter; sludge threshold 200 mg/kg

1 · Identity

Symbol, number

Ba, 56

Oxidation states in water

+2 only, as the large, weakly hydrated Ba²⁺ ion; it forms few complexes and behaves as a heavier calcium that is stopped by sulfate rather than by carbonate.

Note

The element entry gives the metal's reactions and the barite and witherite minerals. This chapter is about Ba²⁺ in solution and BaSO₄ in pipes, filters and sludges.

2 · Occurrence in water

Natural sources

Leaching and erosion of barite and witherite deposits and of igneous and sedimentary rocks; barium enters water primarily from natural sources, and deep rock and drift wells can hold milligram levels (WHO). Solubility rises as pH falls and falls where sulfate or carbonate are high, because BaSO₄ and BaCO₃ precipitate (WHO background document). Deep formation waters are sulfate free and therefore carry barium in quantity.

Anthropogenic sources

Drilling wastes (barite mud) and metal refineries, the sources the US EPA lists; industrial emissions and uses; produced water from unconventional gas, where barium averages 815 mg/L in shale gas and 25 mg/L in coal bed methane waters (Willems 2025); barium chloride dosed deliberately to precipitate radium as Ba(Ra)SO₄ (Clifford).

3 · Speciation

Barium is Ba²⁺ across the whole pH range of natural water, with minor BaSO₄ and BaCO₃ ion pairs; it hydrolyses only above pH 13. The solid that controls it is barite: with 0.003 g/L solubility (WHO background document) any water carrying both barium and sulfate at more than a few milligrams per litre is supersaturated, so barium is either low and sulfate rich, or high and sulfate free, never both. Carbonate takes over as the control only in sulfate free alkaline water (witherite, 0.0014 g/L).

Solubility

BaSO₄ 0.003 g/L at 20 C and BaCO₃ 0.0014 g/L at 20 C, against 370 g/L for BaCl₂ and 49 g/L for Ba(OH)₂ (WHO background document Table 2). The solubility product of barite is about 10⁻10 in the Stumm and Morgan table (from the chapter, not re-read), so 1 mg/L barium is at saturation with about 1 mg/L sulfate at low ionic strength, and brine ionic strength raises it several fold.

Hydrolysis

Negligible: Ba²⁺ is the least hydrolysed of the common divalent cations and BaOH⁺ matters only above pH 13 (Stumm and Morgan chapter 6, from the chapter). Barium hydroxide is a strong base and fully dissociated.

Complexation

Weak ion pairs with sulfate, carbonate and chloride; no significant organic complexation. Barium is carried instead by adsorption onto suspended solids and sediment (WHO background document).

Precipitates

BaSO₄ barite (the scale, the drilling mud and the radium carrier), BaCO₃ witherite (softening sludge), Ba(Ra)SO₄ solid solution. Barium chromate and barium phosphate are analytical, not treatment, solids.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidify with nitric acid, never sulfuric; EPA 200.8 notes that nitric acid holds only minimal barium in solution when free sulfate is present, so a sulfate rich sample that is concentrated or evaporated loses barium as barite. Total versus dissolved barium differ where barite particles are suspended; filter in the field for the dissolved figure.

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

Not carcinogenic or genotoxic; soluble salts cause hypertension, arrhythmia and skeletal muscle paralysis in acute poisoning; nephropathy in animals is the guideline end point; a human study found no effect on hypertension at 10 mg/L but was small and short (WHO). Insoluble barite is essentially non toxic, which is why it is the X-ray contrast meal (element entry).

Bioaccumulation

Not addressed as such in the sources read; barium follows calcium into bone and is retained there, and the dissolved fraction is what matters because BaSO₄ is not bioavailable (Willems 2025).

Ecotoxicity

Chronic Ceriodaphnia dubia reproduction EC₂₀ 0.95 mg/L and EC₅₀ 10.1 mg/L dissolved barium; with 410 mg/L NaCl the EC₂₀ was 10.1 mg/L; toxicity is governed by solubility, since BaSO₄ (0.0025 g/L in that paper) is not bioavailable and hardness raises tolerance (Willems 2025). No EPA aquatic life criterion or EU EQS was read for barium.

Flags

• Occurrence figures are national compilations in the WHO 2016 background document (Netherlands, Canada, Sweden, Norway, Italy, USA); the Illinois 1.1 to 10 mg/L supplies are the extreme, not the norm.
• The produced water barium averages (815 and 25 mg/L) come from the introduction of a toxicity paper, not from a produced water database.
• No seawater barium figure was read; the sulfate rich seawater argument is chemistry, not a measured number.
• The barite solubility product and the hydrolysis statement are cited to Stumm and Morgan chapters from memory of the text, not re-read.
• The Ba(Ra)SO₄ co-precipitation and cation exchange equations are written from Clifford's slide notation; the radium removal percentages are his ranges.
• Abu Dhabi values cover two media (marine 2.0 mg/L, sewer 10 mg/L); other GCC states were not read.
• No quantitative barite scaling source (rates, saturation indices, inhibitor doses) was read; the scaling row is descriptive.

Gaps

• No source read gives barium in seawater, surface water as a survey range, municipal wastewater or industrial wastewater other than produced water.
• Barite scaling kinetics, saturation index practice and low sulfate seawater treatment were not sourced; they belong with the oilfield chapter of the ledger.
• Barium in drilling mud discharges and the offshore barite rules (OSPAR, 40 CFR 435) were not read.
• Barium removal percentages are given by WHO as achievable residuals, not as percentages.
• Sorption of barium on iron and manganese oxides and its behaviour in RO concentrate were not sourced.
• EPA aquatic life criteria and EU EQS were not checked for barium beyond noting none in the tables read.
• The chloride ion pairing, the barium hydroxide dissolution and the radium for barium exchange on impregnated media are written as the mass balances behind statements the WHO background document and the EPA radionuclides manual make in words; no stability constants, solubility products or exchange constants beyond the barite value were read.

Sources

Identity

Name and symbol

Barium, Ba

Atomic number

56 protons

Position

group 2 · period 6 · s-block · alkaline earth metal

CAS number

7440-39-3

Atomic structure

Atomic mass

137.327 u

Electron configuration

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

Electrons per shell

2, 8, 18, 18, 8, 2

Valence electrons

2 outer shell

Physical properties

State at room temperature

Solid

Melting point

1,000 K (726.85 °C)

Boiling point

2,170 K (1,896.85 °C)

Density

3.62 g/cm3

Appearance

silvery gray; with a pale yellow tint

Thermal conductivity

18.4 W/(m·K)

Electrical resistivity

332 nΩ·m at 20 °C

Electrical conductivity

3.01 MS/m

Crystal structure

body-centered cubic

Molar heat capacity

28.07 J/(mol·K)

Chemical properties

Oxidation states

+2

Electronegativity

0.89 (Pauling Scale)

Ionisation energy

5.212 eV
1st 502.9, 2nd 965.2, 3rd 3,600 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 215, covalent 215, van der Waals 268 pm

Ionic radius

Ba²⁺ 135 pm

Reactivity

The most reactive of the common alkaline-earth metals, chemically resembling calcium: it oxidises very easily in air, is decomposed by water and alcohol, and is kept under petroleum or another oxygen-free liquid.

with water

Decomposed by water with vigorous release of hydrogen and heat, giving barium hydroxide: $\ce{Ba + 2 H2O -> Ba(OH)2 + H2}$.

with oxygen, air

Oxidises rapidly in air at room temperature to a dark grey oxide layer: $\ce{2 Ba + O2 -> 2 BaO}$; on heating it can form the peroxide BaO2 and, with nitrogen, the nitride.

with acids

Attacked readily by dilute acids with release of hydrogen: $\ce{Ba + 2 HCl -> BaCl2 + H2}$; sulfuric acid does not react because insoluble barium sulfate passivates the surface.

with halogens

Combines with the halogens to the dihalides: $\ce{Ba + Cl2 -> BaCl2}$.

Typical compounds

• BaSO₄ barium sulfate barite; drilling mud, X-ray contrast meal, insoluble
• BaCO₃ barium carbonate witherite; glass making, former rat poison
• BaO barium oxide fluorescent-lamp electrode coating
• BaCl₂ barium chloride electrolysed for the metal; green fireworks
• Ba(NO₃)₂ barium nitrate green colour in fireworks
• BaTiO₃ barium titanate ferroelectric electroceramic

Occurrence, production and use

Crustal abundance

4.25×102 milligrams per kilogram

Oceanic abundance

1.3×10-2 milligrams per liter

Occurrence and sources

It is found only in combination with other elements, chiefly with sulfate and carbonate and is prepared by electrolysis of the chloride.

• barite (BaSO4) India, China, Morocco, Kazakhstan, Mexico, Iran, Laos, Russia, Turkey; Nevada
• witherite (BaCO3) Cumberland, England; a minor ore
• crustal and oceanic abundance about 456 ppm (BGS figure via RSC); 425 mg/kg crust and 0.013 mg/L seawater (PubChem)

Extraction, production

Barite mining, crushing and grinding

drilling-grade barite is ground and used without chemical processing; barium chemicals (chloride, precipitated sulfate and carbonate, oxide, hydroxide, peroxide) are made from it by routes the sources do not state

Electrolysis of molten barium chloride (metal)

$\ce{BaCl2 (l) -> Ba (l) + Cl2 (g)}$

RSC states the route and barium as product; chlorine is the necessary anode product of a chloride melt and is written on that basis

Aluminothermic reduction of barium oxide (metal)

$\ce{3 BaO (s) + 2 Al (l) -> 3 Ba (g) + Al2O3 (s)}$

RSC states barium oxide heated with aluminium powder gives barium; aluminium oxide as co-product is implied, not named

Uses

Barium is used as a getter, a material that combines with and removes trace gases from vacuum tubes.

Barium sulfate (BaSO4), a common barium compound, is used as a filler for rubber, plastics and resins. It can be combined with zinc oxide (ZnO) to make a white pigment known as lithophone or with sodium sulfate (Na2SO4) to make another white pigment known as blanc fixe. Stones made from impure barium sulfate glow when exposed to light and will glow in the dark for up to six years if intensely heated in the presence of charcoal. These stones, known as Bologna stones, were discovered near Bologna, Italy in the early 1500s and were thought to possess magical properties by alchemists. Although all barium compounds are poisonous, barium sulfate can be safely ingested since it does not dissolve in water. It is also a good absorber of X-rays and, when swallowed, can be used to produce X-ray images of the intestinal tract.

Barium carbonate (BaCO3), another common barium compound, is used in the manufacture of ceramics and some types of glass. It is a component in clay slurries used in drilling oil wells. Barium carbonate is used to purify some chemical solutions and is the primary base material for the manufacture of other barium compounds.

Barium forms several other useful compounds. Barium nitrate (Ba(NO3)2) burns with a bright green color and is used in signal flares and fireworks. Barium chloride (BaCl) is used as a water softener. Barium oxide (BaO) easily absorbs moisture and is used as a desiccant. Barium peroxide (BaO2) forms hydrogen peroxide (H2O2) when it is mixed with water and is used as a bleaching agent that activates when wet. Barium titanate (BaTiO3) is used as a dielectric material in capacitors. Barium ferrite (BaO·6Fe2O3) is used to make magnets.

Barium-137m, a radioactive form of barium produced by the decay of cesium-137, has a relatively short half-life and is commonly used in high school and college physics half-life determination experiments.

The metal is used as a "getter" in vacuum tubes. The most important compounds are the peroxide, chloride, sulfate, carbonate, nitrate, and chlorate. Lithopone, a pigment containing barium sulfate and zinc sulfide, has good covering power, and does not darken in the presence of sulfides. The sulfate, as permanent white is also used in paint, in X-ray diagnostic work, and in glassmaking. Barite is extensively used as a weighing agent in oil well drilling fluids, and is used in making rubber. The carbonate has been used as a rat poison, while the nitrate and chlorate give colors in pyrotechnics. The impure sulfide phosphoresces after exposure to the light. All barium compounds that are water or acid soluble are poisonous. Naturally occurring barium is a mixture of seven stable isotopes. Twenty two other radioactive isotopes are known to exist.

• Oil and gas drilling: ground barite as the weighting agent in drilling fluids; no large-scale substitute more than 90 percent of US barite sales in 2024 (usgs-mcs2025-barite)
• Chemicals (pigments, PVC stabilisers, barium salts): lithopone (barium sulfate with zinc sulfide) and precipitated barium sulfate pigments and fillers; barium stabilisers replacing barium-cadmium stabilisers in flexible PVC; barium chloride, carbonate, oxide, hydroxide and peroxide
• Construction and automotive: filler in paints, plastics and rubber; brake and clutch pads, paint primer; high-density concrete for radiation shielding; cement jackets on subsea pipelines, mould-release compounds
• Medicine: ultrapure barium sulfate as X-ray and CT contrast medium for the gastrointestinal tract
• Glass and pyrotechnics: barium in glassmaking; barium nitrate for green fireworks

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H260 In contact with water releases flammable gases which may ignite spontaneously Substances and mixtures which in contact with water, emit flammable gases
• H261 In contact with water releases flammable gas Substances and mixtures which in contact with water, emit flammable gases
• H301 Toxic if swallowed Acute toxicity, oral
• H314 Causes severe skin burns and eye damage Skin corrosion/irritation
• H315 Causes skin irritation Skin corrosion/irritation
• H318 Causes serious eye damage Serious eye damage/eye 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

Discovery and name

Discovered by

Carl Wilhelm Scheele

Discovered

1772

First isolated

Humphry Davy

Named by

not in sources

Origin of the name

from Greek βαρὺς (barys), meaning 'heavy'

Barium is a metallic element, soft, and when pure is silvery white; it belongs to the alkaline earth group, chemically resembling calcium. The metal oxidizes very easily and should be kept under petroleum or other suitable oxygen-free liquids to exclude air. It is decomposed by water or alcohol.