Strontium

Typical wastewaters

• nuclear fuel cycle releases and weapons test fallout strontium-90 as Sr²⁺, chemically identical to stable strontium in treatment guidance level 10 Bq/L (WHO)
• water softening residuals (ion exchange regenerant brine, lime softening sludge) Sr²⁺ in the sodium chloride regenerant; SrCO₃ co-precipitated with calcite in lime sludge softeners remove 92 to 98 percent across the bed; blending returns strontium to the finished water
• desalination concentrate and cooling tower blowdown Sr²⁺ concentrated with calcium; SrSO₄ (celestite) scale in high sulfate brines cited to the MWH membrane chapter, not re-read
• strontium carbonate, celestite and pyrotechnics manufacture Sr²⁺; the chloride and nitrate are freely soluble named by the element entry; no effluent figure read

1 · Identity

Symbol, number

Sr, 38

Oxidation states in water

+2 only, Sr²⁺, a hardness cation that follows calcium into carbonate, sulfate and phosphate solids and into bone. Strontium-90, half-life about 28 years, a beta emitter with a yttrium-90 daughter, is the regulated radionuclide; strontium-89 is the shorter lived fission product.

Note

The element entry covers celestite, pyrotechnics and magnets; this chapter is the hardness ion and the radionuclide.

2 · Occurrence in water

Natural sources

Leaching of limestone, celestite and strontium bearing sedimentary rocks; the ionic form Sr²⁺ is the exchangeable, water soluble fraction (Health Canada). Under UCMR 3 strontium was found above the 0.3 µg/L reporting level in 62,799 of 62,913 samples (99.8 percent) from all 4,922 systems, and 1,739 samples (2.8 percent) from 286 systems (5.8 percent) exceeded the 1,500 µg/L health reference level. Seawater carries 7.9 mg/L.

Anthropogenic sources

Strontium-90 from weapons test fallout and past releases to the environment, including drinking water sources (WHO chapter 9); strontium carbonate, celestite and pyrotechnics manufacture (element entry); concentration of natural strontium in desalination and cooling tower brines with calcium; ion exchange softener regenerant and lime softening sludge carry the strontium removed from supplies.

3 · Speciation

Strontium is the free Sr²⁺ ion with sulfate and bicarbonate ion pairs; it does not hydrolyse at natural pH and has no redox chemistry. Like calcium it precipitates as the carbonate (strontianite, Ksp 10⁻9.25), the sulfate (celestite, Ksp 10⁻6.46) and the phosphate (Ksp 10⁻27.4) (Health Canada citing Dean 1992 and Najm 2016). In lime softening it co-precipitates with calcite, entering the calcium carbonate lattice and forming strontianite, so its removal tracks calcium removal.

Solubility

SrCO₃ Ksp 10⁻9.25, SrSO₄ Ksp 10⁻6.46, Sr₃(PO₄)₂ Ksp 10⁻27.4 (Health Canada); the chloride and nitrate are freely soluble (element entry). Celestite is far more soluble than barite, so strontium sulfate scale forms only in concentrated brines.

Hydrolysis

None at natural pH; Sr(OH)₂ is a strong base and strontium stays as the free ion to the highest softening pH.

Complexation

Sulfate and bicarbonate ion pairs; strontium is held by cation exchange resins and by inorganic sorbents (zeolites, sodium titanates, silicotitanates) used for radiostrontium (Health Canada). Constants not quoted.

Precipitates

SrCO₃ (strontianite, and in calcite lattice), SrSO₄ (celestite), Sr₃(PO₄)₂.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidify for total strontium; no speciation issue. For treatment assessment sample raw, softened and blended water separately, since blending decides the finished concentration. For strontium-90 the sample volume and the yttrium-90 ingrowth time govern the result.

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

Stable strontium is non toxic at environmental levels and has no biological role; high doses cause adverse bone effects in animals and a few human studies, with the first year of life the most sensitive period (Health Canada MAC 7.0 mg/L). The EPA reference dose is 0.6 mg/kg per day. Strontium-90 lodges in bone and is a bone marrow dose (US Table A critical organ).

Bioaccumulation

Incorporated into bone and shell in place of calcium (element entry); the reason strontium-90 was the fallout nuclide of concern in milk and bone.

Ecotoxicity

Not addressed in the sources read; no US EPA aquatic criterion appears in the table.

Flags

• US occurrence is UCMR 3 screening data for 2013 to 2015; no natural water survey outside the US was read.
• The Ohio plant figures are eight groundwater plants in one high strontium region.
• The celestite scaling statement is cited to the MWH membrane chapter from memory, not re-read.
• The softening equation is a mass balance written here; the papers describe co-precipitation with calcite without printing it.
• The EPA health reference level is quoted as 1,500 µg/L in the 2017 UCMR 3 summary and as 1.5 mg/L in the 2023 paper; the same value.
• Health Canada occurrence data were not extracted; only the MAC and treatment sections were read.
• No GCC standard lists strontium; the row records the absence.

Gaps

• No surface water or non US groundwater concentration range was read.
• No reverse osmosis or nanofiltration rejection figure for strontium was read.
• Strontium-90 analytical detection limits and the Euratom Annex III performance figure were not extracted.
• The EPA 2014 preliminary determination and its health reference level derivation were not read; the Federal Register page did not load.
• Strontium in desalination brines and cooling water as concentrations was not read.
• Other GCC discharge standards were not read.

Sources

Identity

Name and symbol

Strontium, Sr

Atomic number

38 protons

Position

group 2 · period 5 · s-block · alkaline earth metal

CAS number

7440-24-6

Atomic structure

Atomic mass

87.62 u

Electron configuration

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

Electrons per shell

2, 8, 18, 8, 2

Valence electrons

2 outer shell

Physical properties

State at room temperature

Solid

Melting point

1,050 K (776.85 °C)

Boiling point

1,655 K (1,381.85 °C)

Density

2.64 g/cm3

Appearance

silvery white metallic; with a pale yellow tint

Thermal conductivity

35.4 W/(m·K)

Electrical resistivity

132 nΩ·m at 20 °C

Electrical conductivity

7.58 MS/m

Crystal structure

face-centered cubic

Molar heat capacity

26.4 J/(mol·K)

Chemical properties

Oxidation states

+2

Electronegativity

0.95 (Pauling Scale)

Ionisation energy

5.695 eV
1st 549.5, 2nd 1,064.2, 3rd 4,138 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 195, covalent 195, van der Waals 249 pm

Ionic radius

Sr²⁺ 118 pm

Reactivity

An alkaline-earth metal softer and more reactive than calcium, between calcium and barium in its reactions: it tarnishes at once in air, decomposes water, and is stored under kerosene.

with water

Decomposes water more vigorously than calcium, giving strontium hydroxide and hydrogen: $\ce{Sr + 2 H2O -> Sr(OH)2 + H2}$.

with oxygen, air

Freshly cut metal rapidly turns yellow in air as the oxide forms: $\ce{2 Sr + O2 -> 2 SrO}$; the finely divided metal ignites spontaneously, and above 380 C it also takes up nitrogen.

with acids

Reacts vigorously with dilute acids to strontium salts and hydrogen: $\ce{Sr + 2 HCl -> SrCl2 + H2}$.

with halogens

Burns in the halogens to the dihalides: $\ce{Sr + Cl2 -> SrCl2}$.

Typical compounds

• SrCO₃ strontium carbonate strontianite; crimson fireworks, ferrite magnets
• SrSO₄ strontium sulfate celestite, the main ore
• SrO strontium oxide reduced with aluminium to make the metal
• SrCl₂ strontium chloride electrolysed for the metal; toothpaste for sensitive teeth
• Sr(OH)₂ strontium hydroxide strong base from the water reaction
• SrTiO₃ strontium titanate synthetic gem with dispersion above diamond

Occurrence, production and use

Crustal abundance

3.70×102 milligrams per kilogram

Oceanic abundance

7.9 milligrams per liter

Occurrence and sources

• celestite (SrSO4, 43.88 percent Sr) sedimentary evaporite deposits; mined in Iran, Spain, China, Mexico, Argentina
• strontianite (SrCO3) the mineral first found at Strontian, Scotland; a minor ore
• crustal and oceanic abundance about 320 ppm (BGS figure via RSC); 370 mg/kg crust and 7.9 mg/L seawater (PubChem)

Extraction, production

Celestite to strontium carbonate

Celestite is ground and converted to strontium carbonate, the compound from which the nitrate and other salts are derived; neither source states the conversion chemistry

Electrolysis of molten strontium chloride (with potassium chloride flux)

$\ce{SrCl2 (l) -> Sr (l) + Cl2 (g)}$

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

Aluminothermic reduction of strontium oxide under vacuum

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

RSC names strontium oxide and aluminium as reactants and strontium as product; aluminium oxide as the co-product is implied by the reduction, not stated

Uses

Most of the strontium produced today is used in the manufacture of color television picture tubes. It is also used to refine zinc and is combined with iron to make magnets.

Two strontium compounds, strontium carbonate (SrCO3) and strontium nitrate (Sr(NO3)2), burn with a bright, red flame and are used in fireworks and signal flares. Strontium carbonate is also used to make certain kinds of glass and is the base material for making most other strontium compounds.

Strontium-90, a radioactive isotope of strontium, is a common product of nuclear explosions. It has a half-life of about 28.8 years and decays into yttrium-90 through beta decay. Strontium-90 is especially deadly since it has a relatively long half-life, is strongly radioactive and is absorbed by the body, where it accumulates in the skeletal system. The radiation affects the production of new blood cells, which eventually leads to death.

In addition to the medical imaging application described in the image caption above, strontium has found use in producing ferrite magnets and in refining zinc. Strontium titanate is an interesting optical material as it has an extremely high refractive index and an optical dispersion greater than that of diamond. It has been used as a gemstone, but is very soft. It does not occur naturally.

• Electronics (ceramic ferrite magnets): strontium carbonate sintered with iron oxide to make permanent ceramic ferrite magnets about 40 percent of US strontium consumption in 2024 (usgs-mcs2025-strontium)
• Pyrotechnics and signal flares: strontium nitrate for brilliant red colour in fireworks and flares about 40 percent of US consumption in 2024 (usgs-mcs2025-strontium)
• Oil and gas drilling: ground celestite as a weighting agent in drilling fluids, substituting for barite when barite is dear about 2 percent of US consumption in 2024, but the main fate of US celestite imports (usgs-mcs2025-strontium)
• Metallurgy, glass and pigments: electrolytic zinc production; glass; master alloys; pigments and fillers; strontium aluminate glow-in-the-dark paints and plastics other uses together about 18 percent of US consumption in 2024 (usgs-mcs2025-strontium)
• Medicine and consumer products: strontium chloride hexahydrate in toothpaste for sensitive teeth; strontium-90 beta sources for thickness gauges, static elimination and remote power units

Safety, toxicity

GHS classification, signal word Danger

• H260 In contact with water releases flammable gases which may ignite spontaneously Substances and mixtures which in contact with water, emit flammable gases
• 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
• H360D May damage the unborn child Reproductive toxicity
• H250 Catches fire spontaneously if exposed to air Pyrophoric liquids

Discovery and name

Discovered by

William Cruickshank

Discovered

1787

First isolated

Humphry Davy

Named by

not in sources

Origin of the name

after the mineral strontianite, itself named after Strontian, Scotland

Strontium is softer than calcium and decomposes in water more vigorously. It does not absorb nitrogen below 380°C. It should be kept under kerosene to prevent oxidation. Freshly cut strontium has a silvery appearance, but rapidly turns a yellowish color with the formation of the oxide. The finely divided metal ignites spontaneously in air. Volatile strontium salts impart a beautiful crimson color to flames, and these salts are used in pyrotechnics and in the production of flares. Natural strontium is a mixture of four stable isotopes.