Strontium (Sr)

Strontium is named after the village of "Strontian" in Scotland. Adair Crawford in 1790 recognized a new mineral (strontianite) in samples of witherite (a mineral consisting of barium carbonate, BaCO3) from Scotland. It was some time before it was recognized that strontianite contained a new element. Strontianite is now known to consists of strontium carbonate, SrCO3. The element itself was not isolated for a number of years after this when strontium metal was isolated by Davy by electrolysis of a mixture containing strontium chloride and mercuric oxide in 1808. Today, strontium is obtained from two of its most common ores, celestite (SrSO4) and strontianite (SrCO3), by treating them with hydrochloric acid, forming strontium chloride. The strontium chloride, usually mixed with potassium chloride (KCl), is then melted and electrolyzed, forming strontium and chlorine gas (Cl2).

Strontium is a bright silvery metal that is softer than calcium and even more reactive in water; strontium will decompose on contact to produce strontium hydroxide and hydrogen gas. It burns in air to produce both a strontium oxide and strontium nitride, but since it does not react with nitrogen below 380ºC it will only form the oxide spontaneously at room temperature. It should be kept under kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal will ignite 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.

In its pure form strontium is extremely reactive with air and spontaneously combusts. It is therefore considered to be a fire hazard. The human body absorbs strontium as if it were calcium. The stable forms of strontium do not pose a significant health threat, but the radioactive strontium-90 can lead to various bone disorders and diseases, including bone cancer. The strontium unit is used in measuring radioactivity from absorbed strontium-90.

Strontium-90 is a by-product of the fission of uranium and plutonium in nuclear reactors, and in nuclear weapons. Strontium-90 is found in waste from nuclear reactors. It can also contaminate reactor parts and fluids. Large amounts of Sr-90 were produced during atmospheric nuclear weapons tests conducted in the 1950s and 1960s and dispersed worldwide.

Strontium-90 is used as a radioactive tracer in medical and agricultural studies. The heat generated by strontium-90's radioactive decay can be converted to electricity for long-lived, light-weight power supplies. These are often used in remote locations, such as in navigational beacons, weather stations, and space vehicles. Strontium-90 is also used in electron tubes, as a radiation source in industrial thickness gauges, and for the treatment of eye diseases. Controlled amounts of strontium-90 have been used as a treatment for bone cancer.

Everyone is exposed to small amounts of strontium-90, since it is widely dispersed in the environment and the food chain. Dietary intake of Sr-90, however, has steadily fallen over the last 30 years with the suspension of nuclear weapons testing. People who live near or work in nuclear facilities may have increased exposure to Sr-90. The greatest concern would be the exposures from an accident at a nuclear reactor, or an accident involving high-level wastes. People may inhale trace amounts of strontium-90 as a contaminant in dust. But, swallowing Sr-90 with food or water is the primary pathway of intake.

When people ingest Sr-90, about 70-80% of it passes through the body. Virtually all of the remaining 20-30% that is absorbed is deposited in the bone. About 1% is distributed among the blood volume, extracellular fluid, soft tissue, and surface of the bone, where it may stay and decay or be excreted.

Strontium-90 is chemically similar to calcium, and tends to deposit in bone and blood-forming tissue (bone marrow). Thus, strontium-90 is referred to as a "bone seeker." Internal exposure to Sr-90 is linked to bone cancer, cancer of the soft tissue near the bone, and leukemia. Risk of cancer increases with increased exposure to Sr-90. The risk depends on the concentration of Sr-90 in the environment, and on the exposure conditions.

Strontium-90 dispersed in the environment, like that from atmospheric weapons testing, is almost impossible to avoid. You may also be exposed to tiny amounts from nuclear power reactors and certain government facilities. The more serious risk to you (though it is unlikely), is that you may unwittingly encounter an industrial instrument containing a Sr-90 radiation source. This is more likely if you work in specific industries:

  • scrap metal sorting, sales and brokerage
  • metal melting and casting
  • municipal landfill operations.