Both strontium and strontianite are named afterStrontian, a village in Scotland near which the mineral was discovered in 1790 byAdair Crawford andWilliam Cruickshank; it was identified as a new element the next year from its crimson-redflame test color. Strontium was first isolated as a metal in 1808 byHumphry Davy using the then newly discovered process ofelectrolysis. During the 19th century, strontium was mostly used in the production of sugar fromsugar beets (seestrontian process). At the peak of production of televisioncathode-ray tubes, as much as 75% of strontium consumption in the United States was used for the faceplate glass.[9] With the replacement of cathode-ray tubes with other display methods, consumption of strontium has dramatically declined.[9]
While natural strontium (which is mostly theisotope strontium-88) is stable, the syntheticstrontium-90 is radioactive and is one of the most dangerous components ofnuclear fallout, as strontium is absorbed by the body in a similar manner to calcium. Natural stable strontium, on the other hand, is not hazardous to health.
Strontium is adivalent silvery metal with a pale yellow tint whose properties are mostly intermediate between and similar to those of its group neighborscalcium andbarium.[10] It is softer than calcium and harder than barium. Its melting (777 °C) and boiling (1377 °C) points are lower than those of calcium (842 °C and 1484 °C respectively); barium continues this downward trend in the melting point (727 °C), but not in the boiling point (1900 °C). The density of strontium (2.64 g/cm3) is similarly intermediate between those of calcium (1.54 g/cm3) and barium (3.594 g/cm3).[11] Threeallotropes of metallic strontium exist, withtransition points at 235 and 540 °C.[citation needed]
Thestandard electrode potential for the Sr2+/Sr couple is −2.89 V, approximately midway between those of the Ca2+/Ca (−2.84 V) and Ba2+/Ba (−2.92 V) couples, and close to those of the neighboringalkali metals.[12] Strontium is intermediate between calcium and barium in its reactivity toward water, with which it reacts on contact to producestrontium hydroxide andhydrogen gas. Strontium metal burns in air to produce bothstrontium oxide andstrontium nitride, but since it does not react withnitrogen below 380 °C, at room temperature it forms only the oxide spontaneously.[11] Besides the simple oxide SrO, theperoxide SrO2 can be made by direct oxidation of strontium metal under a high pressure of oxygen, and there is some evidence for a yellowsuperoxide Sr(O2)2.[13]Strontium hydroxide, Sr(OH)2, is a strong base, though it is not as strong as the hydroxides of barium or the alkali metals.[14] All four dihalides of strontium are known.[15]
Due to the large size of the heavys-block elements, including strontium, a vast range ofcoordination numbers is known, from 2, 3, or 4 all the way to 22 or 24 in SrCd11 and SrZn13. The Sr2+ ion is quite large, so that high coordination numbers are the rule.[16] The large size of strontium and barium plays a significant part in stabilising strontium complexes withpolydentatemacrocyclic ligands such ascrown ethers: for example, while18-crown-6 forms relatively weak complexes with calcium and the alkali metals, its strontium and barium complexes are much stronger.[17]
Organostrontium compounds contain one or more strontium–carbon bonds. They have been reported as intermediates inBarbier-type reactions.[18][19][20] Although strontium is in the same group as magnesium, andorganomagnesium compounds are very commonly used throughout chemistry, organostrontium compounds are not similarly widespread because they are more difficult to make and more reactive. Organostrontium compounds tend to be more similar to organoeuropium or organosamarium compounds due to the similarionic radii of these elements (Sr2+ 118 pm; Eu2+ 117 pm; Sm2+ 122 pm). Most of these compounds can only be prepared at low temperatures; bulky ligands tend to favor stability. For example, strontium dicyclopentadienyl, Sr(C5H5)2, must be made by directly reacting strontium metal withmercurocene orcyclopentadiene itself; replacing the C5H5 ligand with the bulkier C5(CH3)5 ligand on the other hand increases the compound's solubility, volatility, and kinetic stability.[21]
Because of its extreme reactivity withoxygen and water, strontium occurs naturally only in compounds with other elements, such as in the mineralsstrontianite andcelestine. It is kept under a liquidhydrocarbon such asmineral oil orkerosene to preventoxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal ispyrophoric, meaning that it will ignite spontaneously in air at room temperature. Volatile strontium salts impart a bright red color to flames, and these salts are used inpyrotechnics and in the production offlares.[11] Like calcium and barium, as well as the alkali metals and the divalentlanthanideseuropium andytterbium, strontium metal dissolves directly in liquidammonia to give a dark blue solution of solvated electrons.[10]
Natural strontium is a mixture of four stableisotopes:84Sr,86Sr,87Sr, and88Sr.[11] On these isotopes,88Sr is the most abundant, makes up about 82.6% of all natural strontium, though the abundance varies due to the production ofradiogenic87Sr as the daughter of long-livedbeta-decaying87Rb.[22] This is the basis ofrubidium–strontium dating. Of the unstable isotopes, the primary decay mode of the isotopes lighter than85Sr iselectron capture orpositron emission to isotopes of rubidium, and that of the isotopes heavier than88Sr iselectron emission to isotopes ofyttrium. Of special note are89Sr and90Sr. The former has ahalf-life of 50.6 days and is used to treatbone cancer due to strontium's chemical similarity and hence ability to replace calcium.[23][24] While90Sr (half-life 28.90 years) has been used similarly, it is also an isotope of concern infallout fromnuclear weapons andnuclear accidents due to its production as afission product. Its presence in bones can cause bone cancer, cancer of nearby tissues, andleukemia.[25] The1986 Chernobyl nuclear accident contaminated about 30,000 km2 with greater than 10 kBq/m2 with90Sr, which accounts for about 5% of the90Sr which was in the reactor core.[26]
Strontium is named after the Scottish village ofStrontian (Scottish Gaelic:Sròn an t-Sìthein), where it was discovered in the ores of the lead mines.[27]
In 1790,Adair Crawford, a physician engaged in the preparation of barium, and his colleagueWilliam Cruickshank, recognised that the Strontian ores exhibited properties that differed from those in other "heavy spars" sources.[28] This allowed Crawford to conclude on page 355 "... it is probable indeed, that the scotch mineral is a new species of earth which has not hitherto been sufficiently examined." The physician and mineral collectorFriedrich Gabriel Sulzer analysed together withJohann Friedrich Blumenbach the mineral from Strontian and named it strontianite. He also came to the conclusion that it was distinct from thewitherite and contained a new earth (neue Grunderde).[29] In 1793Thomas Charles Hope, a professor of chemistry at the University of Glasgow studied the mineral[30][31] and proposed the namestrontites.[32][33][34] He confirmed the earlier work of Crawford and recounted: "... Considering it a peculiar earth I thought it necessary to give it an name. I have called it Strontites, from the place it was found; a mode of derivation in my opinion, fully as proper as any quality it may possess, which is the present fashion." The element was eventually isolated by SirHumphry Davy in 1808 by theelectrolysis of a mixture containingstrontium chloride andmercuric oxide, and announced by him in a lecture to the Royal Society on 30 June 1808.[35] In keeping with the naming of the other alkaline earths, he changed the name tostrontium.[36][37][38][39][40]
The first large-scale application of strontium was in the production of sugar fromsugar beet. Although a crystallisation process using strontium hydroxide was patented byAugustin-Pierre Dubrunfaut in 1849[41] the large scale introduction came with the improvement of the process in the early 1870s. The Germansugar industry used the process well into the 20th century. BeforeWorld War I the beet sugar industry used 100,000 to 150,000 tons of strontium hydroxide for thisprocess per year.[42] The strontium hydroxide was recycled in the process, but the demand to substitute losses during production was high enough to create a significant demand initiating mining of strontianite in theMünsterland. The mining of strontianite in Germany ended when mining of thecelestine deposits inGloucestershire started.[43] These mines supplied most of the world strontium supply from 1884 to 1941. Although the celestine deposits in theGranada basin were known for some time the large scale mining did not start before the 1950s.[44]
During atmosphericnuclear weapons testing, it was observed that strontium-90 is one of thenuclear fission products with a relatively high yield. The similarity to calcium and the chance that the strontium-90 might become enriched in bones made research on the metabolism of strontium an important topic.[45][46]
Strontium commonly occurs in nature, being the 15th mostabundant element on Earth (its heavier congener barium being the 14th), estimated to average approximately 360 parts per million in theEarth's crust[47] and is found chiefly as thesulfatemineralcelestine (SrSO4) and thecarbonatestrontianite (SrCO3). Of the two, celestine occurs much more frequently in deposits of sufficient size for mining. Because strontium is used most often in the carbonate form, strontianite would be the more useful of the two common minerals, but few deposits have been discovered that are suitable for development.[48] Because of the way it reacts with air and water, strontium only exists in nature when combined to form minerals. Naturally occurring strontium is stable, but its synthetic isotope Sr-90 is only produced by nuclear fallout.
In groundwater strontium behaves chemically much like calcium. At intermediate to acidicpH Sr2+ is the dominant strontium species. In the presence of calcium ions, strontium commonly formscoprecipitates with calcium minerals such ascalcite and anhydrite at an increased pH. At intermediate to acidic pH, dissolved strontium is bound to soil particles bycation exchange.[49]
The mean strontium content of ocean water is 8 mg/L.[50][51] At a concentration between 82 and 90 μmol/L of strontium, the concentration is considerably lower than the calcium concentration, which is normally between 9.6 and 11.6 mmol/L.[52][53] It is nevertheless much higher than that of barium, 13 μg/L.[11]
The major producers of strontium as celestine as of January 2024 are Spain (200,000 t), Iran (200,000 t), China (80,000 t), Mexico (35,000 t); and Argentina (700 t).[54] Although strontium deposits occur widely in the United States, they have not been mined since 1959.[54]
A large proportion of mined celestine (SrSO4) is converted to the carbonate by two processes. Either the celestine is directly leached with sodium carbonate solution or the celestine is roasted with coal to form the sulfide. The second stage produces a dark-coloured material containing mostlystrontium sulfide. This so-called "black ash" is dissolved in water and filtered. Strontium carbonate is precipitated from the strontium sulfide solution by introduction ofcarbon dioxide.[55] The sulfate isreduced to thesulfide by thecarbothermic reduction:
SrSO4 + 2 C → SrS + 2 CO2
About 300,000 tons are processed in this way annually.[56]
Most of the world's production of strontium used to be consumed in the production of cathode-ray tube (CRT) displays. The glass contained strontium and barium oxide to block X-rays.
Consuming 75% of production, the primary use for strontium was in glass for colour televisioncathode-ray tubes,[56] where it preventedX-ray emission.[57][58] This application for strontium has been declining because CRTs are being replaced by other display methods. This decline has a significant influence on the mining and refining of strontium.[48] All parts of the CRT must absorb X-rays. In the neck and the funnel of the tube, lead glass is used for this purpose, but this type of glass shows a browning effect due to the interaction of the X-rays with the glass. Therefore, the front panel is made from a different glass mixture with strontium and barium to absorb the X-rays. The average values for the glass mixture determined for a recycling study in 2005 is 8.5%strontium oxide and 10%barium oxide.[59]
Because strontium is so similar to calcium, it is incorporated in the bone. All four stable isotopes are incorporated, in roughly the same proportions they are found in nature. However, the actual distribution of the isotopes tends to vary greatly from one geographical location to another. Thus, analyzing the bone of an individual can help determine the region it came from.[60][61] This approach helps to identify the ancient migration patterns and the origin of commingled human remains in battlefield burial sites.[62]
87Sr/86Sr ratios are commonly used to determine the likely provenance areas of sediment in natural systems, especially inmarine andfluvial environments. Dasch (1969) showed that surface sediments of Atlantic displayed87Sr/86Sr ratios that could be regarded as bulk averages of the87Sr/86Sr ratios of geological terrains from adjacent landmasses.[63] A good example of a fluvial-marine system to which Sr isotope provenance studies have been successfully employed is the River Nile-Mediterranean system.[64] Due to the differing ages of the rocks that constitute the majority of theBlue andWhite Nile,catchment areas of the changing provenance of sediment reaching theRiver Nile Delta and East Mediterranean Sea can be discerned through strontium isotopic studies. Such changes are climatically controlled in theLate Quaternary.[64]
Strontium salts are added to fireworks in order to create red colors.
Strontium carbonate and other strontiumsalts are added to fireworks to give a deep red colour.[70] This same effect identifies strontiumcations in theflame test. Fireworks consume about 5% of the world's production.[56] Strontium carbonate is used in the manufacturing of hardferrite magnets.[71][72]
Strontium chloride is sometimes used in toothpastes for sensitive teeth. One popular brand includes 10% total strontium chloride hexahydrate by weight.[73] Small amounts are used in the refining of zinc to remove small amounts of lead impurities.[11] The metal itself has a limited use as agetter, to remove unwanted gases in vacuums by reacting with them, although barium may also be used for this purpose.[12]
The ultra-narrow optical transition between the [Kr]5s21S0 electronicground state and themetastable [Kr]5s5p3P0 excited state of87Sr is one of the leading candidates for the future re-definition of thesecond in terms of an optical transition as opposed to the current definition derived from a microwave transition between differenthyperfine ground states of133Cs.[74] Current opticalatomic clocks operating on this transition already surpass the precision and accuracy of the current definition of the second.[75]
89Sr is the active ingredient inMetastron,[76] aradiopharmaceutical used for bone pain secondary tometastaticbone cancer. The strontium is processed like calcium by the body, preferentially incorporating it into bone at sites of increasedosteogenesis. This localization focuses the radiation exposure on the cancerous lesion.[24]
90Sr has been used as a power source forradioisotope thermoelectric generators (RTGs).90Sr produces approximately 0.93 watts of heat per gram (it is lower for the form of90Sr used in RTGs, which isstrontium fluoride).[77] However,90Sr has one third the lifetime and a lower density than238Pu, another RTG fuel. The main advantage of90Sr is that it is significantly cheaper than238Pu and is found innuclear waste. The latter must be prepared by irradiating237Np with neutrons then separating the modest amounts of238Pu. The principal disadvantage of90Sr is the high energy beta particles produceBremsstrahlung as they encounter nuclei of other nearby heavy atoms such as adjacent strontium. This is mostly in the range of X-rays. Thus strong beta emitters also emit significant secondary X-rays in most cases. This requires significant shielding measures which complicates the design of RTGs using90Sr. TheSoviet Union deployed nearly 1000 of these RTGs on its northern coast as a power source for lighthouses and meteorology stations.[78][79]
Acantharea, a relatively large group of marineradiolarianprotozoa, produce intricate mineralskeletons composed ofstrontium sulfate.[81] In biological systems, calcium is substituted to a small extent by strontium.[82] In the human body, most of the absorbed strontium is deposited in the bones. The ratio of strontium to calcium in human bones is between 1:1000 and 1:2000, roughly in the same range as in the blood serum.[83]
The human body absorbs strontium as if it were its lighter congener calcium. Because the elements are chemically very similar, stable strontium isotopes do not pose a significant health threat. The average human has an intake of about two milligrams of strontium a day.[84] In adults, strontium consumed tends to attach only to the surface of bones, but in children, strontium can replace calcium in the mineral of the growing bones and thus lead to bone growth problems.[85]
Thebiological half-life of strontium in humans has variously been reported as from 14 to 600 days,[86][87] 1,000 days,[88] 18 years,[89] 30 years[90] and, at an upper limit, 49 years.[91] The wide-ranging published biological half-life figures are explained by strontium's complex metabolism within the body. However, by averaging all excretion paths, the overall biological half-life is estimated to be about 18 years.[92] The elimination rate of strontium is strongly affected by age and sex, due to differences inbone metabolism.[93]
The drugstrontium ranelate aidsbone growth, increases bone density, and lessens the incidence of vertebral, peripheral, and hipfractures.[94][95] However, strontium ranelate also increases the risk of venous thromboembolism, pulmonary embolism, and serious cardiovascular disorders, including myocardial infarction. Its use is therefore now restricted.[96] Its beneficial effects are also questionable, since the increased bone density is partially caused by the increased density of strontium over the calcium which it replaces. Strontium alsobioaccumulates in the body.[97] Despite restrictions onstrontium ranelate, strontium is still contained in some supplements.[98][99] There is not much scientific evidence on risks of strontium chloride when taken by mouth. Those with a personal or family history of blood clotting disorders are advised to avoid strontium.[98][99]
Strontium has been shown to inhibit sensory irritation when applied topically to the skin.[100][101] Topically applied, strontium has been shown to accelerate the recovery rate of the epidermal permeability barrier (skin barrier).[102]
Algae has shown selectivity for strontium in studies, where most plants used inbioremediation have not shown selectivity between calcium and strontium, often becoming saturated with calcium, which is greater in quantity and also present in nuclear waste.[103]
Researchers have looked at the bioaccumulation of strontium byScenedesmus spinosus (algae) in simulated wastewater. The study claims a highly selectivebiosorption capacity for strontium ofS. spinosus, suggesting that it may be appropriate for use in treating nuclear wastewater.[105]
A study of the pond algaClosterium moniliferum using non-radioactive strontium found that varying the ratio ofbarium to strontium in water improved strontium selectivity.[103]
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