Technetium

Technetium (pronounced /tɛkˈniːʃɪəm/) is the lightest chemical element with no stable isotope. It has atomic number 43 and is given the symbol Tc. The chemical properties of this silvery grey, crystalline transition metal are intermediate between rhenium and manganese. Its short-lived gamma-emitting nuclear isomer ^99mTc (technetium-99m) is used in nuclear medicine for a wide variety of diagnostic tests. ⁹⁹Tc is used as a gamma ray-free source of beta particles. The pertechnetate ion (TcO₄^-) could eventually be used^[clarify] as a strong anodic corrosion inhibitor for mild steel in closed cooling systems.

Before the element was discovered, many of the properties of element 43 were predicted by Dmitri Mendeleev. Mendeleev noted a gap in his periodic table and called the element ekamanganese. In 1937 its isotope ⁹⁷Tc became the first predominantly artificial element to be produced, hence its name (from the Greek τεχνητός, meaning "artificial"). Most technetium produced on Earth is a by-product of fission of uranium-235 in nuclear reactors and is extracted from nuclear fuel rods. No isotope of technetium has a half-life longer than 4.2 million years (⁹⁸Tc), so its detection in red giants in 1952 helped bolster the theory that stars can produce heavier elements. On Earth, technetium occurs in trace but measurable quantities as a product of spontaneous fission in uranium ore or by neutron capture in molybdenum ores.

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Characteristics

Technetium is a silvery-grey radioactive metal with an appearance similar to platinum. However, it is commonly obtained as a grey powder. Its position in the periodic table is between rhenium and manganese and as predicted by the periodic law its properties are intermediate between those two elements. Technetium is unusual among the lighter elements in that it has no stable isotopes. Only technetium and promethium have no stable isotopes, but are followed by elements which do.

Technetium is therefore extremely rare on Earth. Technetium plays no natural biological role and is not normally found in the human body.

The metal form of technetium slowly tarnishes in moist air. Its oxides are TcO₂ and Tc₂O₇. Under oxidizing conditions technetium (VII) will exist as the pertechnetate ion, TcO₄^-.^[5] Common oxidation states of technetium include 0, +2, +4, +5, +6 and +7.^[6] Technetium will burn in oxygen when in powder form.^[7] It dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but it is not soluble in hydrochloric acid. It has characteristic spectral lines at 363 nm, 403 nm, 410 nm, 426 nm, 430 nm, and 485 nm.^[8]

The metal form is slightly paramagnetic, meaning its magnetic dipoles align with external magnetic fields even though technetium is not normally magnetic.^[9] The crystal structure of the metal is hexagonal close-packed. Pure metallic single-crystal technetium becomes a type II superconductor at 7.46 K; irregular crystals and trace impurities raise this temperature to 11.2 K for 99.9% pure technetium powder.^[10] Below this temperature technetium has a very high magnetic penetration depth, the largest among the elements apart from niobium.^[11]

Technetium is produced in quantity by nuclear fission, and spreads more readily than many radionuclides. In spite of the importance of understanding its toxicity in animals and humans, experimental evidence is scant. It appears to have low chemical toxicity. Its radiological toxicity (per unit of mass) is a function of compound, type of radiation for the isotope in question, and the isotope half-life. Technetium-99m is particularly attractive for medical applications, as the radiation from this isotope is a gamma ray with the same wavelength as X-rays used for common medical diagnostic X-ray applications, giving it adequate penetration while causing minimal damage for a gamma photon. This, plus the extremely short half-life of this metastable nuclear isomer, followed by the relatively long half-life of the daughter isotope Tc-99 which allows it to be eliminated from the body before it decays. This leads to a relatively low dose of administered radiation in biologically dose-equivalent amounts (sieverts) for a typical Tc-99m based nuclear scan (see more on this subject below).^[10]

All isotopes of technetium must be handled carefully. The most common isotope, technetium-99, is a weak beta emitter; such radiation is stopped by the walls of laboratory glassware. Soft X-rays are emitted when the beta particles are stopped, but as long as the body is kept more than 30 cm away these should pose no problem. The primary hazard when working with technetium is inhalation of dust; such radioactive contamination in the lungs can pose a significant cancer risk. For most work, careful handling in a fume hood is sufficient; a glove box is not needed.^[10]

Applications

Nuclear medicine

^99mTc ("m" indicates that this is a metastable nuclear isomer) is used in radioactive isotope medical tests, for example as a radioactive tracer that medical equipment can detect in the body.^[12] It is well suited to the role because it emits readily detectable 140 keV gamma rays, and its half-life is 6.01 hours (meaning that about fifteen sixteenths of it decays to ⁹⁹Tc in 24 hours).^[13] Klaus Schwochau's book Technetium lists 31 radiopharmaceuticals based on ^99mTc for imaging and functional studies of the brain, myocardium, thyroid, lungs, liver, gallbladder, kidneys, skeleton, blood and tumors.^[10]

Immunoscintigraphy incorporates ^99mTc into a monoclonal antibody, an immune system protein capable of binding to cancer cells. A few hours after injection, medical equipment is used to detect the gamma rays emitted by the ^99mTc; higher concentrations indicate where the tumor is. This technique is particularly useful for detecting hard-to-find cancers, such as those affecting the intestine. These modified antibodies are sold by the German company Hoechst (now part of Sanofi-Aventis) under the name "Scintium".^[14]

When ^99mTc is combined with a tin compound it binds to red blood cells and can therefore be used to map circulatory system disorders. It is commonly used to detect gastrointestinal bleeding sites. A pyrophosphate ion with ^99mTc adheres to calcium deposits in damaged heart muscle, making it useful to gauge damage after a heart attack.^[15] The sulfur colloid of ^99mTc is scavenged by the spleen, making it possible to image the structure of the spleen.^[16]

Radiation exposure due to diagnostic treatment involving Tc-99m can be kept low. Because ^99mTc has a short half-life and high energy gamma (allowing small amounts to be easily detected), its quick decay into the far-less radioactive ⁹⁹Tc results in relatively less total radiation dose to the patient, per unit of initial activity after administration. In the form administered in these medical tests (usually pertechnetate) both isotopes are quickly eliminated from the body, generally within a few days.^[15]

Technetium for nuclear medicine purposes is usually extracted from technetium-99m generators. ^95mTc, with a half-life of 61 days, is used as a radioactive tracer to study the movement of technetium in the environment and in plant and animal systems.^[10]

Industrial

Technetium-99 decays almost entirely by beta decay, emitting beta particles with consistent low energies and no accompanying gamma rays. Moreover, its long half-life means that this emission decreases very slowly with time. It can also be extracted to a high chemical and isotopic purity from radioactive waste. For these reasons, it is a NIST standard beta emitter, used for equipment calibration.^[10]

Technetium-99 has also been proposed for use in optoelectric and nanoscale nuclear batteries.^[17]

Chemical

Like rhenium and palladium, technetium can serve as a catalyst. For certain reactions, for example the dehydrogenation of isopropyl alcohol, it is a far more effective catalyst than either rhenium or palladium. Of course, its radioactivity is a major problem in finding safe applications.^[10]

Under certain circumstances, a small concentration (5×10⁻⁵ mol/L) of the pertechnetate ion in water can protect iron and carbon steels from corrosion. For this reason, pertechnetate could find use as an anodic corrosion inhibitor for steel, although technetium's radioactivity poses problems for strictly chemical uses such as these. While (for example) CrO₄²⁻ can also inhibit corrosion, it requires a concentration ten times as high. In one experiment, a test specimen was kept in an aqueous solution of pertechnetate for 20 years and was still uncorroded. The mechanism by which pertechnetate prevents corrosion is not well-understood, but seems to involve the reversible formation of a thin surface layer. One theory holds that the pertechnetate reacts with the steel surface to form a layer of technetium dioxide which prevents further corrosion; the same effect explains how iron powder can be used to remove pertechnetate from water. (Activated carbon can also be used for the same effect.) The effect disappears rapidly if the concentration of pertechnetate falls below the minimum concentration or if too high a concentration of other ions is added.

As noted, the radioactive nature of technetium (3 MBq per liter at the concentrations required) makes this corrosion protection impractical in almost all situations. Nevertheless, corrosion protection by pertechnetate ions was proposed (but never adopted) for use in boiling water reactors.^[10]

History

Search for element 43

  For a number of years there was a gap in the periodic table between molybdenum (element 42) and ruthenium (element 44). Many early researchers were eager to be the first to discover and name the missing element; its location in the table suggested that it should be easier to find than other undiscovered elements. It was first thought to have been found in platinum ores in 1828. It was given the name polinium but it turned out to be impure iridium. Then in 1846 the element ilmenium was claimed to have been discovered but was determined to be impure niobium. This mistake was repeated in 1847 with the "discovery" of pelopium.^[18] Dmitri Mendeleev predicted that this missing element, as part of other predictions, would be chemically similar to manganese and gave it the name ekamanganese.

In 1877, the Russian chemist Serge Kern reported discovering the missing element in platinum ore. Kern named what he thought was the new element davyum, after the noted English chemist Sir Humphry Davy, but it was determined to be a mixture of iridium, rhodium and iron. Another candidate, lucium, followed in 1896 but it was determined to be yttrium. Then in 1908 the Japanese chemist Masataka Ogawa found evidence in the mineral thorianite which he thought indicated the presence of element 43. Ogawa named the element nipponium, after Japan (which is Nippon in Japanese). In 2004 H. K Yoshihara utilized "a record of X-ray spectrum of Ogawa's nipponium sample from thorianite [which] was contained in a photographic plate preserved by his family. The spectrum was read and indicated the absence of the element 43 and the presence of the element 75 (rhenium)."^[19]

German chemists Walter Noddack, Otto Berg and Ida Tacke (later Mrs. Noddack) reported the discovery of element 75 and element 43 in 1925 and named element 43 masurium (after Masuria in eastern Prussia, now in Poland, the region where Walter Noddack's family originated).^[20] The group bombarded columbite with a beam of electrons and deduced element 43 was present by examining X-ray diffraction spectrograms. The wavelength of the X-rays produced is related to the atomic number by a formula derived by Henry Moseley in 1913. The team claimed to detect a faint X-ray signal at a wavelength produced by element 43. Contemporary experimenters could not replicate the discovery, and in fact it was dismissed as an error for many years.^[21][22]

In 1998 John T. Armstrong of the National Institute of Standards and Technology ran "computer simulations" of the 1925 experiments and obtained results quite close to those reported by the Noddack team. He claimed that this was further supported by work published by David Curtis of the Los Alamos National Laboratory measuring the (tiny) natural occurrence of technetium.^[21][23] However, the Noddack's experimental results have never been reproduced, and they were unable to isolate any element 43. Debate still exists as to whether the 1925 team actually did discover element 43.

Official discovery and later history

Discovery of element 43 was finally confirmed in a 1937 experiment at the University of Palermo in Sicily conducted by Carlo Perrier and Emilio Segrè. In the summer of 1936 Segrè and his wife visited the United States, first New York at Columbia University, where he had spent time the previous summer, and then Berkeley at Ernest O. Lawrence's Radiation Laboratory. He persuaded cyclotron inventor Lawrence to let him take back some discarded cyclotron parts that had become radioactive. In early 1937 Lawrence mailed him a molybdenum foil that had been part of the deflector in the cyclotron. Segrè enlisted his experienced chemist colleague Perrier to attempt to prove through comparative chemistry that the molybdenum activity was indeed Z = 43, an element not existent in nature because of its instability against nuclear decay. With considerable difficulty they finally succeeded in isolating three distinct decay periods (90, 80, and 50 days) that eventually turned out to be two isotopes, ⁹⁵Tc and ⁹⁷Tc, of technetium, the name given later by Perrier and Segrè to the first man-made element.^[24] University of Palermo officials wanted them to name their discovery panormium, after the Latin name for Palermo, Panormus. The researchers instead named element 43 after the Greek word τεχνητός, meaning "artificial", since it was the first element to be artificially produced.^[20][18] Segrè returned to Berkeley and immediately sought out Glenn T. Seaborg. They isolated the technetium-99m isotope which is now used in some 10,000,000 medical diagnostic procedures annually.^[25]

In 1952 astronomer Paul W. Merrill in California detected the spectral signature of technetium (in particular, light at 403.1 nm, 423.8 nm, 426.8 nm, and 429.7 nm) in light from S-type red giants.^[10] These massive stars near the end of their lives were rich in this short-lived element, meaning nuclear reactions within the stars must be producing it. This evidence was used to bolster the then unproven theory that stars are where nucleosynthesis of the heavier elements occurs.^[26] More recently, such observations provided evidence that elements were being formed by neutron capture in the s-process.^[10]

Since its discovery, there have been many searches in terrestrial materials for natural sources. In 1962, technetium-99 was isolated and identified in pitchblende from the Belgian Congo in extremely small quantities (about 0.2 ng/kg);^[10] there it originates as a spontaneous fission product of uranium-238. This discovery was made by B.T. Kenna and P.K. Kuroda.^[27] There is also evidence that the Oklo natural nuclear fission reactor produced significant amounts of technetium-99, which has since decayed to ruthenium-99.^[10]

Occurrence and production

Natural production

Since technetium is unstable, only minute traces occur naturally in the Earth's crust as a spontaneous fission product of uranium. In 1999 David Curtis (see above) estimated that a kilogram of uranium contains 1 nanogram (1×10⁻⁹ g) of technetium.^[28] Extraterrestrial technetium was found in some red giant stars (S-, M-, and N-types) that contain an absorption line in their spectrum indicating the presence of this element.^[29]

Byproduct production of Tc-99 in fission wastes

In contrast with the rare natural occurrence, bulk quantities of technetium-99 are produced each year from spent nuclear fuel rods, which contain various fission products. The fission of a gram of uranium-235 in nuclear reactors yields 27 mg of ⁹⁹Tc, giving technetium a fission product yield of 6.1%.^[30] Other fissile isotopes also produce similar yields of technetium,^[10] e.g. 4.9% from uranium-233 or 6.21% from plutonium-239.

It is estimated that up to 1994, about 49,000 TBq (78 metric tons) of technetium was produced in nuclear reactors, which is by far the dominant source of terrestrial technetium.^[31] However, only a fraction of the production is used commercially. As of 2005, technetium-99 is available to holders of an ORNL permit for US$83/g plus packing charges.^[32]

Since the yield of technetium-99 as a product of the nuclear fission of both uranium-235 and plutonium-239 is moderate, it is present in radioactive waste of fission reactors and is produced when a fission bomb is detonated. The amount of artificially produced technetium in the environment exceeds its natural occurrence to a large extent. This is due to release by atmospheric nuclear testing along with the disposal and processing of high-level radioactive waste. Due to its high fission yield and relatively high half-life, technetium-99 is one of the main components of nuclear waste. Its decay, measured in becquerels per amount of spent fuel, is dominant at about 10⁴ to 10⁶ years after the creation of the nuclear waste.^[31]

An estimated 160 TBq (about 250 kg) of technetium-99 was released into the environment up to 1994 by atmospheric nuclear tests.^[31] The amount of technetium-99 from nuclear reactors released into the environment up to 1986 is estimated to be on the order of 1000 TBq (about 1600 kg), primarily by nuclear fuel reprocessing; most of this was discharged into the sea. In recent years, reprocessing methods have improved to reduce emissions, but as of 2005 the primary release of technetium-99 into the environment is by the Sellafield plant, which released an estimated 550 TBq (about 900 kg) from 1995–1999 into the Irish Sea. From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year.^[33]

As a result of nuclear fuel reprocessing, technetium has been discharged into the sea in a number of locations, and some seafood contains tiny but measurable quantities. For example, lobster from west Cumbria contains small amounts of technetium.^[34] The anaerobic, spore-forming bacteria in the Clostridium genus are able to reduce Tc(VII) to Tc(IV). Clostridia bacteria play a role in reducing iron, manganese and uranium, thereby affecting these elements' solubility in soil and sediments. Their ability to reduce technetium may determine a large part of Tc's mobility in industrial wastes and other subsurface environments.^[35]

The long half-life of technetium-99 and its ability to form an anionic species makes it (along with ¹²⁹I) a major concern when considering long-term disposal of high-level radioactive waste. In addition, many of the processes designed to remove fission products from medium-active process streams in reprocessing plants are designed to remove cationic species like caesium (e.g., ¹³⁷Cs) and strontium (e.g., ⁹⁰Sr). Hence the pertechnetate is able to escape through these treatment processes. Current disposal options favor burial in geologically stable rock. The primary danger with such a course is that the waste is likely to come into contact with water, which could leach radioactive contamination into the environment. The anionic pertechnetate and iodide are less able to absorb onto the surfaces of minerals so they are likely to be more mobile.

By comparison plutonium, uranium, and caesium are much more able to bind to soil particles. For this reason, the environmental chemistry of technetium is an active area of research. An alternative disposal method, transmutation, has been demonstrated at CERN for technetium-99. This transmutation process is one in which the technetium (⁹⁹Tc as a metal target) is bombarded with neutrons to form the shortlived ¹⁰⁰Tc (half life = 16 seconds) which decays by beta decay to ruthenium (¹⁰⁰Ru). If recovery of usable ruthenium is a goal, an extremely pure technetium target is needed; if small traces of the minor actinides such as americium and curium are present in the target, they are likely to undergo fission and form more fission products which increase the radioactivity of the irradiated target. The formation of ¹⁰⁶Ru (half life 374 days) from the fresh fission is likely to increase the activity of the final ruthenium metal, which will then require a longer cooling time after irradiation before the ruthenium can be used.

The actual production of technetium-99 from spent nuclear fuel is a long process. During fuel reprocessing, it appears in the waste liquid, which is highly radioactive. After sitting for several years, the radioactivity has fallen to a point where extraction of the long-lived isotopes, including technetium-99, becomes feasible. Several chemical extraction processes are used yielding technetium-99 metal of high purity.^[10]

Neutron activation of molybdenum or other pure elements

The meta stable (a state where the nucleus is in an excited state) isotope ^99mTc is produced as a fission product from the fission of uranium or plutonium in nuclear reactors. Because used fuel is allowed to stand for several years before reprocessing, all ⁹⁹Mo and ^99mTc will have decayed by the time that the fission products are separated from the major actinides in conventional nuclear reprocessing. The PUREX raffinate will contain a high concentration of technetium as TcO₄^- but almost all of this will be ⁹⁹Tc. The vast majority of the ^99mTc used in medical work is formed from ⁹⁹Mo which is formed by the neutron activation of ⁹⁸Mo. ⁹⁹Mo has a half-life of 67 hours, so short-lived ^99mTc (half-life: 6 hours), which results from its decay, is being constantly produced.^[36] The hospital then chemically extracts the technetium from the solution by using a technetium-99m generator ("technetium cow," also occasionally called a molybdenum cow).

The normal technetium cow is an alumina column which contains molybdenum-98; inasmuch as aluminium has a small neutron cross section, it is convenient for an alumina column bearing inactive ⁹⁸Mo to be irradiated with neutrons to make the radioactive Mo-99 column for the technetium cow.^[37] By working in this way, there is no need for the complex chemical steps which would be required to separate molybdenum from a fission product mixture. This alternative method requires that an enriched uranium target be irradiated with neutrons to form ⁹⁹Mo as a fission product, then separated.^[38]

Other technetium isotopes are not produced in significant quantities by fission; when needed, they are manufactured by neutron irradiation of parent isotopes (for example, ⁹⁷Tc can be made by neutron irradiation of ⁹⁶Ru).

Isotopes

Main article: isotopes of technetium

Technetium is one of the two elements in the first 82 that have no stable isotopes (in fact, it is the lowest-numbered element that is exclusively radioactive); the other such element is promethium.^[39] The most stable radioisotopes are ⁹⁸Tc (half-life of 4.2 Ma), ⁹⁷Tc (half-life: 2.6 Ma) and ⁹⁹Tc (half-life: 211.1 ka).^[40]

Twenty-two other radioisotopes have been characterized with atomic masses ranging from 87.933 u (⁸⁸Tc) to 112.931 u (¹¹³Tc). Most of these have half-lives that are less than an hour; the exceptions are ⁹³Tc (half-life: 2.75 hours), ⁹⁴Tc (half-life: 4.883 hours), ⁹⁵Tc (half-life: 20 hours), and ⁹⁶Tc (half-life: 4.28 days).^[40]

Technetium also has numerous meta states. ^97mTc is the most stable, with a half-life of 90.1 days (0.097 MeV). This is followed by ^95mTc (half life: 61 days, 0.038 MeV), and ^99mTc (half-life: 6.01 hours, 0.143 MeV). ^99mTc only emits gamma rays, subsequently decaying to ⁹⁹Tc.^[40]

For isotopes lighter than the most stable isotope, ⁹⁸Tc, the primary decay mode is electron capture, giving molybdenum. For the heavier isotopes, the primary mode is beta emission, giving ruthenium, with the exception that ¹⁰⁰Tc can decay both by beta emission and electron capture.^[40][41]

Technetium-99 is the most common and most readily available isotope, as it is a major product of the fission of uranium-235. One gram of ⁹⁹Tc produces 6.2×10⁸ disintegrations a second (that is, 0.62 GBq/g).^[42]

Stability of technetium isotopes

Technetium and promethium are unusual light elements in that they have no stable isotopes. The reason for this is somewhat complicated. ^[43]

Using the liquid drop model for atomic nuclei, one can derive a semiempirical formula for the binding energy of a nucleus. This formula predicts a "valley of beta stability" along which nuclides do not undergo beta decay. Nuclides that lie "up the walls" of the valley tend to decay by beta decay towards the center (by emitting an electron, emitting a positron, or capturing an electron).

For a fixed odd number of nucleons A, the graph of binding energies vs. atomic number (number of protons) is shaped like a parabola (U-shaped), with the most stable nuclide at the bottom. A single beta decay or electron captures then transforms one nuclide of mass A into the next or preceding one, if the product has a lower binding energy and the difference in energy is sufficient to drive the decay mode. When there is only one parabola, there can be only one stable isotope lying on that parabola.^{[citation needed]}

For a fixed even number of nucleons A, the graph is jagged and is better visualized as two separate parabolas for even and odd atomic numbers, because isotopes with an even number of protons and an even number of neutrons are more stable than isotopes with an odd number of neutrons and an odd number of protons.

When there are two parabolas, that is, when the number of nucleons is even, it can happen (rarely) that there is a stable nucleus with an odd number of neutrons and an odd number of protons (although there are only 4 truly stable examples as opposed to very long-lived: the light nuclei: ²H, ⁶Li, ¹⁰B, ¹⁴N). However, if this happens, there can be no stable isotope with an even number of neutrons and an even number of protons.^{[citation needed]}

For technetium (Z=43), the valley of beta stability is centered at around 98 nucleons. However, for every number of nucleons from 95 to 102, there is already at least one stable nuclide of either molybdenum (Z=42) or ruthenium (Z=44).^{[citation needed]} For the isotopes with odd numbers of nucleons, this immediately rules out a stable isotope of technetium, since there can be only one stable nuclide with a fixed odd number of nucleons. For the isotopes with an even number of nucleons, since technetium has an odd number of protons, any isotope must also have an odd number of neutrons. In such a case, the presence of a stable nuclide having the same number of nucleons and an even number of protons rules out the possibility of a stable nucleus.^[44]

References

Works cited

Notes