Technetium

Template:Pp-move Template:Use dmy dates Template:Infobox technetium Technetium is a chemical element; it has symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. Technetium and promethium are the only radioactive elements whose neighbours in the sense of atomic number are both stable. All available technetium is produced as a synthetic element. Naturally occurring technetium is a spontaneous fission product in uranium ore and thorium ore (the most common source), or the product of neutron capture in molybdenum ores. This silvery gray, crystalline transition metal lies between manganese and rhenium in group 7 of the periodic table, and its chemical properties are intermediate between those of both adjacent elements. The most common naturally occurring isotope is ⁹⁹Tc, in traces only.

Many of technetium's properties had been predicted by Dmitri Mendeleev before it was discovered; Mendeleev noted a gap in his periodic table and gave the undiscovered element the provisional name ekamanganese (Em). In 1937, technetium became the first predominantly artificial element to be produced, hence its name (from the Greek Template:Transliteration, 'artificial', + Template:Nowrap

One short-lived gamma ray–emitting nuclear isomer, technetium-99m, is used in nuclear medicine for a wide variety of tests, such as bone cancer diagnoses. The ground state of the nuclide technetium-99 is used as a gamma ray–free source of beta particles. Long-lived technetium isotopes produced commercially are byproducts of the fission of uranium-235 in nuclear reactors and are extracted from nuclear fuel rods. Because even the longest-lived isotope of technetium has a relatively short half-life (4.21 million years), the 1952 detection of technetium in red giants helped to prove that stars can produce heavier elements.

History

Early assumptions

From the 1860s through 1871, early forms of the periodic table proposed by Dmitri Mendeleev contained a gap between molybdenum (element 42) and ruthenium (element 44). In 1871, Mendeleev predicted this missing element would occupy the empty place below manganese and have similar chemical properties. Mendeleev gave it the provisional name eka-manganese (from eka, the Sanskrit word for one) because it was one place down from the known element manganese.<ref>Template:Cite journal</ref>

Early misidentifications

Many early researchers, both before and after the periodic table was published, 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. This turned out not to be the case, due to technetium's radioactivity.

Year	Claimant	Suggested name	Actual material
1828	Gottfried Osann	Polinium	Iridium
1845	Heinrich Rose	Pelopium<ref name="history-origin">Template:Cite news</ref>	Niobium–tantalum alloy
1847	R. Hermann	Ilmenium<ref>Template:Cite journal</ref>	Niobium–tantalum alloy
1877	Serge Kern	Davyum	Iridium–rhodium–iron alloy
1896	Prosper Barrière	Lucium	Yttrium
1908	Masataka Ogawa	Nipponium	Rhenium, which was the unknown dvi-manganese<ref>Template:Cite journal</ref><ref name=nipponium2022>Template:Cite journal</ref>

Irreproducible results

File:Periodisches System der Elemente (1904-1945, now Gdansk University of Technology).jpg

Template:Lang (Periodic system of the elements) (1904–1945, now at the Gdańsk University of Technology): lack of elements: polonium Template:SupPo (though discovered as early as in 1898 by Maria Sklodowska-Curie), astatine Template:SupAt (1940, in Berkeley), francium Template:SupFr (1939, in France), neptunium Template:SupNp (1940, in Berkeley) and other actinides and lanthanides. Uses old symbols for: argon Template:SupAr (here: A), technetium Template:SupTc (Ma, masurium), xenon Template:SupXe (X), radon Template:SupRn (Em, emanation).

German chemists Walter Noddack, Otto Berg, and Ida Tacke 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).<ref name=multidict/> This name caused significant resentment in the scientific community, because it was interpreted as referring to a series of victories of the German army over the Russian army in the Masuria region during World War I; as the Noddacks remained in their academic positions while the Nazis were in power, suspicions and hostility against their claim for discovering element 43 continued.<ref name=Scerri/> The group bombarded columbite with a beam of electrons and deduced element 43 was present by examining X-ray emission spectrograms.Template:Sfn 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. Later experimenters could not replicate the discovery, and it was dismissed as an error.<ref name="armstrong">Template:Cite journal</ref><ref>Template:Cite news</ref> Still, in 1933, a series of articles on the discovery of elements quoted the name masurium for element 43.<ref>Template:Cite journal</ref> Some more recent attempts have been made to rehabilitate the Noddacks' claims, but they are disproved by Paul Kuroda's study on the amount of technetium that could have been present in the ores they studied: it could not have exceeded Template:Nobr of ore, and thus would have been undetectable by the Noddacks' methods.<ref name=Scerri>Template:Cite book</ref><ref>Template:Cite journal</ref>

Official discovery and later history

The discovery of element 43 was finally confirmed in a 1937 experiment at the University of Palermo in Sicily by Carlo Perrier and Emilio Segrè.<ref>Template:Cite book</ref> In mid-1936, Segrè visited the United States, first Columbia University in New York and then the Lawrence Berkeley National Laboratory in California. He persuaded cyclotron inventor Ernest Lawrence to let him take back some discarded cyclotron parts that had become radioactive. Lawrence mailed him a molybdenum foil that had been part of the deflector in the cyclotron.<ref>Template:Cite book</ref>

Segrè enlisted his colleague Perrier to attempt to prove, through comparative chemistry, that the molybdenum activity was indeed from an element with the atomic number 43. In 1937, they succeeded in isolating the isotopes technetium-95m and technetium-97.<ref name=segre/><ref name=blocks>Template:Harvnb</ref>Template:Disputed inline University of Palermo officials wanted them to name their discovery Template:Lang, after the Latin name for Palermo, Template:Lang. In 1947,<ref name=segre>Template:Cite journal</ref> element 43 was named after the Greek word Template:Transliteration (Template:Lang), meaning 'artificial', since it was the first element to be artificially produced.<ref name=history-origin/><ref name=multidict> Template:Cite web </ref> Segrè returned to Berkeley and met Glenn T. Seaborg. They isolated the metastable isotope technetium-99m, which is now used in some ten million medical diagnostic procedures annually.<ref> Template:Cite book </ref>

In 1952, the astronomer Paul W. Merrill detected the spectral signature of technetium (specifically wavelengths of 403.1 nm, 423.8 nm, 426.2 nm, and 429.7 nm) in light from S-type red giants.<ref>Template:Cite journal</ref> The stars were near the end of their lives but were rich in the short-lived element, which indicated that it was being produced in the stars by nuclear reactions. That evidence bolstered the hypothesis that heavier elements are the product of nucleosynthesis in stars.<ref name=blocks/> More recently, such observations provided evidence that elements are formed by neutron capture in the s-process.<ref name=s8>Template:Harvnb</ref>

Since that discovery, there have been many searches in terrestrial materials for natural sources of technetium. In 1962, technetium-99 was isolated and identified in pitchblende from the Belgian Congo in very small quantities (about 0.2 ng/kg),<ref name=s8/> where it originates as a spontaneous fission product of uranium-238. The natural nuclear fission reactor in Oklo contains evidence that significant amounts of technetium-99 were produced and have since decayed into ruthenium-99.<ref name=s8/>

Characteristics

Physical properties

Technetium is a silvery-gray radioactive metal with an appearance similar to platinum, commonly obtained as a gray powder.Template:Sfn The crystal structure of the bulk pure metal is hexagonal close-packed. Atomic technetium has characteristic emission lines at wavelengths of 363.3 nm, 403.1 nm, 426.2 nm, 429.7 nm, and 485.3 nm.<ref>Template:Cite book</ref> The unit cell parameters of the orthorhombic Tc metal were reported when Tc is contaminated with carbon (Template:Mvar = 0.2805(4), Template:Mvar = 0.4958(8), Template:Mvar = 0.4474(5)·nm for Tc-C with 1.38 wt% C and Template:Mvar = 0.2815(4), Template:Mvar = 0.4963(8), Template:Mvar = 0.4482(5)·nm for Tc-C with 1.96 wt% C ).<ref name="carbide"/> The metal form is slightly paramagnetic, meaning its magnetic dipoles align with external magnetic fields, but will assume random orientations once the field is removed.<ref name=enc>Template:Cite book</ref> Pure, metallic, single-crystal technetium becomes a type-II superconductor at temperatures below Template:Convert.Template:SfnTemplate:Efn Below this temperature, technetium has a very high magnetic penetration depth, greater than any other element except niobium.<ref>Template:Cite conference</ref>

Chemical properties

Technetium is located in group 7 of the periodic table, between rhenium and manganese. As predicted by the periodic law, its chemical properties are between those two elements. Of the two, technetium more closely resembles rhenium, particularly in its chemical inertness and tendency to form covalent bonds.Template:Sfn This is consistent with the tendency of period 5 elements to resemble their counterparts in period 6 more than period 4 due to the lanthanide contraction. Unlike manganese, technetium does not readily form cations (ions with net positive charge). Technetium exhibits nine oxidation states from −1 to +7, with +4, +5, and +7 being the most common.<ref name=LANL/> Technetium dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but not in hydrochloric acid of any concentration.Template:Sfn

Metallic technetium slowly tarnishes in moist air<ref name=LANL>Template:Cite web</ref> and, in powder form, burns in oxygen. When reacting with hydrogen at high pressure, it forms the non-stoichiometric hydride TcHTemplate:Sub<ref name="Zhou 2023">Template:Cite journal</ref> and while reacting with carbon it forms TcTemplate:SubC,<ref name=carbide>Template:Cite journal</ref> with cell parameter 0.398 nm.

Technetium can catalyse the destruction of hydrazine by nitric acid, and this property is due to its multiplicity of valencies.<ref>Template:Cite journal</ref> This caused a problem in the separation of plutonium from uranium in nuclear fuel processing, where hydrazine is used as a protective reductant to keep plutonium in the trivalent rather than the more stable tetravalent state. The problem was exacerbated by the mutually enhanced solvent extraction of technetium and zirconium at the previous stage,<ref>Template:Cite journal</ref> and required a process modification.

Compounds

Pertechnetate and other derivatives

File:Pertechnetate1.svg

Pertechnetate is one of the most available forms of technetium. It is structurally related to permanganate.

The most prevalent form of technetium that is easily accessible is sodium pertechnetate, Na[TcO₄]. The majority of this material is produced by radioactive decay from [⁹⁹MoO₄]²⁻:Template:Sfn<ref name="nuclmed" />

Pertechnetate (Template:Chem) is only weakly hydrated in aqueous solutions,<ref>Template:Cite journal</ref> and it behaves analogously to perchlorate anion, both of which are tetrahedral. Unlike permanganate (Template:Chem), it is only a weak oxidizing agent.

Related to pertechnetate is technetium heptoxide. This pale-yellow, volatile solid is produced by oxidation of Tc metal and related precursors:

It is a molecular metal oxide, analogous to manganese heptoxide. It adopts a centrosymmetric structure with two types of Tc−O bonds with 167 and 184 pm bond lengths.<ref>Template:Cite journal</ref>

Technetium heptoxide hydrolyzes to pertechnetate and pertechnetic acid, depending on the pH:Template:Sfn<ref>Template:Cite book</ref>

HTcO₄ is a strong acid. In concentrated sulfuric acid, [TcO₄]⁻ converts to the octahedral form TcO₃(OH)(H₂O)₂, the conjugate base of the hypothetical triaquo complex [TcO₃(H₂O)₃]⁺.<ref>Template:Cite journal</ref>

Other chalcogenide derivatives

Technetium forms a dioxide,Template:Sfn disulfide, diselenide, and ditelluride. An ill-defined Tc₂S₇ forms upon treating pertechnate with hydrogen sulfide. It thermally decomposes into disulfide and elemental sulfur.Template:Sfn Similarly the dioxide can be produced by reduction of the Tc₂O₇.

Unlike the case for rhenium, a trioxide has not been isolated for technetium. However, TcO₃ has been identified in the gas phase using mass spectrometry.<ref>Template:Cite journal</ref>

Simple hydride and halide complexes

Technetium forms the complex Template:Chem. The potassium salt is isostructural with [[Potassium nonahydridorhenate|Template:Chem]].Template:Sfn At high pressure formation of TcH_1.3 from elements was also reported.<ref name="Zhou 2023"/>

File:Zirconium-tetrachloride-3D-balls-A.png

TcCl₄ forms chain-like structures, similar to the behavior of several other metal tetrachlorides.

The following binary (containing only two elements) technetium halides are known: TcF₆, TcF₅, TcCl₄, TcBr₄, TcBr₃, α-TcCl₃, β-TcCl₃, TcI₃, α-TcCl₂, and β-TcCl₂. The oxidation states range from Tc(VI) to Tc(II). Technetium halides exhibit different structure types, such as molecular octahedral complexes, extended chains, layered sheets, and metal clusters arranged in a three-dimensional network.<ref>Template:Cite thesis</ref><ref name=AS>Template:Cite journal</ref> These compounds are produced by combining the metal and halogen or by less direct reactions.

TcCl₄ is obtained by chlorination of Tc metal or Tc₂O₇. Upon heating, TcCl₄ gives the corresponding Tc(III) and Tc(II) chlorides.<ref name=AS/>

The structure of TcCl₄ is composed of infinite zigzag chains of edge-sharing TcCl₆ octahedra. It is isomorphous to transition metal tetrachlorides of zirconium, hafnium, and platinum.<ref name="AS" />

File:Chloro-containing coordination complexes of technetium (Tc-99).jpg

Chloro-containing coordination complexes of technetium (⁹⁹Tc) in various oxidation states: Tc(III), Tc(IV), Tc(V), and Tc(VI) represented.

Two polymorphs of technetium trichloride exist, α- and β-TcCl₃. The α polymorph is also denoted as Tc₃Cl₉. It adopts a confacial bioctahedral structure.<ref>Template:Cite journal</ref> It is prepared by treating the chloro-acetate Tc₂(O₂CCH₃)₄Cl₂ with HCl. Like Re₃Cl₉, the structure of the α-polymorph consists of triangles with short M-M distances. β-TcCl₃ features octahedral Tc centers, which are organized in pairs, as seen also for molybdenum trichloride. TcBr₃ does not adopt the structure of either trichloride phase. Instead it has the structure of molybdenum tribromide, consisting of chains of confacial octahedra with alternating short and long Tc—Tc contacts. TcI₃ has the same structure as the high temperature phase of TiI₃, featuring chains of confacial octahedra with equal Tc—Tc contacts.<ref name=AS/>

Several anionic technetium halides are known. The binary tetrahalides can be converted to the hexahalides [TcX₆]²⁻ (X = F, Cl, Br, I), which adopt octahedral molecular geometry.<ref name=s8/> More reduced halides form anionic clusters with Tc–Tc bonds. The situation is similar for the related elements of Mo, W, Re. These clusters have the nuclearity Tc₄, Tc₆, Tc₈, and Tc₁₃. The more stable Tc₆ and Tc₈ clusters have prism shapes where vertical pairs of Tc atoms are connected by triple bonds and the planar atoms by single bonds. Every technetium atom makes six bonds, and the remaining valence electrons can be saturated by one axial and two bridging ligand halogen atoms such as chlorine or bromine.<ref>Template:Cite journal</ref>

Coordination and organometallic complexes

File:Tc CNCH2CMe2(OMe) 6Cation.png

Technetium (^99mTc) sestamibi ("Cardiolite") is widely used for imaging of the heart.

Technetium forms a variety of coordination complexes with organic ligands. Many have been well-investigated because of their relevance to nuclear medicine.<ref>Template:Cite journal</ref>

Technetium forms a variety of compounds with Tc–C bonds, i.e. organotechnetium complexes. Prominent members of this class are complexes with CO, arene, and cyclopentadienyl ligands.<ref name=Alberto/> The binary carbonyl Tc₂(CO)₁₀ is a white volatile solid.<ref>Template:Cite journal</ref> In this molecule, two technetium atoms are bound to each other; each atom is surrounded by octahedra of five carbonyl ligands. The bond length between technetium atoms, 303 pm,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> is significantly larger than the distance between two atoms in metallic technetium (272 pm). Similar carbonyls are formed by technetium's congeners, manganese and rhenium.Template:Sfn Interest in organotechnetium compounds has also been motivated by applications in nuclear medicine.<ref name=Alberto>Template:Cite book</ref> Technetium also forms aquo-carbonyl complexes, one prominent complex being [Tc(CO)₃(H₂O)₃]⁺, which are unusual compared to other metal carbonyls.<ref name="Alberto" />

Isotopes

Template:Main Technetium, with atomic number Z = 43, is the lowest-numbered element in the periodic table for which all isotopes are radioactive. The second-lightest exclusively radioactive element, promethium, has atomic number 61.<ref name=LANL/> Atomic nuclei with an odd number of protons are less stable than those with even numbers, even when the total number of nucleons (protons + neutrons) is even,<ref>Template:Cite book</ref> and odd numbered elements have fewer stable isotopes.

The most stable radioactive isotopes are technetium-97 with a half-life of Template:Val million years and technetium-98 with Template:Val million years; current measurements of their half-lives give overlapping confidence intervals corresponding to one standard deviation and therefore do not allow a definite assignment of technetium's most stable isotope. The next most stable isotope is technetium-99, which has a half-life of 211,100 years.Template:NUBASE2020 Thirty-four other radioisotopes have been characterized with mass numbers ranging from 86 to 122.Template:NUBASE2020 Most of these have half-lives that are less than an hour, the exceptions being technetium-93 (2.73 hours), technetium-94 (4.88 hours), technetium-95 (20 hours), and technetium-96 (4.3 days).<ref name=CRCisotopes/>

The primary decay mode for isotopes lighter than technetium-98 (⁹⁸Tc) is electron capture, producing molybdenum (Z = 42).<ref name=NNDC/> For technetium-98 and heavier isotopes, the primary mode is beta emission (the emission of an electron or positron), producing ruthenium (Z = 44), with the exception that technetium-100 can decay both by beta emission and electron capture.<ref name=NNDC> Template:Cite web </ref><ref> Template:Cite book </ref>

Technetium also has numerous nuclear isomers, which are isotopes with one or more excited nucleons. Technetium-97m (^97mTc; "m" stands for metastability) is the most stable, with a half-life of 91 days and excitation energy 0.0965 MeV.<ref name=CRCisotopes> Template:Cite book </ref> This is followed by technetium-95m (61 days, 0.03 MeV), and technetium-99m (6.01 hours, 0.142 MeV).<ref name="CRCisotopes" />

Technetium-99 (⁹⁹Tc) is a major product of the fission of uranium-235 (²³⁵U), making it the most common and most readily available isotope of technetium. One gram of technetium-99 produces Template:Nobr per second (in other words, the specific activity of ⁹⁹Tc is 0.62 GBq/g).<ref name=enc/>

Occurrence and production

Technetium occurs naturally in the Earth's crust in minute concentrations of about 0.003 parts per trillion. Technetium is so rare because the half-lives of ⁹⁷Tc and ⁹⁸Tc are only Template:Nobr More than a thousand of such periods have passed since the formation of the Earth, so the probability of survival of even one atom of primordial technetium is effectively zero. However, small amounts exist as spontaneous fission products in uranium ores. A kilogram of uranium contains an estimated 1 nanogram Template:Nobr, equivalent to ten trillion atoms, of technetium.<ref name=blocks/><ref> Template:Cite journal </ref><ref> Template:Cite journal </ref> Some red giant stars with the spectral types S, M, and N display a spectral absorption line indicating the presence of technetium.Template:Sfn<ref>Template:Cite journal</ref> These red giants are known informally as technetium stars.

Fission waste product

In contrast to 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 technetium-99, giving technetium a fission product yield of 6.1%.<ref name="enc" /> Other fissile isotopes produce similar yields of technetium, such as 4.9% from uranium-233 and 6.21% from plutonium-239.Template:Sfn An estimated 49,000 TBq (78 metric tons) of technetium was produced in nuclear reactors between 1983 and 1994, by far the dominant source of terrestrial technetium.<ref name=yoshihara> Template:Cite book </ref><ref name=leon/> Only a fraction of the production is used commercially.Template:Efn

Technetium-99 is produced by the nuclear fission of both uranium-235 and plutonium-239. It is therefore present in radioactive waste and in the nuclear fallout of fission bomb explosions. Its decay, measured in becquerels per amount of spent fuel, is the dominant contributor to nuclear waste radioactivity after about Template:Nobr after the creation of the nuclear waste.<ref name=yoshihara/> From 1945–1994, an estimated 160 TBq (about 250 kg) of technetium-99 was released into the environment during atmospheric nuclear tests.<ref name=yoshihara/><ref> Template:Cite book </ref> The amount of technetium-99 from nuclear reactors released into the environment up to 1986 is on the order of 1000 TBq (about 1600 kg), primarily by nuclear fuel reprocessing; most of this was discharged into the sea. Reprocessing methods have reduced emissions since then, 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 to 1999 into the Irish Sea.<ref name=leon> Template:Cite journal </ref> From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year.<ref> Template:Cite journal </ref> Discharge of technetium into the sea resulted in contamination of some seafood with minuscule quantities of this element. For example, European lobster and fish from west Cumbria contain about 1 Bq/kg of technetium.<ref> Template:Cite book </ref><ref> Template:Cite journal </ref>Template:Efn

Fission product for commercial use

The metastable isotope technetium-99m is continuously produced as a fission product from the fission of uranium or plutonium in nuclear reactors:

<chem display="block"> ^{238}_{92}U ->[\ce{sf}] ^{137}_{53}I + ^{99}_{39}Y + 2^{1}_{0}n</chem> <chem display="block"> ^{99}_{39}Y ->[\beta^-][1.47\,\ce{s}] ^{99}_{40}Zr ->[\beta^-][2.1\,\ce{s}] ^{99}_{41}Nb ->[\beta^-][15.0\,\ce{s}] ^{99}_{42}Mo ->[\beta^-][65.94\,\ce{h}] ^{99}_{43}Tc ->[\beta^-][211,100\,\ce{y}] ^{99}_{44}Ru</chem>

Because used fuel is allowed to stand for several years before reprocessing, all molybdenum-99 and technetium-99m is decayed by the time that the fission products are separated from the major actinides in conventional nuclear reprocessing. The liquid left after plutonium–uranium extraction (PUREX) contains a high concentration of technetium as Template:Chem but almost all of this is technetium-99, not technetium-99m.Template:Sfn

The vast majority of the technetium-99m used in medical work is produced by irradiating dedicated highly enriched uranium targets in a reactor, extracting molybdenum-99 from the targets in reprocessing facilities,<ref name="nuclmed">Template:Cite journal</ref> and recovering at the diagnostic center the technetium-99m produced upon decay of molybdenum-99.<ref>Template:Cite patent</ref><ref>Template:Cite book</ref> Molybdenum-99 in the form of molybdate Template:Chem is adsorbed onto acid alumina (Template:Chem) in a shielded column chromatograph inside a technetium-99m generator ("technetium cow", also occasionally called a "molybdenum cow"). Molybdenum-99 has a half-life of 67 hours, so short-lived technetium-99m (half-life: 6 hours), which results from its decay, is being constantly produced.<ref name="blocks" /> The soluble pertechnetate Template:Chem can then be chemically extracted by elution using a saline solution. A drawback of this process is that it requires targets containing uranium-235, which are subject to the security precautions of fissile materials.<ref>Template:Cite news</ref><ref>Template:Cite conference</ref>

File:First technetium-99m generator - 1958.jpg

The first technetium-99m generator, unshielded, 1958. A Tc-99m pertechnetate solution is being eluted from Mo-99 molybdate bound to a chromatographic substrate

Almost two-thirds of the world's supply comes from two reactors; the National Research Universal Reactor at Chalk River Laboratories in Ontario, Canada, and the High Flux Reactor at Nuclear Research and Consultancy Group in Petten, Netherlands. All major reactors that produce technetium-99m were built in the 1960s and are close to the end of life. The two new Canadian Multipurpose Applied Physics Lattice Experiment reactors planned and built to produce 200% of the demand of technetium-99m relieved all other producers from building their own reactors. With the cancellation of the already tested reactors in 2008, the future supply of technetium-99m became problematic.<ref>Template:Cite journal</ref>

Waste disposal

The long half-life of technetium-99 and its potential to form anionic species creates a major concern for long-term disposal of radioactive waste. Many of the processes designed to remove fission products in reprocessing plants aim at cationic species such as caesium (e.g., caesium-137) and strontium (e.g., strontium-90). Hence the pertechnetate escapes through those processes. Current disposal options favor burial in continental, geologically stable rock. The primary danger with such practice is the likelihood that the waste will contact water, which could leach radioactive contamination into the environment. The anionic pertechnetate and iodide tend not to adsorb into the surfaces of minerals, and are likely to be washed away. By comparison plutonium, uranium, and caesium tend to bind to soil particles. Technetium could be immobilized by some environments, such as microbial activity in lake bottom sediments,<ref>Template:Cite journal</ref> and the environmental chemistry of technetium is an area of active research.<ref>Template:Cite book</ref>

An alternative disposal method, transmutation, has been demonstrated at CERN for technetium-99. In this process, the technetium (technetium-99 as a metal target) is bombarded with neutrons to form the short-lived technetium-100 (half-life = 16 seconds) which decays by beta decay to stable ruthenium-100. 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 ruthenium-106 (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.<ref>Template:Cite book</ref>

The actual separation of technetium-99 from spent nuclear fuel is a long process. During fuel reprocessing, it comes out as a component of the highly radioactive waste liquid. After sitting for several years, the radioactivity reduces to a level where extraction of the long-lived isotopes, including technetium-99, becomes feasible. A series of chemical processes yields technetium-99 metal of high purity.Template:Sfn

Neutron activation

Molybdenum-99, which decays to form technetium-99m, can be formed by the neutron activation of molybdenum-98.<ref name="IAEA TECDOC-1340">Template:Cite news</ref> When needed, other technetium isotopes are not produced in significant quantities by fission, but are manufactured by neutron irradiation of parent isotopes (for example, technetium-97 can be made by neutron irradiation of ruthenium-96).<ref>Template:Cite book</ref>

Particle accelerators

The feasibility of technetium-99m production with the 22-MeV-proton bombardment of a molybdenum-100 target in medical cyclotrons following the reaction ¹⁰⁰Mo(p,2n)^99mTc was demonstrated in 1971.<ref>Template:Cite journal</ref> The recent shortages of medical technetium-99m reignited the interest in its production by proton bombardment of isotopically enriched (>99.5%) molybdenum-100 targets.<ref name="bbc-20150530">Template:Cite news</ref><ref>Template:Cite journal</ref> Other techniques are being investigated for obtaining molybdenum-99 from molybdenum-100 via (n,2n) or (γ,n) reactions in particle accelerators.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Applications

Nuclear medicine and biology

Upper image: two drop-like features merged at their bottoms; they have a yellow centre and a red rim on a black background. Caption: Graves' Disease Tc-Uptake 16%. Lower image: red dots on black background. Caption: 250 Gy (30mCi) + Prednison.

Technetium scintigraphy of a neck of Graves' disease patient

Technetium-99m ("m" indicates that this is a metastable nuclear isomer) is used in radioactive isotope medical tests. For example, technetium-99m is a radioactive tracer that medical imaging equipment tracks in the human body.<ref name="blocks" /><ref name="bbc-20150530" /> 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 94% of it decays to technetium-99 in 24 hours).<ref name="enc" /> The chemistry of technetium allows it to be bound to a variety of biochemical compounds, each of which determines how it is metabolized and deposited in the body, and this single isotope can be used for a multitude of diagnostic tests. More than 50 common radiopharmaceuticals are based on technetium-99m for imaging and functional studies of the brain, heart muscle, thyroid, lungs, liver, gall bladder, kidneys, skeleton, blood, and tumors.Template:Sfn

The longer-lived isotope, technetium-95m 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.Template:Sfn

Industrial and chemical

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 U.S. National Institute of Standards and Technology (NIST) standard beta emitter, and is used for equipment calibration.Template:Sfn Technetium-99 has also been proposed for optoelectronic devices and nanoscale nuclear batteries.<ref>Template:Cite report</ref>

Like rhenium and palladium, technetium can serve as a catalyst. In processes such as the dehydrogenation of isopropyl alcohol, it is a far more effective catalyst than either rhenium or palladium. However, its radioactivity is a major problem in safe catalytic applications.Template:Sfn

When steel is immersed in water, adding a small concentration (55 ppm) of potassium pertechnetate(VII) to the water protects the steel from corrosion,<ref name=":0">Template:Cite web</ref> even if the temperature is raised to Template:Convert.Template:Sfn For this reason, pertechnetate has been used as an anodic corrosion inhibitor for steel, although technetium's radioactivity poses problems that limit this application to self-contained systems.<ref>Template:Cite book</ref> While (for example) Template:Chem can also inhibit corrosion, it requires a concentration ten times as high. In one experiment, a specimen of carbon steel was kept in an aqueous solution of pertechnetate for 20 years and was still uncorroded.Template:Sfn The mechanism by which pertechnetate prevents corrosion is not well understood, but seems to involve the reversible formation of a thin surface layer (passivation). 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. 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.Template:Sfn

As noted, the radioactive nature of technetium (3 MBq/L at the concentrations required) makes this corrosion protection impractical in almost all situations.<ref name=":0" /> Nevertheless, corrosion protection by pertechnetate ions was proposed (but never adopted) for use in boiling water reactors.Template:Sfn

Precautions and biological effect

Technetium plays no natural biological role and is not normally found in the human body.Template:Sfn Technetium is produced in quantity by nuclear fission, and spreads more readily than many radionuclides. It appears to have low chemical toxicity. For example, no significant change in blood formula, body and organ weights, and food consumption could be detected for rats which ingested up to 15 μg of technetium-99 per gram of food for several weeks.<ref>Template:Cite book</ref> In the body, technetium quickly gets converted to the stable Template:Chem ion, which is highly water-soluble and quickly excreted. The radiological toxicity of technetium (per unit of mass) is a function of compound, type of radiation for the isotope in question, and the isotope's half-life.Template:Sfn

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. 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, and a glove box is not needed.Template:Sfn

Being close to noble metals, technetium is not very susceptible to corrosion, and during biofouling, its ability to self-cleanse has been recorded due to its radiotoxic effect on biota.<ref>Template:Cite journal</ref>

References

Template:Reflist S. Garg and B. Maheshwari, et al., Atomic Data and Nuclear Data Tables 150, 101546 (2023) https://doi.org/10.1016/j.adt.2022.101546

Bibliography

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Further reading

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External links

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