Technetium
Technetium is the lightest of the chemical elements that do not have stable isotopes and the first synthetic element to be found on the periodic table. Its atomic number is 43 and its symbol is Tc. The chemical properties of this silver-gray crystalline transition metal are intermediate to those of rhenium and manganese. Its very short-lived, gamma-emitting nuclear isomer 99mTc is used in nuclear medicine for a wide variety of diagnostic tests. 99Tc is used as a source of beta particles free from gamma ray emission. The pertechnetate anion (TcO4-) is used as an anodic corrosion inhibitor for steels.
Before it was discovered, many of the properties of element 43 were predicted by Dmitri Mendeleev. Mendeleev reserved a space in his periodic table for a hypothetical element that he called eka-manganese. In 1937, the isotope 97Tc became the first predominantly artificially produced element, hence its name (from the Greek τεχνητός, meaning "artificial"). Most of the technetium produced on Earth is obtained as a by-product of the fission of 235U in nuclear reactors and is extracted from nuclear fuel rods. No technetium isotope has a half-life greater than 4.2 million years (the specific case of 98Tc), so its detection in red giants in 1952 helped reinforce the theory that in stars can generate heavy elements. On Earth, technetium occurs in detectable traces as a product of spontaneous fission in uranium ores by the action of neutron capture in molybdenum ores.
Physical and chemical properties
Technetium is a silvery-gray radioactive metal with an appearance similar to platinum metal. However, when obtained it is usually in the form of a grayish powder. Its position on the periodic table is between molybdenum and ruthenium, and as periodic laws predict, its properties are intermediate to those of these two metals. Technetium, like promethium, is unique among the light elements in that it does not possess any stable isotopes (and yet is surrounded by elements that do).
Because of its instability, technetium is extremely rare on Earth. It plays no biological role and, under normal conditions, is not found in the human body. The metallic form of technetium tarnishes rapidly in the presence of moist air.
Its oxides are TcO2 and Tc2O7. Under oxidizing conditions, technetium(VII) exists as the pertechnetate anion, TcO4-. The most common oxidation states of technetium are 0, +2, + 4, +5, +6, and +7. When pulverized, technetium burns in the presence of oxygen. It dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but not in hydrochloric acid. It has characteristic spectral lines at the following wavelengths: 363 nm, 403 nm, 410 nm, 426 nm, 430 nm, and 485 nm.
The metallic form is slightly paramagnetic, that is, its magnetic dipoles align with external magnetic fields, despite the fact that technetium is normally nonmagnetic. close to spheres, magnesium type). An isolated crystal of pure metallic technetium becomes a type II superconductor at a temperature of 7.46 K; the irregularity of the crystals and trace impurities raise this value to 11.2 K for a pulverized technetium of 99.9% purity. Below this temperature, technetium has a very high depth of penetration. magnetic, the largest of all elements after niobium.
Technetium is generated in nuclear fission processes, and it spreads more easily than many other radionuclides. Understanding its toxicity in animals and humans is important, but experimental evidence is scant. It appears to have low chemical toxicity. Its radiological toxicity (per unit mass) varies depending on the compound, the type of radiation of the isotope in question and its half-life. 99mTc is particularly attractive for its medical applications. The maximum radiation that this isotope presents is gamma rays with the same wavelength as the X-rays used for common diagnosis, offering adequate penetration and causing minimal damage. All of this, coupled with the short half-life of its metastable nuclear isomer and the relatively long half-life of the produced 99Tc isotope that allows it to be cleared from the body before it decays, makes a scanner Typical 99mTc nuclear radiation assumes a relatively low dose of delivered radiation. (See more on this topic below)
All isotopes of technetium must be handled with care. The most common of these, 99Tc, is a weak emitter of beta particles; this type of radiation can be stopped by the walls of the laboratory glassware. When they are stopped, low intensity X-rays are emitted, but a separation of about 30 cm is enough for it to affect our body. The main risk when working with technetium is inhalation of the dust; the radioactive contamination this produces in the lungs poses a very significant risk of cancer. For most technetium work, careful handling under a fume hood is usually sufficient; use of a dry chamber with gloves is not required.
Applications
Nuclear Medicine
99mTc (the “m” indicates that it is a metastable nuclear isomer) is the most widely used radioisotope in diagnostic practice, with an estimated 80% of Nuclear medicine procedures use it. It is mainly used in diagnostic procedures for organ function in the human body, for example, as a radioactive tracer that can be detected in the human body by medical equipment. This isotope is well suited for use, since it emits easily detectable gamma rays with an energy of 140 keV, and its half-life is 6.0058 hours (that is, fifteen-sixteenths of the total decay in 24 hours to give rise to 99Tc). The book Technetium, by Klaus Schwochau, lists 31 radiopharmaceuticals based on 99mTc used in functional studies of the brain, myocardium, thyroid gland, lungs, liver, gallbladder, kidneys, skeleton, blood, and tumors.
Immune scintigraphy incorporates 99mTc into a monoclonal antibody, an immune system protein capable of binding to cancer cells. A few hours after the injection, the gamma rays emitted by the 99mTc are detected with the corresponding medical equipment; high concentrations indicate where the tumor is located. This technique is particularly useful for detecting tumors that are difficult to locate, such as those that affect the intestine. These modified antibodies are marketed by the German company Hoechst under the name Scintium.
When 99mTc is combined with a tin compound, it binds to red blood cells and can be used to target disorders of the circulatory system. It is normally used to detect gastrointestinal bleeding. Pyrophosphate ion combined with 99mTc binds to calcium deposits in damaged heart muscle, useful for assessing damage after a heart attack. Sulfur colloid with 99mTc is filtered by the spleen, making it possible to visualize the structure of this organ.
Radiation exposure from diagnostic treatment with 99mTc can be kept at low levels. Due to the short half-life, its rapid decay to give rise to 99Tc - much less radioactive - makes the total radiation dose received by the patient (per unit of initial activity after administration) relatively low. low. As administered, usually as pertechnetate, both isotopes are rapidly cleared from the body within a few days.
Technetium used in nuclear medicine is usually extracted from 99mTc generators. 95mTc, with a half-life of 61 days, is used as a radioactive tracer to study the diffusion of technetium in the environment and in animal and plant systems.
Industrial use
99Tc decays emitting low energy beta particles and no gamma rays. In addition, its long half-life means that its emission decreases very slowly with time. High purity technetium can also be extracted chemical and isotopic from nuclear waste. For all these reasons, 99Tc is a beta emission standard, used for the calibration of scientific equipment.
The possibility of using 99Tc in optoelectric nuclear batteries has been studied.
Chemical use
Like rhenium and palladium, technetium can be used as a catalyst. For some reactions, for example the dehydrogenation of isopropyl alcohol, it is a much more effective catalyst than rhenium or palladium. Of course, its radioactivity is the biggest problem when it comes to finding safe applications.
Under certain circumstances, a small concentration (5 10-5 mol L-1) of the pertechnetate anion in water can protect irons and carbon steels from corrosion. For this reason, pertechnetate can be used as an anodic corrosion inhibitor for steel, but the radioactivity of technetium presents certain problems when using it for strictly chemical applications such as this. Although (for example) the CrO42- anion can also inhibit corrosion, concentrations up to ten times higher are required. In one experiment, a sample was kept in an aqueous pertechnetate solution for 20 years and did not suffer any corrosion. The mechanism by which the pertechnetate anion prevents corrosion is not well understood, but appears to involve the formation of a thin surface layer. One theory holds that pertechnetate reacts with the steel surface to form a layer of technetium dioxide that prevents further corrosion; the same effect explains how powdered iron can be used to remove pertechnetate from water (activated carbon can also be used for this purpose). The effect disappears rapidly if the pertechnetate concentration falls below a minimum or if a high concentration of other ions is added.
Obviously, the radioactive nature of technetium (3 MBq per liter for the required concentration) makes this type of protection impractical in almost all situations. However, corrosion protection using pertechnetate anions has been suggested (although never applied) for use in boiling water reactors.
In the late 1970s, successful electrodeposition of technetium onto various substrates was performed by Lichtenberger at the University of Virginia as part of a research study on the use of weak beta emissions to avoid the biological degradation of marine instrumentation. These studies were frustrated by the low stability in seawater.
History
The search for element 43
For many years there was a vacant space on the periodic table between molybdenum (element 42) and ruthenium (element 44). Many researchers at the time were eager to be the first to discover and name element 43; its location on the table suggested that it must be easier to discover than other items not yet found. In 1828, it was believed to have been found in platinum ores. It was given the name of pollinium, but it finally turned out to be impure iridium. Later in 1846 they again claimed to have discovered the element they named ilmenium, but it was determined to be impure niobium. That mistake was made again in 1847 when he claimed to have discovered the so-called pelopion. Dimitri Mendeleev predicted that element 43 must be chemically similar to manganese, and called it eka - manganese.
In 1877, Russian chemist Serge Kern reported the discovery of the element in a platinum ore. Kern named it davyo, after the prominent English chemist Sir Humphry Davy, but it was determined that it was actually a mixture of iridium, rhodium, and iron. Another candidate, the pike, was next in 1896, but it turned out to be yttrium. Later, in 1908, the Japanese chemist Masataka Ogawa found evidence in a sample of a mineral called thorianite that seemed to indicate the presence of element 43. Ogawa named it niponium, after Japan (Nippon in Japanese). In 2004, H.K. Yoshihara reviewed a copy of the X-ray spectrum of the thorianite sample in which Ogawa found niponium etched on a photographic plate preserved by the Japanese chemist's family. The spectrum was reinterpreted and indicated the presence of element 75 (rhenium), instead of element 43.
German chemists Otto Berg, Walter Noddack and Ida Tacke (the latter two would later marry) reported the discovery of elements 75 and 43 in 1925, naming the latter masurium (after Masuria, in East Prussia, now Polish territory, the region where Noddack's family came from). The chemists bombarded samples of columbite with an electron beam and deduced the presence of element 43 by examining X-ray diffraction patterns The X-ray wavelength is related to the atomic number through an expression deduced by Henry Moseley in 1913. The team claimed to have detected a faint X-ray signal at the wavelength corresponding to element 43. Other researchers Contemporaries have not been able to reproduce this experiment and, in fact, it was regarded as a mistake for many years.
In 1998, John T. Armstrong of the National Institute of Standards and Technology ran computer simulations of the 1925 experiments and found results very similar to those obtained by Noddack's team, and claimed they were supported by work published by David Curtis of Los Alamos National Laboratory on measuring the natural abundance of technetium. However, Noddack's experimental results have never been reproduced, and they were never able to isolate element 43. The idea that Noddack could indeed have obtained samples of technetium was proposed by the Belgian physicist Pieter van Assche. Assche attempted a post hoc analysis of the Noddack data to show that the detection limit of Noddack's analytical method could have been of the order 1000 times lower than the value proposed in his work (10-9). These values were used by Armstrong to simulate the original X-ray spectrum. Armstrong claimed to have obtained results very similar to the original spectrum without making any reference to where the original data was published. In this way, he offered convincing support for the idea that Noddack did indeed identify masurian fission, based on spectral data. However, Gunter Herrmann of the University of Mainz, after careful This study showed that van Assche's arguments had to be developed ad hoc to fit somewhat forcefully with previously established results. In addition, the expected 99Tc content in a typical pitchblende sample (50% uranium) is approximately 10-10 g·(kg of ore) -1 and, since uranium never exceeded 5% (approximately) in the Noddack columbite samples, the amount of element 43 could not exceed 3 10-11 μg ·(kg of ore)-1. It is clear that such a small amount could not be weighed, nor could X-ray spectral lines be obtained from it that could be clearly distinguished from noise. The only way to detect their presence is from radioactivity measurements, a technique that Noddack did not use, but Segrè and Perrier did.
Official discovery and subsequent history
The discovery of element 43 was finally confirmed in an experiment in 1937 carried out at the University of Palermo (Sicily), by Carlo Perrier and Emilio Segrè. In the summer of 1936, Segrè and his wife visited the United States. They were first in New York, at Columbia University, where Segrè had spent the previous summer, and then in Berkeley at the Ernest O. Lawrence Radiation Laboratory. Segrè convinced the inventor of the cyclotron, Lawrence, to give him some of the discarded parts of the cyclotron that had become radioactive. In early 1937, Lawrence sent him a molybdenum sheet that was part of the cyclotron baffle. Segrè encouraged his experienced colleague Perrier to help him try to demonstrate through comparative chemistry that the activity of molybdenum was actually caused by an element with Z = 43, an element that does not exist in nature due to its instability due to nuclear decay. With considerable difficulty, they were able to isolate three distinct decay periods (90, 80, and 50 days) that corresponded to the 95Tc and 97Tc isotopes of technetium, name given later by Perrier and Segrè to the first chemical element synthesized by humans. The University of Palermo officially wanted the element to be named panormium, since the Latin name for Palermo is Panormus. Instead of that name, the researchers decided to name the new element using the Greek word technètos, meaning "artificial", as it was the first artificially produced element. Segrè returned to Berkeley and immediately sought out Glenn T. Seaborg. There they isolated the isotope 99mTc, which is now used in more than 10,000,000 diagnostic medical procedures a year.
In 1952, astronomer Paul W. Merrill in California detected the spectral signal of technetium (specifically, at lengths 403.1 nm, 423.8 nm, 426.8 nm, and 429.7 nm) in the light emitted by S-type red giants. These massive stars near the end of their lives were rich in this short-lived element, which meant that nuclear reactions that take place in stars could generate it. This evidence was used to support the unproven theory that nucleosynthesis of heavy elements occurs in stars. More recently, these observations provided evidence that elements were formed by neutron capture in the S-process.
Since this discovery, attempts have been made to search for natural sources of technetium in terrestrial materials. In 1962, 99Tc was isolated and identified in a sample of pitchblende from the Belgian Congo, at very low concentrations (approximately 0.2 ng kg-1); its presence was due to the spontaneous fission of 238U. This discovery was made by B. T. Kenna and P. K. Kuroda. There is evidence that significant amounts of 99Tc were produced in the natural fission reactor at Oklo, which decayed to 99ru.
Abundance and obtaining
Naturally obtained
Since technetium is unstable, there are only tiny traces in the earth's crust caused by the spontaneous fission of uranium. In 1999, David Curtis (see above) estimated that about 1 ng (10-9 g) of technetium is contained in one kilogram of uranium. Technetium of extraterrestrial origin has been found in red giant stars (types S, M and N) by analyzing the spectrum of the light emitted by them.
By-product in nuclear fission waste
| |||||||||||||||||||||||||||||||||||||||||||||
In contrast to the low natural abundance, large quantities of 99Tc are obtained each year from spent nuclear fuel rods, which contain various fission products. Fission of one gram of the isotope 235U in nuclear reactors produces 27 mg of 99Tc, giving a total technetium yield of 6.1%. Other fissile isotopes they also produce similar yields.
It is estimated that as of 1994, some 78 metric tons of technetium had been produced in nuclear reactors, which are the main source of this element on Earth. However, only a fraction of total technetium production technetium is used commercially. Since 2005, 99Tc has been available to those who have a permit from the competent authority for an approximate price of $83 per gram, plus packaging costs.
Nuclear fission of 235U and 239Pu leaves a moderate yield of technetium (99Tc), so this element is present in radioactive waste from fission reactors, and is also produced after the detonation of a fission bomb. The amount of artificially produced technetium in nature far exceeds the amount of naturally occurring technetium. This is due to the release produced in nuclear tests carried out in the open air, as well as in nuclear waste treatment processes. 99Tc is the major component of nuclear waste, partly because of its relatively long half-life. Its disintegration, measured in becquerels per amount of spent fuel, reaches very important values even between 104 and 106 years after the generation of nuclear waste.
It is estimated that up to the year 1994 some 250 kg of 99Tc have been released into the environment as a result of nuclear tests. The amount of 99Tc released by nuclear reactors until 1986 is estimated to be of the order of 1600 kg, mainly in the reprocessing of nuclear fuel; most of it was dumped into the sea. In recent years, reprocessing methods have improved to reduce emissions, but since 2005 the main source of release of 99Tc to nature is the Sellafield plant, which released about 900 kg between 1995 and 1999 to the Irish Sea. Since 2000, the amount released into the environment has been regulated, limiting it to about 140 kg per year.
As a result of nuclear fuel reprocessing, technetium has been released into the sea at numerous locations, and some shellfish contain small but detectable amounts. For example, the western Cumbrian lobster contains small amounts of this element. Anaerobic bacteria of the genus Clostridium are capable of reducing Tc(VII) to Tc(IV). These bacteria play an important role in the reduction of iron, manganese and uranium, modifying the solubility of these elements in soils and sediments. Its ability to reduce technetium can largely determine the location of industrial waste.
The long half-life of 99Tc and its ability to form anionic species (along with 129I) are two important characteristics to consider when long-term storage of highly radioactive nuclear waste. In addition, many of the methods designed to remove fission products from reprocessing plant process streams are based on removing cationic species such as cesium (for example, 137Cs) and strontium (for example,, the 90Mr). Eliminated these cationic species, the technetium can remain in the form of anionic pertechnatio. Current nuclear waste storage options favor burial in geologically stable rock. The main risk in storage is that the waste is likely to come into contact with water, which could cause environmental spread of radioactive contamination. Anionic pertechnetate and iodide are more difficult to adsorb onto mineral surfaces and are therefore much more mobile.
In comparison, plutonium, uranium, and cesium have much greater ability to bind to soil particles. For this reason, the environmental chemistry of technetium is an active area of research. An alternative method for waste storage, transmutation, was carried out at CERN for 99Tc. In this transmutation process, technetium (99Tc as "white") is bombarded with neutrons forming the isotope 100Tc (half-life = 16 s) which undergoes beta decay to ruthenium (100Ru). A drawback of this process is the need to have a very high purity technetium as a blank. While the presence of traces of other fission products are capable of slightly increasing the activity of the irradiated target, if these traces are of minor actinides (such as americium and curium) a fission process will occur that will give rise to the corresponding fission products. Thus, the presence of a small amount of minor actinides leads to a very high level of radioactivity in the irradiated target. The formation of 106Ru (half-life: 374 days) from fission is capable of increasing the activity of the final metallic ruthenium, which will then require a long cooling time after irradiation to be able to be used.
The actual production of 99Tc from spent nuclear fuel is a long process. During fuel reprocessing, 99Tc appears in the residual liquid, which is highly radioactive. After several years of storage, the radioactivity decays to a point where extraction of long-lived isotopes, including 99Tc, is feasible. Numerous chemical extraction processes are used to obtain high purity 99Tc metal.
Neutron activation of molybdenum or other pure elements
The metastable isotope (the nucleus is in an excited state) 99mTc is generated as a product from the fission of uranium or plutonium in nuclear reactors. Since it is permissible to store used nuclear fuel for years before it is reprocessed, all 99Mo and 99mTc will have decayed when these fission products are separated from the other actinides in conventional nuclear reprocessing. PUREX Raffinate will contain a high concentration of Technetium in the form of TcO4-, the majority being 99Tc. The vast majority of 99mTc used for medical purposes originates from 99Mo which is created from the neutron activation of 98Mo. 99Mo has a half-life of 67 hours, and 99mTc (with a half-life of only 6 hours) arises continuously upon decay. Hospitals then chemically extract the technetium from the solution using a 99mTc generator.
The most common technetium generator is an alumina column containing 98Mo; as aluminum has a small neutron capture cross section, it is convenient for a column of alumina to contain inactive 98Mo to be irradiated with neutrons, giving rise to a column of 99Radioactive Mo, for the technetium generator. By working in this way, there is no need to carry out complex chemical procedures that might require separating the molybdenum from the fission product mixture. This alternative method requires an enriched uranium target to be irradiated with neutrons to form 99Mo as a fission product that is subsequently separated.
Other isotopes of technetium exist, but are not obtained in significant quantities by fission; when needed, they are obtained by neutron irradiation of isotopes of the same family (for example, 97Tc can be produced by neutron irradiation of 96Ru).
Isotopes
Technetium is one of the two elements, among the first 82, that does not have stable isotopes (in fact, it is the element with the lowest atomic number that is exclusively radioactive); the other element is promethium. The most stable radioisotopes of technetium are 98Tc (half-life 4.2 million years), 97Tc (half-life: 4.2 million years), half-life: 2.6 million years) and 99Tc (half-life: 211.1 thousand years).
Twenty-two other radioisotopes with atomic masses ranging from 87.933 u (88Tc) to 112.931 u (113Tc) have been characterized. Most of its half-lives are less than one hour; exceptions are 93Tc (half-life: 2.75 hours), 94Tc (half-life: 4.883 hours), 95 Tc (half-life: 20 hours) and 96Tc (half-life: 4.28 days).
Technetium also has numerous meta-states. 97mTc is the most stable, with a half-life of 90.1 days (0.097 eV). It is followed by 95mTc (half-life: 61 days, 0.038 eV), 99mTc (half-life: 61 days, 0.038 eV), half-life: 6.01 hours, 0.143 eV). 99mTc only emits gamma rays, decaying to 99Tc.
For isotopes lighter than the 98Tc isotope, the primary mode of decay is electron capture, yielding molybdenum. For the heavier isotopes, the primary mode is beta emission, giving rise to ruthenium, with the exception of 100Tc which can decay by both beta emission and electron capture.
99Tc is the most common isotope and the easiest to obtain, since it is the majority product of the fission of 235U. One gram of 99Tc produces 6.2 108 disintegrations per second (this is 0.62 GBq g-1).
Stability of technetium isotopes
Technetium and promethium are unconventional light elements, since they do not have stable isotopes. The reason for this fact is somewhat complicated. Using the liquid drop model for atomic nuclei, a semi-empirical formula for the binding energy of a nucleus can be obtained. This formula predicts a "valley of beta stability" as well as which nuclides do not undergo beta decay. The nuclides that exceed the borders of the valley tend to decay with beta emission, going towards the center of the valley (emitting an electron, a positron, or capturing an electron). For a fixed number of nucleons A, the binding energies are described by one or more parabolas, with the most stable nuclide at the bottom. There can be more than one parabola 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 even number of protons. A single beta emission thus transforms a nuclide of one type into a nuclide of the other type. When there is only one parabola, there can only be one stable isotope whose energy is described by the parabola. When there are two parabolas, that is, when the number of nucleons is even, it can (rarely) happen that there is a stable nucleus with an odd number of neutrons and an odd number of protons (although this only happens in four cases). However, if this happens, there can be no stable isotopes with an even number of neutrons and an even number of protons.
For technetium (Z=43), the valley of beta stability is centered around 98 nucleons. However, for every number of nucleons from 95 to 102, there is already at least one stable nuclide for both molybdenum (Z=42) and ruthenium (Z=44). For isotopes with odd number of nucleons, this immediately precludes the possibility of a stable isotope of technetium, since there can only be one stable nuclide with a fixed odd number of nucleons. For 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 this case, the presence of a stable nuclide with the same number of nucleons and an even number of protons makes it impossible for the nucleus to be stable.
Contenido relacionado
Full wave rectifier
Lipid bilayer
Annex: periodic table