Technetium abundance

This book presents a unified treatment of the chemistry of the elements. At present 112 elements are known, though not all occur in nature of the 92 elements from hydrogen to uranium all except technetium and promethium are found on earth and technetium has been detected in some stars. To these elements a further 20 have been added by artificial nuclear syntheses in the laboratory. Why are there only 90 elements in nature Why do they have their observed abundances and why do their individual isotopes occur with the particular relative abundances observed Indeed, we must also ask to what extent these isotopic abundances commonly vary in nature, thus causing variability in atomic weights and possibly jeopardizing the classical means of determining chemical composition and structure by chemical analysis. 

Technetium then became available in a weighable quantity because of uranium nuclear fission leading to the production of "Tc in nuclear reactors. The total amount of "Tc in the world at the end of 1993 is estimated to be 78 tons, more abundant than rhenium on the earth. 

With autoclave syntheses a high yield of clusters is achieved, and it is possible for researchers to follow the reaction path in solution by gradually changing (from experiment to experiment) the working parameters of the synthesis (temperature, pressure, exposure at working temperatures, etc). All these advantages of the autoclave technique have resulted in an abundance of new forms of technetium clusters (particularly, polynuclear ones) because it has been possible to develop and improve the method of obtaining these compounds. 

Progress in understanding stellar evolution and nucleosynthesis, and the discovery by Merrill (1952) of the unstable element technetium in the S star R Andromedae, demonstrating the occurrence of stellar nucleosynthesis within a few half-lives of Tc (i.e. < about 1 Myr see Fig. 1.8), has led to acceptance of the idea that abundance variations among stars are perfectly natural as a consequence of three main effects (see Fig. 3.37) ... 

Occurrence. The natural abundance of Tc is negligibly small. Technetium is a by-product of the nuclear industry and it is a product of the uranium decay. 

Almost all of the elements heavier than He are synthesized in the interiors of stars. The work of Burbidge et al. (1957) gives the theoretical framework for the synthesis of the elements. The experimental evidence of active nucleosynthesis came from the discovery of the unstable nuclei of technetium in the spectra of red giants (Merrill 1952). The solar elemental and isotopic abundances which are taken from the primitive carbonaceous chondrites constitute the guidelines for testing such models (Anders and Grevesse 1989). A minimum of eight basic processes are required to reproduce the observed compositions. Nucleosynthetic... 

Table A. 1 comprises the stable elements from hydrogen to bismuth with the radioactive elements technetium and promethium omitted. Natural variations in isotopic composition of some elements such as carbon or lead do not allow for more accurate values, a fact also reflected in the accuracy of their relative atomic mass. However, exact masses of the isotopes are not affected by varying abundances. The isotopic masses listed may differ up to some 10 u in other publications.

Technetium is the 76th most abundant element, but it is so rare that it is not found as a stable element on Earth. All of it is artificially produced. Even though natural technetium is so scarce that it is considered not to exist on Earth, it has been identified in the tight spectrum from stars. Using a spectroscope that produces unique tines for each element, scientists are able to view several types of stars. The resulting spectrographs indicate that technetium exists in the stars and thus the universe, but not on Earth as a stable element. 

Technetium is now moderately abundant because it accumulates in the decay products of nuclear power plants. Another isotope, technetium-99, has pharmaceutical applications, particularly for bone scans (Box 17.2). 

Various theories have been advanced to explain the relative abundance of the elements. In recent years knowledge has accumulated of the types of nuclear transformations occurring in stars. A continuous process of synthesis and consumption of elements (Burbidge, Burbidge, Fowler and Hoyle, 1957) accounts for the observed differences in composition of stars of different ages and also such abnormalities as the presence of technetium in S-type stars. 

Fig. 22.1 Relative abundances of the second and third row <5 -block metals in the Earth s crust. The data are plotted on a logarithmic scale, and the units of abundance are parts per 10 . Technetium (group 7) does not occur naturally.

All isotopes of technetium are unstable toward ft decay or electron capture and traces exist in Nature only as fragments from the spontaneous fission of uranium. The element was named technetium by the discoverers of the first radioisotope—Perrier and Segre. Three isotopes have half-lives greater than 105 years, but the only one that has been obtained on a macro scale is "Tc (fi, 2.12xl05 years). Technetium is recovered from waste fission-product solutions after removal of plutonium and uranium. It is an interesting irony that the supply of technetium, which does not exist in Nature, might easily be made to exceed that of Re, which does, because of the increasing number of reactors and the very low ( 10-9%) abundance of Re in the earth s crust. 

The mass spectrometrie determination of technetium is a highly sensitive and frequently applied method. Tc isotopes fonned upon bombardment of a molybdenum target with 22 MeV protons were identified by mass spectrometry. The technetium fraction was separated and purified by anion exchange chromatography and the isotopes (abundance) c (0.5 %), Tc (56.0 %), Tc (17.3 %), and T c (26.7 %) were detected in about 1 pg technetium on a mass spectrometer for the first time (Fig. 7.1.A). As little as 5 10 g of technetium could be measured by mass spectrometry [491. 

The waste contains considerable amounts of Ru, Rh, and Pd, all metals in scarce abundance on earth. These elements are used as catalysts in the chemical industry, for catalytic exhaust cleaning in cars, and as corrosion resistant materials. The United States demand exceeds the domestic production by about a factor of 1(X). The United States would be independent of import from year 2000 if these elements were recovered from the yearly generated spent fuel. This is particularly true if technetium is recovered, since it can often replace platinum. Some of the recovered elements would be radioactive, but the activities would be small enough to make the elements easy to handle. 

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