Technetium (Tc) is the lowest molecular weight element that is exclusively radioactive. Because of its instability, it occurs naturally only very rarely, and is found in but trace amounts in the Earth's crust from fission reactions in uranium ores. It is almost exclusively synthetically produced. The more stable isotopes, Tc-97, Tc-98, and Tc-99, have half-lives of 2.6 million, 4.6 million, and 213,000 years, respectively. These long half-lives make Tc a potentially dangerous source of radiation. [1]
While Tc-97 and Tc-98 exist only in very small quantities, due to the necessity for their artificial synthesis, large amounts of Tc-99 are produced from the fission of uranium (U)-235 or plutonium (Pu)-239 in about 5 % yield with respect to U or Pu. An estimated 78 tonnes of Tc-99 were produced by nuclear reactors between 1983 and 1994. [2] Such large amounts of a radioactive material make the production and disposal of Tc-99 an important environmental consideration, and in 2000, regulations were put in place to limit Tc-99 production to about 140 kg/year. [3]
Its long half-life, and our ability to extract it from radioactive waste with high chemical and isotopic purity, permits Tc-99 to be used for industrial purposes. [4] One such method of isolation involves the cathodic electrodeposition of a TcO2 hydrate onto a thin silver foil from a basic solution of the water soluble ion TcO4- (pertechnetate). Its consistent, low-energy output of β particles makes it ideal for instrument calibration and optoelectronic nuclear batteries. Such batteries may last for decades and provide high energy density, but their prohibitively high prices prevent them from being used very commonly. Tc(VII) may also serve as a corrosion inhibitor and oxidizing agent. However, the industrial uses of Tc far outstrip its production, making the remaining radioactive Tc a danger to both health and environment.
The water solubility of pertechnetate makes its long half-life an ever more important problem. The ion is not only water soluble, but also highly geochemically mobile, permitting facile uptake by plants and aquatic life from Tc-rich soil. [5] Further, the mobile pertechnetate ion is easily fixed in plants into less mobile Tc organics, oxides, or sulfides. Plant-fixed Tc is likely one of the key sources of Tc radiation to humans. Once in the body, pertechnetate is readily transferred to the bloodstream with high efficiency by the intestines and lungs. It deposits in high concentration in the thyroid, stomach wall, and liver, where the emission of β particles may induce cancer.
A key approach to reducing the uptake of Tc by humans is to reduce Tc(VII) levels in plants, and thus in soils, and thus ultimately its water solubility. In order to render it insoluble, Tc(VII), a redox-active element, may be reduced to a lower oxidation state, thereby controlling its levels in plants. Soils contaminated with Tc may also be heated to roughly 1000 °C in order to volatilize the Tc; however, this does not eliminate all of the Tc within a sample, and repeated efforts of volatilization do not substantially lower Tc levels. Most disposal techniques for nuclear waste deal with the removal of cationic species, which are much more common. [6] This makes the elimination of the anionic pertechnetate species more difficult. Transmutation is an alternative disposal method, in which Tc-99 is bombarded with neutrons to form Tc-100, which quickly decays to ruthenium-100.
The large-scale production of Tc-99, in conjunction with its long half-life, makes the removal of this isotope an important problem. While few efficient methods for its removal are currently in place, the development of such methods is an active field of research.
© Koyel X. Bhattacharyya. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Technetium," in CRC Handbook of Chemistry & Physics, 84th Ed., D. R. Lide, ed. (CRC Press, 2004), pp. 4-29.
[2] M. García-León, "Tc-99 in the Environment: Sources, Distribution and Methods," J. Nucl. Radiochem. Sci. 6, 253 (2005).
[3] K. Tagami, "Technetium-99 Behavior in the Terrestrial Environment - Field Observations and Radiotracer Experiments," J. Nucl. Radiochem. Sci. 4, A1 (2003).
[4] K. Schwochau, Technetium: Chemistry and Radiopharmaceutical Applications (Wiley, 2000).
[5] J. D. Harrison and A. Phipps, "Gut Transfer and Doses from Environmental Technetium," J. Radiological Protection 21, 9 (2001).
[6] P. Altomare et al., "Alternative Disposal Concepts for High-level and Transuranic Radioactive Waste Disposal," Mitre Corporation, Technical Report MTR-7718, May 1979.