Artificial respiration of bacteria – Science – Kommersant

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At the Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences, they discovered a bacterium that can breathe technetium, the first artificially synthesized radioactive element.

Researchers of the radioecology and biogeotechnology group of the technetium chemistry laboratory of the Institute of Physical Chemistry, Russian Academy of Sciences, during the study of microbial communities in groundwater contaminated with radionuclides, discovered and isolated in a pure culture the bacterium Shewanella xiamenensis DCB 2-1, which is capable of extremely intensive use of pertechnetate (technetium acid salt) as an oxidizing agent in the process anaerobic respiration.

The ability of microorganisms to use alternating valence metals as electron acceptors in the process of anaerobic respiration has been studied in detail. Among such metals are iron, vanadium, chromium, manganese, uranium, etc. Technetium also belongs to alternating-valent metals. In redox reactions occurring during anaerobic respiration of a microorganism, technetium in the composition of pertechnetate (valency +7) takes electrons and is reduced to dioxide (valency +4).

The discovered strain Shewanella xiamenensis DCB 2–1 is able to live in a solution containing up to 0.01M technetium with an activity of up to 106 Bq / L (which is comparable to radioactive waste of an average activity level), while the process of reduction of pertechnetate to technetium dioxide takes several days. As a result of the reduction, a brown, slightly soluble precipitate of hydrated forms of technetium dioxide precipitates, and if sulfate ions are added to the medium, a black precipitate is formed containing a mixture of iron and technetium sulfides.

In both cases, the formation of biogenic technetium-containing sediments is very promising for the immobilization of technetium in the environment. Currently, the group of radioecology and biogeotechnology of the technetium chemistry laboratory of the Institute of Physical Chemistry of the Russian Academy of Sciences is working on creating a biogeochemical barrier for immobilizing technetium in groundwater using biological products containing the Shewanella xiamenensis DCB 2-1 strain. The study of the genome of this strain made it possible to identify a large number of genes responsible for resistance to heavy metals, which makes it possible to grow a culture in electroplating waste containing copper, zinc and other metals. Using this strain, immobilized in the form of a biofilm on zeolite, studies have been carried out on the possibility of treating wastewater containing heavy metals.

Technetium – the 43rd element of the periodic table, is in the seventh group between molybdenum and ruthenium. For a long time technetium could not be found in nature, and the place of the 43rd element was empty. The futility of searching for technetium in nature is due to its lack of stable isotopes. Natural technetium (99Tc isotope) with a half-life of about 200 thousand years is a product of spontaneous fission of uranium and thorium ores or a product of neutron capture in molybdenum ores, therefore it is found in extremely low concentrations.

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In the middle of the 20th century, technetium was artificially synthesized from a molybdenum target at the Lawrence Berkeley National Laboratory in the United States and became, in fact, the first artificially obtained element.

In Russia, the first weight amounts of technetium (about 60 mg) were isolated by V.I.Spitsyn and A.F. Kuzina in 1957 at the Institute of Physical Chemistry of the Academy of Sciences of the USSR (now the Institute of Physical Chemistry, Russian Academy of Sciences) from molybdenum irradiated at the reactor of the Institute of Physics of the Academy of Sciences of the USSR.

Almost all technetium on Earth is of artificial origin as a fission product of uranium-235 and other fissile nuclei in nuclear reactors of all types. Its content in NPP waste depends on the type of nuclear fuel and can reach 6%. The total accumulation of technetium in all reactors operating on Earth per year exceeds 10 tons.

As a product of irradiated nuclear fuel, technetium creates many problems in waste management. In addition to a significant half-life (200 thousand years), the physicochemical properties of technetium – the high solubility of its compounds under oxidizing conditions, low sorption on the main soil minerals, and the volatility of some compounds – put it in the first group of radiological risk. There is a high risk of technetium migration in ecosystems during the disposal of radioactive waste or in the event of radiation accidents.

One of the methods of disposal of technetium is its cementation to form storage-stable matrices. For this, technetium must be bound in the form of an insoluble compound – for example, dioxide. One way of such binding is offered by the bacterium Shewanella xiamenensis.

Alexey Safonov, Candidate of Chemical Sciences, Senior Researcher, Laboratory of Technetium Chemistry, IPCE RAS

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