Effect of Electron Donor and Solution Chemistry on Products of Dissimilatory Reduction of Technetium by Shewanella putrefaciens

Abstract

To help provide a fundamental basis for use of microbial dissimilatory reduction processes in separating or immobilizing 99 Tc in waste or groundwaters, the effects of electron donor and the presence of the bicarbonate ion on the rate and extent of pertechnetate ion [Tc(VII)O 4 ؊ ] enzymatic reduction by the subsurface metalreducing bacterium Shewanella putrefaciens CN32 were determined, and the forms of aqueous and solid-phase reduction products were evaluated through a combination of high-resolution transmission electron microscopy, X-ray absorption spectroscopy, and thermodynamic calculations. When H 2 served as the electron donor, dissolved Tc(VII) was rapidly reduced to amorphous Tc(IV) hydrous oxide, which was largely associated with the cell in unbuffered 0.85% NaCl and with extracellular particulates (0.2 to 0.001 m) in bicarbonate buffer. Cell-associated Tc was present principally in the periplasm and outside the outer membrane. The reduction rate was much lower when lactate was the electron donor, with extracellular Tc(IV) hydrous oxide the dominant solid-phase reduction product, but in bicarbonate systems much less Tc(IV) was associated directly with the cell and solid-phase Tc(IV) carbonate may have been present. In the presence of carbonate, soluble (<0.001 m) electronegative, Tc(IV) carbonate complexes were also formed that exceeded Tc(VII)O 4 ؊ in electrophoretic mobility. Thermodynamic calculations indicate that the dominant reduced Tc species identified in the experiments would be stable over a range of E h and pH conditions typical of natural waters. Thus, carbonate complexes may represent an important pathway for Tc transport in anaerobic subsurface environments, where it has generally been assumed that Tc mobility is controlled by low-solubility Tc(IV) hydrous oxide and adsorptive, aqueous Tc(IV) hydrolysis products.Technetium (element 43) is present in the environment principally as a result of fallout from nuclear weapons testing, uranium enrichment, nuclear fuel processing, and disposal after pharmaceutical use (34). During nuclear fuel reprocessing, Tc is solubilized from spent fuels and is present in all waste streams (10) principally as the pertechnetate anion [Tc(VII)O 4 Ϫ ]. Over the pH and E h ranges typical of most groundwaters, Tc may exist in oxidation states VII, VI, V or IV, but in the absence of strong complexing agents Tc(VI) and Tc(V) may be expected to disproportionate to Tc(VII) and Tc(IV) (31). The dominant oxidation state under oxic conditions is Tc(VII), which is weakly sorbed by most soils and subsurface sediments at near neutral pH values (15,35). Under anoxic conditions and in the absence of aqueous complexing agents other than OH Ϫ , Tc(IV) is largely immobile because it forms concentration-limiting solid phases and strong surface complexes with hydroxylated surface sites on Al and Fe oxides and clays (5, 9, 24, 31). Organisms capable of anaerobic energy metabolism, including the dissimilatory metal-reducing bacteria (DMRB) Shewanella putrefaciens, Shewanella alga, ...

Developments in Bioremediation of Soils and Sediments Polluted with Metals and Radionuclides. 3. Influence of Chemical Speciation and Bioavailability on Contaminants Immobilization/Mobilization Bio-processes

“…An important implication of these studies is the effect of NOM on the potential transport of the metals by either increasing or decreasing their redox states and solubility and thereby causing their mobilization or immobilization in the subsurface soil Fredrickson et al 2000;Wildung et al 2000). Because of the much smaller size of humic molecules as compared to the size of bacteria, humic substances could allow access to locations from which bacteria are excluded due to size or nutrient limitations and therefore transfer the microbial reducing power to contaminants at such isolated locations.…”

Determination of technetium-99 in environmental samples: A review

“…For TIMS, a special separation procedure is needed to obtain a small sample solution and good precision of isotopic measurements. It is 56 always time consuming and labour intensive. AMS operation is more complex and a well experienced team is required, although a very low detection limit can be obtained.…”

“…In reductive conditions, Tc(IV) is the most stable oxidation state and strongly hydrolyzes in aqueous solutions and is very stable as TcO 2 in the absence of water [54]. In addition, various complexes of Tc(IV) can be formed in the presence of organic or inorganic ligands such as carbonate, EDTA, citrate and natural humic substances [55][56][57][58].…”

Geomicrobiological factors that control uranium mobility in the environment: Update on recent advances in the bioremediation of uranium-contaminated sites

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