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, ...

The absorption characteristics of Cd2' by 10-to 12-day-old soybean plants (Glycine max cv Williams) were investigated with respect to influence of Cd concentration on adsorption to root surfaces, root absorption, transport kinetics and interaction with the nutrient cations Cu2+, Fe2 , Mn2 , and Zn2 . The fraction of nonexchangeable Cd bound to roots remained relatively constant at 20 to 25% of the absorbed fraction at solution concentration of 0.0025 to 0.5 micromolar, and increased to 45% at solution concentration in excess of 0.5 micromolar. The exchangeable fraction represented 1.4 to 32% of the absorbed fraction, and was concentration dependent. Using dinitrophenol as a metabolic inhibitor, the 'metabolically absorbed' frction was shown to represent 75 to 80% of the absorbed fraction at concentration less than 0.5 micromolar, and decreased to 55% at 5 micromolar. At comparatively low Cd concentrations, 0.0025 to micromolar 0. Cadmium Uptake. Evaluation of the absorption behavior of Cd was performed using 10-to 12-d-old plants. Prior to use, plants were transferred from nutrient solutions to 0.5 mM CaCl2 solutions (pH 5.8) for 12 h to allow for desorption of possible interfering ions from root surfaces. Individual plants were then transferred to fresh 0.5 mm CaCl22 and various concentrations of CdCl2. CaCl2 was employed in all uptake and absorption solutions to provide sufficient Ca2`to maintain membrane permeability (20,22). For absorption periods of 60 min or less, 500 ml volumes were employed; for absorption periods of over 60 min, 1-L volumes were employed to limit reduction of total Cd levels in solution to less than 10% during the experiment.Solutions containing CdCl2 levels from 0.0025 to 5.0 liM were traced with carrier-free`°CdCI2, and adjusted to pH 5.8 with KOH. Following the absorption period, shoots were removed and roots were transferred to 0.5 mm CaCl2 solutions containing unlabeled CdCl2 at concentrations 20-fold higher than treatment concentrations. Roots were routinely desorbed using three changes of solution for a total of 2 h; in preliminary studies, desorbed '"Cd was measured in efflux solutions at 30-min intervals following 5-min wash in 0.5 mM CaC12.

Technetium (Tc) may enter the environment as a result of nuclear weapons testing, nuclear power production, nuclear fuel reprocessing, nuclear waste storage, and pharmaceutical use. The isotope 99Tc has a long half‐life and relatively high fission yield, and an understanding of its behavior and effects in the environment is essential to complete assessment of the environmental impact of the nuclear fuel cycle. Although little is known of Tc environmental behavior, the aqueous chemistry of Tc is well understood and limited laboratory investigations have been conducted on its behavior in soils, plants, and waters. On the basis of this current knowledge, the behavior and implications of Tc in the environment are assessed and avenues of research are suggested. The most stable chemical species of Tc in aqueous solution is the pertechnetate ion, TcO4−. As the pertechnetate ion, Tc is highly soluble in water and in soil, but reduced forms (dioxide and sulfide) have limited solubility; sorption is significant in surface soils of high organic C content and low pH and may be significant in subsoils and geologic media under reduced conditions. A fraction of the sorption measured in surface soils may also have been due to immobilization in microbial cells. The pertechnetate ion is readily available to microbiota, algae, and higher plants and is toxic to higher plants at relatively low soil levels (0.1 µg/g). In higher plants, it is transported largely as the pertechnetate ion and toxicity appears to be due to biochemical rather than radiation effects. Uptake and toxicity appear to be due to Tc functioning as a nutrient analog. Preliminary studies indicate that, in contrast to hydrolyzable radionuclides, incorporation of Tc in plants reduces Tc absorption in animals. Ultimately, the mobility and availability of Tc to biota will be governed by the quantity and form emitted and its form and solubility over the long‐term in soils, sediments, and waters. Uptake by biota will be dependent upon the rate and extent of Tc movement across biological membranes and its toxicity. Proper evaluation of these phenomena will be largely dependent on development of an understanding of Tc sorption mechanisms in soils and sediments and the determination of the stable forms of soluble chemical species that occur in the environment over the long term.

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Pertechnetate ion [Tc(VII)O(4) (-)] reduction rate was determined in core samples from a shallow sandy aquifer located on the US Atlantic Coastal Plain. The aquifer is generally low in dissolved O(2) (<1 mg L(-1)) and composed of weakly indurated late Pleistocene sediments differing markedly in physicochemical properties. Thermodynamic calculations, X-ray absorption spectroscopy and statistical analyses were used to establish the dominant reduction mechanisms, constraints on Tc solubility, and the oxidation state, and speciation of sediment reduction products. The extent of Tc(VII) reduction differed markedly between sediments (ranging from 0% to 100% after 10 days of equilibration), with low solubility Tc(IV) hydrous oxide the major solid phase reduction product. The dominant electron donor in the sediments proved to be (0.5 M HCl extractable) Fe(II). Sediment Fe(II)/Tc(VII) concentrations >4.3 were generally sufficient for complete reduction of Tc(VII) added [1-2.5 micromol (dry wt. sediment) g(-1)]. At these Fe(II) concentrations, the Tc (VII) reduction rate exceeded that observed previously for Fe(II)-mediated reduction on isolated solids of geologic or biogenic origin, suggesting that sediment Fe(II) was either more reactive and/or that electron shuttles played a role in sediment Tc(VII) reduction processes. In buried peats, Fe(II) in excess did not result in complete removal of Tc from solution, perhaps because organic complexation of Tc(IV) limited formation of the Tc(IV) hydrous oxide. In some sands exhibiting Fe(II)/Tc(VII) concentrations <1.1, there was presumptive evidence for direct enzymatic reduction of Tc(VII). Addition of organic electron donors (acetate, lactate) resulted in microbial reduction of (up to 35%) Fe(III) and corresponding increases in extractable Fe(II) in sands that exhibited lowest initial Tc(VII) reduction and highest hydraulic conductivities, suggesting that accelerated microbial reduction of Fe(III) could offer a viable means of attenuating mobile Tc(VII) in this type of sediment system.