Tc Species Research Articles

Mixed-ligand technetium(III) complexes with tetradendate/monodendate NS(3)/isocyanide coordination: a new nonpolar technetium chelate system for the design of neutral and lipophilic complexes stable in vivo.

Starting from the tripodal ligand 2,2',2' '-nitrilotris(ethanethiol) (NS(3)) and isocyanides (CNR) as co-ligands, neutral mixed-ligand technetium(III) complexes of the general formulation [Tc(NS(3))(CNR)] have been synthesized and characterized. The (99)Tc complexes can be( )()obtained by a two-step reduction/substitution procedure starting from [TcO(4)](-) via the phosphine-containing precursor complex [Tc(NS(3))(PMe(2)Ph)]. As shown by X-ray structural analyses, the complexes adopt a nearly ideal trigonal-bipyramidal geometry with the trigonal plane formed by the three thiolate sulfurs of the tripodal ligand. The central nitrogen atom of the chelate ligand and the monodendate isocyanides occupy the apical positions. The no-carrier-added preparation of the corresponding (99m)Tc complexes was performed by a one-step procedure starting from (99m)[TcO(4)](-) with stannous chloride as reducing agent. Biodistribution studies in the rat demonstrated for the nonpolar, lipophilic compounds a significant initial brain uptake. In vitro challenge experiments with glutathione clearly indicated that no transchelation reaction occurs. Furthermore, there were no indications for reoxidation of Tc(III) to Tc(V) species or pertechnetate. We propose this type of complexes as a useful tool in the design of lipophilic (99m)Tc or (186)Re/(188)Re radiopharmaceuticals.

The speciation of Tc(IV) in chloride solutions

The speciation of tetravalent technetium has been studied in HCl solutions ranging from 1.0 to 6.0 M. Tc(IV) exists in 6.0 M HCl as the well characterized TcCl62-chloride complex. In 1.0 M HCl, the chloride ligands of TcCl62-are slowly substituted by other ligands such as H2O, OH-or O2-. So TcClmOn(OH)p(H2O)q(4-m-2n-p)+compounds may form with (m + n + p + q) = 6. Several speciation techniques such as UV-visible and Raman spectrometries, X-ray absorption spectroscopy and electrochemistry have enabled to get an insight into this problem. All the results have indicated two most likely species of Tc(IV) after aging for about ten days in 1.0 M HCl to be aquo-chlorocomplexes of technetium and not oxo-chorocomplexes: TcCl5· H2O-and TcCl4· 2 H2O.

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

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 metal-reducing 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 microm) 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 microm) 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.

A dual radiotracer speciation technique with emphasis on probing of artefacts: a case study for technetium and spinach ( Spinacia oleracea L.)

Speciation is required for obtaining information on chemical reactivity, toxicity and bioavailability of an element. In this work, a novel analytical speciation methodology, using a dual radiotracer technique is described. The approach is illustrated with the speciation of 99 Tc , a pure β −-emitter, in spinach plants. The actual leaf concentration of two technetium species, i.e. 99 TcO − 4 and 99 TcX (plant-formed Tc species), is determined as a function of incubation time with 99 TcO − 4 in the substrate. Addition of 95 m TcO − 4 or 99 m TcO − 4 , both containing a γ-emitting radioisotope of technetium, to the substrate allows easy radiotracing, while addition of 99 m TcO − 4 or 95 m TcX yield tracers during the analytical homogenisation procedure makes it possible to correct for procedural losses and redox conversions. For TcO − 4 significant procedural losses were observed, while for TcX procedural recovery was nearly quantitative. Significant procedural conversions of TcO − 4 or TcX were not observed. Speciation of other elements might benefit from this novel methodology.

Chemical forms of technetium in tomato plants; TcO 4−, Tc–cysteine, Tc–glutathione and Tc–proteins

The rapid accumulation of Tc, originating from nuclear power generation, in plants and the phytotoxicity at low Tc levels suggest an interaction between Tc and proteins. In the present study, different chemical forms of Tc in tomato plants were identified using an approach explicitly designed to prevent artefacts. After precipitation of Tc–proteins (80% ammonium sulphate), TcO 4 −, Tc–cysteine, and Tc–glutathione were determined, using reversed-phase ion-pair chromatography. A vast range of Tc–proteins was detected using a gradient of ammonium sulphate precipitation or hydrophobic interaction chromatography. The hypothesis that Tc, as an S-analogue, is organo-metallically incorporated into organic substances appears to be rather unlikely, because all reduced Tc in the leaves was susceptible to ligand-exchange with 2-mercaptoethanol. A ligand exchange route for reduced Tc from proteins to cysteine and glutathione in the leaves was suggested, using Tc species ratios and mathematical modelling.

Characterization of Technetium Species Induced in Spinach

AbstractPlants have the ability to accumulate the long‐lived fission product 99Tc. In this work, an attempt was made to separate and characterize technetium species induced by spinach plants (Spinacia oleracea L.) grown on a TcO−4 containing nutrient solution. Combination of data obtained with selective extraction (ammonium sulfate, acetone, diethyl ether, and 8‐hydroxyquinoline in chloroform) and chromatography (size‐exclusion chromatography and reversed‐phase ion‐pair chromatography) gave us insight into Tc speciation in spinach plants. The following classes of Tc species in spinach leaf homogenate were found after an incubation period of 11 d: TcO−4 (ca. 7%), TcV‐cysteine (ca. 25%), Tc bound to insoluble cell‐wall polysaccharides (ca. 17%), Tc bound to proteins (ca. 26%), and hydrophilic non‐protein Tc species (ca. 25%). These results may yield a new insight into the metabolic pathways of Tc in plants.

Bacterial Reduction and Removal of Technetium from Solution

A novel Phosphorlmager-based technique was used to screen microorganisms for the ability to reduce Tc(VII) (as the pertechnetate ion; TCO4 −), a by-product of the nuclear industry. The photosynthetic eubacteria (cyanobacteria) Coetosphaerium sp., Osciltatoria sp, Microcystis aeruginosa and Anabaena cylindrica were unable to reduce the radionuclide, despite reports in the literature to the contrary. Several anaerobic bacteria were however able to reduce Tc(VII). The facultative anaerobe, Shewanella putrefaciens reduced Tc(VII) to a range of low oxidation state Tc species but did not accumulate the radionuclide. Another facultative anaerobe, Escherichia coli, and the obligate anaerobe Geobacler metallireducens were able to reduce and accumulate substantial quantities of Tc, probably as a low valence insoluble oxide. The enzyme responsible for Tc reduction in E. coli was identified, using genetic and physiological approaches, as the hydrogenase III component of the formate hydrogen Iyase complex. Either hydrogen or formate were used as electron donors for the bioreduction. Resting cells of E. coli, immobilised in a flow through bioreactor, coupled formate oxidation to the reduction and accumulation of Tc, with no loss in reactor efficiency noted for up to 2 weeks (the duration of the experiments). An overview of the development of the bioprocess to laboratory scale is given, possible future developments highlighted and a comparison with competing technologies made.

A Novel PhosphorImager-Based Technique for Monitoring the Microbial Reduction of Technetium

A novel PhosphorImager-based technique which can be used to quantify low concentrations of radionuclides is described. The technique offers several benefits, combining very high sensitivity with containment of the radioisotope in the solid state, thus minimizing disposal procedures. In this study, it was used in conjunction with paper chromatography to quantify different oxidation states of (sup99)Tc in solution. The technique was used to evaluate the potential of anaerobic cultures of Shewanella putrefaciens and Geobacter metallireducens (bacteria with known metal-reducing capabilities) to reduce highly soluble Tc(VII) to insoluble lower-valence species, facilitating its removal from solution. Both organisms reduced Tc(VII), but profiles of Tc species produced in culture supernatants were strain specific. S. putrefaciens produced Tc(V), Tc(IV), and one unidentified species, but no Tc was removed from solution. G. metallireducens removed 70% of the 250 (mu)M Tc added in solution, with trace amounts of Tc(V) and the unidentified species detected in culture supernatants. Possible uses for these organisms in the bioremediation of Tc-contaminated waters are discussed, and other uses of the PhosphorImager technique are highlighted.

Aqueous solvation effect on the prototropic tautomerism of 2‐thiocytosine

AbstractFree energies of solvation were calculated for the four most stable gas‐phase tautomeric forms of 2‐thiocytosine (TC) using a local field SCF procedure with the solvation model SM2. The calculated changes in the free energies for each pair of tautomers reveal that all six possible equilibria tend to produce the thione‐amino TC(1, 8, 8) and TC(3, 8, 8) species. This is in agreement with UV and ionization constant data.

Diffusion of Technetium in Dense Bentonite under Oxidizing and Reducing Conditions

AbstractDiffusion is the principal mechanism by which most contaminants migrate through dense bentonite‐based barrier materials used in many waste‐containment strategies. Diffusion coefficients are, therefore, critical parameters for predicting migration rates of contaminants through these materials. Diffusion coefficients, D, were determined for Tc (99Tc is a relatively long‐lived radionuclide present in high‐level nuclear fuel waste) in compacted, nearly saturated bentonite under both oxidizing and reducing conditions. The D values were measured under oxidizing conditions at 25 or 80 °C in bentonite compacted to dry bulk densities, ρb, ranging from about 0.9 to 1.35 Mg/m3. In a reducing environment, D values were determined at 25 °C and ρb ≃ 1.3 Mg/m3; reducing conditions were established by mixing 1% (w/w) Fe(II)‐silicate with the bentonite and by conducting the experiment under a N2 atmosphere. Under oxidizing conditions at 25 °C, D ranged from 2.3 × 10−10 m2/s at ρb ≃ 0.9 Mg/m3 to 0.73 × 10−10 m2/s at 1.35 Mg/m3. The decrease in D with increasing density is largely attributed to an increase in the tortuosity of the diffusion path as the density increased. At 80 °C, D values were two to three times greater than those obtained at 25 °C at all densities; this is primarily due to a decrease in the viscosity of the saturating solution with increasing temperature. In a reducing environment, D was 6 ± 3 × 10−12 m2/s. The lower D values under reducing conditions are attributed to the reduction of TcO−4[Tc(VII)] to a less mobile Tc(IV) species. The results indicate that the migration of Tc through a bentonite‐based barrier material will be relatively slow in a reducing environment, such as that expected in a nuclear fuel waste disposal vault located deep in plutonic rock; this will mitigate the potential hazard associated with 99Tc in the long‐term disposal of nuclear fuel waste.

Ligand substitution reactions of tetrachlorobis(triphenylphosphine)technetium(IV)

The synthesis and characterization of tetrachlorobis (dimethyl sulfoxide)technetium(IV), tetrachlorobis (pyridine)technetium(IV), trichlorotris(pyridine)- technetium(III), trichlorotris(4-methylpyridine)technetium(III), and trichlorotris(3,5-dimethylpyridine)- technetium(III) are described, as well as the reaction of tetrachlorobis(triphenylphosphine)technetium(IV) with acetonitrile. Fast atom bombardment mass spectrometry, proton nuclear magnetic resonance, and cyclic voltammetry studies of the new complexes are reported. The Tc(IV) and Tc(III) species are paramagnetic, but the latter give well-resolved contact-shifted 1H NMR spectra.

Determination of the oxidation state of Tc in 99Tc(Sn)EHDP, 99mTc(Sn)EHDP, 99Tc(Sn)MDP and 99mTc(Sn)MDP complexes. Characterization of Tc(III)-, Tc(IV)- and Tc(V)EHDP complexes

The oxidation state of complexed Tc was determined, by performing paper and thin-layer chromatography during potentiometric titrations. Both titrations of Sn(II) with TcO 4 − and of TcO 4 − with Sn(II) were performed at pH 2.5, 7.0 and 12.0. Differences were found in the number of electrons transferred to 1 mol of TcO 4 −, n, in the titration solutions at the end point of a direct and a reversed titration. The oxidation state of Tc changes during a titration with the titration degree. It was possible to obtain almost pure Tc(III)-, Tc(IV)- and Tc(V)EHDP complexes, depending on the pH, titration mode and titration degree. These TcEHDP complexes were analyzed by u.v./vis spectrometry, anion-exchange HPLC on Aminex A-28, gel chromatography on Biogel P 4 and extraction with oxine in chloroform. The spectra of the pure Tc(III), Tc(IV) and Tc(V) species were calculated. Adsorption of these complexes on calcium phosphate increased in the order Tc(V) < Tc(IV) < Tc(III). From the chromatographic characterizations it was concluded that in the 99mTc(Sn)EHDP bone scanning agent Tc is tetravalent. An analogous (though less elaborate) investigation was performed on the Tc(Sn)MDP complex.

Solidification/stabilization of technetium in cement-based grouts

Mixed low-level radioactive and chemically hazardous process treatment wastes from the Portsmouth Gaseous Diffusion Plant are stabilized by solidification in cement-based grouts. Conventional portland cement and fly-ash grouts have been shown to be effective for retention of hydrolyzable metals (e.g., lead, cadmium, uranium, and nickel) but are marginally acceptable for retention of radioactive 99Tc, which is present in the waste as the highly mobile pertechnetate anion. Addition of ground blast furnace slag to the grout is shown to reduce the leachability of technetium by several orders of magnitude. The selective effect of slag is believed to be due to its ability to reduce Tc (VII) to the less soluble Tc (IV) species.

Technetium Absorption and Turnover in Monogastric and Polygastric Animals

Technetium (Tc) released into the environment can reach animals in various chemical forms: as pertechnetate (TcO-4) in drinking water or deposited on the surface of vegetables and forage plants, or as Tc bioincorporated into plants and associated with various plant constituents. In addition to being influenced by chemical speciation in the diet, absorption, metabolism, and retention of Tc in animals are modified by the treatment that the alimentary bolus undergoes during its passage through the gastrointestinal tract. This behavior differs markedly between polygastric and monogastric animals. We have, therefore, studied the fate of 99mTc given in the diet either as TcO-4 or bioincorporated into maize in rats (as an example of a monogastric animal) and in sheep (as an example of a polygastric animal). Urine and feces were collected and assayed for Tc activity by gamma spectrometry. Animals were sacrificed at different times after contamination, and the Tc content of tissues was determined. The pattern of absorption, excretion and, to a certain degree, of organ distribution and retention depended on animal species and species of Tc administered. Excretion was by feces and urine, and several metabolic components could be discerned. A component of very short half-time in urine suggests that newly absorbed Tc is more readily excreted than that already bound by tissues. The highest tissue concentrations were found in the thyroid. Retention of Tc was, however, most pronounced in bone and skin. Hair contains considerable amounts of Tc and may serve as a bioindicator of Tc contamination.

Electrochemistry of oxo-technetium(V) complexes containing Schiff base and 8-quinolinol ligands

The electrochemistry of six-coordinate, monooxo technetium(V) complexes containing Schiff base ligands has been studied in acetonitrile and N,N'-dimethylformamide solutions. The complexes have the general formula TcOCl(L/sub B/)/sub 2/ or TcO(L/sub T/)(L/sub B/), where L/sub B/ represents a bidentate-N,O Schiff base ligand or a bidentate-N,O 8-quinolinol ligand and L/sub T/ represents a tridentate-O,N,O Schiff base ligand. Cyclic voltammetry at a platinum-disk electrode, controlled-potential coulometry, and thin-layer spectroelectrochemistry were used to probe both the oxidation and the reduction of these complexes. The results of these studies, and previously reported results on the analogous Re(V) complexes, can be understood within a single general reaction scheme. The salient features of this scheme are (i) one-electron reduction of Tc(V) to Tc(IV), (ii) subsequent loss of a ligand situated cis to the Tc/identical to/O linkage, and (iii) subsequent isomerization of this unstable Tc(IV) product to more stable complex in which the site trans to the Tc/identical to/O linkage is vacant. The Tc(IV) complexes can also be reduced to analogous Tc(III) species, which appear to undergo the same ligand loss and isomerization reactions. The technetium complexes are 400-500 mV easier to reduce than are their rhenium analogues. The 8-quinolinol ligands, and especially the 5-nitro derivative, both thermodynamicallymore » and kinetically stabilize the Tc(IV) and Tc(III) oxidation states. These electrogenerated species are unusual in that they constitute the bulk of the known examples of monomeric Tc(IV) and Tc(III) complexes containing only N- and O-donating ligands. 34 refs., 9 figs., 1 tab.« less

Study of reduction of TcO 4− in 6 M HBr

The reduction of pertechnetate by 6 M HBr takes place in two steps: (i) Tc(VII) → Tc(V) and (ii) Tc(V) → Tc(IV). The kinetics of both reactions were studied. The first step corresponds to a pseudo first order reaction. The second step of the reduction appears to be a combination of a first order with a zero order process. The same Tc(V) species was formed when the TcO 4 − was reduced with either 8.7 M or 6 M HBr. The spectrophotometric characteristics of this species were comparable to those of Rb + and Cs + salts of [TcOBr 5] 2−.

Aquation of K2 TcBr6 in dilute acid solutions

The photochemical aquation of K2 TcBr6 in 2M HRr, 1M HClO4 and 1M H2SO4 has been studied. The absorption spectra of the various Tc(IV) species were measured after the electrophoretic separation. The spectrophotometric changes and the yield of each species as a function of the UV irradiation time were determined. After 25 hours of irradiation of HBr and HClO4 solutions the main species were the cationic ones (∼80%) but in H2SO4 solution the neutral species reached a yield of 90%. The oxidation of Tc(IV) species to TcO4− proceeded more rapidly and extensively in HClO4 than in HBr and H2SO4.

Leaching of Chlorine, Cesium, Strontium and Technetium from Cement-Fixed Intermediate Level Liquid Waste

ABSTRACTThe leaching behaviour of Cl-36, Cs-137, Sr-90 and Tc-99 from cement-fixed intermediate level liquid waste was investigated. Bentonite, MicroSili-ca®, acrylic resin and sodium sulfide were used as additives to a blast furnace cement matrix. In some cases portland cement was used. The liquid (water or waste solution) to cement ratio applied was mostly 0.47. Samples were leached with water or quinary brine (Q-brine).After 250 days the sequence of leachability from additive-free specimen in Q-brine was Cl&gt;Cs&gt;Sr&gt;Tc with little modification in water: Cs&gt;Cl&gt;Sr&gt;Tc. Cl and Cs leaching are diffusion-controlled, Sr and Tc are chemically fixed.Bentonite improves the retardation of Cs significantly, but has no significant effect on Cl, Sr and Tc. MicroSilica® deteriorates the retardation of Cs, but has no effect on Cl, Sr and Tc release.Sulfide was expected to reduce the leachability of Tc by forming insoluble TC2S7. No significant effect was observed, however, because either TC2S7 does not form at the pH value of the cement slurry or, due to a solubility competition, insoluble Tc species also form in the absence of sulfide ions.Acrylic resin does not reduce the leachability of Cs significantly. Irradiation with doses up to 4.4–105 Gy slightly deteriorates Cs retardation.Applying a liquid to cement ratio of 0.35 instead of 0.47 improves the retardation of Cs by a factor of 1.5–4, which may be due to a reduction of porosity in the specimen.

Catalytic and inhibitory effects of technetium on reduction processes

Interactions between technetium and some anthraquinones and tartrazin in the presence of tin(II) are described. It was found that whereas the reductive process between Sn(II) and 1-amino-4-hydroxyanthraquinone is catalyzed by technetium, the reduction of tartrazin is inhibited by this element. Study of such processes seems to indicate that the catalytic effect of technetium on the reduction processes is due to Tc(V) action whereas the inhibitory effect is due to the Tc(IV) species.

Technetium electrochemistry. 3. Spectroelectrochemical studies on the mixed-ligand technetium(III) complexes trans-[Tc(PR2R')2L]+ where L is a tetradentate Schiff base ligand and PR2R' is a monodentate tertiary phosphine ligand

The redox chemistry of a series of eight well-characterized, robust, cationic technetium(III) complexes has been investigated in propylene carbonate by using spectroelectrochemistry with a gold minigrid optically transparent thin-layer electrode. The complexes undergo a reversible 1-equiv reduction of Tc(III) to Tc(II) and a reversible 1-equiv oxidation of Tc(III) to Tc(IV). E/sup 0/' values for the Tc(III)/Tc(II) couple range from -1.11 to -0.69 V (vs. Ag/AgCl/NaCl (3 M)) while E/sup 0/' values for the Tc(IV/Tc(III) couple range from + 0.62 to + 0.79 V (vs. the same reference). The difference between E/sup 0/' /sub IV/III/ and E/sup 0/'/sub III/II/ ranges from 1.5 to 1.75 V, reflecting the large range of stability of the Tc(III) state. The formal potentials of both redox processes depend on the nature of the phosphine and Schiff base ligands. These dependencies are readily explained in terms of ..pi.. back-bonding from low-valent technetium to both the phosphine and Schiff base ligands. All the technetium complexes, including the electrogenerated Tc(II) and Tc(IV) species, exhibit characteristic metal-to-ligand charge-transfer (MTLCT) bands in the visible region of the spectrum. The energies of these MTLCT tranisitions are a function of the oxidation state of the technetium, the nature of the phosphine ligands, andmore » the nature of the Schiff base ligand. The energy of the Tc(III) MTLCT band is linearly related to the redox potential of the Tc(IV)/Tc(III) couple.« less