Reduction of uranium(VI) phosphate during growth of the thermophilic bacterium Thermoterrabacterium ferrireducens.

The thermophilic, gram-positive bacterium Thermoterrabacterium ferrireducens coupled organotrophic growth to the reduction of sparingly soluble U(VI) phosphate. X-ray powder diffraction and X-ray absorption spectroscopy analysis identified the electron acceptor in a defined medium as U(VI) phosphate [uramphite; (NH4)(UO2)(PO4) . 3H2O], while the U(IV)-containing precipitate formed during bacterial growth was identified as ningyoite [CaU(PO4)2 . H2O]. This is the first report of microbial reduction of a largely insoluble U(VI) compound.

Solution chemistry of uranyl ion with iminodiacetate and oxydiacetate: A combined NMR/EXAFS and potentiometry/calorimetry study.

The solution chemistry of uranyl ion with iminodiacetate (IDA) and oxydiacetate (ODA) was investigated using NMR and EXAFS spectroscopies, potentiometry, and calorimetry. From the NMR and EXAFS data and depending on stoichiometry and pH, three types of metal:ligand complex were identified in solution in the pH range 3-7: 1:1 and 1:2 monomers; a 2:2 dimer. From NMR and EXAFS data for the IDA system and previous studies, we propose the three complex types are [UO(2)(IDA)(H(2)O)(2)], [UO(2)(IDA)(2)](2)(-), and [(UO(2))(2)(IDA)(2)(mu-OH)(2)](2)(-). From EXAFS spectroscopy, similar 1:1, 2:2, and 1:2 complexes are found for the ODA system, although (13)C NMR spectroscopy was not a useful probe in this system. For the 1:1 and 1:2 complexes in solution, EXAFS spectroscopy is ambiguous because the data can be fitted with either a long U-N/O(ether) value (ca. 2.9 A) suggesting 1,7-coordination of the ligand or a U-C interaction at a similar distance, consistent with terminal bidentate coordination. However, the NMR data of the IDA system suggest that 1,7-coordination is the more likely. The stability constants of the three complexes were determined by potentiometric titrations; the log beta values are 9.90 +/-, 16.42 +/-, and 10.80 +/- for the 1:1, 1:2, and 2:2 uranyl-IDA complexes, respectively, and 5.77 +/-, 7.84 +/-, and 4.29 +/- for the 1:1, 1:2, and 2:2 uranyl-ODA complexes, respectively. The thermodynamic constants for the complexes were calculated from calorimetric titrations; the enthalpy changes (kJ mol(-)(1)) and entropy changes (J K(-)(1) mol(-)(1)) of complexation for the 1:1, 1:2, and 2:2 complexes respectively are the following. IDA: 12 +/- 2, 230 +/- 8; 8 +/- 2, 151 +/- 9; -33 +/- 3, -283 +/- 11. ODA: 26 +/- 2, 198 +/- 12; 20 +/- 2, 106 +/- 8; -24 +/- 2; -219 +/- 8.

Synthesis and characterization of uranyl compounds with iminodiacetate and oxydiacetate displaying variable denticity.

Eight uranyl compounds containing the dicarboxylate ligands iminodiacetate (IDA) or oxydiacetate (ODA) have been characterized in the solid state. The published polymeric structures for [UO(2)(C(4)H(6)NO(4))(2)] and [UO(2)(C(4)H(4)O(5))](n) have been confirmed, while Ba[UO(2)(C(4)H(5)NO(4))(2)] x 3H(2)O, [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)][UO(2)(C(4)H(4)O(5))(2)] [orthorhombic space group Pnma, a = 10.996(5) A, b = 21.42(1) A, c = 8.700(3) A, Z = 4], and [C(2)H(5)NH(2)(CH(2))(2)NH(2)C(2)H(5)][UO(2)(C(4)H(4)O(5))(2)] [monoclinic space group P2(1)/n, a = 6.857(3) A, b = 9.209(5) A, c = 16.410(7) A, beta = 91.69(3), Z = 2] contain monomeric anions. The distance from the uranium atom to the central heteroatom (O or N) in the ligand varies. Crystallographic study shows that U-heteroatom (O/N) distances fall into two groups, one 2.6-2.7 A in length and one 3.1-3.2 A, the latter implying no bonding interaction. By contrast, EXAFS analysis of bulk samples suggests that either a long U-heteroatom (O/N) distance (2.9 A) or a range of distances may be present. Three possible structural types, two symmetric and one asymmetric, are identified on the basis of these results and on solid-state (13)C NMR spectroscopy. The two ligands in the complex can be 1,4,7-tridentate, giving five-membered rings, or 1,7-bidentate, to form an eight-membered ring. (C(4)H(12)N(2))[(UO(2))(2)(C(4)H(5)NO(4))(2)(OH)(2)] x 8H(2)O [monoclinic space group P2(1)/a, a = 7.955(9) A, b = 24.050(8) A, c = 8.223(6) A, beta = 112.24(6), Z = 2], (C(2)H(10)N(2))[(UO(2))(2)(C(4)H(5)NO(4))(2)(OH)(2)] x 4H(2)O, and (C(6)H(13)N(4))(2)[(UO(2))(2)(C(4)H(4)O(5))(2)(OH)(2)] x 2H(2)O [monoclinic space group C2/m, a = 19.024(9) A, b = 7.462(4) A, c = 2.467(6) A, beta = 107.75(4), Z = 4] have a dimeric structure with two capping tridentate ligands and two mu(2)-hydroxo bridges, giving edge-sharing pentagonal bipyramids.

Plutonium solubility in sediment pore waters.

Using in situ porous cup samplers, dissolved Pu concentrations have been measured over a year in the pore waters from two contrasting sites in the valley of the River Esk, North West England. In saltmarsh sediments, dissolved Pu represents approximately 1 part in 10(6) of the total inventory. The Pu concentration in solution is in the range 1.1-3.5 mBq l-1, varying by a factor of 3 in the course of the year. Most of the changes in dissolved Pu coincide with changes in dissolved Fe and Mn concentrations, with Pu being low in the summer months when Fe and Mn are high. Nevertheless, there are a number of factors which make it unclear as to whether these patterns might be related to seasonal redox changes in the saltmarsh. At the highly organic, reducing reedbed site, the proportion of Pu in solution is typically around 1 part in 10(3), proportionately much higher than in the saltmarsh, giving concentrations ranging between 9.0 and 28.5 mBq l-1, and are apparently maintained by complexation to dissolved organic matter. There is no obvious seasonal pattern at the reedbed site nor is there any relation to any of the dissolved species measured (Fe, Mn, Na, DOC).

Determination of U-236 in sediment samples by accelerator mass spectrometry.

236U is produced only by neutron irradiation of uranium and therefore is potentially useful as a marker for anthropogenic uranium in the environment. Accelerator mass spectrometry (AMS) provides a technique for the determination of very low concentrations of actinide nuclides, and has now been applied to the determination of 236U:235U ratios in an intertidal sediment core collected from the North Irish Sea. Combining measurements of the 238U mass concentrations calculated from alpha spectrometry with 238U:235U ratios from ICP-MS and 236U:235U ratios from AMS has allowed the estimation of the mass concentrations of 236U in the sediments. 236U mass concentrations are in the range 10(-8) to 10(-9) g kg-1, and 236U:238U atom ratios in the range from 10(-5) to 10(-6), well above natural baseline levels. Uncertainties based on propagation of measurement errors were less than +/- 10% although +/- 15% is perhaps a more realistic estimate of overall uncertainty.

Measurement of 237Np in environmental water samples by accelerator mass spectrometry.

Accelerator mass spectrometry (AMS) was used to measure 237Np in environmental water samples extracted from Irish Sea sediments. The samples were of limited volume (approximately 700 ml) and of low activity (0.06-0.79 mBq l-1; 2.30-30.3 pg l-1). AMS proved to have the required sensitivity for measuring these samples, and was in principle capable of measuring much smaller amounts, as low as 0.4 microBq (3.9 x 10(7) atoms). However, the background level in the procedural blanks showed that there was a systematic low level 237Np contamination of each sample, arising from the 239Np yield monitor used in the separations procedure, which effectively increased the detection limit of these analyses.

Determination of radionuclide exchangeability in freshwater systems.

Two freshwater sediments were spiked with 57Co, 85Sr and 134Cs and left for adsorption times ranging from 1 day to over 60 days. Following adsorption, the 'exchangeable' pool of each radionuclide was measured using ammonium acetate extractions and a sequential leach procedure (Tessier et al., 1979), and the results were compared. Exchangeability was found to depend upon the sediment, radionuclide, sorption time and the identity of extracting agent. All three radionuclides showed a shift with increasing adsorption time from regular exchange sites to sites which are sterically hindered, but a fixation within the sediment was only observed for 57Co and 134Cs, with similar ammonium acetate extraction yields for both radionuclides. Misleading results were obtained during the sequential leach procedure due to redistribution and, therefore, the inclusion of a NH4+ leach after the MgCl2 extraction step was suggested. A mathematical model of element speciation was fairly successful at defining the 'exchangeable' fraction, suggesting that the chemically- and mathematically-defined fractions were similar.

X-ray Absorption Spectroscopy of Tricarbonatodioxouranate(V), [UO(2)(CO(3))(3)](5)(-), in Aqueous Solution.

Actinyl ions (AnO(2)(+) and AnO(2)(2+)) are strongly complexed by CO(3)(2)(-) in aqueous solution, and tricarbonato complexes of UO(2)(2+) and NpO(2)(+), in particular, have been widely studied. Cyclic voltammetry shows that, whereas the Np(V)/Np(VI) couple is reversible in aqueous CO(3)(2)(-) solution, the analogous U couple is irreversible, suggesting significant conformational change on one-electron reduction of [UO(2)(CO(3))(3)](4)(-). EXAFS spectroscopy has been used to define the coordination geometry of the reduced species and shows that both axial and equatorial U-O distances increase on reduction, from 1.80 to 1.90 Å and from 2.43 to 2.50 Å, respectively. The basic geometry of the U complex is apparently unchanged by reduction so any conformational change which does occur must be subtle.

Selectivity in the complexation of actinides by humic substances.

The interactions of a range of actinide elements (Th, U, Np, Pu, Am) with humic substances from the Needle's Eye natural analogue site were studied by gel permeation chromatography. Bulk humic substances were isolated by ammonia extraction, followed by dialysis against distilled water and freeze-drying. The gel permeation results suggest that Needle's Eye humic substances can be fractionated into three incompletely resolved fractions with average molecular weights determined by analytical ultracentrifugation around 49 000 for Fraction 1, around 14 700 for Fraction 2 and around 8000 for Fraction 3. Although there are significant differences between the organic matter elution patterns in individual gel permeation experiments, presumably due to differences in column packing, these are much smaller than the differences between metal ions. The uranium that is naturally present in these humic substances is largely bound in the late-eluting fraction. Spikes of the early actinides, including Np and Pu in controlled valency states, have been added to the humic substances, and gel permeation of the spiked humic substances shows that the three humic fractions vary greatly in their effectiveness and selectivity as ligands for early actinides.

A review of radionuclides in tide-washed pastures on the Irish Sea coast in England and Wales and their transfer to food products.

Close to the Sellafield Reprocessing Plant activity concentrations of many radionuclides including the long-lived 137Cs and actinides on tide-washed pastures bordering the Irish Sea are high compared with other terrestrial ecosystems in the United Kingdom. Despite the comparatively high deposition of radionuclides, contamination of agricultural products from tide-washed pastures is lower than would be predicted from studies in terrestrial ecosystems. This is because the main source of contamination in tide-washed pasture, radionuclides associated with sedimentary particles, has low availability for uptake by plant roots and in the gut of ruminants. Generalised Derived Limits (GDLs), published by the National Radiological Protection Board (NRPB) are estimates of environmental radioactivity concentrations which would potentially be radiologically significant. These provide a benchmark against which environmental measurements can be assessed. In the past, several comparisons of radioactivity concentrations in tide-washed pastures with GDLs have been made, but only some of these comparisons are valid since current approaches used to assess doses are not always strictly appropriate to these ecosystems. Directly applicable GDLs are needed, and are currently being developed by the NRPB.

Evidence for the remobilisation of transuranic elements in the terrestrial environment.

The transuranium elements, Np, Pu and Am discharged from the BNFL fuel reprocessing plant at Sellafield have accumulated in the local environment. The processes responsible for their dispersal rely both on physical transportation and their chemical reactivity. The transuranics have a complex chemistry, with multiple oxidation states and a strongly polarising character. In the environment, the particle active III/IV and more mobile VNI oxidation state groups are important and govern their geochemical behaviour and subsequent dispersal.Studies of the behaviour of the transuranics, particularly Pu, in the Irish Sea, have shown that the majority of the radionuclides in the liquid effluent discharged from Sellafield, quickly becomes associated with the marine sediments. Their dispersal and distribution in the environment is then governed primarily by the movement of particulate material and for some sites it has been suggested that sediment profiles preserve the historical record of discharges from the plant.In tidally inundated soils, radionuclide levels are greatly enhanced. These soils are water-logged for long periods of the year, are strongly anoxic and accretion rate are very low. The distribution of Np, Pu and Am in the soil suggests that simple sedimentary accumulation mechanism cannot provide an adequate explanation for the profiles observed. From preliminary studies of soil pore water composition and detailed analysis of the variation of isotopic ratios in the soil cores, it is apparent that a small but significant component of the radionuclide inventory is mobile. In addition, it is clear that the mechanisms responsible for this mobility allows differentiation between the transuranium nuclides.

Development of a laboratory method to predict rapidly the availability of radiocaesium.

A simple extraction procedure has been developed to assess rapidly the probable extent of the transfer of radiocaesium into ruminant food products soon after a nuclear accident. The in vitro extractions were validated against true absorption measurements of different forms of radiocaesium in the sheep gut. Extractions were performed on a range of different radiocaesium sources. Some of these sources were artificial (ionic radiocaesium adsorbed onto bentonite, silica spheres and filter-papers) and others were environmentally contaminated [silt from the Ravenglass Estuary contaminated by effluent from British Nuclear Fuels Limited (BNFL) Sellafield, and upland grass and heather contaminated by Chernobyl fallout]. Laboratory experiments concentrated primarily on the use of simple inorganic extractants in competitive ion-exchange processes. Of the reagents used, 0.1 mol dm-3 stable caesium chloride solution was the most effective extractant. The proportion of radiocaesium extracted by 0.1 mol dm-3 caesium chloride correlated well with measurements of true absorption. Extracting radiocaesium using 0.1 mol dm-3 caesium chloride proved to be an inexpensive and rapid method of predicting the availability of radiocaesium for absorption in the ruminant gut, giving results within 24 h. Further extractions were carried out using cellulase/pepsin simulated digestions and ovine rumen fluid. Results suggested that the availability of radiocaesium from some inorganic sources may be underestimated using such techniques.

Distribution of radiocesium in the soil-plant systems of upland areas of Europe.

The distribution and behavior of Cs in the soil-plant systems at some upland sites in Northeastern Italy, Scotland, and Norway have been investigated. From the limited range of samples taken, there appears to be no dominant physicochemical control on the plant availability of Cs. The presence of micaceous minerals or illitic clays does not significantly inhibit Cs uptake, either because of recycling in the organic surface horizons or because of clay-organic complex formation. Lower plants (bryophytes and lichens) show the highest Cs accumulation. Of the higher plants, ericaceous species take up Cs more than the others.

Chemical reactions of metals with humic material.

Humic substances are chemically very complex materials whose structure and reactions are not fully understood. They are believed to be macromolecules, spanning a wide range of molecular weights, which are formed from quinones and phenolic compounds. They contain a wide variety of functional groups, which may react with metals. Many different physical and chemical procedures have been used to study these interactions, and numerous different reaction mechanisms and products have been postulated. The colloidal properties of humic materials also affects their interactions with metals. Reaction with humic substances profoundly affects the environmental behaviour of metals. Solubility, plant availability and even volatility are all greatly influenced and can be either enhanced or reduced by these reactions.