Biomineralization of Uranium-Phosphates Fueled by Microbial Degradation of Isosaccharinic Acid (ISA).

Geological disposal is the globally preferred long-term solution for higher activity radioactive wastes (HAW) including intermediate level waste (ILW). In a cementitious disposal system, cellulosic waste items present in ILW may undergo alkaline hydrolysis, producing significant quantities of isosaccharinic acid (ISA), a chelating agent for radionuclides. Although microbial degradation of ISA has been demonstrated, its impact upon the fate of radionuclides in a geological disposal facility (GDF) is a topic of ongoing research. This study investigates the fate of U(VI) in pH-neutral, anoxic, microbial enrichment cultures, approaching conditions similar to the far field of a GDF, containing ISA as the sole carbon source, and elevated phosphate concentrations, incubated both (i) under fermentation and (ii) Fe(III)-reducing conditions. In the ISA-fermentation experiment, U(VI) was precipitated as insoluble U(VI)-phosphates, whereas under Fe(III)-reducing conditions, the majority of the uranium was precipitated as reduced U(IV)-phosphates, presumably formed via enzymatic reduction mediated by metal-reducing bacteria, including Geobacter species. Overall, this suggests the establishment of a microbially mediated "bio-barrier" extending into the far field geosphere surrounding a GDF is possible and this biobarrier has the potential to evolve in response to GDF evolution and can have a controlling impact on the fate of radionuclides.

Plutonium(IV) Sorption during Ferrihydrite Nanoparticle Formation.

Understanding interactions between iron (oxyhydr)oxide nanoparticles and plutonium is essential to underpin technology to treat radioactive effluents, in cleanup of land contaminated with radionuclides, and to ensure the safe disposal of radioactive wastes. These interactions include a range of adsorption, precipitation, and incorporation processes. Here, we explore the mechanisms of plutonium sequestration during ferrihydrite precipitation from an acidic solution. The initial 1 M HNO3 solution with Fe(III)(aq) and 242Pu(IV)(aq) underwent controlled hydrolysis via the addition of NaOH to pH 9. The majority of Fe(III)(aq) and Pu(IV)(aq) was removed from solution between pH 2 and 3 during ferrihydrite formation. Analysis of Pu-ferrihydrite by extended X-ray absorption fine structure (EXAFS) spectroscopy showed that Pu(IV) formed an inner-sphere tetradentate complex on the ferrihydrite surface, with minor amounts of PuO2 present. Best fits to the EXAFS data collected from Pu-ferrihydrite samples aged for 2 and 6 months showed no statistically significant change in the Pu(IV)-Fe oxyhydroxide surface complex despite the ferrihydrite undergoing extensive recrystallization to hematite. This suggests the Pu remains strongly sorbed to the iron (oxyhydr)oxide surface and could be retained over extended time periods.

Behaviour and mobility of U and Ra in sediments near an abandoned uranium mine, Cornwall, UK.

Sediment samples were collected from the vicinity of the abandoned South Terras uranium mine in south-west UK and analysed for uranium and (226)Ra to explore their geochemical dispersion. The radioactivity concentrations in the sediment samples were measured using alpha spectrometry for uranium, and gamma spectrometry for radium. Sequential chemical extraction was applied to selected sediments in order to investigate the speciation of the radionuclides and their association with stable elements. The activity ratio of the uranium isotopes was used to explore the mobility of uranium, and scanning electron microscopy (SEM) and electron microprobe analysis (EMPA) were used to characterise the sediments. The radiochemical results identified two locations with enhanced radioactivity, so two samples from these locations were further investigated. The geochemical distribution of the radionuclides in these two samples varies within the five operationally-defined fractions. In one sample, the majority of the uranium was released from the 'carbonate' fraction, followed by the organic fractions. Similarly, in the second sample, the uranium was mainly resealed from the carbonate fraction, although a considerable percentage associated with the resistant fraction. The fractionation trend of radium noticed to show some similarities to that of barium, as expected from the similarity in their chemistries. Geochemical distributions of the stable elements, such as Mn, Ti and As, were different in the enhanced radioactivity samples. The activity ratio of (234)U/(238)U shows different trends in the two sediments, signifying the impact of organic matter and/or the exchange between water and sediment. SEM and EMPA analysis identified uranium-bearing phases in association with potassium, calcium, iron, manganese and arsenic.

Dispersion of U-series natural radionuclides in stream sediments from Edale, UK.

The spatial distribution of (238)U-series radionuclides, specifically 238U, 234U, 230Th and 226Ra, has been determined in stream sediments from Edale, Derbyshire, United Kingdom, to explore the behaviour of U-series radionuclides during weathering. For uranium and thorium, two different extraction methods were used, total dissolution with HNO3/HF in a microwave and leaching with aqua regia. This was followed by radiochemical separation using extraction chromatography, then alpha spectrometry measurement. The total radium contents in the sediments were measured using gamma spectrometry, while the leached fraction was measured in the same way as for uranium and thorium. The total sediment content of uranium and thorium ranges from ∼10 up to ∼200 Bq kg(-1), while the radium specific activity lies between ∼15 and 180 Bq kg(-1). In the aqua regia extractions, the uranium and thorium contents are in the range of ∼5 to ∼100 Bq kg(-1), while the radium specific activities are similar to those measured by total dissolution. All the radionuclides show no correlation with organic matter content. The activity ratios 234U/238U, 230Th/238U and 226Ra/238U were used to determine the degree of radioactive disequilibrium. The data show disequilibrium in most of the sediments, with activity ratios of 234U/238U, 230Th/238U and 226Ra/238U>1, inconsistent with evolution through straightforward weathering processes. Multivariate cluster analysis based on five variables, the specific activities of 238U, 234U, 230Th, 226Ra and loss on ignition, was employed to group the data and identify five distinct clusters. There seems to be a link between high radionuclide concentrations and proximity to landslips.

Is gluconate a good model for isosaccharinate in uranyl(VI) chemistry? A DFT study.

The geometries, relative energies and spectroscopic properties of a range of α-isosaccharinate complexes of uranyl(VI) are studied computationally using ground state and time-dependent density functional theory. The effect of pH is accommodated by varying the number of water and hydroxide ligands accompanying isosaccharinate in the equatorial plane of the uranyl unit. For 1 : 1 complexes, the calculated uranyl ν(asym) stretching frequency decreases as pH increases, in agreement with previous experimental data. Three different isosaccharinate chelating modes are studied. Their relative energies are found to be pH dependent, although the energetic differences between them are not sufficient to exclude the possibility of multiple speciation. At higher pH, the uranyl-ligand interactions are dominated more by the equatorial OH(-) than by the organic ligands. Calculated electronic excitation energies support experiment in finding the lowest energy transitions to be ligand → metal charge transfer. (13)C NMR chemical shifts are calculated for the coordinated isosaccharinate in the high pH mimics, and show good agreement with experimental data, supporting the experimental conclusion that the five-membered chelate ring is favoured at high pH. The effect of increasing the isosaccharinate concentration is modelled by calculating 1 : 2 and 1 : 3 uranyl : α-isosaccharinate complexes. Comparison of the results of the present study with those from our closely related investigation of uranyl(VI)-D-gluconate complexes (Dalton Transactions 40 (2011) 11248) reveals strong similarities in structure, bonding, coordination geometry and electronic excitations, but also differences in ΔG for key ligand replacement reactions, suggesting that caution should be exercised when using gluconate as a thermodynamic model for isosaccharinate in uranyl(vi) chemistry.

Computational investigation of the speciation of uranyl gluconate complexes in aqueous solution.

The geometries, relative energies and spectroscopic properties of a range of D-gluconate complexes of uranyl(VI) are studied computationally using density functional theory. The effect of pH is accommodated by varying the number of water and hydroxide ligands accompanying gluconate in the equatorial plane of the uranyl unit. For 1 : 1 complexes, the calculated uranyl ν(asym) stretching frequency decreases as pH increases, in agreement with previous experimental data. Three different gluconate chelating modes are studied. Their relative energies are found to be pH dependent, although the energetic differences between them are not sufficient to exclude the possibility of multiple speciation. (13)C NMR chemical shifts are calculated for the coordinated gluconate in the high pH mimics, and show good agreement with experimental data, supporting the experimental conclusion that the six-membered chelate ring is favoured at high pH. Attempts to improve the description of the aqueous environment via the addition of second solvation shell water molecules resulted in significantly worse agreement with experiment for ν(asym). The effect of increasing the gluconate concentration is modelled by calculating 1 : 2 and 1 : 3 uranyl : D-gluconate complexes.

Probing the biogeochemical behavior of technetium using a novel nuclear imaging approach.

Dynamic gamma-camera imaging of radiotracer technetium ((99m)Tc) was used to assess the impact of biostimulation of metal-reducing bacteria on technetium mobility at 10(-12) mol L(-1) concentrations in sediments. Addition of the electron donor acetate was used to stimulate a redox profile in sediment columns, from oxic to Fe(III)-reducing conditions. When (99m)Tc was pumped through the columns, real-time gamma-camera imaging combined with geochemical analyses showed technetium was localized in regions containing biogenic Fe(II). In parallel experiments, electron microscopy with energy-dispersive X-ray (EDX) mapping confirmed sediment-bound Tc was associated with iron, while X-ray absorption spectroscopy (XAS) confirmed reduction of Tc(VII) to poorly soluble Tc(IV). Molecular analyses of microbial communities in these experiments supported a direct link between biogenic Fe(II) accumulation and Tc(VII) reductive precipitation, with Fe(III)-reducing bacteria more abundant in technetium immobilization zones. This offers a novel approach to assessing radionuclide mobility at ultratrace concentrations in real-time biogeochemical experiments, and confirms the effectiveness of biostimulation of Fe(III)-reducing bacteria in immobilizing technetium.

Treatment of radioactive wastes: an X-ray absorption spectroscopy study of the reaction of technetium with green rust.

Technetium is a long-lived product of nuclear fission that readily forms the soluble pertechnetate anion [TcO(4)](-). Green rusts (layered hydrous oxides containing both Fe(II) and Fe(III) and with interlayer sulfate or carbonate anions) concentrate >99.8% of 99Tc, present as [TcO(4)](-), from aqueous solution, even in the presence of high concentrations of NaNO(3), a common constituent of radioactive waste streams. The mechanism of removal from solution is apparently reduction and formation of strong Tc(IV) surface complexes. X-ray absorption spectroscopy shows that [TcO(4)](-) is indeed reduced by reaction with both sulfate- and carbonate-form green rusts and is found in a TcO(2)-like environment. On contact with air, the green rusts oxidize to poorly crystalline goethite but the Tc environment is unchanged. There is no increase in Tc solubility associated with oxidation of the host green rust. This behavior suggests that green rusts may be useful in the treatment of Tc-containing waste streams, in groundwater cleanup, and in restricting Tc migration from repositories.