Potential of indigenous bacteria driven U(VI) reduction under relevant deep geological repository (DGR) conditions.

Understanding the biogeochemical U redox processes is crucial for controlling U mobility and toxicity under conditions relevant to deep geological repositories (DGRs). In this study, we examined the microbial reduction of aqueous hexavalent uranium U(VI) [U(VI)aq] by indigenous bacteria in U-contaminated groundwater. Three indigenous bacteria obtained from granitic groundwater at depths of 44-60 m (S1), 92-116 m (S2), and 234-244 m (S3) were used in U(VI)aq bioreduction experiments. The concentration of U(VI)aq was monitored to evaluate its removal efficiency for 24 weeks under anaerobic conditions with the addition of 20 mM sodium acetate. During the anaerobic reaction, U(VI)aq was precipitated in the form of U(IV)-silicate with a particle size >100 nm. The final U(VI)aq removal efficiencies were 37.7%, 43.1%, and 57.8% in S1, S2, and S3 sample, respectively. Incomplete U(VI)aq removal was attributed to the presence of a thermodynamically stable calcium uranyl carbonate complex in the U-contaminated groundwater. High-throughput 16S rRNA gene sequencing analysis revealed the differences in indigenous bacterial communities in response to the depth, which affected to the U(VI)aq removal efficiency. Pseudomonas peli was found to be a common bacterium related to U(VI)aq bioreduction in S1 and S2 samples, while two SRB species, Thermodesulfovibrio yellowstonii and Desulfatirhabdium butyrativorans, played key roles in the bioreduction of U(VI)aq in S3 sample. These results indicate that remediation of U(VI)aq is possible by stimulating the activity of indigenous bacteria in the DGR environment.

Spectroscopic Study into Lanthanide Speciation in Deep Eutectic Solvents.

Deep eutectic solvents are a new class of green solvents that are being explored as an alternative for used nuclear fuel and critical material recycling. However, there is a paucity of knowledge regarding metal behavior in them. This paper explores the underlying chemistry of rare-earth elements in choline chloride-based deep eutectic solvents by using a multi-technique spectroscopic methodology. Results show that speciation is highly dependent on the choice of the hydrogen-bond donor. Collected EXAFS data showed Ln3+ coordination with ethylene glycol and urea in their respective solvents and coordination with chloride in the lactic acid system. Generalized coordination environments were determined to be [LnL4-5], [LnL7-10], and [LnL5-6] in the ethylene glycol, urea, and lactic acid systems, respectively. Collected UV/vis spectra for Nd3+ and Er3+ showed variations with changing solvents, showing that Ln-Cl interactions do not dominate in these systems. Luminescence studies were consistent, showing varying emission spectra with varying solvent systems. The shortest luminescent lifetimes were observed in the choline chloride-ethylene glycol deep eutectic solvent, suggesting coordination through O-H groups. Combining all collected data allowed Eu3+ coordination geometries to be assigned.

Study of Aqueous Am(III)-Aliphatic Dicarboxylate Complexes: Coordination Mode-Dependent Optical Property and Stability Changes.

The thermodynamics of Am(III) complex formation in natural groundwater systems is one of the major topics of research in the field of high-level radioactive waste management. In this study, we investigate the absorption and luminescence properties of aqueous Am(III) complexes with a series of aliphatic dicarboxylates in order to learn the thermodynamic complexation behaviors in relation to binding geometries. The formation of Am(III) complexes with these carboxylate ligands induced distinct red shifts in the absorption spectra, which enabled chemical speciation. The formation constants determined by deconvolution of the absorption spectra showed a linear decrease for the three ligands (oxalate (Ox), malonate (Mal), and succinate (Suc)) and a mild decrease for the remaining ligands (glutarate (Glu) and adipate (Adi)). Time-resolved laser fluorescence spectroscopy (TRLFS) was used to obtain information about the aqua ligand, which indirectly indicated the bidentate bindings of these dicarboxylate ligands. A complementary attenuated total reflectance Fourier transform infrared (ATR-FTIR) study on Eu(III), which is a nonradioactive analogue of Am(III) ion, showed that the coordination modes differ depending on the alkyl chain length. Ox and Mal bind to Am(III) via side-on bidentate bindings with two carboxylate groups, resulting in the formation of stable 5- and 6-membered ring structures, respectively. On the other hand, Suc, Glu, and Adi form end-on bidentate bindings with a single carboxylate group, resulting in a 4-membered ring structure. Density functional theory calculations provided details about the bonding properties and supported the experimentally proposed coordination geometries. This study demonstrates that coordination mode-dependent changes in optical properties occur along with thermodynamic stability changes in Am(III)-dicarboxylate complexes.

Studies of aqueous U(iv) equilibrium and nanoparticle formation kinetics using spectrophotometric reaction modeling analysis.

Hydrolysis of tetravalent uranium (U(iv)) and U(iv)-nanoparticle formation kinetics were examined over a wide range of temperatures using spectrophotometric reaction modeling analysis. The characteristic absorption bands representing U4+, U(OH)3+, and a proposed oxohydroxo species were newly identified in the UV region (190-300 nm). Dynamic absorption band changes in the UV and visible regions (360-800 nm) were explored to reevaluate the binary ion interaction coefficients for U(iv) ions and the thermodynamic constants of the primary hydrolysis reaction, including complexation constants, enthalpy, and entropy. No further hydrolysis equilibrium beyond the formation of U(OH)3+ was identified. Instead, an irreversible transformation of U(iv) ions to U(iv)-nanoparticles (NPs) was found to occur exclusively via the formation of a new intermediate species possessing characteristic absorption bands. The kinetic analysis, based on a two-step, pseudo-first-order reaction model, revealed that the rate of the initial step producing the intermediates is highly temperature-dependent with the measured kinetic energy barrier of ∼188 kJ mol-1. With additional experimental evidence, we conclude that the intermediates are oligomeric oxohydroxo U(iv) species occurring from the condensation of U(iv) ions and simultaneously participating in the nucleation and growth process of UO2(cr)-NPs.

Hydrolysis of trivalent plutonium and solubility of Pu(OH)3 (am) under electrolytic reducing conditions.

The aim of this work is to determine the solubility product of plutonium hydroxide under reducing conditions and to ascertain the stability of Pu(OH)3 (am) in water. Hydrolysis of Pu(iii) and solubility of Pu(OH)3 (am) were investigated at a constant ionic strength of 0.1 M NaClO4. Coulometric titration was adopted to adjust the pH of plutonium solutions, during which the electrolytic reducing conditions maintained the oxidation state of Pu(iii). Chemical speciation for dissolved plutonium was investigated using sensitive spectrophotometry coupled with a liquid waveguide capillary cell. The spectroscopic investigations indicated that dissolved Pu(iv), Pu(v), and Pu(vi) species were ignorable under these experimental conditions. The absorbance of Pu3+ ions decreased due to hydrolysis of Pu(iii) but the absorbance of Pu(iii) hydrolysis species was not distinguishable. The formation constant for the first hydrolysis species (log *ß'1) determined in the present study is -6.62 ± 0.25. The non-crystalline structure of the plutonium precipitate was observed through X-ray diffraction. The solubility product of Pu(OH)3 (am), log *K's,0 is determined to be 15.23 ± 0.50. These results indicate a stronger tendency for the hydrolysis of Pu(iii) and higher stability (lower solubility) of Pu(OH)3 (am) compared to Am(iii).

Factors affecting measurement of channel thickness in asymmetrical flow field-flow fractionation.

Asymmetrical flow field-flow fractionation (AF4) has been considered to be a useful tool for simultaneous separation and characterization of polydisperse macromolecules or colloidal nanoparticles. AF4 analysis requires the knowledge of the channel thickness (w), which is usually measured by injecting a standard with known diffusion coefficient (D) or hydrodynamic diameter (dh). An accurate w determination is a challenge due to its uncertainties arising from the membrane's compressibility, which may vary with experimental condition. In the present study, influence of factors including the size and type of the standard on the measurement of w was systematically investigated. The results revealed that steric effect and the particles-membrane interaction by van der Waals or electrostatic force may result in an error in w measurement.

Retention behavior of microparticles in gravitational field-flow fractionation (GrFFF): effect of ionic strength.

Retention behavior of micron-sized particles in gravitational field-flow fractionation (GrFFF) was studied in this study. Effects of ionic strength and flow rate as well as the viscosity of the GrFFF carrier liquid was investigated on the size-based selectivity (Sd), retention ratio (R), and plate height (H) of micron-sized particles using polystyrene latex beads as model particles. It was found that the retention ratio of microparticles increases with increasing flow rate or the viscosity of the carrier liquid as the particles are forced away from the accumulation wall by increased hydrodynamic lift forces (HLF). On the other hand, the retention time increases (retention ratio decreases) with increasing ionic strength of the carrier liquid at the same flow rate, due to decreased repulsive interaction between the particles and the channel accumulation wall (glass in this study) allowing the particles approach closer to the wall. Results suggest the ionic strength of the carrier liquid plays a critical role in determining retention of microparticles in GrFFF as well as the viscosity or the flow rate of the carrier liquid. It was found that the resolution and the separation time could be improved by increasing the carrier viscosity and by carefully adjusting the ionic strength of the carrier liquid.

Study on steric transition in asymmetrical flow field-flow fractionation and application to characterization of high-energy material.

In field-flow fractionation (FFF), there is the 'steric transition' phenomenon where the sample elution mode changes from the normal to steric/hyperlayer mode. Accurate analysis by FFF requires understanding of the steric transition phenomenon, particularly when the sample has a broad size distribution, for which the effect by combination of different modes may become complicated to interpret. In this study, the steric transition phenomenon in asymmetrical flow FFF (AF4) was studied using polystyrene (PS) latex beads. The retention ratio (R) gradually decreases as the particle size increases (normal mode) and reaches a minimum (Ri) at diameter around 0.5µm, after which R increases with increasing diameter (steric/hyperlayer mode). It was found that the size-based selectivity (Sd) tends to increase as the channel thickness (w) increases. The retention behavior of cyclo-1,3,5-trimethylene-2,4,6-trinitramine (commonly called 'research department explosive' (RDX)) particles in AF4 was investigated by varying experimental parameters including w and flow rates. AF4 showed a good reproducibility in size determination of RDX particles with the relative standard deviation of 4.1%. The reliability of separation obtained by AF4 was evaluated by transmission electron microscopy (TEM).

Quantitative analysis of uranium in aqueous solutions using a semiconductor laser-based spectroscopic method.

A simple analytical method based on the simultaneous measurement of the luminescence of hexavalent uranium ions (U(VI)) and the Raman scattering of water, was investigated for determining the concentration of U(VI) in aqueous solutions. Both spectra were measured using a cw semiconductor laser beam at a center wavelength of 405 nm. The empirical calibration curve for the quantitative analysis of U(VI) was obtained by measuring the ratio of the luminescence intensity of U(VI) at 519 nm to the Raman scattering intensity of water at 469 nm. The limit of detection (LOD) in the parts per billion range and a dynamic range from the LOD up to several hundred parts per million were achieved. The concentration of uranium in groundwater determined by this method is in good agreement with the results determined by kinetic phosphorescence analysis and inductively coupled plasma mass spectrometry.

Evaluation of the behavior of uranium peroxocarbonate complexes in Na-U(VI)-CO3-OH-H2O2 solutions by Raman spectroscopy.

In this work, the formation of uranium species and their stabilities in Na-U(VI)-CO(3)-OH-H(2)O(2) solutions at different pHs are studied by Raman spectroscopy. In this solution, the UO(2)(O(2))(CO(3))(2)(4-) species was formed together with three other uranium species of UO(2)(O(2))(2)(2-), UO(2)(CO(3))(3)(4-), and a speculated uranium species of the uranyl carbonate hydroxide complex, UO(2)(CO(3))(x)(OH)(y)(2-2x-y), which showed remarkable Raman peaks at approximately 769, 848, 811, and 727 cm(-1), respectively. The UO(2)(O(2))(CO(3))(2)(4-) species disappeared at pH conditions where bicarbonate dominated, and its Raman peak could be clearly observed in only a narrow pH range from approximately 9 to 12. When the pH of the solution increased further, the UO(2)(O(2))(CO(3))(2)(4-) species changed to UO(2)(CO(3))(3)(4-) and the UO(2)(CO(3))(x)(OH)(y)(2-2x-y) species. Moreover, the UO(2)(O(2))(CO(3))(2)(4-) species continuously decomposed into uranyl tricarbonate in the carbonate solution at an elevated temperature because of the instability of the peroxide ion, O(2)(2-), in alkaline conditions.

Synthesis and size characterization of silica nanospheres using sedimentation field-flow fractionation (SdFFF).

Silica nanoparticles were synthesized by a conventional emulsion polymerization by mixing ethanol, ammonium hydroxide, water and tetra ethyl orthosilicate (TEOS). A new reaction apparatus was assembled for a large scale synthesis of silica nanospheres in the laboratory, which was designed for uniform mixing of the reactants. The apparatus was equipped with a disc type agitator with six rectangular propellers. The new apparatus allowed high reproducibility in terms of the mean size and the size distribution of the silica nanoparticles with the relative standard deviation of less than about 6%. Sedimentation field-flow fractionation (SdFFF) was employed for determination of the size distribution of the silica nanoparticles. SdFFF provided size-based separation of the silica nanoparticles, with the retention time increasing with the size. When SdFFF analysis was repeated three times for the same sample, the standard deviation was less than 4%, showing reliability of SdFFF in size measurement. SdFFF seems to provide more accurate size distribution than DLS, particularly for those having broad and multimodal size distributions. Change in the agitation speed resulted in significant change in the mean diameter of the silica nanoparticles. Agitation speed of 400 rpm in 3 L reaction vessel yielded silica particles of about 100 nm in diameter, while at 200 rpm in 1 L vessel yielded those of about 500 nm.

Adsorption behavior of Eu(III) on partially Fe(III)- or Ti(IV)-coated silica.

The adsorption behavior of Eu(III) onto silica surface, which was partially coated with Fe(III) or Ti(IV), was investigated to determine Fe(III) or Ti(IV) effects on the surface reaction of lanthanides on mineral surfaces in groundwater. Compared with a parallel uncoated silica, the Fe(III)-coated silica did not enhance the adsorption of Eu(III). However, enhanced adsorption of Eu(III) on the Ti(IV)-coated silica was observed by increasing the amount of Ti(IV) on the silica surface.

Isotope correlations for determining the isotopic composition of plutonium in high burnup pressurized water reactor (PWR) samples.

Correlations among the alpha activity ratios of (238)Pu/((239)Pu+(240)Pu), the alpha specific activities of Pu and the atom % abundances of Pu isotopes were derived for the plutonium samples obtained from high burnup fuel samples from pressurized water reactors. Using the alpha activity ratios of (238)Pu/((239)Pu+(240)Pu) determined by alpha spectrometry, the alpha specific activities of Pu as well as the atom % abundances of the plutonium isotopes in the unknown samples were calculated without depending on mass spectrometry. The calculated alpha specific activities of Pu agreed with those determined by experiment within 2%, and the atom % abundances of the Pu isotopes agreed within 4% for (238)Pu, 5% for (239)Pu, 7% for (240)Pu and 5% for (242)Pu, respectively. In addition, an attempt was made to elucidate a correlation between the fuel burnup and the alpha activity ratio of (238)Pu/((239)Pu+(240)Pu) at a range of the burnup from 35.5 to 62.9 GWd/MtU.

Development of a 756 nm, 3 W injection-locked cw Ti:sapphire laser.

We have developed a 756 nm, 3 W single-frequency cw Ti:sapphire laser by using the technique of injection locking. A cw Ti:sapphire laser in a ring-type configuration was forced to lase unidirectionally by use of an optical diode to prevent a high-power backward laser from disturbing the injection laser. A master laser was amplified by a broad-area laser diode and coupled into a single-mode fiber to generate a 50 mW injection laser with a Gaussian beam profile, which was enough to lock the Ti:sapphire laser at full power of 3 W. Such a high-power single-frequency Ti:sapphire laser enables a watt-level blue or near-ultraviolet single-frequency laser to be generated by frequency doubling.