Aqueous solutions of uranium(VI) as studied by time-resolved emission spectroscopy: a round-robin test

Results of an inter-laboratory round-robin study of the application of time-resolved emission spectroscopy (TRES) to the speciation of uranium(VI) in aqueous media are presented. The round-robin study involved 13 independent laboratories, using various instrumentation and data analysis methods. Samples were prepared based on appropriate speciation diagrams and, in general, were found to be chemically stable for at least six months. Four different types of aqueous uranyl solutions were studied: (1) acidic medium where UO2(2+)aq is the single emitting species, (2) uranyl in the presence of fluoride ions, (3) uranyl in the presence of sulfate ions, and (4) uranyl in aqueous solutions at different pH, promoting the formation of hydrolyzed species. Results between the laboratories are compared in terms of the number of decay components, luminescence lifetimes, and spectral band positions. The successes and limitations of TRES in uranyl analysis and speciation in aqueous solutions are discussed.     

Determination of solubilities of uranium complexes in supercritical CO2 by on-line laser-induced fluorescence

A new approach to the measurement of solubilities in supercritical CO2 is reported, utilizing laser-induced fluorescence. This selective technique was found to be capable of rapid multipoint solubility measurements with high sensitivity (10(-9) M). The sensitivity enables measurements to be performed with small amounts of analytes and at low pressure, 80-130 atm, typically below the range of other methods. Four uranyl complexes were investigated using this method, UO2(TTA)2.H20, UO2(TTA)2.TBP, UO2(TTA)2.TOPO, and UO2(NO3)2.2TBP (where TTA = thenoyltrifluoroacetone, TBP = tributyl phosphate, and TOPO = trioctyl phosphine oxide). Comparison with solubility data previously obtained with UV-visible spectroscopy shows good agreement between the techniques. We have shown that the Chrastil solubility equation is equally valid at lower ScF densities, and consequently, solubility values determined at high pressures can be extrapolated to the low-pressure regions and vice versa for solid materials.     

Structure of Trimethylbenzylammonium and Triethylbenzylammonium Tetrachlorouranilates

Radiochemistry - Synthesis, IR spectroscopic and X-ray structural studies of new chlorine-containing uranyl complexes (C10H16N)2[UO2Cl4] (I) and (C13H22N)2[UO2Cl4]0.5[UO2(NO3)2Cl2]0.5 (II) have...     

Recovery of U(VI) from aqueous media with layered double hydroxides of Zn and Al, intercalated with complexones

The ability of layered double hydroxides (LDHs) with DTPA and EDTA complexones and carbonate ions in the interlayer space to sorb U(VI) was compared. The U(VI) sorption values correlate with the dissociation stability of the U(VI) complexes with these ligands in solution. The synthesized hydrotalcite-like sorbents intercalated with complexones showed high performance in sorption treatment of water to remove U(VI), irrespective of its speciation in natural waters (including di- and tricarbonate complexes). The carbonate form of LDH recovers only cationic uranyl species.     

Extended X-ray absorption fine structure investigation of adsorption and separation phenomena of metal ions in organic resin

Analytical technique using organic resins has already been well-developed, and its applications are employed in various fields; nevertheless, the chemical phenomena occurring inside the resin remain unclear for the most part. In the present study, we apply EXAFS spectroscopy to elucidate the adsorption and separation phenomena of metal ions by organic resin. That is, the chemical species of trivalent lanthanides (Ln(III)) adsorbed in a tertiary pyridine resin from hydrochloric acid and nitric acid solutions have been determined by EXAFS. The results in HCl solutions suggest that Ln(III) ions are partly dehydrated in the resin phase, enabling the pyridine groups of the resin and chloride ions to coordinate to the Ln(III) ions in their primary coordination sphere. On the other hand, Ln(III) ions are tightly coordinated by several nitrate ions in HNO3 solutions and they keep forming the nitrate complex even in the resin phase. The lighter Ln of Nd tends to form an anionic nitrate complex, [Nd(NO3)4.nH2O]-, in the resin phase, while the middle Ln of Sm exists as a cationic nitrate complex, [Sm(NO3)2.nH2O]+, for the most part. On the basis of these EXAFS results, the adsorption and separation mechanisms of the pyridine resin in HCl solutions are interpreted as the direct coordination of pyridine groups to metal ions, while the mechanisms in HNO3 solutions are mainly dominated by the anion-exchange reaction between the protonated pyridine groups and the anionic nitrate complexes of Ln(III). The obtained results demonstrate that the hydration of metal ions weakens, and instead, other complexations are enhanced in the resin phase.     

Uranyl ion extraction with conventional PUREX/TRUEX ligands assessed by electroanalytical chemistry at micro liquid/liquid interfaces

The facilitated ion transfer (FIT) of uranyl or dioxouranium (UO(2)(2+)) was studied electrochemically using a micro interface between two immiscible electrolytic solutions (micro-ITIES) in order to evaluate the complexation stoichiometry and complexation constants (β) of two widely used ligands in spent fuel reprocessing: tributylphosphate (TBP) and octyl(phenyl)-N,N-diisobutylcarbamoylmethyl-phosphine oxide (CMPO). For the first time, discrete interfacial complexation reaction steps of varying uranyl to the two ligands ratios were resolved using the micro-ITIES hosted at the tip of a 25 μm diameter glass capillary. Two stoichiometries for UO(2)NO(3)TBP(n)(+) were determined including n = 3 and 4 with β values of 3.2 × 10(11) and 3.9 × 10(13), respectively. Subsequently, three distinct complexation reactions of CMPO with UO(2)(2+) were discovered corresponding to UO(2)NO(3)CMPO(2)(+), UO(2)NO(3)CMPO(3)(+), and UO(2)CMPO(5)(2+) whose respective complexation constants were determined to be 8.0 × 10(11), 8.8 × 10(14), and 6.5 × 10(32). The participation of nitrate anions in these complexation reactions is also discussed.     

Uranyl-selective electrode based on a new bifunctional derivative combining the synergistic properties of phosphine oxide and ester of phosphoric acid

Ion-selective electrodes based on the bifunctional chelating agent O-methyldihexylphosphine oxide O'-hexyl-2-ethylphosphoric acid (HL) incorporated into a poly(vinyl chloride) membrane were developed. This new derivative is proposed as a single molecular unit combining the overall properties of the synergistic single components, di-2-ethylhexylphosphoric acid and trioctylphosphine oxide. Two different ionophores, HL and its uranyl complex (UO2L2), were studied. The response of the electrodes to uranyl ion was Nernstian for UO2L2 and super-Nernstian for HL ionophores, with detection limits of 3.0 x 10(-6) and 2.0 x 10(-5) M, respectively. Results indicate a more effective interaction with the analyte in the case of having a unique molecule incorporating the two functional groups immobilized into a polymeric membrane, rather than the separated two synergistic ligands. Flow-through tubular electrodes based on both ionophores were also used as potentiometric detectors in flow injection techniques.     

Synthesis and Structure of Uranyl Succinate Complex with Isonicotinic Acid and New Polymorph of Uranyl Succinate Monohydrate

Radiochemistry - Synthesis, IR spectroscopic and X-ray structural studies of crystals [UO2(C4H4O4)(C6H5NO2)2] (I) and a new modification [UO2(C4H4O4)(H2O)] (II), where C4H4O42– are succinate...     

Differential optical absorption spectroscopy instrument for stratospheric balloonborne trace-gas studies

A newly developed UV-visible instrument for differential optical absorption spectroscopic measurements of atmospheric trace gases from balloon platforms is described. Direct solar light at daytime in the near-ultraviolet (320.6-422.6-nm) and the visible (417.6-670.7-nm) spectral ranges can be simultaneously analyzed for the atmospheric column abundances or profiles of O(3), NO(2), NO(3), BrO, OClO, O(4), H(2)O, and possibly other species (HNO(2), IO, CH(2)O). Compared with previously used balloonborne UV-visible spectrometers, the instrument has the superior properties of low mass (42 kg), low power consumption (30 W), decreased spectral drift that is caused by temperature and pressure changes, low detector dark current, and low spectrometer stray light. The three last-named characteristics are achieved by enclosure of the entire spectrometer in a pressurized and thermostated container and by inclusion of separately thermostated photodiode array detectors. The optical setup is simplified to reduce its weight. The spectral stray light is reduced by suppression of the higher-order and zero-order grating reflections by use of light traps and in the UV by addition of a dispersive prism preanalyzer. The major instrumental design characteristics and the instrumental performance as tested in the laboratory and during several stratospheric balloon flights are reported.     

Neptunium(VI) and Plutonium(VI) Pyrazine-2-carboxylates

New Np(VI) and Pu(VI) pyrazine-2-carboxylates isostructural with the uranyl complex [UO₂ · (C₄H₃N₂COO)₂(H₂O)] · 2H₂O were synthesized from aqueous solutions. The unit cell parameters of [NpO₂ · (C₄H₃N₂COO)₂(H₂O)] · 2H₂O and [PuO₂(C₄H₃N₂COO)₂(H₂O)] · 2H₂O were calculated from the powder X-ray patterns. The thermal behavior of the complexes suggests that the Np(VI) compound contains superstoichiometric water molecules, which are also manifested in the IR spectrum. The absorption spectra of the compounds show that the pyrazinecarboxylic acid anion is a strong complexing agent with respect to Np(VI) and Pu(VI).__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 35–38.Original Russian Text Copyright © 2005 by Andreev, Budantseva, Perminov, Fedoseev.     

Complexation of actinides(VI) with furan-2-carboxylic and thiophene-2-carboxylic acids in aqueous solutions and synthesis of crystalline compounds

Crystalline uranyl compounds with furan-2-carboxylic (F2C) and thiophene-2-carboxylic (T2C) acids containing the complex anions [UO₂(OOCC₄H₃O)₃]^? and [UO₂(OOCC₄H₃S)₃]^? were synthesized and studied by single crystal X-ray diffraction. A previously unknown compound of a transuranium element, Pu(VI), with T2C, also containing the complex anions [PuO₂(OOCC₄H₃S)₃]^?, was synthesized. With Pu(VI) taken as example, the complexation of hexavalent actinides with F2C and T2C in aqueous solutions was studied, and the stability of the complexes PuO₂L⁺ (L is F2C or T2C anion) was determined. The Pu(VI) complexes with F2C are slightly more stable than those with T2C but less stable than Pu(VI) monoacetate complexes. The spectral characteristics of the complexes are discussed.     

Chromium speciation by solid phase extraction on Dowex M 4195 chelating resin and determination by atomic absorption spectrometry

A solid phase extraction procedure has been established for chromium speciation in natural water samples prior to determination by atomic absorption spectrometry. The procedure is based on the solid phase extraction of the Cr(VI)- Dowex M 4195 chelating resin. After oxidation of Cr(III) to Cr(VI) by using H2O2, the presented method was applied to the determination of the total chromium. The level of Cr(III) is calculated by difference of total chromium and Cr(VI) levels. The procedure was optimized for some analytical parameters including pH, eluent type, flow rates of sample and eluent, matrix effects, etc. The presented method was applied for the speciation of chromium in natural water samples with satisfactory results (recoveries >95%, RSDs <10%). In the determinations of chromium species, flame atomic absorption spectrometer was used. The results were checked by using NIST SRM 2711 Montana soil and GBW 07603 Bush branched and leaves.     

Extended X-ray absorption fine structure (EXAFS) study of secondary phases in Yb2O3-doped Si3N4 ceramics

Liquid-phase sintered Si3N4 doped with Yb2O3 as a sintering aid was characterized by both transmission electron microscopy and extended X-ray absorption fine structure (EXAFS) measurements. Structural information about the secondary phases was obtained with an emphasis being placed on the evaluation of EXAFS data. Two Si3N4 samples were processed which contained either 5 vol% or 10 vol% Yb2O3 as sintering additive. After sintering, only an amorphous secondary phase was observed in the material doped with 5 vol% Yb2O3. The material with the higher Yb2O3 volume fraction underwent a further heat treatment after the densification, in order to crystallize pockets of the secondary phase. This heat treatment resulted in the formation of Yb2Si2O7 at multi-grain junctions, with however, amorphous phases remaining along the grain and phase boundaries. The EXAFS data obtained from the two doped Si3N4 materials were compared with reference spectra obtained on pure Yb2O3 and synthetic Yb2Si2O7. No residual Yb2O3 was determined in the doped Si3N4 materials, independent of the Yb2O3 volume fraction. Compared to synthetic Yb2Si2O7, the secondary phase formed in the 10 vol% Yb2O3 containing material showed only subtle changes in the EXAFS data. A clear distinction between purely amorphous and a combination of crystalline plus amorphous Yb secondary phases was possible, when both doped Si3N4 materials were compared. However, no distinction between the glass phase present at triple junctions and the amorphous residue along grain/phase boundaries was feasible, since a full numerical data evaluation could not be performed.     

Facilitated transport of uranium(VI) across supported liquid membranes containing T2EHDGA as the carrier extractant

Facilitated transport of uranyl ion from nitric acid feed solutions was investigated across PTFE supported liquid membranes using N,N,N',N'-tetra-2-ethylhexyl-3-pentane-diamide (T2EHDGA) in n-dodecane as the carrier extractant containing 30% iso-decanol as the phase modifier. Solvent extraction studies indicated extraction of species of the type, UO(2)(NO(3))(2)·T2EHDGA. The distribution coefficients increased in the presence of NaNO(3) as compared to equivalent concentration of HNO(3) which was exactly the opposite of what was reported for Am(III)-TODGA extraction system. Supported liquid membrane studies indicated about 11h were required for quantitative transport of U(VI) from a feed of 3M HNO(3) using 0.2M T2EHDGA in n-dodecane containing 30% iso-decanol as the carrier extractant. Effect of various parameters such as feed acidity, T2EHDGA concentration, and nature of the strippant on the transport rate was investigated. The transport was found to be diffusion controlled in the membrane phase and the permeability coefficient was calculated to be (3.20 ± 0.13)× 10(-4)cm/s for the feed composition of 3M HNO(3), receiver phase composition of 0.01 M HNO(3) and membrane carrier phase of 0.2M T2EHDGA in n-dodecane containing 30% iso-decanol. The present results may be useful for the separation of U from lean solutions or radioactive wastes considered hazardous due to the presence of alpha-particle emitting radionuclides.     

Microwave digestion of environmental and natural waters for selenium speciation

A microwave preparation procedure is proposed for selenium speciation in natural and drinking waters. Different chemical reagents were tested, and the conditions for Se speciation were optimized. The effect of the different reagents on various oxidation states of selenium under microwave digestion conditions was investigated. Most of the Se(-II) was converted to selenite when digested with HNO3 and <20% to selenate. The digestion with H2O2/H2SO4 can change most Se species into Se(IV). The concentration of Se(IV) in the samples was then determined by HPLC with a fluorescence detector after derivatization with 2,3-diamino-naphthalene (DAN). The microwave preparation procedure allows Se speciation in water samples. Se(IV) was determined after concentrating the sample under nitrogen protection. The amount of Se(IV) and Se(VI) was measured by adding an equal volume of concentrated hydrochloric acid to water sample to reduce Se(VI) to Se(IV). Then the amount of Se(VI) can be calculated by subtraction. The total selenium can be determined after digestion with H2O2/H2SO4, or after digestion with HNO3 followed by reduction with concentrated hydrochloric acid. Selenium (-II, 0) was calculated by subtracting inorganic Se(IV+VI) from the total. Detection limits of 0.0066 ng and 0.0096 ng Se were obtained for HNO3 and H202/H2SO4 as digestion reagents, respectively. The total Se in the four water samples tested range from 0.20 to 0.90 microg L(-1). Among them the dominant form was Se(VI) with the exception of pond waters where Se(-II) predominated.     

Speciation of copper in the incineration fly ash of a municipal solid waste

The speciation of copper and zinc in the incineration fly ash of a municipal solid waste in Taiwan was investigated in the present work. By the least-squares fitted X-ray absorption near edge structural (XANES) spectroscopy, we found that CuCO3, CuOH2, and CuO (fractions of 0.09, 0.39 and 0.51, respectively) were the main copper species in the fly ash. Quantitative analysis of the extended X-ray absorption fine structural (EXAFS) spectra indicated that the bond distance of Cu-O in the fly ash was 1.96 A with a coordination number (CN) of 3.9 in the first shell of copper. In the second shell, the bond distance and CN of Cu-(O)-Cu were 2.91 A and 2.7, respectively. In addition, speciation of Zn was also examined in the same X-ray absorption energy (8780-9970 eV). The bond distance of Zn-O and Zn-O-Zn were 1.97 and 2.94 A, respectively. However, the Zn-O-Cu structure was not found because of the physically unreasonable sigma(2) (Debye-Waller factor) values in the EXAFS data fitting process.     

P-N bond length alterations monitored by infrared absorption and Fourier transform Raman spectroscopy in combination with density functional theory calculations

Fourier transform (FT) Raman and infrared spectroscopy in combination with density functional theory calculations have been applied to the vibrational characterization of the dimeric zinc diphenylphosphanyl(trimethylsilyl)amide complex [(Me3Si)2NZnPh2PNSiMe3]2 and the ortho-metallated species [Li(o-C6H4PPh2NSiMe3)]2 x Et2O in relation to their parent starting materials diphenylphosphanyl (trimethylsilyl)amine Ph2P-N(H)SiMe3 and iminophosphorane Ph3P=NSiMe3. The spectroscopic changes evidenced in the spectra were correlated with the structural parameters in order to provide insight as to what extent the P-N bond is affected by the coordination to the metal center. The employment of density functional theory (DFT) calculations in addition to these spectroscopic methods offers the possibility of predicting whether the Lewis-basic imido nitrogen atom is involved in coordination not only in the solid state, but also in the gas phase.     

Sorption kinetics and chemical forms of Cd(II) sorbed by thiol-functionalized 2:1 clay minerals

The interaction between Cd(II) in aqueous solution and two 2:1 expandable clay minerals (i.e., montmorillonite and vermiculite), showing different layer charge, was addressed via batch sorption experiments on powdered clay minerals both untreated and amino acid (cysteine) treated. Reaction products were characterized via X-ray powder diffraction (XRDP), chemical analysis (elemental analysis and atomic absorption spectrophotometry), thermal analysis combined with evolved gases mass spectrometry (TGA-MSEGA) and synchrotron-based X-ray absorption spectroscopy via extended X-ray absorption fine structure (EXAFS) characterization. Sorption isotherms for Cd(II) in presence of different substrates, shows that Cd(II) uptake depends both on Cd(II) starting concentration and the nature of the substrate. Thermal decomposition of Cd-cysteine treated clay minerals evidences the evolution of H(2)O, H(2)S, NO(2), SO(2), and N(2)O(3). These results are well consistent with XRDP data collected both at room and at increasing temperature and further stress the influence of the substrate, in particular cysteine, on the interlayer. EXAFS studies suggest that Cd(II) coordinates with oxygen atoms, to give monomer complexes or CdO molecules, either on the mineral surface and/or in the interlayer. For Cd-cysteine complexes EXAFS data agree with the existence of Cd-S clusters, thus suggesting a predominant role of the thiol group in the bonding of Cd with the amino acid.     

Behavior of neptunium ions in organic media

Published data on the effect of organic solvents on the hydrolysis of Np(IV) and redox reactions of Np(IV?CVI) are analyzed. In aqueous-organic solutions, Np(IV) ions undergo hydrolysis at higher acidity than in aqueous solutions. With respect to the effect on hydrolysis, the solvents can be ranked in the order methanol > ethanol > dioxane > acetone > acetonitrile. Dimethyl sulfoxide suppresses the hydrolysis. The Np(V) disproportionation in CH₃OH and CH₃OH + C₆H₆ solutions in the presence of HCl, HDEHP, or TTA and in a TBP solution was studied. The influence of the solution composition, including the H₂O concentration, on the reaction kinetics was examined. The reactions occur faster than in aqueous solutions. The reaction mechanism is the same in all the media: Two solvated Np(V) ions form a complex decomposing upon protonation into Np(IV) and Np(VI). The role of the solvent in the Np(V) reproportionation was examined. In mixed water-ethylene glycol, water-methanol, and water-acetone solvents, with an increase in the fraction of the organic component, the Np(IV) + Np(VI) reaction rate passes through a maximum, which is due to combined effect of two factors: Np(IV) hydrolysis (acceleration) and decrease in [H₂O] (deceleration). In TBP solutions, the Np(IV) + Np(VI) reaction decelerates in proportion to [HNO₃]^?2 and [H₂O]. The course of the Np(VI) + H₂O₂ and Np(IV?CVI) + HNO₂ reactions in TBP differs from that in aqueous solutions. Deceleration of the Np(VI) reduction and acceleration of the Np(V) oxidation, compared to aqueous solutions, are associated with a decrease in the formal potential of the Np(VI)/(V) couple in going from H₂O to TBP. In solutions of KOH in aqueous methanol, Np(VI) rapidly disproportionates to Np(VII) and Np(V). A decrease in the H₂O concentration shifts the equilibrium toward Np(VII).     

Spectroscopic study of the uranyl-acetohydroxamate adduct with tributyl phosphate

The ultraviolet-visible (UV-Vis) and Fourier transform infrared (FT-IR) spectroscopic studies carried out for the system UO2(NO3)/AHA/TBP (uranyl-acetohydroxamate-tributyl phosphate) confirmed the presence of the adduct of UO2(NO3)(AHA) 2TBP with 1:1 stoichiometry for UO2:AHA (acetohydroxamic acid). The spectrum of this complex is identical to the infrared spectrum of the organic phase formed in the uranium distribution experiments with 30% TBP/n-dodecane and AHA present in aqueous phase. Disappearance of the hydroxyl stretching band and a shift in the position of the carbonyl band in the infrared spectra revealed that both the hydroxyl and the carbonyl group of acetohydroxamic acid are involved in the chelate ring with uranium. Also, acetic acid, accrued after acidic hydrolysis of acetohydroxamic acid, was identified in the extraction organic phase.