A 31p and 15N NMR study of the extraction of uranyl nitrate by Di-2-ethylhexyl phosphoric acid

Low temperature ³¹P and ¹⁵N NMR spectroscopy was used to investigate the species forming in the organic layer following the extraction of uranium from nitric acid solutions with di-2-ethylhexyl phosphoric acid. It was found that uranium is extracted from neutral solutions as the 1:2 complex UO₂A₂ regardless of what anion is present. For dilute nitric acid solutions, the uranium is extracted both as associated and mixed nitrato species. As the nitric acid concentration of the aqueous layer increases, the mixed nitrato complex, UO2(NO₃)A·HA, becomes predominant.     

A 31P and 15N NMR study of the synergistic extraction of uranyl nitrate by tributylphosphate and Di-2-ethylhexyl phosphoric acid

Low temperature ³¹P and ¹⁵N NMR spectroscopy was used to investigate the species forming in the organic layer following the extraction of uranium from nitric acid solutions with the synergistic mixture, TBP-DEHPA. It was found that the complex formed during extraction was a mixed nitrato complex UO₂(NO₃)A·TBP. Although the existence of the complex UO₂A₂·χTBP may be demonstrated in model systems, the complex does not form under actual extraction conditions.     

Insights into the interactions of cyanobacteria with uranium

Due to various activities associated with nuclear industry, uranium is migrated to aquatic environments like groundwater, ponds or oceans. Uranium forms stable carbonate complexes in the oxic waters of pH 7–10 which results in a high degree of uranium mobility. Microorganisms employ various mechanisms which significantly influence the mobility and the speciation of uranium in aquatic environments. Uranyl bioremediation studies, this far, have generally focussed on low pH conditions and related to adsorption of positively charged UO₂ ²⁺ onto negatively charged microbial surfaces. Sequestration of anionic uranium species, i.e. [UO₂(CO₃) ₂ ^2? ] and [UO₂(CO₃) ₃ ^4? ] onto microbial surfaces has received only scant attention. Marine cyanobacteria are effective metal adsorbents and represent an important sink for metals in aquatic environment. This article addresses the cyanobacterial interactions with toxic metals in general while stressing on uranium. It focusses on the possible mechanisms employed by cyanobacteria to sequester uranium from aqueous solutions above circumneutral pH where negatively charged uranyl carbonate complexes dominate aqueous uranium speciation. The mechanisms demonstrated by cyanobacteria are important components of biogeochemical cycle of uranium and are useful for the development of appropriate strategies, either to recover or remediate uranium from the aquatic environments.     

Uranyl mixed-ligand complex formation in artificial seawater

Abstract

The stability constants of the mixed uranyl-hydroxo-peroxo-carbonato species have been calculated. By incorporating those and including stability constants and corresponding equilibria into the developed model of seawater species distribution enables the dependence of uranyl species on pH to be evaluated. The calculations show that at seawater conditions (pH = 8), 88.3% of total uranyl-ion is in the form UO₂(CO₃)^4-, 11% is in the form UO₂(CO₃)₂(OH)^3-, and 0.5% in the UO₂(CO₃)(OH)₂^2- mixed-ligand complex form, while in the photic layer, 81.2% of uranyl-ion is in the tricarbonate complex, 10.1% in the form UO₂(CO₃)₂(OH₂)^3-, 7% as UO₂(CO₃)₂(OH)^3-, 0.5% as UO₂(CO₃)(OH)₂^4- and 0.5% as UO₂(O₂)₂^2- species.     

Solvent-extraction complex of uranium(VI) with cis,syn, cis-dicyclohexano-18-crown-6

The extraction of U(VI)with dicyclohexano-18-crown-6 (mixed isomers or isomer A) from HCl medium is effective and selective, and can be used for separating and analysing uranium and thorium. However, little is known of the properties of the extraction complex of uranium with crown-ether in organic phase. In this paper we report the preparation, characteristic and structure of the crystalline extraction complex IaUO₂Cl₂HClH₂O, Iabeing isomer A of dicyclohexano-18-crown-6.After extracting uranium(VI) from aqueous hydrochloric acid solution with Ia in 1,2-dichloroethane, the crystalline product of the extraction complex was prepared from the organic phase by diluting with a non-polar solvent at 25 °C. The content of uranium, crown-ether and HCl was determined. The IR spectrum of the crystals shows that the strong hydronium-crown ether/oxygen hydrogen bond absorption is found in the region 2300–2400 cm⁻¹. The chemical shift in the range 9–12 ppm was observed. The ¹H NMR signal of hydronium protons appears at 9.890 ppm. The results of assay correspond to the formula Ia₂·(H₃O⁺)₂·UO₂Cl₄²⁻.Crystal structure of the extraction complex has been determined by X-ray crystallography. Crystals are monoclinic, space group C_2/c (No. 15) a=32.464, b=10.203, c=21.616 Å, β=119.73° and Z=4. In the complex each of the two H₃O⁺ cations is anchored in the crown-ether cavity by three stronger hydrogen bonds (distances approximately 2.65 Å), whereas uranium forms UO₂Cl₄²⁻ with Cl⁻ as counterion about 8 Å away from the H₃O⁺.     

Infrared spectroscopic studies of aqueous solutions of dioxouranium(VI) and its hydrolysed products and of in situ electro-generated dioxouranium(V)

Aqueous solutions of dioxouranium(VI) (pH range 0 to 4) give rise to bands at 954 and 938 cm⁻¹ attributable to the v₃(MO₂) stretching modes of the UO₂²⁺ and (UO₂)₂(OH)_2²⁺ cations, respectively. A shoulder at 916 cm⁻¹ is assigned to the v₃(MO₂) mode of hydrolysed dioxouranium(VI) species of higher nuclearity. Infrared spectro-electrochemical studies using a thin-layer reflection-absorption cell have facilitated the study of the reduction of aqueous solutions of dioxouranium(VI) to yield dioxouranium(V) which may be further reduced to uranium(IV). The electrogeneration of dioxouranium(V) is monitored by following the increase in intensity of a band at 914 cm⁻¹ which is present in the spectra at potentials between −0.2 and −0.8 V. The dioxouranium(V) species is predominantly in the form UO₂⁺, which may be in solution or incorporated into an insoluble phase of uranium oxides which deposit onto the working electrode. The U^VO bond length is estimated to be 1.76 Å, 0.03 Å longer than the U^VIO bond in aqueous solution. The maximum concentration of UO₂⁺able to be achieved is highly dependent on the pH and is optimum at pH 3.4. Changes in the pH of the solution under study can be monitored by infrared spectroscopy during the course of the reduction by determining the relative concentrations of hydrolysed dioxouranium(VI) species.     

Electrokinetic-enhanced phytoremediation of uranium-contaminated soil using sunflower and Indian mustard

Abstract

Electrokinetic-enhanced phytoremediation is an effective technology to decontaminate heavy metal contaminated soil. In this study, we examined the effects of electrokinetic treatments on plant uptake and bioaccumulation of U from soils with various U sources. Redistribution of uranium in soils as affected by planting and electrokinetic treatments was investigated. The soil was spiked with 100?mg kg^–1 UO₂, UO₃, and UO₂(NO₃)₂. After sunflower and Indian mustard grew for 60 days, 1 voltage of direct-current was applied across the soils for 9 days. The results indicated that U uptake in both plants were significantly enhanced by electrokinetic treatments from soil with UO₃ and UO₂(NO₃)₂. U was more accumulated in roots than in shoots. Electrokinetic treatments were effective on lowering soil pH near the anode region. Overall, uranium (U) removal efficiency reached 3.4–4.3% from soils with UO₃ and uranyl with both plants while that from soil with UO₂ was 0.7–0.8%. Electrokinetic remediation treatment significantly enhanced the U removal efficiency (5–6%) from soils with UO₃ and uranyl but it was 0.8–1.3% from soil with UO₂, indicating significant effects of U species and electrokinetic enhancement on U bioaccumulation. This study implies the potential feasibility of electrokinetic-enhanced phytoremediation of U soils with sunflower and Indian mustard.     

Uranium luminescence in La2Zr2O7: effect of concentration and annealing temperature

The speciation of a particular element in any given matrix is a prerequisite to understanding its solubility and leaching properties. In this context, speciation of uranium in lanthanum zirconate pyrochlore (La₂Zr₂O₇ = LZO), prepared by a low‐temperature combustion route, was carried out using a simple photoluminescence lifetime technique. The LZO matrix is considered to be a potential ceramic host for fixing nuclear and actinide waste products generated during the nuclear fuel cycle. Special emphasis has been given to understanding the dynamics of the uranium species in the host as a function of annealing temperature and concentration. It was found that, in the LZO host, uranium is stabilized as the commonly encountered uranyl species (UO₂²⁺) up to a heat treatment of 500 °C at the surface. Above 500 °C, the uranyl ion is diffused into the matrix as the more symmetric octahedral uranate species (UO₆^6–). The uranate ions thus formed replace the six‐coordinated ‘Zr’ atoms at regular lattice positions. Further, it was observed that concentration quenching takes place beyond 5 mol% of uranium doping. The mechanism of the quenching was found to be a multipolar interaction. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis and characterization of uranyl(VI) and thorium(IV) complexes with phosphine oxides. The crystal structure of UO2(NO3)2(NO2-C6H4Ph2PO)2

Uranyl(VI) and thorium(IV) complexes of the type UO₂(NO₃)₂(L¹)₂, UO₂(NO₃)₂(L²)₂, UO₂(CH₃COO)₂L¹, UO₂(CH₃COO)₂L², Th(NO₃)₄(L¹)₂ and Th(NO₃)₄(L²)₂ (L¹ = (2-nitro)phenyl-bis-phenyl phosphine oxide, L² = triferrocenylphosphine oxide) are reported, together with their physico-chemical properties.The crystal structure of UO₂(NO₃)₂(L¹)₂ is also reported. The crystals are monoclinic, space group P2₁/n with a = 17.78(1), b = 13.88(1), c = 17.37(1) Å, β = 114.8(1)° for Z = 4. The uranium atom is 8-coordinated, the uranyl(VI) group being equatorially surrounded by an irregular hexagon of six oxygen atoms from two trans neutral ligands and two nitrato groups.     

Uranyl tris nitrato as a luminescent probe for trace water detection in acetonitrile

Uranyl tris nitrato i.e. [UO₂(NO₃)₃]^– was formed by adding tetramethylammonium nitrate to uranyl nitrate in acetonitrile medium. The luminescence features of this complex in acetonitrile are very sensitive to water content, which could lead to the use of it as a luminescent probe for water present in acetonitrile. The luminescence intensity ratio of 507 to 467 nm peak of uranyl tris nitrato showed a linear response in the range 0–5% (v/v) water content in acetonitrile. The present method was applied for three synthetic samples of acetonitrile for water detection and the results obtained were compared using Karl Fischer titration. There was a good agreement in the values obtained by both the methods.     

Uranium phosphate biomineralization by fungi

Geoactive soil fungi were investigated for phosphatase‐mediated uranium precipitation during growth on an organic phosphorus source. Aspergillus niger and Paecilomyces javanicus were grown on modified Czapek–Dox medium amended with glycerol 2‐phosphate (G2P) as sole P source and uranium nitrate. Both organisms showed reduced growth on uranium‐containing media but were able to extensively precipitate uranium and phosphorus‐containing minerals on hyphal surfaces, and these were identified by X‐ray powder diffraction as uranyl phosphate species, including potassium uranyl phosphate hydrate (KPUO₆.3H₂O), meta‐ankoleite [(K_1.7Ba_0.2)(UO₂)₂(PO₄)₂.6H₂O], uranyl phosphate hydrate [(UO₂)₃(PO₄)₂.4H₂O], meta‐ankoleite (K(UO₂)(PO₄).3H₂O), uramphite (NH₄UO₂PO₄.3H₂O) and chernikovite [(H₃O)₂(UO₂)₂(PO₄)₂.6H₂O]. Some minerals with a morphology similar to bacterial hydrogen uranyl phosphate were detected on A. niger biomass. Geochemical modelling confirmed the complexity of uranium speciation, and the presence of meta‐ankoleite, uramphite and uranyl phosphate hydrate between pH 3 and 8 closely matched the experimental data, with potassium as the dominant cation. We have therefore demonstrated that fungi can precipitate U‐containing phosphate biominerals when grown with an organic source of P, with the hyphal matrix serving to localize the resultant uranium minerals. The findings throw further light on potential fungal roles in U and P biogeochemistry as well as the application of these mechanisms for element recovery or bioremediation.     

The diffusion coefficients and viscosities of the UO2(NO3)2·2TBP complex in organic solvents

The diffusion coefficients and the solution viscosities of the UO₂(NO₃)₂·2TBP complex in 12 solvents (tributylphosphate, n-hexane, n-dodecane and 9 mixed solvents) at 298 K were determined by use of an analytical ultracentrifuge and an Ubbelohde viscometer. It is shown in the mixed solvents that all systems obeyed the Jones-Dole relation. The product of diffusion coefficient and viscosity is roughly constant, corresponding to an Einstein -Stokes radius of 0.54 nm for the uranium complex.     

Sugar interaction with uranium ion. Synthesis,spectroscopic and stmctural analysis of UO2-sugar complexes containing D-glucuronate moiety

Interaction between D-glucuronic acid and hydrated uranyl salts has been studied in aqueous solution and solid complexes of the type UO₂(D- glucuronate)X·2H₂0 and UO₂(D-glucuronate)₂·2H₂O, where X = CI⁻, Br⁻ or NO₃⁻, are isolated and characterized by means of FT-IR and proton-NMR spectroscopy.On comparison with the structurally identified Ca(D-glucuronate)Br·3H₂O compound, it is concluded that the UO₂²⁺ cation binds to two D- glucuronate moieties in uranylsugar complexes via O6, O5 oxygen atoms (ionized carboxyl group) of the first and O6′, 04 (non-ionized carboxyl group) of the second sugar moiety, whereas in the UO₂(D- glucuronate)₂·2H₂O salt the uranyl ion is bonded to two sugar anions through O6, O6′ oxygen atoms of the ionized carboxyl group, resulting in a six- coordination geometry around the uranium ion. The strong intermolecular hydrogen bonding network of the free acid is rearranged upon sugar metalation and the sugar moiety showed β-anomer conformation both in the free acid and in these uranylsugar complexes.     

Dioxouranium(VI) complexes with 2,6-acetylpyridinebenzoylhydrazones and -semicarbazones

Benzoylhydrazones and semicarbazones derived from 2,6-diacetylpyridine react with common dioxouranium(VI) compounds such as uranyl nitrate or [NBu₄]₂[UO₂Cl₄] to form air-stable complexes. 2,6-Diacetylpyridinebis(benzoylhydrazone) (H₂L¹), 2,6-diacetylpyridinebis(N4-phenylsemicarbazone) (H₂L²) and the asymmetric proligand 2,6-diacetylpyridine(benzoylhydrazone)(N4-phenylsemicarbazone) (H₂L³) give yellow products of the composition [UO₂(L)]. The neutral compounds contain doubly deprotonated ligands and possess uranium atoms with distorted pentagonal-bipyramidal coordination spheres. The equatorial coordination spheres of the metal atoms can be extended by the addition of a monodentate ligand such as pyridine or DMSO. The uranium atoms in the resulting complexes have hexagonal-bipyramidal coordination environments with the oxo ligands in axial positions.X-ray diffraction studies on [UO₂(L¹)(DMSO)], [UO₂(L²)], [UO₂(L²)(DMSO)] and [UO₂(L³)] show relatively short U-O bonds to the benzoylic oxygen atoms between 2.273(6) and 2.368(5) Å. This suggests a preference of these donor sites of the ligands over their imino and amine functionalities (U-N bond lengths: 2.502(7)-2.671(7) Å). The addition of a sixth ligand to the equatorial coordination sphere results in a lengthening of the metal-pyridine bonds.     

Studies on the growth ofThiobacillus ferrooxidans

Summary Uranyl sulphate (0.2–0.9 mM) inhibited ferrous iron oxidation by growing cultures ofThiobacillus ferrooxidans. The addition of 5–100 mM uranium to the cultures caused immediate cessation of carbon dioxide fixation, rapid loss of viability and gradual depression of ferrous iron oxidation. Virtually no uranium was found in washed cells grown in the presence of subtoxic to toxic amounts of uranyl sulphate. Uranium-poisoned organisms appeared plasmolyzed in electron micrographs. Cultures tolerant to 5 mM UO₂ ²⁺ were develoepd by successive subculturing in increased uranium concentrations. The tolerance was maintained during subculturing in uranium-free medium. Frequency of mutants resistant to 1.0 and 1.5 mM UO₂ ²⁺ was 1 per 1.3×10⁶ and 1 per 9.0×10⁸, respectively. The frequency was increased in the presence of 15–150 mM nickel, zinc and manganese. In liquid cultures, bivalent cations and EDTA alleviated the toxicity of 2 mM uranyl sulphate.     

Assessment of the Resistance to Uranium (VI) Exposure by Arthrobacter sp. Isolated from Hanford Site Soil

Production of nuclear fuel has resulted in hazardous waste streams that have contaminated the soil and groundwater. Arthrobacter strains, G975, G968, and G954 were used in the prescreening tests to evaluate their tolerance to UO₂ ²⁺ and investigate bacteria-U(VI) interactions under oxidizing pH-neutral conditions. Experiments have shown G975 is the fastest growing and the most uranium tolerant strain that removed about 90% of uranium from growth media. Atomic Force Microscopy images exhibited an irregular surface structure, which perhaps provided a larger surface area for uranium precipitation. The data indicate that aerobic heterotrophic bacteria may offer a solution to sequestering uranium in oxic conditions, which prevail in the vadose zone.     

Denitrification of nitrate and nitric acid with methanol as carbon source

Summary A methanol/nitrate-medium and anaerobic conditions yielded an enrichment culture which consisted ofHyphomicrobium andParacoccus. This mixed culture proved to be very effective in denitrification of solutions containing high concentrations of nitrate and free nitric acid when grown in a chemostat (D=0.04 h^-1). With 0.1 mol/l nitric acid solution as feed medium the pH in the culture vessel adjusted itself to 5.8. For the reduction of 1 g NO₃–N 2.6 g methanol were consumed and 0.56 g cells were produced.     

The chemistry of uranium dispersion in groundwaters at the Pinhal do Souto Mine,Portugal

Solution studies of H-autunite, H₂(UO₂)₂(PO₄)₂, and its K⁺, Na⁺, Li⁺, Ca²⁺, Mg²⁺, Cu²⁺ and Ni²⁺ analogues, have been carried out and solution stability constants determined. These values are in excellent agreement with those recently reported by other workers at 298.2 K and ionic strength equal to zero. separate field study involving groundwaters collected from the oxidized zone of the Pinhal do Souto mine, Portugal, has been undertaken. Here the principal uranium minerals are autunite, Ca(UO₂)₂(PO₄)₂·nH₂O and torbernite Cu(UO₂)₂(PO₄)₂·nH₂O. Detailed analyses of and calculations of species distributions in these samples indicate that currently circulating groundwaters are undersaturated with respect to these two minerals by two to four orders of magnitude. The mineralogical and geochemical significance of these results is discussed.     

The solvent extraction of metal ions from aqueous solutions containing NNN′N′-Ethylenediaminetetraacetic and related ccids. Part 1. Uranium(IV)

The uranium(IV) complexes [U(EDTA)(H₂O)₂], [U(HOEDTA)]⁺, and [U(DTPA)]^? are well-formed in the pH fange 2–3 ([DTPA]^5- = diethylenetriaminepentaacetate; [HOEDTA]^3-  N-(2-hydroxyethyl)ethylenediaminetriacetate). Of these, only [U(DTPA)]- is extracted from an aqueous phase at pH 2 by the perchlorate salt of the primary amine, Primene JM-T. As the aqueous phase pH was raised, extraction occurred in all three cases and hydrolysed species may be extracted from EDTA and HOEDTA solutions but [U(DTPA)]^? resists hydrolysis. The addition of sulphate had a marked effect on the extraction of U(IV) from EDTA and HOEDTA through the formation of [U(EDTA)(SO₄)(H₂O)]^2- and [U(HOEDTA)(SO₄)(H₂O)_n]^?. The equilibrium constant, log β₁, for: [(U(EDTA)(H₂O)₂] 2 [SO₄]^2? ? [U(EDTA)(SO₄)(H₂O)]^2- 2 H₂O was found to be 2.43 ± 0.04 (I = 1 mol dm^?3, NaClO₄; pH 2.0; 20 °C) from spectrophotometric data.With tri-n-octylphosphine oxide (TOPO) electronic spectroscopy showed that the same U(IV) complex was extracted at pH 2 for Cs₂UCl₆, U(IV)/ HOEDTA, and U(IV)/DTPA and the aminepoly- carboxylates were aqueous phase masking agents but with [U(EDTA)(H₂O)₂] oxidation gave a uranyl(VI) organic phase species.Uranium(IV) is strongly extracted from aqueous solutions of HOEDTA at pH 2 or 3 by bis(2-ethyl- hexyl)phosphoric acid (HBEHP) but less so from EDTA and DTPA. Since U(IV) is completely extracted from Cs₂UCl₆ it could be that the amine- polycarboxylates were aqueous phase masking agents although spectral evidence did not support this.     

Studies of alveolar cell morphometry and mass clearance in the rat lung following inhalation of an enriched uranium dioxide aerosol

Summary Seventy-three rats were exposed to an aerosol of enriched uranium dioxide (UO₂), giving initial lung burdens of 26 to 447 µg at 6 days post-inhalation (PI). At 7 days PI 35 of these rats were further exposed to thermalised neutrons at a fluence of 1 x 10¹² neutrons CM^–2. There was no significant difference between the two groups in the clearance rate of the UO₂ particles from the lung, up to 590 days PI. The particles cleared relatively slowly over this period with a retention halftime in the lung of 160 to 176 days.Transmission electron microscope (TEM) studies of tissue from the alveolar region at 8 days PI showed that inhalation of UO₂ particles significantly increased the sizes of macrophage and type II cells, and the number of macrophage and type I cells. There was also a significant increase in the size of lysosomal granules within the macrophages after exposure to the UO₂ particles. The exposure to UO₂, neutrons and²³⁵U fission fragments had no significant effect on any of the cells above that observed in the animals exposed to UO₂ alone.Additional rats were exposed to the same neutron fluence without prior UO₂ inhalation. The alveolar cells of neutron-only exposed rats were, in size and number, typically no different from those in the completely unexposed control rats.