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About: Uranyl is a(n) research topic. Over the lifetime, 7410 publication(s) have been published within this topic receiving 153992 citation(s). The topic is also known as: Uranyl ion & dioxouranium(2+).

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Journal ArticleDOI
Abstract: Gibbs free energies, enthalpies and entropies of 42 dissolved uranium species and 30 uranium-bearing solid phases have been critically evaluated from the literature and estimated when necessary for 25°C. Application of the data shows that the uranium in natural waters is usually complexed. At typical concentrations of chloride, fluoride, phosphate and sulfate, uranous (U4+) fluoride complexes are important in anoxic waters below pH 3–4. An intermediate Ehs (between about +0.2 and −0.1 V) and pH values 1–7, UO2+ ion may predominate. In oxidized waters, uranyl (U22+) fluoride complexes and uranyl ion predominate below pH 5; from about pH 4 to 7.5, UO2(HPO4)22− is the principal species; while at higher pHs, UO2CO30 and the di- and tri-carbonate complexes predominate. Uraninite [UO2-UO2.25], α-U3O8 and schoepite are the stable uranium oxides and hydroxides in water at 25°C. Coffinite, USiO4 (c), is probably stable relative to UO2(c) when dissolved silica exceeds about 60 ppm (as SiO2). At low Ehs and pH 4–6, the solubilities of stoichiometric crystalline uraninite and coffinite are below roughly 10−4 ppb. But at intermediate Ehs and neutral to alkaline pHs in the presence of phosphate or carbonate, the formation of uranyl phosphate or carbonate complexes can increase the solubilities of these minerals by several orders of magnitude. The uranyl minerals carnotite, tyuyamunite, autunite, potassium autunite and uranophane are least soluble at pHs in the range 5–8.5 and, in the case of carnotite and tyuyamunite, have solubilities as low as 0.2 and 1 ppb uranium, respectively. The autunites and uranophane are usually several orders of magnitude more soluble than this, consistent with their natural occurrences. Sorption of uranyl on to natural materials is maximal in the same pH range of 5–8.5.

1,896 citations

Journal ArticleDOI
Abstract: Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10−5 M, (dissolved uranyl 10−5 to 10−8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO3 solutions containing up to 0.01 M NaHCO3. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10−3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO2OH+ and (UO2)3(OH)+5, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO2OH+ surface complex, and a monodentate, bidentate or tridentate (UO2)3(OH)+5surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10−2 and 10−3 M.

752 citations

Journal ArticleDOI
Abstract: Colloidal hematite (α-Fe2O3) is used as model solid to investigate the kinetic effect of specific adsorption interactions on the chemical reduction of uranyl (UVIO22+) by ferrous iron. Acid–base titrations and Fe(II) and uranyl adsorption experiments are performed on hematite suspensions, under O2- and CO2-free conditions. The results are explained in terms of a constant capacitance surface complexation model of the hematite–aqueous solution interface. Two distinct Fe(II) surface complexes are required to reproduce the data: (≡FeIIIOFeII)+ (or ≡FeIIIOFeII(OH2)n+) and ≡FeIIIOFeIIOH0 (or ≡FeIIIOFeII(OH2)n−1OH0). The latter complex represents a significant fraction of total adsorbed Fe(II) at pH > 6.5. Uranyl binding to the hematite particles is characterized by a sharp adsorption edge between pH 4 and pH 5.5. Because of the absence of competing aqueous carbonate complexes, uranyl remains completely adsorbed at pH > 7. A single mononuclear surface complex accounts for the adsorption of uranyl over the entire range of experimental conditions. Although thermodynamically feasible, no reaction between uranyl and Fe(II) is observed in homogeneous solution at pH 7.5, for periods of up to three days. In hematite suspensions, however, surface-bound uranyl reacts on a time scale of hours. Based on Fourier Transformed Infrared spectra, chemical reduction of U(VI) is inferred to be the mechanism responsible for the disappearance of uranyl. The kinetics of uranyl reduction are quantified by measuring the decrease with time of the concentration of U(VI) extractable from the hematite particles by NaHCO3. In the presence of excess Fe(II), the initial rate of U(VI) reduction exhibits a first-order dependence on the concentration of adsorbed uranyl. The pseudo-first-order rate constant varies with pH (range, 6–7.5) and the total (dissolved + adsorbed) concentration of Fe(II) (range, 2–160 μM). When analyzing the rate data in terms of the calculated surface speciation, the variability of the rate constant can be accounted for entirely by changes in the concentration of the Fe(II) monohydroxo surface complex ≡FeIIIOFeIIOH0. Therefore, the following rate law is derived for the hematite-catalyzed reduction of uranyl by Fe(II), d[U(VI)] dt =−k[≡ Fe III OFe II OH 0 ][U(VI)] ads where the bimolecular rate constant k has a value of 399 ± 25 M−1 min−1 at 25°C. The hydroxo surface complex is the rate-controlling reductant species, because it provides the most favorable coordination environment in which electrons are removed from Fe(II). Natural particulate matter collected in the hypolimnion of a seasonally stratified lake also causes the rapid reduction of uranyl by Fe(II). Ferrihydrite, identified in the particulate matter by X-ray diffraction, is one possible mineral phase accelerating the reaction between U(VI) and Fe(II). At near-neutral pH and total Fe(II) levels less than 1 mM, the pseudo-first-order rate constants of chemical U(VI) reduction, measured in the presence of the hematite and lake particles, are of the same order of magnitude as the highest corresponding rate coefficients for enzymatic U(VI) reduction in bacterial cultures. Hence, based on the results of this study, surface-catalyzed U(VI) reduction by Fe(II) is expected to be a major pathway of uranium immobilization in a wide range of redox-stratified environments.

613 citations

Journal ArticleDOI
Abstract: The crystal structures of uranyl minerals and inorganic uranyl compounds are important for understanding the genesis of U deposits, the interaction of U mine and mill tailings with the environment, transport of actinides in soils and the vadose zone, the performance of geological repositories for nuclear waste, and for the development of advanced materials with novel applications. Over the past decade, the number of inorganic uranyl compounds (including minerals) with known structures has more than doubled, and reconsideration of the structural hierarchy of uranyl compounds is warranted. Here, 368 inorganic crystal structures that contain essential U6+ are considered (of which 89 are minerals). They are arranged on the basis of the topological details of their structural units, which are formed by the polymerization of polyhedra containing higher-valence cations. Overarching structural categories correspond to those based upon isolated polyhedra (8), finite clusters (43), chains (57), sheets (204), and frameworks (56) of polyhedra. Within these categories, structures are organized and compared upon the basis of either their graphical representations, or in the case of sheets involving sharing of edges of polyhedra, upon the topological arrangement of anions within the sheets.

546 citations

Journal ArticleDOI
TL;DR: This work shows that simple, cost-effective, and portable metal sensors can be obtained with similar sensitivity and selectivity as much more expensive and sophisticated analytical instruments.
Abstract: Here, we report a catalytic beacon sensor for uranyl (UO22+) based on an in vitro-selected UO22+-specific DNAzyme. The sensor consists of a DNA enzyme strand with a 3′ quencher and a DNA substrate with a ribonucleotide adenosine (rA) in the middle and a fluorophore and a quencher at the 5′ and 3′ ends, respectively. The presence of UO22+ causes catalytic cleavage of the DNA substrate strand at the rA position and release of the fluorophore and thus dramatic increase of fluorescence intensity. The sensor has a detection limit of 11 parts per trillion (45 pM), a dynamic range up to 400 nM, and selectivity of >1-million-fold over other metal ions. The most interfering metal ion, Th(IV), interacts with the fluorescein fluorophore, causing slightly enhanced fluorescence intensity, with an apparent dissociation constant of ≈230 μM. This sensor rivals the most sensitive analytical instruments for uranium detection, and its application in detecting uranium in contaminated soil samples is also demonstrated. This work shows that simple, cost-effective, and portable metal sensors can be obtained with similar sensitivity and selectivity as much more expensive and sophisticated analytical instruments. Such a sensor will play an important role in environmental remediation of radionuclides such as uranium.

430 citations

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