Uranyl UO

Sedimentary carbonate-fluor-apatite Cas(P04,003)3 is not luminescent under X-rays, and under UV lamp excitation it is characterized by broad structureless bands which are very similar to those encountered in many sedimentary minerals. It was concluded that this luminescence is due to different kinds of water-organic complexes (Tarashchan 1978). For this reason the luminescence properties of the sedimentary apatites is much less informative compared to magmatic apatite and attracted not much attention. 

Carbonate-fluor-apatite accommodates large quantities of trace elements, mainly uranium, which are potential luminescence centers. It has been proposed that uranium may occur in phosphorites in the following forms as a separate uraninite phase as an adsorbed or structurally incorporated uranyl ion as a dominantly replacement for Ca +, to be structurally incorporated 

The steady-state luminescence of water-organic complexes is strong and conceals the weaker characteristic luminescence of uranium containing centers, which can be detected by the difference in decay times only. The reason is that the decay time of water-organic complexes is characterized by two time intervals less then 30 ns and more then 10 ms. Since the uranium centers have decay times in the microseconds range, it is possible to detect them by time-resolved spectroscopy. In the time-delayed laser-induced spectroscopy, the luminescence spectra are recorded at a fixed moment after a laser pulse. These spectra maybe different from the integrated steady-state ones since after a certain time short luminescence will be practically absent. 

Thermal treatment has great influence on liuninescence spectra. Certain cations may change valence and accommodation form during heating, with transformation from a nonluminescent form to a luminescent form. 

The spectral-kinetic parameters of the green laser-induced luminescence of the sedimentary apatites allow its association with emission. The spectra 

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Figure 10.7 Adsorption isotherm describing the adsorption of uranyl (UO +) species onto suspended amorphous ferric hydroxide at pH 7.23 and 25°C. The vertical line denoting saturation with respect to schoepite [U02(0H)2 H2OI has been computed from the pH and dissolved uranyl concentration. The enrichment factor, E.F., equals K,. Reprinted from Geochim. et Cosmochim. Acta, Vol. 53(6), D. Langmuir, Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits, pp. 547-569,...

Sorption of uranyl (UO +) species (the oxidized form of dissolved uranium) by Fe(OH)3 was measured in the laboratory at a constant pH of 7.23 and 25"C. In the experiment, 51.3 mg of ferrihydrite, Fe(OH)3 (weight as Fe) was suspended in 50 ml of a 0.01 M KCl solution. The unpublished experimental data given here are from van der Weijden et al. (1976). Speciation calculations using MINTEQA2 show that the dominant form of dissolved uranyl ( 97%) in these experiments is the UOjfOH) complex. 

This element is usually classed with Fe and Or, or with Ni and Co. It does not, however, form compounds resembling the ferric it forms a series of well-defined uranates, and a series of compounds of the radical uranyl (UO). Standard solutions of its acetate or nitrate are used for the quantitative determination of HjPO. ... 

Actinide Peroxides. Many peroxo compounds of thorium, protactinium, uranium, neptunium, plutonium, and americium are known (82,89). The crystal stmctures of a number of these have been deterrnined. Perhaps the best known are uranium peroxide dihydrate [1344-60-1/, UO 2H20, and, the uranium peroxide tetrahydrate [15737-4-5] UO 4H2O, which are formed when hydrogen peroxide is added to an acid solution of a uranyl salt. 

In TBP extraction, the yeUowcake is dissolved ia nitric acid and extracted with tributyl phosphate ia a kerosene or hexane diluent. The uranyl ion forms the mixed complex U02(N02)2(TBP)2 which is extracted iato the diluent. The purified uranium is then back-extracted iato nitric acid or water, and concentrated. The uranyl nitrate solution is evaporated to uranyl nitrate hexahydrate [13520-83-7], U02(N02)2 6H20. The uranyl nitrate hexahydrate is dehydrated and denitrated duting a pyrolysis step to form uranium trioxide [1344-58-7], UO, as shown ia equation 10. The pyrolysis is most often carried out ia either a batch reactor (Fig. 2) or a fluidized-bed denitrator (Fig. 3). The UO is reduced with hydrogen to uranium dioxide [1344-57-6], UO2 (eq. 11), and converted to uranium tetrafluoride [10049-14-6], UF, with HF at elevated temperatures (eq. 12). The UF can be either reduced to uranium metal or fluotinated to uranium hexafluoride [7783-81-5], UF, for isotope enrichment. The chemistry and operating conditions of the TBP refining process, and conversion to UO, UO2, and ultimately UF have been discussed ia detail (40). 

In practice, uranium ore concentrates are first purified by solvent extraction with tributyl phosphate in kerosene to give uranyl nitrate hexahydrate. The purified uranyl nitrate is then decomposed thermally to UO (eq. 10), which is reduced with H2 to UO2 (eq. 11), which in turn is converted to UF by high temperature hydrofluorination (eq. 12). The UF is then converted to uranium metal with Mg (eq. 19). 

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

The majority of U(V1) coordination chemistry has been explored with the trans-ddo s.o uranyl cation, UO " 2- The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatihty as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned ia the iatroduction have been complexed with U(V1) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples iuclude U02X2L (X = hahde, OR, NO3, RCO2, L = NH3, primary, secondary, and tertiary amines, py n = 2-4), U02(N03)2L (L = en, diamiaobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J j)/H20 (x = 2-5). 

The ion exchange process involves the ability of hexavalent uranium as the uranyl ion, UO+, to form anionic complexes with sulfate ions, SO2-, and carbonate ions, CO2-. In a general way, it may be mentioned that the uranyl ion exits in dynamic equilibrium with its sulfate complexes,... 

Use of the oxidation number and charge number extends the range for radicals for example, UO + uranyl(VI) or uranyl(2+) cation UOJ, uranyl(V) or uranyl(l +) cation. 

Uranium also combines with oxygen in various ratios. For instance, uranium dioxide (UO ) is a brownish-black powder that was once thought to be pure uranium. Uranium trioxide (UOj), a heavy orangish-powder, was once referred to as uranyl oxide. 

Two other actinyl ions, UO (5/°) and NpO (5/ ), are worth mentioning here. Although the uranyl ion contains no unpaired/electrons, its complexes may display a weak temperature-independent paramagnetism because of... 

Three types of vibrational spectra are considered now in minerals electron-hole centers O2 for the blue region and S2 for orange-red, together with UO for green. O2 and S2 centers have been proposed earlier to explain the vibration bands in natural sulfates (Tarashchan 1978). Nevertheless, we checked the S2 luminescence in several minerals and it was found that it is characterized by a short decay of several ns, which is much shorter than in our case. The uranyl possibility for the green lines may be also excluded because the U content of 0.12 ppm in the corresponding sample is low. 

Uranyl oxalate actinometer. This actinometer has a range of 208-435 nm with an average quantum yield of about 0.5. Since the UO + ion acts as a photosensitizer for the oxalate decomposition the light absorption remains constant, but rather long exposures. re required for final accurate oxalate titrations. It is now mainly of histo al interest. 

A special case in the uranyl spectroscopy is formed by Denning s extensive work on the UO + group in Cs2U02Cl4C ([65], and papers cited therein). Later this work was extended to two-photon spectroscopy, a powerful method for a complex with inversion symmetry [66]. Excited state spectroscopy followed soon after [67], The latter experiments show a progression in the excited-state absorption spectrum of the UO + complex in a frequency of 585 cm-1. This is the U-O stretching frequency in the second excited state. In the first excited state it is at 715 cm-1. In this way it is possible to determine that the U-O distance expands from 1.77 A in the ground state to 1.84 A in the first and 1.95 A in the second excited state. This work has been reviewed recently [68]. 

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