Uranium species

Magnetic-susceptibility measurements showed the presence of UF4 in heated samples. Ebert et al. (E14) reported a nominal stoichiometry CisUFg, magnetic-susceptibility measurements indicating partial reduction at room temperature, with 10% of the uranium species present as U(IV). Wide-line NMR demonstrated the presence of both U(VI) and U(IV). 

The incorporation of anions, as for example, S04 , CO2-, etc., makes leaching possible through the formation of stable uranyl (VI) oxyanions. In sulfate leaching, an observation of the potential-pH diagram for the uranium system reveals that uranium species in solution may be in the form of cations U02+, neutral species U02(S04)2 or anions U02(S04)4-. The oxidation of uraninite, U02, in acid solutions, transforming U(IV) to U(VI), yields soluble uranyl sulfate through the reaction as shown below ... 

Alkaline leaching is carried out by using sodium carbonate solution. In this case any U(IV) present in the ore must also be oxidized to U(VI). The uranium species soluble in carbonate leach solutions in the uranyl tricarbonate ion. The formation of this ion by solubilization of a hexavalent uranium mineral such as camotite, or a tetravalent uranium mineral such as uraninite, may be represented by the following reactions ... 

This reaction with TMEDA is temperature dependent. When the reaction of U with benzophenone is carried out at 70 °C, TPE is produced exclusively, with no TPA or DPM seen. As the reaction temperature is increased above 70 °C, the amounts of TPA and DPM produced increase. It appears that th hydrogenated products TPA and DPM arise when benzophenone reacts with U containing uranium hydrides. These hydrides may be formed from either substrates containing acidic hydrogens, or by thermal reaction of the low valent uranium species with coordinated TMEDA. 

At unit activities of the oxidant and reductant, the potential depends only on pH the slope of the line for a plot of potential versus pH is governed by the ratio m/n. Potential-pH diagrams are a concise means to display the redox properties of a system. We will take uranium as an example. The +6, +5, +4, and + 3 oxidation states are known in aqueous solution. The determination of +6 uranium by coulometric titration has been investigated by many workers and the lower oxidation states have all been used as coulometric titrants. Hydrolyzed uranium species exist in a noncomplexing solution, but the chemistry is simplified considerably if the discussion is limited to solutions more acidic than about pH 4. Some of the half-reactions to be considered are listed next with E° vs. NHE ... 

Figure 8 The Eh/pH diagram for the U/02/C02/H20 system at 25 °C with a partial C02 pressure of 10 2bar.27 Solid-solution boundaries are drawn at 10-6 M dissolved uranium species U02am = amorphous U02...

Although the trivalent uranium species of the tetrapyrrolide polyanions display a very high reactivity, no evidence was found for the ability to interact with N2. However, the reduction of the trivalent [ [(—CH2 )5]4-calix[4]tetrapy-rrole U(dme)][K (dme)] with [K-(naphthalenide)] in DME afforded N2cleavage with formation of the mixed-valenceU(IV)-U(V) nitride below (Scheme 13) (36). The highly reactive species that performs the N2 cleavage in this case was not identified since reactions carried out under an Ar atmosphere promoted solvent deoxygenation, as well as depolymerization of polysilanol. 

As reported elsewhere, the experiments were used to demonstrate the major process steps with recycle of the uranium. Most experiments were conducted in an alumina-lined tube furnace with the uranium in an alumina boat. Hydrogen gases were produced and measured. Reduction of CuO to copper metal, determined by weight loss, was the primary method used to measure the amount of reductant produced. X-ray diffraction analysis was used in most cases to confirm the uranium species. 

The results of thermodynamic analysis and experiments led to the UTC shown in Figure 1 that uses valence state transitions and the formation of oxidised and reduced uranium species to split H20 for the production of H2. Some of the chemical reactions are new but most of the chemical reactions are used industrially within the uranium industry. 

These results illustrate the importance of the chemical species of the element present in the deposit with regard to ion emission (and gives insight into the effect of the oxidizing/reducing nature of the ion emitter) but tell little about the actual mechanisms active in the ion emitting process. As an example, the ions could be emitted either from the deposit itself or from an intermediate material that formed as a consequence of the chemical properties, or it could be entirely an interface phenomenon in which the deposit only served as a repository for the uranium species and the supporting filament served as the ionization surface. 

Two surveys consider uranium species in the year 1992 (Grenthe et al. 1992 [NEA 92] and Fuger et al. 1992 [IAEA 92]) lead to quite different interpretations regarding some hexavalent uranium-hydroxo-species. These differences do influence considerably the species distribution of a measured total uranium concentration at neutral and basic pH values (Table 24). 

A redox couple can be separately defined according to a redox sensitive element (in the example U according to N(5)/N(-3) redox couple) that can be given either as total concentration (like U) or as partial concentrations of the respective species (like Fe). The input enforces a calculation of a redox equilibrium of the redox sensitive elements by means of the given redox couple. In this example the standard pE value for the standard redox couple will not be used for this element (in the example of uranium) to calculate the uranium species. 

How do the uranium species change Which species predominate at which pH value What are the effects of the change in uranium species concerning processes of transport and sorption ... 

First of all both solutions, the acid mine water and the groundwater, are defined in the PHREEQC input file and mixed applying the keyword MIX. Then this solution is saved as solution 3 (SAVE SOLUTION) and the job is finished by END. A second job follows, which uses again SOLUTION 2 (groundwater) and SOLUTION 3 (1 1 diluted water) via the key word USE, once again mixes both solutions 1 1, and saves the result as SOLUTION 4, etc. SELECTED OUTPUT facilitates the further data processing in EXCEL by providing the pH values and the molalities of all uranium species. The key word itself has to be repeated for every job, as well as the definition of the desired parameters pH and molalities. 

Yet the file name (e.g. 3 uranium species pHdependent.csv) should only appear within the first SELECTEDOUTPUT block and not be repeated. This way PHREEQC writes the parameters of all modeling in one single SELECTED OUTPUT file. The headline, however, is repeated for every modeling ran. The sub key word reset false , also applied just once within the first SELECTED OUTPUT block, suppresses the standard output for all other modeling, which are written into the same file. 

With seven mixings plus the initial solution of acid mine water and the resulting solution of groundwater the variations in uranium species are as shown in (Fig. 56). 

Fig. 56 Development of uranium species during the mixing of an acid mine water (pH= 2.3) with a groundwater (pH=6.6)...

Equilibrium distributions of uranium species by the model calculation are shown in Figures 1 and 2. Figure 1 is the calculated result of U speciation under 0% CO2 condition. Uranium almost exists as U02 at pHs 5.5 or below. Uranium also exists as a form of UO2OH and (U02)3(0H)s between pH 5 and 6.5. Uranium is precipitated as a form of 0 -U02<0H)2(s) between pH 6 and 9. The distribution percentage of the solid-phase is over 80%. Uranium exists as U02(0H)3 at pHs 9 or above. The dominant solid-phase is uranyl hydroxide because of the CO2 free condition. The distribution of U species under air condition is shown in Figure 2. Uranium exists as U02 at pHs 5.5 or below and as U020H and (U02)3(0H)s between pH 5 and 6.5. Uranium is precipitated as species of 0 -U02(0H)2(s) between 6 and 7.5. The maximum percentage of the solid-phase is 54%. 

Figure 1. Calculated distribution of uranium species in the aqueous and solid-phase of IxlO M solution equilibrated under 0% CO2 condition.

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