Uranium solubility, carbonate

The chemical reactivity of minor elements in seawater is strongly influenced by their specia-tion (see Stumm and Brauner, 1975). For example, the Cu ion is toxic to phytoplankton (Sunda and Guillard, 1976). Uranium (VI) forms the soluble carbonate complex, U02(C03)3, and as a result uranium behaves like an unreactive conservative element in seawater (Ku et ah, 1977). 

The dissolution time for the unreprocessed fuel would be at least 1 million years due to the limited water supply, even if a rapid oxidation of uranium to the hexavalent state and a subse-guent formation of water soluble carbonate complexes are assumed (15). Since the conditions are reducing in the groundwater (see beTow) the dissolution time would probably be several orders of magnitude larger. The unsignificant dissolution of uranium and fission products observed in the Oklo-deposit (16) is an example of a similar extremely slow leaching process in the natural environment. 

The possibility of dissolving the mixed hydroxide in HNOs and obtaining direct extraction of thorium (and uranium) from the nitrate solution has been studied [155,156], but does not seem to be too promising, possibly due to the partial oxidation of tripositive cerium to the tetrapositive state. Kraitzer [157] was able to separate thorium from the mixed hydroxide cake by extracting the cake with sodium carbonate buffer at pH 9.5—10. Thorium was found to form a soluble carbonate complex and a recovery of better than 99% of thorium was claimed after only four extractions. 

The extraction of dates from the uranium and thorium isotope ratios in speleothems depends on a quirk in the geochemistry of these two elements. Uranium is easily oxidized to the state where it usually appears as the U02 ion. The uranyl ion in addition to its intrinsic solubility also forms soluble carbonate complexes that further mobilize the element in karstic ground waters (Langmuir, 1978, 1997). Thorium is firmly locked into the insoluble Th" state and is immobile in ground water (Langmuir... 

Uranium solubility is increased even more in the nitrate microcosms, and one possible explanation is the conversion of acetate to CO2, coupled with nitrate reduction, which would give higher dissolved carbonate concentrations in the nitrate microcosms. However, PHREEQE modelling showed that the higher CO concentration would not greatly affect uranium speciation in solution and is therefore unlikely to account for the enhanced solubility. Alternatively, as nitrate is reduced to ammonium (NH/), which promotes cation exchange, this could lead to displacement of U02 from surface complexes, which are the predominant uranyl species on mineral surfaces. " ... 

When sodium carbonate is present in excess, the uranyl ion forms a soluble carbonate complex, whereas most metals separate as carbonates, basic carbonates, or hydroxides [62] — with the exception of V, Be, and Th, which remain partly in solution together with uranium. If the precipitate contains several metals, double precipitation is necessary. The amount of uranium retained by the precipitate does not exceed a few percent. When a carbonate fusion is used for decomposition of the sample, and the melt is leached with water, uranium passes into solution. 

Factors that may influence the solubility of uranium in a hydro-thermal system are temperature, pressure, oxidation state, pH, activity of complexing anions and partial pressure of such volatile components as carbon dioxide. The solubility of uranium decreases with increase in temperature, so cooling cannot be a possible mechanism of deposition. The effect of pressure on uranium solubility is difficult to evaluate, but at the relatively, low pressures of formation for many hydrothermal deposits the role of pressure is to affect the partial pressure of volatile components only. As pressure decreases, the partial pressure of CO2 decreases, which will decrease the activity of... 

In oxygenated seawater, uranium is thermodynamically predicted to be present in a hexavalent (-b 6) oxidation state, but it can also exist as the tetravalent U(IV) if conditions are sufficiently reducing. Reduced uranium in the +A oxidation state is highly insoluble or particle reactive. In contrast, U(VI) is readily soluble due to the rapid formation of stable inorganic carbonate complexes. According... 

Uranium is readily mobilized in the meteoric environment, principally as the highly soluble uranyl ion (U02 ) and its complexes, the most important of which are the stable carbonate complexes that form in typical groundwaters (pH > 5, pC02 = 10 bar) (Gascoyne 1992b Grenthe et al. 1992 see also Langmuir (1997) for review). Uranium is... 

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 ... 

Uranium mineral first is digested with hot nitric acid. AH uranium and radium compounds dissolve in the acid. The solution is filtered to separate insoluble residues. The acid extract is then treated with sulfate ions to separate radium sulfate, which is co-precipitated with the sulfates of barium, strontium, calcium, and lead. The precipitate is boiled in an aqueous solution of sodium chloride or sodium hydroxide to form water-soluble salts. The solution is filtered and the residue containing radium is washed with boiling water. This residue also contains sulfates of other alkahne earth metals. The sohd sulfate mixture of radium and other alkahne earth metals is fused with sodium carbonate to convert these metals into carbonates. Treatment with hydrochloric acid converts radium and other carbonates into chlorides, all of which are water-soluble. Radium is separated from this solution as its chloride salt by fractional crystallization. Much of the barium, chemically similar to radium, is removed at this stage. Final separation is carried out by treating radium chloride with hydrobromic acid and isolating the bromide by fractional crystallization. 

Uranium(VI) fluoride is a colorless solid which sublimes at 56.2°C it does not exist as a liquid at atmospheric pressure. It is soluble in l,l,2-trichloro-l,2,2-trifluorocthane (Freon 113). hydrogen fluoride, and chlorohydrocarbons, although it will react31 slowly with the latter it cleaves some ethers12-13 and reacts vigorously32 with carbon disulfide to give sulfur tetrafluoride, carbon tetrafluoride, bis(trifluoromethyl) disulfide, and bis(trifluoromethyl) trisulfide. 

The bicarbonate ion, HC03, is a prevalent species in natural waters, ranging in concentrations up to 0.8 X 10 3. As was indicated previously, carbonate ions have the ability to form complexes with plutonium. Starik (39) mentions that, in an investigation of the adsorption of uranium, there was a decrease in the adsorption after reaching a maximum, which was explained by the formation of negative carbonate complexes. Kurbatov and co-workers (20) found that increasing the bicarbonate ion concentration in a UXi (thorium) solution decreased the amount of thorium which formed a colloid and became filterable. This again was believed to be caused by the formation of a soluble complex with the bicarbonate. 

IJmnate. Sodium uranate, uranium yellow, Na2U04, yellow solid, insoluble, formed by reaction of soluble uranyl salt solution and excess sodium carbonate solution. Used (1) in the manufacture of yellowish-green fluorescent glass, (2) in ceramic enamels, (3) as a source of uranium for chemical reactions. 

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