Uranyl complexes carbonates

Owing to the stability of the uranyl carbonate complex, uranium is universally present in seawater at an average concentration of ca. 3.2/rgL with a daughter/parent activity ratio U) of 1.14. " In particulate matter and bottom sediments that are roughly 1 x 10 " years old, the ratio should approach unity (secular equilibrium). The principal source of dissolved uranium to the ocean is from physicochemical weathering on the continents and subsequent transport by rivers. Potentially significant oceanic U sinks include anoxic basins, organic rich sediments, phosphorites and oceanic basalts, metalliferous sediments, carbonate sediments, and saltwater marshes. " ... 

Figure 4. Solubility of uraninite as a function of Eh and PCO2 at pH = 8 and 25°C. The increase of uraninite solubility at high Pco2 results from the formation of uranyl carbonate complexes. [Used with permission of Elsevier Science, from Langmuir (1978) Geochim Cosmochim Acta, Vol. 42, Fig. 15, p. 561].

Goishi and Libby have investigated the extraction of pertechnetate from alkali solutions with pyridine. Later work showed that a better extraction is obtained using a mixture of sodium hydroxide and sodium carbonate as the aqueous phase. Since the uranyl carbonate complex is not extracted into pyridine, this system may be used for the separation of technetium from uranium. Distribution coefficients of fission products in pyridine are given in Table 4. Substituted pyridine such as 2,4-dimethylpyridine or 4-(5-nonyl)pyridine ) are useful for separating technetium from solutions containing appreciable amounts of aluminum nitrate. 

Owing to the stability of the uranyl carbonate complex, uranium is universally present in seawater at an average concentration of ca. 3.2/rgL with a daughter/parent activity ratio of 1.14. " In particulate matter and... 

This paper is devoted to the sorption of uranyl, which exhibits a complex aqueous and surface chemistry. We review briefly the sorption behaviour of An in the environment, and illustrate the variety of environmental processes using published data of uranyl sorption in the Ban-gombe natural reactor zone. After summarizing the general findings of the mechanisms of An sorption, we then focus particularly on the current knowledge of the mechanisms of uranyl sorption. A major area of research is the influence of the aqueous uranyl speciation on the uranyl surface species. Spectroscopic data of U(VI) sorbed onto silica and alumina minerals are examined and used to discuss the role of aqueous uranyl polynuclear species, U02(0H)2 colloids and uranyl-carbonate complexes. The influence of the mineral surface properties on the mechanisms of sorption is also discussed. 

Investigations of uranyl carbonate complexes on an amorphous Al phase using EXAFS... 

Uranyl carbonate complexes have attracted considerable interest in recent years as they are intermediates in the processing of mixed oxide reactor fuels and in extraction of uranium from certain ores using carbonate leaching more topically they can be formed when uranyl ores react with carbonate or bicarbonate ions underground, and can be present in relatively high amounts in groundwaters. The main complex formed in carbonate leaching of uranyl ores is 8 coordinate [1102(003)3], but around pH 6 a cyclic trimer [(002)3(003)6] has been identified. 

In contrast, the chemical toxicity of uranium is more important than its radiological hazard. In body fluids, uranium is present as soluble U(VI) species and is rapidly eliminated from the body (60% within 24 h Goyer and Clarkson (2001)). It is rapidly absorbed from the gastrointestinal tract and moves quickly through the body. The uranyl carbonate complex in plasma is filtered out by the kidney glomerulus, the bicarbonate is reabsorbed by the proximule tubules, and the liberated uranyl ion is concentrated in the tubular cells. This produces systemic toxicity in the form of acute renal damage and renal failure. 

Some uranium ore bodies are high in limestone and dolomitic content which is wasteful on acid employed for leaching. Such sources are successfully treated by an alkaline mixed sodium carbonate and bicarbonate leach, the later being required to buffer the pH so as not to precipitate the uranium. Sorption occurs as the uranyl carbonate complex anion, U02(C03)3 , but elution is with sodium nitrate since acid would cause evolution of carbon dioxide gas. 

A high degree of mobility is conferred on the uranyl ion by the presence of carbon dioxide which allows the formation of stable uranyl carbonate complex ions (Tugarinov, 1975). Table 8.3 lists some complexes which have been referred to as being of great importance in the transport of uranium. Many other complex ions, including organic complexes, are known. The... 

Baturin (1973b) examined the uranium cycle in the Black Sea and Azov Sea and found most transport to be in solution as a uranyl carbonate complex. The Azov Sea is a shallow-water basin in which sediments are often disturbed by storms bottom sediments have a fairly uniform and low (0.3— 2.3 pgg" ) content of uranium. The Black Sea, by contrast, is deep, with H2S-rich bottom waters of low Eh. The bottom water is strongly depleted in uranium, while the sediment is of variable, sometimes high (0.2—23.0 pg g" ) uranium content. The Black Sea sediments are said to receive up to 400 Mg of ur2mium from the water. The residence time of uranium in Azov Sea waters is only 14.5 y, but in Black Sea water, is of the order of 3000—4000... 

Most leaching described in the literature has followed the lines described above. Strongly basic ores, usually those containing abundant carbonate, are not well suited to bacterial leaching since a low pH is not as readily obtained as in other cases. For such ores, leaching with sodium carbonate solution has been used successfully, the uranium being transported as an anionic uranyl carbonate complex (Merritt, 1971). 

For the uranyl-TRUEX system, the average water soluble uranyl concentration was low, 7 X 10 M. It appears that TRUEX bound uranyl ions are not back extracted into the neutral pH simulated groundwater. For uranyl samples deposited as nitrate salts or hydroxides, the solubility in the absence of phosphate is about 1 x 10" M at pH 5 and pH 6, rising to 2 x 10" M at pH 7 and 2.6 x lO" M at pH 8. The increase in solubility at higher pH is a direct result of increased uranyl-carbonate complexation. With the introduction of phosphate, uranyl solubility is reduced at all pH s, as shown in Figure 2. Uranyl solubility is generally lowest for 0.0001 M phosphate, rising... 

At a typical groundwater CO2 pressure of 10 bar, the highly stable uranyl carbonate complexes predominate above about pH 5 (Fig. 13.9). Comparison of Figs. 13.8 and 13.9 indicates that these complexes are stable relative to U(OH)4 under highly reducing conditions. Accordingly, above pH 5, the oxidation of U(lV)(aq) and dissolution of U02(s) can occur at lower Eh values when high carbonate concentrations are present. (The U(IV)-carbonate complexes are unstable relative to U(0H)4 under these conditions and so do not stabilize U(IV)(aq).)... 

Assuming that uranyl carbonate complexes are not adsorbed, using the triple layer (TL) model compute the adsorption of uranyl species by hydrous ferric oxide (HFO) at pH = 7 in the absence of CO2, and for Pcoj = given that total U(VI) = 10 M. Assume the solution... 

Differences in behavior of actinides in the aquatic environment are shown by the data in Table VI. In WOL, we have noted that about 12% of the total Pu in the water column passes a 0.45-vim filter and also passes a 10,000 mol wt membrane filter. Essentially all of this soluble Pu is retained when passed through an anion exchange column. Curium-244 behaves somewhat differently in that about 50% of the activity in WOL water is soluble, but it also behaves anionically. The reasons for a negative charge on Pu and Cm are complex and are not understood, but one possible explanation is the presence of Pu(VI) carbonate complexes, analogous to the soluble uranyl carbonate complexes found in natural waters. Experiments are in progress to determine the valence state of this soluble form of Pu. Another explanation for the observed soluble Pu would be organic complexes. 

Uranyl carbonate complexes, like sodium uranyl tricarbonate, Na4[U02(C03>3], that is obtained when uranium ore is leached with sodium carbonate solutions and ammonium uranyl carbonate (AUC), (NH4)4[U02(C03)3l, that is used to precipitate the uranium in the UCF, are important in the NFC. These carbonates serve to purify the uranium from several metals (like Fe, Al, Cr, Ni, and other metals) that are precipitated as hydroxides or oxycarbonates, as well as aUcaline-earth elements. These purification methods utilize the effect of the ammonium carbonate concentration on the solubility of uranium. Upon heating of AUC to 300°C-500°C, it decomposes to UO3, ammonia, CO2, and water and at temperatures of 700°C-800°C, without air, UO2 may be formed (the ammonia serves as the reducing agent). The solubility of AUC decreases markedly in the presence of ammonium carbonate, for example, from 119.3 g L" at 50°C without ammonium carbonate to 0.5 g L" with 35% ammonium carbonate (Galkin 1966). The carbonate complexes also play a role in biological systems and affect clearance by the blood after exposure to uranium compounds. 

According to Naumov and Mitronova, the decomposition of uranyl carbonate complexes and the simultaneous reduction of uranium proceed the more readily w ith decreasing carbonate ion concentration. In fact, the potential of the couple in the presence of CO3" ions is lower than that in carbonate-free waters—that is, more reducing conditions are required to remove uranium from carbonate-rich water in comparison with waters low in carbonate. 

Uranous complexes tend to be insoluble at low temperatures and at pH 4.5-7. At temperatures above 150°C uranous transport may become dominant. Depending on ligand concentrations, uranous fluoride, phosphate, sulphate and especially hydroxide compounds are important species under these conditions, but uranous carbonate complexes are not. Uranyl species are soluble over a wide range of conditions. In normal groundwater, at temperatures of 25°C, uranyl fluoride complexes are dominant at pH <4, uranyl phosphates at pH 4-7.5 and uranyl di- and tricarbonate complexes at pH >7.5. Uranyl silicate complexes are probably insignificant, and at temperatures near 100°C uranyl hydroxides predominate, whereas uranyl carbonate complexes dissociate. ... 

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