Uranium complexes organic

Ganesh R, KG Robinson, GD Feed, GS Sayler (1997) Reduction of hexavalent uranium from organic complexes by sulfate- and iron-reducing bacteria. Appl Environ Microbiol 63 4385-4391. 

In dealing with solvent extraction as applied to uranium, it may at first be pointed out that uranium can exist in aqueous solution either as a neutral complex, a complex anion, or a cation. The organic solvents used in solvent extraction can accordingly be divided into three classes, according to the type of uranium complex extracted. 

Solubility data for the metal complexes in ScF CO2 are essential in order to develop models for the extraction processes and assess the feasibility of using ScF CO2 to replace organic solvents in conventional extraction processes. ScF CO2 solubility data for uranium complexes with TBP, P iketon or phosphine oxides are of particular interest fcM development of a ScF based process for reprocessing spent nuclear fuels. 

Nguyen-Trung, C., GM. Begun, and D.A. Palmer. 1992. Aqueous uranium complexes. 2. Raman spectroscopic study of the complex formation of the dioxouranium(VI) ion with a variety of inorganic and organic ligands. Inorg. Chem. 31 5280-5287. 

The pregnant organic solvent is stripped by agitation with a strong carbonate solution which removes uranium as the stable U02(C02) 3 aqueous complex. 

Oxygen-Gontaining Organics. Neutral and anionic oxygen-containing organic molecules form a wide variety of complexes with uranium. Much work has focused on alkoxides (222), aryloxides and carboxylates complexes with alcohols, ethers, esters, ketones, aldehydes, ketoenolates, and carbamates are also well known. 

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. " ... 

This removal may also include diffusion of soluble U(VI) from seawater into the sediment via pore water. Uranium-organic matter complexes are also prevalent in the marine environment. Organically bound uranium was found to make up to 20% of the dissolved U concentration in the open ocean." ° Uranium may also be enriched in estuarine colloids and in suspended organic matter within the surface ocean. " Scott" and Maeda and Windom" have suggested the possibility that humic acids can efficiently scavenge uranium in low salinity regions of some estuaries. Finally, sedimentary organic matter can also efficiently complex or adsorb uranium and other radionuclides. 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

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