Tetravalent uranium

Geochemical Nature and Types of Deposits. The cmst of the earth contains approximately 2—3 ppm uranium. AlkaHc igneous rock tends to be more uraniferous than basic and ferromagnesian igneous rocks (10). Elemental uranium oxidizes readily. The solubiHty and distribution of uranium in rocks and ore deposits depend primarily on valence state. The hexavalent uranium ion is highly soluble, the tetravalent ion relatively insoluble. Uraninite, the most common mineral in uranium deposits, contains the tetravalent ion (II). 

Group 3 (IIIB) and Inner Transition-Metal Perchlorates. The rare-earth metal perchlorates of yttrium and lanthanum have been reported (53). Tetravalent cerium perchlorate [14338-93-3] 06(0.04)4, and uranium perchlorate have also been identified (54). 

Thorium has a wide distribution in nature and is present as a tetravalent oxide in a large number of minerals in minor or trace amounts. Thorium is significantly more common in nature than uranium, having an average content in the earth s cmst of approximately 10 ppm. By comparison, Pb is approximately 16 ppm. Thorium has a seawater concentration of <0.5 x 10 . Thorium refined from ores free of uranium would be almost... 

Studies show that the main sites of uranium deposition ate the renal cortex and the Hvet (8). Uranium is also stored in bones deposition in soft tissues is almost negligible. Utanium(VI) is deposited mostly in the kidneys and eliminated with the urine whereas, tetravalent uranium is preferentially deposited in the Hvet and eliminated in the feces. The elimination of uranium absorbed into the blood occurs via the kidneys in urine, and most, - 84%, of it is cleared within 4 to 24 hours (8). 

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... 

Comparison of these results for plutonium with those for other tetravalent metals reveals some interesting facts. Thor-ium(IV), uranium(IV) and neptunium(IV) sulfates have been investigated under hydrothermal hydrolytic conditions. For uranium, the stable phases which have been reported include U(0H)2S0i (2), U60i, (OH)i, (SO.,) 6 (2). U (SOi,) 2 4H20 (23) and IKSO (24). 

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. 

This latter situation affords a good method for separating uranium from plutonium. Hydroxylammonium formate (HAF) and hydrazium formate (NHF) were added to the formic acid to reduce Pu(IV) to Pu(III) to aid in plutonium recovery, although formic acid alone will strip tetravalent actinides, e.g., Th(IV) from 0D[IB]CMP0, once excess HNO3 present in the organic phase is removed. Thus, formic acid with HAF and NHF affords an excellent method for stripping all the actinides from these very powerful CMP extractants. Under the above conditions Am(III) and Cm(III) present in... 

One of the first bed materials was based on the extractant diamyl amylphosphonate (DAAP marketed under the name U-TEVA-Spec ) and was designed for purification of the tetravalent actinides (U (IV), Th (IV), Pu (IV)) and hexavalent uranium (U(VI)). This material is characterized by high (>10-100) distribution coefficients for U and Th in significant (>3 M) concentrations of both nitric and hydrochloric acids, and so is useful for both U and Th purification (Horwitz et al. 1992 Goldstein et al. 1997 Eikenberg et al. 2001a). 

Uranium and thorium partitioning into amphibole were also studied experimentally by Tiepolo et al. (2000a). Under the redox conditions of their experiments (FMQ-2 log units) U was dominantly tetravalent. They find no correlation between Aj, Djh and crystal composition, and, but again find a linear correlation with silica content ... 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

Primary (predominantly tetravalent uranium) Uraninite Ideally U02... 

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 ... 

The extractant is octyl pyrophosphoric acid (OPPA process). The stripping is by concentrated hydrofluoric acid. Yields UF4. Extracts uranium in tetravalent state. It is, therefore, necessary to use metallic iron as a reducing agent. 

The extractant is a commercial mixture of mono- and dioctyl phenyl phosphoric acid (OPPA). It is used in conjuction with tributyl phosphate (TBP). Stripping is by ammonium carbonate solution. The mixture shows synergism. Uranium is extracted in the tetravalent state. The process is much less expensive and possesses a higher extracting power than D2EHPA-TOPO combination. 

Tetravalent tellurium. See Tellurium(IV) Tetravalent tungsten, 25 386 Tetravalent uranium coordination complexes, 25 434 Tetrazene, 10 730... 

As indicated earlier Kraus (9) has recognised three types of hydrolysis products of tetravalent thorium, uranium, plutonium and americium. These products are ... 

In 1942, the Mallinckrodt Chemical Company adapted a diethylether extraction process to purify tons of uranium for the U.S. Manhattan Project [2] later, after an explosion, the process was switched to less volatile extractants. For simultaneous large-scale recovery of the plutonium in the spent fuel elements from the production reactors at Hanford, United States, methyl isobutyl ketone (MIBK) was originally chosen as extractant/solvent in the so-called Redox solvent extraction process. In the British Windscale plant, now Sellafield, another extractant/solvent, dibutylcarbitol (DBC or Butex), was preferred for reprocessing spent nuclear reactor fuels. These early extractants have now been replaced by tributylphosphate [TBP], diluted in an aliphatic hydrocarbon or mixture of such hydrocarbons, following the discovery of Warf [9] in 1945 that TBP separates tetravalent cerium from... 

For vanadium and chromium the first ionization energies are much lower than the first ionization energies of phosphorus and sulphur, respectively. This explains the high heats of formation of VC13 and CrCl3. In uranium, the tetravalent state is more stable than that in tungsten because uranium as an actinide has a different electron configuration. 

The Purex process is used for almost all fuel reprocessing today. Irradiated UO2 fuel is dissolved in HNO3 with the uranium being oxidized to U02(N03)2 and the plutonium oxidized to Pu(NC>3)4. A solution of TBP in a high-boiling hydrocarbon, such as n-dodecane, is used to selectively extract the hexavalent U02(N03)2 and the tetravalent Pu(NC>3)4 from the other actinides and fission products in the aqueous phase. The overall reactions are... 

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