Uranium oxidation states

Abstract uranium isotopes fractionate as a result of nuclear volume effects such that ratios vary as a funotion of uranium oxidation state, being highest in reduced species such as U"" in uraninite. The values of uranium minerals from volcanic-, metasomatic-, unconformity-,... 

H2S interface (Swarzenski etal., 1999b). Such concentration maxima at the redox boundary is also observed for DOC, Sr and Ba. The authors hypothesize that the source of elevated U at the redox boundary must be due to microbial uptake and subsequent release processes. Uranium oxidation state determinations in waters from 1, 22 and 30 m depth reveal that reduced U(IV) is not present in significant abundance, and that the chemical and/or biological reduction of hexavalent uranium is largely inhibited. These results suggest that U, DOC, Sr, Ba, Fe(II), and Mn(II) are greatly modified by direct and indirect microbial transformation reactions which are most concentrated across the redox transition zone in Framvaren Fjord. 

The first member of this family, manganese, exhibits one of the most interesting redox chemistries known thus it has already been discussed in detail above. Technetium exhibits the expected oxidation states, and associated with these are modest emf values. All of the isotopes of technetium are radioactive but Tc has a relatively long half-life (2.14 x 10s years) and is found in nature in small amounts because of the radioactive decay of uranium. Oxidation states of rhenium range from +7 to -3, with some species (e.g., ReOj and ReJ+) unstable with respect to disproportionation. 

The H NMR spectra of the U" species show substantial isotropic shifts, and variable temperature experiments indicate restricted rotation around the U-N bond. The bis(dialkylamides) undergo stepwise migratory CO insertion to yield mono- and bis-carbamoyl derivatives. A large number of organouranium imido complexes with different uranium oxidation states IV and VI have been synthesized (Arney et al. 1992, Arney and Burns 1993, 1995) by both metathesis and direct protonation routes (scheme 16). 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

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). 

Uranium is the fourth element of the actinide (SJ series. In the actinide series the electrons are more effectively shielded by the Is and 7p electrons relative to the 4f electrons (shielded by 6s, 6p) in the lanthanide (4p series. Thus, there is a greater spatial extension of 5f orbitals for actinides than 4f orbitals for lanthanides. This results in a small energy difference between and 5/ 6d7s electronic configurations, and a wider range of oxidation states is... 

Sohd uranium—phosphate complexes have been reported for the IV and VI oxidation states, as well as for compounds containing mixed oxidation states of U(IV) and U(VI). Only a few sohd state stmctures of U(IV) phosphates have been reported, including the metaphosphate U(P03)4, the pyrophosphate U(P202), and the orthophosphate, CaU(PO4)2. The crystal stmcture of orthorhombic CaU(POis similar to anhydrite (194). Compounds of the general formula MU2(PO4)3 have been reported for M = Li, Na, and K, but could not be obtained with the larger Rb and Cs ions (195). In the sohd state, uranium(IV) forms the triclinic metaphosphate, U(P03)4. Each uranium atom is eight-coordinate with square antiprismatic UOg units bridged by... 

Baddeleyite, a naturally occurring zirconium oxide, has been found in the Poco de Caldas region of the states of Sao Paulo and Minas Geraes in Brazil, the Kola Peninsula of the former USSR, and the northeastern Transvaal of the Repubflc of South Africa. BraziUan baddeleyite occurs frequently with zircon, and ore shipments are reported to contain 65—85% zirconium oxide, 12—18% siUca, and 0.5% uranium oxide. Veryhttle of this ore is exported now because all radioactive minerals are under close control of the BraziUan government. 

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... 

Because of the technical importance of solvent extraction, ion-exchange and precipitation processes for the actinides, a major part of their coordination chemistry has been concerned with aqueous solutions, particularly that involving uranium. It is, however, evident that the actinides as a whole have a much stronger tendency to form complexes than the lanthanides and, as a result of the wider range of available oxidation states, their coordination chemistry is more varied. 

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