Uranyl complexes reduction

The previously proposed uptake models were mathematical assumptions and had no physical or chemical basis. Millard and Hedges, on the other hand, considered the chemistry of bone-uranium interactions. With the D-A model, they proposed that U was diffusing into bone as uranyl complexes, and adsorbing to the large surface area presented by the bone mineral hydroxyapatite (Millard and Hedges 1996). Laboratory experiments showed a partition coefficient between uranyl and hydroxyapatite under oxic conditions of 10" -10, demonstrating U uptake in the U state without the need for reduction by protein decay products as proposed by Rae and Ivanovich (1986). 

Compared to studies in acidic media, studies on the electrochemical behavior of 1102 " in basic media are more limited. The report from Morris [52] describing voltammetry results for hydroxo and carbonato uranyl complexes is a recent example. Previous studies have been performed mostly in carbonate and bicarbonate solutions. Wester and Sullivan have studied the reduction of 1102 in these solutions to find an electrochemically irreversible process but disproportionation of U(V)02" " was evidenced only in the bicarbonate solutions [67]. 

The trinuclear complexes containing manganese(II), iron(II) or cobalt(II) as central ions, apart from the two uranyl-centred reductions, gave only irreversible oxidation steps. It is evident that, at variance with (U02)2(dpb)2(py)2, in this case the presence of the central M(II) ion favours the electronic interaction between the two outer uranium(VI) centres. 

They also follow expected effects of added ions (e.g. Li N03") via the reduction of the water activity ("salting-out effect"). The apparent discrepancy comes from our choice to model uranyl by its neutral U02(N03)2 salt instead of U02, in order allow for its possible extraction. In ct, it is very difficult to predict the status of ion pairs in pure solutions (see e.g. ref (53) for lanthanide salts in acetonitrile) and, a fortiori, at liquid-liquid interfaces. The formation of neutral salts may also be too slow to occur at the simulated time scales. Thus, this facet cannot be addressed by our simulations. In this context, the spontaneous formation of uranyl complexes with TBP is remarkable, and points to the importance of (micro)-heterogeneities of the systems and metastable conditions to form the complexes and promote their extraction to organic or supercritical phases. 

Another major processs involved in uranium mineral formation is the reduction of uranyl complexes to species of low solubility. This process may occur in regions where uraniferous groundwater passes into a zone of low E, . The diagrams for the U-O2-CO2-H2O and... 

The reason why the addition of relatively large quantities of inert sulfate salts to uranyl sulfate solutions reduces the corrosiveness of the resulting solutions is not known, but may be due to one of a number of factors, such as the formation of stable complexes, reduction of acidity, changes in oxidizing power, increased viscosity and densit or changes in colloidal properties of the oxide. 

The example of uranyl reduction shows the utility of this approach. The concentrations of the two surface complexes vary strongly with pH, and this variation explains the observed effect of pH on reaction rate, using a single value for the rate constant k+. If we had chosen to let the catalytic rate vary with surface area, according to 17.12, we could not reproduce the pH effect, even using H+ and OH-as promoting and inhibiting species (since the concentration of a surface species depends not only on fluid composition, but the number of surface sites available). We would in this case need to set a separate value for the rate constant at each pH considered, which would be inconvenient. 

The U(IV) chemistry is similar to that of Th4+, except for the difference in the charge/radius ratio of the ions. U4+ solutions are green in color, stable, and slowly oxidized by air to U02+. Solutions of U4+ are generally prepared by reduction of solutions of the uranyl (U02+) ion. U(IV) forms complexes with many anions (C204-,C2H302, C03-, Cl-, and NO3"). The chlorides and bromides of U(IV) are soluble while the fluorides and hydroxides are insoluble. In aqueous solution, U(IV) hydrolyzes via the reaction,... 

The purification of uranium by precipitation may involve the formation of insoluble oxalate or peroxide complexes.50 In the oxalate method the ore concentrate is dissolved in nitric acid to give a uranyl nitrate solution from which uranyl oxalate is precipitated according to equation (101), leaving the bulk of the impurities in solution. This approach is favoured over dissolution in hydrochloric acid and reduction to UIV prior to U(C204)2 precipitation since it is simpler and... 

A large number of other metal complexes have received long and detailed attention, but activity in recent years has revealed few new principles appropriate for discussion here and some systems have been treated in detail elsewhere.2 Included among these are oxalato complex photochemistry where oxidation of the oxalato ligands is coupled to the central metal reduction Ag(I) photochemistry related to imaging systems uranyl ion photochemical reactions coupled to organic oxidations and aquo ion photoredox reactions. Two specific topics have recently emerged as... 

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