Partitioning uranium

We will use the proxy approach for all of the U-series elements, except U and Th for which there are already sufficient experimental data (Table lb). For both of these elements we will discuss 4+ cations only. U also forms 5+ and 6+ cations in oxidizing environments. These are more relevant to aqueous and hydrothermal settings and will not be considered further here. However, in some experiments run at atmospheric pressure in air (e.g., Beattie 1993b), U will occur in one of its oxidized forms. These data are excluded from discussion, although it should be noted that in all minerals discussed here, the uranium partition coefficient will be considerably smaller when uranium is dominantly 5+ and 6+ compared to when it is dominantly 4+. 

Bruns, L. E. "Plutonium - Uranium Partitioning by a Reflux Extraction Flowsheet," in "Proceedings, ISEC 71," Society of Chemical Industry, London, 1971, Vol. 1, p. 186. 

Thorium and uranium contents of apatite vary widely but are normally very high compared to other mantle phases (generally >10 ppm Table 9 Ionov et al, 1997). Apatites in MARID xenoliths tend to have lower uranium (Kramers et al, 1983), possibly due to uranium partitioning into rutile or zircon. Lead contents are the highest reported for the common mantle minerals but U/Pb is generally PUM. 

Irving and Edglngton i have observed a synergistic enhancement of the uranium partition coefficient with trlbutyl-phoBi te (TBP) - or trlbutylphoBphlne oxide (TBPO) - TTA mixtures. The results, versus percent TBP or TBPO In the extractant mixture, are given In figure 58. 

The simple box-type mixer—settler (113) has been used extensively in the UK for the separation and purification of uranium and plutonium (114). In this type of extractor, interstage flow is handled through a partitioned box constmction. Interstage pumping is not needed because the driving force is provided by the density difference between solutions in successive stages (see Plutoniumand plutonium compounds Uraniumand uranium compounds). 

Chemical Separation. A reprocessing facility typically utilizes multiple extraction/reextraction (stripping) cycles for the recovery and purification of uranium and plutonium. For example, a co-decontamination and partitioning cycle is followed by one or more cycles of uranium and plutonium purification. The basic process is illustrated in Figure 3. 

Uranium—Plutonium Partitioning". The uranium and plutonium are separated in the partitioning column by reducing the plutonium to a less extractable valence state. The plutonium nitrate transfers back to the aqueous phase and the uranium remains with the organic. 

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. 

Mineral-Melt Partitioning of Uranium, Thorium and Their Daughters... 

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 ... 

Spinels. There are limited experimental data on uranium and thorium partitioning between magnetite and melt (Nielsen et al. 1994 Blundy and Brooker 2003). Both studies find U and Th to be moderately incompatible. Blundy and Brooker s results for a hydrous dacitic melt at 1 GPa and 1025°C give Du and D h. of approximately 0.004. The accuracy of these values is compromised by the very low concentrations in the crystals and the lack of suitable SIMS secondary standards for these elements in oxide minerals. Nonetheless, these values are within the range of Djh of magnetites at atmospheric pressure 0.003-0.025 (Nielsen et al. 1994). It is difficult to place these values within the context of the lattice strain model, firstly because there are so few systematic experimental studies of trace element partitioning into oxides and secondly because of the compositional diversity of the spinels and their complex intersite cation ordering. 

Beattie P (1993b) Uranium-thorium disequihbria and partitioning on melting of garnet peridotite. Nature 363 63-65... 

Beattie P (1993a) The generation of uranium series disequilibria by partial melting of spinel peridotite constraints from partitioning studies. Earth Planet Sci Lett 117 379-391... 

Blundy J, Wood B (2003) Mineral-melt partitioning of uranium, thorium and their daughters. Rev Mineral Geochem 52 59-123... 

Blatter DL, Carmichael ISE (1998) Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle. Geology 26 1035-1038 Blundy J, Wood B (2003) Mineral-melt partitioning of uranium, thorium and their daughters. Rev Mineral Geochem 52 59-123... 

Lundstrom CC, Shaw H, Ryerson F, Phinney D, Gill J, Williams Q (1994) Compositional controls on on the partitioning of U, Th, Ba, Pb, Sr and Zr between clinopyroxene and haplobasaltic melts implications for uranium series disequilibria in basalts. Earth Planet Sci Lett 128 407-423 Lnndstrom CC (2003) Uranium-series disequilibria in mid-ocean ridge basalts observations and models of basalt genesis. Rev Mineral Geochem 52 175-214... 

Several studies have examined the partitioning of U on particles and colloids. Results from detailed sampling and particle separation in the Amazon estuary shows that most of the uranium at the Amazon River mouth is associated with particles (>0.4 im) and that >90% of the uranium in filtered water (<0.4 im) is transported in a colloidal phases (from a nominal molecular weight of 10 000 MW up to 0.4 im) (Swarzenski et al. 1995 Moore et al. 1996). Mixing diagrams for uranium in different size fractions in the Amazon estuary reveal that uranium in all size fractions clearly display both removal and substantial input during mixing. 

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). 

The relative area of mine solid waste tails (per 100,000 M3 of rock mass) is 0.7-0.8 of the total area. On average, the disturbed areas of uranium ore exploration site are partitioned as follows 32.3% of disturbed land is occupied by dumps, 27.2%, by pits, 20.3%, by industrial areas, 13.3%, by tails, and about 10%, by other types of land disturbance. 

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