Nitrate complexes actinide

The metal ion extraction should increase with the increase in extractant concentration as well as with nitrate ion concentration. With the increasing concentration of nitric acid, beyond a point however, a decrease in metal ion extraction is observed, which is ascribed primarily to the (i) formation of anionic actinide complexes, and (ii) decrease of extractant concentration caused by nitric acid extractant complex formation. The latter is represented as ... 

Complexes of the Actinide(iv) Nitrates and Halides 11.6.1 Thorium Nitrate Complexes... 

For example, halide ions may form soluble actinide complexes without addition of nitric acid vapor. Preliminary tests have shown, however, that uranyl chloride, UO2CI2, is not soluble in either a 6-mole% potassium chioride/94-mole% potassium nitrate melt, or a 5-mole% sodium chloride/95-mole% sodium nitrate melt. Uranyl fluoride, UO2F2, was slightly soluble in a 9-mole% potassium fluoride/91-mole% potassium nitrate system. There was no evidence of solubility of uranyl fluoride in 3.5 mole% sodium fluoride/96.5-mole% sodium nitrate. 

The actinide nitrates whose structures are known are listed in Table 20.9, and a review of actinide complexes has been published by Casellato et al [413]. These are limited mainly to tetravalent Th and to uranyl complexes. In the Th(iv) and Pu(iv) compounds the NO3 ions are bidentate, which allows for large coordination numbers compared to the usual eight-fold coordination of these ions with monodentate ligands. In the uranyl complexes the two close actinyl oxygen atoms limit the available space for other bonded atoms, and the maximum number of equatorial oxygen atoms is six, even for bidentate NO3 ions. In Cs2lJ02(N03)4 all four NO3 ions are attached to the U atom even though space allows only two of them to be bidentate. 

In the case of the monofluorocomplexes of quadrivalent plutonium, it is obvious that the lower values obtained in chloride and nitrate media are due to complexing by these ions these results will not be discussed further. In HCIO4 media the data for the first two fluoride complexes are quite self-consistent and well within the same order of magnitude as these reported for the other quadrivalent actinides (12, 89). An extensive comparison would extend beyond the scope oT tKTs paper. In the case of PuF3+, extrapolation of bi to zero ionic strength is not warranted as such in view of the limited number of data. However, in the case of ThF3+ where the data extend over a very wide range of ionic... 

The apparent failure of trivalent and tetravalent cations to enter plants could result from the interaction of the cations with the phospholipids of the cell membranes. Evidence for such interactions is provided by the use of lanthanum nitrate as a stain for cell membranes (143) while thorium (IV) has been shown to form stable complexes with phospholipid micelles (144). However, it is possible that some plant species may possess ionophores specific to trivalent cations. Thomas (145) has shown that trees such as mockernut hickory can accumulate lanthanides. The proof of the existence of such ionophores in these trees may facilitate the development of safeguards to ensure that the actinides are not readily transported from soil to plants. 

The SETFICS process (Solvent Extraction for Trivalent /-elements Intragroup Separation in CMPO-Complexant System) was initially proposed by research teams of the former Japan Nuclear Cycle Development Institute (JNC, today JAEA) to separate An(III) from PUREX raffinates. It uses a TRUEX solvent (composed of CMPO and TBP, respectively dissolved at 0.2 and 1.2 M in -dodecane) to coextract trivalent actinides and lanthanides, and a sodium nitrate concentrated solution (4 M NaN03) containing DTPA (0.05 M) to selectively strip the TPEs at pH 2 and keep the Ln(III) extracted by the TRUEX solvent (239). However, the DFs for heavy Ln(III) are rather poor. An optimized version of the SETFICS process has recently been proposed as an alternative process to extraction chromatography for the recovery of Am(III) and Cm(III) in the New Extraction System for TRU Recovery (NEXT) process. NEXT basically consists of a front-end crystallization of uranium, a simplified PUREX process using TBP for the recovery of U, Np, and Pu, and a back-end Am(III) + Cm(III) recovery step (240, 241). 

There are a great many actinide nitrate complexes, many of which are important in separation procedures whereby the elements are extracted from aqueous nitric acid into nonpolar solvents. Of these, the U02+ complexes are best characterized they are typically 8-coordinate with two bidentate N03" ions and two neutral ligands (H20, THF, DMSO, and R3PO) forming a distorted equatorial hexagon. 

The mixture is extracted with a counter-current of a solution of TBP in kerosene. Uranium and plutonium are extracted into kerosene as the complexes [U02(N03)2(tbp)2] and [Pu(N03)4(tbp)2], but the other nitrates, of metals such as the lanthanides and actinides beyond Pu, as well as fission products, do not form strong complexes with TBP and stay in the aqueous layer. 

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