Actinides bidentate extraction

Recovery of actinides at the RFP with an organic phosphorous bidentate extractant has been proposed. A conceptual production flow sheet is shown in Figure 3. The bidentate reagent, dihexyl-N, N-diethylcarbamoylmethylenephosphonate (DHDECMP), is especially attractive since it can recover actinides from MSE residues containing aluminum. The cation exchange process is unable to effect actinide purification when aluminum is present. (DHDECMP extracts actinides and lanthanides, but does not extract common RFP contaminants, e.g., aluminum. No lanthanides are used in process streams at RFP.)... 

Upon entry into the WTF, the HLLW is contacted (see Figure l) with the bidentate extractant dihexyl-N,N-diethylcarbamylmethylene phosphonate (CMP) which is diluted to 30 vol with di-isopropyl-benzene. Under high acid conditions (l3.,l ), this extractant will remove all trivalent and tetravalent actinides to some degree from the HLLW. Of the fission products, only Zr, Tc, Ru, Mo, Nb, Y, and Pd extract to any significant extent. These species can be controlled either by subsequent scrubbing of the organic phase or by adjusting extraction conditions so as to reject those fission products that exist as anions to the raffinate. 

Siddall investigated the ability of these compounds to extract Am(III), Ce(III), Pm(III), and HN03 from 0.1 to 12M HN03 solutions. His favorable results led him to suggest such bidentate extractants could be used to remove trivalent actinides from high-level TBP extraction process waste. This idea was later patented (5). Schulz demonstrated in 1973 that DHDECMP was an effective extractant for americium and plutonium from radioactive Hanford generated wastes (6). 

Two techniques appear to be useful for the bidentate extraction of actinides. The first is liquid-liquid solvent extraction, a method which has several advantages. Currently, however, the type of equipment needed (mixer-settlers, centrifugal contactors, etc.) is not available at RFP. We are better equipped to use a column technique. This can be done by sorbing the bidentate extractant on an inert solid support, loading ion exchange columns with the sorbent, and proceeding with column runs (extraction chromatography). 

Since TBP leaves Am(iii) behind in an addic raffinate, an effective extractant is needed to remove Am(iii) from such media. Siddall found that bidentate extractants have this desirable property [44]. Schulz [45] and Mclsaac [46] at the Hanford and Idaho Falls sites, respectively, have recently revived interest in plant-scale application of neutral bidentate organophosphorus extractants, particularly dihexyl-iV,iV-diethylcarbamoyl methylenephosphonate (DHDECMP) and dibutyl-iV,iV-diethylcarbamoyl methylenephosphonate (DBDECMP). The binding of actinide ions to DHDECMP has not been eluddated fully. Horwitz [335] states that in perchloric add the extractant is... 

Sasaki, Y., Umetani, S. 2006. Comparison of four bidentate phosphoric and diamide compounds for the extractability of actinides. Journal of Nuclear Science and Technology 43(7) 794-797. 

Smirnov, I.V. 2007. Anomalous effects in extraction of lanthanides and actinides with bidentate neutral organophosphorous extractants. Role of proton hydrate solvates. Radiochemistry 47(1) 44—54. 

Mclsaac, L. D. and Schulz, W. W. Removal of Actinides from Nuclear Fuel Reprocessing Waste Solution with Bidentate Organophosphorus Extractants in "Transplutonium 1975,"... 

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 triacetate uranyl complex (24) is structurally similar to the trinitrate uranyl complex (6) (three bidentate ligands arranged equa-torially around the uranyl O—U—O axis). It was expected that the visible, near ultraviolet spectrum of the triacetate uranyl complex would be similar to the spectrum of U02(N03)3 as are the spectra of 1102(804)3, 1102(003)3 , and other uranyl complexes which apparently have the same structure (17). The absorption spectrum of uranyl acetate extracted into tri-n-octylamine in xylene from dilute acetic acid is different from the trinitrato uranyl spectrum. This indicates that the triacetate uranyl complex is probably not the species involved. By analogy to the uranyl nitrate system 14), formation of a tetraacetate uranyl complex might be expected. The purpose of this work is to determine the nature of the anionic hexavalent actinide acetate complexes and to identify the species involved in the amine extraction and anion exchange of the hexavalent actinides from acetate systems. 

Polyfunctional phosphinopyridine VV,P-dioxides, (phosphinomethyl)pyridine A,P-dioxides and bis(phosphinomethyl)pyridine VV,P,P-trioxides have been prepared, and selected coordination chemistry with actinide ions has been explored. The phosphinopyridine A,P-dioxides form bidentate chelates with uranyl and Th , and in the solid state these complexes display six-membered chelate rings that appear to be relatively sterically congested." " The solvent extraction properties of these ligands are not unique since they resemble the performance of trialkylphosphine oxides." ... 

XAFS data for complexes of Cm with the HC301 extractant indicate only sulfur donation to the metal in the inner sphere of coordination. HC301 forms 3 1 complexes with the trivalent actinides and are coordinated in a bidentate mode as seen in Figure 91. Data indicates a hexacoordinate structure that resembles Z>3 symmetry in lanthanide dithiophosphinic acid complexes. 

Although a variety of extractants have been studied and tested for their suitability in analytical separations of actinides, perhaps the most recent milestone in the process use of actinide extractants is the application of the bidentate carbamoylmethyl-phosphonates (17,18,19). The pioneering work on these compounds was done 15 years ago by Siddall (20,21). Their principal advantage is their ability to extract the trivalent, tetravalent, and hexavalent actinides from considerably more concentrated nitric acid media than other extractants. 

The trisolvate of the trisnitrato complex of the trivalent metal ion is extracted into the organic phase (yielding a formally nine-coordinate hydrophobic complex assuming nitrate is bidentate). Unlike the extraction of the tetravalent and hexavalent actinides, extremely high concentrations of nitric acid are required for even moderate extraction of the trivalent metal ions. Greater efficiency is observed for this system if alkali-metal salts are used as the aqueous medium as discussed below. 

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