Comparison of Aromatic Dithiophosphinic and Phosphinic Acid Derivatives for Minor Actinide Extraction
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1104-NN04-03
Comparison of Aromatic Dithiophosphinic and Phosphinic Acid Derivatives for Minor Actinide Extraction Mason K. Harrup, Dean R. Peterman, Mitchell R. Greenhalgh, Thomas A. Luther, and John Klaehn Idaho National Laboratory, Idaho Falls, ID, 83415 ABSTRACT A new extractant for the separation of actinide(III) and lanthanide(III), bis(otrifluoromethylphenyl)phosphinic acid (O-PA) was synthesized. The synthetic route employed mirrors one that was employed to produce the sulfur containing analog bis(otrifluoromethylphenyl)dithiophosphinic acid (S-PA). Multinuclear NMR spectroscopy was used for elementary characterization of the new O-PA derivative. This new O-PA extractant was used to perform Am(III)/Eu(III) separations and the results were directly compared to those obtained in identical separation experiments using S-PA, an extractant that is known to exhibit separation factors of ~100,000 at low pH. The separations data are presented and discussed in terms comparing the nature of the oxygen atom as a donor to that of the sulfur atom in extractants that are otherwise identical. INTRODUCTION The coordination chemistry and extraction behavior of the actinides(III) and the lanthanides(III) has been extensively examined, yet is still not completely understood. It has been shown that ligands containing hard donor atoms (e.g., O) do not exhibit any selectivity between An(III) from Ln(III) [1]. However, the introduction of softer donor atoms (e.g., S) into similarly structured ligands does offer a differentiation between An(III) and Ln(III) coordination. Exploiting this difference in coordination behavior, various solvent extraction processes have been developed that utilize soft donor atoms to effect the separation of An(III) from Ln(III) [28]. However, the incorporation of soft donors alone does not ensure efficient separation of An(III) and Ln(III). A prime example of this is the direct comparison of the commercially available extractant Cyanex-301® (bis-(2,4,4-trimethylpentyl)dithiophosphinic acid) to bis(chlorophenyl)dithiophosphinic acid extractants (see Figure 1). Cyanex is an effective agent for the separation of An(III) from Ln(III) in acidic media, yet the chlorophenyl extractants alone do not show any appreciable lanthanide/actinide selectivity [9-11]. The chlorophenyl extractants only afford appreciable lanthanide/actinide selectivity upon the inclusion of oxygen-bearing organophosphorus synergists, such as trioctylphosphinoxide.
P
S
Cl
S
P SH
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Cl Cyanex 301®
Bis(chlorophenyl)dithiophosphinic Acid
Figure 1. Example dithiophosphinic acid actinide extractants.
Recently, new aromatic dithiophosphinic acids (R2PS2H; S-PA) derivatives were synthesized at the Idaho National Laboratory (INL) [12]. Using the INL extraction process, bis(o-trifluoromethylphenyl)dithiophosphinic acid (1; Figure 2) efficiently separates Am(III) directly from Eu(III) with separation factors of ~100,000 at low pH [13-14]. In order to better understand this unusually effective separation behavior, the dioxo analog of 1 h
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