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A thermodynamic model of the distribution of Pu(IV) between aqueous solutions of nitric acid and lithium nitrate and 30 % (by volume) TBP in n-dodecane was developed. The mean activity coefficients of the hydrogen ion, nitrate ion, and water were calculated using Bromley's method of activity coefficients. The computation of the distribution ratios is based on a critical evaluation of the speciation of Pu(IV) under the solution conditions used. Five Pu(IV) species, Pu 4+ , Pu(OH) 3+ , Pu(OH) 2 2+ , Pu(NO 3 ) 3+ , and Pu(NO 3 ) 2 2+ , were considered to be present in (0.1 to 4) mol • L -1 aqueous solutions of HNO 3 . Because of the various extraction capabilities of the different oxidation states of Pu, disproportionation of Pu(IV) is the main factor controlling the distribution of Pu at low acidity. Two different Pu(IV) solvate adducts Pu(NO 3 ) 4 • TBP 2 and Pu(NO 3 ) 4 • TBP 2 • HNO 3 were considered as extracted species over a wide range of experimental conditions, and their extractions constants were determined. The correlation between experimental and calculated data produced a reasonable fit. To determine the extraction constant of hydrolyzed Pu(IV) species for low acid concentrations, additional experimental data on the kinetics of disproportionation of tetravalent plutonium in two phase systems would be necessary.

The separation of adjacent lanthanides continues to be a challenge worldwide because of the similar physical and chemical properties of these elements and a necessity to advance the use of clean-energy applications. Herein, a systematic structure−performance relationship approach toward understanding the effect of N-alkyl group characteristics in diglycolamides (DGAs) on the separation of lanthanides(III) from a hydrochloric acid medium is presented. In addition to the three most extensively studied DGA complexants [N,N,N′,N′-tetra(n-octyl)diglycolamide, TODGA; N,N,N′,N′-tetra(2-ethylhexyl)diglycolamide, TEHDGA; N,N′-dimethyl-N,N′-di(n-octyl)diglycolamide, DMDODGA], 12 new extracting agents with varying substitution patterns were designed to study the interplay of steric and electronic effects that control rare-earth element extraction. Subtle changes in the structure around diglycolamide carbonyl oxygen atoms result in dramatic shifts in the lanthanide extraction strength and selectivity. The effects of the chain length and branching position of N-alkyl substituents in DGAs are elaborated on with the use of experimental, computational, and solutionstructure characterization techniques.

Aqueous complexation of Th(IV), U(IV), Np(IV), Pu(III/IV), and Ce(III/IV) with DTPA was studied by potentiometry, absorption spectrophotometry, and cyclic voltammetry at 1 M ionic strength and 25 °C. The stability constants for the 1:1 complex of each trivalent and tetravalent metal were calculated. From the potentiometric data, we report stability constant values for Ce(III)DTPA, Ce(III)HDTPA, and Th(IV)DTPA of log β(101) = 20.01 ± 0.02, log β(111) = 22.0 ± 0.2, and log β(101) = 29.6 ± 1, respectively. From the absorption spectrophotometry data, we report stability constant values for U(IV)DTPA, Np(IV)DTPA, and Pu(IV)DTPA of log β(101) = 31.8 ± 0.1, 32.3 ± 0.1, and 33.67 ± 0.02, respectively. From the cyclic voltammetry data, we report stability constant values for Ce(IV) and Pu(III) of log β(101) = 34.04 ± 0.04 and 20.58 ± 0.04, respectively. The values obtained in this work are compared and discussed with respect to the ionic radius of each cationic metal.

The aqueous complexation of Nd(III) and Am(III) with anions of citrate was studied by potentiometry, absorption spectrophotometry, microcalorimetry, and X-ray absorption fine structure (XAFS). Using potentiometric titration data fitting the metal-ligand (L) complexes that were identified for Nd(III) were NdHL, NdL, NdHL2, and NdL2; a review of trivalent metal-citrate complexes is also included. Stability constants for these complexes were calculated from potentiometric and spectrophotometric titrations. Microcalorimetric results concluded that the entropy term of complex formation is much more dominant than the enthalpy. XAFS results showed a dependence in the Debye-Waller factor that indicated Nd(iii)-citrate complexation over the pH range of 1.56-6.12.

The complexation of HDEHP, HEH[EHP], T2EHDGA, and TODGA and mixtures thereof in n-dodecane with Nd(III), Pm(III), and Am(III), extracted from pH 3 aqueous media, were analyzed by distribution analysis, IR, UV−vis, and X-ray spectroscopies. Appreciable ternary complex formation is seen among HDEHP with DGA components and to a lesser extent with HEH[EHP]. Ternary complex formation constants were determined by absorption spectroscopy and slope analysis. The effect of the adduct formation on the process performance and on the Ln/An selectivity is discussed. It has been shown that the ternary complexes have the tendency to extract larger trivalent cations as Am and Nd over Eu, thus diminishing the overall Ln/Am selectivity of the process. Also, previously unreported extraction constants were derived for HEH [EHP] with Am and Pm as well as coordination information for Nd-HEH[EHP] and Nd-T2EHDGA complexes.