Jian-Feng Chen

EXTRACTION OF AMERICIUM AND FISSION PRODUCT LANTHANIDES WITH CYANEX 272 AND CYANEX 301

ABSTRACT The extraction of tracer Am(III) with macro amounts of Ce(III), Pr(III), Nd(III), Sm(III) and Eu(III) has been studied using Cyanex 272 and Cyanex 301 in heptane. For extraction by Cyanex 272, the average separation factor of Am/Nd was 0.82, while that for Am/Eu was 0.088. However, for the Cyanex 301 extraction, separation factors of 810 for Am/Nd and 840 for Am/Eu were obtained. The separation values are not affected by the concentration of the extractant but decrease as the loading of the lanthanide increases in the organic phase. The hypersensitive absorption spectrum of the complex of Nd with Cyanex 301 in the heptane solvent indicates the formation of a Nd-S bond. The extractions by HDEHP under the same conditions are compared with that of Cyanex 272 and 301.

Complexation of americium with humic, fulvic and citric acids at high ionic strength

Summary The stability constants of the Am3+ complexes of humic, fulvic and citric acids (HA, FA and Cit) were determined as a function of ionic strength (NaCl) using a solvent extraction technique. At a HA degree of ionization of 0.7 for the carboxylate groups, the Am-HA binding constant, logβ1, varies from 8.3 at I = 0.2 m to 7.2 at I = 6.0 m. The logβ1 of Am-Cit varies from 5.9 at I = 0.3 m to 5.10 at I = 5.0 m. Comparison of the binding constants of Am with HA, FA and Cit shows that, at high ionic strength, logβ1(AmCit) ≈ logβ1(AmFA) ≈ logβ1AmHA-1. Comparison of the values of other metals indicate that the logβ1 values for Cit can serve to estimate HA and FA.

Thermodynamic modeling of actinide complexation with acetate and lactate at high ionic strength

The stability constants of NpO2+, UO22+ Am3+, and Th4+ with acetate and lactate anions has been measured in 0.3–5.0m NaCl media at 25°C by the solvent extraction technique. For the 1:1 complexation, the values of the stability constants increased in the order: NpO2+ < Am3+ < 22+ < Th4+, in accordance with the actinide charge density and reflecting the strongly ionic bonding of the complexes. The Pitzer ionic interaction parameters were calculated and used to estimate the thermodynamic stability constants at I = 0. Because our data were collected mainly in the high ionic strength region values of β(1) were estimated from values reported in the literature. For all stability constants the Pitzer model gives an excellent representation of the data using three interaction parameters β(0), β(1), and Cφ

Complexation of Am(III) by Oxalate in NaClO4 Media

The complexation of Am(III) by oxalate has been investigated in solutions of NaClO{sub 4} up to 9.0 M ionic strength at 25{degrees}C. The dissociation constants of oxalic acid were determined by potentiometric titration, while the stability constants of the Am(III)-oxalate complexation were measured by the solvent extraction technique. A thermodynamic model was constructed to predict the apparent equilibrium constants at different ionic strengths by applying the Pitzer equation using parameters for the Na{sup +}-HOx{sup -}, Na{sup +}-Ox{sup -}, AmOx{sup +}-ClO{sub 4}{sup -}, and Na{sup +}-Am(Ox){sub 2}{sup -} interactions obtained by fitting the data.

Thermodynamic modeling of actinide complexation with oxalate at high ionic strength

A geochemical model describing the solubility of actinides in underground water at the Waste Isolation Pilot Plant (WIPP) Project is under development. The database for this model consists of standard chemical potentials and Pitzer model parameters for hundreds of species that may be present in the WIPP disposal room. Organic ligands used in separation and decontamination processes may be present in the nuclear wastes placed in the WIPP site and could have a significant impact on mobile actinide concentrations. In this work the β1 and β2 stability constants of NpO2+, UO22+, Am3+ and Th4+ with the oxalate anion have been measured in 0.3–5.0M NaCl media at 25 °C by a solvent extraction technique. For the 1:1 complexation, the values of the stability constants increased in the order: NpO2+<Am3+<UO22+<Th4+, in accordance with the actinide charge density and reflecting the strongly ionic bonding of the complexes. The Pitzer ionic interaction model was used to model the data. Because the data were collected mainly in the high ionic strength region, values of β(1) were estimated from these plus literature values. The Pitzer model gives a good representation of the data using three interaction parameters β(0), β(1), and cφ.

Activity coefficients of single electrolytes in concentrated solutions derived from a quasi-lattice model

This paper describes a new model to calculate the mean activity coefficients of dissociated electrolytes in concentrated solutions. It is based on three assumptions: (i) a quasi-lattice arrangements of ions in solution; (ii) a contribution from ion-water interactions to the mean activity coefficients; (iii) a concentration dependence of the dielectric constant. The mean activity coefficients of thirteen strong electrolytes from moderately dilute solutions to saturated solutions are found to correlate well by this model. For dilute solutions, a limiting equation in which only ion-specific parameters are required is proposed. It is suggested that specific ionwater interactions might be the major source of the nonideality of strong electrolyte solutions at high concentrations.

Extraction and spectrophotometric determination of Nd(III), Th(IV) and U(VI) in synthetic brine using Chlorophosphonazo III

A simple method has been developed to determine micromolar concentrations of Nd(III), Th(IV) and U(VI) in brine. The method involves the extractive separation of these cations from brine matrix species by using Chlorophosphonazo III as extractant with 1-butanol as solvent, and the spectrophotometric determination of the concentration of extracted species in the butanol phase. The lowest concentrations that could be determined are estimated to be 5 × 10−7 M for Nd(III) and 2 × 10−7 M for TH(IV) and U(VI).

Solubility, dissociation and complexation with Nd(III) and Th(IV) of oxine, thenoyltrifluoroacetone and 1,10-phenanthroline in 5.0 m NaCl

Equilibria in the system of Nd(III) and Th(IV) with 8-hydroxyquinoline (oxine), thenoyltrifluoroacetone (HTTA) and 1,10-phenanthroline (phen) in 5.0 m NaCl solution have been investigated by spectroscopy and potentiometry. The solubility and deprotonation constants of the three organics were measured to be: pK(s) = 3.09 +/- 0.01, pK(a1) = 5.82 +/-0.02, pK(a2)= 10.00 +/-0.01 for oxine; pK(s) = 2.49 +/- 0.01, pK(a1) = 6.47 +/-0.03 for HTTA; pK(s) = 2.86 +/- 0.02, pK(a2) = 5.82 +/- 0.05 for phen. The stabilities of the corresponding metal complexes are in the order M(oxine) > M(TTA) > M(phen), where M = Nd(III), Th(IV). For all three organic ligands, the Th(IV) complexation is stronger than that of Nd(III).

Solubility of Nd in Brine

The solubility of Nd(III) has been measured in a synthetic brine at pcH 6.4, 8.4, 10.4 and 12.4. The brine consisted predominantly of (Na+K)Cl and MgCl{sub 2}, with an ionic strength of 7.8M (9.4m). The experimental solubility is much less than that estimated from modeling of the species in solution in equilibrium with the Nd solid using S.I.T. The predominant solid compound of Nd (III) at each pcH was determined from X-ray diffraction patterns.

Complexation and Ion Interactions in Am(III)/EDTA/NaCl Ternary System

The complexation of Am(III) by EDTA was investigated over a range of NaCl concentration from 0.30 to 5.0 m by the solvent extraction technique using HDEHP as extractant. The conditional stability constants for the complexation reaction: Am3+ + Y4− = AmY−, were determined to be: log cβ1(I) = 15.1 ± 0.11 (0.3 m), 13.96 ± 0.07 (1.0 m), 14.04 ± 0.09 (2.0 m), 13.76 ± 0.02 (3.0 m), 13.89 ± 0.03 (4.0 m), 14.38 ± 0.05 (5.0 m). The data were analyzed by the Pitzer model and the interaction parameters for the Am3+/Cl−, Na+/Y4−, Na+/HY3−, Na+/H2Y2−, Na+/H3Y−, Na+/AmY− ion pairs as well as the standard chemical potentials of the aqueous species involved were evaluated and are reported.