Sergey I. Sinkov

Solvent Extraction Behavior of Plutonium (IV) Ions in the Presence of Simple Hydroxamic Acids

Abstract Formo‐ and aceto‐hydroxamic acids are very effective reagents for stripping Pu(IV) ions from a tri‐butyl phosphate phase into nitric acid. Distribution data for Pu(IV) in the presence of these hydroxamate ions have been obtained and trends established. The affinity of aceto‐hydroxamic acid for Pu(IV) ions and its selectivity over U(VI) ions is demonstrated by the values of the stability constants in HClO4. These data support the applications of simple hydroxamic acids in advanced Purex‐type solvent extraction systems.

The role of carboxylic acids in TALSQuEAK separations

Recent reports have indicated that Trivalent Actinide–Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes (TALSPEAK)-type separations chemistry can be improved through the replacement of bis-2-ethyl(hexyl) phosphoric acid (HDEHP) and diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) with the weaker reagents 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA), respectively. This modified TALSPEAK has been provided with an adjusted acronym of TALSQuEAK (Trivalent Actinide–Lanthanide Separation using Quicker Extractants and Aqueous Komplexes). Among several benefits, TALSQuEAK chemistry provides more rapid phase transfer kinetics, is less reliant on carboxylic acids to mediate lanthanide extraction, and allows a simplified thermodynamic description of the separations process that generally requires only parameters available in the literature to describe metal transfer. This article focuses on the role of carboxylic acids in aqueous ternary (M-HEDTA-carboxylate) complexes, americium/lanthanide separations, and extraction kinetics. Spectrophotometry (UV-Vis) of the Nd3+ hypersensitive band indicates the presence of aqueous ternary Nd–Lac–HEDTA species (Lac = lactate, K 111 = 1.83 ± 0.01 at 1.0 mol L−1 ionic strength, Nd(HEDTA) + Lac− ⇄ Nd(HEDTA)Lac−). While lower levels (0.1 mol L−1 vs. 1.0 mol L−1) of carboxylic acid will still be necessary to control pH and encourage phase transfer of the heavier lanthanides, application of different carboxylic acids does not have an overwhelming impact on Ln/Am separations or extraction kinetics relative to conventional TALSPEAK separations. TALSQuEAK separations come to equilibrium in two to five minutes depending on the system pH using only 0.1 mol L−1 total lactate or citrate.

Solvent Extraction Behavior of Neptunium (IV) Ions between Nitric Acid and Diluted 30% Tri‐butyl Phosphate in the Presence of Simple Hydroxamic Acids

Abstract Formo‐ and aceto‐hydroxamic acids are very effective reagents for stripping tetravalent actinide ions such as Np(IV) and Pu(IV) ions from a tri‐butyl phosphate phase into nitric acid. Distribution data for Np(IV) in the presence of these hydroxamate ions have now been accumulated and trends established. Stability constants for aceto‐hydroxamate complexes of Np(IV) and Np(V) ions have also been determined in a perchlorate medium, and these reaffirm the affinity of hydroxamate ligands for actinide (IV) ions over actinyl (V,VI) ions.

Lipophilic ternary complexes in liquid–liquid extraction of trivalent lanthanides

The formation of ternary complexes between lanthanide ions [Nd(III) or Eu(III)], octyl(phenyl)-N,N-diisobutyl-carbamoylmethylphosphine oxide (CMPO), and bis-(2-ethylhexyl)phosphoric acid (HDEHP) was probed by liquid–liquid extraction and spectroscopic techniques. Equilibrium modeling of data for the extraction of Nd(III) or Eu(III) from lactic acid media into n-dodecane solutions of CMPO and HDEHP indicates the predominant extracted species are of the type [Ln(AHA)2(A)] and [Ln(CMPO)(AHA)2(A)], where Ln = Nd or Eu and A represents the DEHP− anion. FTIR (for both Eu and Nd) and visible spectrophotometry (in the case of Nd) indicate the formation of the [Ln(CMPO)(A)3] complexes when CMPO is added to n-dodecane solutions of the LnA3 compounds. Both techniques indicate a stronger propensity of CMPO to complex Nd(III) versus Eu(III).

Neodymium(III) Complexes of Dialkylphosphoric and Dialkylphosphonic Acids Relevant to Liquid–Liquid Extraction Systems

The complexes formed during the extraction of neodymium(III) into hydrophobic solvents containing acidic organophosphorus extractants were probed by single-crystal X-ray diffractometry, visible spectrophotometry, and Fourier-transform infrared spectroscopy. The crystal structure of the compound Nd(DMP)3 (1, DMP = dimethyl phosphate) revealed a polymeric arrangement in which each Nd(III) center is surrounded by six DMP oxygen atoms in a pseudo-octahedral environment. Adjacent Nd(III) ions are bridged by (MeO)2POO(-) anions, forming the polymeric network. The diffuse reflectance visible spectrum of 1 is nearly identical to that of the solid that is formed when an n-dodecane solution of di(2-ethylhexyl)phosphoric acid (HA) is saturated with Nd(III), indicating a similar coordination environment around the Nd center in the NdA3 solid. The visible spectrum of the HA solution fully loaded with Nd(III) is very similar to that of the NdA3 material, both displaying hypersensitive bands characteristic of an pseudo-octahedral coordination environment around Nd. These spectral characteristics persisted across a wide range of organic Nd concentrations, suggesting that the pseudo-octahedral coordination environment is maintained from dilute to saturated conditions.

Americium(III) oxidation by copper(III) periodate in nitric acid solution as compared with the action of Bi(V) compounds of sodium, lithium, and potassium

Abstract The oxidative action of a Cu(III) periodate compound toward Am(III) in nitric acid was studied. The extent of oxidation of Am(III) to Am(VI) was investigated using a constant initial Cu(III)-to-Am(III) molar ratio of 10:1 and varying nitric acid concentrations from 0.25 to 3.5 mol/L. From 0.25 to 3 mol/L HNO3, more than 98% of the Am(III) was oxidized to Am(VI); however, at 3.5 mol/L HNO3, the conversion to Am(VI) was only 80%. Increasing the Cu(III)-to-Am(III) molar ratio to 20:1 in 3.5 mol/L HNO3 resulted in 98% conversion to Am(VI). For comparison, oxidation of Am(III) with NaBiO3 was studied at 3.5 mol/L HNO3 and the same stoichiometric excess of Bi(V) oxidant over Am(III) (stoichiometric ratio of 3.33:1). With NaBiO3, the extent of Am(III) conversion to Am(VI) was only 19%, while with the Cu(III) compound this value was found to be about 4 times higher under otherwise identical conditions. Similar results were obtained with other Bi(V) salts. These results show that the Cu(III) periodate compound is a superior oxidant to NaBiO3, yielding rapid conversion to Am(VI) in a homogeneous acidic solution, and is, therefore, an excellent candidate for further development of Am separation systems.

An Advanced TALSPEAK Concept for Separating Minor Actinides. Part 1. Process Optimization and Flowsheet Development

ABSTRACT A solvent extraction system was developed for separating trivalent actinides from lanthanides. This “Advanced TALSPEAK” system uses 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) to extract the lanthanides into an n-dodecane-based solvent; the actinides are retained in a citrate-buffered aqueous phase by complexation to a polyaminocarboxylate ligand. Several aqueous-phase ligands were investigated, and N-(2-hydroxyethyl)ethylenediamine-N,N’,N’-triacetic acid (HEDTA) was chosen for further study. Batch distribution measurements indicate that the separation of americium (Am) from the light lanthanides increases as the pH increases. However, previous investigations indicated that the extraction rates for the heavier lanthanides decrease with increasing pH. Therefore, a balance between these competing effects is required. An aqueous phase at pH 2.6 was chosen for further process development, because this offered optimal separation. Centrifugal-contactor single-stage efficiencies were measured to characterize the system’s performance under flow conditions, and an Advanced TALSPEAK flowsheet was designed.

Development of a Chemistry-Based, Predictive Method for Determining the Amount of Non-Pertechnetate Technetium in the Hanford Tanks: FY 2012 Progress Report

This report describes investigations directed toward understanding the extent of the presence of highly alkaline soluble, non-pertechnetate technetium (n-Tc) in the Hanford Tank supernatants. The goals of this report are to: a) present a review of the available literature relevant to the speciation of technetium in the Hanford tank supernatants, b) attempt to establish a chemically logical correlation between available Hanford tank measurements and the presence of supernatant soluble n-Tc, c) use existing measurement data to estimate the amount of n-Tc in the Hanford tank supernatants, and d) report on any likely, process-friendly methods to eventually sequester soluble n-Tc from Hanford tank supernatants.

Multivariate Analysis for Quantification of Plutonium(IV) in Nitric Acid Based on Absorption Spectra

Development of more effective, reliable, and fast methods for monitoring process streams is a growing opportunity for analytical applications. Many fields can benefit from online monitoring, including the nuclear fuel cycle where improved methods for monitoring radioactive materials will facilitate maintenance of proper safeguards and ensure safe and efficient processing of materials. Online process monitoring with a focus on optical spectroscopy can provide a fast, nondestructive method for monitoring chemical species. However, identification and quantification of species can be hindered by the complexity of the solutions if bands overlap or show condition-dependent spectral features. Plutonium(IV) is one example of a species which displays significant spectral variation with changing nitric acid concentration. Single variate analysis (i.e., Beers Law) is difficult to apply to the quantification of Pu(IV) unless the nitric acid concentration is known and separate calibration curves have been made for all possible acid strengths. Multivariate or chemometric analysis is an approach that allows for the accurate quantification of Pu(IV) without a priori knowledge of nitric acid concentration.

Uranium Metal Analysis via Selective Dissolution

Uranium metal, which is present in sludge held in the Hanford Site K West Basin, can create hazardous hydrogen atmospheres during sludge handling, immobilization, or subsequent transport and storage operations by its oxidation/corrosion in water. A thorough knowledge of the uranium metal concentration in sludge therefore is essential to successful sludge management and waste process design. The goal of this work was to establish a rapid routine analytical method to determine uranium metal concentrations as low as 0.03 wt% in sludge even in the presence of up to 1000-fold higher total uranium concentrations (i.e., up to 30 wt% and more uranium) for samples to be taken during the upcoming sludge characterization campaign and in future analyses for sludge handling and processing. This report describes the experiments and results obtained in developing the selective dissolution technique to determine uranium metal concentration in K Basin sludge.