Mechanism for Solvent Extraction of Lanthanides from Chloride Media by Basic Extractants

The solvent extraction of lanthanides from chloride media to an organic phase containing an anion exchanger in the chloride form is known to show low extraction percentages and small separation factors. The coordination chemistry of the lanthanides in combination with this kind of extractant is poorly understood. Previous work has mainly used solvent extraction based techniques (slope analysis, fittings of the extraction curves) to derive the extraction mechanism of lanthanides from chloride media. In this paper, EXAFS spectra, luminescence lifetimes, excitation and emission spectra, and organic phase loadings of lanthanides in dry, water-saturated and diluted Aliquat 336 chloride or Cyphos IL 101 have been measured. The data show the formation of the hydrated lanthanide ion [Ln(H₂O)_8–9]³⁺ in undiluted and diluted Aliquat 336 and the complex [LnCl₆]^3? in dry Aliquat 336. The presence of the same species [Ln(H₂O)_8–9]³⁺ in the aqueous and in the organic phase explains the small separation factors and the poor selectivities for the separation of mixtures of lanthanides. Changes in separation factors with increasing chloride concentrations can be explained by changes in stability of the lanthanide chloro complexes in the aqueous phase, in combination with the extraction of the hydrated lanthanide ion to the organic phase. Finally, it is shown that the organic phase can be loaded with 107 g·L^?1 of Nd(III) under the optimal conditions.     

Solvent extraction of uranium(VI) by p-tert-butylcalix[n]-arene acetate

The solvent extraction of U(VI) by p-tert-butylcalix[n]-arene acetate (H _n L) (n=4, 6, 8) has been studied. The effects of acidity in aqueous phase and concentration of extractant in organic phase on the distribution ratio were examined. It has been found that the distribution ratio is proportional to [H⁺]⁻² and [H _n L]_(O) and the extracted complex species is UO₂H _n ⁻²L. The equilibrium constants of the extraction reactions have been determined. The reaction mechanism is discussed.     

Die Verteilung von Uran(IV) zwischen wässriger Salpetersäure und organischen Lösungen des Nitratsalzes des sekundären Amins Amberlite LA-1

The nitrate salt solution of the secondary amine Amberlite LA-1 in organic solvents extracts uranium(IV) from aqueous nitric acid solutions. The distribution ratio of uranium(IV) reaches a maximum at an equilibrium nitric acid concentration of 8.5M in the aqueous phase. Addition of n-octanol to the organic phase decreases, and the addition of nitrate to the aqueous phase increases the uranium(IV) distribution ratio. The extraction of uranium(IV) is fast and the equilibrium distribution is reached within less than one minute. At low uranium(IV) concentrations (<6·10⁻³ M) the distribution ratio is independent of the uranium(IV) concentration. At high uranium(IV) loadings of the organic phase an extrapolation gives a mole ratio of amine: uranium(IV)=2∶1. A double logarithmic plot of the dependence of the uranium(IV) distribution ratio vs. the LA-1 concentration in the organic phase gives a curve with a slope of two when polar diluents for LA-1 are used. This slope of two and the shapes of the absorption spectra of the organic phase from 400 to 700 nm make it very probable that uranium(IV) exists in the organic phase as a hexanitrato complex.      

Investigation of Uranium(VI) Extraction Mechanisms from Phosphoric and Sulfuric Media by 31P-NMR

Uranium extraction using DEHCNPB (butyl-1-[N,N-bis(2-ethylhexyl)carbamoyl]nonyl phosphonic acid, a bifunctional cationic extractant) has been studied to better understand mechanism differences depending on the original acidic solution (phosphoric or sulfuric). Solvent extraction batch experiments were carried out and the organic phases were probed using ³¹P-NMR. This technique enabled to demonstrate that phosphoric acid is poorly extracted by DEHCNPB ([H₃PO₄]_org < 2mM), using direct quantification in the organic phase by ³¹P-NMR spectra integration. Moreover, in the presence of uranium in the initial phosphoric acid solution, uranyl extraction by DEHCNPB competes with H₃PO₄ extraction.Average stoichiometries of U(VI)-DEHCNPB complexes in organic phases were also determined using slope analysis on uranium distribution data. Uranium seems to be extracted from a phosphoric medium by two extractant molecules, whereas more than three DEHCNPB on average would be necessary to extract uranium from a sulfuric medium. Thus, uranium is extracted according to different mechanisms depending on the nature of the initial solution.     

Solvent extraction of thorium(IV) with dibutyldithiophosphoric acid in various organic solvents

The extraction of thorium(IV) from perchlorate solutions with di-n-butyldithiophosphoric acid (HBudtp) in various organic solvents occurs through an ion exchange mechanism. The extracted species in the organic phase is an eight-coordinate complex Th(Budtp)₄. The higher values of the distribution ratio obtained in HBudtp-benzene-water system than in HBudtp-n-butanol-water system are explained by higher solubility of the complex species in nonpolar solvents. The position of the extraction curves in the pH-range lower than 0.7 reduces the complexation of thorium(IV) with Budtp^– in the aqueous phase and also the hydrolysis process.     

Liquid-liquid extraction of protactinium(V) using tri-iso-octylamine

Solvent extraction of protactinium with tri-iso-octyl-amine (TIOA) in xylene, benzene, carbon tetrachloride and chloroform from HCl, HF, HNO₃, HClO₄ and H₂SO₄ media was studied using ²³³Pa as a radiotracer. The extraction efficiencies of protactinium were determined as a function of shaking time, concentrations of mineral acids in aqueous phase, extractant concentrations and diluents in organic phase. The extraction mechanism was discussed. The results show that the extracted species in the organic phase is [(R₃N⁻H)_nPa(OH)_xCl _y ^5−x−y ].     

Complexation of plutonium(IV) with sulfate at variable temperatures

The complexation of plutonium(IV) with sulfate at variable temperatures has been investigated by solvent extraction method. A NaBrO₃ solution was used as holding oxidant to maintain the plutonium(IV) oxidation state throughout the experiments. The distribution ratio of Pu(IV) between the organic and aqueous phases was found to decrease as the concentrations of sulfate were increased. Stability constants of the 1:1 and 1:2 Pu(IV)-HSO₄ ⁻ complexes, dominant in the aqueous phase, were calculated from the effect of [HSO₄ ⁻] on the distribution ratio. The enthalpy and entropy of complexation were calculated from the stability constants at different temperatures using the Van’t Hoff equation.     

Thermodynamics of neptunium(V) complexes with phosphate at elevated temperatures

The complexation of Np(V) with phosphate at elevated temperatures was studied by a synergistic extraction method. A mixed buffer solution of TRIS and MES was used to maintain an appropriate pH value during the distribution experiments. The distribution ratio of Np(V) between the organic and aqueous phases was found to decrease as the concentrations of phosphate were increased. Stability constants of the 1:1 and 1:2 Np(V)-HPO₄ ²⁻ complexes, dominant in the aqueous phase under the experimental conditions, were calculated from the effect of [HPO₄ ²⁻] on the distribution ratio. The thermodynamic parameters including enthalpy and entropy of complexation between Np(V) and HPO₄ ²⁻ at 25 °C–55 °C were calculated by the temperature coefficient method.     

On the mechanism of trivalent actinide extraction in the system HDEHP—Lactic acid with DTPA

The dependence of the distribution coefficients on the hydrogen ion concentration and HDEHP concentration in the process of extraction from lactic acid solutions in the presence of DTPA and without it has been investigated. It has been shown that when the lactate ion concentration is higher than 0.01M, Cm is extracted predominantly as the first lactate complex. The presence of DTPA does not change the extraction mechanism. Similarity of the observed regularities for trivalent actinides and lanthanides is confirmed by the extraction of Bk(III), Ce(III), and Eu(III). It is suggested that the nature of carbonic acid used in TALSPEAK-process greatly influences the efficiency of group separation of TPE and lanthanides. It is supported by some experimental data. The extraction and stability constants of the first lactate complex (Cm Lact²⁺) have been found to be: K_e=2, β ₁ ⁰ = 7,7 · 10³. In the practical lactic acid concentration range (1 M) the unextractable complex, Cm Lact₃, is also formed in the aqueous phase. The stability constant of this complex has been found to be β ₃ ⁰ = 1.2 · 10⁷.     

Extraction of sulfuric acid with TOPO

A study on solvent extraction of sulfuric acid by tri-octylphosphine oxide (TOPO) in n-heptane has been made. Extraction coefficients of H₂SO₄ as a function of H₂SO₄ concentration in aqueous phase, and extractant concentrations in organic phase have been studied. The composition of extracted species, equilibrium constants of extraction reaction have been evaluated. These results are important for interpreting extraction equilibrium data of uranium(VI) or other metal ions with TOPO in sulfuric acid media.     

Solvent extraction studies of plutonium(IV) and americium(III) in room temperature ionic liquid (RTIL) by di-2-ethyl hexyl phosphoric acid (HDEHP) as extractant

Solvent extraction of Pu(IV) and Am(III) from aqueous nitric acid into room temperature ionic liquid (RTIL) by an acidic extractant HDEHP (di-2-ethyl hexyl phosphoric acid) was carried out. The D values indicated substantial extraction for Pu(IV) and poor extraction for Am(III) at 1M aqueous nitric acid concentration. However at lower aqueous nitric acid concentrations (pH 3), the Am(III) extraction was found to be quantitative. The least squares analysis of the extraction data for both the actinides ascertained the stoichiometry of the extracted species in the RTIL phase for Pu(IV) and Am(III) as [PuH(DEHP)₂]³⁺, AmH(DEHP)²⁺. From the D values at two temperatures, the thermodynamic parameters of the extraction reaction for Pu(IV) was calculated.     

Solvent extraction of chromium(VI) from base metal ions with diphenyl-2-pyridylmethane as a liquid anion exchanger

The extraction of chromium(VI) from aqueous hydrochloric, nitric and sulfuric acid solutions by diphenyl-2-pyridylmethane(DPPM) dissolved in chloroform has been studied. Chromium(VI) is quantitatively extracted from hydrochloric acid solutions in the range 0.1–1M. With increasing acid concentration, the extraction of chromium diminishes and in concentrated acid solutions practically all the chromium remains in the aqueous phase. The quantitative back-extraction of chromium from the organic phase is possible with HCl or HNO₃ at concentrations higher than 5M through the use of reducing agents. The composition of the extracted chromium(VI) species was studied in solution. The complexes (DPPMH)⁺HCrO ₄ ⁻ and (DPPMH)₂Cr₂O ₇ ⁻ are extracted for tracer and macro amounts of chromium(VI) respectively. The data have been utilized for the separation of chromium(VI) from base metal ions.     

Étude par spectrométrie raman laser de l'extraction liquide-liquide du FeCl3 par le chlorhydrate de trilaurylamine

Raman spectra of aqueous 1 M chloroferrate(III) solutions indicate the presence of FeCl₃ and FeCl₄^- in proportion to the hydrochloric acid concentration, but FeCl₄^- is only present in solutions of acidity higher than 5 M HCl. The liquid-liquid extraction of the aqueous chloroferrate phases ( 1 M HCl) by trilaurylamine hydrochloride in cyclohexane, shows that iron(III) is extracted only in its FeCl₄^- form, even though this species does not appear in the aqueous solutions. A quantitative spectroscopic study of the organic phases by means of the v₁ line of FeCl₄^- at 332 cm^-1 leads to the following conclusions: (a) the plot of the scattering coefficient of the 332 cm^-1 line versus iron(III) concentration shows the presence of two complex species in the organic phase; (b) the distribution curves of the complex species could be calculated. On the basis of the Raman results, an ion-pair type compound is proposed for the two complexes.     

On the influence of some inorganic salts on the partitioning of citric acid between water and organic solutions of tri-n-octylamine: Part I: Methyl isobutyl ketone as organic solvent

Reactive extraction is a commonly applied process to recover carboxylic acids from aqueous solutions. Such processes are nowadays designed using process simulation software. However, the essential prerequisite for such a simulation is the availability of a reliable thermodynamic model for the encountered phase equilibrium. Industrial experience revealed that even very small amounts of a strong electrolyte (e.g., sodium chloride) can considerably reduce the amount of carboxylic acid extracted from the aqueous into the organic phase. This contribution presents new experimental results for the influence of sodium nitrate, sodium chloride, sodium sulfate, sodium citrate and hydrochloric acid on the partitioning of citric acid to the coexisting aqueous/organic liquid phases of the system water + methyl isobutyl ketone (organic solvent) + tri-n-octylamine (chemical extractant) at 25 °C. A detailed discussion of the experimental results reveals that the dramatic decrease of the partition coefficient of carboxylic acid is caused by the chemical loading of the extractant by the inorganic acid, i.e. both acids (the weak carboxylic acid as well as the strong inorganic acid) compete for the sodium ions (in the aqueous phase) and for the amine (in the organic phase). In phase equilibrium the amine is predominantly loaded with the inorganic acid while the sodium salt of the carboxylic acid remains in the aqueous phase. That behavior is described by a thermodynamic framework that is able to predict the complex liquid–liquid equilibrium from information determined exclusively from investigations on subsystems.     

Die Verteilung von Uran(IV) zwischen unverdünntem Tri-n-butylphosphat und wäßrigen Salpetersäurelösungen

The distribution ratios of U(IV) in small concentrations between aqueous nitric acid and undiluted tri-n-butylphosphate phases were determined. U(IV) can well be extracted from aqueous nitric acid solutions. The ratio of distribution increases up to nitric acid concentrations 8n in the aqueous equilibrium phase, passes through a maximum between 8n to 9n and decreases again. Some factors of possible importance for the shape of the curve are being discussed briefly.     

Radiation stability of benzyldibutylamine and benzyldimethyldodecylammonium nitrate in the extraction of lanthanides and americium

The radiation stability was investigated of organic phases containing tertiary benzyldialkylamines and quaternary benzyltrialkylammonium salts which are sultable for the separation of lanthanides and americium from irradiated nuclear fuel. Attention was paid to changes of the extraction properties in Eu(III) and Am(III) extraction. The influence of the individual components forming the organic phase (extractant, solvent, solubilizer and nitric acid) on the decrease of the extraction capacity of the organic phase after irradiation is discussed. The greatest changes in the distribution coefficients D_Eu and D_Am after irradiation were shown for extraction in the presence of nitric acid. As regards the absorbed dose, these systems can be considered as stable in comparison with organophosphorus extractans.     

Extraction studies of trivalent ions from TALSPEAK medium using phsophonic acid-TBP solvent mixture

Indigenously synthesized extractant, phenyl (octyl) phosphonic acid (POPA) in tri-n-butylphosphate (TBP) and dodecane, has been investigated for the separation of americium from trivalent lanthanides in nitric acid medium as well as diethylene triaminepentaacetic acid (DTPA) and lactic acid mixture (TALSPEAK medium). Various experimental parameters like concentration of DTPA, lactic acid, TBP, nitrate ions and pH of the aqueous feed solution have been optimized to obtain the highest separation factor between americium and europium. Bulk actinide–lanthanide separation reagent, tetra (ethylhexyl) diglycolamide (TEHDGA), was equilibrated with simulated solution of americium and lanthanides, equivalent in concentration to the reprocessing waste originating from PHWR spent fuel. DTPA/lactic acid mixture was used to strip the metal ions from the loaded organic phase and re-extracted into POPA in TBP/dodecane to evaluate the separation factor of individual lanthanides with respect to americium. Very good separation factors between americium and trivalent lanthanides were obtained.     

Application of Composite Materials Based on Various Extractants for Isolation of Lanthanide(III) Nitrates from Multicomponent Aqueous Solutions

Extraction of lanthanide(III) nitrates of ceric subgroup was studied by pure tributylphosphate, 50% solution of trialkylmethylammonium nitrate in toluene and composite materials based on porous Teflon and mentioned extractans in the presence of 1–5 mol/dm³ of sodium nitrate in aqueous phase. Effect of additions of sodium sulfate and chloride to nitrate solutions on distribution parameters of lanthanides(III) was determined. The extraction process can be described with taking into account the formation of [Ln(NO₃)₃(S) _j ] (j = 3, 4) in organic phase. The extraction constants were estimated. It was established, that accounting tributylphosphate concentration in the phase of composite, extraction constants and isotherm shape for systems based on tributylphosphate are identical within the error of determination. In the case of trialkylmethylammonium nitrate those for composite are little greater then for liquid extractant.     

Extraction of Hg/II/ with dibutylester of N-/4-antipyryl/-amidophosphoric acid from iodide solutions

A new extraction reagent-dibutylester of N-/4-antipyryl/-amidophosphoric acid/DBAAP/—has been developed and used for the extraction of divalent mercury into chloroform from mixture of sodium iodide and perchloric acid. It was found that the composition of the species extracted into the organic phase depends on the acidity of the aqueous phase. The solvate HgI₂. DBAAP is extracted at low HClO₄ concentrations, an ion-pair, /DBAAPH/⁺.HgI ₃ ^– , is formed and extracted at high concentrations of perchloric acid.