Extraction of U(VI), Th(IV), and REE(III) from nitric acid solutions with (N,N-dialkylcarbamoylmethyl)- and (N-alkylcarbamoylmethyl)phenylphosphinic acids

Extraction of microamounts of U(VI), Th(IV), and REE(III) from HNO₃ solutions with solutions of (N,N-dialkylcarbamoylmethyl)- and (N-alkylcarbamoylmethyl)phenylphosphinic acids in dichloroethane was studied. The stoichiometry of the extractable complexes was determined, and the influence exerted on the efficiency of the U(VI), Th(IV), and REE(III) recovery into the organic phase by the extractant structure, organic diluent, and aqueous phase composition was examined. The possibility of selective recovery and concentration of U(VI), Th(IV), and REE(III) from nitric acid solutions with a complexing sorbent prepared by noncovalent immobilization of the examined polyfunctional organophosphorus acids on a macroporous polymeric matrix was demonstrated.     

Effect of Fe(III) on the extraction of U(VI), Th(IV), and REE(III) from HCl solutions with diphenyl(dibutylcarbamoylmethyl)phosphine oxide

The extraction of U(VI), Th(IV), and REE(III) from HCl solutions with solutions of diphenyl(dibutylcarbamoylmethyl)phosphine oxide in dichloroethane in the presence of Fe(III) was studied. An increase in the Fe concentration in the organic phase leads to a considerable increase in the distribution ratios of U(VI), Th(IV), and REE(III), which is caused by transfer into the organic phase of mixed complexes MCl _m?n (FeCl₄) _n solvated by the extractant molecules. A macroporous styrene-divinylbenzene copolymer impregnated with diphenyl(dibutylcarbamoylmethyl)phosphine oxide can be used for concentrating U(VI), Th(IV), and REE(III) from HCl solutions in the presence of Fe(III).     

Extraction of REE(III), U(VI), and Th(IV) from Nitric Acid Solutions with Carbamoylmethylphosphine Oxides in the Presence of Bis[(trifluoromethyl)sulfonyl]imide Ions

Extraction of microamounts of REE(III), U(VI), and Th(IV) with solutions of carbamoylmethylphosphine oxides (CMPOs) in organic diluents from aqueous HNO₃ solutions containing lithium bis[(trifluoromethyl) sulfonyl]imide (LiTf₂N) was studied. The efficiency of the REE(III), U(VI), and Th(IV) extraction from nitric acid solutions with CMPO solutions considerably increases in the presence of Tf₂N^– ions in the aqueous phase. The stoichiometry of the extractable complexes was determined, and the influence of the structure of the CMPO molecule, kind of organic diluent, and aqueous phase composition on the efficiency of the U(VI), Th(IV), and REE(III) extraction into the organic phase was considered.     

Solubility of mixed-valence U(IV–VI) and Np(IV–V) hydroxides in simulated groundwater and 0.1 M NaClO4 solutions

The kinetics of U(VI) accumulation in the phase of U(IV) hydroxide and of Np(V) in the phase of neptunium(IV) hydroxide, and also the solubility of the formed mixed-valence U(IV)-U(IV) and Np(IV)-Np(V) hydroxides in simulated groundwater (SGW, pH 8.5) and 0.1 M NaClO₄ (pH 6.9) solutions was studied. It was found that the structure of the mixed U(IV–VI) hydroxide obtained by both oxidation of U(IV) hydroxide with atmospheric oxygen and alkaline precipitation from aqueous solution containing simultaneously U(IV) and U(VI) did not affect its solubility at the U(VI) content in the system exceeding 16%. The solubility of mixed-valence U(IV–VI) hydroxides in SGW and 0.1 M NaClO₄ is (3.6±1.9) × 10^?4 and (4.3 ± 1.7) × 10^?4 M, respectively. The mixed Np(IV–V) hydroxide containing from 8 to 90% Np(V) has a peculiar structure controlling its properties. The solubility of the mixed-valence Np(IV–V) hydroxide in SGW [(6.5 ± 1.5) × 10^?6 M] and 0.1 M NaClO₄ [(6.1±2.4) × 10^?6 M] is virtually equal. Its solubility is about three orders of magnitude as high as that of pure Np(OH)₄ (10^?9–10^?8 M), but considerably smaller than that of NpO₂(OH) (～7 × 10^?4 M). The solubility is independent of the preparation procedure [oxidation of Np(OH)₄ with atmospheric oxygen or precipitation from Np(IV) + Np(V) solutions]. The solubility of the mixed-valence Np hydroxide does not increase and even somewhat decreases [to (1.4±0.7) × 10^?6 M] in the course of prolonged storage (for more than a year).     

Extraction of REEs(III), U(VI), and Th(IV) with acidic phosphoryl-containing podands from nitric acid solutions

The extraction of microamounts of REEs(III), U(VI), and Th(IV) from HNO₃ solutions with solutions of acidic phosphoryl-containing podands in 1,2-dichloroethane was studied. The stoichiometry of the extractable complexes was determined. The influence of the extractant structure and aqueous phase composition on the efficiency and selectivity of the recovery of REEs(III), U(VI), and Th(IV) into the organic phase was considered.     

Tc(VII)-catalyzed oxidation of U(IV) with nitric acid in solution of TBP in <Emphasis Type="Italic">n</Emphasis>-dodecane

Oxidation of U(IV) with nitric acid in 30% solution of TBP in n-dodecane is catalyzed by Tc ions; the rate-determining steps are 3U(IV) + 2Tc(VII) → 3U(VI) + 2Tc(IV) and Tc(IV) + Tc(VII) → Tc(V) + Tc(VI). Oxidation of U(IV) is inhibited by the reaction product, HNO₂, which partially binds Tc(IV) ions (TcO²⁺) in an inert complex. The overall rate equation of U(IV) oxidation is-d[U(IV)]/dt = k ₁[U(IV)][Tc][HNO₃]^?3 ? k ₄[U(IV)]²[HNO₂]²[HNO₃]^?1, where k ₁ = 4.8 ± 1.0 mol²1^?2 min^?1 and k ₄ = (2.4 ± 1.0) × 10⁵ 1² mol^?2 min^?1 at 25°C, [H₂O] = 0.4 M ([Tc] is the total Tc concentration in the reaction mixture). Water and U(VI) have no effect on the reaction rate.     

Uranium(IV) Complexes Formed in Extraction with Tributyl Phosphate from Nitric Acid Solutions

Speciation of U(IV) in tributyl phosphate (TBP) solutions prepared by extraction of U(IV) from 2 M HNO₃ was studied. The electronic spectra showed that in the solutions containing from 3 to 60% TBP a mixture of disolvate U(NO₃)₄(TBP)₂ and hydrates with hypothetical formula (TBP) _m ...[U(NO₃) _k · (H₂O) _n ]^(4-k)+ (k = 3 or 4) is formed. Within the 70-100% concentration range, the hexanitrate complex (TBP) _n ...2H₅O₂(H₂O) _p ⁺...[U(NO₃)₆]^{2 -} also appears. In undiluted TBP, as the concentration of uranyl nitrate increases, first the hexanitrate complex and then hydrated complexes of U(IV) gradually disappear. At uranium concentration more than 300 g l^-1, only U(IV) tetranitrate disolvate exists in the organic phase.     

A new approach to mathematical description of the extraction of HNO3, U(VI), and other hexavalent actinides with tributyl phosphate in a mixture with paraffins

The isotherm of uranyl nitrate extraction with TBP in diluent without salting-out agents with the formation of the disolvate is described by a chemical equation with the concentration constant of 10 without correction coefficients in a wide range of component concentrations. The concentration of “free” TBP in the dependence of the extraction of U(VI) microamounts on the acid concentration was calculated using this value, and it was shown that the formal stoichiometric ratio TBP: HNO₃ in the extract is 0.75. To describe this effect, a set of reactions was suggested, taking into account the role of extracted water, and the corresponding concentration equilibrium constants were calculated without corrections. The values obtained differ significantly from those suggested previously. The constants obtained adequately describe the joint extraction of U(VI) and HNO₃ at the acid concentration of 0.3–8 M and extract loading with U(VI) of up to 85% of its capacity. At low extract loadings and acid concentration lower than 1 M, weak hydrolysis of uranyl ion and extraction of U(VI) hydrate solvate were taken into account. For the extraction of Np(VI) and Pu(VI) with TBP in diluent, the extraction constants that do not vary with the U(VI) and HNO₃ concentrations were calculated taking into account the formation of the Np(VI) and Pu(VI) hydrate solvates.     

制备方法对氧化石墨烯氧化程度及对Th(IV)、U(VI)吸附的影响

采用Hummers方法、优化Hummers方法及改进Hummers方法合成氧化石墨烯, 并通过FT-IR、TGA、XRD、XPS、SEM以及元素分析等手段对制备产物进行了表征。结果表明, 利用优化Hummers方法制备得到的氧化石墨烯具有较高的氧化程度。三种产物对Th(IV)、U(VI)的等温吸附实验结果表明, 采用优化Hummers方法制备的氧化石墨烯对Th(IV)的最大吸附量为192.3 mg/g, 相比于Hummers方法制备产物的吸附能力提高了38.5%; 对U(VI)的最大吸附量为156.2 mg/g, 吸附能力提高了28.1%, 三种样品对Th(IV)、U(VI)的吸附都更加符合Langmuir等温吸附模型。此外, 考察了优化Hummers方法制备的氧化石墨烯吸附Th(IV)、U(VI)的动力学和热力学参数, 证实氧化石墨烯吸附Th(IV)、U(VI)符合准二级动力学方程, 是自发吸热行为。     

Preconcentration of U(VI), Th(IV), and REE(III) from nitric acid solutions using carbon nanotubes modified with bis(dioctylphosphinylmethyl)phosphinic acid

The extraction of microamounts of U(VI) and Th(IV) from HNO₃ solutions in the form of complexes with bis(dioctylphosphinylmethyl)phenylphosphinic acid was studied. The stoichiometry of the extractable complexes was studied, and the influence of the extractant structure on the efficiency and selectivity of the U(VI) and Th(IV) extraction was examined. U(VI), Th(IV), and REE(III) can be preconcentrated from nitric acid solutions with a complexing sorbent prepared by noncovalent immobilization of bis(dioctylphosphinylmethyl) phosphinic acid on the surface of carbon nanotubes.     

Extraction of U(VI), Th(IV), and REE(III) from nitrate solutions with diaryl(dialkylcarbamoylalkyl)phosphine oxides containing a dialkylcarbamoylmethyl substituent in the alkylene bridge

The extraction of microamounts of U(VI), Th(IV), and REE(III) from HNO₃ and NH₄NO₃ solutions with solutions of diaryl(dialkylcarbamoylalkyl)phosphine oxides containing a dialkylcarbamoylmethyl substituent in the alkylene bridge was studied. The stoichiometry of the complexes extracted from nitric acid solutions with N,N,N',N'-tetrabutyl-2-(di-p-anisylphosphinyl)butanedioic diamide I was determined. The influence of the extractant structure and aqueous phase composition on the efficiency and selectivity of the extraction of U(VI), Th(IV), and REE(III) into the organic phase was examined. Introduction of the–CH₂C(O)NAlk₂ fragment into the methylene bridge of the diaryl(dialkylcarbamoylmethyl)phosphine oxide molecule considerably enhances the extraction of REE(III) from neutral nitrate solutions. Such modification of the extractant molecule only slightly influences the extraction of REE(III) from nitric acid solutions, but leads to a considerable increase in the U(VI) extraction and to a decrease in the Th(IV) extraction. The selectivity of the extraction of U(VI) and REE(III) is thus considerably increased.     

Extraction of Am(III), U(VI), and Pu(IV) from HCl Solutions with Diphenyl(dibutylcarbamoylmethyl)phosphine Oxide

Extraction of Am(III), U(VI), and Pu(IV) from HCl solutions with solutions of diphenyl(dibutylcarbamoylmethyl)phosphine oxide in dichloroethane and m-nitrobenzotrifluoride was studied. The curves of the Am(III) and U(VI) extraction as a function of the acid concentration pass through a minimum at [HCl] = 1 M, followed by a steep ascent at the acid concentration increased further. The logarithmic dependences of the distribution factors on the extractant concentration are nonlinear, with the slope of the upper portions close to 2 for Am(III) and 1 for U(VI). Pu(IV) is extracted noticeably worse than U(VI). A significant anomalous aryl strengthening effect is observed in HCl solutions.     

Oxidation of U(VI) with oxygen in weakly acidic and neutral solutions

The kinetics of U(IV) oxidation with atmospheric oxygen in solutions with pH 2–7 was studied. In the kinetic curves there is an induction period, which becomes shorter with increasing pH. The induction period is caused by accumulation of U(VI), whose initial presence in the working solution accelerates oxidation. The pseudo-first-order rate constants and bimolecular rate constants of U(IV) oxidation with oxygen were evaluated. The mechanism of U(IV) oxidation is considered. At pH higher than 3, formation of a polymer of hydrolyzed U(IV) with U(VI) plays an important role in oxidation of U(IV), since this prevents formation of U(V). Heating accelerates oxidation of U(IV) at pH 2–2.5, but at a higher pH the process becomes difficultly controllable.     

Redox Kinetics of U,Pu, and Np in TBP Solutions: VI. Reactions of Neptunium and Plutonium with Organic Reducing Agents: Determination of the Final Oxidation States and Reaction Rates

Reactions of Pu(IV) and Np(VI) with organic reducing agents of various types (substituted hydroxylamines, oximes, aldehydes, etc.) in tributyl phosphate solutions containing nitric acid were studied spectrophotometrically. The molar extinction coefficients of neptunium and plutonium in various oxidation states [Np(IV,V,VI), Pu(III,IV,VI)] in TBP solutions were determined as influenced by HNO₃ and H₂O concentrations and temperature. It was found that organic reducing agents at low HNO₃ concentration convert plutonium and neptunium to Pu(III) and Np(V), respectively. With increasing HNO₃ concentration Pu(III) and Np(V) are partly oxidized back to Pu(IV) and Np(VI), respectively, by reaction with nitrous acid. The rate constants of Pu(VI) and Np(VI) reduction and Np(V) oxidation as influenced by concentration of organic reducing agents and HNO₃ were evaluted from the kinetic data.     

Role of U(VI) complexes in the chemiluminescent reaction U(IV) + O3 in H2SO4 solution

Data on the effect of U(VI) on the reaction U(IV) + O₃ in H₂SO₄ solution are analyzed. The chemiluminescence enhancement is caused by the formation of a complex of an excited U(VI) ion with an unexcited U(VI) ion, so-called excimer. The decomposition of the excimer to two U(V) ions and H₂O₂ is followed by the reaction of U(V) with ozone, giving rise to the excited U(VI) ion. Thus, a chain reaction develops.     

Extraction and sorption preconcentration of U(VI), Th(IV), and REE(III) from nitric acid solutions using bis[2-(diphenylphosphinyl)phenoxymethyl]phosphinic acid

Extraction of microamounts of U(VI) and Th(IV) from HNO₃ solutions in the form of complexes with polyfunctional organophosphorus acids was studied. The influence of the extractant structure on the efficiency and selectivity of the extraction of U(VI) and Th(IV) was examined, and the stoichiometry of the extractable complexes was determined. The possibility of preconcentration of U(VI), Th(IV), and REE(III) from nitric acid solutions with a complexing sorbent prepared by noncovalent immobilization of bis[2-(diphenylphosphinyl) phenoxymethyl]phosphinic acid on a macroporous polymeric matrix was demonstrated.     

Stripping of Pu and Tc from tributyl phosphate solutions with carbohydrazide

Stripping of Pu from 30% tributyl phosphate solutions with carbohydrazide was studied. The degree of the Pu stripping drastically decreases with an increase in acidity and increases with an increase in the carbohydrazide concentration and in temperature. The Pu(IV) reduction rate in the two-phase system is sufficiently high to perform the reductive stripping of Pu with carbohydrazides both in mixer-settlers and in centrifugal extractors. Stripping of Tc at high phase ratio (O/W = 30) with carbohydrazide, hydrazine, and U(IV) in a mixture with hydrazine was studied. At low acidity (<1 M HNO₃) and 30?C35°C, carbohydrazide allows more than 80% stripping of Tc from 30% TBP solutions even at high volume ratio of the phases.     

Mathematical modeling of extraction of nitric acid,actinides, and fission products into 30% TBP in halogenated diluents using A.M. Rozen’s model

The effect of a series of halogenated diluents [bis(octafluoroamyloxy)methane, phenyl trifluoromethyl sulfone, methyl dodecafluoroheptyl ether, m-nitrobenzotrifluoride, hexachlorobutadiene, CHCl₃, CCl₄] on the extraction of spent nuclear fuel solution components [HNO₃, U(VI), An(IV), Zr, and Tc] into 30% TBP was studied and mathematically described using improved A.M. Rozen’s model. Comparison with the TBP–paraffins extraction system was made.     

Behavior of Cs, Np(V), Pu(IV), and U(VI) in pore water of bentonite

Sorption of Cs, Pu(IV), Np(V), and U(VI) with bentonite from solutions was studied. Physicochemical species of radionuclides in the liquid phase were determined, the sorption mechanisms were established, and the influence of bentonite colloids on the behavior of radionuclides was studied. It was shown that Cs is sorbed by the ion-exchange mechanism, whereas the sorption of actinides at pH > 5 is governed by the reaction with surface hydroxy groups of betonite, and at pH < 5 the competing processes are ion exchange and complex formation. Reduction of Np(V) and U(VI) to Np(IV) and U(IV) in the solution with Fe(II) compounds present in the system was proved by the extraction method. Various methods of separating the solid phase were used in studying the dependence of the distribution coefficients of Np and Pu on the ratio of pore water and bentonite; it was shown that Np and Pu are sorbed on bentonite colloids.     

Separation of U and Pu by countercurrent chromatography with support-free liquid stationary phase in the TBP white spirit nitric acid system

The article deals with the use of countercurrent chromatography (CCC), support-free partition chromatography, for separation of U and Pu from the organic extract obtained directly by the dissolution of MOX fuel in supercritical CO₂ containing the complex TBP · nHNO₃. White spirit solutions with various TBP concentrations were used as a stationary phase. The effects of the compositions of the stationary and mobile phases on the U/Pu partition efficiency are studied. The CCC method allows the separation of U and Pu under conditions of a TBP concentration gradient in the stationary phase and also of an HNO₃ concentration gradient in the mobile phase. Chromatographic separation first gives the Pu fraction containing 98.9% of Pu and 0.07% of U, and then the U fraction (99.93% of U and 1.1% of Pu). The separation time is 50 min.