Extraction of long-lived radionuclides from nitric acid solutions using an extractant based on dibutyl hydrogen phosphate and chlorinated cobalt dicarbollide

Chlorinated cobalt dicarbollide (CCD), when added to concentrations of 0.25 M to a solution of dibutyl hydrogen phosphate (HDBP) in m-nitrobenzotrifluoride (MNBTF), increases the distribution ratios of trace amounts of Eu and Am without changing the slope (tan α ～ 2) of their dependences on HDBP concentration in the 0–1.5 M range. At [CCD]/([CCD] + DBPA]) = 0.2–0.22, the synergistic effect is observed in the entire range of HDBP concentrations in extraction of these elements from 1.0 and 2.5 M HNO₃. In this case, HDBP suppresses the extraction of Cs with CCD in the area below the synergistic maximum, where antagonism is observed in the extraction of Cs. Polyethylene glycol (PEG, Slovafol-909) was added to the extraction mixture to improve the extraction of Sr. The extremum is attained at its concentration in the solvent with HDBP of ～0.033–0.065 M, which is smaller than that in the absence of HDBP by a factor of 1.5–2.5. With increasing concentration of HDBP in the HDBP-CCD-PEG-MNBTF extraction system, the slopes for Eu and Am are 1.3 and 0.6, whereas the slopes for Cs and Sr decrease nonlinearly and amount to ?1.8 and ?1.3, respectively. With increasing concentration of HNO₃, D for Eu, Am, and Cm decreases in proportion to the HNO₃ concentration to the power of ?3 irrespective of the PEG concentration, and for Cs and Sr, to the power of ?2 in the presence of PEG, whereas in the PEG-free systems the dependences are nonlinear. The synergistic extractant is characterized by higher (by an order of magnitude) solubility of metal solvates as compared to the HDBP-MNBTF system (concentration of Eu in the extractant >0.163 M). The extractant containing HDBP (1.1 M), CCD (0.23 M), and Slovafol-90 (0.065 M) in MNBTF is suggested for combined recovery of rareearth (REE) and transplutonium elements (TPE) and of Cs and Sr from high-level waste (HLW) after reprocessing of spent nuclear fuel (SNF) with high burn-up.     

Synergistic extraction of REE and TPE from HNO<Subscript>3</Subscript> solutions with a mixture of CCD and HDBP ZS (1: 6) in a polar diluent

Extraction of Ce(III), Am(III), and Eu(III) from HNO₃ solutions with acidic zirconium salt of dibutyl hydrogen phosphate (HDBP ZS) at the ratio Zr: HDBP = 1: 6 and with a mixture of HDBP ZS (1: 6) and chlorinated cobalt dicarbollide (CCD) in a polar diluent, mixture of o-chloronitrobenzene and CCl₄ (1: 1), was studied. A synergistic effect was observed at a definite ratio of CCD and HDBP in the mixture. The maximal synergistic effect was observed for a mixture of HDBP ZS with CCD and an HDBP to CCD molar ratio of 6:1. The largest magnitude of the synergistic effect was 5 for all the examined elements. The synergistic effect observed in the HDBP ZS (1: 6)-CCD system in extraction of rare-earth and transplutonium elements can be attributed to interaction of Zr, CCD, and HDBP leading to the formation of a complex acid, which is stronger and more hydrophobic than the initial compounds. The stoichiometry of the extraction of Ce and Eu with such acid corresponds to the stoichiometry of the extraction with HDBP ZS and CCD, and in the extraction of Am it is somewhat different. In the extraction ability, this acid considerably surpasses CCD and somewhat surpasses HDBP ZS. The extractability order of metals corresponds to the order of their extractability with HDBP ZS and is opposite to the order of their extractability with CCD. High extraction ability of the complex extractant allows REE and TPE to be recovered directly from the Purex process raffinate.     

Synergistic extraction of REE and TPE from HNO<Subscript>3</Subscript> solutions with a mixture of CCD and HDPB ZS (1 : 8) in a polar diluent

Extraction of Ce(III), Am(III), and Eu(III) from HNO₃ solutions with an acidic zirconium salt of dibutyl hydrogen phosphate (HDBP ZS) at the Zr: HDBP ratio of 1: 8 and with a mixture of HDBP ZS (1: 8) and chlorinated cobalt dicarbollide (CCD) in an o-chloronitrobenzene-CCl₄ mixture (1: 1) was studied. At a definite molar ratio of CCD and HDBP in a mixture, a considerable synergistic effect is observed, reaching a maximum (30–40 for all the examined elements) at HDBP: CCD = 2: 1. The synergistic effect observed in the system HDBP ZS (1: 8)-CCD in extraction of rare-earth and transplutonium elements can be attributed to interaction of Zr, CCD, and HDBP, leading to the formation of a complex acid that is stronger and more hydrophobic than the starting compounds. The stoichiometry of the extraction of REE and TPE with such an acid corresponds to the stoichiometry of their extraction with HDBP ZS or CCD, and the extraction ability of the system is considerably enhanced relative to the components taken separately. The metal extractabilility series corresponds to the extractability series with HDBP ZS and is opposite to the extractability series with CCD. The Zr atom in the complex extractant binds in the first coordination sphere two CCD anions, two DBP anions, and four HDBP molecules.     

Synergistic extraction of REE and TPE from HNO3 solutions with a CCD-HDBP ZS (1: 12) mixture in a polar diluent

Extraction of Ce(III), Am(III), and Eu(III) from HNO₃ solutions with an acidic zirconium salt (HDBP ZS) of dibutyl hydrogen phosphate (HDBP) at Zr: HDBP = 1: 12 and with a mixture of HDBP ZS with chlorinated cobalt dicarbollide (CCD) in a polar diluent, a 1: 1 o-chloronitro-benzene-CCl₄ mixture, was examined. This system exhibits a substantial synergistic effect, which is at a maximum (30–40 for all the elements examined) at the 1: 1 HDBP to CCD molar ratio. The synergistic effect observed in the HDBP ZS (1: 12)-CCD system in extraction of REE and TPE can be associated with interaction of Zr, CCD, and HDBP, resulting in formation of a complex acid, more hydrophobic and stronger than the initial compounds. The trends in variation of the metal extractability are identical to those for HDBP ZS and opposite to those for CCD. The Zr atom in the complex extractant binds three CCD anions, as well as DBP anion and HDBP molecules.     

Synergistic extraction of REE and TPE from nitric acid solutions with chlorinated cobalt dicarbollide-dibutyl hydrogen phosphate mixture in a polar diluent

The extraction of Ce(III), Am(III), and Eu(III) from nitric acid solutions with chlorinated cobalt dicarbollide (CCD), dibutyl hydrogen phosphate (HDBP), and their mixtures in a polar diluent, m-nitrobenzo-trifluoride, was studied. The extraction with CCD-HDBP mixture is characterized by considerable synergistic effect. Its maximal value is close to 40 at HDBP to CCD molar ratio of 5–8 for all the metals under consideration. We believe that this synergistic effect is caused by formation of a complex acid from HDBP and CCD. This complex acid is more hydrophobic and more acidic than the initial reagents. In extraction of the metals from strongly acidic solutions, the HDHP to CCD ratio corresponding to optimal extraction decreases. This effect can be attributed to displacement of HDBP molecules from the extractable complex with HNO₃ molecules.     

Extraction of HNO3 with solutions of zirconium salt of dibutyl hydrogen phosphate in 30% tributyl phosphate and in xylene

Zirconium salt of dibutyl hydrogen phosphate (ZS HDBP) dissolved in xylene or 30% tributyl phosphate (TBP) + xylene mixture extracts noticeable amounts of HNO₃ at its low concentrations in the aqueous phase, when extraction of HNO₃ with straight HDBP itself is not observed. IR data show that DBP^? and NO ₃ ^? ions compete for coordination to Zr, which also adds from 2 to 4 HDBP molecules. With a further increase in the HDBP concentration, the HDBP and HNO₃ molecules compete for coordination in the outer coordination sphere of zirconium. Inflections in the S-shaped dependence of the HNO₃ concentration in the organic phase on the [Zr]:[HDBP] molar ratio lie within the interval from 1:8 to 1:12, which corresponds to filling of the Zr outer sphere with HDBP molecules. In the presence of TBP, these curves become smooth and have no apparent inflection.     

Extraction of alkaline-earth elements from nitric acid with a solution of HDBP zirconium salt

Extraction of alkaline-earth metals with acidic zirconium salt of dibutyl hydrogen phosphate (HDBP AZS) increases in the order Ba < Sr < Ca. Magnesium is extracted substantially better than Ba, but by an order of magnitude worse than Ca. The maximal recovery of Sr is observed at Zr: HDBP ratio of 1: 9, similarly to extraction of TPE and REE. The recovery of Sr is possible from weakly acidic solutions (<0.5 M HNO₃); as the acidity increases, the Sr distribution coefficient decreases in proportion to [H⁺]^?3. The presence of TBP in the extractant slightly decreases the Sr extraction but simultaneously increases the extractant capacity. The latter effect is substantially less pronounced than for TPE and REE. In the experiment with model solutions on a bench of centrifugal extractors, the Sr decontamination factor from REE and TPE as high as 50–100 was attained.     

Extraction of molybdenum from nitric acid solutions with solutions of dibutyl hydrogen phosphate zirconium salt

The extraction of Mo with a solution of dibutyl hydrogen phosphate (HDBP) zirconium salt (ZS) in xylene and in TBP-xylene and TBP-decane mixtures was studied in comparison with the Mo extraction in similar systems without Zr. The Mo extraction is enhanced to the largest extent (by a factor of 7 with 1.5 M HNO₃) at the HDBP: Zr ratio of 12, but the majority of experiments were performed at the ratio HDBP: Zr = 9, characteristic of the extraction of TPE and REE. The plots of D _Mo vs. HNO₃ concentration in all the cases pass through a minimum at ∼3 M HNO₃, and the minimum is the deeper, the higher the TBP concentration. At the same time, in the system with HDBP ZS at the TBP concentration in the solvent exceeding 50%, further decrease in D _Mo is not observed. The dependence of the Mo distribution ratio D _Mo on the HDBP concentration at a fixed ratio HDBP: Zr = 9, the composition of the saturated systems, and the spectra of the saturated and unsaturated (with respect to Mo) systems suggest formation of mixed HDBP-Zr-Mo complexes. Assumptions concerning their composition and structure were made. According to the IR data, in these complexes in the absence of TBP the nitrate ion is not coordinated to the Mo atom. Bridged structures Mo-O-Mo are not formed either. Addition of TBP leads to a decrease in D _Mo in the extraction with HDBP ZS, but, at the same time, the slope of the dependence of D _Mo on the HDBP concentration increases, and the limiting capacity of the solvent for Mo also somewhat increases. These effects are primarily attributed to an increase in the HNO₃ concentration in the organic phase in the presence of TBP.     

Synergistic extraction of rare earth elements from HNO<Subscript>3</Subscript> solutions with a mixture of chlorinated cobalt dicarbollide and acidic zirconium salt of dibutyl hydrogen phosphate in a polar diluent

Extraction of La and Ln(III) (except Pm) from HNO₃ solutions with mixtures of acidic zirconium salts of dibutyl hydrogen phosphate (ZS HDBP) with chlorinated cobalt dicarbollide (CCD) in a polar diluent, 1: 1 mixture of о-chloronitrobenzene and CCl₄, was studied. A synergistic effect is observed at definite ratios of CCD and ZS HDBP in the mixture. The magnitude of the synergistic effect increases with an increase in the Zr to HDBP molar ratio. The maximal synergistic effect equal to 360 was reached for Dy in the ZS HDBP (1: 6)–CCD extraction system. The maximal REE distribution ratio (D_max) is observed for elements from La to Dy at the HDBP to CCD molar ratio of 2 in all the extraction systems. For elements of the Ho–Lu series, D_max is observed at the HDBP to CCD molar ratio from 2 to 6. The HDBP to CCD molar ratio at which D_max is reached increases with an increase in the Zr content of the extraction system. For some pairs of elements, the separation factor considerably increases in the extractant mixture relative to the extractants taken separately. The largest increase in the separation factor (by a factor of 8.3) was noted for the Tm–Er pair in the CCD–ZS HDBP (1: 6) system. The synergistic effect in some cases is preserved at least up to the HDBP to CCD molar ratio of 670: 1, and at higher ratios the magnitude of the synergistic effect does not exceed the uncertainty of its determination. This fact suggests the formation of so-called hypercomplexes containing tens and hundreds of HDBP molecules in the CCD–ZS HDBP systems, similarly to the CCD–HDBP system. The largest number of HDBP molecules was noted in the Ln–CCD–[(ZS HDBP (1: 6)]_n complexes. The CCD–ZS HDBP (1: 6) and CCD–ZS HDBP (1: 12) systems are promising for the separation of REE and TPE, and small addition of CCD to the ZS HDBP (1: 6) system in a polar diluent allows the development of a cheap solvent efficiently recovering all the REE and TPE from a 3 M HNO₃ solution.     

Effect of TBP on Extraction of REE and TPE from Nitric Acid Solutions with Dibutylphosphoric Acid and Its Zirconium Salt

Solvent effect on extraction of REE and TPE with dibutylphosphoric acid is studied. Over the HNO₃ concentration range 0.1-0.5 M, HDBP is a good extractant for REE and TPE, whereas addition of TBP suppresses the extraction. The effect of excess HDBP is similar to the previously observed effect of TBP on extraction of REE. Evidently, free HDBP, like TBP, can enter the solvation shell of a lanthanide with formation of fairly complex, probably polymeric species.     

Interaction of HDBP with Zr in extraction

Complexes of Zr with dibutyl hydrogen phosphate (HDBP) formed in extraction from 1–6 M HNO₃ solutions were studied by extraction methods and differential IR spectrometry. The study revealed formation of a single complex Zr(NO₃)₂A₂, or “core” (where A is DBP anion). Additional HDBP molecules are linked to the “core” via phosphoryl oxygen atoms to form associates Zr(NO₃)₂A₂(HA) _n with no definite stoichiometry. A differential spectrometric study revealed the band ν_s(POO^?) = 1010 cm^?1, which in the spectra of neat HDBP is obscured by the P-OC vibration frequency. The difference ν_as(POO^?) ? ν_s(POO^?) decreases as the HDBP: Zr ratio is decreased from 6 to 2. Extraction of Eu with the associate Zr(NO₃)₂A₂(HA)₆ occurs owing to interaction with both phosphoryl oxygen atoms of HDBP linked to the “core” by a strong hydrogen bond.     

Separation of Am and Cm by Extraction from Weakly Acidic Nitrate Solutions with Tributyl Phosphate in Isoparaffin Diluent

Trivalent transplutonium (TPE) and rare earth (REE) elements are extracted to more than 80% with 30% TBP in Isopar M from solutions containing 0.06–0.5 M HNO₃ and a salting-out agent, NH₄NO₃, in a concentration of ≥6 M. The elements are stripped from the organic phase with 0.1 M HNO₃. The Am(III)/Eu separation factors vary from 1.8 to 2, which can be used for their extraction separation. The Cm/Am(III) separation factors in 0.06–3 M HNO₃ are in the range from 1.1 to 1.2; therefore, to separate these elements, higher oxidation states of Am, Am(VI) and Am(V), should be used. The effect of various factors on the stability of Am(VI) was examined, and the conditions of the existence of Am(VI) and Am(V) in ≤0.1 M HNO3 solutions containing ~8 M NH₄NO₃ were determined. Curium is extracted with 30% TBP in Isopar M virtually completely, whereas americium only partially (≤30%) passes into the organic phase in the form of Am(III). In the process, high degree of separation of Cm from Am(V) remaining in the aqueous phase is reached (≥99.9%).     

A spectroscopic study of molybdenum extracts in organic solutions of dibutyl hydrogen phosphate in equilibrium with aqueous nitric acid solutions

The Mo extracts prepared by contacting dibutyl hydrogen phosphate (HDBP, HA) dissolved in xylene or CCl₄ with HNO₃ solutions of various concentrations in a wide range of Mo concentrations, and also the third phase formed in the analogous extraction system with tridecane as diluent were examined by IR and UV spectroscopy. The data obtained confirm the conclusion that, at the ratios HDBP: Mo ≥ 4 in the extract, molybdenum is extracted in the form of the acidic salt MoO₂(HA₂)₂. The spectroscopic data obtained at higher extract loading were interpreted assuming the formation of binuclear complexes (HA₂)MoO₂A₂MoO₂(HA₂) and, at HDBP: Mo < 3, of complexes of hydroxomolybdenyl and/or dimolybdenyl, and finally of polymeric complexes with bridging Mo-O-Mo bonds. In strongly acidic media, coextraction of outer-sphere molecular HNO₃ and the presence of coordinated nitrate ion in the extract (probably in the form of molybdenyl nitrate complexes) are observed. A multistep extraction mechanism involving formation in the aqueous phase of an asymmetric monomeric complex MoO₂(OH)(H₂O)(HA₂) was suggested.     

Synergic extraction of Ce(III) from HNO<Subscript>3</Subscript> solutions with a mixture of CCD and HDBP ZS in a polar diluent

The extraction of Ce(III) from HNO₃ solutions with a mixture of chlorinated cobalt dicarbollide (CCD) and dibutyl hydrogen phosphate (HDBP) or its zirconium salt (HDBP ZS) in m-nitrobenzotrifluoride was studied by the method of isomolar series. A considerable synergic effect S was observed, reaching a maximum in a mixture of HDBP with CCD at the molar ratio HDBP : CCD = 7 : 1 (S = 100). The highest Ce(III) distribution ratios D _Ce are observed with mixtures of CCD and HDBP ZS (1 : 6 and 1 : 4). An increase in the HNO₃ concentration causes changes in D _Ce and S to different extents depending on the type of HDBP ZS. The synergic effect was attributed to the formation of coordination compounds that are more hydrophobic than the initial compounds and have stronger bonds of Ce(III) with the ligands.     

Extraction of Am(III) and Rare-Earth Elements from Nitric Acid Solutions with Dibutyl Diphenylphosphinylmethylphosphonate

Extraction of HNO₃ and trace amounts of Am, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu nitrates from aqueous HNO₃ with solutions of dibutyl diphenylphosphinylmethylphosphonate in dichloroethane was studied. The stoichiometry of the extractable solvates was determined, and the apparent extraction constants were calculated.     

Extraction of Mo from solutions in HNO<Subscript>3</Subscript> and other mineral acids with solutions of dibutyl hydrogen phosphate in a diluent

Extraction of Mo from HNO₃ solutions with solutions of HDBP in xylene and CCl₄ in a wide range of Mo concentrations was studied. The Mo distribution ratios are considerably higher with CCl₄ diluent compared to xylene, but the extractant capacity in both cases is the same and corresponds to the ratio HDBP: Mo = 2. The active species in the Mo extraction is the HDBP dimer. In the first step, an acidic molybdenyl salt with HDBP of the composition MoO₂(DBP)₂(HDBP)₂, exhibiting certain secondary extraction properties toward rare-earth elements, is formed in all the cases. The dependence of the Mo extraction on the aqueous solution acidity passes through a minimum at 3–4 M HNO₃. The subsequent increase in the Mo distribution ratios is associated with the simultaneous extraction of HNO₃ (or nitrate ion) whose concentration in the extract is considerably lower than the Mo concentration. With an increase in the loading of the extract with molybdenum, the acidic molybdenum salt of HDBP undergoes restructurization, probably associated with additional coordination of H₂MoO₄ to it. The dependence of the Mo distribution ratio on the acid concentration in the extraction from sulfuric acid solutions passes through a minimum at 3–4 M H₂SO₄, which correlates with the first step of the acid dissociation, and in the extraction from HClO₄ the dependence passes through a minimum and a maximum. In the extraction from hydrochloric acid solutions, the Mo extractability decreases with an increase in the acid concentration, owing to complexation in the aqueous phase. The nature of processes occurring at various loadings of the extract with molybdenum are discussed.     

Extraction of Am and Eu with N,N′-substituted pyridine-2,6-dicarboxamides in fluorinated diluents

Extraction of Am and Eu with N,N′-substituted pyridine-2,6-dicarboxamides in fluorinated diluents was studied. N,N′-Diethyl-N,N′-di(p-tolyl)pyridine-2,6-dicarboxamide (EtTPDA) is readily soluble in fluorinated diluents, and its solutions allow the extraction of TPE and REE from nitric acid solutions. The extraction ability of EtTPDA in fluorinated polar diluents is higher than in CHCl₃. In the extraction with EtTPDA, the Am/Eu selectivity is independent of the extractant concentration and decreases with an increase in the acidity of the medium. Americium is extracted with EtTPDA better than Eu, which allows selective recovery of Am at moderate HNO₃ concentrations.     

Extraction of rare-earth elements from nitric acid solutions with bidentate neutral organophosphorus compounds in the presence of l-butyl-3-methylimidazolium hexafluorophosphate

The distribution of microamounts of La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y nitrates between aqueous HNO₃ solutions and solutions of tetraphenylmethylenediphosphine dioxide, diphenyl(diethylcarbamoylmethyl)phosphine oxide, and dibutyl(diethylcarbamoylmethyl)phosphine oxide in organic solvents in the presence of l-butyl-3-methylimidazolium hexafluorophosphate (BMImPF₆) was studied. The stoichiometry of the extractable complexes was determined. The influence of the HNO₃ concentration and the nature of the extractant and organic solvent on the synergistic effect observed in the REE extraction in the presence of BMImPF₆ in the organic phase was analyzed. The possibility of using a macroporous polymeric sorbent impregnated with a mixture of tetraphenylmethylenediphosphine dioxide and BMImPF₆ for the recovery of REE(III), U(VI), and Th(IV) from HNO₃ solution was demonstrated.     

α-Radiolysis of the tributyl phosphate–hydrocarbon diluent–nitric acid system

α-Radiolysis of tributyl phosphate in Sintin n-paraffin diluent in equilibrium with HNO₃ solutions at single “internal” irradiation from the extracted ²³⁸Pu was studied. The radiation-chemical yields (molecules/100 eV) of butyl hydrogen phosphates (BHP), carboxylic acids, carbonyl compounds, and nitro compounds upon irradiation of 20% TBP in the treated Sintin in equilibrium with 3 M HNO₃ were 0.4 (at dibutyl hydrogen phosphate to monobutyl dihydrogen phosphate ratio HDBP: H₂MBP = 4.3), 1.4, 0.2–0.3, and 0.2–0.3, respectively. The degradation and oxidation processes occur more deeply than under γ-irradiation. A simple volumetric method for determining carboxylic acids in the extract was developed. In the course of irradiation, the Pu(IV) oxidation state in the extract does not change, and its retention is due to the interaction with BHP at the ratio BHP: Pu = 2 in stripping with 0.02 M HNO3 and BHP: Pu = 4 in stripping with Fe(II). The retention can be eliminated by the displacing action of Np(IV).