Parveen K. Verma

Investigations on preferential Pu(IV) extraction over U(VI) by N,N-dihexyloctanamide versus tri-n-butyl phosphate: evidence through small angle neutron scattering and DFT studies.

Straight chain amide N,N-dihexyloctanamide (DHOA) has been found to be a promising alternative extractant to tri-n-butyl phosphate (TBP) for the reprocessing of irradiated uranium- and thorium-based fuels. Unlike TBP, DHOA displays preferential extraction of Pu(IV) over U(VI) at higher acidities (≥3 M HNO3) and poor extraction at lower acidities. Density functional theory (DFT) based calculations have been carried out on the structures and relative binding energies of U(VI) and Pu(IV) with the extractant molecules. These calculations suggest that the differential hardness of the two extractants is responsible for the preferential binding/complexation of TBP to uranyl, whereas the softer DHOA and the bulky nature of the extractant lead to stronger binding/complexation of DHOA to Pu(IV). In conjunction with quantum chemical calculations, small angle neutron scattering (SANS) measurements have also been performed for understanding the stoichiometry of the complex formed that leads to relatively lower extraction of Th(IV) (a model for Pu(IV)) as compared to U(VI) using DHOA and TBP as the extractants. The combined experimental and theoretical studies helped us to understand the superior complexation/extraction behavior of Pu(IV) over U(VI) with DHOA.

An Insight into Third-Phase Formation during the Extraction of Thorium Nitrate: Evidence for Aggregate Formation from Small-Angle Neutron Scattering and Validation by Computational Studies

Small-angle neutron scattering (SANS) studies were carried out to compare the aggregation behavior of 1.1 M solutions of tributyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) dissolved in different deuterated diluents, viz., n-dodecane, chloroform, and benzene, during the extraction of Th(IV) from nitric acid medium. The scattering data was treated using the Baxter sticky spheres model. The third phase formed in the case of DHOA displayed higher aggregation tendency compared to that of TBP. These studies have demonstrated that the nature of the diluents plays an important role in the aggregation behavior of the extracted species (reverse micelles). No third phase was observed in the case of chlorinated and aromatic diluents like chloroform and benzene during the extraction of Th(IV) from nitric acid medium. Theoretical calculations were also performed to gain insights into the binding of thorium nitrate with TBP and DHOA models. These calculations suggest that two to three molecules of both DHOA and TBP strongly coordinate to Th(NO3)4. It is noted that the highly charged Th(IV) cations are screened by nitrates and extractants which enables the interaction of second unit of such complex through noncovalent interactions.

Effect of successive alkylation of N,N-dialkyl amides on the complexation behavior of uranium and thorium: solvent extraction, small angle neutron scattering, and computational studies.

The effect of successive alkylation of the Cα atom adjacent to the carbonyl group in N,N-dialkyl amides (i.e., di(2-ethylhexyl)acetamide (D2EHAA), di(2-ethylhexyl)propionamide (D2EHPRA), di(2-ethylhexyl)isobutyramide (D2EHIBA), and di(2-ethylhexyl)pivalamide (D2EHPVA)) on the extraction behavior of hexavalent uranium (U(VI)) and tetravalent thorium (Th(IV)) ions has been investigated. These studies show that the extraction of Th(IV) is significantly suppressed compared to that of U(VI) with increased branching at the Cα atom adjacent to the carbonyl group. Small angle neutron scattering (SANS) studies showed an increased aggregation tendency in the presence of nitric acid and metal ions. D2EHAA showed more aggregation compared to its branched homologues, which explains its capacity for higher extraction of metal ions. These experimental observations were further supported by density function theory calculations, which provided structural evidence of differential binding affinities of these extractants for uranyl cations. The complexation process is primarily controlled by steric and electronic effects. Quantum chemical calculations showed that local hardness and polarizability can be extremely useful inputs for designing novel extractants relevant to a nuclear fuel cycle.

Optimization Studies for the Recovery of Thorium from Advanced Heavy Water Reactor High Level Waste (AHWR-HLW) Solutions Using Green Solvents

An Advanced Heavy Water Reactor (AHWR) has been specifically designed to exploit Th/233U as fuels. The reprocessing is focused mainly on the recovery of 233U and Pu from the spent fuels leaving bulk of Th (∼100 g/L) in the High Level Waste (HLW) solutions. No systematic attempts have been made so far to identify suitable solvents for the recovery of thorium from high level waste (HLW) solutions generated after AHWR spent fuel reprocessing. Tri-n-butyl phosphate (TBP), though the work horse for nuclear fuel reprocessing as an extractant, suffers from the serious limitation of third-phase formation during the extraction of macro concentrations of thorium. Two straight chain dialkyl amides such as N,N-dihexyl octanamide (DHOA), and N,N-dihexyl decanamide (DHDA) as well as TBP were evaluated for the recovery of the thorium from AHWR-HLW solutions. Attempts were made to identify suitable solvent (extractant + diluent) system and optimize the conditions for the recovery of thorium from HLW solutions. Selectivity of the solvents was examined for thorium extraction over fission products/structural materials under AHWR raffinate solutions. Counter-current centrifugal contactor runs were also carried out on simulated waste solutions to validate the optimized conditions for the recovery of thorium from the simulated AHWR waste solutions.

A revisit of the cation–cation interactions between NpO2+ and UO22+ in nitric acid medium and their impact on separation processes: spectrophotometric and solvent extraction studies

There are contradicting reports on the thermodynamics of cation-cation interactions (CCIs; inner/outer sphere) involving NpO2(+) and UO2(2+). This paper revisits CCIs of NpO2(+) (2 × 10(-4) M) under varying conditions such as reaction time, nitric acid (2 × 10(-3)-4 M HNO3)/uranium (up to 1.2 M) concentrations, and temperature (283-343 K) by spectrophotometric measurements. This study reports for the first time the appearance of a signature peak of Np(IV) (∼964 nm) in addition to NpO2(+) (980 nm) and the NpO2(+)-UO2(2+) complex (992 nm). For a pure NpO2(+) solution at 4 M HNO3, there is a gradual increase in Np(IV) peak intensity with increasing temperature and correspondingly the Np(V) peak diminishes. The CCIs are more favored at higher uranium concentrations. However, the intensity of the 992 nm peak decreases steadily with increasing temperature suggesting the exothermic nature of the complexation process. The thermodynamic data and reported structural studies indicate the formation of an inner-sphere complex under the conditions of this study. In addition, the spectral changes also suggest the formation of Np(IV) even in the presence of uranium at elevated temperatures. Solvent extraction studies using 1.1 M TBP and 1.1 M DHOA solutions in n-dodecane show that NpO2(+)-UO2(2+) complexes are extractable leaving NpO2(+) in the aqueous phase.

Sorption of metal cations on suspended bentonite: effects of pH, ionic strength and complexing anions

Abstract Batch sorption experiments have been carried out to understand the interaction of different metal cations such as Am(III), Eu(III), Sr(II), and Cs(I) with bentonite clay at varying pH (1–9). The effects of other experimental parameters such as ionic strength (0.01–1 M (NaClO4)), clay to metal ion concentration ratio, and the presence of complexing anions such as oxalic acid (ox), carbonate (CO32−), ethylenediaminetetraacetic acid (EDTA), and humic acid (HA) on Eu(III) sorption have also been investigated. The sorption of Eu(III) has been found to be invariant with the change in ionic strength suggesting inner-sphere complexation on the bentonite surface. Near quantitative sorption of Eu(III) and Am(III) has been observed in the entire pH range and there is marginal influence of the presence of 1 · 10–4 M of ox and CO32− on the sorption profile. However, the presence of 1 · 10–4 M EDTA suppresses the sorption of Eu(III) ion onto bentonite. Desorption studies of Eu(III) loaded onto bentonite using varying concentrations of HClO4 (0.01–1.0 M) solutions reveal that higher acidity favors the process. The sorption of Eu(III) on bentonite followed the Langmuir isotherm suggesting monolayer sorption process. The data fitting to D-R isotherm suggested that the Eu(III) sorption on bentonite follows ion exchange mechanism. The sorption capacity of bentonite clay was determined to be 3.8(± 0.1) × 10−4 moles/g using Langmuir and D-R isotherms.

Effect of different complexing ligands on europium uptake from aqueous phase by kaolinite: batch sorption and fluorescence studies

Clay minerals, a ubiquitous part of the geosphere, can interact with the radioactive contaminants present in aqueous media and can alter their pathways in the geochemical cycle. The final fate of these radiotoxic metal ions in the geosphere is decided not only by nearby clay minerals but also by their organic surroundings. In the present paper, Eu(III) sorption and speciation was studied on a kaolinite–water interface in the presence of complexing ligands, such as oxalic acid, citric acid and humic acid. The % sorption was found to be dependent on the ionic strength of media in the lower pH range but independent in higher pH range. This suggested that the Eu(III) sorption follows an ion-exchange (outer-sphere) mechanism up to pH ∼ 6, beyond which surface complexation (inner-sphere) is predominantly responsible for Eu(III) sorption onto kaolinite. The addition of complexing ligands modifies the sorption profiles. Fluorescence studies showed the sorption of Eu(III) as an Eu(III)–oxalate complex onto the kaolinite surface. The effect of the addition sequence of Eu(III) and humic acid on the sorption and speciation of Eu(III) onto the kaolinite surface was investigated and found to affect the sorption behaviour of Eu(III) onto the kaolinite surface. The effect of kaolinite solubility on Eu(III) sorption and desorption was also investigated.

Spectroscopic investigations on sorption of uranium onto suspended bentonite: effects of pH, ionic strength and complexing anions

Abstract Batch sorption experiments were carried out under aerobic conditions to understand the sorption behavior of U(VI) onto bentonite clay under varying pH (2–8) and ionic strength (I = 0.01 – 1 M (NaClO4)) conditions. The influences of different complexing anions (1 · 10–4 M) such as oxalic acid (ox), carbonate (CO32–), citric acid (cit), and humic acid (HA, 10 mg/L) on the sorption behavior were also investigated. The sorption of U(VI) increased with increasing pH up to pH 6 beyond which a decrease was attributed to the formation of anionic carbonate species. Marginal influence of the change in the ionic strength of the medium on the sorption profile of uranium suggested inner-sphere complexation onto the bentonite surface. The presence of humic acid showed interesting sorption profile with varying pH. Initially, there was an enhancement in the sorption with increased pH followed by a plateau and finally a decrease thereafter due to the formation of aqueous U(VI)-humate complexes. Spectroscopic studies such as UV spectrophotometry, luminescence and extended X-ray absorption fine structure (EXAFS) measurements were also performed to understand the changes in aqueous speciation of U(VI) ion. The luminescence yields of different aqueous U(VI) species followed the order: U(VI)Hydroxy > U(VI)HumicAcid > U(VI)carbonate > U(VI)citrate. The lower luminescence yield of U(VI)carbonate complex can be attributed to the strong dynamic quenching by carbonate at room temperature. The U(VI) samples shows two distinct life-time suggesting the presence of the different luminescent U(VI) species. Similar trend was observed for U(VI)-bentonite suspension in presence/absence of the complexing ligands. There was luminescence quenching for the sorbed U(VI) due to surface complexation. These observations were further supported by spectrophotometric measurements. EXAFS spectra of U(VI) samples were recorded in luminescence mode at the U L3 edge. There was shift in the absorbance edge which was attributed to decrease in electron density at U(VI) due to surface or ligand complexation. The R space spectra are mainly dominated by the back-scattering from the axial oxygens in the first shell. The inner-sphere multinuclear complex formation takes place during the U(VI) sorption onto bentonite.

Characterization of the Species Formed during the Extraction of Thorium Employing Tri-n-Butyl Phosphate and N,N-Dihexyl Octanamide as Extractants by Laser Desorption/Ionization Time-of-Flight Mass Spectrometry:

Laser desorption/ionization-time-of-flight mass spectrometry (LDI-ToF-MS) has been applied to identify and characterize the organic phase species formed during the extraction of thorium nitrate by 1.1 M tri-n-butyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) solutions in n-dodecane. The aqueous phase thorium concentrations (at 4 M HNO3) have been suitably chosen to get loaded organic phases with/without third-phase. The extracted species have been characterized for the first time using LDI-ToF-MS. The results show feasibility of the use of this technique for understanding the extraction mechanisms and third-phase formation behavior of different extractants. The different chemical species observed using this technique are consistent with those observed by small-angle neutron scattering (SANS).

Structural investigations on uranium(VI) and thorium(IV) complexation with TBP and DHOA: a spectroscopic study

Spectroscopic studies were carried out to understand the complexation of U(VI) and Th(IV) with tri-butyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) in different non-aqueous solvents. The observations from the various spectroscopic studies (FTIR, TRFS and/or UV-Vis spectroscopy) of Th(IV)–L and U(VI)–L (L: DHOA/TBP) were correlated with the EXAFS data. The combined FTIR and EXAFS studies for Th(IV)–L and U(VI)–L extracted complexes suggested strong nitrate binding, in the presence of the ligand (TBP/DHOA), in the latter case. The peak positions of the phosphoryl oxygen of TBP for Th(IV) and U(VI) binding were at 1178 cm−1 and 1184 cm−1, suggesting stronger complexation with Th(IV) as compared to U(VI). The nitrate ion binding as well as the symmetry of the nitrate ions were in conformity with the EXAFS and FTIR data. The difference in the U(VI) UV-vis and luminescence spectra of the two complexes in a given medium was explained from the symmetry of the formed complexes in the non-aqueous media. The differences in the UV-absorption spectra of the U(VI)–L complexes in n-dodecane were correlated with the difference in the relative arrangement of nitrates and the ligands’ approach in the two cases as seen by the combined study of EXAFS and FTIR. The coordination ability of DHOA was found to be superior to that of TBP for U(VI) in the studied solvents.